摘要
BACKGROUND 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.AIM To investigate the suppressive mechanisms of BRUCE in hepatic fibrosis and HCC development.METHODS 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.RESULTS 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 ofβ-catenin in the three stages of carcinogenesis:Pre-malignancy,tumor initiation,and HCC.This suggests that BRUCE negatively regulatesβ-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.CONCLUSION 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 ofβ-catenin.This study implicates BRUCE as a novel negative regulator of both PKA andβ-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 andβ-catenin.
基金
Supported by NIH(Du CY),No.R21CA241025-01
NIH(Du CY),No.RO1CA158323
NCI RO1 Diversity Supplement(Du CY),No.R01CA158323-05S
National Center for Advancing Translational Sciences of the National Institutes of Health(Du CY),No.2UL1TR001425-05A1
University of Cincinnati Center for Environmental Genetics-NIH/NIEHS Award(Du CY),No.P30 ES006096
Pathways to Cancer Therapeutics T32(Du CY and Vilfranc CL),No.CA117846-12.
