Extracellular signal-regulated kinase activates topoisomerase II alpha through a mechanism independent of phosphorylation

The mitogen-activated protein (MAP) kinases, extracellular signal-related k inase 1 (ERK1) and ERK2, regulate cellular responses by mediating extracell ular growth signals toward cytoplasmic and nuclear targets. A potential tar get for ERK is topoisomerase II alpha, which becomes highly phosphorylated during mitosis and is required for several aspects of nucleic acid metaboli sm, including chromosome condensation and daughter chromosome separation. I n this study, we demonstrated interactions between ERK2 and topoisomerase I I alpha proteins by coimmunoprecipitation from mixtures of purified enzymes and from nuclear extracts. In vitro, diphosphorylated active ERK2 phosphor ylated topoisomerase II alpha and enhanced its specific activity by sevenfo ld, as measured by DNA relaxation assays, whereas unphosphorylated ERK2 had no effect. However, activation of topoisomerase II was also observed with diphosphorylated inactive mutant ERK2, suggesting a mechanism of activation that depends on the phosphorylation state of ERK2 but not on its kinase ac tivity. Nevertheless, activation of ERK by transient transfection of consti tutively active mutant MAP kinase kinase 1 (MKK1) enhanced endogenous topoi somerase II activity by fourfold. Our findings indicate that ERK regulates topoisomerase II alpha in vitro and in vivo, suggesting a potential target for the MKK/ERK pathway in the modulation of chromatin reorganization event s during mitosis and in other phases of the cell cycle.