CSIG-12. TUSC2–ALDH1A3 SIGNALING AXIS IS A TARGETABLE PATHWAY DRIVING STEMNESS AND POOR PROGNOSIS IN GLIOBLASTOMA

Abstract Glioblastoma (GBM) remains among the most aggressive primary brain tumors with limited therapeutic options. Tumor Suppressor Candidate 2 (TUSC2) is recognized as a novel tumor suppressor frequently lost in GBM. However, the precise mechanisms underpinning TUSC2-mediated tumor suppression and its regulation of protein stability have yet to be fully elucidated. This study aimed to identify and characterize TUSC2 interacting proteins and investigate the mechanisms through which TUSC2 regulates tumor suppression in GBM. Our analyses identified a novel interaction between TUSC2 and Aldehyde dehydrogenase 1A3 (ALDH1A3), a protein highly expressed in the glioma stem cell (GSC) population and previously implicated in promoting GBM resistance and stemness. TUSC2 interaction with ALDH1A3 significantly reduced ALDH enzymatic activity, inhibited retinoic acid (RA) synthesis, and subsequently suppressed downstream RA signaling pathways critical for GBM proliferation and stemness. Conversely, loss of TUSC2 resulted in increased ALDH activity, elevated RA synthesis, and enhanced RA signaling, highlighting the critical suppressive role of TUSC2. Further mechanistic exploration revealed TUSC2 undergoes critical post-translational modifications, notably phosphorylation at serine 50 (S50), mediated by protein kinases PKAcβ and PKCα. This phosphorylation event was demonstrated to directly influence TUSC2 protein stability by facilitating its binding with E3 ubiquitin ligases, subsequently leading to its poly-ubiquitination and degradation. Notably, inhibition or mutation of the phosphorylation site significantly stabilized TUSC2 protein, underscoring the therapeutic potential of modulating this phosphorylation event. Collectively, our findings uncover a novel mechanistic axis involving TUSC2-mediated suppression of ALDH1A3 and the retinoic acid signaling pathway in GBM. Furthermore, we delineate a crucial regulatory mechanism whereby TUSC2 protein stability and tumor suppressive functions are intricately controlled by phosphorylation at S50, providing a potential therapeutic target to restore TUSC2 activity and suppress GBM progression.