A role for PKR in hematologic malignancies

The double-stranded RNA-dependent kinase PKR has been described for many years as strictly a pro-apoptotic kinase. Recent data suggest that the main purpose of this kinase is damage control and repair following stress and, if all else fails, apoptosis. Aberrant activation of PKR has been reported in numerous neurodegenerative diseases and cancer. Although a subset of myelodysplastic syndromes (MDS) and chronic lymphocytic leukemia contain low levels of PKR expression and activity, elevated PKR activity and/or expression have been detected in a wide range of hematologic malignancies, from bone marrow failure disorders to acute leukemia. With the recent findings that cancers containing elevated PKR activity are highly sensitive to PKR inhibition, we explore the role of PKR in hematologic malignancies, signal transduction pathways affected by PKR, and how PKR may contribute to leukemic transformation.

Translational control in cancer

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

Normoxic destabilization of ATF-4 depends on proteasomal degradation

AIM

Hypoxia-inducible gene expression is an important physiological adaptive mechanism in response to a decreased oxygen supply. We have recently described an oxygen- and prolyl-4-hydroxylase (PHD)3-dependent stabilization of the activating transcription factor 4 (ATF-4). The aim of the present study was to examine if the normoxic destabilization of ATF-4 is regulated by oxygen-dependent proteasomal degradation.

METHODS

We determined poly-ubiquitination of ATF-4 in normoxia compared to hypoxia by immunoprecipitation and immunoblots. Furthermore, we analysed the expression of the ATF-4 target gene GADD153 as a function of oxygen concentration.

RESULTS

ATF-4 protein levels were not detectable in normoxia. Normoxic degradation correlated with an oxygen-dependent poly-ubiquitination of ATF-4, which was hindered by hypoxic incubation of the cells. As a result of hypoxia, GADD153 was expressed. The hypoxic GADD153 expression was attenuated or increased by transfecting the cells with ATF-4 siRNA or PHD3 siRNA respectively.

CONCLUSION

Our results demonstrate the involvement of oxygen-dependent proteasomal degradation of ATF-4 in the hypoxia-induced expression of GADD153. Taken together, hypoxia/PHD3-regulated stabilization of ATF-4 by hindering oxygen-dependent degradation may play a critical role in linking cell fate decisions to oxygen availability.

eIF2alpha phosphorylation tips the balance to apoptosis during osmotic stress

Regulation of cell volume is of great importance because persistent swelling or shrinkage leads to cell death. Tissues experience hypertonicity in both physiological (kidney medullar cells) and pathological states (hypernatremia). Hypertonicity induces an adaptive gene expression program that leads to cell volume recovery or apoptosis under persistent stress. We show that the commitment to apoptosis is controlled by phosphorylation of the translation initiation factor eIF2alpha, the master regulator of the stress response. Studies with cultured mouse fibroblasts and cortical neurons show that mutants deficient in eIF2alpha phosphorylation are protected from hypertonicity-induced apoptosis. A novel link is revealed between eIF2alpha phosphorylation and the subcellular distribution of the RNA-binding protein heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1). Stress-induced phosphorylation of eIF2alpha promotes apoptosis by inducing the cytoplasmic accumulation of hnRNP A1, which attenuates internal ribosome entry site-mediated translation of anti-apoptotic mRNAs, including Bcl-xL that was studied here. Hypertonic stress induced the eIF2alpha phosphorylation-independent formation of cytoplasmic stress granules (SGs, structures that harbor translationally arrested mRNAs) and the eIF2alpha phosphorylation-dependent accumulation of hnRNP A1 in SGs. The importance of hnRNP A1 was demonstrated by induction of apoptosis in eIF2alpha phosphorylation-deficient cells that express exogenous cytoplasmic hnRNP A1. We propose that eIF2alpha phosphorylation during hypertonic stress promotes apoptosis by sequestration of specific mRNAs in SGs in a process mediated by the cytoplasmic accumulation of hnRNP A1.

Integration of general amino acid control and target of rapamycin (TOR) regulatory pathways in nitrogen assimilation in yeast

Two important nutrient-sensing and regulatory pathways, the general amino acid control (GAAC) and the target of rapamycin (TOR), participate in the control of yeast growth and metabolism during changes in nutrient availability. Amino acid starvation activates the GAAC through Gcn2p phosphorylation of translation factor eIF2 and preferential translation of GCN4, a transcription activator. TOR senses nitrogen availability and regulates transcription factors such as Gln3p. We used microarray analyses to address the integration of the GAAC and TOR pathways in directing the yeast transcriptome during amino acid starvation and rapamycin treatment. We found that GAAC is a major effector of the TOR pathway, with Gcn4p and Gln3p each inducing a similar number of genes during rapamycin treatment. Although Gcn4p activates a common core of 57 genes, the GAAC directs significant variations in the transcriptome during different stresses. In addition to inducing amino acid biosynthetic genes, Gcn4p in conjunction with Gln3p activates genes required for the assimilation of secondary nitrogen sources such as gamma-aminobutyric acid (GABA). Gcn2p activation upon shifting to secondary nitrogen sources is suggested to occur by means of a dual mechanism. First, Gcn2p is induced by the release of TOR repression through a mechanism involving Sit4p protein phosphatase. Second, this eIF2 kinase is activated by select uncharged tRNAs, which were shown to accumulate during the shift to the GABA medium. This study highlights the mechanisms by which the GAAC and TOR pathways are integrated to recognize changing nitrogen availability and direct the transcriptome for optimal growth adaptation.

Deficiency of suppressor enhancer Lin12 1 like (SEL1L) in mice leads to systemic endoplasmic reticulum stress and embryonic lethality

Stress in the endoplasmic reticulum (ER) plays an important causal role in the pathogenesis of several chronic diseases such as Alzheimer, Parkinson, and diabetes mellitus. Insight into the genetic determinants responsible for ER homeostasis will greatly facilitate the development of therapeutic strategies for the treatment of these debilitating diseases. Suppressor enhancer Lin12 1 like (SEL1L) is an ER membrane protein and was thought to be involved in the quality control of secreted proteins. Here we show that the mice homozygous mutant for SEL1L were embryonic lethal. Electron microscopy studies revealed a severely dilated ER in the fetal liver of mutant embryos, indicative of alteration in ER homeostasis. Consistent with this, several ER stress responsive genes were significantly up-regulated in the mutant embryos. Mouse embryonic fibroblast cells deficient in SEL1L exhibited activated unfolded protein response at the basal state, impaired ER-associated protein degradation, and reduced protein secretion. Furthermore, markedly increased apoptosis was observed in the forebrain and dorsal root ganglions of mutant embryos. Taken together, our results demonstrate an essential role for SEL1L in protein quality control during mouse embryonic development.

Molecular mechanisms involved in the adaptation to amino acid limitation in mammals

In mammals, metabolic adaptations are required to cope with episodes of protein deprivation and malnutrition. Consequently, mammals have to adjust physiological functions involved in the adaptation to amino acid availability. Part of this regulation involves the modulation of the expression of numerous genes. In particular, it has been shown that amino acids by themselves can modify the expression of target genes. This review describes the regulation of amino acids homeostasis and the their role as signal molecules. The recent advances in the understanding of the molecular mechanisms involved in the control of mammalian gene expression in response to amino acid limitation will be described.

Eukaryotic stress granules: the ins and outs of translation

The stress response in eukaryotic cells often inhibits translation initiation and leads to the formation of cytoplasmic RNA-protein complexes referred to as stress granules. Stress granules contain nontranslating mRNAs, translation initiation components, and many additional proteins affecting mRNA function. Stress granules have been proposed to affect mRNA translation and stability and have been linked to apoptosis and nuclear processes. Stress granules also interact with P-bodies, another cytoplasmic RNP granule containing nontranslating mRNA, translation repressors, and some mRNA degradation machinery. Together, stress granules and P-bodies reveal a dynamic cycle of distinct biochemical and compartmentalized mRNPs in the cytosol, with implications for the control of mRNA function.

Proteasome inhibitors activate autophagy as a cytoprotective response in human prostate cancer cells

The ubiquitin-proteasome and lysosome-autophagy pathways are the two major intracellular protein degradation systems that work cooperatively to maintain homeostasis. Proteasome inhibitors (PIs) have clinical activity in hematological tumors, and inhibitors of autophagy are also being evaluated as potential antitumor therapies. In this study, we found that chemical PIs and small interfering RNA-mediated knockdown of the proteasome's enzymatic subunits promoted autophagosome formation, stimulated autophagic flux, and upregulated expression of the autophagy-specific genes (ATGs) (ATG5 and ATG7) in some human prostate cancer cells and immortalized mouse embryonic fibroblasts (MEFs). Upregulation of ATG5 and ATG7 only occurred in cells displaying PI-induced phosphorylation of the eukaryotic translation initiation factor 2 alpha (eIF2alpha), an important component of the unfolded protein responses. Furthermore, PIs did not induce autophagy or upregulate ATG5 in MEFs expressing a phosphorylation-deficient mutant form of eIF2alpha. Combined inhibition of autophagy and the proteasome induced an accumulation of intracellular protein aggregates reminiscent of neuronal inclusion bodies and caused more cancer cell death than blocking either degradation pathway alone. Overall, our data show that proteasome inhibition activates autophagy through a phospho-eIF2alpha-dependent mechanism to eliminate protein aggregates and alleviate proteotoxic stress.

Alterations in neuronal gene expression profiles in response to experimental demyelination and axonal transection

The main pathological features of multiple sclerosis, demyelination and axonal transection, are considered to cause reversible and irreversible neurological deficits, respectively. This study aimed to separately analyze the effects of these pathological hallmarks on neuronal gene expression in experimental paradigms. The pontocerebellar pathway was targeted with either lysolecithin-induced chemical demyelination or a complete pathway transection (axonal transection) in rats. Transcriptional changes in the pontocerebellar neurons were investigated with microarrays at days 4, 10 and 37 post-intervention, which was confirmed by immunohistochemistry on protein level. A common as well as unique set of injury-response genes was identified. The increased expression of activating transcription factor 3 (Atf3) and thyrotropin-releasing hormone (Trh) in both injury paradigms was validated by immunohistochemistry. The expression of Atf3 in a patient with Marburg's variant of multiple sclerosis was also detected, also confirming the activation of the Atf3 pathway in a human disease sample. It was concluded that this experimental approach may be useful for the identification of pathways that could be targeted for remyelinative or neuroprotective drug development.

Stress-sensitive regulation of IFRD1 mRNA decay is mediated by an upstream open reading frame

In this report, we demonstrate that cellular stress regulates expression of IFRD1 by a post-transcriptional control mechanism. IFRD1 mRNA and protein are elevated in tunicamycin-treated human kidney epithelial cells via stabilization of the mRNA. IFRD1 mRNA instability in resting cells requires translation of an upstream open reading frame (ORF) that represses translation of the major ORF. During stress response, the mRNA is stabilized via inhibition of translational initiation mediated by phosphorylated eIF2alpha. Translation of the major ORF of IFRD1 involves both leaky scanning at the upstream AUG codon and re-initiation at the major AUG codon and is not altered during stress. Finally, the instability mechanism depends upon UPF1, suggesting that it is related to nonsense-mediated decay. Importantly, the sequence and length of the upstream ORF are critical but do not need to code for a specific peptide. Moreover the sequence environment of the upstream ORF termination site is not an essential feature of instability. These features of decay collectively define a distinct upstream ORF-mediated instability mechanism whereby cellular stress can modulate specific gene expression through alteration of mRNA half-life.

Differential regulation of oestrogen receptor β isoforms by 5' untranslated regions in cancer

Oestrogen receptors (ERs) are critical regulators of the behaviour of many cancers. Despite this, the roles and regulation of one of the two known ERs – ERβ– are poorly understood. This is partly because analyses have been confused by discrepancies between ERβ expression at mRNA and proteins levels, and because ERβ is expressed as several functionally distinct isoforms. We investigated human ERβ 5′ untranslated regions (UTRs) and their influences on ERβ expression and function. We demonstrate that two alternative ERβ 5′UTRs have potent and differential influences on expression acting at the level of translation. We show that their influences are modulated by cellular context and in carcinogenesis, and demonstrate the contributions of both upstream open reading frames and RNA secondary structure. These regulatory mechanisms offer explanations for the non-concordance of ERβ mRNA and protein. Importantly, we also demonstrate that 5′UTRs allow the first reported mechanisms for differential regulation of the expression of the ERβ isoforms 1, 2 and 5, and thereby have critical influences on ERβ function.

Tumor suppression by PTEN requires the activation of the PKR-eIF2alpha phosphorylation pathway

Inhibition of protein synthesis by phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2) at Ser(51) occurs as a result of the activation of a family of kinases in response to various forms of stress. Although some consequences of eIF2alpha phosphorylation are cytoprotective, phosphorylation of eIF2alpha by RNA-dependent protein kinase (PKR) is largely proapoptotic and tumor suppressing. Phosphatase and tensin homolog deleted from chromosome 10 (PTEN) is a tumor suppressor protein that is mutated or deleted in various human cancers, with functions that are mediated through phosphatase-dependent and -independent pathways. Here, we demonstrate that the eIF2alpha phosphorylation pathway is downstream of PTEN. Inactivation of PTEN in human melanoma cells reduced eIF2alpha phosphorylation, whereas reconstitution of PTEN-null human glioblastoma or prostate cancer cells with either wild-type PTEN or phosphatase-defective mutants of PTEN induced PKR activity and eIF2alpha phosphorylation. The antiproliferative and proapoptotic effects of PTEN were compromised in mouse embryonic fibroblasts that lacked PKR or contained a phosphorylation-defective variant of eIF2alpha. Induction of the pathway leading to phosphorylation of eIF2alpha required an intact PDZ-binding motif in PTEN. These findings establish a link between tumor suppression by PTEN and inhibition of protein synthesis that is independent of PTEN's effects on phosphoinositide 3'-kinase signaling.

Mechanical stimulation suppresses phosphorylation of eIF2alpha and PERK-mediated responses to stress to the endoplasmic reticulum

Cellular perturbations such as stress to the endoplasmic reticulum induce an integrated stress response, which activates phosphorylation of eIF2alpha and leads to alleviation of cellular injury or apoptosis. This study investigated the role of mechanical stimulation in the regulation of eIF2alpha and cell death. Mechanical stimulation was applied to mouse ulnae, MC3T3 cells, and mesenchymal stem cells. The results demonstrate that mechanical stimulation reduces phosphorylation of eIF2alpha through inactivation of Perk. Furthermore, flow pre-treatment reduces thapsigargin-induced cell mortality through suppression of phosphorylation of Perk. However, H(2)O(2)-driven cell mortality, which is not mediated by Perk, is not suppressed by mechanical stimulation. Taken together, our observations suggest a pro-survival role of mechanical stimulation in Perk-mediated stress responses.

Initiation factor eIF2-independent mode of c-Src mRNA translation occurs via an internal ribosome entry site

Overexpression and activation of the c-Src protein have been linked to the development of a wide variety of cancers. The molecular mechanism(s) of c-Src overexpression in cancer cells is not clear. We report here an internal ribosome entry site (IRES) in the c-Src mRNA that is constituted by both 5'-noncoding and -coding regions. The inhibition of cap-dependent translation by m(7)GDP in the cell-free translation system or induction of endoplasmic reticulum stress in hepatoma-derived cells resulted in stimulation of the c-Src IRES activities. Sucrose density gradient analyses revealed formation of a stable binary complex between the c-Src IRES and purified HeLa 40 S ribosomal subunit in the absence of initiation factors. We further demonstrate eIF2-independent assembly of 80 S initiation complex on the c-Src IRES. These features of the c-Src IRES appear to be reminiscent of that of hepatitis C virus-like IRESs and translation initiation in prokaryotes. Transfection studies and genetic analysis revealed that the c-Src IRES permitted initiation at the authentic AUG351, which is also used for conventional translation initiation of the c-Src mRNA. Our studies unveiled a novel regulatory mechanism of c-Src synthesis mediated by an IRES element, which exhibits enhanced activity during cellular stress and is likely to cause c-Src overexpression during oncogenesis and metastasis.

Regulation of endoplasmic reticulum stress-induced cell death by ATF4 in neuroectodermal tumor cells

The neuroectodermal tumors neuroblastoma and melanoma represent biologically aggressive and chemoresistant cancers. The chemotherapeutic agents fenretinide and bortezomib induce apoptosis through endoplasmic reticulum (ER) stress in these tumor types. The aim of this study was to test the hypothesis that the early events of ER stress signaling and response pathways induced by fenretinide and bortezomib are mediated by the eukaryotic initiation factor 2alpha (eIF2alpha)-ATF4 signaling pathway. Treatment of neuroblastoma and melanoma cell lines with fenretinide, bortezomib, or thapsigargin resulted in induction of eIF2alpha signaling, characterized by increased expression of phosphorylated eIF2alpha, ATF4, ATF3, and GADD34. These events correlated with induction of the pro-apoptotic protein Noxa. The cytotoxic response, characterized by up-regulation of Noxa and cell death, was dependent on ATF4, but not the ER-related pro-death signaling pathways involving GADD153 or IRE1. Although PERK-dependent phosphorylation of eIF2alpha enhanced ATF4 protein levels during ER stress, cell death in response to fenretinide, bortezomib, or thapsigargin was not abrogated by inhibition of eIF2alpha phosphorylation through PERK knockdown or overexpression of wild-type eIF2alpha. Furthermore, ATF4 induction in response to ER stress was dependent primarily on transcriptional activation, which occurred in a PERK- and phosphorylated eIF2alpha-independent manner. These results demonstrate that ATF4 mediates ER stress-induced cell death of neuroectodermal tumor cells in response to fenretinide or bortezomib. Understanding the complex regulation of cell death pathways in response to ER stress-inducing drugs has the potential to reveal novel therapeutic targets, thus allowing the development of improved treatment strategies to overcome chemoresistance.

Conjugated linoleic acid activates AMP-activated protein kinase and reduces adiposity more effectively when used with metformin in mice

Trans-10, cis-12 (t10c12) conjugated linoleic acid (CLA) reduces lipid levels in adipocytes, but the mechanisms involved are still emerging. The hypotheses of this study were that t10c12 CLA treatment activated AMP-activated protein kinase (AMPK) and that the effectiveness of a low dose of t10c12 CLA would be increased when combined with an AMPK activator. We demonstrated t10c12 CLA, directly or indirectly, activated AMPK and increased the amount of phosphorylated acetyl-CoA carboxylase (ACC) in 3T3-L1 adipocytes. Compound C, a potent inhibitor of AMPK, attenuated the phosphorylation of ACC, integrated stress response (ISR), inflammatory response, reduction in key lipogenic transcription factors, and triglyceride (TG) reduction that otherwise occurred in t10c12 CLA-treated adipocytes. Treatment of adipocytes or mice with a low dose of t10c12 CLA in conjunction with the AMPK activator metformin resulted in more TG loss than treatment with the individual chemicals. Additionally, although an inflammatory response was required for robust TG reduction, the combination of t10c12 CLA with AMPK activators had a similar TG loss with a reduced inflammatory response. A microarray analysis of the transcriptional response to either t10c12 CLA, metformin, or the combination, indicated the responses were very similar, with a correlation coefficient of 0.91 or better for genes in the ISR or lipid-related pathways. Altogether, these results support our hypotheses that t10c12 CLA activates AMPK, directly or indirectly, and that metformin increases the effectiveness of t10c12 CLA in reducing TG amounts in adipocytes.

Differential regulation of CHOP translation by phosphorylated eIF4E under stress conditions

Cells respond to environmental stress by inducing translation of a subset of mRNAs important for survival or apoptosis. CHOP, a downstream transcriptional target of stress-induced ATF4, is also regulated translationally in a uORF-dependent manner under stress. Low concentration of anisomycin induces CHOP expression at both transcriptional and translational levels. To study specifically the translational aspect of CHOP expression, and further clarify the regulatory mechanisms underlying stress-induced translation initiation, we developed a CMV promoter-regulated, uORF^chop-driven reporter platform. Here we show that anisomycin-induced CHOP expression depends on phosphorylated eIF4E/S209 and eIF2α/S51. Contrary to phospho-eIF2α/S51, phospho-eIF4E/S209 is not involved in thapsigargin-induced CHOP expression. Studies using various kinase inhibitors and mutants uncovered that both the p38MAPK-Mnk and mTOR signaling pathways contribute to stress-responsive reporter and CHOP expression. We also demonstrated that anisomycin-induced translation is tightly regulated by partner binding preference of eIF4E. Furthermore, mutating the uORF sequence abolished the anisomycin-induced association of chop mRNA with phospho-eIF4E and polysomes, thus demonstrating the significance of this cis-regulatory element in conferring on the transcript a stress-responsive translational inducibility. Strikingly, although insulin treatment activated ERK-Mnk and mTOR pathways, and consequently eIF4E/S209 phosphorylation, it failed to induce phospho-eIF2α/S51 and reporter translation, thus pinpointing a crucial determinant in stress-responsive translation.

Translational control by RGS2

The regulator of G protein signaling (RGS) proteins are a family of guanosine triphosphatase (GTPase)-accelerating proteins. We have discovered a novel function for RGS2 in the control of protein synthesis. RGS2 was found to bind to eIF2Bepsilon (eukaryotic initiation factor 2B epsilon subunit) and inhibit the translation of messenger RNA (mRNA) into new protein. This effect was not observed for other RGS proteins tested. This novel function of RGS2 is distinct from its ability to regulate G protein-mediated signals and maps to a stretch of 37 amino acid residues within its conserved RGS domain. Moreover, RGS2 was capable of interfering with the eIF2-eIF2B GTPase cycle, which is a requisite step for the initiation of mRNA translation. Collectively, this study has identified a novel role for RGS2 in the control of protein synthesis that is independent of its established RGS domain function.

Metabolic Remodeling Induced by Mitochondrial Aldehyde Stress Stimulates Tolerance to Oxidative Stress in the Heart

Rationale : Aldehyde accumulation is regarded as a pathognomonic feature of oxidative stress–associated cardiovascular disease.

Objective : We investigated how the heart compensates for the accelerated accumulation of aldehydes.

Methods and Results : Aldehyde dehydrogenase 2 (ALDH2) has a major role in aldehyde detoxification in the mitochondria, a major source of aldehydes. Transgenic (Tg) mice carrying an Aldh2 gene with a single nucleotide polymorphism ( Aldh2*2 ) were developed. This polymorphism has a dominant-negative effect and the Tg mice exhibited impaired ALDH activity against a broad range of aldehydes. Despite a shift toward the oxidative state in mitochondrial matrices, Aldh2*2 Tg hearts displayed normal left ventricular function by echocardiography and, because of metabolic remodeling, an unexpected tolerance to oxidative stress induced by ischemia/reperfusion injury. Mitochondrial aldehyde stress stimulated eukaryotic translation initiation factor 2α phosphorylation. Subsequent translational and transcriptional activation of activating transcription factor-4 promoted the expression of enzymes involved in amino acid biosynthesis and transport, ultimately providing precursor amino acids for glutathione biosynthesis. Intracellular glutathione levels were increased 1.37-fold in Aldh2*2 Tg hearts compared with wild-type controls. Heterozygous knockout of Atf4 blunted the increase in intracellular glutathione levels in Aldh2*2 Tg hearts, thereby attenuating the oxidative stress–resistant phenotype. Furthermore, glycolysis and NADPH generation via the pentose phosphate pathway were activated in Aldh2*2 Tg hearts. (NADPH is required for the recycling of oxidized glutathione.)

Conclusions : The findings of the present study indicate that mitochondrial aldehyde stress in the heart induces metabolic remodeling, leading to activation of the glutathione–redox cycle, which confers resistance against acute oxidative stress induced by ischemia/reperfusion.

Protein synthesis and its control in neuronal cells with a focus on vanishing white matter disease

Protein synthesis (also termed mRNA translation) is a key step in the expression of a cell's genetic information, in which the information contained within the coding region of the mRNA is used to direct the synthesis of the new protein, a process that is catalysed by the ribosome. Protein synthesis must be tightly controlled, to ensure the right proteins are made in the right amounts at the right time, and must be accurate, to avoid errors that could lead to the production of defective and potentially damaging proteins. In addition to the ribosome, protein synthesis also requires proteins termed translation factors, which mediate specific steps of the process. The first major stage of mRNA translation is termed ‘initiation’ and involves the recruitment of the ribosome to the mRNA and the identification of the correct start codon to commence translation. In eukaryotic cells, this process requires a set of eIFs (eukaryotic initiation factors). During the second main stage of translation, ‘elongation’, the ribosome traverses the coding region of the mRNA, assembling the new polypeptide: this process requires eEFs (eukaryotic elongation factors). Control of eEF2 is important in certain neurological processes. It is now clear that defects in eIFs or in their control can give rise to a number of diseases. This paper provides an overview of translation initiation and its control mechanisms, particularly those examined in neuronal cells. A major focus concerns an inherited neurological condition termed VHM (vanishing white matter) or CACH (childhood ataxia with central nervous system hypomyelination). VWM/CACH is caused by mutations in the translation initiation factor, eIF2B, a component of the basal translational machinery in all cells.

Dual functions of Rift Valley fever virus NSs protein: inhibition of host mRNA transcription and post-transcriptional downregulation of protein kinase PKR

Rift Valley fever virus (RVFV), which belongs to the genus Phlebovirus, family Bunyaviridae, is a negative-stranded RNA virus carrying a single-stranded, tripartite RNA genome. RVFV is an important zoonotic pathogen transmitted by mosquitoes and causes large outbreaks among ruminants and humans in Africa and the Arabian Peninsula. Human patients develop an acute febrile illness, followed by a fatal hemorrhagic fever, encephalitis, or ocular diseases. A viral nonstructural protein, NSs, is a major viral virulence factor. Past studies showed that NSs suppresses the transcription of host mRNAs, including interferon-beta mRNAs. Here we demonstrated that the NSs protein induced post-transcriptional downregulation of dsRNA-dependent protein kinase (PKR), to prevent phosphorylation of eIF2alpha and promoted viral translation in infected cells. These two biological activities of the NSs most probably have a synergistic effect in suppressing host innate immune functions and facilitate efficient viral replication in infected mammalian hosts.

Mechanisms and implications of reactive oxygen species generation during the unfolded protein response: roles of endoplasmic reticulum oxidoreductases, mitochondrial electron transport, and NADPH oxidase

Cellular mechanisms governing redox homeostasis likely involve their integration with other stresses. Endoplasmic reticulum (ER) stress triggers complex adaptive or proapoptotic signaling defined as the unfolded protein response (UPR), involved in several pathophysiological processes. Since protein folding is highly redox-dependent, convergence between ER stress and oxidative stress has attracted interest. Evidence suggests that ROS production and oxidative stress are not only coincidental to ER stress, but are integral UPR components, being triggered by distinct types of ER stressors and contributing to support proapoptotic, as well as proadaptive UPR signaling. Thus, ROS generation can be upstream or downstream UPR targets and may display a UPR-specific plus a nonspecific component. Enzymatic mechanisms of ROS generation during UPR include: (a) Multiple thiol-disulfide exchanges involving ER oxidoreductases including flavooxidase Ero1 and protein disulfide isomerase (PDI); (b) Mitochondrial electron transport; (c) Nox4 NADPH oxidase complex, particularly Nox4. Understanding the roles of such mechanisms and how they interconnect with the UPR requires more investigation. Integration among such ROS sources may depend on Ca(2+) levels, ROS themselves, and PDI, which associates with NADPH oxidase and regulates its function. Oxidative stress may frequently integrate with a background of ER stress/UPR in several diseases; here we discuss a focus in the vascular system.

The sensing of essential amino acid deficiency in the anterior piriform cortex, that requires the uncharged tRNA/GCN2 pathway, is sensitive to wortmannin but not rapamycin

Animals detect and reject their first essential/indispensable amino acid (IAA) deficient meal within 20min; this IAA sensing requires an intact anterior piriform cortex (APC). In the biochemical responses to IAA deficiency in the APC we have shown that: uncharged tRNA is the primary sensor; IAA transport is increased; and signaling, including the extracellular-regulated kinase (ERK1/2), is activated. The mammalian target of rapamycin (mTOR) is a potential AA sensor and is regulated by AA transport. Previously, the inhibitors, rapamycin for mTOR, wortmannin for phosphoinositide 3 kinase (PI3K) and PD98059 for ERK, each blocked the upregulation of the System A transporter in IAA depleted APC neurons. Here we injected these same inhibitors into the APC and measured intake of an IAA deficient diet. Rapamycin had no effect on the rejection of the IAA deficient diet, but wortmannin increased ERK activation and intake of the deficient diet before 40min and PD98059 acted after 40min to increase the second meal. While the specific wortmannin target involved in blocking the behavioral response remains unclear, we conclude that mTOR is dispensable for sensing IAA deficiency in the APC, and that ERK is associated with the secondary learned responses to IAA deficient diets.

GCN2 protein kinase is required to activate amino acid deprivation responses in mice treated with the anti-cancer agent L-asparaginase

Asparaginase depletes circulating asparagine and glutamine, activating amino acid deprivation responses (AADR) such as phosphorylation of eukaryotic initiation factor 2 (p-eIF2) leading to increased mRNA levels of asparagine synthetase and CCAAT/enhancer-binding protein beta homologous protein (CHOP) and decreased mammalian target of rapamycin complex 1 (mTORC1) signaling. The objectives of this study were to assess the role of the eIF2 kinases and protein kinase R-like endoplasmic reticulum resident kinase (PERK) in controlling AADR to asparaginase and to compare the effects of asparaginase on mTORC1 to that of rapamycin. In experiment 1, asparaginase increased hepatic p-eIF2 in wild-type mice and mice with a liver-specific PERK deletion but not in GCN2 null mice nor in GCN2-PERK double null livers. In experiment 2, wild-type and GCN2 null mice were treated with asparaginase (3 IU per g of body weight), rapamycin (2 mg per kg of body weight), or both. In wild-type mice, asparaginase but not rapamycin increased p-eIF2, p-ERK1/2, p-Akt, and mRNA levels of asparagine synthetase and CHOP in liver. Asparaginase and rapamycin each inhibited mTORC1 signaling in liver and pancreas but maximally together. In GCN2 null livers, all responses to asparaginase were precluded except CHOP mRNA expression, which remained partially elevated. Interestingly, rapamycin blocked CHOP induction by asparaginase in both wild-type and GCN2 null livers. These results indicate that GCN2 is required for activation of AADR to asparaginase in liver. Rapamycin modifies the hepatic AADR to asparaginase by preventing CHOP induction while maximizing inhibition of mTORC1.

Effects of hepatitis C virus envelope glycoprotein unfolded protein response activation on translation and transcription

The hepatitis C virus (HCV) envelope glycoproteins have been shown to cause ER stress and induce the unfolded protein response (UPR). Using a bicistronic reporter, we show that the envelope glycoproteins repressed both cap-dependent and HCV IRES-mediated translation in HeLa cells but displayed a differential repression of cap-dependent translation in Huh-7 cells. In contrast, the envelope glycoproteins repressed E2F transcriptional activity in both HeLa and Huh-7 cells and caused increased accumulation of the underphosphorylated retinoblastoma protein. Expression of the envelope glycoproteins induced eIF2alpha phosphorylation, suggesting a role of the UPR in regulating translation and E2F transcriptional activity. The envelope glycoproteins also enhanced transcriptional activity from the COX-2 promoter and endogenous COX-2 expression in HeLa cells, but not in Huh-7 cells. Together, these results suggest that the envelope glycoproteins may assume more functional roles in viral replication and host cell interactions.

Acute ablation of PERK results in ER dysfunctions followed by reduced insulin secretion and cell proliferation

Background

A deficiency in Perk (EIF2AK3) causes multiple neonatal defects in humans known as the Wolcott Rallison syndrome. Perk KO mice exhibit the same array of defects including permanent neonatal diabetes (PND). PND in mice was previously shown by us to be due to a decrease in beta cell proliferation and insulin secretion. The aim of this study was to determine if acute ablation of PERK in the 832/13 beta cells recapitulates these defects and to identify the primary molecular basis for beta cell dysfunction.

Results

The INS1 832/13 transformed rat beta cell line was transduced with a dominant-negative Perk transgene via an adenoviral vector. AdDNPerk-832/13 beta cells exhibited reduced expression of insulin and MafA mRNAs, reduced insulin secretion, and reduced cell proliferation. Although proinsulin content was reduced in AdDNPerk-832/13 beta cells, proinsulin was abnormally retained in the endoplasmic reticulum. A temporal study of the acute ablation of Perk revealed that the earliest defect seen was induced expression of two ER chaperone proteins, GRP78/BiP and ERp72. The oxidized states of ERp72 and ERp57 were also increased suggesting an imbalance in the redox state of the ER.

Conclusion

Acute ablation of Perk in INS 832/13 beta cells exhibited all of the major defects seen in Perk KO mice and revealed abnormal expression and redox state of key ER chaperone proteins. Dysregulation of ER chaperone/folding enzymes ERp72 and GRP78/BiP occurred early after ablation of PERK function suggesting that changes in ER secretory functions may give rise to the other defects including reduced insulin gene expression, secretion, and cell proliferation.

Regulation of host translational machinery by African swine fever virus

African swine fever virus (ASFV), like other complex DNA viruses, deploys a variety of strategies to evade the host's defence systems, such as inflammatory and immune responses and cell death. Here, we analyse the modifications in the translational machinery induced by ASFV. During ASFV infection, eIF4G and eIF4E are phosphorylated (Ser1108 and Ser209, respectively), whereas 4E-BP1 is hyperphosphorylated at early times post infection and hypophosphorylated after 18 h. Indeed, a potent increase in eIF4F assembly is observed in ASFV-infected cells, which is prevented by rapamycin treatment. Phosphorylation of eIF4E, eIF4GI and 4E-BP1 is important to enhance viral protein production, but is not essential for ASFV infection as observed in rapamycin- or CGP57380-treated cells. Nevertheless, eIF4F components are indispensable for ASFV protein synthesis and virus spread, since eIF4E or eIF4G depletion in COS-7 or Vero cells strongly prevents accumulation of viral proteins and decreases virus titre. In addition, eIF4F is not only activated but also redistributed within the viral factories at early times of infection, while eIF4G and eIF4E are surrounding these areas at late times. In fact, other components of translational machinery such as eIF2α, eIF3b, eIF4E, eEF2 and ribosomal P protein are enriched in areas surrounding ASFV factories. Notably, the mitochondrial network is polarized in ASFV-infected cells co-localizing with ribosomes. Thus, translation and ATP synthesis seem to be coupled and compartmentalized at the periphery of viral factories. At later times after ASFV infection, polyadenylated mRNAs disappear from the cytoplasm of Vero cells, except within the viral factories. The distribution of these pools of mRNAs is similar to the localization of viral late mRNAs. Therefore, degradation of cellular polyadenylated mRNAs and recruitment of the translation machinery to viral factories may contribute to the inhibition of host protein synthesis, facilitating ASFV protein production in infected cells.

African Swine Fever Virus (ASFV) is a large DNA virus that infects different species of swine, causing an acute, highly contagious and often fatal disease. Infection by ASFV is characterized by the absence of a neutralizing immune response, which has so far hampered the development of a conventional vaccine. While a number of reports have been concerned with ASFV genes and mechanisms regulating programmed cell death and immune evasion, nothing is known so far regarding how ASFV replicates in the infected cells. As intracellular parasites, viruses are highly dependent on host translation machinery for synthesizing their own proteins. We have observed that the cellular protein synthesis is strongly inhibited during ASFV infection, while viral proteins are efficiently produced. Furthermore, we here describe the processes by which ASFV activates and redistributes the cellular machinery to synthesize its own proteins. It has been reported that ASFV replicates within discrete cytoplasmic areas known as factories. In this regard, we have identified the presence of important cellular factors involved in the control of protein synthesis, located close to viral factories, together with ribosomes and the mitochondrial network, which represents a sophisticated mechanism of viral control.

Blocking UV-induced eIF2alpha phosphorylation with small molecule inhibitors of GCN2

The eIF2alpha kinase general control non-depressible 2 integrates translation initiation rates to amino acid availability. General control non-depressible 2 also regulates translation initiation during synaptic plasticity and GCN2(-/-) mice show improved memory compared with wild-type mice with a reduced threshold for triggering late long-term potentiation. This property suggests that inhibiting general control non-depressible 2 function might represent a therapeutic avenue for improving memory. We screened for general control non-depressible 2 inhibitors using a small library of known kinase inhibitors and ATP-analogs and identified three compounds--indirubin-3'-monoxime, SP600125 and a SyK inhibitor with activity against general control non-depressible 2. All three compounds inhibit the ability of general control non-depressible 2 to phosphorylate eIF2alphain vitro as well as in vivo following UV-treatment of mouse embryonic fibroblasts. Using computer-assisted modeling, we modeled the binding of the inhibitors in the ATP-binding site of general control non-depressible 2. This work provides the molecular basis for undertaking structure-activity relationships of these compounds in order to develop specific inhibitors of general control non-depressible 2.

Translational isoforms of FOG1 regulate GATA1-interacting complexes

Erythropoietic and megakaryocytic programs are directed by the transcription factor GATA1. Friend of GATA1 (FOG1), a protein interaction partner of GATA1, is critical for GATA1 function in multiple contexts. Previous work has shown that FOG1 recruits two multi-protein complexes, the nucleosome remodeling domain (NuRD) complex and a C-terminal binding protein (CTBP)-containing complex, into association with GATA1 to mediate activation and repression of target genes. To elucidate mechanisms that might differentially regulate the association of FOG1, as well as GATA1, with these two complexes, we characterized a previously unrecognized translational isoform of FOG1. We found that an N-terminally truncated version of FOG1 is produced from an internal ATG and that this isoform, designated FOG1S, lacks the nucleosome remodeling domain-binding domain, altering the complexes with which it interacts. Both isoforms interact with the C-terminal binding protein complex, which we show also contains lysine-specific demethylase 1 (LSD1). FOG1S is preferentially excluded from the nucleus by unknown mechanisms. These data reveal two novel mechanisms for the regulation of GATA1 interaction with FOG1-dependent protein complexes through the production of two translational isoforms with differential interaction profiles and independent nuclear localization controls.

Old target new approach: an alternate NF-kappaB activation pathway via translation inhibition

Activation of the transcription factor NF-kappaB is a highly regulated multi-level process. The critical step during activation is the release from its inhibitor IkappaB, which as any other protein is under the direct influence of translation regulation. In this review, we summarize in detail the current understanding of the impact of translational regulation on NF-kappaB activation. We illustrate a newly developed mechanism of eIF2alpha kinase-mediated IkappaB depletion and subsequent NF-kappaB activation. We also show that the classical NF-kappaB activation pathways occur simultaneously with, and are complemented by, translational down regulation of the inhibitor molecule IkappaB, the importance of one or the other being shifted in accordance with the type and magnitude of the stressing agent or stimuli.

UVB radiation induces persistent activation of ribosome and oxidative phosphorylation pathways

Ultraviolet B (UVB) radiation has strong biological effects and modulates the expression of many genes. The major biological pathways affected by UVB radiation remain controversial. In this work, we used a loop-design microarray approach and applied rigorous statistical analyses to identify differentially regulated genes at 4, 8, 16 or 24 h after UVB irradiation. The most prominent biological categories in lists of differentially regulated gene sets were extracted by functional enrichment analysis. With this approach, we determined that genes participating in two prime cellular processes, the ribosome pathway and the oxidative phosphorylation pathway, were persistently activated after UVB irradiation. Mitochondrial activity assays confirmed increased activity for up to 24 h after UVB irradiation. These results suggest that the persistent activation of ribosome and oxidative phosphorylation pathways may have a key role in UVB-radiation-induced cellular responses. For the first time, the specific cellular pathways that respond to UVB radiation consistently and persistently can be delineated with confidence using a loop-design microarray approach and functional bioinformatics analysis. The results of this study offer further insight into UVB-radiation-induced stress responses.

A regulator of Aspergillus fumigatus extracellular proteolytic activity is dispensable for virulence

Virulence of the fungal pathogen Aspergillus fumigatus is in part based on the saprophytic lifestyle that this mold has evolved. A crucial function for saprophytism resides in secreted proteases that allow assimilation of proteinaceous substrates. The impact of extracellular proteolytic activities on the pathogenesis of aspergillosis, however, remains controversial. In order to address this issue, characterization of a conserved regulatory factor, PrtT, that acts on expression of secreted proteases was pursued. Expression of PrtT appears to be regulated posttranscriptionally, and the existence of an mRNA leader sequence implies translational control via eIF2alpha kinase signaling. Phenotypic classification of a prtTDelta deletion mutant revealed that expression of several major extracellular proteases is PrtT dependent, resulting in the inability to utilize protein as a nutritional source. Certain genes encoding secreted proteases are not regulated by PrtT. Most strikingly, the deletant strain is not attenuated in virulence when tested in a leukopenic mouse model, which makes a strong case for reconsidering any impact of secreted proteases in pulmonary aspergillosis.

Genome-wide analysis of tRNA charging and activation of the eIF2 kinase Gcn2p

When cells are subjected to nutritional stress, uncharged tRNAs accumulate and activate Gcn2p phosphorylation of eukaryotic initiation factor-2 (eIF2) and the general amino acid control pathway. The Gcn2p regulatory domain homologous to histidyl-tRNA synthetases is proposed to bind to uncharged tRNA, directly contributing to activation of Gcn2p. Here we apply a microarray technology to analyze genome-wide changes in tRNA charging in yeast upon activation of Gcn2p in response to amino acid starvation and high salinity, a stress not directly linked to nutritional deficiency. This microarray technology is applicable for all eukaryotic cells. Strains were starved for histidine, leucine, or tryptophan and shown to rapidly induce Gcn2p phosphorylation of eIF2. The relative charging level of all tRNAs was measured before and after starvation, and Gcn2p activation and the intracellular levels of the starved amino acid correlate with the observed decrease in tRNA charging. Interestingly, in some cases, tRNAs not charged with the starved amino acid became deacylated more rapidly than tRNAs charged with the starved amino acid. This increase in uncharged tRNA levels occurred although the intracellular levels for these non-starved amino acids remained unchanged. Additionally, treatment of a wild-type strain with high salinity stress showed transient changes in the charging of several different tRNAs. These results suggest that Gcn2p can be activated by many different tRNA species in the cell. These results also depict a complex cellular relationship between tRNA charging, amino acid availability, and non-nutrient stress. These relationships are best revealed by simultaneous monitoring of the charging level of all tRNAs.

Halofuginone inhibits TH17 cell differentiation by activating the amino acid starvation response

A central challenge for improving autoimmune therapy is preventing inflammatory pathology without inducing generalized immunosuppression. T helper 17 (TH17) cells, characterized by their production of interleukin-17, have emerged as important and broad mediators of autoimmunity. Here we show that the small molecule halofuginone (HF) selectively inhibits mouse and human TH17 differentiation by activating a cytoprotective signaling pathway, the amino acid starvation response (AAR). Inhibition of TH17 differentiation by HF is rescued by the addition of excess amino acids and is mimicked by AAR activation after selective amino acid depletion. HF also induces the AAR in vivo and protects mice from TH17-associated experimental autoimmune encephalomyelitis. These results indicate that the AAR pathway is a potent and selective regulator of inflammatory T cell differentiation in vivo.

Molecular mechanisms involved in farnesol-induced apoptosis

The isoprenoid alcohol farnesol is an effective inducer of cell cycle arrest and apoptosis in a variety of carcinoma cell types. In addition, farnesol has been reported to inhibit tumorigenesis in several animal models suggesting that it functions as a chemopreventative and anti-tumor agent in vivo. A number of different biochemical and cellular processes have been implicated in the growth-inhibitory and apoptosis-inducing effects of farnesol. These include regulation of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase and CTP:phosphocholine cytidylyltransferase alpha (CCTalpha), rate-limiting enzymes in the mevalonate pathway and phosphatidylcholine biosynthesis, respectively, and the generation of reactive oxygen species. In some cell types the action of farnesol is mediated through nuclear receptors, including activation of farnesoid X receptor (FXR) and peroxisome proliferator-activated receptors (PPARs). Recent studies have revealed that induction of endoplasmic reticulum (ER) stress and the subsequent activation of the unfolded protein response (UPR) play a critical role in the induction of apoptosis by farnesol in lung carcinoma cells. This induction was found to be dependent on the activation of the MEK1/2-ERK1/2 pathway. In addition, farnesol induces activation of the NF-kappaB signaling pathway and a number of NF-kappaB target genes. Optimal activation of NF-kappaB was reported to depend on the phosphorylation of p65/RelA by the MEK1/2-MSK1 signaling pathway. In a number of cells farnesol-induced apoptosis was found to be linked to activation of the apoptosome. This review provides an overview of the biochemical and cellular processes regulated by farnesol in relationship to its growth-inhibitory, apoptosis-promoting, and anti-tumor effects.

eIF2alpha kinases GCN2 and PERK modulate transcription and translation of distinct sets of mRNAs in mouse liver

In eukaryotes, selective derepression of mRNA translation through altered utilization of upstream open reading frames (uORF) or internal ribosomal entry sites (IRES) regulatory motifs following exposure to stress is regulated at the initiation stage through the increased phosphorylation of eukaryotic initiation factor 2 on its alpha-subunit (eIF2alpha). While there is only one known eIF2alpha kinase in yeast, general control nonderepressible 2 (GCN2), mammals have evolved to express at least four: GCN2, heme-regulated inhibitor kinase (HRI), double-stranded RNA-activated protein kinase (PKR), and PKR-like endoplasmic reticulum-resident kinase (PERK). So far, the main known distinction among these four kinases is their activation in response to different acute stressors. In the present study, we used the in situ perfused mouse liver model and hybridization array analyses to assess the general translational response to stress regulated by two of these kinases, GCN2 and PERK, and to differentiate between the downstream effects of activating GCN2 versus PERK. The resulting data showed that at least 2.5% of mouse liver mRNAs are subject to derepressed translation following stress. In addition, the data demonstrated that eIF2alpha kinases GCN2 and PERK differentially regulate mRNA transcription and translation, which in the latter case suggests that increased eIF2alpha phosphorylation is not sufficient for derepression of translation. These findings open an avenue for more focused future research toward groups of mRNAs that code for the early cellular stress response proteins.

Elevated ATF4 expression, in the absence of other signals, is sufficient for transcriptional induction via CCAAT enhancer-binding protein-activating transcription factor response elements

Protein limitation in vivo or amino acid deprivation of cells in culture causes a signal transduction cascade consisting of activation of the kinase GCN2 (general control nonderepressible 2), phosphorylation of eukaryotic initiation factor 2, and increased synthesis of activating transcription factor (ATF) 4 by a translational control mechanism. In a self-limiting transcriptional program, ATF4 transiently activates a wide range of downstream target genes involved in transport, cellular metabolism, and other cell functions. Simultaneous activation of other signal transduction pathways by amino acid deprivation led to the question of whether or not the increased abundance of ATF4 alone was sufficient to trigger the transcriptional control mechanisms. Using 293 cells that ectopically express ATF4 in a tetracycline-inducible manner showed that ATF4 target genes were activated in the absence of amino acid deprivation. Ectopic expression of ATF4 alone resulted in effective recruitment of the general transcription machinery, but some reduction in histone modification was observed. These data document that ATF4 alone is sufficient to trigger the amino acid-responsive transcriptional control program. However, the absolute amount of ectopic ATF4 required to achieve the same degree of transcriptional activation observed after amino acid limitation was greater, suggesting that other factors may serve to enhance ATF4 function.

Leucine restriction inhibits chondrocyte proliferation and differentiation through mechanisms both dependent and independent of mTOR signaling

Linear growth in children is sensitive to nutritional status. Amino acids, in particular leucine, have been shown to regulate cell growth, proliferation, and differentiation through the mammalian target of rapamycin (mTOR), a nutrient-sensing protein kinase. Having recently demonstrated a role for mTOR in chondrogenesis, we hypothesized that leucine restriction, acting through mTOR, would inhibit growth plate chondrocyte proliferation and differentiation. The effect of leucine restriction was compared with that of the specific mTOR inhibitor, rapamycin. Leucine restriction produced a dose-dependent inhibition of fetal rat metatarsal explant growth. This was accounted by reduced cell proliferation and hypertrophy but not apoptosis. mTOR activity, as reflected by ribosomal protein S6 phosphorylation, was only partially inhibited by leucine restriction, whereas rapamycin abolished S6 phosphorylation. In chondrogenic ATDC5 cells, leucine restriction inhibited cell number, proteoglycan accumulation, and collagen X expression despite minimal inhibition of mTOR. Microarray analysis demonstrated that the effect of leucine restriction on ATDC5 cell gene expression differed from that of rapamycin. Out of 1,571 genes affected by leucine restriction and 535 genes affected by rapamycin, only 176 genes were affected by both. These findings indicate that the decreased chondrocyte growth and differentiation associated with leucine restriction is only partly attributable to inhibition of mTOR signaling. Thus nutrient restriction appears to directly modulate bone growth through unidentified mTOR-independent mechanisms in addition to the well-characterized mTOR nutrient-sensing pathway.

Cellular stress/the unfolded protein response: relevance to sleep and sleep disorders

Recent transcript profiling and microarray studies are beginning to unveil some of the mysteries of sleep. One of the most important clues has been the identification of the endoplasmic reticulum (ER) resident chaperone, immunoglobulin binding protein (BiP), that increases with sleep deprivation in all species studied. BiP, an ER resident chaperone, is the key cellular marker and master regulator of a signaling pathway called the ER stress response or unfolded protein response. The ER stress response occurs in 3 phases. It is healthy, protective and adaptive when the ER stress is moderate. Failure of the adaptive response leads to the activation of an inflammatory response. When the ER stress burden is great and prolonged, executioner pathways are activated. Collectively this work provides new evidence that modest sleep deprivation induces cellular stress that activates an adaptive response. Aging tilts the response to sleep deprivation from one that is adaptive and protective to one that is maladaptive. Understanding the pathways activated by sleep loss and the mechanisms by which they occur will allow the development of therapies to protect the brain during prolonged wakefulness and specifically in sleep disorders including those associated with aging.

DNA Methyltransferase protein synthesis is reduced in CXXC finger protein 1-deficient embryonic stem cells

CXXC finger protein 1 (CFP1) binds to unmethylated CpG dinucleotides and is required for embryogenesis. CFP1 is also a component of the Setd1A and Setd1B histone H3K4 methyltransferase complexes. Murine embryonic stem (ES) cells lacking CFP1 fail to differentiate, and exhibit a 70% reduction in global genomic cytosine methylation and a 50% reduction in DNA methyltransferase (DNMT1) protein and activity. This study investigated the underlying mechanism for reduced DNMT1 expression in CFP1-deficient ES cells. DNMT1 transcript levels were significantly elevated in ES cells lacking CFP1, despite the observed reduction in DNMT1 protein levels. To address the posttranscriptional mechanisms by which CFP1 regulates DNMT1 protein activity, pulse/chase analyses were carried out, demonstrating a modest reduction in DNMT1 protein half-life in CFP1-deficient ES cells. Additionally, global protein synthesis was decreased in ES cells lacking CFP1, contributing to a reduction in the synthesis of DNMT1 protein. ES cells lacking CFP1 were found to contain elevated levels of phosphorylated eIF2alpha, and an accompanying reduction in translation initiation as revealed by a lower level of polyribosomes. These results reveal a novel role for CFP1 in the regulation of translation initiation, and indicate that loss of CFP1 function leads to decreased DNMT1 protein synthesis and half-life.

PfeIK1, a eukaryotic initiation factor 2alpha kinase of the human malaria parasite Plasmodium falciparum, regulates stress-response to amino-acid starvation

Background

Post-transcriptional control of gene expression is suspected to play an important role in malaria parasites. In yeast and metazoans, part of the stress response is mediated through phosphorylation of eukaryotic translation initiation factor 2α (eIF2α), which results in the selective translation of mRNAs encoding stress-response proteins.

Methods

The impact of starvation on the phosphorylation state of PfeIF2α was examined. Bioinformatic methods were used to identify plasmodial eIF2α kinases. The activity of one of these, PfeIK1, was investigated using recombinant protein with non-physiological substrates and recombinant PfeIF2α. Reverse genetic techniques were used to disrupt the pfeik1 gene.

Results

The data demonstrate that the Plasmodium falciparum eIF2α orthologue is phosphorylated in response to starvation, and provide bioinformatic evidence for the presence of three eIF2α kinases in P. falciparum, only one of which (PfPK4) had been described previously. Evidence is provided that one of the novel eIF2α kinases, PfeIK1, is able to phosphorylate the P. falciparum eIF2α orthologue in vitro. PfeIK1 is not required for asexual or sexual development of the parasite, as shown by the ability of pfeik1^- parasites to develop into sporozoites. However, eIF2α phosphorylation in response to starvation is abolished in pfeik1^- asexual parasites

Conclusion

This study strongly suggests that a mechanism for versatile regulation of translation by several kinases with a similar catalytic domain but distinct regulatory domains, is conserved in P. falciparum.

Eukaryotic initiation factor 6 mediates a continuum between 60S ribosome biogenesis and translation

Eukaryotic ribosome biogenesis and translation are linked processes that limit the rate of cell growth. Although ribosome biogenesis and translation are mainly controlled by distinct factors, eukaryotic initiation factor 6 (eIF6) has been found to regulate both processes. eIF6 is a necessary protein with a unique anti-association activity, which prevents the interaction of 40S ribosomal subunits with 60S subunits through its binding to 60S ribosomes. In the nucleolus, eIF6 is a component of the pre-ribosomal particles and is required for the biogenesis of 60S subunits, whereas in the cytoplasm it mediates translation downstream from growth factors. The translational activity of eIF6 could be due to its anti-association properties, which are regulated by post-translational modifications; whether this anti-association activity is required for the biogenesis and nuclear export of ribosomes is unknown. eIF6 is necessary for tissue-specific growth and oncogene-driven transformation, and could be a new rate-limiting step for the initiation of translation.

Amino acids as regulators of gene expression in mammals: molecular mechanisms

In mammals, the impact of nutrients on gene expression has become an important area of research. Because amino acids have multiple and important functions, their homeostasis has to be finely maintained. However, amino acidemia can be affected in some nutritional conditions and by various forms of stress. Consequently, mammals have to adjust physiological functions involved in the adaptation to amino acid availability. Part of this regulation involves the modulation of numerous gene expression. It has been shown that amino acids by themselves can modify the expression of target genes. This review focuses on the recent advances in the understanding of the mechanisms involved in the control of mammalian gene expression in response to amino acid limitation.