Lung tumors in A/J mice exposed to environmental tobacco smoke: estimated potency and implied human risk

Short-term early exposure to thirdhand cigarette smoke increases lung cancer incidence in mice

Exposure to thirdhand smoke (THS) is a recently described health concern that arises in many indoor environments. However, the carcinogenic potential of THS, a critical consideration in risk assessment, remains untested. Here we investigated the effects of short-term early exposure to THS on lung carcinogenesis in A/J mice. Forty weeks after THS exposure from 4 to 7 weeks of age, the mice had increased incidence of lung adenocarcinoma, tumor size and, multiplicity, compared with controls. In vitro studies using cultured human lung cancer cells showed that THS exposure induced DNA double-strand breaks and increased cell proliferation and colony formation. RNA sequencing analysis revealed that THS exposure induced endoplasmic reticulum stress and activated p53 signaling. Activation of the p53 pathway was confirmed by an increase in its targets p21 and BAX. These data indicate that early exposure to THS is associated with increased lung cancer risk.

Pomegranate Juice Prevents the Formation of Lung Nodules Secondary to Chronic Cigarette Smoke Exposure in an Animal Model

BACKGROUND

Cigarette smoke (CS) induces an oxidative stress, DNA damage, and lung cancer. Pomegranate juice (PJ) possess potent antioxidant activity attributed to its polyphenols. We investigated whether PJ supplementation would prevent the formation of lung nodules, attenuate mitotic activity, and reduce hypoxia-inducible factor-1α (HIF-1α) expression secondary to CS exposure in an animal model.

METHODS

Mice were divided into: Control group, CS group, CS + PJ group, and PJ-only group. CS and CS + PJ were exposed to CS, 5 days per week, for a total of 5 months. Animals were then housed for additional four months. CS + PJ and PJ groups received PJ throughout the experiment period while others received placebo. At the end of the experiment, the incidence of lung nodules was assessed by (1) histological analysis, (2) mitotic activity [measurement of PHH3 antibodies], and (3) measurement of HIF-1α expression.

RESULTS

The incidence of lung nodules was significantly increased in CS. CS exposure significantly increased PHH3 and HIF-1α expression. PJ supplementation attenuated the formation of lung nodules and reduced PHH3 and HIF-1α expression.

CONCLUSION

PJ supplementation significantly decreased the incidence of lung cancer, secondary to CS, prevented the formation of lung nodules, and reduced mitotic activity and HIF-1α expression in an animal model.

Generic Hockey-Stick Model for Estimating Benchmark Dose and Potency: Performance Relative to BMDS and Application to Anthraquinone

Benchmark Dose Model software (BMDS), developed by the U.S. Environmental Protection Agency, involves a growing suite of models and decision rules now widely applied to assess noncancer and cancer risk, yet its statistical performance has never been examined systematically. As typically applied, BMDS also ignores the possibility of reduced risk at low doses ("hormesis"). A simpler, proposed Generic Hockey-Stick (GHS) model also estimates benchmark dose and potency, and additionally characterizes and tests objectively for hormetic trend. Using 100 simulated dichotomous-data sets (5 dose groups, 50 animals/group), sampled from each of seven risk functions, GHS estimators performed about as well or better than BMDS estimators, and a surprising observation was that BMDS mis-specified all of six non-hormetic sampled risk functions most or all of the time. When applied to data on rodent tumors induced by the genotoxic chemical carcinogen anthraquinone (AQ), the GHS model yielded significantly negative estimates of net potency exhibited by the combined rodent data, suggesting that-consistent with the anti-leukemogenic properties of AQ and structurally similar quinones-environmental AQ exposures do not likely increase net cancer risk. In addition to its simplicity and flexibility, the GHS approach offers a unified, consistent approach to quantifying environmental chemical risk.

A/J Mouse As A Model For Lung Tumorigenesis Caused By Tobacco Smoke: Strengths And Weaknesses

Strain A/J mice have successfully been used to develop an animal model for tobacco smoke carcinogenesis. In 18 individual studies, reported by 4 different laboratories, a significant increase in lung tumor multiplicities following exposure from 50 to 170mg/m3 of total suspended tobacco smoke particulates was found in 15 studies (83 %) and a significant increase in lung tumor incidence in 10 studies (56%). However, tumor multiplicities are comparatively low (from an average of 1.1 to 2.8 tumors per lung). From a toxicological standpoint, this indicates that cigarette smoke is a weak animal carcinogen. Although the assay allowed one to detect substantial chemopreventive activity of a mixture of myo-inositol and dexamethasone, it was less successful in showing efficacy for several other agents.

Alveolar macrophage recruitment and activation by chronic second hand smoke exposure in mice

Approximately 15% of cases of COPD occur in non-smokers. Among the potential risk factors for COPD in non-smokers is second-hand smoke (SHS) exposure. However, the Surgeon General reported in 2006 that the evidence linking second hand smoke and COPD is insufficient to infer a causal relationship, largely because current evidence does not establish a biological link. The goal of this study was to determine whether SHS exposure can induce alveolar macrophage recruitment and expression of activation markers that we have previously demonstrated in human smokers and in mouse models of emphysema. To achieve these goals, we studied mice exposed to an ambient mixture of predominantly [89%] sidestream smoke at increasing doses over 3 months. We found that second hand smoke exposure induced a dose-dependent increase in alveolar macrophage recruitment (mean +/- sd; 224,511 +/- 52,330 vs 166,152 +/- 47,989 macrophages/ml of bronchoalveolar lavage in smoke-exposed vs air-exposed controls at 3 months, p = 0.003). We also found increased expression of several markers of alveolar macrophage activation (PLA2g7, dkfzp434l142, Trem-2, and pirin, all p < 0.01 at 3 months) and increased lavage levels of two inflammatory mediators associated with COPD (CCL2 [MCP-1], 58 +/- 12 vs. 43 +/- 22 pg/ml, p = 0.03; and TNFalpha, 138 +/- 43 vs 88 +/- 78 pg/ml, p = 0.04 at 3 months). These findings indicate that second smoke exposure can cause macrophage recruitment and activation, providing a biological link between second-hand smoke exposure and the development of inflammatory processes linked to COPD.

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.

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.

Chronic Oral Exposure to Inorganic Arsenate Interferes with Methylation Status of p16INK4a and RASSF1A and Induces Lung Cancer in A/J Mice

Although inorganic arsenate (iAs(V)) or arsenite (iAs(III)) is clearly a human carcinogen, it has been difficult to produce tumors in rodents. In the present study, we orally administered iAs(V) to A/J mice to examine arsenic carcinogenicity in rodent. A/J mice (male, n = 120) assigned to four groups were given drinking water containing 0, 1, 10, and 100 ppm iAs(V) for 18 months. At the end of experiment, the complete lungs were removed and used for examining histopathology and extracting RNA and DNA. Epigenetic effects of iAs(V) on DNA methylation patterns of p16INK4a and RASSF1A genes were determined by methylation-specific polymerase chain reaction. Changes of p16INK4a and RASSF1A at mRNA and protein levels were examined by reverse transcriptase-polymerase chain reaction and immunohistochemistry. Arsenic was accumulated dose dependently in the lung tissues of iAs(V)-exposed mice. Increase in lung tumor number and lung tumor size was observed in iAs(V)-exposed mice compared to the control. Histopathological examination showed that the rate of poorly differentiated lung adenocarcinoma was much higher in iAs(V)-exposed mice than in the control. Methylation rates appeared to be higher in a dose-related tendency in lung tumors from iAs(V)-exposed mice compared to the control. Lower or loss of p16INK4a and RASSF1A expression was found in lung tumors from iAs(V)-exposed mice, compared to that in nontumor lung tissues from both control and iAs(V)-exposed mice, and this reduced or lost expression was in accordance with hypermethylation of the genes. In conclusion, iAs(V) exposure increased lung tumor incidence and multiplicity in A/J mice. Epigenetic changes of tumor suppressor genes such as p16INK4a and RASSF1A are involved in the iAs(V)-induced lung carcinogenesis.

Pulmonary prostacyclin synthase overexpression chemoprevents tobacco smoke lung carcinogenesis in mice

Increased pulmonary production of prostaglandin I2 (prostacyclin) by lung-specific overexpression of prostacyclin synthase decreases lung tumor incidence and multiplicity in chemically induced murine lung cancer models. We hypothesized that pulmonary prostacyclin synthase overexpression would prevent lung carcinogenesis in tobacco-smoke exposed mice. Murine exposure to tobacco smoke is an established model of inducing pulmonary adenocarcinomas and allows for the testing of potential chemopreventive strategies. Transgenic FVB/N mice with lung-specific prostacyclin synthase overexpression were exposed to mainstream cigarette smoke for 22 weeks and then held unexposed for an additional 20 weeks. All of the exposed animals developed bronchiolitis analogous to the respiratory bronchiolitis seen in human smokers. The transgenic mice, when compared with smoke-exposed transgene negative littermates, had significant decreases in tumor incidence and multiplicity. Significantly fewer transgenics (6 of 15; 40%) developed tumors compared with the tumor incidence in wild-type littermates (16 of 19; 84%; Fisher's exact test, P = 0.012). Tumor multiplicity was also significantly decreased in the transgenic animals (tg+ = 0.4 +/- 0.5 versus wild-type = 1.2 +/- 0.86 tumors/mouse; P < 0.001). Targeted prostaglandin levels at the time of sacrifice revealed significantly elevated prostaglandin I2 levels in the transgenic animals, coupled with significantly decreased prostaglandin E2 levels. Gene expression analysis of isolated type II pneumocytes suggests potential explanations for the observed chemoprevention, with Western blot analysis confirming decreased expression of cytochrome p450 2e1. These studies extend our previous studies and demonstrate that manipulation of prostaglandin production distal to cyclooxygenase significantly reduces lung carcinogenesis in a tobacco smoke exposure model, and gene expression studies show critical alterations in antioxidation, immune response, and cytokine pathways.