Specialized Proteins and Specialized Functions in Ocular Tissues

NQO1 in Tumors

NQO1, a bioactivating enzyme for mitomycin C, was localized immunohistochemically in corneal and conjunctival epithelial tumors, as well as most tissues of human donor eyes. However, Schelonka et al. (p. 1617) found two eyes from a donor carrying a genetic defect in the enzyme showed markedly reduced immunohistochemical staining for the enzyme. NQO1 may bioactivate mitomycin C in epithelial tumors and in normal tissues during glaucoma therapy. Since the prevalence of the genetic defect in the enzyme is between 7% and 20%, some individuals may be resistant to mitomycin C therapy. 

GR, MR, and 11β-HSD in Glaucoma

Corticosteroids induce ocular hypertension. The activity of corticosteroids at the level of their target receptors, GR and MR, is regulated by the 11β-hydroxysteroid dehydrogenases 11β-HSD types 1 and 2. In this study, Stokes et al. (p. 1629) have demonstrated that GR, MR, and 11β-HSDs are present in mammalian ocular tissues responsible for regulation of intraocular pressure (ciliary epithelium and trabecular meshwork) in the human and rat. Alterations in the number or affinity of target receptors or in the activity of the 11β-HSDs may alter the effects of glucocorticoids on intraocular pressure. The authors suggest that investigating whether or not these alterations occur in disorders of intraocular pressure may help to further define the role of glucocorticoids in disorders, such as primary open angle glaucoma, and may lead to new therapeutic approaches in the treatment of this disease. 

VEGF-R2 and Neuropilin-1 in PDR

The expression of particular growth factor receptors, as well as the growth factors themselves, may accelerate the development of proliferative diabetic retinopathy (PDR). Ishida et al. (p. 1649) studied fibrovascular tissues removed at vitrectomy from 22 subjects with PDR. Fourteen of the cases examined simultaneously expressed the vascular endothelial growth factor (VEGF) receptors VEGF-R2 and neuropilin-1 in these membranes. Seven of the fibrovascular membranes that expressed both receptors were the only tissues examined that also expressed VEGF₁₆₅. Membranes that expressed both receptors, and particularly those that also expressed VEGF₁₆₅, came from younger patients and had significantly higher vascular density. VEGF was expressed in glial cells from these membranes. 

PAF and PAF-R in Corneal Inflammation

Platelet activating factor (PAF) is a unique phospholipid with potent inflammatory properties. PAF receptors (PAF-R) in the cornea mediate a wide array of cellular responses. After corneal injury, PAF accumulates in the tissue as a result of its production by corneal cells and inflammatory cells that infiltrate the cornea. Ma and Bazan (p. 1696) have found that corneal epithelial cells respond to injury by upregulating PAF-R gene expression. Further increases in PAF-R expression were found following corneal PAF treatment. More importantly, the authors found that the transcriptional increase of PAF-R by PAF requires injury to the epithelial cells and that specific growth factors are positive candidates in PAF-R upregulation. Thus, the increase in PAF following severe inflammatory conditions may constitute an important feedback mechanism that maintains cellular responses to PAF through the activation of its receptors, thus sustaining the inflammatory process. PAF antagonists could be of therapeutic importance in the prevention of inflammation. 

MMPs in Tears

The matrix metalloproteinase enzyme, MMP-9, and its inhibitor, TIMP-1, are produced by the human corneal epithelium and present in human tear fluid. Sobrin et al. (p. 1703) detected stromelysin-1 (MMP-3), an efficient activator of MMP-9, in the tear fluid of patients with ocular rosacea and corneal epithelial disease, but not in normal tear fluid. MMP-3 activated MMP-9 in corneal epithelial conditioned media and tear fluid. Doxycycline, a commonly used therapy of rosacea, significantly decreased the concentration and activity of MMP-9 produced by the corneal epithelium. 

TIGR/MYOC in TM

Using in situ hybridization, Wang and Johnson (p. 1724) detected TIGR/MYOC expression in trabecular cells of both normal and glaucomatous eyes. In normal fresh eyes, positive hybridization was found in most cells of all three regions: uveal, corneoscleral, and JCT. In contrast, labeling of the endothelial cells of Schlemm’s canal was variable, with only a few positive cells present. Trabeculectomy specimens from glaucoma patients showed labeling in many, but not all, cells of the uveal, corneoscleral, and JCT. In these glaucomatous eyes, labeling of TIGR/MYOC mRNA was neither as common nor as strong as in normal eyes. These results suggest that expression for TIGR/MYOC is not necessarily increased in POAG. 

βig-h3 and Cataract

βig-h3 is a secreted protein associated with the extracellular matrix. Mutated forms of βig-h3 are major constituents of the abnormal deposits found in several hereditary corneal diseases. The expression of βig-h3 is induced by TGF-β in several cell types. Lee et al. (p. 1840) investigated βig-h3 expression in lens epithelial cells obtained from cataract patients. Significant expression of βig-h3 mRNA and protein was detected in anterior polar cataracts (APC), in which lens epithelial cells transform into myofibroblast-like cells, but not in clear lenses or nuclear cataracts. TGF-β, a cytokine implicated in APC formation, significantly increased βig-h3 expression in cultured human lens epithelial B-3 cells. These findings suggest a potential role for βig-h3 protein in pathologic accumulation of extracellular matrix seen in APC. 

Substance P and Iris Contraction

Rings of rat iris were mounted in an organ chamber containing Krebs solution, and isometric tension was recorded. Substance P (SP) produced graded contraction, dependent on external Ca²⁺, in the rat iris. Atropine and tetrodotoxin significantly shifted the SP response to the right. All the natural tachykinin agonists caused concentration-dependent contraction in rat iris. The utilization of selective agonists and antagonists suggests that this contraction response is probably mediated via activation of NK₃ and NK₂ receptors. These results of Grumann-Júnior et al. (p. 1861) show that SP could be the main nonadrenergic and noncholinergic response in rat iris. 

VEGF and Endothelial Cell Transport

VEGF, a cytokine with vascular endothelial-specific permeability and mitogenic effects, is thought to play a major role in the development of diabetic retinopathy (DR). Sone et al. (p. 1876) demonstrate that VEGF increases glucose transport in retinal endothelial cells via its stimulatory effects on protein kinase C and causes translocation of GLUT1, the glucose transporter that mediates glucose entry into the retina, to the endothelial cell plasma membrane. This study extends the repertoire of potentially damaging effects of VEGF in DR: VEGF may increase glucose flux into retinal endothelial cells and thus exacerbate the deleterious effects of hyperglycemia on the retinal microvasculature. 

HGF and Retinal Neovascularization

Hepatocyte growth factor (HGF) is increased in the serum and vitreous of patients with proliferative diabetic retinopathy and stimulates growth and motility in nonocular endothelial cells. Cai et al. (p. 1885) demonstrate that HGF and its receptor are expressed in retinal endothelial cells and stimulate both growth and migration through PKC and PI3 kinase, resulting in MAPK phosphorylation. These results indicate that HGF may have a significant role in mediating retinal endothelial cell proliferation and migration in diabetic retinopathy and begin to elucidate this signal transduction pathway. 

Neurotrophins and Retinoblastoma

The hypothesis that neurotrophins play a role in the malignant growth of retinoblastoma was tested by Wagner et al. (p. 1932) in Y-79 cells. In contrast to previously studied normal retinal precursor cells, Y-79 cells express not only NGF, BDNF, NT-3, and the p75 receptor, but also the high affinity receptors TrkA, TrkB, and TrkC. Proliferation was stimulated by added neurotrophins. Inhibition of protein kinase phosphorylation with K252a blocked proliferation and promoted differentiated properties, such as improved attachment, neurite outgrowth, expression of NF-68, and loss of GFAP and parvalbumin, accompanied by downregulation of TrkB and TrkC, but not TrkA or p75. The authors propose that mitogenic effects of neurotrophins could contribute to retinoblastoma growth. 

E-Cadherin and RPE Polarity

RPE cells have an apically polarized Na/K ATPase, but some RPE cells also express E-cadherin, a cell adhesion protein that can induce a basal sodium pump polarity. Burke et al. (p. 1945) examined the bovine RPE monolayer and found differences in protein expression and distribution among individual RPE cells. Most cells have long apical microvilli where Na/K ATPase was distributed. However, groups of cells with distinct junctional staining for E-/P-cadherin have shorter microvilli and Na/K ATPase distributed on all cell surfaces. Did E-cadherin cause a decrease in apical polarity? If so, why was the polarity not reversed, as in other cells with E-cadherin? 

Bcl-2 and BAG-1 in Photoreceptor Apoptosis

Several mutations within genes involved in photoreception lead to the onset of programmed cell death in photoreceptor cells, culminating in degeneration of the retina. Coexpression of Bcl-2 and BAG-1 in the photoreceptors of mice with an autosomal dominant form of retinitis pigmentosa was found by Eversole-Cire et al. (p. 1953) to produce a synergistic inhibitory effect against photoreceptor cell death. The synergistic effect indicates that Bcl-2 and BAG-1 may function as a complex to prevent photoreceptor cell death and that their relative levels may play a critical role in determining cell survival. Coexpression of Bcl-2 and BAG-1 may serve as a potential means to treat retinal degenerative diseases in the future. 

Adrenomedullin and RPE Proliferation

Retinal pigment epithelial (RPE) cells are known to migrate into the vitreous and proliferate under some pathological conditions, such as proliferative vitreoretinopathy. Udono et al. (p. 1962) show that adrenomedullin (ADM), a potent vasodilator peptide, is secreted by cultured RPE cells and has proliferative effects on these cells. ADM production was augmented by inflammatory cytokines. ADM secreted by RPE cells may stimulate RPE proliferation and that of other cell types in some inflammatory disorders of the eye, including proliferative vitreoretinopathy. The findings raise the possibility that ADM might be a new target to treat inflammatory ocular disorders. 

6B4 Proteoglycan/ Phosphacan in Retinal Development

Inatani et al. (p. 1990) have demonstrated the expression of the mRNA and core protein for 6B4 proteoglycan/phosphacan in rat retina. The core protein is less glycosylated than that of 6B4 proteoglycan in the brain. A significant portion of retinal phosphacan exists in a non-proteoglycan form without chondroitin sulfate side chains. The results show that, in developing rat retina, the expression of phosphacan is regulated both temporally and spatially, suggesting that this proteoglycan may play an important role in differentiation and neural-network formation in the mammalian retina.