Citation: Aoki Y, Aoki M, Jenkins DJA. Sodium-glucose co-transporter 2 inhibitors could improve the bioavailability of vitamin C at the kidney in diabetes treatment. Cell Mol Med 2017, 3:2.
Letter to EditorIt is known that plasma (serum) vitamin C level is lowered in diabetic patients by some mechanisms including renal loss [1]. Vitamin C and glucose filtered from the glomerulus are presumed to be reabsorbed at the renal proximal tubule by sodium-vitamin C co-transporter (SVCT) 1 and sodium-glucose co-transporter (SGLT) 2, respectively [2-4]. In the enterocyte that expresses SVCT 1 and SGLT 1, it was demonstrated in vitro that cellular uptake of vitamin C was inhibited by increasing concentrations of glucose in medium, which was not observed at the presence of phlorizin, a non-specific SGLT inhibitor [2]. If such an effect is applied to renal proximal tubular cells, it is hypothesized that SGLT 2 inhibitors can improve the bioavailability of vitamin C in diabetes treatment.In two cases of diabetic patients treated with empagliflozin, a SGLT 2 inhibitor, in hospitalization, their serum vitamin C levels (normal reference range, 5.5 μg/ml to 16.8 μg/ml) have been observed to increase soon after its administration. In a male patient newly diagnosed with type 2 diabetes (HbA1c, 13.4 %) at age 41 (174 cm, 114 kg), his serum vitamin C level changed from 7.2 μg/ml to 8.1 μg/ml two days after adding 10 mg empagliflozin on 1000 mg metformin. Urinary glucose excretion increased from 15.5 g/day to 148.1 g/day. In a female patient with 12-year duration of type 2 diabetes (HbA1c, 7.4%) and renal insufficiency (serum creatinine, 1.64 mg/dl) at age 67 (155 cm, 99 kg), her serum vitamin C level changed from 7.9 μg/ml to 8.5 μg/ml twelve days after adding 10 mg empagliflozin on 5 mg linagliptin. Urinary glucose excretion increased from 2.9 g/day to 23.8 g/day. Ascorbic acid, a reduced form of vitamin C, or glucose enters cells specifically through SVCT or SGLT, driven by the inward sodium gradient maintained with sodium pump (Na + /K + -ATPase). Dehydroascorbic acid, an oxidized form of vitamin C, enters or exits cells competitively with glucose through facilitative glucose transporters (GLUTs) [3,5]. Taken together, a model of putative cellular transport of vitamin C in the early proximal tubular cells is drawn in Figure 1. Figure 1 A model of putative cellular transport of vitamin C in the early proximal tubular cell and its molecular structures. Vitamin C reabsorbed as ascorbic acid in the brush border membrane through vitamin C transporter (SVCT) 1 is oxidized by reactive oxygen species and exits the cell as dehydroascorbic acid competitively with glucose through facilitative glucose transporter (GLUT) 2 in the basolateral membrane. SVCT 1 and sodium-glucose cotransporter (SGLT) 2 are driven by the inward sodium gradient maintained by sodium pump (Na + /K + -ATPase).Ascorbic acid reabsorbed at the brush border membrane through SVCT 1 is oxidized by reactive oxygen species, and exits the cell as dehydroascorbic acid competitiv...