Indoxylsulfate, a Metabolite of the Microbiome, Has Cytostatic Effects in Breast Cancer via Activation of AHR and PXR Receptors and Induction of Oxidative Stress

Simple Summary The aim of our study was to identify metabolites of bacterial origin that play role in the pathogenesis of breast cancer. We identified indoxylsulfate, a metabolite of the amino acid tryptophan, as a metabolite with cytostatic properties on breast cancer cells. Cytostatic properties were dependent on increasing oxidative stress blocking the capacity of cancer cells to migrate, enter blood vessels and to form metastases. Furthermore, indoxylsulfate reduced the proportions of cancer stem cells that are highly resistant to chemotherapy and have vital role in initiating recurrence. We identified that indoxylsulfate exert its effects through the pregnane-X receptor and the aryl-hydrocarbon receptor. The expression of these receptors decrease with the progression of the disease, furthermore, the expression of these receptors is low in cases with poor prognosis. Changes to bacterial metabolite-elicited signaling, in oncobiosis associated with breast cancer, plays a role in facilitating the progression of the disease. We show that indoxyl-sulfate (IS), a tryptophan metabolite, has cytostatic properties in models of breast cancer. IS supplementation, in concentrations corresponding to the human serum reference range, suppressed tumor infiltration to the surrounding tissues and metastasis formation in a murine model of breast cancer. In cellular models, IS suppressed NRF2 and induced iNOS, leading to induction of oxidative and nitrosative stress, and, consequently, reduction of cell proliferation; enhanced oxidative and nitrosative stress are crucial in the subsequent cytostasis. IS also suppressed epithelial-to-mesenchymal transition vital for suppressing cellular movement and diapedesis. Furthermore, IS rendered cells hypometabolic, leading to a reduction in aldehyde-dehydrogenase positive cells. Pharmacological inhibition of the pregnane-X receptor using CH223191 and the aryl-hydrocarbon receptor using ketoconazole diminished the IS-elicited effects, suggesting that these receptors were the major receptors of IS in these models. Finally, we showed that increased expression of the human enzymes that form IS (Cyp2E1, Sult1A1, and Sult1A2) is associated with better survival in breast cancer, an effect that is lost in triple negative cases. Taken together, IS, similar to indolepropionic acid (another tryptophan metabolite), has cytostatic properties and higher expression of the metabolic machinery responsible for the formation of IS supports survival in breast cancer.