The epidermal growth factor receptor (EGFR; ErbB-1; HER1 in humans) is a transmembrane protein that is a receptor for members of the epidermal growth factor family (EGF family) of extracellular protein ligands. [5]
The epidermal growth factor receptor is a member of the ErbB family of receptors, a subfamily of four closely related receptor tyrosine kinases: EGFR (ErbB-1), HER2/neu (ErbB-2), Her 3 (ErbB-3) and Her 4 (ErbB-4). In many cancer types, mutations affecting EGFR expression or activity could result in cancer. [6]
Epidermal growth factor and its receptor was discovered by Stanley Cohen of Vanderbilt University. Cohen shared the 1986 Nobel Prize in Medicine with Rita Levi-Montalcini for their discovery of growth factors.
Deficient signaling of the EGFR and other receptor tyrosine kinases in humans is associated with diseases such as Alzheimer's, while over-expression is associated with the development of a wide variety of tumors. Interruption of EGFR signalling, either by blocking EGFR binding sites on the extracellular domain of the receptor or by inhibiting intracellular tyrosine kinase activity, can prevent the growth of EGFR-expressing tumours and improve the patient's condition[ citation needed ].
Epidermal growth factor receptor (EGFR) is a transmembrane protein that is activated by binding of its specific ligands, including epidermal growth factor and transforming growth factor alpha (TGF-α). [7] ErbB2 has no known direct activating ligand, and may be in an activated state constitutively or become active upon heterodimerization with other family members such as EGFR. Upon activation by its growth factor ligands, EGFR undergoes a transition from an inactive monomeric form to an active homodimer. [8] – although there is some evidence that preformed inactive dimers may also exist before ligand binding. [9] In addition to forming homodimers after ligand binding, EGFR may pair with another member of the ErbB receptor family, such as ErbB2/Her2/neu, to create an activated heterodimer. There is also evidence to suggest that clusters of activated EGFRs form, although it remains unclear whether this clustering is important for activation itself or occurs subsequent to activation of individual dimers. [10]
EGFR dimerization stimulates its intrinsic intracellular protein-tyrosine kinase activity. As a result, autophosphorylation of several tyrosine (Y) residues in the C-terminal domain of EGFR occurs. These include Y992, Y1045, Y1068, Y1148 and Y1173, as shown in the adjacent diagram. [11] This autophosphorylation elicits downstream activation and signaling by several other proteins that associate with the phosphorylated tyrosines through their own phosphotyrosine-binding SH2 domains. These downstream signaling proteins initiate several signal transduction cascades, principally the MAPK, Akt and JNK pathways, leading to DNA synthesis and cell proliferation. [12] Such proteins modulate phenotypes such as cell migration, adhesion, and proliferation. Activation of the receptor is important for the innate immune response in human skin. Additionally, the kinase domain of the EGFR can cross-phosphorylate the tyrosine residues of other receptors with which it is aggregated and thereby activate itself.
The EGFR is essential for ductal development of the mammary glands, [13] [14] [15] and agonists of the EGFR such as amphiregulin, TGF-α, and heregulin induce both ductal and lobuloalveolar development even in the absence of estrogen and progesterone. [16] [17]
Mutations that lead to EGFR overexpression (known as upregulation or amplification) have been associated with a number of cancers, including adenocarcinoma of the lung (40% of cases), anal cancers, [18] glioblastoma (50%) and epithelian tumors of the head and neck (80–100%). [19] These somatic mutations involving EGFR lead to its constant activation, which produces uncontrolled cell division. [20] In glioblastoma a specific mutation of EGFR, called EGFRvIII, is often observed. [21] Mutations, amplifications or misregulations of EGFR or family members are implicated in about 30% of all epithelial cancers. [22]
Aberrant EGFR signaling has been implicated in psoriasis, eczema and atherosclerosis. [23] [24] However, its exact roles in these conditions are ill-defined.
A single child displaying multi-organ epithelial inflammation was found to have a homozygous loss of function mutation in the EGFR gene. The pathogenicity of the EGFR mutation was supported by in vitro experiments and functional analysis of a skin biopsy. His severe phenotype reflects many previous research findings into EGFR function. His clinical features included a papulopustular rash, dry skin, chronic diarrhoea, abnormalities of hair growth, breathing difficulties and electrolyte imbalances. [25]
EGFR has been shown to play a critical role in TGF-beta1 dependent fibroblast to myofibroblast differentiation. [26] [27] Aberrant persistence of myofibroblasts within tissues can lead to progressive tissue fibrosis, impairing tissue or organ function (e.g. skin hypertrophic or keloid scars, liver cirrhosis, myocardial fibrosis, chronic kidney disease).
The identification of EGFR as an oncogene has led to the development of anticancer therapeutics directed against EGFR (called "EGFR inhibitors", EGFRi), including gefitinib, [28] erlotinib, [29] afatinib, brigatinib and icotinib [30] [31] for lung cancer, and cetuximab for colon cancer. More recently AstraZeneca has developed Osimertinib, a third generation tyrosine kinase inhibitor. [32] [31]
Many therapeutic approaches are aimed at the EGFR. Cetuximab and panitumumab are examples of monoclonal antibody inhibitors. However the former is of the IgG1 type, the latter of the IgG2 type; consequences on antibody-dependent cellular cytotoxicity can be quite different. [33] Other monoclonals in clinical development are zalutumumab, nimotuzumab, and matuzumab. The monoclonal antibodies block the extracellular ligand binding domain. With the binding site blocked, signal molecules can no longer attach there and activate the tyrosine kinase.
Another method is using small molecules to inhibit the EGFR tyrosine kinase, which is on the cytoplasmic side of the receptor. Without kinase activity, EGFR is unable to activate itself, which is a prerequisite for binding of downstream adaptor proteins. Ostensibly by halting the signaling cascade in cells that rely on this pathway for growth, tumor proliferation and migration is diminished. Gefitinib, erlotinib, brigatinib and lapatinib (mixed EGFR and ERBB2 inhibitor) are examples of small molecule kinase inhibitors.
CimaVax-EGF, an active vaccine targeting EGF as the major ligand of EGF, uses a different approach, raising antibodies against EGF itself, thereby denying EGFR-dependent cancers of a proliferative stimulus; [34] it is in use as a cancer therapy against non-small-cell lung carcinoma (the most common form of lung cancer) in Cuba, and is undergoing further trials for possible licensing in Japan, Europe, and the United States. [35]
There are several quantitative methods available that use protein phosphorylation detection to identify EGFR family inhibitors. [36]
New drugs such as osimertinib, gefitinib, erlotinib and brigatinib directly target the EGFR. Patients have been divided into EGFR-positive and EGFR-negative, based upon whether a tissue test shows a mutation. EGFR-positive patients have shown a 60% response rate, which exceeds the response rate for conventional chemotherapy. [37]
However, many patients develop resistance. Two primary sources of resistance are the T790M mutation and MET oncogene. [37] However, as of 2010 there was no consensus of an accepted approach to combat resistance nor FDA approval of a specific combination. Clinical trial phase II results reported for brigatinib targeting the T790M mutation, and brigatinib received Breakthrough Therapy designation status by FDA in Feb. 2015.
The most common adverse effect of EGFR inhibitors, found in more than 90% of patients, is a papulopustular rash that spreads across the face and torso; the rash's presence is correlated with the drug's antitumor effect. [38] In 10% to 15% of patients the effects can be serious and require treatment. [39] [40]
Some tests are aiming at predicting benefit from EGFR treatment, as Veristrat. [41]
Laboratory research using genetically engineered stem cells to target EGFR in mice was reported in 2014 to show promise. [42] EGFR is a well-established target for monoclonal antibodies and specific tyrosine kinase inhibitors. [43]
Imaging agents have been developed which identify EGFR-dependent cancers using labeled EGF. [44] The feasibility of in vivo imaging of EGFR expression has been demonstrated in several studies. [45] [46]
It has been proposed that certain computed tomography findings such as ground-glass opacities, air bronchogram, spiculated margins, vascular convergence, and pleural retraction can predict the presence of EGFR mutation in patients with non-small cell lung cancer. [47]
Epidermal growth factor receptor has been shown to interact with:
In fruitflies, the epidermal growth factor receptor interacts with Spitz. [105]
Epidermal growth factor (EGF) is a protein that stimulates cell growth and differentiation by binding to its receptor, EGFR. Human EGF is 6-kDa and has 53 amino acid residues and three intramolecular disulfide bonds.
Receptor tyrosine-protein kinase erbB-2 is a protein that normally resides in the membranes of cells and is encoded by the ERBB2 gene. ERBB is abbreviated from erythroblastic oncogene B, a gene originally isolated from the avian genome. The human protein is also frequently referred to as HER2 or CD340.
Growth factor receptor-bound protein 7, also known as GRB7, is a protein that in humans is encoded by the GRB7 gene.
The insulin-like growth factor 1 (IGF-1) receptor is a protein found on the surface of human cells. It is a transmembrane receptor that is activated by a hormone called insulin-like growth factor 1 (IGF-1) and by a related hormone called IGF-2. It belongs to the large class of tyrosine kinase receptors. This receptor mediates the effects of IGF-1, which is a polypeptide protein hormone similar in molecular structure to insulin. IGF-1 plays an important role in growth and continues to have anabolic effects in adults – meaning that it can induce hypertrophy of skeletal muscle and other target tissues. Mice lacking the IGF-1 receptor die late in development, and show a dramatic reduction in body mass. This testifies to the strong growth-promoting effect of this receptor.
The MAPK/ERK pathway is a chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell to the DNA in the nucleus of the cell.
Receptor tyrosine kinases (RTKs) are the high-affinity cell surface receptors for many polypeptide growth factors, cytokines, and hormones. Of the 90 unique tyrosine kinase genes identified in the human genome, 58 encode receptor tyrosine kinase proteins. Receptor tyrosine kinases have been shown not only to be key regulators of normal cellular processes but also to have a critical role in the development and progression of many types of cancer. Mutations in receptor tyrosine kinases lead to activation of a series of signalling cascades which have numerous effects on protein expression. The receptors are generally activated by dimerization and substrate presentation. Receptor tyrosine kinases are part of the larger family of protein tyrosine kinases, encompassing the receptor tyrosine kinase proteins which contain a transmembrane domain, as well as the non-receptor tyrosine kinases which do not possess transmembrane domains.
Growth factor receptor-bound protein 2, also known as Grb2, is an adaptor protein involved in signal transduction/cell communication. In humans, the GRB2 protein is encoded by the GRB2 gene.
Transforming growth factor alpha (TGF-α) is a protein that in humans is encoded by the TGFA gene. As a member of the epidermal growth factor (EGF) family, TGF-α is a mitogenic polypeptide. The protein becomes activated when binding to receptors capable of protein kinase activity for cellular signaling.
Son of sevenless homolog 1 is a protein that in humans is encoded by the SOS1 gene.
The ErbB family of proteins contains four receptor tyrosine kinases, structurally related to the epidermal growth factor receptor (EGFR), its first discovered member. In humans, the family includes Her1, Her2 (ErbB2), Her3 (ErbB3), and Her4 (ErbB4). The gene symbol, ErbB, is derived from the name of a viral oncogene to which these receptors are homologous: erythroblastic leukemia viral oncogene. Insufficient ErbB signaling in humans is associated with the development of neurodegenerative diseases, such as multiple sclerosis and Alzheimer's disease, while excessive ErbB signaling is associated with the development of a wide variety of types of solid tumor.
Betacellulin is a protein that in humans is encoded by the BTC gene located on chromosome 4 at locus 4q13-q21. Betacellulin was initially identified as a mitogen. Betacellulin, is a part of an Epidermal Growth Factor (EGF) family and functions as a ligand for the epidermal growth factor receptor (EGFR). The role of betacellulin as an EGF is manifested differently in various tissues, and it has a great effect on nitrogen signaling in retinal pigment epithelial cells and vascular smooth muscle cells. While many studies attest a role for betacellulin in the differentiation of pancreatic β-cells, the last decade witnessed the association of betacellulin with many additional biological processes, ranging from reproduction to the control of neural stem cells. Betacellulin is a member of the EGF family of growth factors. It is synthesized primarily as a transmembrane precursor, which is then processed to mature molecule by proteolytic events.
Cbl is a mammalian gene family. CBL gene, a part of the Cbl family, encodes the protein CBL which is an E3 ubiquitin-protein ligase involved in cell signalling and protein ubiquitination. Mutations to this gene have been implicated in a number of human cancers, particularly acute myeloid leukaemia.
Phospholipase C, gamma 1, also known as PLCG1 and PLCgamma1, is a protein that in humans involved in cell growth, migration, apoptosis, and proliferation. It is encoded by the PLCG1 gene and is part of the PLC superfamily.
Receptor tyrosine-protein kinase erbB-3, also known as HER3, is a membrane bound protein that in humans is encoded by the ERBB3 gene.
Receptor tyrosine-protein kinase erbB-4 is an enzyme that in humans is encoded by the ERBB4 gene. Alternatively spliced variants that encode different protein isoforms have been described; however, not all variants have been fully characterized.
Mitogen-activated protein kinase 7 also known as MAP kinase 7 is an enzyme that in humans is encoded by the MAPK7 gene.
Phosphatidylinositol 3-kinase regulatory subunit beta is an enzyme that in humans is encoded by the PIK3R2 gene.
Sprouty homolog 2 (Drosophila), also known as SPRY2, is a protein which in humans is encoded by the SPRY2 gene.
CBL-B is an E3 ubiquitin-protein ligase that in humans is encoded by the CBLB gene. CBLB is a member of the CBL gene family.
A tyrosine kinase inhibitor (TKI) is a pharmaceutical drug that inhibits tyrosine kinases. Tyrosine kinases are enzymes responsible for the activation of many proteins by signal transduction cascades. The proteins are activated by adding a phosphate group to the protein (phosphorylation), a step that TKIs inhibit. TKIs are typically used as anticancer drugs. For example, they have substantially improved outcomes in chronic myelogenous leukemia. They have also been used to treat other diseases, such as idiopathic pulmonary fibrosis.