Title: Peroxisomes in Dermatology.ppt
1Peroxisomes Peroxisome Proliferator-Activator
Receptors in Dermatology
- M.Y.ABDEL_MAWLA,MD
- Zagazig Faculty of Medicine,Zagzig,EGYPT
2Peroxisomes in Cells
- The trouble with simple things is that one must
- understand them very well !!!
3The Peroxisome
- Single membrane
- Roughly spherical
- 0.2 - 1.7?m
- Composition varies
4Peroxisomes
- Peroxisomes intracellular organelles with
important roles defined in many metabolic
processes. - Peroxisomesin all mamalian cells,including
kertatinocytes - They derive their name from their ability to
produce H2O2 through a group of oxidizing enzymes
which use molecular oxygen to transform their
substrates, releasing H2O2 and OH
5Peroxisomes
- The oxidative stress resulting from H2O2 is known
to stimulate phospholipase D, associated with the
production of phosphatidic acid and
diacylglycerol. - These in turn affect adenylyl cyclase and protein
kinase C, respectively, which can modulate a wide
array of target proteins including plasma
membrane receptors, contractile proteins and
regulatory enzymes.
6Peroxisomes
- Peroxisomes are similar physically to lysosomes,
but they are different in two important ways - First, they are believed to be formed by
self-replication (or perhaps by budding off from
the smooth endoplasmic reticulum) rather than
from the Golgi apparatus. - Second, they contain oxidases rather than
hydrolases.
7Peroxisomes Biogenesis
- The development of peroxisomes involves a series
of events including the recruitment of
(phospho)lipids, synthesis, sorting and
assembly/activation of matrix membrane proteins. - The peroxisomal membrane mainly consists of
phosphatidylcholine phosphatidylethanolamine
and largely resembles that of the endoplasmic
reticulum (ER)
8Peroxisome Biogenesis
9Peroxisomes Biogenesis
- Membrane proteins are synthesized on free
polysomes in the cytosol. - The model discriminates between membrane proteins
essential for the biogenesis of organelles (PMBs)
and those involved in the organellar function
(PMFs, e.g. transporters). - PMBs (e.g. Pex2p, Pex3p, Pex15p and Pex16p) may
initially be transported to the ER and,
eventually vesicle-mediated, be sorted to the
growing peroxisome - PMFs may be transported directly to the organelle
10Peroxisomes Functions
- Peroxisomes play only a minor role in cellular
functions. - Peroxisomes play an important role in regulating
cellular proliferation and differentiation as
well as in the modulation of inflammatory
mediators. - Peroxisomes have broad effects on the metabolism
of lipids, hormones. - Peroxisomes also affect cellular membranes and
adipocyte formation, as well as insulin
sensitivity. - Peroxisomes play a role in aging and
tumorigenesis through their effects on oxidative
stress.
11Peroxisomes Functions
- Peroxisomes may be either catabolic or anabolic.
- Some functions are exclusively peroxisomal
- Others shared with other cellular compartments
(microsomes endoplasmic reticulum, and
mitochondria).
12Peroxisomes Functions in Cell signaling
- N nucleus, MIT mitochondria, MIC microsomes,
Per peroxisome, PKO phosphatidyl choline, - PC phosphosphatidyl choline, AA arachidonic
acid, OX peroxisomal oxidases, - LCFA long-chain fatty acids, MCFA
medium-chain fatty acids, DG diacylglycerol, PA
- phosphatidic acid, CAT catalase, OFR
oxygen free radicals. - Peroxisome functions involved n cell signaling
would potentiate differentiation, while reactive
oxygen species would potentiate tumorigenesis and
aging and catalase and other peroxisomal
scavengers would moderate the effects of reactive
oxygen species
13Peroxisomes Free Radicles
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15Peroxisomal Enzymes
16Key peroxisomal enzymes
17How Peroxisomes to Function?
- Peroxisomes, contribute to key metabolic pathways
of the cell function. - These organelles are able to proliferate
abundantly upon exposure to so-called peroxisome
proliferators. - Peroxisome proliferators may activate PPARs by
binding directly to the receptors,and the
activated PPARs may regulate the expression of
genes involved in lipid metabolism and peroxisome
proliferation - Peroxisome-located enzymes are also regulated by
many molecules including nutrients (fatty acids,
steroids), hormones (T3, retinoids), PPARs and
other nuclear signaling factors.
18How Peroxisomes to Function?
- Peroxisomal proliferator activator receptors
(PPARs) and ligands for these receptors modulate
different peroxisomal functions.
19PPARs Distribution
- PPARs are found mainly in tissues associated with
high fatty acid metabolism. - Thus are expressed mainly in liver,
- They are also found in kidney, muscle, heart,
fat, B and T lymphocytes , vascular smooth
muscle and keratinocytes
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21 Peroxisome Proliferator-Activator Receptors
(PPARs)
- Nuclear hormone receptor superfamily
- Multiple isoforms (?, ? (d), ?)
- Unique tissue distribution
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23Mechanism of PPAR action
?
Co-repressors Co-activators
PPARRXR?
?
?
Modulation of gene transcription
Biological effect
24Mechanism of PPAR action
25PPARs FUNCTIONS
- PPARs play a significant role in many in skin
homeostasis. - On binding of ligand, PPARs undergo a
conformational change enabling them to form a
heterodimer with the 9-cis retinoic acid receptor
(RXR). - PPAR/RXR complex forms a transcription factor
able to bind to DNA at PPREs, which are DNA
sequences specifically capable of recognition
and binding by the DNA-binding component of this
transcription factor.
26PPAR FUNCTIONS
- A large number of PPAR target genes have been
identified with wide-ranging effects on cell
proliferation differentiation, inflammatory
responses and angiogenesis, as well as lipid and
glucose metabolism.
27- Peroxisome proliferator-activated receptors
(PPARs) function as heterodimers with retinoid X
receptors (RXRs) and are activated by specific
ligands they then modulate DNA transcription by
binding to defined nucleotide sequences
(peroxisome proliferator response element, PPRE)
in the promoter region of target genes. Several
cofactors (coactivators or corepressors) mediate
the ability of nuclear receptors to stimulate or
repress the transcription process. (b) The
N-terminus A/B domain contains a
ligand-independent transcriptional activation
domain (AF-1), which can be regulated by
mitogen-activated protein kinase (MAPK)
phosphorylation in a and ? isotypes. The C domain
contains two zinc-finger-like motifs that
specifically bind the PPRE in the regulatory
region of PPAR-responsive genes. The D domain or
hinge region allows conformational changes in the
molecule. The E/F domain consists of the
ligand-binding domain and the ligand-dependent
transcriptional activation domain (AF-2). The
ligand-binding pocket appears to be quite large
in comparison with other nuclear receptors,
allowing the PPARs to interact with a broad range
of natural and synthetic ligands
28ACTIVATION OF PPARs
- Most nuclear hormone receptors exist in an
inactive form complexed with heats hock proteins - PPAR activation and regulation may occur either
following interaction with specific ligands or by
kinase-mediated phosphorylation, following a
variety of extracellular signals. - Following binding of ligand, the PPAR undergoes a
conformational change and becomes able to
interact through the E/F domain with RXR to form
heterodimers. - These dimers will then interact with the cognate
recognition motif (PPREs) in the promoter region
of relevant genes to activate transcription of
target genes.
29ACTIVATION OF PPARs
- PPARs are held in inactive complex
associated with heat shock proteins (HsP).
Following binding with ligand or other activation
signals . - HsP is replaced with retinoic acid receptor
(RXR) forming active transcription factor. - The active PPAR/RXR heterodimer binds to PPAR
response element AGGTCA X AGGTCA to initiate
transcription of relevant genes.
30PPAR? activation.
- Upon ligand binding, the nuclear receptor PPAR?
associates with nuclear receptor RXR as well as
with coactivators and corepressors which are
present in a cell type and state specific
pattern. This complex binds to PPAR response
elements to enhance or repress gene transcription.
31Biological roles of PPARa
PPARa mediates the induction of multiple enzymes
required to mobilize and transport fatty acids
from adipose stores to liver for catabolism.
Basis for therapeutic use in humans to lower
serum lipids.
32Biological roles of PPARb/d
Ligand activation of PPARb/d leads to terminal
differentiation of keratinocytes as shown by four
independent laboratories. Activation of PPARb/d
in skeletal muscle leads to increased catabolism
of fatty acids and improved insulin sensitivity.
33Biological roles of PPARg
The role of PPARg in carcinogenesis is also
controversial. There is evidence that activation
of PPARg can either potentiate or attenuate
cancer, but current consensus favors
attenuation.
34Orchestration of Immune Responses
CELLS Lymphocytes Monocytes/Macs Neutrophils Eosin
ophils Basophils Dendritic cells
MOLECULES Complement Lysozyme Inflammatory
mediators Chemokines Cytokines
TISSUES Thymus Spleen Lymph nodes Blood
Innate immunity Adaptive Immunity
PPARs are found in a number of immune cell types
and there is evidence that they could modulate a
number of different immune responses
35 Role of PPARa in Immune Function
- Expressed in monocytes/macrophages, increased
after treatment with phorbol ester - PPARa ligands induce apoptosis in activated
macrophages - PPARa ligands decrease secretion of MMPs in
LPS-treated monocytes - PPARa ligands decrease NOS activity in
macrophages - BUT
- Natural PPARa ligands increase NOS activity in
macrophages
36 Role of PPARa in Immune Function
- Inflammatory response induced by LTB4 is enhanced
in PPARa-null mice - PPARa ligands can inhibit inflammatory cytokine
production - BUT
- PPARa ligands cause increase in serum TNFa after
LPS
37 Role of PPARa in Immune Function
- Reports suggest that PPARa ligands are
anti-inflammatory but there are also some reports
suggesting that PPARa ligands are pro-inflammatory
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39Peroxisomes Inflammation
- PPAR-a activation transcription factor for
peroxisomal metabolism for arachidonic-derived
proinflammatory eicosanoids. - ligands for this activation are the w-3 fatty
acids, which may explain their antiinflammatory
effects.
40Peroxisomes Inflammation
- PPARa repress NFkB transcription, signal
transducer and activator of transcriptions
(STATs) and AP-1 (Jun/ Fos) transcription and the
downstream signaling mechanism. - These effects result in the decreased production
of inflammatory cytokines, protease production,
and some mechanisms for proliferation and
apoptotic signaling.
41Peroxisomes Inflammation
- Activation of PPARs also modulates endothelial
cell adhesion molecules, and endothelial cells
express both PPARa and PPAR gamma
42Peroxisomes INFLAMMATION
- The peroxisome proliferator-activated receptors
(PPARs) a, ß/d, and ? are ligand-activated
transcription factors belonging to the nuclear
receptor superfamily. - In addition to their regulatory role on lipid and
glucose metabolism, they exert anti-inflammatory
properties. - In skin both PPAR-a and PPAR-ß/d regulate
keratinocyte proliferation/differentiation and
contribute to wound healing. - The 3 PPAR isoforms are expressed by several cell
types recruited into the dermis during
inflammation.
43Effects on angiogenesis
- Vascular endothelial cell growth factor(VEGF) is
a potent endothelial cell-specific mitogen. - PPAR agonists have diverse effects on the
expression of VEGF . - PPAR gamma ligands increase the generation of
VEGF , which might be expected to have
pro-angiogenic effects.
44Peroxisomes and Sterol Metabolism
- The levels of 17b-hydroxy forms of sex steroids
are regulated by the 17b-hydroxysteroid
dehydrogenase (17b- - HSD) family of proteins.
- The type IV enzyme, found primarily in
peroxisomes, possesses a number of unique features
45Peroxisomes and Sterol Metabolism
- 17b-HSD IV is involved in degradation of
branched-chain fatty acids and the side chain of
cholesterol. - Its expression is stimulated by PPARa ligands
46Peroxisomes in Epidermal Differentiation
Proliferation
- A well documented effects of PPARs on gene
transcription associated with lipid metabolism
energy homeostasis. - Roles in epidermal maturation, repair and
angiogenesis are highlighted.
47Peroxisomes in Epidermal Differentiation
Proliferation
- PPARa has a role in barrier development.
- Activators of PPARa, such as clofibrate, oleic
acid and linoleic acid, accelerate the barrier
development. as evidenced by - decreased transepidermal water loss (TEWL)
- increased epidermal stratification
- the appearance of mature lamellae in the
extracellular spaces of a multilayered stratum
corneum. - the induction of b-galactosidase and steroid
sulphatase, - accelerated the expression of profilaggrin, and
its processing to filaggrin, and the expression
of loricrin, a
48Peroxisomes Atopic Dermatitis
- Changes in immunoglobulin-associated
transcription in atopic dermatitis may favour - IgE over secretory immunoglobulin (multimeric
IgM and IgA) expression in AD skin. - Decreased PPAR activity appears common
- to both AD and psoriasis, and reduced
cutaneous IFNa2 transcription also appears
characteristic of AD
49Peroxisomes in Epidermal Differentiation
Proliferation
- PPARa ligands to promote epidermal
differentiation,to restore epidermal homeostasis
in hyperproliferative - mouse epidermis and regulate apoptosis.
- PPARb / gammainduced expression of involucrin
and transglutaminase( markers of keratinocyte
differentiation)
50Wound healing
- PPARa and PPARb / gammaexpression is upregulated
in the keratinocytes at the wound edge of the
damaged skin. - PPARa is re-expressed transiently in this area
during the early inflammatory phase of the
healing. - Delays in wound healing parallel the pattern of
PPAR expression of the respective PPAR isotypes - PPARb upregulation linked to
- proinflammatory cytokines, such as
- interferon, tumour necrosis factor (TNF)-a.
51Wound healing
- PPARb a key mediator ofepidermal effects in
wound healing by converting the extracellular
inflammatory signal into an organized pattern of
gene expression leading to survival, migration
and differentiation of keratinocytes.
52Sebocyte glands
- Activation of PPAR gamma a by their respective
specific ligands, stimulates lipid - droplet accumulation in sebocytes.
- Because increased sebum production is an
important element in the pathogenesis of acne
vulgaris, development PPAR antagonists
interfering selectively with sebum formation may
have implications for the treatment of acne.
53Psoriasis
- The hallmarks of psoriasis are abnormal
differentiation hyperproliferation of
keratinocytes with inflammatory cell
infiltration. These cellular changes are likely
to find their explanation in activated T
lymphocytes infiltrating the skin.
54Psoriasis
- PPARs a critical regulator of cutaneous
homeostasis in psoriasis. - In view of their prodifferentiating,
antiproliferative immunomodulating effects,
PPAR ligands may be interesting compounds for
the treatment of epidermal disorders showing
inflammation,hyperproliferation - aberrant differentiation, such as psoriasis.
55Psoriasis
- PPAR gamma and some of its ligands, including two
lipoxygenase metabolites, are induced by the T
helper (TH)2 cytokine IL-4 in macrophages. - PPAR gamma activation may also be an integral
part of the effecter mechanism for downregulation
TH1 dominate inflammatory reaction such as
psoriasis and rheumatoid arthritis
56Psoriasis
- The expression of both PPARa gamma is decreased
in epidermis , whereas the exact opposite happens
with PPAR dela. - Treatment by troglitazone, a specific PPAR gamma
activator, inhibited the proliferation of both
normal and psoriatic human keratinocytes
57Psoriasis
- There may be an association between
- psoriasis and the genes encoding PPARa or PPAR
gamma
58Effects on angiogenesis
- Vascular endothelial cell growth factor(VEGF) is
a potent endothelial cell-specific mitogen. - PPAR agonists have diverse effects on the
expression of VEGF . - PPAR gamma ligands increase the generation of
VEGF , which might be expected to have
pro-angiogenic effects.
59Human immunodeficiencyvirus-1-protease inhibitor
associatedlipodystrophy
- HIV-1-protease inhibitor-associated lipodystrophy
the result of impaired cellular retinoic acid
binding - protein type I (CRABP-1)-mediated 9-cis
retinoic stimulation of PPARcRXR. - Altered differentiation status of peripheral
adipocytes in HIV-1-infected patients with
protease inhibitor lipodystrophy is associated
with greatly reduced sterol-regulatory
element-binding protein 1c (SREBP1c) mRNA
expression - Reduced SREBP1 expression consequently alters the
PPAR gamma activity, which may lead to
lipodystrophy and to metabolic alterations.
60Peroxisome defects
- There are now many inherited disorders known to
relate to peroxisome defects, frequently with
significant cutaneous manifestations such as - ichthyosis
- Recurrent ulceration,
- alopecia,
- follicular atrophoderma
- And photosensitivity,
- Thus ,it is suggested that modification of
their - activity may be of therapeutic benefit in
the field of dermatology
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64Summary
- Peroxisome proliferator-activated receptors
(PPARs) are ligand-activated transcription
factors that regulate the expression of target
genes involved in many cellular functions
including cell proliferation, differentiation and
immune/inflammation response. - The PPAR subfamily consists of three isotypes
PPARa, PPARß/d and PPAR?, which have all been
identified in keratinocytes. -
65Summary
- PPARß/d is the predominant subtype in human
keratinocytes, whereas PPARa and PPAR? are
expressed at much lower levels and increase
significantly upon keratinocyte differentiation. - PPARß/d is significantly upregulated upon
various conditions that result in keratinocyte
proliferation, and during skin wound healing.
66Summary
- PPARs appear to play an important role in skin
barrier permeability, inhibiting epidermal cell
growth, promoting epidermal terminal
differentiation and regulating skin inflammatory
response by diverse mechanisms. - These proprieties are pointing in the direction
of PPARs being key regulators of skin conditions
characterized by hyperproliferation, inflammatory
infiltrates and aberrant differentiation such as
psoriasis, - They may also have clinical implications in
other inflammatory skin disease (e.g. atopic
dermatitis), proliferative skin disease, wound
healing, acne and protease inhibitor associated
lipodystrophia.
67A Message Home
- Peroxisomes are small cellular organelles that
were almost ignored for years because they were
believed to play only a minor role in cellular
functions. - Peroxisomes play an important role in regulating
cellular proliferation and differentiation as
well as in the modulation of inflammatory
mediators. - Peroxisomes have broad effects on the metabolism
of lipids, hormones,. - Through their effects on lipid metabolism,
peroxisomes also affect cellular membranes and
adipocyte formation, as well as insulin
sensitivity - Peroxisomes play a role in aging and
tumorigenesis through their effects on oxidative
stress.
68THANK YOU