Lung cancer is the leading cause of cancer death in the United States. The majority of lung cancer cases occur in smokers, in whom emphysema frequently develops as a component of chronic obstructive pulmonary disease (COPD). Being a smoker and having emphysema are both risk factors for lung cancer. Nicotine, a large constituent of cigarette smoke, may stimulate the promotion of lung carcinogenesis but the data remain inconsistent. Nicotine and the tobacco carcinogen 4-methylnitrosamino-1-[3-pyridyl]-1-butanone (NNK) can exert their carcinogenicity by binding to the homopentameric α7 nicotinic acetylcholine receptor (α7 nAChR) thereby activating downstream signals important for lung cancer cell proliferation, survival, and metastasis. Nicotine exposure to rats during gestation and lactation induces emphysema development in the offspring. Therefore, we hypothesized that nicotine could induce emphysema development, which would greatly augment lung tumor multiplicity and volume in the NNK-treated mouse lung tumor model. Recently, sirtuin 1 (SIRT1), the closest homolog of yeast Sir2 gene, has been implicated in the underlying pathogenesis of both emphysema and lung carcinogenesis. Thus SIRT1 may be a promising molecular target for the prevention of emphysema and lung cancer.
Despite an inconsistent association between the reduced risk of lung cancer and β-carotene supplementation in smokers, a pooled analysis from seven cohort studies in North America and Europe has identified that β-cryptoxanthin (BCX, a pro-vitamin A carotenoid rich in pumpkin, citrus fruits, papaya, and sweet red peppers) was the only carotenoid for which intake was associated with a lower risk of lung cancer in current smokers. However, the inhibitory effect of BCX on lung tumor formation and promotion has never been studied. We hypothesized that BCX is an effective chemopreventive agent against emphysema and lung tumor development by targeting the α7 nAChR and SIRT1 for its chemopreventive action.
In the first part of the study, we examined the effects of BCX supplementation at 1 and 10 mg kg diet−1 for 18 weeks on the development of spontaneous lung tumors and on NNK-initiated lung tumors (the initiation stage of carcinogenesis) in male A/J mice carrying KRAS mutations. There was a dose-dependent increase of BCX concentrations in the serum without a significant alteration of retinol concentrations. In mice that were not injected with NNK, BCX supplementation did not affect the spontaneous lung tumor multiplicity. Interestingly, compared to the NNK-treated mice, there was about a 52–63% significant reduction of lung tumor multiplicity in the NNK-treated mice supplemented with BCX. The preventive effects of BCX were associated with reduced expression levels of α7 nAChR and transcription factor early growth response-1 (egr-1), and decreased phosphorylation levels of AKT (serine 473) and Bad (serine 136) in the lungs.
In the second part of the study, nicotine injection at 1 mg kg bw −1 three times weekly for 10 weeks greatly increased both emphysema and lung tumor development in the NNK-treated male A/J mice, as compared to the mice treated with NNK alone. This dose of nicotine is comparable to that in nicotine replacement therapy products (e.g. gums, inhalers, lozenges). These nicotine-promoted effects were strongly associated with reduced protein levels of SIRT1, decreased mRNA levels of p53 and RAR-β, increased mRNA levels of IL-6 and phosphorylation levels of AKT, but not with an alteration of the levels of α7 nAChR expression. These changes were also accompanied by the alterations of emphysema-related markers including cyclin D1, p50, MMP-2, elastin, and egr-1.
In the third part of the study, we examined the effects of BCX supplementation at 10 and 20 mg kg diet−1 for halting the nicotine-promoted emphysema and lung tumor development (the promotion stage of carcinogenesis) in the NNK-treated male A/J mice. Compared to the mice treated with both NNK and nicotine, there was roughly an 86–91% reduction of lung tumor multiplicity in mice supplemented with BCX. BCX supplementation also reduced the lung tumor volume and emphysema development in the NNK and nicotine treated mice. These protective effects were associated with reversal of the reduced protein levels of SIRT1, the decreased mRNA levels of p53 and RAR-β, the increased mRNA levels of IL-6 and phosphorylation levels of AKT, and the alterations of the emphysema-related markers.
This thesis provides the first in vivo experimental evidence supporting BCX as an effective chemopreventive agent against tobacco smoke components-related lung tumor development and emphysema. The thesis reports the possible mechanistic basis of BCX chemopreventive action, which involves the down-regulation of α7-nAChR and the up-regulation of SIRT1 expression levels. In addition, the protective effects of BCX were likely to be independent of BCX's vitamin A activity. In conclusion, the study demonstrates the unique potential of BCX against the progression of smoking-related emphysema and lung cancer, which makes a significant contribution towards finding a dietary preventive agent for emphysema and lung cancer.
|Commitee:||Ausman, Lyne M., Choi, Sang-Woon|
|School:||Tufts University, Gerald J. and Dorothy R. Friedman School of Nutrition Science and Policy|
|Department:||Biochemical and Molecular Nutrition|
|School Location:||United States -- Massachusetts|
|Source:||DAI-B 73/08(E), Dissertation Abstracts International|
|Subjects:||Health sciences, Nutrition|
|Keywords:||Beta-cryptoxanthin, Carotenoids, Emphysema, Lung tumor, Mouse, Prevention|
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