Effects of Purple Rice Bran Genotype Kum Doi Saket (Oryza sativa L. ssp. indica) on Skeletal Muscle Glucose Transport System in Diabetic Rats

Abstract

Thai purple glutinous rice genotype Kum Doi Saket (Oryza sativa L. ssp. indica) is a famous local cultivar of rice in the Northern of Thailand that contains a much higher content of anthocyanins and gamma oryzanol in the aleurone layer. Anthocyanins, the major subgroup of flavonoids responsible for the blue, purple and red color plant tissue, have been displayed various biological actions including antioxidant, anti-inflammatory, antihyperlipidemic, and also antihyperglycemic actions. However, little is known regarding its potential effects on skeletal muscle, the major site of glucose disposal. Therefore, this present study aimed to investigate the anti-diabetic effect and to explore the underlying molecular mechanisms of action of this purple glutinous rice bran via focusing on skeletal muscle glucose transport system in both type 1 and type 2 diabetic rat models.

The results showed that purple rice bran supplement (50 g/kg diet) for 8 weeks successfully reduced fasting plasma glucose, HbA1c, triglyceride (TG), and free fatty acid (FFA) levels in both streptozotocin (STZ)-induced type 1 diabetic rats and high-fat (HF) diet with STZ-induced type 2 diabetic rats. Interestingly, the defects in fasting plasma insulin level or HOMA-β index were effectively attenuated in type 1 diabetic rats supplemented with purple rice bran, indicating the less deteriorating of pancreatic β-cells. Moreover, supplementation of purple rice bran improved the whole body insulin sensitivity by significantly reducing the AUCg from oral glucose tolerance test (OGTT) and HOMA-IR index in both type 1 and type 2 diabetic rats. Furthermore, experiments in muscle glucose transport system revealed that purple rice bran supplement enhanced the expression and activation of insulin signaling pathway-related protein including IRS-1, PKCζ/λ, Akt, and also the insulin-independent pathway such as AMPK which resulting an increase in GLUT4 translocation. Therefore, these findings suggested that purple rice bran supplement for 8 weeks exerted an anti-diabetic effect, enhanced insulin secretion and also moderately improved insulin action as well as the AMPK activation on skeletal glucose transport system in type 1 diabetic rats. Whereas, the anti-hyperglycemic effect in type 2 diabetic rats was related to reducing dyslipidemia and tissue TG accumulation, improving insulin sensitivity and also amelioration the defects in insulin signaling pathway (IRS-1/PI3-K/Akt) and AMPK activation, resulting enhanced skeletal glucose transport.

In conclusion, this present study firstly illustrated that one potential site of anti-diabetic effect of purple rice bran genotype Kum Doi Saket is the skeletal glucose transport system. These findings evidence the nutraceutical property of this purple rice bran against the pathogenesis of the metabolic disorders by attenuating insulin resistance or defects of insulin secretion, and dyslipidemia associated to diabetes mellitus.