Evidence of oxidative stress in human corneal diseases

Effects of Vitrectomy and Lensectomy on Older Rhesus Macaques: Oxygen Distribution, Antioxidant Status, and Aqueous Humor Dynamics

Purpose

The purpose of this study is to evaluate effects of vitrectomy (PPV) and lens extraction with intraocular lens implantation (PE/IOL) on molecular oxygen (pO₂) distribution, aqueous humor antioxidant–oxidant balance, aqueous humor dynamics, and histopathologic changes in the trabecular meshwork (TM) in the older macaque monkey.

Methods

Six rhesus monkeys underwent PPV followed by PE/IOL. pO₂, outflow facility, and intraocular pressure (IOP) were measured. Aqueous and vitreous humor specimens were analyzed for antioxidant status and 8-hydroxy-2′-deoxyguanosine (8-OHdG), a marker of oxidative damage. TM specimens were obtained for immunohistochemical and quantitative PCR analysis.

Results

pO₂ at baseline revealed steep gradients in the anterior chamber and low levels in the posterior chamber (PC) and around the lens. Following PPV and PE/IOL, pO₂ significantly increased in the PC, around the IOL, and angle. IOP increased following both surgical interventions, with no change in outflow facility. Histopathologic analysis did not show changes in TM cell quantification, but there was an increase in 8-OHdG. Quantitative PCR did not reveal significant differences in glaucoma-related gene expression. Aqueous and vitreous humor analysis revealed decreased ascorbate and total reactive antioxidant potential and increased 8-OHdG in the aqueous humor only in the surgical eyes.

Conclusions

Oxygen distribution in the older rhesus monkey is similar to humans at baseline and following surgical interventions. Our findings of histopathologic changes of TM oxidative damage and alterations in the oxidant-antioxidant balance suggest a potential correlation of increased oxygen exposure with oxidative stress/damage and the development of open angle glaucoma.

Pathophysiology of Keratoconus: What Do We Know Today

Keratoconus is a common corneal ectasia that leads to progressive visual impairment. Numerous studies have shown abnormal protein expression patterns in keratoconic corneas. However, the specific mechanisms causing this disease remain ambiguous. This review aims to provide an update on morphological studies of the keratoconic cornea, relate these early studies with current findings from proteomic, biochemical and cell culture studies and to postulate possible pathogenic pathways.

A review of keratoconus: Diagnosis, pathophysiology, and genetics

We discuss new approaches to the early detection of keratoconus and recent investigations regarding the nature of its pathophysiology. We review the current evidence for its complex genetics and evaluate the presently identified genes/loci and potential candidate gene/loci. In addition, we highlight current research methodologies that may be used to further elucidate the pathogenesis of keratoconus.

SLC4A11 depletion impairs NRF2 mediated antioxidant signaling and increases reactive oxygen species in human corneal endothelial cells during oxidative stress

Corneal endothelial dystrophy is a progressive disease with gradual loss of vision and characterized by degeneration and dysfunction of corneal endothelial cells. Mutations in SLC4A11, a Na⁺ dependent OH⁻ transporter, cause congenital hereditary endothelial dystrophy (CHED) and Fuchs’ endothelial corneal dystrophy (FECD), the two most common forms of endothelial degeneration. Along with genetic factors, oxidative stress plays a role in pathogenesis of several corneal diseases. In this study we looked into the role of SLC4A11 in antioxidant stress response in human corneal endothelial cells (HCEnC). We found increased expression of SLC4A11 in presence of oxidative stress. Depletion of SLC4A11 using targeted siRNA, caused an increase in reactive oxygen species, cytochrome c, lowered mitochondrial membrane potential, and reduced cell viability during oxidative stress. Moreover, SLC4A11 was found to be necessary for NRF2 mediated antioxidant gene expression in HCEnC. On the other hand, over expression of SLC4A11 reduces reactive oxygen species levels and increases cell viability. Lastly, CHED tissue specimens show evidence of oxidative stress and reduced expression of NRF2. In conclusion, our data suggests a possible role of SLC4A11 in regulating oxidative stress, and might be responsible for both the etiology and treatment of corneal endothelial dystrophy.

Mitochondrial dysfunction and oxidative stress in corneal disease

The cornea is the anterior transparent surface and the main refracting structure of the eye. Mitochondrial dysfunction and oxidative stress are implicated in the pathogenesis of inherited (e.g. Kearns Sayre Syndrome) and acquired corneal diseases (e.g. keratoconus and Fuchs endothelial corneal dystrophy). Both antioxidants and reactive oxygen species are found in the healthy cornea. There is increasing evidence of imbalance in the oxidative balance and mitochondrial function in the cornea in disease states. The cornea is vulnerable to mitochondrial dysfunction and oxidative stress due to its highly exposed position to ultraviolet radiation and high oxygen tension. The corneal endothelium is vulnerable to accumulating mitochondrial DNA (mtDNA) damage due to the post- mitotic nature of endothelial cells, yet their mitochondrial genome is continually replicating and mtDNA mutations can develop and accumulate with age. The unique physiology of the cornea predisposes this structure to oxidative damage, and there is interplay between inherited and acquired mitochondrial dysfunction, oxidative damage and a number of corneal diseases. By targeting mitochondrial dysfunction in corneal disease, emerging treatments may prevent or reduce visual loss.

MicroRNA-29b Overexpression Decreases Extracellular Matrix mRNA and Protein Production in Human Corneal Endothelial Cells

PURPOSE

MicroRNAs are small noncoding RNAs that regulate gene expression at the posttranscriptional level. We reported that levels of microRNA (miR)-29 family are decreased in corneas of patients with Fuchs endothelial corneal dystrophy (FECD). The miR-29 family regulates the production of extracellular matrix (ECM) proteins. Accumulation of ECM proteins in Descemet membrane is an important pathologic change in FECD. In this study, we transfected miR-29b into human corneal endothelial cells and tissues and evaluated ECM protein expression levels.

METHODS

An immortalized Fuchs human corneal endothelial cell line (iFECD) was established by infection of corneal endothelial cells from patients with FECD with hTERT lentivirus. MiR-29b was transfected into iFECD, and the expression levels of ECMs collagen type 1 alpha 1 (COL1A1), collagen type 4 alpha 1 (COL4A1), and laminin gamma 1 (LAMC1) were evaluated with quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) and Western blot. Expression level of LAMC1 protein in miR-29b-transfected donor corneal endothelium was also evaluated by Western blot.

RESULTS

Compared with control, miR-29b expression level after transfection of iFECD was increased to 335.6% (±91.0%), and ECM expression levels were significantly decreased. Compared with control, qRT-PCR demonstrated reduction of ECM to the following levels: COL1A1: 1.9% (±0.4%); COL4A1: 7.1% (±1.7%); and LAMC1: 21.5% (±2.7%). Western blot showed reduced protein expression: COL1A1: 4.8% (±3.2%); COL4A1: 42.5% (±25.0%); and LAMC1: 44.8% (±3.1%). In miR-29b-transfected corneal tissue, LAMC1 protein expression level was decreased to 14.4% (±20.5%).

CONCLUSIONS

Overexpression of miR-29b decreased ECM protein production in human corneal endothelial cells. Thus, miR-29 replacement therapy might be a new treatment strategy for FECD aimed at reducing pathologic production of ECM proteins in Descemet membrane.

Evaluation of Serum Superoxide Dismutase Activity, Malondialdehyde, and Zinc and Copper Levels in Patients With Keratoconus

PURPOSE

The aim of this study was to evaluate the relationship between antioxidant superoxide dismutase (SOD) enzyme activity, malondialdehyde (MDA) as a lipid peroxidation marker, and some trace elements such as zinc (Zn) and copper (Cu) levels in patients with keratoconus.

METHODS

A total of 58 patients with keratoconus and 53 control subjects with similar age and sex were evaluated in this study. The modified Krumeich keratoconus classification was used to divide the patients into 4 stages. Serum SOD activity, MDA, and zinc and copper levels were compared between the patient and control groups.

RESULTS

The median serum SOD activity, MDA, and Zn and Cu levels were 27.2 (42.4-13.7) U/mL, 10.2 (11.9-8.5) nmol/mL, 87.9 (104.6-76.5) μmol/L, and 103.2 (117.9-90.3) μmol/L in the keratoconus group and 26.2 (32.5-14.4) U/mL, 8.8 (11.4-7.1) nmol/mL, 100.5 (121.1-81.8) μmol/L, and 98.4 (120.3-83.4) μmol/L in the control group, respectively. There was a statistically significant difference between the MDA and Zn levels of the keratoconus group and control subjects but not between the respective SOD activities or Cu levels (P = 0.016, P = 0.031, P = 0.440, and P = 0.376, respectively). We found no significant difference between the keratoconus group stages for serum SOD activity, serum MDA, and Zn and Cu levels (P > 0.05), and there was also no significant correlation between the keratoconus group stages and serum SOD activity, serum MDA, and Zn and Cu levels (P > 0.05).

CONCLUSIONS

There is imbalance in the systemic oxidant/antioxidant status where Zn deficiency also plays a role in patients with keratoconus.

Molecular Hydrogen Effectively Heals Alkali-Injured Cornea via Suppression of Oxidative Stress

The aim of this study was to examine the effect of molecular hydrogen (H₂) on the healing of alkali-injured cornea. The effects of the solution of H₂ in phosphate buffered saline (PBS) or PBS alone topically applied on the alkali-injured rabbit cornea with 0.25 M NaOH were investigated using immunohistochemical and biochemical methods. Central corneal thickness taken as an index of corneal hydration was measured with an ultrasonic pachymeter. Results show that irrigation of the damaged eyes with H₂ solution immediately after the injury and then within next five days renewed corneal transparency lost after the injury and reduced corneal hydration increased after the injury to physiological levels within ten days after the injury. In contrast, in injured corneas treated with PBS, the transparency of damaged corneas remained lost and corneal hydration elevated. Later results-on day 20 after the injury-showed that in alkali-injured corneas treated with H₂ solution the expression of proinflammatory cytokines, peroxynitrite, detected by nitrotyrosine residues (NT), and malondialdehyde (MDA) expressions were very low or absent compared to PBS treated injured corneas, where NT and MDA expressions were present. In conclusion, H₂ solution favorably influenced corneal healing after alkali injury via suppression of oxidative stress.

An immunohistochemical study of nitrotyrosine expression in pancreatic islets of cases with increasing duration of type 1 diabetes and without diabetes

Peroxynitrite-induced nitration of cellular proteins has been shown to associate with various human pathologies. The expression of pancreatic nitrotyrosine and its cellular source relative to insulitis were analysed in cases with increasing duration of type 1 diabetes and compared with non-diabetic autoantibody-negative and -positive cases. Pancreatic tail sections from non-diabetic autoantibody-negative cases (Group 1; n = 7), non-diabetic autoantibody-positive cases (Group 2; n = 6), recently diagnosed cases (Group 3; n = 6), 0.25-5 years of diabetes (Group 4; n = 8) and 7-12 years of diabetes (Group 5; n = 6) were immunostained sequentially for nitrotyrosine, insulin and leucocytes. Nitrotyrosine expression was observed in selective beta cells only. In group 1, the percentage of insulin-positive islets with nitrotyrosine ranged from 7.6 to 58.8%. In group 2, it was minimally expressed in 2 cases and was present in 4.7-19.3% of insulin-positive islets in 3 cases and in all islets in 1 case. In group 3, it was absent in 1 case and in the remaining 5 cases, the values were 17.4-85.7%. In group 4, nitrotyrosine was absent in 6 cases and positive in 1.8 and 22.2% of insulin-positive islets in 2 cases. In group 5, the values were 60% (1 case) and 100% (2 cases), being absent in 3 cases, consistent with insulin-negativity. This case analysis shows that nitrotyrosine immunostaining is independent of the presence and severity of insulitis. Variable nitrotyrosine expression is present in some non-diabetic cases. Its increased expression in beta cells of recent-onset and long-standing disease requires further studies to determine whether beta cell nitration plays a pathogenic role during T1D.

Molecular and Histopathological Changes Associated with Keratoconus

Keratoconus (KC) is a corneal thinning disorder that leads to loss of visual acuity through ectasia, opacity, and irregular astigmatism. It is one of the leading indicators for corneal transplantation in the Western countries. KC usually starts at puberty and progresses until the third or fourth decade; however its progression differs among patients. In the keratoconic cornea, all layers except the endothelium have been shown to have histopathological structural changes. Despite numerous studies in the last several decades, the mechanisms of KC development and progression remain unclear. Both genetic and environmental factors may contribute to the pathogenesis of KC. Many previous articles have reviewed the genetic aspects of KC, but in this review we summarize the histopathological features of different layers of cornea and discuss the differentially expressed proteins in the KC-affected cornea. This summary will help emphasize the major molecular defects in KC and identify additional research areas related to KC, potentially opening up possibilities for novel methods of KC prevention and therapeutic intervention.

Genomic strategies to understand causes of keratoconus

Keratoconus (KTCN) is a degenerative disorder of the eye characterized by the conical shape and thinning of the cornea. The abnormal structure of KTCN-affected cornea results in loss of visual acuity. While many studies examine how environmental factors influence disease development, finding the genetic triggers has been a major emphasis of KTCN research. This paper focuses on genomic strategies that were implemented for finding candidate genes, including linkage and association studies, and presents different approaches of mutation screening. The advantages and limitations of particular tools are discussed based on literature and personal experience. Since etiology underlying KTCN is complex, numerous findings indicating heterogeneity of genetic factors involved KTCN etiology are presented.

Dr John Nottingham's 1854 Landmark Treatise on Conical Cornea Considered in the Context of the Current Knowledge of Keratoconus

John Nottingham has been widely credited with the first accurate description of keratoconus in his treatise on conical cornea, published in 1854. Contained within the 270-page treatise are accounts and theories of keratoconus postulated by authors such as Scarpa, von Carion, von Ammon, and Mackenzie, synthesized by Nottingham in a treatise containing his own original observations. Nottingham's work delves deeply into keratoconus, with coverage reminiscent of a modern review, albeit in a far less succinct manner. He extensively describes the epidemiology, clinical presentation, underlying cause, and treatment of keratoconus. However, the concepts put forth are limited largely by the contemporary lack of understanding of the underlying anatomy and physiology of the eye, and the observations, by technological limitations. He postulates a similar treatment algorithm to that used today; optical devices being the management option of choice in the mild stages with surgery being a last resort. None of the surgical methods discussed are used in the modern era, but he does make reference to the possible efficacy of corneal transplantation. Nottingham's treatise was published over 160 years ago, yet his ideas and observations are surprisingly accurate. It is very possible that he was the first person to publish an accurate, comprehensive description of keratoconus.

Corneal Protein Nitration in Experimental Uveitis

Increased expression of inducible nitric oxide synthase (NOS-2) in inflammatory diseases like uveitis suggests that it contributes to the observed pathological state. The aim of this study was to evaluate corneal expression of NOS-2 and corneal protein nitration in a rat model of uveitis. A single injection of intravitreal lipopolysaccharide was used to induce uveitis. Corneal proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and visualized by Coomassie blue staining. Expression of NOS-2 and nitrotyrosine (NO2Tyr) formation were determined via immunohistochemistry and Western blot analysis. Total nitrate/nitrite levels in the vitreous were measured by spectral analysis via the Griess reagent. Immunohistochemical analysis revealed increased corneal NOS-2 and NO2Tyr immunoreactivity in rats with uveitis compared with controls. NOS-2 and NO2Tyr immunoreactivity was observed in and around basal cells in the corneal epithelium. Western blot analysis of corneal lysates showed multiple nitrated protein bands in uveitic rats. Spectrophotometric measurement of total nitrate/nitrite levels in the vitreous affirmed significantly increased levels of nitric oxide generation in uveitis (126 ±2.63 μ M/mg protein) compared with controls (65 ±6.57 μ M/mg protein). The presented data suggests that extensive formation of protein nitration and reactive nitrogen species in the cornea contributes to tissue destruction in uveitis. Hence, selective inhibition of NOS-2 may prevent long-term complications and lead to an improvement in the management of uveitis.

UV-A Irradiation Activates Nrf2-Regulated Antioxidant Defense and Induces p53/Caspase3-Dependent Apoptosis in Corneal Endothelial Cells

Purpose

To examine whether Nrf2-regulated antioxidant defense and p53 are activated in human corneal endothelial cells (CEnCs) by environmental levels of ultraviolet A (UV-A), a known stimulator of oxidative stress.

Methods

Immortalized human CEnCs (HCEnCi) were exposed to UV-A fluences of 2.5, 5, 10, or 25 J/cm², then allowed to recover for 3 to 24 hours. Control HCEnCi did not receive UV-A. Reactive oxygen species (ROS) were measured using H₂DCFDA. Cell cytotoxicity was evaluated by lactate dehydrogenase (LDH) release. Levels of Nrf2, HO-1, NQO-1, p53, and caspase3 were detected by immunnoblotting or real-time PCR. Activated caspase3 was measured by immunoblotting and a fluorescence assay.

Results

Exposure of HCEnCi to 5, 10, and 25 J/cm² UV-A increased ROS levels compared with controls. Nrf2, HO-1, and NQO-1 mRNA increased 1.7- to 3.2-fold at 3 and 6 hours after irradiation with 2.5 and 5 J/cm² UV-A. At 6 hours post irradiation, UV-A (5 J/cm²) enhanced nuclear Nrf2 translocation. At 24 hours post treatment, UV-A (5, 10, and 25 J/cm²) produced a 1.8- to 2.8-fold increase in phospho-p53 and a 2.6- to 6.0-fold increase in activated caspase3 compared with controls, resulting in 20% to 42% cell death.

Conclusions

Lower fluences of UV-A induce Nrf2-regulated antioxidant defense and higher fluences activate p53 and caspase3, indicating that even near-environmental levels of UV-A may affect normal CEnCs. This data suggest that UV-A may especially damage cells deficient in antioxidant defense, and thus may be involved in the etiology of Fuchs' endothelial corneal dystrophy (FECD).

A Case-Control Study on the Oxidative Balance of 50% Autologous Serum Eye Drops

Importance. Autologous serum (AS) eye drops are recommended for severe dry eye in patients with ocular surface disease. No description of the antioxidant balance of AS eye drops has been reported in the literature. Objective. This study sought to evaluate the total reactive antioxidant potential (TRAP) and concentration of reactive oxygen species (ROS) in samples of 50% AS eye drops and their correlations with the demographic characteristics and lifestyle habits of patients with ocular surface disease and healthy controls. Design. This was a case-control study with a 3-month follow-up period. Participants. 16 patients with severe dry eye disease of different etiologies and 17 healthy controls matched by age, gender, and race were included. Results. TRAP and ROS were detected at all evaluated times. There were no differences in the mean ROS (p = 0.429) or TRAP (p = 0.475) levels between cases and controls. No statistically significant differences in the concentrations of ROS or TRAPs were found at 0, 15, or 30 days (p for ROS = 0.087 and p for TRAP = 0.93). Neither the demographic characteristics nor the lifestyle habits were correlated with the oxidative balance of the 50% AS eye drops. Conclusions and Relevance. Both fresh and frozen 50% AS eye drops present antioxidant capacities and ROS in an apparently stable balance. Moreover, patients with ocular surface disease and normal controls produce equivalent AS eye drops in terms of oxidative properties.

Influence of Light Emitting Diode-Derived Blue Light Overexposure on Mouse Ocular Surface

Purpose

To investigate the influence of overexposure to light emitting diode (LED)-derived light with various wavelengths on mouse ocular surface.

Methods

LEDs with various wavelengths were used to irradiate C57BL/6 mice at an energy dose of 50 J/cm², twice a day, for 10 consecutive days. The red, green, and blue groups represented wavelengths of 630 nm, 525 nm, and 410 nm, respectively. The untouched group (UT) was not exposed to LED light and served as the untreated control. Tear volume, tear film break-up time (TBUT), and corneal fluorescein staining scores were measured on days 1, 3, 5, 7, and 10. Levels of interferon (IFN)-γ, interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α were measured in the cornea and conjunctiva using a multiplex immunobead assay at day 10. Levels of malondialdehyde (MDA) were measured with an enzyme-linked immunosorbent assay. Flow cytometry, 2’7’-dichlorofluorescein diacetate (DCF-DA) assay, histologic analysis, immunohistochemistry with 4-hydroxynonenal, and terminal deoxynucleotidyl transferase-mediated dUTP-nick end labeling (TUNEL) staining were also performed.

Results

TBUT of the blue group showed significant decreases at days 7 and 10, compared with the UT and red groups. Corneal fluorescein staining scores significantly increased in the blue group when compared with UT, red, and green groups at days 5, 7, and 10. A significant increase in the corneal levels of IL-1β and IL-6 was observed in the blue group, compared with the other groups. The blue group showed significantly increased reactive oxygen species production in the DCF-DA assay and increased inflammatory T cells in the flow cytometry. A significantly increased TUNEL positive cells was identified in the blue group.

Conclusions

Overexposure to blue light with short wavelengths can induce oxidative damage and apoptosis to the cornea, which may manifest as increased ocular surface inflammation and resultant dry eye.

Ultraviolet Radiation-Induced Corneal Degeneration in 129 Mice

Ultraviolet radiation (UVR) is a risk factor for the development of ocular disease in humans, including acute photokeratitis, chronic corneal spheroidal degeneration, and cataract formation. This report describes the ocular lesions seen in 21 mice chronically exposed to UVR as part of a skin carcinogenicity study. All globes were affected to varying degrees. The primary lesion, not previously reported in UVR-exposed mice, was marked loss of keratocytes relative to age-matched controls. Secondary lesions included corneal stromal thinning, keratoconus, corneal vascularization and fibrosis, keratitis, globe rupture, and phthisis bulbi. In addition, more than 90% of UVR-exposed and unexposed lenses had evidence of cataract formation; this is the first report of the occurrence of spontaneous cataracts in 129 mice. In a subsequent study, apoptotic cells were identified histologically and by cleaved caspase 3 immunoreactivity in the corneal epithelium and, less commonly, in the corneal stroma after acute UVR exposure. Based on this finding, we propose that the loss of keratocytes observed in the chronic study was due to UVR-induced apoptosis.

Protective Role of Hinokitiol Against H2O2-Induced Injury in Human Corneal Epithelium

PURPOSE

We recently found that hinokitiol has anti-inflammatory activity in human corneal epithelial (HCE) cells. Herein, we investigated the protective role of hinokitiol against H₂O₂-induced injury in HCE cells and the mechanisms that underlie its action.

METHODS

HCE cells were incubated with different concentrations of hinokitiol or dimethylsulfoxide (DMSO), which served as a vehicle control, before H₂O₂ stimulus. The cell viability was evaluated using a cell counting kit-8 (CCK-8) assay. TUNEL, phosphorylated histone γH2A.X, cleaved caspase-3 expression analyses, and location of cytochrome c were conducted to detect cell injury and apoptosis. Reactive oxygen species (ROS), catalase (CAT), superoxide dismutase (SOD), methane dicarboxylic aldehyde (MDA), and total antioxidative capacity (T-AOC) were used to determine oxidative stress. Bcl-2 and Bax protein expressions were measured by western blotting.

RESULTS

Hinokitiol significantly improved the cell viability, decreased the apoptosis rate, inhibited DNA damage, and reduced cleaved caspase-3 expression and the leakage of cytochrome c from mimitochondrion to cytoplasm of HCE cells against the oxidative stress induced by H₂O₂. Generation of ROS and MDA and decreased activity of CAT, SOD, and T-AOC were also ameliorated by hinokitiol administration. Moreover, Bcl-2 expression was down-regulated while Bax was up-regulated by H₂O₂ stimulus, which were reversed by hinokitiol application.

CONCLUSION

Hinokitiol protects HCE cells against H₂O₂-induced injury likely by its antioxidant activity and modulating the Bcl-2 signaling pathway.

Polymorphism of the APEX nuclease 1 gene in keratoconus and Fuchs endothelial corneal dystrophy

Human APEX nuclease 1 (APEX1) plays an important role in the repair of oxidative DNA lesions through base excision repair. It may influence the development of oxidative stress-related diseases. The aim of this study was to determine the relationship between the genotypes of the c.444 T>G (rs1130409) and c.-468 T>G (rs1760944) polymorphisms in the APEX1 gene and the occurrence of two oxidative stress-related eye diseases: keratoconus (KC) and Fuchs endothelial corneal dystrophy (FECD). The study involved 250 patients with KC, 209 patients with FECD, and 350 control subjects. All of the patients and control subjects underwent a detailed ophthalmic examination. The polymorphisms were genotyped by mismatch polymerase chain reaction restriction fragment length polymorphism (mismatch PCR-RFLP). We observed that the G/T and T/T genotypes of the c.-468 T>G polymorphism were respectively associated with a decreased occurrence of KC (OR 0.54, 95% CI 0.37-0.95; p = 0.030) and an increased occurrence of KC (OR 1.87, 95% CI 1.06-3.32; p = 0.032). None of these polymorphisms showed any association with FECD. Furthermore, no other association was observed, including haplotypes of the two polymorphisms. Our findings suggest that the c.-468 T>G polymorphism of the APEX1 gene may play a role in the pathogenesis of KC.

Endocrine and Metabolic Pathways Linked to Keratoconus: Implications for the Role of Hormones in the Stromal Microenvironment

Hormones play a critical role in regulating tissue function by promoting cell survival, proliferation, and differentiation. Our study explores the influence of endocrine function in regulating metabolism and inflammatory pathways in Keratoconus (KC), which is a corneal thinning disease associated with reduced stromal deposition. KC is known to be a multifactorial disease with an elusive pathogenesis. We utilized a cross-sectional study analyzing clinical features and saliva samples from sixty-four KC patients and fourteen healthy controls. In order to determine if endocrine function varied between healthy controls and KC, we measured hormone levels in saliva and found significantly increased dehydroepiandrosterone sulfate (DHEA-S) and reduced estrone levels in KC patients compared to healthy controls. We measured significant variations in metabolites associated with pro-inflammatory processes, including myoinositol and 1-methyl-histidine, by targeted mass spectrometry. We also measured significantly increased IL-16 and stem cell factor in KC saliva samples compared to healthy controls, with higher expression of these pro-inflammatory proteins correlating with increased KC clinical grade, corneal curvature, and stromal thinning. Our results identify a novel mechanism linking KC and pro-inflammatory markers and suggest that altered hormone levels modulate metabolism, cytokine, and growth factor expression leading to increased severity of the KC condition.

Tear Film and Serum Prolidase Activity and Oxidative Stress in Patients With Keratoconus

PURPOSE

To determine and compare the serum and tear film prolidase activity (PA) between patients with keratoconus and healthy subjects. Also, we aimed to evaluate the serum oxidative stress level and the correlation with serum PA in patients with keratoconus.

METHODS

This prospective, comparative clinical study included 31 patients with keratoconus and 33 age-matched and sex-matched control subjects. All participants underwent a detailed ophthalmologic examination. Serum and tear samples were obtained from all participants. Tears and serum PA and serum oxidative stress markers were measured.

RESULTS

No significant differences in demographic characteristics were detected between groups (P > 0.05). The serum PA was significantly lower in the keratoconus group than in the control group (895.6 ± 198.7 vs. 1145.9 ± 285.4 U/L, P < 0.001). A tear film comparison showed that PA was lower in the keratoconus group than in the control group; however, this difference was not significant (3075.4 ± 672.2 vs. 3225.8 ± 903.2 U·L⁻¹·g⁻¹ protein, P = 0.45). Oxidative stress markers, such as total oxidant status and oxidative stress index, were found to be significantly higher in the keratoconus group (P < 0.001).

CONCLUSIONS

The serum PA was found to be lower in patients with keratoconus than in the controls. Additionally, serum oxidative stress markers were found to be higher than those of the controls. Thus, prolidase and systemic oxidative stress may have a role in the pathogenesis of keratoconus.

Fuchs endothelial corneal dystrophy: current perspectives

Fuchs endothelial corneal dystrophy (FECD) is the most common corneal dystrophy and frequently results in vision loss. Hallmarks of the disease include loss of corneal endothelial cells and formation of excrescences of Descemet's membrane. Later stages involve all layers of the cornea. Impairment of endothelial barrier and pump function and cell death from oxidative and unfolded protein stress contribute to disease progression. The genetic basis of FECD includes numerous genes and chromosomal loci, although alterations in the transcription factor 4 gene are associated with the majority of cases. Definitive treatment of FECD is corneal transplantation. In this paper, we highlight advances that have been made in understanding FECD's clinical features, pathophysiology, and genetics. We also discuss recent advances in endothelial keratoplasty and potential future treatments.

Keratin 12 missense mutation induces the unfolded protein response and apoptosis in Meesmann epithelial corneal dystrophy

Meesmann epithelial corneal dystrophy (MECD) is a rare autosomal dominant disorder caused by dominant-negative mutations within the KRT3 or KRT12 genes, which encode the cytoskeletal protein keratins K3 and K12, respectively. To investigate the pathomechanism of this disease, we generated and phenotypically characterized a novel knock-in humanized mouse model carrying the severe, MECD-associated, K12-Leu132Pro mutation. Although no overt changes in corneal opacity were detected by slit-lamp examination, the corneas of homozygous mutant mice exhibited histological and ultrastructural epithelial cell fragility phenotypes. An altered keratin expression profile was observed in the cornea of mutant mice, confirmed by western blot, RNA-seq and quantitative real-time polymerase chain reaction. Mass spectrometry (MS) and immunohistochemistry demonstrated a similarly altered keratin profile in corneal tissue from a K12-Leu132Pro MECD patient. The K12-Leu132Pro mutation results in cytoplasmic keratin aggregates. RNA-seq analysis revealed increased chaperone gene expression, and apoptotic unfolded protein response (UPR) markers, CHOP and Caspase 12, were also increased in the MECD mice. Corneal epithelial cell apoptosis was increased 17-fold in the mutant cornea, compared with the wild-type (P < 0.001). This elevation of UPR marker expression was also observed in the human MECD cornea. This is the first reporting of a mouse model for MECD that recapitulates the human disease and is a valuable resource in understanding the pathomechanism of the disease. Although the most severe phenotype is observed in the homozygous mice, this model will still provide a test-bed for therapies not only for corneal dystrophies but also for other keratinopathies caused by similar mutations.

The Role of the Reactive Oxygen Species and Oxidative Stress in the Pathomechanism of the Age-Related Ocular Diseases and Other Pathologies of the Anterior and Posterior Eye Segments in Adults

The reactive oxygen species (ROS) form under normal physiological conditions and may have both beneficial and harmful role. We search the literature and current knowledge in the aspect of ROS participation in the pathogenesis of anterior and posterior eye segment diseases in adults. ROS take part in the pathogenesis of keratoconus, Fuchs endothelial corneal dystrophy, and granular corneal dystrophy type 2, stimulating apoptosis of corneal cells. ROS play a role in the pathogenesis of glaucoma stimulating apoptotic and inflammatory pathways on the level of the trabecular meshwork and promoting retinal ganglion cells apoptosis and glial dysfunction in the posterior eye segment. ROS play a role in the pathogenesis of Leber's hereditary optic neuropathy and traumatic optic neuropathy. ROS induce apoptosis of human lens epithelial cells. ROS promote apoptosis of vascular and neuronal cells and stimulate inflammation and pathological angiogenesis in the course of diabetic retinopathy. ROS are associated with the pathophysiological parainflammation and autophagy process in the course of the age-related macular degeneration.

The Favorable Effect of Mesenchymal Stem Cell Treatment on the Antioxidant Protective Mechanism in the Corneal Epithelium and Renewal of Corneal Optical Properties Changed after Alkali Burns

The aim of this study was to examine whether mesenchymal stem cells (MSCs) and/or corneal limbal epithelial stem cells (LSCs) influence restoration of an antioxidant protective mechanism in the corneal epithelium and renewal of corneal optical properties changed after alkali burns. The injured rabbit corneas (with 0.25 N NaOH) were untreated or treated with nanofiber scaffolds free of stem cells, with nanofiber scaffolds seeded with bone marrow MSCs (BM-MSCs), with adipose tissue MSCs (Ad-MSCs), or with LSCs. On day 15 following the injury, after BM-MSCs or LSCs nanofiber treatment (less after Ad-MSCs treatment) the expression of antioxidant enzymes was restored in the regenerated corneal epithelium and the expressions of matrix metalloproteinase 9 (MMP9), inducible nitric oxide synthase (iNOS), α-smooth muscle actin (α-SMA), transforming growth factor-β1 (TGF-β1), and vascular endothelial factor (VEGF) were low. The central corneal thickness (taken as an index of corneal hydration) increased after the injury and returned to levels before the injury. In injured untreated corneas the epithelium was absent and numerous cells revealed the expressions of iNOS, MMP9, α-SMA, TGF-β1, and VEGF. In conclusion, stem cell treatment accelerated regeneration of the corneal epithelium, restored the antioxidant protective mechanism, and renewed corneal optical properties.

Impact of Corneal Endothelial Dysfunctions on Intraocular Oxygen Levels in Human Eyes

PURPOSE

We studied the implications of corneal endothelial dysfunctions on oxidative stress in the anterior segment via in vivo measurements of oxygen partial pressure (pO2) in the anterior chamber (AC) of human eyes.

METHODS

We recruited 51 patients undergoing cataract surgery and/or endothelial keratoplasty (EK). Endothelial cell density (ECD; n = 33) and central corneal thickness (CCT; n = 41) were measured on patients with relatively clear corneas. Before surgery, an oxygen sensor was introduced into the AC via a peripheral corneal paracentesis. In all patients, seven measurements of pO2 were obtained by positioning the flexible tip near the endothelium at the central cornea, at four cardinal subendothelial locations near the midperipheral cornea, and in the mid-AC and AC angle. In patients with pseudophakia or eyes undergoing cataract surgery, pO2 also was measured near the lens surface and in the posterior chamber.

RESULTS

Consistent with our previous reports, a steep oxygen gradient was noted in the anterior segment of normal controls (n = 24). In patients with endothelial dysfunctions (n = 27), there was a significant increase of pO2 at all five subendothelial locations without a significant increase of pO2 in the AC angle. By regression analyses, subendothelial pO2 correlated inversely with ECD and positively with CCT in patients with endothelial dysfunctions.

CONCLUSIONS

This study demonstrates an even steeper intraocular oxygen gradient in eyes with corneal endothelial dysfunctions. It suggests that the reduced oxygen consumption in corneal endothelial cells may increase oxidative stress in the AC and the existence of an alternative aqueous inflow pathway that maintains a relatively low and constant pO2 at the AC angle.

Lack of association between polymorphisms of the DNA base excision repair genes MUTYH and hOGG1 and keratoconus in a Polish subpopulation

INTRODUCTION

Keratoconus (KC) is a non-inflammatory thinning of the cornea and a leading indication for corneal transplantation. Oxidative stress plays a role in the pathogenesis of this disease. The products of the hOGG1 and MUTYH genes play an important role in the repair of oxidatively modified DNA in the base excision repair pathway. We hypothesized that variability in these genes may change susceptibility to oxidative stress and predispose individuals to the development of KC. We investigated the possible association between the c.977C>G polymorphism of the hOGG1 gene (rs1052133) and the c.972G>C polymorphism of the MUTYH gene (rs3219489) and KC occurrence as well as the modulation of this association by some KC risk factors.

MATERIAL AND METHODS

A total of 205 patients with KC and 220 controls were included in this study. The polymorphisms were genotyped with polymerase chain reaction (PCR) restriction fragment length polymorphism and PCR-confronting two-pair primer techniques. Differences in genotype and allele frequency distributions were evaluated using the χ(2) test, and KC risk was estimated with an unconditional multiple logistic regression with and without adjustment for co-occurrence of visual impairment, allergies, sex and family history for KC.

RESULTS

We did not find any association between the genotypes and combined genotypes of the c.977C>G polymorphism of the hOGG1 gene and the c.972G>C polymorphism of the MUTYH gene and the occurrence of KC.

CONCLUSIONS

Our findings suggest that the c.977C>G-hOGG1 polymorphism and the c.972G>C-MUTYH polymorphism may not be linked with KC occurrence in this Polish subpopulation.

Variation in DNA Base Excision Repair Genes in Fuchs Endothelial Corneal Dystrophy

BACKGROUND Fuchs endothelial corneal dystrophy (FECD) is a corneal disease characterized by abnormalities in the Descemet membrane and the corneal endothelium. The etiology of this disease is poorly understood. An increased level of oxidative DNA damage reported in FECD corneas suggests a role of DNA base excision repair (BER) genes in its pathogenesis. In this work, we searched for the association between variation of the PARP-1, NEIL1, POLG, and XRCC1 genes and FECD occurrence. MATERIAL AND METHODS This study was conducted on 250 FECD patients and 353 controls using polymerase chain reaction-restriction fragment length polymorphism, high-resolution melting analysis, and the TaqMan® SNP Genotyping Assay. RESULTS We observed that the A/A genotype and the A allele of the c.1196A>G polymorphism of the XRCC1 gene were positively correlated with an increased FECD occurrence, whereas the G allele had the opposite effect. A weak association between the C/G genotype of the g.46438521G>C polymorphism of the NEIL1 gene and an increased incidence of FECD was also detected. Haplotypes of both polymorphisms of the XRCC1 were associated with FECD occurrence. No association of the c.2285T>C, c.-1370T>A and c.580C>T polymorphisms of the PARP-1, POLG and XRCC1 genes, respectively, with FECD occurrence was observed. CONCLUSIONS Our results suggest that the c.1196A>G polymorphism in the XRCC1 gene may be an independent genetic risk factor for FECD.

Nonrandom Distribution of miRNAs Genes and Single Nucleotide Variants in Keratoconus Loci

Despite numerous studies, the causes of both development and progression of keratoconus remain elusive. Previous studies of this disorder focused mainly on one or two genetic factors only. However, in the analysis of such complex diseases all potential factors should be taken into consideration. The purpose of this study was a comprehensive analysis of known keratoconus loci to uncover genetic factors involved in this disease causation in the general population, which could be omitted in the original studies. In this investigation genomic data available in various databases and experimental own data were assessed. The lists of single nucleotide variants and miRNA genes localized in reported keratoconus loci were obtained from Ensembl and miRBase, respectively. The potential impact of nonsynonymous amino acid substitutions on protein structure and function was assessed with PolyPhen-2 and SIFT. For selected protein genes the ranking was made to choose those most promising for keratoconus development. Ranking results were based on topological features in the protein-protein interaction network. High specificity for the populations in which the causative sequence variants have been identified was found. In addition, the possibility of links between previously analyzed keratoconus loci was confirmed including miRNA-gene interactions. Identified number of genes associated with oxidative stress and inflammatory agents corroborated the hypothesis of their effect on the disease etiology. Distribution of the numerous sequences variants within both exons and mature miRNA which forces you to search for a broader look at the determinants of keratoconus. Our findings highlight the complexity of the keratoconus genetics.

Keratoconus: an inflammatory disorder?

Keratoconus has been classically defined as a progressive, non-inflammatory condition, which produces a thinning and steepening of the cornea. Its pathophysiological mechanisms have been investigated for a long time. Both genetic and environmental factors have been associated with the disease. Recent studies have shown a significant role of proteolytic enzymes, cytokines, and free radicals; therefore, although keratoconus does not meet all the classic criteria for an inflammatory disease, the lack of inflammation has been questioned. The majority of studies in the tears of patients with keratoconus have found increased levels of interleukin-6 (IL-6), tumor necrosis factor-α(TNF-α), and matrix metalloproteinase (MMP)-9. Eye rubbing, a proven risk factor for keratoconus, has been also shown recently to increase the tear levels of MMP-13, IL-6, and TNF-α. In the tear fluid of patients with ocular rosacea, IL-1α and MMP-9 have been reported to be significantly elevated, and cases of inferior corneal thinning, resembling keratoconus, have been reported. We performed a literature review of published biochemical changes in keratoconus that would support that this could be, at least in part, an inflammatory condition.

The oxidant role of 4-hydroxynonenal in corneal epithelium

4-Hydroxynonenal (4-HNE or HNE) is a main endogenous product of cellular lipid peroxidation in tissues and is reported to play pathogenic roles in eye diseases. Here we investigated the association between 4-HNE and oxidative stress in the corneal epithelium. 4-HNE suppressed the cell viability of human corneal epithelial cells (HCE) in a concentration dependent manner. 4-HNE significantly increased the level of 3-Nitrotyrosine (3-NT), a marker of oxidative stress, in HCE cells and corneal epithelium of rats by immunofluorescent staining and Western blot analysis. To its underlying mechanistic on ROS system, 4-HNE elevated the ROS generation enzyme NADPH oxidase 4 (NOX4) and induced the activation of NF-E2–related factor-2 (NRF2) and its downstream effectors: NAD(P)H dehydrogenase (quinone 1) (NQO1) and glutathione S-transferase P (GSTP). Furthermore, N-acetylcysteine (NAC), an antioxidant and ROS scavenger, antagonized the inhibitory and oxidant effects of 4-HNE on the corneal epithelial cells. In conclusion, 4-HNE plays an oxidant role in the corneal epithelium and this work provides a new strategy for the pathogenesis and treatment of corneal diseases.

Role of tyrosine-sulfated proteins in retinal structure and function

The extracellular matrix (ECM) plays a significant role in cellular and retinal health. The study of retinal tyrosine-sulfated proteins is an important first step toward understanding the role of ECM in retinal health and diseases. These secreted proteins are members of the retinal ECM. Tyrosine sulfation was shown to be necessary for the development of proper retinal structure and function. The importance of tyrosine sulfation is further demonstrated by the evolutionary presence of tyrosylprotein sulfotransferases, enzymes that catalyze proteins' tyrosine sulfation, and the compensatory abilities of these enzymes. Research has identified four tyrosine-sulfated retinal proteins: fibulin 2, vitronectin, complement factor H (CFH), and opticin. Vitronectin and CFH regulate the activation of the complement system and are involved in the etiology of some cases of age-related macular degeneration. Analysis of the role of tyrosine sulfation in fibulin function showed that sulfation influences the protein's ability to regulate growth and migration. Although opticin was recently shown to exhibit anti-angiogenic properties, it is not yet determined what role sulfation plays in that function. Future studies focusing on identifying all of the tyrosine-sulfated retinal proteins would be instrumental in determining the impact of sulfation on retinal protein function in retinal homeostasis and diseases.

The genetic and environmental factors for keratoconus

Keratoconus (KC) is the most common cornea ectatic disorder. It is characterized by a cone-shaped thin cornea leading to myopia, irregular astigmatism, and vision impairment. It affects all ethnic groups and both genders. Both environmental and genetic factors may contribute to its pathogenesis. This review is to summarize the current research development in KC epidemiology and genetic etiology. Environmental factors include but are not limited to eye rubbing, atopy, sun exposure, and geography. Genetic discoveries have been reviewed with evidence from family-based linkage analysis and fine mapping in linkage region, genome-wide association studies, and candidate genes analyses. A number of genes have been discovered at a relatively rapid pace. The detailed molecular mechanism underlying KC pathogenesis will significantly advance our understanding of KC and promote the development of potential therapies.

Study on the Protective Effect of a New Manganese Superoxide Dismutase on the Microvilli of Rabbit Eyes Exposed to UV Radiation

We present a study on the protective effects against UV radiation of a gel formulation containing a new recombinant form of manganese superoxide dismutase on the conjunctiva and corneal epithelia of rabbit eyes. The integrity of the microvilli of both ocular tissues has been considered as an indicator of the health of the tissues. Samples, collected by impression cytology technique, were added of 80 µL of a gel formulation containing superoxide dismutase (2.0 µg/mL) and irradiated with UV rays for 30 minutes and were evaluated with scanning electron microscopy. Wilcoxon test was used to verify the possible occurrence of statistically significant differences between damage for treated and nontreated tissues. Application of gel produces a significant reduction of damage by UV irradiation of ocular epithelia; both epithelia present a significant reduction of damaged microvilli number if treated with the superoxide dismutase gel formulation: the p values (differences between damage found for treated and nontreated both ocular tissues) for conjunctiva and cornea samples were p ≪ 0.01 and p ≪ 0.0001, respectively, at confidence level of 95%. The administration of this gel formulation before UV exposure plays a considerable protective role in ocular tissues of rabbit eye with a significant reduction of the damage.

Antioxidant properties of amniotic membrane: novel observations from a pilot study

OBJECTIVE

Amniotic membrane (AM) is used to manage various debilitated ocular surface conditions. The impact of oxidative stress and free radicals on the ocular surface is increasingly being recognized. Hyaluronic acid (HA) has anti-inflammatory properties and is abundantly present in AM. In this in vitro pilot study, we investigated the potential of AM for intrinsic free radical scavenging properties.

METHODS

Strips of AM were incubated in sealed tubes with hydrogen peroxide (H2O2). After being sonicated, uptake of reactive oxygen species (ROS) was measured by the Amplex Red Hydrogen Peroxide/Peroxidase assay. For comparison, 1630 kDA HA was used.

RESULTS

There was uptake of ROS by all AM samples, which decreased with increasing concentrations of H2O2. Mean ROS uptake for 5 different AMs at 1 hour was significantly greater for 50 μM (83%; SD 11.7, SEM 5.23) compared with 100 μM (67%; SD 20.48, SEM 9.16; p = 0.028; 95% CI 2.8-29.2). The HA comparison group showed similar uptake and trend.

CONCLUSION

This pilot study demonstrates that AM is able to remove ROS from its environment. Demonstrating total antioxidant capacity in AM provides evidence for use as a free radical scavenger. The antioxidant properties of AM and the contribution from HA require more research.

Oxidative stress to the cornea, changes in corneal optical properties, and advances in treatment of corneal oxidative injuries

Oxidative stress is involved in many ocular diseases and injuries. The imbalance between oxidants and antioxidants in favour of oxidants (oxidative stress) leads to the damage and may be highly involved in ocular aging processes. The anterior eye segment and mainly the cornea are directly exposed to noxae of external environment, such as air pollution, radiation, cigarette smoke, vapors or gases from household cleaning products, chemical burns from splashes of industrial chemicals, and danger from potential oxidative damage evoked by them. Oxidative stress may initiate or develop ocular injury resulting in decreased visual acuity or even vision loss. The role of oxidative stress in the pathogenesis of ocular diseases with particular attention to oxidative stress in the cornea and changes in corneal optical properties are discussed. Advances in the treatment of corneal oxidative injuries or diseases are shown.

Systemic Prolidase Activity and Oxidative Stress in Keratoconus

AIM

Our aim was to evaluate the serum prolidase activity, total antioxidant capacity (TAC) and total oxidant status (TOS) in patients with keratoconus.

MATERIAL AND METHOD

A total 69 keratoconus patients and 72 control subjects with similar age and gender were evaluated within the scope of this study. The keratoconus group was divided into four stages with the modified Krumeich classification. Serum prolidase activity, TAC and TOS were measured and compared between the patient and control groups.

RESULTS

The median serum prolidase enzyme activity value was 528.3 (684.1-416.7) U/L in the keratoconus group and 606.2 (812.9-482.3) U/L in the control group. The difference between the groups was statistically significant (p = 0.027). The median TAC value was 1.24 (1.37-1.05) mmol/L in the keratoconus group and 1.29 (1.38-1.18) mmol/L in the control group. The median TOS value was 2 (4-1) μmol/L in the keratoconus group and 3 (4-2) μmol/L in the control group. There was no statistically significant difference between the two groups in terms of TAC or TOS (p = 0.113 and p = 0.366, respectively). There was a positive correlation between TAC and TOS in keratoconus group but not in the control group (r = 0.670, p = 0.001 and r = 0.141, p = 0.241, respectively). No significant relationship was seen between the keratoconus group stages and serum prolidase activity, TAS or TOS (p = 0.894, p = 0.155 and p = 0.381, respectively).

CONCLUSION

In conclusion, a significant relationship was found between decreased serum prolidase activity and keratoconus but there was no significant relationship between keratoconus and serum TAC or TOS.

Tear metabolite changes in keratoconus

While efforts have been made over the years, the exact cause of keratoconus (KC) remains unknown. The aim of this study was to identify alterations in endogenous metabolites in the tears of KC patients compared with age-matched healthy subjects. Three groups were tested: 1) Age-matched controls with no eye disease (N = 15), 2) KC - patients wearing Rigid Gas permeable lenses (N = 16), and 3) KC - No Correction (N = 14). All samples were processed for metabolomics analysis using LC-MS/MS. We identified a total of 296 different metabolites of which >40 were significantly regulated between groups. Glycolysis and gluconeogenesis had significant changes, such as 3-phosphoglycerate and 1,3 diphosphateglycerate. As a result the citric acid cycle (TCA) was also affected with notable changes in Isocitrate, aconitate, malate, and acetylphosphate, up regulated in Group 2 and/or 3. Urea cycle was also affected, especially in Group 3 where ornithine and aspartate were up-regulated by at least 3 fold. The oxidation state was also severely affected. Groups 2 and 3 were under severe oxidative stress causing multiple metabolites to be regulated when compared to Group 1. Group 2 and 3, both showed significant down regulation in GSH-to-GSSG ratio when compared to Group 1. Another indicator of oxidative stress, the ratio of lactate - pyruvate was also affected with Groups 2 and 3 showing at least a 2-fold up regulation. Overall, our data indicate that levels of metabolites related to urea cycle, TCA cycle and oxidative stress are highly altered in KC patients.

Human corneal epithelial cell response to substrate stiffness

It has been reported that mechanical stimulus can affect cellular behavior. While induced differentiation in stem cells and proliferation and directional migration in fibroblasts are reported as responses to mechanical stimuli, little is known about the response of cells from the cornea. In the present study, we investigated whether changes in substrate stiffness (measured by elastic modulus) affected the behavior of human corneal epithelial cells (HCECs). Polyacrylamide substrates with different elastic moduli (compliant, medium and stiff) were prepared and HCECs were cultured on them. HCECs responses, including cell viability, apoptosis, intercellular adhesion molecule-1 (ICAM-1) expression, integrin-α3β1 expression and changes in cytoskeleton structure (actin fibers) and migratory behavior, were studied. No statistically significant cell activation, as measured by ICAM-1 expression, was observed. However, on compliant substrates, a higher number of cells were found to be apoptotic and disrupted actin fibers were observed. Furthermore, cells displayed a statistically significant lower migration speed on compliant substrates when compared with the stiffer substrates. Thus, corneal epithelial cells respond to changes in substrate stiffness, which may have implications in the understanding and perhaps treatment of corneal diseases, such as keratoconus.