Altered GLUT1 Substrate Selectivity in Human Erythropoiesis?

A Cell paper by Montel-Hagen et al. (2008) published last year addresses the comparative physiology of ascorbate recycling in erythrocytes (Montel-Hagen et al., 2008). The authors claim in this paper that GLUT1-mediated glucose transport decreases whereas GLUT1-mediated dehydroascorbate (DHA) transport increases during erythropoi-esis despite markedly enhanced GLUT1 expression. To support these claims, the authors measured the uptake of the phospho-rylatable analog of glucose, 2-deoxy-D-glucose (2DG), at different stages of erythropoiesis in CD34 + human hematopoietic progenitor cells stimulated to differentiate by erythropoietin. They observed that the ratio of GLUT1 mRNA to GAPDH mRNA increased over 12 days of differentiation by 1000-fold; large increases in GLUT1 expression were also observed by immunoblot analysis. A 4-fold decrease in the rate of labeled 2DG accumulation was observed between day 0 and day 8, whereas with DHA uptake (also transported via GLUT1), there is approximately a 3-fold increase in uptake between day 0 and day 8. These apparent differences are ascribed to a large increase in stoma-tin expression during the red blood cell maturation process, which could lead to sequestration of GLUT1 within lipid rafts causing inhibition of glucose transport. In support of this hypothesis, the authors showed that 2DG uptake in erythrocytes from two patients with stomatin deficiency was increased by 50%, whereas DHA uptake was decreased by 40%. Unfortunately, interpretation of these data may be obscured by the method used to monitor 2DG uptake. The authors monitored 2DG uptake in CD34 + cells at room temperature for 30 s at a concentration of 0.5 µM. Uptake of 2DG and DHA in control erythrocytes and in erythro-cytes from patients was measured over incubation times of 10 min (Figure 5C in the Cell paper). At these concentrations and temperatures, sugar equilibration in human erythrocytes occurs within 1–2 s (Carruthers, 1990; Lowe and Walms-ley, 1986). It is therefore difficult to see how a difference in transport in human erythrocytes can be ascertained by this methodology. Rather, these measurements report the cellular 2DG space, which comprises cell water (equilibrated in 1–2 s) and a metabolic sink (ongoing over 10 min). The authors indicated that they observed competitive inhibition of 2DG uptake with 5 mM glucose. This will indeed reduce uptake of label via the GLUT1 glucose transporter but also will reduce uptake of label into the hexose phosphate pool. They also state that uptake of the nonmetabolizable sugar 3-O methylglucose is inhibited by 5 mM glucose …