A bifunctional intronic element regulates the expression of the arginine/lysine transporter Cat-1 via mechanisms involving the purine-rich element binding protein A (Pur alpha)

Identification of amino acid response element of SLC38A9 as an ATF4-binding site in porcine skeletal muscle cells

Amino acids can affect protein synthesis by activating mammalian target of rapamycin complex 1 (mTORC1) signaling pathway. Amino acid transporters SLC38A9 on the lysosomal membrane not only transport amino acids, but also can sense amino acids and activate mTORC1 signaling pathway. Activating transcription factor 4 (ATF4) can promote the expression of amino acid transporters by binding with amino acid response element (AARE). In this study, two AAREs were found in the SLC38A9 promoter region of pig, and both of them bound to ATF4. The AARE in the first intron was located in the core promoter region of SLC38A9. ATF4 regulated mRNA expression level of SLC38A9 in porcine skeletal muscle cells. In the absence of amino acids, the expression of ATF4 decreased and the expression of SLC38A9 increased. After leucine addition, the expression levels of ATF4 and SLC38A9 increased. It suggested that in the absence of amino acids, the expression of SLC38A9 was increased via binding of ATF4 to AARE binding factors in SLC38A9 promoter fragment; after the addition of leucine, ATF4 was activated, resulting in the increase of SLC38A9 expression.

Hepatitis B virus X protein-induced upregulation of CAT-1 stimulates proliferation and inhibits apoptosis in hepatocellular carcinoma cells

The HBx protein of hepatitis B virus (HBV) is widely recognized to be a critical oncoprotein contributing to the development of HBV-related hepatocellular carcinoma (HCC). In addition, cationic amino acid transporter 1 (CAT-1) gene is a target of miR-122. In this study, we found that CAT-1 protein levels were higher in HBV-related HCC carcinomatous tissues than in para-cancerous tumor tissues, and that CAT-1 promoted HCC cell growth, proliferation, and metastasis. Moreover, HBx-induced decreases in Gld2 and miR-122 levels that contributed to the upregulation of CAT-1 in HCC. These results indicate that a Gld2/miR-122/CAT-1 pathway regulated by HBx likely participates in HBV-related hepatocellular carcinogenesis.

Activation of GCN2 kinase by ribosome stalling links translation elongation with translation initiation

Ribosome stalling during translation has recently been shown to cause neurodegeneration, yet the signaling pathways triggered by stalled elongation complexes are unknown. To investigate these pathways we analyzed the brain of C57BL/6J-Gtpbp2^nmf205-/- mice in which neuronal elongation complexes are stalled at AGA codons due to deficiencies in a tRNA^Arg_UCU tRNA and GTPBP2, a mammalian ribosome rescue factor. Increased levels of phosphorylation of eIF2α (Ser51) were detected prior to neurodegeneration in these mice and transcriptome analysis demonstrated activation of ATF4, a key transcription factor in the integrated stress response (ISR) pathway. Genetic experiments showed that this pathway was activated by the eIF2α kinase, GCN2, in an apparent deacylated tRNA-independent fashion. Further we found that the ISR attenuates neurodegeneration in C57BL/6J-Gtpbp2^nmf205-/- mice, underscoring the importance of cellular and stress context on the outcome of activation of this pathway. These results demonstrate the critical interplay between translation elongation and initiation in regulating neuron survival during cellular stress.

DOI:http://dx.doi.org/10.7554/eLife.14295.001

Information stored in DNA is used to make proteins in a two-step process. First, the DNA is copied to make molecules of messenger ribonucleic acid (or messenger RNA for short). Next, machines called ribosomes use the messenger RNAs as templates to assemble chains of amino acids – the building blocks of proteins – in a process called translation. Another type of RNA molecule called transfer RNA carries each amino acid to the ribosomes. If a specific transfer RNA is not available for translation at the right time, the ribosome might stall as it moves along the messenger RNA. At this point, the ribosome needs to be restarted or it will fall off the mRNA without finishing the protein.

In 2014, a group of researchers reported that certain types of brain cells are very sensitive to ribosome stalling, and tend to die if translation does not continue. A protein called GTPBP2 was shown to play an important role in restarting stalled ribosomes in these cells. Here, Ishimura, Nagy et al. – including some of the researchers from the earlier work – investigated the molecular pathways that ribosome stalling triggers in brain cells using mutant mice that lacked the GTPBP2 protein. The experiments show that ribosome stalling activates an enzyme known as GCN2, which was already known to sense other types of malfunctions in cellular processes.

Ishimura, Nagy et al. also show that GCN2 triggers stress responses in the cells by activating a communication system called the ATF4 pathway. This pathway protects the cells from damage, and its absence results in more rapid cell deterioration and death. The next challenges are to understand the exact mechanism by which GCN2 senses stalled ribosomes, and to find out how ribosome stalling causes the death of brain cells.

DOI:http://dx.doi.org/10.7554/eLife.14295.002

Insulin requires A1 adenosine receptors expression to reverse gestational diabetes-increased L-arginine transport in human umbilical vein endothelium

Gestational diabetes mellitus (GDM) associates with increased L-arginine transport and extracellular concentration of adenosine in human umbilical vein endothelial cells (HUVECs). In this study we aim to determine whether insulin reverses GDM-increased L-arginine transport requiring adenosine receptors expression in HUVECs. Primary cultured HUVECs from full-term normal (n = 38) and diet-treated GDM (n = 38) pregnancies were used. Insulin effect was assayed on human cationic amino acid transporter 1 (hCAT1) expression (protein, mRNA, SLC7A1 promoter activity) and activity (initial rates of L-arginine transport) in the absence or presence of adenosine receptors agonists or antagonists. A1 adenosine receptors (A1AR) and A2AAR expression (Western blot, quantitative PCR) was determined. Experiments were done in cells expressing or siRNA-suppressed expression of A1AR or A2AAR. HUVECs from GDM exhibit higher maximal transport capacity (maximal velocity (V max)/apparent Michaelis Menten constant (K m), V max/K m), which is blocked by insulin by reducing the V max to values in cells from normal pregnancies. Insulin also reversed the GDM-associated increase in hCAT-1 protein abundance and mRNA expression, and SLC7A1 promoter activity for the fragment -606 bp from the transcription start point. Insulin effects required A1AR, but not A2AAR expression and activity in this cell type. In the absence of insulin, GDM-increased hCAT-1 expression and activity required A2AAR expression and activity. HUVECs from GDM pregnancies exhibit a differential requirement of A1AR or A2AAR depending on the level of insulin, a phenomenon that represent a condition where adenosine or analogues of this nucleoside could be acting as helpers of insulin biological effects in GDM.

Insulin reverses D-glucose-increased nitric oxide and reactive oxygen species generation in human umbilical vein endothelial cells

Vascular tone is controlled by the L-arginine/nitric oxide (NO) pathway, and NO bioavailability is strongly affected by hyperglycaemia-induced oxidative stress. Insulin leads to high expression and activity of human cationic amino acid transporter 1 (hCAT-1), NO synthesis and vasodilation; thus, a protective role of insulin on high D-glucose-alterations in endothelial function is likely. Vascular reactivity to U46619 (thromboxane A2 mimetic) and calcitonin gene related peptide (CGRP) was measured in KCl preconstricted human umbilical vein rings (wire myography) incubated in normal (5 mmol/L) or high (25 mmol/L) D-glucose. hCAT-1, endothelial NO synthase (eNOS), 42 and 44 kDa mitogen-activated protein kinases (p42/44mapk), protein kinase B/Akt (Akt) expression and activity were determined by western blotting and qRT-PCR, tetrahydrobiopterin (BH4) level was determined by HPLC, and L-arginine transport (0-1000 μmol/L) was measured in response to 5-25 mmol/L D-glucose (0-36 hours) in passage 2 human umbilical vein endothelial cells (HUVECs). Assays were in the absence or presence of insulin and/or apocynin (nicotinamide adenine dinucleotide phosphate-oxidase [NADPH oxidase] inhibitor), tempol or Mn(III)TMPyP (SOD mimetics). High D-glucose increased hCAT-1 expression and activity, which was biphasic (peaks: 6 and 24 hours of incubation). High D-glucose-increased maximal transport velocity was blocked by insulin and correlated with lower hCAT-1 expression and SLC7A1 gene promoter activity. High D-glucose-increased transport parallels higher reactive oxygen species (ROS) and superoxide anion (O2•-) generation, and increased U46619-contraction and reduced CGRP-dilation of vein rings. Insulin and apocynin attenuate ROS and O2•- generation, and restored vascular reactivity to U46619 and CGRP. Insulin, but not apocynin or tempol reversed high D-glucose-increased NO synthesis; however, tempol and Mn(III)TMPyP reversed the high D-glucose-reduced BH4 level. Insulin and tempol blocked the high D-glucose-increased p42/44mapk phosphorylation. Vascular dysfunction caused by high D-glucose is likely attenuated by insulin through the L-arginine/NO and O2•-/NADPH oxidase pathways. These findings are of interest for better understanding vascular dysfunction in states of foetal insulin resistance and hyperglycaemia.

Repeat-mediated genetic and epigenetic changes at the FMR1 locus in the Fragile X-related disorders

The Fragile X-related disorders are a group of genetic conditions that include the neurodegenerative disorder, Fragile X-associated tremor/ataxia syndrome (FXTAS), the fertility disorder, Fragile X-associated primary ovarian insufficiency (FXPOI) and the intellectual disability, Fragile X syndrome (FXS). The pathology in all these diseases is related to the number of CGG/CCG-repeats in the 5′ UTR of the Fragile X mental retardation 1 (FMR1) gene. The repeats are prone to continuous expansion and the increase in repeat number has paradoxical effects on gene expression increasing transcription on mid-sized alleles and decreasing it on longer ones. In some cases the repeats can simultaneously both increase FMR1 mRNA production and decrease the levels of the FMR1 gene product, Fragile X mental retardation 1 protein (FMRP). Since FXTAS and FXPOI result from the deleterious consequences of the expression of elevated levels of FMR1 mRNA and FXS is caused by an FMRP deficiency, the clinical picture is turning out to be more complex than once appreciated. Added complications result from the fact that increasing repeat numbers make the alleles somatically unstable. Thus many individuals have a complex mixture of different sized alleles in different cells. Furthermore, it has become apparent that the eponymous fragile site, once thought to be no more than a useful diagnostic criterion, may have clinical consequences for females who inherit chromosomes that express this site. This review will cover what is currently known about the mechanisms responsible for repeat instability, for the repeat-mediated epigenetic changes that affect expression of the FMR1 gene, and for chromosome fragility. It will also touch on what current and future options are for ameliorating some of these effects.

Salubrinal protects against rotenone-induced SH-SY5Y cell death via ATF4-parkin pathway

Parkinson's disease (PD) is a progressive neurodegenerative disorder, for which there are no effective disease-modifying therapies. Growing evidence from studies in human PD brain, in addition to genetic and toxicological models, indicates that endoplasmic reticulum (ER) stress is a common feature of the disease and contributes to neurodegeneration. We examine whether salubrinal, a ER stress inhibitor, can protect the rotenone-induced SH-SY5Y cell death and explore the mechanisms underlying this protection. Our results demonstrated that rotenone induced a significant ER stress response and caused cell apoptosis, which was inhibited by salubrinal. Activating transcription factor 4 (ATF4), a member of the ATF/CREB family of basic leucine zipper transcription factors, has been implicated in the pathogenesis of neurodegeneration. We showed that salubrinal increased the up-regulation of ATF4 expression. An ATF4 siRNA significantly increased the rotenone cytotoxicity and decreased the salubrinal's protection. Further, we showed that ATF4 siRNA inhibited the expression of parkin, and parkin knockdown similarly aggravated the rotenone cytotoxicity and reduced the salubrinal's protection. Additionally, the protein level of parkin was declined after treatment with rotenone, whereas this reduction was rescued by salubrinal. These findings indicate ATF4-parkin pathway plays an important role in the salubrinal-mediated neuroprotection of rotenone-induced dopaminergic cell death.

Overexpression of arginine transporter CAT-1 is associated with accumulation of L-arginine and cell growth in human colorectal cancer tissue

We previously showed that L-arginine (Arg) accumulates in colorectal cancer tissues. The aim of this study was to investigate the mechanism by which Arg accumulates and determine its biological significance. The concentration of Arg and Citrulline (Cit) in sera and tumor tissues from colorectal cancer (CRC) patients was analyzed by high-performance liquid chromatography (HPLC). The expression of Arg transporters was analyzed by quantitative reverse transcription polymerase chain reaction (qRT-PCR) and immunohistochemical analysis of tissue microarray. We also transfected the colon cancer cell line HCT-116 with siRNA specific for the Arg transporter CAT-1 and measured the induction of apoptosis by flow cytometry and cell proliferation by MTT assay. Consistent with our previous results, serum Arg and Cit concentrations in colorectal cancer patients were significantly lower than those in normal volunteers, while Arg and Cit concentrations in colorectal cancer tissues were significantly higher than in matched adjacent normal colon tissues. Quantitative RT-PCR showed that the CAT-1 gene was highly overexpressed in 70.5% of colorectal cancer tissue samples relative to adjacent normal colon tissues in all 122 patients with colorectal cancer. Immunohistochemical analysis of tissue microarray confirmed that the expression of CAT-1 was higher in all 25 colorectal cancer tissues tested. CAT-1 siRNA significantly induced apoptosis of HCT-116 cells and subsequently inhibited cell growth by 20–50%. Our findings indicate that accumulation of L-Arg and Cit and cell growth in colorectal cancer tissues is associated with over-expression of the Arg transporter gene CAT-1. Our results may be useful for the development of molecular diagnostic tools and targeted therapy for colorectal cancer.

ER-stress-induced transcriptional regulation increases protein synthesis leading to cell death

Protein misfolding in the endoplasmic reticulum (ER) leads to cell death through PERK-mediated phosphorylation of eIF2α, although the mechanism is not understood. ChIP-seq and mRNA-seq of activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP), key transcription factors downstream of p-eIF2α, demonstrated that they interact to directly induce genes encoding protein synthesis and the unfolded protein response, but not apoptosis. Forced expression of ATF4 and CHOP increased protein synthesis and caused ATP depletion, oxidative stress and cell death. The increased protein synthesis and oxidative stress were necessary signals for cell death. We show that eIF2α-phosphorylation-attenuated protein synthesis, and not Atf4 mRNA translation, promotes cell survival. These results show that transcriptional induction through ATF4 and CHOP increases protein synthesis leading to oxidative stress and cell death. The findings suggest that limiting protein synthesis will be therapeutic for diseases caused by protein misfolding in the ER.

Molecular symbiosis of CHOP and C/EBP beta isoform LIP contributes to endoplasmic reticulum stress-induced apoptosis

Induction of the transcription factor CHOP (CCAAT-binding homologous protein; GADD 153) is a critical cellular response for the transcriptional control of endoplasmic reticulum (ER) stress-induced apoptosis. Upon nuclear translocation, CHOP upregulates the transcription of proapoptotic factors and downregulates antiapoptotic genes. Transcriptional activation by CHOP involves heterodimerization with other members of the basic leucine zipper transcription factor (bZIP) family. We show that the bZIP protein C/EBP beta isoform LIP is required for nuclear translocation of CHOP during ER stress. In early ER stress, LIP undergoes proteasomal degradation in the cytoplasmic compartment. During later ER stress, LIP binds CHOP in both cytoplasmic and nuclear compartments and contributes to its nuclear import. By using CHOP-deficient cells and transfections of LIP-expressing vectors in C/EBP beta(-/-) mouse embryonic fibroblasts (MEFs), we show that the LIP-CHOP interaction has a stabilizing role for LIP. At the same time, CHOP uses LIP as a vehicle for nuclear import. LIP-expressing C/EBP beta(-/-) MEFs showed enhanced ER stress-induced apoptosis compared to C/EBP beta-null cells, a finding in agreement with the decreased levels of Bcl-2, a known transcriptional control target of CHOP. In view of the positive effect of CHOP-LIP interaction in mediating their proapoptotic functions, we propose this functional cooperativity as molecular symbiosis between proteins.

Temporal regulation of Cat-1 (cationic amino acid transporter-1) gene transcription during endoplasmic reticulum stress

Expression of the Cat-1 gene (cationic amino acid transporter-1) is induced in proliferating cells and in response to a variety of stress conditions. The expression of the gene is mediated via a TATA-less promoter. In the present study we show that an Sp1 (specificity protein 1)-binding site within a GC-rich region of the Cat-1 gene controls its basal expression and is important for induction of the gene during the UPR (unfolded protein response). We have shown previously that induction of Cat-1 gene expression during the UPR requires phosphorylation of the translation initiation factor eIF2alpha (eukaryotic initiation factor 2alpha) by PERK (protein-kinase-receptor-like endoplasmic reticulum kinase), one of the signalling pathways activated during the UPR. This leads to increased translation of the transcription factor ATF4 (activating transcription factor 4). We also show that a second signalling pathway is required for sustained transcriptional induction of the Cat-1 gene during the UPR, namely activation of IRE1 (inositol-requiring enzyme 1) leading to alternative splicing of the mRNA for the transcription factor XBP1 (X-box-binding protein 1). The resulting XBP1s (spliced XBP1) can bind to an ERSE (endoplasmic-reticulum-stress-response-element), ERSE-II-like, that was identified within the Cat-1 promoter. Surprisingly, eIF2alpha phosphorylation is required for accumulation of XBP1s. We propose that the signalling via phosphorylated eIF2alpha is required for maximum induction of Cat-1 transcription during the UPR by inducing the accumulation of both ATF4 and XBP1s.