Cell Cycle, Apoptosis and Cancer

Question 1

Quiescent cells (in G0)

Answer 1

May reenter the cell cycle at G1 after stimulation by molecules such as growth factors

Hepatocytes, epithelial cells of the kidney tubules

Question 2

Senescent cells (in G0)

Answer 2

Never able to reenter the cell cycle and are unable to proliferate even in the presence of growth factors

Neurons-will never divide again

Question 3

Labile cells

Answer 3

Never enter G0 and are constantly dividing

Gut epithelium, skin, hair follicles, bone marrow

Question 4

How to fully activate CDK’s

Answer 4

Need CDK-activating kinases (CAK)

Question 5

CDK levels during cell cycle

Answer 5

Levels of CDKs are constant throughout the cell cycle

Question 6

Cyclin levels during cell cycle

Answer 6

Levels of individual cyclins vary considerably during cell cycle

Question 7

Phosphorylation of CDKs by CAK does what?

Answer 7

CAK (CDK-activating kinase) fully activates the cyclin-CDK complex

(WEE1 inactivates by phosphorylating)

Question 8

Phosphorylation of CDKs by WEE1 kinase does what?

Answer 8

Inhibits cyclin-CDK activity

CAK (CDK-activating kinase) activates by phosphorylation

Question 9

CDC25

Answer 9

A phosphatase that can remove the phosphate group that was put on by WEE1.

This will reactivate the cyclin-CDK complex

Question 10

Cyclin-CDK complex activities can be modulated by what?

Answer 10

CDK inhibitor proteins (CKIs)

Question 11

2 families of CKIs

Answer 11

• CIP/KIP

- INK4

Question 12

CIP/KIP

Answer 12

1 of 2 families of CKIs (CDK inhibitor proteins)

i.e. p27. Inhibit G1 and S phase cyclin-CDK activities by binding to the cyclin-CDK complex and altering the conformation of the active site to render CDK inactive

Question 13

INK4

Answer 13

1 of 2 families of CKIs (CDK inhibitor proteins)

i.e. p15, p16, p18 etc. Inhibit G1 CDKs (4 and 6) by binding to them and preventing CDK4 and CDK6 from associating with cyclin D

Question 14

How to terminate the activity of cyclin-CDK complexes

Answer 14

Degrade the transiently expressed cyclin proteins through regulated proteolysis. Targeting them for this is done via polyubiquitination and catalyzed by ubiquitin ligases.

Specific ubiquitin ligases can also ubiquitinate CKIs and target them for degradation, thereby releasing the inhibition of S phase cyclin-CDK complexes

Question 15

What does p27 do?

Answer 15

Binds to the cyclin-CDK complex and inactivates the kinase activity of CDK

Question 16

Retinoblastoma (RB) protein

Answer 16

Substrate of G1 and G1/S cyclin-CDK complexes

Used as a tumor-suppressor protein b/c it can arrest the cell cycle at the G1 checkpoint

Question 17

Hypophosphorylated form of RB

Answer 17

Binds to E2F transcription factors. This sequesters E2Fs and prevents them from triggering the transcription of cyclin E (a G1/S phase cyclin) and cyclin A (an S phase cyclin)

Question 18

Hyperphosphorylation of RB

Answer 18

Hyperphosphorylation of RB by the G1 and G1/S CDKs releases E2Fs and allow for the transcription of cyclin E to occur so that cells can transit the late G1 checkpoint and enter the S phase

Question 19

What keeps RB (retinoblastoma) protein in the phosphorylated state?

Answer 19

S phase and M phase cyclin-CDK complexes

Degradation of these cyclins during the later stages of mitosis allows for the dephosphorylation of RB for the next round of the cell cycle

Question 20

What does DNA damage do to p53?

Answer 20

DNA damage appears to stabilize another tumor suppressor protein known as p53

Question 21

In the presence of DNA damage, p53 is _____

Answer 21

p53 is phosphorylated and stabilized

It can stimulate the transcription of many genes including p21 (CIP/KIP family of CKI). Inhibition of cyclin-CDKs by p21 can lead to arrest of cells in G1, S and G2 phases

Question 22

If DNA damage is not fixed, then what happens?

Answer 22

Apoptosis

Question 23

Retinoblastoma and the RB protein

Answer 23

Retinoblastoma is a childhood disease.

Tumors develop due to unchecked division of precursor cells in the immature retina.

Hereditary form affects both eyes
Nonhereditary form affects one eye

In the hereditary form, one copy of the RB gene is mutated or lost in every cell, and the cells become predisposed to becoming cancerous. Cancerous cells contain different somatic mutations in the two copies of the RB gene

Question 24

p53 and cancer

Answer 24

More than 50% of all human cancers exhibit mutations in the p53 gene

Metabolic activation of benzo(a)pyrene (in cigarette smoke) makes a potent mutagen. Activated benzopyrene causes mutations in genes such as p53 by G–>T transversion.

Aflatoxin (in moldy grain and peanuts) also induces G–>T transversions in the p53 gene

Question 25

Apoptosis is caused by what?

Answer 25

A cascade of intracellular proteases called caspases

Question 26

Extrinsic pathway

Answer 26

Initiates outside the cell

“death receptor pathway”

Triggered by binding and activation of an external death ligand (i.e. Fas ligand) to its receptor on the plasma membrane

Adaptor proteins containing death domains (i.e. Fasassociated death domain protein (FADD)) bind to intracellular regions of the activated receptor and recruit procaspase-8

Autocatalysts of procaspase-8 generates caspase-8, which then initiates a caspase cascade involving other caspases (3,6,7)

Question 27

Intrinsic Pathway

Answer 27

Triggers apoptosis inside the cell

“mitochondrial pathway”

Triggered by events such as growth factor withdrawal, DNA damage (which increases p53), and cell cycle defects

(Check notes/picture for pathway diagram)

Question 28

Proto-oncogenes

Answer 28

These encode proteins that promote cell growth and division. Tend to be members of sell-signaling pathways, i.e. growth factors, hormone receptors, signal transducers and TF’s

Such “gain of function” mutations often appear to be inherited in an autosomal dominant fashion

Question 29

How are proto-oncogenes converted to oncogenes?

Answer 29

Via gain of function mutations

Question 30

Proto-oncogenes and HER2

Answer 30

A point mutation that changes Val to Gln

Oncoprotein: NEU

BREAST CANCER

Question 31

EGF and ligand binding doma–>ocoprotein

Answer 31

Deletion in part of gene

Oncoprotein: EGFRvIII

GLIOBLASTOMA

Question 32

How proto-oncogenes are converted to oncogenes (3 mechanisms)

Answer 32

1. A point mutation
2. Gene amplification
3. Chromosomal translocation: genetic rearrangement (e.g. c-MYC, ABL)
   - MYC→Burkitt Lymphoma
   - ABL→myelogenous leukemia (CML)

Question 33

Tumor suppressor genes

Answer 33

Even one good copy of a tumor suppressor gene is usually sufficient to provide normal regulation. Therefore, mutations in tumor suppressor genes appear to be autosomal recessive mutations, and both copies of such genes have to be damaged to make the cells prone to the onset of cancer. These mutations are known as “loss of function”

Thus, oncogenesis results when somatic mutations occur in combination with the inheritance of recessive mutations and so both alleles become dysfunctional.

Question 34

Tumor suppressor gene: p53

Answer 34

Monitors checkpoints of cell cycle

Characteristics of Mutated Protein Product: Observed in >50% of all human tumors

Question 35

Tumor suppressor gene: RB

Answer 35

Regulates G1 phase of cell cycle

Characteristics of Mutated Protein Product: Observed in retinoblastoma

Question 36

Tumor suppressor gene: APC

Answer 36

Regulates cell proliferation

Early event that occurs in the progression toward colon cancer

Question 37

Tumor suppressor gene: BRCA1/BRCA2

Answer 37

Plays a role in DNA repair and apoptosis

Inheritance of a mutated BRCA1 gene increases chance of developing breast cancer by the age of 50 by 30x

Question 38

Traits that appear to be common to most tumor cells

Answer 38

1. Self-sufficiency in growth signals (tumor cells don’t need external growth factors b/c many oncogene products mimic growth signaling)
2. Evading growth suppressors (resist growth inhibitory signals)
3. Activating invasion and metastasis (tumor masses spawn pioneer cells that invade other regions. Metastases are the cause of 90% of cancer deaths)
4. Enabling replicative immortality (cancer cells continue to express telomerase activity)
5. Inducible angiogenesis (cells in tissue need a nearby blood vessel to get their nutrients. Cancer cells can induce the development of new blood vessels)
6. Resisting cell death (cancer cells appear to be defective in both intrinsic/extrinsic pathways of apoptosis)
7. Deregulating cellular energetics (they can upregulate their GLUT1 glucose transporters)
8. Avoiding immune destruction (they appear to be invisible to the body’s T and B lymphocyte immune cells)
9. Tumor promoting inflammation (ROS may accelerate mutagenesis of cancer cells)
10. Genome instability and mutation (mutations lead to faster mutations)

Question 39

Metastatic colon cancer model

Answer 39

“Multihit” model: takes mutations in multiple genes over a period of time for the development of full-blown cancer

Development of metastatic colon carcinoma appears to involve the following events:

1. An early loss of the anaphase-promoting complex (APC) tumor suppressor gene from the long arm of chromosome 5
2. Activation of the K-RAS oncogene (with a Val 12 mutation) on the short arm of chromosome 12
3. Loss of the deleted in colorectal carcinoma (DCC) tumor suppressor gene from the long arm of chromosome 18
4. Loss of the p53 tumor suppressor gene from the short arm of chromosome 17
5. Other events (e.g. stimulated angiogenesis)

Question 40

RAS and cancer

Answer 40

Point mutation changing Glycine → Valine

Result: RAS oncoprotein is perpetually active

Question 41

Recognition domains

Answer 41

Adaptor proteins such as GRB-2 and IRS-1 have special domains known as SH2 domains or PTB domains that recognize and bind to motifs on the receptor that contain phosphorylated tyrosine residues

Question 42

Viral Oncogenesis

Answer 42

It accidentally incorporates an adjacent host proto-oncogene into its genome

Question 43

Fibrosarcomas in chickens

Answer 43

**Rous sarcoma retrovirus (RSV) can cause fibrosarcomas in chickens. The transforming factors introduced by the RSV was v-src, a tyrosine kinase. The v-src gene is the oncogenic form of the normal host proto-oncogene c-SRC

Proto-oncogene: c-SRC
oncogene(mutated): v-src

Question 44

Targeted inhibition of Oncoproteins. What do they use in general?

Answer 44

They target particular oncoproteins with monoclonal antibodies or specific inhibitors

Question 45

Targeted inhibition of Oncoproteins: Herceptin

Answer 45

For breast cancer

Monoclonal antibody (mAb) against HER2/NEU epidermal growth factor (EGF) receptors expressed in the mammary tissue

HER2/NEU are overexpressed

Question 46

Targeted inhibition of Oncoproteins: Erbitux

Answer 46

For colorectal cancer

A monoclonal antibody (mAb) directed against the EGF receptor that is over-expressed in colorectal cancer

Question 47

Targeted inhibition of Oncoproteins: Gleevec

Answer 47

For chronic myelogenous leukemia (CML)

An inhibitor that binds to the active site of the ABL tyrosine kinase and inhibits its activity (prevents phosphorylation). Has little effect on hurting the normal white cells in the blood. Targets an oncoprotein (BCR-ABL) that is unique to cancer cells.

Question 48

G1, G2, G3

Answer 48

G1: fix any DNA damage
G2: verify
G3: ensure chromasomes are good

Question 49

If a cancer destroys all abilities of DNA repair pathways…which principle does this represent?

Mutation in epidermal growth factor…what principle is this?

Answer 49

Genomic instability

Self-sufficiency in growth signals