Transforming growth factor-β1 is up-regulated by podocytes in response to excess intraglomerular passage of proteins: A central pathway in progressive glomerulosclerosis

Chronic diseases of the kidney have a progressive course toward organ failure. Common pathway mechanisms of progressive injury, irrespectively of the etiology of the underlying diseases, include glomerular capillary hypertension and enhanced passage of plasma proteins across the glomerular capillary barrier because of impaired permselective function. These changes are associated with podocyte injury and glomerular sclerosis. Direct evidence for causal roles is lacking, particularly for the link between intraglomerular protein deposition and sclerosing reaction. Because transforming growth factor-β1 (TGF-β1) is the putative central mediator of scarring, we hypothesized that TGF-β1 can be up-regulated by protein overload of podocytes thereby contributing to sclerosis. In rats with renal mass reduction, protein accumulation in podocytes as a consequence of enhanced transcapillary passage preceded podocyte dedifferentiation and injury, increase in TGF-β1 expression in podocytes, and TGF-β1-dependent activation of mesangial cells. Angiotensin-converting enzyme inhibitor prevented both accumulation of plasma proteins and TGF-β1 overexpression in podocytes and sclerosis. Albumin load on podocytes in vitro caused loss of the synaptopodin differentiation marker and enhanced TGF-β1 mRNA and protein. Conditioned medium of albumin-stimulated podocytes induced a sclerosing phenotype in mesangial cells, an effect mimicked by TGF-β1 and blocked by anti-TGF-β1 antibodies. Thus, the passage of excess plasma proteins across the glomerular capillary wall is the trigger of podocyte dysfunction and of a TGF-beta;1-mediated mechanism underlying sclerosis. Agents to reduce TGF-β1, possibly combined with angiotensin blockade, should have priority in novel approaches to treatment of progressive nephropathies.