Myocyte-dependent Regulation of Endothelial Cell Syndecan-4 Expression

Abstract

Syndecan-4 is a unique member of the syndecan gene family that has the ability to bind and activate protein kinase C-␣. 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-␣ (TNF-␣) 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-␣ ؊/؊ knockout mice were able to induce syndecan-4 expression in ECV cells when cultured under hypoxic conditions. In vitro studies demonstrated that TNF-␣ induced endothelial cell syndecan-4 expression by both increasing syndecan-4 gene expression in an NF-B-dependent manner and by prolonging syndecan-4 mRNA half-life. We conclude that TNF-␣ 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.Syndecan-4 is one of the principal heparin sulfate-carrying proteins on the cell surface (1). It is expressed by a number of different cell types including endothelial cells, smooth muscle cells, and cardiac myocytes, although expression in quiescent tissues is fairly low. Syndecans can participate in a variety of biological functions including regulation of blood coagulation, cell adhesion, and low density lipoprotein transport (2, 3). In addition to these activities shared by all syndecans, syndecan-4 cytoplasmic domain can bind to and activate protein kinase C-␣ with the degree of activation regulated by the extent of syndecan-4 cytoplasmic tail phosphorylation (4 -7).Despite these interesting properties, little is known about the regulation of syndecan-4 gene expression. Its levels are increased after various forms of tissue injury including skin wounds (8), vascular wall injury (9), or myocardial infarction (10). The increase in syndecan-4 expression in skin wounds and in the heart following myocardial infarction has been ascribed to the presence of a proline-arginine-rich peptide, PR39. No other factors responsible for this increase have been identified (8, 10). The present study was designed to study the role of cardiac myocytes in the induction of syndecan-4 expression under normal and ischemic (hypoxic) conditions. To this end, we determined the ability of myocyte-or myoblast-conditioned medium to induce syndecan-4 expression in a human endothelial cell line. We find that TNF-␣ 1 secreted by hypoxic but not normal myocytes is able to induce syndecan-4 expression and that the mechanism of this induction involves both transcriptional activati...

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