beta-Carotene and cancer chemoprevention: from epidemiological associations to cellular mechanisms of action
1998
Naves, M.M.V. | Moreno, F.S.
Numerous studies have been conducted in an attempt to clarify the relationship between beta-carotene and cancer chemoprevention, assumed to exist on the basis of an inverse association between fruit and vegetable intake, or blood levels of beta-carotene, and cancer risk, suggested in observational epidemiologic studies. The chemopreventive action of the carotenoid has been observed in both in vitro and in vivo experimental models of carcinogenesis. Several cellular and molecular mechanisms have been proposed to explain the anticarcinogenic effect of beta-carotene, with emphasis on its metabolic conversion to retinoids, its antioxidant action and, more recently, its eventual modulatory action at the gene expression level. On the other hand, in intervention trials, beta-carotene was found to be deleterious to smokers. Some cell mechanisms are indicated here to explain this adverse effect of the carotenoid, causing a probable chronic prooxidant action that potentiates the oxidative state present in the lungs of these individuals during an advanced promotional phase of the carcinogenic process. In this respect, more basic research is needed, especially using an animal model of pulmonary carcinogenesis that will permit to investigate in a distinct manner the effect of the carotenoid on the initiation, promotion and progression steps of carcinogenesis. Furthermore, beta-carotene may be effective as a chemopreventive agent against cancer if chronically present during or before the early stages of carcinogenesis, in a nonoxidative cellular environment and in physiological amounts, in combination with antioxidant substances, as it is naturally found in a diet rich in fruits and vegetables.
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