Stem cell factor attenuates vascular smooth muscle apoptosis and increases intimal hyperplasia after vascular injury

OBJECTIVE - Stem cell factor (SCF) through its cognate receptor, the tyrosine kinase c-kit, promotes survival and biological functions of hematopoietic stem cells and progenitors. However, whether SCF/c-kit interactions exacerbate intimal hyperplasia through attenuating VSMC apoptosis induced by vascular injury has not been thoroughly investigated. METHODS AND RESULTS - VSMCs were stimulated with serum deprivation and H2O2 to induce apoptosis. The transcription of c-kit mRNA and the expression of the c-kit protein by VSMCs were estimated by Q-polymerase chain reaction and Western blotting, respectively. The interactions of SCF and c-kit were investigated by in vitro and in vivo experiments. In vitro, H2O2 stimulation significantly induced apoptosis of VSMCs as evidenced by the 3- and 3.2-fold increases of cleaved caspase-3 compared with those in the control group by Western blot and flow cytometric analyses, respectively (P<0.01). Stimulation of apoptosis also caused 3.5- and 9-fold increases in c-kit mRNA transcription and protein expression, respectively, by VSMCs compared with those in the control group. Administration of SCF (10 to 1000 ng/mL) significantly lowered the amount of cleaved caspase-3 in H2O2-treated VSMCs (P<0.01). Specifically, SCF exerted this effect through activating Akt, followed by increasing Bcl-2 and then inhibiting the release of cytochrome-c from the mitochondria to the cytosol. In vivo, the mouse femoral artery was injured with a wire in SCF mutant (Sl/Sl), c-kit mutant (W/W), and colony control mice. In colony control mice, confocal microscopy demonstrated that the wire-injury generated a remarkable activation of caspase-3 on medial VSMCs, coinciding with upregulation of c-kit expression. The wire-injury also caused an increase in the expression of SCF on surviving medial VSMCs and cells in the adventitia. The upregulated c-kit expression in the vessel wall also facilitated homing by circulating SCF cells. Compared with colony control mice, vascular injury in SCF mutant and c-kit mutant mice caused a higher number of apoptotic VSMCs on day 14 and a lower number of proliferating cells, and resulted in significantly less neointimal formation (P<0.01) on day 28. CONCLUSIONS - The interactions between SCF and the c-kit receptor play an important role in protecting VSMCs against apoptosis and in maintaining intimal hyperplasia after vascular injury.