A novel function of the MA-3 domains in transformation and translation suppressor Pdcd4 is essential for its binding to eukaryotic translation initiation factor 4A

Negative regulation of human U6 snRNA promoter by p38 kinase through Oct-1

Recruitment of Oct-1 protein to the octamer sequence of U6 promoter is critical for optimal transcription by RNA polymerase III. Here we report that p38 kinase inhibitors, SB202190 and SB203580, stimulated U6 promoter activity and this stimulation can be observed only in the presence of octamer sequence. SB202190-treated cell nuclear extract had about 50% increase in Oct-1 binding activity suggesting that the increased U6 promoter activity by p38 kinase inhibitor is mediated through Oct-1. Mutation in octamer sequence significantly reduced the SB202190-stimulated U6 promoter transcription and the distance between octamer and proximal sequence element of U6 promoter is also critical for the p38 kinase inhibitor-stimulated activity. Exogenous Oct-1 expression showed a concentration-dependent activation of U6 promoter that was further stimulated by the p38 kinase inhibitors. When cells were treated with p38 kinase inducer, hydrogen peroxide or phorbol 12-myristate 13-acetate (PMA), U6 promoter activity was down regulated and this inhibition was reversed by p38 kinase inhibitors. Over-expression of p38α kinase down-regulated U6 promoter activity and this inhibition was further enhanced by PMA and p38 kinase inhibitors reversed this inhibition. p38 kinase inhibitor-treated cells had 50% more U6 RNA than the control cells. Taken together, our results show a negative correlation between the p38 kinase levels and Oct-1 binding on U6 promoter, suggesting that U6 promoter is negatively regulated by p38 kinase.

Myostatin inactivation increases myotube size through regulation of translational initiation machinery

Myostatin deficiency leads in skeletal muscle overgrowth but the precise molecular mechanisms underlying this hypertrophy are not well understood. In this study, to gain insight into the role of endogenous myostatin in the translational regulation, we used an in vitro model of cultured satellite cells derived from myostatin knock-out mice. Our results show that myostatin knock-out myotubes are larger than control myotubes and that this phenotype is associated with an increased activation of the Akt/mTOR signaling pathway, a known regulator of muscle hypertrophy. These results demonstrate that hypertrophy due to myostatin deficiency is preserved in vitro and suggest that myostatin deletion results in an increased protein synthesis. Accordingly, the rates of global RNA content, polysome formation and protein synthesis are all increased in myostatin-deficient myotubes while they are counteracted by the addition of recombinant myostatin. We furthermore demonstrated that genetic deletion of myostatin stimulates cap-dependent translation by positively regulating assembly of the translation preinitiation complex. Together the data indicate that myostatin controls muscle hypertrophy in part by regulating protein synthesis initiation rates, that is, translational efficiency.

Tumor suppressor PDCD4 represses internal ribosome entry site-mediated translation of antiapoptotic proteins and is regulated by S6 kinase 2

Apoptosis can be regulated by extracellular signals that are communicated by peptides such as fibroblast growth factor 2 (FGF-2) that have important roles in tumor cell proliferation. The prosurvival effects of FGF-2 are transduced by the activation of the ribosomal protein S6 kinase 2 (S6K2), which increases the expression of the antiapoptotic proteins X chromosome-linked Inhibitor of Apoptosis (XIAP) and Bcl-x(L). We now show that the FGF-2-S6K2 prosurvival signaling is mediated by the tumor suppressor programmed cell death 4 (PDCD4). We demonstrate that PDCD4 specifically binds to the internal ribosome entry site (IRES) elements of both the XIAP and Bcl-x(L) messenger RNAs and represses their translation by inhibiting the formation of the 48S translation initiation complex. Phosphorylation of PDCD4 by activated S6K2 leads to the degradation of PDCD4 and thus the subsequent derepression of XIAP and Bcl-x(L) translation. Our results identify PDCD4 as a specific repressor of the IRES-dependent translation of cellular mRNAs (such as XIAP and Bcl-x(L)) that mediate FGF-2-S6K2 prosurvival signaling and provide further insight into the role of PDCD4 in tumor suppression.

Fine scale analysis of gene expression in Drosophila melanogaster gonads reveals Programmed cell death 4 promotes the differentiation of female germline stem cells

Background

Germline stem cells (GSCs) are present in the gonads of Drosophila females and males, and their proper maintenance, as well as their correct differentiation, is essential for fertility and fecundity. The molecular characterization of factors involved in maintenance and differentiation is a major goal both in Drosophila and stem cell research. While genetic studies have identified many of these key factors, the use of genome-wide expression studies holds the potential to greatly increase our knowledge of these pathways.

Results

Here we report a genome-wide expression study that uses laser cutting microdissection to isolate germline stem cells, somatic niche cells, and early differentiating germ cells from female and male gonads. Analysis of this data, in association with two previously published genome-wide GSC data sets, revealed sets of candidate genes as putatively expressed in specific cell populations. Investigation of one of these genes, CG10990 the Drosophila ortholog of mammalian Programmed cell death 4 (Pdcd4), reveals expression in female and male germline stem cells and early differentiating daughter cells. Functional analysis demonstrates that while it is not essential for oogenesis or spermatogenesis, it does function to promote the differentiation of GSCs in females. Furthermore, in females, Pdcd4 genetically interacts with the key differentiation gene bag of marbles (bam) and the stem cell renewal factor eIF4A, suggesting a possible pathway for its function in differentiation.

Conclusions

We propose that Pdcd4 promotes the differentiation of GSC daughter cells by relieving the eIF4A-mediated inhibition of Bam.

Tumour suppressive function and modulation of programmed cell death 4 (PDCD4) in ovarian cancer

Background

Programmed cell death 4 (PDCD4), originally identified as the neoplastic transformation inhibitor, was attenuated in various cancer types. Our previous study demonstrated a continuous down-regulation of PDCD4 expression in the sequence of normal-borderline-malignant ovarian tissue samples and a significant correlation of PDCD4 expression with disease-free survival. The objective of the current study was to further investigate the function and modulation of PDCD4 in ovarian cancer cells.

Principal Findings

We demonstrated that ectopic PDCD4 expression significantly inhibited cell proliferation by inducing cell cycle arrest at G₁ stage and up-regulation of cell cycle inhibitors of p27 and p21. Cell migration and invasion were also inhibited by PDCD4. PDCD4 over-expressing cells exhibited elevated phosphatase and tensin homolog (PTEN) and inhibited protein kinase B (p-Akt). In addition, the expression of PDCD4 was up-regulated and it was exported to the cytoplasm upon serum withdrawal treatment, but it was rapidly depleted via proteasomal degradation upon serum re-administration. Treatment of a phosphoinositide 3-kinase (PI3K) inhibitor prevented the degradation of PDCD4, indicating the involvement of PI3K-Akt pathway in the modulation of PDCD4.

Conclusion

PDCD4 may play a critical function in arresting cell cycle progression at key checkpoint, thus inhibiting cell proliferation, as well as suppressing tumour metastasis. The PI3K-Akt pathway was implied to be involved in the regulation of PDCD4 degradation in ovarian cancer cells. In response to the stress condition, endogenous PDCD4 was able to shuttle between cell compartments to perform its diverted functions.

Deconvoluting mTOR biology

In metazoans, TOR is an essential protein that functions as a master regulator of cellular growth and proliferation. Over the past decade, there has been an explosion of information about this critical master kinase, ranging from the composition of the TOR protein complex to its ability to act as an integrator of numerous extracellular signals. Unfortunately, this plethora of information has also raised numerous questions regarding TOR function. Currently, the prevailing view is that mammalian TOR (mTOR) exists in at least two molecular complexes, mTORC1 and mTORC2, which are largely defined by the presence of either RAPTOR or RICTOR. However, additional co-factors have been identified for each complex, and their importance in mediating mTOR signals has been incompletely elucidated. Similarly, there are differences in mTOR function that reflect the tissue of origin. In this review, we present an alternative view to mTOR complex formation and function, which envisions mTOR regulation and signal propagation as a reflection of cell type- and basal state-dependent conditions. The re-interpretation of mTOR biology in this framework may facilitate the design of therapies most likely to effectively inhibit this central regulator of cell behavior.

Inhibitors of translation targeting eukaryotic translation initiation factor 4A

The RNA helicases eIF4AI and eIF4AII play key roles in recruiting ribosomes to mRNA templates during eukaryotic translation initiation. Small molecule inhibitors of eIF4AI and eIF4AII have been useful for chemically dissecting their role in translation in vitro and in vivo. Here, we describe a screen performed on a small focused library of kinase inhibitors to identify a novel helicase inhibitor. We describe assays that have been critical for characterizing novel RNA helicase inhibitors.

Hydrogen sulfide inhibits high glucose-induced matrix protein synthesis by activating AMP-activated protein kinase in renal epithelial cells

Hydrogen sulfide, a signaling gas, affects several cell functions. We hypothesized that hydrogen sulfide modulates high glucose (30 mm) stimulation of matrix protein synthesis in glomerular epithelial cells. High glucose stimulation of global protein synthesis, cellular hypertrophy, and matrix laminin and type IV collagen content was inhibited by sodium hydrosulfide (NaHS), an H(2)S donor. High glucose activation of mammalian target of rapamycin (mTOR) complex 1 (mTORC1), shown by phosphorylation of p70S6 kinase and 4E-BP1, was inhibited by NaHS. High glucose stimulated mTORC1 to promote key events in the initiation and elongation phases of mRNA translation: binding of eIF4A to eIF4G, reduction in PDCD4 expression and inhibition of its binding to eIF4A, eEF2 kinase phosphorylation, and dephosphorylation of eEF2; these events were inhibited by NaHS. The role of AMP-activated protein kinase (AMPK), an inhibitor of protein synthesis, was examined. NaHS dose-dependently stimulated AMPK phosphorylation and restored AMPK phosphorylation reduced by high glucose. Compound C, an AMPK inhibitor, abolished NaHS modulation of high glucose effect on events in mRNA translation as well as global and matrix protein synthesis. NaHS induction of AMPK phosphorylation was inhibited by siRNA for calmodulin kinase kinase β, but not LKB1, upstream kinases for AMPK; STO-609, a calmodulin kinase kinase β inhibitor, had the same effect. Renal cortical content of cystathionine β-synthase and cystathionine γ-lyase, hydrogen sulfide-generating enzymes, was significantly reduced in mice with type 1 diabetes or type 2 diabetes, coinciding with renal hypertrophy and matrix accumulation. Hydrogen sulfide is a newly identified modulator of protein synthesis in the kidney, and reduction in its generation may contribute to kidney injury in diabetes.

Transcriptional Regulation of CRD-BP by c-myc: Implications for c-myc Functions

The coding region determinant binding protein, CRD-BP, is a multifunctional RNA binding protein involved in different processes such as mRNA turnover, translation control, and localization. It is mostly expressed in fetal and neonatal tissues, where it regulates many transcripts essential for normal embryonic development. CRD-BP is scarce or absent in normal adult tissues but reactivated and/or overexpressed in various neoplastic and preneoplastic tumors and in most cell lines. Its expression has been associated with the most aggressive form of some cancers. CRD-BP is an important regulator of different genes including a variety of oncogenes or proto-oncogenes (c-myc, β-TrCP1, GLI1, etc.). Regulation of CRD-BP expression is critical for proper control of its targets as its overexpression may play an important role in abnormal cell proliferation, suppression of apoptosis, invasion, and metastasis. Molecular bases of the regulatory mechanisms governing CRD-BP expression are still not completely elucidated. In this article, we have identified c-myc as a novel transcriptional regulator of CRD-BP. We show that c-myc binds to CRD-BP promoter and induces its transcription. This induction of CRD-BP expression contributes to the role of c-myc in the regulation of translation, increase in cell size, and acceleration of cell cycle progression via a mechanism involving upregulation of β-TrCP1 levels and activities and accelerated degradation of PDCD4.

Association of Tumor Suppressor Protein Pdcd4 With Ribosomes Is Mediated by Protein-Protein and Protein-RNA Interactions

The Pdcd4 (programmed cell death gene 4) gene has been implicated as a novel tumor suppressor gene in the development of several types of human cancer. The Pdcd4 protein is believed to act as a translation suppressor of mRNAs containing structured 5' UTRs. Pdcd4 contains 2 copies of so-called MA3 domains that mediate tight interactions with the translation initiation factor eIF4A, resulting in the inhibition of the eIF4A helicase activity. The N-terminal part of Pdcd4, which has been less well characterized, binds RNA in vitro, but as yet, it has not been clear whether RNA binding by Pdcd4 plays a role in vivo. Here, the authors have identified 2 highly conserved clusters of basic amino acid residues that are essential for the RNA binding activity of Pdcd4. They also show that a substantial fraction of Pdcd4 is present, together with small ribosomal subunits, in translation preinitiation complexes. Using mutants that disrupt RNA binding or the Pdcd4-eIF4A interaction, they demonstrate that the ribosomal association of Pdcd4 is dependent on its RNA binding activity as well as on its ability to interact with eIF4A. Their work provides the first direct evidence for an essential role of the Pdcd4 RNA binding activity in vivo and suggests that RNA binding is required for recruiting Pdcd4 to the translation machinery.

Targeting mTOR for the treatment of AML. New agents and new directions

Despite recent advances in the field, the treatment of patients with acute myeloid leukemia (AML) remains challenging and difficult. Although chemotherapeutic agents induce remissions in a large number of patients, many of them eventually relapse and die. A major goal for the development of new approaches for the treatment of AML is to enhance the antileukemic effects of standard chemotherapeutics and to design effective combinations targeting non-overlapping cellular pathways. The PI3K/Akt/mTOR signaling pathway plays a critical role in survival and growth of malignant cells and its targeting has been the focus of extensive work and research efforts over the last two decades. It now appears possible that a major limitation of the first generation of mTOR inhibitors can be overcome by a new class of catalytic inhibitors of mTOR. There is emerging evidence that such compounds target both TORC1 and TORC2 and elicit much more potent responses against early leukemic precursors in vitro. In addition, recent studies have shown that combinations of such agents with cytarabine result in enhanced antileukemic responses in vitro, raising the prospect and potential of use of these agents in combination regimens for the treatment of AML.

Tumor suppressor protein Pdcd4 inhibits translation of p53 mRNA

The tumor suppressor protein Pdcd4 is thought to suppress translation of mRNAs containing structured 5'-UTRs by interacting with translation initiation factor eIF4A and inhibiting its helicase activity. However, natural target mRNAs regulated by Pdcd4 so far are mostly unknown. Here, we identified p53 mRNA as a translational target of Pdcd4. We found that Pdcd4 is associated with p53 mRNA and suppresses its translation. The inhibitory effect of Pdcd4 on the translation of p53 mRNA depends on the ability of Pdcd4 to interact with eIF4A and is mediated by the 5'-UTR of p53 mRNA, which is able to form a stable stem-loop structure. We show that treatment of cells with DNA-damaging agents decreases the expression of Pdcd4. This suggests that translational suppression by Pdcd4 plays a role in maintaining a low level of p53 in unstressed cells and that this suppression is abrogated due to low levels of Pdcd4 after DNA damage. Overall, our work demonstrates for the first time that Pdcd4 is directly involved in translational suppression of a natural mRNA with a 5'-structured UTR and provides novel insight into the translational control of p53 expression.

Transcription and translation in a package deal: the TISU paradigm

The major strategy for cap dependent translation involves ribosomal scanning. In the scanning mechanism the small ribosomal subunit is recruited to the mRNA through the m7G cap and then scans the 5' UTR until it reaches an AUG codon. This short review focuses on a recently discovered alternative strategy of cap-dependent translation that operates without scanning, but nonetheless is highly efficient and accurate. This non-scanning translation is directed by the Translation Initiator of Short 5' UTR (TISU) element. TISU is strictly located close to the 5' end of the mRNA, resulting in a very short 5' UTR. It is present in a sizable number of mammalian genes, many of them with fundamental cellular functions. In addition to its unique translational activity, TISU is also a transcription regulatory element that is specifically enriched in TATA-less promoters. Thus TISU represents a prototype regulatory element that links mammalian transcription to a specific mode of translation initiation.

EIN2 regulates salt stress response and interacts with a MA3 domain-containing protein ECIP1 in Arabidopsis

Ethylene signalling regulates plant growth and development. However, its roles in salt stress response are less known. Here we studied functions of EIN2, a central membrane protein of ethylene signalling, and its interacting protein ECIP1 in salt stress responses. Mutation of EIN2 led to extreme salt sensitivity as revealed by phenotypic and physiological changes, and overexpression of C-terminus of EIN2 suppressed salt sensitivity in ein2-5, indicating that EIN2 is required for salt tolerance. Downstream components EIN3 and EIL1 are also essential for salt tolerance because ein3-1eil1-1 double mutant showed extreme salt-sensitive phenotype. A MA3 domain-containing protein ECIP1 was further identified to interact with EIN2 in yeast two-hybrid assay and GST pull-down assay. Loss-of-function of ECIP1 resulted in enhanced ethylene response but altered salt response during seed germination and plant growth. Double mutant analysis revealed that ein2-1 was epistatic to ecip1, and ecip1 mutation partially suppressed ethylene-insensitivity of etr2-1 and ein4-1. These studies strengthen that interactions between ECIP1 and EIN2 or ethylene receptors regulate ethylene response and stress response.

Silencing of microRNA-21 in vivo ameliorates autoimmune splenomegaly in lupus mice

MicroRNAs (miRNAs) have been implicated in B cell lineage commitment, regulation of T cell differentiation, TCR signalling, regulation of IFN signalling, and numerous other immunological processes. However, their function in autoimmunity, and specifically in systemic lupus erythematosus (SLE), remains poorly understood. B6.Sle123 is a spontaneous genetic mouse model of SLE characterized by autoantibody production, lymphosplenomegaly, and glomerulonephritis. We identified several differentially regulated miRNAs in B and T lymphocytes of B6.Sle123 mice. We found that miR-21 expression in lupus B and T cells is up-regulated and that in vivo silencing of miR-21 using a tiny seed-targeting LNA reversed splenomegaly, one of the cardinal manifestations of autoimmunity in B6.Sle123 mice, and de-repressed PDCD4 expression in vivo and in vitro. In addition, treatment with anti-miR-21 altered CD4/CD8 T cell ratios and reduced Fas receptor-expressing lymphocyte populations. Our study shows that tiny LNAs can be used to efficiently antagonize endogenous miRNAs in peripheral lymphocytes in vivo and in primary lymphocytes cultured ex vivo and can alter the course of a spontaneous genetic disease in mice.

MicroRNAs in Development and Disease

MicroRNAs (miRNAs) are a class of posttranscriptional regulators that have recently introduced an additional level of intricacy to our understanding of gene regulation. There are currently over 10,000 miRNAs that have been identified in a range of species including metazoa, mycetozoa, viridiplantae, and viruses, of which 940, to date, are found in humans. It is estimated that more than 60% of human protein-coding genes harbor miRNA target sites in their 3′ untranslated region and, thus, are potentially regulated by these molecules in health and disease. This review will first briefly describe the discovery, structure, and mode of function of miRNAs in mammalian cells, before elaborating on their roles and significance during development and pathogenesis in the various mammalian organs, while attempting to reconcile their functions with our existing knowledge of their targets. Finally, we will summarize some of the advances made in utilizing miRNAs in therapeutics.

Protein arginine methyltransferase 5 accelerates tumor growth by arginine methylation of the tumor suppressor programmed cell death 4

Programmed cell death 4 (PDCD4) has been described as a tumor suppressor, with high expression correlating with better outcomes in a number of cancer types. Yet a substantial number of cancer patients with high PDCD4 in tumors have poor survival, suggesting that oncogenic pathways may inhibit or change PDCD4 function. Here, we explore the significance of PDCD4 in breast cancer and identify protein arginine methyltransferase 5 (PRMT5) as a cofactor that radically alters PDCD4 function. Specifically, we find that coexpression of PDCD4 and PRMT5 in an orthotopic model of breast cancer causes accelerated tumor growth and that this growth phenotype is dependent on both the catalytic activity of PRMT5 and a site of methylation within the N-terminal region of PDCD4. In agreement with the xenograft model, elevated PDCD4 expression was found to correlate with worse outcome within the cohort of breast cancer patients whose tumors contain higher levels of PRMT5. These results reveal a new cofactor for PDCD4 that alters its tumor suppressor functions and point to the utility of PDCD4/PRMT5 status as both a prognostic biomarker and a potential target for chemotherapy.

Pdcd4 directly binds the coding region of c-myb mRNA and suppresses its translation

Pdcd4 is a novel tumor suppressor protein that functions in the nucleus and the cytoplasm, and appears to be involved in the regulation of transcription and translation. In the cytoplasm, Pdcd4 has been implicated in the suppression of translation of mRNAs containing structured 5'-untranslated regions; however, the mechanisms that recruit Pdcd4 to specific target mRNAs and the identities of these mRNAs are mostly unknown. In this study, we have identified c-myb mRNA as the first natural translational target mRNA of Pdcd4. We have found that translational suppression of c-myb mRNA by Pdcd4 is dependent on sequences located within the c-myb-coding region. Furthermore, we have found that the N-terminal domain of Pdcd4 has an important role in targeting Pdcd4 to c-myb RNA by mediating preferential RNA binding to the Pdcd4-responsive region of c-myb mRNA. Overall, our work demonstrates for the first time that Pdcd4 is directly involved in translational suppression of a natural mRNA and provides the first evidence for a key role of the RNA-binding domain in targeting Pdcd4 to a specific mRNA.

Highly increased maspin expression corresponds with up-regulation of miR-21 in endometrial cancer: a preliminary report

BACKGROUND

Maspin and programmed cell death 4 (Pdcd4) are tumor suppressor genes, and miR-21 is overexpressed in many solid tumors and was proven to negatively regulate a number of tumor suppressor genes including maspin and Pdcd4.The purpose of this study was to investigate the expression of maspin, Pdcd4, and miR-21 and their interrelations with clinicopathologic features in endometrial cancer using a quantitative approach.

METHODS

Maspin, Pdcd4, and miR-21 expressions were evaluated by a real-time polymerase chain reaction in 20 endometrial cancer and 10 normal endometrium samples.

RESULTS

Maspin showed a significantly increased expression in endometrial cancer samples compared with the control group and was up-regulated by a mean factor of 46.54 (SE range, 2.367-1160.26; 95% confidence interval, 0.515-15001, P < 0.0001). Expression of miR-21 was found significantly up-regulated in the sample group in comparison to control group by a mean factor of 2.312 (SE range, 0.741-7.778; 95% confidence interval 0.191-15.0, P = 0.028). No significant differences were present in the expression level of Pdcd4 between endometrial cancer and control groups. Comparison between IA and more advanced International Federation of Gynecology and Obstetrics stages of endometrial cancer in regard to expression levels of maspin, Pdcd4, and miR-21 did not reveal any significant differences. Similarly, no differences were encountered when histopathologic grading, myometrial invasion, age, body mass index, and parity were taken into consideration.

CONCLUSIONS

Association between increased maspin expression and up-regulation of miR-21 in endometrial cancer suggests distinct and tissue-specific relationships of the 2 molecules in this type of malignancy and requires further studies that would reveal its clinical relevance.

Structure of the tandem MA-3 region of Pdcd4 protein and characterization of its interactions with eIF4A and eIF4G: molecular mechanisms of a tumor suppressor

One of the key regulatory points of translation initiation is recruitment of the 43S preinitation complex to the 5' mRNA cap by the eIF4F complex (eIF4A, eIF4E, and eIF4G). The tumor suppressor protein Pdcd4 has been shown to inhibit cap-dependent translation by interacting tightly with the RNA helicase eIF4A via its tandem MA-3 domains. The NMR studies reported here reveal a fairly extensive and well defined interface between the two MA-3 domains in solution, which appears to be stabilized by a network of interdomain salt bridges and hydrogen bonds, and reveals a unique orientation of the two domains. Characterization of the stoichiometry of the Pdcd4-eIF4A complex suggests that under physiological conditions Pdcd4 binds to a single molecule of eIF4A, which involves contacts with both Pdcd4 MA-3 domains. We also show that contacts mediated by a conserved acidic patch on the middle MA-3 domain of Pdcd4 are essential for forming a tight complex with eIF4A in vivo, whereas the equivalent region of the C-terminal MA-3 domain appears to have no role in complex formation in vivo. The formation of a 1:1 eIF4A-Pdcd4 complex in solution is consistent with the reported presence in vivo of only one molecule of eIF4A in the eIF4F complex. Pdcd4 has also been reported to interact directly with the middle region of eIF4G, however, we were unable to obtain any evidence for even a weak, transient direct interaction.

Regulation of mammalian target of rapamycin and mitogen activated protein kinase pathways by BCR-ABL

A large body of evidence has established that BCR-ABL regulates engagement and activation of mammalian target of rapamycin (mTOR) and mitogen activated protein kinase (MAPK) signaling cascades. mTOR-mediated signals, as well as signals transduced by ERK, JNK, and p38 MAPK, are important components of the aberrant signaling induced by BCR-ABL. Such deregulation of mTOR or MAPK pathways contributes to BCR-ABL leukemogenesis, and their targeting with selective inhibitors provides an approach to enhance antileukemic responses and/or overcome leukemic cell resistance in chronic myeloid leukemia (CML) and Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukemia (ALL). This review explores recent advances in our understanding of mTOR and MAPK signaling in BCR-ABL-expressing leukemias and discusses the potential therapeutic targeting of these pathways in CML and Ph+ ALL.

PDCD4 nuclear loss inversely correlates with miR-21 levels in colon carcinogenesis

Programmed cell death 4 (PDCD4) has recently been demonstrated to be a new tumor suppressor gene involved in colon carcinogenesis. PDCD4 immunohistochemical expression was assessed in 300 polypoid lesions of the colon mucosa (50 hyperplastic polyps [HP], 50 serrated adenomas [SA], 50 tubular adenomas with low-grade-intraepithelial neoplasia [LG-IEN], 50 tubular adenomas with high-grade-IEN [HG-IEN]), and in 50 colon adenocarcinomas (CRC). As normal controls, we considered 50 biopsy samples obtained from patients with irritable bowel syndrome (N). We further investigated PDCD4 messenger RNA (mRNA) levels by quantitative real-time polymerase chain reaction (PCR) in a different series of N, LG-IEN, HG-IEN, and CRC biopsy samples. miR-21 expression (an important PDCD4-expression regulator) was also determined by quantitative real-time PCR and in situ hybridization. Normal colocytes and HP featured strong PDCD4 nuclear immunostaining whereas a significantly lower PDCD4 nuclear expression was observed in dysplasia (low- and high-grade adenomas and SA) and invasive CRC. PDCD4 immunostaining and mRNA levels decreased significantly as the phenotypic changes occurring during colon carcinogenesis progressively increased (p < 0.001). As expected, miR-21 expression was significantly upregulated in preneoplastic/neoplastic samples, consistent with PDCD4 downregulation. These results consistently support the use of nuclear PDCD4 immunohistochemical downregulation as a novel biomarker for the diagnosis of dysplastic/neoplastic lesions in colon biopsy samples.

Integrated analysis of mutations, miRNA and mRNA expression in glioblastoma

BACKGROUND

Glioblastoma arises from complex interactions between a variety of genetic alterations and environmental perturbations. Little attention has been paid to understanding how genetic variations, altered gene expression and microRNA (miRNA) expression are integrated into networks which act together to alter regulation and finally lead to the emergence of complex phenotypes and glioblastoma.

RESULTS

We identified association of somatic mutations in 14 genes with glioblastoma, of which 8 genes are newly identified, and association of loss of heterozygosity (LOH) is identified in 11 genes with glioblastoma, of which 9 genes are newly discovered. By gene coexpression network analysis, we identified 15 genes essential to the function of the network, most of which are cancer related genes. We also constructed miRNA coexpression networks and found 19 important miRNAs of which 3 were significantly related to glioblastoma patients' survival. We identified 3,953 predicted miRNA-mRNA pairs, of which 14 were previously verified by experiments in other groups. Using pathway enrichment analysis we also found that the genes in the target network of the top 19 important miRNAs were mainly involved in cancer related signaling pathways, synaptic transmission and nervous systems processes. Finally, we developed new methods to decipher the pathway connecting mutations, expression information and glioblastoma. We identified 4 cis-expression quantitative trait locus (eQTL): TP53, EGFR, NF1 and PIK3C2G; 262 trans eQTL and 26 trans miRNA eQTL for somatic mutation; 2 cis-eQTL: NRAP and EGFR; 409 trans- eQTL and 27 trans- miRNA eQTL for lost of heterozygosity (LOH) mutation.

CONCLUSIONS

Our results demonstrate that integrated analysis of multi-dimensional data has the potential to unravel the mechanism of tumor initiation and progression.

Competitive and noncompetitive binding of eIF4B, eIF4A, and the poly(A) binding protein to wheat translation initiation factor eIFiso4G

Eukaryotic translation initiation factor 4G (eIF4G) functions to organize the assembly of initiation factors required for the recruitment of a 40S ribosomal subunit to an mRNA and for interacting with the poly(A) binding protein (PABP). Many eukaryotes express two highly similar eIF4G isoforms. eIFiso4G, one of two isoforms in plants, is highly divergent and unusually small in size. Unlike animal and yeast eIF4G, the domain organization of plant eIF4G proteins is largely unknown. Consequently, little is known about the conservation of plant eIF4G with those in other eukaryotes. In this study, we show that eIFiso4G is similar to other eIF4G proteins in that there are interaction domains for eIF4A and PABP and we identify, for the first time, the interaction domain for eIF4B. In contrast to previous reports, two eIF4A binding domains in eIFiso4G were identified, similar in number and organization to those of animal eIF4G. The eIFiso4G domain organization does differ, however, in that the N-terminal eIF4A binding domain overlaps with the eIF4B and PABP binding domains. Moreover, the eIF4B and PABP binding domains overlap. PABP and eIF4B compete with eIF4A for binding eIFiso4G in the absence of the C-terminal eIF4A binding domain but not when both eIF4A binding domains are present, suggesting that the C-terminal eIF4A interaction domain functions to stabilize the association of eIF4A with eIFiso4G in the presence of eIF4B or PABP. Competitive binding to eIFiso4G was also observed between eIF4B and PABP. These observations reveal an important function of the C-terminal eIF4A binding domain in maintaining the interaction of multiple partner proteins with eIFiso4G despite the substantial divergence in its size and domain organization.

Involvement of microRNA-21 in mediating chemo-resistance to docetaxel in androgen-independent prostate cancer PC3 cells

AIM

To investigate whether microRNA-21 was involved in mediating the chemoresistance of prostate cancer cells to docetaxel.

METHODS

A microarray technique was used to determine the miRNA profile in docetaxel-resistant PC3 cells. Real-time PCR was used to confirm the array results. miR-21 mimics and inhibitors were synthesized and introduced to cells using Lipofectamine 2000. Cell proliferation was examined with the CCK-8 assay. Luciferase reporter containing PDCD 3'UTR was constructed and the activity was detected by a dual luciferase assay. PDCD4 protein expression was evaluated using Western blot.

RESULTS

A docetaxel-resistant prostate cancer PC3 cell line (PC3R) was established . Using microarrays, miR-21 was found to be up-regulated in PC3R cells. Ectopic expression of miR-21 increased the resistance to docetaxel in PC3 wild type cells. In contrast, silencing of miR-21 in PC3R cells sensitized the cells to docetaxel. The IC(50) values for miR-21-silencing cells and control cells were 28.31 and 35.89 nmol/L, respectively. PDCD4, a direct target gene of miR-21, could mediate chemoresistance to docetaxel in PC3 cells.

CONCLUSION

Our findings suggest that miR-21 contributed to the resistance of PC3 cells to docetaxel, and that targeting miR-21 may offer a promising therapeutic approach in sensitizing prostate cancer to docetaxel treatment.

Pdcd4 repression of lysyl oxidase inhibits hypoxia-induced breast cancer cell invasion

Metastasis to bone, liver and lungs is the primary cause of death in breast cancer patients. Our studies have revealed that the novel tumor suppressor Pdcd4 inhibits breast cancer cell migration and invasion in vitro. Loss of Pdcd4 in human nonmetastatic breast cancer cells increased the expression of lysyl oxidase (LOX) mRNA. LOX is a hypoxia-inducible amine oxidase, the activity of which enhances breast cancer cell invasion in vitro and in vivo. Specific inhibition of LOX activity by beta-aminopropionitrile or small interfering RNA decreased the invasiveness of T47D and MCF7 breast cancer cells attenuated for Pdcd4 function. Most significantly, loss of Pdcd4 augments hypoxia induction of LOX as well. Conversely, overexpression of Pdcd4 significantly reversed the hypoxia induction of LOX expression in T47D cells attenuated for Pdcd4. However, Pdcd4 did not affect hypoxia-inducible factor-1 (HIF-1) protein expression or HIF-1-responsive element-luciferase activity in response to hypoxia, suggesting that Pdcd4 regulation of LOX occurs through an HIF-independent mechanism. Nevertheless, the loss of Pdcd4 early in cancer progression may have an important role in the increased sensitivity of cancer cells to hypoxia through increased LOX activity and concomitant enhanced invasiveness.

Regulation of tumor suppressor PDCD4 by novel protein kinase C isoforms

Transforming growth factor-beta1 (TGF-beta1) induces apoptosis in normal hepatocytes and hepatoma cells. PDCD4 is involved in TGF-beta1-induced apoptosis via the Smad pathway. The tumor promoter 12-O-tetradecanoylphorbor-13-acetate (TPA), a protein kinase C stimulator, inhibits TGF-beta1-induced apoptosis. However, the mechanisms of TPA action on PDCD4 expression remain to be elucidated. Therefore. the regulatory mechanism of PDCD4 expression by PKC was investigated. The treatment of the human hepatoma cell line, Huh7 with TPA suppressed PDCD4 protein expression and TGF-beta1 failed to increase the PDCD4 protein expression. PKC inhibitors Ro-31-8425 or bisindolylmaleimide-1-hydrocholoride (pan-PKC inhibitors) and rottlerin (PKCdelta inhibitor), but not Go6976 (PKCalpha inhibitor), enhanced the induction of PDCD4 protein by TGF-beta1. Furthermore, siRNA-mediated knockdown of PKCdelta and epsilon, but not PKCalpha, augmented the TGF-beta1-stimulated PDCD4 protein expression. However, TPA or pan-PKC inhibitor did not alter the PDCD4 mRNA expression either under basal- and TGF-beta1-treated conditions. The down-regulation of PDCD4 by TPA was restored by treatment with the proteasome inhibitor MG132. These data suggest that two isoforms of PKCs are involved in the regulation of the PDCD4 protein expression related to the proteasomal degradation pathway.

The connectivity map links iron regulatory protein-1-mediated inhibition of hypoxia-inducible factor-2a translation to the anti-inflammatory 15-deoxy-delta12,14-prostaglandin J2

Hypoxia-inducible factors 1 and 2 (HIF1 and HIF2) are heterodimeric transcription factors consisting of alpha regulatory subunits and a constitutively expressed beta subunit. The expression of alpha regulatory subunits is promoted by hypoxia, cancer-associated mutations, and inflammatory cytokines. Thus, HIF1 and HIF2 provide a molecular link between cancer and inflammation. We have recently identified novel small molecules that selectively inhibit translation of the HIF2a message and thereby powerfully inhibit the expression of HIF2a target genes. We report here that Connectivity Map analysis links three of these compounds to the anti-inflammatory cytokine 15-deoxy-Delta(12,14)-prostaglandin J(2) (PGJ(2)). As with our identified compounds, PGJ(2) inhibits translation of the HIF2a message in a mammalian target of rapamycin-independent manner by promoting the binding of iron regulatory protein-1 (IRP1) to a noncanonical iron responsive element (IRE) embedded within the 5'-untranslated region of the HIF2a message. The IRE is necessary and sufficient for mediating the effect. Mutation of the IRE sequence, or downregulation of IRP1 expression, blocks the effect of PGJ(2) on HIF2a translation. This is the first report of an endogenous natural molecule regulating HIF2a translation, and it suggests that part of the anti-inflammatory and putative antineoplastic effects of PGJ(2) may be mediated through inhibition of HIF2a within tumor epithelial cells themselves and/or mesenchymal cells of the tumor microenvironment.

Interaction of Pdcd4 with eIF4E inhibits the metastatic potential of hepatocellular carcinoma

Oxidative stress can contribute to the development of hepatocellular carcinoma (HCC) ability of the carcinoma. It has been found that oxidative stress stimulates the phosphorylation of eIF4E primarily through mitogen-activated protein kinase (MAPK) pathways resulting in increased protein translation. Utilizing specific inhibitors of MAPK pathways (SP600125 for c-Jun amino-terminal kinases [JNKs], PD098059 for extracellular signal-regulated kinases [ERKs], and SB203580 for p38 MAPK), we determined that it is primarily the inhibition of JNK that results in the suppression of the increase of p-eIF4E. We also found that PDCD4 inhibits JNK activity resulting in inhibition of the phosphorylation of c-Jun, one isoform of AP-1. We demonstrated that transfection with PDCD4 or inhibition of JNK by SP600125 alters the expression and phosphorylation of eIF4E in the presence of H(2)O(2). PDCD4 results in a stronger inhibitory effect than SP600125.

Pdcd4, a colon cancer prognostic that is regulated by a microRNA

The novel tumor suppressor Pdcd4 inhibits neoplastic transformation, tumor progression and translation. Furthermore, we and others have recently shown that Pdcd4 suppresses invasion and intravasation, at least in part by suppressing expression of the invasion-related urokinase receptor (u-PAR) gene via the transcription factors Sp1/Sp3. Nevertheless, relatively little is known about mechanisms that regulate Pdcd4 expression in cancer. MicroRNAs (miRNAs) have been recently discovered and shown to be naturally occurring non-coding RNAs that control gene expression via specific sites within the 3'UTR of target miRNAs. This short review will focus on our recent finding that the microRNA miR-21 posttranscriptionally regulates Pdcd4, as well as invasion, intravasation, and metastasis. Furthermore, we will review the first translational and clinical results concerning the prognostic value of Pdcd4, in particular our own data that show Pdcd4 to be a novel and independent prognostic factor in colorectal cancer, and a potential supportive diagnostic tool for discriminating normal colonic tissues from benign adenomas and colorectal carcinomas.

Downregulation of E-cadherin is an essential event in activating beta-catenin/Tcf-dependent transcription and expression of its target genes in Pdcd4 knockdown cells

We previously reported that knock-down of tumor suppressor Pdcd4 (ProgrammedCell Death 4) down-regulates E-cadherin expression and activates β-catenin/Tcf (T cell factor) dependent transcription in colon tumor cells. However, the underlying mechanism of these observations remains unknown. In this study, we demonstrated that knock-down of Pdcd4 down-regulates E-cadherin expression through elevated protein level of Snail. Over-expression of Pdcd4 up-regulates E-cadherin expression and inhibits β-catenin/Tcf dependent transcription. We then showed that knock-down of E-cadherin activates β-catenin/Tcf dependent transcription. Conversely, over-expression of E-cadherin in Pdcd4 knock-down cells inhibits β-catenin/Tcf dependent transcription. In addition, Pdcd4 knock-down stimulates u-PAR and c-Myc expression, while u-PAR and c-Myc expression can be reversed by over-expressing E-cadherin in Pdcd4 knock-down cells. Using chromatin immunoprecipitation, we demonstrated that β-catenin/Tcf4 directly binds to the promoters of u-PAR and c-myc in Pdcd4 knock-down cells. Futhermore, knock-down of u-PAR or c-Myc inhibits invasion in Pdcd4 knock-down cells, suggesting that both u-PAR and c-Myc contribute to invasion induced by Pdcd4 knock-down. Taken together, our data demonstrated that elevated Snail expression by Pdcd4 knock-down leads to down-regulation of E-cadherin resulting in activating β-catenin/Tcf dependent transcription and stimulating the expression of c-Myc and u-PAR, thus providing molecular explanation of how Pdcd4 suppresses tumor invasion.

Disruption of the Pdcd4 tumor suppressor gene in chicken DT40 cells reveals its role in the DNA-damage response

The programmed cell death gene 4 (Pdcd4) gene has been implicated as a new tumor suppressor gene in the development of several types of human cancer. Pdcd4 interacts with the translation initiation factor, eIF4A, and is thought to act as a translation inhibitor. Here, we have used the chicken B-cell line DT40 to disrupt the Pdcd4 gene by homologous recombination. Our study shows that cells lacking a functional Pdcd4 gene are viable and have no obvious defects when cultivated under normal growth conditions. However, Pdcd4 knockout cells show an increased sensitivity to agents that cause DNA damage, such as UV light, etoposide or ethyl-methanesulfonate. In summary, our findings show that Pdcd4 has an important function in the cellular response to DNA damage. Low Pdcd4 expression, which is frequently observed in tumor cells, might therefore contribute to tumorigenesis by disturbing the cellular DNA-damage response.

PDCD4 inhibits the malignant phenotype of ovarian cancer cells

Programmed cell death 4 (PDCD4) is a newly identified tumor suppressor that can inhibit activator protein (AP)-1 activation and protein translation. Our previous studies indicate that lost or reduced PDCD4 expression is associated with the progression of ovarian carcinoma. However, direct evidence that PDCD4 inhibits malignant phenotype of human cancer cells is limited. In the present study, we found that PDCD4 expression in ovarian cancer cell lines (SKOV3, 3AO, and CAOV3) inhibited significantly their proliferation and cell cycle progression, and induced apoptosis. More importantly, up-regulation of PDCD4 expression decreased the colony-forming capacity of ovarian cancer cells in vitro and tumorigenic capacity in mice. These results demonstrate that PDCD4 can suppress the malignant phenotype of ovarian cancer cells, and may represent a novel therapeutic target for the treatment of ovarian cancer.

miR-21: a small multi-faceted RNA

More than 1000 microRNAs (miRNAs) are expressed in human cells, some tissue or cell type specific, others considered as house-keeping molecules. Functions and direct mRNA targets for some miRNAs have been relatively well studied over the last years. Every miRNA potentially regulates the expression of numerous protein-coding genes (tens to hundreds), but it has become increasingly clear that not all miRNAs are equally important; diverse high-throughput screenings of various systems have identified a limited number of key functional miRNAs over and over again. Particular miRNAs emerge as principal regulators that control major cell functions in various physiological and pathophysiological settings. Since its identification 3 years ago as the miRNA most commonly and strongly up-regulated in human brain tumour glioblastoma [1], miR-21 has attracted the attention of researchers in various fields, such as development, oncology, stem cell biology and aging, becoming one of the most studied miRNAs, along with let-7, miR-17-92 cluster ('oncomir-1'), miR-155 and a few others. However, an miR-21 knockout mouse has not yet been generated, and the data about miR-21 functions in normal cells are still very limited. In this review, we summarise the current knowledge of miR-21 functions in human disease, with an emphasis on its regulation, oncogenic role, targets in human cancers, potential as a disease biomarker and novel therapeutic target in oncology.

Reduced expression of an RNA-binding protein by prolactin leads to translational silencing of programmed cell death protein 4 and apoptosis in newt spermatogonia

Recent studies indicate that the balance between cell survival and proapoptotic signals determines which cells commit to life or death. We have shown that the balance between follicle-stimulating hormone and prolactin determines differentiation or apoptosis in 7th generation spermatogonia during newt spermatogenesis; however, the molecular mechanisms specifying their fate are poorly understood. Here we show that the newt RNA-binding protein (nRBP) plays a critical role in determining their fate. nRBP was identified as a clone whose mRNA is decreased by prolactin, resulting in the reduction of the protein, which is otherwise expressed predominantly in the spermatogonia. nRBP protein associated with the mRNA for newt programmed cell death protein 4 (nPdcd4) at the 3'-untranslated region. nRBP reduction increased nPdcd4 mRNA but decreased its protein. In a cell-free system, cytoplasmic extracts containing reduced amounts of nRBP and nPdcd4 protein induced apoptosis, whereas adding nRBP protein to the extracts blocked apoptosis. Furthermore, overexpression of nRBP protected cells from apoptosis, stabilized the chimeric transcript containing the nPdcd4 3'-untranslated region, and accelerated its translation. These data suggest that, in the absence of nRBP, nPdcd4 mRNA is not stabilized and its translation is suppressed, leading to apoptosis in the spermatogonia.

The tumour suppressor Pdcd4: recent advances in the elucidation of function and regulation

Pdcd4 (programmed cell death 4) has been known as a tumour suppressor gene and potential target for anticancer therapies for several years. Initially, Pdcd4 was identified as a gene that is up-regulated during apoptosis, but its precise role still remains to be defined. However, there is increasing evidence that Pdcd4 levels influence transcription, as well as translation, modulate different signal transduction pathways and might act as a tumour suppressor. Interestingly, recent data suggest that Pdcd4 function may depend on cell type and/or genetic background. This review summarizes the current knowledge regarding the function and regulation of Pdcd4.

An aptamer-based biosensor for mammalian initiation factor eukaryotic initiation factor 4A

Aptamers are short single-stranded DNA or RNA sequences that are selected in vitro based on their high affinity to a target molecule. Here we demonstrate that an RNA aptamer selected against eukaryotic initiation factor 4A (eIF4A) serves as an efficient biosensor. The aptamer, when immobilized to resin, purifies eIF4A from crude cell extracts by affinity pull-down, and 32P-labeled aptamer can detect some 300 ng of eIF4A by dot-blot analysis. Moreover, by use of an aptamer-immobilized sensor chip, we developed a surface plasmon resonance assay to detect eIF4A at the nanogram level within whole cell lysates after optimizing sample preparation, thereby showing a real-time sensor for eIF4A in cell extract solution.

Topology and regulation of the human eIF4A/4G/4H helicase complex in translation initiation

The RNA helicase eIF4A plays a key role in unwinding of mRNA and scanning during translation initiation. Free eIF4A is a poor helicase and requires the accessory proteins eIF4G and eIF4H. However, the structure of the helicase complex and the mechanisms of stimulation of eIF4A activity have remained elusive. Here we report the topology of the eIF4A/4G/4H helicase complex, which is built from multiple experimentally observed domain-domain contacts. Remarkably, some of the interactions are continuously rearranged during the ATP binding/hydrolysis cycle of the helicase. We show that the accessory proteins modulate the affinity of eIF4A for ATP by interacting simultaneously with both helicase domains and promoting either the closed, ATP-bound conformation or the open, nucleotide-free conformation. The topology of the complex and the spatial arrangement of the RNA-binding surfaces offer insights into their roles in stimulation of helicase activity and the mechanisms of mRNA unwinding and scanning.

Crystal structure of the eIF4A-PDCD4 complex

Tumor suppressor programmed cell death protein 4 (PDCD4) inhibits the translation initiation factor eIF4A, an RNA helicase that catalyzes the unwinding of secondary structure at the 5'-untranslated region of mRNAs and controls the initiation of translation. Here, we determined the crystal structure of the human eIF4A and PDCD4 complex. The structure reveals that one molecule of PDCD4 binds to the two eIF4A molecules through the two different binding modes. While the two MA3 domains of PDCD4 bind to one eIF4A molecule, the C-terminal MA3 domain alone of the same PDCD4 also interacts with another eIF4A molecule. The eIF4A-PDCD4 complex structure suggests that the MA3 domain(s) of PDCD4 binds perpendicular to the interface of the two domains of eIF4A, preventing the domain closure of eIF4A and blocking the binding of RNA to eIF4A, both of which are required events in the function of eIF4A helicase. The structure, together with biochemical analyses, reveals insights into the inhibition mechanism of eIF4A by PDCD4 and provides a framework for designing chemicals that target eIF4A.

Small-molecule inhibitors of HIF-2a translation link its 5'UTR iron-responsive element to oxygen sensing

Cells transiently adapt to hypoxia by globally decreasing protein translation. However, specific proteins needed to respond to hypoxia evade this translational repression. The mechanisms of this phenomenon remain unclear. We screened for and identified small molecules that selectively decrease HIF-2a translation in an mTOR-independent manner, by enhancing the binding of Iron-Regulatory Protein 1 (IRP1) to a recently reported iron-responsive element (IRE) within the 5'-untranslated region (UTR) of the HIF-2a message. Knocking down the expression of IRP1 by shRNA abolished the effect of the compounds. Hypoxia derepresses HIF-2a translation by disrupting the IRP1-HIF-2a IRE interaction. Thus, this chemical genetic analysis describes a molecular mechanism by which translation of the HIF-2a message is maintained during conditions of cellular hypoxia through inhibition of IRP-1-dependent repression. It also provides the chemical tools for studying this phenomenon.

Structural basis for translational inhibition by the tumour suppressor Pdcd4

Pdcd4 is a tumour suppressor protein. It inhibits translation through interaction with translation initiator eIF4A, resulting in the suppression of neoplastic transformation and tumour invasion. Here, we present the crystal structures of an N-terminal-truncated Pdcd4 in free form and in complex with eIF4A. Upon binding to eIF4A, Pdcd4 undergoes a marked conformational change to form a heterotrimeric complex with eIF4A, with one Pdcd4 binding to two eIF4A molecules in two different modes. The binding of Pdcd4 to eIF4A is required to inhibit the enzymatic activity of eIF4A, translation initiation, and AP-1-dependent transcription. Both MA3 domains are required to efficiently compete with the C-terminal domain of eIF4G (eIF4Gc) for binding to eIF4A whereas a single MA3 is sufficient to inhibit translation. Our structural and mutational analyses reveal that Pdcd4 inhibits translation initiation by trapping eIF4A in an inactive conformation, and blocking its incorporation into the eIF4F complex.

siRNA-mediated knockdown of Pdcd4 expression causes upregulation of p21(Waf1/Cip1) expression

The transformation suppressor gene, programmed cell death gene 4 (Pdcd4), inhibits tumor-promoter-mediated transformation of mouse keratinocytes and has been implicated as a tumor suppressor gene in the development of human cancer. The Pdcd4 protein interacts with translation initiation factors eIF4A and eIF4G and binds to RNA, suggesting that it might be involved in regulating protein translation or other aspects of RNA metabolism. To study the function of Pdcd4 in more detail, we have downregulated Pdcd4 expression in HeLa cells by stable expression of shRNA. We have found that diminished Pdcd4 expression leads to increased expression of p21(Waf1/Cip1) and several other p53-regulated genes. Reporter gene studies demonstrate that Pdcd4 interferes with the activation of p53-responsive promoters genes by p53. Pdcd4 knockdown cells show decreased apoptosis and increased survival after UV irradiation. Taken together, our observations suggest a model in which low Pdcd4 expression after DNA damage favors the survival of cells, which would be eliminated by apoptosis under normal levels of Pdcd4 expression. Our results provide the first evidence that Pdcd4 is important role in the DNA-damage response and suggest that low levels of Pdcd4 expression observed in certain tumor cells contribute to tumorigenesis by affecting the fate of DNA-damaged cells.

Translation initiation factor 4A: a prototype member of dead-box protein family

DEAD-box proteins are characterized by nine conserved helicase motifs. Several different DEAD-box proteins are found in eukaryotes, whereas prokaryotes have small number of these proteins. They play important roles in almost all kinds of RNA metabolism including roles in remodeling ribonuclear protein complexes. These proteins are usually very specific and cannot mutually be replaced. Many of these DEAD-box proteins (but not all) have been shown to have RNA-dependent ATPase and ATP-dependent RNA helicase activities. Many of them have also been shown to contain DNA unwinding activities. Translation initiation factor 4A is the prototype of the DEAD-box family of proteins. Actually, the DEAD-box protein family was discovered on the basis of conserved sequence motifs of eIF4A. Motif II (or Walker B motif) has the amino acids D-E-A-D (Asp-Glu-Ala-Asp), which gave the name to the family. In general, the eIF4A is considered as a helicase that locally melts the secondary structures and makes the RNA accessible to nucleases. It is part of the cap-binding complex eIF4F but is also found to be present in a free form. The biochemical activities of eIF4A are reported to be upregulated by eIF4B, eIF4H and eIF4G. It has been proposed that eIF4A helps to unwind secondary structures in the 5'-untranslated region, which are inhibitory for protein synthesis. In plants, it has been shown to play a unique role in abiotic stress tolerance, which suggests a new pathway to engineer to increase the crop production under the stress conditions.

MicroRNA-21 promotes cell transformation by targeting the programmed cell death 4 gene

MicroRNAs (miRNAs) are small noncoding RNA molecules that negatively control expression of target genes in animals and plants. The microRNA-21 gene (mir-21) has been identified as the only miRNA commonly overexpressed in solid tumors of the lung, breast, stomach, prostate, colon, brain, head and neck, esophagus and pancreas. We initiated a screen to identify miR-21 target genes using a reporter assay and identified a potential miR-21 target in the 3'-UTR of the programmed cell death 4 (PDCD4) gene. We cloned the full-length 3'-UTR of human PDCD4 downstream of a reporter and found that mir-21 downregulated, whereas a modified antisense RNA to miR-21 upregulated reporter activity. Moreover, deletion of the putative miR-21-binding site (miRNA regulatory element, MRE) from the 3'-UTR of PDCD4, or mutations in the MRE abolished the ability of miR-21 to inhibit reporter activity, indicating that this MRE is a critical regulatory region. Western blotting showed that Pdcd4 protein levels were reduced by miR-21 in human and mouse cells, whereas quantitative real-time PCR revealed little difference at the mRNA level, suggesting translational regulation. Finally, overexpression of mir-21 in MCF-7 human breast cancer cells and mouse epidermal JB6 cells promoted soft agar colony formation by downregulating Pdcd4 protein levels. The demonstration that miR-21 promotes cell transformation supports the concept that mir-21 functions as an oncogene by a mechanism that involves translational repression of the tumor suppressor Pdcd4.

PDCD4 inhibits translation initiation by binding to eIF4A using both its MA3 domains

Programmed Cell Death 4 (PDCD4) is a protein known to bind eukaryotic initiation factor 4A (eIF4A), inhibit translation initiation, and act as a tumor suppressor. PDCD4 contains two C-terminal MA3 domains, which are thought to be responsible for its inhibitory function. Here, we analyze the structures and inhibitory functions of these two PDCD4 MA3 domains by x-ray crystallography, NMR, and surface plasmon resonance. We show that both MA3 domains are structurally and functionally very similar and bind specifically to the eIF4A N-terminal domain (eIF4A-NTD) using similar binding interfaces. We found that the PDCD4 MA3 domains compete with the eIF4G MA3 domain and RNA for eIF4A binding. Our data provide evidence that PDCD4 inhibits translation initiation by displacing eIF4G and RNA from eIF4A. The PDCD4 MA3 domains act synergistically to form a tighter and more stable complex with eIF4A, which explains the need for two tandem MA3 domains.

Loss of programmed cell death 4 expression marks adenoma-carcinoma transition, correlates inversely with phosphorylated protein kinase B, and is an independent prognostic factor in resected colorectal cancer

BACKGROUND

Programmed cell death 4 (Pdcd4) inhibits malignant transformation, and initial studies of Pdcd4 suggested the regulation of Pdcd4 localization by protein kinase B (Akt). However, supporting patient tissue data are missing, and the diagnostic/prognostic potential of Pdcd4 rarely has been studied. The objectives of the current were 1) to determine Pdcd4 as a diagnostic marker in the adenoma-carcinoma sequence, 2) to support phosphorylated Akt (pAkt)-mediated Pdcd4 regulation in vivo, and 3) to obtain the first prognostic evidence of Pdcd4 in colorectal cancer.

METHODS

Tumor samples and normal tissues from 71 patients with colorectal cancer who were followed prospectively (median follow-up, 36 months) and 42 adenomas were analyzed for Pdcd4, Akt, and pAkt in immunohistochemical and Western blot analyses.

RESULTS

A significant reduction in Pdcd4 was observed between normal mucosa and adenomas and between adenomas and tumor samples (P < .01 and P < .01, respectively). Normal mucosa demonstrated strong nuclear Pdcd4, which was reduced significantly in adenomas (P < .01) and almost was lost in tumors (P < .01). pAkt was correlated inversely with Pdcd4 and with the transition of Pdcd4 from nucleus to cytoplasm (P < .01). Kaplan-Meier analysis (using the Mantel-Cox log-rank test) indicated a significant correlation between the loss of total and nuclear Pdcd4 in tumors and overall survival (P < .05 and P < .02, respectively) and disease-specific survival (P < .01 and P < .01, respectively). In multivariate analysis, loss of total or nuclear Pdcd4 was an independent predictor of disease-specific or overall survival.

CONCLUSIONS

To the authors' knowledge, this is the first study to demonstrate an independent prognostic impact of Pdcd4 and its expression pattern in colorectal cancer. Data from this study support the regulation of Pdcd4 localization by pAkt in vivo. Pdcd4 immunohistochemistry may be useful as a supportive diagnostic tool for the transition between normal, adenoma, and tumor tissues.