Xenobiotic-Induced Hepatocyte Proliferation Associated with Constitutive Active/Androstane Receptor (CAR) or Peroxisome Proliferator-Activated Receptor α (PPARα) Is Enhanced by Pregnane X Receptor (PXR) Activation in Mice

Abstract

Xenobiotic-responsive nuclear receptors pregnane X receptor (PXR), constitutive active/androstane receptor (CAR) and peroxisome proliferator-activated receptor α (PPARα) play pivotal roles in the metabolic functions of the liver such as xenobiotics detoxification and energy metabolism. While CAR or PPARα activation induces hepatocyte proliferation and hepatocarcinogenesis in rodent models, it remains unclear whether PXR activation also shows such effects. In the present study, we have investigated the role of PXR in the xenobiotic-induced hepatocyte proliferation with or without CAR activation by 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) and phenobarbital, or PPARα activation by Wy-14643 in mice. Treatment with TCPOBOP or phenobarbital increased the percentage of Ki-67-positive nuclei as well as mRNA levels of cell proliferation-related genes in livers as expected. On the other hand, treatment with the PXR activator pregnenolone 16α-carbonitrile (PCN) alone showed no such effects. Surprisingly, PCN co-treatment significantly augmented the hepatocyte proliferation induced by CAR activation with TCPOBOP or phenobarbital in wild-type mice but not in PXR-deficient mice. Intriguingly, PXR activation also augmented the hepatocyte proliferation induced by Wy-14643 treatment. Moreover, PCN treatment increased the RNA content of hepatocytes, suggesting the induction of G0/G1 transition, and reduced mRNA levels of Cdkn1b and Rbl2, encoding suppressors of cell cycle initiation. Our present findings indicate that xenobiotic-induced hepatocyte proliferation mediated by CAR or PPARα is enhanced by PXR co-activation despite that PXR activation alone does not cause the cell proliferation in mouse livers. Thus PXR may play a novel and unique role in the hepatocyte/liver hyperplasia upon exposure to xenobiotics.

The human constitutive androstane receptor promotes the differentiation and maturation of hepatic-like cells

“…In these respects, PXR appears to function in the maintenance of the immature phenotype, rather than a promoter of stem cell differentiation, as is the case for CAR. This finding appears consistent with several previous reports demonstrating that PXR suppresses the proliferation of cancer cells by upregulating cyclin-dependent kinase (CDK) inhibitor p21, that ectopic PXR expression in neuroblastoma cells results in growth suppression (Misawa et al, 2005), and that PXR activation alone has no effect on cell proliferation in mouse liver (Shizu et al, 2013). …”

Acceleration of murine hepatocyte proliferation by imazalil through the activation of nuclear receptor PXR

“…Recently, we have demonstrated that PXR activation accelerates CAR-dependent as well as growth factor-mediated proliferation of murine hepatocytes by increasing the sensitivity to proliferation stimuli through the downregulation of cyclin-dependent kinase inhibitor proteins (Shizu et al, 2013(Shizu et al, , 2016. We thus co-treated mice with TCPOBOP and imazalil to verify whether imazalil treatment could accelerate hepatocyte proliferation induced by TCPOBOP through mPXR activation.…”

“…In addition to this well-known function, recent studies have expanded the roles of PXR, which include the regulation of glucose or lipid metabolisms in the liver (Sui et al, 2011;Chang and Waxman, 2006;Hukkanen, 2012;Gotoh and Negishi, 2015;Spruiell et al, 2014). In addition, we have recently demonstrated that PXR activation enhances hepatocyte proliferation mediated by CAR or peroxisome proliferator-activated receptor α (PPARα) activation: Although treatment of mice with pregnenolone 16α-carbonitrile (PCN), a PXR agonist, alone did not induce hepatocyte proliferation, co-treatment of mice with PCN and CAR or PPARα agonist showed intense hepatocyte proliferation compared to single treatment with CAR or PPARα agonist (Shizu et al, 2013), which probably results from the PXR-mediated down-regulation of cell cycle suppressor genes (Shizu et al, 2016). Because CAR-and PPARα-dependent hepatocyte proliferation is associated with liver cancer in rodents, the exposure to PXR activators might increase the risk of liver cancer, that is, PXR-activating capability of chemicals can be a potential risk of adverse effects.…”

Pregnane X Receptor and Cancer: Context-Specificity is Key

“…However, PXR activation also induces differentiation of osteoblasts and apoptosis of osteoclasts and certain leukemia cells (Austin et al, 2015; Hassen et al, 2014; Igarashi et al, 2007; Kameda et al, 1996; Tabb et al, 2003), suggesting that control of cell proliferation by PXR is likely tissue and cell-specific. A similar theme plays out in cancer cells, in which, PXR differentially regulates cell growth through multiple mechanisms in a variety of cancers, including liver, prostate, breast, ovarian, endometrial, cervical, and colon (Braeuning et al, 2014; Kakehashi et al, 2013; Koutsounas et al, 2013; Luisier et al, 2014; Ma et al, 2015; Niu et al, 2014; Pondugula and Mani, 2013; Qiao et al, 2013; Ross et al, 2010; Rouquie et al, 2014; Shizu et al, 2013; Tinwell et al, 2014). Additionally, PXR is involved in regulating metastasis of cancer cells (Gupta et al, 2008; Masuyama et al, 2007; Wang et al, 2011).…”