Repression of IL-4-Induced Gene Expression by IFN-γ Requires Stat1 Activation

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Abstract
IFN-γ antagonizes many physiological responses mediated by IL-4, including the inhibition of IL-4-induced IgE production. This event is largely mediated at the level of transcription. We observed that the IL-4 response element of the germline epsilon promoter is sufficient to confer IFN-γ-mediated repression onto a reporter construct. The inhibitory effects were observed in both lymphoid and nonlymphoid cell lines. Stat1, which is activated by IFN-γ, cannot recognize the Stat6-specific IL-4 response element in the ε promoter. Hence, competitive DNA binding does not seem to be the underlying mechanism for the inhibitory effect. This is supported by the observation that inhibition is not seen at early time points, but requires prolonged IFN-γ treatment. IFN-γ stimulation results in a loss of IL-4-induced Stat6 tyrosine phosphorylation, nuclear translocation, and DNA binding. Using the fibrosarcoma cell line U3A, which lacks Stat1, we demonstrated that the transcription activation function of Stat1 is required for the IFN-γ-mediated repression. Repression was restored by overexpression of Stat1α, but not Stat1β, in U3A cells. Treatment with IFN-γ, but not IL-4, specifically up-regulates the expression of SOCS-1 (silencer of cytokine signaling), a recently characterized inhibitor of cytokine signaling pathways, such as IL-6 and IFN-γ. Overexpression of SOCS-1 effectively blocks IL-4-induced Stat6 phosphorylation and transcription. This suggests that IFN-γ-mediated repression of IL-4-induced transcription is at least in part mediated by SOCS-1.