Pathway-specific effect of caffeine on protection against UV irradiation-induced apoptosis in corneal epithelial cells

NEAT1 Deficiency Promotes Corneal Epithelial Wound Healing by Activating cAMP Signaling Pathway

Purpose

This study aimed to investigate the role of the long non-coding RNA (lncRNA) NEAT1 in corneal epithelial wound healing in mice.

Methods

The central corneal epithelium of wild-type (WT), MALAT1 knockout (M-KO), NEAT1 knockout (N-KO), and NEAT1 knockdown (N-KD) mice was scraped to evaluate corneal epithelial and nerve regeneration rates. RNA sequencing of the corneal epithelium from WT and N-KO mice was performed 24 hours after debridement to determine the role of NEAT1. Quantitative PCR (qPCR) and ELISA were used to confirm the bioinformatic analysis. The effects of the cAMP signaling pathway were evaluated in N-KO and N-KD mice using SQ22536, an adenylate cyclase inhibitor.

Results

Central corneal epithelial debridement in N-KO mice significantly promoted epithelial and nerve regeneration rates while suppressing inflammatory cell infiltration. Furthermore, the expression of Atp1a2, Ppp1r1b, Calm4, and Cngb1, which are key components of the cAMP signaling pathway, was upregulated in N-KO mice, indicative of its activation. Furthermore, the cAMP pathway inhibitor SQ22536 reversed the accelerated corneal epithelial wound healing in both N-KO and N-KD mice.

Conclusions

NEAT1 deficiency contributes to epithelial repair during corneal wound healing by activating the cAMP signaling pathway, thereby highlighting a potential therapeutic strategy for corneal epithelial diseases.

Caffeine and its main targets of colorectal cancer

Caffeine is a purine alkaloid and is widely consumed in coffee, soda, tea, chocolate and energy drinks. To date, a growing number of studies have indicated that caffeine is associated with many diseases including colorectal cancer. Caffeine exerts its biological activity through binding to adenosine receptors, inhibiting phosphodiesterases, sensitizing calcium channels, antagonizing gamma-aminobutyric acid receptors and stimulating adrenal hormones. Some studies have indicated that caffeine can interact with signaling pathways such as transforming growth factor β, phosphoinositide-3-kinase/AKT/mammalian target of rapamycin and mitogen-activated protein kinase pathways through which caffeine can play an important role in colorectal cancer pathogenesis, metastasis and prognosis. Moreover, caffeine can act as a general antioxidant that protects cells from oxidative stress and also as a regulatory factor of the cell cycle that modulates the DNA repair system. Additionally, as for intestinal homeostasis, through the interaction with receptors and cytokines, caffeine can modulate the immune system mediating its effects on T lymphocytes, B lymphocytes, natural killer cells and macrophages. Furthermore, caffeine can not only directly inhibit species in the gut microbiome, such as Escherichia coli and Candida albicans but also can indirectly exert inhibition by increasing the effects of other antimicrobial drugs. This review summarizes the association between colorectal cancer and caffeine that is being currently studied.

The acute effects of single cup of coffee on ocular biometric parameters in healthy subjects

Purpose

To evaluate ocular biometric changes in healthy subjects after caffeine consumption from a cup of coffee.

Methods

A total of 36 subjects were included in this prospective observational study. Axial length (AL) and anterior segment parameters including aqueous depth (AD), anterior chamber depth (ACD), lens thickness (LT), and central corneal thickness (CCT) were measured with optic biometry, Lenstar LS 900 (Haag-Streit, Inc., Koeniz, Switzerland) before and 1 and 4 h after ingesting a cup of coffee (60 mg caffeine/100 mL).

Results

Mean age of the participants was 30.05 ± 7.43 years (range, 19–45). At baseline, 1st, and 4th hour, AL values were 23.9 ± 1.04 mm, 23.91 ± 1.04 mm, and 23.89 ± 1.04 mm, respectively, and no significant difference was observed (P>0.05). At baseline, 1st, and 4th hour, AD values were 3.06 ± 0.3 mm, 3.11 ± 0.3 mm, and 3.09 ± 0.3 mm, and ACD values were 3.6 ± 0.32, 3.66 ± 0.31, and 3.64 ± 0.31, respectively. AD and ACD values were significantly greater than baseline at 1st and 4th hours following coffee ingestion. Coffee intake caused a significant reduction in LT, compared with baseline and at the 1st and 4th hours which were 3.76 ± 0.28 mm, 3.69 ± 0.32 mm, and 3.72 ± 0.27 mm, respectively. No statistically significant difference was determined in between the 3 measurements in terms of CCT (P>0.05).

Conclusion

Caffeine causes a significant increase in AD and ACD and a significant decrease in LT following oral intake, for at least 4 h.

Role of Bone Morphogenetic Protein 7 (BMP7) in the Modulation of Corneal Stromal and Epithelial Cell Functions

In the cornea, healing of the wounded avascular surface is an intricate process comprising the involvement of epithelial, stromal and neuronal cell interactions. These interactions result to the release of various growth factors that play prominent roles during corneal wound healing response. Bone morphogenetic proteins (BMPs) are unique multi-functional potent growth factors of the transforming growth factor-beta (TGF-β) superfamily. Treatment of corneal epithelial cells with substance P and nerve growth factor resulted to an increase in the expression of BMP7 mRNA. Since BMP7 is known to modulate the process of corneal wound healing, in this present study, we investigated the influence of exogenous rhBMP7 on human corneal epithelial cell and stromal cell (SFs) function. To obtain a high-fidelity expression profiling of activated biomarkers and pathways, transcriptome-wide gene-level expression profiling of epithelial cells in the presence of BMP7 was performed. Gene ontology analysis shows BMP7 stimulation activated TGF-β signaling and cell cycle pathways, whereas biological processes related to cell cycle, microtubule and intermediate filament cytoskeleton organization were significantly impacted in corneal epithelial cells. Scratch wound healing assay showed increased motility and migration of BMP7 treated epithelial cells. BMP7 stimulation studies show activation of MAPK cascade proteins in epithelial cells and SFs. Similarly, a difference in the expression of claudin, Zink finger E-box-binding homeobox 1 was observed along with phosphorylation levels of cofilin in epithelial cells. Stimulation of SFs with BMP7 activated them with increased expression of α-smooth muscle actin. In addition, an elevated phosphorylation of epidermal growth factor receptor following BMP7 stimulation was also observed both in corneal epithelial cells and SFs. Based on our transcriptome analysis data on epithelial cells and the results obtained in SFs, we conclude that BMP7 contributes to epithelial-to-mesenchymal transition-like responses and plays a role equivalent to TGF-β in the course of corneal wound healing.

Caffeine Prevents Blood Retinal Barrier Damage in a Model, In Vitro, of Diabetic Macular Edema

Diabetic macular edema (DME) is the major cause of vision loss in patients affected by diabetic retinopathy. Hyperglycemia and hypoxia represent the key elements in the progression of these pathologies, leading to breakdown of the blood-retinal barrier (BRB). Caffeine, a psychoactive substance largely consumed in the world, is a nonselective antagonist of adenosine receptors (AR) and it possesses a protective effect in various diseases, including eye pathologies. Here, we have investigated the effect of this substance on BRB integrity following exposure to hyperglycemic/hypoxic insult. Retinal pigmented epithelial cells, ARPE-19, have been grown on semi-permeable supports mimicking an experimental model, in vitro, of outer BRB. Caffeine treatment has reduced cell monolayer permeability after exposure to high glucose and desferoxamine as shown by TEER and FITC-dextran permeability assays. This effect is also mediated through the restoration of membrane's tight junction expression, ZO-1. Moreover, we have demonstrated that caffeine is able to prevent outer BRB damage by inhibiting apoptotic cell death induced by hyperglycemic/hypoxic insult since it downregulates the proapoptotic Bax and upregulates the anti-apoptotic Bcl-2 genes. Although further studies are needed to better comprise the beneficial effect of caffeine, we can speculate that it might be used as an innovative drug for DME treatment. J. Cell. Biochem. 118: 2371-2379, 2017. © 2017 Wiley Periodicals, Inc.

Nuclear ferritin mediated regulation of JNK signaling in corneal epithelial cells

Corneal epithelial (CE) cells are exposed to environmental insults (e.g., UV-irradiation), yet they suffer little damage. Our previous studies suggest that chicken CE cells have a novel form of protection involving having ferritin in a nuclear location where it can bind to DNA and sequester free iron. Here we describe another potential nuclear ferritin-mediated protective mechanism: the down-regulation of the JNK signaling pathway. The JNK pathway has been shown by others to promote apoptosis in response to cell damage and also to be activated in CE cell lines following exposure to UV radiation. Here we show in COS7 reporter cell lines that the expression of ferritin in a nuclear localization significantly down-regulates the JNK pathway (p = 5.7 × 10(-6)), but has no effect on the NFkB or the Erk pathways. In organ cultures of embryonic chicken corneas, we observed that inhibiting the synthesis of nuclear ferritin in CE cells, using the iron-chelating molecule deferoxamine, led to an increase in JNK signaling, as measured by phospho-JNK levels compared to CE cells with nuclear ferritin. Furthermore, the chemical inhibition of the JNK pathway using the molecule AS601245 decreased the production of nuclear ferritin. Taken together, these observations suggest that in CE cells a feedback-loop exists in which JNK signaling increases the production of nuclear ferritin and, in turn, nuclear ferritin decreases the activity of the JNK signaling pathway.

Osmotic stress-induced phosphorylation of H2AX by polo-like kinase 3 affects cell cycle progression in human corneal epithelial cells

Increased concentrations of extracellular solutes affect cell function and fate by stimulating cellular responses, such as evoking MAPK cascades, altering cell cycle progression, and causing apoptosis. Our study results here demonstrate that hyperosmotic stress induced H2AX phosphorylation (γH2AX) by an unrevealed kinase cascade involving polo-like kinase 3 (Plk3) in human corneal epithelial (HCE) cells. We found that hyperosmotic stress induced DNA-double strand breaks and increased γH2AX in HCE cells. Phosphorylation of H2AX at serine 139 was catalyzed by hyperosmotic stress-induced activation of Plk3. Plk3 directly interacted with H2AX and was colocalized with γH2AX in the nuclei of hyperosmotic stress-induced cells. Suppression of Plk3 activity by overexpression of a kinase-silencing mutant or by knocking down Plk3 mRNA effectively reduced γH2AX in hyperosmotic stress-induced cells. This was consistent with results that show γH2AX was markedly suppressed in the Plk3(-/-) knock-out mouse corneal epithelial layer in response to hyperosmotic stimulation. The effect of hyperosmotic stress-activated Plk3 and increased γH2AX in cell cycle progression showed an accumulation of G2/M phase, altered population in G1 and S phases, and increased apoptosis. Our results for the first time reveal that hyperosmotic stress-activated Plk3 elicited γH2AX. This Plk3-mediated activation of γH2AX subsequently regulates the cell cycle progression and cell fate.

Ocular injury by transient formaldehyde exposure in a rabbit eye model

Formaldehyde (FA) is frequently used in sterilizing surgical instruments and materials. Exposure to FA is highly concerned for eye tissues. Rabbit corneal epithelial cells were examined for changes after FA exposure. Our results showed that cell survival decreased 7 days after transient 3 min exposure to more than 100 ppm FA by trypan blue staining while MTT assay detected significant decrease at 20 ppm at 24 hours observation. The decrease of cell survival rate was concentration (up to 600 ppm)- and observation time (1–7 day)- dependent. The cell number decreased after 100 ppm FA exposure for more than 10 min at 7-day observation. The FA treated cells showed increased apoptosis/necrosis and cell cycle accumulation at sub G1 phase as well as mitochondria clustering around nucleus. The in vivo rabbit eye exposure for tear production by Schirmer’s test revealed that the FA-induced overproduction of tear also exhibited observation time (1–10 day)- and FA concentration (20–300 ppm for 5 min exposure)-dependent. Activated extracellular signal-regulated kinase (pERK2) in cornea explants by western blotting was reduced and increased c-Jun amino - terminal kinase (JNK) activation (pJNK) in cornea and conjunctiva was evident at 2 month after exposure to 50–200 ppm FA for 5 min. In conclusion, injury to the eye with transient exposure of up to 100 ppm FA for 3 min decreased corneal cell survival while a more sensitive MTT test detected the cell decrease at 20 ppm FA exposure. Morphology changes can be observed even at 5 ppm FA exposure for 3 min at 7 days after. The FA exposure also increased apoptotic/necrotic cells and sub-G1 phase in cell cycle. Long term effect (2 months after exposure) on the eye tissues even after the removal of FA can be observed with persistent JNK activation in cornea and conjunctiva.

Caffeine neuroprotects against dexamethasone-induced anxiety-like behaviour in the Zebrafish (Danio rerio)

The early-life stress has critical impact on brain development which can lead to long-term effects on brain functions during adulthood. It has been reported that caffeine possesses a protective effect in neurodegenerative diseases. Thus, this study investigates the potential of caffeine to protect brain functions from adverse effects due to stress exposure during early-life development in the male zebrafish. In the first part of this study, synthetic glucocorticoid, dexamethasone (DEX) (2-200 mg/L for 24 h) was used to induce stress effects in the zebrafish larvae from 4 to 5 days post-fertilisation (dpf) and the effect of DEX administration on zebrafish larvae on anxiety-like behaviour during adulthood in novel tank test was investigated. Next, the possible protective effect of caffeine pre-treatment (5-50 mg/L for 24 h from 3 to 4dpf) before DEX administration was studied. DEX-treated adult male zebrafish showed higher anxiety levels in behavioural tests, as seen in longer latency to enter the top part of the tank, lower transition numbers between the top and bottom parts with more time spent at the bottom and lesser time spent at the top and lower distance travelled at top part. The effect of DEX on anxiety-like behaviour was dose-dependent. Importantly, adult male zebrafish pre-treated with caffeine before DEX treatment did not show any anxiety-like behaviour. These results show that exposure to stress during early-life leads to anxiety-like behaviour in the adult male zebrafish but pre-treatment with caffeine protects from stress-induced anxiety.

Effect of caffeine on the intraocular pressure in patients with primary open angle glaucoma

Purpose

Coffee and tea are very common nonalcoholic beverages. However, their intake, particularly that of coffee, has been suggested to increase intraocular pressure (IOP) in patients with open angle glaucoma/ocular hypertension. The causative agent has been suggested to be their caffeine content. The objective of this study was to determine if this represents a direct caffeine effect. This study was therefore done using pure caffeine applied directly to the eyes.

Methods

The study was conducted with five human volunteers with open angle glaucoma/ ocular hypertension. IOP was measured using a Perkins applanation tonometer. Eye drops of 1% caffeine were prepared in-home. Following the initial (basal) measurement of the IOP, 50 μL of the eye drop preparation was instilled in the eye at 0-, 4-, and 6-hour intervals. IOPs were measured 30 minutes after each instillation. A second study was also undertaken following the first. In this study, the same patients instilled the eye drops three times per day for 1 week at home and then returned to the clinic on day 7. They were then again treated with caffeine eye drops as above and IOPs measured.

Results

In the 1-day study, the mean basal IOP was 23.6 ± 2.80 mmHg. Thirty minutes after instillation of the drops as described, the pressures were 23.2 ± 1.93, 22.2 ± 1.99, and 22.6 ± 2.31. The basal reading was taken at 10 am and another reading was then taken at 10.30 am. Additional eye drops were instilled at 2 and 8 pm and readings taken 30 minutes after each instillation. In the 1 week study, the basal value was 22.6 ± 2.32. After instillation of the drops as above the values were 23 ± 2.16, 22.4 ± 2.27, and 23 ± 1.94.

Conclusion

Administration of caffeine into the eyes of patients did not have any effect on IOP and it remained relatively unchanged. This was true in the 1-day study as well as in the 1-week study. A cumulative effect was not visible. The results therefore demonstrate that caffeine has no significant effect on IOP in patients with glaucoma. Any effects reported in coffee drinkers may therefore be related to other constituents in coffee, known to be generated pyrolytically from endogenous constituents of coffee beans by roasting at relatively high temperature, combined with the osmotic effects imposed by adequate fluid intake, known to be common in glaucoma patients.

Protective effect of aminophylline against cigarette smoke extract-induced apoptosis in human lung fibroblasts (MRC-5 cells)

Cigarette smoking is the principal cause of chronic obstructive pulmonary disease (COPD), especially emphysema, which is characterized by alveolar wall destruction and airspace enlargement. Apoptosis of lung structural cells is involved in the pathogenesis of COPD. Xanthine derivatives (aminophylline or theophylline) have been used for the treatment of COPD as a bronchodilator. But the effects of xanthine derivatives on apoptosis of the lung structural cells remain poorly understood, even though it is known that theophylline protects against ultraviolet irradiation-induced cell death in corneal epithelial cells. This study was designed to determine whether aminophylline would protect against cigarette smoke extract (CSE)-induced apoptosis in lung fibroblasts. We demonstrated that aminophylline protected against apoptosis of MRC-5 cells at a relatively lower therapeutic range (10 μg/ml), resulting in a significant increase in cell viability occurring at 20% concentration after 8-hr exposure. Annexin staining decreased from 68 ± 4% of the control to 12 ± 2% of aminophylline (10 μg/ml) pre-treatment after 20% CSE exposure for 12 hr (p < 0.05). Aminophylline decreased caspase 3 and 8 activities and nuclear condensation or fragmentation in MRC-5 cells after exposure to 20% CSE for 12 hr compared with control and high levels of aminophylline (>50 μg/ml) pre-treatment. These findings suggest that aminophylline protected apoptosis of MRC-5 cells through the inactivation of caspases 3 and 8 and could be an effective agent to reduce cigarette smoking-induced lung structural cell apoptosis.

Effectiveness of topical caffeine in cataract prevention: studies with galactose cataract

PURPOSE

The primary objective of the study was to investigate the possible inhibition of cataract formation by topical administration of caffeine using the galactosemic rat model. It was hypothesized that caffeine will do so by acting as scavenger of reactive oxygen species known to be generated under hyperglycemic conditions.

METHODS

Cataract was induced by feeding young rats a diet containing 24% galactose for a period of 25 days. A control group of such rats was treated with a placebo eye drop preparation containing hydroxy propyl methyl cellulose as a wetting agent. In the experimental group, the rats were treated with the above preparation mixed with 72 mM caffeine.

RESULTS

Administration of caffeine eye drops was found to significantly inhibit the onset as well as the progress of cataract formation. By day 25 on the galactose diet, all the animals in the control group developed advanced white opacity spread over the entire area of the lens. In the caffeine group, the formation of such opacity remained strikingly inhibited. The lenses remained largely transparent. The transparency data paralleled the higher concentration of glutathione maintained by caffeine treatment. Its levels in the placebo group were 0.8, 0.5, and 0.4 µmoles/g lens wt. on days 5, 15, and 25 against a consistent basal control value of ~3 µmoles/g over the entire period. In the caffeine group, the corresponding values were nearly 3 µmoles/g till day 15, but decreasing to ~2 µmoles/g on day 25. The levels were hence significantly higher than in the caffeine untreated group, remaining relatively closer to the basal controls. In addition, the compound was found effective in inhibiting morphological changes induced by galactose.

CONCLUSIONS

Micromolar amounts of topical caffeine have been found to be significantly effective in inhibiting the formation of galactose cataract, strongly suggesting its possible usefulness against diabetic cataracts. The effects are attributed to its ability to prevent oxidative stress and consequent maintenance of tissue metabolic and transport functions, in addition to preventing the induction of apoptosis.

Hyperosmotic stress-induced ATF-2 activation through Polo-like kinase 3 in human corneal epithelial cells

Elevated extracellular solute concentration (hyperosmotic stress) perturbs cell function and stimulates cell responses by evoking MAPK cascades and activating AP-1 transcription complex resulting in alterations of gene expression, cell cycle arrest, and apoptosis. The results presented here demonstrate that hyperosmotic stress elicited increases in ATF-2 phosphorylation through a novel Polo-like kinase 3 (Plk3) pathway in human corneal epithelial (HCE) cells. We found in hyperosmotic stress-induced HCE cells that Plk3 transferred to the nuclear compartment and was colocalized with ATF-2 in nuclei. Kinase activity of Plk3 was significantly activated by hyperosmotic stimulation. Further downstream, active Plk3 phosphorylated ATF-2 at the Thr-71 site in vivo and in vitro. Overexpression of Plk3 and its mutants enhanced hyperosmotic stress-induced ATF-2 phosphorylation. In contrast, suppression of Plk3 by knocking down Plk3 mRNA effectively diminished the effect of hyperosmotic stress-induced ATF-2 phosphorylation. The effect of hyperosmotic stress-induced activation of Plk3 on ATF-2 transcription factor function was also examined in CRE reporter-overexpressed HCE cells. Our results for the first time reveal that hyperosmotic stress can activate the Plk3 signaling pathway that subsequently regulates the AP-1 complex by directly phosphorylating ATF-2 independent from the effects of JNK and p38 activation.

Tropheryma whipplei, the Whipple's disease bacillus, induces macrophage apoptosis through the extrinsic pathway

Tropheryma whipplei, the etiological agent of Whipple's disease, is an intracellular bacterium that infects macrophages. We previously showed that infection of macrophages results in M2 polarization associated with induction of apoptosis and interleukin (IL)-16 secretion. In patients with Whipple's disease, circulating levels of apoptotic markers and IL-16 are increased and correlate with the activity of the disease. To gain insight into the understanding of the pathophysiology of this rare disease, we examined the molecular pathways involved in T. whipplei-induced apoptosis of human macrophages. Our data showed that apoptosis induction depended on bacterial viability and inhibition of bacterial protein synthesis reduced the apoptotic program elicited by T. whipplei. Induction of apoptosis was also associated with a massive degradation of both pro- and anti-apoptotic mediators. Caspase-specific inhibition experiments revealed that initiator caspases 8 and 10 were required for apoptosis, in contrast to caspases 2 and 9, in spite of cytochrome-c release from mitochondria. Finally, the effector caspases 3 and 6 were mandatory for apoptosis induction. Collectively, these data suggest that T. whipplei induces apoptosis through the extrinsic pathway and that, beside M2 polarization of macrophages, apoptosis induction contributes to bacterial replication and represents a virulence trait of this intracellular pathogen.

Photoprotective effects of glucomannan isolated from Candida utilis

Glucomannans belong to yeast and fungal cell wall polysaccharides with known immunostimulatory and radioprotective effects. However, glucomannan protective effects against pathological consequences of skin exposure to short wavelength solar light, ultraviolet (UV) radiation, are unclear. Herein, a highly branched glucomannan (GM) isolated from the cell wall of Candida utilis, a member of the alpha-(1-->6)-D-mannan group, was tested for its photoprotective effects in an in vitro model of UVB-irradiated human keratinocytes and an in vivo model of UV-induced erythema formation in human volunteers. GM suppressed the UVB-induced decrease of keratinocyte viability, which was connected with the suppression of UVB-induced keratinocyte apoptosis. GM reduced UVB-mediated caspase activation together with suppression of DNA fragment release into the cytoplasm. Furthermore, GM suppressed UVB-induced gene expression of pro-inflammatory markers including nuclear factor kappa B, inducible nitric oxide synthase, interleukins 8 and 1, together with suppression of prostaglandin E2 and interleukin 1alpha protein release. In vivo, GM decreased UV-induced skin erythema formation, which was correlated with a decrease of phosholipase A(2) activity within the stratum corneum. It could be concluded that GM isolated from C. utilis possesses significant photoprotective effects on human keratinocytes in vitro as well as in vivo.

Stress-induced c-Jun activation mediated by Polo-like kinase 3 in corneal epithelial cells

Polo-like kinase 3 (Plk3) activation occurs after exposure to environmental or genotoxic stresses. Plk3 regulates cell fate through regulating cell cycle progression. UV irradiation is one of the major environmental stresses that affect corneal epithelial wound healing. In the present study, we report that UV irradiation activated Plk3 and that Plk3 interacts with AP-1 and c-Jun, which appears to be important to mediate corneal epithelial cell apoptosis after UV irradiation. Recombinant Plk3, as well as Plk3 immunoprecipitated from UV-irradiated cells, phosphorylated c-Jun in vitro. The phosphorylation of c-Jun by Plk3 immunoprecipitates was not altered by the pre-removal of JNK from the cell lysates. In addition, the effect of UV irradiation-induced phosphorylation of c-Jun and apoptosis were not significantly affected by knockdown of JNK mRNA. Co-immunoprecipitation reveals that Plk3 and c-Jun directly interacted with each other. Consistently, Plk3 co-localized with c-Jun to the nucleus after UV irradiation. Further, modulating Plk3 activities by overexpressing Plk3 or its mutants significantly affected UV irradiation-induced c-Jun activity and subsequent apoptosis. Our results thus provide for the first time that Plk3 mediates UV irradiation-induced c-Jun activation by phosphorylating c-Jun, suggesting that Plk3 plays an important role in mediating programmed cell death of corneal epithelial cells after UV irradiation.