01-02-22 - Lipids in Cell Membranes 2 Flashcards
Learning outcomes
- To introduce the concept of cell communication
- To introduce the concept of signal transduction
- To explain the role of phospholipids from the cell membrane in intra- and inter-cell signalling
- To review the role of phospholipase C in cell signalling
- To describe the synthetic pathway of eicosnoids from arachidonic acid liberated from phospholipids
- To explain how eicosanoid synthesis can be targeted to treat inflammatory disorders
What 2 ways do phospholipids participate in cell signalling?
• 2 ways do phospholipids participate in cell signalling
1) Phospholipids participate in cell-cell communication mechanisms by giving rise to intracellular second messengers
2) They also serve as precursors for compounds that are released from cells and act on other cell types, called eicosanoids, which are inflammatory mediators
What are the 6 steps of cell-cell communication?
• 6 steps of cell-cell communication:
1) Synthesis of signal
2) Release of the signalling molecule by the signalling cell – may take place exocytosis, diffusion, or cell-cell contact (signalling molecule may be on cell surface)
3) Transport of the signal to the target cell e.g blood stream, nerve
4) Detection of the signal by a specific receptor protein
5) A change in cellular metabolism, function or development triggered by the receptor-signal complex
6) Removal of the signal or desensitisation (return to rest)
What are 2 examples of extracellular signalling molecules that can act at long range?
What is an example of each?
• extracellular signalling molecules that can act at long range?
1) Endocrine (hormones)
• Hormones released by endocrine cells and carried in the bloodstream to distal target cells
• Example – follicle stimulating hormone (FSH) released from the pituitary acts upon the ovary
2) Neurotransmission
• Network of neurons
• Neuron releases neurotransmitter which is received by its target cell (another neuron)
• Example- breathing: the phrenic and thoracic nerve send impulses form the brain to the diaphragm
What are 2 examples of extracellular signalling molecules that can act at short range?
What is an example of each?
• Extracellular signalling molecules that can act at short range:
1) Paracrine
• Signalling molecules only affect target cells in close proximity to secreting cells
• Examples – somatostatin released by the pancreas cells react locally
• Neurotransmission can also be considered a type of paracrine signalling
2) Autocrine
• Cells respond to substances that they release themselves
• Example – some neurotransmitters and growth factors bind to cells that release them
3) Membrane bound
• Membrane bond proteins can interact to signal
• Example – signalling by T-cells in immune system
What are examples of multiple types of signalling occurring simultaneously?
• Insulin is released form pancreatic beta-cells acts in an autocrine, a paracrine, and an endocrine manner
What 2 ways do signalling molecules affect cell activity?
• Methods of signalling molecules affecting cell activity:
1) Hydrophilic signal transduction
• Signalling molecules act on membrane-bound receptors that control the production of intracellular chemicals (second messengers), which mediate cell activity
2) Hydrophobic signal transduction
• Lipid soluble (hydrophobic) signalling molecules can diffuse through the lipi bilayer and bind to intracellular receptors
What are the 4 different types of receptors found in the cell?
• 4 different types of receptors found in the cell:
1) Ligand gated ion channels (ionotropic receptors)
• Open or close in response to binding to signalling molecule
• Triggers hyperpolarisation or depolarisation through movement of ions across the membrane
• Rapid signalling
2) G-protein couples’ receptors (metabotropic)
• These receptors can signal through a G-protein
• When the receptor is activated by a binding signal molecule, the G-protein goes and interacts with another type of molecule in the cell
• Could be an ion channel, but more often will be an enzyme that creates second messengers, which mediate cell activity
3) Kinase-linked receptors
• Receptors that are enzymes that phosphorylate proteins inside the cell
4) Nuclear receptors
• Hydrophobic signalling molecule that can diffuse through the cell membrane and bind to a receptor in the cell
• This receptor will normally be a transcription factor, meaning proteins have to be produced, and the response may take several hours
What can dictate whether intracellular responses are fast or slow?
- If protein machinery is already present within the cell, signalling will be faster (seconds to minutes)
- If genes need to be transcribed to produce proteins, the cellular response will be slower (minutes to hours)
How are hydrophobic signalling molecules normally transported round the body?
What happens when the signalling molecules get near the cell?
What are 4 examples of lipid soluble molecules?
- Hydrophobic molecules are normally transported round the body through the circulation, but because they are hydrophobic, they would need a transport protein
- When the hydrophobic signalling molecules get near the cell, they are released from the transport protein and diffuse through the membrane
- They then bind to a specific receptor, creating a complex
- This complex is normally a transcription factor that will bind to DNA to invoke synthesis of mRNA, which can be used to synthesise proteins
• Examples of lipid soluble molecules: 1) Cortisol (stress hormone) 2) Oestradiol 3) Testosterone 4) Thyroxine
Describe the steps in the inositol phospholipid signalling pathway
1) A signalling molecule binds to a G-protein-linked receptor, which activates a G-protein α subunit
2) This subunit then activates phospholipase C, which then cleaves PIP2, so that the inositol group and 3 phosphate becomes cleaved, forming IP3 and diacylglycerol
3) IP3 binds to calcium receptors on the ER, causing it to open, allowing calcium to move into the cell cytosol
4) Protein Kinase C (PKC) can then bind to calcium and diacylglycerol and become active
5) PKC can then phosphorylate proteins in the cell and alter their function
When does calcium concentration increase?
What is the role of calcium?
What is an example?
What happens when calcium binds to calmodulin?
- Calcium concentrations increase in the cell in response to IP3 release
- Calcium binds to proteins to regulate their function
- Example - Ca2+/Calmodulin activates proteins/enzymes through direct interaction (e.g. myosin light chain kinase (MLK), which regulates smooth muscle contraction
- When calcium binds, protein interaction sites on calmodulin are accessible
What are 3 steps to Protein Kinase C (PKC) activation?
What are 3 examples of PKC substrates?
What is their role?
• Steps to PKC activation:
1) Calcium released from ER in response toIP3
2) Calcium binds to C domain on PKC
3) This activates the enzyme, allowing it to phosphorylate various substrates
• PKC substrates:
1) Tumour suppressor p53 (transcription factor – prevents tumour formation
2) Cav 1.2 (calcium channel) – heart muscle contraction
3) IKKα (cytokine) – B cell activation (immune function)
What happens to IP3 after signal transduction has occurred?
• After signal transduction has occurred, IP3 is recycled back to the membrane as PIP2
What are eicosanoids (prostanoids)?
What are they also considered to be?
What range do they act at?
How quickly are they degraded? W
hat are the main 3 eicosanoids?
What is the main course of eicosanoids?
- Eicosanoids are inflammatory mediators
- The eicosanoids are local hormones
- Eicosanoids have specific effects on target cells close to their site of formation (autocrine/paracrine mechanisms)
- Eicosanoids are rapidly degraded, so they are not transported to distal sites within the body
• Main 3 eicosanoids:
1) Prostaglandins
2) Thromboxanes
3) Leukotrienes
• The main source of eicosanoids is arachidonic acid, which is a 20-carbon unsaturated fatty acid containing 4 double bonds
What are eicosanoids (prostanoids)?
What are they also considered to be?
What range do they act at?
How quickly are they degraded? W
hat are the main 3 eicosanoids?
What is the main course of eicosanoids?
- Eicosanoids are inflammatory mediators
- The eicosanoids are local hormones
- Eicosanoids have specific effects on target cells close to their site of formation (autocrine/paracrine mechanisms)
- Eicosanoids are rapidly degraded, so they are not transported to distal sites within the body
• Main 3 eicosanoids:
1) Prostaglandins
2) Thromboxanes
3) Leukotrienes
• The main source of eicosanoids is arachidonic acid, which is a 20-carbon unsaturated fatty acid containing 4 double bonds
What is the initial and rate-limiting step in eicosanoid synthesis?
What 4 things is PLA2 activated by?
- The initial and rate-limiting step in eicosanoid synthesis is the liberation (release) of arachidonic acid by phospholipase A2 (PLA2)
- PLA2 is activated by a variety of receptor-mediated signals:
1) Serotonin receptors
2) Glutamate receptor 1
3) Some cytokine receptors
4) Increase in calcium concentration
What 2 ways can arachidonic acid be metabolised to give eicosanoids?
• Arachidonic acid can be metabolised by:
1) Cyclo-oxygenase (COX) and peroxidase to give prostaglandins and thromboxanes
2) Lipoxygenase to give leukotrienes
What are prostaglandins synthesised by?
What 5 things does prostaglandin promote?
- Prostaglandins synthesised in all tissues and cell types
- Prostaglandin promotes:
1) Vasoconstriction/vasodilation (EP1 receptor for vasoconstriction, EP2 receptor for vasodilation)
2) Inhibit/promote platelet aggregation
3) Inflammatory response
4) Thermoregulation
5) Pain
Where are thromboxanes synthesised?
How long-lived are they?
What 3 properties does Thromboxane A2 (TXA2) have?
- Thromboxanes are synthesised in platelets
- They are short lived (autocrine/paracrine)
• Thromboxane A2 (TXA2):
1) Has prothrombotic properties
2) Stimulate aggregation
3) Vasoconstrictor
Why are leukotrienes given this name?
What do some leukotrienes contain?
What 2 other things are leukotrienes involved in?
- ‘Leuko’ because they are synthesised in white blood cells
- ‘trienes’ because they contain a conjugated triene system of double bonds
- Some leukotrienes contain the amino acid cysteine in their structure, which are associated with anaphylactic shock
• Leukotrienes are also involved in:
1) Immune response3
2) Heavily implicated in asthma and allergy – treatment to asthma is targeting these pathways
What is platelet activating factor?
What is it a by-product of?
What 2 places is it synthesised?
What 4 things is PAF involved in?
- Platelet activating factor is not strictly an eicosanoid
- It is a by-product of an arachidonic acid liberation
• PAF is synthesised:
1) In leukocytes (platelets, neutrophils, basophils)
2) Also synthesised by injured tissue (e.g endothelial cells)
• PAF is involved in:
1) Platelet aggregation
2) Vasoconstriction
3) Inflammation
4) Immune response (also anaphylaxis)
What are NSAIDs?
What are 2 examples of NSAIDs?
What 3 things do NSAIDs inhibit?
• NSAIDs are non-steroidal anti-inflammatory drugs
• Examples of NSAIDs:
1) Aspirin
2) Ibuprofen and derivatives of ibuprofen
- Block the hydrophobic channel by which arachidonate enters the cyclooxygenase
- NSAIDs inhibit:
- Cyclooxygenases (COX)
- Formation of prostaglandins involved in fever, pain, and inflammation
- Blood clotting by blocking thromboxane formation in blood platelets
How does aspirin inhibit COX-1?
How is cyclooxygenase activity restored?
- Aspirin irreversible inhibits COX-1 by acetylating a serine hydroxyl group near the active site, preventing arachidonate from binding
- COX-1 has to be resynthesized to restore COX-1 activity
