The Ser/Thr kinase mechanistic target of rapamycin complex 1 (mTORC1) is often aberrantly activated in cancer. Activation of mTORC1 promotes many anabolic processes, including mRNA translation, aerobic glycolysis, de novo lipid synthesis, the pentose phosphate pathway, and de novo nucleotide synthesis. mTORC1 controls aerobic glycolysis and de novo lipid synthesis by regulating two transcription factors, SREBP and HIF1. A novel transcription factor downstream of mTORC1, ATF4, was recently discovered to contribute to the anabolic process of de novo purine synthesis. ATF4 is a well-studied effector of the integrated stress response and has been linked to cancer cell survival upon amino acid deprivation. It is not known whether ATF4 can promote de novo purine synthesis and other anabolic processes in cancer. Understanding this potential dual role for ATF4 could lead to novel insight about how cancer cells are able to proliferate and are reprogrammed for anabolic metabolism in an often nutrient-deprived tumor microenvironment. To determine whether ATF4 was contributing to other anabolic processes downstream of activated mTORC1, we utilized Tsc2-/- and isogenic wild-type Tsc2+/+ mouse embryonic fibroblasts (MEFs). TSC2-deficient MEFs have constitutive, growth-factor independent activation of mTORC1. Strikingly, knockdown of ATF4 in TSC2- deficient MEFs or PTEN-deficient cell lines with aberrant activation of mTORC1 displayed increased transcript levels of genes involved with amino acid charging, transport, and biosynthesis. These changes were also dependent on mTORC1 activation and were reflected at the protein level. Knockdown of ATF4 in Tsc2-/- MEFs also decreased protein synthesis. The goal of this project is to determine whether the mTORC1-ATF4 axis contributes to tumorigenesis in cancers with aberrant activation of mTORC1. To address this question, I propose to: 1) Define the contribution of ATF4 downstream of mTORC1 to amino acid uptake, synthesis, and tRNA charging 2) Determine if the mTORC1-ATF4 axis plays an anabolic role in PTEN-deficient cancer cells 3) Determine the role of ATF4 in promoting PTEN-deficient tumor metabolism and growth in vivo By addressing these aims I can gain knowledge about the biological significance of the mTORC1-ATF4 axis in cancer. Since ATF4 is a transcription factor that is also involved in the nutrient-deprivation response of cancer, these aims could give insight about the interface between nutrient-deprivation and anabolic mTORC1-driven metabolism as tumor progression occurs. Ultimately, this could lead to novel approaches to treat the metabolic dependencies of cancer.
mTORC1 is aberrantly activated in many cancer types to promote anabolic metabolism. Using a variety of in vitro and in vivo techniques, this project will investigate the mTORC1-ATF4 axis in cancer. This proposal will lead to novel insight about interplay between mTORC1-driven anabolic metabolism of cancer cells and the often nutrient-deprived tumor microenvironment.
