Human colorectal cancer cell lines DLD1, HCT116, HT-29, LS513, and RKO were treated with indole-3-carbinol or vehicle. Cell viability was assessed via a fluorescent product assay, and apoptotic activity was assessed via a luminescent signal tied to a ratio of caspase-3 and caspase-7 activity. Gene expression of AHR and CYP1A1 messenger ribonucleic acid (mRNA) was measured using quantitative real-time polymerase chain reaction. Small interfering RNA stable expression lines were established on a HCT116 background using a laboratory-developed transfection protocol as published elsewhere.
Multiple colorectal cancer cell types express increased CYP1A1 mRNA levels (a specific marker of AHR-driven activity) after treatment with I3C, characterizing I3C treatment as agonistic of this pathway. Also, I3C induced a dose-dependent decrease in cell viability as well as inducing apoptosis. Furthermore, using small interfering RNA interference to knockdown AHR responsiveness generated a significant resistance to the chemotherapeutic actions of indole-3-carbinol regarding both cell viability and apoptotic activity.
Some degree of the cytotoxic and proapoptotic effects of indole-3-carbinol on colon cancer cells is dependent on activation of the aryl hydrocarbon receptor. This represents a novel mechanism for the molecular action of indole-3-carbinol and enhances our understanding of its effects in the context of colorectal cancer. Continued preclinical study of both indole-3-carbinol and the aryl hydrocarbon receptor pathway is warranted, which may one day lead to novel diet-derived colon cancer treatments that enlist the AHR.