薬理と治療

Abstract

Objectives　Hypertriglyceridemia contributes to diabetic nephropathy. In UKPDS 74, hypertriglyceridemia was detected as one of the risk factors for diabetic nephropathy. Increased remnant lipoproteins underlie hypertriglyceridemia. We first found that apolipoprotein（apo）E2 is a genetic factor for diabetic nephropathy, and that increase of plasma TG╱remnant lipoproteins caused by apo E2 contributes to diabetic nephropathy. Remnant lipoproteins are taken up by mesangial cells, and then cause renal damage. The frequency of apo E2 is approx. 10％ in Japan and western countries. Diabetic nephropathy is characterized by the accumulation of extracellular matrix protein（type Ⅳ collagen）in the glomerular mesangium. TGF-β stimulates typeⅣ collagen protein synthesis. Incretin-related drugs, which are widely used in diabetic patients, reduce plasma TG╱remnant lipoproteins as well as plasma glucose. There is little information about the effect of incretin on diabetic nephropathy. In the present study we examined whether sitagliptin, DPP-4 inhibitor, which acts through increasing incretin is effective to improve nephropathy in type 2 diabetic patients with apo E2 allele, and examined the effects of GLP-1 on TGF-β and type Ⅳ collagen expression in human mesangial cells（HMCs）loaded with apo E2 remnant lipoproteins. Methods　Type 2 diabetic patients with apo E3╱3 genotype（n＝38）and apo E3╱2 genotype （n＝7）were studied. Patients were treated with sitagliptin（50 mg╱day）over 10 months. HbA1c, plasma TG and remnant cholesterol were compared before and after sitagliptin therapy. Remnant cholesterol was determined as RLP︱cholesterol（normal range＜5.2 mg╱dL）. Urinary albumin-to-creatinine ratio（ACR, normal range＜30 mg╱g cre）was measured as a marker of nephropathy. Remnant lipoproteins were isolated from plasma of a patient with type 2 diabetes and apo E2╱2 genotype by ultracentrifugal method. HMCs loaded with remnant lipoproteins were incubated for 24 h with GLP-1 at the concentration of 0, 100, 300 and 500 nmol╱L. To evaluate the expression of TGF-β, type Ⅳ collagen and GLP-1 receptor m-RNA in HMCs, PCR procedure was performed. Results　In apo E3╱3 patients, 10-month therapy with sitagliptin significantly reduced HbA1c （6.6→6.0％, P＜.001）, plasma TG（129→107 mg╱dL, P＜0.05）and remnant cholesterol （5.5→3.9 mg╱dL, P＜0.001）, and ACR（57.4→34.1 mg╱g cre, P＜0.05）. In apo E3╱2 patients, it reduced HbA1c（6.8→6.0％, P＜0.001）, plasma TG（173→119 mg╱dL, P＜0.05）and remnant cholesterol（8.0→5.0 mg╱dL, P＜0.001）, and ACR（119.0→19.2 mg╱g cre, P＜0.001）. Reduction in HbA1c from baseline at 10 months was not significantly different between apo E3╱3 patients and apo E3╱2 patients, but reduction in plasma TG and remnant cholesterol, and ACR from baseline at 10 months was significantly（P＜0.05）greater in apo E3╱2 patients than in apo E3╱3 patients. The next study using renal mesangium cells was conducted to elucidate the mechanism by which sitagliptin reduces ACR. Apo E2 remnant lipoproteins significantly（P＜0.001）stimulated the expression of TGF-β and type Ⅳ collagen mRNA in HMCs. Exendin-4 which is an incretin mimic and has activity similar to GLP-1 significantly（P＜0.001）suppressed the expression of TGF-β and type Ⅳ collagen mRNA in HMCs loaded with apo E2 remnant lipoproteins in a dose-dependent manner. Exendin︱4 significantly（P＜0.001）stimulated the expression of GLP-1 receptor. Conclusions　Sitagliptin reduced plasma TG╱remnant lipoproteins, and ACR more greatly in apo E2 diabetic patients. GLP-1 suppressed the expression of TGF-β and type Ⅳ collagen in HMCs loaded with apo E2 remnant lipoproteins. It is suggested that incretin is effective to protect and improve diabetic nephropathy associated with apo E2 through its lowering effect of plasma TG╱remnant lipoproteins and╱or its direct effect on renal mesangial cells.