Receptor tyrosine kinases (RTKs) regulate cell proliferation, survival, migration, and differentiation through tyrosine phosphorylation. RTK mutations, amplifications, fusions, or abnormal expression can continuously activate MAPK, PI3K/AKT, STAT, PLC, and other signaling pathways, thereby driving tumorigenesis and influencing molecular classification, targeted therapy, and drug resistance.

Keywords: receptor tyrosine kinase; RTK; EGFR; HER2; FGFR; MET; ALK; RET; MAPK; PI3K/AKT; gene fusion; precision classification; targeted therapy; resistance mechanism

1 Structural Characteristics and Classification of Receptor Tyrosine Kinase Families

1.1 Basic Molecular Structure of RTKs

Receptor tyrosine kinases generally consist of an extracellular ligand-binding region, a single-pass transmembrane region, and an intracellular tyrosine kinase domain. The extracellular region determines ligand recognition and receptor dimerization, the transmembrane helix participates in conformational transmission, and the intracellular kinase domain is responsible for ATP binding, substrate phosphorylation, and signal amplification. Multiple phosphorylatable tyrosine residues are usually located near the kinase domain and within the carboxyl-terminal region. After receptor activation, these sites form docking platforms for signaling proteins containing SH2 or PTB domains.

1.2 Major Classification of RTK Families

RTKs can be classified into multiple families according to their extracellular structures, kinase-domain characteristics, and ligand types. Major families include the ERBB, FGFR, VEGFR, PDGFR, insulin receptor, MET, ALK, ROS1, RET, TRK, KIT, FLT3, TAM, and Eph receptor families. These families differ in tissue expression, dimerization patterns, ligand dependence, and downstream signaling preferences, but all regulate cellular behavior through tyrosine phosphorylation.

1.3 Structural Differences Among RTK Families

The ERBB family includes EGFR, HER2, HER3, and HER4. HER2 lacks a clearly defined direct ligand and serves as an important heterodimerization partner, whereas HER3 has weak intrinsic kinase activity and mainly transmits signals by forming heterodimers with receptors such as HER2. The insulin receptor exists as a preformed dimer linked by disulfide bonds. FGFR complex formation requires both FGF ligands and heparan sulfate, whereas Eph receptors bind membrane-associated Ephrin ligands and form higher-order receptor clusters.

2 Ligand-Dependent Receptor Activation and Tyrosine Phosphorylation

2.1 Ligand Binding and Receptor Dimerization

After a growth factor binds to the extracellular region of an RTK, it may directly cross-link two receptor molecules or expose a dimerization interface that brings the intracellular kinase domains into proximity. Receptors can form homodimers or heterodimers with other members of the same family. Ligand concentration, receptor abundance, and dimer composition influence signal intensity, duration, and downstream pathway preference.

2.2 Activation of the Kinase Domain

After dimerization, the two intracellular kinase domains interact and enter an active state, followed by trans-phosphorylation of tyrosine residues within the activation loop, juxtamembrane region, and carboxyl terminus. Phosphorylation relieves kinase autoinhibition, increases the efficiency of ATP and protein-substrate binding, and creates recruitment sites for intracellular signaling proteins. Some oncogenic mutations stabilize an active receptor conformation in the absence of ligand, resulting in persistent RTK phosphorylation.

2.3 Formation of Signaling Complexes

Phosphorylated tyrosine sites can recruit signaling molecules such as GRB2, SHC, GAB, PI3K, PLCγ, STAT, SRC, and protein tyrosine phosphatases. Different phosphorylation sites recruit different adaptor proteins, allowing the same receptor to connect simultaneously with multiple downstream pathways. After internalization, receptors may continue to signal from early endosomes, enter lysosomes for degradation, or recycle back to the plasma membrane.

2.4 Termination of RTK Signaling

RTK signaling is terminated through dephosphorylation by protein tyrosine phosphatases, receptor ubiquitination, internalization and degradation, and downstream negative-feedback regulation. CBL-mediated ubiquitination promotes lysosomal degradation of certain receptors, whereas DUSP, SPRY, and other negative regulators suppress MAPK signaling. Receptor overexpression, ubiquitination defects, or abnormal endocytosis can prolong RTK signaling.

3 RTK-Mediated MAPK, PI3K/AKT, JAK/STAT, and PLC Signaling Networks

Figure 1. Major downstream signaling networks mediated by receptor tyrosine kinase activation

3.1 RAS-RAF-MEK-ERK Pathway

After RTK phosphorylation, SHC and GRB2 recruit SOS, which promotes conversion of RAS from its GDP-bound state to its GTP-bound state. Activated RAS sequentially activates RAF, MEK, and ERK, thereby regulating MYC, ELK1, FOS, cyclins, and other transcriptional or proliferative factors. Persistent activation of this pathway by RTKs promotes cell-cycle entry, clonal expansion, and tumor growth.

3.2 PI3K-AKT-mTOR Pathway

RTKs can recruit class I PI3K directly or indirectly through adaptors such as GAB, causing conversion of PIP2 into PIP3 and recruitment of PDK1 and AKT to the plasma membrane. Activated AKT suppresses FOXO and proapoptotic proteins while promoting mTORC1 activity, protein synthesis, glucose metabolism, and cell survival. PTEN loss, PIK3CA mutation, or abnormal AKT activation can strengthen this pathway and reduce tumor dependence on upstream RTK signaling.

3.3 JAK/STAT and STAT Signaling

Some RTKs can directly phosphorylate STAT proteins or indirectly activate transcription factors such as STAT3 and STAT5 through JAK or SRC-family kinases. Phosphorylated STAT proteins form dimers and translocate into the nucleus, where they regulate genes associated with survival, inflammation, immune suppression, and stemness. Unlike cytokine receptors, not all RTKs require JAK proteins as intermediates for STAT activation.

3.4 PLCγ-PKC and Calcium Signaling

PLCγ binds phosphorylated RTKs through its SH2 domains and catalyzes hydrolysis of PIP2 into DAG and IP3. DAG activates PKC, whereas IP3 promotes Ca²⁺ release from the endoplasmic reticulum, subsequently regulating CaMK, NFAT, and cytoskeleton-associated signaling. This branch participates in cell migration, angiogenesis, secretion, and tissue remodeling and can interact with the MAPK and PI3K pathways.

3.5 Pathway Crosstalk and Feedback Compensation

RTK downstream pathways are not independent. RAS can regulate PI3K activity, AKT and ERK can jointly influence mTORC1, and SRC connects RTKs with focal adhesion and cytoskeletal signaling. After a targeted drug inhibits one receptor, relief of negative feedback may increase expression of other RTKs or ligands, allowing reactivation of MAPK and PI3K/AKT signaling.

4 Oncogenic Mechanisms of RTK Mutation, Amplification, Fusion, and Abnormal Expression

4.1 Activating Mutations

Activating mutations can occur in the extracellular region, juxtamembrane region, kinase domain, or carboxyl terminus. These mutations can promote dimerization, relieve autoinhibition, stabilize an active conformation, or reduce receptor degradation, resulting in persistent signaling. EGFR kinase-domain mutations, activating KIT and PDGFRA mutations, RET point mutations, and FLT3 internal tandem duplications can all produce ligand-independent activation.

4.2 Gene Amplification and Protein Overexpression

RTK gene amplification can markedly increase receptor density at the plasma membrane, allowing low ligand concentrations to generate strong signals and increasing the probability of ligand-independent receptor clustering. Amplification of ERBB2, EGFR, MET, FGFR, and other genes occurs in multiple solid tumors. Protein overexpression may also result from enhanced transcription, altered RNA stability, or impaired receptor degradation and does not necessarily involve increased gene copy number.

4.3 Gene Fusion and Structural Rearrangement

ALK, ROS1, RET, NTRK, FGFR, and other RTKs can form chimeric proteins with various fusion partners. These fusions generally retain the intracellular RTK kinase domain, whereas the fusion partner provides a dimerization domain or abnormal promoter, resulting in constitutive kinase activity. Fusion proteins often lack the normal extracellular region and transmembrane regulatory structures and therefore no longer depend on ligand stimulation.

4.4 Autocrine and Paracrine Activation

Tumor cells may simultaneously express an RTK and its ligand, creating an autocrine signaling loop. Alternatively, tumor-associated fibroblasts, macrophages, or endothelial cells may produce ligands and generate paracrine activation. HGF-MET, FGF-FGFR, and GAS6-AXL signaling axes can promote invasion, epithelial-mesenchymal transition, immune suppression, and treatment resistance within the tumor microenvironment.

4.5 Receptor Transactivation and Bypass Compensation

Different RTKs can transactivate one another through heterodimerization, SRC-family kinases, or membrane-protein complexes. Functional compensation among HER2, HER3, MET, AXL, and IGF-1R can maintain PI3K/AKT and MAPK signaling after inhibition of a single receptor and provides an important basis for bypass resistance to targeted therapy.

5 Oncological Characteristics of the HER, FGFR, MET, ALK, and Other RTK Families

5.1 HER Family

The HER family includes EGFR, HER2, HER3, and HER4. EGFR mutation or amplification can drive lung cancer, glioma, and head and neck tumors. HER2 amplification, overexpression, or activating mutation is an important classification criterion in subsets of breast cancer, gastric cancer, and other solid tumors. Although HER3 has weak intrinsic kinase activity, it contains multiple PI3K-binding sites and can enhance survival signaling by forming heterodimers with HER2, EGFR, or MET.

5.2 FGFR Family

FGFR1 through FGFR4 form signaling complexes with FGF ligands and heparan sulfate. FGFR amplification, point mutation, and fusion can occur in bladder cancer, cholangiocarcinoma, endometrial cancer, lung cancer, and certain hematologic malignancies. FGFR abnormalities mainly activate MAPK, PI3K/AKT, and PLCγ signaling, and some tumors exhibit dependence on specific FGFR subtypes.

5.3 MET Receptor

MET is the receptor for hepatocyte growth factor (HGF) and regulates cell migration, invasion, tissue repair, and morphogenesis. MET exon 14 skipping, gene amplification, activating mutation, or excessive HGF secretion can all cause abnormal signaling. MET amplification can also act as a bypass resistance mechanism after EGFR inhibitor treatment by restoring HER3-PI3K/AKT signaling and maintaining tumor-cell survival.

5.4 ALK and ROS1

ALK and ROS1 fusions can continuously activate their kinase domains without normal ligand regulation and drive MAPK, PI3K/AKT, and STAT signaling. Fusion partner, fusion breakpoint, and tumor tissue type can influence protein localization, signal intensity, and drug sensitivity. During treatment, secondary kinase-domain mutations, target amplification, and bypass-receptor activation may develop.

5.5 RET and NTRK

RET point mutations, gene fusions, or abnormal expression occur in thyroid tumors and subsets of lung cancer. NTRK1, NTRK2, and NTRK3 fusions can occur across multiple adult and pediatric tumor types. Although these fusions are relatively uncommon overall, they usually function as strong oncogenic drivers and are suitable for tumor-agnostic molecular classification and targeted therapy.

5.6 VEGFR, PDGFR, KIT, FLT3, and the TAM Family

VEGFR primarily regulates tumor angiogenesis and lymphangiogenesis, whereas PDGFR participates in signaling in stromal cells, pericytes, and tumor cells. KIT and FLT3 abnormalities commonly occur in certain stromal tumors and hematologic malignancies. AXL, MERTK, and TYRO3 form the TAM family, which can be activated by GAS6 or PROS1 and participate in epithelial-mesenchymal transition, apoptotic-cell clearance, immune suppression, and resistance to targeted therapy.

5.7 Major Abnormalities and Research Directions of RTK Families

6 Resistance Mechanisms and Combination Strategies for RTK-Targeted Therapy

6.1 Secondary On-Target Alterations

During treatment, secondary mutations can arise within RTK kinase domains and interfere with drug binding. Target-gene amplification, alternative splicing, or changes in fusion structure can also increase receptor signaling. Different inhibitors cover different receptor conformations and resistance mutations, so molecular testing should be repeated after resistance develops.

6.2 Bypass-Receptor Activation

MET, HER2, HER3, AXL, FGFR, IGF-1R, and other receptors can maintain downstream signaling after the primary driver RTK is inhibited. Bypass activation may result from gene amplification, increased ligand production, receptor transcriptional upregulation, or changes in the tumor microenvironment. Combined inhibition of the primary driver and bypass receptor can strengthen pathway suppression but may also increase toxicity and selective pressure.

6.3 Reactivation of Downstream Pathways

Alterations in RAS, BRAF, PIK3CA, AKT, PTEN, and other downstream molecules can bypass upstream RTK inhibition and restore MAPK or PI3K/AKT signaling. Such resistance should be treated experimentally according to the actual pathway alteration using MEK, ERK, PI3K, AKT, or mTOR inhibitors rather than by simply increasing the intensity of upstream RTK inhibition.

6.4 Phenotypic Transformation and Cell-State Changes

Some tumors reduce dependence on the original RTK through epithelial-mesenchymal transition, histological transformation, increased stemness, or lineage reprogramming. Tumor cells may also enter a drug-tolerant persister state and survive transiently without stable genetic mutations. These states are associated with chromatin remodeling, AXL signaling, antiapoptotic proteins, and metabolic adaptation.

6.5 Tumor Microenvironment and Drug Distribution

Tumor-associated fibroblasts, macrophages, and endothelial cells can secrete HGF, FGF, EGF, GAS6, and other ligands that weaken the effects of RTK inhibitors. The blood-brain barrier, tumor interstitial pressure, and drug-efflux transporters can further reduce local drug concentrations. Resistance studies should therefore consider genetic changes, microenvironmental support, and pharmacokinetic factors.

6.6 Combination Intervention Strategies

Combination therapy may involve dual RTK blockade, combined inhibition of an RTK and downstream pathway, combination of RTK-targeted agents with antiangiogenic drugs, or combination with chemotherapy, radiotherapy, or immunotherapy. Rational combinations should be based on resistance mechanisms and biomarkers, and pharmacodynamic indicators should confirm effective inhibition of the relevant signaling axis.

7 Products Related to Aberrant RTK Signaling and Targeted Research

7.1 Recombinant RTK Proteins and Detection Reagents

7.2 RTK-Targeted Antibodies and Phosphorylation Detection Reagents

7.3 RTK Inhibitors, Protein Degraders, and Gene-Intervention Products

Aberrant receptor tyrosine kinase signaling can be continuously activated by mutation, amplification, fusion, or abnormal expression and provides an important molecular basis for tumorigenesis, precision classification, and targeted therapy.

References

[1] Shaw, A. T. et al. First-Line Lorlatinib or Crizotinib in Advanced ALK-Positive Lung Cancer. New England Journal of Medicine 383, 2018–2029 (2020).

[2] Drago, J. Z., Ferraro, E., Abuhadra, N. & Modi, S. Beyond HER2: Targeting the ErbB receptor family in breast cancer. Cancer Treatment Reviews 109, 102436 (2022).

[3] Oñate, G. et al. Survival improvement of patients with FLT3 mutated acute myeloid leukemia: results from a prospective 9 years cohort. Blood Cancer Journal 13, 69 (2023).

[4] Linguanti, F., Abenavoli, E. M., Calabretta, R., Berti, V. & Lopci, E. ImmunoPET Targeting Receptor Tyrosine Kinase: Clinical Applications. Cancers 15 (2023).

[5] Rendell, A. et al. Targeting Tyrosine Kinases in Ovarian Cancer: Small Molecule Inhibitor and Monoclonal Antibody, Where Are We Now? Biomedicines 10 (2022).

[6] Takiar, R. & Phillips, T. J. Durable responses with ALK inhibitors for primary refractory anaplastic lymphoma kinase-positive large B-cell lymphoma. Blood Advances 7, 2912–2916 (2023).

For more related articles, please see below:

[1] Composition, Activation Mechanisms, and Biological Effects of the AKT Pathway

[2] PI3K-Akt-mTORC Signaling

[3] JAK/STAT Inhibitors