Fig. 1.

Biological processes known to be stimulated in an AHR-dependent and/or AHR-independent manner by the tryptophan photoproduct FICZ. (A) FICZ acts as an agonist of AHR, thereby stimulating the canonical AHR-ARNT signaling pathway that leads to rapid and transient enhancement of the transcription of various sets of genes in different types of cells (2, 3, 12, 13, 17, 18). (B) In murine T cells stimulated by IL-27, FICZ promotes the binding of AHR to c-Maf and, consequently, expression of IL-10 and IL-21 (16). (C) FICZ activates human LXRα and LXRβ with EC₅₀ values similar to those of endogenous activators of these same receptors, such as oxysterols (15). (D) FICZ activates L1-RTP, as described by Okudaira et al. (1). (E) Human cytochrome P450 enzymes CYP1A1, CYP1A2, and CYP1B1 convert FICZ to ring-hydroxylated metabolites that are further metabolized via sulfoconjugation (3).