Aryl hydrocarbon receptor/dioxin receptor in human monocytes and macrophages

Aryl hydrocarbon receptor (AhR) belongs to the bHLH/PAS transcription facto r family and is activated by various polycyclic or halogenated aromatic hyd rocarbons, e.g., 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD), 3-methylcho lanthrene (3MC). In the present study, we showed that in U937 cells and hum an macrophages AhR, with its partner cofactor Arnt, is expressed and CYP1A1 mRNA expression is induced in the presence of AhR ligand 3MC. Moreover, we showed that AhR, associating with Arnt, binds to target DNA sequences and activates transcription. Since part of AhR is activated into DNA binding sp ecies in the absence of exogenous ligand and competitive AhR antagonist alp ha -naphthoflavone inhibits this activation process with reducing CYP1A1 mR NA expression levels, the presence of endogenous ligand is indicated.