INTERACTION BETWEEN C-REL AND THE MITOGEN-ACTIVATED PROTEIN-KINASE KINASE KINASE-1 SIGNALING CASCADE IN MEDIATING KAPPA-B ENHANCER ACTIVATION

The Rel family of transcription factors are important mediators of var ious cytokine stimuli such as interleukin (IL)-1, tumor necrosis facto r (TNF)-alpha, and CD28 costimulation in T cell effector responses, Th ese stimuli induce Rel family DNA-binding activity to the kappa B enha ncer and CD28 response elements of many cytokine gene promoters leadin g to cytokine production, Consistent with the importance of Rel family induction during immune responses, c-Rel knockout mice exhibit profou nd defects in T cell functions including IL-2 secretion and T cell pro liferative responses to CD28 plus T cell receptor costimulation. The n ovel protein kinases, c-Jun NH2-terminal kinases (JNKs)/stress-activat ed protein kinases, are also activated by TNF-alpha, IL-1, and CD28 co stimulation. Because of the common regulation of c-Rel and JNK1 by the se agents in T cells, we investigated the role of JNK1 in c-Rel activa tion, We found that MAP kinase kinase kinase (MEKK) 1, a JNK1 activato r, induced transcription from the human immunodeficiency virus-1 long terminal repeat and IL-2R alpha promoters in a kappa B-dependent manne r, Coexpression of I kappa B alpha, a c-Rel inhibitor, inhibited the M EKK1-induced transcriptional activity, JNK1 synergized with MEKK1 in a ctivating transcription from a kappa B-driven heterologous promoter. F urthermore, JNK1 associated with c-Rel in vivo in Jurkat T cells by co immunoprecipitation assays and bound directly to c-Rel in a yeast two- hybrid assay, c-Rel also competed with c-Jun in in vitro kinase assays . However, JNK1 did not phosphorylate c-Rel, NF-kappa B, and I kappa B alpha in vitro, indicating that c-Rel may serve as a docking molecule to allow JNK1 phosphorylation of certain Rel-associated proteins. Tra nsactivation of the IL-2R alpha and HIV-kappa B-driven promoters by c- Rel was augmented by coexpression of MEKK1, These results demonstrate the first significant role for the MEKK1 kinase cascade module in c-Re l-mediated transcription.