ERK1/2 activation mediated by the nutlin-3-induced mitochondrial translocation of p53

Abstract

Nutlin-3 is a small-molecule antagonist of murine double minute 2 (MDM2) that blocks its binding to p53, leading to an increase in p53 protein levels. The tumor suppressor p53 induces growth arrest or apoptosis in response to genotoxic stress. Along with its growth-suppressive effect, it has been reported that p53 stimulates the mitogen-activated protein kinase (MAPK) pathway via the upregulation of heparin-binding epidermal growth factor-like growth factor (HB-EGF), an epidermal growth factor receptor (EGFR) ligand, and discoidin domain receptor 1 (DDR1), a tyrosine kinase receptor, at the transcription level. In this study, we propose a novel mechanism involved in the p53-induced MAPK activation. Nutlin-3 induced the phosphorylation of EGFR, MAPK/ERK kinase (MEK)1/2 and extracellular signal-regulated kinase (ERK)1/2 in U2OS human osteosarcoma cells harboring wild-type p53. This phosphorylation was completely inhibited by p53 siRNA, but not by pifithrin (PFT)-α, an inhibitor of the transcriptional activity of p53. While the nutlin-3-induced EGFR phosphorylation was prevented by the inactivation of ERK1/2, the nutlin-3-induced MEK1/2-ERK1/2 phosphorylation was still observed in the cells in which EGFR phosphorylation was inhibited using EGFR siRNA and AG1478, an inhibitor of EGFR tyrosine kinase. Of note, nutlin-3 caused the accumulation of mitochondrial reactive oxygen species (ROS) and this correlated with the mitochondrial translocation of p53. 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO), a ROS scavenger, prevented the phosphorylation of ERK1/2. PFT-μ, which prevented the mitochondrial localization of p53, suppressed ERK1/2 phosphorylation, as well as ROS accumulation. Finally, we analyzed the effect of ERK1/2 activation on nutlin-3-induced apoptosis. The knockdown of MEK1/2 and ERK1/2 activity using U0126, a MEK inhibitor, or siRNAs, resulted in the enhancement of nutlin-3-induced apoptosis. In addition, TEMPO and PFT-μ also promoted nutlin-3-induced apoptosis. Taking the above findings into account, it can be concluded that nutlin-3 activates ERK1/2 prior to EGFR phosphorylation via ROS generation following the mitochondrial translocation of p53 and that nutlin-3-induced ERK1/2 activation may play a role in protecting U2OS cells from p53-dependent apoptosis, constituting a negative feedback loop for p53-induced apoptosis.