Role of NF-κB in Regulation of PXR-mediated Gene Expression

It is a long-standing observation that inflammatory responses and infections decrease drug metabolism capacity in human and experimental animals. Cytochrome P-450 3A4 cyp304 is responsible for the metabolism of over 50% of current prescription drugs, and cyp3a4 expression is transcriptionally regulated by pregnane X receptor (PXR), which is a ligand-dependent transcription factor. In this study, we report that NF-kappaB activation by lipopolysaccharide and tumor necrosis factor-alpha plays a pivotal role in the suppression of cyp3a4 through interactions of NF-kappaB with the PXR.retinoid X receptor (RXR) complex. Inhibition of NF-kappaB by NF-kappaB-specific suppressor SRIkappaBalpha reversed the suppressive effects of lipopolysaccharide and tumor necrosis factor-alpha. Furthermore, we showed that NF-kappaB p65 disrupted the association of the PXR.RXRalpha complex with DNA sequences as determined by electrophoretic mobility shift assay and chromatin immunoprecipitation assays. NF-kappaB p65 directly interacted with the DNA-binding domain of RXRalpha and may prevent its binding to the consensus DNA sequences, thus inhibiting the transactivation by the PXR.RXRalpha complex. This mechanism of suppression by NF-kappaB activation may be extended to other nuclear receptor-regulated systems where RXRalpha is a dimerization partner.