Accumulating active p53 in the nucleus by inhibition of nuclear export: a novel strategy to promote the p53 tumor suppressor function

A nuclear protein quality control system for elimination of nucleolus-related inclusions

The identification of pathways that control elimination of protein inclusions is essential to understand the cellular response to proteotoxicity, particularly in the nuclear compartment, for which our knowledge is limited. We report that stress-induced nuclear inclusions related to the nucleolus are eliminated upon stress alleviation during the recovery period. This process is independent of autophagy/lysosome and CRM1-mediated nuclear export pathways, but strictly depends on the ubiquitin-activating E1 enzyme, UBA1, and on nuclear proteasomes that are recruited into the formed inclusions. UBA1 activity is essential only for the recovery process but dispensable for nuclear inclusion formation. Furthermore, the E3 ligase HUWE1 and HSP70 are components of the ubiquitin/chaperone systems that promote inclusion elimination. The recovery process also requires RNA Pol I-dependent production of the lncRNA IGS42 during stress. IGS42 localises within the formed inclusions and promotes their elimination by preserving the mobility of resident proteins. These findings reveal a protein quality control system that operates within the nucleus for the elimination of stress-induced nucleolus-related inclusions.

Apoptosis: A Comprehensive Overview of Signaling Pathways, Morphological Changes, and Physiological Significance and Therapeutic Implications

Cell survival and death are intricately governed by apoptosis, a meticulously controlled programmed cell death. Apoptosis is vital in facilitating embryonic development and maintaining tissue homeostasis and immunological functioning. It is a complex interplay of intrinsic and extrinsic signaling pathways that ultimately converges on executing the apoptotic program. The extrinsic pathway is initiated by the binding of death ligands such as TNF-α and Fas to their respective receptors on the cell surface. In contrast, the intrinsic pathway leads to increased permeability of the outer mitochondrial membrane and the release of apoptogenic factors like cytochrome c, which is regulated by the Bcl-2 family of proteins. Once activated, these pathways lead to a cascade of biochemical events, including caspase activation, DNA fragmentation, and the dismantling of cellular components. Dysregulation of apoptosis is implicated in various disorders, such as cancer, autoimmune diseases, neurodegenerative disorders, and cardiovascular diseases. This article focuses on elucidating the molecular mechanisms underlying apoptosis regulation, to develop targeted therapeutic strategies. Modulating apoptotic pathways holds immense potential in cancer treatment, where promoting apoptosis in malignant cells could lead to tumor regression. This article demonstrates the therapeutic potential of targeting apoptosis, providing options for treating cancer and neurological illnesses. The safety and effectiveness of apoptosis-targeting drugs are being assessed in ongoing preclinical and clinical trials (phase I-III), opening the door for more effective therapeutic approaches and better patient outcomes.

Safety and Preliminary Efficacy of Once-Weekly Split-Dose Selinexor in Soft Tissue Sarcoma: Results of the Phase Ib METSSAR Clinical Trial

BACKGROUND

The approved dose of Selinexor, 60 mg twice-weekly, is associated with several clinically relevant toxicities. Preclinical studies show that a sustained-release formulation of selinexor results in a lower toxicity profile.

OBJECTIVE

The phase 1b METSSAR trial assessed the safety and tolerability of an alternative dosing schedule of selinexor (to mimic the sustained-release formulation) in advanced soft tissue sarcoma (STS) patients.

PATIENTS AND METHODS

Selinexor was administered in a split-dose schedule (40 mg, 20 mg, 20 mg in the morning, afternoon, and evening, respectively) on days 1, 8, 15, and 22 of a 28-day cycle, until unacceptable toxicity or disease progression. The primary endpoint was the rate of grade ≥ 3 treatment-related adverse events (TRAEs). Secondary objectives were EORTC QLQ-C30 quality of life (QoL) assessment, and preliminary efficacy.

RESULTS

Twenty patients with 12 STS subtypes were enrolled and received a median of four cycles of treatment. There were no grade ≥ 3 TRAEs. Dysgeusia, nausea, fatigue, and thrombocytopenia were the most common grade ≤ 2 TRAEs. No treatments were discontinued due to TRAE, but four patients (20%) required dose reduction. Median change in global health status (GHS) score from baseline to cycle 2 (by QLQ-C30 v3.0) was - 8.33, and only 39% of patients reported a clinically meaningful decline in GHS score (≥ 10 points). Median symptom scale scores on treatment were increased for fatigue (+12.35), nausea/vomiting (+18.52), and anorexia (+16.67), but reduced for pain (- 3.70). The median progression-free survival (PFS) was 4.0 months (95% confidence interval 1.9-7.5).

CONCLUSIONS

Split-dose once-weekly selinexor was reasonably well tolerated in this heterogeneous group of advanced STS patients with a better, or at least similar, clinician- and patient-reported toxicity profile compared to the standard dosing regimen. Further clinical evaluation is warranted, as better dose delivery can lead to improved antitumor efficacy.

Plasmablastic Lymphoma. A State-of-the-Art Review: Part 2-Focus on Therapy

The objective of this two-part review is to present a current and comprehensive understanding of the diagnosis and management of plasmablastic lymphoma. The first part, which was published previously, focused on the study of epidemiology, etiology, clinicopathological characteristics, differential diagnosis, prognostic variables, and the impact of plasmablastic lymphoma on specific populations. This second part addresses the difficult topic of the treatment of plasmablastic lymphoma, specifically examining both the conventional, consolidated approach and the novel therapeutic strategy.

Novel CDK9 inhibitor oroxylin A promotes wild-type P53 stability and prevents hepatocellular carcinoma progression by disrupting both MDM2 and SIRT1 signaling

Hepatocellular carcinoma (HCC) is one of the most lethal tumours worldwide. However, the effects of first-line sorafenib treatment in advanced HCC fail to prolong patients' survival due to the highly heterogeneous characteristics of HCC etiology. Cyclin-dependent kinase 9 (CDK9) is an important target in the continuous development of cancer therapy. Here, we demonstrate that CDK9 is closely associated with the progression of HCC and can serve as an HCC therapeutic target by modulating the recovery of wild-type p53 (wt-p53) function. We prove that mouse double minute 2 homologue (MDM2) and Sirtuin 1 (SIRT1) are phosphorylated by CDK9 at Ser166 and Ser47, respectively. Inhibition of CDK9 not only reduces the MDM2-mediated ubiquitination and degradation of wt-p53 but also increases wt-p53 stability by suppressing deacetylase activity of SIRT1. Thus, inhibition of CDK9 promotes the wt-p53 stabilization and prevents HCC progression. However, excessive inhibition by high concentrations of specific CDK9 inhibitors counteracts the promotion of p53 stability and reduces their anti-HCC activity because of extreme general transcription repression. The effects of a novel CDK9 inhibitor named oroxylin A (OA) from Scutellaria baicalensis are explored, with the results indicating that OA shows moderate and controlled inhibition of CDK9 activity and expression, and stabilizes wt-p53 by inhibiting CDK9-regulated MDM2 and SIRT1 signaling. These outcomes indicate the high therapeutic potential of OA against HCC and its low toxicity in normal tissue. This study demonstrates a novel mechanism for the regulation of wt-p53 by CDK9 and indicates that OA is a potential candidate for HCC therapy.

Selective inhibition of nuclear export: a promising approach in the shifting treatment paradigms for hematological neoplasms

Novel targeted therapeutics alone or in rational combinations are likely to dominate the future management of various hematological neoplasms. However, the challenges currently faced are the molecular heterogeneity in driver lesions and genetic plasticity leading to multiple resistance pathways. Thus, progress has overall been gradual. For example, despite the advent of targeted agents against actionable drivers like FLT3 in acute myeloid leukemia (AML), the prognosis remains suboptimal in newly diagnosed and dismal in the relapsed/refractory (R/R) setting, due to other molecular abnormalities contributing to inherent and acquired treatment resistance. Nuclear export inhibitors are of keen interest because they can inhibit several active tumorigenic processes simultaneously and also synergize with other targeted drugs and chemotherapy. XPO1 (or CRM1, chromosome maintenance region 1) is one of the most studied exportins involved in transporting critical cargoes, including tumor suppressor proteins like p27, p53, and RB1. Apart from the TSP cargo transport and its role in drug resistance, XPO1 inhibition results in retention of master transcription factors essential for cell differentiation, cell survival, and autophagy, rendering cells more susceptible to the effects of other antineoplastic agents, including targeted therapies. This review will dissect the role of XPO1 inhibition in hematological neoplasms, focusing on mechanistic insights gleaned mainly from work with SINE compounds. Future potential combinatorial strategies will be discussed.

Impact of XPO1 mutations on survival outcomes in metastatic non-small cell lung cancer (NSCLC)

BACKGROUND

Nuclear protein transport is essential in guiding the traffic of important proteins and RNAs between the nucleus and cytoplasm. Export of proteins from the nucleus is mostly regulated by Exportin 1 (XPO1). In cancer, XPO1 is almost universally hyperactive and can promote the export of important tumor suppressors to the cytoplasm. Currently, there are no studies evaluating XPO1 amplifications and mutations in NSCLC and the impact on outcomes.

METHODS

Tumor samples were analyzed using next-generation sequencing (NGS) (NextSeq, 592 Genes), immunohistochemistry (IHC), and whole transcriptome sequencing (WTS, NovaSeq) (Caris Life Sciences, Phoenix, AZ). Survival was extracted from insurance claims data and calculated from time of tissue collection to last contact using Kaplan-Meier estimate.

RESULTS

Among 18,218 NSCLC tumors sequenced, 26 harbored XPO1 mutations and 24 had amplifications. XPO1 mutant tumors were more likely to have high TMB (79% vs. 52%, p = 0.007) and less likely to have high PD-L1 (32% vs. 68%, p = 0.03). KRAS co-mutations were seen in 19% (n = 5) and EGFR co-mutations were rare (n = 2). Among the 17,449 NSCLC tumors with clinical data, there were 24 XPO1 mutant. Comparison of survival between XPO1 mutant and WT showed a negative association with a hazard ratio (HR) of 1.932 (95% CI: 1.144-3.264 p = 0.012). XPO1 amplification was not associated with survival.

CONCLUSIONS

XPO1 pathogenic mutations were associated with a poor survival in NSCLC. Although XPO1 mutations are rare in NSCLC, further studies to assess its associations with treatment responses are warranted.

Incorporating Novel Targeted and Immunotherapeutic Agents in Treatment of B-Cell Lymphomas

The introduction of novel targeted agents and immunotherapeutic modalities into the treatment of B-cell lymphomas has drastically shifted the treatment landscape. In diffuse large B-cell lymphoma, recent approvals of CAR T-cell therapy, the antibody-drug conjugate polatuzumab, and the anti-CD19 monoclonal antibody tafasitamab have provided efficacious options for patients with relapsed and refractory disease. These immunotherapies attempt to harness power from the patient's own immune system to eradicate lymphoma. In chronic lymphocytic leukemia, oral targeted kinase inhibitors such as ibrutinib and acalabrutinib (Bruton tyrosine kinase inhibitors) and venetoclax (BCL2 inhibitor) are now favored over chemoimmunotherapy for upfront treatment because of improved progression-free survival across all subgroups (including high-risk subgroups such as unmutated immunoglobulin variable heavy chain and chromosome 17p deletion). In indolent lymphomas, several PI3K inhibitors are approved for treatment of relapsed disease. However, uptake of these agents has been limited because of toxicity concerns. Combination of lenalidomide and rituximab has been a safe and effective immune modality for patients with refractory indolent lymphomas; it is currently being used as a backbone to bring other targeted agents such as tazemetostat (EZH2 inhibitor) into earlier lines of treatment. In this article, we will review novel commercially available agents in the treatment of relapsed/refractory diffuse large B-cell lymphoma, treatment-naïve chronic lymphocytic leukemia, and relapsed/refractory indolent lymphomas. We will evaluate clinical trials that led to their approval and will provide an outlook into the future novel agents currently under investigation in B-cell malignancies.

A phase 1b trial of selinexor, a first-in-class selective inhibitor of nuclear export (SINE), in combination with doxorubicin in patients with advanced soft tissue sarcomas (STS)

BACKGROUND

Selinexor is a first-in-class selective inhibitor of nuclear export (SINE) compound with single-agent activity in soft tissue sarcoma (STS). The study's aim was to determine the safety and efficacy of selinexor in combination with doxorubicin in patients with locally advanced/metastatic STS.

METHODS

This phase 1b study used a mTPI design. Patients received selinexor at either 60 or 80 mg weekly PO plus doxorubicin (75 mg/m² IV q21 days). Patients with clinical benefit (defined as ≥stable disease via RECIST 1.1) after six cycles of combination treatment received maintenance selinexor until disease progression or unacceptable toxicity. Disease assessments were conducted every two cycles. Pharmacokinetic data were collected on the first three patients per dose level.

RESULTS

Twenty-five patients were enrolled (20 female, ECOG 0/1: 13/12, median age 57 years [range 21-74]). Disease subtypes included leiomyosarcoma (n = 6), malignant peripheral nerve sheath tumour (n = 3) and other sarcomas (n = 16). Three (12%) and 22 (88%) patients were treated at 60 mg and 80 mg of selinexor, respectively. The most common ≥G3 drug-related adverse events (AEs) were haematological, including neutropenia (56%), febrile neutropenia (28%) and anaemia (24%). There were four dose-limiting toxicities (febrile neutropenia (x2), vomiting, fatigue) all at the 80 mg dose level. There was one death secondary to heart failure. Of the 24 evaluable patients, 5 (21%) had a partial response and 15 (63%) had SD as best response. The estimated median progression-free survival (PFS) and overall survival (OS) were 5.5 (95% CI:4.1-5.7) and 10.5 (95% CI:7.5-14) months.

CONCLUSION

In a heterogeneous group of patients with locally advanced/metastatic STS, the combination of selinexor and doxorubicin fulfilled the prespecified boundary for tolerability.

Investigational drugs for the treatment of diffuse large B-cell lymphoma

Introduction: Diffuse large B cell lymphoma (DLBCL) is the most frequent lymphoma in adults. 30-40% DLBCL eventually relapse and 10% are primary refractory, posing an unmet clinical need, especially in patients not eligible for hematopoietic stem cell transplant. Knowledge of DLBCL molecular pathogenesis has identified druggable molecular pathways. Surface antigens can be targeted by novel antibodies and innovative cell therapies. Areas covered: This review illuminates those investigational drugs and cell therapies that are currently in early phase clinical trials for the treatment of DLBCL. New small molecules that modulate the pathways involved in the molecular pathogenesis of DLBCL, monospecific and bispecific monoclonal antibodies, drug-immunoconjugates, and cellular therapies are placed under the spotlight. A futuristic perspective concludes the paper. Expert opinion: A precision medicine strategy based on robust molecular predictors of outcome is desirable in the development of investigational small molecules for DLBCL. Novel monoclonal and bispecific antibodies may be offered to (i) relapsed/refractory patients ineligible for CAR-T cells because of comorbidities, and (ii) younger patients before CAR-T cell infusion to reduce a high tumor burden. A focus on the optimal sequencing of the emerging DLBCL drugs is appropriate and necessary.

Selinexor in patients with relapsed or refractory diffuse large B-cell lymphoma (SADAL): a single-arm, multinational, multicentre, open-label, phase 2 trial

BACKGROUND

Relapsed or refractory diffuse large B-cell lymphoma (DLBCL) is an aggressive cancer with a median overall survival of less than 6 months. We aimed to assess the response to single-agent selinexor, an oral selective inhibitor of nuclear export, in patients with relapsed or refractory DLBCL who had no therapeutic options of potential clinical benefit.

METHODS

SADAL was a multicentre, multinational, open-label, phase 2b study done in 59 sites in 19 countries. Patients aged 18 years or older with pathologically confirmed diffuse large B-cell lymphoma, an Eastern Cooperative Oncology Group performance status of 2 or less, who had received two to five lines of previous therapies, and progressed after or were not candidates for autologous stem-cell transplantation were enrolled. Germinal centre B-cell or non-germinal centre B-cell tumour subtype and double or triple expressor status were determined by immunohistochemistry and double or triple hit status was determined by cytogenetics. Patients received 60 mg selinexor orally on days 1 and 3 weekly until disease progression or unacceptable toxicity. The study was initially designed to evaluate both 60 mg and 100 mg twice-weekly doses of selinexor; however, the 100 mg dose was discontinued in the protocol (version 7.0) on March 29, 2017, when an improved therapeutic window was observed at 60 mg. Primary outcome was overall response rate. The primary outcome and safety were assessed in all patients who received 60 mg selinexor under protocol version 6.0, or enrolled under protocol versions 7.0 or higher and received at least one dose of selinexor. This trial is registered at ClinicalTrials.gov, NCT02227251 (active but not enrolling).

FINDINGS

Between Oct 21, 2015, and Nov 2, 2019, 267 patients were randomly assigned, with 175 allocated to the 60 mg group and 92 to the discontinued 100 mg group. 48 patients assigned to the 60 mg group were excluded due to enrolment before version 6.0 of the protocol; the remaining 127 patients received selinexor 60 mg and were included in analyses of primary outcome and safety. The overall response rate was 28% (36/127; 95% CI 20·7-37·0); 15 (12%) achieved a complete response and 21 (17%) a partial response. The most common grade 3-4 adverse events were thrombocytopenia (n=58), neutropenia (n=31), anaemia (n=28), fatigue (n=14), hyponatraemia (n=10), and nausea (n=8). The most common serious adverse events were pyrexia (n=9), pneumonia (n=6), and sepsis (n=6). There were no deaths judged as related to treatment with selinexor.

INTERPRETATION

Single-drug oral selinexor induced durable responses and had a manageable adverse events profile in patients with relapsed or refractory DLBCL who received at least two lines of previous chemoimmunotherapy. Selinexor could be considered a new oral, non-cytotoxic treatment option in this setting.

FUNDING

Karyopharm Therapeutics Inc.

Recent Synthetic Approaches towards Small Molecule Reactivators of p53

The tumor suppressor protein p53 is often called "the genome guardian" and controls the cell cycle and the integrity of DNA, as well as other important cellular functions. Its main function is to trigger the process of apoptosis in tumor cells, and approximately 50% of all cancers are related to the inactivation of the p53 protein through mutations in the TP53 gene. Due to the association of mutant p53 with cancer therapy resistance, different forms of restoration of p53 have been subject of intense research in recent years. In this sense, this review focus on the main currently adopted approaches for activation and reactivation of p53 tumor suppressor function, focusing on the synthetic approaches that are involved in the development and preparation of such small molecules.

p53-NF-κB Crosstalk in Febrile Temperature-Treated Human Umbilical Cord-Derived Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) are successful for their therapeutic application in immune and inflammatory contexts due to their anti-inflammatory, trophic, and immunomodulatory roles. However, though MSCs have the potential to provide regenerative treatment toward a wide range of devastating diseases, massive cell death of transplanted MSCs remains an obstacle to overcome. The relation between MSCs and inflammation is multifactorial and challenging to comprehend. Fever is a critical component of the inflamed microenvironment. Also, the choice of MSC source could be critical in determining the fate of transplanted cells under stress conditions. Here we investigated the thermosensitivity of Wharton's jelly MSCs (WJ-MSCs) to elevated temperature in the physiological fever range. We explored the effect of febrile range temperature on morphology, viability, proliferation kinetics, and cell cycle status of WJ-MSCs. WJ-MSCs adopted a flattened morphology at 40°C, and our data from proliferation kinetics study using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and apoptosis assays showed that WJ-MSCs had reduced proliferation and viability at 40°C compared with control cultures. There was also a G0/G1 cell cycle arrest, which was further confirmed by messenger RNA (mRNA) levels of genes specific for different stages of cell cycle. On evaluating p53 status, we observed an increase in p53 protein expression and its nuclear localization in WJ-MSCs exposed to 40°C. Its downstream effector p21 too was upregulated. Moreover, this temperature-induced p53 induction was inhibited on exposure to 40°C in the presence of NF-κB pathway inhibitor, pyrrolidinedithiocarbamate (PDTC) or endonuclease-prepared small interfering RNA (esiRNA) targeting p65. Febrile temperature exposure did not affect the senescence status of WJ-MSCs. The MSC-specific surface antigen profile at 40°C was similar to control WJ-MSCs. Our findings suggest that under febrile temperature stress conditions, WJ-MSCs exhibit G0/G1 cell cycle arrest and reduction in viable cell count, while retaining their basic characteristics, with an underlying interplay of p53 and NF-κB pathway.

Targeting p53 as a promising therapeutic option for cancer by re-activating the wt or mutant p53’s tumor suppression

p53 protein, a product of the TP53 tumor suppressor gene, controls the cellular genome’s integrity and is an important regulator of cell cycling, proliferation, apoptosis and metabolism. Mutations of TP53 or inactivation of its gene product are among the first events initiating malignant transformation. The consequent loss of control over the cell cycle, resulting in accelerated cell proliferation and facilitating metabolic reprogramming, gives the initiated (premalignant) cells numerous advantages over healthy cells. Interestingly, p53 status is not only an important marker in cancer diagnosis; it has also become a promising target of personalized therapy. Depending on the TP53 status different therapeutic options have been developed. (Re)-activation of p53 functionality in cancer cells offers promising new alternatives to existing oncological therapies.

Selective inhibition of nuclear export with selinexor in patients with non-Hodgkin lymphoma

Patients with relapsed or refractory (R/R) non-Hodgkin lymphoma (NHL) have a poor prognosis and limited treatment options. We evaluated selinexor, an orally bioavailable, first-in-class inhibitor of the nuclear export protein XPO1, in this phase 1 trial to assess safety and determine a recommended phase 2 dose (RP2D). Seventy-nine patients with various NHL histologies, including diffuse large B-cell lymphoma, Richter's transformation, mantle cell lymphoma, follicular lymphoma, and chronic lymphocytic leukemia, were enrolled. In the dose-escalation phase, patients received 3 to 80 mg/m² of selinexor in 3- or 4-week cycles and were assessed for toxicities, pharmacokinetics, and antitumor activity. In the dose-expansion phase, patients were treated with selinexor at 35 or 60 mg/m² The most common grade 3 to 4 drug-related adverse events were thrombocytopenia (47%), neutropenia (32%), anemia (27%), leukopenia (16%), fatigue (11%), and hyponatremia (10%). Tumor biopsies showed decreases in cell-signaling pathways (Bcl-2, Bcl-6, c-Myc), reduced proliferation (Ki67), nuclear localization of XPO1 cargos (p53, PTEN), and increased apoptosis after treatment. Twenty-two (31%) of the 70 evaluable patients had an objective responses, including 4 complete responses and 18 partial responses, which were observed across a spectrum of NHL subtypes. A dose of 35 mg/m² (60 mg) was identified as the RP2D. These findings suggest that inhibition of XPO1 with oral selinexor at 35 mg/m² is a safe therapy with encouraging and durable anticancer activity in patients with R/R NHL. The trial was registered at www.clinicaltrials.gov as #NCT01607892.

Comparative ultrastructure of CRM1-Nucleolar bodies (CNoBs), Intranucleolar bodies (INBs) and hybrid PML/p62 bodies uncovers new facets of nuclear body dynamic and diversity

In order to gain insights on the nuclear organization in mammalian cells, we characterized ultrastructurally nuclear bodies (NBs) previously described as fluorescent foci. Using high resolution immunoelectron microscopy (I-EM), we provide evidence that CNoBs (CRM1-Nucleolar bodies) and INBs (Intranucleolar bodies) are distinct genuine nucleolar structures in untreated HeLa cells. INBs are fibrillar and concentrate the post-translational modifiers SUMO1 and SUMO-2/3 as strongly as PML bodies. In contrast, the smallest CRM1-labeled CNoBs are vitreous, preferentially located at the periphery of the nucleolus and, intricately linked to the chromatin network. Upon blockage of the CRM1-dependent nuclear export by leptomycin B (LMB), CNoBs disappear while p62/SQSTM1-containing fibrillar nuclear bodies are induced. These p62 bodies are enriched in ubiquitinated proteins. They progressively associate with PML bodies to form hybrid bodies of which PML decorates the periphery while p62/SQSTM1 is centrally-located. Our study is expanding the repertoire of nuclear bodies; revealing a previously unrecognized composite nucleolar landscape and a new mode of interactions between ubiquitous (PML) and stress-induced (p62) nuclear bodies, resulting in the formation of hybrid bodies.

Molecular neuro-oncology and the development of targeted therapeutic strategies for brain tumors Part 4: p53 signaling pathway

Brain tumors are a diverse group of malignancies that remain refractory to conventional treatment approaches. Molecular neuro-oncology has now begun to clarify the transformed phenotype of brain tumors and identify oncogenic pathways that might be amenable to targeted therapy. Loss of the tumor suppressor gene p53 and its encoded protein are the most common genetic events in human cancer and are a frequent occurrence in brain tumors. p53 functions as a transcription factor and is responsible for the transactivation and repression of key genes involved in cell growth, apoptosis and the cell cycle. Mutation of the p53 gene or dysfunction of its signaling pathway are early events in the transformation process of astrocytic gliomas. The majority of mutations are missense and occur in the conserved regions of the gene, within exons 5 through 8. Molecular therapeutic strategies to normalize p53 signaling in cells with mutant p53 include pharmacologic rescue of mutant protein, gene therapy approaches, small-molecule agonists of downstream inhibitory genes, antisense approaches and oncolytic viruses. Other strategies include activation of normal p53 activity, inhibition of mdm2-mediated degradation of p53 and blockade of p53 nuclear export. Further development of targeted therapies designed to restore or enhance p53 function, and evaluation of these new agents in clinical trials, will be needed to improve survival and quality of life for patients with brain tumors.

Selective inhibitors of nuclear export show that CRM1/XPO1 is a target in chronic lymphocytic leukemia

The nuclear export protein XPO1 is overexpressed in cancer, leading to the cytoplasmic mislocalization of multiple tumor suppressor proteins. Existing XPO1-targeting agents lack selectivity and have been associated with significant toxicity. Small molecule selective inhibitors of nuclear export (SINEs) were designed that specifically inhibit XPO1. Genetic experiments and X-ray structures demonstrate that SINE covalently bind to a cysteine residue in the cargo-binding groove of XPO1, thereby inhibiting nuclear export of cargo proteins. The clinical relevance of SINEs was explored in chronic lymphocytic leukemia (CLL), a disease associated with recurrent XPO1 mutations. Evidence is presented that SINEs can restore normal regulation to the majority of the dysregulated pathways in CLL both in vitro and in vivo and induce apoptosis of CLL cells with a favorable therapeutic index, with enhanced killing of genomically high-risk CLL cells that are typically unresponsive to traditional therapies. More importantly, SINE slows disease progression, and improves overall survival in the Eμ-TCL1-SCID mouse model of CLL with minimal weight loss or other toxicities. Together, these findings demonstrate that XPO1 is a valid target in CLL with minimal effects on normal cells and provide a basis for the development of SINEs in CLL and related hematologic malignancies.

An evaluation of small-molecule p53 activators as chemoprotectants ameliorating adverse effects of anticancer drugs in normal cells

Pharmacological activation of wild-type p53 has been found to protect normal cells in culture from cytotoxicity and nuclear aberrations caused by conventional cancer therapeutics. Hence, small-molecule p53 activators could have clinical benefits as chemoprotectants for cancer patients bearing p53-mutant tumors. We have evaluated 16 p53-based cyclotherapy regimes combining p53 activators tenovin-6, leptomycin B, nutlin-3 and low dose actinomycin D, with clinically utilized chemotherapeutic agents (S- and M-phase poisons), vinblastine, vinorelbine, cytosine arabinoside and gemcitabine. All the p53 activators induce reversible cell-cycle arrest in primary human fibroblasts and protect them from both S- and M-phase poisons. Furthermore, studies with p53-mutant cancer cell lines show that nutlin-3 and low dose actinomycin D do not affect the sensitivity of these cells to any of the chemotherapeutics tested. Thus, these two small molecules could be suitable choices for cyclotherapy regimes involving S- or M-phase poisons. In contrast, pre-incubation of p53-mutant cells with tenovin-6 or leptomycin B reduces the efficacy of vinca alkaloids, suggesting that these p53 activators could be effective as chemoprotectants if combined with S- but not M-phase poisons. Discrepancies were observed between the levels of protection detected immediately after treatment and following recovery in fresh medium. This highlights the need to assess both short- and long-term effects when evaluating compounds as potential chemoprotectants for cancer therapy.

Chemotherapeutic sensitization of leptomycin B resistant lung cancer cells by pretreatment with doxorubicin

The development of novel targeted therapies has become an important research focus for lung cancer treatment. Our previous study has shown leptomycin B (LMB) significantly inhibited proliferation of lung cancer cells; however, p53 wild type lung cancer cells were resistant to LMB. Therefore, the objective of this study was to develop and evaluate a novel therapeutic strategy to sensitize LMB-resistant lung cancer cells by combining LMB and doxorubicin (DOX). Among the different treatment regimens, pretreatment with DOX (pre-DOX) and subsequent treatment with LMB to A549 cells significantly decreased the 50% inhibitory concentration (IC50) as compared to that of LMB alone (4.4 nM vs. 10.6 nM, P<0.05). Analysis of cell cycle and apoptosis by flow cytometry further confirmed the cytotoxic data. To investigate molecular mechanisms for this drug combination effects, p53 pathways were analyzed by Western blot, and nuclear proteome was evaluated by two dimensional-difference gel electrophoresis (2D-DIGE) and mass spectrometry. In comparison with control groups, the levels of p53, phospho-p53 (ser15), and p21 proteins were significantly increased while phospho-p53 (Thr55) and survivin were significantly decreased after treatments of pre-DOX and LMB (P<0.05). The 2D-DIGE/MS analysis identified that sequestosome 1 (SQSTM1/p62) had a significant increase in pre-DOX and LMB-treated cells (P<0.05). In conclusion, our results suggest that drug-resistant lung cancer cells with p53 wild type could be sensitized to cell death by scheduled combination treatment of DOX and LMB through activating and restoring p53 as well as potentially other signaling pathway(s) involving sequestosome 1.

p53 and MDM2 are involved in the regulation of osteocalcin gene expression

Osteocalcin (OC) is a major noncollagenous bone matrix protein and an osteoblast marker whose expression is limited to mature osteoblasts during the late differentiation stage. In previous studies we have shown osteosarcomas to lose p53 function with a corresponding loss of osteocalcin gene expression. Introduction of wild type p53 resulted in re expression of the osteocalcin gene. Using gel shift and chromatin immunoprecipitation assays, we have identified a putative p53 binding site within the rat OC promoter region and observed an increase in OC promoter activity when p53 accumulates using a CAT assay. The p53 inducible gene Mdm2 is a well-known downstream regulator of p53 levels. Our results showed a synergistic increase in the OC promoter activity when both p53 and MDM2 were transiently overexpressed. We further demonstrate that p53 is not degraded during overexpression of MDM2 protein. Increased OC expression was observed with concomitantly increased p53, VDR, and MDM2 levels in ROS17/2.8 cells during treatment with differentiation promoting (DP) media, but was significantly decreased when co-treated with DP media and the small molecule inhibitor of MDM2-p53 interaction, Nutlin-3. We have also observed a dramatic increase of the OC promoter activity in the presence of p53 and Mdm2 with inclusion of Cbfa-1 and p300 factors. Our results suggest that under some physiological conditions the oncoprotein MDM2 may cooperate with p53 to regulate the osteocalcin gene during osteoblastic differentiation.

The Karyopherin proteins, Crm1 and Karyopherin beta1, are overexpressed in cervical cancer and are critical for cancer cell survival and proliferation

The Karyopherin proteins are involved in nucleo-cytoplasmic trafficking and are critical for protein and RNA subcellular localization. Recent studies suggest they are important in nuclear envelope component assembly, mitosis and replication. Since these are all critical cellular functions, alterations in the expression of the Karyopherins may have an impact on the biology of cancer cells. In this study, we examined the expression of the Karyopherins, Crm1, Karyopherin beta1 (Kpnbeta1) and Karyopherin alpha2 (Kpnalpha2), in cervical tissue and cell lines. The functional significance of these proteins to cancer cells was investigated using individual siRNAs to inhibit their expression. Microarrays, quantitative RT-PCR and immunofluorescence revealed significantly higher expression of Crm1, Kpnbeta1 and Kpnalpha2 in cervical cancer compared to normal tissue. Expression levels were similarly elevated in cervical cancer cell lines compared to normal cells, and in transformed epithelial and fibroblast cells. Inhibition of Crm1 and Kpnbeta1 in cancer cells significantly reduced cell proliferation, while Kpnalpha2 inhibition had no effect. Noncancer cells were unaffected by the inhibition of Crm1 and Kpnbeta1. The reduction in proliferation of cancer cells was associated with an increase in a subG1 population by cell cycle analysis and Caspase-3/7 assays revealed increased apoptosis. Crm1 and Kpnbeta1 siRNA-induced apoptosis was accompanied by an increase in the levels of growth inhibitory proteins, p53, p27, p21 and p18. Our results demonstrate that Crm1, Kpnbeta1 and Kpnalpha2 are overexpressed in cervical cancer and that inhibiting the expression of Crm1 and Kpnbeta1, not Kpnalpha2, induces cancer cell death, making Crm1 and Kpnbeta1 promising candidates as both biomarkers and potential anticancer therapeutic targets.

Thyroid hormone receptor alpha1 follows a cooperative CRM1/calreticulin-mediated nuclear export pathway

The thyroid hormone receptor alpha1 (TRalpha) exhibits a dual role as an activator or repressor of its target genes in response to thyroid hormone (T(3)). Previously, we have shown that TRalpha, formerly thought to reside solely in the nucleus bound to DNA, actually shuttles rapidly between the nucleus and cytoplasm. An important aspect of the shuttling activity of TRalpha is its ability to exit the nucleus through the nuclear pore complex. TRalpha export is not sensitive to treatment with the CRM1-specific inhibitor leptomycin B (LMB) in heterokaryon assays, suggesting a role for an export receptor other than CRM1. Here, we have used a combined approach of in vivo fluorescence recovery after photobleaching experiments, in vitro permeabilized cell nuclear export assays, and glutathione S-transferase pull-down assays to investigate the export pathway used by TRalpha. We show that, in addition to shuttling in heterokaryons, TRalpha shuttles rapidly in an unfused monokaryon system as well. Furthermore, our data show that TRalpha directly interacts with calreticulin, and point to the intriguing possibility that TRalpha follows a cooperative export pathway in which both calreticulin and CRM1 play a role in facilitating efficient translocation of TRalpha from the nucleus to cytoplasm.

Acute ammonia neurotoxicity in vivo involves increase in cytoplasmic protein P53 without alterations in other markers of apoptosis

Acute intoxication with large ammonia doses leads to activation of NMDA receptors in the brain, resulting in oxidative stress and disturbance of mitochondrial function. Altered mitochondrial function is a crucial step in some mechanisms of cellular apoptosis. This study assesses whether ammonia intoxication in vivo leads to induction of apoptotic markers such as permeability transition pore (PTP) formation, caspase-3, and caspase-9 activation, changes in p53 protein, or cytochrome c release. Acute ammonia intoxication did not affect caspase-9 or caspase-3 activities. The mitochondrial membrane potential also remained unaltered in non-synaptic brain mitochondria after injection of ammonia, indicating that ammonia did not induce PTP formation in brain in vivo. The nuclear level of p53 did not change, whereas its cytoplasmic level increased approximately two-fold. In agreement with the theory that translocation of the p53 from cytosol to nuclei is an essential step for induction of apoptosis we did not find apoptotic nuclei in brain of rats injected with ammonia. This supports the idea that ammonia neurotoxicity does not involve apoptosis and points to impaired p53 transfer from cytoplasm to nuclei as a possible preventer of apoptosis. We did not find any release of cytochrome c from mitochondria to cytosol after ammonia injection. Cytochrome c content was significantly reduced (30%) in brain mitochondria from rats injected with ammonia. This decrease may contribute to the reduced state 3 respiration, decreased respiratory control index, and disturbances in the mitochondrial electron transport chain in brain mitochondria from rats injected with ammonia.

Selective induction of apoptosis by leptomycin B in keratinocytes expressing HPV oncogenes

Human papillomavirus (HPV) infection is strongly associated with the development of anogenital neoplasia, particularly cervical cancer. It has been estimated that 99.7% of all cervical carcinomas are attributable to infection with HPV, and types 16 and 18 account for the vast majority of such cases. Both of these 'high risk' HPV types encode the oncoproteins E6 and E7, which exert multiple effects on many proteins involved in cell-cycle regulation, including p53. The nuclear export protein inhibitor leptomycin B (LMB) has been shown to cause the nuclear sequestration of p53 in cervical carcinoma cells. We demonstrate that LMB induces apoptosis selectively at nanomolar concentrations in primary human keratinocytes (PHKs) expressing HPV oncogenes. Both monolayer and organotypic raft cultures of transduced PHKs were highly susceptible to treatment with LMB. By contrast, although LMB stimulated p53 accumulation in normal PHKs, no significant induction of apoptosis was detected on Western blots or immunostained monolayer/raft cells, or following pulsed exposure to the drug. Furthermore, topical application of microM concentrations of LMB to mouse skin was non-toxic. These data suggest that the topical application of LMB to HPV-infected intra-epithelial lesions may represent a specific and effective therapeutic strategy against HPV-associated anogenital neoplasia.

Restoration of wild-type p53 function in human cancer: relevance for tumor therapy

BACKGROUND

In the majority of human cancers, the tumor suppressor activity of p53 is impaired because of mutational events or interactions with other proteins (ie, MDM2). The loss of p53 function is responsible for increased aggressiveness of cancers, while tumor chemoresistance and radioresistance are dependent upon the expression of mutant p53 proteins.

METHODS

Review of the literature indicates that p53 acts primarily as a transcription factor whose function is subject to a complex and diverse array of covalent post-translational modifications that markedly influence the expression of p53 target genes responsible for cellular responses such as growth arrest, senescence, or apoptosis. The ability of p53 to induce apoptosis in cancer cells is believed essential for cancer therapy.

RESULTS

Numerous data indicate that p53 dependent apoptosis is a relevant factor in determining the efficacy of anticancer treatments. Thus, the development of new strategies for restoration of p53 function in human tumors is considered an important issue. Two main approaches for restoration of p53 function have been pursued that impact anticancer treatments: (a) de novo expression of wild-type p53 (wt-p53) through gene therapy and (b) identification of small molecules reactivating wt-p53 function.

CONCLUSIONS

The extensive body of knowledge acquired has identified manipulations of p53 signaling as a relevant issue for successful therapies. In this context, the recognition of p53 status in cancer cells is significant and would help considerably in the selection of an appropriate therapeutic approach. p53 manipulations for cancer therapy have revealed the need for specificity of p53 activation and ability to spare body tissues. Furthermore, the promising results obtained by using molecules competent to reactivate wt-p53 functions in cancer cells provide the basis for the design of new molecules with lower side effects and higher anti-tumor efficiency. The reexpression and reactivation of p53 protein in human cancer cells would increase tumor susceptibility to radiation or chemotherapy enhancing the efficacy of standard therapeutic protocols.

Evaluation of the molecular mechanisms involved in the gain of function of a Li-Fraumeni TP53 mutation

The TP53 tumor suppressor gene is the most frequent target for genetic alterations in human cancer. TP53 gene alterations may result in the gain of oncogenic functions such as neoangiogenesis and resistance to therapy. The TP53 germ line mutation c.659A>C (p.Y220S) was identified in stored DNA from related patients with Li-Fraumeni syndrome (LFS) who died after developing clinically aggressive tumors. All of the patients were treated with protocols that included doxorubicin hydrochloride (DX) as a pivotal drug. To define the in vitro mutational phenotype of this germ line mutation, we used murine fibroblasts explanted from wild-type (wt) and p53 knockout (KO) mice from the same littermate. p53Y220S and p53R175H fibroblasts, obtained from p53KO fibroblasts transfected with expression vectors encoding the human Y220S and R175H p53 mutants, respectively, exhibited resistance to DX treatment. Moreover, p53Y220S fibroblasts exhibited angiogenetic properties, and after DX treatment, p53Y220S failed to translocate into the nucleus and showed an increase in its cytosolic levels. DX treatment does not influence p53 distribution within the nuclear and cytosolic compartments in p53R175H fibroblasts. Peroxiredoxin II (Prx II), a protein that is involved in eliminating reactive oxygen species (ROS), showed increased expression intensity in p53Y220S fibroblasts after DX treatment, as observed by two-dimensional electrophoresis analysis. Moreover, Thioredoxin (Trx), a protein that cooperates with Prx II, is overexpressed in p53Y220S mutants under basal conditions. These data suggest a relationship between the presence of the p53Y220S mutation and enhanced levels of Prx II and Trx in mutant fibroblasts. Since one of the mechanisms of the DX antitumor effect has been ascribed to production of ROS, future studies will evaluate the involvement of PrxII and Trx in the chemoresistance of p53Y220S fibroblasts to DX.

Histone deacetylase inhibitor-mediated radiosensitization of human cancer cells: class differences and the potential influence of p53

Histone deacetylase inhibitors (HDI) are emerging as potentially useful components of the anticancer armamentarium and as useful tools to dissect mechanistic pathways. HDIs that globally inhibit histone deacetylases (HDAC) have radiosensitizing effects, but the relative contribution of specific HDAC classes remains unclear. Newly characterized HDIs are now available that preferentially inhibit specific HDAC classes, including SK7041 (inhibits class I HDACs) and splitomicin (inhibits class III HDACs). We investigated in human cancer cells the relative radiosensitizations that result from blocking specific HDAC classes. We found that trichostatin A (TSA; inhibitor of both class I and II HDACs) was the most effective radiosensitizer, followed by the class I inhibitor SK7041, whereas splitomicin (inhibitor of class III) had least effect. Interestingly, radiosensitization by TSA in cell lines expressing p53 was more pronounced than in isogenic lines lacking p53. Radiosensitization of cells expressing p53 by TSA was reduced by pifithrin-alpha, a small-molecule inhibitor of p53. In contrast, the radiosensitization by TSA of cells expressing low levels of p53 was enhanced by transfection of wild-type p53-expressing vector or pretreatment with leptomycin B, an inhibitor of nuclear export that increased intracellular levels of p53. These effects on radiosensitization were respectively muted or not seen in cells treated with SK7041 or splitomicin. To our knowledge, this may be among the first systematic investigations of the comparative anticancer effects of inhibiting specific classes of HDACs, with results suggesting differences in the degrees of radiosensitization, which in some cell lines may be influenced by p53 expression.

Molecular cloning of the rice field eel Nup93 with predominant expression in gonad and kidney

Nucleoporins (Nups) are important components of nuclear pore complexes (NPCs). NPCs control gene expression, cells proliferation and differentiation by mediating exchange of cellular signal molecules on both nuclear and cytoplasmic sides. Using subtractive screening, 3'end fragment of Nup93 from the testis cDNA library of the rice field eel was obtained. Full-length cDNA of the gene was further cloned by degenerate PCR and 5'RACE methods. Sequence analysis indicated that the homology of the rice field eel Nup93 were 36.5% with yeast Nic96, 94.6% and 90.5% with Nup93 of zebrafish and human, respectively. Phylogenetic analysis showed that the rice field eel Nup93 fits with Nup93 of the other fishes. Real-time PCR result showed that expression of Nup93 in gonads and kidney were much higher than in other tissues, and different expression quantities among gonads of three sexes were also observed, suggesting that Nup93 may involve in gonad development.

Identification of an Aberrantly Spliced Form of HDMX in Human Tumors: A New Mechanism for HDM2 Stabilization

The HDMX protein is closely related to HDM2 with which it shares different structural domains, particularly the p53 binding domain and the ring finger domain, where the two HDM proteins interact. Several oncogenic forms derived from splicing of HDM2 have been described in cancer. This work aimed at investigating whether analogous forms of HDMX exist in human tumors. Here, we report the characterization of an aberrantly spliced form of HDMX, HDMX211, isolated from the thyroid tumor cell line, ARO. HDMX211 binds and stabilizes the HDM2 protein. Although it lacks the p53 binding domain, HDMX211 also stabilizes p53 by counteracting its degradation by HDM2. However, the resulting p53 is transcriptionally inactive and increasingly associated to its inhibitor HDM2. Expression of HDMX211 strongly enhances the colony-forming ability of human cells in the presence or absence of wild-type p53. Conversely, depletion of HDMX211 by small interfering RNA significantly reduces the growth of ARO cells and increases their sensitivity to chemotherapy. Screening of lung cancer biopsies shows the presence of HDMX211 in samples that overexpress HDM2 protein, supporting a pathologic role for this new protein. This is the first evidence of a variant form of HDMX that has oncogenic potential independently of p53. HDMX211 reveals a new mechanism for overexpression of the oncoprotein HDM2. Most interestingly, it outlines a possible molecular explanation for a yet unclarified tumor phenotype, characterized by simultaneous overexpression of HDM2 and wild-type p53.

SW480, a p53 Double-mutant Cell Line Retains Proficiency for Some p53 Functions

During certain types of cellular stress, the p53 tumor suppressor protein binds to DNA and transactivates a variety of genes that regulate critical responses including apoptosis, cell cycle checkpoints, differentiation, and angiogenesis. In addition, functional p53 is known to be required for efficient nucleotide excision repair (NER) of bulky DNA adducts generated through exposure to environmental mutagens such as UV light. Nonetheless, we previously showed that the model p53-mutated human adenocarcinoma strain SW480 is proficient in the removal of UV-induced cyclobutane pyrimidine dimers (CPD) via NER. We undertook the present study to begin probing the molecular basis for this unexpected repair phenotype. Cytogenetic analysis indicated that SW480 is stable at the chromosomal level, i.e. manifests a karyotypic profile very similar to that revealed for this line as far back as 14 years ago. After fluorescence in situ hybridization (FISH), using a probe complementary to the p53 gene, we found that 98% of the SW480 interphase nuclei contains three copies of the gene, later revealed to be localized on intact short arms of three chromosomes 17. DNA sequence analysis further showed that all three p53 copies in SW480 carry two point mutations (R273H and P309S), and levels of the corresponding mutated p53 protein are about 20-fold higher than in the closely related p53 wild-type strain LoVo. Using an electrophoretic mobility shift assay (EMSA), we demonstrated that R273H/P309S p53 is able to bind with wild-type affinity to its consensus DNA sequence in vitro. Analysis of p21(Cip1/WAF1) expression and in vivo footprinting by ligation-mediated PCR (LMPCR) showed that, in wild-type LoVo cells, an exposure to cellular stress (e.g. UV or ionizing radiation) is necessary for p53 activation of the p21(Cip1/WAF1) promoter. In contrast, the R273H/P309S-mutated p53 protein in SW480 constitutively activates p21(Cip1/WAF1) in the absence of stress through an unknown mechanism. A similar phenomenon whereby mutated p53 in SW480 is able to induce NER-related proteins might explain the normal DNA repair phenotype previously observed in this strain. For now we conclude that, in general, results obtained using SW480 as a p53-deficient cell line should be interpreted very cautiously.

Transcription - guarding the genome by sensing DNA damage

Cells induce the expression of DNA-repair enzymes, activate cell-cycle checkpoints and, under some circumstances, undergo apoptosis in response to DNA-damaging agents. The mechanisms by which these cellular responses are triggered are not well understood, but there is recent evidence that the transcription machinery might be used in DNA-damage surveillance and in triggering DNA-damage responses to suppress mutagenesis. Transcription might also act as a DNA-damage dosimeter where the severity of blockage determines whether or not to induce cell death. Could transcription therefore be a potential therapeutic target for anticancer strategies?

p14ARF expression in invasive breast cancers and ductal carcinoma in situ--relationships to p53 and Hdm2

Introduction

p14^ARF stabilises nuclear p53, with a variable expression of p14^ARF mRNA in breast cancers. In vitro, nuclear p14^ARF binds Hdm2 to block Hdm2-dependent nucleocytoplasmic shuttling of p53, which is required before cytoplasmic degradation of p53. p14^ARF is negatively regulated by p53 and through p53-independent pathways. No studies have yet examined levels of p14^ARF protein expression in breast cancer and their relationship to Hdm2/p53 immunoreactivity or subcellular localisation. Previously, immunohistochemical expression of cytoplasmic p14^ARF, p53 and Hdm2 has been described. HER-2 (c-erbB2/neu) predicts prognosis and interacts with the p14^ARF/Hdm2 pathway to inactivate p14^ARF and to influence Hdm2 activity and localisation. This study examined p14^ARF and p53/Hdm2 expression and subcellular localisation by using immunohistochemistry in a series of invasive ductal breast cancers (IDCs) with concomitant ductal carcinoma in situ (DCIS), to evaluate whether findings in vitro were related to clinicopathological parameters such as HER-2 and their effect on patient outcome.

Methods

The 4C6 anti-p14^ARF monoclonal antibody and Dako Envision Plus system were used to evaluate p14^ARF expression in 103 patients; p53/Hdm2 staining was performed.

Results

p14^ARF was evaluable in 96 patients, with nuclear p14^ARF expression (modified Quick-score ≥ 3) in 79% (n = 76) of IDCs and in associated DCIS in 74 patients. Cytoplasmic p14^ARF was detectable in 23 breast cancers. Nuclear and cytoplasmic p14^ARF showed no correlation with p53 subcellular immunoreactivity. Increasing levels of cytoplasmic p14^ARF were associated with nuclear and cytoplasmic Hdm2 expression (P < 0.001). Subcellular ARF expression was not associated with clinicopathological parameters, and although not an independent prognosticator, these preliminary findings suggest that cytoplasmic p14^ARF might be associated with a better overall survival (P = 0.09; log rank). The association between HER-2 positivity and nuclear p14^ARF (P = 0.038), as well as nuclear Hdm2 (P = 0.019), reflects the in vitro findings of HER-2 interaction with the ARF/Hdm2 pathway. Cytoplasmic p53 and Hdm2 expression might have biological implications, through an association of cytoplasmic p53 with increased tumour proliferation (P = 0.005), and an improved overall survival (P = 0.002, log rank) in cytoplasmic Hdm2-expressing tumours, that independently predict favourable overall survival (P = 0.02) and disease-free survival (P = 0.03).

Conclusions

Nuclear p14^ARF expression is similar in IDCs and DCIS and is associated with Hdm2 immunoreactivity. Nuclear p14^ARF and Hdm2 might be regulated by HER-2. Clearly, our findings in vivo suggest a complexity of p14^ARF/Hdm2 and p53 pathways in which consideration of cytoplasmic p14^ARF and Hdm2 might have tumorigenic implications.

The importance of p53 location: nuclear or cytoplasmic zip code?

The regulation of p53 functions is tightly controlled through several mechanisms including p53 transcription and translation, protein stability, post-translational modifications, and subcellular localization. Despite intensive study of p53, the regulation of p53 subcellular localization although important for its function is still poorly understood. The regulation of p53 localization depends on factors that influence its nuclear import and export, subnuclear localization and cytoplasmic tethering and sequestration. In this review, we will focus on various proteins and modifications that regulate the location and therefore the activity of p53. For example, MDM2 is the most important regulator of p53 nuclear export and degradation. Cytoplasmic p53 associates with the microtubule cytoskeleton and the dynein family of motor proteins; while Parc and mot2 are involved in its cytoplasmic sequestration. Finally, a portion of p53 is localized to the mitochondria as part of the non-transcriptional apoptotic response. In this review we strive to present the most recent data on how the activity of p53 is regulated by its location.

Impaired p53 expression, function, and nuclear localization in calreticulin-deficient cells

The tumor suppressor protein, p53 is a transcription factor that not only activates expression of genes containing the p53 binding site but also can repress the expression of some genes lacking this binding site. Previous studies have shown that overexpression of wild-type p53 leads to apoptosis and cell cycle arrest. DNA damage, such as that caused by UV irradiation, results in p53 stabilization and nuclear localization that subsequently induces apoptosis. Recently, the level of calreticulin (CRT) has been correlated with the rate of apoptosis. Therefore, the aim of this study was to investigate the role of CRT in the regulation of apoptosis via modulating p53 function and expression. Here we show a significant decrease in both basal and DNA damage induced p53 functions in the CRT-deficient cells (crt-/-). This study is the first to demonstrate that CRT function is required for the stability and localization of the p53 protein. By using immuonocytochemical techniques, we showed that observed changes in p53 in the crt-/- cells are due to the nuclear accumulation of Mdm2 (murine double minute gene). These results, lead us to conclude that CRT regulates p53 function by affecting its rate of degradation and nuclear localization.

A chemical genetic screen identifies inhibitors of regulated nuclear export of a Forkhead transcription factor in PTEN-deficient tumor cells

The PI3K/PTEN/Akt signal transduction pathway plays a key role in many tumors. Downstream targets of this pathway include the Forkhead family of transcription factors (FOXO1a, FOXO3a, FOXO4). In PTEN null cells, FOXO1a is inactivated by PI3K-dependent phosphorylation and mislocalization to the cytoplasm, yet still undergoes nucleocytoplasmic shuttling. Since forcible localization of FOXO1a to the nucleus can reverse tumorigenicity of PTEN null cells, a high-content, chemical genetic screen for inhibitors of FOXO1a nuclear export was performed. The compounds detected in the primary screen were retested in secondary assays, and structure-function relationships were identified. Novel general export inhibitors were found that react with CRM1 as well as a number of compounds that inhibit PI3K/Akt signaling, among which are included multiple antagonists of calmodulin signaling.

Prognostic significance of p53 mutation and p53 overexpression in advanced epithelial ovarian cancer: a Gynecologic Oncology Group Study

PURPOSE

The prognostic significance of p53 mutations and overexpression in advanced epithelial ovarian cancers was examined in primary tumors from 125 patients participating in a Gynecologic Oncology Group randomized phase III treatment protocol.

PATIENTS AND METHODS

Mutational analysis of p53 was performed in RNA or genomic DNA extracted from frozen tumor. An immunohistochemistry assay was used to detect p53 overexpression in fixed tumor.

RESULTS

There were 81 patients (74%) with a single mutation, three patients (3%) with two mutations, and 25 patients (23%) lacking a mutation in exons 2 to 11 of p53. Although most mutations occurred within exons 5 to 8, mutations outside this region were observed in 11% of patients. A mutation in exons 2 to 11 of p53 was associated with a short-term improvement in overall survival and progression-free survival. Adjusted Cox modeling demonstrated a 70% reduction in risk of death (P =.014) and a 60% reduction in risk of disease progression (P =.014) for women with such mutations. However, these striking risk reductions increased over time (P <.02) and eventually disappeared with longer follow-up. Overexpression of p53 was observed in 55 patients (100%) with only missense mutation(s), seven patients (32%) with truncation mutations, and eight patients (40%) lacking a mutation in exons 2 to 11. Overexpression of p53 was associated with tumor grade but not with patient outcome.

CONCLUSION

Alterations in p53 are a common event in advanced epithelial ovarian cancer. A mutation in p53, but not overexpression of p53, is associated with a short-term survival benefit. Additional studies are required to define the roles that p53 plays in regulating therapeutic responsiveness and patient outcome.

Improving cancer therapy by non-genotoxic activation of p53

Inactivation of p53 function is a common event in cancer. Approximately 50% of human tumours express mutant p53 and there is evidence that in others, including many childhood tumours, p53 function is impaired in other ways. These defects in p53 function may be due to the alteration of cellular factors that modulate p53 or to the expression of viral oncoproteins. Radiotherapy and many of the chemotherapeutic drugs currently used in cancer treatment are potent activators of p53. However, most of these therapies have a serious drawback, and that is the long-term consequences of their DNA damaging effects. Here, we review the discoveries in p53 research that are most significant to the development of new therapies based on the induction of the transcriptional activity of p53 in a non-genotoxic way and discuss the situations in which this type of approach may be most beneficial.

Nuclear export inhibitor leptomycin B induces the appearance of novel forms of human Mdm2 protein

The nuclear export inhibitor leptomycin B (LMB) prevents the export of proteins from the nucleus to the cytoplasm, protects p53 from Mdm2-mediated degradation and is a very potent inducer of the p53 transcriptional activity. Here we suggest that LMB can also interfere with the degradation of human Mdm2. In the presence of this drug, we observed two novel forms of this protein: a slow mobility form and an amino-terminal fragment with an apparent molecular mass of 32 kDa. The presence of this 32 kDa band is abolished with proteasome inhibitors, indicating that its appearance could be because of limited processing by the proteasome. These results may be useful in understanding the mechanism of degradation of Mdm2 by the proteasome.

Nuclear transport as a target for cell growth

The function of many key proteins and transcription factors involved in cell growth can be regulated by their cellular localization. Such proteins include the tumor suppressor p53 and the nuclear factor kappaB. Although the idea of trapping such proteins in either the nucleus or cytoplasm has been introduced as a potential therapeutic target, only two nuclear transport inhibitors have been reported. Here, we explore the roles of small-molecule inhibitors that cause target proteins to sequester in either the nucleus or cytoplasm. Methods of artificially targeting proteins to the nucleus or cytoplasm using peptide aptamer technology are also discussed.

Proteomic analysis of the mammalian nuclear pore complex

As the sole site of nucleocytoplasmic transport, the nuclear pore complex (NPC) has a vital cellular role. Nonetheless, much remains to be learned about many fundamental aspects of NPC function. To further understand the structure and function of the mammalian NPC, we have completed a proteomic analysis to identify and classify all of its protein components. We used mass spectrometry to identify all proteins present in a biochemically purified NPC fraction. Based on previous characterization, sequence homology, and subcellular localization, 29 of these proteins were classified as nucleoporins, and a further 18 were classified as NPC-associated proteins. Among the 29 nucleoporins were six previously undiscovered nucleoporins and a novel family of WD repeat nucleoporins. One of these WD repeat nucleoporins is ALADIN, the gene mutated in triple-A (or Allgrove) syndrome. Our analysis defines the proteome of the mammalian NPC for the first time and paves the way for a more detailed characterization of NPC structure and function.

Therapeutic exploitation of the p53 pathway

Analysis of the gene encoding p53 could serve to evaluate the effectiveness of a cancer treatment. Mutations in this gene occur in half of all human cancers, and regulation of the protein is defective in a variety of others. Novel strategies that exploit our knowledge of the function and regulation of p53 are being actively investigated. Strategies directed at treating tumours that have p53 mutations include gene therapy, viruses that only replicate in p53 deficient cells, and the search for small molecules that reactivate mutant p53. Potentiating the function of p53 in a non-genotoxic way in tumours that express wildtype protein can be achieved by inhibiting the expression and function of Mdm2 or viral oncoproteins.

A direct interaction between the survival motor neuron protein and p53 and its relationship to spinal muscular atrophy

Mutations in the SMN1 (survival motor neuron 1) gene cause spinal muscular atrophy (SMA). We now show that SMN protein, the SMN1 gene product, interacts directly with the tumor suppressor protein, p53. Pathogenic missense mutations in SMN reduce both self-association and p53 binding by SMN, and the extent of the reductions correlate with disease severity. The inactive, truncated form of SMN produced by the SMN2 gene in SMA patients fails to bind p53 efficiently. SMN and p53 co-localize in nuclear Cajal bodies, but p53 redistributes to the nucleolus in fibroblasts from SMA patients. These results suggest a functional interaction between SMN and p53, and the potential for apoptosis when this interaction is impaired may explain motor neuron death in SMA.

Cocompartmentalization of p53 and Mdm2 is a major determinant for Mdm2-mediated degradation of p53

The product of the Mdm2 oncogene directly interacts with p53 and promotes its ubiquitination and proteasomal degradation. Initial biological studies identified nuclear export sequences (NES), similar to that of the Rev protein from the human immunodeficiency virus, both in Mdm2 and p53. The reported phenotypes resulting from mutation of these NESs, together with results obtained using the nuclear export inhibitor leptomycin B (LMB), have led to a model according to which nuclear export of p53 (via either the NES of Mdm2 or its own NES) is required for efficient p53 degradation. In this study we demonstrate that Mdm2 can promote degradation of p53 in the nucleus or in the cytoplasm, provided both proteins are colocalized. We also investigated if nuclear export is an obligate step on the p53 degradation pathway. We find that (1) when proteasome activity is inhibited, ubiquitinated p53 accumulates in the nucleus and not in the cytoplasm; (2) Mdm2 with a mutated NES can efficiently mediate degradation of wild type p53 or p53 with a mutated NES; (3) the nuclear export inhibitor LMB can increase the steady-state level of p53 by inhibiting Mdm2-mediated ubiquitination of p53; and (4) LMB fails to inhibit Mdm2-mediated degradation of the p53NES mutant, demonstrating that Mdm2-dependent proteolysis of p53 is feasible in the nucleus in the absence of any nuclear export. Therefore, given cocompartmentalization, Mdm2 can promote ubiquitination and proteasomal degradation of p53 with no absolute requirement for nuclear to cytoplasmic transport.

Molecular genetic abnormalities in premalignant lung lesions: biological and clinical implications

Lung cancer is a leading cause of cancer death worldwide; however, despite major advances in cancer treatment during the past two decades, the prognostic outcome of lung cancer patients has improved only minimally. This is largely due to the inadequacy of the traditional screening approach, which detects only well-established overt cancers and fails to identify precursor lesions in premalignant conditions of the bronchial tree. In recent years this situation has fundamentally changed with the identification of molecular abnormalities characteristic of premalignant changes; these concern tumour suppressor genes, loss of heterozygosity at crucial sites and activation of oncogenes. After considering the morphological modifications that occur in premalignant lesions of the bronchial tree, we analyse the alterations occurring in a series of relevant genes: p53 and its functional regulation by MDM2 and p14ARF, p16INK4, p15INK4b, FHIT, as well as LOH at important sites such as 3p, 8p, 9p and 5q. Activation of oncogenes is considered for K-ras, the cyclin D1, the heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNPA2/B1), and finally the c-myc oncogene. The expression of c-myc is influenced strongly by the presence of growth factors (GFs), among which EGF is of prime importance, as well as its receptor coded for by the c-erbB-2 oncogene. Basic knowledge at the molecular level has extremely important clinical implications with regard to early diagnosis, risk assessment and prevention, and therapeutic targets. The novel techniques for early diagnosis and screening of premalignant lung lesions, such as fluorescence bronchoscopy, endobronchial ultrasound, spiral computed tomography combined with precise spatial localization techniques, should basically change the approach to the problems raised by this disease and allow for an increased discovery rate of incipient lesions. Sequential applications will lead to the identification of individuals/populations at high risk, while the availability of accurate 'intermediate end points' will enable the effects of preventive trials to be monitored. Finally, the same molecular abnormalities may serve as 'starting points' for innovative treatments designed to restore the altered functions to normality. Recent developments in our knowledge and understanding of the molecular genetic abnormalities in premalignant lung lesions open an era of hope.