Epidermal Growth Factors (EGF) and Analogs
Definition
Epidermal Growth Factor (EGF) is part of a complex network of growth factors and receptors that together help to modulate the growth of cells. EGF is released by cells, and then is picked up either by the cell itself, stimulating its own growth, or by neighboring cells, stimulating their ability to divide 1.
Related Peptides
Depending on their binding specificities, EGF-related peptides can be subdivided into two classes. The first group of ligands binds to the EGFR (EGF receptors) and includes EGF itself, transforming growth factor (TGF) a, amphiregulin (AR), heparin-binding EGF-like growth factor (HB-EGF), and betacellulin (BTC). Each of these peptides competes with EGF for receptor binding and therefore this family of growth factors is referred to as the EGF agonists. Another family of EGF-related peptides is composed of the neu differentiation factors (NDFs)/heregulins ligands for ErbB-3 and ErbB-4. There are at least 12 different isoforms arising from a single gene by alternative splicing and depending on the sequence of their EGF-like repeat they are classified as either a or ß isoforms. However, despite the large number of NDFs no differences in receptor binding specificities appear to exist: ErbB-3 functions as a low affinity receptor for all NDF isoforms while ErbB-4 serves as a high affinity receptor 2.
Discovery
The discovery of EGF won Stanley Cohen a Nobel Prize in Physiology and Medicine in 1986 and was patented for cosmetic use by Greg Brown in 1989 .
Structural Characteristics
EGF is a small mitogenic protein that is thought to be involved in mechanisms such as normal cell growth, oncogenesis, and wound healing. This protein shows both strong sequential and functional homology with human type-alpha transforming growth factor (Htgf-a), which is a competitor for EGF receptor sites. EGF is a small 53 amino acid residue long protein that contains three disulfide briges. The side chains of residues 13 (Tyr), 41 (Arg), and 47 (Leu) are all thought to play an important role in EGF's functionality 3.
Analogs
Three site-directed mutants of human epidermal growth factor, Leu-26-Gly, Leu-47-Ala, and Ile-23-Thr, were examined for their ability to stimulate the protein-tyrosine kinase activity of the epidermal growth factor receptor. The receptor binding affinities of the mutant growth factors were 20- to 50-fold lower, as compared to wild-type growth factor. At saturating concentrations of growth factor, the velocities of the phosphorylation of exogenously added substrate and receptor autophosphorylation were significantly lower with the mutant analogs, suggesting a partial 'uncoupling' of signal transduction. The mutant analogs were shown to compete directly with the binding of wild-type, resulting in a decrease in growth factor-stimulated kinase activity 4. Six mutants of human EGF, which carry single point substitutions within a surface patch proposed to juxtapose the bound receptor, were prepared and characterized for receptor affinity and mitogenicity. Receptor affinities relative to EGF are G12Q > H16D > Y13W > Q43A ˜ H16A ˜ EGF >> L15A. Notably, the reduced receptor affinity of mutant L15A indicates that Leu15 probably contributes substantially to receptor binding whereas unaltered receptor affinities observed for analogs H16A and Q43A indicate that neither His16 nor Gln43 contributes significantly to this interaction. On the other hand, the observed enhanced receptor affinities of analogs G12Q, Y13W and H16D highlight surface loci where additional productive receptor-binding contacts can be introduced. Interestingly, at acidic pH analog H16A reveals substantially greater receptor affinity than that of EGF, a property which may offer enhanced therapeutic utility in acidic environments in vivo 5.
Mode of Action
Receptors on the surface of the cell bind to EGF and relay the signal inside. When the receptor binds to EGF, it is activated by forming a dimer with other receptors. Four similar receptors have been discovered: the EGF receptor and three variants. These may dimerize with themselves, or mix-and-match to form heterodimers with the other types. The set of growth factors that interacts with these receptors is even more varied, with a dozen or so known examples, including EGF, transforming growth factor-a, and a number of neuregulins. The receptor is composed of a single chain with many functional parts. It is found in the cell membrane, with one portion facing out to receive the message and one portion facing inward to relay the message to the cell machinery. The outer portion forms an EGF-binding domain. It is composed of four articulated parts: two globular parts that grip EGF and two rod-shaped linkers that are rigidified by dozens of cysteine amino acids. When this multi-part domain binds to EGF, it changes shape, releasing one of the long, cysteine-rich sections. This allows the receptor to dimerize with other receptors. This brings the two kinase domains close to one another, allowing them to add phosphates to each other and activating the signaling process. Many hormone receptors act by binding to either side of a hormone, with the hormone in the center. The EGF receptor, surprisingly, forms dimers with the growth factors on opposite sides of the dimer, far from the point of contact between the two receptors6.
Functions
The EGFR signaling pathway is one of the most important pathways that regulate growth, survival, proliferation, and differentiation in mammalian cells6. In animal models of acute renal injury, the administration of epidermal growth factor, insulin-like growth factor I (IGF-I), or hepatocyte growth factor accelerates the restoration of kidney function and the normalization of histology post-acute renal injury and reduces mortality. The mechanisms by which the growth factors act in acute renal failure include the stimulation of anabolism, the maintenance of glomerular filtration, and the enhancement of tubular regeneration7.
References
1. Goodsell DS (2003). The molecular perspective: epidermal growth factor. The Oncologist, 8(5):496–497.
2. Beerli RR, Hynes NE (1996). Epidermal growth factor-related peptides activate distinct subsets of erbb receptors and differ in their biological activities. J Biol Chem., 271:6071-6076.
3. Montelione GT, Wuthrich K, Burgess AW, Nice EC, Wagner G, Gibson KD, Scheraga HA (1992). Solution structure of murine epidermal growth factor determined by NMR spectroscopy and refined by energy minimization with restraints. Biochemistry, 31:236-242.
4. Matsunami RK, Campion SR, Niyogi SK, Stevens A (1990). Analogs of human epidermal growth factor which partially inhibit the growth factor-dependent protein-tyrosine kinase activity of the epidermal growth factor receptor. Febs letters, 264(1):105-108.
5. Mullenbach GT, Chiu CY, Gyenes A, Blaney J, Rosenberg S, Marlowe CK, Brown S, Stratton-Thomas J, Montelione GT, George-Nascimento C, Stauber G (1998). Modification of a receptor-binding surface of epidermal growth factor (EGF): analogs with enhanced receptor affinity at low pH or at neutrality. Protein Engineering, 11(6):473–480.
6. Oda K, Matsuoka Y, Funahashi A, Kitano H (2005). A comprehensive pathway map of epidermal growth factor receptor signaling. Molecular Systems Biology, 1:2005.
7. Hammerman MR, Miller SB (1994). Therapeutic use of growth factors in renal failure. Journal of the American Society of Nephrology, 5:1-11.