β-Klotho and FGF-15/19 inhibit the apical sodium-dependent bile acid transporter in enterocytes and cholangiocytes

Abstract

β-Klotho, a newly described membrane protein, regulates bile acid synthesis. Fibroblast growth factor-15 (FGF-15) and FGF receptor-4 (FGFR4) knockout mice share a similar phenotype with β-Klotho-deficient mice. FGF-15 secretion by the intestine regulates hepatic bile acid biosynthesis. The effects of β-Klotho and FGF-15 on the ileal apical sodium bile transporter (ASBT) are unknown. β-Klotho siRNA treatment of the mouse colon cancer cell line, CT-26, and the human intrahepatic biliary epithelial cells (HIBEC) resulted in upregulation of endogenous ASBT expression that was associated with reduced expression of the farnesoid X receptor (FXR) and the short heterodimer partner (SHP). Silencing β-Klotho activated the ASBT promoter in CT-26, Mz-ChA-1 (human cholangiocarcinoma), and HIBEC cells. Site-directed mutagenesis of liver receptor homolog-1 (mouse) or retinoic acid receptor/retinoid X receptor (RAR/RXR) (human) cis-elements attenuated the basal activity of the ASBT promoter and abrogated its response to β-Klotho silencing. siSHP, siFXR, or dominant-negative FXR treatment also eliminated the β-Klotho response. FGF-15 secretion into cell culture media by CT-26 cells was diminished after siFGF-15 or siβ-Klotho treatment and enhanced by chenodeoxycholic acid. Exogenous FGF-19 repressed ASBT protein expression in mouse ileum, gallbladder, and in HIBEC and repressed ASBT promoter activity in Caco-2, HIBEC, and Mz-ChA-1 cells. Promoter repression was dependent on the expression of FGFR4. These results indicate that both β-Klotho and FGF-15/19 repress ASBT in enterocytes and cholangiocytes. These novel signaling pathways need to be considered in analyzing bile acid homeostasis.