Changes in cytoskeletal actin content, F-actin distribution, and surface morphology during HL-60 cell volume regulation.

Cell volume regulation occurs via the regulated fluxes of ions and solutes across the cell membrane in response to cell volume perturbations under anisotonic conditions. Our earlier studies in human promyelocytic leukemic HL-60 cells showed that volume-dependent changes in total cellular F-actin content occur concomitantly as an inverse function of acute cell volume changes in anisotonic media (Hallows et al., 1991, Am. J. Physiol., 261:C1154-C1161). Although treatment with cytochalasin under anisotonic conditions significantly reduced total cellular F-actin levels, cytochalasin did not significantly affect the ability of cells to undergo normal volume regulation responses, which suggested that these volume-dependent changes in F-actin content may not play a critical role in HL-60 cell volume regulation. To examine more closely the possible role of the actin cytoskeleton in HL-60 cell volume regulation, we quantitated changes in Triton-insoluble cytoskeletal actin in the presence and absence of cytochalasin and also observed changes in F-actin distribution and surface morphology during volume regulation. The quantity of cytoskeletal-associated F-actin, like total F-actin, shifts inversely with initial cell volume changes in anisotonic media; however, subsequent changes in cytoskeletal actin levels during volume regulation are not significant. The soluble F-actin pool in HL-60 cells may thus be more susceptible to the physicochemical effects of shifts in cell volume than the insoluble (cytoskeletal) F-actin pool. Twenty-five micromolar dihydrocytochalasin B (DHB) treatment dramatically lowers cellular cytoskeletal actin levels by approximately 75% under resting (isotonic) conditions, but there are no significant further changes in cytoskeletal actin as cells undergo anisotonic volume regulation in the presence of DHB. These results suggest that volume-dependent changes in the absolute amounts of cytoskeletal-associated F-actin are not critical for HL-60 cell volume regulation. However, because some portions of the actin cytoskeleton are resistant to cytochalasin disruption during volume regulation, a role for the cytoskeleton in the sensing and signaling of HL-60 cell volume regulatory responses cannot be rigorously excluded. Particular F-actin distribution patterns, as observed using confocal fluorescent microscopy, were correlated with particular phases of volume regulation. Also, comparison of cellular F-actin distribution with surface morphology (observed by scanning electronic microscopy) of cells during volume regulation reveals a positive correlation between surface blebs and increased cortical F-actin staining intensity.