Elevated extracellular K+ inhibits apoptosis of corneal epithelial cells exposed to UV-B radiation.
The goal of this study was to determine if the high [K(+)] in tears, 20-25 mM, serves to protect corneal epithelial cells from going into apoptosis after exposure to ambient UV-B radiation. Human corneal-limbal epithelial (HCLE) cells in culture were exposed to UV-B at doses of 50-200 mJ/cm(2) followed by measurement of K(+) channel activation and activity of apoptotic pathways. Patch-clamp recording showed activation of K(+) channels after UV-B exposure at 80 mJ/cm(2) or 150 mJ/cm(2) and a decrease in UV-induced K(+) efflux with increasing [K(+)](o). The UV-activated current was partially blocked by the specific K(+) channel blocker, BDS-1. DNA fragmentation, as measured by the TUNEL assay, was induced after exposure to UV-B at 100-200 mJ/cm(2). DNA fragmentation was significantly decreased when cells were incubated in 25, 50 or 100mM K(o)(+) after exposure to UV-B. The effector caspase, caspase-3, was activated by exposure to UV-B at 50-200 mJ/cm(2), but there was a significant decrease in activation when the cells were incubated in 25, 50 or 100mM K(o)(+) following exposure to UV-B. A decrease in mitochondrial potential, a possible activator of caspase-3, occurred after exposure to UV-B at 100-200 mJ/cm(2). This decrease in mitochondrial potential was prevented by 100mM K(o)(+); however, 25 or 50mM K(o)(+) provided minimal protection. Caspase-9, which is in the pathway from mitochondrial potential change to caspase-3 activation, showed little activation by UV-B radiation. Caspase-8, an initiator caspase that activates caspase-3, was activated by exposure to UV-B at 50-200 mJ/cm(2), and this UV-activation was significantly reduced by 25-100mM K(o)(+). The data show that the physiologically relevant [K(+)](o) of 25 mM can inhibit UV-B induced activation of apoptotic pathways. This suggests that the relatively high [K(+)] in tears reduces loss of K(+) from corneal epithelial cells in response to UV exposure, thereby contributing to the protection of the ocular surface from ambient UV radiation.
Singleton
Katherine R
KR
Department of Biology, Calvin College, Grand Rapids, MI 49546, USA.
Will
David S
DS
Schotanus
Mark P
MP
Haarsma
Loren D
LD
Koetje
Leah R
LR
Bardolph
Susan L
SL
Ubels
John L
JL
D017209:Apoptosis / Q000502:physiology / Q000528:radiation effects*; D020169:Caspases / Q000378:metabolism; D002461:Cell Line, Transformed; D002478:Cells, Cultured; D053938:DNA Fragmentation; D004307:Dose-Response Relationship, Radiation; D004789:Enzyme Activation / Q000187:drug effects; D019573:Epithelium, Corneal / Q000166:cytology / Q000378:metabolism / Q000528:radiation effects*; D006801:Humans; D020287:In Situ Nick-End Labeling; D053078:Membrane Potential, Mitochondrial / Q000502:physiology / Q000528:radiation effects; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000502:physiology* / Q000528:radiation effects; D015398:Signal Transduction / Q000528:radiation effects; D014466:Ultraviolet Rays
D015221:Potassium Channels; D020169:Caspases