Effects of transformation by v-fps on nucleoside transport in Rat-2 fibroblasts

Important cellular nutrients, including nucleosides and hexose sugars, are rapidly taken up by cells, largely through mediated carrier systems. The present study examined nucleoside and hexose transport activity in normal Rat-2 fibroblasts and clonal derivatives that expressed either the wild-type (C10) or a temperature-sensitive mutant (NA9) form of v-fps, a transforming protein-tyrosine kinase. Initial uptake rates (transport) of adenosine, thymidine, 3-O-methylglucose and 2-deoxyglucose were greater in v-fps-transformed cells than in normal cells. Elevated transport rates were seen in cells that expressed the temperature-sensitive mutant v-fps only after growth at a temperature that was permissive for protein-tyrosine kinase activity. Nucleoside transport rates declined with increasing cell density in both normal and v-fps transformed cells. Analysis of the sensitivity of adenosine transport to inhibition by nitrobenzylthioinosine (NBMPR) indicated that Rat-2 fibroblasts, like many other rat cell types, possess at least two nucleoside transport systems, which can be distinguished by differences in sensitivity to NBMPR. Although both transport activities were elevated in v-fps-transformed cells, a greater increase was seen in the NBMPR-sensitive component than in the NBMPR-insensitive component. Mass law analysis of the binding of [3H]NBMPR indicated that transformed cells had either the same number (NA9) or a smaller number (C10) of NBMPR-binding sites than normal cells, and photolabelling of membrane proteins with [3H]NBMPR identified polypeptides with similar electrophoretic mobilities (55-75 kDa) in both normal and transformed cells. Thus transformation by v-fps resulted in an increase in NBMPR-sensitive transport activity which was not related to either the number of NBMPR-binding sites or the apparent molecular mass of NBMPR-binding polypeptides.