Cell Communication: Hormones, Growth factors and Neurotransmitters

• cells can communicate with those right next to them or can communicate with targets at a distance

• communication can be through direct contact = adhesion-based mechanisms, transfer of materials through gap junctions

• or through the production of extracellular factors called signals - e.g. hormones, neurotransmitters, neuropeptides, growth factors

• this is called extracellular signaling

• these compounds exert their effects by binding to the target cells and/or entering the cell

• the ultimate goal is to affect the function of the cell

– through modifying the expression of genes/proteins

-6 steps to extracellular signaling1. synthesis of signal (hormone, NT)2. release of signal (exocytosis)3. transport of signal to target (local? distance?)4. detection of signal by target (binding to receptors)5. change in target cell function6. removal of the signal & loss of effect

-three types of extracellularchemical messengers:1. paracrines (e.g. growth factors)2. neurotransmitters3. hormones

Extracellular Signaling:

Mechanisms

growth factors

hormones, NTs, growth factors

hormones Hormones, growth factors

• most signals produced by cells within the body bind to receptors that are specific for that signal

• most receptors are found on the cell surface• although some can be found within the cell!• binding of the signal (ligand) to the receptor results in a

series of events (signal transduction) within the cell that changes the cells function– e.g. may change the transcription rate of a gene – effects protein

production

Extracellular Signaling: Mechanisms

Signaling within cells

1. Ion channel – binding of the S to the R changes the conformation of the R and allows transit of a specific ion-this allows entry of the signaling ion-responsible for the changing of membrane potentials-once the S is removed the “gate” closes

NTs

2. GPCR – binding of the signal (S) to its R activates the R-the R binds to an adjacent plasma membrane protein = adenylyl cyclase (AC) (adenylate cyclase)-AC then converts ATP to cAMP-cAMP acts as a signal within the cell = second messenger – effects cell activity-BUT some G proteins can inhibit this pathway!!!! (no AC activation, no cAMP production)

HORMONESNTs

GrowthFactors

3. Tyrosine-linked and RTKs – binding of the S to the R causes the R to dimerize (pair-up)-this activates a target (kinase)-this kinase then goes on to activate its target by phosphorylating it (adding a phosphate group)-the way the RTK and the TLK activate their targets are different but the end effect is the same

d) RTKs

Hormones: Mechanisms of Signaling• hormone producing cell = endocrine cell

– e.g. thyroid, pituitary• Autocrine signaling

– cell responds to the hormone it produces• Paracrine signaling

– local action– local hormone (paracrine hormones) act on neighboring cells– autocrines act on same cell that secreted

them• Endocrine signaling

– circulating hormones (endocrine hormones)

– act on distant targets– travel in blood

Types of Hormones

• water-soluble

• lipid -soluble

Lipid-soluble Hormones• Steroids

– lipids derived from cholesterol – made in SER– different functional groups attached to

core of structure provide uniqueness• e.g. cortisol, progesterone, estrogen,

testosterone, aldosterone

• Thyroid hormones– tyrosine ring plus attached iodines– are lipid-soluble

• Retinoic acid– lipids derived from retinol (vitamin A)– regulate proliferation, differentiation and

death of many cell types

• some vitamins can acts a lipid-soluble hormones– e.g. vitamin D

• Nitric oxide (NO)- gas

testosterone aldosterone

cortisol

• Eicosanoids– prostaglandins or leukotrienes

– derived from arachidonic acid (fatty acid)

– AA is converted either into prostaglandin H or into the leukotrienes

– conversion of AA into prostaglandins is regulated by the COX enzymes

– both act in the inflammatory reaction

• e.g. stimulate smooth muscle cells to contract

• e.g. stimulate nerve cells – pain

Lipid-soluble Hormones

• synthesis of steroid hormones from cholesterol backbone requires a series of specific enzymatic reactions that modifies the cholesterol– these enzymes are specific for each steroid made– they are located in specific cell types

• e.g. enzymes for cortisol are located specifically in the adrenal cortex

• not stored – once formed they released by diffusion through the PM into the blood– carrier proteins can be specific or some can pick up any steroid

hormone• e.g. serum albumin – indiscriminate in its steroid • the hormone becomes active once released• therefore the body keeps a balance of bound-inactive steroid

hormones and unbound hormones that rapidly enter the cell• 50% of the water soluble catecholamines are actually bound to

albumin – reason is unclear

• only cholesterol is stored in the cytoplasm

Lipid-soluble Hormones

Water-soluble Hormones• Amine, peptide and protein hormones

– modified amino acids to protein chains– serotonin, melatonin, histamine,

epinephrine, insulin, dopamine– protein hormones – comprised of one or

many polypeptide chains• insulin, glucagon

– peptide hormones – comprised of chains of amino acids

• e.g. growth hormone, oxytocin

– amine hormones – derived from the amino acids tyrosine or tryptophan

• epinephrine (tyrosine and phenylalanine), serotonin (tryptophan), dopamine (tyrosine)

• one subcategory is called the:– catecholamines: epinephrine, norepi.

and dopamine

– can also act as neurotransmitters

insulin

• peptide hormones are synthesized and secreted using the same mechanism that regulates the secretion of any other protein– made as precursors in the ER – called preprohormones

– transport to the Golgi where they are “pruned” to give rise to the active hormone

– packaged and secreted from the Golgi

– stored in the cytoplasm until needed

– secretion is triggered only by specific stimulus

Water-soluble Hormones

Action of Lipid-Soluble Hormones: Endogenous signaling

• Hormone must be carried by a transport protein that allows it to dissolve within the aqueous (watery) environment of the blood plasma

• Hormone diffuses through phospholipid bilayer & into cell

• the receptor is located within the cell (cytoplasm or the nucleus)

• binding of H to R results in its translocation into the nucleus

• the H then binds directly to specific sequences within the DNA = response elements

• this binding turns on/off specific genes – activates or inhibits gene transcription

• if turned on - new mRNA is formed & directs synthesis of new proteins

• new protein alters cell’s activity• if turned off – no new protein results

and the cell’s activity is altered

Action of Lipid-Soluble Hormones– For an animation:

http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter10/animation__mechanism_of_steroid_hormone_action__quiz_1_.html

Action of Lipid-Soluble Hormones

• some lipid-soluble hormone don’t cross the plasma membrane – too large

• therefore they bind with receptors on the cell surface and trigger signaling events within the cells– signal similar to water-soluble hormones– e.g. prostaglandins

• easily travels through the blood - hydrophilic• but cannot diffuse through plasma membrane!• therefore absolutely requires the expression of receptors

on the cell surface – integral membrane proteins that act as first messenger

• the receptor protein activates a series of signaling events within the cells

– e.g. epinephrine binds to receptor and activates an adjacent G-protein in membrane

– G-protein activates adenylate cyclase to convert ATP to cyclic AMP (cAMP) in the cytosol

– cAMP acts as a 2nd messenger– cAMP activates a series of proteins in the cytosol

called kinases– kinases act to phosphorylate their targets – either

activating them or inhibiting them– this speeds up/slows down physiological responses

within the cell– phosphodiesterase inactivates cAMP quickly

• many second messengers are made in cells in response to specific hormones

– e.g. calcium, IP3, DAG• Cell response is turned off unless new hormone

molecules arrive• this mechanism allows for amplification – one H-R

combination can activate two G proteins which activates 4 kinases which activate 16 more kinases etc…….

Action of Water-Soluble Hormones

• so the binding of a hormone to a receptor results in downstream cellular events

• either through direct activity of the receptor (activated by the ligand) or through production of a second messenger– types:

• 1. cAMP: produced by adenylyl cyclase/AC (activated by hormone G protein interaction)

• 2. calcium– -IP3 & DAG

Action of Water-Soluble Hormones

Action of Water-Soluble Hormones

-animations: go to you tube and search“G protein animation” OR-http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter10/animation__second_messenger__camp.html

• cell expresses numerous type of G proteins that interact with the GPCRs– some activate adenylyl cyclase and stimulate

production of cAMP – Gs (G stimulatory)– others inhibit AC – Gi (G inhibitory

Gs protein Adenylyl cyclase

cAMP Second Messenger systems• best studied system: binding of epinephrine to the b2-adrenergic receptor• activates the Gs protein and produces cAMP

– Gs protein is comprised of three subunits– the active subunit is the alpha subunit– however the beta and gamma subunits have signaling roles also– note the Gs subunit cycles between GTP and GDP bound states – called a GTPase protein– the cycling between GTP and GDP helps control its function– ANIMATION: http://www.youtube.com/watch?v=NMeBZlbs2dU

• the ability to bind and hydrolyze GTP determine the function of the Gs subunit

• also the site at which bacterial toxins can affect this signaling path• the hydrolysis of the GTP on the Gs protein is catalysed by the Gs

protein itself

cAMP Second Messenger systems

• cholera stimulates the addition of an ADP onto the Gs protein (takes it from intracellular NAD+)

• the activity of AC is modified also by interactions with the Gi protein• therefore the cell can modify its level of cAMP made by stimulating the GPCRs that

activate either Gs or Gi proteins• the alpha subunit of the Gi protein (Gi) also interacts with AC (at a different location)• this Gi protein is also an GTPase and requires the binding of GTP to become active

and inhibit AC – once GTP is hydrolyzed the protein dissociates the AC inhibition is relieved

– this is the basis of pertussis – the pertussis toxin prevents the hydrolysis of GTP bound to the Gi – leads to prolonged inhibition of AC and drops in intracellular cAMP levels – inhibits cell signaling

cAMP Second Messenger systems

• cAMP phosphorylates a class of kinases called cAMP-dependent protein kinases (PKAs)

• the cell has multiple isoforms of PKAs• the PKA then phosphorylates another downstream

kinase as its target• these kinases can vary from cell type to cell type and

also vary according to the upstream ligand– epinephrine binding activates PKA - activates GPK

(glycogen phophorylase kinase)– insulin activates PKA which then activates acetyl CoA

carboxylase and then pyruvate dehydrogenase

cAMP Second Messenger systems

Kinase cascades permit multienzyme regulation and amplify hormone signals

IP3 and DAG – calcium second messengers

• most intracellular calcium stores are sequestered in the ER or other vesicles

• RTK or GPCR pathways trigger the activation of phospholipase C in the PM

– e.g. hormone-GPCR binding triggers activation of a Gq protein which then activates phospholipase C

• results in production of IP3 and DAG– IP3 diffuses through the cytoplasm

and activates Ca channels within the PM or within the ER to release or allow entry of calcium within the cytoplasm

– increased cytoplasmic calcium activates a class of calcium-dependent kinases called PKCs (protein kinase C) – role for DAG in this step

Phosphorylation of substrates

http://bcs.whfreeman.com/lodish5e/content/cat_010/13010-01.htm?v=chapter&i=13010.01&s=13000&n=00010&o

Water soluble hormones and RTKs

• bind to protein/peptide classes of hormones

– e.g. insulin– e.g. growth factors – EGF,

NGF, bFGF, PDGF• H binding leads to

dimerization of the RTK and activation of the kinase activity endogenous to the receptor

• this activity phosphorylates its target and initiates the downstream signaling cascade

• major initiating protein is called Ras (GTPase)

• activation of the RTK leads to binding of the GTP-bound form of Ras

• this activated Ras than activates multiple downsteam paths

• the major one is called the MAPK pathway

• hormones can utilize more than one receptor and more than one pathway to activate the same target

– e.g. can bind and activate both GPCRs and RTKs

• provides the body with flexibility in its choice of hormone

• also allows two hormones to combine to increase the strength of an event

• or allows one hormone to decrease the cells response while the other hormone is trying to increase it

Water-soluble Hormone Signaling: Mechanisms