Do bile acids signal through FXR and TGR5 to support the intestinal barrier and limit inflammation?

Bile acids serve as sophisticated signaling molecules that interact with specific receptors to maintain intestinal health and balance the immune system. This signaling network is centered on the nuclear receptor FXR and the membrane receptor TGR5, both of which are highly responsive to different components of the bile acid pool.

Mechanistic pathways of bile acid receptors

Bile acids do not just aid in digestion; they function as primary ligands for specialized receptors that trigger complex cellular cascades.

• FXR (Farnesoid X Receptor): Primarily activated by primary bile acids like chenodeoxycholic acid (CDCA), FXR acts as a nuclear transcription factor. Once activated, it migrates to the cell nucleus to regulate genes involved in metabolic homeostasis and inflammatory control.
• TGR5 (GPBAR1): This membrane-bound receptor is most sensitive to secondary bile acids such as lithocholic acid (LCA) and deoxycholic acid (DCA). Its activation triggers a rapid cAMP-dependent signaling pathway (cAMP–PKA axis) that influences energy expenditure and immune cell behavior.

Regulation of intestinal barrier function

The activation of FXR and TGR5 is essential for maintaining the "tightness" of the intestinal lining, preventing the leakage of toxins into the bloodstream.

• Tight junction reinforcement: FXR activation directly upregulates the expression of essential barrier proteins, including zonula occludens-1 (ZO-1), occludin, and claudin-1. It simultaneously suppresses claudin-2, a "leaky" protein that increases permeability.
• Epithelial repair and protection: TGR5 signaling in intestinal L-cells stimulates the secretion of incretin hormones like GLP-1 and GLP-2. These hormones are trophic factors that promote the repair and proliferation of the intestinal epithelium, further strengthening the mucosal barrier.

Modulation of inflammatory immune responses

Both receptors act as powerful brakes on the immune system, particularly within the gut and liver, to prevent excessive inflammation.

• NF-κB inhibition: FXR and TGR5 both antagonize the NF-κB pathway—the central "on switch" for inflammation. FXR achieves this through physical interference with the p65 subunit, while TGR5 uses cAMP-mediated signaling to block NF-κB and the NLRP3 inflammasome.
• Macrophage polarization: Activation of these receptors shifts macrophages from a pro-inflammatory (M1) state to an anti-inflammatory (M2-like) state. This results in a significant reduction in pro-inflammatory cytokines like TNF-α, IL-1β, and IL-6, while increasing the production of the anti-inflammatory cytokine IL-10.

Bottom line

Bile acids act as indispensable signaling molecules through FXR and TGR5 to reinforce the intestinal barrier and suppress systemic inflammation. These pathways work in tandem to maintain mucosal integrity by upregulating tight junction proteins and reprogramming immune cells toward a resolving, anti-inflammatory phenotype.