Induction of BGT-1 and amino acid system A transport activities in endothelial cells exposed to hyperosmolarity

Neutral amino acid transporter SLC38A2 protects renal medulla from hyperosmolarity-induced ferroptosis

Hyperosmolarity of the renal medulla is essential for urine concentration and water homeostasis. However, how renal medullary collecting duct (MCD) cells survive and function under harsh hyperosmotic stress remains unclear. Using RNA-Seq, we identified SLC38A2 as a novel osmoresponsive neutral amino acid transporter in MCD cells. Hyperosmotic stress-induced cell death in MCD cells occurred mainly via ferroptosis, and it was significantly attenuated by SLC38A2 overexpression but worsened by Slc38a2-gene deletion or silencing. Mechanistic studies revealed that the osmoprotective effect of SLC38A2 is dependent on the activation of mTORC1. Moreover, an in vivo study demonstrated that Slc38a2-knockout mice exhibited significantly increased medullary ferroptosis following water restriction. Collectively, these findings reveal that Slc38a2 is an important osmoresponsive gene in the renal medulla and provide novel insights into the critical role of SLC38A2 in protecting MCD cells from hyperosmolarity-induced ferroptosis via the mTORC1 signalling pathway.

PRIMUS: Comprehensive proteomics of mouse intervertebral discs that inform novel biology and relevance to human disease modelling

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Proteomics of healthy mouse IVDs differentiating compartments and spine levels.

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NP cells feature vacuoles with lysosomal, transport and cell–cell communication functions.

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Collagen XII, decorin and other ECM proteins contribute to function of the AF.

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Distinct proteomics between lumbar and tail discs.

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Mouse is a relevant model for human disc biology but care is needed in its use.

Mice are commonly used to study intervertebral disc (IVD) biology and related diseases such as IVD degeneration. Discs from both the lumbar and tail regions are used. However, little is known about compartmental characteristics in the different regions, nor their relevance to the human setting, where a functional IVD unit depends on a homeostatic proteome. Here, we address these major gaps through comprehensive proteomic profiling and in-depth analyses of 8-week-old healthy murine discs, followed by comparisons with human. Leveraging on a dataset of over 2,700 proteins from 31 proteomic profiles, we identified key molecular and cellular differences between disc compartments and spine levels, but not gender. The nucleus pulposus (NP) and annulus fibrosus (AF) compartments differ the most, both in matrisome and cellularity contents. Differences in the matrisome are consistent with the fibrous nature required for tensile strength in the AF and hydration property in the NP. Novel findings for the NP cells included an enrichment in cell junction proteins for cell–cell communication (Cdh2, Dsp and Gja1) and osmoregulation (Slc12a2 and Wnk1). In NP cells, we detected heterogeneity of vacuolar organelles; where about half have potential lysosomal function (Vamp3, Copb2, Lamp1/2, Lamtor1), some contain lipid droplets and others with undefined contents. The AF is enriched in proteins for the oxidative stress responses (Sod3 and Clu). Interestingly, mitochondrial proteins are elevated in the lumbar than tail IVDs that may reflect differences in metabolic requirement. Relative to the human, cellular and structural information are conserved for the AF. Even though the NP is more divergent between mouse and human, there are similarities at the level of cell biology. Further, common cross-species markers were identified for both NP (KRT8/19, CD109) and AF (COL12A1). Overall, mouse is a relevant model to study IVD biology, and an understanding of the limitation will facilitate research planning and data interpretation, maximizing the translation of research findings to human IVDs.

Anti-nociceptive and antioxidant activity of betaine on formalin- and writhing tests induced pain in mice

Pain is a physiological response which is mediated via the central and peripheral nervous system. Betaine, is a methyl glycine derivative and a commonly used nutrient supplement. The main purpose of the current paper is to determine the possible anti-nociceptive and antioxidant activity and sedative effect of betaine in mice. Adult male albino mice were divided into two categories, formalin and writhing tests. In the formalin test, mice were injected with betaine (10, 20 and 30 mg/kg) or morphine (5 mg/kg). For co-injections mice received betaine (30 mg/kg) + naloxone (2 mg/kg) or atropine (1 mg/kg), chlorpheniramine (20 mg/kg), flumazenil (5 mg/kg), cimetidine (12.5 mg/kg) and cyproheptadine (4 mg/kg). Then the formalin test was done and paw licking time was determined. In the writhing test, injections were the same but the animals were injected with acetic acid (0.6 %) and the percentage of writhing inhibition was recorded. At the end of the study, blood antioxidant levels were determined. According to the results, betaine reduced the pain response in a dose-dependent manner. Co-administration of the naloxone + betaine or flumazenil + betaine significantly decreased the anti-nociceptive effect of betaine on the licking and biting time of the injected paw and inhibited the number of writhing movements. Betaine decreased malondialdehyde (MDA) and improved superoxide dismutase (SOD) and glutathione peroxidase (GPx) levels in formalin receiving mice. No adverse locomotion and sedation effect were observed in betaine-treated mice. These findings suggest that betaine has anti-nociceptive and antioxidant activity in mice, and its anti-nociceptive role interacts with opioidergic and GABA receptors.

NFAT5 up-regulates expression of the kidney-specific ubiquitin ligase gene Rnf183 under hypertonic conditions in inner-medullary collecting duct cells

We previously reported that among the 37 RING finger protein (RNF) family members, RNF183 mRNA is specifically expressed in the kidney under normal conditions. However, the mechanism supporting its kidney-specific expression pattern remains unclear. In this study, we elucidated the mechanism of the transcriptional activation of murine Rnf183 in inner-medullary collecting duct cells. Experiments with anti-RNF183 antibody revealed that RNF183 is predominantly expressed in the renal medulla. Among the 37 RNF family members, Rnf183 mRNA expression was specifically increased in hypertonic conditions, a hallmark of the renal medulla. RNF183 up-regulation was consistent with the activation of nuclear factor of activated T cells 5 (NFAT5), a transcription factor essential for adaptation to hypertonic conditions. Accordingly, siRNA-mediated knockdown of NFAT5 down-regulated RNF183 expression. Furthermore, the -3,466 to -3,136-bp region upstream of the mouse Rnf183 promoter containing the NFAT5-binding motif is conserved among mammals. A luciferase-based reporter vector containing the NFAT5-binding site was activated in response to hypertonic stress, but was inhibited by a mutation at the NFAT5-binding site. ChIP assays revealed that the binding of NFAT5 to this DNA site is enhanced by hypertonic stress. Of note, siRNA-mediated RNF183 knockdown increased hypertonicity-induced caspase-3 activation and decreased viability of mIMCD-3 cells. These results indicate that (i) RNF183 is predominantly expressed in the normal renal medulla, (ii) NFAT5 stimulates transcriptional activation of Rnf183 by binding to its cognate binding motif in the Rnf183 promoter, and (iii) RNF183 protects renal medullary cells from hypertonicity-induced apoptosis.

Increased levels of sodium chloride directly increase osteoclastic differentiation and resorption in mice and men

To better understand the association between high salt intake and osteoporosis, we investigated the effect of sodium chloride (NaCl) on mice and human osteoclastogenesis. The results suggest a direct, activating role of NaCl supplementation on bone resorption.

INTRODUCTION

High NaCl intake is associated with increased urinary calcium elimination and parathyroid hormone (PTH) secretion which in turn stimulates the release of calcium from the bone, resulting in increased bone resorption. However, while calciuria after NaCl loading could be shown repeatedly, several studies failed to reveal a significant increase in PTH in response to a high-sodium diet. Another possible explanation that we investigated here could be a direct effect of high-sodium concentration on bone resorption.

METHODS

Mouse bone marrow macrophage and human peripheral blood mononuclear cells (PBMC) driven towards an osteoclastogenesis pathway were cultivated under culture conditions mimicking hypernatremia environments.

RESULTS

In this study, a direct effect of increased NaCl concentrations on mouse osteoclast differentiation and function was observed. Surprisingly, in a human osteoclast culture system, significant increases in the number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts, calcitonin receptor (CTR)-positive osteoclasts, nuclear factor-activated T cells c1 (NFATc1) gene expression, and areal and volumetric resorptions were observed for increasing concentrations of NaCl. This suggests a direct, activating, cell-mediated effect of increased concentrations of NaCl on osteoclasts.

CONCLUSIONS

The reported that enhanced bone resorption after high-sodium diets may not only be secondary to the urinary calcium loss but may also be a direct, cell-mediated effect on osteoclastic resorption. These findings allow us to suggest an explanation for the clinical findings independent of a PTH-mediated regulation.

Effect of ABCG2/BCRP Expression on Efflux and Uptake of Gefitinib in NSCLC Cell Lines

Background

BCRP/ABCG2 emerged as an important multidrug resistance protein, because it confers resistance to several classes of cancer chemotherapeutic agents and to a number of novel molecularly-targeted therapeutics such as tyrosine kinase inhibitors. Gefitinib is an orally active, selective EGFR tyrosine kinase inhibitor used in the treatment of patients with advanced non small cell lung cancer (NSCLC) carrying activating EGFR mutations. Membrane transporters may affect the distribution and accumulation of gefitinib in tumour cells; in particular a reduced intracellular level of the drug may result from poor uptake, enhanced efflux or increased metabolism.

Aim

The present study, performed in a panel of NSCLC cell lines expressing different ABCG2 plasma membrane levels, was designed to investigate the effect of the efflux transporter ABCG2 on intracellular gefitinib accumulation, by dissecting the contribution of uptake and efflux processes.

Methods and Results

Our findings indicate that gefitinib, in lung cancer cells, inhibits ABCG2 activity, as previously reported. In addition, we suggest that ABCG2 silencing or overexpression affects intracellular gefitinib content by modulating the uptake rather than the efflux. Similarly, overexpression of ABCG2 affected the expression of a number of drug transporters, altering the functional activities of nutrient and drug transport systems, in particular inhibiting MPP, glucose and glutamine uptake.

Conclusions

Therefore, we conclude that gefitinib is an inhibitor but not a substrate for ABCG2 and that ABCG2 overexpression may modulate the expression and activity of other transporters involved in the uptake of different substrates into the cells.

Gap junctions favor normal rat kidney epithelial cell adaptation to chronic hypertonicity

Upon hypertonic stress most often resulting from high salinity, cells need to balance their osmotic pressure by accumulating neutral osmolytes called compatible osmolytes like betaine, myo-inositol, and taurine. However, the massive uptake of compatible osmolytes is a slow process compared with other defense mechanisms related to oxidative or heat stress. This is especially critical for cycling cells as they have to double their volume while keeping a hospitable intracellular environment for the molecular machineries. Here we propose that clustered cells can accelerate the supply of compatible osmolytes to cycling cells via the transit, mediated by gap junctions, of compatible osmolytes from arrested to cycling cells. Both experimental results in epithelial normal rat kidney cells and theoretical estimations show that gap junctions indeed play a key role in cell adaptation to chronic hypertonicity. These results can provide basis for a better understanding of the functions of gap junctions in osmoregulation not only for the kidney but also for many other epithelia. In addition to this, we suggest that cancer cells that do not communicate via gap junctions poorly cope with hypertonic environments thus explaining the rare occurrence of cancer coming from the kidney medulla.

Hippocampal betaine/GABA transporter mRNA expression is not regulated by inflammation or dehydration post-status epilepticus

Seizure activity can alter GABA transporter and osmoprotective gene expression, which may be involved in the pathogenesis of epilepsy. However, the response of the betaine/GABA transporter (BGT1) is unknown. The goal of the present study was to compare the expression of BGT1 mRNA to that of other osmoprotective genes and GABA transporters following status epilepticus (SE). The possible contributory role of dehydration and inflammation was also investigated because both have been shown to be involved in the regulation of GABA transporter and/or osmoprotective gene expression. BGT1 mRNA was increased 24 h post-SE, as were osmoprotective genes. BGT1 was decreased 72 h and 4 weeks post-SE, as were the GABA transporter mRNAs. The mRNA values for osmoprotective genes following 24-h water withdrawal were significantly lower than the values obtained 24 h post-SE despite similarities in their plasma osmolality values. BGT1 mRNA was not altered by lipopolysaccharide-induced inflammation while the transcription factor tonicity-responsive enhancer binding protein and the GABA transporters 1 and 3 were. These results suggest that neither plasma osmolality nor inflammation fully account for the changes seen in BGT1 mRNA expression post-SE. However, it is evident that BGT1 mRNA expression is altered by SE and displays a temporal pattern with similarities to both GABA and osmolyte transporters. Further investigation of BGT1 regulation in the brain is warranted.

Choline and betaine in health and disease

Choline is an essential nutrient, but is also formed by de novo synthesis. Choline and its derivatives serve as components of structural lipoproteins, blood and membrane lipids, and as a precursor of the neurotransmitter acetylcholine. Pre-and postnatal choline availability is important for neurodevelopment in rodents. Choline is oxidized to betaine that serves as an osmoregulator and is a substrate in the betaine-homocysteine methyltransferase reaction, which links choline and betaine to the folate-dependent one-carbon metabolism. Choline and betaine are important sources of one-carbon units, in particular, during folate deficiency. Choline or betaine supplementation in humans reduces concentration of total homocysteine (tHcy), and plasma betaine is a strong predictor of plasma tHcy in individuals with low plasma concentration of folate and other B vitamins (B₂, B₆, and B₁₂) in combination TT genotype of the methylenetetrahydrofolate reductase 677 C->T polymorphism. The link to one-carbon metabolism and the recent availability of food composition data have motivated studies on choline and betaine as risk factors of chronic diseases previously studied in relation to folate and homocysteine status. High intake and plasma level of choline in the mother seems to afford reduced risk of neural tube defects. Intake of choline and betaine shows no consistent relation to cancer or cardiovascular risk or risk factors, whereas an unfavorable cardiovascular risk factor profile was associated with high choline and low betaine concentrations in plasma. Thus, choline and betaine showed opposite relations with key components of metabolic syndrome, suggesting a disruption of mitochondrial choline oxidation to betaine as part of the mitochondrial dysfunction in metabolic syndrome.

Ontogeny of the neutral amino acid transporter SNAT1 in the developing rat

System A is a highly regulated, Na+-dependent transporter that accepts neutral amino acids containing short, polar side chains. System A plays an important role during rat development as decreased pup weights are observed in dams infused during gestation with a non-metabolizable System A substrate. Given the potential importance of SNAT1 during development in the rat brain, we examined whether SNAT1 would be present at an earlier gestation during organogenesis in multiple organs by immunohistochemistry and immunoblotting. SNAT1 protein was observed in the developing lungs, intestines, kidneys, heart, pancreas, and skeletal muscle of rats at prenatal days 14, 17, 19, 21, and postnatal day 2 rats. SNAT1 protein expression decreased in the liver and intestine shortly after birth and as the rat matured. SNAT1 expression was constant throughout development in the lungs and kidney and increased in the heart from prenatal day 19 to postnatal day 2. Highest levels of expression in older animals were seen in organs undergoing rapid cell division.

Hyperosmotic stress response: comparison with other cellular stresses

Cellular responses induced by stress are essential for the survival of cells under adverse conditions. These responses, resulting in cell adaptation to the stress, are accomplished by a variety of processes at the molecular level. After an alteration in homeostatic conditions, intracellular signalling processes link the sensing mechanism to adaptive or compensatory changes in gene expression. The ability of cells to adapt to hyperosmotic stress involves early responses in which ions move across cell membranes and late responses characterized by increased synthesis of either membrane transporters essential for uptake of organic osmolytes or of enzymes involved in their synthesis. The goal of these responses is to return the cell to its normal size and maintain cellular homeostasis. The enhanced synthesis of molecular chaperones, such as heat shock proteins, is another important component of the adaptive process that contributes to cell survival. Some responses are common to different stresses, whereas others are specific. In the first part of the review, we illustrate the characteristic and specific features of adaptive response to hypertonicity; we then describe similarities to and differences from other cellular stresses, such as genotoxic agents, nutrient starvation and heat shock.

Large discrepancies in cellular distribution of the tonicity-induced expression of osmoprotective genes and their regulatory transcription factor TonEBP in rat brain

Osmoprotective genes are tonicity-activated genes involved in cellular osmoadaptation to hypertonicity and considered to be regulated by a specific transcription factor called tonicity-responsive enhancer-binding protein (TonEBP). In the brain we had previously established that TonEBP was expressed and tonicity-induced in neurons only. Here we have compared in various brain regions of rats subjected to systemic hypertonicity, the cellular expression of TonEBP through immunocytochemistry and the cellular expression of osmoprotective genes, namely aldose reductase (AR), sodium-dependent myo-inositol transporter (SMIT), betaine/GABA transporter (BGT1) and taurine transporter (TauT), by in situ hybridization using non-radioactive digoxigenin-labeled riboprobes. In neurons where TonEBP was strongly tonicity-induced, AR-mRNA labeling was strongly increased in some subsets (e.g. hippocampus pyramidal cells, cerebellar Purkinje cells and neurons of the hypothalamic magnocellular nuclei) but remained undetectable in some other subsets (e.g. neurons in cerebral cortex). Tonicity-induced AR-mRNA labeling was observed only several hours after the tonicity-induced expression of TonEBP. SMIT-mRNA labeling was tonicity-induced as densely and evenly distributed dots in neuron poor regions (e.g. cerebral cortex layer I and hippocampus stratum lacunosum-moleculare). The tonicity-induced expression of SMIT-mRNA may thus occur in non-neuronal cells, presumably astrocytes, where TonEBP is neither significantly expressed, nor tonicity-induced. In neurons showing a strong tonicity-induced expression of TonEBP, no SMIT-mRNA labeling was observed. BGT1-mRNA and TauT-mRNA labeling could not be detected, even after systemic hypertonicity. The present work reveals large discrepancies between the cellular distribution of the tonicity-induced expression of osmoprotective genes and that of their regulatory transactivator TonEBP. Depending on the cell subsets and the osmoprotective genes, TonEBP may appear insufficient or conversely unnecessary for the tonicity-induced activation of an osmoprotective gene. Altogether our results show that brain cells, even from the same class, activate distinct osmoprotective genes through distinct activation processes to adapt to hypertonicity.

Creatine as a compatible osmolyte in muscle cells exposed to hypertonic stress

Exposure of C2C12 muscle cells to hypertonic stress induced an increase in cell content of creatine transporter mRNA and of creatine transport activity, which peaked after about 24 h incubation at 0.45 osmol (kg H(2)O)(-1). This induction of transport activity was prevented by addition of either cycloheximide, to inhibit protein synthesis, or of actinomycin D, to inhibit RNA synthesis. Creatine uptake by these cells is largely Na(+) dependent and kinetic analysis revealed that its increase under hypertonic conditions resulted from an increase in V(max) of the Na(+)-dependent component, with no significant change in the K(m) value of about 75 mumol l(-1). Quantitative real-time PCR revealed a more than threefold increase in the expression of creatine transporter mRNA in cells exposed to hypertonicity. Creatine supplementation significantly enhanced survival of C2C12 cells incubated under hypertonic conditions and its effect was similar to that obtained with the well known compatible osmolytes, betaine, taurine and myo-inositol. This effect seemed not to be linked to the energy status of the C2C12 cells because hypertonic incubation caused a decrease in their ATP content, with or without the addition of creatine at 20 mmol l(-1) to the medium. This induction of creatine transport activity by hypertonicity is not confined to muscle cells: a similar induction was shown in porcine endothelial cells.

The role of the neutral amino acid transporter SNAT2 in cell volume regulation

Sodium-dependent neutral amino acid transporter-2 (SNAT2), the ubiquitous member of SLC38 family, accounts for the activity of transport system A for neutral amino acids in most mammalian tissues. As the transport process performed by SNAT2 is highly energized, system A substrates, such as glutamine, glycine, proline and alanine, reach high transmembrane gradients and constitute major components of the intracellular amino acid pool. Moreover, through a complex array of exchange fluxes, involving other amino acid transporters, and of metabolic reactions, such as the synthesis of glutamate from glutamine, SNAT2 activity influences the cell content of most amino acids, thus determining the overall size and the composition of the intracellular amino acid pool. As amino acids represent a large fraction of cell organic osmolytes, changes of SNAT2 activity are followed by modifications in both cell amino acids and cell volume. This mechanism is utilized by many cell types to perform an effective regulatory volume increase (RVI) upon hypertonic exposure. Under these conditions, the expression of SNAT2 gene is induced and newly synthesized SNAT2 proteins are preferentially targeted to the cell membrane, leading to a significant increase of system A transport Vmax. In cultured human fibroblasts incubated under hypertonic conditions, the specific silencing of SNAT2 expression, obtained with anti-SNAT2 siRNAs, prevents the increase in system A transport activity, hinders the expansion of intracellular amino acid pool, and significantly delays cell volume recovery. These results demonstrate the pivotal role played by SNAT2 induction in the short-term hypertonic RVI and suggest that neutral amino acids behave as compatible osmolytes in hypertonically stressed cells.

Differential cellular distribution of tonicity-induced expression of transcription factor TonEBP in the rat brain following prolonged systemic hypertonicity

In a previous work performed on cerebral cortex and hippocampus we reported that tonicity-responsive enhancer binding protein (TonEBP), originally identified as a transactivator of osmoprotective genes involved in osmoadaptation of renal cells, was induced in neurons only, but to varying levels, following acute systemic hypertonicity. Whether or not this cellular specificity reflected a unique ability of neurons or a differential time course among brain cells for tonicity-induction of TonEBP was investigated throughout the brain in this study by subjecting the animals to prolonged systemic hypertonicity. In normal rats, TonEBP immunolabeling and TonEBP-mRNA in situ hybridization labeling showed a widespread, uneven and parallel distribution. TonEBP was expressed primarily in the cell nuclei of neurons, where it was heterogeneously distributed in a nucleoplasmic and a granular pool. In rats subjected to prolonged systemic hypertonicity, TonEBP labeling increased in the cell nuclei of neurons only. The tonicity-induced expression of TonEBP for a given cell group of neurons was rather uniform but varied greatly among neuronal cell groups and was positively correlated with the average size of the cell nuclei, as determined by quantitative analysis of digitized images. The detailed distribution of tonicity-induced expression of TonEBP is reported throughout the brain. In normal rats, a very minor proportion of non-neuronal cells, identified as a subset of astrocytes and possibly oligodendrocytes, showed faint nuclear immunolabeling, which however did not increase in hypertonic animals. Ependymocytes, capillary endothelial cells, and microglial cells showed no TonEBP labeling, even in hypertonic animals. Altogether our data indicate that neurons, albeit possibly to a varying extent, are the only brain cells able to use TonEBP-mediated processes for adaptation to a systemic hyperosmotic unbalance.

Increased Levels of Inducible HSP70 in Cells Exposed to Electromagnetic Fields

Because reports in the literature on the effects of electromagnetic fields (EMFs) on expression of the 70-kDa heat-shock protein (HSP70) are somewhat contradictory, we studied the influence of low-frequency EMFs on the accumulation of inducible HSP70 in several cell models. Some of the cell types tested showed increased levels of HSP70 protein when exposed for 24 h to 50 Hz, 680 microT EMFs. In endothelial cells, EMFs alone induced only a poor and transient activation of the heat-shock transcription factor 1 (HSF1); however, neither the level of HSP70 mRNA nor the synthesis of HSP70 appeared to be altered significantly. Accordingly, transfection experiments involving HSP70 promoter showed that gene transcription was not affected. We also noted a marked reduction in proteasome activities in cell extracts exposed to EMFs. Interestingly, the heat-shock-induced levels of HSP70 mRNA and protein were increased by a concomitant weak stressor like EMFs. Taken together, our results indicate that in EMF-exposed endothelial cells, HSP70 gene transcription and translation are unaffected; however, EMFs alone promoted accumulation of the inducible HSP70 protein, probably by increasing its stability, and it enhanced accumulation and translation of the heat-induced HSP70 mRNA when applied in concert with heat shock.

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.

Effect of Hyperosmotic Conditions on the Expression of the Betaine-GABA-Transporter (BGT-1) in Cultured Mouse Astrocytes

The adaptation of cells to hyperosmotic conditions involves accumulation of organic osmolytes to achieve osmotic equilibrium and maintenance of cell volume. The Na+ and Cl(-)-coupled betaine/GABA transporter, designated BGT-1, is responsible for the cellular accumulation of betaine and has been proposed to play a role in osmoregulation in the brain. BGT-1 is also called GAT2 (GABA transporter 2) when referring to the mouse transporter homologue. Using Western Blotting the expression of the mouse GAT2 protein was investigated in astrocyte primary cultures exposed to a growth medium made hyperosmotic (353+/-2.5 mosmol/kg) by adding sodium chloride. A polyclonal anti-BGT-1 antibody revealed the presence of two characteristic bands at 69 and 138 kDa. When astrocytes were grown for 24 h under hyperosmotic conditions GAT2 protein was up-regulated 2-4-fold compared to the level of the isotonic control. Furthermore, the expected dimer of GAT2 was also up-regulated after 24 h under the hyperosmotic conditions. The [3H]GABA uptake was examined in the hyperosmotic treated astrocytes, and characterized using different selective GABA transport inhibitors. The up-regulation of GAT2 protein was not affecting total GABA uptake but the hyperosmotic condition did change total GABA uptake possibly involving GAT1. Immunocytochemical studies revealed cell membrane localization of GAT2 throughout astroglial processes. Taken together, these results indicate that astroglial GAT2 expression and function may be regulated by hyperosmolarity in cultured mouse astrocytes, suggesting a role of GAT2 in osmoregulation in neural cells.

High NaCl increases TonEBP/OREBP mRNA and protein by stabilizing its mRNA

Hypertonicity increases mRNA and protein abundance of the transcription factor tonicity-responsive enhancer/osmotic response element binding protein (TonEBP/OREBP), contributing to increased transcription of downstream osmoprotective genes. Previously, this was attributed to increased transcription of TonEBP/OREBP because no change was found in its mRNA stability. However, there is no direct evidence for increased transcription, and the 3'-untranslated region (UTR) of TonEBP/OREBP contains numerous adenylate/uridylate-rich elements, which can modulate RNA stability. Therefore, we have reinvestigated the effect of hypertonicity on TonEBP/OREBP mRNA stability. We find that, in mouse inner medullary collecting duct cells, raising osmolality from 300 to 500 mosmol/kgH(2)O by adding NaCl increases TonEBP/OREBP mRNA to a peak of 2.3-fold after 4 h, followed by a decline. TonEBP/OREBP protein increases to a sustained peak of 3.0-fold at 8 h. To determine the stability of TonEBP/OREBP mRNA, we measured the rate of its decrease after inhibiting transcription with actinomycin D, finding that it is stabilized for 6 h after addition of NaCl. This stabilization is sufficient to explain the increase in mRNA without any change in transcription. To investigate how hypertonicity stabilizes TonEBP/OREBP mRNA, we tested luciferase reporters containing parts of the TonEBP/OREBP mRNA UTR. Inclusion of both the 5'- and 3'-UTR increases reporter activity, consistent with mRNA stabilization. Surprisingly, however, it is the 5'-UTR that stabilizes; the 3'-UTR, by itself, decreases reporter activity. We concluded that 1) hypertonicity stabilizes TonEBP/OREBP mRNA, contributing to its increase, and 2) stabilization depends on the presence of the 5'-UTR.

Roles of compatible osmolytes and heat shock protein 70 in the induction of tolerance to stresses in porcine endothelial cells

Studies of the responses of porcine pulmonary endothelial cells to acute hypertonic stress have been extended by examining the induction and underlying mechanisms of cell tolerance to both osmotic and heat stresses. Preliminary adaptation of these cells to 0.4osmol (kg H(2)O)(-1) rendered them tolerant either to subsequent severe osmotic stress (0.7osmol (kg H(2)O)(-1)) or to subsequent severe heat shock (50 min at 49 degrees C). In contrast, preliminary exposure of the cells to mild heat shock (44 degrees C for 30 min) induced tolerance only to severe heat shock, not to hyperosmotic stress. Induction of tolerance to heat shock by either procedure correlated with the induced expression of heat shock protein 70 (HSP70). Induction of tolerance to hyperosmotic stress, on the other hand, was associated with the cellular accumulation of osmolytes, such as amino acids, betaine and myo-inositol, and did not correlate with the induced expression of HSP70. It also required a reduction in the final change of osmotic pressure applied to the cells, such that maximum cell shrinkage would not be much more than 40%. In general, therefore, HSP70 and compatible osmolytes have distinct roles in cellular adaptation to these stresses.

Subcellular redistribution of the renal betaine transporter during hypertonic stress

The betaine transporter (BGT1) protects cells in the hypertonic renal inner medulla by mediating uptake and accumulation of the osmolyte betaine. Transcriptional regulation plays an essential role in upregulation of BGT1 transport when renal cells are exposed to hypertonic medium for 24 h. Posttranscriptional regulation of the BGT1 protein is largely unexplored. We have investigated the distribution of BGT1 protein in live cells after transfection with BGT1 tagged with enhanced green fluorescent protein (EGFP). Fusion of EGFP to the NH2 terminus of BGT1 produced a fusion protein (EGFP-BGT) with transport properties identical to normal BGT1, as determined by ion dependence, inhibitor sensitivity, and apparent Km for GABA. Confocal microscopy of EGFP-BGT fluorescence in transfected Madin-Darby canine kidney (MDCK) cells showed that hypertonic stress for 24 h induced a shift in subcellular distribution from cytoplasm to plasma membrane. This was confirmed by colocalization with anti-BGT1 antibody staining. In fibroblasts, transfected EGFP-BGT caused increased transport in response to hypertonic stress. The activation of transport was not accompanied by increased expression of EGFP-BGT, as determined by Western blotting. Membrane insertion of EGFP-BGT protein in MDCK cells began within 2-3 h after onset of hypertonic stress and was blocked by cycloheximide. We conclude that posttranscriptional regulation of BGT1 is essential for adaptation to hypertonic stress and that insertion of BGT1 protein to the plasma membrane may require accessory proteins.

Effects of osmolarity, ions and compatible osmolytes on cell-free protein synthesis

To mimic what might happen in cells exposed to hypertonicity, the effects of increased osmolarity and ionic strength on cell-free protein synthesis have been examined. Translation of globin mRNA by rabbit reticulocyte lysate decreased by 30-60% when osmolality was increased from 0.35 to 0.53 osmol/kg of water by the addition of NaCl, KCl, CH(3)CO(2)Na or CH(3)CO(2)K. In contrast, equivalent additions of the compatible osmolytes betaine or myo -inositol caused a 40-50% increase in the rate of translation, whereas amino acids (50-135 mM) that are transported via system A had no significant effect. Addition of 75 mM KCl caused a dramatic fall in the amount of the 43 S pre-initiation complex, whereas it was totally preserved when osmolarity was similarly increased by the addition of 150 mM betaine. The formation of a non-enzymic initiation complex between rabbit [(3)H]Phe-tRNA, poly(U) and the 80 S ribosomes was unaffected by the addition of 75 mM NaCl or KCl, but translation of the complex decreased by 70%. Density-gradient centrifugation of reticulocyte extracts translating endogenous mRNA revealed that addition of 150 mM betaine had no effect, whereas addition of 75 mM KCl caused a marked decrease in the polysome peak, concomitant with an increase in the proportion of 80 S ribosomes and ribosomal subunits, even when elongation was inhibited with fragment A of diphtheria toxin. These results are consistent with the notion that both initiation and elongation are inhibited by unusually high concentrations of inorganic ions, but not by the compatible osmolytes betaine or myo -inositol.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

Compatible osmolytes modulate the response of porcine endothelial cells to hypertonicity and protect them from apoptosis

Porcine pulmonary arterial endothelial cells accumulated myo-inositol and taurine, as well as betaine, during adaptation to hypertonic stress. The cells grew and maintained their normal morphology during culture in hypertonic (0.5 osmol (kg H(2)O)(-1)) medium that contained osmolytes such as betaine, myo-inositol or taurine at concentrations close to reported physiological values. The cells did not grow well in hypertonic medium depleted of potential compatible osmolytes. After a few days, cell density decreased by about 50 % and many cells rounded up and detached from the plates, their nuclei showing clear apoptotic morphology. The caspase-3 activity of the cells also increased dramatically under these conditions, but remained negligibly low when betaine and myo-inositol were added to the medium. Addition of betaine and myo-inositol to hypertonic medium depleted of compatible osmolytes increased the number of colonies remaining after 12 days of culture; with each solute at 30-100 micromol l(-1) the number increased about sixfold. In the absence of compatible osmolytes, increased mRNA levels and corresponding activities of betaine/gamma-aminobutyric acid transporter (BGT1) and sodium/myo-inositol transporter (SMIT) induced by hypertonicity remained high after 72 h incubation, whereas they were down regulated in the presence of betaine and myo-inositol. Similarly, the down regulation of the amino acid System A transporter (ATA2) was markedly slowed in the absence of compatible osmolytes. We conclude that these compatible osmolytes at concentrations close to physiological values enable the endothelial cells to adapt to hypertonic stress, protecting them from apoptosis, and also modulate the adaptation process.