Epidermal growth factor receptor (EGFR) was initially discovered in 1962 following the identification of the ligand EGF. Following this, the role of EGFR in protein phosphorylation and tumorigenesis has been established, and the EGF-EGFR signaling axis has consequently been the focus of research in oncology and developmental biology. Situated in the plasma membrane, EGFR has been an attractive target for anticancer therapy as it becomes activated upon ligand binding, recruiting a number of downstream molecules that leads to the activation of major pathways implicated in tumor growth, progression, and survival.
EGFR belongs to the ErbB/HER receptor tyrosine kinase family that encompasses HER2 (ErbB2), HER3 (ErbB3), and HER4 (ErbB4).ii Each constituent of the EGFR family shares a basic domain structure including an N-terminal extracellular domain (subdivided in to domains I through IV), a transmembrane domain, an intracellular kinase domain, and a cytoplasmic C-terminal tail containing several phosphorylation sites that serve as signal transduction modules. The binding of one of several ligands to the extracellular ligand-binding domain induces receptor homo-dimerization or hetero-dimerization and results in kinase activation. Among the EGFR family, it should be noted that ErbB2 is an orphan receptor, characterized by ligand-induced hetero-dimerization with any other family member for activation. Research on ErbB3 initially did not show any intrinsic kinase activity, although recent research findings suggest otherwise.
In the initial activation response of HER family constituents and similar receptor tyrosine kinases are downregulation events, involving ligand-stimulated endocytosis of occupied receptor accompanied by receptor ubiquitination and followed by lysosomal degradation of both ligands and receptors. Another possibility is that receptor tyrosine kinases are recycled from endosomes to the plasma membrane. Interestingly, HER2 and HER3 are internalized and targeted to lysosomes less efficiently than EGFR, and increased expression of HER2 or HER3 can have a dominant-negative effect on EGFR downregulation and degradation. Whether degradation and/or recycling events occur will affect the number of receptors and thus downstream signalling, profoundly changing the biological response.
Ligands of EGFR include EGF, transforming growth factor-α, and heparin-binding EGF-like growth factor. Once bound to its ligand, EGFR recruits, phosphorylates and activates all of the following downstream signalling cascades: MAPK, PLC-ϒ/PKC, Ras-Raf-Mek, PI3K-Akt-mTOR/PKB and JAK2-STAT3. EGFR can also mediate cellular processes through the physical interaction with other proteins in the absence of kinase activity or ligand activation.
Several EGFR-targeting small molecule kinase inhibitors and therapeutic antibodies have been approved by the FDA to treat patients with breast cancer, colorectal cancer, non-small cell lung cancer (NSCLC), head and neck-related squamous cell carcinoma and pancreatic cancer. Despite the role that the EGFR signalling pathway plays in several downstream events that lead to tumorigenesis, currently approved EGFR-targeted therapies show only modest effect on most cancer types.
Of the classes of compounds that are in clinical trials and/or are approved for clinical practice use, there are two molecular approaches to target EGFR: (1) monoclonal antibodies, directed against the external ligand-binding site of the receptor (i.e. cetuximab and panitumumab), and (2) small molecule tyrosine kinase inhibitors, directed against intracellular tyrosine kinase domain (i.e. gefitinib, erlotinib, and lapatinib). In either case, both classes of compounds target EGFR homodimers and heterodimers. Evidence of this includes the ability of erlotinib to target both EGFR and HER3. Interestingly, the impact of monoclonal antibodies targeting EGFR or HER2 is observed to impact VEGFR expression. EGFR affects VEGFR activity through the MAPK and PI3K signalling pathways and at least three different transcription factors, STAT3, Sp1 and hypoxia-inducible factors (HIFs).ii A close relative of EGFR is EGFRvIII, found to be localized on the cell-surface where it activates several signalling molecules; overexpression of EGFRvIII has been implicated in malignant gliomas. In tumorigenesis, both EGFR and EGFRvIII are observed to be involved in malignant phenotypes in human cancers.
ErbB proteins are highlighted in oncology research for their ability to drive proliferation, survival, and differentiation. Their overexpression in a variety of human cancers qualifies them as an ideal target for further investigation.
 Han, W., and Lo, H-W. Mini Review: Landscape of EGFR signaling network in human cancers: biology and therapeutic response in relation to receptor subcellular locations. Cancer Letters 2012; 318: 124-134.
 Larsen, A.K., Ouaret, D., El Ouadrani, K., and Petitprez, A. Targeting EGFR and VEGF(R) pathway cross-talk in tumor survival and angiogenesis. Pharmacology and Therapeutics 2011; 131: 80-90.
 Vivancoa, I and Mellinghoff, I.K. Epidermal growth factor receptor inhibitors in oncology. Current Opinion in Oncology 2010; 22: 573–578.