Cell Immortalization and Tumorigenesis

What did Leonard Hayflick observe about cell division limitations?

In primary cells, there is a limit of 60 population doublings before they stop growing and become senescent

How can senescence be visualized?

marker enzyme B-galactosidase (blue) can be seen in aging tissue, hypoxic environments, ROS, radiation and chemotherapy

Can senescence be a good sign?

In cancer, senescence would be beneficial as the cancer cells my temporarliy withdraw from the cell cycle

Increased expression of what two things leads to replicative senescence?

CDK inhibitors - p16^INK4A and p21^cip1

How does the expression of SV40 large T antigen oncoprotein help cancer overcome senescence?

It binds and sequesters pRb and p53

Other than surpassing senescence, what else do cancer cells need to overcome?

Crisis, or wide spread apoptosis of cells.

Both mechanisms are on a hard-wired counting device for population doublings in telomeres

What are telomeres? What is their purpose?

Hexanucleotide sequences (5’ TTAGGG 3’) that extend 5-10 kb in lenth at the ends of chromosomes. They work to protect DNA during replication. As they degrade, the DNA can become damaged and cell death usually occurs.

How are telomeres protected?

Shelterin complexes like TRF2

Shelterin hides the chromosome ends from the DNA damage response system, while the t-loop hides the 3' overhang, preventing the cell from treating chromosome ends as broken DNA

Per cellular generation, how many bp of NT are lost?

50-100 bp

The reintroduction of telomerase contributes to which hallmark of cancer?

Endless replicative potential

The loss of telomeres can lead to different consequences like:

• end-joining of chromosomes

• alterations in chromosome number and distribution

• apoptosis even without p53

What is the mechanism of reactivation of telomerase?

An RNA molecule (hTR) is repeatedly used to synthesize DNA to the 3’ end of telomeres through reverse transcriptase activity (hTERT) and a separate DNA poly finishes.

The expression of telomerase can be seen through the expression of _____.

hTERT - telomerase reverse transcriptase

For tumors that do not express telomerase, what mechanism do they use to induce endless replicative potential?

They lengthen telomeres through the incorporation of other telomere strands (extension and disengagement) to make the strands longer

Specifically seen in osterosarcomas

How do cancer cells use the natural erosion of telomeres to its advantage?

Since the natural erosion of telomeres can eventually damage DNA, sometimes cancer will wait to activate telomerase to see if any advantageous mutations occur naturally.