Lipid Signalling

Question 1

What are lipids used for?

Answer 1

Used as energy storage for energy consumption when needed
Protection of organs
Used as signalling molecules

Question 2

What are second messengers?

Answer 2

They are conduits for information transfer
There is an input signal binding to a receptors and in the cell there is a creation of a new signalling ‘mediator’ or second messenger
They activate a downstream event
Eventually there is degradation of signalling second messenger - net levels of second messenger determine the cellular response

Second messengers can propagate signals between proteins that are at a distance
cAMP, Ca2+ ions and phosphoinositide’s are widely used second messengers

Question 3

What are some lipid derived second messengers?

Answer 3

Many lipid-derived second messengers are produced in membranes

Phospholipase C enzymes release soluble and lipid-attached second messengers in response to diverse inputs
Channels and transporters are modulated by different lipids in addition to inputs from other sources
PI 3-kinase synthesizes PIP3 to modulate cell shape and motility
Phospholipases D (PLD) and A (PLA) enzyme families hydrolyse membrane-bound phospholipids to create other lipid second messengers

Question 4

What are some types of lipid signalling?

Answer 4

Phosphoinositides
Sphingolipids
GPCR activators
Nuclear receptor activators

Question 5

Describe phosphoinositide signalling?

Answer 5

This regulates cell function
Phosphorylation/dephosphorylation of lipid second messengers
E.g. PIP2 to PIP3
Many phosphoinositides affect different aspects of cell proliferation, metabolism (glucose uptake/glycogen synthesis), migration and homeostasis

Question 6

Describe PIP2?

Answer 6

Phosphatidylinositol 4,5-bisphosphate (PIP2)
This is a key phosphoinositide in cell regulation
Glycerol backbone – phosphate – inositol sugar
2 x fatty acyl chains (C18-C20) attached to glycerol backbone
Inositol sugar attached to terminal glycerol carbon

PIP2 can be broken down in order to regulate multiple cellular responses e.g. Signalling, endocytosis, cytoskeleton and ion fluxes
PIP2 hydrolysis by phospholipase C (PLC) to DAG and IP3 is a major signal transduction pathway
DAG and IP3 are second messengers

Question 7

Describe phosphoinositide mediated intracellular signalling pathways?

Answer 7

ATP and phosphatases can change between different phosphoinositides
Inositol phosphorylation and metabolism allows conversion into many different soluble forms
Each form has a potential different regulatory action and biological function
IP3 can target calcium channels in the ER to mediate calcium rise in the cytosol
There for depending on the PIP metabolism = different second messengers

The turnover can have different effects
Changing the rate at which the cell moves/is stationary

Question 8

What do phosphoinositides regulate?

Answer 8

They regulate different membrane events:
Signalling pathways
Membrane protein activity e.g. ion channels
Lipid transfer protein activity
Sculpting or modulating vesicle fission and fusion in membrane trafficking

Question 9

Describe membrane bound phosphoinositides?

Answer 9

Membrane bound phosphoinositides enables effector proteins to be recruited to membranes:
They interact with key enzymes such as G-proteins and triggering signalling
They recruit key protein kinases and phosphatases that trigger intracellular signalling pathways

Question 10

What is PI3K?

Answer 10

Phosphoinositide 3-kinases (PI3K)
This is a key regulator
They all phosphorylate PIP2 -Class 1A - regulatory and catalytic subunit, lipid kinase domain at the C-terminus
Class 1B - similar to 1A (not as big)

Phosphoinositides-modifying enzymes localise to different membranes
Class I PI3K – usually localised to plasma membrane and endosomes
Class II PI3K – localised to Golgi and secretory pathway
Class III PI3K – localised to endosomes and Golgi membrane
PI4K enzymes – localised to Golgi and endosomes
PI5K - regulates G-protein coupled receptors

Question 11

Describe the structure of phosphoinositides?

Answer 11

The heads groups have different structures
This is because there are 5 OH groups that can be potentially phosphorylated

Different membrane lipids bind different protein modules - this changes due to the phosphate head

Question 12

Describe the phosphoinositides bind pleckstrin homology (PH) domains?

Answer 12

The pleckstrin homology (PH) domain is a major protein kinase C substrate in platelets
Found in many signalling proteins that associate with membranes e.g. phospholipase Cg1
Some PH domains bind specific phosphoinositides with high affinity
Low micromolar or nanomolar dissociation constant, Kd

Question 13

Give an overview of phospolipase C and it’s domains?

Answer 13

Involved with internal phospholipids
There are 5 different ones and some with isoforms

PH domain – binds PI lipids
C2 domain – binds membranes in the presence of calcium ions
SH2 domain – binds phosphotyrosine epitopes
SH3 domain – binds polyproline motifs
RasGEF – acts to promote GTP exchange and activation of Ras proto-oncogene
EF hands – binds calcium ions

Question 14

Give an overview of protein kinase C?

Answer 14

This phosphorylates many targets and has a C terminal kinase domain
C1 domain – binds DAG lipid
C2 domain – binds membranes in the presence of calcium ions
C3 domain – binds ATP
C4 domain – binds substrate for phosphorylation by kinase domain

They regulate diverse cellular responses:
PIP2 - cytoskeleton, phagocytosis, exocytosis, endocytosis, ion channels, Ca2+ etc…
PIP3 - protein synthesis, cell survival, glycogen metabolism, gene transcription, cytoskeleton rearrangements, superoxide formation etc…

Question 15

What is the outcome of lipid-related compounds in signal transduction?

Answer 15

They can be broken down into - lipoxins, prostaglandins and leukotrienes
Phosphatidylcholine can be split into lyso-PC and arachidonic acid
Arachidonic acid -> prostaglandins (via COX enzymes) or leukotrienes (via lipoxygenase)

Question 16

Describe some different sphingolipid second messengers?

Answer 16

Ceramide regulates cell stress e.g. apoptosis, senescence, etc.
Targets include Ser/Thr protein phosphatases e.g. PP1 and PP2A
Ceramide-1-phosphate activates phospholipase A2, stimulating production of arachidonic acid
This also regulates membrane trafficking, cell survival, apoptosis, phagocytosis and macrophage degranulation
Glucosylceramide is implicated in post-Golgi trafficking, and drug resistance in cancer (after chemotherapy)

Sphingosine interacts with protein kinase H with regulation of actin dynamics, cytoskeleton, endocytosis, cell cycle and apoptosis
Sphingosine-1-phosphate has effects on endothelial cell growth and movement and inflammation
Also implicated in lymphocyte function and immunity
Could be important in cancer, arthritis, diabetes, and neurodegeneration e.g. multiple sclerosis

Question 17

Describe sphingolipid synthesis and turnover?

Answer 17

Sphingomyelin / Glucosylceramide ceramide
Ceramide Sphingosine Sphingosine-1-phosphate
They are all second messengers

Sphingomyelin signalling effects - apoptosis, proliferation, inflammation and angiogenesis

Question 18

Describe GPCR activators (from lipid-derived compounds)?

Answer 18

Lysophosphatidic acids (LPA) binds with high affinity to GPCRs - (LPA1, LPA2 and LPA3)
Sphinosine-1-phosphate (S1P) activates GPCRs called S1P1 - S1P5
Platelet activating factor binds to a single GPCR called PAFR and activates platelet aggregation, inflammation and anaphylaxis
Endocannabinoids activate cannabinoid receptors (CB1 in central nervous system, CB2 in lymphocytes)
This can cause analgesia, anti-inflammation, tissue protection during ischemia
Palmitolyethanolamide binds to GRP55 and PPARs receptor with potent analgesic and anti-inflammatory properties
Prostaglandins (eicosanoids) can activate 10 types of GPCRs - e.g. DP1, DP2, EP1-EP4, FP, IP1, IP2 and TP
Retinol derivatives such as retinol (vitamin A) binds to rhodopsin (a GPCR) and regulates the visual cycle

Question 19

Describe nuclear activators (from lipid-derived compounds)?

Answer 19

Steroid hormones are a diverse class of steroids that resemble cholesterol

Retinol (vitamin A) can be broken down to retinoic acid which activates nuclear receptors such as retinoic acid receptor (RAR) to control the differentiation and proliferation of many cell types
Important in embryogenesis

Prostaglandins can also activate PPAR family proteins which are nuclear receptors that regulate pro-inflammatory gene expression

Question 20

Describe steroid hormones?

Answer 20

Steroid hormones are a diverse class of molecules which resemble cholesterol
Mammalian steroids are grouped into 5 classes depending receptor recognition
Glucocorticoids, mineralocorticoids, androgens, estrogens and progestogens
Receptors include nuclear receptors, some GPCRs and ion channels