LM 8.1: Cancer Cell Signaling

Question 1

what is the general cause behind cancer?

Answer 1

cancer is a disease of miscommunication between cells

good communication is essential for maintaining homeostasis in multicellular organisms

cancer starts when cells stop communicating, refuse to die and continue to proliferate

cells communicate directly via cell junctions or indirectly via extracellular signal molecules

Question 2

what are some types of extracellular signaling molecules?

Answer 2

hormones

cytokines

chemokines

Question 3

how does extracellular signaling work?

Answer 3

a ligand binds a receptor – the receptor is then activated and relays the signal from the cell surface to the targets inside via a signaling cascade

membrane receptors use many different mechanisms to pass on the messages they receive at the surface to targets inside the cells

the signal gets passed on usually by protein kinases

Question 4

when a ligand binds to a receptor and triggers a response, what is the target inside the cell?

Answer 4

transcription factors in the nucleus

pro-apoptotic factors in the mitochondria

Question 5

what is the Ras pathway?

Answer 5

very important for cell proliferation and survival

Question 6

how does the Ras pathway work?

Answer 6

1. ligand binds to a receptor
2. Ras-GDP is activated into Ras-GTP
3. active Ras-GTP activates Raf kinase
4. Raf activates MEK
5. MEK activates ERK
6. ERK activates AP1 transcription factor in the nucleus

**Raf, MEK and ERK are protein kinases that relay the signal to the TF in the nucleus

Question 7

what inhibits Raf in the Ras pathway?

Answer 7

RKIP

Question 8

what is an oncogene?

Answer 8

something that positively unregulates the Ras pathway

the wild-type version of the oncogene is called a proto-oncogene

Question 9

what is a TSG?

Answer 9

an inhibitor of the Ras pathway

removal of a TSG would lead to an increase in the output of the Ras signaling pathway

Question 10

what causes tumorigenesis?

Answer 10

activation of porto-oncogenes and loss of TSGs plays a pivotal role in carcinogenesis

stepwise accumulation of mutations in key elements in proliferation and survival pathways play a pivotal role in tumorigenesis

Question 11

what type of growth are tumors?

Answer 11

monoclonal growth

a SINGLE cell is transformed from normal to cancerous behavior to become the ancestor of the cells in a tumor mass

human tumors are monoclonal!!

Question 12

what is a polyclonal tumor?

Answer 12

multiple cells cross over the border from normalcy to malignancy to become the ancestors of several genetically distinct subpopulations of cells within a tumor mass

aka different cells mutate and clone and then combine to form a tumor so a tumor is made of different cells

human tumors are NOT polyclonal

Question 13

what are the hallmarks of cancer?

Answer 13

1. self sufficiency in growth signals
2. evading apoptosis
3. insensitivity to anti-growth signals
4. sustained angiogenesis
5. limitless replicative potential
6. tissue invasion & metastasis

Question 14

what are the functions of oncogenes and TSGs?

Answer 14

1. mechanisms that should trigger or carry out a self-destruct program in damaged cells are disabled or overridden
2. cancer cells amplify external growth cues or generate their own
3. cancer cells become deaf to quiescence cues from surrounding tissue
4. tumors invade and metastasize
5. cancer cells evade intrinsic limits on the number of times a normal cell can divide
6. tumors emit signals promoting the development of new blood vessels to deliver oxygen and nutrients

Question 15

what is quiescence?

Answer 15

resting state of the cell characterized by the absence of cell growth and division

Question 16

what are two additional hallmarks of conversion involved in the pathogenesis of some and maybe all cancers?

Answer 16

1. the capability of modify or reprogram cellular metabolism in order to most effectively support neoplastic proliferation
2. cancer cells can evade immunological destruction, in particular by T and B lymphocytes, macrophages and NK cells

these are emerging hallmarks because their capability isn’t fully validated

Question 17

what are two enabling characteristics of tumors?

Answer 17

1. genomic instability gives cancer cells genetic alterations that drive tumor progression
2. tumor-promoting inflammation: innate immune cells designed to fight infections and heal wounds can instead result in their inadvertent support of multiple hallmark capability

Question 18

oncogene vs. proto-oncogene

Answer 18

A proto-oncogene is a normal gene found in the cell that helps with cell proliferation

If an error (mutation) occurs in a proto-oncogene, the gene can become turned on when isn’t supposed to be turned on. If this happens, the proto-oncogene can turn into a malfunctioning gene called an oncogene. Cells will start to grow out of control. Uncontrollable cell growth leads to cancer

Question 19

what mutation is responsible for H-Ras oncogene activation?

Answer 19

a single nucleotide base difference that effected the 12th cofon of the H-ras reading frame

proto-oncogene has a glycine while the oncogene has a valine

glycine –> valine

Question 20

how does Ras become an oncogene?

Answer 20

glycine –> valine “gain of function mutation”

in normal cells, Ras exists as an inactive GDP bound or active GTP bound form

the conversion between the active and inactive forms is facilitated by two separate proteins named GAP (GTPase activating protein) and GEF (GTP exchange factor)

when corrupted by a single “gain of function” mutation, Ras proto-oncogene can be converted to an oncogene that permanently locks into the GTP bound active conformation

active Ras has many targets and most of them are key elements in proliferative and survival signaling pathways

Question 21

how can a mutated growth factor receptor cause uncontrollable cell proliferation?

Answer 21

normally growth factor receptors on the plasma membrane of a cell release signals into the cell interior only when the extracellular domain of the receptor has bound to the appropriate growth factor

if the extracellular domain of certain receptors is deleted because of a mutation in the receptor-encoding gene or alternative splicing of the receptor pre-mRNA

the resulting truncated receptor protein then emits signals into the cell without binding its growth factor ligand

Question 22

which genetic changes can effect protein expression and lead to the activation of proto-oncogenes?

Answer 22

1. gene amplification

2. chromosomal translocation

Question 23

how is the N-myc gene related to childhood neuroblastomas?

Answer 23

N-myc gene is often amplified in childhood neuroblastomas

kids whose tumor cells have minimal of no N-myc amplification have a really good prognosis and minimal clinical events

kids whose tumors have extensive N-myc application have a dramatically poorer prognosis and short survival times after diagnosis

Question 24

how is the C-myc gene related to Burkitt’s lymphomas?

Answer 24

in BL, a chromosomal translocation juxtaposes immunoglobulin genes on chromsomes 2, 14 or 22 with the C-myc gene on chromosome 8

so the translocation puts C-myc gene on chromosome 8 under the control of the immunoglobulin heavy-chain sequences on chromosome 14

the immunoglobulin enhancer sequences direct high, constitutive expression so the normal modulation of C-myc in response to physiologic signals is gotten rid of

the resulting myc oncogene initially makes structurally normal Myc protein but in insanely high amounts

Question 25

what gene is an example of gene amplification?

Answer 25

the N-myc gene related to childhood neuroblastomas

Question 26

what gene is an example of chromosomal translocation?

Answer 26

the C-myc gene related to Burkitt’s lymphomas

Question 27

how is the bcr-abl oncogene formed?

Answer 27

reciprocal chromosomal translocations between chromosome 9 which caries the abl gene and chromosome 22 which carries the bcr gene

results in the formation of fused, hybrid genes that encode hybrid bcr-abl proteins commonly found in chronic myelogenous leukemias (CML)

the reading frame of the bcr gene is fused with the reading frame of the abl gene which causes deregulated firing of the tyrosine kinase of the fusion protein which is responsible for me transforming effects of the newly formed oncoprotein

Question 28

how was the site of action of an oncogene figured out?

Answer 28

by staining with fluorescently labeled antibodies to the various proteins

or by fractioning cells in a centrifuge and then identifying the product found in particular fractions

Question 29

how can microRNA be a cancer gene?

Answer 29

miRNA modulates expression of TSGs and oncogenes

miRNAs operate by either cleaving mRNA or inhibiting translation

over expression of miRNAs by amplification of the miRNA-encoding locus could decrease expression of the target, such as a TSG

under expression of miRNAs by deletion or methylation of the miRNA locus could result in increased expression of a target such as an oncogene

Question 30

how does the cell cycle guard against activation of oncogenes?

Answer 30

the checkpoint machinery can force the cancer cell to exit the cell cycle via the CDK4 inhibitor INK4A and the MDM2 inhibitor ARF

ex. activation of oncogenic RAS leads to up regulation of INK4A which blocks cyclin-D-CDK4 mediated hyperphosphoyrlation of RB which stops the cell cycle
ex. activated MYC oncogene activates ARF which blocks the activity of the p53 inhibitor MDM2 and thereby activates p53

for a tumor to develop, these failsafe mechanisms would have to fail

Question 31

how does the cell cycle protect against an active RAS oncogene?

Answer 31

activation of oncogenic RAS leads to up regulation of INK4A which blocks cyclin-D-CDK4 mediated hyperphosphoyrlation of RB which stops the cell cycle

Question 32

how does the cell cycle protect against an active MYC oncogene?

Answer 32

activated MYC oncogene activates ARF which blocks the activity of the p53 inhibitor MDM2 and thereby activates p53

Question 33

what is oncogene addiction?

Answer 33

cancer cells acquire abnormalities in multiple oncogenes and TSGs

inactivation of a single critical oncogene A can induce cancer cells to differentiate into normal cells or undergo apoptosis

this dependence on aka addiction to oncogene A for maintaining the cancer phenotype provides an Achilles heel for tumors that can be exploited in cancer therapy

Question 34

is the tumorigenic phenotype dominant or recessive?

Answer 34

the cancer phenotype is recessive

cell fusion tests showed the cancer cells derived from most human tumors formed hybrid cells that were non-tumorgenic

Question 35

what are TSGs?

Answer 35

antigrowth genes whose activities are required to constrain or suppress cell proliferation

inactivation of TSG plays an important role in cancer pathogenesis

the presence of one functional copy of the TSG is sufficient to inhibit tumor growth – the diploid state of the human genome requires two successive loss of function mutations to inactive a TSG

in many kinds of cancers the loss of TSG is more important than oncogene activation

Question 36

what is loss of heterozygosity?

Answer 36

the loss of normal function of one allele of a gene in cells in which the other allele was already inactive is called a loss of heterozygosity (LOH)

Question 37

how can LOH be used to find TSGs?

Answer 37

mmm idk ask someone

Question 38

what is RASSF1A?

Answer 38

a TSG promotor

Question 39

what happens when you methylate RASSF1A?

Answer 39

you turn off a TSG

when the CpG island of the RASSF1A tumor suppressor gene is methylated, the TSG is turned off and basically all tumor cells have methylated RASSF1A

Question 40

what are gatekeepers?

Answer 40

TSGs that directly regulate the growth of tumors by inhibiting growth or promoting death

each cell type has only one, or a few, gatekeepers

inactivation of these genes s rate-limiting for the initiation of a tumor

Question 41

what is a caretaker?

Answer 41

TSGs that maintain the integrity of the genome

inactivation of a caretaker gene does not promote tumor initiation directly

inactivation leads to genetic instability which results in increased mutation of all genes

Question 42

is RB a gatekeeper or caretaker?

Answer 42

gatekeeper

hypophosphorylated pRB discourages transcription by attracting HDAC enzymes to pRB-E2F complexes

HDACs prevent transcription by removing acetyl groups from nearby histones so that RNA polymerase can’t get to the chromatin and transcription can’t happen

when the E2Fs are not complexed with pRB they attract histone acetylates which place chromatin in a configuration that is conductive for transcription

Question 43

are the BRCA proteins gatekeepers or caretakers?

Answer 43

caretakers

they are widely expressed in different tissues during the S and G2 phase

we aren’t exactly sure what their normal cellular functions are but they are important for maintaining genomic stability and they’re required for early embryogenesis

Question 44

what happens when there’s a loss of BRCA2 function?

Answer 44

various karyotypic abnormalities like:
- arrowheads

• fusions between chromosomal arms resulting in chromosomal translocations that often lead to bad chromatid pairings at the metaphase of mitosis

fusions are usually caused by unprepared or improperly repaired dsDNA breaks

Question 45

what do BRCA1 and BRCA 2 do?

Answer 45

they play a role in DNA repair

they are scaffolds used to assemble a cohort of other DNA repair proteins into large physical complexes

once assembled, these multi protein complexes help in the repair of dsDNA breaks usually via homology-directed repair (HR)

Question 46

what is tumor suppressor p53?

Answer 46

it was discovered by its association with viral oncoprotein T antigen

elevated p53 in DNA tumor virus SV40-transformed cells cause the immune system to mount an immune response against both large T and the body’s own p53

Question 47

how does the frequency of mutant p53 alleles play a role in human tumor cell genomes?

Answer 47

mutant alleles of p53 are found frequently in commonly occurring human tumors

ex. 119/123 high-grade ovarian serious carcinomas carry mutant p53 alleles

Question 48

how does p53 play a role in the development of cancer

Answer 48

p53 is the master guardian against development of cancer

a variety of cell physiologic stresses like lack of nucleotides, UV radiation, ionizing radiation, oncogene signaling, hypoxia and blockage of transcription can cause a rapid increase in p53 levels

p53 then undergoes post-translational modifications and proceeds to induce a number of response such as cell cycle arrest

DNA repair proteins may be mobilized as well as proteins that antagonize blood vessel formation = block of angiogenesis

other times, p53 can trigger apoptosis

Question 49

what regulates p53 levels?

Answer 49

p53 levels are controlled by two upstream regulators: Mdm2 and p19ARF

mdm2 destroys p53

ARF inhibits mdm2 from acting

excessive activity of E2Fs which is triggered by deregulation of the pRB pathway results in activation of ARF and thus p53

Question 50

what is the body’s reaction if there’s excessive activity of E2Fs?

Answer 50

excessive activity of E2Fs which is triggered by deregulation of the pRB pathway results in activation of ARF and thus p53