Benefits and Risks of Supplementation with Indole Phytochemicals
Susan C. Tilton, Ph.D.
Consumption of cruciferous vegetables, such as broccoli, cauliflower, Brussels sprouts, and cabbage, has been associated with decreased risk of several types of cancer in epidemiological studies. These vegetables contain substantial amounts of sulfur-containing compounds known as glucosinolates, which break down into isothiocyanates and indoles when the vegetables are chopped or chewed. There is strong interest among researchers to evaluate the health effects of these glucosinolate-derived compounds compared to the whole vegetable. Our laboratory is particularly interested in the cancer chemoprotective effects of the indole compounds, indole-3-carbinol (I3C) and its primary acid condensation product in the stomach, 3,3'-diindolylmethane (DIM). We are also interested in determining if there are any possible toxicities associated with long-term consumption of I3C and DIM, which are both currently available over the counter as dietary supplements. Supplement companies suggest an intake of 200-1,200 mg/day, which equates to 3-17 milligrams/ kilogram/day (mg/kg/day) for a 70-kg adult.
From previous studies in our lab and by other scientists, we know that I3C can inhibit the development of cancer in the trout liver and in multiple tissues of rodents, including breast, colon, stomach, lung, and liver. Protection by I3C is typically observed in animals when it is fed in the diet prior to or concurrent with exposure to the carcinogen. Thus, I3C can be considered a “blocking agent” because it blocks the initial steps required for cancer development. It likely does this by inducing enzymes responsible for the metabolism and excretion of the carcinogen. However, when I3C is fed to animals in the diet long-term following exposure to the carcinogen, it actually promotes or enhances the development of some cancers by an unknown mechanism. We have observed cancer promotion by I3C in the trout liver, and subsequent studies found similar promotion by I3C in rat thyroid, uterus, liver, and colon. The contradictory effects of I3C in animal studies raise questions about the safety of long-term supplementation with indole phytochemicals in humans. Therefore, we are currently investigating how I3C enhances cancer development in the rainbow trout model to better understand its potential relevance for humans.
Rainbow trout have been used as a biomedical research model for over 40 years at Oregon State University and in the Linus Pauling Institute by multiple investigators, including Drs. George Bailey, David Williams, and Rod Dashwood. Data from studies on cancer development and progression, carcinogen sensitivity, and responsiveness to dietary supplementation in trout support the use of these animals as a model for human carcinogenesis, particularly in liver cancer. We have also examined genes important for development of trout liver tumors and found they are very similar to genes expressed in liver tumors in humans and other animals.
Recent studies indicate that comparative analyses of gene profiles across diverse species are more likely to highlight functional gene interactions important in key mechanisms of human carcinogenesis. We've applied the same comparative approach to our studies of tumor enhancement by indole phytochemicals in trout.
To examine how I3C may promote liver cancer, we first looked for proteins in the liver that were induced by I3C and could be used as markers for I3C exposure. We found that I3C induces a metabolizing enzyme, CYP1A, which was expected, since we know I3C can induce certain metabolism enzymes in liver. We also observed the unexpected induction of a protein that only responds to estrogens in trout liver, suggesting that I3C may be acting like an estrogen. We know that estrogens can enhance cancer development in the liver of trout and rodents. We also know that some inducers of the CYP1A enzyme can promote liver cancer, so we continued to investigate the possibility that I3C was acting through one of these mechanisms in its ability to promote liver cancer.
Our next step was to take a more global approach by using microarray analysis to examine the expression of several thousand genes in the liver simultaneously after exposure to I3C rather than looking at only a few protein markers. We analyzed gene expression in trout livers after dietary exposure to I3C and DIM (24 and 78 mg/kg/day) and compared the gene expression profile to those of two known liver tumor promoters, 17β-estradiol (endogenous estrogen hormone) and β-naphthoflavone (a known inducer of CYP1A). We found that both I3C and DIM have very similar gene profiles to estradiol, suggesting that they act like estrogens in trout liver and likely promote liver cancer by this mechanism. Interestingly, DIM was a stronger estrogen than I3C. This data led us to question whether DIM would also promote liver cancer in trout similar to estradiol and I3C. Only a few studies have examined the effects of DIM on cancer development in animals, and none have examined the potential for DIM to promote cancer.
We performed a tumor study in trout to examine the effects of DIM on cancer development compared to 17β-estradiol. Feeding the trout either DIM (24 mg/kg/day) or estradiol in the diet long-term following acute exposure to a liver carcinogen resulted in more tumors compared to control animals. Concentrations of DIM lower than 24 mg/kg/day did not significantly increase the number of tumors. We confirmed that DIM was promoting cancer by acting like an estrogen by comparing gene expression in the liver samples over the course of cancer development. At all timepoints examined, gene expression in liver samples from DIM and estradiol-treated animals were very similar, indicating they are working by the same mechanism.
Metastasis is a process in which malignant tumors spread from their primary site to other areas in the body and is associated with a decreased prognosis for treatment and higher mortality. Interestingly, the malignant tumors observed in our study may be less likely to metastasize to other organs and, therefore, may be less aggressive. Therefore, the prospect that dietary indoles may protect against metastasis is a very interesting area of future research for our laboratory. However, these potential benefits need to be weighed against the tumor enhancement that was observed in this study.
Overall, our data indicate that I3C and DIM can promote liver cancer in trout by acting as estrogens, but we don't yet know if this is relevant to other animals.
These results may help explain some of the contradictory data observed on the cancer protective and promoting effects of dietary indole phytochemicals. However, it continues to raise questions about the safety of supplementation with I3C and DIM. Research in our lab will continue to evaluate the relative risks, as well as benefits, of long-term supplementation with dietary indole phytochemicals.
Last updated June, 2006