Schisandrin C Affects Glucose-Stimulated Insulin Secretion in Pancreatic β-Cells and Glucose Uptake in Skeletal Muscle Cells

Abstract

The aim of our study was to investigate the effect of three lignans (schisandrol A, schisandrol B, and schisandrin C) on insulin secretion in rat INS-1 pancreatic β-cells and glucose uptake in mouse C2C12 skeletal muscle cells. Schisandrol A and schisandrin C enhanced insulin secretion in response to high glucose levels with no toxic effects on INS-1 cells. The effect of schisandrin C was superior to that of gliclazide (positive control), a drug commonly used to treat type 2 diabetes (T2D). In addition, western blot analysis showed that the expression of associated proteins, including peroxisome proliferator-activated receptor γ (PPARγ), pancreatic and duodenal homeobox 1 (PDX-1), phosphatidylinositol 3-kinase (PI3K), Akt, and insulin receptor substrate-2 (IRS-2), was increased in INS-1 cells after treatment with schisandrin C. In addition, insulin secretion effect of schisandrin C were enhanced by the Bay K 8644 (L-type Ca²⁺ channel agonist) and glibenclamide (K⁺ channel blocker), were abolished by the nifedipine (L-type Ca²⁺ channel blocker) and diazoxide (K⁺ channel activator). Moreover, schisandrin C enhanced glucose uptake with no toxic effects on C2C12 cells. Western blot analysis showed that the expression of associated proteins, including insulin receptor substrate-1 (IRS-1), AMP-activated protein kinase (AMPK), PI3K, Akt, glucose transporter type 4 (GLUT-4), was increased in C2C12 cells after treatment with schisandrin C. Schisandrin C may improve hyperglycemia by enhancing insulin secretion in pancreatic β-cells and improving glucose uptake into skeletal muscle cells. Our findings may provide evidence that schisandrin C may be beneficial in devising novel anti-T2D strategies.

Keywords: GLUT-4; PDX-1; glucose uptake; glucose-stimulated insulin secretion; schisandrin C.

Figure 1

Effects of three lignans (schisandrol A, schisandrol B, and schisandrin C) on the viability of INS-1 cells. (A) Chemical structures of the compounds. (B–D) MTT assay results of the cell viability of INS-1 cells after 24 h treatment with schisandrol A, schisandrol B, and schisandrin C, compared with the control (0 μM).

Figure 2

Effects of three lignans (schisandrol A, schisandrol B, and schisandrin C) on glucose-stimulated insulin secretion in INS-1 cells. Insulin secretion in INS-1 cells after 1 h incubation with basal (2.8 mM) and stimulant (16.7 mM) concentrations of glucose in the presence or absence of (A–C) schisandrol A, schisandrol B, schisandrin C, and (D) gliclazide (positive control) assessed by insulin secretion assay. The data represent the mean ± S.E.M., n = 3, * p < 0.05 compared with the control.

Figure 3

Effect of schisandrin C on the protein expression levels of peroxisome proliferator-activated receptor γ (PPARγ), pancreatic and duodenal homeobox 1 (PDX-1), phospho-insulin receptor substrate-2 (P-IRS-2) (Ser731), IRS-2, phospho-phosphatidylinositol 3-kinase (P-PI3K), PI3K, phospho-Akt (P-Akt) (Ser473), and Akt. (A) Protein expression levels of PPARγ, PDX-1, P-IRS-2 (Ser731), IRS-2, P-PI3K, PI3K, P-Akt (Ser473), Akt, and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) in INS-1 cells treated or untreated with 2.5 and 5 μM schisandrin C for 24 h. (B–F) Each bar graph presents the densitometric quantification of western blot bands. The data represent the mean ± S.E.M., n = 3, * p < 0.05 compared with the control.

Figure 4

Effect of schisandrin C on ATP/ADP ratio and involvement of L-type Ca²⁺ and K⁺ channels in INS-1 cells. (A) ATP/ADP ratio in INS-1 cells after 1 h incubation with basal (2.8 mM) and stimulant (16.7 mM) concentrations of glucose in the presence or absence of schisandrin C assessed by ADP/ATP ratio assay. (B) Insulin secretion in INS-1 cells after 1 h incubation with basal (2.8 mM) and stimulant (16.7 mM) concentrations of glucose in the presence or absence of schisandrin C, nifedipine (L-type Ca²⁺ channel blocker), and Bay K 8644 (L-type Ca²⁺ channel activator) assessed by insulin secretion assay. (C) Insulin secretion in INS-1 cells after 1 h incubation with basal (2.8 mM) and stimulant (16.7 mM) concentrations of glucose in the presence or absence of schisandrin C, diazoxide (K⁺ channel activator), and glibenclamide (K⁺ channel blocker) assessed by insulin secretion assay. The data represent the mean ± S.E.M., n = 3, * p < 0.05 compared with the control.

Figure 5

Effects of schisandrin C on the glucose uptake in C2C12 cells. (A) A MTT assay result of the cell viability of C2C12 cells after 24 h treatment with schisandrin C compared with the control (0 μM). (B) Glucose uptake in C2C12 cells after 1 h incubation with schisandrin C and 2-(N-(7-Nitrobenz-2-oxa-1,3-diazol-4-yl) amino)-2-Deoxyglucose (2-NBDG) assessed by glucose uptake assay. (C) Effect of schisandrin C on the protein expression levels of phospho-insulin receptor substrate-1 (P-IRS-1), IRS-1, phospho-AMP-activated protein kinase (P-AMPK), AMPK, phospho-phosphatidylinositol 3-kinase (P-PI3K), PI3K, phospho-Akt (P-Akt) (Ser473), Akt, glucose transporter type 4 (GLUT-4), and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) in INS-1 cells treated or untreated with 2.5 and 5 μM schisandrin C for 24 h. (D–H) Each bar graph presents the densitometric quantification of western blot bands. The data represent the mean ± S.E.M., n = 3, * p < 0.05 compared with the control.

Figure 6

Schematic illustration of the effects of schisandrin C on glucose-stimulated insulin secretion in pancreatic β-cells and glucose uptake in skeletal muscle cells.