Contents

Introduction 88

ITCs as Chemopreventive Agents for Lung Tumorigenesis and Structure-Activity

Relationship 88

ITCs as Chemopreventive Agents for Tumorigenesis at Other Organ Sites 89

Mechanisms of Tumor Inhibition by ITCs 91

Chemopreventive Effect of SFN and PEITC Against AOM-Induced Aberrant

Crypt Foci with F344 Rats 92

Inhibition of Growth of Human Prostate Tumors by SFN and PEITC 93

Metabolism, Tissue, Distribution, and Pharmacokinetics of ITCs 94

Rodent Studies 94

Human Studies 96

The Role of Dietary ITCs in the Protection Against Human Cancers 98

Conclusion 100

Acknowledgments 100

References 101

© 2002 by CRC Press LLC

INTRODUCTION

Isothiocyanates (ITCs) are a group of naturally occurring compounds commonly found in the crucifer family, such as watercress, broccoli, radish, and cabbage (Fenwick and Heaney, 1993). The consumption of this family of vegetables has been linked to the reduced risk of certain human cancers (Verhoeven et al., 1996; Steinmetz and Potter, 1991). It has been speculated that ITCs may contribute to the protective effects of cruciferous vegetables. The first study on the inhibition of tumor development by ITCs was published in the 1960s by Sidransky et al. (1966). In the study, it was reported that 1-naphthyl ITC, a synthetic ITC, inhibits liver tumorigen-esis in rats induced by ethionine, N-2-fluorenylacetaminde, and aminoazobenzene. Almost a decade later, Wattenberg (1978) demonstrated that certain dietary aromatic ITCs suppressed benzo(a)pyrene (BaP) and 9,10-dimethyl-1,2-benzanthracene (DMBA)-induced mammary tumorigenesis in animals.

These studies prompted us to initiate a screening assay of dietary-related inhibitors against environmental nitrosamine-induced tumorigenesis in the early 1980s. The screening study marks the beginning of our long-standing effort to investigate the role of ITCs in cancer prevention. Our screening was conducted with the liver microsomes obtained from rats fed diets containing one of 25 different compounds commonly found in fruits and vegetables (Chung et al., 1984). The results demonstrated that benzyl ITC (BITC) and phenethyl ITC (PEITC), occurring primarily in gardencress and watercress, respectively, are two remarkably effective agents in blocking cytochrome P450-mediated activation of various nitrosamines, including the potent and highly specific nicotine-derived lung carcinogen, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) (Chung et al., 1985). These findings led us to focus on BITC and PEITC, and to test their chemopreventive potential in the lung tumor bioassays. This chapter describes our studies on the structural features and the mechanisms of ITC compounds important for tumor inhibition, as well as their metabolic fates in rodents and humans. In addition, our recent studies of sulforaphane (SFN), a major constituent of ITC in broccoli, for its effect on colon aberrant crypt foci (ACF) formation, is presented. Finally, based on analysis of a urinary marker for dietary uptake of ITCs, the potential role of ITCs in reducing lung cancer risk in humans was evaluated in a large prospective study.

ITCs AS CHEMOPREVENTIVE AGENTS FOR LUNG TUMORIGENESIS AND STRUCTURE-ACTIVITY RELATIONSHIP

Over the years we have used the A/J mouse to screen chemopreventive agents for lung tumors. The A/J mouse is a useful model, as a single dose of NNK (10 |imol) induces a significant number of lung adenomas within 16 weeks (Hecht et al., 1989). Our studies showed that the pretreatment of A/J mice with PEITC significantly inhibited the number of lung adenomas per mouse (tumor multiplicity) induced by NNK, whereas the pretreatment with lower homologues BITC and phenyl ITC

(PITC) did not affect lung tumor development (Morse et al., 1989a). A subsequent study showed that PEITC administered in the diet throughout the bioassay also inhibited lung tumorigenesis in F344 rats chronically exposed to NNK (Morse et al., 1989c).

The differences in tumor inhibitory potency by PEITC and its lower homologues in A/J mice indicate the importance of alkyl chain length. This notion was further verified by the structure-activity relationship (SAR) studies showing that the alkyl chain length is indeed a critical feature for the inhibitory activity against lung tumori-genesis, and that the potency of ITCs increase with the increased carbon chain up to six carbons (Morse et al., 1989b). Thus, the newly synthesized 6-phenylhexyl ITC (PHITC) was found to be one of the most potent inhibitors for NNK lung tumorige-nesis so far studied (Chung, 1992). A closer examination of the SAR among 16 ITC compounds demonstrated that the potency of ITCs is correlated directly with lipophilicity, and inversely related to their reactivity toward glutathione (Figure 7.1) (Jiao et al., 1994). Based on SAR data, some of the synthetic diphenyl alkyl ITCs were predicted and found to have remarkable chemopreventive activity for lung tumors.

ITCs AS CHEMOPREVENTIVE AGENTS FOR TUMORIGENESIS AT OTHER ORGAN SITES

The activity of ITCs is not only limited to lung tumorigenesis, as a large number of animal studies have shown that ITCs are chemopreventive for cancer at a variety of other organ sites, including mammary gland, liver, esophagus, bladder, pancreas, and colon (Hecht, 1995; Zhang and Talalay, 1994). Most of these bioassays used protocols involving chronic administration of ITC before, during, and after the carcinogen exposure. Therefore, ITCs as a class appear to be promising and versatile agents toward prevention of chemical carcinogenesis. Among ITCs, SFN is the most abundant that occurs in broccoli and broccoli sprouts. SFN has been shown to inhibit DMBA-induced mammary tumorigenesis (Zhang et al., 1994) and, recently, AOM-induced ACF in the rat colon (Chung et al., 2000).

It should be noted, however, while results of most studies are consistent with the view that ITCs are effective inhibitors independent of the model, a few studies reported the adverse effects of the treatment with certain ITCs. For example, PEITC appears to enhance carcinogen-induced mammary and bladder tumorigenesis (Lubet et al., 1997; Hirose et al., 1998). PHITC, the synthetic compound, given in the diet, appeared to cause an increase in tumor formation in AOM-induced colon tumorige-nesis (Rao et al., 1995). Even though PEITC and phenylpropyl ITC (PPITC) were remarkable inhibitors against the NMBA-induced esophageal tumor, it was found that PHITC caused an increase in tumor formation in this model (Stoner et al., 1995). These results did raise cautions in choosing the appropriate and relevant doses and models for the study of chemoprevention by ITCs, and emphasized the need for more studies to define efficacy and mechanisms.

Figure 7.1 Structures of ITCs screened in lung tumorigenesis in A/J mice and the relationship of their relative potency (4 as the most potent) with lipophilicity (log P) and reactivity toward GSH (Kobs).

MECHANISMS OF TUMOR INHIBITION BY ITCs

Several mechanisms have been proposed and investigated for tumor inhibition by ITCs. The inhibition of NNK-induced lung tumorigenesis by PEITC and other related ITCs was mediated primarily by the inhibition of NNK metabolism. This resulted in a decrease in O6-methylguanine in lung DNA, indicating that ITCs target cytochrome P450s (Morse et al., 1989a; Morse et al., 1991). This mechanism was verified by a number of in vitro and in vivo experiments showing specific binding and inhibition by ITCs of cytochrome p450 isozymes (Smith et al., 1996; Guo et al., 1993). ITCs such as SFN and PEITC are also potent inducers of the phase II enzymes involved in detoxification of carcinogens (Zhang and Taladay, 1994). An interesting observation was that a methylthiol conjugate of SFN, sulforamate, was found to be equally potent in phase II enzyme induction as the parent ITC, but was less toxic (Gerhauser et al., 1997). These results, together with the data of thiol conjugates of BITC and PEITC on the inhibition of lung tumorigenesis, suggest the potential of ITC conjugates as more efficacious chemopreventive agents (Jiao et al., 1997).

Phase II enzyme induction may not be critical in the prevention of nitrosamine tumorigenesis; however, it has been shown to play an important role in carcinogen-esis induced by polycyclic aromatic hydrocarbons and heterocyclic aromatic amines. Many of these studies on phase I and phase II enzyme modulation constitute the early investigations on the mechanisms of ITCs. More recently, however, several laboratories demonstrated in tumor cells that ITCs, including PEITC and SFN, induce MAP kinases and other transcription factors, AP-1, c-jun, and p53, and apoptosis and arrest cell cycle (Chen et al., 1998; Huang et al., 1998; Yu et al., 1998; Gamet-Payrastre et al., 2000). These studies showed that ITC induce apoptosis and these activities may be, in part, related to the induction of phosphorylation of MAP kinases. These results thus suggest yet another potentially important mechanism by which ITCs inhibit tumorigenesis.

Results of our recent bioassay in A/J mice with PEITC and BITC and their NAC (N-acetylcysteine) conjugates administered after a single dose of either NNK or BaP appear to support the mechanisms of induction of apoptosis (Yang et al., 2002). A/J mice fed diets containing BITC or PEITC (5 |imol/g diet), or their NAC conjugates (15 |imol/g diet) for 20 weeks beginning 2 days after initiation with 20 |imol BaP showed decreases in lung tumor multiplicity by 30-40% in all treatment groups. Since ITC was given during the postinitiation stage of lung tumorigenesis, the effects cannot be attributed to the modulation of phase I or phase II enzymes.

We investigated the in vivo mechanisms of the postinitiation tumor inhibition by ITCs or their conjugates in BaP-treated A/J mice. Lung tissues obtained from interim sacrifices during the bioassay showed significant increases in the apoptotic index in lung tissue of BITC-NAC and PEITC-NAC groups at 84 and 140 days, with concomitant down-regulation of Bcl-2. The MAP kinase pathway was activated in lungs of treatment groups. The specific activity of JNK was detected in all treatment groups using a phosphorylation-specific antibody, with higher activity occurring in the BITC-NAC and PEITC-NAC groups. The phosphorylation level of Erk 1 was increased by PEITC-NAC and PEITC, while no significant changes in Erk 2 and p38 MAP kinase activities occurred. Downstream of MAP kinase, AP-1 and p53 were also activated. A gel shift assay showed that the AP-1 binding activity was remarkably increased in lungs from BITC-NAC and PEITC-NAC treatment groups. Phosphorylation of p53 was induced above constitutive levels after ITC treatment and was highest in PEITC-NAC and PEITC groups, but no induction of p53 accumulation was detected in any group. The study is the first in vivo demonstration that dietary ITCs induce MAP kinase activity, AP-1 activity, and p53 phosphorylation.

CHEMOPREVENTIVE EFFECT OF SFN AND PEITC AGAINST AOM-INDUCED ABERRANT CRYPT FOCI WITH F344 RATS

A recent case-control study in Los Angeles showed that high consumption of broccoli reduced the risk of colon cancer, and the protective effect was only found in GST-null individuals (Lin et al., 1998). Since ITCs are metabolically eliminated by GST-mediated conjugation with GSH, these authors suggested that the protection is likely to be attributed to dietary ITCs in broccoli. Surprisingly, there has been no animal data on the effect of SFN, a major ITC in broccoli, in colon tumorigenesis until recently. The scarcity of animal data on SFN has been, for the most part, due to its limited availability and cost, and yet SFN is by far the most abundant ITC (approximately 40-50% of total ITCs) in broccoli. Therefore, we carried out a bioassay to examine the effect of SFN and PEITC on AOM-induced ACF in F344 rats (Chung et al., 2000). Treatment with SFN and PEITC and their NAC conjugates by gavage (5 or 20 |imol, respectively) three doses per week for 8 weeks after AOM dosing resulted in a 30-40% reduction in formation of ACF in F344 rats (Table 7.1).

The dose of the conjugate was four times that of ITCs; however, no significant differences in the inhibition of ACF were found. These results suggest the conjugates render the inhibitory activity via gradual dissociation to parent ITCs. The protection of SFN and PEITC against ACF during the postinitiation phase of colon tumorigen-esis may have important implications in the prevention of human colon cancer. However, since ACF are precancerous lesions, these results are considered preliminary. Nevertheless, the animal bioassay data are consistent with the epidemiological observations and support a potential role of SFN and PEITC in protecting against colon cancer. A preclinical efficacy study in this model is warranted.

Current data from mechanism studies also support SFN in the prevention of colon cancer. Gamet-Payrastre et al. recently showed that SFN induces cell cycle arrest in HT29 human colon cancer cells (Gamet-Payrastre et al., 2000). The growth arrest induced by SFN, followed by cell death via apoptosis, appeared to be associated with expression of cyclins A and B1. Although the mechanisms by which SFN and SFN-NAC inhibited ACF in our bioassay are yet to be investigated, its effects on apoptosis via cell cycle arrest, as demonstrated in human colon cell lines, is certainly a possibility. In addition, NAC, released by dissociation of SFN-NAC in an equilibrium, is a known antioxidant capable of inhibiting mouse fibroblast cell proliferation and locking cells in G1 phase (Sekharam et al., 1998) and induce p53-dependent

TABLE 7.1

Effects of SFN and PEITC on the formation of aberrant crypt foci induced by AOM

Average

Body Number of aberrant crypt foci Weight at

Termination

Body Number of aberrant crypt foci Weight at

Termination

Dose of ITC Compounds (umol)

Treatment Groupa

>4 Crypts

Total

1. AOM

310

52

153

2. AOM^SFN

5

301

30 (42)bc

103 (33)d

3. AOM^SFN-NAC

20

297

31 (40)c

116 (24)e

4. AOM^PEITC

5

306

27 (48)c

100 (35)d

5. AOM^PEITC-NAC

20

313

38 (27)f

113 (26)e

6. SFN

5

309

0

0

7. SFN-NAC

20

312

0

0

8. PEITC

5

333

0

0

9. PEITC-NAC

20

301

0

0

10. Control

320

0

0

aITC compounds are administered during postinitiation phase.

bPercent of inhibition compared to Group 1. cSignificantly different from Group 1 at p < 0.01. dSignificantly different from Group 1 at p < 0.0001 eSignificantly different from Group 1 at p < 0.001. fSignificantly different from Group 1 at p < 0.05.

aITC compounds are administered during postinitiation phase.

bPercent of inhibition compared to Group 1. cSignificantly different from Group 1 at p < 0.01. dSignificantly different from Group 1 at p < 0.0001 eSignificantly different from Group 1 at p < 0.001. fSignificantly different from Group 1 at p < 0.05.

apoptosis in several transformed cell lines (Liu et al., 1998). All these activities may have contributed to the inhibition of ACF in this model.

INHIBITION OF GROWTH OF HUMAN PROSTATE TUMORS BY SFN AND PEITC

Much attention has been given to prostate cancer because of its rapid increase in incidence in recent years. The notion that prostate cancer can be protected by high consumption of cruciferous vegetables has been controversial. A recent case-control study, however, showed that intake of crucifers, but not fruits or other vegetables, lowered the prostate cancer incidence (Cohen et al., 2000). We conducted a study of PEITC-NAC on LNCaP, androgen-dependent, and DU145, androgen-independent, human prostate cancer cell lines (Chiao et al., 2000). At high concentrations, PEITC-NAC caused cytolysis, while at lower concentrations PEITC-NAC mediated a

Dose of ITC Compounds (umol)

dose-dependent growth modulation, with reduction of DNA synthesis and growth rate, inhibition of clonogenicity, and induction of apoptosis in both types of prostate cancer cells. PEITC-NAC decreased cells in S and G2M phases of cell cycle, blocking cells entering replicating phases. In parallel, a significant enhancement of cells expressing the cell cycle regulator p21, as well as its intensity, was seen using a fluorescent antibody technique.

In a similar study, we also investigated the effects of SFN and its NAC conjugate (SFN-NAC) on LNCaP (Chiao et al., 2001). Both SFN and SFN-NAC mediated a dose-dependent growth inhibition and apoptosis. DNA strand breaks were detected in the apoptotic cells; total caspase activity was also elevated. SFN-NAC displayed a weaker activity than SFN in mediating apoptotic cell death. Parallel to apoptosis induction, the agents reduced the expression of cyclin D1 and the entry of G1 cells into S and G2M phases. DNA synthesis and subsequent cell densities were decreased in treated cell cultures. Additionally, SFN and SFN-NAC attenuated the expression of the androgen receptor and PSA production. The results indicate that SFN and SFN-NAC regulate the mechanism of cellular replication and development in human prostate cancer cells. Although tumor bioassays need to be performed, these data do suggest that dietary SFN and PEITC and their thiol conjugates may be active in the prevention of prostate cancer.

METABOLISM, TISSUE DISTRIBUTION, AND PHARMACOKINETICS OF ITCs

RODENT STUDIES

Using 14C-PEITC synthesized in our laboratory with the a-carbon adjacent to the -N=C=S labeled with 14C, the tissue distribution and metabolism in mice treated with PEITC by gavage were studied (Eklind et al., 1990). After a single oral dose of 5 |imol (2 |Ci)/mouse 14C-PEITC in corn oil, a total of 50% of the dose was excreted in the urine within 48 h. Two major urinary metabolites were isolated in the urine and identified as a cyclic mercaptopyruvic acid conjugate and the NAC conjugate (Figure 7.2a). These metabolites accounted for 25 and 10%, respectively, of the administered PEITC dose.

Radioactivity in all major organs was counted, up to 72 h, after dosing of PEITC and was distributed readily in all tissues within 1 h after dosing and persisted up to 8 h. Lungs showed a maximal radioactivity between 4 and 8 h, after dosing, suggesting this time period would be optimal for inhibition. Benzyl ITC (BITC) has a similar metabolic fate as PEITC, and the main metabolite in the urine of rats dosed with BITC is the NAC conjugate (Brusewitz et al., 1977). Kassahun et al. (1997) showed that SFN is also metabolized in rats via GSH conjugation to excrete NAC conjugates in the urine as the major metabolite (>60% of the dose administered). Erucin, the sulfide analog of SFN, was produced as a metabolite, which was subsequently conjugated and excreted via mercapturic acid pathway as an NAC conjugate. These results indicate that a major metabolic pathway for ITCs through GSH conju-

Hplc Detection Degradation Thiols

Figure 7.2 HPLC analysis of a single dose of urinary metabolites of PEITC in (a) A/J mice after oral administration of 14C-PEITC. The upper panel are the UV standards of the cyclic mercaptopyruvic acid conjugate 1 and the NAC conjugate 2 and (b) humans after ingestion of watercress. The HPLC mobile phase used was different for each experiment.

Figure 7.2 HPLC analysis of a single dose of urinary metabolites of PEITC in (a) A/J mice after oral administration of 14C-PEITC. The upper panel are the UV standards of the cyclic mercaptopyruvic acid conjugate 1 and the NAC conjugate 2 and (b) humans after ingestion of watercress. The HPLC mobile phase used was different for each experiment.

gation is likely to be mediated by GST (such as GSTM1) via the mercapturic acid pathway.

We also conducted a disposition and pharmacokinetic study of PEITC and PHITC, two potent inhibitors against NNK-induced lung tumorigenesis, in F344 rats (Conaway et al., 1999). The purpose of this study was to address the question of why

PHITC is about two orders of magnitude more potent than PEITC (Morse et al., 1991). 14C-PEITC and 14C-PHITC were used for these studies. A single gavage dose of 50 |imol/kg (3.71 |Ci/|mol) 14C-PEITC or 50 |imol/kg (6.59 |Ci/|mol) 14C-PHITC in corn oil was administered. After 14C-PEITC dosing, whole blood 14C peaked at 2.9 h, with an elimination half-life of 21.7 h; blood 14C from 14C-PHITC-treated rats peaked at 8.9 h, with elimination half-life of 20.5 h.

In lungs, the target organ, the elimination half-life for 14C-PHITC and its labeled metabolites was more than twice that for 14C-PEITC and its labeled metabolites; the effective dose (area under the curve -AUC) for 14C from PHITC was >2.5 times the AUC of 14C from PEITC in liver, lungs, and several other tissues. During 48 h, approximately 16.5% of the administered dose of 14C-PHITC was expired as 14C-CO2, more than 100 times the 14C-CO2 expired by rats treated with 14C-PEITC. In rats given 14C-PEITC, 88.7±2.2% and 9.9±1.9% of the dose appeared in the urine and feces, respectively, during 48 h; however, rats given 14C-PHITC excreted 7.2±0.8% of the dose of 14C in urine, and 47.4±14.0% in the feces. This study concluded that higher effective doses of PHITC in the lungs and other organs may be the basis, in part, for its greater potency as a chemopreventive agent.

HUMAN STUDIES

The intake of ITCs in humans is primarily through the consumption of cruciferous vegetables. When the vegetables are chewed or chopped, glucosinolates are hydrolyzed by the enzymatic action of myrosinase to yield ITC. An example of ingestion of PEITC from gluconasturtiin is shown in Figure 7.3. The metabolic fate of PEITC in humans, after ingestion of watercress, is somewhat different from that in rodents. Like mice, humans process PEITC and other ITC, such as allyl ITC from mustard, primarily by conjugation with GSH via the mercapturic acid pathway. However, humans do not produce the cyclic mercaptupyruvic acid conjugate, but only excrete the NAC conjugate in urine (Chung et al., 1992; Jiao et al., 1994) (Figure 7.2b). Approximately 50% of the PEITC administered was excreted as the NAC metabolite in the urine within 24 h. The peak of excretion was 4 h after ingestion.

Since most vegetables are consumed after being cooked, and cooking destroys myrosinase, it is important to examine whether dietary glucosinolates are actually converted to ITCs after eating cooked vegetables. A urinary marker, based on a cyclocondensation product formed by the reaction of ITCs with 1,2-benzenedithiol, was used to quantify the uptake of ITCs in humans (Zhang et al., 1992; Chung et al., 1998). Approximately one third of PEITC was excreted as PEITC-NAC after eating a total of 350 g of cooked watercress compared with uncooked watercress. These results indicate that bioavailability of PEITC is markedly compromised by cooking. The fact that the cooked watercress is completely devoid of myrosinase activity for hydrolysis of glucosinolates to ITCs and yet ITC metabolites were still found in urine, suggests that intestinal microflora are likely to be involved in converting gluco-nasturtiin to PEITC after ingesting cooked watercress (Figure 7.4a). The conversion of gluconasturtiin to PEITC upon incubation with a human fecal preparation seems to support this notion (Getahun and Chung, 1999) (Figure 7.4b).

Mercapturic Pathway
Figure 7.3 Metabolism of gluconasturtiin and PEITC in rodents and humans. Hydrolysis of gluconasturtiin by myrosinase to PEITC followed by GSH conjugation and enzymatic degradation via the mercapturic acid pathway.

Similar results on SFN were obtained from another crossover study using cooked and uncooked broccoli (Conaway et al., 2001). In the study, 12 volunteers consumed 200 g of fresh or steamed broccoli. The average 24 h urinary excretion of ITC equivalents amounted to 32.3±12.7% and 10.2±5.9% of the amounts ingested for fresh and steamed broccoli, respectively. About 40% of ITCs in urine occurred as SFN-NAC. Figure 7.5 shows the time course of total ITC excretion up to 24 h after ingestion. Not only a substantial difference in the amount of ITC excreted, but also a small shift to a later time was noted after eating steamed vs. uncooked broccoli. Shapiro et al. (1998) have drawn the same conclusions from a similar study.

1 2 3 4 5 Retention time (min)

Figure 7.4 (a) HPLC analysis of a 24 h human urine sample after eating 350 g of cooked watercress; the peak at 3.2 min is ITC metabolites measured as the 1,2-benzenedithiol cyclocondensation product; (b) conversion of glucosinolates to ITCs upon incubation of juice of cooked watercress with human fecal homogenates.

Figure 7.4 (a) HPLC analysis of a 24 h human urine sample after eating 350 g of cooked watercress; the peak at 3.2 min is ITC metabolites measured as the 1,2-benzenedithiol cyclocondensation product; (b) conversion of glucosinolates to ITCs upon incubation of juice of cooked watercress with human fecal homogenates.

THE ROLE OF DIETARY ITCs IN THE PROTECTION AGAINST HUMAN CANCERS

There is ample evidence from animal studies supporting the potential protective effect of ITCs against cancers, yet there is little known about their roles in human cancers. It is well documented that consumption of cruciferous vegetables reduces

Baseline 0-2 h 2-4 h 4-8 h 8-12 h 12-24 h Time (hours)

Figure 7.5 Time course of urinary excretion of ITC after ingestion of (a) steamed vs. (b) uncooked broccoli in 12 volunteers.

Baseline 0-2 h 2-4 h 4-8 h 8-12 h 12-24 h Time (hours)

Figure 7.5 Time course of urinary excretion of ITC after ingestion of (a) steamed vs. (b) uncooked broccoli in 12 volunteers.

the risk of certain human cancers, including colorectal cancer; however, the exact nature of ingredients contributing to the beneficial effect is still not clear. There are many active compounds in cruciferous vegetables that may contribute to cancer protection.

From a chemoprevention point of view, it is important to know whether the beneficial effects of crucifers come, at least in part, from ITCs. To this end, we have collaborated with Dr. Mimi Yu and colleagues at the Norris Cancer Center of USC to apply the validated urinary marker of dietary ITCs developed in our laboratory (Seow et al., 1998). These studies have allowed us to evaluate the protective role of dietary ITCs in human cancers, such as lung, stomach, esophagus, and colon. The examination of the relationship between the amount of ITCs in urine, collected before diagnosis, of 232 lung cancer patients and 710 matched controls from a cohort of 18,244 men in Shanghai, China, followed from 1986 to 1997, has been recently completed. More than 80% of the cases are current smokers, whereas only 47% of the controls are smokers. The protective effect of dietary ITC against lung cancer was supported by the findings that individuals with detectable levels of ITCs in the urine are less likely to develop lung cancer than those with no detectable ITCs (RR,95% CI,0.60-0.65). More interesting was the observation that the reduction in risk was strongest among individuals genetically deficient in GSTM1 and T1 (RR,95% CI,0.28-0.30). Since GSTM1 is shown to be involved in conjugation of ITCs to eliminate ITCs via the mercapturic acid pathway, these results provide support for the role of ITCs in lowering risk of lung cancer (London et al., 2000).

Thus far, few studies have examined the direct relationship of ITC and human cancer. This study is the first to provide direct evidence to support the role of dietary ITCs in the prevention of human cancer, and it also suggests that the protective effect of ITCs may vary depending on the individual genetic makeup in ITC metabolism. These results warrant future clinical trials to directly evaluate the effects of ITCs on human cancers or other alternative cancer biomarkers.

CONCLUSION

ITCs are natural products that humans consume through eating cruciferous vegetables, such as watercress, broccoli, and cabbage. These compounds are readily taken up and metabolized by tissues in rodents and humans, and exhibit activities against chemical-induced carcinogenesis in various animal models. Evidence from epidemiological studies show that these compounds may play a role in the prevention of certain human cancers, a claim supported by mechanistic data from in vitro and in vivo studies. However, not everyone likes these vegetables (a notable example is former President Bush who does not like broccoli). The extensive evaluation and development of some ITCs as chemopreventive agents in clinical trials presents a practical alternative to the dietary sources.

ACKNOWLEDGMENTS

These studies would not be possible without the collaboration of many colleagues over the years. Specifically, I would like to thank Mark Morse, Karin Eklind, Stephen Hecht, Ding Jiao, Cliff Conaway, Serkadis Getahun, Shantu Amin, Bandaru Reddy, J.W. Chiao, Mimi Yu, Debra Topham, Leonard Liebes, Maria Botero-Omary, and Donald Pusateri.

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Keep Your Weight In Check During The Holidays

Keep Your Weight In Check During The Holidays

A time for giving and receiving, getting closer with the ones we love and marking the end of another year and all the eating also. We eat because the food is yummy and plentiful but we don't usually count calories at this time of year. This book will help you do just this.

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