1. Lecture 23 Signal Transduction 2

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

3. 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

4. Receptor Tyrosine Kinases Control Cell Proliferation and Metabolism

5. Receptor Protein Tyrosine Kinases
(EGF, Epidermal growth factor)
Courtesy: Roger Miesfeld

6. 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.

7. 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

8. 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:

9. Back to Cancer: Receptor Protein Tyrosine Kinases
(EGF, Epidermal growth factor)
Courtesy: Roger Miesfeld

10. 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

11. SOS (Son of Sevenless) connects Grb2 to Ras, activates Ras
SH3 domain
Courtesy: Roger Miesfeld
SOS is a guanine nucleotide exchange factor (GEF)

12. Ras/Src activate phosphorylation cascade
MAP kinase family:
Raf
MEK
ERK

13. GTPase activating proteins (GAP) such as RasGAP bind to Ras and stimulate GTP hydrolysis
Courtesy: Roger Miesfeld
Turn off that signal!!

14. 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)

15. most common Ras activating mutation
glycine to valine mutation at codon 12 (G12V)
disrupts the intrinsic GTPase activity
Courtesy: Roger Miesfeld

16. Insulin signaling is similar to EGF signaling
IRS: insulin receptor substrate proteins
Courtesy: Roger Miesfeld

17. TNF Receptors Activate Proteins That Control Cell Death and Survival

18. 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

19. To be or not to be…
Courtesy: Roger Miesfeld

20. 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.

21. 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