4-Hydroxyisoleucine improves insulin resistance in HepG2 cells by decreasing TNF-α and regulating the expression of insulin signal transduction proteins

Previous studies have indicated that 4-hydroxy-isoleucine (4-HIL) improves insulin resistance, however, the underlying mechanisms remain to be elucidated. In the present study, the molecular mechanisms underlying how 4-HIL improves insulin resistance in hepatocytes were examined. HepG2 cells were co-cultured with insulin and a high glucose concentration to obtain insulin-resistant (IR) HepG2 cells. Insulin sensitivity was determined by measuring the glucose uptake rate. The IR HepG2 cells were treated with different concentrations of 4-HIL to determine its effect on IR Hep2 cells. The levels of tumor necrosis factor-alpha (TNF-alpha) were measured by an enzyme-linked immunosorbent assay and protein levels of TNF-alpha converting enzyme (TACE)/tissue inhibitor of metalloproteinase 3 (TIMP3), insulin receptor substrate (IRS)-1, IRS-2, phosphorylated (p)-IRS-1 (Ser307) and glucose transporter type 4 (GLUT4) were measured by western blot analysis. The results of the present study demonstrated that insulin-induced glucose uptake was reduced in IR HepG2 cells; however, this reduction was reversed by 4-HIL in a dose-dependent manner. 4-HIL achieved this effect by downregulating the expression of TNF-alpha and TACE, and upregulating the expression of TIMP3 in IR HepG2 cells. In addition, 4-HIL increased the expression of the insulin transduction regulators IRS-1 and GLUT4, and decreased the expression of p-IRS-1 (Ser307), without affecting the expression of IRS-2. The present study suggests that 4-HIL improved insulin resistance in HepG2 cells by the following mechanisms: 4-HIL reduced TNF-alpha levels by affecting the protein expression of the TACE/TIMP3 system and 4-HIL stimulated the expression of IRS-1 and GLUT4, but inhibited the expression of p-IRS-1 (Ser307).