microRNA-558 facilitates the expression of hypoxia-inducible factor 2 alpha through binding to 5'-untranslated region in neuroblastoma

Regulation of the HIF switch in human endothelial and cancer cells

Hypoxia-inducible factors (HIFs) are transcription factors that reprogram the transcriptome for cells to survive hypoxic insults and oxidative stress. They are important during embryonic development and reprogram the cells to utilize glycolysis when the oxygen levels are extremely low. This metabolic change facilitates normal cell survival as well as cancer cell survival. The key feature in survival is the transition between acute hypoxia and chronic hypoxia, and this is regulated by the transition between HIF-1 expression and HIF-2/HIF-3 expression. This transition is observed in many human cancers and endothelial cells and referred to as the HIF Switch. Here we discuss the mechanisms involved in the HIF Switch in human endothelial and cancer cells which include mRNA and protein levels of the alpha chains of the HIFs. A major continuing effort in this field is directed towards determining the differences between normal and tumor cell utilization of this important pathway, and how this could lead to potential therapeutic approaches.

Role of non-coding RNAs in neuroblastoma

Neuroblastoma is known as the most prevalent extracranial malignancy in childhood with a neural crest origin. It has been widely accepted that non-coding RNAs (ncRNAs) play important roles in many types of cancer, including glioma and gastrointestinal cancers. They may regulate the cancer gene network. According to recent sequencing and profiling studies, ncRNAs genes are deregulated in human cancers via deletion, amplification, abnormal epigenetic, or transcriptional regulation. Disturbances in the expression of ncRNAs may act either as oncogenes or as anti-tumor suppressor genes, and can lead to the induction of cancer hallmarks. ncRNAs can be secreted from tumor cells inside exosomes, where they can be transferred to other cells to affect their function. However, these topics still need more study to clarify their exact roles, so the present review addresses different roles and functions of ncRNAs in neuroblastoma.

miRNAs: Potential as Biomarkers and Therapeutic Targets for Cancer

MicroRNAs (miRNAs) are single-stranded, non-coding RNA molecules that regulate gene expression post-transcriptionally by binding to messenger RNAs. miRNAs are important regulators of gene expression, and their dysregulation is implicated in many human and canine diseases. Most cancers tested to date have been shown to express altered miRNA levels, which indicates their potential importance in the oncogenic process. Based on this evidence, numerous miRNAs have been suggested as potential cancer biomarkers for both diagnosis and prognosis. miRNA-based therapies have also been tested in different cancers and have provided measurable clinical benefits to patients. In addition, understanding miRNA biogenesis and regulatory mechanisms in cancer can provide important knowledge about resistance to chemotherapies, leading to more personalized cancer treatment. In this review, we comprehensively summarized the importance of miRNA in human and canine cancer research. We discussed the current state of development and potential for the miRNA as both a diagnostic marker and a therapeutic target.

MicroRNAs as prospective biomarkers, therapeutic targets and pharmaceuticals in neuroblastoma

Neuroblastomas, the most prevalent malignant solid neoplasms of childhood, originate from progenitor cells of the sympathetic nervous system. Their genetic causation is diverse and involves multiple molecular mechanisms. This review highlights multiple roles of microRNA in neuroblastoma pathogenesis and discusses the prospects of harnessing these important natural regulator molecules as biomarkers, therapeutic targets and pharmaceuticals in neuroblastoma.

Silencing of Circ_0135889 Restrains Proliferation and Tumorigenicity of Human Neuroblastoma Cells

INTRODUCTION

Neuroblastoma (NB) is the most common extracranial solid tumor in infants and young children. Circular ribonucleic acid (RNA) hsa_circ_0135889 (circ_0135889; hsa_circ argonaute 2 _001) is highly expressed in multiple cancer tissues, including NB. However, its role in tumor progression of NB was unclear.

METHODS

Real-time quantitative polymerace chain reaction was used to detect RNA expression, and western blotting, or immunohistochemistry was used to measure protein expression. Functional experiments were performed using 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, 5-Ethynyl-2'- deoxyuridine, Annexin V-fluorescein isothiocyanate/propidium iodide, and transwell assays, as well as xenograft tumor model. The intermolecular interaction was predicted by online databases and confirmed by dual-luciferase reporter assay and RNA pull-down assay.

RESULTS

Circ_0135889 and neuronal differentiation 1 (NEUROD1) were upregulated whilst microRNA (miR)-127-5p, was downregulated in NB tumors and immortalized NB cells. Silencing of circ_0135889 could suppress cell proliferation, migration and invasion, but enhance apoptosis rate of NB cells in vitro. More importantly, circ_0135889 depletion inhibited xenograft tumor growth of NB cells. Circ_0135889 was a sponge for miR-127-5p, and inhibition of miR-127-5p counteracted the inhibitory impact of circ_0135889 knockdown on the malignant behaviors of NB cells. Moreover, NEUROD1 was a direct target of miR-127-5p, and miR-127-5p exerted the anti-tumor role in NB cells by targeting NEUROD1. Furthermore, circ_0135889 regulated NEUROD1expression by sponging miR-127-5p.

CONCLUSIONS

Circ_0135889 promoted the tumorigenicity of NB by regulating miR-127-5p/NEUROD1 axis, which might provide a promising therapeutic target for NB.

Angioregulatory role of miRNAs and exosomal miRNAs in glioblastoma pathogenesis

Glioblastoma (GB) is a highly aggressive cancer of the central nervous system, occurring in the brain or spinal cord. Many factors such as angiogenesis are associated with GB development. Angiogenesis is a procedure by which the pre-existing blood vessels create new vessels that play an essential role in health and disease, including tumors. Also, angiogenesis is one of the significant factors thought to be responsible for treatment resistance in many tumors, including GB. Hence, an improved understanding of the molecular processes underlying GB angiogenesis will pave the way for developing potential new treatments. Recently, it has been found that microRNAs (miRNAs) and exosomal miRNAs have a crucial role in inducing or inhibiting the angiogenesis process in GB development. A better knowledge of the miRNA's regulation pathway in the angiogenesis process in cancer offers unique mechanistic insight into the mechanism of tumor-associated neovascularization. Because of advancements in miRNA characterization and delivery methods, miRNAs can also be employed in clinical settings as potential biomarkers for anti-angiogenic treatment response as well as therapies targeting tumor angiogenesis. The recent finding and insights about miRNAs' angioregulatory role and exosomal miRNAs in GB are provided throughout the review. Also, we discuss the new concept of miRNAs-based therapies for GB in the future.

Circular RNAs in prostate cancer: Biogenesis,biological functions, and clinical significance

Circular RNAs (circRNAs) are covalently closed RNA molecules that play important regulatory roles in various tumors. Prostate cancer (PCa) is one of the most common malignant tumors in the world, with high morbidity and mortality. In recent years, more and more circRNAs have been found to be abnormally expressed and involved in the occurrence and development of PCa, including cell proliferation, apoptosis, invasion, migration, metastasis, chemotherapy resistance, and radiotherapy resistance. Most of the circRNAs regulate biological behaviors of cancer through a competitive endogenous RNA (ceRNA) regulatory mechanism, and some can exert their functions by binding to proteins. circRNAs are also associated with many clinicopathological features of PCa, including tumor grade, lymph node metastasis, and distant metastasis. In addition, circRNAs are potential diagnostic and prognostic biomarkers for PCa. Considering their critical regulatory roles in the progression of PCa, circRNAs would be the potential therapeutic targets. In this paper, the current research status of circRNAs in PCa is briefly reviewed.

Prediction methods for microRNA targets in bilaterian animals: Toward a better understanding by biologists

MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression at the posttranscriptional level. Because of their wide network of interactions, miRNAs have become the focus of many studies over the past decade, particularly in animal species. To streamline the number of potential wet lab experiments, the use of miRNA target prediction tools is currently the first step undertaken. However, the predictions made may vary considerably depending on the tool used, which is mostly due to the complex and still not fully understood mechanism of action of miRNAs. The discrepancies complicate the choice of the tool for miRNA target prediction. To provide a comprehensive view of this issue, we highlight in this review the main characteristics of miRNA-target interactions in bilaterian animals, describe the prediction models currently used, and provide some insights for the evaluation of predictor performance.

Vascular Dysfunction in Preeclampsia

Preeclampsia is a life-threatening pregnancy-associated cardiovascular disorder characterized by hypertension and proteinuria at 20 weeks of gestation. Though its exact underlying cause is not precisely defined and likely heterogenous, a plethora of research indicates that in some women with preeclampsia, both maternal and placental vascular dysfunction plays a role in the pathogenesis and can persist into the postpartum period. Potential abnormalities include impaired placentation, incomplete spiral artery remodeling, and endothelial damage, which are further propagated by immune factors, mitochondrial stress, and an imbalance of pro- and antiangiogenic substances. While the field has progressed, current gaps in knowledge include detailed initial molecular mechanisms and effective treatment options. Newfound evidence indicates that vasopressin is an early mediator and biomarker of the disorder, and promising future therapeutic avenues include mitigating mitochondrial dysfunction, excess oxidative stress, and the resulting inflammatory state. In this review, we provide a detailed overview of vascular defects present during preeclampsia and connect well-established notions to newer discoveries at the molecular, cellular, and whole-organism levels.

Roles of Non-coding RNAs and Angiogenesis in Glioblastoma

One of the significant hallmarks of cancer is angiogenesis. It has a crucial function in tumor development and metastasis. Thus, angiogenesis has become one of the most exciting targets for drug development in cancer treatment. Here we discuss the regulatory effects on angiogenesis in glioblastoma (GBM) of non-coding RNAs (ncRNAs), including long ncRNA (lncRNA), microRNA (miRNA), and circular RNA (circRNA). These ncRNAs may function in trans or cis forms and modify gene transcription by various mechanisms, including epigenetics. NcRNAs may also serve as crucial regulators of angiogenesis-inducing molecules. These molecules include, metalloproteinases, cytokines, several growth factors (platelet-derived growth factor, vascular endothelial growth factor, fibroblast growth factor, hypoxia-inducible factor-1, and epidermal growth factor), phosphoinositide 3-kinase, mitogen-activated protein kinase, and transforming growth factor signaling pathways.

Non-Coding RNAs Participate in the Pathogenesis of Neuroblastoma

Neuroblastoma is one of the utmost frequent neoplasms during the first year of life. This pediatric cancer is believed to be originated during the embryonic life from the neural crest cells. Previous studies have detected several types of chromosomal aberrations in this tumor. More recent studies have emphasized on expression profiling of neuroblastoma samples to identify the dysregulated genes in this type of cancer. Non-coding RNAs are among the mostly dysregulated genes in this type of cancer. Such dysregulation has been associated with a number of chromosomal aberrations that are frequently detected in neuroblastoma. In this study, we explain the role of non-coding transcripts in the malignant transformation in neuroblastoma and their role as biomarkers for this pediatric cancer.

Circ0001320 inhibits lung cancer cell growth and invasion by regulating TNFAIP1 and TPM1 expression through sponging miR-558

Lung cancer is the most affected malignant tumor in the world, and its specific pathogenesis is still unclear. It has been confirmed that circ0001320 is down-regulated in lung cancer, but its mechanism has not been reported. Further study found that circ0001320 was down-regulated in lung cancer cells, localized in the cytoplasm, and had multiple miR-558 binding sites. Dual-luciferase reporter gene assay, RNA-pull-down, and immunoprecipitation experiments all confirmed that circ0001320 directly bound to miR-558, and then inhibit the expression of miR-558. MiR-558 was up-regulated in lung cancer cells, and bound the downstream target genes TNFAIP1 and TPM1 to inhibit their expression. Western blot showed that circ0001320 significantly up-regulated the protein levels of TNFAIP1 and TPM1, while miR-558 blocked this effect of circ0001320. Circ0001320, TNFAIP1, and TPM1 all inhibited the proliferation and invasion of lung cancer cells and promoted apoptosis, while miR-558 had the opposite effects. After transfection with circ0001320 overexpression vector, miR-558 up-regulation or down-regulation of TNFAIP1, or TPM1 expression significantly reversed the inhibition of cell growth and invasion by circ0001320. Similarly, the expression of TNFAIP1 or TPM1 was down-regulated, while miR-558 expression was inhibited, and the levels of cell proliferation, apoptosis, and invasion did not change significantly. Therefore, these fully show that circ0001320 inhibits the growth and invasion of lung cancer cells through miR-558/TNFAIP1 and TPM1 pathways, which may be closely related markers and therapeutic targets of lung cancer.

Suppression of ABCE1-Mediated mRNA Translation Limits N-MYC-Driven Cancer Progression

The ability of the N-MYC transcription factor to drive cancer progression is well demonstrated in neuroblastoma, the most common extracranial pediatric solid tumor, where MYCN amplification heralds a poor prognosis, with only 11% of high-risk patients surviving past 5 years. However, decades of attempts of direct inhibition of N-MYC or its paralogues has led to the conclusion that this protein is "undruggable." Therefore, targeting pathways upregulated by N-MYC signaling presents an alternative therapeutic approach. Here, we show that MYCN-amplified neuroblastomas are characterized by elevated rates of protein synthesis and that high expression of ABCE1, a translation factor directly upregulated by N-MYC, is itself a strong predictor of poor clinical outcome. Despite the potent ability of N-MYC in heightening protein synthesis and malignant characteristics in cancer cells, suppression of ABCE1 alone selectively negated this effect, returning the rate of translation to baseline levels and significantly reducing the growth, motility, and invasiveness of MYCN-amplified neuroblastoma cells and patient-derived xenograft tumors in vivo. The growth of nonmalignant cells or MYCN-nonamplified neuroblastoma cells remained unaffected by reduced ABCE1, supporting a therapeutic window associated with targeting ABCE1. Neuroblastoma cells with c-MYC overexpression also required ABCE1 to maintain cell proliferation and translation. Taken together, ABCE1-mediated translation constitutes a critical process in the progression of N-MYC-driven and c-MYC-driven cancers that warrants investigations into methods of its therapeutic inhibition. SIGNIFICANCE: These findings demonstrate that N-MYC-driven cancers are reliant on elevated rates of protein synthesis driven by heightened expression of ABCE1, a vulnerability that can be exploited through suppression of ABCE1.

A circRNA derived from linear HIPK3 relieves the neuronal cell apoptosis in spinal cord injury via ceRNA pattern

Spinal cord injury (SCI) is a severe disable symptom and has posed a great health threat to many people. Circ-HIPK3 has been reported to modulate the biological behavior of neuronal cells. Thence, in this study, we explored the mechanism of circ-HIPK3 in affecting functions of neuronal cell in SCI. SCI rat model was constructed to evaluate the apoptosis condition of spinal cord tissue. Meanwhile, 100 μM of CoCl₂ was used to treat AGE1.HN and PC12 cells to induce in vitro SCI model. Functional assays were implemented to investigate the apoptosis of AGE1.HN and PC12 cells. RNase R and Act D treatment were both conducted to verify the circular character of circ-HIPK3. In this study, circ-HIPK3 was found lowly expressed in SCI rat models and AGE1.HN and PC12 cells induced by 100uM of CoCl₂. Meanwhile, inhibited circ-HIPK3 or overexpressed circ-HIPK3 could separately elevate or reduce the apoptosis of AGE1.HN and PC12 cells. Moreover, circ-HIPK3 was identified as the ceRNA against miR-558 to up-regulate DPYSL5. Circ-HIPK3/miR-558/DPYSL5 axis modulated the apoptosis of AGE1.HN and PC12 cells in SCI. In conclusion, circ-HIPK3 relieves the neuronal cell apoptosis through regulating miR-588/DPYSL5 axis in SCI.

Improving Bioinformatics Prediction of microRNA Targets by Ranks Aggregation

microRNAs are noncoding RNAs which downregulate a large number of target mRNAs and modulate cell activity. Despite continued progress, bioinformatics prediction of microRNA targets remains a challenge since available software still suffer from a lack of accuracy and sensitivity. Moreover, these tools show fairly inconsistent results from one another. Thus, in an attempt to circumvent these difficulties, we aggregated all human results of four important prediction algorithms (miRanda, PITA, SVmicrO, and TargetScan) showing additional characteristics in order to rerank them into a single list. Instead of deciding which prediction tool to use, our method clearly helps biologists getting the best microRNA target predictions from all aggregated databases. The resulting database is freely available through a webtool called miRabel¹ which can take either a list of miRNAs, genes, or signaling pathways as search inputs. Receiver operating characteristic curves and precision-recall curves analysis carried out using experimentally validated data and very large data sets show that miRabel significantly improves the prediction of miRNA targets compared to the four algorithms used separately. Moreover, using the same analytical methods, miRabel shows significantly better predictions than other popular algorithms such as MBSTAR, miRWalk, ExprTarget and miRMap. Interestingly, an F-score analysis revealed that miRabel also significantly improves the relevance of the top results. The aggregation of results from different databases is therefore a powerful and generalizable approach to many other species to improve miRNA target predictions. Thus, miRabel is an efficient tool to guide biologists in their search for miRNA targets and integrate them into a biological context.

MicroRNAs in neuroblastoma tumorigenesis, therapy resistance, and disease evolution

Neuroblastoma (NB) deriving from neural crest cells is the most common extra-cranial solid cancer at infancy. NB originates within the peripheral sympathetic ganglia in adrenal medulla and along the midline of the body. Clinically, NB exhibits significant heterogeneity stretching from spontaneous regression to rapid progression to therapy resistance. MicroRNAs (miRNAs, miRs) are small (19-22 nt in length) non-coding RNAs that regulate human gene expression at the post-transcriptional level and are known to regulate cellular signaling, growth, differentiation, death, stemness, and maintenance. Consequently, the function of miRs in tumorigenesis, progression and resistance is of utmost importance for the understanding of dysfunctional cellular pathways that lead to disease evolution, therapy resistance, and poor clinical outcomes. Over the last two decades, much attention has been devoted to understanding the functional roles of miRs in NB biology. This review focuses on highlighting the important implications of miRs within the context of NB disease progression, particularly miRs’ influences on NB disease evolution and therapy resistance. In this review, we discuss the functions of both the “oncomiRs” and “tumor suppressor miRs” in NB progression/therapy resistance. These are the critical components to be considered during the development of novel miR-based therapeutic strategies to counter therapy resistance.

Upregulated long noncoding RNA Linc00261 in pre-eclampsia and its effect on trophoblast invasion and migration via regulating miR-558/TIMP4 signaling pathway

Pre-eclampsia (PE) is a leading cause of maternal and perinatal morbidity and mortality but the exact underlying mechanisms of PE pathogenesis remain elusive. Accumulated data suggested that the long noncoding RNAs (lncRNAs) play important roles in the pathogenesis of PE. The present study identified the changes of lncRNA Linc00261 in PE and its effects on trophoblasts invasion and migration. Our results showed that the expression of Linc00261 was upregulated in placental tissues of PE women compared with those of healthy pregnant women. Overexpression of Linc00261 suppressed cell invasion and migration, induced cell apoptosis, and caused cell-cycle arrest at G0 /G1 phase of HTR-8/SVneo cells; while knockdown of Linc00261 had the opposite effects on the HTR-8/SVneo cells. Mechanistic studies showed Linc00261 functioned as a competing endogenous RNA for miR-558 in HTR-8/SVneo cells, and miR-558 was negatively regulated by Linc00261. The expression level of miR-558 in the PE group was significantly lower than the control group, and the expression level of Linc00261 was negatively correlated with the expression level of miR-558 in the placental tissues of women with PE. Furthermore, miR-558 was found to negatively regulate the expression of TIMP metallopeptidase inhibitor 4 (TIMP4) via targeting the 3' untranslated region in the HTR-8/SVneo cells. Overexpression of miR-558 increased HTR-8/SVneo cell invasion and migration, which was attenuated by TIMP4 overexpression. More importantly, both overexpression of miR-558 and knockdown of TIMP4 partially reversed the suppressive effects of Linc00261 overexpression on cell invasion and migration of HTR-8/SVneo cells. Collectively, our results for the first time showed the upregulation of Linc00261 in the placental tissues of severe PE patients. The mechanistic results indicated that Linc00261 exerted the suppressive effects on the trophoblast invasion and migration via targeting miR-558/TIMP4 axis, which may involve in the pathogenesis of PE.

The Clinicopathologic and Prognostic Value of Hypoxia-Inducible Factor-2α in Cancer Patients: A Systematic Review and Meta-Analysis

Hypoxia-inducible factor-2α (HIF2α) plays an important role in the development of tumors. However, the clinicopathologic and prognostic significance of HIF2α in cancer patients remains controversial. Therefore, we performed a meta-analysis to investigate the relationship between the HIF2α status and clinical outcome in human cancer. Studies were screened online using electronic databases. The pooled risk ratios or hazard ratios (HR) with their 95% confidence intervals (CI) were calculated from available publications. Subgroup analysis, sensitivity analysis, heterogeneity, and publication bias were also conducted. A total of 854 studies with 4,345 patients were obtained in this meta-analysis. The results indicated that the increased expression of HIF2α could predict unfavorable overall survival of cancer patients on both univariate analysis (HR, 1.64; 95% CI, 1.41-1.92, P < 0.001) and multivariate analysis (HR, 2.21; 95% CI, 1.70-2.87, P < 0.001). Moreover, HIF2α overexpression was associated closely with tumor differentiation, tumor-node-metastasis stage, and lymph metastasis. In addition, there was no obvious evidence for significant publication bias in this meta-analysis. Our study indicated that HIF2α might be an indicator of poor prognosis and clinicopathologic features of tumors and could serve as a novel biomarker in human cancer.

LncRNA MALAT1 protects cardiomyocytes from isoproterenol-induced apoptosis through sponging miR-558 to enhance ULK1-mediated protective autophagy

Investigating the molecular mechanisms of myocardial infarction (MI) and subsequent heart failure have gained considerable attention worldwide. Long noncoding RNA (lncRNA) metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) has been previously demonstrated to regulate the proliferation and metastasis of several tumors. However, little is known about the effects of MALAT1 in MI and in regulating the cell date after MI. In our study, first, it was shown that the expression levels of MALAT1 were increased in the MI samples compared with normal tissues using quantitative reverse-transcription polymerase chain reaction. Then, MALAT1 knockdown could significantly decrease the cell viability and increase the apoptotic rates in isoproterenol (ISO)-treated H9C2 cells. In addition, we screened the possible target and found that miR-558 is its direct target using dual luciferase reporter assay, indicating that MALAT1 functioned as decoys sponging miR-558. Transfection of miR-558 mimic decreased the cell viability and enhanced the apoptosis. Furthermore, we revealed that miR-558 could downregulate ULK1 expression and suppressed ISO-induced protective autophagy. Activation of MALAT1/miR-558/ULK1 pathway protected H9C2 cells from ISO-induced mitochondria-dependent apoptosis. Finally, we used MALAT1-knockout mice to further demonstrated that MALAT1 protected cardiomyocytes from apoptosis and partially improved the cardiac functions upon ISO treatment. In conclusion, we elucidated that lncRNA MALAT1 protected cardiomyocytes from ISO-induced apoptosis by sponging miR-558 thus promoting ULK1-dependent autophagy. Targeting lncRNA MALAT1 might become a potential strategy in protecting cardiomyocytes during MI.

Circular RNA circAGO2 drives cancer progression through facilitating HuR-repressed functions of AGO2-miRNA complexes

Argonaute 2 (AGO2), the core component of microRNA (miRNA)-induced silencing complex, plays a compelling role in tumorigenesis and aggressiveness. However, the mechanisms regulating the functions of AGO2 in cancer still remain elusive. Herein, we indentify one intronic circular RNA (circRNA) generated from AGO2 gene (circAGO2) as a novel regulator of AGO2-miRNA complexes and cancer progression. CircAGO2 is up-regulated in gastric cancer, colon cancer, prostate cancer, and neuroblastoma, and is associated with poor prognosis of patients. CircAGO2 promotes the growth, invasion, and metastasis of cancer cells in vitro and in vivo. Mechanistic studies reveal that circAGO2 physically interacts with human antigen R (HuR) protein to facilitate its activation and enrichment on the 3'-untranslated region of target genes, resulting in reduction of AGO2 binding and repression of AGO2/miRNA-mediated gene silencing associated with cancer progression. Pre-clinically, administration of lentivirus-mediated short hairpin RNA targeting circAGO2 inhibits the expression of downstream target genes, and suppresses the tumorigenesis and aggressiveness of xenografts in nude mice. In addition, blocking the interaction between circAGO2 and HuR by cell-penetrating inhibitory peptide represses the tumorigenesis and aggressiveness of cancer cells. Taken together, these results indicate that oncogenic circAGO2 drives cancer progression through facilitating HuR-repressed functions of AGO2-miRNA complexes.

miRNAs regulate the HIF switch during hypoxia: a novel therapeutic target

The decline of oxygen tension in the tissues below the physiological demand leads to the hypoxic adaptive response. This physiological consequence enables cells to recover from this cellular insult. Understanding the cellular pathways that mediate recovery from hypoxia is therefore critical for developing novel therapeutic approaches for cardiovascular diseases and cancer. The master regulators of oxygen homeostasis that control angiogenesis during hypoxia are hypoxia-inducible factors (HIFs). HIF-1 and HIF-2 function as transcriptional regulators and have both unique and overlapping target genes, whereas the role of HIF-3 is less clear. HIF-1 governs the acute adaptation to hypoxia, whereas HIF-2 and HIF-3 expressions begin during chronic hypoxia in human endothelium. When HIF-1 levels decline, HIF-2 and HIF-3 increase. This switch from HIF-1 to HIF-2 and HIF-3 signaling is required in order to adapt the endothelium to prolonged hypoxia. During prolonged hypoxia, the HIF-1 levels and activity are reduced, despite the lack of oxygen-dependent protein degradation. Although numerous protein factors have been proposed to modulate the HIF pathways, their application for HIF-targeted therapy is rather limited. Recently, the miRNAs that endogenously regulate gene expression via the RNA interference (RNAi) pathway have been shown to play critical roles in the hypoxia response pathways. Furthermore, these classes of RNAs provide therapeutic possibilities to selectively target HIFs and thus modulate the HIF switch. Here, we review the significance of the microRNAs on the relationship between the HIFs under both physiological and pathophysiological conditions.

Hypoxia and hypoxia-inducible factors in neuroblastoma

Hypoxia (i.e., low oxygen levels) is a known feature of aggressive tumors. Cells, including tumor cells, respond to conditions of insufficient oxygen by activating a transcriptional program mainly driven by hypoxia-inducible factors (HIF)-1 and HIF-2. Both HIF-1α and HIF-2α expression levels have been shown to correlate to patient outcome in various tumor forms and in neuroblastoma, a solid childhood tumor of the sympathetic nervous system, in particular, HIF-2α marks a subpopulation of immature neural crest-like perivascularly located cells and associates with aggressive disease and distant metastasis. It has for long been recognized that the HIF-α subunits are oxygen-dependently regulated at the post-translational level, via ubiquitination and proteasomal degradation. Evidence of oxygen-independent mechanisms of regulation, transcriptional control of EPAS1/HIF2A and possible cytoplasmic activities of HIF-2α has also emerged during recent years. In this review, we discuss these non-conventional actions of HIF-2α, its putative role as a therapeutic target and the constraints it carries, as well as the importance of HIF-2 activity in a vascularized setting, the so-called pseudo-hypoxic state.