what-when-how
In Depth Tutorials and Information
non-retinal neurons such as hippocampal and spinal-
cord motor neurons.
23,24
PEDF has also been shown to have a strong antiangio-
genic activity. Studies have shown that PEDF is a more
potent endogenous antiangiogenic molecule than endo-
statin, a protein well known for its antiangiogenic activ-
ity.
25,26
In vitro
treatment with PEDF inhibited migration
of cultured endothelial cells, and PEDF also strongly
prevented neovascularization in the rat cornea.
25
Intravitreous or subretinal injection of an adenoviral vec-
tor encoding PEDF reduced choroidal neovasculariza-
tion after laser-induced rupture of Bruch's membrane.
27
Long-term expression of PEDF by adeno-associated virus
(AAV) vector significantly reduced retinal neovascular-
ization through an extended period of time in a mouse
model of ischemic retinopathy.
28,29
Recently, PEDF has
been shown to systemically inhibit the release of endo-
thelial progenitor cells (EPC) from the bone marrow,
which caused a reduction of EPC-mediated retinal neo-
vascularization.
30
Neovascularization plays a critical role
in tumor growth, because new blood vessels provide
vital nutrients to tumor cells. Blocking neovasculariza-
tion is one of the main therapeutic targets for cancer
treatment. Hence, PEDF has been considered as a potent
therapeutic agent against tumor growth.
31
PEDF was
also able to inhibit brain metastases of breast cancer inde-
pendent of its antiangiogenic function.
32
PEDF was also identified as an adipokine secreted by
adipose tissue in human and mouse.
33,34
Adipose tissue
and plasma PEDF levels were increased in the mouse
model of obesity. Plasma PEDF concentration is also
positively associated with human metabolic disease and
type 2 diabetes.
35-38
Acute and prolonged administra-
tion of PEDF decreased insulin sensitivity, while PEDF
neutralization improved insulin sensitivity in the mouse
model of obesity.
33
Interestingly, insulin resistance in
morbidly obese patients is also associated with increased
serum levels of PEDF.
39
Despite correlation between
PEDF and insulin resistance, the molecular mechanism
underlying this relationship has not been studied well.
The human
SERPINF1
, which encodes PEDF, is
localized on chromosome 17 and is composed of eight
exons.
40,41
PEDF is a 418 amino acid polypeptide and puri-
ied as a 50 kDa protein. It includes a 17 amino acid sig-
nal peptide at its N-terminal end, which is important for
its secretion.
19
Plasma PEDF undergoes post-translational
modifications such as the addition of an N-terminal
pyroglutamate blocking group and an N-linked glyco-
sylation at position Asn266.
42
PEDF also possesses three
phosphorylation sites at Ser24, Ser114 and Ser227.
43
Ser24
and Ser114 residues are phosphorylated extracellularly
by casein kinase 2 (CK2), and Ser227 residue is phos-
phorylated intracellularly by protein kinase A (PKA).
Phosphorylation by CK2 at Ser24 and Ser114 is necessary
for neurotrophic activity, and phosphorylation by PKA at
Ser227 is required for antiangiogenic activity of the pro-
tein. Although the primary and spatial structure of PEDF
is homologous to other members of the serpin superfam-
ily, it does not exhibit serine protease inhibitory activity
due to lack of an Ala-rich sequence and Thr in the reac-
tive center loop.
44
Clusters of positively and negatively
charged amino acids are asymmetrically distributed in
PEDF, and this contributes to its binding to extracellular
matrix proteins such as collagen, heparin and glycosami-
noglycans.
45,46
The clusters of negatively charged residues
(Asp and Glu) determine the affinity of PEDF to type I, II
and III collagens, and the clusters of positively charged
amino acids (Arg, Lys and His) are involved in binding of
PEDF to heparin and glycosaminoglycans. Interestingly,
other studies showed that small peptide regions of PEDF
were able to mediate various functions of PEDF.
47,48
Short
34-mer peptide and 44-mer peptide at the N-terminal
region exhibited antiangiogenic and neurotrophic activity
of PEDF (
Figure 17.1
).
A phospholipase-linked membrane protein was
identified as a possible PEDF receptor by yeast two-
hybrid screening.
49
The putative receptor showed tri-
glyceride lipase and acylglycerol transacylase activities.
Interestingly, PEDF binding induced phospholipase
A(2) activity of the receptor. Although the function of
phospholipase-linked membrane proteins in lipid sig-
naling pathways has been well established, the func-
tional relationship between PEDF and the receptor
requires further investigation. In addition to the phos-
pholipase-linked membrane protein, yeast two-hybrid
screening identified two other putative proteins as PEDF
receptors. Bernard et al. found that PEDF binds the
laminin receptor to regulate endothelial cell apoptosis,
migration and proliferation.
50
Anguissola et al. identi-
ied transportin-SR2, a member of the importin-β family,
as a putative PEDF binding partner.
51
The authors iden-
tified a novel nuclear localization signal, which is impor-
tant for PEDF nuclear localization and interaction with
transportin-SR2. But the role of PEDF in the nucleus has
not yet been established.
PEDF has been shown to be involved in several intra-
cellular signaling pathways. First, it protects neuronal
cells from apoptosis by activation of the NF-κB signaling
cascade, which induces the expression of anti-apoptotic
and/or neurotrophic factors.
52
Second, PEDF activates
Fas-Fas ligand (FasL) cell death signaling. PEDF induced
the expression of Fas ligand that triggered endothelial
cell death.
53
Third, PEDF activates p38 mitogen-activated
protein kinase (MAPK), which sequentially induces the
expression of peroxisome proliferator-activated recep-
tor gamma (PPAR-γ) and p53 in human umbilical vein
endothelial cells (HUVECs). This increase in PPAR-γ
and p53 results in PEDF-induced apoptosis of HUVEC.
54
Finally, PEDF also modulates Notch and Wnt signaling
pathways.
55,56
The studies showed that PEDF enhanced
