Does beta-carotene increase, rather than decrease, human lung cancer rates? A large body of observational epidemiologic study has demonstrated that individuals who eat more fruits and vegetables rich in carotenoids and/or who have higher levels of serum beta-carotene have a lower risk of cancer, particularly lung cancer. This inverse relationship has been particularly strong in lung cancer patients with a history of heavy smoking. However, there is contradictory evidence from recent human intervention studies using beta-carotene supplements (20-30 mg per day). An increase in risk of lung cancer among smokers who took beta-carotene supplements was reported in the Alpha Tocopherol, Beta-carotene Cancer Prevention (ATBC) Trial and among smokers and asbestos-exposed workers in the Beta-Carotene and Retinol Efficiency Trial (CARET), but not among male physicians in the United States in the Physicians Health Study (only 11% of whom were current smokers). Whether there is a true hazard associated with beta-carotene has been evaluated in control studies using the ferret. This animal mimics the human tissue metabolism of beta-carotene, and has been used for studies of tobacco smoking and inhalation toxicology. In the first study, ferrets were given a high-dose beta-carotene supplement equivalent to 30 mg per day in humans, and exposed cigarette smoke or both for six months. A strong proliferative response in lung tissue and squamous metaplasia were observed in all beta-carotene-supplemented animals, and this response was enhanced by exposure to tobacco smoke. When compared to the control group, beta-carotene-supplemented animals (with or without smoke exposure) had statistically significantly lower concentrations of retinoic acid in lung tissue, and they exhibited reductions in RAR-beta gene expression (a tumor suppressor gene). Further, ferrets given a high-dose beta-carotene supplement and exposed to tobacco smoke had fourfold elevated expressions of c-jun and c-fos genes. In a second study, ferrets were given either physiological- or pharmacologic-dose beta-carotene supplementations, which were equivalent to 6 mg vs. 30 mg per day in humans, respectively. The animals were exposed to cigarette smoke for six months. The retinoic acid concentration and RAR beta-gene expression were reduced in the lung tissues, whereas the expression of AP1, cyclin D1, and proliferative cell nuclear antigen were greater in the high-dose, beta-carotene-supplemented animals with or without smoke, as well as the smoke-exposed, low-dose, beta-carotene-supplemented animals-but not in the low-dose, beta-carotene-supplemented animals alone, as compared with the control group. Squamous metaplasia was only observed in the lung tissues of high-dose, beta-carotene exposed groups with or without smoke (but not the low-dose beta-carotene plus smoke group, the low-dose beta-carotene-supplemented group, or the control group). These data show that in contrast with the pharmacologic dose of beta-carotene, a physiologic dose of beta-carotene in smoke-exposed ferrets has no detrimental effect-and, in fact, may afford weak protection against lung damage induced by cigarette smoke. Further studies from our laboratory have revealed an instability of the beta-carotene molecule in the lungs of cigarette smoke-exposed ferrets. Oxidized beta-carotene metabolites may play a role in lung carcinogenesis: by inducing carcinogen-bioactivatin enzymes, facilitating the binding of metabolites of benz[a]pyrene to DNA, enhancing retinoic acid metabolism by P450 enzyme induction with subsequent down-regulation of RAR-beta, and acting as pro-oxidants, causing damage to DNA. Ferret studies under highly controlled experimental conditions using high- and low-dose beta-carotene in the presence of alpha tocopherol and ascorbic acid (thereby stabilizing the beta-carotene molecule) showed protective effects against smoke-induced lung squamous metaplasia in ferrets.
