Forskolin-mediated induction of CYP3A1 mRNA expression in primary rat hepatocytes is independent of elevated intracellular cyclic AMP

JS Sidhu and CJ Omiecinski

Department of Environmental Health, University of Washington, Seattle, USA.

Previously, we demonstrated that elevated levels of cyclic AMP (cAMP) repressed phenobarbital (PB)-inducible cytochrome P450 (CYP)2B gene expression in primary rat hepatocyte cultures. Although CYP3A1 induction by PB was similarly repressed by most of the cAMP-enhancing strategies, forskolin additions in particular resulted in marked stimulation of CYP3A1 expression. Here we examined whether this effect was due to forskolin's ability to activate adenylate cyclase. By using a specific ELISA for assessment of intracellular cAMP levels, we determined that forskolin and a water-soluble analog (L858051; 7 beta- desacetyl-7 beta-(N-methylpiperazine)) were equipotent in stimulating adenylate cyclase activity. However, only forskolin and its inactive 1,9-dideoxy analog were active as inducers of CYP3A1. In comparative studies, both dexamethasone and PB were ineffective in stimulating production of intracellular cAMP. Furthermore, treatment of hepatocytes with glucagon, dibutyryl-cAMP, or N6O2'-dibutyryl-cyclic GMP, resulted in no detectable enhancement of CYP3A1 gene expression. These results demonstrated that CYP3A1 induction by forskolin is independent of cAMP, and instead is likely to involve a direct chemical effect of forskolin on the CYP3A1 activation pathway.

Volume 276, Issue 1, pp. 238-245, 01/01/1996
Copyright © 1996 by American Society for Pharmacology and Experimental Therapeutics