Regulation of the cyclin E gene by transcription factor E2F1

The crosstalk between non-coding RNAs and cell-cycle events: A new frontier in cancer therapy

The cell cycle comprises sequential events during which a cell duplicates its genome and divides it into two daughter cells. This process is tightly regulated to ensure that the daughter cell receives identical copied chromosomal DNA and that any errors in the DNA during replication are correctly repaired. Cyclins and their enzyme partners, cyclin-dependent kinases (CDKs), are critical regulators of G- to M-phase transitions during the cell cycle. Mitogenic signals induce the formation of the cyclin/CDK complexes, resulting in phosphorylation and activation of the CDKs. Once activated, cyclin/CDK complexes phosphorylate specific substrates that drive the cell cycle forward. The sequential activation and inactivation of cyclin-CDK complexes are tightly controlled by activating and inactivating phosphorylation events induced by cell-cycle proteins. The non-coding RNAs (ncRNAs), which do not code for proteins, regulate cell-cycle proteins at the transcriptional and translational levels, thereby controlling their expression at different cell-cycle phases. Deregulation of ncRNAs can cause abnormal expression patterns of cell-cycle-regulating proteins, resulting in abnormalities in cell-cycle regulation and cancer development. This review explores how ncRNA dysregulation can disrupt cell division balance and discusses potential therapeutic approaches targeting these ncRNAs to control cell-cycle events in cancer treatment.

A cell cycle regulator, E2F2, and glucocorticoid receptor cooperatively transactivate the bovine alphaherpesvirus 1 immediate early transcription unit 1 promoter

Bovine alphaherpesvirus 1 (BoHV-1) infection causes respiratory tract disorders and immune suppression and may induce bacterial pneumonia. BoHV-1 establishes lifelong latency in sensory neurons after acute infection. Reactivation from latency consistently occurs following stress or intravenous injection of the synthetic corticosteroid dexamethasone (DEX), which mimics stress. The immediate early transcription unit 1 (IEtu1) promoter drives expression of infected cell protein 0 (bICP0) and bICP4, two viral transcriptional regulators necessary for productive infection and reactivation from latency. The IEtu1 promoter contains two glucocorticoid receptor (GR) responsive elements (GREs) that are transactivated by activated GR. GC-rich motifs, including consensus binding sites for specificity protein 1 (Sp1), are in the IEtu1 promoter sequences. E2F family members bind a consensus sequence (TTTCCCGC) and certain specificity protein 1 (Sp1) sites. Consequently, we hypothesized that certain E2F family members activate IEtu1 promoter activity. DEX treatment of latently infected calves increased the number of E2F2+ TG neurons. GR and E2F2, but not E2F1, E2F3a, or E2F3b, cooperatively transactivate a 436-bp cis-regulatory module in the IEtu1 promoter that contains both GREs. A luciferase reporter construct containing a 222-bp fragment downstream of the GREs was transactivated by E2F2 unless two adjacent Sp1 binding sites were mutated. Chromatin immunoprecipitation studies revealed that E2F2 occupied IEtu1 promoter sequences when the BoHV-1 genome was transfected into mouse neuroblastoma (Neuro-2A) or monkey kidney (CV-1) cells. In summary, these findings revealed that GR and E2F2 cooperatively transactivate IEtu1 promoter activity, which is predicted to influence the early stages of BoHV-1 reactivation from latency.

IMPORTANCE

Bovine alpha-herpesvirus 1 (BoHV-1) acute infection in cattle leads to establishment of latency in sensory neurons in the trigeminal ganglia (TG). A synthetic corticosteroid dexamethasone consistently initiates BoHV-1 reactivation in latently infected calves. The BoHV-1 immediate early transcription unit 1 (IEtu1) promoter regulates expression of infected cell protein 0 (bICP0) and bICP4, two viral transcriptional regulators. Hence, the IEtu1 promoter must be activated for the reactivation to occur. The number of TG neurons expressing E2F2, a transcription factor and cell cycle regulator, increased during early stages of reactivation from latency. The glucocorticoid receptor (GR) and E2F2, but not E2F1, E2F3a, or E2F3b, cooperatively transactivated a 436-bp cis-regulatory module (CRM) in the IEtu1 promoter that contains two GR responsive elements. Chromatin immunoprecipitation studies revealed that E2F2 occupies IEtu1 promoter sequences in cultured cells. GR and E2F2 mediate cooperative transactivation of IEtu1 promoter activity, which is predicted to stimulate viral replication following stressful stimuli.

Cell proliferation and Notch signaling coordinate the formation of epithelial folds in the Drosophila leg

The formation of complex three-dimensional organs during development requires precise coordination between patterning networks and mechanical forces. In particular, tissue folding is a crucial process that relies on a combination of local and tissue-wide mechanical forces. Here, we investigate the contribution of cell proliferation to epithelial morphogenesis using the Drosophila leg tarsal folds as a model. We reveal that tissue-wide compression forces generated by cell proliferation, in coordination with the Notch signaling pathway, are essential for the formation of epithelial folds in precise locations along the proximo-distal axis of the leg. As cell numbers increase, compressive stresses arise, promoting the folding of the epithelium and reinforcing the apical constriction of invaginating cells. Additionally, the Notch target dysfusion plays a key function specifying the location of the folds, through the apical accumulation of F-actin and the apico-basal shortening of invaginating cells. These findings provide new insights into the intricate mechanisms involved in epithelial morphogenesis, highlighting the crucial role of tissue-wide forces in shaping a three-dimensional organ in a reproducible manner.

MYC Oncogene: A Druggable Target for Treating Cancers with Natural Products

Various diseases, including cancers, age-associated disorders, and acute liver failure, have been linked to the oncogene, MYC. Animal testing and clinical trials have shown that sustained tumor volume reduction can be achieved when MYC is inactivated, and different combinations of therapeutic agents including MYC inhibitors are currently being developed. In this review, we first provide a summary of the multiple biological functions of the MYC oncoprotein in cancer treatment, highlighting that the equilibrium points of the MYC/MAX, MIZ1/MYC/MAX, and MAD (MNT)/MAX complexes have further potential in cancer treatment that could be used to restrain MYC oncogene expression and its functions in tumorigenesis. We also discuss the multifunctional capacity of MYC in various cellular cancer processes, including its influences on immune response, metabolism, cell cycle, apoptosis, autophagy, pyroptosis, metastasis, angiogenesis, multidrug resistance, and intestinal flora. Moreover, we summarize the MYC therapy patent landscape and emphasize the potential of MYC as a druggable target, using herbal medicine modulators. Finally, we describe pending challenges and future perspectives in biomedical research, involving the development of therapeutic approaches to modulate MYC or its targeted genes. Patients with cancers driven by MYC signaling may benefit from therapies targeting these pathways, which could delay cancerous growth and recover antitumor immune responses.

Coordination of cell cycle and morphogenesis during organ formation

Organ formation requires precise regulation of cell cycle and morphogenetic events. Using the Drosophila embryonic salivary gland (SG) as a model, we uncover the role of the SP1/KLF transcription factor Huckebein (Hkb) in coordinating cell cycle regulation and morphogenesis. The hkb mutant SG exhibits defects in invagination positioning and organ size due to the abnormal death of SG cells. Normal SG development involves distal-to-proximal progression of endoreplication (endocycle), whereas hkb mutant SG cells undergo abnormal cell division, leading to cell death. Hkb represses the expression of key cell cycle and pro-apoptotic genes in the SG. Knockdown of cyclin E or cyclin-dependent kinase 1, or overexpression of fizzy-related rescues most of the morphogenetic defects observed in the hkb mutant SG. These results indicate that Hkb plays a critical role in controlling endoreplication by regulating the transcription of key cell cycle effectors to ensure proper organ formation.

Expanding Roles of the E2F-RB-p53 Pathway in Tumor Suppression

The transcription factor E2F links the RB pathway to the p53 pathway upon loss of function of pRB, thereby playing a pivotal role in the suppression of tumorigenesis. E2F fulfills a major role in cell proliferation by controlling a variety of growth-associated genes. The activity of E2F is controlled by the tumor suppressor pRB, which binds to E2F and actively suppresses target gene expression, thereby restraining cell proliferation. Signaling pathways originating from growth stimulative and growth suppressive signals converge on pRB (the RB pathway) to regulate E2F activity. In most cancers, the function of pRB is compromised by oncogenic mutations, and E2F activity is enhanced, thereby facilitating cell proliferation to promote tumorigenesis. Upon such events, E2F activates the Arf tumor suppressor gene, leading to activation of the tumor suppressor p53 to protect cells from tumorigenesis. ARF inactivates MDM2, which facilitates degradation of p53 through proteasome by ubiquitination (the p53 pathway). P53 suppresses tumorigenesis by inducing cellular senescence or apoptosis. Hence, in almost all cancers, the p53 pathway is also disabled. Here we will introduce the canonical functions of the RB-E2F-p53 pathway first and then the non-classical functions of each component, which may be relevant to cancer biology.

Multiple cullin-associated E3 ligases regulate cyclin D1 protein stability

Cyclin D1 is a key regulator of cell cycle progression, which forms a complex with CDK4/6 to regulate G1/S transition during cell cycle progression. Cyclin D1 has been recognized as an oncogene since it was upregulated in several different types of cancers. It is known that the post-translational regulation of cyclin D1 is controlled by ubiquitination/proteasome degradation system in a phosphorylation-dependent manner. Several cullin-associated F-box E3 ligases have been shown to regulate cyclin D1 degradation; however, it is not known if additional cullin-associated E3 ligases participate in the regulation of cyclin D1 protein stability. In this study, we have screened an siRNA library containing siRNAs specific for 154 ligase subunits, including F-box, SOCS, BTB-containing proteins, and DDB proteins. We found that multiple cullin-associated E3 ligases regulate cyclin D1 activity, including Keap1, DDB2, and WSB2. We found that these E3 ligases interact with cyclin D1, regulate cyclin D1 ubiquitination and proteasome degradation in a phosphorylation-dependent manner. These E3 ligases also control cell cycle progression and cell proliferation through regulation of cyclin D1 protein stability. Our study provides novel insights into the regulatory mechanisms of cyclin D1 protein stability and function.

Insulin Receptor Isoforms and Insulin Growth Factor-like Receptors: Implications in Cell Signaling, Carcinogenesis, and Chemoresistance

This comprehensive review thoroughly explores the intricate involvement of insulin receptor (IR) isoforms and insulin-like growth factor receptors (IGFRs) in the context of the insulin and insulin-like growth factor (IGF) signaling (IIS) pathway. This elaborate system encompasses ligands, receptors, and binding proteins, giving rise to a wide array of functions, including aspects such as carcinogenesis and chemoresistance. Detailed genetic analysis of IR and IGFR structures highlights their distinct isoforms, which arise from alternative splicing and exhibit diverse affinities for ligands. Notably, the overexpression of the IR-A isoform is linked to cancer stemness, tumor development, and resistance to targeted therapies. Similarly, elevated IGFR expression accelerates tumor progression and fosters chemoresistance. The review underscores the intricate interplay between IRs and IGFRs, contributing to resistance against anti-IGFR drugs. Consequently, the dual targeting of both receptors could present a more effective strategy for surmounting chemoresistance. To conclude, this review brings to light the pivotal roles played by IRs and IGFRs in cellular signaling, carcinogenesis, and therapy resistance. By precisely modulating these receptors and their complex signaling pathways, the potential emerges for developing enhanced anti-cancer interventions, ultimately leading to improved patient outcomes.

RBL2-E2F-GCN5 guide cell fate decisions during tissue specification by regulating cell-cycle-dependent fluctuations of non-cell-autonomous signaling

The retinoblastoma family proteins (RBs) and E2F transcription factors are cell-autonomous regulators of cell-cycle progression, but they also impact fate choice in addition to tumor suppression. The range of mechanisms involved remains to be uncovered. Here, we show that RBs, particularly RBL2/p130, repress WNT ligands such as WNT4 and WNT8A, thereby directing ectoderm specification between neural crest to neuroepithelium. RBL2 achieves this function through cell-cycle-dependent cooperation with E2Fs and GCN5 on the regulatory regions of WNT loci, which direct neuroepithelial versus neural crest specification by temporal fluctuations of WNT/β-catenin and DLL/NOTCH signaling activity. Thus, the RB-E2F bona fide cell-autonomous axis controls cell fate decisions, and RBL2 regulates field effects via WNT ligands. This reveals a non-cell-autonomous function of RBL2-E2F in stem cell and tissue progenitor differentiation that has broader implications for cell-cycle-dependent cell fate specification in organogenesis, adult stem cells, tissue homeostasis, and tumorigenesis.

MYCN Amplification, along with Wild-Type RB1 Expression, Enhances CDK4/6 Inhibitors’ Efficacy in Neuroblastoma Cells

Neuroblastoma (NB) is one of the primary causes of death for pediatric malignancies. Given the high heterogeneity in NB’s mutation landscape, optimizing individualized therapies is still challenging. In the context of genomic alterations, MYCN amplification is the most correlated event with poor outcomes. MYCN is involved in the regulation of several cellular mechanisms, including cell cycle. Thus, studying the influence of MYCN overexpression in the G1/S transition checkpoint of the cell cycle may unveil novel druggable targets for the development of personalized therapeutical approaches. Here, we show that high expression of E2F3 and MYCN correlate with poor prognosis in NB despite the RB1 mRNA levels. Moreover, we demonstrate through luciferase reporter assays that MYCN bypasses RB function by incrementing E2F3-responsive promoter activity. We showed that MYCN overexpression leads to RB inactivation by inducing RB hyperphosphorylation during the G1 phase through cell cycle synchronization experiments. Moreover, we generated two MYCN-amplified NB cell lines conditionally knockdown (cKD) for the RB1 gene through a CRISPRi approach. Indeed, RB KD did not affect cell proliferation, whereas cell proliferation was strongly influenced when a non-phosphorylatable RB mutant was expressed. This finding revealed the dispensable role of RB in regulating MYCN-amplified NB’s cell cycle. The described genetic interaction between MYCN and RB1 provides the rationale for using cyclin/CDK complexes inhibitors in NBs carrying MYCN amplification and relatively high levels of RB1 expression.

Encoding and Decoding of p53 Dynamics in Cellular Response to Stresses

In the cellular response to stresses, the tumor suppressor p53 is activated to maintain genomic integrity and fidelity. As a transcription factor, p53 exhibits rich dynamics to allow for discrimination of the type and intensity of stresses and to direct the selective activation of target genes involved in different processes including cell cycle arrest and apoptosis. In this review, we focused on how stresses are encoded into p53 dynamics and how the dynamics are decoded into cellular outcomes. Theoretical modeling may provide a global view of signaling in the p53 network by coupling the encoding and decoding processes. We discussed the significance of modeling in revealing the mechanisms of the transition between p53 dynamic modes. Moreover, we shed light on the crosstalk between the p53 network and other signaling networks. This review may advance the understanding of operating principles of the p53 signaling network comprehensively and provide insights into p53 dynamics-based cancer therapy.

Deregulated E2F Activity as a Cancer-Cell Specific Therapeutic Tool

The transcription factor E2F, the principal target of the tumor suppressor pRB, plays crucial roles in cell proliferation and tumor suppression. In almost all cancers, pRB function is disabled, and E2F activity is enhanced. To specifically target cancer cells, trials have been undertaken to suppress enhanced E2F activity to restrain cell proliferation or selectively kill cancer cells, utilizing enhanced E2F activity. However, these approaches may also impact normal growing cells, since growth stimulation also inactivates pRB and enhances E2F activity. E2F activated upon the loss of pRB control (deregulated E2F) activates tumor suppressor genes, which are not activated by E2F induced by growth stimulation, inducing cellular senescence or apoptosis to protect cells from tumorigenesis. Deregulated E2F activity is tolerated in cancer cells due to inactivation of the ARF-p53 pathway, thus representing a feature unique to cancer cells. Deregulated E2F activity, which activates tumor suppressor genes, is distinct from enhanced E2F activity, which activates growth-related genes, in that deregulated E2F activity does not depend on the heterodimeric partner DP. Indeed, the ARF promoter, which is specifically activated by deregulated E2F, showed higher cancer-cell specific activity, compared to the E2F1 promoter, which is also activated by E2F induced by growth stimulation. Thus, deregulated E2F activity is an attractive potential therapeutic tool to specifically target cancer cells.

Cell cycle exits and U-turns: Quiescence as multiple reversible forms of arrest

Cell proliferation control is essential during development and for maintaining adult tissues. Loss of that control promotes not only oncogenesis when cells proliferate inappropriately but also developmental abnormalities or degeneration when cells fail to proliferate when and where needed. To ensure that cells are produced at the right place and time, an intricate balance of pro-proliferative and anti-proliferative signals impacts the probability that cells undergo cell cycle exit to quiescence, or G₀ phase. This brief review describes recent advances in our understanding of how and when quiescence is initiated and maintained in mammalian cells. We highlight the growing appreciation for quiescence as a collection of context-dependent distinct states.

Cell cycle control by the insulin-like growth factor signal: at the crossroad between cell growth and mitotic regulation

In proliferating cells and tissues a number of checkpoints (G1/S and G2/M) preceding cell division (M-phase) require the signal provided by growth factors present in serum. IGFs (I and II) have been demonstrated to constitute key intrinsic components of the peptidic active fraction of mammalian serum. In vivo genetic ablation studies have shown that the cellular signal triggered by the IGFs through their cellular receptors represents a non-replaceable requirement for cell growth and cell cycle progression. Retroactive and current evaluation of published literature sheds light on the intracellular circuitry activated by these factors providing us with a better picture of the pleiotropic mechanistic actions by which IGFs regulate both cell size and mitogenesis under developmental growth as well as in malignant proliferation. The present work aims to summarize the cumulative knowledge learned from the IGF ligands/receptors and their intracellular signaling transducers towards control of cell size and cell-cycle with particular focus to their actionable circuits in human cancer. Furthermore, we bring novel perspectives on key functional discriminants of the IGF growth-mitogenic pathway allowing re-evaluation on some of its signal components based upon established evidences.

INO80 Is Required for the Cell Cycle Control, Survival, and Differentiation of Mouse ESCs by Transcriptional Regulation

Precise regulation of the cell cycle of embryonic stem cells (ESCs) is critical for their self-maintenance and differentiation. The cell cycle of ESCs differs from that of somatic cells and is different depending on the cell culture conditions. However, the cell cycle regulation in ESCs via epigenetic mechanisms remains unclear. Here, we showed that the ATP-dependent chromatin remodeler Ino80 regulates the cell cycle genes in ESCs under primed conditions. Ino80 loss led to a significantly extended length of the G1-phase in ESCs grown under primed culture conditions. Ino80 directly bound to the transcription start site and regulated the expression of cell cycle-related genes. Furthermore, Ino80 loss induced cell apoptosis. However, the regulatory mechanism of Ino80 in differentiating ESC cycle slightly differed; an extended S-phase was detected in differentiating inducible Ino80 knockout ESCs. RNA-seq analysis of differentiating ESCs revealed that the expression of genes associated with organ development cell cycle is persistently altered in Ino80 knockout cells, suggesting that cell cycle regulation by Ino80 is not limited to undifferentiated ESCs. Therefore, our study establishes the function of Ino80 in ESC cycle via transcriptional regulation, at least partly. Moreover, this Ino80 function may be universal to other cell types.

An Expanded Interplay Network between NF-κB p65 (RelA) and E2F1 Transcription Factors: Roles in Physiology and Pathology

Transcription Factors (TFs) are the main regulators of gene expression, controlling among others cell homeostasis, identity, and fate. TFs may either act synergistically or antagonistically on nearby regulatory elements and their interplay may activate or repress gene expression. The family of NF-κB TFs is among the most important TFs in the regulation of inflammation, immunity, and stress-like responses, while they also control cell growth and survival, and are involved in inflammatory diseases and cancer. The family of E2F TFs are major regulators of cell cycle progression in most cell types. Several studies have suggested the interplay between these two TFs in the regulation of numerous genes controlling several biological processes. In the present study, we compared the genomic binding landscape of NF-κB RelA/p65 subunit and E2F1 TFs, based on high throughput ChIP-seq and RNA-seq data in different cell types. We confirmed that RelA/p65 has a binding profile with a high preference for distal enhancers bearing active chromatin marks which is distinct to that of E2F1, which mostly generates promoter-specific binding. Moreover, the RelA/p65 subunit and E2F1 cistromes have limited overlap and tend to bind chromatin that is in an active state even prior to immunogenic stimulation. Finally, we found that a fraction of the E2F1 cistrome is recruited by NF-κΒ near pro-inflammatory genes following LPS stimulation in immune cell types.

Lysine stimulates the development of the murine mammary gland at puberty via PI3K/AKT/mTOR signalling axis

Lysine is one of the essential amino acids. The effect of lysine on milk protein and milk fat anabolism has been reported, but the effect on mammary glands development has not been studied in detail. The normal development of the mammary glands at puberty is crucial to lactation of mammals. In this study, to explore the effect of lysine on mammary glands development, we fed different concentrations of lysine (0.025%, 0.05%, 0.1%) to pubertal mice and found that the addition of 0.1% lysine to drinking water significantly promoted mammary glands development. Furthermore, we treated mMECs (mouse mammary epithelial cells) with different concentrations of lysine (0, 0.2, 0.4, 0.6, 0.8 and 1 mM) to explore the underlying mechanism, and found that lysine promoted the proliferation of mMECs and development of mammary glands through PI3K/AKT/mTOR signalling pathway in pubertal mice. Overall, the results of this study revealed that lysine activated the PI3K/AKT/mTOR signal axis, elevated protein concentrations of cell proliferation markers, such as PCNA, Cyclin D1 and D3, and enhanced the proliferation of mMECs, finally promoted the murine mammary glands development at puberty.

E2F4 regulates the cell cycle and DNA replication in the silkworm, Bombyx mori

The E2F family of transcription factors is crucial for cell cycle progression and cell fate decisions. Although E2Fs have been widely studied in mammals, there have been few studies performed in insects. Here, we determined the function of E2F4 in the silkworm, Bombyx mori. We demonstrate that E2F proteins are highly conserved among species from lower animals to higher mammals. Overexpression of the BmE2F4 gene led to cell cycle arrest in the G1 phase, whereas interfering with the BmE2F4 mRNA led to accumulation of cells in the S phase. These results indicate that BmE2F4 is important in cell cycle regulation. We also demonstrate that the BmE2F4 gene is involved in DNA replication of BmN-SWU1 cells and DNA synthesis in the silk gland. Furthermore, we identified a protein called Bm14-3-3ζ that can interact with BmE2F4 and allow it to localize in the nucleus. Overexpression of the Bm14-3-3ζ gene led to cell cycle arrest in the G1 phase, while knocking down the gene increased the proportion of cells in S phase. These findings provide important insights into the function of E2F transcription factors and increase our understanding of their involvement in cell cycle regulation.

CDK6 increases glycolysis and suppresses autophagy by mTORC1-HK2 pathway activation in cervical cancer cells

Cervical carcinoma is a leading malignant tumor among women worldwide, characterized by the dysregulation of cell cycle. Cyclin-dependent kinase 6 (CDK6) plays important roles in the cell cycle progression, cell differentiation, and tumorigenesis. However, the role of CDK6 in cervical cancer remains controversial. Here, we found that loss of CDK6 in cervical adenocarcinoma HeLa cell line inhibited cell proliferation but induced apoptosis as well as autophagy, accompanied by attenuated expression of mammalian target of rapamycin complex 1 (mTORC1) and hexokinase 2 (HK2), reduced glycolysis, and production of protein, nucleotide, and lipid. Similarly, we showed that CDK6 knockout inhibited the survival of CDK6-high CaSki but not CDK6-low SiHa cervical cancer cells by regulation of glycolysis and autophagy process. Collectively, our studies indicate that CDK6 is a critical regulator of human cervical cancer cells, especially with high CDK6 level, through its ability to regulate cellular apoptosis and metabolism. Thus, inhibition of CDK6 kinase activity could be a powerful therapeutic avenue used to treat cervical cancers.

Cancer cell cycle dystopia: heterogeneity, plasticity, and therapy

The mammalian cell cycle has been extensively studied regarding cancer etiology, progression, and therapeutic intervention. The canonical cell cycle framework is supported by a plethora of data pointing to a relatively simple linear pathway in which mitogenic signals are integrated in a stepwise fashion to allow progression through G1/S with coordinate actions of cyclin-dependent kinases (CDK)4/6 and CDK2 on the RB tumor suppressor. Recent work on adaptive mechanisms and intrinsic heterogeneous dependencies indicates that G1/S control of the cell cycle is a variable signaling pathway rather than an invariant engine that drives cell division. These alterations can limit the effectiveness of pharmaceutical agents but provide new avenues for therapeutic interventions. These findings support a dystopian view of the cell cycle in cancer where the canonical utopian cell cycle is often not observed. However, recognizing the extent of cell cycle heterogeneity likely creates new opportunities for precision therapeutic approaches specifically targeting these states.

Quantitative profiling of adaptation to cyclin E overproduction

Cyclin E/CDK2 drives cell cycle progression from G1 to S phase. Despite the toxicity of cyclin E overproduction in mammalian cells, the cyclin E gene is overexpressed in some cancers. To further understand how cells can tolerate high cyclin E, we characterized non-transformed epithelial cells subjected to chronic cyclin E overproduction. Cells overproducing cyclin E, but not cyclins D or A, briefly experienced truncated G1 phases followed by a transient period of DNA replication origin underlicensing, replication stress, and impaired proliferation. Individual cells displayed substantial intercellular heterogeneity in cell cycle dynamics and CDK activity. Each phenotype improved rapidly despite high cyclin E-associated activity. Transcriptome analysis revealed adapted cells down-regulated a cohort of G1-regulated genes. Withdrawing cyclin E from adapted cells only partially reversed underlicensing indicating that adaptation is at least partly non-genetic. This study provides evidence that mammalian cyclin E/CDK inhibits origin licensing indirectly through premature S phase onset and provides mechanistic insight into the relationship between CDKs and licensing. It serves as an example of oncogene adaptation that may recapitulate molecular changes during tumorigenesis.

Nuclear respiratory factor 1 promotes the growth of liver hepatocellular carcinoma cells via E2F1 transcriptional activation

Background

Recent studies have shown that functional mitochondria are essential for cancer cells. Nuclear respiratory factor 1 (NRF1) is a transcription factor that activates mitochondrial biogenesis and the expression of the respiratory chain, but little is known about its role and underlying mechanism in liver hepatocellular carcinoma (LIHC).

Methods

NRF1 expression was analyzed via public databases and 24 paired LIHC samples. Clinical-pathological information and follow-up data were collected from 165 patients with LIHC or online datasets. Furthermore, cellular proliferation and the cell cycle were analyzed by MTT, Clone-forming assay and flow cytometric analyses. NRF1 target genes were analyzed by Chromatin immunoprecipitation sequencing (ChIP-Seq). PCR and WB analysis was performed to detect the expression of related genes. ChIP and luciferase activity assays were used to identify NRF1 binding sites.

Results

Our results showed that NRF1 expression was upregulated in LIHC compared to normal tissues. NRF1 expression was associated with tumour size and poor prognosis in patients. Knockdown of NRF1 repressed cell proliferation and overexpression of NRF1 accelerated the G₁/S phase transition. Additionally, data from ChIP-seq pointed out that some NRF1 target genes are involved in the cell cycle. Our findings indicated that NRF1 directly binds to the E2F1 promoter as a transcription factor and regulates its gene expression.

Conclusion

Therefore, this study revealed that NRF1 promotes cancer cell growth via the indirect transcriptional activation of E2F1 and is a potential biomarker in LIHC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12876-022-02260-7.

Dysfunction of lipid storage droplet-2 suppresses endoreplication and induces JNK pathway-mediated apoptotic cell death in Drosophila salivary glands

The lipid storage droplet-2 (LSD-2) protein of Drosophila is a homolog of mammalian perilipin 2, which is essential for promoting lipid accumulation and lipid droplet formation. The function of LSD-2 as a regulator of lipolysis has also been demonstrated. However, other LSD-2 functions remain unclear. To investigate the role of LSD-2, we performed tissue-specific depletion in the salivary glands of Drosophila using a combination of the Gal4-upstream activating sequence system and RNA interference. LSD-2 depletion inhibited the entry of salivary gland cells into the endoreplication cycle and delayed this process by enhancing CycE expression, disrupting the development of this organ. The deficiency of LSD-2 expression enhanced reactive oxygen species production in the salivary gland and promoted JNK-dependent apoptosis by suppressing dMyc expression. This phenomenon did not result from lipolysis. Therefore, LSD-2 is vital for endoreplication cell cycle and cell death programs.

Shifting the Focus of Signaling Abnormalities in Colon Cancer

Simple Summary

The major signaling pathways in colon cancer are WNT, RAS, and TGF-β. Components of these pathways are mutated in the majority of colon cancers, resulting in aberrantly high or low activity of the pathway. The functional consequences of the mutations reflect the behavior of these signaling pathways in intestinal stem cells. To better understand the roles of each pathway, we cover the basic function as well as points of intersection between the different pathways, to describe how they function individually, as well as together, to regulate cell proliferation.

Abstract

Colon cancer tumorigenesis occurs incrementally. The process involves the acquisition of mutations which typically follow an established pattern: activation of WNT signaling, activation of RAS signaling, and inhibition of TGF-β signaling. This arrangement recapitulates, to some degree, the stem cell niche of the intestinal epithelium, which maintains WNT and EGF activity while suppressing TGF-β. The resemblance between the intestinal stem cell environment and colon cancer suggests that the concerted activity of these pathways generates and maintains a potent growth-inducing stimulus. However, each pathway has a myriad of downstream targets, making it difficult to identify which aspects of these pathways are drivers. To address this, we utilize the cell cycle, the ultimate regulator of cell proliferation, as a foundation for cross-pathway integration. We attempt to generate an overview of colon cancer signaling patterns by integrating the major colon cancer signaling pathways in the context of cell replication, specifically, the entrance from G₁ into S-phase.

Nests of dividing neuroblasts sustain interneuron production for the developing human brain

The human cortex contains inhibitory interneurons derived from the medial ganglionic eminence (MGE), a germinal zone in the embryonic ventral forebrain. How this germinal zone generates sufficient interneurons for the human brain remains unclear. We found that the human MGE (hMGE) contains nests of proliferative neuroblasts with ultrastructural and transcriptomic features that distinguish them from other progenitors in the hMGE. When dissociated hMGE cells are transplanted into the neonatal mouse brain, they reform into nests containing proliferating neuroblasts that generate young neurons that migrate extensively into the mouse forebrain and mature into different subtypes of functional interneurons. Together, these results indicate that the nest organization and sustained proliferation of neuroblasts in the hMGE provide a mechanism for the extended production of interneurons for the human forebrain.

Emerging Mechanisms of G1/S Cell Cycle Control by Human and Mouse Cytomegaloviruses

Cytomegaloviruses (CMVs) are among the largest pathogenic viruses in mammals. To enable replication of their long double-stranded DNA genomes, CMVs induce profound changes in cell cycle regulation. A hallmark of CMV cell cycle control is the establishment of an unusual cell cycle arrest at the G₁/S transition, which is characterized by the coexistence of cell cycle stimulatory and inhibitory activities. While CMVs interfere with cellular DNA synthesis and cell division, they activate S-phase-specific gene expression and nucleotide metabolism. This is facilitated by a set of CMV gene products that target master regulators of G₁/S progression such as cyclin E and A kinases, Rb-E2F transcription factors, p53-p21 checkpoint proteins, the APC/C ubiquitin ligase, and the nucleotide hydrolase SAMHD1. While the major themes of cell cycle regulation are well conserved between human and murine CMVs (HCMV and MCMV), there are considerable differences at the level of viral cell cycle effectors and their mechanisms of action. Furthermore, both viruses have evolved unique mechanisms to sense the host cell cycle state and modulate the infection program accordingly. This review provides an overview of conserved and divergent features of G₁/S control by MCMV and HCMV.

Cyclin E/CDK2: DNA Replication, Replication Stress and Genomic Instability

DNA replication must be precisely controlled in order to maintain genome stability. Transition through cell cycle phases is regulated by a family of Cyclin-Dependent Kinases (CDKs) in association with respective cyclin regulatory subunits. In normal cell cycles, E-type cyclins (Cyclin E1 and Cyclin E2, CCNE1 and CCNE2 genes) associate with CDK2 to promote G1/S transition. Cyclin E/CDK2 complex mostly controls cell cycle progression and DNA replication through phosphorylation of specific substrates. Oncogenic activation of Cyclin E/CDK2 complex impairs normal DNA replication, causing replication stress and DNA damage. As a consequence, Cyclin E/CDK2-induced replication stress leads to genomic instability and contributes to human carcinogenesis. In this review, we focus on the main functions of Cyclin E/CDK2 complex in normal DNA replication and the molecular mechanisms by which oncogenic activation of Cyclin E/CDK2 causes replication stress and genomic instability in human cancer.

Total ginsenosides promote the IEC-6 cell proliferation via affecting the regulatory mechanism mediated by polyamines

Epithelial cell proliferation has been demonstrated to be a critical modality for mucosal repair after gastrointestinal mucosal injury. This research aimed to investigate the effect of total ginsenosides upon the proliferation of intestinal epithelial cells (IEC-6), and elucidate its potential mechanisms through polyamine-regulated pathway including the expression of proliferation-related proteins. Total ginsenosides (PGE3) were extracted from Panax ginseng, a Chinese herbal medicine, whose chromatogram was obtained by high performance liquid chromatographic method with evaporative light scattering detection (HPLC-ELSD). The cell proliferation, cell cycle distribution and the level of c-Myc, RhoA, Cdk2 proteins were detected to determine the effects of PGE3 at 25, 50 and100 mg/l doses on IEC-6. Furthermore, rats model of intestinal mucosal injury were induced by the subcutaneous injection of indomethacin, and the effect of Panax ginseng aqueous extracts (PGE1) on intestinal mucosal injury was observed. PGE3 could promote IEC-6 cell proliferation, reduce the proportion of G0/G1 phase cells and elevate the proportion of G2/M + S phase cells, and revert the proliferation and cell cycle arrest induced by DFMO (DL-a-difluoromethylornithine, an inhibitor of polyamines synthesis). PGE3 exposure enhanced the level of c-Myc, RhoA and Cdk2 proteins, and reversed the inhibition of these proteins expression induced by DFMO. The results of gross and pathological scores showed administration of PGE1 significantly alleviated intestinal mucosal injury of rats. Our findings indicate that total ginsenosides promoted the IEC-6 proliferation presumably via its regulation on cell cycle and the expression of proliferation-related proteins regulated by polyamines, and provided a novel perspective for exploring the repair effect of Panax ginseng upon gastrointestinal mucosal injury.

A novel function of HRP-3 in regulating cell cycle progression via the HDAC-E2F1-Cyclin E pathway in lung cancer

To improve the poor survival rate of lung cancer patients, we investigated the role of HDGF‐related protein 3 (HRP‐3) as a potential biomarker for lung cancer. The expression of endogenous HRP‐3 in human lung cancer tissues and xenograft tumor models is indicative of its clinical relevance in lung cancer. Additionally, we demonstrated that HRP‐3 directly binds to the E2F1 promoter on chromatin. Interestingly, HRP‐3 depletion in A549 cells impedes the binding of HRP‐3 to the E2F1 promoter; this in turn hampers the interaction between Histone H3/H4 and HDAC1/2 on the E2F1 promoter, while concomitantly inducing Histone H3/H4 acetylation around the E2F1 promoter. The enhanced Histone H3/H4 acetylation on the E2F1 promoter through HRP‐3 depletion increases the transcription level of E2F1. Furthermore, the increased E2F1 transcription levels lead to the enhanced transcription of Cyclin E, known as the E2F1‐responsive gene, thus inducing S‐phase accumulation. Therefore, our study provides evidence for the utility of HRP‐3 as a biomarker for the prognosis and treatment of lung cancer. Furthermore, we delineated the capacity of HRP‐3 to regulate the E2F1 transcription level via histone deacetylation.

Transcription Factor Homeobox D9 Drives the Malignant Phenotype of HPV18-Positive Cervical Cancer Cells via Binding to the Viral Early Promoter

Simple Summary

Transcription factor homeobox D9 (HOXD9) was previously reported to bind to the P97 promoter of HPV16 to induce viral E6/E7 oncogenes. In this article, we investigated whether HOXD9 regulated the P105 promoter of HPV18 and examined the role of HOXD9 in intracellular signaling of cervical cancer (CC). HOXD9 was directly bound to the P105 promoter and regulated the expression of E6/E7 genes of HPV18. The HOXD9 knockdown suppressed the E6/E7 gene expression in HPV18-positive cervical cancer cells. It decreased the expression of E6, activated the p53 pathway, and induced apoptosis. In addition, downregulation of the E7 gene expression activated the Rb pathway, causing G1 arrest in the cell cycle and markedly suppressing cell proliferation. Our results indicate that HOXD9 has pivotal roles in the proliferation and immortalization of HPV18-positive cervical cancer cells through activating the P105 promoter.

Abstract

Persistent infections with two types of human papillomaviruses (HPV), HPV16 and HPV18, are the most common cause of cervical cancer (CC). Two viral early genes, E6 and E7, are associated with tumor development, and expressions of E6 and E7 are primarily regulated by a single viral promoter: P97 in HPV16 and P105 in HPV18. We previously demonstrated that the homeobox D9 (HOXD9) transcription factor is responsible for the malignancy of HPV16-positive CC cell lines via binding to the P97 promoter. Here, we investigated whether HOXD9 is also involved in the regulation of the P105 promoter using two HPV18-positive CC cell lines, SKG-I and HeLa. Following the HOXD9 knockdown, cell viability was significantly reduced, and E6 expression was suppressed and was accompanied by increased protein levels of P53, while mRNA levels of TP53 did not change. E7 expression was also downregulated and, while mRNA levels of RB1 and E2F were unchanged, mRNA levels of E2F-target genes, MCM2 and PCNA, were decreased, which indicates that the HOXD9 knockdown downregulates E7 expression, thus leading to an inactivation of E2F and the cell-cycle arrest. Chromatin immunoprecipitation and promoter reporter assays confirmed that HOXD9 is directly associated with the P105 promoter. Collectively, our results reveal that HOXD9 drives the HPV18 early promoter activity to promote proliferation and immortalization of the CC cells.

Arsenic trioxide reduces the expression of E2F1, cyclin E, and phosphorylation of PI3K signaling molecules in acute leukemia cells

Arsenic trioxide (ATO) has been used for the treatment of acute promyelocytic leukemia (APL). Although ATO modulates cell cycle progression and apoptosis in APL cells, its exact mechanism of action remains elusive. In this research, we investigated its effects on E2F1, cyclin E, p53, pRb, and PI3K signaling molecules by western blotting, immunocytochemistry and/or confocal imaging. We found that ATO inhibited the proliferation of APL cells through down-regulation of E2F1 and cyclin E expression, and stimulation of pRb. It also reduced the interaction of pRb and E2F1with binding to the E2F1 promoter, by stimulating pRb association. ATO also effected the phosphorylation of pRb at S608 and T373 residues and association of E2F1, pRb, and p53, simultaneously. However, in p53-knockdown NB4 cells, ATO did not significantly reduce E2F1 and cyclin E expression. Our findings demonstrate that ATO inhibits APL cell growth through reduced expression of E2F1, cyclin E, and stimulation of pRb. It also effected both interaction and association of E2F1, pRb, and p53 by phosphorylation of pRb at T373 and S608 residues and reduced phosphorylation of PI3K signaling molecules. This novel mode of action of ATO in APL cells may be useful for designing new APL drugs.

MYC: a multipurpose oncogene with prognostic and therapeutic implications in blood malignancies

MYC oncogene is a transcription factor with a wide array of functions affecting cellular activities such as cell cycle, apoptosis, DNA damage response, and hematopoiesis. Due to the multi-functionality of MYC, its expression is regulated at multiple levels. Deregulation of this oncogene can give rise to a variety of cancers. In this review, MYC regulation and the mechanisms by which MYC adjusts cellular functions and its implication in hematologic malignancies are summarized. Further, we also discuss potential inhibitors of MYC that could be beneficial for treating hematologic malignancies.

HBx-upregulated MAFG-AS1 promotes cell proliferation and migration of hepatoma cells by enhancing MAFG expression and stabilizing nonmuscle myosin IIA

To identify hepatitis B virus (HBV)-related lncRNA(s), we previously examined the transcription profiles of the HBV-transgenic cell line HepG2-4D14 and parental HepG2 cells by RNA deep sequencing and identified 38 upregulated long noncoding RNAs (lncRNAs). In the present study, the lncRNA MAFG-AS1 is investigated in detail because its gene is located adjacent to the MAFG gene, which is an important transcription factor involved in cell proliferation. The level of MAFG-AS1 is significantly higher in HCC tissue than in nontumor tissues. TCGA data show that the expression level of MAFG-AS1 is negatively correlated with survival of HCC patients. GEO cohort data show that compared with healthy tissues, the expression level of MAFG-AS1 is significantly higher in HBV-infected liver tissues. Real-time PCR and luciferase reporter assay data show that HBx can enhance the transcription of MAFG-AS1. Gain-of-function and loss-of-function experiments indicate that MAFG-AS1 promotes proliferation, migration, and invasion of HCC cells. Tumor formation assay results demonstrate that knockdown of MAFG-AS1 significantly inhibits cell proliferation in nude mice. Furthermore, MAFG-AS1 enhances the transcription of adjacent MAFG via E2F1. Additionally, MAFG-AS1 interacts with three subunits (MYH9, MYL12B, and MYL6) of nonmuscle myosin IIA (NM IIA). Knockdown of MAFG-AS1 inhibits ATPase activity of MYH9, interaction of NM IIA subunits, and cell cycle progression. Thus, the lncRNA MAFG-AS1 is upregulated by HBV and promotes proliferation and migration of HCC cells. Our findings suggest that MAFG-AS1 is a potential oncogene that may contribute to HBV-related HCC development.

Gypenoside LI arrests the cell cycle of breast cancer in G0/G1 phase by down-regulating E2F1

ETHNOPHARMACOLOGICAL RELEVANCE

Gynostemma pentaphyllum (Thunb.) Makino, a traditional medicine in China, has been widely used for the treatment of various diseases. Gypenoside LI (Gyp LI) is a major constituent from steamed G. pentaphyllum. Previous studies have shown that gypnenoside LI possess inhibitory effect on the growth of many cancer cells. However, its pharmacological effect in breast cancer and the mechanism have not been reported yet.

AIM OF THE STUDY

To investigate the anti-breast cancer activity of gypenoside LI and underlying mechanisms of gypenoside LI in MDA-MB-231 and MCF-7 cells.

MATERIAL/METHODS

The cytotoxicity of gypenoside LI was determined by MTT, colony-formation and three-dimensional spheroid assay. The migration, cell apoptosis and the cell cycle were investigated through cell morphology observation, flow cytometry analysis and key proteins detection. The anticancer mechanisms of gypenoside LI were detected by RNA sequencing (RNA-seq) and Gene Set Enrichment Analysis (GSEA) transcriptome analysis.

RESULTS

Gypenoside LI inhibited cell proliferation, migration, induced cell apoptosis and cell cycle arrest. Gypenoside LI arrested cell cycle at G0/G1 phase by regulating E2F1. It also inhibited tumor proliferation by regulating the expression of ERCC6L. Interestingly, we found that E2F1 siRNA also down-regulated the expression of ERCC6L. Gypenoside LI showed potential anti-breast cancer cells activity, especially on triple-negative breast cancer cells.

CONCLUSIONS

These data indicate that gypenoside LI could inhibit human breast cancer cells through inhibiting proliferation and migration, inducing apoptosis, arresting cell cycle at G0/G1 phase by regulating E2F1. It could be used as potential multi-target chemopreventive agents for cancer.

Knockdown of CENPW Inhibits Hepatocellular Carcinoma Progression by Inactivating E2F Signaling

Aim:

This study aimed to evaluate the effects of centromere protein W (CENPW, also known as CUG2) in hepatocellular carcinoma (HCC).

Methods:

CENPW expression in HCC tissues and cells was detected by RT-qPCR assay. CCK-8 and colony formation assay were used to assess cell proliferation. Wound healing and Transwell assay was used to detect cell migration and invasion, respectively. The flow cytometry was used to analyze the cell cycle distribution and apoptosis.

Results:

CENPW expression was upregulated in HCC tissues and cells. Knockdown of CENPW inhibited cell proliferation, migration, and invasion and induced the G0/G1 phase arrest and cell apoptosis in HCC cells, which might involve the E2F signaling regulation.

Conclusion:

CENPW acted as an oncogenic role in HCC progression via activation E2F signaling. Our findings may provide new insights into the studying mechanisms of HCC.

Sirtuins-Mediated System-Level Regulation of Mammalian Tissues at the Interface between Metabolism and Cell Cycle: A Systematic Review

Sirtuins are a family of highly conserved NAD+-dependent proteins and this dependency links Sirtuins directly to metabolism. Sirtuins' activity has been shown to extend the lifespan of several organisms and mainly through the post-translational modification of their many target proteins, with deacetylation being the most common modification. The seven mammalian Sirtuins, SIRT1 through SIRT7, have been implicated in regulating physiological responses to metabolism and stress by acting as nutrient sensors, linking environmental and nutrient signals to mammalian metabolic homeostasis. Furthermore, mammalian Sirtuins have been implicated in playing major roles in mammalian pathophysiological conditions such as inflammation, obesity and cancer. Mammalian Sirtuins are expressed heterogeneously among different organs and tissues, and the same holds true for their substrates. Thus, the function of mammalian Sirtuins together with their substrates is expected to vary among tissues. Any therapy depending on Sirtuins could therefore have different local as well as systemic effects. Here, an introduction to processes relevant for the actions of Sirtuins, such as metabolism and cell cycle, will be followed by reasoning on the system-level function of Sirtuins and their substrates in different mammalian tissues. Their involvement in the healthy metabolism and metabolic disorders will be reviewed and critically discussed.

ABT-751 Induces Multiple Anticancer Effects in Urinary Bladder Urothelial Carcinoma-Derived Cells: Highlighting the Induction of Cytostasis through the Inhibition of SKP2 at Both Transcriptional and Post-Translational Levels

The objective was to investigate the anti-cancer effects and underlying molecular mechanisms of cytostasis which were activated by an anti-microtubule drug, ABT-751, in two urinary bladder urothelial carcinoma (UBUC)-derived cell lines, BFTC905 and J82, with distinct genetic backgrounds. A series of in vitro assays demonstrated that ABT-751 induced G₂/M cell cycle arrest, decreased cell number in the S phase of the cell cycle and suppressed colony formation/independent cell growth, accompanied with alterations of the protein levels of several cell cycle regulators. In addition, ABT-751 treatment significantly hurdled cell migration and invasion along with the regulation of epithelial–mesenchymal transition-related proteins. ABT-751 triggered autophagy and apoptosis, downregulated the mechanistic target of rapamycin kinase (MTOR) and upregulated several pro-apoptotic proteins that are involved in extrinsic and intrinsic apoptotic pathways. Inhibition of autophagosome and autolysosome enhanced apoptosis was also observed. Through the inhibition of the NFκB signaling pathway, ABT-751 suppressed S-phase kinase associated protein 2 (SKP2) transcription and subsequent translation by downregulation of active/phospho-AKT serine/threonine kinase 1 (AKT1), component of inhibitor of nuclear factor kappa B kinase complex (CHUK), NFKB inhibitor alpha (NFKBIA), nuclear RELA proto-oncogene, NFκB subunit (RELA) and maintained a strong interaction between NFKBIA and RELA to prevent RELA nuclear translocation for SKP2 transcription. ABT-751 downregulated stable/phospho-SKP2 including pSKP2(S64) and pSKP2(S72), which targeted cyclin-dependent kinase inhibitors for degradation through the inactivation of AKT. Our results suggested that ABT-751 may act as an anti-cancer drug by inhibiting cell migration, invasion yet inducing cell cycle arrest, autophagy and apoptosis in distinct UBUC-derived cells. Particularly, the upstream molecular mechanism of its anticancer effects was identified as ABT-751-induced cytostasis through the inhibition of SKP2 at both transcriptional and post-translational levels to stabilize cyclin dependent kinase inhibitor 1A (CDKN1A) and CDKN1B proteins.

Recent Understanding and Future Directions of Recurrent Corticotroph Tumors

Corticotroph tumors (CTs) are pituitary neoplasms arising from the Tpit lineage, which may or not express adrenocorticotrophic hormone (ACTH). Functioning CTs cause Cushing's disease (CD), which has high morbidity and mortality due to hypercortisolemia. "Non-functioning" or silent CTs (SCT) and the Crooke's cell subtypes do not cause CD and may be asymptomatic until manifested by compressive symptoms and are more frequently found as macroadenoma. Both tend toward more aggressive behavior, recurrence, and a higher rate of malignant transformation to pituitary carcinoma. Tumorigenesis involves genetic, epigenetic, and post-transcriptional disruption of cell-cycle regulators, which increase cell proliferation, POMC overexpression, ACTH transcription, and/or hypersecretion. Furthermore, functioning CTs develop resistance to glucocorticoid-mediated negative feedback on ACTH secretion, through increased expression of testicular orphan nuclear receptor 4 (TR4), heat-shock protein 90 (HSP90), and loss-of-function mutation of CDK5 and ABL enzyme substrate 1 (CABLES1) gene. Overt autonomous hypercortisolemia is difficult to control, and multiple diagnostic studies and therapeutic modalities are commonly required. Cell-cycle regulation depends mainly on p27, cyclin E, cyclin-dependent kinases (CDKs), and the retinoblastoma protein (Rb)/E2F1 transcription factor complex. Gain-of-function mutations of ubiquitin-specific protease (USP) 8, USP48, and BRAF genes may subsequently cause overexpression of epithelial growth factor receptor (EGFR), and enhance POMC transcription, cell proliferation, and tumor growth. Epigenetic changes through micro RNAs and decreased DNA deacetylation by histone deacetylase type 2 (HDAC2), may also affect tumor growth. All the former mechanisms may become interesting therapeutic targets for CTs, aside from temozolomide, currently used for aggressive tumors. Potential therapeutic agents are EGFR inhibitors such as gefitinib and lapatinib, the purine analog R-roscovitine by dissociation of CDK2/Cyclin E complex, the HSP90 inhibitor silibinin (novobiocin), to reduce resistance to glucocorticoid-mediated negative feedback, and BRAF inhibitors vemurafenib and dabrafenib in BRAF V600E positive tumors. This review summarizes the molecular mechanisms related to CTs tumorigenesis, their diagnostic approach, and provides an update of the potential novel therapies, from the lab bench to the clinical translation.

Regulation of E2F1 activity via PKA-mediated phosphorylations

E2F1 becomes activated during the G1 phase of the cell cycle, and posttranslational modifications modulate its activity. Activation of G-protein coupled receptors (GPCR) by many ligands induces the activation of adenylate cyclases and the production of cAMP, which activates the PKA enzyme. Activated PKA elicits its biological effect by phosphorylating the target proteins containing serine or threonine amino acids in the RxxS/T motif. Since PKA activation negatively regulates cell proliferation, we thought that activated PKA would negatively affect the activity of E2F1. In line with this, when we analyzed the amino acid sequence of E2F1, we found 3 hypothetical consensus PKA phosphorylation sites located at 127-130, 232-235, and 361-364 positions and RYET, RLLS, and RMGS sequences. After showing the binding and phosphorylation of E2F1 by PKA, we converted the codons of Threonine-130, Serine-235, and Serine-364 to Alanine and Glutamic acid codons on the eukaryotic E2F1 expression vector we had previously created. We confirmed the phosphorylation of T130, S235, and S364 by developing monoclonal antibodies against phospho-specific forms of these sites and showed that their phosphorylation is cell cycle-dependent. According to our results, PKA-mediated phosphorylation of E2F1 by PKA inhibits proliferation and glucose uptake and induces caspase-3 activation and senescence.

Synthetic lethality of RB1 and aurora A is driven by stathmin-mediated disruption of microtubule dynamics

RB1 mutational inactivation is a cancer driver in various types of cancer including lung cancer, making it an important target for therapeutic exploitation. We performed chemical and genetic vulnerability screens in RB1-isogenic lung cancer pair and herein report that aurora kinase A (AURKA) inhibition is synthetic lethal in RB1-deficient lung cancer. Mechanistically, RB1^−/− cells show unbalanced microtubule dynamics through E2F-mediated upregulation of the microtubule destabilizer stathmin and are hypersensitive to agents targeting microtubule stability. Inhibition of AURKA activity activates stathmin function via reduced phosphorylation and facilitates microtubule destabilization in RB1^−/− cells, heavily impacting the bipolar spindle formation and inducing mitotic cell death selectively in RB1^−/− cells. This study shows that stathmin-mediated disruption of microtubule dynamics is critical to induce synthetic lethality in RB1-deficient cancer and suggests that upstream factors regulating microtubule dynamics, such as AURKA, can be potential therapeutic targets in RB1-deficient cancer.

Retinoblastoma susceptibility gene (RB1) is frequently mutated in lung cancers. Here the authors perform chemical and genetic vulnerability screens and identify aurora A kinase (AURKA) as a synthetic lethal candidate for RB1-deficient lung cancer cells and that AURKA inhibition sensitizes these cells to mitotic cell death.

Effect of TNIK upregulation on JQ1-resistant human colorectal cancer HCT116 cells

JQ1 disrupts the binding of bromodomain and extra-terminal (BET) family of proteins to acetylated histones, modulates the expression of various genes, and inhibits the proliferation of cancer cells. We established two JQ1-resistant sublines from human colorectal cancer HCT116 cells. These resistant cells showed an 8- to 9-fold higher resistance to JQ1, and a 2- to 4-fold higher resistance to various anti-cancer agents, such as doxorubicin, etoposide, mitoxantrone, SN-38, cisplatin, and methotrexate than the parental HCT116 cells. The JQ1-resistant cells expressed higher levels of TRAF2 and NCK-interacting protein kinase (TNIK), cyclin D1 (CCND1), cyclin E1 (CCNE1), and their corresponding mRNAs than the parental cells. TNIK is a regulator of Wnt/β-catenin signaling and is known to transactivate CCND1. Transient transfection of HCT116 cells with a TNIK expression plasmid resulted in the upregulation of cyclin D1, cyclin E1, and their corresponding mRNAs, as well as an increase in CCNE1 promoter activity. Furthermore, luciferase assay revealed that the JQ1-resistant cells showed high CCNE1 promoter activity. These results suggest that TNIK also transactivates CCNE1. Three stable TNIK transfectant clones of HEK293 cells expressed 1.5- to 2-fold higher levels of TNIK, cyclin D1, and cyclin E1 than the parental cells. The 293/TNIK-6 cells, which expressed the highest level of TNIK among the transfectants, showed a 2.3-fold higher resistance to JQ1 than the parental cells. These results suggest the possible involvement of TNIK in cellular resistance to JQ1.

An increase of microRNA-16-1 is associated with the high proliferation of squamous intraepithelial lesions in the presence of the integrated state of HR-HPV in liquid cytology samples

Studies of cervical cancer (CC) have reported that microRNA-16-1 (miR-16-1), which is an oncomiR, is increased in the tissues and cell lines of CC. The aim of the present study was to investigate the association of miRNA-16-1 expression level with squamous cell carcinoma (SCC), the presence of squamous intraepithelial lesions (SIL) and the integration of high-risk human papillomavirus (HR-HPV) DNA. The current study analyzed 80 samples obtained from women by liquid-based cytology, which revealed that 20 were negative for SIL (NSIL) and without HPV, 20 were low-grade SIL (LSIL), 20 were high-grade SIL (HSIL), and 20 were diagnosed as SCC with HR-HPV. The genotyping of the viral DNA was conducted via an INNO-LiPA-HPV array, the expression of miR-16-1 was determined by reverse transcription-quantitative PCR, and the physical state of the HR-HPV was ascertained by in situ hybridization with amplification with tyramide. A total of eight HR-HPV genotypes were distinguished; the most frequent of these being HPV16, followed by multiple infection with HR-HPV (including HPV16). The mixed state of the HR-HPV was observed in 60 and 65% of LSIL and HSIL cases, respectively, while an integrated HR-HPV state was identified in 90% of cases with SCC. The expression level of miR-16-1 increased according to the grade of SIL, and cases with HSIL exhibited a significantly higher miR-16-1 expression level compared with women with NSIL (P<0.001; Table II). It can therefore be determined that the expression of miR-16-1 effects cellular proliferation, due to the viral integration of various HR-HPV genotypes in unique infection or in multiple infection. Thus, the overexpression of miR-16-1 could be monitored in women with LSIL, in order to discard a major lesion.

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.

Restriction point regulation at the crossroads between quiescence and cell proliferation

The coordination of cell proliferation with reversible cell cycle exit into quiescence is crucial for the development of multicellular organisms and for tissue homeostasis in the adult. The decision between quiescence and proliferation occurs at the restriction point, which is widely thought to be located in the G1 phase of the cell cycle, when cells integrate accumulated extracellular and intracellular signals to drive this binary cellular decision. On the molecular level, decision-making is exerted through the activation of cyclin-dependent kinases (CDKs). CDKs phosphorylate the retinoblastoma (Rb) transcriptional repressor to regulate the expression of cell cycle genes. Recently, the classical view of restriction point regulation has been challenged. Here, we review the latest findings on the activation of CDKs, Rb phosphorylation and the nature and position of the restriction point within the cell cycle.

Ailanthone: A novel potential drug for treating human cancer

Cancer is the second leading cause of death after cardiovascular disease. In 2015, >8.7 million people died worldwide due to cancer, and by 2030 this figure is expected to increase to ~13.1 million. Tumor chemotherapy drugs have specific toxicity and side effects, and patients can also develop secondary drug resistance. To prevent and treat cancer, scientists have developed novel drugs with improved antitumor effects and decreased toxicity. Ailanthone (AIL) is a quassinoid extract from the traditional Chinese medicine plant Ailanthus altissima, which is known to have anti-inflammatory and antimalarial effects. An increasing number of studies have focused on AIL due to its antitumor activity. AIL can inhibit cell proliferation and induce apoptosis by up- or downregulating cancer-associated molecules, which ultimately leads to cancer cell death. Antitumor effects of AIL have been observed in melanoma, acute myeloid leukemia, bladder, lung, breast, gastric and prostate cancer and vestibular neurilemmoma. To the best of our knowledge, the present study is the first review to describe the antitumor mechanisms of AIL.

Yes-associated protein (YAP) and transcriptional coactivator with a PDZ-binding motif (TAZ): a nexus between hypoxia and cancer

Hypoxia is a common feature of solid tumors. As transcription factors, hypoxia-inducible factors (HIFs) are the master regulators of the hypoxic microenvironment; their target genes function in tumorigenesis and tumor development. Intriguingly, both yes-associated protein (YAP) and its paralog transcriptional coactivator with a PDZ-binding motif (TAZ) play fundamental roles in the malignant progression of hypoxic tumors. As downstream effectors of the mammalian Hippo pathway, YAP and/or TAZ (YAP/TAZ) are phosphorylated and sequestered in the cytoplasm by the large tumor suppressor kinase 1/2 (LATS1/2)-MOB kinase activator 1 (MOB1) complex, which restricts the transcriptional activity of YAP/TAZ. However, dephosphorylated YAP/TAZ have the ability to translocate to the nucleus where they induce transcription of target genes, most of which are closely related to cancer. Herein we review the tumor-related signaling crosstalk between YAP/TAZ and hypoxia, describe current agents and therapeutic strategies targeting the hypoxia–YAP/TAZ axis, and highlight questions that might have a potential impact in the future.

PRMT5 silencing selectively affects MTAP-deleted mesothelioma: In vitro evidence of a novel promising approach

Malignant mesothelioma (MM) is an aggressive asbestos‐related cancer of the serous membranes. Despite intensive treatment regimens, MM is still a fatal disease, mainly due to the intrinsic resistance to current therapies and the lack of predictive markers and new valuable molecular targets. Protein arginine methyltransferase 5 (PRMT5) inhibition has recently emerged as a potential therapy against methylthioadenosine phosphorylase (MTAP)‐deficient cancers, in which the accumulation of the substrate 5'‐methylthioadenosine (MTA) inhibits PRMT5 activity, thus sensitizing the cells to further PRMT5 inhibition. Considering that the MTAP gene is frequently codeleted with the adjacent cyclin‐dependent kinase inhibitor 2A (CDKN2A) locus in MM, we assessed whether PRMT5 could represent a therapeutic target also for this cancer type. We evaluated PRMT5 expression, the MTAP status and MTA content in normal mesothelial and MM cell lines. We found that both administration of exogenous MTA and stable PRMT5 knock‐down, by short hairpin RNAs (shRNAs), selectively reduced the growth of MTAP‐deleted MM cells. We also observed that PRMT5 knock‐down in MTAP‐deficient MM cells reduced the expression of E2F1 target genes involved in cell cycle progression and of factors implicated in epithelial‐to‐mesenchymal transition. Therefore, PRMT5 targeting could represent a promising new therapeutic strategy against MTAP‐deleted MMs.

Epigenetic Targeting of TERT-Associated Gene Expression Signature in Human Neuroblastoma with TERT Overexpression

Neuroblastoma is a deadly pediatric solid tumor with infrequent recurrent somatic mutations. Particularly, the pathophysiology of tumors without MYCN amplification remains poorly defined. Utilizing an unbiased approach, we performed gene set enrichment analysis of RNA-sequencing data from 498 patients with neuroblastoma and revealed a differentially overexpressed gene signature in MYCN nonamplified neuroblastomas with telomerase reverse transcriptase (TERT) gene overexpression and coordinated activation of oncogenic signaling pathways, including E2Fs, Wnt, Myc, and the DNA repair pathway. Promoter rearrangement of the TERT gene juxtaposes the coding sequence to strong enhancer elements, leading to TERT overexpression and poor prognosis in neuroblastoma, but TERT-associated oncogenic signaling remains unclear. ChIP-seq analysis of the human CLB-GA neuroblastoma cells harboring TERT rearrangement uncovered genome-wide chromatin co-occupancy of Brd4 and H3K27Ac and robust enrichment of H3K36me3 in TERT and multiple TERT-associated genes. Brd4 and cyclin-dependent kinases (CDK) had critical regulatory roles in the expression and chromatin activation of TERT and multiple TERT-associated genes. Epigenetically targeting Brd4 or CDKs with their respective inhibitors suppressed the expression of TERT and multiple TERT-associated genes in neuroblastoma with TERT overexpression or MYCN amplification. ChIP-seq and ChIP-qPCR provided evidence that the CDK inhibitor directly inhibited Brd4 recruitment to activate chromatin globally. Therefore, inhibiting Brd4 and CDK concurrently with AZD5153 and dinaciclib would be most effective in tumor growth suppression, which we demonstrated in neuroblastoma cell lines, primary human cells, and xenografts. In summary, we describe a unique mechanism in neuroblastoma with TERT overexpression and an epigenetically targeted novel therapeutic strategy. SIGNIFICANCE: Epigenetically cotargeting Brd4 and Cdks suppresses human neuroblastoma with TERT overexpression by inhibiting the TERT-associated gene expression networks.

KNK437 restricts the growth and metastasis of colorectal cancer via targeting DNAJA1/CDC45 axis

As an inhibitor of heat shock proteins (HSPs), KNK437 has been reported to play an anti-tumor role in several cancers. But its therapeutic effect and mechanisms in colorectal cancer (CRC) remain unclear. Here, KNK437 sharply inhibited the level of DnaJ heat shock protein family (Hsp40) member A1 (DNAJA1), followed by DNAJB1, but had little effect on the levels of HSP27, HSP105, HSP90, and HSP70 in CRC cells. DNAJA1 promoted CRC cell proliferation in vitro and tumor growth and metastasis in vivo. Mechanistically, DNAJA1 was activated by E2F transcription factor 1 (E2F1) and then promoted cell cycle by stabilizing cell division cycle protein 45 (CDC45), which could be reversed by KNK437. DNAJA1 was significantly upregulated in CRC tissues and positively correlated with serosa invasion, lymph node metastasis. High level of DNAJA1 predicted poor prognosis for CRC patients. Its expression was highly linked with E2F1 and CDC45 in CRC tissues. More importantly, KNK437 significantly suppressed the growth of DNAJA1 expressing tumor in vivo. The combined treatment of KNK437 with 5-FU/L-OHP chemotherapy reduced liver metastasis of CRC. These data reveal a novel mechanism of KNK437 in anti-tumor therapy of CRC and provides a newly therapeutic strategy with potential translation to the CRC patients.