Activation of the Ras/mitogen-activated protein kinase signaling pathway alone is not sufficient to induce glucose uptake in 3T3-L1 adipocytes.
The signal transduction pathway by which insulin stimulates glucose transport is largely unknown, but a role for tyrosine and serine/threonine kinases has been proposed. Since mitogen-activated protein (MAP) kinase is activated by insulin through phosphorylation on both tyrosine and threonine residues, we investigated whether MAP kinase and its upstream regulator, p21ras, are involved in insulin-mediated glucose transport. We did this by examining the time- and dose-dependent stimulation of glucose uptake in relation to the activation of Ras-GTP formation and MAP kinase by thrombin, epidermal growth factor (EGF), and insulin in 3T3-L1 adipocytes. Ras-GTP formation was stimulated transiently by all three agonists, with a peak at 5 to 10 min. Thrombin induced a second peak at approximately 30 min. The activation of p21ras was paralleled by both the phosphorylation and the activation of MAP kinase: transient for insulin and EGF and biphasic for thrombin. However, despite the strong activation of Ras-GTP formation and MAP kinase by EGF and thrombin, glucose uptake was not stimulated by these agonists, in contrast to the eightfold stimulation of 2-deoxy-D-[14C]glucose uptake by insulin. In addition, insulin-mediated glucose transport was not potentiated by thrombin or EGF. Although these results cannot exclude the possibility that p21ras and/or MAP kinase is needed in conjunction with other signaling molecules that are activated by insulin and not by thrombin or EGF, they show that the Ras/MAP kinase signaling pathway alone is not sufficient to induce insulin-mediated glucose transport.
van den Berghe
N
N
Department of Endocrinology, University Hospital Leiden, The Netherlands.
Ouwens
D M
DM
Maassen
J A
JA
van Mackelenbergh
M G
MG
Sips
H C
HC
Krans
H M
HM
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D016475:3T3 Cells; D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D017667:Adipocytes / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism*; D003847:Deoxyglucose / Q000378:metabolism*; D003907:Dexamethasone / Q000494:pharmacology; D004789:Enzyme Activation; D004815:Epidermal Growth Factor / Q000494:pharmacology; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D007328:Insulin / Q000494:pharmacology; D007700:Kinetics; D051379:Mice; D010766:Phosphorylation; D016283:Proto-Oncogene Proteins p21(ras) / Q000378:metabolism*; D015398:Signal Transduction; D013917:Thrombin / Q000494:pharmacology
D007328:Insulin; D006153:Guanosine Diphosphate; D015124:8-Bromo Cyclic Adenosine Monophosphate; D004815:Epidermal Growth Factor; D003907:Dexamethasone; D006160:Guanosine Triphosphate; D003847:Deoxyglucose; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D013917:Thrombin; D016283:Proto-Oncogene Proteins p21(ras); D015056:1-Methyl-3-isobutylxanthine