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Receptor-Ligand Interaction

• Receptors are proteins altering ligand conformation for intracellular signal transduction.
• Strong binding enables transduction, but weak enough for detachment post-mission.
• Receptor-ligand interactions can be reversible via weak forces or irreversible through covalent bonds, such as alpha-bungarotoxin or aspirin.
• Receptor classification relies on location, either in the plasma membrane or inside the cell.
• Receptor examples include Tyrosine Kinase Receptors (RTKs), intracellular receptors, GPCRs, voltage-operated channels, and receptor/ligand-operated channels.

Receptor Tyrosine Kinases (RTKs)

• RTKs, located in the plasma membrane, are growth factor receptors with several types that share common structural elements.
• RTKs include an ectodomain for ligand binding (extracellular), a transmembrane region, a juxtamembrane domain (intracellular), and a C-terminal tail for signaling protein interaction.
• These molecules undergo post-translational modifications.
• Gamma secretase complex cleaves protein and can be subject to mutations causing unwanted cleavage, related to neurological and neurodegenerative disorders like Alzheimer's.
• The RTK family comprises over 100 receptors and ligands, involving cytokines, growth factors, or hormones.
• Approximately 20 RTK families exist.
• EGFR ligands include EGF (mitogenic effect on epithelial cells), TGF-alpha, Amphiregulin, and HB-EGF (mitogenic and chemiotactic effects).
• Neuregulin is a growth factor family that stimulates ErbB RTKs (ErbB1, ErbB2, ErbB3, or ErbB4).
• Neuregulin has 60 isoforms encoded by 6 genes with a conserved intracellular domain and cytoplasmic tail but diverse extracellular domains.

ErbB Receptor Varieties

• ErbB isoforms arise through alternative splicing/mRNA use and function as dimers.
• ErbB2 cannot bind a ligand, rendering its homodimers non-functional.
• ErbB3 can bind a ligand but cannot transduce a signal, making its homodimers non-functional.
• In the PNS, the ErbB2-ErbB3 heterodimer is potent, initiating signaling rapidly and strongly, implicating glial cell differentiation and potentially leading to tumor generation.
• Partnering ErbB4 preferentially with ErbB2 in cells expressing both when ErbB4 transduces a weak signal.

Receptor Activation Steps

• Receptor activation comprises ligand binding and conformational change, receptor dimerization, transphosphorylation, and intracellular substrate recruitment with downstream signal cascade activation.
• Ligand proximity initiates RTK dimerization from their steady-state monomeric forms
• Kinase domains juxtapose upon receptor dimerization in the extracellular region.
• Trans-phosphorylation of serine, tyrosine, or threonine residues stabilizes the complex.

Receptor Dimerization Processes

• PDGFR ligands form dimers before receptor binding to trigger the receptors.
• EGFR ligands bind to a subunit of each receptor
• INSR ligands possess two binding sites that induce a conformational change in an already existing receptor dimer, which lacks functionality.
• FGFR ligands feature two binding sites, requiring two ligands for receptor dimerization, further stabilized by ECM modification.

Recruitment of Intracellular Substrates

• Intracellular proteins are recruited via specific domains (PTB or SH2) in docking proteins, which recognize the phosphorylated receptor and activate downstream signaling.
• PTB domain-containing proteins are involved in phosphorylated receptor binding and triggering signal cascades.
• SH domains can be located in proteins, but also found in enzymes, transcription factors STAT, scaffold proteins, or signaling regulators.

Downstream Signaling Pathways

• MAPK-MEK, PI3K-Akt, PLCgamma-PKC, JAK-STAT, and Rac-Rho pathways have variable outputs depending on the cell type, expression timing, and involved ligands.
• The Ras-MAPK pathway regulates cell proliferation, differentiation, and metabolism, with Ras mutations linked to cancers like pancreatic, colorectal, bladder, stomach, and endometrial cancers.
• Docking protein Grb2 binds the phosphorylated site and recruits Ras.
• SOS phosphorylates Ras, activating the Ras-Raf-MAPK pathway.
• The Ras-Raf-MAPK pathway affects transcriptional regulation by ending in the nucleus, resulting in permanent changes and long-term effects.
• A GTPase called Ras, besides functioning as an oncogene, is regulated by GEF and GAP
• Phosphorylation of the RTK receptor activates GEF to activate Ras
• The PI3K-Akt signaling pathway governs migration, proliferation, survival, angiogenesis, protein, and glycogen synthesis, with mutations leading to cancer.
• Subunits p85 and p110 of PI3K recruit and control the activation of PIP2.
• PIP3, created once PI3K activation, is another docking site to activate a protein, also known as AKT/PKB.
• PLCgamma-PKC contains the regulatory mechanisms for calcium homeostasis by phosphorylating them.
• This includes the regulators IP3 (release of intracellular calcium) and DAG .
• This is accomplished once it recognizes the phosphorylated receptor via an SH2 domain.
• The process of phosphorylation, proliferation, and inflammatory response of the target cell are affected by this passage.
• SH2 domain containing STAT functions as a transcription factor and docking protein.
• Long-term modifications are a result once it dimerizes, reaches the nucleus, and recognizes the phosphorylated receptor.

RTK Signaling Modulation

• Strategies are required to shut down overactive signaling of RTKs to stop the process of constitutive activation that causes cancer.
• Ligand sequestration can sequester RTKs.
• RTK autophosphorylation may be stopped.
• Protein phosphorylation of different proteins like Ras, Raf, MEK, and PI3K can be inhibited by docking.
• Ligand-induced receptor ubiquitination and degradation occurs with signaling endosomes (endosomes and lysosomes).
• Relocalization of the receptor to another location within the cell that does not use ubiquitination results in the signal, or lysosome degradation.
• Diseases can result from aberrant activation of RTKs.
• Causes of pathway abnormality can be autocrine activation, chromosomal translocations, RTK overexpression, and GOF point mutations.
• Monoclonal antibodies are the primarily pharmacological approach used.
• Targeted therapies are favored that focus on patient alterations and disease monitoring.

RTKs in Disease - EGFR

• Deletion of extracellular portion causing carcinomas and pediatric tumors
• Drug treatments: Cetuximab for colorectal cancer, Panitumumab for colorectal cancer, and Lapatinib for mammary carcinoma.
• Can be teamed up with classical treatments like chemotherapies that can enhance a immune response and promote cytotoxicity.

RTKs in Disease - ErbB2

• Breast cancer, but also some lung cancers, results.
• Treatable with a variety of drugs: Trastuzumab and Lapatinib

RTKs in Disease - VEGF

• It induces angiogenesis and appears in bladder (primarily), brain, breast, colon, gastric, ovarian, prostate, head, and neck cancers.
• Drug treatments include: Bevacizumab for colorectal cancer and lung cancer.
• Other drugs include Sunitinib, Axitinib, and Ramucirumab

RTKs in Disease - PDGFR

• The location is primarily low and high grade gliomas in the central nervous system.
• It also shows in ovarian and lung cancers.
• Drug treatments include: Sunitinib and TKI258.

Drug Development Naming

• -omab suffix denotes rodent developed therapies.
• -umab suffix signifies human derived drugs.
• The more effective and precise human antibodies cost more and have decreased side effects.

RTKs - Cancer Drugs and MOA

• Cause cytotoxity leading to tumor destruction pathways
• Affecting tumor survival
• Promote apoptosis
• Deliver cytotoxins
• Generate controlled immune responses to promote cell targeted cytotoxity
• Promote immune action by stimulating certain lymphocytes that promote the immune response.
• Act on new angiogenesis

Therapeutic Approaches

• Scavengers that bind to ligands as they pass
• Block dimerization using antagonists that interferes with ligand use.
• The EGFR- Cetuximad, for example.
• Promote cell lysis by loading the antibodies with chemotherapeutic reagents.
• Use antibodies that affect immune regulators like CD20s

Monoclonal Ab - Problems

• Limited efficiency due to poor penetration and toxicity

Intracellular Receptors

• Must be lipophilic to pass directly across the outer membrane.
• Hormones such as steroid, thyroid, and retinoic acid are common examples.
• A carrier protein is required for necessary bodily areas due to ligand lipophilic nature.
• Binding causes a change exposing co-activator binding sites.
• Alters transcription causing long-term changes for expression.
• They are activators or TF inhibitors.
• Location can be either cytoplasm or directly to NLS
• Hormone binding, DNA binding and Transactivation all make up the final structure.
• In order to prevent translocation unless necessary, the cytoplasmic receptors are kept inactive by HSP proteins.
• The steady level is maintained in the already bonded receptors via bonding to an antagonist.