Urinary cystatin C as a biomarker for diabetic nephropathy and its immunohistochemical localization in kidney in Zucker diabetic fatty (ZDF) rats

Cystatin C, a cysteine protease inhibitor, is a novel biomarker of renal damage. In the present study, we examined the usefulness of urinary cystatin C for the detection of diabetic nephropathy in Zucker diabetic fatty (ZDF) rats compared to other biomarkers (β2-microglobulin, calbindin, clusterin, epidermal growth factor (EGF), alpha-glutathione S-transferase (GST-α), mu-glutathione S-transferase (GST-μ), kidney injury molecule-1 (KIM-1), neutrophil gelatinase-associated lipocalin (NGAL), osteopontin, tissue inhibitor of metalloprotease-1 (TIMP-1), and vascular endothelial growth factor (VEGF). Urinary levels of cystatin C were increased in ZDF rats where renal damage was not histopathologically observed, and then further increased with the progression of renal damage, demonstrating the usefulness of early detection and accurate assessment of diabetic nephropathy. Urinary β2-microglobulin, clusterin, GST-μ, KIM-1, and osteopontin had the potency to detect renal damage in ZDF rats as well as cystatin C. We also investigated immunohistochemical localization of cystatin C in the kidney according to progressive renal damage. Cystatin C expression was mainly observed in the proximal renal tubule in ZDF rats, and hardly changed with progression of nephropathy. When renal damage was remarkable, cystatin C expression was also observed in the tubular lumen of the cortex and medulla, which was considered to be characteristic of renal damage in diabetic nephropathy. In conclusion, urinary cystatin C, β2-microglobulin, clusterin, GST-μ, KIM-1, and osteopontin could be useful biomarkers of diabetic nephropathy in ZDF rats. Immunohistochemical cystatin C expression in the proximal renal tubule was hardly changed by the progression of diabetic nephropathy, but it was newly observed in the tubular lumen when renal damage was remarkable in ZDF rats.