Regulation of Complement C3 Expression by the Bile Acid Receptor FXR

The farnesoid X receptor (FXR; NR1H4) is an intracellular bile acid-sensing transcription factor that plays a critical role in the regulation of synthesis and transport of bile acids as well as lipid metabolism. Although the reciprocal relationship between bile acid and triglyceride levels is well known, the mechanism underlying this link is not clearly defined. In this study, we demonstrate that FXR regulates the expression of at least two secreted factors, complement component C3 and FGF15, the rat ortholog of FGF19, known to influence lipid metabolism. The analysis of the human complement C3 gene reveals the presence of functional FXR response elements in the proximal promoter of C3. Furthermore, rats given a single dose of an FXR agonist exhibit an increase in the plasma concentration of complement C3 protein. These studies demonstrate a mechanism by which FXR, a nuclear receptor with a limited tissue expression pattern, regulates secretion of factors that ultimately can affect lipid metabolism in an endocrine or paracrine manner.

Farnesoid X receptor regulates bile acid-amino acid conjugation

The farnesoid X receptor (FXR; NR1H4) regulates bile acid and lipid homeostasis by acting as an intracellular bile acid-sensing transcription factor. Several identified FXR target genes serve critical roles in the synthesis and transport of bile acids as well as in lipid metabolism. Here we used Affymetrix micro-array and Northern analysis to demonstrate that two enzymes involved in conjugation of bile acids to taurine and glycine, namely bile acid-CoA synthetase (BACS) and bile acid-CoA: amino acid N-acetyltransferase (BAT) are induced by FXR in rat liver. Analysis of the human BACS and BAT genes revealed the presence of functional response elements in the proximal promoter of BACS and in the intronic region between exons 1 and 2 of the BAT gene. The response elements resemble the consensus FXR binding site consisting of two nuclear receptor half-sites organized as an inverted repeat and separated by a single nucleotide (IR-1). These response elements directly bind FXR/retinoid X receptor (RXR) heterodimers and confer the activity of FXR ligands in transient transfection experiments. Further mutational analysis confirms that the IR-1 sequence of the BACS and BAT genes mediate transactivation by FXR/RXR heterodimers. Finally, Fisher rats treated with the synthetic FXR ligand GW4064 clearly show increased transcript levels of both the BACS and BAT mRNA. These studies demonstrate a mechanism by which FXR regulates bile acid amidation, a critical component of the enterohepatic circulation of bile acids.