The molecular basis of the antidiabetic action of quercetin in cultured skeletal muscle cells and hepatocytes - PubMed (original) (raw)

The molecular basis of the antidiabetic action of quercetin in cultured skeletal muscle cells and hepatocytes

Hoda M Eid et al. Pharmacogn Mag. 2015 Jan-Mar.

Abstract

Background: Quercetin is universally distributed in the plant kingdom and is the most abundant flavonoid in the human diet. In a previous study, we have reported that quercetin stimulated glucose uptake in cultured C2C12 skeletal muscle through an insulin-independent mechanism involving adenosine monophosphate-activated protein kinase (AMPK). AMPK is a key regulator of the whole body-energy homeostasis. In skeletal muscle, activation of AMPK increases glucose uptake through the stimulation of the glucose transporter GLUT4 translocation to the plasma membrane. In liver, AMPK decreases glucose production mainly through the downregulation of the key gluconeogenesis enzymes such as phosphoenolpyruvate carboxylase (PEPCK) and Glucose -6-phosphate (G6Pase).

Objective: To study the effect of quercetin on glucose homeostasis in muscle and liver.

Materials and methods: L6 skeletal muscle cells, murine H4IIE and human HepG2 hepatocytes were treated with quercetin (50 μM) for 18 h.

Results: An 18 h treatment with quercetin (50 μM) stimulated AMPK and increased GLUT4 translocation and protein content in cultured rat L6 skeletal muscle cells. On the other hand, we report that quercetin induced hepatic AMPK activation and inhibited G6pase in H4IIE hepatocytes. Finally, we have observed that quercetin exhibited a mild tendency to increase the activity of glycogen synthase (GS), the rate-limiting enzyme of glycogen synthesis, in HepG2 hepatocytes.

Conclusions: Overall, these data demonstrate that quercetin positively influences glucose metabolism in the liver and skeletal muscle, and therefore appear to be a promising therapeutic candidate for the treatment of in type 2 diabetes.

Keywords: Akt; Quercetin; adenosine monophosphate-activated protein kinase; glucose transporter; glucose-6-phosphatase; glycogen synthase; insulin resistance; type 2 diabetes mellitus.

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Conflict of interest statement

Conflict of Interest: None declared.

Figures

Figure 1

Figure 1

Quercetin increases glucose uptake and GLUT4 translocation in L6 GLUT4_myc_ myotubes. Cells were treated with either 50 μM of quercetin, or with vehicle (0.1% DMSO) for 18 h. 100 nM insulin was applied for the last 15 min of the treatment in vehicle-treated cells. (a) Glucose uptake was assessed by the incorporation of 3H-deoxyglucose as described in Materials and Methods. Data are expressed relative to basal uptake observed in vehicle control treated cells (100%). Data represent the mean ± SEM of 3 experiments, each experiment composed of 3-4 replicates per condition. (b) GLUT4 translocation was assessed by measuring cell surface GLUT4_myc_ using an enzyme-linked colorimetric assay, as described in Materials and Methods. Data are expressed relative to basal translocation observed in vehicle control treated cells (1.0). Data represent the mean ± SEM of 3 experiments, each experiment composed of 3-4 replicates per condition. * Indicates a significant (P ≤ 0.05) difference from the vehicle control group and § indicates a significant (P ≤ 0.05) difference from insulin group as assessed by ANOVA

Figure 2

Figure 2

Quercetin increases GLUT4 content in L6 myotubes. Cells were treated with vehicle (0.1% DMSO, 18 h), quercetin (50 μM, 18 h), or AICAR (1mM, 30 min). Immunoblots were probed with an anti-GLUT4 antibody. (a) Representative blots are shown. (b) Data are expressed as GLUT4/β-actin, and are given as mean ± SEM from 3 experiments

Figure 3

Figure 3

Quercetin increases AMPK phosphorylation in L6 myotubes. Shown are representative immunoblots of cells treated with vehicle (0.1% DMSO, 18 h), quercetin (50 μM, 18 h), or AICAR (1 mM, 30 min). Immunoblots were probed with phospho-specific antibodies against AMPK (Thr 172) as described in Materials and Methods section. Immunoblots were probed with β-actin as loading control. (a) Representative immunoblots. (b) Data are expressed as pAMPK/β-actin, and are given as mean ± SEM from 3 experiments

Figure 4

Figure 4

Quercetin treatment inhibits G6Pase activity and stimulated the phosphorylation of AMPK in H4IIE hepatocytes. H4IIE cells were treated with either vehicle (0.1% DMSO, 18 h) or quercetin (50 μM, 18 h). (a) G6Pase activity was assessed by measuring the rate of glucose formation in the presence of a non-limiting amount of G6P as described under “Materials and Methods.” Insulin (100 nM, 18 h) served as the positive control. Results are expressed as mean % change ± SEM, relative to a vehicle-treated control group for three independent experiments of four to six replicates per condition. G6Pase activity data were normalized to total protein content per well. * denotes a significant difference (P ≤ 0.05). (b) Phosphorylation of AMPK was measured by western immunoblot. AICAR (2 mM) applied for 30 min served as the positive control. The upper immunoblot was probed with anti-phospho-AMPK and the lower blot was probed with β-actin as loading control. Blots shown are representative from 3 experiments

Figure 5

Figure 5

Effect of quercetin on glycogen synthase in HepG2. HepG2 cells were incubated for 18 h with either 0.1% DMSO (vehicle), or quercetin (50 μM). Insulin (100 nM) applied for 15 min and served as positive control. GS activity was assayed in the supernatants of cell lysates as described under “Materials and Methods”. Results were expressed as fractional activities (active/total). Values shown are means ± S.E.M. for three different experiments. * denotes a significant difference (P ≤ 0.05) from the vehicle control group

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