Endonuclease activation during apoptosis: The role of cytosolic Ca²⁺ and pH

An axiom of apoptosis is that increases in cytosolic Ca²⁺ activate a Ca²⁺ Mg²⁺-dependent endonuclease. However, when HL-60 human promyelocytic leukemia cells were incubated with the Ca²⁺ ionophore ionomycin in varied extracellular Ca²⁺, DNA digestion was independent of extracellular Ca²⁺. Under these conditions, intracellular Ca²⁺ concentrations did not correlate with the observed DNA digestion. In contrast, intracellular acidification correlated well with DNA digestion. These data indicate that increased intracellular Ca²⁺ is not the primary signal for endonuclease activation in all forms of apoptosis, but that intracellular acidification may be involved. The observed intracellular acidification is consistent with the involvement of deoxyribonuclease II in apoptosis.