Abstract 2657: GSTP mediated S-glutathionylation of GRP78 attributes to multiple myeloma resistance

Abstract

Abstract The proteasome inhibitor bortezomib (Btz) is used in the treatment of multiple myeloma (MM), but its efficacy is restricted by the wide-spread occurrence of resistance. MM cells have the highest rate of secretion of proteins with disulfide bonds and are exquisitely dependent upon redox balance and accurate protein folding. Our results showed that some important redox enzymes, including glutathione transferase P (GSTP) are upregulated in Btz-resistant cells. While GSTP was originally described as a phase II detoxification enzyme, our laboratory has ascribed a thiolase activity catalyzing the forward S-glutathionylation reaction, a post-translational modification (PTM) through reversible addition of glutathione to thiolate anions of cysteines in target proteins. Although primarily cytosolic, GSTP has been reported in the nuclear and mitochondrial compartments. We recently showed that GSTP is localized at high concentrations in the endoplasmic reticulum (ER) and regulates a number of redox active proteins through S-glutathionylation; these include glucose-regulated protein (GRP78). Proteomic analysis revealed that both cys41 and cys 420 of GRP78 were subject to S-glutathionylation and this PTM altered the activity of GRP78, enhancing its holdase activity, while concomitantly decreasing its foldase activity. This alters protein folding and contributes to the Btz-resistant phenotype of MM. Our data also support the principle that drugs that target the ER protein folding machinery significantly alter local redox conditions. Thapsigargin (Thg) is an inhibitor of SERCA, interfering with the functional activities of calcium-dependent chaperones, while tunicamycin (TuM) inhibits N-linked glycosylation. Btz resistant cells showed marked levels of cross-resistance (2-10 fold) to both. Linking such results with GSTP/redox, the absence of GSTP (from GSTP knockout mice) makes both bone marrow derived dendritic and mouse embryonic fibroblast cells more sensitive to Btz, ThG and TuM. The nature of the cross-resistance patterns and the fact that the presence of GSTP in drug naïve cells also confers resistance provides a rational platform for studying redox-based mechanisms as causative in resistance. Moreover, inhibition of GSTP with Telintra restored Btz sensitivity, decreased S-glutathionylation and increased expression of markers of UPR induced apoptosis. We conclude that altered GSTP expression and S-glutathionylation of key ER-localized proteins are significant contributors to drug response and resistance in MM treated with Btz. Citation Format: Zhiwei Ye, Jie Zhang, Kenneth D. Tew, Danyelle M. Townsend. GSTP mediated S-glutathionylation of GRP78 attributes to multiple myeloma resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2657.