Determination of 2-acetylaminofluorene adducts by immunoassay

Antisera elicited in rabbits were used in radioimmunoassay (RIA) and enzyme-linked immunosorbent assay (ELISA) to determine femtomole quantities of deoxyguanosin-(8-yl)-acetylaminofluorene (dg-8-AAF) and deoxyguanosin-(8-yl)-aminofluorene (dg-8-AF). These adducts have been monitored in liver and kidney DNA of male Wistar-Furth rats fed 0.02% or 0.04% 2-acetylaminofluorene (2-AAF) either continuously or for a limited time followed by an interval on control diet. After 24 hr of 0.02% 2-AAF feeding, substantial levels of binding (80 fmole/mug DNA) were observed in liver DNA and increased with time, reaching a plateau of approximately 230 fmole/mug DNA at 30 days and thereafter. During the first week of continuous feeding about 80% of the total C-8 adducts in the liver DNA were deacetylated (dG-8-AF). By 25-60 days, dG-8-AF represented 97-100% of all C-8 adducts as measured by RIA and confirmed by HPLC. Values for C-8 adduct formation in kidney DNA were severalfold lower than in liver and dG-8-AF represented >90% of C-8 adducts at all times studied. In removal or repair experiments, rats were fed 2-AAF for 3, 7 or 28 days, the 2-AAF diet was discontinued and the liver adducts assayed after intervals on control diet. When dietary 2-AAF administration was for 3 or 7 days, removal of adducts was efficient and almost complete by 28 days on control diet, with preferential retention of dG-8-AF. However, when dietary 2-AAF administration was for 28 days, adduct levels were higher, the repair capacity was saturated and the removal of C-8 adducts was not complete after control diet for a 28-day interval. In a preliminary experiment when [(3)H]-2-AAF was fed for 3 days, after 25 days of 0.02% 2-AAF, the rates of newly formed adduct formation and removal were similar to those observed for the initial 3 days of 2-AAF feeding. These results demonstrate the predominance and persistence of dG-8-AF in liver and kidney DNA of 2-AAF-fed rats and suggest that the repair capacity of the whole rat liver was not diminished after 1 month of 2-AAF feeding.

Antibodies elicited against cis-diamminedichloroplatinum(II)-modified DNA are specific for cis-diamminedichloroplatinum(II)-DNA adducts formed in vivo and in vitro

Rabbit antiserum elicited against calf thymus DNA modified to 4.4% (Pt drug/nucleotide ratio = 0.044) with the antitumor drug cis-diamminedichloroplatinum(II) (cis-DDP) contains antibodies specific for the Pt-modified DNA immunogen as well as for Pt-DNA adducts formed in both cultured mouse leukemia L1210 cells and in L1210 cells from the ascites fluid of tumor-bearing mice exposed to cis-DDP. Pt-modified DNA was electrostatically complexed to methylated bovine serum albumin and injected into rabbits. Early bleedings of the derived antiserum were used to establish a competitive enzyme-linked immunosorbent assay (ELISA), which demonstrated specificity for the Pt-modified DNA but not for DNA or the Pt drug alone. In the ELISA, 50% inhibition occurred at a concentration of 0.5 nM Pt (on DNA) as determined by atomic absorption spectroscopy. This value corresponds to a lower limit of detectability of one adduct in 10(7) nucleotides, with 50 micrograms of sample DNA added per microtiter well. DNA isolated from cultured mouse L1210 cells exposed to increasing doses of the Pt drug was found by ELISA to contain from 0.2 to 10.0 fmol of Pt adduct per microgram of DNA. These levels remained stable for up to 4 hr after a 1-hr drug treatment, during which time DNA interstrand crosslinks developed. Thus, the antiserum appears not to be specific for DNA interstrand crosslinks. DNAs from L1210 cells exposed to trans-diamminedichloroplatinum(II) and L-phenylalanine mustard were not recognized in the ELISA. DNA prepared from the ascites cells of mice bearing the L1210 tumor 5 hr after injection of cis-DDP was found to contain about 2 fmol of Pt per microgram of DNA. This work establishes that cis-DDP-DNA adducts prepared in vitro are relevant to the in vivo binding of the Pt drug to its biological target, DNA, and opens new avenues for studying the mechanism of action of the Pt anticancer drugs.

Formation and removal of (guan-8-yl)-DNA-2-acetylaminofluorene adducts in liver and kidney of male rats given dietary 2-acetylaminofluorene

Radioimmunoassay has been used to measure acetylated and deacetylated deoxyguanosine C-8 DNA adducts of 2-acetylaminofluorene in liver and kidney DNA of male Wistar-Furth rats fed a dietary regimen of 0.02 or 0.04% 2-acetylaminofluorene. When adduct formation was monitored during continuous feeding up to 60 days, substantial levels of binding (80 fmol/microgram DNA) were observed in liver by 1 day, and maximum steady-state values averaging 230 fmol/microgram DNA were reached by 30 days. Initially, during the continuous feeding, about 80% of the total C-8 adducts were deacetylated [N-deoxyguanosin(8-yl)aminofluorene], and this proportion increased to about 97% by 15 and 30 days of administration of 0.04 and 0.02% 2-acetylaminofluorene diets, respectively. Levels of C-8 adducts bound to kidney DNA in the same animals averaged 10 to 15% of the liver values with greater than or equal to 80% of these adducts in the deacetylated form. In separate experiments, rats were exposed to 2-acetylaminofluorene for 3, 7, 28, and 112 days; the carcinogen-containing diet was discontinued; and C-8 adducts were monitored during 1, 7, and 28 days of feeding control diet. At both carcinogen doses, after dietary administration for 3 or 7 days, there was a rapid decrease in liver C-8 adducts so that, after 28 days on control diet, 65 to 90% of the original adducts were no longer present in the DNA. In contrast, in animals fed 0.02% 2-acetylaminofluorene for 28 days, there were high levels (70 to 100% of the original C-8 adduct) remaining on the DNA at the end of 28 days on control diet. This apparent loss in capacity for removal of C-8 adducts is discussed in relation to biological and biochemical changes induced in the rat liver during 2-acetylaminofluorene hepatocarcinogens.

Indirect immunofluorescent localization of benzo[a]pyrene adducted to nucleic acids in cultured mouse keratinocyte nuclei

The localization of benzo[a]pyrene-deoxyguanosine adducts was studied by indirect immunofluorescence in cultured BALB/c epidermal cells exposed to (+/-) 7 alpha, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (the anti-isomer) utilizing an antiserum specific for the major benzo[a]pyrene-deoxyguanosine adduct in DNA. This antiserum does not cross-react with benzo[a]pyrene or DNA alone. When cultured keratinocytes were incubated with the carcinogen for 1 h, the immunofluorescence was localized in the nucleus as intense spots on a background of diffuse fluorescence. Fluorescence was absent from cells not exposed to carcinogen and from carcinogen-exposed cells incubated with normal rabbit serum in place of the antiserum. Fluorescence was abolished when the specific antiserum was absorbed with the immunogen DNA prior to incubation with cells, and substantially diminished when exposed cells were preincubated with deoxyribonuclease before the application of the specific antiserum. Incubation of exposed cells with ribonuclease prior to incubation with the specific antiserum removed the bright fluorescent spots and resulted in fluorescent nuclei containing dark sports in similar frequency. Dose-response studies in which benzo[a]pyrene-deoxyguanosine adducts were quantified by enzyme-linked immunosorbent assay and compared with intensity of immunofluorescence demonstrated that decreasing doses of the carcinogen resulted in fewer numbers of adducts as well as proportionally less fluorescence. When cells were exposed to non-toxic doses of the activated carcinogen for 1 h, nuclear fluorescence was detectable in immediately-fixed cells but faded to non-detectable levels when cells were washed and cultured for an additional 24-48 h before fixation.

Detection and quantitation of acetylated and deacetylated N-2-fluorenylacetamide-DNA adducts by radioimmunoassay

Antibody raised in rabbits has been used for the detection of picomole quantities of the major adducts formed upon interaction of activated N-acetoxy-2-fluorenylacetamide (N-AcO-2-FAA) with DNA. By radioimmunoassay the quantitation of N(deoxyguanosin-8-yl)-2-FAA and N(deoxyguanosin-8-yl)-2-fluorenamine and discernment of the proportion of each in a mixture are possible. The antibody does not recognize the minor adduct 3-(deoxyguanosin-N(2)-yl)-2-FAA, 2-FAA, or DNA and is therefore specific for the acetylated and deacetylated C-8 adducts. We used the radioimmunoassay to detect and quantitate these adducts in DNA from several types of cultured cells exposed to 10(-5) M N-AcO-2-FAA. Levels of bound C-8 adducts varied between 100 to 200 fmol/micrograms DNA for all cells investigated. In all cells except primary rat hepatocytes, 95-97% of the C-8 adducts were in the deacetylated form, but in the rat hepatocytes, 80% of the C-8 adducts were acetylated. Our attempts to manipulate the amount and proportion of C-8 adducts bound to the DNA of primary BALB/c epidermal cells met with success in two areas. When cells were exposed to the carcinogen in the absence of serum, total binding and the percentage of acetylation were increased twofold to threefold. Also, in the presence of paraoxon, 99% of the binding and the formation of all deacetylated C-8 adducts were inhibited. We also used the radioimmunoassay to monitor repair of C-8 adducts from the DNA of BALB/c epidermal cells and normal human fibroblasts (YDF line) for 24 hours after removal of the carcinogen-containing culture medium. During this interval, the BALB/c epidermal cells and YDF cells removed approximately 40 and 50%, respectively, of the C-8 adducts from the DNA. These studies demonstrated that carcinogen-DNA adduct antibodies are useful for determining specific adducts in investigations related to aromatic amine carcinogenesis.

Measurement of benzo(a)pyrene-DNA adducts by enzyme immunoassays and radioimmunoassay

Ultrasensitive enzymatic radioimmunoassay (USERIA) was compared to radioimmunoassay and enzyme-linked immunosorbent assay in determining the amount of benzo(a)pyrene [B(a)P] metabolite covalently bound to guanine in DNA. In the USERIA approach, DNA either with or without B(a)P metabolite modification was adsorbed on the wells of microtiter plates, and rabbit antiserum to B(a)P metabolite-modified DNA [B(a)P-DNA] was then added. Antibodies reacted specifically with the B(a)P-DNA attached to the surface of the plate. After reaction between goat anti-rabbit IgG conjugated to alkaline phosphatase and rabbit IgG bound to the solid phase, this specific antigen-antibody reaction was enzymatically amplified by alkaline phosphatase conversion of [3H]adenosine 5'-monophosphate to [3H]adenosine. Following chromatographic separation from [3H]adenosine5'-monophosphate, [3H]adenosine was measured by liquid scintillation counting. The amount of [3H]-adenosine formed was linearly related to the amount of B(a)P-DNA in 10 ng DNA attached to the solid phase. As little as 3 fmol of bound B(a)P metabolite can be detected by noncompetitive USERIA, while 10 fmol of the adducts in 25 microgram DNA [1 B(a)P-DNA adduct per 7 X 10(6) nucleotides] can be measured by the competitive USERIA approach. Under our standard competitive procedure with 1 microgram DNA in the antigen-antibody reaction mixture, USERIA is approximately 500-fold more sensitive than radioimmunoassay and 5-fold more sensitive than enzyme-linked immunosorbent assay for the detection of B(a)P-DNA adducts. These highly sensitive assays should be extremely useful for studies in DNA damage and repair by B(a)P metabolites as well as in studies on environmental exposure to this ubiquitous carcinogen.

Ultrasensitive enzymatic radioimmunoassay (USERIA) detects femtomoles of acetylaminofluorene-DNA adducts

USERIA is a modification of radioimmunoassay and enzyme-liked immunosorbent assay. When compared to these assays, USERIA is 60- and 10-fold, respectively, more sensitive for the detection of adducts in DNA modified by the chemical carcinogen, 2-acetylaminofluorene (AAF). The specific antigen-antibody reaction is enzymatically amplified by an anti-IgG alkaline phosphatase conjugate which converts the substrate, [3H]adenosine-5'-monophosphate, to [3H]adenosine. As few as 2 fmol of AAF-DNA adducts can be detected by a competitive USERIA assay and less than 3 fmol of the specific adduct, N-(deoxyguanosine-8-yl)-AAF, can be detected using a non-competitive USERIA approach. The sensitivity of USERIA should aid efforts to measure carcinogen-DNA adducts in biological specimen samples from humans and experimental animals.

Specific quantification of mouse and human keratin proteins by radioimmunoassay

Mouse epidermal keratin proteins were purified and labelled with 125I by chemical techniques. A radioimmunoassay method was established with a rabbit antibody elicited against the mouse keratin. This assay method was utilized to quantify keratin proteins of mouse and human epidermal extracts, both from intact tissues and cells in culture. As little as 30 ng of mouse keratin (0.5 pmol) was quantifiable.

Quantitation of benzo(a)pyrene-deoxyguanosine adducts by radioimmunoassay

Calf thymus DNA was modified with the benzo(a)pyrene (BP) derivative, (+/-)7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene [(+/-) BPDE I], under conditions which yielded greater than 99% of the binding product in the form of trans-(7R)-N2-(10[7 beta,8 alpha,9 alpha-trihydroxy-7,8,9,10-tetrahydrobenzo(a)pyrene]yl) deoxyguanosine. Rabbits were immunized with modified DNA coupled to methylated bovine serum albumin, and the resulting antiserum was utilized in a competition radioimmunoassay for the quantitation of products of BP covalently bound to DNA. The antiserum was specific for both native and denatured immunogen DNA's as well as for the major isolated BP binding product, but it did not recognize BP, the tetrol of (+/-)BPDE I, or unmodified deoxyguanosine. The modified DNA was assayed in quantities as low as 2 pmol of adduct, a sensitivity sufficient to quantitate the extent of modification of cellular DNA when epidermal cell cultures were exposed either to BP or to (+/-)BPDE I. High-pressure liquid chromatographic analysis of DNA hydrolsates, obtained from epidermal cells exposed to BP or to (+/-)BPDE I, indicated that the major adduct was the same as than on the immunogen DNA. This approach should prove valuable for further studies on the mechanism of carcinogenesis and for monitoring human exposure to this ubiquitous carcinogen.

Formation and removal of specific acetylaminofluorene-DNA adducts in mouse and human cells measured by radioimmunoassay

Rabbit antiserum prepared against N-(guanosin-8-yl)-acetylaminofluorene was utilized in radioimmunoassay to detect formation and removal of C-8 adducts from the DNA of cultured cells exposed to N-acetoxy-2-acetylaminoflorene. The assay was able to quantitate both acetylated and deacetylated C-8 adducts between 0.5 and 5 pmol while the N2 adduct, 3-(deoxyguanosin-N2-yl)acetylaminofluorene, was not detected below 160 pmol. By varying the proportions of acetylated and deacetylated C-8 adducts in the radioimmunoassay, a series of standard curves were developed from which the relative proportion of each adduct could be determined in unknown mixtures. DNA from mouse epidermal cells and human skin fibroblasts exposed to N-acetoxy-2-acetylaminofluorene in culture contained only 3 and 5% respectively, of the C-8 adduct in the acetylated form. Quantitation by radioimmunoassay of total C-8 adducts bound to DNA yielded values approximately 25% lower than total carcinogen binding determined by radiolabeling. When removal of C-8 adducts was followed over a 23-hr, carcinogen-free culture period, mouse and human cells removed 40 and 50%, respectively, of bound acetylated and deacetylated C-8 adducts. These studies demonstrate the versatility of radioimmunoassay as a molecular probe for studies of chemical carcinogens.

Subunit structure of the mouse epidermal keratin filament

The two proteins which are the subunits of mouse epidermal keratin filaments have been isolated from fully differentiated epidermis (stratum corneum), viable differentiating cells and cells grown in culture. The proteins have molecular weights of 68 000 and 60 000, consist of families of very similar species, have common N-terminal (N-acetylserine) and C-terminal (glycine) residues, contain 35--40% alpha-helix and are immunologically cross-reacting. In mixtures, the two proteins polymerize in vitro into native-type keratin filaments that are 70--80 angstrom in diameter, up to 30 micrograms long, possess a characteristic alpha-type X-ray diffraction pattern and contain the subunits in the precise molar ratio of 1 : 2 or 2 : 1.

Nonspecific inhibition of DNA repair synthesis by tumor promoters in human diploid fibroblasts damaged with N-acetoxy-2-acetylaminofluorene

The effects of selected tumor-promoting agents and their nonpromoting analogs on DNA repair synthesis were examined in human diploid fibroblasts (WI-38) damaged with N-acetoxy-2-acetylaminofluorene. Over a range of doses, three promoters (croton oil, 12-O-tetradecanoylphorbol-13-acetate, and anthralin) were found to inhibit DNA repair synthesis while their nonpromoting analogs (phorbol and 1,8-dihydroxyanthraquinone) had little effect. Another tumor promoter, phenol, inhibited DNA repair synthesis only at very high concentrations while an analog, 4-nitrophenol, produced inhibition of DNA repair synthesis at molar concentrations at which phenol had no effect. To investigate the specificity of this phenomenon, the effects of these agents on DNA-replicative synthesis, RNA synthesis, protein synthesis, and cell morphology were evaluated. At equimolar concentrations, tumor promoters were found to inhibit DNA-replicative synthesis as effectively as repair synthesis. RNA and protein synthesis were similarly inhibited over the same range of concentrations. Extensive morphological changes, interpreted as evidence of toxicity, were seen at concentrations of promoters that inhibited the macromolecular syntheses studied. The nonpromoting analogs, with the exception of nitrophenol, had little effect on these processes and showed only slight morphological damage. Thus tumor-promoting agents appeared to inhibit a number of macromolecular synthetic events, including DNA repair synthesis. It is suggested that the effect of tumor promoters on DNA repair synthesis is part of a general response to cellular injury rather than a selective response involving a single metabolic pathway. Furthermore, it is unlikely that the inhibition of repair synthesis represents the major mode of action of promoting agents in the carcinogenic process.

Intragenomal distribution of DNA repair synthesis: repair in satellite and mainband DNA in cultured mouse cells

DNA repair synthesis was examined in mouse satellite and mainband DNA derived from confluent Balb/c 3T3 cells damaged with ultraviolet radiation or N-acetoxy-2-acetylaminofluorene. Two different approaches were used: (i) Contact-inhibited cells were treated with hydroxyurea to reduce replicative synthesis to low levels; and (ii) bromodeoxyuridine was used to label newly replicated DNA in cells that had escaped contact inhibition. DNA was separated into mainband and satellite fractions in Ag(+)-Cs(2)SO(4) gradients. After treatment with either ultraviolet radiation or N-acetoxy-2-acetylaminofluorene, repair synthesis occurred to the same extent in mainband and satellite DNA. Repair synthesis increased over an ultraviolet radiation dose range of 30-200 erg/mm(2), and the extent of repair in the two DNA species was similar at each dose level. An analysis of the separated strands of satellite DNA from ultraviolet-irradiated cells indicated that the extent of repair is closely correlated with the availability of pyrimidines for cyclobutyl dimer formation and provided evidence that repair synthesis occurs at the site of damage. Within the precision of our experiments the results suggest that at least one group of highly repetitive, nontranslated DNA sequences is repaired to about the same extent as the rest of the genome.