Constitutive Androstane Receptor Agonist Initiates Metabolic Activity Required for Hepatocyte Proliferation.

Activation of the constitutive androstane receptor (CAR, NR1I3) by chemical compounds induces liver hyperplasia in rodents. 1,4-Bis[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP), a mouse CAR agonist, is most often used to study chemically induced liver hyperplasia and hepatocyte proliferation in vivo. TCPOBOP is a potent murine liver chemical mitogen, which induces rapid liver hyperplasia in mice independently of liver injury. In recent years, great amount of data has been accumulated on the transcription program that characterizes the TCPOBOP-induced hepatocyte proliferation. However, there are only few data about the metabolic requirements of hepatocytes that divide upon exposure to xenobiotics. In the present study, we have employed liquid chromatography - mass spectrometry technology combined with statistical analysis to investigate metabolite profile of small biomolecules, in order to identify key metabolic changes in the male mouse liver tissue after TCPOBOP administration. Analysis of biochemical pathways of the differentially affected metabolites in the mouse liver demonstrated significant TCPOBOP-mediated enrichment of several processes including those associated with nucleotide metabolism, amino acid metabolism, and energy substrate metabolism. Our findings provide evidence to support the conclusion that the CAR agonist, TCPOBOP, initiates an intracellular program that promotes global coordinated metabolic activities required for hepatocyte proliferation. Our metabolic data might provide novel insight into the biological mechanisms that occur during the TCPOBOP-induced hepatocyte proliferation in mice.

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.

Activation of the Akt pathway by a constitutive androstane receptor agonist results in ß-catenin activation.

It is well known that activating the constitutive androstane receptor (CAR, NR1I3) leads to a significant proliferation of liver cells, which suggests that NR1I3 could be a therapeutic target for the partial resection of this organ. Studies describing NR1I3-mediated proliferative pathways could help determine the possible clinical applications of NR1I3 agonists during liver resection or transplantation. Recently, we reported that liver hyperplasia, which results from NR1I3 activation, is mediated by the activation of the Akt signaling pathway. In this study, we investigated the impact of the Akt signaling pathway on ß-catenin and its role in liver growth. Our findings showed that NR1I3-mediated activation of the Akt pathway results in the nuclear redistribution of ß-catenin and increases hepatocyte proliferation. Inhibiting the Akt pathway using the allosteric inhibitor MK-2206 decreased the amount of ß-catenin in the nucleus and reduced the hepatocyte proliferation induced by a NR1I3 agonist, but promoted hypertrophic liver growth. These findings suggest that NR1I3-mediated hepatocyte proliferation and liver growth are not necessarily linked. Additionally, we found that the proliferation effect of the NR1I3-Akt-ß-catenin signaling pathway is mediated, at least in part, by Cyclic D1 up-regulation. In contrast, the nuclear localization of ß-catenin in response to NR1I3 did not affect the expression of the ß-catenin target cMyc in the liver. In conclusion, the NR1I3-Akt signaling pathway plays a significant role in regulating hepatocyte proliferation, at least in part, by activating ß-catenin.

Promotion of liver growth by CAR is accompanied by Akt pathway activation and FoxM1-Nedd4-mediated repression of PTEN.

Recently, we reported that treatment with the mouse agonist of the constitutive androstane receptor (CAR), 1,4-bis benzene[2-(3,5-dichloropyridyloxy)] (TCPOBOP; a well-known hepatomitogen), reduced PTEN protein levels, leading to Akt activation. Hence, the present study was performed to demonstrate the role of CAR in PTEN regulation and liver growth. Liver hyperplasia caused by CAR activation was confirmed to be mediated by a decrease in PTEN protein level and the activation of the Akt signalling pathway in the liver of mice. Treatment with the CAR agonist decreased the PTEN levels and increased Foxm1 levels, which correlate with the elevated expression of the FoxM1 target gene, Nedd4-1, an E3 ligase involved in PTEN ubiquitination, and the promotion of degradation. The increase in Nedd4-1 levels was accompanied by an increase in CAR-mediated accumulation of Foxm1 on the Nedd4-1 gene promoter. Therefore, these results provide evidence that a notable function of CAR is its liver growth promotion effect, which is accompanied by FoxM1-Nedd4-mediated repression of PTEN and Akt pathway activation.

CAR-mediated repression of Cdkn1a(p21) is accompanied by the Akt activation.

It was shown that CAR participates in the regulation of many cell processes. Thus, the activation of CAR causes a proliferating effect in the liver, which provides grounds to consider CAR as a therapeutic target when having a partial resection of this organ. Even though a lot of work has been done on the function of CAR in regulating hepatocyte proliferation, very little has been done on its complex mediating mechanism. This study, therefore, showed that the liver growth resulting from CAR activation leads to the decline in the level of PTEN protein and subsequent Akt activation in mouse liver. The increase of Akt activation produced by CAR agonist was accompanied by a decrease in the level of Foxo1, which was correlated with decreased expression of Foxo1 target genes, including Cdkn1a(p21). Moreover, the study also demonstrated that there exists a negative regulatory impact of CAR on the relationship between Foxo1 and targeted Cdkn1a(p21) promoter. Therefore, the study results revealed an essential function of CAR-Akt-Foxo1 signalling pathway in controlling hepatocyte proliferation by repressing the cell cycle regulator Cdkn1a (p21).