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

Proteomic analysis reveals a PLK1-dependent G2/M degradation program and a role for AKAP2 in coordinating the mitotic cytoskeleton

Ubiquitination is an essential regulator of cell division. The kinase Polo-like kinase 1 (PLK1) promotes protein degradation at G2/M phase through the E3 ubiquitin ligase Skp1-Cul1-F box (SCF)βTrCP. However, the magnitude to which PLK1 shapes the mitotic proteome is uncharacterized. Combining quantitative proteomics with pharmacologic PLK1 inhibition revealed a widespread, PLK1-dependent program of protein breakdown at G2/M. We validated many PLK1-regulated proteins, including substrates of the cell-cycle E3 SCFCyclin F, demonstrating that PLK1 promotes proteolysis through at least two distinct E3 ligases. We show that the protein-kinase-A-anchoring protein A-kinase anchor protein 2 (AKAP2) is cell-cycle regulated and that its mitotic degradation is dependent on the PLK1/βTrCP signaling axis. Expression of a non-degradable AKAP2 mutant resulted in actin defects and aberrant mitotic spindles, suggesting that AKAP2 degradation coordinates cytoskeletal organization during mitosis. These findings uncover PLK1's far-reaching role in shaping the mitotic proteome post-translationally and have potential implications in malignancies where PLK1 is upregulated.

Cyclin F-EXO1 axis controls cell cycle-dependent execution of double-strand break repair

Ubiquitination is a crucial posttranslational modification required for the proper repair of DNA double-strand breaks (DSBs) induced by ionizing radiation (IR). DSBs are mainly repaired through homologous recombination (HR) when template DNA is present and nonhomologous end joining (NHEJ) in its absence. In addition, microhomology-mediated end joining (MMEJ) and single-strand annealing (SSA) provide backup DSBs repair pathways. However, the mechanisms controlling their use remain poorly understood. By using a high-resolution CRISPR screen of the ubiquitin system after IR, we systematically uncover genes required for cell survival and elucidate a critical role of the E3 ubiquitin ligase SCFcyclin F in cell cycle-dependent DSB repair. We show that SCFcyclin F-mediated EXO1 degradation prevents DNA end resection in mitosis, allowing MMEJ to take place. Moreover, we identify a conserved cyclin F recognition motif, distinct from the one used by other cyclins, with broad implications in cyclin specificity for cell cycle control.

F-box proteins and gastric cancer: an update from functional and regulatory mechanism to therapeutic clinical prospects

Gastric cancer (GC) is a prevalent malignancy characterized by significant morbidity and mortality, yet its underlying pathogenesis remains elusive. The etiology of GC is multifaceted, involving the activation of oncogenes and the inactivation of antioncogenes. The ubiquitin-proteasome system (UPS), responsible for protein degradation and the regulation of physiological and pathological processes, emerges as a pivotal player in GC development. Specifically, the F-box protein (FBP), an integral component of the SKP1-Cullin1-F-box protein (SCF) E3 ligase complex within the UPS, has garnered attention for its prominent role in carcinogenesis, tumor progression, and drug resistance. Dysregulation of several FBPs has recently been observed in GC, underscoring their significance in disease progression. This comprehensive review aims to elucidate the distinctive characteristics of FBPs involved in GC, encompassing their impact on cell proliferation, apoptosis, invasive metastasis, and chemoresistance. Furthermore, we delve into the emerging role of FBPs as downstream target proteins of non-coding RNAs(ncRNAs) in the regulation of gastric carcinogenesis, outlining the potential utility of FBPs as direct therapeutic targets or advanced therapies for GC.

Deubiquitination of CDC6 by OTUD6A promotes tumour progression and chemoresistance

BACKGROUND

CDC6 is an oncogenic protein whose expression level fluctuates during the cell cycle. Although several E3 ubiquitin ligases responsible for the ubiquitin-mediated proteolysis of CDC6 have been identified, the deubiquitination pathway for CDC6 has not been investigated.

METHODS

The proteome-wide deubiquitinase (DUB) screening was used to identify the potential regulator of CDC6. Immunofluorescence, protein half-life and deubiquitination assays were performed to determine the protein stability of CDC6. Gain- and loss-of-function experiments were implemented to analyse the impacts of OUTD6A-CDC6 axis on tumour growth and chemosensitivity in vitro. N-butyl-N-(4-hydroxybutyl) nitrosamine (BBN)-induced conditional Otud6a knockout (CKO) mouse model and tumour xenograft model were performed to analyse the role of OTUD6A-CDC6 axis in vivo. Tissue specimens were used to determine the association between OTUD6A and CDC6.

RESULTS

OTUD6A interacts with, depolyubiquitinates and stabilizes CDC6 by removing K6-, K33-, and K48-linked polyubiquitination. Moreover, OTUD6A promotes cell proliferation and decreases sensitivity to chemotherapy by upregulating CDC6. CKO mice are less prone to BCa tumorigenesis induced by BBN, and knockdown of OTUD6A inhibits tumour progression in vivo. Furthermore, OTUD6A protein level has a positive correlation with CDC6 protein level, and high protein levels of OTUD6A and CDC6 are associated with poor prognosis in patients with bladder cancer.

CONCLUSIONS

We reveal an important yet missing piece of novel DUB governing CDC6 stability. In addition, our findings propose a model for the OTUD6A-CDC6 axis that provides novel insights into cell cycle and chemosensitivity regulation, which may become a potential biomarker and promising drug target for cancer treatment.

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.

RAD51 restricts DNA over-replication from re-activated origins

Eukaryotic cells rely on several mechanisms to ensure that the genome is duplicated precisely once in each cell division cycle, preventing DNA over-replication and genomic instability. Most of these mechanisms limit the activity of origin licensing proteins to prevent the reactivation of origins that have already been used. Here, we have investigated whether additional controls restrict the extension of re-replicated DNA in the event of origin re-activation. In a genetic screening in cells forced to re-activate origins, we found that re-replication is limited by RAD51 and enhanced by FBH1, a RAD51 antagonist. In the presence of chromatin-bound RAD51, forks stemming from re-fired origins are slowed down, leading to frequent events of fork reversal. Eventual re-initiation of DNA synthesis mediated by PRIMPOL creates ssDNA gaps that facilitate the partial elimination of re-duplicated DNA by MRE11 exonuclease. In the absence of RAD51, these controls are abrogated and re-replication forks progress much longer than in normal conditions. Our study uncovers a safeguard mechanism to protect genome stability in the event of origin reactivation.

Cyclin F can alter the turnover of TDP-43

Previously, we demonstrated that the SCFcyclin F complex directly mediates the poly-ubiquitylation of TDP-43, raising the question of whether cyclin F can be used to enhance the turnover of TDP-43. A hurdle to the use of cyclin F, however, is that the overexpression of cyclin F can lead to the initiation of cell death pathways. Accordingly, the aim of this study was to identify and evaluate a less toxic variant of cyclin F. To do so, we first confirmed and validated our previous findings that cyclin F binds to TDP-43 in an atypical manner. Additionally, we demonstrated that mutating the canonical substrate region in cyclin F (to generate cyclin FMRL/AAA) led to reduced binding affinity to known canonical substrates without impacting the interaction between cyclin F and TDP-43. Notably, both wild-type and cyclin FMRL/AAA effectively reduced the abundance of TDP-43 in cultured cells whilst cyclin FMRL/AAA also demonstrated reduced cell death compared to the wild-type control. The decrease in toxicity also led to a reduction in morphological defects in zebrafish embryos. These results suggest that cyclin F can be modified to enhance its targeting of TDP-43, which in turn reduces the toxicity associated with the overexpression of cyclin F. This study provides greater insights into the interaction that occurs between cyclin F and TDP-43 in cells and in vivo.

Inhibitors Targeting the F-BOX Proteins

F-box proteins are involved in multiple cellular processes through ubiquitylation and consequent degradation of targeted substrates. Any significant mutation in F-box protein-mediated proteolysis can cause human malformations. The various cellular processes F-box proteins involved include cell proliferation, apoptosis, invasion, angiogenesis, and metastasis. To target F-box proteins and their associated signaling pathways for cancer treatment, researchers have developed thousands of F-box inhibitors. The most advanced inhibitor of FBW7, NVD-BK M120, is a powerful P13 kinase inhibitor that has been proven to bring about apoptosis in cancerous human lung cells by disrupting levels of the protein known as MCL1. Moreover, F-box Inhibitors have demonstrated their efficacy for treating certain cancers through targeting particular mutated proteins. This paper explores the key studies on how F-box proteins act and their contribution to malignancy development, which fabricates an in-depth perception of inhibitors targeting the F-box proteins and their signaling pathways that eventually isolate the most promising approach to anti-cancer treatments.

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.

Genetic Primary Microcephalies: When Centrosome Dysfunction Dictates Brain and Body Size

Primary microcephalies (PMs) are defects in brain growth that are detectable at or before birth and are responsible for neurodevelopmental disorders. Most are caused by biallelic or, more rarely, dominant mutations in one of the likely hundreds of genes encoding PM proteins, i.e., ubiquitous centrosome or microtubule-associated proteins required for the division of neural progenitor cells in the embryonic brain. Here, we provide an overview of the different types of PMs, i.e., isolated PMs with or without malformations of cortical development and PMs associated with short stature (microcephalic dwarfism) or sensorineural disorders. We present an overview of the genetic, developmental, neurological, and cognitive aspects characterizing the most representative PMs. The analysis of phenotypic similarities and differences among patients has led scientists to elucidate the roles of these PM proteins in humans. Phenotypic similarities indicate possible redundant functions of a few of these proteins, such as ASPM and WDR62, which play roles only in determining brain size and structure. However, the protein pericentrin (PCNT) is equally required for determining brain and body size. Other PM proteins perform both functions, albeit to different degrees. Finally, by comparing phenotypes, we considered the interrelationships among these proteins.

Therapeutic Potential of Nanomedicine in Management of Alzheimer's Disease and Glioma

Neoplasm (Glioblastoma) and Alzheimer's disease (AD) comprise two of the most chronic psychological ailments. Glioblastoma is one of the aggressive and prevalent malignant diseases characterized by rapid growth and invasion resulting from cell migration and degradation of extracellular matrix. While the latter is characterized by extracellular plaques of amyloid and intracellular tangles of tau proteins. Both possess a high degree of resistance to treatment owing to the restricted transport of corresponding drugs to the brain protected by the blood-brain barrier (BBB). Development of optimized therapies using advanced technologies is a great need of today. One such approach is the designing of nanoparticles (NPs) to facilitate the drug delivery at the target site. The present article elaborates the advances in nanomedicines in treatment of both AD as well as Gliomas. The intention of this review is to provide an overview of different types of NPs with their physical properties emphasizing their importance in traversing the BBB and hitting the target site. Further, we discuss the therapeutic applications of these NPs along with their specific targets. Multiple overlapping factors with a common pathway in development of AD and Glioblastoma are discussed in details that will assist the readers in developing the conceptual approach to target the NP for an aging population in the given circumstances with limitations of currently designed NPs, and the challenges to meet and the future perspectives.

The CMG helicase and cancer: a tumor "engine" and weakness with missing mutations

The replicative Cdc45-MCM-GINS (CMG) helicase is a large protein complex that functions in the DNA melting and unwinding steps as a component of replisomes during DNA replication in mammalian cells. Although the CMG performs this important role in cell growth, the CMG is not a simple bystander in cell cycle events. Components of the CMG, specifically the MCM precursors, are also involved in maintaining genomic stability by regulating DNA replication fork speeds, facilitating recovery from replicative stresses, and preventing consequential DNA damage. Given these important functions, MCM/CMG complexes are highly regulated by growth factors such as TGF-ß1 and by signaling factors such as Myc, Cyclin E, and the retinoblastoma protein. Mismanagement of MCM/CMG complexes when these signaling mediators are deregulated, and in the absence of the tumor suppressor protein p53, leads to increased genomic instability and is a contributor to tumorigenic transformation and tumor heterogeneity. The goal of this review is to provide insight into the mechanisms and dynamics by which the CMG is regulated during its assembly and activation in mammalian genomes, and how errors in CMG regulation due to oncogenic changes promote tumorigenesis. Finally, and most importantly, we highlight the emerging understanding of the CMG helicase as an exploitable vulnerability and novel target for therapeutic intervention in cancer.

DNA replication: Mechanisms and therapeutic interventions for diseases

Accurate and integral cellular DNA replication is modulated by multiple replication-associated proteins, which is fundamental to preserve genome stability. Furthermore, replication proteins cooperate with multiple DNA damage factors to deal with replication stress through mechanisms beyond their role in replication. Cancer cells with chronic replication stress exhibit aberrant DNA replication and DNA damage response, providing an exploitable therapeutic target in tumors. Numerous evidence has indicated that posttranslational modifications (PTMs) of replication proteins present distinct functions in DNA replication and respond to replication stress. In addition, abundant replication proteins are involved in tumorigenesis and development, which act as diagnostic and prognostic biomarkers in some tumors, implying these proteins act as therapeutic targets in clinical. Replication-target cancer therapy emerges as the times require. In this context, we outline the current investigation of the DNA replication mechanism, and simultaneously enumerate the aberrant expression of replication proteins as hallmark for various diseases, revealing their therapeutic potential for target therapy. Meanwhile, we also discuss current observations that the novel PTM of replication proteins in response to replication stress, which seems to be a promising strategy to eliminate diseases.

MEKs/ERKs-mediated FBXO1/E2Fs interaction interference modulates G1/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.

Ubiquitin and SUMO as timers during DNA replication

Every time a cell copies its DNA the genetic material is exposed to the acquisition of mutations and genomic alterations that corrupt the information passed on to daughter cells. A tight temporal regulation of DNA replication is necessary to ensure the full copy of the DNA while preventing the appearance of genomic instability. Protein modification by ubiquitin and SUMO constitutes a very complex and versatile system that allows the coordinated control of protein stability, activity and interactome. In chromatin, their action is complemented by the AAA+ ATPase VCP/p97 that recognizes and removes ubiquitylated and SUMOylated factors from specific cellular compartments. The concerted action of the ubiquitin/SUMO system and VCP/p97 determines every step of DNA replication enforcing the ordered activation/inactivation, loading/unloading and stabilization/destabilization of replication factors. Here we analyze the mechanisms used by ubiquitin/SUMO and VCP/p97 to establish molecular timers throughout DNA replication and their relevance in maintaining genome stability. We propose that these PTMs are the main molecular watch of DNA replication from origin recognition to replisome disassembly.

Unscheduled DNA replication in G1 causes genome instability and damage signatures indicative of replication collisions

DNA replicates once per cell cycle. Interfering with the regulation of DNA replication initiation generates genome instability through over-replication and has been linked to early stages of cancer development. Here, we engineer genetic systems in budding yeast to induce unscheduled replication in a G1-like cell cycle state. Unscheduled G1 replication initiates at canonical S-phase origins. We quantifiy the composition of replisomes in G1- and S-phase and identified firing factors, polymerase α, and histone supply as factors that limit replication outside S-phase. G1 replication per se does not trigger cellular checkpoints. Subsequent replication during S-phase, however, results in over-replication and leads to chromosome breaks and chromosome-wide, strand-biased occurrence of RPA-bound single-stranded DNA, indicating head-to-tail replication collisions as a key mechanism generating genome instability upon G1 replication. Low-level, sporadic induction of G1 replication induces an identical response, indicating findings from synthetic systems are applicable to naturally occurring scenarios of unscheduled replication initiation.

Norcantharidin promotes cancer radiosensitization through Cullin1 neddylation-mediated CDC6 protein degradation

Radiotherapy (RT) is a conventional cancer therapeutic modality. However, cancer cells tend to develop radioresistance after a period of treatment. Diagnostic markers and therapeutic targets for radiosensitivity are severely lacking. Our recently published studies demonstrated that the cell division cycle (CDC6) is a critical molecule contributing to radioresistance, and maybe a potential therapeutic target to overcome radioresistance. In the present study, we for the first time reported that Norcantharidin (NCTD), a demethylated form of cantharidin, re-sensitized radioresistant cancer cells to overcome radioresistance, and synergistically promoted irradiation (IR)-induced cell killing and apoptosis by inducing CDC6 protein degradation. Mechanistically, NCTD induced CDC6 protein degradation through the ubiquitin-proteasome pathways. By using small interfering RNA (siRNA) interference or small compound inhibitors, we further determined that NCTD induced CDC6 protein degradation through a neddylation-dependent pathway, but not through Huwe1, Cyclin F, and APC/C-mediated ubiquitin-proteasome pathways. We screened the six most relevant Cullin subunits (CUL1, 2, 3, 4A, 4B, and 5) using siRNAs. The knockdown of Cullin1 but not the other five cullins remarkably elevated CDC6 protein levels. NCTD promoted the binding of Cullin1 to CDC6, thereby promoting CDC6 protein degradation through a Cullin1 neddylation-mediated ubiquitin-proteasome pathway. NCTD can be used in combination with radiotherapy to achieve better anticancer efficacy, or work as a radiosensitizer to overcome cancer radioresistance.

Small Molecule Inhibitor Targeting CDT1/Geminin Protein Complex Promotes DNA Damage and Cell Death in Cancer Cells

DNA replication initiation requires the loading of MCM2-7 complexes at the origins of replication during G1. Replication licensing renders chromatin competent for DNA replication and its tight regulation is essential to prevent aberrant DNA replication and genomic instability. CDT1 is a critical factor of licensing and its activity is controlled by redundant mechanisms, including Geminin, a protein inhibitor of CDT1. Aberrant CDT1 and Geminin expression have been shown to promote tumorigenesis in vivo and are also evident in multiple human tumors. In this study, we developed an in vitro AlphaScreen™ high-throughput screening (HTS) assay for the identification of small-molecule inhibitors targeting the CDT1/Geminin protein complex. Biochemical characterization of the most potent compound, AF615, provided evidence of specific, dose-dependent inhibition of Geminin binding to CDT1 both in-vitro and in cells. Moreover, compound AF615 induces DNA damage, inhibits DNA synthesis and reduces viability selectively in cancer cell lines, and this effect is CDT1-dependent. Taken together, our data suggest that AF615 may serve as a useful compound to elucidate the role of CDT1/Geminin protein complex in replication licensing and origin firing as well as a scaffold for further medicinal chemistry optimisation.

Thermal stress induced hyperglycemia in the blue swimmer crab, Portunus pelagicus

Hyperglycemia is a stress responsible mechanism induced in crustaceans through the secretion of Crustacean Hyperglycemic Hormone (CHH). The effect of thermal shock on the hemolymph CHH levels was studied in P. pelagicus. Crabs were exposed to varying temperatures for 3 h and were then transferred to ambient temperature (28 °C). A higher CHH level of 47.30 ± 2.26 fmol/ml was observed on exposure of crabs to 24 °C, over a recovery period of 3 h. This was reflected with increase in hemolymph glucose causing hyperglycemia and subsequent decrease in hepatopancreas glycogen levels. The results suggest the modulatory role of CHH in producing the energy required for the physiological reparation faced by the crabs during thermal stress.

Coordinating DNA Replication and Mitosis through Ubiquitin/SUMO and CDK1

Post-translational modification of the DNA replication machinery by ubiquitin and SUMO plays key roles in the faithful duplication of the genetic information. Among other functions, ubiquitination and SUMOylation serve as signals for the extraction of factors from chromatin by the AAA ATPase VCP. In addition to the regulation of DNA replication initiation and elongation, we now know that ubiquitination mediates the disassembly of the replisome after DNA replication termination, a process that is essential to preserve genomic stability. Here, we review the recent evidence showing how active DNA replication restricts replisome ubiquitination to prevent the premature disassembly of the DNA replication machinery. Ubiquitination also mediates the removal of the replisome to allow DNA repair. Further, we discuss the interplay between ubiquitin-mediated replisome disassembly and the activation of CDK1 that is required to set up the transition from the S phase to mitosis. We propose the existence of a ubiquitin–CDK1 relay, where the disassembly of terminated replisomes increases CDK1 activity that, in turn, favors the ubiquitination and disassembly of more replisomes. This model has important implications for the mechanism of action of cancer therapies that induce the untimely activation of CDK1, thereby triggering premature replisome disassembly and DNA damage.

The SCF Complex Is Essential to Maintain Genome and Chromosome Stability

The SKP1, CUL1, F-box protein (SCF) complex encompasses a group of 69 SCF E3 ubiquitin ligase complexes that primarily modify protein substrates with poly-ubiquitin chains to target them for proteasomal degradation. These SCF complexes are distinguishable by variable F-box proteins, which determine substrate specificity. Although the function(s) of each individual SCF complex remain largely unknown, those that have been characterized regulate a wide array of cellular processes, including gene transcription and the cell cycle. In this regard, the SCF complex regulates transcription factors that modulate cell signaling and ensures timely degradation of primary cell cycle regulators for accurate replication and segregation of genetic material. SCF complex members are aberrantly expressed in a myriad of cancer types, with altered expression or function of the invariable core SCF components expected to have a greater impact on cancer pathogenesis than that of the F-box proteins. Accordingly, this review describes the normal roles that various SCF complexes have in maintaining genome stability before discussing the impact that aberrant SCF complex expression and/or function have on cancer pathogenesis. Further characterization of the SCF complex functions is essential to identify and develop therapeutic approaches to exploit aberrant SCF complex expression and function.

Increased expression of Cyclin F in liver cancer predicts poor prognosis: A study based on TCGA database

BACKGROUND

Cyclin F (CCNF) dysfunction has been implicated in various forms of cancer, offering a new avenue for understanding the pathogenic mechanisms underlying hepatocellular carcinoma (HCC). We aimed to evaluate the role of CCNF in HCC using publicly available data from The Cancer Genome Atlas (TCGA).

METHOD

We used TCGA data and Gene Expression Omnibus (GEO) data to analyze the differential expression of CCNF between tumor and adjacent tissues and the relationship between CCNF and clinical characteristics. We compared prognosis of patients with HCC with high and low CCNF expression and constructed receiver operating characteristic (ROC) curves. In addition, we also explored the types of gene mutations in relevant groups and conducted Gene Set Enrichment Analysis (GSEA).

RESULTS

The expression of CCNF in liver cancer tissues was significantly increased compared with that in adjacent tissues, and patients with high CCNF expression had a worse prognosis than those with low CCNF expression. Patients with high CCNF expression also had more somatic mutations. High expression of CCNF hampers the prognosis independently. The GSEA showed that the "http://www.gsea-msigdb.org/gsea/msigdb/cards/BIOCARTA_WNT_PATHWAY" Wnt pathway, "http://www.gsea-msigdb.org/gsea/msigdb/cards/BIOCARTA_P53_PATHWAY" P53 pathway, "http://www.gsea-msigdb.org/gsea/msigdb/cards/HALLMARK_PI3K_AKT_MTOR_SIGNALING" PI3K/Akt/mTOR pathway, "http://www.gsea-msigdb.org/gsea/msigdb/cards/HALLMARK_NOTCH_SIGNALING" Notch pathway were enriched in patients with the high CCNF expression phenotype.

CONCLUSION

High CCNF expression can be seen as an independent risk factor for poor survival in HCC. Its expression may serve as a target for the diagnosis and treatment of liver cancer.

Dynamics of replication origin over-activation

Safeguards against excess DNA replication are often dysregulated in cancer, and driving cancer cells towards over-replication is a promising therapeutic strategy. We determined DNA synthesis patterns in cancer cells undergoing partial genome re-replication due to perturbed regulatory interactions (re-replicating cells). These cells exhibited slow replication, increased frequency of replication initiation events, and a skewed initiation pattern that preferentially reactivated early-replicating origins. Unlike in cells exposed to replication stress, which activated a novel group of hitherto unutilized (dormant) replication origins, the preferred re-replicating origins arose from the same pool of potential origins as those activated during normal growth. Mechanistically, the skewed initiation pattern reflected a disproportionate distribution of pre-replication complexes on distinct regions of licensed chromatin prior to replication. This distinct pattern suggests that circumventing the strong inhibitory interactions that normally prevent excess DNA synthesis can occur via at least two pathways, each activating a distinct set of replication origins.

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.

Replication initiation: Implications in genome integrity

In every cell cycle, billions of nucleotides need to be duplicated within hours, with extraordinary precision and accuracy. The molecular mechanism by which cells regulate the replication event is very complicated, and the entire process begins way before the onset of S phase. During the G1 phase of the cell cycle, cells prepare by assembling essential replication factors to establish the pre-replicative complex at origins, sites that dictate where replication would initiate during S phase. During S phase, the replication process is tightly coupled with the DNA repair system to ensure the fidelity of replication. Defects in replication and any error must be recognized by DNA damage response and checkpoint signaling pathways in order to halt the cell cycle before cells are allowed to divide. The coordination of these processes throughout the cell cycle is therefore critical to achieve genomic integrity and prevent diseases. In this review, we focus on the current understanding of how the replication initiation events are regulated to achieve genome stability.

The Ubiquitin Proteasome System in Genome Stability and Cancer

Faithful DNA replication during cellular division is essential to maintain genome stability and cells have developed a sophisticated network of regulatory systems to ensure its integrity. Disruption of these control mechanisms can lead to loss of genomic stability, a key hallmark of cancer. Ubiquitination is one of the most abundant regulatory post-translational modifications and plays a pivotal role in controlling replication progression, repair of DNA and genome stability. Dysregulation of the ubiquitin proteasome system (UPS) can contribute to the initiation and progression of neoplastic transformation. In this review we provide an overview of the UPS and summarize its involvement in replication and replicative stress, along with DNA damage repair. Finally, we discuss how the UPS presents as an emerging source for novel therapeutic interventions aimed at targeting genomic instability, which could be utilized in the treatment and management of cancer.

Control of pre-replicative complex during the division cycle in Chlamydomonas reinhardtii

DNA replication is fundamental to all living organisms. In yeast and animals, it is triggered by an assembly of pre-replicative complex including ORC, CDC6 and MCMs. Cyclin Dependent Kinase (CDK) regulates both assembly and firing of the pre-replicative complex. We tested temperature-sensitive mutants blocking Chlamydomonas DNA replication. The mutants were partially or completely defective in DNA replication and did not produce mitotic spindles. After a long G1, wild type Chlamydomonas cells enter a division phase when it undergoes multiple rapid synchronous divisions ('multiple fission'). Using tagged transgenic strains, we found that MCM4 and MCM6 were localized to the nucleus throughout the entire multiple fission division cycle, except for transient cytoplasmic localization during each mitosis. Chlamydomonas CDC6 was transiently localized in nucleus in early division cycles. CDC6 protein levels were very low, probably due to proteasomal degradation. CDC6 levels were severely reduced by inactivation of CDKA1 (CDK1 ortholog) but not the plant-specific CDKB1. Proteasome inhibition did not detectably increase CDC6 levels in the cdka1 mutant, suggesting that CDKA1 might upregulate CDC6 at the transcriptional level. All of the DNA replication proteins tested were essentially undetectable until late G1. They accumulated specifically during multiple fission and then were degraded as cells completed their terminal divisions. We speculate that loading of origins with the MCM helicase may not occur until the end of the long G1, unlike in the budding yeast system. We also developed a simple assay for salt-resistant chromatin binding of MCM4, and found that tight MCM4 loading was dependent on ORC1, CDC6 and MCM6, but not on RNR1 or CDKB1. These results provide a microbial framework for approaching replication control in the plant kingdom.

Unbiased Label-Free Quantitative Proteomics of Cells Expressing Amyotrophic Lateral Sclerosis (ALS) Mutations in CCNF Reveals Activation of the Apoptosis Pathway: A Workflow to Screen Pathogenic Gene Mutations

The past decade has seen a rapid acceleration in the discovery of new genetic causes of ALS, with more than 20 putative ALS-causing genes now cited. These genes encode proteins that cover a diverse range of molecular functions, including free radical scavenging (e.g., SOD1), regulation of RNA homeostasis (e.g., TDP-43 and FUS), and protein degradation through the ubiquitin-proteasome system (e.g., ubiquilin-2 and cyclin F) and autophagy (TBK1 and sequestosome-1/p62). It is likely that the various initial triggers of disease (either genetic, environmental and/or gene-environment interaction) must converge upon a common set of molecular pathways that underlie ALS pathogenesis. Given the complexity, it is not surprising that a catalog of molecular pathways and proteostasis dysfunctions have been linked to ALS. One of the challenges in ALS research is determining, at the early stage of discovery, whether a new gene mutation is indeed disease-specific, and if it is linked to signaling pathways that trigger neuronal cell death. We have established a proof-of-concept proteogenomic workflow to assess new gene mutations, using CCNF (cyclin F) as an example, in cell culture models to screen whether potential gene candidates fit the criteria of activating apoptosis. This can provide an informative and time-efficient output that can be extended further for validation in a variety of in vitro and in vivo models and/or for mechanistic studies. As a proof-of-concept, we expressed cyclin F mutations (K97R, S195R, S509P, R574Q, S621G) in HEK293 cells for label-free quantitative proteomics that bioinformatically predicted activation of the neuronal cell death pathways, which was validated by immunoblot analysis. Proteomic analysis of induced pluripotent stem cells (iPSCs) derived from patient fibroblasts bearing the S621G mutation showed the same activation of these pathways providing compelling evidence for these candidate gene mutations to be strong candidates for further validation and mechanistic studies (such as E3 enzymatic activity assays, protein-protein and protein-substrate studies, and neuronal apoptosis and aberrant branching measurements in zebrafish). Our proteogenomics approach has great utility and provides a relatively high-throughput screening platform to explore candidate gene mutations for their propensity to cause neuronal cell death, which will guide a researcher for further experimental studies.

Multiple, short protein binding motifs in ORC1 and CDC6 control the initiation of DNA replication

The initiation of DNA replication involves cell cycle-dependent assembly and disassembly of protein complexes, including the origin recognition complex (ORC) and CDC6 AAA⁺ ATPases. We report that multiple short linear protein motifs (SLiMs) within intrinsically disordered regions (IDRs) in ORC1 and CDC6 mediate cyclin-CDK-dependent and independent protein-protein interactions, conditional on the cell cycle phase. A domain within the ORC1 IDR is required for interaction between the ORC1 and CDC6 AAA⁺ domains in G1, whereas the same domain prevents CDC6-ORC1 interaction during mitosis. Then, during late G1, this domain facilitates ORC1 destruction by a SKP2-cyclin A-CDK2-dependent mechanism. During G1, the CDC6 Cy motif cooperates with cyclin E-CDK2 to promote ORC1-CDC6 interactions. The CDC6 IDR regulates self-interaction by ORC1, thereby controlling ORC1 protein levels. Protein phosphatase 1 binds directly to a SLiM in the ORC1 IDR, causing ORC1 de-phosphorylation upon mitotic exit, increasing ORC1 protein, and promoting pre-RC assembly.

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.

Expression of Genomic Instability-Related Molecules: Cyclin F, RRM2 and SPDL1 and Their Prognostic Significance in Pancreatic Adenocarcinoma

In the present study, we aimed to assess the selected components of cell cycle machinery, checkpoint, DNA repair, and synthesis, namely RRM2, cyclin F, and SPDL1 in pancreatic adenocarcinomas (PAC) by in-house immunohistochemistry (IHC) and bioinformatic analysis of public datasets, in terms of expression, correlation with clinicopathological parameters, and patient survival. Sixty eight patients with pancreatic ductal adenocarcinoma (PDAC) were included in our cohort study, and IHC was performed on tissue macroarrays. RNA-Seq-based transcriptome data for 177 PACs were retrieved from the Cancer Genome Atlas (TCGA). We found cyclin F, RRM2, and SPDL1 to be overexpressed at both protein and mRNA levels in tumor tissues compared to respective controls. Based on TCGA dataset, we have demonstrated that CCNF, RRM2, and SPDL1 are potent independent prognostic markers for poor overall survival, both by themselves and even more in combination with each other. Furthermore, high CCNF mRNA expression was associated with features of cancer progression. By contrast, overexpression of cyclin F or SPDL1 proteins denoted a good prognosis in PDAC patients; however, in the case of the former protein, the results did not reach statistical significance. Specifically, high levels of SPDL1 protein emerged as the most powerful independent prognostic factor associated with a better outcome. If validated, the CCNF/RRM2/SPDL1 three-gene panel developed in this study, as well as SPDL1 protein, may provide significant clinical implications for the prognosis prediction of PAC patients.

E2F1: Cause and Consequence of DNA Replication Stress

In mammalian cells, cell cycle entry occurs in response to the correct stimuli and is promoted by the transcriptional activity of E2F family members. E2F proteins regulate the transcription of S phase cyclins and genes required for DNA replication, DNA repair, and apoptosis. The activity of E2F1, the archetypal and most heavily studied E2F family member, is tightly controlled by the DNA damage checkpoints to modulate cell cycle progression and initiate programmed cell death, when required. Altered tumor suppressor and oncogenic signaling pathways often result in direct or indirect interference with E2F1 regulation to ensure higher rates of cell proliferation independently of external cues. Despite a clear link between dysregulated E2F1 activity and cancer progression, literature on the contribution of E2F1 to DNA replication stress phenotypes is somewhat scarce. This review discusses how dysfunctional tumor suppressor and oncogenic signaling pathways promote the disruption of E2F1 transcription and hence of its transcriptional targets, and how such events have the potential to drive DNA replication stress. In addition to the involvement of E2F1 upstream of DNA replication stress, this manuscript also considers the role of E2F1 as a downstream effector of the response to this type of cellular stress. Lastly, the review introduces some reflections on how E2F1 activity is integrated with checkpoint control through post-translational regulation, and proposes an exploitable tumor weakness based on this axis.

Regulation of cell cycle drivers by Cullin-RING ubiquitin ligases

The last decade has revealed new roles for Cullin-RING ubiquitin ligases (CRLs) in a myriad of cellular processes, including cell cycle progression. In addition to CRL1, also named SCF (SKP1-Cullin 1-F box protein), which has been known for decades as an important factor in the regulation of the cell cycle, it is now evident that all eight CRL family members are involved in the intricate cellular pathways driving cell cycle progression. In this review, we summarize the structure of CRLs and their functions in driving the cell cycle. We focus on how CRLs target key proteins for degradation or otherwise alter their functions to control the progression over the various cell cycle phases leading to cell division. We also summarize how CRLs and the anaphase-promoting complex/cyclosome (APC/C) ligase complex closely cooperate to govern efficient cell cycle progression.

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.

The Global Phosphorylation Landscape of SARS-CoV-2 Infection

The causative agent of the coronavirus disease 2019 (COVID-19) pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has infected millions and killed hundreds of thousands of people worldwide, highlighting an urgent need to develop antiviral therapies. Here we present a quantitative mass spectrometry-based phosphoproteomics survey of SARS-CoV-2 infection in Vero E6 cells, revealing dramatic rewiring of phosphorylation on host and viral proteins. SARS-CoV-2 infection promoted casein kinase II (CK2) and p38 MAPK activation, production of diverse cytokines, and shutdown of mitotic kinases, resulting in cell cycle arrest. Infection also stimulated a marked induction of CK2-containing filopodial protrusions possessing budding viral particles. Eighty-seven drugs and compounds were identified by mapping global phosphorylation profiles to dysregulated kinases and pathways. We found pharmacologic inhibition of the p38, CK2, CDK, AXL, and PIKFYVE kinases to possess antiviral efficacy, representing potential COVID-19 therapies.

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.

Pathogenic mutations in the ALS gene CCNF cause cytoplasmic mislocalization of Cyclin F and elevated VCP ATPase activity

Amyotrophic lateral sclerosis (ALS) is an adult-onset motor neuron disease characterized by a progressive decline in motor function. Genetic analyses have identified several genes mutated in ALS patients, and one of them is Cyclin F gene (CCNF), the product of which (Cyclin F) serves as the substrate-binding module of a SKP1-CUL1-F-box protein (SCF) ubiquitin ligase complex. However, the role of Cyclin F in ALS pathogenesis has remained unclear. Here, we show that Cyclin F binds to valosin-containing protein (VCP), which is also reported to be mutated in ALS, and that the two proteins colocalize in the nucleus. VCP was found to bind to the NH2-terminal region of Cyclin F and was not ubiquitylated by SCFCyclin F in transfected cells. Instead, the ATPase activity of VCP was enhanced by Cyclin F in vitro. Furthermore, whereas ALS-associated mutations of CCNF did not affect the stability of Cyclin F or disrupt formation of the SCFCyclin F complex, amino acid substitutions in the VCP binding region increased the binding ability of Cyclin F to VCP and activity of VCP as well as mislocalization of the protein in the cytoplasm. We also provided evidence that the ATPase activity of VCP promotes cytoplasmic aggregation of transactivation responsive region (TAR) DNA-binding protein 43, which is commonly observed in degenerating neurons in ALS patients. Given that mutations of VCP identified in ALS patients also increase its ATPase activity, our results suggest that Cyclin F mutations may contribute to ALS pathogenesis by increasing the ATPase activity of VCP in the cytoplasm, which in turn increases TDP-43 aggregates.

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.

Pisum sativum Defensin 1 Eradicates Mouse Metastatic Lung Nodules from B16F10 Melanoma Cells

Psd1 is a pea plant defensin which can be actively expressed in Pichia pastoris and shows broad antifungal activity. This activity is dependent on fungal membrane glucosylceramide (GlcCer), which is also important for its internalization, nuclear localization, and endoreduplication. Certain cancer cells present a lipid metabolism imbalance resulting in the overexpression of GlcCer in their membrane. In this work, in vitroassays using B16F10 cells showed that labeled fluorescein isothiocyanate FITC-Psd1 internalized into live cultured cells and targeted the nucleus, which underwent fragmentation, exhibiting approximately 60% of cells in the sub-G0/G1 stage. This phenomenon was dependent on GlcCer, and the participation of cyclin-F was suggested. In a murine lung metastatic melanoma model, intravenous injection of Psd1 together with B16F10 cells drastically reduced the number of nodules at concentrations above 0.5 mg/kg. Additionally, the administration of 1 mg/kg Psd1 decreased the number of lung inflammatory cells to near zero without weight loss, unlike animals that received melanoma cells only. It is worth noting that 1 mg/kg Psd1 alone did not provoke inflammation in lung tissue or weight or vital signal losses over 21 days, inferring no whole animal cytotoxicity. These results suggest that Psd1 could be a promising prototype for human lung anti-metastatic melanoma therapy.

F-box proteins and cancer: an update from functional and regulatory mechanism to therapeutic clinical prospects

E3 ubiquitin ligases play a critical role in cellular mechanisms and cancer progression. F-box protein is the core component of the SKP1-cullin 1-F-box (SCF)-type E3 ubiquitin ligase and directly binds to substrates by various specific domains. According to the specific domains, F-box proteins are further classified into three sub-families: 1) F-box with leucine rich amino acid repeats (FBXL); 2) F-box with WD 40 amino acid repeats (FBXW); 3) F-box only with uncharacterized domains (FBXO). Here, we summarize the substrates of F-box proteins, discuss the important molecular mechanism and emerging role of F-box proteins especially from the perspective of cancer development and progression. These findings will shed new light on malignant tumor progression mechanisms, and suggest the potential role of F-box proteins as cancer biomarkers and therapeutic targets for future cancer treatment.

Reduced SKP1 Expression Induces Chromosome Instability through Aberrant Cyclin E1 Protein Turnover

Chromosome instability (CIN), or progressive changes in chromosome numbers, is an enabling feature of many cancers; however, the mechanisms giving rise to CIN remain poorly understood. To expand our mechanistic understanding of the molecular determinants of CIN in humans, we employed a cross-species approach to identify 164 human candidates to screen. Using quantitative imaging microscopy (QuantIM), we show that silencing 148 genes resulted in significant changes in CIN-associated phenotypes in two distinct cellular contexts. Ten genes were prioritized for validation based on cancer patient datasets revealing frequent gene copy number losses and associations with worse patient outcomes. QuantIM determined silencing of each gene-induced CIN, identifying novel roles for each as chromosome stability genes. SKP1 was selected for in-depth analyses as it forms part of SCF (SKP1, CUL1, FBox) complex, an E3 ubiquitin ligase that targets proteins for proteolytic degradation. Remarkably, SKP1 silencing induced increases in replication stress, DNA double strand breaks and chromothriptic events that were ascribed to aberrant increases in Cyclin E1 levels arising from reduced SKP1 expression. Collectively, these data reveal a high degree of evolutionary conservation between human and budding yeast CIN genes and further identify aberrant mechanisms associated with increases in chromothriptic events.

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.

Regulation of Cell Cycle Entry and Exit: A Single Cell Perspective

The transition between proliferating and quiescent states must be carefully regulated to ensure that cells divide to create the cells an organism needs only at the appropriate time and place. Cyclin-dependent kinases (CDKs) are critical for both transitioning cells from one cell cycle state to the next, and for regulating whether cells are proliferating or quiescent. CDKs are regulated by association with cognate cyclins, activating and inhibitory phosphorylation events, and proteins that bind to them and inhibit their activity. The substrates of these kinases, including the retinoblastoma protein, enforce the changes in cell cycle status. Single cell analysis has clarified that competition among factors that activate and inhibit CDK activity leads to the cell's decision to enter the cell cycle, a decision the cell makes before S phase. Signaling pathways that control the activity of CDKs regulate the transition between quiescence and proliferation in stem cells, including stem cells that generate muscle and neurons. © 2020 American Physiological Society. Compr Physiol 10:317-344, 2020.

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

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/CCdh1 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 SCFcyclin 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 SCFcyclin F remains stable during G2. Furthermore, SCFcyclin F can also interact and induce degradation of E2F8. However, this does not require the cyclin domain of SCFcyclin 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.