Therapeutic targeting of DGKA-mediated macropinocytosis in lymphangioleiomyomatosis
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BACKGROUND: Lymphangioleiomyomatosis (LAM) is a rare disease characterized by cystic destruction of the lung. It occurs in 80% of people with Tuberous Sclerosis Complex disorder (TSC), a multisystem, autosomal dominant disorder caused by mutations in tumor suppressor genes TSC1 and TSC2. Spontaneous biallelic mutations in these genes can give rise to sporadic LAM. Mammalian target of rapamycin complex I (mTORC1), a master regulator of cellular anabolic metabolism is hyperactivated in LAM cells. Upregulation of protein synthesis and downregulation of autophagy creates a state of starvation stress that upregulates pathways of extracellular nutrient acquisition. Macropinocytosis, a form of clathrin-independent endocytosis, is upregulated in TSC2-deficient cells. We performed a high-throughput compound screen utilizing a repurposing drug library. We identified that ritanserin, a diacylglycerol kinase alpha (DGKA) inhibitor, synergizes with Chloroquine (CQ) to selectively inhibit proliferation of TSC2-deficient mouse embryonic fibroblasts (MEFs) compared to TSC2+/+ MEFs. OBJECTIVE: We hypothesized that TSC2-deficient cells rely on macropinocytosis to support their growth during the periods of stress and starvation and that ritanserin synergizes with CQ to inhibit proliferation in TSC2-deficient cells by inhibiting macropinocytosis. METHODS: Crystal violet-based proliferation assays were used to monitor the effect of pharmacological and genetic inhibition of DGKA on cell proliferation. Immunoblotting was used to measure the expression levels of TSC2, tS6R, pS6R, Cleaved PARP, Cleaved Caspase 3 and Actin. siRNA induced Htr2a knockdown and shRNA induced DGKA knockdown cell culture models were used to define the dual functions of ritanserin and observe their effects on macropinocytosis and cell proliferation. LC/MS was used to measure cell lipid content and how it changes in response to ritanserin. Fluorophore-labeled BSA and 70-kDa Dextran were used to measure macropinocytosis. Lysotracker was used to measure the number of lysosomes, while DQ-BSA was used to measure lysosomal functionality. RESULTS: TSC2-deficient cells express higher levels and show upregulated activity of DGKA. Genetic and pharmacologic inhibition of DGKA prevents TSC2-deficient cells from acquiring nutrients via macropinocytosis. Phospholipid metabolism is altered in TSC2-deficient cells, marked by the accumulation of phosphatidic acid and ceramides. Treatment with ritanserin leads to the accumulation of diacylglycerol and phospholipids, as well as a reduction in phosphatidic acid. CONCLUSIONS: TSC2-deficient cells rely on macropinocytosis to meet their metabolic needs. Diacylglycerol kinase alpha (DGKA) is required for macropinocytic nutrient uptake. Pharmacologic or genetic inhibition of DGKA creates metabolic stress in TSC2-deficient cells, which ultimately leads to increased apoptotic response to treatment with CQ. This project identifies a novel connection between mTOR signaling, lysosome metabolism and macropinocytosis, and a vulnerability that allows the selective targeting of LAM cells.