Retinoic-acid-induced differentiation of HL-60 cells is associated with biphasic activation of the Na+— K+ pump

Abstract. The human promyelocytic leukemia cell line, HL-60, can be induced to differentiate into granulocyte-like cells when cultured in the presence of 10^-6 M retinoic acid (RA) for several days. Following the addition of RA two kinds of changes occur. First, there are early changes that comprise an increase in the intracellular concentration of sodium ions [Na]_i, which reaches its maximum after 6 h, and an increase in the activity of the Na⁺-pump, which is reflected by an ouabain-sensitive K⁺ influx that peaks at 8 h (170% of the control value) and that occurs without any change in the number of pump molecules, as measured by the binding of ³H-ouabain. Second, beginning after 12 h of culture with RA, a decrease in the number of ouabain-binding sites occurs, this being accompanied by an increase in the number of K⁺ ions actively transported by each site. The effect of modulation of Na⁺ -pump activity on the RA-induced differentiation of HL-60 cells was studied using low, noncytotoxic concentrations of ouabain which, although alone having no differentiating effect, accelerated and potentiated the effect of RA on differentiation. When added in combination. these drugs induced rapid stimulation of the Na⁺ -pump, which reached its peak after 2 h. These results indicate that a concomitant increase in the level of [Na⁺]_i and in the activity of the Na⁺ -pump constitute primary events in the interaction between RA and HL-60 cells, and that cation fluxes may play a role in the initiation of the process of differentiation.