PT  - JOURNAL ARTICLE
AU  - Ritu Rana
AU  - Sherry Coulter
AU  - Harriet Kinyamu
AU  - Joyce A. Goldstein
TI  - RBCK1, an E3 Ubiquitin Ligase, Interacts with and Ubiquinates the Human Pregnane X Receptor
AID  - 10.1124/dmd.112.048728
DP  - 2013 Feb 01
TA  - Drug Metabolism and Disposition
PG  - 398--405
VI  - 41
IP  - 2
4099  - http://dmd.aspetjournals.org/content/41/2/398.short
4100  - http://dmd.aspetjournals.org/content/41/2/398.full
SO  - Drug Metab Dispos2013 Feb 01; 41
AB  - The pregnane X receptor (PXR, NR1I2) plays a pivotal role in the disposition and detoxification of numerous foreign and endogenous chemicals by increasing transcription of numerous target genes, including phase I and II drug-metabolizing enzymes and transporters. In the present study, yeast two-hybrid screening identified an E3 ubiquitin ligase, RBCK1 (Ring-B-box-coiled-coil protein interacting with protein kinase C-1), as a human pregnane X receptor (hPXR)–interacting protein. Coimmunoprecipitation studies confirmed the interaction between RBCK1 and hPXR when both were ectopically expressed in AD-293 cells. Domain mapping studies showed that the interaction between RBCK1 and hPXR involves all RBCK1 domains. We further demonstrate that RBCK1 ubiquitinates hPXR, and this may target hPXR for degradation by the ubiquitin-proteasome pathway. Simultaneous ectopic overexpression of RBCK1 and PXR decreased PXR levels in AD-293 cells, and this decrease was inhibited by the proteasomal inhibitor MG-132 (carbobenzoxy-Leu-Leu-leucinal). Furthermore, overexpression of RBCK1 decreased endogenous levels of PXR in HepG2 cells. Of importance, ectopic overexpression and silencing of endogenous RBCK1 in primary human hepatocytes resulted in a decrease and increase, respectively, in endogenous PXR protein levels and in the induction of PXR target genes by rifampicin. These results suggest that RBCK1 is important for the ubiquitination of PXR and may play a role in its proteasomal degradation.