Interferon α induces rapid tyrosine phosphorylation of the α subunit of its receptor

The mechanisms of generation of second messengers after binding of interferon α (IFNα) to its receptor remain unknown. We have studied the phosphorylation of the α subunit of the IFNα receptor, which is recognized by the monoclonal antibody IFNa receptor 3. Immunoblotting experiments showed that IFNα induced rapid tyrosine phosphorylation of the α subunit in the IFNα-sensitive H-929, U-266, and Daudi cell lines. Immunoprecipitation experiments performed with ³²P-labeled cells showed that the α subunit is phosphorylated before IFNα treatment and that the level of phosphorylation increases after IFNα stimulation. Phosphoamino acid analysis confirmed the IFNα-induced tyrosine phosphorylation and demonstrated that the base-line phosphorylation corresponded to serine phosphorylation that increased 50% upon IFNα treatment. Tyrosine phosphorylation of the α subunit was time- and dose-dependent, further demonstrating the specificity of the process. Phosphorylation of the α subunit of the receptor occurred rapidly after IFNα binding, both at 37 and 4 °C. Exposure of the cells to the tyrosine kinase inhibitor genistein blocked the IFNα-induced tyrosine phosphorylation of this subunit of the IFNα receptor. In contrast H7, a specific protein kinase C inhibitor, and acute and chronic exposure to phorbol esters had no effect on tyrosine phosphorylation, suggesting that protein kinase C does not regulate the tyrosine phosphorylation of the α subunit of the IFNα receptor. No IFNα-induced tyrosine phosphorylation was observed in the IFNα-resistant U-937 cell line that expresses a variant IFNα receptor. Altogether these data suggest that tyrosine phosphorylation of the α subunit may play a role in the signal transduction pathway of IFNα.