Epidermal Growth Factor Receptor (EGFR), also known as ErbB1 or HER1, is member of the ErbB family of receptor tyrosine kinases, which also include ErbB2/HER2, ErbB3/HER3, and ErbB4/HER4. EGFR signaling -mediated by a number of downstream signaling cascades- is responsible for controlling fundamental cellular functions related to survival, growth, proliferation, and apoptosis.
Elevated expression levels of EGFR have been reported in several epithelial tumor types, prompting extensive research into the role of this receptor tyrosine kinase as a potential target for cancer therapy .
EGFR, and the other ErbB family members, are membrane-bound receptor tyrosine kinases. They are comprised of a ligand-binding extracellular domain, a short hydrophobic transmembrane domain, and a cytosolic domain that possesses tyrosine kinase activity.
EGFR is mainly activated by a ligand-dependent mechanism, in which the binding of EGFR-specific ligands lead to conformational changes in the extracellular domain of the EGFR receptor and facilitates its dimerization with another EGFR receptor . EGFR dimerization induces auto-phosphorylation of tyrosine residues in the carboxyl terminal of the receptor , which serve as docking sites for proteins containing Src homology 2 (SH2) domains or phosphotyrosine binding domains (PTB). The binding of these proteins to the receptor triggers distinct downstream signaling cascades that result in specific functions related to cell survival, growth, and proliferation. Six ligands have been identified for EGFR so far, including the well-studied growth factors, epidermal growth factor (EGF) and transforming growth factor alpha (TGF-alpha) .
EGFR is also activated in a ligand-independent manner. This is mainly observed in tumor cells that express defective EGFR proteins lacking the extracellular domain, which results in constitutively active forms of the receptor . In HEp3 human carcinoma, overexpression of the urokinase-type plasminogen activator receptor induces ligand-independent activation of EGFR by facilitating its association with alpha5beta1 integrins. This association is important for triggering the downstream ERK signaling pathway that induces in vivo proliferation of cancer cells .
Some of the major signaling pathways initiated by EGFR activation include: Ras/Raf/mitogen-activated protein kinase pathway, phosphatidylinositol 3-kinase/Akt pathway, phospholipase C-gamma pathway, and signal transducers and activators of transcription (STAT) pathway. The initiation of each of these pathways is dependent on the recruitment and binding of specific signaling proteins to the phosphorylated tyrosine residues on the carboxyl termini of EGFR receptor molecules. Together, these signaling pathways regulate cell survival, growth, proliferation, migration, and apoptosis.
Ras/Raf/mitogen-activated protein kinase pathway
This pathway is initiated by the recruitment and binding of adaptor proteins Grb2 and Sos to EGFR C-terminal tyrosine residues . The formation of this protein complex induces conformational changes in Sos, which facilitates its association with Ras-GDP and its activation to GTP-bound form (Ras-GTP). The pathway proceeds through the enzymatic activation of several downstream effectors, including Raf-1, and eventually results in the phosphorylation and activation of mitogen-activated protein kinases (MAPKs). Activated MAPKs enter the nucleus, where they activate transcription factors for genes involved in cell proliferation .
Phosphatidylinositol 3-kinase (PI3K)/Akt pathway
This pathway is initiated by the association of the regulatory subunit (p85) of PI3K with activated EGFR receptors. However, this association is reliant on the dimerization of EGFR with another ErbB family member, ErbB3/HER3, since EGFR lacks binding sites for p85 and indirectly associates with it via binding sites on HER3. The catalytic subunit (p110) of PI3K generates the second messenger, phosphatidylinositol 3,4,5-triphosphate (PIP3) that leads to the phosphorylation and activation of downstream effectors such as the serine/threonine protein kinase Akt. The activation of the PI3K/Akt pathway leads to the regulation of cell growth, migration, and apoptosis .
Phospholipase C-gamma pathway
Upon binding to activated EGFR, phospholipase C-gamma hydrolyses phosphatidylinositol 4,5-bisphosphate (PIP2) to generate inositol triphosphate (IP3) and diacylglycerol (DAG), which function as secondary messengers for the activation of several signaling pathways .
Signal transducers and activators of transcription (STAT) pathway
This pathway is activated when EGFR phosphotyrosine residues are bound by STAT proteins via their Src homology 2 (SH2) domains. The association induces the dimerization of STAT proteins and results in their nuclear relocation, where they activate the expression of genes involved in cell proliferation . Constitutively active forms of STAT3, a well-studied member of the STAT protein family, have been commonly reported in several cancers, especially breast cancers .
Novel EGFR pathway
In addition to the conventional signaling pathways triggered by activated EGFR, a novel EGFR pathway has been identified, wherein the EGFR family receptors directly translocate to the nucleus and drive the expression of target genes. Nuclear EGFR was first identified in hepatocyte nuclei by scientists studying the regeneration of rat liver , and has since been detected in other cell types such as thyroid and uterine cells, immortalized epithelial cells from kidneys and ovaries, and so on. Other EGFR family members, HER2, HER3, and the carboxyl terminal of HER4 have also been detected in the nuclei of several cell types .
Inside the nucleus, EGFR functions as a transcription cofactor that activates gene expression, with cyclin D1 and iNOS genes being its primary gene targets. EGFR either binds to the ATRS (AT-rich Response Sequence) on target gene promoters or it functions as a cofactor for the transcription factor STAT3. In a recent study that involved an analysis of 130 breast carcinomas, a negative correlation was noted between nuclear EGFR expression and overall survival of patients .
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