EAST 501 Lecture 13: 13- A. Bateman
21 Jun 2018
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13- A. Bateman
19- Somatic growth factors (I)
●Major themes
○Growth factors- introduction
■Secreted proteins.
○Autocrine, paracrine, juxtacrine signaling, the importance of gradients
■Act exclusively on receptors.
○Interactions of GFs with other signals in the tissue microenvironment (notch, hedgehog,
wnt, matricellular proteins)
○Multiple growth factors and microenvironmental signals interact in, as an ex,
angiogenesis
○Growth factors in cell motility, integrin, focal adhesions and CSK.
●Growth factors
○Secreted proteins
○Regulate cell division (positively or negatively)
■Usually positively.
○Also control cell motility, survival, differentiation
○Act through receptor mediated signal cascades
○May be autocrine, paracrine, or endocrine
■Example of endocrine GF = IGF-1 (secreted by the liver and circulates).
■IGF-2 is a paracrine growth factor (made where it has its effect).
■We will mostly see paracrine GFs today.
●Cellular targets of growth factor signaling
○Growth factors act on receptors.
■Once they engage the receptor, and initiate the signal transduction pathways, a
number of downstream effects:
●Transcription → control expression of cyclins (cell division). Allows
GFs to regulate cell cycle entry.
●Cytoskeleton → ability of cells to move. Cells don’t exist on a plate. In
the real body, surrounded by an ECM that is full of proteins. In order to
move, the cell has to detach form the ECM, break open from it, and
move. Regulated by matrix metalloproteinases. Some GFs can then
stimulate reformation of the ECM (stimulate collagen synthesis for
example).
●Translational machinery → new proteins (translation of proteins on
ribosomes)- cell size. If a cell is going to divide, it needs new proteins.
○mTOR pathway is involved. Allows GFs to communicate with
ribosomes and control the rate of mRNA translation.
●Receptor tyrosine kinase growth factor families
○There a very large number of GFs, but tend to orientate themselves into families:
■IGFs
■EGF/ TGF-alpha
■FGFs
■PDGFs
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13- A. Bateman
■VEGFs → growth factor that regulates vascular endothelial cells (important in
blood vessel development and angiogenesis).
■HGF → discovered in the context of liver regeneration, but is actually present
everywhere (also called scatter factor: stimulates cell movement).
■Ephrins → largest class of growth factors.
■Angiopoietins → involved in blood vessel development.
1- Growth factor receptors
●GF receptors
○They are tyrosine kinases.
○Each family of receptor differ in the details of their signaling, but the basic plan is the
same.
■We will use the FGF-receptor as an example.
○These receptors all use the familiar signal transduction pathways of SOS, RAS, RAF,
MAPKs pathway; PI3K pathway, and PLC gamma pathway.
●Some protein docking domains
○When activated, a phosphate is added onto a tyrosine (tyrosine kinase receptors).
○SH2 domain: conserved protein domain originally identified in the Src oncoprotein. Bind
to phosphorylated tyrosine residues on other proteins → allow proteins that contain the
domain to ‘dock’ onto the GF-R after it is tyrosine phosphorylated.
○SH3 domains: can be linked to SH2 domains to create adaptor proteins. Can link a
tyrosine kinase upstream signal to downstream pathways that are engaged by the
adaptor’s SH3 domains.
■Ex = Grb2 adaptor proteins, which connect tyrosine kinase signals to Ras. SH2
domain binds p-Tyr, and has two SH3 domains on either side.
●Some downstream kinases
○Tyrosine kinase: not receptors, but downstream molecules (cytoplasmic proteins). Will
phosphorylate other things. Include the SRC proto oncogene family.
○Serine/threonine kinases: start a Raf → MEK → MAPK or Erk.
○Lipid kinases: activation of GFRs leads to engagement of the lipid kinase PI-3K, which
converts PI4,5-bisphosphate to PI3,4,5-triphosphate → activates protein kinase B (=
Akt).
○Phospholipase C: breaks down PIP2 to give DAG (which activates the Ser/Thr kinase
protein kinase C) and IP3 (which stimulates the release of stored calcium and activation
of Ser/Thr kinase calcium sensitive kinases, such as calmodulin kinase).
●From the receptor to RAS
○Ras is a small GTPase .Activates Raf and sets the MAPK cascade in motion.
○SES is guanine nucleotide exchange factor for RAS.
■Removes the inactivating GDP from GDP-RASE, and allows GTP to bind to
RAS to temporarily turn RAS activity on.
○Small GTPase family: RAS family is involved in cell division, Rho family in cell shape
and movement, Ran family in movement of molecules in and out of the nucleus, Rab
family involved in membrane trafficking, Arf family which regulates vesicle trafficking,
and Rhebs which regulate the mTOR signaling pathway.
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13- A. Bateman
●FGF-receptor signaling pathways
○Gab1 and Gab2 are adaptor proteins. FRS2 as well → same as IRS1 and IRS2.
○Src family
○PLCgamma → once the receptor is phosphorylated, becomes activated, releases DAG
and IP3.
■DAG → activates PKC
■IP3 → increases calcium + activates calmodulin
○Grb2 binds phoshoyaltes FRS2 → linked to SOS → activation of RAS and MAPK
cascade.
○Need to be able to identify:
■A tyrosine kinase (apart form the receptor itself)
■Serine/threonine kinases (six)
■A lipid kinase
■Adaptor proteins
■A GTPase
■A guanine nucleotide exchange factor
●Growth factors and their receptors
○GFs exist in structurally related families.
■Each GF family has its own family of receptors, many ligands/ many receptors.
○Examples:
■EGF-family: 11 ligands and 4 receptors
■FGF-family: 23 ligands and 4 receptors
●EGF family
○All members of the EGF family are defined by the possession of a conserved structural
motif, the EGF-fold.
■Involved in cell proliferation, differentiation and survival, but hard to know
exactly where and why they are necessary.
●Epidermal growth factor family members- multiple ligands
○All members of the EGF family share the 3 beta-turns (some have one EGF motif, some
several).
■But other proteins (like selectins) have this EGF motif but aren’t GFs.
○Subgroups:
■EGF, TGF-alpha, etc → produced by and act on epithelial cells or fibroblasts.
■Neuregulins act in the nervous system.
○All EGF-family ligands are synthesized as a type 1 TM precursor.
■Then, these membrane receptors are broken down to release the active growth
factor.
●Three ways to use an EGF ligand to activate its receptor
○1- The membrane bound precursor (proligand) is cleaved by an enzyme called ADAM to
release the soluble ligand, which will diffuse through the extracellular fluid or blood to
the receptor.
○2- The precursor remains embedded in the cell membrane and activates receptors on
neighbouring cells (juxtacrine signaling).
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Document Summary
Autocrine, paracrine, juxtacrine signaling, the importance of gradients. Interactions of gfs with other signals in the tissue microenvironment (notch, hedgehog, wnt, matricellular proteins) Multiple growth factors and microenvironmental signals interact in, as an ex, angiogenesis. Growth factors in cell motility, integrin, focal adhesions and csk. Example of endocrine gf = igf-1 (secreted by the liver and circulates). Igf-2 is a paracrine growth factor (made where it has its effect). We will mostly see paracrine gfs today. Once they engage the receptor, and initiate the signal transduction pathways, a number of downstream effects: Transcription control expression of cyclins (cell division). Cytoskeleton ability of cells to move. In the real body, surrounded by an ecm that is full of proteins. In order to move, the cell has to detach form the ecm, break open from it, and move. Some gfs can then stimulate reformation of the ecm (stimulate collagen synthesis for example).
