Dual actions of gallic acid and andrographolide trigger AdipoR1 to stimulate insulin secretion in a streptozotocin-induced diabetes rat model

Abstract

Common treatments for the management of diabetes have limitations due to side effects, hence the need for continuous research to discover new remedies with better therapeutic efficacy. Previously, we have reported that the combination treatment of gallic acid (20 mg/kg) and andrographolide (10 mg/kg) for 15 days demonstrated synergistic hypoglycemic activity in the streptozotocin (STZ)-induced insulin-deficient diabetes rat model. Here, we attempt to further elucidate the effect of this combination therapy at the biochemical, histological and molecular levels. Our biochemical analyses showed that the combination treatment significantly increased the serum insulin level and decreased the total cholesterol and triglyceride level of the diabetic animals. Histological examinations of H&E stained pancreas, liver, kidney and adipose tissues of combination-treated diabetic animals showed restoration to the normalcy of the tissues. Besides, the combination treatment significantly enhanced the level of glucose transporter-4 (GLUT4) protein expression in the skeletal muscle of treated diabetic animals compared to single compound treated and untreated diabetic animals. The molecular docking analysis on the interaction of gallic acid and/or andrographolide with the adiponectin receptor 1 (AdipoR1), a key component in the regulation of pancreatic insulin secretion, revealed a greater binding affinity of AdipoR1 to both compounds compared to individual compounds. Taken together, these findings suggest the combination of gallic acid and andrographolide as a potent therapy for the management of diabetes mellitus.

Keywords: AGP, Andrographolide; AdipoR1, Adiponectin receptor 1; Andrographolide; Diabetes mellitus; ELISA, Enzyme-linked immunosorbent assay; GA, Gallic acid; GLUT4, Glucose transporter-4; Gallic acid; H&E, Hematoxylin-eosin; Insulin; PBS, Phosphate buffer saline; STZ, Streptozotocin; TBST, Tris-buffered saline with 0.1% (v/v) Tween-20; adipoR1.

Graphical abstract

Fig. 1

Effect on serum insulin levels after respective treatment for 15 days. Bars are represented as mean ± S.E.M for diabetic control group (n = 5) and other groups (n = 6). Different letters indicate significant differences analyzed by one-way ANOVA followed by the Tukey post hoc test (P < 0.05).

Fig. 2

Light photomicrograph of the pancreatic section from experimental rats stained with H&E. AC = acinar cells; IL = islets of Langerhans. The image is representative of three animals per experimental group (magnification × 200).

Fig. 3

Light photomicrograph of H&E-stained adipose tissue from experimental rats. The image is representative of three animals per experimental group (magnification × 100).

Fig. 4

Light photomicrograph of H&E-stained liver section from experimental rats. CV = central vein; H = hepatocytes; black arrow = sinusoids. The image is representative of three animals per experimental group (magnification × 200).

Fig. 5

Light photomicrograph of H&E-stained kidney section from experimental rats. T = renal tubules; G = glomerulus; black arrow = Bowman's space; arrowhead = vacuolar degeneration of the renal tubules. The image is representative of three animals per experimental group (magnification × 100).

Fig. 6

Effects of gallic acid and andrographolide treatment on GLUT4 protein expression in soleus muscle. (A) shows representative GLUT4 protein bands; (B) shows the relative GLUT4 protein expression normalized by total protein. (Lane 1) Normal control, (Lane 2) diabetic control, (Lane 3) diabetic + gallic acid (20 mg/kg), (Lane 4) diabetic + andrographolide (10 mg/kg), and (Lane 5) diabetic + gallic acid (20 mg/kg) + andrographolide (10 mg/kg). Bars are represented as means ± S.E.M for three independent experiments performed. ^a significantly different from the normal control group (P < 0.05); ^b significantly different from the diabetic control group (P < 0.05).