UV dose-dependent caspase activation in a corneal epithelial cell line

Purpose. To characterize the UVB radiation-dependent patterns of caspase activation and cell death in SV 40 immortalized corneal epithelial cells. Methods. Cell death in immortalized human corneal epithelial cells (T-HCEC) was induced by exposure to low (50 mJ/cm²) and high (450 mJ/cm ²) doses of UVB. Cell death morphology was examined by fluorescence microscopy using the cell death marker propidium iodide (PI). Apoptosis of T-HCEC was analyzed by DNA fragmentation assays, and enzyme activity was measured for caspase 3 and 9 by fluorophotometry. Changes in mitochondrial inner membrane potential were measured by flow cytometry using the fluorescent marker, rhodamine 123. Redistribution of cytochrome c, the upstream trigger of caspase 9, was measured in the cytosol fraction of T-HCEC following irradiation. Results. PI staining revealed a fragmented staining pattern of the nucleus consistent with apoptosis in detached cells irradiated with low-dose UVB, while cells receiving high dose UVB demonstrated round, well bordered staining of the nucleus. Flow cytometry revealed irreversible mitochondrial damage in the high dose group shown by decreased levels of rhodamine 123 fluorescence. Cells in the low-dose group had intact mitochondrial inner membrane potential, increased cytosolic cytochrome c, and showed a significantly higher rate of DNA fragmentation and caspase activation than the high dose group. Conclusion. Low dose UVB caused cytochrome c redistribution, caspase activation and apoptosis of corneal epithelial cells, which was not observed at high irradiation levels of UVB.