Enhanced Expression of Rat Hepatic CYP3A1 and 3A2 by Pyridine (PY)
Cibor, Gregory M.
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Cytochrome P450s (CYPs) are monoxygenases, primarily found in hepatic microsomes, which play major roles in the metabolism of numerous xenobiotics and physiological-occurring compounds. However, it has been reported that elevated levels of CYPs may predispose cells to carcinogenic events, cell death or cell mutation. In particular, CYP3A1 and 3A2 are responsible for the metabolism of a variety of hormones, antibiotics and procarcinogens. Consequently, elevated levels of CYP3A 1/3A2 may initiate carcinogenesis, disrupt hormonal balance andlor render oral contraceptives ineffective. Pyridine (PY) is a constituent of tobacco and tobacco smoke and the PY moiety is present in several therapeutic agents. Previous studies reported that PY induced levels of CYP 1 A 1/2, 28112 and 2E 1. The effects of pyridine on CYP3A 1 and 3A2 expression in rat liver have been examined by enzymatic activity assays and immunoblot analyses. The molecular basis of induction was characterized by Northern blot hybridization analyses using oligonucleotide probes specific for 3A1 or 3A2 mRNA. Erythromycin N-demethylase activity was elevated -4-fold over controls folIowing treatment with 400 mg PY /kg/day for 3 days. CYP3A 1/3A2 protein levels were elevated -3-fold over controls following treatment with 400 mg PY/kg/day for 3 days. Dexamethasone (5 mg/kg/day for 4 days) and phenobarbital (100 mg/kg/day for 3 days) treatments elevated both erythromycin N-demethylase activity and 3A 1/3A2 protein levels 2- and 3-fold, respectively. Northern blot analysis revealed that 3A 1 poly(A)+ RNA levels rapidly increased following treatment with high-dose PY (200 mg/kg/day for 3 days), whereas the 3A2 poly(A)+ RNA levels only slightly increased following treatment with high-dose PY. Phenobarbital treatment (100 mg/kg/day for 3 days) induced both 3A 1 and 3A2 poly(A)+ RNA expression, whereas dexamethasone treatment (5 mg/kg/day for 4 days) induced 3A 1 poly(A)+ RNA expression only. These results suggest that 3A 1 and 3A2 induction by PY occurs primarily as a result of increased mRNA levels. Supported by NIH grant ES05577 (H. K.), CA16954 (P. F. H.) and ES03656 (R. F. N.).