Liver X receptors (LXRs) are members of the nuclear receptor family of ligand-dependent transcription factors which regulate the expression of lipid and cholesterol metabolism genes. Moreover, LXRs and their ligands have been shown to inhibit tumor growth in a variety of cancers. We have previously identified the small molecule compound GAC0001E5 (1E5) as an LXR inverse agonist and a potent inhibitor of pancreatic cancer cells. Transcriptomic and metabolomic studies showed that 1E5 disrupts glutamine metabolism, an essential metabolic pathway commonly reprogrammed during malignant transformation, including in breast cancers. To determine the role of LXRs and potential application of 1E5 in breast cancer, we examined LXR expression in publicly available clinical samples, and found that LXR expression is elevated in breast tumors as compared to normal tissues. In luminal A, endocrine therapy-resistant, and triple-negative breast cancer cells, 1E5 exhibited LXR inverse agonist and “degrader” activity and strongly inhibited cell proliferation and colony formation. Treatments with 1E5 downregulated the transcription of key glutaminolysis genes, and, correspondingly, biochemical assays indicated that 1E5 lowered intracellular glutamate and glutathione levels and increased reactive oxygen species. These results indicate that novel LXR ligand 1E5 is an inhibitor of glutamine metabolism and redox homeostasis in breast cancers and suggest that modulating LXR activity and expression in tumor cells is a promising strategy for targeting metabolic reprogramming in breast cancer therapeutics.

The HER2-positive subtype accounts for approximately one-fifth of all breast cancers. Insensitivity and development of acquired resistance to targeted therapies in some patients contribute to their poor prognosis. HER2 overexpression is associated with metabolic reprogramming, facilitating cancer cell growth and survival. Novel liver X receptor (LXR) ligand GAC0001E5 (1E5) has been shown to inhibit cancer cell proliferation by disrupting glutaminolysis and inducing oxidative stress. In this study, HER2-positive breast cancer cells were treated with 1E5 to determine their potential inhibitory effects and mechanisms of action in HER2-positive breast cancers. Similar to previous observations in other cancer types, 1E5 treatments inhibited LXR activity, expression, and cancer cell proliferation. Expression of fatty acid synthesis genes, including fatty acid synthase (FASN), was downregulated following 1E5 treatment, and results from co-treatment experiments with an FASN inhibitor suggest that the same pathway is targeted by 1E5. Treatments with 1E5 disrupted glutaminolysis and resulted in increased oxidative stress. Strikingly, HER2 transcript and protein levels were both significantly downregulated by 1E5. Taken together, these findings indicate the therapeutic potential of targeting HER2 overexpression and associated metabolic reprogramming via the modulation of LXR in HER2-positive breast cancers.

Liver X receptors (LXRs) are members of the nuclear receptor superfamily of ligand-dependent transcription factors. LXRα is predominantly expressed in metabolic tissues, whereas LXRβ is ubiquitously expressed. Upon ligand binding, they regulate the expression of target genes involved in lipid metabolism, cholesterol homeostasis, and immune responses, including those which function in pathways that are commonly reprogrammed during carcinogenesis. Known LXR ligands include oxysterols and natural and synthetic agonists which upregulate LXR transcriptional activity and target gene expression. Synthetic inverse agonists have also been identified that inhibit LXR activity. While both types of ligands have been shown to inhibit cancer cells and tumor growth either directly or indirectly by modulating the activities of stromal cells within the tumor microenvironment, they appear to target different aspects of cancer metabolism and other cancer hallmarks, including immune evasion. This review summarizes the characterization of LXRs and their ligands and their mechanisms of action in cancer models and discusses the future directions for translating these discoveries into novel cancer therapeutics.

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.