PROJECT SUMMARY/ABSTRACT The sphingolipid rheostat, which is the balance of ceramides and sphingosine-1-phosphate, is critical for regulating cell fate. Ceramides can be converted by ceramidases to sphingosine, which can then be converted by sphingosine kinase 1 to sphingosine-1-phosphate. Shifts toward sphingosine-1-phosphate production may allow cells to evade apoptosis and increase migration, while shifts toward ceramides may favor cell death. Dysregulated sphingolipid metabolism is associated with neurodegenerative disorders and cancer, although the sphingolipid rheostat is less well-studied in neural stem cells or brain tumors. The most common primary, malignant brain tumor is glioblastoma for which standard of care includes surgery, chemotherapy with temozolomide, and radiotherapy. This aggressive regimen fails to eradicate a subset of highly invasive, chemo- and radiotherapy resistant, neural stem cell-like, brain tumor initiating cells (BTICs) and glioblastoma quickly recurs. Radiotherapy can increase acid ceramidase (ASAH1) in BTICs, contributing to a shift in the sphingolipid balance towards sphingosine-1-phosphate. ASAH1 is highly expressed in glioblastoma and is associated with worse survival of glioma patients in The Cancer Genome Atlas data. ASAH1 inhibitors have been shown to increase pro-apoptotic ceramides and block the progression of some cancer types. We seek to determine whether an inhibitor of ASAH1, carmofur, is effective against BTICs derived from parental and temozolomide- resistant patient derived xenografts and to determine the impact of altered sphingolipid metabolism on glioblastoma migration and the brain tumor microenvironment. I anticipate that the studies proposed here will demonstrate that shifting the sphingolipid balance back toward ceramides is an effective therapeutic strategy in glioblastoma.