Epidermal Growth Factor Receptor Signaling

Historically, the growth factors that stimulate RTKs were discovered first, followed by the structural and functional characterization of the RTKs themselves.9,10 Epidermal growth factor (EGF) was initially purified from mouse submaxillary glands in 1962 by Stanley Cohen and was found to stimulate premature eyelid opening and incisor eruption, phenotypes that suggested a role for EGF in the regulation of cellular proliferation.¹¹ In 1978, the epidermal growth factor receptor (EGFR) was identified as the cell surface binding site for EGF.¹² During the next 2 years, tyrosine phosphorylation was identified in cells, followed by the discovery that the viral Src oncogene, which induces transformation of cells in vitro, is itself a tyrosine kinase, underscoring the potential importance of tyrosine phosphorylation for oncogenesis.^13,14 Once the complete sequence of the EGFR protein was elucidated in the 1980s,¹⁵ the amino acid sequence of the receptor cytoplasmic domain was found to be similar to Src, suggesting that EGFR also possessed tyrosine phosphorylation activity. The connection between RTK activation and oncogenesis was further solidified when the amino acid sequence of EGFR was found to be homologous to the avian erythroblastosis virus erbB oncogene, which, when infected into chicken red blood cell precursors, is sufficient to induce erythroleukemia.^16,17 The erbB oncogene encodes a transmembrane protein that lacks the extracellular ligand binding domain of EGFR but possesses a cytoplasmic kinase domain that, when expressed in cells, can signal in a growth factor–independent manner. Subsequent studies have since identified numerous alterations of EGFR and other RTKs within human cancers that enhance proliferation without the need for growth factor stimulation (see specific examples in the second half of each RTK subsection).

The EGFR class of RTKs comprises four receptor proteins encoded by four genes (in parentheses): EGFR (ERBB1), HER2/neu (ERBB2), HER3 (ERBB3), and HER4 (ERBB4). EGFR binds to and is activated by a number of ligands, including EGF, transforming growth factor (TGF)–α, heparin-binding (HB)-EGF, amphiregulin, betacellulin, epiregulin, and epigen.18,19 Growth factor binding promotes either homodimerization or heterodimerization with other human epidermal growth factor receptor (HER) family members, followed by transphosphorylation.²⁰ A ligand for HER2 has not yet been identified; instead, HER2 is activated through heterodimer formation with one of the other three ligand-bound receptors.²¹ Notably, HER2 is the preferred dimerization partner for EGFR, and EGFR-HER2 heterodimers are more stable than EGFR homodimers, remaining at the cell surface for a longer duration and undergoing endocytosis at a lower rate than EGFR homodimers.^22,23 Furthermore, HER2 reduces the dissociation rate of EGF from EGFR, allowing for a more sustained period of EGF-induced signaling.¹⁸ The EGFR and HER2 components of the EGFR-HER2 heterodimer are also more likely to be recycled back to the cell surface than are EGFR homodimers, which are more readily targeted for degradation.²⁴ Additionally, HER2-HER3 heterodimers possess the most potent mitogenic activity among the heterodimer and homodimer HER kinase combinations.²⁵ In contrast to the other HER kinase family members, HER3 does not have intrinsic kinase activity and preferentially forms heterodimers with HER2.²⁶ The ligands for HER3 and HER4 are the neuregulins, including heregulin.

A number of tumor types frequently exhibit alterations within the EGFR family of RTKs. Sustained activation of these pathways can result in oncogene or pathway-addicted tumors, and selective HER kinase inhibitors are now a component of the standard treatment of several malignancies. Alterations within RTKs include the following: activating mutations that result in constitutive activation of the tyrosine kinase; overexpression of the receptor, often because of gene amplification; and elevated levels of RTK ligands that stimulate signaling. EGFR mutations are found in 10% to 50% of non–small cell lung cancers depending on geographic location, with kinase domain deletions (exon 19) and mutations (exon 21) comprising more than 80% of these alterations.27,28 Overexpression of wild-type EGFR as a result of gene amplification has been observed in non–small cell lung, breast, gastric, colorectal, and head and neck cancers.^29,30 Up to 30% of breast cancers display overexpression of HER2, which is an unfavorable prognostic factor, and therapy for these ERBB2-amplified breast cancers is now distinct from that of breast cancers with normal HER2 expression levels.³¹ In glioblastoma multiforme, the extracellular domain of EGFR is frequently deleted, resulting in the expression of a truncated mutant protein (EGFRvIII).^32,33

Numerous targeted agents have been developed that selectively inhibit EGFR-induced signaling (Table 2-1 and Fig. 2-2). These agents include gefitinib and erlotinib, both of which are used for the treatment of non–small cell lung cancers (NSCLCs).³⁴ These agents are particularly effective in patients with lung cancer whose tumors harbor activating EGFR kinase mutations. Cetuximab, a chimeric monoclonal antibody that binds to the extracellular domain of EGFR and competitively inhibits ligand binding, thereby preventing receptor activation, is approved for the treatment of colorectal and head and neck cancers.^37–37 Trastuzumab, a humanized antibody that binds to the extracellular domain of HER2, has been approved by the U.S. Food and Drug Administration (FDA) for the treatment of breast cancers that display HER2 overexpression. Trastuzumab has been shown to be effective both as single-agent therapy and when used in combination with chemotherapy.^38,39 ERBB2 amplification strongly correlates with HER2 protein overexpression, and the presence of either marker predicts for trastuzumab response.⁴⁰ Although the introduction of trastuzumab has resulted in a significant improvement in the survival of patients with HER2-overexpressing breast cancers, drug resistance remains a major clinical problem. Potential resistance mechanisms include concomitant overexpression of other HER kinase family members and/or ligands, phosphatase and tensin homolog (PTEN) loss, and the expression of a truncated HER2 protein lacking the extracellular antibody binding site.⁴¹ Additional HER2-directed agents include lapatinib, a selective tyrosine kinase inhibitor (TKI), and pertuzumab, which binds to a different HER2 epitope (the dimerization domain) than does trastuzumab, resulting in impaired dimer formation.^42,43 Lapatinib is approved by the FDA for use in combination with capecitabine in patients with HER2-overexpressing advanced or metastatic breast cancer who have progressed while undergoing prior therapy with trastuzumab and certain classes of chemotherapy.⁴² Lapatinib also received accelerated approval for use in combination with the aromatase inhibitor letrozole.⁴⁴ Consistent with its activity in patients with breast cancer, the addition of trastuzumab to chemotherapy is associated with improved overall survival compared with chemotherapy alone in patients with HER2-overexpressing esophagogastric cancer.⁴⁵

Insulin and Insulin-Like Growth Factor–1 Receptor Signaling

The insulin and insulin-like growth factor–1 (IGF1) receptor family is dysregulated in multiple malignancies.⁴⁶ The insulin receptor exists as two isoforms encoded by splice variants of the same gene.⁴⁷ Each isoform can dimerize with the other (forming hybrid dimers) or with itself.⁴⁸ The IGF1 receptor (IGF1R) can dimerize with either of the insulin receptor isoforms or with itself, resulting in six different dimer combinations.⁴⁸ The insulin receptor is stimulated by insulin or insulin-like growth factor–2 (IGF2), whereas IGF1R can be activated by either IGF1 or IGF2. Both of these latter ligands can stimulate IGF1R in an autocrine fashion or can be elaborated from distant sites.^49,50 Circulating IGF binding proteins have a similar affinity for IGF1 and IGF2 as IGF1R does and therefore compete for binding to both ligands, thus titrating the amount of free ligand available for IGF1R stimulation.⁵¹ IGF binding protein proteases provide an additional mechanism for controlling ligand levels by increasing the half-life of free ligand available for receptor binding.⁵²

After ligand binding, IGF1R dimerizes and undergoes transphosphorylation, leading to activation of downstream signaling pathways, including both the RAS-RAF-mitogen-activated protein kinase (MAPK) and the PI3K-AKT-mammalian target of rapamycin (mTOR) cascades (see individual sections later in the chapter) (Figs. 2-1 and 2-2).⁵³ Specifically, insulin receptor substrate–1 (IRS1) binds to a phosphotyrosine motif on IGF1R via its SH2 domains and is phosphorylated by IGF1R.⁵⁴ It subsequently recruits PI3K to the plasma membrane, which converts phosphatidylinositol-4,5-bisphosphate (PIP₂) to phosphatidylinositol-3,4,5-triphosphate (PIP₃), which results in AKT and mTOR pathway activation.

Activating mutations of IGF1R do not appear to be common in human cancer. However, amplification of the IGF1R gene locus has been identified in some colon, pancreatic, and lung cancers. Sarcomas often have either increased expression of the IGF1 and IGF2 ligands or decreased insulin-like growth factor binding protein–3 expression (Ewing sarcoma), which results in increased IGF1 levels in the tumor microenvironment.⁵⁵ Gastrointestinal stromal tumors (GISTs) lacking KIT and platelet-derived growth factor receptor (PDGFR) mutations also commonly harbor IGF1R amplification.⁵⁶ AMG479, a monoclonal human antibody targeting IGF1R, has shown promising antitumor activity in patients with Ewing sarcoma.⁵⁵ Phase 2 studies of anti-IGF1R antibodies are also ongoing in multiple sarcoma subtypes. IGF1R overexpression has also been observed in up to 44% of breast tumors and may mediate resistance to HER2-directed therapies.⁵⁷ Clinical trials of IGF1R targeting antibodies in combination with hormonal and anti-HER2 therapy are ongoing.

The insulin receptor family members anaplastic lymphoma kinase (ALK) and ROS1 were also recently implicated in tumorigenesis.58,59 Chromosomal translocations involving ALK and multiple 5′ fusion partners have been identified.⁵⁹ In NSCLC, the EML4 gene is the preferred translocation partner, resulting in the expression of an EML4-ALK fusion protein in 2% to 5% of patients.⁶⁰ Patients with NSCLC who have EML4-ALK fusions often exhibit dramatic radiographic responses to crizotinib, an inhibitor of the ALK, ROS1, and MET tyrosine kinases (Table 2-1).⁶¹ Notably, EML-ALK fusions are found in a mutually exclusive pattern with EGFR kinase domain mutations, suggesting that they have overlapping phenotypic effects. ROS1 gene rearrangements are also found in a small minority of patients with NSCLC.^62,63 Crizotinib has shown promising results both in preclinical models of ROS1-fusion lung cancer and in the clinic.^62,63

Platelet-Derived Growth Factor Receptor Signaling

Platelet-derived growth factor (PDGF) is the ligand for PDGFRs, which stimulate the proliferation and migration of mesenchymal cells, such as oligodendrocyte precursors, vascular smooth muscle cells, and pericytes during embryonic development.⁶⁴ PDGF signaling is also implicated in organ development, including lung and intestinal epithelial folding and glomerular capillary tuft formation. Furthermore, PDGFs promote angiogenesis, wound healing, and erythropoiesis.⁶⁵ Aberrations in the PDGFR pathway result in uncontrolled proliferation and enhanced angiogenesis.

Four isoforms of PDGF have been identified: PDGF-A, -B, -C, and -D.⁶⁶ These isoforms are activated by proteolytic cleavage and assemble into five homodimeric or heterodimeric combinations that bind to and stimulate either PDGFR-α or PDGFR-β. PDGFR-α homodimers inhibit chemotaxis, whereas PDGFR-β homodimers and α/β heterodimers stimulate chemotaxis within fibroblasts and smooth muscle cells.⁶⁷ After dimerization and transphosphorylation, PDGFRs activate signal transduction pathways through recruitment of adaptor proteins containing SH2 domains, most notably the GRB2 protein that in turn binds the guanine nucleotide exchange factor SOS, which subsequently activates RAS.^68,69 Additionally, phosphorylated tyrosine residues also serve as docking sites for kinases containing SH2 domains, including PI3K, phospholipase C-gamma, and SRC, as well as the tyrosine phosphatase SH-containing phosphatase 2 (SHP2) and the signal transducer and activator of transcription (STAT) transcription factor family.⁶⁹

Alterations in PDGFR signaling in cancer include excess autocrine secretion of PDGF (e.g., glioblastoma and sarcomas), gain-of-function mutations that cause constitutive tyrosine kinase activation (e.g., gastrointestinal stromal tumors),⁷⁰ translocation of either the PDGF or PDGFR genes (e.g., dermatofibrosarcoma protuberans, chronic myelomonocytic leukemia, and hypereosinophilic syndrome),^73–73 and PDGFR gene amplification (glioblastoma).⁷⁴ PDGFR-α mutations are found in approximately 10% of KIT wild-type GISTs. The D842V mutation comprises approximately two thirds of PDGFR-α activating mutations and is resistant to inhibition by imatinib, a TKI of KIT, BCR-ABL, and PDGFRs; however, the remaining mutations are sensitive to imatinib therapy.⁶⁷ Dermatofibrosarcoma protuberans is a rare, low-grade cutaneous sarcoma that harbors a chromosome 17;22 translocation that fuses portions of the COL1A1 (collagen 1A1) gene and PDGFB, resulting in overexpression of PDGF-β and subsequent stimulation of PDGFR signaling.⁶⁷ Imatinib has shown significant benefit in patients with recurrent or metastatic dermatofibrosarcoma protuberans and is approved by the FDA for this indication (Table 2-1).⁷⁵

Imatinib is also approved by the FDA for use in patients with KIT mutant GISTs⁷⁶ and Philadelphia chromosome–positive hematologic cancers (see the SRC/BCR-ABL sections that follow). The KIT gene was first identified as the human homolog of the viral oncogene v-Kit responsible for a subtype of feline sarcoma. The KIT RTK is a member of the type III RTK family that includes PDGFR and Fms-like tyrosine kinase–3 (FLT3) (see following paragraph).^77,78 Stem cell factor (SCF) is the ligand for KIT.^79–82 Hot-spot mutations in exons 9 and 11 of KIT have been identified in several tumor types, including GISTs and melanomas.^82–86 Patients with GISTs often have dramatic and durable responses to the kinase inhibitors imatinib and sunitinib (Table 2-1).^76,87

The FLT3 receptor, a third member of the RTK class that includes PDGFR and KIT, is involved in the development of normal hematopoietic cells. It contains an extracellular region composed of five immunoglobulin domains, transmembrane and juxtamembrane domains, and two cytoplasmic tyrosine kinase domains.⁸⁸ FLT3 alterations are common in hematopoietic malignancies. Specifically, internal tandem duplication within exons 14 and 15 of the FLT3 gene is found in up to one third of acute myelogenous leukemias (AMLs).⁸⁹ This duplication likely results in ligand-independent activation of FLT3 and is associated with a poor prognosis in patients with AML. The clinical activity of FLT3 inhibitors has been modest to date, although responses appear to be more common in patients with FLT3/internal tandem duplication AML (Table 2-1).⁸⁹ Interestingly, resistance to FLT3 inhibition in such patients is associated with selection for secondary mutations within the tyrosine kinase domain of FLT3, suggesting a central role of FLT3 in AML pathogenesis.⁸⁹

Fibroblast Growth Factor Receptor Signaling

Fibroblast growth factor receptors (FGFRs) comprise a family of RTKs that regulate cell proliferation, differentiation, and migration, along with selective apoptosis during embryogenesis. Germline mutations of the FGFR genes are the basis of a spectrum of skeletal developmental disorders that are thought to derive from premature differentiation and growth restriction of chondrocytes resulting from dysregulated FGFR pathway activation.⁹⁰ The FGFRs are composed of an extracellular ligand-binding domain, a hydrophobic transmembrane region, and an intracellular tyrosine kinase domain.⁹¹ The extracellular domain is organized into three immunoglobulin (Ig) domains; differential splicing of the second half of the third Ig domain dictates tissue-specific expression of the receptor.

Fibroblast growth factors (FGFs) bind to the extracellular domain of the FGFRs in combination with specific heparan sulfate glycosaminoglycans, inducing FGFR dimerization and transphosphorylation of intracellular tyrosine residues. Eighteen FGF ligands have been identified, and specificity for FGF receptors is based on numerous factors, including tissue-specific FGF ligand and receptor expression, the presence of cell surface molecules that facilitate the interaction between individual FGF ligands and receptors, and the differential binding capability of the ligands themselves for specific FGF receptors.⁹² Subsequent stimulation of the tyrosine kinase domain leads to phosphorylation and activation of multiple downstream signaling proteins in the same manner as previously described for other RTKs. Unique to the FGFR signaling complex is FGFR substrate 2 (FRS2), an adaptor protein that binds to specific phosphotyrosines on the intracellular domain of active FGFR dimers.⁹³ FRS2 is itself phosphorylated by FGFRs and serves as a docking site for the Grb2-Sos adaptor complex that activates the RAS/RAF/MAPK pathway. Phosphorylated FRS2 also recruits GRB2-associated binding protein–1 (GAB1), which activates PI3K. Additionally, phospholipase C–γ binds to phosphorylated FGFR dimers via an SH2 domain, leading to its activation and the cleavage of PIP₂ to form inositol 1,4,5-triphosphate (IP₃) and diacylglycerol (DAG).

FGFR signaling is dysregulated in cancer by multiple mechanisms, including mutational or translocation-induced activation of FGFRs, gene amplification of receptors, and autocrine or paracrine secretion of ligands by tumors. For example, activating mutations within FGFR3 result in constitutive receptor dimerization and subsequent signaling. FGFR3 mutations occur in up to 70% of nonmuscle invasive bladder cancers and in 15% of patients with advanced urothelial tumors.⁹³ Unlike EGFR activating mutations, which predominantly affect the tyrosine kinase domain of the receptor, FGFR3 mutations are commonly located within the extracellular domain and promote ligand-independent receptor dimerization through formation of an aberrant disulfide bridge between two receptor monomers.⁹⁴ Up to 15% of multiple myelomas harbor an intergenic 4;14 translocation between the FGFR3 gene and the immunoglobulin heavy chain locus, which places FGFR3 expression under the highly active heavy chain promoter.⁹⁵ Approximately 10% of diffuse-type gastric cancers display FGFR2 gene amplification, and cell lines with this amplification show ligand-independent pathway activation and sensitivity to selective FGFR inhibitors.⁹⁶ Autocrine and paracrine FGF ligand secretion with resultant pathway activation has been reported to occur in a subset of melanomas and prostate cancers, respectively.^97,98

Multiple FGFR inhibitors are currently being tested in early-phase clinical trials, but most of these compounds are multitargeted TKIs, many of which also potently inhibit members of the vascular endothelial growth factor receptor (VEGFR) and PDGFR families. The close structural similarity between these RTKs has made development of FGFR selective inhibitors challenging, although several such drugs are now entering the clinic.⁹³ Because FGFR3 is a cell surface receptor, it may also be susceptible to monoclonal antibody–mediated inhibition, similar to trastuzumab-mediated inhibition of HER2. To date, anti-FGFR antibodies have not entered clinical testing, although preclinical studies have shown promising antitumor effects in urothelial cancer (both FGFR3 wild-type and mutant) and t(4;14) expressing multiple myeloma cell lines.⁹⁹

RET Signaling

The RET (rearranged during transfection) protein is a membrane-bound RTK with a role in the normal development of the kidney and the enteric nervous system.100,101 RET is expressed predominantly on the surface of neural crest tissues, and glial-derived neurotrophic factors (GDNFs) serve as ligands for RET. GDNFs initially bind to GDNF receptors on the cell surface followed by binding of this complex to RET, RET dimer formation, and subsequent transphosphorylation of tyrosine residues.¹⁰² Tyrosine residues at positions 752 and 928 serve as a docking site for the STAT3 transcription factor. Other phosphorylated residues are recognized by SRC, resulting in activation of focal adhesion kinase, which promotes cell migration and metastatic spread. Additionally, the MAPK, PI3K/AKT, and phospholipase C–γ pathways can be activated by RET to promote cellular proliferation and survival.¹⁰²

Germline RET mutations are the basis for the multiple endocrine neoplasia type 2 syndromes. In patients with multiple endocrine neoplasia type 2 syndrome, familial medullary thyroid carcinomas and other cancers develop.¹⁰³ Sporadic medullary thyroid carcinomas are much more common, and up to 60% of such tumors harbor somatic mutations in RET that are thought to be a driver alteration in this disease. RET inhibitors have shown significant antitumor activity in patients with medullary thyroid cancer, and vandetanib, an oral inhibitor of RET, EGFR, and VEGFR, was recently approved by the FDA for the treatment of patients with advanced medullary thyroid cancer (Table 2-1).¹⁰⁴ A randomized, placebo-controlled, phase 3 study of cabozantinib, an oral, multitargeted TKI that inhibits RET, VEGFR2, and MNNG HOS transforming gene (MET), was also recently conducted in patients with unresectable, locally advanced, or metastatic medullary thyroid carcinoma.¹⁰⁵ This trial documented a statistically significant improvement in median progression-free survival with cabozantinib compared with placebo (11.2 months vs. 4.2 months in the placebo arm, P < .0001).

Vascular Endothelial Growth Factor Signaling

Six vascular endothelial growth factor (VEGF) ligands have been identified, VEGF-A, -B, -C, and -D, along with placental growth factor 1 and 2.¹⁰⁶ VEGF-A has four isoforms produced by alternative gene splicing, with the 165 amino acid length isoform playing a central role in tumor angiogenesis.¹⁰⁷ Specifically, VEGF-A enhances vascular permeability and stimulates endothelial cell proliferation, resulting in new blood vessel formation. VEGFRs are receptor tyrosine kinases that possess a modular structure consisting of an extracellular domain with seven immunoglobulin-like regions, a transmembrane domain, and an intracellular tyrosine kinase domain.¹⁰⁶ VEGF-A, VEGF-B, and placental growth factor all bind VEGFR1, but the exact role of VEGFR1 in tumor angiogenesis has yet to be fully elucidated. In some settings it may act as a decoy receptor that prevents ligand-mediated stimulation of VEGFR2.¹⁰⁸ VEGFR2 has been implicated in the development of vasculature and is considered the primary receptor through which VEGF exerts its angiogenic effects.¹⁰⁸ Binding of ligand to VEGFR2 results in receptor dimerization and transphosphorylation followed by activation of multiple signal transduction cascades.¹⁰⁹

Targeted therapies that inhibit VEGF signaling include antibodies that bind circulating ligand and receptor TKIs (Table 2-1). The humanized monoclonal antibody bevacizumab binds to free VEGF, thereby preventing its association with VEGF receptors. This antibody has been approved by the FDA for use in combination with chemotherapy for patients with metastatic colorectal¹¹⁰ and nonsquamous NSCLCs.^106,111 Bevacizumab also has activity in patients with glioblastoma¹¹² and metastatic renal cell carcinoma, in whom it is often used in combination with interferon (IFN)-α.¹¹³

Sorafenib and sunitinib are multitargeted TKIs with nanomolar potency for VEGFR2 (Table 2-1). Sunitinib is used in the treatment of patients with metastatic renal cell carcinoma, GISTs, and pancreatic neuroendocrine tumors.¹¹⁴ Sorafenib has been approved for the treatment of liver and renal cell cancers.^115,116 Although these agents inhibit multiple kinases, their antitumor effects have been attributed primarily to their antiangiogenic activity. More recently, the TKI pazopanib was approved for the initial treatment of metastatic renal cell carcinoma and in cytokine-pretreated patients,¹¹⁷ and axitinib¹¹⁸ was approved in the second-line setting after failure of prior systemic therapy.

Despite widespread activity in preclinical models, antiangiogenic therapies have shown disappointing activity in several tumor types. A number of resistance mechanisms have been hypothesized to explain the lack of broader clinical activity, including the activation of redundant signaling pathways that promote angiogenesis, the recruitment by tumors of bone marrow–derived endothelial progenitor cells, increased pericyte density around existing blood vessels that enhances vascular growth and survival, and the ability of tumor cells to invade surrounding stroma to co-opt additional blood supply.¹¹⁹ A better understanding of these resistance mechanisms may lead to the development of more effective antiangiogenic therapies in the future.

Hepatocyte Growth Factor Receptor Signaling

The hepatocyte growth factor receptor (HGFR or MET) is encoded by the MET gene.¹²⁰ The MET extracellular domain consists of an α subunit connected by a disulfide bridge to a β subunit that spans the cell membrane.¹²¹ The intracellular portion of the receptor contains a juxtamembrane region that harbors a serine residue that inhibits receptor tyrosine kinase activity upon phosphorylation, as well as a tyrosine kinase domain. A tyrosine residue at position 1003, proximal to the tyrosine kinase domain, serves as an interaction site for the ubiquitin ligase CBL, which marks the receptor for endocytosis and degradation.¹²² Tyrosine residues C-terminal to the kinase domain represent docking sites for adaptor proteins involved in downstream signaling. Upon binding of hepatocyte growth factor (HGF) to the extracellular portion of MET, receptor dimerization occurs, followed by transphosphorylation. A number of adaptor proteins then bind to phosphorylated tyrosine residues, including GRB2 and GAB1, which promotes the activation of the MAPK and PI3K/AKT signaling pathways.¹²³ MET can also activate RAC1/CDC42 and p21-activated kinase, both of which regulate cytoskeletal proteins and integrin expression and activation and thus cell migration.¹²³

Dysregulation of MET signaling can occur through multiple mechanisms, including receptor overexpression, upregulation of HGF, which can activate MET in an autocrine and/or paracrine manner, and tyrosine kinase domain mutations.¹²⁴ Several receptor tyrosine kinases have also been shown to activate MET, including EGFR, HER2, and IGF1R. For example, EGFR activation can stimulate MET signaling, and resistance to EGFR inhibitors in some lung cancers has been shown to stem from co-activation of MET in the setting of gene amplification.¹²⁵ Germline mutations of MET are found in patients with hereditary papillary renal cell carcinomas, and MET overexpression is observed in a significant proportion of sporadic papillary cancers, as well as collecting duct carcinomas.¹²⁶ Multiple other malignancies exhibit aberrations in MET signaling including lung, breast, pancreatic, colon, and gastric cancers. Amplification of MET is associated with a worse prognosis in lung and gastric cancers, and expression of MET and HGF are unfavorable prognostic biomarkers in liver, kidney, colorectal, and gastric cancers.¹²⁷

Inhibitors of MET signaling have been in development for a number of years. These inhibitors include antibodies that target the extracellular domain of the receptor, antibodies that bind to and thus sequester circulating HGF, and small molecule TKIs that are both MET selective or multitargeted inhibitors.¹²⁷ As previously mentioned, concurrent MET activation is a potential mechanism of resistance to EGFR inhibitor therapy in lung cancer, and clinical trials testing combinations of anti-MET antibodies or TKIs plus EGFR TKIs are ongoing. Preliminary results suggest an improvement in progression-free survival with the addition of anti-MET antibodies to an EGFR inhibitor in patients whose tumors display MET overexpression by immunohistochemistry.¹²⁸ Given the limited clinical activity of selective MET inhibitors in most cancer types, predictive biomarkers of MET dependence and therefore response to MET inhibition are a focus of ongoing studies.¹²⁷