Induction of drug metabolism by forskolin: The role of the pregnane X receptor and the protein kinase A signal transduction pathway

An extract of the plant Coleus forskohlii has been used for centuries in Ayurvedic medicine to treat various diseases such as hypothyroidism, heart disease, and respiratory disorders. Additionally, complex herbal mixtures containing this extract are gaining popularity in United States for their putative "fatburning" properties. The active ingredient in C. forskohlii extract is the diterpene compound forskolin. Forskolin is a widely used biochemical tool that activates adenyl cyclase, thereby increasing intracellular concentration of cAMP and thus activating the protein kinase A (PKA) signal transduction pathway. We show herein that both forskolin and its nonadenyl cyclase-activating analog 1,9 dideoxyforskolin induce CYP3A gene expression in primary hepatocytes by functioning as agonists of the pregnane X receptor (PXR). We show that activation of PKA signaling potentiates PXR-mediated induction of CYP3A gene expression in cultured hepatocytes and increases the strength of PXR- coactivator protein-protein interaction in cell-based assays. Kinase assays show that PXR can serve as a substrate for catalytically active PKA in vitro. Our data provide important insights into the molecular mechanism of both the PKA-dependent and - independent effects of forskolin on the expression of drug-metabolizing enzymes in liver. Finally, our data suggest that herbal therapy with C. forskohlii extract should be approached cautiously due to the potential for herb-drug interactions in patients on combination therapy.