Ch. 5: Growth Factors, Receptors, and Cancers Flashcards
signal transduction
- is generally the
transmission of a molecular signal outside of a cell to the inside of a cell. - A way for cells to communicate with
each other. - One cell can produce a ligand which causes a neighboring cell to have a
response.
What do multicellular organisms need to do to divide?
need to talk with each other to decide if a cell needs to divide
What determines division in single cellular organisms?
the decision
to divide is largely dependent upon the
availability of nutrients. If there are adequate nutrients the cells will sense this and divide.
What determines division in multi-cellular organisms?
The decision
to divide is largely dependent upon
signals by nearby cells. Therefore, a cell only divides when it is good for the organism as a whole
What are growth factors?
secreted proteins that are able to stimulate cell
division/growth/proliferation by binding to a specific cell surface receptor
Example of growth factor?
wound healing
- Various growth factors are released into the blood serum as a result
of platelets aggregation/clotting-
PDGF
EGF
TGFB1
VEGF
IGF
FGF2
- Act on diverse cell types to help tissue reconstruction.
- Proliferation of multiple different cell types needs to be coordinated.
Platelet Derived Growth Factor (PDGF) does what in a culture?
activates fibroblasts
What do normal fibroblasts need to grow?
- cannot grow on a dish with just nutrients
- need to be grown with growth factors to stimulate cell division
What does PDGF do?
stimulate cell migration and division
Scratch assay
- Take a pipette tip and scratch away a line of cells in a dish.
- Wait 48 hours, see if cells were able to migrate and divide and fill in that empty space.
Summarize the results of scratch assay on slide 11
PDGF and a working receptor are needed for cells to migrate and divide enough to fill the gap
What is the first step in signal transduction?
Growth factor receptors
EGF
epidermal growth factor
3 domains of EGF?
- extracellular (ectodomain)
- transmembrane
- cytoplasmic domain
SRC
oncogenic gene of Rous Sarcoma Virus
- also a protein kinase
- phosphorylates proteins on the amino acid tyrosine
What does EGF have on its intracellular domain?
tyrosine kinase domain
Structure of EGF receptor
- extracellular domain - binds specific ligands (growth factors)
- trans-membrane domain
- intracellular domain- can phosphorylate protein targets
Significance of EGF structure?
Offers a method for extracellular growth factor to communicate with the interior of a cell.
Growth factor receptors are…
receptor tyrosine kinases (RTK)
RTK
Generally, RTK are used for signals that induce cell division.
A variety of different downstream
changes occur from RTK?
- Cell division
- Change in cell shape
- Cell survival
- Cell metabolism
- Cell motility- ability to migrate
What do growth factor receptors generally do upon ligand binding?
generally dimerize and transphosphorylate
What are growth factors and their receptors often involved in?
tumor pathogenesis
How are growth factors and their receptors
altered to become oncogenes?
- alteration of growth factor receptors
- change in protein structure
- change in cell type expressing ligand
- changes that cause dimerization
Example of alteration of growth factor receptors
Example- Loss of ligand binding domain in v-ErbB
(=HER2/NEU2/ERBB2) in AEV
- Loss of the ligand binding domain causes the intracellular tyrosine kinase domain to be constitutively
active (the intracellular kinase domain is continuously active without the presence of a growth factor).
This causes the cell to continuously be told to proliferate.
Change in protein structure: In a cancer cell, a mutation effecting
protein structure can
cause…
constitutive activation
Change in protein structure: In a normal cell,
growth factor binds
to an RTK and causes…
activation of the
intracellular kinase
domain
Change in cell type expressing ligand: In a normal situation
- In a normal situation,
growth factors are
secreted by separate
cells than the ones
that express the
receptor - Significance:
Cells only proliferate
when other cells tell
them to. In situations
that benefit the
whole organism.
Change in cell type expressing ligand: In a cancer situation
- In a cancer situation,
growth factors are
secreted by the same
cells that express the
receptor. - Significance:
Cancer cells tell themselves to
proliferate, in
situations that are
not beneficial for the
whole organism.
Dependence on
outside signals for
growth is greatly
reduced. - Example: v-sis gene in simian sarcoma virus. Very similar to PDGF.
Changes that cause dimerization: in a normal situation
- In a normal situation:
Fig is a protein that
dimerizes with itself. - Ros RTK move
around the plasma
membrane
Changes that cause dimerization: in a cancer situation
- In a cancer situation:
Fig and Ros become fused.
Ros will dimerize and cause the intracellular domain of the modified RTK to dimerize and phosphorylate and
activate one other. - Example:
ROS gene in glioblastoma becomes fused with Fig gene.
Changes to growth factors and their receptors
helps explain 1 property of transformation:
- Transformed cells can grow without the presence of growth factors
- Most cancers have altered growth factor signaling
Other types of receptors
- cytokine receptors (involved in diverse immune responses)
- notch receptors (activated by neighboring cells and regulates cell fate decisions)
- hedgehog signaling (involved in embryogenesis and maintenance of adult stem cells)
- canonical Wnt signaling increases txn of genes such as MYC
- a subset of GPCRs recruit immune cells to the environment surrounding tumors
What would occur if one signal was continuously on (constitutively active)?
one would always be proliferating at the expense of all other signals
Why are there so many signaling pathways?
so no signals get crossed/all can work properly
What part of the plasma membrane is hydrophobic?
nonpolar phospholipid tails
How do small hydrophobic things get across the plasma membrane?
they freely diffuse
ligands
small and
hydrophobic molecules
(example- steroids)
Nuclear receptors (inside the cell)
- Receptors are transcription factors
- Receptors are active and
can initiate txn when a
ligand is bound to them
How do nuclear receptors know which genes to
regulate?
They contain DNA binding
domains that bind to specific DNA sequences
Hormone response elements (HRE)
Are the specific regions
of DNA a nuclear receptor
can bind to
What steroid hormones are commonly involved in
cancer? What kind of cancers?
- Estrogens, progesterone,
androgens (testosterone) - Involved in breast, ovarian, and prostate carcinomas
Ex of steroid horomone cancer: Estrogen receptor
(ER)
- When there is no estrogen present, ER is bound to co-repressors (proteins that modify
chromatin and inhibit txn). - Estrogen binding causes a change in the 3D shape of the ER so that it can release co-repressors and
bind co-activators (proteins that modify chromatin and increase
txn). - Estrogen receptor functions as a heterodimer. (2 versions that are slightly different that dimerize together to activate transcription)
Inhibition of ER signaling has clinical benefits
- Selective estrogen
receptor modulators
(SERM), such as
tamoxifen, bind ER and
cause the release of co-
activators and the
recruitment of co-
activators. - Used to treat ER+ human
breast carcinomas - 2-fold reduction in BC
relapse
Extracellular matrix
(ECM)
Complex network
of glycoproteins including
collagen, laminin,
proteoglycans, and
fibronectin. Usually found
in the spaces between
cells in most tissues.
What do normal cells need to attach to to survive and proliferate?
extracellular matrix (ECM)
Cancer cells exhibit anchorage ________ growth
independent
If a normal cell is grown on a soft substrate (with no ECM) they will activate…
programmed cells death program (apoptosis) termed anoikis
One of the properties of
transformed cells is that they can continue to survive and proliferate in the absence of
An ECM to attach to (when grown on a soft substrate.
How do cells sense if they are attached to something?
- Integrins
- ligands
What are integrins?
- Large family of
heterodimeric
transmembrane cell surface receptors composed of 𝛼 and 𝛽 subunits. - Cell surface receptors
that sense the ECM that a cell is bound to and transmits this information to the interior of the
cell. - help stabilize tissues
by providing something for cells to attach to
Why are there so many different types of heterodimers? (24 known)
Makes sense, want there to be distinct responses to
different molecules present outside of a cell.
How do integrins attach to intracellular cytoskeleton?
- Integrins associate with actin cytoskeleton (pink).
- Intermediary proteins (actinin, vinculin, and talin) allow these linkages to be formed.
What is downstream of integrin binding to their ECM ligands?
Upon ligand binding, integrins cluster together and form focal adhesions
What are focal adhesions?
multi-protein structures linking intracellular actin bundles with extracellular substrate via integrins. How the cytoskeleton of a cell connects to the ECM.
How do integrins relate to anchorage dependent
growth in normal cells?
- Integrins trigger the release of anti-apoptotic signals (promote cell survival)
- In the absence of integrin
binding to ECM, apoptotic
signals would not be
inhibited and cell death
would occur.
What is the significance of these various receptors? (ECM, integrin, GF)
The cell must have adequate levels of growth factors and the existence of adequate anchoring to specific ECM factors to be able to undergo cell division/proliferation
The presence of an activated RAS oncogene allows cells to…
grow in environments with low levels of growth factors and in an anchorage independent fashion
What did scientists know about RAS at the time?
- RAS is bound to GDP in its inactive state
- A guanine nucleotide exchange factor (GEF) can trigger the release of GDP and the acquisition of GTP
- GTP bound RAS is in its active state
- RAS itself or a GAP (GTPase activating protein) causes the hydrolysis of GTP into GDP + P, allowing re-entry to the off state
RAS oncogenic mutation in Harvey sarcoma virus
- Point mutation (single amino acid substitution) that has lost GTPase activity.
- Once it is pushed into an active state it is unable to turn itself off!
1) What is occurring between ligand
and receptor in A and B?
2) What is Cbl doing in C?
3) What process is occurring in D?
4) Where do ubiquitinated bound
receptors end up?
5) Where do non-ubiquitinated
receptors go (after F)?
(Ch. 5 part 2, slide 26)
- The ligand is binding to the receptor causes dimerization to recruit phosphotyrosine and Cbl
- Cbl is catalyzing the bonding of ubiquitin to the receptor
- Clathrin coats the receptors into a coated pit and brings them into the cell via a vesicle (they are being endocytosed)
- they move to a multivesicular body (sorting endosome) to a lysosome to be broken down. (they get lysed)
- They are moved to a recycling endosome to be recycled and moved back to the surface of the membrane
Ubiquitination play multiple roles in protein
regulation
- histone regulation
- endocytosis
- proteasomal degradation
- DNA repair
What does extracellular domain do?
ligand-binding
what does trans-membrane domian do?
anchor it in the hydrophobic region of the plasma membrane
what does the cytoplasmic/intracellular domain do?
phosphorylates protein targets
