Epidermal Growth Factor receptor (EGFR)
Epidermal Growth Factor receptor (EGFR) is a receptor tyrosine kinase and belongs to ErbB receptor tyrosine kinases family. Aberrant regulation of EGFR has been linked to various cancers.
Some pro-oncogenic signaling pathways are found downstream of the EGFR pathway. For example, activation of EGFR by EGF in Hela cells leads to phosphorylation of 2244 proteins at 6600 sites. As a result, migration, proliferation, growth, and differentiation of cells can be stimulated by the activation of EGFR signaling network.
Aberrant activation of EGFR may lead to increased transcriptional expression and gene amplification. Elevated EGFR level may relate with poor prognosis in cancers such as colorectal cancer, lung cancer and endometrial cancer EGFR level is a determinant factor for tumor size, patient prognosis and relapse of cancer.
For activation of EGFR, ligands are required. There are seven known EGFR ligands. They are EGF, TGF-α, AREG, EREG, BTC, HB-EGF and EPI. Each ligand activates EGFR similarly. Following the ligand binding, EGFR dimerizes and is cross phosphorylated which initiates signaling cascades.
Stimulated EGFR activates three main cascades;
- ERK MAPK,
- AKT-PI3K and
- PLC-γ1-PKC pathways.
Activated EGFR is then internalized by endocytosis into early endosomes. When EGFR is activated by EGF, it is internalized by either clathrin mediated endocytosis (CME) and clathrin independent endocytosis which includes Caveolin mediated endocytosis and micropinocytosis. Nevertheless, main EGFR endocytosis mechanism is CME. Endocytosed EGFR carrying endosome either fuses with lysosomes or is transformed into a retromer structure for recycling to cell surface. CME leads EGFR to be found in endosomes that contain Rab5. After these endosomes become mature, they are recycled back to plasma membrane. If EGFR is found in late endosome which contain Rab7, it is ubiquitinated. Ubiquitinated EGFR is directed to the intraluminal vesicles that forms multivesicular body where EGFR is degraded.
EGFR is activated by binding of EGF. EGF leads to homodimerization and cross phosphorylation of EGFRs. Stimulated EGFR can activate downstream signaling pathways like PI3K/AKT/mTOR, RAS/RAF/MEK/ERK and PLCγ/PKC. These activated pathways have role in cell proliferation, cell migration and tumor formation.
Retromer complex comes into stage while EGFR is sorting to the lysosome. Retromer complex which contain VPS35, VPS26, VPS29 and SNX3 has been linked to the WASH complex (Wiskott–Aldrich syndrome protein and SCAR homolog complex) which is necessary for sorting of EGFR to lysosome. It was also shown that deletion of WASH1 from T cell of transgenic mice resulted defects of EGFR recycling from endosome to cell surface. Close relation between WASH and retromer complex is putting SNX3 in a critical place for EGFR regulation. It was shown that downregulation of SNX3 led to increased EGFR degradation. Although there are extensive studies about EGFR trafficking, the mechanism behind EGFR regulation by SNX3 is not clear yet.
EGFR and EMT
Recent studies have demonstrated that EGF and TGFβ1 can induce EMT. ERK1/2 and PI3K/Akt pathways can potently induce EMT. Expression of SNAIL which is a major transcription factor that leads to EMT is regulated by variety of signaling patterns like PI3K, MAPK, GSK-3β and NFκB pathways. For example, EGFR pathway which is triggered by EGF induces SNAIL expression through preventing the GSK-3β activity. GSK-3β phosphorylates SNAIL and leads to degradation through ubiquitination.
Increased CD44 expression level correlated with increased expression level of EGFR which activates PI3K/Akt in colon cancer cells. In that way, CD44 was linked to invasion and migration.
It is known that, TWIST1 expression is regulated by signaling pathways components such as Akt and WNT-β Catenin. AKT1 leads to increase the phosphorylation of TWIST1 which is required for degradation and ubiquitination. Extensive work on EGFR and EMT indicates that there is a strong association between them.