Proceedings of The Physiological Society

University College Dublin (2009) Proc Physiol Soc 15, C133

Oral Communications

Farnesoid X receptor activation attenuates colonic epithelial secretory function

N. Keating1, S. J. Keely1

1. Molecular Medicine, Royal College of Surgeons, Ireland, Dublin, Ireland.

At pathophysiological (mM) concentrations, bile acids acutely stimulate chloride and fluid secretion across colonic epithelial cells, leading to the onset of diarrhoea. However, our previous studies show that at physiological (µM) concentrations, the predominant colonic bile acid, deoxycholic acid (DCA) chronically inhibits colonic Cl- secretion. However, the underlying mechanisms remain unknown. The Farnesoid X receptor (FXR) is a nuclear hormone receptor that is activated by bile acids. FXR is involved in the regulation of bile-acid biosynthesis and FXR agonists have received much research interest in treatment of diseases associated with bile acid malabsorption. However, to date a role for the FXR in regulating epithelial transport processes has not been reported. To investigate the role of FXR in regulating colonic epithelial Cl- secretion. Cl- secretory responses to the Ca2+-dependent agonist, carbachol (CCh; 100 μM) or the cAMP-dependent agonist, forskolin (FSK; 10 μM), were measured as changes in short circuit current (ISC) across voltage-clamped monolayers of T84 colonic epithelial cells. Immunocytochemistry and laser scanning confocal microscopy were used to examine the subcellular localization of FXR. Results are expressed as mean ± SEM and statistical analyses were made by one way ANOVA and Student Newman Keul’s post-hoc test FXR expression was confirmed in T84 cells by western blotting. Treatment with the FXR agonist, GW4064 (2μM for 24 hrs) induced FXR translocation from the cytosol to the nucleus. GW4064 pretreatment attenuated subsequent secretory responses to CCh and FSK to 54.8 ± 4% and 72.2 ± 3% of those in control cells, respectively (p<0.001, n=14). The effects of GW4064 were concentration-dependent, with antisecretory effects apparent at 0.1 μM. As previously reported, pretreatment of T84 cells with DCA (50μM for 24 hrs) also attenuated secretory responses to CCh and FSK. However, the antisecretory effects of GW4064 and DCA were additive and effects of GW4064, but not DCA, were inhibited by an FXR antagonist, guggulsterone (5μM), suggesting independent mechanisms of action. Furthermore, CCh-stimulated Na+/K+ ATPase activity was not altered by DCA but was inhibited 31.3 ± 6% by GW4064 compared to untreated controls (p<0.01; n=7). These studies demonstrate a novel role for the FXR in downregulating colonic epithelial secretory capacity. This antisecretory effect of FXR activation appears to be distinct from that elicited by DCA and is mediated by inhibition of Na+/K+ ATPase activity. By virtue of their ability to downregulate epithelial secretory function, our data suggest that FXR agonists may have therapeutic potential as anti-diarrhoeal agents.

Where applicable, experiments conform with Society ethical requirements