Unit 2- Growth Factors, Receptors, and Cancer Flashcards
What are growth factors?
- signaling proteins that stimulate cell growth/ differentiation/ survival
Why do we need growth factors?
- cell communication
- to maintain proper tissue architecture
- many cell types > control proliferation, so have the right # of cells
Why do we need growth factor receptors?
- plasma membrane is impermeable to most signaling molecules (h20 soluble)
- proteins/ genes that regulate cell function are located within cell
- receptors transmit extracellular signals > intracellular proteins
How can growth factors contribute to cancer?
- GF/ GFR signaling is tightly controlled
- tumor cells can alter GF production/ GFR signaling
What gene provided the initial clues about cell signaling via growth factors?
Src (v-src = viral src)
- first discovered oncoprotein
- gene from rous sarcoma virus > induced sarcomas in chickens
What happened when cells transformed with v-src?
- ↑ cell proliferation
- altered glucose uptake/ cell shape
- resulted in anchorage-independent growth > tumor formation
What kind of a protein is Src/ how was this determined?
Tyrosine Kinase
- anti-Src antibody phosphorylated
Why is Src unlike other kinases?
- phosphotyrosine rare compared to phosphothreonine/ phosphoserine
- many substrates (>50) can explain pleiotropic effects
What is the common structure of tyrosine kinase receptors?
- at least 1 tyrosine kinase domain in intracellular space
- ligands bind extracellular
- most receptors are monomers/ dimerize when bind ligand
What are gene mutations?
- permanent alteration in DNA sequence that makes up a gene, so sequence differs from what is found in most people
How can mutations impact the function of a protein?
- affect whether full-length/ truncated protein is produced (↑/↓ function)
- influence intracellular localization
- impact whether protein expressed in particular cell
- alter ability of signals to turn on/off protein
- alter protein ability to interact with other cell components
How can mutations lead to deregulation of receptor signaling?
normal receptor > ligand binds > ligand-dependent firing
mutations > ligand-independent firing
What are 3 mechanisms of deregulation of receptor signaling in tumors?
mutation > ligand-independent firing
autocrine signaling
receptor amplification > ↑ # of receptors to ↑ activation
How are tyrosine kinase receptors activated?
Transphosphorylation
> monomers dimerize
What are the Src homology domains? (protein interaction domains)
SH2- allows protein to bind phosphorylated tyrosine residue
SH3- allows protein-protein interactions
What provides protein binding sites?
- receptor phosphorylation
What provides negative feedback of tyrosine kinase signaling?
Phosphatases- remove phosphate from tyrosine
(Kinase-add phosphate/ activate)
What factors promote Ras activation/ inactivation?
GDP-Ras = inactive/ GTP-Ras = active
GEF = Guanine Exchange Factors > promote Ras activation ex) Sos
- Shc/ Grb2 proteins recruit Sos
GAP = GTPase Activating Protein > GTP hydrolysis/ Ras inactivation
How does Ras signaling work?
Ras- tethered/ stuck on plasma membrane
Ras = GTPase (GDP-Ras = inactive/ GTP-Ras = active)
- SOS (a GEF) recruited to Ras > facilitates GDP > GTP exchange > Ras activation
How can a mutation impact Ras signaling?
- oncogenic mutation causes GAP (GTPase activating protein) blockage
> GAP can not inactivate Ras (GTP-Ras overactivated)
What are the major Ras signaling cascades?
MAPK > ↑ transcription factors
Ral > metastasis
AKT/PKB > promotes cell proliferation/ survival/ growth
How does Ras > AKT/PKB signaling work?
PIP2 = phospholipid inactive/ PIP3 = active
- Ras activates PI3K enzyme > PIP2 to PIP3 > AKT/PKB localization to plasma membrane > activation
- PTEN inactivates PIP3 to PIP2 (opposite of PI3K)
What is the result of a loss of PTEN in AKT/ PKB signaling?
- AKT hyperactivation
- PTEN normally inactivates PIP3 > PIP2/ controls AKT/ PKB activation
What does AKT/PKB signaling do?
- stimulates proliferation
- stimulates angiogenesis
- inhibits apoptosis
What are 3 key tyrosine kinase receptors in human cancer?
EGFR2/ HER2 = Epidermal Growth Factor Receptor 2
IGF-IR- Type I Insulin-like Growth Factor Receptor
EGFR1/ EGFR = Epidermal Growth Factor Receptor 1
How was EGFR2/ HER2/ p185neu identified as an oncogene?
- DNA from tumor cells > transform normal rodent cells
HER2 overexpressed in 25-30% of breast cancers > worse prognosis
What is next after establishing a protein is an oncogene?
- General Cell lines/ Cell Culture Systems
- Create Animal Model
What are the pros/ cons of cell line/ cell culture studies?
- isolate tumor cells/ can overexpress gene
- useful to study mechanisms, but environment is artificial (isolated)
What are the pros of animal models?
- inject tumor cells into mice
- use transgenic mouse to overexpress a gene of interest
- use tumor suppressors to knock out gene of interest
- more physiologically relevant environment
- can study tumor growth/ metastasis
How was a transgenic HER2/ neu mice model developed?
- superovulate mouse/ isolate fertilized eggs
- insert expression construct/ inject DNA into pronuclei
MMTV promoter > upstream of gene, so transcription of gene only in specific tissue of interest (mammary gland)
What was the result of trangenic overexpression of neu in mice?
- induces mammary tumors in mice
What drug was developed for HER2/ EGF2 as a therapeutic target?
Herceptin (Traztuzamab) = muMAb4D5 (mouse monoclonal antibody)
- enhanced effects of chemotherapy/ ↑ survival in patients with HER2 overexpression
What is a problem with targeted therapies?
Drug Resistance > tumors non-responsive to Herceptin
- mutation in target (HER2)/ stop expressing HER2
- upregulation of other pathways
What is the IGF family/ IGF-IR?
IGF-IR= type I Insulin-like GFR = Mitogenic/ Anti-Apoptotic
- 6 binding proteins (IGFBP) prevent GF degradation
What pathways does IGF-IR signaling activate?
MAPK/ PI3K AKT/ STATS
How do IGFs/ IGF-IR contribute to breast cancer?
in vitro- IGFs stimulate breast cancer cell proliferation/ inhibit apoptosis
- targeting IGF-IR ↓ breast cancer cell survival
in vivo- ↑ circulating IGF-I associated with ↑ breast cancer risk
- IGF-IR overexpressed in high % of human breast tumors
- although present, the exact role of IGF-IR in tumor initiation/ progression/ metastasis is unclear
What is interesting about IGF-IR signaling-dependent tumors?
How was this studied?
- generation of double transgenic mouse > Dox inducible IGF-IR
- IGF-IR overexpression > induces mammary tumor formation
- high IGF-IR expression in mammary tumors
- remove DOX/ suppress IGF-IR signaling > tumors regress
- tumors induced by IGF-IR rremain dependent on IGF-IR signaling
What are drugs targeting EGFR/ HER1?
Iressa (Gefitinib)/ Tarceva (Erlotinib)
- small molecule EGFR inhibitors
What is interesting about mutations to EGFR/HER1 tumors?
- mutations make receptor more sensitive to drugs (gefitinib/ erlotinib)
- ↑ survival if you have mutation
What is the first-line therapy in patients with EGFR mutation-positive NCSLC?
Erlotinib (Tarceva)
- unfortunately, patients often become resistant
What are integrin receptors?
- transmembrane proteins
- cell-cell adhesions/ receptor binding in ECM (extracellular)
- short cytoplasmic tail/ protein binding cytoskeleton/actin (cytoplasm)
