Regulation of the Nuclear Bile Acid Receptor, Farnesoid X Receptor, by the Omega‐3 Polyunsaturated Fatty Acid, Alpha‐Linolenic Acid, in Colonic Epithelial Cells

Abstract

Introduction and AimsThe intestinal epithelium forms the interface between the body and the luminal contents, facilitating fluid and electrolyte transport, while also acting as a barrier to harmful pathogens. Dysregulation of barrier and transport function is associated with the pathogenesis of a number of conditions, including chronic diarrhoeal and inflammatory bowel diseases (IBD). Previous studies from ours, and other laboratories, have identified the nuclear bile acid receptor, farnesoid x receptor (FXR), as an excellent target for the development of new anti‐diarrhoeal and anti‐inflammatory therapeutics. Polyunsaturated fatty acids (PUFAs), such as alpha‐linolenic acid (ALA), are a group of bioactive phytochemicals, which have been shown in other systems to modulate FXR activity. PUFAs are also thought to be ligands for another group of nuclear receptors, known as the peroxisome proliferator‐activated receptors (PPARs), which have been recently shown to modulate FXR expression. Here, we set out to investigate a possible role for ALA in regulating farnesoid x receptor signalling in colonic epithelial cells.MethodsT84 human colonic epithelial cells were cultured as polarised monolayers on permeable supports and treated bilaterally with alpha‐linolenic acid (ALA) (100 µM) for 1 – 24 hours, in the presence or absence of the FXR agonist, GW4064 (5 µM), or the PPARg antagonist, GW9662 (20 µM). Levels of FXR, FGF19, an index of FXR activation, and ANGPTL4, a PPAR target gene, were measured by qRT‐PCR, western blotting, or ELISA.ResultsTreatment of T84 cells with ALA increased FXR expression in a time dependent fashion. Maximal effects of ALA occurred after 6 hrs when increases in FXR mRNA and protein were 8.65 ± 0.80 fold (n = 4; p < 0.01) and 4.44 ± 0.85 fold (n = 4) of untreated controls, respectively. Moreover, treatment of the cells with ALA potentiated GW4064‐induced expression of the FXR target gene, FGF‐19, at both mRNA and protein levels by 2.66 ± 0.56 (n = 4) and 5.52 ± 1.52 (n = 6), respectively. The effects of ALA on FXR expression and activity were mimicked by the PPARg agonist, rosiglitazone (1 mM). Treatment with ALA also significantly increased mRNA expression of the PPAR target gene, ANGPTL4, by 56.69 ± 5.7 (n = 4; p < 0.01), while the PPARg antagonist, GW9662 (20 µM; 1 hr), inhibited ALA‐induced FXR mRNA expression by 0.44 ± 0.12 fold (n =4) and protein by 0.23 ± 0.06 fold (n = 3).ConclusionThe dietary PUFA, ALA, regulates colonic epithelial FXR expression through mechanisms that involve activation of PPARγ. By virtue of their ability to upregulate FXR expression and activity, PUFAs, such as ALA, have excellent potential for development as FXR‐targeted nutraceuticals for the treatment and prevention of intestinal disease.