4-Hydroxyisoleucine stimulates glucose uptake by increasing surface GLUT4 level in skeletal muscle cells via phosphatidylinositol-3-kinase-dependent pathway

Purpose To determine the effect of 4-Hydroxyisoleucine (4-HIL), an unusual amino acid isolated from the seeds of Trigonella foenum - graecum , on glucose uptake and the translocation of glucose transporter 4 (GLUT4) to plasma membrane in skeletal muscle cells and to investigate the underlying mechanisms of action. Methods Rat skeletal muscle cells (L6-GLUT4 myc ) were treated with 4-HIL, and the effect on glucose uptake was determined by measuring the incorporation of radio-labeled 2-deoxy-[ 3 H]- d -glucose (2-DG) into the cell. Translocation of GLUT4 myc to plasma membrane was measured by an antibody-coupled colorimetric assay. Results The prolonged exposure (16 h) of L6-GLUT4 myc myotubes to 4-HIL caused a substantial increase in the 2-DG uptake and GLUT4 translocation to the cell surface, without changing the total amount of GLUT4 and GLUT1. Cycloheximide treatment reversed the effect of 4-HIL on GLUT4 translocation to the basal level suggesting the requirement of new protein synthesis. The 4-HIL-induced increase in GLUT4 translocation was completely abolished by wortmannin, and 4-HIL significantly increased the basal phosphorylation of AKT (Ser-473), but did not change the mRNA expression of AKT, IRS-1, GLUT4, and GSK3β. Conclusion Results suggest that 4-HIL stimulates glucose uptake in L6-GLUT4 myc myotubes by enhancing translocation of GLUT4 to the cell surface in a PI-3-kinase/AKT-dependent mechanism.