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

Syndecan-4 is a unique member of the syndecan gene family that has the abil ity to bind and activate protein kinase C-alpha. 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) mer e 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 condi tioned medium demonstrated an increased presence of tumor necrosis factor-a lpha (TNF-alpha) in the medium conditioned by hypoxic but not normal myobla sts. Primary cardiac myocytes collected from the wild type C57/129 but not the homozygous TNF-alpha(-/-) knockout mice were able to induce syndecan-4 expression in ECV cells when cultured under hypoxic conditions. In vitro st udies demonstrated that TNF-alpha induced endothelial cell syndecan-4 expre ssion by both increasing syndecan-4 gene expression in an NF-KB-dependent m anner and by prolonging syndecan-4 mRNA half-life. We conclude that TNF-alp ha is the principal factor produced by the ischemic myocytes that is respon sible for induction of endothelial cell syndecan-4 expression and that this requires both transcriptional and posttranscriptional mechanisms.