System L amino acid transporters mediate the movement of bulky neutral amino acids across cell membranes. Until now three proteins that induce system L activity have been identified: LAT1, LAT2, and LAT3. The former two proteins belong to the solute carrier family 7 (SLC7), whereas the latter belongs to SLC43. In the present study we present a new cDNA, designated LAT4, which also mediates system L activity when expressed in Xenopus laevis oocytes. Human LAT4 exhibits 57% identity to human LAT3. Like LAT3, the amino acid transport activity induced by LAT4 is sodium-, chlorideand pH-independent, is not trans-stimulated, and shows two kinetic components. The low affinity component of LAT4 induced activity is sensitive to the sulfhydryl-specific reagent N-ethylmaleimide but not that with high affinity. Mutation in LAT4 of the SLC43 conserved serine 297 to alanine abolishes sensitivity to N-ethylmaleimide. LAT4 activity is detected at the basolateral membrane of PCT kidney cells. In situ hybridization experiments show that LAT4 mRNA is restricted to the epithelial cells of the distal tubule and the collecting duct in the kidney. In the intestine, LAT4 is mainly present in the cells of the crypt.

Arginine is processed by macrophages in response to the cytokines to which these cells are exposed. Th1-type cytokines induce NO synthase 2, which metabolizes arginine into nitrites, while the Th2-type cytokines produce arginase, which converts arginine into polyamines and proline. Activation of bone marrow-derived macrophages by these two types of cytokines increases l-arginine transport only through the y+ system. Analysis of the expression of the genes involved in this system showed that Slc7A1, encoding cationic amino acid transporters (CAT)1, is constitutively expressed and is not modified by activating agents, while Slc7A2, encoding CAT2, is induced during both classical and alternative activation. Macrophages from Slc7A2 knockout mice showed a decrease in l-arginine transport in response to the two kinds of cytokines. However, while NO synthase 2 and arginase expression were unmodified in these cells, the catabolism of arginine was impaired by both pathways, producing smaller amounts of nitrites and also of polyamines and proline. In addition, the induction of Slc7A2 expression was independent of the arginine available and of the enzymes that metabolize it. In conclusion, the increased arginine transport mediated by activators is strongly regulated by CAT2 expression, which could limit the function of macrophages.

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In murine macrophages, as a result of arginine catabolism during activation, citruline is produced under the effect of IFN-c and LPS, and ornithine and polyamines by IL-4 and IL-10. For proliferation, arginine is required from the extracellular medium and is used for protein synthesis. During activation, most arginine (>95% in 6 h) was metabolized, while under proliferation only half was incorporated into proteins. Under basal conditions, this amino acid was preferentially transported by y + L activity. During activation, arginine transport increased drastically (4-5-fold) through y + cationic amino acid transporter (CAT) activity. By contrast, M-CSF induced only a modest increase in uptake (0.5-fold). The increase in arginine transport during activation, but not proliferation, was mediated by the SLC7A2/Cat2 gene. SLC7A1/Cat1 is constitutively expressed, and is not modified by proliferating or activating agents. M-CSF-dependent proliferation was not affected in the macrophages of SLC7A2 knockout mice; however, these cells showed a drastic reduction in the production of citruline or ornithine and polyamines during activation. The data show that a large increase in a specific transport system (CAT2) is necessary for activation-induced arginine metabolism, while arginine is in excess for the requirements of proliferation and a modest increase in transport occurs.Supporting information for this article is available at http://www.wiley-vch.de/contents/jc_2040/2006/35694_s.pdf IntroductionMacrophages originate from undifferentiated stem cells in the bone marrow, and are transported through the blood to all body tissues where, in most cases, they undergo apoptosis [1]. Depending on the stimuli received, in tissues these phagocytes differentiate in numerous cell types including liver Kupffer cells, dermal Langerhans cells, bone osteoclasts, brain microglia, and Cell proliferation and activation require high concentrations of arginine either for protein synthesis or to elaborate either NO in classical activation or to produce polyamines and praline in alternative activation [3]. The extracellular milieu is the main source of arginine and several transport systems are involved in carrying this amino acid across the plasma membrane. Depending on the cell type, a number of transport activities may be expressed [6]. The SLC7 (solute carrier) family is divided into two subgroups, the cationic amino acid transporters (the CAT family, SLC7A1-4) and the glycoprotein-associated amino acid transporters SLC7A5-11, also called light chains or catalytic chains of the hetero(di)meric amino acid transporters. The CAT group includes four members, CAT-1 to CAT-4, whose gene products are SLC7A1 to SLC7A4. The first three members transport cationic L-amino acids, whereas the function of CAT-4 is not known. The hetero(di)meric amino acid transporters family is composed of seven proteins, whose genes are SLC7A5 to SLC7A11, but only y + LAT2, y + LAT1 and b o+ AT transport cationic amino acids [6].Macrophages require arginine to proliferate ...