Cancer Cell Signalling

Amanda Harvey

• Publisher: Wiley
• Year: 2013
• Print ISBN:
  9781119967576
  9781119967569
• eBook ISBN:
  9781118527757
• Edition: 1
• Topic: Medicine
• Subtopic: Oncology

A focused, accessible introduction to this key aspect of cancer biology. It covers the individual cell signalling pathways that are known to be involved in cancer development, and, most important, includes the cross- interactions between the pathways together with the current therapeutic approaches.  This is a 'must-have' for advanced undergraduate and postgraduate students studying and researching within the field of cancer biology.

Chapter 1

Epidermal growth factor receptor family

Amanda Harvey

Biosciences, Brunel University, London

ErbB receptor ligands bind to their respective receptors, initiating the formation of homo- or heterodimers. The intracellular kinase domain of one receptor trans-phosphorylates the intracellular tyrosine residues on the opposite receptor thereby activating downstream signalling pathways.

At a cellular level, ErbB receptor ligands control a number of processes, including cell cycle progression, proliferation, cell death, protein synthesis, metabolism and differentiation. Physiologically this results in regulation of wound healing, neonatal growth and development as well as the development of adult tissues. Alterations in ErbB receptor signalling can result in oncogenesis in response to increased proliferation and decreased cell death as well as up-regulation of processes required for cell metastasis, such as adhesion, migration, invasion and neo-angiogenesis.

1.1 ErbB receptors and their structure

The epidermal growth factor receptor (EGFR/HER1) and the other family members (c-ErbB2/HER2/neu, ErbB3/HER3 and ErbB4/HER4) are 160–190 kDa transmembrane (type 1) receptor tyrosine kinases. They each comprise extracellular ligand binding and cysteine-rich domains, a transmembrane region, a kinase domain and an intracellular C-terminal tail, which contains the multiple tyrosine phosphorylation sites that are required for regulating receptor activation (reviewed in Ferguson, 2008).

EGFR was the first member of the family to be identified; it is a 170 kDa glycoprotein (Carpenter, 1987). Co-purification of the receptor with its growth factor ligand (epidermal growth factor, EGF) was reported in 1979 (McKanna et al., 1979), which followed the discovery of EGF in 1972 (Savage et al., 1972) and Stanley Cohen's pioneering work showing that EGF bound to the surface of cells (Cohen et al., 1975; Carpenter et al., 1975, 1978). HER2 was characterised in the 1980s as a 185 kDa protein (Schechter et al., 1984) and has been shown to be highly homologous to EGFR (Coussens et al., 1985). There are proto-oncogenic and oncogenic forms of HER2 and these differ in sequence by a single amino acid substitution (Bargmann et al., 1986). HER2 is the preferred dimerisation partner of the other three family members and it is always available for dimerisation, as it largely exists in normal cells in a monomeric state (Weiner et al., 1989a).

ErbB3 and ErbB4 were identified as the third and fourth members of the EGFR/ErbB family in the late 1980s based on their sequence homology with EGFR (Plowman et al., 1990, 1993; Kraus et al., 1989). Much of the sequence is conserved between the family members, with the highest degree of homology between each receptor and EGFR being in the kinase domain.

Of the family, EGFR and ErbB4 are the only fully functional members. ErbB3 has minimal kinase activity so the formation of ErbB3 homodimers does not result in active signalling and, as yet, there has been no ligand identified for HER2.

1.2 ErbB ligands

The monomeric growth factor ligands in this peptide family are 45–60 amino acids and they contain six conserved cysteine residues, which are linked by three disulphide bonds. EGF was the first factor to be characterised over 40 years ago (Savage et al., 1972), followed eight years later by transforming growth factor-α (TGFα) (Roberts et al., 1980; Torado et al., 1980). Shortly following its initial discovery, EGF was shown to stimulate DNA synthesis and cell proliferation (Carpenter and Cohen, 1976).

In the 30 years since the discovery of TGFα, the family has grown to over 12 ligands that have different receptor binding preferences and therefore have the ability to regulate different cellular events (Figure 1.1).

Figure 1.1 Schematic representation of the ErbB receptors. All four receptors are depicted and the percentage sequence homology in each domain with the EGFR is indicated. The extracellular region of the receptor has four subdomains, two of which (1 and 3) are involved in ligand binding and two (2 and 4) are cysteine rich and are involved in mediating dimerization. Individual ligands have different binding affinities for specific receptors; note that ErbB2/HER2 does not have a ligand (indicated by ?) and that ErbB3 does not have an active kinase domain (indicated by X), possibly as a result of its reduced homology with EGFR.

EGF, TGFα, epigen and amphiregulin bind to EGFR; epiregulin and heparin binding EGF-like growth factor (HB-EGF) bind to EGFR and HER4; the neuregulins (1–6) have binding preferences for both HER3 and HER4, and betacellulin (BTC) binds to HER2, HER3 and HER4 (reviewed in Eccles, 2011). What is perhaps most striking is that despite appearing to have a functional ligand-binding domain, no ligand has yet been identified that binds HER2 with high affinity, although HER2 will bind with low affinity to the EGF family of ligands.

1.2.1 Ligand production

EGF family ligands are secreted but often require cleavage, unlike ligands for other receptor tyrosine kinases. The ligands are found tethered to the external surface of the cell membrane in pro-forms and require proteolytic cleavage in order to be released. For many ErbB ligands this is carried out by the disintegrin and metalloproteinase, ADAM17 (Hinkle et al., 2004; Sahin et al., 2004 reviewed in Booth and Smith, 2007) via a process that is known as ectodomain shedding. In vivo evidence that ADAM17 acts upstream of EGFR also comes from knock-out mice. Both ADAM17^–/– and EGFR^–/– mice display aberrant developmental phenotypes (Wiesen et al., 1999; Jackson et al., 2003; Yamazaki et al., 2003) and EGFR activation only occurred when ADAM17 and amphiregulin were expressed (Sternlicht et al., 2005).

Once soluble, ligands can activate the receptors in paracrine, autocrine or endocrine fashions. This mechanism forms the basis of some types of signalling cross-talk (Chapter 9).

1.2.2 Effects of ligand binding to receptors

The extracellular domains of the receptors are responsible for ligand binding and facilitate most of the dimerisation events. Many of our insights into the mechanisms of ligand binding and the events involved in receptor dimerisation have come from experimental mutations of the receptors (reviewed in Brennan et al., 2000; Ferguson, 2008). Once the ligand has bound to the receptor, an event that occurs with a 1:1 stoichiometry, a change occurs in the conformation of the receptor that facilitates downstream phosphorylation events and signalling transduction.

A number of groups have presented models for investigating ligand–receptor interactions. When the ligand binds to its receptor, a large domain rearrangement occurs that ultimately results in receptor dimerisation. Dimerisation itself is mediated in part, but not solely, by a dimerisation arm or loop that protrudes from the receptor due to the structural rearrangement that takes place upon ligand binding removing the arm from its intra-molecular tether (Garrett et al., 2002; Ogiso et al., 2002; Ferguson et al., 2003; Greenfield et al., 1989). Exposure of the dimerisation arm initiates the subsequent dimerisation of the receptors with an asymmetric interaction between the intracellular domains (Figure 1.2). In contrast with other signalling pathways (such as IGF, see Chapter 2) ErbB dimerisation involves direct interaction between the receptors, rather than via an association mediated through a divalent ligand that acts as a molecular ‘bridge’ (reviewed in Ferguson, 2008).

Figure 1.2 Schematic representation of the extracellular domain rearrangement leading to receptor dimerization. In a closed confirmation the receptor is inactive. Ligand (EGF) can bind weakly to subdomain 1, which is not enough to induce receptor activation. However on binding to subdomain 3, the receptor confirmation opens up into an extended confirmation allowing ligand binding to both domains 1 and 3 and exposing the dimerization arm. The extended receptor then dimerizes through interactions that are mediated predominantly through subdomain 2 and, to a lesser extent, subdomain 4 (based on Ferguson et al., 2003).

In addition, ligand binding also brings about additional conformational changes that are required for dimerisation including rotation of part of the receptor (Ogiso et al., 2002 and Ferguson et al., 2003). It is clear that the spatial arrangement of the receptors is important in order that additional contact points can be made at the extracellular interface between the two receptors undergoing dimerisation (Ferguson et al., 2003). The arrangement of these contact points could be central in determining the extent of receptor hetero- or homo-dimerisation.

In 2009 Wilson and colleagues hypothesised that different ErbB ligands would stabilise the extracellular regions of the receptors in slightly different conformations (Wilson et al., 2009). This would affec...

Table of contents

• Cover
• Title Page
• Copyright
• List of contributors
• Acknowledgements
• Introduction
• About the companion website
• Chapter 1: Epidermal growth factor receptor family
• Chapter 2: Insulin and the insulin-like growth factor (IGF) family
• Chapter 3: Transforming growth factor-β receptor signalling
• Chapter 4: Wnt signalling
• Chapter 5: Mammalian target of rapamycin (mTOR) signalling
• Chapter 6: c-Met receptor signalling
• Chapter 7: Vascular endothelial growth factor and its receptor family
• Chapter 8: Progesterone receptor signalling in breast cancer models
• Chapter 9: Signalling cross-talk
• Index