At the interface between endoplasmic reticulum and mitochondria: inhibition of SOAT1 as new treatment strategy against adrenocortical carcinoma

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Mitotane is the standard of care for adrenocortical carcinoma (ACC). This rare tumour comes with a very poor prognosis (median survival <3.5 years) and only about 30% of patients benefit from mitotane therapy, which is associated with severe adverse effects. Therefore, it is necessary to increase the accuracy of the therapy and to develop more effective drugs. In the first funding period, we were able to identify for the first time a molecular target of mitotane, namely sterol-O-acyltransferase 1 (SOAT1). Using gene expression analysis, lipid mass spectrometry and functional methods, we have shown that mitotane influences the lipid composition particularly in hormone producing cells of the adrenal cortex, thereby strongly activating the endoplasmic reticulum (ER) stress signalling pathway. This is due to direct inhibition of SOAT1 by mitotane. SOAT1 is located at the interface of the ER and the mitochondria within the so-called mitochondria-associated membranes (MAM). We also characterized the interaction of mitotane with membranes and lipoproteins and identified potentially synergistic drugs. In five sub-projects, we will now continue our efforts: 1) To investigate establish SOAT1 expression as a marker for a personalized therapy with mitotane we will perform SOAT1 specific immunohistochemistry in over 300 clinically very precisely characterized tumour samples. Our hypothesis is that SOAT1 expression is able to predict response to mitotane (both in adjuvant and palliative setting). 2) Following the hypothesis that other SOAT1 inhibitors are more effective and tolerable, we will characterize their effect in ACC cells. Of note, SOAT1 inhibitors were developed in the 1990ies as lipid lowering drugs but development was stopped partly due to adrenal toxicity. Now, we will assess changes in intracellular lipids and metabolites by mass spectrometry, energy metabolism by metabolic flux analyses, apoptosis by flow cytometry and the resulting adaptive processes by gene expression analyses in different cell systems. 3) Since we have noted in preliminary studies that MAMs are of great importance for adrenal steroid synthesis and mitotane action, in the third subproject we will use SOAT inhibitors as a tool to better understand the role of MAMs in lipid-induced ER stress. 4 + 5) In the last two subprojects, we will investigate the hypotheses that mitotane might also inhibit other membrane-bound O-acyltransferases, and that the mitotane-induced ER stress leads to immunogenic cell death by inducing certain signal patterns (danger associated membrane patterns). We are confident that these studies will significantly improve the therapy of ACC patients in the future and might also provide the basis for SOAT1 as a target molecule for the treatment of other tumours.