Growth, ribonucleotide reductase and metals in murine leukemic lymphocytes

Trace metals are essential for the growth and several other properties of human lymphocytes. We studied the effects of media with variable concentrations of three metals (Fe2+, Cu2+, Zn2+), a metal chelator (deferoxamine, DFX) and a cell-growth inhibitor (hydroxyurea) on the growth, intracellular metal concentration and activity of the enzyme ribonucleotide reductase in murine leukemic lymphocytes (L1210). Intracellular concentrations of Fe and Cu fluctuated within narrow limits in normal media, but decreased to very low concentrations in metal-poor media. The intracellular Zn concentration did not vary appreciably. Growth in intact cells decreased by 50%-70% when normal media were replaced by metal-poor media, but returned to control values when media were supplemented with gradually increasing concentrations of Fe and Cu. Fe and Cu had synergistic effects, while Zn had no stimulatory action. Hydroxyurea and DFX both inhibited cell growth, but only DFX inhibition was reversed by addition of metals. The addition of the above metals and inhibitors to the cell extracts produced effects on ribonucleotide reductase activity similar to those observed on the growth of whole cell preparations (stimulation by Fe and Cu, inhibition by Zn, DFX and hydroxyurea). These findings show that (a) the intracellular metal concentration is maintained in a narrow range during cell growth; (b) ribonucleotide reductase activity varies with cell growth; (c) ribonucleotide reductase activity and cell growth increase with Fe and Cu and decrease with Zn and DFX. Our data suggest that (a) Fe, Cu and Zn may have some effect on the growth and ribonucleotide reductase activity of L1210 cells, that (b) Fe, Cu and Zn may operate in a related and interdependent way and that (c) DFX inhibits cell growth probably through inhibition of the reductase activity and chelation of the Fe of its Fe-containing subunit. We conclude that any study on one of these metals should always include the other two and that manipulation of intracellular metals should be investigated as a potential therapeutic modulator of growth in leukemic lymphocytes.