2. Physiology of ErbB Receptors
The EGF system is present in various human organs and plays a significant role in cell proliferation, differentiation, migration and apoptosis during embryogenesis and postnatal development
[39][43][44].
2.1. ErbB Receptors
ErbB receptors are members of the subclass I superfamily of receptor tyrosine kinases (RTKs)
[37][39][45]. In humans, the EGF system consists of the following ErbB receptors: epidermal growth factor receptor (EGFR), ErbB-2, ErbB-3 and ErbB-4
[37][39][44][45][46]. These receptors are trans-membrane glycoproteins that catalyze the transferring of γ phosphate of ATP to hydroxyl groups of tyrosines in target proteins
[47]. However, ErbB-3 has no intrinsic tyrosine kinase activity, and it depends on another ErbB receptor (usually ErbB-2) for intracellular signaling
[45][48].
Regarding their structure, ErbB receptors have an extracellular ligand-binding domain, a transmembrane domain, a short juxtamembrane section, an intracellular bilobed tyrosine kinase domain and a tyrosine-containing C-terminal tail (
Figure 1)
[44][45][46][49].
Figure 1. Schematic structure of ErbB receptors. EC domain: extracellular ligand-binding domain. TM domain: transmembrane domain. JM section: juxtamembrane section. K domain: intracellular bilobed tyrosine kinase domain. CT tail: tyrosine-containing C-terminal tail.
The extracellular ligand-binding domain is divided into four subdomains: L1 (or I), CR1 (or II), L2 (or III) and CR2 (or IV)
[44][46][49]. The leucine-rich subdomains L1 and L2 participate in ligand binding
[44][46][49]. The cysteine-rich subdomains CR1 and CR2 participate in disulfide bond formation, while subdomain CR1 contains a β-hairpin loop and participates in ErbB receptors’ homodimerization and heterodimerization
[44][46][49]. Moreover, the intracellular tyrosine kinase domain is subdivided into two lobes, N and C
[44][46].
2.2. ErbB Ligands
In humans, the EGF system has the following ErbB peptide mediators (ligands): EGF, transforming growth factor-a (TGF-a), amphiregulin (AR), heparin-binding growth factor (HB-EGF), betacellulin (BTC), epigen, epiregulin (EPR), neuregulin-1 (NRG-1), neuregulin-2 (NRG-2), neuregulin-3 (NRG-3), neuregulin-4 (NRG-4), neuroglycan C and tomoregulin
[39][44][45][46]. Ligand binding to the extracellular domain of the ErbB receptor results in conformational changes and induces homodimerization and heterodimerization of receptors
[37][44][45][46]. However, the ErbB-2 receptor fails to bind any ligands
[37][44][45][46].
Based on their affinity for one or more receptors, ErbB ligands could be further classified into the following subgroups:
1. Ligands with binding specificity for EGFR only: EGF, TGF-a and AR
[44][45][46].
2. Ligands with dual binding specificity for EGFR and ErbB4: HB-EGF, BTC and EPR
[44][45][46].
3. Ligands with binding specificity for ErbB-3 only: neuroglycan C
[44][45][46].
4. Ligands with binding specificity for ErbB-4 only: NRG-3, NRG-4 and tomoregulin
[44][45][46].
5. Ligands with dual binding specificity for ErbB-3 and ErbB-4: NRG-1, NRG-2
[44][45][46].
2.3. Receptor Homodimerization and Heterodimerization
There are two distinct conformations of the extracellular ligand-binding domain, based on the activation status of EGFR, ErbB-3 and ErbB-4 receptors:
1. Closed conformation. When ErbB receptors are inactive, there are intramolecular interactions between the cysteine-rich subdomains CR1 and CR2, causing closed conformation of the extracellular ligand-binding domain
[44][45][46][50][51].
2. Open conformation. When ErbB receptors become active, the leucine-rich subdomains L1 and L2 create a ligand-binding pocket, allowing interactions with a single ligand, while the extracellular ligand-binding domain takes an open conformation and the β-hairpin loop dimerization arm of subdomain CR1 is exposed
[44][45][46][50][51].
It seems that there is equilibrium between both conformations of the extracellular ligand-binding domain, related directly to ligand presence and subsequent ligand binding
[50][51][52]. More specifically, ligand binding to the leucine-rich subdomains L1 and L2 stabilises the extracellular ligand-binding domain to an open conformation, exposes the β-hairpin loop dimerization arm of subdomain CR1 and allows receptor homodimerization and heterodimerization
[44][45][46][51][52][53]. Subsequently, ErbB receptor dimerization induces conformational changes of the intracellular bilobed tyrosine kinase domain
[44][45][46][54][55].
In contrast, the extracellular ligand-binding domain of the ErbB-2 receptor has an extended conformation that is not suitable for ligand binding, as there is close proximity of the leucine-rich subdomains L1 and L2, abolishing the ligand-binding site
[44][45][46][56][57][58]. However, the extended conformation of the ErbB-2 receptor is necessary for interaction with other ErbB receptors and subsequent ligand-independent heterodimerization and signaling
[44][45][46][56][57][58]. Moreover, abnormal overexpression of the ErbB-2 receptor permits ligand-independent receptor homodimerization
[44][46][57].
Overall, homodimerization and heterodimerization of ErbB receptors represents an essential part in the pathophysiology of the EGF system signaling network
[44][45][46][54][55]. Furthermore, the ErbB-2 and ErbB-3 heterodimer is the most transforming and mitogenic receptor complex
[59].
2.4. Intracellular Tyrosine Kinase Activation
Following homodimerization and heterodimerization of ErbB receptors, conformational changes of the intracellular tyrosine kinase domain take place, which in turn cause tyrosine kinase activation and phosphorylation of the tyrosine-containing C-terminal tail
[44][45][46][54][55].
As already mentioned, the intracellular tyrosine kinase domain has a bilobed structure, with ATP binding between the N and C lobes
[44][45][46][55]. More specifically, the C-lobe of an intracellular tyrosine kinase domain (activator) allosterically interacts with the N-lobe of another intracellular tyrosine kinase domain (receiver) within the same dimerization pair
[44][45][46][55]. This interaction induces conformational changes in the N-lobe of the receiver tyrosine kinase and finally causes its activation
[44][45][46][55]. Subsequently, the activated receiver tyrosine kinase catalyzes phosphorylation of tyrosine residues in the tyrosine-containing C-terminal tail of the activator tyrosine kinase
[44][45][46][55]. These phosphorylated tyrosine residues serve as docking sites for adaptor proteins, enzymes and various signaling molecules containing Src homology 2 (SH2) and phosphotyrosine binding (PTB) domains
[38][44][45][46][55][60][61].
3. ErbB Receptors in Endometrial Cancer
During the menstrual cycle, there is a wide variation in the profile of ErbB receptors, indicating a central role of the EGF system in the regulation of endometrial cyclical growth and shedding
[62][63].
In EC, the expression of ErbB receptors is significantly different, compared with the premenopausal and postmenopausal endometrium
[62][64][65]. This is mainly because of the increased transcriptional activity of ErbB encoding genes in EC cells
[65].
3.1. Profile of ErbB Receptors in Endometrial Cancer
Overall, EGFR overexpression is reported in 43–67% of unselected EC cases
[65][66][67][68][69][70][71][72][73][74][75][76]. EGFR overexpression is present in approximately 46% of type I EC (endometrioid) cases
[70][72][77]. EGFR overexpression is observed in 34–50% of type II EC (papillary serous, clear cell, undifferentiated) cases
[70][72][77][78][79][80].
ErbB-2 overexpression and ΕrbB-2 gene amplification represents a very rare event in unselected EC cases
[65][70][74][75][76]. However, ΕrbB-2 overexpression and ΕrbB-2 gene amplification are present in only 8–15% and 3% of type I EC cases, respectively
[65][70][77][81][82][83][84]. In contrast, ΕrbB-2 overexpression and ErbB-2 gene amplification are more common in type II EC cases
[70][72][77][78][79][80].
Moreover, the exact frequency of ErbB-2 overexpression and ΕrbB-2 gene amplification in type II EC remains controversial, as there are many racial differences
[70][72][78][85][86]. More specifically, ErbB-2 overexpression and ΕrbB-2 gene amplification are more common in African—American patients with type II EC, when compared with Caucasian individuals
[85][86].
Likewise, ErbB-2 overexpression and ΕrbB-2 gene amplification have significant variations among different histologic subtypes of type II EC
[70][72][78][81][86][87][88]. ErbB-2 overexpression and ΕrbB-2 gene amplification are reported in 18–80% and 17–47% of papillary serous EC cases, respectively
[70][72][81][84][86][87][88], and 33% and 16–50% of clear cell EC cases, respectively
[70][72][81][88].
ErbB-3 overexpression is reported in 30% of unselected EC cases
[62][74]. More specifically, ErbB-3 overexpression is more common in well differentiated tumors when compared with moderately and poorly differentiated ones
[62].
Similarly, ErbB-4 overexpression is reported in 15% of unselected EC cases
[62][74].
Overall, there are some differences in ErbB-2 receptor profile in selected EC patients (EC histologic subtypes and racial—ethnic subgroups)
[64][71][78][85]. ErbB-2 receptor expression is more common in papillary serous and clear cell EC cases
[64][71][78]. This is mainly based on differences in the pathophysiology and clinical behavior of various EC histologic subtypes
[64][71][78].
3.2. Clinical Role in Endometrial Cancer
The relationship of the ErbB receptors profile with disease stage, tumor grade and response to treatment remains controversial in EC cases
[70][74].
In particular, the clinical role of EGFR overexpression has not been studied thoroughly in EC patients
[70][74]. Some studies demonstrate an association between EGFR overexpression and poor clinical outcome, while others report otherwise
[66][67][68][69]. It seems that EGFR overexpression may have a dual role in EC cases
[70]. EGFR overexpression in type I EC is associated with less aggressive disease and more favorable outcomes
[70][72][74][78]. In contrast, EGFR overexpression in type II EC is associated with more aggressive disease and adverse clinical outcomes
[70][72][74][78].
However, the clinical significance of ΕrbB-2 overexpression and ΕrbB-2 gene amplification has been studied extensively in EC patients
[72][78][81][86][87][89][90][91]. ΕrbB-2 overexpression and ΕrbB-2 gene amplification are indicators of a more aggressive disease with reduced response to treatment and less favorable outcomes, especially in patients with type II EC
[72][74][78][81][86][87][89][90][91][92].
Furthermore, the clinical role of ErbB-3 and ErbB-4 overexpression has not been studied extensively in patients with EC
[62][64][71][72][73][74][78].
It becomes apparent that ErbB-2 receptor expression is more common in aggressive EC histologic subtypes (papillary serous and clear cell)
[64][71][78]. This possibly indicates a future role of ErbB-targeted therapies in well-defined EC subgroups with overexpression of ErbB receptors
[71][78][93].