Epidermal Growth Factor (EGF) Pathway

1. What is Epidermal Growth Factor?

Epidermal growth factor (EGF) is a polypeptide with a low molecular weight (6.2 kDa), consisting of 53 amino acids. This signaling molecule binds to its receptor, EGFR, with high affinity, initiating receptor dimerization. This conformational change is pivotal for activating the intracellular tyrosine kinase activity of the receptor, which subsequently phosphorylates tyrosine residues on downstream substrate proteins. This process triggers a cascade of biochemical events critical for cellular proliferation, differentiation, and metabolic regulation.

2. Understanding EGFR

The EGF receptor, known as EGFR or ErbB1/HER1, is part of the ERBB family. It's a transmembrane protein that can hook up with seven different ligands, EGF being one of them. Once bound, this complex can actually move into the nucleus through specific pathways, which sets off a chain of reactions inside the cell.

2.1 Structure and Function

The EGFR protein manifests a tripartite transmembrane structure, each domain of which is integral to it signaling capabilities:

• Extracellular region: This ligand-binding module facilitates receptor activation through induced conformational shifts, enabling homodimerization or heterodimerization with other ERBB family members, thereby priming the receptor for downstream signal transduction.
• Transmembrane region: This region anchors the receptor within the plasma membrane, ensuring proper localization and structural stability, which are critical for receptor function and signaling efficiency.
• Intracellular region: Housing a catalytic tyrosine kinase (TK) domain, this region drives receptor autophosphorylation, a key regulatory step that modulates cellular processes such as proliferation, survival, and differentiation.

Fig.1 EGFR structure, including domains, exons, and respective mutations.^1,4

2.2 ERBB Family Members

The human ERBB family comprises four receptor tyrosine kinases: EGFR (ERBB1/HER1), ERBB2 (HER2), ERBB3 (HER3), and ERBB4 (HER4). These receptors exhibit combinatorial diversity, forming homo- and heterodimers in 28 distinct configurations to respond to environmental stimuli such as ultraviolet radiation.

2.3 EGFR Expression

EGFR is present in a lot of different cells, such as blood vessels, HeLa cells, uterine muscles, skin, and the placenta. In normal cells, EGFR levels are usually below 100,000. However, in cancer cells, this number can jump up to about 2 million. This big increase suggests that high EGFR levels are linked to cell growth, cell death, and tumor development.

Table 1. Characterization of EREG expression in different cancers.

2.4 EGFR Mutations

Mutations in the EGFR are distributed across its extracellular domain, kinase domain, and C-terminal region. Specific cancer often exhibits mutations at characteristic locations. Many EGFR mutations result in constant receptor activation by promoting ligand-free dimerization with ERBB family members. The L858R mutation in the kinase domain is notably common, making up about 45% of these mutations. This "canonical" mutation significantly enhances kinase activity and ATP binding, raising both by 50-fold compared to normal EGFR.

Fig.2 EGFR mutations and other main aberrations in lung and brain tumors.^2,4

3. EGFR Pathway

3.1 Activation

EGFR activation typically occurs following ligand binding. Receptor dimerization marks the beginning of the process, leading to the creation of homodimers or heterodimers with other proteins from the HER family. Cross-phosphorylation of the dimerized receptors subsequently activates the intracellular tyrosine kinase domain, completing receptor activation and initiating downstream signaling cascades.

3.2 Ligand

EGF is a well-known ligand of EGFR. In addition, EGFR can also be activated by six ligands, namely TGF-α, betacellulin (BTC), heparin binding EGF-like growth factor (HB-EGF), epigen (EPI), AREG, and EREG. Although these ligands have different names and sources, they trigger EGFR in essentially the same way: receptor dimerization, trans autophosphorylation, etc. EGFR ligands exhibit dual characteristics of multiple specificity and functional diversity in receptor recognition and signaling:

• A single ligand may interact with one or more receptors in the EGFR family, forming distinct dimer configurations and eliciting varied biological responses.
• Different ligands can also bind to the same receptor, producing differential outcomes.

4. Downstream Pathways

Fig.3 Downstream pathways of EGFR.^3,4

4.1 Ras-Raf-MEK Pathway

The protein Grb2, which binds to growth factor receptors, possesses an SH2 domain. This domain is great at picking out phosphorylated tyrosine residues on activated EGFR dimers. Once it does, Grb2 uses its SH3 domain to bring in the guanine nucleotide exchange factor (SOS). It's like a well - oiled team. SOS then changes Ras's shape, turning inactive Ras-GDP into active Ras-GTP. Activated Ras binds to Raf kinase's N-terminal part, kicking off Raf activation. Once Raf is active, it phosphorylates MEK, which in turn activates ERK. This step-by-step activation controls transcription factors that manage cell growth and change.

4.2 PI3K/AKT Pathway

Phosphoinosine-3 kinase (PI3K) is an intracellular enzyme consisting of p85 and p110. At the same time, PI3K exhibits dual enzymatic functions: serine/threonine (Ser/Thr) kinase and phosphatidylinositol kinase activity. When ligands such as EGF bind to EGFR, p85 is activated and catalyzes the conversion the transition of phosphatidylinositol 4,5-bisphosphate (PIP2) to phosphatidylinositol-3,4,5-trisphosphate (PIP3). It modulates AKT/PKB activity through two distinct regulatory pathways:

• First, it forms a complex with AKT, facilitating AKT translocation to the plasma membrane.
• Subsequently, with the help of AKT activators PDK1 and PDK2, PIP3 fully activates AKT.

Once activated, AKT can phosphorylate downstream proteins such as Bad and Caspase-9, thereby regulating cell proliferation and inhibiting apoptosis.

4.3 JAK/STAT Pathway

When a ligand attaches to EGFR, it's like hitting the start button for JAK1 and JAK2 in the Janus kinase family. Upon activation, these JAKs begin to interact with the STAT proteins. They phosphorylate the C - terminal tyrosine parts of STAT proteins. As soon as STATs are phosphorylated, they part ways with the receptor. Then, using their SH2 domains, they team up to form either homodimers or heterodimers. This teamwork exposes their nuclear localization signals. These new dimers then make their way to the nucleus. Once there, they latch onto specific bits of DNA and change how genes like Myc, Nos2, p21, and cytokines are expressed.

4.4 PLCγ Pathway

Phospholipase Cγ (PLCγ) is a membrane-associated enzyme activated following EGFR dimerization. Think of PLCγ as a catalyst, slicing phosphatidylinositol 4,5-bisphosphate (PIP2) into bits known as IP3 and DAG. IP3 elevates calcium inside cells, creating a ripple effect. At the same time, DAG triggers the activation of protein kinase C (PKC). Once roused, PKC doesn't sit idle—it jumps into action, sending signals down the line to mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase (JNK), impacting everything from cell movement to the complex labyrinths of calcium signaling.

4.5 Nck/PAK Pathway

Then we find the Nck adaptor protein, which connects subtly yet importantly with EGFR. Consider Nck as a networker in the cellular world, helping signals that promote cell migration and survival travel smoothly. Acting akin to a savvy scout, it zeroes in on p21-activated kinase (PAK), setting off a chain reaction. When PAK is on, it sparks MEKK1, eventually stirring up JNK through MKK4/7. At this point, JNK makes its way to the nucleus, where it meets transcription factors c-Fos and c-Jun. This meeting crafts cellular stories through gene expressions tailored to respond to various stimuli.

5. Related Products

Creative Biolabs is a trusted provider of high-quality EGF-related antibodies for research applications. We offer custom antibodies targeting key components of the ERBB family, Ras-Raf-MEK, PI3K/AKT, JAK/STAT, PLCγ, and Nck/PAK Pathway. Our products are designed to work well with multiple techniques like Western blot, immunohistochemistry, and ELISA, thereby assisting researchers in progressing their research.