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      Cystathionine-β-synthase: Molecular Regulation and Pharmacological Inhibition

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          Abstract

          Cystathionine-β-synthase (CBS), the first (and rate-limiting) enzyme in the transsulfuration pathway, is an important mammalian enzyme in health and disease. Its biochemical functions under physiological conditions include the metabolism of homocysteine (a cytotoxic molecule and cardiovascular risk factor) and the generation of hydrogen sulfide (H 2S), a gaseous biological mediator with multiple regulatory roles in the vascular, nervous, and immune system. CBS is up-regulated in several diseases, including Down syndrome and many forms of cancer; in these conditions, the preclinical data indicate that inhibition or inactivation of CBS exerts beneficial effects. This article overviews the current information on the expression, tissue distribution, physiological roles, and biochemistry of CBS, followed by a comprehensive overview of direct and indirect approaches to inhibit the enzyme. Among the small-molecule CBS inhibitors, the review highlights the specificity and selectivity problems related to many of the commonly used “CBS inhibitors” (e.g., aminooxyacetic acid) and provides a comprehensive review of their pharmacological actions under physiological conditions and in various disease models.

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          The cystine/glutamate antiporter system x(c)(-) in health and disease: from molecular mechanisms to novel therapeutic opportunities.

          The antiporter system x(c)(-) imports the amino acid cystine, the oxidized form of cysteine, into cells with a 1:1 counter-transport of glutamate. It is composed of a light chain, xCT, and a heavy chain, 4F2 heavy chain (4F2hc), and, thus, belongs to the family of heterodimeric amino acid transporters. Cysteine is the rate-limiting substrate for the important antioxidant glutathione (GSH) and, along with cystine, it also forms a key redox couple on its own. Glutamate is a major neurotransmitter in the central nervous system (CNS). By phylogenetic analysis, we show that system x(c)(-) is a rather evolutionarily new amino acid transport system. In addition, we summarize the current knowledge regarding the molecular mechanisms that regulate system x(c)(-), including the transcriptional regulation of the xCT light chain, posttranscriptional mechanisms, and pharmacological inhibitors of system x(c)(-). Moreover, the roles of system x(c)(-) in regulating GSH levels, the redox state of the extracellular cystine/cysteine redox couple, and extracellular glutamate levels are discussed. In vitro, glutamate-mediated system x(c)(-) inhibition leads to neuronal cell death, a paradigm called oxidative glutamate toxicity, which has successfully been used to identify neuroprotective compounds. In vivo, xCT has a rather restricted expression pattern with the highest levels in the CNS and parts of the immune system. System x(c)(-) is also present in the eye. Moreover, an elevated expression of xCT has been reported in cancer. We highlight the diverse roles of system x(c)(-) in the regulation of the immune response, in various aspects of cancer and in the eye and the CNS.
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            Amino acid transporter SLC7A11/xCT at the crossroads of regulating redox homeostasis and nutrient dependency of cancer

            Cancer cells often upregulate nutrient transporters to fulfill their increased biosynthetic and bioenergetic needs, and to maintain redox homeostasis. One nutrient transporter frequently overexpressed in human cancers is the cystine/glutamate antiporter solute carrier family 7 member 11 (SLC7A11; also known as xCT). SLC7A11 promotes cystine uptake and glutathione biosynthesis, resulting in protection from oxidative stress and ferroptotic cell death. Recent studies have unexpectedly revealed that SLC7A11 also plays critical roles in glutamine metabolism and regulates the glucose and glutamine dependency of cancer cells. This review discusses the roles of SLC7A11 in regulating the antioxidant response and nutrient dependency of cancer cells, explores our current understanding of SLC7A11 regulation in cancer metabolism, and highlights key open questions for future studies in this emerging research area. A deeper understanding of SLC7A11 in cancer metabolism may identify new therapeutic opportunities to target this important amino acid transporter for cancer treatment.
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              Ultradeep human phosphoproteome reveals a distinct regulatory nature of Tyr and Ser/Thr-based signaling.

              Regulatory protein phosphorylation controls normal and pathophysiological signaling in eukaryotic cells. Despite great advances in mass-spectrometry-based proteomics, the extent, localization, and site-specific stoichiometry of this posttranslational modification (PTM) are unknown. Here, we develop a stringent experimental and computational workflow, capable of mapping more than 50,000 distinct phosphorylated peptides in a single human cancer cell line. We detected more than three-quarters of cellular proteins as phosphoproteins and determined very high stoichiometries in mitosis or growth factor signaling by label-free quantitation. The proportion of phospho-Tyr drastically decreases as coverage of the phosphoproteome increases, whereas Ser/Thr sites saturate only for technical reasons. Tyrosine phosphorylation is maintained at especially low stoichiometric levels in the absence of specific signaling events. Unexpectedly, it is enriched on higher-abundance proteins, and this correlates with the substrate KM values of tyrosine kinases. Our data suggest that P-Tyr should be considered a functionally separate PTM of eukaryotic proteomes. Copyright © 2014 The Authors. Published by Elsevier Inc. All rights reserved.

                Author and article information

                Journal
                Biomolecules
                Biomolecules
                biomolecules
                Biomolecules
                MDPI
                2218-273X
                30 April 2020
                May 2020
                : 10
                : 5
                : 697
                Affiliations
                [1 ]Chair of Pharmacology, Section of Medicine, University of Fribourg, 1702 Fribourg, Switzerland; karim.zuhra@ 123456unifr.ch (K.Z.); fiona.augsburger@ 123456unifr.ch (F.A.)
                [2 ]Department of Pediatrics, University of Colorado Anschutz Medical Campus, Aurora, CO 80045, USA; tomas.majtan@ 123456cuanschutz.edu
                Author notes
                [* ]Correspondence: csaba.szabo@ 123456unifr.ch
                Author information
                https://orcid.org/0000-0003-0186-1817
                https://orcid.org/0000-0001-8357-6766
                https://orcid.org/0000-0002-2751-2124
                https://orcid.org/0000-0003-3110-4235
                Article
                biomolecules-10-00697
                10.3390/biom10050697
                7277093
                32365821
                ed6e623f-1c35-477e-92b0-dc47777d818f
                © 2020 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

                History
                : 14 April 2020
                : 27 April 2020
                Categories
                Review

                hydrogen sulfide,cancer,down syndrome,pharmacology,homocysteine,cystathionine

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