Abstract
In this study, we aimed to investigate a potential role of small heterodimer partner (Shp, a nuclear receptor) in regulation of morphine withdrawal syndrome and to determine the mechanisms thereof. Somatic opiate withdrawal and pharmacokinetic experiments were performed with wild-type (WT) and Shp knockout (Shp-KO) mice. Regulatory effects of Shp on Ugt2b expression were assessed in vitro (using mouse hepatoma Hepa1-6 cells) and in vivo (using Shp-KO mice). Ugt2b mRNA and protein expressions were determined by qPCR and Western blotting, respectively. Microsomal Ugt2b activity was measured with morphine and chloramphenicol. Luciferase reporter, promoter analysis and chromatin immunoprecipitation assays were performed to identify the Hnf1α- and Rev-erbα-binding sites in Ugt2b36 promoter. Protein-protein interactions were explored using co-immunoprecipitation assays. Shp ablation exacerbated morphine withdrawal syndrome in mice. Furthermore, systemic and liver exposures of morphine were elevated in Shp-KO mice due to reduced metabolism. Down-regulation of morphine metabolism was supported by down-regulated expressions of Ugt2b genes in Shp-KO mice. Regulation of Ugt2b genes by Shp was confirmed in mouse hepatoma Hepa1-6 cells. Moreover, Shp positively regulated Ugt2b36 expression through repression of Dec2 and Rev-erbα, two negative regulators of Ugt2b36 enzyme. Rev-erbα repressed Ugt2b36 transcription via direct binding to a specific response element (located at -30/-15 bp) in promoter region of Ugt2b36, whereas Dec2 acted on Ugt2b36 expression via suppression of Hnf1α-transactivation of Ugt2b36 gene. In conclusion, Shp regulated morphine withdrawal syndrome via modulation of Ugt2b expression and detoxification capacity. Targeting Shp may represent a novel approach for management of morphine dependence.