TAZ-induced Cybb contributes to liver tumor formation in non-alcoholic steatohepatitis

Abstract

Background & aims: Non-alcoholic steatohepatitis (NASH) is a leading cause of hepatocellular carcinoma (HCC), but mechanisms linking NASH to eventual tumor formation remain poorly understood. Herein, we investigate the role of TAZ/WWTR1, which is induced in hepatocytes in NASH, in the progression of NASH to HCC.

Methods: The roles of hepatocyte TAZ and its downstream targets were investigated in diet-induced and genetic models of NASH-HCC using gene-targeting, adeno-associated virus 8 (AAV8)-H1-mediated gene silencing, or AAV8-TBG-mediated gene expression. The biochemical signature of the newly elucidated pathway was probed in liver specimens from humans with NASH-HCC.

Results: When hepatocyte-TAZ was silenced in mice with pre-tumor NASH using AAV8-H1-shTaz (short-hairpin Taz), subsequent HCC tumor development was suppressed. In this setting, the tumor-suppressing effect of shTaz was not dependent of TAZ silencing in the tumors themselves and could be dissociated from the NASH-suppressing effects of shTaz. The mechanism linking pre-tumor hepatocyte-TAZ to eventual tumor formation involved TAZ-mediated induction of the NOX2-encoding gene Cybb, which led to NADPH-mediated oxidative DNA damage. As evidence, DNA damage and tumor formation could be suppressed by treatment of pre-tumor NASH mice with AAV8-H1-shCybb; AAV8-TBG-OGG1, encoding the oxidative DNA-repair enzyme 8-oxoguanine glycosylase; or AAV8-TBG-NHEJ1, encoding the dsDNA repair enzyme non-homologous end-joining factor 1. In surrounding non-tumor tissue from human NASH-HCC livers, there were strong correlations between TAZ, NOX2, and oxidative DNA damage.

Conclusions: TAZ in pre-tumor NASH-hepatocytes, via induction of Cybb and NOX2-mediated DNA damage, contributes to subsequent HCC tumor development. These findings illustrate how NASH provides a unique window into the early molecular events that can lead to tumor formation and suggest that NASH therapies targeting TAZ might also prevent NASH-HCC.

Lay summary: Non-alcoholic steatohepatitis (NASH) is emerging as the leading cause of a type of liver cancer called hepatocellular carcinoma (HCC), but molecular events in pre-tumor NASH hepatocytes leading to HCC remain largely unknown. Our study shows that a protein called TAZ in pre-tumor NASH-hepatocytes promotes damage to the DNA of hepatocytes and thereby contributes to eventual HCC. This study reveals a very early event in HCC that is induced in pre-tumor NASH, and the findings suggest that NASH therapies targeting TAZ might also prevent NASH-HCC.

Keywords: NOX2/Cybb; TAZ/WWTR1; hepatocellular carcinoma (HCC); nonalcoholic steatohepatitis (NASH); oxidative DNA damage.