Ubiquitin-mediated control of oncogene and tumor suppressor gene products

Using microRNAs Networks to Understand Pancreatic Cancer-A Literature Review

Pancreatic cancer is a severe disease, challenging to diagnose and treat, and thereby characterized by a poor prognosis and a high mortality rate. Pancreatic ductal adenocarcinoma (PDAC) represents approximately 90% of pancreatic cancer cases, while other cases include neuroendocrine carcinoma. Despite the growing knowledge of the pathophysiology of this cancer, the mortality rate caused by it has not been effectively reduced. Recently, microRNAs have aroused great interest among scientists and clinicians, as they are negative regulators of gene expression, which participate in many processes, including those related to the development of pancreatic cancer. The aim of this review is to show how microRNAs (miRNAs) affect key signaling pathways and related cellular processes in pancreatic cancer development, progression, diagnosis and treatment. We included the results of in vitro studies, animal model of pancreatic cancer and those performed on blood, saliva and tumor tissue isolated from patients suffering from PDAC. Our investigation identified numerous dysregulated miRNAs involved in KRAS, JAK/STAT, PI3/AKT, Wnt/β-catenin and TGF-β signaling pathways participating in cell cycle control, proliferation, differentiation, apoptosis and metastasis. Moreover, some miRNAs (miRNA-23a, miRNA-24, miRNA-29c, miRNA-216a) seem to be engaged in a crosstalk between signaling pathways. Evidence concerning the utility of microRNAs in the diagnosis and therapy of this cancer is poor. Therefore, despite growing knowledge of the involvement of miRNAs in several processes associated with pancreatic cancer, we are beginning to recognize and understand their role and usefulness in clinical practice.

TRIM6 promotes glioma malignant progression by enhancing FOXO3A ubiquitination and degradation

PURPOSE

TRIM6, an E3 ubiquitin ligase with tripartite motif, directly targets protein substrates for degradation through ubiquitination. Studies have shown that TRIM6 plays a significant role in tumor development in various human malignancies. Thus, the aim of this study was to investigate the importance of TRIM6 and its associated mechanism in promoting the progression of glioma.

METHODS

The expression of TRIM6 and its prognostic value in glioma patients were collected from the TCGA and CGGA databases. The effects of TRIM6 on glioma were investigated in vitro by CCK8, colony formation, wound healing, and transwell assays. Co-IP and western blot analysis were used to detect the interaction between TRIM6 and FOXO3A. The effects of TRIM6 were verified in vivo in subcutaneously xenograft models, and tumor size, and immunohistochemical changes were observed.

RESULTS

Our analysis of TRIM6 expression in glioma tissues revealed a high level of expression, and the heightened expression of TRIM6 showed a positive correlation with the unfavorable prognosis among glioma/GBM patients. Through loss-of-function and gain-of-function experiments, we observed a profound impact on the proliferation, invasion, and migration abilities of glioma cells both in vitro and in vivo upon deletion of TRIM6. Conversely, the overexpression of TRIM6 intensified the malignant characteristics of glioma. Additionally, our findings revealed a significant interaction between TRIM6 and FOXO3A, wherein TRIM6 contributed to the destabilization of FOXO3A protein by promoting its ubiquitination and subsequent degradation. Experiments conducted in the rescue study affirmed that the promotion of glioma cell proliferation, invasion, and migration is facilitated by TRIM6 through the suppression of FOXO3A protein levels.

CONCLUSIONS

These observations imply that the TRIM6-FOXO3A axis could potentially serve as an innovative focus for intervening in glioma.

ACE2 in chronic disease and COVID-19: gene regulation and post-translational modification

Angiotensin-converting enzyme 2 (ACE2), a counter regulator of the renin-angiotensin system, provides protection against several chronic diseases. Besides chronic diseases, ACE2 is the host receptor for SARS-CoV or SARS-CoV-2 virus, mediating the first step of virus infection. ACE2 levels are regulated by transcriptional, post-transcriptional, and post-translational regulation or modification. ACE2 transcription is enhanced by transcription factors including Ikaros, HNFs, GATA6, STAT3 or SIRT1, whereas ACE2 transcription is reduced by the transcription factor Brg1-FoxM1 complex or ERRα. ACE2 levels are also regulated by histone modification or miRNA-induced destabilization. The protein kinase AMPK, CK1α, or MAP4K3 phosphorylates ACE2 protein and induces ACE2 protein levels by decreasing its ubiquitination. The ubiquitination of ACE2 is induced by the E3 ubiquitin ligase MDM2 or UBR4 and decreased by the deubiquitinase UCHL1 or USP50. ACE2 protein levels are also increased by the E3 ligase PIAS4-mediated SUMOylation or the methyltransferase PRMT5-mediated ACE2 methylation, whereas ACE2 protein levels are decreased by AP2-mediated lysosomal degradation. ACE2 is downregulated in several human chronic diseases like diabetes, hypertension, or lung injury. In contrast, SARS-CoV-2 upregulates ACE2 levels, enhancing host cell susceptibility to virus infection. Moreover, soluble ACE2 protein and exosomal ACE2 protein facilitate SARS-CoV-2 infection into host cells. In this review, we summarize the gene regulation and post-translational modification of ACE2 in chronic disease and COVID-19. Understanding the regulation and modification of ACE2 may help to develop prevention or treatment strategies for ACE2-mediated diseases.

CDK4/6 inhibitor palbociclib promotes SARS-CoV-2 cell entry by down-regulating SKP2 dependent ACE2 degradation

Coronavirus disease 2019 (COVID-19) outbreak has become a global pandemic. CDK4/6 inhibitor palbociclib was reported to be one of the top-scored repurposed drugs to treat COVID-19. As the receptor for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) entry, expression level of angiotensin-converting enzyme 2 (ACE2) is closely related to SARS-CoV-2 infection. In this study, we demonstrated that palbociclib and other methods could arrest cells in G0/G1 phase and up-regulate ACE2 mRNA and protein levels without altering its subcellular localization. Palbociclib inhibited ubiquitin-proteasome and lysosomal degradation of ACE2 through down-regulating S-phase kinase-associated protein 2 (SKP2). In addition, increased ACE2 expression induced by palbociclib and other cell cycle arresting compounds facilitated pseudotyped SARS-CoV-2 infection. This study suggested that ACE2 expression was down-regulated in proliferating cells. Cell cycle arresting compounds could increase ACE2 expression and facilitate SARS-CoV-2 cell entry, which may not be suitable therapeutic agents for the treatment of SARS-CoV-2 infection.

SOX2 Promotes Invasion in Human Bladder Cancers through MMP2 Upregulation and FOXO1 Downregulation

SOX2, a member of the SRY-related HMG-box (SOX) family, is abnormally expressed in many tumors and associated with cancer stem cell-like properties. Previous reports have shown that SOX2 is a biomarker for cancer stem cells in human bladder cancer (BC), and our most recent study has indicated that the inhibition of SOX2 by anticancer compound ChlA-F attenuates human BC cell invasion. We now investigated the mechanisms through which SOX2 promotes the invasive ability of BC cells. Our studies revealed that SOX2 promoted SKP2 transcription and increased SKP2-accelerated Sp1 protein degradation. As Sp1 is a transcriptionally regulated gene, HUR transcription was thereby attenuated, and, in the absence of HUR, FOXO1 mRNA was degraded fast, which promoted BC cell invasion. In addition, SOX2 promoted BC invasion through the upregulation of nucleolin transcription, which resulted in increased MMP2 mRNA stability and expression. Collectively, our findings show that SOX2 promotes BC invasion through both SKP2-Sp1-HUR-FOXO1 and nucleolin-MMP2 dual axes.

Uncovering Oncogenic Mechanisms of Tumor Suppressor Genes in Breast Cancer Multi-Omics Data

Tumor suppressor genes (TSGs) are essential genes in the development of cancer. While they have many roles in normal cells, mutation and dysregulation of the TSGs result in aberrant molecular processes in cancer cells. Therefore, understanding TSGs and their roles in the oncogenic process is crucial for prevention and treatment of cancer. In this research, multi-omics breast cancer data were used to identify molecular mechanisms of TSGs in breast cancer. Differentially expressed genes and differentially coexpressed genes were identified in four large-scale transcriptomics data from public repositories and multi-omics data analyses of copy number, methylation and gene expression were performed. The results of the analyses were integrated using enrichment analysis and meta-analysis of a p-value summation method. The integrative analysis revealed that TSGs have a significant relationship with genes of gene ontology terms that are related to cell cycle, genome stability, RNA processing and metastasis, indicating the regulatory mechanisms of TSGs on cancer cells. The analysis frame and research results will provide valuable information for the further identification of TSGs in different types of cancers.

HERC5/IFI16/p53 signaling mediates breast cancer cell proliferation and migration

AIMS

This study aims to find differentially expressed ubiquitination-related gene(s) and elucidates their biological significance in breast cancer.

MAIN METHODS

Differentially expressed genes were profiled in MCF-7 and MDA-MB-231 cells by using PCR array method. Abnormal expression of HERC5 was studied in the cells and in breast cancer specimens via Quantitative Real-time PCR and western blot. Cell proliferation and cell migration abilities were evaluated by using cell counting kits, or through colony formation, wound healing and trans-well assays. HERC5 target proteins were investigated via proteomic, co-immunoprecipitation and western blot methods. Down-stream signaling pathways were investigated through gene expression/knockdown methods.

KEY FINDINGS

Huge increase of HERC5 expression was found in MCF-7 and MDA-MB-231 cells, knockdown of which repressed the cell proliferation and migration. HERC5 interacted with IFI16, mediated IFI16 ISGylation at K274 and facilitated IFI16 proteasomal degradation. IFI16 acted as a tumor suppressor and to some extent mediated the HERC5 function in the breast cancer (BC) cells. HERC5 was negatively correlated with IFI16 protein, while IFI16 was positively correlated to p53 expression at mRNA and protein levels, which indicates a novel signaling pathway - HERC5/IFI16/p53. HERC5 expression was increased in glucose-starved BC cells and in human breast cancer tissues, accompanied with the decrease of IFI16 and P53.

SIGNIFICANCE

Our work reveals the abnormal expression of HERC5 and its carcinogenic role in breast cancer cells, which is probably mediated by an HERC5/IFI16/p53 signaling pathway. This work also provides potential diagnostic/therapeutic biomarkers for breast cancer diagnosis and treatment.

NEDD4 degrades TUSC2 to promote glioblastoma progression

Whether tumor suppressor candidate 2 (TUSC2) plays an important role in glioblastoma (GBM) progression is largely unknown. Whether TUSC2 undergoes polyubiquitination is unknown. Herein, we report that TUSC2 protein expression is reduced/lost in GBM compared to normal brain due to protein destabilization; TUSC2 mRNA is equally expressed in both tissues. NEDD4 E3 ubiquitin ligase polyubiquitinates TUSC2 at residue K71, and the TUSC2-K71R mutant is resistant to NEDD4-mediated proteasomal degradation. Analysis of GBM specimens showed NEDD4 protein is highly expressed in GBM and the level is inversely correlated with TUSC2 protein levels. Furthermore, TUSC2 restoration induces apoptosis and inhibits patient-derived glioma stem cells (PD-GSCs) in vitro and in vivo. Conversely, TUSC2-knockout promotes PD-GSCs in vitro and in vivo. RNA-Seq analysis and subsequent validations showed GBM cells with TUSC2-knockout expressed increased Bcl-xL and were more resistant to apoptosis induced by a Bcl-xL-specific BH3 mimetic. A TUSC2-knockout gene signature created from the RNA-seq data predicts poor patient survival. Together, these findings establish that NEDD4-mediated polyubiquitination is a novel mechanism for TUSC2 degradation in GBM and that TUSC2 loss promotes GBM progression in part through Bcl-xL upregulation.

Manuscript Title: Analysis of tripartite motif (TRIM) family gene expression in prostate cancer bone metastases

Tripartite motif (TRIM) family proteins are post-translational protein modifiers with E3-ubiquitin ligase activity, thereby involved in various biological processes. The molecular mechanisms driving prostate cancer (PCa) bone metastasis (BM) are incompletely understood, and targetable genetic alterations are lacking in the majority of cases. Therefore, we aimed to explore the expression and potential functional relevance of 71 TRIM members in bone metastatic PCa. We performed transcriptome analysis of all human TRIM family members and 770 cancer-related genes in 29 localized PCa and 30 PCa BM using Nanostring. KEGG, STRING and Ubibrowser were used for further bioinformatic gene correlation and pathway enrichment analyses. Compared to localized tumors, six TRIMs are under-expressed while nine TRIMs are over-expressed in BM. The differentially expressed TRIM proteins are linked to TNF-, TGFβ-, PI3K/AKT- and HIF-1-signaling, and to features such as proteoglycans, platelet activation, adhesion and ECM-interaction based on correlation to cancer-related genes. The identification of TRIM-specific E3-ligase-substrates revealed insight into functional connections to oncogenes, tumor suppressors and cancer-related pathways including androgen receptor- and TGFβ signaling, cell cycle regulation and splicing. In summary, this is the first study that comprehensively and systematically characterizes the expression of all TRIM members in PCa BM. Our results describe post-translational protein modification as an important regulatory mechanism of oncogenes, tumor suppressors, and pathway molecules in PCa progression. Therefore, this study may provide evidence for novel therapeutic targets, in particular for the treatment or prevention of BM.

Molecular Pathogenesis of Pulmonary Fibrosis, with Focus on Pathways Related to TGF-β and the Ubiquitin-Proteasome Pathway

Idiopathic pulmonary fibrosis (IPF) is a progressive and fatal interstitial lung disease. During the past decade, novel pathogenic mechanisms of IPF have been elucidated that have shifted the concept of IPF from an inflammatory-driven to an epithelial-driven disease. Dysregulated repair responses induced by recurrent epithelial cell damage and excessive extracellular matrix accumulation result in pulmonary fibrosis. Although there is currently no curative therapy for IPF, two medications, pirfenidone and nintedanib, have been introduced based on understanding the pathogenesis of the disease. In this review, we discuss advances in understanding IPF pathogenesis, highlighting epithelial-mesenchymal transition (EMT), the ubiquitin-proteasome system, and endothelial cells. TGF-β is a central regulator involved in EMT and pulmonary fibrosis. HECT-, RING finger-, and U-box-type E3 ubiquitin ligases regulate TGF-β-Smad pathway-mediated EMT via the ubiquitin-proteasome pathway. p27 degradation mediated by the SCF-type E3 ligase, Skp2, contributes to the progression of pulmonary fibrosis by promotion of either mesenchymal fibroblast proliferation, EMT, or both. In addition to fibroblasts as key effector cells in myofibroblast differentiation and extracellular matrix deposition, endothelial cells also play a role in the processes of IPF. Endothelial cells can transform into myofibroblasts; therefore, endothelial-mesenchymal transition can be another source of myofibroblasts.

Targeting DNA Repair Pathways in Hematological Malignancies

DNA repair plays an essential role in protecting cells that are repeatedly exposed to endogenous or exogenous insults that can induce varying degrees of DNA damage. Any defect in DNA repair mechanisms results in multiple genomic changes that ultimately may result in mutation, tumor growth, and/or cell apoptosis. Furthermore, impaired repair mechanisms can also lead to genomic instability, which can initiate tumorigenesis and development of hematological malignancy. This review discusses recent findings and highlights the importance of DNA repair components and the impact of their aberrations on hematological malignancies.

Tumor-intrinsic CD47 signal regulates glycolysis and promotes colorectal cancer cell growth and metastasis

Rationale: CD47 plays a vital role in the immune escape of tumor cells, but its role in regulating immune-unrelated biological processes such as proliferation and metastasis remains unclear. We seek to explore the immune-independent functions of CD47 in colorectal cancer (CRC). Methods: The expression of CD47 in CRC was determined by immunohistochemistry. The biological effect of CD47 signaling on tumor cell proliferation and metastasis was evaluated in vitro and in vivo. RNA sequencing analysis was performed to identify pivotal signaling pathways modulated by CD47. The interaction between CD47 and ENO1 was verified by co-immunoprecipitation (co-IP). The effect of CD47 on glycolytic metabolites was analyzed by seahorse XF and targeted metabolomics. Results: The expression of CD47 was upregulated and correlated to poor prognosis in CRC patients. Functional assays revealed that CD47 promoted CRC cell growth and metastasis in vitro and in vivo. Our mechanistic investigations demonstrated that CD47 interacted with ENO1 and protected it from ubiquitin-mediated degradation, subsequently promoting glycolytic activity and phosphorylation of ERK in CRC cells. Inhibition of ENO1 diminished CD47-mediated cell growth and migration. Clinically, the combined expression of CD47 and ENO1 provided reliable predictive biomarkers for the prognosis of CRC patients. Conclusions: CD47 is overexpressed in CRC, and its expression is associated with poor prognosis. Through stabilizing ENO1, CD47 enhances the aerobic glycolysis and ERK activity in CRC cells, thereby promoting the progression of CRC. Our studies reveal an unconventional role of CD47, suggesting that targeting the CD47-ENO1 axis may provide a novel therapeutic avenue for CRC.

ADAR, the carcinogenesis mechanisms of ADAR and related clinical applications

Adenosine deaminases acting on RNA (ADARs) catalyze the conversion of adenosine (A) to inosine (I) in double-stranded RNA, which can change the codons after transcription. Abnormal ADAR editing is present in a variety of cancers. However, the study of the biological effects of ADARs in cancer is not very deep. Here, we review current important ADAR-mediated editing events, related carcinogenic mechanisms and applications in clinical medicine. Further exploration in ADARs can provide a new direction for cancer treatment.

Harnessing the anti-cancer natural product nimbolide for targeted protein degradation

Nimbolide, a terpenoid natural product derived from the Neem tree, impairs cancer pathogenicity; however, the direct targets and mechanisms by which nimbolide exerts its effects are poorly understood. Here, we used activity-based protein profiling (ABPP) chemoproteomic platforms to discover that nimbolide reacts with a novel functional cysteine crucial for substrate recognition in the E3 ubiquitin ligase RNF114. Nimbolide impairs breast cancer cell proliferation in-part by disrupting RNF114 substrate recognition, leading to inhibition of ubiquitination and degradation of the tumor-suppressors such as p21, resulting in their rapid stabilization. We further demonstrate that nimbolide can be harnessed to recruit RNF114 as an E3 ligase in targeted protein degradation applications and show that synthetically simpler scaffolds are also capable of accessing this unique reactive site. Our study highlights the utility of ABPP platforms in uncovering unique druggable modalities accessed by natural products for cancer therapy and targeted protein degradation applications.

Ubiquitin specific peptidase 5 regulates colorectal cancer cell growth by stabilizing Tu translation elongation factor

Ubiquitin specific peptidase 5 (USP5) is a ubiquitous expressed deubiquitinating enzyme (DUB). It has been shown involved in DNA repair, apoptosis, inflammation, and tumor cell growth. However, the function and molecular mechanism of USP5 in colorectal cancer (CRC) are still unclear. In the present study, we asked how it affected the growth of colorectal cancer cells.

Methods: A shRNA-based high-content screening was performed to identify DUBs affecting the growth of CRC cells. CCK-8 assay and xenografts were used to assess CRC cell growth, survival and tumorigenesis. RT-qPCR, immunoblotting and immunohistochemistry were carried out to quantitate USP5 expression in CRC tissues and cell lines. Immunoprecipitation and mass spectrometry analysis were performed to identify USP5-interacting proteins. Cycloheximide chase was performed to assess Tu translation elongation factor (TUFM) stability. Dual luciferase reporter assay was utilized for USP5 promoter analysis.

Results: We found that USP5 was highly expressed in a group of primary CRC tissues, and the increased USP5 was correlated with clinical stages and shorter overall survival. While USP5 knockdown effectively inhibited CRC cell growth, overexpressed USP5 promoted the growth of CRC cells and made them more resistant to doxorubicin (DOX). TUFM was discovered as a substrate of USP5. USP5 deubiquitinated TUFM and increased its level in CRC cells. Enforced expression of TUFM was able to alleviate the growth inhibition induced by USP5 knockdown. Further analyses showed that EBF transcription factor 1 (EBF1) was a major regulator for USP5 transcription, and DOX inhibited EBF1-USP5-TUFM axis in CRC cells.

Conclusions: USP5 was required for CRC cells and promoted their growth and resistance to chemotherapeutics. TUFM was a USP5 deubiquitinating substrate that mediated the cellular effects of USP5. The transcription of USP5 was regulated by EBF1. Thus, targeting EBF1-USP5-TUFM axis is a potential novel strategy for CRC treatment.

Emerging role of F-box proteins in the regulation of epithelial-mesenchymal transition and stem cells in human cancers

Emerging evidence shows that epithelial-mesenchymal transition (EMT) plays a crucial role in tumor invasion, metastasis, cancer stem cells, and drug resistance. Data obtained thus far have revealed that F-box proteins are critically involved in the regulation of the EMT process and stem cell differentiation in human cancers. In this review, we will briefly describe the role of EMT and stem cells in cell metastasis and drug resistance. We will also highlight how numerous F-box proteins govern the EMT process and stem cell survival by controlling their downstream targets. Additionally, we will discuss whether F-box proteins involved in drug resistance are associated with EMT and cancer stem cells. Targeting these F-box proteins might be a potential therapeutic strategy to reverse EMT and inhibit cancer stem cells and thus overcome drug resistance in human cancers.

Skp2 Expression Is Inhibited by Arsenic Trioxide through the Upregulation of miRNA-330-5p in Pancreatic Cancer Cells

Arsenic trioxide (ATO) has been found to exert its anti-cancer activity in various human malignancies. In our previous report, we have shown that ATO inhibited cell growth and invasion via downregulation of Skp2 in pancreatic cancer (PC) cells. It has been extensively demonstrated that microRNAs (miRNAs) play a pivotal role in tumorigenesis. ATO might induce PC cell apoptosis and regulate Skp2 downregulation through the regulation of miRNAs. One study has demonstrated that miR-330-5p exerts a tumor-suppressive function in PC cell lines. Here, we investigated the role of miRNA-330-5p in ATO-mediated anti-tumor activity and explored whether ATO could regulate miR-330-5p in PC cells. We found that ATO treatment upregulated the expression of miR-330-5p. Moreover, miR-330-5p inhibitor rescued the ATO-mediated tumor-suppressive function. The combination of miR-330-5p mimic with ATO reduced cell growth, motility, and invasion, and enhanced apoptosis to a greater degree in PC cells. This study suggests that the combination of miR-330-5p mimic with ATO may be a potential therapeutic strategy for the treatment of PC.

Arsenic trioxide inhibits Skp2 expression to increase chemosensitivity to gemcitabine in pancreatic cancer cells

The S-phase kinase associated protein 2 (Skp2), a member of the F-box protein family, regulates cell cycle progression and is highly expressed in pancreatic cancer (PC). Recently, we reported that arsenic trioxide (ATO) inhibited cell growth and invasion via downregulation of Skp2 in PC cells. Emerging evidence has revealed that Skp2 plays a crucial role in drug resistance in several kinds of cancers. Here, we determined whether ATO enhanced the sensitivity of PC cell lines to gemcitabine (GEM). We found that the combined treatment of ATO and GEM demonstrated strong antitumor effects in Patu8988 and Panc-1 PC cells. In addition, ATO potentiated the effects of GEM via downregulation of the Skp2 pathway in PC cells. Together, these findings suggested that Skp2 may be a promising therapeutic target to overcome resistance to GEM in PC.

Treatment of Lymphoid and Myeloid Malignancies by Immunomodulatory Drugs

Thalidomide and its derivatives (lenalidomide, pomalidomide, avadomide, iberdomide hydrochoride, CC-885 and CC-90009) form the family of immunomodulatory drugs (IMiDs). Lenalidomide (CC5013, Revlimid®) was approved by the US FDA and the EMA for the treatment of multiple myeloma (MM) patients, low or intermediate-1 risk transfusion-dependent myelodysplastic syndrome (MDS) with chromosome 5q deletion [del(5q)] and relapsed and/or refractory mantle cell lymphoma following bortezomib. Lenalidomide has also been studied in clinical trials and has shown promising activity in chronic lymphocytic leukemia (CLL) and non-Hodgkin lymphoma (NHL). Lenalidomide has anti-inflammatory effects and inhibits angiogenesis. Pomalidomide (CC4047, Imnovid® [EU], Pomalyst® [USA]) was approved for advanced MM insensitive to bortezomib and lenalidomide. Other IMiDs are in phases 1 and 2 of clinical trials. Cereblon (CRBN) seems to have an important role in IMiDs action in both lymphoid and myeloid hematological malignancies. Cereblon acts as the substrate receptor of a cullin-4 really interesting new gene (RING) E3 ubiquitin ligase CRL4CRBN. This E3 ubiquitin ligase in the absence of lenalidomide ubiquitinates CRBN itself and the other components of CRL4CRBN complex. Presence of lenalidomide changes specificity of CRL4CRBN which ubiquitinates two transcription factors, IKZF1 (Ikaros) and IKZF3 (Aiolos), and casein kinase 1α (CK1α) and marks them for degradation in proteasomes. Both these transcription factors (IKZF1 and IKZF3) stimulate proliferation of MM cells and inhibit T cells. Low CRBN level was connected with insensitivity of MM cells to lenalidomide. Lenalidomide decreases expression of protein argonaute-2, which binds to cereblon. Argonaute-2 seems to be an important drug target against IMiDs resistance in MM cells. Lenalidomide decreases also basigin and monocarboxylate transporter 1 in MM cells. MM cells with low expression of Ikaros, Aiolos and basigin are more sensitive to lenalidomide treatment. The CK1α gene (CSNK1A1) is located on 5q32 in commonly deleted region (CDR) in del(5q) MDS. Inhibition of CK1α sensitizes del(5q) MDS cells to lenalidomide. CK1α mediates also survival of malignant plasma cells in MM. Though, inhibition of CK1α is a potential novel therapy not only in del(5q) MDS but also in MM. High level of full length CRBN mRNA in mononuclear cells of bone marrow and of peripheral blood seems to be necessary for successful therapy of del(5q) MDS with lenalidomide. While transfusion independence (TI) after lenalidomide treatment is more than 60% in MDS patients with del(5q), only 25% TI and substantially shorter duration of response with occurrence of neutropenia and thrombocytopenia were achieved in lower risk MDS patients with normal karyotype treated with lenalidomide. Shortage of the biomarkers for lenalidomide response in these MDS patients is the main problem up to now.

Chemical probes of Skp2-mediated p27 ubiquitylation and degradation

Skp2 is a member of the F-box family of proteins that serve as substrate-specific adaptors in Skp1-CUL1-ROC1-F-box (SCF) E3 ubiquitin ligases. Skp2 (Fbxl1) directly binds to the tumor suppressor p27 in the context of the SCF^Skp2 E3 ubiquitin ligase to ubiquitylate and target-phosphorylated p27 for proteasomal degradation. As p27 is a powerful suppressor of growth in a variety of cells, and as Skp2 is also overexpressed in many human cancers, Skp2 is considered an oncogene and an intriguing drug target. However, despite 20 years of investigation, a valid chemical inhibitor of Skp2-mediated degradation of p27 has not been identified. Recently, an increasing number of compounds designed to have this bioactivity have been reported. Here, we conduct a meta-analysis of the evidence regarding bioactivity, structure, and medicinal chemistry in order to evaluate and compare these Skp2 inhibitor compounds. Despite chemically diverse compounds with a wide array of Skp2-mediated p27 ubiquitylation inhibition properties reported by several independent groups, no current chemical probe formally qualifies as a validated pharmaceutical hit compound. This finding suggests that our knowledge of the structural biochemistry of the Skp2-p27 complex remains incomplete and highlights the need for novel modes of inquiry.

SMAR1 inhibits Wnt/β-catenin signaling and prevents colorectal cancer progression

Reduced expression of Scaffold/Matrix Attachment Region Binding Protein 1 (SMAR1) is associated with various cancers resulting in poor prognosis of the diseases. However, the precise underlying mechanism elucidating the loss of SMAR1 requires ongoing study. Here, we show that SMAR1 is highly downregulated during aberrant Wnt3a signaling due to proteasomal degradation and predicted poor prognosis of colorectal cancer. However, substitution mutation (Arginine and Lysine to Alanine) in the D-box elements of SMAR1 viz. "RCHL" and "RQRL" completely abrogated its proteasomal degradation despite Wnt3a activity. SMAR1 inhibited Wnt/β-catenin signaling by recruiting Histone deacetylase-5 to β-catenin promoter resulting in reduced cell migration and invasion. Consequently, reduced tumor sizes in in-vivo NOD-SCID mice were observed that strongly associated with suppression of β-catenin. However, loss of SMAR1 led to enriched H3K9 Acetylation in the β-catenin promoter that further increased Wnt/β-catenin signaling activities and enhanced colorectal cancer progression drastically. Using docking and isothermal titration calorimetric studies we show that small microbial peptides viz. AT-01C and AT-01D derived from Mycobacterium tuberculosis mask the D-box elements of SMAR1. These peptides stabilized SMAR1 expression that further inhibited metastatic SW480 colorectal cancer cell migration and invasion. Drastically reduced subcutaneous tumors were observed in in-vivo NOD-SCID mice upon administration of these peptides (25 mg/kg body weight) intraperitoneally. Taken together our structural studies, in-vitro and in-vivo results strongly suggest that the D-box elements of SMAR1 represent novel druggable targets, where the microbial peptides hold promise as novel colorectal cancer therapeutics.

UBE2T knockdown inhibits gastric cancer progression

Ubiquitin-conjugating enzymes (E2 enzymes) such as UBE2T target proteins for degradation via the proteasome. Here, we examined the effects of UBE2T on the progression of gastric cancer. UBE2T was highly expressed in gastric tumors and gastric cancer cells. siRNA-mediated suppression of UBE2T inhibited gastric cancer cell proliferation and colony formation by promoting cell cycle arrest at G2/M phase and increasing apoptosis. Suppression of UBE2T also attenuated the invasive and metastatic abilities of gastric cancer cells by altering expression of epithelial-mesenchymal transition (EMT)-related factors. A xenograft model in which nude mice were injected with UBE2T knockdown human gastric cancer cells confirmed that suppression of UBE2T also decreased tumor formation and growth in vivo. Expression levels of CCND1, Phospho-GSK3B, WNT family members, and MYC were all affected by UBE2T knockdown. These results suggest that UBE2T plays a critical role in gastric cancer, and that it may serve as a useful prognostic biomarker and therapeutic target in gastric cancer patients.

TRIM71 suppresses tumorigenesis via modulation of Lin28B-let-7-HMGA2 signaling

TRIM71 (tripartite motif-containing 71) belongs to the TRIM-NHL protein family, which plays a conserved role in regulating early development and differentiation. However, the molecular functions of TRIM71 have remained largely unknown. Here, we explored the role of TRIM71 together with modulation of Lin28B-let-7-HMGA2 (high-mobility group AT-hook 2) signaling in tumorigenesis. TRIM71 overexpression opposed Lin28B-induced transformation in primary cells and inhibited tumor formation in a mouse model. Specific knockdown of TRIM71 expression increased cancer cell proliferation and invasion. Conversely, overexpression of wild-type TRIM71 in non-small cell lung carcinoma (NSCLC) cells in which Lin28B-let-7-HMGA2 signaling was conserved decreased both cancer cell phenotypes. More importantly, overexpression of an ubiquitin transfer activity-deficient TRIM71 mutant in NSCLC cells had no effect on proliferation or invasion, regardless of the conservation status of Lin28B-let-7-HMGA2 signaling. The tumorigenic inhibitory action of TRIM71 was antagonized by overexpression of the TRIM71 downstream targets, Lin28B and HMGA2. Furthermore, a bioinformatics analysis revealed that TRIM71 expression was downregulated in various types of cancer tissue from patients. Taken together, these data indicate that TRIM71 acts through post-transcriptional repression of Lin28B and subsequent modulation of let-7-HMGA2 signaling during tumorigenesis to potentially function as a tumor suppressor.

Escitalopram oxalate induces apoptosis in U-87MG cells and autophagy in GBM8401 cells

Glioblastoma multiforme (GBM) is recognized as a most aggressive brain cancer with the worst prognosis and survival time. Owing to the anatomic location of gliomas, surgically removing the tumour is very difficult and avoiding damage to vital brain regions during radiotherapy is impossible. Therefore, therapeutic strategies for malignant glioma must urgently be improved. Recent studies have demonstrated that selective serotonin reuptake inhibitors (SSRIs) have cytotoxic effect on certain cancers. Considering as a more superior SSRI, escitalopram oxalate exhibits favourable tolerability and causes generally mild and temporary adverse events. However, limited information is revealed about the influence of escitalopram oxalate on GBM. Therefore, an attempt was made herein to explore the effects of escitalopram oxalate on GBM. The experimental results revealed that escitalopram oxalate significantly inhibits the proliferation and invasive ability of U‐87MG cells and significantly reduced the expressions of cell cycle inhibitors such as Skp2, P57, P21 and P27. Notably, escitalopram oxalate also induced significant apoptotic cascades in U‐87MG cells and autophagy in GBM8401 cells. An animal study indicated that escitalopram oxalate inhibits the proliferation of xenografted glioblastoma in BALB/c nude mice. These findings implied that escitalopram oxalate may have potential in treatment of glioblastomas.

Dysregulated expression of SKP2 and its role in hematological malignancies

S-phase kinase-associated protein 2 (SKP2) is a well-studied F-box protein and a critical part of the Skp1-Cul1-Fbox (SCF) E3 ligase complex. It controls cell cycle by regulating the expression level of p27 and p21 through ubiquitination and proteasomal degradation. SKP2-mediated loss of p27Kip1 is associated with poor clinical outcome in various types of cancers including hematological malignancies. It is however well established that SKP2 is an oncogene, and its targeting may be an attractive therapeutic strategy for the management of hematological malignancies. In this article, we have highlighted the recent findings from our group and other investigators regarding the role of SKP2 in the pathogenesis of hematological malignancies.

CyclinD1 and p57kip2 as biomarkers in differentiation, metastasis and prognosis of gastric cardia adenocarcinoma

Objective

This study aims to investigate the expression and significance of p57^kip2 and cyclinD1 in gastric cardia adenocarcinoma (GCA). p57^kip2 is a negative regulator in the cell cycle. On the contrary, cyclinD1 is a positive regulator of cell cycle progression.

Methods

Thirty-two cases of GCA tissues and adjacent non-cancerous tissues were collected for this study. Immunohistochemistry and fluorescence qualitative PCR was used to determine the level of p57^kip2 and cyclinD1 in GCA and its adjacent non-cancerous tissues. Furthermore, the correlation between the mRNA/protein and GCA clinical pathologic parameters were analyzed, and the relationship of p57^kip2 and cyclinD1 in GCA were also evaluated.

Results

The expression of p57^kip2 significantly lower in GCA (P = 0.036), and there was a significant correlation in the different degrees of differentiation (P < 0.05). Furthermore, median survival time was 41 months for patients with high mRNA expression of p57^kip2. This was longer compared to patients with low mRNA expression of P57^kip2 (37 months, X² = 4.788, P = 0.029).The expression of cyclinD1 was significantly higher in GCA(P = 0.002), and was significant correlated to clinical stage(P<0.05). Median survival time was 34 months in patients with high mRNA expression of cyclinD1, which was shorter than in patients with low expression of cyclinD1 mRNA (41 months, X² = 4.071, P = 0.044). The protein expression of p57^kip2 was not correlated to the protein expression of cyclinD1 (P = 0.55).

Conclusion

The expression of p57^kip2 and cyclinD1 are likely to suppress or promote the tumorigenesis and progression of GCA.

Cardio-Oncology: Cancer Therapy-related Cardiovascular Complications in a Molecular Targeted Era: New Concepts and Perspectives

Cardio-oncology is a medical discipline that identifies, prevents, and treats the cardiovascular complications related to cancer therapy. Due to the remarkable proliferation of new cancer therapies causing cardiovascular complications, such as hypertension, heart failure, vascular complications, and cardiac arrhythmia, we provide an extensive, comprehensive revision of the most up-to-date scientific information available on the cardiovascular complications associated with the use of newer, novel chemotherapeutic agents, including their reported incidence, suggested pathophysiology, clinical manifestations, potential treatment, and prevention. The authors consider this topic to be relevant for the clinicians since cardiovascular complications associated with the administration of recently approved drugs are relatively underappreciated. The purpose of this article is to provide a state-of-the-art review of cardiovascular complications associated with the use of newer, novel chemotherapeutic agents and targeted therapies, including their reported incidence, suggested pathophysiology, clinical manifestations, potential treatment, and prevention.  Ongoing efforts are needed to provide a better understanding of the frequency, mechanisms of disease, prevention, and treatment of cardiovascular complications induced by the newer, novel chemotherapeutic agents. Development of a cardio-oncology discipline is warranted in order to promote task forces aimed at the creation of oncology patient-centered guidelines for the detection, prevention, and treatment of potential cardiovascular side effects associated with newer cancer therapies.

Associations and prognostic significance of p27Kip1, Jab1 and Skp2 in non-Hodgkin lymphoma

Non-Hodgkin lymphoma (NHL) is a primary tumor arising in lymph nodes and lymphoid tissue. The incidence of NHL is increasing at an annual rate of 3%. The human Jun activation domain-binding protein 1/COP9 signalosome subunit 5 (Jab1/CSN5) is a negative regulator of the cell cycle inhibitor p27^Kip1 and abnormal expression of Jab1 is correlated with reduced p27 expression and associated with advanced tumor stage and poor prognosis in several human cancers. F-box protein S-phase kinase-interacting protein-2 (Skp2), the substrate recognition subunit of the Skp1-Cul1-F-box protein ubiquitin protein ligase complex, is required for the ubiquitination and consequent degradation of p27. The Skp2 protein is overexpressed in several human cancers and is associated with the degree of differentiation and the prognosis. The aim of the present study was to investigate the expression status of p27^Kip1, Jab1 and Skp2 by immunohistochemistry, and assess their prognostic significance in patients with NHL. Immunohistochemical analysis revealed an inverse association between Jab1 and p27 in NHL tissue samples. Kaplan-Meier analysis demonstrated that Jab1 overexpression, Skp2 overexpression and low p27 expression were significantly associated with poor prognosis. Among clinicopathological parameters, overexpression of Jab1 was significantly associated with tumor size and International Prognostic Index (IPI), whereas Skp2 expression was significantly associated with metastasis and IPI. These findings suggest that the overexpression of Jab1 or Skp2 plays an important role in the pathogenesis of NHL. Thus, the expression of p27^Kip1, Jab1 and Skp2 provided a clinical reference for the treatment of NHL.

Definition of a Skp2-c-Myc Pathway to Expand Human Beta-cells

Type 2 diabetes (T2D) is characterized by insulin resistance and reduced functional β-cell mass. Developmental differences, failure of adaptive expansion and loss of β-cells via β-cell death or de-differentiation have emerged as the possible causes of this reduced β-cell mass. We hypothesized that the proliferative response to mitogens of human β-cells from T2D donors is reduced, and that this might contribute to the development and progression of T2D. Here, we demonstrate that the proliferative response of human β-cells from T2D donors in response to cdk6 and cyclin D3 is indeed dramatically impaired. We show that this is accompanied by increased nuclear abundance of the cell cycle inhibitor, p27^kip1. Increasing nuclear abundance of p27^kip1 by adenoviral delivery decreases the proliferative response of β-cells from non-diabetic donors, mimicking T2D β-cells. However, while both p27^kip1 gene silencing and downregulation by Skp2 overexpression increased similarly the proliferative response of human β-cells, only Skp2 was capable of inducing a significant human β-cell expansion. Skp2 was also able to double the proliferative response of T2D β-cells. These studies define c-Myc as a central Skp2 target for the induction of cell cycle entry, expansion and regeneration of human T2D β-cells.

The cilia-regulated proteasome and its role in the development of ciliopathies and cancer

The primary cilium is an essential structure for the mediation of numerous signaling pathways involved in the coordination and regulation of cellular processes essential for the development and maintenance of health. Consequently, ciliary dysfunction results in severe human diseases called ciliopathies. Since many of the cilia-mediated signaling pathways are oncogenic pathways, cilia are linked to cancer. Recent studies demonstrate the existence of a cilia-regulated proteasome and that this proteasome is involved in cancer development via the progression of oncogenic, cilia-mediated signaling. This review article investigates the association between primary cilia and cancer with particular emphasis on the role of the cilia-regulated proteasome.

The G1 phase E3 ubiquitin ligase TRUSS that gets deregulated in human cancers is a novel substrate of the S-phase E3 ubiquitin ligase Skp2

E3 ubiquitin ligases have been implicated in the ubiquitination and proteasome-mediated degradation of several key regulators of cell cycle. Owing to their pleotropic behavior, E3 ubiquitin ligases are tightly regulated both at transcriptional and post-translational levels. The E3 ubiquitin ligase TRUSS (tumor necrosis factor receptor-associated ubiquitous scaffolding and signaling protein) which negatively regulates c-Myc, are found down-regulated in most human cancer cell lines. However, the mechanism of regulation of intracellular levels of TRUSS remains elusive. Here we show that TRUSS is expressed majorly during the G1 phase of cell cycle and its level starts to decline with the expression of S-phase specific E3 ligase Skp2. Enforced expression of Skp2 led to a marked increase in the ubiquitination of TRUSS after its phosphorylation by GSK3β and followed by rapid proteolytic degradation. Our co-immunoprecipitation studies suggested a direct interaction between Skp2 and TRUSS through the LRR motif of Skp2. Interestingly, the human tumor samples that exhibited elevated expression of Skp2, showed relatively poor expression of TRUSS. Further, enforced expression of HBx, the oncoprotein of Hepatitis B virus which is known to stabilize c-Myc and enhance its oncogenic potential, led to the intracellular accumulation of TRUSS as well as c-Myc. Apparently, HBx also interacted with TRUSS which negatively impacted the TRUSS-c-Myc and TRUSS-Skp2 interactions leading to stabilization of TRUSS. Thus, the present study suggests that TRUSS is a novel substrate of E3 ligase Skp2 and that disruption of TRUSS-Skp2 interaction by viral oncoproteins could lead to pathophysiological sequelae.

Fbxw7-associated drug resistance is reversed by induction of terminal differentiation in murine intestinal organoid culture

Colorectal cancer (CRC) is one of the top three cancer-related causes of death worldwide. FBXW7 is a known tumor-suppressor gene, commonly mutated in CRC and in a variety of other epithelial tumors. Low expression of FBXW7 is also associated with poor prognosis. Loss of FBXW7 sensitizes cancer cells to certain drugs, while making them more resistant to other types of chemotherapies. However, is not fully understood how epithelial cells within normal gut and primary tumors respond to potential cancer therapeutics. We have studied genetically engineered mice in which the fbxw7 gene is conditionally knocked-out in the intestine (fbxw7^∆G). To further investigate the mechanism of Fbxw7-action, we grew intestinal crypts from floxed-fbxw7 (fbxw7^fl/fl) and fbxw7^ΔG mice, in a Matrigel-based organoid (mini-gut) culture. The fbxw7^ΔG organoids exhibited rapid budding events in the crypt region. Furthermore, to test organoids for drug response, we exposed day 3 intestinal organoids from fbxw7^fl/fl and fbxw7^∆G mice, to various concentrations of 5-fluorouracil (5-FU) for 72 hours. 5-FU triggers phenotypic differences in organoids including changing shape, survival, resistance, and death. 5-FU however, rescues the drug-resistance phenotype of fbxw7^ΔG through the induction of terminal differentiation. Our results support the hypothesis that a differentiating therapy successfully targets FBXW7-mutated CRC cells.

Ubiquitination-mediated degradation of cell cycle-related proteins by F-box proteins

F-box proteins, subunits of SKP1-cullin 1-F-box protein (SCF) type of E3 ubiquitin ligase complexes, have been validated to play a crucial role in governing various cellular processes such as cell cycle, cell proliferation, apoptosis, migration, invasion and metastasis. Recently, a wealth of evidence has emerged that F-box proteins is critically involved in tumorigenesis in part through governing the ubiquitination and subsequent degradation of cell cycle proteins, and dysregulation of this process leads to aberrant cell cycle progression and ultimately, tumorigenesis. Therefore, in this review, we describe the critical role of F-box proteins in the timely regulation of cell cycle. Moreover, we discuss how F-box proteins involve in tumorigenesis via targeting cell cycle-related proteins using biochemistry studies, engineered mouse models, and pathological gene alternations. We conclude that inhibitors of F-box proteins could have promising therapeutic potentials in part through controlling of aberrant cell cycle progression for cancer therapies.

FBXW7-mutated colorectal cancer cells exhibit aberrant expression of phosphorylated-p53 at Serine-15

FBXW7 mutations occur in a variety of human cancers including colorectal cancer (CRC). Elucidating its mechanism of action has become crucial for cancer therapy; however, it is also complicated by the fact that FBXW7 can influence many pathways due to its role as an E3-ubiquitin ligase in proteasome degradation. FBXW7 and TP53 are tumour suppressors intensively implicated in colorectal carcinogenesis. Deletion mutations in these two genes in animal models mark the progression from adenoma to carcinoma. Although still largely unknown, the last defense mechanism against CRC at the molecular level could be through a synergistic effect of the two genes. The underlying mechanism requires further investigation. In our laboratory, we have used a phospho-kinase profiler array to illustrate a potential molecular link between FBXW7 and p53 in CRC cells. In vitro and in vivo assessments demonstrated aberrant induction of phosphorylated p53 at Serine 15 [phospho-p53(Ser15)] in human FBXW7-deficient CRC cells as compared to their FBXW7-wild-type counterparts. FBXW7 loss in HCT116 cells promoted resistance to oxaliplatin. Immunoblotting data further confirmed that reduction of phospho-p53(Ser15) may contribute to the decreased efficacy of therapy in FBXW7-mutated CRC cells. The findings may suggest the applicability of phospho-p53(Ser15) as an indicative marker of FBXW7-mutations. Phospho-p53(Ser15) regulation by FBXW7 E3-ligase activity could provide important clues for understanding FBXW7 behavior in tumour progression and grounds for its clinical applicability thereafter.

Involvement of F-box proteins in esophageal cancer (Review)

The F-box proteins (FBPs) in esophageal tumorigenesis are pivotal as they govern a broad array of basic physiological responses including cell growth, cell death and DNA damage repair. Esophageal cancer (EC) is a common and highly aggressive cancer worldwide. Aberrant stabilization of crucial proteins participates in esophageal tumorigenesis. Recently, growing evidence has shown that FBPs play a critical role in oncogenesis, invasion, metastasis and prognosis assessment of EC. In this review we summarized published data on the roles of known FBPs, their respective substrates and the key signaling pathways, in the development of EC, aiming to uncover new ways for the rational design of targeted therapies in EC.

SKP2 cooperates with N-Ras or AKT to induce liver tumor development in mice

Mounting evidence indicates that S-Phase Kinase-Associated Protein 2 (SKP2) is overexpressed in human hepatocellular carcinoma (HCC). However, the role of SKP2 in hepatocarcinogenesis remains poorly delineated. To elucidate the function(s) of SKP2 in HCC, we stably overexpressed the SKP2 gene in the mouse liver, either alone or in combination with activated forms of N-Ras (N-RasV12), AKT1 (myr-AKT1), or β-catenin (ΔN90-β-catenin) protooncogenes, via hydrodynamic gene delivery. We found that forced overexpression of SKP2, N-RasV12 or ΔN90-β-catenin alone as well as co-expression of SKP2 and ΔN90-β-catenin did not induce liver tumor development. Overexpression of myr-AKT1 alone led to liver tumor development after long latency. In contrast, co-expression of SKP2 with N-RasV12 or myr-AKT1 resulted in early development of multiple hepatocellular tumors in all SKP2/N-RasV12 and SKP2/myr-AKT1 mice. At the molecular level, preneoplastic and neoplastic liver lesions from SKP2/N-RasV12 and SKP2/myr-AKT1 mice exhibited a strong induction of AKT/mTOR and Ras/MAPK pathways. Noticeably, the tumor suppressor proteins whose levels have been shown to be downregulated by SKP2-dependent degradation in various tumor types, including p27, p57, Dusp1, and Rassf1A were not decreased in liver lesions from SKP2/N-RasV12 and SKP2/myr-AKT1 mice. In human HCC specimens, nuclear translocation of SKP2 was associated with activation of the AKT/mTOR and Ras/MAPK pathways, but not with β-catenin mutation or activation. Altogether, the present data indicate that SKP2 cooperates with N-Ras and AKT proto-oncogenes to promote hepatocarcinogenesis in vivo.

Colorectal Carcinogenesis, Radiation Quality, and the Ubiquitin-Proteasome Pathway

Adult colorectal epithelium undergoes continuous renewal and maintains homeostatic balance through regulated cellular proliferation, differentiation, and migration. The canonical Wnt signaling pathway involving the transcriptional co-activator β-catenin is important for colorectal development and normal epithelial maintenance, and deregulated Wnt/β-catenin signaling has been implicated in colorectal carcinogenesis. Colorectal carcinogenesis has been linked to radiation exposure, and radiation has been demonstrated to alter Wnt/β-catenin signaling, as well as the proteasomal pathway involved in the degradation of the signaling components and thus regulation of β-catenin. The current review discusses recent progresses in our understanding of colorectal carcinogenesis in relation to different types of radiation and roles that radiation quality plays in deregulating β-catenin and ubiquitin-proteasome pathway (UPP) for colorectal cancer initiation and progression.

Wnt/β-catenin signaling inhibits FBXW7 expression by upregulation of microRNA-770 in hepatocellular carcinoma

FBXW7 (F-box and WD repeat domain-containing 7) is the F-box protein component of a Skp1-Cul1-F-box protein-type (SCF-type) ubiquitin ligase. Previous studies have shown that FBXW7 serves as a tumor suppressor and is frequently downregulated in many types of human neoplasms. However, the molecular mechanisms for its downregulation remain poorly understood. Hyperactivation of Wnt/β-catenin signaling pathway is viewed as crucial for tumorigenesis, including hepatocellular carcinoma (HCC). In the present study, we show that protein levels, but not message RNA, of FBXW7 were suppressed by Wnt3a treatment or transfection of a constitutively activated β-catenin in HCC cells. Besides, microRNA-770 was identified as an important downstream target of Wnt/β-catenin signaling, to inhibit FBXW7 expression through targeting its 3'-untranslated region. Thus, our results suggest a previously unknown Wnt/β catenin-miR-770-FBXW7 molecular network in the HCC development.

UV Damage-Induced Phosphorylation of HBO1 Triggers CRL4DDB2-Mediated Degradation To Regulate Cell Proliferation

Histone acetyltransferase binding to ORC-1 (HBO1) is a critically important histone acetyltransferase for forming the prereplicative complex (pre-RC) at the replication origin. Pre-RC formation is completed by loading of the MCM2-7 heterohexameric complex, which functions as a helicase in DNA replication. HBO1 recruited to the replication origin by CDT1 acetylates histone H4 to relax the chromatin conformation and facilitates loading of the MCM complex onto replication origins. However, the acetylation status and mechanism of regulation of histone H3 at replication origins remain elusive. HBO1 positively regulates cell proliferation under normal cell growth conditions. Whether HBO1 regulates proliferation in response to DNA damage is poorly understood. In this study, we demonstrated that HBO1 was degraded after DNA damage to suppress cell proliferation. Ser50 and Ser53 of HBO1 were phosphorylated in an ATM/ATR DNA damage sensor-dependent manner after UV treatment. ATM/ATR-dependently phosphorylated HBO1 preferentially interacted with DDB2 and was ubiquitylated by CRL4(DDB2). Replacement of endogenous HBO1 in Ser50/53Ala mutants maintained acetylation of histone H3K14 and impaired cell cycle regulation in response to UV irradiation. Our findings demonstrate that HBO1 is one of the targets in the DNA damage checkpoint. These results show that ubiquitin-dependent control of the HBO1 protein contributes to cell survival during UV irradiation.

Regulation of Unperturbed DNA Replication by Ubiquitylation

Posttranslational modification of proteins by means of attachment of a small globular protein ubiquitin (i.e., ubiquitylation) represents one of the most abundant and versatile mechanisms of protein regulation employed by eukaryotic cells. Ubiquitylation influences almost every cellular process and its key role in coordination of the DNA damage response is well established. In this review we focus, however, on the ways ubiquitylation controls the process of unperturbed DNA replication. We summarise the accumulated knowledge showing the leading role of ubiquitin driven protein degradation in setting up conditions favourable for replication origin licensing and S-phase entry. Importantly, we also present the emerging major role of ubiquitylation in coordination of the active DNA replication process: preventing re-replication, regulating the progression of DNA replication forks, chromatin re-establishment and disassembly of the replisome at the termination of replication forks.

Reduced ubiquitin-specific protease 9X expression induced by RNA interference inhibits the bioactivity of hepatocellular carcinoma cells

Ubiquitin-specific protease 9X (USP9X) is crucial in many tumor types, but not in hepatocellular carcinoma (HCC). The current study aimed to examine the effects of RNA interference on USP9X expression, and subsequently on the bioactivity of HCC SMMC7721 and HepG2 cells. The protein expression of USP9X in SMMC7721, HepG2 and normal human liver cell line L02 at the cellular level was determined by western blot analysis; USP9X was knocked down by small interfering RNA (siRNA) in HCC SMMC7721 and HepG2 cells. In vitro cell viability was assessed by MTT assay, apoptosis was determined by flow cytometry (FCM) and cell migration was evaluated by Transwell assays. The protein expression of USP9X in SMMC7721 and HepG2 were both significantly higher than that in L02 (P<0.01). The results of western blot demonstrated that the USP9X-siRNA could efficiently inhibit USP9X expression when compared with that of the negative control (NC) group (P<0.01) and MTT assay demonstrated that cell proliferation in USP9X-blocked cells was significantly reduced when compared with that of the NC group (P<0.01). The results of FCM revealed that apoptosis was significantly increased in USP9X-blocked cells when compared with that of the NC group (P<0.01). The results of transwell assay showed that cell migration was significantly inhibited in USP9X-blocked cells when compared with that of the NC group (P<0.01). These results show that expression of USP9X is upregulated in hepatoma cells SMMC7721 and HepG2, and that downregulating USP9X by siRNA may induce cell apoptosis, inhibit cell growth and cell migration in the HCC SMMC7721 and HepG2 cell lines. USP9X may therefore be a potential target for HCC treatment and early detection.

Regulation of GATA-binding protein 2 levels via ubiquitin-dependent degradation by Fbw7: involvement of cyclin B-cyclin-dependent kinase 1-mediated phosphorylation of THR176 in GATA-binding protein 2

A GATA family transcription factor, GATA-binding protein 2 (GATA2), participates in cell growth and differentiation of various cells, such as hematopoietic stem cells. Although its expression level is controlled by transcriptional induction and proteolytic degradation, the responsible E3 ligase has not been identified. Here, we demonstrate that F-box/WD repeat-containing protein 7 (Fbw7/Fbxw7), a component of Skp1, Cullin 1, F-box-containing complex (SCF)-type E3 ligase, is an E3 ligase for GATA2. GATA2 contains a cell division control protein 4 (Cdc4) phosphodegron (CPD), a consensus motif for ubiquitylation by Fbw7, which includes Thr(176). Ectopic expression of Fbw7 destabilized GATA2 and promoted its proteasomal degradation. Substitution of threonine 176 to alanine in GATA2 inhibited binding with Fbw7, and the ubiquitylation and degradation of GATA2 by Fbw7 was suppressed. The CPD kinase, which mediates the phosphorylation of Thr(176), was cyclin B-cyclin-dependent kinase 1 (CDK1). Moreover, depletion of endogenous Fbw7 stabilized endogenous GATA2 in K562 cells. Conditional Fbw7 depletion in mice increased GATA2 levels in hematopoietic stem cells and myeloid progenitors at the early stage. Increased GATA2 levels in Fbw7-conditional knock-out mice were correlated with a decrease in a c-Kit high expressing population of myeloid progenitor cells. Our results suggest that Fbw7 is a bona fide E3 ubiquitin ligase for GATA2 in vivo.

The Importance of Ubiquitin E3 Ligases, SCF and APC/C, in Human Cancers

A normal evolution of the cell-cycle phases consists of multiple consecutive events, which makes it a highly complex process. Its preservation is regulated by Cyclin-Cdks (cyclin-dependent kinases) interactions and protein degradation, which is often controlled by the ubiquitin-mediated proteolysis.

The goal of this review is to emphasize the most important features of the regulation of the cell-cycle involved in cancerogenesis, by presenting the involvement of E3 ubiquitin ligases SCF (Skp1-Cul1-F-box protein) and APC/C (Anaphase-promoting complex/cyclosome) in human malignancies. Also, we discuss the importance of the ubiquitin proteasome pathway blockade in cancer treatment. We know that a better understanding of the regulatory biology of the cell cycle can lead to the development of new target therapies for cancer.