Falk 2010

Modulation of FXR activity via O-linked N-acetylglucosaminylation upon refined sugar feeding in mice

• Marije Boesjes ¹
• Hilde Herrema ¹
• Jurre Hageman ¹
• Folkert Kuipers ^1,2
• Albert K. Groen ^1,2

Laboratories of ¹ Pediatrics and ² Laboratory Medicine, Center for Liver Digestive and Metabolic Diseases, University Medical Center Groningen, Groningen, The Netherlands

Introduction: Dietary refined sugar intake has been shown to decrease bile salt synthesis via yet unknown mechanisms. It was the aim of this study to determine whether and to what extent FXR, the bile salt-activated nuclear receptor, is involved in this phenomenon.

Methods: Male C57BL/6J or intestinal specific FXR knock-out mice were fed a normal chow or a semi synthetic diet in which the carbohydrate fraction consisted solely of dextrose for 2 weeks. After the diet intervention, mice were terminated at 7AM and 7PM. Upon termination plasma and hepatic parameters as well as intestinal and hepatic gene expression patterns were measured. Bile acid kinetic parameters where examined by stable isotope dilution technique. O-linked N-acetylglucosaminylated (O-GlcNac) FXR was immunoprecipitated from FXR transfected HEK293 cells in vitro and detected on Western blot.

Results: Biliary (-33%) and fecal bile salt (-35%) secretion were decreased in dextrose-fed mice compared to chow fed controls. Primary bile salt synthesis was massively (-50%) decreased contributing to a 40% smaller pool size observed in dextrose-fed mice. Interesting, despite anticipated low bile salt levels in the ileum of dextrose-fed mice, ileal expression of the FXR target genes Shp and Fgf15 were significantly increased, particularly in the fed state (7AM), but expression of FXR itself was not altered. Dextrose feeding had no effect on Shp and Fgf15 expression in intestinal FXR knock-out mice. We therefore investigated possible posttranslational modification of FXR induced by dextrose feeding. Preliminary results of in vitro studies indicated that FXR is posttranslationally regulated by O-GlcNac, a well known glucose sensing mechanism.

Discussion/Conclusion: Simple carbohydrate feeding suppresses bile salt synthesis, possibly by the upregulation of intestinal Fgf15 expression via activation of FXR. Since intestinal bile salt concentrations were reduced we postulate that transcriptional activity of FXR was increased via posttranslational O-GlcNac of the protein.