Uptake of 5-methyltetrahydrofolate into PC-3 human prostate cancer cells is carrier-mediated

Uptake of 5-methyltetrahydrofolate into the PC-3 human prostate cancer cells was linear for the first 60 min. There was no difference in the initial rate of uptake in cells incubated in folate-free medium for 24 or 48 hr compared to control cells grown in folate-containing medium. The initial rate of 5-methyltetrahydrofolate uptake showed little dependence on extracellular pH and it was independent of extracellular sodium ions. Transport of 5-methyltetrahydrofolate into PC-3 cells was saturable – Km = 0.74 μM and Vmax = 7.78 nmol/109cells/min and these kinetic constants were not different in cells incubated for 24 hr in folate-free medium (Km = 0.80 ± 0.22, Vmax = 8.52 ± 0.50; P = 0.09, N = 3). Uptake of 5-methyltetrahydrofolate was inhibited by structural analogs with the Ki values being 0.50, 1.79, and 31.8 μM for 5-formyltetrahydrofolate, methotrexate, and folic acid, respectively. Uptake of 5-methyltetrahydrofolate was inhibited by the energy poisons, sodium cyanide, sodium arsenate, p-chloromercuriphenylsulfonate, and sodium azide. Uptake was inhibited by increasing concentrations of sulfate and phosphate ions, suggesting that 5-methyltetrahydrofolate may be transported by an anion-exchange mechanism. These results show that 5-methyltetrahydrofolate is transported into PC-3 prostate cancer cells by a carrier-mediated process.