Thiazolidinedione compounds ameliorate glomerular dysfunction independent of their insulin-sensitizing action in diabetic rats.

Abstract

Thiazolidinedione (TZD) compounds are widely used as oral hypoglycemic agents. Herein, we provide evidence showing that troglitazone, one of the TZD compounds, is able to prevent glomerular dysfunction in diabetic rats through a novel mechanism independent of its insulin-sensitizing action. We examined the effect of troglitazone on functional and biochemical parameters of glomeruli in streptozotocin-induced diabetic rats. Troglitazone was able to prevent not only diabetic glomerular hyperfiltration and albuminuria, but an increase in mRNA expression of extracellular matrix proteins and transforming growth factor-␤1 in glomeruli of diabetic rats, without changing blood glucose levels. Biochemically, an increase in diacylglycerol (DAG) contents and the activation of the protein kinase C (PKC)-extracellular signal-regulated kinase (ERK) pathway in glomeruli of diabetic rats were abrogated by troglitazone. The activation of DAG-PKC-ERK pathways in vitro in mesangial cells cultured under high glucose conditions was also inhibited by troglitazone. Troglitazone enhanced the activities of DAG kinase, which could metabolize DAG to phosphatidic acid, in both glomeruli of diabetic rats and mesangial cells cultured under high glucose conditions. Surprisingly, pioglitazone, another TZD compound without ␣-tocopherol moiety in its structure, also prevented the activation of the DAG-PKC pathway and activated DAG kinase in mesangial cells cultured under high glucose conditions. These results may identify the TZDs as possible new therapeutic agents for diabetic nephropathy that prevent glomerular dysfunction through the inhibition of the DAG-PKC-ERK pathway. Diabetes 49:1022-1032, 2000 T hiazolidinediones (TZDs), novel insulin-sensitizing agents, have been shown to attenuate hyperinsulinemia and hyperglycemia in insulin-resistant diabetic animals and human subjects with type 2 diabetes (1,2). These agents are currently in clinical trials and have been widely used as oral antidiabetic agents in subjects with type 2 diabetes in the U.S. and other countries. Although it is generally believed that TZDs exert insulin-sensitizing activity through the activation of peroxisome proliferator-activated receptor (PPAR)-␥, a member of the PPAR nuclear receptor superfamily (3), the exact mechanism of their action remains to be clarified (4-6). Besides its insulinsensitizing activity, troglitazone, one of the TZD compounds, was found to inhibit high glucose-induced proliferation and migration of vascular smooth muscle cells by inhibiting the activation of protein kinase C (PKC) (7); to inhibit platelet aggregation (8); to reduce balloon injury-induced intimal hyperplasia in rat artery (9); and to suppress both insulin-and angiotensin II-induced increase of DNA synthesis by inhibiting the activation of extracellular signal-regulated kinase (ERK) (10,11). Troglitazone was also shown to ameliorate albuminuria and peripheral neuropathy in streptozotocin (STZ)-induced diabetic rats, regardless of blood glucose levels (12,13). These results indicate ...

Reversal of redox-dependent inhibition of diacylglycerol kinase by antioxidants in mesangial cells exposed to high glucose

“…After washing, total cellular lipids were extracted from the cells according to the method of Bligh and Dyer (14). Total DAG contents were determined with a radio-enzymatic assay kit (Habersham, Arlington Heights, IL, USA) employing DG kinase (Caliches, San Diego, CA, USA), according to the instructions provided by the manufacturer, as previously described (5,8), which quantitatively converts DAG to [γ-32 P]-PA in the presence of [γ-32 P]-ATP (NEN, Boston, MA, USA) according to the instructions provided by the manufacturer. The resulting [γ-32 P]-PA was separated by silica-gel thin-layer plates (EM Separations, Gibbstown, NJ, USA) in a chamber containing chloroform/acetone/methanol/ acetic acid/water (10:4:3:2:1) according to the method of Priess et al (15).…”

“…Intracellular hyperglycemia promotes abnormal DAG accumulation from de novo synthesis, and high levels of DAG induce the activation of PKC. High DAG levels and PKC activation have been demonstrated in vascular tissues including the kidneys of diabetic animals (5-7), as well as in cultured renal vascular cells exposed to high glucose (7)(8)(9)(10). DAG is converted by diacylglycerol kinase (DGK) to phosphatidic acid (PA), and enhanced DGK activity leads to a reduction in excess DAG accumulation induced by high glucose, resulting in reduced PKC activation.…”

“…DAG is converted by diacylglycerol kinase (DGK) to phosphatidic acid (PA), and enhanced DGK activity leads to a reduction in excess DAG accumulation induced by high glucose, resulting in reduced PKC activation. We previously revealed that d-α-tocopherol inhibited DAG accumulation in diabetic renal glomeruli or mesangial cells under high glucose conditions via the enhancement of DGK activity (8,11), leading to the reduction of PKC activation. Notably, Lee et al (9) reported that not only d-α-tocopherol but also probucol increased DGK activity in vascular smooth muscle cells, resulting in an improvement in excess DAG accumulation and increased PKC activation under high glucose conditions.…”

Renal Function in Type 2 Diabetes with Rosiglitazone, Metformin, and Glyburide Monotherapy

“…It is unclear, however, whether different antihyperglycemic agents have specific antialbuminuria effects independent of their blood glucose-lowering effects. Animal data and short-term studies in humans suggest that thiazolidinediones that have anti-inflammatory properties and affect levels of adipokines and cytokines may be more potent at lowering albuminuria and preventing its rise than other oral glucose-lowering agents (5)(6)(7).…”

Therapeutic Modalities in Diabetic Nephropathy: Standard and Emerging Approaches

“…146 Beyond their hypoglycemic actions, PPARЎ agonists exert a number of beneficial effects in diabetes including improvement in endothelial function, 147,148 reduction in pro-atherogenic inflammatory markers 149 and angiotensin-I and -II, 150 down regulation of AT 1 mRNA and protein in vascular smooth muscle cells, 151,152 decrease in urine endothelin-1 secretion, 153 attenuated lipid accumulation and its related injury in mesangial cells, 154,155 and inhibition of glomerular and tubular cell proliferation 156,157 . Several animal [158][159][160][161][162][163] and human studies 153,[164][165][166][167][168][169][170][171][172][173][174][175] using various TZDs have demonstrated a reduction in proteinuria and BP. Unfortunately; most of these studies were of short duration averaging 1-9 months in the animal studies 158-163 and 3-12 months in human studies.…”