Placental growth factor (PlGF) is a member of the vascular endothelial growth factor (VEGF) family of angiogenesis factors. It has the highest expression level in the endothelium as compared to other VEGF family members and plays an important role in pathological neovascularization.1 In humans, four isoforms of PlGF exist (PlGF-1 through 4). Mice only express a single isoform of PlGF which is comparable to human PlGF-2.2 There has been great interest in the physiological function of PlGF. Its global deletion, unlike VEGF-A, is not lethal suggesting minimal role for PlGF during embryonic development.3 Deletion of a single allele of VEGF-A has significant impact on embryonic development suggesting non- overlapping functions for these factors.
The specific functions of the VEGF family members are related to the specificity of these factors for their corresponding cell surface receptors. Although VEGF-A interacts with VEGF receptor-1 (VEGFR-1 also known as flt-1) and VEGF receptor-2 (VEGFR-2 also known as KDR), PlGFs can interact with VEGF-R1 as well as neuropilins.1-2 However, PlGF-1 only interacts with VEGFR-1 and not neuropilins.4 PlGF and other members of the VEGF family are expressed in the retina with distinct regulatory roles during retinal vascular development and regression of the hyaloid vasculature.5-6 In addition, their altered expression plays an important role in ischemia mediated retinal neovascularization.7 Interestingly, only the expression of PlGF is augmented by hyperoxia.6 The exogenous administration of PlGF-1 and exclusive activation of VEGFR-1 protects retinal vasculature from hyperoxia-mediated vaso-obliteration.4 Thus, specific targeting and activation of VEGFR-1 may have therapeutic benefit during oxygen- induced ischemic retinopathy.
Studies attempting to delineate the physiological function of PlGF have resulted in contradictory reports regarding the role of PlGF in pathological neovascularization with some cancers but not others.8-9 It is now somewhat clear that inhibition of PlGF activity using antibodies may be tumor specific and requires functional VEGFR-1 expression.10 However, not all anti-PlGF antibodies show antagonistic activity,8 the reason for which remains unclear. In addition, PlGF may synergize with VEGF under some conditions with significant impact on pathological angiogenesis.3
Studies with PlGF null mice have consistently shown attenuation of choroidal neovascularization (CNV) in the laser model, and antibodies to PlGF or VEGFR-1 are similarly efficacious in inhibiting CNV in this model.8,11 The proangiogenic activity of PlGF in this model may be attributed to its function in recruiting macrophages and their proangiogenic programming in exudative AMD.12 In the current issue of Journal of Ophthalmic and Vision Research, Nourinia and colleagues show that knockdown of PlGF by targeted siRNA, likely in RPE cells, also suppresses CNV in a mouse laser model.13
Age-related macular degeneration (AMD) and CNV are a major cause of blindness in the elderly and VEGF is well known to be a contributing factor. In fact antagonism of VEGF activity, using antibodies to VEGF, has proven effective in preserving vision loss at least in some patients with exudative AMD.14 However, the exact contribution of PlGF to the pathogenesis of AMD and CNV requires further studies. The expression of PlGF has been demonstrated in human neovascular membranes.11 PlGF mRNA expression has also been demonstrated in the intact choroid, and is significantly up-regulated during the course of experimental CNV.11 Thus, it is clear that PlGF plays a significant role in experimental CNV. Increased levels of PlGF have also been reported in eyes of patients with diabetes and interference with its receptor during oxygen-induced ischemic retinopathy (OIR) inhibits neovascularization.15-16 Therefore, targeting PlGF may be a promising strategy for treatment of eye diseases with a neovascular component including diabetic retinopathy, retinopathy of prematurity and exudative AMD.
Retinal pigment epithelial (RPE) cells are a major source of production of various pro- and anti-angiogenic factors including PlGF and thrombospondin-1.17-18 RPE cells not only produce PlGF but also respond to PlGF by enhanced migration and diminished proliferation.17 RPE cells also express Flt-1, KDR, and neuropilins 1 and 2. Thus, PlGF has autocrine and paracrine action in RPE cells. The impact of PlGF deficiency on the phenotype of RPE cells has been examined in knockdown experiments.19 Decreased levels of PlGF minimally impacted the proliferation and migration of RPE cells but attenuated their proangiogenic activity in culture. Knockdown of PlGF in tumor cells also attenuates their angiogenic potential.12,20 Thus, PlGF may be a major regulator of angiogenic potential during ocular vascularization and tumor progression.
The same authors had previously shown that the expression of PlGF in RPE cells can be effectively down-regulated using a siRNA strategy.19 However, it remains to be demonstrated that administration of the siRNA and attenuation of CNV in vivo is associated with reduced levels of PlGF. Unfortunately, very little information is available regarding various ocular cell types responsible for production of PlGF, and potential targets for siRNA knockdown. However, the studies presented by Nourinia et al.13 provide additional support for the important role of PlGF in exudative AMD and its potential for targeting treatment of CNV. Furthermore the synergistic antagonism of PlGF activity, along with that of VEGF, may prove more effective for treatment of exudative AMD and deserves further investigation.
There are still a number of key questions which deserve further consideration. For example the major cellular sources of PlGF, especially during the pathogenesis of AMD, remain unknown. In addition, the detailed underlying mechanisms responsible for changes in the expression of PlGF, and its proangiogenic activity under various pathological conditions remain to be determined. This is further impacted by specific expression of PlGF isoforms and their interactions with their specific set of receptors. The potential heterodimerization of PlGF with various VEGF family members adds additional complexity. Thus, additional work still remains to elucidate various activities of PlGF and its impact on various diseases with a neovascular component.
Footnotes
Conflicts of Interest
None.
REFERENCES
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