Hyperglycemia in both type 1 and 2 diabetes is toxic to pancreatic beta cells and exacerbates the decline in functional beta cell mass in both diseases. Thus, identifying the molecular mechanism for how this glucotoxicity negatively impacts beta cells is essential for identifying new therapeutic targets to treat both forms of diabetes. We identified that EGF receptor feedback inhibitor Mig6 is induced in islets cultured in glucotoxic conditions and that Mig6 promotes apoptosis in beta cells. To define how the adaptor protein Mig6 promotes apoptosis, we sought to identify its interacting partners. Following immunoprecipitation of flag-tagged Mig6 in Ins1-derived 832/13 cells cultured in normal and toxic conditions, mass spectrometric analysis revealed protein phosphatase 2A (PP2A). PP2A controls many cellular functions ranging from metabolism, cell cycle, DNA replication, growth and apoptosis, and PP2A can be exploited by tumor cells to promote cell survival. Thus, we hypothesized Mig6 interacts with PP2A to regulate beta cell apoptosis. We cultured 832/13 cells and mouse islets in low (5 mM) or high glucose (HG, 20 mM) for up to 48h and validated that glucotoxicity increased methylation and decreased phosphorylation of PP2A (indicating activation). HG also increased expression of Mig6, CHOP (ER stress), γH2AX (DNA damage response), and cleaved caspase 3 (CC3; apoptosis). Compared to controls, siRNA-mediated suppression of Mig6 in 832/13 cells increased methylated PP2A and decreased γH2AX and CC3, suggesting Mig6 antagonizes beta cell survival. We generated mice lacking Mig6 in beta cells (Mig6βKO) by crossing Mig6-floxed mice (Mig6fl/fl) with RIP-Cre mice. Beta cell mass and glucose homeostasis were similar between Mig6βKO and control Mig6fl/fl littermates. Islets from Mig6βKO mice had increased methylated PP2A during HG compared to controls. We speculate the Mig6-PP2A interaction limits PP2A activation; hence, disrupting Mig6-PP2A or inhibiting Mig6 could promote compensatory beta cell survival by activating PP2A.
A. Rezaeizadeh: None. B.M. Bauer: None. P.T. Fueger: None.