Essential role of c-myc in ara-C-induced differentiation of human erythroleukemia cells.

The mode of action of the differentiation inducer 1-beta-D-arabinofuranosylcytosine (ara-C) is as yet unknown. We have analyzed the role of c-myc expression in ara-C-induced differentiation of K562 human erythroleukemia cells. Six hours after differentiation induction, c-myc expression is down-regulated transiently. This was followed by the onset of cell differentiation together with a final c-myc 'down-regulation' at 24 h after treatment. This regulation of expression may be caused by hypermethylation of c-myc DNA sequences, which we find to follow a similar pattern. We propose that the loss of c-myc expression might be a necessary prerequisite for ara-C-induced differentiation. This hypothesis was tested by transfecting a constitutive c-myc expression construct into K562 cells prior to ara-C treatment. By following the differentiation parameters cell morphology, benzidine staining (hemoglobin monitored), and alkaline phosphatase activity (presence of mature red blood cell antigen monitored) in cells expressing cotransfected LacZ marker gene, it was demonstrated that the introduction of the c-myc expression plasmid blocked ara-C-induced differentiation. We conclude that loss of c-myc expression mediated the differentiation found in ara-C-treated K562 cells.