The enigma of beta-carotene in carcinogenesis: what can be learned from animal studies.
Beta-carotene and other carotenoids have been thought to have anti-cancer activity, either because of antioxidant activity or because of their ability to be converted to vitamin A. Nevertheless, two large scale intervention studies in humans using high doses of beta-carotene found that beta-carotene supplementation resulted in more lung cancer rather than less lung cancer among smoking and asbestos exposed populations. Studies conducted in the ferret have elucidated molecular mechanisms behind this observation, in that high-dose beta-carotene and smoke exposure in these animals leads to squamous metaplasia, a pre-cancerous lesion in the lung. High dose beta-carotene in the smoke exposed animals was found to give rise to a number of transient oxidative metabolites, which include P450 enzymes that result in the destruction of retinoic acid, and diminished retinoid signaling, and enhanced cell proliferation. In addition, eccentric cleavage beta-carotene metabolites facilitate the binding of smoke derived carcinogens to DNA. In other ferret studies low dose beta-carotene smoke exposure provided mild protection against squamous metaplasia. Thus, it appears that the explanation of the apparent paradoxical effects of beta-carotene on lung cancer is related to dose. The metabolism and breakdown of natural products should be thoroughly investigated in animal models before embarking on large scale intervention trials, particularly when using unusually high doses that greatly exceed normal dietary levels.
Russell
Robert M
RM
Jean Mayer US Department of Agriculture Human Nutrition Research Center on Aging, Tufts University, Boston, MA 02111, USA. rob.russell@tufts.edu
D000818:Animals; D001194:Asbestos; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D019587:Dietary Supplements; D004305:Dose-Response Relationship, Drug; D005289:Ferrets; D006801:Humans; D008168:Lung / Q000473:pathology; D008175:Lung Neoplasms / Q000473:pathology; D009369:Neoplasms; D012907:Smoking; D014212:Tretinoin / Q000378:metabolism; D019207:beta Carotene / Q000008:administration & dosage
D019207:beta Carotene; D001194:Asbestos; D014212:Tretinoin; D003577:Cytochrome P-450 Enzyme System