HETEROGENEITY OF NUCLEOSIDE TRANSPORT IN MAMMALIAN-CELLS - 2 TYPES OF TRANSPORT ACTIVITY IN L1210 AND OTHER CULTURED NEOPLASTIC-CELLS

Abstract

The characteristics of nucleoside transport were examined in L1210 murine leukemia cells and 5 other cultured neoplastic cells. The initial rates of uridine, adenosine and thymidine transport in L1210 cells were only partially inhibited by 1 .mu.M nitrobenzylthioinosine (NBMPR), a potent inhibitor of nucleoside transport in other cells. The IC50 [median inhibitory concentration] for NBMPR inhibition of uridine transport was 5 nM, but 20% of the activity remained insensitive to concentrations as high as 3 .mu.M. Uridine uptake in the presence of 1 .mu.M NBMPR was saturable and was inhibited by other nucleosides, suggesting the participation of an NBMPR-insensitive transport mechanism. There appeared to be little difference in the specificity of NBMPR-sensitive and -insensitive transport for the physiological nucleosides. Uridine, adenosine and thymidine were all substrates for both mechanisms, and the Km values for total and NBMPR-insensitive uridine transport were the same (250 .mu.M). Little difference was found in the ability of several other nucleosides to inhibit total of NBMPR-insensitive uridine transport. In both cases, adenosine was the most effective inhibitor; cytidine and deoxycytidine were the least effective. The 2 transport processes did differ from each other in their sensitivity to p-mercuribenzenesulfonate (pMBS). NBMPR-insensitive uridine transport was inhibited by pMBS with an IC50 < 25 .mu.M; the IC50 for NBMPR-sensitive transport was > 400 .mu.M. Cloning of the parent L1210 cell line indicated that both NBMPR-sensitive and -insensitive transport occurred in the same cell. Both types of uridine transport activity were also observed in 3 other cell lines (RPMI 6410 [human carcinoma], L5178Y [mouse lymphocytic leukemia] and P388 [mouse leukemia]), while 2 lines, S49 [mouse lymphoma] and Walker 256 [rat carcinoma], exhibited only NBMPR-sensitive and -insensitive transport, respectively. The level of NBMPR-insensitive transport was an important determinant in the ability of NBMPR to inhibit uridine uptake over prolonged periods (10 min), with as little as 20% NBMPR-insensitive transport sufficient to render uptake over 10 min virtually insensitive to NBMPR. The existence of these 2 types of nucleoside transport activity in mammalian cells may have important implications in the chemotherapeutic use of transport inhibitors in combination with cytotoxic nucleosides or with inhibitors of pyrimidine and purine biosynthesis.