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

Atg7 senses ATP levels and regulates AKT1-PDCD4 phosphorylation-ubiquitination axis to promote survival during metabolic stress

We report that autophagy-related gene 7 (ATG7) modulates p53 activity to regulate cell cycle and survival during metabolic stress, and that indicates Atg7 is functionally involved in cellular homeostasis in autophagy independent fashion. As a protein translation inhibitor, Programmed cell death 4 (PDCD4) expression is regulated by AKT1 phosphorylation. Here, we find that Atg7 interacts with PDCD4 and AKT1 to regulate AKT1-PDCD4 phosphorylation-ubiquitination axis during metabolic stress. We demonstrate that Atg7 senses decrease of ATP levels to suppress AKT-mediated PDCD4 phosphorylation at Ser67, which inhibits PDCD4 ubiquitinating during metabolic stress. Finally, PDCD4 accumulates and functions as a protein translation inhibitor to conserve energy, thus reducing apoptosis and allowing cells to survive stress periods. These results suggest that the ATP-Atg7-PDCD4 axis acts as a metabolic adaptation pathway which dictates cells to overcome metabolic stress.

Induction of premature senescence and a less-fibrogenic phenotype by programmed cell death 4 knockdown in the human hepatic stellate cell line Lieming Xu-2

Although programmed cell death 4 (PDCD4) was initially reported as a tumor suppressor and has been shown to inhibit cancer cell growth and metastasis, recent studies have demonstrated that loss of PDCD4 expression also induces growth inhibition by inducing apoptosis and/or cellular senescence. At present, the roles of PDCD4 in the activation and profibrogenic properties of myofibroblasts, which are critically involved in organ fibrosis, such as that in the liver, are unclear. We, therefore, investigated the roles of PDCD4 in myofibroblasts using human hepatic stellate cell line Lieming Xu-2 (LX-2). PDCD4 knockdown inhibited LX-2 proliferation and induced a senescent phenotype with increased β-galactosidase staining and p21 expression in a p53-independent manner together with downregulation of the notch signaling mediator RBJ-κ/CSL. During PDCD4 knockdown, alpha smooth muscle actin (α-SMA; an activation marker of myofibroblasts), matrix metalloproteinases MMP-1 and MMP-9, and collagen IV were upregulated, but the expression of collagen1α1 and collagen III was markedly downregulated without any marked change in the expression of tissue inhibitor of metalloproteinase-1 (TIMP-1). These results demonstrated that knockdown of PDCD4 induced the cellular senescence phenotype and activated myofibroblasts while suppressing the profibrogenic phenotype, suggesting roles of PDCD4 in cellular senescence and fibrogenesis in the liver.

Protective Effects of PPARγ on Renal Ischemia-Reperfusion Injury by Regulating miR-21

Renal ischemia-reperfusion injury (RIRI) is a common pathological process that causes kidney injury. Previous studies have indicated that both peroxisome proliferator-activated receptor γ (PPARγ) and microRNA-21 (miR-21) exert protective effects against RIRI. However, their relationship is not well understood. In the present study, we investigated the role of the PPARγ/miR-21/programmed cell death 4 (PDCD4) axis in IRI, both in vivo and in vitro. In vitro cell hypoxia/reoxygenation (H/R) and in vivo RIRI models were established, and cell viability, cell apoptosis, and key molecule expression profiles were analyzed. Our results showed that both PPARγ and miR-21 had protective effects against RIRI to varying degrees, and there was an interaction between PPARγ and miR-21. PPARγ could promote the expression of miR-21 and partially protect against RIRI by reducing the level of the miR-21 target protein (PDCD4). Our findings underscore the potential utility of future clinical investigations of PPARγ activation and targeting of the underlying miR-21/PDCD4/caspase-3 pathway, which may participate in the pathogenesis of human IRI.

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.

The Dysregulation of MicroRNAs in the Development of Cervical Pre-Cancer—An Update

Globally in 2020, an estimated ~600,000 women were diagnosed with and 340,000 women died from cervical cancer. Compared to 2012, the number of cases increased by 7.5% and the number of deaths increased by 17%. MiRNAs are involved in multiple processes in the pathogenesis of cervical cancer. Dysregulation of miRNAs in the pre-stage of cervical cancer is the focus of this review. Here we summarize the dysregulated miRNAs in clinical samples from cervical pre-cancer patients and relate them to the early transformation process owing to human papillomavirus (HPV) infection in the cervical cells. When HPV infects the normal cervical cells, the DNA damage response is initiated with the involvement of HPV’s E1 and E2 proteins. Later, cell proliferation and cell death are affected by the E6 and E7 proteins. We find that the expressions of miRNAs in cervical pre-cancerous tissue revealed by different studies seldom agreed with each other. The discrepancy in sample types, samples’ HPV status, expression measurement, and methods for analysis contributed to the non-aligned results across studies. However, several miRNAs (miR-34a, miR-9, miR-21, miR-145, and miR-375) were found to be dysregulated across multiple studies. In addition, there are hints that the DNA damage response and cell growth response induced by HPV during the early transformation of the cervical cells are related to these miRNAs. Currently, no review articles analyse the relationship between the dysregulated miRNAs in cervical pre-cancerous tissue and their possible roles in the early processes involving HPV’s protein encoded by the early genes and DNA damage response during normal cell transformation. Our review provides insight on spotting miRNAs involved in the early pathogenic processes and pointing out their potential as biomarker targets of cervical pre-cancer.

Dissecting the Roles of PDCD4 in Breast Cancer

The human programmed cell death 4 (PDCD4) gene was mapped at chromosome 10q24 and encodes the PDCD4 protein comprised of 469 amino acids. PDCD4 inhibits protein translation PDCD4 inhibits protein translation to suppress tumor progression, and its expression is frequently decreased in breast cancer. PDCD4 blocks translation initiation complex by binding eIF4A via MA-3 domains or by directly binding 5’ mRNA internal ribosome entry sites with an RNA binding domain to suppress breast cancer progression and proliferation. Numerous regulators and biological processes including non-coding RNAs, proteasomes, estrogen, natural compounds and inflammation control PDCD4 expression in breast cancer. Loss of PDCD4 expression is also responsible for drug resistance in breast cancer. HER2 activation downregulates PDCD4 expression by activating MAPK, AKT, and miR-21 in aromatase inhibitor-resistant breast cancer cells. Moreover, modulating the microRNA/PDCD4 axis maybe an effective strategy for overcoming chemoresistance in breast cancer. Down-regulation of PDCD4 is significantly associated with short overall survival of patients, which suggests that PDCD4 may be an independent prognostic marker for breast cancer.

The mTORC1/S6K/PDCD4/eIF4A Axis Determines Outcome of Mitotic Arrest

mTOR is a serine/threonine kinase and a master regulator of cell growth and proliferation. Raptor, a scaffolding protein that recruits substrates to mTOR complex 1 (mTORC1), is known to be phosphorylated during mitosis, but the significance of this phosphorylation remains largely unknown. Here we show that raptor expression and mTORC1 activity are dramatically reduced in cells arrested in mitosis. Expression of a non-phosphorylatable raptor mutant reactivates mTORC1 and significantly reduces cytotoxicity of the mitotic poison Taxol. This effect is mediated via degradation of PDCD4, a tumor suppressor protein that inhibits eIF4A activity and is negatively regulated by the mTORC1/S6K pathway. Moreover, pharmacological inhibition of eIF4A is able to enhance the effects of Taxol and restore sensitivity in Taxol-resistant cancer cells. These findings indicate that the mTORC1/S6K/PDCD4/eIF4A axis has a pivotal role in the death versus slippage decision during mitotic arrest and may be exploited clinically to treat tumors resistant to anti-mitotic agents.

The CDC37-HSP90 chaperone complex co-translationally degrades the nascent kinase-dead mutant of HIPK2

Homeodomain-interacting protein kinase 2 (HIPK2) is a highly conserved, constitutively active Ser/Thr protein kinase that is involved in various important biological processes. HIPK2 activates itself by auto-phosphorylation during its synthesis, and its activity is mainly controlled through modulation of its expression by ubiquitin-dependent degradation. By comparing the expression of wild-type and kinase-defective HIPK2, we have recently described a novel mechanism of HIPK2 regulation that is based on preferential co-translational degradation of kinase-defective versus wild-type HIPK2. Here, we have addressed this novel regulatory mechanism in more detail by focusing on the possible involvement of chaperones. Our work shows that HIPK2 is a client of the CDC37-HSP90 chaperone complex and points to a novel role of CDC37 in the co-translational degradation of a client protein.

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.

Expression of protein kinase HIPK2 is subject to a quality control mechanism that acts during translation and requires its kinase activity to prevent degradation of nascent HIPK2

HIPK2 is a highly conserved, constitutively active Ser/Thr protein kinase that is involved in a broad spectrum of biological processes. We have previously reported that the expression of HIPK2 is auto-regulated by a mechanism that depends on the activity of its kinase domain, leading to decreased expression of kinase-dead versus wild-type HIPK2. We have now explored this mechanism in more detail. Differential expression of wild-type and kinase-dead HIPK2 is dependent on sequences located in the C-terminal part of HIPK2, but is only observed when this part of HIPK2 is translated together with the defective kinase domain. On their own, both the defective kinase domain and the C-terminal amino acid sequences are expressed at normal levels and independently of kinase activity. Insertion of a 2A-ribosomal skipping sequence into the HIPK2 coding sequence revealed that the differential expression of wild-type and kinase-dead HIPK2 is caused by degradation of nascent kinase-dead HIPK2. Because HIPK2 is constitutively active and auto-activates its kinase domain already during its translation we speculate that the regulatory mechanism discovered here serves as a quality control mechanism that leads to degradation of nascent kinase molecules with defective kinase domains. Overall our work provides insight into a novel auto-regulatory mechanism of HIPK2 expression, thereby adding a new layer of control to the regulation of HIPK2.

PDCD4 controls the G1/S-phase transition in a telomerase-immortalized epithelial cell line and affects the expression level and translation of multiple mRNAs

PDCD4, the protein encoded by the tumor suppressor gene PDCD4 (programmed cell death 4) has been implicated in the control of cellular transcription and translation by modulating the activity of specific transcription factors and suppressing the translation of mRNAs with structured 5′-UTRs. Most studies of human PDCD4 have employed tumor cell lines, possibly resulting in a biased picture of its role in normal cells. Here, we have studied the function of PDCD4 in a telomerase-immortalized human epithelial cell line. We show for the first time that PDCD4 is required for the G1/S-transition, demonstrating its crucial role in the cell cycle. Inhibition of p53-dependent activation of p21^WAF1/CIP1 overrides the requirement for PDCD4 for the G1/S-transition, suggesting that PDCD4 counteracts basal p53 activity to prevent activation of the G1/S checkpoint by p53. Transcriptome and ribosome profiling data show that silencing of PDCD4 changes the expression levels and translation of many mRNAs, providing an unbiased view of the cellular processes that are affected by PDCD4 in an epithelial cell line. Our data identify PDCD4 as a key regulator of cell cycle- and DNA-related functions that are inhibited when it is silenced, suggesting that decreased expression of PDCD4 might contribute to tumor development by compromising genomic integrity.

Development of microRNA-21 mimic nanocarriers for the treatment of cutaneous wounds

Rationale: Of the regulatory microRNAs expressed in the wounded skin, microRNA-21 (miR21) plays a pivotal role in wound repair by stimulating re-epithelialization, an essential feature to facilitate healing and reduce scar formation. Despite their crucial roles in wound healing, synthetic exogenous microRNAs have limited applications owing to the lack of an appropriate delivery system. Herein, we designed an miR21 mimic nanocarrier system using facial amphipathic bile acid-conjugated polyethyleneimines (BA-PEI) for the intracellular and transdermal delivery of synthetic miR21 molecules to accelerate wound repair. Methods: To design miR21 mimic nanocarriers, BA-conjugated PEIs prepared from three different types of BA at molar feed ratios of 1 and 3 were synthesized. The intracellular uptake efficiency of synthetic miR21 mimics was studied using confocal laser scanning microscopy and flow cytometry analysis. The optimized miR21/BA nanocarrier system was used to evaluate the wound healing effects induced by miR21 mimics in human HaCaT keratinocytes in vitro and a murine excisional acute wound model in vivo. Results: The cell uptake efficiency of miR21 complexed with BA-conjugated PEI was dramatically higher than that of miR21 complexed with PEI alone. Deoxycholic acid (DA)-modified PEI at a molar feed ratio of 3:1 (DA3-PEI) showed the highest transfection efficiency for miR21 without any increase in toxicity. After effective transdermal and intracellular delivery of miR21/DA3 nanocarriers, miR21 mimics promoted cell migration and proliferation through the post-transcriptional regulation of programmed cell death protein 4 (PDCD4) and matrix metalloproteinases. Thus, miR21 mimic nanocarriers improved both the rate and quality of wound healing, as evident from enhanced collagen synthesis and accelerated wound re-epithelialization. Conclusion: Our miRNA nanocarrier systems developed using DA3-PEI conjugates may be potentially useful for the delivery of synthetic exogenous miRNAs in various fields.

Petal abscission in roses is associated with the activation of a truncated version of the animal PDCD4 homologue, RbPCD1

Abscission is a developmental process that leads to shedding of organs not needed by the plant. Apart from wall hydrolysis, the cells of the abscission zone (AZ) are also believed to undergo programmed cell death (PCD). We show that ethylene-induced petal abscission in Rosa bourboniana is accompanied with the activation of RbPCD1 (PROGRAMMED CELL DEATH LIKE 1) encoding a protein of 78 amino acids. Its expression increases during natural and ethylene-induced petal abscission. Its transcription in most tissues is up-regulated by ethylene. RbPCD1 shows similarity to the N-terminal domain of animal PDCD4 (PROGRAMMED CELL DEATH PROTEIN 4) proteins that are activated during apoptosis and function as transcriptional and translational repressors. RbPCD1 resides in the nucleus and cytoplasm and acts as a transcriptional repressor. Constitutive expression of RbPCD1 in transgenic Arabidopsis is seedling lethal. Heat-induced expression of RbPCD1 under the soybean heat-shock promoter affects leaf function, inflorescence development, silique formation, seed yield and reduces survival. Nuclear localization of RbPCD1 is necessary for manifestation of its effects. RbPCD1 may be necessary to mediate some of the ethylene-induced changes during abscission and senescence in specific tissues.

PDCD4 Knockdown Induces Senescence in Hepatoma Cells by Up-Regulating the p21 Expression

While the over-expression of tumor suppressor programmed cell death 4 (PDCD4) induces apoptosis, it was recently shown that PDCD4 knockdown also induced apoptosis. In this study, we examined the cell cycle regulators whose activation is affected by PDCD4 knockdown to investigate the contribution of PDCD4 to cell cycle regulation in three types of hepatoma cells: HepG2, Huh7 (mutant p53 and p16-deficient), and Hep3B (p53- and Rb-deficient). PDCD4 knockdown suppressed cell growth in all three cell lines by inhibiting Rb phosphorylation via down-regulating the expression of Rb itself and CDKs, which phosphorylate Rb, and up-regulating the expression of the CDK inhibitor p21 through a p53-independent pathway. We also found that apoptosis was induced in a p53-dependent manner in PDCD4 knockdown HepG2 cells (p53+), although the mechanism of cell death in PDCD4 knockdown Hep3B cells (p53-) was different. Furthermore, PDCD4 knockdown induced cellular senescence characterized by β-galactosidase staining, and p21 knockdown rescued the senescence and cell death as well as the inhibition of Rb phosphorylation induced by PDCD4 knockdown. Thus, PDCD4 is an important cell cycle regulator of hepatoma cells and may be a promising therapeutic target for the treatment of hepatocellular carcinoma.

Evaluation of miRNA-196a2 and apoptosis-related target genes: ANXA1, DFFA and PDCD4 expression in gastrointestinal cancer patients: A pilot study

Previous reports have suggested the significant association of miRNAs aberrant expression with tumor initiation, progression and metastasis in cancer, including gastrointestinal (GI) cancers. The current preliminary study aimed to evaluate the relative expression levels of miR-196a2 and three of its selected apoptosis-related targets; ANXA1, DFFA and PDCD4 in a sample of GI cancer patients. Quantitative real-time PCR for miR-196a2 and its selected mRNA targets, as well as immunohistochemical assay for annexin A1 protein expression were detected in 58 tissues with different GI cancer samples. In addition, correlation with the clinicopathological features and in silico network analysis of the selected molecular markers were analyzed. Stratified analyses by cancer site revealed elevated levels of miR-196a2 and low expression of the selected target genes. Annexin protein expression was positively correlated with its gene expression profile. In colorectal cancer, miR-196a over-expression was negatively correlated with annexin A1 protein expression (r = -0.738, p < 0.001), and both were indicators of unfavorable prognosis in terms of poor differentiation, larger tumor size, and advanced clinical stage. Taken together, aberrant expression of miR-196a2 and the selected apoptosis-related biomarkers might be involved in GI cancer development and progression and could have potential diagnostic and prognostic roles in these types of cancer; particularly colorectal cancer, provided the results experimentally validated and confirmed in larger multi-center studies.

Peroxisome proliferator-activated receptor-γ enhances human pulmonary artery smooth muscle cell apoptosis through microRNA-21 and programmed cell death 4

Pulmonary hypertension (PH) is a progressive disorder whose cellular pathogenesis involves enhanced smooth muscle cell (SMC) proliferation and resistance to apoptosis signals. Existing evidence demonstrates that the tumor suppressor programmed cell death 4 (PDCD4) affects patterns of cell growth and repair responses in the systemic vasculature following experimental injury. In the current study, the regulation PDCD4 and its functional effects on growth and apoptosis susceptibility in pulmonary artery smooth muscle cells were explored. We previously demonstrated that pharmacological activation of the nuclear transcription factor peroxisome proliferator-activated receptor-γ (PPARγ) attenuated hypoxia-induced proliferation of human pulmonary artery smooth muscle cells (HPASMCs) by inhibiting the expression and mitogenic functions of microRNA-21 (miR-21). In the current study, we hypothesize that PPARγ stimulates PDCD4 expression and HPASMC apoptosis by inhibiting miR-21. Our findings demonstrate that PDCD4 is reduced in the mouse lung upon exposure to chronic hypoxia (10% O₂ for 3 wk) and in hypoxia-exposed HPASMCs (1% O₂). HPASMC apoptosis was reduced by hypoxia, by miR-21 overexpression, or by siRNA-mediated PPARγ and PDCD4 depletion. Activation of PPARγ inhibited miR-21 expression and resultant proliferation, while restoring PDCD4 levels and apoptosis to baseline. Additionally, pharmacological activation of PPARγ with rosiglitazone enhanced PDCD4 protein expression and apoptosis in a dose-dependent manner as demonstrated by increased annexin V detection by flow cytometry. Collectively, these findings demonstrate that PPARγ confers growth-inhibitory signals in hypoxia-exposed HPASMCs through suppression of miR-21 and the accompanying derepression of PDCD4 that augments HPASMC susceptibility to undergo apoptosis.

Reduced PDCD4 Expression Promotes Cell Growth Through PI3K/Akt Signaling in Non-Small Cell Lung Cancer

It is largely recognized that PDCD4 is frequently lost in tumors of various origins, including lung cancer, and its loss contributes to tumor progression. However, its role and molecular mechanism remain largely unexplored in non-small cell lung cancer (NSCLC). In this study, downregulated PDCD4 mRNA expression was found in NSCLC tissues compared to their corresponding paracarcinoma tissues and distal paracarcinoma tissues. Induced expression of PDCD4 inhibited cell growth and proliferation and cell cycle transition in vitro. Conversely, knocking down PDCD4 expression promoted cell growth and proliferation. Mechanistically, PDCD4 inactivated PI3K/Akt signaling and its downstream cell cycle factors CCND1 and CDK4 to regulate cell growth in NSCLC. Additionally, PI3K-specific inhibitor Ly294002 suppressed the expression of pPI3K (Tyr458), pAkt (Ser473), CCND1, and CDK4 in PC9-shPDCD4 and A549-shPDCD4 cells. Furthermore, Akt-specific inhibitor MK2206 inhibited the expression of pAkt (Ser473), CCND1, and CDK4 in PC9-shPDCD4 and A549-shPDCD4 cells. Taken together, our study provides evidence that PDCD4 inhibits cell growth through PI3K/Akt signaling in NSCLC and may be a potential therapeutic target for NSCLC.

miR-374a-CCND1-pPI3K/AKT-c-JUN feedback loop modulated by PDCD4 suppresses cell growth, metastasis, and sensitizes nasopharyngeal carcinoma to cisplatin

miR-374a has been reported to function as an oncogene during tumor pathogenesis. In this study, miR-374a is observed to reduce nasopharyngeal carcinoma (NPC) cell proliferation, migration, invasion, metastasis and cisplatin (DDP) resistance in vitro and in vivo. Mechanistic analyses indicate that miR-374a directly targets CCND1 to inactivate pPI3K/pAKT/c-JUN forming a negative feedback loop, as well as suppressing downstream signals related to cell cycle progression and epithelial-mesenchymal transition (EMT). Interestingly, we also observed that miR-374a direct targeting of CCND1 is modulated by tumor suppressor PDCD4 via suppressing pPI3K/pAKT/c-JUN signaling. In clinical specimens, miR-374a was positively and negatively correlated with expression of PDCD4 and CCND1, respectively. Our studies are the first to demonstrate that the miR-374a-CCND1-pPI3K/AKT-c-JUN feedback loop induced by PDCD4 supresses NPC cell growth, metastasis and chemotherapy resistance.

Down-regulation of tumor suppressor PDCD4 expression in endometrium of adenomyosis patients

OBJECTIVE

Adenomyosis is a common benign gynecological disease which has some malignant behaviors. Programmed cell death 4 (PDCD4) is a newly identified tumor suppressor gene which lowly expresses in various cancers. However, the expression status of PDCD4 in endometrium of adenomyosis patients has not been investigated. The aim of this study is to assess the expression levels of PDCD4 in endometrium of normal controls and adenomyosis patients.

METHODS

The expression of PDCD4 in endometrium of normal controls and eutopic or ectopic endometrium of patients with adenomyosis was evaluated with quantitative real-time PCR, western blot and immunohistochemistry. In addition, the levels of serum estradiol and progesterone in normal controls and adenomyosis patients were detected using electrochemiluminescence immunoassay.

RESULTS

The results showed that PDCD4 mainly expressed in the cytoplasma of glandular epithelium of control endometrium and varied during the cycle changes of endometrium, which may be regulated by changing concentrations of progesterone in the menstrual cycle. Compared with the proliferative phase of control endometrium, PDCD4 expression was down-regulated in proliferative phase of eutopic endometrium or ectopic endometrium, and there was no cyclic variation of PDCD4 expression in eutopic endometrium of adenomyosis patients due to progesterone resistance.

CONCLUSION

These results suggest that PDCD4 may be involved in the pathogenesis of adenomyosis, which will provide a novel strategy for the early diagnosis and new therapeutic target of adenomyosis.

Cell Density-Dependent Upregulation of PDCD4 in Keratinocytes and Its Implications for Epidermal Homeostasis and Repair

Programmed cell death 4 (PDCD4) is one multi-functional tumor suppressor inhibiting neoplastic transformation and tumor invasion. The role of PDCD4 in tumorigenesis has attracted more attention and has been systematically elucidated in cutaneous tumors. However, the normal biological function of PDCD4 in skin is still unclear. In this study, for the first time, we find that tumor suppressor PDCD4 is uniquely induced in a cell density-dependent manner in keratinocytes. To determine the potential role of PDCD4 in keratinocyte cell biology, we show that knockdown of PDCD4 by siRNAs can promote cell proliferation in lower cell density and partially impair contact inhibition in confluent HaCaT cells, indicating that PDCD4 serves as an important regulator of keratinocytes proliferation and contact inhibition in vitro. Further, knockdown of PDCD4 can induce upregulation of cyclin D1, one key regulator of the cell cycle. Furthermore, the expression patterns of PDCD4 in normal skin, different hair cycles and the process of wound healing are described in detail in vivo, which suggest a steady-state regulatory role of PDCD4 in epidermal homeostasis and wound healing. These findings provide a novel molecular mechanism for keratinocytes’ biology and indicate that PDCD4 plays a role in epidermal homeostasis.

PDCD4 as a predictor of sensitivity to neoadjuvant chemoradiotherapy in locally advanced rectal cancer patients

OBJECTIVE

The purpose of this study was to examine the role of programmed cell death 4 (PDCD4) expression in predicting tumor response to neoadjuvant chemoradiotherapy and outcomes for patients with locally advanced rectal cancer.

METHODS

Clinicopathological factors and expression of PDCD4 were evaluated in 92 patients with LARC treated with nCRT. After the completion of therapy, 4 cases achieved clinical complete response (cCR), and thus the remaining 88 patients underwent a standardized total mesorectal excision procedure. There were 38 patients (41.3%) with a good response (TRG 3-4) and 54 (58.7%) with a poor one (TRG 0-2).

RESULTS

Immunohistochemical staining analyses showed that patients with high expression of PDCD4 were more sensitive to nCRT than those with low PDCD4 expression (P=0.02). High PDCD4 expression before nCRT and good response (TRG3-4) were significantly associated with improved 5-year disease-free survival and 5-year overall survival (P<0.05). Multivariate analysis demonstrated that the pretreatment PDCD4 expression was an independent prognostic factor.

CONCLUSION

Our study demonstrated that high expression of PDCD4 protein is a useful predictive factor for good tumor response to nCRT and good outcomes in patients with LARC.

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.

MicroRNA-183 promotes proliferation and invasion in oesophageal squamous cell carcinoma by targeting programmed cell death 4

Background:

Dysregulated microRNAs (miRNAs) can serve as oncogenes or suppressors and are associated with many cancers, including oesophageal squamous cell carcinoma (ESCC).

Methods:

An alignment miRNA array was used to identify differentially expressed miRNAs in ESCC tissues. The expression of miR-183 and programmed cell death 4 (PDCD4) in oesophageal tissues from ESCC and early oesophageal carcinoma patients was examined by quantitative reverse transcriptase PCR and western blotting. A luciferase assay was performed to confirm miR-183 target genes. The effects of miR-183 on ESCC cells and the associated mechanisms were established by in vitro experiments.

Results:

We identified 51 upregulated miRNAs and 17 downregulated miRNAs in our array, and miR-183 was one of the most upregulated miRNAs. An inverse correlation between miR-183 and PDCD4 levels was found in ESCC tissues. Upregulated expression of miR-183 was not correlated with tumour stage or lymphatic metastasis in ESCC patients. The luciferase assay confirmed that miR-183 directly interacted with the PDCD4 mRNA 3′-untranslated region in ESCC cells. Overexpression of miR-183 led to decreased PDCD4 protein levels and promoted ESCC cell proliferation and invasion. Inhibition of the PI3K/Akt signalling pathway increased PDCD4 protein levels and decreased miR-183 expression in ESCC cells.

Conclusions:

MiR-183 promotes ESCC cell proliferation and invasion by directly targeting PDCD4, which suggests that it is involved in the pathogenesis of ESCC.

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.

Integrative analysis of differential miRNA and functional study of miR-21 by seed-targeting inhibition in multiple myeloma cells in response to berberine

Background

Berberine is a natural alkaloid derived from a traditional Chinese herbal medicine. It is known to modulate microRNA (miRNA) levels, although the mechanism for this action is unknown. Here, we previously demonstrate that the expression of 87 miRNAs is differentially affected by berberine in multiple myeloma cells. Among 49 miRNAs that are down-regulated, nine act as oncomirs, including miR-21. Integrative analysis showed that 28 of the down-regulated miRNAs participate in tumor protein p53 (TP53) signaling and other cancer pathways. miR-21 is involved in all these pathways, and is one of the most important oncomirs to be affected by berberine in multiple myeloma cells.

Results

We confirmed that berberine down-regulated miRNA-21 expression and significantly up-regulated the expression of programmed cell death 4 (PDCD4), a predicted miR-21 target. Luciferase reporter assays confirmed that PDCD4 was directly regulated by miR-21. Bioinformatic analysis revealed that the miR-21 promoter can be targeted by signal transducer and activator of transcription 3 (STAT3). Down-regulation of interleukin 6 (IL6) by berberine might lead to inhibition of miR-21 transcription through STAT3 down-regulation in multiple myeloma. Furthermore, both berberine and seed-targeting anti-miR-21 oligonucleotide induced apoptosis, G2-phase cell cycle arrest and colony inhibition in multiple myeloma cell lines. Depletion of PDCD4 by short interfering RNA could rescue berberine-induced cytotoxicity in multiple myeloma cells.

Conclusions

Our results suggest that berberine suppresses multiple myeloma cell growth, at least in part, by down-regulating miR-21 levels possibly through IL6/STAT3. This led to increased PDCD4 expression, which is likely to result in suppression of the p53 signaling pathway. These findings may also provide new mechanistic insight into the anti-cancer effects of certain compounds in traditional Chinese herbal medicines.

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.

Atheroprotective pulsatile flow induces ubiquitin-proteasome-mediated degradation of programmed cell death 4 in endothelial cells

Objectives

We recently found low level of tumor suppressor programmed cell death 4 (PDCD4) associated with reduced atherosclerotic plaque area (unpublished). We investigated whether atheroprotective unidirectional pulsatile shear stress affects the expression of PDCD4 in endothelial cells.

Methods and Results

En face co-immunostaining of the mouse aortic arch revealed a low level of PDCD4 in endothelial cells undergoing pulsatile shear stress. Application of unidirectional pulsatile shear stress to human umbilical vein endothelial cells (HUVECs) decreased PDCD4 protein but not mRNA level. Immunoprecipitation revealed that pulsatile shear stress induced the coupling of ubiquitin with PDCD4 expression. The phosphatidyl inositol 3-kinase (PI3K)/Akt pathway was involved in this ubiquitin-proteasome–mediated degradation of PDCD4. Gain of function and loss of function experiments showed that PDCD4 induced turnover (proliferation and apoptosis) of HUVECs. Low PDCD4 level was associated with reduced proliferation but not apoptosis or phosphorylation of endothelial nitric oxide synthase caused by pulsatile shear stress to help maintain the homeostasis of endothelial cells.

Conclusions

Pulsatile shear stress induces ubiquitin-proteasome–mediated degradation of PDCD4 via a PI3K/Akt pathway in HUVECs. PDCD4 induces turnover (proliferation and apoptosis) of HUVECs. Low PDCD4 level is associated with reduced proliferation for maintenance of HUVEC homeostasis under pulsatile shear stress.

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.

Differential effects of PDCD4 depletion on protein synthesis in myoblast and myotubes

BACKGROUND

Reduced muscle mass is a hallmark of metabolic diseases like diabetes and cancer. The mammalian (mechanistic) target of rapamycin complex 1/S6 kinase 1 (mTORC1/S6K1) pathway is critical to the regulation of muscle protein synthesis and mass but its mechanism of action is not completely understood.

RESULTS

Using L6 myotubes, we characterized the regulation of programmed cell death 4 (PDCD4), a recently described substrate of S6K1. The abundance, but not Ser67 phosphorylation, of PDCD4 was sensitive to amino acid and serum deprivation: values in starved cells were 4.5X of control (P < 0.001). Refeeding had opposite effects. Growth factors, compared to amino acids, appeared more critical in regulating PDCD4 abundance. Furthermore, inhibition of mTORC1 or the proteasome prevented the refeeding-associated decrease in PDCD4 abundance. Amino acid and serum deprivation significantly increased PDCD4 binding to eIF4A (P < 0.05); this was reversed during refeeding. PDCD4 depletion by RNA interference had no significant effect on phenylalanine incorporation into myotube mixed proteins in control cells but further suppressed (30%) this measure in nutrient-deprived cells (P < 0.0005). This was not observed in myoblasts. In starved myotubes, PDCD4 depletion further reduced the association of eIF4G with eIF4E.

CONCLUSION

Our data suggest that in myotubes, PDCD4 abundance is sensitive to nutritional manipulation in an mTORC1 and proteasome depended manner. Furthermore, the role of PDCD4 in regulating protein synthesis appears dependent on the developmental state of the cell.

Tumor suppressor PDCD4 modulates miR-184-mediated direct suppression of C-MYC and BCL2 blocking cell growth and survival in nasopharyngeal carcinoma

Programmed cell death 4 (PDCD4), a novel tumor suppressor, inhibits cell proliferation, migration and invasion as well as promotes cell apoptosis in tumors. However, the molecular mechanism of its tumor-suppressive function remains largely unknown in tumors including nasopharyngeal carcinoma (NPC). In this study, downregulated PDCD4 expression was significantly associated with the status of NPC progression and poor prognosis. PDCD4 markedly suppressed the ability of cell proliferation and cell survival by modulating C-MYC-controlled cell cycle and BCL-2-mediated mitochondrion apoptosis resistance signals, and oncogenic transcription factor C-JUN in NPC. Furthermore, miR-184, a tumor-suppressive miRNA modulated by PDCD4 directly targeting BCL2 and C-MYC, participated in PDCD4-mediated suppression of cell proliferation and survival in NPC. Further, we found that PDCD4 decreased the binding of C-Jun to the AP-1 element on the miR-184 promoter regions by PI3K/AKT/JNK/C-Jun pathway and stimulated miR-184 expression. In clinical fresh specimens, reduced PDCD4 mRNA level was positively correlated with miR-184 expression in NPC. Our studies are the first to demonstrate that PDCD4 as tumor suppressor regulated miR-184-mediated direct targeting of BCL2 and C-MYC via PI3K/AKT and JNK/C-Jun pathway attenuating cell proliferation and survival in NPC.

Early dynamic transcriptomic changes during preoperative radiotherapy in patients with rectal cancer: A feasibility study

AIM: To develop novel biomarkers of rectal radiotherapy, we measured gene expression profiles on biopsies taken before and during preoperative radiotherapy.

METHODS: Six patients presenting with a locally advanced rectal cancer (T>T2, N0/Nx, M0) eligible for preoperative radiotherapy (45 Gy in 25 fractions) were selected in a pilot study. Six tumor and 3 normal tissues biopsies were taken before and during radiotherapy, after a dose of 7.2 Gy at a median time of 1 h following irradiation (0:27-2:12). Tumor or normal tissue purity was assessed by a pathologist prior to RNA extraction. Mean RNA content was 23 μg/biopsy (14-37) before radiotherapy and 22.7 μg/biopsy (12-35) during radiotherapy. After RNA amplification, biopsies were analysed with 54K HG-U133A Plus 2.0 Affymetrix expression micro-arrays. Data were normalized according to MAS5 algorithm. A gene expression ratio was calculated as: (gene expression during radiotherapy - gene expression before radiotherapy)/gene expression before radiotherapy. Were selected genes that showed a ratio higher than ± 0.5 in all 6 patients.

RESULTS: Microarray analysis showed that preoperative radiotherapy significantly up-regulated 31 genes and down-regulated 6 genes. According to the Gene Ontology project classification, these genes are involved in protein metabolism (ADAMDEC1; AKAP7; CAPN5; CLIC5; CPE; CREB3L1; NEDD4L; RAB27A), ion transport (AKAP7; ATP2A3; CCL28; CLIC5; F2RL2; NEDD4L; SLC6A8), transcription (AKAP7; CREB3L1; ISX; PABPC1L; TXNIP), signal transduction (CAPN5; F2RL2; RAB27A; TNFRSF11A), cell adhesion (ADAMDEC1; PXDN; SPON1; S100A2), immune response (CCL28; PXDN; TNFRSF11A) and apoptosis (ITM2C; PDCD4; PVT1). Up-regulation of 3 genes (CCL28; CLIC5; PDCD4) was detected by 2 different probes and up-regulation of 2 genes (RAB27A; TXNIP) by 3 probes.

CONCLUSION: Micro-arrays can efficiently assess early transcriptomic changes during preoperative radiotherapy for rectal cancer, and may help better understand tumor radioresistance.

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.

PDCD4 functions as a suppressor for pT2a and pT2b stage gastric cancer

Gastric cancer is one of the leading causes of cancer‑related mortality worldwide. Loss of programmed cell death 4 (PDCD4) expression has been detected in gastric cancer. However, the effects of PDCD4 on pT2 stage gastric cancer remain unclear. The aim of this study was to identify the relationship between PDCD4 expression and clinicopathological features of patients with pT2 stage gastric cancer. In the present study, 122 pT2 stage gastric cancer specimens were subclassified as pT2a and pT2b stage. The levels of PDCD4 mRNA and protein in gastric cancer tissues were lower compared to that in normal tissues as detected by real‑time PCR and western blot analysis, respectively. In addition, both PDCD4 mRNA and protein in pT2b stage gastric cancer were lower when compared to that in pT2a stage gastric cancer. Finally, we used immuno-histochemistry to determine the protein expression and analyzed the relationship between PDCD4 expression and the clinicopathological features of pT2 stage gastric cancer patients. Cumulative survival rate of patients with PDCD4 expression was significantly higher compared to the patients without PDCD4 expression. PDCD4 expression in gastric cancer can be employed to indicate a favorable prognosis for the disease outcome.

PDCD4/miR-21 dysregulation in inflammatory bowel disease-associated carcinogenesis

Inflammatory bowel diseases (IBDs; both ulcerative colitis [UC] and Crohn's colitis [CC]) are well-established predisposing pathological conditions for colorectal cancer (CRC) development. In IBDs, both the endoscopy and the histology assessment of CRC precursors (i.e., dysplasia, also defined as intraepithelial neoplasia) are associated with low interobserver consistency, and no reliable dysplasia-specific biomarker is available. The programmed cell death 4 (PDCD4) tumor suppressor gene is involved in sporadic colorectal oncogenesis, but scanty information is available on its involvement in IBD-associated colorectal oncogenesis. One hundred twenty tissue samples representative of active and inactive IBD and of flat dysplasia were obtained from 30 cases of UC and 30 of CC who undergone colectomy. Twenty additional biopsy samples obtained from patients with irritable bowel syndrome acted as normal controls. PDCD4 expression was assessed by immunohistochemistry; the expression of miR-21 (a major PDCD4 regulator) was investigated by quantitative real-time PCR and in situ hybridization in different series of a hundred samples. Tissue specimens from both controls and inactive IBD consistently featured strong PDCD4 nuclear immunostain; conversely, lower PDCD4 nuclear expression was featured by both active IBD and IBD-associated dysplastic lesions. Significant PDCD4 down-regulation distinguished IBD-associated dysplasia (p < 0.001) versus active IBD. In both active IBD and dysplasia, PDCD4 down-regulation was significantly associated with miR-21 up-regulation. PDCD4 nuclear down-regulation (which parallels miR-21 up-regulation) is involved in the molecular pathway of IBD-associated carcinogenesis. PDCD4 nuclear expression may be usefully applied as ancillary maker in the histological assessment of IBD-associated dysplastic lesions.

Heterogeneous nuclear ribonucleoprotein C1/C2 controls the metastatic potential of glioblastoma by regulating PDCD4

MicroRNAs (miRNAs) have been implicated in the pathogenesis and progression of brain tumors. miR-21 is one of the most highly overexpressed miRNAs in glioblastoma multiforme (GBM), and its level of expression correlates with the tumor grade. Programmed cell death 4 (PDCD4) is a well-known miR-21 target and is frequently downregulated in glioblastomas in accordance with increased miR-21 expression. Downregulation of miR-21 or overexpression of PDCD4 can inhibit metastasis. Here, we investigate the role of heterogeneous nuclear ribonucleoprotein C1/C2 (hnRNPC) in the metastatic potential of the glioblastoma cell line T98G. hnRNPC bound directly to primary miR-21 (pri-miR-21) and promoted miR-21 expression in T98G cells. Silencing of hnRNPC lowered miR-21 levels, in turn increasing the expression of PDCD4, suppressing Akt and p70S6K activation, and inhibiting migratory and invasive activities. Silencing of hnRNPC reduced cell proliferation and enhanced etoposide-induced apoptosis. In support of a role for hnRNPC in the invasiveness of GBM, highly aggressive U87MG cells showed higher hnRNPC expression levels and hnRNPC abundance in tissue arrays and also showed elevated levels as a function of brain tumor grade. Taken together, our data indicate that hnRNPC controls the aggressiveness of GBM cells through the regulation of PDCD4, underscoring the potential usefulness of hnRNPC as a prognostic and therapeutic marker of GBM.

MicroRNAs: molecular features and role in cancer

microRNAs (miRNAs) are small noncoding endogenously produced RNAs that play key roles in controlling the expression of many cellular proteins. Once they are recruited and incorporated into a ribonucleoprotein complex miRISC, they can target specific mRNAs in a miRNA sequence-dependent process and interfere in the translation into proteins of the targeted mRNAs via several mechanisms. Consequently, miRNAs can regulate many cellular pathways and processes. Dysregulation of their physiological roles may largely contribute to disease. In particular, in cancer, miRNAs can be involved in the deregulation of the expression of important genes that play key roles in tumorigenesis, tumor development, and angiogenesis and have oncogenic or tumor suppressor roles. This review focuses on the biogenesis and maturation of miRNAs, their mechanisms of gene regulation, and the way their expression is deregulated in cancer. The involvement of miRNAs in several oncogenic pathways such as angiogenesis and apoptosis, and in the inter-cellular dialog mediated by miRNA-loaded exosomes as well as the development of new therapeutical strategies based on miRNAs will be discussed.

Regulatory effects of programmed cell death 4 (PDCD4) protein in interferon (IFN)-stimulated gene expression and generation of type I IFN responses

The precise mechanisms by which the activation of interferon (IFN) receptors (IFNRs) ultimately controls mRNA translation of specific target genes to induce IFN-dependent biological responses remain ill defined. We provide evidence that IFN-α induces phosphorylation of programmed cell death 4 (PDCD4) protein on Ser67. This IFN-α-dependent phosphorylation is mediated by either the p70 S6 kinase (S6K) or the p90 ribosomal protein S6K (RSK) in a cell-type-specific manner. IFN-dependent phosphorylation of PDCD4 results in downregulation of PDCD4 protein levels as the phosphorylated form of PDCD4 interacts with the ubiquitin ligase β-TRCP (β-transducin repeat-containing protein) and undergoes degradation. This process facilitates IFN-induced eukaryotic translation initiation factor 4A (eIF4A) activity and binding to translation initiation factor eIF4G to promote mRNA translation. Our data establish that PDCD4 degradation ultimately facilitates expression of several ISG protein products that play important roles in the generation of IFN responses, including IFN-stimulated gene 15 (ISG15), p21(WAF1/CIP1), and Schlafen 5 (SLFN5). Moreover, engagement of the RSK/PDCD4 pathway by the type I IFNR is required for the suppressive effects of IFN-α on normal CD34(+) hematopoietic precursors and for antileukemic effects in vitro. Altogether, these findings provide evidence for a unique function of PDCD4 in the type I IFN system and indicate a key regulatory role for this protein in mRNA translation of ISGs and control of IFN responses.

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.

TGF-β-induced miR-21 negatively regulates the antiproliferative activity but has no effect on EMT of TGF-β in HaCaT cells

The transforming growth factor-β (TGF-β) signaling pathway plays important roles in maintaining normal tissue homeostasis, and is tightly controlled by a network of biomolecules. MicroRNAs (miRNAs) are small noncoding RNAs of ∼22 nucleotides that regulate gene expression at posttranscriptional levels. Increasing evidence points to the important role of miRNAs in TGF-β signaling. OncomicroRNA miR-21 has been established as a key regulator of mesenchymal phenotype transition induced by TGF-β. However, the effects of miR-21 on epithelial biology involved in TGF-β signaling pathway such as cytostatic program and epithelial to mesenchymal transition (EMT) processes are unclear. Here we show that miR-21 is upregulated after TGF-β exposure in both growth inhibition and EMT models of HaCaT keratinocytes. To determine the potential roles of miR-21 in TGF-β-induced growth-arrest and EMT models, we showed that ectopic expression of miR-21 overcame TGF-β' growth-inhibitory effect and the knockdown of miR-21 potentialized this effect, but perturbation of miR-21 levels had little effect on EMT. Moreover, TGFBR2, PTEN, PDCD4, and TAp63 were identified as targets of miR-21 in HaCaT cells. And among them, TGFBR2, PTEN, and TAp63 were associated with TGF-β-induced cytostatic program. Thus, our results suggest that miR-21 regulates the ability of epithelial cells to respond to TGF-β, with potential impact on epithelium homeostasis, wound-healing and tumorigenesis.

Signaling by the matrix proteoglycan decorin controls inflammation and cancer through PDCD4 and MicroRNA-21

The mechanisms linking immune responses and inflammation with tumor development are not well understood. Here, we show that the soluble form of the extracellular matrix proteoglycan decorin controls inflammation and tumor growth through PDCD4 (programmed cell death 4) and miR-21 (microRNA-21) by two mechanisms. First, decorin acted as an endogenous ligand of Toll-like receptors 2 and 4 and stimulated production of proinflammatory molecules, including PDCD4, in macrophages. Second, decorin prevented translational repression of PDCD4 by decreasing the activity of transforming growth factor-β1 and the abundance of oncogenic miR-21, a translational inhibitor of PDCD4. Moreover, increased PDCD4 abundance led to decreased release of the anti-inflammatory cytokine interleukin-10, thereby making the cytokine profile more proinflammatory. This pathway operates in both pathogen-mediated and sterile inflammation, as shown here for sepsis and growth retardation of established tumor xenografts, respectively. Decorin was an early response gene evoked by septic inflammation, and protein concentrations of decorin were increased in the plasma of septic patients and mice. In cancer, decorin reduced the abundance of anti-inflammatory molecules and increased that of proinflammatory molecules, thereby shifting the immune response to a proinflammatory state associated with reduced tumor growth. Thus, by stimulating proinflammatory PDCD4 and decreasing the abundance of miR-21, decorin signaling boosts inflammatory activity in sepsis and suppresses tumor growth.

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.

Loss of programmed cell death 4 induces apoptosis by promoting the translation of procaspase-3 mRNA

The programmed cell death 4 (Pdcd4), a translation inhibitor, plays an essential role in tumor suppression, but its role in apoptosis remains unclear. Here we show that Pdcd4 is a critical suppressor of apoptosis by inhibiting the translation of procaspase-3 mRNA. Pdcd4 protein decreased more rapidly through microRNA-mediated translational repression following apoptotic stimuli than did the activation of procaspase-3, cleavage of poly(ADP)ribose polymerase (PARP) by active caspase-3, and nuclear fragmentation. Strikingly, the loss of Pdcd4 by the specific RNA interference increased procaspase-3 expression, leading to its activation and PARP cleavage even without apoptotic stimuli, and sensitized the cells to apoptosis. Thus, our findings provide insight into a novel mechanism for Pdcd4 as a regulator of apoptosis.

The microRNA-21-PDCD4 axis prevents type 1 diabetes by blocking pancreatic beta cell death

Death of pancreatic β cells is a pathological hallmark of type 1 diabetes (T1D). However, the molecular mechanisms of β cell death and its regulation are poorly understood. Here we describe a unique regulatory pathway of β cell death that comprises microRNA-21, its target tumor suppressor PDCD4, and its upstream transcriptional activator nuclear factor-κB (NF-κB). In pancreatic β cells, c-Rel and p65 of the NF-κB family activated the mir21 gene promoter and increased miR-21 RNA levels; miR-21 in turn decreased the level of PDCD4, which is able to induce cell death through the Bax family of apoptotic proteins. Consequently, PDCD4 deficiency in pancreatic β cells renders them resistant to death, and PDCD4 deficiency in NOD or C57BL/6 mice conferred resistance to spontaneous diabetes and diabetes induced by autoimmune T cells or the β cell toxin streptozotocin (STZ). Thus, the NF-κB-microRNA-21-PDCD4 axis plays a crucial role in T1D and represents a unique therapeutic target for treating the disease.

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.