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 flavonoid and a potent inhibitor of mitogen-activated protein kinase kinase (MEK). Concentrations of PD98059 of ≤20 μm were not cytotoxic to cultures of the immortalized human breast epithelial cell line MCF10A. The agent was weakly cytostatic at concentrations of ≥10 μm. In vivo exposure of cultures to ≤20 μm 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 forms of extracellular signal-regulated kinase (IC₅₀ = 1 μm). Treatment of cultures with PD98059 of ≥1 μm 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-dependent manner the accumulation of induced steady state CYP1A1, CYP1B1, and NQO1 mRNAs. The addition of PD98059 to rat liver cytosol just before the addition of TCDD suppressed TCDD binding (IC₅₀ = 4 μm) and aryl hydrocarbon receptor (AHR) transformation (IC₅₀ = 1 μm), as measured by sucrose gradient centrifugation and electrophoretic mobility shift assays. Flavone and flavanone, two closely related structural analogs of PD98059, inhibited AHR transformation by TCDD with IC₅₀ values similar to that obtained with PD98059. However, neither analog was as potent as PD98059 in inhibiting MEK (IC₅₀ ∼ 190 μm for both). These results suggest that PD98059 is a ligand for the AHR and functions as an AHR antagonist at concentrations commonly used to inhibit MEK and signaling processes that entail MEK activation.