Investigation of triamterene as an inhibitor of the TGR5 receptor: identification in cells and animals

Abstract

BackgroundG-protein-coupled bile acid receptor 1 (GPBAR1, also known as TGR5) has been shown to participate in glucose homeostasis. In animal models, a TGR5 agonist increases incretin secretion to reduce hyperglycemia. Many agonists have been developed for clinical use. However, the effects of TGR5 blockade have not been studied extensively, with the exception of studies using TGR5 knockout mice. Therefore, we investigated the potential effect of triamterene on TGR5.MethodsWe transfected the TGR5 gene into cultured Chinese hamster ovary cells (CHO-K1 cells) to express TGR5. Then, we applied a fluorescent indicator to examine the glucose uptake of these transfected cells. In addition, NCI-H716 cells that secrete incretin were also evaluated. Fura-2, a fluorescence indicator, was applied to determine the changes in calcium concentrations. The levels of cyclic adenosine monophosphate (cAMP) and glucagon-like peptide (GLP-1) were estimated using enzyme-linked immunosorbent assay kits. Moreover, rats with streptozotocin (STZ)-induced type 1-like diabetes were used to investigate the effects in vivo.ResultsTriamterene dose dependently inhibits the increase in glucose uptake induced by TGR5 agonists in CHO-K1 cells expressing the TGR5 gene. In cultured NCI-H716 cells, TGR5 activation also increases GLP-1 secretion by increasing calcium levels. Triamterene inhibits the increased calcium levels by TGR5 activation through competitive antagonism. Moreover, the GLP-1 secretion and increased cAMP levels induced by TGR5 activation are both dose dependently reduced by triamterene. However, treatment with KB-R7943 at a dose sufficient to block the Na+/Ca2+ exchanger (NCX) failed to modify the responses to TGR5 activation in NCI-H716 cells or CHO-K1 cells expressing TGR5. Therefore, the inhibitory effects of triamterene on TGR5 activation do not appear to be related to NCX inhibition. Blockade of TGR5 activation by triamterene was further characterized in vivo using the STZ-induced diabetic rats.ConclusionBased on the obtained data, we identified triamterene as a reliable inhibitor of TGR5. Therefore, triamterene can be developed as a clinical inhibitor of TGR5 activation in future studies.

Compound 18 Improves Glucose Tolerance in a Hepatocyte TGR5-dependent Manner in Mice

“…While our data are consistent with previous work demonstrating that TGR5 ablation contributes to metabolic dysregulation in mouse models [ 3 , 19 , 34 , 37 , 49 ], our data are the first to demonstrate that TGR5 signaling specifically within the hepatocyte plays a significant role in whole body glucose regulation. Until now, the role of TGR5 in the maintenance of glucose homeostasis and insulin sensitivity has been attributed to its effects on mitochondrial function in muscle and BAT and/or insulin release from the pancreas, enhanced by enteroendocrine L cell GLP-1 secretion [ 3 , 13 , 14 , 15 , 17 , 20 ]. For example, various TGR5 agonists have been shown to ameliorate glucose intolerance in obese and diabetic mice [ 3 , 13 , 50 , 51 ].…”

“…TGR5 signaling in many of these tissue types has been shown to contribute to glucose regulation [ 3 , 13 ]. For example, in gastrointestinal enteroendocrine L cells, TGR5 signaling promotes glucagon-like peptide-1 (GLP-1) secretion [ 3 , 14 , 15 ]. Furthermore, in mice fed a high fat diet (HFD), TGR5 agonists have been shown to increase in GLP-1 secretion from L cells, which was associated with an improvement in insulin sensitivity and measures of hepatic steatosis [ 16 ].…”

The Dietary Furocoumarin Imperatorin Increases Plasma GLP-1 Levels in Type 1-Like Diabetic Rats

“…Moreover, triamterene also attenuated the effect of imperatorin in a dose-dependent manner. In a recent study, triamterene specifically blocked TGR5 [ 32 ]. Therefore, imperatorin can activate TGR5 in vitro.…”

“…Moreover, this effect of imperatorin was inhibited by triamterene. It has been reported that triamterene possesses a TGR5 blocking-like action as a pleiotropic effect in addition to K + -sparing [ 32 ]. The functional blockade of TGR5 by triamterene supports that imperatorin mediates TGR5.…”

TGR5 Signaling in Hepatic Metabolic Health

“…Initial studies revealed that TGR5 signaling regulates glucose tolerance, inflammation, and energy expenditure [ 9 , 13 , 14 , 15 ], such that TGR5 is now recognized as a potential target for the treatment of metabolic disorders. To this end, much metabolic research regarding TGR5 signaling has focused on TGR5-mediated effects on GLP-1 secretion from enteroendocrine L cells [ 9 , 16 , 17 ], mitochondrial thermogenesis in adipocytes [ 13 , 18 , 19 ], and decreased inflammatory cytokine secretion from immune cells [ 1 , 14 , 15 ].…”