Hypoxia-stimulated cardiac fibroblast production of IL-6 promotes myocardial fibrosis via the TGF-β1 signaling pathway

Interlukin-6 (IL-6) is a multifunctional cytokine produced by several cell types that has a role in fibrosis. Fibroblasts (FBs) maintain this underlying pathogenic change through regulation of IL-6 production; however, its potential functional role in regulating surrounding cellular structural changes during ischemic myocardial remodeling remains unexplored. Here, we generated FBs, cardiomyocytes (CMs), and blood vascular endothelial cells (ECs) from the ventricles of neonatal rats. IL-6 was then overexpressed in FBs and the cells were treated with IL-6 receptor inhibitor (IL6RI), TGF-β1 receptor inhibitor (TβRI), or MMP2/MMP9 inhibitor (MMPI) using monoculture or coculture models under hypoxic conditions. The results indicate that overexpression of IL-6 is sufficient to induce myofibroblastic proliferation, differentiation, and fibrosis, probably via increased TGF-β1-mediated MMP2/MMP3 signaling. The use of IL6RI, TβRI, or MMPI diminished these effects. In addition, IL-6 activated the apoptosis-associated factors Caspase3 and Smad3, and decreased the expression of anti-apoptotic factor Bcl2, resulting in apoptosis of CMs under hypoxic coculture: IL6RI or TβRI inhibited these effects. Unexpectedly, IL-6-overexpressing FBs significantly increased the angiogenesis of ECs, which involved significant increases in the expression of proangiogenic growth factors. Treatment of FBs with IL6RI or TβRI in coculture with ECs reduced the levels of secreted proangiogenic growth factors, and the angiogenesis of ECs was significantly downregulated. Thus, IL-6 functions in ischemic myocardial remodeling through multifunctional reprogramming of hypoxia-associated FBs towards fibrosis via upregulation of the TGF-β1 signaling pathway.