Mechanisms Involved in A/J Mouse Lung Tumorigenesis Induced by Inhalation of an Environmental Tobacco Smoke Surrogate

Reduced Chronic Toxicity and Carcinogenicity in A/J Mice in Response to Life-Time Exposure to Aerosol From a Heated Tobacco Product Compared With Cigarette Smoke

We conducted an inhalation study, in accordance with Organisation for Economic Co-operation and Development Test Guideline 453, exposing A/J mice to tobacco heating system (THS) 2.2 aerosol or 3R4F reference cigarette smoke (CS) for up to 18 months to evaluate chronic toxicity and carcinogenicity. All exposed mice showed lower thymus and spleen weight, blood lymphocyte counts, and serum lipid concentrations than sham mice, most likely because of stress and/or nicotine effects. Unlike THS 2.2 aerosol-exposed mice, CS-exposed mice showed increased heart weight, changes in red blood cell profiles and serum liver function parameters. Similarly, increased pulmonary inflammation, altered lung function, and emphysematous changes were observed only in CS-exposed mice. Histopathological changes in other respiratory tract organs were significantly lower in the THS 2.2 aerosol-exposed groups than in the CS-exposed group. Chronic exposure to THS 2.2 aerosol also did not increase the incidence or multiplicity of bronchioloalveolar adenomas or carcinomas relative to sham, whereas CS exposure did. Male THS 2.2 aerosol-exposed mice had a lower survival rate than sham mice, related to an increased incidence of urogenital issues that appears to be related to congenital factors rather than test item exposure. The lower impact of THS 2.2 aerosol exposure on tumor development and chronic toxicity is consistent with the significantly reduced levels of harmful and potentially harmful constituents in THS 2.2 aerosol relative to CS. The totality of the evidence from this study further supports the risk reduction potential of THS 2.2 for lung diseases in comparison with cigarettes.

Tobacco Smoke–Induced Lung Cancer in Animals—A Challenge to Toxicology (?)

Tobacco smoke is a known human carcinogen that primarily produces malignant lesions in the respiratory tract, although it also affects multiple other sites. A reliable and practical animal model of tobacco smoke–induced lung cancer would be helpful for in studies of product modification and chemoprevention. Over the years, many attempts to reproduce lung cancer in experimental animals exposed to tobacco smoke have been made, most often with negative or only marginally positive results. In hamsters, malignant lesions have been produced in the larynx, but not in the deeper lung. Female rats and female B6C3F1 mice, when exposed over lifetime to tobacco smoke, develop tumors in the nasal passages and also in the lung. Contrary to what is seen in human lung cancers, most rodent tumors are located peripherally and only about half of them show frank malignant features. Distant metastases are extremely rare. Male and female strain A mice exposed to 5 months to tobacco smoke and then kept for another 4 months in air respond to tobacco smoke with increased lung tumor multiplicities. However, the increase over background levels is comparatively small, making it difficult to detect significant differences when the effects of chemopreventive agents are evaluated. On the other hand, biomarkers of exposure and of effect as well as evaluation of putative carcinogenic mechanisms in rats and mice exposed to tobacco smoke allow detection of early events and their modification by different smoke types or chemopreventive agents. The challenge will be to make such data broadly acceptable and accepted in lieu of having to do more and more long term studies involving larger and larger number of animals.

An Updated Review of Inhalation Studies with Cigarette Smoke in Laboratory Animals

Until recently, the published literature on inhalation studies with laboratory animals and cigarette smoke consisted entirely of negative findings, as far as neoplastic disease is concerned. This paper brings readers up to date, with analyses of recent studies that do indeed appear to report success after so many years of failure. The paper consists of a brief analysis of the literature up until a couple of years ago, giving brief, representative examples of inhalation studies with the five main species of laboratory animals that have been used: rat, mouse, hamster, dog, and nonhuman primate. A brief examination of the various technologies used to expose laboratory animals is given, along with an analysis of the histopathology and related toxicology data (specifically, biomarkers of exposure) that have been reported. The paper concludes by briefly mentioning the most recent studies, where positive results have been reported.

Smoking and Lung Cancer: Future Research Directions

The aim of future research in this area is to provide the mechanistic understanding and the tools for effective prevention, early diagnosis, and therapy of lung cancer. With the established causal link between cigarette smoking and the risk of developing lung cancer, the most effective prevention is certainly not to smoke. A much better mechanistic understanding of lung cancer and its variability will support the development and evaluation of potentially reduced risk products for those who maintain smoking as well as for the development of early diagnostic tools and targeted therapies. Because of the complexity of lung cancer and the long duration for its development, nonclinical and clinical research efforts need to complement each other. Recent promising advances in this research area are the understanding of the interaction between genotoxic and epigenetic effects of smoking, the development of laboratory animal models for lung tumorigenesis by smoke inhalation, the unraveling of molecular pathways and signatures in clinical lung cancer research useful for developing diagnostic tools and therapeutic approaches, and the first successful therapy for lung cancer—although less suitable for smokers. The above—in combination with emerging data sets from explorative non-clinical and clinical studies as well as improved modeling approaches—are setting the stage for accelerated progress towards developing successful early diagnostic tools and therapies as well as for the assessment of new consumer products with potentially reduced risk.

Systems toxicology approaches enable mechanistic comparison of spontaneous and cigarette smoke-related lung tumor development in the A/J mouse model

The A/J mouse is highly susceptible to lung tumor induction and has been widely used as a screening model in carcinogenicity testing and chemoprevention studies. However, the A/J mouse model has several disadvantages. Most notably, it develops lung tumors spontaneously. Moreover, there is a considerable gap in our understanding of the underlying mechanisms of pulmonary chemical carcinogenesis in the A/J mouse. Therefore, we examined the differences between spontaneous and cigarette smoke-related lung tumors in the A/J mouse model using mRNA and microRNA (miRNA) profiling. Male A/J mice were exposed whole-body to mainstream cigarette smoke (MS) for 18 months. Gene expression interaction term analysis of lung tumors and surrounding non-tumorous parenchyma samples from animals that were exposed to either 300 mg/m³ MS or sham-exposed to fresh air indicated significant differential expression of 296 genes. Ingenuity Pathway Analysis^® (IPA^®) indicated an overall suppression of the humoral immune response, which was accompanied by a disruption of sphingolipid and glycosaminoglycan metabolism and a deregulation of potentially oncogenic miRNA in tumors of MS-exposed A/J mice. Thus, we propose that MS exposure leads to severe perturbations in pathways essential for tumor recognition by the immune system, thereby potentiating the ability of tumor cells to escape from immune surveillance. Further, exposure to MS appeared to affect expression of miRNA, which have previously been implicated in carcinogenesis and are thought to contribute to tumor progression. Finally, we identified a 50-gene expression signature and show its utility in distinguishing between cigarette smoke-related and spontaneous lung tumors.

Lung inflammatory effects, tumorigenesis, and emphysema development in a long-term inhalation study with cigarette mainstream smoke in mice

Cigarette smoking is the leading cause of lung cancer and chronic obstructive pulmonary disease, yet there is little mechanistic information available in the literature. To improve this, laboratory models for cigarette mainstream smoke (MS) inhalation-induced chronic disease development are needed. The current study investigated the effects of exposing male A/J mice to MS (6h/day, 5 days/week at 150 and 300 mg total particulate matter per cubic meter) for 2.5, 5, 10, and 18 months in selected combinations with postinhalation periods of 0, 4, 8, and 13 months. Histopathological examination of step-serial sections of the lungs revealed nodular hyperplasia of the alveolar epithelium and bronchioloalveolar adenoma and adenocarcinoma. At 18 months, lung tumors were found to be enhanced concentration dependently (up to threefold beyond sham exposure), irrespective of whether MS inhalation had been performed for the complete study duration or was interrupted after 5 or 10 months and followed by postinhalation periods. Morphometric analysis revealed an increase in the extent of emphysematous changes after 5 months of MS inhalation, which did not significantly change over the following 13 months of study duration, irrespective of whether MS exposure was continued or not. These changes were found to be accompanied by a complex pattern of transient and sustained pulmonary inflammatory changes that may contribute to the observed pathogeneses. Data from this study suggest that the A/J mouse model holds considerable promise as a relevant model for investigating smoking-related emphysema and adenocarcinoma development.

Differential carcinogenicity of cigarette smoke in mice exposed either transplacentally, early in life or in adulthood

Cigarette smoke (CS) plays a dominant role in the epidemiology of human cancer. However, it is difficult to reproduce its carcinogenicity in laboratory animals. Recently, we showed that CS becomes a potent carcinogen in mice when exposure starts soon after birth. In our study, we comparatively evaluated the carcinogenic response to mainstream CS in mice at different ages. Neonatal mice were exposed daily for 4 months to CS, starting within 12 hr after birth, and sacrificed at 8 months. Adult mice were exposed for the same time period (3-7 months) and sacrificed at 11 months. Other mice were exposed transplacentally or both transplacentally and early in life. A total of 351 neonatal mice and 80 adult Swiss H mice were used. With varying intensity depending on age, CS induced pulmonary emphysema, bronchial and alveolar epithelial hyperplasia, blood vessel proliferation and hemangiomas and microadenomas in lung as well as parenchymal degeneration of liver. Histopathological alterations of kidney were only observed in mice exposed to CS early in life. Lung adenomas and malignant tumors of various histopathological nature were detected in neonatally exposed mice but not in adults. Transplacental CS induced the formation of lung adenomas in the offspring 8 months after birth. Previous exposure during pregnancy attenuated CS-related alveolar epithelial hyperplasia induced after birth. In conclusion, the carcinogenic response to CS varies depending on the developmental stage. The early postnatal life and the prenatal life are particularly at risk for the later development of CS-related tumors.

A further review of inhalation studies with cigarette smoke and lung cancer in experimental animals, including transgenic mice

CONTEXT

The lack of an effective animal model for pulmonary carcinogenesis in smokers is a continuing problem for researchers trying to design Potentially Reduced Risk Products for those smokers who are either unwilling or unable to quit smoking. The major failing of inhalation assays with cigarette smoke in laboratory animals is that these assays produce only small percentages of animals with pulmonary tumors (e.g. adenomas, with the occasional adenocarcinoma), as opposed to the highly invasive carcinomas (e.g. small cell and squamous cell) seen in smokers.

OBJECTIVE

To update previous reviews on animal models, and to add different types of transgenic (Tg) mice to the review.

METHODS

Reviews were made of articles retrieved from PubMed and elsewhere.

RESULTS

The addition of Tg mice to the arsenal of tests used for the evaluation of the carcinogenic potential of cigarettes did not result in any better understanding of the inability of such testing to reflect the epidemiological evidence for lung cancer in smokers.

CONCLUSION

As in previous reviews on the subject, the best assay providing support for the epidemiology data is still the 5-month whole-body exposure of male A/J mice to a combination of mainstream/sidestream smoke, followed by a 4-month recovery.

Murine lung tumor response after exposure to cigarette mainstream smoke or its particulate and gas/vapor phase fractions

Knowledge on mechanisms of smoking-induced tumorigenesis and on active smoke constituents may improve the development and evaluation of chemopreventive and therapeutic interventions, early diagnostic markers, and new and potentially reduced-risk tobacco products. A suitable laboratory animal disease model of mainstream cigarette smoke inhalation is needed for this purpose. In order to develop such a model, A/J and Swiss SWR/J mouse strains, with a genetic susceptibility to developing lung adenocarcinoma, were whole-body exposed to diluted cigarette mainstream smoke at 0, 120, and 240 mg total particulate matter per m(3) for 6h per day, 5 days per week. Mainstream smoke is the smoke actively inhaled by the smoker. For etiological reasons, parallel exposures to whole smoke fractions (enriched for particulate or gas/vapor phase) were performed at the higher concentration level. After 5 months of smoke inhalation and an additional 4-month post-inhalation period, both mouse strains responded similarly: no increase in lung tumor multiplicity was seen at the end of the inhalation period; however, there was a concentration-dependent tumorigenic response at the end of the post-inhalation period (up to 2-fold beyond control) in mice exposed to the whole smoke or the particulate phase. Tumors were characterized mainly as pulmonary adenomas. At the end of the inhalation period, epithelial hyperplasia, atrophy, and metaplasia were found in the nasal passages and larynx, and cellular and molecular markers of inflammation were found in the bronchoalveolar lavage fluid. These inflammatory effects were mostly resolved by the end of the post-inhalation period. In summary, these mouse strains responded to mainstream smoke inhalation with enhanced pulmonary adenoma formation. The major tumorigenic potency resided in the particulate phase, which is contrary to the findings published for environmental tobacco smoke surrogate inhalation in these mouse models.

Effects of cigarette smoke on the activation of oxidative stress-related transcription factors in female A/J mouse lung

Cigarette smoke contains a high concentration of free radicals and induces oxidative stress in the lung and other tissues. Several transcription factors are known to be activated by oxidative stress, including nuclear factor-kappaB (NF-kappaB), activator protein-1 (AP-1), and hypoxia-inducible factor (HIF). Studies were therefore undertaken to examine whether cigarette smoke could activate these transcription factors, as well as other transcription factors that may be important in lung carcinogenesis. Female A/J mice were exposed to cigarette smoke for 2, 5, 10, 15, 20, 42, or 56 d (6 hr/d, 5 d/wk). Cigarette smoke did not increase NF-kappaB activation at any of these times, but NF-kappaB DNA binding activity was lower after 15 d and 56 d of smoke exposure. The DNA binding activity of AP-1 was lower after 10 d and 56 d but was not changed after 42 d of smoke exposure. The DNA binding activity of HIF was quantitatively increased after 42 d of smoke exposure but decreased after 56 d. Whether the activation of other transcription factors in the lung could be altered after exposure to cigarette smoke was subsequently examined. The DNA binding activities of FoxF2, myc-CF1, RORE, and p53 were examined after 10 d of smoke exposure. The DNA binding activities of FoxF2 and p53 were quantitatively increased, but those of myc-CF1 and RORE were unaffected. These studies show that cigarette smoke exposure leads to quantitative increases in DNA binding activities of FoxF2 and p53, while the activations of NF-kappaB, AP-1, and HIF are largely unaffected or reduced.

Dose-dependent inhibition of tobacco smoke carcinogen-induced lung tumorigenesis in A/J mice by indole-3-carbinol

Recently, we reported inhibition of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) plus benzo(a)pyrene (BaP)-induced lung tumorigenesis in A/J mice by indole-3-carbinol (I3C; 112 micromol/g diet) administered beginning at 50% in the carcinogen treatment phase. In this study, we examined the dose-dependent and postcarcinogen tumor-inhibitory activities of I3C. A mixture of NNK plus BaP (2 micromol each) administered by gavage as eight biweekly doses caused 21.1 +/- 5.2 lung tumors per mouse. Carcinogen-treated mice given diets containing I3C at 1, 10, 30, 71, and 112 micromol/g, beginning at 50% in the carcinogen treatment phase, had 17.9 +/- 6.1, 10.4 +/- 3.7, 9.8 +/- 5.1, 5.2 +/- 4.0, and 2.5 +/- 2.4 lung tumors per mouse, corresponding to reductions by 15%, 51%, 53%, 75%, and 88%, respectively. All reductions, except at the lowest dose level (1 micromol I3C/g diet), were significant (P < 0.001). Similarly, administration of I3C (112 micromol/g diet) beginning 1 week after the last dose of the carcinogen significantly reduced NNK plus BaP-induced lung tumor multiplicity to 5.6 +/- 3.5, corresponding to a reduction by 74%. Analyses of cell proliferation and apoptosis markers revealed that I3C reduced the number of Ki-67-positive cells and expression of proliferating cell nuclear antigen, phospho-Akt, and phospho-BAD and increased cleavage of poly(ADP-ribose) polymerase, suggesting that the lung tumor inhibitory effects of I3C were mediated, at least partly, through inhibition of cell proliferation and induction of apoptosis. These results clearly show the efficacy of I3C in the prevention of tobacco carcinogen-induced lung tumorigenesis in A/J mice and provide a basis for future evaluation of this compound in clinical trials as a chemopreventive agent for current and former smokers.

Preneoplastic and neoplastic lesions in the lung, liver and urinary tract of mice exposed to environmental cigarette smoke and UV light since birth

It is difficult to reproduce the carcinogenicity of cigarette smoke (CS) in animal models. Recently, we showed that exposure of mice to mainstream CS (MCS) for 120 days, starting immediately after birth, resulted in an early and potent carcinogenic response. In parallel, we implemented studies evaluating intermediate biomarkers and tumors in mice exposed to environmental CS (ECS). To this purpose, we used 263 newborn CD-1 mice born from 27 dams. The whole-body exposure to ECS for 120 days, starting within 12 hr after birth, resulted in an early appearance of preneoplastic lesions in lung, which however tended to attenuate after discontinuing exposure. When the experiment was stopped, after 330 days, the number of lung adenomas was higher in ECS-exposed mice as compared to sham-exposed mice, but such increase was statistically significant only in mice co-exposed to smoke and halogen light mimicking solar irradiation. Moreover, exposure to ECS produced extensive histopathological changes, mainly parenchymatous degeneration, in liver. The alterations produced in both lung and liver require that exposure to ECS starts immediately after birth, no effect being observed when exposure started 8 days later. In contrast, induction by ECS of alterations in the urinary tract, such as microadenomas and adenomas in renal pelvis and kidney, papillary hyperplasia of urothelium, and urinary bladder papillomas, were unrelated to the exposure time after birth. The results obtained with ECS cannot be directly compared to those previously obtained with MCS, since the latter involved shorter daily exposures to more massive CS doses.

Combinations of N-Acetyl-S-(N-2-Phenethylthiocarbamoyl)-L-Cysteine and myo-inositol inhibit tobacco carcinogen-induced lung adenocarcinoma in mice

We have previously generated convincing evidence that combinations of N-acetyl-S-(N-2-phenethylthiocarbamoyl)-L-cysteine (PEITC-NAC; 3 micromol/g diet) and myo-inositol (MI; 56 micromol/g diet) were significantly more effective than the individual compounds as inhibitors of tobacco smoke carcinogen-induced lung tumorigenesis in A/J mice. In this study, we further investigated the efficacy of combinations of PEITC-NAC (9 or 15 micromol/g diet) and MI (56 micromol/g diet). Female A/J mice were treated with a mixture of the tobacco smoke carcinogens 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and benzo[a]pyrene by gavage once weekly for 8 weeks. PEITC-NAC plus MI was given in the diet beginning at 1 day after the 4th of eight carcinogen treatments (temporal sequence A) or 1 week after the last carcinogen treatment (temporal sequence B). Regardless of the dose of carcinogen or PEITC-NAC plus MI, or temporal sequence, administration of PEITC-NAC plus MI significantly reduced the multiplicity of gross tumors and, in most instances, adenocarcinoma. PEITC-NAC plus MI was particularly effective against bigger tumors. The observed inhibition of lung tumorigenesis by PEITC-NAC plus MI was attributed, at least partly, to inhibition of cell proliferation and induction of apoptosis. These results clearly show the efficacy of PEITC-NAC plus MI in the prevention of tobacco carcinogen-induced lung adenocarcinoma in A/J mice and provide a basis for future evaluation of PEITC-NAC plus MI in clinical trials as a chemopreventive agent for current and former smokers.

Indole-3-carbinol inhibits 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone plus benzo(a)pyrene-induced lung tumorigenesis in A/J mice and modulates carcinogen-induced alterations in protein levels

We tested the chemopreventive efficacy of indole-3-carbinol (I3C), a constituent of Brassica vegetables, and its major condensation product, 3,3'-diindolylmethane (DIM), against lung tumorigenesis induced by a mixture of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and benzo[a]pyrene (BaP) in A/J mice. The mixture of NNK plus BaP (2 micromol each) was administered by gavage as eight weekly doses, whereas I3C (112 micromol/g diet) and DIM (2 and 30 micromol/g diet in experiments 1 and 2, respectively) were given in the diet for 23 weeks beginning at 50% of carcinogen treatment. I3C reduced NNK plus BaP-induced tumor multiplicity by 78% in experiment 1 and 86% in experiment 2; the respective reductions in tumor multiplicity by DIM were 5% and 66%. Using a quantitative proteomics method, isobaric tags for relative and absolute quantitation (iTRAQ) coupled with mass spectrometry, we identified and quantified at least 250 proteins in lung tissues. Of these proteins, nine showed differences in relative abundance in lung tissues of carcinogen-treated versus untreated mice: fatty acid synthase, transketolase, pulmonary surfactant-associated protein C (SP-C), L-plastin, annexin A1, and haptoglobin increased, whereas transferrin, alpha-1-antitrypsin, and apolipoprotein A-1 decreased. Supplementation of the diet of carcinogen-treated mice with I3C reduced the level of SP-C, L-plastin, annexin A1, and haptoglobin to that of untreated controls. These results were verified using immunoblotting. We show here that tumor-associated signature proteins are increased during NNK plus BaP-induced lung carcinogenesis, and I3C inhibits this effect, suggesting that the lung tumor chemopreventive activity of I3C might be related to modulation of carcinogen-induced alterations in protein levels.

Histological alterations in male A/J mice following nose-only exposure to tobacco smoke

The incidence and multiplicity of grossly observed and microscopic lesions of the respiratory tract of A/J mice exposed nose-only to mainstream smoke (50, 200, or 400 mg total particulate matter/m3 from 2R4F cigarettes) was compared to those of filtered air controls. Animals were necropsied at the end of exposure (5 mo) or following 4 or 7 mo of recovery. Lungs were visually inspected for tumors at all necropsies and examined histopathologically at 9 and 12 mo. At 5 mo no tumors were recorded. No significant elevations in tumor incidence or multiplicity were recorded although at 9 mo multiplicity was elevated in the mid-exposure group (0.90 versus 0.55 tumors per animal for controls). At 12 mo, multiplicity was increased over the 9-mo necropsy at all exposures except 200 mg/m3; however, there were no dose-related trends in multiplicity or incidence. Histopathological alterations included hyperplasia, metaplasia, and inflammation of the nose and larynx and proliferative lesions of the lungs. At 9 mo, the multiplicity of focal lung lesions was 1.4 per animal in controls but averaged 1.0 among smoke-exposed groups. There was an inverse relation (p < .059) between smoke concentration and the percentage of hyperplastic lesions at 9 mo. At 12 mo the high-exposure group had slightly increased multiplicity of 2.3 lesions compared with 1.6 among controls, while the percentage of hyperplasic lesions was similar between groups. Nose-only inhalation of mainstream tobacco smoke resulted in chronic inflammatory changes of the respiratory tract yet failed to produce statistically significant changes in tumor incidence or multiplicity.

Sidestream cigarette smoke toxicity increases with aging and exposure duration

OBJECTIVES

To determine the effects of aging on the toxicity of sidestream tobacco smoke, the complex chemical mixture that enters the air from the lit end of burning cigarettes and constitutes the vast bulk of secondhand smoke.

DESIGN

Statistical analysis of data from controlled experimental exposures of Sprague Dawley rats to fresh and aged (for more than 30 minutes) sidestream smoke for up to 90 days followed by histological sectioning of the respiratory epithelium. The data were obtained from a series of experiments conducted at Philip Morris' formerly secret INBIFO (Institut für Biologische Forschung) laboratory in Germany.

RESULTS

Using total particulate material as the measure of smoke exposure, aging sidestream cigarette smoke for at least 30 minutes increases its toxicity fourfold for 21 day exposures and doubles the toxicity for 90 day exposures, relative to fresh sidestream smoke.

CONCLUSIONS

These results help explain the relatively large biological effects of secondhand smoke compared to equivalent mass doses of mainstream smoke.

Lung tumors in 2 year old strain A/J mice exposed for 6 months to tobacco smoke

Young adult strain A/J mice were exposed for 6 months in a whole-body inhalation chamber to a mixture of 89% sidestream and 11% mainstream cigarette smoke generated from Kentucky 1R4F research cigarettes. Chamber concentrations of smoke constituents were 158mg/m(3) of total suspended particulate matter (TSP). After an additional 4 months in air, some of the animals were killed. Lung tumor multiplicities in the smoke exposed animals were 1.8+/-0.2 versus 0.9+/-0.2 in controls. In animals kept beyond the age of 12 months, lung tumor multiplicities increased in both groups, but remained at all times twice the control values in the smoke exposed animals compared to controls (4.3+/-0.7 vs. 2.1+/-0.5 tumors per lung in 24 months old animals). Histopathology showed that, in 2 year old animals, still about 80% of tumors were of benign nature. No tumors were found in the nasal passages. It was concluded that tobacco smoke exposure not simply accelerates the development of lesions that eventually would have developed spontaneously, but induced de novo formation of lung tumors in A/J mice.

The complexities of an apparently simple lung tumor model: The A/J mouse

A simple animal model of tobacco smoke carcinogenesis works as follows: Strain A/J mice are exposed for 5 months to tobacco smoke. They are then given a 4-month recovery period in air before being killed. Lung surface tumors are counted and lung tumor multiplicity (average number of tumors per lung, including non-tumor bearing animals) is calculated. Results obtained in four different laboratories during the past 8 years have consistently shown significant increases in lung tumor multiplicities in tobacco smoke exposed animals. While inhaling to tobacco smoke, strain A mice (but not some other strains) fail to gain weight and immediately after smoke exposure only have about 75% of control weight; however, when removed into air, they regain weight rapidly up to control levels. The counting of surface tumors only may occasionally underestimate total number of lung tumors and thus yield false negatives. At the end of the experiment, the mice are 1-year old and about 80% of the tumors are adenomas, the remainder adenomas with carcinomatous foci or adenocarcinomas. Tobacco smoke does not increase the percentage of adenocarcinomas. Studies with filtered tobacco smoke have suggested that benzo(a)pyrene or tobacco smoke-specific nitrosamines cannot account for lung carcinogenesis in mice; the most likely single agent to cause lung tumors is 1,3-butadiene. A major disadvantage of the assay is its low statistical power. While it is easy to detect a 70-100% decrease in lung tumor multiplicity caused by a chemopreventive agent using group sizes of 20-30 animals, the detection of smaller reductions (20-50%) would require group sizes in the hundreds. From all available evidence it must be concluded that the complex mixture of tobacco smoke, a known human carcinogen, is a rather weak rodent carcinogen.