Expression of the candidate MCT-1 oncogene in B- and T-cell lymphoid malignancies

To initiate or not to initiate: A critical assessment of eIF2A, eIF2D, and MCT-1·DENR to deliver initiator tRNA to ribosomes

Selection of the correct start codon is critical for high-fidelity protein synthesis. In eukaryotes, this is typically governed by a multitude of initiation factors (eIFs), including eIF2·GTP that directly delivers the initiator tRNA (Met-tRNAi Met ) to the P site of the ribosome. However, numerous reports, some dating back to the early 1970s, have described other initiation factors having high affinity for the initiator tRNA and the ability of delivering it to the ribosome, which has provided a foundation for further work demonstrating non-canonical initiation mechanisms using alternative initiation factors. Here we provide a critical analysis of current understanding of eIF2A, eIF2D, and the MCT-1·DENR dimer, the evidence surrounding their ability to initiate translation, their implications in human disease, and lay out important key questions for the field. This article is categorized under: RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes Translation > Mechanisms Translation > Regulation.

Cyclin B/CDK1 and Cyclin A/CDK2 phosphorylate DENR to promote mitotic protein translation and faithful cell division

DENR and MCTS1 have been identified as oncogenes in several different tumor entities. The heterodimeric DENR·MCTS1 protein complex promotes translation of mRNAs containing upstream Open Reading Frames (uORFs). We show here that DENR is phosphorylated on Serine 73 by Cyclin B/CDK1 and Cyclin A/CDK2 at the onset of mitosis, and then dephosphorylated as cells exit mitosis. Phosphorylation of Ser73 promotes mitotic stability of DENR protein and prevents its cleavage at Asp26. This leads to enhanced translation of mRNAs involved in mitosis. Indeed, we find that roughly 40% of all mRNAs with elevated translation in mitosis are DENR targets. In the absence of DENR or of Ser73 phosphorylation, cells display elevated levels of aberrant mitoses and cell death. This provides a mechanism how the cell cycle regulates translation of a subset of mitotically relevant mRNAs during mitosis.

The cell cycle regulates translation during mitosis by controlling DENR stability. Here, the authors show the non-canonical translation initiation complex DENR·MCTS1 is phosphorylated during mitosis by CDK1 and 2, enabling the translation of genes needed for proper mitotic progression.

MCTS1 promotes invasion and metastasis of oral cancer by modifying the EMT process

Background

The oncogene, malignant T-cell-amplified sequence 1 (MCTS1), has been found to be highly expressed in a variety of cancer cell lines. It has been shown to be involved in cell cycle progression and to confer a growth advantage for lymphomas and breast cancer. Nevertheless, the role of MCTS1 in contributing to the development of oral cancer remains elusive.

Methods

We analyzed the gene expression profiles of MCTS1 in normal oral keratinocytes and cancerous cells. Cellular proliferation, invasion, and migration experiments were performed to detect the effect of MCTS1 on the biological evolution of oral cancer. The in vitro results were verified by the in vivo lymphatic metastasis test. The underlying mechanism of MCTS1 in promoting oral cancer invasion and metastasis correlated with the epithelial-mesenchymal transition (EMT) process as revealed by western blotting.

Results

The results showed that MCTS1 was aberrantly expressed in oral cancer cells. MCTS1 overexpression significantly promoted tumor cell growth, proliferation, migration, and invasion. MCTS1-mediated lymphatic metastasis was verified in vivo using an intraplantar tumor model. Biomarkers associated with EMT progression were positively or negatively regulated upon knockdown or overexpression of MCTS1, respectively.

Conclusions

Higher MCTS1 expression in oral cancer may be connected with an unfavorable prognosis due to involvement of MCTS1. MCTS1 potentiates the growth and proliferation of oral cancer cells and subsequent metastasis by regulating cell cycle and modifying the EMT process.

Keywords

Oral cancer; oncogene; malignant T-cell-amplified sequence 1 (MCTS1); metastasis; invasion.

40S ribosome profiling reveals distinct roles for Tma20/Tma22 (MCT-1/DENR) and Tma64 (eIF2D) in 40S subunit recycling

The recycling of ribosomes at stop codons for use in further rounds of translation is critical for efficient protein synthesis. Removal of the 60S subunit is catalyzed by the ATPase Rli1 (ABCE1) while removal of the 40S is thought to require Tma64 (eIF2D), Tma20 (MCT-1), and Tma22 (DENR). However, it remains unclear how these Tma proteins cause 40S removal and control reinitiation of downstream translation. Here we used a 40S ribosome footprinting strategy to directly observe intermediate steps of ribosome recycling in cells. Deletion of the genes encoding these Tma proteins resulted in broad accumulation of unrecycled 40S subunits at stop codons, directly establishing their role in 40S recycling. Furthermore, the Tma20/Tma22 heterodimer was responsible for a majority of 40S recycling events while Tma64 played a minor role. Introduction of an autism-associated mutation into TMA22 resulted in a loss of 40S recycling activity, linking ribosome recycling and neurological disease.

MCTS1 promotes the development of lung adenocarcinoma by regulating E2F1 expression

Lung adenocarcinoma (LUAD) is the most common subtype of lung cancer that results in the majority of cancer-associated mortality. Multiple copies in T-cell lymphoma-1 (MCTS1) is an oncogene that is expressed at high levels in several types of cancer tissues. However, its exact role and pathomechanism in the development of LUAD remains unknown. Reverse transcription-quantitative PCR analysis was performed to detect MCTS1 expression. Immunohistochemistry analysis was performed to detect MCTS1 expression in LUAD tissues and normal tissues. The MTT, colony formation, EdU, flow cytometry, wound healing and Transwell assays were performed to assess the proliferation, apoptosis, migration and invasion of LUAD cells. Western blot analysis was performed to detect protein expression levels. The present study aimed to investigate the effects of MCTS1 on the progression of LUAD and the potential mechanisms underlying its effects. The results demonstrated that MCTS1 expression was upregulated in LUAD tissues and cells, which was associated with an unfavorable outcome in patients with LUAD. MCTS1 knockdown inhibited LUAD progression by suppressing cell viability and motility, and promoting apoptosis. In addition, E2F1 protein expression was attenuated following MCTS1 knockdown. The silencing MCTS1-induced inhibitory effect on LUAD malignancy was reversed following overexpression of E2F1 by modulating the c-Myc signaling pathway. Taken together, the results of the present study suggest that MCTS1 facilitates cell proliferation and migration, and suppresses apoptosis of LUAD cells by regulating E2F1 expression and the c-Myc signaling pathway.

The Role of the MCTS1 and DENR Proteins in Regulating the Mechanisms Associated with Malignant Cell Transformation

The mutations associated with malignant cell transformation are believed to disrupt the expression of a significant number of normal, non-mutant genes. The proteins encoded by these genes are involved in the regulation of many signaling pathways that are responsible for differentiation and proliferation, as well as sensitivity to apoptotic signals, growth factors, and cytokines. Abnormalities in the balance of signaling pathways can lead to the transformation of a normal cell, which results in tumor formation. Detection of the target genes and the proteins they encode and that are involved in the malignant transformation is one of the major evolutions in anti-cancer biomedicine. Currently, there is an accumulation of data that shed light on the role of the MCTS1 and DENR proteins in oncogenesis.

Targeting translation regulators improves cancer therapy

Deregulation of protein synthesis may be involved in multiple aspects of cancer, such as gene expression, signal transduction and drive specific cell biological responses, resulting in promoting cancer growth, invasion and metastasis. Study the molecular mechanisms about translational control may help us to find more effective anti-cancer drugs and develop novel therapeutic opportunities. Recently, the researchers had focused on targeting translational machinery to overcome cancer, and various small molecular inhibitors targeting translation factors or pathways have been tested in clinical trials and exhibited improving outcomes in several cancer types. There is no doubt that an insight into the class of translation regulation protein would provide new target for pharmacologic intervention and further provide opportunities to develop novel anti-tumor therapeutic interventions. In this review, we summarized the developments of translational control in cancer survival and progression et al, and highlighted the therapeutic approach targeted translation regulation to overcome the cancer.

Tma64/eIF2D, Tma20/MCT-1, and Tma22/DENR Recycle Post-termination 40S Subunits In Vivo

The recycling of ribosomal subunits after translation termination is critical for efficient gene expression. Tma64 (eIF2D), Tma20 (MCT-1), and Tma22 (DENR) function as 40S recycling factors in vitro, but it is unknown whether they perform this function in vivo. Ribosome profiling of tma deletion strains revealed 80S ribosomes queued behind the stop codon, consistent with a block in 40S recycling. We found that unrecycled ribosomes could reinitiate translation at AUG codons in the 3' UTR, as evidenced by peaks in the footprint data and 3' UTR reporter analysis. In vitro translation experiments using reporter mRNAs containing upstream open reading frames (uORFs) further established that reinitiation increased in the absence of these proteins. In some cases, 40S ribosomes appeared to rejoin with 60S subunits and undergo an 80S reinitiation process in 3' UTRs. These results support a crucial role for Tma64, Tma20, and Tma22 in recycling 40S ribosomal subunits at stop codons and translation reinitiation.

DENR-MCTS1 heterodimerization and tRNA recruitment are required for translation reinitiation

The succession of molecular events leading to eukaryotic translation reinitiation—whereby ribosomes terminate translation of a short open reading frame (ORF), resume scanning, and then translate a second ORF on the same mRNA—is not well understood. Density-regulated reinitiation and release factor (DENR) and multiple copies in T-cell lymphoma-1 (MCTS1) are implicated in promoting translation reinitiation both in vitro in translation extracts and in vivo. We present here the crystal structure of MCTS1 bound to a fragment of DENR. Based on this structure, we identify and experimentally validate that DENR residues Glu42, Tyr43, and Tyr46 are important for MCTS1 binding and that MCTS1 residue Phe104 is important for tRNA binding. Mutation of these residues reveals that DENR-MCTS1 dimerization and tRNA binding are both necessary for DENR and MCTS1 to promote translation reinitiation in human cells. These findings thereby link individual residues of DENR and MCTS1 to specific molecular functions of the complex. Since DENR–MCTS1 can bind tRNA in the absence of the ribosome, this suggests the DENR–MCTS1 complex could recruit tRNA to the ribosome during reinitiation analogously to the eukaryotic initiation factor 2 (eIF2) complex in cap-dependent translation.

Usually, eukaryotic ribosomes translate only a single open reading frame (ORF) on an mRNA and then dissociate from the mRNA. In some cases, when there is a short upstream open reading frame (uORF) that precedes the main ORF, ribosomes can translate the uORF, terminate translation, and then undergo a poorly understood process called “translation reinitiation” whereby they resume scanning for another AUG initiation codon and then translate the main ORF. The molecular functions required for translation reinitiation are not known. We previously showed that two noncanonical initiation factors, density-regulated reinitiation and release factor (DENR) and multiple copies in T-cell lymphoma-1 (MCTS1), are involved in this process. We show here, based on a structure of MCTS1 bound to a fragment of DENR, that in order to successfully promote translation reinitiation, DENR and MCTS1 need to dimerize, and they need to bind tRNA. We thereby identify two molecular functions needed for translation reinitiation.

PKC inhibition of sotrastaurin has antitumor activity in diffuse large B-cell lymphoma via regulating the expression of MCT-1

MCT-1 (multiple copies in T-cell lymphoma-1), a novel oncogene, was originally identified in T-cell lymphoma. A recent study has demonstrated that MCT-1 is highly expressed in 85% of diffuse large B-cell lymphomas (DLBCL). PKC (protein kinase C) plays an essential role in signal transduction for multiple biologically active substances for activating cellular functions and proliferation. In this study, we found that the mRNA and protein expression levels of MCT-1 were visibly decreased after knocking down PKC by siRNA in SUDHL-4 and OCI-LY8 DLBCL cell lines. A selective PKC inhibitor, sotrastaurin, effectively inhibited cell proliferation and induced cell apoptosis in a dose- and time-dependent manner. Meanwhile, we also observed that the cell cycle was arrested in the G1 phase in sotrastaurin-treated cells. In addition, MCT-1 was down-regulated in the sotrastaurin treatment group in vivo. Furthermore, we demonstrated that the PKC inhibitor sotrastaurin induced cell apoptosis and cell cycle arrest in DLBCL cells potentially through regulating the expression of MCT-1. Our data suggest that targeting PKC may be a potential therapeutic approach for lymphomas and related malignancies that exhibit high levels of MCT-1 protein.

Identification of transcripts with short stuORFs as targets for DENR•MCTS1-dependent translation in human cells

The non-canonical initiation factors DENR and MCTS1 have been linked to cancer and autism. We recently showed in Drosophila that DENR and MCTS1 regulate translation re-initiation on transcripts containing upstream Open Reading Frames (uORFs) with strong Kozak sequences (stuORFs). Due to the medical relevance of DENR and MCTS1, it is worthwhile identifying the transcripts in human cells that depend on DENR and MCTS1 for their translation. We show here that in humans, as in Drosophila, transcripts with short stuORFs require DENR and MCTS1 for their optimal expression. In contrast to Drosophila, however, the dependence on stuORF length in human cells is very strong, so that only transcripts with very short stuORFs coding for 1 amino acid are dependent on DENR and MCTS1. This identifies circa 100 genes as putative DENR and MCTS1 translational targets. These genes are enriched for neuronal genes and G protein-coupled receptors. The identification of DENR and MCTS1 target transcripts will serve as a basis for future studies aimed at understanding the mechanistic involvement of DENR and MCTS1 in cancer and autism.

CD147 promotes reprogramming of glucose metabolism and cell proliferation in HCC cells by inhibiting the p53-dependent signaling pathway

BACKGROUND & AIMS

Cancer cells exhibit the reprogrammed metabolism characterized by high level of glycolysis even in the presence of oxygen. Aerobic glycolysis, known as the Warburg effect, supplies cancer cells with the substrates required for biomass generation. To date, several intracellular signaling mediators have been identified in metabolic regulation of cancer cells. However, it remains largely ambiguous how molecules on the cell surface are involved in regulation of cancer metabolism.

METHODS

In the current study, we established several HCC cell lines differing in their CD147 (a typical transmembrane glycoprotein) expression status by zinc-finger nuclease and RNAi techniques. Then, we systematically investigated the role of CD147 in the regulation of the Warburg effect in HCC cells and explored the underlying mechanism.

RESULTS

We found that CD147 significantly contributed to the reprogramming of glucose metabolism in HCC cells through a p53-dependent way. CD147 facilitated the cell surface expression of MCT1 and lactate export, which led to activation of the PI3K/Akt/MDM2 pathway and thus increased p53 degradation. The gain/loss-of-function studies demonstrated that while CD147 promoted glycolysis, mediated by p53-dependent upregulation of GLUT1 and activation of PFKL, it inhibited mitochondrial biogenesis and functions, mediated by p53-dependent downregulation of PGC1α, TFAM, and p53R2. Additionally, proliferation of HCC cells was suppressed by blocking CD147 and/or MCT1, which resulted in down-regulation of glucose metabolism.

CONCLUSIONS

We demonstrate that CD147 is a crucial regulator of glucose metabolism.

MCT-1 expression and PTEN deficiency synergistically promote neoplastic multinucleation through the Src/p190B signaling activation

Multinucleation is associated with malignant neoplasms; however, the molecular mechanism underlying the nuclear abnormality remains unclear. Loss or mutation of PTEN promotes the development of malignant tumors. We now demonstrate that increased expression of the oncogene MCT-1 (multiple copies in T-cell malignancy 1) antagonizes PTEN gene presentation, PTEN protein stability and PTEN functional activity, thereby further promoting phosphoinositide 3 kinase/AKT signaling, survival rate and malignancies of the PTEN-deficient cells. In the PTEN-null cancer cells, MCT-1 interacts with p190B and Src in vivo, supporting that they are in proximity of the signaling complexes. MCT-1 overexpression and PTEN loss synergistically augments the Src/p190B signaling function that leads to inhibition of RhoA activity. Under such a condition, the incidence of mitotic catastrophes including spindle multipolarity and cytokinesis failure is enhanced, driving an Src/p190B/RhoA-dependent neoplastic multinucleation. Targeting MCT-1 by the short hairpin RNA markedly represses the Src/p190B function, improves nuclear structures and suppresses xenograft tumorigenicity of the PTEN-null breast cancer cells. Consistent with the oncogenic effects in vitro, clinical evidence has confirmed that MCT-1 gene stimulation is correlated with p190B gene promotion and PTEN gene suppression in human breast cancer. Accordingly, MCT-1 gene induction is recognized as a potential biomarker of breast tumor development. Abrogating MCT-1 function may be a promising stratagem for management of breast cancer involving Src hyperactivation and/or PTEN dysfunction.

Targeting MCT-1 oncogene inhibits Shc pathway and xenograft tumorigenicity

Overexpression of Shc adaptor proteins is associated with mitogenesis, carcinogenesis and metastasis. Multiple copies in T-cell malignancy 1 (MCT-1) oncoprotein promotes cell proliferation, survival and tumorigenic effects. Our current data show that MCT-1 is a novel regulator of Shc-Ras-MEK-ERK signaling and MCT-1 is significantly co-activated with Shc gene in human carcinomas. The knockdown of MCT-1 enhances apoptotic cell death accompanied with the activation of caspases and cleavage of caspase substrates under environmental stress. The cancer cell proliferation, chemo-resistance and tumorigenic capacity are proved to be effectively suppressed by targeting MCT-1. Accordingly, an important linkage between MCT-1 oncogenicity and Shc pathway in tumor development has now been established. Promoting MCT-1 expression by gene hyperactivation may be recognized as a tumor marker and MCT-1 may serve as a molecular target of cancer therapy.

The involvement of MCT-1 oncoprotein in inducing mitotic catastrophe and nuclear abnormalities

Centrosome amplification and chromosome abnormality are frequently identified in neoplasia and tumorigenesis. However, the mechanisms underlying these defects remain unclear. We here identify that MCT-1 is a centrosomal oncoprotein involved in mitosis. Knockdown of MCT-1 protein results in intercellular bridging, chromosome mis-congregation, cytokinesis delay, and mitotic death. Introduction of MCT-1 oncogene into the p53 deficient cells (MCT-1-p53), the mitotic checkpoint kinases and proteins are deregulated synergistically. These biochemical alterations are accompanied with increased frequencies of cytokinesis failure, multi-nucleation, and centrosome amplification in subsequent cell cycle. As a result, the incidences of polyploidy and aneuploidy are progressively induced by prolonged cell cultivation or further promoted by sustained spindle damage on MCT-1-p53 background. These data show that the oncoprotein perturbs centrosome structure and mitotic progression, which provide the molecular aspect of chromsomal abnormality in vitro and the information for understanding the stepwise progression of tumors under oncogenic stress.

The novel anti-MEK small molecule AZD6244 induces BIM-dependent and AKT-independent apoptosis in diffuse large B-cell lymphoma

The RAS/RAF/MEK/ERK signaling pathway has been largely unexplored as a potential therapeutic target in lymphoma. The novel 2nd generation anti-MEK small molecule, AZD6244, down-regulated its direct downstream target, phospho-ERK (pERK) in germinal center and nongerminal center diffuse large B-cell lymphoma (DLBCL) cell lines and primary cells. Similar decreased pERK levels were noted despite constitutive activation (CA) of MEK. Consequently, several lymphoma-related ERK substrates were down-regulated by AZD6244 including MCT-1, c-Myc, Bcl-2, Mcl-1, and CDK1/2. AZD6244 induced time- and dose-dependent antiproliferation and apoptosis in all DLBCL cell lines and fresh/primary cells (IC(50) 100nM-300nM). Furthermore, AZD6244 resulted in significantly less tumor compared with control in an in vivo DLBCL SCID xenograft model. Cell death was associated with cleaved PARP, caspases-8, -9, and -3, and apoptosis was caspase-dependent. In addition, there was stabilization of FoxO3a, activation of BIM and PUMA, and a significant decrease in c-Myc transcripts. Moreover, siRNA knockdown of BIM abrogated AZD6244-related apoptosis, while shRNA knockdown of ERK minimally sensitized cells. Finally, manipulation of AKT with transfection of OCI-LY3 cells with CA-AKT or through chemical inhibition (LY294002) had minimal effect on AZD6244-induced cell death. Altogether, these findings show that the novel anti-MEK agent, AZD6244, induced apoptosis in DLBCL and that cell death was BIM-dependent.

Novel disease targets and management approaches for diffuse large B-cell lymphoma

Diffuse large B-cell lymphoma (DLBCL) responds well to treatment with CHOP and the R-CHOP regimen, but a subset of patients still fail to achieve complete or durable responses. Recent advances in gene expression profiling have led to the identification of three different subtypes of DLBCL, and confirmed that patients with the activated B-cell (ABC) disease subtype are less likely to respond well to CHOP-based regimens than those with germinal centre B-cell-type (GCB) disease. This discovery could herald the use of gene expression profiling to aid treatment decisions in DLBCL, and help identify the most effective management strategies for patients. Treatment options for patients with relapsed or refractory DLBCL are limited and several novel agents are being developed to address this unmet clinical need. Novel agents developed to treat plasma cell disorders such as multiple myeloma have shown promising activity in patients with NHL. Indeed, the immunomodulatory agent lenalidomide and the proteasome inhibitors bortezomib and carfilzomib, as single agents or in combination with chemotherapy, have already demonstrated promising activity in patients with the ABC subtype of DLBCL. One should not be complacent however when applying these agents to new disease types, because dose and drug scheduling can have marked effects on the responses achieved with investigational agents. As more targeted agents are developed, the timing of administration with other agents in clinical trials will become increasingly important to ensure maximal efficacy while minimizing side effects.

The antagonism between MCT-1 and p53 affects the tumorigenic outcomes

Background

MCT-1 oncoprotein accelerates p53 protein degradation via a proteosome pathway. Synergistic promotion of the xenograft tumorigenicity has been demonstrated in circumstance of p53 loss alongside MCT-1 overexpression. However, the molecular regulation between MCT-1 and p53 in tumor development remains ambiguous. We speculate that MCT-1 may counteract p53 through the diverse mechanisms that determine the tumorigenic outcomes.

Results

MCT-1 has now identified as a novel target gene of p53 transcriptional regulation. MCT-1 promoter region contains the response elements reactive with wild-type p53 but not mutant p53. Functional p53 suppresses MCT-1 promoter activity and MCT-1 mRNA stability. In a negative feedback regulation, constitutively expressed MCT-1 decreases p53 promoter function and p53 mRNA stability. The apoptotic events are also significantly prevented by oncogenic MCT-1 in a p53-dependent or a p53-independent fashion, according to the genotoxic mechanism. Moreover, oncogenic MCT-1 promotes the tumorigenicity in mice xenografts of p53-null and p53-positive lung cancer cells. In support of the tumor growth are irrepressible by p53 reactivation in vivo, the inhibitors of p53 (MDM2, Pirh2, and Cop1) are constantly stimulated by MCT-1 oncoprotein.

Conclusions

The oppositions between MCT-1 and p53 are firstly confirmed at multistage processes that include transcription control, mRNA metabolism, and protein expression. MCT-1 oncogenicity can overcome p53 function that persistently advances the tumor development.

Targeting the translational machinery as a novel treatment strategy for hematologic malignancies

The dysregulation of protein synthesis evident in the transformed phenotype has opened up a burgeoning field of research in cancer biology. Translation initiation has recently been shown to be a common downstream target of signal transduction pathways deregulated in cancer and initiated by mutated/overexpressed oncogenes and tumor suppressors. The overexpression and/or activation of proteins involved in translation initiation such as eIF4E, mTOR, and eIF4G have been shown to induce a malignant phenotype. Therefore, understanding the mechanisms that control protein synthesis is emerging as an exciting new research area with significant potential for developing innovative therapies. This review highlights molecules that are activated or dysregulated in hematologic malignancies, and promotes the transformed phenotype through the deregulation of protein synthesis. Targeting these proteins with small molecule inhibitors may constitute a novel therapeutic approach in the treatment of cancer.

Extracellular signal-regulated kinase positively regulates the oncogenic activity of MCT-1 in diffuse large B-cell lymphoma

The MCT-1 oncogene was originally identified from lymphoma cell lines. Herein we establish that MCT-1 is highly expressed in 85% of human diffuse large B-cell lymphomas (DLBCL) and that knocking down MCT-1 by a specific short hairpin RNA in DLBCL cells induces apoptosis, supporting a critical role for MCT-1 in DLBCL cell survival. However, the mechanism underlying MCT-1 regulation is largely unknown. We find that MCT-1 is phosphorylated and up-regulated by extracellular signal-regulated kinase (ERK). Furthermore, by using a small inhibitory molecule targeting ERK, we interrupted MCT-1 phosphorylation and stability. Significantly, cells with distinct levels of MCT-1 protein displayed differential sensitivity to ERK inhibitor-induced apoptosis. Treatment with the ERK inhibitor showed marked in vivo antitumor activity in a human DLBCL xenograft model. Our findings establish a functional molecular interaction between MCT-1 and the MEK/ERK signaling pathway and suggest that the activation of MCT-1 function by its upstream kinase ERK plays an important role in lymphomagenesis.

A screen for retrotransposed imprinted genes reveals an association between X chromosome homology and maternal germ-line methylation

Imprinted genes undergo epigenetic modifications during gametogenesis, which lead to transcriptional silencing of either the maternally or the paternally derived allele in the subsequent generation. Previous work has suggested an association between imprinting and the products of retrotransposition, but the nature of this link is not well defined. In the mouse, three imprinted genes have been described that originated by retrotransposition and overlap CpG islands which undergo methylation during oogenesis. Nap1l5, U2af1-rs1, and Inpp5f_v2 are likely to encode proteins and share two additional genetic properties: they are located within introns of host transcripts and are derived from parental genes on the X chromosome. Using these sequence features alone, we identified Mcts2, a novel candidate imprinted retrogene on mouse Chromosome 2. Mcts2 has been validated as imprinted by demonstrating that it is paternally expressed and undergoes promoter methylation during oogenesis. The orthologous human retrogenes NAP1L5, INPP5F_V2, and MCTS2 are also shown to be paternally expressed, thus delineating novel imprinted loci on human Chromosomes 4, 10, and 20. The striking correlation between imprinting and X chromosome provenance suggests that retrotransposed elements with homology to the X chromosome can be selectively targeted for methylation during mammalian oogenesis.

The conventional view is that DNA carries all of our heritable information and our genes control development into adulthood. The discovery of epigenetics, a term coined to describe effects that are not coded for by DNA sequence, but can nonetheless affect our development and well-being, has added another layer of complexity to our understanding of genetics. One class of genes under epigenetic control are imprinted genes. Mammals inherit two copies of every gene, one from mother and one from father, and in most cases, both are active. However, for a small number of imprinted genes in mammals, only one is active, either the maternal or the paternal copy. Epigenetics amounts to a control system for switching genes on and off appropriately. We focus on a group of little-studied imprinted genes that share features that give clues to their evolutionary origins. These so-called “retrogenes” are protein-coding sequences of DNA that have undergone duplication and jumped into novel locations in the genome. Because of this, it is possible to determine where, and roughly when, many of the imprinted retrogenes originated. This provides an opportunity to study the molecular events that have generated imprinted genes during mammalian evolution.

MCT-1 protein interacts with the cap complex and modulates messenger RNA translational profiles

MCT-1 is an oncogene that was initially identified in a human T cell lymphoma and has been shown to induce cell proliferation as well as activate survival-related pathways. MCT-1 contains the PUA domain, a recently described RNA-binding domain that is found in several tRNA and rRNA modification enzymes. Here, we established that MCT-1 protein interacts with the cap complex through its PUA domain and recruits the density-regulated protein (DENR/DRP), containing the SUI1 translation initiation domain. Through the use of microarray analysis on polysome-associated mRNAs, we showed that up-regulation of MCT-1 was able to modulate the translation profiles of BCL2L2, TFDP1, MRE11A, cyclin D1, and E2F1 mRNAs, despite equivalent levels of mRNAs in the cytoplasm. Our data establish a role for MCT-1 in translational regulation, and support a linkage between translational control and oncogenesis.

Phosphorylation of MCT-1 by p44/42 MAPK is required for its stabilization in response to DNA damage

We discovered a novel oncogene in a T-cell lymphoma cell line, multiple copies in T-cell lymphoma-1 (MCT-1), that has been shown to decrease cell-doubling time, shorten the duration of G(1) transit time and/or G(1)-S transition, and transform NIH3T3 fibroblasts. We subsequently demonstrated that there were significantly increased levels of MCT-1 protein in a subset of primary diffuse large B-cell lymphomas. Levels of MCT-1 protein were shown to be increased after exposure to DNA damaging agents. This increase did not require new protein synthesis, suggesting that post-translational mechanisms were involved. Phosphorylation is one potential mechanism by which the activity of molecules involved in cell cycle/survival is rapidly modulated. The RAS/mitogen-activated/extracellular-regulated kinase (MEK)/extracellular signal-regulated kinases (ERK) pathway plays a prominent role in the regulation of cell growth and proliferation through phosphorylation-dependent regulation of several substrates. The MCT-1 protein is predicted to have numerous putative phosphorylation sites. Using a combination of genetic and pharmacological approaches, we established that phosphorylation of MCT-1 protein by p44/p42 mitogen-activated protein kinases is critical for stabilization of MCT-1 protein and for its ability to promote cell proliferation. Our data suggests that targeting the RAS/MEK/ERK signal transduction cascade may provide a potential therapeutic approach in lymphomas and related malignancies that exhibit high levels of MCT-1 protein.

Platelet activation in Helicobacter pylori-associated idiopathic thrombocytopenic purpura: eradication reduces platelet activation but seldom improves platelet counts

INTRODUCTION

It has been suggested that Helicobacter pylori eradication often increases platelet counts in patients with chronic idiopathic thrombocytopenic purpura (ITP). In addition, H. pylori has been shown to induce platelet activation (CD62p or P-selectin expression) in previous studies. We assessed the response of platelet count and CD62p expression after eradication therapy in patients with ITP and H. pylori infection.

METHODS AND RESULTS

We prospectively studied 15 ITP patients diagnosed with H. pylori infection by serology and breath test. A follow-up breath test was used to document eradication. Two out of 15 patients showed improvement in platelet counts after 6 months, 1 of which may have had drug-induced thrombocytopenia. Overall, certain platelet response rate in our series was 6.7% (1/15). We found that platelet CD62p expression by flow cytometry was elevated in 10/15 (66.7%) H. pylori-infected patients, which is a statistically significant difference when compared with 3/33 (9.1%) control ITP patients seronegative for H. pylori (p = 0.002). In addition, eradication therapy decreased CD62p expression (p = 0.04). However, reduction in platelet activation was not associated with an increase in platelet counts (mean 72.4 x 10(9)/l before and 68.7 after therapy; p = 0.4).

CONCLUSION

In our series, platelet activation was common in ITP patients with H. pylori, and eradication therapy decreased platelet activation but seldom increased platelet counts. Increased platelet CD62p expression is a putative link between chronic infections and atherosclerosis, but further study is needed to clarify the implications of our observation.

Characterization of the MCT-1 pseudogene: identification and implication of its location in a highly amplified region of chromosome 20

The MCT-1 oncogene was initially identified as an amplified gene on chromosome Xq22-24 in a T-cell lymphoma. MCT-1 is over-expressed in a subset of diffuse large B-cell lymphoma (DLBCL), a common form of Non-Hodgkin's Lymphoma (NHL). We have identified a pseudogene for MCT-1 (PsiMCT-1) that is located on chromosome 20q11.2, a region within an amplicon containing several important genes frequently amplified in certain breast and ovarian cancers. Genomic analysis revealed that PsiMCT-1 is a processed pseudogene. Interestingly, both MCT-1 and its pseudogene are located on regions of the genome that are frequently amplified in several different human malignancies. MCT-1 is the oldest known oncogene and its insertion as a pseudogene occurred at a later time point in evolution. Existence of PsiMCT-1 should be considered when analyzing genomic amplification and or expression of MCT-1. Analysis of MCT-1 and PsiMCT-1 might provide clues to cancer genes and their evolution across species.

Systematic identification and functional screens of uncharacterized proteins associated with eukaryotic ribosomal complexes

Translation regulation is a critical means by which cells control growth, division, and apoptosis. To gain further insight into translation and related processes, we performed multifaceted mass spectrometry-based proteomic screens of yeast ribosomal complexes and discovered an association of 77 uncharacterized yeast proteins with ribosomes. Immunoblotting revealed an EDTA-dependent cosedimentation with ribosomes in sucrose gradients for 11 candidate translation-machinery-associated (TMA) proteins. Tandem affinity purification linked one candidate, LSM12, to the RNA processing proteins PBP1 and PBP4. A second candidate, TMA46, interacted with RBG1, a GTPase that interacts with ribosomes. By adapting translation assays to high-throughput screening methods, we showed that null yeast strains harboring deletions for several of the TMA genes had alterations in protein synthesis rates (TMA7 and TMA19), susceptibility to drugs that inhibit translation (TMA7), translation fidelity (TMA20), and polyribosome profiles (TMA7, TMA19, and TMA20). TMA20 has significant sequence homology with the oncogene MCT-1. Expression of human MCT-1 in the Deltatma20 yeast mutant complemented translation-related defects, strongly implying that MCT-1 functions in translation-related processes. Together these findings implicate the TMA proteins and, potentially, their human homologs, in translation related processes.

MCT-1 oncogene contributes to increased in vivo tumorigenicity of MCF7 cells by promotion of angiogenesis and inhibition of apoptosis

Overexpression of a novel oncogene MCT-1 (multiple copies in a T cell malignancy) causes malignant transformation of murine fibroblasts. To establish its role in the pathogenesis of breast cancer in humans, we generated stable transfectants of MCF7 breast cancer cells negative for endogenous MCT-1 (MCF7-MCT-1). Overexpression of MCT-1 in these cells resulted in a slight elevation of estrogen receptor-alpha, and higher rates of DNA synthesis and growth in response to estradiol compared with the empty vector control (MCF7-EV). The pure antiestrogen fulvestrant inhibited the estradiol-stimulated proliferation of MCF7-MCT-1 cells. The MCF7-MCT-1 clones showed increased invasiveness in the presence of 50% serum compared with the MCF7-EV. In a tumor xenograft model, MCT-1-overexpressing cells showed higher take rates and formed significantly larger tumors than MCF7-EV controls. When we examined angiogenic phenotype and molecular mediators of angiogenesis in MCF7-MCT-1 tumors in vivo, we found greater microvascular density and lower apoptosis in the MCF7-MCT-1 tumors compared with MCF7-EV controls accompanied by a dramatic decline in the levels of angiogenesis inhibitor, thrombospondin-1 (TSP1). In vitro, blocking TSP1 in the medium conditioned by MCT-1-negative cells restored its angiogenic potential to that of the MCF7-MCT-1 cells. Conversely, despite an increase in mRNA encoding vascular endothelial growth factor upon MCT-1 overexpression, vascular endothelial growth factor protein levels have not been notably altered. Taken together, our results suggest that MCT-1 may contribute to the pathogenesis and progression of human breast cancer via at least two routes: promotion of angiogenesis through the decline of TSP1 and inhibition of apoptosis.

The MCT-1 oncogene product impairs cell cycle checkpoint control and transforms human mammary epithelial cells

Multiple copies in T-cell maligancy (MCT-1) is a putative oncogene initially identified in a human T-cell lymphoma. Forced expression of MCT-1 has recently been shown to induce cell transformation and proliferation, as well as to activate survival-related PI-3K/AKT pathways protecting cells from apoptosis. MCT-1 protein is stabilized in response to DNA damage. The impact of MCT-1 overexpression on DNA damage response remains unknown. Here, we show that MCT-1 deregulates cell cycle checkpoints. The phosphorylation of genomic stabilizers H2AX and NBS1 are enhanced in MCT-1-overexpressing cells. Forced expression of MCT-1 significantly increases the number of DNA damage-induced foci involving gamma-H2AX and 53BP1. In MCT-1-overexpressing cells, the proportion of S-phase cell population is preferentially increased after exposure to gamma-irradiation compared to controls. Knockdown of endogenous MCT-1 using an siRNA approach attenuates the H2AX phosphorylation and the G1/S checkpoint defect. Furthermore, MCT-1 is capable of transforming immortalized human mammary epithelial cells and promoting genomic instability. These data shed light on the role of MCT-1 in the cellular response to DNA damage and its involvement in malignant transformation.