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Proteasome Inhibition Blocks Ligand-Induced Dynamic Processing and Internalization of Epidermal Growth Factor Receptor via Altered Receptor Ubiquitination and Phosphorylation

¹ Molecular Imaging Center, Mallinckrodt Institute of Radiology and ² Department of Developmental Biology, Washington University School of Medicine, St. Louis, Missouri

Requests for reprints: David Piwnica-Worms, Mallinckrodt Institute of Radiology, Washington University Medical School, 510 South Kingshighway Boulevard, Box 8225, St. Louis, MO 63110. Phone: 314-362-9356; Fax: 314-362-0152; E-mail: piwnica-wormsd{at}mir.wustl.edu.

Key Words: signal transduction • EGFR • ErbB1 • proteasome • posttranslational modification • molecular imaging • bioluminescence • firefly luciferase

Epidermal growth factor (EGF) receptor (EGFR), a member of the EGF superfamily of receptor tyrosine kinases, is a critical regulator of cell growth and an important target for single agent and combination anticancer therapeutics. To further investigate the dynamics of ligand-induced EGFR processing and regulation noninvasively, we developed a chimeric EGFR-firefly luciferase (FLuc) fusion reporter to directly monitor processing of EGFR in real-time. In a stable HeLa cell line expressing the reporter at physiologically relevant levels, bioluminescence imaging continuously monitored reporter dynamics, correlating with the ligand-induced response of endogenous EGFR as determined by Western blot, subcellular localization of an EGFR-green fluorescent protein (GFP) fusion protein, and validated pharmacologic responses. The signaling competency of the reporter was confirmed by gene rescue experiments in EGFR-null cells. Bioluminescence analysis further showed that proteasome inhibition with bortezomib or MG132 attenuated overall ligand-induced degradation of EGFR. In cells expressing EGFR-GFP, pretreatment with proteasome inhibitors trapped essentially all of the receptor at the cell membrane both before and after ligand-induced activation with EGF. Furthermore, proteasome inhibition enhanced receptor ubiquitination in both the basal and ligand-activated states as well as delayed the processing of ligand-activated phosphorylation of the receptor, kinetically correlating with attenuated receptor degradation. These observations point to a potential mechanism for the synergistic therapeutic effects of combination EGFR- and proteasome-targeted therapies. [Cancer Res 2009;69(3):976–83]