The Deubiquitinating Enzyme Otub2 Modulates Pancreatic Beta-Cells Function and Survival

We have previously shown that otubain 2 (OTUB2), a deubiquitinating enzyme, inhibits caspase-3/7 activity in primary human islets; promotes insulin secretion and inhibits cytokine-induced nuclear factor-κB (NFκB) activity. In the present work we show that overexpression of Otub2 in MIN6 cells inhibits NFκB activity and the expression of its target genes MCP-1 and iNOS. Consequently, both the basal and the cytokine-induced apoptosis of cultured MIN6 cells and dispersed human islets were inhibited. Overexpression of Otub2 in MIN6 cells increase the mRNA levels of NKx6.1 and Glut2 and concomitantly increased glucose-stimulated insulin secretion (GSIS) (by 2-3-fold). The beneficial effects of Otub2 on β-cell function was demonstrated by the phenotype of Otub2 ^-/+ and Otub2 ^-/- mice, which manifested impaired glucose tolerance and increased expression of NFkB target genes (e.g. IP-10, MCP-1 and IL-1β). RNAseq analysis of pancreata derived from OTUB2 KO mice revealed reduced expression of genes that down regulate K ⁺ transporters (e.g. Ank2 , Cacna1a and Kcnab1) combined with an increase in oxidative phosphorylation related genes. Given that closure of K ⁺ channels is crucial for insulin secretion, these results could account, at least in part, for the impaired GSIS in the OTUB2 KO mice. Indeed, mass-spectrometry analysis of proteins co-immunoprecipitated with Otub2 revealed the voltage-gated potassium channel subunit Kv9.3 as a major Otub2 binding-partner. Additional binding partners included the Peg3 and Camk2d proteins, which promote NFκB signaling and β-cell death. Hence, by deubiquitinating proteins in complexes that contain Peg3 and Camk2d, Otub2 might inhibit propagation of NFκB signaling and β-cell apoptosis. Collectively our findings implicate Otub2 as a key regulator of β cell function, mainly affecting NFkB signaling and the K ⁺ channels that regulate insulin secretion.