The CXCR4 Signaling In Stem/Progenitor Cell-mediated Cardiovascular Repair

Ischemic heart disease and consequent heart failure remain the leading cause of morbidity and mortality worldwide. Traditional therapies, such as angioplasty and thrombolytic agents, can relieve only the cause of infarction; no existing medication or procedure can effectively replace cardiac scarring with functional contractile tissue. Newer cell therapies that incorporate recently identified populations of progenitor cells (PCs) may regenerate cardiac tissue directly by inducing neovasculogenesis and cardiogenesis. However, early clinical studies achieved only modest efficacy when PCs were injected directly into the injured heart or when pharmacological agents were used to mobilize PCs from the bone marrow (BM) to the circulation. Only a very small number of transplanted PCs are retained in the ischemic myocardium and poor cell retention is one of the primary barriers to the effectiveness of cell therapy. In addition, the current mobilizing agents are not PC-specific; the inflammatory cells co-mobilized from BM deteriorate the microenvironment of the injured tissue and jeopardize PC's regenerative capacity. Thus, techniques that enhance the recruitment and retention of transplanted PCs or techniques that can selectively mobilize endogenous BM PCs are crucial to adequately replenish the resident progenitor cell pool and to maximize its regenerative potential.The CXC chemokine receptor 4 (CXCR4) belongs to the large super-family of G protein-coupled receptors (GPCR), and is involved in a number of biological processes including cardiovascular organogenesis, hematopoiesis, and immune response. Recent reports indicate that the interactions between CXCR4 and its ligand, the CXC chemokine stromal cell-derived factor 1 (SDF-1), critically mediate the ischemia-induced recruitment of bone marrow-derived circulating stem/progenitor cells. In this study, we have investigated the molecular mechanism of CXCR4 signaling in the maintenance and mobilization of BM PCs (Partâ… ) and the usefulness of the induced expression of CXCR4 in the recruitment and retention of cardiosphere-derived PCs to the ischemic myocardium (Partâ…¡). Our results suggest that therapies targeting CXCR4 signaling may enhance the benefit of stem/progenitor cell mediated cardiovascular repair. Rationale:The mobilization of bone-marrow (BM) progenitor cells (PCs) is largely governed by interactions between stromal-cell derived factor 1 (SDF-1) and CXC-chemokine receptor 4 (CXCR4). Ischemic injury disrupts the SDF-1-CXCR4 interaction and releases BM PCs into the peripheral circulation, where the mobilized cells are recruited to the injured tissue and contribute to vessel growth. BM PCs can also be mobilized by the pharmacological CXCR4 antagonist AMD3100, but the other components of the SDF-1-CXCR4 signaling pathway are largely unknown. c-kit, a membrane bound tyrosine-kinase and the receptor for stem cell factor, has also been shown to play a critical role in BM PC mobilization and ischemic tissue repair.Objective:To investigate the functional interaction between SDF-1-CXCR4 signaling and the c-kit activity in BM PC mobilization.Methods and Results:AMD3100 administration failed to mobilize BM PCs in mice defective in c-kit kinase activity or in mice transplanted with BM cells that expressed a constitutively active c-kit mutant. Furthermore, BM levels of phosphorylated c-kit (phospho-c-kit) declined after AMD3100 administration and after CXCR4 deletion. In cells adhering to culture plates coated with vascular cell adhesion molecule 1 (VCAM-1), SDF-1 and SCF increased phospho-c-kit levels, and AMD3100 treatment suppressed SDF-1-induced, but not SCF-induced, c-kit phosphorylation. SDF-1-induced c-kit phosphorylation also required the activation of Src non-receptor tyrosine kinase: pre-treatment of cells with a selective Src inhibitor blocked both c-kit phosphorylation and the interaction between c-kit and phosphorylated Src.Conclusions:These findings indicate that the regulation of BM PC trafficking by SDF-1 and CXCR4 is dependent on Src-mediated c-kit phosphorylation. Rationale:Myocardial infarction (MI) rapidly depletes the endogenous cardiac progenitor-cell pool, and the inefficient recruitment of exogenously administered progenitor cells limits the effectiveness of cardiac cell therapy. Recent reports indicate that interactions between the CXC chemokine stromal cell-derived factor 1 (SDF-1) and its receptor CXC chemokine receptor 4 (CXCR4) critically mediate the ischemia-induced recruitment of bone marrow-derived circulating stem/progenitor cells, but the expression of CXCR4 in cardiac progenitor cells is very low.Objective:Here, we studied the influence of hypoxia on CXCR4 expression in cardiac progenitor cells, on the recruitment of intravenously administered cells to ischemic heart tissue, and on the preservation of heart function in a murine model of MI.Methods and Results:We found that hypoxic preconditioning increased CXCR4 expression in cardiosphere-derived, Lin-/c-kit+ progenitor (CLK) cells and markedly augmented CLK-cell migration (in vitro) and recruitment (in vivo) to the ischemic myocardium. Four weeks after surgically induced MI, infarct size and heart function were significantly better in mice administered hypoxia-preconditioned CLK cells than in mice treated with cells cultured under normoxic conditions. Furthermore, these effects were largely abolished by the addition of a CXCR4 inhibitor.Conclusions:These results indicate that the benefits of hypoxic preconditioning are mediated by the SDF-1/CXCR4 axis, and that therapies targeting this axis may enhance cardiac progenitor cell-based regenerative therapy.