Differential Effects of 17β-Estradiol, Its Metabolites, and Cadmium on Cytotoxicity and Redox-Related Pathways in Doxorubicin-Sensitive and -Resistant Breast Cancer Cell Lines

Background: Breast cancer is the most common malignancy among women and a leading cause of cancer-related deaths globally. Its development involves hormonal, genetic, environmental and inflammatory factors. Among environmental contributors, cadmium (Cd2+), a metalloestrogen known to induce redox imbalance, as well as estrogen metabolites, may exert divergent biological effects. Methods: This study investigated the effects of 17β-estradiol (E2) and its metabolites—2-methoxyestradiol (2-MeOE2) and 4-hydroxyestradiol (4-OHE2)—administered alone or in combination with CdCl2, on estrogen receptor–-positive MCF-7 breast cancer cells and their doxorubicin-resistant cells (MCF-7/DOX). We evaluated cytotoxicity, interaction profiles (synergism/antagonism), and redox-related enzymes associated with drug resistance, including superoxide dismutase 1 (SOD1) and glutathione S-transferase pi (GST-pi). There are no known examples of these types of interactions, especially those involving estrogen metabolites with opposing biological activities—anticancer 2-MeOE2 and procarcinogenic—4-OHE2 in combination with cadmium. Cell viability was assessed after 48 h exposure to individual and combined treatments of CdCl2. Interaction types (synergism/antagonism) were determined via the combination index method. Antioxidative enzymes were evaluated by quantitative and immunocytochemical analysis of SOD1, GST and GST-pi expression. Results: All tested compounds reduced cell viability in a concentration-dependent manner, with CdCl2 showing the highest cytotoxicity. MCF-7 cell lines were generally more sensitive to CdCl2, E2, and 2-MeOE2, whereas MCF-7/DOX cell lines exhibited greater sensitivity to 4-OHE2. Combination studies revealed predominantly antagonistic interactions, particularly for CdCl2 + 2-MeOE2, suggesting a protective redox-modulating effect of this metabolite. Resistant cells consistently displayed higher SOD1 activity and GST-pi expression, indicating enhanced adaptive responses to oxidative stress. Conclusions: Our study underscores the importance of concentration-dependent interactions between environmental Cd2+ and pathways regulated by 17β-estradiol and its metabolites, particularly in the context of cytotoxicity and redox imbalance relevant to breast cancer progression and therapy resistance.