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Previous investigators reported that epiretinal membranes isolated from patients with proliferative vitreoretinopathy (PVR) express various platelet-derived growth factor (PDGF) family members and PDGF receptors (PDGFRs) (Cui, J.Z., Chiu, A., Maberley, D., Ma, P., Samad, A., Matsubara, J.A., 2007. Stage specificity of novel growth factor expression during development of proliferative vitreoretinopathy. Eye 21, 200-208; Robbins, S.G., Mixon, R.N., Wilson, D.J., Hart, C.E., Robertson, J.E., Westra, I., Planck, S.R., Rosenbaum, J.T., 1994. Platelet-derived growth factor ligands and receptors immunolocalized in proliferative retinal diseases. Invest. Ophthalmol. Vis. Sci. 35(10), 3649-3663). Co-expression of ligand and receptor raises the possibility of an autocrine loop, which could be of importance in the pathogenesis of PVR. To begin to address this issue we determined whether the PDGFRs in epiretinal membranes isolated from PVR patient donors were activated. Indeed, immunohistochemical staining (using pan- and phospho-PDGFR antibodies) revealed that both PDGFR subunits were activated. Quantification of these data demonstrated that a greater percentage of cells expressed the PDGFR [alpha] subunit as compared with the [beta] subunit (44 /- 13% versus 32 /- 6.5%). Staining with phospho-PDGFR antibodies indicated that 36 /- 10% of the PDGFR [alpha] subunits were activated, whereas only 16 /- 5.5% of the PDGFR [beta] subunits were activated. Thus, a 2.25 fold greater percentage of the PDGFR [alpha] subunits was activated. Co-staining with diagnostic cell-type antibodies indicated that both retinal pigment epithelial and glial cells expressed activated PDGFR [alpha] subunits. These findings support the recent discovery that PDGF-C is the major vitreal isoform because PDGF-C is 3 times more likely to activate a PDGFR [alpha] subunit as compared with a PDGFR [beta] subunit. We conclude that PDGFRs are activated in epiretinal membranes of patients with PVR, and that the profile of active PDGFR subunits functionally supports the idea that PDGF-C is the predominant PDGF isoform present in the vitreous of patients with PVR. These findings identify PDGF-A, -AB and C as the best therapeutic targets within the PDGF family.

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