Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 519
Assessment of
Carcinogenicity:
Generic Issues and Their
Application to Diesel
Exhaust
DAVID G. KAUFMAN
University of North Carolina
Mechanisms of Carcinogenesis Relevant to Assessment of Mobile
Source Emissions / 520
Experimental Models in Chemical Carcinogenesis / 520 Role of
DNA Replication and Repair / 522 Genetic Effects of Carcinogen
Damage to DNA / 522 Atypical Carcinogens / 524 Promotion,
Cocarcinogenesis, and Enhancement / 525 Multistep
Processes / 526 Variations in Susceptibility / 527 Metabolic
Conversion and Carcinogen Activation / 530
Qualitative Assessments of Carcinogenicity / 530
Epidemiologic Evaluation / 530 Bioassays in Experimental
Animals / 531 Short-Term Tests in Vivo and in Vitro / 533
Methods for Quantitative Extrapolations to Human Cancer
Risk / 534
Estimation of Quantitative Risk in Laboratory
Animals / 534 Extrapolations Among Species / 535 Extrapolations
Among Routes of Administration or Exposure / 535 Extrapolation
to Dose Levels of Human Exposure / 537
Experimental Evidence on Carcinogenicity of Diesel
Exhaust / 539
Short-Term Tests of Activity of Diesel Emissions / 540 Data
on Carcinogenic Activity of Diesel Exhaust Emissions / 541
Quantitative Assessment of the Cancer Risk of Diesel Exhaust in
Humans / 543
Summary / 546
Summary of Research Recommendations: Priorities, Purposes,
and Responsibilities / 547
Summary of Research Recommendations: A Research
Plan / 549
Air Pollution, the Automobile, and Public Health. (it) 1988 by the Health Effects
Institute. National Academy Press, Washington, D.C.
519
OCR for page 520
520
Assessment of Carcinogenicity
Despite our limited understanding of carci-
nogenesis, practical concerns in the "real
world" confront us with the need to assess
the potential significance of diesel exhaust
as a human carcinogen. Such an assessment
requires progressing from fragmentary the-
oretical insights into the process of carcino-
genesis to estimates of the human risk
posed by diesel exhaust. Confounding this
effort is the fact that diesel exhaust is an
imprecisely characterized and inconsis-
tently constituted product composed of
chemicals that may trigger carcinogenesis
individually, cooperatively, or even se-
quentially.
Researchers are now confronting the dif-
ficulties of understanding the etiology and
pathogenesis of multifactorial, multistep
disease processes, and they are just begin-
ning to recognize general principles that
may operate in most typical cases of cancer.
There is awareness of the relationship be-
tween the dose of carcinogens and the
resulting tumor response, and recognition
of the importance of the metabolism of a
carcinogen into reactive intermediates that
may cause damage. Cellular mechanisms
such as DNA replication may provide op-
portunities for carcinogens to transform
genetic information, and targets in DNA
may include specific genes or sites at which
chromosomes are prone to breakage. En-
hancing factors, such as promoters, may
increase the likelihood of cancer develop-
ment. Variations in human susceptibility to
cancer make evaluation of the activity of
specific carcinogens difficult, although it is
clear that certain human tissues or certain
individuals are more susceptible to cancer
than others. Certain familial tendencies or
acquired illnesses are also thought to pre-
dispose people to cancer.
In this chapter, the evidence on the car-
cinogenicity of diesel engine exhausts and
the methods used to make quantitative risk
estimates from these data are evaluated.
Specific evidence concerning carcinogen-
esis of diesel exhaust in experimental sys-
tems is reviewed, and relationships be
. . , . . .
tween t. his Information anc . reviews In
other chapters are identified. Current
knowledge as well as areas of ignorance
influence efforts to estimate human risks by
extrapolation from the experimental data
on animals. A discussion of these issues
serves as an outline for making such esti-
mates in the future.
Mechanisms of Carcinogenesis
Relevant to Assessment of
Mobile Source Emissions
Chemical carcinogenesis is a very complex
topic. Thus, this review is selective in its
consideration of carcinogenesis, focusing
on several general concepts rather than on
specific details. The constructive role of
studying cancer development in animal
models is considered, and certain aspects of
the general principles operating in most
typical cases of carcinogenesis are exam-
ined. The review also touches on unusual
cases that appear not to fit the typical
pattern of cancer development. It considers
the evidence for and the problems associ-
ated with evaluating a disease that develops
as the result of a multistep process. Finally,
the factors that define individual variations
in susceptibility are discussed, and features
of carcinogen metabolism and translocation
are reviewed.
Experimental Models in Chemical
Carcinogenesis
Experimental animal models have been
employed to reproduce tumors of the his-
tologic types and organs of origin that
commonly occur in humans. Such models
permit direct experimental study of factors
that influence the development of the most
common cancers in humans and the mech-
anisms of action of particular carcinogens.
Examples of valuable animal models and
their applications are listed in table 1. Some
unique insights have been derived from
comparisons of the properties of animal
tissues in which the tumor response is a
good model for the human disease, to
tissues of other species in which the re-
sponse is very different from that of hu-
mans.
Studies of particular tissues have been
facilitated by using organ cultures (Saff~otti
OCR for page 521
David G. Kaufman
521
Table 1. Examples of Valuable Animal Models of Human Carcinogcncsis
Rodent
Species
Type of Human
Cancer Modeled References
Application
L)osc/
response
Metabolic
mechanisms
Hormonal
influences
Dietary
influences
Hamster
Rat
Rat
Mouse
Lung
Pancreatic
Breast
Colon
Skin
Saff~otti et al. (1968)
Pour (1984)
Scarpelli ct al. (1984)
Muggins et al. (1961)
Gullino et al. (1975)
Ward et al. (1973)
Rcddy et al. (1974)
Berenblum and Shubik Promotion
(1 947)
and Harris 1979~. In this technique, pieces
of intact tissue representative of the sam-
pled organ are grown in culture. Many
features of the tissue that exist in viva,
including the interrelationship between the
epithelial components and the supportive
cells, are preserved. Such cultures can be
used to assess morphological features, mac-
romolecular synthesis, and responses to
hormones as well as capacity to metabolize
carcinogens and to repair DNA damage.
Use of organ cultures has been a principal
approach used for analysis of properties
related to carcinogenesis in human tissues.
Some of the attractive features of organ
cultures for example, their maintenance
of natural relationships between epithelial
and supportive cells are related to some of
their major shortcomings. In contrast to
cell cultures, organ cultures cannot be
propagated, and material from an individ-
ual human subject is rapidly exhausted.
Cell culture overcomes this limitation be-
cause cells may be propagated in culture.
However, the very process of propagation
exerts a selective pressure, and the cell type
that emerges may be unlike that predomi-
nating in the intact tissue. Nevertheless,
isolated cells have proven very useful in the
study of common and unique features of
carcinogenesis under far more controlled
environmental conditions than is possible
. . .
In an intact an1ma. I.
Although direct experimentation with
the objective of inducing carcinogenesis is
clearly unethical in humans, a broader,
deeper information base is needed on the
properties of human cells and tissues that
relate to carcinogenesis. This goal has been
approached by undertaking culture studies
of human cells and tissues obtained at im-
mediate autopsies or from surgical speci-
mens (Harris and Trump 1983~.
Studying the properties of human tissues
in vitro allows examination of the human
diversity in cancer development. For exam-
ple, in vitro techniques can be used to
explore the individual variability in metab-
olizing carcinogens, repairing DNA dam-
age, responding to various hormones, and
perhaps even to determine the degree to
which various nutrients serve as cofactors
. . .
In carcmogenesls.
Although in vitro carcinogenesis with
human cells in culture is rather new, trans-
formation of normal cells to neoplastic ones
has been accomplished with a number of
cell types. Results from such studies permit
the direct comparison of the stages in the
presumed multistep process of carcinogen-
esis in humans and in animals. For exam-
ple, the apparently greater difficulty in
transforming human cells than animal cells
may parallel the comparative susceptibility
to cancer of these various species. If the
determinants of the various stages in carci-
nogenesis are successfully characterized in
human cells, it may be possible to develop
improved methods for early detection of
preneoplastic or early neoplastic lesions.
Some human tissues have been main-
tained as viable xenotransplants in nude
mice (Valerio et al. 1981~. Such models are
an ethically acceptable method for in viva
study of the process of carcinogenesis in
human tissue (Shimosato et al. 1980~. This
OCR for page 522
522
Assessment of Carcinogenicity
model may provide for direct comparisons
of features of carcinogenesis between hu-
mans and experimental animals that are
commonly used in bioassays. Such infor-
mation would clearly be valuable in de-
termining the risks to humans of agents
demonstrated as carcinogenic in animal
bioassays.
Role of DNA Replication and Repair
It is a well-recognized clinical observation
that cancer typically occurs in tissues that
have a high rate of cell proliferation or in
tissues in which cell proliferation occurs in
response to injury. Conversely, cancer is
extremely rare in adult tissues or cell types
in which cell proliferation does not occur.
It was the opinion of classical pathologists
that chronic irritation or injury was the
etiologic factor for the development of
cancer. Subsequently, a variety of specific
carcinogenic etiologic agents have been rec-
ognized. Nonetheless, cell proliferation
plays a significant role in the evolution of
cancers (Grisham et al. 1983~. This is well
illustrated in the case of liver cancers in-
duced in rats by chemical carcinogens.
Typical liver carcinogens at effectiv'e doses
are also hepatotoxic, and they induce re-
storative hyperplasia to replace cells lost as
the result of the toxicity.
The influence of cell proliferation as a
contributing factor in the development of
cancer presumably results from effects on
the mitotic process and on DNA synthesis.
Replicating DNA is vulnerable for a variety
of reasons. First, replicating DNA is af-
fected to a greater extent by chemical car-
cinogens than is nonreplicating DNA
(Cordeiro-Stone et al. 1982~. Second, rep-
lication of DNA that contains carcinogen
adducts may cause incorporation of incor-
rect nucleotides at sites of altered or excised
bases. Third, some carcinogens may mod-
ify nucleotide precursors, and altered pre-
cursors may be incorporated into DNA.
Fourth, DNA replication itself occurs with
a low, but nonzero, error rate. Situations
that increase cell replication are likely to
cause mutations strictly as the result of
these errors.
Mammalian cells have a number of
mechanisms to repair DNA damage and to
reduce the likelihood of errors during
DNA replication. Treatments of cells or
animals with chemical carcinogens or radi-
ation cause the onset of DNA repair pro-
cesses. In studies in which cell proliferation
has been inhibited and DNA repair has
been allowed to remove some or most
carcinogen-induced DNA adducts, the
transforming effects of the carcinogen dam-
age have been reduced (Ikenaga and Kaku-
naga 1977~. In contrast, in patients with
defective DNA repair processes, such as the
genetically determined syndrome known as
xeroderma pigmentosum, increased inci-
dences of tumors have been observed
(Setlow 1978; Hanawalt and Sarasin 1986~.
Thus, DNA repair processes appear to be
protective against tumor development,
whereas defects of DNA repair appear to be
associated with increased risks of cancer.
There appears to be a critical interrela-
tionship between the repair and replication
of DNA as factors in the etiology of cancer
(Kakunaga 1975~. If DNA replication pro-
ceeds within a damaged region prior to
repair, there is a substantial risk of error-
making during replication, which may
cause a mutation to occur as the result of
alteration of the base sequence of the com-
plementary DNA strand. Of course, this
does not occur if the repair of the damage
precedes replication. Consequently, the re-
lationship in time of the repair and replica-
tion of DNA may be a major determinant
of the potential for the occurrence of mu-
tations and also oresumablv. of carcino-
genes~s.
, ~,,
Genetic Effects of Carcinogen
Damage to DNA
Chemical carcinogens have been shown to
produce a variety of types of DNA damage
that can lead to genetic effects on cells (table
2) (Sarma et al. 1975; Drake and Baltz 1976;
Singer and Grunberger 1983~. Point muta-
tions and frameshift mutations can alter the
regulatory or coding regions of genes. On
a larger scale, carcinogens can directly af-
fect chromatics and chromosomes (Evans
1983~. By still unknown mechanisms, car-
cinogen damage can cause the exchange of
OCR for page 523
David G. Kaufman
523
Table 2. Genetic EEects of Carcinogen
Damage to DNA
Point mutations transitions and transversions
Frameshift mutations small deletions or additions
Mutations at "hot spots"
Chromosomal breakage at "fragile sites"
Recombinations and rearrangements
Sister chromatic exchanges
Translocations of portions of chromosomes
Gene amplification
Aneuploidy
DNA segments between sister chromatics,
and chromosomal breakage that leads to
large deletions or transposition of chromo-
somal segments to other chromosomes. Pre-
sumably, such damage may lead to failures of
mitotic division with unequal distribution of
chromosomes between daughter cells, result-
ing in abnormal DNA content. DNA dam-
age is also thought to be one mechanism for
the amplification of segments of DNA.
The significance of many or all of these
forms of damage to DNA does not concern
the chemical composition of this molecule
but rather its content of genetic informa-
tion. Valuable insights about these genetic
effects, particularly with regard to onco-
genes, have arisen from recent studies in
viral carcinogenesis and molecular biology.
Investigations of the mechanism of cell
transformation by oncogenic retroviruses
have shown that their transforming genes,
designated as oncogenes, are derived from
the coding regions of cellular precursor
genes known as proto-oncogenes (Bishop
1983~. Proto-oncogenes are believed to
play an important, though as yet unknown,
role in normal cellular function or differen-
tiation because they are highly conserved in
widely divergent species from yeast to
humans. Recent studies have shown that
proto-oncogenes can acquire transforming
activity as the result of genetic alterations
that affect their DNA sequence or place
them under abnormal genetic regulation by
chromosomal rearrangements, insertion of
promoters, or gene amplification (Wein-
berg 1985; Barbacid 1986~. The number of
known retroviral oncogenes is quite lim-
ited about two dozen. Even when the
proto-oncogenes from which they are de-
rived and the closely related cellular genes
(for example, N-myc and N-ras) are added
to the sum, the total of retrovirus-related
oncogenes is still small. Although further
studies of human and animal tumors have
identified additional genes with transform-
ing activity, it is not yet possible to esti-
mate the number of cellular genes that have
transforming activity induced by genetic
alteration.
It is well known that mutations occur at
exceptionally high rates at specific sites in
DNA of viruses and other prokaryotic or-
ganisms. This nonhomogeneous effect is
recognized for spontaneous mutations as
well as mutations induced by radiation or
chemicals. The location of these so-called
"hot spots" relates to the specific form of
radiation or chemical carcinogen that in-
duces the mutations. DNA sequence as
well as the structural features of DNA,
including bending and association with
proteins, appears to influence the spectrum
of hot spots. Clearly the DNA sequence in
higher organisms such as mammals is far
less completely defined, and the means for
cataloging the spectra of hot spots in DNA
of these organisms are very limited. None-
theless, some evidence suggests that there
are sites selectively affected by carcinogens
where mutations occur at high frequency.
Fragile sites are locations in chromo-
somes that are particularly prone to break-
age. When cell growth conditions are al-
tered, such as through deprivation of
thymidine and folic acid, chromosomes
have been found to break consistently at the
same sites. These sites are closely related to
sites where chromosomal rearrangements
occur in human cancers (Yunis and Soreng
1984), suggesting that structural peculiari-
ties that make these sites prone to breakage
may be important factors in the develop-
ment of cancers. Another notable point is
the chromosomal location of these fragile
sites relative to several of the known proto-
oncogenes. Although the power of the
scientific methods used to compare the
locations of these sites is not great, the
apparent statistical relationship within the
experimental error of the methods suggests
that some very important feature of cancer
development is related to the structure of
DNA at these sites.
OCR for page 524
524
Assessment of Carcinogenicity
Techniques for identifying subregions
(bands) within chromosomes now allow
abnormal chromosomes in cancer cells to
be examined with far greater resolution and
specificity than previously possible. Sur-
veys of the chromosomal banding patterns
of a wide spectrum of cancer cells have
shown some consistent patterns of chro-
mosomal abnormalities for many different
types of cancer (Sandberg 1983; Mitelman
1986~. For some cancers- for example,
Burkitt's lymphoma there is a very high
degree of consistency in the type of alter-
ation observed. Most Burkitt's lymphomas
show balanced translocations of portions of
specific chromosomes. In other cases, such
as the development of the Philadelphia
chromosome (loss of a portion of the long
arms of chromosome 22) in chronic my-
elogenous leukemia, the appearance of the
chromosomal abnormality accompanies
the chronic phase of the disease.
Another very common feature of cancer
cells is the development of aneuploidy,
with cells having more or less than the
normal diploid number of chromosomes or
an abnormal DNA content. In fact, the
large size and the hyperchromaticity char-
acteristic of cancer cell nuclei are largely
due to the increased DNA content of typ-
ical aneuploid cells. Aneuploidy is pre-
sumed to develop as a consequence of
unequal mitotic divisions during the evolu-
tion of cancer cells. The presence of abnor-
mal mitotic figures is one feature of cancers
used to arrive at a pathological diagnosis.
One of the consequences of the abnormal
chromosomal content of cancer cells is that
particular genes are present in low or high
copy number. One can speculate how the
loss of a normal inhibiting function can
occur with the loss of a chromosome in
hypodiploid cancer cells. Hyperdiploid
cells can greatly overexpress particular gene
products, or they may generate insufficient
inhibitory activity to balance the high copy
number of some cancer-related gene.
Atypical Carcinogens
A number of substances very different from
the typical chemical carcinogens have been
shown to be carcinogenic in humans and in
experimental animals. With atypical carcin-
ogens, carcinogenesis can be induced by
physical agents and chemicals that do not
directly alter DNA. The differences be-
tween these atypical carcinogens and the
common carcinogens challenge the classical
concepts of carcinogenesis and demand the
development of theories of carcinogenesis
that can include their mode of action.
For some time, asbestos has been recog-
nized as carcinogenic, first in humans (Doll
1955) and later confirmed in experimental
animals (Wagner et al. 1973~. When di-
rectly instilled into the pleural cavity of
experimental animals it has been shown to
produce tumors like those that follow as-
bestos exposure in humans (Wagner et al.
1973~. The critical property of asbestos best
associated with carcinogenicity is the phys-
ical dimensions of fibers (Stanton and
Wrench 1972) rather than the chemical
composition of the asbestos or the sub-
stances adsorbed on it. This was confirmed
by showing that glass fibers, prepared in
length and width comparable to asbestos
fibers, were also carcinogenic.
The cellular response to asbestos fibers
and other foreign bodies involves the for-
eign-body inflammatory reaction wherein
the fibers are surrounded by macrophages
and fibroblasts (Brand et al. 1975~. Current
hypotheses suggest that the inflammatory
cells or epithelial cells produce reactive
forms of oxygen molecules which may
affect the DNA of the epithelial cells, and
this damage is fundamental to the carcino-
genic process. Others suggest that asbestos
acts by affecting chromosomal segregation
during mitosis. On a practical level, asbes-
tos is relevant to the topic of mobile source
emissions. It is known that asbestos expo-
sure is associated with mesothelial cancer in
humans. However, in individuals in whom
asbestos exposure is combined with ciga-
rette smoking, the risk of cancer is greatly
increased, and the leading type of cancer is
bronchogenic carcinoma (Selikoff et al.
1968~. It is conceivable that individuals
who have been exposed to asbestos will
represent a group at increased risk from the
combined effects of asbestos and mobile
. .
source em1sslons.
A number of studies have shown that
OCR for page 525
David G. Kaufman
525
unusual substances, functioning as atypical
carcinogens, can produce cancers in exper-
imental animals. Plastic films have been
shown to produce tumors when implanted
into animals. However, when the films
were sufficiently fenestrated, or when they
were ground to a powder, the material was
not carcinogenic. Several metals, in the
forms of ores, refinery process by-prod-
ucts, and ions and salts, have been shown to
be carcinogenic in humans or experimental
animals (International Agency for Research
on Cancer 1980~. Examples include various
forms of arsenic, chrome, and nickel.
A number of reports in recent years have
noted that chemicals, including therapeutic
agents that cause proliferation of peroxi-
somes, are carcinogenic (Ready et al.
1980~. Unlike chemicals such as phenobar-
bital, these agents appear to function as
complete carcinogens rather than just as
promoters. Investigations of examples of
this class of chemicals have shown that they
do not form adducts with DNA.
Several other chemicals and drugs are
carcinogenic in animals or humans, but are
not known to interact with or form adducts
with DNA. Among these are agents that
affect enzymes involved in the metabolism
of DNA precursors or that more directly
affect DNA precursor pools. These prop-
erties make some of these chemicals effec-
tive therapeutic agents for treating cancer.
The action of some of these agents is be-
lieved to be a consequence of imbalances in
DNA precursor pools, which also cause
mutations.
Promotion, Cocarcinogenesis, and
Enhancement
.
Exposure to carcinogens is not the only
determinant of cancer development. Other
substances or other processes can influence
the risk for cancer development, particu-
larly when they complement exposures to
carcinogens or act on animals that have a
high spontaneous tumor incidence. These
factors must be recognized when the obser-
vational data derived from carcinogenicity
tests in animals are being interpreted mech-
anistically. The terms "enhancers" and
"enhancement" describe effects that include
those typically classified as promoters or
cocarcinogens but without attribution of a
mechanism of action.
Promotion is defined operationally,
based on classical experiments in which
tumors were induced in mouse skin by a
two-step treatment protocol (Berenblum
1975~. The first treatment involved the
application of a subcarcinogenic dose of a
strong carcinogen to the mouse skin, fol-
lowed by a prolonged series of applications
of a noncarcinogenic agent. The combined
treatments produced a tumor response,
whereas the same dose of the carcinogen or
the second agent, which has come to be
known as a promoter when used alone, was
ineffective or vastly weaker. The two steps
of the treatment process were designated as
. . . . . .
nltlatlon anc . promotion, ant . have come to
be interpreted as separate events or pro-
cesses in the evolution of cancers.
In contrast to the separate application of
. . . .
initiator anc promoter, cocarclnogens are
agents that enhance the development of
cancers when administered concurrently
with a carcinogen. Cocarcinogens act
through a variety of mechanisms. They
may modify the metabolism of carcinogens
to yield a greater quantity or proportion of
ultimate carcinogenic metabolites. They
may act by causing cell or tissue toxicity
with accompanying accelerated cell prolif-
eration; this in turn may increase the risk of
malignant transformation. They may also
act by interfering with normal defense
mechanisms that function to counteract the
detrimental effects of carcinogens.
Enhancing or inhibiting effects from ex-
posure to a wide variety of substances (for
example, certain constituents of mobile
source emissions), not just exposure to car-
cinogens (such as the possibly carcinogenic
constituents of such mobile source emis-
sions), determine the tumor response. Our
understanding of these effects and the inter-
actions between substances is very limited.
More-specific enhancing effects, in some
cases affecting individual tissues, may come
from exposures to noncarcinogens that ex-
ert a promoting or cocarcinogenic effect.
Individual genetics, prior or concurrent
medical conditions, and diet all may con-
tribute to an individual's specific risk from
OCR for page 526
526
Assessment of Carcinogenicity
a given level of exposure. Such enhancers
presumably increase the effects of other
carcinogens. It is particularly important to
ascertain whether mobile source emissions
contain constituents with enhancing activ-
~ty.
Mobile source emissions may represent a
serious public health problem if they en-
hance carcinogenesis initiated by other
exposures. This is an important general
problem that requires further attention.
Different methods of bioassay from those
used to detect carcinogens will have to be
developed to determine whether these
emissions have enhancing activity. Such
methods are needed to explore the possibil-
ities that enhancing activities are specific in
augmenting the activity of particular types
of carcinogens or that their activity differs
according to tissue sites at which they act.
It is clear that standard carcinogenicity
bioassavs are not cure tests for either can
cer-~nit~at~ng activity or activity as a com-
plete carcinogen. They are phenomeno-
logic studies that associate excess cancers
with particular treatments, but they do not
indicate the mechanism by which the can-
cers are produced. Particularly in the case
of tumors in tissues with a high spontane-
ous tumor incidence in untreated animals,
increases in the incidence of tumors may
reflect a toxic or promoting activity of the
tested chemical.
If this effect is not the result of toxicity
associated with high exposure levels, then
the result may be a demonstration of pro-
motion activity. Such a conclusion might
distinguish these compounds from stan-
dard carcinogens, but it does not indicate
that they are without risk. In view of the
hazard posed by chemicals of this type, it is
important to develop methods to demon-
strate how they cause tumors. Since some
of the constituents of diesel exhaust may
also have this kind of activity, it would be
useful to have the means of identifying and
quantitating these chemicals.
Promoting or enhancing activity may
involve a number of organs and tissues
other than the skin. For this reason it will
be necessary to evaluate the differences in
enhancing effects in different tissues. For
example, 12-0-tetradecanoylphorbol-13
acetate (TPA) is a good promoter for
mouse skin, but is not a good promoter for
rat liver; conversely, phenobarbital is very
effective in rat liver, but not in skin. The
possibility also exists that enhancing activ-
ity relates to the type of initiator. The type
of initiator may determine which tissues or
organs will be sensitized to promoter or
enhancer effects. It is entirely conceivable
that the broad spectrum of chemicals in diesel
exhaust contains enhancing agents with dis-
tinctive patterns of organ selectivity.
To test these hypotheses it will be neces-
sary to examine the enhancing activity of
materials such as diesel exhaust following
an initiating treatment with any of a variety
of carcinogens with a range of organ spec-
ificities. In this manner it may be possible
to develop a standard panel of animal test
models that would have the ability to detect
and quantify promoters and enhancers that
are active in any of a number of tissues.
· Recommendation 1.
. . . .
The role of pro-
moters and enhancers In human carcinogen-
esis should be determined.
Multistep Processes
On the basis of a variety of clinical and
experimental evidence, the development of
cancer is believed to be a multistep process
(Armitage 1985~. Clinical experience has
shown that the incidence of most cancers
rises with age and most are seen to pass
through premalignant stages prior to the
development of clinically overt cancer
(Doll 1971~. The most thoroughly studied
case is that of the multistep evolution of
squamous cell carcinoma of the uterine
cervix. The validity of a multistep interpre-
tation is attested to by the fact that clinical
intervention at an early stage vastly reduces
the incidence of the overt, invasive tumors
of this type. A similar sequence of prema-
lignant lesions of the bronchial epithelium
precedes invasive lung cancer (Auerbach et
al. 1961~. One study followed uranium
miners with repetitive sputum cytologies
for many years (Schreiber et al. 1974~.
Progressive cnanges In Cyrologlc nnalngs
proceeded from squamous metaplasia
through various stages of dysplasia, in situ
.
~. . . . . .. ..
OCR for page 527
David G. Kaufman
527
. . . . .
carcinoma, and Invasive carcinoma over
the course of several years. Subsequently,
comparable sequences of epithelial lesions
have been found to precede overt cancers in
a number of sites (Farber 1984~.
Initiation and promotion in the mouse
skin bioassay is an example of carcinogen-
esis as a two-step process (Berenblum
1975~. Comparatively recent studies have
shown that the process of promotion itself
can be divided into stages (Slaga et al.
1980~. In the case of the evolution of tu-
mors of rat liver, cancer is believed to be
the end result of a process in which foci or
areas of enzyme-altered hepatocytes and
neoplastic nodules precede malignant tu-
mors (Farber 1980~. In the respiratory tract
of hamsters, carcinogen treatment has been
found to cause a progressive sequence of
histologic alterations that culminate in
invasive, malignant tumors (Saff~otti and
Kaufman 1975~. These lesions demonstrate
a spectrum of morphological changes very
close in appearance to the lesions of the
respiratory tract seen in humans. In fact,
the evolving lesions shed cells analogous to
those observed in the cytology preparations
from the uranium miners cited above
(Schreiber et al. 1974~.
Methods to study the transformation of
cells by chemical carcinogens in tissue cul-
ture have been available for about two
decades. These studies first were successful
in rodent embryo and fibroblast cells. More
recently, similar results have been achieved
using rodent epithelial cells, and in the past
few years human cells have also been trans-
formed with chemical carcinogens. A num-
ber of morphological, biological and phe-
notypic changes have often been observed
in these in vitro transformation systems as
the cells progress from the original cell
population to demonstrably malignant
cells. In some cases, for example in studies
using Syrian hamster embryo cells, a spe-
cific sequence of changes in the culture has
been linked to malignancy (Barrett and
Ts'o 1978; Smith and Sager 1982~. With
cultured rat tracheal epithelial cells, a se-
quence of progressive changes in the bio-
logical behavior of carcinogen-treated cells
has been observed (Nettesheim and Barrett
1984~. In this system, the cultured cells can
be evaluated for their relationship to the
morphological alterations observed in vivo
by allowing them to repopulate a trans-
planted rat trachea which had been deepi-
thelialized (Klein-Szanto et al. 1982~. Other
evidence of the multistep nature of trans-
formation found in this system is a two
. . . . . . .
step transformation envoy Ding an ~n~t~at~ng
carcinogen treatment followed by in vitro
promotion with TPA (Steele et al. 1980~.
Studies of the transformation of human
cells in vitro have also shown that several
distinct alterations occur consistently and in
a generally similar order (Kakunaga et al.
1983).
Variations in Susceptibility
Rates of development of spontaneous be-
nign and malignant tumors vary in animals
of different species and strains (Grasso and
Hardy 1974~. Some animals are highly re-
sistant to tumor development, and even
after a long life few will die with tumors. In
contrast, some species and strains of ani-
mals have a very high incidence of cancers,
in some cases 100 percent. In these species
and strains, the type and quantity of these
background tumors are characteristic of the
animal and are presumed not to be the
result of unusual exposures to environmen-
tal factors. Among the animals typically
chosen for carcinogenesis bioassays, mice
have an exceptionally high incidence of
liver tumors, particularly in males. In fe-
male rats, mammary tumors are very com-
mon. Treatment of these animals with
chemical carcinogens results in tumors at
various locations and of types that depend
on the activity and dose of the carcinogen
as well as the route of administration and
other factors. Commonly, these treatments
also affect the incidence and multiplicity of
the tumors characteristic of the untreated
animals, indicative of the sensitivity of
these tissues to transformation.
The human population, in comparison,
appears to have a relatively low back-
ground level of cancer, as determined from
cancer incidence data for certain low-risk
groups in underdeveloped nations or in
specific populations such as Mormons or
Seventh Day Adventists in the United
OCR for page 528
528
Assessment of Carcinogenicity
Table 3. Factors Affecting Human
Susceptibility to Carcinogenesis
E.
xposures to carcinogens
Diet composition and nutritional status
Personal habits including cigarette smoking and
alcohol consumption
Determinants of geographic variations in cancer
development
Genetic diseases or heterozygous carrier states
Acquired illnesses and infections
Unknown factors determining familial
. . .
prec .lsposltlons
Variations in metabolic activation or inactivation of
carcinogens
States in which there are religious restric-
tions on smoking or certain dietary prac-
tices. Despite the low overall cancer rates in
these groups, certain cancers are seen and
these may represent the background tu-
mors of humans. These include leukemias
and lymphomas, soft-tissue sarcomas, skin
tumors, and a low rate of tumors of several
epithelial tissues. Above this background,
several factors appear to affect the suscep-
tibility of humans to the development of
cancers (table 3~.
The incidence in humans of tumors of
various organs differs by country and even
by population group within countries. In
the United States, cancer of the lung is the
most common significant cancer in males
and females, whereas in Egypt and Japan,
cancers of the urinary bladder and stomach,
respectively, are the most common. Within
the United States, there appear to be geo-
graphic differences in incidence of tumors
of various organs (Pickle et al. 1987~.
Clearly, a large proportion of these tumors
are induced rather than spontaneous and are
~ . . . .
01 environmental orlgln.
In contrast to the general population,
there are individuals who are genetically
predisposed to the development of cancers.
Examples of genetic diseases associated
with a high incidence of cancer include
xeroderma pigmentosum, ataxia telangiec-
tasia, familial retinoblastoma, Fanconi's
anemia, Gardner's syndrome, familial
polyposis coli, and many others. Studies
have shown that for the recessively inher-
ited genetic disease ataxia telangiectasia,
close relatives who do not have the disease,
but are heterozygous carriers, also have an
elevated cancer risk (Swift et al. 1976~. In
fact, the heterozygous carriers of the ataxia
. . .
te anglectasla trait may represent up to a
few percent of the human population. At
present, the biological basis for these ge-
netic diseases and their link to cancer are
unknown. However, it is known that there
are defects of DNA repair functions, pre-
sumably different defects, for xeroderma
pigmentosum, ataxia telangiectasia, and
Fanconi's anemia. Familial retinoblastoma
has been shown to be consistent with a
deletion or mutation of chromosome 13.
Familial polyposis cold and Gardner's syn-
drome are associated with abnormalities of
cellular growth control.
These observations provide clues to pos-
sible steps in the presumed multistep pro-
cess of malignant transformation. To the
extent that carrier states for these diseases
are common in the population, these ge-
netic traits may be factors that influence
individual risk for developing cancer (Swift
et al. 19764. It is likely that other genetically
determined factors may influence cancer
risks even if they do not yield recognized
genetic diseases. For example, there may be
genetically determined influences on the
rate or route of metabolism of chemicals.
The racial differences in alcohol metabo-
lism illustrate that such differences occur in
the human population. Individual varia-
tions in other factors, such as those affecting
responses to injury, may also influence
cancer risk. Knowledge about such factors
is limited at present but may be an impor-
tant and useful area for continued research.
There are a variety of illnesses and infec-
tions that predispose people to the devel-
opment of cancer. For examples, hepatitis
B and schistosomiasis of the bladder are
important factors in the causation of liver
and bladder cancers, respectively. Certain
lymphomas are associated with infectious
diseases (for example, human T-cell lym-
photrophic virus types I and III, or Epstein-
Barr virus), and colon cancers are associ-
ated with ulcerative colitis. These diseases
and conditions cause a high level of cell
proliferation in specific target cell popula-
tions which may predispose to cancer de-
velopment in the affected tissue.
OCR for page 529
David G. Kaufman
529
Some tumors of the lung are associated
with scars of the parenchyma. There has
also been speculation that other lung con-
ditions predispose people to lung cancer
development (Kuschner 1985~. Although it
is likely that these conditions predispose
people to lung cancer because of increased
cell proliferation rates, it is also possible
that these conditions affect the capacity of
the lung to clear exogenous materials, in-
cluding potential environmental carcino-
gens.
Omitting the known genetic diseases
that predispose to cancer development, and
in the absence of acquired diseases that are
associated with cancer, there are still a
number of families with an unusually high
incidence of cancer. In most of these fami-
lies there is variable penetrance of tumor
risk with less than 100 percent incidence of
cancer in these populations. In some cases
there are distinct patterns of tumor devel-
opment with particular organs affected to
unusual extents and with different tumors
predominating in males and females. It is
unclear whether these occurrences are pri-
marily the result of unrecognized genetic
diseases or a heterozygous carrier state for a
recessive genetic disease. Alternatively,
these families may develop these cancers
because of elusive environmental factors
passed socially from generation to genera-
tion, such as diet or personal habits.
Clinical and experimental evidence sug-
gest that there are important differences in
susceptibility to cancer among individuals
in the human population. This could be a
very significant factor in efforts to control
specific types of cancer, including any re-
lated to exposure to diesel exhausts. The
population of susceptible individuals may
account for a disproportionate share of
particular types of cancer. It may be pos-
sible to significantly change the overall in-
cidence of specific types of cancer by
identifying susceptible individuals and con-
centrating cancer prevention activities on
this population. It might be possible to
identify individuals for whom specific
types of carcinogens or diesel exhaust rep-
resent a particular hazard and protect them
from such hazards.
We have limited knowledge of biological
and enzymatic factors that determine these
states of unusual susceptibility. More data
are needed about the range of variation of
metabolic processes, DNA repair pro-
cesses, constitutive and induced cell prolif-
eration rates, and responsiveness to hor-
mones in tissues from human subjects.
These data are needed for each tissue in
which cancer is common, and this informa-
tion should be obtained where possible to
determine the variability according to age,
gender, genetic background, and personal
factors such as diet, therapeutic drug use,
and personal habits (for example, cigarette
smoking). Accomplishing these goals will
require development of methods to obtain
human tissues in an acceptable manner.
Furthermore, human subjects will have to
be chosen scientifically so that they are
representative of the population as a whole
or the subpopulations that appear to be at
unusual risk. If these studies are successful,
the next step will be development of meth-
ods to test these characteristics in samples
of tissue that can be obtained from normal
individuals with little or no risk.
On the basis of epidemiologic observa-
tions that there is a range of responses
within populations apparently exposed to
the same levels of a carcinogenic substance,
it is conceivable that there are individual
factors that are major determinants of risk.
Identifying the portion of the population at
exceptional risk and concentrating protec-
tive efforts on that population might have a
major impact on the overall cancer inci-
dence at particular tissue-specific sites.
Such an approach has proven notably effec-
tive in reducing myocardial infarction rates
in individuals with genetic abnormalities of
low-density lipoprotein metabolism and in
individuals with acquired coronary artery
disease.
Developing methods to determine the
elements of individual risk will require
great attention. The development of appro-
priate and acceptable methods for obtain-
ing cells from various body sites with little
or no risk should be included in this
method. Methods to test various cellular
characteristics that have been associated
with the development of cancer will also
have to be devised. Such factors as the
OCR for page 544
544
Assessment of Carcinogenicity
culations for other compounds and the
approach used to make this assessment was
an extrapolation by comparisons and anal-
ogy (Albert et al. 1983; Lewtas et al. 1983~.
Results of skin carcinogenesis and skin ini-
tiation in SENCAR mice were compared
for diesel exhaust extracts and extracts
from gasoline engine exhaust, cigarette
smoke condensate, roofing tar vapors, and
coke oven emissions. Similarly, results of
short-term assays for genotoxicity were
compared for these same compounds.
From these data the comparative potency
of diesel exhaust was estimated on the basis
of the most active diesel extract. To relate
these values to estimates of risk for the
human population, estimates of human
lung cancer risk were made for coke oven
emissions, roofing tar, and cigarette smoke
condensate. For each of these compounds
there were epidemiologic data relating ex-
posure to human cancer and experimental
data in test systems identical to that for
diesel exhausts. The risk per unit quantity
for diesel exhausts was extrapolated by
determining the human risk on the basis of
a unit quantity of organic extractable ma-
terial. The estimated unit risk obtained for
human lung cancer was 0.02-0.60 x 10-4
(lung cancers)/,ug exhaust particulates/m3
of air.
To understand this estimate, it is impor-
tant to recognize the inherent assumptions
of the method (Lewtas et al. 1983~. The
method assumes that the relative potency
~ .
Or carcinogens in one carclnogenesls assay
is directly proportional to that in another
bioassay. Further, this comparability is as-
sumed to apply across biological systems
and species. This assumes that the bioavail-
ability of the active compounds at the tar-
get tissue is proportional even when ex-
trapolations are made between species and
between routes of exposure. As Cuddihy
and McClellan (1983) note, the estimates
derived by this method suggest that ex-
tracts of diesel exhaust particles are not
"orders of magnitude more potent than
other emissions."
This risk assessment has a number of
limitations. These studies and extrapola-
tions are not based on whole, fresh ex-
hausts; the exhausts are not acting on a
.
population exposed to a myriad of other
carcinogens and active compounds unre-
lated to diesel exhaust; and the exhausts are
acting on a homogeneous population
where genetic factors, prior illness, and
personal habits do not influence the suscep-
tibility to these insults. Also, this assess-
ment offers an estimate of risk strictly for
lung cancer, although cancers in other sites
might also be affected. The risk estimate is
also provided as specific risk per unit of
exposure. This specific risk is not very
dissimilar to those for the other materials to
which it was compared experimentally.
Thus, the comparison with the specific risk
for gasoline engines is somewhat mislead-
ing when one considers the fact that diesel
engines may generate one to two orders of
magnitude more particles than a gasoline
engine with a catalytic converter.
More recently, the comparative potency
approach has been used to assess the human
cancer risk associated with diesel exhaust in
a more comprehensive analysis that in-
cludes estimates of population exposures
(Cuddihy et al. 1984; McClellan 1986~.
Those analyses used previously reported
estimates of specific risk of lung cancer
development (lung cancers/,ug diesel par-
ticulates/m3 of air/year) (Albert et al. 1983;
Cuddihy et al. 1983; Harris 1983~. Expo-
sure estimates were based on environmen-
tal concentrations in various locations and
distribution of the population according to
concentration levels and assuming a 20
percent proportion of diesel-powered light-
duty vehicles. Analyses were also based on
the estimates of risk from epidemiologic
studies. From these data, calculated values
for excess lung cancer deaths per year
ranged from 100 to 3,500, a range attribut-
able to an increase to a 20 percent light-
duty diesel-powered fleet. As with the ear-
lier estimate noted above, there are
numerous potential sources of error in
these calculated risk values. Despite these
limitations the risk estimate offers a starting
point for determining the overall potential
for changes in the rate of cancer deaths as a
result of increasing the use of diesel engines
in the U.S. transportation fleet.
Too little generic information exists on
the carcinogenicity of the gaseous and par
OCR for page 545
David G. Kaufman
545
ticulate emissions of mobile sources. Stud-
ies have been performed on representative
emissions generated by particular sources
operating under specific conditions. Scien-
tifically, it is not clear to what extent such
results apply to different engines operating
with different fuel or other different condi-
tions. Further, it is not clear how these
. . .
results relate to the same engine operating
under other conditions or even to the same
engine operating under presumably identi-
cal conditions at a different time. Differ-
ences between individual engines or oper-
ating conditions can result in the generation
of emissions with quantitative and even
qualitative differences in the products
formed. These differences in turn can be the
major determinant of the activity of the
emission in carcinogenicity tests. This sit-
uation is quite unlike the testing of a pure
chemical, in which case there is a reason-
able assurance of repeatability upon retest-
ing.
Given that there is no standard mixture
for mobile source emissions, the question
arises whether these mixtures can be eval-
uated on the basis of quantity and activity
of certain "sentinel" constituents. If these
most active components could be moni-
tored and minimized, then optimum en-
gines and operating conditions might be
selected. Although this idea has merit as a
comparative measure, the assessment of the
actual quantitative risk at any level of these
sentinel compounds may be difficult to
determine. A further complication is the
fact that little is known about the possibil-
ity of interactions between carcinogenic
compounds. It is unlikely that the risk
associated with the mixture of sentinel
compounds is simply the sum of the effects
of the individual compounds. This uncer-
tainty is further amplified when the numer-
ous other constituents of emissions are con-
sidered as influencing the activities of the
sentinel compounds.
Further bioassays are needed to provide a
sufficient body of information about the
carcinogenicity of diesel exhausts in exper-
imental animals. In view of the variability
of diesel engine exhausts due to engine
design, conditions of operation, and the
fuel used, it is necessary to perform more
studies to evaluate the influence of these
variables on tumor responses. Exposure to
diesel alone should be complemented by
studies in which diesel exhaust exposures
follow initiating carcinogen treatment in
each of several organs or tissues. Exposures
should not be limited to a particular frac-
tion of diesel exhaust condensate or to the
particulate material, since the complete ex-
hausts may have additive or inhibitory ef-
fects that would otherwise not be detected.
~ Recommendation 6. Additional stud-
ies should be performed on the carcinoge-
nicity of diesel exhaust.
Assessment of the carcinogenicity of
mixtures poses two conflicting problems.
The first concerns the nature of mixtures
and the fact that each mixture represents a
unique case. The second concerns attempts
to evaluate mixtures by dividing them into
their constituents. In such cases it is difficult
to determine how to reconstitute the effects
of the individual constituents into the ef-
fects of the total mixture.
Mixtures such as the diverse combustion
products in mobile source emissions can be
administered to experimental animals and
tested for carcinogenicity. However, these
mixtures are not intentionally formulated
with precise analytical procedures. They
are the products of a process or source that
may not have exceptional reproducibility.
Thus, the emissions from two different
diesel engines may have quantitative differ-
ences in the products of combustion. Even
the same engine, operating under slightly
different, or even unmodified conditions,
may yield mixtures of products with some
quantitative differences. Despite the slight
quantitative differences in the various com-
bustion products, it is likely that the mix-
tures will have similar qualitative effects:
they will prove to be carcinogenic or they
will not. The magnitude of the carcino-
genic response may be affected by the ac-
tual composition of the mixtures.
Under any condition of operation or
engine design, diesel exhaust is a mixture of
many chemicals. The composition of this
mixture is highly variable and depends on
such factors as the engine, fuel, and oper
OCR for page 546
546
Assessment of Carcinogenicity
ating conditions. It is conceivable that the Summary
interaction of the components of this mix
ture is the determinant of overall carcino
genicity of the complete mixture. There
fore, the only currently valid method to
determine the carcinogenic activity of each
form of diesel exhaust is by a separate
animal bioassay.
However, animal bioassay is not practi
cal for evaluating modifications of diesel
engine design or other aspects of their
operation as they affect carcinogenicity. It
would be valuable to have some method of
estimating changes in carcinogenic activity
based on knowledge about the changes in
composition of diesel exhaust. This would
require a better understanding of the in
teractions of components of complex
mixtures in causing cancer. To learn how
constituents of mixtures interact in carcin
ogenesis, it will be necessary to determine
how carcinogenicity changes with the vari
ation of the concentration of individual
components. Choices of chemicals to study
would presumably be based on the activ
ity of the isolated compound or its rela
tive abundance in the diesel exhaust mix
ture. In addition, constituents that have
demonstrated or are suspected of enhanc
ing (or inhibiting, for that matter) the
action of carcinogens (for example, pro
moters or enhancers) will also need to be
studied.
This problem will require additional
study to evaluate the effects of variation in
the composition of diesel exhaust on the
tumorigenicity of other carcinogens; this
serves as a model of the multiple complex
exposures associated with human environ
ments and lifestyles. Optimally it would be
possible to achieve a reasonable estimate for
the complex exhaust mixture that is based
on measurements of the concentrations of a
certain small number of index compounds.
This hypothesis and experimental approach
should be tested and validated. If it is found
to predict certain levels of carcinogenic
activity, the predictions should be tested bv
performing animal bioassays.
,
· Recommendation 7. Methods should
be developed for assessing the carcinoge
. . . .
nlclty ot mixtures.
This chapter reviews information on mech-
anisms of carcinogenesis and considers fac-
tors that influence the rate of tumor forma-
tion. It also considers the criteria for
identifying a chemical qualitatively as a
carcinogen, and methods that have been
used to extrapolate from these data to
quantitative estimates of cancer risk in hu-
mans. Finally, data have been reviewed
regarding the qualitative assessment of die-
sel exhaust as a carcinogen and the extrap-
olations made using these data to estimate
human cancer risk from diesel exhaust ex-
posure.
The review of current knowledge about
. . . .
carclnogenes1s points out great advances
that have been made in our understanding
of cancer and also reflects the vast remain-
ing ignorance. Cancer development has
come to be recognized as a slowly pro-
gressing, multifactorial, multistep process.
Cell proliferation is known to have one or
several roles in the process, and abnor-
malities of the control of this process are
fundamental features of cancer cells. Fac-
tors such as chronic injury or toxicity
(for example, toxicity that is produced by
high, but nonlethal doses of administered
drugs) can result in elevated rates of cell
proliferation with the attendant increase
in cancer risk. Some chemicals are known
to act as carcinogens by direct effects on
DNA; in some cases, specific mutations
induced by chemicals have been identified.
Other genes whose effects are recognized
in their absence or altered state in certain
genetic diseases predisposing to cancer
have been localized cytogenetically, and
efforts are in progress to isolate the genes.
Studies of atypical carcinogens that do
not have a direct mutagenic effect have
suggested alternate pathways or separate
steps in the pathway to the development
of cancer. Similarly, studies of the role
of promoters in carcinogenesis have
pointed out the multistep nature of the
process, and the potential influence of
factors that may act by selecting cells
with abnormal properties. The human
population is diverse in its genetic back-
ground and its exposures to harmful
OCR for page 547
David G. Kaufman
547
materials. Many chemicals require enzy-
matic activation to become reactive ulti-
mate carcinogenic metabolites; these meta-
bolic processes may be included among the
factors influenced by genetics and environ-
mental exposures, that determine the indi-
vidual variations in susceptibility to cancer.
The list of issues about the process of
carcinogenesis considered in this section
was necessarily incomplete, limited by
space constraints rather than the exhaustion
of important facts.
The section on qualitative evaluations of
carcinogenicity and quantitative estimates
of cancer risks in humans considered the
criteria for designating a chemical as a
carcinogen and how these data are quanti-
fied and extrapolated to estimates of human
cancer risk. The section shows that there
are reasonably well-defined criteria for
judging whether a chemical is a carcinogen.
That is not to say that this evaluation is not
without problems. Weak carcinogenic re-
sponses may be difficult to distinguish from
background levels of tumors. Increased
rates of tumors may be observed as the
result of exposure to promoters of carcino-
genesis; on the basis of a positive tumor
response, the promoter is classified for-
mally as a carcinogen. However, the tumor
response produced by promoters may be
critically dependent on dose and may even
have a threshold level for activity. Conse-
quently, promoters may become classified
as carcinogens although their mode of ac-
tion may be quite different from that of
strong mutagenic carcinogens. The review
of methods for quantitative risk assessment
includes discussions of the several factors
that must be considered in extrapolating
estimates of human cancer risk. The discus-
sion of extrapolation to humans from the
species used in the carcinogenicity tests
includes consideration of differences be
. . . . .
tween species in t :le sensitivity to tumor
formation in particular organs. Also noted
are the considerations that must be given to
account for differences in dose and in the
route of exposure to a compound when
extrapolating from animal tests to estimates
of human risk.
The review of data specifically concerned
with diesel engine exhaust emissions dem
onstrates that these exhausts have biologi-
cal activity. Short-term tests have shown
that diesel engine exhausts are mutagenic
and can cause chromosomal damage. The
activity in these studies was influenced by
the source of the emissions tested, for ex-
ample, the type of engine used. A variety of
studies have evaluated the activity of diesel
engine exhaust as a complete carcinogen
and as an initiator or promoter of carcino-
genesis. Some of the studies failed to pro-
duce positive results or were equivocal.
Positive results, however, have been found
in inhalation studies and in mouse skin
painting and lung adenoma formation as-
says. The results of these studies have been
used with current, though admittedly im-
perfect, risk extrapolation methods, and
values for projected human cancers have
been calculated. The risk for diesel engine
exhaust was calculated to be comparable to
the approximate range found for other car-
cinogenic human exposures such as coke
oven emissions and roofing tar. Within
the limitations of these estimates, diesel
engine exhausts do not appear to be notably
more active than these other materials.
Review of the epidemiologic studies of the
risk of diesel engine exhausts shows that
exposure to these exhausts does not cause a
strong effect like cigarette smoking. How-
ever, because of the limitations of the stud-
ies, it is difficult to conclude conversely that
the carcinogenic activity is negligible or
absent.
Summary of Research
Recommendations: Priorities,
Purposes, and Responsibilities
Many factors must be considered in devel-
oping a research plan that sets priorities for
the pursuit of the various recommended
research goals. For example, these prior-
ities might be selected on the basis of the
unique mission of the Health Effects Insti-
tute, or they might be viewed on the basis
of the more general need for furthering our
knowledge about how to make quantitative
risk assessments. From a practical point of
view, it might be preferable to place the
OCR for page 548
548
Assessment of Carcinogenicity
highest priority on goals that will require
the longest time to accomplish or that are
not getting adequate attention and support
from other sources. It is also reasonable to
place highest priority on goals that might
significantly affect the cancer risks that
might be attributable to diesel engine ex-
haust, even without accomplishing all of
the proposed research goals.
If accomplishment of the unique mission
of the Health Effects Institute is the per-
spective from which priorities are deter-
mined, then highest priority must be given
to performing additional studies on the
carcinogenicity of diesel exhaust (Recom-
mendation 6) and developing methods for
. . . . .
assessing t he carclnogenlclty of mixtures
(Recommendation 7~. It is unlikely that
other sources or organizations will place
comparable emphasis on the direct study of
diesel exhaust as a carcinogen. The issue of
the carcinogenicity of mixtures is a more
general problem, but it is essential for the
evaluation of diesel exhaust, although only
a secondary concern in the evaluation of
many other materials.
If the view is taken that adequate assess-
ment of the hazards of diesel exhaust will
not be possible without obtaining more
knowledge about how to make quantita-
tive risk assessments in general, then prior-
ities might be set somewhat differently. In
this case, the highest priority might be
placed on evaluating the role of toxicity in
carcinogenesis (Recommendation 3) and
gathering critical data for quantitative as-
sessments (Recommendation 4~. By learn-
ing how to make quantitative risk assess-
ments that account for effects of toxicity,
and which involve extrapolations to low
doses, among routes of administration,
anc among species, In a manner more
firmly founded on scientific knowledge,
better estimates of human cancer risks
in general will become possible, and this
will benefit the assessments of diesel ex-
haust.
Another basis for setting priorities might
be consideration of practical issues. For
example, priorities could be set so that the
research goals all might be accomplished in
the shortest time. From this perspective,
highest priority might be placed on goals
that require the longest time to accomplish.
Thus, priority might be given to evaluating
the role of toxicity in carcinogenesis (Rec-
ommendation 3) Lathering critical data for
. .
~ ~ 7
quantitative assessments (Recommenda-
tion 4), and developing methods for assess-
ing the carcinogenicity of mixtures (Rec-
ommendation 7~. Each of these is a
complex problem that will require the per-
formance of long-term studies to accom-
plish, and may require several such studies
in sequence. By beginning these studies at
the earliest time and phasing in other goals
later, it might be possible to have the more
complete body of knowledge with which
to make scientific risk assessments at the
. . .
earliest time.
Another perspective is to place the high-
est priority on goals that are not receiv-
ing adequate attention and support from
other sources. It could be argued that
many or most of the Research Recommen-
dations are not being pursued with the
vigor that might be desired. The conclu-
sion from this, however, is that all of the
Recommendations should be given a high
priority. This point of view may be accu-
rate but it does not contribute to a practical
plan.
Another view might be predicated on the
idea that early availability of certain critical
knowledge might make it possible to affect
the cancer risks from diesel exhaust signif-
icantly even before all of the needed infor-
mation for scientific risk assessments has
been obtained. A possible scenerio that
might fit this perspective would place the
highest priority on developing methods to
identify individuals at high risk (Recom-
mendation 2) and developing acceptable
methods for dosimetry in humans (Recom-
mendation 5~. For example, if one could
identify the individuals who were at high
risk for the development of cancer if they
are excessively exposed to diesel engine
exhaust, then it would be possible to focus
preventive health measures on this group.
If it were possible to carry out dosimetry
on exposed individuals, then preventive
measures might be developed that would
reduce exposure and risk.
OCR for page 549
David G. Kaufman
549
Summary of Research Recommendations:
A Research Plan
From the preceding discussion, it is clear that there are many
ways to assign priorities for the pursuit of the various Research
Recommendations. The following plan considers these different
perspectives in defining a preferred set of priorities.
HIGH PRIORITY
No other organization will commit comparable attention or
resources to the study of diesel engine exhaust, and therefore this
must be done by the Health Effects Institute. One would hope that
research on the scientific problems in making critical extrapolations
in quantitative risk assessment and in validating the process would
be widely supported and actively pursued. Unfortunately, this
need has been clear for some time, yet there has been less progress
in solving this problem than might have been expected. Accom-
plishment of the following two goals will provide the most
urgently needed information to perform better assessments of the
human risks resulting from diesel exhausts.
Recommendation 4 Critical data should be gathered for quantitative assessments.
Recommendation 6 Additional studies should be performed on the carcinogenicity of
diesel exhaust.
MEDIUM PRIORITY
Development of methods for human dosimetry may benefit
from investigator-initiated basic research and even from the re
search of commercial enterprises. Therefore, the pursuit of these
goals may be given somewhat lower priority. A similar lower
priority may be given to the evaluation of the carcinogenicity of
mixtures. This problem is not unique to the assessment of diesel
exhausts and knowledge may be gained from studies supported by
other regulatory programs.
Recommendations Acceptable methods should be developed for dosimetry in
humans.
Recommendation 7 Methods should be developed for assessing the carcinogenicity of
mixtures.
LOW PRIORITY
The remaining recommendations are important but are generic
goals that would improve our general ability to make risk assess
ments. These issues touch on basic research that is being pursued in
investigator-initiated studies. Investigations of this type may also
be pursued by other agencies that are required to make risk
assessments. Thus, although these are important goals, they may
deserve lower priority in this program.
OCR for page 550
550
Assessment of Carcinogenicity
Recommendation 1 The role of promoters and enhancers in human carcinogenesis
should be determined.
Recommendation 2 Methods should be developed to identify individuals at high risk.
Recommendation 3 The role of toxicity in carcinogenesis should be evaluated.
Acknowledgments
The author thanks Dianne Shaw for excel-
lent technical editing and Brigitte Cooke
for skillful secretarial assistance.
References
diesel and related environmental emissions: in vitro
mutagenesis and oncogenic transformation, Envi-
ron. Int. 5:403-409.
Claxton, L. D. 1981. Mutagenic and carcinogenic
potency of diesel and related environmental emis-
sions: Salmonella bioassay, Environ. Int. 5:389-391.
Cordeiro-Stone, M., Topal, M. D., and Kaufman,
D. G. 1982. DNA in proximity to the site of
replication is more alkylated than other nuclear
DNA in S Phase 10T1/2 cells treated with N-
methyl-N-nitrosourea, Carcinogenesis 3:1119-1127.
Cuddihy, R. G., and McClellan, R. O. 1983. Evalu-
ating lung cancer risks from exposures to diesel
engine exhaust, Risk Anal. 3:119-123.
Cuddihy, R. G., Griffith, W. C., and McClellan,
R. O. 1984. Health risks from light-duty diesel
vehicles, Environ. Sci. Technol . 18:14A-21 A.
Curren. R. D.. Kouri. R. E.. Kim C. M.. and
Albert, R. E., Lewtas, J., Nesnow, S., Thorslund,
T. W., and Anderson, E. 1983. Comparative po-
tency method for cancer risk assessment: applica-
tion to diesel particulate emissions, Risk Anal.
3:101-117.
Armitage, P. 1985. Multistage models of carcinogen-
esis, Environ. Health Perspect. 63:195-201.
Auerbach, O., Stout, A. P., Hammond, E. C., and
Garfinkel, L. 1961. Changes in bronchial epithelium
in relation to cigarette smoking and in relation to
lung cancer, N. Engl. J. Med. 265:253-267.
Barbacid, M. 1986. Oncogenes and human cancer:
cause or consequence? Carcinogenesis 7:1037-1042.
Barrett, J. C., and Tsto, P. O. P. 1978. Evidence for
the progressive nature of neoplastic transformation
in vitro, Proc. Natl. Acad. Sci. USA 75:3297-3301.
Berenblum, I. 1975. Sequential aspects of chemical
carcinogenesis: skin. In: Cancer: A Comprehensive
Treatise (F. F. Becker, ed.), pp. 323-344, Plenum
Press, New York.
Berenblum, I., and Shubik, P. 1947. A new, quanti-
tative, approach to the study of the stages of chem-
ical carcinogenesis in the mouse's skin, Br. J. Cancer
1 :383-391.
Bishop, J. M. 1983. Cellular oncogenes and retrovi-
ruses, Ann. Rev. Biochem. 52:301-354.
Brand, K. G., Buoen, L. C., Johnson, K. H., and
Brand, I. 1975. Etiological factors, stages, and the
role of the foreign body in foreign body tumorigen-
esis: a review, Cancer Res. 35:279-286.
Cairns, T. 1980. The EDo~ study: introduction, ob-
jectives, and experimental design, J. Environ.
Pathol. Toxicol. 3:1-7.
Casto, B. C., Hatch, G. G., and Huang, S. L. 1981.
Mutagenic and carcinogenic potency of extracts of
Correspondence should be addressed to David G.
Kaufman, Department of Pathology, School of Med
icine, University of North Carolina, Brinkhous-Bul
litt Building, 228H, Chapel Hill, NC 27514.
, ~7 - - 7
Schechtman, L. M. 1981. Mutagenic and carcino-
genic potency of extracts from diesel related envi-
ronmental emissions: simultaneous morphological
transformation and mutagenesis in BALB/c 3T3
cells, Environ. Int. 5:411-415.
Doll, R. 1955. Mortality from lung cancer in asbestos
workers, Br. J. Ind. 12:81-86.
Doll, R. 1971. The age distribution of cancer: impli-
cations for models of carcinogenesis, J. Roy. Soc.
Med. 134:133-166.
Drake, J. W., and Baltz, R. H. 1976. The biochemis-
try of mutagenesis, Ann. Rev. Biochem. 45:11-37.
Evans, H. J. 1983. Effects on chromosomes of carci-
nogenic rays and chemicals, In: Chromosome Muta-
tions and Neoplasia a German, ed.), pp. 253-279,
A. R. Liss, New York.
Farber, E. 1980. The sequential analysis of liver cancer
induction, Biochim. Biophys. Acta 605:149-166.
Farber, E. 1984. Chemical carcinogenesis: a current
biological perspective, Carcinogenesis 5:1-5.
Gaylor, D. W. 1980. The EDo, study: summary and
conclusions, J. Environ. Pathol. Toxicol. 3:179-183.
Grasso, P., and Hardy, J. 1974. Strain differences in
natural incidence and response to carcinogens, In:
Mouse Hepatic Neoplasia (W. H. Butler and P. M.
Newberne, eds.), pp. 111-132, Elsevier Press,
Amsterdam.
Grisham, J. W., KauLmann, W. K., and Kaufman,
D. G. 1983. The cell cycle and chemical carcinogen-
esis, Sure. Synth. Pathol. Res. 1:49-66.
Gullino, P. M., Pettigrew, H. M., and Grantham,
F. H. 1975. N-Nitrosomethylurea as mammary
gland carcinogen in rats, J. Nat. Cancer Inst.
54:401-414.
Hall, N. E., and Wynder, E. L. 1984. Diesel exhaust
OCR for page 551
David G. Kaufman
551
exposure and lung cancer: a case-control study,
Environ. Res. 34:77-86.
Hanawalt, P. C., and Sarasin, A. 1986. Cancer-prone
hereditary diseases with DNA processing abnor-
malities, Trends Genet. 2:12~129.
Harris, C. C., and Trump, B. F. 1983. Human tissues
and cells in biomedical research, Sure. Synth. Pathol.
Res. 1:16~171.
Harris, J. E. 1983. Diesel emissions and lung cancer,
Risk Anal. 3:83-100.
Heino, M., Ketola, R., and Makela, P. 1978. Work
conditions and health of locomotive engineers. I.
Noise, vibration, thermal climate, diesel exhaust
constituents, ergonomics, Scand. J. Work Environ.
Health 4:3-14.
Heinrich, U., Peters, L., Funcke, W., Pott, F., Mohr,
U., and Stober, W. 1982. Investigation of toxic and
carcinogenic effects of diesel exhaust in long-term
inhalation exposure of rodents, In: Toxicologic Effects
of Emissions~rom Diesel Engines U Lewtas, ed.), pp.
22~242, Elsevier Science Publishing, Inc., Amster-
dam.
Heinrich, U., Pott, F., Mohr, U., and Stober, W.
1985. Experimental methods for the detection of the
carcinogenicity andfor cocarcinogenicity of inhaled
polycyclic-aromatic-hydrocarbon-containing emis-
sions, In: Carcinogenesis, A Comprehensive Survey.
Vol. 8, Cancer of the Respiratory Tract: Predisposing
Factors (M. J. Mass, D. G. Kaufman, J. M. Sieg-
fried, V. E. Steele, and S. Nesnow, eds.), Vol. 8,
pp. 131-146, Raven Press, New York.
Heinrich, U., Muhle, H., Takenaka, S., Ernst, H.,
Fuhst, R., Pott, F., Mohr, U., and Stober, W. 1986.
Chronic effects on the respiratory tract of hamsters,
mice and rats after long-term inhalation of high
concentrations of filtered and unfiltered diesel en-
gine emissions, J. Appl. Toxicol. 6:383-397.
Hueper, W. C. 1955. A Quest into the Environmental
Causes of Carcinoma of the Lung. Public Health
Monograph No. 36, U.S. Department of Health,
Education and Welfare, Public Health Service.
Huggins, C., Grand, L. C., and Brillantes, F. P. 1961.
Mammary cancer induced by a single feeding of
polynuclear hydrocarbons and its suppression, Na-
ture 189:20~207.
Ikenaga, M., and Kakunaga, T. 1977. Excision of
tnitroquinoline 1-oxide damage and transforma-
tion in mouse cells, Cancer Res. 37:3672-3678.
International Agency for Research on Cancer. 1980.
IARC Monographs on the Evaluation of the Carcino-
genic Risks of Chemicals to Humans, Vol. 23, Some
Metals and Metallic Compounds, IARC, Lyon,
France.
Kakunaga, T. 1975. The role of cell division in the
malignant transformation of mouse cells treated
with 3-methylcholanthrene, Cancer Res. 35:1637-
1642.
Kakunaga, T., Crow, J. D., Hamada, H., Hirakawa,
T., and Leavitt, J. 1983. Mechanisms of neoplastic
transformation of human cells, In: Human Carcino-
genesis (C. C. Harris and H. N. Autrup, eds.), pp.
371-399, Academic Press, New York.
Kaplan, I. 1959. Relationship of noxious gases to
carcinoma of the lung in railroad workers, J. Am.
Med. Assoc. 171 :20302043.
Klein-Szanto, A. J. P., Terzaghi, M., Mirkin, L. D.,
Nartin, D., Shiba, M. 1982. Propagation of normal
human epithelial cell populations using an in viva
culture system, Am. J. Pathol. 108:231-239.
Kotin, P., Falk, H. L., and Thomas, M. 1955. Aro-
matic hydrocarbons. III. Presence in particulate
phase of diesel-engine exhausts and the carcinoge-
nicity of exhaust extracts, AMA Arch. Ind. Hyg.
Occup. Med. 11 :113-120.
Kuschner, M. 1985. Perspective on pathologic predis-
position to lung cancer in humans, In: Carcino-
genesis, A Comprehensive Survey. Vol. 8, Cancer of the
Respiratory Tract: Predisposing Factors (M. J. Mass,
D. G. Kaufman, J. M. Siegfried, V. E. Steele, and
S. Nesnow, eds.), pp. 17-21, Raven Press, New
York.
Lewtas, J., Bradow, R. L., Jungers, R. H., Harris,
B. D., Zweidinger, R. B., Cushing, K. M., Gill,
B. E., and Albert, R. E. 1981. Mutagenic and
carcinogenic potency of extracts of diesel and re-
lated environmental emissions: study design, sam-
ple generation, collection and preparation, Environ.
Int. 5:383-387.
Lewtas, J., Nesnow, S., and Albert, R. E. 1983. A
comparative potency method for cancer risk assess-
ment: clarification of the rationale, theoretical basis
and application to diesel particulate emissions, Risk
Anal. 3:133-137.
Li, A. P., and Royer, R. E. 1982. Diesel-exhaust-
particle extract enhancement of chemical-induced
mutagenesis in cultured Chinese hamster ovary
cells: possible interaction of diesel exhaust with
environmental carcinogens, Mutat. Res. 103:349-
355.
Littlefield, N. A., Farmer, J. H., and Gaylor, D. W.
1980. Effects of dose and time in a long-term,
low-dose carcinogenic study, J. Environ. Pathol.
Toxicol . 3:17-34.
Lockard, J. M., Kaur, P., Lee-Stephens, C., Sab-
harwal, P. S., Pereira, M. A., McMillan, L., and
Mattox, J. 1982. Induction of sister-chromatic ex-
changes in human lymphocytes by extracts of par-
ticulate emissions from a diesel engine, Mutat. Res.
104:355-359.
Luepker, R. V., and Smith, M. C. 1978. Mortality in
unionized truck drivers,J. Occup. Med. 20:677-682.
Mauderly, J. L., Jones, R. K., Griff~th, W. C., Hen-
derson, R. F., and McClellan, R. O. 1987. Diesel
exhaust is a pulmonary carcinogen in rats exposed
chronically by inhalation, Fundam. Appl. Toxicol.
9:208-221.
McClellan, R. O. 1986. Health effects of diesel ex-
haust: a case study in risk assessment, Am. Ind. Hyg.
Assoc. J. 47: 1-13.
McClellan, R. O., Bice, D. E., Cuddihy, R. G.,
Gillett, N. A., Henderson, R. F., Jones, R. K.,
Mauderly, J. L., Pickrell, J. A., Shami, S. G., and
Wolff, R. K. 1986a. Health effects of diesel exhaust,
In: Aerosols: Research, Risk Assessment and Control
Strategies (S. D. Lee, T. Schneider, L. D. Grant, and
P. J. Verkerk, eds.), pp. 597-615, Lewis Publishers,
Inc., Chelsea, Mich.
OCR for page 552
552
McClellan, R. O., Mauderly, J. L., Jones, R. K.,
Henderson, R. F., and Wolff, R. K. 1986b. Lung
tumor induction in rats by chronic exposure to
diesel exhaust, Abstract of lecture presented at the
Second International Aerosol Conference, Berlin,
West Germany, September 22-26, 1986.
Menck, H. R., and Henderson, B. E. 1976. Occupa-
tional differences in rates of lung cancer, J. Occup.
Med. 18:797-801.
Merletti, F., Heseltine, E., Saracci, R., Simonato, L.,
Vainio, H., and Wilbourn, J. 1984. Target organs
for carcinogenicity of chemicals and industrial ex-
posures in humans: a review of results in the IARC
Monographs on the Evaluation of the Carcinogenic
Risk of Chemicals to Humans, Cancer Res.
44:2244-2250.
Miller, J. A. 1970. Carcinogenesis by chemicals: an
overview-G. H. A. Clowe's Memorial Lecture,
Cancer Res. 30:559-576.
Mitchell, A. D., Evans, E. L., end Jotz, M. M. 1981.
Mutagenic and carcinogenic potency of extracts of
diesel and related environmental emissions: in vitro
mutagenesis and DNA damage, Environ. Int. 5:
393-401.
Mitelman, F. 1986. Clustering of chromosomal
breakpoints in neoplasia, Cancer Genet. Cytogenet.
19:67-71.
Nachtman, J. P., Xiao-bai, X., Rappaport, S. M.,
Talcott, R. E., and Wei, E. T. 1981. Mutagenic
activity in diesel exhaust particulates, Bull. Environ.
Contam. Toxicol. 27:463-466.
Nesnow, S., Triplett, L. L., and Slaga, T. J. 1981.
Tumorigenesis of diesel exhaust, gasoline exhaust
and related emission extracts on SENCAR mouse
skin, In: Short- Term Bioassays in the Analysis of
Complex Environmental Mixtures II (M. D. Waters,
S. S. Sandhu, J. L. Huisingh, L. Claxton, and S.
Nesnow, eds. ), pp. 277-297, Plenum Publishing
Corp., New York.
Nesnow, S., Triplett, L. L., and Slaga, T. J. 1982.
Comparative tumor-initiating activity of complex
mixtures from environmental particulate emissions
on Sencar mouse skin, J. Nat. Cancer Inst. 68:829-
834.
Nesnow, S., Triplett, L. L., and Slaga, T. J. 1983a.
Mouse skin tumor initiation-promotion and com-
plete carcinogenesis bioassays: mechanisms and bi-
ological activities of emission samples, Environ.
Health Perspect. 47:255-268.
Nesnow, S., Triplett, L. L., and Slaga, T. J. 1983b.
Mouse skin carcinogenesis: application to the anal-
ysis of complex mixtures, In: Short-Term Bioassays
in the Analysis of Complex Environmental Mixtures III
(M. D. Waters, S. S. Sandhu, J. N. Lewtas, L.
Claxton, N. Chernoff, and S. Nesnow, eds.), pp.
367-390, Plenum Publishing Corp., New York.
Nettesheim, P., and Barrett, J. C. 1984. Tracheal
epithelial cell transformation: a model system for
studies on neoplastic progression, Crit. Rev. Toxi-
col. 12:215-239.
Orthoefer, J. G., Moore, W., Kraemer, D., Truman,
F., Crocker, W., and Yang, Y. Y. 1981. Carcino-
genicity of diesel exhaust as tested in strain A mice,
Environ. Int. 5:461-471.
Assessment of Carcinogenicity
Pereira, M. A., Shinozuka, H., and Lombardi, B.
1981. Test of diesel exhaust emissions in the rat liver
foci assay, Environ. Int. 5:455-458.
Pereira, M. A., McMillan, L., Kaur, P., Gulati,
D. K., Sabharwal, P. S. 1982. Effect of diesel
exhaust emissions, particulates and extract on sister
chromatic exchange in transplacentally exposed fe-
tal hamster liver, Environ. Mutagen. 4:215-220.
Pickle, L. P., Mason, T. J., Howard, N., Hoover, R.,
and Fraumeni, J. F. 1987. Atlas of U.S. Cancer
Mortality Among Whites: 195~1980, pp. 1-149, Na-
tional Institutes of Health, Bethesda, Md.
Pitts, J. N., Lokensgard, D. M., Harger, W., Fisher,
T. S., Mejia, V., Schuler,J.J., Scorziell, G. M., and
Katzenstein, Y. A. 1982. Mutagens in diesel exhaust
particulate: identification and direct activities of
6-nitrobenzota~pyrene, 9-nitroanthracene, 1-nitro-
pyrene and 5H-phenanthro [(4,5-BCD)] pyran-5-
one, Mutat. Res. 103(3-6) :241-249.
Pour, P. M. 1984. Histogenesis of exocrine pancreatic
cancer in the hamster model, Environ. Health Per-
spect. 56:229-243.
Raffle, P. A. B. 1957. The health of the worker, Br.J.
Ind. Med. 14:7~80.
Reddy, B. S., Weisburger, J. H., Narisawa, T.,
Wynder, E. L. 1974. Colon carcinogenesis in germ-
free rats with 1,2-dimethylhydrazine and N-
methyl-N'-nitro-N-nitrosoguanidine, Cancer Res.
34:2368-2372.
Reddy, J. K., Azarnoff, D. L., and Hignite, C. E.
1980. Hypolipidemic hepatic peroxi-some prolif-
erators form a novel class of chemical carcinogens,
Nature 283:397-398.
Saffiotti, U., and Harris, C. C. 1979. Carcinogenesis
studies on organ cultures of animal and human
respiratory tissue, In: Carcinogens: Identi~fcation and
Mechanisms of Action (A. C. Griff~n and C. R. Shaw,
eds.), pp. 6~82, Raven Press, New York.
Saff~otti, U., and Kaufman, D. G. 1975. Carcino-
genesis of laryngeal carcinoma, Laryngoscope 85:
454 457.
Saffiotti, U., Cefis, F., and Kolb, L. H. 1968. A
method for the experimental induction of broncho-
genic carcinoma, Cancer Res. 28:100124.
Sandberg, A. A. 1983. A chromosomal hypothesis of
oncogenesis, Cancer Genet. Cytogenet. 8:277-285.
Sarma, D. S. R., Rajalakshmi, S., and Farber, E.
1975. Chemical carcinogenesis: interactions of car-
cinogens with nucleic acids, In: Cancer, A Compre-
hensive Treatise (F. F. Becker, ed. ), Vol. I, pp.
23~287, Plenum Press, New York.
Scarpelli, D. G., Rao, M. S., and Reddy, J. K. 1984.
Studies of pancreatic carcinogenesis in different an-
imal models, Environ. Health Perspect. 56:219-227.
Schenker, M. B. 1980. Diesel exhaust an occupa-
tional carcinogen?J. Occup. Med. 22:41-46.
Schreiber, H., Saccomanno, G., Martin, D. H., and
Brennan, L. 1974. Sequential cytological changes
during development of respiratory tract tumors
induced in hamsters by benzola]pyrene-ferric ox-
ide, Cancer Res. 34:689~98.
Selikoff, I. J., Hammond, E. C., and Churg, J. 1968.
Asbestos exposure, smoking and neoplasia, J. Am.
Med. Assoc. 204: 106-112.
OCR for page 553
David G. Kaufman
553
Setlow, R. B. 1978. Repair deficient human disorders
and cancer, Nature 271:71~715.
Shimosato, Y., Kodama, T., Tamai, S., and Kameya,
T. 1980. Induction of squamous cell carcinoma in
human bronchi transplanted into nude mice, Gann
71 :402007.
Singer, B., and Grunberger, D. 1983. Molecular Biol-
ogy of Mutagens and Carcinogens, pp. 4~219, Plenum
Press, New York.
Slaga, T. J., Fischer, S. M., Nelson, K., and Gleason,
G. L. 1980. Studies on the mechanism of skin tumor
promotion: evidence for several stages in promo-
tion, Proc. Nat. Acad. Sci. USA 77:3659-3663.
Slaga, T. J., Triplett, L. L., and Nesnow, S. 1981.
Mutagenic and carcinogenic potency of extracts of
diesel and related environmental emissions: two-
stage carcinogenesis in skin tumor sensitive mice
(Sencar), Environ. Int. 5:417023.
Smith, B. L., and Sager, R. 1982. Multistep origin of
tumor-forming ability in Chinese hamster embryo
fibroblast cells, Cancer Res. 42:389-396.
Stanton, M. F., and Wrench, C. 1972. Mechanisms of
mesothelioma induction with asbestos and fibrous
glass, J. Nat. Cancer Inst. 49:797-821.
Steele, V. E., Marchok, A. C., and Nettesheim, P.
1980. Enhancement of carcinogenesis in cultured
respiratory tract epithelium by 12-O-tetradecanoyl-
phorbol-13-acetate, Int. J. Cancer 26:34~348.
Swift, M., Sholman, L., Perry, M., and Chase, C.
1976. Malignant neoplasms in the families of pa-
tients with ataxia-telangiectasia, Cancer Res. 36:
209-215.
Valerio, M. G., Fineman, E. L., Bowman, R. L.,
Harris, C. C., Stoner, G. D., Autrup, H., Trump,
B. F., McDowell, E. M., end Jones, R. T. 1981.
Long-term survival of normal adult human tissues
as xenografts in congenitally athymic nude mice, J.
Nat. Cancer Inst. 66:849-858.
Wagner, J. C., Berry, G., and Timbrell, V. 1973.
Mesotheliomata in rats after inoculation with asbes-
tos and other materials, Br. J. Cancer 28:17~185.
Ward, J. M., Yamamato, R. S., and Brown, C. A.
1973. Pathology of intestinal neoplasms and other
lesions in rats exposed to azoxymethane, J. Nat.
Cancer Inst. 51 :1029-1039.
Waxweiller, R. J., Wagner, J. K., and Archer, W. C.
1973. Mortality of potash workers, J. Occup. Med.
15:406-409.
Wegman, D. H., and Peters, J. M. 1978. Oat cell
cancer in selected occupations, J. Occup. Med.
20:79~796.
Weinberg, R. A. 1985. The action of oncogenes in the
cytoplasm and nucleus, Science 230:770-776.
Wolff, R. K., Henderson, R. F., Snipes, M. B., Sun,
J. D., Bond, J. A.; Mitchell, C. E., Mauderly, J. L.,
and McClellan, R. O. 1986. Lung retention of diesel
soot and associated organic compounds, Abstract of
lecture presented at the International Symposium
on Toxicological Effects of Emissions from Diesel
Engines, Tsukuba Science City, Japan, July 2~28,
1986.
Wong, D., Mitchell, C., Wolff, R. K., Mauderly,
J. L., and Jeffrey, A. M. 1986. Identification of
DNA damage as a result of exposure of rats to
diesel engine exhaust, Proc. Am. Assoc. Cancer Res.
27:84.
Yunis, J. J., and Soreng, A. L. 1984. Constitutive
fragile sites and cancer, Science 226:1199-1204.
OCR for page 554
Representative terms from entire chapter:
diesel engine
