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Gene Expression Regulation in the Mouse Liver by Mechanistic Target Of Rapamycin Complexes I and II

The mechanistic target of rapamycin (mTOR) is a key serine/threonine protein kinase that functions in complexes mTORC1 and mTORC2. mTORC1, originally discovered due to its sensitivity towards the mTOR inhibitor rapamycin, responds to extracellular growth factor signaling, WNT signaling, and nutrient abundance via glucose and amino acid-triggered signaling. Downstream effectors of mTORC1 include autophagy, mitochondrial metabolic function, protein synthesis, and ribosome biogenesis. mTORC2, initially discovered as a rapamycin-insensitive complex of mTOR, responds to insulin, growth factor signaling, and inflammatory signaling such as tumor necrosis factor-alpha, with its downstream effectors being Akt, a key serine/threonine kinase that functions in cell division and is frequently dysregulated in many types of cancer, the NFkB pathway, and cytoskeletal reorganization and protein synthesis. Much research has been devoted to mTORC1 signaling, with mTORC2 receiving significantly less attention, despite both complexes’ regulation of key cellular activities and response to rapamycin, as well as to other rapamycin-derived drugs (rapalogs). We have targeted both mTORC1 and mTORC2 for hepatocyte-specific deletion during the gestational period of mice, with the goal of describing mTORC1 and mTORC2 signaling and its perturbation in the adult mouse hepatocyte. Our model has shown that deletion of RAPTOR, the regulatory associated protein of mTOR, and RICTOR, the rapamycin insensitive component of mTOR, in mTORC1 and mTORC2 respectively, leads to widespread effects on the hepatocyte transcriptome. We have found that a subset of genes responds both to Raptor and Rictor knockout, and an analysis of these genes indicates their function in key disorders of the liver, such as non-alcoholic fatty liver disease and hepatocellular carcinoma. Bioinformatic analysis following hepatocyte RNA sequencing of mTORC1 and mTORC2 knockout mice has revealed an unexpected upregulation of genes known to be regulated by these respective complexes. We have also found that cross talk exists between both complexes, in which the knockout of one yields the activation of the other. We have additionally found translationally relevant enrichments following Ingenuity Pathway Analysis (IPA) of RNA sequencing data. These results provide a key mechanistic discovery of mTOR signaling activity, and allow for a better understanding of the potential physiological effects of mTOR inhibition in human patients.