Indole-3-carbinol Induces Tumor Cell Death: Function Follows Form

Background: Even with colonoscopy screening and preventive measures becoming more commonplace, colorectal cancer (CRC) remains the third leading cause of oncologic death in the United States as of 2014. Many chemotherapeutics exist for the treatment of colorectal cancer, though they often come with significant side effect profiles or narrow efficacy ranges in terms of patient profile. Dietary phytochemicals such as glucobrassicin and its metabolite indole-3-carbinol (I3C) have been implicated in tumor prevention in many preclinical models across a variety of gastrointestinal tumors and represent an intriguing new class of natural chemotherapeutics for CRC. I3C has been identified as a ligand of the aryl hydrocarbon receptor (AHR), and we aimed to characterize this AHR activation in relation to its cytotoxic properties.

Methods: 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.

Results: 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.

Conclusions: 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.