Cyclin D1 production in cycling cells depends on ras in a cell-cycle-specific manner

Casticin induces ferroptosis in human osteosarcoma cells through Fe2+ overload and ROS production mediated by HMOX1 and LC3-NCOA4

Osteosarcoma is a primary solid bone malignancy, and surgery + chemotherapy is the most commonly used treatment. However, chemotherapeutic drugs can cause a range of side effects. Casticin, a polymethoxyflavonoid, has anti-tumor therapeutic effects. This study is aim to investigate the anti-osteosarcoma activity of casticin and explore the mechanism. Crystal violet staining, MTT assay, colony formation assay, wound healing assay, transwell assay, hoechst 33,258 staining, and flow cytometry analysis were used to investigate the effects of casticin on proliferation, migration, invasion, and apoptosis of osteosarcoma cells in vitro. The intracellular Fe2+, ROS, MDA, GSH/GSSG content changes were detected using the corresponding assay kits. The mRNA sequencing + bioinformatics analysis and western blot were used to detect the possible mechanism. We found that casticin caused G2/M phase cell cycle arrest in human osteosarcoma cells, inhibited the migration and invasion, and induced cell apoptosis and ferroptosis. Mechanistic studies showed the ferroptosis pathway was enriched stronger than apoptosis. Casticin up-regulated the expression of HMOX1, LC3 and NCOA4, meanwhile it activated MAPK signaling pathways. Animal experiments proved that casticin also inhibited the growth and metastasis of osteosarcoma cell xenograft tumor in vivo. In conclusion, casticin can induce ferroptosis in osteosarcoma cells through Fe2+ overload and ROS production mediated by HMOX1 and LC3-NCOA4. This provides a new strategy for osteosarcoma treatment.

Unveiling the therapeutic potential of thymol from Nigella sativa L. seed: selective anticancer action against human breast cancer cells (MCF-7) through down-regulation of Cyclin D1 and proliferative cell nuclear antigen (PCNA) expressions

BACKGROUND

Breast adenocarcinoma cells (MCF-7) are characterized by the overexpression of apoptotic marker genes and proliferative cell nuclear antigen (PCNA), which promote cancer cell proliferation. Thymol, derived from Nigella sativa (NS), has been investigated for its potential anti-proliferative and anticancer properties, especially its ability to suppress Cyclin D1 and PCNA expression, which are crucial in the proliferation of cancer cells.

METHODS

The cytotoxicity of thymol on MCF-7 cells was assessed using 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide (MTT) and lactate dehydrogenase (LDH) release methods. Thymol was tested at increasing concentrations (0-1000 µM) to evaluate its impact on MCF-7 cell growth. Additionally, Cyclin D1 and PCNA gene expression in thymol-treated and vehicle control groups of MCF-7 were quantified using real-time Polymerase Chain Reaction (RT-qPCR). Protein-ligand interactions were also investigated using the CB-Dock2 server.

RESULTS

Thymol significantly inhibited MCF-7 cell growth, with a 50% inhibition observed at 200 µM. The gene expression of Cyclin D1 and PCNA was down-regulated in the thymol-treated group relative to the vehicle control. The experimental results were verified through protein-ligand interaction investigations.

CONCLUSIONS

Thymol, extracted from NS, demonstrated specific cytotoxic effects on MCF-7 cells by suppressing the expression of Cyclin D1 and PCNA, suggesting its potential as an effective drug for MCF-7. However, additional in vivo research is required to ascertain its efficacy and safety in medical applications.

Differential cell cycle checkpoint evasion by PTEN germline mutations associated with dichotomous phenotypes of cancer versus autism spectrum disorder

Individuals with a PTEN germline mutation receive the molecular diagnosis of PTEN hamartoma tumor syndrome (PHTS). PHTS displays a complex spectrum of clinical phenotypes including harmartomas, predisposition to cancers, and autism spectrum disorder (ASD). Clear-cut genotype-phenotype correlations are yet to be established due to insufficient information on the PTEN function being impacted by mutations. To fill this knowledge gap, we compared functional impacts of two selected missense PTEN mutant alleles, G132D and M134R, each respectively being associated with distinct clinical phenotype, ASD or thyroid cancer without ASD using gene-edited human induced pluripotent stem cells (hiPSCs). In homozygous hiPSCs, PTEN expression was severely reduced by M134R mutation due to shortened protein half-life. G132D suppressed PTEN expression to a lesser extent than Μ134R mutation without altering protein half-life. When challenged with γ-irradiation, G132D heterozygous cells exited radiation-induced G2 arrest earlier than wildtype and M134R heterozygous hiPSCs despite the similar DNA damage levels as the latter two. Immunoblotting analyses suggested that γ-irradiation induced apoptosis in G132D heterozygous cells to lesser degrees than in the hiPSCs of other genotypes. These data suggest that ASD-associated G132D allele promotes genome instability by premature cell cycle reentry with incomplete DNA repair.

Influence of cyclin D1 splicing variants expression on breast cancer chemoresistance via CDK4/CyclinD1-pRB-E2F1 pathway

Cyclin D1 (CCND1), a mediator of cell cycle control, has a G870A polymorphism which results in the formation of two splicing variants: full-length CCND1 (CCND1a) and C-terminally truncated CCND1 species (CCND1b). However, the role of CCND1a and CCND1b variants in cancer chemoresistance remains unknown. Therefore, this study aimed to explore the molecular mechanism of alternative splicing of CCND1 in breast cancer (BC) chemoresistance. To address the contribution of G870A polymorphism to the production of CCND1 variants in BC chemoresistance, we sequenced the G870A polymorphism and analysed the expressions of CCND1a and CCND1b in MCF-7 and MCF-7/ADM cells. In comparison with MCF-7 cells, MCF-7/ADM cells with the A allele could enhance alternative splicing with the increase of SC-35, upregulate the ratio of CCND1b/a at both mRNA and protein levels, and activate the CDK4/CyclinD1-pRB-E2F1 pathway. Furthermore, CCND1b expression and the downstream signalling pathway were analysed through Western blotting and cell cycle in MCF-7/ADM cells with knockdown of CCND1b. Knockdown of CCND1b downregulated the ratio of CCND1b/a, demoted cell proliferation, decelerated cell cycle progression, inhibited the CDK4/CyclinD1-pRB-E2F1 pathway and thereby decreased the chemoresistance of MCF-7/ADM cells. Finally, CCND1 G870A polymorphism, the alternative splicing of CCDN1 was detected through Sequenom Mass ARRAY platform, Sanger sequencing, semi-quantitative RT-PCR, Western blotting and immunohistochemistry in clinical BC specimens. The increase of the ratio of CCND1b/a caused by G870A polymorphism was involved in BC chemoresistance. Thus, these findings revealed that CCND1b/a ratio caused by the polymorphism is involved in BC chemoresistance via CDK4/CyclinD1-pRB-E2F1 pathway.

Age- and cell cycle-related expression patterns of transcription factors and cell cycle regulators in Müller glia

Mammalian Müller glia express transcription factors and cell cycle regulators essential for the function of retinal progenitors, indicating the latent neurogenic capacity; however, the role of these regulators remains unclear. To gain insights into the role of these regulators in Müller glia, we analyzed expression of transcription factors (Pax6, Vsx2 and Nfia) and cell cycle regulators (cyclin D1 and D3) in rodent Müller glia, focusing on their age- and cell cycle-related expression patterns. Expression of Pax6, Vsx2, Nfia and cyclin D3, but not cyclin D1, increased in Müller glia during development. Photoreceptor injury induced cell cycle-associated increase of Vsx2 and cyclin D1, but not Pax6, Nfia, and cyclin D3. In dissociated cultures, cell cycle-associated increase of Pax6 and Vsx2 was observed in Müller glia from P10 mice but not from P21 mice. Nfia levels were highly correlated with EdU incorporation suggesting their activation during S phase progression. Cyclin D1 and D3 were transiently upregulated in G1 phase but downregulated after S phase entry. Our findings revealed previously unknown links between cell cycle progression and regulator protein expression, which likely affect the cell fate decision of proliferating Müller glia.

DNA Polymerase Theta Plays a Critical Role in Pancreatic Cancer Development and Metastasis

Pancreatic ductal adenocarcinoma (PDAC), due to its genomic heterogeneity and lack of effective treatment, despite decades of intensive research, will become the second leading cause of cancer-related deaths by 2030. Step-wise acquisition of mutations, due to genomic instability, is considered to drive the development of PDAC; the KRAS mutation occurs in 95 to 100% of human PDAC, and is already detectable in early premalignant lesions designated as pancreatic intraepithelial neoplasia (PanIN). This mutation is possibly the key event leading to genomic instability and PDAC development. Our study aimed to investigate the role of the error-prone DNA double-strand breaks (DSBs) repair pathway, alt-EJ, in the presence of the KRAS G12D mutation in pancreatic cancer development. Our findings show that oncogenic KRAS contributes to increasing the expression of Polθ, Lig3, and Mre11, key components of alt-EJ in both mouse and human PDAC models. We further confirm increased catalytic activity of alt-EJ in a mouse and human model of PDAC bearing the KRAS G12D mutation. Subsequently, we focused on estimating the impact of alt-EJ inactivation by polymerase theta (Polθ) deletion on pancreatic cancer development, and survival in genetically engineered mouse models (GEMMs) and cancer patients. Here, we show that even though Polθ deficiency does not fully prevent the development of pancreatic cancer, it significantly delays the onset of PanIN formation, prolongs the overall survival of experimental mice, and correlates with the overall survival of pancreatic cancer patients in the TCGA database. Our study clearly demonstrates the role of alt-EJ in the development of PDAC, and alt-EJ may be an attractive therapeutic target for pancreatic cancer patients.

The HPV16E7 Affibody as a Novel Potential Therapeutic Agent for Treating Cervical Cancer Is Likely Internalized through Dynamin and Caveolin-1 Dependent Endocytosis

Affibodies targeting intracellular proteins have a great potential to function as ideal therapeutic agents. However, little is known about how the affibodies enter target cells to interact with intracellular target proteins. We have previously developed the HPV16E7 affibody (Z_HPV16E7384) for HPV16 positive cervical cancer treatment. Here, we explored the underlying mechanisms of Z_HPV16E7384 and found that Z_HPV16E7384 significantly inhibited the proliferation of target cells and induced a G1/S phase cell cycle arrest. Furthermore, Z_HPV16E7384 treatment resulted in the upregulation of retinoblastoma protein (Rb) and downregulation of phosphorylated Rb (pRb), E2F1, cyclin D1, and CDK4 in the target cells. Moreover, treatment with dynamin or the caveolin-1 inhibitor not only significantly suppressed the internalization of Z_HPV16E7384 into target cells but also reversed the regulation of cell cycle factors by Z_HPV16E7384. Overall, these results indicate that Z_HPV16E7384 was likely internalized specifically into target cells through dynamin- and caveolin-1 mediated endocytosis. Z_HPV16E7384 induced the cell cycle arrest in the G1/S phase at least partially by interrupting HPV16E7 binding to and degrading Rb, subsequently leading to the downregulation of E2F1, cyclin D1, CDK4, and pRb, which ultimately inhibited target cell proliferation. These findings provide a rationale of using Z_HPV16E7384 to conduct a clinical trial for target therapy in cervical cancer.

The antitumour activity of C21 steroidal glycosides and their derivatives of Baishouwu: A review

ETHNOPHARMACOLOGICAL RELEVANCE

Baishouwu has been used in China for thousands of years since it was first discovered in the late Tang Dynasty and flourished in the Song and Ming Dynasties. The Chinese herbal medicines named Baishouwu include Cynanchum auriculatum Royle ex Wight., Cynanchum bungei Decne. and Cynanchum wilfordii Hemsl. It is described in the Sign of Materia Medica as "sweet, bitter, reinforce liver and kidney, and non-toxic". It is widely used for nourishing the blood to expel wind, reinforcing liver and kidney, strengthening bones and muscles.

AIM OF THE REVIEW

In this review, the current research status of the C₂₁ steroidal glycosides and their derivatives of Baishouwu for malignant tumours and their anti-tumour mechanisms are discussed. This may lay the ground for potential application of Baishouwu and its active ingredients in the treatment of tumours.

MATERIALS AND METHODS

Scientific databases, including PubMed, Elsevier, Science Direct, Google Scholar, CNKI, WANFANG DATA and VIP were searched to gather data about Baishouwu and its C₂₁ steroidal glycosides and their derivatives.

RESULTS

Prior literature indicates that Baishouwu has important biological activities such as anti-tumour, anti-epileptic, reducing cholesterol, protection of liver and kidney and immunomodulatory, which are of increasing interest, especially its anti-tumour activity. Recent studies demonstrate that the C₂₁ steroidal glycosides of Baishouwu, which have prominent antitumour efficacy, are one of its main active ingredients. Presently, a variety of C₂₁ steroidal glycosides have been isolated from Baishouwu medicinal part, the tuberous root. This review summarizes the various antitumour activities of the C₂₁ steroidal glycosides and their derivatives of Baishouwu.

CONCLUSIONS

In this review, the antitumour effects and mechanisms of total C₂₁ steroidal glycosides and monomers and derivatives of Baishouwu in vitro and in vivo were summarized. Baishouwu can inhibit tumourigenesis by blocking tumour cell cycle progression, regulating numerous signaling pathways, promoting apoptosis, inhibiting tumour cells proliferation and metastasis, improving immunity and so on. This review provides a theoretical basis for inheriting and developing the medical heritage of the motherland, exploring the resources of traditional Chinese medicine for ethnic minorities and clinical rational drug use.

Effect of polystyrene nanoplastics and their degraded forms on stem cell fate

Several studies have examined the effects of micro- and nanoplastics on microbes, cells, and the environment. However, only a few studies have examined their effects-especially, those of their reduced cohesiveness-on cell viability and physiology. We synthesized surfactant-free amine-functionalized polystyrene (PS) nanoparticles (NPs) and PS-NPs with decreased crosslinking density (DPS-NPs) without changing other factors, such as size, shape, and zeta potential and examined their effects on cell viability and physiology. PS- and DPS-NPs exhibited reactive oxygen species (ROS) scavenging activity by upregulating GPX3 expression and downregulating HSP70 (ROS-related gene) and XBP1 (endoplasmic reticulum stress-related gene) expression in human bone marrow-derived mesenchymal stem cells (hBM-MSCs). Additionally, they led to upregulation of MFN2 (mitochondrial fusion related gene) expression and downregulation of FIS1 (mitochondrial fission related gene) expression, indicating enhanced mitochondrial fusion in hBM-MSCs. Cell-cycle analysis revealed that PS- and DPS-NPs increased the proportion of cells in the S phase, indicating that they promoted cell proliferation and, specifically, the adipogenic differentiation of hBM-MSCs. However, the cytotoxicity of DPS-NPs against hBM-MSCs was higher than that of PS-NPs after long-term treatment under adipogenic conditions.

Reciprocal regulation of p21 and Chk1 controls the Cyclin D1-RB pathway to mediate senescence onset after G2 arrest

Senescence is an irreversible proliferation withdrawal that can be initiated after DNA damage-induced cell cycle arrest in G2 phase to prevent genomic instability. Senescence onset in G2 requires p53 and RB family tumour suppressors, but how they are regulated to convert a temporary cell cycle arrest into a permanent one remains unknown. Here, we show that a previously unrecognised balance between the CDK inhibitor p21 and Chk1 controls D-type cyclin-CDK activity during G2 arrest. In non-transformed cells, p21 activates RB in G2 by inhibiting Cyclin D1-CDK2/CDK4. The resulting G2 exit, which precedes appearance of senescence markers, is associated with a mitotic bypass, Chk1 downregulation and DNA damage foci reduction. In p53/RB-proficient cancer cells, compromised G2 exit correlates with sustained Chk1 activity, delayed p21 induction, untimely Cyclin E1 re-expression and genome reduplication. Conversely, Chk1 depletion promotes senescence by inducing p21 binding to Cyclin D1 and Cyclin E1-CDK complexes and down-regulating CDK6, whereas Chk2 knockdown enables RB phosphorylation and delays G2 exit. In conclusion, p21 and Chk2 oppose Chk1 to maintain RB activity, thus promoting DNA damage-induced senescence onset in G2.

Licorice extract inhibits growth of non-small cell lung cancer by down-regulating CDK4-Cyclin D1 complex and increasing CD8+ T cell infiltration

Background

Targeting tumor microenvironment (TME) may provide therapeutic activity and selectivity in treating cancers. Therefore, an improved understanding of the mechanism by which drug targeting TME would enable more informed and effective treatment measures. Glycyrrhiza uralensis Fisch (GUF, licorice), a widely used herb medicine, has shown promising immunomodulatory activity and anti-tumor activity. However, the molecular mechanism of this biological activity has not been fully elaborated.

Methods

Here, potential active compounds and specific targets of licorice that trigger the antitumor immunity were predicted with a systems pharmacology strategy. Flow cytometry technique was used to detect cell cycle profile and CD8⁺ T cell infiltration of licorice treatment. And anti-tumor activity of licorice was evaluated in the C57BL/6 mice.

Results

We reported the G0/G1 growth phase cycle arrest of tumor cells induced by licorice is related to the down-regulation of CDK4-Cyclin D1 complex, which subsequently led to an increased protein abundance of PD-L1. Further, in vivo studies demonstrated that mitigating the outgrowth of NSCLC tumor induced by licorice was reliant on increased antigen presentation and improved CD8⁺ T cell infiltration.

Conclusions

Briefly, our findings improved the understanding of the anti-tumor effects of licorice with the systems pharmacology strategy, thereby promoting the development of natural products in prevention or treatment of cancers.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12935-021-02223-0.

The Involvement of Ubiquitination Machinery in Cell Cycle Regulation and Cancer Progression

The cell cycle is a collection of events by which cellular components such as genetic materials and cytoplasmic components are accurately divided into two daughter cells. The cell cycle transition is primarily driven by the activation of cyclin-dependent kinases (CDKs), which activities are regulated by the ubiquitin-mediated proteolysis of key regulators such as cyclins, CDK inhibitors (CKIs), other kinases and phosphatases. Thus, the ubiquitin-proteasome system (UPS) plays a pivotal role in the regulation of the cell cycle progression via recognition, interaction, and ubiquitination or deubiquitination of key proteins. The illegitimate degradation of tumor suppressor or abnormally high accumulation of oncoproteins often results in deregulation of cell proliferation, genomic instability, and cancer occurrence. In this review, we demonstrate the diversity and complexity of the regulation of UPS machinery of the cell cycle. A profound understanding of the ubiquitination machinery will provide new insights into the regulation of the cell cycle transition, cancer treatment, and the development of anti-cancer drugs.

Expression of PRIP, a phosphatidylinositol 4,5-bisphosphate binding protein, attenuates PI3K/AKT signaling and suppresses tumor growth in a xenograft mouse model

Cancer is characterized by uncontrolled proliferation resulting from aberrant cell cycle progression. The activation of phosphatidylinositol 3-kinase (PI3K)/AKT signaling, a regulatory pathway for the cell cycle, stabilizes cyclin D1 in the G1 phase by inhibiting the activity of glycogen synthase kinase 3β (GSK3β) via phosphorylation. We previously reported that phospholipase C-related catalytically inactive protein (PRIP), a phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] binding protein, regulates PI3K/AKT signaling by competitively inhibiting substrate recognition by PI3K. Therefore, in this study, we investigated whether PRIP is involved in cell cycle progression. PRIP silencing in MCF-7 cells, a human breast cancer cell line, demonstrated PI(3,4,5)P3 signals accumulated at the cell periphery compared to that of the control. This suggests that PRIP reduction enhances PI(3,4,5)P3-mediated signaling. Consistently, PRIP silencing in MCF-7 cells exhibited increased phosphorylation of AKT and GSK3β which resulted in cyclin D1 accumulation. In contrast, the exogenous expression of PRIP in MCF-7 cells evidenced stronger downregulation of AKT and GSK3β phosphorylation, reduced accumulation of cyclin D1, and diminished cell proliferation in comparison to control cells. Flow cytometry analysis indicated that MCF-7 cells stably expressing PRIP attenuate cell cycle progression. Importantly, tumor growth of MCF-7 cells stably expressing PRIP was considerably prevented in an in vivo xenograft mouse model. In conclusion, PRIP expression downregulates PI3K/AKT/GSK3β-mediated cell cycle progression and suppresses tumor growth. Therefore, we propose that PRIP is a new therapeutic target for anticancer therapy.

Mutant p53 drives the loss of heterozygosity by the upregulation of Nek2 in breast cancer cells

BACKGROUND

Mutations in one allele of the TP53 gene in early stages are frequently followed by the loss of the remaining wild-type p53 (wtp53) allele (p53LOH) during tumor progression. Despite the strong notion of p53LOH as a critical step in tumor progression, its oncogenic outcomes that facilitate the selective pressure for p53LOH occurrence were not elucidated.

METHODS

Using MMTV;ErbB2 mouse model of breast cancer carrying heterozygous R172H p53 mutation, we identified a novel gain-of-function (GOF) activity of mutant p53 (mutp53): the exacerbated loss of wtp53 allele in response to γ-irradiation.

RESULTS

As consequences of p53LOH in mutp53 heterozygous cells, we observed profound stabilization of mutp53 protein, the loss of p21 expression, the abrogation of G2/M checkpoint, chromosomal instability, centrosome amplification, and transcriptional upregulation of mitotic kinase Nek2 (a member of Never in Mitosis (NIMA) Kinases family) involved in the regulation of centrosome function. To avoid the mitotic catastrophe in the absence of G2/M checkpoint, cells with centrosome amplification adapt Nek2-mediated centrosomes clustering as pro-survival mutp53 GOF mechanism enabling unrestricted proliferation and clonal expansion of cells with p53LOH. Thus, the clonal dominance of mutp53 cells with p53LOH may represent the mechanism of irradiation-induced p53LOH. We show that pharmacological and genetic ablation of Nek2 decreases centrosome clustering and viability of specifically mutp53 cells with p53LOH.

CONCLUSION

In a heterogeneous tumor population, Nek2 inhibition may alter the selective pressure for p53LOH by contraction of the mutp53 population with p53LOH, thus, preventing the outgrowth of genetically unstable, more aggressive cells.

The PRC2 complex directly regulates the cell cycle and controls proliferation in skeletal muscle

The polycomb repressive complex 2 (PRC2) is an important developmental regulator responsible for the methylation of histone 3 lysine 27 (H3K27). Here, we show that the PRC2 complex regulates the cell cycle in skeletal muscle cells to control proliferation and mitotic exit. Depletions of the catalytic subunit of the PRC2 complex, EZH2, have shown that EZH2 is required for cell viability, suggesting that EZH2 promotes proliferation. We found that EZH2 directly represses both positive and negative cell cycle genes, thus enabling the PRC2 complex to tightly control the cell cycle. We show that modest inhibition or depletion of EZH2 leads to enhanced proliferation and an accumulation of cells in S phase. This effect is mediated by direct repression of cyclin D1 (Ccnd1) and cyclin E1 (Ccne1) by the PRC2 complex. Our results show that PRC2 has pleiotropic effects on proliferation as it serves to restrain cell growth, yet clearly has a function required for cell viability as well. Intriguingly, we also find that the retinoblastoma protein gene (Rb1) is a direct target of the PRC2 complex. However, modest depletion of EZH2 is not sufficient to maintain Rb1 expression, indicating that the PRC2 dependent upregulation of cyclin D1 is sufficient to inhibit Rb1 expression. Taken together, our results show that the PRC2 complex regulates skeletal muscle proliferation in a complex manner that involves the repression of Ccnd1 and Ccne1, thus restraining proliferation, and the repression of Rb1, which is required for mitotic exit and terminal differentiation.

FGF2 modulates simultaneously the mode, the rate of division and the growth fraction in cultures of radial glia

Radial glial progenitors in the mammalian developing neocortex have been shown to follow a deterministic differentiation program restricted to an asymmetric-only mode of division. This feature seems incompatible with their well-known ability to increase in number when cultured in vitro, driven by fibroblast growth factor 2 and other mitogenic signals. The changes in their differentiation dynamics that allow this transition from in vivo asymmetric-only division mode to an in vitro self-renewing culture have not been fully characterized. Here, we combine experiments of radial glia cultures with numerical models and a branching process theoretical formalism to show that fibroblast growth factor 2 has a triple effect by simultaneously increasing the growth fraction, promoting symmetric divisions and shortening the length of the cell cycle. These combined effects partner to establish and sustain a pool of rapidly proliferating radial glial progenitors in vitro. We also show that, in conditions of variable proliferation dynamics, the branching process tool outperforms other commonly used methods based on thymidine analogs, such as BrdU and EdU, in terms of accuracy and reliability.

Highlighted Article: When mode and/or rate of division are changing, a branching process, rather than a thymidine analog method, provides temporal resolution, it is more robust and does not interfere with cell homeostasis.

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.

Temporal integration of mitogen history in mother cells controls proliferation of daughter cells

Multicellular organisms use mitogens to regulate cell proliferation, but how fluctuating mitogenic signals are converted into proliferation-quiescence decisions is poorly understood. In this work, we combined live-cell imaging with temporally controlled perturbations to determine the time scale and mechanisms underlying this system in human cells. Contrary to the textbook model that cells sense mitogen availability only in the G1 cell cycle phase, we find that mitogenic signaling is temporally integrated throughout the entire mother cell cycle and that even a 1-hour lapse in mitogen signaling can influence cell proliferation more than 12 hours later. Protein translation rates serve as the integrator that proportionally converts mitogen history into corresponding levels of cyclin D in the G2 phase of the mother cell, which controls the proliferation-quiescence decision in daughter cells and thereby couples protein production with cell proliferation.

Irradiation induces p53 loss of heterozygosity in breast cancer expressing mutant p53

Mutations in one allele of the TP53 gene in cancer early stages are frequently followed by the loss of the remaining wild-type allele (LOH) during tumor progression. However, the clinical impact of TP53 mutations and p53LOH, especially in the context of genotoxic modalities, remains unclear. Using MMTV;ErbB2 model carrying a heterozygous R172H p53 mutation, we report a previously unidentified oncogenic activity of mutant p53 (mutp53): the exacerbation of p53LOH after irradiation. We show that wild-type p53 allele is partially transcriptionally competent and enables the maintenance of the genomic integrity under normal conditions in mutp53 heterozygous cells. In heterozygous cells γ-irradiation promotes mutp53 stabilization, which suppresses DNA repair and the cell cycle checkpoint allowing cell cycle progression in the presence of inefficiently repaired DNA, consequently increases genomic instability leading to p53LOH. Hence, in mutp53 heterozygous cells, irradiation facilitates the selective pressure for p53LOH that enhances cancer cell fitness and provides the genetic plasticity for acquiring metastatic properties.

JARID2 and the PRC2 complex regulate the cell cycle in skeletal muscle

JARID2 is a noncatalytic member of the polycomb repressive complex 2 (PRC2) which methylates of histone 3 lysine 27 (H3K27). In this work, we show that JARID2 and the PRC2 complex regulate the cell cycle in skeletal muscle cells to control proliferation and mitotic exit. We found that the stable depletion of JARID2 leads to increased proliferation and cell accumulation in S phase. The regulation of the cell cycle by JARID2 is mediated by direct repression of both cyclin D1 and cyclin E1, both of which are targets of PRC2-mediated H3K27 methylation. Intriguingly, we also find that the retinoblastoma protein (RB1) is a direct target of JARID2 and the PRC2 complex. The depletion of JARID2 is not sufficient to activate RB1. However, the ectopic expression of RB1 can suppress cyclin D1 expression in JARID2-depleted cells. Transient depletion of JARID2 in skeletal muscle cells leads to a transient up-regulation of cyclin D1 that is quickly suppressed with no resulting effect on proliferation, Taken together, we show that JARID2 and the PRC2 complex regulate skeletal muscle proliferation in a precise manner that involves the repression of cyclin D1, thus restraining proliferation and repressing RB1, which is required for mitotic exit and terminal differentiation.

Insulinoma-associated protein 1 controls nasopharyngeal carcinoma to radiotherapy by modulating cyclin D1-dependent DNA repair machinery

Insulinoma-associated protein 1 (INSM1), a zinc finger transcriptional factor, is proven to be deregulated in several types of cancers. However, comprehension of the molecular mechanism of INSM1-mediated tumor progression remains poor. Here, we show that the radioresistant nasopharyngeal carcinoma (NPC) patients have higher expressions of INSM1 that correlated with poor prognosis. Genetic manipulation of INSM1 expression sufficiently controls the response of NPC cells to irradiation (IR). Mechanistically, cells exposed to IR, increased intracellular INSM1 competitively disrupts the interaction of cyclin D1 and CDK4 resulting in cell survival by the cyclin D1-dependent DNA repair machinery. Moreover, knockdown of INSM1 sensitives NPC cells to IR in vivo and protects xenograft mice from mortality. Taken together, these results indicate that INSM1 modulates NPC to radiotherapy by controlling cyclin D1-dependent DNA repair machinery that could be manipulated as a novel molecular target for NPC therapy.

Cyclin D-Cdk4,6 Drives Cell-Cycle Progression via the Retinoblastoma Protein's C-Terminal Helix

The cyclin-dependent kinases Cdk4 and Cdk6 form complexes with D-type cyclins to drive cell proliferation. A well-known target of cyclin D-Cdk4,6 is the retinoblastoma protein Rb, which inhibits cell-cycle progression until its inactivation by phosphorylation. However, the role of Rb phosphorylation by cyclin D-Cdk4,6 in cell-cycle progression is unclear because Rb can be phosphorylated by other cyclin-Cdks, and cyclin D-Cdk4,6 has other targets involved in cell division. Here, we show that cyclin D-Cdk4,6 docks one side of an alpha-helix in the Rb C terminus, which is not recognized by cyclins E, A, and B. This helix-based docking mechanism is shared by the p107 and p130 Rb-family members across metazoans. Mutation of the Rb C-terminal helix prevents its phosphorylation, promotes G1 arrest, and enhances Rb's tumor suppressive function. Our work conclusively demonstrates that the cyclin D-Rb interaction drives cell division and expands the diversity of known cyclin-based protein docking mechanisms.

Cyclin D-CDK4 relieves cooperative repression of proliferation and cell cycle gene expression by DREAM and RB

The Retinoblastoma protein (RB) restricts cell cycle gene expression and entry into the cell cycle. The RB-related protein p130 forms the DREAM (DP, RB-like, E2F and MuvB) complex and contributes to repression of cell cycle dependent genes during quiescence. Although both RB and DREAM bind and repress an overlapping set of E2F dependent gene promoters, it remains unclear if they cooperate to restrict cell cycle entry. To test the specific contributions of RB and DREAM, we generated RB and p130 knockout cells in primary human fibroblasts. Knockout of both p130 and RB yielded higher levels of cell cycle gene expression in G0 and G1 cells compared to cells with knockout of RB alone, indicating a role for DREAM and RB in repression of cell cycle genes. We observed that RB played a dominant role in E2F dependent gene repression during mid to late G1 while DREAM activity was more prominant during G0 and early G1. Cyclin D - Cyclin Dependent Kinase 4 (CDK4) dependent phosphorylation of p130 occurred during early G1 and led to the release of p130 and MuvB from E2F4 and decreased p130 and MuvB binding to cell cycle promoters. Specific inhibition of CDK4 activity by palbociclib blocked DREAM complex disassembly during cell cycle entry. In addition, sensitivity to CDK4 inhibition was dependent on RB and an intact DREAM complex in both normal cells as well as in palbociclib-sensitive cancer cell lines. Although RB knockout cells were partially resistant to CDK4 inhibition, RB and p130 double knockout cells were significantly more resistant to palbociclib treatment. These results indicate that DREAM cooperates with RB in repressing E2F dependent gene expression and cell cycle entry and supports a role for DREAM as a therapeutic target in cancer.

Quiescence, an individual journey

Quiescence is operationally characterized as a temporary and reversible proliferation arrest. There are many preconceived ideas about quiescence, quiescent cells being generally viewed as insignificant sleeping G1 cells. In fact, quiescence is central for organism physiology and its dysregulation involved in many pathologies. The quiescent state encompasses very diverse cellular situations depending on the cell type and its environment. This diversity challenges not only quiescence uniformity but also the universality of the molecular mechanisms beyond quiescence regulation. In this mini-perspective, we discuss recent advances in the concept of quiescence, and illustrate that this multifaceted cellular state is gaining increasing attention in many fields of biology.

Ras signals principally via Erk in G1 but cooperates with PI3K/Akt for Cyclin D induction and S-phase entry

Numerous studies exploring oncogenic Ras or manipulating physiological Ras signalling have established an irrefutable role for Ras as driver of cell cycle progression. Despite this wealth of information the precise signalling timeline and effectors engaged by Ras, particularly during G1, remain obscure as approaches for Ras inhibition are slow-acting and ill-suited for charting discrete Ras signalling episodes along the cell cycle. We have developed an approach based on the inducible recruitment of a Ras-GAP that enforces endogenous Ras inhibition within minutes. Applying this strategy to inhibit Ras stepwise in synchronous cell populations revealed that Ras signaling was required well into G1 for Cyclin D induction, pocket protein phosphorylation and S-phase entry, irrespective of whether cells emerged from quiescence or G2/M. Unexpectedly, Erk, and not PI3K/Akt or Ral was activated by Ras at mid-G1, albeit PI3K/Akt signalling was a necessary companion of Ras/Erk for sustaining cyclin-D levels and G1/S transition. Our findings chart mitogenic signaling by endogenous Ras during G1 and identify limited effector engagement restricted to Raf/MEK/Erk as a cogent distinction from oncogenic Ras signalling.

The cell biology of quiescent yeast – a diversity of individual scenarios

Most cells, from unicellular to complex organisms, spend part of their life in quiescence, a temporary non-proliferating state. Although central for a variety of essential processes including tissue homeostasis, development and aging, quiescence is poorly understood. In fact, quiescence encompasses various cellular situations depending on the cell type and the environmental niche. Quiescent cell properties also evolve with time, adding another layer of complexity. Studying quiescence is, above all, limited by the fact that a quiescent cell can be recognized as such only after having proved that it is capable of re-proliferating. Recent cellular biology studies in yeast have reported the relocalization of hundreds of proteins and the reorganization of several cellular machineries upon proliferation cessation. These works have revealed that quiescent cells can display various properties, shedding light on a plethora of individual behaviors. The deciphering of the molecular mechanisms beyond these reorganizations, together with the understanding of their cellular functions, have begun to provide insights into the physiology of quiescent cells. In this Review, we discuss recent findings and emerging concepts in Saccharomyces cerevisiae quiescent cell biology.

CDC25A pathway toward tumorigenesis: Molecular targets of CDC25A in cell-cycle regulation

The cell division cycle 25 (CDC25) phosphatases regulate key transitions between cell-cycle phases during normal cell division, and in the case of DNA damage, they are key targets of the checkpoint machinery that ensure genetic stability. Little is known about the mechanisms underlying dysregulation and downstream targets of CDC25. To understand these mechanisms, we silenced the CDC25A gene in breast cancer cell line MDA-MB-231 and studied downstream targets of CDC25A gene. MDA-MB-231 breast cancer cells were transfected and silenced by CDC25A small interfering RNA. Total messenger RNA (mRNA) was extracted and analyzed by quantitative real-time polymerase chain reaction. CDC25A phosphatase level was visualized by Western blot analysis and was analyzed by 2D electrophoresis and LC-ESI-MS/MS. After CDC25A silencing, cell proliferation reduced, and the expression of 12 proteins changed. These proteins are involved in cell-cycle regulation, programmed cell death, cell differentiation, regulation of gene expression, mRNA editing, protein folding, and cell signaling pathways. Five of these proteins, including ribosomal protein lateral stalk subunit P0, growth factor receptor bound protein 2, pyruvate kinase muscle 2, eukaryotic translation elongation factor 2, and calpain small subunit 1 increase the activity of cyclin D1. Our results suggest that CDC25A controls the cell proliferation and tumorigenesis by a change in expression of proteins involved in cyclin D1 regulation and G1/S transition.

Poly(ADP-Ribose) Polymerase Inhibition Sensitizes Colorectal Cancer-Initiating Cells to Chemotherapy

Colorectal cancer (CRC) remains a leading killer in the U.S. with resistance to treatment as the largest hurdle to cure. Colorectal cancer-initiating cells (CICs) are a self-renewing tumor population that contribute to tumor relapse. Here, we report that patient-derived CICs display relative chemoresistance compared with differentiated progeny. In contrast, conventional cell lines failed model therapeutic resistance. CICs preferentially repaired chemotherapy-induced DNA breaks, prompting us to interrogate DNA damage pathways against which pharmacologic inhibitors have been developed. We found that CICs critically depended on the key single-strand break repair mediator, poly(ADP-ribose) polymerase (PARP), to survive treatment with standard-of-care chemotherapy. Small molecule PARP inhibitors (PARPi) sensitized CICs to chemotherapy and reduced chemotherapy-treated CIC viability, self-renewal, and DNA damage repair. Although PARPi monotherapy failed to kill CICs, combined PARPi therapy with chemotherapy attenuated tumor growth in vivo. Clinical significance of PARPi for CRC patients was supported by elevated PARP levels in colorectal tumors compared with normal colon, with further increases in metastases. Collectively, our results suggest that PARP inhibition serves as a point of fragility for CICs by augmenting therapeutic efficacy of chemotherapy. Stem Cells 2019;37:42-53.

Control of the Restriction Point by Rb and p21

The Restriction Point was originally defined as the moment that cells commit to the cell cycle and was later suggested to coincide with hyperphosphorylation of the retinoblastoma protein (Rb). Current cell cycle models posit that cells exit mitosis into a pre-Restriction Point state, where they have low cyclin-dependent kinase (CDK) activity and hypophosphorylated Rb; passage through the Restriction Point then occurs in late G1. Recent single-cell studies have challenged the current paradigm, raising questions about the location of the Restriction Point and the notion that cells exit mitosis into a pre-Restriction Point state. Here, we use a variety of single-cell techniques to show that both noncancer and cancer cells bifurcate into two subpopulations after anaphase, marked by increasing vs. low CDK2 activity and hyper- vs. hypophosphorylation of Rb. Notably, subpopulations with hyper- and hypophosphorylated Rb are present within minutes after anaphase, delineating one subpopulation that never "uncrosses" the Restriction Point and continues cycling and another subpopulation that exits mitosis into an uncommitted pre-Restriction Point state. We further show that the CDK inhibitor p21 begins rising in G2 in mother cells whose daughters exit mitosis into the pre-Restriction Point, CDK2low state. Furthermore, degradation of p21 coincides with escape from the CDK2low state and passage through the Restriction Point. Together, these data support a model in which only a subset of cells returns to a pre-Restriction Point state after mitosis and where the Restriction Point is sensitive to not only mitogens, but also inherited DNA replication stress via p21.

Hypoxia induces pulmonary fibroblast proliferation through NFAT signaling

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive and typically fatal lung disease with a very low survival rate. Excess accumulation of fibroblasts, myofibroblasts and extracellular matrix creates hypoxic conditions within the lungs, causing asphyxiation. Hypoxia is, therefore, one of the prominent features of IPF. However, there have been few studies concerning the effects of hypoxia on pulmonary fibroblasts. In this study, we investigated the molecular mechanisms of hypoxia-induced lung fibroblast proliferation. Hypoxia increased the proliferation of normal human pulmonary fibroblasts and IPF fibroblasts after exposure for 3–6 days. Cell cycle analysis demonstrated that hypoxia promoted the G1/S phase transition. Hypoxia downregulated cyclin D1 and A2 levels, while it upregulated cyclin E1 protein levels. However, hypoxia had no effect on the protein expression levels of cyclin-dependent kinase 2, 4, and 6. Chemical inhibition of hypoxia-inducible factor (HIF)-2 reduced hypoxia-induced fibroblast proliferation. Moreover, silencing of Nuclear Factor Activated T cell (NFAT) c2 attenuated the hypoxia-mediated fibroblasts proliferation. Hypoxia also induced the nuclear translocation of NFATc2, as determined by immunofluorescence staining. NFAT reporter assays showed that hypoxia-induced NFAT signaling activation is dependent on HIF-2, but not HIF-1. Furthermore, the inhibition or silencing of HIF-2, but not HIF-1, reduced the hypoxia-mediated NFATc2 nuclear translocation. Our studies suggest that hypoxia induces the proliferation of human pulmonary fibroblasts through NFAT signaling and HIF-2.

Competing memories of mitogen and p53 signalling control cell-cycle entry

Regulation of cell proliferation is necessary for immune responses, tissue repair, and upkeep of organ function to maintain human health. When proliferating cells complete mitosis, a fraction of newly born daughter cells immediately enter the next cell cycle, while the remaining cells in the same population exit to a transient or persistent quiescent state. Whether this choice between two cell-cycle pathways is due to natural variability in mitogen signalling or other underlying causes is unknown. Here we show that human cells make this fundamental cell-cycle entry or exit decision based on competing memories of variable mitogen and stress signals. Rather than erasing their signalling history at cell-cycle checkpoints before mitosis, mother cells transmit DNA damage-induced p53 protein and mitogen-induced cyclin D1 (CCND1) mRNA to newly born daughter cells. After mitosis, the transferred CCND1 mRNA and p53 protein induce variable expression of cyclin D1 and the CDK inhibitor p21 that almost exclusively determines cell-cycle commitment in daughter cells. We find that stoichiometric inhibition of cyclin D1-CDK4 activity by p21 controls the retinoblastoma (Rb) and E2F transcription program in an ultrasensitive manner. Thus, daughter cells control the proliferation-quiescence decision by converting the memories of variable mitogen and stress signals into a competition between cyclin D1 and p21 expression. We propose a cell-cycle control principle based on natural variation, memory and competition that maximizes the health of growing cell populations.

CDK4 protein is degraded by anaphase-promoting complex/cyclosome in mitosis and reaccumulates in early G1 phase to initiate a new cell cycle in HeLa cells

CDK4 regulates G₁/S phase transition in the mammalian cell cycle by phosphorylating retinoblastoma family proteins. However, the mechanism underlying the regulation of CDK4 activity is not fully understood. Here, we show that CDK4 protein is degraded by anaphase-promoting complex/cyclosome (APC/C) during metaphase-anaphase transition in HeLa cells, whereas its main regulator, cyclin D1, remains intact but is sequestered in cytoplasm. CDK4 protein reaccumulates in the following G₁ phase and shuttles between the nucleus and the cytoplasm to facilitate the nuclear import of cyclin D1. Without CDK4, cyclin D1 cannot enter the nucleus. Point mutations that disrupt CDK4 and cyclin D1 interaction impair the nuclear import of cyclin D1 and the activity of CDK4. RNAi knockdown of CDK4 also induces cytoplasmic retention of cyclin D1 and G₀/G₁ phase arrest of the cells. Collectively, our data demonstrate that CDK4 protein is degraded in late mitosis and reaccumulates in the following G₁ phase to facilitate the nuclear import of cyclin D1 for activation of CKD4 to initiate a new cell cycle in HeLa cells.

The role of MIF, cyclinD1 and ERK in the development of pulmonary hypertension in broilers

Pulmonary hypertension (PH) is a major disease in the broiler breeding industry. During PH, the pulmonary artery undergoes remodelling, which is caused by pulmonary vascular smooth muscle cell proliferation. CyclinD1 regulates cell proliferation. This study investigated the role of cyclinD1 in the development of PH in broilers, and which bioactivators and signalling pathway are involved in the pathological process. The PH group contained 3-4-week-old broilers with clinical PH, and the healthy group broilers from the same flock without PH. Histopathology indicated pulmonary arterial walls were thicker in the PH group compared with the healthy group. Target gene expressions of macrophage migration inhibitory factor (MIF), extracellular signal-regulated kinase (ERK), and cyclinD1 detected by quantitative real-time PCR were upregulated in the PH group compared with the healthy group. Immunohistochemistry showed MIF, phosphorylated ERK (p-ERK) and cyclinD1 were present on pulmonary vascular walls; MIF was present in the cytoplasm of arterial endothelial cells and smooth muscle cells; p-ERK and cyclinD1 were present in smooth muscle cell cytoplasm. Western blotting demonstrated that MIF, p-ERKand cyclinD1 levels were significantly higher (P < 0.01) in the PH group compared with the healthy group. In summary, increased MIF in PH broiler pulmonary arteries upregulated cyclinD1 via the ERK signalling pathway to induce pulmonary vascular smooth muscle cell proliferation, causing pulmonary artery remodelling and hypertension.

Targeting the AKT/cyclin D1 pathway to overcome intrinsic and acquired radioresistance of tumors for effective radiotherapy

PURPOSE

Radiotherapy (RT) is a powerful tool in the treatment of cancer, having the advantage of preserving normal tissues. Clinical outcomes of RT are significantly improved by technological advances, enabling increased radiation doses directed very specifically to a tumor. However, tumor radioresistance remains a major impediment to effective RT. We have shown that human tumor cells surviving after repeated exposure to fractionated radiation (FR) of X-rays for 1 month have acquired radioresistance through constitutive activation of AKT and downstream cyclin D1 nuclear retention. Tumor radioresistance is also proposed to be an intrinsic characteristic of cancer stem cells (CSC), whose efficient DNA repair is thought to confer this phenotype. We have isolated radioresistant CD133-positive cells following exposure to long-term FR. These cells exhibited the CSC phenotype with activation of the AKT/cyclin D1 pathway. In this review, I summarize our current understanding of the molecular mechanisms underlying tumor radioresistance and propose a strategy for overcoming radioresistance by targeting the AKT/cyclin D1 pathway.

CONCLUSION

Two different mechanisms: acquired radioresistance of surviving tumor cells after RT and intrinsic radioresistance of CSC are associated with tumor radioresistance. Inhibition of the AKT pathway results in radiosensitization of both types of tumor radioresistance.

Getting to S: CDK functions and targets on the path to cell-cycle commitment

How and when eukaryotic cells make the irrevocable commitment to divide remain central questions in the cell-cycle field. Parallel studies in yeast and mammalian cells seemed to suggest analogous control mechanisms operating during the G1 phase—at Start or the restriction (R) point, respectively—to integrate nutritional and developmental signals and decide between distinct cell fates: cell-cycle arrest or exit versus irreversible commitment to a round of division. Recent work has revealed molecular mechanisms underlying this decision-making process in both yeast and mammalian cells but also cast doubt on the nature and timing of cell-cycle commitment in multicellular organisms. These studies suggest an expanded temporal window of mitogen sensing under certain growth conditions, illuminate unexpected obstacles and exit ramps on the path to full cell-cycle commitment, and raise new questions regarding the functions of cyclin-dependent kinases (CDKs) that drive G1 progression and S-phase entry.

Microsatellite alteration and immunohistochemical expression profile of chromosome 9p21 in patients with sporadic renal cell carcinoma following surgical resection

Background

Long-term prognostic significance of loss of heterozygosity on chromosome 9p21 for localized renal cell carcinoma following surgery remains unreported. The study assessed the frequency of deletions of different loci of chromosome 9p along with immunohistochemical profile of proteins in surgically resected renal cancer tissue and correlated this with long-term outcomes.

Methods

DNA was extracted from renal tumours and corresponding normal kidney tissues in prospectively collected samples of 108 patients who underwent surgical resection for clinically localized disease between January 2001 and December 2005, providing a minimum of 9 years follow-up for each participant. After checking quality of DNA, amplified by PCR, loss of heterozygosity (LOH) on chromosome 9p was assessed using 6 microsatellite markers in 77 clear cell carcinoma. Only 5 of the markers showed LOH (D9S1814, D9S916, D9S974, D9S942, and D9S171). Protein expression of p15(INK4b), p16(INK4a), p14(ARF), CAIX, and adipose related protein (ADFP) were demonstrated by immunostaining in normal and cancer tissues. Loss of heterozygosity for microsatellite analysis was correlated with tumour characteristics, recurrence free, cancer specific, and overall survival, including significance of immunohistochemical profile of protein expressions.

Results

The main deletion was found at loci telomeric to CDKN2A region at D9S916. There was a significant correlation between frequency of LOH stage (p = 0.005) and metastases (p = 0.006) suggesting a higher LOH for advanced and aggressive renal cell carcinoma. Most commonly observed LOH in the 3 markers: D9S916, D9S974, and D9S942 were associated with poor survival, and were statistically significant on multivariate analysis. Immunohistochemical expression of p14, p15, and p16 proteins were either low or absent in cancer tissue compared to normal.

Conclusions

Loss of heterozygosity of p921 chromosome is associated with aggressive tumours, and predicts cancer specific or recurrence free survival on long-term follow-up.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2514-8) contains supplementary material, which is available to authorized users.

Evidence for a CDK4-dependent checkpoint in a conditional model of cellular senescence

Cellular senescence, the stable cell cycle arrest elicited by various forms of stress, is an important facet of tumor suppression. Although much is known about the key players in the implementation of senescence, including the pRb and p53 axes and the cyclin dependent kinase inhibitors p16^INK4a and p21^CIP1, many details remain unresolved. In studying conditional senescence in human fibroblasts that express a temperature sensitive SV40 large T-antigen (T-Ag), we uncovered an unexpected role for CDK4. At the permissive temperature, where pRb and p53 are functionally compromised by T-Ag, cyclin D-CDK4 complexes are disrupted by the high p16^INK4a levels and reduced expression of p21^CIP1. In cells arrested at the non-permissive temperature, p21^CIP1 promotes reassembly of cyclin D-CDK4 yet pRb is in a hypo-phosphorylated state, consistent with cell cycle arrest. In exploring whether the reassembled cyclin D-CDK4-p21 complexes are functional, we found that shRNA-mediated knockdown or chemical inhibition of CDK4 prevented the increase in cell size associated with the senescent phenotype by allowing the cells to arrest in G1 rather than G2/M. The data point to a role for CDK4 kinase activity in a G2 checkpoint that contributes to senescence.

Investigating breast cancer cell behavior using tissue engineering scaffolds

Despite early detection through the use of mammograms and aggressive intervention, breast cancer (BC) remains a clinical dilemma. BC can resurge after >10 years of remission. Studies indicate that BC cells (BCCs) with self-renewal and chemoresistance could be involved in dormancy. The majority of studies use in vitro, two-dimensional (2-D) monolayer cultures, which do not recapitulate the in vivo microenvironment. Thus, to determine the effect of three-dimensional (3-D) microenvironment on BCCs, this study fabricated tissue engineering scaffolds made of poly (ε-caprolactone) (PCL) having aligned or random fibers. Random and aligned fibers mimic, respectively, the random and highly organized collagen fibers found in the tumor extracellular matrix. Chemoresistant BCCs were obtained by treating with carboplatin. Western blot analysis of carboplatin resistant (treated) MDA-MB-231 (highly invasive, basal-like) and T47D (low-invasive, luminal) BCCs showed an increase in Bcl-2, Oct-4 and Sox-2, suggesting protection from apoptosis and increase in stem-like markers. Further studies with MDA-MB-231 BCCs seeded on the scaffolds showed little to no change in cell number over time for non-treated BCCs whereas on tissue culture polystyrene (TCP), non-treated BCCs displayed a significant increase in cell number at days 4 and 7 as compared to day 1 (p<0.05). Treated BCCs did not proliferate on TCP and the fibrous scaffolds. Little to no cyclin D1 was expressed for non-treated BCCs on TCP. On fibrous scaffolds, non-treated BCCs stained for cyclin D1 during the 7-day culture period. Treated BCCs expressed cyclin D1 on TCP and fibrous scaffolds during the 7-day culture period. Proliferation, viability and cell cycle analysis indicated that this 3-D culture prompted the aggressive BCCs to adopt a dormant phenotype, while the treated BCCs retained their phenotype. The findings indicate that random and aligned fibrous PCL scaffolds may provide a useful system to study how the 3-D microenvironment affects the behavior of BCCs.

A novel histone deacetylase (HDAC) inhibitor MHY219 induces apoptosis via up-regulation of androgen receptor expression in human prostate cancer cells

Histone deacetylase (HDAC) inhibitors are a new class of anticancer agents that act by inhibiting cancer cell proliferation and inducing apoptosis in various cancer cell lines. To investigate the anticancer effect of a novel histone deacetylase (HDAC) inhibitor MHY219, its efficacy was compared to that of suberoylanilide hydroxamic acid (SAHA) in human prostate cancer cells. The anticancer effects of MHY219 on cell viability, HDAC enzyme activity, cell cycle regulation, apoptosis and other biological assays were performed. MHY219 was shown to enhance the cytotoxicity on DU145 cells (IC₅₀, 0.36 μM) when compared with LNCaP (IC₅₀, 0.97 μM) and PC3 cells (IC₅₀, 5.12 μM). MHY219 showed a potent inhibition of total HDAC activity when compared with SAHA. MHY219 increased histone H3 hyperacetylation and reduced the expression of class I HDACs (1, 2 and 3) in prostate cancer cells. MHY219 effectively increased the sub-G1 fraction of cells through p21 and p27 dependent pathways in DU145 cells. MHY219 significantly induced a G2/M phase arrest in DU145 and PC3 cells and arrested the cell cycle at G0/G1 phase in LNCaP cells. Furthermore, MHY219 effectively increased apoptosis in DU145 and LNCaP cells, but not PC3 cells, according to Annexin V/PI staining and Western blot analysis. These results indicate that MHY219 is a potent HDAC inhibitor that targets regulating multiple aspects of cancer cell death and might have preclinical value in human prostate cancer chemotherapy, warranting further investigation.

Cyclin D1 overexpression perturbs DNA replication and induces replication-associated DNA double-strand breaks in acquired radioresistant cells

Fractionated radiotherapy (RT) is widely used in cancer treatment, because it preserves normal tissues. However, repopulation of radioresistant tumors during fractionated RT limits the efficacy of RT. We recently demonstrated that a moderate level of long-term fractionated radiation confers acquired radioresistance to tumor cells, which is caused by DNA-PK/AKT/GSK3β-mediated cyclin D1 overexpression. The resulting cyclin D1 overexpression leads to forced progression of the cell cycle to S-phase, concomitant with induction of DNA double-strand breaks (DSBs). In this study, we investigated the molecular mechanisms underlying cyclin D1 overexpression-induced DSBs during DNA replication in acquired radioresistant cells. DNA fiber data demonstrated that replication forks progressed slowly in acquired radioresistant cells compared with corresponding parental cells in HepG2 and HeLa cell lines. Slowly progressing replication forks were also observed in HepG2 and HeLa cells that overexpressed a nondegradable cyclin D1 mutant. We also found that knockdown of Mus81 endonuclease, which is responsible for resolving aberrant replication forks, suppressed DSB formation in acquired radioresistant cells. Consequently, Mus81 created DSBs to remove aberrant replication forks in response to replication perturbation triggered by cyclin D1 overexpression. After treating cells with a specific inhibitor for DNA-PK or ATM, apoptosis rates increased in acquired radioresistant cells but not in parental cells by inhibiting the DNA damage response to cyclin D1-mediated DSBs. This suggested that these inhibitors might eradicate acquired radioresistant cells and improve fractionated RT outcomes.

Senescence regulation by mTOR

The senescence program is activated in response to diverse stress stimuli potentially compromising genetic stability and leads to an irreversible cell cycle arrest. The mTOR pathway plays a crucial role in the regulation of cell metabolism and cellular growth. The goal of this chapter is to present evidence linking these two processes, which have one common regulator-the tumor suppressor p53. While the role of mTOR in senescence is still controversial, recent papers have shed new light onto this issue. This review, far from being exhaustive given the complexity of the field, will hopefully stimulate further research in this domain, whose relevance for ageing is becoming increasingly documented.

Three Observations That Have Changed Our Understanding of Cyclin D1 and p27 in Cell Cycle Control

Our understanding of cell cycle control has been based largely upon studies of synchronized cultures, often focused upon the early stages of the cell cycle following stimulation of quiescent cultures. These studies showed that cyclin D1 and p27(Kip1) (p27) each respond to the growth environment of the cell and together control entry into the cell cycle. In contrast, all cell cycle phases were considered in these studies of actively growing cultures, including events leading to withdrawal from the cell cycle. This approach relies upon the techniques of microinjection, quantitative image analysis, and time-lapse microscopy. The results provide critical new detail to our understanding of the roles of cyclin D1 and p27 in cell cycle regulation. Three critical observations resulting from this work will be described here to demonstrate that 1) cyclin D1 levels oscillate through the normal cell cycle, 2) checkpoint kinases are able to suppress cyclin D1 during S phase, and 3) the level of p27 is determined by a dynamic interaction between cyclin D1 and p27 so as to determine the rate of cell cycle progression. Based upon these observations, a model of cell cycle control is presented in which ras activity stimulates cyclin D1 during G2 phase, resulting in commitment of the cell to continued cell cycle progression. During G1 phase, ras activity suppresses the level of p27 protein, most of which is bound to cyclin D1, resulting in regulation of the rate of proliferation. This model predicted the involvement of checkpoint kinases in regulating cyclin D1 and the role of checkpoint kinases in the protection of neural cells against reactive oxygen. The substantiation of these 2 predictions serves as general validation of the model.

Acquired radioresistance of cancer and the AKT/GSK3β/cyclin D1 overexpression cycle

Fractionated radiotherapy (RT) is widely used in cancer therapy for its advantages in the preservation of normal tissues. However, repopulation of surviving tumor cells during fractionated RT limits the efficacy of RT. In fact, repopulating tumors often acquire radioresistance and this is the major cause of failure of RT. We have recently demonstrated that human tumor cells acquire radioresistance when exposed to fractionated radiation (FR) of X-rays every 12 hours for 1 month. The acquired radioresistance was associated with overexpression of cyclin D1, a result of a series of molecular changes; constitutive activation of DNA-PK and AKT with concomitant down-regulation of glycogen synthase kinase-3β (GSK3β) which results in suppression of cyclin D1 proteolysis. Aberrant cyclin D1 overexpression in S-phase induced DNA double strand breaks which activated DNA-PK and established the vicious cycle of cycling D1 overexpression. This overexpression of cyclin D1 is responsible for the radioresistance phenotype of long-term FR cells, since this phenotype was completely abrogated by treatment of FR cells by the API-2, an AKT inhibitor or by a Cdk4 inhibitor. Thus, targeting the AKT/GSK3β/cyclin D1/Cdk4 pathway can be an efficient modality to suppress acquired radioresistance of tumor cells. In this article, I overview the newly discovered molecular mechanisms underlying acquired radioresistance of tumor cells induced by FR, and propose a strategy for eradication of tumors using fractionated RT by overcoming tumor radioresistance.

Acquired radioresistance of human tumor cells by DNA-PK/AKT/GSK3beta-mediated cyclin D1 overexpression

Recurrence is frequently associated with the acquisition of radioresistance by tumors and resulting failures in radiotherapy. We report, in this study, that long-term fractionated radiation (FR) exposures conferred radioresistance to the human tumor cells, HepG2 and HeLa with cyclin D1 overexpression. A positive feedback loop was responsible for the cyclin D1 overexpression in which constitutively active AKT was involved. AKT is known to inactivate glycogen synthase kinase-3beta (GSK3beta), which is essential for the proteasomal degradation of cyclin D1. The resulting cyclin D1 overexpression led to the forced progression of S-phase with the induction of DNA double strand breaks. Cyclin D1-dependent DNA damage activated DNA-dependent protein kinase (DNA-PK), which in turn activated AKT and inactivated GSK3beta, thus completing a positive feedback loop of cyclin D1 overproduction. Cyclin D1 overexpression led to the activation of DNA damage response (DDR) consisted of ataxia telangiectasia mutated (ATM)- and Chk1-dependent DNA damage checkpoint and homologous recombination repair (HRR). Long-term FR cells repaired radiation-induced DNA damage faster than non-FR cells. Thus, acquired radioresistance of long-term FR cells was the result of alterations in DDR mediated by cyclin D1 overexpression. Inhibition of the AKT/GSK3beta/cyclin D1/Cdk4 pathway by the AKT inhibitor, Cdk4 inhibitor or cyclin D1 targeting small interfering RNA (siRNA) suppressed the radioresistance. Present observations give a mechanistic insight for acquired radioresistance of tumor cells by cyclin D1 overexpression, and provide novel therapeutic targets for recurrent radioresistant tumors.

The cell cycle is a redox cycle: linking phase-specific targets to cell fate

Reactive oxygen species (ROS) regulate the strength and duration of signaling through redox-dependent signal transduction pathways via the cyclic oxidation/reduction of cysteine residues in kinases, phosphatases, and other regulatory factors. Signaling circuits may be segregated in organelles or other subcellular domains with distinct redox states, permitting them to respond independently to changes in the oxidation state of two major thiol reductants, glutathione and thioredoxin. Studies in yeast, and in complex eukaryotes, show that oscillations in oxygen consumption, energy metabolism, and redox state are intimately integrated with cell cycle progression. Because signaling pathways play specific roles in different phases of the cell cycle and the hierarchy of redox-dependent regulatory checkpoints changes during cell cycle progression, the effects of ROS on cell fate vary during the cell cycle. In G1, ROS stimulate mitogenic pathways that control the activity of cyclin-dependent kinases (CDKs) and phosphorylation of the retinoblastoma protein (pRB), thereby regulating S-phase entry. In response to oxidative stress, Nrf2 and Foxo3a promote cell survival by inducing the expression of antioxidant enzymes and factors involved in cell cycle withdrawal, such as the cyclin-dependent kinase inhibitor (CKI) p27. In S phase, ROS induce S-phase arrest via PP2A-dependent dephosphorylation of pRB. In precancerous cells, unconstrained mitogenic signaling by activated oncogenes induces replication stress in S phase, which activates the DNA-damage response and induces cell senescence. A number of studies suggest that interactions of ROS with the G1 CDK/CKI network play a fundamental role in senescence, which is considered a barrier to tumorigenesis. Adaptive responses and loss of checkpoint proteins such as p53 and p16(INK4a) allow tumor cells to tolerate constitutive mitogenic signaling and enhanced production of ROS, leading to altered redox status in many fully transformed cells. Alterations in oxidant and energy metabolism of cancer cells have emerged as fertile ground for new therapeutic targets. The present challenge is to identify redox-dependent targets relevant to each cell cycle phase, to understand how these targets control fate decisions, and to describe the mechanisms that link metabolism to cell cycle progression.

DNA replication stalling attenuates tyrosine kinase signaling to suppress S phase progression

Here we report that T cell protein tyrosine phosphatase (TCPTP)-dependent and -independent pathways attenuate the JAK and Src protein tyrosine kinases (PTKs) and STAT3 phosphorylation to suppress cyclin D1 expression and S phase progression in response to DNA replication stress. Cells that lack TCPTP fail to suppress JAK1, Src, and STAT3, allowing for sustained cyclin D1 levels and progression through S phase despite continued replication stress. Cells that bypass the checkpoint undergo aberrant mitoses with lagging chromosomes that stain for the DNA damage marker gamma H2AX. Therefore, inactivating JAK, Src, and STAT3 signaling pathways in response to DNA replication stress may be essential for the suppression of S phase progression and the maintenance of genomic stability.