34. Cell signalling, receptors and kinases

Last updated on March 12, 2020 at 17:51

Summary

  • AMPK is activated when a cell has low [ATP], and activates and inhibits enzymes to try to raise [ATP]
  • mTORC1 is a protein that is activated by growth factors and nutrients, that stimulates cell growth
  • Erythropoietin affects gene expression through the JAK/STAT pathway and MAPK
  • Protein kinase C is activated by many hormones, and regulates the cell cycle and stimulates inflammation
  • HIF-1α is a protein that is activated in hypoxia. It increases glycolysis and inhibits TCA, while trying to make the oxidative phosphorylation more effective. It also activates transcription of erythropoietin and VEGF.
  • ChREBP is carbohydrate response element binding protein, and is stimulated by the end-products of the pentose phosphate pathway and activates anabolic enzymes
  • SREBP is sterol regulatory element binding protein. It activates cholesterol synthesis when cholesterol is low. It’s explained in the cholesterol chapter.
  • FOXO1 is a transcription factor that is inactivated by PKB.

Cell surface receptors

Those hormones which cannot diffuse freely across the cell membrane must bind to proteins which are located on the outside of the cell membrane, so-called cell surface receptors. These cell surface receptors are embedded in the cell membrane, with two important parts: one which is on the extracellular side of the cell membrane and one which is on the intracellular side of the cell membrane.

The hormone binds to the extracellular part of the receptor. When this binding happens, the conformation of the receptor changes. This triggers some change on the intracellular part of the receptor, which initiates a process inside the cell. That process will then initiate another process, and so on. This is how the signal is transmitted from outside the cell to the inside of the cell, without the hormone itself ever entering the cell.

There exists many different types of cell surface receptors:

  • Metabotropic receptors
    • G-protein coupled receptors
    • Receptor tyrosine kinase
    • Receptor guanylyl cyclase
    • Non-receptor tyrosine kinases (not very important right now)
  • Ionotropic receptors
    • Ligand-gated ion channels

Metabotropic receptors are those which act via a second messenger, which is explained below. Ionotropic receptors don’t act via a second messenger but rather by opening ion channels.

G-protein coupled receptors

  • Adrenergic receptors
  • Dopamine receptors
  • Glucagon receptors
  • Anterior pituitary hormone receptors

G-protein coupled receptors are the biggest class of cell surface receptors. Their name comes from the fact that they are coupled (but not bound) to a so-called G-protein on the intracellular side. The G protein is composed of three subunits, one α, one β and one γ.

  1. When glucagon (or any G-protein coupled receptor hormone) binds to its receptor, the receptor undergoes a conformational change.
  2. This change causes the intracellular part of the receptor to bind the G protein.
  3. This binding activates the G protein
  4. The activated G protein binds a GTP
  5. The binding of GTP causes the α subunit of the G protein to dissociate from the other two subunits
  6. The α subunit will then activate a different target protein, which furthers the signal transduction

There are three types of G proteins. The three different types act on different target proteins:

  • Gs protein – acts on the enzyme adenylyl cyclase and stimulates it
  • Gi protein – acts on the enzyme adenylyl cyclase and inhibits it
  • Gq protein – acts on the enzyme phospholipase c and stimulates it

Adenylyl cyclase is an enzyme which converts ATP to cyclic AMP, often abbreviated as cAMP. Phospholipase C is an enzyme which cleaves phospholipids in the cell membrane into two molecules, IP3 and DAG.

cAMP, IP3 and DAG are so-called second messengers. They are small molecules which are synthesizes in large amounts in response to a hormone binding to a receptor, like we’ve seen. These second messengers will then stimulate other proteins, which will change the cell’s behaviour in some way.

cAMP activates a protein called protein kinase A (PKA). IP3 and DAG activate a protein called protein kinase C (PKC). More about these later.

When described stuff that occurs after the binding of the hormone to the receptor, we use the term downstream. Think of the signal transduction as a river. What occurs further down the river occurs downstream. This means that the second messengers act downstream from the receptors, and PKA and PKC act downstream from the second messengers.

Each G protein coupled receptor subtype is coupled to one of the three types of G protein. For example:

  • α1-adrenergic receptor is Gq-protein coupled
  • α2-adrenergic receptor is Gi-protein coupled
  • β-adrenergic receptor is Gs-protein coupled
  • Dopamine D2 receptor is Gi-protein coupled
  • Histamine H1 receptor is Gq-protein coupled
  • Histamine H2 receptor is Gs-protein coupled

Let’s continue the process from above. We left off at step 6, and our example is glucagon and the glucagon receptor, which is Gs-protein coupled.

7. The α subunit of the Gs protein will stimulate adenylyl cyclase

8. Adenylyl cyclase will convert many ATP molecules into many cAMP

9. cAMP will activate many protein kinase A molecules

10. Protein kinase A will change the behaviour of the cell. If the cell in question is a liver cell, the cell will initiate glycogenolysis and gluconeogenesis, and stop glycolysis.

Receptor tyrosine kinases

  • Insulin receptor

The only important receptor tyrosine kinase (RTK) is the insulin receptor. It functions similarly to the G protein coupled receptors, except the receptor is not coupled to a G protein. Instead, when the hormone binds to the receptor, the intracellular part of the receptor phosphorylates itself (autophosphorylates).  Let’s look at the whole process.

  1. Insulin binds to the insulin receptor
  2. The insulin receptor undergoes dimerization, a process where to insulin receptors “bind” together
  3. The two dimerized insulin receptors phosphorylate each other, which is called autophosphorylation
  4. The autophosphorylation causes the intracellular part of the insulin receptor to gain tyrosine kinase activity, meaning that it can phosphorylate tyrosine amino acid residues on other molecules
  5. The insulin receptor phosphorylates and activates many downstream targets, like PI3K, insulin-sensitive kinase (ISK), insulin receptor substrate (IRS)
  6. Each of the downstream target active further downstream processes which alters the behaviour of the cell

More about insulin in its topic.

Receptor guanylyl cyclase

Receptor guanylyl cyclase is special and exists in two forms; one “normal” form which is membrane bound like GPCR and receptor tyrosine kinases, and one “soluble” one, which is not bound to the cell membrane but rather floats around freely inside the cytoplasm.

Membrane-bound receptor guanylyl cyclase:

  • Atrial natriuretic peptide (ANP)
  • Brain natriuretic peptide (BNP)

When ANP or BNP binds to their receptor, the events will be similar to those of the insulin receptor. The only exception is that instead of the intracellular part of the receptor gaining tyrosine kinase activity, it gains guanylyl cyclase activity instead. Guanylyl cyclase converts GTP to cyclic GMP (cGMP), another second messenger. cGMP then activates the so-called protein kinase G (PKG).

Souble guanylyl cyclase:

  • Nitric oxide (NO)

NO is a gas and therefore freely diffuses across cell membranes. This is how it can reach and bind to the soluble guanylyl cyclase, which floats around in the cytoplasm. After the binding guanylyl cyclase will convert GTP to cGMP and activate PKG, just like for the membrane-bound version.

Ligand-gated ion channels

  • Nicotinic acetylcholine receptors
  • GABA-A receptors
  • NMDA glutamate receptors

Ligand-gated ion channels, also called ionotropic receptors, are cell surface receptors which are also ion channels. When the hormone binds to the extracellular part of the ligand-gated ion channel, the ion channel in question opens up, allowing ions to flow into or out of the cell. The exact ion depends on the specific subtype of receptor.

The prototoype ionotropic receptor is the nicotinic acetylcholine receptor. It is one of two types of receptors which acetylcholine binds to, the other being muscarinic acetylcholine receptor. The receptor is present in many places in the body, most notably in the neuromuscular junction and in the ganglia of the autonomic nervous system.

After acetylcholine has bound to the nicotinic acetylcholine receptor, the ion channel will open. The ion channel in question is a non-selective cation channel, meaning that it allows all types of cations to flow into and out of the cell, most importantly Na+ and K+. Sodium will flow into the cell and potassium will flow out. This causes a depolarization of the cell membrane, which transmits the signal further.

Nuclear receptors

  • Type I nuclear receptors
    • Androgen receptor
    • Oestrogen receptor
    • Glucocorticoid receptor
    • Mineralocorticoid receptor
    • Progesterone receptor
  • Type II nuclear receptors
    • Retinoic acid receptor
    • Thyroid hormone receptor

There are two types of nuclear receptors, receptors which are located inside the nuclei. The hormones which bind to these receptors diffuse across the cell membrane and perhaps also the nuclear envelope to reach the receptor located inside the nucleus.

All nuclear receptors are actually ligand-gated transcription factors. This means that when the hormone binds to the nuclear receptor, the hormone-receptor complex will act as a transcription factor, changing DNA transcription in some way.

Type I nuclear receptors are actually located in the cytoplasm. When the hormone diffuses across the cell membrane and enters the cytoplasm, it will bind to the type I nuclear receptor there. This hormone-receptor complex will create a dimer with another hormone-receptor complex. This dimer of two nuclear receptors and two hormones then passes the nuclear envelope through a nuclear pore, and then binds to DNA to begin DNA transcription.

Type II nuclear receptors are found in the nucleus. They exist in a complex with a retinoid X receptor (RXR), and are bound to a corepressor, a molecule that represses its activity. After the hormone enters the nucleus through a nuclear pore, it will bind the nuclear receptor/RXR complex, which will cause the corepressor to dissociate. The complex will then bind RNA polymerase, which will begin transcription.

AMPK

AMP-dependant kinase, or AMPK for short, is an enzyme that is activated when the cell’s concentration of AMP is high (because that means that [ATP] is low), or by exercise. It is also activated by adiponectin, a hormone that is released by adipose tissue during periods of starvation. AMPK then activates many processes that will elevate the level of available energy in the body (ATP), while inactivating many processes that would use ATP. The table below shows what AMPK affects.

Cardiac glycolysis Activated
Glucose uptake through GLUT4 and GLUT1 Activated
Fatty acid oxidation Activated
Fatty acid synthesis Inactivated
Cholesterol synthesis Inactivated
Triacylglycerol synthesis Inactivated
Glycogen synthesis Inactivated
Protein synthesis Inactivated
Insulin secretion Inactivated

AMPK also affects cognitive functions. It will stimulate feeding behaviour, and inhibit behaviour that uses unnecessary energy. By changing the body’s behaviour to stimulate feeding and stimulate processes which yields energy from energy stores (like beta oxidation), AMPK works to increase the concentration of ATP in the cells of the body.

mTORC1 increases cell growth and proliferation

mTORC1 is a protein complex that is activated by high concentration of nutrients and energy, and growth factors. It increases energy production and the activity of the pentose phosphate pathway, to create materials needed for protein synthesis.

It also increases transcription of genes involved in angiogenesis and adipogenesis, to provide materials needed for membranes. Finally, the cell uses these materials to grow and proliferate. Too high or too low activity of mTORC1 causes age-related diseases.

Erythropoietin

EPO is a hormone that stimulates RBC production. The hormone binds to its receptor, EPO receptor. This receptor is bound to a protein called Janus kinase, or JAK. When EPO is bound, JAK is activated. JAK then activates two pathways, the MAPK cascade, and the STAT protein. Both affect gene expression.

Protein kinase C

PKC is a kinase which is activated by Ca2+ and diacylglycerols (DAG). It’s involved in the signalling pathway of many hormones, like oxytocin, angiotensin, histamine, vasopressin, etc.

When activated, PKC is involved in many functions, like learning, memory, regulating cell growth and mediating immune responses. It also regulates transcription through a protein called NF-κB, which also stimulates inflammation.

HIF-1α

Hypoxia-inducible factor 1-α, or HIF-1α, is a protein that is used to sense the concentration of O2 in the cells. It is activated when cells are low on oxygen (hypoxia), and when activated, it activates processes that yield energy without needing oxygen, while inhibiting processes that do need oxygen. It also activates processes like angiogenesis, which is important in hypoxia.

During normoxia

HIF-1α is constantly hydroxylated at a proline residue by O2, a process which requires vitamin C and Fe2+. After hydroxylation, it binds to a protein called Von Hippel–Lindau protein, or pVHL. The HIF-1α-pVHL complex is then ubiquitinated and broken down.

During hypoxia

When O2-saturation is low, HIF-1α can’t be hydroxylated, because hydroxylation requires O2. When not hydroxylates, HIF-1α is a transcription factor. Now that HIF-1α isn’t hydroxylated and broken down it is instead phosphorylated, which prevents it from being broken down, allowing it to do its job as a transcription factor. It will then travel into the nucleus and change gene expression of certain genes.

HIF-1α increases expression of genes that are involved in glycolysis, and lactate dehydrogenase. Because glycolysis doesn’t need O2, but the TCA and oxidative phosphorylation do, glycolysis is activated while the latter two are inhibited. It also inhibits pyruvate dehydrogenase complex.

HIF-1α also modifies complex IV of the oxidative phosphorylation. It switches out one of its subunits with another subunit that is more effective when O2 is low.

The mechanism of how cells use VHL and HIF-1α to sense the oxygen level was the subject for the nobel prize of medicine and physiology in 2019. Cool stuff!

ChREBP

Carbohydrate response element binding protein, or ChREBP, is a transcription factor whose purpose is to stimulate fatty acid and lipid synthesis when the body has enough energy.

It is activated by xylulose 5-phosphate, a product of the pentose phosphate pathway, and PP2A. ChREBP increases expression of pyruvate kinase (in the liver), acetyl-CoA carboxylase and fatty acid synthase in the liver, adipose tissue and the kidney. This increases fatty acid and lipid synthesis.

FOXO

Forkhead box other, or FOXO, is a class of transcription factors. They target genes that negatively regulate cell growth and survival. We know from before that FOXO1 activates PEPCK and G6Phase, but FOXO genes also inhibit expression of enzymes related to glycolysis, pentose phosphate pathway and triacylglycerol synthesis. It can bind another protein called 14-3-3.

NF-κB

Nuclear factor kappa B, or NF-κB, is a transcription factor that is activated by several stimuli, like stress, free radicals, cytokines, infection or UV radiation. It responds to these stimuli by increasing expression of genes related to immune response and inflammation, amongst others. It is a very central component of immunology and pathology and you’re going to hear more about it later.


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33. Insulin

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35. NO and CO as signal molecules

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