In metazoans, most microRNAs imperfectly base-pair with the 3' untranslated region (3'UTR) of target mRNAs and prevent protein accumulation by either repressing translation or inducing mRNA degradation. Examples of specific mRNAs undergoing microRNA-mediated repression are numerous, but whether the repression is a reversible process remains largely unknown. Here we show that cationic amino acid transporter 1 (CAT-1) mRNA and reporters bearing its 3'UTR can be relieved from the microRNA miR-122-induced inhibition in human hepatocarcinoma cells subjected to different stress conditions. The derepression of CAT-1 mRNA is accompanied by its release from cytoplasmic processing bodies and its recruitment to polysomes. The derepression requires binding of HuR, an AU-rich-element binding protein, to the 3'UTR of CAT-1 mRNA. We propose that proteins interacting with the 3'UTR will generally act as modifiers altering the potential of miRNAs to repress gene expression.

Macrophages play a major role in the immune system, both as antimicrobial effector cells and as immunoregulatory cells, which induce, suppress or modulate adaptive immune responses. These key aspects of macrophage biology are fundamentally driven by the phenotype of macrophage arginine metabolism that is prevalent in an evolving or ongoing immune response. M1 macrophages express the enzyme nitric oxide synthase, which metabolizes arginine to nitric oxide (NO) and citrulline. NO can be metabolized to further downstream reactive nitrogen species, while citrulline might be reused for efficient NO synthesis via the citrulline–NO cycle. M2 macrophages are characterized by expression of the enzyme arginase, which hydrolyzes arginine to ornithine and urea. The arginase pathway limits arginine availability for NO synthesis and ornithine itself can further feed into the important downstream pathways of polyamine and proline syntheses, which are important for cellular proliferation and tissue repair. M1 versus M2 polarization leads to opposing outcomes of inflammatory reactions, but depending on the context, M1 and M2 macrophages can be both pro- and anti-inflammatory. Notably, M1/M2 macrophage polarization can be driven by microbial infection or innate danger signals without any influence of adaptive immune cells, secondarily driving the T helper (Th)1/Th2 polarization of the evolving adaptive immune response. Since both arginine metabolic pathways cross-inhibit each other on the level of the respective arginine break-down products and Th1 and Th2 lymphocytes can drive or amplify macrophage M1/M2 dichotomy via cytokine activation, this forms the basis of a self-sustaining M1/M2 polarization of the whole immune response. Understanding the arginine metabolism of M1/M2 macrophage phenotypes is therefore central to find new possibilities to manipulate immune responses in infection, autoimmune diseases, chronic inflammatory conditions, and cancer.

Three isozymes of nitric oxide (NO) synthase (EC 1.14.13.39) have been identified and the cDNAs for these enzymes isolated. In humans, isozymes I (in neuronal and epithelial cells), II (in cytokine-induced cells), and III (in endothelial cells) are encoded for by three different genes located on chromosomes 12, 17, and 7, respectively. The deduced amino acid sequences of the human isozymes show less than 59% identity. Across species, amino acid sequences for each isoform are well conserved (>90% for isoforms I and III, >80% for isoform II). All isoforms use L-arginine and molecular oxygen as substrates and require the cofactors NADPH, 6(7?)-5,6,7,8-tetrahydrobiopterin, flavin adenine dinucleotide, and flavin mononucleotide. They all bind calmodulin and contain heme. Isoform I is constitutively present in central and peripheral neuronal cells and certain epithelial cells. Its activity is regulated by Ca 2+ and calmodulin. Its functions include long-term regulation of synaptic transmission in the central nervous system, central regulation of blood pressure, smooth muscle relaxation, and vasodilatation via peripheral nitrergic nerves. It has also been implicated in T he initial evidence for the production of nitrogen oxides in mammals came from experiments demonstrating nitrate production in germ-free rats. 1This triggered the search for mammalian cells capable of synthesizing nitrogen oxides and in 1985 led to the discovery that macrophages could be induced with lipopolysaccharide to produce significant amounts of both nitrite and nitrate. 2 Further work demonstrated that L-arginine was the substrate for this pathway and that L-citrulline was formed as a coproduct.3 -4 One year later nitric oxide (NO) was identified as the initial product that is subsequently oxidized to nitrite and nitrate. 5 In parallel, Furchgott and coworkers* 7 had discovered endothelium-derived relaxing factor (EDRF). It had been established that, similar to nitrovasodilators, the EDRF-mediated vasodilatation was associated with increased levels of cyclic GMP and activation of cyclic GMP kinase activity in smooth muscle cells 810 and that the EDRF could directly stimulate purified soluble guanyh/1 cyclase. 1112 In 1987 it was concluded that NO can account for the biologic activity of EDRF, 1315 and, analogous to the macrophage system, L-arginine was established as a substrate for EDRF/NO synthesis in endothelial cells. physiological research demonstrated that stimulation of neuronal cells and brain slices with agonists leads to the release of a labile mediator that stimulates guanylyl cyclase and has the properties of NO.1820 During the past 4 years, significant progress has been made elucidating the mechanism of NO synthesis, the NO synthases involved, and the functions of NO in different biologic systems. The present review attempts to summarize this progress with some emphasis on the cardiovascular system. Isozymes of NO SynthaseMany cells are capable of synthesizing NO. Three isozymes of NO synthase (EC 1.14.13.39) have been...

The SLC7 family is divided into two subgroups, the cationic amino acid transporters (the CAT family, SLC7A1-4) and the glycoprotein-associated amino acid transporters (the gpaAT family,

Susceptibility of rodent cells to infection by ecotropic murine leukaemia viruses (MuLV) is determined by binding of the virus envelope to a membrane receptor that has multiple membrane-spanning domains. Cells infected by ecotropic MuLV synthesize envelope protein, gp70, which binds to this receptor, thereby preventing additional infections. The consequences of envelope-MuLV receptor binding for the infected host cell have not been directly determined, partly because the cellular function of the MuLV receptor protein is unknown. Here we report a coincidence in the positions of the first eight putative membrane-spanning domains found in the virus receptor and in two related proteins, the arginine and histidine permeases of Saccharomyces cerevisiae (Fig. 1), but not in any other proteins identified by computer-based sequence comparison of the GenBank data base. Xenopus oocytes injected with receptor-encoding messenger RNA show increased uptake of L-arginine, L-lysine and L-ornithine. The transport properties and the expression pattern of the virus receptor behave in ways previously attributed to y+, the principal transporter of cationic L-amino acids in mammalian cells.