Analysis of responses to chronic stress generated by proteinuria in renal proximal tubular cells.

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It was well known that proteinuria involved in the progression of kidney diseases. We found that proteinuria increased the expression of brain specific protein, glia maturation factor-b (GMF) in the disease model kidney. The expression of GMF caused vulnerability to oxidative injury, leading to apoptosis in cultured renal proximal tubular (PT) cells named mProx24. We also found that albumin generated oxidative stress within 15min, resulting in activation of STAT1 and STAT5. Thus, it was hypothesized that albumin may cause apoptosis in GMF expressing PT cells (GMF-PT) because of oxidative stress. GMF-PT and wild type PT cells were treated with various concentration of albumin (1-30mg/ml) for 72hrs. Cell viability was examined by using MTS assay. It showed that albumin caused a significant and dose-related increase of cell death in GMF-PT cells, but not in wild-type PT cells. The addition of antioxidants N-Acetyl-L-Cysteine, resveratorol, vitamins C and E inhibited cell death of GMF-PT by albumin. Caspase-3 activity was increased in GMF-PT after 24hr treatment of albumin (30mg/ml). It was inhibited by the caspase-3 inhibitor 50 μM Z-VAD-fmk and the p38MAPK inhibitor 10 μM SB203580. These results demonstrated that albumin, a major component of proteinuria, caused apoptosis of GMF expressing PT cells dependent on p38MAPK because of the increased oxidative stress. It was reported that NF-kB played significant roles in apoptosis by inducing anti-apoptotic genes. EMSA demonstrated that expression of GMF decreased the binding activity of NF-kB. We constructed transgenic mice bearing GMF gene (GMF-TG) and have been investigating the effects of GMF expression in vivo.In conclusion, expression of GMF induced apoptosis of renal cells both in vivo and in vitro.