Dependence on the microtubule network and heat

This article has not been copyedited and formatted. The final version may differ from this version. 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 showed 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 hr prior to and during 4 hr of PB-treatment; whereas, the CAR mRNA and protein expression levels were not affected. Furthermore, the transactivation of the PBREM-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 (RAD), the CYP2B mRNA expression induced by PB was also repressed. Taking these into consideration, PB-mediated nuclear translocation of rCAR might be dependent on heat shock protein 90 (Hsp90) as well as microtubule network.