Elsevier

Biochemical Pharmacology

Volume 55, Issue 9, 1 May 1998, Pages 1435-1443
Biochemical Pharmacology

Original Articles
Negative Regulation by Dexamethasone of Fluvastatin-Inducible CYP2B Expression in Primary Cultures of Rat Hepatocytes: Role of CYP3A

https://doi.org/10.1016/S0006-2952(97)00658-8Get rights and content

Abstract

Fluvastatin (Fluva), a synthetic inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, induces CYP2B1/2 in rat liver and primary cultured rat hepatocytes. However, the overall profile of CYP induction, which includes induction of CYP4A, suggests that Fluva is not a typical “phenobarbital (PB)-like” inducer. Several treatments affecting diverse cell signaling pathways have been reported to modify PB-inducible CYP2B expression in primary cultured rat hepatocytes. We examined the effects of selected treatments on the ability of Fluva to induce CYP2B1/2 mRNA. Only dexamethasone (Dex) produced effects on Fluva-inducible CYP2B1/2 mRNA expression that differed from those produced on PB-inducible CYP2B1/2 mRNA expression. Dex concentrations up to 10−7 M of potentiated PB (10−4 M)-mediated CYP2B1/2 mRNA induction, while higher Dex concentrations produced a progressive reduction in PB-induced CYP2B1/2 mRNA levels. By contrast, Dex concentrations up to 10−8 M had no effect on Fluva (3 × 10−5 M)-induced CYP2B1/2 mRNA levels, while Dex concentrations of 10−7 M and higher markedly suppressed Fluva-mediated CYP2B1/2 mRNA induction. The concentrations of several glucocorticoids that produced suppression of Fluva-induced CYP2B1/2 mRNA levels were the same concentrations that induced CYP3A mRNA. Treatment with pregnenolone 16α-carbonitrile also produced a concentration-dependent suppression of Fluva-induced CYP2B1/2 mRNA levels. Dex-mediated suppression of Fluva-induced CYP2B1/2 mRNA was concentration-dependently reversed when hepatocytes were cotreated with troleandomycin, a selective CYP3A inhibitor. The amounts of Fluva detected in culture medium and cells were reduced significantly when hepatocytes were incubated with Dex. However, Dex-mediated suppression of Fluva-induced CYP2B1/2 mRNA expression was not overcome when hepatocytes were incubated with Fluva concentrations greater than 3 × 10−5 M, suggesting that mechanisms other than CYP3A-catalyzed metabolism may contribute to Dex-mediated suppression of Fluva-induced CYP2B1/2 expression.

Section snippets

Materials

Fluva was a gift from the Sandoz Research Institute. Cycloheximide, Dex, dibutyryl cyclic AMP, glucagon, growth hormone (porcine), PB, triamcinolone, triamcinolone acetonide, and TAO were purchased from the Sigma Chemical Co. Prednisolone and 6β-hydroxyprednisolone were purchased from Steraloids. HPLC solvents were purchased from Burdick & Jackson. Vitrogen (95–98% type I collagen with remainder type III collagen) was purchased from Celtrix. Other supplies and reagents were purchased from the

Results

The primary mechanism through which PB or any other agent induces CYP2B1/2 is not known. However, manipulations that modulate the ability of PB to induce CYP2B in primary hepatocyte culture may provide insight into whether PB and Fluva induce CYP2B through a common mechanism. We therefore examined the effects of several of these manipulations on the levels of CYP2B1/22 RNA induction produced in primary cultured rat hepatocytes treated for 24 hr (beginning 48 hr postplating) with PB or Fluva, at

Discussion

Treatment of primary cultured rat hepatocytes with a variety of agents previously reported to modify PB-inducible CYP2B expression generally produced the expected results. Thus, incubation of cultured rat hepatocytes with fetal bovine serum, growth hormone, or cycloheximide suppressed PB-mediated CYP2B induction, as previously reported 4, 5. Also, maintenance of hepatocytes on a Vitrogen substratum in glucocorticoid-deficient medium did not support PB-inducible CYP2B1/2 mRNA induction, in

Acknowledgements

This work was supported by National Institutes of Health Grant HL50710 and, in part, by NIEHS Center Grant ES06639. The authors thank Dr. Melissa Runge-Morris for her constructive comments during the preparation of this manuscript.

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