Objective: To explore the renoprotective effects of (-)-epigallocatechin-3-gallate (EGCG) and its potential mechanism in type 2 diabetic db/db mice.
Methods: 8-week-old db/db mice (6 h fasting plasma glucose >16.7 mmol/L) were allocated randomly into Control group (non-intervention group, n=8), EGCG A group (50 mg·kg(-1)·d(-1,)n=8), EGCG B group (100 mg·kg(-1)·d(-1,)n=8). Before the study and after the intervention (in the 4(th)and 8(th)week), the body weight, the level of fasting plasma glucose, oral glucose tolerance test (OGTT) were measured and 24 h urine samples were collected. 24 h proteinuria was measured by routine chemical method. The levels of angiotensin Ⅱ(AngⅡ), fasting plasma insulin and urinary 8-OHdG were measured with enzyme-linked immunosorbent assay (ELISA). The protein expression levels of angiotensin Ⅱ type 1 receptor (AT-1R), nicotinamide adenine dinucleotide phosphate (NADPH) oxidase subunit P22-phox, NADPH oxidase subunit P47-phox, phospho-extracellular regulated protein kinases (p-Erk1/2), phospho-P38 mitogen-activated protein kinase (p-P38MAPK), phospho -phosphatidylinositol 3-hydroxy kinase (p-PI3K) and phospho-protein kinase B (p-AKT) were determined by Western blot. The renal pathological changes were examined by the method of PAS (periodic acid-Schiff stain).
Results: After 8 weeks of treatment with EGCG, the level of fasting plasma glucose decreased[(14.4±1.0) mmol/L, (14.2±0.7) mmol/L vs. (17.2±0.8) mmol/L]; the level of fasting plasma insulin increased[(13.2±1.2)mU/L, (13.4±1.3) mU/L vs. (9.9±1.0) mU/L]; the area under the curve (AUC) of OGTT decreased[(49.3±1.8) mmol·L(-1)·h(-1,)(44.8±0.7) mmol·L(-1)·h(-1)vs. (60.0±0.8) mmol·L(-1)·h(-1)]; the level of 24 h proteinuria[(8.8±1.0) mg, (8.6±1.1) mg vs. (11.7±1.3) mg]and urinary 8-OHdG[(90±5) ng/d, (78±5) ng/d vs. (118±10) ng/d]decreased; the level of serum Ang-Ⅱ[(498±23) ng/L, (511±19) ng/L vs. (688±17) ng/L]and renal cortex AngⅡ[(367±5) ng/L, (384±10) ng/L vs. (406±7) ng/L]decreased; the expression levels of AT-1R, P22-phox, P47-phox, p-Erk1/2, p-P38MAPK downregulated obviously and the expression levels of p-PI3K, p-AKT increased significantly (P<0.05), and renal pathology improved as compared with the control group. After 8 weeks of treatment with EGCG, the level of urinary 8-OHdG decreased (P=0.007) and the AUC of OGTT also decreased (P=0.01) in EGCG B group when compared with the EGCG A group.
Conclusion: EGCG protects the kidney in diabetic db/db mice via anti-oxidative stress pathway, as well as inhibiting Erk1/2-P38MAPK pathway and improving PI3K-AKT signaling transduction pathway.