Transcript Lecture 23 Signal Transduction 2

Lecture 23
Signal Transduction 2
Major Concepts
• Receptor tyrosine kinases control cell metabolism and
proliferation
– Growth factor signaling through Ras
– Mutated cell signaling genes in cancer cells are called
oncogenes
– Insulin signaling through PI-3 kinase
• TNF receptors activate protein complexes that control
cell death and survival
Chronic myelogenous leukemia (CML) cells
– Chromosomal rearrangement leads to expression of a unique
signaling kinase (Bcr-Abl) required for the leukemia cells to
survive
– Gleevec inhibits Bcr-Abl kinase, cells die through apoptosis
Receptor Tyrosine Kinases Control
Cell Proliferation and Metabolism
Receptor Protein Tyrosine Kinases
(EGF, Epidermal growth factor)
Courtesy: Roger Miesfeld
Receptor Protein Tyrosine Kinases
1) Receptor tyrosine kinases transmit extracellular signals
by ligand-activation of an intrinsic tyrosine kinase
function encoded in the cytoplasmic tail of the receptor.
2) Activation of the intrinsic tyrosine kinase activity
requires receptor dimerization, which is often stimulated,
or at least stabilized, by ligand binding.
3) Autophosphorylation of tyrosine residues within the
receptor creates phosphotyrosine docking sites for
signaling proteins that establish a relay signal between
the receptor and a downstream phosphorylation cascade.
Drosophila eyes and cancer are connected?
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Activated EGF receptor with intrinsic tyrosine kinase
Adaptor protein Grb2 binds EGF receptor dimers
Recruitment of SOS
Activation of Ras (inactivation by RasGAP)
Ras-Raf complex formation
Src phosphorylates Raf
Raf phosphorylates MEK
MEK phosphorylates ERK
ERK dimerizes and then phosphorylates ELK
SRF binds phosphorylated ELK
Initiation of transcription leads to cell proliferation
In “Sevenless” mutant, the R7
photoreceptor does not differentiate
properly, leading to no R7 cell, hence
the name.
SOS (Son of sevenless) is
downstream in pathway, interacts
with Sevenless
BOSS (self explanatory) is ligand
from neighboring cell that binds to
and turns on Sevenless in developing
eye
You can read more at:
http://www.sdbonline.org/fly/aimorph/eye.htm
Back to Cancer:
Receptor Protein Tyrosine Kinases
(EGF, Epidermal growth factor)
Courtesy: Roger Miesfeld
Grb2 binds to phosphotyrosines on
activated EGF receptor (Sevenless)
• Src homology domain, or SH2 domain
• phosphotyrosine binding pocket and a separate
specificity pocket
EGF Receptor
(Sevenless class)
Courtesy: Roger Miesfeld
SOS (Son of Sevenless) connects Grb2 to Ras,
activates Ras
SH3 domain
Courtesy: Roger Miesfeld
SOS is a guanine nucleotide exchange factor (GEF)
Ras/Src activate
phosphorylation
cascade
MAP kinase family:
Raf
MEK
ERK
GTPase activating proteins (GAP) such as
RasGAP bind to Ras and stimulate GTP hydrolysis
Courtesy: Roger Miesfeld
Turn off that signal!!
Oncogenes just turn me on
• Many “oncogenes” (oncology is the study of cancer),
interfere with feedback inhibition of growth factor
signaling pathways.
• Gain of function mutations
• Ras (rat sarcoma virus oncogene) may be involved in
30% of human cancers
• Src (Roux sarcoma virus oncogene)
most common Ras activating mutation
glycine to valine
mutation at codon
12 (G12V)
disrupts the intrinsic
GTPase activity
Courtesy: Roger Miesfeld
Insulin signaling is similar to EGF signaling
IRS: insulin receptor
substrate proteins
Courtesy: Roger Miesfeld
TNF Receptors Activate Proteins That
Control Cell Death and Survival
What in the world is TNF?
• Tumor Necrosis Factor
– an inflammatory cytokine
– a signaling molecule that induces apoptosis
• Apoptosis is cell death through “falling apart”
– Apoptotic bodies
– Engulfed by surrounding cells to clean up debris
• TNF Binds to a trimeric membrane receptor
– Initiates 2 or more pathways, depending on the cell conditions
To be or not to be…
Courtesy: Roger Miesfeld
Signal transduction through TNF receptor family
• Adaptor complex formation
• TNF R1 receptor is activated by TNF-alpha
• 80 amino acid structural motif in the cytoplasmic tail of
receptor called a Death Domain (DD) interacts with DDs
on other proteins
• The fate of the cell rests in the relative abundance (and
activities) of proteins in two separate, but inter-related,
signaling pathways.
Cell survival and cell death
are opposing pathways
FADD binding to
procaspase 8 stimulates
an autocleavage reaction
leading to cell death
TRAF2 binding to
TRADD recruits the
NFkB-inducing
kinase (NIK)
leading to cell
survival
Ratio of FADD/Caspase 8
to TRAF2, RIP and NFkB
determines cell fate
Courtesy: Roger Miesfeld