A role for serine proteases in mediating phorbol ester-induced differentiation of HL-60 cells

Treatment of human HL-60 promyelocytic leukemia cells with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) results in increases in proteolytic activity and maturation toward the monocytic lineage. To investigate the potential roles that different classes of proteases play in the monocytic differentiation of HL-60 cells, cells were treated with phorbol ester in the presence of various serine and cysteine protease inhibitors. The serine protease inhibitors 4-(2-aminoethyl)-benzenesulfonyl fluoride (AEBSF), N-alpha-tosyl-phenylalanine chloromethyl ketone (TPCK), and N-alpha-tosyl-lysine chloromethyl ketone (TLCK) repressed a number of phenotypic markers of monocytic differentiation including surface expression of the CD11b integrin, cell aggregate formation, cell cycle exit, and cell death. CD11b was not detected at the cell surface by FACS analysis up to 24 h after induction of differentiation; however, both CD11b mRNA and protein were present. Downregulation of c-myc mRNA and upregulation of c-fos and egr-1 mRNA and protein, which normally occur during TPA-induced differentiation, were not affected by inclusion of the protease inhibitors. These data indicate that serine proteases specifically mediate many of the phenotypic aspects of TPA-induced monocytic differentiation but are not involved with the induction or repression of differentiation-sensitive transcription factors and suggest that serine protease activity is required for intracellular processing of CD11b.