Distinct pharmacological activities associated with naturally occurring splice variants of the human constitutive androstane receptor

The constitutive androstane receptor (CAR/CAR1, NR1I3) responds to chemical modulators to regulate the expression of many hepatic genes, including those related to all three phases of xenobiotic biotransformation. Recent reports from our laboratory have focused on functional analysis of human CAR variants produced by alternative mRNA splicing. In particular, CAR2 and CAR3 contain insertions of 4‐ and 5‐amino acids, respectively, within their ligand binding domains. Results obtained from specific, quantitative RT‐PCR assays demonstrated that the proportions of CAR1:CAR2:CAR3 transcript levels are relatively consistent among individual hepatocyte preparations derived from 7 different human donors. On average the CAR2 insertion occurred in 28.4% of CAR transcripts and the CAR3 insertion was found in 18.3%. Luciferase reporter/transcription assays were used to differentially assess the CAR variants’ ability to activate a promoter derived from the endogenous CYP2B6 target gene. Our results demonstrate that CAR2 possesses a distinct activation profile relative to the CAR1 ligands, (5‐β)‐pregnane‐3,20‐dione, CITCO, meclizine, and clotrimazole. These data are now being used to design pharmacophores for use in ligand‐based virtual screening studies to identify selective CAR2 ligands. Supported by NIH, GM66411.