Molecular Cell Biology Lecture Notes: RTKs and Insulin Signaling

Lecture Overview

  • Title: Receptor Tyrosine Kinases I

  • Presenter: Mitra Esfandiarei, Ph.D.

  • Date: October 1, 2025

  • Contact: mesfan@midwestern.edu

Learning Objectives

  • Describe the function of a growth factor.

  • Name different types of growth factor receptors.

  • Understand the general structure of monomer and dimer growth factor receptors.

  • Understand the differences between receptors with intrinsic enzyme activity vs. those associated with intracellular kinases (enzyme-linked).

  • Understand the general properties of receptor tyrosine kinases (RTKs).

  • Understand the general mechanism for RTK activation and the role of phospho-tyrosine docking sites in attracting intracellular signaling molecules.

  • Understand the main function of binding domains such as “SH2”, “PTB”, and “SH3” in cell signaling and RTK activation.

  • Understand the general structure of insulin receptors.

  • Describe the role of the insulin signaling pathway in metabolic response, growth, and survival.

  • Distinguish between metabolic vs. mitogenic roles of insulin.

  • Understand different functions of insulin in the human body and targeted tissues.

  • Explain the activation of MAPK signaling by insulin and the role of “SH2” and “SH3” domains in this process.

  • Understand the role of monomeric G protein “Ras” (Ras GTPase) in insulin signaling.

  • Understand the function of Grb2 as an adaptor protein in the MAPK pathway.

  • Understand the role of Sos as a GEF protein in activating Ras-GTPase in response to insulin.

  • Understand the downstream effects of active ERK1/2 (nuclear & cytoplasmic).

  • Draw and explain insulin-mediated activation of the PI3K/Akt signaling pathway step by step, including the roles of “SH2” and “PH” domains.

  • Understand the specific roles of PDK1, Akt, and mTORC2.

  • Explain the main downstream events of Akt activation, focusing on glucose entry (effects on GLUT4), glycogen synthesis (effects on GSK3), and mTORC1 activation.

  • Understand the regulation of glycogen synthase enzyme and glycogen storage by GSK3.

  • Understand the downstream effects of mTORC1 activation on lipid and protein synthesis and glucose metabolism.

Growth Factors

  • Definition: Biologically active molecules (ligands) regulating cellular processes including:

    • Proliferation

    • Differentiation

    • Survival

    • Migration

    • Programmed cell death (apoptosis)

  • Importance:

    • Essential for normal development, tissue repair, and homeostasis.

  • Mechanism:

    • Signaling molecules binding to specific receptors on target cells, initiating intracellular events leading to changes in cell behavior.

  • Mode of action: Secreted by cells, functioning through paracrine and endocrine mechanisms.

Growth Factor Receptors (GFRs)

  • Definition: Membrane-bound receptors that can contain intrinsic enzymatic activities or be enzyme-linked.

  • Structural Domains:

    • Extracellular domain: Binds ligands.

    • Transmembrane domain: Anchors receptor in the cell membrane.

    • Cytoplasmic domain: Contains the enzyme activity.

  • Common Types:

    • Receptor Tyrosine Kinases (RTKs)

    • Receptor Serine/Threonine Kinases

    • Cytokine Receptors

    • G-Protein Coupled Receptors

  • Note: Cytoplasmic domains of receptor tyrosine kinases and serine/threonine kinases have intrinsic enzyme activities.

Receptor Tyrosine Kinases (RTKs)

  • Description: Single subunit receptors (monomer) that form dimers after activation.

  • Associated Growth Factors: Insulin, epidermal growth factor (EGF), fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), and vascular endothelial growth factor (VEGF).

  • Activation Mechanism:

    • Dimer ligand (1 dimer or 2 monomers) triggers activation of RTK.

Mechanism of RTK Activation

  • By dimer ligand: One dimer ligand activates and dimerizes the receptor.

  • By monomer ligand: Two molecules of a monomer ligand are required for activation and receptor dimerization.

General Mechanism of RTK Activation: Overview

  • Two receptors dimerize in response to ligand binding.

  • Intercellular kinase domains auto-phosphorylate and cross-phosphorylate on multiple tyrosine sites, amplifying kinase activity.

  • Newly phosphorylated tyrosine residues create docking sites for intracellular signaling molecules, recruited using PTB and SH2 domains.

  • Recruited signaling molecules activate downstream target proteins, initiating a signaling cascade.

Clinical Relevance: Insulin Signaling

  • Insulin: A critical hormone from beta cells of the pancreas, involved in metabolic response and growth.

  • Functions of Insulin:

    • Glucose uptake

    • Lipogenesis

    • Glycogenesis

    • Systemic cell growth and division.

  • Conditions:

    • Hyperinsulinemia linked with weight gain, insulin resistance, cancer risk, non-alcoholic fatty liver disease, neuropathy.

  • Structure:

    • Monomeric insulin receptor as heterodimer (α2β2) formed after activation.

Downstream Pathways of Insulin Receptor Activation

  1. Phosphatidylinositol 3-kinase/protein kinase B (Akt) pathway.

  2. Mitogen-activated protein kinase (MAPK) pathway.

    • GLUT4 functions to transport glucose into fat and muscle cells.

Activation of MAPK Signaling Pathway

  • Mechanism:

    • Insulin receptor autophosphorylation leads to Grb2 binding through its SH2 domain.

    • Grb2 uses SH3 domain to activate Sos (Ras GEF).

    • Ras (Ras-GTPase) activates downstream proteins (Raf, Mek, Erk).

  • Functions of Active Erk:

    • Activate cytoplasmic signaling for proliferation.

    • Move to the nucleus to activate transcription factors for genes involved in cell proliferation.

Activation of PI3K/Akt Signaling Pathway by Insulin

  • Process Overview:

    • Insulin binds to the RTK, activating intrinsic kinase activity; recruits IRS-1/2 via SH and PTB domains.

    • Phosphorylation of IRS creates docking sites for downstream signaling molecules.

    • PI3K activation by binding to phosphorylated IRS generates PI(3,4,5)P₃.

    • PI(3,4,5)P₃ recruits PDK1 and Akt to the membrane; PDK1 phosphorylates Akt.

    • mTORC2 further phosphorylates Akt for full activation.

Downstream Events of AKT/PKB Activation

  1. GLUT4 Translocation & Glucose Uptake:

    • Akt prompts translocation of GLUT4 to the plasma membrane, enhancing glucose uptake in various tissues.

  2. Glycogen Synthesis & Storage:

    • Akt inhibits GSK-3, promoting glycogen synthase activity for glycogen synthesis.

  3. Regulation of Metabolism:

    • Through mTORC1 activation, regulates protein synthesis, glycolysis, lipid synthesis, and inhibits autophagy.

Insulin Impact on Glycogen Storage: Overview

  • Insulin binds to the receptor → activates IRS → activates PI3K → generates PIP3 → activates PDK1 → phosphorylates how Akt is activated:

    • Akt phosphorylates GSK3, inhibiting it, which allows glycogen synthase to remain active.

    • In the absence of insulin, GSK3 phosphorylates glycogen synthase, reducing glycogen synthesis.

Downstream Effects of Akt Activation: mTORC1

  • Active Akt phosphorylates mTORC1, influencing:

    • Protein synthesis

    • Glycolysis

    • Lipid synthesis

    • Autophagy regulation

    • SREBP-1c activation, enhancing lipogenesis.

Conclusion

  • Receptor Tyrosine Kinases play a central role in cell signaling related to growth factors, metabolism, and overall cell function, influencing various biological processes through complex pathways involving insulin and its signaling mechanisms.