ADP-ribosyltransferase activity in cultured hepatocytes. Interactions with DNA repair

Inhibition of poly(ADP-ribose) polymerase-1 alters expression of mitochondria-related genes in PC12 cells: relevance to mitochondrial homeostasis in neurodegenerative disorders

Alzheimer's disease (AD) is characterized by the release of amyloid beta peptides (Aβ) in the form of monomers/oligomers which may lead to oxidative stress, mitochondria dysfunction, synaptic loss, neuroinflammation and, in consequence, to overactivation of poly(ADP-ribose) polymerase-1 (PARP-1). However, Aβ peptides are also released in the brain ischemia, traumatic injury and in inflammatory response. PARP-1 is suggested to be a promising target in therapy of neurodegenerative disorders. We investigated the impact of PARP-1 inhibition on transcription of mitochondria-related genes in PC12 cells. Moreover, the effect of PARP-1 inhibitor (PJ34) on cells subjected to Aβ oligomers (AβO) - evoked stress was analyzed. Our data demonstrated that inhibition of PARP-1 in PC12 cells enhanced the transcription of genes for antioxidative enzymes (Sod1, Gpx1, Gpx4), activated genes regulating mitochondrial fission/fusion (Mfn1, Mfn2, Dnm1l, Opa1, Fis1), subunits of ETC complexes (mt-Nd1, Sdha, mt-Cytb) and modulated expression of several TFs, enhanced Foxo1 and decreased Nrf1, Stat6, Nfkb1. AβO elevated free radicals concentration, decreased mitochondria membrane potential (MMP) and cell viability after 24h. Gene transcription was not affected by AβO after 24h, but was significantly downregulated after 96h. In AβO stress, PJ34 exerted stimulatory effect on expression of several genes (Gpx1, Gpx4, Opa1, Mfn2, Fis1 and Sdha), decreased transcription of numerous TFs (Nrf1, Tfam, Stat3, Stat6, Trp53, Nfkb1) and prevented oxidative stress. Our results indicated that PARP-1 inhibition significantly enhanced transcription of genes involved in antioxidative defense and in regulation of mitochondria function, but was not able to ameliorate cells viability affected by Aβ.

Going Beyond Common Drug Metabolizing Enzymes: Case Studies of Biotransformation Involving Aldehyde Oxidase, γ-Glutamyl Transpeptidase, Cathepsin B, Flavin-Containing Monooxygenase, and ADP-Ribosyltransferase

The significant roles that cytochrome P450 (P450) and UDP-glucuronosyl transferase (UGT) enzymes play in drug discovery cannot be ignored, and these enzyme systems are commonly examined during drug optimization using liver microsomes or hepatocytes. At the same time, other drug-metabolizing enzymes have a role in the metabolism of drugs and can lead to challenges in drug optimization that could be mitigated if the contributions of these enzymes were better understood. We present examples (mostly from Genentech) of five different non-P450 and non-UGT enzymes that contribute to the metabolic clearance or bioactivation of drugs and drug candidates. Aldehyde oxidase mediates a unique amide hydrolysis of GDC-0834 (N-[3-[6-[4-[(2R)-1,4-dimethyl-3-oxopiperazin-2-yl]anilino]-4-methyl-5-oxopyrazin-2-yl]-2-methylphenyl]-4,5,6,7-tetrahydro-1-benzothiophene-2-carboxamide), leading to high clearance of the drug. Likewise, the rodent-specific ribose conjugation by ADP-ribosyltransferase leads to high clearance of an interleukin-2-inducible T-cell kinase inhibitor. Metabolic reactions by flavin-containing monooxygenases (FMO) are easily mistaken for P450-mediated metabolism such as oxidative defluorination of 4-fluoro-N-methylaniline by FMO. Gamma-glutamyl transpeptidase is involved in the initial hydrolysis of glutathione metabolites, leading to formation of proximate toxins and nephrotoxicity, as is observed with cisplatin in the clinic, or renal toxicity, as is observed with efavirenz in rodents. Finally, cathepsin B is a lysosomal enzyme that is highly expressed in human tumors and has been targeted to release potent cytotoxins, as in the case of brentuximab vedotin. These examples of non-P450- and non-UGT-mediated metabolism show that a more complete understanding of drug metabolizing enzymes allows for better insight into the fate of drugs and improved design strategies of molecules in drug discovery.

Elucidation of the mechanism of ribose conjugation in a pyrazole-containing compound in rodent liver

1. Here we report on the mechanism of ribose conjugation, through NADH as a cofactor, of a pyrazole-containing compound (PT). Incubation of PT in rat liver microsomes supplemented with NADP⁺/H, NAD⁺/H, and β-nicotinamide mononucleotide (NMN) resulted in complete conjugation to the adenine dinucleotide phosphate conjugate (ADP-C), adenine dinucleotide conjugate (AD-C), and 5-phosphoribose conjugate (Rib-C1), respectively. In hepatocytes, PT predominantly formed three ribose conjugates: Rib-C1, the ribose conjugate (Rib-C2), and the carboxylic acid of Rib-C2 (Rib-C3). 2. Phosphatase inhibitors were added to hepatocyte incubations. AD-C was detected in this reaction, which suggests that one of the major pathways for the formation of the ribose conjugates is through NAD⁺/H. When AD-C was incubated with phosphatase, Rib-C1 and Rib-C2 formed. 3. To understand the in vivo relevance of this metabolic pathway, rats were dosed with PT and Rib-C2 was found in the urine. 4. Structure-activity relationship shows that replacement of the distal thiazole group in the PT to a phenyl group abolishes this conjugation. Three amino acid residues in the active site preferentially interact with the sulfur atom in the thiazole of PT. 5. In summary, PT forms direct AD-C in hepatocytes, which is further hydrolyzed by phosphatase to give ribose conjugates.

D-beta-hydroxybutyrate is neuroprotective against hypoxia in serum-free hippocampal primary cultures

Hypoxia decreased survival of cultured rat primary hippocampal neurons in a time dependent manner. Addition of 4 mM Na D-beta-hydroxybutyrate (bHB), a ketone body, protected the cells for 2 hr and maintained the increase in survival compared to that of controls for up to 6 hr. Trypan blue exclusion indicated that acute cell death was reduced markedly after 2-hr exposure to hypoxia in the bHB-treated group. The presence of bHB also decreased the number of neurons exhibiting condensed nuclei visualized by propidium iodide, indicative of apoptosis. The mitochondrial transmembrane potential (Em/c) was maintained for up to 2 hr exposure to hypoxia in the bHB-treated group, whereas the potential in the control group was decreased. Furthermore, cytochrome C release, caspase-3 activation, and poly (ADP-ribose) polymerase (PARP) cleavage were decreased in the bHB-treated group for the first 2 hr of exposure. These findings indicate that ketone bodies may be a candidate for widening the therapeutic window before thrombolytic therapy and at the same time decreasing apoptotic damage in the ischemic penumbra.

Role of poly-(ADP-ribose) synthetase in lipopolysaccharide-induced vascular failure and acute lung injury in pigs

PURPOSE

To assess the contribution of poly (adenosine 5'-diphosphate ribose) synthetase (PARS) to the development of bacterial lipopolysaccharide (LPS)-induced acute lung injury and vascular failure in pigs.

MATERIALS AND METHODS

Four groups of anesthetized, paralyzed, and mechanically ventilated domestic white pigs. Group 1 served as control, whereas Escherichia coli LPS (20 microg/kg/h) was continuously infused in group 2. Group 3 received 20 mg/kg injection of 3-aminobenzamide (a selective inhibitor of PARS activity) 15 minutes before LPS infusion. Only 3-aminobenzamide and not LPS was injected in group 4. All animals were examined for 180 minutes. Systemic and pulmonary hemodynamics and lung mechanics were measured during the experimental period. Lung wet/dry ratio, bronchoalveolar lavage (BAL) protein levels and cell counts and lung nitrotyrosine (footprint of peroxynitrite) immunostaining were also measured in a few animals.

RESULTS

LPS infusion evoked a progressive decline in systemic arterial pressure, a small increase in cardiac output, and biphasic elevation of pulmonary arterial pressure. Lung compliance declined progressively, whereas lung and total respiratory resistance rose significantly after LPS infusion. Prominent nitrotyrosine immunostaining was detected around small airways and pulmonary endothelium of LPS-infused animals. No significant changes in lung wet/dry ratio and BAL protein levels and cell counts were produced by LPS infusion. Pretreatment with 3-aminobenzamide did not alter the systemic and pulmonary hemodynamic responses to LPS infusion but eliminated the rise in pulmonary and total respiratory resistance.

CONCLUSIONS

We concluded that PARS activation plays an important role in the changes of lung mechanics associated with LPS-induced acute lung injury but had no role in vascular failure.

Activation of poly(ADP-ribose) polymerase in the rat hippocampus may contribute to cellular recovery following sublethal transient global ischemia

We have investigated the role of poly(ADP-ribose) polymerase (PARP) activation in rat brain in a model of sublethal transient global ischemia. Adult male rats were subjected to 15 min of ischemia with brain temperature reduced to 34 degrees C, followed by 1, 2, 4, 8, 16, 24, and 72 h of reperfusion. PARP mRNA expression was examined in the hippocampus using quantitative RT-PCR, northern blot analysis, and in situ hybridization. Protein expression was assessed using western blot analysis. PARP enzymatic activity was investigated by measuring nuclear [3H]NAD incorporation. The presence of poly(ADP-ribose) polymers was assessed immunocytochemically. Although PARP mRNA and protein expressions were not altered after ischemia, enzymatic activity was increased 4.37-fold at 1 h (p < 0.05 vs. sham) and 1.73-fold (p < 0.05 vs. sham) at 24 h of reperfusion. Immunostaining demonstrated the presence of poly(ADP-ribose) polymers in CA1 neurons. Cellular NAD+ levels were not significantly altered at any time point. Furthermore, systemic administration of 3-aminobenzamide (30 mg/kg), a PARP inhibitor, prevented the increase in PARP activity at 1 and 24 h of reperfusion, significantly decreased the number of surviving neurons in the hippocampal CA1 region 72 h after ischemia (p < 0.01 vs. sham), and increased DNA single-strand breaks assessed as DNA polymerase I-mediated biotin-dATP nick-translation (PANT)-positive cells (p < 0.01 vs. sham). Furthermore, using an in vitro DNA repair assay, 3-aminobenzamide (30 mg/kg) was shown to block DNA base excision repair activity. These data suggest that the activation of PARP, without subsequent NAD+ depletion, following mild transient ischemia may be neuroprotective in the brain.

Effects of nicotinamide-related agents on the growth of primary rat hepatocytes and formation of small hepatocyte colonies

AIMS/BACKGROUND

We report in this study that, 10 mM nicotinamide can stimulate the proliferation of primary rat hepatocytes in serum-free Dulbecco's modified Eagle's medium supplemented with 10 ng/ml epidermal growth factor and that small hepatocyte colonies appear from 4 to 5 days after plating. We examined the effects of nicotinamide-related agents on the growth and differentiation of primary rat hepatocytes and on the appearance of small hepatocyte colonies.

METHODS

As nicotinamide is an aqueous vitamin named niacin and known to act as an inhibitor of poly (ADP-ribose) polymerase (PARP), we therefore chose to examine the effects on hepatocytes of three nicotinamide-related agents, nicotinic acid (NA) which is also a niacin, 3-aminobenzamide (3-AB) which is a strong inhibitor of PARP but is not a niacin, and 3-acetylpyridine (3-AP) which is a weak inhibitor of PARP and also not a niacin. To examine their effects on the growth of the cells and on the formation of the colony, immunocytochemistry for BrdU was carried out. Expression of albumin, tryptophan 2,3-dioxygenase (TO), and connexin 32 (Cx32) mRNAs were used as marks of hepatic differentiation. Intracellular NAD+ content was also measured.

RESULTS

At concentration of 10 mM, NA could not enhance the proliferation of mature hepatocytes but induced the appearance of small hepatocyte colonies. At concentration of 5 mM, 3-AB enhanced the proliferation of the hepatocytes but did not induce small hepatocyte colonies. On the other hand, although 10 mM 3-AP remarkably inhibited the DNA synthesis of the cells, the expression not only of albumin but also of TO and Cx32 mRNAs in the cells was well maintained for more than one week. The intracellular NAD+ concentration was correlated with the proliferation of the hepatocytes.

CONCLUSION

These results suggest that the intracellular NAD+ content may be correlated with the proliferation of primary hepatocytes and that the supplementation of niacin in the medium may be important for the appearance of small hepatocyte colonies.

The current status of primary hepatocyte culture

Recently, there have been significant advances toward the development of culture conditions that promote proliferation of primary rodent hepatocytes. There are two major methods for the multiplication of hepatocytes in vitro: one is the use of nicotinamide, the other is the use of a nutrient-rich medium. In the medium containing a high concentration of nicotinamide and a growth factor, primary hepatocytes can proliferate well. In this culture condition small mononucleate cells, which are named small hepatocytes, appear and form colonies. Small hepatocytes have a high potential to proliferate while maintaining hepatic characteristics, and can differentiate into mature ones. On the other hand, combining the nutrient-rich medium with 2% DMSO, the proliferated hepatocytes can recover the hepatic differentiated functions and maintain them for a long time. In this review I describe the culture conditions for the proliferation and differentiation of primary hepatocytes and discuss the small hepatocytes, especially their roles in liver growth.

Growth and maturation of small hepatocytes

Proliferation of adult rat hepatocytes is observed in serum-free Dulbecco's modified Eagle's medium (DMEM) supplemented with 10 mmol/L nicotinamide and 10 ng/mL epidermal growth factor (EGF). The proliferating cells are mainly mononucleate and form small cell colonies surrounded by mature hepatocytes. Although these cells in focal colonies have a less-differentiated appearance, immunocytochemically and ultrastructurally they possess hepatic characteristics. The size of small hepatocytes is one-third to half that of mature hepatocytes. Therefore, we call the cells forming a colony, small hepatocytes. The small hepatocytes can be subcultured for several passages. Furthermore, the cells are rich in the supernatant following 50 g centrifugation for 1 min after collagenase liver perfusion. When the cells are cultured in DMEM supplemented with 10% foetal bovine serum, 10 mmol/L nicotinamide, 1 mmol/L ascorbic acid 2-phosphate, 10 ng/mL EGF and 1% dimethyl sulphoxide, each small hepatocyte can clonally proliferate for more than 3 months. A small hepatocyte divides to form a colony and the number of cells reaches more than 100 within 20 days. With time in culture, cells with a large cytoplasm appear within a colony. They have many mitochondria and large peroxisomes with crystalline nucleoids and are typical, mature hepatocytes. Immunoreactivity to connexin 32 and well-developed bile canaliculus structures are often observed in the cell-cell borders. Thus, we suggest that small hepatocytes may be considered to be 'committed progenitor cells' that can further differentiate into mature hepatocytes.

Modulation of DNA repair by various inhibitors of DNA synthesis following 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) induced DNA damage

The tobacco specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is present in tobacco smoke and is hepatocarcinogenic in rats. Its bioactivation in rat hepatocytes leads to methylation and pyridyloxobutylation of DNA. Rat hepatocytes were cultured in serum-free William medium E on collagen-coated dishes. We demonstrated that some enzymes of the base and/or excision-repair pathways were involved in repair of NNK-induced DNA damage, measured by [methyl-3H] thymidine incorporation. Unscheduled DNA synthesis (UDS) induced by N-methyl-N-nitrosourea (MNU), NNK, N'-nitrosonornicotine (NNN) and 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone (NNKOAc) increased 2.9-, 2.8-, 1.5- and 3.5-fold, respectively, suggesting that methylated and/or pyridyloxobutylated-DNA by these four nitroso compounds is repaired by the excision pathway. Moreover, levels of NNK-induced UDS were dose (1-3 mM) and time (1-18 h) dependent. Enzymes involved in the excision repair pathways were selectively inhibited. Inhibitors of DNA topoisomerase I (camptothecin) and topoisomerase II (etoposide, nalidixic acid) did not decrease the induction of UDS, suggesting that topoisomerases are not involved in the repair of NNK-induced damage. While aphidicolin and arabinocytidine (DNA polymerase alpha, delta, epsilon inhibitors) totally inhibited NNK- and NNKOAc-induced UDS, dideoxythymidine (DNA polymerase beta inhibitor) inhibited NNK- and NNKOAc-induced UDS by 40 and 33%, respectively. We conclude that DNA polymerase alpha, delta or epsilon and to a lesser degree polymerase beta are involved in the repair of pyridyloxobutylated DNA. Previous studies showed that inhibition of poly(ADP-ribosyl) polymerase (PARP) by 3-aminobenzamide (3-ab) facilitated DNA ligation. Our results demonstrate that 3-ab increased NNK-induced UDS, but does not affect NNKOAc-induced UDS. These observations suggest that the ligation step is rate limiting in the repair of methylated DNA but not of pyridyloxobutylated DNA.

Induction of dedifferentiated clones of LLC-PK1 cells upon long-term exposure to dichlorovinylcysteine

Dichlorovinylcysteine (DCVC), the key metabolite of the nephrotoxic and nephrocarcinogenic chemicals, trichloroethylene and dichloroacetylene, exerts potent acute cellular toxicity in LLC-PK1 cells (Vamvakas S., Bittner, D., Dekant, W. and Anders, M.W. (1992). Events that precede and that follow S-(1,2-dichlorovinyl)-L-cysteine-induced release of mitochondrial Ca2+ and their association with cytotoxicity to renal cells. Biochem. Pharmacol. 44, 1131-1138). In the present study we investigated whether long-term exposure of LLC-PK1 cells to low, non-cytotoxic concentrations of DCVC results in stable morphological and biochemical dedifferentiation. After 7 weeks exposure to 1 and 5 microM DCVC, morphologically changed single cells were picked under the microscope and cultured in absence of DCVC for 4-8 weeks. In contrast to the physiological cuboidal shape of untreated LLC-PK1 cells, the clones derived from long-term exposure to DCVC consisted of elongated, spindle-shaped cells tending to form irregular borders. Moreover, glucose uptake, pH-dependent ammonia production and dome formation, important indicators of the renal tubule origin of the LLC-PK1 cells, were severely impaired in the clones. In addition to the loss of membrane polarity, the clones exhibited altered composition of the nuclear matrix and intermediate filament proteins by two-dimensional gel electrophoresis, increased poly(ADP-ribosyl)ation of nuclear proteins and enhanced expression of c-fos. The induction of dedifferentiated LLC-PK1 clones with stable characteristics upon long-term exposure to the nephrocarcinogen DCVC may represent a useful in vitro model to study biochemical alterations involved in chronic renal toxicity and carcinogenicity.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Induction of poly(ADP-ribosyl)ation in the kidney after in vivo application of renal carcinogens

Dichlorovinylcysteine, the key metabolite thought to be responsible for the nephrocarcinogenicity of trichloroethene and dichloroacetylene, induces DNA double-strand breaks followed by increased poly(ADP-ribosyl)ation of nuclear proteins in cultured renal cells (Vamvakas et al., 1992, Biochem. Pharmacol. 44, 1131-1138). Poly(ADP-ribosyl)ation represents a post-translational modification of nuclear proteins involved in DNA repair, DNA replication, and modulation of gene expression. The present study investigates the induction of DNA double-strand breaks and poly(ADP-ribosyl)ation in the renal cortex after in vivo administration of several renal carcinogens to male Wistar rats, and the temporal relationship between these two processes. Dichlorovinylcysteine caused a time-dependent increase in the amount of poly(ADP-ribosyl)conjugates in the kidney cortex, which was preceded by increased formation of DNA double-strand breaks. Potassium bromate and ferric nitrilotriacetate, whose nephrocarcinogenicity is thought to result from increased formation of reactive oxygen species, both induced poly(ADP-ribosyl)ation with the concomitant formation of DNA double-strand breaks. Dimethylnitrosamine, an indirect acting methylating agent, and trimethylpentane, a non-genotoxic renal carcinogen, failed to induce poly(ADP-ribosyl)ation or a significant increase in DNA double-strand breaks in the renal cortex. The results indicate that nephrocarcinogens capable of inducing DNA fragmentation also induce post-translational modification of renal proteins via increased poly(ADP-ribosyl)ation.

Differing effects of the inhibition of poly(ADP-ribose) polymerase on the course of oxidative cell injury in hepatocytes and fibroblasts

The effects of the two inhibitors of poly(ADP-ribose) polymerase, 3-aminobenzamide (ABA) and benzamide (BA), on the oxidative killing of L929 mouse fibroblasts and primary cultures of rat hepatocytes were studied. The killing of L929 cells by tert-butyl hydroperoxide (TBHP) occurred by two mechanisms, one sensitive and the other insensitive to the antioxidant N,N'-diphenylphenylene diamine (DPPD). Cell killing by either mechanism was prevented by the ferric iron chelator deferoxamine. ABA and BA prevented the killing of L929 cells that occurred in the presence, but not in the absence, of DPPD. ABA and BA inhibited the activity of poly(ADP-ribose) polymerase by 85%. Protection was accompanied by the sparing of the depletion of both NAD and ATP, but there was no effect of either ABA or BA on the iron-dependent appearance of single-strand breaks in DNA. Depletion of ATP by treating the fibroblasts with 2-deoxyglucose and sodium azide did not result in any loss of viability. H2O2 similarly killed the L929 cells by a mechanism that depended on a source of ferric iron. However, DPPD had no effect on the cell killing, and ABA and BA completely protected the cells in the presence or absence of DPPD. H2O2 caused the appearance of single-strand breaks that were prevented by deferoxamine, but again not by ABA or BA. ABA and deferoxamine reduced, but did not prevent, the depletion of both NAD and ATP occurring with H2O2. With the cultured hepatocytes, ABA and BA inhibited poly(ADP-ribose) polymerase at concentrations that were without effect on either the extent of cell killing or the depletion of NAD occurring with either TBHP, H2O2, or menadione. These data indicate that the relationship between oxidative DNA damage and the genesis of lethal injury is very different in the two types of cells. In the fibroblasts, the appearance of single strand breaks in DNA was accompanied by depletion of NAD and ATP and subsequently by the death of the cells. These events were mediated by the activity of poly(ADP-ribose) polymerase, as inhibition of the enzyme prevented their development. In the hepatocytes, inhibition of poly(ADP-ribose) polymerase was without effect on the oxidative death of the cells.

Effect of procainamide and hydralazine on poly (ADP-ribosylation) in cell lines

The prescription drugs procainamide (PA) and hydralazine (HYD) are associated with the induction of autoimmunity and a clinical syndrome called drug-induced lupus. Since PA- and HYD-induced autoantibodies are directed primarily against histones and histones are prime acceptors of poly (ADP-ribose) (PADPR), we have investigated the effects of PA and HYD on the activity of poly (ADP-ribose) polymerase (PADPRP). Control substances, with structures similar to PA and HYD but not known to induce lupus, included N-acetylprocainamide (NAPA) and the amino acids phenylalanine, tryptophan and proline, and their amide derivatives. Wil-2 cells were incubated in 0.5-50 microM PA, NAPA and HYD, which included therapeutic concentrations of these drugs. The mean enhancement of incorporation of [3H]-nicotinamide adenine dinucleotide (NAD) into PADPR was 1.84 (P = 0.005) with PA, with HYD 1.48 (P = 0.029), and with NAPA 1.38 (P = 0.036). This increase was suppressed by 3-aminobenzamide, an inhibitor of PADPRP activity. Little or no increase in [3H]-NAD incorporation was observed with equivalent concentrations of phenylalanine, phenylalaninamide or tryptophan. However, a 1.29-fold increase was noted with 0.5 microM tryptophanamide, a 1.26-fold increase with 0.5 microM prolinamide and a 1.4-fold increase with 50 microM proline. PA increased PADPRP activity in B- and T-cell lines but not in promyelocytic leukemia or epithelial cell lines. Since poly (ADP-ribosylation) is important in the cellular response to various agents, the increased ADP-ribosylation of intracellular molecules may be a key event in the induction of autoantibodies.

Hydroperoxide-induced oxidative stress alters pyridine nucleotide metabolism in neonatal heart muscle cells

An oxidant burden established by hydrogen peroxide (H2O2) overload may elicit postischemic myocardial damage. We assess herein the influence of H2O2-induced oxidative stress on heart muscle pyridine nucleotide metabolism. Exposure of neonatal rat cardiomyocytes to 50 microM-1.0 mM H2O2 bolus rapidly shifted their pyridine-nucleotide redox balance toward oxidation. At least 30% of the observed NADPH oxidation was independent of glutathione cycle activity and appeared chemical in nature with H2O2 itself, and not a radical metabolite, acting as oxidant. Cell exposure to H2O2 also depleted cardiomyocyte pyridine nucleotides as a consequence of enhanced utilization. The oxidative stress activated one major route of pyridine nucleotide catabolism (i.e., protein ADP-ribosylation) without acute inhibitory effect upon the other (cleavage by NAD glycohydrolase). The limited NAD sparing by metal chelators and inhibitors of ADP-ribosylation reflected pyridine nucleotide utilization for repair of single-strand DNA breaks caused by hydroxyl-like radicals formed intracellularly through iron-dependent H2O2 reduction. Cardiomyocyte NAD depletion during H2O2-induced oxidative stress was independent of cell integrity and lipid peroxidation. The NAD lost after a discrete H2O2 "pulse" was only partly replenished over a 24-h postinjury period. These data demonstrate that cardiomyocyte pyridine nucleotide metabolism is a nonperoxidative injury target that is chronically affected by H2O2 overload. Derangement of myocardial pyridine nucleotide pools due to oxidative stress may contribute to ischemic heart injury in vivo by interfering with cardiac hydrogen metabolism and redox balance.

Specific binding and uptake of extracellular nicotinamide in human leukemic K-562 cells

Extracellular nicotinamide is well recognized as the primary precursor to the cellular synthesis of NAD. NAD is a pivotal molecule in regulating the energy and redox potentials of cells via synthesis of ATP and NAD(P)/NAD(P)H ratios. NAD turnover in cells is very rapid due to NAD catabolism via ADP-ribosylation reactions. These facts suggest that the cellular uptake and transport of nicotinamide may not be a passive process but a highly regulated cellular event. We have utilized radiometric procedures to characterize the uptake of [14C]nicotinamide in human leukemic K-562 cells. At physiologically relevant doses of nicotinamide (< 100 microM), the uptake was saturable with a Km of 2.3 +/- 1.0 microM and a Vmax of about 1.5 +/- 0.5 pmol/10(6) cells/min. Kinetic studies revealed that nicotinamide was first taken up intracellularly and then immediately converted to NAD and 1-methyl nicotinamide. All of the nicotinamide taken up into the cell was bound tightly to plasma membranes (25,000 g pellet) with Kd values between 3.2 and 12.7 microM and a Bmax of 1.56 pmol/10(6) cells. The specificity of nicotinamide binding was demonstrated by competitive inhibition experiments using NAD analogs, nicotinamide derivatives, and agonists or antagonists of plasma membrane receptors. We conclude that there is specific binding of nicotinamide, followed by intracellular uptake and immediate synthesis to NAD.

Nicotinamide analogs and DNA-damaging agents deplete thyroid hormone receptor and c-erbA mRNA levels in pituitary GH1 cells

Incubation of pituitary GH1 cells with N'-methylnicotinamide, nicotinamide and 3-acetylpyridine which inhibit nuclear ADP-ribosylation and/or the cellular concentration of its substrate NAD+ reduced the amount of nuclear thyroid hormone receptors in a time- and dose-dependent manner without altering the affinity of the receptors for the hormone. A transient activation of poly(ADP-ribose)polymerase by methyl methanesulfonate, ultraviolet irradiation or spermine caused a rapid depletion of cellular NAD+ content and was followed by a strong inhibition of ADP-ribosylation. These agents also produced a very rapid and marked reduction of receptor numbers. The decrease of receptors caused by the different compounds is not secondary to a generalized inhibition of protein synthesis or to an alteration in hormone availability. The abundance of c-erbA alpha and beta mRNAs, which encode thyroid hormone receptors, was reduced in cells treated with the compounds that decrease receptor number, thus suggesting that this effect is caused by a decrease in the expression of c-erbA genes.

Oxidized low density lipoproteins elicit DNA fragmentation of cultured lymphoblastoid cells

Lymphoblastoid cell lines continuously pulsed with mildly oxidized low density lipoproteins, exhibited a significant increase of DNA fragmentation induced by oxidized LDL internalized by cells. DNA fragmentation was associated with an increasing number of morphologically characteristic apoptotic cells simultaneously with the increase of cytotoxicity indexes, and the activation of the poly(ADP-ribose) polymerase, a nuclear enzyme stimulated by DNA strand breaks. The potential involvement of these biochemical and morphological changes in atherogenesis is discussed.

Ethanol enhances ADP-ribosylation of protein in rat hepatocytes

Decreases in hepatocyte NAD+ produced by ethanol are only partially explained by the increased conversion of NAD+ to NADH and NADP+. The purpose of this study was to determine whether a mechanism for the ethanol-induced decrease in NAD+ is its increased use in ADP-ribosylation. Exposure of hepatocytes in culture for 2 hr to 100 mmol/L ethanol increased the incorporation of 14C-ribose from prelabeled NAD+ into 14C-ribosylated proteins. Poly (ADP-ribose) polymerase activity was increased by exposure of isolated hepatocytes to 100 mmol/L ethanol for 10 min. In hepatocyte culture, increases in poly (ADP-ribose) polymerase were not detected after exposure to 100 mmol/L ethanol for 10 min or 2 hr but rather occurred at 24 hr. Ethanol exposure of hepatocytes in culture for 2 hr, however, decreased the Km for NAD+ of poly (ADP-ribose) polymerase. Both nicotinamide and 5-aminobenzamide, which are inhibitors of poly (ADP-ribose) polymerase, prevented the decrease in NAD+ produced by 2-hr exposure of hepatocytes in culture to 100 mmol/L ethanol. The effect of ethanol in decreasing DNA synthesis on days 3 and 4 of culture was not reversed by the inhibitors of poly (ADP-ribose) polymerase. These results indicate that increased ADP-ribosylation of hepatocyte proteins is a mechanism for the effect of ethanol in decreasing NAD+.

Biochemical assessment of the genotoxicity of the in vitro interaction between chlordecone and carbon tetrachloride in rat hepatocytes

The genotoxic potential of the administration of carbon tetrachloride alone or carbon tetrachloride to chlordecone-pretreated rats was investigated using an in vivo-in vitro animal model and a battery of biochemical assays to measure DNA repair in rat hepatocytes. Whereas carbon tetrachloride alone was not genotoxic, chlordecone or chlordecone in combination with carbon tetrachloride was genotoxic. The need for further investigation into the mechanism underlying the interaction between chlordecone and carbon tetrachloride is indicated strongly by the results of the present study.

The bicarbonate ion is essential for efficient DNA synthesis by primary cultured rat hepatocytes

Bicarbonate in the culture medium is essential for DNA synthesis of primary cultured rat hepatocytes stimulated by epidermal growth factor (EGF). When primary cultured hepatocytes in supplemented Leibovitz L15 medium were placed in a 100% air incubator, no increase in DNA synthesis was observed even after stimulation by EGF. However, when these cells were cultured with NaHCO3 and EGF and placed in a 5% CO2:95% air incubator, a stimulus of DNA synthesis more than 10-fold greater than in cultures in air only was seen, and many mitotic figures could be identified. Furthermore, NaHCO3 added to supplemented DMEM/F12 medium enhanced the DNA synthesis of primary cultured rat hepatocytes in this medium. The ideal pH of the medium for DNA synthesis of cultured hepatocytes was in the range of 7.6 to 8.0. A dose response of NaHCO3 in several media showed that DNA synthesis of the cells increased as the concentration of NaHCO3 increased and that 25 to 30 mM NaHCO3 in the medium was optimal for the replication of DNA by primary cultured rat hepatocytes.

Stable transformation of the moss Physcomitrella patens

We report the stable transformation of Physcomitrella patens to either G418 or hygromycin B resistance following polyethylene glycol-mediated direct DNA uptake by protoplasts. The method described in this paper was used successfully in independent experiments carried out in our two laboratories. Transformation was assessed by the following criteria: selection of antibiotic-resistant plants, mitotic and meiotic stability of phenotypes after removal of selective pressure and stable transmission of the character to the offspring; Southern hybridisation analysis of genomic DNA to show integration of the plasmid DNA; segregation of the resistance gene following crosses with antibiotic-sensitive strains; and finally Southern hybridisation analysis of both resistant and sensitive progeny. In addition to stable transformants, a heterogeneous class of unstable transformants was obtained.

Direct stimulation of poly(ADP ribose) polymerase in permeabilized cells by double-stranded DNA oligomers

Poly(ADP ribosyl)ation, a post-translational modification of nuclear proteins catalyzed by poly (ADP ribose) polymerase, is an immediate response of most eukaryotic cells to DNA strand breaks and has been implicated in DNA repair and other cellular phenomena associated with DNA strand breakage. Poly(ADP ribose) polymerase activity levels have been frequently assayed by incubating permeabilized cells with radioactively labeled NAD+ as substrate. In such assays enzyme activation has routinely been achieved indirectly by prior exposure of living cells to carcinogens or by adding DNase I to permeabilized cells, thereby introducing strand breaks in chromosomal DNA. Here we show that, as an alternative method, the direct activation of purified poly(ADP ribose) polymerase by double-stranded oligonucleotides (N. A. Berger and S. I. Petzold, 1985, Biochemistry 24, 4352-4355) can be adopted for permeabilized cell systems. The inclusion of a palindromic decameric deoxynucleotide in the reaction buffer stimulated the enzyme activity in permeabilized Molt-3 human lymphoma cells up to 30-fold (at 50 micrograms/ml [corrected] oligonucleotide concentration) in a concentration-dependent manner. The activating effect of oligonucleotides was also evident when ethanol-fixed HeLa cells were postincubated with NAD+ to allow poly(ADP ribose) synthesis to occur in situ, which was detected as specific anti-poly (ADP ribose) immunofluorescence. We conclude that double-stranded oligonucleotides can be conveniently used as chemically and stoichiometrically well-defined poly (ADP ribose) polymerase activators in permeabilized or ethanol-fixed mammalian cells.

Activation of human monocyte-derived macrophages by interferon gamma is accompanied by increase of poly(ADP-ribose) polymerase activity

We have investigated poly(ADP-ribosyl)ation processes in human monocyte-derived macrophages and the effect of the activating cytokine, interferon gamma (IFN-gamma) on these processes. IFN-gamma was shown to increase the activity of poly(ADP-ribose) polymerase in human macrophages. A 2-3-fold enhancement of poly(ADP-ribose) polymerase activity was observed after 3-4 h of incubation with IFN-gamma, whose effects were dose-dependent and maximal at 20-50 U/ml. Staining with anti-poly(ADP-ribose) antibodies and purification of ADP-ribosylated nuclear proteins by affinity chromatography over boronate agarose showed that enhancement of poly(ADP-ribose) polymerase activity by IFN-gamma was accompanied by accumulation of poly(ADP-ribose) polymers in nuclear proteins. The effects of IFN-gamma on poly(ADP-ribose) polymerase activity were not due to an enhanced accumulation of the message for the enzyme, indicating that the activation of the enzyme activity was due to post-transcriptional modifications. IFN-gamma was shown to induce DNA strand breaks in human macrophages. This phenomenon followed the same time-course and was evident with the same doses of IFN-gamma that increased poly(ADP-ribose) polymerase activity. Since poly(ADP-ribose) polymerase is known to require DNA nicks for its activity, the capability of IFN-gamma to induce DNA strand breaks can explain its effects on poly(ADP-ribosyl)ation processes.

Isolation of 8-methoxypsoralen accessible DNA domains from chromatin of intact cells

The chromatin organization of living mammalian cells was probed using 8-methoxypsoralen (MOP). In intact cells, MOP intercalates into DNA domains which are also preferentially accessible to micrococcal nuclease. After UV365 nm irradiation of MOP-treated cells, this chemical forms bifunctional adducts crosslinking the two strands of DNA. Following extraction of cellular DNA, heat denaturation and renaturation at low temperature, the fraction of crosslinked DNA is obtained following enzymatic hydrolysis of unhybridized, non-crosslinked DNA by nuclease S1 treatment. An application of this procedure in the isolation of 8-methoxypsoralen-accessible DNA domains during DNA excision repair is shown.

The maintenance of cytochrome P-450 in rat hepatocyte culture: some applications of liver cell cultures to the study of drug metabolism, toxicity and the induction of the P-450 system

Treatments affecting the loss of cytochrome P-450 in rat hepatocyte culture are reviewed and the way in which these have produced an understanding of the mechanisms involved are discussed extensively. A simple way to prevent the loss of P-450 in hepatocytes is to culture them with 0.5 mM metyrapone which appears to restore the cytochromes' synthesis and degradation to steady state values. Knowledge of this mechanism has led to the formulation of special culture medium and the application of both culture systems to the study of drug metabolism and toxicity are described. Finally the effect of these culture systems on the expression of the multiple forms of cytochrome P-450 are presented to illustrate the potential of cultured hepatocytes in induction studies.

Poly(ADP-ribose) catabolism in mammalian cells exposed to DNA-damaging agents

DNA damage inflicted by the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine, or by UV254nm, stimulated the catabolism of protein-bound poly(ADP-ribose) in the chromatin of cultured hepatocytes. The stimulation was highest at the largest doses of DNA-damaging treatment. As a consequence, the half-life of ADP-ribosyl polymers may drop to less than 41 s. This rapid turnover contrasts with the slow catabolism of a constitutive fraction of polymers exhibiting a half-life of 7.7 h. Our data suggest that post-incisional stimulation of poly(ADP-ribose) biosynthesis in DNA-excision repair is coupled with an adaptation of poly(ADP-ribose) catabolism in mammalian cells.

Mutant cells defective in poly(ADP-ribose) synthesis due to stable alterations in enzyme activity or substrate availability

We used two different approaches to develop cell lines deficient in poly(ADP-ribose) synthesis to help determine the role of this reaction in cellular functions. One approach to this problem was to develop cell lines deficient in enzyme activity; the other approach was to develop cell lines capable of growing with such low nicotinamide adenine dinucleotide (NAD) levels so as to effectively limit substrate availability for poly(ADP-ribose) synthesis. The selection strategy for obtaining cells deficient in activity of poly(ADP-ribose) polymerase was based on the ability of this enzyme to deplete cellular NAD in response to high levels of DNA damage. Using this approach, we first obtained cell lines having 37-82% enzyme activity compared to their parental cells. We now report the development and characterization of two cell lines which were obtained from cells having 37% enzyme activity by two additional rounds of further mutagenization and selection procedures. These new cell lines contain 5-11% enzyme activity compared to the parental V79 cells. In pursuit of the second strategy, to obtain cells which limit poly(ADP-ribose) synthesis by substrate restriction, we have now isolated spontaneous mutants from V79 cells which can grow stably in the absence of free nicotinamide or any of its analogs. These cell lines maintain NAD levels in the range of 1.5-3% of that found in their parental V79 cells grown in complete medium. The pathway of NAD biosynthesis in these NAD-deficient cells is not yet known. Further characterization of these lines showed that under conditions that restricted poly(ADP-ribose) synthesis, they all had prolonged doubling times and increased frequencies of sister chromatid exchanges.

Spermidine level and protein synthesis are coregulated in nonproliferating hepatocytes

The relationship between polyamines and the rate of protein synthesis was investigated in non-proliferating cells: primary cultures of adult rat hepatocytes maintained in serum-free media, and treated with dexamethasone or dexamethasone + insulin. During the second day of culture, polyamine biosynthesis became induced along with the rate of protein synthesis. While the activity of ornithine decarboxylase and the intracellular concentration of putrescine increased only transiently and that of spermine declined, the rise of the protein synthetic rate was paralleled by that of the intracellular spermidine concentration. The polyamine analogue diamino-propanol specifically decreased spermidine content and the protein synthetic rate. The intracellular concentration of spermidine was found subject to tight homeostatic regulation, e.g. not being altered by the addition of up to 1 mM of this polyamine to the culture medium. In contrast, addition of putrescine or spermine led to an increase in their respective intracellular concentrations. These findings indicate that spermidine specifically of the polyamines is involved in protein synthesis in the intact hepatocyte. Moreover, spermidine may mediate part of the trophic action of dexamethasone and insulin upon cultured hepatocytes.

NAD+ depletion and cytotoxicity in isolated hepatocytes

Activation of poly(ADP-ribose)polymerase by DNA damaging agents causes a depletion of intracellular NAD+ and subsequent lowering of ATP pools, which if extensive may lead to cell death. We have studied the cytotoxicity to isolated hepatocytes of dimethyl sulphate, a direct-acting carcinogen and mutagen, hydrogen peroxide, generated by glucose/glucose oxidase, and menadione (2-methyl-1,4-naphthoquinone) in relation to their effects on intracellular NAD+ and ATP levels. Both dimethyl sulphate and glucose/glucose oxidase caused a depletion of NAD+, which was apparently due to an activation of poly(ADP-ribose)polymerase as it was prevented by inhibitors of the polymerase, i.e. 3-aminobenzamide and nicotinamide. This protection of intracellular NAD+ was accompanied by a prevention of the cytotoxicity of both dimethyl sulphate and glucose/glucose oxidase, while it did not alter the decrease in intracellular ATP they induced. This apparent dissociation of effects on ATP from NAD+ does not support the suggestion that activation of poly(ADP-ribose)polymerase leads to a decrease in cellular ATP as a consequence of NAD+ depletion. Menadione also caused a depletion of NAD+ which preceded cytotoxicity, but in contrast to dimethyl sulphate and H2O2 this depletion did not involve poly(ADP-ribose)polymerase as it was not prevented by inhibitors of the enzyme. Our results also indicate that the cytotoxicity of menadione is not mediated by H2O2 alone. Marked depletion of intracellular NAD+ prior to toxicity and a protection against toxicity associated with maintenance of NAD+ suggest a possible role for the maintenance of intracellular NAD+ in cellular integrity.

Sodium butyrate preserves aspects of the differentiated phenotype of normal adult rat hepatocytes in culture

We have determined that sodium butyrate and, to a lesser extent, dimethylsulfoxide (DMSO) and 3-aminobenzamide (3-AB) preserve aspects of the differentiated phenotype of primary cultures of adult rat hepatocytes. The histone deacetylase inhibitor, butyrate, inhibits the increase in gamma-glutamyltranspeptidase (GGT) activity and the decrease in basal tyrosine aminotransferase (TAT) activity normally observed when hepatocytes are cultured under appropriate conditions. The effects of butyrate on GGT and TAT activities are accompanied by parallel changes in GGT and TAT mRNA levels. The poly(ADP)ribose-synthetase inhibitor, 3-aminobenzamide, has effects similar to butyrate on GGT activity and mRNA levels, while both 3-AB and DMSO increase basal TAT activity in cultured hepatocytes. Under appropriate conditions all three agents--butyrate, 3-AB, and DMSO--extend the length of time cultured hepatocytes can be maintained as confluent monolayers. However, under all the conditions studied, butyrate extended the length of time hepatocytes could be maintained as monolayers more than any other treatment used. Butyrate-treated hepatocytes maintained ultrastructural features that were more similar to those of hepatocytes in vivo than hepatocytes treated with any other of the agents tested. Histone acetylation levels of primary cultures of adult rat hepatocytes declined concomitant with the loss of the differentiated phenotype of the cells. These results suggest that histone acetylation may play a role in the changes in gene expression observed when hepatocytes are placed in culture.

Hepatotoxicity and molecular aspects of hepatocyte function in primary culture

1. The application of primary cultures of hepatocytes in testing for hepatotoxicity of drugs is reviewed. 2. Hepatotoxicity results principally from the biotransformation of toxic agents. This process is very complex and specific and involves a powerful system of multigenic isozyme families for both phase I and phase II drug metabolizing reactions. Many of the isozymes are specifically expressed in the liver in relation to the maturation or differentiation state, and are specifically induced, possibly through a complex temporally programmed gene regulation. 3. This highly specific, coordinated, molecular regulation is difficult to maintain in vitro. Isolation of hepatocytes induces a prompt differential decline of liver-specific gene transcription, which leads to preferential loss of the most specific functions, including those of the drug metabolizing isozymes, whereas repair of cell damage remains active. 4. The use of serum-free, hormonally defined media stabilizes specific hepatic functions, but not transcriptional activity, for 4-5 days. Defined media retain active DNA replication but do not permit clonal growth of hepatocytes. Co-culturing hepatocytes with primitive biliary cells prolongs cell survival and their functional capacities for several weeks, including some of the transcriptional activity.

3-Aminobenzamide, an inhibitor of poly(ADP-ribose) polymerase, is a stimulator, not an inhibitor, of DNA repair

An inhibitor of poly(ADP-ribose) synthesis, 3-aminobenzamide (3AB), at low concentrations (0.01-0.1 mM) was found to reduce strand-break frequencies and increase repair replication in human lymphoid cells damaged by methyl methanesulfonate. A concentration of 0.1 mM 3AB was adequate to produce a maximum effect on strand-break frequencies and repair replication. This evidence, together with our previous measurements, demonstrates that 3AB cannot be regarded as an inhibitor of DNA repair; rather, it actually accelerates the ligation of DNA repair patches. Previous considerations of 3AB as a repair inhibitor may have derived from the use of excessive concentrations above 1 mM that may have stimulated additional damage and from the use of ethyl alcohol as a solvent for 3AB. Interpretations of the role of single-strand breaks and poly(ADP-ribose) in DNA repair, differentiation, and gene activity may need reevaluation because they have frequently been based on an erroneous notion of 3AB as a repair inhibitor, when its mode of action is, in fact, more complex.

Poly(ADP-ribose) biosynthesis and suicidal NAD+ depletion following carcinogen exposure of mammalian cells

Hepatocytes were found to be remarkably resistant to suicidal NAD+ depletion due to consumption for chromatin-associated poly(ADP-ribose) biosynthesis, which normally follows infliction of DNA damage in mammalian cells. N-methyl-N'-nitro-N-nitrosoguanidine treatment, which depleted NAD+ levels of confluent fibroblasts to about 40% of controls, did not reduce hepatocellular NAD+ pools, although poly(ADP-ribose) concentrations were concomitantly elevated by 21-fold. This differential behavior, demonstrable also with other carcinogens, can be attributed to the different NAD+ biosynthetic capacities of these cells.

A new highly selective physicochemical assay to measure NAD+ in intact cells

A simple, fast, and highly specific chromatographic method for measuring the content of NAD+ in intact cells has been developed. This procedure involves the separation of NAD+ from the bulk of acid-soluble nucleosides, nucleotides, and other pyridine containing molecules by affinity chromatography on dihydroxyboronyl-Bio-Rex. The boronate purified preparations were utilized for the quantification of NAD+ by strong anion exchange high-pressure liquid chromatography under isocratic conditions using a low salt buffer system. The overall recovery of the method exceeded 80%. This new method was applied to determine the extent of NAD+ consumption in intact hepatocytes following treatment with two different DNA damaging agents. A major advantage of this method is that it allows for the simultaneous determination of poly(ADP-ribose) in the acid-insoluble fraction of the same sample.

Poly(ADP-ribose) synthesis and inhibition of DNA synthesis in Chinese hamster cells treated with methylating agents and thymidine

A possible role of poly(ADP-ribose) synthesis in modulating the response of V79 cells to DNA damage induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and methyl methanesulfonate (MMS) was investigated. Inhibition of [3H]thymidine (dThd) incorporation into DNA and lowering of NAD+ levels in intact cells were employed as parameters of DNA-synthesis inhibition and poly(ADP-ribose) synthesis, respectively. Dose responses of these parameters were studied in cells 2 and 24 h after treatment with the methylating agents in medium with or without dThd. The initial inhibition of DNA synthesis was uniformly associated with stimulation of poly(ADP-ribose) synthesis whether the cells were treated with MNNG or MMS, incubated with or without 20 microM dThd which did not inhibit poly(ADP-ribose) synthesis, or incubated with 3 mM dThd which did inhibit the latter synthesis. By contrast, the DNA-synthesis inhibition detected 24 h after treatment with MNNG was not associated with poly(ADP-ribose) synthesis. These data suggest that (i) the mechanism of this later inhibition of DNA synthesis is different from that of the initial inhibition, (ii) DNA-synthesis inhibition does not stimulate poly(ADP-ribose) synthesis, and (iii) single-strand breaks, resulting from N-methylation of the DNA, stimulate poly(ADP-ribose) synthesis, which may produce the initial inhibition of DNA synthesis. The initial inhibition of DNA synthesis was not uniformly associated with mutagenesis and dThd facilitation of MNNG-induced cytotoxicity and mutagenesis. This indicates that O-methylation of DNA does not stimulate poly(ADP-ribose) synthesis. Our data suggest that, in V79 cells treated with methylating agents, poly(ADP-ribose) synthesis is stimulated by single-strand breaks, inhibits DNA synthesis, and thereby serves to allow time for repair of the DNA prior to replication.

Tumor-promoting barbiturates act on DNA repair of cultured hepatocytes

We have observed that two promoters of liver carcinogenesis, i.e. phenobarbital and barbital, markedly increase DNA-repair synthesis of cultured hepatocytes following treatment with the ultimate carcinogens methyl methanesulfonate, N-methyl-N'-nitro-N-nitrosoguanidine, N-acetoxy-2-acetylaminofluorene, and UV light of 254 nm. Phenobarbital also increased the incorporation rates of deoxynucleoside triphosphates into nuclear DNA of permeabilized hepatocytes following carcinogen treatment. The action of these barbiturates apparently correlates with their potential to promote hepatocarcinogenesis in vivo, since the non-promoting agent barbituric acid did not modify carcinogen-induced repair synthesis. Moreover, the mechanisms of action of tumor-promoting barbiturates is different from the known enhancing action on repair synthesis of inhibitors of nuclear poly(ADP-ribose) biosynthesis.

Isolation of ADP-ribosyltransferase by affinity chromatography

An affinity adsorbent for ADP-ribosyltransferase (EC 2.4.2.30) has been synthesized by coupling 3-aminobenzamide to Sepharose 4B. Using this material, ADP-ribosyltransferase from human placenta has been purified from crude extract to homogeneity within a few hours. The enzyme has an apparent Km for NAD+ of 52 microM. Its molecular mass is 115,000 as determined by gel electrophoresis. The enzyme is DNA dependent and stimulated by histone, its temperature optimum is at 25 degrees C, and its pH optimum is around pH 9. alpha-NAD+, thymidine, caffeine, theophylline, theobromine, 3-methoxybenzamide, and nicotinamide inhibit the enzyme. Purification of ADP-ribosyltransferases from horse, rat, and chicken liver was also achieved with the method described.

Effects of hyperthermia and nicotinamide on DNA repair synthesis, ADP-ribosyl transferase activity, NAD+ and ATP pools, and cytotoxicity in gamma-irradiated human mononuclear leukocytes

Effects of hyperthermia and nicotinamide on ADP-ribosyl transferase activity (ADPRT), unscheduled DNA synthesis (UDS), NAD+- and ATP-pools and cytotoxicity were investigated in gamma-irradiated human mononuclear leukocytes. A significant decrease in radiation-induced UDS after heat treatment for 45 min was found. Nicotinamide increased the UDS levels in irradiated cells, but no effect of hyperthermia on these increased UDS values was observed. In the presence of 2 mM nicotinamide radiation-induced ADPRT activity was reduced to about 50 per cent. However, hyperthermia for 45 min was found to have no effect on the enzyme activity for temperatures below 46 degrees C. Nicotinamide increased the NAD+ pool in unirradiated cells. Damaging the cells with gamma-radiation leads to a severe depletion of the NAD+ pool. The NAD+ pool is restored, however, if the cells repair for 5 h at 37 degrees C. When radiation-damaged cells were treated with hyperthermia, exogenously supplied nicotinamide could not be converted to NAD+ in sufficient amounts to prevent NAD+ depletion. These data indicate that the radiosensitizing effect of heat and nicotinamide could both be explained by effects on the enzyme ADPRT, i.e. nicotinamide by directly blocking the enzyme and hyperthermia by limiting the co-substrate (NAD+).

Cellular regulation of poly(ADP) ribosylation of proteins. I. Comparison of hepatocytes, cultured cells and liver nuclei and the influence of varying concentrations of NAD

The in vitro rates (vinit) of poly(ADP-ribose) polymerase of permeabilized rat hepatocytes and of nuclei, isolated from hepatocytes, did not differ significantly. Incubation beyond 3 min resulted in diminished poly(ADP) ribosylation in hepatocytes compared with nuclei, coinciding with high rates of plasma membrane-associated NAD-glycohydrolase. Cultured cells (Drosophila Kc cells, gliosarcoma 9L, human fibroblasts and mouse spleen lymphocytes) exhibit variations of NAD-glycohydrolase and poly(ADP-ribose) polymerase activities and the assessment of poly(ADP-ribose) polymerase activity in permeabilized cells requires simultaneous assay of NAD-glycohydrolase. In rat liver nuclei during 10 min incubation with 500 microM NAD, 40% of NAD is consumed, 10% ADP-ribose is bound to proteins, and 20% ADP-ribose, 5% AMP and 2.7% adenosine are liberated. As determined by solvent partitioning (Jackowski, G & Kun, E, J biol chem 258 (1983) 12587) [1], the phenol-soluble protein-ADP-ribose fraction represents largely mono(ADP)-ribose protein adducts, whereas the H2O-soluble phase contains poly(ADP)-ribosylated proteins. The quantity of ADP-ribose protein adducts, the chain length of oligomers and the nature of apparent acceptor proteins in liver nuclei vary significantly with the concentration of NAD as substrate. At 500 microM NAD concentration the quantity of ADP-ribose containing adducts was in the nmol per mg DNA range, the polymers are long chains and the acceptor proteins predominantly non-histone proteins. At 0.1 microM NAD as substrate pmol quantities of monomeric ADP-ribose adducts per mg DNA were formed and the main acceptors were sharply discernable on the basis of molecular mass as histones, high mobility non-histone proteins, two protein groups of a mass of 66 and 44 kD respectively, and the poly(ADP-ribose) polymerase enzyme protein of 119 kD mass. Whereas products in the presence of 0.1 microM NAD may indicate acceptors of highest reactivity, protein adducts formed in the presence of 500 microM NAD resemble a pattern found in vivo.

Poly(ADP-ribose) synthesis is involved in the toxic effects of alkylating agents but does not regulate DNA repair

Poly(ADP-ribose) is a nuclear polymer that is synthesized in response to DNA-strand breaks and covalently modifies numerous nuclear proteins. Inhibition of poly(ADP-ribose) polymerase by 3-amino-benzamide in cells exposed to DNA-damaging agents has a variety of cellular effects, including increases in cell killing, frequency of single-strand breaks, repair replication, and sister-chromatid exchange. These increases have been interpreted as an indication that poly(ADP-ribose) polymerization regulates the rate of ligation. Because of slow ligation, continued repair polymerization should therefore generate longer repair patches. Direct measurement of the rate of ligation of intracellular repair patches and of the size of repair patches indicates that they are unchanged when poly(ADP-ribose) polymerization is inhibited. We therefore conclude that poly(ADP-ribose) does not regulate the ligation stage of repair but instead may regulate the activity of intracellular nucleases and other enzymes that can cause additional DNA damage and changes in chromatin structure.

Induction of DNA damage and repair synthesis in freshly isolated rat hepatocytes by the proallatocidin precocene II

The proallatocidin precocene II (6,7-dimethoxy-2,2-dimethyl-2H-benzo-[b]pyran) has previously been shown to induce centrolobular liver necrosis. Here we have examined the ability of precocene II to produce DNA damage in suspensions of freshly isolated rat hepatocytes using the alkaline elution technique with N-methyl-N'-nitro-N-nitrosoguanidine as a positive control. At concentrations (10(-4)-10(-5) M) which did not induce cytotoxicity as judged by the leakage of glutamic-oxaloacetic transaminase, precocene II was capable of producing DNA single-strand breaks. In addition, a dose-dependent DNA repair synthesis (unscheduled DNA synthesis, UDS) was detected in hepatocytes exposed to precocene II. The induction of UDS was measured by incorporation of [3H]thymidine into purified hepatic DNA via a membrane filter retention method and liquid scintillation counting. Hence, results obtained in the present study indicate the potential genotoxicity of precocene II and the utility of DNA damage and repair assays in genetic toxicology.