W32N Kinase-Linked Receptors

• Receptor superfamilies are groups of receptors sharing similar basic molecular structures and signal transduction pathways
• There are four major receptor superfamilies.

Receptor Superfamilies:

• Ligand-gated/Ion channel-linked receptors
• G-protein-coupled receptors
• Kinase-linked receptors
• Intracellular/Nuclear receptors

Enzyme-Linked Receptors:

• These are diverse multi-subunit transmembrane receptor protein complexes
• They contain an intrinsic enzyme activity in their intracellular domain or directly associate with an intracellular enzyme
• Ligand binding induces a conformational change in the receptor protein, transmitted via a transmembrane helix
• This results in activation of an intrinsic/associated enzyme, initiating signaling cascades
• They mediate effects of growth factors, cytokines, and hormones
• They regulate cell growth, proliferation, differentiation, and inflammation

Classification of Enzyme-Linked Receptors:

• There are five main types of enzyme-linked receptors.
• Receptor Tyrosine Kinases (RTKs) include EGFR and IR, containing intrinsic tyrosine kinase activity
• Receptor Serine/Threonine Kinases (e.g., TGF-βR) activate and phosphorylate amino acid residues, containing intrinsic serine/threonine kinase activity
• Tyrosine Kinase-Associated Receptors (Cytokine Receptors) associate with proteins that have tyrosine kinase activity
• Receptor Guanylyl Cyclases such as ANPR contain intrinsic cyclase activity
• Receptor Tyrosine Phosphatases.

Receptor Tyrosine Kinases (RTKs):

• RTKs are cell-surface receptors mediating the actions of polypeptide and protein hormones and growth factors
• They contain an intrinsic tyrosine kinase domain that becomes active upon ligand binding, initiating signaling cascades
• They are key regulators of cell processes such as proliferation, differentiation, survival, metabolism, migration, and cell cycle control
• Mutations in RTKs can cause diseases, including cancers
• There are 58 RTKs in the human genome, classified into 20 families (Type I-XX)

Epidermal Growth Factor Receptors (EGFRs):

• EGFRs are a type of receptor tyrosine kinase
• They regulate cell proliferation, differentiation, growth, survival, and migration
• They mediate the actions of peptide growth factors like EGF and TGF-α
• EGF is synthesized and released from organs like the kidney and submaxillary gland
• EGF/EGFR signaling promotes embryonic development, stem cell regeneration, ion transport regulation, and wound healing
• Dysregulation of EGF/EGFR expression, such as mutation and overexpression, can lead to cancer

EGFRs - Structure and Activation:

• EGFRs have a typical RTK structure.
• An extracellular ligand-binding domain
• A single transmembrane (TM) helix domain
• A juxtamembrane region
• An intracellular catalytic/transcellular tyrosine kinase domain (TKD), adaptor domains with tyrosine kinase residues, and a flexible C-terminal tail
• EGF binding causes dimerization of EGFR monomers, leading to a conformational change and release of cis-autoinhibition
• Trans- and autophosphorylation of tyrosine residues occur in the cytoplasmic domains
• Phosphorylated tyrosine residues act as a platform for recognition and recruitment of adaptor/effector proteins, initiating downstream signaling

EGFR Signaling Pathways:

• Activated EGFRs trigger two main downstream signaling pathways.
• RAS/MAPK
• PI3K/AKT

RAS/MAPK Pathway:

• Grb2, an adaptor protein, binds to activated EGFRs, recruiting and activating GEF (SOS)
• GEF (SOS) activates RAS via GDP exchange for GTP
• RAS-GTP activates RAF kinase (MAPKKK), activating MEK 1 or 2 (MAPKK), then activates ERK 1 or 2 (MAPK)
• MAPK (ERK 1 or 2) phosphorylates and activates effector proteins in the cytoplasm
• MAPK (ERK 1 or 2) translocates to the nucleus, phosphorylates transcription factors (CREB, ELK-1, c-Fos, c-Jun), promoting cell growth, proliferation, differentiation, and survival

PI3K/AKT Pathway:

• Grb2 associates with Gab1 and recruits PI3K to the plasma membrane, leading to PIP2 phosphorylation and PIP3 generation
• PIP3 accumulation co-localizes Akt/Protein Kinase B and phosphoinositide-dependent protein kinase 1 (PDK1) on the plasma membrane
• AKT/PKB is phosphorylated by PDK1 and mTOR complex 2
• Activated AKT translocates to the cytosol phosphorylating target/effector proteins, influencing cell growth, proliferation, motility, and survival

Tyrosine Kinase-Associated Receptors - Cytokine Receptors

• Cytokine Receptors do not have inherent tyrosine kinase activity but transduce signals from cytokines, important for inflammation and immunity
• They often associate with intracellular non-receptor tyrosine protein kinases

Cytokines:

• Cytokines are small proteins produced by immune cells that facilitate cell communication
• They are important in cell development, differentiation, immune, and inflammatory responses
• Cytokines use multiple signaling pathways, including the JAK/STAT pathway

JAK/STAT Pathway:

• Focus is on intracellular signaling through Janus kinase (JAK) and signal transducer & activator of transcription (STAT)
• JAKs 1-3 are intracellular non-receptor tyrosine protein kinases converting extracellular stimuli into cellular processes
• STATs are transcription factors and SH2-domain proteins

The JAK/STAT Signaling Pathway Steps:

• Cytokine binds to its receptor, causing receptor dimerization and transphosphorylation/activation of associated JAKs
• Activated JAKs phosphorylate tyrosine residues in cytokine receptor cytoplasmic domains
• These residues act as docking sites for STATs
• Once docked, STATs are phosphorylated and activated by receptor-associated JAKs
• Phosphorylated STATs dissociate from receptors, dimerize, and translocate to the nucleus to activate gene transcription
• Activated transcription/translation produces proteins that mediate immune responses and inflammation

EGFR & Signaling Pathways - Pathophysiology:

• EGFR and its signaling pathways are important in cancer development
• Examples include fibro-sarcomas, glioblastomas, mammary, ovarian, colorectal, and non-small cell lung cancer
• Oncogenic transformation results from loss of auto-control or increased autocrine signaling
• Two major classes of EGFR-targeted anticancer therapies have been developed
• Humanized monoclonal antibodies against the EGFR extracellular domain which block ligand (EGF) binding, such as cetuximab and panitumumab
• Tyrosine kinase inhibitors (TKIs) are ATP mimetics that bind to the receptor's kinase pocket, blocking ATP binding and signal transduction, examples include erlotinib, gefitinib, and lapatinib

Cytokine Receptors & JAK/STAT Signaling – Pathophysiological Role:

• Dysregulation of cytokine receptor-stimulated JAK/STAT signaling is involved in cancers and immune/inflammatory disorders

• Examples include rheumatoid arthritis, atopic dermatitis, psoriasis, and inflammatory bowel disease

• Increased JAK/STAT activity and decreased activity of intrinsic negative regulators lead to upregulation of pro-proliferative, anti-apoptotic, pro-inflammatory, and immunosuppressive proteins

• JAK inhibitors treat immune/inflammatory disorders

• Tofacitinib (JAK1, 2 & 3 inhibitor) treats rheumatoid arthritis, psoriatic arthritis, and ulcerative colitis

• Baricitinib, Filgotinib, and Upadacitinib (JAK1 inhibitors) treat rheumatoid/psoriatic arthritis and IBD

• Ruxolitinib (JAK1 & 2 inhibitor) treats polycythaemia and myelofibrosis