Human MDM2 isoforms translated differentially on constitutive versus p53-regulated transcripts have distinct functions in the p53/MDM2 and TSG101/MDM2 feedback control loops

HER2 Antibody-Drug Conjugates Are Active against Desmoplastic Small Round Cell Tumor

PURPOSE

Desmoplastic small round cell tumor (DSRCT) is a rare but highly aggressive soft tissue sarcoma that arises in the abdominopelvic cavity of young males. Since the discovery of EWSR1::WT1 fusion as the driver of DSRCT, no actionable genomic alterations have been identified, limiting disease management to a combination of surgery, chemotherapy, and radiation, with very poor outcomes. Herein, we evaluated ERBB2/HER2 expression in DSRCT as a therapeutic target.

EXPERIMENTAL DESIGN

ERBB2/HER2 expression was assessed in clinical samples and patient-derived xenografts (PDX) using RNA sequencing, RT-qPCR, and a newly developed HER2 IHC assay (clone 29D8). Responses to HER2 antibody-drug conjugates (ADC)-trastuzumab deruxtecan (T-DXd) and trastuzumab emtansine-were evaluated in DSRCT PDX, cell line, and organoid models. Drug internalization was demonstrated by live microscopy. Apoptosis was evaluated by Western blotting and caspase activity assays.

RESULTS

ERBB2/HER2 was detectable in DSRCT samples from patients and PDXs, with higher sensitivity RNA assays and improved IHC detectability using clone 29D8. Treatment of ERBB2/HER2-expressing DSRCT PDX, cell line, and organoid models with T-DXd or trastuzumab emtansine resulted in tumor regression. This therapeutic response was long-lasting in T-DXd-treated xenografts and was mediated by rapid HER2 ADC complex internalization and cytotoxicity, triggering p53-mediated apoptosis and growth arrest. Xenograft regression was associated with bystander payload effects triggering global tumor niche responses proportional to HER2 status.

CONCLUSIONS

ERBB2/HER2 is a therapeutic target in DSRCT. HER2 ADCs may represent novel options for managing this exceptionally aggressive sarcoma, possibly fulfilling an urgent and historically unmet need for more effective clinical therapy.

A common cellular response to broad splicing perturbations is characterized by metabolic transcript downregulation driven by the Mdm2-p53 axis

Disruptions in core cellular processes elicit stress responses that drive cell-state changes leading to organismal phenotypes. Perturbations in the splicing machinery cause widespread mis-splicing, resulting in p53-dependent cell-state changes that give rise to cell-type-specific phenotypes and disease. However, a unified framework for how cells respond to splicing perturbations, and how this response manifests itself in nuanced disease phenotypes, has yet to be established. Here, we show that a p53-stabilizing Mdm2 alternative splicing event and the resulting widespread downregulation of metabolic transcripts are common events that arise in response to various splicing perturbations in both cellular and organismal models. Together, our results classify a common cellular response to splicing perturbations, put forth a new mechanism behind the cell-type-specific phenotypes that arise when splicing is broadly disrupted, and lend insight into the pleiotropic nature of the effects of p53 stabilization in disease.

Interruption of p53-MDM2 Interaction by Nutlin-3a in Human Lymphoma Cell Models Initiates a Cell-Dependent Global Effect on Transcriptome and Proteome Level

In most lymphomas, p53 signaling pathway is inactivated by various mechanisms independent to p53 gene mutations or deletions. In many cases, p53 function is largely regulated by alterations in the protein abundance levels by the action of E3 ubiquitin-protein ligase MDM2, targeting p53 to proteasome-mediated degradation. In the present study, an integrating transcriptomics and proteomics analysis was employed to investigate the effect of p53 activation by a small-molecule MDM2-antagonist, nutlin-3a, on three lymphoma cell models following p53 activation. Our analysis revealed a system-wide nutlin-3a-associated effect in all examined lymphoma types, identifying in total of 4037 differentially affected proteins involved in a plethora of pathways, with significant heterogeneity among lymphomas. Our findings include known p53-targets and novel p53 activation effects, involving transcription, translation, or degradation of protein components of pathways, such as a decrease in key members of PI3K/mTOR pathway, heat-shock response, and glycolysis, and an increase in key members of oxidative phoshosphorylation, autophagy and mitochondrial translation. Combined inhibition of HSP90 or PI3K/mTOR pathway with nutlin-3a-mediated p53-activation enhanced the apoptotic effects suggesting a promising strategy against human lymphomas. Integrated omic profiling after p53 activation offered novel insights on the regulatory role specific proteins and pathways may have in lymphomagenesis.

The Prognostic Significance of Spliceosomal Proteins for Patients with Glioblastoma

Glioblastoma (GBM) is considered one of the most aggressive human cancers. Earlier, our group have demonstrated that alternative RNA splicing plays an important role in the regulation of the GBM phenotype. To continue this study, we analyzed the type of RNA splicing and the expression levels of the spliceosomal genes in a large number of tumor tissue samples and patient-derived GBM sphere lines. We demonstrated that the expression level of splicing factors allows dividing GBM patients into groups with different survival prognosis and also reflects the phenotype of the tumor. In addition, we identified the alternative splicing events that may regulate the GBM phenotype. Finally, we for the first time compared the expression profiles of the spliceosomal genes in different regions of the same tumor and identified splicing factors whose expression most significantly correlates with GBM patients' survival. Aforementioned data emphasize the important role of pre-mRNA splicing in GBM progression.

U2af1 is a haplo-essential gene required for hematopoietic cancer cell survival in mice

Somatic mutations in the spliceosome gene U2AF1 are common in patients with myelodysplastic syndromes. U2AF1 mutations that code for the most common amino acid substitutions are always heterozygous, and the retained WT allele is expressed, suggesting that mutant hematopoietic cells may require the residual WT allele to be viable. We show that hematopoiesis and RNA splicing in U2af1 heterozygous knockout mice were similar to those in control mice, but that deletion of the WT allele in U2AF1(S34F) heterozygous mutant-expressing hematopoietic cells (i.e., hemizygous mutant) was lethal. These results confirm that U2AF1 mutant hematopoietic cells are dependent on the expression of WT U2AF1 for survival in vivo and that U2AF1 is a haplo-essential cancer gene. Mutant U2AF1(S34F)-expressing cells were also more sensitive to reduced expression of WT U2AF1 than nonmutant cells. Furthermore, mice transplanted with leukemia cells expressing mutant U2AF1 had significantly reduced tumor burden and improved survival after the WT U2af1 allele was deleted compared with when it was not deleted. These results suggest that selectively targeting the WT U2AF1 allele in heterozygous mutant cells could induce cancer cell death and be a therapeutic strategy for patients harboring U2AF1 mutations.

The exon-junction complex helicase eIF4A3 controls cell fate via coordinated regulation of ribosome biogenesis and translational output

Eukaryotic initiation factor 4A-III (eIF4A3), a core helicase component of the exon junction complex, is essential for splicing, mRNA trafficking, and nonsense-mediated decay processes emerging as targets in cancer therapy. Here, we unravel eIF4A3's tumor-promoting function by demonstrating its role in ribosome biogenesis (RiBi) and p53 (de)regulation. Mechanistically, eIF4A3 resides in nucleoli within the small subunit processome and regulates rRNA processing via R-loop clearance. EIF4A3 depletion induces cell cycle arrest through impaired RiBi checkpoint-mediated p53 induction and reprogrammed translation of cell cycle regulators. Multilevel omics analysis following eIF4A3 depletion pinpoints pathways of cell death regulation and translation of alternative mouse double minute homolog 2 (MDM2) transcript isoforms that control p53. EIF4A3 expression and subnuclear localization among clinical cancer specimens correlate with the RiBi status rendering eIF4A3 an exploitable vulnerability in high-RiBi tumors. We propose a concept of eIF4A3's unexpected role in RiBi, with implications for cancer pathogenesis and treatment.

Development and structural characterisation of human scFv targeting MDM2 spliced variant MDM215kDa

The murine double minute 2 (MDM2) protein is a major negative regulator of the tumour suppressor protein p53. Under normal conditions, MDM2 constantly binds to p53 transactivation domain and/or ubiquinates p53 via its role as E3 ubiquitin ligase to promote p53 degradation as well as nuclear export to maintain p53 levels in cells. Meanwhile, amplification of MDM2 and appearance of MDM2 spliced variants occur in many tumours and normal tissues making it a prognostic indicator for human cancers. The mutation or deletion of p53 protein in half of human cancers inactivates its tumour suppressor activity. However, cancers with wild type p53 have its function effectively inhibited through direct interaction with MDM2 oncoprotein. Here, we described the construction of a MDM2 spliced variant (rMDM2^15kDa) consisting of SWIB/MDM2 domain and its central region for antibody generation. Biopanning with a human naïve scFv library generated four scFv clones specific to rMDM2^15kDa. Additionally, the selected scFv clones were able to bind to the recombinant full length MDM2 (rMDM2-FL). Computational prediction showed that the selected scFv clones potentially bind to exon 7-8 of MDM2 while leaving the MDM2/SWIB domain free for p53 interaction. The developed antibodies exhibit good specificity can be further investigated for downstream biomedical and research applications.

The roles and regulation of MDM2 and MDMX: it is not just about p53

In this review, Klein et al. discuss the p53-independent roles of MDM2 and MDMX. First, they review the structural and functional features of MDM2 and MDMX proteins separately and together that could be relevant to their p53-independent activities. Following this, they summarize how these two proteins are regulated and how they can function in cells that lack p53.

Most well studied as proteins that restrain the p53 tumor suppressor protein, MDM2 and MDMX have rich lives outside of their relationship to p53. There is much to learn about how these two proteins are regulated and how they can function in cells that lack p53. Regulation of MDM2 and MDMX, which takes place at the level of transcription, post-transcription, and protein modification, can be very intricate and is context-dependent. Equally complex are the myriad roles that these two proteins play in cells that lack wild-type p53; while many of these independent outcomes are consistent with oncogenic transformation, in some settings their functions could also be tumor suppressive. Since numerous small molecules that affect MDM2 and MDMX have been developed for therapeutic outcomes, most if not all designed to prevent their restraint of p53, it will be essential to understand how these diverse molecules might affect the p53-independent activities of MDM2 and MDMX.

TSG101 Promotes the Proliferation, Migration, and Invasion of Human Glioma Cells by Regulating the AKT/GSK3β/β-Catenin and RhoC/Cofilin Pathways

The tumor susceptibility gene 101 (TSG101) has been reported to play important roles in the development and progression of several human cancers, such as pancreatic cancer, prostate cancer, and hepatocellular carcinoma. However, its potential roles and underlined mechanisms in human glioma are still needed to be further clarified. This study was designed to assess the expression of TSG101 in glioma patients and its effects on glioma cell proliferation, migration, and invasion. Publicly available data revealed that TSG101 mRNA was significantly upregulated in glioma tissues, and high levels of TSG101 were associated with poor prognosis in glioma patients. Western blot and immunohistochemistry experiments further showed that the expression level of TSG101 protein was significantly upregulated in glioma patients, especially in the patients with high-grade glioma. The functional studies showed that knockdown of TSG101 suppressed the proliferation, migration, and invasion of glioma cells, while overexpression of TSG101 facilitated them. Mechanistic studies indicated that the proliferation, migration, and invasion induced by TSG101 in human glioma were related to AKT/GSK3β/β-catenin and RhoC/Cofilin signaling pathways. In conclusion, the above results suggest that the expression of TSG101 is elevated in glioma patients, which accelerates the proliferation, migration, and invasion of glioma cells by regulating the AKT/GSK3β/β-catenin and RhoC/Cofilin pathways.

MiR-17-5p-mediated endoplasmic reticulum stress promotes acute myocardial ischemia injury through targeting Tsg101

Cardiovascular diseases are the leading cause of death globally, among which acute myocardial infarction (AMI) frequently occurs in the heart and proceeds from myocardium ischemia and endoplasmic reticulum (ER) stress-induced cell death. Numerous studies on miRNAs indicated their potential as diagnostic biomarkers and treatment targets for heart diseases. Our study investigated the role of miR-17-5p and its regulatory mechanisms during AMI. Echocardiography, MTT, flow cytometry assay, evaluation of caspase-3 and lactate dehydrogenase (LDH) activity were conducted to assess cell viability, apoptosis in an MI/R mice model, and an H2O2-induced H9c2 hypoxia cell model, respectively. The expression levels of ER stress response-related biomarkers were detected using qRT-PCR, IHC, and western blotting assays. The binding site of miR-17-5p on Tsg101 mRNA was determined by bioinformatic prediction and luciferase reporter assay. The expression levels of miR-17-5p were notably elevated in MI/R mice and hypoxia cell models, accompanied by enhanced cell apoptosis. Inhibition of miR-17-5p led to decreased apoptosis related to ER stress response in the hypoxia model, which could be counteracted by knockdown of Tsg101 (tumor susceptibility gene 101). Transfection with miR-17-5p mimics downregulated the expression of Tsg101 in H9c2 cells. Luciferase assay demonstrated the binding between miR-17-5p and Tsg101. Moreover, 4-PBA, the inhibitor of the ER stress response, abolished shTsg101 elevated apoptosis in hypoxic H9c2 cells. Our findings investigated the pro-apoptotic role of miR-17-5p during MI/R, disclosed the specific mechanism of miR-17-5p/Tsg101 regulatory axis in ER stress-induced myocardium injury and cardiomyocytes apoptosis, and presented a promising diagnostic biomarker and potential target for therapy of AMI.

Cellular Prion Protein (PrPc): Putative Interacting Partners and Consequences of the Interaction

Cellular prion protein (PrPc) is a small glycosylphosphatidylinositol (GPI) anchored protein most abundantly found in the outer leaflet of the plasma membrane (PM) in the central nervous system (CNS). PrPc misfolding causes neurodegenerative prion diseases in the CNS. PrPc interacts with a wide range of protein partners because of the intrinsically disordered nature of the protein’s N-terminus. Numerous studies have attempted to decipher the physiological role of the prion protein by searching for proteins which interact with PrPc. Biochemical characteristics and biological functions both appear to be affected by interacting protein partners. The key challenge in identifying a potential interacting partner is to demonstrate that binding to a specific ligand is necessary for cellular physiological function or malfunction. In this review, we have summarized the intracellular and extracellular interacting partners of PrPc and potential consequences of their binding. We also briefly describe prion disease-related mutations at the end of this review.

Expression of the Long Noncoding RNA DINO in Human Papillomavirus-Positive Cervical Cancer Cells Reactivates the Dormant TP53 Tumor Suppressor through ATM/CHK2 Signaling

Functional restoration of the TP53 tumor suppressor holds great promise for anticancer therapy. Current strategies are focused on modulating TP53 regulatory proteins. Long noncoding RNAs (lncRNAs) have emerged as important regulators of TP53 as well as modulators of downstream tumor-suppressive transcriptional responses. Unlike many other cancer types, human papillomavirus (HPV)-positive cancer cells retain wild-type TP53 that is rendered dysfunctional by the viral E6 protein. We show that acute expression of the damage-induced long noncoding RNA, DINO, a known TP53 transcriptional target and functional modulator, causes TP53 reactivation in HPV-positive cervical cancer cells. This causes increased vulnerability to standard chemotherapeutics as well as biguanide compounds that cause metabolic stress. Hence, strategies that target DINO may be useful for restoring TP53 tumor suppressor activity in HPV-positive cancers and other tumor types that retain wild-type TP53.

Tumor cells overcome the cytostatic and cytotoxic restraints of TP53 tumor suppressor signaling through a variety of mechanisms. High-risk human papillomavirus (HPV)-positive tumor cells retain wild-type TP53 because the HPV E6/UBE3A ubiquitin ligase complex targets TP53 for proteasomal degradation. While restoration of TP53 in tumor cells holds great promise for cancer therapy, attempts to functionally restore the dormant TP53 tumor suppressor in HPV-positive cancer cells by inhibiting the HPV E6/UBE3A ubiquitin ligase complex have not yet been successful. The damage-induced long noncoding RNA, DINO (DINOL), is a TP53 transcriptional target that has been reported to bind to and stabilize TP53, thereby amplifying TP53 signaling. We show that HPV-positive cervical carcinoma cells contain low levels of DINO because of HPV E6/UBE3A-mediated TP53 degradation. Acute DINO expression overrides HPV16 E6/UBE3A-mediated TP53 degradation, causing TP53 stabilization and increased expression of TP53 transcriptional target genes. This causes a marked sensitization to chemotherapy agents and renders cells vulnerable to metabolic stress. Acute DINO expression in HPV-positive cervical cancer cells induces hallmarks of DNA damage response signaling, and TP53 activation involves ATM/CHK2 signaling. DINO upregulation in response to DNA damage is independent of ATM/CHK2 and can occur in cancer cells that express mutant TP53.

siRNA against TSG101 reduces proliferation and induces G0/G1 arrest in renal cell carcinoma - involvement of c-myc, cyclin E1, and CDK2

Objective

The tumor susceptibility gene 101 (TSG101) is closely associated with various tumor types, but its role in the pathogenesis of renal cell carcinoma (RCC) is still unknown. This study used RNA interference to silence the expression of TSG101 in RCC cell lines and explore the role of TSG101 in RCC.

Methods

Immunohistochemistry and western blot were performed to detect the expression of TSG101 in 15 paired renal tumor samples. A small interfering RNA (siRNA) targeting TSG101 was transfected into A498 and 786-O cell lines. The Cell Counting Kit-8 (CCK-8) assay and colony formation assay were used to observe the changes in cell proliferation after transfection. Flow cytometry was used to detect the effect on the cell cycle. Western blot was conducted to study the changes of related functional proteins.

Results

The expression of TSG101 was higher in RCC tissues than in adjacent normal tissues. The CCK-8 assay showed that the proliferation and colony formation of the A498 and 786-O cell lines were attenuated after suppression of TSG101. Flow cytometry showed that silencing of TSG101 induced G0/G1 arrest. The western blot results revealed that the levels of cell cycle-related proteins (c-myc, cyclin E1 and cyclin-dependent kinase 2 (CDK2)) were markedly decreased in the siRNA groups.

Conclusions

TSG101 promotes proliferation of RCC cells. This positive effect on tumor growth involves activation of c-myc and cyclin E1/CDK2 and their effect on cell cycle distribution.

TSG101 promotes the proliferation, migration and invasion of hepatocellular carcinoma cells by regulating the PEG10

The tumour susceptibility gene 101 (TSG101) is reported to play important roles in the development and progression of several human cancers. However, its potential roles and underlined mechanisms in human hepatocellular carcinoma (HCC) are still needed to be further clarified. In the present study, we reported that knock down of TSG101 suppressed the proliferation, migration and invasion of HCC cells, while overexpression of TSG101 facilitated them. Molecularly, the results revealed that knock down of TSG101 significantly decreased the cell cycle related regulatory factor p53 and p21. In another point, knock down of TSG101 also obviously decreased the level of metallopeptidase inhibitor TIMP1 (Tissue inhibitors of metalloproteinases 1), which results in inhibition of MMP2, MMP7 and MMP9. In contrast, overexpression of TSG101 had opposite effects. The iTRAQ proteomics analysis identified that oncogenic protein PEG10 (Paternally expressed gene 10) might be a potential downstream target of TSG101. Further investigation showed that TSG101 interacted with PEG10 and protected it from proteasomal degradation thereby regulating the expression of p53, p21 and MMPs. Finally, we found that both TSG101 and PEG10 proteins are up-regulated and presented a direct correlation in HCC patients. In conclusion, these results suggest that TSG101 is up-regulated in human HCC patients, which may accelerate the proliferation, migration and invasion of HCC cells through regulating PEG10.

BAI1 Suppresses Medulloblastoma Formation by Protecting p53 from Mdm2-Mediated Degradation

Adhesion G protein-coupled receptors (ADGRs) encompass 33 human transmembrane proteins with long N termini involved in cell-cell and cell-matrix interactions. We show the ADGRB1 gene, which encodes Brain-specific angiogenesis inhibitor 1 (BAI1), is epigenetically silenced in medulloblastomas (MBs) through a methyl-CpG binding protein MBD2-dependent mechanism. Knockout of Adgrb1 in mice augments proliferation of cerebellar granule neuron precursors, and leads to accelerated tumor growth in the Ptch1+/- transgenic MB mouse model. BAI1 prevents Mdm2-mediated p53 polyubiquitination, and its loss substantially reduces p53 levels. Reactivation of BAI1/p53 signaling axis by a brain-permeable MBD2 pathway inhibitor suppresses MB growth in vivo. Altogether, our data define BAI1's physiological role in tumorigenesis and directly couple an ADGR to cancer formation.

DNA damage response signaling does not trigger redistribution of SAMHD1 to nuclear foci

SAMHD1 (Sterile alpha motif and histidine-aspartic acid (HD) domain containing protein 1) is a deoxyribonucleoside triphosphate (dNTP) triphosphohydrolase (dNTPase) that restricts viral replication in infected cells. This protein is also involved in DNA repair by assisting in DNA end resection by homologous recombination (HR) after DNA double-strand break (DSB) induction with camptothecin (CPT) or etoposide (ETO). We showed that a monoclonal anti-SAMHD1 antibody produced against the full-length protein detected an unspecific 50 kDa protein that colocalized with dot-like structures after CPT treatment in HeLa cells. In contrast, a polyclonal anti-SAMHD1 antibody raised against the N-terminus of this protein specifically detected SAMHD1, as shown in Jurkat, HAP1^KO and HEK293T SAMHD1-siRNA cell lysates compared with their respective controls. Our findings showed that SAMHD1 is not localized in dot-like structures under DSB induction in HeLa cells.

Tumor susceptibility gene 101 regulates predisposition to apoptosis via ESCRT machinery accessory proteins

ESCRT proteins are implicated in myriad cellular processes, including endosome formation, fusion of autophagosomes/amphisomes with lysosomes, and apoptosis. The role played by these proteins in either facilitating or protecting against apoptosis is unclear. In this study, while trying to understand how deficiency of Mahogunin RING finger 1 (MGRN1) affects cell viability, we uncovered a novel role for its interactor, the ESCRT-I protein TSG101: it directly participates in mitigating ER stress-mediated apoptosis. The association of TSG101 with ALIX prevents predisposition to apoptosis, whereas ALIX-ALG-2 interaction favors a death phenotype. Altered Ca2+ homeostasis in cells and a simultaneous increase in the protein levels of ALIX and ALG-2 are required to elicit apoptosis by activating ER stress-associated caspase 4/12. We further demonstrate that in the presence of membrane-associated, disease-causing prion protein CtmPrP, increased ALIX and ALG-2 levels are detected along with ER stress markers and associated caspases in transgenic brain lysates and cells. These effects were rescued by overexpression of TSG101. This is significant because MGRN1 deficiency is closely associated with neurodegeneration and prenatal and neonatal mortality, which could be due to excess cell death in selected brain regions or myocardial apoptosis during embryonic development.

HIV-1 Tat potently stabilises Mdm2 and enhances viral replication

Murine double minute 2 (Mdm2) is known to enhance the transactivation potential of human immunodeficiency virus (HIV-1) Tat protein by causing its ubiquitination. However, the regulation of Mdm2 during HIV-1 infection and its implications for viral replication have not been well studied. Here, we show that the Mdm2 protein level increases during HIV-1 infection and this effect is mediated by HIV-1 Tat protein. Tat appears to stabilise Mdm2 at the post-translational level by inducing its phosphorylation at serine-166 position through AKT. Although p53 is one of the key players for Mdm2 induction, Tat-mediated stabilisation of Mdm2 appears to be independent of p53. Moreover, the non-phosphorylatable mutant of Mdm2 (S166A) fails to interact with Tat and shows decreased half-life in the presence of Tat compared with wild-type Mdm2. Furthermore, the non-phosphorylatable mutant of Mdm2 (S166A) is unable to support HIV-1 replication. Thus, HIV-1 Tat appears to stabilise Mdm2, which in turn enhances Tat-mediated viral replication. This study highlights the importance of post-translational modifications of host cellular factors in HIV-1 replication and pathogenesis.

E3 Ubiquitin Ligases Neurobiological Mechanisms: Development to Degeneration

Cells regularly synthesize new proteins to replace old or damaged proteins. Deposition of various aberrant proteins in specific brain regions leads to neurodegeneration and aging. The cellular protein quality control system develop various defense mechanisms against the accumulation of misfolded and aggregated proteins. The mechanisms underlying the selective recognition of specific crucial protein or misfolded proteins are majorly governed by quality control E3 ubiquitin ligases mediated through ubiquitin-proteasome system. Few known E3 ubiquitin ligases have shown prominent neurodevelopmental functions, but their interactions with different developmental proteins play critical roles in neurodevelopmental disorders. Several questions are yet to be understood properly. How E3 ubiquitin ligases determine the specificity and regulate degradation of a particular substrate involved in neuronal proliferation and differentiation is certainly the one, which needs detailed investigations. Another important question is how neurodevelopmental E3 ubiquitin ligases specifically differentiate between their versatile range of substrates and timing of their functional modulations during different phases of development. The premise of this article is to understand how few E3 ubiquitin ligases sense major molecular events, which are crucial for human brain development from its early embryonic stages to throughout adolescence period. A better understanding of these few E3 ubiquitin ligases and their interactions with other potential proteins will provide invaluable insight into disease mechanisms to approach toward therapeutic interventions.

Opposite regulation of MDM2 and MDMX expression in acquisition of mesenchymal phenotype in benign and cancer cells

Plasticity of cancer cells, manifested by transitions between epithelial and mesenchymal phenotypes, represents a challenging issue in the treatment of neoplasias. Both epithelial-mesenchymal transition (EMT) and mesenchymal-epithelial transition (MET) are implicated in the processes of metastasis formation and acquisition of stem cell-like properties. Mouse double minute (MDM) 2 and MDMX are important players in cancer progression, as they act as regulators of p53, but their function in EMT and metastasis may be contradictory. Here, we show that the EMT phenotype in multiple cellular models and in clinical prostate and breast cancer samples is associated with a decrease in MDM2 and increase in MDMX expression. Modulation of EMT-accompanying changes in MDM2 expression in benign and transformed prostate epithelial cells influences their migration capacity and sensitivity to docetaxel. Analysis of putative mechanisms of MDM2 expression control demonstrates that in the context of defective p53 function, MDM2 expression is regulated by EMT-inducing transcription factors Slug and Twist. These results provide an alternative context-specific role of MDM2 in EMT, cell migration, metastasis, and therapy resistance.

Differential targeting of the cyclin-dependent kinase inhibitor, p21CIP1/WAF1, by chelators with anti-proliferative activity in a range of tumor cell-types

Chelators such as 2-hydroxy-1-napthylaldehyde isonicotinoyl hydrazone (311) and di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT) target tumor cell iron pools and inhibit proliferation. These agents also modulate multiple targets, one of which is the cyclin-dependent kinase inhibitor, p21. Hence, this investigation examined the mechanism of action of these compounds in targeting p21. All the chelators up-regulated p21 mRNA in the five tumor cell-types assessed. In contrast, examining their effect on total p21 protein levels, these agents induced either: (1) down-regulation in MCF-7 cells; (2) up-regulation in SK-MEL-28 and CFPAC-1 cells; or (3) had no effect in LNCaP and SK-N-MC cells. The nuclear localization of p21 was also differentially affected by the ligands depending upon the cell-type, with it being decreased in MCF-7 cells, but increased in SK-MEL-28 and CFPAC-1 cells. Further studies assessing the mechanisms responsible for these effects demonstrated that p21 expression was not correlated with p53 status, suggesting a p53-independent mechanism. Considering this, we examined proteins that modulate p21 independently of p53, namely NDRG1, MDM2 and ΔNp63. These studies demonstrated that a dominant negative MDM2 isoform (p75(MDM2)) closely resembled p21 expression in response to chelation in three cell lines. These data suggest MDM2 may be involved in the regulation of p21 by chelators.

Mahogunin regulates fusion between amphisomes/MVBs and lysosomes via ubiquitination of TSG101

Aberrant metabolic forms of the prion protein (PrP), membrane-associated (Ctm)PrP and cytosolic (cyPrP) interact with the cytosolic ubiquitin E3 ligase, Mahogunin Ring Finger-1 (MGRN1) and affect lysosomes. MGRN1 also interacts with and ubiquitinates TSG101, an ESCRT-I protein, involved in endocytosis. We report that MGRN1 modulates macroautophagy. In cultured cells, functional depletion of MGRN1 or overexpression of (Ctm)PrP and cyPrP blocks autophagosome-lysosome fusion, alleviates the autophagic flux and its degradative competence. Concurrently, the degradation of cargo from the endo-lysosomal pathway is also affected. This is significant because catalytic inactivation of MGRN1 alleviates fusion of lysosomes with either autophagosomes (via amphisomes) or late endosomes (either direct or mediated through amphisomes), without drastically perturbing maturation of late endosomes, generation of amphisomes or lysosomal proteolytic activity. The compromised lysosomal fusion events are rescued by overexpression of TSG101 and/or its monoubiquitination in the presence of MGRN1. Thus, for the first time we elucidate that MGRN1 simultaneously modulates both autophagy and heterophagy via ubiquitin-mediated post-translational modification of TSG101.

Mahogunin Ring Finger-1 (MGRN1), a Multifaceted Ubiquitin Ligase: Recent Unraveling of Neurobiological Mechanisms

In healthy cell, inappropriate accumulation of poor or damaged proteins is prevented by cellular quality control system. Autophagy and ubiquitin proteasome system (UPS) provides regular cytoprotection against proteotoxicity induced by abnormal or disruptive proteins. E3 ubiquitin ligases are crucial components in this defense mechanism. Mahogunin Ring Finger-1 (MGRN1), an E3 ubiquitin ligase of the Really Interesting New Gene (RING) finger family, plays a pivotal role in many biological and cellular mechanisms. Previous findings indicate that lack of functions of MGRN1 can cause spongiform neurodegeneration, congenital heart defects, abnormal left-right patterning, and mitochondrial dysfunctions in mice brains. However, the detailed molecular pathomechanism of MGRN1 in cellular functions and diseases is not well known. This article comprehensively represents the molecular nature, characterization, and functions of MGRN1; we also summarize possible beneficiary aspects of this novel E3 ubiquitin ligase. Here, we review recent literature on the role of MGRN1 in the neuro-pathobiological mechanisms, with precise focus on the processes of neurodegeneration, and thereby propose new lines of potential targets for therapeutic intervention.

Transcription factors that interact with p53 and Mdm2

The tumor suppressor p53 is activated upon cellular stresses such as DNA damage, oncogene activation, hypoxia, which transactivates sets of genes that induce DNA repair, cell cycle arrest, apoptosis, or autophagy, playing crucial roles in the prevention of tumor formation. The central regulator of the p53 pathway is Mdm2 which inhibits transcriptional activity, nuclear localization and protein stability. More than 30 cellular p53-binding proteins have been isolated and characterized including Mdm2, Mdm4, p300, BRCA1/2, ATM, ABL and 53BP-1/2. Most of them are nuclear proteins; however, not much is known about p53-binding transcription factors. In this review, we focus on transcription factors that directly interact with p53/Mdm2 through direct binding including Dmp1, E2F1, YB-1 and YY1. Dmp1 and YB-1 bind only to p53 while E2F1 and YY1 bind to both p53 and Mdm2. Dmp1 has been shown to bind to p53 and block all the known functions for Mdm2 on p53 inhibition, providing a secondary mechanism for tumor suppression in Arf-null cells. Although E2F1-p53 binding provides a checkpoint mechanism to silence hyperactive E2F1, YB-1 or YY1 interaction with p53 subverts the activity of p53, contributing to cell cycle progression and tumorigenesis. Thus, the modes and consequences for each protein-protein interaction vary from the viewpoint of tumor development and suppression.

Role of the MDM2 promoter polymorphism (-309T>G) in acute myeloid leukemia development

BACKGROUND

The human homologue of the mouse double minute 2 (MDM2) gene is a negative regulator of Tp53. MDM2-309T>G a functional promoter polymorphism was found to be associated with overexpression thereby attenuation of Tp53 stress response and increased cancer susceptibility. We have planned to evaluate the possible role of MDM2-309T>G polymorphism with risk and response to chemotherapy in AML.

MATERIALS AND METHODS

A total of 223 de novo AML cases and 304 age and sex matched healthy controls were genotyped for the MDM2-309T>G polymorphism through the tetra-primer amplification refractory mutation system (ARMS)-PCR method. In order to assess the functional relationship of -309T>G SNP with MDM2 expression level, we quantified MDM2 mRNA in 30 primary AML blood samples through quantitative RT-PCR. Both the (-309T>G) genotypes and the MDM2 expression were correlated with disease free survival (DFS) rates among patients who have achieved complete remission (CR) after first induction chemotherapy.

RESULTS

MDM2-309T>G polymorphism was significantly associated with AML development (p<0.0001). The presence of either GG genotype or G allele at MDM2-309 confered 1.79 (95% CI: 1.12-2.86; p<0.001) and 1.46 fold (95%CI: 1.14-1.86; p=0.003) increased AML risk. Survival analysis revealed that CR+ve cases with GG genotype had significantly increased DFS rates (16months, p=0.05) compared to CR+ve TT (11 months) and TG (9 months) genotype groups. Further, MDM2 expression was also found to be significantly elevated in GG genotype patients (p=0.0039) and among CR+ve cases (p=0.0036).

CONCLUSIONS

The MDM2-309T>G polymorphism might be involved in AML development and also serve as a good prognostic indicator.

AtMBP-1, an alternative translation product of LOS2, affects abscisic acid responses and is modulated by the E3 ubiquitin ligase AtSAP5

The LOS2 gene in Arabidopsis encodes an enolase with 72% amino acid sequence identity with human ENO1. In mammalian cells, the α-enolase (ENO1) gene encodes both a 48 kDa glycolytic enzyme and a 37 kDa transcriptional suppressor protein that are targeted to different cellular compartments. The tumor suppressor c-myc binding protein (MBP-1), which is alternatively translated from the second start codon of ENO1 transcripts, is preferentially localized in nuclei while α-enolase is found in the cytoplasm. We report here that an Arabidopsis MBP-1-like protein (AtMBP-1) is alternatively translated from full-length LOS2 transcripts using a second start codon. Like mammalian MBP-1, this truncated form of LOS2 has little, if any, enolase activity, indicating that an intact N-terminal region of LOS2 is critical for catalysis. AtMBP-1 has a short half-life in vivo and is stabilized by the proteasome inhibitor MG132, indicating that it is degraded via the ubiquitin-dependent proteasome pathway. Arabidopsis plants that over-express AtMBP-1 are hypersensitive to abscisic acid (ABA) during seed germination and show defects in vegetative growth and lateral stem development. AtMBP-1 interacts directly with the E3 ubiquitin ligase AtSAP5 and co-expression of these proteins resulted in destabilization of AtMBP-1 in vivo and abolished the developmental defects associated with AtMBP-1 over-expression. Thus, AtMBP-1 is as a bona fide alternative translation product of LOS2. Accumulation of this factor is limited by ubiquitin-dependent destabilization, apparently mediated by AtSAP5.

Anticarcinogenic effects of the ethanolic extract of Salix aegyptiaca in colon cancer cells: involvement of Akt/PKB and MAPK pathways

The bark from Salix species of plants has been traditionally consumed for its antiinflammatory properties. Because inflammation frequently accompanies the progress of colorectal cancer (CRC), we have evaluated the anticancer properties of the ethanolic extract from the bark (EEB) of S. aegyptiaca, a Salix species endogenous to the Middle East, using HCT-116 and HT29 CRC cell lines. Fresh bark from S. aegyptiaca was extracted with ethanol, fractionated by solvent-solvent partitioning and the fractions were analyzed by tandem mass spectrometry. Catechin, catechol, and salicin were the most abundant constituents of the extract. Interestingly, EEB showed the highest anticancer effect in the colon cancer cells followed by its fractions in ethyl acetate and water, with catechin, catechol, and salicin showing the least efficacy. EEB could strongly reduce the proliferation of the cancer cells, but not of CCD-18Co, normal colon fibroblast cell line. Accompanying this was cell cycle arrest at G1/S independent of DNA damage in the cancer cells, induction of apoptosis through a p53 dependent pathway and an inhibition of PI3K/Akt and MAP Kinase pathways at levels comparable to known commercial inhibitors. We propose that the combination of the polyphenols and flavonoids in EEB contributes toward its potent anticarcinogenic effects. [Supplementary materials are available for this article. Go to the publisher's online edition of Nutrition and Cancer for the following free supplemental resource(s): Supplementary Figure 1 and Supplementary Figure 2.].

MDM2's social network

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

A 40-bp insertion/deletion polymorphism in the constitutive promoter of MDM2 confers risk for hepatocellular carcinoma in a Chinese population

The pathogenesis of HCC is a multistage process with the involvement of genetic factors. The aim of the present study is to investigate the possible association between a 40-bp insertion/deletion polymorphism (indel) at constitutive promoter of MDM2 and risk of hepatocellular carcinoma (HCC) in a Chinese population. Using 420 HCC patients and 423 control subjects, we genotyped the indel polymorphism (rs3730485) using polymerase chain reaction method. Logistic regression was used to analyze the association between the polymorphism and HCC susceptibility. Under co-dominant model, we found that the ins/del and del/del genotype of indel was associated with a significantly increased risk of HCC compared with its homozygote ins/ins (OR=1.39, 95%C.I.=1.03-1.87; OR=1.68, 95%C.I.=1.03-2.73, respectively). Presence of 40-bp deletion allele of MDM2 seemed to confer higher risk for HCC when compared with non-carriers (OR=1.30, 95%C.I.=1.06-1.60, P=0.011). Further stratification analysis showed that this association was more pronounced in patients with a family history of HCC, early tumor stage and higher serum alpha-fetoprotein (AFP). These findings indicated that the MDM2 indel polymorphism may be a genetic modifier for developing HCC in Chinese population.

Altered expression of Ano1 variants in human diabetic gastroparesis

Diabetes affects many organs including the stomach. Altered number and function of interstitial cells of Cajal (ICC), the gastrointestinal pacemaker cells, underlie a number of gastrointestinal motility disorders, including diabetic gastroparesis. In the muscle layers, ICC selectively express Ano1, thought to underlie classical Ca(2+)-activated Cl(-) currents. Mice homozygous for Ano1 knock-out exhibit abnormal ICC function and motility. Several transcripts for Ano1 are generated by alternative splicing of four exons. Here, we report expression levels of transcripts encoded by alternative splicing of Ano1 gene in gastric muscles of patients with diabetic gastroparesis and nondiabetic control tissues. Expression of mRNA from two alternatively transcribed exons are significantly different between patients and controls. Furthermore, patients with diabetic gastroparesis express mRNA for a previously unknown variant of Ano1. The 5' end of this novel variant lacks exons 1 and 2 and part of exon 3. Expression of this variant in HEK cells produces a decreased density of Ca(2+)-activated Cl(-) currents that exhibit slower kinetics compared with the full-length Ano1. These results identify important changes in expression and splicing of Ano1 in patients with diabetic gastroparesis that alter the electrophysiological properties of the channel. Changes in Ano1 expression in ICC may directly contribute to diabetic gastroparesis.

Functional inactivation of endogenous MDM2 and CHIP by HSP90 causes aberrant stabilization of mutant p53 in human cancer cells

The tight control of wild-type p53 by mainly MDM2 in normal cells is permanently lost in tumors harboring mutant p53, which exhibit dramatic constitutive p53 hyperstabilization that far exceeds that of wild-type p53 tumors. Importantly, mutant p53 hyperstabilization is critical for oncogenic gain of function of mutant p53 in vivo. Current insight into the mechanism of this dysregulation is fragmentary and largely derived from ectopically constructed cell systems. Importantly, mutant p53 knock-in mice established that normal mutant p53 tissues have sufficient enzymatic reserves in MDM2 and other E3 ligases to maintain full control of mutant p53. We find that in human cancer cells, endogenous mutant p53, despite its ability to interact with MDM2, suffers from a profound lack of ubiquitination as the root of its degradation defect. In contrast to wild-type p53, the many mutant p53 proteins which are conformationally aberrant are engaged in complexes with the HSP90 chaperone machinery to prevent its aggregation. In contrast to wild-type p53 cancer cells, we show that in mutant p53 cancer cells, this HSP90 interaction blocks the endogenous MDM2 and CHIP (carboxy-terminus of Hsp70-interacting protein) E3 ligase activity. Interference with HSP90 either by RNA interference against HSF1, the transcriptional regulator of the HSP90 pathway, or by direct knockdown of Hsp90 protein or by pharmacologic inhibition of Hsp90 activity with 17AAG (17-allylamino-17-demethoxygeldanamycin) destroys the complex, liberates mutant p53, and reactivates endogenous MDM2 and CHIP to degrade mutant p53. Of note, 17AAG induces a stronger viability loss in mutant p53 than in wild-type p53 cancer cells. Our data support the rationale that suppression of mutant p53 levels in vivo in established cancers might achieve clinically significant effects.

Transcription factor Nrf2 maintains the basal expression of Mdm2: An implication of the regulation of p53 signaling by Nrf2

Co-operated regulation of oxidative stress-response transcription factors would be an important issue for animals to determine the cell fate under environmental stress. This notion raises a possibility that NF-E2-related factor 2 (Nrf2), which confers cytoprotection against oxidative stress, and p53 can have a direct co-regulation network. In the current study, we have indentified that the expression of murine double minute 2 (Mdm2) is repressed in nrf2-deleted murine embryonic fibroblasts (MEFs). This was confirmed by microarray, RT-PCR, and immunoblot analyses, and further promoter analysis showed that Nrf2 is directly involved in the basal expression of Mdm2 through the antioxidant response element, which is located in the first intron of this gene. This linkage between Nrf2 and Mdm2 appears to cause the accumulation of p53 protein in nrf2-deficent MEFs. In addition, we show that ovarian carcinoma A2780 cells with Nrf2 shRNA expression displayed higher levels of p53 activation in response to hydrogen peroxide treatment, leading to increased cell death. Collectively, our results suggest novel evidence that the inhibition of Nrf2 can suppress Mdm2 expression, which may result in p53 signaling modulation. In addition, this observation supports the concept that Nrf2 inhibition in cancer cells can facilitate apoptotic response upon environmental stress.

The translational response of the human mdm2 gene in HEK293T cells exposed to rapamycin: a role for the 5'-UTRs

Polysomal messenger RNA (mRNA) populations change rapidly in response to alterations in the physiological status of the cell. For this reason, translational regulation, mediated principally at the level of initiation, plays a key role in the maintenance of cellular homeostasis. In an earlier translational profiling study, we followed the impact of rapamycin on polysome re-seeding. Despite the overall negative effect on transcript recruitment, we nonetheless observed that some mRNAs were significantly less affected. Consequently, their relative polysomal occupancy increased in the rapamycin-treated cells. The behaviour of one of these genes, mdm2, has been further analysed. Despite the absence of internal ribosome entry site activity we demonstrate, using a dual reporter assay, that both the reported mdm2 5′-UTRs confer resistance to rapamycin relative to the 5′-UTR of β-actin. This relative resistance is responsive to the downstream targets mTORC1 but did not respond to changes in the La protein, a reported factor acting positively on MDM2 translational expression. Furthermore, extended exposure to rapamycin in the presence of serum increased the steady-state level of the endogenous MDM2 protein. However, this response was effectively reversed when serum levels were reduced. Taken globally, these studies suggest that experimental conditions can dramatically modulate the expressional output during rapamycin exposure.

The Mdm2-p53 relationship evolves: Mdm2 swings both ways as an oncogene and a tumor suppressor

Mdm2 has been well characterized as a negative regulator of the tumor suppressor p53. Recent studies have shown that Mdm2 is activated in response to a variety of oncogenic pathways independent of p53. Although its role as an oncogene via suppression of p53 function remains clear, growing evidence argues for p53-independent effects, as well as the remarkable possibility that Mdm2 has tumor suppressor functions in the appropriate context. Hence, Mdm2 is proving to be a key player in human cancer in its own right, and thus an important target for therapeutic intervention.

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.

ESCRT & Co

Components of the ESCRT (endosomal sorting complex required for transport) machinery mediate endosomal sorting of ubiquitinated membrane proteins. They are key regulators of biological processes important for cell growth and survival, such as growth-factor-mediated signalling and cytokinesis. In addition, enveloped viruses, such as HIV-1, hijack and utilize the ESCRTs for budding during virus release and infection. Obviously, the ESCRT-facilitated pathways require tight regulation, which is partly mediated by a group of interacting proteins, for which our knowledge is growing. In this review we discuss the different ESCRT-modulating proteins and how they influence ESCRT-dependent processes, for example, by acting as positive or negative regulators or by providing temporal and spatial control. A number of the interactors influence the classical ESCRT-mediated process of endosomal cargo sorting, for example, by modulating the interaction between ubiquitinated cargo and the ESCRTs. Certain accessory proteins have been implicated in regulating the activity or steady-state expression levels of the ESCRT components, whereas other interactors control the cellular localization of the ESCRTs, for example, by inducing shuttling between cytosol and nucleus or endosomes. In conclusion, the discovery of novel interactors has and will extend our knowledge of the biological roles of ESCRTs.

Abnormal regulation of TSG101 in mice with spongiform neurodegeneration

Spongiform neurodegeneration is characterized by the appearance of vacuoles throughout the central nervous system. It has many potential causes, but the underlying cellular mechanisms are not well understood. Mice lacking the E3 ubiquitin ligase Mahogunin Ring Finger-1 (MGRN1) develop age-dependent spongiform encephalopathy. We identified an interaction between a "PSAP" motif in MGRN1 and the ubiquitin E2 variant (UEV) domain of TSG101, a component of the endosomal sorting complex required for transport I (ESCRT-I), and demonstrate that MGRN1 multimonoubiquitinates TSG101. We examined the in vivo consequences of loss of MGRN1 on TSG101 expression and function in the mouse brain. The pattern of TSG101 ubiquitination differed in the brains of wild-type mice and Mgrn1 null mutant mice: at 1 month of age, null mutant mice had less ubiquitinated TSG101, while in adults, mutant mice had more ubiquitinated, insoluble TSG101 than wild-type mice. There was an associated increase in epidermal growth factor receptor (EGFR) levels in mutant brains. These results suggest that loss of MGRN1 promotes ubiquitination of TSG101 by other E3s and may prevent its disassociation from endosomal membranes or cause it to form insoluble aggregates. Our data implicate loss of normal TSG101 function in endo-lysosomal trafficking in the pathogenesis of spongiform neurodegeneration in Mgrn1 null mutant mice.

Recent advances in validating MDM2 as a cancer target

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

TSG101, identified by screening a cancer cDNA library and soft agar assay, promotes cell proliferation in human lung cancer

Understanding the genesis and development of tumors is an essential component in cancer research. It is of interest to discover unknown genes that are responsible for cellular transformation. A cDNA library of a highly metastatic lung adenocarcinoma cell line was constructed. This library was introduced into the NIH3T3 mouse embryonic fibroblast cell line to screen for cDNAs that increase anchorage-independent colony formation in soft agar. The expression of TSG101 in lung cancer cell lines and specimens was confirmed using reverse transcription-polymerase chain reaction. The level of TSG101 protein in transfected A549 cells was determined by western blotting. Cell-cycle distribution was analyzed using a FACStar Plus flow cytometer. One of the candidate cDNAs that increases anchorage-independent colony formation was shown to correspond to the TSG101 cDNA sequence. Levels of TSG101 mRNA were higher in lung cancer cell lines and specimens compared to matched normal lung tissues. Ectopic expression of TSG101 in the A549 lung adenocarcinoma cell line increased the numbers of cells in S phase, suggesting an increased cell proliferation rate. These results indicate that TSG101 may induce the malignant phenotype of cells.

Rapamycin increases the p53/MDM2 protein ratio and p53-dependent apoptosis by translational inhibition of mdm2 in cancer cells

Rapamycin, a potential anti-cancer agent, modulates activity of various factors functioning in translation, including eIF4E, an initiation factor selectively regulating expression of a subset of cellular transcripts. We show here that rapamycin suppresses levels of the p53-regulator MDM2 by translational inhibition without affecting mdm2 mRNA expression or protein stability. Rapamycin inhibits translation of mdm2 mRNA from the constitutive P1 promoter, which contains two upstream ORFs (uORFs) in the 5'UTR. Suppression is accompanied by increased hypo-phosphorylation of 4EBP-1, an inhibitory eIF4E binding protein. Ectopic expression of eIF4E abrogates rapamycin-mediated MDM2 inhibition, suggesting that eIF4E is crucial in modulating MDM2 expression in rapamycin-treated cells. Rapamycin administration also results in elevated PUMA expression and PARP cleavage, which is reproduced by siRNA knockdown of eIF4E or MDM2, suggesting that MDM2 suppression by rapamycin stimulates p53-mediated apoptosis. Together, our results define translational regulation of MDM2 expression by eIF4E and provide a molecular mechanism underlying rapamycin-induced p53-dependent apoptosis.

Identification and characterization of two novel isoforms of Pirh2 ubiquitin ligase that negatively regulate p53 independent of RING finger domains

Pirh2 is a newly identified E3 ubiquitin ligase known to inhibit tumor suppressor p53 function via ubiquitination and proteasomal degradation. We have identified two novel Pirh2 splice variants that encode different Pirh2 isoforms and named these Pirh2B and Pirh2C. Accordingly, the full-length protein is now classified as isoform Pirh2A. The central region of Pirh2 harbors a RING finger domain that is critical for its ubiquitin ligase function. The Pirh2B isoform lacks amino acids 171-179, whereas Pirh2C is missing C-terminal amino acids 180-261, which for each isoform results in a RING domain deletion and the abrogation of ubiquitin ligase activity. Our findings further indicate that the Pirh2B isoform but not the Pirh2C isoform is capable of binding to Pirh2A, suggesting that the C-terminal region absent in Pirh2C is critical for Pirh2-Pirh2 interactions. Similar to Pirh2A, both Pirh2B and Pirh2C interact with p53; however, interactions between p53 and Pirh2B appear stronger than those between p53 and Pirh2C. Interestingly, although both Pirh2B and Pirh2C are not able to promote in vitro p53 ubiquitination, both are capable of negatively regulating p53 protein stability and promoting the intracellular ubiquitination of p53. Furthermore, like Pirh2A, both isoforms are able to inhibit p53 transcriptional activity. We have also for the first time demonstrated that Pirh2A as well as the novel isoforms also interact directly with MDM2 within a region encompassing MDM2 acidic and zinc finger domains. It is therefore possible that Pirh2A and the novel Pirh2 isoforms identified in this study may also modulate p53 function by engaging MDM2.

P53 mRNA controls p53 activity by managing Mdm2 functions

The E3 ubiquitin ligase Mdm2 is a focal regulator of p53 tumour suppressor activity. It binds p53, promoting its polyubiquitination and degradation, and also controls p53 synthesis. However, it is not known how this dual function of Mdm2 on p53 synthesis and degradation is achieved. Here we show that the p53 mRNA region encoding the Mdm2-binding site interacts directly with the RING domain of Mdm2. This impairs the E3 ligase activity of Mdm2 and promotes p53 mRNA translation. We also show that introduction of cancer-derived single silent point-mutations in the p53 mRNA weakens its binding to Mdm2 and results in reduced p53 activity. These data are consistent with a mechanism by which changes in silent nucleotides can affect the function of the encoded protein, and indicate that Mdm2-mediated control of p53 synthesis and degradation has evolved in the p53 mRNA sequence and its encoded amino acids.

Induction of PPM1D following DNA-damaging treatments through a conserved p53 response element coincides with a shift in the use of transcription initiation sites

PPM1D (Wip1), a type PP2C phosphatase, is expressed at low levels in most normal tissues but is overexpressed in several types of cancers. In cells containing wild-type p53, the levels of PPM1D mRNA and protein increase following exposure to genotoxic stress, but the mechanism of regulation by p53 was unknown. PPM1D also has been identified as a CREB-regulated gene due to the presence of a cyclic AMP response element (CRE) in the promoter. Transient transfection and chromatin immunoprecipitation experiments in HCT116 cells were used to characterize a conserved p53 response element located in the 5' untranslated region (UTR) of the PPM1D gene that is required for the p53-dependent induction of transcription from the human PPM1D promoter. CREB binding to the CRE contributes to the regulation of basal expression of PPM1D and directs transcription initiation at upstream sites. Following exposure to ultraviolet (UV) or ionizing radiation, the abundance of transcripts with short 5' UTRs increased in cells containing wild-type p53, indicating increased utilization of downstream transcription initiation sites. In cells containing wild-type p53, exposure to UV resulted in increased PPM1D protein levels even when PPM1D mRNA levels remained constant, indicating post-transcriptional regulation of PPM1D protein levels.

Association of HDM2 transcript levels with age of onset and prognosis in soft tissue sarcomas

The p53 stress response is crucial for the prevention of tumor formation. The oncogene HDM2 is one of the key negative regulators of p53 and is a central node in the p53 pathway. P53 and HDM2 form an oscillating feedback loop. HDM2 expression is regulated by different promoters. To evaluate its clinical relevance, we determined the levels of HDM2 transcripts originating from the constitutive P1 and p53-sensitive P2 promoter in 133 soft tissue sarcomas and correlated the results with the age of diagnosis and the patients' outcome. We show that only high levels of the HDM2-P1 transcript but not the P2 transcript are associated with an 11-year earlier age of onset (50.5 years) compared with low P1 levels (61.5 years; P < 0.0001, t test). In addition, low P1 and P2 mRNA expression levels were independent predictors of poor outcome for patients with soft tissue sarcomas (low P1: relative risk, 3.7; P < 0.0001; low P2: relative risk, 2.5; P = 0.001). A change in the expression levels of the HDM2 transcripts originating from the two HDM2 promoters could disrupt the oscillating P53-HDM2 feedback loop in a way that elevated levels of HDM2-P1 transcript are associated with an earlier age of tumor onset and that reduced levels of HDM2-P1 or HDM2-P2 transcripts are correlated with poor prognosis of patients with soft tissue sarcomas.

Alternative translation start sites and hidden coding potential of eukaryotic mRNAs

It is widely suggested that a eukaryotic mRNA typically contains one translation start site and encodes a single functional protein product. However, according to current points of view on translation initiation mechanisms, eukaryotic ribosomes can recognize several alternative translation start sites and the number of experimentally verified examples of alternative translation is growing rapidly. Also, the frequent occurrence of alternative translation events and their functional significance are supported by the results of computational evaluations. The functional role of alternative translation and its contribution to eukaryotic proteome complexity are discussed.

Role of P53-MDM2 negative-feedback in the pathopoiesis of Helicobacter pylori

Murine double minute-2 (mdm2), one of downstream genes of P53, forms a negative feedback loop with P53 to maintain P53 at a low level under normal circumstances. On one hand, P53 activates transcription of mdm2. On the other hand, MDM2 suppresses activity of P53. The negative feedback, which plays an important role in tumor development, is regulated by a variety of factors. At present, Helicobacter pylori (H. pylori) is considered as a key gastrointestinal disease pathogenic factor. Its pathogenic or carcinogenic mechanism has become a hot research issue in recent years, and there have been substantial research on the role of p53 gene networks in H. pylori pathogenic process. And the P53-MDM2 negative feedback may play an important role in this process.