Activation of GLUT1 by metabolic and osmotic stress: potential involvement of AMP-activated protein kinase (AMPK)

In the rat liver epithelial cell line Clone 9, the V_max for glucose uptake is actuely increased by inhibition of oxidative phosphorylation and by osmotic stress. By using a membrane-impermeant photoaffinity labelling reagent together with an isoform-specific antibody, we have, for the first time, provided direct evidence for the involvement of the GLUT1 glucose transporter isoform in this response. Transport stimulation was found to be associated with enhanced accessibility of GLUT1 to its substrate and with photolabelling of formerly `cryptic' exofacial substrate binding sites in GLUT1 molecules. The total amount of cell surface GLUT1 remained constant. The precise mechanism for this binding site `unmasking' is unclear but appears to involve AMP-activated protein kinase: in the current study, osmotic and metabolic stresses were found to result in activation of the α1 isoform of AMP-activated protein kinase, and transport stimulation could be mimicked both by 5-aminoimidazole-4-carboxamide ribonucleoside and by infection of cells with a recombinant adenovirus encoding constitutively active AMP-activated protein kinase. The effect of 5-aminoimidazole-4-carboxamide ribonucleoside, as for metabolic stress, was on the V_max rather than on the K_m for transport and did not affect the cell-surface concentration of GLUT1. The relevant downstream target(s) of AMP-activated protein kinase have not yet been identified, but stimulation of transport by inhibition of oxidative phosphorylation or by 5-aminoimidazole-4-carboxamide ribonucleoside was not prevented by either inhibitors of conventional and novel protein kinase C isoforms or inhibitors of nitric oxide synthase. These enzymes, which have been implicated in stress-regulated pathways in other cell types, are therefore unlikely to play a role in transport regulation by stress in Clone 9 cells.