Chapter 4 Flashcards
(27 cards)
Initiation of cell division
typically signals from outside the cell signals cell division
Initiation of cell division
Transduction of growth signaling involves:
- Growth factors
- growth factor receptors
- Intracellular signal transducers
- Nuclear transcription
Growth Factor Receptors
- Growth factor receptors (GFRs) link signals outside the cell to intracellular regions
- Many GFRs are kinases
-gamma phosphate transferred from ATP/GTP to hydroxyl groups of specific aa in target protein
-Tyrosine or serine/threonine
Epidermal growth factor signaling
- Signaling through (EGF) is widely studied in cancer
- EGF family: family of tyrosine kinases that respond to EGF
- 4 members:
- EGFR; ErbB1; HER1
- ErbB2; HER2; HER2/neu
- ErbB3; HER3
- ErbB4; HER4
- 4 members:
Steps of EGF Signaling
- Growth factor binds to receptor
- Receptor dimerizes
- Autophosphorylation
- Activation of intracellular transducers
- Activation of ser/thr kinase cascade
- Regulation of transcription factors for gene expression
10
Step 1
EGF binds to receptor
the extracellular domains of EGFR form a binding pocket that allows for EGF binding
Step 2
EGFR Dimerizes
-EGF binding to the EGFR causes conformational change that reveals dimerization domain
-Facilitates binding to another, similar domain after that receptor has also bound to EGF.
-Can homodimerize or heterodimerize
Step 3
Autophosphorylation
Conformational change after ligand disrupts intramolecular autoinhibitory interactions
- Result is kinase activation: ATP and substrate can now access the cytoplasmic catalytic kinase domain
- Kinase domain of one receptor
phosphorylates others at multiple
tyrosines and vice versa
- Phosphorylation necessary for
recruiting substrate proteins
Step 3: Autophosphorylation
Kinase activity must be turned off at a certain time:
- Additional phosphorylation triggers conformational change to inhibit ligand binding/kinase activity
- Desphosphororylation by tyrosine phosphatases
- Kinase domain bound by negative regulators
- Receptor endocytosis and degradation
Step 4
Activation of Intracellular Transducers
Some phosphorylated tyrosine residues create high-affinity binding sites for proteins with Src homology domains
- SH2: 100 aas long, recognizes and binds sequences C-terminal to phosphorylated tyrosines
- SH3: 50 aas long, bind to residues on partner proteins.
- E.g., Grb2 - 1 SH2 and SH3 domains
Grb2
SOS
RAS proteins
Grb2: 1 SH2 and 2 SH3 domains
- SH3s interact with SOS
SOS: a guanine nucleotide exchange factor that activates RAS
Ras proteins: GTP-binding proteins that can activate ser/thr kinases
- N-RAS, H-RAS, and K-RAS
- Farnesylation directs RAS to the cell membrane
- Inactivate when bound to GDP
- Active when bound to GTP- can now interact with downstream proteins
-E.G., activates ser/thr kinase raf
Step 4
GTPase activating proteins (GAPs)
Catalyze hydrolysis of GTP to GDP to terminate signal
- RAS proteins have intrinsic GTPase activity for self-regulation
Step 5
Activation of Ser/Thr cascade
Raf: ser/thr kinase activated by RAS; carries signal away from membrane; phosphorylates MEK
- aka MAPKKK
MEK: tyrosine and ser/thr kinase that phosphorylates MAPK
- aka MAPKK
MAPK (mitogen-activated protein kinase): family of ser/thr kinases
- Phosphorylate transcription factors to regulate their activity
Step 6
Regulation of Transcription Factors
MAPKs regulate many transcription factors
-Example:
- AP-1 transcription factors–involved in growth, differentiation and death
-Phosphorylation of Fos alters DNA-
binding activity
- Myc family of transcription factors – regulate expression of genes important for proliferation
- Target N-ras and p53
More than regulating Gene expression..
Cell signaling can also affect cell behavior
-Src: intracellular tyrosine kinase with SH2 and SH3 domains
- activated EGFR interacts with SH2 domain of SRC to induce cell proliferation
- Focal adhesion kinase (FAK) also interacts with SH2 - important for cell shape, adherence, and motility
Oncogenes
(mutated gene whose protein product is produced in higher quantities/has increased activity)
Over 11 oncogenes and 15 tumor suppressors identified
- Examples seen in every type of protein involved in growth factor signaling pathways
(ecDNA is less compact/more accessible
Historical Data
-Discovery of oncogenes came from studies of retroviruses
-Retroviruses inject RNA into host
- End up with a provirus: DNA form a retrovirus that results from reverse transcription
-Integrated randomly into host genome
-Replicated, transcribed, and
translated by the host.
- Translocation of viral RNA
produces viral proteins needed for
synthesis of new viral particles
- Virus can acquire fragments of
genes from host at sites of
integration
-E.g., Rous Sarcoma virus acquired truncated form of c-src
Historical data
(next slide)
Rous (1991) - Rous Sarcoma Viruse and v-scr viral
Bishop and Varmus (1976) - c-src (normal cellular)
Growth Factors as Oncogenes
Platelet-derived growth factor (PDGF): stimulates epithelial cells around wound edge to proliferate and repair damage
- One of the first identified proto-oncogenes
- Oncogene results in cytoplasmic rather than secreted PDGF
-Result: PDGF signaling inappropriately
activated, leading to unregulated
growth
Growth Factors as Oncogenes
slide 2
- EGFR is a proto-oncogene
- EGFR oncogene is a truncated form of EGFR - doesn’t have an extracellular domain
- Result is constitutive activation
- Oncogenic activation of receptor tyrosine kinases occurs through mutations that cause:
- Constitutive tyrosine activation
- Dimerization
- Altered substrate activity
Intracellular Signal Transducers as Oncogenes
- Rasis most commonly mutated oncogene
-Mutations result in loss of GTPase activity of RAS protein- So, RAS-GTP can’t be inactivated to
RAS-GDP
-Result: constitutive activation of RAs
protein
- So, RAS-GTP can’t be inactivated to
- B-Raf point mutation common in melanoma
-Result: constitutive kinase activity, I.
insensitivity to feedback mechanisms
Intracellular Signal Transducers as Oncogenes
ABL
- ABL: Nuclear threonine kinase involved in DNA damage-induced apoptosis
- Translocation places in next to bcr
-BCR-ABL fusion protein maintained in
cytoplasm
-Result: kinase activity constitutively
active in cytoplasm - access to
different substrates
Transcription Factors as Oncogenes
- AP-1 components (Jun and Fos) are proto-oncogenes
- Mutation of fos results in truncation and eliminates a motif involved in mRNA instability
- Result: mRNA with a longer half-life, ultimately leading to inappropriate increase in transcription of AP-1 regulated genes
Targeting Kinases
- Structure of catalytic domains of kinases very similar when active
-but, many specific inhibitors have been created
