Hallmarks of Cancer

What does it mean if a tumor is self-sufficient?

It proliferates in the absence of growth factors

What is required for a cell to pass the R point?

Growth factors (usually in serum)

Do cancer cells need growth factors to pass the R point?

No, they continue through the cell cycle even in the absence of growth factors

How is the Her2 receptor an example of a proto-oncogene receptor that allows for self-sufficiency?

The Her2 enzyme-linked receptor normally binds growth factors; in cancer cells, the Her2 protein being expressed (by an oncogene) develops a mutation that leads to an amino acid substitution, substituting valine for glutamine; this substitution is in the transmembrane domain of the receptor, and this mutation is sufficient to change the structure of the protein such that the receptor becomes constitutively active, so it becomes auto-phosphorylated even in the absence of growth factors; this allows for self-sufficiency because this receptor activates the Ras pathway that leads to expression of Myc, Jun, and Fos, as well as other genes needed for S-phase; once this mutation occurs, the Her2 receptor is now called the Neu oncoprotein

How is the EGF (epidermal growth factor) receptor an example of a proto-oncogene that allows for self-sufficiency?

The EGF enzyme-linked receptor binds growth factors normally; in cancer cells, a deletion mutation results in the oncoprotein expressed lacking the entire extracellular domain; this results in the protein becoming constitutively active and the intracellular domain becomes autophosphorylated; this allows for self-sufficiency because this receptor activates the Ras pathway that leads to expression of Myc, Jun, and Fos, as well as other genes needed for S-phase

What does it mean if a cancer cell exhibits insensitivity?

Anti-growth signals are no longer recognized

How do cancer cells exhibit insensitivity?

In our DNA, there is a gene called p15 that encodes an inhibitor of cell cycle progression; there is also a gene called pAI-1 that encodes an inhibitor for a protease that degrades the ECM; in normal cells, the Smad protein complex allows for the transcription of these genes; Smad normally becomes active when Ser/Thr kinase receptors become activated when TGF-β (transforming growth factor beta) binds to it; TGF-β is an anti-growth signal, so it stops the cell from excessive growth; cancer cells have mutations that interrupt this signaling event, so cell cycle progression is not inhibited (no G1 arrest) and there is degradation of the ECM, allowing the cancer cells to move around and proliferate

How does p15 affect the cell cycle?

p15 encodes a protein that is an inhibitor of cell cycle progression

What is TGF-β?

It is Transforming Growth Factor Beta, and it is an anti-growth signal

What does the gene p15 encode for?

An inhibitor of cell cycle progression

What does the gene pAI-1 encode for?

An inhibitor of a protease that degrades the ECM

In order to remain healthy, what do cells need to be in contact with?

The ECM

How did researchers figure out that cells must be in contact with the ECM to remain healthy?

They took glass slides of different size and covered them in ECM proteins; they placed a single cell on each slide; they found that as the area of the slide got bigger, the amount of apoptosis decreased, and the amount of DNA synthesis increased, suggesting cells on larger slides (more contact with ECM proteins) were more primed to go through cell division; this showed that a cell’s contact with ECM determines rates of division, DNA synthesis, and apoptosis

What is anoikis?

The type of programmed cell death that is triggered by lack of contact with the ECM; this process is important for the excavation of tissues (like in forming a duct) and the prevention of tumors

What would predicted Bcl-2 levels be in cancer cells?

They would have high Bcl-2 levels because Bcl-2 inhibits programmed cell death

What does it mean when a cancer cell exhibits evasion?

They are escaping programmed cell death

What are different ways a cell can exhibit evasion?

• Have high Bcl-2 levels → Bcl-2 inhibits apoptosis

• Have high Mdm2 levels → Mdm2 is the protein that inhibits p53, so p53 won’t be expressed and it won’t transcribe genes needed for apoptosis

What does it mean if a cancer cell exhibits immortality?

It has limitless replicative potential

With cancer, are all cells in the tumor immortal?

Yes, all cells in the tumor have acquired limitless replicative potential, but this does not mean all of them actually have high replicative potential (only a minority do)

How are self-sufficiency, insensitivity, and evasion different from immortality with respect to cancer cells?

Self-sufficiency, insensitivity, and evasion uncouple cells from mechanisms that limit proliferation, but they don’t ensure limitless proliferation, only immortality does that

What is the maximum number of replications a mammalian cell can undergo?

40-50 divisions

What is cell senescence?

These cells are in terminal G₀ state, so they will never divide again; these cells are still viable and have a stable karyotype, they just have no replicative potential; this process is driven by physiological and metabolic stress (accumulation of reactive oxygen species) that damages the cell over time

What will the growth rate of a senescent cell culture look like on a graph?

Over time, the growth rate of the culture will steadily decline until it reaches 0

Is there a time limit to when a cell becomes senescent?

No, it is thought about in terms of replicative age rather than chronological age, so it is more about how many times a cell has divided

What is the Hayflick Limit?

The replicative limit for cells after which they enter terminal G₀ and become senescent; for mammalian cells it is 40-50 divisions

What is the difference between cell senescence and cell crisis?

Cell senescence is defined by the growth rate of the culture (it steadily declines over time) whereas cell crisis is defined by the viability of the cells (replicating, healthy cells that experience a sudden crash in viability)

What is cell crisis?

This has to do with the viability of cells in a culture; these cells replicate normally and are not undergoing senescence, when suddenly the entire culture experiences a crash in viability; these cells are no longer viable and have extreme genomic instability; this is driven by the loss of telomeres, which typically protect chromosomes from fusion and breakage events; this damage in DNA causes the cells to die by apoptosis

What are telomeres?

They are the ends of chromosomes, and they are typically made up of repetitive sequences of DNA that are non-functional; the telomeres form a loop at the very end of the chromosome; the purpose of these telomeres is to protect the functional genes near the ends of chromosomes from being lost during chromosome replication and to prevent breakage and fusion events of chromsomes

Why would the ends of chromosomes shorten every time DNA is replicated if there were no telomeres?

Because the synthesized 5’ strand would always be shorter since the primer is placed there and DNA polymerase has no DNA to attach to to replace the primer on the 5’ end

How are telomeres extended?

By telomerase, which contains a short RNA molecules that acts as a template, allowing the template strand to be extended; this gives allows a new primer to be placed so DNA polymerase something to bind to in order to extend the synthesized strand; telomere ends shorten with each successive replication of DNA, leading to crisis in some cases

What did Barbara McClintock’s work show?

She used FISH (fluorescence in situ hybridization) to show that chromosomal fusion events can occur, and that they are more likely at happen at the ends of chromosomes

Why don’t normal cells have limitless replicative potential if they have the gene for telomerase?

Because telomerase is only really expressed when we are embryos, so our telomeres are formed when we are young and then shorten throughout life

What do chromosomal translocations typically lead to?

They typically lead to programmed cell death, but in some cases they can lead to oncogenesis

How does chronic myelogenous leukemia occur?

It is the result of a chromosomal translocation; breakage occurs on two different chromosomes and then a fusion event occurs, fusing the BCR and ABL genes together, resulting in a hyperactive fusion protein that leads to cancer; two proto-oncogenes become one oncogene

How does Burkitt lymphoma occur?

It is the result of a chromosomal translocation; breakage occurs on two different chromosomes and then a fusion event occurs, putting the Myc gene onto the active region of the other chromosome, leading to excess levels of Myc, which is a transcription factor for genes needed to enter S phase; a proto-oncogene becomes an oncogene

How do cancer cells escape crisis?

By expressing high levels of telomerase, so the ends of the chromosomes do not shorten and trigger crisis

Is telomerase necessary for neoplastic growth?

Yes, telomerase is necessary to double cell population and without telomerase, crisis is triggered, inhibiting neoplastic growth

What is necessary for limitless replicative potential of cells?

Long telomeres

How do the length of telomeres compare between different cell types?

• Somatic cells have reduced telomeres

• Cells in crisis have no telomeres

• Cancer cells have very long telomeres

• Embryonic and stem cells also have long telomeres, but not as long as cancer cells