INTERFERON-GAMMA ACTIVATION OF RAF-1 IS JAK1-DEPENDENT AND P21(RAS)-INDEPENDENT

Signal transduction through the interferon gamma (IFN gamma) receptor involves the formation of a ligand-dependent multimolecular associatio n of receptor chains (alpha and beta), Janus tyrosine kinases (Jak1 an d Jak2), and the transcription factor (signal transducers and activato rs of transcription 1 alpha (STAT1 alpha)) in addition to activation o f mitogen-activated protein kinases (MAPK), Interactions between compo nents of the Jak/STAT cascade and the p21(ras)/Raf-1/MAPK cascade are unexplored, Treatment of HeLa cells with IFN gamma resulted in the rap id and transient activation of Raf-1 and MAPK. Parallel activation of cells resulted in essentially no enhancement of p21(ras) activation de spite marked enhancement after treatment with epidermal growth factor, In HeLa (E1C3) and fibrosarcoma (U4A) cell lines, both of which are d eficient in Jak1 kinase, Raf-1 activation by IFN gamma was absent. Rec onstitution of Raf-1 activity was observed only with kinase active Jak 1 in both cell lines. In COS cells, transient expression of wild type or kinase-inactive Jak1 coimmunoprecipitated with Raf-1, but activatio n of Raf-1 activity was only observed in cells expressing kinase-activ e Jak1. These observations suggest that a kinase-active Jak1 is requir ed for IFN gamma activation of Raf-1 that is p21(ras)-independent.