SOCS-3 (suppressor of cytokine signaling 3) is an intracellular protein that is selectively and rapidly induced by appropriate agonists and that modulates responses of immune cells to cytokines by interfering with the Janus kinase/signal transducer and activator of transcription (Jak/STAT) pathway. On the basis of the observations that interferon γ (IFNγ) up-regulates SOCS-3 gene and protein expression in primary mouse macrophages, J774 macrophage cell line and embryonal fibroblasts, we investigated which sequences of the 5′ SOCS-3 gene are responsive to IFNγ. By promoter deletion analysis we identified a functional IFNγ-responsive element, located at nucleotides -72/-64 upstream from the transcription initiation, whose presence and integrity is necessary to ensure responsiveness to IFNγ. This element contains a STAT consensus binding sequence (SOCS-3/STAT-binding element (SBE)) whose specific mutation totally abolished the responsiveness to IFNγ. In contrast, discrete deletion of other 5' regions of the SOCS-3 promoter did not substantially modify the inducibility by IFNγ. Electromobility shift assay analyses revealed that IFNγ promotes specific DNA binding activities to an oligonucleotide probe containing the SOCS-3/SBE sequence. Even though EFNγ triggered tyrosine phosphorylation of both STAT1 and STAT3 in macrophages and J774 cells, only STAT1 was appropriately activated and thus found to specifically bind to the SOCS-3/SBE oligonucleotide probe. Accordingly, IFNγ-induced SOCS-3 protein expression was not impaired in STAT3-deficient embryonal fibroblasts. Taken together, these results demonstrate that the induction of SOCS-3 by IFNγ depends upon the presence of a STAT-binding element in the SOCS-3 promoter that is specifically activated by STAT1.
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