Glycogen Synthase Kinase-3β and Cathepsin B in Diabetic Endothelial Progenitor Cell Dysfunction: An Old Player Finds a New Partner

Microvascular and macrovascular complications associated with type 2 diabetes are leading causes of morbidity and mortality. In fact, cardiovascular (CV) complications account for a major portion of health care costs in patients with diabetes. Although lifestyle changes and pharmacotherapy aimed at CV disease risk factor modification are the mainstay of medical treatment, revascularization (surgical and endovascular) is often necessary to restore tissue perfusion and function. Indeed, nearly 25% of coronary revascularization procedures performed in the U.S. occur in individuals with diabetes (1). Furthermore, diffuse atherosclerosis, impaired ability to form vascular collaterals, and restenosis contribute to higher rates of repeat revascularization in these patients (1). Consequently, cell-based therapy with bone marrow cells and endothelial progenitor cells (EPCs) aimed at inducing angiogenesis and reendothelialization is a promising strategy that is under active investigation in both preclinical and clinical studies (2,3). EPCs constitute a very small subset of circulating blood cells. They are progenitors of endothelial cells and have the ability to proliferate and differentiate to form perfused blood vessels in vivo and tube-like structures in vitro (4). EPC-induced neovascularization in response to tissue hypoxia and injury is a highly coordinated, temporally regulated, and complex set of events that involves mobilization, migration, and homing of EPCs to the target tissue (5,6). Endothelial injury and hypoxia activate the transcription factor hypoxia-induced factor (HIF) to initiate the expression and release of growth factors and chemokines. These include stromal cell– derived factor 1 (SDF-1), vascular endothelial growth factor (VEGF), c-Kit ligand (or SCF), angiopoietin, and interleukin-8 (IL-8), among others (5,6). Platelet aggregation leads to high levels of platelet-derived SDF-1 at the site of endothelial injury (7). EPCs are retained in the bone marrow in distinct niches by their interaction with stromal cells. Circulating SDF-1 and VEGF stimulate production of nitric oxide (NO) by endothelial NO synthase, thereby activating matrix metalloproteinase-9 (MMP-9) (6). In turn, enhanced MMP-9 activity disrupts EPCstromal cell interaction to mobilize EPCs from the marrow. Concentration gradients of SDF-1 direct circulating EPCs to the site of injury (7). Increased surface expression of integrin b2 and selectins (selectins E and P) on the endothelium interact with specific ligands on EPCs to recruit and home EPCs (5,6). These interrelationships are shown in Fig. 1. Phosphatidylinositol-3 kinase (PI3-K) and protein kinase B (Akt) activation not only stimulate NO production, but they also inhibit glycogen synthase kinase-3b (GSK3b) (8). Similarly, activation of canonical Wnt signaling inactivates GSK3b (9). Wnts are secreted glycoproteins known to regulate hematopoiesis and stem cell function (9). In the unstimulated cell, GSK3b phosphorylates and accelerates degradation of HIF-1a and b-catenin (9,10). Inhibition of GSK3b leads to cytosolic accumulation and nuclear translocation of these transcription factors in a manner that increases EPC survival, proliferation, differentiation, mobilization, and adhesion (11–13). EPCs