Yes‐Associated Protein Is Crucial for Constitutive Androstane Receptor‐Driven Hepatocyte Proliferation But Not for Induction of Drug Metabolism Genes in Mice

Abstract

Constitutive androstane receptor (CAR) agonists, such as TCPOBOP, are known to cause robust hepatocyte proliferation and hepatomegaly in mice along with induction of drug metabolism genes, without any associated liver injury. Yes‐associated protein (Yap) is a key transcription regulator that tightly controls organ size including that of liver. Ours and other previous studies suggested increased nuclear localization and activation of Yap after TCPOBOP‐treatment in mice and potential role of Yap in CAR‐driven proliferative response. Here, we investigated a direct role of Yap in CAR‐driven hepatomegaly and hepatocyte proliferation using hepatocyte‐specific Yap‐KO mice. AAV8‐TBG‐CRE vector was injected to Yap‐floxed mice for achieving hepatocyte‐specific Yap deletion followed by TCPOBOP‐treatment. Yap deletion did not decrease protein expression of CAR or CAR‐driven induction of drug metabolism genes (including Cyp2b10, Cyp2c55 and Ugt1a1). However, Yap deletion substantially reduced TCPOBOP‐induced hepatocyte proliferation. TCPOBOP‐driven cell cycle activation was disrupted in Yap‐KO mice due to delayed (and decreased) induction of cyclin D1 and higher expression of p21, resulting in decreased phosphorylation of retinoblastoma (Rb) protein. Further, induction of other cyclins, which are sequentially involved in progression through cell cycle (including cyclin E1, A2 and B1) and important mitotic regulators (such as aurora B kinase and polo‐like kinase 1) was remarkably reduced in Yap‐KO mice. Microarray analysis revealed that 26% of TCPOBOP‐responsive genes mainly related to proliferation, but not to drug metabolism, were altered by Yap deletion. Yap regulated these proliferation genes via alerting expression of Myc and Foxm1, two critical transcriptional regulators of CAR‐mediated hepatocyte proliferation. Conclusion: Our study revealed an important role of Yap signaling in CAR‐driven hepatocyte proliferation; however, CAR‐driven induction of drug metabolism genes was independent of Yap.