CENPA promotes glutamine metabolism and tumor progression by up-regulating SLC38A1 in endometrial cancer

Glutamine addiction is a significant hallmark of metabolic reprogramming in tumors and is crucial to the progression of cancer. Nevertheless, the regulatory mechanisms of glutamine metabolism in endometrial cancer (EC) remains elusive. In this research, we found that elevated expression of CENPA and solute carrier family 38 member 1 (SLC38A1) were firmly associated with worse clinical stage and unfavorable outcomes in EC patients. In addition, ectopic overexpression or silencing of CENPA could either enhance or diminish glutamine metabolism and tumor progression in EC. Mechanistically, CENPA directly regulated the transcriptional activity of the target gene, SLC38A1, leading to enhanced glutamine uptake and metabolism, thereby promoting EC progression. Notably, a prognostic model utilizing the expression levels of CENPA and SLC38A1 genes independently emerged as a prognostic factor for EC. More importantly, CENPA and SLC38A1 were significantly elevated and positively correlated, as well as indicative of poor prognosis in multiple cancers. In brief, our study confirmed that CENPA is a critical transcription factor involved in glutamine metabolism and tumor progression through modulating SLC38A1. This revelation suggests that targeting CENPA could be an appealing therapeutic approach to address pan-cancer glutamine addiction.

circTADA2A inhibited SLC38A1 expression and suppresses melanoma progression through the prevention of CNBP trans-activation

BACKGROUND

CircTADA2A has been demonstrated to play critical roles in the occurrence and development of human cancer. However, the expression pattern and biological mechanisms of circTADA2A in melanoma remains largely unknown.

METHODS

CircTADA2A were detected by quantitative real-time RT-PCR (qRT-PCR) and validated by Sanger sequencing. Function of circTADA2A and its protein partner in melanoma cells was investigated using RNA interference and overexpression assays. Interaction of circTADA2A, CCHC-type zinc finger nucleic acid binding protein (CNBP) and solute carrier family 38 member 1 (SLC38A1) was confirmed by RNA immunoprecipitation, RNA pull-down, and dual-luciferase reporter assay. The expression of genes and proteins were detected by qRT-PCR and western blot assays.

RESULTS

Data from the investigation showed that a novel circRNA (circTADA2A, hsa_circ_0043278) was markedly downregulated in melanoma cells. Functionally, circTADA2A repressed cell proliferation, migration, invasion in melanoma cells. Mechanistically, circTADA2A interacted with CNBP, acting to suppress the binding of CNBP to the SLC38A1 promoter and subsequently restrained SLC38A1 transcription, which resulting in repression of melanoma progression.

CONCLUSIONS

CircTADA2A suppresses melanoma progression by regulating CNBP/SLC38A1 axis, indicating a potential therapeutic target in melanoma.

CHOP upregulation and dysregulation of the mature form of the SNAT2 amino acid transporter in the placentas from small for gestational age newborns

BACKGROUND

The placentas from newborns that are small for gestational age (SGA; birth weight < -2 SD for gestational age) may display multiple pathological characteristics. A key determinant of fetal growth and, therefore, birth weight is placental amino acid transport, which is under the control of the serine/threonine kinase mechanistic target of rapamycin (mTOR). The effects of endoplasmic reticulum (ER) stress on the mTOR pathway and the levels of amino acid transporters are not well established.

METHODS

Placentas from SGA and appropriate for gestational age (AGA) newborns and the human placental BeWo cell line exposed to the ER stressor tunicamycin were used.

RESULTS

We detected a significant increase in the levels of C/EBP homologous protein (CHOP) in the placentas from SGA newborns compared with those from AGA newborns, while the levels of other ER stress markers were barely affected. In addition, placental mTOR Complex 1 (mTORC1) activity and the levels of the mature form of the amino acid transporter sodium-coupled neutral amino acid transporter 2 (SNAT2) were also reduced in the SGA group. Interestingly, CHOP has been reported to upregulate growth arrest and DNA damage-inducible protein 34 (GADD34), which in turn suppresses mTORC1 activity. The GADD34 inhibitor guanabenz attenuated the increase in CHOP protein levels and the reduction in mTORC1 activity caused by the ER stressor tunicamycin in the human placental cell line BeWo, but it did not recover mature SNAT2 protein levels, which might be reduced as a result of defective glycosylation.

CONCLUSIONS

Collectively, these data reveal that GADD34A activity and glycosylation are key factors controlling mTORC1 signaling and mature SNAT2 levels in trophoblasts, respectively, and might contribute to the SGA condition. Video Abstract.

Stress-induced perturbations in intracellular amino acids reprogram mRNA translation in osmoadaptation independently of the ISR

The integrated stress response (ISR) plays a pivotal role in adaptation of translation machinery to cellular stress. Here, we demonstrate an ISR-independent osmoadaptation mechanism involving reprogramming of translation via coordinated but independent actions of mTOR and plasma membrane amino acid transporter SNAT2. This biphasic response entails reduced global protein synthesis and mTOR signaling followed by translation of SNAT2. Induction of SNAT2 leads to accumulation of amino acids and reactivation of mTOR and global protein synthesis, paralleled by partial reversal of the early-phase, stress-induced translatome. We propose SNAT2 functions as a molecular switch between inhibition of protein synthesis and establishment of an osmoadaptive translation program involving the formation of cytoplasmic condensates of SNAT2-regulated RNA-binding proteins DDX3X and FUS. In summary, we define key roles of SNAT2 in osmotolerance.

ChREBP downregulates SNAT2 amino acid transporter expression through interactions with SMRT in response to a high-carbohydrate diet

Carbohydrate responsive element-binding protein (ChREBP) has been identified as a primary transcription factor that maintains energy homeostasis through transcriptional regulation of glycolytic, lipogenic, and gluconeogenic enzymes in response to a high-carbohydrate diet. Amino acids are important substrates for gluconeogenesis, but nevertheless, knowledge is lacking about whether this transcription factor regulates genes involved in the transport or use of these metabolites. Here, we demonstrate that ChREBP represses the expression of the amino acid transporter sodium-coupled neutral amino acid transporter 2 (SNAT2) in response to a high-sucrose diet in rats by binding to a carbohydrate response element (ChoRE) site located -160 bp upstream of the transcriptional start site in the SNAT2 promoter region. Additionally, immunoprecipitation assays revealed that ChREBP and silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) interact with each other, as part of the complex that repress SNAT2 expression. The interaction between these proteins was confirmed by an in vivo chromatin immunoprecipitation assay. These findings suggest that glucogenic amino acid uptake by the liver is controlled by ChREBP through the repression of SNAT2 expression in rats consuming a high-carbohydrate diet.NEW & NOTEWORTHY This study highlights the key role of carbohydrate responsive element-binding protein (ChREBP) in the fine-tuned regulation between glucose and amino acid metabolism in the liver via regulation of the amino acid transporter sodium-coupled neutral amino acid transporter 2 (SNAT2) expression after the consumption of a high-carbohydrate diet. ChREBP binds to a carbohydrate response element (ChoRE) site in the SNAT2 promoter region and recruits silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) corepressor to reduce SNAT2 transcription. This study revealed that ChREBP prevents the uptake of glucogenic amino acids upon the consumption of a high-carbohydrate diet.

Inhibition of the glutamine transporter SNAT1 confers neuroprotection in mice by modulating the mTOR-autophagy system

The pathophysiological role of mammalian target of rapamycin complex 1 (mTORC1) in neurodegenerative diseases is established, but possible therapeutic targets responsible for its activation in neurons must be explored. Here we identified solute carrier family 38a member 1 (SNAT1, Slc38a1) as a positive regulator of mTORC1 in neurons. Slc38a1flox/flox and Synapsin I-Cre mice were crossed to generate mutant mice in which Slc38a1 was selectively deleted in neurons. Measurement of 2,3,5-triphenyltetrazolium chloride (TTC) or the MAP2-negative area in a mouse model of middle cerebral artery occlusion (MCAO) revealed that Slc38a1 deficiency decreased infarct size. We found a transient increase in the phosphorylation of p70S6k1 (pp70S6k1) and a suppressive effect of rapamycin on infarct size in MCAO mice. Autophagy inhibitors completely mitigated the suppressive effect of SNAT1 deficiency on neuronal cell death under in vitro stroke culture conditions. These results demonstrate that SNAT1 promoted ischemic brain damage via mTOR-autophagy system.

Increased SLC38A4 Amino Acid Transporter Expression in Human Pancreatic α-Cells After Glucagon Receptor Inhibition

Plasma amino acids and their transporters constitute an important part of the feedback loop between the liver and pancreatic α-cell function, and glucagon regulates hepatic amino acid turnover. Disruption of hepatic glucagon receptor action activates the loop and results in high plasma amino acids and hypersecretion of glucagon associated with α-cell hyperplasia. In the present study, we report a technique to rescue implanted human pancreatic islets from the mouse kidney capsule. Using this model, we have demonstrated that expression of the amino acid transporter SLC38A4 increases in α-cells after administration of a glucagon receptor blocking antibody. The increase in SLC38A4 expression and associated α-cell proliferation was dependent on mechanistic target of rapamycin pathway. We confirmed increased α-cell proliferation and expression of SLC38A4 in pancreas sections from patients with glucagon cell hyperplasia and neoplasia (GCHN) with loss-of-function mutations in the glucagon receptor. Collectively, using a technique to rescue implanted human islets from the kidney capsule in mice and pancreas sections from patients with GCHN, we found that expression of SLC38A4 was increased under conditions of disrupted glucagon receptor signaling. These data provide support for the existence of a liver-human α-cell endocrine feedback loop.

MicroRNA-593-3p regulates insulin-promoted glucose consumption by targeting Slc38a1 and CLIP3

Insulin plays an important role in the regulation of glucose metabolism. However, the molecular mechanisms involved are not entirely clarified. In this context, we found that miR-593-3p negatively regulates insulin-regulated glucose metabolism in hepatocellular carcinoma HepG2 and Bel7402 cells. We then identified Slc38a1 and CLIP3 as novel targets of miR-593-3p. Further studies demonstrated that Slc38a1 and CLIP3 mediate insulin-regulated glucose metabolism. Interestingly, we also demonstrated that miR-593-3p expression was negatively associated with Slc38a1 and CLIP3 expression in insulin-treated HepG2 cells, and insulin-induced Slc38a1 and CLIP3 expression via downregulation of miR-593-3p. Taken together, this study indicates that inhibition of miRNA-593-3p by insulin promotes glucose metabolism through the regulation of Slc38a1 and CLIP3 expression, and provides a new insight into the role and mechanism of insulin-induced glycolysis.

DNA-Binding Motif of the Imprinted Transcription Factor PEG3

Peg3 is an imprinted gene that is predicted to encode a DNA-binding zinc finger protein. This was previously demonstrated through Chromatin ImmunoPrecipitation-based Sequencing experiments. In the current study, we reanalyzed the previous ChIP-Seq results and further characterized the DNA-binding motif of PEG3. According to the results, PEG3 binds to the promoters and enhancers of a subset of genes that are closely associated with the known functions of Peg3. Some of these identified targets include Tufm, Mrpl45, Cry2, Per1, Slc25a29 and Slc38a2. With this set of targets, we derived a DNA-binding motif of PEG3, 5'-GTGGCAGT-3', which also provides a tabulated matrix that can be used for predicting other unknown genomic targets. Among the newly identified targets, we analyzed in detail the two loci, Slc38a2 and Slc38a4, which are known to be involved in neutral amino acid transport. The results indicated that PEG3 likely functions as a transcriptional repressor for these two loci. Overall, the current study provides a set of genomic targets and also redefines the DNA-binding motif for the imprinted transcription factor PEG3.

Characterization and Regulation of the Amino Acid Transporter SNAT2 in the Small Intestine of Piglets

The sodium-dependent neutral amino acid transporter 2 (SNAT2), which has dual transport/receptor functions, is well documented in eukaryotes and some mammalian systems, but has not yet been verified in piglets. The objective of this study was to investigate the characteristics and regulation of SNAT2 in the small intestine of piglets. The 1,521-bp porcine full cDNA sequence of SNAT2 (KC769999) from the small intestine of piglets was cloned. The open reading frame of cDNA encodes 506 deduced amino acid residues with a calculated molecular mass of 56.08 kDa and an isoelectric point (pI) of 7.16. Sequence alignment and phylogenetic analysis revealed that SNAT2 is highly evolutionarily conserved in mammals. SNAT2 mRNA can be detected in the duodenum, jejunum and ileum by real-time quantitative PCR. During the suckling period from days 1 to 21, the duodenum had the highest abundance of SNAT2 mRNA among the three segments of the small intestine. There was a significant decrease in the expression of SNAT2 mRNA in the duodenal and jejunal mucosa and in the expression of SNAT2 protein in the jejunal and ileal mucosa on day 1 after weaning (P < 0.05). Studies with enterocytes in vitro showed that amino acid starvation and supplementation with glutamate, arginine or leucine enhanced, while supplementation with glutamine reduced, SNAT2 mRNA expression (P < 0.05). These results regarding the characteristics and regulation of SNAT2 should help to provide some information to further clarify its roles in the absorption of amino acids and signal transduction in the porcine small intestine.

Regulation of glutamine carrier proteins by RNF5 determines breast cancer response to ER stress-inducing chemotherapies

Many tumor cells are fueled by altered metabolism and increased glutamine (Gln) dependence. We identify regulation of the L-glutamine carrier proteins SLC1A5 and SLC38A2 (SLC1A5/38A2) by the ubiquitin ligase RNF5. Paclitaxel-induced ER stress to breast cancer (BCa) cells promotes RNF5 association, ubiquitination, and degradation of SLC1A5/38A2. This decreases Gln uptake, levels of TCA cycle components, mTOR signaling, and proliferation while increasing autophagy and cell death. Rnf5-deficient MMTV-PyMT mammary tumors were less differentiated and showed elevated SLC1A5 expression. Whereas RNF5 depletion in MDA-MB-231 cells promoted tumorigenesis and abolished paclitaxel responsiveness, SLC1A5/38A2 knockdown elicited opposing effects. Inverse RNF5(hi)/SLC1A5/38A2(lo) expression was associated with positive prognosis in BCa. Thus, RNF5 control of Gln uptake underlies BCa response to chemotherapies.

Plasmodium falciparum malaria elicits inflammatory responses that dysregulate placental amino acid transport

Placental malaria (PM) can lead to poor neonatal outcomes, including low birthweight due to fetal growth restriction (FGR), especially when associated with local inflammation (intervillositis or IV). The pathogenesis of PM-associated FGR is largely unknown, but in idiopathic FGR, impaired transplacental amino acid transport, especially through the system A group of amino acid transporters, has been implicated. We hypothesized that PM-associated FGR could result from impairment of transplacental amino acid transport triggered by IV. In a cohort of Malawian women and their infants, the expression and activity of system A (measured by Na⁺-dependent ¹⁴C-MeAIB uptake) were reduced in PM, especially when associated with IV, compared to uninfected placentas. In an in vitro model of PM with IV, placental cells exposed to monocyte/infected erythrocytes conditioned medium showed decreased system A activity. Amino acid concentrations analyzed by reversed phase ultra performance liquid chromatography in paired maternal and cord plasmas revealed specific alterations of amino acid transport by PM, especially with IV. Overall, our data suggest that the fetoplacental unit responds to PM by altering its placental amino acid transport to maintain adequate fetal growth. However, IV more profoundly compromises placental amino acid transport function, leading to FGR. Our study offers the first pathogenetic explanation for FGR in PM.

Malaria infection during pregnancy can cause fetal growth restriction and low birthweight associated with high infant mortality and morbidity rates. The pathogenesis of fetal growth restriction in placental malaria is largely unknown, but in other pathological pregnancies, impaired transplacental amino acid transport has been implicated. In a cohort of Malawian women and their infants, we found that placental malaria, especially when associated with local inflammation, was associated with decreased expression and activity of an important group of amino acid placental transporters. Using an in vitro model of placental malaria with local inflammation, we discovered that maternal monocyte products could impair the activity of amino acid transporters on placental cells. Amino acid concentrations in paired maternal and cord plasmas revealed specific alterations of amino acid transport by placental malaria, especially with local inflammation. Overall, our data suggest that, more than malaria infection per se, the local inflammation it triggers compromises placental amino acid transport function, leading to fetal growth restriction. Greater understanding of the mechanisms involved, combined with interventions to improve fetal growth in malaria, are important priorities in areas of the world where the co-existence of malaria and maternal malnutrition threatens the health and lives of millions of young babies.

Association of the polymorphisms 292 C>T and 1304 G>A in the SLC38A4 gene with hyperglycaemia

BACKGROUND

The SLC38A4 gene is related to system 'A' activity, which seems to be related to impaired gluconeogenesis. The objective of this study was to determine whether the 292 C>T and 1304 G>A polymorphisms of SLC38A4 gene are associated with hyperglycaemia in humans.

METHODS

A total of 227 individuals were enrolled in a case-control study, in which hyperglycaemia was defined by plasma glucose levels ≥95 mg/dL. Genotyping was carried out by using real-time polymerase chain reaction.

RESULTS

The frequency of mutant alleles of SLC38A4 gene for single-nucleotide polymorphism (SNP) 1304 G>A was 23.6% and 30.2% for SNP 292 C>T. The frequency of allele T for the SNP 292 C>T in the case and control groups did not show significant differences, whereas the frequency of allele A for the SNP 1304 G>A was significantly higher in the case group than in the control group (p = 0.04). In the logistic regression analysis, the SNP 1304 G>A [odds ratio (OR) 1.78; 95%CI 1.04-3.05, p = 0.03] but not SNP 292 C>T (OR 1.41; 95%CI 0.80-2.47, p = 0.23) showed a significant association with hyperglycaemia. After adjusting by body mass index, waist circumference and triglycerides, the SNP 1304 G>A remained significantly associated with hyperglycaemia (OR 2.13; 95%CI 1.18-3.83, p = 0.03). Pair wise linkage disequilibrium showed correlation (D' > 0.82) between 292 C>T and 1304 G>A SNPs. Haplotype association with hyperglycaemia also showed significant association between both homozygous mutant alleles (A/T) and hyperglycaemia (OR 1.68; 95%CI 1.01-2.79, p = 0.048).

CONCLUSIONS

Our results suggest that mutant allele A for SNP 1304 G>A of SLC38A4 gene is associated with hyperglycaemia.

The specific contributions of cohesin-SA1 to cohesion and gene expression: implications for cancer and development

Besides its well-established role in sister chromatid cohesion, cohesin has recently emerged as major player in the organization of interphase chromatin. Such important function is related to its ability to entrap two DNA segments also in cis, thereby facilitating long-range DNA looping which is crucial for transcriptional regulation, organization of replication factories and V(D)J recombination. Vertebrate somatic cells have two different versions of cohesin, containing Smc1, Smc3, Rad21/Scc1 and either SA1 or SA2, but their functional specificity has been largely ignored. We recently generated a knockout mouse model for the gene encoding SA1, and found that this protein is essential to complete embryonic development. Cohesin-SA1 mediates cohesion at telomeres, which is required for their replication. Telomere defects in SA1- deficient cells provoke chromosome segregation errors resulting in aneuploidy despite robust centromere cohesion. This aneuploidy could explain why heterozygous animals have an earlier onset of tumorigenesis. In addition, the genome-wide distribution of cohesin changes dramatically in the absence of SA1, and the complex shows reduced accumulation at promoters and CTCF sites. As a consequence, gene expression is altered, leading to downregulation of biological processes related to a developmental disorder linked to cohesin function, the Cornelia de Lange Syndrome (CdLS). These results point out a prominent role of cohesin-SA1 in transcriptional regulation, with clear implications in the etiology of CdLS.

Amino acid sensing by enteroendocrine STC-1 cells: role of the Na+-coupled neutral amino acid transporter 2

The results presented here show that STC-1 cells, a model of intestinal endocrine cells, respond to a broad range of amino acids, including l-proline, l-serine, l-alanine, l-methionine, l-glycine, l-histidine, and alpha-methyl-amino-isobutyric acid (MeAIB) with a rapid increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)). We sought to identify the mechanism by which amino acids induce Ca(2+) signaling in these cells. Several lines of evidence suggest that amino acid transport through the Na(+)-coupled neutral amino acid transporter 2 (SNAT2) is a major mechanism by which amino acids induced Ca(2+) signaling in STC-1 cells: 1) the amino acid efficacy profile for inducing Ca(2+) signaling in STC-1 cells closely matches the amino acid specificity of SNAT2; 2) amino acid-induced Ca(2+) signaling in STC-1 cells was suppressed by removing Na(+) from the medium; 3) the nonmetabolized synthetic substrate of amino acid transport MeAIB produced a marked increase in [Ca(2+)](i); 4) transfection of small interfering RNA targeting SNAT2 produced a marked decrease in Ca(2+) signaling in response to l-proline in STC-1 cells; 5) amino acid-induced increase in [Ca(2+)](i) was associated with membrane depolarization and mediated by Ca(2+) influx, since it depended on extracellular Ca(2+); 6) the increase in [Ca(2+)](i) in response to l-proline, l-alanine, or MeAIB was abrogated by either nifedipine (1-10 muM) or nitrendipine (1 muM), which block L-type voltage-sensitive Ca(2+) channels. We hypothesize that the inward current of Na(+) associated with the function of SNAT2 leads to membrane depolarization and activation of voltage-sensitive Ca(2+) channels that mediate Ca(2+) influx, thereby leading to an increase in the [Ca(2+)](i) in enteroendocrine STC-1 cells.

Tertiary active transport of amino acids reconstituted by coexpression of System A and L transporters in Xenopus oocytes

The System L transporter facilitates cellular import of large neutral amino acids (AAs) such as Leu, a potent activator of the intracellular target of rapamycin (TOR) pathway, which signals for cell growth. System L is an AA exchanger, proposed to accumulate certain AAs by coupling to dissipation of concentration gradient(s) of exchange substrates generated by secondary active AA transporters such as System A (SNAT2). We addressed the hypothesis that this type of coupling (termed tertiary active transport) acts as an indirect mechanism to extend the range of AA stimulating TOR to those transported by both Systems A and L (e.g., Gln) through downstream enhancement of Leu accumulation. System A overexpression enabled Xenopus oocytes to accumulate substrate AAs (notably Ser, Gln, Ala, Pro, Met; totaling 2.6 nmol/oocyte) from medium containing a physiological AA mixture at plasma concentrations. Net accumulation of System L (4F2hc-xLAT1) substrates from this medium by System L-overexpressing oocytes was increased by 90% (from 0.7 to 1.35 nmol/oocyte; mainly Leu, Ile) when Systems A and L were coexpressed, coincident with a decline in accumulation of specific System A substrates (Gln, Ser, Met), as expected if the latter were also System L substrates and functional coupling of the transport Systems occurred. AA flux coupling was confirmed as trans-stimulation of Leu influx in System L-expressing oocytes by Gln injection (0.5 nmol/oocyte). The observed changes in Leu accumulation are sufficient to activate the TOR pathway in oocytes, although intracellular AA metabolism limits the potential for AA accumulation by tertiary active transport in this system.

Amino acid transport systems beta and A in fetal T lymphocytes in intrauterine growth restriction and with tumor necrosis factor-alpha treatment

Intrauterine growth restriction (IUGR) is associated with reduced activity of placental amino acid transport systems beta and A. Whether this phenotype is maintained in fetal cells outside the placenta is unknown. In IUGR, cord blood tumor necrosis factor (TNF)-alpha concentrations are raised, potentially influencing amino acid transport in fetal cells. We used fetal T lymphocytes as a model to study systems beta and A amino acid transporters in IUGR compared with normal pregnancy. We also studied the effect of TNF-alpha on amino acid transporter activity. In fetal lymphocytes from IUGR pregnancies, taurine transporter mRNA expression encoding system beta transporter was reduced, but there was no change in system beta activity. No significant differences were observed in system A mRNA expression (encoding SNAT1 and SNAT2) or system A activity between the two groups. After 24 or 48 h TNF-alpha treatment, fetal T lymphocytes from normal pregnancies showed no significant change in system A or system beta activity, although cell viability was compromised. This study represents the first characterization of amino acid transport in a fetal cell outside the placenta in IUGR. We conclude that the reduced amino acid transporter activity found in placenta in IUGR is not a feature of all fetal cells.

Expression of D-serine and glycine transporters in the prefrontal cortex and cerebellum in schizophrenia

The NMDA receptor co-agonists D-serine and glycine are thought to contribute to glutamatergic dysfunction in schizophrenia. They are removed from the synapse by specific neuronal and glial transporters, the status of which is clearly relevant to theories of D-serine and glycine function in the disorder. D-serine is primarily transported by Asc-1, and glycine by GlyT1 but maybe also by SNAT2. As a first step to addressing this issue, we studied Asc-1, GlyT1 and SNAT2 expression in dorsolateral prefrontal cortex and cerebellum of 18 subjects with schizophrenia and 20 controls, using immunoblotting and in situ hybridization. Asc-1 protein and SNAT2 mRNA were decreased in schizophrenia in both regions. GlyT1 mRNA and protein, and Asc-1 mRNA, were not altered. Antipsychotic administration for 14 days did not alter expression of the genes in rat brain. Unchanged GlyT1 suggests that glycine transport is not markedly affected in schizophrenia, and therefore that increased synaptic removal is not the basis for the putative deficit in glycine modulation of NMDA receptors in the disorder. Lowered Asc-1 in schizophrenia implies that D-serine reuptake is reduced, perhaps as a response to decreased synaptic D-serine availability. However, this interpretation remains speculative. Further investigations will be valuable in the evaluation of these transporters as potential therapeutic targets in psychosis.

Upregulation of the glutamine transporter through transactivation mediated by cAMP/protein kinase A signals toward exacerbation of vulnerability to oxidative stress in rat neocortical astrocytes

In the present study, we have evaluated the possible functionality in astrocytes of the glutamine (Gln) transporter (GlnT) known to predominate in neurons for the neurotransmitter pool of glutamate. Sustained exposure to the adenylyl cyclase activator forskolin for 24 h led to a significant increase in mRNA expression of GlnT among different membrane transporters capable of transporting Gln, with an increase in [(3)H]Gln accumulation sensitive to a system A transporter inhibitor, in cultured rat neocortical astrocytes, but not neurons. Forskolin drastically stimulated GlnT promoter activity in a manner sensitive to a protein kinase A (PKA) inhibitor in rat astrocytic C6 glioma cells, while deletion mutation analysis revealed that the stimulation was mediated by a cAMP responsive element (CRE)/activator protein-1 (AP-1) like site located on GlnT gene promoter. Forskolin drastically stimulated the promoter activity in a fashion sensitive to a PKA inhibitor in C6 glioma cells transfected with a CRE or AP-1 reporter plasmid, in association with the phosphorylation of CRE binding protein on serine133. Transient overexpression of GlnT significantly exacerbated the cytotoxicity of hydrogen peroxide in cultured astrocytes. These results suggest that GlnT expression is upregulated by cAMP/PKA signals for subsequent exacerbation of the vulnerability to oxidative stress in astrocytes.

Activation of a system A amino acid transporter, ATA1/SLC38A1, in human hepatocellular carcinoma and preneoplastic liver tissues

The expression of amino acid transporter (AT) mRNAs including A system (ATA1/SNAT1/SLC38A1, ATA2/SNAT2/SLC38A2 and ATA3/SNAT3/SLC38A4), L system (LAT1/SLC7A5 and LAT2/SLC7A8), and y+ (CAT2/SLC7A2) genes, were compared among hepatocellular carcinoma (HCC) and non-cancerous liver cells. Among them the ATA1 mRNA expression was significantly elevated in all HCC cell lines (HepG2, HLF, HuH7 and JHH4) examined compared with normal liver tissue. We further discovered that the expression of ATA1 mRNA was significantly activated in HCC tissues and also elevated in pre-malignant cirrhotic livers from HCC patients, compared with normal livers from non-HCC patients. The ATA1 protein was extensively accumulated in the cytoplasm of pre-malignant liver and most HCCs, while being weak or undetectably low in normal liver tissues. SiRNA-mediated suppression of endogenous ATA1 lowered the viability of HepG2 cells. Thus, the activation of ATA1 confers growth and survival advantages in pre-malignant and malignant liver lesions.

Distinct sensor pathways in the hierarchical control of SNAT2, a putative amino acid transceptor, by amino acid availability

Mammalian nutrient sensors are novel targets for therapeutic intervention in disease states such as insulin resistance and muscle wasting; however, the proteins responsible for this important task are largely uncharacterized. To address this issue we have dissected an amino acid (AA) sensor/effector regulon that controls the expression of the System A amino acid transporter SNAT2 in mammalian cells, a paradigm nutrient-responsive process, and found evidence for the convergence of at least two sensor/effector pathways. During AA withdrawal, JNK is activated and induces the expression of SNAT2 in L6 myotubes by stimulating an intronic nutrient-sensitive domain. A sensor for large neutral AA (e.g. Tyr, Gln) inhibits JNK activation and SNAT2 up-regulation. Additionally, shRNA and transporter chimeras demonstrate that SNAT2 provides a repressive signal for gene transcription during AA sufficiency, thus echoing AA sensing by transceptor (transporter-receptor) orthologues in yeast (Gap1/Ssy1) and Drosophila (PATH). Furthermore, the SNAT2 protein is stabilized during AA withdrawal.

Expression and adaptive regulation of amino acid transport system A in a placental cell line under amino acid restriction

Trans-placental transport of amino acids is vital for the developing fetus. Using the BeWo cell line as a placental model, we investigated the effect of restricting amino acid availability on amino acid transport system type A. BeWo cells were cultured either in amino acid-depleted (without non-essential amino acids) or control media for 1, 3, 5 or 6 h. System A function was analysed using alpha(methyl-amino)isobutyric acid (MeAIB) transcellular transport studies. Transporter (sodium coupled neutral amino acid transporter (SNAT1/2)) expression was analysed at mRNA and protein level by Northern and Western blotting respectively. Localisation was carried out using immunocytochemistry. MeAIB transcellular transport was significantly (P < 0.05) increased by incubation of the cells in amino acid-depleted medium for 1 h, and longer incubation times caused further increases in the rate of transfer. However, the initial response was not accompanied by an increase in SNAT2 mRNA; this occurred only after 3 h and further increased for the rest of the 6-h incubation. Similarly, it took several hours for a significant increase in SNAT2 protein expression. In contrast, relocalisation of existing SNAT2 transporters occurred within 30 min of amino acid restriction and continued throughout the 6-h incubation. When the cells were incubated in medium with even lower amino acid levels (without non-essential plus 0.5 x essential amino acids), SNAT2 mRNA levels showed further significant (P < 0.0001) up-regulation. However, incubation of cells in depleted medium for 6 h caused a significant (P = 0.014) decrease in the expression of SNAT1 mRNA. System L type amino acid transporter 2 (LAT2) expression was not changed by amino acid restriction, indicating that the responses seen in the system A transporters were not a general cell response. These data have shown that placental cells adapt in vitro to nutritional stress and have identified the physiological, biochemical and genomic mechanisms involved.

The Effects of Hypoxia-Ischemia on Neutral Amino Acid Transporters in the Developing Rat Brain

The neutral amino acid transporters SNAT1-3 and ASCT1 play critical roles in the recycling of glutamine, and subsequently glutamate, via the glutamine-glutamate cycle. Hypoxia-ischemia was induced in rat pups using the Rice-Vannucci model. Brains were harvested at 1 h, 24 h and 7 days after ischemia. The expression of NAATs was evaluated using immunoblotting, real-time PCR, and immunohistochemistry. Results were compared with age-matched controls and shams. SNAT1 mRNA decreased at 1 h after injury in both hemispheres when compared with the control animals and correlated with a decrease in protein expression at 24 h in the hippocampus and cortex. SNAT1 protein expression increased globally at 7 days after injury and specifically in the hippocampus. Finally, SNAT2 and 3 demonstrated subtle changes in various brain regions after injury. These data suggest that neutral amino acid transporters remain largely intact after hypoxia-ischemia.

Assessment of genomic imprinting of SLC38A4, NNAT, NAP1L5, and H19 in cattle

Background

At present, few imprinted genes have been reported in cattle compared to human and mouse. Comparative expression analysis and imprinting status are powerful tools for investigating the biological significance of genomic imprinting and studying the regulation mechanisms of imprinted genes. The objective of this study was to assess the imprinting status and pattern of expression of the SLC38A4, NNAT, NAP1L5, and H19 genes in bovine tissues.

Results

A polymorphism-based approach was used to assess the imprinting status of four bovine genes in a total of 75 tissue types obtained from 12 fetuses and their dams. In contrast to mouse Slc38a4, which is imprinted in a tissue-specific manner, we found that SLC38A4 is not imprinted in cattle, and we found it expressed in all adult tissues examined. Two single nucleotide polymorphisms (SNPs) were identified in NNAT and used to distinguish between monoallelic and biallelic expression in fetal and adult tissues. The two transcripts of NNAT showed paternal expression like their orthologues in human and mouse. However, in contrast to human and mouse, NNAT was expressed in a wide range of tissues, both fetal and adult. Expression analysis of NAP1L5 in five heterozygous fetuses showed that the gene was paternally expressed in all examined tissues, in contrast to mouse where imprinting is tissue-specific. H19 was found to be maternally expressed like its orthologues in human, sheep, and mouse.

Conclusion

This is the first report on the imprinting status of SLC38A4, NAP1L5, and on the expression patterns of the two transcripts of NNAT in cattle. It is of interest that the imprinting of NAP1L5, NNAT, and H19 appears to be conserved between mouse and cow, although the tissue distribution of expression differs. In contrast, the imprinting of SLC38A4 appears to be species-specific.

IGF regulation of neutral amino acid transport in the BeWo choriocarcinoma cell line (b30 clone): evidence for MAP kinase-dependent and MAP kinase-independent mechanisms

OBJECTIVE

IGF-1 and IGF-1 receptors are major determinants of fetal growth and are expressed primarily on the maternal-facing surface of the syncytiotrophoblast cell membrane in the human placenta. IGF-1 regulates fetal growth, in part, by regulating amino acid transport across the placenta. The objective of these studies was to study the role of IGF-1 and its signaling pathway in regulating neutral amino acid transport in a human trophoblast cell culture model.

DESIGN

The regulation of neutral amino acid transport by IGF-1 was studied in cultured BeWo(b30) choriocarcinoma cells using the non-metabolizing amino acid analog, [(3)H]-alpha-aminoisobutyric acid (AIB). Transport in the absence of Na was used to distinguish system L from total AIB transport. Similarly, Na-dependent transport in the presence of excess methyl-AIB (MeAIB) permitted discrimination of systems A (MeAIB-sensitive) and ASC (MeAIB-insensitive). Specific inhibitors of intracellular signaling pathways were then used to determine the signaling pathway utilized by IGFs to regulate each amino acid transport system. Specificity of inhibition was assessed using specific markers of p70 S6 kinase activity and MAP kinase activation.

RESULTS

Maximal stimulating concentrations of IGF-I (100 ng/ml) stimulated AIB transport by 30-40% exclusively through system A. Wortmannin (100 nM), an inhibitor of PI-3-kinase activity, inhibited all IGF-I-stimulated transport. Rapamycin (100 ng/ml), an inhibitor of p70 S6 kinase, and bisindolylmaleimide, an inhibitor of protein kinase C (PKC), had no effect. PD-098059 (50 miccroM), an inhibitor of MAP kinase activation, inhibited 20-30% of basal AIB transport but did not inhibit IGF-I-stimulated transport under the conditions studied. IGF-1 did not increase steady state mRNA levels of the system A transporters, SNAT1 and SNAT2, suggesting IGF-1 stimulates transport via post-transcriptional mechanisms.

CONCLUSIONS

These data demonstrate that IGF-I stimulates neutral amino acid transport system A by a PI3-kinase dependent, post-transcriptional pathway in the BeWo(b30) cell line. Additionally, system A activity appear to be sensitive to MAP kinase-dependent pathways not regulated by IGFs.

Cortisol stimulates system A amino acid transport and SNAT2 expression in a human placental cell line (BeWo)

Both placental system A activity and fetal plasma cortisol concentrations are associated with intrauterine growth retardation, but it is not known if these factors are mechanistically related. Previous functional studies using hepatoma cells and fibroblasts produced conflicting results regarding the regulation of system A by cortisol. Using the b30 BeWo choriocarcinoma cell line, we investigated the regulation of system A by cortisol. System A function was analyzed using methyl amino isobutyric acid (MeAIB) transcellular transport studies. Transporter expression [system A transporter (SNAT)1/2] was studied at the mRNA and protein levels using Northern and Western blotting, respectively. Localization was carried out using immunocytochemistry. The [(14)C]MeAIB transfer rate across BeWo monolayers after preincubation with cortisol for 24 h was significantly increased compared with control. This was associated with a relocalization of the SNAT2 transporter at lower cortisol levels and significant upregulation of mRNA and protein expression levels at cortisol levels >1 microM. This is the first study to show functional and molecular regulation of system A by cortisol in BeWo cells. It is also the first study to identify which system A isoform is regulated. These results suggest that cortisol may be involved in upregulation of system A in the placenta to ensure sufficient amino acid supply to the developing fetus.

Functional expression of A glutamine transporter responsive to down-regulation by lipopolysaccharide through reduced promoter activity in cultured rat neocortical astrocytes

The prevailing view is that the glutamine (Gln) transporter (GlnT/ATA1/SAT1/SNAT1) is a member of the system A transporter superfamily with the ability to fuel the glutamate/Gln cycle at nerve terminals in glutamatergic neurons. Semiquantitative reverse transcription-polymerase chain reaction revealed similarly high expression of mRNA for GlnT by rat brain neocortical astrocytes as well as neurons, with progressively lower expression by cerebellar astrocytes, hippocampal astrocytes, and whole-brain microglia in culture. [(3)H]Gln was accumulated in a temperature-dependent manner with a saturable profile in both cultured neocortical neurons and astrocytes, whereas biochemical and pharmacological analyses on [(3)H]Gln accumulation revealed the expression of both system A and system L transporters by cultured neocortical neurons and astrocytes. Exposure to lipopolysaccharide (LPS) for 24 hr resulted in a significant decrease in both GlnT mRNA expression and [(3)H]Gln accumulation, with a concomitant drastic increase in nitrite formation in cultured neocortical astrocytes. Moreover, LPS significantly inhibited the promoter activity of GlnT in the astrocytic cell line C6 glioma cells as well as primary rat neocortical astrocytes in culture. These results suggest that activation by LPS would lead to down-regulation of the expression of GlnT responsible for the incorporation of extracellular Gln into intracellular spaces across plasma membranes through the inhibition of its promoter activity in cultured rat neocortical astrocytes.

Characterization of the amino acid response element within the human sodium-coupled neutral amino acid transporter 2 (SNAT2) System A transporter gene

The neutral amino acid transport activity, System A, is enhanced by amino acid limitation of mammalian cells. Of the three gene products that encode System A activity, the one that exhibits this regulation is SNAT2 (sodium-coupled neutral amino acid transporter 2). Fibroblasts that are deficient in the amino acid response pathway exhibited little or no induction of SNAT2 mRNA. Synthesis of SNAT2 mRNA increased within 1-2 h after amino acid removal from HepG2 human hepatoma cells. The amino acid responsive SNAT2 genomic element that mediates the regulation has been localized to the first intron. Increased binding of selected members of the ATF (activating transcription factor) and C/EBP (CCAAT/enhancer-binding protein) families to the intronic enhancer was established both in vitro and in vivo. In contrast, there was no significant association of these factors with the SNAT2 promoter. Expression of exogenous individual ATF and C/EBP proteins documented that specific family members are associated with either activation or repression of SNAT2 transcription. Chromatin immunoprecipitation analysis established in vivo that amino acid deprivation led to increased RNA polymerase II recruitment to the SNAT2 promoter.

Selective decrease of SN1(SNAT3) mRNA expression in human and rat glioma cells adapted to grow in acidic medium

The system N glutamine (Gln) transporter SN1(SNAT3) is overexpressed in human malignant glioma cells in situ as compared to the adjacent brain tissue or metastases from different organs [Sidoryk, M., Matyja, E., Dybel, A., Zielińska, M., Bogucki, J., Jaskólski, D.J., Liberski, P.P., Kowalczyk, P., Albrecht, J., 2004]. Increased expression of a glutamine transporter SNAT3 is a marker of malignant gliomas. NeuroReport 15, 575-578], but its role in tumor growth as compared to the other Gln transporters is unknown. One of the profound, growth-promoting effects of glial tumor in situ is acidification of the extracellular space. In the kidney SN1(SNAT3) mRNA participates in the adaptation to acidosis. In this study therefore, expression of mRNAs coding for SN1(SNAT3) and other Gln transporters was measured in human (T98G) and rat (C6) glioma cells incubated for 4h in an acidic medium (AI) (pH 6.5). MTT assay revealed no cell loss in AI cells, and intracellular pH (pHi) as measured by a fluorescent probe (BCECF-AM) was slightly alkaline in C6 and T98G cells, indicating that the cells have adapted to AI. AI significantly decreased the SN1(SNAT3) mRNA expression in C6 (a 60% decrease) and T98G cells (a 50% decrease). The decrease retreated in C6 cells 4h after transferring them back to the neutral medium. The expression of ASCT2 mRNA (system ASC), ATA1 mRNA (system A) and SN2(SNAT5) mRNA (system N) were not affected by AI in either of the cell lines. [(3)H]Gln uptake in C6 or T98G cells grown in neutral medium was mainly mediated by system ASCT2: system N contributed to only approximately 7% of the uptake. AI did not affect the total Gln uptake, and only slightly decreased the system N-mediated component of the uptake. Hence, SN1(SNAT3) does not seem to be involved in the adaptation of cultured glioma cells to acidic millieu.

Amino acid starvation induces the SNAT2 neutral amino acid transporter by a mechanism that involves eukaryotic initiation factor 2alpha phosphorylation and cap-independent translation

Nutritional stress caused by amino acid starvation involves a coordinated cellular response that includes the global decrease of protein synthesis and the increased production of cell defense proteins. Part of this response is the induction of transport system A for neutral amino acids that leads to the recovery of cell volume and amino acid levels once extracellular amino acid availability is restored. Hypertonic stress also increases system A activity as a mechanism to promote a rapid recovery of cell volume. Both a starvation-dependent and a hypertonic increase of system A transport activity are due to the induction of SNAT2, the ubiquitous member of SLC38 family. The molecular mechanisms underlying SNAT2 induction were investigated in tissue culture cells. We show that the increase in system A transport activity and SNAT2 mRNA levels upon amino acid starvation were blunted in cells with a mutant eIF2alpha that cannot be phosphorylated. In contrast, the induction of system A activity and SNAT2 mRNA levels by hypertonic stress were independent of eIF2alpha phosphorylation. The translational control of the SNAT2 mRNA during amino acid starvation was also investigated. It is shown that the 5'-untranslated region contains an internal ribosome entry site that is constitutively active in amino acid-fed and -deficient cells and in a cell-free system. We also show that amino acid starvation caused a 2.5-fold increase in mRNA and protein expression from a reporter construct containing both the SNAT2 intronic amino acid response element and the SNAT2-untranslated region. We conclude that the adaptive response of system A activity to amino acid starvation requires eukaryotic initiation factor 2alpha phosphorylation, increased gene transcription, and internal ribosome entry site-mediated translation. In contrast, the response to hypertonic stress does not involve eukaryotic initiation factor 2alpha phosphorylation, suggesting that SNAT2 expression can be modulated by specific signaling pathways in response to different stresses.

Expression and function of glutamine transporters SN1 (SNAT3) and SN2 (SNAT5) in retinal Müller cells

PURPOSE

The expression and function of the glutamine transporters ATA1 and ATA2 (isoforms of system A), SN1 and SN2 (isoforms of system N), and LAT1 and LAT2 (isoforms of system L) were investigated in Müller cells in a rat Müller cell line (rMC1) and primary cultures of mouse Müller cells.

METHODS

Glutamine uptake in rMC1 cells and primary Müller cells was measured. The relative contributions of various transport systems to glutamine uptake were determined based on the differential substrate specificities and Na(+) dependence of individual transport systems. RT-PCR was used to analyze the expression of transporter-specific mRNAs.

RESULTS

Three different transport systems participated in glutamine uptake in rMC1 cells: system L (Na(+)-independent), system A (Na(+)-dependent and alpha-(methylamino)isobutyric acid [MeAIB]-sensitive), and system N (Na(+)-dependent and MeAIB-insensitive). System N was the principal contributor (approximately 70%); the contributions of systems A and L were relatively lesser (20% and <10%, respectively). The functional features of Na(+)-dependent and MeAIB-insensitive glutamine uptake were similar to the known characteristics of clones of SN1 and SN2. Glutamine uptake in primary Müller cells behaved in a manner similar to that in rMC1 cells. mRNA transcripts specific for ATA1, ATA2, SN1, SN2, LAT1, and LAT2 were expressed in Müller cells.

CONCLUSIONS

System N (SN1 as well as SN2) is responsible for most of the glutamine uptake in Müller cells. Because system N is capable of mediating the release of glutamine from the cells, its abundant expression in Müller cells is of importance in the handling of glutamine in the retina.

Adaptation of nutrient supply to fetal demand in the mouse involves interaction between the Igf2 gene and placental transporter systems

The mammalian fetus is unique in its dependence during gestation on the supply of maternal nutrients through the placenta. Maternal supply and fetal demand for nutrients need to be fine tuned for healthy growth and development of the fetus along its genetic trajectory. An altered balance between supply and demand can lead to deviations from this trajectory with long-term consequences for health. We have previously shown that in a knockout lacking the imprinted placental-specific Igf2 transcript (P0), growth of the placenta is compromised from early gestation but fetal growth is normal until late gestation, suggesting functional adaptation of the placenta to meet the fetal demands. Here, we show that placental transport of glucose and amino acids are increased in the Igf2 P0(+/-) null and that this up-regulation of transport occurs, at least in part, through increased expression of the transporter genes Slc2a3 and Slc38a4, the imprinted member of the System A amino acid transporter gene family. Decreasing fetal demand genetically by removal of fetal Igf2 abolished up-regulation of both transport systems and reduced placental System A amino acid transport activity and expression of Slc38a2 in late gestation. Our results provide direct evidence that the placenta can respond to fetal demand signals through regulation of expression of specific placental transport systems. Thus, crosstalk between an imprinted growth demand gene (Igf2) and placental supply transporter genes (Slc38a4, Slc38a2, and Slc2a3) may be a component of the genetic control of nutrient supply and demand during mammalian development.

SNAT4 isoform of system A amino acid transporter is expressed in human placenta

The system A amino acid transporter is encoded by three members of the Slc38 gene family, giving rise to three subtypes: Na+-coupled neutral amino acid transporter (SNAT)1, SNAT2, and SNAT4. SNAT2 is expressed ubiquitously in mammalian tissues; SNAT1 is predominantly expressed in heart, brain, and placenta; and SNAT4 is reported to be expressed solely by the liver. In the placenta, system A has an essential role in the supply of neutral amino acids needed for fetal growth. In the present study, we examined expression and localization of SNAT1, SNAT2, and SNAT4 in human placenta during gestation. Real-time quantitative PCR was used to examine steady-state levels of system A subtype mRNA in early (6-10 wk) and late (10-13 wk) first-trimester and full-term (38-40 wk) placentas. We detected mRNA for all three isoforms from early gestation onward. There were no differences in SNAT1 and SNAT2 mRNA expression with gestation. However, SNAT4 mRNA expression was significantly higher early in the first trimester compared with the full-term placenta (P < 0.01). We next investigated SNAT4 protein expression in human placenta. In contrast to the observation for gene expression, Western blot analysis revealed that SNAT4 protein expression was significantly higher at term compared with the first trimester (P < 0.05). Immunohistochemistry and Western blot analysis showed that SNAT4 is localized to the microvillous and basal plasma membranes of the syncytiotrophoblast, suggesting a role for this isoform of system A in amino acid transport across the placenta. This study therefore provides the first evidence of SNAT4 mRNA and protein expression in the human placenta, both at the first trimester and at full term.

SNAT2 silencing prevents the osmotic induction of transport system A and hinders cell recovery from hypertonic stress

Under hypertonic conditions the induction of SLC38A2/SNAT2 leads to the stimulation of transport system A and to the increase in the cell content of amino acids. In hypertonically stressed human fibroblasts transfection with two siRNAs for SNAT2 suppressed the increase in SNAT2 mRNA and the stimulation of system A transport activity. Under the same condition, the expansion of the intracellular amino acid pool was significantly lowered and cell volume recovery markedly delayed. It is concluded that the up-regulation of SNAT2 is essential for the rapid restoration of cell volume after hypertonic stress.

Mouse Oocytes Regulate Metabolic Cooperativity Between Granulosa Cells and Oocytes: Amino Acid Transport1

A search for genes expressed more highly in mouse cumulus cells than mural granulosa cells by subtraction hybridization yielded Slc38a3. SLC38A3 is a sodium-coupled neutral amino acid transporter having substrate preference for l-glutamate, l-histidine, and l-alanine. Detectable levels of Slc38a3 mRNA were found by in situ hybridization in granulosa cells of large preantral follicles, but levels were higher in all granulosa cells of small antral follicles; expression became limited to cumulus cells of large antral follicles. Expression of Slc38a3 mRNA in granulosa cells was promoted by fully grown oocytes from antral follicles but not by growing oocytes from preantral follicles. Fully grown oocytes were dependent on cumulus cells for uptake of l-alanine and l-histidine but not l-leucine. Fully grown but not growing oocytes secreted one or more paracrine factors that promoted cumulus cell uptake of all three amino acids but of l-alanine and l-histidine to a much greater extent than l-leucine. Uptake of l-leucine appeared dependent primarily on contact-mediated signals from fully grown oocytes. Fully grown oocytes also promoted elevated levels of Slc38a3 mRNA and l-alanine transport by preantral granulosa cells, but growing oocytes did not. Therefore, fully grown oocytes secrete one or more paracrine factors that promote cumulus cell uptake of amino acids that oocytes themselves transport poorly. These amino acids are likely transferred to oocytes via gap junctions. Thus, oocytes use paracrine signals to promote their own development via metabolic cooperativity with cumulus cells. The ability of oocytes to mediate this cooperativity is developmentally regulated and acquired only in later stages of oocyte development.

Functional expression and adaptive regulation of Na+-dependent neutral amino acid transporter SNAT2/ATA2 in normal human astrocytes under amino acid starved condition

We reported here the functional characteristics of Na+ -dependent neutral amino acid transport system A in normal human astrocytes and its adaptive regulation, a process in which amino acid starvation induces the transport activity. Reverse transcription-PCR revealed that the system A transporter subtype, SNAT2/ATA2, is only expressed in these cells. The other two known system A transporter subtypes, SNAT1/ATA1 and SNAT4/ATA3, could not be detected. Na+ -dependent uptake of alpha-(methylamino)isobutyric acid, a specific model substrate for system A, was pH-sensitive and saturable with a Michaelis-Menten constant of 0.22 +/- 0.03 mM. Exposures of human astrocytes to amino acid-free medium increased the system A activity and the steady-state levels of SNAT2/ATA2 mRNA in an exposure time-dependent manner. This stimulatory effect was attenuated significantly by actinomycin D, an inhibitor of RNA synthesis, and cycloheximide, an inhibitor of protein synthesis. Taken collectively, these data show that chronic exposure (6 h) of the cells to the amino acid-free medium increases the system A activity most likely by enhancing de novo synthesis of the transporter protein and consequently increasing the density of the transporter protein in the plasma membrane.

Cyclic strain stimulates L-proline transport in vascular smooth muscle cells

BACKGROUND

The increase in vessel wall strain in hypertension contributes to arterial remodeling by stimulating vascular smooth muscle cell (SMC) proliferation and collagen synthesis. Because L-proline is essential for the synthesis of collagen and cell growth, we examined whether cyclic strain regulates the transcellular transport of L-proline by vascular SMC.

METHODS

Cultured rat aortic SMCs were subjected to mechanical strain using the Flexercell 3000 Strain Unit.

RESULTS

Cyclic strain increased L-proline transport in a time- and strain-degree-dependent manner that was inhibited by cycloheximide or actinomycin D. Kinetic studies indicated that cyclic strain-induced L-proline uptake was mediated by an increase in transport capacity independent of any change in the affinity for L-proline. Cyclic strain stimulated the expression of system A amino acid transporter 2 mRNA in a time-dependent fashion that paralleled the increase in L-proline transport. Cyclic strain also induced the release of transforming growth factor-beta1 in a time- and strain-dependent manner. Moreover, conditioned media from SMCs exposed to cyclic strain stimulated the transport of L-proline in control, static SMCs and this was significantly attenuated by a transforming growth factor-beta1 neutralizing antibody.

CONCLUSIONS

These results demonstrate that cyclic strain stimulates L-proline transport by inducing system A amino acid transporter 2 gene expression through the autocrine release of transforming growth factor-beta1. The ability of cyclic strain to induce system A amino acid transporter 2 expression may promote arterial remodeling in hypertension by providing vascular SMCs with the necessary intracellular levels of L-proline required for collagen synthesis and cell growth.

Increased expression of a glutamine transporter SNAT3 is a marker of malignant gliomas

Glutamine (Gln) is a growth determinant in neoplastic tissues. We analysed by RT-PCR the expression of mRNAs coding for the human variants of Gln transporters: ASCT2 (system ASC), SNAT1 [ATA1] (system A), SNAT3 [SN1] and SNAT5 [SN2] (system N), in samples of human malignant gliomas WHO grades III/IV (anaplastic astrocytoma and glioblastoma), glioma-derived cell cultures, brain metastases from peripheral organs, and control brain tissue. SNAT3 mRNA showed a 3-5 times stronger expression in gliomas than in metastases or control tissue, and was virtually absent from glioma cultures. Native glioblastoma immunostained positively with anti-SNAT3 antibody. The expression of ASCT2 mRNA, but not SNAT5 or SNAT1 mRNAs, was increased in all neoplastic tissues studied. Hence, increased expression of SNAT3 is a marker of primary malignant gliomas in situ.

DNA Hypomethylation and Ovarian Cancer Biology

Hypomethylation of some portions of the genome and hypermethylation of others are very frequent in human cancer. The hypomethylation often involves satellite 2 (Sat2) DNA in the juxtacentromeric (centromere-adjacent) region of chromosome 1. In this study, we analyzed methylation in centromeric and juxtacentromeric satellite DNA in 115 ovarian cancers, 26 non-neoplastic ovarian specimens, and various normal somatic tissue standards. We found that hypomethylation of both types of satellite DNA in ovarian samples increased significantly from non-neoplastic toward cancer tissue. Furthermore, strong hypomethylation was significantly more prevalent in tumors of advanced stage or high grade. Importantly, extensive hypomethylation of Sat2 DNA in chromosome 1 was a highly significant marker of poor prognosis (relative risk for relapse, 4.1, and death, 9.4) and more informative than tumor grade or stage. Also, comparing methylation of satellite DNA and 15 5' gene regions, which are often hypermethylated in cancer or implicated in ovarian carcinogenesis, we generally found no positive or negative association between methylation changes in satellite DNA and in the gene regions. However, hypermethylation at two loci, CDH13 (at 16q24) and RNR1 (at 13p12), was correlated strongly with lower levels of Sat2 hypomethylation. The CDH13/Sat2 epigenetic correlation was seen also in breast cancers. We conclude that satellite DNA hypomethylation is an important issue in ovarian carcinogenesis as demonstrated by: (a) an increase from non-neoplastic tissue toward ovarian cancer; (b) an increase within the ovarian cancer group toward advanced grade and stage; and (c) the finding that strong hypomethylation was an independent marker of poor prognosis.

Sodium-coupled neutral amino acid (System N/A) transporters of the SLC38 gene family

The sodium-coupled neutral amino acid transporters (SNAT) of the SLC38 gene family resemble the classically-described System A and System N transport activities in terms of their functional properties and patterns of regulation. Transport of small, aliphatic amino acids by System A subtypes (SNAT1, SNAT2, and SNAT4) is rheogenic and pH sensitive. The System N subtypes SNAT3 and SNAT5 also countertransport H(+), which may be key to their operation in reverse, and have narrower substrate profiles than do the System A subtypes. Glutamine emerges as a favored substrate throughout the family, except for SNAT4. The SLC38 transporters undoubtedly play many physiological roles including the transfer of glutamine from astrocyte to neuron in the CNS, ammonia detoxification and gluconeogenesis in the liver, and the renal response to acidosis. Probing their regulation has revealed additional roles, and recent work has considered SLC38 transporters as therapeutic targets in neoplasia.

Transcriptional control of the human sodium-coupled neutral amino acid transporter system A gene by amino acid availability is mediated by an intronic element

System A amino acid transporter (SNAT2) gene expression is up-regulated at the transcriptional level in response to amino acid deprivation. Functional analysis of genomic fragments 5' upstream of the transcription start site, for both human and mouse SNAT2 genes showed that these regions exhibit promoter activity, but were amino acid unresponsive. However, when the human and mouse constructs were extended to include intron 1, it was observed that the rate of transcription was increased following amino acid deprivation. Deletion analysis of the human gene identified an intron 1 sequence spanning 54 nucleotides that was sufficient for conferring amino acid-dependent regulation to a minimal SNAT2 promoter. Alignment of the corresponding region from the human, mouse, and rat genomes revealed three highly conserved sequences. From site-directed mutagenesis, it was concluded that one of these sites functions as an amino acid response element (AARE) to regulate transcription. The core sequence of this site is identical to the AARE in the human CHOP gene. The SNAT2 AARE, along with a nearby conserved CAAT box, has enhancer activity in that it functions in an orientation and position independent manner, and it confers regulated transcription to a heterologous promoter.

Ontogeny of the neutral amino acid transporter SAT1/ATA1 in rat brain

The glutamine-glutamate/GABA cycle is critical for the developing brain as glutamatergic neurotransmission is important for neuronal survival and drives synaptogenesis and activity-dependent synaptic plasticity. GABAergic transmission may be essential for the formation of neural circuits. Recently a cDNA encoding a brain-enriched System A transporter (SAT1/ATA1), has been identified which may provide glutamine to neurons for the biosynthesis of neurotransmitters glutamate and gamma-aminobutyric acid (GABA). In this study, we have examined the developmental expression pattern of SAT1/ATA1 protein in rat brain by immunohistochemistry. We find that SAT1/ATA1 was present in the developing rat brain at all gestational ages examined including prenatal days 17 and 19 and postnatal days 2, 10, 14, and adult. SAT1/ATA1 immunoreactivity was seen in the neocortex, hippocampus, and neuroepithelium at the earliest time point examined, prenatal day 17. SAT1/ATA1 was prominent in the striatum, the hippocampus and the cortex in the postnatal animals. In adults, SAT1/ATA1 was limited to the cell body region while in developing animals SAT1/ATA1 protein was found in neuronal processes. These results contribute to our understanding of the relationship between the cycling of glutamate and glutamine between astrocytes and glia and the pathophysiological conditions that occur in hypoxic ischemic encephalopathy.

The osmoregulatory and the amino acid-regulated responses of system A are mediated by different signal transduction pathways

The osmotic response of system A for neutral amino acid transport has been related to the adaptive response of this transport system to amino acid starvation. In a previous study (Ruiz-Montasell, B., M. Gómez-Angelats, F.J. Casado, A. Felipe, J.D. McGivan, and M. Pastor-Anglada. 1994. Proc. Natl. Acad. Sci. USA. 91:9569-9573), a model was proposed in which both responses were mediated by different mechanisms. The recent cloning of several isoforms of system A as well as the elucidation of a variety of signal transduction pathways involved in stress responses allow to test this model. SAT2 mRNA levels increased after amino acid deprivation but not after hyperosmotic shock. Inhibition of p38 activity or transfection with a dominant negative p38 did not alter the response to amino acid starvation but partially blocked the hypertonicity response. Inhibition of the ERK pathway resulted in full inhibition of the adaptive response of system A and no increase in SAT2 mRNA levels, without modifying the response to hyperosmolarity. Similar results were obtained after transfection with a dominant negative JNK1. The CDK2 inhibitor peptide-II decreased the osmotic response in a dose-dependent manner but did not have any effect on the adaptive response of system A. In summary, the previously proposed model of up-regulation of system A after hypertonic shock or after amino acid starvation by separate mechanisms is now confirmed and the two signal transduction pathways have been identified. The involvement of a CDK-cyclin complex in the osmotic response of system A links the activity of this transporter to the increase in cell volume previous to the entry in a new cell division cycle.

Identification of novel imprinted genes in a genome-wide screen for maternal methylation

A characteristic of imprinted genes is that the maternal and paternal alleles show differences in methylation. To perform a genome-wide screen for novel imprinted loci, we applied methylation-sensitive representational difference analysis (Me-RDA) to parthenogenetic mouse embryos, to identify differentially methylated regions (DMRs) methylated specifically on the maternal allele. We isolated a total of 26 distinct clones from known and novel DMRs and identified three novel imprinted genes. Nap1l5 is located on proximal chromosome 6 and encodes a protein with homology with nucleosome assembly proteins (NAPs); it has tissue-specific imprinting with expression from the paternal allele. We identified two DMRs on chromosome 15, a chromosome that was not thought to contain imprinted loci, and demonstrated that each is associated with a paternally expressed transcript. Peg13 gives rise to a noncoding RNA that is highly expressed in the brain and imprinted in all tissues examined. A DMR was also identified at the chromosome 15 Slc38a4 gene, which encodes a system A amino acid transporter; we show that Slc38a4 is imprinted in a tissue-specific manner. Interestingly, two of the three novel genes identified in this screen are located within the introns of other genes; their identification indicates that such "microimprinted" domains may be more common than previously thought.

The osmoprotective function of the NFAT5 transcription factor in T cell development and activation

The NFAT5/TonEBP transcription factor, a recently identified rel/NF-kappaB family member, activates transcription of osmocompensatory genes in response to extracellular hyperosmotic stress. However, the function of NFAT5 under isosmotic conditions present in vivo remains unknown. Here we demonstrate that NFAT5 is necessary for optimal T cell development in vivo and allows for optimal cell growth ex vivo under conditions associated with osmotic stress. Transgenic mice expressing an inhibitory form of NFAT5 in developing and mature T cells exhibited a 30% reduction in thymic cellularity evenly distributed among thymic subsets, consistent with the uniform expression and nuclear localization of NFAT5 in each subset. This was associated with a 25% reduction in peripheral CD4(+) T cells and a 50% reduction in CD8(+) T cells. While transgenic T cells exhibited no impairment in cell growth or cytokine production under normal culture conditions, impaired cell growth was observed under both hyperosmotic conditions and isosmotic conditions associated with osmotic stress. Transgenic thymocytes also demonstrated increased sensitivity to osmotic stress. Consistent with this, the system A amino acid transporter gene ATA2 exhibited NFAT5 dependence under hypertonic conditions but not in response to amino acid deprivation. Expression of the TNF-alpha gene, a putative NFAT5 target, was not altered in transgenic T cells. These results not only demonstrate an osmoprotective function for NFAT5 in primary cells but also show that NFAT5 is necessary for optimal thymic development in vivo, suggesting that developing thymocytes within the thymic microenvironment are subject to an osmotic stress that is effectively countered by NFAT5-dependent responses.

Stabilization of hsp70 mRNA on prolonged cell exposure to hypertonicity

Prolonged exposure of 3T3 cells to 0.5 osM hypertonic medium induced the accumulation of hsp70 mRNAs. This increase in mRNA levels required active protein synthesis. A weak and transient activation of heat shock factor 1 (HSF1) was noted, but it was temporally uncoupled to the accumulation of the hsp70 mRNAs. Nuclear run-on assay and transfection experiments showed that hsp70 gene transcription was not affected by hypertonicity. ActD chase experiments showed that during hypertonic treatment, degradation of hsp70 mRNAs was markedly reduced. This effect did not appear to be a general phenomenon since the increase in mRNA level of another gene induced by hypertonicity (ATA2 transporter) was scarcely due to RNA stabilization. These findings suggest that hypertonic treatment increases the production of hsp70 protein in 3T3 cells via a stabilization of its corresponding mRNA.