Abstract

Many cell types transport vitamin C solely in its oxidized form, dehydroascorbic acid, through facilitative glucose transporters. These cells accumulate large intracellular concentrations of vitamin C by reducing dehydroascorbic acid to ascorbate, a form that is trapped intracellularly. Certain specialized cells can transport vitamin C in its reduced form, ascorbate, through a sodium-dependent cotransporter. We found that normal human melanocytes and human malignant melanoma cells are able to transport vitamin C using both mechanisms. Melanoma cell lines transported dehydroascorbic acid at a rate that was at least 10 times greater than the rate of transport by melanocytes, whereas both melanoma cells and melanocytes transported ascorbate with similar efficiency. Dehydroascorbic acid transport was inhibited by deoxyglucose and cytochalasin B, indicating the direct participation of facilitative glucose transporters in the transport of oxidized vitamin C. Melanoma cells accumulated intracellular vitamin C concentrations that were up to 100 times greater than the corresponding extracellular dehydroascorbic acid concentrations, whereas intracellular accumulation of vitamin C by melanocytes never exceeded the extracellular level of dehydroascorbic acid. Melanoma cells transported dehydroascorbic acid through at least two different transporters, each with a distinct K(m), a finding that agreed well with the presence of several glucose transporter isoforms in these cells. Only one kinetic component of ascorbate uptake was identified in both melanocytes and melanoma cells, and ascorbate transport was sodium dependent and inhibited by ouabain. Both cell types were able to accumulate intracellular concentrations of vitamin C that were greater than the extracellular ascorbate concentrations. The data indicate that melanoma cells and normal melanocytes transport vitamin C using two different transport systems. The transport of dehydroascurbic acid is mediated by a facilitated mechanism via glucose transporters, whereas transport of ascorbic acid involves a sodium-ascorbate cotransporter. The differential capacity of melanoma cells to transport the oxidized form of vitamin C reflects the increased expression of facilitative transporters associated with the malignant phenotype.