Introduction
The constitutive androstane receptor (CAR, NR3I1) is a
xenobiotic-sensing nuclear receptor and a member of the nuclear receptor
superfamily (Konno, Negishi & Kodama, 2008). Like other nuclear
receptors, CAR is composed of a DNA-binding domain, a large
carboxy-terminal ligand-binding domain (LBD), and a poorly conserved
amino-terminal domain (Willson & Kliewer, 2002). CAR is localized in
the cytoplasm by forming a complex with HSP90 and the cytoplasmic CAR
retention protein (CCRP). Once activated, CAR dissociates from the
cytoplasmic complex and is translocated into the nucleus where it forms
a heterodimer with retinoid X receptor alpha (RXRα, ΝΡ2Β1) and activates
transcription of its target genes (Kodama & Negishi, 2006).
CAR activates transcription of its target genes encoding various enzymes
and proteins involved in drug metabolism and transport when exposure to
xenobiotics (Konno, Negishi & Kodama, 2008; Wei, Zhang, Egan-Hafley,
Liang & Moore, 2000). In addition to xenobiotics, CAR can also regulate
the elimination of toxic endobiotics such as bilirubin and bile acids,
so one major function of CAR is detoxification (Eloranta &
Kullak-Ublick, 2005; Sugatani et al., 2002). However, recent studies
have also revealed a key role for CAR in various physiological and
pathophysiological processes occurring in the liver including
gluconeogenesis, metabolism of fatty acids, metabolism of lipids,
hormonal regulation, proliferation of hepatocytes, and
hepatocarcinogenesis (Kachaylo, Pustylnyak, Lyakhovich & Gulyaeva,
2011). CAR is also involved in metabolic diseases including jaundice,
cholestasis, thyroid homeostasis and obesity (Dash et al., 2014;
Qatanani & Moore, 2005).
TCPOBOP (1,4-Bis [2-(3,5-dichloropyridyloxy)] benzene), a strong
agonist of CAR, was reported to induce robust hepatocyte proliferation
and hepatomegaly in mice livers (Costa, Kalinchenko, Tan & Wang, 2005).
Moreover, the rapid proliferation response induced by TCPOBOP can
protect liver from failure even after massive tissue loss by 91%
hepatectomy (Tschuor et al., 2016). The mechanism of TCPOBOP-induced
hepatomegaly was studied in previous reports. C-Myc and FOXM1 mediated
proliferative programs are key mediators of TCPOBOP-CAR induced direct
liver hyperplasia (Blanco-Bose et al., 2008). β-catenin (CTNNB1)
deficiency decreases the impact of CAR activation on hepatocytes
proliferation (Ganzenberg, Singh & Braeuning, 2013). Disruption of EGFR
and MET signaling leads to a dramatic impairment of the TCPOBOP-induced
proliferative response without altering CAR activation (Bhushan et al.,
2019).
Yes-associated protein (YAP) is a key downstream factor of Hippo
signaling pathway, which is a potent regulator of organ size and tissue
homeostasis (Kowalik et al., 2011). When Hippo signaling is ON, YAP is
phosphorylated by LATS1/2 and binding with 14-3-3 in cytoplasm. When
Hippo signaling is OFF, YAP can be dephosphorylated and translocated
into nucleus where it binds with TEAD to activate transcription of genes
involved in cell survival, growth, and proliferation (Patel, Camargo &
Yimlamai, 2017). According to previous studies, YAP increases organ size
and causes aberrant tissue expansion in mice. YAP activation reversibly
increases liver size more than 4-fold (Camargo et al., 2007). Removal ofMst1 and Mst2 (the upstream of YAP) in liver, which
activates YAP, results in significant liver enlargement and dysplasia
(Song et al., 2010). In addition, YAP also participates in liver
regeneration after partial hepatectomy (PHx). Hepatocyte nuclear
accumulation of YAP increased in parallel with hepatocyte proliferative
activity after PHx. Deleting Yap from hepatocytes blocked the
nuclear accumulation of pSmad2 and EMT-like response, as well as their
proliferation (Oh, Swiderska-Syn, Jewell, Premont & Diehl, 2018).
Inhibition of MST1 and MST2 augments liver repair and regeneration by
activating the downstream effector YAP and promoting cell growth (Fan et
al., 2016).
Most recently, YAP/TEAD activation was found to participate in
CAR-dependent proliferation of murine hepatocytes (Abe et al., 2018).
However, this study used an in vitro cell-based system and an
animal experiment of verteporfin (an inhibitor of YAP/TEAD interaction)
treatment. The mechanistic influence of CAR activation on YAP signaling
has not been clarified and the relationship between CAR and YAP remains
unknown. Therefore, the current study aims to elucidate the role of YAP
in CAR-induced hepatomegaly using liver-specific YAP-deficient mice, to
investigate whether YAP is associated with CAR-promoted liver
regeneration, and further explore the relationship between CAR and YAP.