Small Molecule Liver X Receptor Modulator GAC0001E5 Targets Mechanisms of Endocrine Resistance in Estrogen Receptor-Positive Breast Cancer Cells

Endocrine therapy is an effective and common treatment strategy for estrogen receptor (ER)-positive breast cancers. However, the development of endocrine resistance, through genetic mutations and epigenetic alterations, in about 40% of treated patients remains a significant therapeutic challenge. Liver X receptors (LXRs) are nuclear receptors that regulate lipid metabolism and cholesterol homeostasis and have been implicated in metabolic reprogramming in breast cancers and other malignancies. We previously identified a novel LXR ligand GAC0001E5 (1E5), with potent antiproliferative activity across breast cancer subtypes. Here, we investigate its mechanisms of action in responsive (MCF-7) and endocrine-resistant (MCF-7-TamR) ER-positive breast cancer cells. Treatment with 1E5 resulted in the downregulation of LXR and its target genes, and significantly reduced ERα expression and the expression of ER-responsive genes. Aberrant expression of androgen receptor (AR) and human epidermal growth factor receptor 2 (HER2), both implicated in endocrine resistance, were downregulated following 1E5 treatment. siRNA-mediated knockdown of LXR expression only partially recapitulated the actions of 1E5, suggesting the involvement of LXR-dependent and independent mechanisms. Collectively, these findings reveal potential crosstalk between LXR and the genetic and epigenetic regulation of pathways involved in endocrine response and alternative signaling mechanisms, highlighting potential targets in endocrine-resistant breast cancer.