Interactions of beta-carotene with cigarette smoke in vitro

Abstract

Recent intervention trials reported that smokers given dietary β-carotene supplementation were at increased risk of lung cancer and overall mortality. These results were unexpected based on previous observational epidemiology, which suggested β-carotene would decrease the onset of lung cancer in these individuals. The mechanism by which β-carotene supplementation results in increased lung cancer among smokers has yet to be defined. However, one proposed explanation is that β-carotene acts as a prooxidant in lungs exposed to cigarette smoke and exacerbates oxidative damage. This project has examined the consequences resulting from interactions of cigarette smoke with β-carotene in model systems. A novel β-carotene oxidation product, 4-nitro-β-carotene has been identified from smoke oxidation of beta-carotene in solution. This product has been isolated by both reverse-phase and cyano-column HPLC, and characterized by UV-vis spectroscopy, APCI-LC-MS, and NMR spectrometry. This class of products includes cis and all-trans isomers of 4-nitro-β-carotene. A new method for measuring primary products of lipid peroxidation has been developed. This method is specific and sensitive for the determination of 9- and 13-hydroxy fatty acid methyl esters of octadecadienoic acid as pentafluorobenzoyl esters by GC-ECD. These compounds are derived from the first stable products of linoleic acid oxidation. Using the method discussed above, the interactions between β-carotene and cigarette smoke were examined in a liposomal model system. DLPC liposomes showed little differences in oxidative damage with or without β-carotene incorporation. This result was consistent with gas-phase or whole smoke exposures. The effect of β-carotene on the oxidation of other antioxidants was also examined. Both the lipid soluble antioxidant α-tocopherol and the water soluble antioxidant ascorbate showed lower oxidation rates due to smoke exposure in the presence of β-carotene than without. These data indicate that β-carotene does not have prooxidant effects in this system. β-carotene is oxidized by cigarette smoke in model systems. 4-Nitro-β-carotene is one of several classes of products resulting from cigarette smoke oxidation of β-carotene. β-Carotene incorporation did not increase oxidation of lipid or other antioxidants upon smoke exposure. It is unlikely a prooxidant effect of β-carotene is responsible for increased lung cancer observed in recent intervention trials.