Ceramide-induced endoplasmic reticulum stress reveals a targetable vulnerability in endocrine therapy-resistant breast cancer.

Despite the success of endocrine therapy (ET) in treating hormone receptor-positive breast cancer, a significant proportion of patients relapse during or after treatment, making ET resistance a major clinical challenge. Previously we have shown that ET-resistant breast cancer cells exhibit reduced ceramide levels and an increased sensitivity to ceramide-induced cell death. Here, we demonstrate that ceramides induce a distinct transcriptional reprogramming in ET-resistant cells, characterized by upregulation of endoplasmic reticulum stress (EnRS) pathways. Ceramide-induced EnRS is PERK-dependent and functionally linked to cell death in multiple models of ET resistance. Using a photoactivatable ceramide probe, we identify TRAM1 as a functionally important ceramide-interacting protein (CIP) in ET-resistant cells that correlates with worse relapse-free survival and a more aggressive breast cancer phenotype in luminal breast cancer patients. Additionally, knockdown of TRAM1 phenocopies ceramide action in ET resistance, thereby suggesting its role in mediating ceramide-induced lethal actions in ET resistance. Together, our findings reveal that ET-resistant breast cancer cells are highly sensitive to PERK-mediated EnRS relative to ET-sensitive cells. Ceramides, likely via interactions with CIPs such as TRAM1, lead to PERK activation and consequential cell death in the ET-resistant breast cancer models. This sensitivity to ceramide-induced EnRS and cell death is a vulnerability that could be taken advantage of to treat ET-resistant breast cancer. Implications: This study elucidates the functional relevance of ceramide depletion in endocrine therapy-resistant breast cancer cells.