Abnormal expression of MDM-2 in breast carcinomas

Association of MDM2 Overexpression in Ameloblastomas with MDM2 Amplification and BRAFV600E Expression

Ameloblastoma is a rare tumor but represents the most common odontogenic neoplasm. It is localized in the jaws and, although it is a benign, slow-growing tumor, it has an aggressive local behavior and high recurrence rate. Therefore, alternative treatment options or complementary to surgery have been evaluated, with the most promising one among them being a targeted therapy with the v-Raf murine sarcoma viral oncogene homologue B (BRAF), as in ameloblastoma the activating mutation V600E in BRAF is common. Studies in other tumors have shown that the synchronous inhibition of BRAF and human murine double minute 2 homologue (MDM2 or HDM2) protein is more effective than BRAF monotherapy, particularly in the presence of wild type p53 (WTp53). To investigate the MDM2 protein expression and gene amplification in ameloblastoma, in association with BRAFV600E and p53 expression. Forty-four cases of ameloblastoma fixed in 10% buffered formalin and embedded in paraffin were examined for MDM2 overexpression and BRAFV600E and p53 expression by immunohistochemistry, and for MDM2 ploidy with fluorescence in situ hybridization. Sixteen of forty-four (36.36%) cases of ameloblastoma showed MDM2 overexpression. Seven of sixteen MDM2-positive ameloblastomas (43.75%) were BRAFV600E positive and fifteen of sixteen MDM2-positive ameloblastomas (93.75%) were p53 negative. All MDM2 overexpressing tumors did not show copy number alterations for MDM2. Overexpression of MDM2 in ameloblastomas is not associated with MDM2 amplification, but most probably with MAPK activation and WTp53 expression. Further verification of those findings could form the basis for the use of MDM2 expression as a marker of MAPK activation in ameloblastomas and the trial of dual BRAF/MDM2 inhibition in the management of MDM2-overexpressing/BRAFV600E-positive/WTp53 ameloblastomas.

Intracellular CYTL1, a novel tumor suppressor, stabilizes NDUFV1 to inhibit metabolic reprogramming in breast cancer

Loss-of-function mutations frequently occur in tumor suppressor genes, i.e., p53, during the malignant progression of various cancers. Whether any intrinsic suppressor carries a rare mutation is largely unknown. Here, we demonstrate that intracellular cytokine-like protein 1 (CYTL1) plays a key role in preventing the robust glycolytic switching characteristic of breast cancer. A low intracellular CYTL1 level, not its mutation, is required for metabolic reprogramming. Breast cancer cells expressing an intracellular form of CYTL1 lacking a 1-22 aa signal peptide, ΔCYTL1, show significantly attenuated glucose uptake and lactate production, which is linked to the inhibition of cell growth and metastasis in vitro and in vivo. Mechanistically, CYTL1 competitively binds the N-terminal sequence of NDUFV1 to block MDM2-mediated degradation by the proteasome, leading to the stability of the NDUFV1 protein. In addition to inducing increased NAD⁺ levels, NDUFV1 interacts with Src to attenuate LDHA phosphorylation at tyrosine 10 and reduce lactate production. Our results reveal, for the first time, that CYTL1 is a novel tumor suppressor. Its function in reversing metabolic reprogramming toward glycolysis may be very important for the development of novel antitumor strategies.

Naturally Sourced CDK Inhibitors and Current Trends in Structure-Based Synthetic Anticancer Drug Design by Crystallography

Cyclin-dependent kinases (CDKs) are the chief regulators in cell proliferation; the kinase activities are largely regulated by their interactions with CDK inhibitors (CKIs) and Cyclins. The association of different CDKs with CDKIs and Cyclins at the cell-cycle checkpoints of different stages of mitotic cell cycle function act more likely as the molecular switches that regulate different transcriptional events required for progression through the cell cycle. A fine balance in response to extracellular and intracellular signals is highly maintained in the orchestrated function of CDKs along with Cyclins and CDKIs for normal cell proliferation. This fine-tuning in mitotic cell cycle progression sometimes gets lost due to dysregulation of CDKs. The aberrant functioning of the CDKIs is therefore studied for its contributions as a vital hallmark of cancers. It has attracted our focus to maneuver cancer therapy. Hence, several synthetic CDKIs and their crystallography-based drug design have been explained to understand their mode of action with CDKs. Since most of the synthetic drugs function by inhibiting the CDK4/6 kinases by competitively binding to their ATP binding cleft, these synthetic drugs are reported to attack the normal, healthy growing cells adjacent to the cancer cells leading to the decrease in the life span of the cancer patients. The quest for traditional natural medicines may have a great impact on the treatment of cancer. Therefore, in the present studies, a search for naturally sourced CDK inhibitors has been briefly focused. Additionally, some synthetic crystallography-based drug design has been explained to elucidate different avenues to develop better anticancer chemotherapeutics, converting natural scaffolds into inhibitors of the CDK mediated abnormal signal transduction with lesser side effects.

Feasibility of Developing Radiotracers for MDM2: Synthesis and Preliminary Evaluation of an 18F-Labeled Analogue of the MDM2 Inhibitor SP-141

Murine double minute 2 (MDM2), a negative regulator of the p53 tumor suppressor protein, is overexpressed in several human cancers. Herein we investigate the feasibility of developing ¹⁸F-labeled compounds based on the small molecule inhibitor SP-141 for imaging tumor MDM2 expression levels with positron emission tomography (PET). Three nonradioactive fluorinated SP-141 analogues, 1–3, were synthesized, and their binding to the MDM2 protein was analyzed by surface plasmon resonance (SPR). One of these, a fluoroethoxy analogue, was labeled with fluorine-18 (¹⁸F) using ¹⁸F-fluorethyl bromide to provide [¹⁸F]1 and evaluated in vitro and in vivo. SPR analysis confirmed the binding of the fluorinated analogues to MDM2 at 1.25–20 µM concentrations. Cell uptake studies revealed high uptake (67.5–71.4%/mg protein) and specificity of [¹⁸F]1 in MCF7 and HepG2 cells. The uptake of [¹⁸F]1 in these cells could be modulated using 100 µM SP-141, potentially reflecting changes in MDM2 expression because of p53 activation by SP-141. [¹⁸F]1 exhibited stable uptake and retention in HepG2 tumor xenografts (~3 %ID/g) in vivo, but poor clearance from blood and other normal tissues, yielding low tumor-to-background ratios (<2) at 2 h post injection. Our results suggest that [¹⁸F]1 has suboptimal characteristics for in vivo evaluation as a PET tracer for MDM2, but warrant radiolabeling and assessment of the other fluorinated analogues synthesized in this work, 2 and 3, and potentially other molecular scaffolds for developing MDM2 targeted radiotracers.

Synthesis, Antitumor Evaluation, Molecular Modeling and Quantitative Structure-Activity Relationship (QSAR) of Novel 2-[(4-Amino-6-N-substituted-1,3,5-triazin-2-yl)methylthio]-4-chloro-5-methyl-N-(1H-benzo[d]imidazol-2(3H)-ylidene)Benzenesulfonamides

A series of novel 2-[(4-amino-6-R2-1,3,5-triazin-2-yl)methylthio]-4-chloro-5-methyl-N-(5-R1-1H-benzo[d]imidazol-2(3H)-ylidene)benzenesulfonamides 6-49 was synthesized by the reaction of 5-substituted ethyl 2-{5-R1-2-[N-(5-chloro-1H-benzo[d]imidazol-2(3H)-ylidene)sulfamoyl]-4-methylphenylthio}acetate with appropriate biguanide hydrochlorides. The most active compounds, 22 and 46, showed significant cytotoxic activity and selectivity against colon (HCT-116), breast (MCF-7) and cervical cancer (HeLa) cell lines (IC50: 7-11 µM; 15-24 µM and 11-18 µM), respectively. Further QSAR (Quantitative Structure-Activity Relationships) studies on the cytotoxic activity of investigated compounds toward HCT-116, MCF-7 and HeLa were performed by using different topological (2D) and conformational (3D) molecular descriptors based on the stepwise multiple linear regression technique (MLR). The QSAR studies allowed us to make three statistically significant and predictive models for them. Moreover, the molecular docking studies were carried out to evaluate the possible binding mode of the most active compounds, 22 and 46, within the active site of the MDM2 protein.

Targeting cell membrane HDM2: A novel therapeutic approach for acute myeloid leukemia

The E3 ligase human double minute 2 (HDM2) regulates the activity of the tumor suppressor protein p53. A p53-independent HDM2 expression has been reported on the membrane of cancer cells but not on that of normal cells. Herein, we first showed that membrane HDM2 (mHDM2) is exclusively expressed on human and mouse AML blasts, including leukemia stem cell (LSC)-enriched subpopulations, but not on normal hematopoietic stem cells (HSCs). Higher mHDM2 levels in AML blasts were associated with leukemia-initiating capacity, quiescence, and chemoresistance. We also showed that a synthetic peptide PNC-27 binds to mHDM2 and enhances the interaction of mHDM2 and E-cadherin on the cell membrane; in turn, E-cadherin ubiquitination and degradation lead to membrane damage and cell death of AML blasts by necrobiosis. PNC-27 treatment in vivo resulted in a significant killing of both AML "bulk" blasts and LSCs, as demonstrated respectively in primary and secondary transplant experiments, using both human and murine AML models. Notably, PNC-27 spares normal HSC activity, as demonstrated in primary and secondary BM transplant experiments of wild-type mice. We concluded that mHDM2 represents a novel and unique therapeutic target, and targeting mHDM2 using PNC-27 selectively kills AML cells, including LSCs, with minimal off-target hematopoietic toxicity.

MDM2 Integrates Cellular Respiration and Apoptotic Signaling through NDUFS1 and the Mitochondrial Network

Signaling diversity and subsequent complexity in higher eukaryotes is partially explained by one gene encoding a polypeptide with multiple biochemical functions in different cellular contexts. For example, mouse double minute 2 (MDM2) is functionally characterized as both an oncogene and a tumor suppressor, yet this dual classification confounds the cell biology and clinical literatures. Identified via complementary biochemical, organellar, and cellular approaches, we report that MDM2 negatively regulates NADH:ubiquinone oxidoreductase 75 kDa Fe-S protein 1 (NDUFS1), leading to decreased mitochondrial respiration, marked oxidative stress, and commitment to the mitochondrial pathway of apoptosis. MDM2 directly binds and sequesters NDUFS1, preventing its mitochondrial localization and ultimately causing complex I and supercomplex destabilization and inefficiency of oxidative phosphorylation. The MDM2 amino-terminal region is sufficient to bind NDUFS1, alter supercomplex assembly, and induce apoptosis. Finally, this pathway is independent of p53, and several mitochondrial phenotypes are observed in Drosophila and murine models expressing transgenic Mdm2.

Aberrant splicing of the DMP1-ARF-MDM2-p53 pathway in cancer

Alternative splicing (AS) of mRNA precursors is a ubiquitous mechanism for generating numerous transcripts with different activities from one genomic locus in mammalian cells. The gene products from a single locus can thus have similar, dominant-negative or even opposing functions. Aberrant AS has been found in cancer to express proteins that promote cell growth, local invasion and metastasis. This review will focus on the aberrant splicing of tumor suppressor/oncogenes that belong to the DMP1-ARF-MDM2-p53 pathway. Our recent study shows that the DMP1 locus generates both tumor-suppressive DMP1α (p53-dependent) and oncogenic DMP1β (p53-independent) splice variants, and the DMP1β/α ratio increases with neoplastic transformation of breast epithelial cells. This process is associated with high DMP1β protein expression and shorter survival of breast cancer (BC) patients. Accumulating pieces of evidence show that ARF is frequently inactivated by aberrant splicing in human cancers, demonstrating its involvement in human malignancies. Splice variants from the MDM2 locus promote cell growth in culture and accelerate tumorigenesis in vivo. Human cancers expressing these splice variants are associated with advanced stage/metastasis, and thus have negative clinical impacts. Although they lack most of the p53-binding domain, their activities are mostly dependent on p53 since they bind to wild-type MDM2. The p53 locus produces splice isoforms that have either favorable (β/γ at the C-terminus) or negative impact (Δ40, Δ133 at the N-terminus) on patients' survival. As the oncogenic AS products from these loci are expressed only in cancer cells, they may eventually become targets for molecular therapies.

Estrogen receptor alpha (ERα/ESR1) mediates the p53-independent overexpression of MDM4/MDMX and MDM2 in human breast cancer

MDM2 and MDM4 are heterodimeric, non-redundant oncoproteins that potently inhibit the p53 tumor suppressor protein. MDM2 and MDM4 also enhance the tumorigenicity of breast cancer cells in in vitro and in vivo models and are overexpressed in primary human breast cancers. Prior studies have characterized Estrogen Receptor Alpha (ERα/ESR1) as a regulator of MDM2 expression and an MDM2- and p53-interacting protein. However, similar crosstalk between ERα and MDM4 has not been investigated. Moreover, signaling pathways that mediate the overexpression of MDM4 in human breast cancer remain to be elucidated. Using the Cancer Genome Atlas (TCGA) breast invasive carcinoma patient cohort, we have analyzed correlations between ERα status and MDM4 and MDM2 expression in primary, treatment-naïve, invasive breast carcinoma samples. We report that the expression of MDM4 and MDM2 is elevated in primary human breast cancers of luminal A/B subtypes and associates with ERα-positive disease, independently of p53 mutation status. Furthermore, in cell culture models, ERα positively regulates MDM4 and MDM2 expression via p53-independent mechanisms, and these effects can be blocked by the clinically-relevant endocrine therapies fulvestrant and tamoxifen. Additionally, ERα also positively regulates p53 expression. Lastly, we report that endogenous MDM4 negatively regulates ERα expression and forms a protein complex with ERα in breast cancer cell lines and primary human breast tumor tissue. This suggests direct signaling crosstalk and negative feedback loops between ERα and MDM4 expression in breast cancer cells. Collectively, these novel findings implicate ERα as a central component of the p53-MDM2-MDM4 signaling axis in human breast cancer.

Molecular biomarkers in colorectal carcinoma

Colorectal cancer is a tumor with increasing incidence which represents one of the first leading causes of death worldwide. Gene alterations described for colorectal cancer include genome instability (microsatellite and chromosomal instability), CpG islands methylator phenotype, microRNA, histone modification, protein biomarkers, gene mutations (RAS, BRAF, PI3K, TP53, PTEN) and polymorphisms (APC, CTNNB1, DCC). In this article, biomarkers with prognostic value commonly found in colorectal cancer will be reviewed.

Regulation of the DNA damage response by ubiquitin conjugation

In response to DNA damage, cells activate a highly conserved and complex kinase-based signaling network, commonly referred to as the DNA damage response (DDR), to safeguard genomic integrity. The DDR consists of a set of tightly regulated events, including detection of DNA damage, accumulation of DNA repair factors at the site of damage, and finally physical repair of the lesion. Upon overwhelming damage the DDR provokes detrimental cellular actions by involving the apoptotic machinery and inducing a coordinated demise of the damaged cells (DNA damage-induced apoptosis, DDIA). These diverse actions involve transcriptional activation of several genes that govern the DDR. Moreover, recent observations highlighted the role of ubiquitylation in orchestrating the DDR, providing a dynamic cellular regulatory circuit helping to guarantee genomic stability and cellular homeostasis (Popovic et al., 2014). One of the hallmarks of human cancer is genomic instability (Hanahan and Weinberg, 2011). Not surprisingly, deregulation of the DDR can lead to human diseases, including cancer, and can induce resistance to genotoxic anti-cancer therapy (Lord and Ashworth, 2012). Here, we summarize the role of ubiquitin-signaling in the DDR with special emphasis on its role in cancer and highlight the therapeutic value of the ubiquitin-conjugation machinery as a target in anti-cancer treatment strategy.

Fulvestrant treatment alters MDM2 protein turnover and sensitivity of human breast carcinoma cells to chemotherapeutic drugs

The human homologue of mouse double minute 2 (MDM2) is overexpressed in tumors and contributes to tumorigenesis through inhibition of p53 activity. We investigated the effect of the anti-estrogen fulvestrant on MDM2 expression and sensitivity of estrogen receptor positive human breast cancer cell lines to chemotherapeutics. Fulvestrant down-regulated MDM2 through increased protein turnover. Fulvestrant blocked estrogen-dependent up-regulation of MDM2 and decreased basal expression of MDM2 in the absence of estradiol. As combinations of fulvestrant with doxorubicin, etoposide or paclitaxel were synergistic, altering cell cycle distribution and increasing cell death, this provides rationale for testing combinatorial chemotherapy with fulvestrant as a novel therapeutic strategy for patients with advanced breast cancer.

A 40-bp insertion/deletion polymorphism of Murine Double Minute2 (MDM2) increased the risk of breast cancer in Zahedan, Southeast Iran

BACKGROUND

MDM2 (Murine Double Minute2) is an oncoprotein that inhibits the P53 activity. Overexpression of MDM2 gene has been reported in several human tumors. In the present study, we aimed to evaluate the impact of 40-bp insertion/deletion (ins/del) polymorphism on the promoter of MDM2 and susceptibility to breast cancer in a sample of Iranian population.

METHODS

This case-control study was carried out on 236 patients with breast cancer and 203 healthy individuals. Genomic DNA was extracted from the whole blood by the salting-out method. The 40-bp ins/del polymorphism was determined by using polymerase chain reaction.

RESULTS

The findings indicated that MDM2 ins/del variant increased the risk of breast cancer in co-dominant- (odds ratio [OR] = 2.09, 95% CI = 1.14-3.85, P = 0.018, del/del vs. ins/ins), dominant- (OR = 1.49, 95% CI = 1.02-2.18, P = 0.038, ins/del + del/del vs. ins/ins), and recessive- (OR = 1.86, 95% CI = 1.03-3.34, P = 0.038, del/del vs. ins/ins + ins/del) tested inheritance models. The del allele increased the risk of breast cancer (OR = 1.48, 95% CI = 1.11-1.98, P = 0.008) compared with ins allele.

CONCLUSIONS

Our result revealed that 40-bp ins/del polymorphism in the promoter of MDM2 increased the risk of breast cancer in an Iranian population. Further investigations with larger sample sizes and diverse ethnicities are needed to verify our findings.

Subcellular localization of Mdm2 expression and prognosis of breast cancer

BACKGROUND

Mouse double minute 2 (Mdm2) is a negative regulator of the tumor suppressor p53. The p53-Mdm2 pathway may play a role in cancer development and prognosis, although the role of p53-Mdm2 in breast cancer remains unclear.

METHODS

p53 and Mdm2 expressions were determined by immunohistochemistry of tissue microarrays of 865 breast cancer patients who underwent surgery. Clinicopathological characteristics and survival data were analyzed. Mdm2 expression was categorized into four groups: negative, cytoplasm positive, nucleus positive, and concurrent nuclear and cytoplasm positive (N+&C+).

RESULTS

Negative, cytoplasm-positive, nucleus-positive, and N+&C+ expressions of Mdm2 were observed in 59.2, 10.9, 27.8, and 2.1 % of patients, respectively. The N+&C+ group was associated with larger tumor size, higher grade, negativity for estrogen and progesterone receptors, HER2 positivity, high Ki-67 index, p53 positivity, and triple negative breast cancer. p53-positive tumors showed poorer overall survival than p53-negative tumors. The nucleus-positive and N+&C+ groups showed poorer disease-free survival than the negative and cytoplasm-positive groups. In multivariate analysis, nuclear Mdm2 expression including the N+&C+ group was significantly related to poor prognosis.

CONCLUSIONS

Concurrent nuclear and cytoplasmic Mdm2 expression was an independent prognostic factor in patients with breast cancer. Subcellular localization of Mdm2 expression should be considered in the evaluation of Mdm2 in breast cancer.

Prognostic significance of p16INK4a/p53 in Tunisian patients with breast carcinoma

Infiltrating ductal carcinoma (IDC) of the breast is a result of genetic alterations that affect the regulation of the cell cycle check-point and apoptosis. The aim of the present study was analysis using immunohistochemical localization of mouse double minute-2 (mdm2), p16INK4a, p53, bax and bcl-2 markers in Tunisian patients with breast IDC and to determine if there was correlation with the major clinico-pathological parameters and with survival of patients. We showed that the expression of p53, p16INK4a, mdm2, bcl-2, and bax was observed in 46.3%, 20.7%, 38%, 50% and 11.9% of cases, respectively. Statistical analysis revealed that positive expression of mdm2 was associated with larger tumors (P=0.013), whereas bax positivity was more prevalent in younger patients and in tumors of smaller size (P=0.008 and P=0.012 respectively). Furthermore, the expression of p16INK4a correlated with advanced grade (P<0.0001), triple negative tumors (ER-/PR-/HER2-, P=0.001) and mdm2 expression (P=0.017). The absence of nuclear p53 accumulation was predictive of good prognosis as well as when it was associated with negative expression of p16INK4a. Our findings suggest that among the biomarkers tested, p16INK4a might have a useful clinical and prognostic significance in infiltrating ductal carcinoma of the breast.

Clinical activity of mammalian target of rapamycin inhibitors in solid tumors

The phosphatidylinositol 3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) pathway is vital for cell metabolism, growth, and proliferation. mTOR is frequently upregulated in many tumor types and hence has become an important target in cancer treatment. Sirolimus and its derivatives (rapalogs) interact with the intracellular receptor FK506 binding protein 12 (FKBP12), forming a complex with high affinity for mTOR and thus disrupting its activity. Rapalogs are being evaluated extensively in cancer patients with different formulations and schedules. Significant clinical activity has led to their approval for the treatment of kidney cancer, mantle cell lymphoma, and subependymal giant cell astrocytoma; however, despite increasing knowledge about cancer cell biology, their activity in other malignancies is unclear. Further research is needed to identify optimal dosage, administration and targeted combination as well as the subset of patients likely to respond to mTOR/PI3K inhibition. This review focuses on a discussion of the pathway, its implications in cancer biology and results of clinical trials of rapalogs alone or in combination, organizing them by common malignancy type.

MDM2 regulates estrogen receptor α and estrogen responsiveness in breast cancer cells

Murine double minute clone 2 (MDM2) is a multifunctional protein, which modulates nuclear receptor-mediated transactivation. In this study, we show that MDM2 significantly enhanced estrogen receptor α (ERα) and ERα/specificity protein-mediated transactivation in MCF-7 and ZR-75 breast cancer cells. This was demonstrated by both MDM2 overexpression and knockdown experiments by RNA interference. ERα interacted with wild-type MDM2 and deletion mutants of MDM2 containing amino acids 1-342 (C-terminal deletion) and 134-490 (N-terminal deletion), but not 134-342. In contrast, only wild-type but not mutant MDM2 enhanced ERα-mediated transactivation. Protein-protein interactions in vitro were 17β-estradiol (E(2)) independent, whereas fluorescent resonance energy transfer experiments in living cells showed that E(2) enhanced ERα-MDM2 interactions. Subsequent RNA interference and mammalian two-hybrid experiments suggested that MDM2 did not directly interact with endogenous coactivators such as the steroid receptor coactivators but played a role in enhancing ERα-mediating gene expression and estrogen responsiveness through interactions with ERα.

MDM2 SNP309 is associated with endometrial cancer susceptibility: a meta-analysis

Epidemiological studies have investigated the association between MDM2 promoter SNP 309 (T/G) and endometrial cancer susceptibility. However, the results are still controversial. To obtain a more precise estimate of the relationship, we conducted a meta-analysis of 1,001 cases and 1,889 controls from 6 published case-control studies (one of five articles contains two studies) to estimate the effect of SNP309 on endometrial cancer risk. The strength of association between MDM2 SNP309 and endometrial cancer susceptibility was assessed by calculating pooled odds ratios (ORs) with 95% confidence intervals (CIs). When all the eligible studies were pooled in the meta-analysis, we found that elevated endometrial cancer risk was significantly associated with GG variant genotype, however, heterozygous genotype TG seemed to be only a minor modifier on endometrial cancer risk (for GG vs. TT, OR = 1.54, 95% CI = 1.21-1.95, P = 0.0004; for TG vs. TT, OR = 0.96, 95% CI = 0.81-1.14, P = 0.66; for dominant model, OR = 1.09, 95% CI = 0.93-1.29, P = 0.29; for recessive model, OR = 1.65, 95% CI = 1.33-2.04, P < 0.00001). Overall, the meta-analysis suggested that the GG genotype of MDM2 SNP309 was significantly associated with the increased endometrial cancer risk.

Regulation of MDM4 (MDMX) function by p76(MDM2): a new facet in the control of p53 activity

Under basal growth conditions, p53 function is tightly controlled by the members of MDM family, MDM2 and MDM4. The Mdm2 gene codes, in addition to the full-length p90(MDM2), for a short protein, p76(MDM2) that lacks the p53-binding domain. Despite this property and at variance with p90(MDM2), this protein acts positively toward p53, although the molecular mechanism remains elusive. Here, we report that p76(MDM2) antagonizes MDM4 inhibitory function. We show that p76(MDM2) possesses intrinsic ubiquitinating and degrading activity, and through these activities controls MDM4 levels. Furthermore, the presence of p76(MDM2) decreases the association of MDM4 with p53 and p90(MDM2), and antagonizes p53 degradation by the heterodimer MDM4/p90(MDM2). The p76(MDM2)-mediated regulation of MDM4 occurs in the cytoplasm, under basal growth conditions. Conversely, upon DNA damage, phosphorylation of MDM4Ser403 dissociates p76(MDM2) and prevents MDM4 degradation. The overall negative control of MDM4 by p76(MDM2) reflects on p53 function as p76(MDM2) impairs MDM4-mediated inhibition of p53 activity. In agreement with the positive role of p76(MDM2) toward p53, the p76(MDM2)/p90(MDM2) ratio significantly decreases in a group of thyroid tumor samples compared with normal counterparts. Overall, these findings reveal a new mechanism in the control of p53 basal activity that may account for the distinct sensitivity of tissues to stress signals depending on the balance among MDM proteins. Moreover, these data suggest an oncosuppressive function for a product of the Mdm2 gene.

Proteasome system of protein degradation and processing

In eukaryotic cells, degradation of most intracellular proteins is realized by proteasomes. The substrates for proteolysis are selected by the fact that the gate to the proteolytic chamber of the proteasome is usually closed, and only proteins carrying a special "label" can get into it. A polyubiquitin chain plays the role of the "label": degradation affects proteins conjugated with a ubiquitin (Ub) chain that consists at minimum of four molecules. Upon entering the proteasome channel, the polypeptide chain of the protein unfolds and stretches along it, being hydrolyzed to short peptides. Ubiquitin per se does not get into the proteasome, but, after destruction of the "labeled" molecule, it is released and labels another molecule. This process has been named "Ub-dependent protein degradation". In this review we systematize current data on the Ub-proteasome system, describe in detail proteasome structure, the ubiquitination system, and the classical ATP/Ub-dependent mechanism of protein degradation, as well as try to focus readers' attention on the existence of alternative mechanisms of proteasomal degradation and processing of proteins. Data on damages of the proteasome system that lead to the development of different diseases are given separately.

Signaling to p53: ribosomal proteins find their way

Inherently disparate cell growth and division, which are intimately coupled through a delicate network of intracellular and extracellular signaling, require ribosomal biogenesis. A number of events imparting instability to ribosomal biogenesis can cause nucleolar stress. In response to this stress, several ribosomal proteins bind to MDM2 and block MDM2-mediated p53 ubiquitination and degradation, resulting in p53-dependent cell cycle arrest. By doing so, the ribosomal proteins play a crucial role in connecting deregulated cell growth with inhibition of cell division. The ribosomal protein-MDM2-p53 signaling pathway provides a molecular switch that may constitute a surveillance network monitoring the integrity of ribosomal biogenesis.

Effect of TP53 Arg72Pro and MDM2 SNP309 polymorphisms on the risk of high-grade osteosarcoma development and survival

PURPOSE

The germ-line polymorphisms TP53 Arg72Pro and MDM2 SNP309 T>G are risk factors for tumor development and affect response to chemotherapy and survival in several cancers, but their prognostic and predictive value in patients with high-grade osteosarcomas is not yet defined. The purpose of this study was to investigate the effect of the TP53 Arg72Pro and the MDM2 SNP309 on the risk of osteosarcoma development and survival.

EXPERIMENTAL DESIGN

The relative risk to develop osteosarcomas and the overall survival associated to TP53 Arg72Pro and MDM2 SNP309 polymorphisms were investigated in 201 patients. Correlations with event-free survival (EFS) were analyzed in a homogeneous subgroup of 130 patients with high-grade osteosarcomas of the limbs, nonmetastatic at diagnosis, which underwent neoadjuvant chemotherapy.

RESULTS

Multivariate analysis showed that the MDM2 polymorphism T309G was associated with an increased risk of developing osteosarcomas [GG versus TT; odds ratio, 2.09; 95% confidence interval (95% CI), 1.15-3.78]. A case/control gender approach evidenced a significant increased risk only for female osteosarcoma patients (GG versus TT; odds ratio, 4.26; 95% CI, 1.61-11.25). Subjects carrying the TP53 Arg72Pro polymorphism were found to have a significantly increased death risk (Pro/Pro versus Arg/Arg; hazard ratio, 2.90; 95% CI, 1.28-6.66). In the subgroup of 130 high-grade osteosarcomas, the TP53 Arg72Pro was an independent marker of EFS (Pro/Pro versus Arg/Arg; hazard ratio, 2.67; 95% CI, 1.17-6.11).

CONCLUSION

The study provides evidence supporting the association of MDM2 SNP309 with high-grade osteosarcoma risk in females and shows that TP53 Arg72Pro has a prognostic value for overall survival and EFS in osteosarcoma patients.

Normal human mammary epithelial cells proliferate rapidly in the presence of elevated levels of the tumor suppressors p53 and p21(WAF1/CIP1)

In normal cells, p53 protein is maintained at low levels, but the levels increase after stress or inappropriate growth signals to coordinate growth arrest or apoptosis. Human mammary epithelial cells (HMECs) are unusual in that they exhibit two phases of growth. The second growth phase, referred to as post-selection, follows a period of temporary growth arrest and is characterized by the absence of p16(INK4a) (also known as CDK4I and p16-INK4a) expression. Previously, we observed that post-selection HMECs have elevated levels of p53. Exogenous p16(INK4a) expression decreased levels of both p53 transcript and protein, and this effect was inhibited by nutlin-3a, indicating that p16(INK4a) can regulate p53 expression by affecting both p53 transcription and Mdm2-dependent degradation of p53. The p53 in post-selection HMECs was wild type and, as expected, increased p53 expression was associated with elevated p21(WAF1/CIP1) and Mdm2 levels; the p53 response to DNA damage seemed normal. Despite elevated levels of wild-type p53 and p21(WAF1/CIP1), post-selection cells grew more rapidly than their pre-selection HMEC precursors. We found that the post-selection HMECs contain a truncated Mdm2 protein (p60), which presumably lacks the p53 ubiquitylation domain. We propose that the increased levels of p53 in post-selection HMECs are due to the presence of an Mdm2 fragment that binds p53 but does not result in its degradation.

Identification of HDM2 as a regulator of VEGF expression in cancer cells

Overexpression of vascular endothelial growth factor (VEGF) and the extent of neoangiogenesis are closely correlated with tumor development and cancer metastases. To assess whether VEGF mediated angiogenesis is regulated by HDM2, we treated the GI-101A and HL-60 cells with HDM2 gene specific antisense phosphorothioate oligodeoxynucleotide (AS5). The antisense treatment resulted in a significant reduction of both basal as well as phorbol 12,13-dibutyrate (PDB) and Diethylstilbestrol (DES) induced VEGF mRNA and protein expressions. Furthermore, when the Human Umbilical Vein Endothelial Cells (HUVECs) were exposed to medium obtained from AS5 transfected GI-101A and HL-60 cells, the angiogenesis was significantly reduced compared to the controls in the in vitro angiogenesis assay. On the contrary, the medium obtained from PDB treated cells that expressed HDM2 and VEGF at a higher level showed an increase in the tube formation by HUVEC. Thus, our present study suggests that modulation of HDM2 expression could play an important role in tumor angiogenesis and the metastatic process via transcriptional regulation of VEGF.

MDM2 promoter polymorphism SNP309 contributes to tumor susceptibility: evidence from 21 case-control studies

Since the identification of a well-characterized functional polymorphism named SNP309 in MDM2, abundant studies were published in the last 2 years to evaluate the association between SNP309 and tumor risk in diverse populations. However, the results remain conflicting rather than conclusive. Because a single study may have been underpowered to detect the effect of low-penetrance genes, a quantitative synthesis to accumulate data from different studies may provide better evidence on the association of genetic variant with tumor susceptibility. We conducted a meta-analysis on 14,770 cases with different tumor types and 14,524 controls from 25 published case-control studies to estimate the effect of SNP309 on tumor risk, as well as to quantify the potential between-study heterogeneity. We found that variant homozygote 309GG was associated with a significantly increased risk of all types of tumors [homozygote comparison: odds ratio (OR), 1.17, 95% confidential interval (95% CI), 1.04-1.33, P = 0.0002 for heterogeneity test; recessive model comparison: OR, 1.15, 95% CI, 1.03-1.28, P = 0.0005 for heterogeneity test]. Tumor type and ethnicity contributed to the substantial heterogeneity (69.5% for homozygote comparison and 77.2% for recessive model comparison). The analyses suggest that MDM2 SNP309 serves as a low-penetrance susceptibility tumor marker. Further large studies incorporate quantitative detection of different p53-responsible environmental stresses, p53 mutation status, and also functional genetic variants in p53-MDM2-related genes are warranted.

Association of TP53 codon 72 polymorphism and the outcome of adjuvant therapy in breast cancer patients

INTRODUCTION

Single-nucleotide polymorphisms (SNPs) in codon 72 of the TP53 (also known as p53) gene (rs1042522) and in the promoter region of the MDM2 gene (SNP309; rs2279744) have been suggested to play roles in many cancers. We investigated whether these SNPs were associated with patient outcome and the effect of adjuvant systemic therapy.

METHODS

The genotypes of TP53 codon 72 and MDM2 SNP309 were defined among 557 primary Japanese breast cancer patients (median follow-up, 61.7 months). The effects of several variables on survival were tested by Cox's proportional hazards regression analysis.

RESULTS

We showed that the Pro/Pro genotype of TP53 codon 72 was associated with poorer disease-free survival (DFS) than other genotypes by Kaplan-Meier analysis (P = 0.049) and multivariate Cox's proportional hazards regression analysis (P = 0.047, risk ratio of recurrence = 1.67), whereas MDM2 SNP309 status was not associated with DFS. The association of the Pro/Pro TP53 genotype with poorer DFS was especially significant in patients who received adjuvant chemotherapy (P = 0.009). In contrast, among the patients who had received adjuvant hormonal therapy or no adjuvant systemic therapy, TP53 codon 72 genotype was not associated with DFS.

CONCLUSION

The Pro/Pro genotype of TP53 codon 72 appears to be an independent prognostic marker in breast cancer patients.

A Single Nucleotide Polymorphism in theMDM2Gene Disrupts the Oscillation of p53 and MDM2 Levels in Cells

Oscillations of both p53 and MDM2 proteins have been observed in cells after exposure to stress. A mathematical model describing these oscillations predicted that oscillations occur only at selected levels of p53 and MDM2 proteins. This model prediction suggests that oscillations will disappear in cells containing high levels of MDM2 as observed with a single nucleotide polymorphism in the MDM2 gene (SNP309). The effect of SNP309 upon the p53-MDM2 oscillation was examined in various human cell lines and the oscillations were observed in the cells with at least one wild-type allele for SNP309 (T/T or T/G) but not in cells homozygous for SNP309 (G/G). Furthermore, estrogen preferentially stimulated the transcription of MDM2 from SNP309 G allele and increased the levels of MDM2 protein in estrogen-responsive cells homozygous for SNP309 (G/G). These results suggest the possibility that SNP309 G allele may contribute to gender-specific tumorigenesis through further elevating the MDM2 levels and disrupting the p53-MDM2 oscillation. Furthermore, using the H1299-HW24 cells expressing wild-type p53 under a tetracycline-regulated promoter, the p53-MDM2 oscillation was observed only when p53 levels were in a specific range, and DNA damage was found to be necessary for triggering the p53-MDM2 oscillation. This study shows that higher levels of MDM2 in cells homozygous for SNP309 (G/G) do not permit coordinated p53-MDM2 oscillation after stress, which might contribute to decreased efficiency of the p53 pathway and correlates with a clinical phenotype (i.e., the development of cancers at earlier age of onset in female).

Genetic and expression aberrations of E3 ubiquitin ligases in human breast cancer

Recent studies revealed that E3 ubiquitin ligases play important roles in breast carcinogenesis. Clinical research studies have found that (epi)-genetic (deletion, amplification, mutation, and promoter methylation) and expression aberration of E3s are frequent in human breast cancer. Furthermore, many studies have suggested that many E3s are either oncogenes or tumor suppressor genes in breast cancer. In this review, we provide a comprehensive summary of E3s, which have genetic and/or expression aberration in breast cancer. Most cancer-related E3s regulate the cell cycle, p53, transcription, DNA repair, cell signaling, or apoptosis. An understanding of the oncogenic potential of the E3s may facilitate identifying and developing individual E3s as diagnosis markers and drug targets in breast cancer.

No association of the MDM2 SNP309 polymorphism with risk of breast or ovarian cancer

A functional T to G germline polymorphism in the promoter region of MDM2 (SNP309) has been reported to profoundly accelerate tumor formation suggesting that it may also represent a powerful cancer predisposing allele. To investigate the role of SNP309 in cancer predisposition we undertook a case-control study of this polymorphism among 351 women diagnosed with breast cancer, 302 women diagnosed with ovarian and 258 female controls from a British population. The GG genotype was not associated with either breast cancer (OR 1.04, 95% CI 0.67-1.60) or ovarian cancer (OR 0.86, 95% CI 0.53-1.37). This study has found no evidence that the GG genotype of the MDM2 SNP309 polymorphism is associated with early onset or familial breast cancer or with ovarian cancer.

Polymorphisms in the MDM2 promoter and risk of breast cancer: a case-control analysis in a Chinese population

MDM2 is a phosphoprotein that interacts with P53 and inhibits its activity. Recently, a T/G substitution (SNP309) in the promoter of MDM2 was identified and has been demonstrated to be associated with an increased MDM2 expression and a significantly earlier age of onset of several tumors, including breast cancer. To test the hypothesis that this functional variant in the MDM2 promoter is associated with risk of breast cancer, we conducted a molecular epidemiological study of 366 breast cancer cases (BC), 263 patients with benign breast diseases (BBD) and 605 cancer-free controls in China, in which we genotyped this T/G variant and another common insertion/deletion polymorphism (Del1518) in the MDM2 promoter and evaluated the associations between these two polymorphisms and breast cancer risk. We found that the variant allele frequencies of these two polymorphisms were not statistically different between the cases and controls (SNP309G: 0.500, 0.542, and 0.506 in BC, BBD, and controls, respectively, and Del1518-: 0.296, 0.308, and 0.297 in BC, BBD, and controls, respectively). Logistic regression analyses revealed that the variant genotypes of both MDM2 SNP309 and Del1518 polymorphisms were not significantly associated with risk of breast cancer (adjusted OR, 1.03; 95% CI, 0.74-1.42 for SNP309 TG and GG; and adjusted OR, 1.09; 95% CI, 0.83-1.43 for Del1518 +/- and -/-). These findings suggest that these two MDM2 promoter variants may not play a major role in the etiology of breast cancer.

MDM2 SNP309 Accelerates Tumor Formation in a Gender-Specific and Hormone-Dependent Manner

The importance of the p53 stress response pathway in the suppression of tumor formation is well documented. In a previous report, a single nucleotide polymorphism (SNP309 T/G) was found in the promoter of the MDM2 gene resulting in higher levels of MDM2 RNA and protein and, consequently, in the attenuation of the p53 pathway both in vitro and in vivo. As the SNP309 locus is found in a region of the MDM2 promoter, which is regulated by hormonal signaling pathways, and the G-allele of SNP309 increases the affinity of a well-described cotranscriptional activator of nuclear hormone receptors (i.e., Sp1), the hypothesis that the SNP309 locus could alter the effects of hormones on tumorigenesis was tested in vivo in humans. Data obtained from patients with three different sporadic cancers, from four independent case studies, support this hypothesis, providing an example for the genetic basis of gender differences in cancer and showing that the genotype at a specific locus can affect how hormones, like estrogen, affect tumorigenesis in humans.

Influence of the MDM2 single nucleotide polymorphism SNP309 on tumour development in BRCA1 mutation carriers

Background

The MDM2 gene encodes a negative regulator of the p53 tumour suppressor protein. A single nucleotide polymorphism (SNP) in the MDM2 promoter (a T to G exchange at nucleotide 309) has been reported to produce accelerated tumour formation in individuals with inherited p53 mutations. We have investigated the effect of the MDM2 SNP309 on clinical outcome in a cohort of patients with germline mutations of BRCA1.

Methods

Genomic DNA was obtained for 102 healthy controls and 116 patients with established pathogenic mutations of BRCA1 and Pyrosequencing technology™ was used to determine the genotype at the MDM2 SNP309 locus.

Results

The polymorphism was present in 52.9% of the controls (G/T in 37.3% and G/G in 15.6%) and 58.6% of the BRCA1 mutation carriers (47.4% G/T and 11.2% G/G). Incidence of malignancy in female BRCA1 carriers was not significantly higher in SNP309 carriers than in wildtype (T/T) individuals (72.7% vs. 75.6%, p = 1.00). Mean age of diagnosis of first breast cancer was 41.2 years in the SNP309 G/G genotype carriers, 38.6 years in those with the SNP309 G/T genotype and 39.0 years in wildtype subjects (p = 0.80).

Conclusion

We found no evidence that the MDM2 SNP309 accelerates tumour development in carriers of known pathogenic germline mutations of BRCA1.

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.

Progesterone inhibition of MDM2 p90 protein in MCF-7 human breast cancer cell line is dependent on p53 levels

The mdm2 gene encodes several protein isoforms with different molecular weights (p90, p80, p76 and p57). MDM2 p90 (usually considered to be the major MDM2 protein) binds to and inactivates P53. We have recently shown that growth inhibition of MCF-7 human breast cancer cells by progesterone is associated with P53 down-regulation. In this work, we analyzed the expression pattern of MDM2 proteins in three human breast cancer cell lines by western blotting with anti-MDM2 antibodies. We found a prominent expression of MDM2 p57 protein in cell lines which have non-functional P53 protein (T47D and MDA-MB-231) as compared to the p90, p80 isoforms, whereas p90 was the major protein isoform in MCF-7 cells that contain functional P53 protein. When MCF-7 cells were treated with 100 nM of progesterone, MDM2 p90 was inhibited but the highly expressed MDM2 p57 isoform was not. The inhibition of MDM2 p90 protein by progesterone was abrogated in MCF-7 cells transfected with a P53 expressing vector. To our knowledge, this is the first report linking progesterone-induced growth inhibition with down-regulation of the MDM2 protein. We present evidence that reestablishing of P53 expression by transient transfection of P53 cDNA in these cells enhances the expression level of MDM2 p90 isoform. The data indicate that expression of MDM2 p90 is regulated through a P53-dependent pathway in response to progesterone.

MEK-ERK signaling controls Hdm2 oncoprotein expression by regulating hdm2 mRNA export to the cytoplasm

The physical and functional interaction between the transcription factor p53 and its negative regulatory partner protein Hdm2 (Mdm2 in mouse) is a key point of convergence of multiple signaling pathways that regulates cell proliferation and survival. hdm2 mRNA transcription is induced by p53, forming the basis of an auto-regulatory feedback loop. Growth and survival factor-activated Ras-Raf-MEK-ERK signaling can also regulate Hdm2 expression independently of p53, contributing to the pro-survival effect of these factors. In murine fibroblasts, this occurs through the regulation of mdm2 mRNA transcription. Here we show that, in human breast cancer epithelial cells, MEK-dependent regulation of Hdm2 expression also occurs at a post-transcriptional level. Pharmacological blockade of MEK activity in T47D cells inhibits Hdm2 protein synthesis by 80-90%. This occurs in the absence of changes in the expression of the major hdm2-P1 mRNA transcript and only an approximately 40% reduction in hdm2-P2 transcript levels. The amounts of both transcripts that are associated with polyribosomes and are, hence, being actively translated are reduced by >80% by the MEK inhibitor, U0126. We show here that this is due to the inhibition of hdm2 mRNA export from the nucleus when MEK activity is inhibited. In MCF-7 breast cancer cells that express wild-type p53, Hdm2 is required to suppress p53-dependent transcription when MEK kinase is active. Regulation of the nuclear export of hdm2 mRNA provides, therefore, a mechanism whereby mitogen-stimulated cells avoid p53-dependent cell cycle arrest or apoptosis by maintaining the dynamic equilibrium of the Hdm2-p53 feedback loop.

Combined targeting of epidermal growth factor receptor and MDM2 by gefitinib and antisense MDM2 cooperatively inhibit hormone-independent prostate cancer

PURPOSE

The epidermal growth factor receptor (EGFR) may play a relevant role in the progression, hormone therapy resistance, and prognosis of prostate cancer patients. Also MDM2, a negative p53 regulator that interacts with retinoblastoma (Rb), E2F, p19(arf) and the ras-mitogen-activated protein kinase(MAPK) cascade plays an important role in prostate cancer progression and prognosis. On the basis of the EGFR and MDM2 role in integrating signaling pathways critical for prostate cancer progression, we investigated whether their selective combined blockade may have a cooperative antitumor effect in prostate cancer. For this purpose, we have used the EGFR tyrosine kinase inhibitor gefitinib (ZD1839, Iressa) and a second generation hybrid oligonucleotide antisense MDM2 (AS-MDM2), respectively.

EXPERIMENTAL DESIGN

Gefitinib and AS-MDM2 were administered to hormone-refractory and hormone-dependent human prostate cancer cells in vitro and to mice bearing tumor xenografts, evaluating the effects on growth, apoptosis, and protein expression, in vitro and in vivo.

RESULTS

We demonstrated that the combination of gefitinib and AS-MDM2 synergistically inhibits the growth of hormone-independent prostate cancer cells in vitro. This effect is accompanied by the inhibition of MDM2, phosphorylated Akt (pAkt), phosphorylated MAPK (pMAPK), and vascular endothelial growth factor (VEGF) expression and by Rb hypophosphorylation. The combination of the two agents in nude mice bearing the same hormone-independent tumors caused a potent cooperative antitumor effect. Tumor samples analysis confirmed the inhibition of MDM2, pAkt, pMAPK, VEGF, and basic fibroblast growth factor expression.

CONCLUSIONS

This study shows that EGFR and MDM2 play a critical role in the growth of prostate cancer, especially hormone-dependent, and that their combined blockade by gefitinib and AS-MDM2 causes a cooperative antitumor effect, supporting the clinical development of this therapeutic strategy.

COP1, the negative regulator of p53, is overexpressed in breast and ovarian adenocarcinomas

The tumor suppressor protein p53 plays a central role in protecting normal cells from undergoing transformation. Thus, it is fitting that cancer cells selectively dampen the p53 response to gain a selective growth advantage. In fact, the p53 gene is the most commonly mutated tumor suppressor gene in human cancers, and if the gene is not mutated, then other components of the p53 pathways are skewed to dampen the p53 response to stress. We recently identified COP1 as a novel and critical negative regulator of p53. COP1 is a RING finger-containing protein that targets p53 for degradation to the proteasome and is necessary for p53 turnover in normal and cancer cells. However, the association between COP1 and cancer remains to be determined. We performed expression analysis of COP1 in ovarian and breast cancer tissue microarrays. COP1 is significantly overexpressed in 81% (25 of 32) of breast and 44% (76 of 171) of ovarian adenocarcinoma as assessed by in situ hybridization and immunohistochemistry. Overexpression of COP1 correlated with a striking decrease in steady state p53 protein levels and attenuation of the downstream target gene, p21, in cancers that retain a wild-type p53 gene status. Overall, these results suggest that overexpression of COP1 contributes to the accelerated degradation of p53 protein in cancers and attenuates the tumor suppressor function of p53.

Over-expression of human UREB1 in colorectal cancer: HECT domain of human UREB1 inhibits the activity of tumor suppressor p53 protein

Many fundamental processes, including oncogenesis, have implicated HECT domain proteins with ubiquitin ligase activity. The protein human upstream regulatory element binding protein 1 (hUREB1) is a HECT domain protein whose function is not defined yet. Here, we investigate the function of hUREB1 as a ubiquitin-protein ligase in human colorectal cells. Ectopic expression of the HECT domain of hUREB1 reduces the protein level and transcriptional activity of the p53 tumor suppressor, which is abrogated by the deletion in the HECT domain or point mutations in the essential residues of the HECT domain. The ubiquitination and destabilization of p53 is observed in cells treated with the protease inhibitor MG132, implying that the HECT domain of hUREB1 suppresses the transcriptional activity of p53 through a ubiquitin-dependent degradation pathway. Based on the results of Northern blot analysis, RT-PCR, and immunohistochemical analyses, the over-expression of hUREB1 is associated with colorectal carcinoma. Moreover, protein levels of hUREB1 and p53 were inversely correlated. These findings suggest that hUREB1 can function, at least in part, as a negative regulator of p53 during the colorectal carcinoma progression through the ubiquitination pathway mediated by the HECT domain.

Targeting the ubiquitin-proteasome pathway in breast cancer

The 26S proteasome is an adenosine triphosphate-dependent multicatalytic protease that is responsible for most nonlysosomal intracellular protein degradation. To be selected for proteasomal degradation, proteins must be previously tagged with a polyubiquitin chain, which is then recognized by the proteasome; the ubiquitin chain is removed by isopeptidases and the protein is hydrolysed to small polypeptides. In addition to removing damaged/unnecessary proteins, the proteasome is also an important mechanism of regulation of some key regulatory proteins and their inhibitors. This regulation is crucial for the control of many cellular processes, including activation of transcription factors, cell cycle progression, and apoptosis. The critical role of the ubiquitin-proteasome pathway in tumor cells has led to the investigation of proteasome inhibition as a potential anticancer therapy. The dipeptide boronic acid analogue bortezomib, formerly known as PS-341, is a potent, highly selective, and reversible proteasome inhibitor. The first drug of this class to be used in the clinical setting, it has recently been approved by the US Food and Drug Administration for the treatment of relapsed and refractory multiple myeloma and is currently being tested in clinical trials for the treatment of a wide variety of malignancies. This article provides a summary of the biology of the ubiquitin-proteasome pathway, reviews the available preclinical and clinical data of proteasome inhibition as a therapeutic strategy in breast cancer, and discusses future combination regimens involving bortezomib.

The ubiquitin-mediated protein degradation pathway in cancer: therapeutic implications

The highly conserved eukaryotic ubiquitin-proteasome system (UP-S) plays a pivotal role in protein homeostasis and is critical in regulating normal and cancer-related cellular processes. The hierarchical nature of the UP-S provides a rich source of molecular targets for specific intervention and has therefore arisen as a promising approach to innovative anticancer therapies. The first in class proteasome inhibitory agent Bortezomib (Velcade) has recently obtained regulatory approval for the treatment of multiple myeloma. Ubiquitin-mediated degradation is a complex process that is comprised of well defined steps involving ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s) and ubiquitin ligases (E3s). Although a single E1 activates the ubiquitin conjugation machinery, a large number of E2 conjugating enzymes and E3 ligases are now known to exist. Proteins tagged with ubiquitin are subsequently recognised by the proteasome for digestion and fragmentation. The enzymatic nature, multitude of E3s and their specific substrate recognition predestines them as therapeutic targets. This article will review known inhibitors of the proteasome and their molecular mechanisms as well as ongoing developments and promising avenues for targeting substrate-specific E3 ligases that are likely to yield a new class of therapeutics that will serve and complement the armamentarium of anticancer drugs.

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.

P53 and breast cancer

p53 mutation is the most common genetic abnormality found so far in human cancer, and in breast cancer p53 mutation/alteration is seen in up to 50% of primary carcinomas. Together with the increasing knowledge of the characteristics and understanding of the role of p53 over the last two decades, attention in recent years has been focused on how this knowledge can be used in clinical settings for patient care and management in terms of analyzing p53 as a potential marker for studying the relationship between p53 expression and tumour development, progression and outcome; and designing alternative treatment strategies specifically aimed at restoring normal p53 function.

MDM2 and Prognosis

The cellular stress response pathway regulated by the p53 tumor suppressor is critical to the maintenance of genomic integrity and to the prevention of oncogenic transformation. Intracellular levels of p53 are tightly regulated by an autoregulatory feedback loop comprised of p53 and MDM2. It might be predicted that disruption of this loop, either through p53 mutation or overexpression of MDM2, would be a negative prognostic marker for cancer development, likelihood of relapse, or response to therapy. In fact, although MDM2 overexpression is common in cancer, it can be both a positive and a negative predictor of outcome in different tumors, and its significance as a biomarker remains controversial. Data from a number of different tumor types are reviewed for the predictive significance of MDM2 expression, along with evidence for different mechanisms of MDM2 overexpression in these different tumors.

In light of the biological complexities underlying the p53-MDM2 loop, it is, perhaps, not surprising that no simple paradigm exists that is generally applicable. Much work remains to be done to elucidate the basic mechanisms underlying the physical interactions between the two proteins, the role of protein modifications in altering those interactions, and also the genetic and transcriptional deregulations by which protein levels are altered in human cancers. Only in this way will truly biologically relevant predictive factors emerge.

MDM2 and Its Splice Variant Messenger RNAs: Expression in Tumors and Down-Regulation Using Antisense Oligonucleotides

Alternative splicing has an important role in expanding protein diversity. An example of a gene with more than one transcript is the MDM2 oncogene. To date, more than 40 different splice variants have been isolated from both tumor and normal tissues. Here, we review what is known about the alteration of MDM2 mRNA expression, focusing on alternative splicing and potential functions of different MDM2 isoforms. We also discuss the progress that has been made in the development of antisense oligonucleotides targeted to MDM2 for use as a potential cancer therapy.

Mdm2: A regulator of cell growth and death

The interplay between Mdm2 and p53 represents one of the better-known paradigms of the relationship between an oncogene and a tumor suppressor gene. The Mdm2 protein is a key regulator of cell growth and death and plays a pivotal role in the transformation of normal cells into tumor cells, the hallmark of an oncogene. The primary role of Mdm2 under nonstressed conditions is to target the degradation ofthe tumor suppressor protein p53. In response to stress, however, p53 is not affected by Mdm2 and functions as a transcription factor that induces the transcription of Mdm2 as well as of genes involved in growth control or apoptosis. The effect of Mdm2 on the regulation of cell growth and death depends on p53 but also on a growing number of p53-independent targets. This overview summarizes our current understanding of Mdm2 and p53 regulation, function, and interaction in normal and tumor states.

The cell cycle: a review of regulation, deregulation and therapeutic targets in cancer

The cell cycle is controlled by numerous mechanisms ensuring correct cell division. This review will focus on these mechanisms, i.e. regulation of cyclin-dependent kinases (CDK) by cyclins, CDK inhibitors and phosphorylating events. The quality checkpoints activated after DNA damage are also discussed. The complexity of the regulation of the cell cycle is also reflected in the different alterations leading to aberrant cell proliferation and development of cancer. Consequently, targeting the cell cycle in general and CDK in particular presents unique opportunities for drug discovery. This review provides an overview of deregulation of the cell cycle in cancer. Different families of known CDK inhibitors acting by ATP competition are also discussed. Currently, at least three compounds with CDK inhibitory activity (flavopiridol, UCN-01, roscovitine) have entered clinical trials.

Deregulation of Cdc2 kinase induces caspase-3 activation and apoptosis

Progression of the cell cycle and control of apoptosis are tightly linked processes. It has been reported that manifestation of apoptosis requires cdc2 kinase activity yet the mechanism(s) of which is largely unclear. In an attempt to study the role of human MDM2 (HDM2) in interphase and mitosis, we employed the Xenopus cell-free system to study HDM2 protein stability. Interestingly, HDM2 is specifically cleaved in Xenopus mitotic extracts but not in the interphase extracts. We demonstrate that HDM2 cleavage is dependent on caspase-3 and that activation of cdc2 kinase results in caspase-3 activation in the Xenopus cell-free system. Furthermore, expression of cdc2 kinase in mammalian cells leads to activation of caspase-3 and apoptosis. Taken together, these data indicate that deregulation of cdc2 kinase activity can trigger apoptotic machinery that leads to caspase-3 activation and apoptosis.

Molecular alterations in sporadic breast cancer

Breast cancer is a genetic disease. Like other human cancers, it is thought to occur as the result of progressive accumulation of genetic aberrations. These aberrations result in a deviation of the gene expression profiles from that of the normal progenitor cell. In up to 99% of cases, breast cancer is due to solely somatic genetic aberrations without germ-line ones. Considerable progress have already been made in understanding the genetic mechanisms underlying the development and progression of breast cancer. Several extensively studied genes are now well known to be involved. Unfortunately, our ability to make clinically useful interventions on the basis of these data is limited. Because of the involvement of multiple genes and complex pathways in a single cancer cell, the molecular dysfunctioning underlying breast cancer remains to be completely clarified. In a next future, studying the global gene expression of different types of tumors will allow the development of expression profiles unique for a breast cancer, its stage and prognostic category, leading to diagnostic assays and the identification of new therapeutic targets.