TNFα-induced DLK activation contributes to apoptosis in the beta-cell line HIT

Reduction in beta-cell mass and function contributes to the pathogenesis of diabetes mellitus type 2. The proinflammatory cytokines tumor necrosis factor (TNF)alpha and interleukin (IL)-1 beta have been implicated in the pathogenesis of this disease. Overexpression of the dual leucine zipper kinase (DLK) inhibits beta-cell function and induces apoptosis in the beta-cell line HIT. In the present study, it was investigated whether TNF alpha or IL-1 beta stimulates DLK enzymatic activity. Immunoblot analysis, transient transfection with luciferase reporter gene assays, and immunofluorescence were used. In contrast to IL-1 beta, TNF alpha stimulated DLK kinase activity, which was dependent on the c-Jun N-terminal kinase (JNK). Furthermore, DLK contributed to TNF alpha-induced JNK phosphorylation. The phosphorylation of DLK on Ser-302 within the activation loop was required for DLK to stimulate JNK and to inhibit CREB-dependent gene transcription. TNF alpha induced apoptosis in a time- and concentration-dependent manner and inhibited CREB-directed gene transcription in HIT cells. The reduction of endogenous DLK by small interfering or small hairpin RNA attenuated TNF alpha's effects on apoptosis and CREB-dependent transcription. These data suggest that TNF alpha induces beta-cell apoptosis through activation of DLK thereby inhibiting the beta-cell protective transcription factor CREB. Furthermore, activation of DLK by a well-known diabetic risk factor supports the role of DLK in the pathogenesis of diabetes mellitus. Thus, the inhibition of DLK might prevent or retard the pathogenesis of diabetes mellitus type 2.