Type 2 diabetes mellitus (T2DM), characterized by chronic hyperglycemia, is a prevalent metabolic disease caused by decreased insulin sensitivity or insufficient insulin to overcome insulin resistance (IR). Current T2DM treatments often involve multiple chemical drugs with side effects, prompting interest in natural products with fewer adverse effects. Anthocyanins, a group of water-soluble plant compounds, have shown potential in alleviating glucose metabolism disorders in T2DM. Insulin exerts its effects by binding to receptors, activating downstream proteins that enhance cellular insulin sensitivity. Protein tyrosine phosphatase 1B (PTP1B) negatively regulates insulin signaling by inhibiting insulin receptor substrate 2 (IRS-2) phosphorylation. This study focuses on cyanidin-3-O-glucoside (C3G), an anthocyanin rich in red bayberry, a traditional Chinese medicine, to investigate its mechanism in regulating glucose metabolism by alleviating IR.

Previous studies have shown the anti-diabetic effects of anthocyanins from various sources, such as mulberry and purple corn. Mulberry anthocyanin extract reversed IR and enhanced glucose metabolism in HepG2 cells by regulating the PI3K/AKT signaling pathway. Purple corn anthocyanins ameliorated tumor necrosis factor-α-induced IR by activating insulin signaling and enhancing GLUT4 translocation in 3T3-L1 adipocytes. Aronia melanocarpa anthocyanin extracts increased glucose uptake by modulating GLUT4 and upregulating the p-GSK-3β(Ser9)/GSK-3β ratio to strengthen glycogen synthesis. These studies suggest that anthocyanins have a beneficial impact on T2DM by influencing IR. This study aimed to investigate the mechanisms by which C3G, a major anthocyanin in red bayberry, improves glucose metabolism and alleviates insulin resistance.

The study employed both in vitro and in vivo approaches. For in vitro experiments, HepG2 and L02 liver cell lines were used to create an insulin resistance (IR) model by incubating them with palmitic acid (PA) and high glucose. C3G was then added at various concentrations, and effects on glucose consumption, glucose uptake, glycogen synthesis, and insulin sensitivity were assessed using assays including MTT, glucose detection kit, 2-NBDG fluorescence, and glycogen staining. Western blotting and qRT-PCR were used to analyze the expression of PTP1B and p-IRS-2. For in vivo experiments, C57BL/6J (control) and db/db (diabetic) mice were gavaged with C3G (150 mg/kg/day) or water for six weeks. Fasting blood glucose, oral glucose tolerance test (OGTT), and homeostasis model assessment-insulin resistance (HOMA-IR) were conducted to evaluate the effects of C3G on glucose metabolism and insulin resistance. Statistical analysis was performed using one-way ANOVA, Duncan's multiple range test, and t-test.

In vitro, C3G significantly increased glucose consumption, glucose uptake, and glycogen synthesis in insulin-resistant HepG2 and L02 cells. C3G also enhanced insulin sensitivity in these cells. Mechanistically, C3G inhibited the increased expression of PTP1B and consequently increased the phosphorylation of IRS-2 in IR cells. In vivo, C3G administration to diabetic db/db mice for six weeks significantly reduced fasting blood glucose, improved glucose tolerance (as demonstrated by OGTT), and lowered HOMA-IR scores, indicating an improvement in insulin resistance.

The findings of this study strongly support the hypothesis that C3G effectively alleviates insulin resistance, thus improving glucose metabolism. The use of two distinct liver cell lines strengthens the generalizability of the findings to liver cells in general. The in vivo results using db/db mice confirm the in vitro findings, providing strong evidence for the therapeutic potential of C3G in T2DM. The mechanism appears to involve the modulation of PTP1B and IRS-2 phosphorylation, consistent with previous research demonstrating the role of PTP1B as a negative regulator of insulin signaling. The study's findings offer valuable insight into the use of natural dietary bioactive components for managing T2DM.

This study demonstrated that C3G, an anthocyanin from red bayberry, alleviates insulin resistance in vitro and in vivo, improving glucose metabolism in T2DM. The mechanism involves regulation of PTP1B and p-IRS-2. C3G shows promise as a dietary supplement for managing T2DM. Future research should investigate the long-term effects of C3G and explore other potential therapeutic targets and mechanisms of action.

The study's in vivo OGTT results lacked a direct comparison between the db/db+water group and C57BL/6J control group, limiting definitive conclusions about the extent of glucose intolerance. Further research incorporating additional measures of insulin sensitivity (e.g., insulin tolerance test and hyperinsulinemic-euglycemic clamp) and molecular markers related to IR pathways is needed to strengthen the in vivo findings.