ACTIVATION OF STAT4 BY IL-12 AND IFN-ALPHA - EVIDENCE FOR THE INVOLVEMENT OF LIGAND-INDUCED TYROSINE AND SERINE PHOSPHORYLATION

The immunoregulatory cytokine IL-12 plays a central role in cell-media ted immune responses through its effects on NK cells and T lymphocytes . While IL-12 is known to share some functions with other cytokines, s uch as IFN-LU, it also maintains distinct roles, such as its ability t o induce Th1 differentiation, The molecular basis for these unique and overlapping functions is not well understood, IL-12 has previously be en shown to induce tyrosine phosphorylation and DNA-binding of STAT3 a nd STAT4, members of the signal transducers and activators of transcri ption (STAT) family, Because STAT4 has only been shown to be activated in response to IL-12, this specificity has been suggested to be a bas is for the unique actions of IL-12, In this study, we demonstrated tha t STAT4 activation by IL-12 is not unique; IL-12 and IFN-alpha, but no t IFN-gamma, induced tyrosine phosphorylation and DNA binding of STAT4 , Since tyrosine and serine phosphorylation of STAT1 have previously b een shown to be important in IFN-alpha-mediated signaling, we also inv estigated IL-12- and IFN-alpha-induced serine phosphorylation of STAT4 , We demonstrated that both cytokines induced serine phosphorylation, This modification was not required for DNA binding, but may be importa nt in STAT-mediated transcription, Thus, STAT4 activation was not IL-1 2 specific, and IL-12 and IFN-alpha activated STAT4 through both tyros ine and serine phosphorylation. These findings have significant implic ations for understanding the role of STAT4 in mediating biologic funct ions; specifically, the data argue that the unique effects of IL-12 ca nnot be solely explained by STAT4 activation.