Chemoprevention of cancer by isothiocyanates, modifiers of carcinogen metabolism.
(1/820)
Substantial quantities of isothiocyanates are released upon consumption of normal amounts of a number of cruciferous vegetables. Some of these naturally occurring isothiocyanates such as phenethyl isothiocyanate (PEITC), benzyl isothiocyanate (BITC) and sulforaphane are effective inhibitors of cancer induction in rodents treated with carcinogens. A large amount of data demonstrate that isothiocyanates act as cancer chemopreventive agents by favorably modifying carcinogen metabolism via inhibition of Phase 1 enzymes and/or induction of Phase 2 enzymes. These effects are quite specific, depending on the structure of the isothiocyanate and carcinogen. One of the most thoroughly studied examples of isothiocyanate inhibition of rodent carcinogenesis is inhibition of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced lung tumorigenesis by PEITC. This occurs because PEITC blocks the metabolic activation of NNK, resulting in increased urinary excretion of detoxified metabolites. Similar effects on NNK metabolism have been observed in smokers who consumed watercress, a source of PEITC. On the basis of these observations and knowledge of the carcinogenic constituents of cigarette smoke, a strategy for chemoprevention of lung cancer can be developed. (+info)
Diffusion of dialkylnitrosamines into the rat esophagus as a factor in esophageal carcinogenesis.
(2/820)
To indicate how readily nitrosamines (NAms) diffuse into the esophagus, we measured diffusion rate (flux) through rat esophagus of dialkyl-NAms using side-by-side diffusion apparatuses. Mucosal and serosal flux at 37 degrees C of two NAms, each at 50 microM, was followed for 90 min by gas chromatography-thermal energy analysis of NAms in the receiver chamber. Mucosal flux of one or two NAms at a time gave identical results. Mucosal flux was highest for the strong esophageal carcinogens methyl-n-amyl-NAm (MNAN) and methylbenzyl-NAm. Mucosal esophageal flux of 11 NAms was 18-280 times faster and flux of two NAms through skin was 13-28 times faster than that predicted for skin from the molecular weights and octanol:water partition coefficients, which were also measured. Mucosal: serosal flux ratio was correlated (P < 0.05) with esophageal carcinogenicity and molecular weight. For seven NAms tested for carcinogenicity by Druckrey et al. [(1967) Z. Krebsforsch., 69, 103-201], mucosal flux was correlated with esophageal carcinogenicity with borderline significance (P = 0.07). The MNAN:dipropyl-NAm ratio for mucosal esophageal flux was unaffected when rats were treated with phenethylisothiocyanate and was similar to that for forestomach, indicating no involvement by cytochromes P450. Mucosal esophageal flux of MNAN and dimethyl-NAm was reduced by >90% on enzymic removal of the stratum corneum, was unaffected by 0.1 mM verapamil and was inhibited 67-94% by 1.0 mM KCN and 82-93% by 0.23% ethanol. NAm flux through rat skin and jejunum was 5-17% of that through esophagus. Flux through skin increased 5-13 times after enzymic or mechanical removal of the epidermis; the histology probably explained this difference from esophagus. Hence, NAms could be quite rapidly absorbed by human esophagus when NAm-containing foods or beverages are swallowed, the esophageal carcinogenicity of NAms may be partly determined by their esophageal flux and NAm flux probably occurs by passive diffusion. (+info)
Conversion of glucosinolates to isothiocyanates in humans after ingestion of cooked watercress.
(3/820)
Isothiocyanates (ITCs), major constituents of cruciferous vegetables, can inhibit tumorigenesis in rodents by modulating the metabolism of carcinogens. ITCs that occur as glucosinolates are released by myrosinase-mediated hydrolysis when raw vegetables are chopped or chewed. However, because cruciferous vegetables are commonly consumed by humans after being cooked, it is important to examine whether dietary glucosinolates are converted to ITCs after cooked cruciferous vegetables in which myrosinase is deactivated have been consumed. This information is useful for evaluating the potential role of ITCs in cruciferous vegetables in the protection against human cancers. A urinary marker, based on a cyclocondensation product formed by the reaction of ITCs and their conjugates with 1,2-benzenedithiol, was used to quantify the uptake of dietary ITCs in humans. At breakfast and lunch, nine volunteers consumed a total of 350 g of cooked watercress in which the myrosinase activity was completely deactivated. On the basis of the analysis of ITCs in the cooked watercress upon adding exogenous myrosinase, the amount of glucosinolates ingested by each subject was estimated to be 475 micromol. The 24-h urine samples showed that the total urinary excretion of ITC conjugates in the subjects ranged from 5.6 to 34.8 micromol, corresponding to 1.2-7.3% of the total amount ingested. On the basis of our previous results that approximately 50% of dietary ITCs were excreted in the urine as conjugates, these values represent the minimal in vivo conversion of glucosinolates to ITCs. For purposes of comparison, we carried out a second experiment in which 150 g of uncooked watercress were consumed. The percentage of urinary ITC conjugates excreted in this study ranged from 17.2 to 77.7% of the total ingested ITCs. These results indicate that glucosinolates are converted to ITCs in humans after ingestion of cooked watercress, in which the myrosinase has been completely inactivated. The extent of conversion, however, is considerably less than that after ingesting uncooked vegetables. Furthermore, upon incubation of the cooked watercress juice with fresh human feces under anaerobic conditions, approximately 18% of glucosinolates was hydrolyzed to ITCs in 2 h. These results suggest that the microflora in the intestinal tract are a likely source for the hydrolysis of glucosinolates to ITCs in humans. (+info)
Role of a mitogen-activated protein kinase pathway in the induction of phase II detoxifying enzymes by chemicals.
(4/820)
Mitogen-activated protein kinase (MAPK) cascades are activated by diverse extracellular signals and participate in the regulation of an array of cellular programs. In this study, we investigated the roles of MAPKs in the induction of phase II detoxifying enzymes by chemicals. Treatment of human hepatoma (HepG2) and murine hepatoma (Hepa1c1c7) cells with tert-butylhydroquinone (tBHQ) or sulforaphane (SUL), two potent phase II enzyme inducers, stimulated the activity of extracellular signal-regulated protein kinase 2 (ERK2) but not c-Jun N-terminal kinase 1. tBHQ and SUL also activated MAPK kinase. Inhibition of MAPK kinase with its inhibitor, PD98059, abolished ERK2 activation and impaired the induction of quinone reductase, a phase II detoxifying enzyme, and antioxidant response element (ARE)-linked reporter gene by tBHQ and SUL. Overexpression of a dominant-negative mutant of ERK2 also attenuated tBHQ and SUL induction of ARE reporter gene activity. Interestingly, although expression of Ras and its mutant forms showed distinct effects on basal ARE reporter gene activity, they did not affect the activation of reporter gene by the inducers. Furthermore, a dominant-negative mutant of Ras had little effect on ERK2 activation by tBHQ and SUL, implicating a Ras-independent mechanism. Indeed, both tBHQ and SUL were able to stimulate Raf-1 kinase activity in vivo as well as in vitro. Thus, our results indicate that the induction of ARE-dependent phase II detoxifying enzymes is mediated by a MAPK pathway, which may involve direct activation of Raf-1 by the inducers. (+info)
Effects of watercress consumption on urinary metabolites of nicotine in smokers.
(5/820)
The effects of watercress consumption on the metabolism of nicotine in smokers were examined. Watercress is a rich source of phenethyl isothiocyanate (PEITC), an effective chemopreventive agent for cancers of the lung and esophagus induced in rodents by nitrosamines, including the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. PEITC is believed to inhibit nitrosamine carcinogenesis in rodents by inhibiting specific cytochrome P450 (P450) enzymes. Among the P450s involved in the activation of these nitrosamines are members of the 2A family. P450 2A6 is believed to be involved in the metabolism of both nicotine and its major metabolite cotinine. Therefore, we hypothesized that watercress consumption might inhibit nicotine and cotinine metabolism in smokers. The urine samples analyzed in this study were the same ones that we used in an earlier study (S. S. Hecht et al., Cancer Epidemiol. Biomark. Prev., 4: 877-884, 1995), in which we showed that watercress consumption increased levels of two metabolites of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone: NNAL and its glucuronide NNAL-Gluc. This increase was attributed either to inhibition of cytochromes P450 or induction of glucuronidation. In the present study, we quantified urinary nicotine and seven of its metabolites. There were no effects of watercress consumption on levels of nicotine, cotinine, trans-3'-hydroxycotinine, 4-oxo-4-(3-pyridyl)butanoic acid, or 4-hydroxy-4-(3-pyridyl)butanoic acid, indicating either that watercress ingestion has little effect on the oxidative metabolism of nicotine (presumably by P450 2A6 or other P450 enzymes) or that these enzymes are not important for nicotine and cotinine metabolism in smokers. However, watercress consumption resulted in a significant increase compared to baseline levels of the glucuronides of cotinine (25%, P = 0.031) and trans-3'-hydroxycotinine (33%, P = 0.043) during the period when it was consumed and in a nonsignificant increase in levels of the glucuronide of nicotine. These levels returned to baseline values after the watercress consumption period. There was a correlation between increases in levels of the glucuronides of trans-3'-hydroxycotinine and NNAL in the same subjects, suggesting the involvement of a common enzyme. Thus, the results of this study suggest that PEITC or another component of watercress induces UDP-glucuronosyltransferase activity in humans. (+info)
Genotoxic effects of benzyl isothiocyanate, a natural chemopreventive agent.
(6/820)
Benzyl isothiocyanate (BITC) is contained in cruciferous plants which are part of the human diet. Numerous reports indicate that BITC prevents chemically induced cancer in laboratory animals and it has been postulated that BITC might also be chemoprotective in humans. On the other hand, evidence is accumulating that this compound is a potent genotoxin in mammalian cells by itself. To further elucidate the potential hazards of BITC, we investigated its genotoxic effects in different in vitro genotoxicity tests and in animal models. In in vitro experiments [differential DNA repair assay with Escherichia coli, micronucleus assay with human HepG2 cells and single cell gel electrophoresis (SCGE) assay with hepatocytes and gastrointestinal tract cells] pronounced dose-dependent genotoxic effects were found at low dose levels (+info)
Isothiocyanates and freeze-dried strawberries as inhibitors of esophageal cancer.
(7/820)
A group of arylalkyl isothiocyanates were tested for their abilities to inhibit tumorigenicity and DNA methylation induced by the esophageal-specific carcinogen, N-nitrosomethylbenzylamine (NMBA) in the F344 rat esophagus. Phenylpropyl isothiocyanate (PPITC) was more potent than either phenylethyl isothiocyanate (PEITC) or benzyl isothiocyanate (BITC). Phenylbutyl isothiocyanate (PBITC), however, had a lesser inhibitory effect on esophageal tumorigenesis, and phenylhexyl isothiocyanate (PHITC) actually enhanced esophageal tumorigenesis. Thus, the two- and three-carbon isothiocyanates were more effective inhibitors of NMBA-esophageal carcinogenesis than the longer chain isothiocyanates. The effects of the isothiocyanates on tumorigenesis were well correlated as to their effects on DNA adduct formation. The most likely mechanism of inhibition of tumorigenesis by these isothiocyanates is via inhibition of the cytochrome P450 enzymes responsible for the metabolic activation of NMBA in rat esophagus. A freeze-dried strawberry preparation was also evaluated for its ability to inhibit NMBA-esophageal tumorigenesis. It proved to be an effective inhibitor, although not as potent as either PEITC or PPITC. The inhibitory effect of the berries could not be attributed solely to the content of the chemopreventive agent, ellagic acid, in the berries. (+info)
p38 mitogen-activated protein kinase negatively regulates the induction of phase II drug-metabolizing enzymes that detoxify carcinogens.
(8/820)
Phase II drug-metabolizing enzymes, such as glutathione S-transferase and quinone reductase, play an important role in the detoxification of chemical carcinogens. The induction of these detoxifying enzymes by a variety of agents occurs at the transcriptional level and is regulated by a cis-acting element, called the antioxidant response element (ARE) or electrophile-response element. In this study, we identified a signaling kinase pathway that negatively regulates ARE-mediated gene expression. Treatment of human hepatoma HepG2 and murine hepatoma Hepa1c1c7 cells with tert-butylhydroquinone (tBHQ) stimulated the activity of p38, a member of mitogen-activated protein kinase family. Inhibition of p38 activation by its inhibitor, SB203580, enhanced the induction of quinone reductase activity and the activation of ARE reporter gene by tBHQ. In contrast, SB202474, a negative analog of SB203580, had little effect. Consistent with this result, interfering with the p38 kinase pathway by overexpression of a dominant-negative mutant of p38 or MKK3, an immediate upstream regulator of p38, potentiated the activation of the ARE reporter gene by tBHQ, whereas the wild types of p38 and MKK3 diminished such activation. In addition, inhibition of p38 activity augmented the induction of ARE reporter gene activity by tert-butylhydroxyanisole, sulforaphane, and beta-naphthoflavone. Thus, p38 kinase pathway functions as a negative regulator in the ARE-mediated induction of phase II detoxifying enzymes. (+info)