The beneficial effects of green tea polyphenols (GTP) against metabolic syndrome and type 2 diabetes by suppressing appetite and nutrient absorption have been well reported. However the direct effects and mechanisms of GTP on glucose and lipid metabolism remain to be elucidated. Since the liver is an important organ involved in glucose and lipid metabolism, we examined the effects and mechanisms of GTP on glycogen synthesis and lipogenesis in HepG2 cells. Concentrations of GTP containing 68% naturally occurring (−)-epigallocatechin-3-gallate (EGCG) were incubated in HepG2 cells with high glucose (30 mM) under 100 nM of insulin stimulation for 24 h. GTP enhanced glycogen synthesis in a dose-dependent manner. 10 μM of EGCG significantly increased glycogen synthesis by 2fold (P < 0.05) compared with insulin alone. Western blotting revealed that phosphorylation of Ser9 glycogen synthase kinase 3β and Ser641 glycogen synthase was significantly increased in GTP-treated HepG2 cells compared with nontreated cells. 10 μM of EGCG also significantly inhibited lipogenesis (P < 0.01). We further demonstrated that this mechanism involves enhanced expression of phosphorylated AMP-activated protein kinase α and acetyl-CoA carboxylase in HepG2 cells. Our results showed that GTP is capable of enhancing insulin-mediated glucose and lipid metabolism by regulating enzymes involved in glycogen synthesis and lipogenesis.

This study was carried out to determine the effect of saponins of Panax notoginseng (SPN), a naturally occurring cardiovascular agent, on: (1) glucose uptake, (2) GLUT4 translocation and (3) glycogen synthesis in 3T3-L1 adipocytes. Electrospray ionization-Mass spectrometry (ESI-MS) was used to determine the structural characterization of the major active components of SPN. 3T3-L1 adipocytes were cultured and treated with 100 nM insulin alone or with 10, 50 and 100 microg/ml of SPN. [(3)H]2-deoxyglucose glucose uptake, GLUT4 immunofluorescence imaging and glycogen synthesis assay were carried out to determine the effects of SPN on glucose metabolism. Under insulin stimulation, SPN significantly increased glucose uptake in a dose-dependent manner; 50 microg/ml of SPN increased glucose uptake by 64% (p < 0.001). Immunofluorescence imaging and analysis have revealed that 50 and 100 microg/ml of SPN increased GLUT4 in the plasma membrane by 3-fold and 6-fold respectively (p < 0.001). Furthermore, the incorporation of D-[U-(14)C] glucose into glycogen was enhanced by 53% in 3T3-L1 cells treated with 100 microg/ml of SPN (p < 0.01 vs. insulin stimulation alone). SPN, a naturally occurring agent used to treat ischemic cardio-cerebral vascular disease in China, enhanced insulin-stimulated glucose uptake and glycogen synthesis in adipocytes. The results of this study indicate that SPN may have a therapeutic potential for hyperglycaemia in type 2 diabetes.

Recent studies have demonstrated the effects of green tea polyphenols (GTP) and epigallocatechin-3-gallate (EGCG) on obesity. However, high doses of EGCG have also exhibited cytotoxicity. The aim of this study was to compare total GTP with purified EGCG on cytotoxicity, and to investigate the effects and the molecular mechanism of total GTP and EGCG on adipogenesis. Cytotoxicity was determined by cell viability assay. For the adipogenesis study, 3T3-L1 preadipocytes were incubated with three doses of GTP (1, 10, and 100 μg/ml) and the effect of EGCG (6.8 μg/ml) was compared with 10 μg/ml GTP containing 68% EGCG. Oil Red O staining and triglyceride content assay were carried out 10 days after differentiation and treatment. Adipogenic regulators CCAAT element binding protein α (C/EBPα), peroxisome proliferator-activated receptor gamma (PPARγ) and sterol regulatory element-binding protein-1c (SREBP-1c) were determined by qRT-PCR and immunoblotting. GTP at 1000 μg/ml and EGCG (68 and 680 μg/ml) significantly affected cell viability. Purified EGCG had greater cytotoxicity than corresponding doses of GTP. About 10 μg/ml of GTP showed stronger reduction in triglyceride accumulation than EGCG treatment. Transcriptional factors of C/EBPα, SREBP-1c and PPARγ were markedly decreased in both GTP and EGCG-treated cells and GTP exhibited stronger inhibitory effects on C/EBPα and PPARγ protein expression than EGCG (p < 0.05). In conclusion, total GTP exerted greater inhibitory effects than purified EGCG on adipogenesis through down-regulating the adipogenic factor C/EBPα, SREBP-1c and PPARγ expression. These findings support that a polyphenol mixture is safer and more effective than EGCG alone for preventing obesity and obesity-related chronic diseases.