Tissue kallikrein promotes cardiac neovascularization by enhancing endothelial progenitor cell functional capacity

Tissue kallikrein (TK) has been demonstrated to improve neovasculogenesis after myocardial infarction (MI). In the present study, we examined the role and underlying mechanisms of TK in peripheral endothelial progenitor cell (EPC) function. Peripheral blood-derived mononuclear cells containing EPCs were isolated from rat. The in vitro effects of TK on EPC differentiation, apoptosis, migration, and vascular tube formation capacity were studied in the presence or absence of TK, kinin B(2) receptor antagonist (icatibant), and phosphatidylinositol-3 kinase inhibitor (LY294002). Apoptosis was evaluated by flow-cytometry analysis using Annexin V-FITC/PI staining, as well as western-blot analysis of Akt phosphorylation and cleaved caspase-3. Using an MI mouse model, we then examined the in vivo effects of human TK gene adenoviral vector (Ad.hTK) administration on the number of CD34(+)Flk-1(+) progenitors in the peripheral circulation, heart tissue, extent of vasculogenesis, and heart function. Administration of TK significantly increased the number of Dil-LDL/UEA-lectin double-positive early EPCs, as well as their migration and tube formation properties in vitro. Transduction of TK in cultured EPCs attenuated apoptosis induced by hypoxia and led to an increase in Akt phosphorylation and a decrease in cleaved caspase-3 levels. The beneficial effects of TK were blocked by pretreatment with icatibant and LY294002. The expression of recombinant human TK in the ischemic mouse heart significantly improved cardiac contractility and reduced infarct size 7 days after gene delivery. Compared with the Ad.Null group, Ad.hTK reduced mortality and preserved left ventricular function by increasing the number of CD34(+)Flk-1(+) EPCs and promoting the growth of capillaries and arterioles in the peri-infarct myocardium. These data provide direct evidence that TK promotes vessel growth by increasing the number of EPCs and enhancing their functional properties through the kinin B(2) receptor-Akt signaling pathway.