Uptake of nucleosides in density-inhibited cultures of 3T3 cells

Abstract

AN increase in the rate of nucleoside incorporation into acid-soluble pools is one of the earliest events seen when fibroblasts arrested in the G₁/G₀ or A state¹ are stimulated to grow by the addition of serum^2–5. The stimulation of uridine uptake is preceeded by a lag phase of several minutes⁶ and is not prevented by inhibitors of protein synthesis⁷. The uptake of nucleosides by mammalian cells seems to proceed in two steps: (1) the nucleosides are transferred rapidly across the plasma membrane by a facilitated diffusion mechanism⁸ and (2) the intracellular nucleosides are phosphorylated by specific nucleoside kinases and the nucleosides formed are trapped intracellularly⁹. Although the regulation of nucleoside uptake has been studied in some detail^3–7, it is not known which of these two steps is activated by growth-promoting agents. We have investigated which step of nucleoside uptake is stimulated by serum in density-inhibited cultures of 3T3 cells. The results indicate that phosphorylation is the rate-limiting step in nucleoside uptake by quiescent cells. Growth stimulation by serum enhances the rate of phosphorylation, but apparently not that of transport.