Transient fluctuations of intracellular zinc ions in cell proliferation

Zinc is essential for cell proliferation, differentiation, and viability. When zinc becomes limited for cultured cells, DNA synthesis ceases and the cell cycle is arrested. The molecular mechanisms of actions of zinc are believed to involve changes in the availability of zinc(II) ions (Zn2+). By employing a fluorescent Zn2+ probe, FluoZin-3 acetoxymethyl ester, intracellular Zn2+ concentrations were measured in undifferentiated and in nerve growth factor (NGF)-differentiated rat pheochromocytoma (PC12) cells. Intracellular Zn2+ concentrations are pico- to nanomolar in PC12 cells and are higher in the differentiated than in the undifferentiated cells. When following cellular Zn2+ concentrations for 48 h after the removal of serum, a condition that is known to cause cell cycle arrest, Zn2+ concentrations decrease after 30 min but, remarkably, increase after 1 h, and then decrease again to about one half of the initial concentration. Cell proliferation, measured by an MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay, decreases after both serum starvation and zinc chelation. Two peaks of Zn2+ concentrations occur within one cell cycle: one early in the G1 phase and the other in the late G1/S phase. Thus, fluctuations of intracellular Zn2+ concentrations and established modulation of phosphorylation signaling, via an inhibition of protein tyrosine phosphatases at commensurately low Zn2+ concentrations, suggest a role for Zn2+ in the control of the cell cycle. Interventions targeted at these picomolar Zn2+ fluctuations may be a way of controlling cell growth in hyperplasia, neoplasia, and diseases associated with aberrant differentiation.