SUN-216 Investigating the Role of the Liver X Receptor in Potentiating Mitotane Therapy in Adrenocortical Carcinoma

Introduction: Adrenocortical Carcinoma is a rare aggressive cancer which carries a poor prognosis. Adjuvant mitotane improves survival but is limited by a narrow therapeutic window and severe adverse effects. Liver X receptors (LXRs), part of the nuclear receptor superfamily are highly expressed in adrenal tissue and mediate transcellular and intracellular cholesterol homeostasis. We hypothesise that LXRα inhibition increases toxic lipid accumulation in adrenocortical cancer cells and potentiates the adrenolytic effect of mitotane. Methodology: ATCC-H295R and MUC1 ACC cells and were pre-treated with the LXRα inverse agonist SR9243 5µM and antagonist GSK2033 5µM followed by mitotane treatment (20, 40, 50μΜ) for 6 hours. Cholesterol-methyl-β-cyclodextrin treatment was carried out 1hr prior to mitotane. H295R cells were transfected with a LXR⍺ dominant negative construct using lipofectamine. Cell death was assessed using annexin/PI staining and proliferation using MTT assay. Free cholesterol (FC) levels were assayed using filipin staining and lipid droplets via BODIPY® and analysed on the Amnis ImageStream® imaging cytometer. Downstream targets ABCA1 and ABCG1 were evaluated by qRT-PCR. Lipid droplet associated proteins PLIN 1-4 and hormone sensitive lipase (HSL) expression were evaluated using western blotting. Results: Downstream reduction of ABCA1 and ABCG1 expression confirmed LXRα blockade. Mitotane effectively induced dose-dependent H295R apoptotic cell death which was potentiated pharmacologically and genetically by LXR⍺ inhibition. In line with these findings, cholesterol-methyl-β-cyclodextrin treatment increased cell death in H295R and MUC1 cells. In addition to inducing cell death, LXR⍺ inhibition decreased proliferation of both cell lines. An increase in FC and a decrease in cholesterol esters was observed following mitotane treatment in H295R cells. This was accompanied by decreased lipid droplet numbers confirmed by lower expression of lipid droplet associated proteins, PLIN1-3. These effects were potentiated when mitotane was combined with LXRα inhibition. We demonstrate increased HSL activity, which was associated with higher SOAT-1 expression and increasing toxic FC accumulation. Investigation of lipid droplet content BODIPY® of both cell lines showed H295R cells preferentially store cholesterol esters and MUC1 cells store triacylglycerides.Conclusion: We propose a mechanism for enhancing mitotane’s efficacy as an adrenolytic through increased free cholesterol via LXR⍺inhibition. Targeting the LXRα, its putative ligands, or associated lipid mediators may present a novel therapeutic approach in the setting of primary and metastatic ACC.