HER3 strengthens PI3K/AKT survival signaling by forming heterodimers with receptors such as HER2 and EGFR and contributes to bypass compensation, maintenance of drug resistance, and tumor progression under multiple therapeutic pressures.

Keywords: HER3; ERBB3; HER2; EGFR; NRG1; heterodimerization; PI3K/AKT; therapeutic resistance; bispecific antibody; antibody-drug conjugate

1 Molecular Structure and Receptor Family Characteristics of HER3

1.1 Position of HER3 Within the Receptor Family

HER3 is encoded by the ERBB3 gene and belongs to the ERBB receptor tyrosine kinase family, which also includes EGFR, HER2, and HER4. HER3 contains an extracellular ligand-binding region, a single transmembrane region, an intracellular pseudokinase domain, and a carboxyl-terminal signaling region. However, its kinase catalytic activity is substantially weaker than that of other ERBB receptors, and it primarily depends on dimerization partners for phosphorylation.

1.2 HER3 Extracellular Structure and Ligand Binding

The extracellular region of HER3 consists of four domains, designated I-IV. Binding of NRG1 or NRG2 converts HER3 from a closed conformation to a dimerization-competent conformation, promoting the formation of heterodimers with HER2, EGFR, or HER4. HER3 signaling intensity is mainly determined by ligand abundance, the number of receptors on the cell surface, and the kinase activity of the dimerization partner.

1.3 HER3 Pseudokinase Domain

The intracellular kinase domain of HER3 lacks some of the key residues required for efficient catalysis and cannot independently generate strong phosphorylation signals. Nevertheless, this domain can contribute to stabilization of the heterodimer conformation and facilitate activation of HER2 or EGFR. Therefore, HER3 is not merely a passive phosphorylation substrate.

1.4 Carboxyl-Terminal Signaling Sites of HER3

The carboxyl terminus of HER3 contains multiple tyrosine phosphorylation sites, several of which contain YXXM motifs that directly recruit the PI3K regulatory subunit p85. This structural feature makes HER3 one of the strongest ERBB family receptors for coupling to the PI3K/AKT pathway. Despite its weak intrinsic kinase activity, HER3 can generate substantial prosurvival signaling.

1.5 HER3 Membrane Expression and Intracellular Trafficking

HER3 membrane abundance is jointly regulated by ERBB3 transcription, receptor internalization, membrane recycling, and lysosomal degradation. Targeted therapies can induce increased HER3 expression or renewed enrichment at the cell surface, allowing tumor cells to restore downstream signaling. HER3 membrane localization and internalization capacity also directly influence the delivery efficiency of antibodies and antibody-drug conjugates.

2 HER3 Heterodimerization and Downstream Signal Transduction

2.1 HER2-HER3 Heterodimer

HER2 possesses strong kinase activity, whereas HER3 contains multiple PI3K-binding sites. The HER2-HER3 heterodimer therefore combines efficient receptor phosphorylation with strong PI3K recruitment. In HER2-amplified tumors, HER2-HER3 is an important signaling complex that maintains cell proliferation, metabolism, and resistance to apoptosis.

2.2 EGFR-HER3 Heterodimer

In tumors with EGFR mutations or high EGFR expression, EGFR can phosphorylate HER3 and efficiently transmit signals to the PI3K/AKT pathway through HER3. After EGFR inhibition, increased HER3 expression, receptor reorganization, or compensation by other kinases can restore HER3 phosphorylation and establish a drug-resistant bypass pathway.

2.3 HER4-HER3 Heterodimer

HER4 can form heterodimers with HER3 after NRG stimulation, and the resulting effects depend on tissue type and HER4 splice isoform. This complex can regulate PI3K/AKT, MAPK, and differentiation-associated signaling and has functional roles in breast, neural, and certain endocrine tumors.

2.4 PI3K/AKT/mTOR Signaling Axis

Phosphorylated HER3 recruits p85 through its YXXM motifs and activates PI3K, promoting PIP3 formation and subsequently initiating PDK1-, AKT-, and mTOR-associated signaling. This pathway suppresses apoptosis, promotes protein synthesis and metabolic adaptation, and supports tumor-cell survival under hypoxia, nutrient limitation, and drug pressure.

2.5 RAS/RAF/MEK/ERK Signaling Axis

HER3 heterodimers can also influence the RAS/RAF/MEK/ERK pathway through adaptor proteins such as SHC and GRB2. HER3 has weaker direct recruitment capacity for the MAPK pathway than for PI3K, but both pathways can be activated simultaneously when HER2, EGFR, or NRG signaling is strong.

Table 1 Major HER3 Heterodimers and Their Signaling Characteristics

3 Abnormal HER3 Expression and Tumor Development

3.1 HER3 Overexpression

HER3 expression can be elevated in solid tumors including lung, breast, gastric, colorectal, ovarian, and prostate cancers. HER3 overexpression generally does not independently drive tumorigenesis but can increase cellular responses to HER2, EGFR, and NRG ligands and enhance PI3K/AKT signaling output.

3.2 ERBB3 Mutations

Functional mutations can occur in the extracellular region and pseudokinase domain of ERBB3. Some mutations enhance HER3-HER2 binding, reduce ligand dependence, or increase downstream signaling activity. The functional significance of an ERBB3 mutation must be interpreted together with the status of HER2, EGFR, and the PI3K pathway.

3.3 NRG1 Fusions and Ligand-Driven Signaling

NRG1 fusions generally retain the EGF-like domain capable of binding HER3 and can continuously promote HER3-HER2 dimerization. This mechanism occurs in subsets of lung, pancreatic, and cholangiocarcinomas. The core abnormality is persistent ligand-driven HER2-HER3 signaling rather than amplification of the HER3 gene itself.

3.4 Tumor Cell Proliferation and Survival

HER3 suppresses apoptosis, promotes protein synthesis, and enhances metabolic adaptation through PI3K/AKT signaling. HER3 activity increases tumor-cell tolerance to hypoxia, nutrient restriction, endoplasmic reticulum stress, and drug-induced injury, allowing residual cells to survive after treatment.

3.5 Invasion and Metastasis

HER3 signaling can regulate actin remodeling, cell adhesion, chemotaxis, and matrix degradation. HER3-mediated AKT and cytoskeletal signaling promotes tumor-cell migration, invasion, and distant colonization, particularly in HER2-high or NRG-rich microenvironments.

4 HER3-Mediated Bypass Activation and Therapeutic Resistance

4.1 Resistance to EGFR-TKIs

After EGFR inhibition, tumor cells can upregulate HER3 or restore HER3 phosphorylation through HER2, MET, or other receptors, thereby reactivating PI3K/AKT signaling. Even when EGFR-MAPK signaling is suppressed, tumor cells may continue to survive through the HER3-AKT pathway.

4.2 Resistance to Anti-HER2 Therapy

When HER2 inhibition reduces AKT activity, inhibition of FOXO transcription factors is relieved, promoting ERBB3 transcription. Increased HER3 can associate with residual HER2, EGFR, or other receptors and reactivate PI3K/AKT signaling, reducing the sustained effects of HER2 antibodies and HER2 kinase inhibitors.

4.3 Resistance to PI3K, AKT, and mTOR Inhibitors

Inhibition of the PI3K/AKT/mTOR pathway can relieve negative feedback on upstream receptors, leading to increased expression of HER3, HER2, IGF1R, and other receptors. Renewed recruitment of PI3K by HER3 can weaken pathway inhibition and produce feedback-mediated resistance.

4.4 Resistance to Endocrine Therapy

In hormone receptor-positive breast cancer, NRG-HER3 signaling can activate AKT and ERK and reduce tumor-cell dependence on estrogen receptor signaling. Crosstalk between HER3 and ER signaling can promote resistance to tamoxifen, fulvestrant, and aromatase inhibitors.

4.5 Chemotherapy and Radiotherapy Tolerance

HER3-AKT signaling enhances anti-apoptotic activity, DNA damage repair, and metabolic adaptation, thereby reducing tumor-cell sensitivity to chemotherapy and radiotherapy. Increased HER3 after treatment may result either from enrichment of HER3-high clones or from treatment-induced adaptive changes.

Figure 1. HER3 heterodimerization and RTK bypass signaling mediate downstream activation and therapy resistance

5 Role of HER3 in Lung Cancer, Breast Cancer, and Other Solid Tumors

5.1 EGFR-Mutant Non-Small Cell Lung Cancer

HER3 is an important receptor connecting mutant EGFR to the PI3K/AKT pathway. After EGFR-TKI treatment, HER3 can maintain resistant cells through increased expression, HER2- or MET-mediated bypass activation, and receptor reorganization. HER3 membrane expression and internalization also make it an important delivery target for antibody-drug conjugates.

5.2 HER2-Positive Breast Cancer

HER2-amplified breast cancer is highly dependent on HER2-HER3 dimers. Persistent HER3 activation can reduce the activity of trastuzumab, pertuzumab, and HER2 kinase inhibitors and promote feedback restoration after PI3K pathway inhibition.

5.3 Hormone Receptor-Positive Breast Cancer

NRG-HER3 signaling can bypass ER dependence, increase AKT and ERK activity, and promote resistance to endocrine therapy. Bypass signaling is stronger when HER3 activation coexists with PIK3CA mutations, PTEN loss, or upregulation of growth factor receptors.

5.4 Triple-Negative Breast Cancer

Some triple-negative breast cancers upregulate HER3 under therapeutic pressure or in response to microenvironmental NRG stimulation. HER3 mainly maintains PI3K/AKT survival signaling and reduces the effects of PI3K inhibitors, AKT inhibitors, and chemotherapeutic agents.

5.5 Gastric and Gastroesophageal Junction Cancers

HER3 can cooperate with HER2 or EGFR to maintain gastric cancer signaling. Feedback activation of HER3 in HER2-amplified tumors is an important mechanism of resistance to anti-HER2 therapy. HER3 membrane expression can also be used to evaluate targeted delivery strategies.

NRG1 fusion-positive lung, pancreatic, and cholangiocarcinomas are generally dependent on HER2-HER3 signaling. These tumors are more appropriately classified across cancer types according to the fusion gene rather than selected for treatment solely on the basis of HER3 expression.

6 Therapeutic Strategies Using HER3 Monoclonal and Bispecific Antibodies

6.1 HER3 Monoclonal Antibodies

HER3 monoclonal antibodies can block NRG binding, restrict heterodimerization, induce receptor internalization, or promote HER3 degradation. Antibodies such as Patritumab, Seribantumab, Lumretuzumab, and Barecetamab can be used to investigate ligand-dependent HER3 activation and signaling changes after HER3 blockade.

6.2 Combination Therapy With HER3 Antibodies

HER3 antibodies can be combined with EGFR-TKIs, HER2 inhibitors, PI3K inhibitors, or endocrine therapies. The main objective of these combinations is to inhibit the primary driver receptor while simultaneously blocking treatment-induced HER3 feedback activation.

6.3 HER2-HER3 Bispecific Antibodies

HER2-HER3 bispecific antibodies simultaneously bind HER2 and HER3, restrict receptor dimerization, and block NRG-driven signaling. This strategy is particularly applicable to NRG1 fusion-positive tumors or tumors highly dependent on HER2-HER3 signaling.

6.4 EGFR-HER3 Bispecific Antibodies

EGFR-HER3 bispecific antibodies can simultaneously reduce EGFR-MAPK proliferative signaling and HER3-PI3K survival signaling. Duligotuzumab and Izalontamab can be used to study tumorigenic signaling and bypass resistance involving both EGFR and HER3.

6.5 Advantages and Limitations of Bispecific Antibodies

Bispecific antibodies can simultaneously control two critical nodes in a receptor network and reduce bypass activation following inhibition of a single receptor. However, efficacy still depends on receptor coexpression, ligand status, and downstream pathway dependence and cannot be predicted solely from HER3 positivity.

7 Mechanism and Clinical Translation of HER3-Targeted Antibody-Drug Conjugates

7.1 Basic Structure of HER3-Targeted ADCs

HER3-targeted antibody-drug conjugates consist of a HER3 antibody, a linker, and a cytotoxic payload. After the antibody binds membrane HER3, the complex is internalized, and the payload is released in endosomes or lysosomes to induce cell death. This mechanism does not require HER3 to be the primary oncogenic driver but does require sufficient membrane expression and internalization capacity.

7.2 Patritumab Deruxtecan

Patritumab deruxtecan consists of a HER3 antibody, a cleavable linker, and a topoisomerase I inhibitor payload. Its activity depends on HER3-mediated internalization and intracellular payload release and can be used to study EGFR-mutant lung cancers with diverse resistance mechanisms after EGFR-TKI therapy.

7.3 Bystander Effect

Some HER3 ADC payloads are membrane-permeable and can diffuse from HER3-high cells into adjacent HER3-low cells, producing a bystander effect. This can help overcome heterogeneous HER3 expression but may also increase damage to normal tissues.

7.4 Mechanisms of Resistance to HER3 ADCs

Resistance to HER3 ADCs may result from reduced HER3 membrane expression, decreased receptor internalization, abnormal lysosomal processing, increased payload efflux, or enhanced DNA damage repair. Total HER3 expression does not fully reflect ADC sensitivity, and membrane localization, internalization efficiency, and payload response must also be considered.

7.5 Safety and Optimization of HER3 ADCs

Adverse effects of HER3 ADCs are mainly related to the cytotoxic payload and include myelosuppression, gastrointestinal reactions, fatigue, and interstitial lung injury. Optimization of antibody epitopes, linker stability, drug-to-antibody ratio, and payload type is central to improving the therapeutic window.

Table 2 Major HER3-Targeted Strategies

8 Biomarkers and Efficacy Evaluation for HER3-Targeted Therapy

8.1 HER3 Membrane Expression

HER3 immunohistochemistry can evaluate the proportion and intensity of HER3 membrane expression in tumor cells and provides a basic indicator of antibody binding and ADC delivery. Increased total HER3 protein alone does not demonstrate HER3 signaling dependence and cannot directly predict ADC sensitivity.

Phosphorylated HER3 more directly reflects receptor signaling activity than total HER3. Combined measurement of p-HER3, p-AKT, and p-S6 can determine whether the HER3 network is active and whether signaling has recovered after targeted treatment.

8.3 NRG1 Fusion and Ligand Status

NRG1 fusion is an important biomarker for identifying ligand-driven HER2-HER3 signaling. Fusion testing should preferentially use methods capable of detecting fusion transcripts. Increased NRG1 protein or mRNA alone cannot replace fusion analysis.

8.4 Status of the ERBB Receptor Network

HER2 amplification, EGFR mutation, ERBB3 mutation, and MET activation alter HER3 dimerization partners and signaling dependence. Evaluation of HER3-targeted efficacy must therefore include the status of HER2, EGFR, HER4, and other bypass receptors.

8.5 PI3K Pathway Status

PIK3CA mutations, PTEN loss, AKT activation, and RAS pathway abnormalities can affect signaling responses after HER3 blockade. If downstream pathways have acquired autonomous activation, HER3 inhibition alone is unlikely to produce a sustained effect.

8.6 ADC Trafficking and Payload Sensitivity

In addition to membrane expression, evaluation of HER3 ADCs should include receptor internalization, lysosomal trafficking, drug efflux, and DNA repair capacity. Tumors with similar HER3 expression may show different drug responses because of differences in intracellular processing.

8.7 Pharmacodynamic Evaluation

After HER3-targeted treatment, decreases in p-HER3, p-AKT, and p-S6 can be measured together with Ki-67, EdU, γH2AX, Caspase-3/7, and Annexin V to evaluate proliferation inhibition, DNA damage, and apoptosis. A single cell-viability indicator is insufficient to explain the specific mechanism of action.

Table 3 Major Evaluation Indicators for HER3-Targeted Therapy

9 Products Related to HER3 Targets, Ligands, Antibodies, and Pathway Inhibition

9.1 Products for HER3 Targets, Ligands, and Antibody Intervention

9.2 HER3-Related Pathway Inhibitors

HER3 is an important node connecting HER2, EGFR, and NRG ligand signaling to the PI3K/AKT survival network. HER3-targeted studies should simultaneously evaluate dimerization partners, ligand-driven signaling, receptor membrane expression, treatment-induced feedback, and ADC internalization capacity to distinguish true HER3 dependence from general receptor coexpression.

For more related articles, please see below:

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

[2] Antibody-drug Conjugates: A Comprehensive Guide

[3] Antibody-coupled drug ADCs: Precise guidance, cancer destruction