Receptor Tyrosine Kinases 2 Flashcards
Diabetes:
Type 1 diabetes:
- Early onset
- Autoimmune disease caused by the destruction of
insulin producing cells in the pancreas
Type 2 diabetes:
- Later onset
- Caused by insulin resistance
- Linked to obesity
Insulin receptor:
The insulin receptor is the product of a single gene, INSR
The receptor is translated as a pro-receptor with an N-terminal signal peptide
The signal peptide directs the pro-receptor into the endoplasmic reticulum, where the signal peptide is cleaved off
The receptor then moves through the ER to the Golgi body. Here the protein is further processed by the protease “furin” to give rise to the a and b subunits
These subunits move via vesicles to the cell membrane
Activated by insulin, IGF (insulin like growth factor) -1 and 2
Insulin receptor activation:
The IGF-1 receptor is similar to the insulin receptor.
The IGF-2 receptor does not have a large cytoplasmic domain and cannot activate signaling
Can get 1/2 of insulin receptor (alpha + beta) with 1/2 of IGF1 receptor (alpha + beta) to form tetramer that can be activated by both insulin and IGF1
The receptor can bind up to two insulin molecules; binding is allosteric
Ligand binding results in a conformational change; this is thought to allow the two intracellular kinase domains to interact resulting in the phosphorylation of tyrosine residues in this domain.
This allows the recruitment of adaptor proteins (e.g. IRS1) that can bind phosphotyrosine
The PI 3-Kinase and MAPK pathways are the most established signaling pathways downstream of insulin and IGF.
Of these, work on insulin and IGF was key to uncovering the details of the PI3k-Art pathway
Insulin and glycogen synthase:
Insulin increases the production of glycogen in the liver
This requires an increase in glycogen synthase activity
Glycogen synthase can be inhibited by phosphorylation by the kinase GSK3
Insulin inhibits the ability of GSK3 to phosphorylate glycogen synthase; thus allowing dephosphorylation of glycogen synthase and increased glycogen synthesis
Understanding the regulation of GSK3 led to the description of the PI 3 kinase – Akt pathway:
- In reality recent work has shown that GSK3 is a relative small component of the regulation of glycogen synthase, and its allosteric activation by glucose is the more important pathway in vivo
GSK3:
GSK3 (glycogen synthase kinase 3):
Many substrates now described, plays roles in diverse processes including cell survival and development
In addition to insulin/IGF signaling also acts in the wnt pathway
Multiple regulator inputs
2 mammalian isoforms
Akt phosphorylates GSK3 downstream of insulin:
Members of the AGC kinase family
3 different Akt genes present in mammalian cells
All 3 Akt isoforms have a very similar domain structure, similar substrate specificity, and seem to play similar roles in vivo
Both the PH domain and phosphorylation play critical roles in the activation of Akt
PH Domain, Kinase Domain, Hydrophobic Motif
PH domains:
Around 250 PH domain containing proteins in the human genome
Small globular structure (about 120 amino acids) with a b sandwich and a helical domains
Bind to phosphoinsitides – eg PI (4,5)P2, PI (3,4,5)P3 and PI (3,4) P2
Different PH domains can have specific phosphoinsitide binding profiles
Akt PH domain binds to either PI (3,4)P2 or PI (3,4,5)P3
Activation of Akt:
Activation of Akt is a two step process
1st Akt must be recruited to the cells membrane via its PH domain
Akt is the phosphorylated on two key sites:
- T308 in the kinase activation loop
- S473 in the hydrophobic motif
T308 is phosphorylated by PDK1
S473 is phosphorylated by the mTORC2 complex
T308 phosphorylation is required for activity,
S473 phosphorylation helps stabilize the active conformation and protects T308 from dephosphorylation
Phosphorylation of both sites is required for maximal activity
PIP3 is generated by the action of PI3Ks
PI3K:
Phosphatidylinositol (PI) can be phosphorylated on its head group at the 3, 4 or 5 position
These phosphorylations are carried out by specific lipid kinases, these are named for the position on the ring they phosphorylate
Phosphorylation of the 3 position is carried out by a PI 3 kinase
3 classes of PI 3 kinase, each with multiple isoforms (Class I, II, and III)
Class IA PI3K is the major PI3K downstream of RTKs,
Its preferred substrate in cells is PI (4,5) P2.
Class I PI3K activation:
- In the absence of stimulation, the p105 PI3K subunit (catalytic subunit) is inhibited by the regulatory p85 subunit. (has 2 SH2 domains and a variant SH2 domain that mediates interaction with the catalytic subunit)
- Insulin binding to is receptor results in autophosphorylation of the tyrosine kinase domains
- This allows IRS1 to bind to the receptor via its PTB domain
- The receptor tyrosine kinase then phosphorylates IRS1
- PI3K is then recruited via the SH2 domains in the p85 subunit
- This relieves the autoinhibition of the catalytic subunit by the regulatory subunit, and brings the catalytic subunit to the membrane
- PIP3 is then generated via phosphorylation of PIP2
PKB phosphorylation of the transcription factors FOXO1 and FOXO3:
PKB phosphorylates FOXO1 and FOXO3 on 3 sites. This creates a binding site for 14-3-3 proteins.
13-3-3 binding stops the nuclear import of FOXOs
PKB activation therefore inhibits the transcription of FOXO dependent gnenes by localising FOXO to the cytoplasm
PKB phosphorylation of the transcription factors FOXO1 and FOXO3:
PKB phosphorylates FOXO1 and FOXO3 on 3 sites. This creates a binding site for 14-3-3 proteins.
13-3-3 binding stops the nuclear import of FOXOs
PKB activation therefore inhibits the transcription of FOXO dependent genes by localising FOXO to the cytoplasm
