Transdifferentiation of Endothelial and Renal Tubular Epithelial Cells into Myofibroblast-Like Cells under in vitro Conditions: A Morphological Analysis

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Abstract
Renal fibrosis is a hallmark of progressive kidney disease and is characterized by an accumulation of extracellular-matrix-synthesizing cells in the glomerulus and tubulointerstitium. This population of myofibroblast-like cells (MFLC) is heterogeneous. It has been experimentally shown, for example, that tubular epithelial cells could change their phenotype into MFLC under certain circumstances, a process called epithelial-mesenchymal transdifferentiation. However, MFLC may also originate from other sources. Therefore, we examined whether endothelial cells (EDC) are able to transdifferentiate into MFLC in vitro. We compared potential differences between syngeneic tubular epithelial cells (EPC) and EDC during transdifferentiation into MFLC using bovine and porcine EDC isolated from pulmonary arteries, and glomerular capillaries. Renal tubular EPC were prepared from bovine renal cortical tissue by collagenase digestion and isolation from homogeneous cell monolayers. Bovine renal tubular EPC stained positive for cytokeratin. Furthermore, tubular EPC selectively incoporated labeled bovine serum albumin, a typical property of differentiated renal tubular cells. EDC were characterized by the absence of epithelial markers (e.g. cytokeratin), but stained positive for vWF. The transdifferentiation of EDC into MFLC occurs sequentially in two steps: First, by a rapid reversible transformation in postconfluent or clonal cultures without the need of cytokine stimulation and second, by a prolonged secondary step in the presence of the transformation-accelerating cytokines and the absence of adherently growing EDC. Thus, EDC that are able to sprout can also irreversibly transdifferentiate into MFLC. On the other hand, prolonged incubation of EPC in the presence of cytokines such as transforming growth factor-β1 and tumor necrosis factor-α leads only to a very small number of MFLC without the ability to further proliferate. Our in vitro data suggest that EDC can more easily transdifferentiate into MFLC than syngeneic renal tubular EPC.