Functional Inhibitory Cross-talk between Constitutive Androstane Receptor and Hepatic Nuclear Factor-4 in Hepatic Lipid/Glucose Metabolism Is Mediated by Competition for Binding to the DR1 Motif and to the Common Coactivators, GRIP-1 and PGC-1α

Abstract

The role of the constitutive androstane receptor (CAR) in xenobiotic metabolism by inducing expression of cytochromes P450 is well known, but CAR has also been implicated in the down-regulation of key genes involved in bile acid synthesis, gluconeogenesis, and fatty acid ␤-oxidation by largely unknown mechanisms. Because a key hepatic factor, hepatic nuclear factor-4 (HNF-4), is crucial for the expression of many of these genes, we examined whether CAR could suppress HNF-4 transactivation. Expression of CAR inhibited HNF-4 transactivation of CYP7A1, a key gene in bile acid synthesis, in HepG2 cells, and mutation of the DNA binding domain of CAR impaired this inhibition. Gel shift assays revealed that CAR competes with HNF-4 for binding to the DR1 motif in the CYP7A1 promoter. TCPOBOP, a CAR agonist that increases the interaction of CAR with coactivators, potentiated CAR inhibition of HNF-4 transactivation. Furthermore, inhibition by CAR was reversed by expression of increasing amounts of GRIP-1 or PGC-1␣, indicating that CAR competes with HNF-4 for these coactivators. Treatment of mice with phenobarbital or TCPOBOP resulted in decreased hepatic mRNA levels of the reported genes down-regulated by CAR, including Cyp7a1 and Pepck. In vivo recruitment of endogenous CAR to the promoters of Cyp7a1 and Pepck was detected in mouse liver after phenobarbital treatment, whereas association of HNF-4 and coactivators, GRIP-1, p300, and PGC-1␣, with these promoters was significantly decreased. Our data suggest that CAR inhibits HNF-4 activity by competing with HNF-4 for binding to the DR1 motif and to the common coactivators, GRIP-1 and PGC-1␣, which may be a general mechanism by which CAR down-regulates key genes in hepatic lipid and glucose metabolism.

CCAAT/Enhancer-binding Protein α (C/EBPα) and Hepatocyte Nuclear Factor 4α (HNF4α) Synergistically Cooperate with Constitutive Androstane Receptor to Transactivate the Human Cytochrome P450 2B6 (CYP2B6) Gene

“…These results are in agreement with studies showing that CAR can inhibit HNF4␣ activity in particular promoters (e.g. CYP7A1) by competing for binding to common coactivators, such as PGC1␣ (54). Therefore, it is feasible that the transfection of PGC1␣ along with CAR leads to nonfunctional PGC1␣-CAR complexes, which disrupt the synergistic interaction among CAR, C/EBP␣, and HNF4␣ in the CYP2B6 promoter.…”

Di-(2-ethylhexyl)-phthalate (DEHP) activates the constitutive androstane receptor (CAR): A novel signalling pathway sensitive to phthalates

“…CAR and PXR are also increasingly recognized as important regulators of genes involved in lipid and glucose homeostasis (for review: [70,71]. In particular, CAR activation reduces the expression of critical genes involved in fatty acid oxidation [72], bile acid synthesis and gluconeogenesis [73]. Further studies should evaluate the potential role of these regulations in mediating metabolic disruption by phthalates in light of their potential to activate both PXR and CAR.…”

Deciphering the roles of the constitutive androstane receptor in energy metabolism

“…In the first model, CAR competes with forkhead box protein O1 (FoxO1) and HNF4α on the insulin responsive sequence (IRS) and DR1 element, respectively, on the promoters of Pepck and G6pase [92] . In the second model, CAR competitively binds to SRC2/GRIP1 and PGC1α, which are two coactivators of HNF4α, thus diminishing the expression of gluconeogenic genes [93] . HNF4α-mediated gluconeogenic transactivation is also controlled by nuclear HNF4α exclusion.…”