Evidence linking glycated albumin to altered glomerular nephrin and VEGF expression, proteinuria, and diabetic nephropathy

Background. Albumin modified by Amadori-glucose adducts has been linked to the development of diabetic nephropathy through its ability, independent of hyperglycemia, to activate protein kinase C-beta (PKC-beta), up-regulate the transforming growth factor-beta (TGF-beta) system, and stimulate expression of extracellular matrix proteins in glomerular cells, and by the demonstration that reducing the burden of glycated albumin ameliorates renal structural and functional abnormalities in the db/db mouse. Methods. To probe whether the salutary effects consequent to lowering glycated albumin, which include reduction of albuminuria, relate to an influence of the Amadori-modified protein on nephrin, the podocyte protein critical to regulation of protein excretion, and on the angiogenic vascular endothelial growth factor (VEGF), which induces microvascular permeability, diabetic db/db mice were treated with a small molecule that inhibits the nonenzymatic glycation of albumin. Results. Compared to nondiabetic db/m mice, diabetic controls exhibited increased urinary excretion of albumin and type IV collagen, elevated renal TGF-beta 1 protein levels, reduced glomerular nephrin immunofluorescence and nephrin protein by immunoblotting, and increased glomerular VEGF immunostaining and renal VEGF protein content. Diabetic animals receiving test compound showed significant lowering of proteinuria, normalization of renal TGF-beta 1 protein, and significant restoration of altered glomerular nephrin and VEGF expression. Conclusion. The findings causally implicate the increased glycated albumin associated with the diabetic state in the abnormal renal nephrin and VEGF expression found in diabetes, thereby promoting proteinuria and glomerulosclerosis.