Rescue of K562 cells from MDM2-modulated p53-dependent apoptosis by growthfactor-induced differentiation

The wild-type human MDM2 protooncogene was tested for its ability to modula te apoptotic activity of the de novo expressed p53 tumor suppressor gene in K562 cells. We also studied the role of some cytokines in this phenomenon. K562, a human myeloid leukemia cell Line, does not express p53 at the mRNA or protein level. In this study, we stably transfected K562 with eukaryoti c vectors containing either normal p53 cDNA (pC53-SN3) or mutated p53 (143( Val-->Ala)) cDNA (pC53-SCX3). Transfectants expressing WT p53 or those expr essing mutant p53 are called K562 SN and K562 SM respectively. Many leukemi c cell lines undergo apoptosis when de novo WT p53 is expressed alone. In c ontrast, while the resulting clones (K562 SN and K562 SM) expressed p53, th ey did not undergo apoptosis. However, when treated with MDM2 mRNA antisens e (MDM2 AS) oligodeoxynucleotides (ODNs), K562 SN demonstrated apoptotic fe atures at both molecular and morphological levels. No change was observed w hen the other clones (K562 and K562 SM) were treated with MDM2 AS. Apoptosi s induced in this manner was associated with a relatively small increase in intracellular calcium. [Ca2+](i). Cells cultured in medium previously supp lemented with recombinant human (rh) interleukin (IL)-3 and rh-erythropoiet in (Epo) did not undergo apoptosis. Moreover, K562 SN cells were induced to differentiate. This differentiation was evaluated by measuring hemoglobin (Hb) level in cellular extracted proteins and by analyzing erythroid colony number and morphology. High Hb synthesis was obtained when K562 SN cells w ere cultured with cytokines (IL-3 + Epo) combined with MDM2 AS. Our results are consistent with the hypothesis that the function of the proto-oncogene MDM2 is to provide a 'feedback' mechanism for the p53-dependent pathway of apoptosis that could be shunted toward differentiation. ((C) Elsevier, Par is).