1 Signaling pathways

SIGNALING PATHWAYS

  • Importance of understanding signaling pathways in cellular processes

  • Examples to link previous topics on cellular signaling

LEARNING OUTCOMES

  • Describe the basic organization of signaling pathways

  • Explain key molecular events during signaling pathways using examples

  • Make connections between cellular signaling and other processes (e.g. cell division)

  • Predict consequences of defects in signaling components related to diseases

Mitogenic Signal Transduction Pathways

  • Example: Ras signaling pathway

Signaling Cascade

  • Example: Epidermal Growth Factor (EGF) signaling cascade

  • Phosphorylation is a key post-translational modification

  • Many components in signaling pathways are kinases

  • Transcription factors (TFs) like c-myc can be involved

  • Consideration of gain-of-function vs loss-of-function mutations

Growth Factor Signaling

  • EGFR dimerization and phosphorylation initiates signaling

  • Recruitment of signaling molecules (Grb2, SOS)

  • SOS recruits Ras, activating downstream kinases (Raf, MEK)

  • The cascade ultimately phosphorylates transcription factors to transcribe cell cycle genes like c-myc

Signaling and Entry into the Cell Cycle

  • Cytoplasmic signaling leads to an increase in c-myc, affecting transcription

  • Serum contains mitogenic signals that activate receptor tyrosine kinases

  • Key molecules include Grb, Ras, Raf, MEK, and MAPK involved in the cascade leading to gene transcription

Cell Cycle Regulation

  • Increased synthesis of cyclin D, CDK4, CDK6 mediated by signaling

  • Transcription of cyclin E, CDK2 pushes cell progression through the cell cycle

Ras as a Molecule in Pathways

  • Ras is a small GTPase critical in signaling pathways

  • It is modified (farnesylated) for membrane localization

Switching Ras On and Off

  • GEF (GTP exchange factor) promotes conversion of Ras-GDP to Ras-GTP (active)

  • SOS acts as a GEF for Ras

  • GAP (GTPase activating protein) hydrolyzes GTP to switch Ras off

RAS - An Important Molecule

  • Ras connects various signaling pathways and is affected by post-translational modifications

  • Mutations in Ras lead to significant issues including cancers

RAS Isoforms

  • Four isoforms: HRAS, NRAS, KRAS4A, KRAS4B

  • High homology but different localization and trafficking

RAS and Membrane Localization

  • Post-translational modifications enable Ras to anchor to the plasma membrane

RAS Farnesylation

  • CAAX motif at C-terminus dictates farnesylation by FTase

  • Geranylgeranyltransferase can act on some Ras isoforms under certain conditions

Mutations in Mitogenic Signaling Pathway

  • Types of mutations affecting growth factors, receptors, and signaling components

  • Importance of understanding these mutations in relation to cancer development

RAS Mutations Impact

  • Mutant RAS forms cannot be switched off, leading to uncontrolled cell division

  • Constitutive activation of RAS due to mutations (like G12) is prevalent in cancers

Transformation Assay Techniques

  • Anchorage-independent growth as a sign of carcinogenesis

  • Soft agar colony formation assay for evaluating malignant transformation in vitro

Tumor Formation Studies

  • Example studies on NIH 3T3 cells and H-Ras mutations

  • Correlation of mutations with tumor volume and behavior in vivo

RAS Mutations in Cancer

  • Graphs showing frequencies of RAS mutations across different organ types

  • Common mutations like G12D and G12V noted in various cancers

RAS as a Drug Target

  • Challenges in targeting Ras due to complexity and mutations

  • Exploration of alternative drugs targeting Ras modifications

Signaling Pathway in Adult Stem Cells

  • Wnt pathway as an important signaling mechanism for maintaining stem cells

Cancer and Cell Division

  • Understanding how cancer arises from uncontrolled division in stem cells

  • Importance of stem cells in tissue repair and normal cellular turnover

Wnt Signaling Pathway in Colon Stem Cells

  • Wnt signaling is crucial for division and maintenance of stem cell populations in the colon

Wnt Pathway and Cancer

  • Inactivation of APC or AXIN is often the first step in colon cancer development

Wnt Target Genes in Cancer

  • Summary of genes affected by the Wnt pathway

  • Importance of these genes in cancer biology and stem cell function

Summary of Key Points

  • Recap of the organization of Ras and Wnt signaling pathways

  • Connections between cellular signaling, division, and cancer development

  • Understanding the consequences of signaling component defects and mutations.

Here are some possible multiple choice questions based on the learning outcomes for the signaling pathways notes:

  1. Which of the following best describes the basic organization of signaling pathways?A) Linear pathways onlyB) Branching and interconnected pathwaysC) Circular pathways exclusivelyD) Randomized signaling without structureAnswer: B) Branching and interconnected pathways

  2. What is the function of phosphorylation in signaling pathways?A) It inhibits all signaling eventsB) It serves as a key post-translational modificationC) It solely activates transcription factorsD) It only involves GTP hydrolysisAnswer: B) It serves as a key post-translational modification

  3. Which molecule is crucial for the activation of downstream kinases in signaling pathways?A) c-mycB) RasC) EGFD) MEKAnswer: B) Ras

  4. What happens when a mutation affects Ras in signaling pathways?A) It always leads to enhanced cell divisionB) It has no effect on cell behaviorC) It can lead to either loss or gain of functionD) It simplifies the signaling cascadeAnswer: C) It can lead to either loss or gain of function

  5. In the context of tumor formation, what does anchorage-independent growth indicate?A) Normal cellular behaviorB) Malignant transformationC) A healthy signaling pathwayD) Decreased cell metabolismAnswer: B) Malignant transformation

These questions can help assess understanding of the key concepts related to signaling pathways and their implications in cellular processes and diseases.