Mitogenic Enhancement of Purine Base Phosphoribosylation in Swiss Mouse 3T3 Cells

Increased uptake of guanosine and the purine bases, adenine, hypoxanthine, and guanine, occurs within minutes after addition of fresh serum or purified growth-promoting agents to density-arrested cultures of Swiss 3T3, Swiss 3T6, and tertiary mouse embryo fibroblasts. Enhancement of uptake is maximal by about 50 min, is potentiated by combinations of growth promoters, and involves a process distinguishable from that of the enhanced uridine uptake on the basis of time course, pattern of growth-factor responsiveness and the failure of uridine to inhibit purine uptake. Enhanced purine uptake in 3T3 cells results from stimulation of the phosphoribosylation of purine bases and consequent trapping of nucleotide derivatives rather than from increased rates of membrane transport of the purine compounds. Increased availability of 5-phosphoribosyl 1-pyrophosphate (PRPP), identifiable within 30 min of serum addition, provides a likely explanation for increased purine base phosphoribosylation. Mitogenic stimulation of purine uptake is unaffected by cycloheximide but is markedly reduced in sodium-free medium. Enhanced purine uptake does not clearly depend upon changes in concentrations of effectors of intracellular PRPP synthesis. Nevertheless, when the possibility of direct mitogenic activation of PRPP synthase was studied, no differences were found in activities of this enzyme in extracts from stimulated and unstimulated 3T3 cells and mouse embryo fibroblasts.