Project Summary Serine metabolism is frequently dysregulated in cancer, but the underlying mechanisms remain unclear. Serine can be generated de novo or taken up exogenously. These pathways often do not compensate for one another, suggesting that flux through the synthesis pathway has serine independent effects. Studies are conflicting regarding the role of serine in regulating the mechanistic target of rapamycin complex 1 (mTORC1), a central integrator of metabolism, growth factor signaling and translational output during tumorigenesis. Even less is known about how this metabolic axis contributes to tumor initiation, which increasingly appears to be driven by adult stem cells. Previous work from the Fuchs lab has demonstrated that ectopic activation of the Sox2 gene in epidermal stem cells (EpdSCs) induces oncogenic stress. SOX2 is expressed by and required for the tumor- initiating stem cells of squamous cell carcinomas (SCCs), among the most common and life-threatening cancers. Interestingly, SOX2+ epidermal stem cells globally repress translation and activate an alternative translational program that enables them to drive tumorigenesis. Given that mTORC1 has been implicated in both stress translation and oncogenic serine catabolism, I am specifically interested in using this model to probe how serine metabolism converges on stem cell oncogenesis. Using primary cultured stem cells, I have discovered that SOX2+ EpdSCs downregulate de novo synthesis in favor of exogenous serine uptake. Moreover, repressing serine synthesis is critical to sustain mTORC1 signaling in SOX2+ cells, establishing a previously unappreciated potential explanation underlying why synthesis and uptake are not interchangeable. Thus, based upon this data, my central hypothesis that exogenous serine is necessary for mTORC1 dependent gene regulation during stem cell driven tumorigenesis. In this study, I will (1) dissect the functional role of exogenous serine on tumor initiation and cancer stem cell function in vivo, and (2) interrogate the mechanism by which serine synthesis regulates mTORC1. To test the importance of serine uptake, I will measure tumorigenic growth when mice are fed serine free diet, tracking stem cell proliferation, phenotype and functional ability via immunofluorescence, flow cytometry and serial limiting dilution studies. To establish a mechanistic link between de novo synthesis repression and mTORC1, I will use CRISPR-Cas9 to generate cells deficient in serine synthesis and perform steady state and heavy isotope labeling metabolomics on these cells with and without serine and glycine. I will focus on the ability of de novo serine synthesis to limit branched chain amino acid synthesis, which directly feeds into mTORC1. Subsequently, I will use CRISPR-Cas9 to ablate branched chain amino acid synthesis in SCC cells deficient for serine synthesis and track tumorigenesis and mTORC1 signaling. These studies will provide insights into how metabolism might regulate stemness during tumor initiation and implicate serine as a therapeutically targetable metabolic liability of cancer stem cells. !