Review
Cancer-preventive isothiocyanates: measurement of human exposure and mechanism of action

https://doi.org/10.1016/j.mrfmmm.2004.04.017Get rights and content

Abstract

Numerous studies in rodents have documented the cancer-preventive activity of a significant number of isothiocyanates (ITCs), the majority of which occur in plants, especially in cruciferous vegetables. Dietary ITCs may play an important role in the prevention of human cancers. Several recent epidemiological studies have already shown that dietary consumption of ITCs inversely correlates with the risk of developing lung, breast and colon cancers. ITCs are principally metabolized through the mercapturic acid pathway in vivo, giving rise to N-acetylcysteine conjugates, which are excreted in the urine. Analytical methods have been developed to allow detection of ITCs and their metabolites formed in the mercapturic acid pathway. Studies show that total urinary level of ITC equivalent is an excellent biomarker of human exposure to ITCs. Moreover, these methods also have made it possible to learn the bioavailability of ITCs from cruciferous vegetables. ITCs possess multiple anticarcinogenic mechanisms, including inhibition of carcinogen-activating enzymes, induction of carcinogen-detoxifying enzymes, increase of apoptosis, arrest of cell cycle progression, as well as several other mechanisms that are not yet fully described. These mechanisms, which are discussed in detail in this review, illustrate the remarkable ability of ITCs to inhibit cancer development—effective against both developing and developed cancer cells.

Introduction

Isothiocyanates (ITCs) are a family of compounds derived almost exclusively from plants, although marine sponges and fungi also have been reported to produce a few ITCs [1]. They are synthesized and stored as glucosinolates (β-thioglucoside N-hydroxysulfates, GS) in plants and are released when damage to plant tissue occurs. The conversion is catalyzed by myrosinase (thioglucoside glucohydrolase, EC 3.2.3.1), an enzyme that coexists with but is physically separated from GS in normal plants (Fig. 1). However, it is also known that under certain conditions hydrolysis of GS may lead to non-ITC products, including thiocyanates, nitriles, epithionitriles, indoles, and oxazolidine-2-thiones. Extensive information on myrosinase, GS hydrolysis, and product formation is available elsewhere [1], [2], [3], [4], [5], [6]. Moreover, GS that escape the plant myrosinase may be hydrolyzed in the intestinal tract, as the microflora are known to possess myrosinase activity [6], [7], [8]. Although at least 120 different GS have been identified in various plants [1], the majority of which likely produce ITCs, only a small number of ITCs may be commonly consumed by humans. Cruciferous vegetables are the principle dietary source of ITCs, but the types of crucifers frequently consumed by humans are limited. Examples of popular crucifers that are particularly rich in certain ITCs include mustard and horseradish—allyl-ITC (AITC) [9], watercress—phenethyl-ITC (PEITC) [10], dikon—dehydroerucin [1], [11], and broccoli and broccoli sprouts—sulforaphane (SF) [12], [13] (see Fig. 2 for chemical structure).

To date, the most important known biological activity of ITCs is their ability to inhibit cancer development. There is convincing evidence that certain natural ITCs, such as AITC, benzyl ITC (BITC), PEITC and SF, as well as a number of synthetic analogs, are effective inhibitors of chemically induced tumors in one or more organ sites of rodents, including the bladder, colon, esophagus, mammary glands, pancreas, and stomach. Numerous studies have been performed in this area, and many comprehensive review articles have been written [14], [15], [16], [17], [18]. Moreover, in agreement with the rodent results, several recent epidemiological studies already show that dietary intake of ITCs is inversely correlated with cancer risk of several organ sites, as discussed in Section 4. These findings have generated much enthusiasm about ITCs as potential cancer-preventive agents in humans. Much of the molecular basis of the cancer-preventive activity of these compounds has been learned, as described in Section 5, which indicates that ITCs not only inhibit the development of cancer cells, but also eliminate established cancer cells. For example, both AITC and SF significantly inhibited the growth of PC-3 human prostate cancer cell xenografts in mice [19], [20].

All ITCs are characterized by the presence of an single bondNdouble bondCdouble bondS group, whose central carbon often is highly electrophilic. The biological activities of ITCs, perhaps their toxic effects as well, may be primarily mediated through the reaction of this carbon atom with cellular nucleophilic targets. In contrast, GS are not electrophilic, and there is no clear evidence that intact GS are cancer-preventive. It is believed that the side chains of ITCs may play secondary roles, e.g., affecting the electrophilicity of the single bondNdouble bondCdouble bondS group, altering the steric hindrance to the reactive carbon atom, and controlling the lipophilicity of the molecule. The single bondNdouble bondCdouble bondS group also governs the metabolism of ITCs. ITCs are metabolized in vivo principally by the mercapturic acid pathway: an initial conjugation through the single bondNdouble bondCdouble bondS group with glutathione (GSH), which takes place spontaneously but is further promoted by glutathione transferases (GST) [21], [22], [23], gives rise to the corresponding conjugates. The GSH conjugates then undergo further enzymatic modifications (modifications of the GSH portion) to form sequentially the cysteinylglycine-, cysteine-, and N-acetylcysteine (NAC)-conjugates, which are excreted in urine [24] (Fig. 3).

Section snippets

The cyclocondensation assay

Recent development of a highly sensitive and quantitative method, namely the cyclocondensation assay, for measuring ITCs and their mercapturic acid pathway metabolites [25] (Fig. 4), has provided a valuable tool to better understand human consumption of dietary ITCs, their metabolism and disposal in vivo, and cell and tissue exposure to these compounds. This assay was developed based on the discovery of the almost universal ability of the central carbon of ITCs to undergo successive

Measuring human uptake of dietary ITCs

In agreement with the observation that ITCs are readily absorbed, principally metabolized in vivo through the mercapturic acid pathway, and excreted in urine as NAC–ITCs, many studies have shown that dietary ITC intake can be measured by detection of its NAC conjugate in the urine. For example, when BITC was administered orally to humans, 54% of the dose (14.4 mg) was recovered in the urine as NAC–BITC. NAC–BITC was excreted rapidly; maximum excretion occurred 2–6 h after and was essentially

Evidence of cancer-preventive activity of ITCs in humans

A few studies on the relationship between exposure to ITCs and human cancer risk have been reported, although there have been no studies on the effect of dietary ITCs on human bladder cancer.

London et al. [45] conducted a case–control study in which a cohort of 18,244 men in Shanghai, China was followed from 1986 to 1997. Each participant provided a single-void urine sample at the start of the study. Of the 232 individuals that subsequently developed lung cancer and 710 matched controls, 89.7

The molecular basis of cancer-preventive effects of ITCs

It has been well established that ITCs can inhibit cancer development through multiple mechanisms, including: (i) protecting DNA by modulating carcinogen-metabolizing enzymes; (ii) reducing oxidative stress by elevating and maintaining cellular antioxidants; (iii) inhibiting cell proliferation, thereby retarding or eliminating clonal expansion of initiated, transformed, and/or neoplastic cells. Other effects, including anti-inflammation, anti-infection, and perhaps induction of differentiation

Concluding remarks

ITCs, many of which are commonly present in the human diet and are derived from cruciferous vegetables, are highly promising cancer-preventive agents. Analytical methods have been developed to allow assessment of human exposure to dietary ITCs, and to understand the bioavailability of these compounds from dietary sources. There is epidemiological evidence that dietary ITCs may indeed provide protection against human cancers. ITCs possess multi-faceted cancer-preventive mechanisms, capable of

Acknowledgement

I would like to thank my coworkers Jun Li, Joseph D. Paonessa, and Li Tang for critical reading of this manuscript. This work was supported in part by National Cancer Institute grants CA 80962 and CA100623.

References (124)

  • W.G. Kirlin et al.

    Dietary compounds that induce cancer preventive phase 2 enzymes activate apoptosis at comparable doses in HT29 colon carcinoma cells

    J. Nutr.

    (1999)
  • L. Payen et al.

    Reactive oxygen species-related induction of multidrug resistance-associated protein 2 expression in primary hepatocytes exposed to sulforaphane

    Biochem. Biophys. Res. Commun.

    (2001)
  • T.H. Rushmore et al.

    The antioxidant responsive element

    J. Biol. Chem.

    (1991)
  • K. Itoh et al.

    An Nrf2/small Maf heterodimer mediates the induction of Phase II detoxifying enzyme genes through antioxidant response elements

    Biochem. Biophys. Res. Commun.

    (1997)
  • E.D. Owuor et al.

    Antioxidants and oxidants regulated signal transduction pathways

    Biochem. Pharm.

    (2002)
  • J.W. Fahey et al.

    Antioxidant functions of sulforaphane: a potent inducer of phase II detoxification enzymes

    Food Chem. Toxicol.

    (1999)
  • A.T. Dinkova-Kostova et al.

    Persuasive evidence that quinone reductase type 1 (DT diaphorase) protects cells against the toxicity of electrophiles and reactive forms of oxygen

    Free Rad. Biol. Med.

    (2000)
  • K. Xu et al.

    Involvement of glutathione metabolism in the cytotoxicity of the phenethyl isothiocyanate and its cysteine conjugate to human leukemia cells in vitro

    Biochem. Pharm.

    (2001)
  • Y. Nakamura et al.

    Involvement of the mitochondrial death pathway in chemopreventive benzyl isothiocyanate-induced apoptosis

    J. Biol. Chem.

    (2002)
  • M. Murata et al.

    Mechanism of oxidative DNA damage induced by carcinogenic allyl isothiocyanate

    Free Rad. Biol. Med.

    (2000)
  • S.R.R. Musk et al.

    On the cytotoxicity and genotoxicity of allyl and phenethyl isothiocyanates and their parent glucosinolates sinigrin and gluconasturtiin

    Mut. Res.

    (1995)
  • S.R.R. Musk et al.

    Cytotoxic and clastogenic effects of benzyl isothiocyanate towards cultured mammalian cells

    Food Chem. Toxicol.

    (1995)
  • J.K. Dunnick et al.

    Carcinogenesis bioassay of allyl isothiocyanate

    Fundam. Appl. Toxicol.

    (1982)
  • A. Adesida et al.

    Inhibition of human leukaemia 60 cell growth by mercapturic acid metabolites of phenylethyl isothiocyanates

    Food Chem. Toxicol.

    (1996)
  • V. Lambrix et al.

    The Arabidopsis epithiospecifier protein promotes the hydrolysis of glucosinolates to nitriles and influences trichoplusia ni herbivory

    Plant Cell

    (2001)
  • G.R. Fenwick et al.

    Glucosinolates and their breakdown products in food and food plants

    CRC Crit. Rev. Food Sci. Nutr.

    (1983)
  • A.M. Bones et al.

    The myrosinase–glucosinolate system, its organization and biochemistry

    Physiol. Plant.

    (1996)
  • L. Rask et al.

    Myrosinase: gene family evolution and herbivore defense in Brassicaceae

    Plant Mol. Biol.

    (2000)
  • S. Rabot et al.

    Rape-seed meal toxicity in gnotobiotic rats: influence of a whole human faecal flora or single human strains of Escherichia coli and Bacteroides vulgatus

    Brit. J. Nutr.

    (1993)
  • T.A. Shapiro et al.

    Human metabolism and excretion of cancer chemoprotective glucosinolates and isothiocyanates of cruciferous vegetables

    Cancer Epidemiol. Biomark. Prev.

    (1998)
  • S.M. Getahun et al.

    Conversion of glucosinolates to isothiocyanates in humans after ingestion of cooked watercress

    Cancer Epidemiol. Biomark. Prev.

    (1999)
  • Y. Uematsu et al.

    Determination of isothiocyanates and related compounds in mustard and horseradish extract used as natural food additives

    Shokuhin Eiseigaku Zassihi

    (2002)
  • F.L. Chung et al.

    Quantitation of human uptake of the anticarcinogen phenethyl isothiocyanate after a watercress meal

    Cancer Epidemiol. Biomark. Prev.

    (1992)
  • Y. Nakamura et al.

    4-(Methylthio)-3-butenyl isothiocyanate, a principal antimutagen in daikon (Raphanus sativus; Japanese white radish)

    J. Agric. Food Chem.

    (2001)
  • Y. Zhang et al.

    A major inducer of anticarcinogenic protective enzymes from broccoli: isolation and elucidation of structure

    Proc. Natl. Acad. Sci. USA

    (1992)
  • J.W. Fahey et al.

    Broccoli sprouts: an exceptionally rich source of inducers of enzymes that protect against chemical carcinogens

    Proc. Natl. Acad. Sci. USA

    (1997)
  • Y. Zhang et al.

    Anticarcinogenic activities of organic isothiocyanates: chemistry and mechanisms

    Cancer Res.

    (1994)
  • P. Talalay et al.

    Chemoprotection against cancer by isothiocyanates and glucosinolates

    Biochem. Soc. Trans.

    (1996)
  • S.S. Hecht

    Chemoprevention by isothiocyanates

    J. Cell Biochem.

    (1995)
  • S.S. Hecht

    Inhibition of carcinogenesis by isothiocyanates

    Drug Metab. Rev.

    (2000)
  • C.C. Conaway et al.

    Isothiocyanates as cancer chemopreventive agents: their biological activities and metabolism in rodents and humans

    Curr. Drug Metab.

    (2002)
  • S.K. Srivastava et al.

    Allyl isothiocyanate, a constituent of cruciferous vegetables, inhibits growth of PC-3 human prostate cancer xenografts in vivo

    Carcinogenesis

    (2003)
  • A.V. Singh et al.

    Sulforaphane induces caspase-mediated apoptosis in cultured PC-3 human prostate cancer cells and retards growth of PC-3 xenografts in vivo

    Carcinogenesis

    (2004)
  • R.H. Kolm et al.

    Isothiocyanates as substrates for human glutathione transferases: structure–activity studies

    Biochem. J.

    (1995)
  • D. Meyer et al.

    Forward and reverse catalysis and product sequestration by human glutathione S-transferase in the reaction of GSH with dietary aralkyl isothiocyanates

    Biochem. J.

    (1995)
  • G. Brusewitz et al.

    The metabolism of benzyl isothiocyanate and its cysteine conjugate

    Biochem. J.

    (1977)
  • F.-L. Chung et al.

    A urinary biomarker for uptake of dietary isothiocyanates in humans

    Cancer Epidemiol. Biomark. Prev.

    (1998)
  • M. Vermeulen et al.

    Analysis of isothiocyanate metcapturic acids in urine: a biomarker for cruciferous vegetable intake

    J. Agric. Food Chem.

    (2003)
  • C.C. Conaway et al.

    Decomposition rates of isothiocyanate conjugates determine their activity as inhibitors of cytochrome P450 enzymes

    Chem. Res. Toxicol.

    (2001)
  • K. Kassahun et al.

    Biotransformation of the naturally occurring isothiocyanate sulforaphane in the rat: identification of phase 1 metabolites and glutathione conjugates

    Chem. Res. Toxicol.

    (1997)
  • Cited by (299)

    • Cruciferous vegetables: a mine of phytonutrients for functional and nutraceutical enrichment

      2022, Current Advances for Development of Functional Foods Modulating Inflammation and Oxidative Stress
    • Leveraging Horseradish's Bioactive Substances for Sustainable Agricultural Development

      2023, International Journal of Sustainable Development and Planning
    View all citing articles on Scopus
    View full text