16
Cell Signaling
16 Cell Signaling
• Signaling Molecules and Their Receptors
• G Proteins and Cyclic AMP Signaling
• Tyrosine Kinases and Signaling by MAP
Kinase, PI 3-Kinase, and Phospholipase
C/Calcium Pathways
• Receptors Coupled to Transcription Factors
• Signaling Dynamics and Networks
Introduction
All cells receive and respond to signals
from their environment.
Bacteria and unicellular eukaryotes
respond to environmental signals and to
signaling molecules secreted by other
cells for mating and other
communication.
Introduction
In multicellular organisms, cell–cell
communication is highly sophisticated.
Each cell must be carefully regulated to
meet the needs of the whole organism.
A variety of signaling molecules are
secreted or expressed on the surface of
one cell, and bind to receptors
expressed by other cells.
Introduction
Binding of signal molecules to receptors
initiates a series of reactions that
regulate all aspects of cell behavior.
Many cancers arise from problems in
signaling pathways that control normal
cell proliferation.
Much of our understanding of cell
signaling has come from the study of
cancer cells.
Signaling Molecules and Their Receptors
Signaling molecules range in complexity
from simple gases to proteins.
Some carry signals over long distances;
others act locally.
They also differ in modes of action:
Some cross the plasma membrane and
bind to intracellular receptors; others
bind to receptors on the cell surface.
Signaling Molecules and Their Receptors
Modes of cell signaling include:
• Direct cell–cell signaling—direct
interaction of a cell with its neighbor,
(e.g., via integrins and cadherins).
• Signaling by secreted molecules—
three categories are based on the
distance over which signals are
transmitted.
Figure 16.1 Modes of cell–cell signaling (Part 1)
Signaling Molecules and Their Receptors
Endocrine signaling
Signaling molecules (hormones) are
secreted by specialized endocrine cells
and carried through the circulation to
target cells at distant body sites.
Example: estrogen
Figure 16.1 Modes of cell–cell signaling (Part 2)
Signaling Molecules and Their Receptors
Paracrine signaling
Molecules released by one cell act on
neighboring target cells.
Example: neurotransmitters
Figure 16.1 Modes of cell–cell signaling (Part 3)
Signaling Molecules and Their Receptors
Autocrine signaling
Cells respond to signaling molecules
that they themselves produce.
Example: T lymphocytes respond to
antigens by making a growth factor that
drives their own proliferation, thereby
amplifying the immune response.
Figure 16.1 Modes of cell–cell signaling (Part 4)
Signaling Molecules and Their Receptors
Abnormal autocrine signaling often
contributes to cancer.
A cancer cell produces a growth factor to
which it also responds, thereby
continuously driving its own
unregulated proliferation.
Signaling Molecules and Their Receptors
Receptors may be located on the cell
surface or inside the cell.
Intracellular receptors respond to small
hydrophobic molecules that can diffuse
across the plasma membrane.
Examples: Steroid hormones, thyroid
hormone, vitamin D3, and retinoic acid.
Signaling Molecules and Their Receptors
Steroid hormones are synthesized from
cholesterol:
• Testosterone, estrogen, and
progesterone are the sex steroids,
produced by the gonads.
Figure 16.2 Structure of steroid hormones, thyroid hormone, vitamin D3, and retinoic acid
Signaling Molecules and Their Receptors
Corticosteroids from the adrenal gland:
• Glucocorticoids—stimulate
production of glucose.
• Mineralocorticoids—act on the
kidneys to regulate salt and water
balance.
Signaling Molecules and Their Receptors
• Ecdysone is an insect hormone that
triggers metamorphosis of larvae to
adults.
• Brassinosteroids are plant steroid
hormones that control several
processes, including cell growth and
differentiation.
Signaling Molecules and Their Receptors
Thyroid hormone: synthesized from
tyrosine in the thyroid gland; important
in development and metabolism.
Vitamin D3 regulates Ca2+ metabolism
and bone growth.
Retinoic acid and retinoids:
synthesized from vitamin A; important
in vertebrate development.
Signaling Molecules and Their Receptors
Receptors for these molecules are
members of the nuclear receptor
superfamily.
They are transcription factors that have
domains for ligand binding, DNA
binding, and transcriptional activation.
The steroid hormones and related
molecules directly regulate gene
expression.
Signaling Molecules and Their Receptors
Ligand binding has different effects on
different receptors.
Some nuclear receptors are inactive in the
absence of hormone:
• Glucocorticoid receptor is bound to
Hsp90 chaperones in the absence of
hormone.
• Glucocorticoid binding displaces Hsp90
and leads to binding of regulatory DNA
sequences.
Figure 16.3 Glucocorticoid action
Signaling Molecules and Their Receptors
Hormone binding can alter the activity of
a receptor:
• In the absence of hormone, thyroid
hormone receptor is associated with
a corepressor complex and
represses transcription of target
genes.
• Hormone binding results in activation
of transcription.
Figure 16.4 Gene regulation by the thyroid hormone receptor
Signaling Molecules and Their Receptors
Nitric oxide (NO) is a paracrine signaling
molecule in the nervous, immune, and
circulatory systems.
It can cross the plasma membrane and
alter the activity of enzymes.
NO is synthesized from arginine. Its
action is local, because it is extremely
unstable, with a half-life of only a few
seconds.
Figure 16.5 Synthesis of nitric oxide
Signaling Molecules and Their Receptors
The main target of NO is guanylyl cyclase.
NO binding stimulates synthesis of cyclic
GMP (a second messenger).
A second messenger is a molecule that
relays a signal from a receptor to a target
inside the cell.
Signaling Molecules and Their Receptors
NO can signal dilation of blood vessels:
Neurotransmitters act on endothelial cells
to stimulate NO synthesis.
NO diffuses to smooth muscle cells and
stimulates cGMP production. cGMP
induces muscle cell relaxation and blood
vessel dilation.
Signaling Molecules and Their Receptors
Carbon monoxide (CO), also functions
as a signaling molecule in the nervous
system.
It is related to NO and acts similarly as a
neurotransmitter and mediator of blood
vessel dilation.
Signaling Molecules and Their Receptors
Neurotransmitters carry signals
between neurons or from neurons to
other cells.
They are released when an action
potential arrives at the end of a neuron.
The neurotransmitters then diffuse
across the synaptic cleft and bind to
receptors on the target cell surface.
Figure 16.6 Structure of representative neurotransmitters
Signaling Molecules and Their Receptors
Because neurotransmitters are
hydrophilic; they can’t cross plasma
membranes and must bind to cell
surface receptors.
Many neurotransmitter receptors are
ligand-gated ion channels.
Neurotransmitter binding opens the
channels.
Signaling Molecules and Their Receptors
Other neurotransmitter receptors are
coupled to G proteins—a major group
of signaling molecules that link cell
surface receptors to intracellular
responses.
Signaling Molecules and Their Receptors
Peptide signaling molecules include
peptide hormones, neuropeptides, and
polypeptide growth factors.
Peptide hormones include insulin,
glucagon, and pituitary gland hormones
(e.g., growth hormone, folliclestimulating hormone, prolactin).
Table 16.1 Representative Peptide Hormones, Neuropeptides, and Polypeptide Growth Factors
Signaling Molecules and Their Receptors
Neuropeptides are secreted by some
neurons.
Enkephalins and endorphins act as
neurotransmitters and as
neurohormones—natural analgesics
that decrease pain responses; they bind
to the same receptors on brain cells as
morphine does.
Signaling Molecules and Their Receptors
Nerve growth factor (NGF) is a
member of the neurotrophin family
that regulates development and
survival of neurons.
Epidermal growth factor (EGF)
stimulates cell proliferation. It is the
prototype for the study of growth
factors.
Figure 16.7 Structure of epidermal growth factor (EGF)
Signaling Molecules and Their Receptors
Platelet-derived growth factor (PDGF)
is stored in blood platelets and
released during blood clotting at the
site of a wound.
It stimulates proliferation of fibroblasts,
contributing to regrowth of the
damaged tissue.
Signaling Molecules and Their Receptors
Cytokines regulate development and
differentiation of blood cells and
activities of lymphocytes during the
immune response.
Membrane-anchored growth factors
remain with the plasma membrane and
function as signaling molecules in
direct cell–cell interactions.
Signaling Molecules and Their Receptors
Peptide hormones, neuropeptides, and
growth factors can’t cross the plasma
membranes of target cells, so they act
by binding to cell surface receptors.
Abnormalities in growth factor signaling
are the basis for many diseases,
including many cancers.
Signaling Molecules and Their Receptors
Eicosanoids: lipid signaling molecules
that include prostaglandins,
prostacyclin, thromboxanes, and
leukotrienes.
They break down rapidly, acting in
autocrine or paracrine pathways.
Figure 16.8 Synthesis and structure of eicosanoids
Signaling Molecules and Their Receptors
Eicosanoids are synthesized from
arachidonic acid.
Arachidonic acid is converted to
prostaglandin H2, catalyzed by
cyclooxygenase.
This enzyme is the target of aspirin and
other nonsteroidal anti-inflammatory
drugs (NSAIDs).
Signaling Molecules and Their Receptors
Inhibiting synthesis of the prostaglandins
reduces inflammation and pain.
By inhibiting synthesis of thromboxane,
aspirin reduces platelet aggregation
and blood clotting; thus, small daily
doses of aspirin are often prescribed
for prevention of strokes.
Signaling Molecules and Their Receptors
Aspirin and NSAIDs have also been
found to reduce the frequency of colon
cancer, apparently by inhibiting
synthesis of prostaglandins that
stimulate cell proliferation.
Signaling Molecules and Their Receptors
There are two forms of cyclooxygenase:
• COX-1 results in normal production
of prostaglandins.
• COX-2 results in increased
prostaglandin production associated
with inflammation and disease.
Some drugs selectively inhibit COX-2.
Signaling Molecules and Their Receptors
Plant hormones:
• Gibberellins—stem elongation
• Auxins—cell elongation
• Ethylene—fruit ripening
• Cytokinins—cell division
• Abscisic acid—onset of dormancy
Figure 16.9 Plant hormones
Signaling Molecules and Their Receptors
Auxins induce plant cell elongation by
weakening the cell wall. They also
regulate aspects of plant development,
including cell division and differentiation.
The other plant hormones likewise have
multiple effects.
Newly identified plant hormones include
nitric oxide and brassinosteroids.
Signaling Molecules and Their Receptors
Signaling pathways of some plant
hormones use mechanisms similar to
those in animal cells.
Others pathways are unique to plants.
Auxin controls gene expression by
binding to and activating a receptor
associated with a ubiquitin ligase.
Figure 16.10 Auxin signaling
G Proteins and Cyclic AMP Signaling
Most ligands responsible for cell–cell
signaling bind to surface receptors on
target cells.
Intracellular signal transduction: The
surface receptors regulate intracellular
enzymes, which then transmit signals
from the receptor to a series of
additional intracellular targets.
G Proteins and Cyclic AMP Signaling
The targets of signaling pathways
frequently include transcription factors.
Ligand binding to a receptor initiates a
chain of intracellular reactions,
ultimately reaching the nucleus and
altering gene expression.
G Proteins and Cyclic AMP Signaling
G protein-coupled receptors are the
largest family of cell surface receptors.
Signals are transmitted via guanine
nucleotide-binding proteins (G proteins).
The receptors have seven membranespanning α helices.
Figure 16.11 Structure of a G protein-coupled receptor
G Proteins and Cyclic AMP Signaling
Binding of a ligand induces a
conformational change that allows the
cytosolic domain to activate a G protein
on the inner face of the plasma
membrane.
The activated G protein then dissociates
from the receptor and carries the signal
to an intracellular target.
G Proteins and Cyclic AMP Signaling
G proteins were discovered during
studies of cyclic AMP (cAMP), a
second messenger that mediates
responses to many hormones.
A G protein is an intermediary in
adenylyl cyclase activation, which
synthesizes cAMP.
Figure 16.12 Hormonal activation of adenylyl cyclase
G Proteins and Cyclic AMP Signaling
G proteins have three subunits
designated α, β, and γ.
They are called heterotrimeric G
proteins to distinguish them from other
guanine nucleotide-binding proteins,
such as the Ras proteins.
G Proteins and Cyclic AMP Signaling
The α subunit binds guanine, which
regulates G protein activity.
In the inactive state, α is bound to GDP
in a complex with β and γ.
Homone binding to the receptor causes
exchange of GTP for GDP. The α and
βγ complex then dissociate from the
receptor and interact with their targets.
Figure 16.13 Regulation of G proteins
G Proteins and Cyclic AMP Signaling
A large array of G proteins connect
receptors to distinct targets.
In addition to enzyme regulation, G
proteins can also regulate ion
channels.
• Example: action of the
neurotransmitter acetylcholine on
heart muscle.
G Proteins and Cyclic AMP Signaling
Heart muscle cells have acetylcholine
receptors that are G protein-coupled.
The α subunit of this G protein (Gi)
inhibits adenylyl cyclase.
The Gi βγ subunits open K+ channels in
the plasma membrane, which slows
heart muscle contraction.
G Proteins and Cyclic AMP Signaling
A large family of G protein-coupled
receptors are responsible for odor
detection and recognition.
Genes encoding odorant receptors were
cloned in 1991 by Buck and Axel.
Odorant receptors are encoded by a
family of hundreds of genes.
Key Experiment, Ch. 16, p. 613 (3)
G Proteins and Cyclic AMP Signaling
The role of cAMP as a second
messenger was discovered in 1958 by
Sutherland in studies of epinephrine,
which signals the breakdown of
glycogen to glucose in muscle cells.
cAMP is formed from ATP by adenylyl
cyclase and degraded to AMP by
cAMP phosphodiesterase.
Figure 16.14 Synthesis and degradation of cAMP
G Proteins and Cyclic AMP Signaling
Effects of cAMP are mediated by cAMPdependent protein kinase, or protein
kinase A.
Inactive form has two regulatory and two
catalytic subunits. cAMP binds to the
regulatory subunits, which dissociate.
The free catalytic subunits can then
phosphorylate serine on target proteins.
Figure 16.15 Regulation of protein kinase A
G Proteins and Cyclic AMP Signaling
In glycogen metabolism, protein kinase A
phosphorylates two enzymes:
• Phosphorylase kinase is activated,
and in turn activates glycogen
phosphorylase, which catalyzes
glycogen breakdown.
• Glycogen synthase is inactivated, so
glycogen synthesis is blocked.
Figure 16.16 Regulation of glycogen metabolism by epinephrine
G Proteins and Cyclic AMP Signaling
Signal amplification: Binding of a
hormone molecule leads to activation of
many intracellular target enzymes.
• Example: Each molecule of
epinephrine activates one receptor.
• Each receptor may activate up to
100 molecules of Gs.
G Proteins and Cyclic AMP Signaling
• Gs then stimulates adenylyl cyclase,
which catalyzes synthesis of many
molecules of cAMP.
• Each molecule of protein kinase A
phosphorylates many molecules of
phosphorylase kinase, which
phosphorylate many molecules of
glycogen phosphorylase.
G Proteins and Cyclic AMP Signaling
In many animal cells, increases in cAMP
activate transcription of genes that have a
regulatory sequence called cAMP
response element (CRE).
The free catalytic subunit of protein kinase
A goes to the nucleus and phosphorylates
transcription factor CREB (CRE-binding
protein).
G Proteins and Cyclic AMP Signaling
Phosphorylation of CREB leads to
recruitment of coactivators and
expression of cAMP-inducible genes.
Regulation of gene expression by cAMP
plays important roles in many aspects of
cell behavior.
Figure 16.17 Cyclic AMP-inducible gene expression
G Proteins and Cyclic AMP Signaling
Protein kinases don’t function in isolation.
Protein phosphorylation is rapidly
reversed by protein phosphatases,
which terminate responses initiated by
receptor activation of protein kinases.
Figure 16.18 Regulation of protein phosphorylation by protein kinase A and protein phosphatase 1
G Proteins and Cyclic AMP Signaling
cAMP can also directly regulate ion
channels:
It is a second messenger in sensing
smells—odorant receptors are G
protein-coupled. They stimulate adenylyl
cyclase, leading to increased cAMP.
cAMP opens Na+ channels in the plasma
membrane, leading to initiation of a
nerve impulse.
G Proteins and Cyclic AMP Signaling
Cyclic GMP (cGMP) is another important
second messenger.
cGMP is formed from GTP by guanylyl
cyclases and degraded to GMP by a
phosphodiesterase.
cGMP mediates biological responses,
such as blood vessel dilation.
G Proteins and Cyclic AMP Signaling
In the vertebrate eye, cGMP is the
second messenger that converts visual
signals to nerve impulses.
The photoreceptor in retinal rod cells is a
G protein-coupled receptor called
rhodopsin.
Rhodopsin is activated when light is
absorbed by the associated molecule
11-cis-retinal, which isomerizes to alltrans-retinal.
G Proteins and Cyclic AMP Signaling
Rhodopsin then activates the G protein
transducin.
Transducin stimulates cGMP
phosphodiesterase, leading to
decreased levels of cGMP.
cGMP levels are translated to nerve
impulses by a direct effect of cGMP on
ion channels.
Figure 16.19 Role of cGMP in photoreception
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Other cell surface receptors are directly
linked to intracellular enzymes.
The largest family of these are the
tyrosine kinases, which phosphorylate
their substrates on tyrosine residues.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Receptor tyrosine kinases
Includes the receptors for most
polypeptide growth factors.
The human genome encodes 58
receptor tyrosine kinases, including the
receptors for EGF, NGF, PDGF,
insulin, and many other growth factors.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
All receptor tyrosine kinases have:
• An N-terminal extracellular ligandbinding domain
• One transmembrane α helix
• A cytosolic C-terminal domain with
protein-tyrosine kinase activity
Figure 16.20 Organization of receptor tyrosine kinases
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Binding of ligands (growth factors) to the
extracellular domains activates the
cytosolic kinase domains.
This results in phosphorylation of both
the receptors and intracellular target
proteins that propagate the signal.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
The first step is ligand-induced receptor
dimerization.
This results in receptor
autophosphorylation, as the two
polypeptide chains cross-phosphorylate
each other.
Figure 16.21 Dimerization and autophosphorylation of receptor tyrosine kinases
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Autophosphorylation has two roles:
• Phosphorylation of tyrosine in the
catalytic domain increases protein
kinase activity.
• Phosphorylation of tyrosine outside the
catalytic domain creates binding sites
for other proteins that transmit signals
downstream from the activated
receptors.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Downstream signaling molecules have
domains, such as SH2, that bind to
specific phosphotyrosine-containing
peptides of the activated receptors.
SH2 domains were first recognized in
tyrosine kinases related to Src, the
oncogenic protein of Rous sarcoma
virus.
Figure 16.22 Association of downstream signaling molecules with receptor tyrosine kinases
Figure 16.23 Complex between an SH2 domain and a phosphotyrosine peptide
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Nonreceptor tyrosine kinases stimulate
intracellular tyrosine kinases with which
they are noncovalently associated.
The cytokine receptor superfamily
includes receptors for most cytokines
and some polypeptide hormones.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
The structure of cytokine receptors is
similar to receptor tyrosine kinases, but
the cytosolic domains have no catalytic
activity.
Ligand binding induces dimerization of
receptors, and cross-phosphorylation of
associated nonreceptor tyrosine kinases.
Figure 16.24 Activation of nonreceptor tyrosine kinases (Part 1)
Figure 16.24 Activation of nonreceptor tyrosine kinases (Part 2)
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
The activated kinases then
phosphorylate the receptor.
This provides phosphotyrosine-binding
sites for recruitment of downstream
signaling molecules with SH2 domains.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
JAK/STAT pathway:
The kinases associated with cytokine
receptors belong to the Janus kinase
(JAK) family.
Key targets of JAK kinases are STAT
proteins (signal transducers and
activators of transcription). STAT
proteins are transcription factors with
SH2 domains.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
STAT proteins are inactive in the cytosol
until cytokine receptors are stimulated.
Then they bind to phosphotyrosine sites
on the receptor, and are
phosphorylated by JAK.
The phosphorylated STAT proteins then
dimerize and translocate to the
nucleus.
Figure 16.25 The JAK/STAT pathway
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Additional nonreceptor tyrosine kinases
belong to the Src family.
These kinases play key roles in signaling
downstream of cytokine receptors,
receptor tyrosine kinases, antigen
receptors on B and T lymphocytes, and
receptors involved in cell–cell and cell–
matrix interactions.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
In addition to attaching cells to the
extracellular matrix, integrins also serve
as receptors that activate intracellular
signaling pathways.
One mode of signaling involves
activation of a nonreceptor tyrosine
kinase called FAK (focal adhesion
kinase).
Figure 16.26 Integrin signaling
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
MAP Kinase pathway:
The MAP kinase pathway is a cascade of
protein kinases that is highly conserved
in evolution, found in all eukaryotic cells.
MAP kinases (mitogen-activated protein
kinases) are serine/threonine kinases.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
MAP kinases initially found in mammalian
cells belong to the ERK (extracellular
signal-regulated kinase) family.
The role of ERK signaling emerged from
studies of Ras proteins, first identified as
the oncogenic proteins of viruses that
cause sarcomas in rats.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Ras proteins are guanine nucleotidebinding proteins that alternate between
inactive GDP-bound and active GTPbound forms.
Ras is activated by guanine nucleotide
exchange factors (GEFs) that stimulate
exchange of GDP for GTP.
Figure 16.27 Regulation of Ras proteins
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Ras-GTP activity is terminated by GTP
hydrolysis, stimulated by interaction of
Ras-GTP with GTPase-activating
proteins (GAPs).
Mutations of ras genes in cancers inhibit
GTP hydrolysis, so the Ras proteins
remain continuously in the active GTPbound form, driving proliferation of cancer
cells.
Molecular Medicine, Ch. 16, p. 625
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Autophosphorylation of receptor tyrosine
kinase receptors leads to binding of Ras
GEFs.
GEFs interact with Ras proteins and
stimulate exchange of GDP for GTP,
forming the active Ras-GTP complex.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Activation of Ras leads to activation of
Raf protein serine/threonine kinase.
Raf phosphorylates and activates a
second protein kinase, MEK (MAP
kinase/ERK kinase).
ERK phosphorylates a variety of target
proteins.
Figure 16.28 Activation of Ras, Raf and ERK downstream of receptor tyrosine kinases
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Some activated ERK goes to the nucleus,
where it regulates transcription factors
by phosphorylation.
Figure 16.29 Induction of immediate-early genes by ERK
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
A primary response to growth factor
stimulation is rapid transcription of
immediate-early genes.
This is mediated by a regulatory sequence
called the serum response element
(SRE), which is recognized by
transcription factors including the serum
response factor (SRF) and Elk-1.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Many immediate-early genes encode
transcription factors.
Their induction leads to altered
expression of a battery of other
downstream genes called secondary
response genes.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Yeasts and mammalian cells have
multiple MAP kinase pathways.
Each cascade consists of three protein
kinases: a terminal MAP kinase and two
upstream kinases (analogous to Raf and
MEK).
Mammalian MAP kinases include ERK,
JNK, and p38 kinases.
Figure 16.30 Pathways of MAP kinase activation in mammalian cells
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Specificity of MAP kinase signaling is
maintained partly by physical
association on scaffold proteins.
Example: KSR scaffold protein organizes
ERK and its upstream activators Raf and
MEK into a signaling cassette.
Figure 16.31 A scaffold protein for the ERK MAP kinase cascade
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
PI 3-kinase/Akt pathway:
Based on a second messenger derived
from the membrane phospholipid
phosphatidylinositol 4,5bisphosphate (PIP2).
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
PIP2 is phosphorylated by
phosphatidylinositide (PI) 3-kinase to
yield the second messenger
phosphatidylinositol 3,4,5trisphosphate (PIP3).
PI 3-kinase is recruited to activated
receptor tyrosine kinases via its SH2
domain.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
PIP3 targets a serine/threonine kinase
called Akt via its pleckstrin homology
(PH) domain.
Akt is phosphorylated and activated by
another protein kinase (PDK1).
Activation of Akt also requires
phosphorylation by protein kinase
mTORC2, which is also stimulated by
growth factors.
Figure 16.32 The PI 3-kinase/Akt pathway
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Activated Akt phosphorylates several
target proteins, transcription factors, and
other protein kinases.
Transcription factors include members of
the FOXO family.
Akt phosphorylation of FOXO sequesters
it in inactive form in the cytosol.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
If growth factors are not present, Akt is
not active, and FOXO travels to the
nucleus, where it stimulates transcription
of genes that inhibit cell proliferation or
induce cell death.
Figure 16.33 Regulation of FOXO
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Protein kinase GSK-3 is also inhibited by
Akt phosphorylation.
GSK-3 targets include the translation
initiation factor eIF2B.
Phosphorylation of eIF2B leads to a
global downregulation of translation
initiation.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
The mTOR pathway couples control of
protein synthesis to availability of growth
factors, nutrients, and energy.
The mTORC1 complex is activated
downstream of Akt and regulates cell
size by controlling protein synthesis.
Figure 16.34 The mTOR pathway
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
mTORC1 phosphorylates two targets that
regulate protein synthesis:
• S6 kinase controls translation by
phosphorylating ribosomal protein S6.
• eIF4E binding protein-1 (4E-BP1). If
mTORC1 is not present, 4E-BP1
interferes with initiation factors.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
If mTORC1 phosphorylates 4E-BP1, it
prevents interaction with eIF4E, leading
to increased rates of translation
initiation.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
mTORC1 also inhibits protein degradation
by regulating autophagy.
When cells are starved of nutrients,
mTORC1 activity decreases.
This stimulates autophagy and allows
cells to degrade nonessential proteins
so the amino acids can be reutilized.
Figure 16.35 Regulation of autophagy by mTOR
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Phospholipase C/Calcium Pathway:
Hydrolysis of PIP2 by phospholipase C
produces two second messengers:
• Diacylglycerol (DAG)
• Inositol 1,4,5-trisphosphate (IP3)
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Phospholipase C-γ (PLC-γ) binds to
activated receptor tyrosine kinases via
its SH2 domains.
Tyrosine phosphorylation increases PLCγ activity, stimulating hydrolysis of PIP2.
DAG and IP3 stimulate downstream
pathways.
Figure 16.36 Activation of phospholipase C by tyrosine kinases
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
DAG remains associated with the plasma
membrane and activates
serine/threonine kinases of the protein
kinase C family.
IP3 binds to receptors that are ligandgated Ca2+ channels in the ER. Opening
these channels allows Ca2+ to move out
of the ER.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
One of the major Ca2+-binding proteins
that mediates the effects of Ca2+ is
calmodulin, which is activated when
Ca2+ concentration increases.
Ca2+/calmodulin then binds to target
proteins, including protein kinases.
Figure 16.37 Function of calmodulin
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
One example of a Ca2+/calmodulindependent protein kinase is myosin
light-chain kinase, which signals actinmyosin contraction by phosphorylating
one of the myosin light chains.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Members of the CaM kinase family are
also activated by Ca2+/calmodulin.
They phosphorylate metabolic enzymes,
ion channels, and transcription factors.
One form of CaM kinase regulates
synthesis and release of
neurotransmitters.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
CaM kinases can also regulate gene
expression by phosphorylating
transcription factors.
CREB is phosphorylated by CaM kinase
and also by protein kinase A.
This illustrates one of many intersections
between the Ca2+ and cAMP signaling
pathways, which function coordinately to
regulate many cellular responses.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
Ca2+ is also increased by uptake of
extracellular Ca2+ by regulated channels
in the plasma membrane.
In electrically excitable cells of nerve and
muscle, voltage-gated Ca2+ channels
are opened by membrane
depolarization.
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
The resulting increase in intracellular Ca2+
signals further release of Ca2+ from the
ER by opening Ca2+ channels
(ryanodine receptors) in the ER
membrane.
One effect of higher Ca2+ is to trigger
release of neurotransmitters.
Figure 16.38 Regulation of intracellular Ca2+ in electrically excitable cells
Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and
Phospholipase C/Calcium Pathways
In muscle cells, ryanodine receptors in
the SR may also be opened directly in
response to membrane depolarization.
Ca2+ is a versatile second messenger that
controls a wide range of cellular
processes.
Receptors Coupled to Transcription Factors
Several other types of growth factor
receptors are directly coupled to
transcription factors.
TGF-β/Smad pathway:
• Receptors for transforming growth
factor β (TGF-β) are
serine/threonine kinases, which
directly phosphorylate transcription
factors of the Smad family.
Receptors Coupled to Transcription Factors
The receptors are dimers of type I and II
polypeptides that associate following
ligand binding.
Type II phosphorylates type I, which then
phosphorylates a Smad protein.
Smad complexes translocate to the
nucleus and stimulate expression of
target genes.
Figure 16.39 Signaling from TGF-b receptors
Receptors Coupled to Transcription Factors
There are at least 30 different members
of the TGF-β family in humans, which
elicit different responses in their target
cells.
There are seven different type I receptors
and five type II receptors, which lead to
activation of eight different members of
the Smad family.
Receptors Coupled to Transcription Factors
NF-κB pathways:
The NF-κB family are transcription factors.
One pathway is downstream of the
receptor for tumor necrosis factor (a
cytokine that induces inflammation and
cell death).
The Toll-like receptors recognize
molecules associated with pathogenic
bacteria and viruses.
Receptors Coupled to Transcription Factors
In unstimulated cells, NF-κB proteins are
bound to inhibitory IκB proteins and are
inactive.
Activation of receptors activates IκB
kinase, which phosphorylates IκB,
eventually freeing NF-κB to move to the
nucleus and induce expression of target
genes.
Figure 16.40 NF-kB signaling from the TNF receptor
Receptors Coupled to Transcription Factors
Hedgehog and Wnt pathways:
The Hedgehog and Wnt pathways are
connected signaling systems that play
key roles in determining cell fate during
embryonic development and in
regulating proliferation of stem cells in
adult tissues.
Receptors Coupled to Transcription Factors
The Hedgehog receptor (Patched) inhibits
a second transmembrane protein
(Smoothened).
Binding of Hedgehog inhibits Patched,
which activates Smoothened, which
initiates a signaling pathway leading to
activation of transcription factors Ci
(Drosophila) or Gli (mammals).
Figure 16.41 Hedgehog signaling
Receptors Coupled to Transcription Factors
Wnt proteins are growth factors that bind
to receptors of the Frizzled and LRP
families.
Signaling from Frizzled and LRP leads to
stabilization of β-catenin, a
transcriptional activator.
Figure 16.42 The Wnt pathway
Receptors Coupled to Transcription Factors
Notch pathway:
A highly conserved pathway that controls
cell fate during animal development.
Notch is a receptor for direct cell-cell
signaling by transmembrane proteins
(e.g., Delta) on neighboring cells.
Receptors Coupled to Transcription Factors
Binding of Delta leads to cleavage of
Notch by γ-secretase.
This releases the Notch intracellular
domain, which translocates to the
nucleus and interacts with the CSL
transcription factor to induce gene
expression.
Figure 16.43 Notch signaling
Signaling Dynamics and Networks
Signaling pathways don’t operate in
isolation.
Intracellular signal transduction is really
an integrated network of connected
pathways.
Computational modeling of these
networks is currently a major challenge
in cell biology.
Signaling Dynamics and Networks
Activity of signaling pathways is controlled
by feedback loops.
Example: the NF-κB pathway is a
negative feedback loop:
NF-κB is activated by phosphorylation
and degradation of IκB, but a gene
activated by NF-κB encodes IκB,
generating a feedback loop that inhibits
NF-κB activity.
Figure 16.44 Feedback inhibition of NF-kB
Signaling Dynamics and Networks
This regulation is critical: extent and
duration of NF-κB activity can determine
the transcriptional response of the cell.
Some target genes are induced by
transient NF-κB activity (30–60 min), but
induction of other genes requires several
hours of sustained NF-κB signaling.
Signaling Dynamics and Networks
In response to nerve growth factor (NGF),
transient activation of ERK (30–60 min)
stimulates cell proliferation.
But sustained activation of ERK (2–3 hrs)
induces differentiation of NGF-treated
cells into neurons.
Signaling Dynamics and Networks
Crosstalk is the interaction between
signaling pathways.
Example: Extensive crosstalk between
the PI 3-kinase/Akt/mTORC1 and
Ras/Raf/MEK/ERK pathways.
The crosstalk includes both positive and
negative regulation, which helps
coordinate their activities within the cell.
Figure 16.45 Crosstalk between the ERK and PI 3-kinase signaling pathways
Signaling Dynamics and Networks
Example of regulation of signaling
duration combined with crosstalk:
• G protein-coupled receptors linked to
MAP kinase signaling by β-arrestins.
• Activity of the receptors is turned off
as a result of phosphorylation by
GRKs and association of β-arrestin
with the phosphorylated receptor.
Figure 16.46 Crosstalk between G protein-coupled receptors and ERK signaling by b-arrestin
Signaling Dynamics and Networks
β-arrestins also act as signaling molecules
to stimulate downstream pathways,
including nonreceptor tyrosine kinases
(e.g., Src) and MAP kinase pathways.
β-arrestin serves as a scaffold protein for
Raf-MEK-ERK signaling, linking this MAP
kinase pathway to G protein-coupled
receptors.
Signaling Dynamics and Networks
Multiple signaling pathways interact with
one another to form signaling networks
within the cell.
A full understanding of cell signaling must
include development of network models
that predict the dynamic behavior of the
interconnected signaling pathways.