3) Hormone Action Flashcards
What often occurs during the transmission of a signal? What does that result in?
- The signal is exponentially amplified
- Results in an on or off stage
Why must we possess specific receptors?
- Because there is a multitude of hormone signalling
- There cannot be interference among different receptors
What are target proteins?
Key regulatory proteins that determine the properties of a cell
Based on the signal receptor, what are the two main classes of pathways?
1) Pathways with cell-surface receptors
2) Pathways with intracellular receptors
What are the characteristics of cell-surface receptor ligands?
- Receive hydrophilic molecules (soluble in the extracellular fluid)
- Molecules bind to the outside of the cell (cell-surface receptor)
What are the characteristics of intracellular receptor ligands?
Receive lipophilic molecules, which partition themselves into the plasma membrane
What must hydrophobic molecules possess to travel in the blood? What allows them to interact with intracellular receptors?
- Must possess a carrier protein
- Dissociation from the carrier protein allows them to diffuse into the cell
What are the three structures that form a cell-surface receptor?
- Ectodomain
- Hydrophobic transmembrane domain
- Cytoplasmic domain (endodomain)
What is the ectodomain associated with?
- Amino terminus
- Area where the hormone binds
What is the hydrophobic transmembrane domain composed of?
- Domain that crosses the lipid bilayer
- Alpha helix
- Stretch of hydrophobic amino acids that are soluble in the lipid membrane
What is the function of the cytoplasmic domain of the cell-surface receptor?
- The endodomain activates the signal transduction mechanism
- The “lock and key” mechanism of the receptor and hormone results in the change in the transmembrane domain, activating a change in the cytoplasmic domain
How many amino acids are needed to cross the lipid bilayer membrane?
25 amino acids
Which end of the cell-surface receptor is associated with the amino end of the peptide? Which end of the cell-surface receptor is associated with the carboxyl end of the peptide?
- Amino (NH2): Ectodomain
- Carboxyl (COOH): Endodomain
Are the three domains of the cell-surface receptor functionally dependent or independent?
- Independent
- They are interchangeable
What are structural characteristics of the ectodomain of cell-surface receptors?
- Amino end of the peptide
- Rich in cysteine residues (S-S bonds for folding)
- Often glycosylated
What is the function of a free ectodomain?
May circulate as a hormone binding protein
What equilibrium exists in terms of circulating GH?
Equilibrium between GH bound to the ectodomain, and GH in the aqueous phase
How are ectodomains implicated in Grave’s disease?
- The ectodomain cleaved from the TSH receptor may induce antibodies
- Antibodies bind to the receptor and mimic TSH action
- Causes hyperthyroidism
What are conformational changes during a signalling cascade induced by?
- Phosphorylation of proteins
- Binding between proteins
Many signalling proteins are activated by phosphorylation at which amino acids?
- Serine
- Threonine
- Tyrosine
Which amino acid may be phosphorylated, and serves as a docking site for other proteins to interact?
Tyrosine (aromatic)
Which group is replaced by a phosphate group during phosphorylation?
A polar hydroxyl group is replaced by a phosphate group
What is the phosphate donor for protein phosphorylation?
ATP
Many signalling proteins are _____ that are activated (or inactivated) by phosphorylation.
kinases
What is the function of activated kinases?
- Phosphorylate other signalling proteins
- Induce signal amplification
What do activated kinases use as a phosphate donor?
- ATP
- They do NOT use their own phosphate group
How may phosphorylation be reversed?
Phosphatases
What are the three advantages of phosphorylation of proteins as a controlling mechanism for signal transduction?
- Rapid
- Reversible
- Easy to relay signals
Why is protein phosphorylation rapid?
Does not require new protein synthesis or protein degradation
Why is protein phosphorylation reversible?
Easily reversed by the action of protein phosphatases
Why does protein phosphorylation make it easy to relay signals?
Phosphorylation on tyrosine, threonine, or serine create binding sites for other proteins
___% of all cellular proteins are phosphorylated.
10
Which phosphorylated amino acids are more abundant? Which is less abundant?
- Phosphorylated serines are threonines are more abundant than phosphorylated tyrosines
- 100:1 ratio
The phosphorylation of which amino acid occurs at the beginning of a signal cascade?
Tyrosine
The intracellular domain of many receptors have or induce ________ _______ activity that is activated by a hormone binding to the receptor.
tyrosine kinase
What serves as a docking site for downstream signal proteins?
Phosphorylated tyrosines
What is the amino acid sequence that mediates docking to phosphorylated tyrosine? What is it diagnostic for?
- SH2 and SH3 domains
- Diagnostic for proteins involved in the signalling cascade
What are the three major types of cell-surface receptors?
1) Intrinsic tyrosine kinase activity
2) Recruited tyrosine kinase activity
3) G-protein coupled
What is the mechanism for intrinsic tyrosine kinase activity?
- The intracellular domain of the receptor itself is an inactive tyrosine kinase
- When a hormone binds to the ectodermal domain of the receptor, a conformation change occurs, allowing for the activation of the tyrosine kinase
What is the mechanism for recruited tyrosine kinase activity?
When a hormone binds to the ectodermal domain of the receptor, an activated tyrosine kinase is brought to the complex
What is the characteristic structure of GPCRs?
- 7 domain transmembrane region
- Associated with the inside of the cellular membrane
What is the characteristic structure of tyrosine kinase coupled cell-surface receptors?
Peptide chain crosses the cell membrane once
What are examples of intrinsic tyrosine kinase activity?
- Insulin
- IGF-1
How many domains compose the insulin receptor? What are the chains that compose the insulin receptor?
- Heterotetrameric
- 2 alpha and 2 beta chains
How is the heterotetrameric structure of the insulin receptor held together?
Disulfide bonds
How many precursor proteins are processed to form the insulin receptor?
A single precursor protein
How many insulin receptors are there per cell?
100 and 200 000 receptors per cell
Which tissues possess the largest number of insulin receptors?
- Adipocytes
- Hepatocytes
What receptor does the insulin receptor have homology with?
IGF-1
What is the sequence of events after insulin binding?
1) Autophosphorylation of intracellular domain of receptor
2) Docking and phosphorylation of IRS-1 and IRS-2
3) Activation of the two major signal pathways
Which two major signal pathways are activated by insulin binding to its receptor?
1) PI-3 kinase pathway
2) MAPK pathway
Where does autophosphorylation of the insulin receptor occur?
- Endodomain
- At tyrosine residues
What are the two primary functions of the PI-3K pathway?
1) Maintenance of active (dephosphorylated) glycogen synthase to increase glycogen synthesis
2) Movement of GLUT4 to the outer cell membrane to increase glucose uptake
What is the mechanism of the PI-3K pathway?
1) Phosphorylated IRS-1 activates PI-3K
2) PI-3K converts PIP2 to PIP3
3) PKB, bound to PIP3, is phosphorylated and activated
4) PKB phosphorylates GSK3, inactivating it
5) Inactive GSK3 cannot convert glycogen synthase to its inactive form, so GS remains active
6) PKB stimulates the movement of GLUT4 to the membrane
What is MAPK? What is its function?
- Mitogen activated kinase
- Central to many cellular responses
- Target proteins may be regulatory genes of cell division
What is the function of the MAPK pathway?
- Change of gene expression profile
- Increased division in some cells
What is the mechanism of the MAPK pathway?
1) Phosphorylated IRS-1 binds to Grb2, which binds to SOS, which binds to Ras
2) Ras causes GDP release and GTP binding
3) Activated Ras binds and activates Raf-1
4) Raf-1 phosphorylates MEK
5) MEK phosphorylates MAPK
6) MAPK moves into the nucleus and phosphorylates transcription factors
What are the metabolic effects of insulin?
- Increase glucose uptake
- Increase glycogen synthesis
- Increase glycolysis, Acetyl-CoA production
- Increase fatty acid synthesis
- Increase TG synthesis
What is the net effect of insulin?
Converts excess blood glucose into two forms of storage
What two forms of storage arise from the effects of insulin?
1) Glycogen in the liver and muscle
2) Triacylglycerols in adipose tissue
What do defects in the insulin receptor/signalling pathway lead to?
Insulin resistance syndromes
How many mutants have been identified in the insulin receptor alone?
Over 50
What are symptoms of insulin resistance syndromes?
- Impaired glucose metabolism
- Raised insulin concentrations
What is a mild insulin resistance syndrome?
- Type A insulin resistance
- Loss of certain abilities to control glucose levels, but may be lived with
What is a medium insulin resistance syndrome?
- Rabson-Mendenhall syndrome
- Receptor partially functioning
What is a severe insulin resistance syndrome? What are the effects?
- Leprachaunism/Donahues syndrome
- Severe intra-uterine growth retardation
- Most patients are dead within their first year
Recruited tyrosine kinase activity receptors are a group of what type of receptors? How many members are there?
- Cytokine/hematopoietic receptors
- Over 20 members
What are the structural characteristics of recruited tyrosine kinase activity receptors?
- 4 alpha-helices
- Homology of the ectodomain
How does the cytoplasmic region of recruited tyrosine kinase activity receptors vary?
Cytoplasmic domain varies but one region (Box 1) is conserved
What is Box 1 involved in?
Triggering cell division
What are the best known recruited tyrosine kinase activity receptors?
- Growth hormone
- Prolactin
- Leptin
How many binding sites does GH possess? What does the binding to its receptor form? What does that result in?
- GH has two binding sites
- Binds sequentially to two receptor molecules, resulting in homodimerization
- Initiates signal transduction
Apart from GH, homo dimer formation is also observed in which hormones?
Prolactin and EPO
What is the mechanism for GH signalling (recruited tyrosine kinase activity receptors)?
1) GH binds to receptor 1, and subsequently receptor 2, forming a homodimer complex
2) The GH-dimeric receptor complex recruits and activates JAK-2
3) JAK-2 cross-phosphorylate each other, the cytoplasmic domain of the receptor, and other proteins
Which pathways are activated through GH signalling?
- GLUT4 (through IRS-1)
- STAT pathway
What three pathways are activated from the binding of GH to its receptor?
1) Activation of transcription regulatory proteins (STAT)
2) Activation of the MAPK pathway
3) Activation of PI-3K pathway
What is responsible for the insulin-like metabolic effects of GH?
The activation of the PI-3K pathway
How does the MAPK pathway differ when it is activated by GH, as opposed to insulin?
Activation by GH is not dependent on IRS-1
What is the mechanism for the STAT pathway, activated by GH?
1) STATs dock on the receptor, and JAKs phosphorylate them
2) STATs dissociate from the receptor and dimerize via their SH2 domain
3) STATs migrate to the nucleus, bind to DNA, and other gene regulatory proteins
STATs dissociate from the receptor and dimerize via their ____ domain.
SH2
What does STAT stand for?
Signal Transduction Activation of Transcription
Which proteins must dimerize to migrate to the nucleus to form a transcription complex?
STAT proteins
What is Laron syndrome?
- Severely impaired growth
- Normal or elevated levels of GH
What is the cause of Laron syndrome?
- Defective GH receptor
- Large deletions in exons coding for the GH receptor
GPCRs bind and activate G-proteins bound to the (inner/outer) cell membrane.
inner membrane
The activation of G-proteins leads to the generation of second messengers. What are the three?
- cAMP
- DAG
- IP3
What is the importance of secondary messengers?
- Important links in the signalling chain
- Amplify hormonal signals by orders of magnitude
What is the most numerous class of receptors? How many do they possess?
- GPCRs
- Over 140 members
What are examples of non-endocrine signals that act via G-proteins?
- Glutamate (neurotransmitter)
- Thrombin
- Odorants
What are examples of endocrine hormone receptors that act via G-proteins?
- TRH
- GnRH
- TSH
- LH
- FSH
- ACTH
What is the diagnostic feature of a GPCR?
- Crosses the membrane 7 times
- The seven transmembrane alpha-helices form a pore
What occurs to GPCRs when GTP is bound? What occurs to GPCRs when GDP is bound?
- GTP bound: G-protein on
- GDP bound: G-protein off
What are the two substrates that activated G-proteins may activate?
- Adenylate cyclase
- Phospholipase C
What are the four different types of G-proteins?
- Gs
- Gi
- Gq
- G0
What results from the activation of Gs?
- Activation of adenylate cyclase
- Increase in cAMP
What results from the activation of Gi?
- Inhibition of adenylate cyclase
- Decrease in cAMP
What results from the activation of Gq?
- Activation of phospholipase C
- Increase in calcium and DAG
What is the structure of a G-protein?
- Trimeric structure
- Alpha, beta, and gamma subunits
What are the functional units of G-proteins?
- G-alpha
- G-beta/gamma heterodimer
What occurs when GTP substitutes GDP on the alpha subunit of the G-protein?
- Destabilization of the trimeric complex
- G-alpha protein is free to dissociate
- Alpha subunit migrates in the cell membrane to activate a signal transduction pathway
What displaces the G-alpha subunit of a G-protein?
GDP for GTP exchange
What results from the activation of G0?
Activates ion channels
How many types of G-proteins may hormones use?
Hormones may use more than one type of G-protein
Does the use of G-proteins vary?
Yes, they vary during development
How may G-protein use vary?
- Depending on the concentration of the hormone, or in different tissues
- If you are ingesting a substantial quantity of hormone from drugs, this may affect G-protein use and activate other unwanted pathways
What is McCune Albright syndrome?
- Gain of Gs(alpha) function
- G-signalling pathway is on despite the lack of the hormone stimulant
- Characterized by breast development, precocial puberty associated with ovarian cysts
What is familial male precocious puberty?
High testosterone is produced by Leydig cells despite the absence of LH and FSH
Are the levels of FSH and LH low or high in familial male precocious puberty?
- Undetectable levels of LH and FSH
- However, the testes are still producing testosterone (independent of GnRH release signalling)
How many isoforms of adenylate cyclase exist? How do they differ?
- Over 10 isoforms
- Differ in their interaction with G-proteins
What are structural characteristics of adenylate cyclase?
- Membrane-bound
- 2 sets of 6 membrane-spanning domains
- 2 cytoplasmic domains
What may explain how different cells modulate the same hormonal signal, in terms of adenylate cyclase?
Different isoforms of adenylate cyclase differ in their interaction with G-proteins
What is the function of adenylate cyclase?
- Catalyzes the conversion of ATP to cAMP
- Removes two inorganic phosphate molecules
What is the major function of cAMP?
cAMP activates the cAMP-dependent kinase (PKA)
How does cAMP activate PKA? How many cAMP molecules are required?
- 2 cAMP molecules are required
- cAMP binds to the inactive kinase tetramer, and displaces the inhibitory complex
- Produces active PKA
What is the structure of inactive PKA?
Inactive kinase tetramer
What protein does PKA phosphorylate? What is the result?
- PKA phosphorylates CREB, which binds to CRE on various genes to form part of a transcription complex
- Results in the transcription of a gene
What amino acid residues does PKA phosphorylate on CREB?
Serines and threonines
What is CREB?
- Transcription factors
- Switches on genes by binding to CRE sequence of DNA to induce transcription
How is the signal generated by PKA inactivated?
- The signal is terminated by phosphodiesterase
- Phosphodiesterase hydrolyzes cAMP
- Regulatory inhibitors may now bind to PKA
What is the mechanism of the PLC-coupled receptor?
1) Hormone binds to the GPCR, activating the Gq subunit
2) Gq activates PLC, which cleaves PIP2 to produce DAG and IP3
3) DAG activates PKC, which phosphorylates proteins
4) IP3 results in the efflux of calcium from the ER
What second messengers are generated when PLC is activated?
PIP2 is cleaved to generated DAG and IP3
What is PIP2?
Minor phospholipid of the cell membrane
What is the primary action of DAG?
Activation of PKC
What is the primary action of IP3?
Release of calcium from the ER
What are the downstream effects of the PLC-coupled receptor pathway?
Increased calcium activates many enzymes, including kinases (phosphorylation cascades)
Why is the number of hormonal receptors constantly in flux?
- Change with cell
- Change with development or cell differentiation
What percentage of occupancy of a receptor stimulates maximal response? What is the function of extra receptors?
- 3% occupancy of a receptor stimulates maximal response
- Extra receptors maintain a buffer for further physiological stimulation
How may non-physiological levels of hormone affect the regulation of membrane receptors?
- May down-regulate the membrane receptor
- The cell no longer responds and time is required to restore normal receptor levels
How is the receptor-hormone complex inactivated after cell signalling is initiated?
- Uncoupling
- Endocytosis
Which types of hormones bind to intracellular receptors? (2)
- Steroid hormones
- Thyroid hormones
Where are intracellular receptors located?
- Cytoplasm
- Nucleus
Why is the response to hormones utilizing intracellular receptors slow?
Since transcription and translation of proteins is necessary
What is the mechanism of steroid hormone action?
1) Free hormone diffuses through the cell membrane
2) Hormone binds to the receptor in the cytosol
3) Hormone-receptor complex translocates into the nucleus to initiate transcription
What are orphan receptors?
Receptors for which the ligand has not been identified
How many peptides compose steroid and thyroid receptors?
A single peptide
What are the three domains of steroid and thyroid receptors?
- Hormone-specific binding domain
- Highly conserved DNA-binding domain
- Transcription-activating domain
What is AF1?
- Transcription-activating domain
- Hypervariable
- Not dependent on the type of hormone
What is AF2?
- Hormone-specific binding domain
- Region responsible for hormone-dependent transcription activation at the C-terminus (AF2)
What are the two classes of intracellular receptors?
1) Cytosolic (e.g. steroids)
2) Nuclear
Are the domains of intracellular receptors interchangeable?
Yes
Which intracellular receptor domain is hypervariable?
- AF1
- Transcription-activating domain
How are class I intracellular receptors activated? What binds to DNA?
- DNA is bound by zinc finger domains, which keep transcription inactive
- When a steroid binds to the recognition sequence, it changes the conformation of the sequence
- The DNA binding site is unmasked by hormone binding
What is the DNA-binding domain of class I receptors?
Two loops that form zinc fingers
What do class I intracellular receptors form complexes with? Where is this complex partitioned?
- Heat-shock proteins
- Partitioned between the cytoplasm and nucleus
How do class II intracellular receptors function?
- They are already bound to DNA
- Activated by hormone binding
- The dimers bind to specific DNA sequences, which enhance the transcription from the promoter
Class I steroid hormone receptors form what kind of units?
Homodimers
How are the half-sites of class I steroid hormone receptors arranged?
Palindromically
Class II receptors form what kind of units?
- Heterodimers
- Homodimers
- Monomers
How are the half-sites of class II receptors arranged?
Direct repeat configuration
What are the effects of androgen insensitivity syndrome? What are the molecular defects?
- Male genotype, but female phenotype
- Decrease in receptor number, androgen binding
What are the effects of defects in estrogen receptors?
- Usually lethal
- Estrogen resistance
What may nuclear receptors modify?
Hormones acting via nuclear receptors may be modified by the target cell
What are examples of hormones acting via nuclear receptors that are modified by target cells?
- Conversion of thyroxin to the active T3
- Deactivation of cortisol to cortisone
What is the inactive form of cortisol? What cells convert cortisol to its inactive form? Why?
- Cortisone
- Aldosterone-responsive kidney cells
- Cortisol cross-reacts with the aldosterone receptor
- The conversion to cortisone “protects” cells from inappropriate activation of the aldosterone receptor by cortisol
How does the hypothalamus modify testosterone?
Converts testosterone to estradiol via aromatase, as they have E2 receptors
How does the prostate modify testosterone?
Converts testosterone to DHT via 5-alpha-reductase
What does the cellular response to a hormone depend on?
1) Determined by multiple signals received at the same time (network)
2) The response depends on the cell type
