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Cell Signaling
Lecture 9
JAK/STAT Pathway
The JAK-STAT system consists of three
main components:
 (1) a receptor
 (2) Janus kinase (JAK)
 (3) Signal Transducer and Activator of
Transcription (STAT).
 transduce a multitude of signals for
development and homeostasis in animals,
from humans to flies.
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JAK/STAT pathway
JAK activation stimulates:
1. cell proliferation,
2. differentiation
3. cell migration
4. apoptosis.
 These cellular events are critical to
hematopoiesis, immune development,
mammary gland development and
lactation, adipogenesis, sexually
dimorphic growth and other processes.
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Mutations in JAK/STAT pathway
Mutations that constitutively activate or fail
to regulate JAK signaling properly cause
inflammatory disease, erythrocytosis,
gigantism and an array of leukemias.
 Mutations which slow down or stop
JAK/STAT parthway disrupts the process of
hematopoiesis, immune development,
mammary gland development and lactation,
adipogenesis, sexually dimorphic growth and
other processes.
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Cytokines contain 160 amino acids
 Example 1: Interlukins, essential for proliferation
and functioning of T cells and antibody producing B
cells of the immune system.
T cells: A lymphocyte (WBCs) that matures in the
thymus
 Example 2: Interferon, produced and secreted
by certain cell type following virus infection
 Example 3: Erythropoietin, triggers production
of RBCs and by inducing division and
differentiation of eryhtroid progenitor cells in the
bone marrow
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Erythropoietin and formation of RBCs
Optimal red blood cell (RBC) production requires both erythropoietin (as
the controlling factor) and iron (as the raw material). Several factors can
impair RBC production; inhibit iron availability, and/or shorten RBC life
span . BFU-E, burst-forming unit erythroid; CFU-E, colony-forming unit
erythroid.
JAK/STAT pathway
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Intracellular activation of JAK/STAT occurs when ligand binding
induces the multimerization of receptor subunits.
For some ligands, such as erythropoietin and growth hormone, the
receptor subunits are bound as homodimers while, for others, such
as interferons and interleukins, the receptor subunits are
heteromultimers.
For signal propagation through either homodimers or
heteromultimers, the cytoplasmic domains of two receptor subunits
must be associated with JAK tyrosine kinases.
JAKs are distinctive in that they have kinase-homologous domains at
the C-terminus.
The first is a non-catalytic regulatory domain, whereas the second
has tyrosine kinase activity.
In mammals, the JAK family comprises four members: JAK1, JAK2,
JAK 3 and Tyk2.
JAK activation occurs upon ligand-mediated receptor
multimerization because two JAKs are brought into close proximity,
allowing trans-phosphorylation.
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The activated JAKs subsequently phosphorylate additional
targets, including both the receptors and the major
substrates, STATs.
STATs are transcription factors that reside in the cytoplasm
until activated.
The seven mammalian STATs bear a conserved tyrosine
residue near the C-terminus that is phosphorylated by JAKs.
This phosphotyrosine permits the dimerization of STATs
through interaction with a conserved SH2 domain.
Phosphorylated STATs enter the nucleus by a mechanism
that is dependent on importin α-5 (also called
nucleoprotein interactor 1) and the Ran nuclear import
pathway.
Once in the nucleus, dimerized STATs bind specific
regulatory sequences to activate or repress transcription of
target genes.
Thus the JAK/STAT cascade provides a direct mechanism to
translate an extracellular signal into a transcriptional
response.
JAK/STAT pathway
Negative regulators of JAK/STAT pathway
there are three major classes of negative
regulator:
1.SOCS (suppressors of cytokine signaling)
2.PIAS (protein inhibitors of activated stats)
3.PTPs (protein tyrosine phosphatases)
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PTP (protein tyrosine phosphatases)
The simplest are the protein tyrosine
phosphatases, which reverse the activity of
the JAKs.
 The best characterized of these is SHP-1, the
product of the mouse motheaten gene.
 SHP-1 contains two SH2 domains and can
bind to either phosphorylated JAKs or
phosphorylated receptors to facilitate
dephosphorylation of these activated
signaling molecules.
 Other tyrosine phosphatases, such as CD45,
appear to have a role in regulating JAK/STAT
signaling through a subset of receptors.
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SOCS (suppressors of cytokine signaling)
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SOCS proteins are a family of at least eight members containing an
SH2 domain and a SOCS box at the C-terminus.
In addition, a small kinase inhibitory region located N-terminal to the
SH2 domain has been identified for SOCS1 and SOCS3.
The SOCS complete a simple negative feedback loop in the JAK/STAT
circuitry:
activated STATs stimulate transcription of the SOCS genes and the
resulting SOCS proteins bind phosphorylated JAKs and their
receptors to turn off the pathway.
The SOCS can affect their negative regulation by three means.
First, by binding phosphotyrosines on the receptors, SOCS physically
block the recruitment of signal transducers, such as STATs, to the
receptor.
Second, SOCS proteins can bind directly to JAKs or to the receptors
to specifically inhibit JAK kinase activity.
Third, SOCS interact with the elongin BC complex and cullin 2,
facilitating the ubiquitination of JAKs and, presumably, the receptors.
Ubiquitination of these targets decreases their stability by targeting
them for proteasomal degradation.
PIAS (protein inhibitors of activated stats)
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PIAS1, PIAS3, PIASx and PIASy.
These proteins have a Zn-binding RING-finger
domain in the central portion, a well-conserved
SAP (SAF-A/Acinus/PIAS) domain at the Nterminus, and a less-well-conserved carboxyl
domain.
The latter domains are involved in target protein
binding.
The PIAS proteins bind to activated STAT dimers
and prevent them from binding DNA. The
mechanism by which PIAS proteins act remains
unclear.
Review Paper
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http://jcs.biologists.org/content/117/8/128
1.full
Receptor Tyrosine Kinases
Regulate cell proliferation, differentiation,
cell survival and collular metabolism
 The signaling molecules that activate RTK
are soluble or membrane bound peptide
or protein hormones
 Examples; Nerve growth factor (NGF)
 Platelets-derived growth factor (PDGF)
 Fibroblast growth factor (FGF)
 Epidermal growth factor (EGF)
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Like cytokine receptors, RTK signal
through a protein tyrosine kinase
 Unlike cytokine receptors, which
associate with a separate cytosolic kinase
protein (JAK), RTKs have an intrinsic
kinase as part of their cytosolic domain
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Ligand Binding Leads to Autophosphorylation of
RTKs
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All RTKs comprise an extracellular domain containing a ligand-binding
site, a single hydrophobictransmembrane α helix, and a cytosolic
domain that includes a region with protein-tyrosine kinase activity.
Binding of ligand causes most RTKs to dimerize;
the protein kinase of each receptor monomer then phosphorylates a
distinct set of tyrosine residues in the cytosolic domain of its dimer
partner, a process termed autophosphorylation.
Autophosphorylation occurs in two stages.
First, tyrosine residues in the phosphorylation lip near the catalytic
site are phosphorylated.
This leads to a conformational change that facilitates binding of ATP in
some receptors (e.g., the insulin receptor) and binding of protein
substrates in other receptors (e.g., FGF receptor).
The receptor kinase activity then phosphorylates other sites in the
cytosolic domain; the resulting phosphotyrosines serve as docking
sites for other proteins involved in RTK-mediated signal transduction.
Activation of RTK
Types of RTK
HER1: EGF, HB-EGF, TGFα
 HER2: does not directly bind a ligand,
forms heterodimers with ligand-bound
HER1, HER3, HER4
 HER3: Neuregulins 1 and 2 (NRG),
lacks a functional kinase domain and
can signal only when complexed with
HER2
 HER4: NRG1, NRG2, HB-EGF
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Down-regulation of RTK Signaling
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Endocytosis and Lysosomal Degradation
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Overexpression of HER2 occurs in breats
cancer