TBX3 functions as a tumor suppressor downstream of activated CTNNB1 mutants during hepatocarcinogenesis.

Abstract Background & aims Gain of function (GOF) mutation of CTNNB1 gene is one of the most frequent genetic events in hepatocellular carcinoma (HCC). T-box transcription factor 3 (TBX3) is a liver-specific target of the Wnt/β-Catenin pathway and thought to be an oncogene mediating activated β-Catenin driven HCC formation. Methods We evaluated TBX3 expression pattern in human HCC specimens. Tbx3 was conditionally knocked out in murine HCC models by hydrodynamic tail vein injection of Cre together with c-Met and ΔN90-β-Catenin (c-Met/β-Catenin) in Tbx3flox/flox mice. TBX3 was overexpressed in human HCC cell lines to investigate the functions of TBX3 in vitro. Results A bimodal expression pattern of TBX3 in human HCC samples was detected: high expression of TBX3 in GOF CTNNB1 HCC and downregulation of TBX3 in non-CTNNB1 mutant tumors. High expression of TBX3 was associated with increased differentiation and decreased tumor growth gene expression signatures. Using Tbx3flox/flox mice, we found that ablation of Tbx3 significantly accelerates c-Met/β-Catenin driven HCC formation. Moreover, Tbx3(-) HCC demonstrated increased YAP/TAZ activity. The accelerated tumor growth induced by loss of TBX3 in c-Met/β-Catenin mouse HCC was successfully prevented by overexpression of LATS2, which inhibited YAP/TAZ activity. In human HCC cell lines, overexpression of TBX3 inhibited HCC cell growth as well as YAP/TAZ activation. A negative correlation between TBX3 and YAP/TAZ target genes was observed in human HCC samples. Mechanistically, phospholipase D1 (PLD1), a known positive regulator of YAP/TAZ, was identified as a novel transcriptional target repressed by TBX3. Conclusion Our study suggests that TBX3 is induced by GOF CTNNB1 mutants and suppresses HCC growth by inactivating PLD1, thus leading to the inhibition of YAP/TAZ oncogenes.