Intra-islet inflammation in type 2 diabetes: evidence that the E3 ligase TRAF2 is a key protective factor in beta cells

Background and aims: Activation of the JNK and NF-κB signaling pathways are associated with beta cell failure and apoptosis. TNFα-receptor associated factor 2 (TRAF2), an E3 ubiquitin ligase, mediates activation of both these pathways. We predicted that genetic deletion of TRAF2 in beta cells would improve beta cell function and prevent diabetes; an idea previously untested in islets. Materials and methods: We generated beta cell specific TRAF2 knockout mice (bTRAF2), examined TNFα signaling and subjected mice to a diet-induced obesity model. Results: bTRAF2 islets appeared normal, showed robust insulin labeling, and bTRAF2 mice displayed normal glucose tolerance at 8 weeks of age. Analyzing TNFα induced signaling ex vivo, bTRAF2 islets displayed delayed degradation of IκBα compared to wild type (WT). Phosphorylation of JNK was delayed, and also profoundly prolonged. Deleting TRAF2 in Min6 beta cells using siRNA emulated delayed and prolonged JNK activation. Further, we found increased processing of p100 to p52 in unstimulated bTRAF2 islets indicating cell intrinsic activation of the non-canonical NF-κB pathway. bTRAF2 islets were hyper-responsive to TNFα stimulation compared to WT showing 3-7 fold (**P<0.005) increased gene expression for CXCL10, ICAM1 and TNFα, genes regulated by the noncanonical pathway. Also the basal levels for these genes were increased ~2.5 fold (*P<0.05) compared to WT islets. In vivo, bTRAF2 mice subjected to an i.p. GTT showed a trend towards worsened glucose tolerance at 16 weeks of age (mean AUC±SEM: WT: 969.7±41.71, bTRAF2: 1181±80.13; n≥12; *P<0.05). When fed a high-fat diet for 8 weeks, glucose intolerance in bTRAF2 versus WT mice was greatly exacerbated (mean AUC±SEM: WT: 1420±37.8, bTRAF2: 1903±108.7; n≥12; ***P<0.001). This data was further corroborated by a trend towards lower fasting blood insulin levels and a defect in first phase insulin secretion in bTRAF2 mice determined by i.v. GTT (mean AUC±SEM: WT: 2.47±0.16, bTRAF2: 1.96±0.15; n≥10; *P<0.05). Intriguingly, TRAF2 mRNA levels were reduced in WT islets isolated from high-fat diet fed mice versus islets from chow fed mice. Conclusion: Thus, mice lacking TRAF2 in islets exhibit severe defects in glucose control and insulin secretion under a high fat feeding regimen and bTRAF2 islets exhibit a hyper-inflammatory profile. At the molecular level TRAF2 controls the cellular tempo of JNK and NF-κB activation and reigns in the non-canonical NF-κB pathway. Loss of this control circuit exacerbates diabetes suggesting that TRAF2 functions as a key protective factor in beta cells.