Molecular Cell Biology Lecture #19: Receptor Serine-Threonine Kinase & Non-Receptor Tyrosine Kinase

Molecular Cell Biology Lecture #19: Receptor Serine-Threonine Kinase & Non-Receptor Tyrosine Kinase

Instructor: Mitra Esfandiarei, Ph.D.
Contact: mesfan@midwestern.edu
Date: October 8, 2025

Learning Objectives

• Understand Differences in Receptors:

  • Differentiate between receptors with intrinsic enzyme activity and those associated with intracellular kinases.

• IL-6 Receptor Activation & JAK/STAT Pathway:

  • Describe the downstream pathway activated by IL-6 receptor and the implications in hepatocellular carcinoma.

  • Explain the mechanism of STAT1/2 activation following IL-6 receptor engagement.

• Role of Intracellular Kinases:

  • Understand how intracellular kinases like JAK transduce signals initiated by ligands binding to receptors without intrinsic enzyme activity (example: IL-6 receptor).

• Receptor Serine/Threonine Kinases:

  • Understand the structure and activation mechanisms of receptor serine/threonine kinases.

• TGF-β Signaling:

  • Explain TGF-β binding to its receptors and the subsequent activation pathways.

  • Describe the distinct roles of type 1 and type 2 TGF-β receptors.

  • Draw the Smad2/3/4 activation cascade and discuss Smad-7's inhibitory role in TGF-β signaling.

  • Understand downstream effects of TGF-β signaling.

• Link Between Fructose, Inflammation & ROS:

  • Describe the effects of excessive fructose consumption, including its impact on oxidative stress and inflammation in adipocytes, gut, and liver.

• NFkB Signaling Pathway:

  • Identify main activators of the NFkB pathway and their role in inflammatory responses.

  • Explain how inflammatory cytokines like TNF-alpha induce NFkB pathway activation.

Enzyme-Coupled Receptors (Cytokine Receptors)

• Characteristics:

  • Receptors without intrinsic enzyme activity.

  • Rely on other intracellular enzymes (kinases or phosphatases) for signal transduction.

  • Important in immune and inflammatory responses.

JAK/STAT Signaling Pathway & Clinical Relevance: Hepatocellular Carcinoma

• IL-6 Details:

  • Ligand: Interleukin-6 (IL-6)

  • Source: Macrophages (specifically Kupffer cells in the liver)

  • Target Cells: Endothelial cells and hepatocytes

  • Receptor: IL-6 receptor

  • End Effects: Proliferation & angiogenesis related to cancer progression.

• Clinical Significance:

  • Hepatocellular carcinoma (HCC) ranks as the third leading cause of cancer-related deaths globally.

  • IL-6 is crucial for HCC development; its binding to IL-6R activates JAK/STAT pathway, promoting tumor angiogenesis and hepatocyte proliferation.

Activation of the JAK/STAT Pathway by IL-6

1. Components of the JAK/STAT Pathway:

   • Receptor Dimer: The IL-6 receptor itself has no intrinsic enzyme activity.

   • Cytoplasmic Tyrosine Kinase: Janus kinase (JAK).

   • Transcription Factor: Signal Transducer & Activator of Transcription (STAT).

2. Mechanism:

   • IL-6 binds to IL-6R, leading to receptor dimerization.

   • Dimerization activates associated JAKs, which cross-phosphorylate to enhance activity.

   • Activated JAKs phosphorylate specific tyrosine residues on IL-6R, providing docking sites for STAT1/2.

   • JAKs then phosphorylate STAT1 and STAT2, prompting them to dimerize.

   • These dimers translocate to the nucleus and bind to DNA sequences to stimulate transcription of target genes relevant to proliferation and inflammation.

   • Clinical Applications: FDA-approved JAK inhibitors (e.g., ruxolitinib, tofacitinib) target metastatic pancreatic cancer and myelofibrosis.

Transforming Growth Factor Beta (TGF-β) and Pulmonary Fibrosis

• TGF-β Details:

  • Ligand: Transforming Growth Factor Beta (TGF-β)

  • Source: Myofibroblasts, Mesenchymal cells

  • Target Cells: Fibroblasts

  • Receptors: TGF-β Receptor 1 (TGFβR-1) and TGF-β Receptor 2 (TGFβR-2)

  • End Effects: Collagen deposition and fibrosis.

• Clinical Relevance:

  • Pulmonary fibrosis is a chronic lung condition characterized by lung tissue damage and scarring, reducing lung elasticity and making breathing difficult.

  • TGF-β is a key factor stimulating fibroblast activity and collagen deposition in the lungs.

TGF-β Signaling Pathway

1. Step I - TGF-β Binding:

   • TGF-β binds to TGFβR-2 (a serine/threonine kinase receptor).

   • Binding recruits and phosphorylates TGFβR-1, activating it to trigger downstream signaling.

2. Step II - Smad Activation:

   • Activated TβRI phosphorylates Smad2/3 (receptor-regulated Smads).

   • Phosphorylated Smads bind to Smad4, forming the Smad2/3/4 transcription factor complex.

3. Step III - Nucleus Regulation:

   • Smad2/3/4 translocate to the nucleus, influencing the transcription of target genes associated with ECM production, inflammation, reactive oxygen species (ROS) production, and apoptosis regulation.

   • Smad7 Role: Functions as an inhibitory feedback regulator, blocking Smad2/3/4 translocation to the nucleus.

Summary of Growth Factors & Cytokines

Receptors That Signal via Cytoplasmic Adaptor Proteins

• Many immune response and inflammation-regulating receptors do not possess intrinsic enzymatic activity.

• Rely on adaptor proteins to recruit downstream signaling molecules and propagate signals into the cell.

General Principles:

• Lack of enzymatic activity differentiates these from receptor tyrosine kinases (RTKs).

• Cytoplasmic adaptor proteins serve as scaffolds, connecting receptors to kinases and other signaling complexes.

• Signal Diversification: Adaptor proteins allow a single receptor to incur multiple cellular responses through diverse effector recruitment.

Fructose, Inflammation, and Cellular Stress Pathways

• Excessive fructose metabolism occurs primarily in the liver, bypassing key glycolytic regulatory steps, leading to increased acetyl-CoA and triglyceride production.

• Accumulation of fat induces stress responses and activates inflammatory pathways like NFκB and JNK, resulting in hepatic steatosis.

• Fructose increases gut permeability, allowing bacterial endotoxins (e.g., LPS) to enter circulation, which activates TLR4 in hepatocytes, exacerbating inflammation.

• High fructose levels can also stimulate the JNK pathway, linking to insulin resistance and additional inflammatory cytokine production.

Fructose and Uric Acid Production

• Fructose is primarily metabolized in the liver, phosphorylated by fructokinase to form fructose-1-phosphate (F1P).

• Fructokinase operates unregulated, allowing rapid phosphorylation of fructose and significant depletion of hepatic ATP.

• Low ATP levels promote the action of adenylate kinase (myokinase), converting ADP and AMP, with AMP further degrading into uric acid via xanthine oxidase.

• Elevated uric acid contributes to oxidative stress and may precipitate inflammation conditions like gout and kidney stones.

NFκB Signaling Pathway Related to Oxidative Stress & Inflammation

• NFκB proteins act as transcription regulators for inflammatory and innate immune responses to oxidative stress and injury.

• Receptor activation leading to NFκB activation is mediated by:

  • Toll-like receptors (TLRs)

  • Interleukin-1 receptors

  • Tumor necrosis factor alpha receptors

• Activation cascades involving multiprotein ubiquitinylation and phosphorylation release NFkB from its inhibitory protein (IkB), allowing NFkB to translocate to the nucleus and promote transcription of inflammation-related genes.

• Cytokines such as TNFα1 trigger IKK-NEMO complex recruitment, initiating NFkB signaling.

This lecture encompasses detailed mechanisms of cell signaling pathways, key molecules involved, and their relevance to clinical conditions, particularly focusing on inflammation, cancer progression, and metabolic disorders related to fructose consumption. Understanding these pathways is essential for exploring potential therapeutic interventions for diseases influenced by these molecular mechanisms.