Phosphorylated PTTG1 switches its subcellular distribution and promotes β-catenin stabilization and subsequent transcription activity

The Wnt/β-catenin signaling is usually abnormally activated in hepatocellular carcinoma (HCC), and pituitary tumor-transforming gene 1 (PTTG1) has been found to be highly expressed in HCC. However, the specific mechanism of PTTG1 pathogenesis remains poorly understood. Here, we found that PTTG1 is a bona fide β-catenin binding protein. PTTG1 positively regulates Wnt/β-catenin signaling by inhibiting the destruction complex assembly, promoting β-catenin stabilization and subsequent nuclear localization. Moreover, the subcellular distribution of PTTG1 was regulated by its phosphorylation status. Among them, PP2A induced PTTG1 dephosphorylation at Ser165/171 residues and prevented PTTG1 translocation into the nucleus, but these effects were effectively reversed by PP2A inhibitor okadaic acid (OA). Interestingly, we found that PTTG1 decreased Ser9 phosphorylation-inactivation of GSK3β by competitively binding to PP2A with GSK3β, indirectly leading to cytoplasmic β-catenin stabilization. Finally, PTTG1 was highly expressed in HCC and associated with poor patient prognosis. PTTG1 could promote the proliferative and metastasis of HCC cells. Overall, our results indicated that PTTG1 plays a crucial role in stabilizing β-catenin and facilitating its nuclear accumulation, leading to aberrant activation of Wnt/β-catenin signaling and providing a feasible therapeutic target for human HCC.