Pigment epithelium-derived factor (PEDF) was originally discovered by Joyce Tombran-Tink and Lincoln Johnson in the late 1980s.[8][9] This group was studying human
retinal cell development by identifying secreted factors produced by the
retinal pigmented epithelium (RPE), a layer of cells that supports the retina. Upon noticing RPE produced a factor that promoted the differentiation of primitive retinal cells into cells of a
neuronal phenotype, they set out to determine the identity of the factor. They isolated proteins unique to RPE cells and tested the individual proteins for neurotrophic function, meaning promoting a neuronal phenotype. A neurotrophic protein around 50 kilodaltons (kDa) was identified and temporarily named RPE-54 before being officially termed pigment epithelium-derived factor.
Soon thereafter, the same laboratory
sequenced the PEDF protein and compared it to a human fetal eye
library.[7] They found that PEDF was a previously uncharacterized protein and a member of the
serpin (serine protease inhibitor) family.
Gene
The
gene encoding human PEDF was localized to the 17th
chromosome at position 17p13.1.[10] The human PEDF gene is around 15.6kb, and the
mRNA transcript is around 1.5kb.[11] Immediately upstream of the PEDF gene lies a 200bp
promoter region with putative binding sites for the
transcription factorsHNF4,
CHOP, and
USF. The PEDF gene consists of 8 exons and 7 introns.
The PEDF gene is present in vertebrates from human to fish, but not present in sea squirts, worms, or fruit flies.[11] Sea squirts express several serpin genes, suggesting that the PEDF gene may have arisen from another serpin family member after the evolution of vertebral animals. The gene most homologous to PEDF is its adjacent neighbor on chromosome 17,
SerpinF2.
Protein
The PEDF protein is a secreted protein of roughly 50kDa size and 418 amino acids in length.[5] The N-terminus contains a leader sequence responsible for protein secretion out of the cell at
residues 1-19. A 34-mer fragment of PEDF (residues 24-57) was shown to have
antiangiogenic properties, and a 44-mer (residues 58-101) was shown to have neurotrophic properties.[12] A
BLAST search reveals a putative receptor binding site exists between residues 75-124. A
nuclear localization sequence (NLS) exists about 150 amino acids into the protein. The additional molecular weight is partly due to a single
glycosylation site at residue 285.[13] Near the C-terminus at residues 365-390 lies the reactive center loop (RCL) which is normally involved in serine protease inhibitor activity; however, in PEDF this region does not retain the inhibitory function.[5][14]
In 2001, the
crystal structure of PEDF was successfully generated.[15] The PEDF structure includes 3 beta sheets and 10 alpha helices. This discovery demonstrated that PEDF has an asymmetrical charge distribution across the whole protein. One side of the protein is heavily basic and the other side is heavily acidic, leading to a polar 3-D structure. They proposed that the basic side of the protein contains a heparin binding site.
Secreted PEDF binds a receptor on the cell surface termed
PEDF-R.[20] PEDF-R has
phospholipase A2 activity which liberates fatty acids from glycerolipids. PEDF enhances
gamma-secretase activity, leading to the cleavage of the
VEGF receptor 1 (VEGFR-1) transmembrane domain.[21] This action interferes with VEGF signaling thereby inhibiting angiogenesis.
Laminin receptor is also a target for PEDF, and the interaction occurs between residues 24-57 of PEDF, a region known to regulate antiangiogenic function.[22]
PEDF induces
PPAR-gamma expression which in turn induces
p53, a tumor suppressor gene involved in
cell cycle regulation and
apoptosis.[23]Thrombospondin, an antiangiogenic protein, is upregulated by PEDF.[24] PEDF stimulates several other well known signaling cascades such as the
Ras pathway, the
NF-κB pathway, and extrinsic apoptosis cascades.[25]
Function
PEDF has a variety of functions including antiangiogenic, antitumorigenic, and neurotrophic properties.[26]Endothelial cellmigration is inhibited by PEDF.[27] PEDF suppresses retinal
neovascularization and endothelial
cell proliferation.[28][29] The antiangiogenic residues 24-57 were shown to be sufficient at inhibiting angiogenesis.[30]
PEDF is also responsible for apoptosis of endothelial cells either through the
p38 MAPK pathway[31] or through the
FAS/FASL pathway[32] Antiangiogenic function is also conferred by PEDF through inhibition of both
VEGFR-1[21] and
VEGFR-2.[33]
The antitumorigenic effects of PEDF are not only due to inhibition of supporting vasculature, but also due to effects on the cancer cells themselves. PEDF was shown to inhibit cancer cell proliferation and increase apoptosis via the FAS/FASL pathway.[34] VEGF expression by cancer cells is inhibited by PEDF.[35]
PEDF also displays neurotrophic functions. Retinoblastoma cells differentiate into neurons due to the presence of PEDF.[9] Expression of PEDF in the human retina is found at 7.4 weeks of gestation, suggesting it may play a role in retinal neuron differentiation.[36]
Clinical significance
PEDF, a protein with many functions, has been suggested to play a clinical role in dry eye, choroidal neovascularization, cardiovascular disease, diabetes, diabetic macular edema, osteogenesis imperfecta and cancer.[37][26][28][30][38][39] As an antiangiogenic protein, PEDF may help suppress unwanted neovascularization of the eye. Molecules that shift the balance towards PEDF and away from VEGF may prove useful tools in both choroidal neovascularization and preventing cancer
metastasis formation.[16][40][41]
^Tombran-Tink J, Johnson LV (Aug 1989). "Neuronal differentiation of retinoblastoma cells induced by medium conditioned by human RPE cells". Investigative Ophthalmology & Visual Science. 30 (8): 1700–7.
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^Tombran-Tink J, Pawar H, Swaroop A, Rodriguez I, Chader GJ (Jan 1994). "Localization of the gene for pigment epithelium-derived factor (PEDF) to chromosome 17p13.1 and expression in cultured human retinoblastoma cells". Genomics. 19 (2): 266–72.
doi:
10.1006/geno.1994.1057.
hdl:2027.42/31831.
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^Takenaka K, Yamagishi S, Jinnouchi Y, Nakamura K, Matsui T, Imaizumi T (Nov 2005). "Pigment epithelium-derived factor (PEDF)-induced apoptosis and inhibition of vascular endothelial growth factor (VEGF) expression in MG63 human osteosarcoma cells". Life Sciences. 77 (25): 3231–41.
doi:
10.1016/j.lfs.2005.05.048.
PMID15985268.
^Karakousis PC, John SK, Behling KC, Surace EM, Smith JE, Hendrickson A, Tang WX, Bennett J, Milam AH (Jun 2001). "Localization of pigment epithelium derived factor (PEDF) in developing and adult human ocular tissues". Molecular Vision. 7: 154–63.
PMID11438800.
^Tong JP, Yao YF (Mar 2006). "Contribution of VEGF and PEDF to choroidal angiogenesis: a need for balanced expressions". Clinical Biochemistry. 39 (3): 267–76.
doi:
10.1016/j.clinbiochem.2005.11.013.
PMID16409998.