To Örebro University

BACKGROUND: IL-6 classic signaling is linked to anti-inflammatory functions while the trans-signaling is associated with pro-inflammatory responses. Classic signaling is induced via membrane-bound IL-6 receptor (IL-6R) whereas trans-signaling requires prior binding of IL-6 to the soluble IL-6R. In both cases, association with the signal transducing gp130 receptor is compulsory. However, differences in the downstream signaling mechanisms of IL-6 classic- versus trans-signaling remains largely elusive.

METHODS: In this study, we used flow cytometry, quantitative PCR, ELISA and immuno-blotting techniques to investigate IL-6 classic and trans-signaling mechanisms in Human Umbilical Vein Endothelial Cells (HUVECs).

RESULTS: We show that both IL-6R and gp130 are expressed on the surface of human vascular endothelial cells, and that the expression is affected by pro-inflammatory stimuli. In contrast to IL-6 classic signaling, IL-6 trans-signaling induces the release of the pro-inflammatory chemokine Monocyte Chemoattractant Protein-1 (MCP-1) from human vascular endothelial cells. In addition, we reveal that the classic signaling induces activation of the JAK/STAT3 pathway while trans-signaling also activates the PI3K/AKT and the MEK/ERK pathways. Furthermore, we demonstrate that MCP-1 induction by IL-6 trans-signaling requires simultaneous activation of the JAK/STAT3 and PI3K/AKT pathways.

CONCLUSIONS: Collectively, our study reports molecular differences in IL-6 classic- and trans-signaling in human vascular endothelial cells; and elucidates the pathways which mediate MCP-1 induction by IL-6 trans-signaling.

Sprouting angiogenesis is the formation of new capillaries from existing vessels in response to tissue hypoxia due to growth/development, repair/healing, and also chronic inflammation. In this study, we aimed to elucidate the effect of IL-6, a pleiotropic cytokine with both pro-inflammatory and anti-inflammatory functions, in regulating the sprouting angiogenic response of endothelial cells (ECs). We found that activation of IL-6 trans-signaling inhibited the migration, proliferation, and tube formation ability of ECs. In addition, inhibition of the autocrine IL-6 classic-signaling by depleting endogenous IL-6 from ECs impaired their tube formation ability. At the molecular level, we found that IL-6 trans-signaling in ECs upregulated established endogenous anti-angiogenic factors such as CXCL10 and SERPINF1 while at the same time downregulated known endogenous pro-angiogenic factors such as cKIT and CXCL8. Furthermore, prior activation of ECs by IL-6 trans-signaling alters their response to vascular endothelial growth factor-A (VEGF-A), causing an increased p38, but decreased Erk1/2 phosphorylation. Collectively, our data demonstrated the dual facets of IL-6 in regulating the sprouting angiogenic function of ECs. In addition, we shed light on molecular mechanisms behind the IL-6 trans-signaling mediated impairment of endothelial sprouting angiogenic response.