MAPK kinase kinase-1 is essential for cytokine-induced c-Jun NH2-terminal kinase and nuclear factor-κB activation in human pancreatic islet cells

OBJECTIVE-The transcription factor nuclear factor-kappa B (NF-kappa B) and the mitogen-activated protein kinases (MAPKs) c-Jun NH2-terminal kinase (JNK) 1/2 are known to play decisive roles in cytokine-induced damage of rodent P-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 MAPK 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 beta TC-6 cells, achieved by lipofection and cell sorting, and MEKK-1 downregulation in beta TC-6 cells and human islet cells, achieved by diced-small interfering RNA 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, inhibitor of kappa B (IKB) degradation, and cell death. Downregulation of MEKK-1 in human islet cells provoked opposite effects, i.e., attenuation of cytokine-induced JNK and MKK4 activation, I kappa B stability, and a less pronounced NF-kappa B translocation. beta TC-6 cells with a down-regulated 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-kappa B activation, and this event is necessary for iNOS expression and cell death.