Poly(ADP-ribosyl)ation of chromatin in an in-vitro poly(ADP-ribose)-turnover system.

This paper describes the effect of an in-vitro poly(ADP-ribose) turnover system on the poly(ADP-ribosyl)ation of chromatin. Both poly(ADP-ribose)polymerase and poly(ADP-ribose)glycohydrolase were highly purified and used in 4 different turnover systems: non-turnover, slow, medium and fast turnover. These turnover systems were designed to reflect possible turnover conditions in intact cells. The major protein acceptors for poly(ADP-ribose) are histones and the polymerase itself, a process referred to as automodification. The level of poly(ADP-ribose) modification of polymerase, histone H1 and core histones has been measured. The size of the polymer for each of the 3 groups of acceptor proteins has been determined by gel electrophoresis. After many turnover cycles at medium and fast turnover, the histones (H1 and core) become the main poly(ADP-ribose) acceptor proteins. The rate at which steady-state polymer levels are reached and the total accumulation of polymer in a given turnover system are both inversely proportional to the amount of glycohydrolase present. Furthermore, increasing amounts of glycohydrolase in the turnover systems reduces average polymer size. The polymer synthesized in the medium and fast turnover systems is degraded by glycohydrolase in a biphasic fashion and in these systems the half-life of polymer agreed with results found in intact cells. Our results show that the relative levels of polymerase and glycohydrolase activities can regulate the proportional poly(ADP-ribose) distribution on chromatin-associated acceptor proteins during steady-state turnover conditions. The patterns of modification of polymerase and histones under turnover conditions agree with in vivo observations.     

Poly(ADP-ribose) accessibility to poly(ADP-ribose) glycohydrolase activity on poly(ADP-ribosyl)ated nucleosomal proteins

Hydrolysis of protein-bound 32P-labelled poly(ADP-ribose) by poly(ADP-ribose) glycohydrolase shows that there is differential accessibility of poly(ADP-ribosyl)ated proteins in chromatin to poly(ADP-ribose) glycohydrolase. The rapid hydrolysis of hyper(ADP-ribosyl)ated forms of histone H1 indicates the absence of an H1 dimer complex of histone molecules. When the pattern of hydrolysis of poly(ADP-ribosyl)ated histones was analyzed it was found that poly(ADP-ribose) attached to histone H2B is more resistant than the polymer attached to histone H1 or H2A or protein A24. Polymer hydrolysis of the acceptors, which had been labelled at high substrate concentrations (greater than or equal to 10 microM), indicate that the only high molecular weight acceptor protein is poly(ADP-ribose) polymerase and that little processing of the enzyme occurs. Finally, electron microscopic evidence shows that hyper(ADP-ribosyl)ated poly(ADP-ribose) polymerase, which is dissociated from its DNA-enzyme complex, binds again to DNA after poly(ADP-ribose) glycohydrolase action.     

Zinc-binding domain of poly(ADP-ribose)polymerase participates in the recognition of single strand breaks on DNA

Poly(ADP-ribose)polymerase is a chromatin-associated enzyme of eukaryotic cell nuclei that catalyses the covalent attachment of ADP-ribose units from NAD+ to various nuclear acceptor proteins. This post-translational modification has been postulated to influence several chromatin functions, particularly those where nicking and rejoining of DNA occur. Poly(ADP-ribosyl)ation reactions are strictly dependent upon the presence of interruptions on DNA. We have recently demonstrated that the DNA-binding domain of the protein containing two putative "zinc-fingers" binds DNA in a zinc-dependent manner. The basis for the recognition of the DNA strand breaks by this enzyme, and more precisely, its 29,000 Mr N-terminal part, which contains the metal binding sites, needed to be clarified. DNA probes harbouring a single strand interruption at a defined position were constructed from synthetic oligonucleotides. DNase I protection studies show that poly(ADP-ribose)polymerase specifically binds to a DNA single-strand break by its metal-binding domain depending upon the presence of Zn(II). These results support the idea that the enzyme participates to the maintenance of DNA integrity in eukaryotes.     

A method for determining oligo- and poly(ADP-ribosyl)ated enzymes and proteins in vitro

A new method to determine oligo- and poly(ADP-ribosyl)ated enzymes and proteins in vitro has been developed. This method is based on the facts that in Mg2+-depleted condition automodification of poly(ADP-ribose)polymerase is minimized and exogenously added acceptor protein is oligo(ADP-ribosyl)ated predominantly, and in Mg2+-fortified conditions the exogenous acceptor can be poly(ADP-ribosyl)ated. When 13 proteins, including several enzymes, were subjected to this system, dimeric bovine seminal RNase and micrococcal nuclease were found to be oligo(ADP-ribosyl)ated under Mg2+-depleted conditions but their activity was unchanged. Under Mg2+-fortified conditions however, the RNase was deactivated concomitantly with its extensive poly(ADP-ribosyl)ation. When dimeric bovine seminal RNase was monomerized in advance by treatment with dithiothreitol and urea, the enzyme lost ADP-ribose-accepting ability in spite of a significant residual enzyme activity. As used here successfully, the Mg2+-depleted and Mg2+-fortified ADP-ribosylation and subsequent chromatographic analysis of various proteins and enzymes might be an useful method for proving their oligo- and poly(ADP-ribosyl)ation.     

Inhibition of poly(ADP-ribosyl)ation by overexpressing the poly(ADP-ribose) polymerase DNA-binding domain in mammalian cells

Poly(ADP-ribose) polymerase specifically recognizes DNA strand breaks by its DNA-binding domain. DNA binding activates the enzyme to catalyze the formation of poly(ADP-ribose) utilizing NAD as substrate. By a molecular genetic approach we set out to inhibit this enzyme activity in a highly specific manner, thus avoiding the inherent side effects of NAD analogs which have been used extensively as enzyme inhibitors. cDNA sequences coding for the human poly(ADP-ribose) polymerase DNA-binding domain were subcloned into eucaryotic expression plasmids and transiently transfected into monkey cells. Cells were fixed with ethanol followed by incubation with NAD. Indirect double immunofluorescence to detect both overexpressed protein and poly(ADP-ribose) in situ revealed that overexpression of the DNA-binding domain greatly inhibited poly(ADP-ribosyl)ation catalyzed by the resident enzyme during NAD postincubation. The same inhibition was observed when transfected cells were treated with N-methyl-N'-nitro-N-nitrosoguanidine to induce DNA strand breaks in vivo and subjected to trichloroacetic acid/ethanol fixation and subsequent immunofluorescence analysis, a novel method we developed for the in situ detection of polymer synthesis in intact cells. This molecular genetic approach may prove to be a selective and efficient tool to investigate possible functions of poly(ADP-ribosyl)ation in living cells.     

Importance of poly(ADP-ribose) glycohydrolase in the control of poly(ADP-ribose) metabolism.

Poly(ADP-ribosyl)ation is a posttranslational modification that alters the functions of the acceptor proteins and is catalyzed by the poly(ADP-ribose) polymerase (PARP) family of enzymes. Following DNA damage, activated poly(ADP-ribose) polymerase-1 (PARP-1) catalyzes the elongation and branching of poly(ADP-ribose) (pADPr) covalently attached to nuclear target proteins. Although the biological role of poly(ADP-ribosyl)ation has not yet been defined, it has been implicated in many important cellular processes such as DNA repair and replication, modulation of chromatin structure, and apoptosis. The transient nature and modulation of poly(ADP-ribosyl)ation depend on the activity of a unique cytoplasmic enzyme called poly(ADP-ribose) glycohydrolase which hydrolyzes pADPr bound to acceptor proteins in free ADP-ribose residues. While the PARP homologues have been recently reviewed, there are relatively scarce data about PARG in the literature. Here we summarize the latest advances in the PARG field, addressing the question of its putative nucleo-cytoplasmic shuttling that could enable the tight regulation of pADPr metabolism. This would contribute to the elucidation of the biological significance of poly(ADP-ribosyl)ation.     

Poly(ADP-ribosyl)ation of terminal deoxynucleotidyl transferase in vitro

The activity of purified bovine thymus terminal deoxynucleotidyl transferase was markedly inhibited when the enzyme was incubated in a poly(ADP-ribose)-synthesizing system containing purified bovine thymus poly(ADP-ribose) polymerase, NAD+, Mg2+ and DNA. All of these four components were indispensable for the inhibition. The inhibitors of poly(ADP-ribose) polymerase counteracted the observed inhibition of the transferase. Under a Mg2+-depleted and acceptor-dependent ADP-ribosylating reaction condition [Tanaka, Y., Hashida, T., Yoshihara, H. and Yoshihara, K. (1979) J. Biol. Chem. 254, 12433-12438], the addition of terminal transferase to the reaction mixture stimulated the enzyme reaction in a dose-dependent manner, suggesting that the transferase is functioning as an acceptor for ADP-ribose. Electrophoretic analyses of the reaction products clearly indicated that the transferase molecule itself was oligo (ADP-ribosyl)ated. When the product was further incubated in the Mg2+-fortified reaction mixture, the activity of terminal transferase markedly decreased with increase in the apparent molecular size of the enzyme, indicating that an extensive elongation of poly(ADP-ribose) bound to the transferase is essential for the observed inhibition. Free poly(ADP-ribose) and the polymer bound to poly(ADP-ribose) polymerase were ineffective on the activity of the transferase. All of these results indicate that the observed inhibition of terminal transferase is caused by the poly(ADP-ribosyl)ation of the transferase itself.     

Chain length analysis of ADP-ribose polymers generated by poly(ADP-ribose) polymerase (PARP) as a function of beta-NAD+ and enzyme concentrations

Bireactant autopoly(ADP-ribosyl)ation of poly(ADP-ribose) polymerase (PARP) (EC 2.4.2.30) was carried out by using either increasing concentrations of beta-NAD+ (donor substrate) at a fixed protein concentration or increasing concentrations of PARP (acceptor substrate) at a fixed beta-NAD+ concentration. The [32P]ADP-ribose polymers synthesized were chemically detached from PARP by alkaline hydrolysis of the monoester bond between the carboxylate moiety of Glu and the polymer. Nucleic acid-like polymers were then analyzed by high-resolution polyacrylamide gel electrophoresis and autoradiography. The ADP-ribose chain lengths observed displayed substrate concentration-dependent elongation from 0.2 microM to 2 mM beta-NAD+. Similar results were observed at fixed concentrations of 4.5, 9, 18, 27, and 36 nM PARP. Therefore, we conclude that the concentration of the ADP-ribose donor substrate determines the average chain length of the polymer synthesized. In contrast, the polymer size was unaltered when the concentration of PARP was varied from 4.5 to 18 nM at a fixed beta-NAD+ concentration. However, when PARP concentrations > 18 nM were used, the total amount of monomeric ADP-ribose produced was noticeably less. Therefore, we conclude that high concentrations of PARP lead to acceptor substrate inhibition at the level of the ADP-ribose chain initiation reaction.     

Quantitative studies of inhibitors of ADP-ribosylation in vitro and in vivo

The ADP-ribosyl moiety of NAD+ is consumed in reactions catalyzed by three classes of enzymes: poly(ADP-ribose) polymerase, protein mono(ADP-ribosyl)transferases, and NAD+ glycohydrolases. In this study, we have evaluated the selectivity of compounds originally identified as inhibitors of poly(ADP-ribose) polymerase on members of the three classes of enzymes. The 50% inhibitory concentration (IC50) of more than 20 compounds was determined in vitro for both poly(ADP-ribose) polymerase and mono(ADP-ribosyl)transferase A in an assay containing 300 microM NAD+. Of the compounds tested, benzamide was the most potent inhibitor of poly(ADP-ribose) polymerase with an IC50 of 3.3 microM. The IC50 for benzamide for mono(ADP-ribosyl)transferase A was 4.1 mM, and similar values were observed for four additional cellular mono(ADP-ribosyl)transferases. The IC50 for NAD+ glycohydrolase for benzamide was approximately 40 mM. For seven of the best inhibitors, inhibition of poly(ADP-ribose) polymerase in intact C3H1OT1/2 cells was studied as a function of the inhibitor concentration of the culture medium, and the concentration for 50% inhibition (culture medium IC50) was determined. Culture medium IC50 values for benzamide and its derivatives were very similar to in vitro IC50 values. For other inhibitors, such as nicotinamide, 5-methyl-nicotinamide, and 5-bromodeoxyuridine, culture medium IC50 values were 3-5-fold higher than in vitro IC50 values. These results suggest that micromolar levels of the benzamides in the culture medium should allow selective inhibition of poly(ADP-ribose) metabolism in intact cells. Furthermore, comparative quantitative inhibition studies should prove useful for assigning the biological effects of these inhibitors as an effect on either poly(ADP-ribose) or mono(ADP-ribose) metabolism.     

Molecular and biochemical features of poly (ADP-ribose) metabolism

In the past five years, poly(ADP-ribosyl)ation has developed greatly with the help of molecular biology and the improvement of biochemical techniques. In this article, we describe the physico-chemical properties of the enzymes responsible for the synthesis and degradation of poly(ADP-ribose), respectively poly(ADP-ribose) polymerase and poly(ADP-ribose) glycohydrolase. We then discuss the possible roles of this polymer in DNA repair and replication as well as in cellular differentiation and transformation. Finally, we put forward various hypotheses in order to better define the function of this polymer found only in eucaryotes. (Mol Cell Biochem122: 171–193, 1993)Abbreviations 3-AB 3-Aminobenzamide - 3-MBA 3-Methoxybenzamide - AADH Amino Acid Dehydrogenase - CAT Chloramphenicol Acetyl Transferase - DHFR Dihydrofolate Reductase - DMS Dimethylsulfate - DMSO Dimethylsulfoxide - DNase Deoxyribonuclease - HMG High Mobility Group - kb Kilobase - kDa Kilodalton - LMG Low Mobility Group - MNNG Methylnitroso Nitroguanidine - MNU Methylnitrosourea - NAD Nicotinamide Adenine Dinucleotide - NLS Nuclear Localization System - NTP Nucleotide Triphosphate - pADPR Poly (ADP-ribose) - PARP Poly(ADP-ribose) Polymerase - PHA Phytohemaglutinin - PMA Phorbol Myristate Acetate - PRAMP Phosphoribosyl AMP - (PR)₂AMP Diphosphoribosyl AMP - RNase Ribonuclease - SCE Sister Chromatid Exchange - TPA Tetradecanoyl Phorbol-13-Acetate - UV Ultra-Violet     

Identification of a protease inhibitor from rat peritoneal macrophages as poly(ADP-ribose)

Rat peritoneal macrophages contain a chymotrypsin-like protease and its specific inhibitor, both being associated with chromatin of the cells. The inhibitor was separated from the protease by gel filtration through a Sephadex G-75 column, further treated with trypsin, DNase and RNase, and then purified successively on Sephadex G-75, Sephadex G-25, and dihydroxyboryl Bio-Gel P-60 columns. The purified inhibitor had a molecular weight in the range from 2,000 to 3,500 and an absorption maximum at 260 nm at pH 7.0. When the inhibitor was digested by snake venom phosphodiesterase, the inhibitory potency was lost, yielding 5'-AMP and 2'-(5'-phosphoribosyl)-5'-AMP as the digestion products which were identified by high pressure liquid chromatography. The inhibitory potency was neutralized specifically by anti-poly(ADP-ribose) antiserum. The profile of inhibition by the isolated inhibitor was nearly identical with that produced by authentic poly(ADP-ribose). It was therefore concluded that the inhibitor isolated was identical with poly(ADP-ribose), whose chain length ranged from 4 to 7 ADP-ribosyl units. This is the first demonstration that a intracellular protease inhibitor can be endogenous poly(ADP-ribose).     

trans-dominant inhibition of poly(ADP-ribosyl)ation sensitizes cells against gamma-irradiation and N-methyl-N'-nitro-N-nitrosoguanidine but does not limit DNA replication of a polyomavirus replicon.

Poly(ADP-ribosyl)ation is a posttranslational modification of nuclear proteins catalyzed by poly(ADP-ribose) polymerase (PARP; EC 2.4.2.30), with NAD+ serving as the substrate. PARP is strongly activated upon recognition of DNA strand breaks by its DNA-binding domain. Experiments with low-molecular-weight inhibitors of PARP have led to the view that PARP activity plays a role in DNA repair and possibly also in DNA replication, cell proliferation, and differentiation. Accumulating evidence for nonspecific inhibitor effects prompted us to develop a molecular genetic system to inhibit PARP in living cells, i.e., to overexpress selectively the DNA-binding domain of PARP as a dominant negative mutant. Here we report on a cell culture system which allows inducible, high-level expression of the DNA-binding domain. Induction of this domain leads to about 90% reduction of poly(ADP-ribose) accumulation after gamma-irradiation and sensitizes cells to the cytotoxic effect of gamma-irradiation and of N-methyl-N'-nitro-N-nitrosoguanidine. In contrast, induction does not affect normal cellular proliferation or the replication of a transfected polyomavirus replicon. Thus, trans-dominant inhibition of the poly(ADP-ribose) accumulation occurring after gamma-irradiation or N-methyl-N'-nitro-N-nitrosoguanidine is specifically associated with a disturbance of the cellular recovery from the inflicted damage.     

Inhibition of DNA polymerase alpha, DNA polymerase beta, terminal deoxynucleotidyl transferase, and DNA ligase II by poly(ADP-ribosyl)ation reaction in vitro

Incubation of DNA polymerase alpha, DNA polymerase beta, terminal deoxynucleotidyl transferase, or DNA ligase II in a reconstituted poly(ADP-ribosyl)ating enzyme system markedly suppressed the activity of these enzymes. Components required for poly(ADP-ribose) synthesis including poly(ADP-ribose) polymerase, NAD+, DNA, and Mg2+ were all essential for the observed suppression. Purified poly(ADP-ribose) itself, however, was slightly inhibitory to all of these enzymes. Furthermore, the suppressed activities of DNA polymerase alpha, DNA polymerase beta, and terminal deoxynucleotidyl transferase were largely restored (3 to 4-fold stimulation was observed) by a mild alkaline treatment, a procedure known to hydrolyze alkaline-labile ester linkage between poly(ADP-ribose) and an acceptor protein. All of these results strongly suggest that the four nuclear enzymes were inhibited as a result of poly(ADP-ribosyl)ation of either the enzyme molecule itself or some regulatory proteins of these enzymes.     

Molecular mechanism of poly(ADP-ribosyl)ation by PARP1 and identification of lysine residues as ADP-ribose acceptor sites

Poly(ADP-ribose) polymerase 1 (PARP1) synthesizes poly(ADP-ribose) (PAR) using nicotinamide adenine dinucleotide (NAD) as a substrate. Despite intensive research on the cellular functions of PARP1, the molecular mechanism of PAR formation has not been comprehensively understood. In this study, we elucidate the molecular mechanisms of poly(ADP-ribosyl)ation and identify PAR acceptor sites. Generation of different chimera proteins revealed that the amino-terminal domains of PARP1, PARP2 and PARP3 cooperate tightly with their corresponding catalytic domains. The DNA-dependent interaction between the amino-terminal DNA-binding domain and the catalytic domain of PARP1 increased V_max and decreased the K_m for NAD. Furthermore, we show that glutamic acid residues in the auto-modification domain of PARP1 are not required for PAR formation. Instead, we identify individual lysine residues as acceptor sites for ADP-ribosylation. Together, our findings provide novel mechanistic insights into PAR synthesis with significant relevance for the different biological functions of PARP family members.     

Preferential Perinuclear Localization of Poly(ADP-ribose) Glycohydrolase

The transient nature of poly(ADP-ribosyl)ation, a posttranslational modification of nuclear proteins, is achieved by the enzyme poly(ADP-ribose) glycohydrolase (PARG) which hydrolyzes the poly(ADP-ribose) polymer into free ADP-ribose residues. To investigate the molecular size and localization of PARG, we developed a specific polyclonal antibody directed against the bovine PARG carboxy-terminal region. We found that PARG purified from bovine thymus was recognized as a 59-kDa protein, while Western blot analysis of total cell extracts revealed the presence of a unique 110-kDa protein. This 110-kDa PARG was mostly found in postnuclear extracts, whereas it was barely detectable in the nuclear fractions of COS7 cells. Further analysis by immunofluorescence revealed a cytoplasmic perinuclear distribution of PARG in COS7 cells overexpressing the bovine PARG cDNA. These results provide direct evidence that PARG is primarily a cytoplasmic enzyme and suggest that a very low amount of intranuclear PARG is required for poly(ADP-ribose) turnover.     

Inhibitors and activators of ADP-ribosylation reactions

ADP-ribosylation reaction, that is the transfer of the ADP-ribose moiety of NAD⁺ to acceptor protein, is catalyzed by two classes of ADP-ribosyltransferases,i.e., poly(ADP-ribose) synthetase and mono (ADP-ribosyl)transferases. These two types differ not only in the number of transferring ADP-ribose units but also in the acceptor amino acid(s) and protein. Their in hibitors, particularly those of poly(ADP-ribose) synthetase, have been successfully employed in studies on biological functions of the enzymes and other related fields of research. Recently, we found many potent and specific inhibitors of poly-(ADP-ribose) synthetase, and broadened their chemical as well as biochemical variety. More recently, we found several potent inhibitors of arginine-specific mono(ADP-ribosyl)transferases and activators of poly(ADP-ribose) synthetase.     

The effect of poly(ADP-ribosyl)ation on native and H1-depleted chromatin. A role of poly(ADP-ribosyl)ation on core nucleosome structure

The effect of poly(ADP-ribosyl)ation on native and H1-depleted chromatin was analyzed by gel electrophoresis, electron microscopy, and velocity sedimentation. In parallel, the interaction of automodified poly(ADP-ribose) polymerase with native and H1-depleted chromatin was analyzed. In H1-depleted chromatin histone H2B becomes the major poly(ADP-ribose) histone acceptor protein, whereas in native chromatin histone H1 was the major histone acceptor. Poly(ADP-ribosyl)ation of H1-depleted chromatin prevented the recondensation of polynucleosomes reconstituted with exogenous histone H1. This is probably due to the presence of modified poly(ADP-ribose) polymerase and hyper(ADP-ribosyl)ated histone H2B. Indeed, about 40% of the modified enzyme remained associated with H1-depleted chromatin, while less than 1% of the modified enzyme was bound to native chromatin. The influence of poly(ADP-ribosyl)ation on the chromatin conformation was also studied at the level of nucleosome in using monoclonal and polyclonal antibodies specific for individual histones and synthetic peptides of histones. In native chromatin incubated in the presence of Mg2+ there was a drop in the accessibility of histone epitopes to monoclonal and polyclonal antibodies whereas upon poly(ADP-ribosyl)ation their accessibility was found to remain even in the presence of Mg2+. In poly(ADP-ribosyl)ated H1-depleted chromatin an increased accessibility of some histone tails to antibodies was observed.     

Rapid regulation of telomere length is mediated by poly(ADP-ribose) polymerase-1

Shelterin/telosome is a multi-protein complex at mammalian telomeres, anchored to the double-stranded region by the telomeric-repeat binding factors-1 and -2. In vitro modification of these proteins by poly(ADP-ribosyl)ation through poly(ADP-ribose) polymerases-5 (tankyrases) and -1/-2, respectively, impairs binding. Thereafter, at least telomeric-repeat binding factor-1 is degraded by the proteasome. We show that pharmacological inhibition of poly(ADP-ribose) polymerase activity in cells from two different species leads to rapid decrease in median telomere length and stabilization at a lower setting. Specific knockdown of poly(ADP-ribose) polymerase-1 by RNA interference had the same effect. The length of the single-stranded telomeric overhang as well as telomerase activity were not affected. Release of inhibition led to a fast re-gain in telomere length to control levels in cells expressing active telomerase. We conclude that poly(ADP-ribose) polymerase-1 activity and probably its interplay with telomeric-repeat binding factor-2 is an important determinant in telomere regulation. Our findings reinforce the link between poly(ADP-ribosyl)ation and aging/longevity and also impact on the use of poly(ADP-ribose) polymerase inhibitors in tumor therapy.