Structural basis for inhibition of translation by the tumor suppressor Pdcd4

Human tumor suppressor protein Pdcd4 binds at the mRNA entry channel in the 40S small ribosomal subunit

Translation is regulated mainly in the initiation step, and its dysregulation is implicated in many human diseases. Several proteins have been found to regulate translational initiation, including Pdcd4 (programmed cell death gene 4). Pdcd4 is a tumor suppressor protein that prevents cell growth, invasion, and metastasis. It is downregulated in most tumor cells, while global translation in the cell is upregulated. To understand the mechanisms underlying translational control by Pdcd4, we used single-particle cryo-electron microscopy to determine the structure of human Pdcd4 bound to 40S small ribosomal subunit, including Pdcd4-40S and Pdcd4-40S-eIF4A-eIF3-eIF1 complexes. The structures reveal the binding site of Pdcd4 at the mRNA entry site in the 40S, where the C-terminal domain (CTD) interacts with eIF4A at the mRNA entry site, while the N-terminal domain (NTD) is inserted into the mRNA channel and decoding site. The structures, together with quantitative binding and in vitro translation assays, shed light on the critical role of the NTD for the recruitment of Pdcd4 to the ribosomal complex and suggest a model whereby Pdcd4 blocks the eIF4F-independent role of eIF4A during recruitment and scanning of the 5' UTR of mRNA.

MicroRNA-21 in urologic cancers: from molecular mechanisms to clinical implications

The three most common kinds of urologic malignancies are prostate, bladder, and kidney cancer, which typically cause substantial morbidity and mortality. Early detection and effective treatment are essential due to their high fatality rates. As a result, there is an urgent need for innovative research to improve the clinical management of patients with urologic cancers. A type of small noncoding RNAs of 22 nucleotides, microRNAs (miRNAs) are well-known for their important roles in a variety of developmental processes. Among these, microRNA-21 (miR-21) stands out as a commonly studied miRNA with implications in tumorigenesis and cancer development, particularly in urological tumors. Recent research has shed light on the dysregulation of miR-21 in urological tumors, offering insights into its potential as a prognostic, diagnostic, and therapeutic tool. This review delves into the pathogenesis of miR-21 in prostate, bladder, and renal cancers, its utility as a cancer biomarker, and the therapeutic possibilities of targeting miR-21.

The Role and Interactions of Programmed Cell Death 4 and its Regulation by microRNA in Transformed Cells of the Gastrointestinal Tract

Data from GLOBOCAN 2020 estimates that there were 19.3 million new cases of cancer and 10.0 million cancer-related deaths in 2020 and that this is predicted to increase by 47% in 2040. The combined burden of cancers of the gastrointestinal (GI) tract, including oesophageal-, gastric- and colorectal cancers, resulted in 22.6% of the cancer-related deaths in 2020 and 18.7% of new diagnosed cases. Understanding the aetiology of GI tract cancers should have a major impact on future therapies and lessen this substantial burden of disease. Many cancers of the GI tract have suppression of the tumour suppressor Programmed Cell Death 4 (PDCD4) and this has been linked to the expression of microRNAs which bind to the untranslated region of PDCD4 mRNA and either inhibit translation or target the mRNA for degradation. This review highlights the properties of PDCD4 and documents the evidence for the regulation of PDCD4 expression by microRNAs in cancers of the GI tract.

Targeted inhibition of eIF4A suppresses B-cell receptor-induced translation and expression of MYC and MCL1 in chronic lymphocytic leukemia cells

Signaling via the B-cell receptor (BCR) is a key driver and therapeutic target in chronic lymphocytic leukemia (CLL). BCR stimulation of CLL cells induces expression of eIF4A, an initiation factor important for translation of multiple oncoproteins, and reduces expression of PDCD4, a natural inhibitor of eIF4A, suggesting that eIF4A may be a critical nexus controlling protein expression downstream of the BCR in these cells. We, therefore, investigated the effect of eIF4A inhibitors (eIF4Ai) on BCR-induced responses. We demonstrated that eIF4Ai (silvestrol and rocaglamide A) reduced anti-IgM-induced global mRNA translation in CLL cells and also inhibited accumulation of MYC and MCL1, key drivers of proliferation and survival, respectively, without effects on upstream signaling responses (ERK1/2 and AKT phosphorylation). Analysis of normal naïve and non-switched memory B cells, likely counterparts of the two main subsets of CLL, demonstrated that basal RNA translation was higher in memory B cells, but was similarly increased and susceptible to eIF4Ai-mediated inhibition in both. We probed the fate of MYC mRNA in eIF4Ai-treated CLL cells and found that eIF4Ai caused a profound accumulation of MYC mRNA in anti-IgM treated cells. This was mediated by MYC mRNA stabilization and was not observed for MCL1 mRNA. Following drug wash-out, MYC mRNA levels declined but without substantial MYC protein accumulation, indicating that stabilized MYC mRNA remained blocked from translation. In conclusion, BCR-induced regulation of eIF4A may be a critical signal-dependent nexus for therapeutic attack in CLL and other B-cell malignancies, especially those dependent on MYC and/or MCL1.

Molecular cloning, inducible expression with SGIV and Vibrio alginolyticus challenge, and function analysis of Epinephelus coioides PDCD4

Programmed cell death 4 (PDCD4) in mammals, a gene closely associated with apoptosis, is involved in many biological processes, such as cell aging, differentiation, regulation of cell cycle, and inflammatory response. In this study, grouper Epinephelus coioides PDCD4, EcPDCD4-1 and EcPDCD4-2, were obtained. The open reading frame (ORF) of EcPDCD4-1 is 1413 bp encoding 470 amino acids with a molecular mass of 52.39 kDa and a theoretical pI of 5.33. The ORF of EcPDCD4-2 is 1410 bp encoding 469 amino acids with a molecular mass of 52.29 kDa and a theoretical pI of 5.29. Both EcPDCD4-1 and EcPDCD4-2 proteins contain two conserved MA3 domains, and their mRNA were detected in all eight tissues of E. coioides by quantitative real-time PCR (qRT-PCR) with the highest expression in liver. The expressions of two EcPDCD4s were significantly up-regulated after Singapore grouper iridovirus (SGIV) or Vibrio alginolyticus infection. In addition, over-expression of EcPDCD4-1 or EcPDCD4-2 can inhibit the activity of the nuclear factor-κB (NF-κB) and activator protein-1 (AP-1), and regulate SGIV-induced apoptosis. The results demonstrated that EcPDCD4s might play important roles in E. coioides tissues during pathogen-caused inflammation.

Programmed cell death factor 4 (PDCD4), a novel therapy target for metabolic diseases besides cancer

Programmed cell death factor 4 (PDCD4) is originally described as a tumor suppressor gene that exerts antineoplastic effects by promoting apoptosis and inhibiting tumor cell proliferation, invasion, and metastasis. Several investigations have probed the aberrant expression of PDCD4 with the progression of metabolic diseases, such as polycystic ovary syndrome (PCOS), obesity, diabetes, and atherosclerosis. It has been ascertained that PDCD4 causes glucose and lipid metabolism disorders, insulin resistance, oxidative stress, chronic inflammatory response, and gut flora disorders to regulate the progression of metabolic diseases. This review aims to summarize the latest researches to uncover the structure, expression regulation, and biological functions of PDCD4 and to elucidate the regulatory mechanism of the development of tumors and metabolic diseases. This review has emphasized the understanding of the PDCD4 role and to provide new ideas for the research, diagnosis, and treatment of tumors and metabolic diseases.

The Organizing Principles of Eukaryotic Ribosome Recruitment

The stage at which ribosomes are recruited to messenger RNAs (mRNAs) is an elaborate and highly regulated phase of protein synthesis. Upon completion of this step, a ribosome is positioned at an appropriate initiation codon and primed to synthesize the encoded polypeptide product. In most circumstances, this step commits the ribosome to translate the mRNA. We summarize the knowledge regarding the initiation factors implicated in this activity as well as review different mechanisms by which this process is conducted.

Upf proteins: highly conserved factors involved in nonsense mRNA mediated decay

Over 10% of genetic diseases are caused by mutations that introduce a premature termination codon in protein-coding mRNA. Nonsense-mediated mRNA decay (NMD) is an essential cellular pathway that degrades these mRNAs to prevent the accumulation of harmful partial protein products. NMD machinery is also increasingly appreciated to play a role in other essential cellular functions, including telomere homeostasis and the regulation of normal mRNA turnover, and is misregulated in numerous cancers. Hence, understanding and designing therapeutics targeting NMD is an important goal in biomedical science. The central regulator of NMD, the Upf1 protein, interacts with translation termination factors and contextual factors to initiate NMD specifically on mRNAs containing PTCs. The molecular details of how these contextual factors affect Upf1 function remain poorly understood. Here, we review plausible models for the NMD pathway and the evidence for the variety of roles NMD machinery may play in different cellular processes.

Programmed cell death 4 mechanism of action: The model to be updated?

Programmed cell death 4 (Pdcd4) is frequently suppressed in tumors of various origins and its suppression correlates with tumor progression. Pdcd4 inhibits cap-dependent translation from mRNAs with highly structured 5'-regions through interaction with the eukaryotic translation initiation factor 4A (eIF4A) helicase and a target transcript. Decrease in Pdcd4 protein is believed to provide a relief of otherwise suppressed eIF4A-dependent translation of proteins facilitating tumor progression. However, it remains unknown if lowered Pdcd4 levels in cells suffices to cause a relief in translation inhibition through appearance of the Pdcd4-free translation-competent eIF4A protein, or more complex and selective mechanisms are involved. Here we showed that eIF4A1, the eIF4A isoform involved in translation, significantly over-represents Pdcd4 both in cancerous and normal cells. This observation excludes the possibility that cytoplasmic Pdcd4 can efficiently exert its translation suppression function owing to excess of eIF4A, with Pdcd4-free eIF4A being in excess over Pdcd4-bound translation-incompetent eIF4A, thus leaving translation from Pdcd4 mRNA targets unaffected. This contradiction is resumed in the proposed model, which supposes initial complexing between Pdcd4 and its target mRNAs in the nucleus, with subsequent transport of translation-incompetent, Pdcd4-bound target mRNAs into the cytoplasm. Noteworthy, loss of nuclear Pdcd4 in cancer cells was reported to correlate with tumor progression, which supports the proposed model of Pdcd4 functioning.

Crystal structure of the PEG-bound SH3 domain of myosin IB from Entamoeba histolytica reveals its mode of ligand recognition

The versatility in the recognition of various interacting proteins by the SH3 domain drives a variety of cellular functions. Here, the crystal structure of the C-terminal SH3 domain of myosin IB from Entamoeba histolytica (EhMySH3) is reported at a resolution of 1.7 Å in native and PEG-bound states. Comparisons with other structures indicated that the PEG molecules occupy protein-protein interaction pockets similar to those occupied by the peptides in other peptide-bound SH3-domain structures. Also, analysis of the PEG-bound EhMySH3 structure led to the recognition of two additional pockets, apart from the conventional polyproline and specificity pockets, that are important for ligand interaction. Molecular-docking studies combined with various comparisons revealed structural similarity between EhMySH3 and the SH3 domain of β-Pix, and this similarity led to the prediction that EhMySH3 preferentially binds targets containing type II-like PXXP motifs. These studies expand the understanding of the EhMySH3 domain and provide extensive structural knowledge, which is expected to help in predicting the interacting partners which function together with myosin IB during phagocytosis in E. histolytica infections.

Downregulation of PDCD4 by miR-21 suppresses tumor transformation and proliferation in a nude mouse renal cancer model

Programmed cell death 4 (PDCD4) is known to suppress neoplastic transformation, cell proliferation and metastasis, and to be downregulated by microRNA-21 (miR-21) in renal cell carcinoma (RCC) cell lines and tissues. The aim of the present study was to investigate the roles of and association between PDCD4 and miR-21 in a nude mouse renal cancer model. A total of 24 BALB/c male nude mice were randomly assigned into the following three groups: Negative control (NC; n=8), miR-21 inhibitor (n=8) and miR-21 mimic (n=8). Subsequently, renal cell adenocarcinoma 786-O cells were subcutaneously transplanted into the armpits of the mice, which were then injected daily with NC small interfering (si)RNA, precursor-miR-21 (mimic) or anti-miR-21 (inhibitor). Tumors were removed from the mice and weighed 16 days following 786-O cell transplantation. In addition, the expression of miR-21 and PDCD4 mRNA in cancer tissues was analyzed using reverse transcription-quantitative PCR. The expression of PDCD4 protein in cancer tissues was also examined using immunohistochemistry and western blotting. Furthermore, 786-O cells were transfected with PDCD4 siRNA or NC siRNA, and the effects of silencing PDCD4 on tumor cell growth, proliferation and invasion were investigated using soft agar colony formation, EdU cell proliferation assay and Transwell migration and invasion assays. Another 16 BALB/c male nude mice were randomly assigned into two groups as follows: NC (n=8) and PDCD4 siRNA (n=8). The 786-O cells were subcutaneously transplanted into the armpits of the mice, which were subsequently injected daily with NC siRNA or PDCD4 siRNA. The tumors were removed and weighed 16 days following transplantation. Compared with the NC group, tumor weight in the miR-21 mimic group was significantly increased. By contrast, tumor weight in the miR-21 inhibitor group was significantly decreased. Similar to the results observed in human renal cancer tissue and cell lines, miR-21 expression in the nude mouse renal cancer models was significantly upregulated in the miR-21 mimic group compared with the NC group, while it was significantly lower in the miR-21 inhibitor group. Furthermore, there was a significant reduction in PDCD4 protein levels in the miR-21 mimic group and a significant increase in the miR-21 inhibitor group compared with the NC, whereas PDCD4 mRNA expression was not significantly altered. In the EdU proliferation assay, the mean percentage of new cells that incorporated EdU was 28.6% in the NC siRNA group and significantly increased to 44.7% in PDCD4 siRNA transfected cells. In the soft agar colony formation assay, Transwell and migration and invasion assays, a significant increase in colony formation, migration and invasion capacity in PDCD4 siRNA-transfected cells was observed compared with the NC. Furthermore, compared with the NC group, tumor weight in the PDCD4 siRNA group was significantly increased. Similar to the results observed in human renal cancer tissue and cell lines, miR-21 promoted cancer cell hyperplasia and proliferation, and post-transcriptionally downregulated PDCD4 protein expression, in the nude mouse renal cancer model. The results of the present study and previous studies indicate that PDCD4 and miR-21 serve an important role in renal cancer. Thus, increasing PDCD4 expression or inhibiting miR-21 expression may constitute effective novel therapeutic strategies for the treatment of renal cancer.

Pdcd4 restrains the self-renewal and white-to-beige transdifferentiation of adipose-derived stem cells

The stemness maintenance of adipose-derived stem cells (ADSCs) is important for adipose homeostasis and energy balance. Programmed cell death 4 (Pdcd4) has been demonstrated to be involved in the development of obesity, but its possible roles in ADSC function and adipogenic capacity remain unclear. In this study, we demonstrate that Pdcd4 is a key controller that limits the self-renewal and white-to-beige transdifferentiation of ADSCs. Pdcd4 deficiency in mice caused stemness enhancement of ADSCs as evidenced by increased expression of CD105, CD90, Nanog and Oct4 on ADSCs, together with enhanced in situ proliferation in adipose tissues. Pdcd4 deficiency promoted proliferation, colony formation of ADSCs and drove more ADSCs entering the S phase accompanied by AKT activation and cyclinD1 upregulation. Blockade of AKT signaling in Pdcd4-deficient ADSCs led to a marked decline in cyclinD1, S-phase entry and cell proliferation, revealing AKT as a target for repressing ADSC self-renewal by Pdcd4. Intriguingly, depletion of Pdcd4 promoted the transdifferentiation of ADSCs into beige adipocytes. A reduction in lipid contents and expression levels of white adipocyte markers including C/EBPα, PPAR-γ, adiponectin and αP2 was detected in Pdcd4-deficient ADSCs during white adipogenic differentiation, substituted by typical beige adipocyte characteristics including small, multilocular lipid droplets and UCP1 expression. More lactate produced by Pdcd4-deficient ADSCs might be an important contributor to the expression of UCP1 and white-to-beige transdifferentiation. In addition, an elevation of UCP1 expression was confirmed in white adipose tissues from Pdcd4-deficient mice upon high-fat diet, which displayed increased energy expenditure and resistance to obesity as compared with wild-type obese mice. These findings provide evidences that Pdcd4 produces unfavorable influences on ADSC stemness, which contribute to adipose dysfunction, obesity and metabolic syndromes, thereby proposing Pdcd4 as a potential intervening target for regulating ADSC function.

MicroRNAs block assembly of eIF4F translation initiation complex in Drosophila

miRNAs silence their complementary target mRNAs by translational repression as well as by poly(A) shortening and mRNA decay. In Drosophila, miRNAs are typically incorporated into Argonaute1 (Ago1) to form the effector complex called RNA-induced silencing complex (RISC). Ago1-RISC associates with a scaffold protein GW182, which recruits additional silencing factors. We have previously shown that miRNAs repress translation initiation by blocking formation of the 48S and 80S ribosomal complexes. However, it remains unclear how ribosome recruitment is impeded. Here, we examined the assembly of translation initiation factors on the target mRNA under repression. We show that Ago1-RISC induces dissociation of eIF4A, a DEAD-box RNA helicase, from the target mRNA without affecting 5' cap recognition by eIF4E in a manner independent of GW182. In contrast, direct tethering of GW182 promotes dissociation of both eIF4E and eIF4A. We propose that miRNAs act to block the assembly of the eIF4F complex during translation initiation.

The role of IRES trans-acting factors in carcinogenesis

Regulation of protein expression through RNA metabolism is a key aspect of cellular homeostasis. Upon specific cellular stresses, distinct transcripts are selectively controlled to modify protein output in order to quickly and appropriately respond to stress. Reprogramming of the translation machinery is one node of this strict control that typically consists of an attenuation of the global, cap-dependent translation and accompanying switch to alternative mechanisms of translation initiation, such as internal ribosome entry site (IRES)-mediated initiation. In cancer, many aspects of the RNA metabolism are frequently misregulated to provide cancer cells with a growth and survival advantage. This includes changes in the expression and function of RNA binding proteins termed IRES trans-acting factors (ITAFs) that are central to IRES translation. In this review, we will examine select emerging, as well as established, ITAFs with important roles in cancer initiation and progression, and in particular their role in IRES-mediated translation. This article is part of a Special Issue entitled: Translation and Cancer.

An evolutionarily conserved interaction of tumor suppressor protein Pdcd4 with the poly(A)-binding protein contributes to translation suppression by Pdcd4

The tumor suppressor protein programmed cell death 4 (Pdcd4) has been implicated in the translational regulation of specific mRNAs, however, the identities of the natural Pdcd4 target mRNAs and the mechanisms by which Pdcd4 affects their translation are not well understood. Pdcd4 binds to the eukaryotic translation initiation factor eIF4A and inhibits its helicase activity, which has suggested that Pdcd4 suppresses translation initiation of mRNAs containing structured 5′-untranslated regions. Recent work has revealed a second inhibitory mechanism, which is eIF4A-independent and involves direct RNA-binding of Pdcd4 to the target mRNAs. We have now identified the poly(A)-binding protein (PABP) as a novel direct interaction partner of Pdcd4. The ability to interact with PABP is shared between human and Drosophila Pdcd4, indicating that it has been highly conserved during evolution. Mutants of Pdcd4 that have lost the ability to interact with PABP fail to stably associate with ribosomal complexes in sucrose density gradients and to suppress translation, as exemplified by c-myb mRNA. Overall, our work identifies PABP as a novel functionally relevant Pdcd4 interaction partner that contributes to the regulation of translation by Pdcd4.

Enhanced arginine methylation of programmed cell death 4 protein during nutrient deprivation promotes tumor cell viability

The role of programmed cell death 4 (PDCD4) in tumor biology is context-dependent. PDCD4 is described as a tumor suppressor, but its coexpression with protein arginine methyltransferase 5 (PRMT5) promotes accelerated tumor growth. Here, we report that PDCD4 is methylated during nutrient deprivation. Methylation occurs because of increased stability of PDCD4 protein as well as increased activity of PRMT5 toward PDCD4. During nutrient deprivation, levels of methylated PDCD4 promote cell viability, which is dependent on an enhanced interaction with eIF4A. Upon recovery from nutrient deprivation, levels of methylated PDCD4 are regulated by phosphorylation, which controls both the localization and stability of methylated PDCD4. This study reveals that, in response to particular environmental cues, the role of PDCD4 is up-regulated and is advantageous for cell viability. These findings suggest that the methylated form of PDCD4 promotes tumor viability during nutrient deprivation, ultimately allowing the tumor to grow more aggressively.

A novel mechanism for the control of translation of specific mRNAs by tumor suppressor protein Pdcd4: inhibition of translation elongation

The tumor suppressor gene Pdcd4 (programmed cell death gene 4) has drawn considerable attention because its downregulation is involved in the development of several types of cancer. Because Pdcd4 interacts with the translation initiation factor eIF4A and inhibits its helicase activity, Pdcd4 has been implicated in the translational suppression of cellular mRNAs containing structured 5'-untranslated regions. However, Pdcd4's role in translation regulation is still poorly understood, because only very few physiological Pdcd4 target mRNAs are known. By using a Pdcd4-deficient clone of the chicken B-cell line DT40, we have discovered that the mRNA of the A-myb proto-oncogene is a novel Pdcd4 target RNA whose translation is suppressed by Pdcd4. Interestingly, the inhibitory effect of Pdcd4 is independent of the Pdcd4-eIF4A interaction, but is dependent on an RNA-binding domain at the N terminus of Pdcd4 and on sequences located within the coding region of A-myb mRNA, indicating that Pdcd4 suppresses A-myb translation by a novel mechanism. Our data show that the Pdcd4 RNA-binding domain preferentially recognizes an RNA secondary structure element formed by the part of the A-myb coding region that mediates Pdcd4-dependent suppression. Previously, we have shown that Pdcd4 also suppresses the translation of the c-myb mRNA by a similar mechanism involving binding of Pdcd4 to RNA secondary structure formed by the c-myb coding region. Surprisingly, our data show that Pdcd4 exerts its inhibitory activity only when the target region of Pdcd4 in A-myb and c-myb mRNA is itself translated, consistent with a mechanism in which Pdcd4 suppresses translation by interfering with translation elongation. Taken together, our work reveals a novel mechanism by which Pdcd4 affects the translational of cellular RNAs. Furthermore, as c-myb and A-myb are members of the Myb proto-oncogene family whose deregulation has been implicated in tumorigenesis, inhibiting their translation might contribute to the tumor-suppressive activity of Pdcd4.

Tumor suppressor PDCD4 inhibits NF-κB-dependent transcription in human glioblastoma cells by direct interaction with p65

PDCD4 is a tumor suppressor induced by apoptotic stimuli that regulates both translation and transcription. Previously, we showed that overexpression of PDCD4 leads to decreased anchorage-independent growth in glioblastoma (GBM)-derived cell lines and decreased tumor growth in a GBM xenograft model. In inflammatory cells, PDCD4 stimulates tumor necrosis factor-induced activation of the transcription factor NF-κB, an oncogenic driver in many cancer sites. However, the effect of PDCD4 on NF-κB transcriptional activity in most cancers including GBM is still unknown. We studied the effect of PDCD4 on NF-κB-dependent transcriptional activity in GBM by stably overexpressing PDCD4 in U251 and LN229 cells. Stable PDCD4 expression inhibits NF-κB transcriptional activation measured by a luciferase reporter. The molecular mechanism by which PDCD4 inhibits NF-κB transcriptional activation does not involve inhibited expression of NF-κB p65 or p50 proteins. PDCD4 does not inhibit pathways upstream of NF-κB including the activation of IKKα and IKKβ kinases or degradation of IκBα, events needed for nuclear transport of p65 and p50. PDCD4 overexpression does inhibit localization of p65 but not p50 in the nucleus. PDCD4 protein interacts preferentially with p65 protein as shown by co-immunoprecipitation and confocal imaging. PDCD4 overexpression inhibits the mRNA expression of two NF-κB target genes in a p65-dependent manner. These results suggest that PDCD4 can significantly inhibit NF-κB activity in GBM cells by a mechanism that involves direct or indirect protein-protein interaction independent of the expected mRNA-selective translational inhibition. These findings offer novel opportunities for NF-κB-targeted interventions to prevent or treat cancer.

The unique evolution of the programmed cell death 4 protein in plants

BACKGROUND

The programmed cell death 4 (PDCD4) protein is induced in animals during apoptosis and functions to inhibit translation and tumor promoter-induced neoplastic transformation. PDCD4 is composed of two MA3 domains that share similarity with the single MA3 domain present in the eukaryotic translation initiation factor (eIF) 4G, which serves as a scaffold protein to assemble several initiation factors needed for the recruitment of the 40S ribosomal subunit to an mRNA. Although eIF4A is an ATP-dependent RNA helicase that binds the MA3 domain of eIF4G to promote translation initiation, binding of eIF4A to the MA3 domains of PDCD4 inhibits protein synthesis. Genes encoding PDCD4 are present in many lower eukaryotes and in plants, but PDCD4 in higher plants is unique in that it contains four MA3 domains and has been implicated in ethylene signaling and abiotic stress responses. Here, we examine the evolution of PDCD4 in plants.

RESULTS

In older algal lineages, PDCD4 contains two MA3 domains similar to the homolog in animals. By the appearance of early land plants, however, PDCD4 is composed of four MA3 domains which likely is the result of a duplication of the two MA3 domain form of the protein. Evidence from fresh water algae, from which land plants evolved, suggests that the duplication event occurred prior to the colonization of land. PDCD4 in more recently evolved chlorophytes also contains four MA3 domains but this may have resulted from an independent duplication event. Expansion and divergence of the PDCD4 gene family occurred during land plant evolution with the appearance of a distinct gene member following the evolution of basal angiosperms.

CONCLUSIONS

The appearance of a unique form of PDCD4 in plants correlates with the appearance of components of the ethylene signaling pathway, suggesting that it may represent the adaptation of an existing protein involved in programmed cell death to one that functions in abiotic stress responses through hormone signaling.

Distinct roles of different fragments of PDCD4 in regulating the metastatic behavior of B16 melanoma cells

Melanoma is an aggressive cutaneous malignancy. In this study, we demonstrated that the levels of the programmed cell death 4 (PDCD4) protein and mRNA were lower in tumor tissues compared with normal tissues. In order to further investigate the effects of PDCD4 and its fragments in B16 melanoma cells, we established B16 clones with expression of different PDCD4 fragments. Intact PDCD4, PDCD4∆164‑469 and PDCD4∆327-440 expression, respectively, decreased proliferation and migration and increased apoptosis in B16 cells in vitro. We found that intact PDCD4, PDCD4∆164-469 or PDCD4∆327-440 can inhibit the activity of MMP-2 and the expression of CXCR4. However, PDCD4∆164-275 showed no effects on B16 cells. These results may prove helpful for the development of novel therapies for melanoma treatment.

Tumor suppressor protein Pdcd4 interacts with Daxx and modulates the stability of Daxx and the Hipk2-dependent phosphorylation of p53 at serine 46

The tumor suppressor protein Pdcd4 is a nuclear/cytoplasmic shuttling protein that has been implicated in the development of several types of human cancer. In the nucleus, Pdcd4 affects the transcription of specific genes by modulating the activity of several transcription factors. We have identified the Daxx protein as a novel interaction partner of Pdcd4. Daxx is a scaffold protein with roles in diverse processes, including transcriptional regulation, DNA-damage signaling, apoptosis and chromatin remodeling. We show that the interaction of both proteins is mediated by the N-terminal domain of Pdcd4 and the central part of Daxx, and that binding to Pdcd4 stimulates the degradation of Daxx, presumably by disrupting the interaction of Daxx with the de-ubiquitinylating enzyme Hausp. Daxx has previously been shown to serve as a scaffold for protein kinase Hipk2 and tumor suppressor protein p53 and to stimulate the phosphorylation of p53 at serine 46 (Ser-46) in response to genotoxic stress. We show that Pdcd4 also disrupts the Daxx–Hipk2 interaction and inhibits the phosphorylation of p53. We also show that ultraviolet irradiation decreases the expression of Pdcd4. Taken together, our results support a model in which Pdcd4 serves to suppress the phosphorylation of p53 in the absence of DNA damage, while the suppressive effect of Pdcd4 is abrogated after DNA damage owing to the decrease of Pdcd4. Overall, our data demonstrate that Pdcd4 is a novel modulator of Daxx function and provide evidence for a role of Pdcd4 in restraining p53 activity in unstressed cells.

Assembly of the SLIP1-SLBP complex on histone mRNA requires heterodimerization and sequential binding of SLBP followed by SLIP1

The SLIP1-SLBP complex activates translation of replication-dependent histone mRNAs. In this report, we describe how the activity of the SLIP1-SLBP complex is modulated by phosphorylation and oligomerization. Biophysical characterization of the free proteins shows that whereas SLIP1 is a homodimer that does not bind RNA, human SLBP is an intrinsically disordered protein that is phosphorylated at 23 Ser/Thr sites when expressed in a eukaryotic expression system such as baculovirus. The bacterially expressed unphosphorylated SLIP1-SLBP complex forms a 2:2 high-affinity (K(D) < 0.9 nM) heterotetramer that is also incapable of binding histone mRNA. In contrast, phosphorylated SLBP from baculovirus has a weak affinity (K(D) ~3 μM) for SLIP1. Sequential binding of phosphorylated SLBP to the histone mRNA stem-loop motif followed by association with SLIP1 is required to form an "active" ternary complex. Phosphorylation of SLBP at Thr171 promotes dissociation of the heterotetramer to the SLIP1-SLBP heterodimer. Using alanine scanning mutagenesis, we demonstrate that the binding site on SLIP1 for SLBP lies close to the dimer interface. A single-point mutant near the SLIP1 homodimer interface abolished interaction with SLBP in vitro and reduced the abundance of histone mRNA in vivo. On the basis of these biophysical studies, we propose that oligomerization and SLBP phosphorylation may regulate the SLBP-SLIP1 complex in vivo. SLIP1 may act to sequester SLBP in vivo, protecting it from proteolytic degradation as an inactive heterotetramer, or alternatively, formation of the SLIP1-SLBP heterotetramer may facilitate removal of SLBP from the histone mRNA prior to histone mRNA degradation.

Molecular profiling of prostatic acinar morphogenesis identifies PDCD4 and KLF6 as tissue architecture-specific prognostic markers in prostate cancer

Histopathological classification of human prostate cancer (PCA) relies on the morphological assessment of tissue specimens but has limited prognostic value. To address this deficiency, we performed comparative transcriptome analysis of human prostatic acini generated in a three-dimensional basement membrane that recapitulates the differentiated morphological characteristics and gene expression profile of a human prostate glandular epithelial tissue. We then applied an acinar morphogenesis-specific gene profile to two independent cohorts of patients with PCA (total n = 79) and found that those with tumors expressing this profile, which we designated acini-like tumors, had a significantly lower risk of postoperative relapse compared with those tumors with a lower correlation (hazard ratio, 0.078; log-rank test P = 0.009). Multivariate analyses showed superior prognostic prediction performance using this classification system compared with clinical criteria and Gleason scores. We prioritized the genes in this profile and identified programmed cell death protein 4 (PDCD4) and Kruppel-like factor 6 (KLF6) as critical regulators and surrogate markers of prostatic tissue architectures, which form a gene signature that robustly predicts clinical prognosis with a remarkable accuracy in several large series of PCA tumors (total n = 161; concordance index, 0.913 to 0.951). Thus, by exploiting the genomic program associated with prostate glandular differentiation, we identified acini-like PCA and related molecular markers that significantly enhance prognostic prediction of human PCA.

High-resolution structure of Bombyx mori lipoprotein 7: crystallographic determination of the identity of the protein and its potential role in detoxification

Three crystal structures of a lipoprotein (Bmlp7) of unknown function, a member of the 30 kDa lipoprotein family from mulberry silkworm (Bombyx mori L.) haemolymph, have been determined. The 1.33 Å resolution structure is an excellent example of how a precise crystallographic study can contribute to protein identification. The correct sequence of this haemolymph-isolated protein was assigned thanks to superb-quality electron-density maps. Two unexpected cadmium cations were found in this crystal structure [Bmlp7-I(Cd)] and their presence may be connected to a detoxification mechanism in this insect. For a comparison of the metal-binding sites, the crystal structure of a platinum complex (Bmlp7-Pt) was also solved at 1.94 Å resolution. The third (2.50 Å resolution) structure, of the native protein harvested in a different season (Bmlp7-II), corresponds to a different polymorph with an altered pattern of intermolecular interactions and with a total absence of cadmium ions and highlights the possible involvement of Bmlp7 in the response to environmental pollution. The N-terminal domain of Bmlp7 has a fold resembling a clockwise spiral created by six helices and can be classified as a VHS domain. The C-terminal domain is folded as a β-trefoil. The biological function of Bmlp7 is unknown, but its structural homology to sugar-binding proteins suggests that, in analogy to other 30 kDa haemolymph lipoproteins, it could play a role as an anti-apoptotic factor or function in the immune response of the insect to fungal infections.

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.

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.

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.

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.

Downregulation of Pdcd4 by mir-21 facilitates glioblastoma proliferation in vivo

MicroRNAs (miRNAs) are small, noncoding RNAs that play a critical role in developmental and physiological processes and are implicated in the pathogenesis of several human diseases, including cancer. They function by regulating target gene expression post-transcriptionally. In this study, we examined the role of oncogenic mir-21 in the pathogenesis of glioblastoma, the most aggressive form of primary brain tumor. We have previously reported that mir-21 is expressed at higher levels in primary glioblastoma-tissue and glioblastoma-derived cell lines than in normal brain tissue. We demonstrate that downregulation of mir-21 in glioblastoma-derived cell lines results in increased expression of its target, programmed cell death 4 (Pdcd4), a known tumor-suppressor gene. In addition, our data indicate that either downregulation of mir-21 or overexpression of its target, Pdcd4, in glioblastoma-derived cell lines leads to decreased proliferation, increased apoptosis, and decreased colony formation in soft agar. Using a glioblastoma xenograft model in immune-deficient nude mice, we observe that glioblastoma-derived cell lines in which mir-21 levels are downregulated or Pdcd4 is over-expressed exhibit decreased tumor formation and growth. Significantly, tumors grow when the glioblastoma-derived cell lines are transfected with anti-mir-21 and siRNA to Pdcd4, confirming that the tumor growth is specifically regulated by Pdcd4. These critical in vivo findings demonstrate an important functional linkage between mir-21 and Pdcd4 and further elucidate the molecular mechanisms by which the known high level of mir-21 expression in glioblastoma can attribute to tumorigenesis--namely, inhibition of Pdcd4 and its tumor-suppressive functions.

Actinopolysporins A-C and tubercidin as a Pdcd4 stabilizer from the halophilic actinomycete Actinopolyspora erythraea YIM 90600

Our current natural product program utilizes new actinomycetes originating from unexplored and underexplored ecological niches, employing cytotoxicity against a selected panel of cancer cell lines as the preliminary screen to identify hit strains for natural product dereplication, followed by mechanism-based assays of the purified natural products to discover potential anticancer drug leads. Three new linear polyketides, actinopolysporins A (1), B (2), and C (3), along with the known antineoplastic antibiotic tubercidin (4), were isolated from the halophilic actinomycete Actinopolyspora erythraea YIM 90600, and the structures of the new compounds were elucidated on the basis of spectroscopic data interpretation. All four compounds were assayed for their ability to stabilize the tumor suppressor programmed cell death protein 4 (Pdcd4), which is known to antagonize critical events in oncogenic pathways. Only 4 significantly inhibited proteasomal degradation of a model Pdcd4-luciferase fusion protein, with an IC50 of 0.88±0.09 μM, unveiling a novel biological activity for this well-studied natural product.

Glucose depletion inhibits translation initiation via eIF4A loss and subsequent 48S preinitiation complex accumulation, while the pentose phosphate pathway is coordinately up-regulated

The mechanism and consequences of the translational inhibition caused by glucose depletion in yeast are characterized. eIF4A is lost from the preinitiation complex, and the pentose phosphate pathway is translationally up-regulated, allowing an efficient transition to the new conditions.

Cellular stress can globally inhibit translation initiation, and glucose removal from yeast causes one of the most dramatic effects in terms of rapidity and scale. Here we show that the same rapid inhibition occurs during yeast growth as glucose levels diminish. We characterize this novel regulation showing that it involves alterations within the 48S preinitiation complex. In particular, the interaction between eIF4A and eIF4G is destabilized, leading to a temporary stabilization of the eIF3–eIF4G interaction on the 48S complex. Under such conditions, specific mRNAs that are important for the adaptation to the new conditions must continue to be translated. We have determined which mRNAs remain translated early after glucose starvation. These experiments enable us to provide a physiological context for this translational regulation by ascribing defined functions that are translationally maintained or up-regulated. Overrepresented in this class of mRNA are those involved in carbohydrate metabolism, including several mRNAs from the pentose phosphate pathway. Our data support a hypothesis that a concerted preemptive activation of the pentose phosphate pathway, which targets both mRNA transcription and translation, is important for the transition from fermentative to respiratory growth in yeast.

Inflammation-induced loss of Pdcd4 is mediated by phosphorylation-dependent degradation

The tumor suppressor programmed cell death 4 (Pdcd4) is lost in various tumor tissues. Loss of Pdcd4 has been associated with increased tumorigenic potential and tumor progression. While various mechanisms of Pdcd4 regulation have been described, the effect of an inflammatory tumor microenvironment on Pdcd4 protein expression has not been characterized so far. In the present study, we aimed to elucidate the molecular mechanisms of Pdcd4 protein regulation in tumor cells under inflammatory conditions. 12-O-tetradecanoylphorbol 13-acetate-induced differentiation of human U937 monocytes increased the expression and secretion of inflammatory cytokines such as tumor necrosis factor α, interleukin (IL)-6 and IL-8. Exposure to conditioned medium (CM) of these activated macrophages markedly decreased Pdcd4 protein expression in various tumor cells. Similarly, indirect coculture with such activated U937 monocyte-derived macrophages resulted in the loss of Pdcd4 protein in tumor cells. Decreased Pdcd4 protein levels were attributable to enhanced proteasomal degradation, diminishing Pdcd4 protein half-life. Proteasomal degradation required activation of phosphatidylinositol-3-kinase (PI3K)-mammalian target of rapamycin (mTOR) signaling. Since macrophage-CM sufficed to induce Pdcd4 degradation, Pdcd4 downregulation was determined to be an indirect unidirectional effect of the macrophages on the tumor cells. Pdcd4 protein expression was also attenuated in vivo in mouse colon tissue in response to dextran sodium sulfate-induced colitis. In summary, we characterized PI3K-mTOR-dependent proteasome-mediated Pdcd4 degradation in tumor cells in the inflammatory tumor microenvironment. Consequently, stabilization of Pdcd4 protein could provide a promising novel avenue for therapeutics targeting inflammation-associated tumors.

Inhibitors of translation initiation as cancer therapeutics

Deregulated translation initiation is implicated extensively in cancer initiation and progression. Several translation initiation factors cooperate with known oncogenes, are elevated in human tumors and have been implicated in drug resistance. Consequently, there is a great deal of interest in targeting this process to develop new chemotherapeutics, especially since clinical trial results have been mixed when targeting upstream pathways, such as the mammalian target of rapamycin. Several inhibitors have been characterized over the last 5 years that target the ribosome recruitment phase (eukaryotic initiation factor [eIF]4E [antisense oligonucleotides and 4EGI-1] or eIF4A [pateamine A, hippuristanol and silvestrol]), some of which demonstrate activity in preclinical cancer models. The promise of these inhibitors as chemotherapeutics highlights the importance of targeting this pathway and supports efforts aimed at identifying the most susceptible targets. In addition, the framework in which translation inhibitors would be best employed (i.e., as single agents or as adjuvant therapy) in the clinic remains to be explored systematically. Small-molecule inhibitors of translation initiation are validating the idea that protein synthesis is a legitimate target for curtailing tumor growth.

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.

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.

RNA helicases at work: binding and rearranging

RNA helicases are ubiquitous, highly conserved enzymes that participate in nearly all aspects of RNA metabolism. These proteins bind or remodel RNA or RNA-protein complexes in an ATP-dependent fashion. How RNA helicases physically perform their cellular tasks has been a longstanding question, but in recent years, intriguing models have started to link structure, mechanism and biological function for some RNA helicases. This review outlines our current view on major structural and mechanistic themes of RNA helicase function, and on emerging physical models for cellular roles of these enzymes.

Programmed cell death 4 loss increases tumor cell invasion and is regulated by miR-21 in oral squamous cell carcinoma

BACKGROUND

The tumor suppressor Programmed Cell Death 4 (PDCD4) has been found to be under-expressed in several cancers and associated with disease progression and metastasis. There are no current studies characterizing PDCD4 expression and its clinical relevance in Oral Squamous Cell Carcinoma (OSCC). Since nodal metastasis is a major prognostic factor in OSCC, we focused on determining whether PDCD4 under-expression was associated with patient nodal status and had functional relevance in OSCC invasion. We also examined PDCD4 regulation by microRNA 21 (miR-21) in OSCC.

RESULTS

PDCD4 mRNA expression levels were assessed in 50 OSCCs and 25 normal oral tissues. PDCD4 was under-expressed in 43/50 (86%) OSCCs, with significantly reduced mRNA levels in patients with nodal metastasis (p = 0.0027), and marginally associated with T3-T4 tumor stage (p = 0.054). PDCD4 protein expression was assessed, by immunohistochemistry (IHC), in 28/50 OSCCs and adjacent normal tissues; PDCD4 protein was absent/under-expressed in 25/28 (89%) OSCCs, and marginally associated with nodal metastasis (p = 0.059). A matrigel invasion assay showed that PDCD4 expression suppressed invasion, and siRNA-mediated PDCD4 loss was associated with increased invasive potential of oral carcinoma cells. Furthermore, we showed that miR-21 levels were increased in PDCD4-negative tumors, and that PDCD4 expression may be down-regulated in OSCC by direct binding of miR-21 to the 3'UTR PDCD4 mRNA.

CONCLUSIONS

Our data show an association between the loss of PDCD4 expression, tumorigenesis and invasion in OSCC, and also identify a mechanism of PDCD4 down-regulation by microRNA-21 in oral carcinoma. PDCD4 association with nodal metastasis and invasion suggests that PDCD4 may be a clinically relevant biomarker with prognostic value in OSCC.

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.

Translational control in cancer

Remarkable progress has been made in defining a new understanding of the role of mRNA translation and protein synthesis in human cancer. Translational control is a crucial component of cancer development and progression, directing both global control of protein synthesis and selective translation of specific mRNAs that promote tumour cell survival, angiogenesis, transformation, invasion and metastasis. Translational control of cancer is multifaceted, involving alterations in translation factor levels and activities unique to different types of cancers, disease stages and the tumour microenvironment. Several clinical efforts are underway to target specific components of the translation apparatus or unique mRNA translation elements for cancer therapeutics.

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.

Development of a high-throughput cell-based reporter assay to identify stabilizers of tumor suppressor Pdcd4

The novel tumor suppressor Pdcd4 affects tumorigenesis by inhibiting translation. Pdcd4 is phosphorylated and subsequently lost by proteasomal degradation in response to tumor-promoting conditions. Here, the authors describe the development of a reporter cell system to monitor the stability of Pdcd4. The phosphorylation-dependent degradation domain ("target") or an adjacent ("off-target") region of Pdcd4 was cloned into a luciferase expression system. The target constructs were responsive to Pdcd4 degrading conditions (e.g., TPA, p70(S6K1) overactivation), whereas the off-target constructs remained stable. The system was optimized for and shown to be reliable in a high-throughput compatible 384-well format. Screening of 15,275 pure compounds resulted in a hit rate of 0.30% (>50% inhibition of TPA-induced loss of signal, confirmed by reassay). Among the hits were inhibitors of previously identified critical signaling events for TPA-induced Pdcd4 degradation. One compound was identified to be nonspecific using the off-target control cell line. Screening of 135,678 natural product extracts yielded 42 confirmed, specific hits. Z' averaged 0.58 across 446 plates. Further characterization of active natural products and synthetic compounds is expected to identify novel Pdcd4 stabilizers that may be useful in targeting translation to prevent or treat cancers.

Housekeeping genes in prostate tumorigenesis

Housekeeping (HK) genes are involved in basic cellular functions and tend to be constitutively expressed across various tissues and conditions. A number of studies have analyzed the value of HK genes as an internal standard for assessing gene expression, but the role of HK genes in cancer development has never been specifically addressed. In this study, we sought to evaluate the expression of HK genes during prostate tumorigenesis. We performed a meta-analysis of gene expression during the transition from normal prostate (NP) to localized prostate cancer (LPC) (i.e., NP > LPC) and from localized to metastatic prostate cancer (MPC) (i.e., LPC > MPC). We found that HK genes are more likely to be differentially expressed during prostate tumorigenesis than is the average gene in the human genome, suggesting that prostate tumorigenesis is driven by modulation of the expression of HK genes. Cell-cycle genes and proliferation markers were up-regulated in both NP > LPC and LPC > MPC transitions. We also found that the genes encoding ribosomal proteins were up-regulated in the NP > LPC and down-regulated in the LPC > MPC transition. The expression of heat shock proteins was up-regulated during the LPC > MPC transition, suggesting that in its advanced stages, prostate tumor is under cellular stress. The results of these analyses suggest that during prostate tumorigenesis, there is a period when the tumor is under cellular stress and, therefore, may be the most vulnerable and responsive to treatment.

Ribosomal protein S6 kinase from TOP mRNAs to cell size

Ribosomal protein S6 kinase (S6K) has been implicated in the phosphorylation of multiple substrates and is subject to activation by a wide variety of signals that converge at mammalian target of rapamycin (mTOR). In the course of the search for its physiological role, it was proposed that S6K activation and ribosomal protein S6 (rpS6) phosphorylation account for the translational activation of a subgroup of transcripts, the TOP mRNAs. The structural hallmark of these mRNAs is an oligopyrimidine tract at their 5'-terminus, known as the 5'-TOP motif. TOP mRNAs consists of about 90 members that encode multiple components of the translational machinery, such as ribosomal proteins and translation factors. The translation efficiency of TOP mRNAs indeed correlates with S6K activation and rpS6 phosphorylation, yet recent biochemical and genetic studies have established that, although S6K and TOP mRNAs respond to similar signals and are regulated by mTOR, they maintain no cause and effect relationship. Instead, S6K is primarily involved in regulation of cell size, and affects glucose homeostasis, but is dispensable for global protein synthesis, whereas translational efficiency of TOP mRNAs is a determinant of the cellular protein synthesis capacity. Despite extensive studies of their function and mode of regulation, the mechanism underlying the effect of S6K on the cell size, as well as the trans-acting factor that mediates the translational control of TOP mRNAs, still await their identification.

Cell signaling in protein synthesis ribosome biogenesis and translation initiation and elongation

Protein synthesis is a highly energy-consuming process that must be tightly regulated. Signal transduction cascades respond to extracellular and intracellular cues to phosphorylate proteins involved in ribosomal biogenesis and translation initiation and elongation. These phosphorylation events regulate the timing and rate of translation of both specific and total mRNAs. Alterations in this regulation can result in dysfunction and disease. While many signaling pathways intersect to control protein synthesis, the mTOR and MAPK pathways appear to be key players. This chapter briefly reviews the mTOR and MAPK pathways and then focuses on individual phosphorylation events that directly control ribosome biogenesis and translation.

The molecular basis of translational control

Our current understanding of eukaryotic protein synthesis has emerged from many years of biochemical, genetic and biophysical approaches. Significant insight into the molecular details of the mechanism has been obtained, although there are clearly many aspects of the process that remain to be resolved. Importantly, our understanding of the mechanism has identified a number of key stages in the pathway that contribute to the regulation of general and gene-specific translation. Not surprisingly, translational control is now widely accepted to play a role in aspects of cell stress, growth, development, synaptic function, aging, and disease. This chapter reviews the mechanism of eukaryotic protein synthesis and its relevance to translational control.