Growth factor localization in choroidal neovascular membranes of age-related macular degeneration

Because several polypeptide growth factors are known to influence capillary endothelial cell mitogenesis, the authors investigated the presence of some of these molecules in choroidal neovascular membranes (CNVMs) removed surgically from human subjects with age-related macular degeneration (ARMD).

The authors performed immunoelectron microscopic studies on surgically removed submacular CNVMs from nine subjects with ARMD and from one subject with ARMD whose eye was studied after death. These were compared with retinal pigment epithelial (RPE) and choroidal tissue from eight normal subjects whose eyes were received after death and one received after massive trauma.

RPE cells from the CNVMs were strongly immunoreactive for acidic and basic fibroblast growth factor (aFGF and bFGF) and for transforming growth factor beta (TGF beta). Some of the immunoreactivity was intracytoplasmic, but most was intralysosomal. In addition, some choriocapillary endothelial cells located close to the RPE layer in these CNVMs were immunopositive for bFGF and for FGF receptor. Reaction product for these two substances was located at regular intervals along the endothelial plasma membrane on both the anteluminal and the luminal side of the cells, suggesting a physiological reaction between the growth factor and its receptor. Choriocapillary endothelial cells deeper within the stroma were unreactive to bFGF and FGF receptor antibodies. There was little immunoreactivity for the growth factors in RPE or choriocapillary endothelial cells from normal eyes. The aFGF and bFGF immunoreactivity was highly specific because aFGF positivity was abolished when the antibody was incubated with 10(-6) M aFGF but not a with the same concentration of bFGF, whereas bFGF immunoreactivity was abolished by incubation of the antibody with bFGF but not with aFGF. RPE cells from normal eyes and from eyes affected by ARMD showed strong cytoplasmic immunoreactivity to antibodies for cytoplasmic retinaldehyde-binding protein and superoxide dismutase and weak reactivity to antibodies for vimentin.

These results are consistent with the hypothesis that one or both FGFs are causally related to the development of choroidal neovascularization. The authors have reported similar observations in experimental choroidal neovascularization in pigmented rats after red krypton laser photocoagulation. TGF beta may serve to modulate the effects of these mitogens. The authors suggest that growth factor production is induced in RPE cells after physical or chemical damage. Because of the damage to these cells, FGF molecules can be released from the cells despite the absence of a "signal sequence" in the DNA coding for FGF production.