Tyrosine phosphorylation-and epidermal growth factor-dependent regulation of the sodium-coupled amino acid transporter B0 in the human placental choriocarcinoma cell line JAR

Mechanisms of Regulation of Transporters of Amino Acid Absorption in Inflammatory Bowel Diseases

Intestinal absorption of dietary amino acids/peptides is essential for protein homeostasis, which in turn is crucial for maintaining health as well as restoration of health from significant diseases. Dietary amino acids/peptides are absorbed by unique transporter processes present in the brush border membrane of absorptive villus cells, which line the entire length of the intestine. To date, the only nutrient absorptive system described in the secretory crypt cells in the mammalian intestine is the one that absorbs the amino acid glutamine. Majority of the amino acid transporters are sodium dependent and therefore require basolateral membrane Na-K-ATPase to maintain an efficient transcellular Na gradient for their activity. These transport processes are tightly regulated by various cellular and molecular mechanisms that facilitate their optimal activity during normal physiological processes. Malabsorption of amino acids, recently described in pathophysiological states such as in inflammatory bowel disease (IBD), is undoubtedly responsible for the debilitating symptoms of IBD such as malnutrition, weight loss and ultimately a failure to thrive. Also recently, in vivo models of IBD and in vitro studies have demonstrated that specific immune-inflammatory mediators/pathways regulate specific amino acid transporters. This provides possibilities to derive novel nutrition and immune-based treatment options for conditions such as IBD. © 2020 American Physiological Society. Compr Physiol 10:673-686, 2020.

EGF-activated PI3K/Akt signalling coordinates leucine uptake by regulating LAT3 expression in prostate cancer

Background

Growth factors, such as EGF, activate the PI3K/Akt/mTORC1 signalling pathway, which regulates a distinct program of protein synthesis leading to cell growth. This pathway relies on mTORC1 sensing sufficient levels of intracellular amino acids, such as leucine, which are required for mTORC1 activation. However, it is currently unknown whether there is a direct link between these external growth signals and intracellular amino acid levels. In primary prostate cancer cells, intracellular leucine levels are regulated by L-type amino acid transporter 3 (LAT3/SLC43A1), and we therefore investigated whether LAT3 is regulated by growth factor signalling.

Methods

To investigate how PI3K/Akt signalling regulates leucine transport, prostate cancer cells were treated with different PI3K/Akt inhibitors, or stable knock down of LAT3 by shRNA, followed by analysis of leucine uptake, western blotting, immunofluorescent staining and proximity ligation assay.

Results

Inhibition of PI3K/Akt signalling significantly reduced leucine transport in LNCaP and PC-3 human prostate cancer cell lines, while growth factor addition significantly increased leucine uptake. These effects appeared to be mediated by LAT3 transport, as LAT3 knockdown blocked leucine uptake, and was not rescued by growth factor activation or further inhibited by signalling pathway inhibition. We further demonstrated that EGF significantly increased LAT3 protein levels when Akt was phosphorylated, and that Akt and LAT3 co-localised on the plasma membrane in EGF-activated LNCaP cells. These effects were likely due to stabilisation of LAT3 protein levels on the plasma membrane, with EGF treatment preventing ubiquitin-mediated LAT3 degradation.

Conclusion

Growth factor-activated PI3K/Akt signalling pathway regulates leucine transport through LAT3 in prostate cancer cell lines. These data support a direct link between growth factor and amino acid uptake, providing a mechanism by which the cells rapidly coordinate amino acid uptake for cell growth.

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0400-0) contains supplementary material, which is available to authorized users.

Interaction of aurintricarboxylic acid (ATA) with four nucleic acid binding proteins DNase I, RNase A, reverse transcriptase and Taq polymerase

In the investigation of interaction of aurintricarboxylic acid (ATA) with four biologically important proteins we observed inhibition of enzymatic activity of DNase I, RNase A, M-MLV reverse transcriptase and Taq polymerase by ATA in vitro assay. As the telomerase reverse transcriptase (TERT) is the main catalytic subunit of telomerase holoenzyme, we also monitored effect of ATA on telomerase activity in vivo and observed dose-dependent inhibition of telomerase activity in Chinese hamster V79 cells treated with ATA. Direct association of ATA with DNase I (K(d)=9.019 microM)), RNase A (K(d)=2.33 microM) reverse transcriptase (K(d)=0.255 microM) and Taq polymerase (K(d)=81.97 microM) was further shown by tryptophan fluorescence quenching studies. Such association altered the three-dimensional conformation of DNase I, RNase A and Taq polymerase as detected by circular dichroism. We propose ATA inhibits enzymatic activity of the four proteins through interfering with DNA or RNA binding to the respective proteins either competitively or allosterically, i.e. by perturbing three-dimensional structure of enzymes.

Taxol induced apoptosis regulates amino acid transport in breast cancer cells

A major outcome from Taxol treatment is induction of tumor cell apoptosis. However, metabolic responses to Taxol-induced apoptosis are poorly understood. In this study, we hypothesize that alterations in specific amino acid transporters may affect the Taxol-induced apoptosis in breast cancer cells. In this case, the activity of the given transporter may serve as a biomarker that could provide a biological assessment of response to drug treatment. We have examined the mechanisms responsible for Taxol-induced neutral amino acid uptake by breast cancer cells, such as MCF-7, BT474, MDAMB231 and T47D. The biochemical and molecular studies include: (1) growth-inhibition (MTT); (2) transport kinetics: (3) substrate-specific inhibition; (4) effect of thiol-modifying agents NEM and NPM; (5) gene expression of amino acid transporters; and (6) apoptotic assays. Our data show that Taxol treatment of MCF-7 cells induced a transient increase in Na(+)-dependent transport of the neutral amino acid transporter B0 at both gene and protein level. This increase was attenuated by blocking the transporter in the presence of high concentrations of the substrate amino acid. Other neutral amino acid transporters such as ATA2 (System A) and ASC were not altered. Amino acid starvation resulted in the expected up-regulation of System A (ATA2) gene, but not for B0 and ASC. B0 was significantly down regulated. Taxol treatment had no significant effect on the uptake of arginine and glutamate as measured by System y(+) and X(-) (GC) respectively. Tunel assays and FACS cell cycle analysis demonstrated that both Taxol- and doxorubicin-induced upregulation of B0 transporter gene with accompanying increase in cell apoptosis, could be reversed partially by blocking the B0 transporter with high concentration of alanine, and/or by inhibiting the caspase pathway. Both Taxol and doxorubicin treatment caused a significant decrease in S-phase of the cell cycle. However, Taxol-induced an increase primarily in the G2 fraction while doxorubicin caused increase in G1/G0 together with a small increase in G2. In summary, our study showed that Taxol induced apoptosis in several breast cancer cells results in activation of amino acid transporter System B0 at both gene and protein level. Similar response was observed with another chemotherapeutic agent Doxorubicin, suggesting that this increase is in response to apoptosis, and not only due to changes in cell cycle related events.

Amino acid transporters ASCT2 and LAT1 in cancer: partners in crime?

Relative to other neutral amino acid transporters, the expression levels of ASCT2 and LAT1, are coordinately elevated in a wide spectrum of primary human cancers, suggesting that they are frequently co-opted to support the "tumor metabolome". Each has recently been shown to play important roles in the growth and survival of cancer cell lines, making them potential targets for cancer therapy. The properties and putative relationship of these two amino acid exchangers are discussed in the context of their demonstrated utility in cancer biology, including cellular growth and survival signaling and integrated links to the mammalian target-of-rapamycin (mTOR) kinase.

Placental transport of leucine in a porcine model of low birth weight

Low birth weight is a major factor in neonatal morbidity and mortality in humans and domestic species and is a predictor of physiological disorders in adulthood. This study utilised the naturally occurring variation in pig fetal size within a uterus to test the hypothesis that placental amino acid transport capability is associated with fetal growth. Leucine uptake by trophoblast vesicles prepared from placentas supplying an average-sized fetus and the smallest fetus in the uterus was assessed. On days 45 and 65 of gestation, uptake of leucine by the porcine placenta was predominantly sodium independent and was inhibited by the non-metabolised leucine analogue 2-amino-2-norbornane-carboxylic acid, indicating that uptake occurs via system L. By day 100 the uptake of leucine by placentas supplying average-sized fetuses had changed from being predominantly sodium independent to involving both sodium-dependent (system B0) and -independent (system L) pathways. This change was not seen in placentas supplying the smallest fetus, which continued to display predominantly sodium-independent uptake. In conclusion, these data show gestational- and fetal size-dependent changes in the transport of leucine across the porcine placenta.

Syncytin: the major regulator of trophoblast fusion? Recent developments and hypotheses on its action

Syncytin is a membrane protein derived from the envelope gene of an endogenous retrovirus of the HERV-W family. The gene appears to be almost exclusively expressed in placenta; the protein was found in particular in syncytiotrophoblast. After transfection into various cell types it has proven to be a very fusogenic protein, inducing the formation of syncytia. Therefore, the question rises as to whether syncytin is responsible for the fusion process of villous cytotrophoblast into syncytiotrophoblast in vivo. If so, how is this fusion process regulated if syncytin is found all over the syncytiotrophoblast? Can this process be regulated through local or temporal changes in syncytin expression, or is syncytin merely one factor in a cascade of events leading to fusion limited at some other level? This review will try to summarize the published data on the regulation of fusion in trophoblast models as well as on the localization and regulation of syncytin expression and of its presumed receptors. Assuming that syncytin is the key factor inducing trophoblast fusion, a number of models will be presented by which syncytin and/or its receptors might regulate this process. In some of the hypotheses proposed, local coexpression of syncytin and receptor, leading to blocking of one factor by the other, is of functional relevance.

Growth hormone and epidermal growth factor together enhance amino acid transport systems B0,+ and A in remnant small intestine after massive enterectomy

BACKGROUND

Sodium-dependent brush-border nutrient transport is decreased 2 weeks after massive enterectomy. This down-regulation is ameliorated by a 1-week infusion of parenteral growth hormone (GH) and epidermal growth factor (EGF) started 1 week after resection. We hypothesize that glutamine (GLN) transport will be enhanced by earlier and longer growth factor infusion, with differential effects on the Na(+)-dependent GLN transport systems A, B(0,+), and B(0)/ASCT2.

MATERIALS AND METHODS

New Zealand White rabbits underwent 70% small bowel resection then immediately received parenteral EGF, GH, both EGF and GH, or neither for 2 weeks. Na(+)-dependent 3H-GLN uptake by jejunal and ileal brush-border membrane vesicles was measured and the contribution of systems A, B(0,+), and B(0) was then determined by competitive inhibition. Data were analyzed using one-way analysis of variance.

RESULTS

In nonresected animals, the relative contribution of the systems was similar in jejunum (A 9%, B(0,+) 20%, and B(0) 71%) and ileum (A 13%, B(0,+) 27%, and B(0) 60%). Na(+)-dependent GLN uptake was reduced by one half in resected untreated controls, primarily because of decreased B(0) activity. EGF or GH alone did not affect Na(+)-dependent GLN transport, but, as a combination, there was increased uptake in the residual ileum and jejunum by 144% and 150%, respectively, over resected controls (P < 0.05). This was twice that achieved by delayed and shorter-duration combination treatment. This augmentation was a result of a 6.1-8.2-fold increase in system A as well as a 3.8-3.9-fold enhancement of system B(0,+) activity in remnant ileum and jejunum (P < 0.01).

CONCLUSIONS

Parenteral EGF and GH, given in combination for 2 weeks immediately after massive enterectomy, synergistically enhance GLN uptake by systems A and B(0,+).

Isoforms of amino acid transporters in placental syncytiotrophoblast: plasma membrane localization and potential role in maternal/fetal transport

Many cell proteins exist as isoforms arising either from gene duplication or alternate RNA splicing. There is growing evidence that isoforms with different, but closely related, functional characteristics are often directed to discrete cellular locations. Thus, specialized functions may be carried out by proteins of similar evolutionary origin in different membrane compartments. In polarized epithelial cells, this mechanism allows the cell to control amino acid transport independently at each of its specialized apical and basolateral plasma membrane domains. Investigations of isoform localization in these membranes have generally been performed in epithelia other than the placental trophoblast.This review of placental amino acid transporter isoforms first provides an overview of their properties and preliminary plasma membrane localization. We then discuss studies suggesting various roles of isoform localization in trophoblast function. To provide insights into the molecular basis of this localization in trophoblast, we present a review of current knowledge of plasma membrane protein localization as derived from investigations with a widely used epithelial model cell line. Finally, we discuss a potential approach using cultured trophoblast-derived cells for studies of transporter isoform localization and function. We hope that this review will stimulate investigation of the properties of trophoblast transporter isoforms, their membrane localization and their contribution to the cellular mechanism of maternal-fetal nutrient transport.

Growth hormone and epidermal growth factor together enhance amino acid transport systems B(0,+) and A in remnant small intestine after massive enterectomy

BACKGROUND

Sodium-dependent brush border nutrient transport is decreased 2 weeks after massive enterectomy. This downregulation is ameliorated by a 1-week infusion of parenteral growth hormone (GH) and epidermal growth factor (EGF) started 1 week after resection. We hypothesized that glutamine (GLN) transport would be enhanced by earlier and longer growth factor infusion, with differential effects on the Na(+)-dependent GLN transport systems A, B(0,+), and B0/ASCT2.

MATERIALS AND METHODS

New Zealand White rabbits underwent 70% small bowel resection then immediately received parenteral EGF, GH, both, or neither for 2 weeks. Na(+)-dependent 3H-GLN uptake by jejunal and ileal brush-border membrane vesicles was measured and the contribution of systems A, B(0,+), and B0 then determined by competitive inhibition. Data were analyzed using one-way analysis of variance.

RESULTS

In nonresected animals, the relative contribution of the systems was similar in jejunum (A, 9%, B(0,+), 20%; and B0, 71%) and ileum (A, 13%; B(0,+), 27%; and B0, 60%). Na(+)-dependent GLN uptake was reduced by half in resected, untreated controls, primarily because of decreased B(0) activity. EGF or GH alone did not affect Na(+)-dependent GLN transport, but as a combination, increased uptake in the residual ileum and jejunum by 144% and 150%, respectively, over resected controls (P<0.05). This was twice that achieved by delayed and shorter-duration combination treatment. This augmentation was due to a 6.1- to 8.2-fold increase in system A as well as a 3.8- to 3.9-fold enhancement of system B(0,+) activity in remnant ileum and jejunum (P<0.01).

CONCLUSIONS

Parenteral EGF and GH, given in combination for 2 weeks immediately after massive enterectomy, synergistically enhance GLN uptake by systems A and B(0,+).

Characterisation and cloning of a Na(+)-dependent broad-specificity neutral amino acid transporter from NBL-1 cells: a novel member of the ASC/B(0) transporter family

Na(+)-dependent neutral amino acid transport into the bovine renal epithelial cell line NBL-1 is catalysed by a broad-specificity transporter originally termed System B(0). This transporter is shown to differ in specificity from the B(0) transporter cloned from JAR cells [J. Biol. Chem. 271 (1996) 18657] in that it interacts much more strongly with phenylalanine. Using probes designed to conserved transmembrane regions of the ASC/B(0) transporter family we have isolated a cDNA encoding the NBL-1 cell System B(0) transporter. When expressed in Xenopus oocytes the clone catalysed Na(+)-dependent alanine uptake which was inhibited by glutamine, leucine and phenylalanine. However, the clone did not catalyse Na(+)-dependent phenylalanine transport, again as in NBL-1 cells. The clone encoded a protein of 539 amino acids; the predicted transmembrane domains were almost identical in sequence to those of the other members of the B(0)/ASC transporter family. Comparison of the sequences of NBL-1 and JAR cell transporters showed some differences near the N-terminus, C-terminus and in the loop between helices 3 and 4. The NBL-1 B(0) transporter is not the same as the renal brush border membrane transporter since it does not transport phenylalanine. Differences in specificity in this protein family arise from relatively small differences in amino acid sequence.

Growth factor regulation of enterocyte nutrient transport during intestinal adaptation

BACKGROUND

Intestinal adaptation occurs in response to injury or alteration in nutrient availability. It is both morphologic and physiologic in nature and can be mediated by growth factors and nutrients. Pathologic conditions such as short-bowel syndrome and inflammatory bowel disease lead to derangements in nutrient absorption that may exceed the body's regenerative and adaptive capacity. Failure to fully adapt often results in long-term dependence on parenteral nutrition, leading to decreased quality of life and excessive medical expenses. The therapeutic use of appropriate growth factors may increase the adaptive capabilities of the gut.

DATA SOURCE

Medline and current literature review.

CONCLUSIONS

The major known nutrient transporters present in the gut and the mechanisms by which growth factors alter transport activity during intestinal adaptation are summarized. Growth factors have the potential to improve nutrient absorption in some bowel diseases.

Amino acid transporters in the human placenta

Amino acid transport across the human placenta is active, mediated by specific transporters in syncytiotrophoblast plasma membranes. Using functional criteria such as substrate specificity and sodium dependence, approximately 15 transport systems have been identified in the human placenta. Recently, the area of molecular biology of amino acid transporters has evolved rapidly and at least 25 cDNA clones coding for mammalian amino acid transporters or transporter subunits have been identified. The primary objective of this review is to integrate the available functional data on placental amino acid transport systems with recent molecular information on mammalian amino acid transporters. Furthermore, models for the mechanisms for net materno-fetal transfer of amino acids are discussed. Finally, the evidence to suggest that alterations in placental amino acid transport systems may play a crucial role in the regulation of fetal growth are presented briefly.

Transport of large neutral amino acids into BeWo cells

BeWo choriocarcinoma cells were cultured onto solid microcarrier beads, packed into columns and superfused. Unidirectional influx of l -phenylalanine (l -phe) and l -leucine (l -leu) across the microvillous border of the cells was studied using a rapid paired-tracer dilution technique. Influx of l -phe and l -leu comprised both saturable and non-saturable components. K(m)values for l -phe and l -leu were 0.57+/-0.01 m m and 0.05+/-0.01 m m, respectively, with V(max)values of 120.4+/-0.5 nmol/mg/min and 41. 7+/-0.2 nmol/mg/min. Non-saturable uptake components were 29.0+/-0.1 nmol/mg/m m and 37.9+/-0.1 nmol/mg/min/m m respectively. l -leu uptake was found to be sodium-independent. The uptake of l -[(3)H]phe was strongly inhibited (90-100 per cent) by unlabelled l -phe, d -phe, l -leu or 2-aminoendobicyclo-[2,2, 1]-heptane-2-carboxylic acid (BCH) but not by l -arginine (l -arg) or methyl alpha-aminoisobutric acid (Me-AIB). Pre-incubation of Bewo cultures for 24 h in the presence of an additional 1.2 m ml -phe (simulating maternal phenylketonuria) significantly reduced both the K(m)and V(max)components of l -phe influx. l -arg (2 m m) had no effect on l -leu influx whereas 2 m ml -phe completely inhibited saturable l -leu influx. These data suggest that the microvillous border of differentiated BeWo cells transport large neutral amino acids predominantly via system L rather than by B(0) or y(+)L transporters.

The RD114/simian type D retrovirus receptor is a neutral amino acid transporter

The RD114/simian type D retroviruses, which include the feline endogenous retrovirus RD114, all strains of simian immunosuppressive type D retroviruses, the avian reticuloendotheliosis group including spleen necrosis virus, and baboon endogenous virus, use a common cell-surface receptor for cell entry. We have used a retroviral cDNA library approach, involving transfer and expression of cDNAs from highly infectable HeLa cells to nonpermissive NIH 3T3 mouse cells, to clone and identify this receptor. The cloned cDNA, denoted RDR, is an allele of the previously cloned neutral amino acid transporter ATB0 (SLC1A5). Both RDR and ATB0 serve as retrovirus receptors and both show specific transport of neutral amino acids. We have localized the receptor by radiation hybrid mapping to a region of about 500-kb pairs on the long arm of human chromosome 19 at q13.3. Infection of cells with RD114/type D retroviruses results in impaired amino acid transport, suggesting a mechanism for virus toxicity and immunosuppression. The identification and functional characterization of this retrovirus receptor provide insight into the retrovirus life cycle and pathogenesis and will be an important tool for optimization of gene therapy using vectors derived from RD114/type D retroviruses.

Aurintricarboxylic acid inhibits apoptosis and supports proliferation in a haemopoietic growth-factor dependent myeloid cell line

The actions of the nuclease inhibitor aurintricarboxylic acid (ATA) were investigated in the growth-factor dependent murine myeloid cell line NSF-60. NSF-60 cells proliferate in response to interleukin-3 (IL-3) and undergo apoptosis when deprived of exogenous IL-3, as demonstrated by the appearance of characteristic DNA 'ladders' following agarose gel electrophoresis. ATA, at concentrations between 5 and 25 microM, inhibited apoptosis in growth-factor deprived cells as demonstrated by inhibition of DNA fragmentation and increased cell survival. ATA at a concentration of 25 microM supported proliferation of the cell line in the absence of exogenous growth-factor. Both ATA and IL-3 increased protein phosphorylation in this cell line. ATA and IL-3 induced proliferation was inhibited by the kinase inhibitors genistein, staurosporine and H-7. These findings suggest that, in NSF-60, ATA is not acting exclusively as an endonuclease inhibitor and that protein phosphorylation is involved in the mechanism of action of ATA in this cell line.