Actin polymerization and depolymerization during apoptosis in HL-60 cells.

Little is known about the biochemical "machinery" responsible for the morphological features of apoptosis, although the cytoskeleton is presumed to be involved. Using flow cytometry, polyacrylamide gel electrophoresis, and fluorescence microscopy, we show that apoptosis induced by ultraviolet (UV) irradiation or 80 micrograms/ml etoposide correlates with early transient polymerization and later depolymerization of filamentous (F)-actin and dramatic changes in visible microfilament organization. Depolymerization of F-actin began before the formation of apoptotic bodies and was ultimately composed of decreases in both the detergent-insoluble (40%) and detergent-soluble (50%) pools of F-actin. Dihydrocytochalasin B (H2CB), which blocked apoptotic body formation, depolymerized F-actin in the detergent-insoluble pool only. Visually, H2CB treatment disrupted microfilament organization, resulting in short, brightly stained microfilaments dispersed throughout the cytoplasm. In contrast, apoptotic cells contained a network of fine microfilaments with bright staining concentrated at the site of apoptotic body formation. Together, these results suggest that reorganization of the microfilament network is necessary for the formation of apoptotic bodies and that depolymerization of F-actin may also be a necessary component of the process of apoptosis.