Evidence for estrogenic contamination of the MAPK inhibitor PD98059

blocks phosphorylation and activation of MAPK proteins, ERK1 and ERK2. In t he course of examining the effect of PD98059 on estrogen-induced transcript ion of reporter genes in a human breast cancer cell line and in yeast, we f ound that two of four different batches of PD98059 produced estrogenic effe cts in a dose-dependent manner. In a competitive binding assay, these prepa rations of PD98059 displaced radiolabeled estradiol from ER alpha. Furtherm ore, in the yeast assay, addition of a coactivator protein, AIB1, enhanced the transcriptional effect of PD98059, indicating that it induces receptor- coactivator interactions. Although concentrations of PD98059 required to ac tivate ER alpha in these experimental systems are 10(4)-to 10(5) higher tha n the concentration of estradiol required to do the same, the concentration s required to block MAPK activation are well above those which would produc e maximal estrogenic effects. Thus, when PD98059 is used in estrogen-respon sive cells, contaminating estrogenic activity may confound interpretation o f experimental results.