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-arabinofura nosylcytosine (ara-C) is as yet unknown. We have analyzed the role of c-myc expression in ara-C-induced differentiation of K562 human erythr oleukemia cells. Six hours after differentiation induction, c-myc expr ession is down-regulated transiently. This was followed by the onset o f cell differentiation together with a final c-myc 'down-regulation' a t 24 h after treatment. This regulation of expression may be caused by hypermethylation of c-myc DNA sequences, which we find to follow a si milar pattern. We propose that the loss of c-myc expression might be a necessary prerequisite for ara-C-induced differentiation. This hypoth esis was tested by transfecting a constitutive c-myc expression constr uct into K562 cells prior to ara-C treatment. By following the differe ntiation parameters cell morphology, benzidine staining (hemoglobin mo nitored), and alkaline phosphatase activity (presence of mature red bl ood cell antigen monitored) in cells expressing cotransfected LacZ mar ker gene, it was demonstrated that the introduction of the c-myc expre ssion plasmid blocked ara-C-induced differentiation. We conclude that loss of c-myc expression mediated the differentiation found in ara-C-t reated K562 cells.