Network perspectives on HDM2 inhibitor chemotherapy combinations

Spiro-oxindoles as a Promising Class of Small Molecule Inhibitors of p53-MDM2 Interaction Useful in Targeted Cancer Therapy

As a result of the toxicity of currently available anticancer drugs and the inefficiency of chemotherapeutic treatments, the design and discovery of effective and selective antitumor agents continues to be a hot topic in organic medicinal chemistry. Targeted therapy is a newer type of cancer treatment that uses drugs designed to interfere with specific molecules necessary for tumor growth and progression. This review explains the mechanism of regulation of p53 (tumor suppressor protein) by MDM2 and illustrates the role of targeting p53-MDM2 protein-protein interaction using small molecules as a new cancer therapeutic strategy. Spirocyclic oxindoles or spiro-oxindoles, with a rigid heterocyclic ring fused at the 3-position of the oxindole core with varied substitution around it, are the most efficacious class of small molecules which inhibit cell proliferation and induce apoptosis in cancer cells, leading to complete tumor growth regression without affecting activities of normal cells. In this review, we present a comprehensive account of the systematic development of and recent progress in diverse spiro-oxindole derivatives active as potent selective inhibitors of p53-MDM2 interaction with special emphasis on spiro-pyrrolidinyl oxindoles (the MI series), their mechanism of action, and structure-activity relationship. This review will help in understanding the molecular mechanism of p53 reactivation by spiro-oxindoles in tumor tissues and also facilitates the design and exploration of more potent analogues with high efficacy and low side effects for the treatment of cancer. Recent progress in spiro-oxindole derivatives as potent small molecule inhibitors of p53-MDM2 interaction, useful as anticancer agents, is described with reference to their mechanism of action and structure-activity relationship.

Targeting p53-MDM2-MDMX loop for cancer therapy

The tumor suppressor p53 plays a central role in anti-tumorigenesis and cancer therapy. It has been described as "the guardian of the genome", because it is essential for conserving genomic stability by preventing mutation, and its mutation and inactivation are highly related to all human cancers. Two important p53 regulators, MDM2 and MDMX, inactivate p53 by directly inhibiting its transcriptional activity and mediating its ubiquitination in a feedback fashion, as their genes are also the transcriptional targets of p53. On account of the importance of the p53-MDM2-MDMX loop in the initiation and development of wild type p53-containing tumors, intensive studies over the past decade have been aiming to identify small molecules or peptides that could specifically target individual protein molecules of this pathway for developing better anti-cancer therapeutics. In this chapter, we review the approaches for screening and discovering efficient and selective MDM2 inhibitors with emphasis on the most advanced synthetic small molecules that interfere with the p53-MDM2 interaction and are currently on Phase I clinical trials. Other therapeutically useful strategies targeting this loop, which potentially improve the prospects of cancer therapy and prevention, will also be discussed briefly.

Regulation of the HDM2-p53 pathway by ribosomal protein L6 in response to ribosomal stress

The HDM2-p53 loop is crucial for monitoring p53 level and human pathologies. Therefore, identification of novel molecules involved in this regulatory loop is necessary for understanding the dynamic regulation of p53 and treatment of human diseases. Here, we characterized that the ribosomal protein L6 binds to and suppresses the E3 ubiquitin ligase activity of HDM2, and subsequently attenuates HDM2-mediated p53 polyubiquitination and degradation. The enhanced p53 activity further slows down cell cycle progression and leads to cell growth inhibition. Conversely, the level of p53 is dramatically decreased upon the depletion of RPL6, indicating that RPL6 is essential for p53 stabilization. We also found that RPL6 translocalizes from the nucleolus to nucleoplasm under ribosomal stress, which facilitates its binding with HDM2. The interaction of RPL6 and HDM2 drives HDM2-mediated RPL6 polyubiquitination and proteasomal degradation. Longer treatment of actinomycin D increases RPL6 ubiquitination and destabilizes RPL6, and thereby putatively attenuates p53 response until the level of L6 subsides. Therefore, RPL6 and HDM2 form an autoregulatory feedback loop to monitor the level of p53 in response to ribosomal stress. Together, our study identifies the crucial function of RPL6 in regulating HDM2-p53 pathway, which highlights the importance of RPL6 in human genetic diseases and cancers.

HDM2 antagonist MI-219 (spiro-oxindole), but not Nutlin-3 (cis-imidazoline), regulates p53 through enhanced HDM2 autoubiquitination and degradation in human malignant B-cell lymphomas

Background

Lymphomas frequently retain wild-type (wt) p53 function but overexpress HDM2, thereby compromising p53 activity. Therefore, lymphoma is a suitable model for studying the therapeutic value of disrupting the HDM2-p53 interaction by small-molecule inhibitors (SMIs). HDM2 have been developed and are under various stages of preclinical and clinical investigation. Previously, we examined the anti-lymphoma activity of MI-319, the laboratory grade of a new class of HDM2 SMI, the spiro-oxindole, in follicular lymphoma. Since then, MI-219, the clinical grade has become readily available. This study further examines the preclinical effects and mechanisms of MI-219 in a panel of human lymphoma cell lines as well as a cohort of patient-derived B-lymphcytes for its potential clinical use.

Results

Preclinical assessment of MI-219 was evaluated by means of an in vitro and ex vivo approach and compared to Nutlin-3, the gold standard. Characterization of p53 activity and stability were assessed by quantitative PCR, Western blot, and immunoprecipitation. Biological outcome was measured using Trypan blue exclusion assay, Annexin V/PI, PARP and caspase-3 cleavage. Surprisingly, the overall biological effects of Nutlin-3 were more delayed (48 h) while MI-219 triggered an earlier response (12-24 h), predominantly in the form of apoptotic cell death. Using a cell free autoubiquitination assay, neither agent interfered with HDM2 E3 ligase function. MI-219 was more effective in upregulating wt-p53 stabilization compared to Nutlin-3. MI-219, but not Nutlin-3, enhanced the autoubiquitination and degradation of HDM2.

Conclusions

Our data reveals unexpected differences between MI-219 and the well-studied Nutlin-3 in lymphoma cell lines and patient samples. We suggest a novel mechanism for MI-219 that alters the functional activity of HDM2 through enhanced autoubiquitination and degradation. Additionally, this mechanism appears to correspond to biological outcome. Our results provide evidence that different classes of HDM2 SMIs elicit molecular events that extend beyond HDM2-p53 dissociation which may be of biological and potentially therapeutic importance.

Aberrant epigenetic grooming of miRNAs in pancreatic cancer: a systems biology perspective

Pancreatic cancer (PC) is a complex disease harboring a myriad of genetic and epigenetic changes. The dismal survival of patients diagnosed with PC is in part due to de novo and acquired resistance to conventional therapeutics, resulting from deregulated signaling including aberrant expression of small nc miRNAs. Emerging research in this area has lead to the identification and characterization of deregulated miRNAs, which have generated a renewed interest and hope in that novel targeting of miRNAs may lead to a better clinical outcome for patients diagnosed with PC. However, recent evidence suggests that miRNAs are also under a highly coordinated system of epigenetic regulation emphasizing the fact that the design of miRNAs as targeted therapy may not be as simple as originally anticipated. For a successful miRNA-based therapeutic regimen, a holistic integrated approach may be required to take into account because of these emerging epigenetic regulatory mechanisms. In this article, we will discuss miRNA epigenetics, it's significance in PC and the use of a systems science to identify these aberrant epigenetically groomed miRNAs, and we believe that such knowledge would likely benefit further research to realize the dream of miRNA-based targeted therapy for human malignancies.

Network modeling of MDM2 inhibitor-oxaliplatin combination reveals biological synergy in wt-p53 solid tumors

Earlier we had shown that the MDM2 inhibitor (MI-219) belonging to the spiro-oxindole family can synergistically enhance the efficacy of platinum chemotherapeutics leading to 50% tumor free survival in a genetically complex pancreatic ductal adenocarcinoma (PDAC) xenograft model. In this report, we have taken a systems and network modeling approach in order to understand central mechanisms behind MI219-oxaliplatin synergy with validation in PDAC, colon and breast cancer cell lines. Microarray profiling of drug treatments (MI-219, oxaliplatin or their combination) in capan-2 cells reveal a similar unique set of gene alterations that is duplicated in other solid tumor cells. As single agent, MI-219 or oxaliplatin induced alterations in 48 and 761 genes respectively. The combination treatment resulted in 767 gene alterations with emergence of 286 synergy unique genes. Ingenuity network modeling of combination and synergy unique genes showed the crucial role of five key local networks CREB, CARF, EGR1, NF-kB and E Cadherin. The network signatures were validated at the protein level in all three cell lines. Individually silencing central nodes in these five hubs resulted in abrogation of MI-219-oxaliplatin activity confirming their critical role in aiding p53 mediated apoptotic response. We anticipate that our MI219-oxaliplatin network blueprints can be clinically translated in the rationale design and application of this unique therapeutic combination in a genetically pre-defined subset of patients.