Discovery and biological characterization of 1-(1H-indol-3-yl)-9H-pyrido[3,4-b]indole as an aryl hydrocarbon receptor activator generated by photoactivation of tryptophan by sunlight

► Sunlight has an important role in generating bioactive molecules that can activate the AHR. ► Identification of 1-(1H-indol-3-yl)-9H-pyrido[3,4-b]indole (IPI) as a new AHR-inducing tryptophan (TRP) photoproduct. ► IPI induces AHR-mediated P450 activity (EROD) and suppresses glucose output. ► IPI, a prominent component of photoactivated TRP, induces EROD additively with FICZ or TCDD. ► IPI was detected in tissue culture medium in ordinary ambient light conditions.