Ca2+ and phorbol ester synergistically induce HL-60 differentiation.

Exposure of HL-60 cells to subthreshold concentrations of TPA caused monocytic differentiation only when cells were cotreated with the Ca2+ ionophore A23187. Phorbol ester dose-response curves for growth arrest and enzymatic markers of differentiation were shifted to lower concentrations when the ionophore was present. Expression of a monocyte/granulocyte cell surface antigen also occurred only when cells were treated with both agents. Similar effects were seen with other active but not inactive phorbol esters and with another Ca2+ ionophore. The Ca2+ component of phosphoinositide-based signalling may thus play a role in HL-60 differentiation.