Resveratrol inhibits collagen-induced platelet stimulation through suppressing NADPH oxidase and oxidative inactivation of SH2 domain-containing protein tyrosine phosphatase-2

Reactive oxygen species (ROS) produced upon collagen stimulation are implicated in propagating various platelet-activating pathways. Among ROS-producing enzymes, NADPH oxidase (NOX) is largely responsible for collagen receptor-dependent ROS production. Therefore, NOX has been proposed as a novel target for the development of antiplatelet agent. We here investigate whether resveratrol inhibits collagen-induced NOX activation and further examine the effects of resveratrol on ROS-dependent signaling pathways in collagen-stimulated platelets. Collagen-induced superoxide anion production in platelets was inhibited by resveratrol. Resveratrol suppressed collagen-induced phosphorylation of p47(phox), a major regulatory subunit of NOX. Correlated with the inhibitory effects on NOX, resveratrol protected SH2 domain-containing protein tyrosine phosphatase-2 (SHP-2) from ROS-mediated inactivation and subsequently attenuated the specific tyrosine phosphorylation of key components (spleen tyrosine kinase, Vav1, Bruton's tyrosine kinase, and phospholipase Cγ2) for collagen receptor signaling cascades. Resveratrol also inhibited downstream responses such as cytosolic calcium elevation, P-selectin surface exposure, and integrin-αIIbβ3 activation. Furthermore, resveratrol inhibited platelet aggregation and adhesion in response to collagen. The antiplatelet effects of resveratrol through the inhibition of NOX-derived ROS production and subsequent oxidative inactivation of SHP-2 suggest that resveratrol is a potential compound for prevention and treatment of thrombovascular diseases.

Kaempferol suppresses collagen-induced platelet activation by inhibiting NADPH oxidase and protecting SHP-2 from oxidative inactivation

Reactive oxygen species (ROS) generated upon collagen stimulation act as second messengers to propagate various platelet-activating events. Among the ROS-generating enzymes, NADPH oxidase (NOX) plays a prominent role in platelet activation. Thus, NOX has been suggested as a novel target for anti-platelet drug development. Although kaempferol has been identified as a NOX inhibitor, the influence of kaempferol on the activation of platelets and the underlying mechanism have never been investigated. Here, we studied the effects of kaempferol on NOX activation, ROS-dependent signaling pathways, and functional responses in collagen-stimulated platelets. Superoxide anion generation stimulated by collagen was significantly inhibited by kaempferol in a concentration-dependent manner. More importantly, kaempferol directly bound p47(phox), a major regulatory subunit of NOX, and significantly inhibited collagen-induced phosphorylation of p47(phox) and NOX activation. In accordance with the inhibition of NOX, ROS-dependent inactivation of SH2 domain-containing protein tyrosine phosphatase-2 (SHP-2) was potently protected by kaempferol. Subsequently, the specific tyrosine phosphorylation of key components (Syk, Vav1, Btk, and PLCγ2) of collagen receptor signaling pathways was suppressed by kaempferol. Kaempferol also attenuated downstream responses, including cytosolic calcium elevation, P-selectin surface exposure, and integrin-αIIbβ3 activation. Ultimately, kaempferol inhibited platelet aggregation and adhesion in response to collagen in vitro and prolonged in vivo thrombotic response in carotid arteries of mice. This study shows that kaempferol impairs collagen-induced platelet activation through inhibition of NOX-derived ROS production and subsequent oxidative inactivation of SHP-2. This effect suggests that kaempferol has therapeutic potential for the prevention and treatment of thrombovascular diseases.

Peroxiredoxin II is an antioxidant enzyme that negatively regulates collagen-stimulated platelet function

Collagen-induced platelet signaling is mediated by binding to the primary receptor glycoprotein VI (GPVI). Reactive oxygen species produced in response to collagen have been found to be responsible for the propagation of GPVI signaling pathways in platelets. Therefore, it has been suggested that antioxidant enzymes could down-regulate GPVI-stimulated platelet activation. Although the antioxidant enzyme peroxiredoxin II (PrxII) has emerged as having a role in negatively regulating signaling through various receptors by eliminating H2O2 generated upon receptor stimulation, the function of PrxII in collagen-stimulated platelets is not known. We tested the hypothesis that PrxII negatively regulates collagen-stimulated platelet activation. We analyzed PrxII-deficient murine platelets. PrxII deficiency enhanced GPVI-mediated platelet activation through the defective elimination of H2O2 and the impaired protection of SH2 domain-containing tyrosine phosphatase 2 (SHP-2) against oxidative inactivation, which resulted in increased tyrosine phosphorylation of key components for the GPVI signaling cascade, including Syk, Btk, and phospholipase Cγ2. Interestingly, PrxII-mediated antioxidative protection of SHP-2 appeared to occur in the lipid rafts. PrxII-deficient platelets exhibited increased adhesion and aggregation upon collagen stimulation. Furthermore, in vivo experiments demonstrated that PrxII deficiency facilitated platelet-dependent thrombus formation in injured carotid arteries. This study reveals that PrxII functions as a protective antioxidant enzyme against collagen-stimulated platelet activation and platelet-dependent thrombosis.

Reactive oxygen species play a critical role in collagen-induced platelet activation via SHP-2 oxidation

AIMS

The collagen-stimulated generation of reactive oxygen species (ROS) regulates signal transduction in platelets, although the mechanism is unclear. The major targets of ROS include protein tyrosine phosphatases (PTPs). ROS-mediated oxidation of the active cysteine site in PTPs abrogates the PTP catalytic activity. The aim of this study was to elucidate whether collagen-induced ROS generation leads to PTP oxidation, which promotes platelet stimulation.

RESULTS

SH2 domain-containing PTP-2 (SHP-2) is oxidized in platelets by ROS produced upon collagen stimulation. The oxidative inactivation of SHP-2 leads to the enhanced tyrosine phosphorylation of spleen tyrosine kinase (Syk), Vav1, and Bruton's tyrosine kinase (Btk) in the linker for the activation of T cells signaling complex, which promotes the tyrosine phosphorylation-mediated activation of phospholipase Cγ2 (PLCγ2). Moreover, we found that, relative to wild-type platelets, platelets derived from glutathione peroxidase 1 (GPx1)/catalase double-deficient mice showed enhanced cellular ROS levels, oxidative inactivation of SHP-2, and tyrosine phosphorylation of Syk, Vav1, Btk, and PLCγ2 in response to collagen, which subsequently led to increased intracellular calcium levels, degranulation, and integrin αIIbβ3 activation. Consistent with these findings, GPx1/catalase double-deficiency accelerated the thrombotic response in FeCl3-injured carotid arteries.

INNOVATION

The present study is the first to demonstrate that SHP-2 is targeted by ROS produced in collagen-stimulated platelets and suggests that a novel mechanism for the regulation of platelet activation by ROS is due to oxidative inactivation of SHP-2.

CONCLUSION

We conclude that collagen-induced ROS production leads to SHP-2 oxidation, which promotes platelet activation by upregulating tyrosine phosphorylation-based signal transduction.

The effects of 1-nitropyrene, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and 7,12-dimethylbenz[a]anthracene on 8-hydroxy-2'-deoxyguanosine levels in the rat mammary gland and modulation by dietary 1,4-phenylenebis(methylene) selenocyanate

Humans are exposed to 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 1-nitropyrene (1-NP) via several environmental sources and both are known mammary carcinogens in rodents, with the former being more potent (K. El-Bayoumy, Y.-H. Chae, P. Upadhyaya, A. Rivenson, K. Kurtzke, B. Reddy, S.S. Hecht, Comparative tumorigenicity of benzo[a]pyrene, 1-nitropyrene, and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine administered by gavage to female CD rats, Carcinogenesis 16 (1995) 431-434). Following their metabolic activation, both carcinogens are known to bind covalently to DNA. However, it remains to be determined whether these carcinogens can also induce DNA-base oxidation. Our goal was to determine the effects of PhIP and 1-NP on the levels of 8-hydroxy-2'-deoxyguanosine (8-OHdG; a marker of oxidative DNA damage) in rat mammary glands and to evaluate the effect of the chemopreventive agent 1,4-phenylenebis(-methylene)selenocyanate (p-XSC) as an inhibitor of such damage. As an established potent mammary carcinogen, the synthetic 7,12-dimethylbenz[a]anthracene (DMBA) was included in this study. Female CD rats were fed a high-fat AIN-76A diet (23.5% corn oil) supplemented with p-XSC (10 ppm as selenium) or unsupplemented control diet for 1 week. At 50 days of age, each rat (12 rats/group) was gavaged with either PhIP (22 mg (100 micromol) per rat) or I-NP (20 mg (80 micromol) per rat) in trioctanoin (0.5 ml), DMBA (5 mg (20 micromol) per rat] in olive oil (0.2 ml), or the corresponding vehicle. Rats were sacrificed 6 and 24 h after carcinogen treatment (six rats per time point). Mammary fat pads were excised and DNA was isolated and enzymatically hydrolyzed. The hydrolysates were analyzed for 8-OHdG using HPLC with EC detection. PhIP significantly increased the levels of 8-OHdG by 83% after 6 h (P < 0.05), but the increase (47%) at the 24 h point was not significant. p-XSC alone had no effect on the levels of 8-OHdG. However, the elevation of 8-OHdG caused by PhIP at 6 h was significantly inhibited by p-XSC to levels similar to those measured in rats treated with the vehicle only (P < 0.05). p-XSC had no effect on PhIP-induced 8-OHdG at 24 h. I -NP had no effect on the levels of 8-OHdG at either time point. Levels of 8-OHdG were increased by 22% 6 h after DMBA administration and, significantly, rose to 84% at 24 h (P < 0.01); at either time point, this elevation was not inhibited by p-XSC. Although the mechanisms remain to be determined, to our knowledge, this is the first report demonstrating that PhIP and DMBA are capable of enhancing 8-OHdG levels in the rat mammary tissue in vivo.

Saccharomyces boulardii upgrades cellular adaptation after proximal enterectomy in rats

BACKGROUND

Saccharomyces boulardii is a non-pathogenic yeast which exerts trophic effects on human and rat small intestinal mucosa.

AIMS

To examine the effects of S boulardii on ileal adaptation after proximal enterectomy in rats.

METHODS

Wistar rats, aged eight weeks, underwent 60% proximal resection or transection and received by orogastric intubation either 1 mg/g body wt per day lyophilised S boulardii or the vehicle for seven days. The effects on ileal mucosal adaptation were assessed eight days after surgery.

RESULTS

Compared with transection, resection resulted in mucosal hyperplasia with significant decreases in the specific and total activities of sucrase, lactase, and maltase. Treatment of resected animals with S boulardii had no effect on mucosal hyperplasia but did upgrade disaccharidase activities to the levels of the transected group. Enzyme stimulation by S boulardii was associated with significant increases in diamine oxidase activity and mucosal polyamine concentrations. Likewise, sodium dependent D-glucose uptake by brush border membrane vesicles, measured as a function of time and glucose concentration in the incubation medium, was significantly (p<0.05) increased by 81% and three times respectively in the resected group treated with S boulardii. In agreement with this, expression of the sodium/glucose cotransporter-1 in brush border membranes of resected rats treated with S boulardii was enhanced twofold compared with resected controls.

CONCLUSION

Oral administration of S boulardii soon after proximal enterectomy improves functional adaptation of the remnant ileum.

Comparative effects of phenylenebis(methylene)selenocyanate isomers on xenobiotic metabolizing enzymes in organs of female CD rats

The cancer chemopreventive agent 1,4-phenylenebis(methylene)selenocyanate (p-XSC) inhibits various chemically induced tumors in laboratory animals. We examined the effects of p-XSC and its o- and m-isomers on xenobiotic metabolizing enzymes in vivo. Six-week-old female CD rats were given diets containing o-, m- or p-XSC (5 or 15 p.p.m. as Se), or equimolar amounts (30 or 90 micromol/kg) of 1,4-phenylenebis(methylene)thiocyanate (p-XTC, the sulfur analog of p-XSC) for 1 week. At termination, substrate-specific assays for enzymes of xenobiotic metabolism in various organs were performed. Overall, o-XSC was a more potent enzyme inducer than m- or p-XSC. In hepatic microsomes, o-XSC significantly induced CYP2E1 as detected by increased N-nitrosodimethylamine N-demethylase activity and also by western blot. The activities of CYP1A1 (ethoxyresorufin-O-dealkylase) and CYP1A2 (methoxyresorufin-O-dealkylase) were not affected, but a significant decrease in the activity of CYP2B1 (pentoxyresorufin-O-dealkylase) was observed at the 15 p.p.m. Se level of o-XSC. With the m- and p-XSC isomers or with p-XTC, no significant effect on phase I enzymes was noted. Hepatic UDP-glucuronosyltransferase activities were increased 1.5- to 2-fold by all three XSC isomers at the higher dose level (15 p.p.m. Se), but not by p-XTC; o-XSC again was the most effective. All three XSC isomers were found to increase the alpha, mu and pi isozymes of glutathione S-transferases in the liver, kidney, lung, colon and mammary gland to varying degrees. The XSC isomers also significantly increased glutathione peroxidase in the colon and mammary gland. Although o-XSC was the most powerful in stimulating the enzyme activities, especially in the liver, atomic absorption spectrometry showed that the selenium levels were highest in organs of rats given p-XSC. Thus, the level of tissue distribution of the XSC isomers and/or their metabolite(s) does not correlate with their effects on enzyme activities. The present study demonstrates that individual XSC isomers are capable of modulating specific phase I and/or phase II enzymes involved in the activation and/or detoxification of chemical carcinogens, and provides some mechanistic basis for the cancer chemopreventive efficacy of these organoselenium compounds at the stage of tumor initiation.

Comparative metabolism of 1-, 2-, and 4-nitropyrene by human hepatic and pulmonary microsomes

Determining the capability of humans to metabolize the mononitropyrene (mono-NP) isomers 1-, 2-, and 4-NP and understanding which human cytochrome P450 (P450) enzymes are involved in their activation and/or detoxification is important in the assessment of individual susceptibility to these environmental carcinogens. We compared the ability of 15 human hepatic and 8 pulmonary microsomal samples to metabolize each of the three isomers. Human hepatic microsomes were competent in metabolizing all three isomers. Qualitatively similar metabolic patterns were observed, although at much lower levels, upon incubating mono-NP with pulmonary microsomes. Ring-oxidized metabolites (phenols and trans-dihydrodiols) were produced from all three isomers. However, the nitroreductive metabolism leading to the formation of aminopyrene was evident only with 4-NP. The role of specific P450 enzymes in the human hepatic microsomal metabolism of mono-NP was investigated by correlating the P450-dependent catalytic activities in each microsomal sample with the levels of individual metabolites formed by the same microsomes and by examining the effects of agents that can either inhibit or stimulate specific P450 enzymes in mono-NP metabolism. On the basis of these studies, we attribute most of the hepatic microsomal metabolism of 1- and 4-NP to P450 3A4, although a minor role for P450 1A2 cannot be ruled out. Specifically, P450 3A4 was responsible for the formation of 3-hydroxy-1nitropyrene from 1-NP and the formation of trans-9,10-dihydro-9,10dihydroxy-4-nitropyrene, 9(10)-hydroxy-4-nitropyrene, and 4-aminopyrene from 4-NP. None of the P450 enzymes examined (P450s 3A4, 1A2, 2E1, 2A6, 2D6, and 2C9) appeared to be involved in catalyzing the formation of trans-4,5-dihydro-4,5-dihydroxy-2-nitropyrene and 6-hydroxy-2-nitropyrene from 2-NP in human hepatic microsomes. These results, the first report on the comparative metabolism of mono-NP in humans, clearly demonstrate that the role of specific human P450 enzymes in catalyzing oxidative and reductive pathways of mono-NP is dependent upon the position of the nitro group.

Nitroreduction of 4-nitropyrene is primarily responsible for DNA adduct formation in the mammary gland of female CD rats

We determined whether DNA adducts derived from 4-nitropyrene (4-NP) are formed via nitroreduction or ring oxidation. DNA adduct markers derived from both pathways were prepared and, consequently, were compared with those obtained in vivo in rats treated with 4-NP. Following in vitro reaction of 9,10-epoxy-9,10-dihydro-4-nitropyrene (4-NP-9,10-epoxide), an intermediate metabolite derived from ring oxidation of 4-NP, with calf thymus DNA (average level of binding in two determinations was 8.5 nmol/mg of DNA), DNA was enzymatically hydrolyzed to deoxyribonucleosides and the DNA hydrolysates were analyzed by HPLC. Electrospray mass and 1H NMR spectra of the major products indicated that these adducts are deoxyguanosine (dG) derivatives that resulted from N2-dG substitution at the 9- or 10-position of the pyrene nucleus. However, these adducts were not detected in vivo in the rat mammary gland and liver following the administration of 4-NP. Nitroreduction of 4-NP catalyzed by xanthine oxidase in the presence of DNA resulted in three major putative DNA adducts (level of binding of 12.0 +/- 1.1 nmol/mg of DNA, n = 4) designated as peak 1 (46%), peak 2 (25%), and peak 3 (17%). Although peak 1 was further resolved into peaks 1a and 1b, both were unstable and gradually decomposed to peak 2, and the latter was unequivocally identified as pyrene-4,5-dione. On the basis of electrospray mass spectral analysis, peak 3 was tentatively identified as a deoxyinosine-derived 4-aminopyrene adduct. None of the adducts derived from nitroreduction of 4-NP catalyzed by xanthine oxidase coeluted with the synthetic standard N-(deoxyguanosin-8-yl)-4-aminopyrene prepared by reacting dG with N-acetoxy-4-aminopyrene. Nevertheless, HPLC analysis of the hydrolysates of liver and mammary DNA obtained from rats treated with [3H]-4-NP yielded four radioactive peaks, all of which coeluted with the markers derived from the nitroreduction pathway. These results indicate that nitroreduction is primarily responsible for DNA adduct formation in the liver and, especially, in the mammary gland which is the organ susceptible to carcinogenesis by this environmental agent.

Comparative metabolism and DNA binding of 1-, 2-, and 4-nitropyrene in rats

The metabolism and DNA binding studies of mono-NP isomers under identical conditions were conducted, as an initial investigation, in order to provide an understanding for the higher carcinogenic activity of 4-NP in the rat mammary gland. Urinary and fecal excretion patterns of 4-NP and 1-NP 24 h following administration to female CD rats (i.p.; 24 mg/kg body weight; 1.55 mCi/rat) were similar but higher than those of 2-NP. The identified metabolites were formed via nitroreduction and ring oxidation pathways. Neither the excretion patterns nor the nature of the metabolites readily explained why the mammary tumorigenic activity of these three isomers varied. Although overall levels of mono-NP bound to liver DNA did not account for the observed differences in the biological activity, further HPLC analysis of the liver DNA hydrolysates showed that only 4-NP had yielded putative multiple DNA adducts; none were detected in the case of 1-NP and 2-NP. 1-, 2-, and 4-NP were found to bind to mammary DNA at levels of 0.6, 0.3, and 2.1 pmol/mg DNA, respectively. The structure of DNA adducts in the mammary gland and in the liver of female CD rats following the i.p. administration of 4-NP has not been identified. Collectively, the results of this preliminary study indicate that the difference in levels of DNA binding in the mammary gland in vivo may reflect why 4-NP has higher carcinogenic activity.

Chemoprevention of mammary cancer by diallyl selenide, a novel organoselenium compound

Previous research has demonstrated that structurally distinctive organoselenium compounds are superior to the corresponding sulfur analogs in cancer prevention. The present study was designed to extend this observation to diallyl selenide (DASe), a volatile synthetic compound, and diallyl sulfide (DAS), a flavor component of garlic. Their anticarcinogenic activities were evaluated using the 7,12-dimethylbenz(a)-anthracene (DMBA)-induced mammary tumor model. Rats were gavaged three times with DASe (6 or 12 mumol/kg body wt) or DAS (300, 900 or 1,800 mumol/kg) at 96, 48 and 24 hours before DMBA treatment. Significant tumor inhibition was found with the two doses of DASe and the highest dose of DAS. Based on these results, DASe appears to be at least 300 times more active than DAS. Analysis of total DMBA-DNA binding and individual DNA adducts in the mammary gland and liver showed that DASe had no effect on these parameters, suggesting that DASe might influence some unknown risk-associated events other than carcinogen activation/detoxification. Although the mechanism of action of DASe remains to be elucidated, its potential relevance to natural products will be discussed in the context of the chemistry of selenium-enriched garlic which has been reported to be effective in cancer protection in several studies.

Metabolism and DNA binding of the environmental colon carcinogen 6-nitrochrysene in rats

The environmental contaminant 6-nitrochrysene (6-NC) has been shown to induce adenomas and adenocarcinomas in the colons of rats. The present study aimed at providing a better understanding of mechanisms that are responsible for this effect. Three female CD rats were injected i.p. with [3,4,9,10-3H]6-NC [9 mumol/rat (346 microCi/rat)], and urine and feces were collected daily for 3 days. In the first 24 h, radioactivity corresponding to 1.3% of the dose was excreted in the urine, whereas 23.0% was recovered in the feces. After 3 days, the total excretions in urine and feces were 2.8% and 34.9% of the dose, respectively. Radioactivity measured in various organs 3 days after injection of [3,4,9,10-3H]6-NC amounted to 24.8% of the administered dose. Fecal metabolites were identified, based on comparison of their chromatographic characteristics with those of standards, as trans-1,2-dihydro-1,2-dihydroxy-6-NC, chrysene-5,6-quinone, and 6-aminochrysene (6-AC); the structure of the latter was further confirmed by mass spectrometry and UV spectral analysis. Metabolites identified in the urine were 6-AC, trans-1,2-dihydro-1,2-dihydroxy-6-NC, and trans-9,10-dihydro-9,10-dihydroxy-6-NC in free forms and also as glucuronide and/or sulfate conjugates. The 32P-postlabeling assay was used to determine the metabolic pathways that were leading to DNA adduct formation in the target (colon) and nontarget (liver, lung, and mammary tissues) organs of female CD rats injected with 6-NC under conditions identical to those of the bioassay (total, 14.8 mumol/rat; single i.p. injections on days 1, 8, 15, 22 and 29). Twenty-four h after the last carcinogen administration, the levels of the adduct derived from trans-1,2-dihydro-1,2-dihydroxy-6-AC were higher than those derived from N-hydroxy-6-AC in all organs examined; however, the highest levels of DNA adducts were found in the lung and not in the target organ, the colon. Although the role of each adduct in colon carcinogenesis needs to be determined, the results favor the ring oxidation and nitroreduction combination pathway as the primary contributor to the activation of 6-NC as a colon carcinogen in the rat.

Lack of tumorigenicity of cholesterol epoxides and estrone-3,4-quinone in the rat mammary gland

The purpose of this study is to test the long-standing hypothesis that endogenous agents found in human breast fluid and in plasma are potential initiators of breast cancer. Therefore, we evaluated the tumorigenicity in the mammary glands of female CD rats of cholestan-5 alpha,6 alpha-epoxy-3 beta-ol (cholesterol-alpha-epoxide), cholestan-5 beta,6 beta-epoxy-3 beta-ol (cholesterol-beta-epoxide), and 1,5(10)estradiene-3,14,17-trione (estrone-3,4-quinone). As a positive control, trans-3,4-dihydroxy-anti-1,2-epoxy-1,2,3,4-tetrahydrobenzo[c]phenanthren e (BcPDE) was used. Rats were fed a high-fat AIN-76A diet (23.5% corn oil) to mimic the Western dietary composition. Because literature data suggest that the endogenous agents tested in this study are weak electrophiles, the total doses of cholesterol epoxides (12.3 mumol/rat) and of estrone-3,4-quinone (30 mumol/rat)were 10- and 25- fold higher, respectively, than that of BcPDE (1.2 mumol/rat). Each agent was dissolved in DMSO, and one-sixth of the total dose was injected under each of six nipples on the right side. The thoracic glands of the rat were treated at 30 days of age, and those located in the inguinal area were treated on the following day. The experiment was terminated at 44 weeks after treatment. Consistent with our previous study, BcPDE was a strong mammary carcinogen. However, there were no differences between rats treated with DMSO alone or those receiving DMSO containing cholesterol-alpha-epoxide, cholesterol-beta-epoxide, or estrone-3,4-quinone. The results of this study clearly indicate, for the first time, that metabolites derived from cholesterol and estrone lack tumorigenic activity in the rat mammary gland, at least under the conditions of the present protocol.

Comparative tumorigenicity of benzo[a]pyrene, 1-nitropyrene and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine administered by gavage to female CD rats

Agents that are ubiquitous in the environment and are known inducers of mammary cancer in rodents can be regarded as potential causes of human cancer and need to be evaluated more completely. Therefore, the purpose of this study was to determine under identical conditions the relative carcinogenic potency in the mammary glands of rats of benzo[a]pyrene (B[a]P), 1-nitropyrene (1-NP) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Thirty-day-old female CD rats were gavaged once weekly for 8 weeks with B[a]P, 1-NP or PhIP. Each compound was given at 50 mumol/rat/week in 0.5 ml trioctanoin for a total dose of 400 mumol/rat. Forty-one weeks after the last carcinogen administration, rats were killed. In the 1-NP-treated rats, treatment elicited primarily benign tumors. In contrast, the B[a]P- and PhIP-treated rats developed both malignant and benign tumors. The incidence of adenocarcinomas in rats treated with B[a]P or PhIP was comparable and significantly higher than that in animals receiving trioctanoin only. The incidence of benign tumors (fibroadenomas, desmoplastic adenomas and adenomas) observed in animals treated with B[a]P or 1-NP was comparable and significantly higher than that in animals given PhIP or trioctanoin. This is the first report describing the carcinogenic activity of PhIP, given by gavage, in the mammary gland of CD rats and ranking the carcinogenic potency observed under identical conditions, of three agents (B[a]P congruent to PhIP > 1-NP) that are prevalent in the human environment.

Chemoprevention of cancer by organoselenium compounds

A major research goal of our laboratories is the development of new organoselenium cancer chemopreventive agents with less toxicity compared to some of the historical selenium compounds, such as sodium selenite. Ideally, such agents would be employed to inhibit tumor development in different organs caused by a variety of chemical carcinogens, particularly those present in the human environment. A series of organoselenium compounds has been synthesized and evaluated for their chemopreventive efficacy in vivo. Parallel to these studies, short-term in vitro and in vivo assays were employed to understand the mechanism of action and to rapidly evaluate their efficacy in eventual long-term preclinical investigations. We demonstrated that one of the most effective of these organoselenium compounds, 1,4-phenylenebis(methylene)selenocyanate (p-XSC, Fig. 1), is capable of inhibiting tumors in the mammary glands, colon, and lung of laboratory animals. Dietary p-XSC inhibited mammary tumor development induced by 7,12-dimethylbenz(a)anthracene (DMBA) during both the initiation and post-initiation phases of carcinogenesis in female CD rats. p-XSC inhibited DMBA-DNA adduct formation in the mammary glands. In collaboration with other laboratories, we demonstrated that p-XSC inhibited thymidine kinase in mammary tumor cell lines derived from both humans and rats. Employing mammary carcinoma cell lines, p-XSC was also shown to inhibit cell growth and induce a dose-dependent increase in cell death by apoptosis. In these assays p-XSC appears superior to selenite and to its sulfur analog, 1,4-phenylenebis(methylene)thiocyanate. Dietary p-XSC decreased colon tumor induction by azoxymethane in F344 rats during both phases of carcinogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of trans-1,2-dihydro-1,2-dihydroxy-6-aminochrysene to genotoxic metabolites by rat and human cytochromes P450

In order to address the hypothesis that 6-aminochrysene (6-AC) is converted to genotoxic products by cytochrome P450 enzymes via two activation pathways (N-hydroxylation and epoxidation), the activation of 6-AC and trans-1,2-dihydro-1,2-dihydroxy-6-aminochrysene (6-AC-diol) to genotoxic metabolites was examined in rat and human liver microsomal cytochrome P450 enzymes using Salmonella typhimurium TA1535/pSK1002 and TA1535/pSK1002/pNM12 (NM2009) as tester strains. The latter bacteria, an O-acetyltransferase-overexpressing strain, was highly sensitive to metabolites derived from activation of 6-AC, but not those from 6-AC-diol, using liver microsomes from phenobarbital-treated rats or a reconstituted monooxygenase system containing P4502B1 or -2B2, thus suggesting the roles of P450 and acetyltransferase systems in the activation process. 6-AC-diol, on the other hand, was activated very efficiently by liver microsomes prepared from beta-naphthoflavone-treated rats or a reconstituted system containing P4501A1 or -1A2; the activation reaction is considered to proceed through diol-epoxide formation. The contribution of rat P4501A enzymes towards activation of 6-AC-diol was confirmed by the inhibitory effects on the activation process of alpha-naphthoflavone, a specific inhibitor of P4501A-related activities, and antibodies raised against purified P4501A1 and -1A2. In humans, P4501A2 was found to be the major enzyme involved in the activation of 6-AC-diol to genotoxic metabolites while the parent compound 6-AC was activated mainly by P4503A4. Experiments using recombinant P450 proteins expressed in human lymphoblastoid cell lines showed that human P4501A1 could also activate 6-AC-diol to reactive metabolites at almost the same rate measured with P4501A2. In addition, P4502B6 was found to efficiently catalyze the activation of 6-AC to genotoxic metabolites, and P4503A4 was active in the activation of 6-AC-diol as well as 6-AC. Addition of purified rat epoxide hydrolase to the incubation mixture containing purified rat P4501A1 or microsomes expressing human P4501A1 caused inhibition of activation of 6-AC-diol. These results suggest the existence of different enzymatic activation pathways for 6-AC and 6-AC-diol. The former carcinogen may be N-hydroxylated principally by P4502B enzymes in rats and P4503A4 and -2B6 in humans and activation to its ultimate metabolites may proceed through esterification of the N-hydroxy metabolites by an N-acetyltransferase. The 6-AC-diol is metabolized to its ultimate diolepoxide product by P4501A enzymes in rat and human liver microsomes. P4503A4 (humans) and P4503A2 (rats) may also contribute to some extent in the activation of 6-AC-diol, albeit at lower rates than those of P4501A enzymes.

Induction of mammary cancer by 6-nitrochrysene in female CD rats

6-Nitrochrysene (6-NC) is a pollutant generated in diesel exhaust. In order to study its propensity to induce mammary cancer, we injected 6-NC into the mammary glands of female CD rats. 4-Nitropyrene (4-NP), the most active mammary carcinogen among the mononitropyrene isomers, was used as a positive control (K. Imaida et al., Cancer Res., 51: 2902-2907, 1991). A total of 2.04 mumol of each compound in dimethyl sulfoxide was injected into each of the six mammary glands on the left side of weanling rats. The corresponding glands on the right side received injections of dimethyl sulfoxide. The thoracic glands were treated on Day 1 and those located in the inguinal area were treated on Day 2. Rats were sacrificed after 43 wk. 6-NC induced fibroadenomas, adenocarcinomas, and spindle cell sarcomas of the mammary glands in a high percentage of the rats. The numbers of animals with mammary tumors and the numbers of malignant tumors were significantly higher in the group treated with 6-NC than in those receiving 4-NP or dimethyl sulfoxide alone. The results of this study, taken together with those of previous bioassays, demonstrate the remarkable activity of 6-NC as a mammary, colon, and lung carcinogen in rodents.

Roles of human hepatic and pulmonary cytochrome P450 enzymes in the metabolism of the environmental carcinogen 6-nitrochrysene

6-Nitrochrysene is remarkably tumorigenic in the lung and liver of newborn mice and approximates the activities of certain ultimate carcinogenic metabolites of polycyclic aromatic hydrocarbons. Previous studies have indicated that the major metabolic activation pathway of 6-nitrochrysene in newborn mice is initially through the formation of the proximate tumorigen trans-1,2-dihydro-1,2-dihydroxy-6-aminochrysene with subsequent formation of 1,2-dihydroxy-3,4-epoxy-1,2,3,4-tetrahydro-6-aminochrysene. In order to provide information on the possible risk associated with human exposure to 6-nitrochrysene, the ability of human hepatic and pulmonary microsomes to metabolize 6-nitrochrysene was investigated. The major metabolites identified in 11 hepatic microsomes were trans-1,2-dihydro-1,2-dihydroxy-6-nitrochrysene, trans-9,10-dihydro-9,10-dihydroxy-6-nitrochrysene, trans-1,2-dihydro-1,2-dihydroxy-6-aminochrysene, 6-aminochrysene, and chrysene-5,6-quinone. Following the incubations of 6-nitrochrysene with 11 different human pulmonary microsomes, qualitatively similar metabolic patterns were obtained, although quantitative differences were evident. These results demonstrated that human liver and lung are capable of metabolizing 6-nitrochrysene to known potent carcinogenic metabolites via ring oxidation and nitroreduction. In an attempt to define the roles of individual human hepatic P450 involved in the metabolism of 6-nitrochrysene, the catalytic activities known to be associated with a specific P450 were analyzed and compared with the levels of each metabolite of 6-nitrochrysene formed with the same microsomes. Rates of phenacetin O-deethylation (P450 1A2) and nifedipine oxidation (P450 3A4) were well correlated with the rates of formation of trans-1,2-dihydro-1,2-dihydroxy-6-nitrochrysene and 6-aminochrysene, respectively. Inhibition studies with specific P450 inhibitors and antibodies further support the view that P450 1A2 and P450 3A4 are the major forms responsible for the formation of trans-1,2-dihydro-1,2-dihydroxy-6-nitrochrysene and 6-aminochrysene, respectively, in human liver. Further metabolism of trans-1,2-dihydro-1,2-dihydroxy-6-nitrochrysene appears to require P450 3A4. In the human lung, P450 1A1 appears to play a major role in the metabolism of 6-nitrochrysene to trans-1,2-dihydro-1,2-dihydroxy-6-nitrochrysene. These results provide some requisite knowledge for evaluating human susceptibility to 6-nitrochrysene.

Inhibition of 7,12-dimethylbenz(a)anthracene-induced tumors and DNA adduct formation in the mammary glands of female Sprague-Dawley rats by the synthetic organoselenium compound, 1,4-phenylenebis(methylene)selenocyanate

We synthesized a novel organoselenium compound, 1,4-phenylenebis(methylene)selenocyanate (XSC), possessing low toxicity by comparison with inorganic Na2SeO3, and several other synthetic organoselenium compounds (K. El-Bayoumy, Cancer Res., 45: 3631-3636, 1985). We tested the effect of XSC treatment during the initiation phase on 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary carcinoma formation. A semipurified high-fat diet containing 80 ppm of XSC (40 ppm as selenium) was fed to 6-wk-old virgin female Sprague-Dawley rats for 2 wk, starting 1 wk before and ending 1 wk after carcinogen treatment. At 7 wk of age, rats were given a single dose of DMBA (5 mg) in 0.2 ml of olive oil by gastric intubation; the experiment was terminated 16 wk later. The development of mammary tumors in those rats that received XSC-supplemented diets was significantly inhibited when compared with the control group (fed the same diet without XSC supplements). This was evident from tumor incidence (percentage of tumor-bearing rats, 88 versus 20) and multiplicity of tumors (mean number of tumors/rats, 3.96 versus 0.28). The finding that XSC acts as a chemopreventive agent in the DMBA mammary tumor model prompted us to examine the effect of dietary XSC on DMBA-DNA binding in both the liver and mammary tissue under conditions identical to those described above for the bioassay. Rats (four/group) were killed 6, 24, 48, and 168 h after [3H]DMBA (5 mg/rat; specific activity, 51.2 mCi/mM) administration. Liver and mammary tissue were obtained and DNA was isolated. Dietary XSC was found to inhibit total DMBA-DNA binding in the mammary tissue, but not in the liver. The most profound effect was observed at early time points, i.e., 24 to 48 h after [3H]DMBA administration. The inhibition in total binding was attributed to a reduction in the formation of the three major adducts derived from bay-region diol-epoxides of DMBA; these were identified as anti-diol-epoxide:deoxyguanosine, syn-diol-epoxide:deoxyadenosine, and anti-diol-epoxide:deoxyadenosine adducts on the basis of their chromatographic characteristics on high-pressure liquid chromatography and on a boronate affinity column. The inhibition of the DMBA-DNA binding in the target tissue provides a plausible explanation for the chemopreventive effect of XSC during the initiation stage of carcinogenesis.

Effects of synthetic and naturally occurring flavonoids on metabolic activation of benzo[a]pyrene in hamster embryo cell cultures

Biochanin A, an isoflavone, has previously been shown to inhibit the metabolic activation of the carcinogen benzo[a]pyrene (B[a]P) to metabolites that bind to DNA in hamster embryo cells and are mutagenic in Chinese hamster V79 cells. To determine the structural features required for this activity and to attempt to find more effective inhibitors, a series of synthetic and naturally occurring flavonids were tested for their ability to modulate B[a]P metabolism in hamster embryo cell cultures. The observed structure-activity relationships indicate that the structural features of flavonoids important for effective inhibition of B[a]P metabolism in hamster embryo cells are the presence of two hydroxyl, two methoxyl, or methyl and hydroxyl substituents at the 5- and 7-positions and a 2,3-double bond. Flavones are slightly better inhibitors of B[a]P metabolism than the corresponding isoflavones. A substituent at the 4'-position is not essential for inhibition of B bdP metabolism. The presence of a hydroxyl group at position 3 slightly enhances activity. Apigenin, acacetin and kaempferide are effective inhibitors of B[a]P-induced mutagenesis in a hamster embryo cell-mediated V79 cell mutation assay. However, apigenin is cytotoxic at the inhibitory dose, whereas acacetin and kaempferide are not. These results suggest that acacetin and kaempferide are promising candidates for in vivo testing as potential chemopreventive agents.

Effects of biochanin A on metabolism, DNA binding and mutagenicity of benzo[a]pyrene in mammalian cell cultures

The search for potential chemopreventive agents from higher plants based upon alteration of benzo[a]pyrene (B[a]P) metabolism in cell cultures resulted in isolation of the isoflavone biochanin A. The mechanisms by which biochanin A inhibits the metabolic activation of B[a]P were investigated in hamster embryo cell cultures. Biochanin A treatment inhibited the metabolism of B[a]P to water-soluble metabolites. B[a]P-9,10-diol and B[a]P-7,8-diol by 44, 60 and 52% respectively. Biochanin A inhibited the formation of glucuronide conjugates from 3-OH-B[a]P and 9-OH-B[a]P. Biochanin A also inhibited, in a dose-dependent manner, oxidation of B[a]P by homogenate (S-9) of Aroclor 1254-induced rat liver. Exposure of hamster embryo cells to biochanin A and [3H]B[a]P resulted in a decrease in the total level of [3H]B[a]P bound to DNA compared with the control groups at all time points studied between 24 and 120 h. This decrease was due to reduction in the formation of DNA adducts from both (+)-anti-B[a]P-diolepoxide and (+)-syn-B[a]P-diolepoxide. In a hamster embryo cell-mediated V79 cell mutation assay, biochanin A treatment resulted in a dose-dependent reduction in the number of B[a]P-induced mutants. These results indicate that biochanin A inhibits metabolic activation of B[a]P to mutagenic intermediates and warrants further investigation as a potential chemopreventive agent.

Effects of synthetic and naturally occurring flavonoids on benzo[a]pyrene metabolism by hepatic microsomes prepared from rats treated with cytochrome P-450 inducers

Activity-directed fractionation of Trifolium pratense resulted in isolation of the isoflavone biochanin A, a potent inhibitor of metabolic activation of the carcinogen benzo[a]pyrene (B[a]P) in cells in culture. To determine the structural features required for maximal inhibition of cytochrome P-450 mediated metabolism of B[a]P, the inhibitory potencies of 23 flavonoids on metabolism of B[a]P to water-soluble derivatives were examined in liver S-9 homogenate from rats induced with Aroclor 1254. Flavones were much more efficient inhibitors than their corresponding isoflavone or flavanone analogs. Most flavonols were as effective inhibitors as their flavone analogs with the exception of kaempferide. Flavones with two hydroxyl or two methoxyl groups at positions 5 and 7 were the most active. Although all eight flavonoids tested effectively inhibited B[a]P metabolism by beta-naphthoflavone-induced microsomes, none were very effective inhibitors of B[a]P metabolism by phenobarbitol-induced microsomes, and only three were effective inhibitors of B[a]P metabolism by microsomes from non-induced rats. These results indicate that flavones or flavonols that contain free 5- and 7-hydroxyls are potent inhibitors of P-450 induced by beta-naphthoflavone (P-450IA1 and/or P-450IA2) and may potentially be useful as chemopreventive agents against hydrocarbon-induced carcinogenesis.

Use of a mammalian cell culture benzo(a)pyrene metabolism assay for the detection of potential anticarcinogens from natural products: inhibition of metabolism by biochanin A, an isoflavone from Trifolium pratense L

Based on the epidemiological evidence for a relationship between consumption of certain foods and decreased cancer incidence in humans, an assay was developed to screen and fractionate plant extracts for chemopreventive potential. This assay measures effects on the metabolism of [3H]benzo(a)pyrene [B(a)P] in hamster embryo cell cultures. Screening of several plant extracts has generated a number of activity leads. The 95% ethyl alcohol extract of one of these actives, Trifolium pratense L. Leguminosae, red clover, significantly inhibited the metabolism of B(a)P and decreased the level of binding of B(a)P to DNA by 30 to 40%. Using activity-directed fractionation by solvent partitioning and then silica gel chromatography, a major active compound was isolated and identified as the isoflavone, biochanin A. The pure compound decreased the metabolism of B(a)P by 54% in comparison to control cultures and decreased B(a)P-DNA binding by 37 to 50% at a dose of 25 micrograms/ml. These studies demonstrate that the hydrocarbon metabolism assay can detect and guide the fractionation of potential anticarcinogens from plants. The ability of the isoflavone biochanin A to inhibit carcinogen activation in cells in culture suggests that in vivo studies of this compound as a potential chemopreventive agent are warranted.

Glucolimnanthin, a plant glucosinolate, increases the metabolism and DNA binding of benzo[a]pyrene in hamster embryo cell cultures

Glucosinolates are common components of cruciferous vegetables that can be hydrolyzed during food processing to yield isothiocyanates, some of which have been shown to inhibit the induction of mammary tumors in rats by 7,12-dimethyl-benz[a]anthracene. To determine how intact glucosinolates affect the metabolism of benzo[a]pyrene (B[a]P) in mammalian cells in culture, the effects of a series of glucosinolates on the metabolism and DNA binding of B[a]P were investigated in early passage Syrian hamster embryo cell cultures. Glucolimnanthin, a glucosinolate from Limnanthes douglasii increased the amount of B[a]P metabolized by the hamster embryo cell cultures during a 24 h exposure. The glucolimnanthin-treated cultures contained a higher proportion of B[a]P-phenol glucuronides and other water-soluble metabolites than control cultures. Cotreatment with glucolimnanthan and [3H]B[a]P for 24 h resulted in a greater than 2-fold increase in the amount of B[a]P bound to DNA and a 3-fold increase in the amount of deoxyguanosine adduct formed by reaction of 7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydroB[a]P [(+)-anti-B[a]PDE]. The glucolimnanthin was metabolized essentially completely within 24 h. An increase in B[a]P metabolism similar to that caused by glucolimnanthin was induced by cotreatment of hamster embryo cell cultures with m-methoxybenzyl isothiocyanate, a metabolite that can be formed from glucolimnanthin by enzymatic hydrolysis. These results indicate that the glucosinolate glucolimnanthin can increase the metabolic activation of B[a]P in mammalian cells in culture.