Folate in skin cancer prevention

Skin, the largest, most exposed organ of the body, provides a protective interface between humans and the environment. One of its primary roles is protection against exposure to sunlight, a major source of skin damage where the UV radiation (UVR) component functions as a complete carcinogen. Melanin pigmentation and the evolution of dark skin is an adaptive protective mechanism against high levels of UVR exposure. Recently, the hypothesis that skin pigmentation balances folate preservation and Vitamin D production has emerged. Both micronutrients are essential for reproductive success. Photodegradation of bioactive folates suggests a mechanism for the increased tendency of populations of low melanin pigmentation residing in areas of high UV exposure to develop skin cancers. Folate is proposed as a cancer prevention target for its role in providing precursors for DNA repair and replication, as well as its ability to promote genomic integrity through the generation of methyl groups needed for control of gene expression. The cancer prevention potential of folate has been demonstrated by large-scale epidemiological and nutritional studies indicating that decreased folate status increases the risk of developing certain cancers. While folate deficiency has been extensively documented by analysis of human plasma, folate status within skin has not been widely investigated. Nevertheless, inefficient delivery of micronutrients to skin and photolysis of folate argue that documented folate deficiencies will be present if not exacerbated in skin. Our studies indicate a critical role for folate in skin and the potential to protect sun exposed skin by effective topical delivery as a strategy for cancer prevention.

Niacin: vitamin and antidyslipidemic drug

Niacin is defined collectively as nicotinamide and nicotinic acid, both of which fulfill the vitamin functions of niacin carried out by the bioactive forms NAD(P). In the last few decades numerous new enzymes that consume NAD(P) as substrates have been identified. The functions of these enzymes are emerging as exciting paradigm shifts, even though they are in early stages of discovery. The recent identification of the nicotinic acid receptor has allowed distinction of the drug-like roles of nicotinic acid from its vitamin functions, specifically in modulating blood lipid levels and undesirable side effects such as skin vasodilation and the more rare hepatic toxicities. This information has led to a new strategy for drug delivery for niacin, which, if successful, could have a major impact on human health through decreasing risk for cardiovascular disease. Understanding the many other effects of niacin has much broader potential for disease intervention and treatment in numerous diseases including cancer.

Optimizing the energy status of skin cells during solar radiation

Ionizing- and ultraviolet-radiation cause cell damage or death by directly altering DNA and protein structures and by production of reactive oxygen species (ROS) and reactive carbonyl species (RCS). These processes disrupt cellular energy metabolism at multiple levels. The formation of DNA strand breaks activates signaling pathways that consume NAD, which can lead to the depletion of cellular ATP. Poly(ADP)-ribose polymerase (PARP-1) is the enzyme responsible for much of the NAD degradation following DNA damage, although numerous other PARPs have been discovered recently that await functional characterization. Studies on mouse epidermis in vivo and on human cells in culture have shown that UV-B radiation provokes the transient degradation of NAD and the synthesis of ADP-ribose polymers by PARP-1. This enzyme functions as a component of a DNA damage surveillance network in eukaryotic cells to determine the fate of cells following genotoxic stress. Additionally, the activation of PARP-1 results in the activation of a nuclear proteasome that degrades damaged nuclear proteins including histones. Identifying approaches to optimize these responses while maintaining the energy status of cells is likely to be very important in minimizing the deleterious effects of solar radiation on skin.

NMR studies and semi-empirical energy calculations for cyclic ADP-ribose

A possible pH-dependent conformational switch was investigated for cyclic ADP-ribose. NMR signals for the exchangeable protons were observed in H2O at low temperature, but there was no direct evidence for the protonation of N-3 at neutral pH that has previously been postulated. MNDO calculations indicated that pH dependent 31P chemical shift changes are attributable to protonation of the phosphate adjacent to the N-1 of adenine, and not due to trans-annular hydrogen bonding with a protonated N-3.

Restoration of fertility in young obese (Lep(ob) Lep(ob)) male mice with low dose recombinant mouse leptin treatment

OBJECTIVE

We investigated the effects of low-dose leptin treatment on restoration of fertility in young adult male leptin deficient obese mice.

EXPERIMENTAL DESIGN AND RESULTS

MMTV-TGF-alpha Lep(ob) Lep(ob) mice (8--10 weeks old) were treated with recombinant mouse leptin. In experiment 1, four mice (5 microg/g body weight leptin followed by 2.5 microg/g) lost weight and impregnated females (number of pregnancies/number of females, 3/6, 5/6, 5/10, 4/10). In experiment 2, Leptin-Obese (2.5 microg/g) and Control-Lean mice weighed significantly less than Control-Obese mice. Epididymal pad weights of Control-Obese mice were the heaviest, followed by those of Leptin-Obese mice, and Control-Lean mice were the lightest. Testes weight was greater in Control-Lean vs Control-Obese mice. Leptin-Obese mice had testes weight not significantly different from either control group. Four of five Leptin-Obese mice impregnated females (4/10, 5/10, 2/10, 5/12, 0/10).

CONCLUSIONS

These results indicate that low-dosage mouse recombinant leptin treatment restored fertility to young Lep(ob) Lep(ob) male mice. Although body weights of Leptin-Obese mice were similar to those of lean age-matched mice, epididymal fat pad weights were heavier. International Journal of Obesity (2001) 25, 95-97

Histone carbonylation in vivo and in vitro

Non-enzymic damage to nuclear proteins has potentially severe consequences for the maintenance of genomic integrity. Introduction of carbonyl groups into histones in vivo and in vitro was assessed by Western blot immunoassay and reductive incorporation of tritium from radiolabelled NaBH(4) (sodium borohydride). Histone H1 extracted from bovine thymus, liver and spleen was found to contain significantly elevated amounts of protein-bound carbonyl groups as compared with core histones. The carbonyl content of nuclear proteins of rat pheochromocytoma cells (PC12 cells) was not greatly increased following oxidative stress induced by H(2)O(2), but was significantly increased following alkylating stress induced by N-methyl-N'-nitro-N-nitrosoguanidine or by combined oxidative and alkylating stress. Free ADP-ribose, a reducing sugar generated in the nucleus in proportion to DNA strand breaks, was shown to be a potent histone H1 carbonylating agent in isolated PC12 cell nuclei. Studies of the mechanism of histone H1 modification by ADP-ribose indicate that carbonylation involves formation of a stable acyclic ketoamine. Our results demonstrate preferential histone H1 carbonylation in vivo, with potentially important consequences for chromatin structure and function.

Formation of a protein-bound pyrazinium free radical cation during glycation of histone H1

Glycation, the nonenzymatic reaction between protein amino groups and reducing sugars, induces protein damage that has been linked to several pathological conditions, especially diabetes, and general aging. Here we describe the direct identification of a protein-bound free radical formed during early glycation of histone H1 in vitro. Earlier EPR analysis of thermal browning reactions between free amino acids and reducing sugars has implicated the sugar fragmentation product glycolaldehyde in the generation of a 1,4-disubstituted pyrazinium free radical cation. In order to evaluate the potential formation of this radical in vivo, the early glycation of BSA, lysozyme, and histone H1 by several sugars (D-glucose, D-ribose, ADP-ribose, glycolaldehyde) under conditions of physiological pH and temperature was examined by EPR. The pyrazinium free radical cation was identified on histone H1 glycated by glycolaldehyde (g = 2.00539, aN = 8.01 [2N], aH = 5.26 [4H], aH = 2.72 [4H]), or ADP-ribose. Reaction of glycoaldehyde with poly-L-lysine produced an identical signal, whereas reaction with BSA or lysozyme produced only a minor unresolved singlet signal. In the absence of oxygen the signal was stable over several days. Our results raise the possibility that pyrazinium radicals may form during glycation of histone H1 in vivo.

Molecular heterogeneity and regulation of poly(ADP-ribose) glycohydrolase

We have recently described the isolation and characterization of bovine cDNA encoding poly(ADP-ribose) glycohydrolase (PARG). We describe here the preparation and characterization of antibodies to PARG. These antibodies have been used to demonstrate the presence of multiple forms of PARG in tissue and cell extracts from bovine, rat, mouse, and insects. Our results indicate that multiple forms of PARG previously reported could result from a single gene. Analysis of PARG in cells in which poly(ADP-ribose) polymerase (PARP) has been genetically inactivated indicates that the cellular content of PARG is regulated independently of PARP.

Poly(ADP-ribose) polymerase null mouse cells synthesize ADP-ribose polymers

Poly(ADP-ribose) polymerase (PARP) (EC 2.4.2.30), the only enzyme known to synthesize ADP-ribose polymers from NAD+, is activated in response to DNA strand breaks and functions in the maintenance of genomic integrity. Mice homozygous for a disrupted gene encoding PARP are viable but have severe sensitivity to gamma-radiation and alkylating agents. We demonstrate here that both 3T3 and primary embryo cells derived from PARP-/- mice synthesized ADP-ribose polymers following treatment with the DNA-damaging agent, N-methyl-N'-nitro-N-nitrosoguanidine, despite the fact that no PARP protein was detected in these cells. ADP-ribose polymers isolated from PARP-/- cells were indistinguishable from that of PARP+/+ cells by several criteria. First, they bound to a boronate resin selective for ADP-ribose polymers. Second, treatment of polymers with snake venom phosphodiesterase and alkaline phosphatase yielded ribosyladenosine, a nucleoside diagnostic for the unique ribosyl-ribosyl linkages of ADP-ribose polymers. Third, they were digested by treatment with recombinant poly(ADP-ribose) glycohydrolase, an enzyme highly specific for ADP-ribose polymers. Collectively, these data demonstrate that ADP-ribose polymers are formed in PARP-/- cells in a DNA damage-dependent manner. Because the PARP gene has been disrupted, these results suggest the presence of a previously unreported activity capable of synthesizing ADP-ribose polymers in PARP-/- cells.

Inhibition of the intrinsic NAD+ glycohydrolase activity of CD38 by carbocyclic NAD analogues

Carba-NAD and pseudocarba-NAD are carbocyclic analogues of NAD+ in which a 2,3-dihydroxycyclopentane methanol replaces the beta-d-ribonucleotide ring of the nicotinamide riboside moiety of NAD+ [Slama and Simmons (1988) Biochemistry 27, 183-193]. These carbocyclic NAD+ analogues, related to each other as diastereomers, have been tested as inhibitors of the intrinsic NAD+ glycohydrolase activity of human CD38, dog spleen NAD+ glycohydrolase, mouse CD38 and Aplysia californica cADP-ribose synthetase. Pseudocarba-NAD, the carbocyclic dinucleotide in which l-2,3-dihydroxycyclopentane methanol replaces the d-ribose of the nicotinamide riboside moiety of NAD+, was found to be the more potent inhibitor. Pseudocarba-NAD was shown to inhibit the intrinsic NAD+ glycohydrolase activity of human CD38 competitively, with Ki=148 microM determined for the recombinant extracellular protein domain and Ki=180 microM determined for the native protein expressed as a cell-surface enzyme on cultured Jurkat cells. Pseudocarba-NAD was shown to be a non-competitive inhibitor of the purified dog spleen NAD+ glycohydrolase, with Kis=47 miroM and Kii=198 microM. Neither pseudocarba-NAD nor carba-NAD inhibited mouse CD38 or Aplysia californica cADP-ribose synthetase significantly at concentrations up to 1 mM. The results underscore significant species differences in the sensitivity of these enzymes to inhibition, and indicate that pseudocarba-NAD will be useful as an inhibitor of the enzymic activity of human but not mouse CD38 in studies using cultured cells.

Measurement of adenosine concentration in aqueous and vitreous

PURPOSE

The release of adenosine by the ischemic retina may be an initial signal in the development of ischemic macular edema and neovascularization. The levels of adenosine have never been quantified in ocular fluids. In this study, a technique was developed for in vivo measurement of the concentration of adenosine in aqueous and vitreous.

METHODS

Aqueous and vitreous samples were obtained from bovine eyes after death and from live porcine eyes with the subject under general anesthesia. Samples from live eyes were immediately incubated in the sampling syringe with pentoxifylline, erythro-9-(2-hydroxy-3-nonyl) adenine, and dipyridamole to prevent synthesis or degradation of adenosine during the collection procedure, filtered, and flash-frozen in liquid nitrogen. All samples were then filtered and purified on phenylboronate agarose columns and incubated with chloroacetaldehyde to convert the adenosine present in the sample to the fluorescent derivative 1,N6-ethenoadenosine. The 1,N6-ethenoadenosine was separated by high-pressure liquid chromatography and then measured by fluorometry.

RESULTS

Levels of adenosine as low as 0.5 pmole could be detected with this procedure, compared with 20 pmoles by UV detection. By using this technique to measure adenosine levels in the eyes of normal weanling domestic pigs, it was determined that the adenosine concentration in the aqueous was 321.3 +/- 164.9 nM and in the vitreous was 210.8 +/- 41.5 nM.

CONCLUSIONS

The conversion of adenine-containing compounds to fluorescent 1,N6-etheno derivatives offers analytical advantages of selectivity and sensitivity for the quantitative determination of these compounds, with the fluorometric detection providing substantially greater sensitivity than direct detection by UV absorption. The levels obtained in vivo from anesthetized but otherwise healthy pigs presumably reflected basal aqueous and vitreous adenosine levels under the described conditions. This method should be useful in investigating more directly the role of adenosine in models of retinal or ocular ischemia in vivo and in measuring adenosine levels in vitreous or aqueous samples from human patients.

Syntheses of photoactive analogues of adenosine diphosphate (hydroxymethyl)pyrrolidinediol and photoaffinity labeling of poly(ADP-ribose) glycohydrolase

Two isomeric azidoadenosyl analogues of adenosine diphosphate (hydroxymethyl)pyrrolidinediol [ADP-HPD; Slama, J. T., et al. (1995) J. Med. Chem. 38, 389-393] were synthesized as photoaffinity labels for poly(ADP-ribose) glycohydrolase. 8-Azidoadenosine diphosphate (hydroxymethyl)pyrrolidinediol (8-N3-ADP-HPD) inhibited the enzyme activity by 50% at ca. 1 microM, a concentration 80-fold lower than that where the isomeric 2-azidoadenosine diphosphate (hydroxymethyl)pyrrolidinediol did. [alpha-32P]-8-N3-ADP-HPD was therefore synthesized and used to photoderivatize poly(ADP-ribose) glycohydrolase. Irradiation of recombinant poly(ADP-ribose) glycohydrolase and low concentrations of [alpha-32P]-8-N3-ADP-HPD with short-wave UV light resulted in the covalent incorporation of the photoprobe into the protein, as demonstrated by gel electrophoresis followed by autoradiography or acid precipitation of the protein followed by scintillation counting. No photoincorporation occurred in the absence of UV light. The photoincorporation saturated at low concentrations of the photoprobe and photoprotection was observed in the presence of low concentrations of ADP-HPD, an indication of the specificity of the photoinsertion reaction. These results demonstrate that [alpha-32P]-8-N3-ADP-HPD can be used to specifically covalently photoderivatize the enzyme to characterize the polypetides that constitute the ADP-HPD binding site of poly(ADP-ribose) glycohydrolase. The photoincorporation reaction was further used to determine the ability of ADP-ribose polymers of varying size to compete with [alpha-32P]-8-N3-ADP-HPD for binding to the enzyme. Photoincorporation of [alpha-32P]-8-N3-ADP-HPD was inhibited by 80% in the presence of low concentrations of short, unbranched ADP-ribose oligomers (5-15 ADP-ribose units in length). No similar photoprotection was afforded by the addition of a high-molecular weight highly branched polymer. These results indicate that the photolabel shares a binding site with the short, linear polymer, but not with the long, highly branched polymer.

Natural occurrence of 2'-phospho-cyclic ADP ribose in mammalian tissues

2'-Phospho-cyclic ADP-ribose (P-cADPR) is a newly identified Ca2+-mobilizing agent derived from NADP that stimulates intracellular Ca2+ release by a mechanism distinct from inositol 1, 4, 5-trisphosphate. In this report, we show that P-cADPR is an endogenous metabolite in bovine tissues with basal levels ranging from 17.6 to 89.5 fmol/mg protein. The natural occurrence of this Ca2+-mobilizing nucleotide provides a potential link between NADP(H) metabolism and regulation of Ca2+ homeostasis.

Isolation and characterization of the cDNA encoding bovine poly(ADP-ribose) glycohydrolase

The synthesis and rapid turnover of ADP-ribose polymers is an immediate cellular response to DNA damage. We report here the isolation and characterization of cDNA encoding poly(ADP-ribose) glycohydrolase (PARG), the enzyme responsible for polymer turnover. PARG was isolated from bovine thymus, yielding a protein of approximately 59 kDa. Based on the sequence of oligopeptides derived from the enzyme, polymerase chain reaction products and partial cDNA clones were isolated and used to construct a putative full-length cDNA. The cDNA of approximately 4.1 kilobase pairs predicted expression of a protein of approximately 111 kDa, nearly twice the size of the isolated protein. A single transcript of approximately 4. 3 kilobase pairs was detected in bovine kidney poly(A)+ RNA, consistent with expression of a protein of 111 kDa. Expression of the cDNA in Escherichia coli resulted in an enzymatically active protein of 111 kDa and an active fragment of 59 kDa. Analysis of restriction endonuclease fragments from bovine DNA by Southern hybridization indicated that PARG is encoded by a single copy gene. Taken together, the results indicate that previous reports of multiple PARGs can be explained by proteolysis of an 111-kDa enzyme. The deduced amino acid sequence of the bovine PARG shares little or no homology with other known proteins. However, it contains a putative bipartite nuclear location signal as would be predicted for a nuclear protein. The availability of cDNA clones for PARG should facilitate structure-function studies of the enzyme and its involvement in cellular responses to genomic damage.

Photoaffinity labelling of human poly(ADP-ribose) polymerase catalytic domain

Photoaffinity labelling of the human poly(ADP-ribose) polymerase (PARP) catalytic domain (40 kDa) with the NAD+ photoaffinity analogue 2-azido-[alpha-32P]NAD+ has been used to identify NAD+-binding residues. In the presence of UV, photo-insertion of the analogue was observed with a stoichiometry of 0.73 mol of 2-azido-[alpha-32P]NAD+ per mol of catalytic domain. Competition experiments indicated that 3-aminobenzamide strongly protected the insertion site. Residues binding the adenine ring of NAD+ were identified by trypsin digestion and boronate affinity chromatography in combination with reverse-phase HPLC. Two major NAD+-binding residues, Trp1014 of peptide Thr1011-Trp1014 and Lys893 of peptide Ile979-Lys893, were identified. The site-directed mutagenesis of these two residues revealed that Lys893, but not Trp1014, is critical for activity. The close positioning of Lys893 near the adenine ring of NAD+ has been confirmed by the recently solved crystallographic structure of the chicken PARP catalytic domain [Ruf, Menissier-de Murcia, de Murcia and Schulz (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 7481-7485].

ADP-ribose in glycation and glycoxidation reactions

Glycation is initiated by reaction of a reducing sugar with a protein amino group to generate a Schiff base adduct. Following an Amadori rearrangement to form a ketoamine adduct, a complex chemistry involving oxidation often leads to protein glycoxidation products referred to as advanced glycosylation end products (AGE). The AGE include protein carboxymethyllysine (CML) residues and a heterogeneous group of complex modifications characterized by high fluorescence and protein-protein cross links. The sugar sources for the glycoxidation of intracellular proteins are not well defined but pentoses have been implicated because they are efficient precursors for the formation of the fluorescent AGE, pentosidine. ADP-ribose, generated from NAD by ADP-ribose transfer reactions, is a likely intracellular source of a reducing pentose moiety. Incubation of ADP-ribose with histones results in the formation of ketoamine glycation conjugates and also leads to the rapid formation of protein CML residues, histone H1 dimers, and highly fluorescent products with properties similar to the AGE. ADP-ribose is much more efficient than other possible pentose donors for glycation and glycoxidation of protein amino groups. Recently developed methods that differentiate nonenzymic modifications of proteins by ADP-ribose from enzymic modifications now allow investigations to establish whether some protein modifications by monomers of ADP-ribose in vivo represent glycation and glycoxidation.

Intramolecular ADP-ribose transfer reactions and calcium signalling. Potential role of 2'-phospho-cyclic ADP-ribose in oxidative stress

Intramolecular ADP-ribose transfer reactions result in the formation of cyclic ADP-ribose (cADPR) and 2'-phospho-cyclic ADP-ribose (P-cADPR) from NAD and NADP, respectively. The potent Ca2+ releasing activity of these cyclic nucleotides has led to the postulation that they function as second messengers of Ca2+ signalling. The synthesis and hydrolysis of cADPR and P-cADPR are catalyzed by NAD(P) glycohydrolases, but the metabolic signals that regulate their metabolism are poorly understood. To investigate the physiological roles of cADPR and P-cADPR, it is essential to have methods that allow the routine measurement of these nucleotides in cellular systems. As described here, a sensitive and selective radioimmunoassay (RIA) for cADPR has been adapted to search for the natural occurrence of P-cADPR in mammalian tissues. Perchloric acid extracts prepared from bovine tissues and purified by anion exchange chromatography were found to contain immunoreactive material which was identified as P-cADPR. P-cADPR may play an important role in oxidative stress as a link between NADP(H) metabolism and alteration of intracellular Ca2+ homeostasis.

Glycation and glycoxidation of histones by ADP-ribose

The reaction of long lived proteins with reducing sugars has been implicated in the pathophysiology of aging and age-related diseases. A likely intranuclear source of reducing sugar is ADP-ribose, which is generated following DNA damage from the turnover of ADP-ribose polymers. In this study, ADP-ribose has been shown to be a potent histone glycation and glycoxidation agent in vitro. Incubation of ADP-ribose with histones H1, H2A, H2B, and H4 at pH 7.5 resulted in the formation of ketoamine glycation conjugates. Incubation of histone H1 with ADP-ribose also rapidly resulted in the formation of protein carboxymethyllysine residues, protein-protein cross-links, and highly fluorescent products with properties similar to the advanced glycosylation end product pentosidine. The formation of glycoxidation products was related to the degradation of ketoamine glycation conjugates by two different pathways. One pathway resulted in the formation of protein carboxymethyllysine residues and release of an ADP moiety containing a glyceric acid fragment. A second pathway resulted in the release of ADP, and it is postulated that this pathway is involved in the formation of histone-histone cross-links and fluorescent advanced glycosylation end products.

2'-Phospho-cyclic ADP-ribose, a calcium-mobilizing agent derived from NADP

Cyclic adenosine diphosphoribose (cADPR), a metabolite of NAD, appears to modulate changes in intracellular free Ca2+ levels by activation of ryanodine-sensitive Ca2+ channels. We report here that an ADPR cyclase purified from Aplysia californica readily catalyzes the conversion of NADP to 2'-phospho-cyclic adenosine diphosphoribose (2'-P-cADPR), cyclized at N-1 of the adenine moiety. An enzyme from canine spleen previously shown to contain NAD glycohydrolase, ADPR cyclase, and cADPR hydrolase activities also utilized NADP and 2'-P-cADPR as substrates. The apparent Km value for NADP was 1.6 microM compared with 9.9 microM for NAD, and the Vmax with NADP was twice that with NAD, indicating that 2'-P-cADPR is a likely metabolite in mammalian cells. 2'-P-cADPR was as active as cADPR in eliciting Ca2+ release from rat brain microsomes, but was unable to elicit Ca2+ release following conversion to 2'-P-ADPR by the action of canine spleen NAD glycohydrolase. 2'-P-cADPR and 1-D-myo-inositol 1,4,5-trisphosphate (IP3) appear to act by distinct mechanisms as microsomes desensitized to IP3 still released Ca2+ in response to 2'-P-cADPR and vice versa. Also, inhibition of IP3-induced Ca2+ release by heparin had no effect on release by 2'-P-cADPR. Both 2'-P-cADPR and cADPR appear to act by a similar mechanism based on similar kinetics of Ca2+ release, similar dose-response curves, cross-desensitization, and partial inhibition of release by procaine. The results of this study suggest that 2'-P-cADPR may function as a new component of Ca2+ signaling and a possible link between NADP metabolism and Ca2+ homeostasis.

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

Adenosine diphosphate (hydroxymethyl)pyrrolidinediol (ADP-HPD), a nitrogen-in-the-ring analog of ADP-ribose, was recently shown to be a potent and specific inhibitor of poly(ADP-ribose) glycohydrolase. Analysis of the inhibition kinetics of the hydrolase by ADP-HPD using the method of Lineweaver and Burk yields a noncompetitive double-reciprocal plot. Both the intercept (1/V) versus [inhibitor] replot and the slope (Km/V) versus [inhibitor] replot are hyperbolic, indicating partial noncompetitive inhibition. Inhibitor dissociation constants Kii = 52 nM and Kis = 80 nM were determined for ADP-HPD by analysis of the intercept versus [inhibitor] and slope versus [inhibitor] replots. These results show that although ADP-HPD is extremely potent in inhibiting poly(ADP-ribose) glycohydrolase, its effectiveness is limited by its partial inhibition. ADP-HPD was significantly less potent as an inhibitor of the NAD glycohydrolase from Bungarus fasciatus venom. Analysis of the inhibition kinetics using the Lineweaver and Burk method indicated that ADP-HPD was a linear-competitive inhibitor of the NAD glycohydrolase with a Ki of 94 microM. The results indicate that at low concentration ADP-HPD will be a selective inhibitor of poly(ADP-ribose) glycohydrolase; however, complete inactivation of the activity will be difficult to obtain.

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.