An inhibitor of p38 mitogen-activated protein kinase prevents insulin-stimulated glucose transport but not glucose transporter translocation in 3T3-L1 adipocytes and L6 myotubes

The precise mechanisms underlying insulin-stimulated glucose transport stil l require investigation. Here we assessed the effect of SB203580, an inhibi tor of the p38 MAP kinase family, on insulin-stimulated glucose transport i n 3T3-L1 adipocytes and L6 myotubes, We found that SB203580, but not its in active analogue (SB202474), prevented insulin-stimulated glucose transport in both cell types with an IC50 similar to that for inhibition of p38 MAP k inase (0.6 mu M). Basal glucose uptake was not affected. Moreover, SB203580 added only during the transport assay did not inhibit basal or insulin-sti mulated transport. SB203580 did not inhibit insulin-stimulated translocatio n of the glucose transporters GLUT1 or GLUT4 in 3T3-L1 adipocytes as assess ed by immunoblotting of subcellular fractions or by immunofluorescence of m embrane lawns. L6 muscle cells expressing GLUT4 tagged on an extracellular domain with a Myc epitope (GLUT4myc) were used to assess the functional ins ertion of GLUT4 into the plasma membrane. SB203580 did not affect the insul in-induced gain in GLUT4myc exposure at the cell surface but largely reduce d the stimulation of glucose uptake. SB203580 had no effect on insulin-depe ndent insulin receptor substrate-1 phosphorylation, association of the p85 subunit of phosphatidylinositol 3-kinase with insulin receptor substrate-1, nor on pho sphatidylinositol 3-kinase, Akt1, Akt2, or Akt3 activities in 3 T3-L1 adipocytes, In conclusion, in the presence of SB203580, insulin cause d normal translocation and cell surface membrane insertion of glucose trans porters without stimulating glucose transport. We propose that insulin stim ulates two independent signals contributing to stimulation of glucose trans port: phosphatidylinositol 3-kinase leads to glucose transporter translocat ion and a pathway involving p38 MAP kinase leads to activation of the recru ited glucose transporter at the membrane.