CDK1 substitutes for mTOR kinase to activate mitotic cap-dependent protein translation

Rapamycin ameliorates immune-mediated aplastic anemia by inhibiting the proliferation and metabolism of T cells

Aplastic anemia (AA) is a serious blood system disease that threatens human health. At present, the main cause of this disease is believed to be immune hyperfunction. However, the specific metabolic mode involved in the occurrence of lymphocytes in AA is still unknown. In addition, whether rapamycin, a specific blocker of the mTOR signaling pathway, plays a therapeutic role by inhibiting lymphocyte metabolism remains unclear. We induced an AA mouse model through the classical immune-mediated pathway and simultaneously administered rapamycin intervention therapy. First, the AA-associated phenotypic changes and the efficacy of rapamycin in the treatment of AA were discussed. Second, the proliferation and metabolic pathway of bone marrow (BM) lymphocytes in AA and the effect of rapamycin on this process were determined. Finally, the expression levels of mTOR pathway-related proteins were analyzed. By inhibiting the mTOR signaling pathway, rapamycin could ameliorate the phenotype of the immune-mediated AA model and inhibit the proliferation of T cells by preventing cell cycle transition from G0 to G1 phase. Moreover, we found that mitochondrial oxidative phosphorylation is involved in the metabolic reprogramming of T cells in AA and that rapamycin can inhibit this process. We confirmed that mitochondrial oxidative phosphorylation is involved in the metabolic reprogramming of T cells in AA and further extended the mechanism of rapamycin in treating AA by inhibiting the mTOR signaling pathway. This viewpoint may provide a new therapeutic idea for clinical applications.

Effect of the Large and Small T-Antigens of Human Polyomaviruses on Signaling Pathways

Viruses are intracellular parasites that require a permissive host cell to express the viral genome and to produce new progeny virus particles. However, not all viral infections are productive and some viruses can induce carcinogenesis. Irrespective of the type of infection (productive or neoplastic), viruses hijack the host cell machinery to permit optimal viral replication or to transform the infected cell into a tumor cell. One mechanism viruses employ to reprogram the host cell is through interference with signaling pathways. Polyomaviruses are naked, double-stranded DNA viruses whose genome encodes the regulatory proteins large T-antigen and small t-antigen, and structural proteins that form the capsid. The large T-antigens and small t-antigens can interfere with several host signaling pathways. In this case, we review the interplay between the large T-antigens and small t-antigens with host signaling pathways and the biological consequences of these interactions.

Targeting mTOR for cancer therapy

Mechanistic target of rapamycin (mTOR) is a protein kinase regulating cell growth, survival, metabolism, and immunity. mTOR is usually assembled into several complexes such as mTOR complex 1/2 (mTORC1/2). In cooperation with raptor, rictor, LST8, and mSin1, key components in mTORC1 or mTORC2, mTOR catalyzes the phosphorylation of multiple targets such as ribosomal protein S6 kinase β-1 (S6K1), eukaryotic translation initiation factor 4E binding protein 1 (4E-BP1), Akt, protein kinase C (PKC), and type-I insulin-like growth factor receptor (IGF-IR), thereby regulating protein synthesis, nutrients metabolism, growth factor signaling, cell growth, and migration. Activation of mTOR promotes tumor growth and metastasis. Many mTOR inhibitors have been developed to treat cancer. While some of the mTOR inhibitors have been approved to treat human cancer, more mTOR inhibitors are being evaluated in clinical trials. Here, we update recent advances in exploring mTOR signaling and the development of mTOR inhibitors for cancer therapy. In addition, we discuss the mechanisms underlying the resistance to mTOR inhibitors in cancer cells.

Merkel Cell Carcinoma in the HIV-1/AIDS Patient

Merkel cell carcinoma (MCC) is a highly aggressive, primary neuroendocrine cancer of the skin. The majority of MCC cases are associated with the recently discovered Merkel cell polyomavirus (MCPyV), while the remaining are caused by ultraviolet (UV) light-induced mutations from excessive sunlight exposure. The risk of developing MCC is much higher in the white population relative to all other races. Approximately 10% of all patients with MCC have some form of immunosuppression including HIV-1/AIDS, chronic inflammatory conditions, solid organ transplantation, or hematological malignancies. The age of onset of MCC is lower and the mortality is higher in immunosuppressed individuals than in immune-competent patients. It is plausible that HIV-1/AIDS predisposes to virus-positive MCC, but it should be noted that HIV-1/AIDS increases the risk for developing of UV-induced skin cancers such as cutaneous squamous cell carcinoma and basal cell carcinoma and therefore may also increase the risk for virus-negative MCC. Surgical management is considered standard of care for localized Merkel cell carcinoma with current recommendations advising a wide local excision of the lesion. Most international guidelines support the use of local adjuvant radiotherapy coupled with tumor staging to improve the frequency of cure. For advanced, metastatic, and recurrent MCC, checkpoint blockade inhibitors targeting PD-1 and PD-L1 have shown remarkable activity including durable long-term. MCC in patients living with HIV-1/AIDS are treated with similar modalities as HIV-1 uninfected individuals with MCC.

Cell-Cycle-Dependent Regulation of Translation: New Interpretations of Old Observations in Light of New Approaches

It is a long-standing view that global translation varies during the cell cycle and is much lower in mitosis than in other cell-cycle phases. However, the central papers in the literature are not in agreement about the extent of downregulation in mitosis, ranging from a dramatic decrease to only a marginal reduction. Herein, it is argued that the discrepancy derives from technical challenges. Cell-cycle-dependent variations are most conveniently studied in synchronized cells, but the synchronization methods by themselves often evoke stress responses that, in turn, affect translation rates. Further, it is argued that previously reported cell-cycle-dependent changes in the global translation rate to a large extent reflect responses to the synchronization methods. Recent findings strongly suggest that the global translation rate is not regulated in a cell-cycle-dependent manner. Novel techniques allowing a genome-wide analysis of translational profiles suggest that the extent and importance of selective translational regulation associated with cell-cycle transitions have been underestimated. Therefore, the main question is which messenger RNAs (mRNAs) are translated, rather than whether the global translation rate is decreased.

Mitosis-related phosphorylation of the eukaryotic translation suppressor 4E-BP1 and its interaction with eukaryotic translation initiation factor 4E (eIF4E)

Eukaryotic translation initiation factor 4E (eIF4E)–binding protein 1 (4E-BP1) inhibits cap-dependent translation in eukaryotes by competing with eIF4G for an interaction with eIF4E. Phosphorylation at Ser-83 of 4E-BP1 occurs during mitosis through the activity of cyclin-dependent kinase 1 (CDK1)/cyclin B rather than through canonical mTOR kinase activity. Here, we investigated the interaction of eIF4E with 4E-BP1 or eIF4G during interphase and mitosis. We observed that 4E-BP1 and eIF4G bind eIF4E at similar levels during interphase and mitosis. The most highly phosphorylated mitotic 4E-BP1 isoform (δ) did not interact with eIF4E, whereas a distinct 4E-BP1 phospho-isoform, EB-γ, phosphorylated at Thr-70, Ser-83, and Ser-101, bound to eIF4E during mitosis. Two-dimensional gel electrophoretic analysis corroborated the identity of the phosphorylation marks on the eIF4E-bound 4E-BP1 isoforms and uncovered a population of phosphorylated 4E-BP1 molecules lacking Thr-37/Thr-46–priming phosphorylation. Moreover, proximity ligation assays for phospho-4E-BP1 and eIF4E revealed different in situ interactions during interphase and mitosis. The eIF4E:eIF4G interaction was not inhibited but rather increased in mitotic cells, consistent with active translation initiation during mitosis. Phosphodefective substitution of 4E-BP1 at Ser-83 did not change global translation or individual mRNA translation profiles as measured by single-cell nascent protein synthesis and eIF4G RNA immunoprecipitation sequencing. Mitotic 5′-terminal oligopyrimidine RNA translation was active and, unlike interphase translation, resistant to mTOR inhibition. Our findings reveal the phosphorylation profiles of 4E-BP1 isoforms and their interactions with eIF4E throughout the cell cycle and indicate that 4E-BP1 does not specifically inhibit translation initiation during mitosis.

Eukaryotic initiation factor 4E-binding protein as an oncogene in breast cancer

BACKGROUND

Eukaryotic Initiation Factor 4E-Binding Protein (EIF4EBP1, 4EBP1) is overexpressed in many human cancers including breast cancer, yet the role of 4EBP1 in breast cancer remains understudied. Despite the known role of 4EBP1 as a negative regulator of cap-dependent protein translation, 4EBP1 is predicted to be an essential driving oncogene in many cancer cell lines in vitro, and can act as a driver of cancer cell proliferation. EIF4EBP1 is located within the 8p11-p12 genomic locus, which is frequently amplified in breast cancer and is known to predict poor prognosis and resistance to endocrine therapy.

METHODS

Here we evaluated the effect of 4EBP1 targeting using shRNA knock-down of expression of 4EBP1, as well as response to the mTORC targeted drug everolimus in cell lines representing different breast cancer subtypes, including breast cancer cells with the 8p11-p12 amplicon, to better define a context and mechanism for oncogenic 4EBP1.

RESULTS

Using a genome-scale shRNA screen on the SUM panel of breast cancer cell lines, we found 4EBP1 to be a strong hit in the 8p11 amplified SUM-44 cells, which have amplification and overexpression of 4EBP1. We then found that knock-down of 4EBP1 resulted in dramatic reductions in cell proliferation in 8p11 amplified breast cancer cells as well as in other luminal breast cancer cell lines, but had little or no effect on the proliferation of immortalized but non-tumorigenic human mammary epithelial cells. Kaplan-Meier analysis of EIF4EBP1 expression in breast cancer patients demonstrated that overexpression of this gene was associated with reduced relapse free patient survival across all breast tumor subtypes.

CONCLUSIONS

These results are consistent with an oncogenic role of 4EBP1 in luminal breast cancer and suggests a role for this protein in cell proliferation distinct from its more well-known role as a regulator of cap-dependent translation.

The biology and treatment of Merkel cell carcinoma: current understanding and research priorities

Merkel cell carcinoma (MCC) is a rare and aggressive skin cancer associated with advanced age and immunosuppression. Over the past decade, an association has been discovered between MCC and either integration of the Merkel cell polyomavirus, which likely drives tumorigenesis, or somatic mutations owing to ultraviolet-induced DNA damage. Both virus-positive and virus-negative MCCs are immunogenic, and inhibition of the programmed cell death protein 1 (PD-1)–programmed cell death 1 ligand 1 (PD-L1) immune checkpoint has proved to be highly effective in treating patients with metastatic MCC; however, not all patients have a durable response to immunotherapy. Despite these rapid advances in the understanding and management of patients with MCC, many basic, translational and clinical research questions remain unanswered. In March 2018, an International Workshop on Merkel Cell Carcinoma Research was held at the US National Cancer Institute, at which academic, government and industry experts met to identify the highest-priority research questions. Here, we review the biology and treatment of MCC and report the consensus-based recommendations agreed upon during the workshop.

Merkel cell carcinoma (MCC) is a rare and aggressive form of nonmelanoma skin cancer. The availability of immune checkpoint inhibition has improved the outcomes of a subset of patients with MCC, although many unmet needs continue to exist. In this Consensus Statement, the authors summarize developments in our understanding of MCC while also providing consensus recommendations for future research.

Mammalian cell growth dynamics in mitosis

The extent and dynamics of animal cell biomass accumulation during mitosis are unknown, primarily because growth has not been quantified with sufficient precision and temporal resolution. Using the suspended microchannel resonator and protein synthesis assays, we quantify mass accumulation and translation rates between mitotic stages on a single-cell level. For various animal cell types, growth rates in prophase are commensurate with or higher than interphase growth rates. Growth is only stopped as cells approach metaphase-to-anaphase transition and growth resumes in late cytokinesis. Mitotic arrests stop growth independently of arresting mechanism. For mouse lymphoblast cells, growth in prophase is promoted by CDK1 through increased phosphorylation of 4E-BP1 and cap-dependent protein synthesis. Inhibition of CDK1-driven mitotic translation reduces daughter cell growth. Overall, our measurements counter the traditional dogma that growth during mitosis is negligible and provide insight into antimitotic cancer chemotherapies.

Measuring Protein Synthesis during Cell Cycle by Azidohomoalanine (AHA) Labeling and Flow Cytometric Analysis

Protein synthesis is one of the most fundamental biological processes to maintain cellular proteostasis. Azidohomoalaine (AHA) is a non-radioactive and "clickable" amino acid analog of methionine which can be incorporated into newly synthesized proteins. Thus, AHA-labeled nascent proteins can be detected and quantified through fluorescent labeling by "click" chemistry. Here we describe a protocol to measure protein synthesis by AHA labeling and flow cytometry. Taking advantage of gating different cell populations, we provide a typical example of the flow cytometric-based analysis of protein synthesis during the cell cycle. While we used mouse B cells in this protocol this method can be readily applied to any cell types and organisms.

Iron-dependent CDK1 activity promotes lung carcinogenesis via activation of the GP130/STAT3 signaling pathway

Iron dysregulation is associated with several diseases, including lung cancer, but the underlying mechanism is yet unknown. Iron directly binds CDK1, which is upregulated in several cancers, thereby promoting JAK1 phosphorylation and activation of STAT3 signaling to promote colorectal carcinogenesis. This study aimed to investigate the role of iron/CDK1/STAT3 signaling in lung carcinogenesis. We found that iron-dependent CDK1 activity upregulated IL-6 receptor subunit GP130 post-transcriptionally via phosphorylation of 4E-BP1, which is critical for activation of JAK/STAT3 signaling. CDK1 and STAT3 are essential for iron-mediated colony formation in lung cancer cell lines. CDK1 knockdown and iron chelator DFO decreased tumorigenicity and GP130/STAT3 signaling in vivo. Moreover, CDK1/GP130/STAT3 signaling were elevated in lung cancer tissues compared with adjacent normal lung tissues. Altogether, the present results suggest that CDK1 inhibition and iron deprivation are potential strategies to target GP130/STAT3 signaling to suppress lung cancer.

Polyomaviruses of the skin: integrating molecular and clinical advances in an emerging class of viruses

BACKGROUND

Human polyomaviruses (HPyVs) are small, nonenveloped, double-stranded DNA viruses that express tumour antigen proteins. Fourteen species of polyomaviruses have been discovered in humans, and since the 2008 discovery of the first cutaneous polyomavirus - Merkel cell polyomavirus (MCPyV) - six more species have been detected in the skin: trichodysplasia spinulosa-associated polyomavirus (TSPyV), HPyV6, HPyV7, HPyV9, HPyV10 and HPyV13. Of these cutaneous species, only MCPyV, TSPyV, HPyV6 and HPyV7 have been definitively associated with diseases of the skin, most commonly in immunocompromised individuals. MCPyV is a predominant aetiology in Merkel cell carcinomas. TSPyV is one of the aetiological factors of trichodysplasia spinulosa. HPyV6 and HPyV7 have been recently linked to pruritic skin eruptions. The roles of HPyV9, HPyV10 and HPyV13 in pathogenesis, if any, are still unknown, but their molecular features have provided some insight into their functional biology.

RESULTS

In this review, we summarize the known molecular mechanisms, clinical presentation and targeted therapies of each of the eight cutaneous HPyVs.

CONCLUSIONS

We hope that heightened awareness and clinical recognition of HPyVs will lead to increased reports of HPyV-associated diseases and, consequently, a more robust understanding of how to diagnose and treat these conditions.

Merkel cell polyomavirus Tumor antigens expressed in Merkel cell carcinoma function independently of the ubiquitin ligases Fbw7 and β-TrCP

Merkel cell polyomavirus (MCPyV) accounts for 80% of all Merkel cell carcinoma (MCC) cases through expression of two viral oncoproteins: the truncated large T antigen (LT-t) and small T antigen (ST). MCPyV ST is thought to be the main driver of cellular transformation and has also been shown to increase LT protein levels through the activity of its Large-T Stabilization Domain (LSD). The ST LSD was reported to bind and sequester several ubiquitin ligases, including Fbw7 and β-TrCP, and thereby stabilize LT-t and several other Fbw7 targets including c-Myc and cyclin E. Therefore, the ST LSD is thought to contribute to transformation by promoting the accumulation of these oncoproteins. Targets of Fbw7 and β-TrCP contain well-defined, conserved, phospho-degrons. However, as neither MCPyV LT, LT-t nor ST contain the canonical Fbw7 phospho-degron, we sought to further investigate the proposed model of ST stabilization of LT-t and transformation. In this study, we provide several lines of evidence that fail to support a specific interaction between MCPyV T antigens and Fbw7 or β-TrCP by co-immunoprecipitation or functional consequence. Although MCPyV ST does indeed increase LT protein levels through its Large-T Stabilization domain (LSD), this is accomplished independently of Fbw7. Therefore, our study indicates a need for further investigation into the role and mechanism(s) of MCPyV T antigens in viral replication, latency, transformation, and tumorigenesis.

Merkel cell carcinoma (MCC) is a very aggressive and deadly neuroendocrine skin cancer. Merkel cell polyomavirus (MCPyV) contributes to the development and maintenance of 80% of MCCs through the expression of its truncated large tumor antigen (LT-t) and small tumor antigen (ST). MCPyV ST is thought to be primarily responsible for transformation and tumorigenesis through many mechanisms including stabilization of MCPyV LT-t and other cellular proteins involved in proliferation such as c-Myc. As c-Myc is a known target substrate, and MCPyV LT-t is a proposed target substrate, of the ubiquitin ligase Fbw7, it is currently thought that ST stabilizes these proteins and transforms cells through binding and perturbing the function of Fbw7. However, neither MCPyV LT-t nor ST contain a canonical Fbw7 degron sequence necessary for this interaction. MCPyV LT-t, found in MCCs, does not bind to, nor is targeted by, Fbw7. However, an ill-defined, unidirectional interaction between MCPyV LT, ST, and Fbw7 was observed, but had no functional consequence. Therefore, this study calls for further investigation into the mechanism(s) by which MCPyV ST leads to the development and maintenance of MCC.

Pleiotropic Effects of PPARD Accelerate Colorectal Tumorigenesis, Progression, and Invasion

APC mutations activate aberrant β-catenin signaling to drive initiation of colorectal cancer; however, colorectal cancer progression requires additional molecular mechanisms. PPAR-delta (PPARD), a downstream target of β-catenin, is upregulated in colorectal cancer. However, promotion of intestinal tumorigenesis following deletion of PPARD in Apc^min mice has raised questions about the effects of PPARD on aberrant β-catenin activation and colorectal cancer. In this study, we used mouse models of PPARD overexpression or deletion combined with APC mutation (Apc^Δ580 ) in intestinal epithelial cells (IEC) to elucidate the contributions of PPARD in colorectal cancer. Overexpression or deletion of PPARD in IEC augmented or suppressed β-catenin activation via up- or downregulation of BMP7/TAK1 signaling and strongly promoted or suppressed colorectal cancer, respectively. Depletion of PPARD in human colorectal cancer organoid cells inhibited BMP7/β-catenin signaling and suppressed organoid self-renewal. Treatment with PPARD agonist GW501516 enhanced colorectal cancer tumorigenesis in Apc^Δ580 mice, whereas treatment with PPARD antagonist GSK3787 suppressed tumorigenesis. PPARD expression was significantly higher in human colorectal cancer-invasive fronts versus their paired tumor centers and adenomas. Reverse-phase protein microarray and validation studies identified PPARD-mediated upregulation of other proinvasive pathways: connexin 43, PDGFRβ, AKT1, EIF4G1, and CDK1. Our data demonstrate that PPARD strongly potentiates multiple tumorigenic pathways to promote colorectal cancer progression and invasiveness. SIGNIFICANCE: These findings address long-standing, important, and unresolved questions related to the potential role of PPARD in APC mutation-dependent colorectal tumorigenesis by showing PPARD activation enhances APC mutation-dependent tumorigenesis.

MiR-375 Regulation of LDHB Plays Distinct Roles in Polyomavirus-Positive and -Negative Merkel Cell Carcinoma

MicroRNA-375 (miR-375) is deregulated in multiple tumor types and regulates important targets involved in tumorigenesis and metastasis. This miRNA is highly expressed in Merkel cell carcinoma (MCC) compared to normal skin and other non-MCC skin cancers, and its expression is high in Merkel cell polyomavirus (MCPyV)-positive (MCPyV+) and low in MCPyV-negative (MCPyV-) MCC tumors. In this study, we characterized the function and target of miR-375 in MCPyV+ and MCPyV- MCC cell lines. Ectopic expression of miR-375 in MCPyV- MCC cells resulted in decreased cell proliferation and migration, as well as increased cell apoptosis and cell cycle arrest. However, in MCPyV+ MCC cells, inhibition of miR-375 expression reduced cell growth and induced apoptosis. Additionally, the expression of lactate dehydrogenase B (LDHB), a known target of miR-375, was inversely correlated with miR-375. Silencing of LDHB reduced cell growth in MCPyV- cell lines, while its silencing in MCPyV+ cell lines rescued the cell growth effect mediated by miR-375 inhibition. Together, our results suggest dual roles of miR-375 and LDHB in MCPyV and non-MCPyV-associated MCCs. We propose that LDHB could be a therapeutic target in MCC and different strategies should be applied in virus- and non-virus-associated MCCs.

Phosphorylation of the Ribosomal Protein RPL12/uL11 Affects Translation during Mitosis

Emerging evidence indicates that heterogeneity in ribosome composition can give rise to specialized functions. Until now, research mainly focused on differences in core ribosomal proteins and associated factors. The effect of posttranslational modifications has not been studied systematically. Analyzing ribosome heterogeneity is challenging because individual proteins can be part of different subcomplexes (40S, 60S, 80S, and polysomes). Here we develop polysome proteome profiling to obtain unbiased proteomic maps across ribosomal subcomplexes. Our method combines extensive fractionation by sucrose gradient centrifugation with quantitative mass spectrometry. The high resolution of the profiles allows us to assign proteins to specific subcomplexes. Phosphoproteomics on the fractions reveals that phosphorylation of serine 38 in RPL12/uL11, a known mitotic CDK1 substrate, is strongly depleted in polysomes. Follow-up experiments confirm that RPL12/uL11 phosphorylation regulates the translation of specific subsets of mRNAs during mitosis. Together, our results show that posttranslational modification of ribosomal proteins can regulate translation.

Regulation of global translation during the cell cycle

It is generally accepted that global translation varies during the cell cycle and is low during mitosis. However, addressing this issue is challenging because it involves cell synchronization, which evokes stress responses that, in turn, affect translation rates. Here, we have used two approaches to measure global translation rates in different cell-cycle phases. First, synchrony in different cell-cycle phases was obtained involving the same stress, by using temperature-sensitive mutants. Second, translation and DNA content were measured by flow cytometry in exponentially growing, single cells. We found no major variation in global translation rates through the cell cycle in either fission yeast or mammalian cells. We also measured phosphorylation of eukaryotic initiation factor-2α, an event that is thought to downregulate global translation in mitosis. In contrast with the prevailing view, eIF2α phosphorylation correlated poorly with downregulation of global translation and ectopically induced eIF2α phosphorylation inhibited global translation only at high levels.

Therapeutic Opportunities in Eukaryotic Translation

The ability to block biological processes with selective small molecules provides advantages distinct from most other experimental approaches. These include rapid time to onset, swift reversibility, ability to probe activities in manners that cannot be accessed by genetic means, and the potential to be further developed as therapeutic agents. Small molecule inhibitors can also be used to alter expression and activity without affecting the stoichiometry of interacting partners. These tenets have been especially evident in the field of translation. Small molecule inhibitors were instrumental in enabling investigators to capture short-lived complexes and characterize specific steps of protein synthesis. In addition, several drugs that are the mainstay of modern antimicrobial drug therapy are potent inhibitors of prokaryotic translation. Currently, there is much interest in targeting eukaryotic translation as decades of research have revealed that deregulated protein synthesis in cancer cells represents a targetable vulnerability. In addition to being potential therapeutics, small molecules that manipulate translation have also been shown to influence cognitive processes such as memory. In this review, we focus on small molecule modulators that target the eukaryotic translation initiation apparatus and provide an update on their potential application to the treatment of disease.

Signaling Pathways Involved in the Regulation of mRNA Translation

Translation is a key step in the regulation of gene expression and one of the most energy-consuming processes in the cell. In response to various stimuli, multiple signaling pathways converge on the translational machinery to regulate its function. To date, the roles of phosphoinositide 3-kinase (PI3K)/AKT and the mitogen-activated protein kinase (MAPK) pathways in the regulation of translation are among the best understood. Both pathways engage the mechanistic target of rapamycin (mTOR) to regulate a variety of components of the translational machinery. While these pathways regulate protein synthesis in homeostasis, their dysregulation results in aberrant translation leading to human diseases, including diabetes, neurological disorders, and cancer. Here we review the roles of the PI3K/AKT and MAPK pathways in the regulation of mRNA translation. We also highlight additional signaling mechanisms that have recently emerged as regulators of the translational apparatus.

Co-translational protein targeting facilitates centrosomal recruitment of PCNT during centrosome maturation in vertebrates

As microtubule-organizing centers of animal cells, centrosomes guide the formation of the bipolar spindle that segregates chromosomes during mitosis. At mitosis onset, centrosomes maximize microtubule-organizing activity by rapidly expanding the pericentriolar material (PCM). This process is in part driven by the large PCM protein pericentrin (PCNT), as its level increases at the PCM and helps recruit additional PCM components. However, the mechanism underlying the timely centrosomal enrichment of PCNT remains unclear. Here, we show that PCNT is delivered co-translationally to centrosomes during early mitosis by cytoplasmic dynein, as evidenced by centrosomal enrichment of PCNT mRNA, its translation near centrosomes, and requirement of intact polysomes for PCNT mRNA localization. Additionally, the microtubule minus-end regulator, ASPM, is also targeted co-translationally to mitotic spindle poles. Together, these findings suggest that co-translational targeting of cytoplasmic proteins to specific subcellular destinations may be a generalized protein targeting mechanism.

Before a cell divides, it creates a copy of its genetic material (DNA) and evenly distributes it between the new ‘daughter’ cells with the help of a complex called the mitotic spindle. This complex is made of long cable-like protein chains called microtubules.

To ensure that each daughter cell receives an equal amount of DNA, structures known as centrosomes organize the microtubules during the division process. Centrosomes have two rigid cores, called centrioles, which are surrounded by a matrix of proteins called the pericentriolar material. It is from this material that the microtubules are organized.

The pericentriolar material is a dynamic structure and changes its size by assembling and disassembling its protein components. The larger the pericentriolar material, the more microtubules can form. Before a cell divides, it rapidly expands in a process called centrosome maturation. A protein called pericentrin initiates the maturation by helping to recruit other proteins to the centrosome. Pericentrin molecules are large, and it takes the cell between 10 and 20 minutes to make each one. Nevertheless, the cell can produce and deliver large quantities of pericentrin to the centrosome in a matter of minutes. We do not yet know how this happens.

To investigate this further, Sepulveda, Antkowiak, Brust-Mascher et al. used advanced microscopy to study zebrafish embryos and human cells grown in the laboratory. The results showed that cells build and transport pericentrin at the same time. Cells use messenger RNA molecules as templates to build proteins. These feed into protein factories called ribosomes, which assemble the building blocks in the correct order. Rather than waiting for the pericentrin production to finish, the cell moves the active factories to the centrosome with the help of a molecular motor called dynein. By the time the pericentrin molecules are completely made by ribosomes, they are already at the centrosome, ready to help with the recruitment of other proteins during centrosome maturation.

These findings improve our understanding of centrosome maturation. The next step is to find out how the cell coordinates this process with the recruitment of other proteins to the centrosome. It is also possible that the cell uses similar processes to deliver other proteins to different parts of the cell.

ATP depletion during mitotic arrest induces mitotic slippage and APC/CCdh1-dependent cyclin B1 degradation

ATP depletion inhibits cell cycle progression, especially during the G1 phase and the G2 to M transition. However, the effect of ATP depletion on mitotic progression remains unclear. We observed that the reduction of ATP after prometaphase by simultaneous treatment with 2-deoxyglucose and NaN₃ did not arrest mitotic progression. Interestingly, ATP depletion during nocodazole-induced prometaphase arrest resulted in mitotic slippage, as indicated by a reduction in mitotic cells, APC/C-dependent degradation of cyclin B1, increased cell attachment, and increased nuclear membrane reassembly. Additionally, cells successfully progressed through the cell cycle after mitotic slippage, as indicated by EdU incorporation and time-lapse imaging. Although degradation of cyclin B during normal mitotic progression is primarily regulated by APC/C^Cdc20, we observed an unexpected decrease in Cdc20 prior to degradation of cyclin B during mitotic slippage. This decrease in Cdc20 was followed by a change in the binding partner preference of APC/C from Cdc20 to Cdh1; consequently, APC/C^Cdh1, but not APC/C^Cdc20, facilitated cyclin B degradation following ATP depletion. Pulse-chase analysis revealed that ATP depletion significantly abrogated global translation, including the translation of Cdc20 and Cdh1. Additionally, the half-life of Cdh1 was much longer than that of Cdc20. These data suggest that ATP depletion during mitotic arrest induces mitotic slippage facilitated by APC/C^Cdh1-dependent cyclin B degradation, which follows a decrease in Cdc20 resulting from reduced global translation and the differences in the half-lives of the Cdc20 and Cdh1 proteins.

An investigation into the effects of cellular energy depletion reveals a potential mechanism by which tumors evade chemotherapy. Adenosine triphosphate (ATP) is the primary energetic currency for many biological processes, and ATP depletion generally stalls the cell cycle that regulates proliferation. However, researchers led by Jae-Ho Lee of South Korea’s Ajou University School of Medicine discovered that ATP-depleted cells can sometimes bypass roadblocks in the cell division process. Before dividing, cells synthesize duplicates of every chromosome, and Lee’s team treated cells with chemotherapy agents that stall cell division by preventing separation of these duplicates. Surprisingly, subsequent ATP depletion allowed these cells to bypass this arrested state and re-enter the cell cycle, albeit with twice as much DNA as normal. Since many cancerous cells experience ATP depletion, this ‘escape hatch’ could help tumors survive treatment.

Merkel cell polyomavirus and Merkel cell carcinoma

Merkel cell polyomavirus (MCPyV) causes the highly aggressive and relatively rare skin cancer known as Merkel cell carcinoma (MCC). MCPyV also causes a lifelong yet relatively innocuous infection and is one of 14 distinct human polyomaviruses species. Although polyomaviruses typically do not cause illness in healthy individuals, several can cause catastrophic diseases in immunocompromised hosts. MCPyV is the only polyomavirus clearly associated with human cancer. How MCPyV causes MCC and what oncogenic events must transpire to enable this virus to cause MCC is the focus of this essay.This article is part of the themed issue 'Human oncogenic viruses'.

Meeting the meiotic challenge: Specializations in mammalian oocyte spindle formation

Oocytes uniquely accumulate cytoplasmic constituents to support early embryogenesis. This unique specialization is accompanied by acquisition of a large size and by execution of asymmetric meiotic divisions that preserve precious ooplasm through the expulsion of minimal size polar bodies. While often taken for granted, these basic features of oogenesis necessitate unique specializations of the meiotic apparatus. These include a chromatin-sourced RanGTP gradient that restricts spindle size by defining a spatial domain where meiotic spindles form, acentriolar centrosomes that rely on microtubule organizing centers to form spindle poles, and an actin-based mechanism for asymmetric spindle positioning. Additionally, localized protein synthesis to support spindle formation is achieved in the spindle forming region, whilst protein synthesis is reduced elsewhere in the ooplasm. This is achieved through enrichment of spindle-related mRNAs in the spindle forming region combined with local PLK1-mediated phosphorylation and inactivation of the translational repressor EIF4EBP1. This allows PLK1 to function as an important regulatory nexus through which endogenous and exogenous signals can impact spindle formation and function, and highlights the important role that PLK1 may have in maintaining oocyte quality and fertility.

Osteopontin contributes to effective neutrophil recruitment, IL-1β production and apoptosis in Aspergillus fumigatus keratitis

Fungal keratitis is a major cause of corneal ulcers, resulting in significant visual impairment and blindness. A phosphorylated glycoprotein secreted by immunocompetent cells, osteopontin (OPN) mediates cluster formation of the host fungal receptors and enhances the phagocytosis and clearance of pathogenic fungi. However, whether OPN production and function occurs in fungal keratitis is unknown. OPN expression in Aspergillus fumigatus keratitis patient corneas was assessed by quantitative polymerase chain reaction (qRT-PCR) and immunofluorescence. Human neutrophils, THP-1 macrophages and corneal epithelial cells (HCECs) stimulated with A. fumigatus were utilized for in vitro experiments. Mouse models of A. fumigatus keratitis were developed by intrastromal injection for in vivo experiments. Using siRNAs, neutralizing antibodies, recombinant proteins and inhibitors, the production and role of OPN in A. fumigatus infection was assessed by clinical evaluation, qRT-PCR, immunofluorescence, western blotting and bioluminescence image acquisition. We observed increased corneal OPN expression in A. fumigatus keratitis patients and mouse models compared to controls. OPN production in response to A. fumigatus infection was dependent on LOX-1 and Erk1/2. Compared to controls, OPN knockdown impaired proinflammatory cytokine IL-1β production, which was dependent on 4E-BP1. OPN knockdown decreased myeloperoxidase levels, and resulted in decreased neutrophil recruitment, higher fungal load and increased apoptosis in mouse A. fumigatus keratitis. Our results indicate that OPN is a critical component of the antifungal immune response and is essential for effective neutrophil recruitment, inflammatory cytokine production and apoptosis in A. fumigatus keratitis.

PLK1 regulates spindle association of phosphorylated eukaryotic translation initiation factor 4E-binding protein and spindle function in mouse oocytes

Oocyte meiotic spindles are associated with spindle-enriched mRNAs, phosphorylated ribosome protein S6, and phosphorylated variants of the key translational regulator, eukaryotic translation initiation factor 4E-binding protein 1 (eIF4E-BP1), consistent with translational control of localized mRNAs by eIF4E-BP1 in facilitating spindle formation and stability. Using specific kinase inhibitors, we determined which kinases regulate phosphorylation status of eIF4E-BP1 associated with meiotic spindles in mouse oocytes and effects of kinase inhibition on chromosome congression and spindle formation. Neither ataxia telangiectasia-mutated kinase nor mechanistic target of rapamycin inhibition significantly affected phosphorylation status of spindle-associated eIF4E-BP1 at the phosphorylation sites examined. Spindle-associated phospho-eIF4E-BP1, spindle formation, and chromosome congression were strongly disrupted by polo-like kinase I (PLK1) inhibition at both metaphase I (MI) and MII. In addition, direct inhibition of eIF4E-BP1 via 4EGI led to spindle defects at MI, indicating a direct role for eIF4E-BP1 phosphorylation in meiotic spindle formation. PLK1 also regulated microtubule dynamics throughout the ooplasm, indicating likely coordination between spindle dynamics and broader ooplasm cytoskeletal dynamics. Because diverse upstream signaling pathways converge on PLK1, these results implicate PLK1 as a major regulatory nexus coupling endogenous and exogenous signals via eIF4E-BP1 to the regulation of spindle formation and stability.

Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a rare but highly aggressive skin cancer with neuroendocrine features. MCC pathogenesis is associated with either the presence of Merkel cell polyomavirus or chronic exposure to ultraviolet light (UV), which can cause a characteristic pattern of multiple DNA mutations. Notably, in the Northern hemisphere, the majority of MCC cases are of viral aetiology; by contrast, in areas with high UV exposure, UV-mediated carcinogenesis is predominant. The two aetiologies share similar clinical, histopathological and prognostic characteristics. MCC presents with a solitary cutaneous or subcutaneous nodule, most frequently in sun-exposed areas. In fact, UV exposure is probably involved in both viral-mediated and non-viral-mediated carcinogenesis, by contributing to immunosuppression or DNA damage, respectively. Confirmation of diagnosis relies on analyses of histological features and immunological marker expression profiles of the lesion. At primary diagnosis, loco-regional metastases are already present in ∼30% of patients. Excision of the tumour is the first-line therapy; if not feasible, radiotherapy can often effectively control the disease. Chemotherapy was the only alternative in advanced-stage or refractory MCC until several clinical trials demonstrated the efficacy of immune-checkpoint inhibitors.

Discovery of a class of diheteroaromatic amines as orally bioavailable CDK1/4/6 inhibitors

The discovery of a class of diheteroaromatic amines based on LY2835219 as cyclin-dependent kinase (CDK1/4/6) inhibitors was described. The series was found to have much more improved CDK1 inhibition and potent in vitro anti-proliferative effects against cancer cell lines. The synthesis and structure-activity relationship studies of these compounds were reported. One promising compound was selected to evaluate as a novel lead compound after in vitro and in vivo profiling.

Merkel cell polyomavirus recruits MYCL to the EP400 complex to promote oncogenesis

Merkel cell carcinoma (MCC) frequently contains integrated copies of Merkel cell polyomavirus DNA that express a truncated form of Large T antigen (LT) and an intact Small T antigen (ST). While LT binds RB and inactivates its tumor suppressor function, it is less clear how ST contributes to MCC tumorigenesis. Here we show that ST binds specifically to the MYC homolog MYCL (L-MYC) and recruits it to the 15-component EP400 histone acetyltransferase and chromatin remodeling complex. We performed a large-scale immunoprecipitation for ST and identified co-precipitating proteins by mass spectrometry. In addition to protein phosphatase 2A (PP2A) subunits, we identified MYCL and its heterodimeric partner MAX plus the EP400 complex. Immunoprecipitation for MAX and EP400 complex components confirmed their association with ST. We determined that the ST-MYCL-EP400 complex binds together to specific gene promoters and activates their expression by integrating chromatin immunoprecipitation with sequencing (ChIP-seq) and RNA-seq. MYCL and EP400 were required for maintenance of cell viability and cooperated with ST to promote gene expression in MCC cell lines. A genome-wide CRISPR-Cas9 screen confirmed the requirement for MYCL and EP400 in MCPyV-positive MCC cell lines. We demonstrate that ST can activate gene expression in a EP400 and MYCL dependent manner and this activity contributes to cellular transformation and generation of induced pluripotent stem cells.

Quantitative phosphoproteomic analysis reveals system-wide signaling pathways regulated by site-specific phosphorylation of Keratin-8 in skin squamous cell carcinoma derived cell line

Keratin 8/18, a simple epithelia specific keratin pair, is often aberrantly expressed in squamous cell carcinomas (SCC) where its expression is correlated with increased invasion and poor prognosis. Majority of Keratin 8 (K8) functions are governed by its phosphorylation at Serine⁷³ (head-domain) and Serine⁴³¹ (tail-domain) residues. Although, deregulation of K8 phosphorylation is associated with progression of different carcinomas, its role in skin-SCC and the underlying mechanism is obscure. In this direction, we performed tandem mass tag-based quantitative phosphoproteomics by expressing K8 wild type, phosphodead, and phosphomimetic mutants in K8-deficient A431 cells. Further analysis of our phosphoproteomics data showed a significant proportion of total phosphoproteome associated with migratory, proliferative, and invasive potential of these cells to be differentially phosphorylated. Differential phosphorylation of CDK1^T14,Y15 , EIF4EBP1^T46,T50 , EIF4B^S422 , AKT1S1^T246,S247 , CTTN1^{T401,S405,Y421} , and CAP1^S307/309 in K8-S73A/D mutant and CTTN1^{T401,S405,Y421} , BUB1B^S1043 , and CARHSP1^S30,S32 in K8-S431A/D mutants as well as some anonymous phosphosites including MYC^S176 , ZYX^S344 , and PNN^S692 could be potential candidates associated with K8 phosphorylation mediated tumorigenicity. Biochemical validation followed by phenotypic analysis further confirmed our quantitative phosphoproteomics data. In conclusion, our study provides the first global picture of K8 site-specific phosphorylation function in neoplastic progression of A431 cells and suggests various potential starting points for further mechanistic studies.

Merkel cell polyomavirus small T antigen induces genome instability by E3 ubiquitin ligase targeting

The formation of a bipolar mitotic spindle is an essential process for the equal segregation of duplicated DNA into two daughter cells during mitosis. As a result of deregulated cellular signaling pathways, cancer cells often suffer a loss of genome integrity that might etiologically contribute to carcinogenesis. Merkel cell polyomavirus (MCV) small T (sT) oncoprotein induces centrosome overduplication, aneuploidy, chromosome breakage and the formation of micronuclei by targeting cellular ligases through a sT domain that also inhibits MCV large T oncoprotein turnover. These results provide important insight as to how centrosome number and chromosomal stability can be affected by the E3 ligase targeting capacity of viral oncoproteins such as MCV sT, which may contribute to Merkel cell carcinogenesis.

Regulation of Poly(A) Tail and Translation during the Somatic Cell Cycle

Poly(A) tails are critical for mRNA stability and translation. However, recent studies have challenged this view, showing that poly(A) tail length and translation efficiency are decoupled in non-embryonic cells. Using TAIL-seq and ribosome profiling, we investigate poly(A) tail dynamics and translational control in the somatic cell cycle. We find dramatic changes in poly(A) tail lengths of cell-cycle regulatory genes like CDK1, TOP2A, and FBXO5, explaining their translational repression in M phase. We also find that poly(A) tail length is coupled to translation when the poly(A) tail is <20 nucleotides. However, as most genes have >20 nucleotide poly(A) tails, their translation is regulated mainly via poly(A) tail length-independent mechanisms during the cell cycle. Specifically, we find that terminal oligopyrimidine (TOP) tract-containing transcripts escape global translational suppression in M phase and are actively translated. Our quantitative and comprehensive data provide a revised view of translational control in the somatic cell cycle.

Regulation of 4E-BP1 activity in the mammalian oocyte

Fully grown mammalian oocytes utilize transcripts synthetized and stored during earlier development. RNA localization followed by a local translation is a mechanism responsible for the regulation of spatial and temporal gene expression. Here we show that the mouse oocyte contains 3 forms of cap-dependent translational repressor expressed on the mRNA level: 4E-BP1, 4E-BP2 and 4E-BP3. However, only 4E-BP1 is present as a protein in oocytes, it becomes inactivated by phosphorylation after nuclear envelope breakdown and as such it promotes cap-dependent translation after NEBD. Phosphorylation of 4E-BP1 can be seen in the oocytes after resumption of meiosis but it is not detected in the surrounding cumulus cells, indicating that 4E-BP1 promotes translation at a specific cell cycle stage. Our immunofluorescence analyses of 4E-BP1 in oocytes during meiosis I showed an even localization of global 4E-BP1, as well as of its 4E-BP1 (Thr37/46) phosphorylated form. On the other hand, 4E-BP1 phosphorylated on Ser65 is localized at the spindle poles, and 4E-BP1 phosphorylated on Thr70 localizes on the spindle. We further show that the main positive regulators of 4E-BP1 phosphorylation after NEBD are mTOR and CDK1 kinases, but not PLK1 kinase. CDK1 exerts its activity toward 4E-BP1 phosphorylation via phosphorylation and activation of mTOR. Moreover, both CDK1 and phosphorylated mTOR co-localize with 4E-BP1 phosphorylated on Thr70 on the spindle at the onset of meiotic resumption. Expression of the dominant negative 4E-BP1 mutant adversely affects translation and results in spindle abnormality. Taken together, our results show that the phosphorylation of 4E-BP1 promotes translation at the onset of meiosis to support the spindle assembly and suggest an important role of CDK1 and mTOR kinases in this process. We also show that the mTOR regulatory pathway is present in human oocytes and is likely to function in a similar way as in mouse oocytes.

Protein-mediated viral latency is a novel mechanism for Merkel cell polyomavirus persistence

Viral latency, in which a virus genome does not replicate independently of the host cell genome and produces no infectious particles, is required for long-term virus persistence. There is no known latency mechanism for chronic small DNA virus infections. Merkel cell polyomavirus (MCV) causes an aggressive skin cancer after prolonged infection and requires an active large T (LT) phosphoprotein helicase to replicate. We show that evolutionarily conserved MCV LT phosphorylation sites are constitutively recognized by cellular Fbw7, βTrCP, and Skp2 Skp-F-box-cullin (SCF) E3 ubiquitin ligases, which degrade and suppress steady-state LT protein levels. Knockdown of each of these E3 ligases enhances LT stability and promotes MCV genome replication. Mutations at two of these phosphoreceptor sites [serine (S)220 and S239] in the full viral genome increase LT levels and promote MCV virion production and transmission, which can be neutralized with anti-capsid antibody. Virus activation is not mediated by viral gene transactivation, given that these mutations do not increase late gene transcription in the absence of genome replication. Mechanistic target of rapamycin inhibition by either nutrient starvation or use of an active site inhibitor reduces Skp2 levels and stabilizes LT, leading to enhanced MCV replication and transmission. MCV can sense stresses in its intracellular environment, such as nutrient loss, through SCF E3 ligase activities, and responds by initiating active viral transmission. Protein-mediated viral latency through cellular SCF E3 ligase targeting of viral replication proteins is a unique form of latency that may promote chronic viral persistence for some small DNA and RNA viruses.

A researcher's guide to the galaxy of IRESs

The idea of internal initiation is frequently exploited to explain the peculiar translation properties or unusual features of some eukaryotic mRNAs. In this review, we summarize the methods and arguments most commonly used to address cases of translation governed by internal ribosome entry sites (IRESs). Frequent mistakes are revealed. We explain why "cap-independent" does not readily mean "IRES-dependent" and why the presence of a long and highly structured 5' untranslated region (5'UTR) or translation under stress conditions cannot be regarded as an argument for appealing to internal initiation. We carefully describe the known pitfalls and limitations of the bicistronic assay and artefacts of some commercially available in vitro translation systems. We explain why plasmid DNA transfection should not be used in IRES studies and which control experiments are unavoidable if someone decides to use it anyway. Finally, we propose a workflow for the validation of IRES activity, including fast and simple experiments based on a single genetic construct with a sequence of interest.

Overexpression of TNKS1BP1 in lung cancers and its involvement in homologous recombination pathway of DNA double-strand breaks

TNKS1BP1 is a member of the poly(ADP‐ribose) polymerase (PARP) superfamily. Our previous studies have demonstrated that TNKS1BP1 plays an important role in DNA damage response. But whether and how TNKS1BP1 associates with cancer is still not clear. Here, we found that TNKS1BP1 was upregulated in human lung adenocarcinoma (LAC) tissues, and was associated with poor overall survival (OS) in LAC patients. Dysregulation of TNKS1BP1 affected the sensitivity of A549 cells to several DNA damage agents including cisplatin, bleomycin, and ionizing radiation. Mechanically, overexpression of TNKS1BP1 increased the accumulation of S phase cells, which was accompanied by a decrease in M phase cells. More importantly, we found TNKS1BP1 regulated genome stability, mainly through affecting the homologous recombination pathway of DNA double‐strand breaks by inhibiting the RAD51 foci formation. Overall, our study indicates that, in LAC, aberrant expressions of TNKS1BP1 are common events, and overexpression of TNKS1BP1 might affect outcomes of cancer patients to chemotherapy and radiotherapy.

Stabilization of 4E-BP1 by PI3K kinase and its involvement in CHK2 phosphorylation in the cellular response to radiation

Objectives: 4E-BP1 is a family member of eIF4E binding proteins (4E-BPs) which act as the suppressors of cap-dependent translation of RNA via competitively associating with cap-bound eIF4E. RNA translation regulation is an important manner to control the cellular responses to a series of stress conditions such as ionizing radiation (IR)-induced DNA damage response and cell cycle controlling. This study aimed to determine the mechanism of 4E-BP1 stabilization and its potential downstream target(s) in the response to IR. Methods: PI3Ks kinase inhibitors were used to determine the signaling control of 4E-BP1 phosphorylation and protein stability. shRNA strategy was employed to silence the expression of 4E-BP1 in HeLa and HepG2 cells, and determine its effect on the irradiation-induced CHK2 phosphorylation. The protein degradation/stability was investigated by western blotting on the condition of blocking novel protein synthesis by cycloheximide (CHX). Results: The phosphorylation of 4E-BP1 at Thr37/46 was significantly increased in both HepG2 and HeLa cells by ionizing radiation. Depression of 4E-BP1 by shRNA strategy resulted in an incomplete G2 arrest at the early stage of 2 hours post-irradiation, as well as a higher accumulation of mitotic cells at 10 and 12 hours post-irradiation as compared to the control cells. Consistently, the CHK2 phosphorylation at Thr68 induced by IR was also attenuated by silencing 4E-BP1 expression. Both PI3K and DNA-PKcs kinase inhibitors significantly decreased the protein level of 4E-BP1, which was associated with the accelerated degradation mediated by ubiquitination-proteasome pathway. Conclusion: PI3K kinase activity is necessary for maintaining 4E-BP1 stability. Our results also suggest 4E-BP1 a novel biological role of regulating cell cycle G2 checkpoint in responding to IR stress in association with controlling CHK2 phosphorylation.

Translational Regulation in the Mammalian Oocyte

Fully grown oocytes arrest meiosis at prophase I and deposit maternal RNAs. A subset of maternal transcripts is stored in a dormant state in the oocyte, and the timely driven translation of specific mRNAs guides meiotic progression, the oocyte-embryo transition, and early embryo development. In the absence of transcription, the regulation of gene expression in oocytes is controlled almost exclusively at the level of transcriptome and proteome stabilization and at the level of protein synthesis.This chapter focuses on the recent findings on RNA distribution related to the temporal and spatial translational control of the meiotic cycle progression in mammalian oocytes. We discuss the most relevant mechanisms involved in the organization of the oocyte's maternal transcriptome storage and localization, and the regulation of translation, in correlation with the regulation of oocyte meiotic progression.

Insights into the mechanisms of eukaryotic translation gained with ribosome profiling

The development of Ribosome Profiling (RiboSeq) has revolutionized functional genomics. RiboSeq is based on capturing and sequencing of the mRNA fragments enclosed within the translating ribosome and it thereby provides a ‘snapshot’ of ribosome positions at the transcriptome wide level. Although the method is predominantly used for analysis of differential gene expression and discovery of novel translated ORFs, the RiboSeq data can also be a rich source of information about molecular mechanisms of polypeptide synthesis and translational control. This review will focus on how recent findings made with RiboSeq have revealed important details of the molecular mechanisms of translation in eukaryotes. These include mRNA translation sensitivity to drugs affecting translation initiation and elongation, the roles of upstream ORFs in response to stress, the dynamics of elongation and termination as well as details of intrinsic ribosome behavior on the mRNA after translation termination. As the RiboSeq method is still at a relatively early stage we will also discuss the implications of RiboSeq artifacts on data interpretation.

Merkel Cell Polyomavirus Small T Antigen Promotes Pro-Glycolytic Metabolic Perturbations Required for Transformation

Merkel cell polyomavirus (MCPyV) is an etiological agent of Merkel cell carcinoma (MCC), a highly aggressive skin cancer. The MCPyV small tumor antigen (ST) is required for maintenance of MCC and can transform normal cells. To gain insight into cellular perturbations induced by MCPyV ST, we performed transcriptome analysis of normal human fibroblasts with inducible expression of ST. MCPyV ST dynamically alters the cellular transcriptome with increased levels of glycolytic genes, including the monocarboxylate lactate transporter SLC16A1 (MCT1). Extracellular flux analysis revealed increased lactate export reflecting elevated aerobic glycolysis in ST expressing cells. Inhibition of MCT1 activity suppressed the growth of MCC cell lines and impaired MCPyV-dependent transformation of IMR90 cells. Both NF-κB and MYC have been shown to regulate MCT1 expression. While MYC was required for MCT1 induction, MCPyV-induced MCT1 levels decreased following knockdown of the NF-κB subunit RelA, supporting a synergistic activity between MCPyV and MYC in regulating MCT1 levels. Several MCC lines had high levels of MYCL and MYCN but not MYC. Increased levels of MYCL was more effective than MYC or MYCN in increasing extracellular acidification in MCC cells. Our results demonstrate the effects of MCPyV ST on the cellular transcriptome and reveal that transformation is dependent, at least in part, on elevated aerobic glycolysis.

In 2008, Merkel cell polyomavirus (MCPyV) was identified as clonally integrated in a majority of Merkel cell carcinomas (MCC), a rare but highly aggressive neuroendocrine carcinoma of the skin. Since then, studies have highlighted the roles of the MCPyV T antigens in promoting and sustaining MCC oncogenesis. In particular, MCPyV small T antigen (ST) has oncogenic activity in vivo and in vitro. We performed transcriptome analysis of normal human fibroblasts with inducible expression of MCPyV ST and observed significant alterations in levels of metabolic pathway genes, particularly those involved in glycolysis. MCT1, a major monocarboxylate transporter, was rapidly induced following ST expression and inhibition of MCT1 activity reduced the ST growth promoting and transforming activities. The metabolic perturbations induced by this oncogenic human polyomavirus reflect a potent transforming mechanism of MCPyV ST.

Mitotic protein kinase CDK1 phosphorylation of mRNA translation regulator 4E-BP1 Ser83 may contribute to cell transformation

Mammalian target of rapamycin (mTOR)-directed eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1) phosphorylation promotes cap-dependent translation and tumorigenesis. During mitosis, cyclin-dependent kinase 1 (CDK1) substitutes for mTOR and fully phosphorylates 4E-BP1 at canonical sites (T37, T46, S65, and T70) and the noncanonical S83 site, resulting in a mitosis-specific hyperphosphorylated δ isoform. Colocalization studies with a phospho-S83 specific antibody indicate that 4E-BP1 S83 phosphorylation accumulates at centrosomes during prophase, peaks at metaphase, and decreases through telophase. Although S83 phosphorylation of 4E-BP1 does not affect general cap-dependent translation, expression of an alanine substitution mutant 4E-BP1.S83A partially reverses rodent cell transformation induced by Merkel cell polyomavirus small T antigen viral oncoprotein. In contrast to inhibitory mTOR 4E-BP1 phosphorylation, these findings suggest that mitotic CDK1-directed phosphorylation of δ-4E-BP1 may yield a gain of function, distinct from translation regulation, that may be important in tumorigenesis and mitotic centrosome function.

Merkel cell polyomavirus T antigens promote cell proliferation and inflammatory cytokine gene expression

Merkel cell polyomavirus (MCV) is clonally integrated in over 80 % of Merkel cell carcinomas and mediates tumour development through the expression of viral oncoproteins, the large T (LT) and small T antigens (sT). Viral integration is associated with signature mutations in the T-antigen locus that result in deletions of C-terminal replicative functions of the LT antigen. Despite these truncations, the LT LXCXE retinoblastoma (Rb) pocket protein family binding domain is retained, and the entire sT isoform is maintained intact. To investigate the ability of MCV oncoproteins to regulate host gene expression, we performed microarray analysis on cells stably expressing tumour-derived LT, tumour-derived LT along with sT, and tumour-derived LT with a mutated Rb interaction domain. Gene expression alterations in the presence of tumour-derived LT could be classified into three main groups: genes that are involved in the cell cycle (specifically the G1/S transition), genes involved in DNA replication and genes involved in cellular movement. The LXCXE mutant LT largely reversed gene expression alterations detected with the WT tumour-derived LT, while co-expression of sT did not significantly affect these patterns of gene expression. LXCXE-dependent upregulation of cyclin E and CDK2 correlated with increased proliferation in tumour-derived LT-expressing cells. Tumour-derived LT and tumour-derived LT plus sT increased expression of multiple cytokines and chemokines, which resulted in elevated levels of secreted IL-8. We concluded that, in human fibroblasts, the LXCXE motif of tumour-derived LT enhances cellular proliferation and upregulates cell cycle and immune signalling gene transcription.

Cell type-specific control of protein synthesis and proliferation by FGF-dependent signaling to the translation repressor 4E-BP

Regulation of protein synthesis plays a vital role in posttranscriptional modulation of gene expression. Translational control most commonly targets the initiation of protein synthesis: loading 40S ribosome complexes onto mRNA and AUG start codon recognition. This step is initiated by eukaryotic initiation factor 4E (eIF4E) (the m7GTP cap-binding protein), whose binding to eIF4G (a scaffolding subunit) and eIF4A (an ATP-dependent RNA helicase) leads to assembly of active eIF4F complex. The ability of eIF4E to recognize the cap is prevented by its binding to eIF4E binding protein (4E-BP), which thereby inhibits cap-dependent translation by sequestering eIF4E. The 4E-BP activity is, in turn, inhibited by mTORC1 [mTOR (the mechanistic target of rapamycin) complex 1] mediated phosphorylation. Here, we define a previously unidentified mechanism of mTOR-independent 4E-BP1 regulation that is used by chondrocytes upon FGF signaling. Chondrocytes are responsible for the formation of the skeleton long bones. Unlike the majority of cell types where FGF signaling triggers proliferation, chondrocytes respond to FGF with inhibition. We establish that FGF specifically suppresses protein synthesis in chondrocytes, but not in any other cells of mesenchymal origin. Furthermore, 4E-BP1 repressor activity is necessary not only for suppression of protein synthesis, but also for FGF-induced cell-cycle arrest. Importantly, FGF-induced changes in the 4E-BP1 activity observed in cell culture are likewise detected in vivo and reflect the action of FGF signaling on downstream targets during bone development. Thus, our findings demonstrate that FGF signaling differentially impacts protein synthesis through either stimulation or repression, in a cell-type-dependent manner, with 4E-BP1 being a key player.

Adapting the Stress Response: Viral Subversion of the mTOR Signaling Pathway

The mammalian target of rapamycin (mTOR) is a central regulator of gene expression, translation and various metabolic processes. Multiple extracellular (growth factors) and intracellular (energy status) molecular signals as well as a variety of stressors are integrated into the mTOR pathway. Viral infection is a significant stress that can activate, reduce or even suppress the mTOR signaling pathway. Consequently, viruses have evolved a plethora of different mechanisms to attack and co-opt the mTOR pathway in order to make the host cell a hospitable environment for replication. A more comprehensive knowledge of different viral interactions may provide fruitful targets for new antiviral drugs.

Merkel Cell Polyomavirus Small T Antigen Induces Cancer and Embryonic Merkel Cell Proliferation in a Transgenic Mouse Model

Merkel cell polyomavirus (MCV) causes the majority of human Merkel cell carcinomas (MCC) and encodes a small T (sT) antigen that transforms immortalized rodent fibroblasts in vitro. To develop a mouse model for MCV sT-induced carcinogenesis, we generated transgenic mice with a flox-stop-flox MCV sT sequence homologously recombined at the ROSA locus (ROSAsT), allowing Cre-mediated, conditional MCV sT expression. Standard tamoxifen (TMX) administration to adult UbcCreERT2; ROSAsT mice, in which Cre is ubiquitously expressed, resulted in MCV sT expression in multiple organs that was uniformly lethal within 5 days. Conversely, most adult UbcCreERT2; ROSAsT mice survived low-dose tamoxifen administration but developed ear lobe dermal hyperkeratosis and hypergranulosis. Simultaneous MCV sT expression and conditional homozygous p53 deletion generated multi-focal, poorly-differentiated, highly anaplastic tumors in the spleens and livers of mice after 60 days of TMX treatment. Mouse embryonic fibroblasts from these mice induced to express MCV sT exhibited anchorage-independent cell growth. To examine Merkel cell pathology, MCV sT expression was also induced during mid-embryogenesis in Merkel cells of Atoh1CreERT2/+; ROSAsT mice, which lead to significantly increased Merkel cell numbers in touch domes at late embryonic ages that normalized postnatally. Tamoxifen administration to adult Atoh1CreERT2/+; ROSAsT and Atoh1CreERT2/+; ROSAsT; p53flox/flox mice had no effects on Merkel cell numbers and did not induce tumor formation. Taken together, these results show that MCV sT stimulates progenitor Merkel cell proliferation in embryonic mice and is a bona fide viral oncoprotein that induces full cancer cell transformation in the p53-null setting.

Regulation of mRNA translation during mitosis

Passage through mitosis is driven by precisely-timed changes in transcriptional regulation and protein degradation. However, the importance of translational regulation during mitosis remains poorly understood. Here, using ribosome profiling, we find both a global translational repression and identified ∼200 mRNAs that undergo specific translational regulation at mitotic entry. In contrast, few changes in mRNA abundance are observed, indicating that regulation of translation is the primary mechanism of modulating protein expression during mitosis. Interestingly, 91% of the mRNAs that undergo gene-specific regulation in mitosis are translationally repressed, rather than activated. One of the most pronounced translationally-repressed genes is Emi1, an inhibitor of the anaphase promoting complex (APC) which is degraded during mitosis. We show that full APC activation requires translational repression of Emi1 in addition to its degradation. These results identify gene-specific translational repression as a means of controlling the mitotic proteome, which may complement post-translational mechanisms for inactivating protein function.

DOI:http://dx.doi.org/10.7554/eLife.07957.001

The human body contains billions of cells, most of which formed via a process called mitosis in which a single cell divides to produce two new daughter cells. Actively dividing cells pass through a series of events (or phases) that are collectively known as the cell cycle. These phases allow the cell to grow in size, copy its genetic material, and then make preparations for cell division before taking the final decision to divide.

Many proteins are involved in regulating the cell cycle and each protein has a particular role in specific phases. The levels of these proteins in cells may change during the cycle, which is often crucial to allow the cell to progress to the next phase. For example, cells need a group of proteins called the anaphase-promoting complex (or APC for short) to destroy other specific proteins at the end of mitosis.

Another way in which the amount of protein in a cell can be adjusted is by controlling how much new protein is made during a process known as translation. During this process, a molecule called a messenger RNA (mRNA)—which contains information copied from a particular gene—is used as a template to assemble a new protein. However, it is not clear whether regulation of translation is involved in control of the cell division.

Tanenbaum et al. now address this question using a technique called ribosome profiling to measure the translation of individual mRNA molecules. The experiments analysed the changes in protein production before, during and after mitosis. The overall level of translation of all the mRNAs was about 35% lower during mitosis. However, some mRNAs in particular experienced a very large reduction in the level of translation (between three- and ten-fold less than the levels before mitosis).

One example of an mRNA whose translation is turned off in mitosis is the mRNA that makes a protein called Emi1. It is known from previous work that Emi1 inhibits the activity of the APC. Therefore, Emi1 needs to be inactivated in mitosis so that the APC can become active and promote progression to the next phase of the cell cycle. It was previously shown that Emi1 is destroyed during mitosis to allow the APC to operate. Tanenbaum et al. found that translation of the Emi1 mRNA must also be suppressed during mitosis in order to keep Emi1 protein levels very low and allow the APC to become fully active. These findings uncover a new role for the control of protein production in regulating the cell cycle. The next challenge will be to find out whether suppression of translation is also used in other biological systems where proteins need to be rapidly inactivated.

DOI:http://dx.doi.org/10.7554/eLife.07957.002