Triple negative breast cancer (TNBC), characterized by tumors that lack expression of the estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2), carries a poor prognosis. While recently approved poly ADP ribose polymerase (PARP) inhibitor olaparib shows favorable activity in patients with TNBC harboring tumors deficient in DNA repair enzyme BRCA1, patients with TNBC possessing BRCA1-proficient tumors are largely unresponsive to olaparib. Emerging evidence suggests that small molecules that activate aryl hydrocarbon receptor (AhR) signaling have the capacity to confer anticancer actions. 3,3′-diindolylmethane (DIM), an AhR ligand, is the major metabolite of indole-3-carbinol (I3C) found in cruciferous vegetables such as broccoli. Patients often consume natural product-derived AhR ligands while undergoing chemotherapy. It is therefore crucial to enhance our understanding of nutraceutical-pharmaceutical interactions. We previously demonstrated the ability of synthetic AhR ligands to induce the expression of putative tumor suppressor cytoglobin (CYGB) and inhibit Akt signaling in breast cancer cells. Since PARP inhibitor PJ34 has been shown to synergize with Akt inhibitors we hypothesize that DIM induces CYGB expression and synergizes with PJ34 to inhibit TNBC cell growth. Using the Alamar Blue assay, we found that DIM substantially enhanced the sensitivity of TNBC BRCA1 proficient cells to PJ34, though knockdown of CYGB did not significantly influence the responsiveness of cells to DIM. Chromatin immunoprecipitation followed by next-generation sequencing data revealed that DIM promoted the binding of AhR to the CYGB promoter in breast cancer cells. Finally, DIM induced CYGB gene expression in TNBC cells as determined by quantitative PCR. Taken together, our data suggest that DIM upregulates CYGB in TNBC cells and synergizes with PJ34 to confer anticancer actions in TNBC. Our data provide a rationale for incorporating the use of natural product-derived AhR ligands as a strategy to enhance PARP inhibitor efficacy in patients with TNBC that have BRCA1-proficient tumors.

Citation Format: Jonathan V. Wooten, Nicole Mavingire, Leah Rowland, Jason Matthews, Eileen Brantley. 3,3'-diindolylmethane induces cytoglobin expression and synergizes with poly ADP ribose polymerase inhibitor PJ34 to inhibit triple negative breast cancer cell growth [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 16.