Effects of Traditional Chinese Medicinal Plants on Anti-insulin Resistance Bioactivity of DXMS-Induced Insulin Resistant HepG2 Cells

Experimental cell models of insulin resistance: overview and appraisal

Insulin resistance, a key factor in the development of type 2 diabetes mellitus (T2DM), is defined as a defect in insulin-mediated control of glucose metabolism in tissues such as liver, fat and muscle. Insulin resistance is a driving force behind various metabolic diseases, such as T2DM, hyperlipidemia, hypertension, coronary heart disease and fatty liver. Therefore, improving insulin sensitivity can be considered as an effective strategy for the prevention and treatment of these complex metabolic diseases. Cell-based models are extensively employed for the study of pathological mechanisms and drug screening, particularly in relation to insulin resistance in T2DM. Currently, numerous methods are available for the establishment of in vitro insulin resistance models, a comprehensive review of these models is required and can serve as an excellent introduction or understanding for researchers undertaking studies in this filed. This review examines and discusses the primary methods for establishing and evaluating insulin resistance cell models. Furthermore, it highlights key issues and suggestions on cell selection, establishment, evaluation and drug screening of insulin resistance, thereby providing valuable references for the future research efforts.

Structural characterization of Euglena gracilis polysaccharide and its in vitro hypoglycemic effects by alleviating insulin resistance

Diabetes mellitus, characterized by hyperglycemia and insulin resistance, is a disorder of the endocrine metabolic system which has emerged as a common chronic disease worldwide. Euglena gracilis polysaccharides have ideal development potential in the treatment of diabetes. However, their structure and bioactivity are largely unclear. A novel purified water-soluble polysaccharide (EGP-2A-2A) from E. gracilis with a molecular weight of 130.8 kDa consisted of xylose, rhamnose, galactose, fucose, glucose, arabinose, and glucosamine hydrochloride. The SEM image for EGP-2A-2A suggested a rough surface with the presence of globule-like protrusions. Methylation and NMR spectral analyses revealed that EGP-2A-2A was mainly composed of →6)-β-D-Galp-(1 → 2)-α-D-Glcp-(1 → 2)-α-L-Rhap-(1 → 3)-α-L-Araf-(1 → 6)-β-D-Galp-(1 → 3)-α-D-Araf-(1 → 3)-α-L-Rhap-(1 → 4)-β-D-Xylp-(1 → 6)-β-D-Galp-(1 → with complex branching structure. EGP-2A-2A significantly increased glucose consumption and glycogen content in IR-HeoG2 cells and modulates glucose metabolism disorders by regulating PI3K, AKT, and GLUT4 signaling pathways. EGP-2A-2A significantly suppressed TC, TG, and LDL-c levels, and enhanced that of HDL-c. EGP-2A-2A ameliorated abnormalities caused by disorders of glucose metabolism and the hypoglycemic activity of EGP-2A-2A may be mainly positively related to its high glucose content and the β-configuration in the main chain. These results suggested that EGP-2A-2A played an important role in alleviating disorders of glucose metabolism through insulin resistance and has the potential for development as a novel functional food with nutritional and health benefits.

Quercetin-3-O-α-L-arabinopyranosyl-(1→2)-β-D-glucopyranoside Isolated from Eucommia ulmoides Leaf Relieves Insulin Resistance in HepG2 Cells via the IRS-1/PI3K/Akt/GSK-3β Pathway

For nearly 2000 years, Eucommia ulmoides Oliver (EUO) has been utilized in traditional Chinese medicine (TCM) throughout China. Flavonoids present in bark and leaves of EUO are responsible for their antioxidant, anti-inflammatory, antitumor, anti-osteoporosis, hypoglycemic, hypolipidemic, antibacterial, and antiviral properties, but the main bioactive compound has not been established yet. In this study, we isolated and identified quercetin glycoside (QAG) from EUO leaves (EUOL) and preliminarily explored its molecular mechanism in improving insulin resistance (IR). The results showed that QAG increased uptake of glucose as well as glycogen production in the palmitic acid (PA)-induced HepG2 cells in a dose-dependent way. Further, we observed that QAG increases glucose transporters 2 and 4 (GLUT2 and GLUT4) expression and suppresses the phosphorylation of insulin receptor substrate (IRS)-1 at serine⁶¹², thus promoting the expression of phosphatidylinositol-3-kinase (PI3K) at tyrosine⁴⁵⁸ and tyrosine¹⁹⁹, as well as protein kinase B (Akt) and glycogen synthase kinase (GSK)-3β at serine⁴⁷³ and serine⁹, respectively. The influence posed by QAG on the improvement of uptake of glucose was significantly inhibited by LY294002, a PI3K inhibitor. In addition, the molecular docking result showed that QAG could bind to insulin receptors. In summary, our data established that QAG improved IR as demonstrated by the increased uptake of glucose and glycogen production through a signaling pathway called IRS-1/PI3K/Akt/GSK-3β.

Aqueous extract of Scrophularia ningpoensis improves insulin sensitivity through AMPK-mediated inhibition of the NLRP3 inflammasome

BACKGROUND

Scrophularia ningpoensis Hemsl. is a commonly used medicinal plant in China for the treatment of diabetes mellitus (DM), but its mechanism of action remains poorly described. Type 2 diabetes mellitus (T2DM) accounts for > 90% of all DM cases and is characterized by insulin resistance.

PURPOSE

The aim of this study was to investigate whether the insulin sensitivity can be improved by treatment with aqueous extract of S. ningpoensis (AESN) and further explore its mechanism(s) of activity.

METHODS

Primary mouse hepatocytes and human HepG2 hepatocytes were used to investigate the effects of AESN on cell viability, AMP-activated protein kinase (AMPK) activation and glucose output under normal culture conditions. To mimic hyperglycemia and insulin resistance in vitro, hepatocytes were exposed to high glucose (HG), and the influences of AESN on AMPK phosphorylation, NLRP3 inflammation activation, insulin signaling, lipid accumulation and glucose output were investigated. Increasing doses of AESN (50, 100 and 200 mg/kg/day) were administered by gavage to db/db mice for 8 weeks, and then biochemical analysis and histopathological examinations were performed.

RESULTS

AESN significantly activated AMPK and inhibited glucose output in hepatocytes, but did not impact cell viability under normal culture conditions. Moreover, in HG-treated hepatocytes, AESN protected against aberrant AMPK activity, NLRP3 inflammasome activation, insulin signaling, and lipid accumulation. AMPK inhibition abolished the regulatory effects of AESN on the NLRP3 inflammasome, insulin signaling, lipid accumulation, and glucose output of hepatocytes following HG exposure. Furthermore, AESN administration reduced blood glucose and serum insulin levels, improved lipid profiles and insulin resistance, and corrected the aberrant AMPK activity and NLRP3 inflammasome activation in liver tissues.

CONCLUSION

AESN improves insulin sensitivity via AMPK-mediated NLRP3 inflammasome inhibition.

3,3′,4,5′-Tetramethoxy-trans-stilbene Improves Insulin Resistance by Activating the IRS/PI3K/Akt Pathway and Inhibiting Oxidative Stress

The potential anti-diabetic effect of resveratrol derivative, 3,3′,4,5′-tetramethoxy-trans-stilbene (3,3′,4,5′-TMS) and its underlying mechanism in high glucose (HG) and dexamethasone (DXMS)-stimulated insulin-resistant HepG2 cells (IR-HepG2) were investigated. 3,3′,4,5′-TMS did not reduce the cell viability of IR-HepG2 cells at the concentrations of 0.5–10 µM. 3,3′,4,5′-TMS increased the potential of glucose consumption and glycogen synthesis in a concentration-dependent manner in IR-HepG2 cells. 3,3′,4,5′-TMS ameliorated insulin resistance by enhancing the phosphorylation of glycogen synthase kinase 3 beta (GSK3β), inhibiting phosphorylation of insulin receptor substrate-1 (IRS-1), and activating phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) pathway in IR-HepG2 cells. Furthermore, 3,3′,4,5′-TMS significantly suppressed levels of reactive oxygen species (ROS) with up-regulation of nuclear factor erythroid 2-related factor 2 (Nrf2) expression. To conclude, the beneficial effect of 3,3′,4,5′-TMS against insulin resistance to increase glucose consumption and glycogen synthesis was mediated through activation of IRS/PI3K/Akt signaling pathways in the IR-HepG2 cells, accomplished with anti-oxidative activity through up-regulation of Nrf2.

The genus Cuscuta (Convolvolaceac): An updated review on indigenous uses, phytochemistry, and pharmacology

Cuscuta, commonly known as dodder, is a genus of family convolvolaceace. Approximately 170 species of Cuscuta are extensively distributed in temperate and subtropical areas of the world. Species of this genus are widely used as essential constituents in functional foods and traditional medicinal systems. Various parts of many members of Cuscuta have been found efficacious against a variety of diseases. Phytochemical investigations have confirmed presence of biologically active moieties such as flavonoids, alkaloids, lignans, saponines, phenolics, tannins, and fatty acids. Pharmacological studies and traditional uses of these plants have proved that they are effective antibacterial, antioxidant, antiostioporotic, hepatoprotective, anti-inflammatory, antitumor, antipyretic, antihypertensive, analgesic, anti hair fall, and antisteriogenic agents.

Resveratrol affects hepatic gluconeogenesis via histone deacetylase 4

PURPOSE

The aim of this study was to determine whether resveratrol (Rev) affects the expression, phosphorylation, and nuclear and cytoplasmic distribution of histone deacetylase 4 (HDAC4), which in turn affects gluconeogenesis in hepatocytes under an insulin-resistant state.

MATERIALS AND METHODS

HepG2 cells were treated with 0.25 mmol/L palmitic acid (PA) to establish an insulin resistance model. The cells were divided into five groups: control, PA, PA + Rev 100 µM, PA + Rev 50 µM, and PA + Rev 20 µM. After treatment for 24 hours, mRNA and protein expression levels of gluconeogenesis pathway-related molecules and HDAC4 were examined. Next, HepG2 cells were transfected with siRNA-HDAC4. The cells were divided into control, PA, PA + Rev 20 µM, PA + Rev 20 µM +siRNA-HDAC4 negative control, and PA + Rev 20 µM +siRNA-HDAC4 knockdown groups to determine the expression of gluconeogenesis pathway proteins.

RESULTS

Compared with the control group, the gluconeogenesis pathway-related molecules, glucose-6-phosphatase catalytic subunit (G6PC), phosphoenolpyruvate carboxykinase 1 (PCK1) and forkhead box protein O1 (FOXO1), were increased, and the phosphorylation of FOXO1 decreased after PA treatment. The p-HDAC4 level decreased with the increase in HDAC4 in the nucleus and the decrease in HDAC4 in the cytoplasm in the PA group. Treatment with Rev 20 µM suppressed gluconeogenesis and promoted HDAC4 shuttling into the cytoplasm from the nucleus. However, 100 and Rev 50 µM exerted the opposite effects. Finally, after HDAC4 knockdown, the expression levels of the key gluconeogenesis molecules, G6PC, PCK1, and FOXO1, were increased, and p-FOXO1 was decreased, indicating that gluconeogenesis was enhanced.

CONCLUSION

A low concentration of Rev inhibited gluconeogenesis under insulin-resistance conditions via translocation of HDAC4 from the nucleus to the cytoplasm.

Qizhi Jiangtang Jiaonang Improves Insulin Signaling and Reduces Inflammatory Cytokine Secretion and Reactive Oxygen Species Formation in Insulin Resistant HepG2 Cells

We analyzed the effects of a traditional Chinese medicine, Qizhi Jiangtang Jiaonang (QJJ), on insulin resistance (IR) in vitro. After an in vitro model of IR was established by treating human liver cancer cells (HepG2 cells) with palmitic acid, the cells were then treated with various concentrations of QJJ. Treatment with 400 µM palmitic acid for 24 h induced IR in HepG2 cells. The survival rate for HepG2 cells in the IR group was significantly lower than that of the untreated control group (P < 0.001); however, QJJ restored HepG2 cell survival (P < 0.001). As compared with HepG2 cells in the IR group, QJJ at all doses analyzed significantly increased glucose consumption (all P < 0.05). Moreover, treatment with all the QJJ doses significantly reduced the mean intracellular reactive oxygen species levels as compared with the IR group (all P < 0.05). Furthermore, high-dose QJJ reduced both TNF-α and IL-6 levels as compared to the IR group (all P < 0.05). QJJ ameliorated the altered PI3K, GLUT4, and RAGE expression observed with IR. In conclusion, QJJ can improve IR in HepG2 cells, which may be mediated through the IRS-1/PI3K/GLUT4 signaling pathway as well as regulation of NF-κB-mediated inflammation and oxidative stress.