SOX9 inhibits β-TrCP-mediated protein degradation to promote nuclear GLI1 expression and cancer stem cell properties

The high mobility group box protein SOX9 and the GLI1 transcription factor play protumorigenic roles in pancreatic ductal adenocarcinoma (PDA). In Kras transgenic mice, each of these factors are crucial for the development of PDA precursor lesions. SOX9 transcription is directly regulated by GLI1, but how SOX9 functions downstream of GLI1 is unclear. We observed positive feedback, such that SOX9-deficient PDA cells have severely repressed levels of endogenous GLI1, attributed to loss of GLI1 protein stability. SOX9 associated with the F-box domain of the SKP1/CUL1/F-box (SCF) E3 ubiquitin ligase component, beta-TrCP (also known as F-box/WD repeat-containing protein 1A), and suppressed its association with SKP1 and GLI1, a substrate of SCF-beta-TrCP. SOX9 also tethered beta-TrCP within the nucleus and promoted its degradation. SOX9 bound to beta-TrCP through the SOX9 C-terminal PQA/S domain that mediates transcriptional activation. Suppression of beta-TrCP in SOX9-deficient PDA cells restored GLI1 levels and promoted SOX9-dependent cancer stem cell properties. These studies identify SOX9-GLI1 positive feedback as a major determinant of GLI1 protein stability and implicate beta-TrCP as a latent SOX9-bound tumor suppressor with the potential to degrade oncogenic proteins in tumor cells.