Upregulated expression of the genes encoding translation initiation factors eIF-4E and eIF-2alpha in transformed cells

The MYC oncogene - the grand orchestrator of cancer growth and immune evasion

The MYC proto-oncogenes encode a family of transcription factors that are among the most commonly activated oncoproteins in human neoplasias. Indeed, MYC aberrations or upregulation of MYC-related pathways by alternate mechanisms occur in the vast majority of cancers. MYC proteins are master regulators of cellular programmes. Thus, cancers with MYC activation elicit many of the hallmarks of cancer required for autonomous neoplastic growth. In preclinical models, MYC inactivation can result in sustained tumour regression, a phenomenon that has been attributed to oncogene addiction. Many therapeutic agents that directly target MYC are under development; however, to date, their clinical efficacy remains to be demonstrated. In the past few years, studies have demonstrated that MYC signalling can enable tumour cells to dysregulate their microenvironment and evade the host immune response. Herein, we discuss how MYC pathways not only dictate cancer cell pathophysiology but also suppress the host immune response against that cancer. We also propose that therapies targeting the MYC pathway will be key to reversing cancerous growth and restoring antitumour immune responses in patients with MYC-driven cancers.

Targeted Toxin Gene Therapy Of Breast Cancer Stem Cells Using CXCR1 Promoter And bFGF 5'UTR

Background

Breast cancer stem cells (BCSCs) are cells with a higher ability to metastasis and resistance to conventional treatments. They have a phenotype of (CD44high/CD24low) and the unlimited ability for proliferation. Development of strategies to target the BCSC population may lead to the establishment of more effective cancer therapies. Pseudomonas exotoxin A (PE) is a potent cytotoxic protein. CXCR1 promoter provides BCSC and HER2 specificity on transcription level. 5′UTR of the basic fibroblast growth factor-2 (bFGF 5ʹUTR) provides tumor specificity on translation level. Here, we utilized a mutant form of PE encoding DNA “PE38”, CXCR1 promoter and bFGF 5ʹUTR to target BCSCs.

Methods

The stemness of SK-BR-3, MDA-MB-231 and MCF10A cell lines were evaluated based on the expression of the CD44high/CD24low stem cell signature and the ability to form mammospheres. Then, the cell lines were transfected with constructs encoding luciferase/PE38 under the control of the CMV/CXCR1 promoter with or without bFGF 5′UTR. Luciferase protein expression was evaluated using dual-luciferase reporter assay. PE38 transcript expression was measured by real-time PCR, and the cytotoxic effect of PE38 protein expression was determined by MTT assay.

Results

The percentage of CD44high/CD24low population did not correlate to mammosphere forming efficiency (MFE). Given that the percentage of CD44 high/CD24 low is not a conclusive BCSC profile, we based our work on the mammosphere assay. However, in comparison with MCF10A, the two tumorigenic cell lines had higher MFE, probably due to their higher BCSC content. Reporter assay and real-time PCR results demonstrated that CXCR1 promoter combined with bFGF 5ʹUTR increased BCSC-specific gene expression. Meanwhile, tightly regulated expression of PE38 using these two gene regulatory elements resulted in high levels of cell death in the two tumorigenic cell lines while having little toxicity toward normal MCF10A.

Conclusion

Our data show that PE38, CXCR1 promoter and bFGF 5ʹUTR in combination can be considered as a promising tool for killer gene therapy of breast cancer.

Translational control mechanisms in cutaneous malignant melanoma: the role of eIF2α

BACKGROUND

Melanoma cells develop adaptive responses in order to cope with particular conditions of tumor microenvironment, characterized by stress conditions and deregulated proliferation. Recently, the interplay between the stress response and the gene expression programs leading to metastatic spread has been reported.

METHODS

We evaluated levels and localization of eIF2α/peIF2α in V600BRAF and wtBRAF metastatic melanoma cell lines by means of western blot and confocal microscopy analyses. Furthermore, we performed a sequence analyses and structure and dynamics studies of eIF2α protein to reveal the role of eIF2α and its correlations in different pathways involved in the invasive phase of melanoma.

RESULTS

We found peIF2α both in cytoplasm and nucleus. Nuclear localization was more represented in V600BRAF melanoma cell lines. Our studies on eIF2α protein sequence indicated the presence of a predicted bipartite NLS as well as a nuclear export signal NES and an S1 domain, typical of RNA interacting proteins. Furthermore, we found high levels of transcription factor EB (TFEB), a component of the MiT/TFE family, and low β-catenin levels in V600BRAF cells.

CONCLUSIONS

Based on our results, we suggest that peIF2α nuclear localization can be crucial in ER stress response and in driving the metastatic spread of melanoma, through lysosomal signaling and Wnt/β-catenin pathway. In conclusion, this is the first evidence of nuclear localization of peIF2α, representing a possible target for future therapeutic approaches for metastatic melanoma.

Mammalian MYC Proteins and Cancer

The MYC family of proteins is a group of basic-helix-loop-helix-leucine zipper transcription factors that feature prominently in cancer. Overexpression of MYC is observed in the vast majority of human malignancies and promotes an extraordinary set of changes that impact cell proliferation, growth, metabolism, DNA replication, cell cycle progression, cell adhesion, differentiation, and metastasis. The purpose of this review is to introduce the reader to the mammalian family of MYC proteins, highlight important functional properties that endow them with their potent oncogenic potential, describe their mechanisms of action and of deregulation in cancer cells, and discuss efforts to target the unique properties of MYC, and of MYC-driven tumors, to treat cancer.

Cell death/proliferation roles for nc886, a non-coding RNA, in the protein kinase R pathway in cholangiocarcinoma

We have recently identified nc886 (pre-miR-886 or vtRNA2-1) as a novel type of non-coding RNA that inhibits activation of protein kinase R (PKR). PKR's pro-apoptotic role through eukaryotic initiation factor 2 α (eIF2α) phosphorylation is well established in the host defense against viral infection. Paradoxically, some cancer patients have elevated PKR activity; however, its cause and consequence are not understood. Initially, we evaluated the expression of nc886, PKR and eIF2α in non-malignant cholangiocyte and cholangiocarcinoma (CCA) cells. nc886 is repressed in CCA cells and this repression is the cause of PKR's activation therein. nc886 alone is necessary and sufficient for suppression of PKR via direct physical interaction. Consistently, artificial suppression of nc886 in cholangiocyte cells activates the canonical PKR/eIF2α cell death pathway, suggesting a potential significance of the nc886 suppression and the consequent PKR activation in eliminating pre-malignant cells during tumorigenesis. In comparison, active PKR in CCA cells does not induce phospho-eIF2α nor apoptosis, but promotes the pro-survival nuclear factor-κB pathway. Thus, PKR has a dual life or death role during tumorigenesis. Similarly to the CCA cell lines, nc886 tends to be decreased but PKR tends to be activated in our clinical samples from CCA patients. Collectively from our data, we propose a tumor surveillance model for nc886's role in the PKR pathway during tumorigenesis.

PKCδ Regulates Translation Initiation through PKR and eIF2α in Response to Retinoic Acid in Acute Myeloid Leukemia Cells

Translation initiation and activity of eukaryotic initiation factor-alpha (eIF2α), the rate-limiting step of translation initiation, is often overactivated in malignant cells. Here, we investigated the regulation and role of eIF2α in acute promyelocytic (APL) and acute myeloid leukemia (AML) cells in response to all-trans retinoic acid (ATRA) and arsenic trioxide (ATO), the front-line therapies in APL. ATRA and ATO induce Ser-51 phosphorylation (inactivation) of eIF2α, through the induction of protein kinase C delta (PKCδ) and PKR, but not other eIF2α kinases, such as GCN2 and PERK in APL (NB4) and AML cells (HL60, U937, and THP-1). Inhibition of eIF2α reduced the expression of cellular proteins that are involved in apoptosis (DAP5/p97), cell cycle (p21Waf1/Cip1), differentiation (TG2) and induced those regulating proliferation (c-myc) and survival (p70S6K). PI3K/Akt/mTOR pathway is involved in regulation of eIF2α through PKCδ/PKR axis. PKCδ and p-eIF2α protein expression levels revealed a significant association between the reduced levels of PKCδ (P = 0.0378) and peIF2 (P = 0.0041) and relapses in AML patients (n = 47). In conclusion, our study provides the first evidence that PKCδ regulates/inhibits eIF2α through induction of PKR in AML cells and reveals a novel signaling mechanism regulating translation initiation.

eIF4F suppression in breast cancer affects maintenance and progression

Levels of eukaryotic initiation factor 4E (eIF4E) are frequently elevated in human cancers and in some instances have been associated with poor prognosis and outcome. Here we utilize transgenic and allograft breast cancer models to demonstrate that increased mammalian target of rapamycin (mTOR) signalling can be a significant contributor to breast cancer progression in vivo. Suppressing mTOR activity, as well as levels and activity of the downstream translation regulators, eIF4E and eIF4A, delayed breast cancer progression, onset of associated pulmonary metastasis in vivo and breast cancer cell invasion and migration in vitro. Translation of vascular endothelial growth factor (VEGF), matrix metallopeptidase 9 (MMP9) and cyclin D1 mRNAs, which encode products associated with the metastatic phenotype, is inhibited upon eIF4E suppression. Our results indicate that the mTOR/eIF4F axis is an important contributor to tumor maintenance and progression programs in breast cancer. Targeting this pathway may be of therapeutic benefit.

Emerging therapeutics targeting mRNA translation

A defining feature of many cancers is deregulated translational control. Typically, this occurs at the level of recruitment of the 40S ribosomes to the 5'-cap of cellular messenger RNAs (mRNAs), the rate-limiting step of protein synthesis, which is controlled by the heterotrimeric eukaryotic initiation complex eIF4F. Thus, eIF4F in particular, and translation initiation in general, represent an exploitable vulnerability and unique opportunity for therapeutic intervention in many transformed cells. In this article, we discuss the development, mode of action and biological activity of a number of small-molecule inhibitors that interrupt PI3K/mTOR signaling control of eIF4F assembly, as well as compounds that more directly block eIF4F activity.

Translational regulation in nutrigenomics

The emergence of genome-wide analysis to interrogate cellular DNA, RNA, and protein content has revolutionized the study of the control network that mediates cellular homeostasis. Nutrigenomics addresses the effect of nutrients on gene expression, which provides a basis for understanding the biological activity of dietary components. Translation of mRNAs represents the last step of genetic flow and primarily defines the proteome. Translational regulation is thus critical for gene expression, in particular, under nutrient excess or deficiency. Until recently, it was unclear how the global effects of translational control are influenced by nutrient signaling. An emerging concept of translational reprogramming addresses how to maintain the expression of specific proteins during pathophysiological conditions by translation of selective mRNAs. Here we describe recent advances in our understanding of translational control, nutrient signaling, and their dysregulation in aging and cancer. The mechanistic understanding of translational regulation in response to different nutrient conditions may help identify potential dietary and therapeutic targets to improve human health.

Systematic analysis of the antiproliferative effects of novel and standard anticancer agents in rhabdoid tumor cell lines

Rhabdoid tumors are highly aggressive pediatric malignancies. Although the prognosis of children with rhabdoid tumors has improved, it still remains dismal and long-term survivors suffer from severe side effects of current therapeutic approaches. The objective of our study was to explore the toxicity of standard and novel anticancer drugs against rhabdoid tumors in vitro and to prioritize them for future preclinical and clinical studies. Antitumor activity of 10 standard anticancer drugs (doxorubicin, idarubicin, mitoxantrone, actinomycin D, temozolomide, carmustine, oxaliplatin, vinorelbine, methotrexate, thiotepa), five target-specific drugs (sorafenib, imatinib, roscovitine, rapamycin, ciglitazone) and two herbal compounds (curcumin and apigenin) was assessed by a modified 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) cell proliferation assay on three rhabdoid tumor cell lines, A204, G401, and BT16, derived from different anatomical sites. Comparable with their high clinical activity, anthracyclines inhibited tumor cell proliferation by 50% (GI50) in the nanomolar range. Actinomycin D exhibited the lowest GI50 values overall ranging from 2.8x10(-6) nmol/l for G401 to 3.8 nmol/l for A204 cells while thiotepa was the only alkylating drug that inhibited tumor cell growth in clinically relevant concentrations. Target-specific drugs, such as sorafenib, roscovitine, and rapamycin, showed promising results as well. In this report, we show for the first time that apigenin and curcumin effectively inhibit rhabdoid tumor cell growth. Supporting earlier reports we conclude that cyclin D1 seems to be an excellent target in the treatment of rhabdoid tumors. Idarubicin or mitoxantrone represent potent alternatives to doxorubicin, and vinorelbine may substitute vincristine in future clinical trials.

PERK regulates the proliferation and development of insulin-secreting beta-cell tumors in the endocrine pancreas of mice

BACKGROUND

PERK eIF2alpha kinase is required for the proliferation of the insulin-secreting beta- cells as well as insulin synthesis and secretion. In addition, PERK signaling has been found to be an important factor in determining growth and angiogenesis of specific types of tumors, and was attributed to PERK-dependent regulation of the hypoxic stress response. In this report we examine the role of PERK in regulating proliferation and angiogenesis of transformed beta-cells in the development of insulinomas.

METHODOLOGY

The SV40 Large T-antigen (Tag) was genetically introduced into the insulin secreting beta-cells of Perk KO mice under the control of an inducible promoter. Tumor growth and the related parameters of cell proliferation were measured. In late stage insulinomas the degree of vascularity was determined.

PRINCIPAL FINDINGS

The formation and growth of insulinomas in Perk-deficient mice was dramatically ablated with much fewer tumors, which averaged 38-fold smaller than seen in wild-type control mice. Beta-cell proliferation was ablated in Perk-deficient mice associated with reduced tumor growth. In the small number of large encapsulated insulinomas that developed in Perk-deficient mice, we found a dramatic reduction in tumor vascularity compared to similar sized insulinomas in wild-type mice. Although insulinoma growth in Perk-deficient mice was largely impaired, beta-cell mass was increased sufficiently by T-antigen induction to rescue the hypoinsulinemia and diabetes in these mice.

CONCLUSIONS

We conclude that PERK has two roles in the development of beta-cell insulinomas, first to support rapid cell proliferation during the initial transition to islet hyperplasia and later to promote angiogenesis during the progression to late-stage encapsulated tumors.

Curcumin modulates eukaryotic initiation factors in human lung adenocarcinoma epithelial cells

Curcumin, a polyphenolic compound, is the active component of Curcuma longa and has been extensively investigated as an anticancer drug that modulates multiple pathways. Eukaryotic initiation factors (eIFs) have been known to play important roles in translation initiation, which controls cell growth and proliferation. Little is known about the effects of curcumin on eIFs in lung cancer. The objective of this study was to exam the curcumin cytotoxic effect and modulation of two major rate-limiting translation initiation factors, including eIF2α and eIF4E protein expression levels in lung adenocarcinoma epithelial cell line A549. Cytotoxicity was measured by MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay and protein changes were determined by Western blot. A549 cells were treated with 0-240 μM curcumin for 4-96 h. The inhibitory effects of curcumin on cytotoxicity were dose- and time-dependent (P < 0.001). The 50% inhibitory curcumin concentrations (IC50s) at 24, 48, 72, and 96 h were 93, 65, 40, and 24 μM, respectively. Protein expressions of eIF2α, eIF4E, Phospho-4E-BP1 were down-regulated, while Phospho-eIF2α and Phospho-eIF4E were up-regulated after A549 cells were treated with 20 and 40 μM curcumin for 24 h. In addition, the effects of curcumin on these protein expression changes followed a significant dose-response (P < 0.05, trend test). These findings suggest that curcumin could reduce cell viability through prohibiting the initiation of protein synthesis by modulating eIF2α and eIF4E.

Reduced eukaryotic initiation factor 2Bepsilon-subunit expression suppresses the transformed phenotype of cells overexpressing the protein

Eukaryotic initiation factor 2B (eIF2B), a five-subunit guanine nucleotide exchange factor, plays a key role in the regulation of mRNA translation. Expression of its epsilon-subunit is specifically up-regulated in certain conditions associated with increased cell growth. Therefore, the purpose of the present study was to examine the effect of repressing eIF2Bepsilon expression on growth rate, protein synthesis, and other characteristics of two tumorigenic cell lines that display up-regulated expression of the epsilon-subunit. Experiments were designed to compare spontaneously transformed fibroblasts to transformed mouse embryonic fibroblasts infected with a lentivirus containing a short hairpin RNA directed against eIF2Bepsilon. Cells expressing the short hairpin RNA displayed a reduction in eIF2Bepsilon abundance to 30% of the value observed in uninfected transformed mouse embryonic fibroblasts, with no change in the expression of any of the other four subunits. The repression of eIF2Bepsilon expression was accompanied by reductions in guanine nucleotide exchange factor activity and global rates of protein synthesis. Moreover, repressed eIF2Bepsilon expression led to marked reductions in cell growth rate in culture, colony formation in soft agar, and tumor progression in nude mice. Similar results were obtained in MCF-7 human breast cancer cells in which eIF2Bepsilon expression was repressed through transient transfection with a small interfering RNA directed against the epsilon-subunit. Overall, the results support a role for eIF2Bepsilon in the regulation of cell growth and suggest that it might represent a therapeutic target for the treatment of human cancer.

c-Myc and eIF4F are components of a feedforward loop that links transcription and translation

The Myc/Max/Mad family of transcription factors and the eukaryotic initiation factor 4F (eIF4F) complex play fundamental roles in regulating cell growth, proliferation, differentiation, and oncogenic transformation. eIF4F is involved in the recruitment of ribosomes to mRNAs and is thought to generally be the rate-limiting phase of translation. Here, we show that c-Myc directly activates transcription of the three subunits of eIF4F (eIF4E, eIF4AI, and eIF4GI). These transcriptional effects are mediated through canonical E-boxes (5'CACGTG3') present in the promoters of these genes. In addition, the c-Myc antagonist Mad1 down-regulates the expression of eIF4F subunits. We also show that MycER activation stimulates protein synthesis at the level of translation initiation. Increased eIF4F levels result in stimulation of c-Myc mRNA translation specifically, as assessed by quantitative reverse transcription-PCR. We use a murine model of lymphomagenesis to show the expression of eIF4F subunits is also up-regulated by c-Myc in vivo. Our results suggest the presence of a feedforward loop involving c-Myc and eIF4F that serves to link transcription and translation and that could contribute to the effects of c-Myc on cell proliferation and neoplastic growth.

Integrin-dependent translational control: Implication in cancer progression

The importance of translational control in cancer progression has been underscored by a number of recent studies. However, little is known how cancer cells maintain their high efficiency of translation. Here, we summarize studies that support the role of integrins in translational control, especially at the initiation step, and discuss the various mechanisms by which integrins regulate the recruitment of translational machinery. This review also examines the hypothesis that integrins contribute to various aspects of cancer progression such as proliferation, survival, angiogenesis, and invasion through translational control.

Death-associated protein 5 (DAP5/p97/NAT1) contributes to retinoic acid-induced granulocytic differentiation and arsenic trioxide-induced apoptosis in acute promyelocytic leukemia

All-trans-retinoic acid (ATRA) and arsenic trioxide (ATO) induce differentiation and apoptosis in acute promyelocytic leukemia (APL) cells. Here we investigated the role and regulation of death-associated protein-5 (DAP5/p97/NAT1), a novel inhibitor of translational initiation, in APL cell differentiation and apoptosis. We found that ATRA markedly induced DAP5/p97 protein and gene expression and nuclear translocation during terminal differentiation of APL (NB4) and HL60 cells but not differentiation-resistant cells (NB4.R1 and HL60R), which express very low levels of DAP5/p97. At the differentiation inducing concentrations, ATO (<0.5 microM), dimethyl sulfoxide, 1,25-dihydroxy-vitamin-D3, and phorbol-12-myristate 13-acetate also significantly induced DAP5/p97 expression in NB4 cells. However, ATO administered at apoptotic doses (1-2 microM) induced expression of DAP5/p86, a proapoptotic derivative of DAP5/p97. ATRA and ATO-induced expression of DAP5/p97 was associated with inhibition of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. Furthermore, DAP5/p97 expression was upregulated by inhibition of the PI3K/Akt/mammalian target of rapamycin (mTOR) pathway via LY294002 and via rapamycin. Finally, knockdown of DAP5/p97 expression by small interfering RNA inhibited ATRA-induced granulocytic differentiation and ATO-induced apoptosis. Together, our data reveal new roles for DAP5/p97 in ATRA-induced differentiation and ATO-induced apoptosis in APL and suggest a novel regulatory mechanism by which PI3K/Akt/mTOR pathway inhibition mediates ATRA- and ATO-induced expression of DAP5/p97.

Expression of the translation initiation factors eIF-4E and eIF-2* is frequently increased in neoplastic cells of Hodgkin lymphoma

The rate of protein synthesis is regulated in part by 2 key translation initiation factors, eIF-4E and eIF-2*. The expression and activity of both factors are increased transiently when normal resting cells are stimulated to proliferate, but they are constitutively elevated in oncogene-transformed cultured cells. Overexpression of either initiation factor induces a tumorigenic phenotype in rodent cells. We have shown earlier that expression of both eIF-4E and eIF-2* is increased in non-Hodgkin lymphomas (non-HLs). In this study, we performed an immunohistochemical survey of these translation initiation factors in neoplastic cells of HL. We also used Western blot to addressed the possibility that eIF-4E increases expression of NFkappaB. Our results indicate that both eIF-4E and eIF-2* are strongly expressed in neoplastic cells of HL in most cases examined as compared with weak or undetectable expression in most surrounding lymphocytes. An increase in eIF-4E expression may lead to constitutively high expression of NFkappaB, a transcription factor implicated in resistance to apoptosis and induction of cytokine gene expression in various cells, including neoplastic cells of HL.

mTOR and cancer therapy

Proteins regulating the mammalian target of rapamycin (mTOR), as well as some of the targets of the mTOR kinase, are overexpressed or mutated in cancer. Rapamycin, the naturally occurring inhibitor of mTOR, along with a number of recently developed rapamycin analogs (rapalogs) consisting of synthetically derived compounds containing minor chemical modifications to the parent structure, inhibit the growth of cell lines derived from multiple tumor types in vitro, and tumor models in vivo. Results from clinical trials indicate that the rapalogs may be useful for the treatment of subsets of certain types of cancer. The sporadic responses from the initial clinical trials, based on the hypothesis of general translation inhibition of cancer cells are now beginning to be understood owing to a more complete understanding of the dynamics of mTOR regulation and the function of mTOR in the tumor microenvironment. This review will summarize the preclinical and clinical data and recent discoveries of the function of mTOR in cancer and growth regulation.

Physical and functional interaction of the p14ARF tumor suppressor with ribosomes

Alterations in the p14(ARF) tumor suppressor are frequent in many human cancers and are associated with susceptibility to melanoma, pancreatic cancer, and nervous system tumors. In addition to its p53-regulatory functions, p14(ARF) has been shown to influence ribosome biogenesis and to regulate the endoribonuclease B23, but there remains considerable controversy about its nucleolar role. We sought to clarify the activities of p14(ARF) by studying its interaction with ribosomes. We show that p14(ARF) and B23 interact within the nucleolar 60 S preribosomal particle and that this interaction does not require rRNA. In contrast to previous reports, we found that expression of p14(ARF) does not significantly alter ribosome biogenesis but inhibits polysome formation and protein translation in vivo. These results suggest a ribosome-dependent p14(ARF) pathway that regulates cell growth and thus complements p53-dependent p14(ARF) functions.

1,25-Dihydroxyvitamin D3 suppresses gene expression of eukaryotic translation initiation factor 2 in human promyelocytic leukemia HL-60 cells

The physiologically active metabolite of vitamin D(3), 1alpha,25-dihydroxyvitamin D(3) (DVD), is a potent inducer of cell differentiation in human myeloid leukemia cells. In the present study, we examined changes in gene expression during DVD-induced cell differentiation of promyelocytic HL-60 cells employing a DNA microarray technique. The results identified 7 up-regulated and 9 down-regulated genes with a change greater than 1.5-fold after the DVD-treatment for both 2 and 6 days. Seven of these genes were further examined by reverse transcription-polymerase chain reaction (RT-PCR). The results showed that findings obtained from the DNA microarray analysis and RT-PCR are generally comparable with each other. Gene expression of the subunits of eukaryotic translation initiation factor 2 was then examined by methods including RT-PCR and real-time PCR. The results indicated the suppression of these genes, suggesting a linkage to differentiation-associated growth inhibition of these cells.

In silico whole-genome scanning of cancer-associated nonsynonymous SNPs and molecular characterization of a dynein light chain tumour variant

Last decade has led to the accumulation of large amounts of data on cancer genetics, opening an unprecedented access to the mapping of cancer genes in the human genome. Single-nucleotide polymorphisms (SNPs), the most common form of DNA variation in humans, emerge as an invaluable tool for cancer association studies. These genotypic markers can be used to assay how alleles of candidate genes correlate with the malignant phenotype, and may provide new clues into the genetic modifications that characterize cancer onset. In this cancer-oriented study, we detail an SNP mining strategy based on the analysis of expressed sequence tags among publicly available databases. Our whole-genome approach provides a comprehensive and unbiased description of nonsynonymous SNPs (nsSNPs) in tumoral versus normal tissues. To gain further insights into the possible relationships between genetic variation and altered phenotype, locations of a subset of nsSNPs were mapped onto protein domains known to be critical for protein function. Computational methods were also used to predict the potential impact of these cancer-associated nsSNPs on protein structure and function. We illustrate our approach through the detailed biochemical and structural characterization of a previously unknown cancer-associated mutation (G79C) affecting the 8 kDa dynein light chain (DNCL1).

Cancer-specific gene therapy

Cancer cells transcriptionally activate many genes that are important for uncontrolled proliferation and cell death. Deregulated transcriptional machinery in tumor cells usually consists of increased expression/activity of transcription factors. Ideally, cancer-specific killing can be achieved by delivering a therapeutic gene under the control of the DNA elements that can be activated by transcription factors that are overexpressed and/or constitutively activated in cancer cells. Additionally, tumor-specific translation of tumor-killing genes has been also exploited in cancer gene therapy. Based on these rationales, cancer-specific expression of a therapeutic gene has emerged as a potentially successful approach for cancer gene therapy. To achieve tumor-specific expression, cancer-specific vectors are generally composed of promoters, enhancers, and/or 5'-UTR that are responsive to tumor-specific transcription factors. A number of cancer-specific promoters have been reported, such as those of probasin, human telomerase reverse transcriptase, survivin, ceruloplasmin, HER-2, osteocalcin, and carcinoembryonic antigen. Evidences suggest that the enhancer element targeted by beta-catenin can be useful to target colon cancer cells. The 5'-UTR of the basic fibroblast growth factor-2 has been reported to provide tumor specificity. Moreover, a variety of therapeutic genes demonstrated direct antitumor effects such as those encoding proapoptotic proteins p53, E1A, p202, PEA3, BAX, Bik, and prodrug metabolizing enzymes, namely thymidine kinase and cytosine deaminase. As cancerous cells of different origins vary significantly in their genetic, transcriptional/translational, and cellular profiles, the success of a cancer gene therapy will not be promised unless it is carefully designed based on the biology of a specific tumor type. Thus, tremendous research efforts have been focused on the development of non-viral vectors that selectively target various tumors resulting in minimal toxicity in the normal tissues. Significant progresses were also made in the exploitation of various novel apoptotic, cytotoxic genes as therapeutic tools that suppress the growth of different tumors. Together, these recent advances provide rationales for future clinical testing of transcriptionally targeted non-viral vectors in cancer patients.

Comparative studies on cellular gene regulation by HIV-1 based vectors: implications for quality control of vector production

Among the recent gene therapy protocols, vectors that can stably express transgenes, for example, HIV-1-based vectors, are particularly desirable. There have been no direct reports on insertional mutagenesis by lentiviral vectors; however, the severe pathogenic nature of their parental virus (HIV-1) is still a major safety concern surrounding these vectors and prevents the progress with their clinical application. We reason that by investigating the host response we shall be able to assess the safety and potential effects of the vectors on targeted cells and understand the interaction between vectors and the host. For this, two major sets of experiments were conducted. Initially, we used cDNA microarray methodology to examine cellular gene profile in human primary umbilical cord endothelial cells (HUVECs) after HIV-1-based VSV-G/GFP vector transduction and observed a modest effect of HIV-1-based vectors on HUVECs. The represented functional categories include transcription and translation factors, tumour antigens, complement factors and signal transduction factors. Some of the differentially expressed genes, for example, Clusterin, CD151, Ku antigen and eIF4gamma, could have oncogenic potential. In the second approach, we systematically compared five different viral vectors, that is, HIV-1-based VSV-G/Empty, VSV-G/GFP, VSV-G/puro, Amph/GFP and MLV-based Amph/Laz, for the effects of individual viral components on cellular gene regulation. Our comparative results demonstrated a regulatory function of Gag/Pol proteins on cellular gene expression. The significance of our findings in relation to the safety of HIV-1 vectors and the importance of quality control of vector production will be presented and discussed.

RNA polymerases I and III, growth control and cancer

Transcription of rRNA and tRNA genes by RNA polymerases I and III is essential for sustained protein synthesis and is therefore a fundamental determinant of the capacity of a cell to grow. When cell growth is not required, this transcription is repressed by retinoblastoma protein, p53 and ARF. However, inactivation of these tumour suppressors in cancers deregulates RNA polymerases I and III, and oncoproteins such as Myc can stimulate these systems further. Such events might have a significant impact on the growth potential of tumours.

Lost in translation: dysregulation of cap-dependent translation and cancer

Activation of the phosphatidylinositol 3' kinase-Akt pathway has long been associated with malignant transformation and antiapoptotic signaling. Mutations downstream of Akt that activate the TOR kinase are found in tumor-prone syndromes, while overexpression of translation initiation complex components, such as eIF4E, occurs frequently in human cancer. However, direct roles for TOR signaling or eIF4E overexpression, in the genesis of cancer, have been lacking. Recent papers, including one by in this issue of Cancer Cell, clearly establish that dysregulation of cap-dependent translation confers malignant characteristics and induces cancer by suppressing apoptosis, underscoring the potential of therapeutics that selectively target the Akt-TOR-eIF4E pathway.

Targets and mechanisms for the regulation of translation in malignant transformation

There is increasing evidence that deregulation of gene expression at the level of mRNA translation can contribute to cell transformation and the malignant phenotype. Two steps in the pathway of polypeptide chain initiation, viz. the assembly of the 43S initiation complex catalysed by polypeptide chain initiation factor eIF2 and the binding of eIF4E to eIF4G during the recruitment of mRNA to the ribosome, have been shown to be likely targets for changes associated with tumorigenesis. The activity of eIF2 is controlled by changes in phosphorylation of the alpha subunit of this factor. The availability of eIF4E for binding to eIF4G is regulated by the phosphorylation of a small family of eIF4E-binding proteins (the 4E-BPs). The activities of the protein kinases and/or phosphatases responsible for the (de)phosphorylation of these substrates may in turn be controlled by cellular and viral oncogenes and tumour-suppressor genes. This review will describe recent aspects of the mechanisms involved, with particular emphasis on the regulation of the eIF2 alpha kinase PKR and the control of 4E-BP phosphorylation by viral gene products, growth-inhibitory cytokines and the tumour-suppressor protein p53.

The role of c-myc in regulation of translation initiation

Translation initiation is important for the regulation of both cell growth and cell division. It is uniquely poised to coordinate overall cell proliferation by its effects on both growth and division. A number of translation initiation factors are transcriptional targets of c-myc in a variety of assays. In particular, the mRNA cap-binding protein eIF4E has a myc-binding sequence in its promoter that is myc responsive in reporter assays and contains a high-affinity myc-binding site in chromosome immunoprecipitation experiments. Several differential expression screens have demonstrated altered levels of eIF4E, along with several other translation initiation factors, in response to alterations of c-myc levels. The potential for eIF4E and other translational control elements to mediate myc's transforming functions is particularly important because eIF4E is itself a known oncogenic factor. The ability of translation initiation factors to affect both cell division control and cell growth control coincides with myc's remarkable effects on both cell growth and cell division.

The role of translation in neoplastic transformation from a pathologist's point of view

Increased cell proliferation, which is a hallmark of aggressive malignant neoplasms, requires a general increase in protein synthesis and a specific increase in the synthesis of replication-promoting proteins. Transient increase in the general protein synthesis rate, as well as preferential translation of specific mRNAs coding for growth promoting proteins (e.g. cyclin D1), takes place during normal mitogenic response. A number of extensively studied growth signal transduction pathways (Ras, PI3K, MAPK, mTOR-dependent pathways) activate the function and expression of various components of the translational machinery. In abnormal situations, constitutive activation of signal transduction pathways (e.g. oncogenic activation of Ras or Myc) leads to continuous upregulation of key elements of translational machinery. On the other hand, tumor suppressor genes (p53, pRb) downregulate ribosomal and tRNA synthesis, and their inactivation results in uncontrolled production of these translational components. During recent years, a significant effort has been dedicated to determining whether expression of translation factors is increased in human tumors using clinical biopsy specimens. The results of these studies indicate that expression of particular translation initiation factors is not always increased in human neoplasms. The pattern of expression is characteristic for a particular tumor type. For example, eIF-4E is usually increased in bronchioloalveolar carcinomas but not in squamous cell carcinomas of the lung. Interestingly, in certain highly proliferative and aggressive neoplasms (e.g. squamous cell carcinoma of the lung, melanoma), the expression of eIF-4E is barely detectable. These findings suggest that mechanisms for increasing general protein synthesis in various neoplasms differ significantly. Finally, the possibility of qualitative alterations in the translational machinery, rather than a simple increase in the activity of its components, is discussed along with the possibility of targeting those qualitative differences for tumor therapy.

Postgenomic global analysis of translational control induced by oncogenic signaling

It is commonly assumed that developmental and oncogenic signaling achieve their phenotypic effects primarily by directly regulating the transcriptional profile of cells. However, there is growing evidence that the direct effect on transcription may be overshadowed by differential effects on the translational efficiency of specific existing mRNA species. Global analysis of this effect using microarrays indicates that this mechanism of controlling protein production provides a highly specific, robust, and rapid response to oncogenic and developmental stimuli. The mRNAs so affected encode proteins involved in cell-cell interaction, signal transduction, and growth control. Furthermore, a large number of transcription factors capable of secondarily rearranging the transcriptional profile of the cell are controlled at this level as well. To what degree this translational control is either necessary or sufficient for tumor formation or maintenance remains to be determined.

Expression of translation initiation factor eIF-2alpha is increased in benign and malignant melanocytic and colonic epithelial neoplasms

BACKGROUND

Stimulation of resting cells by growth factors leads to an increase in the rate of protein synthesis, which is necessary for cell growth and division. Translation initiation factor eIF-2alpha is one of the key translation factors mediating the effects of growth factors on protein synthesis. In normal cells, expression of eIF-2alpha is increased transiently, but its levels are elevated constitutively in oncogene-transformed cells. Overexpression of constitutively active eIF-2alpha in rodent cells makes them tumorigenic. In this article, the authors report their findings on the increased expression of eIF-2alpha in human benign and malignant neoplasms originating from melanocytes and colonic epithelium.

METHODS

Immunohistochemistry was used to analyze the expression of eIF-2alpha, eIF-4E, and cyclin D1 in melanocytic nevi and melanomas and the expression of eIF-2alpha in colonic adenomas and carcinomas.

RESULTS

The authors found that the expression of eIF-2alpha was increased markedly in both benign and malignant neoplasms of melanocytes and colonic epithelium.

CONCLUSIONS

Increased expression of eIF-2alpha took place in both benign and malignant neoplasms of melanocytes and colonic epithelium. These findings suggest that elevated expression of this translation initiation factor may contribute to tumor initiation and progression but that it is not sufficient for establishing a malignant phenotype in the tumors analyzed in this study.

Direct activation of RNA polymerase III transcription by c-Myc

The proto-oncogene product c-Myc has a direct role in both metazoan cell growth and division. RNA polymerase III (pol III) is involved in the generation of transfer RNA and 5S ribosomal RNA, and these molecules must be produced in bulk to meet the need for protein synthesis in growing cells. We demonstrate here that c-Myc binds to TFIIIB, a pol III-specific general transcription factor, and directly activates pol III transcription. Chromatin immunoprecipitation reveals that endogenous c-Myc is present at tRNA and 5S rRNA genes in cultured mammalian cells. These results suggest that activation of pol III may have a role in the ability of c-Myc to stimulate cell growth.

Translational control of gene expression and disease

In the past decade, translational control has been shown to be crucial in the regulation of gene expression. Research in this field has progressed rapidly, revealing new control mechanisms and adding constantly to the list of translationally regulated genes. There is accumulating evidence that translational control plays a primary role in cell-cycle progression and cell differentiation, as well as in the induction of specific cellular functions. Recently, the aetiologies of several human diseases have been linked with mutations in genes of the translational control machinery, highlighting the significance of this regulatory mechanism. In addition, deregulation of translation is associated with a wide range of cancers. Current research focuses on novel therapeutic strategies that target translational control, a promising concept in the treatment of human diseases.

Selective killing of cancer cells based on translational control of a suicide gene

The translation initiation factor, eIF4E, is commonly overexpressed in solid tumors. This elevation allows for efficient translation of mRNA that are normally repressed by their 5' untranslated region, many of which encode growth-promoting proteins. This property was exploited to modulate the synthesis of HTK at the translational level to selectively kill cancer cells. Various breast cancer cell lines can efficiently synthesize HTK from the translationally regulated mRNA, whereas normal cells cannot. Accordingly, only cancer cells were killed at low concentrations of ganciclovir. By altering the expression of eIF4E, it was possible to modulate the sensitivity of various cell lines to ganciclovir.

The nuclear function of angiogenin in endothelial cells is related to rRNA production

Angiogenin is a potent angiogenic protein whose inhibition is known to prevent human tumor growth in athymic mice. It is secreted by both tumor and normal cells; and interacts with endothelial and smooth muscle cells to induce a wide range of cellular responses including cell migration and invasion, proliferation, and formation of tubular structures. Angiogenin is rapidly endocytosed and translocated to the cell nucleus where it accumulates in the nucleolus and binds to DNA. Although nuclear translocation is necessary for its angiogenic activity, the nuclear function of angiogenin is unclear. Here we report that exogenous angiogenin enhances the production of 45S rRNA in endothelial cells, and reduction of endogenous angiogenin inhibits its transcription. In a nuclear run-on assay, angiogenin stimulates RNA synthesis including that containing the initiation site sequences of 45S rRNA. This suggests that the nuclear function of angiogenin relates to its capacity to induce rRNA synthesis. Because rRNA transcription is essential for the synthesis of new ribosomes that are necessary for protein translation and cell growth, inhibition of angiogenin-stimulated transcription of rRNA may inhibit angiogenesis and therefore, would serve as a molecular target for therapeutic intervention.

Expression of eukaryotic translation initiation factors 4E and 2alpha correlates with the progression of thyroid carcinoma

Cell growth and proliferation depend on protein synthesis that is regulated, in part, by two eukaryotic translation initiation factors, eIF-4E and eIF-2alpha. These factors are transiently increased as normal cells respond to growth factors and are constitutively elevated in transformed cells. In cultured cells, eIF-4E facilitates cell cycle progression by increasing the expression of cell cycle promoting proteins including cyclin D1. Our previous study revealed elevated cyclin D1 expression in histologically more aggressive thyroid carcinomas as compared to conventional papillary carcinoma. We hypothesized that the increased cyclin D1 expression might correlate with increased eIF-4E expression. We, therefore studied the expression of eIF-4E by immunohistochemistry in 25 cases of conventional papillary carcinoma (CPC) and 28 cases of aggressive thyroid carcinomas (ATC), the latter included 11 tall cell/columnar cell variant of papillary carcinoma, 5 insular carcinomas, and 12 anaplastic carcinomas. We also analyzed the expression of eIF-2a in the same samples as this factor is usually regulated similarly to eIF-4E in cell culture models. Of the 25 CPC, 13 were eIF-4E positive (11 weakly and 2 strongly), and 19 were eIF-2a positive (14 weakly and 5 strongly). Conversely, of the 28 ATC, 25 were eIF-4E positive (4 weakly and 21 strongly), and 23 were eIF-2alpha positive (4 weakly and 19 strongly). There was a significantly increased expression of both eIF-4E (p < 0.001) and eIF-2alpha (p < 0.001) in ATC compared to CPC, suggesting that these translation initiation factors may play a role in the progression of thyroid cancer.

Expression of eukaryotic translation initiation factors 4E and 2alpha is increased frequently in bronchioloalveolar but not in squamous cell carcinomas of the lung

BACKGROUND

When resting cells are stimulated by growth factors, an increase in protein synthesis follows that depends in part on two key eukaryotic translation initiation factors, 4E and 2alpha (eIF-4E and eIF-2alpha, respectively). In the normal cell, expression and activity of both factors are increased transiently, whereas they become elevated constitutively in oncogene-transformed cultured cells, and overexpression of either initiation factor in rodent cells makes them tumorigenic. In this study, the authors investigated an association between the expression of these translation initiation factors and lung carcinogenesis.

METHODS

The authors analyzed the expression of the protein synthesis initiation factors eIF-4E and eIF-2alpha by immunohistochemistry in bronchioloalveolar (BA) and squamous cell (SC) carcinomas of the lung. Western blot analysis was performed to validate the specificity of antibodies in detecting their cognate proteins.

RESULTS

Both eIF-4E and eIF-2alpha were increased frequently in BA carcinomas, whereas only rarely did SC carcinomas demonstrate elevation of these translation initiation factors. An analysis of cyclin D1 expression did not show a strict correlation with the expression of eIF-4E and eIF-2alpha.

CONCLUSIONS

Increased expression of either one or both translation initiation factors may facilitate accelerated growth and division of neoplastic cells in BA carcinoma of the lung. However, the current findings suggest a possibility that increased cell growth and proliferation in SC carcinoma may be achieved through a mechanism independent of increases in eIF-4E and eIF-2alpha expression.

Transcriptional regulation of the rat eIF4E gene in cardiac muscle cells: the role of specific elements in the promoter region

Eukaryotic initiation factor 4E (eIF4E) binds to the 7-methylguanosine cap of mRNA and facilitates binding of mRNA to the 40 S ribosome, a rate-limiting step in translation initiation. The expression of eIF4E mRNA and protein increases during growth of cardiac muscle cells (cardiocytes) in vitro. To examine transcriptional regulation of the rat eIF4E gene, 2.1 kB of the rat eIF4E promoter region was cloned and the contribution of specific elements in regulating transcription was determined in primary cultures of rat cardiocytes and in a murine C(2)C(12) myoblast cell line. Sequence analysis of the rat eIF4E promoter revealed 80% sequence similarity with human eIF4E. A putative distal E-box was found at -230 bp and a proximal E-box was located at -77 bp upstream of the transcription start site. Consensus AP-1 motifs were found at -839 and -901 bp and designated as the proximal AP-1 site and distal AP-1 site, respectively. Transfection of reporter gene constructs into cardiocytes showed that deletion of the region between -633 and -318 bp produced a 3-fold increase in basal transcription as compared to the 2.1 kB eIF4E promoter construct. Further deletion of the distal E-box region had no effect on transcription as compared with the 2.1 kB promoter, but deletion of both E-boxes eliminated transcriptional activity. Similar results were obtained in C(2)C(12) myoblasts. To further investigate transcriptional regulation, point mutations were made in the 2.1 kB eIF4E promoter. Mutation of either the distal or proximal E-box had minimal effects on activity in either cell type, but mutation of the distal AP-1 site significantly reduced eIF4E promoter activity by 66+/-4% in cardiocytes. In C(2)C(12) myoblasts, mutating the distal AP-1 site reduced activity by 30+/-4% We conclude that both E-boxes are required for maximal basal activity of the eIF4E promoter, and that the distal AP-1 motif may activate transcription.

Eukaryotic initiation factor 3 p110 mRNA is overexpressed in testicular seminomas

Testicular germ cell tumors are important neoplasms and seminoma accounts for 40 to 50% of these tumors. Little is known concerning the molecular events underlying the development of these malignancies. In the present study we used a modified differential display approach to compare gene expression between seminoma and normal testicular parenchyma, both of which are mixed tissues. We first analyzed mRNA (cDNA) expression by differential display and then directly used differentially expressed cDNAs for the synthesis of radiolabeled riboprobes to attribute differential expression to specific cell types in tissue sections by in situ hybridization. Using this approach along with real-time quantitative reverse transcriptase-polymerase chain reaction analysis we found an overexpression of eukaryotic initiation factor 3 p110 mRNA (EIF3S8) in seminoma cells compared to normal germ cells of testicular tubules. The elF3-p110 subunit may promote seminoma development by generally increasing translation leading to enhanced cellular growth and division rates.

A genetic screen to identify genes that rescue the slow growth phenotype of c-myc null fibroblasts

The c-myc gene is frequently over-expressed in human cancers and is involved in regulation of proliferation, differentiation and apoptosis. c-Myc is a transcription factor that acts primarily by regulating the expression of other genes. However, it has been very difficult to identify bona fide c-Myc target genes that explain its diverse biological activities. The recent generation of c-myc deficient Rat1A fibroblasts with a profound and stable growth defect provides a new system to search for genes that can substitute for c-myc in proliferation. In this study, we have attempted to identify genes that rescue the slow growth phenotype of c-myc null cells through introduction of a series of potent cell cycle regulatory genes and several retroviral cDNA expression libraries. None of the candidate genes tested, including SV40 T-antigen and adenovirus E1A, caused reversal of the c-myc null growth defect. Furthermore, extensive screens with high-complexity retroviral cDNA libraries from three different tissue sources revealed that only c-myc and N-myc rescued the c-myc null slow-growth phenotype. Our data support the notion that there are no functional equivalents of the myc family of proto-oncogenes and also suggest that there are no c-Myc-activated genes that alone can substitute for c-Myc in control of cell proliferation.

Modulation of molecular mechanisms involved in protein synthesis machinery as a new tool for the control of cell proliferation

In the past years, the attention of scientists has focused mainly on the study of the genetic information and alterations that regulate eukaryotic cell proliferation and that lead to neoplastic transformation. All therapeutic strategies against cancer are, to date, directed at DNA either with cytotoxic drugs or gene therapy. Little or no interest has been aroused by protein synthesis mechanisms. However, an increasing body of data is emerging about the involvement of translational processes and factors in control of cell proliferation, indicating that protein synthesis can be an additional target for anticancer strategies. In this paper we review the novel insights on the biochemical and molecular events leading to protein biosynthesis and we describe their involvement in cell proliferation and tumorigenesis. A possible mechanistic explanation is given by the interactions that occur between protein synthesis machinery and the proliferative signal transduction pathways and that are therefore suitable targets for indirect modulation of protein synthesis. We briefly describe the molecular tools used to block protein synthesis and the attempts made at increasing their efficacy. Finally, we propose a new multimodal strategy against cancer based on the simultaneous intervention on protein synthesis and signal transduction.

4E binding protein 1 expression is inversely correlated to the progression of gastrointestinal cancers

Several components of the eukaryotic protein synthesis apparatus have been associated with oncogenic transformation of cells. Overexpression of the initiation factor eIF4E occurs in a variety of human tumours. The aim of this study was to determine the level of expression and the phosphorylation state of eIF4E and 4E-binding protein 1 (4E-BP1) in gastrointestinal cancer, and to ascertain whether or not these factors can be used as diagnostic or prognostic markers within this type of cancer. The eIF4E levels were significantly higher in tumours compared with normal tissue (51. 5+/-4.4 vs 30.9+/-2.5 arbitrary units (A.U.)/mg of protein, p<0.001). However, phosphorylated eIF4E did not change in stomach cancers and decreased in colorectal cancers (67.1+/-1.2 vs 60.8+/-2.8%, p<0.05). 4E-BP1 expression increased in most of the gastrointestinal cancers studied. In addition, an inverse correlation between 4E-BP1 elevation and N and M stages was found, showing significant higher elevation of 4E-BP1 in Node-negative patients (11.21+/-5.74 vs 4. 03+/-2.36 n-fold, p<0.05) as well as in patients without distant metastasis (8.41+/-3.29 vs 0.97+/-0.35 n-fold, p<0.05). These results suggest that 4E-BP1 could function as a tumour suppressor. Moreover, the data show a significant dephosphorylation of 4E-BP1 in gastrointestinal tumours that correlated with an increase in the association of 4E-BP1 and eIF4E indicating a lower availability to eIF4E to recruit to the ribosomes. Our results support a possible role of 4E-BP1 as a prognostic factor in gastrointestinal carcinoma.

Levels, phosphorylation status and cellular localization of translational factor eIF2 in gastrointestinal carcinomas

The level of expression and the phosphorylation status of the alpha subunit of initiation factor 2 (eIF2alpha) protein have been determined by comparing samples from human stomach, colon and sigma-rectum carcinomas with normal tissue from the same patients. The unphosphorylated and phosphorylated levels of cytoplasmic eIF2alpha, as well as the percentage of phosphorylated factor over the total, were significantly higher in stomach, colon and sigma-rectum tumours compared with normal tissue. The expression of this factor was also studied by using immunocytochemical methods, where redistribution towards the nucleus in tumour cells as compared with normal tissue was observed. Our results support a likely implication of eIF2alpha in gastrointestinal cancer.

RNA polymerase III transcription: its control by tumor suppressors and its deregulation by transforming agents

The level of RNA polymerase (pol) III transcription is tightly linked to the rate of growth; it is low in resting cells and increases following mitogenic stimulation. When mammalian cells begin to proliferate, maximal pol III activity is reached shortly before the G1/S transition; it then remains high throughout S and G2 phases. Recent data suggest that the retinoblastoma protein RB and its relatives p107 and p130 may be largely responsible for this pattern of expression. During G0 and early G1 phase, RB and p130 bind and repress the pol III-specific factor TFIIIB; shortly before S phase they dissociate from TFIIIB, allowing transcription to increase. At the end of interphase, when cells enter mitosis, pol III transcription is again suppressed; this mitotic repression is achieved through direct phosphorylation of TFIIIB. Thus, pol III transcription levels fluctuate as mammalian cells cycle, being high in S and G2 phases and low during mitosis and early G1. In addition to this cyclic regulation, TFIIIB can be bound and repressed by the tumor suppressor p53. Conversely, it is a target for activation by several viruses, including SV40, HBV, and HTLV-1. Some viruses also increase the activity of a second pol III-specific factor called TFIIIC. A large proportion of transformed and tumor cell types express abnormally high levels of pol III products. This may be explained, at least in part, by the very high frequency with which RB and p53 become inactivated during neoplastic transformation; loss of function of these cardinal tumor suppressors may release TFIIIB from key restraints that operate in normal cells.

Control of cell growth by c-Myc in the absence of cell division

The c-Myc protein (Myc) is a transcription factor, and deregulated expression of the c-myc gene (myc) is frequently found in tumours. In Burkitt's lymphoma (BL), myc is transcriptionally activated by chromosomal translocation. We have used a B-cell line called P493-6 that carries a conditional myc allele to elucidate the role of Myc in the proliferation of BL cells. Regulation of proliferation involves the coordination of cell growth (accumulation of cell mass) and cell division [1] [2] [3]. Here, we show that division of P493-6 cells was strictly dependent on the expression of the conditional myc allele and the presence of foetal calf serum (FCS). More importantly, cell growth was regulated by Myc without FCS: Myc alone induced an increase in cell size and positively regulated protein synthesis. An increase in protein synthesis is thought to be one of the causes of cell mass increase. Furthermore, Myc stimulated metabolic activities, as indicated by the acidification of culture medium and the activation of mitochondrial enzymes. Our results confirm the model that Myc is involved in the regulation of cell growth [4] and provide, for the first time, direct evidence that Myc induces cell growth, that is, an increase in cell size, uncoupled from cell division.

Cell cycle progression and proliferation despite 4BP-1 dephosphorylation

Proliferation and cell cycle progression in response to growth factors require de novo protein synthesis. It has been proposed that binding of the eukaryotic translation initiation factor 4E (eIF-4E) to the inhibitory protein 4BP-1 blocks translation by preventing access of eIF-4G to the 5' cap of the mRNA. The signal for translation initiation is thought to involve phosphorylation of 4BP-1, which causes it to dissociate from eIF-4E and allows eIF-4G to localize to the 5' cap. It has been suggested that the ability of the macrolide antibiotic rapamycin to inhibit 4BP-1 phosphorylation is responsible for the potent antiproliferative property of this drug. We now show that rapamycin-resistant cells exhibited normal proliferation despite dephosphorylation of 4BP-1 that allows it to bind to eIF-4E. Moreover, despite rapamycin-induced dephosphorylation of 4BP-1, eIF-4E-eIF-4G complexes (eIF-4F) were still detected. In contrast, amino acid withdrawal, which caused a similar degree of 4BP-1 dephosphorylation, resulted in dissociation of the eIF-4E-eIF-4G complex. Thus, 4BP-1 dephosphorylation is not equivalent to eIF-4E inactivation and does not explain the antiproliferative property of rapamycin.

Expression of the eukaryotic translation initiation factors 4E and 2alpha in non-Hodgkin's lymphomas

Transition of cells from quiescence to proliferation requires an increase in the rate of protein synthesis, which is regulated in part by two key translation initiation factors, 4E and 2alpha. The expression and activity of both factors are increased transiently when normal resting cells are stimulated to proliferate. They are constitutively elevated in oncogene transformed cultured cells, and overexpression of either initiation factor in rodent cells makes them tumorigenic. In this study we investigate an association between the expression of translation initiation factors and lymphomagenesis. We have analyzed the expression of the protein synthesis initiation factors 4E and 2alpha by immunohistochemistry in reactive lymph nodes and several types of non-Hodgkin's lymphoma representing a wide range of clinical behaviors based on the Revised European-American Lymphoma behavioral classification. The study included 7 benign lymph nodes with follicular hyperplasia, 26 indolent lymphomas (6 marginal zone lymphomas, 7 small lymphocytic lymphomas, and 13 follicular lymphomas, grades 1 and 2), 16 moderately aggressive lymphomas (8 mantle cell lymphomas and 8 follicular lymphomas, grade 3), 24 aggressive lymphomas (14 large-B-cell lymphomas and 10 anaplastic large-cell lymphomas), and 15 highly aggressive lymphomas (7 lymphoblastic lymphomas and 8 Burkitt's lymphomas). Strong expression of initiation factors 4E and 2alpha was demonstrated in the germinal centers of reactive follicles. Minimal or no expression was seen in the mantle zones and surrounding paracortices, indicating that high expression of initiation factors 4E and 2alpha is associated with the active proliferation of lymphocytes. Most cases of aggressive and highly aggressive lymphomas showed strong expression of initiation factors 4E and 2alpha, in contrast to the cases of indolent and moderately aggressive lymphoma, in which their expression was intermediate between the germinal centers and the mantles of reactive follicles. A positive correlation was found between the expression of both initiation factors 4E and 2alpha and the Revised European-American Lymphoma behavior classification (P < 0.05). Thus, constitutively increased expression of initiation factors 4E and 2alpha may play an important role in the development of lymphomas and is correlated with their biological aggressiveness.

The role of c-myc in cellular growth control

Cell division is coupled to cell growth. Since some c-myc target genes are regulators of cell growth while others function in cell division pathways, c-myc is apparently poised at the interface of these processes. Cell culture systems have shown specific myc-associated growth phenotypes. Increased cell growth precedes DNA synthesis after myc activation in cells expressing myc-estrogen receptor fuson constructs and cells lacking c-myc exhibit a marked loss of protein synthesis. A number of candidate c-myc target genes regulate processes required for cell growth including rRNA transcription and processing, ribosomal protein transcription and translation, and translation initiation. These interactions all have the potential to account for the growth phenotypes in c-myc mutant cells. The ability of translation initiation factors, including eIF4E, to transform cells makes them particularly interesting targets of c-myc. Further evaluation of these target genes will provide important insights into growth control and c-myc's functions in cellular proliferation.

Upregulation of protein synthesis initiation factor eIF-4E is an early event during colon carcinogenesis

A general increase in protein synthesis and a specific increase in the synthesis of growth-promoting proteins are necessary for mitogenesis. Regulation of protein synthesis, as well as preferential translation of some mRNAs coding for growth promoting proteins (e.g. cyclin D1), involves the essential protein synthesis initiation factor eIF-4E. This factor is induced by various oncoproteins, and, when overexpressed, it can transform cultured cells. In this report we explore the roles of eIF-4E in human neoplastic disorders of the colon and in the regulation of general and specific protein synthesis. We find that eIF-4E is increased in colon adenomas and carcinomas, and this increase is accompanied in most but not all cases by elevation of cyclin D1 levels. While general protein synthesis is increased by eIF-4E overexpression in cultured cells, only a small proportion of proteins is preferentially upregulated by eIF-4E, as revealed by two-dimensional gel electrophoresis. These results are consistent with the view that eIF-4E plays a role in carcinogenesis by increasing general protein synthesis and by preferentially upregulating a subset of putative growth promoting proteins. Our results, taken together with the recent findings that c-myc transcription is negatively regulated by APC and our earlier data on transcriptional activation of eIF-4E expression by c-Myc suggest that eIF-4E is a downstream target of the APC/beta-catenin/Tcf-4 pathway, and is strongly involved in colon tumorigenesis.

Coordination of cell growth with cell division

Proliferating cells must increase their mass coordinately with cell division. Recent evidence suggests that coupling of cell growth with cell division might be achieved by making synthesis of activators of cell division particularly sensitive to the capacity of the cell's protein synthesis machinery.