Phorbol ester induction of leukemic cell differentiation is a membrane-mediated process.

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Phorbol esters are potent inducers of macrophage-like differentiation in the HL-60 promyelocytic leukemia cell line. The sequence of events by which they bring about this transition is poorly understood. However, it is known that phorbol esters bind to the surface membrane of HL-60 cells and to various other cells as well. Our studies were directed toward determining the biologic importance of this membrane association. [3H]Phorbol dibutyrate (PBu2) was specifically bound by HL-60 cells with a Kd of 23 nM and with 1.9 X 10(5) binding sites for [3H]PBu2 per cell. There was no internalization of bound [3H]PBu2. Specific binding was fully reversible upon washing in fresh medium, and [3H]PBu2 added thereafter bound normally to its receptor. Within 10 min of binding, PBu2 stimulated [14C]choline incorporation into phosphatidylcholine, with a rapid return to normal upon removal of the PBu2. Membrane-bound PBu2 progressively inhibited DNA synthesis, with 70% inhibition by 8 hr. This process was interrupted if the PBu2 was removed, and little recovery of DNA synthesis occurred in previously inhibited cells. Between 8 and 16 hr, PBu2 induced adherence of cells to plastic, but only in those cells in which phosphatidylcholine synthesis was stimulated, and this process was also interrupted if PBu2 was removed prior to 16 hr. Similarly, nonspecific esterase, which develops after 72 hr of incubation, was induced in cells exposed to PBu2 for the initial 16 hr but not in cells exposed for 5 hr. These studies demonstrate that phorbol esters exert their effects while retained at the cell surface. Inhibition of cell growth and the acquisition of surface and enzymatic properties that characterize macrophages are separable events, each of which proceeds through a receptor-mediated, transmembrane process. The stimulation of phosphatidylcholine synthesis appears to be a part of that process.

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