lecture 19
Lecture 19 – Signal Transduction and Cell Signaling, Part 3
Major Topics Covered in Lecture
Cytokines, Cytokine Receptors, and the JAK/STAT Signaling Pathway (Section 16.4)
The TGF-β Family of Growth Factors, Their Receptor Serine Kinases, and the Smad Transcription Factors They Activate (Section 16.5)
Signal Transduction Pathways That Utilize Regulated, Site-Specific Protein Cleavage: Notch/Delta and EGF Precursors (Section 16.6)
Signal Transduction Pathways That Utilize Proteasomal Degradation of Signaling Components: Wnt, Hedgehog, and the Many Hormones That Activate NF-κB (Section 16.7)
Important Concepts to Remember about Phosphoinositide Signaling Pathways
RTKs and Cytokine Receptors
Initiate the IP3/DAG signaling pathway by activating Phospholipase C.
Initiate another phosphoinositide pathway by activating PI-3 kinase leading to the formation of PI 3-phosphates.
Signaling Complexes
PI 3-phosphates bind PH domains in various proteins, forming signaling complexes that activate PKB (Akt).
Cell Survival
Activated PKB promotes survival by directly inactivating pro-apoptotic proteins and synthesizing anti-apoptotic proteins.
Role of PTEN
PTEN phosphatase hydrolyzes the 3-phosphate in PI 3-phosphates, thereby inactivating the signaling pathways.
The Phosphoinositide 3-kinase Pathway (PI3K)
Definition
A family of Serine/Threonine kinases (greater than 15 human members).
Promotes cell growth and survival.
Diverse Ligands
Major ligands include Insulin-like growth factors among others.
Recruitment Mechanism
Activated receptor recruits PI3K to the plasma membrane, allowing PI3K to phosphorylate inositol phospholipids.
Activation of Signaling Pathways
Signaling Pathways via Phosphorylated Phospholipid Intermediates
Involves phosphatidylinositol (PI) derivatives, affecting long-term gene expression.
PI-3 Kinase Reactions
PI3K adds a 3-phosphate to:
PI(4)P transforming it to PI(3,4)P2
PI(4,5)P2 transforming it to PI(3,4,5)P3
Role of Protein Kinase B (PKB)
Docking Sites
Phosphorylated inositol phospholipids serve as docking sites for specific intracellular signaling proteins; a primary one being PKB (Akt).
Activation Mechanism of PKB
Action of other kinases at the membrane: PDK1 and PDK2 drive full activation of PKB by phosphorylation.
Function of Activated PKB
Targets a range of cytoplasmic proteins, enhancing their activities through phosphorylation and promoting cell survival.
Recruitment and Activation of Protein Kinase B (PKB) in PI-3 Kinase Pathways
In Unstimulated Cells
PKB resides in the cytosol, with its PH domain inhibiting its catalytic kinase domain.
Mechanism in Hormone Stimulated Cells
Step 1: Hormone stimulation activates PI-3 kinase to form PI 3-phosphates.
Step 2: The 3-phosphate group binds the PH domains of PKB (Akt) and PDK2, partially activating PKB.
Step 3: PDK1 and PDK2 sequentially phosphorylate specific serines in PKB, fully activating it to induce cellular responses.
Negative Regulation
The PTEN phosphatase dephosphorylates the 3-phosphate, negatively regulating the PI-3 kinase pathway.
Clinical Implication: PTEN deletion is found in various advanced human cancers, leading to uncontrolled cell growth.
Important Concepts about Signaling Pathways Controlled by Ubiquitinylation and Protein Degradation: Wnt, Hedgehog, and NF-κB
Irreversible Signaling Pathways
Ubiquitinylation and proteolysis of target proteins lead to irreversible or slow reversal of signaling pathways.
Wnt Signaling
Controls critical developmental processes, including brain development and organogenesis.
Hedgehog Signaling
Acts as a morphogen affecting various developmental events.
NF-κB
It is a master transcriptional regulator, especially important in the mammalian immune system.
Ubiquitin- and Proteasome-Mediated Protein Degradation
Role of Protein Degradation
Regulates the lifespan of intracellular proteins, which can vary significantly; some degrade within minutes while others may persist through the life of an organism.
Polyubiquitinylation
Targets proteins for proteasomal degradation; facilitated by E3 ubiquitin ligases that attach ubiquitin to target proteins.
Canonical Wnt Signaling Pathway
Absence of Wnt
TCF transcription factor is inhibited from activating target genes since it associates with transcriptional repressors like Groucho.
β-catenin is phosphorylated by kinases CK1 and GSK3, followed by degradation via ubiquitinylation.
Presence of Wnt
Wnt binding to receptor Frizzled and co-receptor LRP leads to phosphorylation of LRP and disruption of the β-catenin degradation complex.
Free β-catenin accumulates in the cell, translocates into the nucleus, binds to TCF, and activates gene expression.
Clinical Relevance
90% of human colon cancers show hyperactivity in the Wnt signaling pathway due to elevated free β-catenin levels.
Processing of Hedgehog (Hh) Precursor Protein
Definition of Morphogen
A signaling molecule acting directly on cells, creating specific responses based on local concentration gradients.
Hh Precursor Processing
Produces a 45-kDa precursor that undergoes autoproteolysis, involving:
Nucleophilic attack by cysteine 258 on glycine 257, forming a thioester intermediate.
The product retains cholesterol modification essential for tethering Hh to the plasma membrane.
Hedgehog Signaling in Vertebrates
Primary Cilia Function
Hedgehog signaling operates at primary cilia in vertebrates, crucial for cellular communication.
Absence of Hedgehog (-Hh)
Patch (Ptc) prevents Smo from entering the membrane, leading to Gli phosphorylation and the formation of the repressor GliR.
Presence of Hedgehog (+Hh)
Hh binding to Ptc results in Smo activation and Gli liberation to activate gene expression in the nucleus.
Mechanism of Signal Transmission
The active Gli* accumulates and modifies target gene expression.
Activation of the NF-κB Signaling Pathway
Characteristics of NF-κB
Dimeric transcription factor composed of p50 and p65 subunits, otherwise bound to inhibitor I-κBα in resting cells.
Activation Steps
Receptor activation phosphorylates I-κBα, leading to its ubiquitinylation and degradation.
This reveals the nuclear localization signals (NLS) of NF-κB, allowing its translocation to the nucleus to induce gene expression.
Important Concepts about Notch/Delta Signaling Pathway
Role of Cleavage
Growth factors and signaling proteins are released by matrix metalloprotease cleavage, crucial for the Notch/Delta pathway.
Interaction Process Between Notch and Delta
Stepwise engagements leading to Notch cleavage and activity within the nucleus, determining cellular fates during development.
Inappropriate Cleavage of APP in Alzheimer’s Disease
Cleavage Enzymes Involved
ADAM 10 and γ-secretase generate harmless peptides, while β-secretase and γ-secretase result in harmful amyloid aggregates tied to Alzheimer’s.
Integration of Cellular Responses to Multiple Signaling Pathways
Cell Signals
Cells experience diverse signals activating unique pathways influenced by expressed receptors.
Cross-regulation
Kinases exhibit extensive interactions among pathway proteins, enhancing the complexity of cellular responses.
Pathway Integration
Common features exist between varied pathways; signals can activate pathways uniquely in different cell types.
Points to Keep in Mind
Cells are bombarded with numerous signals, and the cellular responses are dependent on available receptors.
Pathways might intersect, allowing different signals to activate the same pathway or leading to complex responses depending on the cell type.
Lecture Conclusion and Next Steps
Next Lecture: Cell Organization and Movement 1: Microfilaments
Reading Assignment: Chapter 17, pages 752-795
Reminder about available adaptive quiz on Achieve site, open until Thursday (Nov 6th) at 8:00 am.