Activation of steroid and xenobiotic receptor (SXR, NR1I2) and its orthologs in laboratory, toxicologic, and genome model species

Abstract

Background: Nuclear receptor subfamily 1, group I, member 2 (NR1I2), commonly known as steroid and xenobiotic receptor (SXR) in humans, is a key ligand-dependent transcription factor responsible for the regulation of xenobiotic, steroid, and bile acid metabolism. The ligand-binding domain is principally responsible for species-specific activation of NR1I2 in response to xenobiotic exposure.

Objectives: Our objective in this study was to create a common framework for screening NR1I2 orthologs from a variety of model species against environmentally relevant xenobiotics and to evaluate the results in light of using these species as predictors of xenobiotic disposition and for assessment of environmental health risk.

Methods: Sixteen chimeric fusion plasmid vectors expressing the Gal4 DNA-binding domain and species-specific NR1I2 ligand-binding domain were screened for activation against a spectrum of 27 xenobiotic compounds using a standardized cotransfection receptor activation assay.

Results: NR1I2 orthologs were activated by various ligands in a dose-dependent manner. Closely related species show broadly similar patterns of activation; however, considerable variation to individual compounds exists, even among species varying in only a few amino acid residues.

Conclusions: Interspecies variation in NR1I2 activation by various ligands can be screened through the use of in vitro NR1I2 activation assays and should be taken into account when choosing appropriate animal models for assessing environmental health risk.

Keywords: PXR; SXR; endocrine disruption; metabolism; pesticides; phthalates; xenobiotics.

Figure 1

Alignment of amino acid sequences of NR1I2 ortholog LBDs. Shaded regions correspond to amino acid residues of the LBD that have been shown to interact with xenobiotic ligands in human SXR (Chrencik et al. 2005; Watkins et al. 2001, 2003). The boxed regions represent the helix 1–3 insert that distinguishes functionally divergent members of the NR1I subfamily (Moore et al. 2002). J. macaque, Japanese macaque; C.E. macaque, crab-eating macaque.

Figure 2

Nonrooted neighbor-joining tree of NR1I2 orthologs and mammalian NR1I3 ligand-binding domains (A), and the percent amino acid identities of NR1I2 otholog ligand-binding domains (B). Abbreviations: FHM, fathead minnow; J. macaque, Japanese macaque; C.E. macaque, crab-eating macaque.