Bile Acids (BAs) have emerged as important gut signaling molecules in aspects of metabolic homeostasis through activation of the nuclear farnesoid X receptor (FXR), and the g-protein bile acid receptor 5, TGR5; potentiating FGF-19 and GLP-1 release, respectively. Enteroendocrine L-cells (GLP-1 producing) are regulators of appetite and glucose homeostasis in the gut. Whether dual expression of both BA pathway receptors, TGR5 and FXR, occurs in L-cells remains controversial, and incompletely validated due to the methodically challenging nature of studying L-cells (<1% gut epithelial cells) using whole colonic tissue.
L-cells were studied in whole colonic tissue and from Flourescence-Activated Cell Sorting (FACS)-isolated GLP-1+ cells from colonic biopsies obtained during a flexible sigmoidoscopy in 3 patients. Expression of GLP-1 and either FXR, or TGR5 was studied using immunofluorescence (IF) techniques, and confocal microscopy. NCI-H716 cells, an in vitro model of human L-cells, were stimulated with conjugated bile acids (CBAS;1mM), a TGR5 agonist,or Chenodeoxycholic acid (CDCA;1mM), an FXR agonist; and GLP-1 and FGF-19 protein and mRNA expression was measured.
TGR5 and FXR were expressed in a population of GLP-1 positive cells in whole human colon, and surrounding colonocytes. The majority of the FACS-isolated, GLP-1(+) cells expressed TGR5 (73%) compared to GLP-1(-) cells (16%). H716 cells also express FXR and TGR5 and when treated with CBAS had significantly increased GLP-1 secretion at 2, 4 and 6 hours, but no significant effects on FGF19. mRNA expression of FGF-19 was increased 11-fold after treatment with CDCA compared to CBAS and 25-fold compared to controls. mRNA expression of GCG was decreased 3-fold in CDCA treatment compared to CBAS and control.
A population of human colonic L-cells expresses machinery for both FXR and TGR5 BA pathways. Elements of both pathways are differentially expressed upon treatment with BAs in an in vitro model of human L-cells.