The Ah receptor is a Ligand-activated transcription factor that mediates ma
ny of the biological actions of a large class of environmental compounds. S
upport for a role of the Ah receptor in normal physiology also has been rep
orted, but an endogenous regulating ligand has not been identified. We have
examined candidate endogenous lipophilic substances and report here the ab
ility of the arachidonic acid metabolite, lipoxin A(4), to bind to and acti
vate 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 n
M, lipoxin A(4) produced a half-maximum displacement(EC50) of [H-3]TCDD bin
ding. Results of Northern blot analyses indicated a transient increase in m
RNA levels of the Ah receptor-responsive gene CYP1A1, which peaked at 4 h,
consistent with the kinetics observed for lipoxin A(4)-induced CYP1A1 enzym
e activity. Further, lipoxin A(4) was found to be a competitive inhibitor f
or the CYP1A1 enzyme, with a calculated K-i = 1.1 mu M. These results estab
lish lipoxin as a new class of Ah receptor ligand, one that differs dramati
cally from classical Ah receptor ligands.