UNCOUPLING OF CELL-CYCLE ARREST FROM THE EXPRESSION OF MONOCYTIC DIFFERENTIATION MARKERS IN HL60 CELL VARIANTS

Differentiation generally leads to cell cycle arrest, Human leukemia H L60 cells respond to the presence of 1,25-dihydroxyvitamin D-3 (1,25D( 3)) by expressing a number of markers of the monocyte/macrophage pheno type and become arrested predominantly in the G1 phase of the cell cyc le, We have recently reported a series (A) of 1,25D(3)-resistant varia nts of HL60 cells which proliferate in the presence of 1,25D(3) and do not express differentiation markers (Exp. Cell Res. 224, 312, 1996), We now describe another series (B) of such variants, which differ from A series cells grown in similar concentrations of 1,25D(3) in that th ey express the CD14 antigen and nonspecific esterase, characteristic o f the monocyte, while continuing to proliferate and they develop hypot etraploid DNA (4C) content at higher concentrations of ambient 1,25D(3 ) than the A series cells, Cells in the B series with 4C DNA content ( 100B and 200B) also differed from the A series 4C cells by the absence of DNA binding by the full-length Sp1 transcription factor. However, B series cells resembled the A series cells in exhibiting faster growt h rates than the parental HL60 cells and showed high levels of vitamin D receptor and retinoid receptor X proteins, These results show that the initial steps in the 1,25D(3) signaling pathway are intact in B se ries resistant cells and lead to the appearance of early markers of mo nocytic differentiation. However, the progression to subsequent events which comprise terminal differentiation and cell cycle arrest is halt ed during the adaptation to the presence of 1,25D(3) in these cells, T hus, the availability of these variant cells should provide a system f or studying the link between differentiation and cell cycle arrest. (C ) 1997 Academic Press.