Lecture 5: Hormone Signaling Pathways Flashcards
Why is only a small amount of hormone necessary to generate a signal?
The effect of hormone binding to its receptor is magnified via amplification.
Once the ligand binds to its receptor - the receptor complex can do what to cellular pathways?
- Activate or inhibit cellular pathway (s) that elicit a particular cellular response (enzyme activity, gene expression)
Differentiate endocrine, paracrine, autocrine, and juxtacrine signaling; give an example of a molecule that participates in each one?
Endocrine: signaling molecule released by a cell distant to target and transported via bloodstream to target. i.e., epinephrine
Paracrine: signaling molecule released by one cell type and diffused to a neighboring target cells of different cell type i.e. testosterone
Autocrine: signaling molecule acts on the same cell type as the secreting cells themselves i.e. IL-1
Juxtacrine: signaling molecule stays attached to the secreting cell and binds to a receptor on an adjacent target cell i.e. heparin-binding epidermal growth factor
Can a signaling molecule be used in more than one type of signaling?
Yes, some molecules may participate in more than one type of signaling
What are the receptor involved in hydrophilic hormone signaling; what does the signaling molecule-receptor complex initiate?
- GPCRs
- RTKs
- Initiates production of second messenger molecules inside cell
- Both found on the surface of target cells, since the signaling molecule won’t be able to diffuse throught the membrane
What are the receptor types for lipophillic hormone signaling (differentiate between the two); binding to the receptor does what?
- Cytoplasmic receptors: exist as inactive complex. Upon binding, the hormone-receptor complex translocates to nucleus where it binds to a specific DNA sequence called the hormone response element (HRE)
- Nuclear receptors: already present in nucleus bound to DNA. The hormone allows for interaction w/ additional proteins and activates the complex
- Signaling molecule-receptor complex acts as a transcription factor
Differentiate between hydrophilic and lipophilic medications?
Hydrophilic: have short 1/2-lives (seconds to mins.). Given a the time of need, like epinephrine.
Lipophilic: have long 1/2-lives (hours to days). Need to take daily, like oral contrapceptives.
What are lipophilic signaling molecules?
- Steroid hormones: progesterone, estradiol, testosterone, cortisol
- Thyroid hormone: thyroxine (T4)
- Retinoids: retinol, retinoic acid
What are some hydrophilic signaling molecules?
- AA derivatives: histamine, serotonin, melatonin, dopamine, NE, epi
- Acetylcholine
- Polypeptides: insulin, glucagon, cytokines, TSH
Explain the general mechanism for GPCR signaling
- Trimeric G-proteins contain three subunits (α, β, γ)
- Inactive G protein has GDP bound to α subunit, and to become active G protein must exchange GDP for GTP using GEF, and α-subunit separates from beta and gamma subunits
- To return to inactive state, intrinsic GTPase activity of G protein hydrolyzes bound GTP into GDP + Pi w/ help from GTPase-activating protein (GAP)
What occurs when a signaling molecule binds to a GPCR using Gs pathway?
- Stimulates adenyly cyclase, which increases cAMP that can activate PKA.
- PKA will phosphorylate target proteins to alter their activity
What occurs when a signaling molecule binds to a GPCR using Gt pathway?
- Light hits a GPCR leading to the stimulation of cGMP phosphodiesterase, which converts cGMP –> 5-GMP, halting any action being promoted by cGMP
What occurs when a signaling molecule binds to a GPCR using Gi pathway?
- Signaling molecule binds causing the inhibition of adenylyl cyclase so NO cAMP is produced, and PKA is NOT activated
What occurs when a signaling molecule binds to a GPCR using Gq pathway?
- Activates PLC, which produces the second messengers DAG and IP3
- IP3 leads to increased Ca2+
- DAG leads to the activation of PKC, which phosphorylates target proteins to alter their activities
Epinephrine, histamine, and NE bind to what type of receptors, activating which GPCR pathways?
- Epinephrine binds β-adrenergic receptor activating Gs
- Histamine binds H2-receptor activating Gs
- Epinephrine/NE bind α-adrenergic receptor activating Gi
Dopamine, ACh, and light bind to what receptors utilizing which GPCR pathway?
- Dopamine binds D2-receptor utilizing the Gi pathway
- ACh binds the M3-receptor utilizing the Gq pathway
- Light activates rhodopsin receptor utilizing Gt pathway
What is the structure of insulin?
- Two polypeptide chains referred to as the A chain (21 AA’s) and B chain (30 AA’s)
- Linked together by two disulfide bridges (S-S)
Describe the basic process underlying insulin synthesis and secretion
- Glucose upregulates preproinsulin mRNA
- Translated into preproinsulin protein in the ribosome
- Cleaved by a protease to form proinsulin
- Folded and transported to Golgi to be packaged into granules
- C-chain cleaved to form A-chain + B-chain insulin structure
How does glucose cause the release of insulin granules from the pancreatic β-cell?
- Glucose enters the cell via GLUT 2 and will undergo glycolysis –> PDH –> TCA producing ATP/ADP
- ATP/ADP caused the closing of ATP-sensitive K+ channels, which lead to the opening of Ca2+ channels
- Opening of the Ca2+ channels stimulate the release of insulin granules into the blood
Describe the mechanism upon insulin binding the RTK that involves the RAS pathway.
- Insulin binds RTK, receptor becomes dimerized and autophosphorylates tyrosine residues.
- IRS-1 is recruited and becomes phosphorylated, attracting the scaffolding protein called GRB-2, which activates the RAS pathway
- RAS pathway phosphorylates nuclear proteins causing an increase in the transcription of Glucokinase and increased glucose uptake
Describe the mechanism upon insulin binding the RTK that involves the RAS-independent pathway.
- Insulin binds RTK, receptors dimerizes, autophosphorylates tyrosine residues.
- IRS-1 is recruited and phosphorylated, then recruits PI 3-kinase, which activates PKB via phosphorylation
- PKB phosphorylates many nuclear proteins and trigger the translocation of GLUT4 to plasma membrane; activation of glycogen synthase
- Leads to increased glucose uptake and glycogen synthesis
What are the 3 major domains of nuclear receptors and function of each?
1) Activation Function 1 Domain (AF1) - modify the conformation of the entire receptor
2) DNA binding domain (DBD) - binds to regulatory sequences on DNA called hormone response element (HRE)
3) Ligand binding domain (LBD) - binds to various molecules (agonist or antagonist) which regulates ligand-dependent activation of receptor
What is the primary (early) response to steroid hormone vs. secondary (delayed) response?
- Ligand binds, conformational change occurs, and will either come to nucleus to interact w/ DNA (cytoplasmic receptor) or form active complex to the HRE (nuclear recptor)
- Activating gene expression will induce synthesis of primary-response proteins, which can go on to block primary-response genes and turn on secondary-response genes, mediating the secondary (delayed) response.
Explain the binding of estrogen to the estrogen receptors (ERα,ERβ)
- Estrogen receptor located in the nucleus, upon binding of estrogen undergoes a conformaional change, which involve the binding of many nuclear proteins
- Results in activation of HAT, an enzyme that modifies chromatin structure.
- Recruits proteins that comprise the general transcription apparatus, leading to enhances transcription to form mRNA
How does Tamoxifen affect the mechanism of estrogen signaling and what is it commonly used for?
- Breast cancer drug similar in structure to that of estrogen.
- Tamoxifen, an antagonist, is metabolized by CYP450 in liver to 4-hydroxy-tamoxifen, which has slightly different structure than estrogen so binds receptor in different place
- Different downstream proteins are activated and bind the complex and the end result is the activation of Histone Deacetylase (HDAC), which keeps DNA in chromatin form and inhibits transcription
- Effective cancer drug because it stops the proliferation of cancer cells
