Lecture 12 Catalytic Steroid Hormone Signaling Flashcards
(41 cards)
1
Q
How can we instruct the cell to initiate a specific
transcriptional program?
A
Cellular signaling or signal transduction pathways
2
Q
Mechanisms that control catalytic and steroid hormone receptor signaling
A
- Receptor tyrosine kinases signaling
- Non-receptor tyrosine kinase signaling
- STAT protein signaling
- Insulin signaling
- Nuclear receptor signaling
3
Q
Basic Steps in Cell Signal/Transduct Pathways
A
- Extracell stim
- Extracell Receptor Domain
- Intracell Receptor Domain
- Signaling Mediators
- Transcriptional Response
4
Q
Catalytic receptors: Receptor tyrosine kinases (RTKs)
A
Kinases
Phosphatases
ATPases and GTPases
5
Q
Kinases
A
- Phosphorylate substrates (Ser/Thr/Tyr)
- Use ATP and produce ADP
- Add a phosphate group to a substrate protein
6
Q
Phosphatases
A
Remove a phosphate group from a substrate protein
7
Q
ATPases and GTPases
A
Hydrolyze ATP or GTP
And produce ADP or GDP and (Pi)
8
Q
Receptor tyrosine kinases: Structure
A
- Extracellular N-terminal:
* Ligand-binding region - Transmembrane domain:
* Hydrophobic domain that traverses the membrane - Intracellular C-terminal domain:
* Tyr kinase catalytic domain
* “Dormant” tyrosine kinase activity
* “Effector” domain - Receptor Activation:
* Ligand binding
* Dimerization
* Activation of the “dormant” Tyr
kinase activity
* Adaptor proteins recognize phospho-tyrosine residues, amplify and transduce the signal
9
Q
Receptor tyrosine kinases: Activation
A
- Growth factor or hormone binds to the extracellular portions of receptor chains.
- Tyrosine kinase domains of each chain activate each other.
- Tyrosine residues on opposite chains serve as substrates for each of the tyrosine kinase domains. As a consequence, phosphorylated tyrosine residues are present on each of the receptor cytoplasmic tails.
- SH2-containing adaptor proteins dock with phosphotyrosines on the cytoplasmic tails of the receptor chains.
10
Q
Tyrosine residues on opposite chains serve as substrates for each of the tyrosine kinase domains. As a consequence, phosphorylated tyrosine residues are present on each of the receptor cytoplasmic tails.
A
- Ligand binding and dimerization
- Auto-phosphorylation:
* The receptor phosphorylates itself - Cross-phosphorylation:
* The receptor phosphorylate its
dimerization partner
* Phospho-tyrosine - Phospho-Tyrosine becomes a “docking site” for
adaptor proteins
* Adaptor proteins recognize phospho-tyrosine
residues (SH2 domain), amplify and transduce the
signal
11
Q
RTK: Adaptor Proteins
A
- Adaptor proteins:
* Recognize a specific motif, in this case phosphorylated tyrosine residues - Src Homology domains (SH domains)
* Proteins have SH domains within their structures
* SH domains exist in many proteins - SH1 domain
* Tyrosine kinase catalytic domain - SH2 domain
* Recognizes and binds to phosphorylated-tyrosine residues - SH3 domain
* Often binds to proteins with SH2 domains bound to phosphorylated tyrosine residues. Recognition domain is associated with poly-proline region.
12
Q
SRC Kinases
A
3 domains
SH2 and SH3 mediate protein to protein interactions
SH1 is catalytic domain
13
Q
Receptor tyrosine kinases (RTKs)
A
Serve as an “ON/OFF switch”
* Phosphorylate specifically Tyrosine residues
* Fleeting modification
* Possess a “dormant” Tyr kinase
* less common, but very important
* Many are growth factors, and thus can be ONCOGENIC!
14
Q
Examples of RTK
A
Insulin receptor,
Transforming Growth Factor
(TGF)
Epidermal Growth Factor (EGF) Platelet Derived Growth Factor (PDGF)
15
Q
Ras signaling (Ras)
A
- Oncogene (Rat Sarcoma virus)
- Small GTP-binding protein
- Membrane-bound
- Highly mutated in cancers
(“always ON” GTP-bound
mutations) - Very important oncogene!
Mutated in ~25% tumors
16
Q
Ras activation
A
- Active (GTP-bound)
- Inactive (GDP-bound)
- Requires a GEF to exchange GDP
for GTP and become active - Requires a GTPase activating
protein (GAP) to hydrolyze GTP
and become inactive
17
Q
Ras Signaling
A
- Ras is membrane-bound and
GDP-bound (inactive) - Upon RTK activation, Grb2
(SH2-domain) binds to the
activated RTK - SOS (GEF) binds Grb2
(through SH3) - GDP-Ras (inactive) binds to
Grb2-SOS complex and GDP
for GTP exchange occurs - GTP-Ras (active) binds to and
activates the Raf kinase
6.This kinase cascade results in
MAPK (ERK) phosphorylation
and translocation into the
nucleus for transcriptional
activation of cell proliferation
signal.
18
Q
Adaptor proteins: STATs
A
Signal Transducers and
Activators of Transcription
- SH2 domain-containing protein
- STATs are also substrates for the RTKs
- Tyr-phosphorylated STATs dimerize and translocate into the nucleus where they act as transcription factors
19
Q
Adaptor proteins: PI3 kinases
A
Phosphatidylinositol 3 kinase
- SH2 and SH3 domain-containing protein
- Phosphorylate membrane inositol
phospholipids - PIP3 a docking site for other signaling molecules (AKT kinase)
- Signal cell growth and
survival and are associated with
cancers!
20
Q
What counteracts most effects of kinases?
A
Phosphatases
Ex. PTEN
21
Q
Bad
A
induces cell death, but phospho-Bad is inactive, thus promoting cell survival! PTEN induces
cell death by disrupting the inactivation of Bad!
PTEN: pot target in prostate cancer
22
Q
Non-receptor tyrosine kinases
A
Some receptors do not possess their own RTK activity
- Receptors without intrinsic tyrosine kinase activity associate with cytoplasmic non-RTKs, which phosphorylate tyrosine residues on their cytoplasmic tail
Includes:
1. Src non-receptor Tyrosine Kinase
2. Janus Kinases (JAKs)
23
Q
Src non-receptor Tyrosine Kinase
A
- First non-receptor Tyrosine Kinase discovered
- At least 8 members of the family
24
Q
Janus Kinases (JAKs)
A
- “Dormant” cytoplasmic tyrosine kinases
- Activated by certain hormones & cytokines
- JAKs phosphorylate Tyr residues the intracellular regions of receptors,
where SH2 domain-containing proteins bind - STATs bind these phospho-tyrosine residues and become themselves phosphorylated by JAKs
(JAK-STAT pathway)
25
JAK-STAT pathway
1. Ligand binds and causes receptor dimerization.
2. JAKs phosphorylate tyrosines on the receptor.
3. SH2 STATs bind to phosphorylated tyrosines, and JAKs phosphorylate STATs.
4. Phosphorylated STATs dimerize, move to the nucleus, and activate gene transcription.
26
Diabetes Mellitus (DM)
Hyperglycemia (>126 mg/dL FBG)
Type I: doesn’t produce insulin
Type II: develop insulin resistance
Hormones that control glucose homeostasis:
* Insulin
* Glucagon
27
Consequences of uncontrolled diabetes
* Neuropathy (nerve damage)
* Retinopathy (blindness)
* Circulation problems (limb amputations)
* Nephropathy (kidney damage)
* Cardiovascular disease
* Stroke
28
RTK: Insulin receptor
1. Most-studied RTK with multiple polypeptides.
2. Protein chains linked before ligand binding.
3. Insulin binding activates RTK and receptor/IRS phosphorylation.
4. IRS proteins activate Ras, STATs, and PI3K pathways.
5. Tissue-specific IRS expression directs specific transcriptional responses.
29
Catalytic receptor signaling
Summary
1. Most receptors are single-chain proteins that dimerize when ligands bind.
2. Dimerization triggers auto- and cross-phosphorylation on tyrosines.
3. Phosphorylated tyrosines act as docking sites for SH2 proteins.
4. Some receptors lack RTK activity but activate other tyrosine kinases (e.g., Src, JAK).
5. These receptors rely on cytoplasmic kinases to phosphorylate their tyrosines.
30
Steroid receptor (or nuclear receptor) signaling
Steroid hormone receptors (Nuclear Receptors):
Act as ligand-activated transcription factors!!!
Hormones derived from cholesterol
Typical NR signaling/effects takes from hours (in cell) to days (in tissue)
These receptors are intracellular!!
31
Steroid Signaling
Classical Signaling----> Nuclear Initiated Steroid Signaling (NISS)
Rapid Signaling -----> Member Initiated Steroid Signaling
32
Type I Receptors
Recruit transcriptional activators upon ligand binding
Sex Hormone Receptor
(andro,estro, proges)
Glucocorticoid Receptor
Mineralcorticoid Receptor
33
Type II Receptors
Are in a complex with transcriptional
repressors and upon ligand binding, they interact with transcriptional activators
Vitamin A
Vitamin D
Retinoid
Thyroid Hormone Receptor
34
Steroid hormones
Derived from cholesterol
Pass through membranes
1. Cortisol:
* Stress hormone
* In adrenal cortex
* Induces gluconeogenesis
2. Estrogens
* Female sex hormone
* In ovaries
* Controls menstrual cycle and
female secondary sexual
characteristics
3. Androgens
* Male sex hormone
* In testes
* Controls male secondary sexual
characteristics and anabolism
35
Steroid Hormone Receptors (NRs) Structure
*Transactivation domain:
Gene regulatory domain that interacts with transcriptional co-activators or co-repressors
* DNA binding domain:
Region of the receptor that interacts with DNA sequences (HREs)
* Ligand binding domain:
Region of the receptor that interacts with a given hormone
* Ligand binding induces a conformational change in the
NR that stimulates its translocation to the nucleus and binding to HRE to act as a transcription factor
36
Steroid Hormone Receptors
(NRs) Signaling:
1. Ligand binds in the cytoplasm.
2. Receptor moves to the nucleus.
3. Receptor-ligand complex binds to HREs on DNA.
4. Complex recruits co-activators or repressors to control transcription.
5. Receptors can be nuclear or cytoplasmic (all intracellular).
6. Some receptors repress but activate transcription when bound by ligand (Type II).
37
Steroid Receptor Signaling
1. Cortisol binds to its receptor: When cortisol (a steroid hormone) binds to the glucocorticoid receptor (GR), it changes the shape of the receptor.
2. GR dimerization and interaction with DNA: The receptor forms a dimer (pairs up) and moves to the nucleus, where it binds to specific DNA sequences called glucocorticoid response elements (GRE).
3. Gene transcription begins:
The receptor recruits coactivators to the basal transcription complex, which helps start the transcription of target genes, leading to gene expression.
38
Two types of steroid signaling
Classical/Nuclear-Initiated Steroid Signaling (NISS):
Steroid binds to its receptor in the cytoplasm and affects gene transcription over hours or days.
Membrane-Initiated Steroid Signaling (MISS):
Steroids act rapidly (seconds or minutes) via receptors on the membrane.
39
Membrane-initiated signaling
The steroid receptor on the membrane interacts with other proteins, such as kinases (e.g., Src, Ras), to activate signaling pathways quickly.
40
Medical applications of steroid hormone signaling
Estrogen
Estrogen-dependent Breast
Cancer:
* Breast cancers require ER signaling in order to grow
* Receptor antagonists inhibit
growth
* Tamoxifen is drug (ER antagonist) used against types of breast cancers
5 years
41
Medical applications of steroid hormone signaling
Androgen
Prostate cancers require AR
signaling in order to grow
* Receptor antagonists can inhibit
growth
* Flutamide is drug (AR antagonist) used against prostate cancers
* PSA (Prostate Specific Antigen)
test: The PSA gene is regulated by
AR and elevated levels suggest
increased prostate growth
signaling!
