A Lanosteryl Triterpene from Protorhus longifolia Improves Glucose Tolerance and Pancreatic Beta Cell Ultrastructure in Type 2 Diabetic Rats

Abstract

Type 2 diabetes remains one of the leading causes of death worldwide. Persistent hyperglycemia within a diabetic state is implicated in the generation of oxidative stress and aggravated inflammation that is responsible for accelerated modification of pancreatic beta cell structure. Here we investigated whether a lanosteryl triterpene, methyl-3β-hydroxylanosta-9,24-dien-21-oate (RA-3), isolated from Protorhus longifolia can improve glucose tolerance and pancreatic beta cell ultrastructure by reducing oxidative stress and inflammation in high fat diet and streptozotocin-induced type 2 diabetes in rats. In addition to impaired glucose tolerance, the untreated diabetic rats showed increased fasting plasma glucose and C-peptide levels. These untreated diabetic rats further demonstrated raised cholesterol, interleukin-6 (IL-6), and lipid peroxidation levels as well as a destroyed beta cell ultrastructure. Treatment with RA-3 was as effective as metformin in improving glucose tolerance and antioxidant effect in the diabetic rats. Interestingly, RA-3 displayed a slightly more enhanced effect than metformin in reducing elevated IL-6 levels and in improving beta cell ultrastructure. Although the involved molecular mechanisms remain to be established, RA-3 demonstrates a strong potential to improve pancreatic beta cell ultrastructure by attenuating impaired glucose tolerance, reducing oxidative stress and inflammation.

Keywords: Protorhus longifolia; antioxidants; hyperglycemia; hyperlipidemia; inflammation; oxidative stress; pancreatic beta cells; triterpenes; type 2 diabetes.

Figure 1

The chemical structure of methyl-3β-hydroxylanosta-9,24-dien-21-oate (RA-3).

Figure 2

Oral glucose tolerance tests (A) and area under the curve (AUC) (B) in high fat diet and streptozotocin-induced type 2 diabetic rats treated with RA-3 and metformin (positive control). The untreated diabetic group presented with a significant increase in fasting plasma glucose levels (*** p ≤ 0.0001) compared to the non-diabetic rats and diabetic rats treated with RA-3 and metformin. ^### p ≤ 0.001 vs. diabetic control. Results are expressed as the mean ± SEM and each treatment group contained at least five rats. One way analysis of variance (ANOVA), followed by a Tukey post-hoc test (Graph Pad Prism version 5.03) were used to determine statistical differences. The values were considered statistically significant where p ≤ 0.05.

Figure 2

Oral glucose tolerance tests (A) and area under the curve (AUC) (B) in high fat diet and streptozotocin-induced type 2 diabetic rats treated with RA-3 and metformin (positive control). The untreated diabetic group presented with a significant increase in fasting plasma glucose levels (*** p ≤ 0.0001) compared to the non-diabetic rats and diabetic rats treated with RA-3 and metformin. ^### p ≤ 0.001 vs. diabetic control. Results are expressed as the mean ± SEM and each treatment group contained at least five rats. One way analysis of variance (ANOVA), followed by a Tukey post-hoc test (Graph Pad Prism version 5.03) were used to determine statistical differences. The values were considered statistically significant where p ≤ 0.05.

Figure 3

The effect of RA-3 on (A) plasma cholesterol and (B) serum interleukin-6 (IL-6) levels in the high fat diet and streptozotocin-induced type 2 diabetic rats. Results are expressed as the mean ± SEM and each treatment group contained at least five rats. * p ≤ 0.05, *** p ≤ 0.0001 vs. non-diabetic control, ^# p ≤ 0.05, ^### p ≤ 0.001 vs. diabetic control. One way analysis of variance (ANOVA), followed by a Tukey post-hoc test (Graph Pad Prism version 5.03) were used to determine statistical differences. The values were considered statistically significant where p ≤ 0.05.

Figure 4

The effect of RA-3 on pancreatic beta cell ultrastructure in the high fat diet and streptozotocin-induced type 2 diabetic rats. (i) Normal structure of pancreatic islets from non-diabetic rats; (ii) A destructed structure as well as a reduced size of a pancreatic islets in untreated diabetic rats; (iii) An improved structure of pancreatic islets with some debris in diabetic rats treated with RA-3; (iv) A reduced size of pancreatic islets with debris of cells in diabetic rats treated with metformin. (L)-designates a normal structure of islets of Langerhans, (E) exocrine portion of pancreatic tissue, and an arrow shows destructed and condensed endocrine cells while a circle illustrates debris of destructed cells. NB: The indicator size for each image is 60 µm.