Published online Mar 27, 2021. doi: 10.4254/wjh.v13.i3.343
Peer-review started: December 12, 2020
First decision: January 7, 2021
Revised: January 15, 2021
Accepted: March 8, 2021
Article in press: March 8, 2021
Published online: March 27, 2021
BIR repeat-containing ubiquitin conjugating enzyme (BRUCE) is a liver tumor suppressor, which is downregulated in a large number of patients with liver diseases. BRUCE facilitates DNA damage repair to protect the mouse liver against the hepatocarcinogen diethylnitrosamine (DEN)-dependent acute liver injury and carcinogenesis. While there exists an established pathologic connection between fibrosis and hepatocellular carcinoma (HCC), DEN exposure alone does not induce robust hepatic fibrosis. Further studies are warranted to identify new suppressive mechanisms contributing to DEN-induced fibrosis and HCC.
To investigate the suppressive mechanisms of BRUCE in hepatic fibrosis and HCC development.
Male C57/BL6/J control mice [loxp/Loxp; albumin-cre (Alb-cre)-] and BRUCE Alb-Cre KO mice (loxp/Loxp; Alb-Cre+) were injected with a single dose of DEN at postnatal day 15 and sacrificed at different time points to examine liver disease progression.
By using a liver-specific BRUCE knockout (LKO) mouse model, we found that BRUCE deficiency, in conjunction with DEN exposure, induced hepatic fibrosis in both premalignant as well as malignant stages, thus recapitulating the chronic fibrosis background often observed in HCC patients. Activated in fibrosis and HCC, β-catenin activity depends on its stabilization and subsequent translocation to the nucleus. Interestingly, we observed that livers from BRUCE KO mice demonstrated an increased nuclear accumulation and elevated activity of β-catenin in the three stages of carcinogenesis: Pre-malignancy, tumor initiation, and HCC. This suggests that BRUCE negatively regulates β-catenin activity during liver disease progression. β-catenin can be activated by phosphorylation by protein kinases, such as protein kinase A (PKA), which phosphorylates it at Ser-675 (pSer-675-β-catenin). Mechanistically, BRUCE and PKA were colocalized in the cytoplasm of hepatocytes where PKA activity is maintained at the basal level. However, in BRUCE deficient mouse livers or a human liver cancer cell line, both PKA activity and pSer-675-β-catenin levels were observed to be elevated.
Our data support a “BRUCE-PKA-β-catenin” signaling axis in the mouse liver. The BRUCE interaction with PKA in hepatocytes suppresses PKA-dependent phosphorylation and activation of β-catenin. This study implicates BRUCE as a novel negative regulator of both PKA and β-catenin in chronic liver disease progression. Furthermore, BRUCE-liver specific KO mice serve as a promising model for understanding hepatic fibrosis and HCC in patients with aberrant activation of PKA and β-catenin.
Core Tip: Upon diethylnitrosamine (DEN) exposure, BIR repeat-containing ubiquitin conjugating enzyme (BRUCE) liver-deficiency accelerates chronic liver diseases such as fibrosis and hepatocellular carcinoma (HCC) in mice. Our previous study established the role of BRUCE in the protection of the liver against DEN-induced liver injury and subsequent disease progression. Here we report a chronic fibrosis background induced by hepatic BRUCE knockout in mice that recapitulates the fibrosis background in HCC patients. We also report a BRUCE-dependent suppression of β-catenin activity through the suppression of protein kinase A (PKA) activity. This study provides a therapeutic potential involving the inhibition of PKA and β-catenin activities in patients with liver disease that carry BRUCE inactivating mutations.