Max and inhibitory c-Myc mutants induce erythroid differentiation and resistance to apoptosis in human myeloid leukemia cells

We have used the human leukemia cell line K562 as a model to study the role of c-myc in differentiation and apoptosis, We have generated stable transfectants of K562 constitutively expressing two c-Myc inhibitory mutants: D106-143, that carries a deletion in the transactivation domain of the protein, and In373, that carries an insertion in the DNA-interacting region, We show here that In373 is able to compete with c-Myc for Max binding and to inhibit the transformation activity of c-Myc, K562 cells can differentiate towards erythroid or myelomonocytic lineages, K562 transfected with c-myc mutants showed a higher expression of erythroid differentiation markers, without any detectable effects in the myelomonocytic differentiation, We also transfected K562 cells with a zinc-inducible max gene, Ectopic Max overexpression resulted in an increased erythroid differentiation, thus reproducing the effects of c-myc inhibitory mutants, We also studied the role of c-myc mutants and max in apoptosis of K562 induced by okadaic acid, a protein phosphatases inhibitor, The expression of D106-143 and In373 c-myc mutants and the overexpression of max reduced the apoptosis mediated by okadaic acid, The common biochemical activity of D106-143 and In373 is to bind Max and hence to titrate out c-Myc to form pen-functional Myc/Max Max diners, Similarly, Max overexpression would decrease the relative levels of c-Myc/Max with respect to Max/Max, The results support a model where a threshold of functional c-Myc/Max is required to maintain K562 cells in an undifferentiated state and to undergo drug-mediated apoptosis.