PHLPP Phosphatase Regulation of EGFR Expression

Abstract

The PH domain leucine‐rich repeat protein phosphatase (PHLPP) opposes growth factor‐dependent signaling by specifically dephosphorylating Akt and protein kinase C on a key regulatory site, the hydrophobic phosphorylation motif. Here we show that PHLPP also suppresses the amplitude of the Ras/ERK pathway by a distinct mechanism which controls the cellular levels of EGF receptor (EGFR). Depletion of PHLPP1 or PHLPP2 in normal or cancer cell lines results in dramatically enhanced EGF‐mediated activation of Ras, MEK and ERK. The increased amplitude of ERK pathway signaling correlates with a robust increase in signaling‐competent EGFR protein levels. Live‐cell imaging of EGFR kinase activity using a genetically‐encoded FRET reporter reveals that depletion of PHLPP isoforms results in a dramatic increase in EGF‐stimulated EGFR kinase activity. QPCR analysis reveals that EGF receptor mRNA levels are an order of magnitude higher in MEFs lacking PHLPP1 compared to wild‐type MEFs. Thus, PHLPP isoforms control the levels of signaling –competent EGFR by decreasing mRNA levels of the receptor. The finding that PHLPP isoforms control EGF receptor levels establish these phosphatases as key regulators of two critical oncogenic pathways, both the PI3K pathway via effects on Akt and, more globally, growth factor signaling by effects on levels of signaling‐competent receptor.