Mechanisms and consequences of TGF-ß overexpression by podocytes in progressive podocyte disease

Abstract

In patients with progressive podocyte disease, such as focal segmental glomerulosclerosis (FSGS) and membranous nephropathy, upregulation of transforming growth factor-ß (TGF-ß) is observed in podocytes. Mechanical pressure or biomechanical strain in podocytopathies may cause overexpression of TGF-ß and angiotensin II (Ang II). Oxidative stress induced by Ang II may activate the latent TGF-ß, which then activates Smads and Ras/extracellular signal-regulated kinase (ERK) signaling pathways in podocytes. Enhanced TGF-ß activity in podocytes may lead to thickening of the glomerular basement membrane (GBM) by overproduction of GBM proteins and impaired GBM degradation in podocyte disease. It may also lead to podocyte apoptosis and detachment from the GBM, and epithelial-mesenchymal transition (EMT) of podocytes, initiating the development of glomerulosclerosis. Furthermore, activated TGF-ß/Smad signaling by podocytes may induce connective tissue growth factor and vascular endothelial growth factor overexpression, which could act as a paracrine effector mechanism on mesangial cells to stimulate mesangial matrix synthesis. In proliferative podocytopathies, such as cellular or collapsing FSGS, TGF-ß-induced ERK activation may play a role in podocyte proliferation, possibly via TGF-ß-induced EMT of podocytes. Collectively, these data bring new mechanistic insights into our understanding of the TGF-ß overexpression by podocytes in progressive podocyte disease.

Fig. 1

Effects of biomechanical strain on the induction of transforming growth factor-ß (TGF-ß) and angiotensin II (Ang II) in podocytes in patients with progressive podocyte disease. Ang II does not directly stimulate TGF-ß production in podocytes, yet activates NADPH oxidase to produce reactive oxygen species, leading to the activation of latent TGF-ß

Fig. 2

Hypothetical pathway for transforming growth factor-ß (TGF-ß)-induced glomerular basement membrane (GBM) thickening, mesangial matrix expansion, podocyte loss, podocyte proliferation, and epithelial-mesenchymal transition (EMT) by podocytes in progressive podocyte disease. CTGF connective tissue growth factor, ERK extracellular signal-regulated kinase, ILK integrin-linked kinase, PI3K phosphatidyl inositol-3-kinase, VEGF vascular endothelial growth factor