Loss of TRB3 Alters Dynamics of MLK3-JNK Signaling and Inhibits Cytokine-activated Pancreatic Beta Cell Death

Background: MLK3 induces TRB3 to inhibit AKT and compromise beta cell survival. Results: AKT regulates novel phosphorylation, ubiquitination, and degradation of MLK3, explaining why TRB3(-/-) islets display attenuated MLK3/JNK activation and beta cell death. Conclusion: TRB3 functionally cooperates with the JNK module to fine-tune inflammatory signaling in beta cells. Significance: Even modest modulation of signaling dynamics can alter inflammatory outcomes. Disabling cellular defense mechanisms is essential for induction of apoptosis. We have previously shown that cytokine-mediated activation of the MAP3K MLK3 stabilizes TRB3 protein levels to inhibit AKT and compromise beta cell survival. Here, we show that genetic deletion of TRB3 results in basal activation of AKT, preserves mitochondrial integrity, and confers resistance against cytokine-induced pancreatic beta cell death. Mechanistically, we find that TRB3 stabilizes MLK3, most likely by suppressing AKT-directed phosphorylation, ubiquitination, and proteasomal degradation of MLK3. Accordingly, TRB3(-/-) islets show a decrease in both the amplitude and duration of cytokine-stimulated MLK3 induction and JNK activation. It is well known that JNK signaling is facilitated by a feed forward loop of sequential kinase phosphorylation and is reinforced by a mutual stabilization of the module components. The failure of TRB3(-/-) islets to mount an optimal JNK activation response, coupled with the ability of TRB3 to engage and maintain steady state levels of MLK3, recasts TRB3 as an integral functional component of the JNK module in pancreatic beta cells.