SOCS3 limits endotoxin-induced endothelial dysfunction by blocking a required autocrine interleukin 6 signal in human umbilical vein endothelial cells

Increased circulating levels of soluble interleukin (IL)-6 receptor (sIL-6R) are commonly observed during inflammatory responses, allowing for IL-6 signaling to occur in cells that express the ubiquitous receptor subunit gp130 but not IL-6R, such as endothelial cells. Activation of Toll-like receptor (TLR)-4 or the tumor necrosis factor (TNF) receptor leads to NF-{kappa}B-dependent increases in endothelial IL-6 expression. Thus, we hypothesize that danger signals may induce autocrine IL-6 signaling within the endothelium via sIL-6R-mediated trans-signaling. In support of this hypothesis, we recently demonstrated that conditional deletion in the endothelium of the IL-6 signaling inhibitor SOCS3 leads to rapid mortality in mice challenged with the TLR-4 agonist endotoxin through increases in vascular leakage, thrombosis, leukocyte adhesion, and a type I-like interferon response. Here, we sought to directly test a role for sIL-6R in LPS-treated human umbilical vein endothelial cells. We show that cotreatment with sIL-6R dramatically increases the loss of barrier function and the expression of COX2 and tissue factor mRNA levels induced by LPS. This co-treatment led to a strong activation of STAT1 and STAT3 while not affecting LPS-induced activation of p38 and NF-{kappa}B signaling. Similar results were obtained when sIL-6R was added to a TNF challenge. JAK inhibition by pretreatment with ruxolitinib or by SOCS3 overexpression blunted LPS and sIL-6R synergistic effects, while SOCS3 knockdown further increased the response. Together, these findings demonstrate that IL-6 signaling downstream of NF-kB activation leads to a strong endothelial activation and may explain the acute endotheliopathy observed during critical illness.