Identification of a novel Calpain-2-SRC feed-back loop as necessity for β-Catenin accumulation and signaling activation in hepatocellular carcinoma

Rapid progression is the major cause of the poor prognosis of hepatocellular carcinoma (HCC); however, the underlying mechanism remained unclear. Here, we found Calpain-2 (CAPN2), a well-established protease that accelerates tumor progression in several malignancies, is overexpressed in HCC and acts as an independent predictor for poor outcomes. Furthermore, CAPN2 promoted the proliferation and invasion of HCC, and showed a positive correlation with the levels of invasion-related markers. Mechanistically, a novel CAPN2-SRC positive regulatory loop was identified upstream of β-catenin to prevent its ubiquitination and degradation, and subsequently promoted HCC progression: CAPN2 could proteolyze PTP1B to form a truncation of approximately 42 kDa with increased phosphatase activity, resulting in reduced SRC Y530 phosphorylation and increased SRC kinase activity; meanwhile, CAPN2 itself was a bone fide substrate of SRC that was primarily phosphorylated at Y625 by SRC and exhibited increased proteolysis activity upon phosphorylation. Interestingly, the CAPN2-SRC loop could not only restrain most of cytoplasmic β-catenin degradation by inhibiting GSK3β pathway, but also prevented TRIM33-induced nuclear β-catenin degradation even in β-catenin-mutant cells. Present study identified a CAPN2-SRC positive loop responsible for intracellular β-catenin accumulation and signaling activation, and targeting CAPN2 protease activity might be a promising approach for preventing HCC progression.