Albumin modified by Amadori glucose adducts activates mesangial cell type IV collagen gene transcription

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Abstract
The direct relationship between elevated glucose concentrations and accelerated protein glycation has implicated increased glycation as a potential mechanistic link between hyperglycemia and the pathogenesis of diabetic nephropathy. Albumin modified by Amadori glucose adducts has been shown to stimulate collagen secretion by mesangial cellsin vitro, and to contribute to the overproduction of glomerular mesangial matrixin vivo. To delineate mechanisms responsible for these effects, we examined the influence of glycated albumin on transcriptional activation of the α1 (IV) collagen gene in renal glomerular mesangial cells. These experiments used a stably transfected reporter mesangial cell line that exhibits responses to media manipulations that are directionally parallel with those of non-transformed mesangial cells, and that expresses luciferase driven by 5′-flanking and first intron regions of the α1 (IV) collagen gene. In these cells, purified glycated albumin stimulated collagen IV gene transcription, whereas glucose-free albumin did not. Further, glycated albumin induced a significant increase in mesangial cell collagen IV mRNA, assessed by Northern blot analysis and quantified by calculation of the ratio of collagen IV mRNA to 18S ribosomal RNA after densitometric scanning. The stimulation of collagen gene transcription and mRNA expression were both prevented by monoclonal antibodies known to specifically recognize Amadori-modified albumin. The findings indicate that glycated albumin promotes mesangial cell transcriptional activation and mRNA expression of the α1 (IV) collagen gene and further implicate increased glycated albumin in diabetes in the pathogenesis of diabetic nephropathy.