A System for Monocytic Differentiation of Leukemic Cells HL 60 by a Short Exposure to 1,25-Dihydroxycholecalciferol

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Abstract
The human proomyelocytic cell line HL 60 was induced to differentiate toward more mature myeloid or monocytic forms by a variety of agents. This process required several days of exposure to the inducer, thus making it difficult to identify the early cellular changes which were fundamental to the differentiation program and to relate the induction to phases of the cell cycle. To study the kinetics of leukemic cell differentiation a system for the induction of rapid monocytic maturation was developed in a subpopulation of HL 60 cells. The cells were exposed to 10-7 M 1,25-dihydroxycholecalciferol for 4 h in serum-free medium. Subsequent incubation in a complete medium resulted in cellular differentiation recognizable by several criteria (phagocytosis, nonspecific esterase reaction, adherence to substratum, cell morphology) beginning at 10 h from the exposure to the inducer. Approximately 20 h later 30-40% of the cells in culture showed the differentiated phenotype and were capable of phagocytosis. The proportion differentiated cells in culture decreased thereafter. This system was utilized to study the expression of c-myc oncogene in relation to the kinetics of maturation and the inhibition of the expression of this gene preceded the onset of phenotypic differentiation by .apprx. 8 h was transient, and was accompanied by a brief retardation of cell proliferation, which resumed the normal rate within 24 h of the exposure to the inducer.