Differential Modulation of Farnesoid X Receptor Signaling Pathway by the Thiazolidinediones

Abstract

Thiazolidinediones (TZD), including troglitazone, rosiglitazone, and pioglitazone, are agonists of peroxisome proliferator-activated receptor (PPAR)-␥ and belong to a class of insulin-sensitizing drugs for type 2 diabetes mellitus. However, memberspecific, PPAR␥-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 ␣-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 dockingguided 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, PPAR␥-independent activities and toxicity.Thiazolidinediones (TZD), including troglitazone, rosiglitazone, and pioglitazone, are a class of insulin-sensitizing drugs to treat type 2 diabetes mellitus. Such therapeutic effect of TZDs is achieved through activating nuclear receptor peroxisome proliferator-activated receptor (PPAR)-␥, which is directly involved in the regulation of genes controlling glucose homeostasis and lipid metabolism. Studies also show that TZDs exhibit other important activities, such as cardiovascular, hypertension, and anticancer effects in a PPAR␥-dependent or -independent manner (Blanquicett et al., 2008;Rizos et al., 2008).

The Role of Bile Salt Export Pump Gene Repression in Drug-Induced Cholestatic Liver Toxicity

“…2C, at higher concentrations, both lopinavir and troglitazone significantly reduced the FXR activity that was preinduced by CDCA, a known endogenous activator of FXR. Intriguingly, a previous report revealed that in Huh7 cells, although troglitazone (10 mM) moderately induces the basal expression of BSEP, it dose dependently represses CDCA-induced BSEP mRNA in a FXR-dependent manner (Kaimal et al, 2009). In contrast, benzbromarone, bosentan, or glimepiride exhibited only negligible effects on FXR activity (Fig.…”

Farnesoid X Receptor Agonists: What They Are and How They Might Be Used in Treating Liver Disease

“…One provocative study found that troglitazone can bind the FXR ligand binding domain. This effect was postulated to be mediated through the tocopherol side chain that is unique to troglitazone among the thiazolidinediones and this inhibitory interaction of troglitazone has been postulated to explain its idiosyncratic hepatotoxicity [13].…”

Mechanistic insights into isoform-dependent and species-specific regulation of bile salt export pump by farnesoid X receptor

“…Human, mouse, and rat bsep promoter reporters and mutants, including phBSEP (-2.6kb), pmBSEP(-2.6kb), prBSEP(-2kb), phBSEP(-125b), and phBSEP(-2.6kb)-IR1a-Mut, were prepared previously ( 18,28,29 ). Human pTK-3xhIR1a, mouse/rat pTK3xm/rIR1a, and pTK-3xIR1b reporters were constructed by cloning three copies of the corresponding element into pTK-Luc vector.…”

“…Mutations were introduced into the templates using a QuickChange site-directed mutagenesis kit according to the manufacturer's manual (Stratagene) ( 29 ). Human BSEP promoter mutants phBSEP(-2.6kb)-IR1b-Mut, phBSEP(-2.6kb)-IR1ab-Mut, and phBSEP(-2.6kb)-ER2-Mut were prepared by mutating the IR1b, IR1a and IR1b, and half ER2 (everted repeat separated by two nucleotides) sites using phBSEP(-2.6kb) as a template.…”