Hormone Signaling Pathways Flashcards
Hormones are messenger molecules synthesized and secreted by _______ cells. They are released into ______ or the blood stream and exert their influence by binding receptors on cells of remote target tissues
Endocrine
ECF
Hormones binding their receptors results in activation of signal ______ and/or alteration in ______ expression ultimately leading to cell type-specific response
Only a small amount is required to alter cell metabolism because the effect is greatly magnified via signal ________
Transduction; gene
Amplification
Multistep process of hormone signaling
Biosynthesis Storage Secretion Transport to target tissue/cells Recognition and binding to receptors Activation of signal transduction pathway Amplification and relay of signal Cellular response Degradation
4 types of cell signaling
Endocrine
Paracrine
Autocrine
Juxtacrine
___________ signaling = signaling molecule released by a cell distant from target cell and transported via blood to target cell
Endocrine
___________ signaling = signaling molecule acts on same cell type as secreting cell itself (ex. IL-1)
Autocrine
___________ signaling = signaling molecule released by one cell type and diffuses to neighboring target cell of a different type
Paracrine
___________ signaling = signaling molecule stays attached to secreting cell and binds receptor on adjacent target cell via connexons (ex. Heparin-binding epidermal growth factor)
Juxtacrine
T/f: some signaling molecules participate in more than one type of signaling
True
Which of the following is typical of testosterone?
A. Endocrine signaling
B. Paracrine signaling
C. Autocrine signaling
D. Juxtacrine signaling
B. Paracrine signaling
Which of the following is typical of epinephrine?
A. Endocrine signaling
B. Paracrine signaling
C. Autocrine signaling
D. Juxtacrine signaling
A. Endocrine signaling
Which of the following are hydrophilic hormones?
A. Epinephrine B. Insulin C. Steroid hormones D. Glucagon E. Thyroid hormone F. Retinoids
A. Epinephrine
B. Insulin
D. Glucagon
Which of the following are lipophilic hormones?
A. Epinephrine B. Insulin C. Steroid hormones D. Glucagon E. Thyroid hormone F. Retinoids
C. Steroid hormones
E. Thyroid hormones
F. Retinoids
What types of receptors are involved in hydrophilic hormone signaling?
GPCRs
RTKs
Lipophilic hormones pass through the plasma membrane of the target cell and bind to either ______ receptors or ______ receptors, either way regulating the transcription of specific genes
Cytoplasmic
Nuclear
Describe lipophilic hormone signaling via cytoplasmic receptors
They exist in an inactive complex with HSP90. Upon binding to signal, the HSP dissociates
The hormone receptor 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
Describe lipophilic hormone signaling via nuclear receptors
They are already present in the nucleus bound to DNA. The hormone signal activates the complex and allows for interactions with additional proteins
Epinephrine is a ______ signal that binds a _______ to cause cellular response
Hydrophilic; GPCR
Insulin is a ______ signal that binds a _______ to cause cellular response
Hydrophilic; RTK
What is the difference between lipophilic and hydrophilic medications?
Hydrophilic have SHORT half lives (e.g. epinephrine); given at time of need
Lipophilic have long half lives (e.g. oral contraceptives); taken daily
GPCR signaling occurs via trimeric G proteins containing 3 subunits (_____, _____, ____)
An inactive G protein has GDP bound to its _____ subunit, which is attached to the other 2 subunits.
To become active, the G protein must exchange its GDP for a GTP, this occurs via the action of _______
The active GTP-bound ____ subunit separates from the other 2 to potentiate activation signal
Alpha, beta, gamma
Alpha
GEF (guanine nucleotide exchange factor)
Alpha
For a GPCR to return to its inactive state, the intrinsic GTPase activity of the G protein hydrolyzes its bound GTP to GDP and Pi
This action is accelerated by a ______
GAP (GTPase-activating protein)
Describe 4 different GPCR signaling variations: Gs, Gt, Gi, Gq
Gs = stimulates adenylate cyclase, forms cAMP, activates PKA, phosphorylates targets
Gt = stimulates hydrolysis of cGMP by phosphodiesterase
Gi = inhibits adenylate cyclase
Gq = stimulates PLC —> DAG and IP3 —> PKC and calcium/calmodulin
Epinephrine binds beta-adrenergic GPCR, activating _____ subunit
What is the physiologic response?
Gs
Relaxation of bronchial and intestinal smooth muscle, contraction of heart muscle, increased breakdown of TAGs in adipose tissue, increased breakdown of glycogen in liver and muscle, increased glycolysis in muscle
[epinephrine is a non-selective agonist of all adrenergic receptors and results in multpile GPCR signaling pathways]
Histamine binds ______ GPCR, activating _____ subunit
What is the physiologic response?
Histamine H2; Gs
Bronchoconstriction and symptoms of allergic reaction (e.g. itchy, watery eyes)
Epinephrine/Norepinephrine binds alpha-adrenergic GPCR, activating _____ subunit
What is the physiologic response?
Gi
Constriction of smooth muscle
Dopamine binds ______ GPCR, activating _____ subunit
What is the physiologic response?
Dopamine D2; Gi
Increased heart rate
Acetylcholine binds ______ GPCR, activating _____ subunit
What is the physiologic response?
Muscarinic ACh M3; Gq
Bronchoconstriction and stimulation of salivary glands
Light binds ______ GPCR, activating _____ subunit
What is the physiologic response?
Rhodopsin; Gt
Vision
RTKs have an extracellular domain that binds the ligand/signaling molecule, a single ________ transmembrane domain, and an intracellular domain with _____ activity
Alpha-helical; tyrosine-kinase
Typical ligands of RTKs are growth factors. What is the result of ligand binding to RTK extracellular domain?
Dimerization
The dimerized receptor then phosphorylates tyrosine residues
Dimerized RTKs phosphorylate tyrosine residues. What effect does this have on signaling process?
Phosphotyrosines are recognized by adaptor and docking proteins which activate downstream signaling pathways (RAS dependent and RAS independent) - both of which trigger phosphorylation of specific protein targets in cytoplasm/nucleus leading to alterations in gene transcription and protein activity
Are MAPK pathway signals RAS-dependent or RAS-independent?
RAS-dependent
[RAS-independent include other kinases]
How is RTK signaling terminated?
Multiple mechanisms: degradation of ligand by EC proteases, ligand-induced endocytosis of receptor and its degradation, RAS inactivation, dephosphorylation of protein targets by phosphatases
Primary structure of insulin
2 peptide chains: A chain and B chain, linked together by 2 disulfide bridges, and an additional disulfide is formed within the A chain
In most species the A chain of insulin consists of ____ amino acids and the B chain of ____ amino acids
21; 30
The secondary and tertiary structure of insulin involves 6 insulin molecules assembled in a ______ with 3-fold symmetry with _____ in the center, connected to the polypeptide via ________
Hexamer; zinc; histidines
Inactive insulin is stored in the body as a ______, while the active form is the _____
Hexamer; monomer
Insulin synthesis and secretion:
Glucose upregulates ______ mRNA, which is translated into ______ protein. This gets translocated into _____ lumen
Preproinsulin; preproinsulin; ER
Describe insulin synthesis and secretion once the preproinsulin protein has been translocated to the ER lumen
It is cleaved by a protease to form proinsulin, folded and transported to golgi, packaged into clathrin coated granules, cleaved by proteases to form insulin and C peptide, forms mature granules containing hexameric crystallized insulin (3dimers), insulin + C peptide released together by beta cells of pancreas
After glucose stimulation, insulin is released in what 2 distinct phases?
First phase = rapid but transient; comes from limited pool of granules referred to as readily releasable pool (RRP) - <5%
Second phase = sustained; comes from larger pool referred to as reserve pool. Granules in this pool must undergo mobilization before they can gain release competence - >95%
Regulation of insulin synthesis and secretion:
Glucose binds ______ receptor, resulting in _______ enzyme forming G1P
Insulin granules are released by opening of ______ channels in cell membrane, as well as closing of ATP-sensitive _____ channels when energy in cell increases
GLUT2; glucokinase
Ca++; K+
Describe Ras-dependent insulin signaling
Insulin binds to RTK which exists as preformed dimer, causing autophosphorylation of tyrosine residues
Phosphotyrosine residues are recognized and bound by IRS-1, which is phosphorylated on its tyrosine by insulin receptor
Phosphorylated IRS-1 is recognized and bound by adaptor protein GRB-2, initiating activation of Ras and Map kinase pathway
Results in phosphorylation of nuclear proteins that increase transcription of glucokinase
Glucokinase phosphorylates glucose in the first step of glycolysis and glycogen synthesis
Describe Ras-independent insulin signaling
Insulin binds RTK which exists as a preformed dimer, causing autophosphorylation of the tyrosin residues
Phosphotyrosine residues recognized and bound by IRS-1
IRS-1 phosphorylated on its tyrosin by insulin receptor
Phosphorylated IRS-1 recruits PI3kinase, which phosphorylates phosphoinositides to form PIP3
PIP3 act as second messengers, stimulating recruitment of PKB to the membrane and its activation by phosphorylation
Active PKB (serine threonine kinase aka Akt) phosphorylates and alters IC proteins to stimulate glucose uptake and storage (upregulates GLUT4, inhibits glycogen synthase kinase via phosphorylation)
What quantifiable lab parameter is measured as the amount of glucose cleared from the blood in response to a fixed dose of insulin?
Insulin resistance
What types of conditions might lead to insulin resistance?
Defects in insulin signaling (receptor activation to translocation of GLUT4)
Mutations in insulin receptor
Increased phosphorylation of serine (via ser/thr kinase) instead of tyrosine in IR and IRS - inhibits activation and signaling
Insulin resistance may result d/t increased phosphorylation of serine via ser/thr kinase instead of tyrosine in the IR and IRS - which inhibits activation and signaling
Phosphorylation of IRS is needed for recruitment of PI3 kinase but ser/thr phosphorylation appears to inactivate the IRS 1 and 2, leading to degradation
What activates the Ser/Thr kinase to cause this effect?
Cytokines Free fatty acids DAG Ceramide Inflammatory molecules
What hormone is released from the pancreas under conditions of hunger
Glucagon
How does glucagon increase blood sugar
Promoting breakdown of glycogen in liver and inhibiting glycogen synthesis
How does epinephrine affect glycogen breakdown
Stimulates it by promoting glucagon secretion
Upon depletion of glycogen stores, the steroid hormone _____ stimulates gluconeogenesis by inducing transcription of enzymes involved in this path
Cortisol
Nuclear receptors are a large family of receptors classified based on ligands they bind. What are the 3 general classifications?
Classic NRs - ligands are lipophilic hormones like glucocorticoids, mineralocorticoids, estrogen, progesterone, androgens
Orphan NRs - other group of receptors discovered by DNA sequencing (ligands unknown)
Adopted NRs - orphan NRs whose ligands have been discovered like retinoids, thyroid hormones, vitamin D, xenobiotics, androstane, etc.
NRs may be localized in what 2 locations
Nucleus
Cytosol
What are some diseases involving abberrations in NR signaling?
Reproductive disorders Cancer Diabetes Inflammation Cardiovascular disorders Obesity
NRs share the same architecture
What are the 3 major domains?
Activation 1 domain (AF1)
DNA binding domain (DBD)
Ligand binding domain (LBD)
Describe the activation function 1 domain (AF1) of NRs
Independent of ligand binding, can modify the conformation of entire receptor
Describe the DND binding domain (DBD) of NRs
Highly conserved; binds regulatory sequences on DNA called hormone response element (HRE), upstream of target gene
Describe Ligand Binding domain (LBD) of NRs
Binds various molecules (agonist or antagonist) which regulates ligand-dependent activation of the receptor
Upon ligand binding within LBD, conformational changes occur allowing recruitment and binding of coactivators or corepressors that regulate transcription
T/F: NRs exist as homo or heterodimers
True
2 major types of estrogen receptor
ER-alpha
ER-beta
The 2 major types of estrogen receptors, ER-alpha and beta, are products of 2 separate genes located on different chromosomes
Their structure is the almost the same with some differences. Both are estrogen-dependent ________ ________
Transcription factors
ER-alpha was the first estrogen receptor discovered. Where is it most abundantly expressed?
Female reproductive tract (uterus, vagina, ovaries)
Mammary gland, hypothalamus, endothelial cells, vascular smooth muscle
Where is ER-beta most abundantly expressed?
Prostate and ovaries
Lower expression in lung, brain, bone, vasculature
T/F:
Many cells express both ER-alpha and beta
They dimerize to form either homo or heterodimers, and the final biological effect is mediated by the ratio of the 2 forms
True
Both ER-alpha and beta are expressed in breast cancers, although _____ is believed to be the predominant form responsible for growth regulation
ER-alpha
Estrogen acts as an _______ at the ER, upregulating ______activity and ______ transcription
Agonist
HAT
Activating
Tamoxifen acts as an _______ at the ER. It is first converted to 4-hydroxy-tamoxifen via _______ metabolism, which then upregulates _____ activity and ______ transcription
Antagonist
CytP450
HDAC
Inhibist
Molecular MOA of nuclear estrogen receptor:
ER exists as a _____ within the nucleus
The ligand binds the receptor, causing conformational change facilitating _________ with specific estrogen response element sequences in DNA
The ER-DNA complex recruits _______ that modify chromatin structure and SRC-1, which further alters chromatin structure by virtue of its _______ activity
Chromatin remodeling facilitates binding of other coactivators to promoter of target genes and recruit proteins that comprise the general _______ apparatus, leading to enhanced transcription to form mRNA
Monomer
Dimerization
Co-activators; HAT
Transcription
T/F; Although it is thought that ER exists as a monomer within the nucleus, some evidence suggests it can be located in the cytosol and estrogen binding causes dimerization and translocation to the nucleus
True
Antagonists like tamoxifen recruit HDACs which act on histone proteins to stabilize nucleosome structure; how do things like tamoxifen prevent transcription?
By stabilizing the nucleosome structure, they prevent interaction with the general transcription apparatus and prevent transcription
Some ERs are considered non-genomic, where are their receptors located and what type of receptors are they?
Some in PM
Some in caveolae (cholesterol enriched domains in PM)
There are GPCR and RTK type non-genomic ERs
How are non-genomic ER effects mediated?
Through metabolic changes as well as changes in gene expression
Production of gonadal hormones testosterone and estrogen are under the control of the ______ and _______
Hypothalamus; anterior pituitary
Cortisol, aldosterone, and androgens are steroid hormones synthesized in the ________
Adrenal cortex
Estrogens and progesterone are steroid hormones synthesized in _____
Ovaries
What steroid hormone is synthesized in the placenta?
Progesterone
Is histamine lipophilic or hydrophilic? What type of receptor does it bind?
Hydrophilic
Binds GPCR
Differentiate type I vs. type II nuclear receptors
Type I - reside in cytoplasm with ligand-mediated translocation to the nucleus
Type II - reside in nucleus to mediate active repression of gene expression until hormone binding comes in to change the response
Estrogen receptors bind to core inverted repeat DNA sequences using 2 _________ that are ________
Zinc-fingers; palindromes
What happens to the co-activators or co-repressors when a hormone comes along to change the gene expression?
They are degraded in proteosome after being ubiquitinated
4 types of reversible post translational modifications on nuclear receptors
Phosphorylation
Acetylation
Ubiquitination
Sumoylation
Ubiquitination, sumoylation, and acetylation typically compete for ______ residues on NRs
Lysine (K)
Phosphorylation is a PTM that typically occurs at _____ residues on NRs
S/T
