Importance of the MKK6/p38 pathway for interleukin-12-induced STAT4 serinephosphorylation and transcriptional activity

Interleukin-12 (IL-12) is a key immunoregulatory cytokine that promotes Th1 differentiation and cell-mediated immune responses, The transcription fact or STAT4 (signal transducer and activator of transcription 4) is an importa nt element in mediating IL-12 signals, as evidenced by the fact that STAT4( -/-) mice display impaired responsiveness to IL-12 and deficient Th1 differ entiation. STAT4 is inducibly phosphorylated on tyrosine and serine in resp onse to IL-12, but the kinase(s) responsible for the latter event is unknow n. Here we show that IL-12 induces STAT4 phosphorylation on serine 721 and that mutation of serine 721 interferes with STAT4 transcriptional activity. In addition, we show that mutation of tyrosine 693 abrogates IL-12-induced STAT4 tyrosine phosphorylation and transcriptional activity. Although the site surrounding serine 721 is an optimum consensus sequence for mitogen-ac tivated family of protein kinases (MAPKs)-mediated phosphorylation, we demo nstrate that IL-12 does not induce extracellular signal-regulated kinase (E RK) or c-Jun N-terminal kinase (JNK) activation in T and natural killer (NK ) cells and that IL-12-induced STAT4 transcriptional activity is not affect ed by these kinases, Rather, we show that IL-12 induces p38 activation, Mor eover, we demonstrate that p38 alpha and its upstream activator, MKK6, phos phorylate STAT4 on serine 721, and are required for STAT4 full transcriptio nal activity induced by IL-12, establishing the MKK6/p38 alpha/STAT4 pathwa y as an important mediator of IL-12 actions. (C) 2000 by The American Socie ty of Hematology.