Cellular Senescence Flashcards
What is the difference between growth between primary cells and transformed cells?
primary cells grow for a limited time in culture then will stop growing- don’t die just persist without dividing whereas transfromed cells continue to grow
How did Hayflick indicate senscence?
described that cells have a finite replicative capacity which despite adequte space; nutrient and growth factors fail to proliferate and remain viable
What was the first molecular explanation for senscence?
telomere shortening
What are the telomere repeats?
TTAGGG repeats
What is the function of telomeric cap ?
represses DNA damage repair pathway which would normally respond to the loss of TTAGGG repeats
What happens to the telomere cap with progressive telomere shortening?
there is not enough telomere for the cap to bind exposing DNA damage resulting in sustained DNA damage signalling
What are the two results of sustained DNA damage signalling?
apoptosis or cellular senscence
What is the function of cellular senescence in the context of telomere shortening?
cell must stop dividing or will start getting DNA damage in the genome, not telomeres and risk of transformation
How is senescence related to iPS reprogramming?
the majority of cells undergo sensecence as recognise the factors provided as oncogenic which provides a significant challenge in the field
How is the effect of cytotoxic drugs induced senscence important?
in lymphomas this is one of the most important reasons for why cytotoxic drugs work as they induce senscence
What happens if you give a primary cell an oncogene?
will undergo lots of proliferation then will recognise this and undergo senescence?
What are the different factors leading to senescence?
telomere shortening; serial culture; cytotoxic drugs; oxidative stress; DNA damage; oncegene activation; iPS reprogramming
What is the overall function of cellular senescence?
maintain genomic stability
What is the first thing seen in response to a senscence stimulus?
develop lots of DNA damage response foci
Why is there upregulation of cylin-kinase dependent regulators in senescence?
to stop the cell cycle
What cyclin-kinase dependent regulators are upregulated in senescence?
p21; p53; p16
What happens to the DNA in cells undergoing senescence?
reorganise nuclear chromatin and nuclear lamina to change gene transcription
What do you see in senescent cells as a result of chromatin reorganisation?
SAHF- senescence associated heterochomatin foci
What happens to the lysosomes in senscent cells?
increased lysosome activity
What is the marker of increased lysosome activity in senescence
SA b-Gal
How do you know SA b-Gal isn’t a driver of senescence?
if knock it out, still get senescence- not a driver but a marker
What is the result of the altered protein metabolism in senescence?
releas of SASP- senescence associated secretory phenotype; also see altered mitochondrial function which is needed to produce lots more proteins
What are SASPs?
collection of things that can cross membrane- paracrine activity which can force other cells to senesce; activate inflammation programs; give GF to other cells to promote transformation
What was the first indication that SASP can promote transformation?
if culture pre-neoplastic prostate cells with senescent cells there is increased transformation
How is senscence related to cancer?
one of hte most powerful barriers to cancer progression; all cancers have to have mutated p21; p16; p53 or something controlling them to develop)
Why might be the reason for high levels of senscent cells being found in premalignant lesiosn?
if constantly transcribing lots of p21 or p16, this leads to mutations and transformation
How is p53 controlled?
phosphorylation
What negatively inhibits p53?
MDM2
What happens when MDM2 is inhibited or deleted?
there is phosphorylation of p53 which stabilises it
What does the phosphorylated form of p53 bind to?
promoter for CDKN1A
What does CDKN1A encode?
p21
What does p21 negatively regulate?
CDK4 and CDK2
What is the effect of inhibiting CD4 and CD2?
Rb cannot be phosphorylated and therefore remains bound to E2F and transcription is repressed
What is the function of E2F?
drives proliferation
What encodes the p16 gene?
INK4A
What controls the expression of INK4A?
PRC1
What CDKs does p16 inhibit?
CDK4 and CDK6
What happens if p53 or p16 are deleted?
completely block growth arrest
What is the phenotype of senescent cells?
growth arrest; apoptosis resistant; altered gene expression
How are senescent cells cleared?
recruitment of immune cells to mediate clearance
What shows that it is immune mediated clearance that results in senescent cell removal rather than death?
immunodeficient mice do not clear
What happens to immunodeficient mice who don’t clear senescent cells ?
grow into tumours
How do senescent cells recruit immune cells?
SASP
What receptors do senescent cells upregulate to ensure their clearance?
upregulate HLA-antigens and activatory NK ligands
Give examples of activatory NK ligands?
MICA; MICB; ULBP1
What is antagonist pleiotrophy?
when something is beneficial early in life but detrimental later in life
How is senescence detrimental in older?
stem cells and their progenitors undergo senescence limiting the stem cell pool resultin in decreased effector numbers
Which type of haematopoietic stem cells are particularly affected?
loss of lymphocytes but maintain transcription of myeloid cells (AML onset is 65)
In addition to depltion of the stem cell pool, what other detrimental effects do senescent cells have?
SASP changes microenvironment which can include promoting tumours
What is senescence in T cells?
have completely different phenotype, as do T cells in older people- maybe not true senescence but on the spctrum
What is the result of senescent T cells?
have different antigen recognition- pro-autoimmune? importnat in RA?
What shows that the effect of senescence is not just in depletion of the stem cell pool?
if remove the senescent cells, can improve the function of the tissue
Give an example of a drug that preferentially targets senescent cells?
ABT263
What is the benefit seen in mice given ABT263?
rejuvenation of aged HSCs
What is the role of senescent cells in wound healing?
if remove, there is impaired regerenation
What demonstrates that senescent cells are critical in contributing to ageing phenotype?
clearance of p16INK4 cells delays ageing associated disorders
What other diseases has senescence been implicated in?
many others espeically age-associated pathologies. Clinical trial in depleting senescent cells in pulmonary fibrosis /
How can senescence be used in cancer treatment?
if can induce senescence in cancer cells (downstream of p53 etc mutations) can induce grwoth arrest and upregulation of NK ligands so cancer is cleared
How is senescence used embryologically?
remodelling in utero
