Effect of dietary beta-carotene on psoralen-induced phototoxicity

Effects of beta-carotene and alpha-tocopherol on photogenotoxicity induced by 8-methoxypsoralen: the role of oxygen

The protective effect of beta-carotene (beta-C) and alpha-tocopherol (alpha-T), singularly and in equimolar mixtures, toward the photomutagenicity induced by 8-methoxypsoralen (8-MOP), at different oxygen partial pressure (pO2), was evaluated in two different experimental models: Salmonella typhimurium TA102 and Saccharomyces cerevisiae D7. After phototreatment with 8-MOP, the results show a lethal effect under hypoxic conditions in both experimental model systems, an increase in revertants associated to the pO2 increase in S. typhimurium TA102, and a decrease in revertants and convertants associated to the pO2 increase in S. cerevisiae D7. In S. typhimurium TA102, in atmospheric condition, beta-C and alpha-T (1.86 or 18.6 microM) show a protective effect only at the higher dosage. Alpha-T was more protective than beta-C. The equimolar mixtures show an antimutagenic effect at both dosage used with a synergistic effect at lower dosage and an additive antimutagenic activity at higher dosage. An inhibition of the spontaneous mutagenicity by mixtures at higher dosage was also observed. The results obtained in S. typhimurium TA102 show an antimutagenic effects of beta-C, alpha-T and their mixture at 190 mmHg pO2, confirming the data obtained in air condition. At 380 mmHg pO2, alpha-T and the mixture show a significant antimutagenic activity; at 570 mmHg pO2, only alpha-T is protective. At 760 mmHg pO2, no protective effect was observed by the two antioxidants, and beta-C increases the photomutagenicity induced by 8-MOP. In S. cerevisiae D7 a protective effect was only observed at 380 mmHg pO2 with the mixture. No antigenotoxic effect was found in the other experimental conditions, even if the uptake of the two antioxidants was confirmed by HPLC. Our results underline the role of oxygen in the photomutagenicity induced by 8-MOP and in the antimutagenic activity of beta-C and alpha-T. This is the first report confirming in a cellular experimental model the data obtained in some chemical systems: the protective effect of beta-C only at low pO2 and the synergistic effect of mixture of beta-C and alpha-T.

Free radicals as carcinogens and their quenchers as anticarcinogens

An oxygen dependent signal was detected, late in the 1950s by electron spin resonance (ESR) in a saline solution of hematoporphyrin (Hp) excited by light. This signal expressed a free radical consisting of 'some kind of an association between Hp and oxygen', that Smaller et al. called 'oxyradical' (HpOO.). It soon opened a new level of understanding in carcinogenesis triggered by photodynamic substances, including Hp itself, polycyclic hydrocarbons (PCHs), as well as any carcinogen involving molecular species activated by radiation and/or metabolic reaction. Early in the 1960s, this prompted the discovery of benzo(a)pyrene (BP) photocarcinogenic enhancement (BP-PCE) in mice, probably due to an increase in free oxygen radical generation following correct light exposure. This assumption was confirmed in 1980 by the fact that mice orally loaded with antioxidants and radical quenchers, such as beta-carotene (BC) and cantaxanthin (CX), were protected against BP-PCE at 100% and against total BP carcinogenicity at more than 60%. These achievements were presented as the bases of the current explosion of interest in biology and medicine in building up the new field of chemoprevention against cancer and other chronic diseases by supplementation with antioxidant vitamins, retinoids and especially carotenoids and their synergistic association. The relevant findings of this research obtained in the last decade in in vitro and in vivo experiments as well as human interventions are reported and discussed with personal contributions.

Mechanisms of photodynamic effects of furocoumarins

The photosensitizing action of furocoumarins on biological systems occurs by both an oxygen-independent pathway, which involves the photoaddition of the sensitizer to nucleic acids, proteins and lipids, and an oxygen-dependent pathway, which includes furocoumarins in the category of photodynamic sensitizers. The photodynamic action of furocoumarins, as studied using isolated biomolecules, human erythrocytes and human skin, appears to involve both activated oxygen species (singlet oxygen, superoxide anion, and hydroxyl radicals) and radical species formed by electron transfer from or to photoexcited furocoumarins. Another oxygen-dependent process involves the formation of photo-oxidized furocoumarin derivatives, which can react in the dark with several substrates (in particular, membrane components), causing an irreversible damage of cells. The latter type of process is temperature dependent. The relative importance of the different photosensitization mechanisms under various experimental conditions is discussed.