AbstractErbB family members represent important biomarkers and drug targets for modern precision therapy. They have gained considerable importance as paradigms for oncoprotein addiction and personalized medicine. This review summarizes the current understanding of ErbB proteins in cell signalling and cancer and describes the molecular rationale of prominent cases of ErbB oncoprotein addiction in different cancer types. In addition, we have highlighted experimental technologies for the development of innovative cancer cell models that accurately predicted clinical ErbB drug efficacies. In the future, such cancer models might facilitate the identification and validation of physiologically relevant novel forms of oncoprotein and non-oncoprotein addiction or synthetic lethality. The identification of genotype-drug response relationships will further advance personalized oncology and improve drug efficacy in the clinic. Finally, we review the most important drugs targeting ErbB family members that are under investigation in clinical trials or that made their way already into clinical routine. Taken together, the functional characterization of ErbB oncoproteins have significantly increased our knowledge on predictive biomarkers, oncoprotein addiction and patient stratification and treatment. View Full-Text
Keywords: ErbB family; oncogene addiction; synthetic lethality; drug discovery; tumour modeling; 3D cell culture; personalized medicine; precision therapyErbB family; oncogene addiction; synthetic lethality; drug discovery; tumour modeling; 3D cell culture; personalized medicine; precision therapy►▼ Figures
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Jacobi, N.; Seeboeck, R.; Hofmann, E.; Eger, A. ErbB Family Signalling: A Paradigm for Oncogene Addiction and Personalized Oncology. Cancers2017, 9, 33.
Jacobi N, Seeboeck R, Hofmann E, Eger A. ErbB Family Signalling: A Paradigm for Oncogene Addiction and Personalized Oncology. Cancers. 2017; 9(4):33.Chicago/Turabian Style
Jacobi, Nico; Seeboeck, Rita; Hofmann, Elisabeth; Eger, Andreas. 2017. "ErbB Family Signalling: A Paradigm for Oncogene Addiction and Personalized Oncology." Cancers 9, no. 4: 33.
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1. Signal Transduction of ErbB Receptor Tyrosine Kinases
The ErbB receptor tyrosine kinase family consists of four cell surface receptors, ErbB1/EGFR/HER1, ErbB2/HER2, ErbB3/HER3 and ErbB4/HER4 . Under normal physiological conditions, ErbB receptor activation is controlled by spatial and temporal expression of their ligands . Extracellular binding of ligands to their cognate receptors induces the formation of active homo- or heterodimers . Seven growth factors are known to bind EGFR i.e., epidermal growth factor (EGF), epigen (EPG), transforming growth factor alpha (TGFA), amphiregulin (AREG), betacellulin (BTC), heparin binding epidermal growth factor (HB-EGF), and epiregulin (EPR). Two ligands selectively bind to ErbB3, Neuregulin (Nrg 1 and 2) and seven ligands interact with ErbB4 (BTC, HB-EGF, EPR, Nrg1-4). ErbB2 lacks a ligand binding domain and can be activated by heterodimerization with other ErbB proteins . In addition, there are numerous reports showing that ErbB2 can also homodimerize and function as an active receptor and oncogenic driver when being a homodimer [5,6,7]. ErbB3 contains no functional kinase domain, but rather displays several tyrosine phosphorylation sites that provide binding sites for signalling proteins that mediate activation of downstream effector molecules such as the Akt/PKB pathway . In general, ligand-induced receptor homo- or heterodimerization triggers cross-autophosphorylation and the assembly of diverse signalling molecules at the sites of the receptor dimer and activation of downstream effector circuits [1,9]. Downstream signalling networks controlled by ErbB activation consist of several interconnected and overlapping modules [1,2,4,10]. The ErbB effectors include the PI3K-Akt-mTOR pathway, the RAS-RAF-MEK-ERK pathway and the phospholipase C gamma (PLCγ) pathway . These signalling cascades regulate a vast variety of physiological events including cell proliferation, apoptosis, angiogenesis, cell adhesion and motility, embryonic development, and organogenesis [10,11,12]. Particularly, EGFR and ErbB2 proteins hyperactivate theses pathways in a broad range of cancers [13,14,15,16]. The evolutionary conserved PI3K-Akt-mTOR cascade is strongly activated by ErbB3 and regulates survival, growth, and proliferation [8,17]. mTOR is crucial for the development of different human malignancies such as brain, breast, colon, liver, lung, ovary and stomach cancer . Akt phosphorylates and inhibits the tumour suppressor TSC2 and thereby indirectly activates RHEB, which in turn is a positive regulator of mTOR . mTOR stimulates in a nutrient and energy-sensing manner the canonical mRNA translation via activation of S6 kinase 1 and suppression of 4E-BP1 . However, there is accumulating evidence that mTOR is also activated independently from ErbB receptors . The importance of mTOR for tumour cell growth is now widely accepted and several agents are available or under investigation that selectively target mTOR . Examples of such drugs are the rapamycin derivatives or analogs (rapalogs) RAD001, BEZ235, CCI-779 and INK-128, which are putative candidates for combination therapies with ErbB receptor inhibitors .
A second major signalling pathway induced by ErbB family members is the RAS-MAP-kinase pathway involving RAF, MEK and ERK. The pathway contributes to cell survival, cell growth and proliferation [22,23,24]. In this regard, the small GTPase RAS acts as a central signalling node that can activate many different downstream effector proteins [25,26,27]. RAS and particularly the subtype KRAS was one of the first molecular biomarkers used for predictive diagnostics in personalized oncology. The clinical need for KRAS mutation testing is largely associated with the use of anti-EGFR antibody therapy for patients with advanced colorectal or lung cancer . There is a high frequency of KRAS mutations in colon tumours that can cause resistance to EGFR inhibitors . Mutations in KRAS mainly occur in codons 12 and 13 impairing the intrinsic GTPase activity of KRAS. As a result, KRAS gets locked in a GTP-bound, active state, and constitutively triggers downstream signalling events, irrespective of upstream EGFR activity [30,31,32]. MAPK signalling results in the activation of the dimeric AP1 transcription factor composed of c-JUN and c-FOS. AP1 promotes tumourigenesis at different levels including epithelial to mesenchymal transition and cancer cell invasion and metastasis, angiogenesis, and cell proliferation and survival [33,34,35,36,37,38,39]. Furthermore, ErbB signalling can activate various other effector molecules such as PLCγ, STATs and SRC [9,40,41,42,43,44,45].
The proto-oncogene c-MET is another tyrosine kinase receptor acting independently but redundantly to ErbB family members. It is activated by hepatocyte growth factor/Scatter factor (HGF) and, similar to ErbB signalling, activates PI3K, MAP-kinase and STAT signalling [46,47,48,49,50]. The diverse array of intracellular signalling networks initiated by ErbB proteins is driving tumour progression in almost all solid cancers in humans. Hence, a new generation of drugs that selectively target the ErbB oncoproteins has demonstrated impressive therapeutic efficacy in the clinic [1,3,51].
2. ErbB Proteins and Oncogene Addiction
A decade has elapsed since the concept of oncogene addiction has first emerged . It postulates, that despite the vast number of genetic and epigenetic changes in cancer cells, some tumours rely on the activity of a single dominant oncogene for growth and survival. Inhibition of the hyperactive oncoprotein is sufficient to halt the neoplastic growth, and cause differentiation or death of cancer cells . Hence, oncogene addiction is generally considered to be the Achilles’ heel of cancer. In addition to oncogenic drivers, tumour cells may also evolve a dependency on cellular switches that are interconnected with oncogenic drivers, but do not work as oncoproteins on their own, irrespective of their mutational status. This phenomenon is commonly known as non-oncogene addiction [54,55]. Finding and targeting the critical driver molecules is a primary goal of present precision medicine [54,56,57,58]. Successful targeting and inactivation of specific driver proteins would cause a systemic failure in tumour cell physiology. However, in many cases the inactivation of a single oncoprotein is not sufficient to kill the cancer cells. Here, synthetic lethality provides a conceptual framework for the development of cancer-specific cytotoxic agents. Two genes are synthetic lethal if the deficiency of either alone is compatible with viability but the deficiencies of both leads to death. So, targeting a gene that is synthetic lethal should primarily kill cancer cells and spare the normal tissues [58,59,60].
In several tumour entities, ErbB family members have been found to be essential for cancer cell proliferation and survival [51,61,62,63,64,65]. The tumours exhibited unique expression and mutation profiles of ErbB genes, all of which had a specific impact on cancer cell differentiation, proliferation, migration, and survival. In many cases the survival of the cancer cells was strictly dependent on the mutant or overexpressed ErbB family member. The inhibition of specific ErbB proteins with low molecular weight tyrosine kinase inhibitors (TKI) or antibodies was often sufficient to cause cancer cell death [66,67]. On the contrary, certain mutations of ErbB proteins conferred resistance to treatment [48,54,68,69,70,71,72,73].
EGFR has four mutational hotspots within its tyrosine kinase domain in exons 18, 19, 20 and 21. As best described in lung cancer, single nucleotide mutations as well as deletions and insertions are associated with increased drug sensitivity (i.e., G719m, E709m, L861Q, L858R, exon 19 deletions and/or insertions) . Specific mutations elevate EGFR activity, often by increasing the binding affinities for dimerization or ATP interaction . In such cases, the treatments with selective drugs such as gefitinib and erlotinib have shown impressive clinical efficacies [76,77,78,79]. On the other hand, the most important alteration that has been frequently associated with acquired resistance is the point mutation T790M in exon 20 [68,72,73,80]. It occurs first and foremost in advanced tumours that have lost normal regulatory feedback circuits and use mutated EGFR to constitutively drive proliferation and survival . Effective targeting of T790M mutated EGFR has been a research challenge of the last decade. With the development of osimertinib, a powerful precision drug is now available . A doubling of progression free survival time from 4.2 to 8.2 months could be achieved in lung cancer patients positive for the T790M mutation [81,82]. Also mutations in the ectodomain of the EGFR can have a strong impact on cancer progression. In glioblastoma multiforme a characteristic deletion of 267 amino acids is often detected in the extracellular domain (EGFRvIII) . The mutated receptor is unable to bind to ligand and yet constitutively activates mitogenic, anti-apoptotic and pro-invasive signalling pathways. The deletion also alters internalisation and degradation of the EGFR. The lack of expression of EGFRvIII in normal tissue makes it a first-rate drug target for precision medicine .
ErbB2 has been intensively studied in breast cancer and is known to be overexpressed in other cancer types as well, including urinary bladder, lung, digestive tract, endometrial and cervical cancer . The ErbB2 oncogene is located on chromosome 17q12. Gene amplification is the primary mode of ErbB2 receptor overexpression and is a major driver of tumour development and progression in a subset of breast cancers. An ErbB2 amplification occurs in about 15–20% of breast cancers [15,84,85]. Metastatic ErbB2-positive breast cancer correlates with increased aggressiveness, poor prognosis, and short overall survival time . The overexpressed receptor represents a paradigm for oncoprotein addiction, and with that a valuable predictive biomarker and therapeutic target. Current American Society of Clinical Oncology (ASCO) guidelines mandate that the ErbB status is evaluated in every invasive breast cancer to select the appropriate therapy, either at the time of diagnosis or recurrence [87,88]. Breast tumours overexpressing ErbB2 show a significant response to ErbB2 targeting agents such as trastuzumab or lapatinib [89,90,91,92]. However, in contrast to breast cancer, the treatment of ErbB2 overexpressing tumours of the endometrium and stomach did not yield comparable positive results [93,94]. Furthermore, activating mutations of ErbB2 were identified few years ago [95,96]. Immunohistochemistry and FISH demonstrated that most cancer cells containing such mutations were not overexpressing the ErbB2 protein. The afflicted cancer cells exhibited different sensitivities towards anti-cancer agents [95,96]. These findings have a profound impact on the clinical management of cancer. Tumours found to harbour ErbB2 mutations may display addiction to ErbB2 signalling and sensitivity towards ErbB2 tyrosine kinase inhibitors . If the correlation between ErbB2 mutations and drug responsiveness is confirmed in prospective clinical trials the screening for ErbB2 mutations in breast tumours will be mandatory before starting the therapy.
In the last years, ErbB3 signalling has gained considerable attention in cancer research. ErbB3 predominantly forms a heterodimer with ErbB2 and was found to be critically involved in tumour initiation and progression and is now considered one of the most active signalling dimers of the ErbB family in cancer [98,99,100]. Consistent with the findings of other groups, we could recently identify two distinct breast cancer populations that expressed either high (~60% of tumours) or low (~40% of tumours) levels of ErbB3. Interestingly, the highest ErbB3 expression was detected in ErbB2-positive specimen. Increased co-expression of ErbB2 and ErbB3 might critically influence oncogenic signalling, oncogene addiction and thereby responsiveness to TKIs . Somatic mutations in the ErbB3 gene occur in approximately 10% of colon and gastric cancers and increased activity may also be involved in melanoma formation [17,101]. Inhibition of ErbB3 with low molecular weight inhibitors is difficult enterprise, as ErbB3 is missing the kinase domain and TKIs are therefore non-effective. Alternatively, various ErbB3 targeting antibodies are currently under investigation which might block heterodimerization with other ErbB proteins [101,102].
The involvement of ErbB4 in carcinogenesis has been less well addressed so far [103,104]. Williams et al. showed that this gene is overexpressed in colon cancer and postulated that it might promote carcinogenesis . Mutations of the ErbB4 gene are not very frequent but some activating alterations were described in the kinase (D931Y and K935I) as well as extracellular (Y285C and D595V) domain and were suggested as putative drug targets .
Taken together, many cases of oncoprotein addiction of the ErbB family have been identified in the last decades. These were instrumental for the development of targeted cancer therapies and useful as predictive biomarkers in the clinic. However, cancer is a complex and multi-faceted disease and acquired drug resistance as well as non-oncogene addiction and synthetic lethality suggest that combination therapies might be the most effective remedies in the future [107,108,109,110,111,112,113,114,115,116]. For example, it has been shown that c-MET can compensate for the loss of EGFR signalling [49,117]. Hence, the effective inhibition of both receptors by combination therapies might overcome the shortcomings of single treatments [118