Transcript When cell junctions fail, cells can wander, but to be dangerously

Metastasis
How the breakdowns of normal cell
adhesions and stasis make cancers
much more dangerous
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Metastasis is important
for cancer medicine
Survival data show that
prognosis gets worse
when distant metastasis
has occurred.
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Fraction of all cancer patients surviving months after
diagnosis, based on the presence (+) or
absence (-) of distant metastasis
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Therefore we need to understand
metastasis
Cell adhesion is a complex process that
depends on many different molecules
Destruction of the cell adhesion machinery
can result from “loss of function” mutations
in one or more of the relevant adhesion
molecules
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• Both cell-cell interactions and cell-matrix
interactions play an important role during the
invasive cascade.
• Connections through cell-adhesion molecules, integrins,
and cadherins, stabilize tissue integrity
• Loss or alteration of these cell surface proteins is
associated with increased metastatic potential.
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Cadherins
 Cadherins are transmembrane glycoproteins that mediate
extracellular calcium-dependent cell-cell interactions
 E-(epithelial) cadherin, the most extensively studied, is
involved in epithelial cell-cell communication
 Bind the cytoskeleton of one cell to that of its neighbors,
forming a unit.
 This coupling contributes
to the mechanical
integrity of a tissue.
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Localization of Cadherin revealed by fusing this
protein with the Green Fluorescent Protein (GFP)
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• Cells also bind to one
another by cell adhesion
molecules that do not
bind Ca.
• Here are 4 CAMs
intercellular (cell
adhesion molecule),
showing their
extracellular folds,
membrane spanning
domains, and ability to
dimerize, and form cellcell bonds.
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Desmosomes
• Make mechanical links
between the intermediate
filaments of one cell and its
neighbors.
• This contributes significantly
to the mechanical strength
of epithelia.
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Epithelial cells bound together by
Desmosomes
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Integrins
• Integrins are a family of
transmembrane glycoproteins
that are expressed by the cell
as α/β heterodimers
• Membrane proteins that bind
to the Extracellular Matrix
(ECM).
• The integrins make bonds
between the actin
cytoskeleton and the fibers of
the ECM, such as collagen and
fibronectin.
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 Originally identified as cell adhesion
molecules,
 Recognized as signaling molecules for
regulation of apoptosis, gene expression,
cell proliferation, invasion and metastasis,
and angiogenesis
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Assembling all the intercellular junctions
into one diagram
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What happens when cell junctions are broken
or weakened?
Watching the direct result of mutations that induce
metastasis is not easy to do, because you don’t
know when the mutation will occur
Thus, we can either compare properties of
metastatic cells with those of normal cells or set up
experiments that knock out known components of
cell adhesion and see how their loss compares with
the onset of metastasis
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 The junctions between normal
cells are strong, but loss of
normal adhesion molecules
can reduce the strength of
cell-cell interactions.
 In (a-c) significant force is
required to pull normal cells
apart.
 In (d-f) metastatic cells are
seen to require much less
force to pull them apart.
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Cell junctions can fail as a result of mutations
that alter CAMs
A loss-of-function allele of a Cadherin, a
Desmocollin, or a Ca++-independent CAM
can lead a cell to a lose its adhesion to
proper neighbors and start to wander
The wandering cell must then get from the
local stroma to more distant sites for true
metastasis growth
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The most common LOF mutations for
carcinomas are in Cadherins
In metastatic prostatic carcinomas, cadherins are
down-regulated, as measured by PCR
This down-regulation is often a result of a single
nucleotide mutation in the DNA 5’ to the structural gene,
presumably its promotor
Snail is a transcription factor that down-regulates
cadherin expression. Its hyper-activity can lead to
lowered levels of CAMs
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Loss of adhesion is necessary, but not
sufficient for metastatic behavior
To get to a site of metastatic growth, a
cells must also wander from its initial
location
At first site, cell wandering would appear to
be a novel property of cancer cells.
There are many circumstances in which cell
migration is a normal and even essential
part of cell behavior
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When cell junctions are broken by a wounding
process, cell migration is induced as a normal
part of wound healing
In vivo, our ability to close a cut depends on
cell migration and proliferation
These processes can be modeled in cell
culture
Confluent layers of epithelial and fibroblastic
cells are comparatively static
With wounding, cells become motile and
division will often ensue.
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Breaking important cell-cell junctions can occur
by LOF mutations, but it takes several losses to
let a cell loose
The probability of acquiring one mutation in a
normal mammalian cell is about 1 in 100 million cell
divisions
For a mutation to become homozygous, additional
improbable events must occur
Look, therefore, for simple ways to bring about
complex events, like loss of cell adhesion
The easiest is to secrete proteases that cut the
proteins that normally hold cells together.
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Cadherin function can be down-regulated
by post-translational effects
Calpain, an intracellular protease, can cut
cadherin down to a 100 kD polypeptide that
lacks its binding sites for beta-catinin
Extracellular proteases, like metaloprotease can cut cadherin and related CAMs,
reducing their effectiveness in holding
tissues together
All such effects promote metastasis
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Mutations that lead to the secretion of
proteases are particularly dangerous
Collagenases will loosen and degrade some ECM
There are several metaloproteases that are
important for normal tissue morphogenesis and
remodeling; examples: stromelysins 1 and 3.
Over expression and secretion of such enzymes
can degrade cadherins and other cell adhesion
molecules, and can modify basal lamellas making it
easier for circulating cells to pass through
Overexpression of Stromelysin 1 is sufficient to
make a cell metastatic. Thus, one GOF mutation
can turn a hypertrophic cell into a metastatic cell
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When cell junctions fail, cells can wander, but to
be dangerously metastatic they do not wander
without logic
First job: cross the BL. For this, proteases
(particularly collagenases) are essential
Once the cell has gotten into the stroma,
(connective tissue below the BL) there are
pathways that can facilitate cell migration
Blood vessels are one such conduit
Lymph vessels are another
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The lymph vessels, or “lymphatics” provide a
network of pathways for metastatic movements
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Epithelial cells that
can cross the BL and
migrate through
connective tissue can
also enter blood
vessels and follow a
fast-track to distant
locations
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Travel in blood is, however, dangerous for
the metastatic cell
Fast flow of blood implies sheer, which can destroy
the metastatic cells.
Indeed, correlations between tumor cells in blood
and frequency of metastatic growth are not good.
This emphasizes the difficulties faced by tumor
cells in accomplishing “extravasation”
Formation of clots (involving platelet activation) can
be important for extravasation, but endothelial cells
have receptors for cell surface molecules that can
bind a metastatic cell in place, giving it time to pass
out of the capillary, just as in normal inflammation
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Best evidence for blood as a pathway for
metastasis is some of the common patterns of
metastasis
Tumors in the lung commonly metastasize to
the brain. Blood flows from the lung to the
heart (return of “pulmonary circulation loop”,
then a significant amount goes to the brain.
Intestinal tumors frequently metastasize to
the liver (remember the hepatic portal
system)
Thus, blood is likely to play an important part
in the “logic of metastasis”
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Once the tumor cells has escaped from the
vasculature, a site of metastatic growth can be
established
Affinity of the metastatic cell for local ECM
may provide another chemical logic that
helps to define sites of frequent metastasis
CAMs that grant cell-cell adhesion specificity
can also help to define where metastatic
growth will occur
The hospitality of an environment for new
growth (degree of vascularization, etc.) can
also make a difference
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