Identification of Andrographolide as an Agonist of Bile Acid TGR5 Receptor in a Cell Line to Demonstrate the Reduction in Hyperglycemia in Type-1 Diabetic Rats

Andrographolide (ADG) is contained in bitter plants, and its effects are widely thought to be associated with taste receptors. The current study used animal studies and cell lines to investigate the role of ADG in diabetic models. The Takeda G-protein-coupled receptor (TGR5) was directly influenced by ADG, and this boosted GLP-1 synthesis in CHO-K1 cells transfected with the TGR5 gene. However, this was not seen in TGR5-mutant cells. The human intestinal L-cell line NCI-H716 showed an increase in GLP-1 production in response to ADG. In NCI-H716 cells, the TGR5 inhibitor triamterene reduced the effects of ADG, including the rise in TGR5 mRNA levels that ADG caused. Additionally, as with the antihyperglycemic impact in type-1 diabetic rats, the increase in plasma-active GLP-1 level caused by ADG was enhanced by a DPP-4 inhibitor. The recovery of the hypoglycemic effect in diabetic rats and the increase in plasma GLP-1 caused by ADG were both suppressed by TGR5 blockers. As a result, after activating TGR5, ADG may boost GLP-1 synthesis in diabetic rats, enhancing glucose homeostasis. In Min-6 cells, a pancreatic cell line grown in culture, ADG-induced insulin secretion was also examined. Blocking GLP-1 receptors had little impact, suggesting that ADG directly affects TGR5 activity in Min-6 cells. A TGR5 mRNA level experiment in Min-6 cells further confirmed that TGR5 is activated by ADG. The current study revealed a novel finding suggesting that ADG may activate TGR5 in diabetic rats in a way that results in enhanced insulin and GLP-1 production, which may be helpful for future research and therapies.