EGF Receptor Members EGF receptor members: Vertibrates: EGFR; ErbB2; ErbB3; ErbB4 C. elegans: Let-23 Drosophila: DER C. elegans and Drosophila have served as useful model systems for studying the signaling processes triggered by the EGF receptor.
RTKs have 3 functional regions : 1. N-terminal extracellular region w/ 1 or more ligand-binding sites 2. Single transmembrane -helix domain 3. C-terminal cytoplasmic region w/ catalytic domain + phosphorylation sites Lodish (2004) Fig 14-5 Recall:
EGF receptor structure ErbB3--has impaired kinase activity, and only signals when heterodimerizing with another receptor type. Bogdan et al. 2000
Ligands: Activation of the EGF receptor Roepstorff et al.2008 SpicesLigands C. elegansLin-3, a soluble TGFa-like ligand. DrosophilaSpitz, Vein, Grken, Keren HumanMore than dozen ligands ErbB2doesn’t bind any EGF- like ligands on its own, but can heterodimerize with other EGF-bound receptors. ErbB3has impaired kinase activity, and only signals when heterodimerizing with another receptor type.
cis- &/or trans- phosphorylation Receptor tyrosine kinase activation by dimerization Lodish (2004) Fig 14-5 only trans- phosphorylation Recall:
Hubbard et al. (2009) Activation of the EGF receptor In the cytoplasm, the two tyrosine kinase domains form an asymmetric dimer, with the c-terminal lobe. Juxtamembrane region of EGFR stabilizes formation of the asymmetric kinase dimer. EGFR is not activated by autophosphorylation of the activation loop.
Negative regulator of EGFR signaling Bogdan et al. 2000
Models of Signaling in Membrane Rafts Model 1a: - raft-associated receptors activated by dimerization. Model 1b: - dimerization increases affinity for raft. Model 2: - clustering of multiple rafts triggers signaling Simons & Toomre 2000 Recall:
Lipid rafts and EGFR methyl-B-cyclodextrin cholesterol depletion inhibition of EGF-stimulated PI turnover. BUT, led to an enhancement of EGF-stimulated MAP kinase activity. Lipid rafts disruption appears to have both positive and negative effects on receptor tyrosine kinase-mediated signaling
Lipid rafts and EGFR methyl-B-cyclodextrin cholesterol depletion EGF receptor function is suppressed when the receptor is localized to lipid rafts. The enhancement of EGF binding and receptor autophosphorylation An increase in intrinsic receptor kinase activity.
A new role of Egfr as a transcription factor Wen, 2010
A new role of Egfr in cancer Engelman etal. 2008
EGFR endocytosis Ligand-induced endocytosis of EGFR Novel way of EGFR endocytosis
Negative regulator of EGFR signaling Bogdan et al. 2000
Grb-dependent pathway: Grb2-Cb1 complex is recruited to C-terminal of EGFR. Adds mono-or polyubiquitins to EGFR. Activated EGFR is transported to clathrin coated pits. Roepstorff et al. 2008 Clathrin-mediated EGFR endocytosis
Grb-independent pathway Cb1 can mediate ubiquination of “X” internalization of EGFR. EGFR can directly interact with AP-2 Sorkin et al.2008
Clathrin-mediated EGFR endocytosis A, EGFR is recycled back to the plasma membrane. B, EGFR will be degraded in lysosomes.
Clathrin-mediated EGFR endocytosis & signaling Signal transduction molecules affect membrane trafficking. Membrane trafficking can regulate signal transduction events. Signal transduction Membrane trafficking AKT and ERK require CME, EGFR-activated DNA synthesis depends on functional CME. The initial phase of EGFR signaling is endocyotosis-independent, but Later events require clathrin-mediated EGFR Endocytosis(CME).
Clathrin-independent EGFR endocytosis Under conditions of receptor overexpression and/or high ligand concentrations, clathrin-independent internalization determines the overall rate of the EGFR uptake into the cell A-431 cells COS cells Several types of cells Formation of micro-and macropinocytic vesiles Large vesicular structures Vesicular-tubular endocytic compartment originated from the plasma membrane dorsal ruffles
The novel function of EGFR dimerization in internalization Inhibition of EGFR kinase activation did not block EGF-induced EGFR internalization. Absence of EGFR dimerization, EGF binding can not stimulate EGFR endocytosis. EGFR dimerization can mediate the EGF-induced cellular process independently of its role in activating EGFR tyrosien kinase.
Clinic Therapy Monoclonal antibodies (mAbs)directed against the EGFR extracellular domain Small molecule tyrosine kinase inhibitors (TKIs)directed against the tyrosine kinase domain.
EGFR inhibitor https://www. medscape.com Natural inhibitors include potato carboxypeptidase inhibitor(PCI), which contains a small cysteine- rich module, called a T-knot scaffold, that is shared by several different protein families, including the EGF family.
New possible therapies Nuclear EGFR and EGFR-targeted Therapy Therapy based on EGFR kinase-indepenent activity
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