Gundelia tournefortii: Fractionation, Chemical Composition and GLUT4 Translocation Enhancement in Muscle Cell Line

Type 2 diabetes (T2D) is a chronic metabolic disease, which could affect the daily life of patients and increase their risk of developing other diseases. Synthetic anti-diabetic drugs usually show severe side effects. In the last few decades, plant-derived drugs have been intensively studied, particularly because of a rapid development of the instruments used in analytical chemistry. We tested the efficacy of Gundelia tournefortii L. (GT) in increasing the translocation of glucose transporter-4 (GLUT4) to the myocyte plasma membrane (PM), as a main strategy to manage T2D. In this study, GT methanol extract was sub-fractionated into 10 samples using flash chromatography. The toxicity of the fractions on L6 muscle cells, stably expressing GLUTmyc, was evaluated using the MTT assay. The efficacy with which GLUT4 was attached to the L6 PM was evaluated at non-toxic concentrations. Fraction 6 was the most effective, as it stimulated GLUT4 translocation in the absence and presence of insulin, 3.5 and 5.2 times (at 250 μg/mL), respectively. Fraction 1 and 3 showed no significant effects on GLUT4 translocation, while other fractions increased GLUT4 translocation up to 2.0 times. Gas chromatography–mass spectrometry of silylated fractions revealed 98 distinct compounds. Among those compounds, 25 were considered anti-diabetic and glucose disposal agents. These findings suggest that GT methanol sub-fractions exert an anti-diabetic effect by modulating GLUT4 translocation in L6 muscle cells, and indicate the potential of GT extracts as novel therapeutic agents for T2D.

Long-term treatment with insulin and retinoic acid increased glucose utilization in L6 muscle cells via glycogenesis

The skeletal muscle regulates glucose homeostasis. Here, the effects of vitamin A metabolites including retinoic acid (RA) alone, and in combination with insulin, on glucose utilization were investigated in rat L6 muscle cells during the differentiation process. L6 cells were treated with differentiation medium containing retinol, retinal, RA, and (or) insulin. The glucose levels and pH values in the medium were measured every 2 days. The expression levels of insulin signaling and glycogen synthesis proteins, as well as glycogen content were determined. Retinal and RA reduced the glucose content and pH levels in the medium of the L6 cells. RA acted synergistically with insulin to reduce glucose and pH levels in the medium. The RA- and insulin-mediated reduction of glucose in the medium only occurred when glucose levels were at or above 15 mmol/L. Insulin-induced phosphorylation of Akt Thr308 was further enhanced by RA treatment through the activation of retinoic acid receptor. RA acted synergistically with insulin to phosphorylate glycogen synthase kinase 3β, and dephosphorylate glycogen synthase (GS), which was associated with increases in the protein and mRNA levels of GS. Increases in glycogen content were induced by insulin, and was further enhanced in the presence of RA. We conclude that activation of the RA signaling pathway enhanced insulin-induced glucose utilization in differentiating L6 cells through increases in glycogenesis.

6-shogaol suppresses oxidative damage in L6 muscle cells

Ginger (Zingiber Officinale Roscoe) has been known reduce muscle pain after exercise, and 6-shogaol {(E)-1-(4-Hydroxy-3-methoxyphenyl)dec-4-en-3-one)} is the major essential oil contained in ginger. In this study, the protective effect of 6-shogaol on L6 muscle cells against oxidative damage was measured. 6-shagol inhibited the damage of L6 cell induced by H2O2, and allowed the increase in mRNA and protein expression levels of intracellular HO-1 and NRF2. 6-shogaol also reduced the production of intracellular ROS. These results suggested that 6-shagol effectively inhibits oxidative damage of skeletal muscle cell.

The Effects of Insulin and Retinoic Acid on the Content of Glucose Transporter 4 in Different Preparation Lysates of L6 Muscle Cells (P15-013-19)

Objectives Glucose enters the skeletal muscle cells through glucose transporters (GLUTs), a process that is stimulated by insulin through the movement of GLUT4 to the cell membrane. Here, we studied the effects of retinoic acid (RA) and insulin on GLUT4 expression in L6 muscle cells. Methods Rat L6 muscle cells were induced to differentiation after confluency. Cells were incubated in medium containing 2% HS containing in the absence or presence of 1 μM RA without or with 10 nM insulin for 4 or 6 days with replacement of fresh media every two days. Cells were lyzed and prepared in the following ways. 1) Cells were lyzed in lysis buffer (50 mM HEPES pH 7.5, 10 mM EDTA, 10% Glycerol, 2% NP-40 and 2% Triton X-100) on ice for 20 minutes, vortex cells every 10 minutes to help lyse completely. After that, the lysate was centrifuged at 12,000 x g for 15 minutes at 4°C for the collection of both the supernatant and pellet. 2) Cells were directly lyzed in 1 x SDS loading buffer (designated as total cell lysate here). The GLUT4 protein levels in those samples were determined via Western Blot using anti-GLUT4 antibody (#07-1404, C-terminus) from EMD Millipore Corp. Results On day 4 or 6, the supernatants of cells treated with RA + insulin had a significantly less GLUT4 expression that that of the control group. Additional, on day 4, the supernatant of cells treated with insulin alone also had less GLUT4 protein than that of the control group. The pellets of cells on day 4 treated with RA had a significantly less GLUT4 protein level than that of the control group. On day 4 or 6, the GLUT4 protein levels in total cell lysates were not significantly different among the four treatment groups. Conclusions The current lysis buffer allowed us to observe that the treatments of RA and/or insulin can significantly affect GLUT4 protein levels in the supernatants. Given the fact that a significant amount of GLUT4 protein remained in the pellets, the lysis buffer used here could not completely solubilize differentiated L6 cells, suggesting that cautions lysis buffers and methods were chosen carefully to prepare L6 cells for the analysis of GLUT4 protein. Additionally, it also indicates that the differentially solubilized fractions probably provide us tools to study the effects of RA and insulin on the subcellular movement of GLUT4 in L6 muscle cells. Funding Sources The University of Tennessee, Knoxville.

Gundelia tournefortiiAntidiabetic Efficacy: Chemical Composition and GLUT4 Translocation

In the presentin vitrostudy, we tested the chemical composition, cytotoxicity and antidiabetic activity of two distinct extracts of wild Artichoke-like vegetable,Gundelia tournefortii: methanol and hexane. GC/MS phytochemical analysis ofG. tournefortiimethanol and hexane extracts revealed 39 compounds reported here for the first time inG. tournefortiiout of the 45 detected compounds. Only Stigmasterol was present in both extracts. The efficacy ofG. tournefortiiextracts in enhancing glucose transporter 4 (GLUT4) translocation to the plasma membrane (PM) was tested in L6 muscle cells stably expressing myc-tagged GLUT4 (L6-GLUT4myc) using cell-ELISA test. Results obtained here indicate that methanol and hexane extracts were safe up to 250 μg/ml as measured with MTT and the LDH leakage assays. The methanol extract was the most efficient in GLUT4 translocation enhancement. It increased GLUT4 translocation at 63 μg/ml 1.5- and 2-fold relative to the control in the absence and presence of insulin, respectively. These findings indicate thatG. tournefortiipossesses antidiabetic activity in part by enhancing GLUT4 translocation to the PM in skeletal muscle.