Current Cancer Drug Targets - Volume 5, Issue 1, 2005

It was with great pleasure that I agreed to be guest editor for this special issue of Current Cancer Drug Targets. The MDM2 oncogene is important for cancer initiation, progression and response to therapy, and new information about MDM2 is being uncovered every month. Since its discovery over thirteen years ago, MDM2 has become a widely-researched molecule, generating thousands of papers about its role in various stages of cancer. Although the p53-dependent pathways have been extensively investigated for many years, new information about the effects of this interaction continues to be revealed. More recently, the p53-independent activity of MDM2 has increasingly attracted more researchers. Because of the sheer amount of research detailing both the p53-dependent and -independent pathways, new reviews about MDM2 are both timely and important. I trust that you will find the reviews in this issue of Current Cancer Drug Targets to be as informative and thought provoking as I did. The topics addressed span the range of MDM2 research, from the historical perspective to the latest discoveries about splice variations, therapeutic targeting, and proteins interacting with MDM2. Bond, Hu and Levine illustrate in great detail what has been discovered about the p53-dependent effects of MDM2, and the p53 / MDM2 relationship. Although the existence of the auto-regulatory loop between MDM2 and p53 has been known for many years, new proteins depending on the interaction are still being discovered, and the complex relationship between p53 and MDM2 is finally becoming clear. Their review is an excellent introduction to the MDM2 field, from the viewpoint of the protein's earliest known function. On the other hand, the review by Zhang and Zhang discusses the many and diverse newly discovered p53-independent interactions of MDM2. These include a variety of proteins involved in signal transduction guiding the cell cycle and apoptosis, as well as other cellular processes. This article is sure to spark the interest of many readers, because MDM2 interacts with so many different proteins and is involved in nearly every important cellular pathway. The review by Harris encompasses the most up-to-date information on the alternative splicing of MDM2. Not only does MDM2 interact with a large number of proteins, there also exist splice variants of the protein, which may have other interactions, or different effects on the same proteins. The article by Rayburn, Zhang, He and Wang addresses the clinical relevance of MDM2 expression. Even before it was known that MDM2 had any effects on proteins other that p53, MDM2 was known to be an indicator for worse prognosis in cancer patients. This review describes the effects of MDM2 expression in various human cancers, and addresses the role of MDM2 in various stages of cancer development and progression, as well as therapeutic intervention. In this issue, three additional articles provide a comprehensive review on intervention approaches to inhibiting MDM2 functions. The review by Zhang, Wang and Agrawal provides a historic perspective and brief summary on antisense approaches to knocking down MDM2 expression, and the underlying molecular mechanisms behind the antisense technology. This approach has been shown to be effective in several in vitro and in vivo cancer models as a novel cancer therapy used alone or in combination with conventional chemotherapy and radiation therapy. The review by Tortora and colleagues focuses on the chemosensitization effects of this antisense oligonucleotide. Buolamwini and coauthors summarize the strategic approaches to developing small molecules as MDM2 inhibitors, which can serve as an example for rational drug design. In short, I hope that you will find the reviews contained within these pages to be interesting and helpful. I would like to thank Elizabeth Rayburn, who was instrumental in the editing and organization of the manuscripts, and thank the authors, whose contributions make this issue possible, and reviewers, whose critical comments and suggestions have significantly improved the quality of the articles.

Twelve years ago, the Mdm2 oncogene was shown to bind to and inhibit the tumor suppressor protein, p53. During the past 12 years, both genetic and biochemical studies have demonstrated that Mdm2 is a key negative regulator of the tumor suppressor p53. Mdm2 and p53 form an oscillating auto-regulatory feedback loop, which is tightly controlled to allow the appropriate response to environmental stresses in order to suppress tumor formation. When Mdm2 activity is inappropriately heightened, as it is in many human tumors, p53 activity is attenuated and tumor susceptibility arises. The p53 gene is mutated in 50% of all human tumors, but in those tumors that retain wild type p53, inhibiting Mdm2 activity could activate p53 tumor suppression and therefore provide a therapeutic strategy for the treatment of cancer.

The feed-back auto-regulatory loop between p53 and MDM2 has been extensively investigated. MDM2 is under the transcriptional control of p53, and MDM2 acts as a negative regulator of p53. There is increasing evidence, however, supporting the notion that MDM2 has activities independent of p53. In the absence of p53, MDM2 may retain its role in cell cycle control, differentiation, cell fate determination, DNA repair, transcription regulation, signal transduction of steroid receptors, cellular response to hypoxia, internalization of surface receptors, and other processes. MDM2 also has oncogenic transformational activities independent of p53. Moreover, anti-MDM2 antisense oligonucleotides have in vitro and in vivo antitumor activity and chemosensitizing and radiosensitizing effects in several human cancer models, regardless of their p53 status. In this article, the p53 independent activities of MDM2 and its interactions with various cellular proteins are considered. The studies reviewed provide a basis for developing novel MDM2 inhibitors as a therapy against human malignancies.

MDM2 splice variants have now been identified in many different tumor types, and their expression has been associated with advanced disease. However, published data concerning their function is contradictory, and therefore their role in tumorigenesis and their potential as a therapeutic target are unclear. Expression of a specific splice variant, MDM2-B, in a transgenic mouse model results in tumor development; and expression of several splice variants has been shown to enhance tumor formation in Em-myc transgenic mice. However, expression of similar variants in vitro results in growth inhibition, an observation inconsistent with a transformed phenotype. The observed growth inhibition is p53-dependent, resulting from the binding of splice variants with an intact C-terminal RING finger domain to full-length MDM2 protein. In doing so, p53 can no longer bind MDM2, and p53 activity is elevated. Subsequent inactivation of p53 or p53-mediated apoptosis could contribute to the MDM2 splice variant-mediated tumorigenesis observed in vivo. However, MDM2 splice variants, like full-length MDM2, probably display p53-independent activities. Therefore, the potential for MDM2 splice variants as therapeutic targets will be dependent upon their phenotype within specific tumor types.

The human homologue of the mouse double minute 2 (MDM2) oncogene is overexpressed in more than forty different types of malignancies, including solid tumors, sarcomas and leukemias. Because of its prevalent expression and its interactions with p53 and other signaling molecules, MDM2 plays a central role in cancer development and progression. The expression of this oncoprotein is being studied by researchers world-wide, and the amount of data published about it is increasing exponentially. Although there are some conflicting data about the effects of MDM2 expression in individual cancers, the overall evidence is convincing, indicating that increased MDM2 expression is related to a worse clinical prognosis. There is an increased likelihood of distant metastases, as well as a decreased response to therapeutic intervention in MDM2-positive cancers. MDM2 may also serve as a diagnostic marker, not only for cancer stage, but to differentiate between similar cancers. MDM2 may also be associated with drug resistance in cancer chemotherapy. These findings make studying the oncoprotein necessary to aid in our understanding of cancer development, to identify novel cancer drug targets, and to increase the efficacy of cancer therapy.

The MDM2 oncogene has been suggested as a novel target for cancer therapy, based on the following observations: 1) MDM2 is overexpressed in many human cancers, including breast, colon, and prostate cancer; 2) high MDM2 levels are associated with poor prognosis in patients with cancer; 3) MDM2 overexpression is associated with advanced cancer phenotypes such as metastatic tumors and hormone-independent tumors; 4) MDM2 overexpression is associated with tumor resistance to chemotherapy and radiation therapy; and 5) inhibiting MDM2 expression or function results in tumor growth inhibition and regression. There are many options for inhibiting MDM2 function, including the use of gene silencing technologies, antibodies, peptides and small molecules. Considering the complexity of MDM2 functions, we have chosen to use gene silencing technologies including antisense oligonucleotides and RNA interference. In this article, we summarize the investigation of the antisense technology for inhibiting MDM2 expression. Antisense mixed-backbone oligonucleotides (MBO) specifically inhibit MDM2 expression in a dose- and time-dependent manner, resulting in significant anti-tumor activity in vitro and in vivo. The MBO also potentiates the therapeutic effects of chemotherapeutic agents and radiation therapy in various tumors, through both p53-dependent and p53-independent mechanisms, indicating that MDM2 inhibitors have a broad spectrum of anti-tumor activity in human cancers, regardless of p53 status. These results provide a basis for clinical evaluation of antisense anti- MDM2 oligonucleotides as chemosensitizers and radiosensitizers. In addition, the MBO has been successfully used to identify novel functions of MDM2.

The MDM2 oncogene is overexpressed in many human cancers, including sarcomas, certain hematologic malignancies, and breast, colon and prostate cancers. The p53-MDM2 interaction pathway has been suggested as a novel target for cancer therapy. To that end, several strategies have been explored, including the use of small polypeptides targeted to the MDM2-p53 binding domain, anti-MDM2 antisense oligonucleotides, and natural agents. Different generations of anti-human-MDM2 oligonucleotides have been tested in in vitro and in vivo human cancer models, revealing specific inhibition of MDM2 expression and significant antitumor activity. Use of antisense oligos potentiated the effects of growth inhibition, p53 activation and p21 induction by several chemotherapeutic agents. Increased therapeutic effectiveness of chemotherapeutic drugs in human cancer cell lines carrying p53 mutations or deletions have shown the ability of MDM2 inhibitors to act as chemosensitizers in various types of tumors through both p53-dependent and p53-independent mechanisms. Inhibiting MDM2 appears to also have a role in radiation therapy for human cancer, regardless of p53 status, providing a rationale for the development of a new class of radiosensitizers. Moreover, MDM2 antisense oligonucleotides potentiate the effect of epidermal growth factor receptor (EGFR) inhibitors by affecting in vitro and in vivo proliferation, apoptosis and protein expression in hormonerefractory and hormone-dependent human prostate cancer cells. These data support the development, among other MDM2 inhibitors, of anti-MDM2 antisense oligonucleotides as a novel class of anticancer agents, and suggest a potentially relevant role for the oligonucleotides when integrated with conventional treatments and / or other signaling inhibitors in novel therapeutic strategies.

In this early phase of the new era of molecularly targeted patient friendly cancer chemotherapy, there is a need for novel viable anticancer molecular targets. The MDM2 oncoprotein has been validated as a potential target for cancer drug development. MDM2 amplification and / or overexpression occur in a wide variety of human cancers, several of which can be treated experimentally with MDM2 antagonists. MDM2 interacts primarily with the p53 tumor suppressor protein in an autoregulatory negative feedback loop to attenuate p53's cell cycle arrest and apoptosis functions. Inhibition of the p53-MDM2 interaction has been shown to cause selective cancer cell death, as well as sensitize cancer cells to chemotherapy or radiation effects. Consequently, this interaction has been the main focus of anticancer drug discovery targeted to MDM2. The promotion of the proteasomal degradation of the p53 protein by MDM2 is central to its repression of the tumor suppressor functions of p53, and many proteins impinge upon this activity, either enhancing or inhibiting it. MDM2 also has oncogenic activity independent of its interaction with p53, but this has so far not been explored for drug discovery. Among the approaches for targeting MDM2 for cancer therapy, small molecule antagonists have recently featured as effective anticancer agents in experimental models, although the repertoire is currently limited and none has yet entered human clinical trials. Small molecules that have been reported to disrupt the p53-MDM2 binding, thereby enhancing p53 activity to elicit anticancer effects include the following: synthetic chalcones, norbornane derivatives, cis-imidazoline derivatives (Nutlins), a pyrazolidinedione sulfonamide and 1,4-benzodiazepine-2,5-diones, as well as tryptophan derivatives. In addition to compounds disrupting p53pMDM2 binding, three compounds have been discovered that are effective in inhibiting the E3 ligase activity of MDM2 towards p53, and should serve as leads for drug discovery targeting this aspect of the p53-MDM2 interaction as well. These compounds were discovered from library screening and / or structure-based rational drug design strategies.