Date of Degree
Biochemistry | Cancer Biology
Phosphatidic Acid, LPAAT, metabolism
Mammalian target of Rapamycin (mTOR) is a protein kinase that integrates nutrient and growth factor signals to promote cellular growth and proliferation. mTOR exists in two complexes - mTORC1 and mTORC2 that are distinguished by their binding partners and signaling inputs. mTORC1 is responsive to growth factors, amino acids and glucose and is associated with Raptor; whereas, mTORC2 is responsive primarily to growth factors and is associated with Rictor. Raptor and Rictor confer substrate specificity to mTORC1 and mTORC2 respectively. Phosphatidic acid (PA), a lipid second messenger and a central metabolite for membrane phospholipid biosynthesis, is required for the stability and activation of both mTORC1 and mTORC2. The negatively charged head group of PA interacts with positively charged Lys and Arg residues in the FRB (FK-506 binding protein–12 (FKBP12)-rapamycin binding) domain of mTOR and stabilizes the mTOR complexes. The PA-binding site on mTOR is highly conserved from yeast to humans – indicating the importance of PA for mTOR function.
While much is known about the role of amino acid sensing by mTOR, much less is known how mTOR responds to other essential nutrients needed for cell growth. An under-appreciated component of serum needed for cell growth is lipids that are used for the synthesis of membranes and organelles. Because of an increased utilization of exogenous lipids by KRas-driven cancer cells, we examined the effect of exogenously supplied lipids on mTORC1 and mTORC2 in KRas-driven cancer cells. We demonstrate that both mTORC1 and mTORC2 are responsive to dietary unsaturated fatty acids through the de novo synthesis of PA. Fatty acids entering the cell are activated through an ATP dependent linking of Coenzyme A (CoA) by Acyl-CoA synthetase, and thereby shunting them towards de novo PA synthesis. We find that a specific isoform of Acyl-CoA synthetase longchain (ACSL)-5 is overexpressed in KRas-driven cancer cells. Genetic ablation of ACSL5 inhibits oleate-mediated activation of mTORC1 and mTORC2 through reduction in levels of PA. Further, inhibition of ACSL5 leads to a G1 cell cycle arrest in Ras driven cells. The activation of mTOR by oleic acid was also dependent on lysophosphatidic acid acyltransferase that adds the CoA-charged fatty acid to 1-acylglycerol-3-phosphate to generate PA. Of significance, the glycerol-3-phosphate that gets acylated to PA was derived by reduction of the glycolytic intermediate dihydroxyacetone phosphate – indicating that glucose is also sensed by mTOR via PA. In response to lipids, we observed that mTORC2 co-localizes with the mitochondrial fraction where it activates downstream targets to regulate mitochondrial metabolism. While it has long been appreciated that mTOR is a sensor of amino acids, this study reveals that mTOR also senses the presence of lipids and glucose via production of PA.
Menon, Deepak, "Lipid Sensing by Mammalian Target of Rapamycin" (2017). CUNY Academic Works.