Signaling in Metabolic Regulation Flashcards
What are growth factor receptors?
There is signal transduction through cAMP pathway or the IP3 pathway, these are part of the GPCRs.
There is also another family of receptors called tyrosine kinase receptors, which have tyrosine kinase activity.
- They are largely single transmembrane domain receptors and have a very large number of functions.
- Activation of the receptor leads to activation of the tyrosine kinase, because it is a kinase it will result in phosphorylation. This can lead to activation of multiple signaling pathways.
The receptors for a given growth factor, can activate different pathways in different tissues, despite the receptor being the same -> tissue specific.
Examples of growth factor receptors
3 examples of Receptor Tyrosine Kinase
2 examples of JAK/STAT
example of Receptor Serine/Threonine kinase
Receptor Tyrosine Kinase
• Insulin
• Epidermal growth factor (EGF)
• Platelet derived growth factor (PDGF)
JAK/STAT
• Growth hormone
• Interferon receptors
Receptor Serine/Threonine kinase
• Transforming growth factor (TGFβ)
Enzyme linked receptors
what are the involved in the regulation of? (5)
They are involved in regulation of • Cell growth • Division • Differentiation • Survival • Migration
Inappropriate activation is associated with disease, particularly cancer
The appearance of a typical tyrosine kinase receptor molecule
protein number?
structure?
exception to this?
where is the binding site?
where is the enzyme activity?
Generally, they are single transmembrane domain proteins (with an exception of insulin). They have a binding site on the extracellular domain and then enzyme activity on the cytosolic part of the molecule.
Binding of an agonist to the receptor result in changes in the intracellular domain enabling it to activate a range of different pathways.
tyrosine kinase receptor molecule
Process after agonist binding
what happens after an agonist binds? why does this happen?
what are 2 ways this can happen?
what kind of interactions are these? what does it allow?
A very important step after the binding of the agonist is that the two receptor molecules dimerise (come together). This is so they can activate each other.
There are two ways in which this can be done; ligand mediated, or receptor mediated
Ligand mediated – Could be that agonist binds to one receptor and another to the other and the two agonists come together also
Receptor mediated – Could be that binding of agonist causes conformational change in molecules leading to dimerization
Remember these are not covalent interactions, they are transient, the process allows the bringing together of the tyrosine kinase activities associated with the intracellular part of the receptor.
- The receptor then autophosphorylates the adjacent tyrosines, but it does this in a very specific manner.
receptor then autophosphorylates the adjacent tyrosines
what 2 molecules bind to the tyrosing kinase? what will thr tyrosine activity do? if there is multiple, what happens?
what is the important thing about this process? what does it form? what does this allow?
what can this lead to?
Two EGF bind to each tyrosine kinase molecules and then these come together. The tyrosine activity will then only phosphorylate tyrosines that are in a specific context/sequence (motif). There is not general phosphorylation of tyrosines.
There may be multiple of these sequences in the intracellular domain, so there may be multiple phosphorylations (so one receptor can be phosphorylated many times).
The important thing about this now phosphorylated tyrosine is that it will form part an SH2 domain motif.
-> This is a site that is recognised by other proteins (with an SH2 domain), allowing other proteins to be recruited to the receptor.
This recruitment can lead to their direct activation or that it facilitates the recruitment of further proteins.
So proteins containing the SH2 domain will bind to the SH2 domain motif which will then set of a cascade of events. The details of the cascade do vary from one agonist to the next
In our example we are using EGF:
EGF cascade
what binds to the SH2 domain motif? what does this activate?
why does this allow fine regulation?
what is a the ras protein associated with? type of protein?
what does it bind when inactive? binds to when active?
The Grb-2 proteins bind to the SH2 domain motif, then the Grb-2 also through SH2 domains activates another protein, which itself then activates another protein.
This allows very fine regulation because you can regulate any one of these proteins to modulate the end result.
The RAS protein (associated with cancer) which a type of G-protein, binds GDP when inactive. On activation it exchanges the GDP for GTP and then is activated.
It is important to remember that activation of EGF receptor can potentially activate many pathways, through recruitment of various proteins.
Each of these pathways can have a different physiological effect e.g. growth, motility. All the pathways can and do interact.
More about RAS
what pathway is it related to for G proteins?
inactive bound to?
active bound to? with the help of?
what does the activated molecule have/enzyme? what does it do? speed? what aids this?
why does the cell have this?
what does GAP do?
The RAS G protein is important because of its implication in certain cancers.
Ras is one of a number of small G protein
Related to the Ga subunit of cAMP
Inactive GDP bound
Active it is replaced by GTP helped by Guanine nucleotide exchange factor (GEF)
There is first the exchange of GDP -> GTP when the protein is activated. This then recruits other proteins, in this case the protein RAF. The whole process is aided (sped up) by the protein guanine nucleotide exchange factor (GEF).
The GTP binding proteins have an activity called GTPase activity, which remove phosphates of the RAS protein, inactivating it. However, this is usually very slow. But, there is a protein that aids this called GAP (GTP-activating protein), which helps in the removal of the phosphate.
The cell has this in order to down-regulate this process so there isn’t overstimulation of the cell.
So GAP enhances inactivation. Once it has removed the phosphate, the complex falls apart and then waits in the membrane for further activation.
So this is one of the means by which receptor tyrosine kinase activity can be regulated.
Mutated Ras is found in 30% of tumours
Summary of tyrosine kinase receptor activation
The RTK (receptor tyrosine kinases) is inactive in the absence of epidermal growth factor, binding of the growth factor to the receptor leads to dimerisation, autophosphorylation of tyrosine residues and activation of proteins.
RTK inactivation
what leads to inactivation? what does this?
when are these activated?
where may these enzymes lead to prolonged activation?
what is special about RAS?
As activation of these receptors leads to phosphorylation, dephosphorylation leads to inactivation. Phosphatases do this.
These phosphatases are activated as a result of the receptor activation, in some cancers the genes for these enzymes are mutated = prolonged activation.
Small g-proteins such as Ras have intrinsic GTPase activity so auto-regulate themselves.
EGF Receptor Family and Cancer
failure to refualt EGF receptor activity leads to?
what is the reaosn for this?
if EGF receptor mutates, what can happen?
what can RAS lose leadinf to inappropriate growth?
what can happen to phosphatase?
what targets the egf receptor? what is this?
Failure to regulate EGF receptor activity is implicated in a number of cancers. The reason for this is because of mutations in part of the cascade, so in the receptor, enzyme, g-protein etc. this then removes a level of control of the process by the cell.
If EGF receptor is mutated and can dimerise in absence of EGF, then it will no longer be dependent on EGF.
Ras may lose its GTPase activity, so it can no longer turn off its activity, leading to inappropriate growth
Loss of phosphatase activity means you cannot inactivate the receptor, leading to excessive stimulation of the downstream pathways.
Herceptin is a monoclonal antibody that targets the EGF receptor blocking its action
Insulin Receptor
what is this? what is special about it? what does it consist of?
when insulin binds, what happens? what is brought together? what do these have? hence effect?
what does it recruit after activation?
how is this protein recruited? effect of this?
The insulin receptor is also a tyrosine kinase type receptor, but it has a different format. It consists of two tyrosine kinase receptor molecules that are linked, so it is already a dimer.
-> It consists of two beta chains and alpha chain linked by disulphide bridges.
When insulin binds, there is a conformational change which brings together the two beta chains. The beta chains have tyrosine kinase activity so can auto-phosphorylate tyrosine residues on the adjacent beta chain.
Insulin receptors are also special in the proteins that it recruits upon activation, one of the most important is the insulin receptor substrate (IRS).
This is a protein recruited through the phosphorylated tyrosines on the beta chain with SH2 domains on the IRS -> phosphorylation. Causing generation of more phosphorylated tyrosines which can then go on to recruit more proteins.
Insulin Receptor
IRS activity
what does IRS activate? what does this lead on to activate?
what can this do? (3)
what else does the insulin receptor recurit? (2)
In the example above, IRS activates PI3K, which leads on to activate protein kinase B. PKB can then do a number of things including stimulating glycogen synthesis, protein synthesis and increased expression of GLUT transporters on cell surface membrane.
The insulin receptor also recruits other proteins such as Ras and PLC (involved in generation of IP3)
So, the insulin receptor is on liver -> adipocytes and muscle and helps reduce blood glucose.
Receptor Tyrosine Kinase Summary
6 key things
- Are a superfamily of receptors, consisting of a ligand binding domain and protein tyrosine kinase domain
- They primarily control cell growth and differentiation
- Each consists of a single transmembrane protein with exception of insulin receptor
- Activation leads to phosphorylation of specific tyrosine residues in an amino acid sequence
- Phosphotyrosine interacts with proteins that contain SH2 domains (src-homology domain)
- SH2- containing proteins include PI-PLC and PI3K which can go on to generate secondary messengers
Growth Hormone Receptor
how is it different to tyrosine kinase receptors? what are growth hormone recptors important for?
what does binding of growth hormone to its receptor lead to? what is different here?
what can this activated growth hormone not do? what helps with this? what does it allow?
once this happens, what does it provide? what is one of these? what happens to it and where does it go? why?
Differently to the tyrosine kinase receptors, growth hormone receptors do not have intrinsic tyrosine kinase activity within the intracellular domain. Note they are important for cytokine binding.
Binding of growth hormone to its receptor leads to dimerisation, but here only one growth hormone molecule needs to bind to facilitate dimersiation/activation of the receptor.
So, this activated growth hormone receptor does not have the ability to phosphorylate tyrosines on its own. This tyrosine kinase activity is provided by the recruitment of a protein called JAK.
The receptor is activated, the JAK is recruited and there is dimerisation, then this JAK gives the tyrosine kinase activity and then causes phosphorylation of the adjacent chain.
This phosphorylation of tyrosines then provides a site to which other proteins can be recruited, one of these proteins is called STAT (signal transducer and activator of transcription). STAT is then dimerised (stabilising it) and then transported to the nucleus where it causes regulation of various genes via transcription
