Lecture 20 Flashcards
Cell Signaling II
receptor tyrosine kinase (RTK) signaling molecules
- the RTKs is a family of membrane receptors with intrinsic protein kinase activity that also transduce extracellular signals
- ligand-binding domain on the extracellular and kinase activity on the intracelluar domains
- around 60 RTKs in humans
- single pass membrane protein -> receptor that acts as an enzyme examples: insulin receptor, platelet derived growth factor receptors
pg 485
insulin signaling as a prototype RTK
- insulin is the ligand
- receptor encoded by a single gene, then the protein is cleaved to form two separate subunits which come back together at cell surface
- unique among RTKs in that is the only known RTK that assumes a dimer formation in the absence of a ligand
- upon insulin binding, each monomer phosphorylates its partner at several tyrosine residues in the cytoplasmic domain
- activates 2 separate but interlocking pathways, the PI3K/Akt pathway and the MAPK pathway
- one subunit phosphorylates the other through transphosphorylation (autophosphorylation)
- ligand binds to receptor -> conformational change -> autophosphorylation of tyrosine
pg 486
the MAPK pathway - responsible for mitogenic effects of insulin
mitogenic mechanism -> uses insulin as ligand for cell growth and cell division
* MAPK = mitogenic activating protein kinases
1. insulin receptor binds insulin and undergoes autophosphorylation on its Tyr residues
2. insulin receptor phosphorylates IRS-1 on its Tyr residues
3. SH2 domain of Grb2 binds to P-Tyr of IRS-1; SOS binds to Grb2, then to Ras causing GDP release and GTP binding to Ras (a G protein)
4. activated Ras binds and activates Raf-1 (a kinase)
5. Raf-1 phosphorylates MEK, MEK phosphorylates ERK (MEK & ERK are MAPK)
6. ERK moves into nucleus and phosphorylates nuclear transcription factors (like Elk1)
7. Elk1 joins SRF to stimulate transcription and translation of genes needed for cell division
pg 487
regulation of glucose and glycogen metabolism by insulin: blood sugar and homeostasis
PI3K pathway -> involved in glucose metabolism and glycogen building activities
1. IRS1 (phosphorylated by insulin receptor) activates PI3K by binding to its SH2 domain, PI3K converts PIP2 to PIP3
2. PKB (protein kinase B, aka AKT) bound to PIP3 is phosphorylated by PDK1. PKB then phosphorylates GSK3 on a Ser residue (inactivating it)
3. GSK3 cannot convert glycogen synthase to its inactive form by phosphorylation, so it remains active
4. synthesis of glycogen from glucose is accelerated
5. PKB stimulates movement of glucose transporter GLUT4 from internal membrane vesicles to the plasma membrane increasing the uptake of glucose
pg 488
PTEN negatively regulates growth factor signaling inhibiting cancers
- PTEN (phosphatase and tensin homologue) directly interferes with the AKT pathway by dephosphorylating PIP3 back to PIP2 (which is no longer capable of recruiting PDK)
- inactivating mutations in PTEN place individuals at greatly increased risk of cancer
- example of MAPK impact; cancer drugs use PTEN homologues to decrease AKT production and control cell divison
pg 490
regulation of cell survival by insulin and other growth factors
- activated Akt phosphorylates and inhibits a protein called Bad which itself stimulates apoptosis -> non-phosphorylated Bad activity promotes apoptosis, when Bad becomes phosphorylated in response to growth factor signaling, it is inactivated and is therefore unable to stimulate apoptosis
- therefore, insulin signaling inhibits an activator of apoptosis, leading to enhanced cell survival
- Akt also phosphorylates Rheb, which activates TOR, which activates cell growth (increased protein syntheis and cell volume -> hypertrophy)
- Akt can influence growth of cell and tissues through Rheb and TOR pathway
pg 491
JAK-STAT pathway
- JAK = just another kinase, STAT = signal transducers and activators of transcription
- ligand-induced receptor dimerization
- JAK recruited to intracellular domains of receptors only after dimerization
- JAK then phosphorylates the receptor, which creates temporary docking sites for STATs (JAK recruits STATs)
- JAK then phosphorylates STATs
- STATs dissociate from the receptor, dimerize, enter nucleus to stimulate transcription of target genes
pg 492
JAK-STAT pathway of erythropoietin (EPO)
- important molecule released by the kidney
- pathway induces erythropoiesis
- treates hypoxia related conditions
pg 492
fibroblast growth factor receptor 3
FGFR3 is an intrinsic tyrosine kinase which suppresses chondrocyte proliferation and differentiation; mutations in FGFR3 or excess FGFR3 can lead to achondroplasia (short stature)
pg 492
hijacking of GPCRs by insulin signaling
- insulin potently antagonizes several GPCRs, especially the β-adrenergic receptor
- other GPCRs are also phosphorylated by insulin signaling, which creates a new docking site for a protein called “SHC”
- SHC can initiate the MAPK pathway independently of IRS-1; in this way, insulin boosts MAPK even further in some cell types
pg 494
mutations in Ras and GAPs induce cancers
- inactivating mutations in Ras are common in many types of cancers; if, for instance, a missense mutation in the active site of Ras kills its activity, it can’t hydrolyze its bound GTP and becomes constitutively active
- loss of function mutations in GAPs can have a similar effect because GAPs normally act to greatly accelerate the rate of GTP hydrolysis in Ras and related proteins
- GEF and Ras keep GTP and GDP in balance; mutations in Ras lead to loss of hydrolysis and uncontrolled growth, proliferation, and migration
pg 495
VEGF signaling in cancer and inhibitors
- vascular endothelial growth factor
- VEGF’s normal function is to create new blood vessels during development, new blood vessels after injury, muscle following exercise, and new vessels to bypass blocked vessels
- Solid cancers cannot grow beyond a limited size without an adequate blood supply; cancers that can express VEGF are able to grow and metastasize.
- Overexpression of VEGF can cause vascular disease in the retina of the eye and other parts of the body.
- Drugs such as bevacizumab can inhibit VEGF and control or slow those diseases.
pg 496
HER2 pathway in breast cancer and effective inhibitors
- HER2 is a membrane tyrosine kinase and oncogene that is overexpressed and amplified in about 20% of breast cancers.
- When activated it provides the cell with potent proliferative and anti-apoptosis signals and it is the major driver of tumor development and progression for this subset of breast cancer.
- Human epidermal growth factor receptor 2 (Erbb gene)
- lapatinib (drug) binds to intracellular domain of HER2 receptor and prevents ATP transfer required for kinase to work
- trastuzumab (drug) binds to extracellular domain and blocks dimerization of receptor therefore blocking all activity
pg 497
