Vitamin D Receptor (VDR) and Retinoid X Receptor (RXR) α are ligand-inducible transcription factors. The activities of VDR and RXR are regulated by various posttranslational modifications. SUMOylation is the covalent attachment of Small Ubiquitin-like MOdifier (SUMO) to a specific lysine residue of a target protein, and the process is reversible by the action of SENtrin/SUMO proteases (SENP). The objective of this thesis is to illustrate the impact of reversible SUMOylation on the VDR/RXR mediated signalling. Our current findings showed that overexpression of SENP1 and SENP2 enhanced the transactivation activity of both receptors. Increased SENP1 levels are correlated with an increased expression of endogenous vitamin D (1,25D) target gene CYP24A1. Conversely, CYP24A1 was down regulated by si-RNA knockdown of SENP1 in Caco-2 cells. Further investigation of the SUMOylation status of the receptors revealed that VDR appeared preferentially SUMOylated by SUMO2, while RXRα is SUMOylated by SUMO1, 2 and 3. The modification of RXRα and VDR is facilitated by a SUMO E3 ligase PIAS4 [Protein Inhibitor of the Activated STAT (Signal Transducer and Activator of Transcription) 4]. Overexpression of PIAS4 caused a repression on RXRα transactivation, and results suggested such an effect is dependent on the SUMO E3 ligase activity. Moreover, our data revealed that RXRα has another SUMO acceptor site in additional to the reported SUMOylation site K108. In summary, our findings suggest that reversible SUMOylation is a novel regulatory mechanism for VDR-mediated signalling, that possibly modulates through VDR and RXR transcriptional functions. Future study of the mechanisms involving SENP/PIAS will better elucidate the functions of VDR and RXRα containing complexes.
A study of the role of reversible SUMOylation on vitamin D receptor/retinoid X receptor α mediated signalling
Lee, W. (Author). May 2011
Student thesis: Doctoral Thesis