Potentiating the cellular targeting and anti-tumor activity of Dp44mT via binding to human serum albumin: two saturable mechanisms of Dp44mT uptake by cells
The mechanisms of iron (Fe) and transferrin (Tf) uptake by the human melanoma cell line, SK-MEL-28, have been investigated using chelators and metabolic probes. These data provide evidence for two saturable processes of Fe uptake from Tf, namely, specific receptor-mediated endocytosis and a second nonspecific, non-receptor-mediated mechanisms which saturated with respect to Fe uptake at a Tf concentration of approximately 0.3 mg/ml. In contrast to Fe uptake, Tf uptake increased linearly up to at least 1 mg/ml. Furthermore, under the culture conditions used, the second nonspecific, non-receptor-mediated mechanism was the most important process in terms of quantitative Fe uptake. Two concentrations of Tf-125I-59Fe (0.01 and 0.1 mg/ml) were used in order to characterise the specific and nonspecific Fe uptake pathways. Membrane permeable chelators were equally effective at both Tf concentrations, whereas membrane impermeable chelators were significantly (P < 0.001) more effective at reducing the internalisation of Fe at the higher Tf concentration, consistent with a mechanism of Fe uptake which occurred at a site in contact with the extracellular medium. The oxidoreductase inhibitor, amiloride, only slightly inhibited Fe uptake at the higher Tf concentration, suggesting that the second nonspecific process was not mediated by a diferric Tf reductase. Three lysosomotrophic agents and the endocytosis inhibitor, phenylglyoxal, markedly reduced Fe uptake at both Tf concentrations, and it is concluded that a saturable process consistent with receptor-mediated endocytosis of Tf occurred at the lower Tf concentration, while the predominant mechanism of Fe uptake at high Tf concentrations was a second saturable process consistent with adsorptive pinocytosis.