Protein mono-ADP-ribosylation is a reversible post-translational modification of cellular proteins. This scheme of amino-acid modification is used not only by bacterial toxins to attack host cells, but also by endogenous ADP-ribosyltransferases (ARTs) in mammalian cells. These latter ARTs include members of three different families of proteins: the well characterised arginine-specific ecto-enzymes (ARTCs), two sirtuins, and some members of the poly(ADP-ribose) polymerase (PARP/ARTD) family. In the present study, we demonstrate that human ARTC1 is localised to the endoplasmic reticulum (ER), in contrast to the previously characterised ARTC proteins, which are typical GPI-anchored ecto-enzymes. Moreover, using the "macro domain" cognitive binding module to identify ADP-ribosylated proteins, we show here that the ER luminal chaperone GRP78/BiP (glucose-regulated protein of 78 kDa/immunoglobulin heavy-chain-binding protein) is a cellular target of human ARTC1 and hamster ARTC2. We further developed a procedure to visualise ADP-ribosylated proteins using immunofluorescence. With this approach, in cells overexpressing ARTC1, we detected staining of the ER that co-localises with GRP78/BiP, thus confirming that this modification occurs in living cells. In line with the key role of GRP78/BiP in the ER stress response system, we provide evidence here that ARTC1 is activated during the ER stress response, which results in acute ADP-ribosylation of GRP78/BiP paralleling translational inhibition. Thus, this identification of ARTC1 as a regulator of GRP78/BiP defines a novel, previously unsuspected, player in GRP78-mediated ER stress responses.

During the development, progression and dissemination of neoplastic lesions, cancer cells can hijack normal pathways and mechanisms. This includes the control of the function of cellular proteins through reversible post-translational modifications, such as ADP-ribosylation, phosphorylation, and acetylation. In the case of mono-ADP-ribosylation and poly-ADP-ribosylation, the addition of one or several units of ADP-ribose to target proteins occurs via two families of enzymes that can generate ADP-ribosylated proteins: the diphtheria toxin-like ADP-ribosyltransferase (ARTD) family, comprising 17 different proteins that are either poly-ADP-ribosyltransferases or mono-ADP-ribosyltransferases or inactive enzymes; and the clostridial toxin-like ADP-ribosyltransferase family, with four human members, two of which are active mono-ADP-ribosyltransferases, and two of which are enzymatically inactive. In line with a central role for poly-ADP-ribose polymerase 1 in response to DNA damage, specific inhibitors of this enzyme have been developed as anticancer therapeutics and evaluated in several clinical trials. Recently, in combination with the discovery of a large number of enzymes that can catalyse mono-ADP-ribosylation, the role of this modification has been linked to human diseases, such as inflammation, diabetes, neurodegeneration, and cancer, thus revealing the need for the development of specific ARTD inhibitors. This will provide a better understanding of the roles of these enzymes in human physiology and pathology, so that they can be targeted in the future to generate new and efficacious drugs. This review summarizes our present knowledge of the ARTD enzymes that are involved in mono-ADP-ribosylation reactions and that have roles in cancer biology. In particular, the well-documented role of macro-containing ARTD8 in lymphoma and the putative role of ARTD15 in cancer are discussed. The ADP-ribosylation reactionsDuring the development, progression and dissemination of neoplastic lesions, cancer cells can hijack normal intracellular pathways and mechanisms. These can include the pathways involved in intercellular communication, control of transcription, and control of protein localization, function and degradation by Abbreviations ARH, ADP-ribosyl hydrolase; ARTC, clostridial toxin-like ADP-ribosyltransferase; ARTD, diphtheria toxin-like ADP-ribosyltransferase; BAL B-aggressive lymphoma; BRCA1, breast cancer type 1; BRCA2, breast cancer type 2; DLBCL, diffuse large B-cell lymphoma; HDAC2, histone deacetylase 2; HDAC3, histone deacetylase 3; IFN, interferon; IL, interleukin; Kapa, importin a; Kapb1, importin b1/karyopherin b1; mH2A, macro-H2A; PAR, poly-ADP-ribose; PARG, poly-ADP-ribose glycohydrolase; PARP, poly-ADP-ribose polymerase; Stat6, signal transducer and activator of transcription; UPR, unfolded-protein response.

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Poly-ADP-ribosylation is a post-translational modification that occurs in multicellular organisms, including plants and some lower unicellular eukaryotes. The founding member of the PARP family is PARP1. To date, 17 members of the PARP family have been identified, which differ from each other in terms of domain organization, transmodification targets, cellular localization, and biological functions. In recent years, considering structural and biochemical features of the different members of the PARP family, a new classification has been proposed. Thus, enzymes firstly classified as PARP are now named diphtheria-toxin-like ARTs, abbreviated to ARTDs, in accordance with the prototype bacterial toxin that their structural aspects resemble, with numbers indicating the different proteins of the family. The 17 human ARTD enzymes can be divided on the basis of their catalytic activity into polymerases (ARTD1-6), mono-ADP-ribosyl-transferases (ARTD7-17), and the inactive ARTD13. In recent years, ADP-ribosylation was intensively studied, and research was dominated by studies focusing on the role of this modification and its implication on various cellular processes. The aim of this review is to provide a general overview of the ARTD enzymes, with a special focus on mono-ARTDs.

The post-translational modifications of proteins by mono- and poly-ADP-ribosylation involve the cleavage of βNAD⁺, with the release of its nicotinamide moiety, accompanied by the transfer of a single (mono) or several (poly) ADP-ribose molecules from βNAD⁺ to a specific amino-acid residue of various cellular proteins. Thus, both mono- and poly-ADP-ribosylation are NAD⁺-consuming reactions. ADP-ribosylation reactions have been reported to have important roles in the nucleus, and in mitochondrial activity. Distinct subcellular NAD⁺ pools have been identified, not only in the nucleus and the mitochondria, but also in the endoplasmic reticulum and peroxisomes. Recent reports have shed new light on the correlation between NAD⁺-dependent ADP-ribosylation reactions and the endoplasmic reticulum. We have demonstrated that ARTD15/PARP16 is a novel mono-ADP-ribosyltransferase with a new intracellular location, as it is associated with the endoplasmic reticulum. The endoplasmic reticulum is a membranous network of tubules, vesicles, and cisternae that are interconnected in the cytoplasm of eukaryotic cells. This intracellular compartment is responsible for many cellular functions, including facilitation of protein folding and assembly, biosynthesis of lipids, storage of intracellular Ca²⁺, and transport of proteins. ARTD15 might have a role in both the nucleo-cytoplasmic shuttling, through importinβ1 mono-ADP-ribosylation, and in the unfolded protein response through its ability to ADP-ribosylate two components of this pathway: PERK and IRE1. This review summarizes our present knowledge of the enzymes and targets involved in ADP-ribosylation reactions, with special regard to the novel regulatory reactions that occurs at the level of the endoplasmic reticulum, and that can affect the function of this organelle.

Protein ADP-ribosylation is a reversible post-translational modification of cellular proteins that is catalysed by enzymes that transfer one (mono) or several (poly) units of ADP-ribose from β-NAD + to a specific amino acid of the target protein. The most studied member of the ADP-ribosyltransferase family is PARP1 (also known as ADP-ribosyltransferase diphtheria toxin-like 1, ARTD1), which is directly activated by DNA strand breaks and is involved in DNA damage repair, chromatin remodelling and transcriptional regulation. Much less is known about the further 16 members of this family. Among these, ARTD10/PARP10 has been previously characterised as a mono-ADP-ribosyltransferase with a role in cell proliferation and in NF-kB signalling. In the present study, we identified the glycolytic enzyme GAPDH as an interactor and a novel cellular target for ARTD10/PARP10. Moreover, we detected the co-localisation of GAPDH and ARTD10/PARP10 in well-defined cytosolic bodies, which we show here to be membrane-free, rounded structures using immunogold labelling and electron microscopy. Using the cognitive binding module macro domain to visualise ADP-ribosylated proteins by immunofluorescence microscopy in cells over-expressing the ARTD10/PARP10 enzyme, we show that the staining of the ARTD10/PARP10-dependent cytosolic bodies was lost when the cells were treated with compounds that inhibit ARTD10/PARP10, either by directly inhibiting the enzyme or by reducing the cellular NAD + levels. In parallel, the same treatment affected the co-localisation of GAPDH and ARTD10/PARP10, as GAPDH disappeared from the cytosolic cell bodies, which indicates that its presence there depends on the catalytic activity of ARTD10/PARP10. In line with this, in cells over-expressing the ARTD10/PARP10 catalytic domain alone, which we show here to form stress granules, GAPDH was recruited into stress granules. These data identify ARTD10/PARP10 as the enzyme that modifies and recruits GAPDH into cytosolic structures.

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