IL-12 selectively regulates STAT4 via phosphatidylinositol 3-kinase and Ras-independent signal transduction pathways

IL-12 is an important immunomodulatory cytokine that induces tyrosine phosp horylation and activation of the signal transducer and activator of transcr iption (STAT)4. IL-12 induces sustained activation and nuclear translocatio n of STAT4 and this regulatory process is coupled to both tyrosine and seri ne phosphorylation of this molecule, IL-12-activated tyrosine kinases are t he Janus kinases Jak2 and Tyk2 but little is known about IL-12 regulation o f serine kinases. The object of the present study was to explore the role o f mitogen-activated protein kinases (MAPK) Erk1 and Erk2 and phosphatidylin ositol 3-kinase (PI3K) in STAT4 regulation. Here we show that the IL-12-ind uced STAT4 serine kinase is not sensitive to inhibitors of the PI3K or MAPK Erk1,2. Moreover, IL-12 activation of STAT4 in human peripheral blood-deri ved T cells is not accompanied by stimulation of the Ras guanine nucleotide binding cycle or stimulation of MAPK Erk1,2 or initiation of the PI3K sign aling pathways. IL-12 is unable to initiate the serine phosphorylation of S TAT 1 and 3. This reveals that the STAT1, 3 and 4 serine kinases are not co ordinately regulated in human T cells and that IL-12 must regulate serine p hosphorylation of STAT4 by a kinase network distinct to the MAPK Erk1,2 or PI3K pathways.