Download Signal Transduction 1. Describe how epinephrine coordinately

Dr. Roger Miesfeld Bioc 460 Spring 2004
Signal Transduction
1. Describe how epinephrine coordinately controls glycogen degradation and glycogen
synthesis in response to imminent danger. What molecular mechanism regulates the on/off
activity of the two key enzymes in glycogen metabolism?
2. Epidermal growth factor (EGF) hormone signaling stimulates cell division by activating an
intracellular phosphorylation cascade mediated by the GTP binding protein Ras. Explain why
mutations in Ras that block its intrinsic GTPase activity result in cancer cell division even when
hormone is not bound to the EGF receptor.
3. Name the two peptide hormones that are most responsible for regulating blood glucose levels
in humans and briefly describe the primary function of each.
4. The phosphatidylinositol 3-kinase (PI-3K) pathway is activated by insulin signaling in liver
cells. Describe the key signaling events in this pathway, beginning with insulin-stimulation of
insulin receptor tyrosine kinase activity, that lead to glucose uptake.
5. How are G protein-mediated hormone signals turned off when hormone levels decrease
outside the cell?
6. Why is Gleevac (STI-571) such an effective anti-cancer drug for the treatment of chronic
myelogenous leukemia (CML) without the side effects of other drugs?
7. What role do receptor tyrosine kinases have in cancer?
8. How does the biochemical function of an oncogenic Ras protein differ from a normal Ras
protein with regard to signal transduction and cell growth control?
9. What explains the observation that the phosphoenolpyruvate carboxykinase (PEPCK) gene is
induced by glucocorticoids in liver cells, but not in bronchial epithelial cells, even though
glucocorticoids and the glucocorticoid receptors are functional in both cell types?
10. Why does treatment with the steroid antagonist tamoxifen block estrogen signaling in breast
cancer cells but not inhibit gluconeogenesis in the liver?
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Dr. Roger Miesfeld Bioc 460 Spring 2004
Answers
1. Epinephrine binding to the epinephrine receptor (beta-adrenergic receptor) stimulates
adenylate cyclase and production of cyclic AMP (through G protein coupled receptor
mechanism). Protein kinase A is activated by cyclic AMP binding which leads to activation of
glycogen phosphorylase (via activation of phosphorylase kinase) and inactivation of glycogen
synthase. Thus, epinephrine coordinately activates glycogen degradation and inhibits glycogen
synthesis. The mechanism of this on/off switch is phosphorylation control.
2. EGF signaling normally stimulates exchange of GDP for GTP to produce the active form of
Ras (Ras-GTP), and cleavage of GTP by the intrinsic Ras GTPase converts it back to the
inactive Ras-GDP form. However, mutations in the Ras GTPase that block Ras inactivation
result in formation of a constitutive "activated" Ras even in the absence of upstream EGF
signaling.
3. Insulin is the "fed state" hormone and it stimulates glucose uptake and glycogen synthesis in
liver and muscle (insulin also inhibits gluconeogenesis in liver. Glucagon is the "hunger
hormone" and it stimulates glycogen breakdown and gluconeogenesis in the liver (glucagon
also inhibits glycogen and fatty acid synthesis.
4. Insulin binding to the insulin receptor activates an intrinsic tyrosine kinase activity which leads
to phosphorylation of IRS protein (insulin receptor substrate). IRS binds to and activates PI-3K
which phosphorylates PIP2 (phosphatidylinositol 4,5-bisphosphate) located on the cytoplasmic
side of the plasma membrane to generate PIP3 (phosphatidylinositol 3,4,5-triphosphate). PIP3
is an anchor for the PH (pleckstrin homology) domain proteins PDK1 (phosphoinositoldependent kinase) and PKB (protein kinase B) which results in the phosphorylation and
activation of PKB by PDK1. Phosphorylation of PKB causes it to dissociate from PIP3 and
diffuse through the cytosol where it activates GLUT4 (glucose transporter) translocation to the
membrane, and regulates enzymes controlling glycogen synthesis.
5. G protein-mediated hormone signals are turned off by the intrinsic GTPase activity of G
proteins which converts the active GTP-G protein complex into an inactive GDP-G protein
complex. In the absence of further hormone stimulation, the GDP-bound G alpha subunit
remains in the inactive state associated with beta + gamma subunits.
6. Gleevac is a specific inhibitor of the Bcr-Abl kinase which is a oncogenic fusion protein
expressed in CML leukemia cells. Since the genetic mutation that leads to the production of the
Bcr-Abl fusion protein does not occur in normal cells, and Gleevac does not bind to the normal
c-Abl kinase, the drug is highly specific and has very few side effects.
7. Normally receptor tyrosine kinases transmit extracellular hormone signals to the cytosol by
stimulating a kinase cascade. If the tyrosine receptor kinase is defective, as it often is in cancer
cells, inappropriate signals are sent to the cell stimulating cell division even in the absence of
hormones. These defective tyrosine receptor kinases are often truncated due to mutations.
8. Oncogenic ras protein lacks intrinsic GTPase activity and therefore is always in the GTP
bound active conformation resulting in constitutive signal transduction causing uncontrolled cell
growth. Normal ras protein becomes inactivated by GTP hydrolysis after the removal of growth
factor signals.
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Dr. Roger Miesfeld Bioc 460 Spring 2004
9. Liver cells contain tissue-specific proteins (transcription factors called HNFs) that are required
for PEPCK regulation by the glucocorticoid receptor. Bronchial epithelial cells do not contain
these proteins (and chromatin in the vicinity of the PEPCK gene is in an inactive condensed
state which prevents glucocorticoid receptor binding).
10. Tamoxifen is an estrogen antagonist that binds with high affinity to the estrogen receptor
and blocks estrogen signaling in breast cancer cells. Tamoxifen does not bind to the
glucocorticoid receptor which controls gluconeogenesis in the liver.
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