Activation of fractalkine/CX3CR1 by vascular endothelial cells induces angiogenesis through VEGF-A/KDR and reverses hindlimb ischaemia.

Aims: The present study investigated the detailed mechanism by which fractalkine (Fkn), a CX3C chemokine, induces angiogenesis and its functional implication in alleviating ischaemia in vivo.

Methods And Results: Fkn induced new vessel formation on the excised rat aorta and chick chorioallantoic membrane (CAM) through CX3CR1 activation. Immunoblotting analysis, promoter assay and electrophoretic mobility shift assay showed that Fkn upregulated hypoxia-inducible factor-1 alpha (HIF-1alpha) by cultured human aortic endothelial cells (ECs), which in turn induced mRNA and protein levels of vascular endothelial growth factor (VEGF)-A through a p42/44 mitogen-activated protein kinase pathway. In vivo Fkn-induced angiogenesis on CAM was completely blocked by functional inhibition of VEGF receptor 2 kinase insert domain-containing receptor (KDR) and Rho GTPase. C57/BL6 mice with CX3CR1(-/-) bone marrow-derived cells developed angiogenesis in the implanted Fkn-mixed Matrigel plug, suggesting CX3CR1 activation in vascular ECs is sufficient for Fkn-induced angiogenesis in vivo. The condition of rat hindlimb ischaemia, which rapidly stimulated mRNA expression of both Fkn and VEGF-A, was significantly alleviated by the injection of whole-length Fkn protein.

Conclusion: Fkn-induced activation of CX3CR1 by ECs leads to in vivo angiogenesis through two sequential steps: the induction of HIF-1alpha and VEGF-A gene expression by CX3CR1 activation and the subsequent VEGF-A/KDR-induced angiogenesis. The potent induction of angiogenesis by Fkn can be used as a therapeutic strategy for alleviating peripheral ischaemia.