PD98059 IS AN EQUIPOTENT ANTAGONIST OF THE ARYL-HYDROCARBON RECEPTOR AND INHIBITOR OF MITOGEN-ACTIVATED PROTEIN-KINASE KINASE

PD98059 2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one] is a flavon oid and a potent inhibitor of mitogen-activated protein kinase kinase (MEK). Concentrations of PD98059 of less than or equal to 20 mu M were not cytotoxic to cultures of the immortalized human breast epithelial cell line MCF10A. The agent was weakly cytostatic at concentrations o f greater than or equal to 10 mu M. In vivo exposure of cultures to le ss than or equal to 20 mu M PD98059 for 2-22 hr did not affect overall extracellular signal-regulated kinase contents; however, exposure to PD98059 resulted in a rapid loss (>95%) of the dually phosphorylated f orms of extracellular signal-regulated kinase (IC50 = 1 mu M). Treatme nt of cultures with PD98059 of greater than or equal to 1 mu M either at the time of addition or up to 48 hr before the addition of 2,3,7,8- tetrachlorodibenzo-p-dioxin (TCDD) suppressed in a concentration-depen dent manner the accumulation of induced steady state CYP1A1, CYP1B1, a n NQO1 mRNAs. The addition of PD98059 to rat liver cytosol just before the addition of TCDD suppressed TCDD binding (IC50 = 4 mu M) and aryl hydrocarbon receptor (AHR) transformation (IC50 = 1 mu M), as measure d by sucrose gradient centrifugation and electrophoretic mobility shif t assays. Flavone and flavanone, two closely related structural analog s of PD98059, inhibited AHR transformation by TCDD with IC50 values si milar to that obtained with PD98059. However, neither analog was as po tent as PD98059 in inhibiting MEK (IC50 similar to 190 mu M for both). These results suggest that PD98059 is a ligand for the AHR and functi ons as an AHR antagonist at concentrations commonly used to inhibit ME K and signaling processes that entail MEK activation.