MAX AND INHIBITORY C-MYC MUTANTS INDUCE ERYTHROID-DIFFERENTIATION ANDRESISTANCE 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 sta ble transfectants of K562 constitutively expressing two c-Myc inhibito ry mutants: D106-143, that carries a deletion in the transactivation d omain 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-My c, K562 cells can differentiate towards erythroid or myelomonocytic li neages, 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 phosp hatases inhibitor, The expression of D106-143 and In373 c-myc mutants and the overexpression of max reduced the apoptosis mediated by okadai c acid, The common biochemical activity of D106-143 and In373 is to bi nd Max and hence to titrate out c-Myc to form pen-functional Myc/Max M ax diners, Similarly, Max overexpression would decrease the relative l evels of c-Myc/Max with respect to Max/Max, The results support a mode l where a threshold of functional c-Myc/Max is required to maintain K5 62 cells in an undifferentiated state and to undergo drug-mediated apo ptosis.