Lecture 23 Signal Transduction 2

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Lecture 23Signal Transduction 2
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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

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• 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

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Receptor Tyrosine Kinases Control Cell
Proliferation and Metabolism
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Receptor Protein Tyrosine Kinases
(EGF, Epidermal growth factor)
Courtesy Roger Miesfeld
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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.

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Drosophila eyes and cancer are connected?

• 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

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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
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Back to CancerReceptor Protein Tyrosine Kinases
(EGF, Epidermal growth factor)
Courtesy Roger Miesfeld
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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
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SOS (Son of Sevenless) connects Grb2 to Ras,
activates Ras
SH3 domain
Courtesy Roger Miesfeld
SOS is a guanine nucleotide exchange factor (GEF)
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Ras/Src activatephosphorylation cascade
MAP kinase family Raf MEK ERK
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GTPase activating proteins (GAP) such as RasGAP
bind to Ras and stimulate GTP hydrolysis
Courtesy Roger Miesfeld
Turn off that signal!!
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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)

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most common Ras activating mutation

• glycine to valine mutation at codon 12 (G12V)
• disrupts the intrinsic GTPase activity

Courtesy Roger Miesfeld
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Insulin signaling is similar to EGF signaling
IRS insulin receptor substrate proteins
Courtesy Roger Miesfeld
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TNF Receptors Activate Proteins That Control Cell
Death and Survival
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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

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To be or not to be
Courtesy Roger Miesfeld
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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.

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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