MOLECULAR BIOLOGY • CELL SIGNALING • GROWTH FACTORS

HGF Signaling: Pathway, MET Receptor Activation and Biological Functions

Discover how hepatocyte growth factor activates the MET receptor and regulates cell proliferation, migration, survival, tissue regeneration and cancer-associated processes.

1. Introduction to HGF Signaling

Hepatocyte growth factor (HGF), also known as scatter factor (SF), is a multifunctional signaling protein that regulates several important biological processes, including cell growth, migration, survival and tissue repair.

HGF exerts its biological effects primarily through binding to its specific cell-surface receptor, MET, also called c-MET. MET is a receptor tyrosine kinase (RTK) encoded by the MET proto-oncogene.

When HGF binds to MET, the receptor becomes activated through phosphorylation and recruits intracellular signaling proteins. These interactions initiate several downstream pathways, including:

• PI3K/AKT/mTOR signaling.
• RAS/RAF/MEK/ERK signaling.
• STAT3 signaling.
• SRC/FAK signaling.
• RAC1 and other Rho-family GTPase pathways.

Through these signaling networks, HGF/MET controls cellular responses such as proliferation, survival, motility, morphogenesis and tissue regeneration.

2. What Is Hepatocyte Growth Factor (HGF)?

HGF is a secreted protein belonging to the plasminogen-related growth factor family. It is produced by several cell types, particularly mesenchymal cells, and acts mainly through paracrine signaling, although autocrine signaling can also occur in certain biological and pathological contexts.

HGF is synthesized as an inactive single-chain precursor called pro-HGF. Proteolytic cleavage converts pro-HGF into its biologically active two-chain form, composed of an alpha chain and a beta chain linked by disulfide bonds.

Main Biological Roles of HGF

• Promotes cell proliferation in responsive cell types.
• Stimulates cell migration and motility.
• Supports cell survival under selected stress conditions.
• Contributes to tissue regeneration and wound repair.
• Regulates epithelial morphogenesis and branching processes.
• Participates in embryonic development.

Although HGF was initially identified through its ability to stimulate hepatocyte growth, subsequent research demonstrated that its activity extends to numerous epithelial, endothelial and other cell populations.

3. Understanding the MET Receptor

MET is a transmembrane receptor tyrosine kinase expressed in several tissues. It is synthesized as a precursor and processed into an extracellular alpha chain and a transmembrane beta chain connected by disulfide bonds.

The receptor contains three major structural regions:

3.1 Extracellular Region

The extracellular portion of MET participates in ligand recognition and receptor interactions. It is responsible for binding HGF and initiating receptor activation.

3.2 Transmembrane Region

This region anchors MET in the plasma membrane and connects the extracellular ligand-binding region to the intracellular signaling machinery.

3.3 Intracellular Tyrosine Kinase Domain

The intracellular region contains the kinase domain responsible for phosphorylation of specific tyrosine residues. These phosphorylation events create docking sites for intracellular adaptor proteins and signaling molecules.

4. How Is the HGF/MET Pathway Activated?

4.1 HGF Binding to MET

HGF binds to the extracellular region of MET. This interaction promotes receptor activation and the organization of MET molecules into an active signaling complex.

4.2 Receptor Dimerization and Activation

HGF binding promotes MET receptor dimerization or ligand-induced receptor rearrangement, allowing the intracellular kinase domains to phosphorylate one another through trans-autophosphorylation.

4.3 MET Autophosphorylation

Phosphorylation of Y1234 and Y1235 within the kinase activation loop increases MET kinase activity. Additional phosphorylation at Y1349 and Y1356 forms a multifunctional docking platform.

4.4 Recruitment of Adaptor Proteins

Phosphorylated MET recruits adaptor and effector proteins, including GAB1, GRB2, SHC, SRC and PI3K-associated components. These proteins connect receptor activation to downstream intracellular signaling pathways.

5. HGF/MET Signaling Pathway Diagram

The following simplified diagram illustrates the major steps in HGF/MET signal transduction.

6. Major Downstream Signaling Pathways

6.1 PI3K/AKT/mTOR Pathway

One of the major downstream branches of HGF/MET signaling is the phosphoinositide 3-kinase (PI3K)/AKT pathway.

Following MET activation, PI3K is recruited to the receptor-associated signaling complex. PI3K generates phosphatidylinositol-3,4,5-trisphosphate (PIP3), which facilitates the recruitment and activation of AKT.

Activated AKT regulates proteins involved in:

• Cell survival and resistance to apoptosis.
• Cell growth and metabolism.
• Protein synthesis through mTOR-related signaling.
• Cellular responses to growth factors.

6.2 RAS/RAF/MEK/ERK Pathway

HGF/MET can activate the RAS/MAPK signaling cascade through adaptor proteins such as GRB2 and SOS.

Activated RAS stimulates RAF, which phosphorylates MEK1/2. MEK subsequently activates ERK1/2, allowing the regulation of transcription factors and proteins involved in cell proliferation, differentiation and gene expression.

6.3 STAT3 Signaling

Signal transducer and activator of transcription 3 (STAT3) is another important downstream mediator of MET signaling.

Following phosphorylation, STAT3 can form dimers and translocate into the nucleus, where it regulates the transcription of genes involved in cell survival, proliferation and tissue remodeling.

6.4 SRC/FAK Signaling

SRC and focal adhesion kinase (FAK) participate in signaling events associated with cell adhesion, cytoskeletal organization and migration.

This branch helps cells coordinate interactions with the extracellular matrix and supports the movement of cells during tissue repair and morphogenesis.

6.5 RAC1 and Rho GTPase Signaling

RAC1 and other Rho-family GTPases regulate actin cytoskeleton remodeling, membrane protrusions and cell polarity. These processes are essential for HGF-induced cell motility and changes in cellular morphology.

7. Biological Functions of HGF Signaling

7.1 Cell Proliferation

HGF can stimulate cell proliferation by activating signaling pathways such as RAS/MAPK and PI3K/AKT. These pathways regulate cell cycle progression and the expression of genes associated with growth.

7.2 Cell Migration and Motility

HGF is well known for its ability to stimulate cell motility. MET activation regulates cytoskeletal rearrangement, focal adhesion dynamics and cell polarity, enabling cells to move in response to environmental signals.

7.3 Cell Survival

Through PI3K/AKT and other signaling branches, HGF/MET can promote cell survival and protect certain cell types against apoptosis under specific conditions.

7.4 Tissue Repair and Regeneration

HGF/MET signaling contributes to tissue repair following injury. It can support epithelial cell migration, proliferation and survival, helping restore damaged tissue structures.

7.5 Embryonic Development and Morphogenesis

During embryonic development, HGF/MET signaling contributes to the migration of specific cell populations, organ formation and branching morphogenesis. These activities require coordinated changes in cell movement, adhesion and organization.

7.6 Angiogenesis

HGF can influence endothelial cell behavior and vascular remodeling. Its effects on cell migration, survival and interactions with the surrounding tissue contribute to angiogenesis in selected physiological and pathological contexts.

8. HGF/MET Signaling in Cancer Research

Although HGF/MET signaling is essential for normal development and tissue homeostasis, abnormal pathway activation can contribute to cancer progression.

Excessive or uncontrolled MET signaling may result from several molecular mechanisms:

• MET gene amplification.
• Activating mutations in the MET gene.
• Increased MET receptor expression.
• Excessive HGF production in the tumor microenvironment.
• Abnormal regulation of receptor degradation.
• Interactions with other receptor tyrosine kinases.

Tumor Cell Proliferation

Persistent MET activation can stimulate PI3K/AKT and RAS/MAPK pathways, promoting tumor cell growth in susceptible cancer models.

Invasion and Metastasis

HGF/MET can stimulate cell motility, extracellular matrix interactions and invasive growth. These processes may contribute to tumor invasion and metastatic dissemination.

Angiogenesis and Tumor Microenvironment

HGF/MET signaling can influence communication between tumor cells, stromal cells and vascular cells. These interactions may support tumor development and progression.

Resistance to Targeted Therapies

MET activation can provide alternative survival signaling in certain cancer models, contributing to resistance to therapies targeting other receptor tyrosine kinases. This makes the HGF/MET axis an important subject in cancer biomarker and therapeutic research.

9. Regulation and Downregulation of MET Signaling

HGF/MET signaling is controlled by multiple regulatory mechanisms that prevent excessive or prolonged receptor activation.

HGF Activation

Pro-HGF must undergo proteolytic processing to become biologically active. Regulation of this activation step controls the availability of active ligand.

Receptor Internalization

After activation, MET can be internalized from the cell surface. Internalization influences receptor signaling duration and intracellular trafficking.

Ubiquitination and Degradation

The E3 ubiquitin ligase CBL can interact with phosphorylated MET through regulatory adaptor mechanisms. Ubiquitination contributes to receptor sorting and degradation, helping terminate signaling.

Negative Regulation and Cross-Talk

Phosphatases, receptor trafficking proteins and interactions with other signaling systems also influence MET activity. These mechanisms regulate the strength and duration of downstream responses.

10. Laboratory Techniques to Study HGF/MET Signaling

Several molecular and biochemical methods are used to investigate HGF/MET activation, receptor phosphorylation and downstream biological responses.

10.1 Western Blotting

Western blotting is widely used to analyze total MET and phosphorylated MET. Phospho-specific antibodies targeting sites such as p-MET Y1234/Y1235 can help evaluate receptor activation.

Researchers may also examine downstream markers such as phosphorylated AKT, ERK1/2 and STAT3.

10.2 ELISA

ELISA can be used to quantify HGF in biological samples, including cell culture supernatants and selected tissue or fluid samples, depending on assay validation.

10.3 RT-qPCR

Reverse transcription quantitative PCR can measure changes in HGF and MET gene expression, as well as downstream genes associated with pathway activation.

10.4 Immunofluorescence and Immunohistochemistry

These techniques help investigate MET expression and cellular localization in cultured cells or tissue sections. They can also provide information about receptor distribution in different tissue compartments.

10.5 Cell Migration and Invasion Assays

Wound-healing assays and Transwell migration assays are commonly used to investigate HGF-induced cell motility. Matrigel-based invasion assays can help evaluate invasive behavior in appropriate experimental models.

10.6 Phosphoproteomic Analysis

Phosphoproteomics can provide a broader view of MET-associated signaling by identifying phosphorylation changes across multiple proteins and pathways.

11. Therapeutic Research Targeting HGF/MET

Because abnormal HGF/MET signaling is associated with several diseases, this pathway has become an important target in biomedical and pharmaceutical research.

Research strategies include:

• Small-molecule inhibitors targeting MET kinase activity.
• Antibodies directed against MET or HGF.
• Approaches that interfere with ligand-receptor interactions.
• Biomarker studies to identify MET-dependent tumors.
• Combination strategies designed to overcome compensatory signaling.

Therapeutic development requires careful consideration of pathway selectivity, molecular alterations, patient selection and potential resistance mechanisms.

13. Frequently Asked Questions

14. Scientific References and Further Reading

The following peer-reviewed publications provide further information about HGF, MET receptor structure, signaling mechanisms, tissue regeneration and cancer biology.