Activation of ERK1 and ERK2 protein kinases has been implicated in diverse cellular processes, including the control of cell proliferation and cell differentiation (Marshall  Cell 80:179). In human myeloblastoid leukemia HL60 cells rapid (ca. 15 min) but transient activation of ERK1/2 has been reported following induction of macrophage/monocyte differentiation by phorbol esters, or by very high (10(-6) M) concentrations of 1,25-dihydroxyvitamin D(3) (1,25D3), while retinoic acid-induced granulocytic differentiation was accompanied by sustained activation of ERK1/2. We report here that monocytic differentiation of HL60 cells induced by moderate (10(-9) to 10(-7) M) concentrations of 1,25D3 could be divided into at least two stages. In the first phase, which lasts 24-48 h, the cells continued in the normal cell cycle while expressing markers of monocytic phenotype, such as CD14. In the next phase the onset of G1 cell cycle block became apparent and expression of CD11b was prominent, indicating a more mature myeloid phenotype. The first phase was characterized by high levels of ERKs activated by phosphorylation, and these decreased as the cells entered the second phase, while the levels of p27/Kip1 increased at that time. Serum-starved or PD98059-treated HL60 cells had reduced growth rate and slower differentiation, but the G1 block also coincided with decreased levels of activated ERK1/2. The data suggest that the MEK/ERK pathway maintains cell proliferation during 1,25D3-induced monocytic differentiation of HL60 cells, but that ERK1/2 activity becomes suppressed during the later stages of differentiation, and the consequent G1 block leads to "terminal" differentiation.
Copyright 2001 Wiley-Liss, Inc.