Blood vessel growth and stability are under the exquisite control of a network of pro- and anti-angiogenic factors. Disruption of the balance between these factors is a characteristic of tumor growth and many vascular diseases. Endogenous angiogenesis inhibitors, particularly those that act broadly at the earliest stages, are excellent pharmacological tools in combating pathogenic vessel growth. PEDF (Pigment Epithelium Derived Factor) is a potent and broadly acting neurotrophic factor that promotes survival of neurons in many regions of the CNS (Central Nervous System) from degeneration caused by serum withdrawal or glutamate cytotoxicity and oxidative damage (Ref.1). Glutamate neurotoxicity involves an increase in intracellular calcium resulting from the opening of NMDA (N-Methyl D-Aspartate) channels.
PEDF is synthesized and secreted by RPE (Retinal Pigment Epithelial) cells in early embryogenesis and is present in the ECM (Extracellular Matrix) between the RPE cells and the neural retina. It is structurally related to the serpin family of serine proteases, and is widely expressed in the developing and adult nervous systems. The structural domains that mediate the biological effects of PEDF have not been identified with precision. The PEDF Receptor is a transmembrane protein having an extracellular ligand-binding domain, a transmembrane domain and an intracellular domain. It shares some homology with an orphan receptor identified in the liver and the protein known as Adiponutrin. Protection against glutamate excitotoxicity in cerebellar granule neurons by PEDF involves activation of the NF-KappaB (Nuclear Factor-KappaB) signaling cascade and the subsequent expression of antiapoptotic and neuroprotective genes by activating IP3 (Inositol (1, 4, 5) Triphosphate) and Akt kinase pathways. PEDF-treated neurons contain predominantly p65/p50 heterodimers and p50/p50 homodimers of NF-KappaB. NF-KappaB in turn induces the expression of antiapoptotic genes and/or neurotrophic factors in cerebellar granule cells. PEDF triggers increased phosphorylation and degradation of I-KappaB-Alpha, I-KappaB-Beta (Inhibitor of Kappa Light Chain Gene Enhancer in B-Cells), decreased levels of IKK proteins, activation of NF-KappaB, nuclear translocation of p65/p50 heterodimers and p50/p50 homodimers and increased NF-KappaB DNA binding activity as part of its mechanism to protect immature cerebellar granule cells against apoptosis induced by low K+ (Ref.2). This sequence of intracellular events leads to changes in the expression of specific genes that have a key role in cell survival. PEDF also increases the expression of FasL and activates a transduction cascade that promotes endothelial cell death. Fas/FasL death cascade restricts the effects of PEDF on endothelial cells and halts their progression toward apoptosis by specifically interfering with the activation of Caspase8
. PEDF also eliminates the VEGF (Vascular Endothelial cell Growth Factor)-induced upregulation of FLIP1 (FLICE Inhibitory Protein), a factor that interferes with Caspase8
activation and increases resistance to the apoptotic effects of FasL. NF-KappaB also upregulates FLIP1 and FLIP1 promotes in turn the activation of NF-KappaB and
ERK1/2 (Extracellular Signal Regulated Kinase) signaling cascades (Ref.3).
ERK1/2 is involved in the regulation of cell growth and differentiation, but when improperly activated contribute to malignant transformation. The specific components of the ERK cascade vary greatly among different stimuli, but the architecture of the pathway usually includes a set of adaptors like SHC (SH2-containing Protein), GRB2 (Growth Factor Receptor Bound protein-2), SOS, etc. transducing the signal to small GTP binding proteins (Ras, Rap1), which in turn activate the core unit of the cascade composed of Raf, MEK1,2 (MAPK/ERK Kinase), and
ERK1/2. An activated ERK dimmer can regulate targets in the cytosol and also translocate to the nucleus where it phosphorylates a variety of transcription factors including TCF (Ternary Complex Factor), Elk1, c-Jun, c-Fos, SRF (Serum Response Factor), etc thereby regulating gene expression. In addition to acting on neurons, PEDF can also affect the survival of other cells such as glia. More importantly, it is a potent antiangiogenic factor. PEDF induces an increase in the anti-apoptotic genes Bcl2 (B-Cell CLL/Lymphoma-2), BclXL, or SOD/Mn-SOD (Superoxide Dismutase) only in immature cerebellar granule cells but causes a long-lasting induction of the genes for NGF (Nerve Growth Factor), BDNF (Brain Derived Neurotrophic Factor), and GDNF (Glial-cell-line-derived Neurotrophic Factor). The expression of PEDF seems to decline with age, and alterations on the availability of this factor might contribute to the development of different neurological conditions (Ref.4).
The signaling cascades that PEDF activates indicate its involvement in events that can lead to both survival and cell death. PEDF inhibits neovascularization in the corneal pocket assay and decreases migration in an endothelial cell. PEDF protein levels in the eye are responsive to ambient oxygen tension with protein levels increasing with increasing oxygen. In fetal and adult human eye tissue, PEDF is expressed by the cornea, ciliary body, and cells of the inner and outer retina. PEDF secreted by these cells accumulates in avascular spaces of the eye such as the aqueous humor, vitreous humor, and the interphotoreceptor matrix where it acts as a major inhibitor of angiogenesis (Ref.5). The activity of PEDF equals or supersedes that of other anti-angiogenic factors, including Angiostatin, Endostatin and Thrombospondin-1. In addition, PEDF has the potential to promote the survival of neurons and affect their differentiation. More recently the anti-angiogenic activity of PEDF has received much attention because blood-vessel growth in avascular compartments of the eye is a leading cause of blindness. A number of other studies have also demonstrated the neuronal-survival effect of PEDF. PEDF regulates normal pancreas and prostate mass. Its androgen sensitivity makes PEDF a likely contributor to the anticancer effects of androgen ablation. PEDF appears to both induce and inhibit apoptosis; the regulation of apoptosis by PEDF is likely to be regulated through its interaction with other proteins and/or by post-translational modifications (Ref.6).