Pathophysiologic glomerulotubular growth factor link

Circumstantial evidence from clinical and pathologic correlations in patients with glomerular diseases and proteinuria suggest that glomerular protein ultrafiltration contributes to tubulointerstitial injury. A series of studies was performed to examine the hypothesis that in rats with adriamycin-induced nephropathy or with diabetic nephropathy (but not in normal rats) high molecular wt. growth factors are ultrafiltered into tubular fluid and act on tubular cells through apical membrane receptors. Analysis of proximal tubular fluid that was collected by nephron micropuncture indicates ultrafiltration of IGF-I, TGF-beta and HGF. Respective receptors are also expressed in apical membranes in some parts of the nephron as examined by immunohistochemistry. In vitro cell culture experiments using proximal tubular fluid obtained from rats with experimental glomerular diseases indicate that ultrafiltered IGF-I may contribute to increased distal tubular Na-absorption. Indirect evidence also suggests that this growth factor may increase the secretion of collagen types I and IV in proximal tubular cells. TGF-beta and HGF cause increased expression and basolateral secretion of MCP-1 in proximal tubular and collecting duct cells. There may be other biologic effects on tubules that are caused by apical exposure to ultrafiltered growth factors. These studies suggest that the glomerular ultrafiltration of bioactive proteins causes or contributes to tubulo-interstitial pathology in glomerular proteinuria.