Map kinase and jak stat pathway cellular and molecular pharmacology

1. MAP kinase and
JAK-STAT pathway
D.THILAGAVATHI
M.PHARM 1ST SEMESTER
DEPARTMENT OF PHARMACOLOGY
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2. Contents
MAP kinase pathway
Requirement for signal transduction
RAS activation
Kinase cascade
Inactivation of RAS
Activation of transcription factors by MAP
kinase
JAK STAT pathway
Mechanism
Movement of STAT from cytosol to nucleus
Integration with other signalling pathway
Regulation
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3. MAP kinase
Map kinase is an enzyme that translocate the signal to the nucleus and
activates many transcriptional factors by phosphorylating many different
proteins
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4. MAP kinase pathway
Signal passes from activated RAS to cascade of protein kinases
This cascade transmits signals downstream from activated RAS protein to MAP kinase
Then MAP kinase translocate the signal to the nucleus and activates transcriptional
factors
This whole process is called “MAP kinase pathway”
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6. Requirement for signal transduction
Signal – that is to be passed
Receptor- to which the ligand bind
Adaptor proteins – form link between membrane bound receptor and protein is to be
activated
Protein cascade – lead to the activation of transcriptional factors
Transcriptional factors – which carry out transcription process
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7. RAS activation
Epidermal growth factor receptor (EGFR) is a receptor linked tyrosine kinase which is
activated by extracellular ligand Epidermal growth factor (EGF)
Binding of EGR to EGFR leads to the EGFR phosphorylation on tyrosine residues
Docking proteins such as GRB2 contain an SH2 domain binds to the phosphotyrosine
residue of the receptor
Now SOs protein (guanine nucleotide exchange factor) binds to the SH2 domain of the
GRB2 protein and forms GRB2 SOs complex.
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8. RAS activation contd.,
This GRB2 SOs complex promotes the phosphorylation so the GDP in RAS is replaced
with GTP.
Now that the GDP in RAS is converted to GTP, the RAS become active
This activated RAS starts the kinase cascade
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11. Kinase cascade
Activated RAS activates the RAF kinase
RAF kinase phosphorylates and activate MEK (MEK1 and MEK2)
MEK phosphorylates and activate MAPK
RAF and MAPK (ERK) are serine/threonine selective protein kinases
MEK- serine/tyrosine/threonine kinase
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13. Inactivation of RAS
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14. Activation of transcription factors by
MAP-kinase
Early response genes (genes which are induced before cells enter the S-phase and
replicate their DNA )
One such gene encodes the transcription factor “C-Fos”
C-Fos with other factors induces expression of many genes encoding proteins which
are necessary for cells to progress through the cell cycle
Enhancers that regulate the C-Fos genes contain a serum response factor (SRF) and
ternary complex factor (TCF)
By modifying these two factors MAPK induces transcription of C-Fos
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15. Activation of transcription factors by
MAP-kinase contd.,
In Cytosol:
MAPK phosphorylates and activates a kinase P90 RSK, which translocate to the nucleus
and phosphorylates a specific serine in SRF
In Nucleus:
MPAK directly phosphorylates specific serine in TCF
Association of phosphorylated TCF with two molecules of phosphorylated SRF forms
an active trimeric factor
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16. Activation of transcription factors by
MAP-kinase contd.,
Scaffold proteins
Scaffold strategy prevent cross-talk between different MAPK modules
Atleast 5 parallel MAPK modules can operate in a mammalian cell
These modules make use of atleast 12 MAP kinases, 7 MAP kinase kinases and 7 MAP
kinas kinase kinase
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18. JAK-STAT pathway
JAK Janus kinase, STAT – Signal Transducer and Activation of Transcription
This signalling pathway transmits information from chemical signals outside the cell,
which cause DNA transcription and activity
4 JAK proteins – JAK1, JAK 2, JAK 3, TYK2
JAKs contains a FERM domain, an SH2 related domain, a kinase domain and a
pseudokinase domain
Kinase domain allows JAK to phosphorylate proteins
7 STAT proteins STAT1, STAT2, STAT3, STAT4, STAT 5A, STAT 5B, STAT 6
STAT contain many different domains, most conserved region is the SH2 domain
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19. JAK-STAT pathway mechanism
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20. JAK-STAT pathway mechanism
Agonist induced dimerization alters the intracellular domain conformation to increase
the affinity for JAK
On binding, JAK gets activated and phosphorylates tyrosine residues of receptor
Binding of STAT and its phosphorylation by JAK
Phosphorylation STAT dimerize and translocate to the nucleus to regulate gene
transcription
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21. Movement of STATs from cytosol to the
nucleus
STAT dimers have to pass through nucleus pore complex (NPCs) which is present along the
nucleus envelope
To enable STATs to move into the nucleus, an amino acid sequence on STATs, called nuclear
localisation signal (NLS) is bound by proteins called importins
After entering, a protein called Ran binds to the importins releasing them from the STAT dimer
STAT dimer is then free in the nucleus
Specific STAT – specific importin proteins
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22. Integration with other signalling
pathways
JAKs phosphorylate cytokine receptors which can bind Grb2 protein
Grb2 activates Sos proteins MAPK signalling
Phosphorylated cytokine receptors ca also bound by P13k
P13k activates AKT
Thus JAK-STAT is able to interconnect with other cell-signalling pathways such as P13k/
AKT/ mTOR/ pathway
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24. Regulation
1. Protein inhibitors of activated STATs (PIAS)
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25. Regulation contd.,
2. Protein tyrosine phosphatases (PTPs)
3 major PTPs – SHP-1, HP-2, CD45
3. Suppressors of cytokine signalling (SoCS)
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