The induction of cytochrome P4502E1 by nitrogen- and sulfur-containing heterocycles: expression and molecular regulation.

Several structurally related sulfur- and nitrogen-containing heterocycles including thiazole, pyrazine, pyridazine, pyrimidine, thiophene, and triazole, which are present in tobacco, tobacco smoke, and certain foods, have been employed with the goal of characterizing the effects of these agents on the inhibition and expression of P4502E1 in hepatic tissue and on the molecular level regulatory events governing enhanced expression. The results of this study reveal that whereas the binding constants of these compounds to 2E1 moderately correlated with the percentage inhibition of metabolic activity in vitro (r = 0.66), neither inhibition of metabolic activity nor binding to P4502E1 correlated with relative induction of P4502E1 levels (r = 0.07 and 0.03, respectively). Thiazole, which produced the greatest inhibition of metabolic activity (88%) and exhibited the highest binding affinity for P4502E1 (35 microM), induced P4502E1 approximately fourfold. In contrast, pyrazine and pyridazine, which only marginally inhibited metabolic activity (54 and 41%, respectively), and weakly bound 2E1 (73 and 384 microM, respectively), increased P4502E1 levels approximately four- and fivefold, respectively. A common feature associated with these inducers, however, was the substantial decrease in hepatic P4502E1 poly(A)+ RNA levels in treated animals relative to untreated animals. Slot and Northern blot hybridization analyses revealed an approximately 80% decrease in P4502E1 poly(A)+ RNA levels at 48 hr following treatment of rats with thiazole, and at 24 hr following treatment of animals with either pyrazine or pyridazine, relative to controls. P4502E1 poly(A)+ RNA levels appeared to increase gradually, returning to levels which approximated 60% of the P4502E1 poly(A)+ RNA levels present in untreated animals at 48 and 72 hr following treatment with pyrazine or pyridazine, respectively. The results of these experiments show that thiazole, pyrazine, and pyridazine induce P4502E1 in rats, that the induction of 2E1 is associated with a concomitant decrease in 2E1 poly(A)+ RNA levels, and that these agents differentially affect the expression of P4502E1.