HCAR3 and inflammatory bowel disease: Furthermore, it was indicated that enzymes responsible for tryptophan (Trp) degradation via the kynurenine pathway (indoleamine 2,3-dioxygenase 1 [IDO1], kynureninase [KYNU], interleukin-4-induced protein 1 [IL4I1], kynurenine 3-monooxygenase [KMO], and tryptophan 2,3-dioxygenase [TDO2]), receptor of Trp metabolites (HCAR3), and enzymes catalyzing NAD+ turnover (nicotinamide phosphoribosyltransferase [NAMPT], NNMT, PARP9, and CD38) were synchronously coregulated in IBD but not in intestinal malignancies [25].