Abstract
Uncontrolled diabetes results in enhanced expression of cytochrome P-450 (CYP)2E1, CYP2B, CYP3A, and CYP4A. Because of the simultaneous and confounding metabolic and hormonal changes that occur in vivo as a consequence of diabetes, primary cultured rat hepatocytes provide an excellent model system for examination of the effects of insulin on P-450 expression and on xenobiotic-mediated P-450 expression. In the present study, we examined the effects of insulin on pyridine-, phenobarbital-, and ciprofibrate-mediated expression of CYP2E1, CYP2B, CYP3A, and CYP4A in primary cultured rat hepatocytes. Pyridine addition to primary rat hepatocytes cultured in the presence of 1 nM insulin or in the absence of insulin resulted in a 3.5-fold and 3-fold enhancement in CYP2E1 protein expression, respectively, in the absence of any pyridine-mediated increase in mRNA expression. In contrast, hepatocytes cultured in the standard concentration of 1 μM insulin resulted in only a 2-fold increase in protein expression. Thus, the fold-induction of CYP2E1 protein in response to pyridine was 1.5- to 1.8-fold greater in either the absence of insulin or in the presence of 1 nM insulin, respectively, than that monitored in the presence of 1 μM insulin. To examine whether insulin effects on xenobiotic-mediated CYP2E1 expression were selective, insulin effects on xenobiotic-mediated expression of transcriptionally regulated CYP2B, CYP3A, and CYP4A were examined. Pyridine- or phenobarbital-mediated induction of CYP2B mRNA and protein expression in hepatocytes was suppressed by as much as 80% at lower insulin levels (0 and 1 nM), relative to the level monitored in the presence of 1 μM insulin. Omitting insulin from the medium resulted in a 50% decrease in CYP3A mRNA levels in response to phenobarbital treatment and a 30% decrease in CYP4A mRNA levels in response to ciprofibrate treatment, relative to the level obtained in response to these treatments in the presence of 1 μM insulin. The results of this study demonstrate that decreasing the insulin level in the primary hepatocyte culture medium enhanced xenobiotic-mediated CYP2E1 expression, whereas lower insulin levels suppressed xenobiotic-mediated CYP2B, CYP3A, and CYP4A expression in this cell culture system.
Footnotes
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Send reprint requests to: Dr. Raymond F. Novak, Institute of Chemical Toxicology, Wayne State University, 2727 Second Avenue, Room 4000, Detroit, MI 48201. E-mail: r.novak{at}wayne.edu
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↵1 This work was supported by Grant ES03656 and the Environmental Health Sciences Center Grant P30 ES06639 from the National Institute of Environmental Health Sciences.
- Abbreviations:
- CYP
- cytochrome P-450
- PYR
- pyridine
- PB
- sodium phenobarbital
- CIPRO
- ciprofibrate
- Received August 21, 1998.
- Accepted January 11, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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