7 - Growth Factors, Oncogenes, and Cancer Flashcards
What decisions do cells make in the cell cycle?
Whether to proceed to S phase, wait at G1 phase, or stop at G0 phase
What helps determine the decision in the cell cycle?
Cell signaling
What was used to study how cell signaling was involved in the cell cycle?
By studying unregulated cell signaling and cell growth (cancer)
What are some properties of malignant cells?
Not responsive to influences that normally cause cells to stop growth and division
What conditions will cause normal cells to stop growing?
Lack of growth factors, and when cells touch neighboring cells
What is contact inhibition?
Stopping of cell growth when it touches other cells
What do normal cells look like in a dish?
Thin monolayer
What do malignant cells look like in a dish?
Clumps (foci)
What is the phenotype of normal fibroblasts?
Flat, many extensions, contact inhibition
What is the phenotype of malignant fibroblasts?
Rounded, few extensions, no contact inhibition
What is tumorigenesis?
The development of a malignant tumor
How does cancer start?
Uncontrolled proliferation of a single cell
How does tumorigenesis occur?
Through a cumulative progression of genetic alterations
What happens to cells as they progress through tumorigenesis?
Cells become less responsive to growth regulation, and better able to invade normal tissues
What is the first step of tumorigenesis?
Formation of a benign tumor
What is a benign tumor?
A tumor composed of cells that proliferate uncontrollably but cannot metastasize
What does it mean to metastasize?
Leave ECM to go to the bloodstream (spread)
What are the genes involved in carcinogenesis responsible for?
Cell cycle regulation, cell adhesion, and DNA repair
Why is the sequence of when genes mutate important in cancer?
Sequence influences how the cancer develops
What did Peyton Rous do?
Isolated the first tumor-causing animal virus
True or false: Peyton Rous received the Nobel Prize shortly after his discovery of Rous sarcoma virus
False: it took nearly 50 years after his discover in 1911 to receive the Nobel Prize in 1966
What happened in the 1960s that helped in studying cancer?
Molecular biology was being used to understand how viruses contain genetic material, which could be used to infect mammalian cells
What led to the identification of many cancer causing genes in humans?
Investigations of abnormal cell growth due to viral infection
What did Baltimore, Dulbecco, and Temin discover about viruses?
They have RNA (not DNA) as genetic material, and had reverse transcriptase (RNA -> DNA)
What did Varmus and Bishop do?
Discovered the first oncogene, src
What is an oncogene?
A gene that creates oncology (cancer)
How was it proven that src caused tumors?
Adding vsrc led to a tumor, and removing vsrc stopped tumor growth
True or false: normal cells also contain src
True: c-src is a proto-oncogene
What is a proto-oncogene?
A gene that could become an oncogene when mutated
What is the origin of oncogenes in viruses?
Viruses take this DNA information from hosts
What did Erikson, Sefton, and Hunter do?
Study what src did
How was the function of src discovered?
Used radioactive ATP, and a 2D gel to find the difference between v-src lines and normal cell lines
What was different in the v-src 2D gel compared to the normal 2D gel?
Only tyrosine was phosphorylated by src
What was the conclusion of the 2D gels of v-src and normal cell lines?
Src was a tyrosine kinase
What is the function of src?
Tyrosine kinase
How much more active is v-src compared to c-src?
3x more
What was the significance of finding the function of src?
First evidence that protein kinases may play a role in cell growth
What cell signaling pathways are regulated by proto-oncogenes and tumor suppressor genes?
Apoptosis, proliferation, immortalization, and senescence
What is immortalization involved with?
The regulation of telomerase
What is senescence?
Cells are metabolically active but are non-dividing
What do proto-oncogenes do?
Promote cell survival / proliferation
What mutations occur with proto-oncogenes in cancer?
Gain of function mutations (unregulated cell growth / proliferation)
What do tumor suppressor genes do?
Inhibit cell survival or proliferation
What mutations occur with tumor suppressor genes in cancer?
Loss of function mutations (allow unregulated cell growth / proliferation)
What do caretaker genes do?
Repair or prevent DNA damage
What mutations occur with caretaker genes in cancer?
Loss of function mutations (allow mutations to accumulate)
Why does cancer increase as you age?
More time to accumulate mutations
What is colon cancer a good example of?
Cancers can have very specific sequences of mutations to develop
What is the structure of c-src?
Kinase domain (binds to ATP), C-terminal SH2 domain, and N-terminal SH3 domain
What does SH2/SH3 stand for?
src homology 2/3 domains
How is c-src inhibited?
Autoinhibition. SH3 domain binds to proline rich sequence, inhibiting kinase domain, and Phospho-Y527 binds to SH2 domain to inhibit kinase domain
How does SH3 inhibit c-src?
Binds to proline rich sequences to inhibit kinase domain
How does SH2 inhibit c-src?
Binds to Phospho-Tyr527 to inhibit kinase domain
What happens when Tyr527 in c-src is phosphorylated?
Binds to SH2 domain, inhibiting c-src
What happens when Tyr527 in c-src loses a phosphate?
Releases SH2 and SH3, activating the kinase
What is the difference in protein structure between c-src and v-src?
No Y527
What is the difference in genes between c-src and v-src?
v-src is a C-terminal truncation mutation
Why is v-src always active?
It has no Y527 to bind to SH2 to inhibit the kinase domain. Thus, it is always active
What question was raised by studying how viruses caused cancer?
If viruses use genetics to cause cancers, can human genes do the same thing?
How can a proto-oncogene be mutated into an oncogene?
Through mutation in coding sequence, gene amplification, or chromosome rearrangement
How does a mutation in the coding sequence affect a proto-oncogene?
Produces hyperactive protein in a normal amount
How does a gene amplification affect a proto-oncogene?
Normal protein is overproduced
How does a chromosome rearrangement affect a proto-oncogene?
Can be either a hyperactive protein, or overproduced
How can a chromosome rearrangement create a hyperactive protein from an oncogene?
Nearby regulatory DNA sequence
How can a chromosome rearrangement create overproduced proteins from an oncogene?
Fusion to an actively transcribed gene
What did Robert Weinberg do?
Discover human cancer genes
What techniques did Robert Weinberg use?
Insert human oncogene into mouse fibroblasts
What was required to perform Weinberg’s studies?
1980’s technology to introduce DNA into cells
How does DNA transfection work?
Add smaller and smaller fragments of cancer DNA until the gene is cloned
What is a phage?
A virus that infects bacteria
How were phages used in Weinberg’s studies?
Each phage had a segment of DNA to be injected into E. Coli
How come you only get a single gene at a time with DNA transfection?
It is an inefficient process, so only a small proportion of cells take up the cut DNA (and the right DNA)
How was the human DNA separated from mouse DNA in Weinberg’s studies?
By using an Alu probe
What are Alu sequences?
Repetitive DNA sequences found in humans, and not mice
How were Alu sequences used in Weinberg’s studies?
Used to probe the human oncogene from the mouse DNA
What protein did Weinberg discover?
Ras
What is ras?
A small monomeric G-protein
What does ras do?
Activate downstream responses when bound to GTP, inactive when bound to GDP
What is the difference between ras and other G-proteins, such as Gs?
Ras is monomeric, and Gs is trimeric
What is the significance of ras in cancer?
Ras is mutated in many cancers
What is needed for ras to properly function?
GEFs and GAPs
What are the GEFs for trimeric G-proteins?
The receptors themselves
What does an inhibitory ras mutation do?
Can’t bind to GEF to exchange GDP for GTP, thus staying inactive
What does an excitatory ras mutation do?
Blocks ability of GAP / ras to hydrolyze GTP into GDP, thus staying active
What are some downstream effectors of ras?
Raf, MEK/MAP kinases, etc.
Generally, what does ras signal for?
Proliferation (stimulate cell cycle)
What are the characteristics of ras oncoprotein?
Higher affinity for GTP, and lower GTPase activity (stuck in “on” position)
True or false: growth factor is needed for oncogenic ras to function
False: no growth factor is needed since ras is always on
What happens when src* is active or ras* is active?
Cells divide
What happens when src* is active and ras* is inhibited?
Cells do not divide
What happens when src* is inhibited and ras* is active?
Cells divide
What is the simplest relationship between src* and ras*?
src -> ras (downstream in a common signaling pathway)
What is the GEF for ras?
SOS
What is the GAP for ras?
RasGAP
What is the problem with src -> ras?
Normally, src is not the major upstream effector of ras
When does src -> ras?
When src is mutated to be constantly active (phosphorylate things it shouldn’t)
What does src activate (when it is mutated)?
SOS (GEF for ras)
What did Stanley Cohen do?
Looked at growth factors to find upstream effectors of ras
What does EGF stand for?
Epidermal growth factor
What happens when EGF is added to a cell?
The cell will divide
What happens when EGF and anti-ras is added to a cell?
The cell will not divide
What is the conclusion of adding EGF and anti-ras to a cell?
EGF and ras must be in the same pathway (ras inhibition stopped growth factors)
What does EGFR stand for?
Epidermal growth factor receptor
What is EGFR?
A receptor tyrosine kinase that can autophosphorylate
How many membrane passes are in EGFR?
One transmembrane region
How does EGFR work?
Binding of a ligand causes dimerization, which activates receptors through autophosphorylation
What happens once a tyrosine kinase receptor is activated?
Phospho-tyrosines act as docking sites for other proteins to continue signal
What binds to activated EGFR?
SOS (and GRB2)
What does GRB2 do?
Coupled SOS to phospho-tyrosines on activated EGFR
True or false: receptor tyrosine kinases can only act in one pathway
False: the many phospho-tyrosines can act as docking sites for proteins in many pathways at once
True or false: multiple receptor tyrosine kinase pathways are independent
False: they can also converge
What gene encodes EGFR?
erb-B
What does erb-B do?
Encode for EGFR
What does v-erb-B look like?
Partial deletion
What does v-EGFR look like?
No extracellular ligand binding site
Why is v-EGFR always activated?
No extracellular domain leads to receptors being always dimerized, and thus activated
What does EGF activate?
EGFR
What are some oncogenes in the receptor tyrosine kinase / ras / MAP kinase cascade?
sis (PDGF, growth factor), erb-B (EGFR, receptor), ras (G-protein, and modulators), raf (MAPKKK, kinase), transcription factors (myc, fos, jun, ets)
What do many proto-oncogenes encode for?
Cell surface receptors
What mutation effects will lead to constitutive activation of receptor tyrosine kinases?
Ligand-independent dimerization, or overproduction
What is Her2?
A receptor tyrosine kinase
What is the difference in structure between c-Her2 and v-Her2?
c-Her2 has a val, while v-Her2 has a gln
What is the significance of the val-gln mutation in Her2?
Keeps receptors dimerized, and thus keeps signal on
What can be used to treat oncogenic receptors?
Monoclonal antibodies
What can receptor tyrosine kinases signal for?
Cell growth, cytoskeleton, anchorage-dependent growth, contact inhibition, metabolism, etc.
What is the first step of cancer?
Initiation (a mutation gives one cell an advantage)
What is the second step of cancer?
Promotion (a second mutation increases the advantage)
What is the third step of cancer?
Progression (chromosomal instability, invasive)
What does Gleevec do?
Binds to oncogenic kinase through competitive inhibition
What was the first discovered tumor suppressor gene?
Retinoblastoma
What does RB stand for?
Retinoblastoma
What is RB?
A tumor suppressor gene
What is sporadic RB?
Two separate mutations leading to loss of RB
What is familial RB?
One allele passed from parent, and another allele mutated, leading to loss of RB
How can a cancerous phenotype be suppressed (for RB)?
By adding wild-type RB
What strategies are being used to combat cancer?
Immunotherapy, targeting cancer proteins, angiogenesis
What is passive immunotherapy?
Uses patient’s own antibodies to respond to tumor cells
What is active immunotherapy?
Uses patient’s own immune system to target malignant cells
How does active immunotherapy work?
Have cells display tumor proteins to be marked by immune system
What is angiogenesis?
Formation of new blood vessels
Why do tumors need angiogenesis?
Constant division is metabolically costly (needs nutrients through vasculature)
What signal is used in angiogenesis?
VEGF
How does mutating RasGAP (structurally) interact with Ras?
Mutated RasGAP cannot bind to ras to increase its GTPase activity, thus keeping ras GTP-bound and always on
What do cancer drugs impact?
Any rapid growth
What is meant by “collateral damage” (for cancer drugs)?
Many cancer drugs have side effects that need to be taken into consideration
Why do adverse reactions arise from monoclonal antibody therapies?
Immune system is recruited, which can lead to many reactions
True or false: Gleevac works for all receptors that it is meant for
False: there can be changes in the receptor that make it resistant to Gleevac (desensitization)
Are oncogene mutations dominant or recessive?
Dominant
Why are oncogene mutations dominant?
One copy is enough to keep the signal turned on (only one copy is needed to be mutated for an effect)
Are tumor suppressor gene mutations dominant or recessive?
Recessive
Why are tumor suppressor gene mutations recessive?
One copy is enough to keep the signal turned off (need both copies mutated for an effect)
How can src activate SOS when overactive, and not under normal conditions?
Localization can be overcome with active mutations (overlapping and intersecting pathways)
True or false: monoclonal antibody therapy is constant
True: you need to keep having treatment to keep the cells under control
What environmental conditions often lead to cancer?
DNA damage (radiation, ultraviolet light, etc.)
True or false: cancer treatment is unprediactable
True: therapies are often very personalized
True or false: c-src is a major upstream affector of ras
False: it normally does not activate SOS
True or false: v-src is a major upstream affector of ras
True: v-src is always on, which makes it able to activate SOS
Why is it unlikely to reverse the mutation of c-src to v-src?
The truncation means that part of the gene is missing
How do cancer cells avoid treatment?
Rapid division with an unstable genome leads to many mutations
How does telomerase impact cancer cells?
Have just enough to maintain telomeres (not significantly longer) and maintain immortality
