ALDH2 Deficiency Drives Proliferative Mammary Morphogenesis and Epithelial Cell Stemness via Oxidative Stress and Estrogen Receptor Activation

Alcohol consumption has been linked to breast cancer, partly due to the accumulation of toxic aldehydes like acetaldehyde, a carcinogenic byproduct of ethanol metabolism. ALDH2, a key mitochondrial enzyme, detoxifies acetaldehyde and other harmful aldehydes that drive oxidative stress, DNA damage, and hormonal dysregulation, key factors in carcinogenesis. Despite the possible link between alcohol consumption and breast cancer risk, little is known about how ALDH2 deficiency itself, independent of alcohol exposure, affects mammary gland biology and cancer susceptibility. Genetic variants leading to ALDH2 deficiency are highly prevalent in East Asian populations, where individuals carrying inactive ALDH2 alleles experience impaired aldehyde detoxification. While these individuals are at increased risk for alcohol-related cancers, the impact of ALDH2 deficiency on mammary gland development and homeostasis in the absence of alcohol exposure remains unexplored. To address this, we utilized a C57BL/6-based ALDH2 knockout (Aldh2-/-) mouse model to investigate its effects on mammary proliferation and development. Our findings revealed that Aldh2-/- mice exhibited hyperproliferative mammary glands, characterized by increased epithelial cell density, ductal expansion, and elevated Ki67+ cells. Flow cytometry analysis demonstrated a rise in luminal and basal epithelial subpopulations, alongside enhanced mammary epithelial stemness, as evidenced by increased mammosphere formation and colony-forming efficiency. At the molecular level, ALDH2 deficiency activated oxidative stress pathways, marked by elevated 8-OHdG, p38 MAPK, NF-{kappa}B, and Nrf2 signaling, alongside DNA damage responses involving p53 and H2A.X. Importantly, we also identified a previously unrecognized upregulation of RANKL in Aldh2-/- mammary glands, implicating the RANK/RANKL axis as a critical mediator linking aldehyde stress to NF-{kappa}B/p38 MAPK activation and enhanced mammary stemness. Furthermore, hormonal dysregulation was observed, with a significant increase in ER and PR expression and phosphorylation. Dysregulated ER signaling was further linked to enhanced erbB3 activation and downstream signaling, including the cyclin D1-pRb-E2F1 axis. Our results suggest that the accumulation of endogenous aldehydes, independent of alcohol exposure, profoundly alters mammary morphogenesis, epithelial repopulation, and stemness. Mechanistically, this occurs through oxidative stress activation and DNA damage pathways, leading to metabolic changes and upregulation of estrogen receptor and receptor tyrosine kinase signaling. This study highlights for the first time the potential role of ALDH2 deficiency in increasing mammary tissue susceptibility to oncogenic factors and breast cancer risk.