Catalytically Active TYK2 Is Essential for Interferon-β-mediated Phosphorylation of STAT3 and Interferon-α Receptor-1 (IFNAR-1) but Not for Activation of Phosphoinositol 3-Kinase

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TYK2, a Janus kinase, plays both structural and catalytic roles in type I interferon (IFN) signaling. We recently reported (Rani, M. R. S., Gauzzi, C., Pellegrini, S., Fish, E., Wei, T., and Ransohoff, R. M. (1999) J. Biol. Chem. 274, 1891–1897) that catalytically active TYK2 was necessary for IFN-β to induce the β-R1 gene. We now report IFN-β-mediated activation of STATs and other components in U1 (TYK2-null) cell lines that were complemented with kinase-negative (U1.KR930) or wild-type TYK2 (U1.wt). We found that IFN-β induced phosphorylation on tyrosine of STAT3 in U1.wt cells but not in U1.KR930 cells, whereas STAT1 and STAT2 were activated in both cell lines. Additionally, IFN-β-mediated phosphorylation of interferon-α receptor-1 (IFNAR-1) was defective in IFN-β treated U1.KR930 cells, but evident in U1.wt cells. In U1A-derived cells, the p85/p110 phosphoinositol 3-kinase isoform was associated with IFNAR-1 but not STAT3, and the association was ligand-independent. Further, IFN-β treatment stimulated IFNAR-1-associated phosphoinositol kinase activity equally in either U1.wt or U1.KR930 cells. Our results indicate that catalytically active TYK2 is required for IFN-β-mediated tyrosine phosphorylation of STAT3 and IFNAR-1 in intact cells.