Lipoxin A4:: A new class of ligand for the Ah receptor

The Ah receptor is a Ligand-activated transcription factor that mediates many of the biological actions of a large class of environmental compounds. Support for a role of the Ah receptor in normal physiology also has been reported, but an endogenous regulating ligand has not been identified. We have examined candidate endogenous lipophilic substances and report here the ability of the arachidonic acid metabolite, lipoxin A(4), to bind to and activate the Ah receptor in Hepa-1 cells. Lipoxin A(4) produced a concentration-dependent response in a DRE-driven CAT reporter construct, with a greater than 10-fold increase in CAT activity at 0.3 mu M. Lipoxin A(4) transformed the Ah receptor to an active DRE-binding form in a concentration-dependent manner as indicated by gel mobility shift analysis. Results of Ah receptor competitive binding experiments indicated that at a concentration of 100 nM, lipoxin A(4) produced a half-maximum displacement(EC50) of [H-3]TCDD binding. Results of Northern blot analyses indicated a transient increase in mRNA levels of the Ah receptor-responsive gene CYP1A1, which peaked at 4 h, consistent with the kinetics observed for lipoxin A(4)-induced CYP1A1 enzyme activity. Further, lipoxin A(4) was found to be a competitive inhibitor for the CYP1A1 enzyme, with a calculated K-i = 1.1 mu M. These results establish lipoxin as a new class of Ah receptor ligand, one that differs dramatically from classical Ah receptor ligands.