Activation of Peroxisome Proliferator-activated Receptor-{delta} Enhances Regenerative Capacity of Human Endothelial Progenitor Cells by Stimulating Biosynthesis of Tetrahydrobiopterin

The mechanisms underlying the regenerative capacity of endothelial progenitor cells (EPCs) are not fully understood. We hypothesized that biosynthesis of tetrahydrobiopterin is an important mechanism responsible for the stimulatory effects of peroxisome proliferator-activated receptor-δ (PPARδ) activation on regenerative function of human EPCs. Treatment of human EPCs with a selective PPARδ agonist GW501516 for 24 hours increased the levels of mRNA, protein, and enzymatic activity of GTP cyclohydrolase I (GTPCH I), as well as the production of tetrahydrobiopterin. The effects of GW501516 were mediated by suppression of PTEN expression, thereby increasing phosphorylation of AKT. The AKT signaling also mediated GW501516-induced phosphorylation of endothelial NO synthase. In addition, activation of PPARδ significantly enhanced proliferation of EPCs. This effect was abolished by the GTPCH I inhibitor, 2,4-diamino-6-hydroxypyrimidine, or genetic inactivation of GTPCH I with small interfering RNA but not by inhibition of endothelial NO synthase with N(G)-nitro-l-arginine methyl ester. Supplementation of NO did not reverse 2,4-diamino-6-hydroxypyrimidine-inhibited 5-bromodeoxyuridine incorporation. Furthermore, transplantation of human EPCs stimulated re-endothelialization in a mouse model of carotid artery injury. Pretreatment of EPCs with GW501516 significantly enhanced the ability of transplanted EPCs to repair denuded endothelium. GTPCH I-small interfering RNA transfection significantly inhibited in vivo regenerative capacity of EPCs stimulated with GW501516. Thus, in human EPCs, activation of PPARδ stimulates expression and activity of GTPCH I and biosynthesis of tetrahydrobiopterin via PTEN-AKT signaling pathway. This effect enhances the regenerative function of EPCs.