Bile acids also function as signaling molecules through interaction with several receptor systems (Figure 1). They serve as ligands for the nuclear transcription factor farnesoid X receptor (FXR), which forms a heterodimeric complex with retinoid X receptor α (RXRα) that binds to an inverted repeat sequence in gene promoters (Hoeke et al., 2014). They are also agonists for a cellular receptor, the G protein-coupled bile acid receptor 1 (GPBAR1 or TGR5), to mediate signaling via the generation of cyclic adenosine monophosphate (cAMP) by adenylate cyclase, which stimulates cAMP-dependent protein kinase A (PKA) and phosphorylation of the cAMP response element binding protein (CREB) transcription factor (Hodge and Nunez, 2016; Schonewille et al., 2016).