RESUMEN
The orphan nuclear receptor pregnane X receptor regulates enzymes and transport proteins involved in the detoxification and clearance of numerous endobiotic and xenobiotic compounds, including pharmaceutical agents. Multiple alternatively spliced pregnane X receptor isoforms have been identified which are significantly expressed in humans and mice (up to 30% of the total pregnane X receptor transcript), however, little is known about their biological action. We explored functional differences between the major mouse pregnane X receptor isoforms mPXR(431) and mPXR(Delta171-211) that lacks 41 amino acids adjacent to the ligand-binding pocket. Transient transfection assays showed that mPXR(Delta171-211) reduced the basal transcription of cytochrome P450 3A4 and the drug transporter P-glycoprotein/Multi Drug Resistance Protein 1 and directly repressed the regulatory effects of mPXR(431) on these genes. Replacement of the mPXR(Delta171-211) DNA-binding domain with that of GAL4 showed mPXR(Delta171-211) retained its repressive role independent of binding to PXR responsive elements located within the cytochrome P450 3A4 and Multi Drug Resistance Protein 1 regulatory regions. Use of the histone deacetylase inhibitor, trichostatin A, demonstrated that the repressive function of mPXR(Delta171-211) acts independently of histone acetylation state. Protein interaction assays revealed mPXR(Delta171-211) and mPXR(431) differentially bind the obligatory heterodimer partner retinoid X receptor. Furthermore, mPXR(431) and mPXR(Delta171-211) proteins could heterodimerize. These studies demonstrate that the variant mouse PXR isoform, mPXR(Delta171-211), has a distinct repressive function from mPXR(431) in regulating genes encoding important drug metabolizing enzymes and transport proteins.
Asunto(s)
Miembro 1 de la Subfamilia B de Casetes de Unión a ATP/metabolismo , Empalme Alternativo , Citocromo P-450 CYP3A/metabolismo , Regulación hacia Abajo , Receptores de Esteroides/genética , Receptores de Esteroides/fisiología , Miembro 1 de la Subfamilia B de Casetes de Unión a ATP/genética , Secuencia de Aminoácidos , Animales , Línea Celular , Chlorocebus aethiops , Citocromo P-450 CYP3A/genética , Proteínas de Unión al ADN , Inhibidores de Histona Desacetilasas/farmacología , Humanos , Masculino , Ratones , Especificidad de Órganos , Receptor X de Pregnano , Dominios y Motivos de Interacción de Proteínas , Isoformas de Proteínas/genética , Isoformas de Proteínas/fisiología , Multimerización de Proteína , Receptores de Esteroides/química , Proteínas Recombinantes de Fusión , Elementos Reguladores de la Transcripción/genética , Receptores X Retinoide/metabolismo , Alineación de Secuencia , Eliminación de SecuenciaRESUMEN
The Pregnane X Receptor (PXR) is pivotal for the body's response to toxic xenobiotics and endogenous metabolites. By acting as a ligand-activated transcription factor, PXR regulates all stages of xenobiotic metabolism and transport and is responsible for important inductive drug interactions. Screening assays to assess the PXR activation potential of new and existing drugs are becoming integral components of drug discovery programs. PXR is also involved in lipid homeostasis providing opportunities for treatments based on PXR agonists for diseases involving aberrant cholesterol and bile acid levels. The expression of PXR in many other tissues besides liver and intestine suggest PXR may have additional protective functions in the body, which contribute to disease outcomes in diverse clinical situations with potential for novel therapeutic approaches.
