Epidermal Growth Factor (EGF) & Receptor Proteins

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Epidermal Growth Factor (EGF) & Receptor Proteins

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Epidermal Growth Factor (EGF) & Receptor Proteins Background

Ligament injuries result in the release of growth factors that initiate the wound healing process. After injury, accumulated platelets are known to secrete growth factors, including Platelet-Derived Growth Factor (PDGF), Epidermal Growth Factor (EGF), and Transforming Growth Factor-β (TGF-β). Among these growth factors, EGF has been shown to play an important role in the repair processes after injury by promoting cell proliferation and collagen synthesis in acute wounds. EGF has also been demonstrated to attract fibroblasts and inflammatory cells to wounded sites, as well as regulate the migration, proliferation and differentiation of fibroblasts. Clinical trials for human chronic wound therapeutics have also illustrated that the addition of topical EGF shortens ligament healing time. Further, both in vitro and in vivo studies have shown that EGF can accelerate and improve wound healing by binding to the EGF receptor (EGFR) and triggering subsequent EGFR signaling.

The EGFR, as a Receptor Tyrosine Kinase (RTK), is a globular protein of 170kDa and 1186 amino acids, including epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor (PDGFR), fibroblast growth factor receptor (FGFR), Met (hepatocyte growth factor receptor), insulin receptor (IR), and many others. Similar to other growth factor receptors, it is composed of three sections: an extracellular EGF or ligand-binding domain, a small hydrophobic membrane sequence, and a cytoplasmic domain that encodes the tyrosine kinase. RTKs are transmembrane receptors with an extracellular portion on the cell surface for ligand binding, and an intracellular, cytosolic portion used for activation of internal signaling cascades. The cytosolic portion contains tyrosine residues that are targets for the tyrosine kinase portion of the receptor. EGFR is activated when the EGF ligand binds to its extracellular portion, and causes phosphorylation.

Signal transduction pathways are initiated when activated EGFR directly or indirectly recruits various signaling proteins, like Shc, Grb2, PLC-γ, Src and PI3K. Due to the functional diversity of signaling proteins, the activation of these signaling proteins may lead to the assembly of different multicomponent signaling complexes and subsequent activation of multiple signaling pathways, including Ras pathway, PLC-γ1 pathway and PI3K pathway. As these pathways are linked together directly or indirectly, activation of EGFR actually stimulates a signaling network. Signal transduetion probably involves a cascade of protein kinases acting on various intracellular targets to produce the multiple mitogenic effects of EGF. These intracellular targets ultimately should increase the transcription of proto-oncogenes like c-fos and c-jun, which are early responders to EGF stimulation. They are probably involved in the increased expression of other genes. Second messenger signaling includes the IP3 increased intracellular calcium ion, rapid cytoplasmic alkalinization of the cell (due to the plasma membrane Na+/H+ antiporter by DAG), and prostaglandin E2 (PGE2) production; even G proteins could be involved in mediating the EGF signal in some cell types. EGF also activates casein kinase II which in turn phosphorylates nuclear proteins.
Abnormal EGF/TGFα metabolism may play a role in viral and non-viral proliferative skin diseases including benign disorders and skin cancers, as well as aging. In a number of benign skin diseases the EGF receptors are increased due to their persistent expression throughout the different layers of the epidermis. For example, EGF receptors are increased 2-4-fold in psoriasis. In addition, the coexpression of TGFα and high levels of EGF receptor by some human tumors implies that this deregulation may be part of a multistep process that confers a selective growth advantage to some human tumor cells.

Epidermal growth factor is a member of the family of EGF-like molecules. This family also includes TGFα, the poxvirus growth factors, and amphiregulin, which are all encoded by separate genes. TGF is found to be structurally related to EGF: both conserve cysteines, have an antigenic relationship to EGF, and have binding capacity for the EGF receptor now called the EGF/TGFα receptor.
Viruses also produce EGF-like growth factors that may affect the growth and development of skin by binding to the EGF receptor and increasing cell proliferation. Examples include vaccinia virus (the cause of cowpox), molluscum contagiosum, and the Shope fibroma virus (the cause of certain fibrosarcomas). The vaccinia virus uses the EGF receptor to bind to and infect cells because pretreating target cells with EGF or synthetic antagonists of EGF before adding the virus prevents productive infections. Based on these in vitro findings, there are suggested roles for the viral growth factors in both the initiation and maintenance of natural viral infections.

Related literatures
1. Yates R A, Nanney L B, Gates R E, et al. Epidermal growth factor and related growth factors[J]. International journal of dermatology, 1991, 30(10): 687-694.
2. Gibbs S, Silva Pinto A N, Murli S, et al. Epidermal growth factor and keratinocyte growth factor differentially regulate epidermal migration, growth, and differentiation[J]. Wound Repair and Regeneration, 2000, 8(3): 192-203.
3. Carpenter G, Cohen S. Epidermal growth factor[J]. Annual review of biochemistry, 1979, 48(1): 193-216.

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