Glomerular epithelial cells transform to myofibroblasts: early but not late removal of TGF-β1 reverses transformation

Studies were carried out to determine whether epithelial–mesenchymal transformation (EMT), well described in renal tubular epithelial cells, also occurs in glomerular epithelial cells and whether it is reversible. To this effect, cultured glomerular epithelial cells were incubated with TGF-β1 and their transformation into myofibroblasts was studied. At 4 days, the cells altered their phenotype, as shown by a change in shape, an increase in intracellular staining for α-smooth muscle actin (α-SMA), a decrease in membrane staining for cytokeratin, and an increase in matrix deposition. Changing the medium after 4 days by excluding TGF-β1 and adding fetal bovine serum (FBS) [as a source of epidermal growth factor (EGF) and other growth factors] caused the cells to revert to their original epithelial phenotype. By contrast, when the medium was changed in the same manner after 8 days of exposure to TGF-β1, the cells did not revert but remained myofibroblastic. Staining the cells for expression of EGF receptor before and after exposure to TGF-β1 caused this receptor, originally present on the plasma membrane, to become partly intracellular after 4 days of TGF-β1 exposure and completely intracellular after 8 days of TGF-β1 exposure. Kidney sections from 2 models of renal mass reduction were stained. Loss of the epithelial marker (podocalyxin) staining and the acquisition of α-SMA staining was observed in the glomeruli. It is concluded that EMT takes place in glomerular epithelial cells in vivo and in vitro. In cultured glomerular epithelial cells, the process can be reversed by early, but not late intervention. It seems that TGF-β1 exposure progressively downregulates the EGF receptor on the membrane, rendering the cell refractory to EGF signals critical for maintaining the epithelial phenotype.