Characterization of lethal action of near-ultraviolet on retinal pigment epithelial cells in vitro.

The lethal effect of near ultraviolet (NUV) with low intensity on cultured RPE cells has been investigated. RPE cultures with various cell densities were exposed to NUV (peaking at 365 nm) with or without ambient oxygen in phenol-red-free Dulbecco's PBS containing Ca2+, Mg2+ and glucose (PBS+). The cell viability was determined by dye exclusion and was expressed as cell death ratio (CDR, dead cells/total cells). When RPE cells at 5 x 10(3) cells/cm2, a non-contact low density, were irradiated either at a fixed irradiance (900 microW/cm2) with different exposure times (4 to 8h) or vice versa (8 h with irradiance from 430 to 900 microW/cm2), the change of CDR represented a similar linear function. The replotted data from both the time- and the irradiance-dependent curves indicated that the killing of RPE cells is dependent on the total energy dose of NUV. When a single NUV energy (19.44 J/cm2) was used, CDR was RPE cell density dependent. At confluence, NUV at the highest dosage tested (26 J/cm2) did not show any lethality. An oxygen-free condition abolished the NUV lethality on RPE cells even though the RPE cells were at a non-contact state. The presence of an antioxidant enzyme, catalase, in oxygen-saturated PBS+ protected RPE cells against NUV killing, but superoxide dismutase did not protect the RPE cells against NUV killing. These findings demonstrate that NUV possesses a lethal effect on RPE cells in vitro. Two key factors determine the magnitude and nature of this lethal effect: first, total NUV energy dose determines the nature of NUV's lethal effect; second, RPE growth conditions suggest the importance of cell-cell interaction in protecting these cells from NUV injury. Because an oxygen-free condition abolishes NUV lethality, it suggests that the oxidative stress is directly related to NUV lethal action. The selective inhibition by catalase of NUV killing of RPE cells suggests that the killing is oxidative species specific. NUV radiation might be highly risky to RPE viability in vivo, especially when the integrity of the RPE layer has been lost.