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Mechanisms of hormone action
Hormones are chemical messengers synthesized byorganisms that initiate biological responses by bindingwith high affinity and specificity to target cell receptorswithin the same individual
Endogenous substance
High affinity and specificity of binding to specificreceptors on target cells
Initiates biological response
Function of hormones
HOMEOSTASIS
Reproduction
Growth and development
Maintenance of internal environment
Production, utilization and storage of energy
Life of a hormone
Hormone transport
protein
Chemical natureof hormones
Can be divided into 3groups
Amino acid derivatives
Peptide hormones
Lipid derivatives
Amino acid derivatives
Derivatives of tyrosine
Catecholamines (epinephrine, dopamine)
Thyroid hormones (dipeptides) Tryptophan derivative
Melatonin
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Peptide hormones
Glycoproteins from anterior pituitary
thyroid-stimulating hormone (TSH)
luteinizing hormone (LH)
follicle-stimulating hormone (FSH)
Peptides and small proteins
Digestivetract hormones
Pituitary hormones
Pancreatic hormones
2 classes of lipid derived hormones
Steroid hormones:
derived from cholesterol
2 groups
with the intact steroid ring (adrenal and gonadalsteroids)
with the steroid ring cleaved (metabolites of vit D)
Eicosanoids:
derived fromarachidonic acid
Catecholamines
Molecules with catechol group
Hormonal regulators
Dopamine in hypothalamusinhibits prolactin secretion
Epinephrine (adrenaline) stressreaction
Synthesized from aa phenylalanineor tyrosine in enzymatic reactions
Synthesis of catecholamines
Synthesized in cytosol
Packaged in vesicles andexocytosed
Water soluble, do not needtransport proteins
Work from the outside fromthe cell (bind to surfacereceptors)
Synthesis of melatonin (indoleamine)
Tryptophan to serotonin
NAT (N-acetyltransferase)is activated only in the dark
Works on surfacereceptors
Peptide hormone synthesis
Synthesized on the ribosomes attached to rough ER
All of them have ER targeting sequence on the N-terminus (signal peptide)
Synthesized as prohormones
Inactive molecules converted to active hormones inER and Golgi, sometimes after secretion
Exocytosed from the cell
Work from the outside from the cell (bind to surfacereceptors)
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Splice variants
Alternative processing of mRNA can result in splicevariants of the same hormone
Availability of transcription factors can affect hormoneproduction
Cell biology vs. endocrinology
Preprohormone full sequence of the peptide
Prohormone peptide minus signal sequence
Can (and usually does) undergoes additional proteolyticcleavage in Golgi
Hormone convertase
Hormone biologicallyactive product
Posttranslational processing
Signal peptide removal
Folding (ER)
Formation of disulfide bonds
Glycosylation (ER)
Posttranslational processing
Clevage (Golgi)
Sometimes multiple copies or even differenthormones are produced from the same prehormone
ProTRH 6 repeats in humans
5 repeats in rats
Peptide homology
Neurohypophyseal hormones
Peptide homology
Glycoproteins of anterior pituitary
Alpha subunit identical in all 3 TSH, LH and FSH
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Shape matters
Insulin and IGF (insulin like growth factor), a realgrowth hormone
Peptide hormone transport
Usually water soluble
Transported in plasma - require no specific carriermechanisms
Signaling process
Recognition of signal
Receptors
Transduction
Change of external signal into intracellular message
Effect
Modification of cell behavior
Hormone receptors
Molecules within or on the surface of target cells that bindhormones with high affinity and specificity and therebyinitiate and mediate biological responses
Hormones will only produce the response in cells thatexpress the receptors for this particular hormone (targetcells)
ONLY target cellsrespond to hormone
Cells that do not havereceptors for the hormone
ignore the hormone
Properties of the hormone-receptorinteractions
Tissue specificity - each
organ has a unique set
of hormone receptors
Peptide and amine receptors
Surface receptors a.k.a transmembrane receptors
Peptides and amines cannot cross the membrane
When activated, a receptor on the surface passes thesignal to intracellular second messengers or directly tocellular effectors to produce biological response
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Many families of cell surface receptors
Based on homology and signaling strategy
The same ligand can bind to two or more differentfamilies!!!
Multiple splice variants (1 and 2 adrenergic receptors)can be tissue specific
G protein coupled receptors
Use G-proteins as molecular switch to turn on enzymesproducing intracellular second messengers
G protein coupled receptors = GPCR
Ligand binding to the receptor activates a signaltransduction cascade that comprises
G protein molecular switch
Enzyme that produces second messengers
Second messengers
Target protein - effector
But not necessary all steps are involved!!!!
Molecular properties of G protein coupledreceptors
A.k.a. serpentine receptors
Seven transmembrane regions of 22-24 hydrophobicresidues
N-terminus faces outside (ligand binding domain)
C-terminus faces cytosol
A cytosolic loop betweenhelices 5 and 6 is the placefor interaction withG protein
G proteins
Membrane bound heterotrimeric proteins consisting of 3subunits , ,
Coupled to surface receptors
Molecular switches
Use the exchange and hydrolysis of nucleotides(GTP/GDP) to transduce the signal from the surfacereceptors to intracellular effectors
G protein cycle
When G protein is inactive it is bound to GDP and existsas a trimer
The exchange of GDP for GTP activates G protein
G protein dissociates into two subunits: and dimer
GTP is bound to subunit
Subunit has an intrinsic GTPase activity and hydrolysesGTP to GDP
This process terminates the signal
and reassociate
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What the G proteins can do?
Activate enzymes to produce second messengers
Activate transcription factors
Modulate ion channels, pumps and exchangers
Affect cytoskeleton
Modulate enzymes
Adrenaline signaling
Amplification of signal by secondmessengers
Differential regulation of adenylatecyclase
Activated by Gs
Inhibited by Gi
Activated CREB binds to CREsequence and stimulatestranscription
CREB needs to bephosphorylated atserine 133
Only genes that have CRE sequenceare activated by those receptors
Regulation of transcription by cAMPkinase
CREB links cAMP signals to transcription
Only genes that have CRE sequence in front ofthem are activated by these receptors
CREB needs to be phosphorylated at serine 133
Interacts with a co-activator CBP/P300 Activated CREB binds to CRE sequence
CBP/P300 links CREB to transcription factors
and stimulates transcription
Second messengers
cAMP is not the only second messenger initiatedby GPCRs
IP3 (inositol 1,4,5 trisphosphate) and DAG
(diacylglycerol), are the second messengers for Gproteins from the Gq family
They are made by phospholipase C (PLC) that breaksphoshatidylinositol 4,5 bisphosphate (PIP2) to IP3and DAG
Several other second messenger are derived frommembrane lipids
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PIP2 breakdown
IP3 increases intracellular calcium levels via the releasefrom intracellular stores
DAG activates protein kinase C (PKC)
IP3 as a second messenger
Calcium is also intracellular secondmessenger
Regulation of hormonesecretion
Regulation of transcriptionthrough Ca- calmodulinkinase
Protein kinase C (PKC) signaling
Serine/threonine kinase
Activated by DAG
Phosphorylates various cellular effectors
Activates transcription factors AP-1 (c-fos and c-jun areboth protooncogenes)
Other lipidmessengers Ceramide signaling
Product of sphingomyelin cycle
Sphingomyelins do not have glycerol backbone
Second messenger in TNF- signaling and stimulation ofapoptosis
Increase in prostaglandin biosynthesis
Activation of transcription factor NF b
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Catalytic receptors with intrinsicenzymatic activity
Ligand binding causes activation of enzymatic activity ofthe receptor (receptor itself is an enzyme)
Tyrosine kinase
Guanylyl cyclase
Phosphatase
Modification of cellular activity
Receptor tyrosine kinases
Ligand binding causes dimerization of the receptor
This activates enzymatic activity of kinase domain andphosphorylation of theother subunit
Phosphorylated tyrosineis recognized by moleculeswith SH2 domain that willpropagate the signal toother cellular effectors
RTKs
Most RTK are monomers when notcrosslinked by ligands
Insulin receptor stays as a dimer butligand binding is necessary forphosphorylation
Signaling by RTK
Activation of enzymes
Activation of Mitogen Activated Protein Kinase pathways(MAPK pathways)
Enzymes activated by RTKHow do RTKs activate MAPK pathways andaffect transcription?
Phosphotyrosine residues on the kinase interact withadapter proteins
Transmit a signal to Ras, a monomeric G protein
(molecular switch) Ras passes the signal to downstream components
Most often - Mitogen Activated Protein Kinase pathways(MAPK pathways)
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MAP kinase regulates the activity oftranscription factors Signaling strategies
Receptors that are linked to cytoplasmic enzymes
Cytokine receptors (tyrosine kinase-linked)
Have the capacity to activate cytosolic tyrosine kinases
Receptor itself lacks kinase activity
Activated kinasephosphorylatescellular substrates
Tyrosine kinase-linked receptors
Have the capacity to activate cytosolic tyrosine kinases
Ligand binding causes dimerization of the receptor
Activation of cytosolic tyrosine kinase
Receptor itself lacks kinase activity
Activated kinase phosphorylates cellular substrates second messengers
Receptors that activate intracellulartyrosine kinases
Tyrosine kinase-linked receptors
Signal to nucleus through the JAK-STAT pathway (signaltransducers and activators of transcription)
Signaling by members of the cytokinereceptor family
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Ion channel receptors
Ligand gated ion channels
Binding of a ligand changes the conformation of thereceptor and opens channel pore
Ions move through the pore
Results in changes of the cell excitability
Termination of signaling
Binding a ligand activates the endocytosis of thereceptors
In endosomes liganddissociates from the receptorbased on the pH gradient
Receptors got recycledback to the membrane
Cellular mechanisms of steroidhormone action
Steroid hormones
Synthesized from cholesterol in enzymatic reactions incytosol
Lipid soluble
Bind to intracellular receptors
Synthesis of steroid hormones
Will be discussed later when we talk about adrenals
Steroid hormone transport
Lipid soluble hormones require transport proteins
albumin and transthyretin (prealbumin)
specific transport molecules (thyroxine-bindingglobulin)
only unbound hormone can enter the cell !!!
Steroid and thyroid hormones are 99% attached tospecial transport proteins
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Intracellular receptors
Receptors for hormones that are able to enter the cell
Ligand activated transcription factors
Localized in nucleus or cytoplasm
Structure of nuclear receptors
Superfamily of ligand-activated transcription factors
Bind to specific DNA sequences as dimers
Similar structure and high homology
Two highly conserved regions
Domain structure of nuclear receptors
Highest homology region
Mechanism of action of nuclear receptors
In the absence ofhormone the DNA bindingdomain is bound tochaperones (mostly hspfamily)
Binding of a hormonecauses dissociation of hspfrom a receptor andexposure of DNA bindingdomain
Hormones that bind to intracellularreceptors
All hormones that can cross the membrane
Small hydrophobic molecules
Steroid hormones Thyroid hormones
1,25-dihydroxycholecalciferol
Retinoic acid
Hormones that bind to intracellularreceptors
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Steroid hormone action
http://www.maxanim.com/biochemistry/Steroid%20Hormone/Steroid%20Hormone.swf
Lipid soluble hormones effects
Induce transcription and translation
Alter transcription of specific genes
Exert mostly long term effects - growth anddifferentiation, new protein synthesis
Regulation of gene transcription
When not bound to the hormone receptors stay boundto chaperones (mostly hsp family)
Binding of a hormone causes dissociation of hsp from areceptor and eexposure of zinc fingers
Activated receptors bind to DNA
Interact predominantly with specific genomic sequences- hormone responsive elements (HRE)
Localized in the 5 flanking regions of target genes
Regulation of gene expression byhomodimer receptors
Recruitment of a co-activator complex
Stabilization of preinitiation complex at TATA box
Binding of TFIIB
Binding ofpolymerase
Regulation of gene expression byheterodimer receptors
Regulation of gene expression bycytoplasmic receptors
Glucocorticoid receptors are localized in the cytoplams
In the absence of hormone cytoplasmic receptor isbound to hsp90
Ligand binding displaces hsp90 complex
Receptor ligand complex translocates to the nucleusand binds to Hormone Response Element on DNA
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Regulation of gene expression byglucocorticoid receptor
Eicosanoids
Derivatives of arachidonic acid
2 groups prostaglandins and leukotrienes
Prostaglandins are produced by COX enzyme (Coxinhibitors are NSID)
Important in coordinating tissue responses to injury ordisease
Are important paracrine factors