Sentrin/SUMO Specific Proteases as Novel Tissue-Selective Modulators of Vitamin D Receptor-Mediated Signaling

Vitamin D receptor (VDR) is a substrate for modification with small ubiquitin-like modifier (SUMO). To further assess the role
of reversible SUMOylation within the vitamin D hormonal response, we evaluated the effects of sentrin/SUMO-specific
proteases (SENPs) that can function to remove small ubiquitin-like modifier (SUMO) from target proteins upon the activities
of VDR and related receptors. We report that SENP1 and SENP2 strikingly potentiate ligand-mediated transactivation of VDR
and also its heterodimeric partner, retinoid X receptor (RXRa) with depletion of cellular SENP1 significantly diminishing the
hormonal responsiveness of the endogenous vitamin D target gene CYP24A1. We find that SENP-directed modulation of
VDR activity is cell line-dependent, achieving potent modulatory effects in Caco-2 and HEK-293 cells, while in MCF-7 cells
the vitamin D signal is unaffected by any tested SENP. In support of their function as novel modulators of the vitamin D hormonal pathway we demonstrate that both SENP1 and SENP2 can interact with VDR and reverse its modification with SUMO2. In a preliminary analysis we identify lysine 91, a residue known to be critical for formation and DNA binding of the VDR-RXR heterodimer, as a minor SUMO acceptor site within VDR. In combination, our results support a repressor function for SUMOylation of VDR and reveal SENPs as a novel class of VDR/RXR co-regulatory protein that significantly modulate the vitamin D response and which could also have important impact upon the functionality of both RXR-containing homo and heterodimers.