Molecular and functional analysis of glutamine uptake in human hepatoma and liver-derived cells

Human hepatoma cells take up glutamine at rates severalfold faster than the system N-mediated transport rates observed in normal human hepatocytes. Amino acid inhibition, kinetic, Northern blotting, RT-PCR, and restriction enzyme analyses collectively identified the transporter responsible in six human hepatoma cell lines as amino acid transporter B(0) (ATB(0)), the human ortholog of rodent ASCT2. The majority of glutamine uptake in liver fibroblasts and an immortalized human liver epithelial cell line (THLE-5B) was also mediated by ATB(0). The 2.9-kb ATB(0) mRNA was equally expressed in all cell lines, whereas expression of the system A transporters ATA2 and ATA3 was variable. In contrast, the system N isoforms (SN1 and SN2) were expressed only in well-differentiated hepatomas. ATB(0) mRNA was also expressed in cirrhotic liver and adult and pediatric liver cancer biopsies but was not detectable in isolated human hepatocytes or fetal liver. Although the growth of all hepatomas was glutamine dependent, competitive inhibition of ATB(0)-mediated glutamine uptake blocked proliferation only in poorly differentiated cells lacking SN1 or SN2 expression and exhibiting low glutamine synthetase mRNA levels.     

ASCT2 silencing regulates mammalian target-of-rapamycin growth and survival signaling in human hepatoma cells

System ASC amino acid transporter-2 (ASCT2) was previously demonstrated to be essential for human hepatoma cell growth and survival, as its silencing via inducible antisense RNA expression results in complete apoptosis within 48 h by a mechanism that transcends its role in amino acid delivery. To gain mechanistic insights into the reliance of cancerous liver cells on ASCT2, the aim of this study was to determine the early consequences of its silencing on the growth and survival signaling that presage apoptosis. Induced antisense ASCT2 RNA in SK-Hep1 cells led to >90% suppression of ASCT2 mRNA by 6 h and inhibition of mammalian target-of-rapamycin (mTOR)/raptor (mTOR complex-1; mTORC1) signaling by 8 h, as manifested by diminished p70 ribosomal protein S6 kinase-1 and eukaryotic initiation factor-4E (eIF4E) binding protein-1 phosphorylation, while protein synthesis rates declined by nearly 50% despite no measurable decreases in the cap binding protein eIF4G or cellular ribosomal protein content. Depressed mTORC1 signaling occurred before detectable reduction in ASCT2 activity but coincided with a 30% decline in total cellular ASCT2 protein. By 12 h after ASCT2 silencing, further decrements were observed in protein synthesis rates and ASCT2 protein and activity, each by 50%, while signaling from mTOR/rictor (mTOR complex-2; mTORC2) was stimulated as indexed by enhanced phosphorylation of the Akt/PKB kinase on serine-473 and of its proapoptotic substrate Bad on serine-136. These results suggest that ASCT2 silencing inhibits mTORC1 signaling to the translational machinery followed by an mTORC2-initiated survival response, establishing a link between amino acid transporter expression and mTOR function. amino acid transport; hepatocellular carcinoma; apoptosis; protein synthesis     

UV-induced apoptosis is mediated independent of caspase-9 in MCF-7 cells: a model for cytochrome c resistance

The importance of the mitochondria in UV-induced apoptosis has become increasingly apparent. Following DNA damage cytochrome c and other pro-apoptotic factors are released from the mitochondria, allowing for formation of the apoptosome and subsequent cleavage and activation of caspase-9. Active caspase-9 then activates downstream caspases-3 and/or -7, which in turn cleave poly(ADP)-ribose polymerase (PARP) and other down-stream targets, resulting in apoptosis. In an effort to understand the mechanisms of Akt-mediated cell survival in breast cancer, we studied the effects of insulin-like growth factor (IGF)-I treatment on UV-treated MCF-7 human breast cancer cells. Apoptosis was induced in MCF-7 cells after UV treatment, as measured by caspase-7 and PARP cleavage, and IGF-I co-treatment protected against this response. Surprisingly caspase-9 cleavage was unchanged with UV and/or IGF-I treatment. Using MCF-7 cells overexpressing caspase-3 we have shown that resistance of caspase-9 to cleavage was not altered by the expression of caspase-3. Furthermore, overexpression of caspase-9 did not enhance PARP or caspase-7 cleavage after UV treatment. Because caspase-8 was activated with UV treatment alone, we believe that UV-induced apoptosis in MCF-7 cells occurs independently of cytochrome c and caspase-9, supporting the existence of a cytoplasmic inhibitor of cytochrome c in MCF-7 cells. We anticipate that such inhibitors may be overexpressed in cancer cells, allowing for treatment resistance.     

Targeting ASCT2-mediated glutamine metabolism inhibits proliferation and promotes apoptosis of pancreatic cancer cells

Some tumor cells have a high rate of glutamine uptake and exhibit glutamine addiction. Alanine-serine cysteine-preferring transporter 2 (ASCT2) is a major mediator of glutamine supply in many tumor cells, but the underlying effects and mechanisms of ASCT2 in pancreatic cancer (PC) are largely unknown. Our results show that ASCT2 expression is significantly higher in PC than in normal pancreatic duct cells and pancreas. Utilizing the Kaplan–Meier Plotter database, a high expression of SLC1A5 mRNA was significantly associated with poor overall survival (OS) in patients with PC. shRNA-mediated inhibition of ASCT2 function in vitro can significantly decrease glutamine consumption, α-ketoglutarate (α-KG) production and ATP generation and increase the reactive oxygen species (ROS) level. Moreover, the antioxidant N-acetylcysteine partially attenuated the increase in the ROS levels and reduced ATP generation. These data suggest that ASCT2 mediates glutamine metabolism and maintains redox homeostasis in PC. To further investigate whether ASCT2 is involved in PC cell growth, we blocked ASCT2 activity with the ASCT2 inhibitor l-γ-glutamyl-p-nitroanilide (GPNA) and silenced the expression of ASCT2 with specific shRNAs. We found that the growth of PC cells was significantly inhibited. Additionally, knockdown of ASCT2 induced apoptosis through the Akt/mTOR signaling pathway. Furthermore, the loss of ASCT2 in BxPC-3 cell xenografts significantly inhibited tumor growth in vivo, and this effect was associated with an increase in cleaved caspase-3 expression and a decrease in Ki67 staining. Taken together, our results show that ASCT2 may be utilized as a putative therapeutic target for PC.     

Daidzein Induced Apoptosis via Down-Regulation of Bcl-2/Bax and Triggering of the Mitochondrial Pathway in BGC-823 Cells

Daidzein belongs to the group of isoflavones, found in a wide variety of plant-derived foods, especially in soybeans and soy-based foods. In this study, the effect of daidzein on human gastric carcinoma cells (BGC-823) and its mechanism were investigated. MTT assay was applied in the detection of the inhibitory effects of daidzein on cell proliferation. Hoechst–propidium iodide staining and flow cytometry were used to examine the apoptosis as well as the mitochondrial transmembrane potential. Western blotting was performed to detect the expression of apoptosis-associated proteins: cleaved PARP, cleaved caspase-9, cleaved caspase-3, Bcl-2, and Bax. Daidzein significantly inhibited the growth and proliferation of human gastric carcinoma cells (BGC-823) in a concentration- and time-dependent manner. Furthermore, it was found that an insult of daidzein to BGC-823 cells caused them to die by disruption of mitochondrial transmembrane potential, demonstrated not only by staining dead cells for phosphatidylserine but also by the up-regulation (cleaved PARP, cleaved caspase-9, cleaved caspase-3, Bax) and down-regulation (Bcl-2) of proteins associated with apoptosis and survival; whereas, the pan-caspase inhibitor z-VAD-fmk could partially rescue cells against damage of daidzein. Taken together, the results of this study demonstrate that daidzein significantly induces apoptosis via a mitochondrial pathway. Specifically, daidzein induced a change in the Bax/Bcl-2 ratios and activation of caspases-3 and -9 and the cleavage of PARP. Therefore, daidzein has the potential for use as a therapeutic agent for the treatment of gastric carcinoma.     

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

We describe the regulation mechanisms of the Na(+)-dependent neutral amino acid transporter ASCT2 via nitric oxide (NO) in the human intestinal cell line, Caco-2. Exposure of Caco-2 cells to S-nitrosothiol, such as S-nitroso-N-acetyl-DL-penicillamine (SNAP) and S-nitrosoglutathione, and the NO-donor, NOC12, concentration- and time-dependently increased Na(+)-dependent alanine uptake. Kinetic analyses indicated that SNAP increases the maximal velocity (V(max)) of Na(+)-dependent alanine uptake in Caco-2 cells without affecting the Michaelis-Menten constant (K(t)). The stimulatory effect was partially eliminated by actinomycin D and cycloheximide. Increased Na(+)-dependent alanine uptake by SNAP was partially abolished by the NO scavengers, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide sodium salt (carboxy-PTIO) and N-(dithiocarboxy)sarcosine disodium salts (DTCS), as well as the NADPH oxidase inhibitor, diphenyleneiodonium. RT-PCR revealed that Caco-2 cells expressed the Na(+)-dependent neutral amino acid transporter ASCT2, but not the other Na(+)-dependent neutral amino acid transporters ATB(0,+) and B(0)AT1. These results suggested that functional up-regulation of ASCT2 by SNAP might be partially associated with an increase in the density of transporter protein via de novo synthesis.     

Interaction of tryptophan derivatives with SLC6A14 (ATB0,+) reveals the potential of the transporter as a drug target for cancer chemotherapy

ATB(0,+) [SLC6A14 (solute carrier family 6 member 14)] is an Na(+)/Cl(-)-coupled amino acid transporter whose expression is upregulated in cancer. 1-Methyltryptophan is an inducer of immune surveillance against tumour cells through its ability to inhibit indoleamine dioxygenase. In the present study, we investigated the role of ATB(0,+) in the uptake of 1-methyltryptophan as a potential mechanism for entry of this putative anticancer drug into tumour cells. These studies show that 1-methyltryptophan is a transportable substrate for ATB(0,+). The transport process is Na(+)/Cl(-)-dependent with an Na(+)/Cl(-)/1-methyltryptophan stoichiometry of 2:1:1. Evaluation of other derivatives of tryptophan has led to identification of alpha-methyltryptophan as a blocker, not a transportable substrate, for ATB(0,+). ATB(0,+) can transport 18 of the 20 proteinogenic amino acids. alpha-Methyltryptophan blocks the transport function of ATB(0,+) with an IC(50) value of approximately 250 muM under conditions simulating normal plasma concentrations of all these 18 amino acids. These results suggest that alpha-methyltryptophan may induce amino acid deprivation in cells which depend on the transporter for their amino acid nutrition. Screening of several mammary epithelial cell lines shows that ATB(0,+) is expressed robustly in some cancer cell lines, but not in all; in contrast, non-malignant cell lines do not express the transporter. Treatment of ATB(0,+)-positive tumour cells with alpha-methyltryptophan leads to suppression of their colony-forming ability, whereas ATB(0,+)-negative cell lines are not affected. The blockade of ATB(0,+) in these cells with alpha-methyltryptophan is associated with cell cycle arrest. These studies reveal the potential of ATB(0,+) as a drug target for cancer chemotherapy.     

Apoptosis induction by gamma-tocotrienol in human hepatoma Hep3B cells

We evaluated the antitumor activity of tocotrienol (T3) on human hepatoma Hep3B cells. At first, we examined the effect of T3 on the proliferation of human hepatoma Hep3B cells and found that gamma-T3 inhibited cell proliferation at lower concentrations and shorter treatment times than alpha-T3. Then, we examined the effect of gamma-T3 apoptosis induction and found that gamma-T3 induced poly (ADP-ribose) polymerase (PARP) cleavage and stimulated a rise in caspase-3 activity. In addition, gamma-T3 stimulated a rise in caspase-8 and caspase-9 activities. We also found that gamma-T3-induced apoptotic cell death was accompanied by up-regulation of Bax and a rise in the fragments of Bid and caspase-8. These data indicate that gamma-T3 induced apoptosis in Hep3B cells and that caspase-8 and caspase-9 were involved in apoptosis induction. Moreover, these results suggest that Bax and Bid regulated apoptosis induction by gamma-T3.     

UCN-01 (7-hydroxystauorsporine) blocks PMA-induced maturation and reciprocally promotes apoptosis in human myelomonocytic leukemia cells (U937)

Interactions between the protein kinase inhibitor UCN-01 and the PKC activator phorbol ester (PMA) have been examined in relation to differentiation and apoptosis in human myelomonocytic leukemia cells (U937). Coadministratation of 100 nM UCN-01 with a low concentration of PMA e.g., 2 nM, inhibited rather than promoted differentiation, reflected by reduced surface expression of the monocytic maturation marker CD11b and diminished cell adherence. Instead, administration of UCN-01 with PMA led to a marked increase in mitochondrial injury (e.g, cytochrome c release), activation of caspases-3 and -8, Bid cleavage, PARP degradation, and apoptosis, accompanied by a substantial reduction in viability and clonogenic survival. These phenomena were associated with multiple perturbations in cell cycle regulatory events, including abrogation of p21(CIP1) induction, p27(KIP1) cleavage, down-regulation of cyclin D1, dephosphorylation (activation) of p34cdc2, and degradation of underphosphorylated pRb. Potentiation of PMA-mediated apoptosis was partially mimicked by caffeine suggesting the involvement of Chk1 in the potentiation of apoptosis. Induction of cell death by UCN-01 and PMA was increased in cells stably expressing a p21(CIP1) mRNA antisense construct, suggesting that p21(CIP1) expression may protect cells from the lethal effects of this drug combination. Finally, ectopic expression of a Bcl-2 but not dominant-negative caspase-8 protected cells from UCN-01/PMA-mediated apoptosis, suggesting the lethal effects of this combination primarily involves the mitochondrial rather than the TNF-related extrinsic apoptotic pathway. Taken together, these findings suggest that UCN-01 disrupts a variety of cell cycle events in leukemic cells exposed to the maturation-inducing agent PMA, causing cells to engage an apoptotic rather than a differentiation-related program.     

Identification of the promoter elements involved in the stimulation of ASCT2 expression by glutamine availability in HepG2 cells and the probable involvement of FXR/RXR dimers

Expression of the glutamine transport protein ASCT2 in the human hepatoma cell line HepG2 is increased when cells are cultured in the presence of glutamine and this has been shown to be due to stimulation of the ASCT2 promoter. Analysis of a number of promoter constructs localised the activation site to be between bases -653 and -543. Gel shift assays identified an IR-1 repeat within a 24bp region of this sequence which bound at least two nuclear proteins. Protein binding to this site was significantly higher in cells grown in glutamine-containing medium than when glutamine was absent. The identity of the higher molecular weight species binding to this promoter element was likely to be FXR/RXR dimers. Simultaneous overexpression of FXR and RXR increased the promoter activity in cells grown without glutamine to the same extent as did glutamine addition; the effects of glutamine and FXR/RXR expression were not additive. Mutagenesis of the FXR/RXR binding site in the promoter construct abolished glutamine and FXR/RXR stimulation. Real-time PCR showed levels of FXR mRNA were significantly increased in response to glutamine. The activity of the FXR promoter was also increased in response to glutamine. These results show that the stimulation of ASCT2 expression in response to glutamine in part involves binding of FXR/RXR to the ASCT2 promoter.     

The nucleoside analog sangivamycin induces apoptotic cell death in breast carcinoma MCF7/adriamycin-resistant cells via protein kinase Cdelta and JNK activation

Sangivamycin has shown a potent antiproliferative activity against a variety of human cancers. However, little is known about the mechanism of action underlying its antitumor activity. Here we demonstrate that sangivamycin has differential antitumor effects in drug-sensitive MCF7/wild type (WT) cells, causing growth arrest, and in multidrug-resistant MCF7/adriamycin-resistant (ADR) human breast carcinoma cells, causing massive apoptotic cell death. Comparisons between the effects of sangivamycin on these two cell lines allowed us to identify the mechanism underlying the apoptotic antitumor effect. Fluorescence-activated cell sorter analysis indicated that sangivamycin induced cell cycle arrest in the G(2)/M phase in MCF7/ADR cells. A marked induction of c-Jun expression as well as phosphorylation of c-Jun and JNK was observed after sangivamycin treatment of MCF7/ADR cells but not MCF7/WT cells. Sangivamycin also induced cleavage of lamin A and poly(ADP-ribose) polymerase (PARP) in MCF7/ADR cells, probably via activation of caspase-6, -7, and -9. Pretreatment with a caspase-9-specific inhibitor or pan-caspase inhibitor abolished sangivamycin-induced cleavage of lamin A and PARP but not sangivamycin induction of c-Jun expression and phosphorylation. Pretreatment of MCF7/ADR cells with SP600125, a specific inhibitor of JNK, or with rottlerin, a specific inhibitor of protein kinase Cdelta (PKCdelta), significantly reduced the sangivamycin-induced apoptosis and almost completely abolished sangivamycin-induced phosphorylation of c-Jun and cleavage of lamin A and PARP. Transfection of MCF7/ADR cells with PKCdelta small interfering RNAs or PKCdelta antibody or rottlerin pretreatment significantly suppressed the phosphorylation of JNK. Taken together, our data suggest that sangivamycin induces mitochondria-mediated apoptotic cell death of MCF7/ADR cells via activation of JNK in a protein kinase Cdelta-dependent manner.     

p38 MAP kinase regulates benzo(a)pyrene-induced apoptosis through the regulation of p53 activation

Polycyclic aromatic hydrocarbons, such as benzo(a)pyrene (BaP), are widespread in the environment and cause untoward effects, including carcinogenesis, in mammalian cells. However, the molecular mechanism of apoptosis by BaP is remained to be elusive. Pharmacological inhibition of p38 kinase markedly inhibited the BaP-induced cytotoxicity, which was proven as apoptosis characterized by an increase in sub-G(0)/G(1) fraction of DNA content, ladder-pattern fragmentation of genomic DNA, and catalytic activation of caspase-3 with PARP cleavage. Our data also demonstrated that activation of caspase-3 was accompanied with activation of caspase-9 and mitochondrial dysfunction, which was also apparently suppressed by pretreatment with p38 kinase inhibitors. Also, pharmacological inhibition of p38 markedly inhibited the phosphorylation, accumulated expression, and transactivation activity of p53 in BaP-treated cells. Adenoviral overexpression of human p53 (wild-type) further augmented in increase of PARP cleavage and the sub-G(0)/G(1) fraction of DNA content. Furthermore, p53 mediated apoptotic activity in BaP-treated cells was inhibited by p38 kinase inhibitor. The current data collectively indicate that BaP induces apoptosis of Hepa1c1c7 cells via activation of p53-related signaling, which was, in part, regulated by p38 kinase.     

Degradomics Reveals That Cleavage Specificity Profiles of Caspase-2 and Effector Caspases Are Alike

Caspase-2 is considered an initiator caspase because its long prodomain contains a CARD domain that allows its recruitment and activation in several complexes by homotypic death domain-fold interactions. Because little is known about the function and specificity of caspase-2 and its physiological substrates, we compared the cleavage specificity profile of recombinant human caspase-2 with those of caspase-3 and -7 by analyzing cell lysates using N-terminal COmbined FRActional DIagonal Chromatography (COFRADIC). Substrate analysis of the 68 cleavage sites identified in 61 proteins revealed that the protease specificities of human caspases-2, -3, and -7 largely overlap, revealing the DEVD↓G consensus cleavage sequence. We confirmed that Asp⁵⁶³ in eukaryotic translation initiation factor 4B (eIF4B) is a cleavage site preferred by caspase-2 not only in COFRADIC setup but also upon co-expression in HEK 293T cells. These results demonstrate that activated human caspase-2 shares remarkably overlapping protease specificity with the prototype apoptotic executioner caspases-3 and -7, suggesting that caspase-2 could function as a proapoptotic caspase once released from the activating complex.     

Lobetyolin induces apoptosis of colon cancer cells by inhibiting glutamine metabolism

The purpose of the present study was to evaluate the anti-cancer property of Lobetyolin on colorectal cancer and explore its potential mechanism. Lobetyolin was incubated with HCT-116 cells in the absence or presence of ASCT2 inhibitor Benser or p53 inhibitor Pifithrin-α. The levels of glutamine, glutamic acid, α-ketoglutarate, ATP and GSH were determined to measure the glutamine metabolism. Annexin V-FITC/PI staining and TUNEL assay were applied to estimate the apoptotic condition. The levels of ASCT2 were examined by RT-qPCR, Western blot and immunofluorescence staining. The expressions of cleaved-caspase-3, caspase-3, cleaved-caspase-7, caspase-7, cleaved-PARP, PARP, p53, p21, bax and survivin were detected using Western blot analysis. As a result, the treatment with Lobetyolin effectively induced apoptosis and glutamine metabolism in HCT-116 cells through ASCT2 signalling. The inhibition of ASCT2 reduced the glutamine-related biomarkers and augmented the apoptotic process. We further found that the effect of Lobetyolin on HCT-116 was related to the expressions of p21 and bax, and transportation of p53 to nucleus. The inhibition of p53 by Pifithrin-α promoted the inhibitory effect of Lobetyolin on ASCT2-mediated apoptosis. Lobetyolin also exerted anti-cancer property in nude mice. In conclusion, the present work suggested that Lobetyolin could induce the apoptosis via the inhibition of ASCT2-mediated glutamine metabolism, which was possibly governed by p53.     

UCN-01 (7-Hydroxystauorsporine) Blocks PMA-Induced Maturation and Reciprocally Promotes Apoptosis in Human Myelomonocytic Leukemia Cells (U937)

Interactions between the protein kinase inhibitor UCN-01 and the PKC activator phorbol ester (PMA) have been examined in relation to differentiation and apoptosis in human myelomonocytic leukemia cells (U937). Coadministratation of 100 nM UCN-01 with a low concentration of PMA e.g., 2 nM, inhibited rather than promoted differentiation, reflected by reduced surface expression of the monocytic maturation marker CD11b and diminished cell adherence. Instead, administration of UCN-01 with PMA led to a marked increase in mitochondrial injury (e.g, cytochrome c release), activation of caspases-3 and -8, Bid cleavage, PARP degradation, and apoptosis, accompanied by a substantial reduction in viability and clonogenic survival. These phenomena were associated with multiple perturbations in cell cycle regulatory events, including abrogation of p21CIP1 induction, p27KIP1 cleavage, down-regulation of cyclin D1, dephosphorylation (activation) of p34cdc2, and degradation of underphosphorylated pRb. Potentiation of PMA-mediated apoptosis was partially mimicked by caffeine suggesting the involvement of Chk1 in the potentiation of apoptosis. Induction of cell death by UCN-01 and PMA was increased in cells stably expressing a p21CIP1 mRNA antisense construct, suggesting that p21CIP1 expression may protect cells from the lethal effects of this drug combination. Finally, ectopic expression of a Bcl-2 but not dominant-negative caspase-8 protected cells from UCN-01/PMA-mediated apoptosis, suggesting the lethal effects of this combination primarily involves the mitochondrial rather than the TNF-related extrinsic apoptotic pathway. Taken together, these findings suggest that UCN-01 disrupts a variety of cell cycle events in leukemic cells exposed to the maturation-inducing agent PMA, causing cells to engage an apoptotic rather than a differentiation-related program.

Key Words:

PMA, UCN-01, Differentiation, Apoptosis     

Asparanin A induces G2/M cell cycle arrest and apoptosis in human hepatocellular carcinoma HepG2 cells

We recently established that asparanin A, a steroidal saponin extracted from Asparagus officinalis L., is an active cytotoxic component. The molecular mechanisms by which asparanin A exerts its cytotoxic activity are currently unknown. In this study, we show that asparanin A induces G₂/M phase arrest and apoptosis in human hepatocellular carcinoma HepG2 cells. Following treatment of HepG2 cells with asparanin A, cell cycle-related proteins such as cyclin A, Cdk1 and Cdk4 were down-regulated, while p21^WAF1/Cip1 and p-Cdk1 (Thr14/Tyr15) were up-regulated. Additionally, we observed poly (ADP-ribose) polymerase (PARP) cleavage and activation of caspase-3, caspase-8 and caspase-9. The expression ratio of Bax/Bcl-2 was increased in the treated cells, where Bax was also up-regulated. We also found that the expression of p53, a modulator of p21^WAF1/Cip1 and Bax, was not affected in asparanin A-treated cells. Collectively, our findings demonstrate that asparanin A induces cell cycle arrest and triggers apoptosis via a p53-independent manner in HepG2 cells. These data indicate that asparanin A shows promise as a preventive and/or therapeutic agent against human hepatoma.     

High-affinity glutamate transporter GLAST/EAAT1 regulates cell surface expression of glutamine/neutral amino acid transporter ASCT2 in human fetal astrocytes

Neutral amino acid transporter ASCT2, together with high-affinity glutamate transporters, belongs to the SLC1 gene family of Na(+)-dependent solute carriers and is one of the major transporters of glutamine in cultured astrocytes. Besides glutamine and other high-affinity substrates--alanine, serine, cysteine or threonine, ASCT2 can also translocate protonated glutamate. The present study elucidated substrate-dependent trafficking of ASCT2 in differentiated primary cultures of human fetal astrocytes. The differentiation induced by 8-bromo-cAMP caused dramatic up-regulation of two co-localized and functionally linked astroglial proteins--glutamate transporter GLAST, that is the only high-affinity router of glutamate into cultured astrocytes, and glutamine synthetase (GS), a cytosolic enzyme that converts at least a part of the arriving glutamate into glutamine. In order to distinguish individual intracellular effects of these two substrates on ASCT2, in some cultures glutamine synthetase was effectively knocked down using siRNA silencing technique. In control conditions, regardless of GS levels, almost the entire ASCT2 immunoreactivity was restricted to the cytosol. Both glutamine and alanine, though to different extents, induced partial redistribution of ASCT2 from the cytosolic compartment to the plasma membrane. However, in cultures with high GS expression, micromolar concentrations of glutamate exhibited more pronounced effect on ASCT2 trafficking than the preferred substrates of this carrier. In contrast, glutamate had no effect on ASCT2 distribution in cultures devoid of GS. D-Aspartate, a metabolically inert substrate effectively transported by GLAST, had no effect in any cell culture utilized. It seems that intracellular glutamine produced by GS from glutamate that, in turn, is supplied by GLAST, is a more potent inducer of ASCT2 trafficking to the cell surface than the ASCT2-mediated translocation of extracellular substrates. At lower pH values (6.2-6.7), the cell surface pool of ASCT2 was significantly larger than at physiological pH. In addition, high concentrations of glutamate, independently from GLAST or glutamate receptor activation, induced further arrival of ASCT2 to the plasma membrane. The pH-dependent functional activation of ASCT2 and the ASCT2-mediated glutamate uptake may play important roles during ischemic acidosis or synaptic activity-induced local acidification.     

Amino acid transport is down-regulated in ischemic human intestinal epithelial cells

Amino acid transport across the plasma membrane is essential for supplying enterocytes with amino acids for cellular metabolism. We studied amino acid transport during ischemic conditions using human intestinal epithelial cell line Caco-2. Cells were incubated under nutrient-deprived (phosphate-buffered saline, PBS), hypoxic, and ischemic (PBS+hypoxia) conditions. Ischemia resulted in a significant decrease in glutamine transport by a mechanism that decreased V(max) without affecting K(m). The expression of system ATB degrees (glutamine transporter) mRNA decreased in the ischemic and nutrient-deprived groups, suggesting that the down-regulation of glutamine transport is due to modification of expression of the ATB degrees gene. The transport of glutamate and leucine, DNA synthesis, and intracellular glutathione also decreased in the ischemic group. These findings throw some light on the mechanism of intestinal epithelial damage during ischemia.