Abstract

The expression and molecular regulation of the cytochrome P450IA (P450IA) gene subfamily have been examined in rat hepatic tissue after treatment with pyridine. The microsomal ethoxyresorufin O-deethylase activity, which has been shown to be specific for the P450IA subfamily, was increased approximately 2- and 3.5-fold over control values at 10 and 16 hr, respectively, after a single dose of pyridine (100 mg/kg, intraperitoneally). P450IA1 protein expression was also elevated in a time-dependent manner, with a maximal increase in P450IA1 protein being seen at approximately 16 hr after a single dose of pyridine (100 mg/kg, intraperitoneally), as detected by immunoblot analysis using a monoclonal antibody that detects both P450IA1 and P450IA2. The immunochemically detectable level of P450IA1 decreased to that of control at 48 hr after treatment. Oligonucleotide probes specific for P450IA1 and P450IA2 mRNA were used in hybridization analyses to examine mRNA levels of P450IA1 and P450IA2, respectively. The level of P450IA1 mRNA in poly(A)+ mRNA was increased approximately 3- and 2-fold at 5 and 12 hr, respectively, after a single injection of pyridine, as evidenced by both slot blot and Northern blot analyses. A lesser increase (approximately 1.5-2-fold) in P450IA2 mRNA was also seen at 5 and 12 hr after treatment. The P450IA1 and P450IA2 mRNA levels returned to control values at 48 hr after pyridine administration. These results were compared with those produced by 3-methylcholanthrene at 5 hr after treatment. A multiplex polymerase chain reaction assay was also used to monitor simultaneously the changes in P450IA1, P450IA2, and P450IIE1 mRNA levels, and the results showed induction of P450IA1, in agreement with the results of slot and Northern blot analyses. In summary, metabolic activity assays, immunochemical detection, and Northern and slot blot analyses provide evidence to support the conclusion that pyridine modulates the expression of the P450IA gene subfamily and does so by elevating P450IA1 and P450IA2 mRNAs, through either transcriptional activation or increased mRNA stabilization. These results are in sharp contrast to P450IIE1 induction by pyridine, which appears to proceed through increased translational efficiency. Thus, pyridine, which is present in tobacco and tobacco smoke, is capable of simultaneously elevating multiple forms of P450 that are active in carcinogen metabolism.