Current Diabetes Reviews, 2005, 1, 41-58 41 1573-3998/05 $50.00+.00 © 2005 Bentham Science Publishers Ltd. Endothelial Progenitor Cells and Vascular Biology in Diabetes Mellitus: Current Knowledge and Future Perspectives Gian P. Fadini*, Carlo Agostini** and Angelo Avogaro* Department of Clinical and Experimental Medicine, * Division of Metabolic Diseases, ** Clinical Immunology and Hematology, University of Padova, School of Medicine Abstract: A growing amount of evidence demonstrates that Endothelial Progenitor Cells (EPCs) are involved in adult neovasculogenesis and maintenance of vascular integrity. EPC decrease and dysfunction are related to atherosclerosis and cardiovascular disease (CVD), and it has been proposed that the level of circulating EPCs may be used as a surrogate index of cumulative cardiovascular risk. Moreover, many experimental approaches reveal that exogenous EPC injection stimulates blood flow recovery in critical limb and myocardial ischemia, providing a new therapeutic tool for CVD. Diabetes Mellitus is a clinical condition characterized by a high incidence of CVD and is indeed associated with alterations in EPC physiology. In this review we focus on the relationships between EPCs and vascular biology, with particular regard to Diabetes Mellitus and future therapeutical implications. Keywords: Endothelium, Diabetes, Angiogenesis. INTRODUCTION Studies on endothelial progenitor cells (EPCs) began in 1997, when Asahara and colleagues isolated putative progenitors implicated in adult neovasculogenesis, challenging the paradigm that de novo vessel formation exclusively occurs during embryonic development [1,2]. Subsequent studies addressed EPC pathophysiology in various clinical conditions associated with cardiovascular risk (CVD), and revealed that circulating progenitor cells are likely to be involved in many aspects of atherosclerotic disease [3], while a role for EPCs in tumoral angiogenesis and wound repair have also been postulated [4]. The discovery that a subtype of peripheral blood mononuclear cells (PBMCs) can differentiate into endothelial cells [6] has led to a new field of cardiovascular science, recently including some clinical trials of cellular therapy for limb and myocardial ischemia [7]. Endothelial Progenitor Cells Circulating EPCs are thought to be a subset of bone marrow-derived PBMCs, expressing immature surface markers common to hematopoietic stem cells, such as CD34 and CD133, and endothelial lineage markers [8]. EPCs can be isolated from peripheral, umbilical cord, and bone marrow blood [8]. CD34 represents a marker of immature stem cells that is often used to characterize EPCs together with other surface antigens. However, as CD34 is also *Address correspondence to this author at the Department of Clinical and Experimental Medicine, Division of Metabolic Diseases, University of Padova, School of Medicine, Via Giustiniani 2, 35128 Padova, Italy; Tel: 0039-049-8212178; Fax: 0039-049-8754179; E-mail: angelo.avogaro@ unipd.it expressed at lower levels on mature endothelial cells, most recent studies used CD133, a marker of more immature hematopoietic stem cells that is now considered the best surface marker to define, identify and isolate circulating EPCs. CD133 (also known as AC133 or prominin) is a highly conserved antigen with unknown biological activity, which is expressed on hematopoietic stem cells, but not on mature endothelial cells and monocytes [9-12]. Even if the exact phenotype of EPCs has not been definitively established yet, there is general agreement for the use of at least one additional marker reflecting endothelial commitment: the most used is Vascular Endothelial Growth Factor Receptor-2 (VEGFR-2 or KDR), while others are Platelet-Endothelial Cells Adhesion Molecule-1 (PECAM-1 or CD31), Vascular Endothelial-cadherin, von Willebrand Factor, c-kit, Tie-2 and VEGFR-1 [13]. In most published studies, the amounts of circulating EPCs are determined by a culture method [14-18]: EPCs are defined as fibronectin adherent peripheral blood-derived cells uptaking acetilated LDL and binding Ulex-selectin in culture, and then further characterized by the expression of surface markers. At present there is no general agreement on the methods to define EPCs, and different studies used different way of identification and isolation, sometimes making results not easily concordant and comparable. However, it should be noted that the culture method is time-dependent and that results vary largely depending on culture conditions. In fact endothelial cells cultured from peripheral blood do not correspond to the actual population of circulating EPCs, but include mature circulating endothelial cells, shed off the vessel wall, and monocyte/macrophage-derived cells assuming endothelial phenotype in culture [19]. Moreover EPC count in vitro does not provide information on the absolute number of