Hydrogen peroxide sensitivity connects the activity of COX5A and NPR3 to the regulation of YAP1 expression

Reactive oxygen species (ROS) are among the most severe types of cellular stressors with the ability to damage essential cellular biomolecules. Excess levels of ROS are correlated with multiple pathophysiological conditions including neurodegeneration, diabetes, atherosclerosis, and cancer. Failure to regulate the severely imbalanced levels of ROS can ultimately lead to cell death, highlighting the importance of investigating the molecular mechanisms involved in the detoxification procedures that counteract the effects of these compounds in living organisms. One of the most abundant forms of ROS is H2 O2 , mainly produced by the electron transport chain in the mitochondria. Numerous genes have been identified as essential to the process of cellular detoxification. Yeast YAP1, which is homologous to mammalian AP-1 type transcriptional factors, has a key role in oxidative detoxification by upregulating the expression of antioxidant genes in yeast. The current study reveals novel functions for COX5A and NPR3 in H2 O2 -induced stress by demonstrating that their deletions result in a sensitive phenotype. Our follow-up investigations indicate that COX5A and NPR3 regulate the expression of YAP1 through an alternative mode of translation initiation. These novel gene functions expand our understanding of the regulation of gene expression and defense mechanism of yeast against oxidative stress.

Patient and family experiences of lysosomal storage diseases in Canada: A qualitative interview study

Canadian patients and families affected by rare genetic lysosomal storage diseases (LSDs) suffer from numerous challenges related to disease management, including issues navigating healthcare and social support services, access to orphan drugs, and intensive treatment regimens. These challenges significantly impact people's quality of life, yet they remain obscure and have not been the subject of comprehensive analysis. Thus, we conducted qualitative interviews with Canadian patients and caregivers living with LSDs to advance current understanding of their experiences with rare-disease (RD) management and health systems navigation to support patient-focused RD policies and programs and improve the health outcomes of the 2.8 million Canadians affected by RDs. This study employed a qualitative descriptive research design with inductive thematic analysis. The study data were collected using semi-structured interviews. Thirty Canadian participants were interviewed in person or remotely via video chat to allow for an interactive discussion and the acquisition of rich data related to the insights and perceptions of people with LSDs. Between April and November 2019, 30 participants (16 patients and 14 caregivers) with experiences with nine types of LSDs and living in seven Canadian provinces were interviewed. Five themes were identified using comprehensive thematic analysis. These themes were the complexity of the diagnosis process; navigation of healthcare systems; psychological, social, and financial implications of LSDs; access to social support services; and access to orphan drugs. Our findings reveal that patients' access to appropriate healthcare and social services is subject to significant delays and lacks care coordination. The process of accessing orphan drugs in Canada is extremely complex and convoluted. The study results also illuminate experiences of RD stigma when navigating healthcare and social support systems. Our study offers new insights into the complex nature and extensive needs of Canadians with LSDs that are currently unmet. The management of these complex diseases requires holistic patient care and support beyond having access to orphan drugs. Our findings highlight the importance of bridging existing gaps between health and social care for RD patients. Policymakers should utilize these results when developing the forthcoming national RD strategy.

TRNT-1 Deficiency Is Associated with Loss of tRNA Integrity and Imbalance of Distinct Proteins

Mitochondrial diseases are a group of heterogeneous disorders caused by dysfunctional mitochondria. Interestingly, a large proportion of mitochondrial diseases are caused by defects in genes associated with tRNA metabolism. We recently discovered that partial loss-of-function mutations in tRNA Nucleotidyl Transferase 1 (TRNT1), the nuclear gene encoding the CCA-adding enzyme essential for modifying both nuclear and mitochondrial tRNAs, causes a multisystemic and clinically heterogenous disease termed SIFD (sideroblastic anemia with B-cell immunodeficiency, periodic fevers, and developmental delay; SIFD). However, it is not clear how mutations in a general and essential protein like TRNT1 cause disease with such clinically broad but unique symptomatology and tissue involvement. Using biochemical, cell, and mass spectrometry approaches, we demonstrate that TRNT1 deficiency is associated with sensitivity to oxidative stress, which is due to exacerbated, angiogenin-dependent cleavage of tRNAs. Furthermore, reduced levels of TRNT1 lead to phosphorylation of Eukaryotic Translation Initiation Factor 2 Subunit Alpha (eIF2α), increased reactive oxygen species (ROS) production, and changes in the abundance of distinct proteins. Our data suggest that the observed variable SIFD phenotypes are likely due to dysregulation of tRNA maturation and abundance, which in turn negatively affects the translation of distinct proteins.

A Correlation between 3'-UTR of OXA1 Gene and Yeast Mitochondrial Translation

Mitochondria possess their own DNA (mtDNA) and are capable of carrying out their transcription and translation. Although protein synthesis can take place in mitochondria, the majority of the proteins in mitochondria have nuclear origin. 3' and 5' untranslated regions of mRNAs (3'-UTR and 5'-UTR, respectively) are thought to play key roles in directing and regulating the activity of mitochondria mRNAs. Here we investigate the association between the presence of 3'-UTR from OXA1 gene on a prokaryotic reporter mRNA and mitochondrial translation in yeast. OXA1 is a nuclear gene that codes for mitochondrial inner membrane insertion protein and its 3'-UTR is shown to direct its mRNA toward mitochondria. It is not clear, however, if this mRNA may also be translated by mitochondria. In the current study, using a β-galactosidase reporter gene, we provide genetic evidence for a correlation between the presence of 3'-UTR of OXA1 on an mRNA and mitochondrial translation in yeast.

Diverse roles of heterogeneous nuclear ribonucleoproteins in viral life cycle

Understanding the host-virus interactions helps to decipher the viral replication strategies and pathogenesis. Viruses have limited genetic content and rely significantly on their host cell to establish a successful infection. Viruses depend on the host for a broad spectrum of cellular RNA-binding proteins (RBPs) throughout their life cycle. One of the major RBP families is the heterogeneous nuclear ribonucleoproteins (hnRNPs) family. hnRNPs are typically localized in the nucleus, where they are forming complexes with pre-mRNAs and contribute to many aspects of nucleic acid metabolism. hnRNPs contain RNA binding motifs and frequently function as RNA chaperones involved in pre-mRNA processing, RNA splicing, and export. Many hnRNPs shuttle between the nucleus and the cytoplasm and influence cytoplasmic processes such as mRNA stability, localization, and translation. The interactions between the hnRNPs and viral components are well-known. They are critical for processing viral nucleic acids and proteins and, therefore, impact the success of the viral infection. This review discusses the molecular mechanisms by which hnRNPs interact with and regulate each stage of the viral life cycle, such as replication, splicing, translation, and assembly of virus progeny. In addition, we expand on the role of hnRNPs in the antiviral response and as potential targets for antiviral drug research and development.

Mitochondrial disease registries worldwide: A scoping review

BACKGROUND

Mitochondrial diseases are a large group of genetically heterogeneous and clinically diverse disorders. Diagnosis often takes many years for which treatment may not exist. Registries are often used to conduct research, establish natural disease progression, engage the patient community, and develop best disease management practices. In Canada, there are limited centralized registries for mitochondrial disease patients, presenting a challenge for patients and professionals.

OBJECTIVE

To support the creation of such a registry, a systematic scoping review was conducted to map the landscape of mitochondrial disease patient registries worldwide, with a focus on registry design and challenges. Furthermore, it addresses a knowledge gap by providing a narrative synthesis of published literature that describes these registries.

METHODS

Arksey and O'Malley's methodological framework was followed to systematically search English-language literature in PubMed and CINAHL describing the designs of mitochondrial disease patient registries, supplemented by a grey literature search. Data were extracted in Microsoft Excel. Stakeholder consultations were also performed with patient caregivers, advocates, and researchers to provide perspectives beyond those found in the literature. These data were thematically analyzed and were reported in accordance with the PRISMA-ScR reporting guidelines.

RESULTS

A total of 17 articles were identified describing 13 unique registries located in North America, Europe, Australia, and West Asia. These papers described the registries' designs, their strengths, and weaknesses, as well as their tangible outcomes such as facilitating recruitment for research and supporting epidemiological studies.

CONCLUSION

Based on our findings in this review, recommendations were formulated. These include establishing registry objectives, respecting patients and their roles in the registry, adopting international data standards, data evaluations, and considerations to privacy legislation, among others. These recommendations could be used to support designing a future Canadian mitochondrial disease patient registry, and to further research directly engaging these registries worldwide.

AMPK-related protein kinase ARK5 regulates subcellular localization of RNA-binding protein hnRNP A1 during hypertonic stress

The heterogeneous nuclear ribonucleoprotein hnRNP A1 is a nucleocytoplasmic-shuttling RNA-binding protein that plays an important role in nucleic acid metabolism and gene expression regulation. The function of hnRNP A1 is determined in part by its specific location within the cell. Although some work has been done to elucidate the signaling pathways that regulate the cellular localization of hnRNP A1, the precise mechanism(s), including physiological and pathophysiological conditions that alter hnRNP A1 localization, are not known. We previously conducted an unbiased RNAi-based kinome-wide screen to identify kinases that regulate hnRNP A1 localization during hypertonic stress. One of the hits from this screen is AMPK-related protein kinase 5 (ARK5). Here, we validate ARK5 as the kinase responsible for controlling hnRNP A1 subcellular localization in response to hypertonic stress. We find using immunoprecipitation and in vitro kinase assay methods that ARK5 directly interacts with and phosphorylates hnRNP A1 on serine residues within the F-peptide region. We further show that the M9 motif of hnRNP A1 is essential for the ARK5-hnRNP A1 interaction and subsequent phosphorylation. In addition, the silencing of ARK5 increases the expression of anti-apoptotic protein Bcl-xL and consequently delays caspase activation during hypertonic stress. Our results indicate that ARK5 phosphorylates hnRNP A1 and regulates its subcellular localization during hypertonic stress.

Screen time is independently associated with serum brain-derived neurotrophic factor (BDNF) in youth with obesity

Low levels of brain derived-neurotrophic factor (BDNF) and excessive screen exposure are risk factors for neurocognitive deficits and obesity in youth, but the relationship between screen time and BDNF remains unknown. This study examined whether duration and/or type of sedentary screen time behaviour (TV viewing, video games, recreational computer use) are associated with serum BDNF levels in youth with obesity. The sample consisted of 250 inactive, postpubertal adolescents with obesity (172 females/78 males, aged 15.5 ± 1.4 years) at the baseline assessment of the Healthy Eating, Aerobic, Resistance Training in Youth Study. After controlling for self-reported age, sex, race, parental education, puberty stage, physical activity, and diet, higher total screen exposure was significantly associated with lower serum BDNF levels (β = -0.21, p = 0.002). TV viewing was the only type of screen behaviour that was associated with BDNF levels (β = -0.22, p = 0.001). Higher exposure to traditional forms of screen time was independently associated with lower serum BDNF levels, and this association appears to be driven primarily by TV viewing. Future intervention research is needed to determine whether limiting screen time is an effective way to increase BDNF and associated health benefits in a high-risk population of youth with obesity. Trial Registration: ClinicalTrials.Gov NCT00195858. Novelty: This study is the first to show that recreational screen time is inversely associated with serum BDNF levels. The inverse association between screen time and BDNF is driven primarily by TV viewing, indicating the type of screen might matter.

Lithium Chloride Sensitivity in Yeast and Regulation of Translation

For decades, lithium chloride (LiCl) has been used as a treatment option for those living with bipolar disorder (BD). As a result, many studies have been conducted to examine its mode of action, toxicity, and downstream cellular responses. We know that LiCl is able to affect cell signaling and signaling transduction pathways through protein kinase C and glycogen synthase kinase-3, which are considered to be important in regulating gene expression at the translational level. However, additional downstream effects require further investigation, especially in translation pathway. In yeast, LiCl treatment affects the expression, and thus the activity, of PGM2, a phosphoglucomutase involved in sugar metabolism. Inhibition of PGM2 leads to the accumulation of intermediate metabolites of galactose metabolism causing cell toxicity. However, it is not fully understood how LiCl affects gene expression in this matter. In this study, we identified three genes, NAM7, PUS2, and RPL27B, which increase yeast LiCl sensitivity when deleted. We further demonstrate that NAM7, PUS2, and RPL27B influence translation and exert their activity through the 5′-Untranslated region (5′-UTR) of PGM2 mRNA in yeast.

Sensitivity of yeast to lithium chloride connects the activity of YTA6 and YPR096C to translation of structured mRNAs

Lithium Chloride (LiCl) toxicity, mode of action and cellular responses have been the subject of active investigations over the past decades. In yeast, LiCl treatment is reported to reduce the activity and alters the expression of PGM2, a gene that encodes a phosphoglucomutase involved in sugar metabolism. Reduced activity of phosphoglucomutase in the presence of galactose causes an accumulation of intermediate metabolites of galactose metabolism leading to a number of phenotypes including growth defect. In the current study, we identify two understudied yeast genes, YTA6 and YPR096C that when deleted, cell sensitivity to LiCl is increased when galactose is used as a carbon source. The 5’-UTR of PGM2 mRNA is structured. Using this region, we show that YTA6 and YPR096C influence the translation of PGM2 mRNA.

Diseases Associated with Defects in tRNA CCA Addition

tRNA nucleotidyl transferase 1 (TRNT1) is an essential enzyme catalyzing the addition of terminal cytosine-cytosine-adenosine (CCA) trinucleotides to all mature tRNAs, which is necessary for aminoacylation. It was recently discovered that partial loss-of-function mutations in TRNT1 are associated with various, seemingly unrelated human diseases including sideroblastic anemia with B-cell immunodeficiency, periodic fevers and developmental delay (SIFD), retinitis pigmentosa with erythrocyte microcytosis, and progressive B-cell immunodeficiency. In addition, even within the same disease, the severity and range of the symptoms vary greatly, suggesting a broad, pleiotropic impact of imparting TRNT1 function on diverse cellular systems. Here, we describe the current state of knowledge of the TRNT1 function and the phenotypes associated with mutations in TRNT1.

Identification of therapeutics that target eEF1A2 and upregulate utrophin A translation in dystrophic muscles

Up-regulation of utrophin in muscles represents a promising therapeutic strategy for the treatment of Duchenne Muscular Dystrophy. We previously demonstrated that eEF1A2 associates with the 5’UTR of utrophin A to promote IRES-dependent translation. Here, we examine whether eEF1A2 directly regulates utrophin A expression and identify via an ELISA-based high-throughput screen, FDA-approved drugs that upregulate both eEF1A2 and utrophin A. Our results show that transient overexpression of eEF1A2 in mouse muscles causes an increase in IRES-mediated translation of utrophin A. Through the assessment of our screen, we reveal 7 classes of FDA-approved drugs that increase eEF1A2 and utrophin A protein levels. Treatment of mdx mice with the 2 top leads results in multiple improvements of the dystrophic phenotype. Here, we report that IRES-mediated translation of utrophin A via eEF1A2 is a critical mechanism of regulating utrophin A expression and reveal the potential of repurposed drugs for treating DMD via this pathway.

One potential approach for the treatment of Duchenne muscular dysrophy is to increase expression of the dystrophin homolog utrophin. Here, the authors show that eEF1A2 regulates utrophin expression, and show that 2 FDA-approved drugs upregulate eEIF1A2 and utrophin level in mice, leading to improvement of the dystrophic phenotype.

Characterizing Cellular Responses During Oncolytic Maraba Virus Infection

The rising demand for powerful oncolytic virotherapy agents has led to the identification of Maraba virus, one of the most potent oncolytic viruses from Rhabdoviridae family which displays high selectivity for killing malignant cells and low cytotoxicity in normal cells. Although the virus is readied to be used for clinical trials, the interactions between the virus and the host cells is still unclear. Using a newly developed interferon-sensitive mutant Maraba virus (MG1), we have identified two key regulators of global translation (4E-BP1 and eIF2α) as being involved in the regulation of protein synthesis in the infected cells. Despite the translational arrest upon viral stress, we showed an up-regulation of anti-apoptotic Bcl-xL protein that provides a survival benefit for the host cell, yet facilitates effective viral propagation. Given the fact that eIF5B canonically regulates 60S ribosome subunit end joining and is able to replace the role of eIF2 in delivering initiator tRNA to the 40S ribosome subunit upon the phosphorylation of eIF2α we have tested whether eIF5B mediates the translation of target mRNAs during MG1 infection. Our results show that the inhibition of eIF5B significantly down-regulates the level of Bcl-xL steady-state mRNA, thus indirectly attenuates viral propagation.

Effects of aerobic training, resistance training, or both on brain-derived neurotrophic factor in adolescents with obesity: The hearty randomized controlled trial

Brain derived neurotrophic factor (BDNF) is a protein that plays a critical role in modulating cognition in animals and humans. Aerobic exercise often increases BDNF in adults, but effects of this exercise modality and others among adolescents remain uncertain. This study examined the effects of aerobic training, resistance training, and combined training on resting serum BDNF levels in adolescents with overweight and obesity. After a 4-week pre-randomization treatment, 304 post-pubertal, adolescents with overweight or obesity (70% females) aged 14-18 years were randomized to one of four groups for 22 weeks: aerobic training (N = 75), resistance training (N = 78), combined aerobic and resistance training (N = 75), or non-exercising control (N = 76). All participants received dietary counseling targeting a daily energy deficit of 250 kcal. The exercise prescription was 4 times per week, progressing to 45 min/session for the aerobic and resistance groups and 90 min/session for the combined group. Resting serum BDNF levels were measured at baseline and 6-months. Results showed that in both intention-to-treat (ITT) and per protocol (≥70% adherence to prescribed sessions) analyses, there were no significant within- or between-group changes in BDNF. Findings indicate that aerobic training, resistance training or their combination did change serum BDNF levels in adolescents with overweight and obesity.

TRIAL REGISTRATION

ClinicalTrials.Gov NCT00195858 http://clinicaltrials.gov/show/NCT00195858, September 12, 2005 (Funded by the Canadian Institutes of Health Research).

Oncolytic virus synergizes with Smac mimetic compounds to induce rhabdomyosarcoma cell death in a syngeneic murine model

Rhabdomyosarcoma (RMS), a neoplasm characterized by undifferentiated myoblasts, is the most common soft tissue tumour in children. Therapeutic resistance is common in RMS and is often caused by acquired defects in the cellular apoptotic program. Smac mimetic compounds (SMCs) are a novel class of inhibitor of apoptosis (IAP) antagonists that are currently under clinical development as cancer therapeutics. We previously reported that cIAP1 is overexpressed in human primary RMS tumours and in patient-derived RMS cell lines where it drives resistance to apoptosis. In this study, we investigated whether inflammatory cytokine production triggered by activators of innate immunity synergizes with LCL161 to induce bystander killing of RMS cells in vitro and in vivo. Indeed, we show that innate immune stimuli (oncolytic virus (VSVΔ51-GFP), interferon γ (IFNγ), and tumour necrosis factor-like weak inducer of apoptosis (TWEAK)) combine with SMCs in vitro to reduce cell viability in the Kym-1 RMS cancer cell line. Other human RMS cell lines (RH36, RH41, RD, RH18, RH28, and RH30) and the murine RMS cell line 76-9 are resistant to treatment with LCL161 alone or in combination with immune stimulants in in vitro cell viability assays. In contrast, we report that the combination of LCL161 and VSVΔ51-GFP reduces tumour volume and prolongs survival in a 76-9 syngeneic murine model. Our results support further exploration of the combined use of IAP antagonists and innate immune stimuli as a therapeutic approach for RMS cancers.

The sensitivity of the yeast, Saccharomyces cerevisiae, to acetic acid is influenced by DOM34 and RPL36A

The presence of acetic acid during industrial alcohol fermentation reduces the yield of fermentation by imposing additional stress on the yeast cells. The biology of cellular responses to stress has been a subject of vigorous investigations. Although much has been learned, details of some of these responses remain poorly understood. Members of heat shock chaperone HSP proteins have been linked to acetic acid and heat shock stress responses in yeast. Both acetic acid and heat shock have been identified to trigger different cellular responses including reduction of global protein synthesis and induction of programmed cell death. Yeast HSC82 and HSP82 code for two important heat shock proteins that together account for 1–2% of total cellular proteins. Both proteins have been linked to responses to acetic acid and heat shock. In contrast to the overall rate of protein synthesis which is reduced, the expression of HSC82 and HSP82 is induced in response to acetic acid stress. In the current study we identified two yeast genes DOM34 and RPL36A that are linked to acetic acid and heat shock sensitivity. We investigated the influence of these genes on the expression of HSP proteins. Our observations suggest that Dom34 and RPL36A influence translation in a CAP-independent manner.

Engaging Cell Death Pathways for the Treatment of Rhabdomyosarcoma

Rhabdomyosarcoma (RMS), a malignant neoplasm of presumed mesenchymal origin, is the most common soft tissue cancer of childhood. Despite aggressive treatment, resistance to current therapies remains a challenge. The success of most cytotoxic chemotherapies requires intact programmed cell death (apoptosis) pathways. Defects in the cellular apoptotic program play a key role in the pathogenesis of RMS and contribute to chemotherapeutic resistance to current treatments. Targeting and engaging apoptotic pathways using small-molecule IAP antagonists, death-inducing ligands, reestablishing pannexin channel expression and activity, immunotherapies, or a combination of these approaches is expected to improve outcomes in RMS patients. There is a clear need to better understand the molecular basis of apoptotic resistance in RMS, which may provide an opportunity to identify the patients most likely to benefit from targeted treatments, and for the discovery of novel therapies.

ERK8 is a novel HuR kinase that regulates tumour suppressor PDCD4 through a miR-21 dependent mechanism

Programmed cell death 4 (PDCD4) is a tumour suppressor implicated in cancer development and progression and was recently identified as a repressor of cap-independent translation of specific genes involved in the regulation of apoptosis. We show that the RNA-binding protein HuR binds to the PDCD4 3′UTR to protect it from miR-21-induced silencing. However, following H₂O₂ treatment, PDCD4 mRNA is degraded via miR-21 binding. Importantly, we identify HuR as a novel substrate of the ERK8 kinase pathway in response to H₂O₂ treatment. We show that phosphorylation of HuR by ERK8 prevents it from binding to PDCD4 mRNA and allows miR-21-mediated degradation of PDCD4.

Elevated levels of ribosomal proteins eL36 and eL42 control expression of Hsp90 in rhabdomyosarcoma

Mammalian 90 kDa heat shock protein (Hsp90) is a ubiquitous molecular chaperone whose expression is selectively upregulated during stress, although the precise control mechanism of this increase is yet to be fully elucidated. We used polysome profiling to show that Hsp90α mRNA is selectively translated, while global translation is inhibited during heat stress. Furthermore, we have identified 2 ribosomal proteins, eL36 and eL42 that modulate Hsp90α expression under both normal and heat shock conditions. Importantly, we noted that expression of eL36 and eL42 is elevated in a panel of human rhabdomyosarcomas where it drives high expression of Hsp90 and modulates sensitivity of these cells to an Hsp90 inhibitor 17-AAG.

DNM1L-related mitochondrial fission defect presenting as refractory epilepsy

Mitochondrial fission and fusion are dynamic processes vital to mitochondrial quality control and the maintenance of cellular respiration. In dividing mitochondria, membrane scission is accomplished by a dynamin-related GTPase, DNM1L, that oligomerizes at the site of fission and constricts in a GTP-dependent manner. There is only a single previous report of DNM1L-related clinical disease: a female neonate with encephalopathy due to defective mitochondrial and peroxisomal fission (EMPF; OMIM #614388), a lethal disorder characterized by cerebral dysgenesis, seizures, lactic acidosis, elevated very long chain fatty acids, and abnormally elongated mitochondria and peroxisomes. Here, we describe a second individual, diagnosed via whole-exome sequencing, who presented with developmental delay, refractory epilepsy, prolonged survival, and no evidence of mitochondrial or peroxisomal dysfunction on standard screening investigations in blood and urine. EEG was nonspecific, showing background slowing with frequent epileptiform activity at the frontal and central head regions. Electron microscopy of skeletal muscle showed subtle, nonspecific abnormalities of cristal organization, and confocal microscopy of patient fibroblasts showed striking hyperfusion of the mitochondrial network. A panel of further bioenergetic studies in patient fibroblasts showed no significant differences versus controls. The proband's de novo DNM1L variant, NM_012062.4:c.1085G>A; NP_036192.2:p.(Gly362Asp), falls within the middle (oligomerization) domain of DNM1L, implying a likely dominant-negative mechanism. This disorder, which presents nonspecifically and affords few diagnostic clues, can be diagnosed by means of DNM1L sequencing and/or confocal microscopy.

Impaired activity of CCA-adding enzyme TRNT1 impacts OXPHOS complexes and cellular respiration in SIFD patient-derived fibroblasts

BACKGROUND

SIFD (Sideroblastic anemia with B-cell immunodeficiency, periodic fevers, and developmental delay) is a novel form of congenital sideroblastic anemia associated with B-cell immunodeficiency, periodic fevers, and developmental delay caused by mutations in the CCA-adding enzyme TRNT1, but the precise molecular pathophysiology is not known.

RESULTS

We show that the disease causing mutations in patient-derived fibroblasts do not affect subcellular localization of TRNT1 and show no gross morphological differences when compared to control cells. Analysis of cellular respiration and oxidative phosphorylation (OXPHOS) complexes demonstrates that both basal and maximal respiration rates are decreased in patient cells, which may be attributed to an observed decrease in the abundance of select proteins of the OXPHOS complexes.

CONCLUSIONS

Our data provides further insight into cellular pathophysiology of SIFD.

Bacterial DNA Protects Monocytic Cells against HIV-Vpr-Induced Mitochondrial Membrane Depolarization

Monocytes and macrophages are important HIV reservoirs, as they exhibit marked resistance to apoptosis upon infection. However, the mechanism underlying resistance to apoptosis in these cells is poorly understood. Using HIV-viral protein R-52-96 aa peptide (Vpr), we show that primary monocytes and THP-1 cells treated with Vpr are highly susceptible to mitochondrial depolarization, but develop resistance following stimulation with bacterial DNA or CpG oligodeoxynucleotide. We have shown that Vpr-induced mitochondrial depolarization is mediated by TNFR-associated factor-1 (TRAF-1) and TRAF-2 degradation and subsequent activation of caspase-8, Bid, and Bax. To provide the mechanism governing such resistance to mitochondrial depolarization, our results show that prior stimulation with CpG oligodeoxynucleotide or Escherichia coli DNA prevented: 1) TRAF-1/2 downregulation; 2) activation of caspase-8, Bid, and Bax; and 3) subsequent mitochondrial depolarization and release of apoptosis-inducing factor and cytochrome c Furthermore, this protection was mediated by upregulation of antiapoptotic protein (c-IAP-2) through calmodulin-dependent kinase-II activation. Thus, c-IAP-2 may prevent Vpr-mediated mitochondrial depolarization through stabilizing TRAF-1/2 expression and sequential inhibition of caspase-8, Bid, and Bax.

Cellular mRNA recruits the ribosome via eIF3-PABP bridge to initiate internal translation

IRES-mediated translation of key cell fate regulating genes has been implicated in tumorigenesis. Concerted action of canonical eukaryotic initiation factors and IRES transacting factors (ITAFs) was shown to regulate cellular IRES mediated translation; however, the precise molecular mechanism of ribosome recruitment to cellular IRESes remains unclear. Here we show that the X-linked inhibitor of apoptosis (XIAP) IRES operates in an evolutionary conserved viral like mode and the structural integrity, particularly in the vicinity of AUG, is critical for ribosome recruitment. The binding of eIF3 together with PABP potentiates ribosome recruitment to the IRES. Our data support the model in which eIF3 binds directly to the XIAP IRES RNA in a structure-dependent manner and acts as a scaffold for IRES RNA, PABP and the 40S ribosome.

Could the eIF2α-Independent Translation Be the Achilles Heel of Cancer?

Eukaryotic initiation factor eIF2 is a key component of the ternary complex whose role is to deliver initiator tRNA into the ribosome. A variety of stimuli, both physiologic and pathophysiologic activate eIF2 kinases that phosphorylate the α subunit of eIF2, preventing it from forming the ternary complex, thus attenuating cellular protein synthesis. Paradoxically, in cancer cells, the phosphorylation of eIF2α is associated with activation of survival pathways. This review explores the recently emerged novel mechanism of eIF2α-independent translation initiation. This mechanism, which appears to be shared by some RNA viruses and Internal Ribosome Entry Site-containing cellular mRNAs and utilizes auxiliary proteins, such as eIF5B, eIF2D, and MCT-1, is responsible for the selective translation of cancer-associated genes and could represent a weak point amenable to specific targeting for the treatment of cancer.

Homozygous mutation in the eukaryotic translation initiation factor 2alpha phosphatase gene, PPP1R15B, is associated with severe microcephaly, short stature and intellectual disability

Protein translation is an essential cellular process initiated by the association of a methionyl-tRNA with the translation initiation factor eIF2. The Met-tRNA/eIF2 complex then associates with the small ribosomal subunit, other translation factors and mRNA, which together comprise the translational initiation complex. This process is regulated by the phosphorylation status of the α subunit of eIF2 (eIF2α); phosphorylated eIF2α attenuates protein translation. Here, we report a consanguineous family with severe microcephaly, short stature, hypoplastic brainstem and cord, delayed myelination and intellectual disability in two siblings. Whole-exome sequencing identified a homozygous missense mutation, c.1972G>A; p.Arg658Cys, in protein phosphatase 1, regulatory subunit 15b (PPP1R15B), a protein which functions with the PPP1C phosphatase to maintain dephosphorylated eIF2α in unstressed cells. The p.R658C PPP1R15B mutation is located within the PPP1C binding site. We show that patient cells have greatly diminished levels of PPP1R15B-PPP1C interaction, which results in increased eIF2α phosphorylation and resistance to cellular stress. Finally, we find that patient cells have elevated levels of PPP1R15B mRNA and protein, suggesting activation of a compensatory program aimed at restoring cellular homeostasis which is ineffective due to PPP1R15B alteration. PPP1R15B now joins the expanding list of translation-associated proteins which when mutated cause rare genetic diseases.

DAP5 associates with eIF2β and eIF4AI to promote Internal Ribosome Entry Site driven translation

Initiation is a highly regulated rate-limiting step of mRNA translation. During cap-dependent translation, the cap-binding protein eIF4E recruits the mRNA to the ribosome. Specific elements in the 5'UTR of some mRNAs referred to as Internal Ribosome Entry Sites (IRESes) allow direct association of the mRNA with the ribosome without the requirement for eIF4E. Cap-independent initiation permits translation of a subset of cellular and viral mRNAs under conditions wherein cap-dependent translation is inhibited, such as stress, mitosis and viral infection. DAP5 is an eIF4G homolog that has been proposed to regulate both cap-dependent and cap-independent translation. Herein, we demonstrate that DAP5 associates with eIF2β and eIF4AI to stimulate IRES-dependent translation of cellular mRNAs. In contrast, DAP5 is dispensable for cap-dependent translation. These findings provide the first mechanistic insights into the function of DAP5 as a selective regulator of cap-independent translation.

Assessment of selective mRNA translation in mammalian cells by polysome profiling

Regulation of protein synthesis represents a key control point in cellular response to stress. In particular, discreet RNA regulatory elements were shown to allow to selective translation of specific mRNAs, which typically encode for proteins required for a particular stress response. Identification of these mRNAs, as well as the characterization of regulatory mechanisms responsible for selective translation has been at the forefront of molecular biology for some time. Polysome profiling is a cornerstone method in these studies. The goal of polysome profiling is to capture mRNA translation by immobilizing actively translating ribosomes on different transcripts and separate the resulting polyribosomes by ultracentrifugation on a sucrose gradient, thus allowing for a distinction between highly translated transcripts and poorly translated ones. These can then be further characterized by traditional biochemical and molecular biology methods. Importantly, combining polysome profiling with high throughput genomic approaches allows for a large scale analysis of translational regulation.

HuR controls mitochondrial morphology through the regulation of BclxL translation

Bcl_xL is a key prosurvival factor that in addition to controlling mitochondrial membrane permeability regulates mitochondrial network dynamics. The expression of Bcl_xL is regulated at the level of transcription, splicing and selective translation. In this study, we show that the RNA-binding protein HuR, which is known to orchestrate an anti-apoptotic cellular program, functions as a translational repressor of Bcl_xL. We show that HuR binds directly to the 5`UTR of Bcl_xL, and represses Bcl_xL translation through the inhibition of its internal ribosome entry site (IRES). Reduction of HuR levels leads to the derepression of Bcl_xL translation and subsequent rearrangement of the mitochondrial network. Our results place Bcl_xL into the HuR-regulated operon and provide further insight into the regulation of cellular stress response by HuR.

hnRNPA1 couples nuclear export and translation of specific mRNAs downstream of FGF-2/S6K2 signalling

The increased cap-independent translation of anti-apoptotic proteins is involved in the development of drug resistance in lung cancer but signalling events regulating this are poorly understood. Fibroblast growth factor 2 (FGF-2) signalling-induced S6 kinase 2 (S6K2) activation is necessary, but the downstream mediator(s) coupling this kinase to the translational response is unknown. Here, we show that S6K2 binds and phosphorylates hnRNPA1 on novel Ser4/6 sites, increasing its association with BCL-XL and XIAP mRNAs to promote their nuclear export. In the cytoplasm, phosphoS4/6-hnRNPA1 dissociates from these mRNAs de-repressing their IRES-mediated translation. This correlates with the phosphorylation-dependent association of hnRNPA1 with 14-3-3 leading to hnRNPA1 sumoylation on K183 and its re-import into the nucleus. A non-phosphorylatible, S4/6A mutant prevented these processes, hindering the pro-survival activity of FGF-2/S6K2 signalling. Interestingly, immunohistochemical staining of lung and breast cancer tissue samples demonstrated that increased S6K2 expression correlates with decreased cytoplasmic hnRNPA1 and increased BCL-XL expression. In short, phosphorylation on novel N-term sites of hnRNPA1 promotes translation of anti-apoptotic proteins and is indispensable for the pro-survival effects of FGF-2.

Abstract 4256: Characterization of the cellular inhibitor of apoptosis 1 (cIAP1) IRES trans-acting factors and their contribution to apoptotic resistance in rhabdomyosarcomas

The cellular inhibitor of apoptosis protein 1 (cIAP1) is a key regulator of the NFκB signaling pathway and of caspase-8 mediated cell death in mammalian cells. cIAP1 expression is regulated at the translation level via an internal ribosome entry site (IRES). We have previously identified and characterized two IRES trans-acting factors - NF45 and IGF2BP1 - that bind specifically to the cIAP1 IRES and regulates its activity in the context of cell division and apoptosis.

In this study, we characterized the minimal requirement of the cIAP1 IRES for these IRES trans-acting factors as well as other canonical translation factors. We also studied the contribution of cIAP1 expression regulation by these IRES trans-acting factors on the apoptotic resistance of rhabdomyosarcoma cancer cells. In particular, we show that the IRES trans-acting factor IGF2BP1 is overexpressed in a panel of rhabdomyosarcoma primary tumors and cell lines where it drives cIAP1 IRES-mediated translation, contributing to increased resistance to cell death. We also show that targeting cIAP1 by either IGF2BP1 knock-down or IAP antagonists delay RMS tumour growth in the presence of TNFα and improve survival in mice.

Citation Format: Mame Daro Faye, Tyson E. Graber, Shawn Beug, Xiao Xiang, Benjamin Wild, Stephanie Langlois, Kyle N. Cowan, Robert G. Korneluk, Martin Holcik. Characterization of the cellular inhibitor of apoptosis 1 (cIAP1) IRES trans-acting factors and their contribution to apoptotic resistance in rhabdomyosarcomas. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4256. doi:10.1158/1538-7445.AM2014-4256

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.

Loss of PDCD4 contributes to enhanced chemoresistance in Glioblastoma multiforme through de-repression of Bcl-xL translation

Glioblastoma multiforme (GBM) is the most common and aggressive form of tumor of the central nervous system. Despite significant efforts to improve treatments, patient survival rarely exceeds 18 months largely due to the highly chemoresistant nature of these tumors. Importantly, misregulation of the apoptotic machinery plays a key role in the development of drug resistance. We previously demonstrated that Bcl-xL, an important anti-apoptotic protein, is regulated at the level of translation by the tumor suppressor programmed cell death 4 (PDCD4). We report here a strong correlation between low expression of PDCD4 and high expression of Bcl-xL in adult de novo GBM, GBM tumor initiating cells, and established GBM cell lines. Importantly, high Bcl-xL expression correlated significantly with poor progression and patient survival. We demonstrate that re-expression of PDCD4 in GBM cells down-regulated Bcl-xL expression and decreased cell viability. Finally, we show that direct inhibition of Bcl-xL by small molecule antagonist ABT-737 sensitizes GBM cells to doxorubicin. Our results identify Bcl-xL as a novel marker of GBM chemoresistance and advocate for the combined use of Bcl-xL antagonists and existing chemotherapeutics as a treatment option for this aggressive tumor.

VPAC2 receptor agonist BAY 55-9837 increases SMN protein levels and moderates disease phenotype in severe spinal muscular atrophy mouse models

Background

Spinal Muscular Atrophy (SMA) is one of the most common inherited causes of infant death and is caused by the loss of functional survival motor neuron (SMN) protein due to mutations or deletion in the SMN1 gene. One of the treatment strategies for SMA is to induce the expression of the protein from the homologous SMN2 gene, a rescuing paralog for SMA.

Methods and results

Here we demonstrate the promise of pharmacological modulation of SMN2 gene by BAY 55-9837, an agonist of the vasoactive intestinal peptide receptor 2 (VPAC2), a member of G protein coupled receptor family. Treatment with BAY 55-9837 lead to induction of SMN protein levels via activation of MAPK14 or p38 pathway in vitro. Importantly, BAY 55-9837 also ameliorated disease phenotype in severe SMA mouse models.

Conclusion

Our findings suggest the VPAC2 pathway is a potential SMA therapeutic target.

Loss of cellular inhibitor of apoptosis protein 2 reduces atherosclerosis in atherogenic apoE-/- C57BL/6 mice on high-fat diet

Background

Cellular inhibitor of apoptosis protein 2 (cIAP2) is predicted to participate in atherosclerosis; however, its direct role in atherosclerosis development has not been investigated. We aimed to examine and assess the loss of cIAP2 on atherosclerosis lesion development.

Methods and Results

We used apoE^−/− C57BL/6 male mice, either cIAP2^−/− or cIAP2^+/+. At 8 weeks, mice were fed a high‐fat diet (HFD) for 4 and 12 weeks. Aortic root was serially sectioned and stained with Sudan IV, CD68, α‐actin, and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL). cIAP2^−/− mice displayed a significant decrease in atherosclerotic lesion's macrophage number after 4 weeks of HFD. Similarly, decrease in lesion area at 4 and 12 weeks HFD was detected by use of en face analysis (cIAP2^−/− 0.58±0.37% versus cIAP2^+/+ 1.51±0.79% [P=0.0056]); (cIAP2^−/− 9.34±4.88% versus cIAP2^+/+ 17.65±6.24% [P=0.0019]). Aortic root lesion area after 4 and 12 weeks of HFD also decreased (cIAP2^−/− 0.0328±0.014 mm² versus cIAP2^+/+ 0.0515±0.021 mm² [P=0.022]); (cIAP2^−/− 0.3614±0.1157 mm² versus cIAP2^+/+ 0.4901±0.125 mm² [P=0.065]). TUNEL analysis after 4 and 12 weeks of HFD showed a 2.5‐fold increase in TUNEL+ cells (cIAP2^−/− 4.47±2.26% versus cIAP2^+/+ 1.74±0.98% [P=0.036]); (cIAP2^−/− 2.39±0.75% versus cIAP2^+/+ 1.29±0.47% [P=0.032]). Smooth muscle cell content in cIAP2^−/− mice was 3.075±3.3% compared with cIAP2^+/+ with 0.085±0.1% (P=0.0071).

Conclusions

Results uncover a key role for cIAP2 in atherosclerotic lesion development, and targeting it may represent a novel therapeutic strategy.

Involvement of yeast HSP90 isoforms in response to stress and cell death induced by acetic acid

Acetic acid-induced apoptosis in yeast is accompanied by an impairment of the general protein synthesis machinery, yet paradoxically also by the up-regulation of the two isoforms of the heat shock protein 90 (HSP90) chaperone family, Hsc82p and Hsp82p. Herein, we show that impairment of cap-dependent translation initiation induced by acetic acid is caused by the phosphorylation and inactivation of eIF2α by Gcn2p kinase. A microarray analysis of polysome-associated mRNAs engaged in translation in acetic acid challenged cells further revealed that HSP90 mRNAs are over-represented in this polysome fraction suggesting preferential translation of HSP90 upon acetic acid treatment. The relevance of HSP90 isoform translation during programmed cell death (PCD) was unveiled using genetic and pharmacological abrogation of HSP90, which suggests opposing roles for HSP90 isoforms in cell survival and death. Hsc82p appears to promote survival and its deletion leads to necrotic cell death, while Hsp82p is a pro-death molecule involved in acetic acid-induced apoptosis. Therefore, HSP90 isoforms have distinct roles in the control of cell fate during PCD and their selective translation regulates cellular response to acetic acid stress.

Celecoxib increases SMN and survival in a severe spinal muscular atrophy mouse model via p38 pathway activation

The loss of functional Survival Motor Neuron (SMN) protein due to mutations or deletion in the SMN1 gene causes autosomal recessive neurodegenerative spinal muscle atrophy (SMA). A potential treatment strategy for SMA is to upregulate the amount of SMN protein originating from the highly homologous SMN2 gene, compensating in part for the absence of the functional SMN1 gene. We have previously shown that in vitro activation of the p38 pathway stabilizes and increases SMN mRNA levels leading to increased SMN protein levels. In this report, we explore the impact of the p38 activating, FDA-approved, blood brain barrier permeating compound celecoxib on SMN levels in vitro and in a mouse model of SMA. We demonstrate a significant induction of SMN protein levels in human and mouse neuronal cells upon treatment with celecoxib. We show that activation of the p38 pathway by low doses celecoxib increases SMN protein in a HuR protein-dependent manner. Furthermore, celecoxib treatment induces SMN expression in brain and spinal cord samples of wild-type mice in vivo. Critically, celecoxib treatment increased SMN levels, improved motor function and enhanced survival in a severe SMA mouse model. Our results identify low dose celecoxib as a potential new member of the SMA therapeutic armamentarium.

A novel cis-acting element from the 3'UTR of DNA damage-binding protein 2 mRNA links transcriptional and post-transcriptional regulation of gene expression

The DNA damage-binding protein 2 (DDB2) is an adapter protein that can direct a modular Cul4-DDB1-RING E3 Ligase complex to sites of ultraviolet light-induced DNA damage to ubiquitinate substrates during nucleotide excision repair. The DDB2 transcript is ultraviolet-inducible; therefore, its regulation is likely important for its function. Curiously, the DDB2 mRNA is reportedly short-lived, but the transcript does not contain any previously characterized cis-acting determinants of mRNA stability in its 3' untranslated region (3'UTR). Here, we used a tetracycline regulated d2EGFP reporter construct containing specific 3'UTR sequences from DDB2 to identify novel cis-acting elements that regulate mRNA stability. Synthetic 3'UTRs corresponding to sequences as short as 25 nucleotides from the central region of the 3'UTR of DDB2 were sufficient to accelerate decay of the heterologous reporter mRNA. Conversely, these same 3'UTRs led to more rapid induction of the reporter mRNA, export of the message to the cytoplasm and the subsequent accumulation of the encoded reporter protein, indicating that this newly identified cis-acting element affects transcriptional and post-transciptional processes. These results provide clear evidence that nuclear and cytoplasmic processing of the DDB2 mRNA is inextricably linked.

Abstract 827: Identification of the insulin-like growth factor 2 mRNA binding protein (IGF2BP1) as an important regulator of cIAP1 translation and apoptosis in rhabdomyosarcomas

Rhabdomyosarcoma, a neoplasm characterized by undifferentiated myoblats-like cells, is the most common soft tissue sarcoma of childhood and represents 3-4% of all childhood cancers. IGF2BP1 is an oncofetal protein that was first identified in the rhabdomyosarcoma RD cell line and was shown to be overexpressed in a variety of cancers. Our group has previously identified IGF2BP1 as a potential translation modulator of the cellular inhibitor of apoptosis protein 1 (cIAP1), a key regulator of the NFκB signaling pathway and of caspase-8 mediated cell death in mammalian cells. In this study, we report that IGF2BP1 and cIAP1 expression is upregulated in a panel of rhabdomyosarcoma cell lines. We also show that IGF2BP1 is a positive regulator of cIAP1 translation, specifically through an internal ribosome entry site (IRES) mechanism. Finally, we report that altering the levels of cIAP1 in two rhabdomyosarcoma cell lines, RH36 and RH41, either by IGF2BP1 knock-down or by a Smac mimetic coumpound, sensitizes these cells to TNFα-mediated cell death. Our results identify IGF2BP1 and cIAP1 as important regulators of apoptosis in rhabdomyosarcomas.

Citation Format: Mame Daro Faye, Tyson Graber, Stephanie Langlois, Kyle Cowan, Martin Holcik. Identification of the insulin-like growth factor 2 mRNA binding protein (IGF2BP1) as an important regulator of cIAP1 translation and apoptosis in rhabdomyosarcomas. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 827. doi:10.1158/1538-7445.AM2013-827

Abstract 1991: PDCD4 de-repression of Bcl-xl IRES-mediated translation leads to enhanced chemo-resistance of glioblastoma multiforme tumors

Glioblastoma multiforme (GBM) is the most prevalent form of tumor of the central nervous system with an average survival time of less than one year. Given the location and characteristics of GBM only limited treatment options are available, thus understanding the mechanisms of GBM formation or progression may lead to the development of novel treatment options. Strikingly, a common feature of GBM is the loss of expression of the tumor suppressor programmed cell death 4 (PDCD4) which correlates with adverse outcomes for patients in response to chemo- or radiation therapy. Through both transcriptional and translational regulation, PDCD4 has been shown to regulate many proteins within the cell, although the precise mechanism of target selection is not known. We have hypothesised that silencing of PDCD4 in glioblastomas leads to an altered expression of apoptosis-regulating proteins resulting in the suppression of apoptotic signals and enhanced chemo-resistance. Indeed, we show that low levels of PDCD4 correlate with an increase in the anti-apoptotic Bcl-xL protein. Importantly, Bcl-xL is regulated at the level of protein synthesis through the internal ribosome entry site (IRES) in its 5’ untranslated region (UTR). We further show that PDCD4 regulates the activity of the Bcl-xL IRES, and is therefore an IRES specific translational regulator. Importantly, reintroduction of PDCD4 into glioblastoma cells or directly inhibiting Bcl-xL with the chemical inhibitor ABT-737 sensitizes these cells to doxorubicin, thus enhancing cell death. In summary, our work provides a novel role for PDCD4 as a translational regulator of Bcl-xL as well as a potential therapeutic option for the treatment of GBM.

Citation Format: Urszula Liwak, Lindsay E. Jordan, Federico Roncaroli, Martin Holcik. PDCD4 de-repression of Bcl-xl IRES-mediated translation leads to enhanced chemo-resistance of glioblastoma multiforme tumors. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1991. doi:10.1158/1538-7445.AM2013-1991

Translation control in apoptosis

Regulation of protein synthesis, although known for many decades, has only recently begun to be recognized as a critical control mechanism for the maintenance of cellular homeostasis and cellular stress response. One of the key advantages of translational control is the ability of cells to rapidly reprogram the protein output in response to internal or external triggers. This is particularly important during cellular response to stress that may lead to apoptosis by providing cells with a fine tuning mechanism that tips the balance between cell survival or apoptosis. In the following review we highlight several distinct mechanisms of translation control and provide specific examples of translational control during apoptosis. This article is part of a Special Issue entitled "Apoptosis: Four Decades Later".

An oxygen-regulated switch in the protein synthesis machinery

Protein synthesis involves the translation of ribonucleic acid information into proteins, the building blocks of life. The initial step of protein synthesis consists of the eukaryotic translation initiation factor 4E (eIF4E) binding to the 7-methylguanosine (m⁷-GpppG) 5′cap of mRNAs^1,2. Low oxygen tension (hypoxia) represses cap-mediated translation by sequestering eIF4E through mammalian target of rapamycin (mTOR)-dependent mechanisms^3–6. While the internal ribosome entry site is an alternative translation initiation mechanism, this pathway alone cannot account for the translational capacity of hypoxic cells^7,8. This raises a fundamental question in biology as to how proteins are synthesized in periods of oxygen scarcity and eIF4E inhibition⁹. Here, we uncover an oxygen-regulated translation initiation complex that mediates selective cap-dependent protein synthesis. Hypoxia stimulates the formation of a complex that includes the oxygen-regulated hypoxia-inducible factor 2α (HIF-2α), the RNA binding protein RBM4 and the cap-binding eIF4E2, an eIF4E homologue. PAR-CLIP¹⁰ analysis identified an RNA hypoxia response element (rHRE) that recruits this complex to a wide array mRNAs, including the epidermal growth factor receptor (EGFR). Once assembled at the rHRE, HIF-2α/RBM4/eIF4E2 captures the 5′cap and targets mRNAs to polysomes for active translation thereby evading hypoxia-induced repression of protein synthesis. These findings demonstrate that cells have evolved a program whereby oxygen tension switches the basic translation initiation machinery.

IRES-mediated translation of cellular messenger RNA operates in eIF2α- independent manner during stress

Physiological and pathophysiological stress attenuates global translation via phosphorylation of eIF2α. This in turn leads to the reprogramming of gene expression that is required for adaptive stress response. One class of cellular messenger RNAs whose translation was reported to be insensitive to eIF2α phosphorylation-mediated repression of translation is that harboring an Internal Ribosome Entry Site (IRES). IRES-mediated translation of several apoptosis-regulating genes increases in response to hypoxia, serum deprivation or gamma irradiation and promotes tumor cell survival and chemoresistance. However, the molecular mechanism that allows IRES-mediated translation to continue in an eIF2α-independent manner is not known. Here we have used the X-chromosome linked Inhibitor of Apoptosis, XIAP, IRES to address this question. Using toeprinting assay, western blot analysis and polysomal profiling we show that the XIAP IRES supports cap-independent translation when eIF2α is phosphorylated both in vitro and in vivo. During normal growth condition eIF2α-dependent translation on the IRES is preferred. However, IRES-mediated translation switches to eIF5B-dependent mode when eIF2α is phosphorylated as a consequence of cellular stress.

Prolactin increases SMN expression and survival in a mouse model of severe spinal muscular atrophy via the STAT5 pathway

Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disease that is characterized by the loss of motor neurons, resulting in progressive muscle atrophy. It is caused by the loss of functional survival motor neuron (SMN) protein due to mutations or deletion in the SMN1 gene. A potential treatment strategy for SMA is to upregulate levels of SMN protein. Several agents that activate STAT5 in human and mouse cell lines enhance SMN expression from the SMN2 gene and can compensate, at least in part, for the loss of production of a functional protein from SMN1. Here, we have shown that prolactin (PRL) increases SMN levels via activation of the STAT5 pathway. PRL increased SMN mRNA and protein levels in cultured human and mouse neuronal cells. Administration of STAT5-specific siRNA blocked the effects of PRL, indicating that the PRL-induced transcriptional upregulation of the SMN-encoding gene was mediated by activation of STAT5. Furthermore, systemic administration of PRL to WT mice induced SMN expression in the brain and spinal cord. Critically, PRL treatment increased SMN levels, improved motor function, and enhanced survival in a mouse model of severe SMA. Our results confirm earlier work suggesting STAT5 pathway activators as potential therapeutic compounds for the treatment of SMA and identify PRL as one such promising agent.

Abstract 2113: Programmed cell death 4 (PDCD4) regulates apoptotic resistance of human gliomas

Glioblastoma multiforme (GBM) is the most prevalent form of tumors of the central nervous system with an average survival rate of less than one year. Given the location and characteristics of GBM only limited treatment options are available, thus understanding the mechanisms of GBM formation or progression may lead to the development of novel treatment options. Strikingly, a common feature of GBM is the loss of expression of the tumor suppressor programmed cell death gene 4 (PDCD4), which correlates with a poor prognosis. Through both transcriptional and translational regulation, PDCD4 has been shown to regulate many proteins within the cell, although the precise mechanism of target selection is not known. We have hypothesised that silencing of PDCD4 in glioblastomas leads to an altered expression of apoptosis-regulating proteins resulting in the suppression of apoptotic signals and enhanced chemo-resistance of glioblastomas. Indeed, we show that low levels of PDCD4 correlate with an increase in the anti-apoptotic XIAP and Bcl-xL proteins, and decrease in pro-apoptotic Apaf-1. Importantly, these proteins are regulated at the level of protein synthesis through the internal ribosome entry sites (IRESs) in the 5’ untranslated region (UTR) of their respective mRNAs. IRES elements allow for translation to occur through a cap-independent mechanism during times of cellular stress when global translation is attenuated. We further show that PDCD4 regulates the activity of Bcl-xL, XIAP and Apaf-1 IRES and is therefore an IRES specific translational regulator. We further dissect the signalling pathways that regulate PDCD4 levels and show that activation of S6K2 results in the proteolytic degradation of PDCD4 and concomitant upregulation of XIAP and Bcl-xL, and downregulation of Apaf-1 levels. Importantly, reintroduction of PDCD4 into glioblastoma cell lines results in a reduction in both XIAP and Bcl-xL protein levels and enhanced cell death. In summary, our work provides a novel role for PDCD4 as a translational regulator of XIAP, Bcl-xL and Apaf-1, as well as a mechanistic understanding into the regulation of PDCD4 expression. Our research offers insight into the biology of glioblastomas in the hopes that we can identify new pathways as novel targets for anticancer therapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2113. doi:10.1158/1538-7445.AM2011-2113

Phosphorylation of eIF2α at serine 51 is an important determinant of cell survival and adaptation to glucose deficiency

Glucose deficiency leads to the induction of eIF2α phosphorylation at serine 51, which results in a global inhibition of protein synthesis. Phosphorylation of eIF2α is an adaptive process that establishes a cytoprotective state in glucose-deficient cells, with possible implications in biological responses that interfere with glucose metabolism.

Various forms of stress induce pathways that converge on the phosphorylation of the alpha (α) subunit of eukaryotic translation initiation factor eIF2 at serine 51 (S51), a modification that results in a global inhibition of protein synthesis. In many cases eIF2α phosphorylation is a biological response that facilitates cells to cope with stressful environments. Glucose deficiency, an important form of stress, is associated with an induction of apoptosis. Herein, we demonstrate that eIF2α phosphorylation is a key step in maintaining a balance between the life and death of a glucose-deficient cell. That is, eIF2α phosphorylation acts as a molecular switch that shifts cells from a proapoptotic to a cytoprotective state in response to prolonged glucose deficiency. This adaptation process is associated with the timely expression of proteins and activation of pathways with significant contributions to cell survival and adaptation including the X-linked inhibitor of apoptosis protein (XIAP). We also show that among the eIF2α kinases GCN2 plays a proapoptotic role whereas PERK and PKR play a cytoprotective one in response to glucose deficiency. Our data demonstrate that eIF2α phosphorylation is a significant determinant of survival and adaptation of glucose-deficient cells with possible important implications in biological processes that interfere with glucose metabolism.