The nuclear receptors CAR and PXR activate hepatic genes in response to therapeutic drugs and xenobiotics, leading to the induction of drug-metabolizing enzymes, such as cytochrome P450. Insulin inhibits the ability of FOXO1 to express genes encoding gluconeogenic enzymes. Induction by drugs is known to be decreased by insulin, whereas gluconeogenic activity is often repressed by treatment with certain drugs, such as phenobarbital (PB). Performing cell-based transfection assays with drug-responsive and insulin-responsive enhancers, glutathione S-transferase pull down, RNA interference (RNAi), and mouse primary hepatocytes, we examined the molecular mechanism by which nuclear receptors and FOXO1 could coordinately regulate both enzyme pathways. FOXO1 was found to be a coactivator to CAR-and PXR-mediated transcription. In contrast, CAR and PXR, acting as corepressors, downregulated FOXO1-mediated transcription in the presence of their activators, such as 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) and pregnenolone 16␣-carbonitrile, respectively. A constitutively active mutant of the insulin-responsive protein kinase Akt, but not the kinase-negative mutant, effectively blocked FOXO1 activity in cell-based assays. Thus, insulin could repress the receptors by activating the Akt-FOXO1 signal, whereas drugs could interfere with FOXO1-mediated transcription by activating CAR and/or PXR. Treatment with TCPOBOP or PB decreased the levels of phosphoenolpyruvate carboxykinase 1 mRNA in mice but not in Car ؊/؊ mice. We conclude that FOXO1 and the nuclear receptors reciprocally coregulate their target genes, modulating both drug metabolism and gluconeogenesis.Liver plays a major role in the metabolism of therapeutic drugs and environmental contaminants. It is endowed with a mechanism to induce hepatic enzymes, leading to increased detoxification and elimination of those xenobiotics. Drug induction is generally regulated by transcriptional activation of hepatic genes encoding drug-metabolizing enzymes, such as cytochrome P450s (CYPs) and specific transferases. Acting as the principal transcription factors which also form complexes with RXR, the nuclear receptors CAR and PXR play a central role in induction by binding to the phenobarbital (PB)-and xenobiotic-responsive enhancer modules PBREM and XREM (5,6,11,12,18,46,53), respectively, and activating transcription of their target genes, such as CYP genes, in response to a distinct but overlapping group of xenobiotics (23, 36, 50). However, induction is heavily influenced by endocrine conditions; glucocorticoid hormone, for example, augments CYP induction by PB (31). In contrast, insulin is known to repress the induction of drug-metabolizing activity by certain drugs and in diabetic livers (44,49,56). Insulin treatment either eliminated or significantly reduced PB induction of CYP2B in rat primary hepatocytes (17,42,58). Hepatic CYP2B, CYP3A, and CYP4A were increased in experimentally generated diabetic rats and mice and were reduced to normal levels by insulin treatmen...

Nuclear receptors constitutive active/androstane receptor (CAR) and pregnane X receptor (PXR) were originally characterized as transcription factors regulating the hepatic genes that encode drug metabolizing enzymes. Recent works have now revealed that these nuclear receptors also play the critical roles in modulating hepatic energy metabolism. While CAR and PXR directly bind to their response sequences phenobarbital-responsive enhancer module (PBREM) and xenobiotic responsive enhancer module (XREM) in the promoter of target genes to increase drug metabolism, the receptors also cross talk with various hormone responsive transcription factors such as forkhead box O1 (FoxO1), forkhead box A2 (FoxA2), cAMP-response element binding protein, and peroxisome proliferator activated receptor gamma coactivator 1alpha (PGC 1alpha) to decrease energy metabolism through down-regulating gluconeogenesis, fatty acid oxidation and ketogenesis and up-regulating lipogenesis. In addition, CAR modulates thyroid hormone activity by regulating type 1 deiodinase in the regenerating liver. Thus, CAR and PXR are now placed at the crossroad where both xenobiotics and endogenous stimuli co-regulate liver function.

The nuclear PXR (pregnane X receptor) was originally characterized as a key transcription factor that activated hepatic genes encoding drug-metabolizing enzymes. We have now demonstrated that PXR also represses glucagon-activated transcription of the G6Pase (glucose-6-phosphatase) gene by directly binding to CREB [CRE (cAMP-response element)-binding protein]. Adenoviral-mediated expression of human PXR (hPXR) and its activation by rifampicin strongly repressed cAMP-dependent induction of the endogenous G6Pase gene in Huh7 cells. Using the -259 bp G6Pase promoter construct in cell-based transcription assays, repression by hPXR of PKA (cAMP-dependent protein kinase)-mediated promoter activation was delineated to CRE sites. GST (glutathione transferase) pull-down and immunoprecipitation assays were employed to show that PXR binds directly to CREB, while gel-shift assays were used to demonstrate that this binding prevents CREB interaction with the CRE. These results are consistent with the hypothesis that PXR represses the transcription of the G6Pase gene by inhibiting the DNA-binding ability of CREB. In support of this hypothesis, treatment with the mouse PXR activator PCN (pregnenolone 16alpha-carbonitrile) repressed cAMP-dependent induction of the G6Pase gene in primary hepatocytes prepared from wild-type, but not from PXR-knockout, mice, and also in the liver of fasting wild-type, but not PXR-knockout, mice. Moreover, ChIP (chromatin immunoprecipitation) assays were performed to show a decreased CREB binding to the G6Pase promoter in fasting wild-type mice after PCN treatment. Thus drug activation of PXR can repress the transcriptional activity of CREB, down-regulating gluconeogenesis.

Pregnane X receptor (PXR), acting as a xenobiotic-activated transcription factor, regulates the hepatic metabolism of therapeutics as well as endobiotics such as steroid hormones. Given our finding that PXR activation by rifampicin (RIF) represses the estrogen sulfotransferase (SULT1E1) gene in human primary hepatocytes and hepatocellular carcinoma Huh7 cells, here we have investigated the molecular mechanism of this repression. First the PXR-responsive enhancer was delineated to a 100 bp sequence (−1000/−901), which contains three half sites that constitute the overlapping direct repeat 1 (DR1) and direct repeat 2 (DR2) motifs and two forkhead factor binding sites. siRNA knockdown, chromatin immunoprecipitation and chromatin conformation capture assays were employed to demonstrate that hepatocyte nuclear factor 4α (HNF4α) bound to the PXR-responsive enhancer, and activated the enhancer by looping its position close to the proximal promoter. Upon activation by RIF, PXR indirectly interacted with the enhancer, decreasing the interaction with HNF4α and dissolving the looped SULT1E1 promoter with deacetylation of histone 3. Removal of the DR sites from the enhancer hampers the ability of HNF4α to loop the promoter and that of PXR to repress the promoter activity. Thus, PXR represses human SULT1E1, possibly attenuating the inactivation of estrogen.

Pregnane X receptor (PXR) was originally characterized as a transcription factor that induces hepatic drug metabolism by activating cytochrome P450 genes. Here we have now demonstrated a novel function of PXR, that of eliciting p38 mitogenactivated protein kinase (MAPK) phosphorylation for cell migration. Upon xenobiotic activation of ectopic human PXR, human hepatocellular carcinoma HepG2 cells were found to exhibit increased phosphorylation of p38 MAPK and to subsequently change morphology and migrate. p38 MAPK was responsible for the regulation of these morphological changes and cell migration because the p38 MAPK inhibitor SB239063 repressed both. Prior to this phosphorylation, PXR directly activated the early response GADD45␤ gene by binding to a distal direct repeat 4 site of the GADD45␤ promoter. Ectopic expression of GADD45␤ increased p38 MAPK phosphorylation, whereas siRNA knockdown of GADD45␤ decreased the PXR-induced p38 MAPK phosphorylation, confirming that GADD45␤ can regulate PXR-induced p38 MAPK phosphorylation in HepG2 cells. These results indicate that PXR activates the GADD45␤ gene, increasing p38 MAPK phosphorylation, and leading HepG2 cells to change morphology and migrate. The GADD45␤ gene is a direct target for PXR, eliciting cell signals to regulate various cellular functions.

In the early 1960s, phenobarbital (PB) was shown to induce hepatic drug metabolism and the induction was implicated in the molecular mechanism of drug tolerance development. Since then, it has become evident that PB not only induces drug metabolism, but also triggers pleiotropic effects on liver function, such as cell growth and communication, proliferation of the endoplasmic reticulum, tumor promotion, glucose metabolism, steroid/ thyroid hormone metabolism, and bile acid synthesis. Upon activation by PB and numerous PB-type inducers, the nuclear receptor CAR mediates those pleiotropic actions by regulating various hepatic genes, utilizing multiple regulatory mechanisms.

Constitutive androstane receptor (CAR) is a xenobiotic-responsive nuclear receptor that is highly expressed in the liver. CAR activation induces hepatocyte proliferation and hepatocarcinogenesis in rodents, but the mechanisms remain unclear. In this study, we investigated the association of CAR-dependent cell proliferation with Yes-associated protein (YAP), which is a transcriptional cofactor controlling organ size and cell growth through the interaction with various transcriptional factors including TEA domain family member (TEAD). In mouse livers, 1,4-bis-(2-[3,5-dichloropyridyloxy])benzene (TCPOBOP) (a mouse CAR [mCAR] activator) treatment increased the nuclear YAP accumulation and mRNA levels of YAP target genes as well as cell-cycle related genes along with liver hypertrophy and verteporfin (an inhibitor of YAP/TEAD interaction) cotreatment tended to attenuate them. Furthermore, in cell-based reporter gene assays, CAR activation enhanced the YAP/TEAD-dependent transcription. To investigate the role of YAP/TEAD activation in the CAR-dependent hepatocyte proliferation, we sought to establish an in vitro system completely reproducing CAR-dependent cell proliferation. Since CAR was only slightly expressed in cultured mouse primary hepatocytes compared with mouse livers and no proliferation was observed after treatment with TCPOBOP, we overexpressed CAR using mCAR expressing adenovirus (Ad-mCAR-V5) in mouse primary hepatocytes. Ad-mCAR-V5 infection and TCPOBOP treatment induced hepatocyte proliferation. Similar results were obtained with immortalized normal mouse hepatocytes as well. In the established in vitro system, CAR-dependent proliferation was strongly inhibited by Yap knockdown and completely abolished by verteporfin treatment. Our present results obtained in in vivo and in vitro experiments suggest that YAP/TEAD activation plays key roles in CAR-dependent proliferation of murine hepatocytes.

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.