Heterogeneity and cancer stem cells
what are the common cancer stem cell signaling pathways?
- TGFbeta - SMAD - WNT - beta- catenin - WNT PCP - Hedgehog - JAK STAT - PDGFR - Notch all of these pathways are the developmental pathways
what are the MET effectors?
-adhesion -cortical actin microfilaments
what are the two pathways a newly transformed mesenchymal cell can take?
-can escape tumor site and move into the blood -can leave the blood and enter the metastatic site
what is the clinical evidence that EMT occurs in cancer?
-cancer cells at the invasive edge express mesenchymal markers (these cells are more likely to escape and enter into blood circulation) -tumor expression of mesenchymal cell properties predicts poor prognosis and resistance to treatment - the tissue structure of cancers becomes more and more disorganized as they proggress
clinical evidence for cancer stem cells
-cancer stem cell frequency and phenotype correlate positively with patient outcome (presence of CSC phenotype leads to poor prognosis, poor survival, and presence of metastasis) - presence of Cancer stem cell gene expression leads to invasive behaviour by the cancer - when using chemotherapy and radiotherapy, it is the CSC that survive and are resistant to treatment
what is the key difference between cancer stem cells and epithelial-mesenchymal transition?
-cancer stem cells there is one type of cell giving rise to another type of cell, and there is no going back -EMT there is a reversible epigenetic plasticity, such that the cells can interchange between themselves
what are the two major pathways that are though to cause tumor heterogeneity?
-clonal evolution -cancer stem cell hypothesis
what are the positive interactions that can occur between two clones within a tumor?
-commensalism (where on cell benefits another cell but it doesnt not harm the cell) -synergism (both cells benefit off one another giving eachother new properties to make them stronger, good characteristics not seen in the individual clone population) -mutualism (both benefit each other equally)
what are the negative interactions that can occur between two tumor clones
-competition (fighting for nutrients) -amensalism (one clone inhibits the other) -predation (one cell inhibits the other and benefits from it) -parasitism (when one cell actively feeds off another cell and causes it harm)
the cancer stem cell hyothesis
-each cancer cell contains only a small minority of cells that can give rise to a cancer cel
explain how a epithelial cell transforms into a mesenchymal cell?
-epithelial cell is exposed to an increase in mesenchymal transcription factors, this causes the epithelial cell to lose its tight junction -the epithelial cell then loses its "lose" junctions(adheren junctions and desmosome) -the epithelial cell is now a mesenchymal cell this all occurs at the tumor site
epithelial cell origin
-epithelial cells are highly structured -epithelial cells are polarized (top side and a bottom side) -epithelial cells are tightly bound to the basement membrane -epithelial cells are tightly bound to eachother (tight junctions) without specific contact points the cell cannot survive
what is the source of heterogeneity
-extrinsic factors (cell microenvironment) -intrinsic factors (mutations and epigenetics)
what are the different mechanisms of clonal evolution?
-genetic mechanisms -epigenetic mechanisms -expression mechanisms -activation mechanisms
what are the EMT effectors?
-growth factors -cytokines -Extracellular matrix
what are the key factors that are found in the cancer stem cell niche?
-growth factors (macrophages, fibroblasts, adipocytes) -factors that inhibit the immune function (macrophages) -factors that encourage dedifferentiation (macrophages, fibroblasts -proteases that are controlling the matrix around the nich (granulocytes)
what is the experimental evidence for EMT in cancer
-if you take cancer cells and put them into a dish the cells become more migratory and motile (cells will move around) -if cancer cells are placed in a 3D matrix (with collagen, fibrin ect.) cells will regain polarity and expression of epithelial markers -cultered cells will decreased epithelial markers (E-cadherin) and increase mesenchymal markers (N-cadherin) over time you can basically bring the cancer cells through the process of EMT in a dish
what are the 4 different ways therapies can target cancer stem cells?
-increase the renewal rate: increasing the proliferation of the cancer stem cells increases their sensitivity to the drugs -increase cell death (if the CSC just died thatd be great) -force the stem cells to differentiate: differentiated cells have different properties -change the niche: by effecting the microenvironment around the CSC, less HIF1 for example
what are the characteristics of EMT?
-loss of cell to cell tight junctions -cytoskeletel reorganization -loss of polarity -becoming motile epithelial phenotype progessively loses epithelial markers and gains mesenchymal markers and becomes a mesenchymal phenotype
what types of cells secrete growth factors for the CSC nich?
-macrophages -fibroblasts -adipocytes
what type of cells secrete dedifferentiation factors for the CSC niche?
-macrophages -fibrpblasts
cross talk between cancer stem cell and EMT may occur in the stem cell niche
-multifactorial and bidirectional -intercellular communication (eg, hormone levels can affect cancer cells) local and distant -regulation of differentiation and self-renewal
what can result when you have a heterogeneous cooperative primary tumor
-one population of clones can metastasize (due to the help from other clones present in the primary tumor) -multiple metastasizes in the same organ (each population of clone is working together -different populations of metastasize to different secondary location (different clones are producing factors that are helping eachother and not hurting eachother)
what are the two therapy escape strategies that a cancer clone population has?
-resistance of cancer stem cells to therapy (activation of DNA repair, resistance to apoptosis etc..) -acquire the same properties as cancer stem cells
what are the 3 things that only cancer stem cells can do?
-self -renewal -proliferate indefinitely -can differentiate into all other tumor subclones (differentiated cells continue to evolve and proliferate in response to environmental pressures)
what are the properties of cancer stem cells?
-slow cycling/ senescence (which means they are resistant to therapy) -efflux transporters expression (if you give them a drug they will just spit it out) -free radical scavengers (good at getting rid of free radicals, so there will be less DNA damage) -reduced immunogenicity (less visible to the immune system) -very high capacity for DNA repair (all types of DNA repair) -increased anti-apoptotic protein expression (cancer wants the cancer stem cells to survive for a long time)
what are the two types of cancer heterogeneity?
-spatial heterogeneity -temporal heterogeneity
cancer stem cell markers
-stem cells express their own characteristic markers -cells expressing no lineage markers but high stem cell markers are much more efficient at producing tumors (than lineage positive cells)
explain how a mesenchymal cell transforms back into an epithelial cell
-the mesenchymal cell is exposed to an increase in epithelial transcription factors, there is an initial E-cadherin adhesive contact -MET continues (Rho-GTPase activation, cortical-actin cytoskeleton reorganization, adherens junctions assemble) -MET continues a desmosomes associate -tight junctions form and cell becomes polarized and becomes an epithelial cell this all occurs at the metastatic site
what is the experimental evidence for cancer stem cells
-there is evidence for self renewal (serial xenotransplantation and keep getting a heterogeneous tumor each time) - CSC accumulate mutations over time (newly arising and divergen populations dont lose initial mutation) -expression of known tissue specific stem cell markers
what are the 3 main hypothesis where cancer stem cells are thought to have come from?
1) cancer stem cells share many features with normal stem cells, so it is thought that cancer stem cells are just normal stem cells gone wrong 2) cancer stem cells arise from transit amplifying cells/ progenitor cells, these cells divide much more rapidly than stem cells so it is thought that these cells are the source for the mutations that give rise to cancer stem cells - somatic stem cells give rise to cancer stem cells by terminally differentiated become differentiated and become a cancer stem cell (tumor suppressor inactivation, oncogene activation) environmental triggers are likely the key in this process there is no strong evidence for which of these is the actual source of cancer stem cells
explain how a cancer cell travels to a distant metastatic site?
1) cell de-differentiation 2) cell is able to enter the blood stream (because it is de-differentiated) 3)cell enters distant metastatic site and re-differentiates (if the cancer started in the colon, the cell will re-differentiate into a colon cell)
both _______ and _________ diversity influence cellular composition of a tumor
1) genetic 2) functional "subclones"
what are the two criteria for a cell to be considered a cancer stem cell?
1) must posses a high reproductive capacity (generate tumors in multiple xenotransplantations) 2) recreate heterogeneous xenotransplantations (new tumors formed must have phenotypic and genetic heterogeneity of the parent tumor)
when is EMT used in non-cancerous cells?
EMT is required for embryogenesis and wound healing (when you get a cut, you need the skin cells to migrate together, to do this they need to under go EMT)
what transcription factor is essential for maintaining cancer stem cells?
HIF1 (hypoxia induced factor) hypoxia induced the transcription factor HIF1
do the different clones within a tumor interact with eachother
YES! can be positive or negative
most solid tumors are _______?
adenocarcinomas; a malignant tumor formed from grandular structures in epithelial tissue
what controls the transition of an epithelial cell to a mesenchymal cell?
an increase in mesenchymal transcription factors
the "gooder" clone strategy
another therapeutic strategy: when you target the population of clones that other cells in the tumor used to survive, by targeting and wiping out this population of clones, there will be a much stronger effect on the tumor that is you were to target the selfish clone within a tumor (wiping out the selfish clones would shrink the tumor initially, but would grow back very fast afterwards) there is reccurrence regardless if you target the good clones or the selfish clones, but the recurrence is delayed after targeting the "gooder" clone
where does MET occur?
at the site of metastasis, the cancer cell must re-differentiate so that is can stay at the metastatic site
how do we know that EMT is an epigenetic mechanism?
because it is a reversible process there is de-differentiation as well as re-differentiation
in theory what should happen once you treat a tumor with the CSC population depleted?
cancer regression (with combination of specific and non-specific therapies)
in theory what should happen if the stochastic mutation theory is true once you treat a tumor with the CSC population depleted?
cancer would relapse due to stochastic mutations or reversibility of non-CSC, cells that were more differentiated, could become less differentiated and more CSC like
lineage proteins/markers
cells that lack proteins characteristic of other cells, they are called lineage negative cells
epigenetic mechanism of clonal evolution
changes in a cell that can be reversed. one day the cell could change its phenotype, then 2 months later it can revert back to its original phenotype, which ever phenotype is present when the cell divides gets passed on to the daughter cell (methylation patterns are shared between parent and daughter cell) if the cells are constantly changing there is heterpgeneity
EMT
epithelial -mesenchymal transition, the process by which a polarized epithelial cell transitions into a motile mesenchymal cell
what process supports the epigenetic clonal evolution mechanism?
epithelial mesenchymal transition
what type of cell secretese proteases for the CSC niche?
granulocytes
the cancer stem cell niche is ______
hypoxic
what is the main difference between the clonal evolution model and the cancer stem cell hypothesis?
in clonal evolution, all the cells are able to proliferate, but in the cancer stem cells are cells that can actually re-grow a tumor from scratch
what controls the process of MET?
increase in epithelial transcription factors
how does a cancer cell evolve to become heterogeneous?
it starts off with one initial dominant clone that shares the same mutations, then more and more mutations occur, and then you will get one dominant clone will re-surface
what types of cells secrete immune inhibitors for the CSC nich?
macrophages
therapeutic strategies for targeting cancer stem cells
make the cancer stem cells less cancer stem cell like, by modifying properties that make them resistant
how many routes can clonal evolution take to proceed?
many different routes, the idea is you are slowly evolving the tumor and different phenotypes are arising due to different selective pressures
what is MET?
mesenchymal to epithelial transition, this occurs during re-differentiation
what is the therapeutic strategy for targeting cancer stem cells in tumors that have temporal heterogeneity?
multiple drugs at different times
what is the therapeutic strategy for targeting cancer stem cells in tumors that have spatial heterogeneity?
multiple drugs at once
cancer stem cell hypothesis
only the cancer stem cells (CSC) within a tumor have intrinsic tumorigenic potential, other cells that are not cancer stem cells could never produce heterogenous cancer themselves every stem cell when it divides forms an identical stem cell and a more differentiated cell, only the stem cell can go on and produce a tumor
what is the key to functional diversity?
proliferation, any alteration in proliferation, if it gives the cell a proliferation advantage, that is what is going to allow them to become a dominant clone
what do cancer stem cells lack?
proteins that are characteristic of other cells
activation mechanism of clonal evolution
rapid an non-heritable activation of a signaling cascade, the presence of a growth factor, its going to come in and trigger the cell to change something in its biology that is going to allow it to survive a particular selective event, this is not heritable
expression mechanism of clonal evolution
rapid and non-heritable, by changing expression levels of different singalling pathways or tanscription factors within the cell, and this is not heritable, but it is very rapid, this will allow the cell to adapt to a rapid change in the tumor microenvironment
HIF1 alpha
represses a negative feedback loop in the Notch pathways HIF1 alpha inhibits the negative regulation of the Notch pathway, so it is increasing the Notch signalling Notch signaling promotes proliferative signaling during neurogenesis
plasticity
the ability for one cell to change its phenotype and become another type of cell "plastic surgery"
what results when you have a primary tumor that is non-cooperative or homogeneous ?
the cancer will have a low level of metastasis, because there are not different clones present (as in heterogeneous tumors) that can help advance the cancer
which cells in a tumor are most likely to escape and enter the blood stream?
the cells at the edge of a tumor, they express more mesenchymal markers
dynamic stemness model
the exact same as the cancer stem cell model except that now there are arrows that go backwards, so cells that become more differentiated after they are produces also have to capacity to become less differentiated
de-differentiation
the gradual loss of epithelial cell characteristics as a cancer cell progresses in order for a cancer cells to escape their microenvironment, they need lose basically all of their epithelial features
how does heterogeneity correlate with patient prognosis?
the more heterogeneous the tumor, typically the worse the prognosis
temporal heterogeneity
the slow evolution of the tumor throughout its life, many clones are present in a tumor over its lifetime
clonal evolution is know as which model?
the stochastic model
therapeutic implication of CSC
the treatment needs to target as many cancer cell populations as possible within a tumor, once you introduce a therapy to cancer, it provides a very strong selective pressure for resistant CSC clones (basically helps certain populations of cancer cells)
genetic mechanism of clonal evolution
there are genetic mechanisms that can act on these cells, that can change their cancer causing potential. any genetic alteration made is permanent. the more genetic differences between areas of the tumor the more heterogenous the tumor
which of the two pathways for heterogeneity is correct?
there is controversy over which of the two mechanism (clonal evolution/ stochastic model or cancer stem cell hypothesis) is correct because there are very different implications for diagnosis and treatment
what is a benefit of cancer stem cells?
they can be used in vitro to regenerate tissues for studying organ function, this can aid in detecting the sensitivity to treatment of certain tissues.
what is the normal progression of a tumor in terms of identity?
to look less and less like the tissue that it originates from
what is the result of the common cancer stem cell signalling pathways?
transcription factors that are being expressed that are associated with either EMT or with stem cells
clonal evolution
when all cells in a tumor have an equal chance of developing mutations that could allow them to out compete their neighbours. each cell has the same intrinsic rate of mutation and susceptibility to mutational events. random events determine which cells will actually go on to divide. depending on the selective pressures in the different areas of the tumor, different mutations will become dominant in different areas of the tumor
how does spatial heterogeneity occur?
when there are different selective pressures in different areas of the tumor there are different areas in the cell that are exposed to different environments
how does temporal heterogeneity occur?
when there the selective presssures on a tumor vary over both short and long term
combination of CSC and stochastic model
when you have cancer stem cells that are being acted upon and acquire mutations, these mutations lead to clones which also have stem cell properties, those stem cells can then give rise to other types of stem cells
spatial heterogeneity
when you have one tumor that has different clones in it at one times, there are many clones present in different location of a tumor at one point in time
how do they test if a cancer cell is a cancer stem cell?
xenotransplantation, by taking tumor cells from a human into an immunocompromised mouse
do spatial and temporal heterogeneity occur at the same time?
yes
