Activation of Glut1 glucose transporter in response to inhibition of oxidative phosphorylation. Role of sites of mitochondrial inhibition and mechanism of Glut1 activation

We have previously shown that exposure of Clone 9 cells to hypoxia, cyanide, or azide results in an acute stimulation of glucose transport that is largely mediated by 'activation' of glucose transporter (Glut1) sites preexisting in the plasma membrane. However, it is not known whether inhibition of oxidative phosphorylation only at its terminal step, or at any of its steps, leads to the glucose transport response. Hence, the effect of azide (5 mM), rotenone (1 μM), rotenone (1 μM) plus thenoyltrifluoroacetone (TTFA) (5 μM), antimycin A (0.3 μM), dinitrophenol (0.25 mM), carbonyl cyanide m-chlorophenylhydrazone (CCCP) (2.5 μM), and oligomycin B (0.15 μM) on glucose transport was determined. All of the above agents elicited a similar ~4-fold stimulation of cytochalasin B (CB)-inhibitable 3-O-methyl glucose (3-OMG) uptake in Clone 9 cells. The stimulatory effect of azide on 3-OMG uptake was not inhibited by antioxidants 2-mercaptopropionyl glycine (1.2 mM) and 1,10-phenanthroline (40 μM), while, in contrast, the antioxidants attenuated the stimulation of glucose transport in response to 250 μM H₂O₂ by ~50%. To differentiate between an increase in the number of functional Glut1 sites in the plasma membrane (in the absence of 'translocation') versus an increase in the 'intrinsic activity' of Glut1, the effect of azide on the energy of activation (E(a)) of glucose transport was measured. The E(a) was determined by measuring the rate of CB-inhibitable 3- OMG uptake at 24.0, 28.0, 35.0, and 40°C. The E(a) of control Clone 9 cells and of cells exposed to 10 mM azide for 2 h was 32,530 ± 1830 and 31,220 ± 600 J/mol, respectively (P > 0.1), while the rate of CB-inhibitable 3-OMG uptake was 9.3 ± 0.7-fold higher in azide-treated cells. It is concluded that (i) inhibition of oxidative phosphorylation, at any of its steps, leads to a stimulation of glucose transport, and (ii) the mechanism of stimulation of glucose transport in response to azide appears to be predominately mediated by an apparent increase in the number of functional Glut1 sites in the plasma membrane (instead of an increase in their 'intrinsic activity'), suggesting an 'unmasking' mechanism.