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 known ubiquitin conjugase/Ligase hybrid that has been shown to inhibit apoptosis, regulate efficient DNA repair, and most recently promote tumor suppression in the liver. Our group previously showed that upon liver injury with diethylnitrosamine (DEN), loss of hepatic BRUCE promoted fibrosis and exacerbated hepatocellular carcinoma (HCC) development in mice.
About 80% of HCCs develop in fibrotic or cirrhotic livers, demonstrating the importance of understanding liver fibrosis as a factor contributing to hepatic malignancy. Identifying mechanisms that can regulate both fibrosis and HCC development simultaneously provides the possibility of opening therapeutic windows for treating fibrosis and HCC. Considering that over 50% of human HCCs have aberrant β-catenin mutations, targeting the Wnt/β-catenin has shown much promise. The key upstream regulators of this pathway that suppress fibrosis and HCC development remain elusive.
The objective of this study was to evaluate the mechanisms of BRUCE in inhibiting hepatic fibrosis and HCC upon liver injury induction.
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. Mice were sacrificed at various time points to examine liver disease progression and liver biopsies were used in the analyses of the proposed mechanism.
Based on the exacerbation of fibrosis and HCC phenotypes observed in the liver-specific BRUCE knockout (LKO) mice that we previously reported, we hypothesized that, “the onset of fibrosis and tumorigenesis are likely earlier events in LKO mice”. In the present study, we found that upon DEN-induction, BRUCE LKO livers developed fibrosis as early as after 6 mo of exposure. Additionally, the LKO mice developed tumors as early as 8-months after exposure compared to the WT tumor onset after 10 mo of DEN exposure. Furthermore, we observed increased accumulation of β-catenin, including its activity in LKO liver samples. The phosphorylation of β-catenin was determined by measuring nuclear levels of total β-catenin, and Ser-675 phosphorylated β-catenin. Additionally, the activity of protein kinase A (PKA), one of the upstream kinases that phosphorylates β-catenin at Ser-675, was found to be increased in both BRUCE-deficient mouse livers and a human liver cancer cell line. More importantly, BRUCE and PKA were found to be colocalized in the cytoplasm of hepatocytes.
In conclusion, this study further demonstrated BRUCE’s liver tumor suppressive function, by identifying the early onset of tumorigenesis in LKO mice. Furthermore, the current study elucidated a novel role of BRUCE in the negative regulation of PKA activity in order to negatively regulate β-catenin stabilization and activity. Together, BRUCE’s regulation of β-catenin through PKA, is a likely mechanism used to suppress hepatic diseases, such as fibrosis and HCC.
While further investigation is warranted, this study revealed the novel role of BRUCE in hepatic regulation of β-catenin upon liver injury. Further establishing BRUCE’s regulation of PKA activity can possibly provide more promising therapeutic approaches for treating liver disease patients with aberrant expression of BRUCE and β-catenin.