mechanisms of hormone actionhsh zfdhshsb

7/28/2019 Mechanisms of Hormone Actionhsh zfdhshsb

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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

mechanisms of hormone actionhsh zfdhshsb

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