EGF Pathway

Overview of EGF Pathway

Epidermal growth factor (EGF) siganling regulates a variety of biological processes, including cell proliferation, differentiation, motility, and survival. EGFR, also known as ErbB1 / HER1, is a receptor tyrosine kinase that belongs to the ErbB family (RTK). Additionally, the epidermal growth factor receptor family comprises ErbB2 (Neu, HER2), ErbB3 (HER3), and ErbB4 (HER4) (HER4).

These transmembrane proteins are activated by ligands such as epidermal growth factor (EGF), transforming growth factor-alpha (TGF-), epigen, amphiregulin, betacellulin, heparin-binding EGF, and epiregulin in order to initiate a series of reactions that span the cell's membrane to the nucleus.

EGF (Epidermal Growth Factor) is a small protein containing 53 amino acids that plays a role in normal cell growth, oncogenesis, and wound healing. This protein is highly similar in sequence and function to hTGF-Alpha (human type-Alpha Transforming Growth Factor), a competitor for EGF receptor sites. EGF binds to a highly specific, low-affinity receptor on the surface of responsive cells called EGFR (Epidermal growth factor receptor). EGFR is a member of the ErbB (Erythroblastic Leukemia Viral Oncogene Homolog) family of receptor tyrosine kinases, a subfamily comprised of four closely related receptor tyrosine kinases: EGFR (ErbB1), Her2/c-neu (ErbB2), Her3 (ErbB3), and Her4 (ErbB4) (ErbB4). The EGFR forms homo- or heterodimers with other family members in response to toxic environmental stimuli such as ultraviolet irradiation or receptor occupation by EGF. EGF binding to the extracellular domain of EGFR results in receptor dimerization, activation of the intrinsic PTK (Protein Tyrosine Kinase), tyrosine autophosphorylation, and recruitment of various signaling proteins to these autophosphorylation sites, which are predominantly located in the receptor's C-terminal tail.

GRB2 is a critical component of EGFR-Ras signaling. GRB2's SH2 (Src Homology-2) domain can bind directly to the activated EGFR's phosphotyrosines 1068 and 1086 or indirectly via the tyrosine-phosphorylated adaptor protein SHC. GRB2's SH3 domains are constitutively associated with SOS (Son of Sevenless), a Ras GTPase exchange factor. Apart from SOS, GRB2 SH3 domains can interact with a variety of proteins, including Dynamin and Cbl (c-Cbl, Cbl-b, and Cbl-3), which are both involved in the regulation of EGFR endocytosis. When the GRB2 and SOS complex binds to the EGFR, SOS is brought into close proximity to Ras, resulting in Ras GTP loading and subsequent activation of Ras effectors such as Raf kinases and PI3K. (Phosphatidylinositol 3-Kinase). Raf initiates a cascade of phosphorylation reactions, which include the phosphorylation and activation of MEKs (MAPK/ERK Kinases) and ERKs (Extracellular Signal-Regulated Kinases). The docking protein GAB1 may also be involved in EGF-induced PI3K stimulation (GRB2-Associated Binder-1). GAB1 is a docking protein that facilitates the recruitment of PI3K and other effector proteins in response to the activation of a large number of RTKs (Receptor Tyrosine Kinases). After activation, PI3K phosphorylates membrane-bound PIP2 (Phosphatidylinositol (4,5)-bisphosphate) to form PIP3 (Phosphatidylinositol-3,4,5-trisphosphate). PIP3 binds to the PH domain of Akt, anchoring it to the plasma membrane and allowing it to be phosphorylated and activated by PDK1 (Phosphoinositide-Dependent Kinase-1). Then, Akt phosphorylates a variety of substrates and participates in cell survival ). The G1 isoform of PLC is one of the prominent enzymes activated by EGFR (Phospholipase-C-Gamma1).

Dok2 (Docking Protein-2) also associates with the EGFR and is phosphorylated on the tyrosine kinase in response to EGF stimulation. Dok2 binding to the EGFR via its PTB (Phosphotyrosine Binding) domain results in a decrease in MAPK (Mitogen-Activated Protein Kinase) activation. Dok2's ability to inhibit MAPK activation induced by EGF is independent of its ability to recruit RasGAP, a known MAPK inhibitor, implying an alternate Dok2-mediated pathway. Dok2 interacts with c-Src and the inhibitory kinase SFK (Src Family Kinase), Csk. Dok2 associates constitutively with c-Src via an SH3-dependent interaction, which is required for Dok2 to inhibit c-Src activity and decrease MAPK and Akt/PKB activity (Ref.6). One of the critical signaling events triggered by EGFR is STAT tyrosine phosphorylation. Stimulation of EGFR induces STAT1 and STAT3 tyrosine phosphorylation and initiates the formation of STAT1 and STAT3 complexes with JAK1 (Janus Kinase-1) and JAK2 (Janus Kinase-2) (Janus Kinase-2). JAKs are required for EGFR interaction with STAT1 and STAT3. Following that, the STATs translocate to the nucleus and become involved in gene transcription.

Dok2 (Docking Protein-2) also associates with the EGFR and is phosphorylated on the tyrosine kinase in response to EGF stimulation. Dok2 binding to the EGFR via its PTB (Phosphotyrosine Binding) domain results in a decrease in MAPK (Mitogen-Activated Protein Kinase) activation. Dok2's ability to inhibit MAPK activation induced by EGF is independent of its ability to recruit RasGAP, a known MAPK inhibitor, implying an alternate Dok2-mediated pathway. Dok2 interacts with c-Src and the inhibitory kinase SFK (Src Family Kinase), Csk. Dok2 associates constitutively with c-Src via an SH3-dependent interaction, which is required for Dok2 to inhibit c-Src activity and decrease MAPK and Akt/PKB activity (Ref.6). One of the critical signaling events triggered by EGFR is STAT tyrosine phosphorylation. Stimulation of EGFR induces STAT1 and STAT3 tyrosine phosphorylation and initiates the formation of STAT1 and STAT3 complexes with JAK1 (Janus Kinase-1) and JAK2 (Janus Kinase-2) (Janus Kinase-2). JAKs are required for EGFR interaction with STAT1 and STAT3. Following that, the STATs translocate to the nucleus and become involved in gene transcription.

E3B1 (EPS8 Binding Protein)/ABI1 (Abl-Interactor-1) is an EGFR signaling protein. E3B1/ABI1 interacts with EPS8 (Epidermal Growth Factor Receptor Pathway Substrate-8), a fibroblast-derived protein that is phosphorylated in response to EGF. Surprisingly, E3B1/ABI1 acts as a negative regulator of the growth factor receptor-mediated cellular response. EPS8 interacts with E3B1 or RNTRE via its SH3 domain (Related to the N-terminus of tre). By forming a complex with E3B1 and SOS1, EPS8 facilitates the transfer of signals between Ras and Rac. SOS1, a bifunctional GEF (Guanine nucleotide Exchange Factor), activates Ras in vivo and acts as a Rac-GEF in vitro when co-expressed with EPS8 and E3B1. RNTRE, on the other hand, acts on Rab5A (Rab5A, a member of the RAS oncogene family) by forming a complex with EPS8 and inhibiting the EGFR's internalization (Ref.8). Cbl is another important EGFR substrate that associates with the receptor in response to EGF stimulation and forms complexes with a number of signaling proteins involved in EGF-mediated cell growth. Due to the fact that Nck is known to bind activated EGFR via its SH2 domain and Cbl via its SH3 domain, Nck represented another potential adaptor that could mediate Cbl-EGFR association. The SH3-SH3-SH3-SH2 adapter protein Nck connects EGFR to downstream signaling pathways, including the p21CDC42/Rac-activated kinase cascade, SOS-activated Ras signaling, and human WASP-mediated actin cytoskeleton changes. Additionally, Nck induces the activation of PAK1 (p21/CDC42/Rac1-Activated Kinase-1). The association of Nck and PAK1 has been demonstrated to occur via PAK1's first N-terminal polyproline domain and Nck's SH3 domain. PAK activation by Nck results in the activation of JNKs (c-Jun Kinases) via MEKK1 (MAP/ERK Kinase Kinase-1) and MKK4/7 (MAP Kinase Kinase-4/7), respectively. Once activated, JNKs migrate into the nucleus, where they phosphorylate transcription factors such as c-Fos and c-Jun. Additionally, EGFR activates Vav (Oncogene Vav).