Monoclonal Antibodies for the Detection of a Specific Cyclic DNA Adduct Derived from ω-6 Polyunsaturated Fatty Acids.

Lipid peroxidation of polyunsaturated fatty acids (PUFAs) is an endogenous source of α,ß-unsaturated aldehydes that react with DNA producing a variety of cyclic adducts. The mutagenic cyclic adducts, specifically those derived from oxidation of ω-6 PUFAs, may contribute to the cancer promoting activities associated with ω-6 PUFAs. ( E)-4-Hydroxy-2-nonenal (HNE) is a unique product of ω-6 PUFAs oxidation. HNE reacts with deoxyguanosine (dG) yielding mutagenic 1, N2-propanodeoxyguanosine adducts (HNE-dG). Earlier studies showed HNE can also be oxidized to its epoxide (EH), and EH can react with deoxyadenosine (dA) forming the well-studied εdA and the substituted etheno adducts. Using a liquid chromatography-based tandem mass spectroscopic (LC-MS/MS) method, we previously reported the detection of EH-derived 7-(1',2'-dihydroxyheptyl)-1, N6-ethenodeoxyadenosine (DHHεdA) as a novel endogenous background adduct in DNA from rodent and human tissues. The formation, repair, and mutagenicity of DHHεdA and its biological consequences in cells have not been investigated. To understand the roles of DHHεdA in carcinogenesis, it is important to develop an immuno-based assay to detect DHHεdA in cells and tissues. In this study we describe the development of monoclonal antibodies specifically against DHHεdA and its application to detect DHHεdA in human cells.

An endogenous DNA adduct as a prognostic biomarker for hepatocarcinogenesis and its prevention by Theaphenon E in mice.

Hepatocellular carcinoma (HCC) is the third leading cause of cancer-related deaths worldwide, mainly because of its poor prognosis. A valid mechanism-based prognostic biomarker is urgently needed. γ-hydroxy-1,N2 -propanodeoxyguanosine (γ-OHPdG) is an endogenously formed mutagenic DNA adduct derived from lipid peroxidation. We examined the relationship of γ-OHPdG with hepatocarcinogenesis in two animal models and its potential role as a prognostic biomarker for recurrence in HCC patients. Bioassays were conducted in xeroderma pigmentosum group A knockout mice and diethylnitrosamine-injected mice, both prone to HCC development. γ-OHPdG levels in the livers of these animals were determined. The effects of antioxidant treatments on γ-OHPdG and hepatocarcinogenesis were examined. Using two independent sets of HCC specimens from patients, we examined the relationship between γ-OHPdG and survival or recurrence-free survival. γ-OHPdG levels in liver DNA showed an age-dependent increase and consistently correlated with HCC development in all three animal models. Theaphenon E treatment significantly decreased γ-OHPdG levels in the liver DNA of xeroderma pigmentosum group A knockout mice and remarkably reduced HCC incidence in these mice to 14% from 100% in the controls. It also effectively inhibited HCC development in the diethylnitrosamine-injected mice. Using clinical samples from two groups of patients, our study revealed that higher levels of γ-OHPdG are strongly associated with low survival (P < 0.0001) and low recurrence-free survival (P = 0.007). CONCLUSION: These results support γ-OHPdG as a mechanism-based, biologically relevant biomarker for predicting the risk of HCC and its recurrence. (Hepatology 2018;67:159-170).

Aldo-keto reductase 1B10 protects human colon cells from DNA damage induced by electrophilic carbonyl compounds.

Electrophilic carbonyl compounds are highly cytotoxic and genotoxic. Aldo-keto reductase 1B10 (AKR1B10) is an enzyme catalyzing reduction of carbonyl compounds to less toxic alcoholic forms. This study presents novel evidence that AKR1B10 protects colon cells from DNA damage induced by electrophilic carbonyl compounds. AKR1B10 is specifically expressed in epithelial cells of the human colon, but this study found that AKR1B10 expression was lost or markedly diminished in colorectal cancer, precancerous tissues, and a notable portion of normal adjacent tissues (NAT). SiRNA-mediated silencing of AKR1B10 in colon cancer cells HCT-8 enhanced cytotoxicity of acrolein and HNE, whereas ectopic expression of AKR1B10 in colon cancer cells RKO prevented the host cells against carbonyl cytotoxicity. Furthermore, siRNA-mediated AKR1B10 silencing led to DNA breaks and activation of γ-H2AX protein, a marker of DNA double strand breaks, particularly in the exposure of HNE (10 µM). In the AKR1B10 silenced HCT-8 cells, hypoxanthine-guanine phosphoribosyl transferase (HPRT) mutant frequency increased by 26.8 times at basal level and by 33.5 times in the presence of 10 µM HNE when compared to vector control cells. In these cells, the cyclic acrolein-deoxyguanosine adducts levels were increased by over 10 times. These findings were confirmed by pharmacological inhibition of AKR1B10 activity by Epalrestat. Taken together, these data suggest that AKR1B10 is a critical protein that protects host cells from DNA damage induced by electrophilic carbonyl compounds. AKR1B10 deficiency in the colon may be an important pathogenic factor in disease progression and carcinogenesis. © 2016 Wiley Periodicals, Inc.

Nucleotide excision repair deficiency increases levels of acrolein-derived cyclic DNA adduct and sensitizes cells to apoptosis induced by docosahexaenoic acid and acrolein.

The acrolein derived cyclic 1,N(2)-propanodeoxyguanosine adduct (Acr-dG), formed primarily from ω-3 polyunsaturated fatty acids such as docosahexaenoic acid (DHA) under oxidative conditions, while proven to be mutagenic, is potentially involved in DHA-induced apoptosis. The latter may contribute to the chemopreventive effects of DHA. Previous studies have shown that the levels of Acr-dG are correlated with apoptosis induction in HT29 cells treated with DHA. Because Acr-dG is shown to be repaired by the nucleotide excision repair (NER) pathway, to further investigate the role of Acr-dG in apoptosis, in this study, NER-deficient XPA and its isogenic NER-proficient XAN1 cells were treated with DHA. The Acr-dG levels and apoptosis were sharply increased in XPA cells, but not in XAN1 cells when treated with 125µM of DHA. Because DHA can induce formation of various DNA damage, to specifically investigate the role of Acr-dG in apoptosis induction, we treated XPA knockdown HCT116+ch3 cells with acrolein. The levels of both Acr-dG and apoptosis induction increased significantly in the XPA knockdown cells. These results clearly demonstrate that NER deficiency induces higher levels of Acr-dG in cells treated with DHA or acrolein and sensitizes cells to undergo apoptosis in a correlative manner. Collectively, these results support that Acr-dG, a ubiquitously formed mutagenic oxidative DNA adduct, plays a role in DHA-induced apoptosis and suggest that it could serve as a biomarker for the cancer preventive effects of DHA.

In vivo detection of a novel endogenous etheno-DNA adduct derived from arachidonic acid and the effects of antioxidants on its formation.

Previous studies showed that 7-(1',2'-dihydroxyheptyl)-substituted etheno DNA adducts are products of reactions with the epoxide of (E)-4-hydroxy-2-nonenal, an oxidation product of ω-6 polyunsaturated fatty acids (PUFAs). In this work, we report the detection of 7-(1',2'-dihydroxyheptyl)-1,N(6)-ethenodeoxyadenosine (DHHedA) in rodent and human tissues by two independent methods: a (32)P-postlabeling/HPLC method and an isotope dilution liquid chromatography-electrospray ionization-tandem mass spectrometry method, demonstrating for the first time that DHHedA is a background DNA lesion in vivo. We showed that DHHedA can be formed upon incubation of arachidonic acid with deoxyadenosine, supporting the notion that ω-6 PUFAs are the endogenous source of DHHedA formation. Because cyclic adducts are derived from the oxidation of PUFAs, we subsequently examined the effects of antioxidants, α-lipoic acid, Polyphenon E, and vitamin E, on the formation of DHHedA and γ-hydroxy-1,N(2)-propanodeoxyguanosine (γ-OHPdG), a widely studied acrolein-derived adduct arising from oxidized PUFAs, in the livers of Long Evans Cinnamon (LEC) rats. LEC rats are afflicted with elevated lipid peroxidation and prone to the development of hepatocellular carcinomas. The results showed that although the survival of LEC rats was increased significantly by α-lipoic acid, none of the antioxidants inhibited the formation of DHHedA, and only Polyphenon E decreased the formation of γ-OHPdG. In contrast, vitamin E caused a significant increase in the formation of both γ-OHPdG and DHHedA in the livers of LEC rats.

Repair kinetics of acrolein- and (E)-4-hydroxy-2-nonenal-derived DNA adducts in human colon cell extracts.

ω-3 and ω-6 polyunsaturated fatty acids (PUFAs) play a role in the pathogenesis of colon cancer. Upon oxidation, PUFAs generate α,ß-unsaturated aldehydes or enals, such as acrolein (Acr) and (E)-4-hydroxy-2-nonenal (HNE), which can form cyclic adducts of deoxyguanosine (Acr-dG and HNE-dG, respectively) in DNA. Both Acr-dG and HNE-dG adducts have been detected in human and animal tissues and are potentially mutagenic and carcinogenic. In vivo levels of Acr-dG in DNA are at least two orders of magnitude higher than those of HNE-dG. In addition to the facile reaction with Acr, the higher levels of Acr-dG than HNE-dG in vivo may be due to a lower rate of repair. Previous studies have shown that HNE-dG adducts are repaired by the NER pathway (Choudhury et al. [42]). We hypothesize that Acr-dG adducts are repaired at a slower rate than HNE-dG and that HNE-dG in DNA may influence the repair of Acr-dG. In this study, using a DNA repair synthesis assay and a LC-MS/MS method, we showed that Acr-dG in a plasmid DNA is repaired by NER proteins, but it is repaired at a much slower rate than HNE-dG in human colon cell extracts, and the slow repair of Acr-dG is likely due to poor recognition/excision of the lesions in DNA. Furthermore, using a plasmid DNA containing both adducts we found the repair of Acr-dG is significantly inhibited by HNE-dG, however, the repair of HNE-dG is not much affected by Acr-dG. This study demonstrates that the NER repair efficiencies of the two major structurally-related in vivo cyclic DNA adducts from lipid oxidation vary greatly. More importantly, the repair of Acr-dG can be significantly retarded by the presence of HNE-dG in DNA. Therefore, this study provides a mechanistic explanation for the higher levels of Acr-dG than HNE-dG observed in tissue DNA.

Effect of CpG methylation at different sequence context on acrolein- and BPDE-DNA binding and mutagenesis.

Acrolein (Acr), an α,ß-unsaturated aldehyde, is abundant in tobacco smoke and cooking and exhaust fumes. Acr induces mutagenic α- and γ- hydroxy-1,N(2)-cyclic propano-deoxyguanosine adducts in normal human bronchial epithelial cells. Our earlier work has found that Acr-induced DNA damage preferentially occurs at lung cancer p53 mutational hotspots that contain CpG sites and that methylation at CpG sites enhances Acr-DNA binding at these sites. Based on these results, we hypothesized that this enhancement of Acr-DNA binding leads to p53 mutational hotspots in lung cancer. In this study, using a shuttle vector supF system, we tested this hypothesis by determining the effect of CpG methylation on Acr-DNA binding and the mutations in human lung fibroblasts. We found that CpG methylation enhances Acr-induced mutations significantly. Although CpG methylation enhances Acr-DNA binging at all CpG sites, it enhances mutations at selective--TCGA--sites. Similarly, we found that CpG methylation enhances benzo(a)pyrene diol epoxide binding at all -CpG- sites. However, the methylated CpG sequences in which benzo(a)pyrene diol epoxide-induced mutations are enhanced are different from the CpG sequences in which Acr-induced mutations are enhanced. CpG methylation greatly increases Acr-induced G to T and G to A mutation frequency to levels similar to these types of mutations found in the CpG sites in the p53 gene in tobacco smoke-related lung cancer. These results indicate that both CpG sequence context and the chemical nature of the carcinogens are crucial factors for determining the effect of CpG methylation on mutagenesis.

Detection of acrolein-derived cyclic DNA adducts in human cells by monoclonal antibodies.

Acrolein (Acr) is a ubiquitous environmental pollutant found in cigarette smoke and automobile exhaust. It can also be produced endogenously by oxidation of polyunsaturated fatty acids. The Acr-derived 1,N(2)-propanodeoxyguanosine (Acr-dG) adducts in DNA are mutagenic lesions that are potentially involved in human cancers. In this study, monoclonal antibodies were raised against Acr-dG adducts and characterized using ELISA. They showed strong reactivity and specificity toward Acr-dG, weaker reactivity toward crotonaldehyde- and trans-4-hydroxy-2-nonenal-derived 1,N(2)-propanodeoxyguanosines, and weak or no reactivity toward 1,N(6)-ethenodeoxyadenosine and 8-oxo-deoxyguanosine. Using these antibodies, we developed assays to detect Acr-dG in vivo: first, a simple and quick FACS-based assay for detecting these adducts directly in cells; second, a highly sensitive direct ELISA assay for measuring Acr-dG in cells and tissues using only 1 µg of DNA without DNA digestion and sample enrichment; and third, a competitive ELISA for better quantitative measurement of Acr-dG levels in DNA samples. The assays were validated using Acr-treated HT29 cell DNA samples or calf thymus DNA, and the results were confirmed by LC-MS/MS-MRM. An immunohistochemical assay was also developed to detect and visualize Acr-dG in HT29 cells as well as in human oral cells. These antibody-based methods provide useful tools for the studies of Acr-dG as a cancer biomarker and of the molecular mechanisms by which cells respond to Acr-dG as a ubiquitous DNA lesion.

High-throughput, quantitative analysis of acrolein-derived DNA adducts in human oral cells by immunohistochemistry.

Acrolein (Acr) is a ubiquitous environmental pollutant as well as an endogenous compound. Acrolein-derived 1,N(2)-propanodeoxyguanosines (Acr-dG) are exocyclic DNA adducts formed following exposure to cigarette smoke or from lipid peroxidation. Acr-dG is mutagenic and potentially carcinogenic and may represent a useful biomarker for the early detection of cancers related to smoking or other oxidative conditions, such as chronic inflammation. In this study, we have developed a high-throughput, automated method using a HistoRx PM-2000 imaging system combined with MetaMorph software for quantifying Acr-dG adducts in human oral cells by immunohistochemical detection using a monoclonal antibody recently developed by our laboratory. This method was validated in a cell culture system using BEAS-2B human bronchial epithelial cells treated with known concentrations of Acr. The results were further verified by quantitative analysis of Acr-dG in DNA of BEAS-2B cells using a liquid chromatography/tandem mass spectrometry/multiple-reaction monitoring method. The automated method is a quicker, more accurate method than manual evaluation of counting cells expressing Acr-dG and quantifying fluorescence intensity. It may be applied to other antibodies that are used for immunohistochemical detection in tissues as well as cell lines, primary cultures, and other cell types.

Acrolein-derived DNA adduct formation in human colon cancer cells: its role in apoptosis induction by docosahexaenoic acid.

The apoptotic effects of docosahexaenoic acid (DHA) and other omega-3 polyunsaturated fatty acids (PUFAs) have been documented in cell and animal studies. The molecular mechanism by which DHA induces apoptosis is unclear. Although there is no direct evidence, some studies have suggested that DNA damage generated through lipid peroxidation may be involved. Our previous studies showed that DHA, because it has a high degree of unsaturation, can give rise to the acrolein-derived 1,N(2)-propanodeoxyguanosine (Acr-dG) as a major class of DNA adducts via lipid oxidation. As a first step to investigate the possible role of oxidative DNA damage in apoptosis induced by DHA, we examined the relationships between oxidative DNA damage and apoptosis caused by DHA in human colon cancer HT-29 cells. Apoptosis and oxidative DNA damage, including Acr-dG and 8-oxo-deoxyguanosine (8-oxo-dG) formation, in cells treated with DHA and omega-6 PUFAs, including arachidonic acid (AA) and linoleic acid (LA), were measured. DHA induced apoptosis in a dose- and time-dependent manner with a concentration range from 0 to 300 microM as indicated by increased caspase-3 activity and PARP cleavage. In contrast, AA and LA had little or no effect at these concentrations. The Acr-dG levels were increased in HT-29 cells treated with DHA at 240 and 300 microM, and the increases were correlated with the induction of apoptosis at these concentrations, while no significant changes were observed for 8-oxo-dG. Because proteins may compete with DNA to react with acrolein, we then examined the effects of BSA on DHA-induced apoptosis and oxidative DNA damage. The addition of BSA to HT-29 cell culture media significantly decreases Acr-dG levels with a concomitant decrease in the apoptosis induced by DHA. The reduced Acr-dG formation is attributed to the reaction of BSA with acrolein as indicated by increased levels of total protein carbonyls. Similar correlations between Acr-dG formation and apoptosis were observed in HT-29 cells directly incubated with 0-200 microM acrolein. Additionally, DHA treatment increased the level of DNA strand breaks and caused cell cycle arrested at G1 phase. Taken together, these results demonstrate the parallel relationships between Acr-dG level and apoptosis in HT-29 cells, suggesting that the formation of Acr-dG in cellular DNA may contribute to apoptosis induced by DHA.

Detection of the acrolein-derived cyclic DNA adduct by a quantitative 32P-postlabeling/solid-phase extraction/HPLC method: blocking its artifact formation with glutathione.

Acrolein (Acr), a hazardous air pollutant, reacts readily with deoxyguanosine (dG) in DNA to produce cyclic 1, N2-propanodeoxyguanosine adducts (Acr-dG). Studies demonstrate that these adducts are detected in vivo and may play a role in mutagenesis and carcinogenesis. In the study described here, a quantitative 32P-postlabeling/solid-phase extraction/HPLC method was developed by optimizing the solid-phase extraction and the 32P-postlabeling conditions for analysis of Acr-dG in DNA samples with a detection limit of 0.1 fmol. It was found that Acr-dG can form as an artifact during the assay. Evidence obtained from mass spectrometry indicates that the Acr in water used in the assay is a likely source of artifact formation of Acr-dG. The formation of Acr-dG as an artifact can be effectively blocked by adding glutathione (GSH) to the DNA sample to be analyzed. In addition, Acr-dG was detected as a contaminant in the commercial dG and dT 3'-monophosphate samples. Finally, this method was used to detect Acr-dG in calf thymus and human colon HT29 cell DNA with an excellent linear quantitative relationship.

A solid-phase extraction/high-performance liquid chromatography-based (32)P-postlabeling method for detection of cyclic 1,N(2)-propanodeoxyguanosine adducts derived from enals.

The cyclic 1,N(2)-propanodeoxyguanosine (PdG) adducts are Michael addition products from reactions of deoxyguanosine (dG) with enals, including acrolein (Acr), crotonaldehyde (Cro), pentenal (Pen), heptenal (Hep), and 4-hydroxy-2-nonenal (HNE). Although this is a general reaction, only the PdG adducts derived from Acr, Cro, and HNE have been detected in vivo as endogenous DNA lesions. Our previous in vitro study demonstrated that PdG adducts of Acr, Cro, and Pen are predominantly derived from oxidation of omega-3 polyunsaturated fatty acids (PUFAs), whereas the long-chain Hep and HNE adducts are from omega-6 PUFAs. PdG adducts are important because they represent a new class of endogenous promutagenic DNA lesions with potential roles in carcinogenesis. Earlier, we developed a (32)P-postlabeling method for detecting PdG adducts from Acr and Cro and a modified method for the long-chain HNE adducts. Both methods require multiple high-performance liquid chromatography steps and, in some cases, time-consuming thin-layer chromatography for purification. There is a lack of a single, versatile, and efficient method for simultaneous detection of all five enal-derived PdG adducts. In this paper, we report an improved (32)P-postlabeling method which permits detection of Acr, Cro, Pen, Hep, and HNE adducts in a single DNA sample. This method relies on solid-phase extraction for adduct enrichment before and after (32)P-labeling; all five PdG adducts were converted to the ring-opened derivatives for confirmation of identities and quantification. The method was validated using the synthetic adducts and enal-modified DNA and was finally applied to rat liver DNA and rat liver DNA samples spiked with different amount of standards. The detection limit was determined to be as low as 0.5 fmol in 80 microg DNA, corresponding to 9 adducts/10(9) dG.

Glutathione depletion enhances the formation of endogenous cyclic DNA adducts derived from t-4-hydroxy-2-nonenal in rat liver.

Earlier, we detected the cyclic adducts of deoxyguanosine (dG) derived from t-4-hydroxy-2-nonenal (HNE), a long chain alpha,beta-unsaturated aldehyde (enal) product from oxidation of omega-6 polyunsaturated fatty acids, in tissue DNA of rats and humans as endogenous DNA damage. Recent evidence implicates the cyclic HNE adducts in human liver carcinogenesis. Because glutathione (GSH) protects against oxidative stress, we undertook a study to examine the effect of GSH depletion on the HNE-derived cyclic adducts in vivo. Four F344 rats were administered L-buthionine-(S,R)-sulfoximine (BSO), a potent inhibitor of GSH biosynthesis, at 10 mM in drinking water for 2 weeks. Rats in the control group were given water only. Livers were harvested, and each liver was divided into portions for GSH and DNA adduct analyses. The BSO treatment depleted hepatic GSH by 77%; the GSH levels were reduced from 6.3 +/- 0.3 in the control rats to 1.5 +/- 0.1 micromol/g tissues in the treated group. The formation of HNE-dG adducts, analyzed by an HPLC-based 32P-postlabeling assay, was increased by 4-fold, from 6.2 +/- 2.2 nmol/mol dG in liver DNA of control rats to 28.5 +/- 16.1 nmol/mol dG in the rats treated with BSO (p <0.05). The formation of 8-oxodG in liver DNA was also increased as a result of BSO treatment, although the increase was not statistically significant. These results further support the endogenous origin of HNE-dG adducts and, more importantly, indicate a critical role that GSH plays in protecting against in vivo formation of the promutagenic cyclic DNA adducts derived from HNE.

Repair kinetics of trans-4-hydroxynonenal-induced cyclic 1,N2-propanodeoxyguanine DNA adducts by human cell nuclear extracts.

trans-4-Hydroxynonenal (HNE) is a major peroxidation product of omega-6 polyunsaturated fatty acids. The reaction of HNE with DNA produces four diastereomeric 1,N(2)-gamma-hydroxypropano adducts of deoxyguanosine (HNE-dG); background levels of these adducts have been detected in tissues of animals and humans. There is evidence to suggest that these adducts are mutagenic and involved in liver carcinogenesis in patients with Wilson's disease and in other human cancers. Here, we present biochemical evidence that in human cell nuclear extracts the HNE-dG adducts are repaired by the nucleotide excision repair (NER) pathway. To investigate the recognition and repair of HNE-dG adducts in human cell extracts, we prepared plasmid DNA substrates modified by HNE. [(32)P]-Postlabeling/HPLC determined that the HNE-dG adduct levels were approximately 1200/10(6) dG of plasmid DNA substrate. We used this substrate in an in vitro repair-synthesis assay to study the complete repair of HNE-induced DNA adducts in cell-free extracts. We observed that nuclear extracts from HeLa cells incorporated a significant amount of alpha[(32)P]dCTP in DNA that contained HNE-dG adducts by comparison with UV-irradiated DNA as the positive control. Such repair synthesis for UV damage or HNE-dG adducts did not occur in XPA cell nuclear extracts that lack the capacity for NER. However, XPA cells complemented with XPA protein restored repair synthesis for both of these adducts. To verify that HNE-dG adducts in DNA were indeed repaired, we measured HNE-dG adducts in the post-repaired DNA substrates by the [(32)P]-postlabeling/HPLC method, showing that 50-60% of HNE-dG adducts were removed from the HeLa cell nuclear extracts after 3 h at 30 degrees C. The repair kinetics indicated that the excision rate is faster than the rate of gap-filling/DNA synthesis. Furthermore, the HNE-dG adduct isomers 2 and 4 appeared to be repaired more efficiently at early time points than isomers 1 and 3.

Formation of trans-4-hydroxy-2-nonenal- and other enal-derived cyclic DNA adducts from omega-3 and omega-6 polyunsaturated fatty acids and their roles in DNA repair and human p53 gene mutation.

The cyclic 1,N(2)-propanodeoxyguanosine adducts, derived from alpha,beta-unsaturated aldehydes or enals, including acrolein (Acr), crotonaldehyde (Cro), and trans-4-hydroxy-2-nonenal (HNE), have been detected as endogenous DNA lesions in rodent and human tissues. Collective evidence has indicated that the oxidative metabolism of polyunsaturated fatty acids (PUFAs) is an important pathway for endogenous formation of these adducts. In a recent study, we examined the specific role of different types of fatty acids, omega-3 and omega-6 PUFAs, in the formation of cyclic adducts of Acr, Cro, and HNE. Our studies showed that the incubation of deoxyguanosine 5'-monophosphate with omega-3 or omega-6 fatty acids under oxidative conditions in the presence of ferrous sulfate yielded different amounts of Acr, Cro, and HNE adducts, depending on the types of fatty acids. We observed that Acr- and Cro-dG adducts are primarily formed from omega-3, and the adducts derived from longer chain enals, such as HNE, were detected exclusively from omega-6 fatty acids. Acr adducts are also formed from omega-6 fatty acids, but to a lesser extent; the yields of Acr adducts are proportional to the number of double bonds present in the PUFAs. Two previously unknown cyclic adducts, one from pentenal and the other from heptenal, were detected as products from omega-3 and omega-6 fatty acids, respectively. Because omega-6 PUFAs are known to be involved in the promotion of tumorigenesis, we investigated the role of HNE adducts in p53 gene mutation by mapping the HNE binding to the human p53 gene with UvrABC nuclease and determined the formation of HNE-dG adducts in the gene. The results showed that HNE-dG adducts are preferentially formed in a sequence-specific manner at the third base of codon 249 in the p53 gene, a mutational hotspot in human cancers. The DNA repair study using plasmid DNA containing HNE-dG adducts as a substrate in HeLa cell extracts showed that HNE adducts are readily repaired, and that nucleotide excision repair appears to be a major pathway involved. Together, results of these studies provide a better understanding of the involvement of different PUFAs in DNA damage and their possible roles in tumorigenesis.

The major lipid peroxidation product, trans-4-hydroxy-2-nonenal, preferentially forms DNA adducts at codon 249 of human p53 gene, a unique mutational hotspot in hepatocellular carcinoma.

Trans-4-hydroxy-2-nonenal (4-HNE), a major electrophilic by-product of lipid peroxidation, is able to interact with DNA to form exocyclic guanine adducts. 4-HNE is a mutagen and a significant amount of 4-HNE-guanine adduct has been detected in normal cells. Recently, it has been reported that exposure of the wild-type p53 human lymphoblastoid cell line to 4-HNE causes a high frequency of G to T transversion mutations at the third base of codon 249 (-AGG*-) in the p53 gene, a mutational hotspot in human cancers, particularly hepatocellular carcinoma. These findings raise a possibility that 4-HNE could be an important etiological agent for human cancers that have a mutation at codon 249 of the p53 gene. However, to date, the sequence specificity of 4-HNE-DNA binding remains unclear due to the lack of methodology. To address this question, we have developed a method, using UvrABC nuclease, a nucleotide excision repair enzyme complex isolated from Escherichia coli, to map the distribution of 4-HNE-DNA adducts in human p53 gene at the nucleotide sequence level. We found that 4-HNE-DNA adducts are preferentially formed at the third base of codon 249 in the p53 gene. The preferential binding of 4-HNE was also observed at codon 174, which has the same sequence and the same nearest neighbor sequences (-GAGG*C-) as codon 249. These results suggest that 4-HNE may be an important etiological agent for human cancers that have a mutation at codon 249 of the p53 gene.

Formation of cyclic deoxyguanosine adducts from omega-3 and omega-6 polyunsaturated fatty acids under oxidative conditions.

The discovery of the cyclic 1,N(2)-propanodeoxyguanosine adducts of acrolein (Acr), crotonaldehyde (Cro), and t-4-hydroxy-2-nonenal (HNE) as endogenous DNA lesions from lipid peroxidation has raised questions regarding the role of different types of fatty acids as sources for their formation. In this study, we carried out reactions at pH 7 and 37 degrees C with deoxyguanosine 5'-monophosphate and omega-3 polyunsaturated fatty acids (PUFAs), including docosahexaenoic acid (DHA), linolenic acid (LNA), and eicosapentaenoic acid (EPA); or omega-6 PUFAs, including linoleic acid (LA) and arachidonic acid (AA), each in the presence of ferrous sulfate. The formation of Acr, Cro, and HNE-derived 1,N(2)-propanodeoxyguanosine adducts (Acr-, Cro-, and HNE-dG) in the incubation mixture was determined by reversed-phase HPLC analysis. The results showed that Acr and Cro adducts are primarily derived from omega-3 PUFAs, although Acr adducts are also formed, to a lesser extent, from oxidized AA and LA. HNE-dG adducts were detected exclusively in incubations with AA. The kinetics of the formation of these adducts was determined during incubations for 2 weeks and 5 days. The rate of Acr adduct formation was about 5-10-fold that of Cro adducts, depending on the type of PUFAs, and the rate of formation of HNE adducts from AA was also considerably slower than that of Acr adducts. Unlike other cyclic adducts, the formation of Acr adducts was independent of types of PUFAs, but its yield was proportional to the number of double bonds in the fatty acid. Only one of the isomeric Acr adducts was detected, and its stereoselective formation is consistent with that observed previously in vivo. Two previously unknown cyclic adducts, one derived from pentenal and the other from heptenal, were also detected as products from omega-3 and omega-6 fatty acids, respectively. This study demonstrated the specificity for the formation of the cyclic adducts of Acr, Cro, and HNE and other related enals by oxidation of omega-3 and omega-6 PUFAs. These results may be important for the understanding of the specific roles of different types of fatty acids in tumorigenesis.