Cancer Stem Cells Induce Proline Synthesis through FAM3C-Mediated SPIN1 Stabilization to Attenuate ROS Stress

Tumors reprogram amino acid metabolism to dysregulate cellular growth through multiple mechanisms. Here, we find that glioblastoma (GBM) stem cells (GSCs) contain increased proline levels relative to differentiated GBM cells (DGCs) through increased proline synthesis. We identified FAM3C (FAM3 Metabolism Regulating Signaling Molecule C) as an upstream driver of proline synthetic enzyme expression. The core stem cell transcription factor, SOX2, drove GSC-specific expression of FAM3C. FAM3C is secreted, but, in GSCs, intracellular FAM3C directly bound histone 3 lysine 4 trimethyl lysine 9 trimethyl (H3K4me3K9me3) reader, SPIN1 (Spindlin1), to prevent SPIN1 lysosomal degradation and promote epigenetic regulation of proline synthesis. Proline synthesis depletes reactive oxygen species (ROS); genetic targeting of FAM3C attenuated ROS scavenging, while SPIN1 overexpression restored ROS levels. Molecular docking screening identified tucatinib as a brain penetrant pharmacologic disruptor of FAM3C-SPIN1 interactions, promoting SPIN1 degradation and reducing intracellular proline levels. Thus, the SOX2-FAM3C-SPIN1 axis activates proline metabolism in GBM and targeted through repurposing of tucatinib.