IDENTIFICATION OF A CELL-TYPE-SPECIFIC AND E2F-INDEPENDENT MECHANISM FOR REPRESSION OF CDC2 TRANSCRIPTION

Human myeloid leukemia cells, such as HL60, U937, and THP1 cells, unde rgo macrophage differentiation and growth arrest following treatment w ith the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Surp risingly, we find that growth of a significant percentage of THP1 cell s is arrested in the G(2), phase of the cell cycle. G(2) arrest correl ates with cell-specific repression of the gene encoding p34(cdc2), a c rucial regulator of G(2)/M progression. Intriguingly, TPA-mediated rep ression of the cdc2 promoter was independent of the transcription fact or E2F, distinguishing this pathway from mechanisms responsible for re pression of cdc2 transcription in response to serum starvation. The re gion of the cdc2 promoter required for repression was located from bp -22 to -2 from the major transcriptional start site. This sequence, wh ich we term the R box, directs the uncoupling of the basal promoter fr om upstream activators following TPA treatment. Analysis of THP1 nucle ar proteins revealed a 55-kDa protein that was induced by TPA and inte racted with the cdc2 promoter in an R-box dependent manner. These obse rvations provide evidence for the existence of cell-type and promoter- specific pathways for the assembly of stable transcriptional initiatio n complexes that function to differentially regulate the expression of cell cycle control genes in mammalian cells.