Akt-Axin1/TNKS-Tiam1-Rac1 mediates insulin-stimulated GLUT4 translocation in skeletal muscle cells

It is known that insulin stimulates skeletal muscle glucose uptake via the InsR-IRS-PI3K pathway. The signaling downstream of PI3K is divided into the Akt-AS160-Rabs branch and the Rac1-actin cytoskeleton branches. These two signaling branches jointly mediate the effect of insulin to promote GLUT4 transporters to transport glucose into the cell. The scaffolding protein Axin1 plays a crucial role in maintaining glucose homeostasis and TNKS, a member of the PARP family, is involved in insulin-stimulated GLUT4 translocation. However, the specific roles of Axin1 and TNKS and their relationship are elusive in insulin-stimulated skeletal muscle cell glucose uptake. Here, we showed that insulin up-regulated the protein levels of Axin1 and TNKS in an Akt-dependent manner in C2C12 skeletal muscle cells. Knockdown of Axin1 inhibited insulin-stimulated GLUT4myc translocation in C2C12-GLUT4myc myotubes. Both over-expression Axin1 and TNKS activity inhibitor XAV939 enhanced insulin-stimulated GLUT4myc translocation. XAV939 up-regulated Axin1 and TNKS protein levels. Knockdown or over-expression of Axin1 down- or up-regulated the protein level of TNKS, respectively. Axin1 interacted with TNKS which was enhanced by insulin. Knockdown of Axin1 inhibited insulin-induced the phosphorylation of the Rac1 target protein PAK. Over-expression of Axin1 and XAV939 increased insulin-phosphorylated PAK. Up- and down-regulation of Axin1 and XAV939 had no effects on the phosphorylation of Akt and AS160. Insulin increased the Rac1-GEF Tiam1 protein levels. Knockdown of Tiam1 diminished insulin-stimulated PAK phosphorylation and GLUT4myc translocation. Knockdown of Axin1 inhibited insulin-induced Tiam1 expression, while over-expression of Axin1 and XAV939 had the opposite effect. In summary, our results suggest that an Akt-Axin1/TNKS-Tiam1-Rac1 signaling pathway mediates insulin-stimulated GLUT4 translocation in skeletal muscle cells.