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KIT

CD117-positive cells contributing to cardiac cell turnover in normal and pathological conditions have recently been described in adult human heart. Since the precise spatial and temporal expression of extracellular matrix proteins and their receptors is critical for organ formation, we compared the distribution of cardiac primitive CD117-positive cells in the human adult normal and pathological hearts with ischemic cardiomyopathy, with respect to localization and expression of laminin and integrin isoforms. In the pathological hearts, CD117-positive cells were significantly more numerous than in the normal hearts. They were localized mainly in the atria and were up to 38-fold more numerous in the subepicardium than in the myocardium. Compared with normal hearts, most CD117-positive cells in the subepicardium of pathological hearts were alpha(6) integrin-positive. Laminin-1, typical of developing heart, was found predominantly in the subepicardium of adult heart. Immunoblotting revealed its highest expression in the normal atrium and pathological left ventricle. Both laminin isoforms reduced apoptosis and increased proliferation and migration of CD117-positive cells in vitro with respect to control, but the effects of laminin-1 significantly outweighed those of laminin-2. Signaling mediated by alpha(6) integrin was implicated in the migration and protection from apoptosis, as documented by transfection with specific small interfering RNA. These data reveal that the increase in the number of cardiac CD117-positive cells and the expression of laminin-1 are observed in ischemic cardiomyopathy. Subepicardial localization of CD117-positive cells and expression of laminin-1 and alpha(6) integrin subunits may all correspond to the activation of regeneration involving an epithelial-mesenchymal transition recently described in adult heart.

epithelial-mesenchymal transition is implicated in the remodelling of tissues during development and in the adult life. In the heart, it gives origin to progenitors of fibroblasts, coronary endothelium, smooth muscle cells, and cardiomyocytes. Moreover, epicardially-derived cells determine myocardial wall thickness and Purkinje fibre network. Recently, the presence of numerous cardiac stem cells in the subepicardium of the adult human heart has been described and the hypothesis that epicardially-derived cells can contribute to the population of cardiac stem cells in the adult heart has been advanced. In an effort to test this hypothesis and establish a possible link between epicardium, epicardially-derived cells and cardiac stem cells in the adult human heart we have examined epicardial mesothelial cells in the normal and pathological adult human heart with ischemic cardiomyopathy in vivo and we have induced and documented their epithelial-mesenchymal transition in vitro. Noticeably, epicardial cells were missing from the surface of pathological hearts and the cells with the expression of epithelial and mesenchymal markers populated thick subepicardial space. When the fragments of epicardium from the normal hearts were cultured on the specific substrate formed by extracellular matrix derived from cardiac fibroblasts, we obtained the outgrowth of the epithelial sheet with the mRNA and protein expression characteristic of epicardium. TGFbeta induced cellular and molecular changes typical of epithelial-mesenchymal transition. Moreover, the epicardially-derived cells expressed CD117 antigen. Thus, this study provides evidence that cardiac stem cells can originate from epithelial-mesenchymal transition of the epicardial cells in the adult human heart.CI - Copyright (c) 2010 Elsevier Ltd. All rights reserved.