Studies of lipoxygenases in the epithelium of cultured bovine cornea using an air interface model

Epithelial lipoxygenases of bovine cornea were investigated in organ culture models. Subcellular fractions of the epithelium were incubated with(14)C-labelled arachidonate and the metabolites were analysed. Bovine corneal epithelial cells contain 15-lipoxygenase type 2 and 12-lipoxygenases of the leukocyte and the platelet types. The 15-lipoxygenase activity was prominent in the cytosolic fraction. Twelve- and 15-lipoxygenases occurred in the microsomal fraction, where the 15-lipoxygenase activity appeared to be favoured by low protein levels. The lipoxygenase activities strongly declined within 24 hr when the cornea was covered with cell culture medium, but were maintained with high activity in an air interface organ culture model for at least 72 hr. Cultured corneas were studied in pairs in the air interface model under influence of inflammatory stimuli. The epithelial 15- and 12-lipoxygenase activities were only slightly augmented by treatment with 12-O-tetradecanoyl-phorbol-13-acetate (10 microM, 8-72 hr), and remained unchanged after treatment with lipopolysaccharide (1-100 microgram ml(-1), 8-72 hr) or UV irradiation (301 nm, 0.17 J cm(-2); 8-24 hr). In some experiments, 5-lipoxygenase activity was detectable, as judged from liquid chromatography-mass spectrometry and chiral chromatography. Reverse transcription-polymerase chain reaction and Northern blot analysis were therefore used to identify mRNA of 5-lipoxygenase and related enzymes in bovine epithelium. 5-Lipoxygenase was detected as an amplicon of 695 bp, which had 91% nucleotide sequence identity with human 5-lipoxygenase and by Northern blot as a 3.0 kb mRNA. Leukotriene A(4)hydrolase was detected with the same techniques. The amino acid sequence of a 612 bp fragment was 90% identical with human leukotriene A(4)hydrolase and the size of the mRNA was 2.7 kb. The two enzymes were also detected in human corneal epithelium by reverse transcription-polymerase chain reaction.

Inhibitory effect of PGE2 on EGF-induced MAP kinase activity and rabbit corneal epithelial proliferation

PURPOSE

To determine in rabbit corneal epithelial cells in culture whether epidermal growth factor (EGF)-induced increases in prostaglandin (PG) E2 production inhibit both the extracellular signal-regulated kinase 2 (Erk-2), a mitogen-activated protein kinase (MAPK), cascade activation, and the mitogenic response to this growth factor.

METHODS

Serum starvation for 24 to 36 hours was used to synchronize cultures of SV40-transformed rabbit corneal epithelial (RCE) cells. The effects of exogenous PGE2, inhibition of PGE2 synthesis, and modulation of protein kinase A (PKA) activity on EGF-induced Erk-2 activation were assessed by immunoprecipitation, kinase assays, and Western blot analysis. PGE2 synthesis was measured by using enzyme-linked immunosorbent assay. [3H]-Thymidine incorporation was used to measure RCE cell proliferation rates.

RESULTS

EGF (5 ng/ml) significantly increased PGE2 production in a time-dependent manner up to 94%+/-8% after 3 hours. EGF-induced PGE2 production was suppressed by AACOCF3, a phospholipase A2 (cPLA2) inhibitor. EGF-induced Erk-2 activation reached a maximal level at 15 minutes, followed by a decline toward the control level after 3 hours. In the presence of either PGE2 (50 microg/ml) or 8-CPT-cAMP (100 microM), the EGF-induced Erk-2 activation was lessened. PKA was activated by applications of EGF or PGE2 and suppressed by AACOCF3. On the other hand, either inhibition of PGE2 production with AACOCF3 or H-89, a PKA inhibitor, enhanced EGF-induced Erk-2 activity. Raf-1 activity was stimulated by EGF to maximal activity at 5 minutes and returned toward its control level after 60 minutes. As with the dependence of Erk-2 activity on PKA activity, in the presence of H-89, the EGF-induced Raf-1 activation was significantly enhanced. DNA synthesis was increased 59%+/-5% (n = 4) after EGF stimulation, indicating a mitogenic effect of EGF in RCE cells. Inhibition of cPLA2 activity with AACOCF3 increased DNA synthesis in RCE cells by another 64% relative to the effect of EGF alone. In contrast, with either PGE2 or 8-CPT-cAMP present the mitogenic response to EGF was totally suppressed.

CONCLUSIONS

EGF-induced increases in PGE2 production dampened the mitogenic response to this growth factor. This suppression appears to be a consequence of PGE2-elicited increases in PKA activity, which leads to inhibition of EGF-induced activation of MAPK cascades at the level of Raf-1 and further affects downstream events including Erk-2. These results indicate that the mitogenic response to EGF in vivo in the proliferating basal cell layer may be dependent on the level of its PKA activity.

Regulation of MMP-9 activity in human tear fluid and corneal epithelial culture supernatant

PURPOSE

To evaluate human corneal epithelial culture supernatant and tear fluid for the presence of activators and inhibitors of matrix metalloproteinase (MMP)-9, MMP-3, and tissue inhibitor of metalloproteinase (TIMP)-1, respectively, and to evaluate the effect of MMP-3 on the activation of MMP-9 in these specimens.

METHODS

Unstimulated tear fluid was collected from patients with ocular rosacea and normal control subjects. Levels of MMP-9, MMP-3, and TIMP-1 were determined by enzyme-linked immunosorbent assay (ELISA) and/or immunoblot analysis. Supernatants from primary human corneal epithelial cultures and human tear fluid were incubated with MMP-3. Cultured epithelial cells and their supernatants were also treated with doxycycline before MMP-3 was added. Gelatin zymography was used to identify activated 82-kDa MMP-9. MMP-9 activity was assessed with a commercial MMP-9 activity assay system.

RESULTS

MMP-9 and TIMP-1 were detected at significantly higher concentrations in rosacea-affected than in normal tear fluids. MMP-3 was detected exclusively in the tear fluid of patients with ocular rosacea who had corneal epithelial disease. Treatment of the supernatant and tear fluid with MMP-3 resulted in two bands with molecular weights of 92 kDa and 82 kDa, representing pro-MMP-9 and activated MMP-9, respectively. Doxycycline added to the conditioned media did not affect activation of MMP-9 by MMP-3. However, 24-hour treatment of corneal epithelial cultures with doxycycline resulted in a lower concentration and activity of MMP-9 in their supernatants.

CONCLUSIONS

MMP-9 and TIMP-1 are produced by the human corneal epithelium and are present in tear fluid. MMP-3 alone is sufficient to activate MMP-9 on the ocular surface. Doxycycline does not directly inhibit this activation by MMP-3, but it decreases MMP-9 activity when added to corneal epithelial cultures.

Epidermal growth factor stimulates phospholipase cgamma1 in cultured rabbit corneal epithelial cells

Phospholipase Cgamma1 (PLCgamma1) catalyses hydrolysis of phosphatidylinositol 4,5-bisphosphate to generate diacylglycerol and inositol 1,4,5-trisphosphate (IP(3)), two second messengers which play important roles in cell proliferation and differentiation. The purpose of the current study was to identify PLCgamma1 in corneal epithelial cells and investigate whether epidermal growth factor (EGF) stimulates the activity of this enzyme. Addition of EGF to [(3)H]myo-inositol-labeled, cultured corneal epithelial cells stimulated production of IP(3), indicating activation of PLC. Western immunoblot analysis and an in vitro assay of PLC activity revealed that EGF activates gamma1 isoform of PLC, which is localized predominantly in the cytosolic fraction of the epithelial cells. EGF receptors were detected in the epithelial cells by EGF receptor antibody. Addition of EGF to the cells caused tyrosine phosphorylation of the receptors, translocation of PLCgamma1 from cytosol to plasma membrane, and phosphorylation of the enzyme at tyrosine residues. Addition of tyrphostin A-25, an inhibitor of receptor tyrosine kinase, attenuated the tyrosine phosphorylation of PLCgamma1 as well as its enzyme activity. These findings suggest that EGF stimulates PLCgamma1 in rabbit corneal epithelial cells, and that this effect is probably mediated by tyrosine phosphorylation of the enzyme.

Regulation of cyclooxygenase-2 by hypoxia and peroxisome proliferators in the corneal epithelium

Hypoxic injury provokes inflammation of many tissues including the ocular surface. In rabbit corneal epithelial cells, both peroxisome proliferator-activated receptor (PPAR)-inducible cytochrome P450 4B1 and cyclooxygenase-2 (COX-2) mRNAs were increased by hypoxia. PPAR alpha and beta but not gamma mRNAs were detected in these cells. The PPAR activator, WY-14,643 increased COX-2 expression. Similarly, non-steroidal anti-inflammatory drugs with the ability to activate PPARs induced COX-2 independently of prostaglandin synthesis inhibition. COX-2 protein overexpression by hypoxia and PPAR activation was not associated with a parallel increase in prostaglandin E(2) accumulation. However, the enzyme regained full catalytic activity when: 1) hypoxic cells were re-exposed to normoxic conditions in the presence of heme and arachidonic acid, and 2) WY-14,643-treated cells were depleted of intracellular GSH. Consistent with previous observations showing that the corneal production of cytochrome P450-derived inflammatory eicosanoids is elevated by hypoxia and inflammation, the current data suggest that hypoxic injury is a model of inflammation in which molecules other than COX-derived arachidonic acid metabolites play a major proinflammatory role. This study also suggests that increased cellular GSH may be the mechanism responsible for the characteristic dissociation of PPAR-induced COX-2 expression and activity. Moreover, we provide new insights into the commonly observed lack of efficacy of classical non-steroidal anti-inflammatory drugs in the treatment of hypoxia-related ocular surface inflammation.

Ecto-diadenosine 5',5'''-P1,P4-tetraphosphate (Ap4A)-hydrolase is expressed as an ectoenzyme in a variety of mammalian and human cells and adds new aspects to the turnover of Ap4A

Ap4A and other dinucleotides participate in the regulation of hemostasis and blood pressure control. With the exception of two previously reported surface anchored ectoAp4A-hydrolases on bovine aortic endothelial and chromaffine cells, all Ap4A-hydrolases reported are intracellular or freely soluble. We demonstrated that ectoAp4A-hydrolases are present on a broad variety of cell types of different species: rat mesangial, bovine corneal epithelial, human Hep-G2 and peridontal cells. Ectoenzyme properties were evaluated on rat mesangium cells. Chromatography of purified plasma membranes on Sephacel 300 resulted in enrichment of ectoAp4A-hydrolase and in separation from ectoATPase. In contrast to ATPase, Ap4A-hydrolase was stable at room temperature. EctoAp4A-hydrolase also recognized ATP as substrate, and therefore is not highly specific. The molecular weight was 180 kD. Unlike ectoAMPase ectoAp4A-hydrolase was not attached via a glycosyl-phosphatidylinositol (GPI)-moiety. Concentrations of PI-PLC 10-100-fold higher than effective for ectoAMPase cleavage (10-100 mU/ml) plus extensively extended incubation times up to eight hours did not result in cleavage of ectoAp4A-hydrolase. The enzyme ectoAp4A-hydrolase might presage a direction for pharmaceutical manipulation in the control of blood pressure and hemostasis.

Expression of phosphatidylinositol 3-kinase during EGF-stimulated wound repair in rabbit corneal epithelium

PURPOSE

To investigate the effect of epidermal growth factor (EGF) on the induction of phosphatidylinositol 3-kinase (PI 3- kinase) gene expression during rabbit corneal epithelial wound repair.

METHODS

Epithelial wounds (6 mm in size) were created in rabbit corneas and EGF (2 microg) applied every 8 hours to one eye, and the other eye served as a control. The wound repair was monitored by staining the tissue with fluorescein followed by photography. The wound area was quantified with a computer program. At different time intervals, the rabbits were killed and the corneal epithelium used for estimation of PI 3-kinase activity, western blot analysis, or reverse transcription-polymerase chain reaction (RT-PCR). For in situ hybridization, the whole corneas were sectioned and the sections processed with PI 3-kinase mRNA probes.

RESULTS

In the untreated eye, the epithelial wound progressively healed in a time-dependent manner, with 75% of the wound closed at 48 hours post wounding. Application of EGF to the corneal epithelium further stimulated wound repair at all time intervals, and the wound was completely closed at 48 hours. Analysis of PI 3-kinase showed a time-dependent increase in its enzyme activity that was maximally increased at 36 hours, the time when the wound was nearly closed. Western blot analysis revealed increased amounts of PI 3- kinase protein during the course of wound repair. Analysis of RT-PCR products from epithelial tissues, taken at different times during wound repair, showed increased PI 3-kinase expression that was maximum at 48 hours post wounding. A visible increase in PI 3-kinase gene expression was also detected by in situ hybridization during the course of the wound repair. This expression was increased maximally by EGF at 48 hours post wounding.

CONCLUSIONS

The results indicate a temporal correlation between increased activation and expression of PI 3- kinase and the epithelial wound repair. Topical application of EGF further stimulates the activity and expression of PI 3- kinase. It is suggested that PI 3- kinase and its products may play a role in EGF-induced cell proliferation during corneal epithelial wound repair.

Identification of a cytosolic NADP+-dependent isocitrate dehydrogenase that is preferentially expressed in bovine corneal epithelium. A corneal epithelial crystallin

Recently, metabolic enzymes have been observed in both the lens and corneal epithelium at levels greatly exceeding what is necessary for normal metabolic functions. These proteins have been termed taxon-specific crystallins and are thought to play a role in maintaining tissue transparency. We report here that cytosolic NADP+-dependent isocitrate dehydrogenase (ICDH) represents a new corneal crystallin. Using suppression subtractive hybridization, we identified a gene (with a deduced amino acid sequence that showed 94% identity to rat cytosolic NADP+-dependent ICDH) that is preferentially expressed in bovine corneal epithelium. Northern blots established that its mRNA level in the corneal epithelium was 31-, 39-, 133-, 230-, and 929-fold more than in the liver, bladder epithelium, stomach epithelium, brain, and heart, respectively. This mRNA was detected primarily in corneal epithelial basal cells by in situ hybridization. SDS-polyacrylamide gel electrophoresis, two-dimensional gel analysis, and Western blotting showed that this protein was overexpressed in the corneal epithelium, constituting approximately 13% of the total soluble bovine corneal epithelial proteins. Enzyme assays showed a corresponding overabundance of this protein in bovine corneal epithelium. Taken together, these data indicate that bovine cytosolic ICDH fulfills the criteria for a corneal epithelial crystallin and may be involved in maintaining corneal epithelial transparency.

[Membrane-bound carbonic anhydrase (CA IV) in human corneal epithelium and endothelium]

PURPOSE

Active HCO3- transport through the corneal endothelial cell layer causes a dehydration of the corneal stroma and is thought to be driven by Na/K- and HCO3(-)-dependent ATPase as well as an electro-genic Na/HCO3- cotransport. Transmembrane bicarbonate transport has also been associated with the recently characterised membrane-anchored isoform of carbonic anhydrase (CA IV) in various tissues. We investigated the localisation of CA IV in human fresh and cultured epi- and endothelium at the light- (LM) and electron-microscopic (EM) level.

METHODS

Postmortem corneas were obtained within 12 hours of death, stored in formaldehyde and sectioned in paraffin. LM immunohisto-chemistry was performed using the purified gamma-globulin fraction of a polyclonal chicken antibody against CA IV isolated from human kidneys. Epi- and endothelial cell cultures were grown in uncoated flasks under standard conditions and processed both for LM and EM immunohistochemistry using the same antibody.

RESULTS

Lightmicroscopy of fresh tissue showed membrane staining for CA IV in the whole circumference of the endothelium. Little staining was also observed in some cells of the basal cell layer of the epithelium. Immunohistochemical staining at the EM level was confined to the cell surface of confluent cultures of both epi- and endothelial cells.

CONCLUSION

The localisation of CA IV to the cell surface of fresh and cultured corneal endothelium suggests the presence of a membrane-bound ion exchange mechanism which may be important for HCO3- transport and corneal hydration. Compromising this mechanism by treatment with local carbonicanhydrase inhibitors may be of clinical importance in selected endothelial disease.

Matrix metalloproteinases in epithelia from human recurrent corneal erosion

PURPOSE

To assay for the presence of matrix metalloproteinases (MMPs) in human corneal epithelium affected by recurrent erosion compared with that in normal corneal epithelium.

METHODS

Corneal epithelial debridement samples were obtained from 13 patients with recurrent epithelial erosion. For control specimens, epithelia were obtained from healthy patients undergoing photorefractive keratectomy. Zymography was performed on all samples to identify MMPs. Immunolocalization of MMP-2, laminin, and collagen type VII was determined in two samples with human recurrent epithelial erosion and compared with that in control epithelium.

RESULTS

Twelve of 13 erosion samples showed MMP-2 enzymatic activity; one of the 12 also showed MMP-9 activity. Only one erosion sample showed no MMP enzymatic activity. All normal control specimens were negative for MMP. Immunohistochemical analysis of two recurrent erosion samples showed MMP-2 presence in basal cells, whereas, in normal epithelium it was not detected. One sample with epithelial erosion showed laminin localization in basal epithelial cells and basal lamina. Type VII collagen localized in basal epithelial cells only in this sample. A second erosion sample showed localization of laminin and type VII collagen in basal epithelial cells only. Normal corneal epithelium showed presence of laminin and type VII collagen in basal epithelium and basal lamina.

CONCLUSIONS

Matrix metalloproteinase-2 expression is upregulated in human epithelia affected by recurrent erosion compared with that in normal control samples. Immunolocalization studies suggest that this enzyme is concentrated in basal epithelial cells where it may play an important role in degradation of the epithelial anchoring system and the recurrent epithelial slippage and erosion observed in these patients.

Immortalized human corneal epithelial cells for ocular toxicity and inflammation studies

PURPOSE

To develop a metabolically competent, human immortalized corneal epithelial cell line for use in toxicity and inflammation studies.

METHODS

Primary corneal epithelial cells (P-CEPI) were immortalized by a recombinant simian virus (SV)40 T antigen retroviral vector defective for viral replication. The cells were grown in serum-free medium with the addition of bovine pituitary extract, cloned at passage 15 and one of the best-growing clones, CEPI-17-CL4, was extensively characterized for differentiation and metabolic characteristics of the human corneal epithelium. Methods used were immunostaining, reverse transcription-polymerase chain reaction (RT-PCR), northern blot analysis, and enzyme assays.

RESULTS

The CEPI-17-CL4 cells showed a typical cobblestone morphology, grew to more than 200 passages and expressed the SV40 T antigen in the nucleus of every cell. Immunofluorescence staining for CEPI-17-CL4 cells was strongly positive for keratins (K)8, K18, and K19 and vimentin; weakly positive for K3, K13, and K17; and negative for K4, K7, and K14. Expression of cytokines (interleukin [IL]-1alpha, IL-1beta, IL-6, IL-8, tumor necrosis factor-alpha, and IL-ra), growth factors (transforming growth factor [TGF]-alpha, epidermal growth factors [EGF], EGF receptor [EGFR], TGF-beta1, TGF-beta2, and platelet-derived growth factor-beta) and cytochrome P450 enzymes (1A1, 2C, 2E1, and 3A5) was similar in CEPI-17-CL4 cells and human corneal epithelial samples obtained in biopsy. The CEPI-17-CL4 cells were metabolically competent for enzymes glutathione S-transferase, quinone reductase, aflatoxin aldehyde reductase, glutathione peroxidase, glutathione reductase, superoxide dismutase, and catalase.

CONCLUSIONS

The CEPI-17-CL4 cells are truly immortal and express an extensive array of cytokines, growth factors, and metabolic enzymes that resemble the original tissue. These characteristics, which remain stable up to high passage, will allow reproducible, mechanistic studies on toxicity, inflammation, and wound healing.

Regulation of gelatinase B production in corneal cells is independent of autocrine IL-1alpha

PURPOSE

The matrix metalloproteinase gelatinase B is synthesized by cells at the leading edge of the corneal epithelium migrating to heal a wound. Recent data from the authors' laboratory suggest that excessive synthesis contributes to repair defects. The goal of the study reported here was to investigate mechanisms controlling gelatinase B production by corneal epithelial cells.

METHODS

Freshly isolated cultures of corneal epithelial cells and early passage stromal fibroblasts from rabbit were used for these studies.

RESULTS

In a previous study, it was found that the cytokine interleukin (IL)-1alpha is released into the culture medium of corneal epithelial cells more efficiently when they are plated at low density with limited cell-cell contact than when plated at high density. In this study, we show that production of gelatinase B by these cells is similarly affected by cell plating density. However, it is further demonstrated that these two events are not dependent on one another but occur in parallel: IL-1alpha does not regulate gelatinase B production (synthesis), nor was there evidence that any other secreted autocrine cytokine acts as mediator. Instead, our data suggest that gelatinase B production is downregulated directly by high cell density and indicate a connection to the level of protein kinase C activity. Nevertheless, the anticancer agent suramin, which blocks collagenase synthesis by interfering with autocrine cytokine-receptor interactions, still inhibits synthesis of gelatinase B.

CONCLUSIONS

Unlike collagenase synthesis by corneal stromal fibroblasts, production (synthesis) of gelatinase B does not appear to be controlled by secreted autocrine cytokines but can still be inhibited by suramin. Suramin may make an effective therapeutic agent for controlling pathologic overproduction of gelatinase B in corneal ulcers.

Corneal epithelial wound healing increases the expression but not long lasting activation of the p85alpha subunit of phosphatidylinositol-3 kinase

PURPOSE

Corneal epithelial wound healing is a complex process involving several growth factors whose interaction with tyrosine kinase receptors (RTK) leads to the recruitment of enzymes coupled to second messengers that propagate and amplify growth factor-induced signals inside the cells. Phosphatidylinositol-3 kinase (PI-3K) is one such enzyme. Here we have investigated changes in PI-3K activity and expression during re-epithelialization after in vivo and in vitro corneal injury.

METHODS

For the in vivo model, epithelium was collected from rabbit corneas at different stages of wound healing after complete de-epithelialization. For in vitro studies, after 7 mm central scrape wounds were applied, rabbit corneas were maintained in organ culture. Immunoprecipitation and Western blot using anti-p85alpha antibodies were employed to determine PI-3K activity and expression of the p85alpha regulatory subunit of PI-3K. Two specific PI-3K inhibitors, Wortmannin and LY 294002 were used to study the effect of PI-3K activity on corneal epithelial wound healing.

RESULTS

Two to four days after in vivo corneal epithelial wound healing, there was a 6-8 fold increase in the expression of the p85alpha subunit of PI-3K. By 8 days, the expression of p85alpha was similar to non-injured tissue. Increased expression of the 85kDa protein was observed mainly in the membrane fraction. Similarly, the expression of PI-3K was increased 24h after injured corneas were maintained in organ culture. Increase of p85alpha was confined to the wound region and surrounding area. No concomitant increase in PI-3K activity was observed in any of the wound models. Forty-eight hours after the central injury, Wortmannin and LY294002 inhibited wound healing by about 50%.

CONCLUSIONS

Association of most of the increased p85alpha with the membrane fraction and no detectable increase in PI-3K activity during corneal re-epithelialization indicates that PI-3K activation is transitory. The results also suggest a mechanism of down regulation of the enzyme to avoid uncontrollable growth and cellular hypertrophy after growth factor stimulation during wound healing.

Immunolocalization and gene expression of matrilysin during corneal wound healing

PURPOSE

To investigate the protein level of matrilysin and stromelysin-1 and the gene expression of matrilysin in rat corneas after excimer keratectomy using immunofluorescence staining, reverse transcriptase-polymerase Chain Reaction (RT-PCR), and in situ hybridization.

METHODS

Rat corneas were treated with 3-mm excimer laser keratectomy (193-nm ArF). Unwounded corneas served as controls. Confocal microscopy was used to immunolocalize matrilysin protein at 6 hours, 1 day, 3 days, 1 week, 4 weeks, and 8 weeks after surgery. RT-PCR was performed to analyze matrilysin mRNA in unwounded controls and in 3-day wounded corneas. In situ hybridization was performed to localize matrilysin mRNA.

RESULTS

Matrilysin was immunolocalized to the epithelial layers of unwounded and wounded corneas, predominantly to the leading edge at 6 hours and 1 day after wounding and to the epithelial layer at 3 to 7 days after surgery. Stromelysin-1 was expressed in the deep stromal layer in the first 3 days after wounding and in the area of newly synthesized stromal matrix 1 week after surgery. Upregulation of matrilysin expression 3 days after wounding was confirmed by RT-PCR. In situ hybridization revealed that the gene expression of matrilysin in rat corneas was upregulated during the migration phase (6 hours, 1 day) and that matrilysin mRNA was predominantly localized to the basal cell layers during the subsequent cell proliferation phase (7 and 14 days).

CONCLUSIONS

Basal epithelial cells express matrilysin during the migration proliferation phase of corneal wound healing after excimer keratectomy.

Ex vivo transduction of corneal epithelial progenitor cells using a retroviral vector

PURPOSE

To transduce corneal epithelial progenitor cells with a reporter gene using a retroviral vector and follow their progeny in vitro and in vivo.

METHODS

Using a lacZ-producing retroviral vector, rabbit keratolimbal explants were transduced ex vivo, autografted onto their original sites, and assessed for lacZ-producing cells in the cornea throughout a 6-month period. Four autografts served as control samples, having received no vector. Experimental and control rabbits were euthanized and corneas with scleral rims harvested, weekly for 4 weeks and then monthly for 6 months. The corneas were first stained with 5-bromo-4-chloro-3-indolyl galactopyranoside (X-gal) as wholemounts and then sectioned for histology and immunohistochemistry to examine lacZ-positive cell outgrowth. Three additional transduced explants were observed in culture. These explants were transferred to new culture dishes every week for 9 weeks. The previously occupied culture dish was stained for lacZ to detect transduced epithelial cells, and the number of lacZ-positive cells was quantitated.

RESULTS

LacZ-positive cells were found in the corneas of 18 of 20 eyes in which virally transduced keratolimbal autografts had been implanted. The cells were epithelial in nature, originated from the limbus, and were found in colonies throughout the epithelial layer of the cornea. The appearance of lacZ-positive cells in four of five corneas harvested after 6 months showed long-term transgene expression consistent with transduction of corneal epithelial stem cells. In vitro, the number of lacZ-positive cells migrating from the keratolimbal autografts decreased rapidly during the first 4 weeks and then remained stable through week 9.

CONCLUSIONS

This study shows that a retroviral vector can effectively transduce corneal epithelial progenitor cells, shown by the long-term appearance of transduced cells on the cornea in vivo and the stable production of lacZ-positive cells in vitro. The appearance and disappearance of labeled cells is consistent with the initial transduction of stem cells and transient amplifying cells.

Selective changes in protein kinase C (PKC) isoform expression in rabbit corneal epithelium during wound healing. Inhibition of corneal epithelial repair by PKCalpha antisense

Protein kinase C (PKC) isoforms display different sensitivities to modulators, tissue specificities and subcellular localizations. PKCalpha increases during rabbit corneal epithelial wound healing. Here we report differential expression of PKC isoforms in the cornea of rabbits at 1, 2, 4 and 8 days during re-epithelization. Cytosolic, membrane and detergent-insoluble fractions from epithelium were analysed by Western blot using monoclonal antibodies against the different PKC isoforms. We have identified PKCalpha, gamma, epsilon, mu and iota. PKCalpha and gamma were expressed only in the cytosolic fraction, with the expression of PKCalpha markedly increasing 4 days after injury. Corneas cultured in the presence of rabbit-specific PKCalpha antisense showed a greater than 50% inhibition of wound closure, compared to controls. The PKCepsilon and mu were expressed in the soluble, as well as in the membrane fraction. Additionally, 12% of PKCmu was found attached to the detergent insoluble fraction. The expression of the membrane-bound PKCepsilon and mu isoforms decreased between 1 and 2 days following injury. Only 10% of the PKCiota expressed in corneal epithelium was membrane bound, but between 4 and 8 days after de-epithelization, the expression in this fraction increased three-fold. Our results suggest that changes in the expression and distribution within the various fractions of selective isoforms of PKC after injury could be involved in events leading to wound healing and that PKCalpha is a key modulator in rabbit corneal wound repair.

Gelatinase B and A expression after laser in situ keratomileusis and photorefractive keratectomy

OBJECTIVE

To compare the expression of gelatinases in the corneal epithelium and stroma after laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK).

METHODS

Rabbit eyes were treated with LASIK (n=11), PRK (n=12), or corneal flap construction (n=12); 4 eyes served as unwounded controls. Zymography was performed on the central epithelium and the stroma 1, 3, and 7 days after surgery to determine the expression of gelatinases.

RESULTS

Epithelial expression of gelatinase B in the LASIK group (0%-25%) was lower than that in the PRK group at all time points (50%-100%) and was identical to the corneal flap group. Stromal expression of gelatinases A and B was similar after LASIK and PRK, but was minimal after corneal flap construction at all time points. Epithelial expression of gelatinase A was similar for the first 3 days after LASIK and PRK but not thereafter.

CONCLUSIONS

Gelatinase B epithelial expression was up-regulated after PRK but not after LASIK. Gelatinase B stromal expression was up-regulated after both procedures.

CLINICAL RELEVANCE

Differences in wound healing and subepithelial scarring after these 2 procedures may be related to gelatinase B.

Hypoxia exerts cell-type-specific effects on expression of the class 3 aldehyde dehydrogenase gene

The Class 3 aldehyde dehydrogenase gene (ALDH3) is expressed differentially in a tissue-specific manner, occurring constitutively in some tissues and in others as a result of xenobiotic induction via the Ah receptor/ARNT pathway. ARNT is also involved in regulating gene expression in response to hypoxia. It dimerizes with hypoxia-inducible factor 1 alpha (HIF-1 alpha) and enhances expression of hypoxia-responsive genes. To determine if ARNT plays a role in regulating ALDH3 in response to low oxygen tension, we studied the effects of 1% oxygen and the hypoxia mimic cobalt chloride on constitutive and inducible ALDH3 expression in rat hepatoma cells and rat corneal epithelial cells. Hypoxia sharply down-regulates constitutive ALDH3 expression in corneal epithelial cells. Likewise, aromatic hydrocarbon-induced ALDH3 expression in H4-II-EC3 cells is significantly reduced by hypoxia. In contrast, hypoxia has no effect on constitutive or aromatic hydrocarbon-inducible ALDH3 expression in HTC cells. Our data indicate that hypoxia exerts cell type-specific effects on both constitutive and induced ALDH3 expression.

Recovery of corneal nerve morphology following laser in situ keratomileusis

Morphological changes in the corneal nerves after laser in situ keratomileusis (LASIK) were investigated and the changes were compared with those observed after creation of the keratectomy flap without subsequent photoablation. After creating the hinged flap, a multizone excimer laser photoablation (myopic correction from 6.00 to 6.66 D; diameter 6 mm) was performed on 27 rabbit corneas. Seven of these 27 rabbits received an automated keratectomy without laser photoablation on the fellow eye. A histochemical acetylcholinesterase reaction was used to demonstrate the changes in the morphology of the corneal nerves 3 days, 2.5 and 5 months after the operations. In all specimens the deepest stromal nerve bundles showed normal morphology. Cut nerve trunks were found at the wound margins and at the level of the flap interphase in the stromal bed. At 3 days, both epithelial and basal epithelial/subepithelial nerves were found at the hinge of the flap but the rest of the flap showed a major loss of epithelial, basal epithelial/subepithelial and superficial stromal nerves. A few new regenerating thin nerve fibers were found to emerge from the cut stromal nerve trunks. They appeared to pass the wound margin into the flap area below the epithelium. At 2.5 and 5 months an increasing number of regenerating nerve leashes were observed to emerge from the cut stromal nerve trunks. They appeared to send anastomosing fibers among the neighboring stromal nerves. By this time the epithelial, basal epithelial/subepithelial and anterior stromal innervation had gained an almost normal nerve density and architecture. In the corneas with the flap only, the epithelial innervation was slightly better spared in the center of the flap, and the stromal changes were somewhat less prominent compared with the LASIK corneas.

A Rho-associated protein kinase: differentially distributed in limbal and corneal epithelia

PURPOSE

The authors have developed monoclonal antibodies (mAbs) to characterize the sequential biochemical changes in corneal epithelial cells after they differentiate from stem cells, located in the limbus, and migrate centripetally to follow the pathway of terminal differentiation. The purpose of this study was to identify a protein (recognized by mAb HE1/11F) with increased expression associated with the transition of the limbal epithelium to corneal epithelium.

METHODS

The distribution and identification of the protein(s) were performed using an indirect immunohistochemical staining technique and a western blot analysis, respectively. A rabbit corneal epithelial cDNA library, constructed in the Uni-Zap XR vector, was screened with mAb HE1/11F to select cDNA clones expressing polypeptide(s) recognized by this mAb. Additional overlapping cDNA clones were obtained from a primer extension cDNA library to determine the sequence of the complete open reading frame encoding the protein recognized by mAb HE1/11F.

RESULTS

Rabbit corneal epithelium exhibited strong immunostaining with mAb HE1/11F, however, the limbal epithelial cells stained weakly. HE1/11F recognized 160-kDa (HEBM1) and 100-kDa (HEBM2) polypeptides in the corneal epithelial extracts. The amino acid sequence of the protein deduced from the nucleotide sequence of the cDNA exhibited a close homology to that of a RhoA (Ras-related small GTPase)-associated serine-threonine kinase (ROCK-I or Rho-associated coiled-coil kinase). A 160-kDa RhoA-binding polypeptide with a molecular mass similar to that of HEBM1 and ROCK-I was detected in the corneal epithelial extracts. These findings strongly suggested that HEBM1 was rabbit ROCK-I. The identity of HEBM1 was further confirmed from the reactivity of mAb HE1/11F with ROCK-I immunoprecipitated from rabbit corneal epithelial extracts using anti-ROCK-I antibodies.

CONCLUSIONS

The increased expression of a protein identified as ROCK-I from cDNA analyses is associated with rabbit corneal epithelial differentiation and transition from the limbal to corneal surface. Therefore, a RhoA signaling pathway is likely to be associated with corneal epithelial differentiation (maturation). A close homology among the cDNA sequences of rabbit, mouse, rat, and human ROCK-I indicates that this RhoA-associated kinase is a well-conserved protein.

Differentiation-dependent expression of alpha-2,3-sialyltransferase in rabbit corneal epithelium

PURPOSE

Lectin studies have shown that in the rabbit corneal epithelium, alpha-2,3 sialylation of O-linked glycans differentiates limbal and corneal epithelial cell phenotypes. Because sialic acid can be regulated at the level of the expression of sialyltransferases (STs), the purpose of the present study was to analyze the expression of alpha-2,3STs in this tissue.

METHODS

Reverse transcription-polymerase chain reaction (RT-PCR) was used to generate ST cDNA from total rabbit corneal epithelium RNA using primers selected from the sequences of three previously cloned STs capable of catalyzing the transfer of sialic acid to O-linked oligosaccharides, human placental Galbeta-1,3GalNAc-Galbeta-1,4GluNAcalpha-2,3ST (STZ), and mouse brain Galbeta-1,3GalNAcalpha-2,3ST types I and II (ST3Gal I and ST3Gal II). Tissue distribution of mRNA was assayed by fluorescent in situ hybridization. A synthetic peptide whose sequence was deduced from a cloned cDNA fragment was synthesized and used to prepare an anti-ST goat antiserum. The molecular weights of immunodetectable polypeptides and their distribution in cryostat sections of the limbocorneal area were investigated by western blot analysis and indirect immunofluorescence, respectively.

RESULTS

RT-PCR yielded cDNA of expected basepair length for STZ and ST3(Gal II. The rabbit STZ cDNA was 86% identical with its human equivalent. Its mRNA was confined to the cornea, mainly in basal epithelial cells, and was not expressed in the limbus. Western blot analysis identified a band at 37 kDa whose binding was abolished by preincubation of the antiserum with the immunization peptide. Immunohistologic analysis revealed the presence of immunoreactive epitopes in all basal cells of the cornea but not in the limbus.

CONCLUSIONS

STZ mRNA and the enzyme itself are expressed in the basal layer of the corneal epithelium but are absent in the limbus. This enzyme's de novo expression seems thus responsible for the differential expression of alpha-2,3 sialylation along the limbocorneal differentiation axes. At least one more alpha-2,3ST is also present in the epithelium.

Reactive oxygen species (ROS) generated by xanthine oxidase in the corneal epithelium and their potential participation in the damage of the corneal epithelium after prolonged use of contact lenses in rabbits

Prolonged use of contact lenses (for 14 days) evoked an imbalance between the activity of xanthine oxidase (an enzyme belonging to reactive oxygen species-generating oxidases) and catalase (an enzyme belonging to reactive oxygen species-scavenging oxidases) in the corneal epithelium of rabbits. The activity of catalase decreased, while xanthine oxidase activity was very high. Of other enzymes studied in the corneal epithelium, the activities of xanthine oxidoreductase, glucoso-6-phosphate dehydrogenase and succinate dehydrogenase were decreased. In contrast, the activities of lactate dehydrogenase and lysosomal hydrolases (acid beta-galactosidase, dipeptidyl peptidase II) were increased and appeared in animals sacrificed immediately after contact lens removal. In rabbits sacrificed later (after 1 h), an additional increase of lactate dehydrogenase and lysosomal hydrolase activities developed in the superficial layers of the corneal epithelium. Catalase supplementation during use of contact lenses prevented both the significant decrease of catalase activity in the corneal epithelium and the development of additional epithelial damage. In contrast, topical treatment with 3-aminotriazole (an inhibitor of catalase) resulted in the nearly complete loss of catalase activity in the corneal epithelium and the appearance of more serious epithelial damage. We conclude that ROS generated by xanthine oxidase induce additional damage of the corneal epithelium related to the use of contact lenses.

Epidermal growth factor stimulates phospholipase D independent of phospholipase C, protein kinase C or phosphatidylinositol-3 kinase activation in immortalized rabbit corneal epithelial cells

PURPOSE

Activation of phospholipase D (PLD) is believed to be an important signaling pathway involved in cell growth and differentiation in several tissues, in response to a variety of mitogens. The aim of the present study was to investigate the effect of epidermal growth factor (EGF) on PLD activity in rabbit corneal epithelial cells (RCEC). We have also examined whether the EGF effect is dependent on concurrent activation of phospholipase C (PLC), protein kinase C (PKC) or phosphatidylinositol 3-kinase (PI-3-kinase) in these cells.

METHODS

RCEC, immortalized with adenovirus SV-40, were cultured until they became confluent. The cells were labeled with [3H]myristic acid and incubated with or without EGF or other agents for specified time intervals. PLD activity was measured by quantifying [3H]phosphatidylethanol in cells incubated in the presence of ethanol. PLC activity was determined by measuring the radioactivity in inositol trisphosphate in myo[3H]inositol-labeled RCEC. PI 3-kinase activity was assessed by measuring the production of PIP3 in 32P-labeled cells.

RESULTS

Addition of EGF to RCEC stimulated PLD activity in a time- and dose-dependent manner. The maximal effect was observed with 150 ng/ml EGF and at 10 min of incubation. The PLD activity was also stimulated when phorbol myristate acetate (PMA) was added to the cells. Treatment of the cells with EGF stimulated PLC activity which was inhibited by U73122, a PLC inhibitor. Under the same experimental conditions, the inhibitor had no effect on EGF-stimulated PLD activity. Down-regulation of PKC or treatment of the cells with RO31-8220, a PKC inhibitor, inhibited the PMA- but not EGF-stimulated PLD activity. Incubation of the cells with wortmannin, a PI 3-kinase inhibitor, abolished the EGF-stimulated PI 3-kinase activity, but potentiated the EGF-stimulated PLD activity. The EGF effect was inhibited by treatment of the cells with tyrphostin B42, a receptor tyrosine kinase inhibitor.

CONCLUSIONS

These results indicate that EGF stimulates PLD activity in RCEC by a mechanism that involves tyrosine phosphorylation of a protein(s) in the cascade of biochemical reactions initiated by EGF-receptor interaction, and it is not dependent on concurrent activation of PKC, PLC, or PI 3-kinase in these cells.