Cell cycle-independent down-regulation of BCL-2 protein expression in differentiating HL-60 cells

To understand the relationship between bcl-2 protein expression and the cel l cycle during the processes of differentiation, we examined bcl-2 protein levels expressed during cell cycle phases in differentiating HL-60 human le ukemia cells by using a two-color flow cytometric method. In exponentially proliferating HL-60 cells bcl-2 protein was constitutively expressed throug hout the cell cycle phases, but a small population of G0/G1 cells expressed decreased levels of protein as compared with other cell cycle phases. HL-6 0 cells can be induced to differentiate to granulocytic pathway by retinoic acid or dimethylsulfoxide, and to monocytic/macrophagic pathway by 1, 25 d ihydroxyvitamin D3 or 12-O-tetradecanoylphorbol-13-acetate. During treatmen t with any of these inducing agents, bcl-2 protein expression was time-depe ndently down-regulated after 24h. A two-color flow cytometric analysis reve aled that this down-regulation occurred throughout cell cycle phases, indic ating that bcl-2 protein expression was down-regulated in cell cycle-indepe ndent manner during induction of differentiation in HL-60 cells. To our kno wledge, this is the first demonstration suggesting that the regulation of b cl-2 protein expression is not related to the cell cycle during induction o f differentiation in HL-60 cells.