Hormone Signaling Pathways

Question 1

hormones

Answer 1

a. Messenger mcs
b. Synthesized and secreted by specialized cells called endocrine cells
c. Released into ECF or bloodstream
d. Exert influence by binding to specific Rs on the cells of remote target tissues
e. Result in activation of signal transduction mechanisms that ultimately lead sto cell type specific responses
f. Small amt of H required to alter cell metabolism
g. Effect greatly magnified via amplification of signal

Question 2

steps in cell signaling

Answer 2

a. In response to a stimulus, a signaling cell synthesizes and secretes a signaling mc—lipid soluble or water soluble
b. The signaling mc transported to target cell where it binds to a specific R protein
c. Signaling mc-R complex activates or inhibits cellular pathways that elicit a particular cellular response (metabolism, gene expression)
d. Signal terminated by removal of the signaling mc and/or R or inactivation of the signaling events triggered by the signaling mc-R complex
i. Receptors are either on the cell membrane or inside the cell in the cytoplasm
1. Hydrophilic mcs cross the membrane to the R on the inside
a. Can be cytosolic/nuclear but eventually goes into the nucleus
2. Hydrophobic mcs bind to Rs on cell membrane

Question 3

endocrine

Answer 3

a. signaling mc released by a cell distant from the target cell and transported via the bloodstream to target cell
i. Ex: epinephrine in the adrenal medulla which has many effects

Question 4

paracrine

Answer 4

a. signaling mc released by one cell type and diffuses to a neighboring target cell of a different cell type
i. Ex: testosterone

Question 5

autocrine

Answer 5

a. Autocrine—signaling mc acts on the same cell type as the secreting cells themselves
i. Ex: interleukin 1

Question 6

juxtacrine

Answer 6

a. Juxtacrine—signaling mc stays attached to the secreting cell and binds to a R on an adjacent target cell
i. Ex: heparin binding epidermal growth factor

Question 7

hydrophilic H signaling

Answer 7

a. Hydrophilic Hs cannot penetrate the plasma membrane
b. Interact with specific Rs at the cell surface
c. Ex: epinephrine, insulin, glucagon, etc.
d. Signaling mc—R complex initiates production of 2nd messenger inside cell
i. This trigger cellular response
e. Rs involved in hydrophilic H signaling
i. G protein coupled Rs (GPCRs)
ii. R tyrosine kinases (RTKs)

Question 8

lipophilic H signaling

Answer 8

a. Lipophilic H passes thru plasma membrane of target cell
b. Ex: steroid Hs, thyroid Hs, and retinoids
c. H ligand binds to specific R proteins inside the cell
d. Signaling mc—R complex acts as a transcription factor
e. Family of DNA binding transcription factors
i. Cytoplasmic Rs—exist in an inactive complex with HSP90 (chaperone protein)
1. Upon binding to signal, HSP dissociates
2. H-R complex translocates to nucleus where it binds to a specific DNA sequence called the hormone response element (HRE) in the promoter region of specific genes
ii. Nuclear Rs—already present in the nucleus bound to DNA
1. The H signal activates the complex and allows for interactions with additional proteins

Question 9

hydrophilic medications

Answer 9

a. have short half lives
i. ie. Epinephrine—contained in autoinjections used to treat severe acute allergic rxn that may lead to anaphylactic shock
1. given at time of need

Question 10

lipophilic medication

Answer 10

a. have long half lives
i. ie. Oral contraceptives—contain ethinyl estradiol, a derivative of estradiol
1. need to take daily

Question 11

GPCR and adenylate cyclase

Answer 11

1. inding of a signaling mc to a GPCR causes activation of Gs which stimulates or Gi which inhibits the adenylate cyclase pathway
   a. activated adenylate cyclase converts ATP to cAMP, a second messenger that regulates the activity of protein kinase A (PKA)
   i. enzymatically inactive PKA exists as a tetrameric complex containing 2 regulatory subunits and 2 catalytic subunits
   ii. binding of cAMP to the regulatory subunit causes the complex to dissociate and the free, active catalytic subunits causes phosphorylate target proteins
   iii. cAMP is hydrolyzed into AMP by the action of phosphodiesterase an enzyme that is inhibited by caffeine

Question 12

Gq signaling

Answer 12

1. signaling via Gq, phospholipase C (PLC), and protein kinase C (PKC)
   a. binding of a signaling mc to a GPCR triggers the activation of Gq which stimulates the cleavage of phosphatidyl inositol 4,5 bisphosphate (PIP2) by PLC to yield IP3 and DAG
   i. IP3—causes the release of Ca from the ER and SR into the cytosol
   b. An inc in the cytoplasmic Ca causes the cytosolic enzyme PKC to translocate to the plasma membrane where it is activated by DAG
   c. Ca also binds to the cytosolic protein calmodulin which forms a complex that activates Ca-calmodulin dependent proteins which include Ca-calmodulin dependent protein kinase (CaM kinase) and myosin light chain kinase (MLC)
   d. Activated CaM kinase phosphorylates target proteins to alter their activities
   i. whereas activated MLC kinase phosphorylates MLCSM contracts

Question 13

receptor tyrosine kinase and insulin

Answer 13

i. insulin H (ligand) binds to its R (RTK) to regulate glucose metabolism
ii. primary structure of insulin
1. composed of 2 peptide chains referred to as the A chain and B chain
a. they are linked together by 2 disulfide bridges and an additional disulfide is formed within the A chain
2. in most species, the A chain consists of 21 AAs and the B chain of 30 AAs

Question 14

synthesis and secretion of insulin

Answer 14

a. preproinsulin mRNA transcription and translation (protein synthesis)
b. preproinsulin has a N terminal hydrophobic signal peptide which interacts with the signal recognition particle that directs it to the ER
i. translocated into the lumen of the ER
c. cleaved by a protease to form proinsulin
d. proinsulin folds into correct conformation and 3 disulfides added
i. ER chaperone proteins help in this
ii. Transported into Golgi
iii. Packaged into immature clathrin coated granules
iv. Cleaved by proteases to form insulin and C peptide
e. The immature granules then become mature granules containing hexameric crystallized insulin
f. Insulin + C peptide released together

Question 15

regulation of the insulin synthesis and secretion

Answer 15

a. After glucose stimulation, insulin granules exhibit 2 characteristic phases
i. First—rapidly initiated but transient
ii. Second—sustained
b. The granules are divided into 2 different pools:
i. Readily releasable pool–95% which is responsible for second phase
1. Granules in this pool must undergo mobilization before they can gain release competence

Question 16

RAS dependent signaling

Answer 16

i. Insulin binds to RTK which exists as a preformed dimer
ii. Causes autophosphorylation of the R’s tyrosine residues
iii. Phosphotyrosine residues recognized and bound by protein called insulin receptor substrate 1 (IRS-1)
iv. IRS-1 phosphorylated on its tyrosine by insulin R
v. Phosphorylated IRS-1 recognized and bound by adaptor protein GRB2 initiating the transcription of RAS and the MAP kinase pathway
vi. Result in phosphorylation of nuclear proteins that increase the transcription of glucokinase
vii. Glucokinase phosphorylates glc in the first step of glycolysis and glycogen synthesis
viii. IRS2 is similar to 1 but has different specificity for adaptor proteins

Question 17

RAS independent signaling

Answer 17

i. Insulin binds to RTK which exists as a preformed dimer
ii. Causes autophosphorylation of the R’s tyrosine residues
iii. Phosphotyrosine residues recognized and bound by IRS 1
iv. IRS 1 phosphorylated on its tyrosine by insulin R
v. Phosphorylated IRS1 recruits PI3 kinase
vi. PI3 kinase phosphorylates phosphoinositides to form PIP3
vii. These membrane bound phosphoinositides act as second messengers
1. Stimulate the recruitment of PKB to membrane and its activation via phosphorylation
viii. Active PKB (serine threonine kinase) also known as Akt phosphorylates and alters the activity of several intracellular proteins
ix. Has stimulatory effects on glc uptake and storage
1. For example, PKB plays a role in the insulin induced mvmt of glc transporter (GLUT4) from cytoplasm to plasma membrane of muscle and adipose cells
x. PKB also promoted glycogen synthesis by phosphorylating and inhibiting glycogen synthase kinase 3 (GSK3)

Question 18

insulin resistance

Answer 18

1. Insulin resistance
   a. Measured as the amount of glc cleared from the blood in response to a dose of insulin
   b. Failure of normal amts of insulin to elicit the expected response is referred to as insulin resistance
   i. Mechanism not understood
   c. Defects in insulin signaling—R activation to translocation of GLUT4 transporters in adipose and skeletal M tissue
   i. >75 different mutations in insulin R identified—impair fcn of R
   d. defects in insulin binding domain on the extracellular side—severe resistance
   i. variations in intracellular domains also cause severe resistance
   e. inc phosphorylation of serine via ser/thr kinase instead of tyrosine in the IR and the IRSinhibits activation and signaling
   f. phosphorylation of IRS needed for recruitment of PI3 kinase but Ser/Thr phsophorylaiton appears to inactivate the IRS1/2 leading to degradation

Question 19

effects of insulin on glucose metabolism

Answer 19

a. under fed conditions, insulin lowers blood glc by promoting glycogen synthesis, stimulating glycolysis, and inhibiting the activity and synthesis of enzymes for gluconeogenesis
b. a deficiency in insulin production (type I diabetes) or insensitivity to insulin (type II diabetes) results in elevated blood glc

Question 20

effects of glucagon on glucose metabolism

Answer 20

a. under conditions of glc deficiency, pancreas releases glucagon
glucagon inc blood sugar by promoting breakdown of glycogen

Question 21

effects of epinephrine on glucose metabolism

Answer 21

a. epinephrine also stimulates breakdown of glycogen by promoting glucagon secretion

Question 22

effects of cortisol on glucose metabolism

Answer 22

a. when the glycogen stores depleted, then steroid H cortisol stimulates gluconeogenesis by inducing transcription of enzymes involved in this pathway

Question 23

nuclear receptors

Answer 23

-lipophilic pathway
i. large superfamily of Rs—include:
1. nuclear H receptors
2. orphan Rs
ii. classification based on ligands they bind
1. classic steroid nuclear R
a. ligands are lipophilic Hs
b. ex: glucocorticoids, mineralocorticoids, estrogen, progesterone, and androgens
2. orphan receptors
a. other group of Rs discovered by DNA sequencing
iii. ligands for several have been discovered since, so now called adopted orphan Rs (retinoids, thyroid Hs, vitamin D, xenobiotics, androstane)
iv. may be localized in nucleus or in sytosol
v. after ligand binding, they translocate to nucleus and influence gene expression
vi. serve as Rs and effectors for the signal
vii. important drug targets
dz involving aberrations in NR signaling: reproductive disorders, cancer, diabetes, inflammation, CV disorders, obesity

Question 24

molecular structure of NRs

Answer 24

1. same architecture—3 domains
   a. activation fcn 1 domain (AF1)
   i. independent of ligand binding
   ii. can modify the conformation of the entire R
   b. DNA binding domain (DBD)
   i. Highly conserved
   ii. Binds to regulatory sequences on DNA called H response element (HRE), upstream of target gene
   c. Ligand binding domain (LBD)
   i. Binds to various mcs (agonist/antagonist) which regulates ligand dependent activation of R
   ii. Upon ligand binding a region within LBD (AF2) undergoes conformational changes allowing recruitment and binding of coactivators or corepressors that regulate transcription
   NRs exist as homo or heterodimers

Question 25

structural domains of NRs

Answer 25

1. Inactive R has a ligand binding domain bound to an inhibitory protein
2. Active R has the ligand binding domain bound to the ligand and then bound with the transcription activating domain to the coactivator proteins

Question 26

primary and secondary response to steroid H

Answer 26

i. Primary
1. Steroid H binds to steroid R which complexes to activate primary response genes
a. Causes induced synthesis of primary response proteins
ii. Secondary
1. Primary response protein shuts off primary response genes
2. Primary response protein turns on secondary response genes
3. Secondary response proteins are forms from the DNA

Question 27

estrogen R

Answer 27

i. Two major types—ER(Alpha) and (beta)
1. Products of 2 separate genes located on different chromosomes
2. Structures are similar with some differences
3. Both are estrogen dependent TFs
ii. Differential tissue expression, influence lots of target genes
iii. Mediates a variety of differential biological effects
iv. KO mice of the 2 forms have different phenotypes
v. ER (beta) can substitute for ER (alpha) in some biologic pathways
vi. Selective ligands have been developed but not in clinical use yet
vii. ER(alpha)
1. First discovered
2. Expressed most abundantly in the female reproductive tract (uterus, vagina, ovaries)
3. Also in mammary gland, hypothalamus, endothelial cells, and vascular SM
viii. ER (beta)
1. Expressed most abundantly in the prostate and ovaries, with lower expression in lung, brain, bone, and vasculature
ix. Many cells express both ER (alpha) and (beta)
x. Dimerize to form either homodimers or heterodimers
xi. Final biological effect mediated by the ratio of 2 forms
1. Disruption in the ratio leads to diseases
2. Most ligands have differential affinity for the 2 forms
xii. Both ER (alpha) and (beta) are expressed in breast cancers, although ER (alpha) is believed to be the predominant form responsible for growth regulation

Question 28

Molecular mechanism of action of nuclear estrogen R

Answer 28

1. ER exists as a monomer within the nucleus
2. Ligand binds to the R
   a. Causes a ligand directed conformational change which facilitates dimerization and interaction with specific estrogen response element sequences in DNA
   b. ER DNA complex recruits co activators such as SWI/SNF that modify chromatin structure, and co-activators such as steroid R co activator (SRC1)
   i. SRC1 has histone acetyltransferase activity that further alters chromatin structure
   c. Chromatin remodeling facilitates binding of other co activators to promoter of target genes
   d. Additionally it recruits proteins that comprise the general transcription apparatus leading to mRNA synthesis
   e. Some evidence that ER located in cytosol.
   i. Estrogen binding causes dimerization and translocation to nucleus

Question 29

action of tamoxifen

Answer 29

a. Antagonists (see pic above, where it is labeled antagonist) such as tamoxifen also bind to ER but produce a different R conformation
i. This conformation also facilitates dimerization and interaction with DNA
b. A different set of proteins called co-repressors, such as nuclear H R co repressor (NcoR) are recruited to the complex
i. NcoR further recruits proteins such as histone deacetylase I (HDAC1) that acts on histone proteins to stabilize nucleosome structure and prevent interaction with the GTA

Question 30

non genomic ER signaling pathways

Answer 30

1. Some ERs located in the plasma membrane
2. Some present in caveolae (cholesterol enriched domains in the plasma membrane)
3. GPCR and RTK type
4. Effects mediated through metabolic changes as well as changes in gene expression