Abstract: |
Granulocyte-macrophage colony-stimulating factor (GM-CSF) stimulates cellular glucose uptake by decreasing the apparent Km for substrate transport through facilitative glucose transporters on the plasma membrane. Little is known about this signal transduction pathway and the role of the α subunit of the GM-CSF receptor (αGMR) in modulating transporter activity. We examined the function of phosphatidylinositol 3-kinase (PI 3-kinase) in GM-CSF-stimulated glucose uptake and found that PI 3-kinase inhibitors, wortmannin and LY294002, completely blocked the GM-CSF-dependent increase of glucose uptake in Xenopus oocytes expressing the low affinity αGMR and in human cells expressing the high affinity αβGMR complex. We identified a Src homology 3 domain-binding motif in αGMR at residues 358-361 as a potential interaction site for the PI 3-kinase regulatory subunit, p85. Physical evidence for p85 binding to αGMR was obtained by co-immunoprecipitation with antibodies to αGMR and p85, and an αGMR mutant with alteration of the Src homology 3 binding domain lost the ability to bind p85. Experiments with a construct eliminating most of the intracellular portion of αGMR showed a 50% reduction in GM-CSF-stimulated glucose uptake with residual activity blocked by wortmannin. Searching for a proximally generated diffusible factor capable of activating PI 3-kinase, we identified hydrogen peroxide (H2O2), generated by ligand or antibody binding to αGMR, as the initiating factor. Catalase treatment abrogated GM-CSF- or anti-αGMR antibody-stimulated glucose uptake in αGMR-expressing oocytes, and H2O2 activated PI 3-kinase and led to some stimulation of glucose uptake in uninjected oocytes. Human myeloid cell lines and primary explant human lymphocytes expressing high affinity GM-CSF receptors responded to αGMR antibody with increased glucose uptake. These results identify the early events in the stimulation of glucose uptake by GM-CSF as involving local H2O2 generation and requiring PI 3-kinase activation. Our findings also provide a mechanistic explanation for signaling through the isolated α subunit of the GM-CSF receptor. |