Specialized signaling pathways
1: RTK associated pathways
Dr. Tania A. Shakoori
Your Case
• 50 year old woman works
in a pesticide factory.
Presents with bleeding in
urine. After investigations
you discover she has
renal cell carcinoma
• You decide to include a
new drug in her
treatment called
Sorafenib- which is
receptor tyrosine kinase
inhibitor
So what is this pathway?
KEY CONCEPTS
Receptor Tyrosine Kinases and Activation of Ras
■ RTKs which bind to peptide and protein hormones, may exist as
preformed dimers or dimerize during binding to ligands.
■ Ligand binding leads to activation of the intrinsic protein tyrosine
kinase activity of the receptor and phosphorylation of tyrosine
residues in its cytosolic domain . The activated receptor also can
phosphorylate other protein substrates.
■ Ras is an intracellular GTPase switch protein that acts downstream
from most RTKs. Like G, Ras cycles between an inactive GDP-bound
form and an active GTP-bound
form. Ras cycling requires the assistance of two proteins, a guanine
nucleotide–exchange factor (GEF) and a GTPase-activating protein
(GAP).
■ RTKs are linked indirectly to Ras via two proteins:
GRB2, an adapter protein, and Sos, which has GEF activity.
KEY CONCEPTS
Receptor Tyrosine Kinases and Activation of Ras
■ GRB2 binds to a phosphotyrosine in activated RTKs and also Sos,
thereby bringing Sos close to membrane-bound RasGDP
and activating its nucleotide exchange activity.
■ Binding of Sos to inactive Ras causes a large conformational
change that permits release of GDP and binding of GTP, forming
active Ras. GAP, which accelerates GTP hydrolysis, is localized near
Ras GTP by binding to activated RTKs.
■ Normally, Ras activation and the subsequent cellular response
require ligand binding to an RTK or a cytokine receptor. In cells
that contain a constitutively active Ras, the cellular response
occurs in the absence of ligand binding.
MAP kinase pathway
• In mammalian cells all receptor tyrosine
kinases (RTKs), as well as most cytokine
receptors, appear to utilize a highly conserved
signal-transduction pathway in which the
signal induced by ligand binding is carried via
GRB2 and Sos to Ras, leading to its activation
Induction of gene
transcription by activated
MAP kinase
• (Especisally transcription factor cFos & c-Juninduces expression
of many genes encoding proteins
necessary for cells to progress
through the cell cycle. )
KEY CONCEPTS: MAP Kinase Pathways
• ■ Activated Ras triggers a kinase cascade in which
Raf, MEK, and MAP kinase are sequentially
phosphorylated and thus activated. Activated
MAP kinase translocates to the nucleus
• ■ Phosphorylation activates MAP kinases and
many other protein kinases involved in signaltransduction pathways.
• ■ Activation of MAP kinase following stimulation
of a growth factor receptor leads to
phosphorylation and activation of transcription
factors which promote transcription of various
early-response genes.
• In mitogen activated pathway the activation of
Ras is countered by:
a)
b)
c)
d)
Protein Kinase C
GTPase activating protein
Phophotidyl inositol
Inositol triphosphate
Summary
• Video: just copy and paste this link to get to
the video
• https://drive.google.com/file/d/0B8lwCh5HN
0GTckFodWhaZWVaQnc/view?usp=sharing
Akt Pathway
Recruitment and activation of protein kinase B (PKB) in PI-3 kinase pathways. In
unstimulated cells, PKB is in the cytosol with its PH domain bound to the catalytic
domain, inhibiting its activity. Hormone stimulation leads to activation of PI-3
kinase and subsequent formation of phosphatidylinositol (PI) 3-phosphates (see
Figure 14-26). The 3-phosphate groups serve as docking sites on the plasma
membrane for the PH domain of PKB and another kinase, PDK1. Full activation of
PKB requires phosphorylation both in the activation lip and at the
C-terminus by PDK1.
Phosphoinositides as Signal
Transducers
• ■ Activated protein kinase B (Akt) promotes survival
of many cells by directly inactivating several proapoptotic proteins and down-regulating expression
of others.
• ■ Signaling via the PI-3 kinase pathway is terminated
by the PTEN phosphatase. Loss of PTEN, a common
occurrence in human tumors, promotes cell survival
and proliferation.
Protein synthesis function of Akt
• Akt Rheb mtor S6K (translation
factor) binds to large subunit of ribosome
translation
Lowers concentration of FOXO
• FOXO is a tumor suppressor protein (inhibits
proliferation of cells)
• Akt activated phosphorylates FOXO which
is a substrate for enzyme ubiquitin ligase
ubiquitinilated FOXO formed destroyed by a
complex of proteases called proteosome
• So what will be the effect of AKT signalling?
• Excessive signaling
• Decreased signaling?
Insulin and Akt
• The principal mechanism by which insulin causes
a reduction of the blood glucose level is by
increasing import of glucose by fat and muscle
cells. This effect is mediated by protein kinase B,
which through mechanisms that are not fully
understood causes movement of the GLUT4
glucose transporter from intracellular
membranes to the cell surface. The resulting
increased influx of glucose into these cells lowers
blood glucose levels.
Summary
• Just copy paste the following to get to the
video:
• https://drive.google.com/file/d/0B8lwCh5HN
0GTem1TV0lyOHNlSjQ/view?usp=sharing
Question Which signaling pathway mediates the following
cellular responses:
stimulation of GLUT 4 translocation to the plasma membrane,
inhibition of glycogen synthase kinase 3 (GSK3) and induction of
triglyceride synthesis?
A Ras/mitogen activated protein (MAP) kinase
B PI-3 kinase – Akt/protein kinase B (Akt/PKB)
C AMP kinase (AMPK)
D Mammalian target of rapamycin (mTOR)