| Incretin are hormone peptides released from the endocrine cells of the gastrointestinal tract and neurons.They affect the energy metabolism through regulating appetite,blood sugar,gastrointestinal motility and growth,exocrine cell secretion and fat cell function.Glucose-dependent insulinotropic peptide(GIP)is one of the important incretins in the body.It is secreted by the proximal small intestinal K cells,and can promote insulin secretion and the proliferation and survival of islet β cells.Improvement of the function of GIP and its receptors can maintain islet function or insulin sensitivity,which is considered as a new strategy for the treatment of type 2 diabetes.Saponins from Camellia sinensis seeds have got a lot of attention for their biological activities such as regulating gastrointestinal function,anti-cancer,anti-inflammatory,anti-microbial,anti-oxidation,neuroprotection,hypolipidemic,and surface activity.However,there are few studies on in-depth mechanisms of their bioactivities.In this paper,mouse model and mouse intestinal endocrine STC-1 cells were used to study the molecular mechanism of C.sinensis seed saponins and Theasaponin E1 on GIP secretion in mice.The results are as follows:(1)Tea seed saponins and Theasaponin E1 could stimulate GIP secretion in vitro and in vivo:Mice were administered with toatal tea seed saponins by single gavage at the dose of 65mg/kg BW,and the GIP mRNA and protein exprression at different time points were determined by real-time quantitative PCR(RT-PCR)and enzyme linked immunosorbent assay(ELISA).The results showed that the mRNA and protein expression of GIP both increased during the first several hours,and then decreased.The highest levels of mRNA and protein occurred at 1 and 3 hours respectively.Moreover,Theasaponin E1 could enhance the GIP mRNA and protein expression in STC-1 cells in a dose-dependent manner.(2)Sodium-glucose cotransporter-1(SGLT1)plays an important role in the secretion of GIP induced by tea seed saponin:simultaneous treatment of STC-1 cells with SGLT-1 inhibitor phlorizin and Theasaponin E,could inhibit the induction of intracellular calcium accumulation and GIP mRNA expression by Theasaponin E1,indicating that Theasaponin E1 induces GIP expression at least partly through activating SGLT1.(3)TGR5 plays an important role in saponin-induced GIP secretion:Western Blot and ELISA analysis showed that Theasaponin E1 can increase TGR5 protein expression and intracellular cAMP levels in STC-1 cells.Knockout of TGR5 by RNA silencing can reduce the GIP mRNA expression treated by Theasaponin E1.This result indicatd that TGR5 played an important role in the regulation of GIP expression by Theasaponin E1.In summary,this study found that tea seed saponin can stimulate the expression of GIP through SGLT-1 and TGR5-related pathways,suggesting that this kind of substances are hopeful to be researched and applied as functional food and medicines for regulating and improving metabolic diseases such as type 2 diabetes. |
