Mechanism of interaction between antineoplastic agents and natural differentiation factors in the induction of human leukemic cell maturation.

DNA-specific agents have the capacity to induce the maturation of ML-1 human myeloblastic leukemia cells to monocyte/macrophages if adequate concentrations of fetal bovine serum or mitogen-stimulated human leukocyte-conditioned medium (CM) are present in the culture medium. Fetal bovine serum and CM contain specific differentiation-inducing factors that, in conjunction with the drugs, bring about cell maturation. To examine the mechanism by which this interactive effect occurs, ML-1 cells were exposed to actinomycin D or daunomycin in various combinations with CM, using concentrations at which neither the drug nor CM, when applied individually, induced maturation to a significant extent. Pretreatment for 3 days with drug followed by treatment for 3 days with CM caused maturation of 75% of the cells, as determined by the appearance of Fc receptors. Other markers of differentiation, including alpha-napthyl acetate esterase, acid phosphatase, and morphology, also reflected the increase in maturation. The simultaneous application of drug and CM was equally effective in inducing differentiation. Pretreatment of the cells with CM followed by treatment with drug failed to induce maturation, whereas pretreatment with CM followed by a second application of CM caused the expression of Fc receptors in 62% of the cells. In contrast, pretreatment with drug followed by a second application of drug did not induce differentiation significantly. These results indicate that the drug sensitizes the cells to respond to concentrations of CM to which they would otherwise be refractive. The drug-induced sensitization is reversible. At sensitizing drug concentrations, cell viability was preserved but, as measured by radiolabeling for 1 h, total RNA synthesis was decreased by 38% and mRNA synthesis by 87%. At these drug concentrations, the synthesis of mRNA specified by all seven oncogenes examined (myb, myc, abl, fos, N-ras, sis, erb B) was decreased by 15-60%. The administration of CM subsequent to drug caused a further decrease of some mRNA levels (c-myb, c-myc) but increased the level of others (c-fos). The drug-induced lowering of mRNA levels is considered to inhibit the synthesis of proteins specifically required for G1-S transit and maintenance of the proliferation program, amplifying, thereby, the maturation signal emitted by the differentiation factors present in serum and in CM. As a result, expression of the maturation program is initiated.