Changes in cell kinetics associated with differentiation of a human promyelocytic cell line (HL60)

Abstract. Terminal cell differentiation results in an irreversible arrest in the G₁ phase of the cell cycle and loss of the capacity for cell renewal. In the murine erythroleukaemia cell line (MELC), commitment to erythroid differentiation was found also to be preceded by an early, transient, phase of inhibition of growth due to prolongation of the G₁ phase. We determined the effect of differentiation-inducing agents on the growth kinetics of a human promyelocytic cell line (HL60) which undergoes differentiation into mature granulocyte. At concentrations of inducers optimal for cell differentiation, an early, transient stimulation of cell multiplication was found. DNA synthesis was enhanced in HL60 cells as early as 5 hr after exposure to inducer. Nevertheless, HL60 cell maturation eventually also resulted in a loss of the multiplication ability. The duration of exposure to inducer required for irreversible loss of the potential for self-renewal was determined by the fall in the cloning efficiency of induced cells; the results indicate that it preceded the switch-off of the replication mechanism; the majority of the cells lost their ability to form large colonies at the time of peak DNA synthesis and were able to complete an additional two to three cell cycles at a rate similar to uninduced cells. These changes occurred before HL60 cells became committed and might play a pivotal role in the process of cell differentiation.