Selenium and Glucosinolates in Cruciferous Vegetables: Metabolic Interactions and Implications for Cancer Chemoprevention in Humans

Cancer is a leading cause of death in North America, and poor diet is suspected as a causative factor in 1/3 of all cancer deaths. Epidemiologic studies have shown that a diet rich in cruciferous vegetables such as broccoli is protective against many cancers. Broccoli contains sulforaphane (SI), a substance that activates promoter elements in certain phase II detoxification enzymes of animals, and upregulation of these genes may increase the rate of clearance of potential carcinogens. Broccoli also accumulates the mineral selenium (Se) and clinical trials have demonstrated that Se may decrease the incidence of prostate and lung cancer. A series of studies have examined the interaction of Se and Sf in broccoli, and the benefit to the health of the animal that consumes broccoli. Selenium-enhanced broccoli reduces the incidence of colon cancer in rats, but increasing the Se content of broccoli results in a corresponding decrease in the Sf content. Sf and Se also interact in the animal that consumes broccoli to upregulate the antioxidant enzyme thioredoxin reductase (TR). TR activity has been shown to be related to oxidative stress in the cell. These studies demonstrate potential problems that may be encountered if a single substance in a plant is targeted for enhancement. They further show that modified plant foods may cause unintended metabolic interactions in the animal that consumes them. INTRODUCTION Cancer is a leading cause of death in the U.S., and the American Cancer Society estimates that 1/3 of all cancer deaths are related to dietary factors (American Cancer Society, 1999). Lack of anti-carcinogenic compounds in our diet (e.g. tocopherols, ascorbic acid, selenium, polyphenolics and isothiocyanates) may contribute more to our risk of developing cancer than the presence of dietary carcinogenic compounds (e.g. heterocyclic amines and aflatoxin). This hypothesis is supported by studies reporting a reduction in cancer risk with increased intake of fruits and vegetables (Heber and Bowerman, 2001). The primary cancer protective benefit of such a dietary change may not be associated with decreased fat or increased fiber intake as much as it is with the increased intake of nonnutritive dietary components that have biological activity such as destruction of oxygen radicals and activation of beneficial enzyme systems. Epidemiologic study reports have found an inverse relationship between the intake of cruciferous vegetables and the risk of lung, pancreas, bladder, prostate, thyroid, skin, stomach and colon cancer cancer (Verhoeven et al., 1996), and this association may be stronger than the association between cancer risk and fruit and vegetable intake in general (Michaud et al., 1999). Consequently, the National Research Council, Committee on Diet, Nutrition and Cancer (1982) has specifically recommended that Americans increase Consumption of cruciferous vegetables (National research council, 1982). Crucifers contain multiple compounds that may have biological activity for chemoprevention; among the most studied are glucosinolates (GS) (Fenwick et al., 1983), precursor compounds that may be converted in the gut to isothiocyanates. Other potential bioactive components include flavonoids such as quercetin (Williamson et al., 1996), minerals such as selenium (Se) (Finley et al., 2000b), S-methyl cysteine sulfoxide and 1,2Proc. JSt is on Hum. Health Effects of F&V Ed.: Y.Desjardins 171 Acta Hurt. 744, iSH5 2007 dithiole-3-thione (Jeffery and Jarrell, 2001). Our laboratory has extensively studied the interaction of two potentially anti-carcinogenic compounds in broccoli (Brass/ca oleracea) the glucosinolate breakdown product, sulforaphane (St), and the essential mineral selenium (Se). We have shown that enhancing broccoli with selenium results in reduction of Sf (and vice-versa) and we have further shown that when an animal consumes broccoli, the two compounds interact to regulate the expression of genes that may be involved in preventing oxidative stress and carcinogenesis. Glucosinolates, Sf and Chemoprotective Mechanisms The chemical structures of GS are similar in all the plants in which they are present (>3000 crucifer species). Their basic structure consists of a f3-D-thioglucose group, a sulfonated oxime group and a side chain derived from methionine, phenylalanine tryptophane or branched-chain amino acids. The sulfate group of a GS molecule is strongly acidic and plants accumulate GS by sequestering them as potassium salts in plant vacuoles (for a complete review see (Keck and Finley, 2004)). More than 120 GS have been characterized, but no essential role for GS in plant metabolism has been found. The content and form of GS within a plant depends on the particular variety. Glucobrassicin and glucoraphanin are generally found in high concentrations in broccoli (0.1-2.8 and 0.8-21.7 mmol/g DW, respectively) and constitute as much as 95% of the total amount of GS (Kushad et al., 1999). In contrast, Brussels sprouts, cabbage and cauliflower contain little or no glucoraphanin. Glucosinolates are not bioactive in the animal that consumes them until they have been enzymatically hydrolyzed, either by the endogenous myrosinase (released during the disruption of the plant cell by harvesting, processing or mastication) or by the action of gut microflora (Rabot et al., 1993). Hydrolysis produces of glucoraphanin produces Sf. A primary mechanism by which glucosinolates may inhibit carcinogenesis is by alteration of phase I and II enzyme detoxification pathways (Zhang and Talalay, 1998), leading to decreased activation of pro-carcinogens and increased excretion of carcinogens. Some compounds activate only phase 11 enzymes through a transcriptional regulatory element common to multiple genes such as glutathione-S-transferase Al, quinone reductase (QR) and thioredoxin reductase (TR); this element is termed the Anti-oxidant Response Element (ARE; (see Finley, 2003 for review.) The ARE responds to a number of chemicals with structures similar to Michael reaction acceptors, but the most powerful dietary-derived constituent is Sf (Zhang et al., 1992; Morimitsu et al., 2002). In addition to their detoxification functions, many of these enzymes also have powerful antioxidant properties. For example quinone reductase (Favreau and Pickett, 1993) catalyzes the reduction of quinones and prevents redox cycling and oxidative stress; gamma-glutamylcysteine synthetase catalyzes the rate limiting step in glutathione bio-synthesis (Jeyapaul and Jaiswal, 2000), one of the most ubiquitous circulating anti-oxidants; TR is the enzyme that reduces thioredoxin and the ascorbyl radical (Mustacich and Powis, 2000). Selenium and Cancer Protection A randomized, placebo-controlled human clinical trial demonstrated that the micronutrient Se protects against several common cancers. Compared to placebo controls, supplementation of humans with 200 ptg Se/d as Se-enriched-yeast significantly reduced overall cancer mortality (Duffield-Lilljco et al., 2002), prostate cancer (Duffield-Lillico et al., 2003) and lung cancer (Reid et al., 2002). The reduction of prostate cancer risk by selenium is also substantiated by numerous epidemiologic studies of selenium intake and prostate cancer risk. Selenium is found covalently bound to many diverse molecules (see Fig. I for a picture of overall selenium metabolism), and the efficacy of a particular source of selenium for inhibition of carcinogenesis depends in part on the chemical form. In general, it now appears that forms of Se that are metabolized through the methylation pathway in preparation for excretion in the urine are the most efficacious. A secondary consequence of metabolism through the methylation pathway is that such forms of Se are not readily