Abstract 3345: PP1 phosphatase complex at the hub of the integrated stress response: A potential therapeutic target in multiple myeloma

Abstract Protein phosphatase-1 (PP1) is a class of eukaryotic enzyme that catalyzes more than half of the phosphoserine/threonine dephosphorylation reactions, and plays essential roles in cell division, glycogen metabolism and protein synthesis. The substrate specificity of PP1 holoenzyme complex is mediated by PP1-interacting proteins and its catalytic subunits. Of note, PPP1R15B/CReP and PPP1R15A/GADD34 are regulatory subunits of PP1 complex that regulates global protein synthesis and mRNA translation through eIF2α dephosphorylation. Notably, PPP1R15B is constitutively expressed, whereas the expression of PPP1R15A is stress-inducible. Previous studies showed that multiple myeloma (MM), a hematological malignancy arising from immunoglobulin-secreting plasma cells, is characterized by continual ER stress and high dependency on the unfolded protein response (UPR). The integrated stress response (ISR) activated by eIF2α phosphorylation is the fastest reaction pathway of the UPR; however, its functional role and therapeutic potential in MM remains largely unknown. In this study, we performed gene expression analysis and super-enhancer (SE) profiling in MM cell lines, primary patient samples, normal CD138+ plasma cells and memory B cells. we found that PPP1R15B is highly expressed in MM cells as compared to healthy controls, but did not observe significant upregulation of PPP1R15A in MM cells. In addition, higher expression of PPP1R15B predicted poor overall survival of MM patients, suggesting its clinical relevance in MM pathogenesis. Depletion of PPP1R15B significantly reduced cell viability, proliferation and induced G2/M phase cell cycle arrest. Gene Ontology analysis of downregulated genes in response to PPP1R15B knockdown indicates significant association with the UPR pathway. Immunoblot analysis showed activation of ER stress pro-apoptotic executors PUMA, NOXA, and cleavage of caspase-12 in PPP1R15B knockdown cells. Furthermore, we examined the therapeutic effects of eIF2α dephosphorylation inhibitors, Salubrinal and Raphin1, on MM cells. Raphin1 is a selective inhibitor of PPP1R15B that binds strongly to the PPP1R15B-PP1c complex, whereas Salubrinal inhibits both PPP1R15A-PP1c and PPP1R15B-PP1c complex. Our data showed that both Salubrinal and Raphin1 inhibited MM cell proliferation in a dose-dependent manner, while sparing normal plasma cells. MM cell lines with elevated PPP1R15B expression level were more sensitive to Salubrinal or Raphin1 than those with lower PPP1R15B level. Similar changes in the expression patterns of ISR-related genes were observed in response to eIF2α dephosphorylation inhibitors. Taken together, our study suggests that targeting PPP1R15B-PP1c complex or maintaining eIF2α hyperphosphorylation may serve as a new therapeutic approach in MM, which warrants further investigation. Citation Format: Sinan Xiong, Jianbiao Zhou, Tze King Tan, Sabrina Hui-Min Toh, Tae-Hoon Chung, Takaomi Sanda, Wee Joo Chng. PP1 phosphatase complex at the hub of the integrated stress response: A potential therapeutic target in multiple myeloma abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 3345.