Epithelial-mesenchymal transdifferentiation of renal tubular epithelial cells induced by urinary proteins requires the activation of PKC-α and βI isozymes

Proteinuria is a common feature for almost all glomerular diseases and reflects the severity of the glomerular lesion. The presence of a large amount of proteins in tubular fluid, however, may also contribute to the development of RIF (renal interstitial fibrosis). Endocytosis of albumin in proximal tubular cells triggers PKC (protein kinase C)-dependent generation of reactive oxygen species and secretion of chemokines. As a family including 12 isozymes, which PKC isozymes participate in RIF is still unclear. EMT (epithelial-mesenchymal transdifferentiation) of RTECs (renal tubular epithelial cells) plays a crucial role in the progress of RIF induced by proteinuria. In the present study, we investigated the role of classical PKC isozymes in the proteinuria-induced EMT of RTECs. Employing immunochemical staining, we found that PKC-alpha, -beta I and -beta II were expressed in glomerulus and in RTECs in both normal and diseased renal tissues, while PKC-gamma was only expressed in podocytes in the glomerulus. Treatment of HK-2 cells with extracted urinary proteins resulted in EMT, as evidenced by morphological changes, decreased E-cadherin expression, increased alpha-SMA (alpha-smooth muscle actin) expression, as well as production of type I collagen and fibronectin. Western blot analysis of PKC isozymes in the cytosolic compared with membrane fraction revealed translocation of PKC-alpha and -beta I, but not PKC-beta II, in HK-2 cells undergoing EMT. Pretreatment with selective PKC-alpha inhibitor G-6976 or PKC-beta inhibitor significantly attenuated EMT induced by urinary proteins. In summary, the present study suggested that PKC-alpha and -beta I play critical roles in the EMT of RTECs in response to urinary proteins.