AHR and renal fibrosis: Indoxyl sulfate is a representative protein-bound uraemic toxin, the excessive accumulation of which promotes CKD progression by activating both the canonical AhR and non-canonical AhR signaling pathways [61, 62], whereas indole-3-propionic acid could attenuate renal fibrosis by suppressing indoxyl sulfate-induced AhR expression [63].