Inhibition of the HDM-2 E3 Ligase Induces Types I and II Cell Death in Wild-Type and Mutant p53 Models of Multiple Myeloma, and Acts Synergistically in Both with ABT-737.

The ubiquitin-proteasome pathway has been validated as a therapeutic target for multiple myeloma by the incorporation of proteasome inhibitors such as bortezomib and carfilzomib into our standards of care against this disease. A minority of patients achieve complete remission with single-agent therapy, however, and proteasome inhibitors may induce on- and off-target anti-apoptotic effects and clinical toxicities that limit their utility. Unlike proteasome inhibitors, which have a broad impact on intracellular proteolysis, E3 ligases are responsible for ubiquitination of only a small subset of proteins. Our hypothesis proposed that inhibition of the Human double minute (HDM)-2 E3 ligase, which in part is responsible for p53 ubiquitination, could be a more rational therapeutic target for myeloma, where p53 mutations are less common than is the case in solid tumors.

We used Nutlin-3a and MI-63, which bind to the p53 binding pocket of HDM-2, as prototypic agents, and studied their activity against both wild-type (wt) and mutant (mut) p53 cell line models of multiple myeloma. These studies were supported by the M. D. Anderson Cancer Center SPORE in Multiple Myeloma.

Inhibition of HDM-2 in wt p53 (MM1.S, H929, MOLP-8) cells with Nutlin-3a or MI-63 reduced viability, with a median inhibitory concentration (IC₅₀) in the 0.5–2 μM range. In wt cells, genomic and proteomic studies showed that MI-63 induced transcription of downstream gene targets including p21, PUMA and Bax, and promoted accumulation of p53, p21, p27, and Bax. This was accompanied by cell cycle arrest at G₁, and induction of apoptosis as demonstrated by Annexin V staining and caspase activation. Interestingly, these agents also showed activity against mut p53 cells such as RPMI 8226 and U266, though with higher IC₅₀'s of 30–40 μM. Knockdown of HDM-2 sensitized mut p53 myeloma cells to MI-63, suggesting that this agent was inducing cell death in an on-target manner. In p53 mutant cells, MI-63 induced both type I cell death and type II, autophagic cell death, as determined by conversion of microtubule-associated protein 1 light chain 3, and activation of transcription of autophagy (ATG) genes ATG3 and 5. Since autophagy can be a mechanism of cell survival or cell death, we tested the impact of the autophagy inhibitors chloroquine and 3-methyladenine, and found that they reduced the impact of MI-63 on viability. Moreover, knockdown of ATG5 helped rescue cells as well, supporting the possibility that autophagic activation was a mechanism of cell death in these myeloma models. Synergistic anti-myeloma effects were seen when MI-63 was combined with the BH3 mimetic ABT-737, with decreased viability and enhanced apoptosis, accompanied by increased Bax activation and caspase cleavage. Notably, ABT-737 also enhanced the effects of MI-63 on mut p53 myeloma models, where increased levels of both type I and II programmed cell death were seen.

Together, our findings support the possibility that HDM-2 is a promising therapeutic target for multiple myeloma, and also provide a rationale for translation of HDM-2 inhibitors alone, and in combination with ABT-737, to the clinic.

Leopold:Ascenta Therapeutics: Employment, Equity Ownership.