L3: Transformed cells and replicative immunity part 2 Flashcards
evidence for co-operation of oncogenes
Evidence for co-operation of oncogenes
Epidemiology suggests that multiple changes are needed (eg. incidence rates vs age)
Tumour progression argues for multiple changes (tumours start benign and get progressively more malignant with time)
Two activated oncogenes not sufficient also need additional changes eg. tumour suppressor gene loss (eg. p53 or Rb)
In lab- know two oncogenes isnt enough. Need to lose tumour suppressive proteins aswell.
functions of proto-oncogenes
Functions of proto-oncogenes
Signal transduction pathways involved in promoting cell growth
Regulation of apoptosis
Regulation of differentiation
Regulation of cellular life-span
Changes in these activities is how cancer cells awuire sustained proliferative signalling, eventually causes detectable, palpable ???
enabling replicative immortality
Another hallmark of cancer cells: enabling replicative immortality
For tumours to form cancer cells must escape the limits
of proliferative potential
From early studies we know that cancer cells acquire the ability to multiply many more times than is seen in normal biology
So what is the cell machinery that limits cell proliferation
and how do cancer cells over come this ?
what limits cell proliferation?
What limits cell proliferation?
Cells enter replicative senescence – halt in cell proliferation but
retain cell viability and tissue integrity - we see that expression of
cell cycle inhibitors p16INK4A and p21Cip1 are increased in these cells.
Cells enter crisis – death by apoptosis > based on generational clock. Where things get so chronic/critical that cells activate apoptotic pathways.
telomeres?
Telomeres
Want to limit no. of times cell divides as that limits risk that there will be a significant meaningful change mistake in the replication (in a coding sequence)
Chromosomal DNA is extremely long and the ends are quite unstable
and prone to fusing - that can cause genetic mistakes
Telomeres (repeated hexonucleotide sequences) sit at the ends and
act to prevent fusion and protect the chromosomal DNA
Telomeres also help to maintain the integrity of
coding sequence as DNA replication machinery (polymerase?)
is not good at getting right to the ends of the
chromosome
Often lose very ends of chromosomes when replicating dna, catastrophic if losing ends of coding sequence
Telomeres: sit at the ends of chromosomal dna, prevent chromosomes from fusing to eachother and protect that chromosomal dna from not being replicated accurately. Maintain integrity of coding sequence.
telomerase?
telomerase
Telomere tandem repeats (about 15 kb, in germ cells), are maintained by an enzyme called telomerase (TERT reverse transcriptase).- puts these repeats at the end of chromosomal dna
Telomerase is expressed in germ cells, and some stem cells, but is absent (or very low) in most somatic tissues
Telomere length decreases with time in cell culture, and with age in vivo
Progressive telomere shortening with each cell cycle may limit proliferative potential
Really key- this telomerase activity is an embryogenic activity, happens in some germ and stem cells but gone/low in somatic tissue. No telomerase activity in adult tissue. Everytime dna replicates, as little less replication everytime, lose telomeres.
telomere shortening in normal adult cell
Telomere shortening in normal adult cell
Telomeres shorten until reach cell crisis. Mechanisms in cell that recognises dna damage and triggers response that triggers cell apoptosis. Cells become immortalised if telomeres start to get added back on.
short telomeres
Short telomeres can induce sensescence or apoptosis
= Short or dysfunctional telomeres can induce a DNA-damage response
Leads to induction of the tumour suppressor, p53 - induces inhibitors and causing cell cycle arrest and apoptosis for the dna damage response
p53 (transcription factor) induces expression of the cyclin-dependent kinase inhibitor (CDKI) p21
P21 causes cell cycle arrest
what happens to cancer cells
Re expression of telomerase. 85% of human tumour express telomerase
Evidence = Transfection of normal human fibroblasts with the catalytic subunit of human telomerase (hTERT) causes telomere lengthening and immortalization
Bodnar et al. (1998)
Immortalized cells (inc. 80-90% of cancers) generally reactivate expression of telomerase, and regain long telomeres
If take transfection and give telomase, can immortalise those cells.
GRAPH
The cells without telomerase tail off, stop dividing
Cells with telomerase have no idea how much they have been dividing as telomerase keeps putting telomeres on the ends of chromosomal dna lose the generational clock and counting mechanism
Very prevalent in tumour biology but often used as a tool in cellular research when making new cell lines
So cancer cells become immortal
hTERT mutations
hTERT mutations
Point mutations in the
promoter have been
documented in human
tumours. -228 and -250
elevate rate of
transcription –
found in 83%
glioblastoma
tumour samples
Protein telomerase is not different. But point mutations in promoter means it gets turned back on. In adult tissue- promoter suppressed so no telomerase expression. Point muations elavate the rate of transcription of telomerase so telomerase itself does not change. Example of inappropriate expression.
