Drug Discovery: LXR Inverse Agonists in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is a disease for which therapeutic interventions are scarce. Currently, the effective treatments available are chemotherapies that offer minimal improvements, especially in later stages of the disease. Novel therapeutic modalities are desperately needed in the treatment of PDAC. The liver X receptor (LXR) is a ligand-dependent transcription factor that regulates the expression of genes involved in lipid metabolism, inflammation, and immune responses. Synthetic LXR agonists have been shown to be effective in reducing growth of various cancers, but their clinical use has been stymied by side-effects. Inverse agonists are a newly discovered class of LXR ligands that have been shown to possess antiproliferative effects in cancer cells and may offer a way to circumvent the side-effects of LXR agonists. A recent screening of a small molecule LXR ligand library in PDAC cells led to the discovery of two inverse agonists: GAC0001E5 (1E5) and GAC0003A4 (3A4). A multi-omic analysis, including RNA-sequencing (RNA-seq) and metabolite mass spectrometry (metabolomics) was conducted following treatments with both inverse agonists and the agonist GW3965 (GW) to ascertain the mechanisms involved in their antiproliferative effects in PDAC cells. RNA-seq results show that PDAC cells harboring mutations in the KRAS oncogene are much more responsive at the transcriptional level to LXR inverse agonists than an agonist. Lipid metabolism was extensively repressed by the inverse agonists, including cholesterol, fatty acid, and phospholipid synthesis. Metabolomics
results showed perturbations in several metabolic pathways, amino acid metabolism, cholesterol metabolism, and phospholipid metabolism. Integration of transcriptomic and metabolomic data indicated disruptions in glycolysis, the TCA cycle, and fatty acid metabolism occurred at both the gene and metabolite levels. Mechanistic investigations of 3A4 revealed disruptions in cholesterol and ceramide homeostasis were involved in its antiproliferative effects. Additionally, a dual-death phenotype comprised of apoptosis and necroptosis was found to be induced by 3A4. A computational approach was taken to explore the ligand-receptor interactions of inverse agonists and the LXR ligand-binding domain. Inverse agonists were shown to cause distinct changes in receptor conformation that could be used for the design of inverse agonists with increased specificity and potency.