Download Lipid rafts

Receptors of fatty acids and
endocannabinoids; lipid rafts
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Receptors of fatty acids
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1) Peroxisomal proliferator-activated receptors (PPAR)
– nuclear
2) Free-fatty acid-activated receptors (FFAR)
– on the cell surface, coupled to G proteins
The PPAR family of receptors
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3 isoforms:
– PPAR
– PPARδ/
– PPARγ
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(oxidized
LDL)
Regulation of lipid metabolism: they bind to response
elements to stimulate
transcription of FA oxidation
and lipid synthesis genes
They bind to DNA as heterodimers with the retinoid X
receptor (RXR, activated by
9-cis retinoic acid)
(PPAR responsive element)
Ligands of PPARs
Synthetic
Natural
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Major natural
ligands: free fatty
acids with long
chains (> 12C),
particularly polyunsaturated FAs
HODE=hydroxyoctadecadienoic acid
Effects of PPAR
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PPARα regulates expression
of various genes implicated
in lipid oxidation, mainly in
the liver and oxidative
muscles, such as the heart
PPARγ is involved in
lipogenesis and terminal
differentiation of adipocytes
in white adipose tissue
PPAR
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Expression is high in the tissues with high rates of FA oxidation:
liver, heart, skeletal muscle, kidney, brown fat
Function: regulation of the
– cellular uptake
– activation
– β-oxidation of FA: stimulates the peroxisomal β-oxidation
pathway profoundly, mitochondrial to a lesser extent
PPAR expression is upregulated during fasting and stress (i.e.
when FAs are released from the adipose tissue)
Lipid-lowering drugs of the fibrate class (bezafibrate) are potent
activators of PPAR
PPARα target genes
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LPL
fatty acid transport protein (FATP), fatty acid translocase (FAT)
acyl-CoA synthetase
enzymes of peroxisomal β-oxidation (acyl-CoA oxidase, thiolase)
enzymes involved in mitochondrial β-oxidation (CPT1, acyl-CoA
dehydrogenase)
enzymes of ketogenesis: 3-hydroxy-3-methylglutaryl-CoA synthase
PPAR/δ
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Ubiquitous; high expression in brain, adipose tissue, skin
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Particularly abundant during development (such as in CNS)
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Suggested to be involved in the differentiation of cells within the
CNS, myelinization, and lipid metabolism in brain
May be also implicated in basic cellular functions, such as
membrane lipid synthesis and turnover
Similarly to PPAR, it stimulates the expression of proteins of FA
oxidation (heart)
PPARγ
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3 isoforms:
– PPARγ1 – in wide range of tissues
– PPARγ2 – predominantly in adipose tissue
– PPARγ3 – only in adipose tissue, macrophages, colon
Functions:
– regulation of adipocyte differentiation
– regulation of lipid anabolism: in the adipose tissue, they regulate
the expression of LPL, FATP…
PPARγ are targeted by anti-diabetic substances of the thiazolidinedione class (glitazones, e.g. rosiglitazone) that enhance the tissue
sensitivity to insulin and reduce the plasma levels of FA and Glc
The FFAR family of receptors
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Localized to the plasma membrane (unlike PPARs) and coupled
to G proteins
Ligand spectrum includes FA chain lengths down to one (unlike
PPARs)
3 isoforms:
– FFA1R
– FFA2R
– FFA3R
Roles of FFARs
FFA1R
FFA2R
FFA3R
Expression pancreatic islets, liver, monocytes, adipose tissue,
skeletal muscle, heart neutrophils
pancreas,
immune cells
Ligands
medium- (down to
10C) to long-chain FA
short-chain
FA (C1-C6)
similar to FFA2R
Effects
regulation of insulin
secretion
control of
control of leptin
chemotaxis production
Endocannabinoids
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Biologically active lipophilic substances that activate cannabinoid
receptors
Derivatives of arachidonic acid, which are generated from
membrane phospholipids in response to stimuli
Two best-characterized:
Synthesis of anandamide
(PE)
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The reaction is initiated by activating
neurotransmitter receptors and/or by
elevated intracellular Ca2+
Synthesis of
2-arachidonoylglycerol
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a)
b)
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Also linked to elevated
intracellular Ca2+
2 pathways:
– phospholipase C +
diacylglycerol lipase
– phospholipase A1 +
phospholipase C (specific
for lyso-PI)
Function
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Paracrine mediators (rapidly eliminated through uptake into cells
and enzymatic hydrolysis)
Produced mainly in the:
– nervous system
– immune system
Anandamide levels in tissues – very low and in some cases, it does
not act as a full agonist  physiological significance questionable
On the other hand, 2-AG is a full agonist at the CB1 as well as CB2
receptor and the levels in tissues are much higher
Cannabinoid receptors
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Targeted by THC
(Δ9-tetrahydrocannabinol)
THC
– CB1 – most abundant in the CNS, also present in immune cells,
lung, small intestine, uterus, testis…
– CB2 – found mostly in the immune system (leukocytes, spleen,
lymph nodes)
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7-transmembrane, coupled to Gi/Go proteins  inhibition of
adenylyl cyclase…
Physiological roles
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2-AG from stimulated neurons attenuates neurotransmitter release
by decreasing intracellular Ca2+ (?calming the excitation of neurons
to prevent cell death?)
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2-AG suppresses long-term potentiation
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?Role in immune response? (CB2 – in the immune system)
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CB1 stimulation inhibits proliferation of human breast cancer cells
Lipid rafts
Plasma membrane:
NOT absolutely homogeneous!
Lipid rafts
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Resistant to mild detergents
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Highly dynamic, densely organized
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(10-200 nm)
Sphingolipid- and cholesterol-rich
microdomains of the plasma
membrane containing a variety of
signalling and transport proteins
This environment fits for transport,
conformational changes of signal
transducers, but also for pathogen
entry into the host cell (HIV, Ebola)
A model of a lipid raft
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Cylindrical glycerophospholipids (GPLs)
form the disordered Lc
phase of the membrane
Ordered Lo phase – raft:
in the outer leaflet,
cholesterol fills the voids
between sphingolipids
(SM); in the inner
leaflet, chol. fills the
voids between selected
(pyramidal) GPLs
This organization
rigidifies the membrane
Proteins in lipid rafts
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Raft resident proteins are often
GPI (glycosylphosphatidylinositol)
anchored:
Rafts contain:
– receptors (Fas, MHCI, II)
– signalling molecules (tyrosin
kinases, G proteins)
– transporters (GLUT4, FAT)
Lipid rafts facilitate
TCR-mediated T cell activation
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TCR moves to rafts during T-cell
activation
Formation of the TCR-MHC
complexes and aggregation of
co-stimulatory molecules in lipid
rafts, raft aggregation
Tyr phosphorylation and
recruitment of signalling
proteins
Caveolae
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Types of rafts that are rich in proteins of the caveolin family
(caveolin-1,-2,-3)
Caveolin-1, integral membrane protein, forms oligomers that
associate with each other and form a pit in the membrane
Proposed roles:
– signalling
– transport
– pathogen entry (SV40, E. Coli)
Raft-mediated HIV entry into the
host cell – a model
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gp120 (viral), CD4, and
sphingolipids (SLs) form a
complex in the raft area; SLs
stabilize HIV on the cell surface
The raft floats on the cell
surface to the co-receptor for
the CD4–gp120 complex
SLs facilitate the conformational changes of gp120 that lead
to the shedding of gp120 →
release of the N-terminus of
viral gp41 (initially buried in a
gp120 pocket)
This „fusion peptide“ penetrates into the plasma membrane
Rafts in prion infection
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The interaction of PrPC with lipid
rafts (sphingolipids) might stabilize
the ‘normal’ conformation of PrPC
These interactions should be
destabilized when exogenous (shed
from an infected cell) PrPSc is
inserted in the vicinity of PrPC
Formation of PrPC/PrPSc complex
(probably by coalescence of both
rafts), PrPC → PrPSc conversion
Propagation on the cell surface