Effects of Cell-Permeable Ceramides and Tumor Necrosis Factor-α on Insulin Signaling and Glucose Uptake in 3T3-L1 Adipocytes

Incubation of 3T3-L1 adipocytes with C₂- and C₆- ceramides (N-acetyl- and N-hexanoylsphingosines) but not dihydro-C₂-ceramide increased 2-deoxyglucose uptake in the absence of insulin. This effect was inhibited by PD 98059, LY 294002, and rapamycin, which block the activation of mitogen-activated protein kinase, phosphatidylinositol (PI) 3-kinase, and ribosomal S6 kinase, respectively. Long-term increases in PI 3-kinase activity associated with insulin receptor substrate 1 (IRS-1) increased GLUT1 and GLUT4 concentrations in plasma membranes. This together with increased GLUT1 (but not GLUT4) synthesis explains the increase in non-insulin-dependent glucose uptake. C₂-ceramide inhibited insulin-stimulated glucose uptake after 2 h by decreasing insulin-induced translocation of GLUT1 and GLUT4 to plasma membranes. This occurred when there was no increase in basal glucose uptake or decrease in activation of IRS-1 or PI 3-kinase. Incubation for 24 h with tumor necrosis factorα (TNF-α) but not C₂-ceramide decreased the concentration and insulin-induced tyrosine phosphorylation of IRS-1 in this experimental system. Cell-permeable ceramides mimic some effects of TNF-α, especially in stimulating basal glucose uptake. We identified a site for inhibiting insulin-stimulated glucose uptake that is downstream of PI 3-kinase. Our work provides further mechanisms for the effects of TNF-α and ceramides in increasing non-insulin-dependent glucose uptake and decreasing insulin-stimulated uptake in vivo.