Intracellular acidification is associated with, but not required for caspase activation, DNA fragmentation or...

Apoptosis is characterized by DNA digestion mediated by either a Ca2+/Mg2+-dependent endonuclease or the acid-activated deoxyribonuclease II (DNase II). However, DNA digestion frequently does not correlate with changes in Ca2+ whereas intracellular acidification is a consistent marker of apoptosis. To confirm the role of low pH in regulating DNA digestion, ML-I cells were damaged with etoposide then incubated at various extracellular pH (pH,). When pH, was 8.1, DNA digestion still occurred, and intracellular pH still decreased but only to 7.2, a pH at which DNase LT is inactive. In contrast, low pH, inhibited the DNA digestion and apoptosis induced by etoposide. An upstream event in apoptosis is the activation of proteases known as caspases. The activity of caspases was inhibited at low pH, demonstrating that the pH-sensitive step is upstream of caspase action. Similar results have been obtained in other models of apoptosis. Hence, both DNase II and Ca2+/Mg2+-dependent endonuclease appear unlikely to cause DNA digestion in apoptosis, unless their ion dependence is modified by, for example, proteolytic cleavage.