FAT4 loss initiates hepatocarcinogenesis through the switching of canonical to noncanonical WNT signaling pathways

Mutation and downregulation of FAT atypical cadherin 4 (FAT4) are frequently detected in HCC, suggesting a tumor suppressor role of FAT4. However, the underlying molecular mechanism remains elusive. CRISPR-Cas9 system was used to knockout FAT4 (FAT4-KO) in a normal human hepatic cell line L02 to investigate the impact of FAT4 loss on the development of HCC. RNA-sequencing and xenograft mouse model were used to study gene expression and tumorigenesis, respectively. The mechanistic basis of FAT4 loss on hepatocarcinogenesis was elucidated using in vitro experiments. We found that FAT4-KO disrupted cell-cell adhesion, induced epithelial-mesenchymal transition, and increased expression of extracellular matrix components. FAT4-KO is sufficient for tumor initiation in a xenograft mouse model. RNA-sequencing of FAT4-KO cells identified PAK6-mediated WNT/β-catenin signaling to promote tumor growth. Suppression of PAK6 led to β-catenin shuttling out of the nucleus for ubiquitin-dependent degradation and constrained tumor growth. Further, RNA-sequencing of amassed FAT4-KO cells identified activation of WNT5A and ROR2. The noncanonical WNT5A/ROR2 signaling has no effect on β-catenin and its target genes (CCND1 and c-Myc) expression. Instead, we observed downregulation of receptors for WNT/β-catenin signaling, suggesting the shifting of β-catenin-dependent to β-catenin-independent pathways as tumor progression depends on its receptor expression. Both PAK6 and WNT5A could induce the expression of extracellular matrix glycoprotein, laminin subunit alpha 4. Laminin subunit alpha 4 upregulation in HCC correlated with poor patient survival. Our data show that FAT4 loss is sufficient to drive HCC development through the switching of canonical to noncanonical Wingless-type signaling pathways. The findings may provide a mechanistic basis for an in-depth study of the two pathways in the early and late stages of HCC for precise treatment.