Abstract

Objective: The transcription factor NF-κB and the MAP kinases JNK1/2 are known to play decisive roles in cytokine-induced damage of rodent β-cells. The upstream events by which these factors are activated in response to cytokines are, however, uncharacterized. The aim of the present investigation was to elucidate a putative role of the MAP kinase kinase kinase-1 (MEKK-1) in cytokine-induced signaling.

Research Design and Methods: To establish a functional role of MEKK-1, the effects of transient MEKK-1 overexpression in βTC-6 cells, achieved by lipofection and cell sorting, and MEKK-1 downregulation in βTC-6 cells and human islet cells, achieved by diced-siRNA treatment, were studied.

Results: We observed that overexpression of wild type MEKK-1, but not of a kinase dead MEKK-1 mutant, resulted in potentiation of cytokine-induced JNK activation, IκB degradation, NF-κB translocation and cell death. Downregulation of MEKK-1 in human islet cell provoked opposite effects, i.e. attenuation of cytokine-induced JNK and MKK4 activation, IκB stability and a less pronounced NF-κB translocation. βTC-6 cells with a downregulated MEKK-1 expression displayed also a weaker cytokine-induced iNOS expression and lower cell death rates. Also primary mouse islet cells with downregulated MEKK-1 expression were protected against cytokine-induced cell death.

Conclusions: MEKK-1 mediates cytokine-induced JNK- and NF-κB activation and this event is necessary for iNOS expression and cell death.