Selective enhancing effect of early mitotic inhibitor 1 (Emi1) depletion on the sensitivity of doxorubicin or X-ray treatment in human cancer cells

Inhibitors of Rho kinases (ROCK) induce multiple mitotic defects and synthetic lethality in BRCA2-deficient cells

The trapping of Poly-ADP-ribose polymerase (PARP) on DNA caused by PARP inhibitors (PARPi) triggers acute DNA replication stress and synthetic lethality (SL) in BRCA2-deficient cells. Hence, DNA damage is accepted as a prerequisite for SL in BRCA2-deficient cells. In contrast, here we show that inhibiting ROCK in BRCA2-deficient cells triggers SL independently from acute replication stress. Such SL is preceded by polyploidy and binucleation resulting from cytokinesis failure. Such initial mitosis abnormalities are followed by other M phase defects, including anaphase bridges and abnormal mitotic figures associated with multipolar spindles, supernumerary centrosomes and multinucleation. SL was also triggered by inhibiting Citron Rho-interacting kinase, another enzyme that, similarly to ROCK, regulates cytokinesis. Together, these observations demonstrate that cytokinesis failure triggers mitotic abnormalities and SL in BRCA2-deficient cells. Furthermore, the prevention of mitotic entry by depletion of Early mitotic inhibitor 1 (EMI1) augmented the survival of BRCA2-deficient cells treated with ROCK inhibitors, thus reinforcing the association between M phase and cell death in BRCA2-deficient cells. This novel SL differs from the one triggered by PARPi and uncovers mitosis as an Achilles heel of BRCA2-deficient cells.

The role of Fbxo5 in the development of human malignant tumors

Fbxo5 (F-Box only protein 5), as a substrate recognition subunit of SCF (SKP1-Cullin1-Fbox) protein, plays a crucial role in various cellular processes through ubiquitination and degradation of multiple proteins. In recent years, many studies have pointed out that Fbxo5 is critically involved in carcinogenesis. Moreover, targeting Fbxo5 could have a therapeutic potential for cancer therapy. This review focuses on the functions of Fbxo5 in various types of human malignancies and its underlying molecular mechanisms. This review might lay the foundation for enhancing future investigation on Fbxo5 functions in cancer development and progression.

Intricate Regulatory Mechanisms of the Anaphase-Promoting Complex/Cyclosome and Its Role in Chromatin Regulation

The ubiquitin (Ub)-proteasome system is vital to nearly every biological process in eukaryotes. Specifically, the conjugation of Ub to target proteins by Ub ligases, such as the Anaphase-Promoting Complex/Cyclosome (APC/C), is paramount for cell cycle transitions as it leads to the irreversible destruction of cell cycle regulators by the proteasome. Through this activity, the RING Ub ligase APC/C governs mitosis, G1, and numerous aspects of neurobiology. Pioneering cryo-EM, biochemical reconstitution, and cell-based studies have illuminated many aspects of the conformational dynamics of this large, multi-subunit complex and the sophisticated regulation of APC/C function. More recent studies have revealed new mechanisms that selectively dictate APC/C activity and explore additional pathways that are controlled by APC/C-mediated ubiquitination, including an intimate relationship with chromatin regulation. These tasks go beyond the traditional cell cycle role historically ascribed to the APC/C. Here, we review these novel findings, examine the mechanistic implications of APC/C regulation, and discuss the role of the APC/C in previously unappreciated signaling pathways.

RNF8 promotes high linear energy transfer carbon-ion-induced DNA double-stranded break repair in serum-starved human cells

The cell-killing effect of radiotherapy largely depends on unrepaired DNA double-stranded breaks (DSBs) or lethal chromosome aberrations induced by DSBs. Thus, the capability of DSB repair is critically important for the cancer-cell-killing effect of ionizing radiation. Here, we investigated the involvement of the DNA damage signaling factors ataxia telangiectasia mutated (ATM), ring finger protein 8 (RNF8), and RNF168 in quiescent G0/G1 cells, which are expressed in the majority of cell populations in tumors, after high linear energy transfer (LET) carbon-ion irradiation. Interestingly, ATM inhibition caused a substantial DSB repair defect after high-LET carbon-ion irradiation. Similarly, RNF8 or RNF168 depletion caused a substantial DSB repair defect. ATM inhibition did not exert an additive effect in RNF8-depleted cells, suggesting that ATM and RNF8 function in the same pathway. Importantly, we found that the RNF8 RING mutant showed a similar DSB repair defect, suggesting the requirement of ubiquitin ligase activity in this repair pathway. The RNF8 FHA domain was also required for DSB repair in this axis. Furthermore, the p53-binding protein 1 (53BP1), which is an important downstream factor in RNF8-dependent DSB repair, was also required for this repair. Importantly, either ATM inhibition or RNF8 depletion increased the frequency of chromosomal breaks, but reduced dicentric chromosome formation, demonstrating that ATM/RNF8 is required for the rejoining of DSB ends for the formation of dicentric chromosomes. Finally, we showed that RNF8 depletion augmented radiosensitivity after high-LET carbon-ion irradiation. This study suggests that the inhibition of RNF8 activity or its downstream pathway may augment the efficacy of high-LET carbon-ion therapy.

An upstream open reading frame regulates vasculogenic mimicry of glioma via ZNRD1-AS1/miR-499a-5p/ELF1/EMI1 pathway

Increasing evidence has suggested that gliomas can supply blood through vasculogenic mimicry. In this study, the expression and function of ZNRD1‐AS1‐144aa‐uORF (144aa‐uORF) and some non‐coding RNAs in gliomas were assessed. Real‐time quantitative PCR or Western blot was used to discover the expression of 144aa‐uORF, ZNRD1‐AS1, miR‐499a‐5p, ELF1 and EMI1 in gliomas. In addition, RIP and RNA pull‐down assays were applied to explore the interrelationship between 144aa‐uORF and ZNRD1‐AS1. The role of the 144aa‐uORF\ZNRD1‐AS1\miR‐499a‐5p\ELF1\EMI1 axis in vasculogenic mimicry formation of gliomas was analysed. This study illustrates the reduced expression of the 144aa‐uORF in glioma tissues and cells. Up‐regulation of 144aa‐uORF inhibits proliferation, migration, invasion and vasculogenic mimicry formation within glioma cells. The up‐regulated 144aa‐uORF can increase the degradation of ZNRD1‐AS1 through the nonsense‐mediated RNA decay (NMD) pathway. Knockdown of ZNRD1‐AS1 inhibits vasculogenic mimicry in glioma cells by modulating miR‐499a‐5p. At the same time, miR‐499a‐5p is down‐regulated and has a tumour‐suppressive effect in gliomas. In addition, ZNRD1‐AS1 serves as a competitive endogenous RNA (ceRNA) and regulates the expression of ELF1 by binding to miR‐499a‐5p. Notably, ELF1 binds to the promoter region of EMI1 and up‐regulates EMI1 expression, while simultaneously promoting vasculogenic mimicry in glioma cells. This study suggests that the 144aa‐uORF\ZNRD1‐AS1\miR‐499a‐5p\ELF1\EMI1 axis takes key part in regulating the formation of vasculogenic mimicry in gliomas and may provide a potential target for glioma treatment.

Emerging roles of F-box proteins in cancer drug resistance

Chemotherapy continues to be a major treatment strategy for various human malignancies. However, the frequent emergence of chemoresistance compromises chemotherapy efficacy leading to poor prognosis. Thus, overcoming drug resistance is pivotal to achieve enhanced therapy efficacy in various cancers. Although increased evidence has revealed that reduced drug uptake, increased drug efflux, drug target protein alterations, drug sequestration in organelles, enhanced drug metabolism, impaired DNA repair systems, and anti-apoptotic mechanisms, are critically involved in drug resistance, the detailed resistance mechanisms have not been fully elucidated in distinct cancers. Recently, F-box protein (FBPs), key subunits in Skp1-Cullin1-F-box protein (SCF) E3 ligase complexes, have been found to play critical roles in carcinogenesis, tumor progression, and drug resistance through degradation of their downstream substrates. Therefore, in this review, we describe the functions of FBPs that are involved in drug resistance and discuss how FBPs contribute to the development of cancer drug resistance. Furthermore, we propose that targeting FBPs might be a promising strategy to overcome drug resistance and achieve better treatment outcome in cancer patients. Lastly, we state the limitations and challenges of using FBPs to overcome chemotherapeutic drug resistance in various cancers.

The Identification of Potential Therapeutic Targets for Cutaneous Squamous Cell Carcinoma

We performed a small interfering RNA screen to identify targets for cutaneous squamous cell carcinoma (cSCC) therapy in the ubiquitin/ubiquitin-like system. We provide evidence for selective anti-cSCC activity of knockdown of the E3 ubiquitin ligase MARCH4, the ATPase p97/VCP, the deubiquitinating enzyme USP8, the cullin-RING ligase (CRL) 4 substrate receptor CDT2/DTL, and components of the anaphase-promoting complex/cyclosome (APC/C). Specifically attenuating CRL4^CDT2 by CDT2 knockdown can be more potent in killing cSCC cells than targeting CRLs or CRL4s in general by RBX1 or DDB1 depletion. Suppression of the APC/C or forced APC/C activation by targeting its repressor EMI1 are both potential therapeutic approaches. We observed that cSCC cells can be selectively killed by small-molecule inhibitors of USP8 (DUBs-IN-3/compound 22c) and the NEDD8 E1 activating enzyme/CRLs (MLN4924/pevonedistat). A substantial proportion of cSCC cell lines are very highly MLN4924-sensitive. Pathways that respond to defects in proteostasis are involved in the anti-cSCC activity of p97 suppression. Targeting USP8 can reduce the expression of growth factor receptors that participate in cSCC development. EMI1 and CDT2 depletion can selectively cause DNA re-replication and DNA damage in cSCC cells.

The Anaphase Promoting Complex/Cyclosome (APC/C): A Versatile E3 Ubiquitin Ligase

In the present chapter we discuss the essential roles of the human E3 ubiquitin ligase Anaphase Promoting Complex/Cyclosome (APC/C) in mitosis as well as the emerging evidence of important APC/C roles in cellular processes beyond cell division control such as regulation of genomic integrity and cell differentiation of the nervous system. We consider the potential incipient role of APC/C dysregulation in the pathophysiology of the neurological disorder Alzheimer's disease (AD). We also discuss how certain Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA) viruses take control of the host's cell division regulatory system through harnessing APC/C ubiquitin ligase activity and hypothesise the plausible molecular mechanisms underpinning virus manipulation of the APC/C. We also examine how defects in the function of this multisubunit protein assembly drive abnormal cell proliferation and lastly argue the potential of APC/C as a promising therapeutic target for the development of innovative therapies for the treatment of chronic malignancies such as cancer.

The F-Box Domain-Dependent Activity of EMI1 Regulates PARPi Sensitivity in Triple-Negative Breast Cancers

The BRCA1-BRCA2-RAD51 axis is essential for homologous recombination repair (HRR) and is frequently disrupted in breast cancers. PARP inhibitors (PARPis) are used clinically to treat BRCA-mutated breast tumors. Using a genetic screen, we identified EMI1 as a modulator of PARPi sensitivity in triple-negative breast cancer (TNBC) cells. This function requires the F-box domain of EMI1, through which EMI1 assembles a canonical SCF ubiquitin ligase complex that constitutively targets RAD51 for degradation. In response to genotoxic stress, CHK1-mediated phosphorylation of RAD51 counteracts EMI1-dependent degradation by enhancing RAD51's affinity for BRCA2, leading to RAD51 accumulation. Inhibition of RAD51 degradation restores HRR in BRCA1-depleted cells. Human breast cancer samples display an inverse correlation between EMI1 and RAD51 protein levels. A subset of BRCA1-deficient TNBC cells develop resistance to PARPi by downregulating EMI1 and restoring RAD51-dependent HRR. Notably, reconstitution of EMI1 expression reestablishes PARPi sensitivity both in cellular systems and in an orthotopic mouse model.

Identification of aberrantly expressed F-box proteins in squamous-cell lung carcinoma

PURPOSE

F-box proteins, as components of the Skp1-Cullin 1-F-box protein (SCF) E3 ubiquitin ligase, can specifically bind to substrates and regulate multiple tumor behaviors. However, the role of F-box proteins in squamous-cell lung carcinoma (SqCLC) has not been established.

METHODS

We identified the differentially expressed F-box protein-encoding genes in SqCLC by analyzing data from The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) databases. Prognosis data were evaluated using the Kaplan-Meier (KM) plotter website. The FBXO5 and FBXO45 mRNA levels were analyzed by real time RT-PCR. The impact of the inhibition of these genes with si-RNA on apoptosis and migration was also investigated.

RESULTS

The FBXO45 and FBXO5 genes were significantly up-regulated in SqCLC compared with normal lung (p values = 0.002 and 0.025, respectively). FBXO45 was significantly elevated in each tumorigenic step, including dysplasia, in situ and SqCLC. The RT-PCR analysis results showed that FBXO5 and FBXO45 were elevated in cancer tissues (p values = 0.024 and 0.004, respectively). Overexpression of FBXO5 and FBXO45 was associated with shorter overall survival (OS) in the SqCLC patients from the K-M plotter database [FBXO5 HR: 1.53 (1.03-2.28), p = 0.036]; [FBXO45 HR: 1.47 (1.03-2.08), p = 0.030]. The GO and KEGG pathway analysis showed that FBXO5 and FBXO45 were associated with cell cycle and adhesion, respectively. Knockdown of FBXO5 leads to increased apoptosis, while knockdown of FBXO45 facilitates the process of epithelial-mesenchymal transition (EMT).

CONCLUSIONS

Our results provide evidence that FBXO45 and FBXO5 may play a key role in tumorigenesis and prognosis of SqCLC.

F-box proteins involved in cancer-associated drug resistance

The ubiquitin proteasome system (UPS) regulated human biological processes through the appropriate and efficient proteolysis of cellular proteins. F-box proteins are the vital components of SKP1-CUL1-FBP (SCF)-type E3 ubiquitin ligases that determine substrate specificity. As F-box proteins have the ability to control the degradation of several crucial protein targets associated with drug resistance, the dysregulation of these proteins may lead to induction of chemoresistance in cancer cells. Chemotherapy is one of the most conventional therapeutic approaches of treatment of patients with cancer. However, its exclusive application in clinical settings is restricted due to the development of chemoresistance, which typically results treatment failure. Therefore, overcoming drug resistance is considered as one of the most critical issues that researchers and clinician associated with oncology face. The present review serves to provide a comprehensive overview of F-box proteins and their possible targets as well as their correlation with the chemoresistance and chemosensitization of cancer cells. The article also presents an integrated representation of the complex regulatory mechanisms responsible for chemoresistance, which may lay the foundation to explore sensible candidate drugs for therapeutic intervention.

Inhibition of Anaphase-Promoting Complex by Silence APC/CCdh1 to Enhance Radiosensitivity of Nasopharyngeal Carcinoma Cells

The aim of this study was to investigate the possibility of APC/C^Cdh1 as a potential therapeutic target in the radiosensitivity of nasopharyngeal carcinoma (NPC) cell CNE-1, and explain the role of APC subunits after silence of Cdh1 combined with radiotherapy. Transfection with Cdh1 shRNA significantly increased the radiosensitivity of CNE-1 cells and the radiation enhancement ratio (RER) of sh-Cdh1 cells was 1.76. Knockdown of Cdh1 in CNE-1 cells increased irradiation induced apoptosis and G2/M phase cell cycle arrest. The levels of CDC20 and CylinB1 increased and the levels of Ku70 and APC3 decreased after irradiation. APC/C^Cdh1 is involved in regulation of radiosensitivity in human NPC CNE-1 cells. Our study may provide a promising therapeutic strategy for NPC by targeting Cdh1. J. Cell. Biochem. 118: 3150-3157, 2017. © 2016 Wiley Periodicals, Inc.

Ameloblastin induces tumor suppressive phenotype and enhances chemosensitivity to doxorubicin via Src-Stat3 inactivation in osteosarcoma

Ameloblastin (AMBN), the most abundant non-amelogenin enamel matrix protein, plays a role in ameloblast differentiation. Previously, we found that AMBN promoted osteogenic differentiation via the interaction between CD63 and integrin β1, leading to the inactivation of Src; however, how AMBN affects the malignant behavior of osteosarcoma is still unclear. Osteosarcoma affects the bone and is associated with poor prognosis because of the high rate of pulmonary metastases and drug resistance. Here we demonstrated that stable overexpression of AMBN induced apoptosis and suppressed colony formation and cell migration via the inactivation of Src-Stat3 pathway in human osteosarcoma cells. Moreover, AMBN induced chemosensitivity to doxorubicin. Thus, AMBN induced a tumor suppressive phenotype and chemosensitivity to doxorubicin via the AMBN-Src-Stat3 axis in osteosarcoma. Indeed, immunohistochemical expression of AMBN was significantly correlated with better outcome of osteosarcoma patients. Our findings suggest that AMBN can be a new prognostic marker and therapeutic target for osteosarcoma combined with conventional doxorubicin treatment.

Topoisomerase 2 Alpha Cooperates with Androgen Receptor to Contribute to Prostate Cancer Progression

Overexpression of TOP2A is associated with risk of systemic progression in prostate cancer patients, and higher levels of TOP2A were found in hormone-resistant cases. To elucidate the mechanism by which high levels of TOP2A contribute to tumor progression we generated TOP2A overexpressing prostate cancer cell lines. We show that TOP2A promotes tumor aggressiveness by inducing chromosomal rearrangements of genes that contribute to a more invasive phenotype. Anti-androgen treatment alone was ineffective in killing TOP2A overexpressing cells due to activation of an androgen receptor network. TOP2A poisons killed tumor cells more efficiently early in the progression course, while at later stages they provided greater benefit when combined with anti-androgen therapy. Mechanistically, we find that TOP2A enhances androgen signaling by facilitating transcription of androgen responsive genes, thereby promoting tumor cell growth. These studies revealed a relationship between TOP2A and androgen receptor signaling pathway that contributes to prostate cancer progression and confers sensitivity to treatments.

Ectopic expression of cancer/testis antigen SSX2 induces DNA damage and promotes genomic instability

SSX cancer/testis antigens are frequently expressed in melanoma tumors and represent attractive targets for immunotherapy, but their role in melanoma tumorigenesis has remained elusive. Here, we investigated the cellular effects of SSX2 expression. In A375 melanoma cells, SSX2 expression resulted in an increased DNA content and enlargement of cell nuclei, suggestive of replication aberrations. The cells further displayed signs of DNA damage and genomic instability, associated with p53-mediated G1 cell cycle arrest and a late apoptotic response. These results suggest a model wherein SSX2-mediated replication stress translates into mitotic defects and genomic instability. Arrest of cell growth and induction of DNA double-strand breaks was also observed in MCF7 breast cancer cells in response to SSX2 expression. Additionally, MCF7 cells with ectopic SSX2 expression demonstrated typical signs of senescence (i.e. an irregular and enlarged cell shape, enhanced β-galactosidase activity and DNA double-strand breaks). Since replication defects, DNA damage and senescence are interconnected and well-documented effects of oncogene expression, we tested the oncogenic potential of SSX2. Importantly, knockdown of SSX2 expression in melanoma cell lines demonstrated that SSX2 supports the growth of melanoma cells. Our results reveal two important phenotypes of ectopic SSX2 expression that may drive/support tumorigenesis: First, immediate induction of genomic instability, and second, long-term support of tumor cell growth.