Sennoside A induces GLP-1 secretion through activation of the ERK1/2 pathway in L-cells

Abstract Background Glucagon-like peptide-1 (GLP-1) is secreted from the intestinal L-cells to stimulate insulin secretion in the control of blood glucose. Sennoside A (SA), derived from Rhubarb extract of traditional Chinese medicine, is often used to treat constipation and lose weight. Our previous study suggests that SA can increase the plasma GLP-1 level in a mouse model of type 2 diabetes. However, the mechanism of SA activity remains unknown. This issue was addressed in this study. Methods C57bl/6 mice were divided randomly into four groups at n = 12. Group one was used as a control group without drug treatment. The other three groups were treated with (SA) at three dosages: a low dose (15 mg/kg/day), medium dose (30mg/kg/day) or high dose (45 mg/kg/day). SA was delivered into the mice through drinking water. Bodyweight was monitored. After treatment, blood glucose was assayed by OGTT. Plasma GLP-1 and insulin were determined at 15 mins of oral glucose challenge. Colon tissues were collected for mRNA or western blot analysis. Immunofluorescence staining assays was used to evaluate the number of β-cells and L-cells. NCI-H716 cells were employed to investigate the mechanism of SA-induced GLP-1 secretion, and the cells were subjected to western blot analysis. In the study of extracellular signal-regulated kinases 1/2 (ERK1/2) pathway, NCI-H716 cells were pretreated with ERK1/2 inhibitors (PD98059, 50 μM) for 30 min in the presence of SA (100 μM). Results In the current study, SA can reduce body weight during 5 weeks of weight monitoring and improve OGTT in C57BL/6 mice on the Chow diet. Furthermore, plasma GLP-1 was significantly elevated in the mouse treated by SA at the dosage of 45 mg/kg/day. The SA activity was supported by improving glucose-induced insulin secretion. Meanwhile, increased expression of EKR1/2 and prohormone convertase 1/3 (PC1/3) proteins was observed in the large intestine of SA-treated mice. The number of L-cells was not altered in each group. In the NCI-H716 cells, GLP-1 secretion was induced by SA with activation of the ERK1/2 pathway and elevation of PC1/3 protein. The SA effect was blocked by the ERK1/2 inhibitor. These data suggest that SA induced GLP-1 secretion in L-cells through activation of the ERK1/2 pathway in the mouse intestine. Conclusion Our study provides direct evidence that SA interacts with L cells for GLP-1 secretion. The data suggest that the SA effect is dependent on the ERK1/2 signaling pathway. Therefore, SA is a new drug candidate for the treatment of type 2 diabetes by induction of GLP-1 secretion.