Geniposide Regulates GLUT2 Endocytosis And Its Relative Molecular Mechanisms In Pancreatic Beta Cells

Type 2 diabetes mellitus(T2DM)is a common chronic metabolic disorder characterized by persistent hyperglycemia and insulin resistance.Glucose-stimulated insulin secretion(GSIS)plays a critical role on maintaining blood glucose balance and the physiological functions of pancreatic β cells.Our previous works demonstrated that,with the activation of glucagon-like peptide 1 receptor(GLP-1R),geniposide promoted glucose uptake,metabolism and GSIS in the presence of low or middle concentrations of glucose,but on the contrary,geniposide significantly attenuated the uptake and metabolism of glucose in the presence of high concentration of glucose in pancreatic beta cells.As we all knew that under the condition of high glucose,continued high blood sugar required pancreatic beta cells to secrete more insulin to maintain the balance of blood sugar,which enhanced the oxidative damage and dysfunction of pancreatic beta cells,and even apoptosis.So,Attenuating GSIS in the presence of high glucose might be benefit for the pancreatic beta cells,and delay the development of T2 DM and its relative complications.But unfortunately,the relative mechanisms of geniposide regulating the uptake of glucose and GSIS keep to be explored.Together with the references and our previous data,we supposed that geniposide regulating GSIS might be involved in its role on the endocytosis of glucose transporter 2(GLUT2),and then affecting the uptake of glucose in pancreatic beta cells.In this study,we design to analyze the effect of geniposide on the endocytosis of GLUT2 and its relative mechanisms in the presence or low or high concentrations of glucose,to clarify the phenomenon of geniposide regulating GSIS,and supply more evidence for the application of geniposide on the treatment of T2 DM in future.Firstly,we determined the influence of geniposide on the expression and endocytosis in the presence of low or high concentration of glucose,the results indicated that after treated with geniposide for 12 or 24 h geniposide enhanced the expression of GLUT2,but no significant effect in short time,and in a short-time treatment(1 h),geniposide significantly inhibited the endocytosis of GLUT2 in the presence of low glucose(5 mM),but on the contrary,in the presence of high glucose(25 mM),geniposide accelerated the endocytosis of GLUT2.This result is consistent with its role on GSIS in pancreatic beta cells,suggested that geniposide regulating GSIS might be involved in its role on GLUT2 endocytosis.And then,we determined the influence of geniposide on the expression of relativegenes regulating GLUT2 endocytosis,included GnT-IVa(N-acetylglucosaminyltransferase IVa),galectin-9 and clathrin,in pancreatic beta cells.The results indicated that after treated with geniposide,the levels of mRNA and protein,including GnT-IVa and Galectin-9,were increased noticeably,but there is no significantly effect on the expression of clathrin.But in a short-time treatment(1 h),geniposide regulated the protein of GnT-IVa,but not galectin-9.At the same time,after treated with geniposide for 1 h,we analyzed the effect of geniposide on the glycosylation of GLUT2 in INS-1 cells,data demonstrated that treatment with geniposide for 1 h enhanced the glycosylation of GLUT2 in the presence of 5 or 11 mM glucose,but in the presence of 25 mM glucose,geniposide attenuated the glycosylated level of GLUT2 protein.Furthermore,knockdown the expression of GnT-IVa with siRNA transfection inhibited the effect of geniposide on the endocytosis of GLUT2 and uptake of glucose in pancreatic beta cells.All these data suggested that GnT-IVa played an essential role on geniposide regulating the endocytosis of GLUT2 and uptake of glucose in pancreatic beta cells,and data from this work clarify the molecular mechanisms of geniposide regulating GSIS,provide some preliminary evidence for the application of geniposide on clinic in future,and supply some novel targets and strategies for the prevention and treatment of T2DM.