Eleutheroside E, An Active Component of Eleutherococcus senticosus, Ameliorates Insulin Resistance in Type 2 Diabetic db/db Mice

Abstract

Eleutheroside E (EE), a principal component of Eleutherococcus senticosus (ES), has anti-inflammatory and protective effects in ischemia heart. However, it is unknown whether it ameliorates insulin resistance and reduces hyperglycemia in diabetes. This study investigated the effect of EE-containing ES extracts, as well as EE, on hyperglycemia and insulin resistance in db/db mice. EE increased the insulin-provoked glucose uptake in C2C12 myotubes. Moreover, EE improved TNF- α -induced suppression of glucose uptake in 3T3-L1 adipocytes. Five-week-old db/db mice were fed a diet consisting of ES extract or EE for 5 weeks. Both were effective in improving serum lipid profiles and significantly decreased blood glucose and serum insulin levels. ES and EE supplementation effectively attenuated HOMA-IR. Glucose tolerance and insulin tolerance tests showed that EE increased insulin sensitivity. Immunohistochemical staining indicated that ES and EE protected pancreatic alpha and beta cells from diabetic damage. In addition, ES and EE improved hepatic glucose metabolism by upregulating glycolysis and downregulating gluconeogenesis in obese type 2 diabetic mice. These data suggest that EE mediates the hyperglycemic effects of ES by regulating insulin signaling and glucose utilization. The beneficial effects of EE may provide an effective and powerful strategy to alleviate diabetes.

Figure 1

Representative HPLC chromatogram of the E. senticosus (ES) extract and its functional standard compounds. (a) HPLC chromatogram of the ES extract. (b) HPLC chromatogram of the major compounds including syringin, chlorogenic acid, eleutheroside E, and isoflaxidin.

Figure 2

The effect of functional ES components on glucose uptake in C2C12 myotubes and insulin-resistant 3T3-L1 cells. (a) C2C12 myotubes were treated for 24 h with 10 μM of each compound and glucose uptake was measured in basal and insulin exposure conditions. *P < 0.05 versus basal control. ^# P < 0.05 versus insulin-treated control. CTL, control; SY, syringin; EE, eleutheroside E; ISO, isoflaxidin. (b) For insulin resistance conditions, 20 ng/mL TNF-α was added for 6 h in differentiated 3T3-L1 cells. After treatment with 10 μM EE for 24 h, glucose uptake was measured. *P < 0.05 versus control. Results are the mean ± SD of 6 wells in each group. Each experiment was repeated at least three times.

Figure 3

The effects of ESL, ESH, and EE on body weight, fasting blood glucose levels, and impaired glucose tolerance in db/db mice. (a) Body weight measurements during the experimental period. (b) Blood glucose levels after 4-hour fast. After feeding the experimental diet for 5 weeks, IPGTT (c) and IPITT (d) were performed. The area under curve (AUC) during IPGTT is also shown. Each value represents the mean ± SEM (n = 8). *P < 0.05 versus diabetes mellitus (DM) group. **P < 0.01 versus DM group.

Figure 4

The effect of ESL and EE on pancreatic insulin and glucagon production. (a) Pancreatic tissue sections from DM, ESL, and EE group were stained with hematoxylin and eosin, anti-insulin, and antiglucagon antibodies, respectively. The scale bar is 100 μm for each panel. (b) Relative β-cell volume in the pancreas was calculated as described in Materials and Methods. *P < 0.05 versus diabetes mellitus (DM) group.

Figure 5

The effect of ESL (a) and EE (b) on insulin signaling in skeletal muscles. Analyses of insulin-induced phosphorylation of IRβ, AKT, and P70S6K were performed by western blot. Following an overnight fast, mice were either sacrificed for or injected with 5 U/kg of insulin. Five minutes after injection, muscle tissues were collected and total protein was analyzed. The phosphorylation site in each protein is indicated.

Figure 6

The effect of ESL and EE on hepatic glucose metabolism. (a) mRNA expression of the glycolysis-related genes, glucokinase and 6-phosphofructokinase, is shown. (b) mRNA expression of gluconeogenesis-related genes, G6Pase and PEPCK, is shown. After normalization of each gene to the 18S gene, mRNA levels are expressed as the percentage of diabetic mice. Values indicate the mean ± SEM. *P < 0.05 versus DM group. **P < 0.01 versus DM group.