1. 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 OR 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

2. Cell Communication: Hormones, Growth factors and Neurotransmitters
-6 steps to extracellular signaling:
synthesis of signal (hormone, NT)
release of signal (exocytosis)
transport of signal to target (local? distance?)
detection of signal by target (binding to receptors)
change in target cell function
removal of the signal & loss of effect
-three types of extracellular
chemical messengers:
paracrines (e.g. growth factors)
neurotransmitters
hormones

3. Extracellular Signaling: Mechanisms
growth factors
hormones, NTs, growth factors
hormones

4. Extracellular Signaling: Mechanisms
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

5. Extracellular Signaling: Mechanisms
NTs
Ion channel – ion channel (transmembrane protein) acts as a receptor for a signal (ligand)
- binding of the ligand to the ion channel changes the conformation of the receptor and allows transit of a specific ion through it
-this allows entry of the signaling ion
-responsible for the changing of membrane potentials
-once the signal is removed the channel closes -many ion channels open and close through a portion of the
protein that acts like a “gate”

6. e.g. most common - adenylyl cyclase or adenylate cyclase (AC)
HORMONES
Neurotransmitters
2. G-protein Coupled Receptor (GPCR) – binding of the signal (S) to its Receptor activates the R
-the R binds to and activates an adjacent plasma membrane protein = called a G protein
-the G protein effects the activity of another plasma membrane protein called an Effector (E)
e.g. most common - adenylyl cyclase or adenylate cyclase (AC)
-Effector then creates a second message within the cell
e.g. AC converts ATP to cAMP (cyclic AMP)
-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)
5 players in this mechanism:
Hormone (1st messanger)
Receptor = GPCR
G protein (stimulatory or inhibitory)
Effector – adenylate cyclase, PLC
Second messanger = cAMP, calcium

7. 3. Receptor Tyrosine-linked kinases (RTKs)
d) Receptor Tyrosine-linked Kinases = Growth factors
GF binding
Receptor dimerization
& activation
Activation of kinases
3. Receptor Tyrosine-linked kinases (RTKs)
– binding of the signal to the Receptor causes the R to dimerize (pair-up)
-the R activates enzymes called kinases
-an activated kinase then goes on to activate another target by phosphorylating it (adding a phosphate group)
KINASE
CASCADE
Growth
Factors
KINASE = protein that phosphorylates a Target protein
-binds and breaks down ATP into ADP + Pi
-kinase takes this Pi and attaches it to its
target
-this activates the target
-most of the time the target is another kinase
-this second kinase phosphorylates its target = kinase cascade results

8. Hormones: Mechanisms of Signaling
hormone producing cell = endocrine cell
can use three mechanisms
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

9. Types of Hormones
water-soluble
lipid -soluble

10. 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
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

11. Lipid-soluble Hormones
Eicosanoids
prostaglandins or leukotrienes
derived from the fatty acid called arachidonic acid
arachidonic acid is used as a starting point for making either prostaglandins or leukotrienes
both act in the inflammatory reaction
e.g. stimulate smooth muscle cells to contract
e.g. stimulate nerve cells – pain
conversion of arachidonic acid into prostaglandins is regulated by the COX enzymes
these enzymes are targeted by anti-inflammatories like aspirin, ibuprofen, Vioxx

12. 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
not stored – once formed they released by diffusion through into the blood
require binding to a carrier protein to be transported in the blood
carrier proteins can be specific or some can pick up any steroid hormone
only cholesterol is stored in the cytoplasm

13. Water-soluble Hormones
Amine, peptide and protein hormones
range from 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 and norepinephrine(tyrosine), serotonin (tryptophan), dopamine (tyrosine)
one subcategory is called the catecholamines: epinephrine, norepi. and dopamine
can also act as neurotransmitters
insulin

14. Water-soluble Hormones
water-soluble 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

15. Action of Lipid-Soluble Hormones: Endogenous signaling
Lipid-soluble hormone must be carried by a carrier/transport protein that allows it to dissolve within the aqueous environment of the blood plasma
Hormone diffuses through phospholipid bilayer & into target cell
the Receptor is located within the cell (cytoplasm or the nucleus)
binding of H to R in the cytoplasm results in its translocation into the nucleus
the H then binds directly to specific sequences within the DNA = response elements
this binding turns on specific genes – activates or inhibits gene transcription
new mRNA is formed & directs synthesis of new proteins
new protein alters cell’s activity

16. Action of Lipid-Soluble Hormones
For an animation:

17. 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 and leukotrienes

18. Action of Water-Soluble Hormones
easily travels through the blood - hydrophilic
but cannot diffuse through plasma membrane!
therefore they require the expression of Receptors on the cell surface – integral membrane proteins
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
protein cascade results

19. cAMP activates its target = kinase
kinases act to phosphorylate their targets
kinase cascade results
physiological response occurs within the cell
phosphodiesterase inactivates cAMP quickly
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…….

20. Kinase cascades amplify hormone signals

21. Action of Water-Soluble Hormones
-animations: go to you tube and search
“G protein animation” OR -

23. 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 of second messengers:
1. cAMP: produced by adenylate cyclase/AC (activated by hormone G protein interaction)
2. calcium
-IP3 & DAG

24. G proteins
cell expresses numerous type of G proteins that interact with the GPCRs
some activate adenylate cyclase and stimulate production of cAMP – Gs (G stimulatory)
others inhibit AC – Gi (G inhibitory)
Gs protein
Adenylate cyclase

25. cAMP Second Messenger systems
best studied system: binding of epinephrine to the b2-adrenergic receptor
activates a Gs protein and produces cAMP
Gs protein is comprised of three subunits – alpha, beta, gamma
the active subunit is the alpha subunit
however the beta and gamma subunits have signaling roles also
note the Gsa subunit cycles between GTP and GDP bound states – called a GTPase protein
the cycling between GTP and GDP helps control its function
ANIMATION:

26. cAMP Second Messenger systems
the ability to bind and hydrolyze GTP determines the function of the Gsa subunit
also the site at which bacterial toxins can affect this signaling path
the hydrolysis of the GTP on the Gsa protein is catalysed by the Gsa protein itself
cholera “locks” the Gs alpha subunit into its GTP-bound “ON” state
persistent activation of adenylate cyclase
severe disruption of water absorption by GI tract
can be fatal within hours

27. cAMP Second Messenger systems
the activity of AC can be modified also by interactions with a 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 (Gia) also interacts with AC (at a different location)
prevents its activity – no second messenger made
this Gia 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

28. 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

29. different effects of cAMP in different cells is due to the kinase cascade that PKA starts

30. IP3 and DAG – Calcium as a second messenger
Phosphorylation of substrates
most intracellular calcium stores are sequestered in the ER or other vesicles
RTK or GPCR pathways trigger the activation of Phospholipase C/PLC – located in the plasma membrane
PLC breaks down a phospholipid called PIP2
results in production of inositol triphosphate/IP3 and diacylglycerol/DAG
IP3 diffuses through the cytoplasm and activates Ca2+ channels on the ER - release of calcium within the cytoplasm
IP3 can also open these channels in the PM and allow Ca2+ to diffuse in
increased cytoplasmic calcium activates a class of calcium-dependent kinases called PKCs (protein kinase C) – role for DAG in this step
PKCs then activate kinase cascades

31. Water soluble hormones and RTKs
bind to protein/peptide classes of hormones and growth factors
Receptor is called a receptor-tyrosine kinase/RTK
signal binding leads to dimerization of the RTK
this activates the RTK
activated RTK binds and activates several “adaptor proteins”
adaptor protein’s job is to activate Ras (GTPase)
this activated Ras then activates multiple downstream kinase cascades
the major one is called the MAPK pathway
Growth factors
Peptide hormones

32. Water-soluble Hormone Signaling: Mechanisms
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
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