FIGURE 5. AhR ligands. The AhR was originally identified as the receptor for polycyclic aromatic hydrocarbons (PAHs). The most potent of these compounds is 2,3,7,8-TCDD. Other synthetic ligands include 2,3,7,8-tetrachlorodibenzofuran (TCDF) and 3-methylcholanthrene (3-MC). Data indicate that the AhR binds to and is activated by a variety of unrelated ligands (reviewed in Denison et al., 2002). Some natural ligands for the AhR include lipoxin A4 (an arachidonic acid metabolite), indirubin and indigo (components of human urine and blood), bilirubin (a heme degradation product), and FICZ (6-formylindolo[3,2-b]carbazole, a tryptophan photoproduct). In addition, several synthetic retinoids also effectively bind to the AhR (Soprano and Soprano, 2003; Soprano et al., 2001).
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