DNA damage-induced [Zn2+]i transients:: correlation with cell cycle arrest and apoptosis in lymphoma cells

Reactive changes in free intracellular zinc cation concentration ([Zn2+](i)) were monitored, using the fluorescent probe Zinquin, in human lymphoma cells exposed to the DNA-damaging agent VP-16. Two-photon excitation microscopy showed that Zinquin-Zn2+ forms complexes in cytoplasmic vesicles. [Zn2+](i) increased in both p53(wt) (wild type) and p53(mut) (mutant) cells after exposure to low drug doses. In p53(mut) cells noncompetent for DNA damage-induced apoptosis, elevated [Zn2+](i) was maintained at higher drug doses, unlike competent p53(wt) cells that showed a collapse of the transient before apoptosis. In p53(wt) cells, the [Zn2+](i) rise paralleled an increase in p53 and bax-to-bcl-2 ratio but preceded an increase in p21(WAF1), active cell cycle arrest in G(2), or nuclear fragmentation. Reducing [Zn2+](i), using N,N,N',N'-tetrakis(2-pyridylmethyl) ethylenediamine, caused rapid apoptosis in both p53(wt) and p53(mut) cells, although cotreatment with VP-16 exacerbated apoptosis only in p53(wt) cells. This may reflect changed thresholds for proapoptotic caspase-3 activation in competent cells. We conclude that the DNA damage-induced transient is p53-independent up to a damage threshold, beyond which competent cells reduce [Zn2+](i) before apoptosis. Early stress responses in p53(wt) cells take place in an environment of enhanced Zn2+ availability.