Dependence on the Microtubule Network and 90-kDa Heat Shock Protein of Phenobarbital-Induced Nuclear Translocation of the Rat Constitutive Androstane Receptor

Abstract

The role of the microtubule network in the constitutive androstane receptor (CAR)-mediated transactivation of CYP2B induced by phenobarbital (PB) in rat primary hepatocytes was investigated using the microtubule-disrupting agent nocodazole (NCZ). In human hepatocytes, it was reported that CAR mRNA expression was decreased by a microtubule-disrupting agent through the inhibition of glucocorticoid receptor (GR)-mediated transactivation. However, in the present study, we show that the rat CAR gene was unaffected by the GR-mediated pathway in rat primary hepatocytes treated with NCZ. The PB-induced expression of CYP2B mRNA was repressed in the presence of NCZ for 2 h before and during 4 h of PB treatment, whereas the CAR mRNA and protein expression levels were not affected. Furthermore, the transactivation of the PB-responsive enhancer module-luciferase reporter gene and the nuclear transport of CAR induced by PB were also repressed in the presence of NCZ. Based on these findings, microtubular integrity might be required for PB-induced nuclear translocation of CAR in rat primary hepatocytes. In the same procedures, except that NCZ was replaced with radicicol, the CYP2B mRNA expression induced by PB was also repressed. Taking these into consideration, PB-mediated nuclear translocation of rCAR might be dependent on the 90-kDa heat shock protein as well as the microtubule network.

Quantitative High-Throughput Identification of Drugs as Modulators of Human Constitutive Androstane Receptor

“…Of the 115 agonists identified, 11 are known CAR activators or CYP2B6 inducers including the antimalarial artemisinin, the antibiotics triclocarban, and the antifungal tolnaftate 29 30 31 . Notably, nocodazole, a hCAR agonist from the current study, was reported previously as a mCAR deactivator and repressed PB-induced expression of Cyp2b10 32 . These findings together indicate nocodazole may function as an agonist of hCAR but an antagonist of mCAR.…”

Deciphering the roles of the constitutive androstane receptor in energy metabolism

“…First, the HSP90-CAR complex recruits PP2A in the presence of phenobarbital, indicating that the ternary complex is necessary for CAR de-phosphorylation, which is a critical event for CAR nuclear translocation [24] . Second, the CAR-HSP90-CCRP ternary complex is associated with the cellular skeleton, and inhibition of HSP90 or disruption of the microtubule network disturbs the nuclear translocation of CAR, further demonstrating that the complex is essential for the nuclear translocation of CAR [26] . Third, the accumulation of CAR in the cytoplasm is not derived from nuclear exclusion because CCRP does not affect the nuclear content of CAR [25] .…”

Carboxylesterases: Pharmacological Inhibition Regulated Expression and Transcriptional Involvement of Nuclear Receptors and other Transcription Factors

“…In addition to a DNA binding domain, this transcription factor has a ligand binding domain that interacts with agonist, antagonist and inverse agonist. CAR normally resides in the cytosol and are complexed with a variety of proteins such as heat shock protein 90 (HSP90) [131]. Interaction with a ligand or dephosphorylation leads to nuclear translocation.…”