Induction of differentiation in HL60 leukaemic cells: a cell cycle dependent all-or-none event

The human leukaemia cell line (HL60) shows a limited capacity to differentiate spontaneously, but this property can be greatly enhanced by chemical inducers. Sodium butyrate induced differentiation in virtually 100% of HL60 cells over a four-day interval to cells with multiple phenotypic markers of monocytes. Clonogenic analysis in agar demonstrated that differentiated cells (either spontaneous or induced) irreversibly lost clonogenic potential. This appeared to be an all-or-none process with unaffected cells exhibiting unaltered clonogeneity. A study of the kinetics of colony formation showed that most, if not all, cells completed one division in the presence of butyrate and sometimes several divisions before loss of proliferative potential. Despite the uniform spectrum of cell cycle states present in HL60 cultures when butyrate was added, all differentiated cells were shown to be arrested in G1. Evidence was obtained suggesting that the 'switch' into the differentiation pathway occurred during a restricted stage of the cell cycle, either late in the cycle (G2-M) or early in G1.