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‑hydroxyisoleucine (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‑α (TNF‑α) were measured by an enzyme‑linked immunosorbent assay and protein levels of TNF‑α 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‑α 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‑α 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).