CATs, a family of three distinct mammalian cationic amino acid transporters

Arginine limitation drives a directed codon-dependent DNA sequence evolution response in colorectal cancer cells

Utilization of specific codons varies between organisms. Cancer represents a model for understanding DNA sequence evolution and could reveal causal factors underlying codon evolution. We found that across human cancer, arginine codons are frequently mutated to other codons. Moreover, arginine limitation-a feature of tumor microenvironments-is sufficient to induce arginine codon-switching mutations in human colon cancer cells. Such DNA codon switching events encode mutant proteins with arginine residue substitutions. Mechanistically, arginine limitation caused rapid reduction of arginine transfer RNAs and the stalling of ribosomes over arginine codons. Such selective pressure against arginine codon translation induced an adaptive proteomic shift toward low-arginine codon-containing genes, including specific amino acid transporters, and caused mutational evolution away from arginine codons-reducing translational bottlenecks that occurred during arginine starvation. Thus, environmental availability of a specific amino acid can influence DNA sequence evolution away from its cognate codons and generate altered proteins.

Screening of commonly prescribed drugs for effects on the CAT1-mediated transport of L-arginine and arginine derivatives

The cationic amino acid transporter 1 (CAT1/SLC7A1) plays a key role in the cellular uptake or export of l-arginine and some of its derivatives. This study investigated the effect of 113 chemically diverse and commonly used drugs (at 20 and 200 µM) on the CAT1-mediated cellular uptake of l-arginine, l-homoarginine, and asymmetric dimethylarginine (ADMA). Twenty-three (20%) of the tested substances showed weak inhibitory or stimulatory effects, but only verapamil showed consistent inhibitory effects on CAT1-mediated transport of all tested substrates.

Arginine-dependent immune responses

A growing body of evidence indicates that, over the course of evolution of the immune system, arginine has been selected as a node for the regulation of immune responses. An appropriate supply of arginine has long been associated with the improvement of immune responses. In addition to being a building block for protein synthesis, arginine serves as a substrate for distinct metabolic pathways that profoundly affect immune cell biology; especially macrophage, dendritic cell and T cell immunobiology. Arginine availability, synthesis, and catabolism are highly interrelated aspects of immune responses and their fine-tuning can dictate divergent pro-inflammatory or anti-inflammatory immune outcomes. Here, we review the organismal pathways of arginine metabolism in humans and rodents, as essential modulators of the availability of this semi-essential amino acid for immune cells. We subsequently review well-established and novel findings on the functional impact of arginine biosynthetic and catabolic pathways on the main immune cell lineages. Finally, as arginine has emerged as a molecule impacting on a plethora of immune functions, we integrate key notions on how the disruption or perversion of arginine metabolism is implicated in pathologies ranging from infectious diseases to autoimmunity and cancer.

Transport of L-Arginine Related Cardiovascular Risk Markers

L-arginine and its derivatives, asymmetric and symmetric dimethylarginine (ADMA and SDMA) and L-homoarginine, have emerged as cardiovascular biomarkers linked to cardiovascular outcomes and various metabolic and functional pathways such as NO-mediated endothelial function. Cellular uptake and efflux of L-arginine and its derivatives are facilitated by transport proteins. In this respect the cationic amino acid transporters CAT1 and CAT2 (SLC7A1 and SLC7A2) and the system y⁺L amino acid transporters (SLC7A6 and SLC7A7) have been most extensively investigated, so far, but the number of transporters shown to mediate the transport of L-arginine and its derivatives is constantly increasing. In the present review we assess the growing body of evidence regarding the function, expression, and clinical relevance of these transporters and their possible relation to cardiovascular diseases.

Molecular Determinants for Nitric Oxide Regulation of the Murine Cationic Amino Acid Transporter CAT-2A

Cationic amino acid transporters (CATs) supply cells with essential and semiessential dibasic amino acids. Among them, l-arginine is the substrate for nitric oxide synthases (NOS) to produce nitric oxide (NO), a key signaling molecule and second messenger. In cardiac preparations, we showed that NO acutely and directly modulates transport activity by noncompetitively inhibiting these CATs. We hypothesize that this NO regulation occurs through modification of cysteine residues in CAT proteins. Homology modeling and a computational chemistry approach identified Cys³⁴⁷ as one of two putative targets for NO binding, of 15 Cys residues present in the low-affinity mouse CAT-2A (mCAT-2A). To test this prediction, mammalian cell lines overexpressing mCAT-2A were used for site-directed mutagenesis and uptake studies. When Cys³⁴⁷ was replaced with alanine (Cys³⁴⁷Ala), mCAT-2A became insensitive to inhibition by NO donors. In addition, the transport capacity of this variant decreased by >50% compared to that of the control, without affecting membrane expression levels or apparent affinities for the transported amino acids. Interestingly, replacing Cys³⁴⁷ with serine (Cys³⁴⁷Ser) restored uptake levels to those of the control while retaining NO insensitivity. Other Cys residues, when replaced with Ala, still produced a NO-sensitive CAT-2A. In cells co-expressing NOS and mCAT-2A, exposure to extracellular l-arginine inhibited the uptake activity of control mCAT-2A, via NO production, but not that of the Cys³⁴⁷Ser variant. Thus, the -SH moiety of Cys³⁴⁷ is largely responsible for mCAT-2A inhibition by NO. Because of the endogenous NO effect, this modulation is likely to be physiologically relevant and a potential intervention point for therapeutics.

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

Expression Profile of Cationic Amino Acid Transporters in Rats with Endotoxin-Induced Uveitis

Purpose. The transcellular arginine transportation via cationic amino acid transporter (CAT) is the rate-limiting step in nitric oxide (NO) synthesis, which is crucial in intraocular inflammation. In this study, CAT isoforms and inducible nitric oxide synthase (iNOS) expression was investigated in endotoxin-induced uveitis (EIU). Methods. EIU was induced in Lewis rats by lipopolysaccharide (LPS) injection. In the treatment group, the rats were injected intraperitoneally with the proteasome inhibitor bortezomib before EIU induction. After 24 hours, leukocyte quantification, NO measurement of the aqueous humor, and histopathological examination were evaluated. The expression of CAT isoforms and iNOS was determined by reverse transcription-polymerase chain reaction, western blotting, and immunofluorescence staining. Nuclear factor-kappa B (NF-κB) binding activity was evaluated by electrophoretic mobility shift assay. The mouse macrophage cell line RAW 264.7 was used to validate the in vivo findings. Results. LPS significantly stimulated iNOS, CAT-2A, and CAT-2B mRNA and protein expression but did not affect CAT-1 in EIU rats and RAW 264.7 cells. Bortezomib attenuated inflammation and inhibited iNOS, CAT-2A, and CAT-2B expression through NF-κB inhibition. Conclusions. CAT-2 and iNOS, but not CAT-1, are specifically involved in EIU. NF-κB is essential in the induction of CAT-2 and iNOS in EIU.

Identification and functional characterization of a novel arginine/ornithine transporter, a member of a cationic amino acid transporter subfamily in the Trypanosoma cruzi genome

Background

Trypanosoma cruzi, the etiological agent of Chagas disease, is auxotrophic for arginine. It obtains this amino acid from the host through transporters expressed on the plasma membrane and on the membranes of intracellular compartments. A few cationic amino acid transporters have been characterized at the molecular level, such as the novel intracellular arginine/ornithine transporter, TcCAT1.1, a member of the TcCAT subfamily that is composed of four almost identical open reading frames in the T. cruzi genome.

Methods

The functional characterization of the TcCAT1.1 isoform was performed in two heterologous expression systems. TcCAT subfamily expression was evaluated by real-time PCR in polysomal RNA fractions, and the cellular localization of TcCAT1.1 fused to EGFP was performed by confocal and immunoelectron microscopy.

Results

In the S. cerevisiae expression system, TcCAT1.1 showed high affinity for arginine (K_m = 0.085 ± 0.04 mM) and low affinity for ornithine (K_m = 1.7 ± 0.2 mM). Xenopus laevis oocytes expressing TcCAT1.1 showed a 7-fold increase in arginine uptake when they were pre-loaded with arginine, indicating that transport is enhanced by substrates on the trans side of the membrane (trans-stimulation). Oocytes that were pre-loaded with [³H]-arginine displayed a 16-fold higher efflux of [³H]-arginine compared with that of the control. Analysis of polysomal RNA fractions demonstrated that the expression of members of the arginine transporter TcCAT subfamily is upregulated under nutritional stress and that this upregulation precedes metacyclogenesis. To investigate the cellular localization of the transporter, EGFP was fused to TcCAT1.1, and fluorescence microscopy and immunocytochemistry revealed the intracellular labeling of vesicles in the anterior region, in a network of tubules and vesicles.

Conclusions

TcCAT1.1 is a novel arginine/ornithine transporter, an exchanger expressed in intracellular compartments that is physiologically involved in arginine homeostasis throughout the T. cruzi life cycle. The properties and estimated kinetic parameters of TcCAT1.1 can be extended to other members of the TcCAT subfamily.

Regulation of CAT: Cationic amino acid transporter gene expression

The majority of mammalian cationic amino acid transport is mediated by the transport system y(+) which facilitates Na(+) independent cationic amino acid (arginine, lysine, & ornithine) transport and Na(+) dependent zwitterionic amino acid (glutamine & homoserine) transport. Other transport systems y(+)L, b(0,+) and B(0,+) also mediate cationic amino acid transport. Their broad substrate specificities and overlapping expression patterns confound biochemical analysis. The isolation of cDNA clones has permitted an analysis of their regulation and opens the opportunity to define the role of each protein in specific cell types. Two genes,Cat1 andCat2 encode transporters with properties similar to the y(+) transport system. Thecat2 gene from the mouse encodes two distinct proteins. mCAT2, and mCAT2A via alternate splicing; each protein has distinctly different transport properties. The regulation of mCAT1, mCAT2 and mCAT2A proteins are reviewed here. The implications of this gene specific regulation on cationic amino acid transport is discussed.

Heparin normalizes allergen-induced nitric oxide deficiency and airway hyperresponsiveness

It has been established that polycations cause airway hyperresponsiveness (AHR) to methacholine by inducing a deficiency of constitutive nitric oxide synthase (cNOS)-derived bronchodilating nitric oxide (NO). Since a deficiency of cNOS-derived NO also contributes to allergen-induced AHR after the early asthmatic reaction (EAR) and since this AHR is associated with the release of polycationic proteins from infiltrated eosinophils in the airways, we hypothesized that endogenous polycations underlie or at least contribute to the allergen-induced NO deficiency and AHR. Using a guinea-pig model of allergic asthma, we addressed this hypothesis by examining the effect of the polyanion heparin, acting as a polycation antagonist, on the responsiveness to methacholine of isolated perfused tracheae from unchallenged control animals and from animals 6 h after ovalbumin challenge, that is, after the EAR. A 2.0-fold AHR (P<0.001) to intraluminal administration of methacholine was observed in airways from allergen-challenged animals compared to control. Incubation of these airways with 250 U ml(-1) heparin completely normalized the observed hyperresponsiveness (P<0.001), whereas the responsiveness to methacholine of airways from unchallenged control animals was not affected. The effect of heparin on airways from allergen-challenged guinea-pigs was dose-dependently (0.1 and 1.0 mM) reversed by the NOS inhibitor L-NAME (P<0.01). These results indicate that endogenous (presumably eosinophil-derived) polycations are involved in allergen-induced NO deficiency and AHR after the EAR, probably by inhibition of l-arginine transport.

Inhibition of nitric oxide synthase abrogates lipopolysaccharides-induced up-regulation of L-arginine uptake in rat alveolar macrophages

It was tested whether the inducible nitric oxide synthase (iNOS) pathway might be involved in lipopolysaccharides-(LPS)-induced up-regulation of L-arginine transport in rat alveolar macrophages (AM). AM were cultured in absence or presence of LPS. Nitrite accumulation was determined in culture media and cells were used to study [3H]-L-arginine uptake or to isolate RNA for RT - PCR. Culture in presence of LPS (1 microg ml(-1), 20 h) caused 11 fold increase of nitrite accumulation and 2.5 fold increase of [3H]-L-arginine uptake. The inducible NO synthase (iNOS) inhibitor 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine (AMT) present alone during culture had only marginal effects on [3H]-L-arginine uptake. However, AMT present during culture additionally to LPS, suppressed LPS-induced nitrite accumulation and LPS-stimulated [3H]-L-arginine uptake in the same concentration-dependent manner. AMT present only for the last 30 min of the culture period had similar effects on [3H]-L-arginine uptake. AMT present only during the uptake period also inhibited LPS-stimulated [3H]-L-arginine uptake, but with lower potency. The inhibitory effect of AMT could not be opposed by the NO releasing compound DETA NONOate. LPS caused an up-regulation of the mRNA for the cationic amino acid transporter CAT-2B, and this effect was not affected by AMT. AMT (100 microM) did not affect L-arginine transport studied by electrophysiological techniques in Xenopus laevis oocytes expressing either the human cationic amino acid transporter hCAT-1 or hCAT-2B. In conclusion, iNOS inhibition in rat AM abolished LPS-activated L-arginine uptake. This effect appears to be caused by reduced flow of L-arginine through the iNOS pathway.

The transport activity of the human cationic amino acid transporter hCAT-1 is downregulated by activation of protein kinase C

1. The human cationic amino acid transporter hCAT-1 contains several consensus sequences for phosphorylation by protein kinase C (PKC). This study investigates the effect of PKC activation on hCAT-1-mediated transport. 2. When expressed in Xenopus laevis oocytes, hCAT-1-mediated L-arginine transport was reduced to 44+/-3% after a 30 min treatment of the oocytes with 100 nM phorbol-12-myristate-13-acetate (PMA). 4 alpha-phorbol-12,13-didecanoate (4 alpha-PDD, 100 nM) had no effect. 3. In EA.hy926 endothelial cells, maximal inhibition of hCAT-1-mediated L-arginine transport (to 3 -- 11% of control) was observed after treatment of the cells with 100 nM PMA for 4 h. A 20 -- 30 h exposure of the cells to 100 nM PMA led to the recovery of the L-arginine uptake rate that was now resistant to a second application of PMA. Phorbol-12,13-dibutyrate had similar effects as PMA, whereas 4 alpha-PDD had no effect. One microM bisindolylmaleimide I reduced the PMA effect significantly. 4. Interestingly, a 4 h treatment with 100 nM PMA increased the expression of hCAT-1 mRNA 3 -- 5 fold. hCAT-1 protein levels were unchanged for up to 4 h after PMA treatment and then increased slightly between 8 -- 28 h. 5. It is concluded that PMA downregulates the intrinsic activity of hCAT-1 by a pathway involving protein kinase C.

Activation of L-arginine transport by protein kinase C in rabbit, rat and mouse alveolar macrophages

1. The role of protein kinase C in controlling L-arginine transport in alveolar macrophages was investigated. 2. L-[3H]Arginine uptake in rabbit alveolar macrophages declined by 80 % after 20 h in culture. 4beta-Phorbol 12-myristate 13-acetate (PMA), but not 4alpha-phorbol 12-myristate 13-acetate (alpha-PMA), present during 20 h culture, enhanced L-[3H]arginine uptake more than 10-fold. Staurosporine and chelerythrine opposed this effect. 3. L-[3H]Arginine uptake was saturable and blockable by L-lysine. After PMA treatment Vmax was increased more than 5-fold and Km was reduced from 0.65 to 0.32 mM. 4. Time course experiments showed that PMA increased L-[3H]arginine uptake almost maximally within 2 h. This short-term effect was not affected by cycloheximide or actinomycin D. 5. L-[3H]Arginine uptake and its stimulation by PMA was also observed in sodium-free medium. 6. L-Leucine (0.1 mM) inhibited L-[3H]arginine uptake by 50 % in sodium-containing medium, but not in sodium-free medium. At 1 mM, L-leucine caused significant inhibition in sodium-free medium also. L-Leucine showed similar effects on PMA-treated cells. 7. N-Ethylmaleimide (200 microM, 10 min) reduced L-[3H]arginine uptake by 70 % in control cells, but had no effect on PMA-treated (20 or 2 h) cells. 8. In alveolar macrophages, multiple transport systems are involved in L-arginine uptake, which is markedly stimulated by protein kinase C, probably by modulation of the activity of already expressed cationic amino acid transporters.