The phosphatase inhibitor calyculin antagonizes the rapid initiation of apoptosis by photodynamic therapy.

DNA fragmentation and internucleosomal cleavage were rapidly initiated after lysosomal photodamage to murine leukemia cells, with apoptotic chromatin and DNA 'ladders' detected within 30 min after irradiation. Apoptosis was inhibited by concurrent exposure of cells to a 10 nM concentration of the serine-threonine phosphatase inhibitor calyculin A and promoted by a serine/threonine kinase inhibitor. These results indicate that a late stage in apoptosis requires serine/threonine dephosphorylation.