Cyclin F suppresses B-Myb activity to promote cell cycle checkpoint control

Eugenol as a potential adjuvant therapy for gingival squamous cell carcinoma

Adoption of plant-derived compounds for the management of oral cancer is encouraged by the scientific community due to emerging chemoresistance and conventional treatments adverse effects. Considering that very few studies investigated eugenol clinical relevance for gingival carcinoma, we ought to explore its selectivity and performance according to aggressiveness level. For this purpose, non-oncogenic human oral epithelial cells (GMSM-K) were used together with the Tongue (SCC-9) and Gingival (Ca9-22) squamous cell carcinoma lines to assess key tumorigenesis processes. Overall, eugenol inhibited cell proliferation and colony formation while inducing cytotoxicity in cancer cells as compared to normal counterparts. The recorded effect was greater in gingival carcinoma and appears to be mediated through apoptosis induction and promotion of p21/p27/cyclin D1 modulation and subsequent Ca9-22 cell cycle arrest at the G0/G1 phase, in a p53-independent manner. At these levels, distinct genetic profiles were uncovered for both cell lines by QPCR array. Moreover, it seems that our active component limited Ca9-22 and SCC-9 cell migration respectively through MMP1/3 downregulation and stimulation of inactive MMPs complex formation. Finally, Ca9-22 behaviour appears to be mainly modulated by the P38/STAT5/NFkB pathways. In summary, we can disclose that eugenol is cancer selective and that its mediated anti-cancer mechanisms vary according to the cell line with gingival squamous cell carcinoma being more sensitive to this phytotherapy agent.

B-Myb deficiency boosts bortezomib-induced immunogenic cell death in colorectal cancer

B-Myb has received considerable attention for its critical tumorigenic function of supporting DNA repair. However, its modulatory effects on chemotherapy and immunotherapy have rarely been reported in colorectal cancer. Bortezomib (BTZ) is a novel compound with chemotherapeutic and immunotherapeutic effects, but it fails to work in colorectal cancer with high B-Myb expression. The present study was designed to investigate whether B-Myb deletion in colorectal cancer could potentiate the immune efficacy of BTZ against colorectal cancer and to clarify the underlying mechanism. Stable B-Myb knockdown was induced in colorectal cancer cells, which increased apoptosis of the cancer cells relative to the control group in vitro and in vivo. We found that BTZ exhibited more favourable efficacy in B-Myb-defective colorectal cancer cells and tumor-bearing mice. BTZ treatment led to differential expression of genes enriched in the p53 signaling pathway promoted more powerful downstream DNA damage, and arrested cell cycle in B-Myb-defective colorectal cancer. In contrast, recovery of B-Myb in B-Myb-defective colorectal cancer cells abated BTZ-related DNA damage, cell cycle arrest, and anticancer efficacy. Moreover, BTZ promoted DNA damage-associated enhancement of immunogenicity, as indicated by potentiated expression of HMGB1 and HSP90 in B-Myb-defective cells, thereby driving M1 polarization of macrophages. Collectively, B-Myb deletion in colorectal cancer facilitates the immunogenic death of cancer cells, thereby further promoting the immune efficacy of BTZ by amplifying DNA damage. The present work provides an effective molecular target for colorectal cancer immunotherapy with BTZ.

Cyclin F, Neurodegeneration, and the Pathogenesis of ALS/FTD

Amyotrophic lateral sclerosis (ALS) is the most common form of motor neuron disease and is characterized by the degeneration of upper and lower motor neurons of the brain and spinal cord. ALS is also linked clinically, genetically, and pathologically to a form of dementia known as frontotemporal dementia (FTD). Identifying gene mutations that cause ALS/FTD has provided valuable insight into the disease process. Several ALS/FTD-causing mutations occur within proteins with roles in protein clearance systems. This includes ALS/FTD mutations in CCNF, which encodes the protein cyclin F: a component of a multiprotein E3 ubiquitin ligase that mediates the ubiquitylation of substrates for their timely degradation. In this review, we provide an update on the link between ALS/FTD CCNF mutations and neurodegeneration.

Keratin 17 Impacts Global Gene Expression and Controls G2/M Cell Cycle Transition in Ionizing Radiation-Induced Skin Damage

Keratin 17 (K17) is a cytoskeletal protein that is part of the intermediate filaments in epidermal keratinocytes. In K17-/- mice, ionizing radiation induced more severe hair follicle damage, whereas the epidermal inflammatory response was attenuated compared with that in wild-type mice. Both p53 and K17 have a major impact on global gene expression because over 70% of the differentially expressed genes in the skin of wild-type mice showed no expression change in p53-/- or K17-/- skin after ionizing radiation. K17 does not interfere with the dynamics of p53 activation; rather, global p53 binding in the genome is altered in K17-/- mice. The absence of K17 leads to aberrant cell cycle progression and mitotic catastrophe in epidermal keratinocytes, which is due to nuclear retention, thus reducing the degradation of B-Myb, a key regulator of the G2/M cell cycle transition. These results expand our understanding of the role of K17 in regulating global gene expression and ionizing radiation-induced skin damage.

Perfluorohexanesulfonic acid (PFHxS) induces oxidative stress and causes developmental toxicities in zebrafish embryos

Perfluorohexanesulfonic acid (PFHxS) is a short-chain perfluoroalkyl substance widely used to replace the banned perfluorooctanesulfonic acid (PFOS) in different industrial and household products. It has currently been identified in the environment and human bodies; nonetheless, the possible toxicities are not well-known. Zebrafish have been used as a toxicant screening model due to their fast and transparent developmental processes. In this study, zebrafish embryos were exposed to PFHxS for five days, and various experiments were performed to monitor the developmental and cellular processes. Liquid chromatography-mass spectrometry (LC/MS) analysis confirmed that PFHxS was absorbed and accumulated in the zebrafish embryos. We reported that 2.5 µM or higher PFHxS exposure induced phenotypic abnormalities, marked by developmental delay in the mid-hind brain boundary and yolk sac edema. Additionally, larvae exposed to PFHxS displayed facial malformation due to the reduction of neural crest cell expression. RNA sequencing analysis further identified 4643 differentiated expressed transcripts in 5 µM PFHxS-exposed 5-days post fertilization (5-dpf) larvae. Bioinformatics analysis revealed that glucose metabolism, lipid metabolism, as well as oxidative stress were enriched in the PFHxS-exposed larvae. To validate these findings, a series of biological experiments were conducted. PFHxS exposure led to a nearly 4-fold increase in reactive oxygen species, possibly due to hyperglycemia and impaired glutathione balance. The Oil Red O' staining and qPCR analysis strengthens the notions that lipid metabolism was disrupted, leading to lipid accumulation, lipid peroxidation, and malondialdehyde formation. All these alterations ultimately affected cell cycle events, resulting in S and G2/M cell cycle arrest. In conclusion, our study demonstrated that PFHxS could accumulate and induce various developmental toxicities in aquatic life, and such data might assist the government to accelerate the regulatory policy on PFHxS usage.

The E3 Ubiquitin Ligase SCF Cyclin F Promotes Sequestosome-1/p62 Insolubility and Foci Formation and is Dysregulated in ALS and FTD Pathogenesis

Amyotrophic lateral sclerosis (ALS)- and frontotemporal dementia (FTD)-linked mutations in CCNF have been shown to cause dysregulation to protein homeostasis. CCNF encodes for cyclin F, which is part of the cyclin F-E3 ligase complex SCFcyclinF known to ubiquitylate substrates for proteasomal degradation. In this study, we identified a function of cyclin F to regulate substrate solubility and show how cyclin F mechanistically underlies ALS and FTD disease pathogenesis. We demonstrated that ALS and FTD-associated protein sequestosome-1/p62 (p62) was a canonical substrate of cyclin F which was ubiquitylated by the SCFcyclinF complex. We found that SCFcyclin F ubiquitylated p62 at lysine(K)281, and that K281 regulated the propensity of p62 to aggregate. Further, cyclin F expression promoted the aggregation of p62 into the insoluble fraction, which corresponded to an increased number of p62 foci. Notably, ALS and FTD-linked mutant cyclin F p.S621G aberrantly ubiquitylated p62, dysregulated p62 solubility in neuronal-like cells, patient-derived fibroblasts and induced pluripotent stem cells and dysregulated p62 foci formation. Consistently, motor neurons from patient spinal cord tissue exhibited increased p62 ubiquitylation. We suggest that the p.S621G mutation impairs the functions of cyclin F to promote p62 foci formation and shift p62 into the insoluble fraction, which may be associated to aberrant mutant cyclin F-mediated ubiquitylation of p62. Given that p62 dysregulation is common across the ALS and FTD spectrum, our study provides insights into p62 regulation and demonstrates that ALS and FTD-linked cyclin F mutant p.S621G can drive p62 pathogenesis associated with ALS and FTD.

Application of Novel Transcription Factor Machine Learning Model and Targeted Drug Combination Therapy Strategy in Triple Negative Breast Cancer

Transcription factors (TFs) have been shown to play a key role in the occurrence and development of tumors, including triple-negative breast cancer (TNBC), with a worse prognosis. Machine learning is widely used for establishing prediction models and screening key tumor drivers. Current studies lack TF integration in TNBC, so targeted research on TF prognostic models and targeted drugs is beneficial to improve clinical translational application. The purpose of this study was to use the Least Absolute Shrinkage and Selection Operator to build a prognostic TFs model after cohort normalization based on housekeeping gene expression levels. Potential targeted drugs were then screened on the basis of molecular docking, and a multi-drug combination strategy was used for both in vivo and in vitro experimental studies. The machine learning model of TFs built by E2F8, FOXM1, and MYBL2 has broad applicability, with an AUC value of up to 0.877 at one year. As a high-risk clinical factor, its abnormal disorder may lead to upregulation of the activity of pathways related to cell proliferation. This model can also be used to predict the adverse effects of immunotherapy in patients with TNBC. Molecular docking was used to screen three drugs that target TFs: Trichostatin A (TSA), Doxorubicin (DOX), and Calcitriol. In vitro and in vivo experiments showed that TSA + DOX was able to effectively reduce DOX dosage, and TSA + DOX + Calcitriol may be able to effectively reduce the toxic side effects of DOX on the heart. In conclusion, the machine learning model based on three TFs provides new biomarkers for clinical and prognostic diagnosis of TNBC, and the combination targeted drug strategy offers a novel research perspective for TNBC treatment.

MEKs/ERKs-mediated FBXO1/E2Fs interaction interference modulates G(1)/S cell cycle transition and cancer cell proliferation

E2F 1, 2, and 3a, (refer to as E2Fs) are a subfamily of E2F transcription factor family that play essential roles in cell-cycle progression, DNA replication, DNA repair, apoptosis, and differentiation. Although the transcriptional regulation of E2Fs has focused on pocket protein retinoblastoma protein complex, recent studies indicate that post-translational modification and stability regulation of E2Fs play key roles in diverse cellular processes. In this study, we found that FBXO1, a component of S-phase kinase-associated protein 1 (SKP1)-cullin 1-F-box protein (SCF) complex, is an E2Fs binding partner. Furthermore, FBXO1 to E2Fs binding induced K48 ubiquitination and subsequent proteasomal degradation of E2Fs. Binding domain analysis indicated that the Arg (R)/Ile (I) and R/Val (V) motifs, which are located in the dimerization domain of E2Fs, of E2F 1 and 3a and E2F2, respectively, acted as degron motifs (DMs) for FBXO1. Notably, RI/AA or RV/AA mutation in the DMs reduced FBXO1-mediated ubiquitination and prolonged the half-lives of E2Fs. Importantly, the stabilities of E2Fs were affected by phosphorylation of threonine residues located near RI and RV residues of DMs. Phosphorylation prediction database analysis and specific inhibitor analysis revealed that MEK/ERK signaling molecules play key roles in FBXO1/E2Fs' interaction and modulate E2F protein turnover. Moreover, both elevated E2Fs protein levels by knockdown of FBXO1 and decreased E2Fs protein levels by sh-E2F3a delayed G₁/S cell cycle transition, resulting in inhibition of cancer cell proliferation. These results demonstrated that FBXO1-E2Fs axis-mediated precise E2Fs stability regulation plays a key role in cell proliferation via G₁/S cell cycle transition.

MYBL2 regulates de novo purine synthesis by transcriptionally activating IMPDH1 in hepatocellular carcinoma cells

BACKGROUND

Metabolic reprogramming is a hallmark of cancer, alteration of nucleotide metabolism of hepatocellular carcinoma (HCC) is not well-understood. MYBL2 regulates cell cycle progression and hepatocarcinogenesis, its role in metabolic regulation remains elusive.

PATIENTS AND METHODS

Copy number, mRNA and protein level of MYBL2 and IMPDH1 were analyzed in HCC, and correlated with patient survival. Chromatin Immunoprecipitation sequencing (Chip-seq) and Chromatin Immunoprecipitation quantitative polymerase chain reaction (ChIP-qPCR) were used to explore the relationship between MYBL2 and IMPDH1. Metabolomics were used to analyze how MYBL2 affected purine metabolism. The regulating effect of MYBL2 in HCC was further validated in vivo using xenograft models.

RESULTS

The Results showed that copy-number alterations of MYBL2 occur in about 10% of human HCC. Expression of MYBL2, IMPDH1, or combination of both were significantly upregulated and associated with poor prognosis in HCC. Correlation, ChIP-seq and ChIP-qPCR analysis revealed that MYBL2 activates transcription of IMPDH1, while knock-out of MYBL2 retarded IMPDH1 expression and inhibited proliferation of HCC cells. Metabolomic analysis post knocking-out of MYBL2 demonstrated that it was essential in de novo purine synthesis, especially guanine nucleotides. In vivo analysis using xenograft tumors also revealed MYBL2 regulated purine synthesis by regulating IMPDH1, and thus, influencing tumor progression.

CONCLUSION

MYBL2 is a key regulator of purine synthesis and promotes HCC progression by transcriptionally activating IMPDH1, it could be a potential candidate for targeted therapy for HCC.

Deep transfer learning of cancer drug responses by integrating bulk and single-cell RNA-seq data

Drug screening data from massive bulk gene expression databases can be analyzed to determine the optimal clinical application of cancer drugs. The growing amount of single-cell RNA sequencing (scRNA-seq) data also provides insights into improving therapeutic effectiveness by helping to study the heterogeneity of drug responses for cancer cell subpopulations. Developing computational approaches to predict and interpret cancer drug response in single-cell data collected from clinical samples can be very useful. We propose scDEAL, a deep transfer learning framework for cancer drug response prediction at the single-cell level by integrating large-scale bulk cell-line data. The highlight in scDEAL involves harmonizing drug-related bulk RNA-seq data with scRNA-seq data and transferring the model trained on bulk RNA-seq data to predict drug responses in scRNA-seq. Another feature of scDEAL is the integrated gradient feature interpretation to infer the signature genes of drug resistance mechanisms. We benchmark scDEAL on six scRNA-seq datasets and demonstrate its model interpretability via three case studies focusing on drug response label prediction, gene signature identification, and pseudotime analysis. We believe that scDEAL could help study cell reprogramming, drug selection, and repurposing for improving therapeutic efficacy.

Circular RNA in multiple myeloma: A new target for therapeutic intervention

Circular RNAs (circRNAs) are RNA molecules with a stable closed-loop structure that are found in a variety of organisms. CircRNAs are highly stable and conserved, and they play important roles in transcriptional regulation and splicing. Multiple Myeloma (MM) is a malignant proliferative disease for which there are currently no effective and comprehensive treatments. Numerous circRNAs may contribute to the development and progression of MM by acting as oncogenes or regulators. Due to the unique function of circRNAs, they have a high potential for regulating the biological functions (including proliferation and apoptosis) of MM cells, and their expression levels and molecular mechanism are closely related to their diagnostic value, therapeutic sensitivity, and clinical prognosis of MM patients. In this review, we aim to provide a detailed overview of the structure and function of circRNAs and demonstrate the potential therapeutic value and potential mechanism of circRNAs in MM via experiments and clinical trials.

Cancer cells use self-inflicted DNA breaks to evade growth limits imposed by genotoxic stress

Genotoxic therapy such as radiation serves as a frontline cancer treatment, yet acquired resistance that leads to tumor reoccurrence is frequent. We found that cancer cells maintain viability during irradiation by reversibly increasing genome-wide DNA breaks, thereby limiting premature mitotic progression. We identify caspase-activated DNase (CAD) as the nuclease inflicting these de novo DNA lesions at defined loci, which are in proximity to chromatin-modifying CCCTC-binding factor (CTCF) sites. CAD nuclease activity is governed through phosphorylation by DNA damage response kinases, independent of caspase activity. In turn, loss of CAD activity impairs cell fate decisions, rendering cancer cells vulnerable to radiation-induced DNA double-strand breaks. Our observations highlight a cancer-selective survival adaptation, whereby tumor cells deploy regulated DNA breaks to delimit the detrimental effects of therapy-evoked DNA damage.

Cyclin F drives proliferation through SCF-dependent degradation of the retinoblastoma-like tumor suppressor p130/RBL2

Cell cycle gene expression programs fuel proliferation and are universally dysregulated in cancer. The retinoblastoma (RB)-family of proteins, RB1, RBL1/p107, and RBL2/p130, coordinately represses cell cycle gene expression, inhibiting proliferation, and suppressing tumorigenesis. Phosphorylation of RB-family proteins by cyclin-dependent kinases is firmly established. Like phosphorylation, ubiquitination is essential to cell cycle control, and numerous proliferative regulators, tumor suppressors, and oncoproteins are ubiquitinated. However, little is known about the role of ubiquitin signaling in controlling RB-family proteins. A systems genetics analysis of CRISPR/Cas9 screens suggested the potential regulation of the RB-network by cyclin F, a substrate recognition receptor for the SCF family of E3 ligases. We demonstrate that RBL2/p130 is a direct substrate of SCF^{cyclin F}. We map a cyclin F regulatory site to a flexible linker in the p130 pocket domain, and show that this site mediates binding, stability, and ubiquitination. Expression of a mutant version of p130, which cannot be ubiquitinated, severely impaired proliferative capacity and cell cycle progression. Consistently, we observed reduced expression of cell cycle gene transcripts, as well a reduced abundance of cell cycle proteins, analyzed by quantitative, iterative immunofluorescent imaging. These data suggest a key role for SCF^{cyclin F} in the CDK-RB network and raise the possibility that aberrant p130 degradation could dysregulate the cell cycle in human cancers.

A Novel Signature of CCNF-Associated E3 Ligases Collaborate and Counter Each Other in Breast Cancer

Simple Summary

The dysregulation of UPS exacerbates the tumor microenvironment and drives malignant transformation. As the largest family of E3 ligases, the SCF^F-boxes promotes BRCA progression. FBXL8 was recently identified to be a novel SCF E3 ligase that potently promotes BRCA. Here, we profiled the transcriptome of BRCA patient tissues by global NGS RNA-Seq and TCGA database analyses. A signature of four SCF^F-box E3 ligases (FBXL8, FBXO43, FBXO15, CCNF) was found to be pivotal for BRCA advancement. Knockdown of FBXL8 and FBXO43 reduced cancer cell viability and proliferation, suggesting their pro-tumorigenic roles. However, the overexpression of CCNF inhibited cancer cell progression, indicating its anti-tumorigenic role. FBXL8 and FZR1 pulled down CCNF, and double knockdown of FBXL8 and FZR1 caused CCNF accumulation. Additionally, CCNF partnered with a pro-tumorigenic factor, RRM2, and overexpression of CCNF reduced RRM2. Our findings suggest a potential for drugging CCNF in co-modulatory partnership with FBXL8 and FZR1, for anti-BRCA therapy.

Abstract

Breast cancer (BRCA) malignancy causes major fatalities amongst women worldwide. SCF (Skp1-cullin-F-box proteins) E3 ubiquitin ligases are the most well-known members of the ubiquitination–proteasome system (UPS), which promotes cancer initiation and progression. Recently, we demonstrated that FBXL8, a novel F-box protein (SCF^F-boxes) of SCF E3 ligase, accelerates BRCA advancement and metastasis. Since SCF^F-boxes is a key component of E3 ligases, we hypothesized that other SCF^F-boxes besides FBXL8 probably collaborate in regulating breast carcinogenesis. In this study, we retrospectively profiled the transcriptome of BRCA tissues and found a notable upregulation of four SCF^F-box E3 ligases (FBXL8, FBXO43, FBXO15, and CCNF) in the carcinoma tissues. Similar to FBXL8, the knockdown of FBXO43 reduced cancer cell viability and proliferation, suggesting its pro-tumorigenic role. The overexpression of CCNF inhibited cancer cell progression, indicating its anti-tumorigenic role. Unexpectedly, CCNF protein was markedly downregulated in BRCA tissues, although its mRNA level was high. We showed that both E3 ligases, FBXL8 and FZR1, pulled down CCNF. Double knockdown of FBXL8 and FZR1 caused CCNF accumulation. On the other hand, CCNF itself pulled down a tumorigenic factor, RRM2, and CCNF overexpression reduced RRM2. Altogether, we propose a signature network of E3 ligases that collaboratively modulates CCNF anti-cancer activity. There is potential to target BRCA through modulation of the partnership axes of (i) CCNF-FBXL8, (ii) CCNF-FZR1, and (iii) CCNF-RRM2, particularly, via CCNF overexpression and activation and FBXL8/FZR1 suppression.

ALS/FTD-causing mutation in cyclin F causes the dysregulation of SFPQ

Previously, we identified missense mutations in CCNF that are causative of familial and sporadic amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Hallmark features of these diseases include the build-up of insoluble protein aggregates as well as the mislocalization of proteins such as transactive response DNA binding protein 43 kDa (TDP-43). In recent years, the dysregulation of SFPQ (splicing factor proline and glutamine rich) has also emerged as a pathological hallmark of ALS/FTD. CCNF encodes for the protein cyclin F, a substrate recognition component of an E3 ubiquitin ligase. We have previously shown that ALS/FTD-linked mutations in CCNF cause disruptions to overall protein homeostasis that leads to a build-up of K48-linked ubiquitylated proteins as well as defects in autophagic machinery. To investigate further processes that may be affected by cyclin F, we used a protein-proximity ligation method, known as Biotin Identification (BioID), standard immunoprecipitations and mass spectrometry to identify novel interaction partners of cyclin F and infer further process that may be affected by the ALS/FTD-causing mutation. Results demonstrate that cyclin F closely associates with proteins involved with RNA metabolism as well as a number of RNA-binding proteins previously linked to ALS/FTD, including SFPQ. Notably, the overexpression of cyclin F(S621G) led to the aggregation and altered subcellular distribution of SFPQ in human embryonic kidney (HEK293) cells, while leading to altered degradation in primary neurons. Overall, our data links ALS/FTD-causing mutations in CCNF to converging pathological features of ALS/FTD and provides a link between defective protein degradation systems and the pathological accumulation of a protein involved in RNA processing and metabolism.

MMB-FOXM1-driven premature mitosis is required for CHK1 inhibitor sensitivity

To identify genes whose loss confers resistance to CHK1 inhibitors, we perform genome-wide CRISPR-Cas9 screens in non-small-cell lung cancer (NSCLC) cell lines treated with the CHK1 inhibitor prexasertib (CHK1i). Five of the top six hits of the screens, MYBL2 (B-MYB), LIN54, FOXM1, cyclin A2 (CCNA2), and CDC25B, are cell-cycle-regulated genes that contribute to entry into mitosis. Knockout of MMB-FOXM1 complex components LIN54 and FOXM1 reduce CHK1i-induced DNA replication stress markers and premature mitosis during Late S phase. Activation of a feedback loop between the MMB-FOXM1 complex and CDK1 is required for CHK1i-induced premature mitosis in Late S phase and subsequent replication catastrophe, indicating that dysregulation of the S to M transition is necessary for CHK1 inhibitor sensitivity. These findings provide mechanistic insights into small molecule inhibitors currently studied in clinical trials and provide rationale for combination therapies.

Branigan et al., by using genome-wide CRISPR screens, identify the MMB-FOXM1 complex as being required for CHK1 inhibitor (CHK1i) sensitivity. Their study shows that CHK1i-induced premature activation of the G2/M transcriptional program by this complex triggers a breakdown in the separation of DNA synthesis and mitosis, leading to replication catastrophe.

circRNA circ-MYBL2 is a novel tumor suppressor and potential biomarker in multiple myeloma

Currently, multiple myeloma (MM) is still an incurable disease. Deciphering its pathogenesis will bring new targets for clinical diagnosis and treatment. In the present study, we identified a MM-associated circular RNA (circRNA), circ-MYBL2, which was dramatically decreased in MM tissue and serum samples in comparison to normal samples. Low circ-MYBL2 level was closely correlated with high clinical stage and unfavorable outcome, and serum circ-MYBL2 had excellent accuracy in diagnosing MM. Exogenous circ-MYBL2 expression notably repressed MM cell viability, DNA synthesis and cell cycle progression. Further exploration revealed that circ-MYBL2 exerted the tumor-inhibiting effect by affecting the phosphorylation level of its linear isoform, in which circ-MYBL2 facilitated the binding of Cyclin F to MYBL2, dampening MYBL2 phosphorylation and activation, thereby inhibiting the transcription of a number of well-known proliferation-related oncogenes. Importantly, overexpression of circ-MYBL2 significantly reduced the tumor size of subcutaneous xenografts in nude mice. Taken together, our data unveil a regulatory mechanism linking circ-MYBL2 and its host gene mediated by Cyclin F, providing a potential diagnostic, prognostic and therapeutic target for MM patients.

Cyclin F Downregulation Affects Epithelial-Mesenchymal Transition Increasing Proliferation and Migration of the A-375 Melanoma Cell Line

Background

Cyclins are well-known cell cycle regulators. The activation of cyclin-dependent kinases by cyclins allows orchestration of the complicated cell cycle machinery and drives the cell from the G1 phase to the end of the mitotic phase. In recent years, it has become evident that cyclins are involved in processes beyond the cell cycle. Cyclin F does not activate CDKs but forms part of the Skp1-Cul1-F-box (SCF) complex where it is responsible for protein target recognition and subsequent degradation in a proteasome-dependent manner.

Results

Here, we report that the downregulation of cyclin F in the A-375 melanoma cell line increases cell viability and colony formation in a cell cycle independent manner. Lower levels of cyclin F do not appear to affect the cell cycle, based on flow cytometry measuring BrdU incorporation and propidium iodide staining. By means of immunofluorescence staining and Western blot analysis, we observed changes in cell morphology-related markers which suggested ongoing epithelial-mesenchymal transition (EMT) in response to cyclin F downregulation. Increases in vimentin and N-cadherin protein levels, decreases in levels of epithelial markers such as ZO-1, along with changes in morphology to a spindle-like shape with the appearance of actin stress fibers, are all hallmarks of EMT. These changes are associated with increased invasive and migratory potential, based on 2D migration assays. Moreover, we observe an increase in RhoABC, talin and paxillin levels, the proteins involved in controlling cell signaling and motility. Lastly, upon knocking down cyclin F expression, we observed a decrease in thrombospondin-1 expression, suggesting a role of cyclin F in angiogenesis.

Conclusion

Cyclin F depletion induces proliferation and EMT processes in the A-375 melanoma model.

Chatterboxes: the structural and functional diversity of cyclins

Proteins of the cyclin family have divergent sequences and execute diverse roles within the cell while sharing a common fold: the cyclin box domain. Structural studies of cyclins have played a key role in our characterization and understanding of cellular processes that they control, though to date only ten of the 29 CDK-activating cyclins have been structurally characterized by X-ray crystallography or cryo-electron microscopy with or without their cognate kinases. In this review, we survey the available structures of human cyclins, highlighting their molecular features in the context of their cellular roles. We pay particular attention to how cyclin activity is regulated through fine control of degradation motif recognition and ubiquitination. Finally, we discuss the emergent roles of cyclins independent of their roles as cyclin-dependent protein kinase activators, demonstrating the cyclin box domain to be a versatile and generalized scaffolding domain for protein-protein interactions across the cellular machinery.

Intramolecular interaction of B-MYB is regulated through Ser-577 phosphorylation

The transcription factor B-MYB is an important regulator of cell cycle-related processes that is activated by step-wise phosphorylation of multiple sites by cyclin-dependent kinases (CDKs) and conformational changes induced by the peptidylprolyl cis/trans isomerase Pin1. Here, we show that a conserved amino acid sequence around Ser-577 in the C-terminal part of B-MYB is able to interact with the B-MYB DNA-binding domain. Phosphorylation of Ser-577 disrupts this interaction and is regulated by the interplay of CDKs and the phosphatase CDC14B. Deletion of sequences surrounding Ser-577 hyperactivates the transactivation potential of B-MYB, decreases its proteolytic stability, and causes cell cycle defects. Overall, we show for the first time that B-MYB can undergo an intramolecular interaction that is controlled by the phosphorylation state of Ser-577.

Cyclin F and KIF20A, FOXM1 target genes, increase proliferation and invasion of ovarian cancer cells

Increased expression of FOXM1 is observed in a variety of human malignancies. The downstream target genes of FOXM1 involved in tumorigenesis and development are not fully elucidated in ovarian cancer. Here, we identified Cyclin F, a substrate recognition subunit of SCF (Skp1-Cul1-F-box protein) complex, and Kinesin Family Member 20A (KIF20A) were transcriptionally regulated by FOXM1 in ovarian cancer. Accordingly, Cyclin F and KIF20A were commonly overexpressed in ovarian cancer. Functionally, forced expression of Cyclin F or KIF20A significantly enhanced while knockdown of them decreased proliferation and invasion of ovarian cancer cells. Importantly, high levels of Cyclin F and KIF20A correlated with poor prognosis in patients with ovarian cancer. Our findings indicate that Cyclin F and KIF20A are functional targets of FOXM1 which might be potential drug targets in ovarian cancer.

Complex Cartography: Regulation of E2F Transcription Factors by Cyclin F and Ubiquitin

The E2F family of transcriptional regulators sits at the center of cell cycle gene expression and plays vital roles in normal and cancer cell cycles. Whereas control of E2Fs by the retinoblastoma family of proteins is well established, much less is known about their regulation by ubiquitin pathways. Recent studies placed the Skp1-Cul1-F-box-protein (SCF) family of E3 ubiquitin ligases with the F-box protein Cyclin F at the center of E2F regulation, demonstrating temporal proteolysis of both activator and atypical repressor E2Fs. Importantly, these E2F members, in particular activator E2F1 and repressors E2F7 and E2F8, form a feedback circuit at the crossroads of cell cycle and cell death. Moreover, Cyclin F functions in a reciprocal circuit with the cell cycle E3 ligase anaphase-promoting complex/cyclosome (APC/C), which also controls E2F7 and E2F8. This review focuses on the complex contours of feedback within this circuit, highlighting the deep crosstalk between E2F, SCF-Cyclin F, and APC/C in regulating the oscillator underlying human cell cycles.

Characterization of the zinc finger proteins ZMYM2 and ZMYM4 as novel B-MYB binding proteins

B-MYB, a highly conserved member of the MYB transcription factor family, is expressed ubiquitously in proliferating cells and plays key roles in important cell cycle-related processes, such as control of G2/M-phase transcription, cytokinesis, G1/S-phase progression and DNA-damage reponse. Deregulation of B-MYB function is characteristic of several types of tumor cells, underlining its oncogenic potential. To gain a better understanding of the functions of B-MYB we have employed affinity purification coupled to mass spectrometry to discover novel B-MYB interacting proteins. Here we have identified the zinc-finger proteins ZMYM2 and ZMYM4 as novel B-MYB binding proteins. ZMYM4 is a poorly studied protein whose initial characterization reported here shows that it is highly SUMOylated and that its interaction with B-MYB is stimulated upon induction of DNA damage. Unlike knockdown of B-MYB, which causes G2/M arrest and defective cytokinesis in HEK293 cells, knockdown of ZMYM2 or ZMYM4 have no obvious effects on the cell cycle of these cells. By contrast, knockdown of ZMYM2 strongly impaired the G1/S-phase progression of HepG2 cells, suggesting that ZMYM2, like B-MYB, is required for entry into S-phase in these cells. Overall, our work identifies two novel B-MYB binding partners with possible functions in the DNA-damage response and the G1/S-transition.

F-Box Proteins and Cancer

Controlled protein degradation is essential for the operation of a variety of cellular processes including cell division, growth, and differentiation. Identification of the relations between ubiquitin ligases and their substrates is key to understanding the molecular basis of cancer development and to the discovery of novel targets for cancer therapeutics. F-box proteins function as the substrate recognition subunits of S-phase kinase-associated protein 1 (SKP1)−Cullin1 (CUL1)−F-box protein (SCF) ubiquitin ligase complexes. Here, we summarize the roles of specific F-box proteins that have been shown to function as tumor promoters or suppressors. We also highlight proto-oncoproteins that are targeted for ubiquitylation by multiple F-box proteins, and discuss how these F-box proteins are deployed to regulate their cognate substrates in various situations.

Cyclin F is involved in response to cisplatin treatment in melanoma cell lines

Cyclin F is a non-canonical cyclin which is a part of the SKP1-CUL1-F-box protein (SCF) E3 ubiquitin-protein ligase complex. Cyclin F is responsible for target recognition, ubiquitination, and degradation of various molecular targets. This protein also controls genome stability through the degradation of ribonucleotide reductase subunit M2 (RRM2). In the present study, the difference between cyclin F expression in cell lines derived from primary and metastatic melanoma, A375 and RPMI-7951, respectively, were investigated using a western blot analysis and flow cytometry assays. A decrease in cyclin F expression in the A375 cells and an increase in RPMI-7951 cells after cisplatin treatment were observed. These changes may be related to a mutation in p53 in the RPMI-7951 cell line. Flow cytometry was conducted to observe that the RPMI-7951 cell line exhibited greater susceptibility to cisplatin, associated with lack of proper cell cycle control. Therefore, it is possible that cyclin F may modulate drug response in melanoma. The presented data describe cyclin F as a new potential factor that contributes to drug resistance in melanoma patients.

β-TrCP- and Casein Kinase II-Mediated Degradation of Cyclin F Controls Timely Mitotic Progression

Orderly progressions of events in the cell division cycle are necessary to ensure the replication of DNA and cell division. Checkpoint systems allow the accurate execution of each cell-cycle phase. The precise regulation of the levels of cyclin proteins is fundamental to coordinate cell division with checkpoints, avoiding genome instability. Cyclin F has important functions in regulating the cell cycle during the G2 checkpoint; however, the mechanisms underlying the regulation of cyclin F are poorly understood. Here, we observe that cyclin F is regulated by proteolysis through β-TrCP. β-TrCP recognizes cyclin F through a non-canonical degron site (TSGXXS) after its phosphorylation by casein kinase II. The degradation of cyclin F mediated by β-TrCP occurs at the G2/M transition. This event is required to promote mitotic progression and favors the activation of a transcriptional program required for mitosis.

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β-TrCP1 and β-TrCP2 interact with cyclin F and control cyclin F levels during mitosis

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A TSGXXS motif is necessary for β-TrCP1 and β-TrCP2 binding to cyclin F

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CKIIα phosphorylates cyclin F at S704 within the TSGXXS motif

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β-TrCP-mediated degradation of cyclin F promotes mitotic progression via B-Myb

DNA replication and mitotic entry: A brake model for cell cycle progression

Lemmens and Lindqvist discuss how DNA replication and mitosis are coordinated and propose a cell cycle model controlled by brakes.

The core function of the cell cycle is to duplicate the genome and divide the duplicated DNA into two daughter cells. These processes need to be carefully coordinated, as cell division before DNA replication is complete leads to genome instability and cell death. Recent observations show that DNA replication, far from being only a consequence of cell cycle progression, plays a key role in coordinating cell cycle activities. DNA replication, through checkpoint kinase signaling, restricts the activity of cyclin-dependent kinases (CDKs) that promote cell division. The S/G2 transition is therefore emerging as a crucial regulatory step to determine the timing of mitosis. Here we discuss recent observations that redefine the coupling between DNA replication and cell division and incorporate these insights into an updated cell cycle model for human cells. We propose a cell cycle model based on a single trigger and sequential releases of three molecular brakes that determine the kinetics of CDK activation.

Cathepsin L induced PC-12 cell apoptosis via activation of B-Myb and regulation of cell cycle proteins

Cathepsin L (CTSL), a cysteine protease, is responsible for the degradation of a variety of proteins. It is known to participate in neuronal apoptosis associated with abnormal cell cycle. However, the mechanisms underlying CTSL-induced cell apoptosis remain largely unclear. We reported here that rotenone caused an activation of CTSL expression in PC-12 cells, while knockdown of CTSL by small interfering RNAs or its inhibitor reduced the rotenone-induced cell cycle arrest and apoptosis. Moreover, elevation of CTSL and increased-apoptosis were accompanied by induction of B-Myb, a crucial cell cycle regulator. We found that B-Myb was increased in rotenone-treated PC-12 cells and knockdown of B-Myb ameliorated rotenone-stimulated cell apoptosis. Further analysis demonstrated that CTSL influenced the expression of B-Myb as suppression of CTSL activity led to a decreased B-Myb expression, whereas overexpression of CTSL resulted in B-Myb induction. Reduction of B-Myb in CTSL-overexpressing cells revealed that regulation of cell cycle-related proteins, including cyclin A and cyclin B1, through CTSL was mediated by the transcription factor B-Myb. In addition, we demonstrated that the B-Myb target, Bim, and its regulator, Egr-1, which was also associated with CTSL closely, were both involved in rotenone-induced apoptosis in PC-12 cells. Our data not only revealed the role of CTSL in rotenone-induced neuronal apoptosis, but also indicated the involvement of B-Myb in CTSL-related cell cycle regulation.

E2F1 proteolysis via SCF-cyclin F underlies synthetic lethality between cyclin F loss and Chk1 inhibition

Cyclins are central engines of cell cycle progression in conjunction with cyclin-dependent kinases (CDKs). Among the different cyclins controlling cell cycle progression, cyclin F does not partner with a CDK, but instead forms via its F-box domain an SCF (Skp1-Cul1-F-box)-type E3 ubiquitin ligase module. Although various substrates of cyclin F have been identified, the vulnerabilities of cells lacking cyclin F are not known. Thus, we assessed viability of cells lacking cyclin F upon challenging them with more than 180 different kinase inhibitors. The screen revealed a striking synthetic lethality between Chk1 inhibition and cyclin F loss. Chk1 inhibition in cells lacking cyclin F leads to DNA replication catastrophe. Replication catastrophe depends on accumulation of the transcription factor E2F1 in cyclin F-depleted cells. We find that SCF-cyclin F controls E2F1 ubiquitylation and degradation during the G2/M phase of the cell cycle and upon challenging cells with Chk1 inhibitors. Thus, Cyclin F restricts E2F1 activity during the cell cycle and upon checkpoint inhibition to prevent DNA replication stress. Our findings pave the way for patient selection in the clinical use of checkpoint inhibitors.

Cullin Ring Ubiquitin Ligases (CRLs) in Cancer: Responses to Ionizing Radiation (IR) Treatment

Treatment with ionizing radiation (IR) remains the cornerstone of therapy for multiple cancer types, including disseminated and aggressive diseases in the palliative setting. Radiotherapy efficacy could be improved in combination with drugs that regulate the ubiquitin-proteasome system (UPS), many of which are currently being tested in clinical trials. The UPS operates through the covalent attachment of ATP-activated ubiquitin molecules onto substrates following the transfer of ubiquitin from an E1, to an E2, and then to the substrate via an E3 enzyme. The specificity of ubiquitin ligation is dictated by E3 ligases, which select substrates to be ubiquitylated. Among the E3s, cullin ring ubiquitin ligases (CRLs) represent prototypical multi-subunit E3s, which use the cullin subunit as a central assembling scaffold. CRLs have crucial roles in controlling the cell cycle, hypoxia signaling, reactive oxygen species clearance and DNA repair; pivotal factors regulating the cancer and normal tissue response to IR. Here, we summarize the findings on the involvement of CRLs in the response of cancer cells to IR, and we discuss the therapeutic approaches to target the CRLs which could be exploited in the clinic.

E2F transcription regulation: an orphan cyclin enters the stage

Marked cyclin protein oscillations over the cell cycle ensure tight regulation of all cell cycle transitions. Despite expression patterns closely mirroring those of cyclin A, cyclin F has long been regarded as an odd outlier within the cyclin family. Constituting part of an E3 ubiquitin ligase, its main role was seen as highly restricted to timely degradation of very few key substrates to ensure termination of one error-free round of replication. Now, a recent series of studies suggests that cyclin F has very similar roles as its closest relatives, merely mediated through a very different mechanism.

Cyclin F-dependent degradation of E2F7 is critical for DNA repair and G2-phase progression

E2F7 and E2F8 act as tumor suppressors via transcriptional repression of genes involved in S-phase entry and progression. Previously, we demonstrated that these atypical E2Fs are degraded by APC/C^{C dh1} during G1 phase of the cell cycle. However, the mechanism driving the downregulation of atypical E2Fs during G2 phase is unknown. Here, we show that E2F7 is targeted for degradation by the E3 ubiquitin ligase SCF^{cyclin F} during G2. Cyclin F binds via its cyclin domain to a conserved C-terminal CY motif on E2F7. An E2F7 mutant unable to interact with SCF^{cyclin F} remains stable during G2. Furthermore, SCF^{cyclin F} can also interact and induce degradation of E2F8. However, this does not require the cyclin domain of SCF^{cyclin F} nor the CY motifs in the C-terminus of E2F8, implying a different regulatory mechanism than for E2F7. Importantly, depletion of cyclin F causes an atypical-E2F-dependent delay of the G2/M transition, accompanied by reduced expression of E2F target genes involved in DNA repair. Live cell imaging of DNA damage revealed that cyclin F-dependent regulation of atypical E2Fs is critical for efficient DNA repair and cell cycle progression.

Cyclin F Controls Cell-Cycle Transcriptional Outputs by Directing the Degradation of the Three Activator E2Fs

E2F1, E2F2, and E2F3A, the three activators of the E2F family of transcription factors, are key regulators of the G1/S transition, promoting transcription of hundreds of genes critical for cell-cycle progression. We found that during late S and in G2, the degradation of all three activator E2Fs is controlled by cyclin F, the substrate receptor of 1 of 69 human SCF ubiquitin ligase complexes. E2F1, E2F2, and E2F3A interact with the cyclin box of cyclin F via their conserved N-terminal cyclin binding motifs. In the short term, E2F mutants unable to bind cyclin F remain stable throughout the cell cycle, induce unscheduled transcription in G2 and mitosis, and promote faster entry into the next S phase. However, in the long term, they impair cell fitness. We propose that by restricting E2F activity to the S phase, cyclin F controls one of the main and most critical transcriptional engines of the cell cycle.

Activation of the oncogenic transcription factor B-Myb via multisite phosphorylation and prolyl cis/trans isomerization

The oncogenic transcription factor B-Myb is an essential regulator of late cell cycle genes whose activation by phosphorylation is still poorly understood. We describe a stepwise phosphorylation mechanism of B-Myb, which involves sequential phosphorylations mediated by cyclin-dependent kinase (Cdk) and Polo-like kinase 1 (Plk1) and Pin1-facilitated peptidyl-prolyl cis/trans isomerization. Our data suggest a model in which initial Cdk-dependent phosphorylation of B-Myb enables subsequent Pin1 binding and Pin1-induced conformational changes of B-Myb. This, in turn, initiates further phosphorylation of Cdk-phosphosites, enabling Plk1 docking and subsequent Plk1-mediated phosphorylation of B-Myb to finally allow B-Myb to stimulate transcription of late cell cycle genes. Our observations reveal novel mechanistic hierarchies of B-Myb phosphorylation and activation and uncover regulatory principles that might also apply to other Myb family members. Strikingly, overexpression of B-Myb and of factors mediating its activation strongly correlates with adverse prognoses for tumor patients, emphasizing B-Myb's role in tumorigenesis.

Placenta-specific 9, a putative secretory protein, induces G2/M arrest and inhibits the proliferation of human embryonic hepatic cells

Background: Placenta-specific 9 (Plac9) is a putative secreted protein that was first discovered in the context of embryogenesis. The expression pattern of Plac9 during embryogenesis, together with the results of recent reports, suggest that Plac9 may play a role in the liver development. The present study was conducted to investigate the secretory characteristics of Plac9 and its potential role in liver cell physiology. Methods: Immunofluorescence was employed to identify the subcellular distribution of Plac9. Cellular proliferative activity was analyzed by MTT assay and cell colony formation. The cell cycle distribution of Plac9 was analyzed by flow cytometry, and a functional analysis was performed using L02 cells following their stable infection with a lentivirus over-expressing Plac9. Results:Plac9 is a novel protein that is localized to the cytoplasm and may be secreted through the classic endoplasmic reticulum-Golgi route. The overexpression of Plac9 inhibits cell growth and induces G2/M phase arrest. Conclusion: Our findings reveal a novel role for Plac9 in regulating cell growth.

ALS Genes in the Genomic Era and their Implications for FTD

Amyotrophic lateral sclerosis (ALS) is a complex neurodegenerative disease, characterized genetically by a disproportionately large contribution of rare genetic variation. Driven by advances in massive parallel sequencing and applied on large patient-control cohorts, systematic identification of these rare variants that make up the genetic architecture of ALS became feasible. In this review paper, we present a comprehensive overview of recently proposed ALS genes that were identified based on rare genetic variants (TBK1, CHCHD10, TUBA4A, CCNF, MATR3, NEK1, C21orf2, ANXA11, TIA1) and their potential relevance to frontotemporal dementia genetic etiology. As more causal and risk genes are identified, it has become apparent that affected individuals can carry multiple disease-associated variants. In light of this observation, we discuss the oligogenic architecture of ALS. To end, we highlight emerging key molecular processes and opportunities for therapy.

The long non-coding RNA EPB41L4A-AS2 inhibits tumor proliferation and is associated with favorable prognoses in breast cancer and other solid tumors

EPB41L4A-AS2 is a novel long non-coding RNA of unknown function. In this study, we investigated the expression of EPB41L4A-AS2 in breast cancer tissues and evaluated its relationship with the clinicopathological features and prognosis of patients with breast cancer. This entailed conducting a meta-analysis and prognosis validation study using two cohorts from the Gene Expression Omnibus (GEO). In addition, we assessed EPB41L4A-AS2 expression and its relationship with the clinicopathological features of renal and lung cancers using the Cancer Genome Atlas cohort and a GEO dataset. We also clarified the role of EPB41L4A-AS2 expression in mediating cancer cell proliferation in breast, renal, and lung cancer cell lines transfected with an EPB41L4A-AS2 expression vector. We found that high EPB41L4A-AS2 expression is associated with favorable disease outcomes. Gene ontology enrichment analysis revealed that EPB41L4A-AS2 may be involved in processes associated with tumor biology. Finally, overexpression of EPB41L4A-AS2 inhibited tumor cell proliferation in breast, renal, and lung cancer cell lines. Our clinical and in vitro results suggest that EPB41L4A-AS2 inhibits solid tumor formation and that evaluation of this long non-coding RNA may have prognostic value in the clinical management of such malignancies.

Genome-wide profiling of nardilysin target genes reveals its role in epigenetic regulation and cell cycle progression

Post-translational histone modifications, such as acetylation and methylation, are prerequisites for transcriptional regulation. The metalloendopeptidase nardilysin (Nrdc) is a H3K4me2-binding protein that controls thermoregulation and β-cell functions through its transcriptional coregulator function. We herein combined high-throughput ChIP-seq and RNA-seq to achieve the first genome-wide identification of Nrdc target genes. A ChIP-seq analysis of immortalized mouse embryo fibroblasts (iMEF) identified 4053 Nrdc-binding sites, most of which were located in proximal promoter sites (2587 Nrdc-binding genes). Global H3K4me2 levels at Nrdc-binding promoters slightly increased, while H3K9ac levels decreased in the absence of Nrdc. Among Nrdc-binding genes, a comparative RNA-seq analysis identified 448 candidates for Nrdc target genes, among which cell cycle-related genes were significantly enriched. We confirmed decreased mRNA and H3K9ac levels at the promoters of individual genes in Nrdc-deficient iMEF, which were restored by the ectopic introduction of Nrdc. Reduced mRNA levels, but not H3K9ac levels were fully restored by the reintroduction of the peptidase-dead mutant of Nrdc. Furthermore, Nrdc promoted cell cycle progression at multiple stages, which enhanced cell proliferation in vivo. Collectively, our integrative studies emphasize the importance of Nrdc for maintaining a proper epigenetic status and cell growth.

Investigating CCNF mutations in a Taiwanese cohort with amyotrophic lateral sclerosis

Mutations in the cyclin F gene (CCNF) have been recently identified in a small number of patients with amyotrophic lateral sclerosis (ALS) and/or frontotemporal dementia, and their role in patients with ALS in Taiwan remains elusive. The aim of this study was to elucidate the frequency and spectrum of CCNF mutations in a Taiwanese ALS cohort of Han Chinese origin. Mutational analyses of the CCNF gene were performed using Sanger sequencing in a cohort of 255 unrelated patients with ALS. Among these patients, the genetic diagnoses of 204 patients remained unclear after mutations in SOD1, C9ORF72, TARDBP, FUS, ATXN2, OPTN, VCP, UBQLN2, SQSTM1, PFN1, HNRNPA1, HNRNPA2B1, MATR3, CHCHD10, TUBA4A, and TKB1 had been investigated. Two novel heterozygous missense mutations in CCNF, p.S222P (c.664T>C) and p.S532R (c.1596C>T), were identified; 1 in each patient with apparently sporadic ALS. In vitro functional study demonstrated that both mutations result in a general and cyclin F-mediated ubiquitin-proteasome pathway dysfunction. The frequency of CCNF mutations in ALS patients in Taiwan is, therefore, approximately 0.8% (2/255). These findings expand the mutational spectrum of CCNF and also emphasize the pathogenic role of CCNF mutations in ALS.

Cell cycle transcription control: DREAM/MuvB and RB-E2F complexes

The precise timing of cell cycle gene expression is critical for the control of cell proliferation; de-regulation of this timing promotes the formation of cancer and leads to defects during differentiation and development. Entry into and progression through S phase requires expression of genes coding for proteins that function in DNA replication. Expression of a distinct set of genes is essential to pass through mitosis and cytokinesis. Expression of these groups of cell cycle-dependent genes is regulated by the RB pocket protein family, the E2F transcription factor family, and MuvB complexes together with B-MYB and FOXM1. Distinct combinations of these transcription factors promote the transcription of the two major groups of cell cycle genes that are maximally expressed either in S phase (G1/S) or in mitosis (G2/M). In this review, we discuss recent work that has started to uncover the molecular mechanisms controlling the precisely timed expression of these genes at specific cell cycle phases, as well as the repression of the genes when a cell exits the cell cycle.

MYBL2 (B-Myb): a central regulator of cell proliferation, cell survival and differentiation involved in tumorigenesis

Limitless cell proliferation, evasion from apoptosis, dedifferentiation, metastatic spread and therapy resistance: all these properties of a cancer cell contribute to its malignant phenotype and affect patient outcome. MYBL2 (alias B-Myb) is a transcription factor of the MYB transcription factor family and a physiological regulator of cell cycle progression, cell survival and cell differentiation. When deregulated in cancer cells, MYBL2 mediates the deregulation of these properties. In fact, MYBL2 is overexpressed and associated with poor patient outcome in numerous cancer entities. MYBL2 and players of its downstream transcriptional network can be used as prognostic and/or predictive biomarkers as well as potential therapeutic targets to offer less toxic and more specific anti-cancer therapies in future. In this review, we summarize current knowledge on the physiological roles of MYBL2 and highlight the impact of its deregulation on cancer initiation and progression.

Selective targeting p53WT lung cancer cells harboring homozygous p53 Arg72 by an inhibitor of CypA

TP53 plays essential roles in tumor initiation and progression, and is frequently mutated in cancer. However, pharmacological stabilization and reactivation of p53 have not been actively explored for targeted cancer therapies. Herein, we identify a novel Cyclophilin A (CypA) small molecule inhibitor (HL001) that induces non-small cell lung cancer (NSCLC) cell cycle arrest and apoptosis via restoring p53 expression. We find that HL001 stabilizes p53 through inhibiting the MDM2-mediated p53 ubiquitination. Further mechanistic studies reveal that the downregulation of G3BP1 and the induction of reactive oxygen species and DNA damage by HL001 contribute to p53 stabilization. Surprisingly, HL001 selectively suppresses tumor growth in p53 wild-type NSCLC harboring Arg72 homozygous alleles (p53-72R) through disrupting interaction between MDM2 and p53-72R in a CypA-dependent manner. Moreover, combining HL001 with cisplatin synergistically enhance tumor regression in orthotopic NSCLC mouse model. Collectively, this study demonstrates that pharmacologic inhibition of CypA offers a potential therapeutic strategy via specific activation of p53-72R in NSCLC.

Cyclin F/FBXO1 Interacts with HIV-1 Viral Infectivity Factor (Vif) and Restricts Progeny Virion Infectivity by Ubiquitination and Proteasomal Degradation of Vif Protein through SCFcyclin F E3 Ligase Machinery

Cyclin F protein, also known as FBXO1, is the largest among all cyclins and oscillates in the cell cycle like other cyclins. Apart from being a G₂/M cyclin, cyclin F functions as the substrate-binding subunit of SCF^{cyclin F} E3 ubiquitin ligase. In a gene expression analysis performed to identify novel gene modulations associated with cell cycle dysregulation during HIV-1 infection in CD4⁺ T cells, we observed down-regulation of the cyclin F gene (CCNF). Later, using gene overexpression and knockdown studies, we identified cyclin F as negatively influencing HIV-1 viral infectivity without any significant impact on virus production. Subsequently, we found that cyclin F negatively regulates the expression of viral protein Vif (viral infectivity factor) at the protein level. We also identified a novel host-pathogen interaction between cyclin F and Vif protein in T cells during HIV-1 infection. Mutational analysis of a cyclin F-specific amino acid motif in the C-terminal region of Vif indicated rescue of the protein from cyclin F-mediated down-regulation. Subsequently, we showed that Vif is a novel substrate of the SCF^{cyclin F} E3 ligase, where cyclin F mediates the ubiquitination and proteasomal degradation of Vif through physical interaction. Finally, we showed that cyclin F augments APOBEC3G expression through degradation of Vif to regulate infectivity of progeny virions. Taken together, our results demonstrate that cyclin F is a novel F-box protein that functions as an intrinsic cellular regulator of HIV-1 Vif and has a negative regulatory effect on the maintenance of viral infectivity by restoring APOBEC3G expression.

Interplay with the Mre11-Rad50-Nbs1 complex and phosphorylation by GSK3β implicate human B-Myb in DNA-damage signaling

B-Myb, a highly conserved member of the Myb transcription factor family, is expressed ubiquitously in proliferating cells and controls the cell cycle dependent transcription of G2/M-phase genes. Deregulation of B-Myb has been implicated in oncogenesis and loss of genomic stability. We have identified B-Myb as a novel interaction partner of the Mre11-Rad50-Nbs1 (MRN) complex, a key player in the repair of DNA double strand breaks. We show that B-Myb directly interacts with the Nbs1 subunit of the MRN complex and is recruited transiently to DNA-damage sites. In response to DNA-damage B-Myb is phosphorylated by protein kinase GSK3β and released from the MRN complex. A B-Myb mutant that cannot be phosphorylated by GSK3β disturbs the regulation of pro-mitotic B-Myb target genes and leads to inappropriate mitotic entry in response to DNA-damage. Overall, our work suggests a novel function of B-Myb in the cellular DNA-damage signalling.

Downregulation of B-myb promotes senescence via the ROS-mediated p53/p21 pathway, in vascular endothelial cells

OBJECTIVES

To reveal whether B-myb is involved in preventing senescence of vascular endothelial cells, and if so, to identify possible mechanisms for it.

MATERIALS AND METHODS

C57/BL6 male mice and primary human aortic endothelial cells (HAECs) were used. Bleomycin was applied to induce stress-related premature senescence. B-myb knockdown was achieved using an siRNA technique and cell senescence was assessed using the senescence-associated β-galactosidase (SA-β-gal) assay. Intracellular reactive oxygen species (ROS) production was analysed using an ROS assay kit and cell proliferation was evaluated using KFluor488 EdU kit. Capillary tube network formation was determined by Matrigel assay. Expressions of mRNA and protein levels were detected by real-time PCR and western blotting.

RESULTS

B-myb expression significantly decreased, while p53 and p21 expressions increased in the aortas of aged mice. This expression pattern was also found in replicative senescent HAECs and senescent HAECs induced by bleomycin. B-myb knockdown resulted in upregulation of p22^phox , ROS accumulation and cell senescence of HAECs. Downregulation of B-myb significantly inhibited cell proliferation and capillary tube network formation and activated the p53/p21 signalling pathway. Blocking ROS production or inhibiting p53 activation remarkably attenuated SA-β-gal activity and delayed cell senescence induced by B-myb-silencing.

CONCLUSION

Downregulation of B-myb induced senescence by upregulation of p22^phox and activation of the ROS/p53/p21 pathway, in our vascular endothelial cells, suggesting that B-myb may be a novel candidate for regulating cell senescence to protect against endothelial senescence-related cardiovascular diseases.

Cyclin F-Mediated Degradation of SLBP Limits H2A.X Accumulation and Apoptosis upon Genotoxic Stress in G2

SLBP (stem-loop binding protein) is a highly conserved factor necessary for the processing, translation, and degradation of H2AFX and canonical histone mRNAs. We identified the F-box protein cyclin F, a substrate recognition subunit of an SCF (Skp1-Cul1-F-box protein) complex, as the G2 ubiquitin ligase for SLBP. SLBP interacts with cyclin F via an atypical CY motif, and mutation of this motif prevents SLBP degradation in G2. Expression of an SLBP stable mutant results in increased loading of H2AFX mRNA onto polyribosomes, resulting in increased expression of H2A.X (encoded by H2AFX). Upon genotoxic stress in G2, high levels of H2A.X lead to persistent γH2A.X signaling, high levels of H2A.X phosphorylated on Tyr142, high levels of p53, and induction of apoptosis. We propose that cyclin F co-evolved with the appearance of stem-loops in vertebrate H2AFX mRNA to mediate SLBP degradation, thereby limiting H2A.X synthesis and cell death upon genotoxic stress.

ATM/ATR-mediated phosphorylation of PALB2 promotes RAD51 function

DNA damage activates the ATM and ATR kinases that coordinate checkpoint and DNA repair pathways. An essential step in homology-directed repair (HDR) of DNA breaks is the formation of RAD51 nucleofilaments mediated by PALB2-BRCA2; however, roles of ATM and ATR in this critical step of HDR are poorly understood. Here, we show that PALB2 is markedly phosphorylated in response to genotoxic stresses such as ionizing radiation and hydroxyurea. This response is mediated by the ATM and ATR kinases through three N-terminal S/Q-sites in PALB2, the consensus target sites for ATM and ATR Importantly, a phospho-deficient PALB2 mutant is unable to support proper RAD51 foci formation, a key PALB2 regulated repair event, whereas a phospho-mimicking PALB2 version supports RAD51 foci formation. Moreover, phospho-deficient PALB2 is less potent in HDR than wild-type PALB2. Further, this mutation reveals a separation in PALB2 function, as the PALB2-dependent checkpoint response is normal in cells expressing the phospho-deficient PALB2 mutant. Collectively, our findings highlight a critical importance of PALB2 phosphorylation as a novel regulatory step in genome maintenance after genotoxic stress.

SCF(Cyclin F)-dependent degradation of CDC6 suppresses DNA re-replication

Maintenance of genome stability requires that DNA is replicated precisely once per cell cycle. This is believed to be achieved by limiting replication origin licensing and thereby restricting the firing of each replication origin to once per cell cycle. CDC6 is essential for eukaryotic replication origin licensing, however, it is poorly understood how CDC6 activity is constrained in higher eukaryotes. Here we report that the SCF^{Cyclin F} ubiquitin ligase complex prevents DNA re-replication by targeting CDC6 for proteasomal degradation late in the cell cycle. We show that CDC6 and Cyclin F interact through defined sequence motifs that promote CDC6 ubiquitylation and degradation. Absence of Cyclin F or expression of a stable mutant of CDC6 promotes re-replication and genome instability in cells lacking the CDT1 inhibitor Geminin. Together, our work reveals a novel SCF^{Cyclin F}-mediated mechanism required for precise once per cell cycle replication.

To ensure genome stability, cells need to restrict DNA replication to once per cell cycle. Here the authors show that Cyclin F interacts with and targets the licensing factor CDC6 for degradation, preventing re-firing of replication origins.