4. Hormone Signaling Pathways Flashcards
(44 cards)
1
Q
Hormones are __________ molecules made and secreted by _______ cells.
- Released into blood.
- Bind to specific receptors
- Result in: __________.
A
Hormones are MESSENGER molecules made and secreted by ENDOCRINE cells.
- Released into blood.
- Bind to specific receptors
- Result in: SIGNAL TRANSDUCTION, ALTER GENE EXPRESSION, CAUSING A. CELL-TYPE SPECIFIC RESPONSE.
2
Q
We only need a small amount of hormone to have an effect. Why?
A
Their effect is magnified by amplification!
3
Q
What are the steps in hormone signaling? (9)
A
- Biosynthesis
- Storage
- Secretion
- Transport to target tissue/cell
- Recognize and bind to R
- Activation of signal transduction path (on switch)
- Amplification
- Cellular response
- Degradation (off switch)
4
Q
How is the signal terminated?
A
- Removal of a signaling molecule/receptor
- Attenuation/inactivation of signaling events.
5
Q
What are the general signaling steps?
A
- Signaling cell makes and secretes a signaling molecule (ligand) d/t a stimulus.
- Signaling molecule (ligand) binds to receptor on target cell.
- Ligand + receptor activates or inhibits cellular pathway to cause a response.
- Signal terminated.
6
Q
Describe the last type of signaling:
- Endocrine signaling- signaling molecule is released to a cell far away via blood. Ex. EPI
- Paracrine signaling- signaling molecule is released by one cell type and binds to neighboring cell of a different type. Ex. Testosterone
-
Autocrine signaling- signaling molecule acts on the same cell type as the secreting cell. Ex. IL-1
4.
A
Juxtacrine signaling- signaling molecule stays attached to the secreted cell and binds to a receptor on an adjacent target cell. Ex. Heparin-binding epidermal GF.
7
Q
Do signaling molecules only participate in ONE type of signaling?
A
NO. Some signaling molecules can participate in more than 1 type of signaling.
8
Q
Hydrophilic hormones (water soluble) cannot pass the phospholipid bilayer, and thus need receptors on the cell surface. When it binds to the receptor, it does what?
A
initiates production of second messenger in the cell to cause a cell response.
9
Q
Hydrophobic hormones (such as the steroid hormones), by contrast, can pass directly into the cell and have an effect there. This receptor could either be cytosolic – forming a complex that translocates to the nucleus, or already in the nucleus. Both act like a what?
A
A transcription factor, regulating transcription.
10
Q
Are cytoplasmic receptors always active?
A
No. They exist as an inactive complex with HSP 90. When ligand binds, HSP 90 dissociates. The ligand+ receptor -> nucleus and binds to DNA sequence called the hormone response element (HRE) in the promotor.
11
Q
Examples of hydrophilic hormones include:
A
- AA derived (melatonin, DA, NE, EPI, Histamine, 5HT) ,
- Those from lipid metabolism (ACh)
- Polpeptides such as insulin, glucagon, TSH
12
Q
Examples of hydrophobic hormones include:
A
- steroid hormones (include cortisol, aldosterone and vitamin D),
- thyroid hormones (thyroxine)
- retinoids.
13
Q
What is the difference between hydrophilic medications and lipophilic medications?
A
- Hydrophilic medications have short half-lives that range from seconds- minutes. Examples are EPI and insulin.
- Lipophilic medications have long-half lives, from hours- days. Examples are oral contraceptives.
14
Q
Describe the activation/inactivation cycle of trimeric G-protein.
A
GCPR is a trimeric G-protein with 3 subunits (a, b, y)
Activation:
- Inactive G protein has GDP bound to alpha subunit. To become active, it exchanges GDP -> GTP via GEF (guanine nucleotide exchange factor). Once active, GTP-bound alpha subunit separates from b and y subunits.
Inactivation:
- Intrinsic GTPase activity of G-protein hydrolyzes GTP-> GDP and Pi. GAP (GTPase activating protein) accelerates this activity.
15
Q
What are the 4 types of G-proteins and what is their mechanism of action?
A
-
Gs: ATP -> cAMP via adenylate cyclase (AC)
- cAMP -> activates PKA
- PKA phosphorylates protein
- Gt: light activates cGMP PDE to convert cGMP-> 5’GMP
-
Gq: signal activates PLC to activate PIP2 -> DAG and IP3.
- IP3 will cause the release of Ca2+ from ER/SR.
- Ca and DAG activate PKC-> phosphorylates
- Ca+ also forms a complex with calmodulin (ca2+-calmodulin complex) which activates proteins.
- Gi: cAMP and PKA are not activated.
16
Q
Gs examples:
A
- EPI is a non-selective agonist of all adrenergic receptors and can undergo multiple GCPR pathways. It can bind to B-adrenergic receptor
- Histamine binds to H2 receptors and causes bronchoconstriction and sx of allergic reactions
17
Q
Gi examples:
A
- EPI/NE bind to a-adrenergic receptor and cause constriction of smooth muscle.
- DA binds to D2 receptors and causes increase HR.
18
Q
Gq examples:
A
- ACh binds to M3 receptors and causes bronchoconstriction and stimulation of salivary glands
19
Q
Gt examples:
A
Light binds to Gt receptor and influences vision.
20
Q
What is the structure of a RTK?
A
- Extracellular domain that binds ligand
- Single alpha helical transmembrane domain
- Intracellular domain with TK activity.
21
Q
What are the steps of RTK activity?
A
- Ligand binds to extracellular domain
- Dimerization
- Dimerized receptor phosphorylates tyrosine residues
- Adaptor and docking proteins recognize the phosphotyrosines
- Activates downstream signaling: RAS dependent (MAPK) and independent.
- Both trigger phosphorylation of a specific protein in the cytoplasm/nucleus -> alter gene transcription and protein activity.
- Signal terminated by
- Degradation of ligand by extracellular proteases
- Ligand-induced endocytosis of receptor and degradation
- RAS inactivation
- Dephosphorylation
22
Q
How is inactive insulin stored in the body?
A
Hexamer.
Active form is a monomer.
23
Q
How is insulin made?
A
- Begins as preproinsulin mRNA
- Translated into preproinsulin protein
- Translated to ER lumen where it is cleaved by a protease to make proinsulin
- Folded and goes to Golgi.
- Packaged into granules
- Cleaved by proteases to make [insulin and C peptide]
- Released together from beta cells.
24
Q
In the blood we see a rise in glucose, this causes insulin to be secreted.
How is it secreted?
A
Insulin is released in two phases:
- a first rapid but transient phase from our readily releasable pool (5%)
- a second sustained phase from our reserve pool, where granules have to undergo mobilization before they can be released.
25
How do we release insulin from the pancreatic B cell?
1. Glucose is going to bind to **GLUT2 R** on the pancreatic B cell and move in via faciltated diffusion
2. Glucose goes inside and Glu--\> G6P via glucokinase.
3. G6P will undergo glycolysis, TCA cycle and oxidation--\> ATP
4. ATP will close KATP channels
5. Cell depolarizes
6. Depolarization reaches the VGCa2+ channels and allows Ca2+ influx
7. Triggers exocytosis of secretory vesicle
8. insulin and C peptide are released
26
What is the target of **sulfonylurea drugs** to treat type 2 non-insulin dependent DB?
**KATP channels; increase insulin secretion**
27
**Describe RAS-dependent insulin signaling.**
1. **_Insulin_** binds to insulin receptor, a **_RTK_**, which is already dimerized.
2. Tyrosine residues are autophosphorylated
3. **_IRS-1 (insulin receptor substrate-1)_** binds to phosphorylated receptor tyrosine kinase
4. Insulin receptor phosphorylates IRS-1 on its tyrosine.
5. IRS-1 then recruits an adapter protein called **_GRB-2,_** which activates the RAS and MAP kinase pathway.
6. Multiple proteins are phosphorylated which leads gene transcription – specifically _↑ transcription of_ glucokinase.
7. This results in increased _glucose uptake_ and _glycogen synthesis._
28
Describe **RAS-independent insulin** signaling:
1. **Insulin** binds to insulin receptor, a RTK, which is already dimerized.
2. Tyrosine residues are autophosphorylated
3. **IRS-1 (insulin receptor substrate-1)** binds to phosphorylated receptor tyrosine kinase
4. Insulin receptor phosphorylates IRS-1 on its tyrosine.
5. IRS-1 then recruits an adapter protein called **PI 3-kinase** (phosphoionositide-3-kinase).
6. P1 3-kinase phosphorylated phosphoinositides to make **PIP2** and **PIP3**.
7. PIP2 and PIP3 act as second messengers and recruit **PKB** and activate it via phosphorylation.
8. RESULT: ALTER PROTEIN AND ENZYME ACTIVITY
9. 1. increase in **GLUT4**'s movement to the plasma membrane,
2. + glycogen synthase.
10. Both result in an increase _in glucose uptake_ and _glycogen synthesis._
29
**How can we terminate insulin signaling?**
1. Insulin receptor complex is internalized in target cells via endocytosis. There it is degraded by proteases or recycled back into membrane.
2. Negative feedback
3. Insulin can increase rate of degradation
30
**How can we test insulin resistance?**
**Measure the amount of glucose cleared from blood in response to a fixed dose of insulin. If insulin does not cause the expected response -\> insulin resistance.**
31
**Why does insulin resistance occur?**
NO CLUE. Possible explanations:
1. _Downregulation_ of insulin receptor
2. _Defects_ in the insulin receptor (mutation)
3. _Defects_ in insulin binding
4. _Defects_ in IRS1 and IRS2
5. _Phophorylation of serine/threonine kinase_ instead of tyrosine of IRS, which inhibits its activation and prevents activation of PI-3 kinase.
6. **Serine threonine kinase is activated** by cytokines, FFA, DAG, ceramind, inflammatory molcules.
32
**Nuclear Receptors** include _________ receptors and _______ receptors that bind lipophilic hormones such as \_\_\_\_\_\_\_\_\_\_, \_\_\_\_\_\_\_\_\_\_, \_\_\_\_\_\_\_, _______ and \_\_\_\_\_\_\_\_\_. **They are _receptors_ and _effectors_ for the signal.**
Nuclear Receptors include **nuclear receptors** and **orphan receptors** that bind lipophilic hormones such as **glucocorticoids**, **mineralocorticoids**, **estrogen**, **progesterone** and **androgens**. They are receptors and effectors for the signal.
33
Where are nuclear receptors located?
**Nucleus or cytosol.**
34
What are NR important for?
**DRUG TARGETS.**
35
**What is the architecture NRs?**
1. **LGB** (ligand binding domain)
2. **AF1 domain (activation function 1 domain) aka transcription activating domain**
3. **DBD** (DNA binding domain)
36
What happens when a ligand binds to **LBD**?
When it binds, the LBD undergoes conformational change and recruits and binds _coactivators_ and _corepressors_ that regulate transcription.
37
What does **AF-I** do?
**modifies the conformation of the entire receptor**
38
What does **DBD** do?
highly conserved and binds to **HRE** (hormone response element) in the nucleus.
39
**How do nuclear receptors change before and after binding of a ligand?**
* Before the ligand binds, the nuclear receptor is surrounded by inhibitory proteins.
* After the ligand binds, the inhibitory proteins dissociate, the activation function domain (AF 1 / "transcription activating domain") changes confirmation, and co-activator proteins bind.
* The nuclear receptor complex then binds to the receptor binding element of the HRE and alters gene expression.
40
**Describe the _primary and secondary responses_ to steroid hormones.**
* In the **primary (early) response,** primary response genes are activated by the steroid hormone receptor complex, and _synthesis of primary response proteins is induced._
* In the **secondary (delayed) response**, the _primary response proteins_ shut off the primary response genes if they persist long enough and activate the secondary response genes to transcribe the _secondary response proteins._
* Both the primary and secondary response proteins can have an effect – testosterone, for example, _increases spermatogenesis in the short-term_, but increases to _form secondary sexual characteristics as a secondary response long-term._
41
**What are the two types of estrogen receptors?**
**ER-alpha** and **ER-beta**, which are both _estrogen-dependent transcription factors._
42
**Where are both ER-alpha and ER-beta located?**
* **ER-alpha** -\> female reproductive tract (uterus, vagina and ovaries) and mammary glands, hypothalamus, endothelial cells and vascular smooth muscle.
* **ER-beta -\>** ovaries and prostate with lower expression in the lung, brain, bone and vasculatire.
43
**Describe the binding onto estrogen receptors.**
* The estrogen receptor is already present in the nucleus (however, there is some evidence that ER is located in the cytosol and binding causes dimerization and translocation -\> nucleus).
* When estrogen binds to the estrogen receptor, it **dimerizes**.
* This allows it to **bind to the estrogen response element (ERE)** on DNA and recruit co-activators that modify chromatin structure via **histone acetyltransferase (HAT).**
* Loosens up chromatin and activates transcription.
* Also, it recruits proteins that make up the **GTA** (general transcription apparatus), causing enhanced transcription to form mRNA.
44
**Describe the importance of tamoxifen.**
Tamoxifen is a **estrogen receptor ANT.**
**Tamoxifen**, which is used to treat breast cancer, is metabolized by cytochrome P450 to 4-hydroxy-tamoxifen, which promotes _histone deacetylase activity_ (HDAC1), which tightens chromatin and prevents interaction with GTA, thus, preventing transcription of cancerous cells.
