Platelet factor 4 disrupts the intracellular signalling cascade induced by vascular endothelial growth factor by both KDR dependent and independent mechanisms

The mechanism by which the CXC chemokine platelet factor 4 (PF-4) inhibits endothelial cell proliferation is unclear. The heparin-binding domains of PF-4 have been reported to prevent vascular endothelial growth factor 165 (VEGF₁₆₅) and fibroblast growth factor 2 (FGF2) from interacting with their receptors. However, other studies have suggested that PF-4 acts via heparin-binding independent interactions. Here, we compared the effects of PF-4 on the signalling events involved in the proliferation induced by VEGF₁₆₅, which binds heparin, and by VEGF₁₂₁, which does not. Activation of the VEGF receptor, KDR, and phospholipase Cγ (PLCγ) was unaffected in conditions in which PF-4 inhibited VEGF₁₂₁-induced DNA synthesis. In contrast, VEGF₁₆₅-induced phosphorylation of KDR and PLCγ was partially inhibited by PF-4. These observations are consistent with PF-4 affecting the binding of VEGF₁₆₅, but not that of VEGF₁₂₁, to KDR. PF-4 also strongly inhibited the VEGF₁₆₅- and VEGF₁₂₁-induced mitogen-activated protein (MAP) kinase signalling pathways comprising Raf1, MEK1/2 and ERK1/2: for VEGF₁₆₅ it interacts directly or upstream from Raf1; for VEGF₁₂₁, it acts downstream from PLCγ. Finally, the mechanism by which PF-4 may inhibit the endothelial cell proliferation induced by both VEGF₁₂₁ and VEGF₁₆₅, involving disruption of the MAP kinase signalling pathway downstream from KDR did not seem to involve CXCR3B activation.