Geniposide and Gentiopicroside Suppress Hepatic Gluconeogenesis via Regulation of AKT-FOXO1 Pathway.

BACKGROUND: Hepatic gluconeogenesis plays an important role in regulating fasting plasma glucose levels and is a target of anti-diabetic drugs. Several kinds of iridoid glucosides exhibit hypoglycemic effect, whereas the mechanism was not clear. AIM OF THE STUDY: In this study, the effects of geniposide and gentiopicroside, two natural iridoid glucosides, on hepatic gluconeogenesis were investigated. METHODS: Glucose uptake assay, MTT assay, q-PCR, luciferase assay and western blot assay were performed to investigate the pharmacological effect of geniposide and gentiopicroside on human liver cell line L02. Thereby the fast blood glucose and intraperitoneal glucose tolerance were measured in high fat diet induced hyperglycemic mice after geniposide or gentiopicroside administration. RESULTS: The results showed that geniposide and gentiopicroside inhibited the transcription of G6PC and PEPCK in L02 cells and in mice. Additional experimental data indicated that these two compounds were able to inhibit the transcriptional activity of FOXO1 by inducing phosphorylation of AKT at Ser473. Furthermore, we found that these two compounds alleviated high fat diet induced hyperglycemia in mice. CONCLUSIONS: Geniposide and gentiopicroside might reduce blood glucose and suppress hepatic gluconeogenesis by regulating the AKT-FOXO1 pathway, and the potential use of these two iridoid glucosides as anti-diabetic agents merits further in-depth exploration.

Antimicrobial mechanism of theaflavins: They target 1-deoxy-D-xylulose 5-phosphate reductoisomerase, the key enzyme of the MEP terpenoid biosynthetic pathway.

1-Deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) is the first committed enzyme in the 2-methyl-D-erythritol 4-phosphate (MEP) terpenoid biosynthetic pathway and is also a validated antimicrobial target. Theaflavins, which are polyphenolic compounds isolated from fermented tea, possess a wide range of pharmacological activities, especially an antibacterial effect, but little has been reported on their modes of antimicrobial action. To uncover the antibacterial mechanism of theaflavins and to seek new DXR inhibitors from natural sources, the DXR inhibitory activity of theaflavins were investigated in this study. The results show that all four theaflavin compounds could specifically suppress the activity of DXR, with theaflavin displaying the lowest effect against DXR (IC50 162.1 µM) and theaflavin-3,3'-digallate exhibiting the highest (IC50 14.9 µM). Moreover, determination of inhibition kinetics of the theaflavins demonstrates that they are non-competitive inhibitors of DXR against 1-deoxy-D-xylulose 5-phosphate (DXP) and un-competitive inhibitors with respect to NADPH. The possible interactions between DXR and the theaflavins were simulated via docking experiments.

Inhibition of green tea and the catechins against 1-deoxy-d-xylulose 5-phosphate reductoisomerase, the key enzyme of the MEP terpenoid biosynthetic pathway.

1-Deoxy-d-xylulose 5-phosphate reductoisomerase (DXR) is the first committed enzyme in the MEP terpenoid biosynthetic pathway and also a validated antimicrobial target. Green tea which is rich in polyphenolic components such as the catechins, possesses a plenty of pharmacological activities, in particular an antibacterial effect. To uncover the antibacterial mechanism of green tea and to seek new DXR inhibitors from natural sources, the DXR inhibitory activity of green tea and its main antimicrobial catechins were investigated in this study. The results show that the raw extract of green tea and its ethyl acetate fraction are able to suppress DXR activity explicitly. Further determination of the DXR inhibitory capacity of eight catechin compounds demonstrates that the most active compound is gallocatechin gallate that is able to inhibit around 50% activity of DXR at 25µM. Based on these data, the primary structure-activity relationship of the catechins against DXR is discussed. This study would be very helpful to elucidate the antimicrobial mechanism of green tea and the catechins and also would be very useful to direct the rational utilization of them as food additives.

Cloning and characterization of GST fusion tag stabilized large subunit of Escherichia coli acetohydroxyacid synthase I.

There are three acetohydroxyacid synthase (AHAS, EC 4.1.3.18) isozymes (I, II, and III) in the enterobacteria Escherichia coli among which AHAS I is the most active. Its large subunit (LSU) possesses full catalytic machinery, but is unstable in the absence of the small subunit (SSU). To get applicable LSU of AHAS I, we prepared and characterized in this study the polypeptide as a His-tagged (His-LSU) and a glutathione S-transferase (GST)-tagged (GST-LSU) fusion protein, respectively. The results showed that the His-LSU is unstable, whereas the GST-LSU displays excellent stability. This phenomenon suggests that the GST polypeptide fusion tag could stabilize the target protein when compared with histidine tag. It is the first time that the stabilizing effect of the GST tag was observed. Further characterization of the GST-LSU protein indicated that it possesses the basic functions of AHAS I with a specific activity of 20.8 µmol min(-1) mg(-1) and a Km value for pyruvate of 0.95 mM. These observations imply that introduction of the GST fusion tag to LSU of AHAS I does not affect the function of the protein. The possible reasons that the GST fusion tag could make the LSU stable are initially discussed.