Pregnane X Receptor PXR Activates the GADD45β Gene, Eliciting the p38 MAPK Signal and Cell Migration

Pregnane X receptor (PXR) was originally characterized as a transcription factor that induces hepatic drug metabolism by activating cytochrome P450 genes. Here we have now demonstrated a novel function of PXR, that of eliciting p38 mitogen-activated protein kinase (MAPK) phosphorylation for cell migration. Upon xenobiotic activation of ectopic human PXR, human hepatocellular carcinoma HepG2 cells were found to exhibit increased phosphorylation of p38 MAPK and to subsequently change morphology and migrate. p38 MAPK was responsible for the regulation of these morphological changes and cell migration because the p38 MAPK inhibitor SB239063 repressed both. Prior to this phosphorylation, PXR directly activated the early response GADD45 beta gene by binding to a distal direct repeat 4 site of the GADD45 beta promoter. Ectopic expression of GADD45 beta increased p38 MAPK phosphorylation, whereas siRNA knockdown of GADD45 beta decreased the PXR-induced p38 MAPK phosphorylation, confirming that GADD45 beta can regulate PXR-induced p38 MAPK phosphorylation in HepG2 cells. These results indicate that PXR activates the GADD45 beta gene, increasing p38 MAPK phosphorylation, and leading HepG2 cells to change morphology and migrate. The GADD45 beta gene is a direct target for PXR, eliciting cell signals to regulate various cellular functions.