Yes‐Associated Protein in Kupffer Cells Enhances the Production of Proinflammatory Cytokines and Promotes the Development of Nonalcoholic Steatohepatitis

Background and Aims Yes‐associated protein (YAP) plays an important role in hepatocarcinogenesis, although the potential role of YAP in non‐neoplastic liver diseases remains largely unknown. We report herein that YAP in Kupffer cells (KCs) enhances the production of proinflammatory cytokines and promotes the development of nonalcoholic steatohepatitis (NASH). Our data show that the expression of YAP is significantly increased in KCs of wild‐type mice fed a high‐fat diet (HFD). Approach and Results We generated mice with macrophage/monocyte‐specific deletion of YAP (YAP ϕKO ) or Toll‐like receptor 4 (TLR4; TLR4 ϕKO ), and animals were fed an HFD or treated with lipopolysaccharide (LPS). Our data showed that YAP ϕKO mice fed an HFD exhibited lower serum alanine aminotransferase (ALT)/aspartate aminotransferase (AST) levels and less hepatic inflammation when compared to their littermate controls. LPS treatment induced accumulation of YAP in KCs in vitro and in mice, which was prevented by macrophage/monocyte‐specific deletion of TLR4 (TLR4 ϕKO ). LPS transcriptionally activates YAP through activator protein 1 in macrophages/KCs. LPS‐induced YAP further enhances expression of proinflammatory cytokines (including monocyte chemoattractant protein 1, tumor necrosis factor alpha, and interleukin 6) through YAP association with the TEA domain–binding motif in the promoter region of inflammatory cytokines. Forced overexpression of active YAP (YAP5SA) in KCs enhanced the production of proinflammatory cytokines. Treatment of HFD‐fed mice with verteporfin inhibited KC activation, reduced liver inflammation, and decreased serum ALT/AST levels. Analyses of liver tissues from NASH patients reveal that YAP is increased in KCs and that level of YAP in human liver tissues is positively correlated with expression of proinflammatory cytokines. Conclusions This study describes an important role of YAP in KCs for regulation of liver inflammation in NASH. Our findings suggest that inhibition of YAP may represent an effective therapeutic strategy for NASH treatment.