Therefore, this observation may indicate that RBBO supplement could improve insulin sensitivity, protecting pancreas islet β-cells as well as stimulating insulin secretion. Moreover, the improved serum insulin in RBBO supplemented groups indicated that bioactive compounds such as tocotrienol, γ-oryzanol or other active compounds, particularly ferulic acid, or β-carotene contained in Riceberry oil might stimulate insulin secretion from the remnant pancreatic β-cells and/or regenerated β-cells (Pari and Latha, 2002; Canas et al., 2012). The lowest serum insulin concentration found in the metformin group could be explained by metformin controlling or decreasing fasting blood glucose by limiting hepatic glucose production rather than promoting cells sensitivity to insulin (He et al., 2009). That study also was consistent with our data which found that insulin value in DM rats treated with metformin was significantly lower than those of the control and other experimental groups. In addition, Zhou et al., (2001) demonstrated that metformin enhanced phosphorylation and activation of adenosine monophosphate protein kinase (AMPK), which helped to regulate lipid and glucose metabolism. Metformin activated AMPK in hepatocytes, and thereby, the reduction of acetyl-CoA carboxylase (ACC) activity and the increase in fatty acid oxidation, resulting in decrease in hepatic glucose production. Results from IPGTT indicated that diabetic rats treated with metformin had the greatest reduction of AUC for glucose by -25.58% while the AUC of rats supplemented with RBBO ranged between -10.41% to -20.83% as compared to DM. The glycemic improvement might be due to the increased stimulation of uptake and utilization of glucose in the peripheral tissues, meanwhile metformin also acted to reduce hepatic glucose production (Zhou et al., 2001). This suggestion was consistent with our finding that diabetic rats treated with metformin had the lowest AUC for glucose as well as serum insulin level. Recently, Fang et al., (2010) investigated the mechanism of tocotrienols on reducing blood glucose by acting through peroxisome proliferator-activated receptor (PPAR) modulators. PPARs are transcriptional factors that regulate the expression of genes in lipid and carbohydrate metabolism. The binding of tocotrienols to PPAR will stimulate the expression of glucose transporter 4 (GLUT4) which contributes to lowering blood glucose by promoting glucose uptake into the peripheral cells (Aggarwal et al., 2010). The study of Fang et al., (2010) was consistent with our data of GLUT 4 protein transporter which found that all the RBBO supplemented groups significantly increased GLUT 4 protein transporter when compared to untreated diabetic group, p<0.05, especially at low dose; 5% RBBO was similar to the values of normal control groups as shown in Figure 1. The increment in GLUT 4 protein level in RBBO supplemented groups, especially at low dose of RBBO might be explained by the action of various bioactive compounds contained in Riceberry oil, particularly tocotrienol, tran-β-carotene, and especially γ-oryzanol which is composed of phytosterols esters of trans-ferulic acid or other plant sterols; triterpenes that enhanced cellular insulin dependent glucose uptake. In the case of insulin resistance or insulin deficiency, it causes low response in signaling GLUT4 to take up glucose (Huang and Czech, 2007). This was consistent with our study which found that the untreated diabetic group had the lowest GLUT 4 value. However, our results did not agree with another previous study which showed that plasma insulin concentration was lower in rats fed the rice bran oil (RBO) diet than that of the diabetes control group and the plasma glucose value did not differ among the experimental groups (Chen and Cheng, 2006). The difference in the results of the present study with the previous study might be due to the differences in rice varieties, some bioactive compounds or the extraction process of the rice bran oil. Generally, STZ-induced diabetes rats had increased blood glucose, hemoglobin A1c, total cholesterol, triglyceride, LDL-C, and decreased HDL-C (Rajasekaran et al., 2006). After supplementation of diabetic rats with RBBO or metformin, there was improvement in serum lipid profile compared to untreated diabetic rats. All RBBO supplemented groups or metformin treated groups had lower serum total cholesterol, LDL-cholesterol and TG concentration than the untreated diabetes group (DMC). That might be due to the effect of various bioactive compounds contained in RBBO such as γ-oryzanol, coenzyme Q10, tocopherol, tocotrienols, polyphenol, and some phenolic acids including ferulic acid, p-coumaric acid and diferulate (Lin and Lai, 2011; Yawadio et al., 2007). A study by Chen and Cheng (2006) demonstrated that feeding of 10% and 15% rice bran oil to type 2 diabetic rats for 4 weeks significantly reduced serum triglyceride and LDL-C. Several studies reported that γ-tocotrienol in rice bran oil could improve the lipid profile through the increases in LDL-receptor and HMG-CoA reductase mRNA expression (Chen and Cheng, 2006; Chou et al., 2009). Bioactive compounds in rice bran oil such as γ-oryzanol and ferulic acids or other anti-oxidative compounds might be active in lowering lipid level by reduction of cholesterol in plasma and in liver; Son et al., (2010) demonstrated that feeding γ-oryzanol and ferulic acids to hypercholesterolemic mice for 7 weeks significantly lowered plasma and hepatic lipid profile by increasing fecal cholesterol and triglyceride excretion (Yawadio et al., 2007; Son et al., 2010; Guo et al., 2007). Sakatani et al. (2005) indicated that diabetes is associated with alterations in lipid profile such as the regulation of plasma or tissue lipid levels which led to an increase in the risk of micro or macrovascular diseases and related diabetes complications (Pushparaj et al., 2007; Zhang et al., 2010). Therefore, those reports are consistent with our results that RBBO might be helpful to prevent diabetic complications through improving dyslipidemia.
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