kandpal signal transduction Flashcards
Cholera Toxin
Stimulatory G-proteins (Gs) activate adenylate cyclase –> increase cAMP
Cholera toxin modifies alpha-subunit of Gs (ADP-ribosylation of GTP-bound alpha of G-protein):
ribosylated alpha remains active.
Adenylate cyclase is active
cAMP levels rise
CFTR (cystic fibrosis transmembrane conductance regulator) remains continuously activated
Efflux of Cl- and water into the gut.
Diarrhea and vomiting
Modify GTP bound alpha subunit–>alpha remains active because GTP cannot be hydrolyzed
also activates adenylate cyclase--> elevated cAMP
CFTR: Cl- channel is cAMP gated ==> more water into the gut==> symptoms.
Pertusis Toxin
Whooping Cough
Inhibitory G-proteins (Gi) inhibit adenylate cyclase activity
Under normal conditions, activated a subunit (GTP bound- a) will inhibit adenylate activity, which will reduce the levels of cAMP.
What does pertusis toxin do?
Pertusis toxin modifies GDP-bound a subunit:
Modified a can’t be activated by exchange of GDP for GTP==> a subunit remains inactive: NO inhibition of adenylate cyclase
==> Levels of cAMP remain ELEVATED
==> Activation of CFTR==> more Cl- in tissue
Her2 and Herceptin
Her2 is epidermal growth factor receptor 2.
Maps to 17q12-q21, and amplification of this genes has been observed in ~30% of breast tumors.
Her2 tumors are associated with poor prognosis.
Her2 expression status has been helpful in determining treatment options.
Herceptin, an antibody to Her2, has shown encouraging results in clinical management of breast cancer.
treat Her2 pt’s with herceptin
Her2=proliferation and anti-apoptosis proteins.
Herceptin= an antibody to Her2==> binds and prevents the binding of ligand to these receptors==> no intracellular signaling –> no proliferation and anti-apoptosis–> stop tumor growth
Retinoblastoma
Retinoblastoma is a tumor of retina with an incidence of 1 in 20,0000.
Familial: tumors in both eyes (autosomal recessive)
Sporadic: tumors in only one eye
Knudson conclusions: 2 hit hypothesis
Rate of familial tumor appearance consistent with a single random genetic hit (from parents)
Rate of sporadic tumor appearance consistent with two random genetic hits
if one mutation, takes shorter time for cell to develop a second mutation
Rb tumors show loss of heterozygosity explained by esterase D. Rb locus on chromosome 13q14 also encodes the enzyme esterase D. Esterase D has two isoforms or alleles that migrate differentially on electrophoresis gels. tumor=only see one isoform--> no active form of Rb. lost heterozygosity==> tumor formation
Rb function: G1 progression
For S-phase initiation, cells transcribe specific genes mediated by transcription factor E2F
E2F activity is regulated by retinoblastoma protein (Rb), which is an inhibitor of cell cycle progression.
p53 (Tumor Suppressor)
binding of cyclins to cdk’s is required for their activation. when activated, will activate their target proteins by phosphorylation –> phosphorylated regulators transcribe genes required for the S and M phase of the cell cycle.
during cell cycle arrest, activity of cdk’s is inhibited by inhibitors such as p21, which is regulated by p53
P53 senses genetic damage in the cell and arrests cell cycle.
Guardian of genome.
If the damage can not be repaired, it may trigger cell apoptosis.
P53 must be disabled for sustained growth of a pre-malignant cell.
found as mutated in many tumors (ovary, esophagus, colorectal, etc)
normal: p53 sense damage and activates kinases to phosphorylate p53
without kinases, degraded by mdm2
with kinases, phosphorylated—> into nucleus–> transcribe p21–> inhibit Cdk–> arrest cell cycle (cell cycle arrest, DNA repair, block of angiogenesis, apoptosis)
when damaged p53==> no stop in cell cycle
Tumor Suppressor Protein APC
Mutated in Familial Colon Cancer (Polyps in Colon are Characteristic to this Cancer)
Wnt Signaling and APC (adenomatous polyposis coli):Familial Colon Cancer or Familial Adenomatous Polyposis (FAP)
Wnt proteins are tethered tightly to extracellular matrix (ECM).
These growth factors activate frizzled receptors (multipass transmembrane proteins).
normal: no Wnt==> frizzled==> GSK 3beta is attached to beta catenin and is degraded–> not used for transcription
with Wnt==> inhibit the ability of GSK3beta to interact with beta catenin==> cannot bind and get phosphorylated and degraded==> promotes proliferation
mutated APC, cannot allow binding of beta catenin–> free from complex (like in Wnt signaling) allow for cell proliferation
Viagra
uses the pathway NO signaling
- normally, cGMP in smooth muscle is degraded by phosphodiesterase.
- Viagra inhibits this enzyme (PDE-5) in erectile tissue
- This leads to accumulation of cGMP, leading to sustained relaxation and increase in blood flow to the erectile tissue.
cGMP binds PKG and which activates it and allows for the production of IP3 (affects Ca2+ channels)
nitroglycerine
- Same pathway of NO signaling.
- Nitroglycerine is converted to NO and then shuttled into the pathway or nitroglycerine itself modifies the rxn of cyclic GMP.
- Either way, it triggers relaxation of smooth muscle cells.
smell
activated Golf receptors stimulate adenylate cyclase–> increase in cAMP–> open ion gated channels—> increase in Na+ influx and depolarization
visual transduction
retina has cones for color vision and rods for dark (black and white) vision
*fastest G-protein mediated response
in rods in the dark, rhodopsin is not activated–> cGMP gated Na+ channels open in the dark when cGMP levels are high
light activated rhodopsin activates Gt (transducin) (conformational change cis–> trans) that then activates cGMP phosphodiesterase –> cGMP=hydrolyzed–> lowers [cGMP] and closes Na+ channels (hyper polarization)
negative feedback loops to get rods back to resting level.
- RK phosphorylates cytosolic tail of activated rhodopsin–>Phosphorylated rhodopsin can’t activate transducin
- Arrestin binds to phosphorylated rhodopsin–>More inhibition of rhodopsin activity
- RGS binds to activated transducer
- -> GTP is converted to GDP (inactivation of transducin)
Ras
oncogene that is activated in 30% of human tumors
monomeric G-protein with inherent GTPase activity
GTP hydrolysis converts active (bound to GTP)–> inactive Ras (bound to GDP)
this conversion of active–> inactive Ras is regulated by GEF and GAP
to activate Ras: 1. activate GEF or 2. inhibit GAP
activated RTK allows Grb-2 adaptor binding to it, which then binds RasGEF–> activates Ras
MAP kinase pathway activated–> cascade of phosphorylation reactions occur.
cell proliferation genes are turned ON by either changes in protein activity or gene expression
