Repeated UVR exposures cause keratocyte resistance to apoptosis and hyaluronan accumulation in the rabbit cornea

Abstract

ABSTRACT.Purpose: To evaluate hyaluronan (HA) production and level of apoptosis of corneal cells after repeated UVR exposures. Methods: Fifteen albino rabbit corneas were exposed to 310 nm UVR at a dose that causes biomicroscopically significant keratitis (0.47 J/cm 2 ). Nine rabbits received a single dose of UVR. Six rabbits were irradiated 3 times at 7-day intervals. Rabbits exposed to the single dose of UVR, were sacrificed 24 hours, 7 and 14 days after irradiation. Rabbits exposed to the repeated doses of UVR, were sacrificed 24 hours and 14 days after the last irradiation. The corneal tissue specimens were processed for histological analysis using specific staining for HA, and the TdT-dUTP terminal nick-end labeling (TUNEL) assay. Results: Corneas exposed to a single UVR dose showed extensive positive TU-NEL staining 24 hours after exposure. Almost all basal epithelial cells, keratocytes throughout the entire thickness of the stroma, and endothelial cells were TUNEL-positive. No HA was found 24 hours after exposure. Extracellular HA staining of high intensity was found at day 7 throughout the entire central stroma, except the anterior one-fourth. At day 14 only a faint HA staining was detected in the posterior stroma, close to Descemet's membrane. Corneas exposed to repeated UVR doses showed at 24 hours positive TUNEL staining only in epithelial cells and in very few stromal cells. The majority of stromal cells and endothelial cells were unaffected. At the same time HA staining of very high intensity was found both at 24 hours and day 14, and it was evenly distributed throughout the entire thickness of the stroma. Conclusion: Repeated UVR exposures lead to increased production and accumulation of HA in the corneal stroma. The repopulated keratocytes are much more resistant to apoptosis than the native ones. HA accumulation may be a sign of long-term changes in the cornea.

Pathogenetic mechanisms and treatment options for ophthalmic pterygium: Trends and perspectives (Review)

“…It is likely that this finding is due to a disturbed balance between pro-apoptotic proteins (such as bax) and anti-apoptotic proteins (such as bcl-2) in the epithelium of pterygium, in favour of the latter (58). Again, differences in the apoptotic status between different epithelial cell layers have been detected with apoptosis remaining active at the basal epithelium of pterygium but not at more superficial epithelial layers (3,59).…”

The effect of single and repeated UVB radiation on rabbit cornea

“…The harmful effect of UVB radiation on corneal tissue has already been widely described [4,7,24,25,36], especially the most common acute effect, photokeratitis [4, 22,23,27,32]. Chronic UVR exposition of the eye, likely to increase with ozone depletion, includes a variety of corneal and conjunctival pathologies, e.g.…”

“…The tissue is thus exposed to oxidative stress and the biochemical status of the cell is severely affected [4,37]. Moreover, as an additional harmful effect of UVB radiation, apoptosis has been frequently observed [24,25]. Both mechanisms enhance the energy demand of the tissue and cause important metabolic changes.…”

“…Previous studies of UVR cumulative effects on corneal tissue were based on biomicroscopic observations [7, 25,40,41], but description of accompanying alterations in the metabolic profile is missing. However, since our research group has introduced HR-MAS 1 H NMR spectroscopy into the animal eye research [28], the metabolic profile of the eye segments can be easily analysed [28,29,[33][34][35][36], and the effect of the repeated UV radiation exposure can be evaluated.…”

“…The cumulative effect of multiple short pulses or repeated long exposures has been recently examined [7,25,40,41] and found to be an important factor in evaluating the UVR effects on the cornea. Based on biomicroscopic observations of the corneal tissue, two threshold exposures, separated by less than 8 h, produced more extensive damage than a single threshold exposure [7].…”