MEKK2 is required for T-cell receptor signals in JNK activation and interleukin-2 gene expression

The c-Jun N-terminal kinases (JNKs) are members of the mitogen-activated pr otein kinase (MAPK) gene family and are essential for cell proliferation, d ifferentiation, and apoptosis. Previously we found that activation of JNK i n T-cells required costimulation of both T-cell receptor and auxiliary rece ptors such as CD28, In this study, we cloned a full-length human MEK kinase (MEKK) 2 cDNA from Jurkat T-cells and demonstrated that it was a major ups tream MAPK kinase kinase for the JNK cascade in T-cells. The human MEKK2 cD NA encoded a polypeptide of 619 amino acids and was the human counterpart o f the reported murine MEKK2, It was 94% homologous with human and murine ME KK3 at the catalytic domains and 60% homologous at the N-terminal noncataly tic region. Northern blot analysis showed that MEKK2 was ubiquitously expre ssed, with the highest level in peripheral blood leukocytes, In T cells, ME KK2 was found to be a strong activator of JNK but not of extracellular sign al-regulated kinase MAPKs and to activate JNK-dependent AP-1 reporter gene expression. MEKK2 also synergized with anti-CD3 antibody to activate JNK in T cells, and stimulation of T cells led to induction of MEKK2 tyrosine pho sphorylation, Significantly, the JNK activation induced by anti-CDS and ant i-CD28 antibodies, but not by 12-O-tetradecanoylphorbol-13-acetate and Ca2 ionophore A23187, was inhibited by dominant negative MEKK2 mutants. AP-1 a nd interleukin-2 reporter gene induction in T-cells was also inhibited by d ominant negative MEKK2 mutants. Taken together, our results showed that hum an MEKK2 is a key signaling molecule for T-cell receptor/CD3-mediated JNK M APK activation and interleukin-2 gene expression.