Treatment Therapies

Targeted therapies

The epidermal growth factor receptor (EGFR)

Growth factors in the cellular environment and cancer

Cells in all tissues secrete soluble growth factors into the extracellular environment. These factors can then directly regulate the function of cells either within the same tissue (autocrine or paracrine control) or tissues in other organs (endocrine control). However, some growth factors are first expressed as inactive precursors. These precursors can be part of structural molecules on the surface of cells or in the extracellular matrix. The active growth factor is then released by the action of proteases or specific interactions with matrix components.¹ Growth factors interact with cellular receptors to elicit a response. Thus, the response of a cell to the array of factors within its environment will depend upon the number and type of receptors it expresses. Growth factors are known to play an important role in tumour growth and survival.²

The human epidermal receptor (HER)

Epidermal growth factor (EGF) was one of the first growth factors to be described.³ Its mitogenic effect is mediated by binding to the HER1/EGFR. This receptor is one of a group of closely related receptors now referred to as the HER’s.⁴ There are four members of the HER family, and they have certain protein sequences in common and a structural homology (Figure 1):

• HER1 (EGFR, erbB1)
• HER2 (erbB2, neu)
• HER3 (erbB3)
• HER4 (erbB4).

Figure 1. The HER family of receptors.

Each receptor is a transmembrane protein and consists of three distinct domains: the extracellular region, a transmembrane section and the intracellular domain.⁵ The extracellular region confers the ligand binding specificity of the receptor, although HER2 has no known natural ligand. The hydrophobic transmembrane section links the extracellular and intracellular domains, and anchors the receptor within the cell membrane. The intracellular domain usually has TK activity (except for HER3). Phosphorylation of the intracellular site triggers downstream signalling via the TK moiety.⁶

HER ligands

This superfamily of ligands consists of a large range of proteins (secreted or membrane-bound) that have a wide variety of functions. These proteins include: proteases, clotting factors, adhesion molecules, extracellular matrix proteins, and peptide growth factors. Examples of the peptide growth factors are listed in Table 2. The peptide growth factors share a 50-amino acid ‘EGF-homologous’ region that is prerequisite for binding and activation of the HER. The most abundant ligands for EGFR are EGF and TGF-a.

Table 2. Examples of peptide ligands of the HER family

Activation of HER1/EGFR

In the inactive state, HERs are present in the cell membrane as single-protein monomers. Binding of a ligand to the extracelluar domain activates the receptors by inducing the formation of dimers consisting of two receptors in close association. The HER1/EGFR can form homodimers (i.e. two HER1/EGFR receptors) or heterodimers (combination with HER2, HER3 or HER4 – Figure 2). Dimerisation leads to a conformational change in the receptor that activates the TK domain by autophosphorylation tyrosine residues in up to five distinct sites. All of the functions for HER1/EGFR, with the exception of ligand binding are dependent on activation of the TK domain.⁷

Figure 2. HER1/EGFR dimerisation.

The phosphorylated sites on the intracellular domain of the HER dimers are specific binding sites for downstream signalling proteins. These proteins also undergo phosphorylation leading to conformational changes and subsequent activation.

HER1/EGFR signal transduction

Diversity

HER1/EGFR activation can produce a range of effects in cells depending on the activating ligand and the type of homo/heterodimer formed. Dimerisation with HER2 is the most common, although HER1/EGFR can also form heterodimers with HER3 or HER4.⁷ Heterodimers of HER1/EGFR and HER2 are relatively very stable, thereby resulting in prolonged downstream signalling and enhanced cellular responses. The diversity of secondary signalling results from specific differences in the peptide sequence of the intracellular domains as well as the tyrosine residues that are autophosphorylated in each of the four HERs. Nonetheless, there is a high degree of overlap in the downstream signalling proteins that bind to the HERs and the signalling pathways activated.

Downstream signalling

The activated HER1/EGFR homo/heterodimer binds to a wide variety of intracellular molecules, which trigger downstream signalling pathways (Figure 3). These multiple pathways are very complex and interconnected.⁸ The activation of a specific signalling pathway depends upon the ligand and the type of dimer.^9,10

Figure 3. Effects of HER1/EGFR activation.

The two major signalling pathways induced by HER1/EGFR are the mitogenactivated protein kinase (MAPK) pathway, and the PI3-Akt pathway.⁷ The MAPK pathway is important for the regulation of cell proliferation. The P13-AKT pathway is predominantly involved in the regulation of cell death and survival. It has been demonstrated that the stimulation of these two pathways may result in resistance to chemotherapy.^11,12 Other downstream effects of activated HER1/EGFR include:

• Migration – physical movement of the cell within the tissues.
• Adhesion – interaction of the cell with the extracellular matrix.
• Invasion – ability to pass through tissue barriers such as the walls of the blood vessels.
• Angiogenesis – stimulation of new blood vessel growth

Overall, all these downstream signalling pathways of HER1/EGFR can contribute to the transformation, survival and growth of cancer cells.

Signal termination

Many receptor-signalling systems have a means of turning off the receptor activation once adequate levels of stimulation have been achieved. Binding of EGF or TGF-a to HER1/EGFR induces the dimerised receptors to cluster on the cell surface. The dimers are then internalised into the cytoplasm via membrane bound vesicles (endocytosis). Cytoplasmic enzymes degrade the receptors and the ligands, thereby inactivating the ligandreceptor complexes. This process is dependent on TK activity, and mutant receptors lacking TK activity do not undergo degradation within the cytoplasm and are recycled to the cell surface. The rate of endocytosis and degradation (or recycling) depends upon the composition of the dimer. For example, HER1/EGFR homodimers are internalised and degraded rapidly leading to a marked decrease in downstream signalling. However, heterodimerisation of HER1/EGFR with HER2 decreases the rate of endocytosis, thus prolonging activity.¹³

References:
1. Flaumenhaft R, Rifkin DB. The extracellular regulation of growth factor action. Mol Biol Cell 1992;3:1057–65.
2. Aaronson SA. Growth factors and cancer. Science 1991;254:1146–53.
3. Cohen S. The epidermal growth factor (EGF). Cancer 1983;51:1787–91.
4. Salomon DS, Bradt R, Ciardiello F, Normanno N. Epidermal growth factor related peptides and their receptors in human malignancies. Crit Rev Oncol Hematol 1995;19:183–232.
5. Anderson NG, Ahmad T. ErbB receptor tyrosine kinase inhibitors as therapeutic agents. Front Biosci 2002;7:1926–40.
6. Deb TB, Su L, Wong L, et al. Epidermal growth factor (EGF) receptor kinas-independent signalling by EGF. J Biol Chem 2001;276:15554–60.
7. Mendelsohn J, Baselga J. The EGF receptor family as targets for cancer therapy. Oncogene 2000;19:6550–65.
8. Olayioye MA, Neve RM, Lane HA, Hynes NE. The ErbB signaling network: receptor heterodimerization in development and cancer. EMBO J 2000;19:3159–67.
9. Yarden Y, Slikowski MX. Untangling the erbB signalling network. Nature Rev Mol Cell Biol 2001;2:127–37.
10. Riese DJ II, Stern DF. Specificity within the EGF family/ErbB receptor family signaling network. Bioessays 1998;20: 41–8.
11. Kraus AC, Ferber I, Bachmann SO, et al. In vitro chemo- and radio-resistance in small cell lung cancer correlates with celladhesion and constitutive activation of AKT and MAP kinase pathways. Oncogene 2002;21:8683–95.
12. Janmaat ML, Kruyt FA, Rodriguez JA, Giaccone G. Response to epidermal growth factor receptor inhibitors in non-small-cell lung cancer cells: limited antiproliferative effects and absence of apoptosis associated with persistent activity of extracellular signal-regulated kinase or Akt kinase pathways. Clin Cancer Res 2003;9:2316–26.
13. Lenferink AE, Pinkas-Kramarski R, van de Poll ML, et al. Differential endocytic routing of homo- and hetero-dimeric ErbB tyrosine kinases confers signalling superiority to receptor heterodimers. EMBO J 1998;17:3385–97.

Treatment Therapies

• Overview
• Chemotherapy
• Targeted therapies
  • Cancer Cell Biology
  • EGFR
  • Rationale for HER1/EGFR
• Current Therapy - Erlotinib