Strategies for therapeutic targeting of the p53 pathway in cancer

From actinic keratosis to cutaneous squamous cell carcinoma: the key pathogenesis and treatments

Cutaneous squamous cell carcinoma (cSCC) is the second most common non-melanoma skin cancer, among which 82% arise from actinic keratosis (AK) characterized by lesions of epidermal keratinocyte dysplasia. It is of great significance to uncover the progression mechanisms from AK to cSCC, which will facilitate the early therapeutic intervention of AK before malignant transformation. Thus, more and more studies are trying to ascertain the potential transformation mechanisms through multi-omics, including genetics, transcriptomics, and epigenetics. In this review, we gave an overview of the specific biomarkers and signaling pathways that may be involved in the pathogenesis from AK to cSCC, pointing out future possible molecular therapies for the early intervention of AK and cSCC. We also discussed current interventions on AK and cSCC, together with future perspectives.

Synthesis and biological assessment of indole derivatives containing penta-heterocycles scaffold as novel anticancer agents towards A549 and K562 cells

Herein, a new series of 2-chloro-N-(5-(2-oxoindolin-3-yl)-4H-pyrazol-3-yl) acetamide derivatives containing 1,3,4-thiadiazole (10a-i) and 4H-1,2,4-triazol-4-amine (11a-r) moiety was designed, synthesised as novel anticancer agents. The antiproliferative activity values indicated that compound 10 b stood as the most potent derivative with IC₅₀ values of 12.0 nM and 10 nM against A549 and K562 cells, respectively. Mechanism investigation and docking studies of 10 b indicated that it possessed good apoptosis characteristic and dose-dependent growth arrest of A549 and K562 cells, blocked cell cycle into G2/M phase. Interestingly, 10 b suppressed the growth of A549 and K562 cells via modulation of EGFR and p53-MDM2 mediated pathway.

GingerenoneA overcomes dexamethasone resistance by activating apoptosis and inhibiting cell proliferation in pediatric T-ALL cells

Plant-based combination strategies have been widely considered in cancer therapy to attenuate chemotherapeutics side effects. The anti-leukemic effect of the whole ginger extract was previously portrayed by our team, and the current study is centered around the cytotoxicity and mechanism of action of a phenolic subsidiary of ginger, GingerenoneA, on pediatric acute lymphoblastic leukemia. GingernoneA imposed, dose-dependently, inhibitory effects on the viability of T and B leukemia cell lines confirmed by MTT assays. Resistance to Dexamethasone, a mostly used chemotherapeutic in acute lymphoblastic leukemia treatments, was overcome by GingernoneA. A synergistic effect of Dexamethasone and GingrenoneA on T leukemia cell lines and patient primary cells was confirmed. Annexin-V/PI and acridine orange/ethidium bromide staining illustrated dose-dependent apoptosis in CCRF-CEM cells developed by GingerenoneA. The intrinsic and extrinsic apoptosis induction and antiproliferative attribution of GingerenoneA were validated by western blot and qPCR. Despite the supposed loss of function in CCRF-CEM cells, TP53 showed increased expression levels and functional activity upon treatment with GingernoneA. Bioinformatic studies revealed the conceivable impact of GingerenoneA on the reactivity of mutant P53 through its binding to Cys124. Our findings may provide novel strategies for therapeutic intervention to ameliorate pALL outcomes.

Soluble peptidoglycan fragments produced by Limosilactobacillus fermentum with antiproliferative activity are suitable for potential therapeutic development: A preliminary report

Currently, the use of probiotic strains and their products represents a promising innovative approach as an antagonist treatment against many human diseases. Previous studies showed that a strain of Limosilactobacillus fermentum (LAC92), previously defined as Lactobacillus fermentum, exhibited a suitable amensalistic property. The present study aimed to purify the active components from LAC92 to evaluate the biological properties of soluble peptidoglycan fragments (SPFs). The cell-free supernatant (CFS) and bacterial cells were separated after 48 h of growth in MRS medium broth and treated for isolation of SPFs. Antimicrobial activity and proliferation analysis on the human cell line HTC116 were performed using technologies such as xCELLigence, count and viability, and clonogenic analysis. MALDI-MS investigation and docking analysis were performed to determine the molecular structure and hypothetical mode of action, respectively. Our results showed that the antimicrobial activity was mainly due to SPFs. Moreover, the results obtained when investigating the SPF effect on the cell line HCT116 showed substantial preliminary evidence, suggesting their significant cytostatic and quite antiproliferative properties. Although MALDI was unable to identify the molecular structure, it was subsequently revealed by analysis of the bacterial genome. The amino acid structure is called peptide 92. Furthermore, we confirmed by molecular docking studies the interaction of peptide 92 with MDM2 protein, the negative regulator of p53. This study showed that SPFs from the LAC92 strain exerted anticancer effects on the human colon cancer HCT116 cell line via antiproliferation and inducing apoptosis. These findings indicated that this probiotic strain might be a potential candidate for applications in functional products in the future. Further examination is needed to understand the specific advantages of this probiotic strain and improve its functional features to confirm these data. Moreover, deeper research on peptide 92 could increase our knowledge and help us understand if it will be possible to apply to specific diseases such as CRC.

Evaluating a therapeutic window for precision medicine by integrating genomic profiles and p53 network dynamics

The response variation to anti-cancer drugs originates from complex intracellular network dynamics of cancer. Such dynamic networks present challenges to determining optimal drug targets and stratifying cancer patients for precision medicine, although several cancer genome studies provided insights into the molecular characteristics of cancer. Here, we introduce a network dynamics-based approach based on attractor landscape analysis to evaluate the therapeutic window of a drug from cancer signaling networks combined with genomic profiles. This approach allows for effective screening of drug targets to explore potential target combinations for enhancing the therapeutic window of drug responses. We also effectively stratify patients into desired/undesired response groups using critical genomic determinants, which are network-specific origins of variability to drug response, and their dominance relationship. Our methods provide a viable and quantitative framework to connect genotype information to the phenotypes of drug response with regard to network dynamics determining the therapeutic window.

Molecular characteristics of uveal melanoma and intraocular tumors

Malignant melanomas within the eye present different types of metabolic and metastatic behavior. Uveal melanoma (UM) affects a quarter of a million individuals in the USA; however, the molecular pathogenesis is not well understood. Although UV radiation is a risk factor in cutaneous melanomas, it is not crucial for UM progression. Apart from chromosomal abnormalities, numerous major tumorigenic signaling pathways, including the PI3K/Akt, MAPK/ERK, Ras-association domain family 1 isoform A and Yes-associated protein/transcriptional co-activator with PDZ-binding motif signaling pathways, are associated with intraocular tumors. The present review describes the current insights regarding these signaling pathways that regulate the cell cycle and apoptosis, and could be used as potential targets for the treatment of UMs.

Hepatocellular Senescence: Immunosurveillance and Future Senescence-Induced Therapy in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the third leading cause of cancer-related deaths worldwide. The lack of effective targeted drugs has become a challenge on treating HCC patients. Cellular senescence is closely linked to the occurrence, development, and therapy of tumor. Induction of cellular senescence and further activation of immune surveillance provides a new strategy to develop HCC targeted drugs, that is, senescence-induced therapy for HCC. Precancerous hepatocytes or HCC cells can be induced into senescent cells, subsequently producing senescence-associated secretory phenotype (SASP) factors. SASP factors recruit and activate various types of immune cells, including T cells, NK cells, macrophages, and their subtypes, which carry out the role of immune surveillance and elimination of senescent cells, ultimately preventing the occurrence of HCC or inhibiting the progression of HCC. Specific interventions in several checkpoints of senescence-mediated therapy will make positive contributions to suppress tumorigenesis and progression of HCC, for instance, by applying small molecular compounds to induce cellular senescence or selecting cytokines/chemokines to activate immunosurveillance, supplementing adoptive immunocytes to remove senescent cells, and screening chemical drugs to induce apoptosis of senescent cells or accelerate clearance of senescent cells. These interventional checkpoints become potential chemotherapeutic targets in senescence-induced therapy for HCC. In this review, we focus on the frontiers of senescence-induced therapy and discuss senescent characteristics of hepatocytes during hepatocarcinogenesis as well as the roles and mechanisms of senescent cell induction and clearance, and cellular senescence-related immunosurveillance during the formation and progression of HCC.

Tumor-Suppressor p53TAD1-60 Forms a Fuzzy Complex with Metastasis-Associated S100A4: Structural Insights and Dynamics by an NMR/MD Approach

Conformationally flexible protein complexes represent a major challenge for structural and dynamical studies. We present herein a method based on a hybrid NMR/MD approach to characterize the complex formed between the disordered p53TAD1-60 and the metastasis-associated S100A4. Disorder-to-order transitions of both TAD1 and TAD2 subdomains upon interaction is detected. Still, p53TAD1-60 remains highly flexible in the bound form, with residues L26, M40, and W53 being anchored to identical hydrophobic pockets of the S100A4 monomer chains. In the resulting "fuzzy" complex, the clamp-like binding of p53TAD1-60 relies on specific hydrophobic anchors and on the existence of extended flexible segments. Our results demonstrate that structural and dynamical NMR parameters (cumulative Δδ, SSP, temperature coefficients, relaxation time, hetNOE) combined with MD simulations can be used to build a structural model even if, due to high flexibility, the classical solution structure calculation is not possible.

Antitumor Activity of New Olivacine Derivatives

Olivacine is an alkaloid-containing pyridocarbazole structure. It is isolated from the bark of the evergreen timber tree, Aspidosperma olivaceum. Its well-documented anticancer activity led to the synthesis of new derivatives, which are semisynthetic and fully synthetic pyridocarbazoles. This study aimed to evaluate the potential antineoplastic activity of four newly synthesized olivacine derivatives. Multidrug resistance is a common phenomenon causing failure in the chemotherapy of many tumors. It is mainly related to increased function of P-glycoprotein, an efflux pump removing cytostatic out of the cells. The cell lines used in the study were colorectal carcinoma cell lines: LoVo (doxorubicin-sensitive) and LoVo/DX (doxorubicin-resistant). The NHDF cell line was used to assess cell viability. First, the cells were incubated with olivacine derivatives. In the next step, the following assays were performed: DCF-DA assay, MTT assay, rhodamine 123 assay, detection of apoptosis, proliferation inhibition-mitotic index. The tested compounds showed higher antineoplastic potential and lower toxicity than the reference compound ellipticine. The results indicate that the new olivacine derivatives are good candidates for future anticancer drugs.

Effect of new olivacine derivatives on p53 protein level

BACKGROUND

The p53 protein is a transcription factor for many genes, including genes involved in inhibiting cell proliferation and inducing apoptosis in genotoxically damaged and tumor-transformed cells. In more than 55% of cases of human cancers, loss of the essential function of p53 protein is found. In numerous reports, it has been shown that small molecules (chemical compounds) can restore the suppressor function of the mutant p53 protein in tumor cells. The aim of this study was to evaluate the potential anticancer activity of three newly synthesized olivacine derivatives.

METHODS

The study was performed using two cell lines-CCRF/CEM (containing the mutant p53 protein) and A549 (containing a non-mutant, wild-type p53 protein). The cells were incubated with olivacine derivatives for 18 h and then assays were carried out: measurement of the amount of p53 and p21 proteins, detection of apoptosis, cell cycle analysis, and rhodamine 123 accumulation assay (evaluation of P-glycoprotein inhibition). Multiple-criteria decision analysis was used to compare the anticancer activity of the tested compounds.

RESULTS

Each tested compound caused the reconstitution of suppressor activity of the p53 protein in cells with the mutant protein. In addition, one of the compounds showed significant antitumor activity in both wild-type and mutant cells. For all compounds, a stronger effect on the level of the p53 protein was observed than for the reference compound-ellipticine.

CONCLUSIONS

The observed effects of the tested new olivacine derivatives (pyridocarbazoles) suggest that they are good candidates for new anticancer drugs.

Development of a DNA biosensor based on MCM41 modified screen-printed graphite electrode for the study of the short sequence of the p53 tumor suppressor gene in hybridization and its interaction with the flutamide drug using hemin as the electrochemical label

Electrochemical DNA biosensors are particularly attractive because of their high sensitivity, suitability and compatibility with miniaturization in nucleic acid technology.

The Localization of p53 in the Crayfish Mechanoreceptor Neurons and Its Role in Axotomy-Induced Death of Satellite Glial Cells Remote from the Axon Transection Site

Neuron and glia death after axon transection is regulated by various signaling proteins. Protein p53 is a key regulator of diverse cell functions including stress response, DNA repair, proliferation, and apoptosis. We showed that p53 was overexpressed in crayfish ganglia after bilateral axotomy. In the isolated crayfish stretch receptor, a simple natural neuroglial preparation, which consists of a single mechanoreceptor neuron (MRN) enveloped by glial cells, p53 regulated axotomy-induced death of glial cells remote from the axon transection site. In MRN, p53 immunofluorescence was highest in the nucleolus and in the narrow cytoplasmic ring around the nucleus; its levels in the nucleus and cytoplasm were lower. After axotomy, p53 accumulated in the neuronal perikaryon. Its immunofluorescence also increased in the neuronal and glial nuclei. However, p53 immunofluorescence in the most of neuronal nucleoli disappeared. Axotomy-induced apoptosis of remote glial cells increased in the presence of p53 activators WR-1065 and nutlin-3 but reduced by pifithrin-α that inhibits transcriptional activity of p53. Pifithrin-μ that inhibits p53 effect on mitochondria increased axotomy-induced apoptosis of remote glial cells but reduced their necrosis. Therefore, axotomy-induced apoptosis of remote glial cells was associated with p53 effect on transcription processes, whereas glial necrosis was rather associated with transcription-independent p53 effect on mitochondria. Apparently, the fate of remote glial cells in the axotomized crayfish stretch receptor is determined by the balance between different modalities of p53 activity.

Genetic alterations in anaplastic thyroid carcinoma and targeted therapies

Thyroid cancer is the most common type of endocrine malignancy, and its incidence is increasing. Anaplastic thyroid cancer (ATC), referring to undifferentiated subtypes, is considered to be aggressive and associated with poor prognosis. Conventional therapies, including surgery, chemotherapy and radioiodine therapy, have been used for ATC, but these do not provide any significant reduction of the overall mortality rate. The tumorigenesis, development, dedifferentiation and metastasis of ATC are closely associated with the activation of various tyrosine cascades and inactivation of tumor suppressor genes, including B-Raf proto-oncogene, serine/threonine kinase^V600E, phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit α,tumor protein 53 mutations and telomerase reverse transcriptase mutation. These pathways exert their functions individually or through a complex network. Identification of these mutations may provide a deeper understanding of ATC. A variety of tyrosine kinase inhibitors have been successfully employed for controlling ATC growth in vitro and in xenografts. Certain novel compounds are still in clinical trials. Multi-kinase inhibitors provide a novel approach with great potential. This systematic review determined the prevalence of the major genetic alterations and their inhibitors in ATC.

Network dynamics-based cancer panel stratification for systemic prediction of anticancer drug response

Cancer is a complex disease involving multiple genomic alterations that disrupt the dynamic response of signaling networks. The heterogeneous nature of cancer, which results in highly variable drug response, is a major obstacle to developing effective cancer therapy. Previous studies of cancer therapeutic response mostly focus on static analysis of genome-wide alterations, thus they are unable to unravel the dynamic, network-specific origin of variation. Here we present a network dynamics-based approach to integrate cancer genomics with dynamics of biological network for drug response prediction and design of drug combination. We select the p53 network as an example and analyze its cancer-specific state transition dynamics under distinct anticancer drug treatments by attractor landscape analysis. Our results not only enable stratification of cancer into distinct drug response groups, but also reveal network-specific drug targets that maximize p53 network-mediated cell death, providing a basis to design combinatorial therapeutic strategies for distinct cancer genomic subtypes.

Genomic alterations underlie the variability of drug responses between cancers, but our mechanistic understanding is limited. Here the authors use the p53 network to study how rewiring of signalling networks by genomic alterations impact their dynamic response to pharmacological perturbation.

MicroRNA-21 versus microRNA-34: Lung cancer promoting and inhibitory microRNAs analysed in silico and in vitro and their clinical impact

MicroRNAs are well-known strong RNA regulators modulating whole functional units in complex signaling networks. Regarding clinical application, they have potential as biomarkers for prognosis, diagnosis, and therapy. In this review, we focus on two microRNAs centrally involved in lung cancer progression. MicroRNA-21 promotes and microRNA-34 inhibits cancer progression. We elucidate here involved pathways and imbed these antagonistic microRNAs in a network of interactions, stressing their cancer microRNA biology, followed by experimental and bioinformatics analysis of such microRNAs and their targets. This background is then illuminated from a clinical perspective on microRNA-21 and microRNA-34 as general examples for the complex microRNA biology in lung cancer and its diagnostic value. Moreover, we discuss the immense potential that microRNAs such as microRNA-21 and microRNA-34 imply by their broad regulatory effects. These should be explored for novel therapeutic strategies in the clinic.

The Inhibitory Effect of Ginger Extract on Ovarian Cancer Cell Line; Application of Systems Biology

Purpose: Ginger is a natural compound with anti-cancer properties. The effects of ginger and its mechanism on ovarian cancer and its cell line model, SKOV-3, are unclear. In this study, we have evaluated the effect of ginger extract on SKOV-3. Methods: SKOV-3 cells were incubated with ginger extract for 24, 48 and 72 hours. Cell toxicity assay was performed. Different data mining algorithms were applied to highlight the most important features contributing to ginger inhibition on the SKOV-3 cell proliferation. Moreover, Real-Time PCR was performed to assay p53, p21 and bcl-2 genes expression. For co-expression meta-analysis of p53, mutual ranking (MR) index and transformation to Z-values (Z distribution) were applied on available transcriptome data in NCBI GEO data repository. Results: The ginger extract significantly inhibited cancer growth in ovarian cancer cell line. The most important attribute was 60 µg/ml concentration which received weights higher than 0.50, 0.75 and 0.95 by 90%, 80% and 50% of feature selection models, respectively. The expression level of p53 was increased sharply in response to ginger treatment. Systems biology analysis and meta-analysis of deposited expression value in NCBI based on rank of correlation and Z-transformation approach unraveled the key co-expressed genes and co-expressed network of P53, as the key transcription factor induced by ginger extract. High co-expression between P53 and the other apoptosis-inducing proteins such as CASP2 and DEDD was noticeable, suggesting the molecular mechanism underpinning of ginger action. Conclusion: We found that the ginger extract has anticancer properties through p53 pathway to induce apoptosis.

Bacterial Carriers for Glioblastoma Therapy

Treatment of aggressive glioblastoma brain tumors is challenging, largely due to diffusion barriers preventing efficient drug dosing to tumors. To overcome these barriers, bacterial carriers that are actively motile and programmed to migrate and localize to tumor zones were designed. These carriers can induce apoptosis via hypoxia-controlled expression of a tumor suppressor protein p53 and a pro-apoptotic drug, Azurin. In a xenograft model of human glioblastoma in rats, bacterial carrier therapy conferred a significant survival benefit with 19% overall long-term survival of >100 days in treated animals relative to a median survival of 26 days in control untreated animals. Histological and proteomic analyses were performed to elucidate the safety and efficacy of these carriers, showing an absence of systemic toxicity and a restored neural environment in treated responders. In the treated non-responders, proteomic analysis revealed competing mechanisms of pro-apoptotic and drug-resistant activity. This bacterial carrier opens a versatile avenue to overcome diffusion barriers in glioblastoma by virtue of its active motility in extracellular space and can lead to tailored therapies via tumor-specific expression of tumoricidal proteins.

Piperlongumine exerts cytotoxic effects against cancer cells with mutant p53 proteins at least in part by restoring the biological functions of the tumor suppressor

Piperlongumine (PL), a small molecule alkaloid present in black pepper (Piper longum), has been reported to kill tumor cells irrespective of their p53 gene status, however, the mechanisms involved are unknown. Since p53 is a redox-sensitive protein, we hypothesized that the redox imbalance induced by PL may affect the structure and/or function of the mutant p53 protein and promote cell death. We used two human colon cancer cell lines, the HT29 and SW620 which harbor the R273H DNA contact abrogatory mutation in p53. PL treatment induced significant ROS production and protein glutathionylation with a concomitant increase in Nrf-2 expression in both cell lines. Surprisingly, immunoprecipitation with wt-p53 specific antibodies (PAb1620) or direct western blotting showed a progressive generation of wild-type-like p53 protein along with a loss of its mutant counterpart in PL-treated HT29 and SW620 cells. Moreover, the EMSA and DNA-affinity blotting revealed a time-dependent restoration of DNA-binding for the mutant p53, which was accompanied by the induction of p53 target genes, MDM2 and Bax. PL, while cytotoxic by itself, also increased the cell killing by many anticancer drugs. In nude mice bearing the HT29 tumors, PL alone (7.5 mg/kg daily) produced a 40% decrease in tumor volume, which was accompanied by diminished intratumoral mutant p53 protein levels. The antitumor efficacy of BCNU or doxorubicin in HT29 xenografts was highly potentiated by PL, followed by expression of apoptotic proteins. These clinically-relevant findings suggest that PL-induced oxidative milieu facilitates a weak functional restoration of mutant p53 through protein glutathionylation and contributes to the increased drug sensitivity.

Integrin α5β1 and p53 convergent pathways in the control of anti-apoptotic proteins PEA-15 and survivin in high-grade glioma

Integrin α5β1 expression is correlated with a worse prognosis in high-grade glioma. We previously unraveled a negative crosstalk between integrin α5β1 and p53 pathway, which was proposed to be part of the resistance of glioblastoma to chemotherapies. The restoration of p53 tumor-suppressor function is under intensive investigations for cancer therapy. However, p53-dependent apoptosis is not always achieved by p53-reactivating compounds such as Nutlin-3a, although full transcriptional activity of p53 could be obtained. Here we investigated whether integrin α5β1 functional inhibition or repression could sensitize glioma cells to Nutlin-3a-induced p53-dependent apoptosis. We discovered that α5β1 integrin-specific blocking antibodies or small RGD-like antagonists in association with Nutlin-3a triggered a caspase (Casp) 8/Casp 3-dependent strong apoptosis in glioma cells expressing a functional p53. We deciphered the molecular mechanisms involved and we showed the crucial role of two anti-apoptotic proteins, phosphoprotein enriched in astrocytes 15 (PEA-15) and survivin in glioma cell apoptotic outcome. PEA-15 is under α5β1 integrin/AKT (protein kinase B) control and survivin is a p53-repressed target. Moreover, interconnections between integrin and p53 pathways were revealed. Indeed PEA-15 repression by specific small-interfering RNA (siRNA)-activated p53 pathway to repress survivin and conversely survivin repression by specific siRNA decreased α5β1 integrin expression. This pro-apoptotic loop could be generalized to several glioma cell lines, whatever their p53 status, inasmuch PEA-15 and survivin protein levels were decreased. Our findings identify a novel mechanism whereby inhibition of α5β1 integrin and activation of p53 modulates two anti-apoptotic proteins crucially involved in the apoptotic answer of glioma cells. Importantly, our results suggest that high-grade glioma expressing high level of α5β1 integrin may benefit from associated therapies including integrin antagonists and repressors of survivin expression.

Nutlin-3 sensitizes nasopharyngeal carcinoma cells to cisplatin-induced cytotoxicity

The small-molecule inhibitor of p53-Mdm2 interaction, Nutlin-3, is known to be effective against cancers expressing wild-type (wt) p53. p53 mutations are rare in nasopharyngeal carcinoma (NPC), hence targeting disruption of p53-Mdm2 interaction to reactivate p53 may offer a promising therapeutic strategy for NPC. In the present study, the effects of Nutlin-3 alone or in combination with cisplatin, a standard chemotherapeutic agent, were tested on C666-1 cells, an Epstein-Barr virus (EBV)-positive NPC cell line bearing wt p53. Treatment with Nutlin-3 activated the p53 pathway and sensitized NPC cells to the cytotoxic effects of cisplatin. The combined treatment also markedly suppressed soft agar colony growth formation and increased apoptosis of NPC cells. The effect of Nutlin-3 on NPC cells was inhibited by knockdown of p53, suggesting that its effect was p53-dependent. Extended treatment with increasing concentrations of Nutlin-3 did not result in emergence of p53 mutations in the C666-1 cells. Collectively, the present study revealed supportive evidence of the effectiveness of combining cisplatin and Nutlin-3 as a potential therapy against NPC.

MiR-34a inhibits migration and invasion by regulating the SIRT1/p53 pathway in human SW480 cells

MicroRNA-34a (miR-34a) is a direct transcriptional target of p53, and is downregulated in several different types of cancer. However, the underlying mechanism of the miR-34a effects in colorectal cancer is not well understood. In this study, we explored the role of miR-34a in cell invasion, migration, and apoptosis. Transient overexpression of miR-34a in SW480 cells caused a severe decrease in cell migration and invasion (both, p<0.05) compared to the control groups. Combining miR-34a transfection with 5-fluorouracil (5-FU) treatment further enhanced the inhibition in SW480 cell migration and invasion (both, p<0.05) compared to 5-FU treatment alone. These cellular changes were associated with upregulation of acetylated‑p53 (ac-p53) and p21 and downregulation of sirtuin 1 (SIRT1). These data demonstrate that miR-34a regulates the expression of a number of critical proteins involved in apoptosis, proliferation and the response to chemotherapy. In summary, miR-34a increases the sensitivity of colon cancer cells to 5-FU treatment through specific regulation of the SIRT1/p53 pathway.

Small molecule MIRA-1 induces in vitro and in vivo anti-myeloma activity and synergizes with current anti-myeloma agents

Background:

Small molecule MIRA-1 induced mutant p53-dependent apoptosis in several types of solid tumours. However, anti-tumour activity of MIRA-1 in haematological malignancies including multiple myeloma (MM) is unknown. In this study, we evaluated the effect of MIRA-1 in MM.

Methods:

We examined the anti-tumour activity of MIRA-1 alone or in combination with current anti-myeloma agents in a panel of MM cell lines, primary MM samples, and in a mouse xenograft model of MM.

Results:

MIRA-1 treatment resulted in the inhibition of viability, colony formation, and migration and increase in apoptosis of MM cells irrespective of p53 status accompanied by upregulation of Puma and Bax and downregulation of Mcl-1 and c-Myc. Genetic knockdown of p53 did not abrogate apoptotic response of MIRA-1. MIRA-1 triggered activation of PERK and IRE-α leading to splicing of XBP1 indicating an association of endoplasmic reticulum stress response. Furthermore, combined treatment of MIRA-1 with dexamethasone, doxorubicin or velcade displayed synergistic response in MM cells. Importantly, MIRA-1 alone or in combination with dexamethasone retarded tumour growth and prolonged survival without showing any untoward toxicity in the mice bearing MM tumour.

Conclusions:

Our data provide the preclinical framework for clinical evaluation of MIRA-1 as a novel therapeutic agent to improve patient outcome in MM.

Murine double minute 2 siRNA and wild-type p53 gene therapy enhances sensitivity of the SKOV3/DDP ovarian cancer cell line to cisplatin chemotherapy in vitro and in vivo

SKOV3/DDP cells urgently require an efficient therapy to improve drug resistance. Here we show a critical role for cisplatin combined with gene therapy, using transfection of a p53 gene/MDM2-siRNA plasmid, in improving cisplatin sensitivity of SKOV3/DDP cells with a strong inhibition of tumor cell growth in vitro and in vivo. The effects may be associated with enhancement of intracellular platinum accumulation via decreased MDR1/P-gp and improvement of apoptotic resistance via increased P53, PUMA and NOXA expression. The combined therapy may efficiently inhibit cell invasion and migration via deceased HIF-1, VEGF, MMP-9 and MMP-2 to suppress malignant progression. These results indicate that cisplatin chemotherapy combined with targeting the MDM2/p53 axis is an attractive strategy to treat SKOV3/DDP cancer.

Disarming mutant p53 oncogenic function

In the last decade intensive research has confirmed the long standing hypothesis that some p53 point mutants acquire novel activities able to cooperate with oncogenic mechanisms. Particular attention has attracted the ability of several such mutants to actively promote the development of aggressive and metastatic tumors in vivo. This knowledge opens a new dimension on rational therapy design, suggesting novel strategies based on pharmacological manipulation of those neomorphic activities. P53 point mutants have several characteristics that make them attractive targets for anti-cancer therapies. Remarkably, mutant p53 has been found predominantly in tumor cells and may act pleiotropically by interfering with a variety of cellular processes. Therefore, drugs targeting mutant p53 may selectively affect tumor cells, inactivating simultaneously several mechanisms of tumor promotion. Moreover, the high frequency of missense mutations on the p53 gene suggests that interfering with mutant p53 function may provide a valuable approach for the development of efficient therapies able to target a wide range of tumor types.

Induction of p53 expression and apoptosis by a recombinant dual-target MDM2/MDMX inhibitory protein in wild-type p53 breast cancer cells

The tumor suppressor gene p53 is often inactivated in breast cancer cells due to gene mutation or overexpression of its repressors (such as murine double minute 2 and murine double minute X). Inhibitors of murine double minute 2 (MDM2) and murine double minute X (MDMX) could lead to tumor suppression by restoration of p53 activity and such an approach is a promising strategy for future control of breast cancer. This study aimed to investigate the feasibility of the recombinant MDM2 and MDMX inhibitory protein in control of breast cancer in vitro. A cell-permeable dual-target MDM2/MDMX inhibitory protein was expressed in E. coli and incubated with p53 wild-type breast cancer cells. The data showed that this recombinant MDM2/MDMX inhibitory protein reduced the viability of MCF-7 and ZR-75-30 breast cancer cell lines and promoted cell cycle arrest and apoptosis by activation and stabilization of the p53 protein. Mechanistically, this MDM2/MDMX inhibitory protein increased the expression of p21, Bax and puma proteins, and inhibitory expression of MDM2 and MDMX proteins. This recombinant protein showed a better in vitro effect than that of nutlin-3α, a small molecule MDM2 inhibitor. The data further support the hypothesis that targeting of the p53 gene pathway could effectively control breast cancer.

PRIMA-1Met/APR-246 displays high antitumor activity in multiple myeloma by induction of p73 and Noxa

Targeting p53 by the small-molecule PRIMA-1(Met)/APR-246 has shown promising preclinical activity in various cancer types. However, the mechanism of PRIMA-1(Met)-induced apoptosis is not completely understood and its effect on multiple myeloma cells is unknown. In this study, we evaluated antitumor effect of PRIMA-1(Met) alone or its combination with current antimyeloma agents in multiple myeloma cell lines, patient samples, and a mouse xenograft model. Results of our study showed that PRIMA-1(Met) decreased the viability of multiple myeloma cells irrespective of p53 status, with limited cytotoxicity toward normal hematopoietic cells. Treatment of multiple myeloma cells with PRIMA-1(Met) resulted in induction of apoptosis, inhibition of colony formation, and migration. PRIMA-1(Met) restored wild-type conformation of mutant p53 and induced activation of p73 upregulating Noxa and downregulating Mcl-1 without significant modulation of p53 level. siRNA-mediated silencing of p53 showed a little effect on apoptotic response of PRIMA-1(Met), whereas knockdown of p73 led to substantial attenuation of apoptotic activity in multiple myeloma cells, indicating that PRIMA-1(Met)-induced apoptosis is, at least in part, p73-dependent. Importantly, PRIMA-1(Met) delayed tumor growth and prolonged survival of mice bearing multiple myeloma tumor. Furthermore, combined treatment of PRIMA-1(Met) with dexamethasone or doxorubicin displayed synergistic effects in both multiple myeloma cell lines and primary multiple myeloma samples. Consistent with our in vitro observations, cotreatment with PRIMA-1(Met) and dexamethasone resulted in enhanced antitumor activity in vivo. Our study for the first time shows antimyeloma activity of PRIMA-1(Met) and provides the rationale for its clinical evaluation in patients with multiple myeloma, including the high-risk group with p53 mutation/deletion.

Enhanced therapeutic effect of cisplatin on the prostate cancer in tumor-bearing mice by transfecting the attenuated Salmonella carrying a plasmid co-expressing p53 gene and mdm2 siRNA

Prostate cancer urgently needs an efficient therapy. Here we demonstrated that cisplatin combined with gene therapy by transfecting the attenuated Salmonella that carry a plasmid containing p53 gene and MDM2 siRNA provided a super-synergistic effect on the inhibition of prostate cancer growth in vivo. This synergistic therapy was associated with the induction of apoptotic cell death with a decreased Bcl2 to Bax expression ratio and increased expression of cleaved caspase 3 and caspase 9 in the prostate cancer xenograft. These results indicate that cisplatin-chemotherapy in combination with targeting the MDM2/p53 axis is an attractive strategy to treat prostate cancer.

Co-transduction of ribosomal protein L23 enhances the therapeutic efficacy of adenoviral-mediated p53 gene transfer in human gastric cancer

Induction of murine double minute 2 (MDM2) expression is thought to be a determinant of resistance to p53 gene therapy for cancer. Previous studies have revealed that ribosomal protein L23 (RPL23) inhibits MDM2-mediated p53 degradation through direct binding to MDM2. In addition, ectopically expressed RPL23 was reported to interact with MDM2 in both the nucleus and cytoplasm, by which RPL23 indirectly inhibited MDM2-p53 binding. Based on the known molecular properties of the RPL23 protein, it was speculated that co-transduction of RPL23 may protect wild‑type p53 protein from MDM2-mediated inactivation and, thus, improve the effect of delivering therapeutic exogenous p53. To test this hypothesis, we constructed a bicistronic adenoviral vector expressing both the RPL23 and p53 genes (Ad-RPL23/p53) and compared its tumor-suppressor activity in human gastric cancer with that of a single gene vector for p53 (Ad-p53). In the in vivo and in vitro experiments, we observed that treatment with Ad-RPL23/p53 resulted in a stronger antitumor response compared to that obtained using Ad-p53. Moreover, the antitumor response of the bicistronic adenovirus was obtained not only in MGC803 cells (endogenous mutant p53) but also in MKN45 cells (endogenous wild‑type p53) which were initially resistant to p53 gene transfer, indicating that co-transduction of RPL23 also expanded the utility of p53 gene therapy. Furthermore, in an orthotopic nude mouse model of human gastric cancer, we found that the survival benefit was greater after Ad-RPL23/p53 treatment than after Ad-p53. Taken together, the data presented here demonstrate that co-transduction of RPL23 enhances the therapeutic efficacy of adenoviral-mediated p53 gene transfer in models of human gastric cancer and support the use of this strategy for cancer treatment.

Licochalcone A enhances geldanamycin-induced apoptosis through reactive oxygen species-mediated caspase activation

BACKGROUND AND PURPOSE

Geldanamycin and licochalcone A induce apoptosis in cancer cells. However, whether the combination of geldanamycin and licochalcone A-induced apoptosis in epithelial ovarian cancer cells is mediated by the formation of reactive oxygen species, leading to the activation of apoptotic caspase, has not been studied.

EXPERIMENTAL APPROACH

Using the human epithelial ovarian carcinoma cell lines OVCAR-3 and SK-OV-3, we investigated the promoting effect of licochalcone A on geldanamycin-induced apoptosis.

RESULTS

Geldanamycin induced changes in apoptosis-related protein levels, loss of the mitochondrial transmembrane potential, release of cytochrome c, activation of caspases, cleavage of PARP-1, formation of reactive oxygen species and depletion of glutathione (GSH). Licochalcone A enhanced geldanamycin-induced apoptosis-related protein activation, formation of reactive oxygen species, caspase activation and cell death. The combined effect was inhibited by the addition of oxidant scavengers.

CONCLUSIONS

Licochalcone A may potentiate the apoptotic effect of geldanamycin on ovarian carcinoma cell lines by the activation of the caspase-8- and Bid-dependent pathways and the mitochondria-mediated apoptotic pathway. The apoptosis-promoting effect of licochalcone A may be mediated by its stimulatory action on the formation of reactive oxygen species and the depletion of GSH, which results in the activation of caspases.

Brefeldin a induces apoptosis by activating the mitochondrial and death receptor pathways and inhibits focal adhesion kinase-mediated cell invasion

Brefeldin A induces apoptosis in various cancer cells; however, the apoptotic process in cancer cells exposed to brefeldin A remains unclear. In addition, it is unclear whether brefeldin A-induced apoptosis is mediated by the formation of reactive oxygen species. Furthermore, the effect of brefeldin A on the invasion and migration of human epithelial ovarian cancer cells has not been studied. Therefore, we investigated the effect of brefeldin A on apoptosis, cell adhesion and migration using the human epithelial ovarian carcinoma cell lines OVCAR-3 and SK-OV-3. The results suggest that brefeldin A may induce apoptotic cell death in ovarian carcinoma cell lines by activating the mitochondrial pathway and the caspase-8- and Bid-dependent pathways. The apoptotic effect of brefeldin A seems to be mediated by formation of reactive oxygen species and depletion of GSH, which results in the activation of apoptotic caspases. Brefeldin A inhibited foetal bovine serum-induced adhesion and migration of OVCAR-3 cells. Brefeldin A may prevent the foetal bovine serum-induced cell adhesion and migration by limiting the focal adhesion kinase-dependent activation of cytoskeletal-associated components.

p28, a first in class peptide inhibitor of cop1 binding to p53

BACKGROUND

A 28 amino-acid (aa) cell-penetrating peptide (p28) derived from azurin, a redox protein secreted from the opportunistic pathogen Pseudomonas aeruginosa, produces a post-translational increase in p53 in cancer cells by inhibiting its ubiquitination.

METHODS

In silico computational simulations were used to predict motifs within the p53 DNA-binding domain (DBD) as potential sites for p28 binding. In vitro direct and competitive pull-down studies as well as western blot and RT-PCR analyses were used to validate predictions.

RESULTS

The L1 loop (aa 112-124), a region within the S7-S8 loop (aa 214-236) and T140, P142, Q144, W146, R282 and L289 of the p53DBD were identified as potential sites for p28 binding. p28 decreased the level of the E3 ligase COP1 >80%, in p53wt and p53mut cells with no decrease in COP1 in p53dom/neg or p53null cells. Brief increases in the expression of the E3 ligases, TOPORS, Pirh2 and HDM2 (human double minute 2) in p53wt and p53mut cells were in response to sustained increases in p53.

CONCLUSION

These data identify the specific motifs within the DBD of p53 that bind p28 and suggest that p28 inhibition of COP1 binding results in the sustained, post-translational increase in p53 levels and subsequent inhibition of cancer cell growth independent of an HDM2 pathway.

Identification of a small molecule 1,4-bis-[4-(3-phenoxy-propoxy)-but-2-ynyl]-piperazine as a novel inhibitor of the transcription factor p53

AIM

To identify novel small compound inhibitor of p53 protein.

METHODS

Mouse embryonic fibroblasts (MEF) and mouse embryonic stem (ES) cells were tested. Cell proliferation rate was determined using a Cell Proliferation Kit. The mRNA and protein levels of p53-related genes were measured using real-time PCR and Western blotting, respectively. Global response in the p53 signaling network was analyzed using Illumina whole-genome expression BeadChips.

RESULTS

Treatment of MEF cells with a small molecule 1,4-bis-[4-(3-phenoxy-propoxy)-but-2-ynyl]-piperazine (G5) at 10 μmol/L for 24 h markedly reduced the mRNA and protein levels of the p53 downstream genes MDM2 and p21. In G5-treated ES cells, a total of 372 differentially expressed genes were identified, and 18 among them were direct downstream genes of p53; 6 out of 9 p53-repressed genes were upregulated, and 5 out of 9 p53-activated genes were downregulated. In both MEF cells and ES cells, treatment of with G5 (10 μmol/L) up to 48 h neither affected the proliferation rate nor caused morphological alterations.

CONCLUSION

G5 inhibits p53 activity and simultaneously preserves the normal growth and proliferation of cells, therefore is a new compound for studies of p53-mediated cell manipulation.

The ubiquitin proteasome system - implications for cell cycle control and the targeted treatment of cancer

Two families of E3 ubiquitin ligases are prominent in cell cycle regulation and mediate the timely and precise ubiquitin-proteasome-dependent degradation of key cell cycle proteins: the SCF (Skp1/Cul1/F-box protein) complex and the APC/C (anaphase promoting complex or cyclosome). While certain SCF ligases drive cell cycle progression throughout the cell cycle, APC/C (in complex with either of two substrate recruiting proteins: Cdc20 and Cdh1) orchestrates exit from mitosis (APC/C(Cdc20)) and establishes a stable G1 phase (APC/C(Cdh1)). Upon DNA damage or perturbation of the normal cell cycle, both ligases are involved in checkpoint activation. Mechanistic insight into these processes has significantly improved over the last ten years, largely due to a better understanding of APC/C and the functional characterization of multiple F-box proteins, the variable substrate recruiting components of SCF ligases. Here, we review the role of SCF- and APC/C-mediated ubiquitylation in the normal and perturbed cell cycle and discuss potential clinical implications of SCF and APC/C functions. This article is part of a Special Issue entitled: Ubiquitin-Proteasome System. Guest Editors: Thomas Sommer and Dieter H. Wolf.

Senescence and aging: the critical roles of p53

p53 functions as a transcription factor involved in cell-cycle control, DNA repair, apoptosis and cellular stress responses. However, besides inducing cell growth arrest and apoptosis, p53 activation also modulates cellular senescence and organismal aging. Senescence is an irreversible cell-cycle arrest that has a crucial role both in aging and as a robust physiological antitumor response, which counteracts oncogenic insults. Therefore, via the regulation of senescence, p53 contributes to tumor growth suppression, in a manner strictly dependent by its expression and cellular context. In this review, we focus on the recent advances on the contribution of p53 to cellular senescence and its implication for cancer therapy, and we will discuss p53's impact on animal lifespan. Moreover, we describe p53-mediated regulation of several physiological pathways that could mediate its role in both senescence and aging.

Selective inhibition of cell death in malignant vs normal B-cell precursors: implications for cAMP in development and treatment of BCP-ALL

B-cell precursor acute lymphoblastic leukemia (BCP-ALL) is the most commonly occurring pediatric cancer. Despite its relatively good prognosis, there is a steady search for strategies to improve treatment effects and prevent the undesired side effects on normal cells. In the present paper, we demonstrate a differential effect of cyclic adenosine monophosphate (cAMP) signaling between normal BCPs and BCP-ALL blasts, pointing to a potential therapeutic window allowing for manipulation of cAMP signaling in the treatment of BCP-ALL. By studying primary cells collected from pediatric BCP-ALL patients and healthy controls, we found that cAMP profoundly decreased basal and DNA damage-induced p53 levels and cell death in malignant cells, whereas normal BCP counterparts displayed slightly augmented cell death when exposed to cAMP-increasing agents. We did not find evidence for a selection process involving generation of increased basal cAMP levels in BCP-ALL cells, but we demonstrate that paracrine signaling involving prostaglandin E2-induced cAMP generation has the potential to suppress p53 activation and cell death induction. The selective inhibitory effect of cAMP signaling on DNA damage-induced cell death in BCP-ALL cells appears to be an acquired trait associated with malignant transformation, potentially allowing the use of inhibitors of this pathway for directed killing of the malignant blasts.

FOXM1 (Forkhead box M1) in tumorigenesis: overexpression in human cancer, implication in tumorigenesis, oncogenic functions, tumor-suppressive properties, and target of anticancer therapy

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor and is also intimately involved in tumorigenesis. FOXM1 stimulates cell proliferation and cell cycle progression by promoting the entry into S-phase and M-phase. Additionally, FOXM1 is required for proper execution of mitosis. In accordance with its role in stimulation of cell proliferation, FOXM1 exhibits a proliferation-specific expression pattern and its expression is regulated by proliferation and anti-proliferation signals as well as by proto-oncoproteins and tumor suppressors. Since these factors are often mutated, overexpressed, or lost in human cancer, the normal control of the foxm1 expression by them provides the basis for deregulated FOXM1 expression in tumors. Accordingly, FOXM1 is overexpressed in many types of human cancer. FOXM1 is intimately involved in tumorigenesis, because it contributes to oncogenic transformation and participates in tumor initiation, growth, and progression, including positive effects on angiogenesis, migration, invasion, epithelial-mesenchymal transition, metastasis, recruitment of tumor-associated macrophages, tumor-associated lung inflammation, self-renewal capacity of cancer cells, prevention of premature cellular senescence, and chemotherapeutic drug resistance. However, in the context of urethane-induced lung tumorigenesis, FOXM1 has an unexpected tumor suppressor role in endothelial cells because it limits pulmonary inflammation and canonical Wnt signaling in epithelial lung cells, thereby restricting carcinogenesis. Accordingly, FOXM1 plays a role in homologous recombination repair of DNA double-strand breaks and maintenance of genomic stability, that is, prevention of polyploidy and aneuploidy. The implication of FOXM1 in tumorigenesis makes it an attractive target for anticancer therapy, and several antitumor drugs have been reported to decrease FOXM1 expression.

A novel anticancer therapy that simultaneously targets aberrant p53 and Notch activities in tumors

Notch signaling pathway plays an important role in tumorigenesis by maintaining the activity of self-renewal of cancer stem cells, and therefore, it is hypothesized that interference of Notch signaling may inhibit tumor formation and progression. H101 is a recombinant oncolytic adenovirus that is cytolytic in cells lacking intact p53, but it is unable to eradicate caner stem cells. In this study, we tested a new strategy of tumor gene therapy by combining a Notch1-siRNA with H101 oncolytic adenovirus. In HeLa-S3 tumor cells, the combined therapy blocked the Notch pathway and induced apoptosis in tumors that are p53-inactive. In nude mice bearing xenograft tumors derived from HeLa-S3 cells, the combination of H101/Notch1-siRNA therapies inhibited tumor growth. Moreover, Notch1-siRNA increased Hexon gene expression at both the transcriptional and the translational levels, and promoted H101 replication in tumors, thereby enhancing the oncolytic activity of H101. These data demonstrate the feasibility to combine H101 p53-targted oncolysis and anti-Notch siRNA activities as a novel anti-cancer therapy.

Combined effect of Hsp90 inhibitor geldanamycin and parthenolide via reactive oxygen species-mediated apoptotic process on epithelial ovarian cancer cells

Hsp90 inhibitor geldanamycin and parthenolide have been shown to induce apoptosis in cancer cells. However, the combined effect of geldanamycin and parthenolide on epithelial ovarian cancer cells has not been studied. In respect of cell death process, we investigated the promoting effect of parthenolide on geldanamycin-induced apoptosis in the human epithelial ovarian carcinoma cell lines OVCAR-3 and SK-OV-3. Geldanamycin induced a decrease in Bid, Bcl-2, Bcl-xL and survivin protein levels; an increase in Bax and tumour suppressor p53 levels; loss of the mitochondrial transmembrane potential; cytochrome c release; activation of caspases (-8, -9 and -3); cleavage of PARP-1; and increase in the reactive oxygen species formation. Parthenolide enhanced geldanamycin-induced changes in the apoptosis-related protein levels, reactive oxygen species formation, nuclear damage and cell death. The combined effect was inhibited by the addition of oxidant scavengers. The results suggest that parthenolide may potentiate the apoptotic effect of geldanamycin on ovarian carcinoma cell lines by the activation of the caspase-8- and Bid-dependent pathway and the mitochondria-mediated apoptotic pathway. The apoptosis-promoting effect seems to be mediated by the stimulatory effect on the formation of reactive oxygen species.

Guanylate cyclase activator YC-1 potentiates apoptotic effect of licochalcone A on human epithelial ovarian carcinoma cells via activation of death receptor and mitochondrial pathways

Natural phenol licorice compounds have been shown to induce apoptosis in cancer cells. 3-(5'-Hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1) may enhance the sensitivity of cancer cells to anticancer drugs. However, the combined effect of licochalcone A and YC-1 on cell death in ovarian cancer cells has not been studied. We assessed the combined effect of licochalcone A and YC-1 on apoptosis in human epithelial ovarian carcinoma cell lines in relation to the cell death process. In the OVCAR-3 and SK-OV-3 cell lines, licochalocone A induced a decrease in Bid, Bcl-2, Bcl-xL and survivin protein levels; an increase in Bax levels; loss of the mitochondrial transmembrane potential; cytochrome c release; activation of caspases (-8, -9 and -3); cleavage of PARP-1; and an increase in the tumor suppressor p53 levels. YC-1 enhanced licochalcone A-induced apoptosis-related protein activation, nuclear damage and cell death. These results suggest that YC-1 may potentiate the apoptotic effect of licochalcone A on ovarian carcinoma cell lines by increasing the activation of the caspase-8- and Bid-dependent pathway and the mitochondria-mediated apoptotic pathway, leading to caspase activation. The combination of licochalcone A and YC-1 may confer a benefit in the treatment of human epithelial ovarian adenocarcinoma.

Specific domains of nucleolin interact with Hdm2 and antagonize Hdm2-mediated p53 ubiquitination

Nucleolin is an abundant multifunctional nucleolar protein with defined roles in ribosomal RNA processing, RNA polymerase I catalyzed transcription and the regulation of apoptosis. Earlier we reported that human nucleolin binds to the p53 antagonist human double minute 2 (Hdm2) as determined by reciprocal co-immunoprecipitation assays using cell lysates. We also demonstrated that nucleolin antagonizes Hdm2-mediated degradation of p53. Here, we identify specific domains of nucleolin and Hdm2 proteins that support mutual interaction and investigate the implications of complex formation on p53 ubiquitination and protein levels. Our data indicate that the nucleolin N-terminus as well as the central RNA-binding domain (RBD) are predominantly involved in binding to Hdm2. The nucleolin RBD robustly bound to the NLS/NES (nuclear localization and export signals) domain of Hdm2 in vitro, while the N-terminus of nucleolin preferentially associated with the Hdm2 RING (really interesting new gene) domain expressed in cells. We further demonstrate that the C-terminal glycine-arginine rich domain of nucleolin serves as the predominant binding domain for direct interaction with p53. While overexpression of nucleolin or its various domains had no significant effect on Hdm2 auto-ubiquitination, the nucleolin RBD antagonized the Hdm2 E3 ligase activity against p53, leading to p53 stabilization. Conversely, the adjacent glycine-arginine rich domain of nucleolin interacted with p53 causing a modest stimulatory effect on p53 ubiquitination. These data suggest that changes in nucleolin conformation can alter the availabilities of such domains in vivo to modulate the overall impact of nucleolin on Hdm2 activity and hence on p53 stability.