Background: -Gingerol, a major component of Zingiber officinale, was previously reported to ameliorate hyperglycemia in type 2 diabetic mice. Endocrine signaling is involved in insulin secretion and is perturbed in db/db Type-2 diabetic mice. -Gingerol was reported to restore the disrupted endocrine signaling in rodents. In this current study on Leprdb/db diabetic mice, we investigated the involvement of endocrine pathway in the insulin secretagogue activity of -Gingerol and the mechanism(s) through which -Gingerol ameliorates hyperglycemia.
Methods: Leprdb/db type 2 diabetic mice were orally administered a daily dose of -Gingerol (200 mg/kg) for 28 days. We measured the plasma levels of different endocrine hormones in fasting and fed conditions. GLP-1 levels were modulated using pharmacological approaches, and cAMP/PKA pathway for insulin secretion was assessed by qRT-PCR and ELISA in isolated pancreatic islets. Total skeletal muscle and its membrane fractions were used to measure glycogen synthase 1 level and Glut4 expression and protein levels.
Results: 4-weeks treatment of -Gingerol dramatically increased glucose-stimulated insulin secretion and improved glucose tolerance. Plasma GLP-1 was found to be significantly elevated in the treated mice. Pharmacological intervention of GLP-1 levels regulated the effect of -Gingerol on insulin secretion. Mechanistically, -Gingerol treatment upregulated and activated cAMP, PKA, and CREB in the pancreatic islets, which are critical components of GLP-1-mediated insulin secretion pathway. -Gingerol upregulated both Rab27a GTPase and its effector protein Slp4-a expression in isolated islets, which regulates the exocytosis of insulin-containing dense-core granules. -Gingerol treatment improved skeletal glycogen storage by increased glycogen synthase 1 activity. Additionally, GLUT4 transporters were highly abundant in the membrane of the skeletal myocytes, which could be explained by the increased expression of Rab8 and Rab10 GTPases that are responsible for GLUT4 vesicle fusion to the membrane.
Conclusions: Collectively, our study reports that GLP-1 mediates the insulinotropic activity of -Gingerol, and -Gingerol treatment facilitates glucose disposal in skeletal muscles through increased activity of glycogen synthase 1 and enhanced cell surface presentation of GLUT4 transporters.
Keywords: GLP-1; GLUT4; Glycogen synthase 1; Leprdb/db mice; Rab10; Rab27a; Rab8; Type 2 diabetes; -Gingerol.