Regulation of the expression of enzymes involved in the replication of DNA in chemically induced monocytic/macrophagic differentiation of HL-60 leukemia cells

The expression of a number of housekeeping enzymes of DNA biosynthesis was measured in HL-60 promyelocytic leukemia cells undergoing monocytic/macrophagic differentiation following treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) or 1α,25-dihydroxyvitamin D₃ (vitamin D₃). Progressive decreases in the steady-state levels of the mRNAs for thymidylate synthase, topoisomerase II, and hypoxanthine-guanine phosphoribosyltransferase occurred following exposure to TPA or vitamin D₃. In contrast, the steady-state levels of the mRNAs for thymidine kinase, topoisomerase I, and DNA polymerase-α did not decrease until days 3-5 of treatment with vitamin D₃ and then progressively declined thereafter. The mRNAs for thymidine kinase and topoisomerase I decreased slightly and the mRNA for DNA polymerase-α by 30-40%, and then remained constant between days 1 to 3 of treatment with the phorbol ester. The M2 subunit of ribonucleotide reductase exhibited an even greater difference, with no change in the steady-state concentration of mRNA over 3 days of exposure to TPA or vitamin D₃. On days 5-7 of treatment with vitamin D₃, essentially complete loss of the expression of the mRNA for the M2 subunit of ribonucleotide reductase occurred. Measurement of the enzymatic activities of thymidylate synthase and thymidine kinase in cells exposed to either of the inducers of maturation corroborated the findings at the level of the mRNAs, with corresponding decreases in the activity of these enzymes. The results indicate that the down-regulation of the expression of housekeeping enzymes of DNA replication occurs as late events in HL-60 cells undergoing monocytic/macrophagic differentiation, implying that the decreases in their gene expression are the result of the termination of proliferation rather than an initiating event in the cessation of DNA biosynthesis.