Stat1-independent induction of SOCS-3 by interferon-γ is mediated by sustained activation of Stat3 in mouse embryonic fibroblasts

Using microarray technology, we previously demonstrated that IFN-γ induces suppressor of cytokine signaling-3 (SOCS-3) in Stat1−/− mouse embryonic fibroblasts and bone marrow-derived macrophages. In this study, we have investigated the mechanism by which SOCS-3 is induced by Stat1-independent signal transduction pathway. Tyrosine kinases Jak1 and Jak2 are required for SOCS-3 induction by IFN-γ in mouse embryonic fibroblasts. IFN-γ stimulated strong and sustained activation of Stat1 whereas Stat3 activation was weak and transient in wild-type fibroblasts. In contrast, Stat3 is activated strongly and in a sustained manner in Stat1−/− fibroblasts. The Src kinase inhibitor SU6656 suppressed IFN-γ activation of Stat3 in both wild-type and Stat1−/− fibroblasts. However, SU6656 inhibited IFN-γ induction of SOCS-3 completely in Stat1−/− but not in wild-type fibroblasts. Knock down of Stat3 by short interfering RNA abrogated Stat3 activation and SOCS-3 induction by IFN-γ in Stat1−/− fibroblasts. In human fibrosarcoma cell line 2fTGH, IFN-γ activated Stat1 but not Stat3. SOCS-3 induction by IFN-γ is strictly Stat1-dependent. The Stat1 docking site is required for SOCS-3 induction by IFN-γ in human lung adenocarcinoma cells. We propose a model in which sustained activation of Stat1 or Stat3 mediates SOCS-3 induction by IFN-γ in wild-type and Stat1−/− mouse embryonic fibroblasts, respectively.