Anemia is common in diabetes
Anemia is common in diabetes, with multiple causes (Thomas et al., 2003, 2006; Craig et al., 2005). Low RBC ascorbate might contribute to anemia, possibly via low grade chronic hemolysis produced by RBC rigidity. Future studies of diabetic subjects will include measurement of cell-free hemoglobin and reticulocyte counts in relation to ascorbate. Future studies will also address whether increased RBC osmotic fragility caused by low ascorbate has consequences to hemoglobin function. Consistent with the famine from feast hypothesis (Fig. 8), it has been reported that low vitamin C plasma concentrations occur in diabetes (Will and Byers, 1996; Chen et al., 2006). However, prior associations between vitamin C and diabetes did not account mechanistically for vascular complications, other than by generalized oxidative damage. Except for the dataset presented here, there has been no recognition that humans with diabetes have lower than expected vitamin C concentrations in their RBCs. Unfortunately, most prior existing datasets for plasma vitamin C concentrations in diabetic subjects are confounded by assay artifacts; by prior inabilities to measure RBC ascorbate; and by inabilities to measure dehydroascorbic order Cy3.5 hydrazide directly in plasma or RBCs (Will and Byers, 1996; Li et al., 2012; Dhariwal et al., 1991; Lykkesfeldt, 2000). Surprisingly, modern hematology textbooks do not even describe that there is vitamin C in RBCs (Kaushansky et al., 2010; Greer et al., 2013). To our knowledge, the data here displaying RBC vs plasma vitamin C concentrations are the most comprehensive available to date in healthy and diabetic humans. Previous investigators found no link between dehydroascorbic acid transport and glucose in mouse and human RBCs (Montel-Hagen et al., 2008a), but the findings may have been due to use of radiolabel without any direct ascorbate measurements; dehydroascorbic acid degradation (Rumsey et al., 1997); flawed kinetics assumptions (Rumsey et al., 1997; Carruthers and Naftalin, 2009); metabolism of glucose and the analog 2-deoxyglucose (Carruthers et al., 2009); and lack of accounting for GLUT transactivation (Cloherty et al., 1996). Based on findings on human RBCs, high co-expression of GLUT 1 with stomatin was interpreted to enhance DHA transport but negatively modulate glucose uptake specifically in human RBCs (Montel-Hagen et al., 2008b). Recent data using RBCs and inside out vesicles do not support a role for a stomatin-regulated pool of GLUT 1 that preferentially transports DHA rather than glucose. Instead, DHA and glucose were transported similarly by GLUT 1 (Rumsey et al., 1997; Sage and Carruthers, 2014). The presence of Glut 4 in mouse RBCs was previously reported (Montel-Hagen et al., 2008b), and confirmed in the results here. While Glut-dependent DHA uptake was concluded to be absent from mature murine RBCs (Montel-Hagen et al., 2008b), the findings in the present manuscript suggest this conclusion is incorrect. In the current experiments we have not assessed dehydroascorbic acid transport activity of all 14 identified GLUTs (Mueckler and Thorens, 2013). In previous experiments, Xenopus oocytes were microinjected with cRNAs for GLUTs 1–12 to assess DHA transport activity. GLUTs 1 and 3 were at least 10 fold more active than other transporters (Rumsey et al., 1997, 2000; Corpe et al., 2013). Data here are consistent with these reports. There are no data available describing DHA transport by GLUTs 13 and 14. GLUT 13 is expressed predominantly in brain, and transports myo-inositol but not glucose. Although GLUT 14 may have arisen as a gene duplication of GLUT 3, there is no rodent homolog, and GLUT 14 is expressed mainly in testis. Relying on identities of GLUTs in human RBCs, GLUT transport activity for DHA, and known distribution of GLUTs 1–14, we and others conclude that in human RBCs DHA is transported by GLUT 1 (Montel-Hagen et al., 2008b; Sage and Carruthers, 2014). Based on similar reasons, we conclude that mouse RBCs utilize Glut 3 and Glut 4 for DHA transport, but it is unclear which transporter predominates.