Differential roles of ERK and Akt pathways in regulation of EGFR-mediated signaling and motility in prostate cancer cells

Abstract

Upregulation of epidermal growth factor receptor (EGFR) and subsequent increases in extracellular-regulated kinase (ERK) and Akt signaling are implicated in prostate cancer progression. Impaired endocytic downregulation of EGFR also contributes to oncogenic phenotypes such as metastasis. Thus, understanding the roles of divergent signaling pathways in the regulation of EGFR trafficking and EGFR-driven invasive migration may enable the development of more effective therapies. In this study, we use the human prostate cancer cell lines, DU145 and PC3, to investigate the effects of both the ERK and Akt pathways on epidermal growth factor (EGF)-mediated EGFR signaling, trafficking and cell motility. We show that DU145 and PC3 cells overexpress EGFR and migrate in a ligand (EGF)-dependent manner. Next, we show that pharmacological inhibition of ERK (but not Akt) signaling enhances EGF-induced EGFR activation, ubiquitination and downregulation, and may lead to enhanced receptor turnover. These findings negatively correlate with ERK-mediated threonine phosphorylation of EGFR, implicating it as a possible mechanism. Further, we uncover that EGF promotes disassembly of cell-cell junctions, downregulation of E-cadherin and upregulation of the transcriptional repressor, Snail, typical characteristics of epithelial-mesenchymal transition (EMT). These effects are dependent on activation of Akt, as inhibition of Akt signaling abolishes EGF/EGFR-driven cell migration and EMT. Knockdown of endogenous Snail also prevents EGFR-mediated downregulation of E-cadherin, EMT and cell migration. Surprisingly, inhibition of the ERK pathway augments EGFR-dependent motility, occurring concomitantly with elevation of EGF-induced Akt activity. Collectively, our results suggest that EGF-triggered ERK activation has profound feedback on EGFR signaling and trafficking by EGFR threonine phosphorylation, and Akt has a pivotal role in EGFR-mediated cell migration by activating EMT. More important, our results also suggest that therapeutic targeting of ERK signaling may have undesirable outcomes (for example, augmenting EGFR-driven motility).