external factors controlling cell division Flashcards
what is cell behaviour
the term used to describe the way cells interact with their external environment and their reactions to this, particularly proliferative and motile responses of cells.
what external influences are detected by the cell *
chemical - hormones, GF, ion concentrations, ECM, molecules on other cells, nutrients, dissolved gas (O2/CO2) concentrations
physical - mechanical stress (movement), temperature (for us regulated narrowly, so doesnt have a massive effect), the topography/layout of the ECM and other cells
what external factors can influence cell division *
all of the factors that are detected - eg if dont have enough nutrients, proliferation can slow down
however with respect to cancer - GF, cell-cell adhesion, cell-ECM adhesion
describe cell spreading *
not a passive gravity, driven process - cells will spread even when upside down - it is controlled by the cytoskeleton
energy is required to modulate cell adhesion and teh cytoskeleton during spreading
it varies betweebn cell type
as cells spread, they lose their blebs
how do we know that adhesion influences cell proliferation *
microengineer surfaces with patches that were adhesive of various sizes, on a non-adhesive background - therefore cells could only spread on the patch
compared to cells in agar (cells dont proliferate in agar - they cant sense it, so feel like they are in nothing)
GF were present
cells in agar - very small probability that they would enetr S phase and proliferate
on adhesive patch the probability of proliferating increased with the size of the patch
therefore, we know cells require to be binded to the ECM to be competent to respond to GF
how do we know the importance of cell spreading *
fibronectin (a molecule where cells would adhere) was put in a single dot and compared to the same amount of fibronectin in different spots
in single patch - cell dies by apoptosis
when fibronectin is spread out - cell can bridge over the non-adhesive surface = proliferation and survival
therefore cell survival depends on the arrangement of the contact
what is important about the cell-ECM adhesion that allows proliferation and survival *
cells need to be attached to ECM and allowed to spread to begin protein synthesis and proliferation
therefore attachment to ECM may be required for cell survival - this is anchorage dependance
describe how a cell phenotype can be determined by the composition of the matrix *
in interstitial matrix ie with Type 1 collagen - mammary epi doesnt differentiate into secretory cells - instead lose ball of cells are formed without normal cell-cell junctions
in basal lamina matrix ie type 4 collagen and laminin - mammary cells organise into organoids and are polarised, differentiated and secretory
how does a cell recieve information about its surroundings from its adhesion to ECM *
cells have receptors (membrane proteins) on their surface which bind specifically to ECM molecules
the cytoplasmic domain of these molecules are linked to the cytoskeleton
therefore there is mechanical continuity between the ECM and interior of cell
describe integrins and their role in sensing external env *
they are heterodimer complexes of a and b subunits that associate extracellularly by their head regions - ligands bind here
each of the leg regions spans the membrane - short cytoplasmic tail
there are>20 a/B combinationss that bind to specific short peptide sequence of ECM proteins
eg a5B1 binds to arg-gly-asp this is tehe RGD sequence
peptide sequences (eg RGD) are found in >1 ECM eg RGD is in fibronectin, vitronectin, fibrinogen and others
most integrins link to cytoskelton via actin binding proteins ie link proteins (ecept a6B4 integrin complex in helidesmosomes linked to cytokeratin [intermediate filament] network)
the integrin complexes cluster to form local adhesions (or hemidesmosomes - a6B4) - the clusters are involved in signal transduction
some integrins bind to specific adhesion molecules on other cells eg in immune system and clotting
explain signalling to and from the ECM receptors *
integrins can transduce signals - ECM binding to an integrin complex can stimulate the complex to produce a signal inside the cell - this is ‘outside in’ integrin signalling
describe the conformational changes to integrin during signal transduction *
they can be flexed or extended - switching between forms alter ability to bind to ligands - therefore cell-ECM adhesion and hence the signalling can be switched off
when flexed - legs are bent
in extended conformation, the legs are open
describe ‘outside in’ signalling *
a cell can recieve information about its surroundings from adhesion to ECM
integrins recruit proteins that help with signalling and recruiting cytoskeleton - these proteins cluster - they recruit src (important in signalling) and Fak (phosphorylates src and influences its proliferative capacity)
the composition of the ECM will determine which integrin binds and therefore what signals the cell recieves - altering the phenotype of the cell
focal adhesions sense the mechanical properties of their surroundings - when the proteins experience force they open up, changing their properties ie they can recurit differnet proteins
the amount of force that is generated at an adhesion depends on the force generated by the cytoskeleton and the stiffness of the ECM
describe inside-out signalling *
a signal generated inside the cell (eg because of hormone binding) can act on integrin complex to alter the integrin’s affinity for ECM - ie activates the receptor
this occurs in inflammation/blood clotying - switches on adhesion of circulating leukocytes, and switches on the integrins on platelets otherwise they would be adhesive all the time
describe the conformational changes to an integrin complex during activation and signalling *
low affinity - bent conformation = weak or no binding to ligand
high affinity - extended conformation = strong binding to ligand
in some cells might always be in high affinty/always a signal, in others low is prefered until signal
signal from inside the cell acts on the low affinity integrin complex to promote the switch to the extended high affinity conformation - this is inside out activation - switches adhesion on
ECM ligand binds causes the further opening of the leds - exposes the binding sites for the recruitment of cytoplasmic signalling molecules - this is outside in activation
describe the density dependance of cell division *
at high cell density the cells compete for growth factors
when cells in culture form a confluent monolayer - they stop proliferating and slow down other metabolic activities
this used to be known as contact-inhibition
however - if you add more GF to a certain area proliferation is stimulated again and cells grow on top of each other
this indicates it is competition for GF not cell-cell contact that inhibits division
what mechanism to GF use to alter gene expression and proliferation of cells *
ERK pathway - MAPK
what are the signals that control proliferation of tissue cells *
GF - depends on density
ECM - depends on anchorage
they activate identical signalling pathways - MAPK
however, individually they are weak/transient
together activation is strong
therefore the separate pathways act synergistically
what are the different types if contact interaction between cells *
short term - transient interactions between cell which dont form stable cell-cell junctions
long term - stable interactions resulting in formation of cell-cell junctions
describe how cell to cell contact between non-epi cells differs from epithelial cells *
epi cells - their job is to form junctions and layers
non-epi - when collide they dont form cell-cell contacts - tehy repel each other by paralysing mptility at the contact site - this promotes formation of a motile front at another site (ie form lamellipod) and the cell moves in the opposite direction
this is contact inhibition of locomotion and is responsible for preventing multilayering of cells in culture/in vivo
describe long-term cell-cell contacts *
some cells stringly adhere and form specific cell-cell junctions (adherens junctions, desmosomes, tight junctions, gap junctions)
this happens in epithelial cells and endothelial cells - they form layers, and neurons forming synapses
junctions in epi: continuous belts (zonula) or as discrete spots (macula) also desmosomes and gap junctions
descripe contact induced epithelial spreading *
contact between epi cells leads o the mutual induction of spreading and actin polymerisation - so the total spread area of contacted cells is more than the sum of the 2 separated cells - this could cause a stable monomer
how does cell-cell adhesion affect proliferation *
the molecules involved in cell to cell junctions are Ca dependant
if you remove the ca there are no junctions - this activates MAPK and decreases p17KIP1 = high proliferation
if ca is added again - cell to cell junctions form = inactivated MAPK = increased p17KIP1 = low proliferation because p27 is an inhibitor of proliferation
get the same effect if add adhesion blocking Ab = high proliferation, remove Ab = low prolif
describe the molecular organisation of teh adherens molecules (cell-cell junctions) *
have cadherins that have linking molecules that link them to the cytoskeleton
cadherins are Ca dependant, homophilic cell adhesion molecules - they bind to identical molecules on adjacent cells
inside the cell molecules inc B-catenin and a-catenin link cadherin to the actin filament
describe adenomatous polyposis coli (APC) (
inherited colon cancer - there are many inherited family forms
the APC gene-product is a protein involved in the degredation of B-catenin which links teh cadherin junctions to the cytoskeleton
when B-catenin is bound - the plasma membrane is involved in forming cell junctions so is sequested
when cytoplasmic B-catenin is increased - if APC complex is active - b-catenin is rapidly degraded; if APC inactive (in cancer) - B-catenin associates with a molecule called LEF-1 forming a TF taht influences gene trasncription = proliferation
other than B-capetin - what adhesion associated pathways influence contact induce inhibition of proliferation *
clustering of cadherins after cell-cell contact is known to alter the activation of small GTPases eg Rac is activated and Rho is inhibited - this can iduce proliferation
some GF receptors are associated with cell-cell junctions - this reduces their capacity to promote proliferation
what happens if a cell loses its behavioural constraints *
they proliferate uncontrollablu - lose density dependance of proliferation
are less adherent and will multilayer - lose contact inhibition of locomotion and anchorage deopendance
epithelia break down cell-cell contacts
not Hayflick limited - express telomerase
ie they become cancer
describe the loss of contact inhibition in cancer cells *
they pile up so form a solid tumour and invasion (ploughing through otehr tissues)
what are proto-oncogenes *
receoptors, signalling intermediates and signalling targets
if the genes coding for a component of a signalling pathway is mutated so that the protein is constituitively actibe - the pathway will be permenantly on
this is the mechanism of short-circuitig leading to uncontrolled proliferation asa result of loss of GF dependance
definition of an oncogene *
mutant gene which promotes uncontrolled cell proliferation
definition of a proto-oncogene *
normal cellular gene corresponding to the oncogene
examples of proto-oncogenes and their oncogene *
EGFR
Ras (signalling intermediate) - V12Ras - mutation - in 30% of all cencers - in 90% of pancreatic cancers
c-Raf (signalling intermediate) - v-Raf - deletion of regulatory domain
c-Jun (TF) - v-Jun - deletion of a regulatory domain
mechanism for uncontrolled proliferation of tissue cells *
if both GF pathway and ECM pathway are switched on all time - no longer need the external signals = proliferation w/o the normal constraints
if only have 1 mutation - wont be fully uncontrolled - need both
describe benign tumours *
encapsulated and dont invade
describe malignant tumours *
invade and metastasise - cause the most problems
describe local invasion and met *
major feature of cancer is its ability to spread
most human cancers are carcinomas (epithelial)
in order to metastasise - cells must break away form tumour, travel in bv or lymph, lodge at a distant site, leave the vessel, ultimately establish a secondary tumour
cell-cell adhesion must be broken down
the cells must be motile
degradation of ECM must occur - matrix metaloproteinase (MMP) levels increased in order to break thorugh basal lamina and ECM
lose contact inhibition of locomotion to travel
the degree of cell-cell adhesion is an indicator of how differentiated the primary tumour is, and indicates its invasiveness and the prognosis
examples of GF *
EGF - epidermal GF
PDGF - platelet derived GF
