Recycling of Vitamin C by a Bystander Effect

Abstract

Human cells transport dehydroascorbic acid through facilitative glucose transporters, in apparent contradiction with evidence indicating that vitamin C is present in human blood only as ascorbic acid. On the other hand, activated host defense cells undergoing the oxidative burst show increased vitamin C accumulation. We analyzed the role of the oxidative burst and the glucose transporters on vitamin C recycling in an in vitro system consisting of activated host-defense cells co-cultured with human cell lines and primary cells. We asked whether human cells can acquire vitamin C by a "bystander effect" by taking up dehydroascorbic acid generated from extracellular ascorbic acid by neighboring cells undergoing the oxidative burst. As activated cells, we used HL-60 neutrophils and normal human neutrophils activated with phorbol 12 myristate 13-acetate. As bystander cells, we used immortalized cell lines and primary cultures of human epithelial and endothelial cells. Activated cells produced superoxide anions that oxidized extracellular ascorbic acid to dehydroascorbic acid. At the same time, there was a marked increase in vitamin C uptake by the bystander cells that was blocked by superoxide dismutase but not by catalase and was inhibited by the glucose transporter inhibitor cytochalasin B. Only ascorbic acid was accumulated intracellularly by the bystander cells. Glucose partially blocked vitamin C uptake by the bystander cells, although it increased superoxide production by the activated cells. We conclude that the local production of superoxide anions by activated cells causes the oxidation of extracellular ascorbic acid to dehydroascorbic acid, which is then transported by neighboring cells through the glucose transporters and immediately reduced to ascorbic acid intracellularly. In addition to causing increased intracellular concentrations of ascorbic acid with likely associated enhanced antioxidant defense mechanisms, the bystander effect may allow the recycling of vitamin C in vivo, which may contribute to the low daily requirements of the vitamin in humans.

Depletion of Intracellular Ascorbate by the Carcinogenic Metals Nickel and Cobalt Results in the Induction of Hypoxic Stress

“…DHA is transported into and out of cells by sodium-independent glucose transporters (GLUT 1 and GLUT 3) (37, 38), whereas ascorbic acid enters cells through sodium-dependent ascorbic acid transporters (39 -41). Inside the cell, DHA is immediately reduced to ascorbate (42). The intracellular level of ascorbate is therefore regulated by its oxidation status, uptake, and efflux.…”

Antioxidant supplementation influences the neutrophil tocopherol associated protein expression, but not the inflammatory response to exercise

“…Vitamin C is transported into neutrophils mainly as dehydroascorbate. Once transported, dehydroascorbate is immediately reduced in the cell to ascorbate [18]. Vitamin E uptake from plasma by neutrophils is yet unknown, but recently the expression of tocopherol associated protein (TAP) with α-tocopherol specific binding, has been elucidated in leukocytes [19,20] This protein could participate in vitamin E transportion or accumulation into cells, as it occurs in the liver [21].…”

Inflammation in the vascular bed: Importance of vitamin C

“…Albeit circumstantial, these arguments suggest that active ascorbate transport is required to maintain the high ascorbate concentrations in cells of the vascular bed. However, DHA uptake could be important when ascorbate is rapidly oxidized outside cells, such as during the respiratory burst of phagocytic cells (Nualart et al, 2003;Washko & Levine, 1992).…”