Changes in the Expression of Cytochrome P450s 2B1, 2B2, 2E1, and 2C11 in Response to Daily Aromatic Hydrocarbon Treatment

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Treatment of rats with ethylbenzene (EB) modulates the hepatic expression of many P450s, with those induced after a single intraperitoneal hydrocarbon injection differing from those induced after more prolonged (3 day) administration. The goals of the current studies are (1) to characterize the induction response after prolonged hydrocarbon exposure, (2) to explain why the elevation of these P450s is attenuated after continued treatment, and (3) to determine how P450 2B protein remains elevated without an elevation of P450 2B1/2 RNA. P450 2C11 protein was decreased after a single EB injection and remained depressed throughout the treatment period. P450 2C11 RNA was only decreased with prolonged, but not acute treatment. P450 2E1 was induced after a single EB injection; however, the initial induction was attenuated with more prolonged treatment. P450 2B1 and P450 2B2 RNAs exhibited a similar response, being elevated after acute administration, but returned to control levels with prolonged EB administration. Interestingly, P450 2B protein levels remained elevated despite the decrease in P450 2B1 and P450 2B2 RNA to control levels. We then tested the possibility that the multiphasic induction pattern of P450 2E1 and P450 2B1/2 RNA was due to differences in the pharmacokinetics of EB. The disappearance of EB with time was measured in rats that were either (1) untreated, (2) pretreated with EB for 1 day, or (3) pretreated with EB for 3 days. These results demonstrated that prior hydrocarbon exposure caused an increase in EB clearance, which decreased the overall levels of EB in the body. Consequently, EB levels were sufficiently diminished to decrease EB's effectiveness as an inducer leading to the decrease in P450 2E1 protein and P450 2B1 and P450 2B2 RNA after continued EB administration. A further consequence of the decreased overall EB concentration is that the hydrocarbon was capable of producing only a transient elevation of P450 2B1 RNA levels. This transient elevation appears to be sufficient to maintain elevated P450 2B protein.