Chemotaxin-dependent translocation of immunoreactive ADP-ribosyltransferase-1 to the surface of human neutrophil polymorphs

Mono-ADP-Ribosylation Catalyzed by Arginine-Specific ADP-Ribosyltransferases

Methods are described for determination of arginine-specific mono-ADP-ribosyltransferase activity of purified proteins and intact cells by monitoring the transfer of ADP-ribose from NAD⁺ to a model substrate, e.g., arginine, agmatine, and peptide (human neutrophil peptide-1 [HNP1]), and for the nonenzymatic hydrolysis of ADP-ribose-arginine to ornithine, a noncoded amino acid. In addition, preparation of purified ADP-ribosylarginine is included as a control substrate for ADP-ribosylation reactions.

Complex roles of members of the ADP-ribosyl transferase super family in immune defences: looking beyond PARP1

ADP ribosylation has been recently recognised as an important posttranslational modification regulating numerous cellular processes. This enzymatic activity is shared by two major families of enzymes, the extracellular ADP-ribosyl-transferases, or ecto-ARTS and the poly-ADP-ribosyltranferases, whose denomination derives from the capacity of its founding member, PARP1, to synthesise large linear or branched polymers of ADP-ribose on target proteins. This latter post-translational modification has recently attracted much interest based on its role in the cellular response to genotoxic and oxidative stress. Accordingly, a series of PARP-specific pharmacological inhibitors have demonstrated cell survival and anti-inflammatory properties in vivo, promoting a renewed interest in the potential immunoregulatory role of this gene family. More recently, the role of ADP-ribosylation in regulating several aspects of intracellular signalling and gene transcription has been uncovered, in particular within cells of the immune system, revealing the potential immunomodulatory role of several members of this family in addition to PARP1. We review herein the experimental evidence illustrating the complex role played by this gene family in regulating multiple aspects of the immune response, including cell survival, cytokine gene transcription and antiviral innate defences. In particular, the unexpected potential anti-inflammatory role of members of this family (including in particular PARP5a, 5b and PARP14) will be briefly discussed, raising some concern on the use of pan-specific PARP inhibitors to treat chronic inflammatory diseases.

Review: NAD +: a modulator of immune functions

Latterly, nicotinamide adenine dinucleotide (NAD+) has emerged as a molecule with versatile functions and of enormous impact on the maintenance of cell integrity. Besides playing key roles in almost all major aspects of energy metabolism, there is mounting evidence that NAD+ and its degradation products affect various biological activities including calcium homeostasis, gene transcription, DNA repair, and intercellular communication. This review is aimed at giving a brief insight into the life cycle of NAD+ in the cell, referring to synthesis, action and degradation aspects. With respect to their immunological relevance, the importance and function of the major NAD+ metabolizing enzymes, namely CD38/CD157, ADP-ribosyltransferases (ARTs), poly-ADP-ribose-polymerases (PARPs), and sirtuins are summarized and roles of NAD+ and its main degradation product adenosine 5'-diphosphoribose (ADPR) in cell signaling are discussed. In addition, an outline of the variety of immunological processes depending on the activity of nicotinamide phosphoribosyltransferase (Nampt), the key enzyme of the salvage pathway of NAD+ synthesis, is presented. Taken together, an efficient supply of NAD+ seems to be a crucial need for a multitude of cell functions, underlining the yet only partly revealed potency of this small molecule to influence cell fate.

Basal and inducible expression of the thiol-sensitive ART2.1 ecto-ADP-ribosyltransferase in myeloid and lymphoid leukocytes

ADP-ribosylation of cell surface proteins in mammalian cells is a post-translational modification by which ecto-ADP-ribosyltransferases (ARTs) transfer ADP-ribose from extracellular NAD to protein targets. The ART2 locus at murine chromosome 7 encompasses the tandem Art2a and Art2b genes that encode the distinct ART2.1 and ART2.2 proteins. Although both ecto-enzymes share 80% sequence identity, ART2.1 activity is uniquely regulated by an allosteric disulfide bond that is reducible in the presence of extracellular thiols, such as cysteine and glutathione, that accumulate in hypoxic and ischemic tissues. Previous studies have characterized the expression of ART2.1 and ART2.2 in murine T lymphocytes but not in other major classes of lymphoid and myeloid leukocytes. Here, we describe the expression of ART2.1 activity in a wide range of freshly isolated or tissue-cultured murine myeloid and lymphoid leukocytes. Spleen-derived macrophages, dendritic cells (DC), and B cells constitutively express ART2.1 as their predominant ART while spleen T cells express both ART2.1 and the thiol-independent ART2.2 isoform. Although bone-marrow-derived macrophages (BMDM) and dendritic cells (BMDC) constitutively express ART2.1 at low levels, it is markedly up-regulated when these cells are stimulated in vitro with IFNbeta or IFNgamma. ART2.1 expression and activity in splenic B cells is modestly up-regulated during incubation in vitro for 24 h, a condition that promotes B cell apoptosis. This increase in ART2.1 is attenuated by IL-4 (a B cell survival factor), but is not affected by IFNbeta/gamma, suggesting a possible induction of ART2.1 as an ancillary response to B cell apoptosis. In contrast, ART2.1 and ART2.2, which are highly expressed in freshly isolated splenic T cells, are markedly down-regulated when purified T cells are incubated in vitro for 12-24 h. Studies with the BW5147 mouse thymocyte line verified basal expression of ART2.1 and ART2.2, as in primary spleen T cells, and demonstrated that both isoforms can be up-regulated when T cells are maintained in the presence of IFNs. Comparison of the surface proteins which are ADP-ribosylated by ART2.1 in the different leukocyte subtypes indicated both shared and cell-specific proteins as ART2.1 substrates. The LFA-1 integrin, a major target for ART2.2 in T cells, is also ADP-ribosylated by the ART2.1 expressed in macrophages. Thus, ART2.1, in contrast to ART2.2, is expressed in a broad range of myeloid and lymphoid leukocytes. The thiol redox-sensitive nature of this ecto-enzyme suggests an involvement in purinergic signaling that occurs in the combined context of inflammation and hypoxia/ischemia.

ADP-ribosyltransferase-specific modification of human neutrophil peptide-1

Epithelial cells lining human airways and cells recruited to airways participate in the innate immune response in part by releasing human neutrophil peptides (HNP). Arginine-specific ADP-ribosyltransferases (ART) on the surface of these cells can catalyze the transfer of ADP-ribose from NAD to proteins. We reported that ART1, a mammalian ADP-ribosyltransferase, present in epithelial cells lining the human airway, modified HNP-1, altering its function. ADP-ribosylated HNP-1 was identified in bronchoalveolar lavage fluid (BALF) from patients with asthma, idiopathic pulmonary fibrosis, or a history of smoking (and having two common polymorphic forms of ART1 that differ in activity), but not in normal volunteers or patients with lymphangioleiomyomatosis. Modified HNP-1 was not found in the sputum of cystic fibrosis patients or in leukocyte granules of normal volunteers. The finding of ADP-ribosyl-HNP-1 in BALF but not in leukocyte granules suggests that the modification occurred in the airway. Most of the HNP-1 in the BALF from individuals with a history of smoking was, in fact, mono- or di-ADP-ribosylated. ART1 synthesized in Escherichia coli, glycosylphosphatidylinositol-anchored ART1 released with phosphatidylinositol-specific phospholipase C from transfected NMU cells, or ART1 expressed endogenously on C2C12 myotubes modified arginine 14 on HNP-1 with a secondary site on arginine 24. ADP-ribosylation of HNP-1 by ART1 was substantially greater than that by ART3, ART4, ART5, Pseudomonas aeruginosa exoenzyme S, or cholera toxin A subunit. Mouse ART2, which is an NAD:arginine ADP-ribosyltransferase, was able to modify HNP-1, but to a lesser extent than ART1. Although HNP-1 was not modified to a significant degree by ART5, it inhibited ART5 as well as ART1 activities. Human beta-defensin-1 (HBD1) was a poor transferase substrate. Reduction of the cysteine-rich defensins enhanced their ability to serve as ADP-ribose acceptors. We conclude that ADP-ribosylation of HNP-1 appears to be primarily an activity of ART1 and occurs in inflammatory conditions and disease.

Inactivation of platelet-derived growth factor-BB following modification by ADP-ribosyltransferase

1. Arginine-specific ADP-ribosyltransferase (ART1) is expressed on the surface of a number of cell types, and catalyses the transfer of ADP-ribose from NAD(+) to target proteins. We investigated whether extracellular proteins such as growth factors may serve as substrates for this enzyme, with subsequent alteration in their biological activity. Experiments were performed with rat skeletal muscle membranes and V79 Chinese hamster lung fibroblasts with doxycycline-inducible expression of human ART. 2. From a panel of growth factors, platelet-derived growth factor-BB (PDGF-BB) was found to be the best substrate for ART1, whereas the structural homologue PDGF-AA was not a substrate. Under conditions of maximum labelling 5 mol ADP-ribose was incorporated per mol of PDGF-BB. 3. Purified (ADP-ribosyl)-PDGF-BB did not stimulate a mitogenic or chemotactic response in human pulmonary smooth muscle cells, and showed a reduced capacity to bind to PDGF receptors in competition binding experiments, when compared to unmodified PDGF-BB. 4. PDGF-dependent [(3)H-methyl]-thymidine incorporation was measured in the ART1-transfected fibroblast cell line at physiological concentrations of PDGF-BB, and without addition of extracellular NAD(+). Fibroblasts expressing human ART1 at the cell surface showed reduced mitogenic responses to PDGF-BB, but not to PDGF-AA. This loss of mitogenic response in cells expressing ART1 activity was reversed by the addition of agmatine (an ART1 substrate). 5. In conclusion, we propose that PDGF-BB-dependent signalling may be regulated by post-translational modification of the growth factor by ART1 at the cell surface. This has been demonstrated in membranes of rat skeletal muscle, and the reaction confirmed in ART1-transfected fibroblasts.