Abstract
Conversion of cholesterol to bile acids in the liver is initiated by the rate-limiting enzyme cholesterol 7α-hydroxylase (CYP7A1) and excretion of bile acids from the liver is mediated by the bile salt export pump (BSEP). The expression of CYP7A1 and BSEP is coordinately regulated by a negative feedback and positive feed-forward mechanism, respectively, through bile acid-mediated activation of farsenoid X receptor (FXR). It is well established that hypolipidemic agent guggulsterone is an FXR antagonist and down-regulates FXR target genes. In this study, however, we have demonstrated that guggulsterone synergistically induced the expression of BSEP in cells treated with FXR agonist bile acids. A dissection study located in the BSEP promoter an activating protein (AP)-1 site supporting the action of guggulsterone. Deletion or mutation of the AP-1 element was diminished, whereas insertion of the AP-1 element into a heterologous promoter enhanced activation of the promoter by guggulsterone. Selective c-Jun N-terminal kinase and extracellular signal-regulated kinase inhibitors markedly decreased the transactivation, suggesting an involvement of AP-1 activation pathway in the up-regulation of BSEP by guggulsterone. Consistent with its FXR antagonism, guggulsterone antagonized bile acid-mediated transactivation of BSEP promoter when the AP-1 element was disrupted. In conclusion, guggulsterone regulates BSEP expression through composite mechanisms, and the transactivation through the AP-1 element is dominant over the FXR-mediated antagonism. The up-regulation of BSEP expression by guggulsterone without activating FXR pathway as an FXR agonist to suppress CYP7A1 expression represents a possible mechanism for guggulsterone-mediated hypolipidemic effect.
Footnotes
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This work was partially supported by the New Investigator Program for pharmacy faculty, American Association of Colleges of Pharmacy; by the Rhode Island Foundation; by the Rhode Island-IDeA Network of Biomedical Research Excellence (Grant P20 RR016457 from National Center for Research Resources/National Institutes of Health); and by the National Institutes of Health (Grants R01GM61988, R01ES07965, and F05AT003019).
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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.106.113837.
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ABBREVIATIONS: CYP7A1, cholesterol 7α-hydroxylase; BSEP, bile salt export pump; FXR, farnesoid X receptor; CDCA, chenodeoxycholic acid; SHP, small heterodimer partner; LRH, liver receptor homolog; AP, activating protein; U0126, 1,4-diamino-2,3-dicyano-1,4-bis(methylthio)butadiene; SB203580, 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole; DMSO, dimethyl sulfoxide; PCR, polymerase chain reaction; GuRE, guggulsterone-responsive element; LRHRE, LRH-1-responsive element; NQO1, NAD(P)H quinine oxidoreductase 1; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; FXRE, FXR element; JNK, c-Jun N-terminal kinase; ERK, extracellular signal-regulated kinase; SP600125, 1,9-pyazoloanthrone anthrapyrazolone; U0126, 1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene; SB203580, 4-(4-fliorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole.
- Received September 11, 2006.
- Accepted November 27, 2006.
- The American Society for Pharmacology and Experimental Therapeutics
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