Mdm2 is critically and continuously required to suppress lethal p53 activity in vivo

Therapeutic targeting of p53 by small molecules

Aberrant p53 function is one of the major requirements for tumor development. Reactivation of p53 function by small molecules is a promising strategy to combat cancer due to potent tumor suppressor activities of p53. Recent developments in p53 biology reveal that manipulation of p53 function might pave way to a long cancer-free life. A number of small molecules which rescue p53 function by different mechanisms, acting upstream of p53 or targeting the p53 protein itself have been identified. Notably, these molecules trigger different biological outcomes, suggesting that it might be feasible to direct p53-mediated response in a desired way. In this review I discuss the latest developments in the search for small molecules which rescue p53 function by targeting the p53 protein.

Mdm2-mediated ubiquitylation: p53 and beyond

The really interesting genes (RING)-finger-containing oncoprotein, Mdm2, is a promising drug target for cancer therapy. A key Mdm2 function is to promote ubiquitylation and proteasomal-dependent degradation of the tumor suppressor protein p53. Recent reports provide novel important insights into Mdm2-mediated regulation of p53 and how the physical and functional interactions between these two proteins are regulated. Moreover, a p53-independent role of Mdm2 has recently been confirmed by genetic data. These advances and their potential implications for the development of new cancer therapeutic strategies form the focus of this review.

The multiple levels of regulation by p53 ubiquitination

p53 is a central integrator of a plethora of signals and outputs these signals in the form of tumor suppression. It is well accepted that ubiquitination plays a major part in p53 regulation. Nonetheless, the molecular mechanisms by which p53 activity is controlled by ubiquitination are complex. Mdm2, a RING oncoprotein, was once thought to be the sole E3 ubiquitin ligase for p53, however recent studies have shown that p53 is stabilized but still degraded in the cells of Mdm2-null mice. Although the essential role of Mdm2 in p53 regulation is well established, there are an increasing number of other E3 ligases implicated in Mdm2-independent regulation of p53 by ubiquitination. The different types of ubiquitination on p53 by various E3 ligases have been linked to its differential effects on p53-mediated stress responses. In addition to proteasome-mediated degradation, ubiquitination of p53 acts as signals for degradation-independent functions, such as nuclear export. The function of ubiquitinated p53 varies in the nucleus and cytosol underlying the many potential contributions ubiquitinated p53 may have in promoting cell proliferation or death. Thus, p53 requires multiple layers of regulatory control to ensure correct temporal and spatial functions.

The p53 orchestra: Mdm2 and Mdmx set the tone

The activities of p53 cover diverse aspects of cell biology, including cell cycle control, apoptosis, metabolism, fertility, differentiation and cellular reprogramming. Although loss of p53 function engenders tumor susceptibility, hyperactivation of p53 is lethal. Therefore, p53 activity must be strictly regulated to maintain normal tissue homeostasis. Critical for the control of p53 function are its two main negative regulators: Mdm2 and Mdmx. Recent reports have provided insight into the complex mechanisms that regulate these two proteins and have revealed novel functions for each. Here, we review and evaluate models of Mdm2- and Mdmx-dependent regulation of p53 activity. Both Mdm2 and Mdmx receive input from numerous signaling pathways and interact with many proteins in addition to p53. Therefore, we also consider roles for Mdm2 and Mdmx in additional cancer-related networks, including Notch signaling and the epithelial-to-mesenchymal transition.

Inhibition of the transcriptional function of p53 by EWS-Fli1 chimeric protein in Ewing Family Tumors

The chromosomal translocation t(11;22)(q24;q12) generates the EWS-Fli1 fusion gene, which contributes to the development of Ewing Family Tumors (EFTs). Although p53 mutations are found only in 5-20% of EFTs, the p53 pathway is thought to be abrogated in EFTs. The role of EWS-Fli1 in the p53 pathway in the tumor is still poorly understood. In this study, using immunoprecipitation and co-localization, we show that EWS-Fli1 interacts with p53 within the nucleus in vivo. The introduction of EWS-Fli1 resulted in significant reduction of promoter activities and mRNA levels of p21 and mdm2, meanwhile it canceled p53-dependent growth suppression. In contrast, knockdown of EWS-Fli1 expression mediated by small interfering RNAs (siRNA) also augmented the induction of p21 and mdm2 in response to DNA damage. Furthermore, using serial deletion constructs of the EWS-Fli1 fusion protein, we determined that EWS-Fli1 binding to p53 as well as inhibition of p21 and mdm2 promoter activities was mediated by its N-terminal domain (amino acid residues 65-109). These observations suggest that the N-terminal region of EWS-Fli1 might associate with p53 and impair its transcriptional activity, subsequently inhibiting the expression of its downstream genes. These results might provide new insight into the oncogenesis of EFTs by EWS-Fli1 via the inhibition of p53 function.

Apoptosis in cancer: key molecular signaling pathways and therapy targets

Apoptosis is a physiological process vital for embryologic development and the maintenance of homeostasis in multicellular organisms, but it is also involved in a wide range of pathological processes, including cancer. In mammalian cells, apoptosis has been divided into two major pathways: the extrinsic pathway, activated by proapoptotic receptor signals at the cellular surface, and the intrinsic pathway, which involves the disruption of mitochondrial membrane integrity. Although many of the proteins vital for apoptosis have been identified, the molecular pathways of cellular death still remain to be elucidated. This review provides references concerning the apoptotic molecules, their interactions, the mechanisms involved in apoptosis resistance, and also the modulation of apoptosis for the treatment of cancer.

MK2 regulates the early stages of skin tumor promotion

The association between inflammation and tumorigenesis is well recognized. Mitogen-activated protein kinase-activated protein kinase-2 (MK2) is known to play a pivotal role in inflammatory processes. Here, we studied the effect of MK2-deficiency and tumor necrosis factor (TNF)-alpha-deficiency on skin tumor development in mice using the two-stage chemical carcinogenesis model. We found that MK2(-/-) mice developed significantly fewer skin tumors compared with both TNF-alpha(-/-) and wild-type mice when induced by initiation with 7,12-dimethylbenz[a]anthracene (DMBA) and by promotion with 12-O-tetradecanoylphorbol-13-acetate (TPA). The TPA-induced inflammatory response was reduced in both, TNF-alpha(-/-) mice and MK2(-/-) mice, but most pronounced in TNF-alpha(-/-) mice, indicating that a reduced inflammatory response was not the only explanation for the inhibited tumorigenesis seen in MK2(-/-) mice. Interestingly, increased numbers of apoptotic cells were detected in the epidermis of MK2(-/-) mice compared with TNF-alpha(-/-) and wild-type mice, suggesting an additional role of MK2 in the regulation of apoptosis. This was further supported by: (i) increased levels of the tumor suppressor protein p53 in MK2(-/-) mice after DMBA/TPA treatment compared with controls, (ii) reduced phosphorylation (activation) of the negative p53 regulator, murine double minute 2 in MK2(-)(/-) mouse keratinocytes in vitro and (iii) a significant decrease in the DMBA/TPA induced apoptosis in cultured MK2(-/-) keratinocytes transfected with p53 small interfering RNA. Taken together, these findings demonstrate a dual role of MK2 in the early stages of tumor promotion through regulation of both the inflammatory response and apoptosis of DNA-damaged cells. These results also identify MK2 as a putative target for future skin carcinoma therapy.

Ubiquitin Family Members in the Regulation of the Tumor Suppressor p53

It is commonly assumed that the p53 tumor suppressor pathway is deregulated in most if not all human cancers. Thus, the past two decades have witnessed intense efforts to identify and characterize the growth-suppressive properties of p53 as well as the proteins and mechanisms involved in regulating p53 activity. In retrospect, it may therefore not be surprising that p53 was one of the very first mammalian proteins that were identified as physiologically relevant substrate proteins of the ubiquitin-proteasome system. Since then, plenty of evidence has been accumulated that p53 is in part controlled by canonical (i.e., resulting in proteasome-mediated degradation) and noncanonical (i.e., nonproteolytic) ubiquitination and by modification with the ubiquitin family members SUMO-1 and NED 8. In this chapter, we will largely neglect the plethora of mechanisms that have been reported to be involved in the regulation of p53 ubiquitination but will focus on the enzymes and components of the respective conjugation systems that have been implicated in p53 modification and how the respective modifications (ubiquitin, SUMO-1, NED 8) may impinge on p53 activity.

New targets for the treatment of follicular lymphoma

The last two decades have witnessed striking advances in our understanding of the biological factors underlying the development of Follicular lymphoma (FL). Development of newer treatment approaches have improved the outlook for many individuals with these disorders; however, with these advances come new questions. Given the long-term survival of patients with FL, drugs with favourable side-effect profile and minimal long-term risks are desired. FL is incurable with current treatment modalities. It often runs an indolent course with multiple relapses and progressively shorter intervals of remission. The identification of new targets and development of novel targeted therapies is imperative to exploit the biology of FL while inherently preventing relapse and prolonging survival. This review summarizes the growing body of knowledge regarding novel therapeutic targets, enabling the concept of individualized targeted therapy for the treatment of FL.

Mouse models of p53 functions

Studies in mice have yielded invaluable insight into our understanding of the p53 pathway. Mouse models with activated p53, no p53, and mutant p53 have queried the role of p53 in development and tumorigenesis. In these models, p53 is activated and stabilized via redundant posttranslational modifications. On activation, p53 initiates two major responses: inhibition of proliferation (via cell-cycle arrest, quiescence, senescence, and differentiation) and induction of apoptosis. Importantly, these responses are cell-type and tumor-type-specific. The analysis of mutant p53 alleles has established a gain-of-function role for p53 mutants in metastasis. The development of additional models that can precisely time the oncogenic events in single cells will provide further insight into the evolution of tumors, the importance of the stroma, and the cooperating events that lead to disruption of the p53 pathway. Ultimately, these models should serve to study the effects of novel drugs on tumor response as well as normal homeostasis.

Targeting the ubiquitin-proteasome system to activate wild-type p53 for cancer therapy

Ubiquitination plays a key role in regulating the tumour suppressor p53. It targets p53 for degradation by the 26S proteasome. The ubiquitin pathway also regulates the activity and localisation of p53. Ubiquitination requires ubiquitin-activating and -conjugating enzymes and ubiquitin ligases. In addition, ubiquitination can be reversed by the action of deubiquitinating enzymes. Here we give an overview of the role of components of the ubiquitin-proteasome system in the regulation of p53 and review progress in targeting these proteins to activate wild-type p53 for the treatment of cancer.

The regulation of MDM2 by multisite phosphorylation--opportunities for molecular-based intervention to target tumours?

The p53 tumour suppressor is a tightly controlled transcription factor that coordinates a broad programme of gene expression in response to various cellular stresses leading to the outcomes of growth arrest, senescence, or apoptosis. MDM2 is an E3 ubiquitin ligase that plays a key role in maintaining p53 at critical physiological levels by targeting it for proteasome-mediated degradation. Expression of the MDM2 gene is p53-dependent and thus p53 and MDM2 operate within a negative feedback loop in which p53 controls the levels of its own regulator. Induction and activation of p53 involves mainly the uncoupling of p53 from its negative regulators, principally MDM2 and MDMX, an MDM2-related and -interacting protein that inhibits p53 transactivation function. MDM2 is tightly regulated through various mechanisms including gene expression, protein turnover (mediated by auto-ubiquitylation), protein-protein interaction with key regulators, and post-translational modification, mainly, but not exclusively, by multisite phosphorylation. The purpose of the present article is to review our current knowledge of the signalling mechanisms that focus on MDM2, and indeed MDMX, through both phosphorylation mechanisms and peptide-docking events and to consider the wider implications of these regulatory events in the context of coordinated regulation of the p53 response. This analysis also provides an opportunity to consider the signalling pathways regulating MDM2 as potential targets for non-genotoxic therapies aimed at restoring p53 function in tumour cells.

20 years studying p53 functions in genetically engineered mice

Cell and molecular biological studies of p53 functions over the past 30 years have been complemented in the past 20 years by studies that use genetically engineered mice. As expected, mice that have mutant Trp53 alleles usually develop cancers of various types more rapidly than their counterparts that have wild-type Trp53 genes. These mouse studies have been instrumental in providing important new insights into p53 tumour suppressor function. Such studies have been facilitated by the development of increasingly sophisticated genetic engineering approaches, which allow the more precise manipulation of p53 structure and function in a mammalian model.

p53--a Jack of all trades but master of none

Cancers are rare because their evolution is actively restrained by a range of tumour suppressors. Of these p53 seems unusually crucial as either it or its attendant upstream or downstream pathways are inactivated in virtually all cancers. p53 is an evolutionarily ancient coordinator of metazoan stress responses. Its role in tumour suppression is likely to be a relatively recent adaptation, which is only necessary when large, long-lived organisms acquired the sufficient size and somatic regenerative capacity to necessitate specific mechanisms to reign in rogue proliferating cells. However, such evolutionary reappropriation of this venerable transcription factor entails compromises that restrict its efficacy as a tumour suppressor.

Determination of nutlin-3a in murine plasma by liquid chromatography electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS)

A sensitive and precise LC-ESI-MS/MS method for determination of nutlin-3a in murine plasma using ketoconazole as an internal standard was developed and validated. Plasma nutlin-3a samples were prepared by either a simple protein precipitation (PP) for the high concentration range (10-20,000ng/mL) or by liquid-liquid extraction (LLE) for the low concentration range (0.25-300ng/mL). Nutlin-3a and ketoconazole were separated on a modified C18 analytical column (4microm, 75mmx2mm) with an isocratic mobile phase (acetonitrile/5mM HCOONH(4)=70/30, v/v). The retention times of nutlin-3a and ketoconazole were 1.14 and 1.45min. Detection was achieved by a tandem MS system, monitoring m/z 582/99 and m/z 532/82 for nutlin-3a and ketoconazole, respectively. The PP method was linear in a range of 10-20,000ng/mL (R(2)>or=0.993) and the LLE method was linear in a range of 0.25-300ng/mL (R(2)>or=0.992). The mean recoveries for PP and LLE were 24% and 78%, respectively. Within-day and between-day precisions were <or=4.5% for PP and were <or=4.9% for LLE. Within-day and between-day accuracies (% error) ranged from 4.8 to -7.9 for PP, and from -0.2 to -8.4 for LLE. The two extraction methods produced equivalent results, allowing use of both within the same study. This method has been applied to the measurement of nutlin-3a concentrations in murine plasma samples obtained from a preclinical pharmacokinetic study.

Posttranslational modification of p53: cooperative integrators of function

The p53 protein is modified by as many as 50 individual posttranslational modifications. Many of these occur in response to genotoxic or nongenotoxic stresses and show interdependence, such that one or more modifications can nucleate subsequent events. This interdependent nature suggests a pathway that operates through multiple cooperative events as opposed to distinct functions for individual, isolated modifications. This concept, supported by recent investigations, which provide exquisite detail as to how various modifications mediate precise protein-protein interactions in a cooperative manner, may explain why knockin mice expressing p53 proteins substituted at one or just a few sites of modification typically show only subtle effects on p53 function. The present article focuses on recent, exciting progress and develops the idea that the impact of modification on p53 function is achieved through collective and integrated events.

p53 and the regulation of hepatocyte apoptosis: implications for disease pathogenesis

The interplay between p53 and apoptosis in diseases such as cancer, neurodegeneration, ischemia and atherosclerosis underscores the need to understand the complexity of p53 networks. Here, we highlight recent studies of p53-induced apoptosis in human diseases, with a focus on the modulation of liver cell apoptosis. In addition, recent work has provided new insights into mechanisms underlying the antiapoptotic functions of the endogenous bile acid ursodeoxycholic acid (UDCA), suggesting that the finely tuned, complex control of p53 by Mdm2 is a key step in the UDCA modulation of deregulated, p53-triggered apoptosis. The effect of targeting cell death signaling proteins has been established in preclinical models of human diseases. Finally, we review recent therapeutic strategies and clinical applications of targeted agents, with a particular emphasis on the potential use of UDCA.

Tumour suppression by p53: a role for the DNA damage response?

Loss of p53 function occurs during the development of most, if not all, tumour types. This paves the way for genomic instability, tumour-associated changes in metabolism, insensitivity to apoptotic signals, invasiveness and motility. However, the nature of the causal link between early tumorigenic events and the induction of the p53-mediated checkpoints that constitute a barrier to tumour progression remains uncertain. This Review considers the role of the DNA damage response, which is activated during the early stages of tumour development, in mobilizing the tumour suppression function of p53. The relationship between these events and oncogene-induced p53 activation through the ARF pathway is also discussed.

Activation of p53 by Nutlin-3a, an antagonist of MDM2, induces apoptosis and cellular senescence in adult T-cell leukemia cells

It has been reported that the induction of cellular senescence through p53 activation is an effective strategy in tumor regression. Unfortunately, however, tumors including adult T-cell leukemia/lymphoma (ATL) have disadvantages such as p53 mutations and a lack of p16(INK4a) and/or p14(ARF). In this study we characterized Nutlin-3a-induced cell death in 16 leukemia/lymphoma cell lines. Eight cell lines, including six ATL-related cell lines, had wild-type p53 and Nutlin-3a-activated p53, and the cell lines underwent apoptosis or cell-cycle arrest, whereas eight cell lines with mutated p53 were resistant. Interestingly, senescence-associated-beta-galactosidase (SA-beta-gal) staining revealed that only ATL-related cell lines with wild-type p53 showed cellular senescence, although they lack both p16(INK4a) and p14(ARF). These results indicate that cellular senescence is an important event in p53-dependent cell death in ATL cells and is inducible without p16(INK4a) and p14(ARF). Furthermore, knockdown of Tp53-induced glycolysis and apoptosis regulator (TIGAR), a novel target gene of p53, by small interfering RNA(siRNA) indicated its important role in the induction of cellular senescence. As many patients with ATL carry wild-type p53, our study suggests that p53 activation by Nutlin-3a is a promising strategy in ATL. We also found synergism with a combination of Nutlin-3a and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), suggesting the application of Nutlin-3a-based therapy to be broader than expected.

Small-molecule inhibitor of USP7/HAUSP ubiquitin protease stabilizes and activates p53 in cells

Deregulation of the ubiquitin/proteasome system has been implicated in the pathogenesis of many human diseases, including cancer. Ubiquitin-specific proteases (USP) are cysteine proteases involved in the deubiquitination of protein substrates. Functional connections between USP7 and essential viral proteins and oncogenic pathways, such as the p53/Mdm2 and phosphatidylinositol 3-kinase/protein kinase B networks, strongly suggest that the targeting of USP7 with small-molecule inhibitors may be useful for the treatment of cancers and viral diseases. Using high-throughput screening, we have discovered HBX 41,108, a small-molecule compound that inhibits USP7 deubiquitinating activity with an IC(50) in the submicromolar range. Kinetics data indicate an uncompetitive reversible inhibition mechanism. HBX 41,108 was shown to affect USP7-mediated p53 deubiquitination in vitro and in cells. As RNA interference-mediated USP7 silencing in cancer cells, HBX 41,108 treatment stabilized p53, activated the transcription of a p53 target gene without inducing genotoxic stress, and inhibited cancer cell growth. Finally, HBX 41,108 induced p53-dependent apoptosis as shown in p53 wild-type and null isogenic cancer cell lines. We thus report the identification of the first lead-like inhibitor against USP7, providing a structural basis for the development of new anticancer drugs.

Blinded by the Light: The Growing Complexity of p53

While the tumor suppressor functions of p53 have long been recognized, the contribution of p53 to numerous other aspects of disease and normal life is only now being appreciated. This burgeoning range of responses to p53 is reflected by an increasing variety of mechanisms through which p53 can function, although the ability to activate transcription remains key to p53's modus operandi. Control of p53's transcriptional activity is crucial for determining which p53 response is activated, a decision we must understand if we are to exploit efficiently the next generation of drugs that selectively activate or inhibit p53.

HIPK2 regulation by MDM2 determines tumor cell response to the p53-reactivating drugs nutlin-3 and RITA

In the past few years, much effort has been devoted to show the single-target specificity of nongenotoxic, p53 reactivating compounds. However, the divergent biological responses induced by the different compounds, even in the same tumor cells, demand additional mechanistic insights, whose knowledge may lead to improved drug design or selection of the most potent drug combinations. To address the molecular mechanism underlying induction of mitotic arrest versus clinically more desirable apoptosis, we took advantage of two MDM2 antagonists, Nutlin-3 and RITA, which respectively produce these two outcomes. We show that, along with p53 reactivation, the proapoptotic p53-activator HIPK2 is degraded by MDM2 in Nutlin-3-treated cells, but activated by transiently reduced MDM2 levels in RITA-treated ones. Gain- and loss-of-function experiments revealed the functional significance of MDM2-mediated HIPK2 regulation in cell decision between mitotic arrest and apoptosis in both types of p53 reactivation. These data indicate that strategies of p53 reactivation by MDM2 inhibition should also take into consideration MDM2 targets other than p53, such as the apoptosis activator HIPK2.

Differential regulation of p53 and p21 by MKRN1 E3 ligase controls cell cycle arrest and apoptosis

Makorin Ring Finger Protein 1 (MKRN1) is a transcriptional co-regulator and an E3 ligase. Here, we show that MKRN1 simultaneously functions as a differentially negative regulator of p53 and p21. In normal conditions, MKRN1 could destabilize both p53 and p21 through ubiquitination and proteasome-dependent degradation. As a result, depletion of MKRN1 induced growth arrest through activation of p53 and p21. Interestingly, MKRN1 used earlier unknown sites, K291 and K292, for p53 ubiquitination and subsequent degradation. Under severe stress conditions, however, MKRN1 primarily induced the efficient degradation of p21. This regulatory process contributed to the acceleration of DNA damage-induced apoptosis by eliminating p21. MKRN1 depletion diminished adriamycin or ultraviolet-induced cell death, whereas ectopic expression of MKRN1 facilitated apoptosis. Furthermore, MKRN1 stable cell lines that constantly produced low levels of p53 and p21 exhibited stabilization of p53, but not p21, with increased cell death on DNA damage. Our results indicate that MKRN1 exhibits dual functions of keeping cells alive by suppressing p53 under normal conditions and stimulating cell death by repressing p21 under stress conditions.

Implications of endoplasmic reticulum stress, the unfolded protein response and apoptosis for molecular cancer therapy. Part I: targeting p53, Mdm2, GADD153/CHOP, GRP78/BiP and heat shock proteins

BACKGROUND

In eukaryotes, endoplasmic reticulum stress (ERS) and the unfolded protein response (UPR) are coordinately regulated to maintain steady-state levels and activities of various cellular proteins to ensure cell survival.

OBJECTIVE

This review (Part I of II) focuses on specific ERS and UPR signalling regulators, their expression in the cancer phenotype and apoptosis, and proposes how their implication in these processes can be rationalised into proteasome inhibition, apoptosis induction and the development of more efficacious targeted molecular cancer therapies.

METHOD

In this review, we contextualise many ERS and UPR client proteins that are deregulated or mutated in cancers and show links between ERS and the UPR, their implication in oncogenic transformation, tumour progression and escape from immune surveillance, apoptosis inhibition, angiogenesis, metastasis, acquired drug resistance and poor cancer prognosis.

CONCLUSION

Evasion of programmed cell death or apoptosis is a hallmark of cancer that enables tumour cells to proliferate uncontrollably. Successful eradication of cancer cells through targeting ERS- and UPR-associated proteins to induce apoptosis is currently being pursued as a central tenet of anticancer drug discovery.

Modes of p53 regulation

The traditional view of p53 activation includes three steps-p53 stabilization, DNA binding, and transcriptional activation. However, recent studies indicate that each step of p53 activation is more complex than originally anticipated. Moreover, both genetic studies in mice and in vitro studies with purified components suggest that the classical model may not be sufficient to explain all aspects of p53 activation in vivo. To reconcile these differences, we propose that antirepression, the release of p53 from repression by factors such as Mdm2 and MdmX, is a key step in the physiological activation of p53.

Modifications of p53: competing for the lysines

The p53 tumour suppressor protein is subject to numerous post-translational modifications, which coalesce in various combinations and patterns to regulate its activity. In addition to a multitude of phosphorylated serines and threonines, many of the lysine residues in p53 can be modified to regulate activity, stability and subcellular localization of the protein. This complexity is amplified by the variety of modifications that can target the same lysine residue - often with opposing effects on p53 function.

Functions of p53 in metabolism and invasion

The p53 protein is an important tumour suppressor that is inactivated in many human cancers. Understanding how p53 is regulated and the downstream consequences of p53 function is helping us to devise novel therapies based on the reactivation of p53. Such approaches may be useful in the treatment of cancer, but a growing understanding of a role for p53 in other conditions suggests that modulation of p53 may have broader applications.

Small-molecule inhibitors of the MDM2-p53 protein-protein interaction to reactivate p53 function: a novel approach for cancer therapy

Tumor suppressor p53 is an attractive cancer therapeutic target because it can be functionally activated to eradicate tumors. Direct gene alterations in p53 or interaction between p53 and MDM2 proteins are two alternative mechanisms for the inactivation of p53 function. Designing small molecules to block the MDM2-p53 interaction and reactivate the p53 function is a promising therapeutic strategy for the treatment of cancers retaining wild-type p53. This review will highlight recent advances in the design and development of small-molecule inhibitors of the MDM2-p53 interaction as new cancer therapies. A number of these small-molecule inhibitors, such as analogs of MI-219 and Nutlin-3, have progressed to advanced preclinical development or early phase clinical trials.

Targeting Mdm2 and Mdmx in cancer therapy: better living through medicinal chemistry?

Genomic and proteomic profiling of human tumor samples and tumor-derived cell lines are essential for the realization of personalized therapy in oncology. Identification of the changes required for tumor initiation or maintenance will likely provide new targets for small-molecule and biological therapeutics. For example, inactivation of the p53 tumor suppressor pathway occurs in most human cancers. Although this can be due to frank p53 gene mutation, almost half of all cancers retain the wild-type p53 allele, indicating that the pathway is disabled by other means. Alternate mechanisms include deletion or epigenetic inactivation of the p53-positive regulator arf, methylation of the p53 promoter, or elevated expression of the p53 regulators Mdm2 and Mdmx. This review discusses current models of p53 regulation by Mdm2 and Mdmx and presents the rationale for design of future Mdmx-specific therapeutics based on our knowledge of its structure and biological functions.

Targeting tumor cells expressing p53 with a water-soluble inhibitor of Hdm2

The tumor suppressor protein p53 is a potent inducer of apoptosis in transformed cells. Hdm2 is an ubiquitin ligase (E3) that acts as a major regulator of p53 by promoting its ubiquitylation and proteasomal degradation. For this reason, inhibiting the E3 activity of Hdm2 has been proposed as a therapeutic approach for cancers expressing wild-type p53. We previously identified a family of small molecules (HLI98s, 7-nitro-10-aryl-5-deazaflavins) that inhibit the E3 activity of Hdm2, increase cellular p53, and selectively kill transformed cells expressing wild-type p53. However, issues of both potency and solubility in aqueous solution limit the utility of the HLI98s. Here, we report that a highly soluble derivative of the HLI98s, which has a 5-dimethylaminopropylamino side chain but lacks the 10-aryl group (HLI373), has greater potency than the HLI98s in stabilizing Hdm2 and p53, activating p53-dependent transcription, and inducing cell death. Furthermore, we show that HLI373 is effective in inducing apoptosis of several tumor cells lines that are sensitive to DNA-damaging agents. These results suggest that HLI373 could serve as a potential lead for developing cancer therapeutics based on inhibition of the ubiquitin ligase activity of Hdm2.

Isoquinolin-1-one inhibitors of the MDM2-p53 interaction

p53 has been at the centre of attention for drug design since the discovery of its growth-suppressive and pro-apoptotic activity. Herein we report the design and characterisation of a new class of isoquinolinone inhibitors of the MDM2-p53 interaction. Our identification of druglike and selective inhibitors of this protein-protein interaction included a straightforward in silico compound-selection process, a recently reported NMR spectroscopic approach for studying the MDM2-p53 interaction, and selectivity screening assays using cells with the same genetic background. The selected inhibitors were all able to induce apoptosis and the expression of p53-related genes, but only the isoquinolin-1-one-based inhibitors stabilised p53. Our NMR experiments give a persuading explanation for these results, showing that isoquinolin-1-one derivates are able to dissociate the preformed MDM2-p53 complex in vitro, releasing a folded and soluble p53. The joint application of these methods provides a framework for the discovery of protein interaction inhibitors as a promising starting point for further drug design.

Targeting the MDM2-p53 interaction for cancer therapy

p53 is a powerful tumor suppressor and is an attractive cancer therapeutic target because it can be functionally activated to eradicate tumors. The gene encoding p53 protein is mutated or deleted in half of human cancers, which inactivates its tumor suppressor activity. In the remaining cancers with wild-type p53 status, its function is effectively inhibited through direct interaction with the human murine double minute 2 (MDM2) oncoprotein. Blocking the MDM2-p53 interaction to reactivate the p53 function is a promising cancer therapeutic strategy. This review will highlight the advances in the design and development of small-molecule inhibitors of the MDM2-p53 interaction as a cancer therapeutic approach.

p53 family in development

The p53 family network is a unique cellular processor that integrates information from various pathways and determines cellular choices between proliferation, replication arrest/repair, differentiation, senescence, or apoptosis. The most studied role of the p53 family is the regulation of stress response and tumor suppression. By removing damaged cells from the proliferating pool, p53 family members preserve the integrity of the genome. In addition to this well recognized role, recent data implicate the p53 protein family in a broader role of controlling cell proliferation, differentiation and death. Members of the p53 protein family with opposing activity perform coordination of these processes. Imbalance of p53 protein family may contribute to a significant proportion of congenital developmental abnormalities in humans.

The intestinal epithelium compensates for p53-mediated cell death and guarantees organismal survival

Mdm2 is the major inhibitor of the p53 tumor suppressor. Loss of Mdm2 in mice or in specific tissues of the mouse always yields p53-dependent lethal phenotypes. However, the role of Mdm2 in tissues with high turnover capacity is unknown. We have engineered mice lacking Mdm2 in the intestinal epithelium using the Cre/LoxP system. Loss of Mdm2 (Mdm2(intDelta)) results in viable animals, but neonates display multiple intestinal abnormalities such as hyperplasia, enterocyte vacuolization, and inflammation. These defects correlate with a drastic increase in p53-dependent apoptosis in highly proliferative and differentiated cells. Unexpectedly, the observed phenotypes disappear with age. The tissue selects against Mdm2-null cells and increases its proliferative capacity. Additionally, the intestinal stem and progenitor cell populations are enriched leading to an increase in crypt fission events. Enhanced proliferation is achieved by activation of the canonical Wnt and EGFR-mediated Ras/MAPK pathways. While Mdm2 is a critical inhibitor of p53 in the intestinal epithelium, the tissue employs a series of processes that compensate for cell death.

Does control of mutant p53 by Mdm2 complicate cancer therapy?

Missense mutant forms of p53 are expressed at high levels in some human cancers and may contribute to oncogenesis. In this issue of Genes & Development, Terzian and colleagues (pp. 1337-1344) describe a mutant p53 knock-in mouse in which normal tissues and some tumors have low levels of mutant p53 protein unless Mdm2 or p16(INK4A) are absent. Once stabilized, mutant p53 promotes metastasis. Therefore, therapies that release p53 from Mdm2 might have unwanted consequences when cells have sustained a mutation in p53.

Acetylation is indispensable for p53 activation

The activation of the tumor suppressor p53 facilitates the cellular response to genotoxic stress; however, the p53 response can only be executed if its interaction with its inhibitor Mdm2 is abolished. There have been conflicting reports on the question of whether p53 posttranslational modifications, such as phosphorylation or acetylation, are essential or only play a subtle, fine-tuning role in the p53 response. Thus, it remains unclear whether p53 modification is absolutely required for its activation. We have now identified all major acetylation sites of p53. Although unacetylated p53 retains its ability to induce the p53-Mdm2 feedback loop, loss of acetylation completely abolishes p53-dependent growth arrest and apoptosis. Notably, acetylation of p53 abrogates Mdm2-mediated repression by blocking the recruitment of Mdm2 to p53-responsive promoters, which leads to p53 activation independent of its phosphorylation status. Our study identifies p53 acetylation as an indispensable event that destabilizes the p53-Mdm2 interaction and enables the p53-mediated stress response.

Acetylation is indispensable for p53 activation

The activation of the tumor suppressor p53 facilitates the cellular response to genotoxic stress; however, the p53 response can only be executed if its interaction with its inhibitor Mdm2 is abolished. There have been conflicting reports on the question of whether p53 posttranslational modifications, such as phosphorylation or acetylation, are essential or only play a subtle, fine-tuning role in the p53 response. Thus, it remains unclear whether p53 modification is absolutely required for its activation. We have now identified all major acetylation sites of p53. Although unacetylated p53 retains its ability to induce the p53-Mdm2 feedback loop, loss of acetylation completely abolishes p53-dependent growth arrest and apoptosis. Notably, acetylation of p53 abrogates Mdm2-mediated repression by blocking the recruitment of Mdm2 to p53-responsive promoters, which leads to p53 activation independent of its phosphorylation status. Our study identifies p53 acetylation as an indispensable event that destabilizes the p53-Mdm2 interaction and enables the p53-mediated stress response.

Temporal activation of p53 by a specific MDM2 inhibitor is selectively toxic to tumors and leads to complete tumor growth inhibition

We have designed MI-219 as a potent, highly selective and orally active small-molecule inhibitor of the MDM2-p53 interaction. MI-219 binds to human MDM2 with a K(i) value of 5 nM and is 10,000-fold selective for MDM2 over MDMX. It disrupts the MDM2-p53 interaction and activates the p53 pathway in cells with wild-type p53, which leads to induction of cell cycle arrest in all cells and selective apoptosis in tumor cells. MI-219 stimulates rapid but transient p53 activation in established tumor xenograft tissues, resulting in inhibition of cell proliferation, induction of apoptosis, and complete tumor growth inhibition. MI-219 activates p53 in normal tissues with minimal p53 accumulation and is not toxic to animals. MI-219 warrants clinical investigation as a new agent for cancer treatment.

Hepatic IGFBP1 is a prosurvival factor that binds to BAK, protects the liver from apoptosis, and antagonizes the proapoptotic actions of p53 at mitochondria

Liver is generally refractory to apoptosis induced by the p53 tumor suppressor protein, but the molecular basis remains poorly understood. Here we show that p53 transcriptional activation leads to enhanced expression of hepatic IGFBP1 (insulin-like growth factor-binding protein-1). Exhibiting a previously unanticipated role, a portion of intracellular IGFBP1 protein localizes to mitochondria where it binds to the proapoptotic protein BAK and hinders BAK activation and apoptosis induction. Interestingly, in many cells and tissues p53 also has a direct apoptotic function at mitochondria that includes BAK binding and activation. When IGFBP1 is in a complex with BAK, formation of a proapoptotic p53/BAK complex and apoptosis induction are impaired, both in cultured cells and in liver. In contrast, livers of IGFBP1-deficient mice exhibit spontaneous apoptosis that is accompanied by p53 mitochondrial accumulation and evidence of BAK oligomerization. These data support the importance of BAK as a mediator of p53's mitochondrial function. The results also identify IGFBP1 as a negative regulator of the BAK-dependent pathway of apoptosis, whose expression integrates the transcriptional and mitochondrial functions of the p53 tumor suppressor protein.

Efficient p53 activation and apoptosis by simultaneous disruption of binding to MDM2 and MDMX

The p53 tumor suppressor plays a key role in protection against malignant transformation. MDM2 and MDMX are important regulators of the transcriptional activity and stability of p53 by binding to its NH(2) terminus. Recent studies suggest that inhibition of both MDM2 and MDMX is necessary for robust activation of p53 in certain tumor cells. However, small-molecule MDM2 inhibitors such as Nutlin fail to inhibit MDMX despite significant homology between the two proteins. The therapeutic efficacy of such compounds may be compromised by MDMX overexpression. To evaluate the feasibility and biological effects of simultaneously disrupting p53 binding to MDM2 and MDMX, we used phage display to identify a novel peptide that can inhibit p53 interactions with MDM2 (IC(50) = 10 nmol/L) and MDMX (IC(50) = 100 nmol/L). Expression of a scaffold protein (thioredoxin) displaying this peptide sequence by adenovirus disrupts both MDM2 and MDMX interaction with p53, resulting in efficient p53 activation, cell cycle arrest, and apoptosis of tumor cells overexpressing MDM2 and MDMX. Intratumoral injection of the adenovirus also induces growth suppression of tumor xenografts in mice in a p53-dependent fashion. These results show the therapeutic potential of targeting both MDM2 and MDMX in cancer, and provide a novel structural motif for the design of potent p53 activators.

Two faces of p53: aging and tumor suppression

The p53 tumor suppressor protein, often termed guardian of the genome, integrates diverse physiological signals in mammalian cells. In response to stress signals, perhaps the best studied of which is the response to DNA damage, p53 becomes functionally active and triggers either a transient cell cycle arrest, cell death (apoptosis) or permanent cell cycle arrest (cellular senescence). Both apoptosis and cellular senescence are potent tumor suppressor mechanisms that irreversibly prevent damaged cells from undergoing neoplastic transformation. However, both processes can also deplete renewable tissues of proliferation-competent progenitor or stem cells. Such depletion, in turn, can compromise the structure and function of tissues, which is a hallmark of aging. Moreover, whereas apoptotic cells are by definition eliminated from tissues, senescent cells can persist, acquire altered functions, and thus alter tissue microenvironments in ways that can promote both cancer and aging phenotypes. Recent evidence suggests that increased p53 activity can, at least under some circumstances, promote organismal aging. Here, we discuss the role of p53 as a key regulator of the DNA damage responses, and discuss how p53 integrates the outcome of the DNA damage response to optimally balance tumor suppression and longevity.

Shaping genetic alterations in human cancer: the p53 mutation paradigm

p53 mutations are found in 50% of human cancers. Molecular epidemiology has shown strong correlations between the spectrum of p53 mutations and exposure to exogenous carcinogens. This spectrum is influenced quantitatively and qualitatively by various upstream genetic filters that modulate carcinogen activation, detoxification, and/or DNA repair. In this review, we will discuss how other factors such as tissue specificity, SNP of genes associated with the p53 pathway, other genetic alterations, or p53 mutant heterogeneity can act as a second set of downstream filters that also have a profound impact on the spectrum of p53 mutations.

Targeted inactivation of Mdm2 RING finger E3 ubiquitin ligase activity in the mouse reveals mechanistic insights into p53 regulation

It is believed that Mdm2 suppresses p53 in two ways: transcriptional inhibition by direct binding, and degradation via its E3 ligase activity. To study these functions physiologically, we generated mice bearing a single-residue substitution (C462A) abolishing the E3 function without affecting p53 binding. Unexpectedly, homozygous mutant mice died before E7.5, and deletion of p53 rescued the lethality. Furthermore, reintroducing a switchable p53 by crossing with p53ER(TAM) mice surprisingly demonstrated that the mutant Mdm2(C462A) was rapidly degraded in a manner indistinguishable from that of the wild-type Mdm2. Hence, our data indicate that (1) the Mdm2-p53 physical interaction, without Mdm2-mediated p53 ubiquitination, cannot control p53 activity sufficiently to allow early mouse embryonic development, and (2) Mdm2's E3 function is not required for Mdm2 degradation.

p53 status dictates responses of B lymphomas to monotherapy with proteasome inhibitors

The proapoptotic function of p53 is thought to underlie most anticancer modalities and is also activated in response to oncogenic insults, such as overexpression of the Myc oncoprotein. Here we generated tractable B lymphomas using retroviral transduction of the MYC oncogene into hematopoietic cells with 2 knock-in alleles encoding a fusion between p53 and 4-hydroxytamoxifen (4OHT) receptor (p53ER(TAM)). In these polyclonal tumors, Myc is the only oncogenic lesion, and p53ER(TAM) status can be rapidly toggled between "off" and "on" with 4OHT, provided that the Trp53 promoter has been independently activated. Although 4OHT can trigger widespread apoptosis and overt tumor regression even in the absence of DNA-damaging agents, in tumors with high levels of Mdm2 these responses are blunted. However, cotreatment with proteasome inhibitors fully restores therapeutic effects in vivo. Similarly, human Burkitt lymphomas with wild-type p53 and overexpression of Hdm2 are highly sensitive to proteasome inhibitors, unless p53 levels are reduced using the HPV-E6 ubiquitin ligase. Therefore, proteasome inhibitors could be highly effective as a monotherapy against Myc-induced lymphomas, with no need for adjuvant chemotherapy or radiation therapy. On the other hand, their efficacy is crucially dependent on the wild-type p53 status of the tumor, placing important restrictions on patient selection.

Opportunities for improving the therapeutic ratio for patients with sarcoma

Sarcomas are mesenchymal cancers, which, in many cases, have distinctive molecular features. Limb-sparing surgery delivered at specialised sarcoma centres as part of a multidisciplinary approach has become the standard treatment for most patients and usually provides excellent local control. Preoperative treatment with chemotherapy is most common for patients with bone sarcomas. The ideal sequence of surgery and radiation for local management of soft-tissue sarcoma remains controversial on the basis of early versus late treatment complications, although preoperative radiation can provide the best results for improved long-term function. New methods for radiation delivery and tumour sensitisation might provide further improvements. However, metastatic disease is common, and conventional chemotherapy provides for only a narrow therapeutic window outside of a few responsive pathological subtypes. Targeting underlying molecular events in specific sarcomas can provide for dramatic benefits, as has been seen with imatinib treatment for gastrointestinal stromal tumours and dermatofibrosarcoma protuberans. Trials of agents targeting the cell cycle and angiogenesis in soft-tissue sarcomas, and of those targeting osteoclasts in bone sarcomas, are currently underway. Biological data and preclinical studies support trials using inhibitors of hedgehog signalling in chondrosarcoma, inhibitors of wnt/beta-catenin in osteosarcoma and aggressive fibromatosis, and inhibitors of histone deacetylases in synovial sarcoma and Ewing sarcoma. Pharmacogenetic approaches will be needed to identify individual determinants of response and outcome in order to maximise the benefits of targeting specific molecular events and keep side-effects to a minimum. Research in stem-cell biology and nanotechnology holds promise for additional novel treatment options in the future.

Loss of Mdm4 results in p53-dependent dilated cardiomyopathy

BACKGROUND

Although several loci for familial dilated cardiomyopathy (DCM) have been mapped, the origin of a large percentage of DCM remains unclear. Mdm2, a p53-negative regulator, protects cardiomyocytes from ischemic and reperfusion-induced cell death. Mdm4, a homolog of Mdm2, inhibits p53 activity in numerous cell types. It is unknown whether Mdm4 plays a role in the inhibition of p53 in fully differentiated tissues such as adult cardiomyocytes and whether this role is associated with DCM.

METHODS AND RESULTS

The conditional knockout of Mdm4 in the heart by use of cardiomyocyte-specific Cre (alphaMyHC-Cre) allele does not result in any developmental defects. With time, however, mice with deletion of Mdm4 in the adult heart developed DCM and had a median survival of 234 days. More interestingly, the onset of DCM occurs significantly earlier in male mice than in female mice, which mimics human DCM disease. DCM in Mdm4 mutant mice was caused by loss of cardiomyocytes by apoptosis, and it was p53-dose dependent.

CONCLUSION

Activity of p53 was inhibited by Mdm4 even in the fully differentiated cardiomyocyte. Elevated apoptosis mediated by the p53 pathway in cardiomyocytes may be a mechanism for DCM.

Dangerous habits of a security guard: the two faces of p53 as a drug target

Being most well-known tumor suppressor that is inactivated in tumors more frequently than any other gene, p53 has been recently recognized as a major player in a variety of pathologies caused by acute stresses of tissues that is responsible for massive cell loss from apoptosis. This created a controversial situation when effective treatment of acute pathology requires inhibition of a major cancer preventive factor that has been traditionally viewed as a target for therapeutic activation. Here we briefly review specific aspects of this problem and discuss the ways of its pharmacological resolution based on detailed knowledge of molecular mechanisms of p53 regulation and activity.

MDM2 and MDM4: p53 regulators as targets in anticancer therapy

The gene TP53, encoding transcription factor p53, is mutated or deleted in half of human cancers, demonstrating the crucial role of p53 in tumor suppression. Importantly, p53 inactivation in cancers can also result from the amplification/overexpression of its specific inhibitors MDM2 and MDM4 (also known as MDMX). The presence of wild-type p53 in those tumors with MDM2 or MDM4 overexpression stimulates the search for new therapeutic agents to selectively reactivate it. This short survey highlights recent insights into MDM2 and MDM4 regulatory functions and their implications for the design of future p53-based anticancer strategies. We now know that MDM2 and MDM4 inhibit p53 in distinct and complementary ways: MDM4 regulates p53 activity, while MDM2 mainly regulates p53 stability. Upon DNA damage, MDM2-dependent degradation of itself and MDM4 contribute significantly to p53 stabilization and activation. These and other data imply that the combined use of MDM2 and MDM4 antagonists in cancer cells expressing wild-type p53 should activate p53 more significantly than agents that only antagonize MDM2, resulting in more effective anti-tumor activity.

Classic and novel roles of p53: prospects for anticancer therapy

The tumor suppressor p53 is a transcription factor that is frequently inactivated in human tumors. Therefore, restoring its function has been considered an attractive approach to restrain cancer. Typically, p53-dependent growth arrest, senescence and apoptosis of tumor cells have been attributed to transcriptional activity of nuclear p53. Notably, wild-type p53 gain-of-function enhances cancer resistance in the mouse, but it also accelerates aging in some models, possibly due to altered p53 activity. Therefore, the emerging evidence of mitochondrial transcription-independent activities of p53 has raised high expectations. Here, we review new developments in transcription-dependent and transcription-independent p53 functions, recent advances in targeting p53 for cancer treatment and the pitfalls of moving from the laboratory research to the clinical setting.