Awakening guardian angels: drugging the p53 pathway

Counteracting MDM2-induced HIPK2 downregulation restores HIPK2/p53 apoptotic signaling in cancer cells

Homeodomain-interacting protein kinase-2 (HIPK2) is a crucial regulator of p53 apoptotic function by phosphorylating serine 46 (Ser46) in response to DNA damage. In tumors with wild-type p53, its tumor suppressor function is often impaired by MDM2 overexpression that targets p53 for proteasomal degradation. Likewise, MDM2 targets HIPK2 for protein degradation impairing p53-apoptotic function. Here we report that zinc antagonised MDM2-induced HIPK2 degradation as well as p53 ubiquitination. The zinc inhibitory effect on MDM2 activity leads to HIPK2-induced p53Ser46 phosphorylation and p53 pro-apoptotic transcriptional activity. These results suggest that zinc derivatives are potential molecules to target the MDM2-induced HIPK2/p53 inhibition.

Oncogene-induced senescence: the bright and dark side of the response

In late 1990s, it was shown that activated oncogenes are able to induce senescence. Since then large leaps in understanding this phenomenon have been achieved. There is substantial evidence supporting oncogene-induced senescence (OIS) as a potent antitumor barrier in vivo. Multiple pathways participating in cell cycle regulation, DNA damage signaling, immune response, and bioenergetics regulate the process. Despite its beneficial effects the senescent cell is thought to promote carcinogenesis and age-related disease in a nonautonomous manner. Here, we highlight the works dealing with all these aspects and discuss the studies proposing therapeutic exploitation of OIS.

MI-219-zinc combination: a new paradigm in MDM2 inhibitor-based therapy

Zinc plays a crucial role in the biology of p53 in that p53 binds to DNA through a structurally complex domain stabilized by zinc atom. The p53 negative regulator MDM2 protein also carries a C-terminal RING domain that coordinates two zinc atoms which are responsible for p53 nuclear export and proteasomal degradation. In this clinically translatable study, we explored the critical role of zinc on p53 re-activation by MDM2-inhibitor MI-219 in colon and breast cancer cells. ZnCl₂ enhanced MI-219 activity (MTT, apoptosis and colony formation), and chelation of zinc not only blocked the activity of MI-219, it also suppressed re-activation of the p53 and its downstream effector molecules p21^WAF1 and Bax. TPEN, a specific zinc chelator but not Bapta-AM, a calcium chelator, blocked MI-219-induced apoptosis. Nuclear localization is a pre-requisite for proper functioning of p53 and our results confirm that TPEN and not Bapta-AM could abrogate p53 nuclear localization and interfered with p53 transcriptional activation. Addition of zinc suppressed the known p53 feedback MDM2 activation which could be restored by TPEN. Co-immunoprecipitation studies verified that MI-219-mediated MDM2-p53 disruption could be suppressed by TPEN and restored by zinc. As such, single agent therapies that target MDM2 inhibition, without supplemental zinc, may not be optimal in certain patients due to the less recognized mild zinc deficiency among the “at risk population” as in the elderly which are more prone to cancers. Therefore, use of supplemental zinc with MI-219 will benefit the overall efficacy of MDM2 inhibitors and this potent combination warrants further investigation.

Disparate chromatin landscapes and kinetics of inactivation impact differential regulation of p53 target genes

The p53 transcription factor regulates the expression of genes involved in cellular responses to stress, including cell cycle arrest and apoptosis. The p53 transcriptional program is extremely malleable, with target gene expression varying in a stress- and cell type-specific fashion. The molecular mechanisms underlying differential p53 target gene expression remain elusive. Here we provide evidence for gene-specific mechanisms affecting expression of three important p53 target genes. First we show that transcription of the apoptotic gene PUMA is regulated through intragenic chromatin boundaries, as revealed by distinct histone modification territories that correlate with binding of the insulator factors CTCF, Cohesins and USF1/2. Interestingly, this mode of regulation produces an evolutionary conserved long non-coding RNA of unknown function. Second, we demonstrate that the kinetics of transcriptional competence of the cell cycle arrest gene p21 and the apoptotic gene FAS are markedly different in vivo, as predicted by recent biochemical dissection of their core promoter elements in vitro. After a pulse of p53 activity in cells, assembly of the transcriptional apparatus on p21 is rapidly reversed, while FAS transcriptional activation is more sustained. Collectively these data add to a growing list of p53-autonomous mechanisms that impact differential regulation of p53 target genes.

Oncogenes and tumor suppressor genes

Breast cancer progression involves multiple genetic events, which can activate dominant-acting oncogenes and disrupt the function of specific tumor suppressor genes. This article describes several key oncogene and tumor suppressor signaling networks that have been implicated in breast cancer progression. Among the tumor suppressors, the article emphasizes BRCA1/2 and p53 tumor suppressors. In addition to these well characterized tumor suppressors, the article highlights the importance of PTEN tumor suppressor in counteracting PI3K signaling from activated oncogenes such as ErbB2. This article discusses the use of mouse models of human breast that recapitulate the key genetic events involved in the initiation and progression of breast cancer. Finally, the therapeutic potential of targeting these key tumor suppressor and oncogene signaling networks is discussed.

Reactivation of p53 by novel MDM2 inhibitors: implications for pancreatic cancer therapy

The present study is the first to show in pancreatic cancer (PC) the growth inhibition and apoptosis by novel MDM2 inhibitors (MI-319 & 219) through reactivation of p53 pathway. Our results highlight two new secondary targets of MDM2 inhibitor 'SIRT1' and Ku70. SIRT1 which has a role in ageing and cancer and is known to regulate p53 signaling through acetylation. Ku70 is a key component of non-homologous end joining machinery in the DNA damage pathway and is known to regulate apoptosis by blocking Bax entry into mitochondria. Growth inhibition and apoptosis by MI-219, MI-319 was accompanied by increase in levels of p53 along with p21(WAF1) and the proapoptotic Puma. SiRNA against p21(WAF1) abrogated the growth inhibition of PC cells confirming p21(WAF1) as a key player downstream of activated p53. Immunoprecipitation-western blot analysis revealed reduced association of MDM2-p53 interaction in drug exposed PC cells. In combination studies, the inhibitors synergistically augmented anti-tumor effects of therapeutic drug gemcitabine both in terms of cell growth inhibition as well as apoptosis. Surface plasmon resonance studies confirmed strong binding between MI-319 and Ku70 (K(D) 170 nM). Western blot revealed suppression of SIRT1 and Ku70 with simultaneous upregulation of acetyl-p53 (Lys379) and Bax. Co-Immunoprecipitation studies confirmed that MI-319 could disrupt Ku70-Bax and SIRT1-Bax interaction. Further, using wt-p53 xenograft of Capan-2, we found that oral administration of MI-319 at 300 mg/kg for 14 days resulted in significant tumor growth inhibition without any observed toxicity to the animals. No tumor inhibition was found in mut-p53 BxPC-3 xenografts. In light of our results, the inhibitors of MDM2 warrant clinical investigation as new agents for PC treatment.

Dysregulation of microRNAs in cancer: playing with fire

MicroRNAs have emerged as key post-transcriptional regulators of gene expression, involved in various physiological and pathological processes. It was found that several miRNAs are directly involved in human cancers, including lung, breast, brain, liver, colon cancer and leukemia. In addition, some miRNAs may function as oncogenes or tumor suppressors in tumor development. Furthermore, a widespread down-regulation of miRNAs is commonly observed in human cancers and promotes cellular transformation and tumorigenesis. More than 50% of miRNA genes are located in cancer-associated genomic regions or in fragile sites, frequently amplified or deleted in human cancer, suggesting an important role in malignant transformation. A better understanding of the miRNA regulation and misexpression in cancer may ultimately yield further insight into the molecular mechanisms of tumorigenesis and new therapeutic strategies may arise against cancer. Here, we discuss the occurrence of the deregulated expression of miRNAs in human cancers and their importance in the tumorigenic process.

Selectivity of oncolytic viral replication prevents antiviral immune response and toxicity, but does not improve antitumoral immunity

Oncolytic infection elicits antitumoral immunity, but the impact of tumor-selective replication on the balance between antiviral and antitumoral immune responses has not yet been investigated. To address this question, we constructed the highly tumor-selective adenovirus Ad-p53T whose replication in target tumor cells is governed by aberrant telomerase activity and transcriptional p53 dysfunction. Telomerase-dependent or nonselective adenoviruses were constructed as isogenic controls. Following infection of mice with the nonselective adenovirus, viral DNA and mRNA levels correlated with strong stimulation of innate immune response genes and severe liver toxicity, whereas telomerase-/p53-specific replication did not trigger innate immunity and prevented liver damage. Compared to telomerase-dependent or unselective viral replication, telomerase-/p53-specific virotherapy significantly decreased antiviral CD8-specific immune responses and antiviral cytotoxicity in vivo. Consistent with our hypothesis, telomerase-selective replication led to intermediate results in these experiments. Remarkably, all viruses efficiently lysed tumors and induced a therapeutically effective tumor-directed CD8 cytotoxicity. In immunocompetent mice with extended lung metastases burden, treatment of subcutaneous primary tumors with Ad-p53T significantly prolonged survival by inhibition of lung metastases, whereas unselective viral replication resulted in death by liver failure. In summary, the degree of tumor selectivity of viral replication marginally influences antitumoral immune responses, but is a major determinant of antivector immunity and systemic toxicity.

Tracing the protectors path from the germ line to the genome

One of the basic principles that nature uses in evolution is to recycle successful concepts and create new functions by modifying existing units. This conservatism in evolution has resulted in an astonishingly high sequence identity of genes, even between evolutionarily distant species such as the nematode Caenorhabditis elegans and Homo sapiens. The recycling of successful concepts in conjunction with gene duplication events has also led to the existence of highly homologous proteins within the genome of many species. Often, these homologous proteins show similar, yet distinct functions that, in combination with their individual tissue distribution, define their specific physiological role. One prominent example is the p53 protein family, which consists of p53, p63, and p73. Recent advances in understanding the specific biological functions of these members have shed some light onto the evolution of this crucial protein family, from a germ line-specific quality-control factor to a somatic tumor suppressor. Furthermore, structures of the oligomerization domains of the mammalian paralogs, p53 and p73, and invertebrate orthologs, CEP-1 and DMP53, have delineated evolutionary changes and revealed that the oligomerization domain of p53 lacks additional stabilizing structural elements present in all other p53 family members. This suggests that p53 is the most recent evolutionary member of this protein family and predicts a mechanism for p53 activation.

Histone deacetylase inhibitors enhance the anticancer activity of nutlin-3 and induce p53 hyperacetylation and downregulation of MDM2 and MDM4 gene expression

Nutlin-3, a small-molecule MDM2 inhibitor, restores p53 function and is, thus, an appealing candidate for the treatment of cancers retaining wild-type p53. However, nutlin-3 applied as single agent may be insufficient for cancer therapy. Therefore, we explored whether the anticancer activity of nutlin-3 could be enhanced by combination with histone deacetylase inhibitors (HDACi), i.e. vorinostat, sodium butyrate, MS-275 and apicidin. We found that nutlin-3 and HDACi cooperated to induce cell death in the p53 wild-type cell lines A549 and A2780, but not in the p53 null cell line PC-3, as assessed by Alamar Blue assay and flow cytometric analyses of propidium iodide uptake and mitochondrial depolarization. Combination index analysis showed that the effect was synergistic. For comparison, we tested nutlin-3 in combination with paclitaxel, revealing that nutlin-3 antagonized the cytotoxic activity of paclitaxel. To shed light on the underlying mechanism of the synergistic action of nutlin-3 and HDACi, we determined the acetylation status of p53 by immunoblotting and the mRNA levels of MDM2 and MDM4 by real-time RT-PCR. We observed vorinostat to induce p53 hyperacetylation, to reduce the constitutive gene expression of MDM2 and MDM4, and to counteract the nutlin-3-induced upregulation of MDM2 gene expression. In conclusion, our study shows that HDACi amplify the antitumor activity of nutlin-3-possibly by inducing p53 hyperacetylation and/or MDM2 and/or MDM4 downregulation-suggesting that treatment with a combination of nutlin-3 and HDACi may be an effective strategy for treating tumors with wild-type p53.

Cdk5 phosphorylates non-genotoxically overexpressed p53 following inhibition of PP2A to induce cell cycle arrest/apoptosis and inhibits tumor progression

Background

p53 is the most studied tumor suppressor and its overexpression may or may not cause cell death depending upon the genetic background of the cells. p53 is degraded by human papillomavirus (HPV) E6 protein in cervical carcinoma. Several stress activated kinases are known to phosphorylate p53 and, among them cyclin dependent kinase 5 (Cdk5) is one of the kinase studied in neuronal cell system. Recently, the involvement of Cdk5 in phosphorylating p53 has been shown in certain cancer types. Phosphorylation at specific serine residues in p53 is essential for it to cause cell growth inhibition. Activation of p53 under non stress conditions is poorly understood. Therefore, the activation of p53 and detection of upstream kinases that phosphorylate non-genotoxically overexpressed p53 will be of therapeutic importance for cancer treatment.

Results

To determine the non-genotoxic effect of p53; Tet-On system was utilized and p53 inducible HPV-positive HeLa cells were developed. p53 overexpression in HPV-positive cells did not induce cell cycle arrest or apoptosis. However, we demonstrate that overexpressed p53 can be activated to upregulate p21 and Bax which causes G2 arrest and apoptosis, by inhibiting protein phosphatase 2A. Additionally, we report that the upstream kinase cyclin dependent kinase 5 interacts with p53 to phosphorylate it at Serine20 and Serine46 residues thereby promoting its recruitment on p21 and bax promoters. Upregulation and translocation of Bax causes apoptosis through intrinsic mitochondrial pathway. Interestingly, overexpressed activated p53 specifically inhibits cell-growth and causes regression in vivo tumor growth as well.

Conclusion

Present study details the mechanism of activation of p53 and puts forth the possibility of p53 gene therapy to work in HPV positive cervical carcinoma.

Deconstructing nucleotide binding activity of the Mdm2 RING domain

Mdm2, a central negative regulator of the p53 tumor suppressor, possesses a Really Interesting New Gene (RING) domain within its C-terminus. In addition to E3 ubiquitin ligase activity, the Mdm2 RING preferentially binds adenine base nucleotides, and such binding leads to a conformational change in the Mdm2 C-terminus. Here, we present further biochemical analysis of the nucleotide–Mdm2 interaction. We have found that MdmX, an Mdm2 family member with high sequence homology, binds adenine nucleotides with similar affinity and specificity as Mdm2, suggesting that residues involved in nucleotide binding may be conserved between the two proteins and adenosine triphosphate (ATP) binding may have similar functional consequences for both Mdm family members. By generating and testing a series of proteins with deletions and substitution mutations within the Mdm2 RING, we mapped the specific adenine nucleotide binding region of Mdm2 to residues 429–484, encompassing the minimal RING domain. Using a series of ATP derivatives, we demonstrate that phosphate coordination by the Mdm2 P-loop contributes to, but is not primarily responsible for, ATP binding. Additionally, we have identified the 2′ and 3′ hydroxyls of the ribose and the C6 amino group of the adenine base moiety as being essential for binding.

D-peptide inhibitors of the p53-MDM2 interaction for targeted molecular therapy of malignant neoplasms

The oncoproteins MDM2 and MDMX negatively regulate the activity and stability of the tumor suppressor protein p53, conferring tumor development and survival. Antagonists targeting the p53-binding domains of MDM2 and MDMX kill tumor cells both in vitro and in vivo by reactivating the p53 pathway, promising a class of antitumor agents for cancer therapy. Aided by native chemical ligation and mirror image phage display, we recently identified a D-peptide inhibitor of the p53-MDM2 interaction termed (D)PMI-alpha (TNWYANLEKLLR) that competes with p53 for MDM2 binding at an affinity of 219 nM. Increased selection stringency resulted in a distinct D-peptide inhibitor termed (D)PMI-gamma (DWWPLAFEALLR) that binds MDM2 at an affinity of 53 nM. Structural studies coupled with mutational analysis verified the mode of action of these D-peptides as MDM2-dependent p53 activators. Despite being resistant to proteolysis, both (D)PMI-alpha and (D)PMI-gamma failed to actively traverse the cell membrane and, when conjugated to a cationic cell-penetrating peptide, were indiscriminately cytotoxic independently of p53 status. When encapsulated in liposomes decorated with an integrin-targeting cyclic-RGD peptide, however, (D)PMI-alpha exerted potent p53-dependent growth inhibitory activity against human glioblastoma in cell cultures and nude mouse xenograft models. Our findings validate D-peptide antagonists of MDM2 as a class of p53 activators for targeted molecular therapy of malignant neoplasms harboring WT p53 and elevated levels of MDM2.

Drug discovery and mutant p53

Missense mutations in the p53 gene are commonly selected for in developing human cancer cells. These diverse mutations in p53 can inactivate its normal sequence-specific DNA-binding and transactivation function, but these mutations can also stabilize a mutant form of p53 with pro-oncogenic potential. Recent multi-disciplinary advances have demonstrated exciting and unexpected potential in therapeutically targeting the mutant p53 pathway, including: the development of biophysical models to explain how mutations inactivate p53 and strategies for refolding and reactivation of mutant p53, the ability of mutant p53 protein to escape MDM2-mediated degradation in human cancers, and the growing 'interactome' of mutant p53 that begins to explain how the mutant p53 protein can contribute to diverse oncogenic and pro-metastatic signaling. Our rapidly accumulating knowledge on mutant p53-signaling pathways will facilitate drug discovery programmes in the challenging area of protein-protein interactions and mutant protein conformational control.

p53 loss promotes acute myeloid leukemia by enabling aberrant self-renewal

The p53 tumor suppressor limits proliferation in response to cellular stress through several mechanisms. Here, we test whether the recently described ability of p53 to limit stem cell self-renewal suppresses tumorigenesis in acute myeloid leukemia (AML), an aggressive cancer in which p53 mutations are associated with drug resistance and adverse outcome. Our approach combined mosaic mouse models, Cre-lox technology, and in vivo RNAi to disable p53 and simultaneously activate endogenous Kras(G12D)-a common AML lesion that promotes proliferation but not self-renewal. We show that p53 inactivation strongly cooperates with oncogenic Kras(G12D) to induce aggressive AML, while both lesions on their own induce T-cell malignancies with long latency. This synergy is based on a pivotal role of p53 in limiting aberrant self-renewal of myeloid progenitor cells, such that loss of p53 counters the deleterious effects of oncogenic Kras on these cells and enables them to self-renew indefinitely. Consequently, myeloid progenitor cells expressing oncogenic Kras and lacking p53 become leukemia-initiating cells, resembling cancer stem cells capable of maintaining AML in vivo. Our results establish an efficient new strategy for interrogating oncogene cooperation, and provide strong evidence that the ability of p53 to limit aberrant self-renewal contributes to its tumor suppressor activity.

Limitations of peptide retro-inverso isomerization in molecular mimicry

A retro-inverso peptide is made up of d-amino acids in a reversed sequence and, when extended, assumes a side chain topology similar to that of its parent molecule but with inverted amide peptide bonds. Despite their limited success as antigenic mimicry, retro-inverso isomers generally fail to emulate the protein-binding activities of their parent peptides of an alpha-helical nature. In studying the interaction between the tumor suppressor protein p53 and its negative regulator MDM2, Sakurai et al. (Sakurai, K., Chung, H. S., and Kahne, D. (2004) J. Am. Chem. Soc. 126, 16288-16289) made a surprising finding that the retro-inverso isomer of p53(15-29) retained the same binding activity as the wild type peptide as determined by inhibition enzyme-linked immunosorbent assay. The authors attributed the unusual outcome to the ability of the D-peptide to adopt a right-handed helical conformation upon MDM2 binding. Using a battery of biochemical and biophysical tools, we found that retro-inverso isomerization diminished p53 (15-29) binding to MDM2 or MDMX by 3.2-3.3 kcal/mol. Similar results were replicated with the C-terminal domain of HIV-1 capsid protein (3.0 kcal/mol) and the Src homology 3 domain of Abl tyrosine kinase (3.4 kcal/mol). CD and NMR spectroscopic as well as x-ray crystallographic studies showed that D-peptide ligands of MDM2 invariably adopted left-handed helical conformations in both free and bound states. Our findings reinforce that the retro-inverso strategy works poorly in molecular mimicry of biologically active helical peptides, due to inherent differences at the secondary and tertiary structure levels between an l-peptide and its retro-inverso isomer despite their similar side chain topologies at the primary structure level.

Regulation of the p53 pathway by ubiquitin and related proteins

The p53 tumour suppressor protein is subject to many levels of control, including modification with ubiquitin and related proteins such as SUMO and NEDD8. These modifications regulate p53 at a number of levels, including control of protein turnover, alterations in sub-cellular localization and changes in the ability to regulate gene expression. Numerous E3 ligases that can mediate these modifications of p53 have been described, some of which promote conjugation with more than one ubiquitin-like protein. Understanding the complexity of this mechanism of p53 regulation will help in the development of therapeutic drugs that function to modulate these events.

Meiotic recombination provokes functional activation of the p53 regulatory network

The evolutionary appearance of p53 protein probably preceded its role in tumor suppression, suggesting that there may be unappreciated functions for this protein. Using genetic reporters as proxies to follow in vivo activation of the p53 network in Drosophila, we discovered that the process of meiotic recombination instigates programmed activation of p53 in the germ line. Specifically, double-stranded breaks in DNA generated by the topoisomerase Spo11 provoked functional p53 activity, which was prolonged in cells defective for meiotic DNA repair. This intrinsic stimulus for the p53 regulatory network is highly conserved because Spo11-dependent activation of p53 also occurs in mice. Our findings establish a physiological role for p53 in meiosis and suggest that tumor-suppressive functions may have been co-opted from primordial activities linked to recombination.

The choice between p53-induced senescence and quiescence is determined in part by the mTOR pathway

Transient induction of p53 can cause reversible quiescence and irreversible senescence. Using nutlin-3a (a small molecule that activates p53 without causing DNA damage), we have previously identified cell lines in which nutlin-3a caused quiescence. Importantly, nutlin-3a caused quiescence by actively suppressing the senescence program (while still causing cell cycle arrest). Noteworthy, in these cells nutlin-3a inhibited the mTOR (mammalian Target of Rapamycin) pathway, which is known to be involved in the senescence program. Here we showed that shRNA-mediated knockdown of TSC2, a negative regulator of mTOR, partially converted quiescence into senescence in these nutlin-arrested cells. In accord, in melanoma cell lines and mouse embryo fibroblasts, which easily undergo senescence in response to p53 activation, nutlin-3a failed to inhibit mTOR. In these senescence-prone cells, the mTOR inhibitor rapamycin converted nutlin-3a-induced senescence into quiescence. We conclude that status of the mTOR pathway can determine, at least in part, the choice between senescence and quiescence in p53-arrested cells.

Familial predisposition to adrenocortical tumors: clinical and biological features and management strategies

The incidence of adrenocortical tumors (ACTs) is increased in several familial cancer syndromes resulting from abnormalities in genes that encode transcription factors implicated in cell proliferation, differentiation, senescence, apoptosis, and genomic instability. These include P53, MEN1, APC, and PRKAR1A. Adenomas are the most common ACTs, but adrenocortical carcinomas occur rarely as well. The clinical manifestations of ACTs, which result from increased secretion of adrenocortical hormones, are similar in the familial and sporadic forms of the disease. However, their management may differ because of unique aspects of the constitutional syndromes. The analysis of gene expression profiles of ACTs in these constitutional syndromes have contributed to our understanding of adrenal tumorigenesis and revealed new molecular diagnostic and prognostic markers and candidate genes for targeted therapies. This chapter summarizes the clinical and biological features, pathogenesis, and management strategies for ACTs that develop in patients with familial cancer syndrome.

MDM4 binds ligands via a mechanism in which disordered regions become structured

MDM2 and MDM4 are proteins involved in regulating the tumour suppressor p53. MDM2/4 and p53 interact through their N-terminal domains and disrupting this interaction is a potential anticancer strategy. The MDM2-p53 interaction is structurally and biophysically well characterised, whereas equivalent studies on MDM4 are hampered by aggregation of the protein. Here we present the NMR characterization of MDM4 (14-111) both free and in complexes with peptide and small-molecule ligands. MDM4 is more dynamic in its apo state than is MDM2, with parts of the protein being unstructured. These regions become structured upon binding of a ligand. MDM4 appears to bind its ligand through conformational selection and/or an induced fit mechanism; this might influence rational design of MDM4 inhibitors.

Regulation of p53 activity by HIPK2: molecular mechanisms and therapeutical implications in human cancer cells

The p53 protein is the most studied tumor suppressor and the p53 pathway has been shown to mediate cellular stress responses that are disrupted when cancer develops. After DNA damage, p53 is activated as transcription factor to directly induce the expression of target genes involved in cell-cycle arrest, DNA repair, senescence and, importantly, apoptosis. Post-translational modifications of p53 are essential for the activation of p53 and for selection of target genes. The tumor suppressor homeodomain-interacting protein kinase-2 (HIPK2) is a crucial regulator of p53 apoptotic function by phosphorylating its N-terminal serine 46 (Ser46) and facilitating Lys382 acetylation at the C-terminus. HIPK2 is activated by numerous genotoxic agents and can be deregulated in tumors by several conditions including hypoxia. Recent findings suggest that HIPK2 active/inactive protein can affect p53 function in multiple and unexpected ways. This makes p53 as well as HIPK2 interesting targets for cancer therapy. Hence, understanding the role of HIPK2 as p53 activator may provide important insights in the process of tumor progression, and may also serve as the crucial point in the diagnostic and therapeutical aspects of cancer.

Pharmacological reactivation of mutant p53: from protein structure to the cancer patient

The p53 tumor suppressor pathway blocks tumor development by triggering apoptosis or cellular senescence in response to oncogenic stress. A large fraction of human tumors carry p53 mutations that disrupt DNA binding of p53 and transcriptional regulation of target genes. Reconstitution of wild-type p53 in vivo triggers rapid elimination of tumors. Therefore, pharmacological reactivation of mutant p53 is a promising strategy for novel cancer therapy. Several approaches for identification of small molecules that target mutant p53 have been applied, including rational design and screening of chemical libraries. The compound PhiKan083 binds with high affinity to a crevice created by the Y220C mutation in p53 and stabilizes the mutant protein. The compound PRIMA-1 (p53 reactivation and induction of massive apoptosis) restores wild-type conformation to mutant p53 by binding to the core and induces apoptosis in human tumor cells. The PRIMA-1 analog APR-246 is currently tested in a clinical trial. Successful development of mutant p53-reactivating anticancer drugs should have a major impact on the treatment of cancer.

Radiation-induced cell death mechanisms

The main goal when treating malignancies with radiation therapy is to deprive tumor cells of their reproductive potential. One approach to achieve this is by inducing tumor cell apoptosis. Accumulating evidences suggest that induction of apoptosis alone is insufficient to account for the therapeutic effect of radiotherapy. It has become obvious in the last few years that inhibition of the proliferative capacity of malignant cells following irradiation, especially with solid tumors, can occur via alternative cell death modalities or permanent cell cycle arrests, i.e., senescence. In this review, apoptosis and mitotic catastrophe, the two major cell deaths induced by radiation, are described and dissected in terms of activating mechanisms. Furthermore, treatment-induced senescence and its relevance for the outcome of radiotherapy of cancer will be discussed. The importance of p53 for the induction and execution of these different types of cell deaths is highlighted. The efficiency of radiotherapy and radioimmunotherapy has much to gain by understanding the cell death mechanisms that are induced in tumor cells following irradiation. Strategies to use specific inhibitors that will manipulate key molecules in these pathways in combination with radiation might potentiate therapy and enhance tumor cell kill.

Paradoxical suppression of cellular senescence by p53

The tumor suppressor p53 is a canonical inducer of cellular senescence (irreversible loss of proliferative potential and senescent morphology). p53 can also cause reversible arrest without senescent morphology, which has usually been interpreted as failure of p53 to induce senescence. Here we demonstrate that p53-induced quiescence actually results from suppression of senescence by p53. In previous studies, suppression of senescence by p53 was masked by p53-induced cell cycle arrest. Here, we separated these two activities by inducing senescence through overexpression of p21 and then testing the effect of p53 on senescence. We found that in p21-arrested cells, p53 converted senescence into quiescence. Suppression of senescence by p53 required its transactivation function. Like rapamycin, which is known to suppress senescence, p53 inhibited the mTOR pathway. We suggest that, while inducing cell cycle arrest, p53 may simultaneously suppress the senescence program, thus causing quiescence and that suppression of senescence and induction of cell cycle arrest are distinct functions of p53. Thus, in spite of its ability to induce cell cycle arrest, p53 can act as a suppressor of cellular senescence.

Current strategies to target p53 in cancer

Tumor suppressor p53 is a transcription factor that guards the genome stability and normal cell growth. Stresses like DNA damage, oncogenic assault will turn on p53 function which leads to cell cycle arrest for DNA repair, senescence for permanent growth arrest or apoptosis for programmed cell death. At the late stage of cancer progression, p53 is hijacked in all forms of tumors either trapped in the negative regulator such as MDM2/viral proteins or directly mutated/deleted. Re-introduction of a functional p53 alone has been proven to induce tumor regression robustly. Also, an active p53 pathway is essential for effective chemo- or radio-therapy. The emerging cyclotherapy in which p53 acts as a chemoprotectant of normal tissues further expands the utility of p53 activators. Functionally, it is unquestionable that drugging p53 will render tumor-specific intervention. One direct method is to deliver the functional wild-type (wt) p53 to tumors via gene therapy. The small molecule strategies consist of activation of p53 family member such as p73, manipulating p53 posttranslational modulators to increase wt p53 protein levels, protein-protein interaction inhibitors to free wt p53 from MDM2 or viral protein, and restoring p53 function to mutant p53 by direct modulation of its conformation. Although most of the current pre-clinical leads are in microM range and need further optimization, the success in proving that small molecules can reactivate p53 marks the beginning of the clinical development of p53-based cancer therapy.

p53 transcriptional pathways in breast cancer: the good, the bad and the complex

A p53 network immunohistochemically-based signature to discriminate between good and poor prognosis breast cancer would have clinical relevance, given the key role of p53 in malignancy and response to therapy. Utilizing a five-protein signature of p53/mdm2/mdm4/bcl2/p21 discriminates good-prognosis and poor-prognosis patient groups, based on the functionality of the p53 network. However, the relationship of this five-protein signature to p53 mutation, the wide range of breast cancer therapies now in use and the over-70 age group remain uncertain. Nonetheless, confirmation of the signature in two independent series suggests that this approach should be considered in further case series and in the context of clinical trials.

Systematic mutational analysis of peptide inhibition of the p53-MDM2/MDMX interactions

Inhibition of the interaction between the tumor suppressor protein p53 and its negative regulators MDM2 and MDMX is of great interest in cancer biology and drug design. We previously reported a potent duodecimal peptide inhibitor, termed PMI (TSFAEYWNLLSP), of the p53-MDM2 and -MDMX interactions. PMI competes with p53 for MDM2 and MDMX binding at an affinity roughly 2 orders of magnitude higher than that of (17-28)p53 (ETFSDLWKLLPE) of the same length; both peptides adopt nearly identical alpha-helical conformations in the complexes, where the three highlighted hydrophobic residues Phe, Trp, and Leu dominate PMI or (17-28)p53 binding to MDM2 and MDMX. To elucidate the molecular determinants for PMI activity and specificity, we performed a systematic Ala scanning mutational analysis of PMI and (17-28)p53. The binding affinities for MDM2 and MDMX of a total of 35 peptides including 10 truncation analogs were quantified, affording a complete dissection of energetic contributions of individual residues of PMI and (17-28)p53 to MDM2 and MDMX association. Importantly, the N8A mutation turned PMI into the most potent dual-specific antagonist of MDM2 and MDMX reported to date, registering respective K(d) values of 490 pM and 2.4 nM. The co-crystal structure of N8A-PMI-(25-109)MDM2 was determined at 1.95 A, affirming that high-affinity peptide binding to MDM2/MDMX necessitates, in addition to optimized intermolecular interactions, enhanced helix stability or propensity contributed by non-contact residues. The powerful empirical binding data and crystal structures present a unique opportunity for computational studies of peptide inhibition of the p53-MDM2/MDMX interactions.

Understanding and managing ultra high-risk chronic lymphocytic leukemia

Modern treatment approaches such as chemoimmunotherapy (e.g., fludarabine/cyclophosphamide/rituximab or FCR) are highly effective in the majority of chronic lymphocytic leukemia (CLL) patients. However, there remains a small but challenging subgroup of patients who show ultra high-risk genetics (17p deletion, TP53 mutation) and/or poor response to chemoimmunotherapy. The median life expectancy of these patients is below 2 to 3 years with standard regimens. Accordingly, CLL with the 17p deletion (and likely also with sole TP53 mutation) should be treated with alternative strategies. While p53 defects appear to play a central role in our understanding of this ultra high-risk group, at least half of the cases will not be predictable based on existing prognostic models. Current treatment approaches for patients with p53 defects or poor response to chemoimmunotherapy should rely on agents acting independently of p53, such as alemtuzumab, lenalidomide, flavopiridol, and a growing number of novel compounds (or combinations thereof) currently available in clinical trials. Poor survival times of patients with ultra high-risk CLL suggest that eligible patients should be offered consolidation with reduced-intensity allogeneic stem-cell transplantation or experimental approaches in clinical trials.

Senescence in tumours: evidence from mice and humans

The importance of cellular senescence, which is a stress response that stably blocks proliferation, is increasingly being recognized. Senescence is prevalent in pre-malignant tumours, and progression to malignancy requires evading senescence. Malignant tumours, however, may still undergo senescence owing to interventions that restore tumour suppressor function or inactivate oncogenes. Senescent tumour cells can be cleared by immune cells, which may result in efficient tumour regression. Standard chemotherapy also has the potential to induce senescence, which may partly underlie its therapeutic activity. Although these concepts are well supported in mouse models, translating them to clinical oncology remains a challenge.

Small-molecule inhibitors of the p53-MDM2 interaction

The p53 tumor suppressor is controlled by MDM2, which binds p53 and negatively regulates its transcriptional activity and stability. Many tumors overproduce MDM2 to impair p53 function. Therefore, restoration of p53 activity by inhibiting the p53-MDM2 binding represents an attractive novel approach to cancer therapy. Recently developed potent and selective small-molecule antagonists of the p53-MDM2 interaction have been used to demonstrate the proof-of-concept for this approach. These compounds interact specifically with the p53-binding pocket of MDM2 and release p53 from negative control. Treatment of cancer cells expressing wild-type p53 stabilize p53 and activate the p53 pathway, leading to cell cycle arrest and apoptosis. In mice-bearing established human tumor xenografts, MDM2 antagonists caused tumor inhibition and regression at nontoxic concentrations, suggesting that they may have a therapeutic utility in the treatment of cancer. An increasing number of MDM2 antagonists are being generated and some of them have entered clinical trials. Here, we review this class of emerging drugs with an emphasis on small molecules that inhibit the p53-MDM2 interaction.