CHEMICALLY INDUCED LUNG CANCER

Cigarette smoking represents the prominent cause of lung cancer.
More than 20 lung carcinogens have been identified in cigarette
smoke, and they can act as initiators and/or promoters of
lung cancer by accelerating tumor onset and increasing tumor
multiplicity. 5 Although 85% of lung cancers are thought to be
as a result of cigarette smoking, individuals exposed to asbestos,
arsenic, nickel, radiation, and those with pulmonary fibrosis are
also at increased risk. 6 For this reason, chemically and carcinogen-
induced animal models of lung cancer have been developed.
They are used not only to identify possible molecules and/or
environmental factors able to induce lung cancer but also to
study the early stage of carcinogenesis and cancer progression.
Chemically and/or carcinogen-induced lung tumors have been
described in various species, including dogs, cats, ferrets, and
mice. 6 The susceptibility of mice to develop chemically and/or
carcinogen-induced lung cancer is strain dependent. Mouse
strains have been categorized into sensitive, intermediate, and
resistant, 7 based on the time of occurrence of lung tumors after
chemical exposure, and on the number of tumors. The A/J
mouse strain belongs to the sensitive group with at least 20 times
higher susceptibility than resistant strains like C57BL/6J and
C3H/HeJ or intermediate strains like BALB/c. The propensity
of these sensitive strains to develop lung tumors strongly correlates
with a polymorphism in the second intron of the K- ras
gene. 8 The polymorphism is a 37-nucleotide– intronic sequence
that is tandemly duplicated in resistant strains and is present as
a single copy in sensitive strains. This polymorphism seems to
confer different nuclear protein-binding abilities that may influence
gene expression. 9 In addition to the K- ras gene, the pulmonary
adenoma susceptibility 1 (Pas1) locus on chromosome 6
has been implicated in the development of lung cancer in the
A/J strain. 10 Based on the fact that both the Pas1 and K- ras
genes map on chromosome 6, 11 it has been proposed that the
Pas1 gene could be identical to the K- ras oncogene, and natural
and/or chemically induced point mutation in Pas1 gene could
lead to lung tumors similarly to what observed with the K- ras
gene. 8 When A/J mice are exposed to urethane, mutations in
either one of these polymorphic loci occur, resulting in the development
of benign lung adenoma within few months from
the exposure. 12 Some of these tumors can progress to adenocarcinomas
with histopathology similar to that seen in humans. 13
More recently, Stathopoulos and colleagues 14 have elegantly
shown that epithelial nuclear factor (NF)- B activation
facilitates urethane- induced lung carcinogenesis. In this context,
mouse strains susceptible to lung tumor formation (i.e., FVB,
BALB/c) exhibited early NF- B activation and inflammation
in the lungs after urethane treatment. In contrast, the resistant
strain C57B6 failed to activate NF- B or induce lung inflammation.
Interestingly, selective NF- B inhibition resulted in
increased apoptosis of airway epithelial cells after urethane exposure,
highly suggesting that NF- B signaling in airway epithelium
is integral to chemically induced tumorigenesis. 14
Another common example of carcinogen-induced lung
neoplasia is the classical two-step initiation/promotion model.
Injection of the initiator 3-methylcholanthrene, followed by
various exposures to the promoter butylated hydroxytoluene,
has been shown to induce adenocarcinoma in BALB/c mice
within 16 weeks from the exposure to the initiator. The first
stage in this two-stage carcinogenesis procedure (initiation)
induces an irreversible lesion in the DNA of a single cell, while
the second stage (promotion) initiates cell clonally expands.
Mice are also used to assess carcinogenic activity of various
chemicals, including benzopyrenes, metals, nitrosamines,
and polyaromatic hydrocarbons. 12 Biochemical effects of lung
chemicals can be detected within hours or days of their administration.
In strains susceptible to lung tumorigenesis, exposure
to chemicals leads to rapid formation of hyperplastic foci
in the bronchioles and alveoli. All or some of these foci then
evolve into microscopic adenomas, and few months later, some
of these adenomas display the nuclear atypia and invasiveness
of adenocarcinomas in situ. 15 In contrast, exposure of chemicals
in resistant strains only leads to the development of few
adenomas and/or papillary tumors within several months from
the original exposure.
Given the high incidence of cigarette-induced lung cancer,
many animal models have been used to determine the effect of
cigarette smoke on lung cancer incidence, formation, and progression.
3 Despite the vast research conducted, it is debatable
whether the measured response to cigarette smoke in animal
species for assessing carcinogenic potential in humans reflects
the strong epidemiological evidence in human smokers. In a
recent review article, Coggins3 points out some of the pitfalls
related to exposure of animal models to cigarette smoke. In this
context, the cigarettes used in many studies were unfiltered
and had very high yields; thus being very different from the
cigarettes commonly smoked today. In addition, whereas some
studies used nose-only smoking machines, others used wholebody
exposures. Moreover, whereas certain animal models were
exposed to single cigarettes, others were exposed to rotating
carousels. Finally, when dogs were used for studies, invasive
tracheotomy technique was used to facilitate smoke breathing.
As results, often lung necrosis and/or inflammation with no
apparent neoplasm were evident. Thus, these studies not only
do not entirely mimic the exposure of smoke as observed in
humans, but also do not recapitulate the events of lung cancer
initiation/formation. Lastly, studies performed in rats and mice
exposure for lifetime to cigarette smoke suggested that, although
both species developed alveolar epithelial hyperplasia, alveolar
adenomas, and alveolar carcinomas, the incidence of all three
were more evident in the rats. Thus, mice might not represent
the most suitable model for smoke-mediated lung cancer. 16
Mice, however, have been successfully used to study susceptibility
to lung cancer following exposure to environmental
agents such as radiation and viruses. 17,18 Although these
two nonchemical models have the advantage that they are not
strain dependent, they present the major disadvantage that
tumors develop in various organs, beside the lungs, making
the analysis of primary versus potential metastatic lung cancer
more difficult to evaluate.
The observation that sensitive mice are more susceptible
to chemically induced lung cancer became the basis for quantitative
carcinogenicity bioassay 12 and screening systems for
chemopreventive agents. 19 Cancer chemoprevention can be
defined as the use of agents able to prevent, inhibit, or reverse
the process of carcinogenesis. 20 Various anti-inflammatory
drugs such as indomethacin or aspirin have been used to lower
chemically induced lung tumor development and multiplicity,
21 and specific cyclooxygenase-2 inhibitors have been used
to reduce the growth of adenocarcinoma after treatment with
carcinogens. 15 In addition, pretreatment of mice with drugs
able to inhibit DNA methylation showed chemopreventive
efficacy in primary mouse lung tumors induced by nitrosamines
(see Chapter 7). 22 Finally, the effect of natural products
can be tested for their preventive effect on carcinogen-induced
lung cancer. Kohno and colleagues 23 identified chemopreventive
factors (i.e., beta-cryptoxanthin and hesperidin) in commercial
mandarin juice, able to suppress lung cancer initiated
with nitrosamines in A/J mice. In addition, pretreatment with
perillyl alcohol, a naturally occurring monoterpene found in
lavender, cherries, and mint, 1 week before lung tumor initiation
with nitrosamines, significantly reduced tumor incidence
and tumor multiplicity, strongly suggesting that this monoterpene
is an effective chemopreventive compound in mouse lung
tumor bioassay. 24
In summary, chemically and/or carcinogen-induced lung
cancer models offer the major advantage that the induction of
lung tumors is highly reproducible, and they can also be used
to screen potential carcinogens as well as to identify chemopreventive
agents. There are, however, disadvantages of these
models. In particular, they are time consuming, strain dependent,
lead primarily to the development of non–small cell
lung carcinoma (NSCLC), allow the detection of lung tumors
at late stage of progression, and lead to the development of
tumors with low metastatic potential. Moreover, for study related
to cigarette-smoke–induced lung cancer, mice might not
represent the best animal models available. Finally, administration
of chemicals or carcinogens can yield to various different
tumor cell types, many of which might not be directly relevant
to human lung cancer.