Inactivation of E2F3 results in centrosome amplification

CDK2 inhibition disorders centrosome stoichiometry and alters cellular outcomes in aneuploid cancer cells

Cyclin-dependent Kinase 2 (CDK2) inhibition prevents supernumerary centrosome clustering. This causes multipolarity, anaphase catastrophe and apoptotic death of aneuploid cancers. This study elucidated how CDK2 antagonism affected centrosome stoichiometry. Focused ion beam scanning electron microscopy (FIB-SEM) and immunofluorescent imaging were used. Studies interrogated multipolar mitosis after pharmacologic or genetic repression of CDK2. CDK2/9 antagonism with CYC065 (Fadraciclib)-treatment disordered centrosome stoichiometry in aneuploid cancer cells, preventing centrosome clustering. This caused ring-like chromosomes or multipolar cancer cells to form before onset of cell death. Intriguingly, CDK2 inhibition caused a statistically significant increase in single centrioles rather than intact centrosomes with two centrioles in cancer cells having chromosome rings or multipolarity. Statistically significant alterations in centrosome stoichiometry were undetected in other mitotic cancer cells. To confirm this pharmacodynamic effect, CDK2 but not CDK9 siRNA-mediated knockdown augmented cancer cells with chromosome ring or multipolarity formation. Notably, engineered gain of CDK2, but not CDK9 expression, reversed emergence of cancer cells with chromosome rings or multipolarity, despite CYC065-treatment. In marked contrast, CDK2 inhibition of primary human alveolar epithelial cells did not confer statistically significant increases of cells with ring-like chromosomes or multipolarity. Hence, CDK2 antagonism caused differential effects in malignant versus normal alveolar epithelial cells. Translational relevance was confirmed by CYC065-treatment of syngeneic lung cancers in mice. Mitotic figures in tumors exhibited chromosome rings or multipolarity. Thus, CDK2 inhibition preferentially disorders centrosome stoichiometry in cancer cells. Engaging this disruption is a strategy to explore against aneuploid cancers in future clinical trials.

The chromatin remodeling protein CHD-1 and the EFL-1/DPL-1 transcription factor cooperatively down regulate CDK-2 to control SAS-6 levels and centriole number

Centrioles are submicron-scale, barrel-shaped organelles typically found in pairs, and play important roles in ciliogenesis and bipolar spindle assembly. In general, successful execution of centriole-dependent processes is highly reliant on the ability of the cell to stringently control centriole number. This in turn is mainly achieved through the precise duplication of centrioles during each S phase. Aberrations in centriole duplication disrupt spindle assembly and cilia-based signaling and have been linked to cancer, primary microcephaly and a variety of growth disorders. Studies aimed at understanding how centriole duplication is controlled have mainly focused on the post-translational regulation of two key components of this pathway: the master regulatory kinase ZYG-1/Plk4 and the scaffold component SAS-6. In contrast, how transcriptional control mechanisms might contribute to this process have not been well explored. Here we show that the chromatin remodeling protein CHD-1 contributes to the regulation of centriole duplication in the C. elegans embryo. Specifically, we find that loss of CHD-1 or inactivation of its ATPase activity can restore embryonic viability and centriole duplication to a strain expressing insufficient ZYG-1 activity. Interestingly, loss of CHD-1 is associated with increases in the levels of two ZYG-1-binding partners: SPD-2, the centriole receptor for ZYG-1 and SAS-6. Finally, we explore transcriptional regulatory networks governing centriole duplication and find that CHD-1 and a second transcription factor, EFL-1/DPL-1 cooperate to down regulate expression of CDK-2, which in turn promotes SAS-6 protein levels. Disruption of this regulatory network results in the overexpression of SAS-6 and the production of extra centrioles.

Centrioles are cellular constituents that play an important role in cell reproduction, signaling and movement. To properly function, centrioles must be present in the cell at precise numbers. Errors in maintaining centriole number result in cell division defects and diseases such as cancer and microcephaly. How the cell maintains proper centriole copy number is not entirely understood. Here we show that two transcription factors, EFL-1/DPL-1 and CHD-1 cooperate to reduce expression of CDK-2, a master regulator of the cell cycle. We find that CDK-2 in turn promotes expression of SAS-6, a major building block of centrioles. When EFL-1/DPL-1 and CHD-1 are inhibited, CDK-2 is overexpressed. This leads to increased levels of SAS-6 and excess centrioles. Our work thus demonstrates a novel mechanism for controlling centriole number and is thus relevant to those human diseases caused by defects in centriole copy number control.

Long noncoding RNA plasmacytoma variant translocation 1 promotes progression of colorectal cancer by sponging microRNA-152-3p and regulating E2F3/MAPK8 signaling

The purpose of this study was to investigate the pathogenesis of colorectal cancer (CRC) and the effects of the long noncoding RNA plasmacytoma variant translocation 1 (PVT1) on CRC progression. Bioinformatics analysis verified PVT1 expression in tumor and normal tissues. Quantitative PCR and western blotting were used to measure mRNA and protein levels, respectively. The MTT, Transwell, colony formation, and in vivo assays were used to assess the effects of PVT1 on proliferation, migration, and invasion by CRC cells. Both PVT1 and microRNA (miR)-152-3p were shown to be colocalized in CRC cells using FISH assay. The target genes of miR-152-3p were predicted and verified by bioinformatics analysis, luciferase assay, and RNA pull-down assay. The ChIP assay revealed that E2F3 binds with the promoter of MAPK8. We found that PVT1 was overexpressed in CRC specimens, and its expression was higher in CRC cells than normal intestinal cells. Overexpression of PVT1 enhanced the proliferation, migration, and invasion of CRC cells, whereas PVT1 knockdown inhibited these processes. MicroRNA-152-3p was a target of PVT1, and E2F3 was a target of miR-152-3p. Rescue experiments confirmed the interaction between miR-152-3p and PVT1 and between miR-152-3p and E2F3. Luciferase and ChIP assay results confirmed that E2F3 modulates the transcriptional activation of MAPK8. Long noncoding RNA PVT1 activated E2F3 signaling by sponging miR-152-3p. The PVT1/miR-152-3p/E2F3/MAPK8 axis promoted CRC progression.

E2F3 drives the epithelial-to-mesenchymal transition, cell invasion, and metastasis in breast cancer

E2F3 is a transcription factor that may initiate tumorigenesis if overexpressed. Previously, we demonstrated that E2F3 mRNA is overexpressed in breast cancer and that E2F3 overexpression results in centrosome amplification and unregulated mitosis, which can promote aneuploidy and chromosome instability to initiate and sustain tumors. Further, we demonstrated that E2F3 leads to overexpression of the mitotic regulator Shugoshin-1, which until recently had unknown roles in cancer. This study aims to evaluate the roles of E2F3 and Shugoshin-1 in breast cancer metastatic potential. Here we demonstrated that E2F3 and Shugoshin-1 silencing leads to reduced cell invasion and migration in two mesenchymal triple-negative breast cancer (TNBC) cell lines (MDA-MB-231 and Hs578t). Moreover, E2F3 and Shugoshin-1 modulate the expression of epithelial-to-mesenchymal transition-associated genes such as Snail, E-Cadherin, and multiple matrix metalloproteinases. Furthermore, E2F3 depletion leads to reductions in tumor growth and metastasis in NOD-scid Gamma mice. Results from this study suggest a key role for E2F3 and a novel role for Shugoshin-1 in metastatic progression. These results can further help in the improvement of TNBC targeted therapies by interfering with pathways that intersect with the E2F3 and Shugoshin-1 signaling pathways.

The Nek2 centrosome-mitotic kinase contributes to the mesenchymal state, cell invasion, and migration of triple-negative breast cancer cells

Nek2 (NIMA-related kinase 2) is a serine/threonine-protein kinase that localizes to centrosomes and kinetochores, controlling centrosome separation, chromosome attachments to kinetochores, and the spindle assembly checkpoint. These processes prevent centrosome amplification (CA), mitotic dysfunction, and chromosome instability (CIN). Our group and others have suggested that Nek2 maintains high levels of CA/CIN, tumor growth, and drug resistance. We identified that Nek2 overexpression correlates with poor survival of breast cancer. However, the mechanisms driving these phenotypes are unknown. We now report that overexpression of Nek2 in MCF10A cells drives CA/CIN and aneuploidy. Besides, enhanced levels of Nek2 results in larger 3D acinar structures, but could not initiate tumors in a p53+/+ or a p53-/- xenograft model. Nek2 overexpression induced the epithelial-to-mesenchymal transition (EMT) while its downregulation reduced the expression of the mesenchymal marker vimentin. Furthermore, either siRNA-mediated downregulation or INH6's chemical inhibition of Nek2 in MDA-MB-231 and Hs578t cells showed important EMT changes and decreased invasion and migration. We also showed that Slug and Zeb1 are involved in Nek2 mediated EMT, invasion, and migration. Besides its role in CA/CIN, Nek2 contributes to breast cancer progression through a novel EMT mediated mechanism.

Principal Postulates of Centrosomal Biology. Version 2020

The centrosome, which consists of two centrioles surrounded by pericentriolar material, is a unique structure that has retained its main features in organisms of various taxonomic groups from unicellular algae to mammals over one billion years of evolution. In addition to the most noticeable function of organizing the microtubule system in mitosis and interphase, the centrosome performs many other cell functions. In particular, centrioles are the basis for the formation of sensitive primary cilia and motile cilia and flagella. Another principal function of centrosomes is the concentration in one place of regulatory proteins responsible for the cell's progression along the cell cycle. Despite the existing exceptions, the functioning of the centrosome is subject to general principles, which are discussed in this review.

Role of E2Fs and mitotic regulators controlled by E2Fs in the epithelial to mesenchymal transition

The epithelial-to-mesenchymal transition (EMT) is a complex cellular process in which epithelial cells acquire mesenchymal properties. EMT occurs in three biological settings: development, wound healing and fibrosis, and tumor progression. Despite occurring in three independent biological settings, EMT signaling shares some molecular mechanisms that allow epithelial cells to de-differentiate and acquire mesenchymal characteristics that confer cells invasive and migratory capacity to distant sites. Here we summarize the molecular mechanism that delineates EMT and we will focus on the role of E2 promoter binding factors (E2Fs) in EMT during tumor progression. Since the E2Fs are presently undruggable due to their control in numerous pivotal cellular functions and due to the lack of selectivity against individual E2Fs, we will also discuss the role of three mitotic regulators and/or mitotic kinases controlled by the E2Fs (NEK2, Mps1/TTK, and SGO1) in EMT that can be useful as drug targets.

Impact statement

The study of the epithelial to mesenchymal transition (EMT) is an active area of research since it is one of the early intermediates to invasion and metastasis—a state of the cancer cells that ultimately kills many cancer patients. We will present in this review that besides their canonical roles as regulators of proliferation, unregulated expression of the E2F transcription factors may contribute to cancer initiation and progression to metastasis by signaling centrosome amplification, chromosome instability, and EMT. Since our discovery that the E2F activators control centrosome amplification and mitosis in cancer cells, we have identified centrosome and mitotic regulators that may represent actionable targets against EMT and metastasis in cancer cells. This is impactful to all of the cancer patients in which the Cdk/Rb/E2F pathway is deregulated, which has been estimated to be most cancer patients with solid tumors.

The broken cycle: E2F dysfunction in cancer

The cyclin-dependent kinase (CDK)-RB-E2F axis forms the core transcriptional machinery driving cell cycle progression, dictating the timing and fidelity of genome replication and ensuring genetic material is accurately passed through each cell division cycle. The ultimate effectors of this axis are members of a family of eight distinct E2F genes encoding transcriptional activators and repressors. E2F transcriptional activity is tightly regulated throughout the cell cycle via transcriptional and translational regulation, post-translational modifications, protein degradation, binding to cofactors and subcellular localization. Alterations in one or more key components of this axis (CDKs, cyclins, CDK inhibitors and the RB family of proteins) occur in virtually all cancers and result in heightened oncogenic E2F activity, leading to uncontrolled proliferation. In this Review, we discuss the activities of E2F proteins with an emphasis on the newest atypical E2F family members, the specific and redundant functions of E2F proteins, how misexpression of E2F transcriptional targets promotes cancer and both current and developing therapeutic strategies being used to target this oncogenic pathway.

Breaking Bad: Uncoupling of Modularity in Centriole Biogenesis and the Generation of Excess Centrioles in Cancer

Centrosomes are tiny yet complex cytoplasmic structures that perform a variety of roles related to their ability to act as microtubule-organizing centers. Like the genome, centrosomes are single copy structures that undergo a precise semi-conservative replication once each cell cycle. Precise replication of the centrosome is essential for genome integrity, because the duplicated centrosomes will serve as the poles of a bipolar mitotic spindle, and any number of centrosomes other than two will lead to an aberrant spindle that mis-segregates chromosomes. Indeed, excess centrosomes are observed in a variety of human tumors where they generate abnormal spindles in situ that are thought to participate in tumorigenesis by driving genomic instability. At the heart of the centrosome is a pair of centrioles, and at the heart of centrosome duplication is the replication of this centriole pair. Centriole replication proceeds through a complex macromolecular assembly process. However, while centrosomes may contain as many as 500 proteins, only a handful of proteins have been shown to be essential for centriole replication. Our observations suggest that centriole replication is a modular, bottom-up process that we envision akin to building a house; the proper site of assembly is identified, a foundation is assembled at that site, and subsequent modules are added on top of the foundation. Here, we discuss the data underlying our view of modularity in the centriole assembly process, and suggest that non-essential centriole assembly factors take on greater importance in cancer cells due to their function in coordination between centriole modules, using the Monopolar spindles 1 protein kinase and its substrate Centrin 2 to illustrate our model.

Centrosome aberrations and chromosome instability contribute to tumorigenesis and intra-tumor heterogeneity

Centrosomes serve as the major microtubule organizing centers in cells and thereby contribute to cell shape, polarity, and motility. Also, centrosomes ensure equal chromosome segregation during mitosis. Centrosome aberrations arise when the centrosome cycle is deregulated, or as a result of cytokinesis failure. A long-standing postulate is that centrosome aberrations are involved in the initiation and progression of cancer. However, this notion has been a subject of controversy because until recently the relationship has been correlative. Recently, it was shown that numerical or structural centrosome aberrations can initiate tumors in certain tissues in mice, as well as invasion. Particularly, we will focus on centrosome amplification and chromosome instability as drivers of intra-tumor heterogeneity and their consequences in cancer. We will also discuss briefly the controversies surrounding this theory to highlight the fact that the role of both centrosome amplification and chromosome instability in cancer is highly context-dependent. Further, we will discuss single-cell sequencing as a novel technique to understand intra-tumor heterogeneity and some therapeutic approaches to target chromosome instability.

miRNA-34b is directly involved in the aging of macrophages

BACKGROUND

MicroRNAs (miRNAs) are a class of short noncoding RNA that play important regulatory roles in living organisms. These RNA molecules are implicated in the development and progression of malignant diseases such as cancer and are closely associated with cell aging. Findings demonstrating that microRNA is associated with aging in macrophages have nevertheless rarely been reported.

AIMS

This study's objective was to investigate if miRNA-34 is linked to aging process of macrophages.

METHODS

We built a cell aging model in mouse RAW264.7 macrophages using D-galactose and determined the expression levels of miRNA-34a, miRNA-34b, and miRNA-34c in aging and normal macrophages by fluorescence quantitative polymerase chain reaction (q-PCR). We predicted a target gene of miRNA-34 using biological information techniques and constructed the recombinant plasmid pGL3-E2f3 for the putative target gene E2f3.

RESULTS

The expression level of miRNA-34b was 5.23 times higher in aging macrophages than in normal macrophages. The luciferase activity decreased by nearly 50 % in cells transfected with miRNA-34b mimics, while no significant decrease in luciferase activity was noted in cells transfected with the miRNA-34b inhibitor or unrelated sequences.

DISCUSSION

Our findings provide the groundwork for further research into the molecular mechanisms whereby miRNA-34b regulates the aging of macrophages.

CONCLUSIONS

miRNA-34b is associated with the aging of RAW264.7 macrophages, and E2f3 is a target gene of miRNA-34b.

Centrosome - a promising anti-cancer target

The centrosome, an organelle discovered >100 years ago, is the main microtubule-organizing center in mammalian organisms. The centrosome is composed of a pair of centrioles surrounded by the pericentriolar material (PMC) and plays a major role in the regulation of cell cycle transitions (G1-S, G2-M, and metaphase-anaphase), ensuring the normality of cell division. Hundreds of proteins found in the centrosome exert a variety of roles, including microtubule dynamics, nucleation, and kinetochore–microtubule attachments that allow correct chromosome alignment and segregation. Errors in these processes lead to structural (shape, size, number, position, and composition), functional (abnormal microtubule nucleation and disorganized spindles), and numerical (centrosome amplification [CA]) centrosome aberrations causing aneuploidy and genomic instability. Compelling data demonstrate that centrosomes are implicated in cancer, because there are important oncogenic and tumor suppressor proteins that are localized in this organelle and drive centrosome aberrations. Centrosome defects have been found in pre-neoplasias and tumors from breast, ovaries, prostate, head and neck, lung, liver, and bladder among many others. Several drugs/compounds against centrosomal proteins have shown promising results. Other drugs have higher toxicity with modest or no benefits, and there are more recently developed agents being tested in clinical trials. All of this emerging evidence suggests that targeting centrosome aberrations may be a future avenue for therapeutic intervention in cancer research.

Silencing of E2F3 suppresses tumor growth of Her2+ breast cancer cells by restricting mitosis

The E2F transcriptional activators E2F1, E2F2 and E2F3a regulate many important cellular processes, including DNA replication, apoptosis and centrosome duplication. Previously, we demonstrated that silencing E2F1 or E2F3 suppresses centrosome amplification (CA) and chromosome instability (CIN) in Her2+ breast cancer cells without markedly altering proliferation. However, it is unknown whether and how silencing a single E2F activator, E2F3, affects malignancy of human breast cancer cells. Thus, we injected HCC1954 Her2+ breast cancer cells silenced for E2F3 into mammary fat pads of immunodeficient mice and demonstrated that loss of E2F3 retards tumor growth. Surprisingly, silencing of E2F3 led to significant reductions in mitotic indices relative to vector controls, while the percentage of cells undergoing S phase were not affected. Nek2 is a mitotic kinase commonly upregulated in breast cancers and a critical regulator of Cdk4- or E2F-mediated CA. In this report, we found that Nek2 overexpression rescued back the CA caused by silencing of shE2F3. However, the effects of Nek2 overexpression in affecting tumor growth rates of shE2F3 and shE2F3; GFP cells were inconclusive. Taken together, our results indicate that E2F3 silencing decreases mammary tumor growth by reducing percentage of cells undergoing mitosis.

The E2F-DP1 Transcription Factor Complex Regulates Centriole Duplication in Caenorhabditis elegans

Centrioles play critical roles in the organization of microtubule-based structures, from the mitotic spindle to cilia and flagella. In order to properly execute their various functions, centrioles are subjected to stringent copy number control. Central to this control mechanism is a precise duplication event that takes place during S phase of the cell cycle and involves the assembly of a single daughter centriole in association with each mother centriole . Recent studies have revealed that posttranslational control of the master regulator Plk4/ZYG-1 kinase and its downstream effector SAS-6 is key to ensuring production of a single daughter centriole. In contrast, relatively little is known about how centriole duplication is regulated at a transcriptional level. Here we show that the transcription factor complex EFL-1-DPL-1 both positively and negatively controls centriole duplication in the Caenorhabditis elegans embryo. Specifically, we find that down regulation of EFL-1-DPL-1 can restore centriole duplication in a zyg-1 hypomorphic mutant and that suppression of the zyg-1 mutant phenotype is accompanied by an increase in SAS-6 protein levels. Further, we find evidence that EFL-1-DPL-1 promotes the transcription of zyg-1 and other centriole duplication genes. Our results provide evidence that in a single tissue type, EFL-1-DPL-1 sets the balance between positive and negative regulators of centriole assembly and thus may be part of a homeostatic mechanism that governs centriole assembly.

E2F activators signal and maintain centrosome amplification in breast cancer cells

Centrosomes ensure accurate chromosome segregation by directing spindle bipolarity. Loss of centrosome regulation results in centrosome amplification, multipolar mitosis and aneuploidy. Since centrosome amplification is common in premalignant lesions and breast tumors, it is proposed to play a central role in breast tumorigenesis, a hypothesis that remains to be tested. The coordination between the cell and centrosome cycles is of paramount importance to maintain normal centrosome numbers, and the E2Fs may be responsible for regulating these cycles. However, the role of E2F activators in centrosome amplification is unclear. Because E2Fs are deregulated in Her2(+) cells displaying centrosome amplification, we addressed whether they signal this abnormal process. Knockdown of E2F1 or E2F3 in Her2(+) cells decreased centrosome amplification without significantly affecting cell cycle progression, whereas the overexpression of E2F1, E2F2, or E2F3 increased centrosome amplification in MCF10A mammary epithelial cells. Our results revealed that E2Fs affect the expression of proteins, including Nek2 and Plk4, known to influence the cell/centrosome cycles and mitosis. Downregulation of E2F3 resulted in cell death and delays/blocks in cytokinesis, which was reversed by Nek2 overexpression. Nek2 overexpression enhanced centrosome amplification in Her2(+) breast cancer cells silenced for E2F3, revealing a role for the E2F activators in maintaining centrosome amplification in part through Nek2.

Coregulation of FANCA and BRCA1 in human cells

Fanconi anemia (FA) is a genetically heterogeneous syndrome associated with increased cancer predisposition. The underlying genes govern the FA pathway which functions to protect the genome during the S-phase of the cell cycle. While upregulation of FA genes has been linked to chemotherapy resistance, little is known about their regulation in response to proliferative stimuli. The purpose of this study was to examine how FA genes are regulated, especially in relation to the cell cycle, in order to reveal their possible participation in biochemical networks. Expression of 14 FA genes was monitored in two human cell-cycle models and in two RB1/E2F pathway-associated primary cancers, retinoblastoma and basal breast cancer. In silico studies were performed to further evaluate coregulation and identify connected networks and diseases. Only FANCA was consistently induced over 2-fold; FANCF failed to exhibit any regulatory fluctuations. Two tools exploiting public data sets indicated coregulation of FANCA with BRCA1. Upregulation of FANCA and BRCA1 correlated with upregulation of E2F3. Genes coregulated with both FANCA and BRCA1 were enriched for MeSH-Term id(s) genomic instability, microcephaly, and Bloom syndrome, and enriched for the cellular component centrosome. The regulation of FA genes appears highly divergent. In RB1-linked tumors, upregulation of FA network genes was associated with reduced expression of FANCF. FANCA and BRCA1 may jointly act in a subnetwork - supporting vital function(s) at the subcellular level (centrosome) as well as at the level of embryonic development (mechanisms controlling head circumference).

PP2A-B55β antagonizes cyclin E1 proteolysis and promotes its dysregulation in cancer

Cyclin E1 regulates the initiation of S-phase in cellular division. However, in many cancers, cyclin E1 is aberrantly overexpressed and this molecular phenotype correlates with increased tumor aggressiveness and poor patient survival. The molecular cause(s) of cyclin E1 abnormalities in cancers is poorly understood. Here, we show that cyclin E1 overexpression in cancer is promoted by dysregulation of the protein phosphatase PP2A-B55β. PP2A-B55β targets the N- and C-terminal phosphodegrons of cyclin E1 for dephosphorylation, thus protecting it from degradation mediated by the SCF(Fbxw7) ubiquitin ligase. Augmented B55β expression stabilizes cyclin E1 and promotes its overexpression in cancer-derived cell lines and breast tumors. Conversely, B55β ablation enforces the degradation of cyclin E1 and inhibits cancer cell proliferation in vitro and tumor formation in vivo. Therefore, PP2A-B55β promotes cyclin E1 overexpression by antagonizing its degradation and its inhibition could represent a therapeutic mechanism for abrogating cyclin E1 function in cancers.

Silencing CDK4 radiosensitizes breast cancer cells by promoting apoptosis

Background

The discovery of molecular markers associated with various breast cancer subtypes has greatly improved the treatment and outcome of breast cancer patients. Unfortunately, breast cancer cells acquire resistance to various therapies. Mounting evidence suggests that resistance is rooted in the deregulation of the G1 phase regulatory machinery.

Methods

To address whether deregulation of the G1 phase regulatory machinery contributes to radiotherapy resistance, the MCF10A immortalized human mammary epithelial cell line, ER-PR-Her2+ and ER-PR-Her2- breast cancer cell lines were irradiated. Colony formation assays measured radioresistance, while immunocytochemistry, Western blots, and flow cytometry measured the cell cycle, DNA replication, mitosis, apoptosis, and DNA breaks.

Results

Molecular markers common to all cell lines were overexpressed, including cyclin A1 and cyclin D1, which impinge on CDK2 and CDK4 activities, respectively. We addressed their potential role in radioresistance by generating cell lines stably expressing small hairpin RNAs (shRNA) against CDK2 and CDK4. None of the cell lines knocked down for CDK2 displayed radiosensitization. In contrast, all cell lines knocked down for CDK4 were significantly radiosensitized, and a CDK4/CDK6 inhibitor sensitized MDA-MB-468 to radiation induced apoptosis. Our data showed that silencing CDK4 significantly increases radiation induced cell apoptosis in cell lines without significantly altering cell cycle progression, or DNA repair after irradiation. Our results indicate lower levels of phospho-Bad at ser136 upon CDK4 silencing and ionizing radiation, which has been shown to signal apoptosis.

Conclusion

Based on our data we conclude that knockdown of CDK4 activity sensitizes breast cancer cells to radiation by activating apoptosis pathways.

Cdk4 and nek2 signal binucleation and centrosome amplification in a her2+ breast cancer model

Centrosome amplification (CA) is a contributor to carcinogenesis, generating aneuploidy, and chromosome instability. Previous work shows that breast adenocarcinomas have a higher frequency of centrosome defects compared to normal breast tissues. Abnormal centrosome phenotypes are found in pre-malignant lesions, suggesting an early role in breast carcinogenesis. However, the role of CA in breast cancers remains elusive. Identification of pathways and regulatory molecules involved in the generation of CA is essential to understanding its role in breast tumorigenesis. We established a breast cancer model of CA using Her2-positive cells. Our goal was to identify centrosome cycle molecules that are deregulated by aberrant Her2 signaling and the mechanisms driving CA. Our results show some Her2+ breast cancer cell lines harbor both CA and binucleation. Abolishing the expression of Cdk4 abrogated both CA and binucleation in these cells. We also found the source of binucleation in these cells to be defective cytokinesis that is normalized by downregulation of Cdk4. Protein levels of Nek2 diminish upon Cdk4 knockdown and vice versa, suggesting a molecular connection between Cdk4 and Nek2. Knockdown of Nek2 reduces CA and binucleation in this model while its overexpression further enhances centrosome amplification. We conclude that CA is modulated through Cdk4 and Nek2 signaling and that binucleation is a likely source of CA in Her2+ breast cancer cells.

An integrated view of cyclin E function and regulation

Cancers of diverse cell lineages express high levels of cyclin E, and in various studies, cyclin E overexpression correlates with increased tumor aggression. One way that normal control of cyclin E expression is disabled in cancer cells is via loss-of-function mutations sustained by FBXW7. This gene encodes the Fbw7 tumor suppressor protein that provides substrate specificity for a ubiquitin ligase complex that targets multiple oncoproteins for degradation. Numerous other mechanisms besides Fbw7 mutations can deregulate cyclin E expression and activity in cancer cells. Recent reports demonstrate that inappropriate cyclin E expression may have far-reaching biological consequences for cell physiology, including altering gene expression programs governing proliferation, differentiation, survival and senescence. In this review, we discuss the function of mammalian cyclin E in the context of these new data as well as the complex network that connects cyclin E functions to the cellular controls regulating its expression and activity.

The SNF2-like helicase HELLS mediates E2F3-dependent transcription and cellular transformation

The activating E2F-transcription factors are best known for their dependence on the Retinoblastoma protein and their role in cellular proliferation. E2F3 is uniquely amplified in specific human tumours where its expression is inversely correlated with the survival of patients. Here, E2F3B interaction partners were identified by mass spectrometric analysis. We show that the SNF2-like helicase HELLS interacts with E2F3A in vivo and cooperates with its oncogenic functions. Depletion of HELLS severely perturbs the induction of E2F-target genes, hinders cell-cycle re-entry and growth. Using chromatin immmunoprecipitation coupled to sequencing, we identified genome-wide targets of HELLS and E2F3A/B. HELLS binds promoters of active genes, including the trithorax-related MLL1, and co-regulates E2F3-dependent genes. Strikingly, just as E2F3, HELLS is overexpressed in human tumours including prostate cancer, indicating that either factor may contribute to the malignant progression of tumours. Our work reveals that HELLS is important for E2F3 in tumour cell proliferation.

Hypoxia-inducible factor 1 is activated by dysregulated cyclin E during mammary epithelial morphogenesis

Increased cyclin E expression has been identified in human tumors of diverse histologies, and in studies of primary breast cancers, high cyclin E is associated with poor prognosis. We have studied dysregulated cyclin E in epithelial tissues using organotypic cultures of human mammary epithelial cells and a murine model. We unexpectedly discovered that dysregulated cyclin E impairs normal acinar morphogenesis in vitro, and this is associated with the induction of p21(Cip1), p27(Kip1), and cellular senescence. Cyclin E-induced morphogenesis arrest is dependent upon hypoxia-inducible factor 1α (HIF-1α), which itself is induced by high cyclin E both in cultured mammary acini and in mammary epithelial tissues in a mouse model of deregulated cyclin E expression. We next determined that E2F activity directly regulates and is required for induction of HIF1A by cyclin E. Additionally, we found that cyclin E deregulation in mammary acini decreases, in an E2F-independent manner, expression of the EGLN1 prolyl hydroxylase that regulates HIF-1α degradation within the VHL ubiquitin ligase pathway. Together, our findings reveal a direct link between cyclin E and HIF-1 activities in mammary epithelial cells and implicate HIF-1 as a mediator of proliferation-independent phenotypes associated with high cyclin E expression in some human breast cancers.

Pathogenesis of focal segmental glomerular sclerosis in a girl with the partial deletion of chromosome 6p

Focal segmental glomerular sclerosis (FSGS) is a leading cause of the nephrotic syndrome and characterized by the sclerosing lesions that affect one or more segments of some glomeruli. We encountered a female patient with a partial deletion of chromosome 6p, who presented proteinuria at age 3 years. Detailed chromosomal analysis disclosed an interstitial deletion of 6p: del(6)(p22.1p22.3). No abnormality such as hydronephrosis or renal agenesis was disclosed by imaging, but FSGS was present in a renal biopsy specimen. The patient is currently 11 years old and shows mental retardation with mild deterioration in the renal function. To address the defective genes in the present patient, we carried out comparative genomic hybridization (CGH), showing that E2F3 on chromosome 6p is absent in this patient. E2F3, a member of the E2F family transcription factors, inhibits expression of vascular endothelial growth factor (VEGF) and induces apoptosis during vascular development. The deletion of E2F3 was also detected by employing a PCR method, suggesting that glomerular architecture had been compromised in this patient. Serum VEGF concentrations were elevated to 177 ± 21.4 pg/mL (upper limit of 33.3 pg/mL), when she was 6 years old, associated with the enhanced expression of VEGF in glomeruli. These findings suggest that the dysregulation of VEGF synthesis caused by the deletion of E2F3 may be associated with development of FSGS. In conclusion, among patients with idiopathic FSGS, an abnormality of E2F3 may exist on chromosome 6p. Therefore, one might consider chromosomal analyses in children with FSGS who have mental retardation.

Overexpression of miR-210, a downstream target of HIF1α, causes centrosome amplification in renal carcinoma cells

MiR-210 is significantly up-regulated in clear cell renal cell carcinoma (CCC), but the mechanism and biological consequences of miR-210 up-regulation are poorly understood. Here, we show that miR-210 is highly expressed in renal carcinoma cell lines and that its expression is clearly correlated with accumulation of hypoxia-inducible factor 1α (HIF1α) under normoxia as well as hypoxia, suggesting that miR-210 up-regulation in renal carcinoma cells is most likely due to accumulation of HIF1α. To reveal the effects of miR-210 up-regulation, the miR-210 precursor was transfected into renal carcinoma cells. After transfection, the cells accumulated at the G2/M phase of the cell cycle and their viability was decreased, suggesting that miR-210 overexpression may trigger an event that hinders normal cell division. Immunocytochemistry demonstrated a multipolar spindle accompanied by centrosome amplification in cells overexpressing miR-210. It has been reported that centrosome amplification induces chromosome mis-segregation, finally leading to chromosome instability and aneuploidy. Indeed, the proportion of aneuploid cells (>4n) was increased in miR-210 overexpressed cells. By using the TargetScan and PicTar algorithms, E2F3 was identified as one of the possible targets of miR-210 and was suppressed at the protein level by miR-210. Moreover, the proportion of aneuploid cells was increased in E2F3 siRNA transfected cells. On the basis of these results, we propose that miR-210 up-regulation due to HIF1α accumulation may induce aneuploidy via E2F3 down-regulation at least in part, and may play a role in tumourigenesis and/or progression of CCC.

The G1 phase Cdks regulate the centrosome cycle and mediate oncogene-dependent centrosome amplification

Because centrosome amplification generates aneuploidy and since centrosome amplification is ubiquitous in human tumors, a strong case is made for centrosome amplification being a major force in tumor biogenesis. Various evidence showing that oncogenes and altered tumor suppressors lead to centrosome amplification and aneuploidy suggests that oncogenes and altered tumor suppressors are a major source of genomic instability in tumors, and that they generate those abnormal processes to initiate and sustain tumorigenesis. We discuss how altered tumor suppressors and oncogenes utilize the cell cycle regulatory machinery to signal centrosome amplification and aneuploidy.

The Ras oncogene signals centrosome amplification in mammary epithelial cells through cyclin D1/Cdk4 and Nek2

Centrosome amplification (CA) contributes to carcinogenesis by generating aneuploidy. Elevated frequencies of CA in most benign breast lesions and primary tumors suggest a causative role for CA in breast cancers. Clearly, identifying which and how altered signal transduction pathways contribute to CA is crucial to breast cancer control. Although a causative and cooperative role for c-Myc and Ras in mammary tumorigenesis is well documented, their ability to generate CA during mammary tumor initiation remains unexplored. To answer that question, K-Ras(G12D) and c-Myc were induced in mouse mammary glands. Although CA was observed in mammary tumors initiated by c-Myc or K-Ras(G12D), it was detected only in premalignant mammary lesions expressing K-Ras(G12D). CA, both in vivo and in vitro, was associated with increased expression of the centrosome-regulatory proteins, cyclin D1 and Nek2. Abolishing the expression of cyclin D1, Cdk4 or Nek2 in MCF10A human mammary epithelial cells expressing H-Ras(G12V) abrogated Ras-induced CA, whereas silencing cyclin E1 or B2 had no effect. Thus, we conclude that CA precedes mammary tumorigenesis, and interfering with centrosome-regulatory targets suppresses CA.

Cdk2 and Cdk4 regulate the centrosome cycle and are critical mediators of centrosome amplification in p53-null cells

The two mitotic centrosomes direct spindle bipolarity to maintain euploidy. Centrosome amplification-the acquisition of > or =3 centrosomes-generates multipolar mitoses, aneuploidy, and chromosome instability to promote cancer biogenesis. While much evidence suggests that Cdk2 is the major conductor of the centrosome cycle and that it mediates centrosome amplification induced by various altered tumor suppressors, the role played by Cdk4 in a normal or deregulated centrosome cycle is unknown. Using a gene knockout approach, we report that Cdk2 and Cdk4 are critical to the centrosome cycle, since centrosome separation and duplication are premature in Cdk2(-)(/)(-) mouse embryonic fibroblasts (MEFs) and are compromised in Cdk4(-)(/)(-) MEFs. Additionally, ablation of Cdk4 or Cdk2 abrogates centrosome amplification and chromosome instability in p53-null MEFs. Absence of Cdk2 or Cdk4 prevents centrosome amplification by abrogating excessive centriole duplication. Furthermore, hyperactive Cdk2 and Cdk4 deregulate the licensing of the centrosome duplication cycle in p53-null cells by hyperphosphorylating nucleophosmin (NPM) at Thr199, as evidenced by observations that ablation of Cdk2, Cdk4, or both Cdk2 and Cdk4 abrogates that excessive phosphorylation. Since a mutant form of NPM lacking the G(1) Cdk phosphorylation site (NPM(T199A)) prevents centrosome amplification to the same extent as ablation of Cdk2 or Cdk4, we conclude that the Cdk2/Cdk4/NPM pathway is a major guardian of centrosome dysfunction and genomic integrity.

E2F3 is a mediator of DNA damage-induced apoptosis

The E2F transcription factors have emerged as critical apoptotic effectors. Herein we report that the E2F family member E2F3a can be induced by DNA damage through transcriptional and posttranslational mechanisms. We demonstrate that the posttranslational induction of human E2F3a is dependent on the checkpoint kinases. Moreover, we show that human E2F3a is a substrate for the checkpoint kinases (chk kinases) and that mutation of the chk phosphorylation site eliminates the DNA damage inducibility of the protein. Furthermore, we demonstrate that E2F1 and E2F2 are transcriptionally induced by DNA damage in an E2f3-dependent manner. Finally, using both in vitro and in vivo approaches, we establish that E2f3 is required for DNA damage-induced apoptosis. Thus, our data reveal the novel ability of E2f3 to function as a master regulator of the DNA damage response.

Orc1 controls centriole and centrosome copy number in human cells

Centrosomes, each containing a pair of centrioles, organize microtubules in animal cells, particularly during mitosis. DNA and centrosomes are normally duplicated once before cell division to maintain optimal genome integrity. We report a new role for the Orc1 protein, a subunit of the origin recognition complex (ORC) that is a key component of the DNA replication licensing machinery, in controlling centriole and centrosome copy number in human cells, independent of its role in DNA replication. Cyclin A promotes Orc1 localization to centrosomes where Orc1 prevents Cyclin E-dependent reduplication of both centrioles and centrosomes in a single cell division cycle. The data suggest that Orc1 is a regulator of centriole and centrosome reduplication as well as the initiation of DNA replication.

MCPH1/BRIT1 cooperates with E2F1 in the activation of checkpoint, DNA repair and apoptosis

Microcephalin (MCPH1) has a crucial role in the DNA damage response by promoting the expression of Checkpoint kinase 1 (CHK1) and Breast cancer susceptibility gene 1 (BRCA1); however, the mechanism of this regulation remains unclear. Here, we show that MCPH1 regulates CHK1 and BRCA1 through the interaction with E2F1 on the promoters of both genes. MCPH1 also regulates other E2F target genes involved in DNA repair and apoptosis such as RAD51, DDB2, TOPBP1, p73 and caspases. MCPH1 interacts with E2F1 on the p73 promoter, and regulates p73 induction and E2F1-induced apoptosis as a result of DNA damage. MCPH1 forms oligomers through the second and third BRCT domains. An MCPH1 mutant containing only its oligomerization domain has a dominant-negative role by blocking MCPH1 binding to E2F1. It also inhibits p73 induction in DNA damage and E2F1-dependent apoptosis. Taken together, MCPH1 cooperates with E2F1 to regulate genes involved in DNA repair, checkpoint and apoptosis, and might participate in the maintenance of genomic integrity.

Comparative analysis of E2F family member oncogenic activity

The E2F family of transcription factors consists of nine members with both distinct and overlapping functions. These factors are situated downstream of growth factor signaling cascades, where they play a central role in cell growth and proliferation through their ability to regulate genes involved in cell cycle progression. For this reason, it is likely that the members of the E2F family play a critical role during oncogenesis. Consistent with this idea is the observation that some tumors exhibit deregulated expression of E2F proteins. In order to systematically compare the oncogenic capacity of these family members, we stably over-expressed E2F1 through 6 in non-transformed 3T3 fibroblasts and assessed the ability of these transgenic cell lines to grow under conditions of low serum, as well as to form colonies in soft agar. Our results show that these six E2F family members can be divided into three groups that exhibit differential oncogenic capacity. The first group consists of E2F2 and E2F3a, both of which have strong oncogenic capacity. The second group consists of E2F1 and E2F6, which were neutral in our assays when compared to control cells transduced with vector alone. The third group consists of E2F4 and E2F5, which generally act to repress E2F-responsive genes, and in our assays demonstrated a strong capacity to inhibit transformation. Our results imply that the pattern of expression of these six E2F family members in a cell could exert a strong influence over its susceptibility to oncogenic transformation.

Preventing the degradation of mps1 at centrosomes is sufficient to cause centrosome reduplication in human cells

Supernumerary centrosomes promote the assembly of abnormal mitotic spindles in many human tumors. In human cells, overexpression of the cyclin-dependent kinase (Cdk)2 partner cyclin A during a prolonged S phase produces extra centrosomes, called centrosome reduplication. Cdk2 activity protects the Mps1 protein kinase from proteasome-mediated degradation, and we demonstrate here that Mps1 mediates cyclin A-dependent centrosome reduplication. Overexpression of cyclin A or a brief proteasome inhibition increases the centrosomal levels of Mps1, whereas depletion of Cdk2 leads to the proteasome-dependent loss of Mps1 from centrosomes only. When a Cdk2 phosphorylation site within Mps1 (T468) is mutated to alanine, Mps1 cannot accumulate at centrosomes or participate in centrosome duplication. In contrast, phosphomimetic mutations at T468 or deletion of the region surrounding T468 prevent the proteasome-dependent removal of Mps1 from centrosomes in the absence of Cdk2 activity. Moreover, cyclin A-dependent centrosome reduplication requires Mps1, and these stabilizing Mps1 mutations cause centrosome reduplication, bypassing cyclin A. Together, our data demonstrate that the region surrounding T468 contains a motif that regulates the accumulation of Mps1 at centrosomes. We suggest that phosphorylation of T468 attenuates the degradation of Mps1 at centrosomes and that preventing this degradation is necessary and sufficient to cause centrosome reduplication in human cells.

Lats2 is an essential mitotic regulator required for the coordination of cell division

Tumor suppressor Lats2 is a member of the conserved Dbf2 kinase family. It localizes to the centrosome and has been implicated in regulation of the cell cycle and apoptosis. However, the in vivo function of this kinase remains unclear. Here, we show that complete disruption of the gene encoding Lats2 in mice causes developmental defects in the nervous system and embryonic lethality. Furthermore, mutant cells derived from total LATS2-knock-out embryos exhibit mitotic defects including centrosome fragmentation and cytokinesis defects, followed by nuclear enlargement and multinucleation. We show that the Mob1 family, a regulator of mitotic exit, associates with Lats2 to induce its activation. We also show that the complete LATS2-knock-out cells exhibit an acceleration of exit from mitosis and marked down-regulation of critical mitotic regulators. These results suggest that Lats2 plays an essential mitotic role in coordinating accurate cytokinesis completion, governing the stabilization of other mitotic regulators.

Selective requirements for E2f3 in the development and tumorigenicity of Rb-deficient chimeric tissues

The tumor suppressor function of the retinoblastoma protein pRB is largely dependent upon its capacity to inhibit the E2F transcription factors and thereby cell proliferation. Attempts to study the interplay between pRB and the E2Fs have been hampered by the prenatal death of Rb; E2f nullizygous mice. In this study, we isolated Rb; E2f3 mutant embryonic stem cells and generated Rb(-/-); E2f3(-/-) chimeric mice, thus bypassing the lethality of the Rb(-/-); E2f3(-/-) germ line mutant mice. We show that loss of E2F3 has opposing effects on two of the known developmental defects arising in Rb(-/-) chimeras; it suppresses the formation of cataracts while aggravating the retinal dysplasia. This model system also allows us to assess how E2f3 status influences tumor formation in Rb(-/-) tissues. We find that E2f3 is dispensable for the development of pRB-deficient pituitary and thyroid tumors. In contrast, E2f3 inactivation completely suppresses the pulmonary neuroendocrine hyperplasia arising in Rb(-/-) chimeric mice. This hyperproliferative state is thought to represent the preneoplastic lesion of small-cell lung carcinoma. Therefore, our observation highlights a potential role for E2F3 in the early stages of this tumor type.

Origins and consequences of centrosome aberrations in human cancers

Recent years have seen a revival of interest in the possible contribution of centrosomes to the development of human cancers. The underlying hypothesis, formulated almost 100 years ago (Boveri T. The origin of malignant tumors; Baltimore, MD: Williams and Wilkins, 1929.), states that numerical and/or structural centrosome abnormalities will cause chromosome [corrected] missegregation. In addition, centrosome abnormalities are expected to affect cell shape, polarity, and motility. Thus, deregulation of centrosome number and function may foster both chromosomal instability and loss of tissue architecture--2 of the most common phenotypes associated with solid human tumors. In support of the role of centrosome deregulation in tumorigenesis, centrosome aberrations have been observed in early, premalignant lesions. Moreover, they are frequent in many different types of common tumors and their prominence often correlates with poor clinical outcome. This review addresses the origins of centrosome aberrations in human tumors as well as the expected impact of centrosome aberrations on cell fate and tumor development.

Centrosome abnormalities are frequently observed in non-small-cell lung cancer and are associated with aneuploidy and cyclin E overexpression

Centrosome abnormalities are observed in human cancers and have been associated with aneuploidy, a driving force in tumour progression. However, the exact pathways that tend to cause centrosome abnormalities have not been fully elucidated in human tumours. Using a series of 68 non-small-cell lung carcinomas and an array of in vitro experiments, the relationship between centrosome abnormalities, aneuploidy, and the status of key G1 to S-phase transition cell-cycle molecules, involved in the regulation of centrosome duplication, was investigated. Centrosome amplification and structural abnormalities were common (53%), were strongly related to aneuploidy, and, surprisingly, were even seen in adjacent hyperplastic regions, suggesting the possibility that these are early lesions in lung carcinogenesis. Cyclin E and E2F1 overexpression, but not p53 mutation, was observed to correlate with centrosome abnormalities in vivo (p = 0.029 and p = 0.015, respectively). This was further strengthened by the observation that cyclin E was specifically present in the nucleus and/or cytoplasm of the cells that contained centrosome aberrations. The cytoplasmic cyclin E signal may be attributed, in part, to the presence of truncated low-molecular-weight isoforms of cyclin E. In order to isolate the effect of cyclin E on the appearance of centrosome abnormalities, a U2OS tetracycline-repressible cyclin E cell line that has a normal centrosome profile by default was used. With this system, it was confirmed in vitro that persistent cyclin E overexpression is sufficient to cause the appearance of centrosome abnormalities.

Does aneuploidy cause cancer?

Aneuploidy has been recognized as a common characteristic of cancer cells for >100 years. Aneuploidy frequently results from errors of the mitotic checkpoint, the major cell cycle control mechanism that acts to prevent chromosome missegregation. The mitotic checkpoint is often compromised in human tumors, although not as a result of germline mutations in genes encoding checkpoint proteins. Less obviously, aneuploidy of whole chromosomes rapidly results from mutations in genes encoding several tumor suppressors and DNA mismatch repair proteins, suggesting cooperation between mechanisms of tumorigenesis that were previously thought to act independently. Cumulatively, the current evidence suggests that aneuploidy promotes tumorigenesis, at least at low frequency, but a definitive test has not yet been reported.

Coactivator-associated arginine methyltransferase 1 (CARM1) is a positive regulator of the Cyclin E1 gene

The Cyclin E1 gene (CCNE1) is an ideal model to explore the mechanisms that control the transcription of cell cycle-regulated genes whose expression rises transiently before entry into S phase. E2F-dependent regulation of the CCNE1 promoter was shown to correlate with changes in the level of H3-K9 acetylation/methylation of nucleosomal histones positioned at the transcriptional start site region. Here we show that, upon growth stimulation, the same region is subject to variations of H3-R17 and H3-R26 methylation that correlate with the recruitment of coactivator-associated arginine methyltransferase 1 (CARM1) onto the CCNE1 and DHFR promoters. Accordingly, CARM1-deficient cells lack these modifications and present lowered levels and altered kinetics of CCNE1 and DHFR mRNA expression. Consistently, reporter gene assays demonstrate that CARM1 functions as a transcriptional coactivator for their E2F1/DP1-stimulated expression. CARM1 recruitment at the CCNE1 gene requires activator E2Fs and ACTR, a member of the p160 coactivator family that is frequently overexpressed in human breast cancer. Finally, we show that grade-3 breast tumors present coelevated mRNA levels of ACTR and CARM1, along with their transcriptional target CCNE1. All together, our results indicate that CARM1 is an important regulator of the CCNE1 gene.

Nucleophosmin and cancer

NPM1 is a crucial gene to consider in the context of the genetics and biology of cancer. NPM1 is frequently overexpressed, mutated, rearranged and deleted in human cancer. Traditionally regarded as a tumour marker and a putative proto-oncogene, it has now also been attributed with tumour-suppressor functions. Therefore, NPM can contribute to oncogenesis through many mechanisms. The aim of this review is to analyse the role of NPM in cancer, and examine how deregulated NPM activity (either gain or loss of function) can contribute to tumorigenesis.

Pin1 regulates centrosome duplication, and its overexpression induces centrosome amplification, chromosome instability, and oncogenesis

Phosphorylation on Ser/Thr-Pro motifs is a major mechanism regulating many events involved in cell proliferation and transformation, including centrosome duplication, whose defects have been implicated in oncogenesis. Certain phosphorylated Ser/Thr-Pro motifs can exist in two distinct conformations whose conversion in certain proteins is catalyzed specifically by the prolyl isomerase Pin1. Pin1 is prevalently overexpressed in human cancers and is important for the activation of multiple oncogenic pathways, and its deletion suppresses the ability of certain oncogenes to induce cancer in mice. However, little is known about the role of Pin1 in centrosome duplication and the significance of Pin1 overexpression in cancer development in vivo. Here we show that Pin1 overexpression correlates with centrosome amplification in human breast cancer tissues. Furthermore, Pin1 localizes to and copurifies with centrosomes in interphase but not mitotic cells. Moreover, Pin1 ablation in mouse embryonic fibroblasts drastically delays centrosome duplication without affecting DNA synthesis and Pin1 inhibition also suppresses centrosome amplification in S-arrested CHO cells. In contrast, overexpression of Pin1 drives centrosome duplication and accumulation, resulting in chromosome missegregation, aneuploidy, and transformation in nontransformed NIH 3T3 cells. More importantly, transgenic overexpression of Pin1 in mouse mammary glands also potently induces centrosome amplification, eventually leading to mammary hyperplasia and malignant mammary tumors with overamplified centrosomes. These results demonstrate for the first time that the phosphorylation-specific isomerase Pin1 regulates centrosome duplication and its deregulation can induce centrosome amplification, chromosome instability, and oncogenesis.

Disruption of protein kinase a regulation causes immortalization and dysregulation of D-type cyclins

Phosphorylation is a key event in cell cycle control, and dysregulation of this process is observed in many tumors, including those associated with specific inherited neoplasia syndromes. We have shown previously that patients with the autosomal dominant tumor predisposition Carney complex carry inactivating mutations in the PRKAR1A gene, which encodes the type 1A regulatory subunit of protein kinase A (PKA), the cyclic AMP-dependent protein kinase. This defect was associated with dysregulation of PKA signaling, and genetic analysis has suggested that complete loss of the gene may be required for tumorigenesis. To determine the mechanism by which dysregulation of PKA causes tumor formation, we generated in vitro primary mouse cells lacking the Prkar1a protein. We report that this genetic disruption of PKA regulation causes constitutive PKA activation and immortalization of primary mouse embryonic fibroblasts (MEFs). At the molecular level, knockout of Prkar1a leads to up-regulation of D-type cyclins, and this increase occurs independently of other pathways known to increase cyclin D levels. Despite the immortalized phenotype, known mediators of cellular senescence (e.g., p53 and p19ARF) seem to remain intact in Prkar1a-/- MEFs. Mechanistically, cyclin D1 mRNA levels are not altered in the knockout cells, but protein half-life is markedly increased. Using this model, we provide the first direct genetic evidence that dysregulation of PKA promotes important steps in tumorigenesis, and that cyclin D1 is an essential target of PKA.

Deregulation of the centrosome cycle and the origin of chromosomal instability in cancer

Although we have begun to tap into the mechanisms behind Boveri's initial observation that supernumerary centrosomes cause chromosome missegregation in sea urchin eggs, there is still much left to discover with regard to chromosomal instability in cancer. Many of the molecular players involved in regulation of the centrosome and cell cycles, and the coupling of the two cycles to produce a bipolar mitotic spindle have been identified. One theme that has become apparent is that cross talk and interrelatedness of the pathways serve to provide redundant mechanisms to maintain genomic integrity. In spite of this, cells occasionally fall prey to insults that initiate and maintain the chromosomal instability that results in viable malignant tumours. Deregulation of centrosome structure is an integral aspect of the origin of chromosomal instability in many cancers. There are numerous routes to centrosome amplification including: environmental insults such as ionising radiation and exposure to estrogen (Li et al., 2005); failure of cytokinesis; and activating mutations in key regulators of centrosome structure and function. There are two models for initiation of centrosome amplification (Figure 2). In the first, centrosome duplication and chromosome replication remain coupled and cells enter G2 with 4N chromosomes and duplicated centrosomes. However, these cells may fail to complete mitosis, and thus reenter G1 as tetraploid cells with amplified centrosomes. In the second, the centrosome cycle is uncoupled from chromosome replication and cells go through one or more rounds of centriole/centrosome duplication in the absence of chromosome replication. If these cells then go through chromosome replication accompanied by another round of centrosome duplication, cells complete G2 with 4N chromosomes and more than 2 centrosomes, and therefore are predisposed to generate multipolar mitotic spindles. Fragmentation of centrosomes due to ionising radiation is a variation of the second model. Once centrosome amplification is present, even in a diploid cell, that cell has the potential to yield viable aneuploid progeny. The telophase cell in Figure 3C illustrates this scenario. In a normal telophase configuration, the total number of chromosomes is 92 (resulting from the segregation of 46 pairs of chromatids), with each daughter nucleus containing 46 individual chromosomes. Based on the number of kinetochore signals present, the lower nucleus in Figure 3C has approximately 28 chromosomes, and the elongate upper nucleus has approximately 60, for a total of 88. Due to superimposition of kinetochores in this maximum projection image, 88 is an underestimate of the actual number of kinetochores and is not significantly different from the expected total of 92. A cell resulting from the lower nucleus with only around 28 chromosomes would probably not be viable, much as Boveri's experiments indicated. However, the upper nucleus with at least 60 chromosomes could be viable. This cell would enter G1 as hypotriploid (69 chromosomes = triploid) with 2 centrosomes. During S and G2, the centrosomes and chromosomes would double, and the following mitosis could be tetrapolar with a 6N chromosome content. When centrosome amplification is accompanied by permissive lapses in cell cycle checkpoints, the potential for malignant growth is present. These lapses could result from specific genetic mutations and amplifications, epigenetic gene silencing, or from massive chromosomal instability caused by the centrosome amplification. Centrosome amplification, therefore, can serve to exacerbate and/or generate genetic instabilities associated with cancers.

Characterization of centrosomal association of nucleophosmin/B23 linked to Crm1 activity

Nucleophosmin (NPM)/B23 is a multifunctional protein, involving in a wide variety of basic cellular processes, including ribosome assembly, DNA duplication, nucleocytoplasmic trafficking, and centrosome duplication. It has previously been shown that NPM/B23 localizes to centrosomes, and dissociate from centrosomes upon phosphorylation by Cdk2/cyclin E. However, detail characterization of centrosomal association of NPM/B23 has been hampered by the lack of appropriate antibodies that efficiently detects centrosomally localized NPM/B23, as well as by apparent loss of natural behavior of NPM/B23 when tagged with fluorescent proteins. Here, by the use of newly generated anti-NPM/B23 antibody, we conducted a careful analysis of centrosomal localization of NPM/B23. We found that NPM/B23 localizes between the paired centrioles of unduplicated centrosomes, suggesting the role of NPM/B23 in the centriole pairing. Upon initiation of centrosome duplication, some NPM/B23 proteins remain at mother centrioles of the parental centriole pairs. We further found that inhibition of Crm1 nuclear export receptor results in both accumulation of cyclin E at centrosomes and efficient dissociation of NPM/B23 from centrosomes.

Inactivation of the Snf5 tumor suppressor stimulates cell cycle progression and cooperates with p53 loss in oncogenic transformation

Snf5 (Ini1/Baf47/Smarcb1), a core member of the Swi/Snf chromatin remodeling complex, is a potent tumor suppressor whose mechanism of action is largely unknown. Biallelic loss of Snf5 leads to the onset of aggressive cancers in both humans and mice. We have developed an innovative and widely applicable analytical technique for cross-species validation of cancer models and show that the gene expression profiles of our Snf5 murine models closely resemble those of human Snf5-deficient rhabdoid tumors. We exploit this system to produce what we believe to be the first report documenting the effects on gene expression of inactivating a Swi/Snf subunit in normal mammalian cells and to identify the transcriptional pathways regulated by Snf5. We demonstrate that the tumor suppressor activity of Snf5 depends on its regulation of cell cycle progression; Snf5 inactivation leads to aberrant up-regulation of E2F targets and increased levels of p53 that are accompanied by apoptosis, polyploidy, and growth arrest. Further, conditional mouse models demonstrate that inactivation of p16Ink4a or Rb (retinoblastoma) does not accelerate tumor formation in Snf5 conditional mice, whereas mutation of p53 leads to a dramatic acceleration of tumor formation.

Involvement of E2F transcription factor family in cancer

The E2F family of transcription factors is a central modulator of important cellular events, including cell cycle progression, apoptosis and DNA damage response. The role of E2F family members in various human malignancies is yet unclear and may provide vital clues to the diagnosis, prognosis and therapy of cancer patients. In this review we provide a brief but concise overview of E2F function and its putative role in the most common human tumour types.

Centrosome-targeting region of CG-NAP causes centrosome amplification by recruiting cyclin E-cdk2 complex

Centrosome duplication occurs once per cell cycle and is thought to be triggered by cyclin E-cdk2. However, it is largely unknown how the duplication is regulated. Here, we found that the expression of the centrosome-targeting region of CG-NAP (centrosome and Golgi-localized PKN-associated protein), which we designate as CG-NAP/D, increased the number of centrosomes in Chinese hamster ovary (CHO)-K1 cells. The amplified centrosomes co-localized with centrosome markers gamma-tubulin, centrin-2 and kendrin as well as endogenous CG-NAP. When CG-NAP/D was dislocated from centrosomes by deleting the centrosome-targeting domain or by fusing with a membrane-targeting sequence, centrosome amplification was suppressed. CG-NAP/D interacted with exogenously expressed cyclin E, which co-localized at centrosomes. The immunoprecipitates of CG-NAP/D exhibited histone H1 kinase activity, suggesting the co-immunoprecipitation of active cyclin-cdk complexes. Furthermore, centrosome fractions prepared from cells expressing CG-NAP/D contained increased amount of cdk2 compared with those from control cells. Centrosome amplification by CG-NAP/D was suppressed by co-expression of a mutant cyclin E unable to interact with cdk2. These results suggest that CG-NAP/D causes centrosome amplification by anchoring excess amount of cyclin E-cdk2 to centrosomes and, possibly, CG-NAP participates in centrosome duplication by recruiting cyclin E-cdk2 to centrosomes in normal cell cycle.

Loss of the forkhead transcription factor FoxM1 causes centrosome amplification and mitotic catastrophe

Expression of the forkhead transcription factor FoxM1 correlates with proliferative status in a variety of normal and transformed cell types. Elevated expression of FoxM1 has been noted in both hepatocellular carcinoma and basal cell carcinoma. However, whether FoxM1 expression is essential for the viability of transformed cells is unknown. We report here that the expression of FoxM1 is significantly elevated in primary breast cancer. Microarray analysis shows that FoxM1 regulates genes that are essential for faithful chromosome segregation and mitosis, including Nek2, KIF20A, and CENP-A. Loss of FoxM1 expression generates mitotic spindle defects, delays cells in mitosis, and induces mitotic catastrophe. Time-lapse microscopy indicates that depletion of FoxM1 generates cells that enter mitosis but are unable to complete cell division, resulting in either mitotic catastrophe or endoreduplication. These findings indicate that FoxM1 depletion causes cell death due to mitotic catastrophe and that inhibiting FoxM1 represents a therapeutic strategy to target breast cancer.

The RB protein family in retinal development and retinoblastoma: new insights from new mouse models

The Rb gene was isolated almost 20 years ago, but fundamental questions regarding its role in retinal development and retinoblastoma remain. What is the normal function of RB protein in retinogenesis? What is the cell-of-origin of retinoblastoma? Why do retinoblastoma tumors have recurrent genetic lesions other than Rb inactivation? Why is retinoblastoma not induced by defects in cell cycle regulators other than Rb? Why is the retina so sensitive to Rb loss? Recently developed conditional Rb knockout models provide new insight into some of these issues. The data suggest that RB protein may not control the rate of progenitor division, but is critical for cell cycle exit when dividing retinal progenitors differentiate into postmitotic transition cells. This finding focuses attention on the ectopically dividing transition cell, rather than the progenitor, as the cell-of-origin. Cell-specific analyses in the RB-deficient retina reveal that ectopically dividing photoreceptors, bipolar and ganglion cells die, but amacrine, horizontal and Muller cells survive and stop dividing when they terminally differentiate. Rare amacrine transition cells escape cell cycle exit and generate tumors. These data suggest that post-Rb mutations are required to overcome growth arrest associated with terminal differentiation, rather than apoptosis as previously suggested. To explain why perturbing cell cycle regulators other than RB does not initiate retinoblastoma, we speculate that mutations in other components of the RB pathway perturb cell cycle arrest, but only RB loss triggers genome instability in retinal transition cells, which may be critical to facilitate post-Rb mutations necessary for transformation. Cell-specific differences in the effect of Rb loss on genome stability may contribute to the tremendous sensitivity of retinal transition cells to tumorigenesis. The new mouse models of retinoblastoma will be invaluable for testing these possibilities.

The E2F transcriptional network: old acquaintances with new faces

The E2 factor (E2F) family of transcription factors are downstream targets of the retinoblastoma protein. E2F factors have been known for several years to be important regulators of S-phase entry. Recent studies have improved our understanding of the molecular mechanisms of action used by this transcriptional network. In addition, they have given us an appreciation of the fact that E2F has functions that reach beyond G1/S control and impact cell proliferation in several different ways. The discovery of new family members with unusual properties, the unexpected phenotypes of mutant animals, a diverse collection of biological activities, a large number of new putative target genes and the new modes of transcriptional regulation have all contributed to an increasingly complex view of E2F function. In this review, we will discuss these recent developments and describe how they are beginning to shape a new and revised picture of the E2F transcriptional program.