糖尿病性腎症治療の新規ターゲット探索 : 高インスリン血症、HIF-1α、ならびにメグシンが糸球体と尿細管間質に及ぼす影響とその機序 The development of novel therapeutic targets for diabetic nephropathy : hyperinsulinemia, HIF-1α, and megsin

博士論文

Diabetic nephropathy is the most common cause of end-stage renal failure in developed countries. Strict glycemic control or blood pressure control of diabetic patients with nephropathy result in delay of dialysis onset. However, there is no therapeutic agent at the moment to regress the kidney injury in diabetic nephropathy. Thus, the development of novel therapeutics for diabetic nephropathy is an intensively investigated topic. In the present study, the mechanisms of the progression of diabetic nephropathy was investigated, using type II diabetic nephropathy rat model SHR/NDmcr-cp and in vitro cultured renal cells. First, the renoprotective effects achieved by an insulin-sensitizer, pioglitazone, or by insulin were compared in SHR/NDmcr-cp. Pioglitazone markedly attenuated hyperinsulinemia and renal dysfunction in contrast with insulin treatment. Although renal accumulation of pentosidine and the local oxidative stress were both reduced to a similar extent by pioglitazone and insulin, the enhanced TGF-β expression was suppressed only by pioglitazone, suggesting that hyperinsulinemia up-regulated TGF-β expression. These data suggest that hyperinsulinemia together with a stimulated TGF-β expression significantly contribute to the incidence and deterioration of diabetic nephropathy. Second, the renoprotective effects of cobalt chloride, which enhances a key defensive factor against hypoxia, i.e. HIF, were evaluated in SHR/NDmcr-cp. The renal expression of TGF-β and CTGF, renal accumulation of pentosidine, and the local oxidative stress were reduced, and significant renoprotection was observed by cobalt treatment. Cobalt treatment up-regulated renal HIF-1α expression, increased the expression of HIF-regulated genes, e.g. erythropoietin, VEGF, and HO-1, and protected peritubular capillaries. Thus, cobalt achieved renal protection via the activation of HIF-1α signals, and its effect was attributed to prevention of renal hypoxia. Third, the role of megsin, a member of the serine protease inhibitor superfamily, in the decreased activities of MMP-2, MMP-9 and plasmin was investigated. High-glucose up-regulated megsin RNA expression in the kidney of SHR/NDmcr-cp as well as in vitro in cultured mesangial cells (RMC). The functional studies in RMC showed that megsin potentially inhibits total enzymatic activities of MMP-2, MMP-9 and plasmin, indicating decreased degradation of mesangial matrix. An anti-megsin neutralizing antibody restored the reduced activities of MMP-2 and MMP-9 by megsin in RMC. These data suggest that hyperglycemia induces up-regulation of megsin which, in turn, inhibits activities of plasmin, MMP-2 and MMP-9, potentially contributing to mesangial matrix accumulation in diabetic nephropathy. In conclusion, this study shows that the etiology of diabetic nephropathy involves hyperinsulinemia due to the increased expression of TGF-β that occurs through several distinct pathways including activation of HIF-1α signals and up-regulation of megsin. These findings provide new insights into searching and identifing therapeutic targets for diavetic nephropathy.