Differentiation and retrodifferentiation of human myeloid leukemia cells is associated with reversible induction of cell cycle-regulatory genes.

Treatment of human myeloid leukemia cells (HL-60, U-937, THP-1) with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) is associated with differentiation along the monocytic lineage. This induction by TPA is characterized in part by growth arrest and the appearance of differentiated monocytic phenotype. The present studies demonstrate that myeloid leukemia cells exit the cell cycle to G0-G1 between 24 and 36 h following TPA treatment. This G0-G1 arrest was accompanied by down-regulation of the cell cycle-regulatory genes cdc2, cyclin A, cyclin B, and cdc25. Similar findings were obtained for histones H1 and H4. Cell cycle progression of synchronized U-937 cells revealed low to undetectable mRNA levels for these genes in G1 and maximal transcription in G2-M phase. Results obtained from mRNA half-life studies demonstrate that the stability of cdc2, cyclin A, cyclin B, and cdc25 transcripts is similar in control and TPA-treated U-937 cells. Nuclear run-on assays demonstrated down-regulation of histone gene transcription, while there was no signal detectable for the cell cycle-regulatory genes. The present findings also demonstrate that long term culture of TPA-differentiated U-937 cells is associated with a decrease in G0-G1-arrested cells and an increase of cells in S and G2-M after 25 days. This reentry into the cell cycle was accompanied by loss of adherence, down-regulation of markers for the monocytic phenotype, and induction of the cell cycle-regulatory genes. This process of retrodifferentiation was completed after 36 days when patterns of cell cycle-regulatory and histone gene expression were identical to that in untreated U-937 cells.