Concomitant inhibition of MDM2 and Bcl-2 protein function synergistically induce mitochondrial apoptosis in AML.

Disruption of Mdm2-p53 interaction activates p53 signaling, disrupts the balance of antiapoptotic and proapoptotic Bcl-2 family proteins and induces apoptosis in acute myeloid leukemia (AML). Overexpression of Bcl-2 may inhibit this effect. Thus, functional inactivation of antiapoptotic Bcl-2 proteins may enhance apoptogenic effects of Mdm2 inhibition. We here investigate the potential therapeutic utility of combined targeting of Mdm2 by Nutlin-3a and Bcl-2 by ABT-737, recently developed inhibitors of protein-protein interactions. Nutlin-3a and ABT-737 induced Bax conformational change and mitochondrial apoptosis in AML cells in a strikingly synergistic fashion. Nutlin-3a induced p53-mediated apoptosis predominantly in S and G2/M cells, while cells in G1 were protected through induction of p21. In contrast, ABT-737 induced apoptosis predominantly in G1, the cell cycle phase with the lowest Bcl-2 protein levels and Bcl-2/Bax ratios. In addition, Bcl-2 phosphorylation on Ser70 was absent in G1 but detectable in G2/M, thus lower Bcl-2 levels and absence of Bcl-2 phosphorylation appeared to facilitate ABT-737-induced apoptosis of G1 cells. The complementary effects of Nutlin-3a and ABT-737 in different cell cycle phases could, in part, account for their synergistic activity. Our data suggest that combined targeting of Mdm2 and Bcl-2 proteins could offer considerable therapeutic promise in AML.