Abstract

Background: The ubiquitin-proteasome pathway has been validated as a target for non-Hodgkin lymphoma (NHL) with the recent approval of bortezomib for mantle cell lymphoma (MCL). In addition to anti-tumor activity, however, proteasome inhibitors have pleiotropic effects, including activation of an anti-apoptotic heat shock protein response, and their use clinically is complicated by toxicities such as peripheral neuropathy. By targeting E3 ubiquitin ligases, which are involved in ubiquitination of only a small subset of cellular proteins, it may be possible to achieve more specific anti-tumor effects with a better therapeutic index. One such attractive target is HDM-2, which is responsible for ubiquitination of the p53 tumor suppressor.

Methods: To evaluate the therapeutic potential of agents targeting HDM-2, we studied the impact of the small molecule MI-63, an inhibitor of the HDM-2-p53 interaction, in both p53 wild-type and -mutant cell line models.

Results: Treatment of wild-type p53 MCL, NHL, and acute lymphocytic leukemia (ALL) cell lines with MI-63 induced a dose- and time-dependent inhibition of proliferation, with an IC50 in the 1.0–5.0 μM range. This was associated with G1/S cell cycle arrest, and apoptosis mediated by caspases-3, 8 and 9. MI-63 induced accumulation and phosphorylation of p53 at serine 15 and 37, and also enhanced HDM-2 levels. Multiple p53 target genes were induced, including p21Cip1 and p53-upregulated modulator of apoptosis (PUMA), resulting in cleavage of poly-ADP-ribose-polymerase (PARP). MI-63 also decreased the levels of the ribonucleotide reductase subunit R2, and caused a corresponding increase in the R2p53 subunit. MI-63 also decreased the levels of E2F. Cell lines expressing certain p53 mutants were sensitive to the effects of MI-63, resulting in apoptosis. Cells without p53 expression were less sensitive to MI-63, but at higher drug concentrations proliferation was still inhibited, indicating a possible impact on HDM-2-mediated but p53-independent cell death pathways. Primary human umbilical cord vein endothelial cell growth was also inhibited and cells failed to recover after extended exposure to MI-63, whereas primary PBMC’s were unaffected by MI-63. Combinations of MI-63 with the molecularly targeted chemotherapy agents bortezomib and rapamycin were synergistic, with mean CI values of 0.88 and 0.6 respectively. The conventional chemotherapy agents doxorubicin and cisplatin were less effective at inducing synergism, with mean CI values of 1.06 and 0.9 respectively. Pretreatment of cells with MI-63 followed by chemotherapy was antagonistic with all agents used, while treatment with a chemotherapeutic first followed by MI-63 was additive to synergistic, indicating a sequence-dependent interaction.

Conclusions: Inhibition of the HDM-2-p53 interaction is a promising approach both by itself, and in combination with currently used chemotherapeutics, against lymphoid malignancies, providing a rationale for translation of such agents into the clinic.

Author notes

Disclosure: Employment: Authors Yang and Bruey-Sedano are employees of Ascenta Therapeutics, Inc.