Molecular mechanisms involved in the regulation of amino acid transport

Macrophage signaling in HIV-1 infection

The human immunodeficiency virus-1 (HIV-1) is a member of the lentivirus genus. The virus does not rely exclusively on the host cell machinery, but also on viral proteins that act as molecular switches during the viral life cycle which play significant functions in viral pathogenesis, notably by modulating cell signaling. The role of HIV-1 proteins (Nef, Tat, Vpr, and gp120) in modulating macrophage signaling has been recently unveiled. Accessory, regulatory, and structural HIV-1 proteins interact with signaling pathways in infected macrophages. In addition, exogenous Nef, Tat, Vpr, and gp120 proteins have been detected in the serum of HIV-1 infected patients. Possibly, these proteins are released by infected/apoptotic cells. Exogenous accessory regulatory HIV-1 proteins are able to enter macrophages and modulate cellular machineries including those that affect viral transcription. Furthermore HIV-1 proteins, e.g., gp120, may exert their effects by interacting with cell surface membrane receptors, especially chemokine co-receptors. By activating the signaling pathways such as NF-kappaB, MAP kinase (MAPK) and JAK/STAT, HIV-1 proteins promote viral replication by stimulating transcription from the long terminal repeat (LTR) in infected macrophages; they are also involved in macrophage-mediated bystander T cell apoptosis. The role of HIV-1 proteins in the modulation of macrophage signaling will be discussed in regard to the formation of viral reservoirs and macrophage-mediated T cell apoptosis during HIV-1 infection.

EAAC1 is expressed in rat and human prostate epithelial cells; functions as a high-affinity L-aspartate transporter; and is regulated by prolactin and testosterone

Background

Prostate epithelial cells accumulate a high level of aspartate that is utilized as a substrate for their unique function of production and secretion of enormously high levels of citrate. In most mammalian cells aspartate is synthesized; and, therefore is a non-essential amino acid. In contrast, in citrate-producing prostate cells, aspartate is an essential amino acid that must be derived from circulation. The prostate intracellular/extracellular conditions present a 40:1 concentration gradient. Therefore, these cells must possess a plasma membrane-associated aspartate uptake transport process to achieve their functional activity. In earlier kinetic studies we identified the existence of a unique Na+-dependent high-affinity L-aspartate transport process in rat prostate secretory epithelial cells. The present report is concerned with the identification of this putative L-aspartate transporter in rat and human prostate cells.

Results

The studies show for the first time that EAAC1 is expressed in normal rat prostate epithelial cells, in normal and hyperplastic human prostate glands, and in human malignant prostate cell lines. EAAC1 expression and high-affinity L-aspartate transport are correspondingly down-regulated by EAAC1 siRNA knock down. Exposure of prostate cells to physiological levels of prolactin or testosterone results in an up-regulation of EAAC1 expression and a corresponding increase in the high-affinity transport of L-aspartate into the cells.

Conclusion

This study shows that EAAC1 functions as the high-affinity L-aspartate transporter that is responsible for the uptake and accumulation of aspartate in prostate cells. In other cells (predominantly excitable tissue cells), EAAC1 has been reported to function as a glutamate transporter rather than as an aspartate transporter. The regulation of EAAC1 expression and L-aspartate transport by testosterone and prolactin is consistent with their regulation of citrate production in prostate cells. The identification of EAAC1 as the high-affinity L-aspartate transporter now permits studies to elucidate the mechanism of hormonal regulation of EAAC1 gene expression, and to investigate the mechanism by which the cellular environment effects the functioning of EAAC1 as an aspartate transporter or as a glutamate transporter.

Multiple actions of the human immunodeficiency virus type-1 Tat protein on microglial cell functions

The human immunodeficiency virus type-1 (HIV-1) regulatory protein Tat is produced in the early phase of infection and is essential for virus replication. Together with other viral products, Tat has been implicated in the pathogenesis of HIV-1-associated dementia (HAD). As HIV-1 infection in the brain is very limited and macrophage/microglial cells are the only cellular type productively infected by the virus, it has been proposed that many of the viral neurotoxic effects are mediated by microglial products. We and others have shown that Tat affects the functional state of microglial cells, supporting the hypothesis that activated microglia play a role in the neuropathology associated with HIV-1 infection. This review describes the experimental evidence indicating that Tat stimulates microglia to synthesize potentially neurotoxic molecules, including proinflammatory cytokines and free radicals, and interferes with molecular mechanisms controlling cAMP levels, intracellular [Ca2+], and ion channel expression.

Functional and molecular characterization of rat intestinal prolidase

Genetic deficiency of prolidase can lead to severe problems in child development, including mental retardation. However, the exact pathogenesis of the disease is unclear. To understand the enzyme's physiologic functions, we studied the regulation of rat intestinal prolidase. The results indicated that 1) the activities of intestinal prolidase and its kinetic parameters (Km and Vmax) are site-dependent; 2) the jejunal prolidase activity was the most sensitive to the dietary restriction, and the duodenal and jejunal but not colonic kinetic parameters changed with dietary restriction; 3) the pH activity profile of jejunal prolidase at 24 h postfeeding was different from that at 48 h postfeeding, whereas the inhibition profiles of prolidase were qualitatively independent of dietary restriction; and 4) old-aged rats have lower prolidase activities in the small intestine. We also purified rat intestinal prolidase I to homogeneity. The characterization study indicated that the purified rat intestinal prolidase I is fairly similar to prolidase I from other species with a molecular weight of 116,000, which consisted of two monomers, 58,000 D each. The purified prolidase I has a Km value of 178 microM and a Vmax value of 601 micromol x min-1. mg protein-1. Screening of a rat intestinal cDNA library produced a 1.8-kb fragment that encodes the rat intestinal prolidase. This enzyme has 494 deduced amino acid sequence, which is 96% or 86% identical to mouse or human erythrocyte prolidase I. This represents the first report of a successful attempt to purify and clone an intestinal prolidase and of investigation to study prolidase regulation by diet.

Altered outward-rectifying K(+) current reveals microglial activation induced by HIV-1 Tat protein

Microglial cells are believed to be one of the key elements in the development of the HIV-related neuropathology. Not only can microglial cells be productively infected by the virus, but they are also sensitive to viral proteins. Among them, the HIV-1 regulatory protein Tat, which was shown to have neurotoxic activity, is able to promote some proinflammatory functions of microglia. Considering that microglial activation goes along with a change of ion channel profile, we aimed to study whether Tat could influence microglial electrophysiology. When microglial cultures obtained from neonatal rats were treated with Tat (> or = 100 ng/ml), whole-cell recording showed the appearance of a large outwardly rectifying current (OR) virtually absent in untreated control cells. According to voltage dependence of the kinetic variables, K(+) permeability, and pharmacological sensitivity, the Tat-induced current was due to the presence of functional Kv1.3 channels. The effect of Tat was abolished by specific anti-Tat polyclonal antibody and by heat denaturation of Tat protein, confirming that the OR enhancement was due to the viral protein. Interestingly, the OR current induced by Tat was largely prevented by two inhibitors of the transcription factor NF-kappaB, TPCK and SN50, which suggests an involvement of NF-kappaB in the effect of the viral protein. The relatively high dose of Tat needed to observe an effect (> or = 100 ng/ml) might indicate that the action of Tat required entrance of the protein into the cell, rather than being mediated by a membrane receptor. In conclusion, the HIV-1 protein Tat is able to enhance OR K(+) current in rat microglia through a mechanism involving the activation of NF-kappaB. We propose that such effect of Tat could be part of the process of microglial activation known to take place in the brain of persons with neuro-AIDS.

Control of expression of the gene for the arginine transporter Cat-1 in rat liver cells by glucocorticoids and insulin

Hepatic arginine and lysine uptake is partly regulated by changes in the transport activity of a group of cell surface proteins exhibiting properties of the transport system y+. The Cat-1 gene encodes a sodium-independent high-affinity cationic amino acid transporter of the y+ system which is nearly undetectable in the quiescent liver. In this paper we investigate the regulation of expression of Cat-1 in the quiescent rat liver by glucocorticoids and insulin, two hormones which play a critical role in amino acid dependent pathways of hepatic metabolism. Injection of insulin and glucocorticoids resulted in a rapid (15-30 min, 4-5 fold) increase in transcription which returned to basal levels within 4 hours. In contrast to the rapid single peak of transcriptional induction of the Cat-1 gene, the accumulation of the Cat-1 mRNAs occurred transiently with two peaks, the first at 30 minutes and the second at 2-4 hours following hormone treatment. These data indicate that expression of the Cat-1 gene in the quiescent liver can be transiently induced by both transcriptional and post-transcriptional mechanisms. In FTO2B rat hepatoma cells, expression of the gene is constitutive and accumulation of Cat-1 mRNAs in response to dexamethasone and insulin was dependent on transcription and protein synthesis. Furthermore, the accumulation of the basal level of the Cat-1 mRNAs was reduced by 70%, upon treatment of cells with inhibitors of protein synthesis for 6 h, when the transcription rate of the gene did not decrease significantly. We conclude the following: (i) under normal physiologic conditions, expression of the Cat-1 gene in the quiescent liver is negligible, probably to prevent unnecessary transport and metabolism of arginine by the hepatic arginase in the hepatocytes. (ii) in the cases when hepatic cationic amino acid transport is needed, such as following feeding, cellular growth and illness, glucocorticoids and insulin induce expression of the Cat-1 gene in liver cells through induction of transcription and stabilization of the mRNA. (iii) constitutive Cat-1 mRNA accumulation in rat hepatoma cells depends on protein synthesis through a labile regulated factor. Overall, constitutive expression of Cat-1 is associated with hepatic cellular growth and transformation.

Formation and fate of tyrosine. Intracellular partitioning of newly synthesized tyrosine in mammalian liver

Tyrosine in an hepatocyte is transported from the plasma, synthesized from phenylalanine, or released during protein turnover. Effects of phenylalanine and tyrosine on the formation and fate (partitioning) of tyrosine from the different sources were examined in primary rat hepatocyte cultures. Rates of tyrosine degradation, transport, incorporation into and release from protein, and synthesis from phenylalanine were measured as well as the intracellular dilution of labeled tyrosine and phenylalanine incorporated into protein. We found tyrosine had little effect on phenylalanine hydroxylation over a wide range of conditions, that transported tyrosine and tyrosine from phenylalanine are in different metabolic pools, and that there appears to be channeling of newly synthesized tyrosine during degradation. In addition, under some conditions, intracellular partitioning of tyrosine is determined by tyrosine concentration. Specifically, if extracellular tyrosine is low and phenylalanine is at a normal plasma level, tyrosine use in protein synthesis takes precedence over tyrosine degradation or export. It is proposed that the mechanism controlling this is kinetic, based on relative rates of tyrosyl-tRNA formation and tyrosine degradation and export. A quantitative model of tyrosine and phenylalanine in-flow and out-flow in hepatocytes is given, incorporating tyrosine synthesis, degradation, plasma membrane transport, and tyrosine and phenylalanine use and release during protein turnover.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.

Interleukin-1 beta and interleukin-6 stimulate 2-methylaminoisobutyric acid uptake in HepG2 cells

The metabolic response to inflammation involves an increased uptake of amino acids in the liver. It has been suggested that cytokines, such as interleukin-1 beta and interleukin-6, could be involved in this increased amino acid uptake. We investigated the role of these two inflammatory cytokines in regulating hepatic amino acid transport systems in the human hepatoma cell line, HepG2. Uptake of methylaminoisobutyric acid, the most specific known substrate of system A, and of glutamine, both transported by other sodium-dependent transport systems ASC and N, was assayed after incubation of the cells for various times with cytokines, using the cluster-tray method. Interleukin-1 beta and interleukin-6 (1000 U/ml) stimulated methylaminoisobutyric acid uptake by 36 +/- 6 and 41 +/- 4%, respectively (per cent +/- SD, n > or = 6). Under our experimental conditions, these cytokines had no effect on glutamine uptake. The stimulatory effect on methylaminoisobutyric acid uptake was not increased by combining the cytokines or by the presence of dexamethasone. The cytokine effect was abolished by cycloheximide, suggesting the involvement of de novo protein synthesis in this activation of transport system A. These data demonstrate that, in our culture conditions, interleukin-1 beta and interleukin-6 indirectly exert a stimulatory effect on methylaminoisobutyric acid transport in HepG2 cells.

Molecular sites of regulation of expression of the rat cationic amino acid transporter gene

Cat-1 is a protein with a dual function, a high affinity, low capacity cationic amino acid transporter of the y+ system and the receptor for the ecotropic retrovirus. We have suggested that Cat-1 is required in the regenerating liver for the transport of cationic amino acids and polyamines in the late G1 phase, a process that is essential for liver cells to enter mitosis. In our earlier studies we had shown that the cat-1 gene is silent in the quiescent liver but is induced in response to hormones, insulin, and glucocorticoids, and partial hepatectomy. Here we demonstrate that cat-1 is a classic delayed early growth response gene in the regenerating liver, since induction of its expression is sensitive to cycloheximide, indicating that protein synthesis is required. The peak of accumulation of the cat-1 mRNA (9-fold) by 3 h was not associated with increased transcriptional activity of the cat-1 gene in the regenerating liver, indicating post-transcriptional regulation of expression of this gene. Induction of the cat-1 gene results in the accumulation of two mRNA species (7.9 and 3.4 kilobase pairs (kb)). Both mRNAs hybridize with the previously described rat cat-1/2.9-kb cDNA clone. However, the 3' end of a longer rat cat-1 cDNA (rat cat-1/6.5-kb) hybridizes only to the 7.9-kb mRNA transcript. Sequence analysis of this clone indicated that the two mRNA species result from the use of alternative polyadenylation signals. The 6. 5-kb clone contains a number of AT-rich mRNA destabilizing sequences which is reflected in the half-life of the cat-1 mRNAs (90 min for 7. 9-kb mRNA and 250 min for 3.4-kb mRNA). Treatment of rats with cycloheximide superinduces the level of the 7.9-kb cat-1 mRNA in the kidney, spleen, and brain, but not in the liver, suggesting that cell type-specific labile factors are involved in its regulation. We conclude that the need for protein synthesis for induction of the cat-1 mRNA, the short lived nature of the mRNAs, and the multiple sites for regulation of gene expression indicate a tight control of expression of the cat-1 gene within the regenerating liver and suggest that y+ cationic amino acid transport in liver cells is regulated at the molecular level.

Association of hepatic system A amino acid transporter with the membrane-cytoskeletal proteins ankyrin and fodrin

System A activity is a highly regulated mechanism for the active transport of zwitterionic amino acids into mammalian cells. Monoclonal antibodies generated against a previously unidentified rat liver plasma membrane-associated protein were shown to immunoprecipitate solubilized System A transport activity. The immunoreactive protein was later determined by immunoblotting and peptide microsequencing to be rat liver alpha-fodrin (non-erythroid spectrin). Antibody against ankyrin, a protein that often serves as a bridge between integral membrane proteins and fodrin, also immunoprecipitated System A transport activity. Fractionation of solubilized plasma membrane proteins on sucrose gradients revealed that the System A transporter co-migrated as a complex with fodrin and ankyrin, even in the presence of detergent and urea. In contrast, the System N amino acid transporter does not co-migrate with ankyrin and fodrin, nor does the anti-fodrin antibody immunoprecipitate System N activity. The present data are the first to demonstrate an association between an organic solute transporter and the membranocytoskeletal proteins ankyrin and fodrin.

Expressed human hippocampal ASCT1 amino acid transporter exhibits a pH-dependent change in substrate specificity

In mammalian cells, the basal Na+-dependent uptake for many of the neutral amino acids is mediated by a transport activity designated System ASC. A cloned human brain cDNA sequence, ASCT1, encodes a Na+-dependent neutral amino acid transport activity that exhibits a substrate specificity similar to that commonly associated with System ASC. However, the characteristics of ASC activity varies significantly between cell types and not all tissues contain detectable levels of ASCT1 mRNA. A unique property of System ASC activity is an altered substrate selectivity such that at pH values below 7.4 anionic amino acids function as inhibitors and substrates. The experiments in this report were designed to determine if the cloned ASCT1 transporter exhibited this pH-dependent anionic transport. Following transfection of HeLa cells with the ASCT1 cDNA, transport strongly favored neutral zwitterionic) amino acids when uptake was measured at a physiologic pH value of 7.5. However, lowering the assay pH to 5.5 significantly enhanced the interaction of the ASCT1 carrier with anionic amino acids such as cysteate, in a pH-dependent manner. The apparent pK for the titratable group was in the range of 6.5-7.0. These results provide evidence that the human brain ASCT1 transporter exhibits the most distinguishing characteristic known for System ASC and provides a model system to investigate the molecular basis for this shift in substrate acceptance.

Eukaryotic gene expression: metabolite control by amino acids

Our understanding of the metabolite control in mammalian cells lags far behind that in prokaryotes. This is particularly true for amino-acid-dependent gene expression. Few proteins have been identified for which synthesis is selectively regulated by amino-acid availability, and the mechanisms for control of transcription and translation in response to changes in amino-acid availability have not yet been elucidated. The intimate relationship between amino-acid supply and the fundamental cellular process of protein synthesis makes amino-acid-dependent control of gene expression particularly important. Future studies should provide important insight into amino-acid and other nutrient signaling pathways, and their impact on cellular growth and metabolism.

Angiotensin II stimulates system y+ and cationic amino acid transporter gene expression in cultured vascular smooth muscle cells

The effect of angiotensin II (Ang II) on the transport of cationic amino acids has been examined in vascular smooth muscle cells (VSMC) isolated from rat aortae. Ang II stimulated the uptake rates of radiolabeled arginine and lysine in a time- and concentration-dependent manner. The stimulated arginine uptake could be blocked by pretreatments with cycloheximide and actinomycin D or co-treatment with valsartan, an antagonist specific for Ang II receptor subtype-1. The modulation by Ang II was bidirectional as the efflux of arginine was also stimulated, 5-fold over basal. Using reverse transcription-coupled polymerase chain reaction methodology, a partial cDNA with 94% sequence identity to that of cationic amino acid transporter subtype-1 (CAT-1) of mouse fibroblasts was obtained from VSMC. This sequence also exhibited 14 base changes compared with the sequence of ecotropic retrovirus receptor (ERR)/CAT-1 from rat hepatoma. Northern analyses with this partial CAT-1 cDNA and CAT-2 cDNA of mouse T-lymphocytes showed that Ang II rapidly stimulated the expression of both CAT-1 and CAT-2 in VSMC. Both signals peaked at 2 h after exposure to Ang II. The CAT-1 signal decayed over the next 6 h to levels 3-fold above basal, which are maintained up until 24 h. The induced CAT-2 mRNA concentration also decayed rapidly but increased again between 16 and 24 h to levels comparable with those observed at 2 h.

Amino acid transport systems modulate human tumor cell growth and invasion: a working hypothesis

Interactions between the extracellular matrix (ECM) and the neoplastic cells they envelop are thought to play a fundamental role in those cells' ability to invade, one of the key events in the metastatic cascade. Cellular transport of amino acids, in turn, is known to be mediated by functionally distinct membrane transport systems and is modulated by substrate bioavailability in the microenvironment. We postulate that certain advantages enjoyed by a neoplastic cell population over their normal counterparts (for example, increased proliferating capability and invasiveness across ECM barriers) are linked to changes in the cells' differential control of amino acid transport (aaT) via host ECM-tumor cell generated signals. Our studies suggest that active transport of neutral amino acids modulates a cells' functional behavior among phenotypically distinct human transformed cell types, irrespective of whether they are categorized as a sarcoma, melanoma, or carcinoma. We present preliminary laboratory evidence which has lead us to formulate a series of working hypotheses as follows: 1. aaT systems operating in both non-transformed and transformed human cells exhibit differential transport kinetics; 2. adaptive regulation of certain amino acids via cell-specific aaT systems alters a cell's ability to invade human ECM; and 3. aaT induction involves changes both at the cellular and molecular levels. This report, therefore, provides experimental support, and suggests a possible mechanism, to explain how neutral amino acids, acting as nutrient signalling factors (along with other biologic elements) within the cell milieu, have the capability of regulating the phenotypic nature of human neoplastic cells.

Peptide transporter function and prolidase activities in Caco-2 cells: a lack of coordinated expression

Peptide transport and prolidase activities were measured to determine whether the expression of these two components of protein nutrition are coordinately regulated; i.e., whether an increase in the peptide transporter function will necessarily lead to a higher prolidase activity, or vice versa. The results indicated that peptide transporter function and prolidase activity respond differently to cell differentiation and feeding schedules. The results also indicated that peptide transport and prolidase activities were different in two Caco-2 cell "clones", with S-K cells transported peptides at higher rates but had lower total prolidase activities, when compared to ATCC cells. These results suggest that the expression of the peptide transporter function and prolidase activity is not coordinated. In addition, both the transporter and the prolidase activities affected the overall transport of Phe when given as the dipeptide Phe-Pro, supporting the notion that intestinal absorption of peptides is an essential component of amino acid absorption. In conclusion, the evidence suggests that the peptide transporter function and prolidase activity are not coordinately expressed by the human intestinal Caco-2 cells.

Impairment of glucagon-induced hepatic system A activity by short-term ethanol administration in the rat

BACKGROUND/AIMS

System A is a membrane-bound, hormonally regulated carrier of amino acids that is induced by liver regeneration and impaired by ethanol. The mechanism of ethanol inhibition of system A is unknown; this study examines the effects of ethanol on the subcellular expression of system A activity following hormonal induction.

METHODS

Following hormonal treatment and short-term ethanol administration to rats, isolated liver Golgi and plasma membrane vesicles were examined for system A transport, and the kinetic parameters were determined.

RESULTS

Four hours after ethanol administration, the initial rate of system A activity was depressed 30% +/- 9% and 19% +/- 7% into Golgi and plasma membrane vesicles, respectively. The affinity constant of 2-(methylamino)-isobutyric acid uptake was unchanged between control and ethanol-treated vesicles, regardless of their subcellular origin. However, the maximal velocity of system A transport decreased from 1030 to 850 pmol.mg-1 protein.10 s-1 in Golgi vesicles and from 740 to 355 pmol.mg-1 protein.10 s-1 in plasma membrane vesicles.

CONCLUSIONS

Ethanol impairs hormonally induced system A activity in Golgi as well as in the plasma membrane vesicles. Ethanol potentially reduces glucagon induction of system A activity through an impairment of carrier biosynthesis or expression.

Characterization of system L and system y+ amino acid transport activity in cultured vascular smooth muscle cells

The uptake of L-leucine and L-lysine into vascular smooth muscle cells cultured from the aortas of rats has been investigated. Both amino acids are taken up by saturable systems that are independent of the presence of a Na+ gradient and can be stimulated in trans by neutral bulky amino acids for leucine and cationic amino acids for lysine. Leucine uptake is inhibited competitively in cis by several neutral amino acids, whereas lysine uptake is inhibited strongly by other cationic amino acids but also significantly by neutral amino acids such as leucine. The leucine inhibition is noncompetitive. Cells preloaded with leucine and lysine could also export these amino acids and the rate of efflux was stimulated by the presence of appropriate amino acids in trans. These data are all consistent with leucine being transported largely if not entirely by System L and lysine by the System y+ transporter.

Intestinal amino acid absorption during sepsis

Sepsis has been shown to cause a decrease in mesenteric blood flow in association with ultrastructural changes in the small intestine and impaired immune, barrier, and metabolic functions of the gut. These impairments in the structure and function of the gastrointestinal tract may have a detrimental effect on the morbidity and mortality of sepsis. Two recent studies have shown that the ability of the small intestine to absorb amino acids is also impaired during sepsis, but the systemic and cellular mechanisms of this impairment are not known. Release of cytokines induced by systemic bacteria or endotoxin may lead to a reduction in the synthesis of transporter proteins by the enterocyte at a time when there is reduced availability of both luminal (because of anorexia) and circulating (because of reduced mesenteric blood flow) substrates. Future research needs to investigate the systemic and local mediation of the sepsis-induced reduction in intestinal amino acid absorption and the possibility of correcting the defect by the administration of enteral nutrients, hormones, or drugs.

Neutral amino acid transport in an androgen-dependent epithelium

1. Transport of neutral amino acids by the isolated seminal vesicle epithelium of normal and gonadectomized guinea pigs has been investigated by measurement of the uptake of 2-amino[1-14C]-isobutyric acid and 2-methylamino[1-14C]isobutyric acid. 2. The Vmax for Na-dependent and -independent transport of both amino acids was reduced by gonadectomy but the general transport characteristics appeared to be unchanged by this treatment. 3. The most likely explanation of the decreased transport is the loss of transporter molecules associated with the tissue regression that follows rapidly on gonadectomy.

Transport of a large neutral amino acid in a human intestinal epithelial cell line (Caco-2): uptake and efflux of phenylalanine

The processes of L-phenylalanine (Phe) uptake and efflux from the apical (AP) and basolateral (BL) sides of an intestinal epithelial cell line (Caco-2) were investigated to further characterize the mechanism of transcellular transport of this amino acid. The results indicated that the initial uptake rates of Phe were saturable with a Km of 2.7 mM for AP uptake and 0.18 mM for BL uptake. Unlike the uptake, the initial efflux rates were shown to be proportional to the intracellular concentrations of Phe. Based on these kinetic studies and determination of other characteristics (e.g., Na+ dependency) of the uptake and efflux processes, it was concluded that AP uptake, BL uptake and BL efflux were distinctly different. This suggests that either different carriers or a different combination of carriers are responsible for the transmembrane transport of this amino acid. When the results of kinetic studies of Phe uptake and efflux were used to determine the rate-limiting step in the AP-to-BL transcellular transport of this amino acid, it was concluded that the BL efflux is the rate-limiting step in the transcellular transport of Phe in the Caco-2 cell monolayers.