Hormone Signaling Pathways

Question 1

• Steps in hormone signaling

Answer 1

1. Biosynthesis
2. Storage
3. Secretion
4. Transport to target tissue/cell
5. Recognition and binding to receptors
6. Activation of signal transduction pathway (on switch)
7. Amplification and relay of signal
8. Cellular response
9. Degredation (off switch)

Question 2

• _ and _ are the major pathways of hormones

Answer 2

• Endocrine
• Paracrine
• Note that some molecules can participate in more than one type of signaling

Question 3

• What are the two classes of hormones? What are some examples?

Answer 3

• Water soluble:
  
  • Epi
  • Insulin
  • Glucagon
  • GH
• Lipid soluble:
  
  • Estrogen
  • Testosterone
  • Cortisol
  • 1,25 Dihydroxy cholecalciferol

Question 4

• Lipophillic hormone signaling involves the _ complex acting as a transcription factor
• What are the two types of receptors?

Answer 4

• Hormone receptor complex
• Cytoplasmic receptors- bind to HRE promoter of specific genes
• Nuclear receptors- already present in nucleus bound to DNA; interactions with additional proteins activates the complex
• Both regulate transcription of specific genes

Question 5

• _ medications have short half lives (seconds to minutes)
• _ medications have long half lives (hours-days)

Answer 5

• Hydrophillic-short half lives (EX: Epi Pen)
• Hydrophobic-long half lives (EX: Birth Control)

Question 6

• What are the two cell surface receptors used for hydrophillic hormonal signaling?

Answer 6

• GPCR
• Receptor Tyrosine Kinase

Question 7

• MOA for GPCR

Answer 7

• Trimeric G protein (with alpha beta and gamma subunit) is inactive when bound to GDP
• GEF enzyme exchanges GDP for GTP
• GTP binds alpha subunit and dissociated from beta and gamma
• To become inactive, intrinsic GTPase activity, together with GAP hydrolyzes GTP to GDP, which can now bind back with beta and gamma subunits

Question 8

• Variations in GPCR signaling:
  
  • Gs- stimulates _
  • Gi-inhibits _
  • Gq- stimulates _
  • Gq- activates _

Answer 8

• Gs-stimulates adenylyl cyclase, increases cAMP, PKA levels
• Gi-inhibits adenylyl cyclase, decreases cAMP and PKA
• Gt-stimulates cGMP phosphodiesterase
• Gq-Activates PLC, which increases IP3/DAG/Ca2+ levels

Question 9

• Epinephrine, via beta adrenergic receptor, activates _ and stimulates relaxation of bronchial and intestinal smooth muscle, contraction of heart, breakdown of triacylglycerols, increases breakdown of glycogen in liver and muscle, glycolysis in muscle
• Epineprhine, via alpha adrenergic receptors, activates _ protein and causes contraction of smooth muscle

Answer 9

• Gs
• Gi

Question 10

• Histamine activates _ protein on H2 receptor to cause bronchoconstriction and allergic reaction symptoms

Answer 10

• Gs

Question 11

• Dopamine activates _ protein on D2 dopamine receptor to increase HR

Answer 11

• Gi

Question 12

• ACh activates _ protein on M2 receptors to stimulate bronchoconstriction and stimulation of salivary glands

Answer 12

• Gq

Question 13

• Light via _ protein on rhodopsin induces vision

Answer 13

• Gt

Question 14

• RTK
  
  • What are the different domains?
  • Function?

Answer 14

• Domains:
  
  • Extracellular domain: binds ligand
  • Transcellular domain
  • Intracellular domain: Tyrosine kinases
• MOA:
  
  • Binding of ligand causes dimerization
  • Dimerized receptor phosphorylates tyrosine residues
  • Phospho-tyrosines recognized by adaptor and docking proteins which activate RAS and RAS independent pathways
  • Trigger phosphorylation of specific protein targers in cytoplasm/nucleus that alters gene transcription
  • Terminated by many mechanisms (dephosphorylation, inactivation of RAS, degradation of ligand, etc)

Question 15

• Insulin is composed of an _ and _ chain
• Linked together via _ bridges
• Inactive form is stored as a _ with Zinc in the center and connected to polypeptides via _
• Active form is a _

Answer 15

• A and B
• Disulfide
• Hexamer, Histamines
• Monomer

Question 16

• Insulin synthesis and secretion MOA

Answer 16

1. Preproinsulin mRNA
2. Translated into preproinsulin protein
3. Translocation to ER lumen
4. Cleaved by protease to form proinsulin
5. Folded and transported to Golgi
6. Packaged into immature granules
7. Cleaved by proteases to form insulin and C peptide
8. Immature granules become mature (contain 3 crystallized dimers)
9. Insulin and C peptide released together

Question 17

• Two pools of insulin granules

Answer 17

1. Readily releasable (<5%)
2. Reserve pool (>95%)
   
   1. Must undergo mobilization before they can gain release competence

Question 18

• Regulation of insulin secretion

Answer 18

1. Glucose enters cell via GLUT 2 facilitated diffusion
2. Glucokinase converts to G6P
3. Synthesis of ATP
4. Closing of ATP-Sensitive K+ channel
5. Membrane Depolarization
6. Opening of Ca2+ channels
7. Release of insulin granules

Question 19

• RAS dependent insulin signaling

Answer 19

• Insulin binds RTK on muscle (already a dimer)
• Autophosphorylation of tyrosine residues
• IRS-1 phosphorylated
• IRS-1 recognized by GRB-2 initiating RAS and MAPK pathway
• Results in phosphorylation of nuclear proteins that increase transcription of glucokinase

Question 20

• RAS Independent Insulin Signaling

Answer 20

• Insulin binds RTK
• Autophosphorylation of tyrosine residues
• IRS-1 phosphorylated
• Recruits PI3 kinase
• P13 converted to PIP2 and PIP3
• PIP2 and PIP3 recruit PKB and activate its phosphorylation
• PKB stimulates:
  
  • Movement of GLUT4 of muscle and adipose cells to their PMs
  • Promotes glycogen synthesis by phosphorylating and inhibiting glycogen synthase kinase (GSK-3)

Question 21

• Termination of insulin signaling

Answer 21

• Insulin receptor endocytosed
  
  • Degraded or recycles
• Can also down regulate receptors by:
  
  • Decreasing rate of synthesis
  • Increasing rate of degradation

Question 22

• Insulin resistance:
  
  • ​Possible causes?

Answer 22

• Downregulation of insulin receptor
• Defects in insulin signaling (affects translocation of GLUT4 to PM in adipose and muscle cells)
• Defects in insulin receptors (mutations)
• Defects in IRS 1 and IRS 2
  
  • ​Phosphorylation of SERINE instead of TYROSINE (via ser/thr kinase) inhibits recruitment of PI3 Kinase
• Ser/THr kinase activated by cytokines, FFAs, DAGs, ceramide, inflammatory molecules

Question 23

• Nuclear receptors are classified based on the _ that they bind

Answer 23

• Ligand
• EX:
  
  • Nuclear receptors-ligands are lipophilic hormones
  • Orphan receptors-ligands unknown
  • Adopted-retinoids, vit D, thyroid hormones, xenobiotics, androstane

Question 24

• Molecular Structure of Nuclear Receptors and MOA

Answer 24

• Structure:
  
  • Ligand binding domain (LBD)
  • Activation Function 1 Domain (AF1)-transcription activating domain
  • DNA binding domain (DBD)
• MOA:
  
  • LBD binds molecules that regulate ligand dependent activation of receptor
  • LBD undergoes conformational changes allowing recruitment and binding of co-activators or co-repressors that activate gene transcription
  • DBD binds HRE (regulatory sequence on the DNA) upstream of target gene
    *

Question 25

* Primary versus secondary response to steroid hormones \*\*

Answer 25

Question 26

* **Estrogen receptor= _ receptor** * **\_ main types** * **Both are _ transcription factors** * **Where are each receptor type found?**

Answer 26

* Nuclear * 2 (Alpha and Beta) * Estrogen-dependent * Alpha * Uterus * Vagina * Ovaries * Mammary glands * Hypothalamus * Endothelial Cells * Vascular Smooth Muscle * Beta * Ovaries * Prostate * Lung * Brian * Bone * Vasculature * Many cells express both alpha and beta ERs

Question 27

* ***_MOA of Estrogen Receptor_***

Answer 27

* Estrogen agonist recruits proteins that promote HAT, which opens up chromatin for gene transcription (also recruits GTA-general transcription apparatus) leading to enhanced transcription to form mRNA * Estrogen receptor antagonist (EX: Tamoxifen) binds ER and produces different conformation, recruits corepressors and HDAC (acts on histones to pack chromatin tight) and prevent GTA recruitment and gene transcription