Control of DNA replication licensing in a cell cycle

Regulation of cancer stem cells by CXCL1, a chemokine whose secretion is controlled by MCM2

BACKGROUND

A high expression pattern of minichromosome maintenance 2 (MCM2) has been observed in various cancers. MCM2 is a protein involved in the cell cycle and plays a role in cancer growth and differentiation by binding to six members of the MCM subfamily. The MCM protein family includes MCM2 through MCM7.

METHODS

MCM2 has shown high expression in both lung cancer stem cells (LCSCs) and glioma stem cells (GSCs). We investigated the characteristics of CSCs and the regulation of the epithelial-to-mesenchymal transition (EMT) phenomenon in LCSCs and GSCs by MCM2. Additionally, we explored secreted factors regulated by MCM2.

RESULTS

There was a significant difference in survival rates between lung cancer patients and brain cancer patients based on MCM2 expression. MCM2 was found to regulate both markers and regulatory proteins in LCSCs. Moreover, MCM2 is thought to be involved in cancer metastasis by regulating cell migration and invasion, not limited to lung cancer but also identified in glioma. Among chemokines, chemokine (C-X-C motif) ligand 1 (CXCL1) was found to be regulated by MCM2.

CONCLUSIONS

MCM2 not only participates in the cell cycle but also affects cancer cell growth by regulating the external microenvironment to create a favorable environment for cells. MCM2 is highly expressed in malignant carcinomas, including CSCs, and contributes to the malignancy of various cancers. Therefore, MCM2 may represent a crucial target for cancer therapeutics.

The cell cycle revisited: DNA replication past S phase preserves genome integrity

Accurate and complete DNA duplication is critical for maintaining genome integrity. Multiple mechanisms regulate when and where DNA replication takes place, to ensure that the entire genome is duplicated once and only once per cell cycle. Although the bulk of the genome is copied during the S phase of the cell cycle, increasing evidence suggests that parts of the genome are replicated in G2 or mitosis, in a last attempt to secure that daughter cells inherit an accurate copy of parental DNA. Remaining unreplicated gaps may be passed down to progeny and replicated in the next G1 or S phase. These findings challenge the long-established view that genome duplication occurs strictly during the S phase, bridging DNA replication to DNA repair and providing novel therapeutic strategies for cancer treatment.

Genome maintenance meets mechanobiology

Genome stability is key for healthy cells in healthy organisms, and deregulated maintenance of genome integrity is a hallmark of aging and of age-associated diseases including cancer and neurodegeneration. To maintain a stable genome, genome surveillance and repair pathways are closely intertwined with cell cycle regulation and with DNA transactions that occur during transcription and DNA replication. Coordination of these processes across different time and length scales involves dynamic changes of chromatin topology, clustering of fragile genomic regions and repair factors into nuclear repair centers, mobilization of the nuclear cytoskeleton, and activation of cell cycle checkpoints. Here, we provide a general overview of cell cycle regulation and of the processes involved in genome duplication in human cells, followed by an introduction to replication stress and to the cellular responses elicited by perturbed DNA synthesis. We discuss fragile genomic regions that experience high levels of replication stress, with a particular focus on telomere fragility caused by replication stress at the ends of linear chromosomes. Using alternative lengthening of telomeres (ALT) in cancer cells and ALT-associated PML bodies (APBs) as examples of replication stress-associated clustered DNA damage, we discuss compartmentalization of DNA repair reactions and the role of protein properties implicated in phase separation. Finally, we highlight emerging connections between DNA repair and mechanobiology and discuss how biomolecular condensates, components of the nuclear cytoskeleton, and interfaces between membrane-bound organelles and membraneless macromolecular condensates may cooperate to coordinate genome maintenance in space and time.

The testosterone paradox of advanced prostate cancer: mechanistic insights and clinical implications

The discovery of the benefits of castration for prostate cancer treatment in 1941 led to androgen deprivation therapy, which remains a mainstay of the treatment of men with advanced prostate cancer. However, as early as this original publication, the inevitable development of castration-resistant prostate cancer was recognized. Resistance first manifests as a sustained rise in the androgen-responsive gene, PSA, consistent with reactivation of the androgen receptor axis. Evaluation of clinical specimens demonstrates that castration-resistant prostate cancer cells remain addicted to androgen signalling and adapt to chronic low-testosterone states. Paradoxically, results of several studies have suggested that treatment with supraphysiological levels of testosterone can retard prostate cancer growth. Insights from these studies have been used to investigate administration of supraphysiological testosterone to patients with prostate cancer for clinical benefits, a strategy that is termed bipolar androgen therapy (BAT). BAT involves rapid cycling from supraphysiological back to near-castration testosterone levels over a 4-week cycle. Understanding how BAT works at the molecular and cellular levels might help to rationalize combining BAT with other agents to achieve increased efficacy and tumour responses.

SPOP is essential for DNA replication licensing through maintaining translation of CDT1 and CDC6 in HaCaT cells

Speckle-type pox virus and zinc finger (POZ) protein (SPOP), a substrate recognition receptor for the cullin-3/RING ubiquitin E3 complex, leads to the ubiquitination of >40 of its target substrates. Since a variety of point mutations in the substrate-binding domain of SPOP have been identified in cancers, including prostate and endometrial cancers, the pathological roles of those cancer-associated SPOP mutants have been extensively elucidated. In this study, we evaluated the cellular functions of wild-type SPOP in non-cancerous human keratinocyte-derived HaCaT cells expressing wild-type SPOP gene. SPOP knockdown using siRNA in HaCaT cells dramatically reduced cell growth and arrested their cell cycles at G1/S phase. The expression of DNA replication licensing factors CDT1 and CDC6 in HaCaT cells drastically decreased on SPOP knockdown as their translation was inhibited. CDT1 and CDC6 downregulation induced p21 expression without p53 activation. Our results suggest that SPOP is essential for DNA replication licensing in non-cancerous keratinocyte HaCaT cells.

Cell cycle deregulation in neurodegenerative diseases

Background: Cell cycle is critical for a wide range of cellular processes such as proliferation, differentiation and apoptosis in dividing cells. Neurons are postmitotic cells which have withdrawn from the cell division cycle. Recent data show us that inappropriate activation of cell cycle regulators including cyclins, cyclin dependent kinases (CDKs) and endogenous cyclin dependent kinase inhibitors (CDKIs) may take part in the aetiology of neurodegenerative diseases. However, the mechanisms for cell cycle reentry in neurodegenerative disease remain unclear.Methods: Electronic databases such as Pubmed, Science Direct, Directory of Open Access Journals, PLOS were searched for relevant articles.Conclusion: The present work reviews basic aspects of cell cycle mechanism, as well as the evidence showing the expression of cell cycle proteins in neurodegenerative disease. We provide a brief summary of these findings and hope to highlight the interaction between the cell cycle reentry and neurodegenerative diseases. Moreover, we outline the possible signaling pathways. However more understanding of the mechanism of cell cycle is of great importance. Because these represents an alternative target for therapeutic interventions, leading to novel treatments of neurodegenerative diseases.

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.

In silico investigation of the role of vitamins in cancer therapy through inhibition of MCM7 oncoprotein

An overabundance of MCM7 protein, a component of the minichromosome maintenance complex that normally initiates DNA replication, has been reported to cause different types of cancers with aggressive malignancy. Inhibition of MCM7 may lead to a significant reduction in cancer-associated cell proliferation. Despite such significance of MCM7 in cancer, the protein structure is yet to be resolved experimentally. This significantly halts the structure-guided ligand designing for cancer therapy targeting the MCM7. The present study aims to resolve the tertiary structure of MCM7 and repurpose the FDA-approved clinically used drugs for cancer therapy by targeting MCM7 protein. The secondary and 3D structures of MCM7 were generated using multiple bioinformatics tools, including the Self-Optimized Prediction Method with Alignment (SOPMA), SWISS-MODEL, and I-TASSER. The reliability of the modeled structure was assessed using PROCHECK. Initially, a structure-guided virtual screening was performed on the approved drug library to identify potential hits against MCM7. The detailed molecular mechanism of receptor interactions of the identified hits was evaluated using extensive molecular dynamics simulation. The results from this study reveal an intriguing discovery of the potential of ergocalciferol (vitamin D2), cholecalciferol (vitamin D3), ergosterol (precursor of vitamin D2) and menaquinone (vitamin K2) as oncoprotein inhibitors for cancer therapy via inhibition of MCM7.

Identification, expression and subcellular localization of Orc1 in the microsporidian Nosema bombycis

As a typical species of microsporidium, Nosema bombycis is the pathogen causing the pébrine disease of silkworm. Rapid proliferation of N. bombycis in host cells requires replication of genetic material. As eukaryotic origin recognition protein, origin recognition complex (ORC) plays an important role in regulating DNA replication, and Orc1 is a key subunit of the origin recognition complex. In this study, we identified the Orc1 in the microsporidian N. bombycis (NbOrc1) for the first time. The NbOrc1 gene contains a complete ORF of 987 bp in length that encodes a 328 amino acid polypeptide. Indirect immunofluorescence results showed that NbOrc1 were colocalized with Nbactin and NbSAS-6 in the nuclei of N. bombycis. Subsequently, we further identified the interaction between the NbOrc1 and Nbactin by CO-IP and Western blot. These results imply that Orc1 may be involved in the proliferation of the microsporidian N. bombycis through interacting with actin.

Symbiosis with Dinoflagellates Alters Cnidarian Cell-Cycle Gene Expression

In the cnidarian-dinoflagellate symbiosis, hosts show altered expression of genes involved in growth and proliferation when in the symbiotic state, but little is known about the molecular mechanisms that underlie the host’s altered growth rate. Using tissue-specific transcriptomics, we determined how symbiosis affects expression of cell cycle-associated genes, in the model symbiotic cnidarian Exaiptasia diaphana (Aiptasia). The presence of symbionts within the gastrodermis elicited cell-cycle arrest in the G1 phase in a larger proportion of host cells compared with the aposymbiotic gastrodermis. The symbiotic gastrodermis also showed a reduction in the amount of cells synthesizing their DNA and progressing through mitosis when compared with the aposymbiotic gastrodermis. Host apoptotic inhibitors (Mdm2) were elevated, while host apoptotic sensitizers (c-Myc) were depressed, in the symbiotic gastrodermis when compared with the aposymbiotic gastrodermis and epidermis of symbiotic anemones, respectively. This indicates that the presence of symbionts negatively regulates host apoptosis, possibly contributing to their persistence within the host. Transcripts (ATM/ATR) associated with DNA damage were also downregulated in symbiotic gastrodermal tissues. In epidermal cells, a single gene (Mob1) required for mitotic completion was upregulated in symbiotic compared with aposymbiotic anemones, suggesting that the presence of symbionts in the gastrodermis stimulates host cell division in the epidermis. To further corroborate this hypothesis, we performed microscopic analysis using an S-phase indicator (EdU), allowing us to evaluate cell cycling in host cells. Our results confirmed that there were significantly more proliferating host cells in both the gastrodermis and epidermis in the symbiotic state compared with the aposymbiotic state. Furthermore, when comparing between tissue layers in the presence of symbionts, the epidermis had significantly more proliferating host cells than the symbiont-containing gastrodermis. These results contribute to our understanding of the influence of symbionts on the mechanisms of cnidarian cell proliferation and mechanisms associated with symbiont maintenance.

Precisely Detecting the Telomerase Activities by an AIEgen Probe with Dual Signal Outputs after Cell-Cycle Synchronization

By maintaining the telomere lengths, telomerase can make the tumor cells avoid the apoptosis, thus, achieving the cell immortalization. In the past, a series of telomerase detection systems have been developed through utilizing the unique characteristic of telomerase extended primer. However, fluctuation of telomerase activity, along with the cell cycle progression, leads to ambiguous detection results. Here, we reported a dual signal output detection strategy based on cell-cycle synchronization for precisely detecting telomerase activities by using a new AIEgen probe SSNB. Experimental and simulating calculation results demonstrated that positively charged SSNB could interact with DNA through the electrostatic interaction and π-π interaction, as well as the hydrogen bonds. The aggregation of SSNB caused by the extended template strand primer (TP) could be observed in tumor cells, thus, indicating the telomerase activity in various cell lines. Furthermore, after cell cycle synchronization, it was found that the telomerase activity in the S phase was the highest, no matter from the fluorescence intensity or the ROS generation situation. Dual signal outputs of SSNB verified the significance and necessity of cell-cycle synchronization detection for telomerase activity. This strategy could open a new window for the biotargets of which activity is variational in time dimension.

CRL4Cdt2 Ubiquitin Ligase, A Genome Caretaker Controlled by Cdt2 Binding to PCNA and DNA

The ubiquitin ligase CRL4Cdt2 plays a vital role in preserving genomic integrity by regulating essential proteins during S phase and after DNA damage. Deregulation of CRL4Cdt2 during the cell cycle can cause DNA re-replication, which correlates with malignant transformation and tumor growth. CRL4Cdt2 regulates a broad spectrum of cell cycle substrates for ubiquitination and proteolysis, including Cdc10-dependent transcript 1 or Chromatin licensing and DNA replication factor 1 (Cdt1), histone H4K20 mono-methyltransferase (Set8) and cyclin-dependent kinase inhibitor 1 (p21), which regulate DNA replication. However, the mechanism it operates via its substrate receptor, Cdc10-dependent transcript 2 (Cdt2), is not fully understood. This review describes the essential features of the N-terminal and C-terminal parts of Cdt2 that regulate CRL4 ubiquitination activity, including the substrate recognition domain, intrinsically disordered region (IDR), phosphorylation sites, the PCNA-interacting protein-box (PIP) box motif and the DNA binding domain. Drugs targeting these specific domains of Cdt2 could have potential for the treatment of cancer.

Geminin is essential for DNA re-replication in the silk gland cells of silkworms

Endoreplication, known as endocycles or endoreduplication, is a cell cycle variant in which the genomic DNA is re-replicated without mitosis leading to polyploidy. Endoreplication is essential for the development and functioning of the different organs in animals and plants. Deletion of Geminin, a DNA replication licensing inhibitor, causes DNA re-replication or damage. However, the role of Geminin in endoreplication is still unclear. Here, we studied the role of Geminin in the endoreplication of the silk gland cells of silkworms by constructing two transgenic silkworm strains, including BmGeminin1-overexpression and BmGeminin1-RNA interference. Interference of BmGeminin1 led to body weight gain, increased silk gland volume, increased DNA content, and enhanced DNA re-replication activity relative to wild-type Dazao. Meanwhile, overexpression of BmGeminin1 showed an opposite phenotype compared to the BmGem1-RNAi strain. Furthermore, RNA-sequencing of the transgenic strains was carried out to explore how BmGeminin1 regulates DNA re-replication. Our data demonstrated a vital role of Geminin in the regulation of endoreplication in the silk gland of silkworms.

Non-metabolic function of MTHFD2 activates CDK2 in bladder cancer

Bladder cancer is a common tumor with a high recurrence rate and high fatality rate, and its mechanism of occurrence and development remains unclear. Many proteins and metabolites reprogram at different stages of tumor development to support tumor cell growth. The moonlighting effect happens when a protein performs multiple functions simultaneously in a cell. In this study, we identified a metabolic protein, MTHFD2, which participates in the cell cycle by binding to CDK2 in bladder cancer. MTHFD2 has been shown to affect bladder cancer cell growth, which is independent of its metabolic function. We found that MTHFD2 was involved in cell cycle regulation and could encourage cell cycle progression by activating CDK2 and sequentially affecting E2F1 activation. In addition, moonlighting MTHFD2 might be regulated by the dynamics of the mitochondria. In conclusion, MTHFD2 localizes in the nucleus to perform a distinct function of catalyzing metabolic reactions. Moreover, the nuclear MTHFD2 activates CDK2 and promotes bladder cancer cell growth by modulating the cell cycle.

Characterization of microRNA expression in B cells derived from Japanese black cattle naturally infected with bovine leukemia virus by deep sequencing

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis (EBL), a malignant B cell lymphoma. However, the mechanisms of BLV-associated lymphomagenesis remain poorly understood. Here, after deep sequencing, we performed comparative analyses of B cell microRNAs (miRNAs) in cattle infected with BLV and those without BLV. In BLV-infected cattle, BLV-derived miRNAs (blv-miRNAs) accounted for 38% of all miRNAs in B cells. Four of these blv-miRNAs (blv-miR-B1-5p, blv-miR-B2-5p, blv-miR-B4-3p, and blv-miR-B5-5p) had highly significant positive correlations with BLV proviral load (PVL). The read counts of 90 host-derived miRNAs (bta-miRNAs) were significantly down-regulated in BLV-infected cattle compared to those in uninfected cattle. Only bta-miR-375 had a positive correlation with PVL in BLV-infected cattle and was highly expressed in the B cell lymphoma tissue of EBL cattle. There were a few bta-miRNAs that correlated with BLV tax/rex gene expression; however, BLV AS1 expression had a significant negative correlation with many of the down-regulated bta-miRNAs that are important for tumor development and/or tumor suppression. These results suggest that BLV promotes lymphomagenesis via AS1 and blv-miRNAs, rather than tax/rex, by down-regulating the expression of bta-miRNAs that have a tumor-suppressing function, and this downregulation is linked to increased PVL.

Comprehensive analysis of pathological changes in hip joint capsule of patients with developmental dysplasia of the hip

Aims

Developmental dysplasia of the hip (DDH) is a complex musculoskeletal disease that occurs mostly in children. This study aimed to investigate the molecular changes in the hip joint capsule of patients with DDH.

Methods

High-throughput sequencing was used to identify genes that were differentially expressed in hip joint capsules between healthy controls and DDH patients. Biological assays including cell cycle, viability, apoptosis, immunofluorescence, reverse transcription polymerase chain reaction (RT-PCR), and western blotting were performed to determine the roles of the differentially expressed genes in DDH pathology.

Results

More than 1,000 genes were differentially expressed in hip joint capsules between healthy controls and DDH. Both gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that extracellular matrix (ECM) modifications, muscle system processes, and cell proliferation were markedly influenced by the differentially expressed genes. Expression of Collagen Type I Alpha 1 Chain (COL1A1), COL3A1, matrix metalloproteinase-1 (MMP1), MMP3, MMP9, and MMP13 was downregulated in DDH, with the loss of collagen fibres in the joint capsule. Expression of transforming growth factor beta 1 (TGF-β1) was downregulated, while that of TGF-β2, Mothers against decapentaplegic homolog 3 (SMAD3), and WNT11 were upregulated in DDH, and alpha smooth muscle actin (αSMA), a key myofibroblast marker, showed marginal increase. In vitro studies showed that fibroblast proliferation was suppressed in DDH, which was associated with cell cycle arrest in G0/G1 and G2/M phases. Cell cycle regulators including Cyclin B1 (CCNB1), Cyclin E2 (CCNE2), Cyclin A2 (CCNA2), Cyclin-dependent kinase 1 (CDK1), E2F1, cell division cycle 6 (CDC6), and CDC7 were downregulated in DDH.

Conclusion

DDH is associated with the loss of collagen fibres and fibroblasts, which may cause loose joint capsule formation. However, the degree of differentiation of fibroblasts to myofibroblasts needs further study.

Cite this article: Bone Joint Res 2021;10(9):558–570.

Aggregation-induced emission luminogens reveal cell cycle-dependent telomerase activity in cancer cells

Telomerase acts as an important biomarker for tumor identification, and synthesizes telomeric repeats at the end of chromosome telomeres during the replicative phase of the cell cycle; thus, the expression level of telomerase changes as the cell cycle progresses. TERT mRNA expression and telomerase activity were significantly increased in over 80% of human cancers from tissue specimens. Although many efforts have been made in detecting the activity of TERT mRNA and active telomerase, the heterogeneous behavior of the cell cycle was overlooked, which might affect the accuracy of the detection results. Herein, the AIEgen-based biosensing systems of PyTPA-DNA and Silole-R were developed to detect the cellular level of TERT mRNA and telomerase in different cell cycles. As a result, the fluorescence signal of cancer cells gradually increased from G0/G1, G1/S to S phase. In contrast, both cancer cells arrested at G2/M phase and normal cells exhibited negligible fluorescence intensities. Compared to normal tissues, malignant tumor samples demonstrated a significant turn-on fluorescence signal. Furthermore, the transcriptomics profiling revealed that tumor biomarkers changed as the cell cycle progressed and biomarkers of CA9, TK1 and EGFR were more abundantly expressed at early S stage. In this vein, our study presented advanced biosensing tools for more accurate analysis of the cell-cycle-dependent activity of TERT mRNA and active telomerase in clinical tissue samples.

Pharmacoinformatics and molecular dynamics simulation-based phytochemical screening of neem plant (Azadiractha indica) against human cancer by targeting MCM7 protein

Minichromosome maintenance complex component 7 (MCM7) belongs to the minichromosome maintenance family that is important for the initiation of eukaryotic DNA replication. Overexpression of the MCM7 protein is relative to cellular proliferation and responsible for aggressive malignancy in various cancers. Mechanistically, inhibition of MCM7 significantly reduces the cellular proliferation associated with cancer. To date, no effective small molecular candidate has been identified that can block the progression of cancer induced by the MCM7 protein. Therefore, the study has been designed to identify small molecular-like natural drug candidates against aggressive malignancy associated with various cancers by targeting MCM7 protein. To identify potential compounds against the targeted protein a comprehensive in silico drug design including molecular docking, ADME (Absorption, Distribution, Metabolism and Excretion), toxicity, and molecular dynamics (MD) simulation approaches has been applied. Seventy phytochemicals isolated from the neem tree (Azadiractha indica) were retrieved and screened against MCM7 protein by using the molecular docking simulation method, where the top four compounds have been chosen for further evaluation based on their binding affinities. Analysis of ADME and toxicity properties reveals the efficacy and safety of the selected four compounds. To validate the stability of the protein-ligand complex structure MD simulations approach has also been performed to the protein-ligand complex structure, which confirmed the stability of the selected three compounds including CAS ID:105377-74-0, CID:12308716 and CID:10505484 to the binding site of the protein. In the study, a comprehensive data screening process has performed based on the docking, ADMET properties, and MD simulation approaches, which found a good value of the selected four compounds against the targeted MCM7 protein and indicates as a promising and effective human anticancer agent.

Transcriptome Analysis of Caco-2 Cells upon the Exposure of Mycotoxin Deoxynivalenol and Its Acetylated Derivatives

Deoxynivalenol (DON), 3-acetyldeoxynivalenol (3-ADON) and 15-acetyldeoxynivalenol (15-ADON) are type B trichothecenes; one of the major pollutants in food and feed products. Although the toxicity of DON has been well documented, information on the toxicity of its acetylated derivative remains incomplete. To acquire more detailed insight into 3-ADON and 15-ADON, Caco-2 cells under 0.5 µM DON, 3-ADON and 15-ADON treatment for 24 h were subjected to RNA-seq analysis. In the present study, 2656, 3132 and 2425 differentially expressed genes (DEGs) were selected, respectively, and were enriched utilizing the Kyoto Encyclopedia of Genes and Genomes (KEGG) and the Gene Ontology (GO) database. The upregulation of ataxia-telangiectasia mutated kinase (ATM), WEE1 homolog 2 (WEE2) and downregulation of proliferating cell nuclear antigen (PCNA), minichromosome maintenance (MCMs), cyclin dependent kinase (CDKs), and E2Fs indicate that the three toxins induced DNA damage, inhibition of DNA replication and cell cycle arrest in Caco-2 cells. Additionally, the upregulation of sestrin (SENEs) and NEIL1 implied that the reason for DNA damage may be attributable to oxidative stress. Our study provides insight into the toxic mechanism of 3-ADON and 15-ADON.

Prospect of reprogramming replication licensing for cancer drug development

Eukaryotic chromosomal DNA replication is preceded by replication licensing which involves the identification of the origin of replication by origin recognition complex (ORC). The ORC loads pre-replication complexes (pre-RCs) through a series of tightly regulated mechanisms where the ORC interacts with Cdc6 to recruit cdt1-MCM2-7 complexes to the origin of replication. In eukaryotes, adherence to regulatory mechanisms of the replication program is required to ensure that all daughter cells carry the exact copy of genetic material as the parent cell. Failure of which leads to the development of genome instability syndromes like cancer, diabetes, etc. In an event of such occurrence, preventing cells from carrying the defaulted genetic material and passing it to other cells hinges on the regulation of chromosomal DNA replication. Thus, understanding the mechanisms underpinning chromosomal DNA replication and particularly replication licensing can expose druggable enzymes, effector molecules, and secondary messengers that can be targeted for diagnosis and therapeutic purposes. Effectively drugging these molecular markers to reprogram pre-replication events can be used to control the fate of chromosomal DNA replication for the treatment of genome instability disorders and in this case, cancer. This review discusses available knowledge of replication licensing in the contest of molecular drug discovery for the treatment of cancer.

OsMre11 Is Required for Mitosis during Rice Growth and Development

Meiotic recombination 11 (Mre11) is a relatively conserved nuclease in various species. Mre11 plays important roles in meiosis and DNA damage repair in yeast, humans and Arabidopsis, but little research has been done on mitotic DNA replication and repair in rice. Here, it was found that Mre11 was an extensively expressed gene among the various tissues and organs of rice, and loss-of-function of Mre11 resulted in severe defects of vegetative and reproductive growth, including dwarf plants, abnormally developed male and female gametes, and completely abortive seeds. The decreased number of cells in the apical meristem and the appearance of chromosomal fragments and bridges during the mitotic cell cycle in rice mre11 mutant roots revealed an essential role of OsMre11. Further research showed that DNA replication was suppressed, and a large number of DNA strand breaks occurred during the mitotic cell cycle of rice mre11 mutants. The expression of OsMre11 was up-regulated with the treatment of hydroxyurea and methyl methanesulfonate. Moreover, OsMre11 could form a complex with OsRad50 and OsNbs1, and they might function together in non-homologous end joining and homologous recombination repair pathways. These results indicated that OsMre11 plays vital roles in DNA replication and damage repair of the mitotic cell cycle, which ensure the development and fertility of rice by maintaining genome stability.

Phylogenetic analysis of cell-cycle regulatory proteins within the Symbiodiniaceae

In oligotrophic waters, cnidarian hosts rely on symbiosis with their photosynthetic dinoflagellate partners (family Symbiodiniaceae) to obtain the nutrients they need to grow, reproduce and survive. For this symbiosis to persist, the host must regulate the growth and proliferation of its symbionts. One of the proposed regulatory mechanisms is arrest of the symbiont cell cycle in the G₁ phase, though the cellular mechanisms involved remain unknown. Cell-cycle progression in eukaryotes is controlled by the conserved family of cyclin-dependent kinases (CDKs) and their partner cyclins. We identified CDKs and cyclins in different Symbiodiniaceae species and examined their relationship to homologs in other eukaryotes. Cyclin proteins related to eumetazoan cell-cycle-related cyclins A, B, D, G/I and Y, and transcriptional cyclin L, were identified in the Symbiodiniaceae, alongside several alveolate-specific cyclin A/B proteins, and proteins related to protist P/U-type cyclins and apicomplexan cyclins. The largest expansion of Symbiodiniaceae cyclins was in the P/U-type cyclin groups. Proteins related to eumetazoan cell-cycle-related CDKs (CDK1) were identified as well as transcription-related CDKs. The largest expansion of CDK groups was, however, in alveolate-specific groups which comprised 11 distinct CDK groups (CDKA-J) with CDKB being the most widely distributed CDK protein. As a result of its phylogenetic position, conservation across Symbiodiniaceae species, and the presence of the canonical CDK motif, CDKB emerged as a likely candidate for a Saccharomyces cerevisiae Cdc28/Pho85-like homolog in Symbiodiniaceae. Similar to cyclins, two CDK-groups found in Symbiodiniaceae species were solely associated with apicomplexan taxa. A comparison of Breviolum minutum CDK and cyclin gene expression between free-living and symbiotic states showed that several alveolate-specific CDKs and two P/U-type cyclins exhibited altered expression in hospite, suggesting that symbiosis influences the cell cycle of symbionts on a molecular level. These results highlight the divergence of Symbiodiniaceae cell-cycle proteins across species. These results have important implications for host control of the symbiont cell cycle in novel cnidarian–dinoflagellate symbioses.

From clocks to dominoes: lessons on cell cycle remodelling from embryonic stem cells

Cell division is a fundamental cellular process and the evolutionarily conserved networks that control cell division cycles adapt during development, tissue regeneration, cell de-differentiation and reprogramming, and a variety of pathological conditions. Embryonic development is a prime example of such versatility: fast, clock-like divisions hallmarking embryonic cells at early developmental stages become slower and controlled during cellular differentiation and lineage specification. In this review, we compare and contrast the unique cell cycle of mouse and human embryonic stem cells with that of early embryonic cells and of differentiated cells. We propose that embryonic stem cells provide an extraordinarily useful model system to understand cell cycle remodelling during embryonic-to-somatic transitions. We discuss how cell cycle networks help sustain embryonic stem cell pluripotency and self-renewal and how they safeguard cell identity and proper cell number in differentiated cells. Finally, we highlight the incredible diversity in cell cycle regulation within mammals and discuss the implications of studying cell cycle remodelling for understanding healthy and disease states.

Characterization of cyclin dependent kinase-7 and its differential response to grafting challenge in the black shell colored selected line of pearl oyster Pinctada fucata martensii

Cyclin dependent kinase-7 (Cdk-7) is a protein kinase associated with regulating the cell cycle, cell differentiation and proliferation, apoptosis and inflammatory response. This study characterized the full cDNA sequence of Cdk-7 in Pinctada fucata martensii (PmCdk-7). A full length sequence of 1473bp with an open reading frame (ORF) of 915bp and encodes a 304aa, 5'-UTR of 58bp and a 3'-UTR of 500bp was obtained. The construed amino acid sequence of PmCdk-7 comprised of a Serine/Threonine protein kinases, catalytic (S_TKc) domain with a protein kinases ATP-binding region signature (14-38aa) and the serine/Threonine protein kinases active-site signature (129-141aa) within the domain. Tissue distribution analysis revealed a high relative mRNA expression of PmCdk-7 within haemocytes. Following the insertion operation (grafting), the relative expression levels of PmCdk-7 in the haemocyte was expressed differentially among the studied groups; the black shell colored selected line (BS) and the control group (CG). High expression was recorded between 12 h and 5d with a peak at 3d suggesting a heightened level of DNA replication and inflammatory response during the pearl-sac formation and this expression was higher in BS than CS showcasing, the heightened immune capacity of BS to grafting operation. Immune stimulation experiment with bacterial endotoxin and a viral mimic revealed PmCdk-7 response to pathogenic stress. The results from our study showed that PmCdk-7 performs a vital function during the cell cycle by aiding DNA replication and also aid response to inflammations generated due to the incision from the grafting operation and long exposure to immune-stimulants (pathogens).

HRness in Breast and Ovarian Cancers

Ovarian and breast cancers are currently defined by the main pathways involved in the tumorigenesis. The majority are carcinomas, originating from epithelial cells that are in constant division and subjected to cyclical variations of the estrogen stimulus during the female hormonal cycle, therefore being vulnerable to DNA damage. A portion of breast and ovarian carcinomas arises in the context of DNA repair defects, in which genetic instability is the backdrop for cancer initiation and progression. For these tumors, DNA repair deficiency is now increasingly recognized as a target for therapeutics. In hereditary breast/ovarian cancers (HBOC), tumors with BRCA1/2 mutations present an impairment of DNA repair by homologous recombination (HR). For many years, BRCA1/2 mutations were only screened on germline DNA, but now they are also searched at the tumor level to personalize treatment. The reason of the inactivation of this pathway remains uncertain for most cases, even in the presence of a HR-deficient signature. Evidence indicates that identifying the mechanism of HR inactivation should improve both genetic counseling and therapeutic response, since they can be useful as new biomarkers of response.

Bipolar androgen therapy in prostate cancer: Current evidences and future perspectives

Testosterone suppression by androgen deprivation therapy is the cornerstone of prostate cancer treatment. New-generation hormone therapies improved overall survival in castration-resistant prostate cancer. More recent trials showed a further increase in overall survival when enzalutamide or abiraterone are associated with androgen deprivation therapy in hormone-sensitive disease. However, a higher clonal pressure may lead to the upregulation of alternative pathways for cancer progression and to dedifferentiated diseases that would probably respond poorly to subsequent treatments. In this contest, new strategies that could be able to delay or even revert resistance are needed. The bipolar androgen therapy is an under-investigation treatment that consists in periodical oscillation between castration levels and supraphysiological levels of testosterone in order to prevent the adaptation of prostate cancer cells to a low-androgen environment. This review aims to underline the biological rationale of bipolar androgen therapy and gather evidences from the most recent clinical trials.

Quantitative proteomic analysis provides insights into the algicidal mechanism of Halobacillus sp. P1 against the marine diatom Skeletonema costatum

Algicidal behavior is a common interaction between marine microalgae and bacteria, especially in the dissipation phase of algal blooms. The marine bacterium Halobacillus sp. P1 was previously isolated and exhibits high algicidal activity against the diatom Skeletonema costatum. However, little is known about the mechanism underlying this algicidal process. Here, a tandem mass tag (TMT)-based proteomic approach was coupled with physiological analysis to investigate the cellular responses of S. costatum when treated with P1 culture supernatant. Among the 4582 proteins identified, 82 and 437 proteins were differentially expressed after treatment for 12 and 24 h, respectively. The proteomic results were in accordance with the results of verification by parallel reaction monitoring (PRM) assays. Proteins involved in reactive oxygen species scavenging, protein degradation and transport were upregulated, while proteins participating in nitrogen metabolism, protein translation, photosynthetic pigment biosynthesis and cell cycle regulation were significantly downregulated (p-value ≤0.05), corresponding to the increasing malondialdehyde content and the decreasing nitrogen, protein and chlorophyll a contents. A nutrient competitive relationship might exist between the bacterium P1 and S. costatum, and the inhibition of nitrogen metabolism by the P1 culture supernatant might be the key lethal factor that results in the dysfunction of S. costatum metabolism. Our study sheds light on the algicidal mechanism of P1 at the molecular level and provides new insights into algae-bacteria interactions.

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.

The tumor suppressor FBXO31 preserves genomic integrity by regulating DNA replication and segregation through precise control of cyclin A levels

F-box protein 31 (FBXO31) is a reported putative tumor suppressor, and its inactivation due to loss of heterozygosity is associated with cancers of different origins. An emerging body of literature has documented FBXO31's role in preserving genome integrity following DNA damage and in the cell cycle. However, knowledge regarding the role of FBXO31 during normal cell-cycle progression is restricted to its functions during the G₂/M phase. Interestingly, FBXO31 levels remain high even during the early G₁ phase, a crucial stage for preparing the cells for DNA replication. Therefore, we sought to investigate the functions of FBXO31 during the G₁ phase of the cell cycle. Here, using flow cytometric, biochemical, and immunofluorescence techniques, we show that FBXO31 is essential for maintaining optimum expression of the cell-cycle protein cyclin A for efficient cell-cycle progression. Stable FBXO31 knockdown led to atypical accumulation of cyclin A during the G₁ phase, driving premature DNA replication and compromised loading of the minichromosome maintenance complex, resulting in replication from fewer origins and DNA double-strand breaks. Because of these inherent defects in replication, FBXO31-knockdown cells were hypersensitive to replication stress-inducing agents and displayed pronounced genomic instability. Upon entering mitosis, the cells defective in DNA replication exhibited a delay in the prometaphase-to-metaphase transition and anaphase defects such as lagging and bridging chromosomes. In conclusion, our findings establish that FBXO31 plays a pivotal role in preserving genomic integrity by maintaining low cyclin A levels during the G₁ phase for faithful genome duplication and segregation.

5-hydroxymethylcytosine Marks Mammalian Origins Acting as a Barrier to Replication

In most mammalian cells, DNA replication occurs once, and only once between cell divisions. Replication initiation is a highly regulated process with redundant mechanisms that prevent errant initiation events. In lower eukaryotes, replication is initiated from a defined consensus sequence, whereas a consensus sequence delineating mammalian origin of replication has not been identified. Here we show that 5-hydroxymethylcytosine (5hmC) is present at mammalian replication origins. Our data support the hypothesis that 5hmC has a role in cell cycle regulation. We show that 5hmC level is inversely proportional to proliferation; indeed, 5hmC negatively influences cell division by increasing the time a cell resides in G1. Our data suggest that 5hmC recruits replication-licensing factors, then is removed prior to or during origin firing. Later we propose that TET2, the enzyme catalyzing 5mC to 5hmC conversion, acts as barrier to rereplication. In a broader context, our results significantly advance the understating of 5hmC involvement in cell proliferation and disease states.

Transcriptome analysis of pancreatic cancer cell response to treatment with grape seed proanthocyanidins

Grape seed proanthocyanidins (GSPs) have been demonstrated to exhibit potential chemotherapeutic efficacy against various cancer types. To determine the underlying molecular mechanisms involved in GSP-induced apoptosis, the present study prepared pancreatic cancer (PC) cells samples, S3, S12 and S24, which were treated with 20 µg/ml GSPs for 3, 12 and 24 h, respectively. Control cell samples, C3, C12 and C24, were also prepared. Using RNA-sequencing, transcriptome comparisons were performed, which identified 966, 3,543 and 4,944 differentially-expressed genes (DEGs) in S3 vs. C3, S12 vs. C12 and S24 vs. C24, respectively. Gene Ontology analysis of the DEGs, revealed that treatment with GSPs is associated with disruption of the cell cycle (CC) in PC cells. Additionally, disruption of transcription, DNA replication and DNA repair were associated with GSP-treatment in PC cells. Network analysis demonstrated that the common DEGs involved in the CC, transcription, DNA replication and DNA repair were integrated, and served essential roles in the control of CC progression in cancer cells. In summary, GSPs may exhibit a potential chemotherapeutic effect on PC cell proliferation.

DNA replication licensing proteins: Saints and sinners in cancer

DNA replication is all-or-none process in the cell, meaning, once the DNA replication begins it proceeds to completion. Hence, to achieve maximum control of DNA replication, eukaryotic cells employ a multi-subunit initiator protein complex known as "pre-replication complex or DNA replication licensing complex (DNA replication LC). This complex involves multiple proteins which are origin-recognition complex family proteins, cell division cycle-6, chromatin licensing and DNA replication factor 1, and minichromosome maintenance family proteins. Higher-expression of DNA replication LC proteins appears to be an early event during development of cancer since it has been a common hallmark observed in a wide variety of cancers such as oesophageal, laryngeal, pulmonary, mammary, colorectal, renal, urothelial etc. However, the exact mechanisms leading to the abnormally high expression of DNA replication LC have not been clearly deciphered. Increased expression of DNA replication LC leads to licensing and/or firing of multiple origins thereby inducing replication stress and genomic instability. Therapeutic approaches where the reduction in the activity of DNA replication LC was achieved either by siRNA or shRNA techniques, have shown increased sensitivity of cancer cell lines towards the anti-cancer drugs such as cisplatin, 5-Fluorouracil, hydroxyurea etc. Thus, the expression level of DNA replication LC within the cell determines a cell's fate thereby creating a paradox where DNA replication LC acts as both "Saint" and "Sinner". With a potential to increase sensitivity to chemotherapy drugs, DNA replication LC proteins have prospective clinical importance in fighting cancer. Hence, in this review, we will shed light on importance of DNA replication LC with an aim to use DNA replication LC in diagnosis and prognosis of cancer in patients as well as possible therapeutic targets for cancer therapy.

Effects of Sepantronium Bromide (YM-155) on the Whole Transcriptome of MDA-MB-231 Cells: Highlight on Impaired ATR/ATM Fanconi Anemia DNA Damage Response

Sepantronium bromide (YM-155) is believed to elicit apoptosis and mitotic arrest in tumor cells by reducing (BIRC5, survivin) mRNA. In this study, we monitored changes in survivin mRNA and protein after treating MDA-MB-231 cells with YM-155 concurrent with evaluation of whole transcriptomic (WT) mRNA and long intergenic non-coding RNA at 2 time points: 8 h sub-lethal (83 ng/mL) and 20 h at the LC50 (14.6 ng/mL). The data show a tight association between cell death and the precipitating loss of survivin protein and mRNA (-2.67 fold-change (FC), p<0.001) at 20 h, questioning if the decline in survivin is attributed to cell death or drug impact. The meager loss of survivin mRNA was overshadowed by enormous differential change to the WT in both magnitude and significance for over 2000 differentially up/down-regulated transcripts: (+22 FC to -12 FC, p<0.001). The data show YM-155 to up-regulate transcripts in control of circadian rhythm (NOCT, PER, BHLHe40, NFIL3), tumor suppression (SIK1, FOSB), histone methylation (KDM6B) and negative feedback of NF-kappa B signaling (TNFAIP3). Down-regulated transcripts by YM-155 include glucuronidase (GUSBP3), numerous micro-RNAs, DNA damage repair elements (CENPI, POLQ, RAD54B) and the most affected system was the ataxia-telangiectasia mutated (ATM)/Fanconi anemia E3 monoubiquitin ligase core complexes (FANC transcripts - A/B/E/F/G/M), FANC2, FANCI, BRCA1, BRCA2, RAD51, PALB2 gene and ATR (ATM- and Rad3-Related) pathway. In conclusion, these findings suggest that a primary target of YM-155 is the loss of replicative DNA repair systems.

Arabidopsis replication factor C4 is critical for DNA replication during the mitotic cell cycle

Replication factor C (RFC) is a conserved eukaryotic complex consisting of RFC1/2/3/4/5. It plays important roles in DNA replication and the cell cycle in yeast and fruit fly. However, it is not very clear how RFC subunits function in higher plants, except for the Arabidopsis (At) subunits AtRFC1 and AtRFC3. In this study, we investigated the functions of AtRFC4 and found that loss of function of AtRFC4 led to an early sporophyte lethality that initiated as early as the elongated zygote stage, all defective embryos arrested at the two- to four-cell embryo proper stage, and the endosperm possessed six to eight free nuclei. Complementation of rfc4-1/+ with AtRFC4 expression driven through the embryo-specific DD45pro and ABI3pro or the endosperm-specific FIS2pro could not completely restore the defective embryo or endosperm, whereas a combination of these three promoters in rfc4-1/+ enabled the aborted ovules to develop into viable seeds. This suggests that AtRFC4 functions simultaneously in endosperm and embryo and that the proliferation of endosperm is critical for embryo maturation. Assays of DNA content in rfc4-1/+ verified that DNA replication was disrupted in endosperm and embryo, resulting in blocked mitosis. Moreover, we observed a decreased proportion of late S-phase and M-phase cells in the rfc4-1/-^{FIS2;DD45;ABI3pro::AtRFC4} seedlings, suggesting that incomplete DNA replication triggered cell cycle arrest in cells of the root apical meristem. Therefore, we conclude that AtRFC4 is a crucial gene for DNA replication.

3D TECA hydrogel reduces cellular senescence and enhances fibroblasts migration in wound healing

This study was designed to investigate the effect of 3D TECA hydrogel on the inflammatory-induced senescence marker, and to assess the influence of the gel on the periodontal ligament fibroblasts (PDLFs) migration in wound healing in vitro. PDLFs were cultured with 20 ng/ml TNF-α to induce inflammation in the presence and absence of 50 µM 3D TECA gel for 14 d. The gel effect on the senescence maker secretory associated-β-galactosidase (SA-β-gal) activity was measured by a histochemical staining. Chromatin condensation and DNA synthesis of the cells were assessed by 4′,6-diamidino-2-phenylindole and 5-ethynyl-2′-deoxyuridine fluorescent staining respectively. For evaluating fibroblasts migration, scratch wound healing assay and Pro-Plus Imaging software were used. The activity of senescence marker, SA-β-gal, was positive in the samples with TNF-α-induced inflammation. SA-β-gal percentage is suppressed (>65%, P < 0.05) in the treated cells with TECA gel as compared to the non-treated cells. Chromatin foci were obvious in the non-treated samples. DNA synthesis was markedly recognized by the fluorescent staining in the treated compared to non-treated cultures. Scratch wound test indicated that the cells migration rate was significantly higher (14.9 µm²/h, P < 0.05) in the treated versus (11 µm²/h) for control PDLFs. The new formula of 3D TECA suppresses the inflammatory-mediated cellular senescence and enhanced fibroblasts proliferation and migration. Therefore, 3D TECA may be used as an adjunct to accelerate repair and healing of periodontal tissues.

Cell cycle pathway dysregulation in human keratinocytes during chronic exposure to low arsenite

BACKGROUND

Arsenic is naturally prevalent in the earth's crust and widely distributed in air and water. Chronic low arsenic exposure is associated with several cancers in vivo, including skin cancer, and with transformation in vitro of cell lines including immortalized human keratinocytes (HaCaT). Arsenic also is associated with cell cycle dysregulation at different exposure levels in multiple cell lines. In this work, we analyzed gene expression in HaCaT cells to gain an understanding of gene expression changes contributing to transformation at an early time point.

METHODS

HaCaT cells were exposed to 0 or 100nM NaAsO2 for 7weeks. Total RNA was purified and analyzed by microarray hybridization. Differential expression with fold change≥|1.5| and p-value≤0.05 was determined using Partek Genomic Suite™ and pathway and network analyses using MetaCore™ software (FDR≤0.05). Cell cycle analysis was performed using flow cytometry.

RESULTS

644 mRNAs were differentially expressed. Cell cycle/cell cycle regulation pathways predominated in the list of dysregulated pathways. Genes involved in replication origin licensing were enriched in the network. Cell cycle assay analysis showed an increase in G2/M compartment in arsenite-exposed cells.

CONCLUSIONS

Arsenite exposure induced differential gene expression indicating dysregulation of cell cycle control, which was confirmed by cell cycle analysis. The results suggest that cell cycle dysregulation is an early event in transformation manifested in cells unable to transit G2/M efficiently. Further study at later time points will reveal additional changes in gene expression related to transformation processes.

The prognostic significance of Cdc6 and Cdt1 in breast cancer

DNA replication is a critical step in cell proliferation. Overexpression of MCM2-7 genes correlated with poor prognosis in breast cancer patients. However, the roles of Cdc6 and Cdt1, which work with MCMs to regulate DNA replication, in breast cancers are largely unknown. In the present study, we have shown that the expression levels of Cdc6 and Cdt1 were both significantly correlated with an increasing number of MCM2-7 genes overexpression. Both Cdc6 and Cdt1, when expressed in a high level, alone or in combination, were significantly associated with poorer survival in the breast cancer patient cohort (n = 1441). In line with this finding, the expression of Cdc6 and Cdt1 was upregulated in breast cancer cells compared to normal breast epithelial cells. Expression of Cdc6 and Cdt1 was significantly higher in ER negative breast cancer, and was suppressed when ER signalling was inhibited either by tamoxifen in vitro or letrozole in human subjects. Importantly, breast cancer patients who responded to letrozole expressed significantly lower Cdc6 than those patients who did not respond. Our results suggest that Cdc6 is a potential prognostic marker and therapeutic target in breast cancer patients.

AEE788 Inhibits Basal Body Assembly and Blocks DNA Replication in the African Trypanosome

Trypanosoma brucei causes human African trypanosomiasis (HAT). The pyrrolopyrimidine AEE788 (a hit for anti-HAT drug discovery) associates with three trypanosome protein kinases. Herein we delineate the effects of AEE788 on T. brucei using chemical biology strategies. AEE788 treatment inhibits DNA replication in the kinetoplast (mitochondrial nucleoid) and nucleus. In addition, AEE788 blocks duplication of the basal body and the bilobe without affecting mitosis. Thus, AEE788 prevents entry into the S-phase of the cell division cycle. To study the kinetics of early events in trypanosome division, we employed an "AEE788 block and release" protocol to stage entry into the S-phase. A time-course of DNA synthesis (nuclear and kinetoplast DNA), duplication of organelles (basal body, bilobe, kinetoplast, nucleus), and cytokinesis was obtained. Unexpected findings include the following: 1) basal body and bilobe duplication are concurrent; 2) maturation of probasal bodies, marked by TbRP2 recruitment, is coupled with nascent basal body assembly, monitored by localization of TbSAS6 at newly forming basal bodies; and 3) kinetoplast division is observed in G2 after completion of nuclear DNA synthesis. Prolonged exposure of trypanosomes to AEE788 inhibited transferrin endocytosis, altered cell morphology, and decreased cell viability. To discover putative effectors for the pleiotropic effects of AEE788, proteome-wide changes in protein phosphorylation induced by the drug were determined. Putative effectors include an SR protein kinase, bilobe proteins, TbSAS4, TbRP2, and BILBO-1. Loss of function of one or more of these effectors can, from published literature, explain the polypharmacology of AEE788 on trypanosome biology.

Regulation of Cell Division

The challenging task of mitotic cell divisions is to generate two genetically identical daughter cells from a single precursor cell. To accomplish this task, a complex regulatory network evolved, which ensures that all events critical for the duplication of cellular contents and their subsequent segregation occur in the correct order, at specific intervals and with the highest possible fidelity. Transitions between cell cycle stages are triggered by changes in the phosphorylation state and levels of components of the cell cycle machinery. Entry into S-phase and M-phase are mediated by cyclin-dependent kinases (Cdks), serine-threonine kinases that require a regulatory cyclin subunit for their activity. Resetting the system to the interphase state is mediated by protein phosphatases (PPs) that counteract Cdks by dephosphorylating their substrates. To avoid futile cycles of phosphorylation and dephosphorylation, Cdks and PPs must be regulated in a manner such that their activities are mutually exclusive.

A Dual Inhibitory Mechanism Sufficient to Maintain Cell-Cycle-Restricted CENP-A Assembly

Chromatin featuring the H3 variant CENP-A at the centromere is critical for its mitotic function and epigenetic maintenance. Assembly of centromeric chromatin is restricted to G1 phase through inhibitory action of Cdk1/2 kinases in other phases of the cell cycle. Here, we identify the two key targets sufficient to maintain cell-cycle control of CENP-A assembly. We uncovered a single phosphorylation site in the licensing factor M18BP1 and a cyclin A binding site in the CENP-A chaperone, HJURP, that mediated specific inhibitory phosphorylation. Simultaneous expression of mutant proteins lacking these residues results in complete uncoupling from the cell cycle. Consequently, CENP-A assembly is fully recapitulated under high Cdk activities, indistinguishable from G1 assembly. We find that Cdk-mediated inhibition is exerted by sequestering active factors away from the centromere. Finally, we show that displacement of M18BP1 from the centromere is critical for the assembly mechanism of CENP-A.

Simvastatin exerts anti-hepatitis B virus activity by inhibiting expression of minichromosome maintenance protein 7 in HepG2.2.15 cells

Simvastatin (SIM), a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, has been reported to inhibit the activity of hepatitis B virus (HBV), however, the mechanism underlying its antiviral function remains unknown. Minichromosome maintenance (MCM) 7, a component of the MCM complex, has been reported to act as an important host factor aiding virus genome replication in host cells. The present study demonstrated that downregulation of MCM7 inhibited the expression of proteins transferred by adenoviral vectors. This suggests an association between MCM7 and viral DNA expression. Thus, the current study aimed to investigate whether SIM affected MCM7 expression. Notably, the results of the present study indicated that following exposure to SIM the protein expression levels of MCM7 in HepG2.2.15, a human HBV‑transfected liver cell line, was decreased. In addition, the HBV DNA replication in the cell line was suppressed. As quantitative polymerase chain reaction experiments demonstrated that SIM did not downregulate the mRNA expression level of MCM7, the current study further investigated whether SIM affects the translation of MCM7. Western blot experiments indicated that SIM improved the activation of eukaryotic initiation factor‑2α (eIF2α), a protein synthesis initiation factor, and upregulated the upstream factors of eIF2α, protein kinase RNA‑like endoplasmic reticulum kinase, which is regulated by the liver kinase B1 (LKB1)‑AMP‑activated protein kinase (AMPK) signaling pathway. These results indicated that SIM induced HBV downregulation via an MCM‑dependent mechanism, and SIM may inhibit MCM7 expression by increasing the phosphorylation of eIF2α, which is mediated by the LKB1-AMPK signaling pathway.

Expression of Sam68 Associates with Neuronal Apoptosis and Reactive Astrocytes After Spinal Cord Injury

Src-associated in mitosis (Sam68; 68 kDa) is a novel RNA-binding protein that belongs to the signal transduction and activation of RNA family involved in various biological processes. However, the expression and roles of Sam68 in the central nervous system remain unknown. In the present study, we performed a spinal cord injury (SCI) model in adult rats and found a significant increase of Sam68 protein levels in this model, which reached a peak at day 3 and then gradually returned to normal levels at day 14 after SCI. We use immunohistochemistry analysis revealing a widespread distribution of Sam68 in the spinal cord. In addition, double-immunofluorescence staining showed that Sam68 immunoreactivity was found predominantly in neurons and astrocytes. Moreover, colocalization of Sam68/active caspase-3 has been respectively detected in neuronal nuclei, and colocalization of Sam68/PCNA has been detected in glial fibrillary acidic protein. In vitro, we found that depletion of Sam68 by short interfering RNA inhibits neuronal apoptosis and astrocyte proliferation and decreases cyclin D1 protein levels. In conclusion, this is the first study to find the Sam68 expression in SCI. Our results suggest that Sam68 might be illustrated in the apoptosis of neurons and proliferation of astrocytes after SCI. This research will provide new drug targets for clinical treatment of SCI.

Strategic role of the ubiquitin-dependent segregase p97 (VCP or Cdc48) in DNA replication

Genome amplification (DNA synthesis) is one of the most demanding cellular processes in all proliferative cells. The DNA replication machinery (also known as the replisome) orchestrates genome amplification during S-phase of the cell cycle. Genetic material is particularly vulnerable to various events that can challenge the replisome during its assembly, activation (firing), progression (elongation) and disassembly from chromatin (termination). Any disturbance of the replisome leads to stalling of the DNA replication fork and firing of dormant replication origins, a process known as DNA replication stress. DNA replication stress is considered to be one of the main causes of sporadic cancers and other pathologies related to tissue degeneration and ageing. The mechanisms of replisome assembly and elongation during DNA synthesis are well understood. However, once DNA synthesis is complete, the process of replisome disassembly, and its removal from chromatin, remains unclear. In recent years, a growing body of evidence has alluded to a central role in replisome regulation for the ubiquitin-dependent protein segregase p97, also known as valosin-containing protein (VCP) in metazoans and Cdc48 in lower eukaryotes. By orchestrating the spatiotemporal turnover of the replisome, p97 plays an essential role in DNA replication. In this review, we will summarise our current knowledge about how p97 controls the replisome from replication initiation, to elongation and finally termination. We will also further examine the more recent findings concerning the role of p97 and how mutations in p97 cofactors, also known as adaptors, cause DNA replication stress induced genomic instability that leads to cancer and accelerated ageing. To our knowledge, this is the first comprehensive review concerning the mechanisms involved in the regulation of DNA replication by p97.

Overexpression of CDT1 Is a Predictor of Poor Survival in Patients with Hepatocellular Carcinoma

BACKGROUND

Genomic instability is a common feature in hepatocellular carcinoma. Deregulation of replication licensing factors has been shown to trigger DNA damage response contributing to genomic instability. Overexpression of DNA replication licensing factors chromatin licensing and DNA replication factor 1 (CDT1) and minichromosome maintenance complex component 7 (MCM7) has been previously reported in several human cancers. The aim of the present study was to evaluate the expression and prognostic significance of CDT1 and MCM7 in association with DNA damage response markers and p53 in patients with hepatocellular carcinoma.

METHODS

Expression of CDT1, MCM7, p-H2A histone family member X (H2AX), phospho-ataxia telangiectasia-mutated (ATM)/ataxia telangiectasia rad3-related (ATR) substrate, and p53 was evaluated by immunohistochemistry on formalin-fixed paraffin-embedded surgical specimens from 111 patients who underwent hepatectomy for hepatocellular carcinoma. Statistical analysis was performed to evaluate associations between the studied proteins, clinicopathological parameters, and patient survival.

RESULTS

CDT1 expression correlated with p-H2AX (p = 0.038), while MCM7 correlated with p-H2AX and phospho-ATM/ATR substrate (p < 0.001). Increased CDT1 expression was associated with higher tumor grade (p = 0.006) and tumor-node-metastasis (TNM) stage (p = 0.033). High CDT1 expression correlated significantly with reduced overall survival (60.8 and 26.5 % vs 82.8 and 53.0 %, for low CDT1 expression, at 2 and 5 years, respectively, p = 0.012) and was identified by multivariate analysis as an independent predictor of poor overall survival (p = 0.049).

CONCLUSIONS

Overexpression of CDT1 and MCM7 in hepatocellular carcinoma correlates with DNA damage response, and CDT1 overexpression is a significant prognostic biomarker in hepatocellular carcinoma.

KSHV Genome Replication and Maintenance

Kaposi's sarcoma associated herpesvirus (KSHV) or human herpesvirus 8 (HHV8) is a major etiological agent for multiple severe malignancies in immune-compromised patients. KSHV establishes lifetime persistence in the infected individuals and displays two distinct life cycles, generally a prolonged passive latent, and a short productive or lytic cycle. During latent phase, the viral episome is tethered to the host chromosome and replicates once during every cell division. Latency-associated nuclear antigen (LANA) is a predominant multifunctional nuclear protein expressed during latency, which plays a central role in episome tethering, replication and perpetual segregation of the episomes during cell division. LANA binds cooperatively to LANA binding sites (LBS) within the terminal repeat (TR) region of the viral episome as well as to the cellular nucleosomal proteins to tether viral episome to the host chromosome. LANA has been shown to modulate multiple cellular signaling pathways and recruits various cellular proteins such as chromatin modifying enzymes, replication factors, transcription factors, and cellular mitotic framework to maintain a successful latent infection. Although, many other regions within the KSHV genome can initiate replication, KSHV TR is important for latent DNA replication and possible segregation of the replicated episomes. Binding of LANA to LBS favors the recruitment of various replication factors to initiate LANA dependent DNA replication. In this review, we discuss the molecular mechanisms relevant to KSHV genome replication, segregation, and maintenance of latency.

Spatiotemporal Expression of EAPP Modulates Neuronal Apoptosis and Reactive Astrogliosis After Spinal Cord Injury

E2F-associated phosphoprotein (EAPP) is a novel E2F binding protein that interacts with the activating members of the E2F transcription factors family and involved in various biological processes. However, the expression and function of EAPP in central nervous system (CNS) are still unknown. In this study, we performed an acute spinal cord injury (SCI) model in adult rats, we found that EAPP protein levels were significantly increased and reached a peak at day 3, and then gradually returned to normal level at day 14 after spinal cord injury and we observed that the expression of EAPP is enhanced in the gray and white matter. Spatially, increased levels of EAPP were striking in neurons and astrocytes. Moreover, colocalization of EAPP/active caspase-3 was detected in neurons, and colocalization of EAPP/proliferating cell nuclear antigen (PCNA) was detected in astrocytes after spinal cord injury. These results indicated that EAPP might play an important role in neuronal apoptosis and reactive astrogliosis. Furthermore in vitro, EAPP depletion by siRNA inhibited astrocyte proliferation, migration and CDK4/cyclinD1 expression. Meanwhile, EAPP knockdown also reduce neuronal apoptosis and cell cycle related proteins. Which indicated that EAPP might integrate cell cycle progression and play a crucial role in cell proliferation and apoptosis. Taken together, we speculated that EAPP was involved in biochemical and physiological responses after SCI.

Identification and Characterization of the Cyclin-Dependent Kinases Gene Family in Silkworm, Bombyx mori

Cyclin-dependent protein kinases (CDKs) play key roles at different checkpoint regulations of the eukaryotic cell cycle. However, only few studies of lepidoptera CDK family proteins have been reported so far. In this study, we performed the cDNA sequencing of 10 members of the CDK family in Bombyx mori. Gene structure analysis suggested that CDK12 and CDC2L1 owned two and three isoforms, respectively. Phylogenetic analysis showed that CDK genes in different species were highly conserved, implying that they evolved independently even before the split between vertebrates and invertebrates. We found that the expression levels of BmCDKs in 13 tissues of fifth-instar day 3 larvae were different: CDK1, CDK7, and CDK9 had a high level of expression, whereas CDK4 was low-level expressed and was detected only in the testes and fat body cells. Similar expression profiles of BmCDKs during embryo development were obtained. Among the variants of CDK12, CDK12 transcript variant A had the highest expression, and the expression of CDC2L1 transcript variant A was the highest among the variants of CDC2L1. It was shown from the RNAi experiments that the silencing of CDK1, CDK10, CDK12, and CDC2L1 could influence the cells from G0/G1 to S phase transition.