Plasma membrane gamma-glutamyltransferase activity facilitates the uptake of vitamin C in melanoma cells.
Adequate cellular transport of ascorbic acid (AA) and its oxidation product dehydroascorbate (DHA) is assured through specific carriers. It was shown that vitamin C is taken up as DHA by most cell types, including cancer cells, via the facilitative GLUT transporters. Thus, AA oxidation to DHA can be considered a mechanism favoring vitamin C uptake and intracellular accumulation. We have investigated whether such an AA-oxidizing action might be provided by plasma membrane gamma-glutamyltransferase (GGT), previously shown to function as an autocrine source of prooxidants. The process was studied using two distinct human metastatic melanoma clones. It was observed that the Me665/2/60 clone, expressing high levels of membrane GGT activity, was capable of effecting the oxidation of extracellular AA, accompanied by a marked increase of intracellular AA levels. The phenomenon was not observed with Me665/2/21 cells, possessing only traces of membrane GGT. On the other hand, AA oxidation and stimulation of cellular uptake were indeed observed after transfection of 2/21 cells with cDNA coding for GGT. The mechanism of GGT-mediated AA oxidation was investigated in acellular systems, including GGT and its substrate glutathione. The process was observed in the presence of redox-active chelated iron(II) and of transferrin or ferritin, i.e., two physiological iron sources. Thus, membrane GGT activity-often expressed at high levels in human malignancies-can oxidize extracellular AA and promote its uptake efficiently.
Corti
Alessandro
A
Department of Experimental Pathology B.M.I.E., University of Pisa Medical School, Via Roma 55, 56126 Pisa, Italy.
Raggi
Chiara
C
Franzini
Maria
M
Paolicchi
Aldo
A
Pompella
Alfonso
A
Casini
Alessandro F
AF
D001205:Ascorbic Acid / Q000032:analysis / Q000378:metabolism*; D001692:Biological Transport / Q000502:physiology; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000201:enzymology* / Q000502:physiology; D005978:Glutathione / Q000378:metabolism; D006801:Humans; D007502:Iron Chelating Agents / Q000494:pharmacology; D008545:Melanoma / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism; D010084:Oxidation-Reduction / Q000187:drug effects; D012878:Skin Neoplasms / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism; D014162:Transfection; D014168:Transferrin / Q000494:pharmacology / Q000502:physiology; D005723:gamma-Glutamyltransferase / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*
D007502:Iron Chelating Agents; D014168:Transferrin; D005723:gamma-Glutamyltransferase; D005978:Glutathione; D001205:Ascorbic Acid