PROX1 suppresses vitamin K-induced transcriptional activity of steroid and xenobiotic receptor

Steroid and Xenobiotic Receptor (SXR) belongs to nuclear receptor superfamily. It was shown that secondary bile acids such as lithocholic acid and several chemical compounds such as rifampicin could be ligands for this receptor. Recently, we have demonstrated that vitamin K2 also serves as a ligand for SXR and activation of SXR by vitamin K2 suppressed proliferation and motility of hepatocellular carcinoma (HCC) cells. To analyze function of SXR in HCC cells, we overexpressed exogenous SXR double-tagged with FLAG and HA in a HCC cell line, HepG2 cells, and purified SXR-binding molecules by immunoprecipitation from the nuclear extracts of these cells. Several binding molecules were identified by TOF-MS analyses. One of the SXR-binding molecules was a transcription factor PROX1. We confirmed the interaction of PROX1 and SXR in HEK293 cells. Then, we have shown that AF2 domain of SXR is necessary for binding with PROX1. We further demonstrated that PROX1 negatively regulated the transcriptional activity of SXR by promoter analyses of SXR target gene. These results suggest that PROX1 could negatively regulate SXR signals in some tumor cells, such as HCC cells, where both SXR and PROX1 are expressed.