Ligand-induced phosphorylation of the human interferon-γ receptor: Dependence on the presence of a functionally active receptor

Phosphorylation of the human interferon-γ (IFN-γ) receptor was studied in three cell lines of distinct lineages using radiophosphate labeling techniques. Receptors from unstimulated Colo-205 displayed a low level of constitutive phosphorylation which was enhanced 5.3-fold following exposure of the cells to either purified recombinant human IFNγ or phorbol myristate acetate. Enhanced receptor phosphorylation was specific, dose- and time-dependent, reversible, and affected only serine and threonine residues. Increased phosphorylation was observed only when cells were treated with human IFN-γ or phorbol myristate acetate and not with murine IFN-γ, human IFNα, human tumor necrosis factor-α, or epidermal growth factor. The biologic relevance of IFN-γ receptor phosphorylation was suggested by three additional observations; 1) there was a close correlation between the extent of receptor phosphorylation and the magnitude of the cellular response induced, 2) TNFα concomitantly enhanced both IFN-γ-dependent HLA-DR expression and IFNγ-dependent receptor phosphorylation on Colo-205, and 3) phosphorylation of functionally inactive recombinant murine IFN-γ receptors expressed on transfected human 293 or Colo-205 cells was not induced by murine IFN-γ but was induced by the homologous human ligand. These results suggest that phosphorylation of the IFN-γ receptor is an important step in the development of IFN-γ-dependent cellular responses and indicates that phosphorylation requires a functionally active receptor.