Defining the molecular basis of Arf and Hdm2 interactions

Bivalent binding of p14ARF to MDM2 RING and acidic domains inhibits E3 ligase function

ARF tumor suppressor protein is a key regulator of the MDM2-p53 signaling axis. ARF interferes with MDM2-mediated ubiquitination and degradation of p53 by sequestering MDM2 in the nucleolus and preventing MDM2-p53 interaction and nuclear export of p53. Moreover, ARF also directly inhibits MDM2 ubiquitin ligase (E3) activity, but the mechanism remains elusive. Here, we apply nuclear magnetic resonance and biochemical analyses to uncover the mechanism of ARF-mediated inhibition of MDM2 E3 activity. We show that MDM2 acidic and zinc finger domains (AD-ZnF) form a weak intramolecular interaction with the RING domain, where the binding site overlaps with the E2∼ubiquitin binding surface and thereby partially reduces MDM2 E3 activity. Binding of human N-terminal 32 residues of p14ARF to the acidic domain of MDM2 strengthens the AD-ZnF-RING domain interaction. Furthermore, the N-terminal RxFxV motifs of p14ARF participate directly in the MDM2 RING domain interaction. This bivalent binding mode of p14ARF to MDM2 acidic and RING domains restricts E2∼ubiquitin recruitment and massively hinders MDM2 E3 activity. These findings elucidate the mechanism by which ARF inhibits MDM2 E3 activity.

Regulation of stability and inhibitory activity of the tumor suppressor SEF through casein-kinase II-mediated phosphorylation

Inflammation and cancer are intimately linked. A key mediator of inflammation is the transcription-factor NF-κB/RelA:p50. SEF (also known as IL-17RD) is a feedback antagonist of NF-κB/RelA:p50 that is emerging as an important link between inflammation and cancer. SEF acts as a buffer to prevent excessive NF-κB activity by sequestering NF-κB/RelA:p50 in the cytoplasm of unstimulated cells, and consequently attenuating the NF-κB response upon pro-inflammatory cytokine stimulation. SEF contributes to cancer progression also via modulating other signaling pathways, including those triggered by growth-factors. Despite its important role in human physiology and pathology, mechanisms that regulate SEF biochemical properties and inhibitory activity are unknown. Here we show that human SEF is an intrinsically labile protein that is stabilized via CK2-mediated phosphorylation, and identified the residues whom phosphorylation by CK2 stabilizes hSEF. Unlike endogenous SEF, ectopic SEF was rapidly degraded when overexpressed but was stabilized in the presence of excess CK2, suggesting a mechanism for limiting SEF levels depending upon CK2 processivity. Additionally, phosphorylation by CK2 potentiated hSef interaction with NF-κB in cell-free binding assays. Most importantly, we identified a CK2 phosphorylation site that was indispensable for SEF inhibition of pro-inflammatory cytokine signaling but was not required for SEF inhibition of growth-factor signaling. To our knowledge, this is the first demonstration of post-translational modifications that regulate SEF at multiple levels to optimize its inhibitory activity in a specific signaling context. These findings may facilitate the design of SEF variants for treating cytokine-dependent pathologies, including cancer and chronic inflammation.

Post-Translational Regulation of ARF: Perspective in Cancer

Tumorigenesis can be induced by various stresses that cause aberrant DNA mutations and unhindered cell proliferation. Under such conditions, normal cells autonomously induce defense mechanisms, thereby stimulating tumor suppressor activation. ARF, encoded by the CDKN2a locus, is one of the most frequently mutated or deleted tumor suppressors in human cancer. The safeguard roles of ARF in tumorigenesis are mainly mediated via the MDM2-p53 axis, which plays a prominent role in tumor suppression. Under normal conditions, low p53 expression is stringently regulated by its target gene, MDM2 E3 ligase, which induces p53 degradation in a ubiquitin-proteasome-dependent manner. Oncogenic signals induced by MYC, RAS, and E2Fs trap MDM2 in the inhibited state by inducing ARF expression as a safeguard measure, thereby activating the tumor-suppressive function of p53. In addition to the MDM2-p53 axis, ARF can also interact with diverse proteins and regulate various cellular functions, such as cellular senescence, apoptosis, and anoikis, in a p53-independent manner. As the evidence indicating ARF as a key tumor suppressor has been accumulated, there is growing evidence that ARF is sophisticatedly fine-tuned by the diverse factors through transcriptional and post-translational regulatory mechanisms. In this review, we mainly focused on how cancer cells employ transcriptional and post-translational regulatory mechanisms to manipulate ARF activities to circumvent the tumor-suppressive function of ARF. We further discussed the clinical implications of ARF in human cancer.

On the relationship status for Arf and NPM1 - it's complicated

Arf levels are tightly regulated in cells and correlate with the level of ribosome biogenesis and proliferative status of cells. Through multivalent interactions with NPM1 - a regulator of ribosome biogenesis, and Mdm2 - a regulator of cellular fate, Arf integrates within the nucleolar matrix, altering its structure, dynamics and function and therefore modulates the cell cycle.

Direct detection of carbon and nitrogen nuclei for high-resolution analysis of intrinsically disordered proteins using NMR spectroscopy

Nuclear magnetic resonance spectroscopy (NMR) is a powerful technique for characterizing the structural and dynamic properties of intrinsically disordered proteins and protein regions (IDPs & IDRs). However, the application of NMR to IDPs has been limited by poor chemical shift dispersion in two-dimensional (2D) ¹H-¹⁵N heteronuclear correlation spectra. Among the various detection schemes available for heteronuclear correlation spectroscopy, ¹³C direct-detection has become a mainstay for investigations of IDPs owing to the favorable chemical shift dispersion in 2D ¹³C'-¹⁵N correlation spectra. Recent advances in cryoprobe technology have enhanced the sensitivity for direct detection of both ¹³C and ¹⁵N resonances at high magnetic field strengths, thus prompting the development of ¹⁵N direct-detect experiments to complement established ¹³C-detection experiments. However, the application of ¹⁵N-detection has not been widely explored for IDPs. Here we compare ¹H, ¹³C, and ¹⁵N detection schemes for a variety of 2D heteronuclear correlation spectra and evaluate their performance on the basis of resolution, chemical shift dispersion, and sensitivity. We performed experiments with a variety of disordered systems ranging in size and complexity; from a small IDR (99 amino acids), to a large low complexity IDR (185 amino acids), and finally a ∼73 kDa folded homopentameric protein that also contains disordered regions (133 amino acids/monomer). We conclude that, while requiring high sample concentration and long acquisition times, ¹⁵N-detection often offers enhanced resolution over other detection schemes in studies of disordered protein regions with low complexity sequences.

Identification of a novel nucleophosmin-interaction motif in the tumor suppressor p14arf

The tumor suppressor p14arf interacts, in response to oncogenic signals, with the p53 E3-ubiquitin ligase HDM2, thereby resulting in p53 stabilization and activation. In addition, it also exerts tumor-suppressive functions in p53-independent contexts. The activities of p14arf are regulated by the nucleolar chaperone nucleophosmin (NPM1), which controls its levels and cellular localization. In acute myeloid leukemia with mutations in the NPM1 gene, mutated NPM1 aberrantly translocates in the cytosol carrying with itself p14arf that is subsequently degraded, thus impairing the p14arf-HDM2-p53 axis. In this work we investigated the complex between these two proteins by means of NMR and other techniques. We identified a novel NPM1-interacting motif in the C-terminal region of p14arf, which corresponds to its predicted nucleolar localization signal. This motif recognizes a specific region of the NPM1 N-terminal domain and, upon binding, the two proteins form soluble high molecular weight complexes. By NMR, we identified critical residues on both proteins involved in the interaction. Collectively, our data provide a structural framework to rationalize the overall assembly of the p14arf-NPM1 supramolecular complexes. A number of p14arf cancer-associated mutations cluster in this motif and their effect on the interaction with NPM1 was also analyzed.

Surface Plasmon Resonance Sensing of Biorecognition Interactions within the Tumor Suppressor p53 Network

Surface Plasmon Resonance (SPR) is a powerful technique to study the kinetics of biomolecules undergoing biorecognition processes, particularly suited for protein-protein interactions of biomedical interest. The potentiality of SPR was exploited to sense the interactions occurring within the network of the tumor suppressor p53, which is crucial for maintaining genome integrity and whose function is inactivated, mainly by down regulation or by mutation, in the majority of human tumors. This study includes p53 down-regulators, p53 mutants and also the p53 family members, p63 and p73, which could vicariate p53 protective function. Furthermore, the application of SPR was extended to sense the interaction of p53 with anti-cancer drugs, which might restore p53 function. An extended review of previous published work and unpublished kinetic data is provided, dealing with the interaction between the p53 family members, or their mutants and two anticancer molecules, Azurin and its cell-penetrating peptide, p28. All the kinetic results are discussed in connection with those obtained by a complementary approach operating at the single molecule level, namely Atomic Force Spectroscopy and the related literature data. The overview of the SPR kinetic results may significantly contribute to a deeper understanding of the interactions within p53 network, also in the perspective of designing suitable anticancer drugs.

Dynamic Protein Interaction Networks and New Structural Paradigms in Signaling

Understanding signaling and other complex biological processes requires elucidating the critical roles of intrinsically disordered proteins (IDPs) and regions (IDRs), which represent ∼30% of the proteome and enable unique regulatory mechanisms. In this review, we describe the structural heterogeneity of disordered proteins that underpins these mechanisms and the latest progress in obtaining structural descriptions of conformational ensembles of disordered proteins that are needed for linking structure and dynamics to function. We describe the diverse interactions of IDPs that can have unusual characteristics such as "ultrasensitivity" and "regulated folding and unfolding". We also summarize the mounting data showing that large-scale assembly and protein phase separation occurs within a variety of signaling complexes and cellular structures. In addition, we discuss efforts to therapeutically target disordered proteins with small molecules. Overall, we interpret the remodeling of disordered state ensembles due to binding and post-translational modifications within an expanded framework for allostery that provides significant insights into how disordered proteins transmit biological information.

The role of structural disorder in cell cycle regulation, related clinical proteomics, disease development and drug targeting

Understanding the molecular mechanisms of the regulation of cell cycle is a central issue in molecular cell biology, due to its fundamental role in the existence of cells. The regulatory circuits that make decisions on when a cell should divide are very complex and particularly subtly balanced in eukaryotes, in which the harmony of many different cells in an organism is essential for life. Several hundred proteins are involved in these processes, and a great deal of studies attests that most of them have functionally relevant intrinsic structural disorder. Structural disorder imparts many functional advantages on these proteins, and we discuss it in detail that it is involved in all key steps from signaling through the cell membrane to regulating transcription of proteins that execute timely responses to an ever-changing environment.

A Conserved Deubiquitinating Enzyme Uses Intrinsically Disordered Regions to Scaffold Multiple Protein Interaction Sites

In the canonical view of protein function, it is generally accepted that the three-dimensional structure of a protein determines its function. However, the past decade has seen a dramatic growth in the identification of proteins with extensive intrinsically disordered regions (IDRs), which are conformationally plastic and do not appear to adopt single three-dimensional structures. One current paradigm for IDR function is that disorder enables IDRs to adopt multiple conformations, expanding the ability of a protein to interact with a wide variety of disparate proteins. The capacity for many interactions is an important feature of proteins that occupy the hubs of protein networks, in particular protein-modifying enzymes that usually have a broad spectrum of substrates. One such protein modification is ubiquitination, where ubiquitin is attached to proteins through ubiquitin ligases (E3s) and removed through deubiquitinating enzymes. Numerous proteomic studies have found that thousands of proteins are dynamically regulated by cycles of ubiquitination and deubiquitination. Thus, how these enzymes target their wide array of substrates is of considerable importance for understanding the function of the cell's diverse ubiquitination networks. Here, we characterize a yeast deubiquitinating enzyme, Ubp10, that possesses IDRs flanking its catalytic protease domain. We show that Ubp10 possesses multiple, distinct binding modules within its IDRs that are necessary and sufficient for directing protein interactions important for Ubp10's known roles in gene silencing and ribosome biogenesis. The human homolog of Ubp10, USP36, also has IDRs flanking its catalytic domain, and these IDRs similarly contain binding modules important for protein interactions. This work highlights the significant protein interaction scaffolding abilities of IDRs in the regulation of dynamic protein ubiquitination.

UBTD1 induces cellular senescence through an UBTD1-Mdm2/p53 positive feedback loop

The tumour suppressor p53 plays an important role in tumourigenesis. Besides inducing apoptosis, it regulates cellular senescence, which constitutes an important barrier to tumourigenesis. The mechanism of regulation of cellular senescence by p53 and its downstream pathway are poorly understood. Here, we report that the ubiquitin domain-containing 1 (UBTD1) gene, a new downstream target of p53, induces cellular senescence and acts as a novel tumour suppressor by a mechanism that depends on p53. Expression of UBTD1 increased upon cellular senescence induced by serial passageing of cultures, as well as by exposure to DNA-damageing drugs that induce premature senescence. Over-expression of UBTD1 induces senescence in human fibroblasts and cancer cells and attenuation of the transformed phenotype in cancer cells. UBTD1 is down-regulated in gastric and colorectal cancer tissues, and its lower expression correlates with a more aggressive phenotype and worse prognosis. Multivariate analysis revealed that UBTD1 expression was an independent prognostic factor for gastric cancer patients. Furthermore, UBTD1 increased the stability of p53 protein, by promoting the degradation of Mdm2 protein. Importantly, UBTD1 and p53 function mutually depend on each other in regulating cellular senescence and proliferation. Thus, our data suggest that, upon DNA damage, p53 induction by UBTD1 creates a positive feedback mechanism to further increase p53 expression. Our results establish UBTD1 as a regulator of cellular senescence that mediates p53 function, and provide insights into the mechanism of Mdm2 inhibition that impacts p53 dynamics during cellular senescence and tumourigenesis.

Circular dichroism (CD) analyses of protein-protein interactions

Circular dichroism (CD) spectroscopy is a useful technique for studying protein-protein interactions in solution. CD in the far ultraviolet region (178-260 nm) arises from the amides of the protein backbone and is sensitive to the conformation of the protein. Thus, CD can determine whether there are changes in the conformation of proteins when they interact. Changes in the conformation of the protein complexes as a function of temperature or added denaturants, compared to the individual proteins, can be used to determine binding constants. CD bands in the near ultraviolet (350-260 nm) and visible regions arise from aromatic amino acid side chains and prosthetic groups. There are often changes in these regions when proteins bind to each other. Because CD is a quantitative technique, these changes are directly proportional to the amount of the protein-protein complexes formed and thus also can be used to estimate binding constants.

The ARF tumor suppressor prevents chromosomal instability and ensures mitotic checkpoint fidelity through regulation of Aurora B

The ARF tumor suppressor is best known for its role in stabilizing p53. This study identifies p53-independent functions of ARF in chromosome segregation and the mitotic checkpoint. Mitotic defects caused by loss of ARF are recapitulated by Aurora B overexpression and rescued by partial depletion of Aurora B.

The ARF tumor suppressor is part of the CDKN2A locus and is mutated or undetectable in numerous cancers. The best-characterized role for ARF is in stabilizing p53 in response to cellular stress. However, ARF has tumor suppressive functions outside this pathway that have not been fully defined. Primary mouse embryonic fibroblasts (MEFs) lacking the ARF tumor suppressor contain abnormal numbers of chromosomes. However, no role for ARF in cell division has previously been proposed. Here we demonstrate a novel, p53-independent role for ARF in the mitotic checkpoint. Consistent with this, loss of ARF results in aneuploidy in vitro and in vivo. ARF^−/− MEFs exhibit mitotic defects including misaligned and lagging chromosomes, multipolar spindles, and increased tetraploidy. ARF^−/− cells exhibit overexpression of Mad2, BubR1, and Aurora B, but only overexpression of Aurora B phenocopies mitotic defects observed in ARF^−/− MEFs. Restoring Aurora B to near-normal levels rescues mitotic phenotypes in cells lacking ARF. Our results define an unexpected role for ARF in chromosome segregation and mitotic checkpoint function. They further establish maintenance of chromosomal stability as one of the additional tumor-suppressive functions of ARF and offer a molecular explanation for the common up-regulation of Aurora B in human cancers.

Autoactivation of the MDM2 E3 ligase by intramolecular interaction

The RING domain ubiquitin E3 ligase MDM2 is a key regulator of p53 degradation and a mediator of signals that stabilize p53. The current understanding of the mechanisms by which MDM2 posttranslational modifications and protein binding cause p53 stabilization remains incomplete. Here we present evidence that the MDM2 central acidic region is critical for activating RING domain E3 ligase activity. A 30-amino-acid minimal region of the acidic domain binds to the RING domain through intramolecular interactions and stimulates the catalytic function of the RING domain in promoting ubiquitin release from charged E2. The minimal activation sequence is also the binding site for the ARF tumor suppressor, which inhibits ubiquitination of p53. The acidic domain-RING domain intramolecular interaction is modulated by ATM-mediated phosphorylation near the RING domain or by binding of ARF. These results suggest that MDM2 phosphorylation and association with protein regulators share a mechanism in inhibiting the E3 ligase function and stabilizing p53 and suggest that targeting the MDM2 autoactivation mechanism may be useful for therapeutic modulation of p53 levels.

Structural polymorphism in the N-terminal oligomerization domain of NPM1

Nucleophosmin (NPM1) is a multifunctional phospho-protein with critical roles in ribosome biogenesis, tumor suppression, and nucleolar stress response. Here we show that the N-terminal oligomerization domain of NPM1 (Npm-N) exhibits structural polymorphism by populating conformational states ranging from a highly ordered, folded pentamer to a highly disordered monomer. The monomer-pentamer equilibrium is modulated by posttranslational modification and protein binding. Phosphorylation drives the equilibrium in favor of monomeric forms, and this effect can be reversed by Npm-N binding to its interaction partners. We have identified a short, arginine-rich linear motif in NPM1 binding partners that mediates Npm-N oligomerization. We propose that the diverse functional repertoire associated with NPM1 is controlled through a regulated unfolding mechanism signaled through posttranslational modifications and intermolecular interactions.

Targeting p53-MDM2-MDMX loop for cancer therapy

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

MDM2's social network

MDM2 is considered a hub protein due to its capacity to interact with a large number of different partners of which p53 is most well described. MDM2 is an E3 ubiquitin ligase, and many, but not all, of its interactions relate directly to this activity, such as substrates, adaptors or bridges, promoters, inhibitors or complementary factors. Some interactions serve regulatory functions that in response to cellular stresses control the localisation and functions of MDM2 including protein kinases, ribosomal proteins and proteases. Moreover, interactions with nucleotides serve other functions such as mRNA to regulate protein synthesis and DNA to control transcription. To perform such a pleiotropic panorama of different functions, MDM2 is subjected to a multitude of post-translational modifications and is expressed in different isoforms. The large and diverse interactome is made possible due to the plasticity of MDM2 and in this review we have listed the MDM2 interactions until now and we will discuss how this multifaceted protein can interact with such a variety of substrates to provide a key intermediary role in different signalling pathways.

Unravelling mechanisms of cisplatin sensitivity and resistance in testicular cancer

Testicular cancer is the most frequent solid malignant tumour type in men 20-40 years of age. At the time of diagnosis up to 50% of the patients suffer from metastatic disease. In contrast to most other metastatic solid tumours, the majority of metastatic testicular cancer patients can be cured with highly effective cisplatin-based chemotherapy. This review aims to summarise the current knowledge on response to chemotherapy and the biological basis of cisplatin-induced apoptosis in testicular cancer. The frequent presence of wild-type TP53 and the low levels of p53 in complex with the p53 negative feed-back regulator MDM2 contribute to cisplatin sensitivity. Moreover, the high levels of the pluripotency regulator Oct4 and as a consequence of Oct4 expression high levels of miR-17/106b seed family and pro-apoptotic Noxa and the low levels of cytoplasmic p21 (WAF1/Cip1) appear to be causative for the exquisite sensitivity to cisplatin-based therapy of testicular cancer. However, resistance of testicular cancer to cisplatin-based therapy does occur and can be mediated through aberrant levels of the above mentioned key players. Drugs targeting these key players showed, at least pre-clinically, a sensitising effect to cisplatin treatment. Further clinical development of such treatment strategies will lead to new treatment options for platinum-resistant testicular cancers.

Functional diversity and structural disorder in the human ubiquitination pathway

The ubiquitin-proteasome system plays a central role in cellular regulation and protein quality control (PQC). The system is built as a pyramid of increasing complexity, with two E1 (ubiquitin activating), few dozen E2 (ubiquitin conjugating) and several hundred E3 (ubiquitin ligase) enzymes. By collecting and analyzing E3 sequences from the KEGG BRITE database and literature, we assembled a coherent dataset of 563 human E3s and analyzed their various physical features. We found an increase in structural disorder of the system with multiple disorder predictors (IUPred – E1: 5.97%, E2: 17.74%, E3: 20.03%). E3s that can bind E2 and substrate simultaneously (single subunit E3, ssE3) have significantly higher disorder (22.98%) than E3s in which E2 binding (multi RING-finger, mRF, 0.62%), scaffolding (6.01%) and substrate binding (adaptor/substrate recognition subunits, 17.33%) functions are separated. In ssE3s, the disorder was localized in the substrate/adaptor binding domains, whereas the E2-binding RING/HECT-domains were structured. To demonstrate the involvement of disorder in E3 function, we applied normal modes and molecular dynamics analyses to show how a disordered and highly flexible linker in human CBL (an E3 that acts as a regulator of several tyrosine kinase-mediated signalling pathways) facilitates long-range conformational changes bringing substrate and E2-binding domains towards each other and thus assisting in ubiquitin transfer. E3s with multiple interaction partners (as evidenced by data in STRING) also possess elevated levels of disorder (hubs, 22.90% vs. non-hubs, 18.36%). Furthermore, a search in PDB uncovered 21 distinct human E3 interactions, in 7 of which the disordered region of E3s undergoes induced folding (or mutual induced folding) in the presence of the partner. In conclusion, our data highlights the primary role of structural disorder in the functions of E3 ligases that manifests itself in the substrate/adaptor binding functions as well as the mechanism of ubiquitin transfer by long-range conformational transitions.

In vitro selection of mutant HDM2 resistant to Nutlin inhibition

HDM2 binds to the p53 tumour suppressor and targets it for proteosomal degradation. Presently in clinical trials, the small molecule Nutlin-3A competitively binds to HDM2 and abrogates its repressive function. Using a novel in vitro selection methodology, we simulated the emergence of resistance by evolving HDM2 mutants capable of binding p53 in the presence of Nutlin concentrations that inhibit the wild-type HDM2-p53 interaction. The in vitro phenotypes were recapitulated in ex vivo assays measuring both p53 transactivation function and the direct p53-HDM2 interaction in the presence of Nutlin. Mutations conferring drug resistance were not confined to the N-terminal p53/Nutlin–binding domain, and were additionally seen in the acidic, zinc finger and RING domains. Mechanistic insights gleaned from this broad spectrum of mutations will aid in future drug design and further our understanding of the complex p53-HDM2 interaction.

ARF represses androgen receptor transactivation in prostate cancer

Androgen receptor (AR) signaling is essential for prostate cancer (PCa) development in humans. The initiation of prostate malignancy and progression to a castration-resistant stage are largely contributed by the modulation of AR activity through its coregulatory proteins. We and others previously reported that p14 alternative reading frame (ARF) expression is positively correlated with the disease progression and severity of PCa. Here, we provide evidence that p14ARF physically interacts with AR and functions as an AR corespressor in both an androgen-dependent and androgen-independent manner. Endogenous ARF (p14ARF in human and p19ARF in mouse) and AR colocalize in both human PCa cells in vitro and PCa tissues of mouse and human in vivo. Overexpression of p14ARF in PCa cells significantly attenuates the activities of androgen response region (ARR2)-probasin and prostate-specific antigen (PSA) promoters. The forced expression of p14ARF in cells resulted in a suppression of PSA and NK transcription factor locus 1 (NKX3.1) expression. Conversely, knockdown of endogenous p14ARF in human PCa cells with short hairpin RNA enhanced AR transactivation activities in a dose-dependent and p53-independent manner. Furthermore, we demonstrated that p14ARF binds to both the N-terminal domain and the ligand-binding domain of AR, and the human double minute 2 (HDM2)-binding motif of p14ARF is required for the interaction of p14ARF and AR proteins. p14ARF perturbs the androgen-induced interaction between the N terminus and C terminus of AR. Most importantly, we observed that the expression of PSA is reversely correlated with p14ARF in human prostate tissues. Taken together, our results reveal a novel function of ARF in modulation of AR transactivation in PCa.

Pro- and anti-apoptotic effects of p53 in cisplatin-treated human testicular cancer are cell context-dependent

In murine testicular cancer (TC) cells wild-type p53 contributes to sensitivity to DNA-damaging drugs in a dose-dependent way. In human TC, however, the role of wild-type p53 functionality in chemotherapeutic response remains elusive. We analyzed functionality of wild-type p53 in cisplatin sensitivity in the human TC setting using a p53 short interfering (si)RNA approach. The cisplatin-sensitive TC cell line (Tera), the subline with acquired cisplatin resistance (Tera-CP) and a panel of intrinsically resistant TC cell lines (Scha and 2102EP), all expressing wild-type p53, were used. p53 and p53 transcriptional targets MDM2 and p21 (Waf1/Cip1) (p21) were expressed in a p53 transactivation-dependent way in all TC cell lines. Following cisplatin exposure, expression levels of p53 increased, with a subsequent increase in MDM2 and p21 mRNA and protein levels and Fas cell membrane levels. Downregulation of p53 with siRNA lowered cisplatin-induced apoptosis in Tera and Tera-CP, which was associated with a diminished Fas membrane expression. In contrast, p53 suppression augmented cisplatin-induced apoptosis in Scha and 2102EP and concomitantly strongly suppressed MDM2 and p21 mRNA and protein expression. Our results indicate that p53 is involved in transactivation of pro- and anti-apoptotic genes in untreated and cisplatin-treated TC cells, but subtle differences are present between TC cell lines. The opposite role of p53 in cisplatin-induced apoptosis among TC cell lines demonstrates the importance of the cellular context for the p53 transactivation phenotype in TC cells.

Intrinsically disordered proteins undergo and assist folding transitions in the proteome

The common notion in the protein world holds that proteins are synthesized as a linear polypeptide chain, followed by folding into a unique, functional 3D-structure. As outlined in many articles of this volume, this is in fact the case for a great proportion of the proteome. Many proteins and protein domains, however, are intrinsically disordered (IDPs), i.e., they cannot fold on their own, but often undergo a folding transition in the presence of a binding partner. This binding-induced folding process shows strong conceptual parallels with the folding of globular proteins, in a sense that it can proceed via two routes, either induction of the folded conformation from an initial random state or selection of a pre-formed state already present in the ensemble. In addition, we show that IDPs not only undergo folding themselves, they also assist the folding process of other proteins as chaperones, and even contribute to the quality control processes of the cell, in which irreparably misfolded proteins are recognized and tagged for proteasomal degradation. These various mechanisms suggest that structural disorder, in a biological context, is linked with protein folding in several ways, in which both the IDP and its partner may undergo reciprocal structural transitions.

RING1B ubiquitination and stability are regulated by ARF

The activity and stability of the E3 ubiquitin ligase RING1B are controlled by the ubiquitin system. Self-ubiquitination of RING1B, generating K6, K27 and K48-based mixed polyubiquitin chains, is a prerequisite for its activity as an E3 ligase for histone H2A. Monoubiquitination of histone H2A is one of the hallmarks of Polycomb-mediated gene silencing. The destruction of RING1B however, is mediated through K48 polyubiquitination catalyzed by the ubiquitin ligase E6-AP. Both forms of ubiquitination of RING1B are mutually exclusive and therefore the balance between them may constitute a point of regulation of Polycomb-mediated gene repression. Here we identify ARF as a regulator of RING1B ubiquitination. ARF appears to selectively prevent RING1B self-ubiquitination, probably allowing more efficient E6-AP-mediated ubiquitination and subsequent degradation of RING1B. By binding to the RING domain of RING1B, ARF disrupts RING1B homodimerization, providing a potential mechanism for its effect on RING1B self-ubiquitination.

Natural product ginsenoside 25-OCH3-PPD inhibits breast cancer growth and metastasis through down-regulating MDM2

Although ginseng and related herbs have a long history of utility for various health benefits, their application in cancer therapy and underlying mechanisms of action are not fully understood. Our recent work has shown that 20(S)-25-methoxyl-dammarane-3β, 12β, 20-triol (25-OCH₃-PPD), a newly identified ginsenoside from Panax notoginseng, exerts activities against a variety of cancer cells in vitro and in vivo. This study was designed to investigate its anti-breast cancer activity and the underlying mechanisms of action. We observed that 25-OCH₃-PPD decreased the survival of breast cancer cells by induction of apoptosis and G1 phase arrest and inhibited the growth of breast cancer xenografts in vivo. We further demonstrated that, in a dose- and time-dependent manner, 25-OCH₃-PPD inhibited MDM2 expression at both transcriptional and post-translational levels in human breast cancer cells with various p53 statuses (wild type and mutant). Moreover, 25-OCH₃-PPD inhibited in vitro cell migration, reduced the expression of epithelial-to-mesenchymal transition (EMT) markers, and prevented in vivo metastasis of breast cancer. In summary, 25-OCH₃-PPD is a potential therapeutic and anti-metastatic agent for human breast cancer through down-regulating MDM2. Further preclinical and clinical development of this agent is warranted.

MDM2 protein-mediated ubiquitination of numb protein: identification of a second physiological substrate of MDM2 that employs a dual-site docking mechanism

The E3 ubiquitin ligase, MDM2, uses a dual-site mechanism to ubiquitinate and degrade the tumor suppressor protein p53, involving interactions with the N-terminal hydrophobic pocket and the acidic domain of MDM2. The results presented here demonstrate that MDM2 also uses this same dual-site mechanism to bind to the cell fate determinant NUMB with both the N-terminal hydrophobic pocket and the acidic domain of MDM2 also involved in forming the interaction with NUMB. Furthermore, the acidic domain interactions are crucial for MDM2-mediated ubiquitination of NUMB. Contrary to p53, where two separate domains form the interface with MDM2, only one region within the phosphotyrosine binding domain of NUMB (amino acids 113-148) mediates binding to both these regions of MDM2. By binding to both domains on MDM2, NUMB disrupts the MDM2-p53 complex and MDM2-catalyzed ubiquitination of p53. Therefore, we have identified the mechanism NUMB uses to regulate the steady-state levels of the p53 in cells. By targeting the acidic domain of MDM2 using acid domain-binding ligands we can overcome MDM2-mediated ubiquitination and degradation of NUMB impacting on the stabilization of p53 in cells. Furthermore, delivery of MDM2 acid domain-binding ligands to cancer cells promotes p53-dependent growth arrest and the induction of apoptosis. This highlights the dual-site mechanism of MDM2 on another physiological substrate and identifies the acid domain as well as N terminus as a potential target for small molecules that inhibit MDM2.

P14ARF inhibits human glioblastoma-induced angiogenesis by upregulating the expression of TIMP3

Malignant gliomas are the most common and the most lethal primary brain tumors in adults. Among malignant gliomas, 60%-80% show loss of P14ARF tumor suppressor activity due to somatic alterations of the INK4A/ARF genetic locus. The tumor suppressor activity of P14ARF is in part a result of its ability to prevent the degradation of P53 by binding to and sequestering HDM2. However, the subsequent finding of P14ARF loss in conjunction with TP53 gene loss in some tumors suggests the protein may have other P53-independent tumor suppressor functions. Here, we report what we believe to be a novel tumor suppressor function for P14ARF as an inhibitor of tumor-induced angiogenesis. We found that P14ARF mediates antiangiogenic effects by upregulating expression of tissue inhibitor of metalloproteinase-3 (TIMP3) in a P53-independent fashion. Mechanistically, this regulation occurred at the gene transcription level and was controlled by HDM2-SP1 interplay, where P14ARF relieved a dominant negative interaction of HDM2 with SP1. P14ARF-induced expression of TIMP3 inhibited endothelial cell migration and vessel formation in response to angiogenic stimuli produced by cancer cells. The discovery of this angiogenesis regulatory pathway may provide new insights into P53-independent P14ARF tumor-suppressive mechanisms that have implications for the development of novel therapies directed at tumors and other diseases characterized by vascular pathology.

MicroRNAs as a novel cellular senescence regulator

Cellular senescence is a program activated in normal cells in response to various types of stresses and is manifested by permanent arrest of cell cycle. Cellular senescence is closely related to tumor suppression, and may contribute to the ageing of organisms. The complex senescence cell phenotype has many different mechanisms. Recent studies have provided important insights regarding the role played by miRNAs during cellular senescence as a novel molecular mechanism. In this article, we will review the latest advances in the identification and validation of senescence-regulatory miRNAs and the possible mechanisms.

Abnormal MDMX degradation in tumor cells due to ARF deficiency

MDMX is a hetero dimeric partner of MDM2 and a critical regulator of p53. MDMX level is generally elevated in tumors with wild type p53 and contributes to p53 inactivation. MDMX degradation is controlled in part by MDM2-mediated ubiquitination. Here we show that MDMX turnover is highly responsive to changes in MDM2 level in non-transformed cells, but not in tumor cells. We found that loss of ARF expression, which occurs in most tumors with wild type p53, significantly reduces MDMX sensitivity to MDM2. Restoration of ARF expression in tumor cells enables MDM2 to degrade MDMX in a dose-dependent fashion. ARF binds to MDM2 and stimulates a second-site interaction between the central region of MDM2 and MDMX, thus increases MDMX-MDM2 binding and MDMX ubiquitination. These results reveal an important abnormality in the p53 regulatory pathway as a consequence of ARF deficiency. Loss of ARF during tumor development not only prevents p53 stabilization by proliferative stress, but also causes accumulation of MDMX that compromises p53 activity. This phenomenon may reduce the clinical efficacy of MDM2-specific inhibitors by preventing MDMX down regulation.

Insulin receptor tyrosine kinase substrate enhances low levels of MDM2-mediated p53 ubiquitination

The tumor suppressor p53 controls multiple cellular functions including DNA repair, cell cycle arrest and apoptosis. MDM2-mediated p53 ubiquitination affects both degradation and cytoplasmic localization of p53. Several cofactors are known to modulate MDM2-mediated p53 ubiquitination and proteasomal degradation. Here we show that IRTKS, a novel IRSp53-like protein inhibited p53-induced apoptosis and depressed its transcription activity. IRTKS bound directly to p53 and increased p53 ubiquitination and cytoplasmic localization. Further studies revealed that IRTKS interacted with MDM2 and promoted low levels of MDM2-mediated p53 ubiquitination in vitro and in vivo. In unstressed cells with low levels of MDM2, IRTKS was found to stabilize the interaction of p53 and MDM2. In stressed cells, IRTKS dissociated from p53, and high levels of MDM2 induced by p53 activation mediate IRTKS poly-ubiquitination and subsequent proteasomal degradation. These data suggest that IRTKS is a novel regulator of p53, modulating low level of MDM2-mediated p53 ubiquitination in unstressed cells.

ZNF668 functions as a tumor suppressor by regulating p53 stability and function in breast cancer

Genome-wide sequencing studies in breast cancer have recently identified frequent mutations in the zinc finger protein 668 (ZNF668), the function of which is undefined. Here, we report that ZNF668 is a nucleolar protein that physically interacts with and regulates p53 and its negative regulator MDM2. Through MDM2 binding, ZNF668 regulated autoubiquitination of MDM2 and its ability to mediate p53 ubiquitination and degradation. ZNF668 deficiency also impaired DNA damage-induced stabilization of p53. RNA interference-mediated knockdown of ZNF668 was sufficient to transform normal mammary epithelial cells. ZNF668 effectively suppressed breast cancer cell proliferation in vitro and tumorigenicity in vivo. Taken together, our studies identify ZNF668 as a novel breast tumor suppressor gene that functions in regulating p53 stability.

Expanding the proteome: disordered and alternatively folded proteins

Proteins provide much of the scaffolding for life, as well as undertaking a variety of essential catalytic reactions. These characteristic functions have led us to presuppose that proteins are in general functional only when well structured and correctly folded. As we begin to explore the repertoire of possible protein sequences inherent in the human and other genomes, two stark facts that belie this supposition become clear: firstly, the number of apparent open reading frames in the human genome is significantly smaller than appears to be necessary to code for all of the diverse proteins in higher organisms, and secondly that a significant proportion of the protein sequences that would be coded by the genome would not be expected to form stable three-dimensional (3D) structures. Clearly the genome must include coding for a multitude of alternative forms of proteins, some of which may be partly or fully disordered or incompletely structured in their functional states. At the same time as this likelihood was recognized, experimental studies also began to uncover examples of important protein molecules and domains that were incompletely structured or completely disordered in solution, yet remained perfectly functional. In the ensuing years, we have seen an explosion of experimental and genome-annotation studies that have mapped the extent of the intrinsic disorder phenomenon and explored the possible biological rationales for its widespread occurrence. Answers to the question 'why would a particular domain need to be unstructured?' are as varied as the systems where such domains are found. This review provides a survey of recent new directions in this field, and includes an evaluation of the role not only of intrinsically disordered proteins but also of partially structured and highly dynamic members of the disorder-order continuum.

Cyclopentenyl cytosine induces senescence in breast cancer cells through the nucleolar stress response and activation of p53

The induction of senescence has emerged as a potentially important contributor to the effects of chemotherapeutic agents against tumors. We have demonstrated that depletion of CTP induced by cyclopentenyl cytosine (CPEC; NSC 375575), a specific inhibitor of the enzyme CTP synthetase, induces irreversible growth arrest and senescence characterized by altered morphology and expression of senescence-associated β-galactosidase activity in MCF-7 breast cancer cells expressing wild-type p53. In contrast, differentiation in the absence of senescence resulted from CPEC treatment in MDA-MB-231 breast cancer cells that express a mutated p53. Both senescence of MCF-7 cells and differentiation of MDA-MB-231 cells were prevented by repletion of CTP through the cytidine salvage pathway. Senescence in MCF-7 cells was associated with a G(2)- and S-phase arrest, whereas differentiation in MDA-MB-231 cells was associated with arrest in G(1) phase at 5 days. Mechanistic studies revealed that CTP depletion induced a rapid translocation of nucleolar proteins, including nucleostemin and nucleolin into the nucleoplasm. This nucleolar stress response resulted in a sustained elevation of p53 and the p53 target genes, p21 and Mdm2, in cells with wild-type p53. Furthermore, short interfering RNA-induced knockdown of p53 in MCF-7 cells treated with CPEC prevented cellular senescence and increased apoptotic cell death. We conclude that CTP depletion and the resulting nucleolar stress response results in a senescence-like growth arrest through activation of p53, whereas cells with mutated p53 undergo differentiation or apoptotic cell death.

Inhibition of p53 DNA binding function by the MDM2 protein acidic domain

MDM2 regulates p53 predominantly by promoting p53 ubiquitination. However, ubiquitination-independent mechanisms of MDM2 have also been implicated. Here we show that MDM2 inhibits p53 DNA binding activity in vitro and in vivo. MDM2 binding promotes p53 to adopt a mutant-like conformation, losing reactivity to antibody Pab1620, while exposing the Pab240 epitope. The acidic domain of MDM2 is required to induce p53 conformational change and inhibit p53 DNA binding. Alternate reading frame binding to the MDM2 acidic domain restores p53 wild type conformation and rescues DNA binding activity. Furthermore, histone methyl transferase SUV39H1 binding to the MDM2 acidic domain also restores p53 wild type conformation and allows p53-MDM2-SUV39H1 complex to bind DNA. These results provide further evidence for an ubiquitination-independent mechanism of p53 regulation by MDM2 and reveal how MDM2-interacting repressors gain access to p53 target promoters and repress transcription. Furthermore, we show that the MDM2 inhibitor Nutlin cooperates with the proteasome inhibitor Bortezomib by stimulating p53 DNA binding and transcriptional activity, providing a rationale for combination therapy using proteasome and MDM2 inhibitors.

Nucleolar proteomics and viral infection

Recent advances in proteomics have been combined with traditional methods for isolation of nucleoli from mammalian and plant cells. This approach has confirmed the growing body of data showing a wide role for the nucleolus in eukaryotic cell biology beyond ribosome generation into many areas of cell function from regulation of the cell cycle, modulation of the cell stress response to innate immune responses. This has been reflected in the growing body of evidence that viruses specifically target the nucleolus by sequestering cellular nucleolar proteins or by targeting viral proteins to the nucleolus in order to maximise viral replication. This review covers those key areas and looks at the latest approaches using high‐throughput quantitative proteomics of the nucleolus in virus infected cells to gain an insight into the role of this fascinating compartment in viral infection.

MeCP2: structure and functionThis paper is one of a selection of papers published in a Special Issue entitled 31st Annual International Asilomar Chromatin and Chromosomes Conference, and has undergone the Journal’s usual peer review process

Despite a vast body of literature linking chromatin structure to regulation of gene expression, the role of architectural proteins in higher order chromatin transitions required for transcription activation and repression has remained an under-studied field. To demonstrate the current knowledge of the role of such proteins, we have focused our attention on the methylated DNA binding and chromatin-associated protein MeCP2. Structural studies using chromatin assembled in vitro have revealed that MeCP2 can associate with nucleosomes in an N-terminus dependent manner and efficiently condense nucleosome arrays. The present review attempts to match MeCP2 structural domains, or lack thereof, and specific chromatin features needed for the proper recruitment of MeCP2 to its multiple functions as either activator or repressor. We specifically focused on MeCP2’s role in Rett syndrome, a neurological disorder associated with specific MeCP2 mutations.

Nucleophosmin and its complex network: a possible therapeutic target in hematological diseases

Nucleophosmin (NPM, also known as B23, numatrin or NO38) is a ubiquitously expressed phosphoprotein belonging to the nucleoplasmin family of chaperones. NPM is mainly localized in the nucleolus where it exerts many of its functions, but a proportion of the protein continuously shuttles between the nucleus and the cytoplasm. A growing number of cellular proteins have been described as physical interactors of NPM, and consequently, NPM is thought to have a relevant role in diverse cellular functions, including ribosome biogenesis, centrosome duplication, DNA repair and response to stress. NPM has been implicated in the pathogenesis of several human malignancies and intriguingly, it has been described both as an activating oncogene and a tumor suppressor, depending on cell type and protein levels. In fact, increased NPM expression is associated with different types of solid tumors whereas an impairment of NPM function is characteristic of a subgroup of hematolologic malignancies. A large body of experimental evidence links the deregulation of specific NPM functions to cellular transformation, yet the molecular mechanisms through which NPM contributes to tumorigenesis remain elusive. In this review, we have summarized current knowledge concerning NPM functions, and attempted to interpret its multifaceted and sometimes apparently contradictory activities in the context of both normal cellular homeostasis and neoplastic transformation.

APR-246 exhibits anti-leukemic activity and synergism with conventional chemotherapeutic drugs in acute myeloid leukemia cells

BACKGROUND

APR-246 belongs to a new generation of the compounds that restore normal p53 function in cells with mutated or wild type p53. The purpose of this study was to examine the effects of APR-246 alone and in combination with other drugs in acute myeloid leukemia (AML) cells.

METHODS

Primary leukemic cells from patients with AML and AML cell lines were studied with respect to cytotoxic and apoptotic effects and mechanism of action of APR-246, alone and in combination with Ara-C, daunorubicin and fludarabine.

RESULTS

APR-246 showed dose-dependent cytotoxic and apoptotic effects in AML cell lines as well as in primary AML patient cells. Cells from patients with TP53 mutation and complex karyotype were more resistant to conventional drugs while these factors did not significantly affect the sensitivity to APR-246. APR-246 increased active caspase-3, upregulated p53 protein levels, and increased the bax/bcl-2 ratio independently of TP53 mutational status in patient cells sensitive to APR-246. AML cells with high p14(ARF) expression were significantly more sensitive to APR-246. APR-246 induced significant synergistic effects in combination with conventional chemotherapeutic agents. Pre-incubation with APR-246 induced more synergistic effects compared to other schedules. In patient cells, pronounced synergism was found when combining APR-246 with danuorubicin.

CONCLUSION

We conclude that APR-246 is effective in AML cells irrespectively of TP53 mutational status and that it has promising properties for combination studies in AML.

Mimicry of protein function with cell-penetrating peptides

Proteins are essential components of cellular processes inside cells, and their interactions between each other and with genes are important for the normal physiological functioning of cells as well as for disease states. Modulating protein interactions by different means can potentially control these interactions and restore normal function to diseased cells. The ways to do so are multiple, and such efforts often begin with knowledge of potential target proteins in order to devise mediators that retain the function of the original protein, i.e., mimic the protein functions. An alternative strategy is to utilize protein mimics to inhibit target proteins rather than restoring the activity of a protein. The vast majority of protein -mimics exploited to date have been designed to inhibit the activity of oncogenes or activate tumor suppressors for the purpose of tumor therapy. These protein mimics are usually based on small organic compounds or peptides, derived from interaction surfaces of the proteins, and in some cases, full proteins have been exploited. Although peptides and proteins are naturally highly specific and efficient inside cells, they suffer from low bioavailability resulting from their inability to enter cells. One strategy increasingly employed to facilitate the internalization of peptides and proteins has been to chemically conjugate them to cell-penetrating peptides (CPP) or to recombinantly express protein-CPP fusion constructs.This chapter provides an overview of some of the aspects of perturbing and mimicking protein interactions using peptides and proteins and CPP as transport vectors.

DNA damage response to the Mdm2 inhibitor nutlin-3

Mdm2 inhibitors represent a promising class of p53 activating compounds that may be useful in cancer treatment and prevention. However, the consequences of pharmacological p53 activation are not entirely clear. We observed that Nutlin-3 triggered a DNA damage response in azoxymethane-induced mouse AJ02-NM(0) colon cancer cells, characterized by the phosphorylation of H2AX (at Ser-139) and p53 (at Ser-15). The DNA damage response was highest in cells showing robust p53 stabilization, it could be triggered by the active but not the inactive Nutlin-3 enantiomer, and it was also activated by another pharmacological Mdm2 inhibitor (Caylin-1). Quantification of gamma H2AX-positive cells following Nutlin-3 exposure showed that approximately 17% of cells in late S and G2/M were mounting a DNA damage response (compared to a approximately 50% response to 5-fluorouracil). Nutlin-3 treatment caused the formation of double-strand DNA strand breaks, promoted the formation of micronuclei, accentuated strand breakage induced by doxorubicin and sensitized the mouse colon cancer cells to DNA break-inducing topoisomerase II inhibitors. Although the HCT116 colon cancer cells did not mount a significant DNA damage response following Nutlin-3 treatment, Nutlin-3 enhanced the DNA damage response to the nucleotide synthesis inhibitor hydroxyurea in a p53-dependent manner. Finally, p21 deletion also sensitized HCT116 cells to the Nutlin-3-induced DNA damage response, suggesting that cell cycle checkpoint abnormalities may promote this response. We propose that p53 activation by Mdm2 inhibitors can result in the slowing of double-stranded DNA repair. Although this effect may suppress illegitimate homologous recombination repair, it may also increase the risk of clastogenic events.

Regulation of p53--insights into a complex process

The p53 protein is one of the most important tumor suppressor proteins. Normally, the p53 protein is in a latent state. However, when its activity is required, e.g. upon DNA damage, nucleotide depletion or hypoxia, p53 becomes rapidly activated and initiates transcription of pro-apoptotic and cell cycle arrest-inducing target genes. The activity of p53 is regulated both by protein abundance and by post-translational modifications of pre-existing p53 molecules. In the 30 years of p53 research, a plethora of modifications and interaction partners that modulate p53's abundance and activity have been identified and new ones are continuously discovered. This review will summarize our current knowledge on the regulation of p53 abundance and activity.

Polo-like kinase-1 phosphorylates MDM2 at Ser260 and stimulates MDM2-mediated p53 turnover

The E3 ubiqutin ligase, murne double-minute clone 2 (MDM2), promotes the degradation of p53 under normal homeostatic conditions. Several serine residues within the acidic domain of MDM2 are phosphorylated to maintain its activity but become hypo-phosphorylated following DNA damage, leading to inactivation of MDM2 and induction of p53. However, the signalling pathways that mediate these phosphorylation events are not fully understood. Here we show that the oncogenic and cell cycle-regulatory protein kinase, polo-like kinase-1 (PLK1), phosphorylates MDM2 at one of these residues, Ser260, and stimulates MDM2-mediated turnover of p53. These data are consistent with the idea that deregulation of PLK1 during tumourigenesis may help suppress p53 function.

ARF antagonizes the ability of Miz-1 to inhibit p53-mediated transactivation

Although Myc-interacting zinc-finger protein-1 (Miz-1) is known to be a poxvirus and zinc-finger (POZ) transcription factor required for Myc transcriptional repression, additional regulatory function of Miz-1 is less well understood. Using a yeast two-hybrid screen, we identified human alternate reading frame (ARF) protein as a novel interaction partner of Miz-1. The zinc-finger domain of Miz-1 is involved in its binding to ARF. In addition, we found that Miz-1 was able to interact with p53 through its DNA-binding domain, thus to diminish the binding of p53 to its target promoter and inhibit p53-mediated gene transcription. Interestingly, the Miz-1-regulated p53 transcriptional suppression does not require the presence of ARF or Mdm2. Importantly, ARF and p53 were found to competitively bind to Miz-1 in regulating p53-mediated transcription, and this conclusion was verified by both in vitro binding assay and competitive chromatin immunoprecipitation assay using a bona fide p53 endogenous Bax and Puma promoters. Thus, our study reveals that Miz-1 acts as a p53 suppressor by interfering with p53 DNA-binding ability, and ARF is able to counteract the suppression of Miz-1 on p53 by direct binding to Miz-1, suggesting that Miz-1 is a novel mediator in the ARF-p53 pathway.

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.

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.

Recent advances in validating MDM2 as a cancer target

The MDM2 oncogene is overexpressed in various human cancers. Its expression correlates with the phenotypes of high-grade, late-stage, and more resistant tumors. The auto-regulatory loop between MDM2 and the tumor suppressor p53 has long been considered the epitome of a rational target for cancer therapy. As such, many novel agents have been generated to interfere with the interaction of the two proteins, which results in the activation of p53. Among these agents are several small molecule inhibitors synthesized based upon the crystal structures of the MDM2-p53 complex. With use of high-throughput screening, several specific and effective agents for inhibition of the protein-protein interaction were discovered. Recent investigations, however, have demonstrated that many proteins regulate the MDM2-p53 interaction, and that MDM2 may have p53-independent oncogenic functions. In order for novel MDM2 inhibitors to be translated to the clinic, it is necessary to obtain a better understanding of the regulation of MDM2 and of the MDM2-p53 interaction. In particular, the implications of various interactions between certain regulator(s) and MDM2/p53 under different circumstances need to be elucidated to determine which pathway(s) represent the best targets for therapy. Targeting both MDM2 itself and regulators of MDM2 and the MDM2-p53 interaction, or use of MDM2 inhibitors in combination with conventional treatments, may improve prospects for tumor eradication.

Melanoma and genetics

As the incidence of malignant melanoma continues to increase and with the completion of the sequencing of the human genome, there have been increasing efforts to identify the "melanoma gene(s)." Although some patients and families have significantly increased risks due to genetic predisposition, most melanoma cases are sporadic and likely result from low to moderate risk genetic factors. This review focuses on the genes that cover the greatest risk of developing melanoma. It is important to remember that many--if not most--cases of melanoma are the result of undiscovered variants. The strongest genetic risk for the development of melanoma results from heritable alterations in cyclin-dependent kinase inhibitor 2A (CDKN2A) gene, which encodes two separate but related proteins, p16/INK4a and p14/ARF. These proteins help regulate cell division and apoptosis, both of which are necessary to maintain cellular homeostasis. Other important genes include CDK4/6 and retinoblastoma (RB1), which encode downstream proteins in the same pathway as p16/INK4a and p14/ARF. Finally, we discuss the relative importance of the melanocortin 1 receptor (MC1R) gene as a moderate risk factor for melanoma. Although great advances have been made in understanding the molecular basis and genetic predisposition of melanoma, many questions still remain to be answered. Someday soon, it will be possible to predict a patient's risk of melanoma by DNA analysis; however, it is important to reconcile our tremendous technologic capabilities with documented clinical utility.

Role of Mdm2 acid domain interactions in recognition and ubiquitination of the transcription factor IRF-2

Mdm2 (murine double minute 2)-mediated ubiquitination of the p53 tumour suppressor requires interaction of the ligase at two distinct binding sites that form general multiprotein-docking sites for the p53 protein. The first Mdm2-binding site resides in the transactivation domain of p53 and is an allosteric effector site for Mdm2-mediated p53 ubiquitination; the second site requires the acid domain of Mdm2 to recognize a 'ubiquitination signal' within p53's DNA-binding core. In order to expand on fundamental requirements for a protein to function as an Mdm2 substrate and the role of the acid domain in recognition, we have carried out a bioinformatics search for open reading frames that have homology with the Mdm2-docking sites in p53. IRF-2 [IFN (interferon) regulatory factor-2], an IFN-regulated transcription factor, has been identified as an Mdm2-binding protein and substrate requiring interactions with both the hydrophobic pocket and the acid domain of Mdm2. Mutation of either of the two Mdm2-binding sites on IRF-2 can attenuate substrate ubiquitination, confirming the requirement of a dual-site substrate interaction mechanism. Ligands that bind to the hydrophobic pocket are not sufficient to inhibit Mdm2 E3-ligase activity. Rather, acid domain-binding ligands act as E3-ligase inhibitors, lending additional support to the idea that the acid domain of Mdm2 is key to understanding its mechanism of action. The ability of Mdm2 and IRF-2 to form a complex in cells complements the biochemical assays and together establishes a novel substrate with which to develop insights into E3-ubiquitin ligase-substrate interactions in vitro and in cells.

Residues in the alternative reading frame tumor suppressor that influence its stability and p53-independent activities

The Alternative Reading Frame (ARF) protein suppresses tumorigenesis through p53-dependent and p53-independent pathways. Most of ARF's anti-proliferative activity is conferred by sequences in its first exon. Previous work showed specific amino acid changes occurred in that region during primate evolution, so we programmed those changes into human p14ARF to assay their functional impact. Two human p14ARF residues (Ala(14) and Thr(31)) were found to destabilize the protein while two others (Val(24) and Ala(41)) promoted more efficient p53 stabilization and activation. Despite those effects, all modified p14ARF forms displayed robust p53-dependent anti-proliferative activity demonstrating there are no significant biological differences in p53-mediated growth suppression associated with simian versus human p14ARF residues. In contrast, p53-independent p14ARF function was considerably altered by several residue changes. Val(24) was required for p53-independent growth suppression whereas multiple residues (Val(24), Thr(31), Ala(41) and His(60)) enabled p14ARF to block or reverse the inherent chromosomal instability of p53-null MEFs. Together, these data pinpoint specific residues outside of established p14ARF functional domains that influence its expression and signaling activities. Most intriguingly, this work reveals a novel and direct role for p14ARF in the p53-independent maintenance of genomic stability.

Study on the spatial architecture of p53, MDM2, and p14ARF containing complexes

We have developed a surface plasmon resonance (SPR)-based immunocapture approach to study multimeric protein-protein complexes. A composition and spatial architecture of protein complexes that contained GST-tagged p53, p14ARF, and MDM2 was examined by the developed approach. Obtained results verified that the p53 protein possesses two binding sites for MDM2. Ternary complexes containing p14ARF, MDM2, and p53 proteins could only be formed when MDM2 protein functions as a bridging molecule. That was confirmed by immunoprecipitation and immunostaining.