Myocyte-dependent regulation of endothelial cell syndecan-4 expression: Role of TNF-α

Abstract

Syndecan-4 is a unique member of the syndecan gene family that has the ability to bind and activate protein kinase C-α. Whereas increased syndecan- 4 levels have been noted in ischemic hearts, little is known regarding the regulation of its expression. To investigate the role of cardiac myocytes in induction of syndecan-4 expression, human endothelial cells (ECV304) were exposed to a medium conditioned by primary mouse cardiac myocytes or H9c2 cells. The medium conditioned by hypoxic but not normal myocytes was able to induce syndecan-4 expression in ECV cells. Western analysis of the conditioned medium demonstrated an increased presence of tumor necrosis factor-α (TNF-α) in the medium conditioned by hypoxic but not normal myoblasts. Primary cardiac myocytes collected from the wild type C57/129 but not the homozygous TNF-α(-/-) knockout mice were able to induce syndecan-4 expression in ECV cells when cultured under hypoxic conditions. In vitro studies demonstrated that TNF-α induced endothelial cell syndecan-4 expression by both increasing syndecan-4 gene expression in an NF-κB- dependent manner and by prolonging syndecan-4 mRNA half-life. We conclude that TNF-α is the principal factor produced by the ischemic myocytes that is responsible for induction of endothelial cell syndecan-4 expression and that this requires both transcriptional and posttranscriptional mechanisms.