Activation of Raf-1 by interferon γ and oncostatin M requires expression of the Stat1 transcription factor

A primary signaling cascade responsible for the ex pression of cytokine-stimulated immediate early genes involves the activation of the Jak/Stat pathway. In addition to being tyrosine-phosphorylated, several signal transducers and activators of transcription (Stats), including Stat1 alpha, Stat3, and Stat4, are phosphorylated on a conserved serine residue, which is a consensus phosphorylation site for mitogen-activated protein kinases (MAPKs), Serine phosphorylation of Stat1 alpha is required for maximal transcriptional activation of early response genes by interferon gamma (IFN gamma) as well as the antiviral and antigrowth actions of this cytokine. Incubation of cells with either IFN gamma or oncostatin M (OSM) activates Raf-1, a serine/threonine kinase responsible for the ultimate activation of p42 MAPK. To examine whether any of the signaling components that are required for activation of the Jak/Stat pathway are also necessary for activation of Raf-1 by IFNs and OSM, we examined activation of Raf-1 in cell lines that are deficient in either Stat1 alpha or Stat2. Unexpectedly, incubation of Stat1-deficient, but not Stat2-deficient cells with IFN gamma or OSM for 5 min displayed no increase in Raf-1 activity. In peripheral blood lymphocytes Raf-1 was associated with Stat1, and this interaction was disrupted after incubation of cells with IFN gamma, Stat1-negative cells reconstituted with either Stat1 alpha or Stat1 alpha with a point mutation in the site where it is serine-phosphorylated displayed normal activation of Raf-1 by IFN gamma and OSM, However, activation of Raf-1 was not observed in lines that expressed Stat1 alpha containing a mutation in its tyrosine phosphorylation site or in its SH2 domain. These results provide the first example of a novel role of Stat1 alpha not as a transcription factor, but as a protein which may function to scaffold signaling components required for activation of the distinct Raf/MEK/MAPK signaling cascade.