We used the promyelocytic leukemic cell line HL-60 to explore the molecular mechanisms regulating stimulus-induced actin polymerization in myeloid cells. HL-60 cells express very few chemotactic peptide receptors in their undifferentiated state and fail to undergo actin polymerization when stimulated with the chemotactic peptide N-formylmethionyl-leucyl-phenylalanine (FMLP). However, when the cells were induced to differentiate with dibutyryl cyclic AMP (dbcAMP) or retinoic acid, they acquired the ability to undergo actin polymerization on stimulation with FMLP or phorbol myristate acetate. Kinetic experiments revealed that in the first 48 h of retinoic acid treatment there was no increase in the chemotactic peptide receptors on HL-60 cells, but the cells were capable of undergoing actin polymerization on stimulation with FMLP. Similarly, treatment with dbcAMP showed no increase in chemotactic peptide receptors until 24 h but stimulus-induced actin polymerization was demonstrable as early as 4 h after the treatment. In addition, with dbcAMP-treated cells the magnitude of stimulus-induced actin polymerization showed large variation depending on the duration of exposure to the drug. Dual-label studies using propidium iodide to measure DNA content and NBD-phallacidin to measure the F-actin content revealed that these variations were not related to the stages of cell cycle. Cells in all stages of the cell cycle responded to stimulus-induced actin polymerization, but the magnitude of the response appeared to be more in cells in G2/M phase. The observations reported here indicate that the small number of chemotactic peptide receptors present on HL-60 cells are adequate to mount an actin polymerization response, provided the required intracellular mechanisms exist. Differentiation-inducing agents, therefore, must cause changes within the cell, such as induction of actin-binding proteins, to cause actin polymerization following FMLP stimulation. The HL-60 system serves as a useful model for studying the molecular mechanisms regulating stimulus-induced actin polymerization in human neutrophils.
This work was supported by funds from Veterans Administration Medical Research Program, NIH Grant R29 DK 38472, and the American Federation for Aging Research.