The Prognostic Impact of Intratumoral Aryl Hydrocarbon Receptor in Primary Breast Cancer Depends on the Type of Endocrine Therapy: A Population-Based Cohort Study

Pre- and Postoperative Antioxidant Use, Aryl Hydrocarbon Receptor (AhR) Activation and Clinical Outcome in Different Treatment Groups of Breast Cancer Patients

BACKGROUND

Cancer patients often use antioxidants that may interact with adjuvant treatments. The purpose was to investigate pre- and postoperative antioxidant use in relation to clinicopathological characteristics and prognosis in different breast cancer treatment groups.

METHODS AND PATIENTS

Pre- and postoperative antioxidant (vitamin A, C, E, carotenoids, or Q10) or multivitamin use was self-reported by patients from Lund (n = 1855) and Helsingborg (n=478), Sweden. Patients were followed for up to 15 years. Clinical data were obtained from patient charts. The aryl hydrocarbon receptor (AhR) was evaluated in tumor tissue arrays from 915 patients from Lund and with Western blot in MCF-7 and MDA-MB-231 cells.

RESULTS

About 10% of patients used antioxidants. Nuclear AhR (AhRnuc) positivity was twice as common in preoperative antioxidant users compared to non-users. In mechanistic studies vitamin C increased AhR levels and its downstream target CYP1B1, indicating AhR activation. There were significant interactions between tumor AhRnuc status and preoperative antioxidant use in relation to clinical outcome. In all patients, antioxidant use (other than multivitamins) at both visits was associated with poorer prognosis, while use only at the follow-up visit was associated with better prognosis, compared with no use at either visit.

CONCLUSION

The clinical impact of antioxidants depended on antioxidant type, timing of use, and tumor AhR activation. Antioxidants may influence clinical outcome by activation of the master regulator AhR in addition to interference with free radicals. Further studies are needed to identify breast patients that might improve or worsen their prognosis when using antioxidants postoperatively.

Unveiling the mechanistic role of the Aryl hydrocarbon receptor in environmentally induced Breast cancer

The aryl hydrocarbon receptor (AhR) is a crucial cytosolic evolutionary conserved ligand-activated transcription factor and a pleiotropic signal transducer. The biosensor activity of the AhR is attributed to the promiscuity of its ligand-binding domain. Evidence suggests exposure to environmental toxins such as polycyclic aromatic hydrocarbons, polychlorinated biphenyls and halogenated aromatic hydrocarbons activates the AhR signaling pathway. The constitutive activation of the receptor signaling system leads to multiple health adversities and enhances the risk of several cancers, including breast cancer (BC). This review evaluates several mechanisms that integrate the tumor-inducing property of such environmental contaminants with the AhR pathway assisting in BC tumorigenesis, progress and metastasis. Intriguingly, immune evasion is identified as a prominent hallmark in BC. Several emerging pieces of evidence have identified AhR as a potent immunosuppressive effector in several cancers. Through AhR signaling pathways, some tumors can avoid immune detection. Thus the relevance of AhR in the immunomodulation of breast tumors and its putative mode of action in the breast tumor microenvironment are discussed in this review. Additionally, the work also explores BC stemness and its associated inflammation in response to several environmental cues. The review elucidates the context-dependent ambiguous behavior of AhR either as an oncogene or a tumor suppressor with respect to its ligand. Conclusively, this holistic piece of literature attempts to potentiate AhR as a promising pharmacological target in BC and updates on the therapeutic manipulation of its various exogenous and endogenous ligands.

The Role of the Aryl Hydrocarbon Receptor (AhR) and Its Ligands in Breast Cancer

Breast cancer is a complex disease which is defined by numerous cellular and molecular markers that can be used to develop more targeted and successful therapies. The aryl hydrocarbon receptor (AhR) is overexpressed in many breast tumor sub-types, including estrogen receptor -positive (ER+) tumors; however, the prognostic value of the AhR for breast cancer patient survival is not consistent between studies. Moreover, the functional role of the AhR in various breast cancer cell lines is also variable and exhibits both tumor promoter- and tumor suppressor- like activity and the AhR is expressed in both ER-positive and ER-negative cells/tumors. There is strong evidence demonstrating inhibitory AhR-Rα crosstalk where various AhR ligands induce ER degradation. It has also been reported that different structural classes of AhR ligands, including halogenated aromatics, polynuclear aromatics, synthetic drugs and other pharmaceuticals, health promoting phytochemical-derived natural products and endogenous AhR-active compounds inhibit one or more of breast cancer cell proliferation, survival, migration/invasion, and metastasis. AhR-dependent mechanisms for the inhibition of breast cancer by AhR agonists are variable and include the downregulation of multiple genes/gene products such as CXCR4, MMPs, CXCL12, SOX4 and the modulation of microRNA levels. Some AhR ligands, such as aminoflavone, have been investigated in clinical trials for their anticancer activity against breast cancer. In contrast, several publications have reported that AhR agonists and antagonists enhance and inhibit mammary carcinogenesis, respectively, and differences between the anticancer activities of AhR agonists in breast cancer may be due in part to cell context and ligand structure. However, there are reports showing that the same AhR ligand in the same breast cancer cell line gives opposite results. These differences need to be resolved in order to further develop and take advantage of promising agents that inhibit mammary carcinogenesis by targeting the AhR.