CYTOCHROME-P450 2A OF NASAL EPITHELIUM - REGULATION AND ROLE IN CARCINOGEN METABOLISM

In this study, we found that rat nasal coumarin-7-hydroxylase (COH) ac tivity was two orders of magnitude higher than rat hepatic COH activit y and could be induced by adding coumarin to the rats' drinking water. In western blot analysis, an anti-cytochrome P450 (Cyp) 2a-5 (mouse l iver COH) antibody recognized a sharp band in the microsomal fraction of rat nasal epithelium but not of the liver; the band comigrated with Cyp2a-5. The intensity of the band was increased by the coumarin trea tment. Similarly, in northern blot analysis, a cDNA probe specific for Cyp2a-5 recognized an mRNA in the nasal epithelium having the same si ze as mouse liver Cyp2a-5 mRNA; however, no hybridizable mRNA was reco gnized in liver preparations. Unlike the protein level, the level of t he mRNA was not increased by coumarin. When northern blot analyses wer e performed with two oligoprobes specific for rat lung CYP2A3, an mRNA of similar size to Cyp2a-5 mRNA was recognized. In immunoinhibition a nalysis, anti-Cyp2a-5 antibody inhibited rat nasal COH activity and af latoxin B-1 (AFB(1)) metabolism completely. It inhibited N-nitrosodiet hylamine (NDEA) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NN K) metabolism by 80-90%. In contrast, the hepatic metabolism of the fo ur compounds was not affected by the antibody. When coumarin instead o f anti-Cyp2a-5 antibody was used, a strong but variable inhibition of the nasal metabolism of AFB(1), NDEA, and NNK was seen. The results su ggest that an enzyme or enzymes similar to mouse liver Cyp2a-5, one of which may be CYP2A3, is expressed at high levels in rat nasal epithel ium but not in the liver and that its expression is increased by couma rin, an odorant and a substrate of Cyp2a-5. The increase probably occu rs by protein stabilization or stimulation of translation. The results also show that the enzyme has a key role in the nasal metabolism of t hree well-known carcinogens, AFB(1), NDEA, and NNK and may therefore b e an important contributing factor in nasal carcinogenesis. (C) 1995 W iley-Liss, Inc.