Introduction and objectives The systemic inflammatory response syndrome (SIRS) which complicates most cases of surgery necessitating cardiopulmonary bypass (snCPB) is associated with endothelial cell injury and activation of the receptor for advanced glycation end products (RAGE). We hypothesised that RAGE activation leads to endothelial cell damage and that plasma levels of RAGE ligands, S100A8/A9 and S100A12, increased following snCPB will be positively associated with raised plasma syndecan-1 shed from damaged endothelial cells. We also hypothesised that S100 proteins and for comparison TNFα directly modulate syndecan-1 expression in cultured endothelial cells.

Methods Enzyme-linked immunosorbent assay measurements of syndecan-1, S100A8/A9 and S100A12 in plasma samples collected from patients pre- and post-snCPB (n=12); real-time-PCR determination of syndecan-1 expression and the house-keeping gene, GAPDH, in human umbilical vein endothelial cells (HUVEC) following incubation with TNFα (20ng/ml), S100A8/A9 (2µg/ml) and S100A12 (2µg/ml) for 3 to 24h.

Results Plasma levels of syndecan-1, S100A12 and S100A8/A9 levels increased following snCPB. Post-snCPB levels of syndecan-1 (86.1±16.2ng/ml) correlated, positively (r2=0.437, p=0.019; and r2=0.729, p=0.0004, respectively) with post-snCPB levels of S100A8/A9 (4.5±0.6ng/ml) and S100A12 (92.9±22.8ng/ml). In cultured HUVEC, TNFα significantly (p<0.01, n=3) decreased syndecan-1 mRNA expression by 75% at 6h and expression remained suppressed at 24h. By contrast, neither S100A8/A9 nor S100A12, under the conditions investigated in this study, significantly altered syndecan-1 mRNA expression.

Conclusion A positive association between post-operative plasma levels of RAGE ligands, S100A8/A9 and S100A12, and syndecan-1 is suggestive of a link between RAGE activation and endothelial injury, key feature of SIRS following snCPB. However, in cultured endothelial cells only TNFα and not S100A8/A9 or S100A12 decreased syndecan-1 mRNA expression; where decreased expression is indicative of reduced endothelial protective function. Possible explanations for the differences with S100A8/A9 and S100A12 in vivo and in vitro are that effects on syndecan-1 shedding in patients undergoing snCPB are indirect; and/or that in vitro, pre-activation of endothelial cells is required to up-regulate RAGE expression in order for S100 proteins to modulate syndecan-1 expression. These questions are the subject of on-going studies into the role of RAGE activation in endothelial dysfunction in SIRS.