Carrier-mediated Transport of Oligopeptides in the Human Fibrosarcoma Cell Line HT1080¹

To explore the feasibility of targeting human tumor cells via their transport systems, dipeptide uptake was studied in the human fibrosarcoma cell line HT1080 and the human fibroblast cell line IMR-90 by the use of hydrolysis-resistant glycylsarcosine (Gly-Sar). The uptake of [¹⁴C]Gly-Sar into HT1080 was time dependent. Kinetic analysis of the concentration dependence of the initial rate of [¹⁴C]Gly-Sar uptake showed that a carrier-mediated transport system with a K_m of 11.4 ± 3.3 mm and V_max of 26.8 ± 4.0 (nmol/15 min/mg protein) and a nonsaturable component (k_d of 0.80 µl/15 min/mg protein) were responsible for the dipeptide uptake by HT1080 cells. The optimal pH for the maximal uptake was around 6.0. [¹⁴C]Gly-Sar uptake was inhibited by various di- and tripeptides and peptide-mimetic drugs, such as bestatin and cefadroxil. [¹⁴C]Gly-Sar uptake was not affected by the presence of amino acids or tetra- or pentapeptides. The uptake of cefadroxil was reduced significantly by unlabeled Gly-Sar. Moreover, Gly-Gly and Gly-Leu produced an increase in the apparent K_m of the uptake of Gly-Sar without altering V_max. On the other hand, dipeptide uptake by IMR-90, which is a normal diploid cell line (not malignant), showed no saturable transport. These results suggest that HT1080 cells take up dipeptides via a pH-dependent transporter. This is the first report showing that a dipeptide transport system, which is similar but not identical to the well-characterized oligopeptide transporters PepT1 and PepT2, exists in fibroblast-derived tumor cells but not in normal fibroblasts. The present finding could be the basis of a novel strategy for the specific delivery of oligopeptide-mimetic anticancer drugs into tumor cells.