Mg2+ as activator of uridine phosphorylation in coordination with other cellular responses to growth factors.

The divalent cation ionophore A23187 facilitates the manipulation of intracellular Mg2+ without increasing the general permeability of the cell. The uptake of uridine into cells is limited by its rate of intracellular phosphorylation that increases within minutes after the addition of growth factors. In the experiments described here, the rate of uridine uptake in ionophore-treated cells stimulated by either serum or insulin depended on the extracellular and intracellular concentrations of Mg2+ and was independent of the extracellular Ca2+ concentration. In very high concentrations of Mg2+ (50 mM), ionophore-treated cells take up uridine as fast, in the absence of growth factors as in their presence, demonstrating that Mg2+ can replace the growth factor requirement for the stimulation of uridine uptake. In contrast, thymidine uptake, which also is limited by its rate of intracellular phosphorylation, showed no early response to either growth factors or Mg2+ concentration, which is consistent with the 10-fold lower Mg2+ requirement of thymidine kinase compared with uridine kinase. The feedback inhibition of uridine kinase by UTP and CTP in cell-free extracts was alleviated by increased Mg2+ concentration. The results support the thesis that the increased uptake of uridine in cells treated with growth factors is determined by a membrane-induced increase in intracellular free Mg2+. Such increase would also accelerate the rate of translation-initiation and other coordinate responses that, unlike increased uridine uptake, are essential for cell proliferation. The rate of uridine uptake is suggested as a direct indicator of free cytosolic Mg2+ that drives the shift from quiescence to proliferation.