PMA-induced activation of the p42/44ERK- and p38RK-MAP kinase cascades in HL-60 cells is PKC dependent but not essential for differentiation to the macrophage-like phenotype.

The signaling mechanisms leading to phorbol ester myristate (PMA)-induced differentiation of HL-60 cells to the macrophagelike phenotype were investigated by using different protein kinase inhibitors. The protein kinase C inhibitor Ro 31-8220 specifically blocks PMA-induced differentiation, activation of the p42/44ERK- and p38RK-MAP kinase cascades and Hsp27-phosphorylation in HL-60 cells. Because Ro 31-8220 does not inhibit activation of the MAP kinase cascades by protein kinase C (PKC)-independent signals such as epidermal growth factor (EGF), heat shock, or anisomycin in these cells, only PMA-induced activation of the MAP kinases can be downstream of PKC. The MEK1 inhibitor PD 098059 and the p38RK inhibitor SB 203580 also were used to analyze whether the PMA-induced PKC-dependent activation of MAP kinases is involved in the differentiation process. Under certain conditions, PD 098059 can completely block the PMA-induced activation of the p42ERK as monitored by imunoprecipitation kinase assay by using the substrate myelin basic protein. SB 203580 specifically inhibits activation of p38RK as judged by MAPKAP kinase 2 activity against the substrate Hsp27 and also blocks Hsp27 phosphorylation in the cells. In contrast, neither PD 098059 nor SB 203580 nor both inhibitors together prevent PMA-induced differentiation of the HL-60 cells to the macrophagelike phenotype. The results suggest the existence of a diversification of PMA-induced signaling in HL-60 cells downstream of PKC, leading to activation of MAP kinases that are not essential for differentiation and to phosphorylation of other, so far unidentified, targets responsible for differentiation. J. Cell. Physiol. 173:310–318, 1997. © 1997 Wiley-Liss, Inc.