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.

Protein modification by ADP-ribose via acid-labile linkages

As substrate for protein-mono-ADP-ribosyltransferases, NAD has been shown to be the donor of ADP-ribose to many different nucleophiles found in proteins. This post-translational modification of proteins has been implicated in the regulation of membrane-associated processes including signal transduction, muscle cell differentiation, and protein trafficking and secretion. Described here is the preparation and chemical characterization of low molecular weight conjugates that were used as models for an acetal linkage between ADP-ribose and the hydroxyl group of a protein acceptor such as serine, threonine, tyrosine, hydroxyproline, or hydroxylysine residues. Model conjugates of ADP-ribose containing an acetal linkage were prepared, their structures were established by NMR, and the chemical stability of the linkage to ADP-ribose was studied and compared to the other known ADP-ribosyl-amino acid linkages. The rapid release of intact ADP-ribose from the acetal model conjugates in 44% formic acid distinguished them chemically from all the other known ADP-ribosyl-amino acid modifications. Rat liver proteins were shown to be modified by ADP-ribose in vivo by acid-labile linkages, providing evidence for a new class of endogenous ADP-ribose modification of animal cell proteins. The amount of modification was approximately 16 pmol of ADP-ribose per mg of total protein, and proteins modified by acid-labile linkages were detected in all subcellular fractions examined, suggesting that the scope of this modification in vivo is broad.

Specific inhibition of poly(ADP-ribose) glycohydrolase by adenosine diphosphate (hydroxymethyl)pyrrolidinediol

Adenosine diphosphate (hydroxymethyl)pyrrolidinediol (ADP-HPD), an NH analog of ADP-ribose, was chemically synthesized and shown to be a potent and specific inhibitor of poly-(ADP-ribose) glycohydrolase. The synthetic starting material was the protected pyrrolidine, (2R,3R,4S)-1-(benzyloxycarbonyl)-2-(hydroxymethyl)pyrrolidine-3,4-diol 3,4-O-isopropylidene acetal. This starting pyrrolidine was phosphorylated, coupled to adenosine 5'-monophosphate, and deprotected, yielding the title inhibitor ADP-HPD. ADP-HDP was shown to inhibit the activity of poly(ADP-ribose) glycohydrolase by 50% (IC50) at 0.12 microM, a value 1000-times lower than the IC50 of the product, ADP-ribose. The NAD glycohydrolase from Bungarus fasciatus venom was less sensitive to inhibition by ADP-HPD, exhibiting an IC50 of 260 microM. ADP-HPD did not inhibit either poly(ADP-ribose) polymerase or NAD:arginine mono(ADP-ribosyl)-transferase A at inhibitor concentrations up to 1 mM. At low ADP-HPD concentration, inhibition was therefore shown to be highly specific for poly(ADP-ribose) glycohydrolase, the hydrolytic enzyme in the metabolism of ADP-ribose polymers.

Evaluating the role of niacin in human carcinogenesis

Our understanding of the role of ADP-ribose polymer metabolism in limiting carcinogenic events and the dependence of this metabolism on cellular NAD levels predicts that niacin deficiency leading to reduced NAD levels may enhance carcinogenesis. This prediction has led us to initiate studies to evaluate the potential of niacin as a preventive factor in human cancer. The first approach involves development of a method to assess biochemically niacin status in humans using intracellular NAD derived from whole blood, primarily erythrocytes, as the relevant marker of niacin status. We have shown that erythrocyte NAD content varies by as much as 12-fold within a population and can be modulated readily by supplementation. A second approach to testing this hypothesis involves understanding the relationship of dietary niacin, circulating levels of NAD precursors (nicotinamide and nicotinic acid) and NAD in target tissues for human cancer. Current analytical methods for quantification of plasma levels of nicotinic acid and nicotinamide following intake in the dietary range are not sufficient. Thus, we have developed a GC-MS method for the rapid, sensitive, and selective determination of both nicotinamide and nicotinic acid in plasma. These methods will now allow assessment of niacin metabolism in humans that could lead to a new understanding of niacin in prevention of cancer.

NAD glycohydrolases and the metabolism of cyclic ADP-ribose

Cyclic ADP-ribose is a recently discovered metabolite of NAD that functions in cellular calcium signalling. The discovery that NAD glycohydrolases can catalyze the synthesis and hydrolysis of cyclic ADP-ribose has renewed interest in this class of ADP-ribose transferring enzymes that were discovered over 50 years ago.

Glycation of proteins by ADP-ribose

Numerous metabolic pathways generate free ADP-ribose at many locations within cells. The metabolic fates of this nucleotide are poorly understood and measurement of it in situ is technically difficult at present. Yet considerable evidence has accumulated implicating that protein glycation by ADP-ribose can occur. This evidence is reviewed here along with recent developments in characterizing the chemistry of this reaction and the application of this information to the identification of this posttranslational modification in protein in situ.

NAD glycohydrolases: a possible function in calcium homeostasis

NAD glycohydrolases are the longest known enzymes that catalyze ADP-ribose transfer. The function of these ubiquitous, membrane-bound enzymes has been a long standing puzzle. The NAD glycohydrolases are briefly reviewed in light of the discovery by our laboratory that NAD glycohydrolases are bifunctional enzymes that can catalyze both the synthesis and hydrolysis of cyclic ADP-ribose, a putative second messenger of calcium homeostasis.

Increased sensitivity to DNA-alkylating agents in CHO mutants with decreased poly(ADP-ribose) polymerase activity

Using a replica-plating procedure and a 32P-NAD+ permeable cell-screening assay, we have isolated a CHO mutant, PADR-9, which displays approximately 17% of the wild-type level of poly(ADP-ribose) polymerase activity. Biochemical analysis of the mutant using activity, Western, and Northern blot techniques indicate that relative to its parent cell, the mutant's enzyme activity, antibody recognition, and mRNA levels have been reduced to approximately the same extent. These results are consistent with a mutation in the PADR-9 cell which has resulted in a reduction in enzyme synthesis due to reduced mRNA synthesis and/or stability. Relative to wild-type CHO cells, the PADR-9 mutant has increased sensitivity to killing by DNA-alkylating agents but has normal gamma-ray sensitivity. Correlation between a decrease in poly(ADP-ribose) polymerase activity and an increased sensitivity to DNA-alkylating agents suggests that poly(ADP-ribose) synthesis may be important in the repair and/or induction of DNA damage produced by these agents.

Modification of cardiac membrane adenylate cyclase activity and Gs alpha by NAD and endogenous ADP-ribosyltransferase

The mechanism by which NAD stimulates cardiac adenylate cyclase was investigated. In highly purified canine cardiac sarcolemma, NAD stimulated adenylate cyclase activity in the presence of agents which activate Gs (i.e. 5 mM AlF4-, 10 microM GTP gamma S, 10 microM GppNHp or isoproterenol plus 2 nM GTP gamma S). Furthermore, the EC50 of isoproterenol to stimulate adenylate cyclase was reduced in the presence of NAD. In membranes incubated with [32P]-NAD, AlF4-, 10 microM GTP gamma S or isoproterenol plus 2 nM GTP gamma S produced a selective increase in the radiolabeling of a single 45-kDa protein which was identified as Gs alpha by immunoprecipitation. Cholera toxin catalysed radiolabeling of the same protein. Neutral hydroxylamine released [32P]-ADP-ribose from Gs alpha prelabeled in the presence of AlF4- and [32P]-NAD indicating that an arginine residue on Gs alpha was modified by an endogenous ADP-ribosyltransferase. ADP-ribosyltransferase inhibitors, novobiocin, vitamin K1 or 3-aminobenzamide, inhibited AlF4- stimulated ADP-ribosylation of Gs alpha and NAD potentiation of adenylate cyclase with similar efficacies. The activity responsible for NAD potentiation of adenylate cyclase and ADP-ribosylation of Gs alpha was not removed under hypotonic or hypertonic conditions and therefore appears to be tightly membrane bound. Collectively, these observations indicate that canine cardiac sarcolemma possess an ADP-ribosyltransferase which may constitutively catalyse transfer of an ADP-ribose to activated Gs alpha.

Synthesis and degradation of cyclic ADP-ribose by NAD glycohydrolases

Cyclic adenosine diphosphoribose (cADPR), a recently discovered metabolite of nicotinamide adenine dinucleotide (NAD), is a potent calcium-releasing agent postulated to be a new second messenger. An enzyme that catalyzes the synthesis of cADPR from NAD and the hydrolysis of cADPR to ADP-ribose (ADPR) was purified to homogeneity from canine spleen microsomes. The net conversion of NAD to ADPR categorizes this enzyme as an NAD glycohydrolase. NAD glycohydrolases are ubiquitous membrane-bound enzymes that have been known for many years but whose function has not been identified. The results presented here suggest that these enzymes may function in the regulation of calcium homeostasis by the ability to synthesize and degrade cADPR.

Position of cyclization in cyclic ADP-ribose

Cyclic adenosine diphosphoribose (cADPR) is a putative second messenger of calcium homeostasis synthesized from NAD by cleavage of the nicotinamide-ribose bond and cyclization of the ribose to the adenine ring. In this study, the ultraviolet absorption spectra of cADPR have been studied as a function of pH and compared to other compounds containing an adenine ring with substitutions at known positions. The results support a structure for cADPR in which cyclization is to position 1 of the adenine ring, rather than to N6, as has been previously proposed.

Evidence for unusual stability of ADP-ribosyl linkage to membrane proteins of a murine leukemic cell line

A set of membrane proteins from murine myelomonocytic leukemia cell line WEHI-3BD+ was found to be ADP-ribosylated. Both this ADP-ribosylation reaction and granulocytic differentiation in response to recombinant G-CSF were inhibited approximately 50% by 2 mM benzamide, suggesting a correlation between these two processes. The stability of the ADP-ribosyl linkage was examined under a series of chemical release conditions and was found to resemble none of the known ADP-ribosyl-amino acid linkages. This chemical modification may represent a new class of mono(ADP-ribosyl) modifications of proteins.

Protein glycation by ADP-ribose: studies of model conjugates

Protein glycation by hexoses has been implicated in the pathophysiology of a number of diseases as well as the aging process. Studies of ADP-ribose polymer metabolism have shown that free ADP-ribose is generated at high rates in the cell nucleus following DNA damage. Protein glycation by ADP-ribose has been reported although the chemistry is not understood. Described here is the synthesis and characterization of model conjugates for protein glycation of lysine residues by ADP-ribose. Two stable conjugates derived from ADP-ribose and n-butylamine were isolated and characterized. Both conjugates were shown to be ketoamines derived from a Schiff base by an Amadori rearrangement. The chemical stability of the ketamines allowed them to be differentiated from all classes of enzymic protein modification by ADP-ribose. Further, their chemical properties suggest that a previous report of histone H1 modification in carcinogen treated cells was due to glycation by ADP-ribose.

A biomarker for the assessment of niacin nutriture as a potential preventive factor in carcinogenesis

The study of protective cellular responses to DNA damage has led to the working hypothesis that optimal niacin nutriture is a preventive factor in cancer. Described here is the development of a biomarker for determining niacin status termed Niacin Number. The combination of this biomarker with diet and cancer epidemiology will allow evaluation of the possible role of this nutrient in cancer risk.

High resolution fractionation and characterization of ADP-ribose polymers

Methods are described for the high resolution fractionation and characterization of ADP-ribose polymers. Polymers prepared in vitro using purified poly(ADP-ribose) polymerase were isolated free from interfering nucleic acids and salts using dihydroxyboronyl-Bio-Rex 70 chromatography and fractionated using anion exchange high-pressure liquid chromatography. The homogeneity of isolated polymer fractions was characterized by gel electrophoresis and polymer size was determined by analysis following enzymatic digestion to nucleosides. The method allows isolation of oligomers up to 50 mer as single species and larger polymers can be isolated free from oligomers according to size and branching frequency. The ability to isolate individual species of ADP-ribose polymers should prove useful for the study of the polymers and their noncovalent interactions with other components of chromatin. Microheterogeneity of individual oligomers was studied and shown to be due to differences at the protein proximal ends resulting from the chemical method of release of polymers from protein. The method also was applied to fractionate polymers generated in intact cultured mouse cells in response to treatment with the carcinogen N-methyl-N'-nitro-N-nitrosoguanidine.

Differences in the poly(ADP-ribosyl)ation patterns of ICP4, the herpes simplex virus major regulatory protein, in infected cells and in isolated nuclei

Infected-cell protein 4 (ICP4), the major regulatory protein in herpes simplex viruses 1 and 2, was previously reported to accept 32P from [32P]NAD in isolated nuclei. This modification was attributed to poly(ADP-ribosyl)ation (C. M. Preston and E. L. Notarianni, Virology 131:492-501, 1983). We determined that an antibody specific for poly(ADP-ribose) reacts with ICP4 extracted from infected cells, electrophoretically separated in denaturing gels, and electrically transferred to nitrocellulose. Our results indicate that all forms of ICP4 observed in one-dimensional gel electrophoresis are poly(ADP-ribosyl)ated. Poly(ADP-ribose) on ICP4 extracted from infected cells was resistant to cleavage by purified poly(ADP-ribose) glycohydrolase unless ICP4 was in a denatured state. Poly(ADP-ribose) added to ICP4 in isolated nuclei was sensitive to this enzyme. This result indicates that the two processes are distinct and may involve different sites on the ICP4 molecule.

Effect of hyperthermia in vitro and in vivo on adenine and pyridine nucleotide pools in human peripheral lymphocytes

Hyperthermia has been shown in vitro and in vivo to potentiate the effects of ionizing irradiation. Previous studies found that hyperthermia alters the metabolism of adenosine diphosphate (ADP)-ribose polymers required for recovery from DNA damage and that poly(ADP-ribose) polymerase activity is very sensitive to cellular nicotinamide-adenine dinucleotide (NAD) levels. Thus, the effect of 41.8 degrees C hyperthermia in vitro and in vivo on NAD and adenosine triphosphate (ATP) levels was studied in human peripheral lymphocytes. In vitro studies showed significant decreases in oxidized NAD (NAD+) and ATP levels after heating that simulated a clinical whole-body hyperthermia (WBH) treatment. This nucleotide depletion could not be attributed to nucleotide leakage or increased enzymatic NAD+ consumption. As the reduction of NAD observed was sufficient to decrease poly(ADP-ribose)polymerase activity by 50%, the studies were extended to clinica cases. Cellular NAD+ and ATP were measured in previously stored lymphocytes obtained from four patients before and after WBH; a statistically significant decrease in NAD+ was observed after WBH which quantitatively agreed with the in vitro results. Based on these results a prospective study was done in three patients; NAD+ was extracted immediately on sample collection, and the kinetics of WBH-induced NAD depletion were studied. These data, which agree quantitatively with the laboratory results, are presented.

An affinity matrix for the purification of poly(ADP-ribose) glycohydrolase

The preparation of quantities of poly(ADP-ribose) glycohydrolase sufficient for detailed structural and enzymatic characterizations has been difficult due to the very low tissue content of the enzyme and its lability in late stages of purification. To date, the only purification of this enzyme to apparent homogeneity has involved a procedure requiring 6 column chromatographic steps. Described here is the preparation of an affinity matrix which consists of ADP-ribose polymers bound to dihydroxyboronyl sepharose. An application is described for the purification of poly(ADP-ribose) glycohydrolase from calf thymus in which a single rapid affinity step was used to replace 3 column chromatographic steps yielding enzyme of greater than 90% purity with a 3 fold increase in yield. This matrix should also prove useful for other studies of ADP-ribose polymer metabolism and related clinical conditions.

Modification of plasma membrane protein cysteine residues by ADP-ribose in vivo

Proteins can be post-translationally modified by ADP-ribose. Previously, two classes of ADP-ribosyl protein linkages have been detected in vivo which have chemical properties indistinguishable from ADP-ribosyl arginine and ADP-ribosyl glutamate or aspartate. Reported here is the detection of a third class of endogenous ADP-ribosyl protein linkage. This class is chemically indistinguishable from ADP-ribose linked to cysteine residues by a thioglycosidic bond. The distribution of ADP-ribosyl cysteine residues was studied in subcellular fractions of rat liver. Proteins modified on cysteine were detected only in the plasma membrane fraction. Pertussis toxin is known to disrupt signal transduction of ADP-ribosylation of cysteine residues of plasma membrane GTP binding proteins. The results described here raise the interesting possibility that the endogenous modification of plasma membrane protein cysteine residues may be involved in signal transduction.

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.

Labeling methods for the study of poly- and mono(ADP-ribose) metabolism in cultured cells

Methods are described for the radiolabeling and determination of NAD+, poly(ADP-ribose), and protein-bound monomers of ADP-ribose in cultured mammalian cells. The adenine nucleotide pools of confluent monolayer cell cultures are radiolabeled using high-specific-activity [3H]adenine. Following any desired experimental manipulation, cultures are treated with trichloroacetic acid. Radiolabel in NAD+ can be rapidly determined from the acid-soluble fraction using dihydroxyboronyl Sepharose (DHB-Sepharose). The acid-insoluble material can be analyzed for radiolabeled polymers of ADP-ribose and protein-bound monomers of ADP-ribose. Polymers are separated from interfering material using dihydroxyboronyl-Bio-Rex 70 (DHB-Bio-Rex). Protein-bound monomers are separated from noncovalently bound ADP-ribose and different classes of (ADP-ribosyl) protein linkages are released by specific chemical treatments. The released ADP-ribose is then separated from interfering materials using DHB-Bio-Rex and DHB-Sepharose. Control experiments have demonstrated the sensitivity, selectivity, and precision of the methods. Major advantages of the methods are that they allow many simultaneous determinations and all components can be determined from material derived from a single dish of cultured cells. The methods should prove useful for detailed studies of the metabolism of both protein-bound monomers and polymers of ADP-ribose in cultured mammalian cells.

Mechanism of alteration of poly(adenosine diphosphate-ribose) metabolism by hyperthermia

The effects of hyperthermia on adenine nucleotide metabolism including NAD and poly(ADP-ribose) have been studied in confluent cultures of C3H10T1/2 cells. Cells replated immediately following hyperthermic treatment showed only 9% survival relative to controls while after a 24-h recovery period at 37 degrees C survival was 87% of control. Hyperthermic treatment caused no detectable effect on total cellular levels of either NAD or ATP but produced a prolonged increase in cellular content of poly(ADP-ribose). Studies of the mechanism of this effect show that a major alteration of poly(ADP-ribose) metabolism caused by hyperthermia involves a decrease in the rate of turnover of polymers of ADP-ribose. Normal polymer turnover rates were restored during recovery at 37 degrees C even in the presence of cyclohexamide. The results argue that poly(ADP-ribose) glycohydrolase activity is reversibly altered by hyperthermia. Inhibition of poly(ADP-ribose) synthesis following hyperthermia delays recovery of normal rates of protein synthesis and recovery of the ability of the cells to plate and form colonies.

Effect of hyperthermia on poly(adenosine diphosphate-ribose) glycohydrolase

The effects of supranormal temperature on the activity of poly(ADP-ribose) glycohydrolase were studied by assaying the enzyme in cell extracts derived from cells subjected to hyperthermia and comparing with extracts that were heated in vitro. The enzyme activity was reduced by both hyperthermic treatment of cells and by heating of cell extracts; however greater reductions were observed when intact cells were subjected to hyperthermia. The additional reduction observed when intact cells were heated was reversed when cells were allowed to recover at 37 degrees C following hyperthermia. We postulate that hyperthermia alters poly(ADP-ribose) glycohydrolase activity by two mechanisms, an irreversible thermal denaturation of the enzyme and a reversible metabolic alteration. Changes in poly(ADP-ribose) glycohydrolase activity can account in full for the observed alterations of poly(ADP-ribose) metabolism that occur following hyperthermia.

Stimulation of mono (ADP-ribosyl)ation by reduced extracellular calcium levels in human fibroblasts

Lowering extracellular calcium in cultures of human diploid fibroblast-like cells caused a rapid depletion of NAD pools. This loss of NAD was reversed by restoring extracellular Ca2+ and was inhibited by 3-aminobenzamide, an inhibitor of ADP-ribosyl transfer reactions. The concentrations of 3-aminobenzamide needed to inhibit the loss of NAD were consistent with those required to inhibit cellular mono(ADP-ribosyl) rather than poly(ADP-ribosyl) reactions. Calcium depletion did not inhibit the biosynthesis of NAD. These results suggest that mono(ADP-ribosyl)ation is involved in the regulation of cellular Ca2+ levels.