DRES-18. SUMO1 AND VALOSIN-CONTAINING PROTEIN REGULATE RETINOID RECEPTOR PROTEIN TURNOVER– A PROCESS DISRUPTED IN GLIOBLASTOMA Free

Resistance to therapeutic use of retinoids in glioblastoma (GBM) has been observed for over 20 years; however, the exact mechanism of resistance remains unknown. To better understand retinoic acid resistance in GBM, we studied the turnover mechanism of retinoid receptor proteins in normal neural stem cells and glioma stem-like cells (GSCs). Currently, it is believed that cellular stress induces global sumoylation of proteins in glioma, yet the precise role of sumoylation is not fully understood.

Protein expression and posttranslational modification (PTM) of the endogenous retinoid receptors were analyzed using Western blots, immunoprecipitations, and siRNA. The discovery of the novel binding partner of retinoid receptors was achieved using immunoprecipitation and mass spectrometry. Promoter luciferase assays were used to measure transcriptional activities. RESULTS AND CONCLUSIONS: Our studies reveal that sumoylation of retinoid receptors occurs in both normal neural stem cells and GSCs; however, protein turnover of the receptor is disrupted in glioma. We show that sumoylation is a PTM required for proteasomal degradation of retinoid receptors. Degradation via the proteasomal pathway is necessary for receptor protein turnover and transcriptional activity. We also identify that the valosin-containing protein (VCP/p97/Cdc48) participates in the PTM of retinoid receptors and impacts the transcriptional activity. The defect in glioma occurs after the sumo modification step and results in the accumulation of high molecular weight forms of the receptors that fail to get degraded. Our findings expand our understanding of the turnover mechanism of nuclear receptors in normal cells. In addition, our findings provide a mechanism for the retinoic acid resistance in glioma cells that involves the disruption of protein turnover and decrease in transcriptional activity. Our studies suggest that the use of combinatory therapies that target retinoid receptors and induce proteasomal degradation of the receptors to ensure protein turnover may be a more effective approach.