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 microl/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.