ACTIN POLYMERIZATION AND DEPOLYMERIZATION DURING APOPTOSIS IN HL-60 CELLS

Little is known about the biochemical ''machinery'' responsible for th e morphological features of apoptosis, although the cytoskeleton is pr esumed to be involved. Using flow cytometry, polyacrylamide gel electr ophoresis, and fluorescence microscopy, we show that apoptosis induced by ultraviolet (UV) irradiation or 80 mu g/ml etoposide correlates wi th early transient polymerization and later depolymerization of filame ntous (F)-actin and dramatic changes in visible microfilament organiza tion. Depolymerization of F-actin began before the formation of apopto tic bodies and was ultimately composed of decreases in both the deterg ent-insoluble (40%) and detergent-soluble (50%) pools of F-actin. Dihy drocytochalasin B (H2CB), which blocked apoptotic body formation, depo lymerized F-actin in the detergent-insoluble pool only. Visually, H2CB treatment disrupted microfilament organization, resulting in short, b rightly stained microfilaments dispersed throughout the cytoplasm. In contrast, apoptotic cells contained a network of fine microfilaments w ith bright staining concentrated at the site of apoptotic body formati on. Together, these results suggest that reorganization of the microfi lament 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.