Lovastatin Inhibits Transforming Growth Factor-b 1 Expression in Diabetic Rat Glomeruli and Cultured Rat Mesangial Cells

Diabetic nephropathy is a leading cause of end-stage renal disease and is characterized by excessive deposition of extracellular matrix (ECM) proteins in the glomeruli. Transforming growth factorb (TGF-b) is the major mediator of excessive accumulation of ECM proteins in diabetic nephropathy through upregulation of genes encoding ECM proteins as well as downregulation of genes for ECM-degrading enzymes. It has been shown that lovastatin, an inhibitor of 3-hydroxy3-methylglutaryl CoA reductase, delays the onset and progression of different models of experimental nephropathy. To evaluate the effect of lovastatin on the development and progression of diabetic nephropathy, streptozotocin-induced diabetic rats were studied for 12 mo. In untreated diabetic rats, there were significant increases in blood glucose, urine albumin excretion, kidney weight, glomerular volume, and TGFb1 mRNA expression in the glomeruli compared with normal control rats treated with citrate buffer only. Treatment with lovastatin in diabetic rats significantly suppressed the increase in urine albumin excretion, kidney weight, glomerular volume, and TGFb1 mRNA expression despite high blood glucose levels. To elucidate the mechanisms of the renal effects of lovastatin, rat mesangial cells were cultured under control (5.5 mM) or high (30 mM) glucose with lovastatin alone, mevalonate alone, or with both. Under high glucose, TGFb1 and fibronectin mRNA and proteins were upregulated. These high glucose-induced changes were suppressed by lovastatin (10 mM) and nearly completely restored by mevalonate (100 mM). These results suggest that lovastatin has a direct cellular effect independent of a cholesterol-lowering effect and delays the onset and progression of diabetic nephropathy, at least in part, through suppression of glomerular expression of TGFb1. Excessive deposition of extracellular matrix (ECM) proteins in the glomeruli and subsequent mesangial expansion are the main structural alterations in diabetic nephropathy (1). Both in vitro and in vivo studies have shown that high glucose stimulates expression of genes encoding ECM proteins in tubular epithelial cells as well as mesangial cells (2–5). These effects are largely mediated by upregulation of transforming growth factor-b (TGF-b) expression and/or activation of TGFb (5–7). TGF-b induces accumulation of ECM proteins by both stimulating synthesis of ECM proteins (8) and decreasing activity of ECM-degrading enzymes (8,9). For these reasons, TGFb has been proposed as the therapeutic target in diabetic nephropathy (10). It has been shown that the inhibition of 3-hydroxy-3-methylglutaryl CoA (HMG CoA) reductase with lovastatin is associated with amelioration of albuminuria and glomerulosclerosis in 5/6 nephrectomized rats (11,12), obese Zucker rats (13), and streptozotocin-induced diabetic uninephrectomized rats (14). Because lovastatin also reduces serum cholesterol levels in these experimental models, the beneficial effect of lovastatin has generally been attributed to a decrease in circulating levels of cholesterol. There is evidence, however, that the effect of lovastatin on glomerular structure and function may result, at east in part, from a direct action at the cellular level. Lovastatin has been shown to inhibit mesangial cell proliferation (15) and induce apoptosis in cultured mesangial cells (16). Lovastatin has been shown to inhibit NFkB activation (17) and monocyte chemoattractant protein-1 production (18), and modulate the plasminogen activator-plasmin system (19). We have demonstrated that lovastatin effectively suppresses TGF-b1 mRNA expression in diabetic rat glomerul i 2 d after induction of diabetes (20). These studies suggest that the beneficial effect of lovastatin on diabetic nephropathy may be due to its direct effect on TGFb and ECM synthesis. However, there is no long-term study showing that lovastatin prevents the renal manifestations of diabetic nephropathy or suppresses TGF-b expression and accumulation of ECM in diabetic glomeruli. Therefore, we explored the effects of lovastatin on the expression of TGFb1 in both streptozotocin-induced diabetic rat glomeruli and in rat mesangial cells cultured under high glucose concentration and assessed the effect of lovastatin on the expression of fibronectin in these cells. Materials and Methods Experimental Animals Male Sprague Dawley rats weighing 200 to 250 g were used. Diabetes was induced by a single tail vein injection of 65 mg/kg body Received April 6, 1999. Accepted June 25, 1999. Correspondence to Dr. Hi Bahl Lee, Hyonam Kidney Laboratory, Soon Chun Hyang University, 657 Hannam Dong, Yongsan Ku, Seoul 140-743, Korea. Phone:182 2 709 9171,182 2 792 6657; Fax: 182 2 792 5812; E-mail: