ENI - Hormones and Cell signalling Flashcards
Define autocrine signalling
Affects cell producing the hormone i.e. feeds back on self e.g. growth factors
Define paracrine signalling
Diffuses short distances to affect cells nearby e.g. neurotransmitters
Define endocrine signalling
Acts on target cells disant from site of synthesis e.g. hormones
Describe the functions of autocrine signalling
- Most effective when performed simultaneously by neighbouring cells of same type
- Encourages large groups of identical cells to make same developmental decisions
- For this to work, adjecent cells must have the receptor required in order for signal amplification to occur
What are the consequences of signalling?
- Signal to survive
- Divide
- Differentiate
- Cell death
- Carry out specialised function
Compare lipid and water soluble hormones
- Lipid: require proteins for transport, bind to intracellular receptors
- Water: move freely through blood, bind to extracellular receptors
Compare cytokines and growth factors
- Growth factors: polypeptides that promote cell growth/proliferation
- Cytokines: factors associated with blood cells
- But no absolute distinction
What is the function of growth factors and cytokines?
- Control fundamental processes
- E.g. cell division (proliferation)
- Cell differentiation
- Apoptosis
Describe lipid soluble hormones
- Not soluble in blood
- Transported in blood by carrier proteins
- Diffuse through palsma memrbae
- Bind to intracellular protein, taken to site of action
- Alters expression of genes at level of nucleus
Give examples of lipid soluble hormones
- Steroid hormones
- Androgens
- Aldosterone
Outline the steps required in order for lipid soluble horones to initiate their action
- Bind to receptors
- Induce receptor shape change
- Bind to specific portion of DNA
- Regulate transcription of that part of DNA
- Increase or decrease
Desribe lipid soluble hormone receptors
- All structurally related, part of nuclear receptor superfamily
- Bind to DNA as homo or heterodimers
- Short DNA binding domain with zinc finger
What are the regions of lipid-soluble hormone receptors?
- Transcription-activating domain
- DNA binding domain
- Hormone binding domain
Describe the activation of the lipid-hormone receptor
- In inactive state, bound to inhibitory proteins
- Binding of ligand causes inhbitory proteins to dissociate
- Coactivator proteins bind to receptor’s transcription-activating domain
- Increases gene transcription
Describe water-soluble hormones
- Easily travel in blood
- Bind to cell surface receptors
- Initiate intracellular cascade
- Results in series of intracellular events
- may be inhibitory or stimulatory
- Induce signal transduction pathways involving variety of molecules
List the intracellular signal transduction pathways triggered by water soluble hormones
- Adenyl cyclase (cyclic AMP)
- Guanyl cyclase (cyclic GMP)
- Phospholipase C, IP3 and DAG
- Tyrosine kinase
- Ion channels
What are the possible effects of water-soluble hormones binding to receptor protein?
- Cellular metabolism can change
- Proteins affecting gene expression altered
- Amount or location of cytoskeleton altered
What hormones does adenyl cyclase respond to?
- ACTH
- ADH
- FSH
- LH
- TSH
What hormones does guanylyl cyclase response to?
ANP (is rare)
What hormones does phospholipase C respond to?
Hypothalamic hormones TRH and GnRH
What hormones does tyrosine kinase respond to?
- Insulin
- Prolactin
- Growth hormone
- IGF
What hormones do ion channels respond to with respect to second messenger signalling of water-soluble hormones?
Hormones using multiple pathways
Give an overview of the intracellular signalling pathway for water soluble hormones
- Signal molecule binds to receptor protein
- Binding leads to change in shape, conveys change in structure through transmembrane domain to intracellular domain
- Triggers effects in cell
- Activates intracellular signalling pathway
- Mediated by series of signalling proteins
- One or more of these interacts with target protein
- Alters target protein so helps change behaviour of cell
What are the 3 components of membrane receptors?
- External domain (binds ligand/hormone)
- Transmembrane domain
- Cytoplasmic/intracellular domain
What domain of the membrane receptor changes shape to convey the signal?
Intracellular (through action of transmembrane domain, passing signal from extra to intracellular)
What is the effect of the conformational change of the intracellular domain of the membrane receptor?
- Stimulates signalling pathway
- Usually activation of dormant enzymatic activity, often protein kinase
- Receptor phosphorylates intracellular proteins to further signal transduction
What is the advantage of the intracellular signalling pathways?
- Different hormones act on different receptors
- Having different signalling pathways that integrate on a few points allows generation of a response that will be most advantagous i.e. one of the 4 basic outcomes
- The aim is to integrate different signals simultaneously
Describe the receptors for growth factors
- Have enzyme activity
- Tyrosine kinase
- Initiate signal that is propagated through cell by phosphorylation
Explain how the same signal can have different effects in different tissues/cells
- Bind to similar receptors
- Intracellular signals produced interpreted differently
- E.g. in heart Ach reduces rate and force of contraction, in salivary glands stimulates secretion and in skeletal muscle cell stimulates contraction
Define half life
Teh time taken for the concentration of a signalling molecule to fal by half
What factors determine the hald life of a signalling molecule?
- Rate of destruction or removal of molecules
- Turnover rate (which depends on promptness of response when signal turned on)
Compare the half life of molecules that take a long time to synthesise vs molecules that take a short time to synthesise
- Long: long half life
- Short: short half life
What are the 4 main classes of cell surface receptors?
- Ion-channel-linked receptors
- G-protein linked
- Tyrosin kinase linked
- Receptors with intrinsic enzymatic activity
What are ion-channel-linked receptors usually used for?
Rapid synaptic signalling e.g.post-synaptic membranes, neuromuscular junction, nicotinic acetylcholine receptor, GABA receptors
Describe how ion-channel-linked receptors work
- Signal molecule (NT) binds to receptor
- Opens (or closes) ion channel (change in conformation)
- Ions move into cell by passive diffusion
- Excitability of post-synaptic cell altered
Describe the structure of G-protein coupled receptors
- 7 transmembrane receptor
- Single polypeptide chain with central hydrophobic region
- Spans plasma membrane 7 times, 7 loops
Explain how G-protein linked receptors work
- When ligand binds to 7 transmembrane receptor, changes shape of intracellular loop
- Allows recruitment and activation of G protein
- This activates enzyme
What is a G protein?
Guanine nucleotide binding protein (signal transduction protein). Composed of 3 different proteins
Explain how G proteins are activated
- Ligand binding to receptor recruits G protein to loop
- GTP replaces GDP causing protein to dissociate into 2 units
- End up with 2 activated pathways: G-alpha and G-beta
Outline the types of actions G proteins can have on adenylate cyclase and how
- Can activate or inhibit
- Speciaised proteins (Stimulatory or Inhibitory) and interaction depending on which receptor is activated
Describe the process of stimulatory effects mediated by the adenylate cyclase system and give an example
- Sub-uit of G protein activates adenylate cyclase
- ATP converted to cAMP catalysed by adenylate cyclase
- cAMP activates protein kinase A
- pKa is part of cascade of intracellular phosphorylations
- Epinephrine acting through adrenergic-B-receptor
Describe signal amplification as a feature of the adenylate cyclase system
- Single hormone molecule can result in production of many molecules of cAMP
- Very efficient means of amplifying receptor hormone interation and original signal
How can G proteins be inhibitory? Give an example
- Inhibit adenylate cyclase
- Thus prevent conversion of ATP to cAMP
- E.g. norepinephrine acting through alpha-2 receptor on pre-synaptic nerve
What are the 2 types of enzyme-linked receptors?
- Function directly as enzymes
- Directly associated with enzymes they activate
Describe how the tyrosine-kinase linked receptors work
- Ligands bind, cross-linking
- Tyrosine residues on intracellular portion of receptor
- Tyrosine kinase activated when ligand binds
- Causes phosphorylation of tyrosine residues
- Phosphate groups act as docking sites for the signalling proteins to bind to
What are the different signal transduction pathways from tyrosine kinase receptors?
- Ras signal transduction pathway
- Phosphoionositide 3-kinase (PI 3-kinase) pathway
- Phospholipase C
What signalling trasnduction pathway is used by insulin?
- Tyrosine-kinase-linked receptor
- Phosphoionositide 3-kinase (PI 3-kinase) transduction pathway
What are second messengers?
Small intracellular signalling molecules, used to confer signal through cell
What are the functions of second messengers?
- Amplify signal
- Scaffolds
- relays
- Adaptors
- Modulators
How do second messengers carry out their functions?
Bind to and alter the behaviour of larger intracellular proteins
What are the key processes in cell signalling?
- Protein phosphorylation
- Reversal of protein phosphorylation
What are enzymes called that act to phosphorylate proteins?
Protein kinases
What are enzymes called that act to dephosphorylate proteins?
Phosphoprotein phosphatases
What are the 2 types of protein phosphorylation?
- Phosphorylation of serine/threonine residues
- Phosphorylation of tyrosine residues
What on the amino acids is phosphate added to in phosphorylation?
Hydroxyl group
Name the 6 classes of enzyme linked receptors
- Receptor tyrosine kinases
- Tyrosine-kinase associated receptors
- Receptor-like tyrosine phosphatases
- Receptor serine/threonine kinases
- Receptor guanyl cyclases
- Histidine-kinase-associated receptors
Compare receptor tyrosine kinases and tyrosine-kinase associated receptors
- RTKs: phosphorylate tyrosines on intracellular signalling molecules
- Associated: associate with intracellular proteins with tyrosine kinase activity
Describe how receptor-like tyrosine phosphatases work
Remove phosphate groups from tyrosine on intracellular proteins
Describe how receptor serine/threonine kinases work
Phosphorylate serines or threonines on regulatory proteins
Describe how receptor guanyl cyclases work
Catalyse production of cytosolic cGMP
Describe how histidine-kinase-associated receptors work
- 2 component system
- Kinase phosphorylates itself and passes on phosphate to intracellular signalling protein
What is the function of intracellular molecular switches?
- Required to turn proteins and signals from activate to inactive and vice versa
- Signalling pathway must receover when not needed
How do molecular switches work and what are the 2 main classes?
- Gain or loss of phosphate groups
- 2 classes: phosphorylation or GTP binding
Desribe the phosphorylation/dephosphorylation molecular switches
- Signalling protein activated by addition of phosphate and inactivated by removal or vice versa
- Kinases add, phosphatases remove
Describe the GTP binding molecular switch mechanism
- When GDP bound, protein inactive
- When signal in, GDP exchanged for GTP to activated
- When signal out, dephosphorylation of GTP and so inactivated again as GDP bound again
How can cells adjust their sensitivity to a signal?
- Cell response proportional to the signal
- Response may depend on binding of more than one intracellular molecule to a target molecule
- Modulation of receptor activity and density
Give the types of target cell desensitisation
- Receptor sequestration
- Receptor down-regulation
- Receptor inactivation
- Inactivation of signalling protein
- Production of inhibitory protein
Describe receptor sequestration
Ligand binding to cell-surface receptor induces their endocytosis and temporary storage within an endosome (sequestion)
Describe receptor down-regulation in target cell desensitisation to a target molecule
- Ligand binds to receptor, induces their endocytosis
- Receptor destroyed in lysosome
Describe the methods of desensitisation of a target cell involving phosphorylation
- 2 ways
- Either phosphorylate receptor and temprarily silence intracellular domain
- Change protein involved in transducing signal
How does the production of inhibitors desensitise target cells to signal molecules?
Blocks transduction process
What are the 3 classifications of hormones?
- Classical (genuine) hormones
- Neurohormones
- Local hormones
Describe the classical hormones
- Secreted by endocrine cells
- Diffuse into blood
- Transported by transporters via blood to target tissue
- e.g. cortisol
Describe neurohoromones
- Synthesised by neuroendocrine cells
- Secreted from nerve terminals
- Diffuse into blood vessels and transported
- E.g. Corticotropin-releasing hormone (CRH)
Describe local hormones
- Diffusion of hormone into interstitial fluid
- Paracrine function and autocrine function
What are the different types of transport proteins for lipid soluble hormones?
- Specific globulins
- Non-specific
Give examples of specific globulins
- Cortisol binding protein/globulin (CBG)
- Vitamin D binding globulin
- Thyroid binding globulin
Give examples of non-specific transport proteins
- Albumin (steroids)
- Pre-albumin (T3 and T4)
Where are transport proteins produced and degraded?
In the liver
What does total hormone concentration measure?
Bound and free (active) hormone
What is the amount of free hormone affected by?
- The binding protein concentration
- Hormone level
Give the hormone binding protein functions
- Serve as hormone reservoir
- Hormone buffers
- Reduce hormone loss
Explain how hormone binding proteins act as hormone reservoirs
- Free hormone diffuses into cells
- Reduces free concentration
- Released from bound in order to replace
- Leads to equilibrium
Explain how hormone binding proteins act as hormone buffers
- Respond to increased/decreased hormone secretion
- Only use around 50% of binding capacity in normal situations so always have empty slots
- Means that a small increase in hormone will not necessarily stimulate tissues
Explain how hormone binding proteins act to reduce hormone loss
- Prevent excretion via the kidney
- Proteins cannot pass through the glomerulus into the ultrafiltrate and so are retained
- Free hormones lost
What are the elimination mechanisms for hormones?
- Enzymatic degradation
- Within target cells after binding
- Via liver and kidney
How does elimination affect function of a hormone?
- Speed of removal affects hormone’s ability to have fast or slow action
- Fast action if removed quickly
- Slow action if removed slowly
Explain why epinephrine has a fast action
- Rapid release from adrenal medulla
- Immediate action on tissues and fast return to normal (short half life)
- Water soluble hormone, binds receptors and the enzymatically degraded by cell
Explain why thyroxine (T4) has a slow action
- Circulates as bound mostly
- Bound ressitant to degradation
- Once delivered to cell converted to T3 which enters cell and alters gene expression (which takes time)
- Or converted to water soluble compounds by liver and excreted in urine
- More steps to go through to elieict action and more steps for elimination so takes longer
What are the main factors affecting hormone responses?
- Variation by tissue
- Variation by time
- Variation by dose
- Status of target tissue
Explain how variation by tissue affects hormone responses
- Tissues respond differently to same hormone
- E.g. T4
- increases cellular metabolism. Most effect in heart and gastric mucosa, some effect in skeletal muscle and no effectin spleen and brain
- Due to lack/differing density in receptos
- Different levels of machinery in different cells alters ability and degree to which a tissue can respond
Explain how variation by time affects hormone responses
- Intital response to hormone may differ from delayed response
- Timing of response varies by species, age, specific hormone
- Older patietns respond less will to retinoids
- Hormone formualtion also affects its half life (and thus time time it is able to cause response)
Explain how variation by dose affects hormone responses
- Responses associated with hormone is for physiological range
- Hormone levels outside physiological range may elicit different responses e.g. steroid therapy
- Short term vs long term treatment
- Long term may lead to suppression of own production
Explain how status of target tissue affects hormone responses
- With overstimulation, target tissues hypertrophy
- With understimulation will atrophy
- i.e. ability and degree to which tissue can and will respond
What tests can be used to measure hormones?
- ELISA
- RIA
- Clearance rate
- Secretion rate
Explain how clearance rate can be measured
- Radioactively labelled hormone
- Or halt secretion
Explain how secretetion rate can be measured
If concentration remains stable, then assume that concentration = clearance
What is bound hormone representative of?
The amount being produced
What is free hormone representative of?
The active amount, usually very low for lipid soluble
What hormones are produced by the heart, intestine, kidney and placenta?
- Heart: atrial natriuretic peptide (ANP)
- Intestine: cholecystokinin, gastrin
- Kidney: calcitriol, EPO, renin
- Placenta: chorionic gonadotrophins
