β-catenin signaling, the constitutive androstane receptor and their mutual interactions

Differential expression levels of β-catenin are associated with invasive behavior of both functional and non-functional pituitary neuroendocrine tumor (PitNET)

BACKGROUND

Although research continues to elucidate the molecular mechanism underlying pituitary tumor pathogenesis, limited information is available on the potential role and expression profile of β-catenin in functional and non-functional pituitary neuroendocrine tumors (PitNETs).

METHODS AND RESULTS

In the current study, 104 pituitary samples (tumors and cadaveric healthy pituitary tissues) were included and the gene and protein expression levels of β-catenin were assessed by Real-Time PCR and immunohistochemistry, respectively. The correlation between expression level of β-catenin and tumor invasive feature and size as well as patient age, gender, and hormonal level was measured. The data showed that PitNET samples expressed higher levels of the β-catenin gene and protein compared to healthy pituitary tissues. Although there was no difference in β-catenin expression level between non-functioning (NF-PitNETs) and growth hormone-producing tumors (GH-PitNETs), both tumor types showed significantly elevated β-catenin levels compared to healthy pituitary tissues. The high level of β-catenin in the invasive functional and non-functional tumors is indicative of the association of β-catenin with PitNETs invasion. The expression pattern of the β-catenin gene and protein was consistently and significantly associated with these tumor types. The correlation between β-catenin and insulin-like growth factor 1 (IGF-1) in GH-PitNETs indicates the potential relevance of β-catenin and IGF-1 for GH-PitNETs.

CONCLUSIONS

The simultaneous increase in the expression of β-catenin gene and protein level in PitNET tissues and their relationship to the tumor severity indicates the possible contributing role of β-catenin and its underlying signaling mediators in PitNET pathogenesis.

Promotion of NR1I3-mediated liver growth is accompanied by STAT3 activation

BACKGROUND

The constitutive androstane receptor (CAR, NR1I3)-mediated mechanisms regulating hepatocyte proliferation and growth of the liver did not yet experience complete elucidation. We investigated whether STAT3 could be activated in vivo by NR1I3 signaling in mouse liver.

METHODS AND RESULTS

Using Western blot analysis, immunofluorescence assays and real-time PCR we established the state of STAT3 activation when it comes to the mouse liver subsequent to treatment ofNR1I3 agonist,1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP). STAT3 nuclear relocation and hepatocyte growth were both induced by NR1I3-mediated phosphorylation of STAT3. Moreover, the NR1I3-STAT3 signaling pathway's proliferation impact was facilitated, partly, by cMyc and Cyclin D1 upregulation.

CONCLUSIONS

This work's evidence demonstrates that NR1I3-pushed STAT3 activation contributes to TCPOBOP-induced liver growth and hepatocyte proliferation, at least in part, through its molecular targets cMyc and CyclinD1.

Off-target lipid metabolism disruption by the mouse constitutive androstane receptor ligand TCPOBOP in humanized mice

The constitutive androstane receptor (CAR) controls xenobiotic clearance, regulates liver glucose, lipid metabolism, and energy homeostasis. These functions have been mainly discovered using the prototypical mouse-specific CAR ligand TCPOBOP in wild-type or CAR null mice. However, TCPOBOP is reported to result in some off-target metabolic effects in CAR null mice. In this study, we compared the metabolic effects of TCPOBOP using lipidomic, transcriptomic, and proteomic analyses in wild-type and humanized CAR-PXR-CYP3A4/3A7 mice. In the model, human CAR retains its constitutive activity in metabolism regulation; however, it is not activated by TCPOBOB. Notably, we observed that TCPOBOP affected lipid homeostasis by elevating serum and liver triglyceride levels and promoted hepatocyte hypertrophy in humanized CAR mice. Hepatic lipidomic analysis revealed a significant accumulation of triglycerides and decrease of its metabolites in humanized CAR mice. RNA-seq analysis has shown divergent gene expression levels in wild-type and humanized CAR mice. Gene expression regulation in humanized mice is mainly involved in lipid metabolic processes and in the PPAR, leptin, thyroid, and circadian clock pathways. In contrast, CAR activation by TCPOBOP in wild-type mice reduced liver and plasma triglyceride levels and induced a typical transcriptomic proliferative response in the liver. In summary, we identified TCPOBOP as a disruptor of lipid metabolism in humanized CAR mice. The divergent effects of TCPOBOP in humanized mice in comparison with the prototypical CAR-mediated response in WT mice warrant the use of appropriate model ligands and humanized animal models during the testing of endocrine disruption and the characterization of adverse outcome pathways.

Protein Kinase N Family Negatively Regulates Constitutive Androstane Receptor-Mediated Transcriptional Induction of Cytochrome P450 2b10 in the Livers of Mice

In receptor-type transcription factors-mediated cytochrome P450 (P450) induction, few studies have attempted to clarify the roles of protein kinase N (PKN) in the transcriptional regulation of P450s. This study aimed to examine the involvement of PKN in the transcriptional regulation of P450s by receptor-type transcription factors, including the aryl hydrocarbon receptor, constitutive androstane receptor (CAR), and pregnane X receptor. The mRNA and protein levels and metabolic activity of P450s in the livers of wild-type (WT) and double-mutant (D) mice harboring both PKN1 kinase-negative knock-in and PKN3 knockout mutations [PKN1 ^T778A/T778A; PKN3 ^-/-] were determined after treatment with activators for receptor-type transcription factors. mRNA and protein levels and metabolic activity of CYP2B10 were significantly higher in D mice treated with the CAR activator phenobarbital (PB) but not with 1,4-bis((3,5-dichloropyridin-2-yl)oxy)benzene compared with WT mice. We examined the CAR-dependent pathway regulated by PKN after PB treatment because the extent of CYP2B10 induction in WT and D mice was notably different in response to treatment with different CAR activators. The mRNA levels of Cyp2b10 in primary hepatocytes from WT and D mice treated with PB alone or in combination with Src kinase inhibitor 1 (SKI-1) or U0126 (a mitogen-activated protein kinase inhibitor) were evaluated. Treatment of hepatocytes from D mice with the combination of PB with U0126 but not SKI-1 significantly increased the mRNA levels of Cyp2b10 compared with those from the corresponding WT mice. These findings suggest that PKN may have inhibitory effects on the Src-receptor for activated C kinase 1 (RACK1) pathway in the CAR-mediated induction of Cyp2b10 in mice livers. SIGNIFICANCE STATEMENT: This is the first report of involvement of PKN in the transcriptional regulation of P450s. The elucidation of mechanisms responsible for induction of P450s could help optimize the pharmacotherapy and improve drug development. We examined whether the mRNA and protein levels and activities of P450s were altered in double-mutant mice harboring both PKN1 kinase-negative knock-in and PKN3 knockout mutations. PKN1/3 negatively regulates CAR-mediated induction of Cyp2b10 through phosphorylation of a signaling molecule in the Src-RACK1 pathway.

Xenobiotic Receptor CAR Is Highly Induced in Psoriasis and Promotes Keratinocyte Proliferation

Psoriasis is a chronic inflammatory skin disease with abnormal epidermal proliferation. Xenobiotics contribute to the pathogenesis of psoriasis. The mechanism linking xenobiotic stimuli with epidermal proliferation remains largely unknown. In this study, we investigated the role of CAR, a nuclear receptor (NR1I3) responsible for xenobiotics detoxification. We showed that CAR and its target genes were induced in the lesions from patients with psoriasis and imiquimod-treated mice. Proinflammatory cytokines (IL-17A, IL-22, oncostatin M, IL-1α, and TNF-α) synergistically increased the expressions of CAR and its target genes in both human and mouse keratinocytes. Overexpression of CAR promoted the G1/S transition by regulating cyclin E and c-Myc expressions, whereas the silencing of CAR attenuated it. Importantly, a selective CAR agonist 6-(4-chlorophenyl)imidazo(2,1-b)(1,3)thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime or the proinflammatory cytokines induced cyclin E and c-Myc, which were largely blocked by clotrimazole, a selective CAR antagonist, or CAR small interfering RNA. In addition, we showed that topical application of 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene, a selective agonist for mouse CAR, exacerbated the IMQ-induced psoriasis lesions with increased expressions of proliferative and inflammatory markers. In contrast, Car-knockout mice developed significantly milder lesions. In conclusion, these results showed that CAR plays a pathogenic role and, potentially, may be a target for the treatment of psoriasis.

Critical evaluation of the human relevance of the mode of action for rodent liver tumor formation by activators of the constitutive androstane receptor (CAR)

Many nongenotoxic chemicals have been shown to produce liver tumors in mice and/or rats by a mode of action (MOA) involving activation of the constitutive androstane receptor (CAR). Studies with phenobarbital (PB) and other compounds have identified the key events for this MOA: CAR activation; increased hepatocellular proliferation; altered foci formation; and ultimately the development of adenomas/carcinomas. In terms of human relevance, the pivotal species difference is that CAR activators are mitogenic agents in mouse and rat hepatocytes, but they do not stimulate increased hepatocellular proliferation in humans. This conclusion is supported by substantial in vitro studies with cultured rodent and human hepatocytes and also by in vivo studies with chimeric mice with human hepatocytes. Examination of the literature reveals many similarities in the hepatic effects and species differences between activators of rodent CAR and the peroxisome proliferator-activated receptor alpha (PPARα), with PPARα activators also not being mitogenic agents in human hepatocytes. Overall, a critical analysis of the available data demonstrates that the established MOA for rodent liver tumor formation by PB and other CAR activators is qualitatively not plausible for humans. This conclusion is supported by data from several human epidemiology studies.

Role of the constitutive androstane receptor (CAR) in human liver cancer

BACKGROUND

The constitutive androstane receptor (CAR) is a member of the nuclear receptor superfamily (subfamily 1, group I, member 3, also known as NR1I3) that is almost exclusively expressed in the liver. CAR interacts with key signalling pathways such as those involved in drug, energy and bilirubin metabolism. In mouse models, activation of CAR leads to tumorigenesis by inducing pro-proliferative and anti-apoptotic signalling. However, many previous reports have shown species differences between CAR activity in animal models and humans. Recent studies have demonstrated that the mode of action of CAR in rodent liver tumorigenesis is not applicable to humans. Despite this, many studies still continue to study the role of CAR in animal models, hence, there is a need to further explore the role of CAR in human diseases particularly cancers. While there is limited evidence for a role of CAR in human cancers, some studies have proposed a tumour-suppressive role of CAR in liver cancer. In addition, recent studies exploring CAR in human livers demonstrated a hepato-protective role for CAR in and more specifically, its ability to drive differentiation and liver regeneration. This review will discuss the role of CAR in liver cancer, with a focus on species differences and its emerging, tumour-suppressive role in liver cancer and its role in the regulation of liver cancer stem cells.