Abstract 115: S100A6 Regulates Endothelial Cell Cycle Progression via Modulation of the STAT1 Signaling Pathway

Abstract

S100A6, also known as Calcyclin, is a 10.5kDa Ca 2+ -binding protein that belongs to the S100 protein family. We have found S100A6 to be highly expressed throughout all vascular layers during the remodeling process in pig coronary arteries after stenting and rat carotid arteries post balloon dilatation injury. It was the objective of this study to decipher S100A6’s function in vascular cells. Abundant S100A6 expression was confirmed in vitro in human umbilical vein endothelial cells (HUVEC) and human umbilical artery smooth muscle cells (SMC). Upon serum stimulation, a significant increase of S100A6 protein levels was observed in both cell types. S100A6 depletion due to siRNA transfection lead to a profoundly reduced proliferation rate in response to growth factors (e.g. VEGF-A), shown by stagnating cell counts and decreased EdU incorporation in HUVEC and SMC. Of note, S100A6 depletion caused a significant migration deficit of SMC. Also, reduced S100A6 levels in HUVEC lead to an increase in cellular senescence, as measured by the expression of senescence associated β-galactosidase expression. To decipher the molecular mechanisms that lead to the phenotype of S100A6 depleted vascular cells, a time-resolved gene expression analysis was carried out in HUVECs and revealed S100A6 to be in control of antiproliferative signaling pathways, while typical pro-proliferative signaling, e.g. the MAPK pathway, was not disrupted. S100A6 depletion caused increased expression and activation of the STAT1 signaling pathway. With increased STAT1 activity, interferon-inducible protein (e.g. IFITM1) expression was upregulated and lead to stabilization and increased expression of the p53 pathway, specifically p21. We propose that S100A6, found to co-localize with the VEGFR2 in a Ca 2+ -dependent manner, controls antiproliferative signaling by inhibition of the STAT1-phosphorylation by the VEGFR2 and suppression of the STAT1-p53 signaling cascade in human endothelial cells. Endothelialization is a hallmark of vascular healing after stenting and balloon dilatation, for example, while it is of utmost importance to block neointima formation. Understanding these signaling pathways is critical to better direct optimization of interventional therapies.