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

Background and Aims Constitutive androstane receptor (CAR) agonists, such as 1,4‐bis [2‐(3,5‐dichloropyridyloxy)] benzene (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. Our and other previous studies suggested increased nuclear localization and activation of Yap after TCPOBOP treatment in mice and the 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‐knockout (KO) mice. Approach and Results Adeno‐associated virus 8‐thyroxine binding globulin promoter‐Cre recombinase 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 cytochrome P450 [Cyp] 2b10, Cyp2c55, and UDP‐glucuronosyltransferase 1a1 [Ugt1a1]). However, Yap deletion substantially reduced TCPOBOP‐induced hepatocyte proliferation. TCPOBOP‐driven cell cycle activation was disrupted in Yap‐KO mice because of delayed (and decreased) induction of cyclin D1 and higher expression of p21, resulting in decreased phosphorylation of retinoblastoma protein. Furthermore, the 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 that were mainly related to proliferation, but not to drug metabolism, were altered by Yap deletion. Yap regulated these proliferation genes through alerting expression of Myc and forkhead box protein M1, two critical transcriptional regulators of CAR‐mediated hepatocyte proliferation. Conclusions 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.