PHOTOSENSITIZATION OF RETINAL-PIGMENT EPITHELIUM BY PROTOPORPHYRIN-IX

Background: Clinical evidence of injury to the retinal pigment epithel ium is an important feature of age-related macular degeneration, but t he mechanism of this injury is unknown. Blue-light-dependent activatio n of the blood-borne photosensitizer protoporphyrin IX is known to pro duce free radicals which may damage cells and tissues. This study was undertaken to determine the effect of blue light and protoporphyrin IX on retinal pigment epithelial cells in vitro. Methods: Third-passage porcine retinal pigment epithelial cells were plated in six-well cultu re plates at 100 000 cells/well and grown to confluence. Retinal pigme nt epithelial cells were then incubated in culture media with and with out 35 mu g/dl protoporphyrin IX and exposed to low intensity (118 mu W/cm(2)) blue, blue-free, or full-spectrum white light in an irradiati ng incubator for 16 h on/8 h off cycles for 7 days. Some of the wells were shielded from light (dark controls), Retinal pigment epithelial c ells were examined by light microscopy and were trypsinized and counte d after 7 days, Results: White light with and without protoporphyrin I X and protoporphyrin IX in dark conditions did not decrease the retina l pigment epithelial cell count significantly. Blue light alone and bl ue light with protoporphyrin IX decreased the cell count by 22 +/- 4% and 35 +/- 3% compared to the controls, respectively. Conclusion: Blue wavelength light without exogenous protoporphyrin IX has a cytotoxic effect on confluent cultures of retinal pigment epithelium, suggesting that endogenous photosensitizers may be present in retinal pigment ep ithelial cells. Protoporphyrin IX has an additive cytotoxic effect in the presence of blue light, suggesting that this photosensitizer is ca pable of mediating blue-light-induced retinal pigment epithelial damag e, Since protoporphyrin IX is present in blood and tissue fluids, and the retina is chronically exposed to light, protoporphyrin IX-mediated free radical formation may occur in vivo and may play a role in retin al pigment epithelial changes that occur early in the pathogenesis of age-related macular degeneration.