RESUMO
Forskolin and cAMP have been shown to have paradoxical effects in the regulation of expression levels of mRNA of cytochrome P450 3A (CYP3A) family members. We demonstrate in this study that forskolin upregulated the promoter for CYP3A4 independent of its ability to increase cAMP levels. This activity was explained showing forskolin directly activated the pregnane-X-receptor, a known regulator of CYP3A genes.
Assuntos
Colforsina/farmacologia , AMP Cíclico/fisiologia , Sistema Enzimático do Citocromo P-450/biossíntese , 8-Bromo Monofosfato de Adenosina Cíclica/farmacologia , Linhagem Celular , Citocromo P-450 CYP3A , Sistema Enzimático do Citocromo P-450/genética , Ativação Enzimática , Histona Acetiltransferases , Humanos , Ligantes , Coativador 1 de Receptor Nuclear , Receptor de Pregnano X , RNA Mensageiro/biossíntese , Receptores Citoplasmáticos e Nucleares/metabolismo , Receptores de Esteroides/metabolismo , Fatores de Transcrição/metabolismoRESUMO
Natural products have been identified as ligands for a number of members of the nuclear hormone receptor (NHR) superfamily. Often these natural products are used as dietary supplements to treat myriad ailments ranging from perimenopausal hot flashes to hypercholesterolemia and reduced cognitive function. Examples of some natural product ligands for NHRs include genestein (estrogen receptors NR3A1 and NR3A2), guggulsterone (farnesoid X receptor NR1H4), and St. John's wort (pregnane X receptor, NR1I2). In this study, we identified the first nonoxysterol natural product that functions as a ligand for the liver X receptor (LXRalpha and LXRbeta; NR1H3, NR1H2), a NHR that acts as the receptor for oxysterols and plays a key role in regulation of cholesterol metabolism and transport as well as glucose metabolism. We show that paxilline, a fungal metabolite, is an efficacious agonist of both LXRalpha and LXRbeta in biochemical and in vitro cell-based assays. Paxilline binds directly to both receptors and is an activator of LXR-dependent transcription in cell-based reporter assays. We also demonstrate that paxilline binding to the receptors results in efficient activation of transcription of two physiological LXR target genes, ABCA1 and SREBP. The discovery of paxilline, the first reported nonoxysterol natural product ligand of the LXRs, may provide insight into the mechanism of ligand recognition by these receptors and reaffirms the utility of examining natural product libraries for identifying novel NHR ligands.