DNA fragmentation is a hallmark of apoptosis that is induced by apoptotic stimuli in various cell types. Apoptotic signal pathways, which eventually cause DNA fragmentation, are largely mediated by the family of cysteinyl aspartate‐specific protease caspases. Caspases mediate apoptotic signal transduction by cleavage of apoptosis‐implicated proteins and the caspases themselves. In the process of caspase activation, reversible protein phosphorylation plays an important role. The activation of various proteins is regulated by phosphorylation and dephosphorylation, both upstream and downstream of caspase activation. Many kinases/phosphatases are involved in the control of cell survival and death, including the mitogen‐activated protein kinase signal transduction pathways. Reversible protein phosphorylation is involved in the widespread regulation of cellular signal transduction and apoptotic processes. Therefore, phosphatase/kinase inhibitors are commonly used as apoptosis inducers/inhibitors. Whether protein phosphorylation induces apoptosis depends on many factors, such as the type of phosphorylated protein, the degree of activation and the influence of other proteins. Phosphorylation signaling pathways are intricately interrelated; it was previously shown that either induction or inhibition of phosphorylation causes cell death. Determination of the relationship between protein and phosphorylation helps to reveal how apoptosis is regulated. Here we discuss DNA fragmentation and protein phosphorylation, focusing on caspase and serine/threonine protein phosphatase activation.

In this study, we investigated the induction of apoptosis by ultrasound in the presence of a photochemically active chlorine, mono-l-aspartyl chlorin e6 (NPe6). HL-60 cells were exposed to ultrasound for up to 3 min in the presence and absence of NPe6, and the induction of apoptosis was examined by analyzing cell morphology, DNA fragmentation, and caspase-3 activity. Cells treated with 80 µ µ µ µM NPe6 and ultrasound clearly showed membrane blebbing and cell shrinkage, whereas significant morphologic changes were not observed in cells exposed to either ultrasound alone, at the same intensity, or NPe6 alone. Also, DNA ladder formation and caspase-3 activation were observed in cells treated with both ultrasound and NPe6 but not in cells treated with ultrasound or NPe6 alone. In addition, NPe6 substantially enhanced nitroxide generation by ultrasound in the same acoustical arrangement. Sonodynamically-induced apoptosis, caspase-3 activation, and nitroxide generation were significantly suppressed by histidine. These results suggest that the combination of ultrasound and NPe6 sonochemically induces apoptosis as well as necrosis in HL-60 cells. They further suggest that some ultrasonically-generated active species, deactivatable by histidine, are the major mediators to induce the observed apoptosis. (Cancer Sci 2008; 99: 166-172)

DNA fragmentation is a hallmark of apoptosis that occurs in a variety of cell types; however, it remains unclear whether caspase‐3 is required for its induction. To investigate this, we produced caspase‐3 knockout Chinese hamster ovary (CHO)‐K1 cells and examined the effects of gene knockout and treatment with caspase‐3 inhibitors. Okadaic acid (OA) is a potent inhibitor of the serine/threonine protein phosphatases (PPs) PP1 and PP2A, which induce apoptotic cellular reactions. Treatment of caspase‐3−/− cells with OA induced DNA fragmentation, indicating that caspase‐3 is not an essential requirement. However, in the presence of benzyloxycarbonyl‐Asp‐Glu‐Val‐Asp (OMe) fluoromethylketone (z‐DEVD‐fmk), DNA fragmentation occurred in CHO‐K1 cells but not in caspase‐3−/− cells, suggesting that caspase‐3 is involved in OA‐induced DNA fragmentation that does not utilize DEVDase activity. In the absence of caspase‐3, DEVDase activity may play an important role. In addition, OA‐induced DNA fragmentation was reduced but not blocked in CHO‐K1 cells, suggesting that caspase‐3 is involved in caspase‐independent OA‐induced DNA fragmentation. Furthermore, OA‐induced cleavage of caspase‐3 and DNA fragmentation were blocked by pretreatment with the wide‐ranging serine protease inhibitor 4‐(2‐aminoethyl)‐benzenesulfonyl fluoride hydrochloride. These results suggest that serine proteases regulate DNA fragmentation upstream of caspase‐3.