Interleukin 8 is differently expressed and modulated by PAR‐1 activation in early and late endothelial progenitor cells

The proinflammatory chemokine interleukin 8 exerts potent angiogenic effects on endothelial cells by interacting with its receptors CXCR1 and CXCR2. As thrombin is also a potent inflammatory factor, and as endothelial progenitor cells (EPC) express functional PAR‐1 thrombin receptor, we examined whether PAR‐1 stimulation interferes with the IL‐8 pathway in EPC. EPC were obtained from adult blood (AB) and cord blood (CB). The effect of PAR‐1 stimulation by the peptide SFLLRN on IL‐8, CXCR1 and CXCR2 expression was examined by RTQ‐PCR and at the protein level in AB and CB late EPC and in AB early EPC. Specific siRNA was used to knock down PAR‐1 expression. The IL‐8 gene was expressed strongly in AB early EPC and moderately in late EPC. In contrast, CXCR1 and CXCR2 gene expression was restricted to AB early EPC. The IL‐8 level in AB early EPC conditioned medium was high in basal conditions and did not change after PAR‐1 activation. By contrast, IL‐8 secretion by late EPC was low in basal conditions and strongly up‐regulated upon PAR‐1 activation. PAR‐1 activation induced a number of genes involved in activating protein‐1 (AP‐1) and nuclear factor (NF)‐κB pathways. Conditioned medium of PAR‐1‐activated late EPC enhanced the migratory potential of early EPC, and this effect was abrogated by blocking IL‐8. Target‐specific siRNA‐induced PAR‐1 knockdown, and fully inhibited PAR‐1‐induced IL‐8 synthesis. In conclusion, PAR‐1 activation induces IL‐8 synthesis by late EPC. This could potentially enhance cooperation between late and early EPC during neovascularization, through a paracrine effect.