TTPAL promotes colorectal tumorigenesis by stabilizing TRIP6 to activate Wnt/β-catenin signaling

Copy number alterations are crucial for the development of colorectal cancer (CRC). Our whole genome analysis identified tocopherol alpha transfer protein-like (TTPAL) as preferentially amplified in CRC. Here we demonstrate that frequent copy number gain of TTPAL leads to gene overexpression in CRC from a Chinese cohort (n=102), which was further validated by a TCGA cohort (n=376). High expression of TTPAL was significantly associated with shortened survival in CRC patients. TTPAL promoted cell viability and clonogenicity, accelerated cell cycle progression, inhibited cell apoptosis, increased cell migration/invasion ability in vitro, and promoted tumorigenicity and cancer metastasis in vivo. TTPAL significantly activated Wnt signaling and increased β-catenin activation and protein expression of cyclin D1 and c-Myc. Co-immunoprecipitation followed by mass spectrometry identified thyroid receptor-interacting protein 6 (TRIP6) as a direct downstream effector of TTPAL. Depletion of TRIP6 significantly abolished the effects of TTPAL on cell proliferation and Wnt activation. Direct binding of TTPAL with TRIP6 in the cytoplasm inhibited ubiquitin-mediated degradation of TRIP6; subsequently increased levels of TRIP6 displaced β-catenin from the tumor suppressor MAGI1 via competitive binding. This sequence of events allows β-catenin to enter the nucleus and promote oncogenic Wnt/β-catenin signaling. In conclusion, TTPAL is commonly overexpressed in CRC due to copy number gain, which promotes colorectal tumorigenesis by activating Wnt/β-catenin signaling via stabilization of TRIP6. TTPAL overexpression may serve as an independent new biomarker for the prognosis of CRC patients.