IFNγ-dependent SOCS3 expression inhibits IL-6-induced STAT3 phosphorylation and differentially affects IL-6 mediated transcriptional responses in endothelial cells

IL-6 has proand anti-inflammatory effects and is involved in endothelial cell (EC) dysfunction. The anti-inflammatory effects of IL-6 are mediated by signal transducer and activator of transcription-3 (STAT3), which is importantly controlled by suppressor of cytokine signaling 3 (SOCS3). Therefore, cytokines that modulate SOCS3 expression might inhibit the anti-inflammatory effects of IL-6. We hypothesized that in EC, interferon-γ (IFNγ)-induced SOCS3 expression leads to inhibition of IL-6-induced STAT3 activation and IL-6-dependent expression of anti-, but not pro-inflammatory, target genes. IFNγ activated STAT1 and STAT3 and increased SOCS3 expression in EC. IL-6 only activated STAT3 and induced SOCS3 expression. IFNγ pretreatment of EC inhibited IL-6-induced STAT3 activation accompanied by increased SOCS3 protein. Inhibition of SOCS3 expression, using costimulation, Act-D, and small interfering RNA (siRNA), subsequently implicated the importance of IFNγ-induced SOCS3 in this phenomenon. Pretreatment of EC with IFNγ also affected the transcriptional program induced by IL-6. We identified 1) IL-6 antiinflammatory target genes that were inhibited by IFNγ, 2) IFNγ-target genes of pro-inflammatory nature that were increased in response to IL-6 in the presence of IFNγ, and 3) a set of target genes that were increased upon IL-6 or IFNγ alone, or combined IFNγ and IL-6. In summary, by increasing SOCS3 expression in EC, IFNγ can selectively inhibit STAT3-dependent IL-6 signaling. This in turn leads to decreased expression of some EC protective genes. In contrast, other genes of pro-inflammatory nature are not inhibited or even increased. This IFNγ-induced shift in IL-6 signaling to a pro-inflammatory phenotype could represent a novel mechanism involved in EC dysfunction.