Biological Activities

Effects on biotransformation enzymes

Biotransformation enzymes play major roles in the metabolism and elimination of many biologically active compounds, including physiologic regulators (e.g., estrogens), drugs, and environmental chemicals (xenobiotics; e.g., carcinogens, toxins). In general, phase I metabolizing enzymes, including the cytochrome P450 (CYP) family, catalyze reactions that increase the reactivity of hydrophobic (fat-soluble) compounds, which prepares them for reactions catalyzed by phase II detoxifying enzymes. Reactions catalyzed by phase II enzymes usually increase water solubility and promote the elimination of these compounds.

Aryl hydrocarbon receptor (AhR) pathway

I3C and some I3C condensation products can bind to a protein in the cytoplasm of cells called the aryl hydrocarbon receptor (AhR) (Figure 3). In fact, ICZ is one of the most potent ligands for the AhR known with an affinity approaching that of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). I3C acid condensation products, as well as indoles and their acid condensation products formed from tryptophan metabolism, appear to be important endogenous ligands for the AhR. Binding allows AhR to enter the nucleus where it forms a complex with the AhR nuclear translocator (Arnt) protein. This AhR/Arnt complex binds to specific DNA sequences, known as xenobiotic response elements (XRE), in the regulatory regions (promoters) of target genes, especially those involved in xenobiotic metabolism. The promoters of genes coding for a number of CYP enzymes and several phase II enzymes contain XREs. Microarray gene expression profiling of I3C- or DIM-treated human prostate cancer cells showed that both compounds upregulated the phase I enzyme, CYP1A1, and the phase II enzymes, glutathione S-transferase theta-1 (GST q1) and aldo-keto reductase. Another study in human prostate cancer cells demonstrated that the removal of AhR abolished I3C- or DIM-induced CYP1A1 mRNA expression. The expression of CYP1A1 and CYP1A2 was also upregulated in human primary liver cells challenged with DIM. Further, I3C and DIM have been found to interfere with CYP activities involved in estrogen metabolism (see Anti-estrogenic activities).

Increasing the activity of biotransformation enzymes is generally considered a beneficial effect because the elimination of potential carcinogens or toxins is enhanced. However, there is a potential for adverse effects because some procarcinogensrequire biotransformation by phase I enzymes to become active carcinogens.

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