Abstract
Thiazolidinediones (TZD), including troglitazone, rosiglitazone, and pioglitazone, are agonists of peroxisome proliferator-activated receptor (PPAR)-γ and belong to a class of insulin-sensitizing drugs for type 2 diabetes mellitus. However, member-specific, PPARγ-independent activities and toxicity have been reported, especially for troglitazone. Currently, the underlying mechanisms are not fully understood. In this study, we demonstrated that troglitazone but not rosiglitazone or pioglitazone modulated expression of farnesoid X receptor (FXR) target genes bile salt export pump (BSEP) and small heterodimer partner (SHP) in Huh-7 cells. More specifically, troglitazone acted as a partial agonist of FXR to weakly increase BSEP and SHP expression but functioned as a potent antagonist to significantly suppress bile acid-induced expression. Consistent with the finding, troglitazone partially induced but markedly antagonized bile acid-mediated BSEP promoter transactivation. However, such modulating effects were not detected with rosiglitazone or pioglitazone. Using the crystal structure of ligand-bound FXR ligand binding domain (LBD), molecular docking predicted that troglitazone, but not rosiglitazone or pioglitazone, could form a stable complex with FXR LBD. The specific α-tocopherol side chain of troglitazone significantly contributed to the formation of such a stable complex through extensive interactions with FXR LBD. The docking model was further validated by functional analyses of a series of docking-guided FXR mutants. In summary, the data demonstrated that troglitazone, but not rosiglitazone or pioglitazone, was an FXR modulator and potently antagonized bile acid-induced expression of FXR target genes. Such differential modulation of FXR signaling pathway by TZDs may represent one of the mechanisms for member-specific, PPARγ-independent activities and toxicity.
Footnotes
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This work was supported in part by the National Institutes of Health National Center for Research Resources [Grant P20-RR016457] (the Rhode Island-Institutional Development Awards Network of Biomedical Research Excellence grant); and by the Rhode Island Foundation [Medical Research Grant 20052653]. B.Y. is supported by the National Institutes of Health [Grants R01GM61988, R01ES07965].
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.109.151233.
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ABBREVIATIONS: TZD, thiazolidinedione; PPAR, peroxisome proliferator-activated receptor; LBD, ligand binding domain; FXR, farnesoid X receptor; CYP7A1, cholesterol 7α-hydroxylase; BSEP, bile salt export pump; SHP, small heterodimer partner; CDCA, chenodeoxycholic acid; DMSO, dimethyl sulfoxide; DMEM, Dulbecco's modified Eagle's medium; kb, kilobase(s); FXRE, farnesoid X receptor responsive element; IR1, inverted repeat with one nucleotide spacing; PPRE, peroxisome proliferator-activated receptor-responsive element; PCR, polymerase chain reaction; wt, wild type; mut, mutant; Luc, luciferase; GW4064, 3-(2,6-dichlorophenyl)-4-(3′-carboxy-2-chloro-stilben-4-yl)-oxymethyl-5-isopropylisoxazole.
- Received January 21, 2009.
- Accepted April 13, 2009.
- The American Society for Pharmacology and Experimental Therapeutics
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