Ceramide- and oxidant-induced insulin resistance involve loss of insulin-dependent Rac-activation and actin remodeling in muscle cells.

In muscle cells, insulin elicits recruitment of the glucose transporter GLUT4 to the plasma membrane. This process engages sequential signaling from insulin receptor substrate (IRS)-1 to phosphatidylinositol (PI) 3-kinase and the serine/threonine kinase Akt. GLUT4 translocation also requires an Akt-independent but PI 3-kinase-and Rac-dependent remodeling of filamentous actin. Although IRS-1 phosphorylation is often reduced in insulin-resistant states in vivo, several conditions eliciting insulin resistance in cell culture spare this early step. Here, we show that insulin-dependent Rac activation and its consequent actin remodeling were abolished upon exposure of L6 myotubes beginning at doses of C2-ceramide or oxidant-producing glucose oxidase as low as 12.5 micromol/l and 12.5 mU/ml, respectively. At 25 micromol/l and 25 mU/ml, glucose oxidase and C2-ceramide markedly reduced GLUT4 translocation and glucose uptake and lowered Akt phosphorylation on Ser473 and Thr308, yet they affected neither IRS-1 tyrosine phosphorylation nor its association with p85 and PI 3-kinase activity. Small interfering RNA-dependent Rac1 knockdown prevented actin remodeling and GLUT4 translocation but spared Akt phosphorylation, suggesting that Rac and actin remodeling do not contribute to overall Akt activation. We propose that ceramide and oxidative stress can each affect two independent arms of insulin signaling to GLUT4 at distinct steps, Rac-GTP loading and Akt phosphorylation.