Phosphomimetic mutants of pigment epithelium-derived factor with enhanced antiangiogenic activity as potent anticancer agents

Pigment epithelium-derived factor (PEDF) is an endogenous inhibitor of angiogenesis and a promising anticancer agent capable of suppressing solid tumor growth in animal cancer models. We have previously shown that PEDF can be phosphorylated and that distinct phosphorylation states of this factor differentially regulate its physiologic function. Here, we report that phosphomimetic mutants of PEDF, which possess significantly increased antiangiogenic activity, are much more efficient than wild-type (WT) PEDF in inhibiting growth and neovascularization in MDA-MB-231 (breast cancer), HCT116 (colon cancer), and U87-MG (glioblastoma) xenograft models. Importantly, the antitumor activity of the phosphomimetic mutants is comparable with that of the established antiangiogenic agent bevacizumab. However, unlike bevacizumab, these compounds act in a vascular endothelial growth factor (VEGF)-independent manner, as they do not affect the levels of VEGF-A mRNA and VEGF receptor 2 phosphorylation. Further immunohistochemical analysis revealed that PEDF mutants affect mainly tumor-residing endothelial cells and prevent the formation of intratumoral vascular network by facilitating endothelial cell apoptosis. It was also found that PEDF mutants reduce survival of endothelial cells in culture much better than WT-PEDF, an effect that is apparent even in the presence of VEGF or basic fibroblast growth factor, and promote much stronger endothelial cell apoptosis. On the other hand, PEDF and its mutants did not affect survival of cultured cancer cells, indicating that the antiangiogenic activity of these agents is the foremost element of the observed antitumor effect. These findings have specific implications on improving the properties of WT-PEDF, which is currently in preclinical development, and encourage the development of PEDF mutants as specific, neovascularization-targeting anticancer agents.