Control of HL-60 cell differentiation lineage specificity, a late event occurring after precommitment

Terminal cell differentiation of HL-60 promyelocytic leukemia cells results when they are continuously exposed to retinoic acid. This process involves an intermediate regulatory state, the precommitment memory state, which occurs before onset of differentiation or growth arrest in G0. The cellular processes occurring prior to onset of terminal differentiation can be resolved into early events anteceding development of the precommitment memory state and late events subsequent to it. While it has been suggested that retinoic acid induced early events regulate G1/0 specific growth arrest associated with terminal differentiation, the significance of induced late events is not known. Exploiting the capability of HL-60 cells to undergo either myeloid or monocytic differentiation in response to different inducers, the present studies examine the response of HL-60 cells to the sequential application of myeloid and monocytic inducers prior to onset of terminal differentiation. The results indicate that the precommitment state induced by retinoic acid is not differentiation lineage specific. Sequential application first of retinoic acid, a myeloid inducer, and then of 1,25-dihydroxyvitamin D3, a monocytic inducer, and vice versa, show that cellular choice of a specific differentiation lineage is regulated by late inducer driven events. The data support a two-step model for induction of terminal differentiation where early events anteceding precommitment regulate growth arrest and late events subsequent to precommitment regulate choice of a specific differentiation lineage. The results are of potential significance to the use of differentiation-inducing agents in chemotherapy. The potential toxicity of prolonged exposure to a single inducer might thus be mitigated by sequential brief exposures to different inducers.