Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC198
The nuclear bile acid receptor, farnesoid X receptor, inhibits CFTR expression and Cl- secretion in colonic epithelial cells.
M. S. Mroz1, N. Keating1, J. B. Ward1, S. J. Keely1
1. RCSI, Dublin, Ireland.
Cl- secretion, the primary driving force for fluid secretion in the intestine, can become dysregulated in many pathological conditions, leading to onset of diarrhoea. However, drugs which directly target epithelial transport proteins for treatment of diarrhoeal diseases are still lacking. We have previously shown that activation of the nuclear bile acid receptor, farnesoid X receptor (FXR), potently inhibits epithelial Cl- secretion in vitro and in vivo, an effect which is mediated by a decrease in Na+-K+-ATPase activity, without altering cellular expression of the protein. Here, we sought to investigate the effects of FXR activation on the activity and expression of CFTR, the primary exit pathway for Cl- in epithelial cells. Methods: GW4064 was used as a prototypical FXR agonist. T84 cell monolayers were mounted in Ussing chambers and Cl- secretion was measured as changes in short-circuit current. Protein expression was measured by RT-PCR and western blotting. Data are presented as mean ± SEM and were statistically analysed by ANOVA or Students t-tests as appropriate. Results: As previously reported, GW4064 (5 μM) inhibited Cl- secretory responses to the Ca2+- and cAMP-dependent agonists, carbachol (CCh) and forskolin (FSK) to 63 ± 8 (n = 8; p < 0.01) and 60 ± 7% (n = 8; p < 0.001) of control values, respectively. Under experimental conditions that specifically isolate Cl- currents across the apical membrane, FSK-induced responses were attenuated 90 ± 2% by the CFTR inhibitor, CFTR-inh-172 (10 μM) (n = 4), confirming these cAMP-dependent currents to be mediated by CFTR. Interestingly, GW4064 (5 μM) also decreased CFTR-mediated currents to 76 ± 5% (n = 8; p < 0.01) of those in control cells, measured 5 min after FSK stimulation. Furthermore, western blot and RT-PCR analysis revealed that GW4064 (5 μM) inhibited CFTR protein expression to 33 ± 9% (n = 4; p < 0.01) of that in control cells without altering CFTR mRNA levels (n = 3). Finally, GW4064 did not affect basal or FSK-stimulated cAMP levels in T84 cell monolayers which were 78 ± 21 and 96 ± 2% of those in control cells, respectively (n = 4). Conclusions: These data reveal a novel role for FXR in regulating the expression and activity of epithelial CFTR Cl- channels and have important implications for our understanding of the role of bile acids in regulating intestinal fluid and electrolyte transport. Our data suggest that by virtue of its ability to inhibit multiple components of the Cl- secretory pathway, FXR represents a good target for the development of new anti-diarrhoeal drugs.
Where applicable, experiments conform with Society ethical requirements