Anti-diabetic effects of cinnamaldehyde and berberine and their impacts on retinol-binding protein 4 expression in rats with type 2 diabetes mellitus

Background: Retinol binding protein 4 (RBP4), as an adipocyte secreted cytokine, was recently found to be inversely correlated with expression of glucose transporter 4 (GLUT4) in insulin resistance (IR) state and to have an intimate relationship with IR and type 2 diabetes mellitus (T2DM). The present study aimed to evaluate the anti-diabetic efficacy of cinnamaldehyde (Cin), berberine (Ber), and metformin (Met) as well as their impacts on the RBP4-GLUT4 system. Methods: Rat models of T2DM were established by combination of intraperitoneal injection of low-dose streptozotocin and high fat diet induction. Rats were divided into five groups: the control group, the diabetes group, the diabetes+Ber group, the diabetes+Cin group, and the diabetes+Met group. Western blotting was used to detect the serum or tissue RBP4 and GLUT4 protein levels. Results: After treatment for four weeks, both Cin and Ber displayed significant hypolipidemic, hypoglycemic, and insulin sensitizing functions (P <0.01) compared with the control group. Their effects on lowering fasting plasma glucose (FPG), low density lipoprotein-cholesterol (LDL-C) and homeostasis model assessment of insulin resistance (HOMA-IR) seem even better than that of Met. Cin and Ber markedly lowered serum RBP4 levels and up-regulated the expression of tissue GLUT4 protein, and Cin seemed more notable in affecting these two proteins. Conclusions: Both Cin and Ber display an exciting anti-diabetic efficacy in this study and may be of great value for the treatment of type 2 diabetes. Their mechanisms involve the RBP4-GLUT4 system, during which the serum RBP4 levels are lowered and the expression of tissue GLUT4 protein is up-regulated.