Using Salmonid Microarrays to Understand the Dietary Modulation of Carcinogenesis in Rainbow Trout

TOXICOLOGICAL SCIENCES 90(1), 1–4 (2006) doi:10.1093/toxsci/kfj093 TOXICOLOGICAL HIGHLIGHT Using Salmonid Microarrays to Understand the Dietary Modulation of Carcinogenesis in Rainbow Trout Evan P. Gallagher1 Received December 22, 2005; accepted January 3, 2006 The highlighted article in this issue by Tilton et al. (2005a) is an innovative approach to evaluate the modulation of estrogen receptor (ER) and aryl hydrocarbon (Ah)-receptor pathways as mechanisms underlying indole-3-carbinol (I3C) tumor promotion in rainbow trout (Onchorhynchus mykiss). I3C and its major in vivo component 3,3#-diindolylmethane (DIM) are potent tumor promoters that appear to target both of the aforementioned receptor pathways. However, the relative importance of I3C modulation of ER and AhR-dependent pathways in the promotion of rainbow trout hepatocarcinogenesis has not been established. Previously, researchers within this group reported that I3C promotes aflatoxin B1 (AFB1)-induced trout hepatocarcinogenesis post-initiation at low concentrations in the diet that induce the expression of vitellogenin, a downstream marker for the activation of ER-dependent pathways in fish. Furthermore, the promotional effects of I3C on AFB1 hepatocarcinogenesis in rainbow trout occur at concentrations that differentially induce vitellogenin, but not CYP1A expression. Interestingly, higher I3C concentrations induce the expression of both CYP1A and vitellogenin. Thus, the relative induction of vitellogenin and CYP1A expression, which are respective markers for activation of fish ER and AhRmediated gene expression, suggest that these pathways may be important for tumor promotion by dietary I3C in trout. Understanding the complexities of I3C-mediated tumor promotion is essential from several perspectives. For example, there is the obvious need to increase our basic understanding of dietary modulation of carcinogenesis. In addition, I3C also exhibits significant antioxidant and cancer chemoprotective effects under certain experimental conditions and in certain models which have led to its recent marketing as a dietary supplement, as well as its development as a possible chemopreventive agent in humans. Key Words: indole-3-carbinol; 3,3#-diindolylmethane; tumor promotion; aflatoxin B1; rainbow trout; toxicogenomics. 1 To whom correspondence should be addressed at Department of Environmental Occupational Health Sciences, 4225 Roosevelt Way, Suite 100, Seattle, WA 98105–6099. Fax: (206) 685-4696. E-mail: . The utility of rainbow trout as an experimental model to study the mechanisms of dietary carcinogenesis has been firmly established via an impressive array of studies conducted at Oregon State University over the past 40 years, and largely through the NIEHS-funded Marine and Freshwater Biomedical Center. In this regard, Drs. Sinnhuber and Bailey recognized early on the promise of the rainbow trout as a model of exquisite sensitivity for cancer studies, subsequently publishing a number of reports arising from studies designed to reduce the risk of aflatoxin B1 (AFB1) liver cancer. The strengths of the trout model reside in its sensitivity to several classes of carcinogens, including AFB1, and well-described tumor pathology (Williams et al., 2003). In addition, trout exhibit very low spontaneous tumor backgrounds over the typical 9–12 month period it requires to produce tumors. Relative to rodents, trout have a low cost associated with husbandry for large-scale cancer studies. The ability to rear and maintain large numbers of clonally-derived trout have enabled Bailey and colleagues to design and conduct large-scale tumor studies requiring thousands of animals and that address statistically challenging dose-response questions. The development of clonal trout lines and triploid individuals has lent new approaches to the study of cancer genetics in this experimental model. An example of the experimental utility of the trout as a cancer model is evidenced by a report that identified indole-3-carbinol (I3C) as a tumor promoter for AFB1-initiated liver cancer at doses near those recommended for dietary supplementation in humans (Oganesian et al., 1999). A subsequent report of the largest animal tumor study ever conducted, which included 42,000 trout and which addressed dibenzo[a,l]pyrene (DBP) carcinogenesis to a 1:5,000 incidence, resulted in data predicting that a DBP dose producing 1 in 106 cancers is 1,000-fold higher than predicted by the conservative linear model (Williams et al., 2003). When similar studies are confirmed with other carcinogens, including those with genotoxic and potentially nongenotoxic mechanisms of action and with other cellular targets, the rainbow trout model has the ability to greatly impact the utilization of animal tumor data in the context of human risk assessment. Ó The Author 2006. Published by Oxford University Press on behalf of the Society of Toxicology. All rights reserved. For Permissions, please email: Department of Environmental and Occupational Health Sciences, University of Washington, Seattle, Washington 98105 2 GALLAGHER vegetables and some of their specific compounds have been shown to modulate carcinogenesis in animals and humans. Among these agents is glucobrassican, which is hydrolyzed by endogenous plant enzymes to release I3C. This compound has been widely studied and has been shown to have chemopreventive effects against development of chemically induced tumors in several species and using a variety of initiation agents including AFB1 (Bailey et al., 1987), polycyclic aromatic hydrocarbons (Grubbs et al., 1995) nitroso-compounds (Grubbs et al., 1995), and heterocyclic aromatic amines (Mori et al., 1999). In addition, there is evidence to indicate that dietary I3C prevents the development of estrogen-enhanced cancers including breast (Grubbs et al., 1995) and cervical cancers (Yuan et al., 1999) in animals. This broad range of I3C cancer chemoprotection in various species, disease states, and organ sites, as well as its potent antioxidant activities (Shertzer and Senft, 2000), has supported studies targeting the development of I3C as a cancer chemopreventative agent, and also its consumer marketing as a dietary supplement. However, despite its demonstrated chemoprotective effects, dietary I3C has paradoxically been found to promote tumor formation in multiple organs in rodents (Kim et al., 1997) and trout (Oganesian et al., 1999). In the case of AFB1-induced liver cancer in rainbow trout, the chemopreventive properties of I3C occur when the compound is administered in the diet concurrent, or prior to exposure to carcinogenic agents, thus appearing to blocking the initiation of DNA injury (Dashwood et al., 1991). In contrast, the tumor promoting effects of I3C occur appears to involve prolonged exposure when the compound is administered after the chemical initiation of DNA injury (Dashwood et al., 1991). Furthermore, it is possible that the promotional pote (...truncated)