Phorbol ester-induced macrophage-like differentiation of human promyelocytic leukemia (HL-60) cells occurs independently of transferrin availability.

Human promyelocytic leukemia (HL-60) cells can be induced to differentiate into macrophage-like cells by the tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA). Addition of this agent to HL-60 cells causes a rapid internalization of surface transferrin receptor, followed by long-term receptor down-regulation at the level of gene expression. These effects precede the inhibition of proliferation and the acquisition of differentiation markers, and it has been suggested that transferrin receptor down-regulation may play a mediating role in these later events. Here we show that HL-60 cells will grow indefinitely in serum-free medium supplemented with either 5 micrograms ml-1 transferrin or 300 microM ferric citrate and that TPA inhibits cell proliferation (assayed by cell density and rate of thymidine incorporation) and induces macrophage-like differentiation (assayed by induction of cell adhesion and increased nonspecific esterase activity) with identical dose curves in both media. Furthermore, a neutralizing anti-transferrin antibody completely inhibits transferrin-dependent cell proliferation but has no effect on differentiation in the presence or absence of transferrin. We conclude that TPA-induced down-regulation of transferrin binding and internalization does not mediate the subsequent growth arrest and differentiation of HL-60 cells.