LM 8.1: Cancer Cell Signaling Flashcards
what is the general cause behind cancer?
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
what are some types of extracellular signaling molecules?
hormones
cytokines
chemokines
how does extracellular signaling work?
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
when a ligand binds to a receptor and triggers a response, what is the target inside the cell?
transcription factors in the nucleus
pro-apoptotic factors in the mitochondria
what is the Ras pathway?
very important for cell proliferation and survival
how does the Ras pathway work?
- ligand binds to a receptor
- Ras-GDP is activated into Ras-GTP
- active Ras-GTP activates Raf kinase
- Raf activates MEK
- MEK activates ERK
- 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
what inhibits Raf in the Ras pathway?
RKIP
what is an oncogene?
something that positively unregulates the Ras pathway
the wild-type version of the oncogene is called a proto-oncogene
what is a TSG?
an inhibitor of the Ras pathway
removal of a TSG would lead to an increase in the output of the Ras signaling pathway
what causes tumorigenesis?
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
what type of growth are tumors?
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!!
what is a polyclonal tumor?
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
what are the hallmarks of cancer?
- self sufficiency in growth signals
- evading apoptosis
- insensitivity to anti-growth signals
- sustained angiogenesis
- limitless replicative potential
- tissue invasion & metastasis
what are the functions of oncogenes and TSGs?
- mechanisms that should trigger or carry out a self-destruct program in damaged cells are disabled or overridden
- cancer cells amplify external growth cues or generate their own
- cancer cells become deaf to quiescence cues from surrounding tissue
- tumors invade and metastasize
- cancer cells evade intrinsic limits on the number of times a normal cell can divide
- tumors emit signals promoting the development of new blood vessels to deliver oxygen and nutrients
what is quiescence?
resting state of the cell characterized by the absence of cell growth and division
what are two additional hallmarks of conversion involved in the pathogenesis of some and maybe all cancers?
- the capability of modify or reprogram cellular metabolism in order to most effectively support neoplastic proliferation
- 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
what are two enabling characteristics of tumors?
- genomic instability gives cancer cells genetic alterations that drive tumor progression
- tumor-promoting inflammation: innate immune cells designed to fight infections and heal wounds can instead result in their inadvertent support of multiple hallmark capability
oncogene vs. proto-oncogene
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
what mutation is responsible for H-Ras oncogene activation?
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
how does Ras become an oncogene?
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
how can a mutated growth factor receptor cause uncontrollable cell proliferation?
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
which genetic changes can effect protein expression and lead to the activation of proto-oncogenes?
- gene amplification
2. chromosomal translocation
how is the N-myc gene related to childhood neuroblastomas?
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
how is the C-myc gene related to Burkitt’s lymphomas?
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
what gene is an example of gene amplification?
the N-myc gene related to childhood neuroblastomas
what gene is an example of chromosomal translocation?
the C-myc gene related to Burkitt’s lymphomas
how is the bcr-abl oncogene formed?
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
how was the site of action of an oncogene figured out?
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
how can microRNA be a cancer gene?
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
how does the cell cycle guard against activation of oncogenes?
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
how does the cell cycle protect against an active RAS oncogene?
activation of oncogenic RAS leads to up regulation of INK4A which blocks cyclin-D-CDK4 mediated hyperphosphoyrlation of RB which stops the cell cycle
how does the cell cycle protect against an active MYC oncogene?
activated MYC oncogene activates ARF which blocks the activity of the p53 inhibitor MDM2 and thereby activates p53
what is oncogene addiction?
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
is the tumorigenic phenotype dominant or recessive?
the cancer phenotype is recessive
cell fusion tests showed the cancer cells derived from most human tumors formed hybrid cells that were non-tumorgenic
what are TSGs?
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
what is loss of heterozygosity?
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)
how can LOH be used to find TSGs?
mmm idk ask someone
what is RASSF1A?
a TSG promotor
what happens when you methylate RASSF1A?
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
what are gatekeepers?
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
what is a caretaker?
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
is RB a gatekeeper or caretaker?
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
are the BRCA proteins gatekeepers or caretakers?
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
what happens when there’s a loss of BRCA2 function?
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
what do BRCA1 and BRCA 2 do?
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)
what is tumor suppressor p53?
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
how does the frequency of mutant p53 alleles play a role in human tumor cell genomes?
mutant alleles of p53 are found frequently in commonly occurring human tumors
ex. 119/123 high-grade ovarian serious carcinomas carry mutant p53 alleles
how does p53 play a role in the development of cancer
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
what regulates p53 levels?
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
what is the body’s reaction if there’s excessive activity of E2Fs?
excessive activity of E2Fs which is triggered by deregulation of the pRB pathway results in activation of ARF and thus p53
