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.

Inhibition of DNA synthesis by an electrophilic metabolite of benzo[a]pyrene.

Mitogen-stimulated scheduled DNA synthesis and DNA excision repair in human lymphocytes, as well as DNA polymerase a activity in a cell-free system, were inhibited by an electrophilic metabolite of benzo[a]pyrene. This metabolite, (+/-)-anti-(7r,8t)-dihydroxy-(9,10t)-epoxy-7,8,9,10-tetrahyd robenzo[a]pyrene (BPDE), covalently binds to cellular macromolecules and is mutagenic, carcinogenic, and cytotoxic. Human lymphocytes treated with BPDE at concentrations greater than 500-800 ng/ml showed decreases in both mitogen-stimulated DNA synthesis and excision repair of damaged DNA but did not exhibit overt cytotoxicity (excluded trypan blue and maintained an adenylate charge of greater than 0.7). Formation of, and total concentration of, BPDE-DNA adducts was not correlated with inhibition of DNA synthesis. DNA polymerase alpha studies using a cell-free system showed that enzymatic activity was not diminished when purified polymerase was treated with BPDE prior to the addition of template DNA. When the template DNA concentration was varied, BPDE inhibition of enzyme activity was uncompetitive. BPDE inhibition of enzyme activity was found to be noncompetitive when concentrations of dATP, dCTP, or dTTP were varied and competitive when the concentration of dGTP was varied. The data indicate that BPDE competitively inhibits interaction of dGTP with the template-DNA polymerase alpha complex.

Benzo[a]pyrene uptake by lymph: a possible transport mode for immunosuppressive chemicals.

Benzo[a]pyrene, a lipophilic promutagen, reached maximal concentrations in the thoracic duct lymphatic circulation within 2 h after gastric instillation. Benzo[a]pyrene in lymph obtained by thoracic duct cannulation decreased to approximately control levels within 4 h after treatment. When lymph was not allowed to enter the blood vascular circulation, serum levels of benzo[a]pyrene increased very slowly, suggesting minimal mesenteric blood vascular absorption of the lipophilic hydrocarbon. Benzo[a]pyrene partitions into lymph lipoproteins as a function of the lipoprotein concentration. Data suggest that low-density lipoproteins may take up benzo[a]pyrene more efficiently than do very low-density or high-density lipoproteins, and that lymph components other than lipoproteins do not take up and transport benzo[a]pyrene. We propose that lipophilic xenobiotic compounds interact with cells of the immune system via lymphatic lipoprotein transport of potentially mutagenic, carcinogenic, or immunosuppressive agents.

Human lymphocytes treated with r-7,t-8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene require low-density lipoproteins for DNA excision repair.

Human lymphocytes which were non-mitogen-stimulated, and which were depleted of lipoproteins, were found to be deficient in DNA excision repair typically initiated in these cells in response to treatment with a direct-acting polynuclear aromatic hydrocarbon carcinogen. Lymphocytes either depleted of lipoproteins or supplemented with human low-density lipoproteins formed DNA-carcinogen adducts which were not chromatographically distinguishable. The state of lipoprotein depletion did not alter lymphocyte uptake of thymidine from the medium. Lymphocytes which were depleted of lipoproteins, treated with carcinogen, and subsequently supplemented with low-density lipoproteins, regained the ability to engage in DNA excision repair. The data suggest that either low-density lipoprotein(s), or a component(s) of low-density lipoprotein(s), is required by human lymphocytes in order to initiate excision repair of carcinogen-damaged DNA.

Binding of benzo[a]pyrene and intracellular transport of a bound electrophilic benzo[a]pyrene metabolite by lipoproteins.

Human serum lipoproteins were isolated by means of size exclusion h.p.l.c. Non-covalent uptake of [3H]benzo[a]pyrene was quantitated for fractions collected from the effluent of a liquid chromatographic separation of human serum, and was found to directly correlate with the lipoprotein concentration. An electrophilic benzo[a]pyrene metabolite, [3H]trans 7,8-dihydrodiol-9,10-epoxybenzo[a]pyrene, non-covalently associated with low density lipoproteins was transferred to human lymphocytes in vitro and bound acid-precipitable nucleic acids of the lymphocytes as a function of time. Benzo[a]pyrene metabolite binding to lymphocyte DNA was demonstrated by means of CsCl density gradient analysis. Non-mitogen-stimulated lymphocytes exposed to very low concentrations of carcinogen in the presence of low density lipoprotein demonstrated [3H]thymidine incorporation; without the concomitant addition of low density lipoprotein the low concentrations of carcinogen did not stimulate [3H]thymidine incorporation.

Reduction of nicotinamide adenine dinucleotide levels by ultimate carcinogens in human lymphocytes.

The effect of several classes of DNA-damaging chemicals and closely related compounds on cellular nicotinamide adenine dinucleotide (NAD) levels was studied in freshly isolated peripheral human lymphocytes. Of the 21 compounds examined, 7 were direct DNA-damaging agents and 14 were non-DNA-damaging compounds or required metabolic activation to casue DNA damage. Rapid lowering of cellular NAD levels was caused by each of the direct DNA-damaging chemicals examined in this study including N-methyl-N'-nitr-N-nitrosoguanidine, methyl methanesulfonate, N-acetoxy-2-acetylaminofluorene, 7-bromomethylbenz(a)anthracene, and the benzo(a)pyrene derivatives, r-7,t-8-dihydroxy-9, 10-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene and benzo(a)pyrene-4,5-epoxide. The indirect-acting carcinogen 2-acetylaminofluorene, 13 polycyclic aromatic hydrocarbons, and derivatives that were non-DNA-damaging did not cause lowering of NAD. The results suggest a general correlation between DNA damage and acute lowering of cellular NAD pools.