Functional regulation of Na+-dependent neutral amino acid transporter ASCT2 by S-nitrosothiols and nitric oxide in Caco-2 cells

P2X7 Receptor Augments LPS-Induced Nitrosative Stress by Regulating Nrf2 and GSH Levels in the Mouse Hippocampus

P2X7 receptor (P2X7R) regulates inducible nitric oxide synthase (iNOS) expression/activity in response to various harmful insults. Since P2X7R deletion paradoxically decreases the basal glutathione (GSH) level in the mouse hippocampus, it is likely that P2X7R may increase the demand for GSH for the maintenance of the intracellular redox state or affect other antioxidant defense systems. Therefore, the present study was designed to elucidate whether P2X7R affects nuclear factor-erythroid 2-related factor 2 (Nrf2) activity/expression and GSH synthesis under nitrosative stress in response to lipopolysaccharide (LPS)-induced neuroinflammation. In the present study, P2X7R deletion attenuated iNOS upregulation and Nrf2 degradation induced by LPS. Compatible with iNOS induction, P2X7R deletion decreased S-nitrosylated (SNO)-cysteine production under physiological and post-LPS treated conditions. P2X7R deletion also ameliorated the decreases in GSH, glutathione synthetase, GS and ASCT2 levels concomitant with the reduced S-nitrosylations of GS and ASCT2 following LPS treatment. Furthermore, LPS upregulated cystine:glutamate transporter (xCT) and glutaminase in P2X7R^+/+ mice, which were abrogated by P2X7R deletion. LPS did not affect GCLC level in both P2X7R^+/+ and P2X7R^−/− mice. Therefore, our findings indicate that P2X7R may augment LPS-induced neuroinflammation by leading to Nrf2 degradation, aberrant glutamate-glutamine cycle and impaired cystine/cysteine uptake, which would inhibit GSH biosynthesis. Therefore, we suggest that the targeting of P2X7R, which would exert nitrosative stress with iNOS in a positive feedback manner, may be one of the important therapeutic strategies of nitrosative stress under pathophysiological conditions.

Leucine stimulates ASCT2 amino acid transporter expression in porcine jejunal epithelial cell line (IPEC-J2) through PI3K/Akt/mTOR and ERK signaling pathways

Leucine has been shown to influence intestinal protein metabolism, cell proliferation and migration. Furthermore, our previous study demonstrated that branched-chain amino acids could modulate the intestinal amino acid and peptide transporters in vivo. As the possible mechanisms are still largely unknown, in the present work, we studied the transcriptional and translational regulation of leucine on amino acid transporter production in IPEC-J2 cells and the signaling pathways involved. Treatment of IPEC-J2 cells with 7.5 mM leucine enhanced the mRNA expression of the Na(+)-neutral AA exchanger 2 (ASCT2) and 4F2 heavy chain (4F2hc) and caused an increase in ASCT2 protein expression. Leucine also activated phosphorylation of 4E-BP1 and eIF4E through the phosphorylation of mTOR, Akt and ERK signaling pathways in IPEC-J2 cells. Pre-treatment of IPEC-J2 cells with inhibitors of mTOR and Akt (rapamycin and wortmannin) or an inhibitor of ERK (PD098059) for 30 min before leucine treatment attenuated the positive effect of leucine in enhancing the protein abundance of ASCT2. These results demonstrate that leucine could up-regulate the expression of the amino acid transporters (ASCT2) through transcriptional and translational regulation by ERK and PI3K/Akt/mTOR activation.

Solute carriers (SLCs) in cancer

During tumor progression cells acquire an altered metabolism, either as a cause or as a consequence of an increased need of energy and nutrients. All four major classes of macromolecules are affected: carbohydrates, proteins, lipids and nucleic acids. As a result of the changed needs, solute carriers (SLCs) which are the major transporters of these molecules are differently expressed. This renders them important targets in the treatment of cancer. Blocking or activating SLCs is one possible therapeutic strategy. For example, some SLCs are upregulated in tumor cells due to the increased demand for energy and nutritional needs. Thus, blocking them and turning off the delivery of fuel or nutrients could be one way to interfere with tumor progression. Specific drug delivery to cancer cells via transporters is another approach. Some SLCs are also interesting as chemosensitizing targets because blocking or activating them may result in an altered response to chemotherapy. In this review we summarize the roles of SLCs in cancer therapy and specifically their potential as direct or indirect targets, as drug carriers or as chemosensitizing targets.

Renal amino acid transport systems and essential hypertension

Several clinical and animal studies suggest that "blood pressure goes with the kidney," that is, a normotensive recipient of a kidney genetically programmed for hypertension will develop hypertension. Intrarenal dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport. The candidate transport systems for L-DOPA, the source for dopamine, include the sodium-dependent systems B(0), B(0,+), and y(+)L, and the sodium-independent systems L (LAT1 and LAT2) and b(0,+). Renal LAT2 is overexpressed in the prehypertensive spontaneously hypertensive rat (SHR), which might contribute to enhanced L-DOPA uptake in the proximal tubule and increased dopamine production, as an attempt to overcome the defect in D1 receptor function. On the other hand, it has been recently reported that impaired arginine transport contributes to low renal nitric oxide bioavailability observed in the SHR renal medulla. Here we review the importance of renal amino acid transporters in the kidney and highlight pathophysiological changes in the expression and regulation of these transporters in essential hypertension. The study of the regulation of renal amino acid transporters may help to define the underlying mechanisms predisposing individuals to an increased risk for development of hypertension.

Biphasic regulation of P-glycoprotein function and expression by NO donors in Caco-2 cells

AIM

To investigate the effects of nitric oxide (NO) donors on the function and expression of P-glycoprotein (P-gp) in Caco-2 cells.

METHODS

Caco-2 cells were exposed to NO donors for designated times. P-gp function and expression were assessed using Rhodamine123 uptake assay and Western blotting, respectively. Intracellular reactive oxygen species (iROS) and intracellular reactive nitrogen species (iRNS) levels were measured using ROS and RNS assay kits, respectively.

RESULTS

Exposure of Caco-2 cells to 0.1 or 2 mmol/L of sodium nitroprusside (SNP) affected the function and expression of P-gp in concentration- and time-dependent manners. A short-term (4 h) exposure reduced P-gp function and expression accompanied with significantly increased levels of iROS and iRNS. In contrast, a long-term (24 h) exposure stimulated the P-gp function and expression. The stimulatory effects of 2 mmol/L SNP was less profound as compared to those caused by 0.1 mmol/L SNP. The other NO donors SIN-1 and SNAP showed similar effects. Neither the NO scavenger PTIO (2 mmol/L) nor soluble guanylate cyclase inhibitor ODQ (50 μmol/L) reversed the SNP-induced alteration of P-gp function. On the other hand, free radical scavengers ascorbate, glutathione and uric acid (2 mmol/L for each), PKC inhibitor chelerythrine (5 μmol/L), PI3K/Akt inhibitor wortmannin (1 μmol/L) and p38 MAPK inhibitor SB203580 (10 μmol/L) reversed the upregulation of P-gp function by the long-term exposure to SNP, but these agents had no effect on the impaired P-gp function following the short-term exposure to SNP.

CONCLUSION

NO donors time-dependently regulate P-gp function and expression in Caco-2 cells: short-term exposure impairs P-gp function and expression, whereas long-term exposure stimulates P-gp function and expression. The regulation occurs via a NO-independent mechanism.

High glucose decreases expression and activity of p-glycoprotein in cultured human retinal pigment epithelium possibly through iNOS induction

Inhibition of p-glycoprotein under hyperglycemic conditions has been reported in various barrier tissues including blood-brain barrier, intestine, and kidney, and has been linked to significant clinical complications. However, whether this is also true for the outer blood-retinal barrier constituted by retinal pigment epithelium, or has a role in pathogenesis of diabetic retinopathy is not yet clear. In this study, using cultured human retinal pigment epithelium cell line D407, we found that high glucose exposure induced a significant decrease in p-glycoprotein expression both at mRNA and at protein levels, accompanied by an attenuated p-glycoprotein activity determined by intracellular rhodamine 123 retention. In marked contrast, the expressions of both mRNA and protein levels of inducible nitrate oxide synthase (iNOS) increased, and were accompanied by increased extracellular nitrate/nitrite production by Griess reaction. In addition, mRNA levels of nuclear receptors revealed a decreased expression of pregnane X receptor after the exposure of high glucose. However, the subsequent alterations in production of nitrate/nitrite, functional expression of p-glycoprotein, and mRNA levels of pregnane X receptor were partially blocked when pretreated with S,S′-1,3-phenylene-bis(1,2-ethanediyl)-bis-isothiourea•2HBr (PBITU), a selective iNOS inhibitor. Moreover, the effects of PBITU were antagonized with the addition of L-arginine, a substrate for NO synthesis. Our in vitro results suggest for the first time that iNOS induction plays a novel role in decreased p-glycoprotein expression and transport function at the human outer blood-retinal barrier under hyperglycemic conditions and further support the concept of inhibiting iNOS pathway as a therapeutic strategy for diabetic retinopathy.

Cisplatin upregulates glutamine transport in human intestinal epithelial cells: the protective mechanism of glutamine on intestinal mucosa after chemotherapy

BACKGROUND

Glutamine (GLN) prevents the intestinal mucosal injury induced by chemotherapy, although the mechanism of this protective action has not yet been elucidated. Amino acid transport across the plasma membrane is essential for supplying enterocytes with amino acids for cellular metabolism. It was hypothesized that chemotherapy stimulates GLN transport, which enables GLN to be used more efficiently as a metabolic fuel.

METHODS

A rat model was used to examine the effect of enteral GLN on intestinal mucosal injury induced by intraperitoneal injection of cisplatin (7.0 mg/kg of body weight). The effects of cisplatin on amino acid transport and the expression of messenger RNA and protein were evaluated by real-time polymerase chain reaction and Western blot analysis, respectively, in the human intestinal epithelial cell line Caco-2. The effects of cisplatin on glutaminase activity and intracellular glutathione were also studied.

RESULTS

GLN prevented mucosal atrophy induced by cisplatin in rats. In Caco-2 cells, cisplatin significantly increased GLN transport and the expression of GLN transporter ASCT2 messenger RNA and protein. Leucine, but not glutamate, transport significantly increased in the cisplatin-treated group due to the increase in LAT1 (leucine transporter) protein expression. Glutaminase activity and intracellular glutathione increased significantly in the cisplatin-treated group.

CONCLUSIONS

Bolus enteral GLN prevents intestinal mucosal injury induced by cisplatin in rats, as demonstrated by increased GLN transport and increased GLN transporter expression after cisplatin administration.

Human endogenous retroviruses and multiple sclerosis: innocent bystanders or disease determinants?

Human endogenous retroviruses (HERVs) constitute 5–8% of human genomic DNA and are replication incompetent despite expression of individual HERV genes from different chromosomal loci depending on the specific tissue. Several HERV genes have been detected as transcripts and proteins in the central nervous system, frequently in the context of neuroinflammation. The HERV-W family has received substantial attention in large part because of associations with diverse syndromes including multiple sclerosis (MS) and several psychiatric disorders. A HERV-W-related retroelement, multiple sclerosis retrovirus (MSRV), has been reported in MS patients to be both a biomarker as well as an effector of aberrant immune responses. HERV-H and HERV-K have also been implicated in MS and other neurological diseases but await delineation of their contributions to disease. The HERV-W envelope-encoded glycosylated protein, syncytin-1, is encoded by chromosome 7q21 and exhibits increased glial expression within MS lesions. Overexpression of syncytin-1 in glia induces endoplasmic reticulum stress leading to neuroinflammation and the induction of free radicals, which damage proximate cells. Syncytin-1's receptor, ASCT1 is a neutral amino acid transporter expressed on glia and is suppressed in white matter of MS patients. Of interest, antioxidants ameliorate syncytin-1's neuropathogenic effects raising the possibility of using these agents as therapeutics for neuroinflammatory diseases. Given the multiple insertion sites of HERV genes as complete and incomplete open reading frames, together with their differing capacity to be expressed and the complexities of individual HERVs as both disease markers and bioactive effectors, HERV biology is a compelling area for understanding neuropathogenic mechanisms and developing new therapeutic strategies.

Intestinal gaboxadol absorption via PAT1 (SLC36A1): modified absorption in vivo following co-administration of L-tryptophan

BACKGROUND AND PURPOSE

Gaboxadol has been in development for treatment of chronic pain and insomnia. The clinical use of gaboxadol has revealed that adverse effects seem related to peak serum concentrations. The aim of this study was to investigate the mechanism of intestinal absorption of gaboxadol in vitro and in vivo.

EXPERIMENTAL APPROACH

In vitro transport investigations were performed in Caco-2 cell monolayers. In vivo pharmacokinetic investigations were conducted in beagle dogs. Gaboxadol doses of 2.5 mg.kg(-1) were given either as an intravenous injection (1.0 mL.kg(-1)) or as an oral solution (5.0 mL.kg(-1)).

KEY RESULTS

Gaboxadol may be a substrate of the human proton-coupled amino acid transporter, hPAT1 and it inhibited the hPAT1-mediated L-[(3)H]proline uptake in Caco-2 cell monolayers with an inhibition constant K(i) of 6.6 mmol.L(-1). The transepithelial transport of gaboxadol was polarized in the apical to basolateral direction, and was dependent on gaboxadol concentration and pH of the apical buffer solution. In beagle dogs, the absorption of gaboxadol was almost complete (absolute bioavailability, F(a), of 85.3%) and T(max) was 0.46 h. Oral co-administration with 2.5-150 mg.kg(-1) of the PAT1 inhibitor, L-tryptophan, significantly decreased the absorption rate constant, k(a), and C(max), and increased T(max) of gaboxadol, whereas the area under the curve and clearance of gaboxadol were constant.

CONCLUSIONS AND IMPLICATIONS

The absorption of gaboxadol across the luminal membrane of the small intestinal enterocytes is probably mediated by PAT1. This knowledge is useful for reducing gaboxadol absorption rates in order to decrease peak plasma concentrations.

In human entrocytes, GLN transport and ASCT2 surface expression induced by short-term EGF are MAPK, PI3K, and Rho-dependent

Glutamine, a key nutrient for the enterocyte, is transported among other proteins by ASCT2. Epidermal growth factor (EGF) augments intestinal adaptation. We hypothesized that short-term treatment of human enterocytes with EGF enhances glutamine transport by increasing membranal ASCT2. To elucidate EGF-induced mechanisms, monolayers of C2(BBe)1 w/wo siRho transfection were treated w/wo EGF and w/wo tyrphostin AG1478 (AG1478), wortmanin, or PD98059. Total and system-specific (3)H-glutamine transports were determined w/wo 5 mmol/l amino acid inhibitors. Total and membranal ASCT2 proteins were measured by Westerns. EGF doubled glutamine transport by increasing B(0)/ASCT2 and B(0,+) activities. Despite the doubling of membranal ASCT2 protein with EGF treatment, total ASCT2 did not change. The increases in B(0)/ASCT2 activity and ASCT2 protein were eliminated by AG1478, PD98059, wortmanin, and siRho, while transport by B(0,+) was inhibited only by PD98059 and siRho. Thus, differential pathways are involved in EGF-induced increase in B(0)/ASCT2 glutamine transport and membranal ASCT2 compared to those involved in B(0,+) activity.

Genomic regulation of intestinal amino acid transporters by aldosterone

Overexpression of renal LAT2, a Na+ -independent L-amino acid transporter, in spontaneous hypertensive rats (SHR) is organ specific and precedes the onset of hypertension (Pinho et al., Hypertension, 42:613-618, 2003). However, the expression of LAT2 correlates negatively with plasma aldosterone levels after high sodium intake (Pinho et al., Am J Physiol Ren Physiol 292:F1452-F1463, 2007). The present study evaluated the expression of Na+ -independent LAT1, LAT2, and 4F2hc and Na+ -dependent ASCT2 amino acid transporters in the intestine of normotensive Wistar rats chronically treated with aldosterone. In conditions of high salt intake, to keep endogenous aldosterone to a minimum, rats were implanted with aldosterone or spironolactone tablets. In aldosterone-treated and aldosterone + spironolactone-treated rats, aldosterone plasma levels were increased by fourfold. At the protein level, aldosterone treatment significantly increased LAT1 (62%), LAT2 (49%), 4F2hc (48%), and ASCT2 (65%) expression. The effect of aldosterone upon LAT1, LAT2, 4F2hc, and ASCT2 protein abundance was completely reversed by spironolactone. Aldosterone significantly increased intestinal LAT2 and 4F2hc mRNA levels (27% and 35% increase, respectively), with no changes in LAT1 and ASCT2 transcript levels. In conclusion, increases in intestinal Na+ -independent LAT1 and LAT2 and Na+ -dependent ASCT2 transcript and protein abundance during chronic treatment with aldosterone occur through a spironolactone-sensitive genomic mechanism.

Na+-dependent neutral amino acid transporter ASCT2 is downregulated in seriously traumatized human intestinal epithelial cells

OBJECTIVE

Serious trauma to the body often is associated with changes in protein metabolism in multiple organs and tissues. Clinically, the catabolic response results in a generalized negative nitrogen balance. Nutrition support has been an important component of the care of seriously traumatized patients. However, during states of severe trauma, enterocyte transport function remains unclear. This study aims to quantitate the Na+-dependent neutral amino acid transport and expression of its transporter in traumatically injured Caco-2 cell lines.

MATERIALS AND METHODS

Transport and transporter of Na+-dependent neutral amino acid in Caco-2 cell lines were characterized. Then the cell lines were cultured under hypoxic, nutrient-deprived, and ischemic conditions for 1, 2, 4, and 6 hours. After severe trauma was performed, we investigated the transport of Na+-dependent neutral amino acids and the expression of transporter protein and mRNA in apical membrane vesicles.

RESULTS

Among the neutral amino acid transporters, only ASCT2 mRNA was amplified successfully. Under nutrient-deprived and ischemic conditions, transport of L-alanine and L-glutamine decreased significantly compared with control (P < 0.01), whereas hypoxia had no significant effect. The changes were associated with a decrease in maximum transport velocity without an influence on transport affinity. Expression of relative transporter proteins and mRNA decreased significantly compared with control (P < 0.01).

CONCLUSIONS

Na+-dependent neutral amino acid transport and its key transporter are differently regulated during state of traumatic injury. It may be of use to provide some strategies targeting the special nutrient requirements and transport capabilities of seriously traumatized patients.

Functional induction of P-glycoprotein in the blood-brain barrier of streptozotocin-induced diabetic rats: evidence for the involvement of nuclear factor-kappaB, a nitrosative stress-sensitive transcription factor, in the regulation

The objective of this study was to investigate the transport kinetics of cyclosporin A, a well known substrate for P-glycoprotein (P-gp), across the blood-brain barrier (BBB), and the expression of the transporter in the brain of streptozotocin-induced diabetic rats. The in vivo transport clearance of cyclosporin A was significantly reduced in diabetic rats compared with that in the control. The decreased transport was associated with the increased level of mRNA and the protein for P-glycoprotein in the rat brain. The functional activity of the efflux transporter in mouse brain capillary endothelial (MBEC4) cells, an in vitro model of the BBB, was also stimulated when slow nitric oxide (NO)-releasing donors were present, whereas the stimulation was absent in the case of rapid NO-releasing donors (e.g., S-nitroso-N-acetyl-dl-penicillamine and diethylenetriamine). The stimulatory effect was highest for sodium nitroprusside (SNP) and the functional induction associated with the increased mRNA and protein level of the transporter. The pretreatment of the cell with SNP along with ascorbate, methylene blue, or superoxide dismutase attenuated the induction of function and expression for P-glycoprotein, suggesting that the reaction product between superoxide and NO is involved in the induction of function and expression. The level of nuclear translocation of nuclear factor-kappaB (NF-kappaB) and DNA binding activity of nuclear extracts to the NF-kappaB consensus oligonucleotide was increased in MBEC4 cells pretreated with SNP. Taken together, these observations suggest that nitrosative stress leads to the up-regulation of the message for the efflux transporter and, ultimately, to the enhanced function, probably via a NF-kappaB-dependent mechanism.

The human endogenous retrovirus envelope glycoprotein, syncytin-1, regulates neuroinflammation and its receptor expression in multiple sclerosis: a role for endoplasmic reticulum chaperones in astrocytes

Retroviral envelopes are pathogenic glycoproteins which cause neuroinflammation, neurodegeneration, and endoplasmic reticulum stress responses. The human endogenous retrovirus (HERV-W) envelope protein, Syncytin-1, is highly expressed in CNS glia of individuals with multiple sclerosis (MS). In this study, we investigated the mechanisms by which Syncytin-1 mediated neuroimmune activation and oligodendrocytes damage. In brain tissue from individuals with MS, ASCT1, a receptor for Syncytin-1 and a neutral amino acid transporter, was selectively suppressed in astrocytes (p < 0.05). Syncytin-1 induced the expression of the endoplasmic reticulum stress sensor, old astrocyte specifically induced substance (OASIS), in cultured astrocytes, similar to findings in MS brains. Overexpression of OASIS in astrocytes increased inducible NO synthase expression but concurrently down-regulated ASCT1 (p < 0.01). Treatment of astrocytes with a NO donor enhanced expression of early growth response 1, with an ensuing reduction in ASCT1 expression (p < 0.05). Small-interfering RNA molecules targeting Syncytin-1 selectively down-regulated its expression, preventing the suppression of ASCT1 and the release of oligodendrocyte cytotoxins by astrocytes. A Syncytin-1-transgenic mouse expressing Syncytin-1 under the glial fibrillary acidic protein promoter demonstrated neuroinflammation, ASCT1 suppression, and diminished levels of myelin proteins in the corpus callosum, consistent with observations in CNS tissues from MS patients together with neurobehavioral abnormalities compared with wild-type littermates (p < 0.05). Thus, Syncytin-1 initiated an OASIS-mediated suppression of ASCT1 in astrocytes through the induction of inducible NO synthase with ensuing oligodendrocyte injury. These studies provide new insights into the role of HERV-mediated neuroinflammation and its contribution to an autoimmune disease.

Underexpression of the Na+-dependent neutral amino acid transporter ASCT2 in the spontaneously hypertensive rat kidney

This study examined the inward transport of l-[14C]alanine, an ASCT2 preferential substrate, in monolayers of immortalized renal proximal tubular epithelial (PTE) cells from Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. The expression of ASCT2 in WKY and SHR PTE cells and kidney cortices from WKY and SHR was also evaluated. l-[14C]alanine uptake was highly dependent on extracellular Na+. Replacement of NaCl by LiCl or choline chloride abolished transport activity in SHR and WKY PTE cells. In the presence of the system L inhibitor BCH, Na+-dependent l-alanine uptake in WKY and SHR PTE cells was inhibited by alanine, serine, and cysteine, which is consistent with amino acid transport through ASCT2. The saturable component of Na+-dependent l-alanine transport under Vmax conditions in SHR PTE cells was one-half of that in WKY PTE cells, with similar Km values. Differences in magnitude of Na+-dependent l-alanine uptake through ASCT2 between WKY and SHR PTE cells correlated positively with differences in ASCT2 protein expression, this being more abundant in WKY PTE cells. Abundance of ASCT2 transcript and protein in kidney cortices of SHR rats was also lower than that in normotensive WKY rats. In conclusion, immortalized SHR and WKY PTE cells take up l-alanine mainly through a high-affinity Na+-dependent amino acid transporter, with functional features of ASCT2 transport. The activity and expression of the ASCT2 transporter were considerably lower in the SHR cells.

Resistance to the skeletal muscle injury expressed by repeated treatment with compound A that has HMG-CoA reductase inhibitory activity

It has been noted that chemical-induced initial insult is sometimes no longer detected in examinations after additional consecutive treatments, suggesting that the target organs acquire resistance to the chemical toxicity. In this study, whether acquired resistance to the skeletal muscle toxicity is observed during repeated treatment of a toxic dose of Compound A that has a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitory activity was examined. F344 male rats (7-weeks old) were given a mixed diet with 0.12% Compound A (corresponding to approximately 100 mg/kg/day) for up to 56 days. Blood samples were obtained from the tail vein periodically during the dosing period, and utilized for the measurement of creatine kinase (CK) as a marker of skeletal muscle injury. In the necropsies on Days 4, 8, 11, 28, 42 and 56, the skeletal muscles from the rectus femoris were removed for histopathology or gene expression analysis. A satellite group was provided to measure the plasma concentrations of Compound A and M1, the active metabolite of Compound A. CK levels increased from Day 9 and reached approximately 30 times those of the controls on Day 12. Histopathology of the skeletal muscle on Day 11 revealed severe necrosis of the muscle fibers. However, in spite of continuous treatments to the damaged rats, the CK levels decreased after that and returned to normal levels on Day 18. No skeletal muscle injury was observed on Days 42 and 56. There were no marked differences in the exposure levels of Compound A and M1 between Days 8 (prior to CK elevation) and 28 (post CK elevation). As for the most significant changes in the gene expression analysis for the skeletal muscle on Days 42 and 56, the probe for IkappaBa, which is known as an inhibitor for nuclear factor-kappaB (NF-kappaB), increased 2-fold compared to the control. Furthermore, an increased probe for CCAAT/enhancer-binding protein (C/EBP) delta, a transcriptional factor, and a decreased probe for cAMP-response element-binding protein (CBP)/p300, a transcriptional coactivator, were also noted significantly on Day 56. These changes in the gene expression analysis suggested suppressed NF-kappaB-mediated transactivation, which was responsible for the protective effects on the muscle injury. Based on the present findings, the resistance to skeletal muscle injury observed in this study may be attributable to the suppressed NF-kappaB-mediated transactivation, but not to the decreased exposure to toxicants.

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.

Functional and molecular identification of sodium-coupled dicarboxylate transporters in rat primary cultured cerebrocortical astrocytes and neurons

Na+-coupled carboxylate transporters (NaCs) mediate the uptake of tricarboxylic acid cycle intermediates in mammalian tissues. Of these transporters, NaC3 (formerly known as Na+-coupled dicarboxylate transporter 3, NaDC3/SDCT2) and NaC2 (formerly known as Na+-coupled citrate transporter, NaCT) have been shown to be expressed in brain. There is, however, little information available on the precise distribution and function of both transporters in the CNS. In the present study, we investigated the functional characteristics of Na+-dependent succinate and citrate transport in primary cultures of astrocytes and neurons from rat cerebral cortex. Uptake of succinate was Na+ dependent, Li+ sensitive and saturable with a Michaelis constant (Kt) value of 28.4 microM in rat astrocytes. Na+ activation kinetics revealed that the Na+ to succinate stoichiometry was 3:1 and the concentration of Na+ necessary for half-maximal transport was 53 mM. Although uptake of citrate in astrocytes was also Na+ dependent and saturable, its Kt value was significantly higher (approximately 1.2 mM) than that of succinate. Unlabeled succinate (2 mM) inhibited Na+-dependent [14C]succinate (18 microM) and [14C]citrate (4.5 microM) transport completely, whereas unlabeled citrate inhibited Na+-dependent [14C]succinate uptake more weakly. Interestingly, N-acetyl-L-aspartate, which is the second most abundant amino acid in the nervous system, also completely inhibited Na+-dependent succinate transport in rat astrocytes. The inhibition constant (Ki) for the inhibition of [14C]succinate uptake by unlabeled succinate, N-acetyl-L-aspartate and citrate was 15.9, 155 and 764 microM respectively. In primary cultures of neurons, uptake of citrate was also Na+ dependent and saturable with a Kt value of 16.2 microM, which was different from that observed in astrocytes, suggesting that different Na+-dependent citrate transport systems are expressed in neurons and astrocytes. RT-PCR and immunocytochemistry revealed that NaC3 and NaC2 are expressed in cerebrocortical astrocytes and neurons respectively. These results are in good agreement with our previous reports on the brain distribution pattern of NaC2 and NaC3 mRNA using in situ hybridization. This is the first report of the differential expression of different NaCs in astrocytes and neurons. These transporters might play important roles in the trafficking of tricarboxylic acid cycle intermediates and related metabolites between glia and neurons.

Functional Expression of Taurine Transporter and its Up-Regulation in Developing Neurons from Mouse Cerebral Cortex

PURPOSE

In the present study, we investigate the characteristics of taurine transport in primary cultures of neurons from mouse cerebral cortex to understand the possibility that taurine might attenuate the effects of central nervous system drugs.

METHODS

Primary cultured neurons from mouse cerebral cortex were used to determine the transport characteristics of taurine. The expression of taurine transporter (TAUT) in mouse neurons was determined by use of reverse transcriptase-polymerase chain reaction and Western blotting.

RESULTS

In vitro transport of taurine in mouse cerebrocortical neurons at day 9 was Na+-dependent and saturable with a Michaelis-Menten constant (Kt) of 10.6 +/- 4.1 microM and a maximum velocity (Vmax) of 6.68 +/- 0.85 nmol/mg protein/10 min. Na+ and Cl- activation kinetics revealed that the Na+-to-Cl(-)-to-taurine stoichiometry was 2:1:1. Na+-dependent [3H]-taurine transport was competitively inhibited by beta-alanine with an inhibitory constant (Ki) of 47.4 +/- 6.5 microM. Gamma-aminobutyric acid also inhibited Na+-dependent [3H]-taurine transport with relatively low affinity (Ki = 273 +/- 71 microM). TAUT mRNA was detected in mouse primary cultured neurons, and TAUT protein was also expressed at approximately 70 kDa. Na+-dependent taurine transport activity was increased with developing neurons and corresponded with the increasing mRNA and protein level of TAUT.

CONCLUSIONS

The present study revealed that Na+/Cl(-)-coupled taurine transporter TAUT is responsible for taurine uptake in mouse cerebrocortical neurons, and that the expression of TAUT is increased with developing cerebrocortical neurons.

Functional characterization of Zn2(+)-sensitive GABA transporter expressed in primary cultures of astrocytes from rat cerebral cortex

The extracellular levels of gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the mammalian cerebral cortex, are regulated by specific high-affinity Na(+)/Cl(-) dependent transporters (GATs). GAT1 mainly expressed in cerebrocortical neurons is thought to play an important role for clearance of GABA in the extracellular fluid, whereas there is a little information available for pharmacological importance for astrocytic GABA transporters. In the present study, we therefore described the functional characterization of GABA transport in primary cultures of astrocytes from rat cerebral cortex and the identification of GABA transporter subtype(s). GABA transport was Na(+) and Cl(-) dependent and saturable with a Michaelis constant (K(t)) of 9.3+/-2.8 microM. Na(+)- and Cl(-)- activation kinetics revealed that the Na(+)-Cl(-)-to-GABA stoichiometry was 2:1:1 and concentrations of Na(+) and Cl(-) necessary for half-maximal transport (K(0.5)(Na) and K(0.5)(Cl)) were 78+/-28 mM and 9.6+/-2.6 mM, respectively. Na(+)-dependent GABA transport was competitively inhibited by various GABA transport inhibitors, especially GAT2- or GAT3-selective inhibitor. In addition, Zn(2+), which has been reported to be a potent inhibitor of GAT3, was found to have a significantly but partially inhibitory effect on the Na(+)-dependent GABA transport in a concentration-dependent manner. Furthermore, reverse transcription-PCR and Western blot analyses revealed that GAT2 and GAT3 are expressed in primary cultures of astrocytes. These results clearly showed that zinc is a useful reagent for separating GAT3 activity from GAT1- and GAT2-activities in CNS. To our knowledge, the present study represents the first report on the inhibitory effect of zinc on the Na(+)-dependent GABA transport in rat cerebrocortical astrocytes.

Functional characterization of Na+-independent choline transport in primary cultures of neurons from mouse cerebral cortex

We report here the functional characteristics of Na+-independent choline transport system in primary cultures of neurons from mouse cerebral cortex. Na+-independent choline transport was saturable with a Michaelis constant (Kt) of 26.7+/-1.2 microM and a maximal velocity (Vmax) of 1.04+/-0.02 nmol/mg protein/10 min. Choline uptake was significantly influenced by extracellular pH and by membrane depolarization. This uptake system was inhibited by various organic cations including unlabeled choline, guanidine, diphenhydramine and the choline analog hemicholinium-3. However, the prototypical organic cation tetraethylammonium and cimetidine showed very little affinity for the Na+-independent choline uptake system in neurons. These results indicate that mouse cerebrocortical neurons express a Na+-independent, high-affinity choline transport system. RT-PCR revealed that choline transporter-like protein 1 (CTL1) and its spliced variant CTL1a, which have been reported to be novel Na+-independent choline transporter, are expressed in mouse cerebrocortical neurons. The Na+-independent transport properties of choline in mouse neurons is similar or identical to that of CTL1 and/or CTL1a. This choline transport system seems to have relevance not only for neuronal physiology but also for the uptake of pharmacologically important organic cation drugs.