Uncontrolled Cell Cycle Research Articles

Dissenting degradation: Deubiquitinases in cell cycle and cancer.

Since its discovery forty years ago, protein ubiquitination has been an ever-expanding field. Virtually all biological processes are controlled by the post-translational conjugation of ubiquitin onto target proteins. In addition, since ubiquitin controls substrate degradation through the action of hundreds of enzymes, many of which represent attractive therapeutic candidates, harnessing the ubiquitin system to reshape proteomes holds great promise for improving disease outcomes. Among the numerous physiological functions controlled by ubiquitin, the cell cycle is among the most critical. Indeed, the discovery that the key drivers of cell cycle progression are regulated by the ubiquitin-proteasome system (UPS) epitomizes the connection between ubiquitin signaling and proliferation. Since cancer is a disease of uncontrolled cell cycle progression and proliferation, targeting the UPS to stop cancer cells from cycling and proliferating holds enormous therapeutic potential. Ubiquitination is reversible, and ubiquitin is removed from substrates by catalytic proteases termed deubiquitinases or DUBs. While ubiquitination is tightly linked to proliferation and cancer, the role of DUBs represents a layer of complexity in this landscape that remains poorly captured. Due to their ability to remodel the proteome by altering protein degradation dynamics, DUBs play an important and underappreciated role in the cell cycle and proliferation of both normal and cancer cells. Moreover, due to their enzymatic protease activity and an open ubiquitin binding pocket, DUBs are likely to be important in the future of cancer treatment, since they are among the most druggable enzymes in the UPS. In this review we summarize new and important findings linking DUBs to cell cycle and proliferation, as well as to the etiology and treatment of cancer. We also highlight new advances in developing pharmacological approaches to attack DUBs for therapeutic benefit.

The Inhibitory Mechanisms of Tumor PD-L1 Expression by Natural Bioactive Gallic Acid in Non-Small-Cell Lung Cancer (NSCLC) Cells.

Non-small-cell lung cancer (NSCLC) is the most common lung cancer subtype and accounts for more than 80% of all lung cancer cases. Epidermal growth factor receptor (EGFR) phosphorylation by binding growth factors such as EGF activates downstream prooncogenic signaling pathways including KRAS-ERK, JAK-STAT, and PI3K-AKT. These pathways promote the tumor progression of NSCLC by inducing uncontrolled cell cycle, proliferation, migration, and programmed death-ligand 1 (PD-L1) expression. New cytotoxic drugs have facilitated considerable progress in NSCLC treatment, but side effects are still a significant cause of mortality. Gallic acid (3,4,5-trihydroxybenzoic acid; GA) is a phenolic natural compound, isolated from plant derivatives, that has been reported to show anticancer effects. We demonstrated the tumor-suppressive effect of GA, which induced the decrease of PD-L1 expression through binding to EGFR in NSCLC. This binding inhibited the phosphorylation of EGFR, subsequently inducing the inhibition of PI3K and AKT phosphorylation, which triggered the activation of p53. The p53-dependent upregulation of miR-34a induced PD-L1 downregulation. Further, we revealed the combination effect of GA and anti-PD-1 monoclonal antibody in an NSCLC-cell and peripheral blood mononuclear–cell coculture system. We propose a novel therapeutic application of GA for immunotherapy and chemotherapy in NSCLC.

MCMs in Cancer: Prognostic Potential and Mechanisms.

Enabling replicative immortality and uncontrolled cell cycle are hallmarks of cancer cells. Minichromosome maintenance proteins (MCMs) exhibit helicase activity in replication initiation and play vital roles in controlling replication times within a cell cycle. Overexpressed MCMs are detected in various cancerous tissues and cancer cell lines. Previous studies have proposed MCMs as promising proliferation markers in cancers, while the prognostic values remain controversial and the underlying mechanisms remain unascertained. This review provides an overview of the significant findings regarding the cellular and tumorigenic functions of the MCM family. Besides, current evidence of the prognostic roles of MCMs is retrospectively reviewed. This work also offers insight into the mechanisms of MCMs prompting carcinogenesis and adverse prognosis, providing information for future research. Finally, MCMs in liver cancer are specifically discussed, and future perspectives are provided.

1679 Polyposis With a Mutation in the CHEK2 Tumor Suppressor Gene

INTRODUCTION: Fifteen to twenty percent of colorectal cancer (CRC) is familial and genetic alterations are commonly encountered. Genetic testing is recommended in patients with a personal history of young onset CRC, significant family history of CRC, or high polyp burden, defined as having greater than 10 adenomas. Mutations in CHEK2 are associated with a variety of cancers, including CRC. However there is limited literature describing CHEK2mutations and polyposis. CASE DESCRIPTION/METHODS: A 47-year-old male with a personal history of thyroid cancer was evaluated for a six-month history of diarrhea. Family history was notable for multiple relatives on both sides with thyroid, breast, pancreatic, or colon cancer. Given progressive symptoms and family history, he underwent colonoscopy revealing 15 polyps varying in size, diffusely spread throughout his colon. Pathology revealed three polyps were tubulovillous adenomas with high-grade dysplasia, one polyp revealed focal invasive adenocarcinoma, and the remaining eleven polyps were low-risk tubular adenomas. Given the patient’s age, polyp burden, and complex cancer history, there was high suspicion for a genetic etiology of his thyroid cancer, colon polyps and CRC. He was referred for a genetic evaluation. A multi-gene cancer panel identified a heterozygous mutation, c.1072C > T (p.Gln358*) in CHEK2. DISCUSSION: Checkpoint kinase 2 (CHEK2) is a critical tumor suppressor gene that serves as an upstream regulator of p53. In response to DNA damage, CHEK2 prevents cell maturation and progression into mitosis. Malfunctioning leads to uncontrolled cell cycle progression without apoptosis. CHEK2 is a multi-organ cancer susceptibility gene linked to increased risk of breast, colon, thyroid and prostate cancer. In addition to a personal history of young onset colon cancer, the patient had a personal history of polyposis. This case report raises the question of whether CHEK2 is also a polyposis gene as this finding has important implications for carrier screening. Genetic evaluation for CHEK2, along with other CRC/polyposis genes, should be considered in young patients with a high-grade dysplastic adenoma, large polyp burden, or a significant personal or familial cancer history. Screening guidelines remain variable at this time and patients found to have a pathologic CHEK2mutation should receive a multidisciplinary approach for cancer screening. Further studies are needed to evaluate the association of polyposis and CHEK2 mutations.

MTOR Signaling Upregulates CDC6 via Suppressing miR-3178 and Promotes the Loading of DNA Replication Helicase

mTOR signaling pathway is deregulated in most cancers and uncontrolled cell cycle progression is a hallmark of cancer cell. However, the precise molecular mechanisms of the regulation of DNA replication and chromatin metabolism by mTOR signaling are largely unknown. We herein report that mTOR signaling promotes the loading of MCM2-7 helicase onto chromatin and upregulates DNA replication licensing factor CDC6. Pharmacological inhibition of mTOR kinase resulted in CHK1 checkpoint activation and decreased MCM2-7 replication helicase and PCNA associated with chromatins. Further pharmacological and genetic studies demonstrated CDC6 is positively controlled by mTORC1-S6K1 and mTORC2 signaling. miRNA screening revealed mTOR signaling suppresses miR-3178 thereby upregulating CDC6. Analysis of TCGA data found that CDC6 is overexpressed in most cancers and associates with the poor survival of cancer patients. Our findings suggest that mTOR signaling may control DNA replication origin licensing and replisome stability thereby cell cycle progression through CDC6 regulation.

Structure Activity Relationship of Xanthones for Inhibition of Cyclin Dependent Kinase 4 from Mangosteen (Garcinia Mangostana L.

The mangosteen fruit is a popular Southeast Asian fruit consumed for centuries. There have been a variety of xanthones isolated from the fruit, bark, roots and leaves with each having unique chemical and physical properties. Previously, the most abundant xanthone α-mangostin has been shown to inhibit CDK4. Herein we describe the role of selected xanthones from the mangosteen inhibiting CDK4. The evidence we provide here is that key functional groups are required to inhibit the CDK4 protein to prevent the phosphorylation of downstream targets critical to inhibiting uncontrolled cell cycle progression. To define the properties of xanthones for inhibiting CDK4 we utilized a cell free biochemical assay to identify inhibitors of CDK4. The following xanthones were used for the analysis: α-mangostin, β-mangostin, γ-mangostin, gartanin, 8-desoxygartanin, garcinone C and garcinone D, 9-hydroxycalabaxanthone, and 3-isomangostin These results further substantiate the unique pharmacological properties of individual xanthones and how a mixture of xanthones may be responsible for a multi-targeted effect in cell based pharmacology systems.

CREPT/RPRD1B associates with Aurora B to regulate Cyclin B1 expression for accelerating the G2/M transition in gastric cancer

Gastric cancer, like most of other cancers, has an uncontrolled cell cycle regulated by cyclins and cyclin-dependent kinases (CDKs). In this study, we reported that gastric cancer cells showed an accelerated G2/M transition promoted by CREPT/RPRD1B and Aurora kinase B (Aurora B). We found that CREPT/RPRD1B and Aurora B were coordinately expressed during the cell cycle in gastric cancer cells. Deletion of CREPT/RPRD1B disturbed the cell progression and extended the length of cell cycle, leading to a significant accumulation of mitotic cells. Mechanistically, we revealed that CREPT/RPRD1B interacted with Aurora B to regulate the expression of Cyclin B1 in gastric cancer cells. Interestingly, Aurora B phosphorylates S145 in a well-conserved motif of CREPT/RPRD1B. We proposed that phosphorylation of CREPT/RPRD1B by Aurora B is required for promoting the transcription of Cyclin B1, which is critical for the regulation of gastric tumorigenesis. Our study provides a mechanism by which gastric tumor cells maintain their high proliferation rate via coordination of Aurora B and CREPT/RPRD1B on the expression of Cyclin B1. Targeting the interaction of Aurora B and CREPT/RPRD1B might be a strategy for anti-gastric cancer therapy in the future.

Therapeutic Effect of Quinacrine, an Antiprotozoan Drug, by Selective Suppression of p-CHK1/2 in p53-Negative Malignant Cancers.

Quinacrine (QNC), antiprotozoan drug commonly used against Malaria and Giardiasis, has been recently tried for rheumatics and prion diseases via drug repositioning. In addition, several reports suggest antitumor effects of QNC through suppression of NF-κB and activation of p53. This study demonstrates the anticancer effect of QNC via a novel pathway through the elimination of checkpoint kinase 1/2 (Chk1/2) under p53-inactivated conditions. Inhibition of p53 by PFT-α or siRNA promotes QNC-induced apoptosis in normal fibroblast and p53-intact cancer cells. Considering that Chk1/2 kinases exert an essential role in the control of cell cycle, inhibition of Chk1/2 by QNC may induce cell death via uncontrolled cell cycle progression. Indeed, QNC reduces Chk1/2 expression under p53-impaired cancer cells and induces cell death in the G2-M phase. QNC increases the binding between p-Chk1/2 and β-TrCP and promotes proteasome-dependent degradation. Moreover, QNC treatment displayed antitumor effects in a Villin-Cre;p53+/LSL-R172H intestinal cancer mouse model system as well as HCT116 p53-/- xenografts.Implications: QNC has been used for the past over 70 years without obvious side effects, as such it is a plausible drug candidate for relapsed cancers, small-cell lung cancer, breast cancer as well as various p53-inactivated human malignancies. Mol Cancer Res; 16(6); 935-46. ©2018 AACR.

Mechanisms of the CDK4/6 inhibitor palbociclib (PD 0332991) and its future application in cancer treatment (Review).

An uncontrolled cell cycle is an obvious marker of tumor cells. The G1‑Sphase is an important restriction point in the normal cell cycle, but in cancer cells the restriction function is reduced, leading to uncontrolled cell proliferation. Two cyclin‑dependent kinases (CDKs), CDK4 and CDK6, play a crucial role in the G1‑Sphase transition. Inhibitors of CDK4/6 are presently the subjects of numerous studies, and PD 0332991, an inhibitor of CDK4/6, has been used to treat hormone receptor (HR)‑positive, advanced‑stage breast cancer. This inhibitor has also been studied in other cancers, such as lung cancer. In this review, we will discuss the regulation of the normal cell cycle transition from G1 to Sphase, the most promising inhibitor of CDK4/6, PD 0332991, as applied in different cancers, and finally we propose a mechanism of acquired resistance as well as the incredible potential for CDK4/6 inhibitors in the treatment of cancer. Briefly, we assert that, going forward, a new treatment pattern for cancer may be a combination therapy with a cell cycle inhibitor and a molecular targeted drug.

Targeting a non-oncogene addiction to the ATR/CHK1 axis for the treatment of small cell lung cancer

Small cell lung cancer (SCLC) is a difficult to treat subtype of lung cancer. One of the hallmarks of SCLC is its almost uniform chemotherapy sensitivity. However, chemotherapy response is typically transient and patients frequently succumb to SCLC within a year following diagnosis. We performed a transcriptome analysis of the major human lung cancer entities. We show a significant overexpression of genes involved in the DNA damage response, specifically in SCLC. Particularly CHEK1, which encodes for the cell cycle checkpoint kinase CHK1, is significantly overexpressed in SCLC, compared to lung adenocarcinoma. In line with uncontrolled cell cycle progression in SCLC, we find that CDC25A, B and C mRNAs are expressed at significantly higher levels in SCLC, compared to lung adenocarcinoma. We next profiled the efficacy of compounds targeting CHK1 and ATR. Both, ATR- and CHK1 inhibitors induce genotoxic damage and apoptosis in human and murine SCLC cell lines, but not in lung adenocarcinoma cells. We further demonstrate that murine SCLC tumors were highly sensitive to ATR- and CHK1 inhibitors, while KrasG12D-driven murine lung adenocarcinomas were resistant against these compounds and displayed continued growth under therapy. Altogether, our data indicate that SCLC displays an actionable dependence on ATR/CHK1-mediated cell cycle checkpoints.

DNA Damage as a Driver for Growth Delay: Chromosome Instability Syndromes with Intrauterine Growth Retardation.

DNA is constantly exposed to endogenous and exogenous mutagenic stimuli that are capable of producing diverse lesions. In order to protect the integrity of the genetic material, a wide array of DNA repair systems that can target each specific lesion has evolved. Despite the availability of several repair pathways, a common general program known as the DNA damage response (DDR) is stimulated to promote lesion detection, signaling, and repair in order to maintain genetic integrity. The genes that participate in these pathways are subject to mutation; a loss in their function would result in impaired DNA repair and genomic instability. When the DDR is constitutionally altered, every cell of the organism, starting from development, will show DNA damage and subsequent genomic instability. The cellular response to this is either uncontrolled proliferation and cell cycle deregulation that ensues overgrowth, or apoptosis and senescence that result in tissue hypoplasia. These diverging growth abnormalities can clinically translate as cancer or growth retardation; both features can be found in chromosome instability syndromes (CIS). The analysis of the clinical, cellular, and molecular phenotypes of CIS with intrauterine growth retardation allows inferring that replication alteration is their unifying feature.

Abstract A10: A Cdk4-dependent phosphorylation threshold regulates the cell cycle entry decision

Abstract Cyclin-dependent kinases (Cdk's) are major cell cycle regulators and uncontrolled cell cycle entry is a hallmark of cancer. Cyclin D, the regulatory subunit of Cdk4, is misregulated in about 75% of human malignancies, leading to hyperactive Cdk4 and uncontrolled cell cycle entry. In sharp contrast, the total loss of Cyclin D-mediated activity results in permanent cell cycle arrest. Several functions of Cdk4 during cell cycle entry have been identified; however, the underlying mechanism for cell cycle arrest upon its inactivation remains unanswered. Here we employed CRISPR/Cas9 and rAAV-mediated gene editing to visualize Restriction Point passage in highly synchronized single cells and find that cyclin D:Cdk4/6 complexes perform a critical switch-like function that results in activation of cyclin E:Cdk2 complexes. Taken together, our data indicate that while cyclin E:Cdk2 is the rate-limiting step for transition across the Restriction Point, it is dependent on cyclin D:Cdk4/6 phosphorylation of an as of yet unknown molecule. Citation Format: Manuel Kaulich, Steven Dowdy. A Cdk4-dependent phosphorylation threshold regulates the cell cycle entry decision. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Cancer Cell Cycle - Tumor Progression and Therapeutic Response; Feb 28-Mar 2, 2016; Orlando, FL. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(11_Suppl):Abstract nr A10.

Abstract 1241: Podophyllotoxin acetate (PA) as an anti-cancer drug candidate regulates various malignancy traits of non-small cell lung cancer

Abstract Here, we reported that podophyllotoxin acetate (PA) showed various aniti-cancer effects: induction of several cell death pathways, inhibition of γ-ionizing radiation (IR)-induced cancer invasion, chemo-sensitizer effect with topoisomerase (TOP) inhibitors against non-small cell lung cancer (NSCLC) cell lines as follows. (1) Stimulation of cell death via various mechanisms. First, PA dose-dependently induced cell cycle arrest at G2/M phase, as shown by accumulation of the mitosis-related proteins, p21, survivin and aurora B. This arrest was due to the PA-induced inhibition of microtubule polymerization that related to DNA damage (reflected by accumulation of γ-H2AX) resulting induction of intrinsic/extrinsic damage apoptotic pathways. PA also activated the pro-apoptotic ER stress pathway, as evidenced by increased expression levels of BiP, CHOP, IRE1-a, phospho-PERK, and phospho-JNK. Next, PA induced autophagy that reflected the expression of beclin-1, Atg3, Atg5 and Atg7, and the cleavage of LC3. (2) Inhibition of IR-induced cancer invasion. IR increased the invasion/migration of A549 cells, and this effect was decreased by PA treatment that the expressions/activities of matrix metalloprotase (MMP) -2, MMP-9, and vimentin, suggesting that PA could block the IR-induced epithelial-mesenchymal transition (EMT). In this study, we also identified IR-induced new signaling pathway, EGFR - p38/ERK - CREB-1/STAT3 - EMT pathway, that was inhibited by PA. (3) Chemo-sensitizer effect conjugated with TOP inhibitors - etoposide (Eto) and camptothecin (Cpt). Combination Index (CI) values of the combinations showed synergistic effects between PA and topoisomerase inhibitors. Combination with PA and Eto/Cpt promotes disruption of dynamics of actin filaments, which subsequently enhanced apoptotic cell death that accompanied to increased phosphorylation of p38. We also observed these combinations also inhibited activation of CREB-1, synergistically. Therefore, this study indicated that PA functions as a chemosensitizer by enhancing apoptosis through activation of p38/caspases apoptotic axis and suppression of CREB-1.Taken together; these results suggest PA is a new anti-cancer drug candidate that could control several malignancy traits of cancer including uncontrolled cell cycle and IR-induced invasion. Citation Format: Jeong Hyun Cho, Wan Gi Hong, Sang-Gu Hwang, Hong-Duck Um, Jong Kuk Park. Podophyllotoxin acetate (PA) as an anti-cancer drug candidate regulates various malignancy traits of non-small cell lung cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1241.

A Potential Linkage Between the JC and BK Polyomaviruses and Brain and Urinary Tract Tumors: A Review of the Literature

JC virus (JCV) and BK virus (BKV) belong to the Polyomaviridae family and were the first human polyomaviruses (PyVs) discovered. Both JCV and BKV remain latent in the kidneys and can reactivate in immunocompromised hosts, causing progressive multifocal leukoencephalopathy (PML) and nephropathy, respectively. PyVs induce cell transformation in vitro and in animal models, where they infect non-permissive cells. The molecular mechanisms of tumor formation are based on the large tumor antigen (T Ag) protein, which is able to bind and inactivate pRb and p53, inducing uncontrolled cell cycle progression. Many studies on clinical samples also indicate a positive association between JCV and BKV and human solid tumors; however no direct involvement of PyVs in the development of human cancers has been demonstrated. In this review we focused on the potential association between JCV and BKV with brain and urinary tract tumors, respectively.

P19-INK4d inhibits neuroblastoma cell growth, induces differentiation and is hypermethylated and downregulated in MYCN-amplified neuroblastomas.

Uncontrolled cell cycle entry, resulting from deregulated CDK-RB1-E2F pathway activity, is a crucial determinant of neuroblastoma cell malignancy. Here we identify neuroblastoma-suppressive functions of the p19-INK4d CDK inhibitor and uncover mechanisms of its repression in high-risk neuroblastomas. Reduced p19-INK4d expression was associated with poor event-free and overall survival and neuroblastoma risk factors including amplified MYCN in a set of 478 primary neuroblastomas. High MYCN expression repressed p19-INK4d mRNA and protein levels in different neuroblastoma cell models with conditional MYCN expression. MassARRAY and 450K methylation analyses of 105 primary neuroblastomas uncovered a differentially methylated region within p19-INK4d. Hypermethylation of this region was associated with reduced p19-INK4d expression. In accordance, p19-INK4d expression was activated upon treatment with the demethylating agent, 2'-deoxy-5-azacytidine, in neuroblastoma cell lines. Ectopic p19-INK4d expression decreased viability, clonogenicity and the capacity for anchorage-independent growth of neuroblastoma cells, and shifted the cell cycle towards the G1/0 phase. p19-INK4d also induced neurite-like processes and markers of neuronal differentiation. Moreover, neuroblastoma cell differentiation, induced by all-trans retinoic acid or NGF-NTRK1-signaling, activated p19-INK4d expression. Our findings pinpoint p19-INK4d as a neuroblastoma suppressor and provide evidence for MYCN-mediated repression and for epigenetic silencing of p19-INK4d by DNA hypermethylation in high-risk neuroblastomas.

Targeting the epigenetic machinery of cancer cells

Cancer is characterized by uncontrolled cell growth and the acquisition of metastatic properties. In most cases, the activation of oncogenes and/or deactivation of tumour suppressor genes lead to uncontrolled cell cycle progression and inactivation of apoptotic mechanisms. Although the underlying mechanisms of carcinogenesis remain unknown, increasing evidence links aberrant regulation of methylation to tumourigenesis. In addition to the methylation of DNA and histones, methylation of nonhistone proteins, such as transcription factors, is also implicated in the biology and development of cancer. Because the metabolic cycling of methionine is a key pathway for many of these methylating reactions, strategies to target the epigenetic machinery of cancer cells could result in novel and efficient anticancer therapies. The application of these new epigenetic therapies could be of utility in the promotion of E2F1-dependent apoptosis in cancer cells, in avoiding metastatic pathways and/or in sensitizing tumour cells to radiotherapy.

G0S2–A new player in leukemia

G0S2–A new player in leukemia

C-JUN N-terminal kinase (JNK) is activated and contributes to tumor cell proliferation in classical Hodgkin lymphoma.

c-JUN N-terminal Kinase (JNK) is activated/phosphorylated by upstream MAPK kinases (MKK), and, in turn, phosphorylates and activates its major substrate c-JUN, a member of the activator protein-1 (AP-1) transcription factors. c-JUN is overexpressed and activated in Hodgkin and Reed Sternberg cells (HRS) of classical Hodgkin lymphoma (cHL), however, the mechanism of its activation remains unknown. JNK activation was immunohistochemically assessed in 60 cases of HL and in a control group of 151 B-cell non-Hodgkin lymphomas. The biologic effects of JNK activation in cultured HRS cells were investigated using colony formation, cell growth and viability assays and cell cycle analysis by flow cytometry. Western blotting was used to assess protein levels. p-JNK was expressed in 90% of HL, 83% of Burkitt lymphomas, 28% of mantle cell lymphomas, 23% of diffuse large B-cell lymphomas, 19% of follicular lymphomas, and 18% of extranodal marginal zone lymphomas of MALT type. None of the 48 cases of chronic lymphocytic leukemia/small lymphocytic lymphoma and 18 cases of plasma cell myeloma showed JNK phosphorylation (P < 001, Kruskall-Wallis test). Pharmacological inhibition of JNK activity in cultured HRS cells resulted in a significant decrease of cell growth, which was associated with cell cycle arrest at the G2/M phase. The cell cycle effects were linked to deactivation of c-JUN and upregulation of its known target, the cyclin-dependent kinase inhibitor p21. JNK is highly activated in HRS cells, and may contribute to uncontrolled cell cycle progression and proliferation of tumor cells in cHL.

Abstract A57: Plasticity of the translational landscape during the mammalian cell cycle: Implications and new insights for cancer cell proliferation

Abstract Deregulated cell proliferation is a defining feature of cancer cells. Normally, an intricate regulatory network ensures proper cell cycle progression during development and for tissue homeostasis. Although many steps in gene regulation imbue specificity to this program, the translational landscape of gene expression underlying cell cycle progression remains poorly understood. To address this outstanding question, we have conducted the first genome-wide analysis of translational regulation using ribosome profiling during each phase of the cell cycle. We have uncovered widespread translational regulation of hundreds of mRNAs that provides a rapid, robust, and precise mechanism for gene regulation that underlies the cell cycle program. These include a number of genes involved in key biological process and hallmarks of cancer cell growth and tumor development including cellular proliferation, cellular metabolism, and safeguarding genomic integrity. We have further characterized a novel network of translational regulation coordinating the expression of the cohesin complex, which is important for proper chromosomal segregation, regulating transcription, and has recently been identified as a common target of mutation in a number of tumor types. Mechanistically, we show that the 5'UTRs of several cohesin mRNAs are sufficient to direct their cell cycle dependent pattern of expression. Thereby, our studies highlight an intricate relationship in translational control between the machineries responsible for maintaining the organization of the genome and protein synthesis. Moreover, we discovered that translational specificity during cell cycle phases coordinates the mTOR signaling pathway. Specifically, we uncovered that translation of RICTOR, a key component of mTORC2, is induced during S-phase, resulting in an increase in mTORC2 signaling during this phase of the cell cycle. Furthermore, we showed that rictor-/- MEFs exhibited a marked decrease in S-phase cells compared to wild-type MEFs, suggesting that RICTOR may be required to prevent premature exit from S-phase, ensuring that cells complete DNA replication. Together, our findings pinpoint that translational regulation of RICTOR underlies cell cycle-dependent mTORC2 activity, which may be a critical determinant of the mammalian cell cycle program. Our findings highlight both the prevalence and dynamic nature of translational regulation during cell cycle progression. This work further suggests a multifaceted mechanism for differential regulation of transcript-specific translational control in the accurate execution of distinct steps of the cell cycle. Current studies are unraveling novel cis-regulatory sequences underlying the molecular mechanisms governing the translational regulation of mRNA networks during the cell cycle and determining how distinct oncogenic insults, which directly impact the translational machinery, alter the expression patterns of these networks leading to uncontrolled cell cycle progression. Citation Format: Craig R. Stumpf, Melissa V. Moreno, Adam B. Olshen, Barry S. Taylor, Davide Ruggero. Plasticity of the translational landscape during the mammalian cell cycle: Implications and new insights for cancer cell proliferation. [abstract]. In: Proceedings of the Third AACR International Conference on Frontiers in Basic Cancer Research; Sep 18-22, 2013; National Harbor, MD. Philadelphia (PA): AACR; Cancer Res 2013;73(19 Suppl):Abstract nr A57.

CREPT Accelerates Tumorigenesis by Regulating the Transcription of Cell-Cycle-Related Genes

Tumorigenesis is caused by an uncontrolled cell cycle and the altered expression of many genes. Here, we report a gene CREPT that is preferentially expressed in diverse human tumors. Overexpression of CREPT accelerates tumor growth, whereas depletion of CREPT demonstrates a reversed effect. CREPT regulates cyclin D1 expression by binding to its promoter, enhancing its transcription both in vivo and in vitro, and interacting with RNA polymerase II (RNAPII). Interestingly, CREPT promotes the formation of a chromatin loop and prevents RNAPII from reading through the 3' end termination site of the gene. Our findings reveal a mechanism where CREPT increases cyclin D1 transcription during tumorigenesis, through enhancing the recruitment of RNAPII to the promoter region, possibly, as well as chromatin looping.