Theaflavin 3′-gallate activates G protein-coupled receptor 55 (GPR55) and enhances GLP-1 secretion via Ca2+/CaMKII/ERK signaling in enteroendocrine STC-1 cells, mitigating postprandial hyperglycemia in mice

Abstract

The antihyperglycemic effect of black tea is well-known, and theaflavins (TFs) are considered active compounds. It is, however, unclear whether glucagon-like peptide-1 (GLP-1) is involved in the antihyperglycemic effects of TFs. We demonstrate that TFs suppress postprandial hyperglycemia by stimulating GLP-1 secretion in mice. In STC-1 cells, theaflavin 3′-gallate (TF2B), possessing a galloyl group at the 3′-position, showed the strongest effect on GLP-1 secretion among the four TFs. TF2B activated G protein-coupled receptor 55 (GPR55) and was confirmed to bind to the receptor, notably exhibiting the highest binding affinity. Moreover, GPR55 antagonist canceled TF2B-induced GLP-1 secretion. Downstream, TF2B increased intracellular Ca²⁺ levels and activated the Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated kinases 1/2 (ERK1/2) pathways. Inhibitors of Ca²⁺ signaling, CaMKII, and ERK pathways abolished TF2B-stimulated GLP-1 secretion. These findings suggest that TF2B from black tea prevents hyperglycemia through GPR55-dependent stimulation of GLP-1 secretion via Ca²⁺–Ca²⁺/CaMKII and ERK1/2 pathways.