Selective in vivo inhibition of mitogen-activated protein kinase activation using cell-permeable peptides

The extracellular signal-regulated kinase (ERK), a member of the mitogen-activated protein kinases (MAPKs), is essential for cellular proliferation and differentiation, and thus there exists great interest to develop specific and selective inhibitors of this enzyme. Whereas small molecule inhibitors PD098095 and U0126 have been used to study MAPK/ERK kinase (MEK), their target selectivity has been questioned recently. The cross-reactivity of ATP-directed inhibitors with other protein kinases prompted us to develop structure-based selective peptide inhibitors of ERK activation. Based on a MEK1-derived peptide, we developed inhibitors of ERK activation in vitro and in vivo. The inclusion of either an alkyl moiety or a membrane-translocating peptide sequence facilitated the cellular uptake of the peptide inhibitor and prevented ERK activation in 4-phorbol 12-myristate 13-acetate-stimulated NIH 3T3 cells or nerve growth factor-treated PC12 cells in a concentration-dependent manner. In addition, cell-permeable peptides inhibited ERK-mediated activation of the transcriptional activity of ELK1. The peptides did not have an inhibitory effect on the activity of two other closely related classes of MAPKs, c-Jun amino-terminal kinase or p38 protein kinase. Thus, these peptides may serve as valuable tools for investigating ERK activation and for selective investigation of ERK-mediated responses. With the knowledge of other kinase interacting domains, it would be possible to design cell-permeable inhibitors for investigating diverse cellular signaling mechanisms and for possible therapeutic applications.