J. Ferrell

OCULAR INFLAMMATORY EFFECTS OF INTRAVITREALLY INJECTED TUMOR NECROSIS FACTOR-ALPHA AND ENDOTOXIN

Intravitreal injection of human recombinant tumor necrosis factor-alpha (TNF) induced inflammation in the rabbit eye characterized by dilation of blood vessels in the iris, disruption of the blood-ocular barriers, infiltration of inflammatory cells into the anterior chamber, and accumulation of prostaglandin E in intraocular fluids. Inflammation first appeared on day 1, increased on day 2, and remained elevated on day 7. The inflammatory eell infiltrate in the anterior segment of the eye was largely monocytic on days 1 and 2; by day 7 large numbers of lymphocytes were also present. TNF-induced ocular inflammation therefore differed from that reported for intravitreally injected endotoxin in terms of time course and the types of inflammatory cells in the aqueous humor. In a series of experiments in which combinations of TNF and endotoxin were used, intravitreal injection of TNF, 24 h after a low dose ofEscherichia coli endotoxin, produced no more inflammation than that produced by TNF following an injection of endotoxin vehicle. However, if TNF was injected 24 h before endotoxin, the resulting inflammation was greater than that observed in animals given TNF followed by endotoxin vehicle.

Iron metabolism in the eye: A review

This review article covers all aspects of iron metabolism, which include studies of iron levels within the eye and the processes used to maintain normal levels of iron in ocular tissues. In addition, the involvement of iron in ocular pathology is explored. In each section there is a short introduction to a specific metabolic process responsible for iron homeostasis, which for the most part has been studied in non-ocular tissues. This is followed by a summary of our current knowledge of the process in ocular tissues.

Iron Regulates L-Cystine Uptake and Glutathione Levels in Lens Epithelial and Retinal Pigment Epithelial Cells by Its Effect on Cytosolic Aconitase

PURPOSE The authors previously published the novel finding that iron regulates L-glutamate synthesis and accumulation in the cell-conditioned medium (CCM) by increasing cytosolic aconitase activity in cultured lens epithelial cells (LECs), retinal pigment epithelial (RPE) cells, and neurons. The present study was designed to determine whether iron-induced L-glutamate accumulation in the CCM regulates L-cystine uptake and glutathione (GSH) levels through the aconitase pathway in LECs and RPE cells. METHODS The presence of xCT, the light chain of X(c)(-), a glutamate/cystine antiporter, was analyzed by RT-PCR, immunoblotting, and immunocytochemistry. Uptake of L-[(35)S]cystine and L-[(3)H]glutamate was measured in the presence or absence of transporter inhibitors. L-cystine uptake and intracellular GSH concentration were measured in the presence or absence of iron-saturated transferrin, the iron chelator dipyridyl (DP), or oxalomalic acid (OMA), an aconitase inhibitor. RESULTS LECs and RPE cells express xCT, as evidenced by RT-PCR analysis and immunoblotting. xCT was localized by immunocytochemistry. The authors found that the iron-induced increase in L-glutamate availability increased L-cystine uptake, with subsequent increases in GSH levels. In addition, L-glutamate production, L-cystine uptake, and GSH concentration were inhibited by OMA and DP, indicating a central role for iron-regulated aconitase activity in GSH synthesis in LECs and RPE cells. CONCLUSIONS These results demonstrate for the first time that iron regulates L-cystine uptake and the downstream production of GSH in two mammalian cell types. It is possible that the increase in intracellular antioxidant concentration induced by iron serves as a protective mechanism against the well-established capacity of iron to induce oxidative damage.

Dimethylthiourea inhibits the inflammatory response to intravitreally-injected endotoxin.

Dimethylthiourea, a potent scavenger of toxic oxygen metabolites such as the hydroxyl radical, hypochlorous acid, and hydrogen peroxide, was tested for its ability to inhibit an experimentally induced inflammatory response. Inflammation was induced in one eye of male New Zealand white rabbits by intravitreal injection of 10 ng Escherichia coli endotoxin; the contralateral eye received an equal volume of pyrogen-free saline vehicle. Dimethylthiourea was administered intraperitoneally to these animals at 0, 300, 450 and 600 mg kg-1. At 24 h post-endotoxin injection, all vehicle-injected eyes appeared normal with the exception of a small, but significant increase in aqueous humor protein concentration in the 600 mg kg-1 dimethylthiourea group. In endotoxin-injected eyes, treatment with dimethylthiourea, especially at the highest dose, significantly reduced iridal hyperemia, aqueous humor cell number and protein and prostaglandin-E concentrations, and the ex vivo release of prostaglandin-E from the lens. The ability of dimethylthiourea to significantly inhibit the inflammatory response to intravitreally-injected endotoxin suggests that toxic oxygen metabolites may play an important role in the initiation and/or propagation of this form of acute anterior uveitis. Furthermore, the data are consistent with an important interaction between toxic oxygen and arachidonic acid metabolites.

Altered Ferritin Subunit Composition: Change in Iron Metabolism in Lens Epithelial Cells and Downstream Effects on Glutathione Levels and VEGF Secretion

PURPOSE The iron storage protein ferritin is necessary for the safe storage of iron and for protection against the production of iron-catalyzed oxidative damage. Ferritin is composed of 24 subunits of two types: heavy (H) and light (L). The ratio of these subunits is tissue specific, and alteration of this ratio can have profound effects on iron storage and availability. In the present study, siRNA for each of the chains was used to alter the ferritin H:L chain ratio and to determine the effect of these changes on ferritin synthesis, iron metabolism, and downstream effects on iron-responsive pathways in canine lens epithelial cells. METHODS Primary cultures of canine lens epithelial cells were used. The cells were transfected with custom-made siRNA for canine ferritin H- and L-chains. De novo ferritin synthesis was determined by labeling newly synthesized ferritin chains with 35S-methionine, immunoprecipitation, and separation by SDS-PAGE. Iron uptake into cells and incorporation into ferritin was measured by incubating the cells with 59Fe-labeled transferrin. Western blot analysis was used to determine the presence of transferrin receptor, and ELISA was used to determine total ferritin concentration. Ferritin localization in the cells was determined by immunofluorescence labeling. VEGF, glutathione secretion levels, and cystine uptake were measured. RESULTS FHsiRNA decreased ferritin H-chain synthesis, but doubled ferritin L-chain synthesis. FLsiRNA decreased both ferritin H- and L-chain synthesis. The degradation of ferritin H-chain was blocked by both siRNAs, whereas only FHsiRNA blocked the degradation of ferritin L-chain, which caused significant accumulation of ferritin L-chain in the cells. This excess ferritin L-chain was found in inclusion bodies, some of which were co-localized with lysosomes. Iron storage in ferritin was greatly reduced by FHsiRNA, resulting in increased iron availability, as noted by a decrease in transferrin receptor levels and iron uptake from transferrin. Increased iron availability also increased cystine uptake and glutathione concentration and decreased nuclear translocation of hypoxia-inducible factor 1-alpha and vascular endothelial growth factor (VEGF) accumulation in the cell-conditioned medium. CONCLUSIONS Most of the effects of altering the ferritin H:L ratio with the specific siRNAs were due to changes in the availability of iron in a labile pool. They caused significant changes in iron uptake and storage, the rate of ferritin synthesis and degradation, the secretion of VEGF, and the levels of glutathione in cultured lens epithelial cells. These profound effects clearly demonstrate that maintenance of a specific H:L ratio is part of a basic cellular homeostatic mechanism.

Ceruloplasmin alters intracellular iron regulated proteins and pathways: Ferritin, transferrin receptor, glutamate and hypoxia-inducible factor-1α

Ceruloplasmin (Cp) is a ferroxidase important to the regulation of both systemic and intracellular iron levels. Cp has a critical role in iron metabolism in the brain and retina as shown in patients with aceruloplasminemia and in Cp-/-hep-/y mice where iron accumulates and neural and retinal degeneration ensue. We have previously shown that cultured lens epithelial cells (LEC) secrete Cp. The purpose of the current study was to determine if cultured retinal pigmented epithelial cells (RPE) also secrete Cp. In addition, the effects of exogenously added Cp on iron regulated proteins and pathways, ferritin, transferrin receptor, glutamate secretion and levels of hypoxia-inducible factor-1α in the nucleus were determined. Like LEC, RPE secrete Cp. Cp was found diffusely distributed within both cultured LEC and RPE, but the cell membranes had more intense staining. Exogenously added Cp caused an increase in ferritin levels in both cell types and increased secretion of glutamate. The Cp-induced increase in glutamate secretion was inhibited by both the aconitase inhibitor oxalomalic acid as well as iron chelators. As predicted by the canonical view of the iron regulatory protein (IRP) as the predominant controller of cellular iron status these results indicate that there is an increase in available iron (called the labile iron pool (LIP)) in the cytoplasm. However, both transferrin receptor (TfR) and nuclear levels of HIF-1α were increased and these results point to a decrease in available iron. Such confounding results have been found in other systems and indicate that there is a much more complex regulation of intracellularly available iron (LIP) and its downstream effects on cell metabolism. Importantly, the Cp increased production and secretion of the neurotransmitter, glutamate, is a substantive finding of clinical relevance because of the neural and retinal degeneration found in aceruloplasminemia patients. This finding and Cp-induced nuclear translocation of the hypoxia-inducible factor-1 (HIF1) subunit HIF-1α adds novel information to the list of critical pathways impacted by Cp.

Mediators of the ocular inflammatory response to interleukin-1β plus tumor necrosis factor-α

Abstract• Background: Intravitreal injection of marginally inflammatory doses of interleukin-1β and tumor necrosis factor-α (IL-1 β/TNFα) has been shown to produce intraocular inflammation distinctly different from that induced by higher intravitreal doses of either IL-1 or TNFα. Since cyclooxygenase inhibitors and platelet-activating factor (PAF)-receptor antagonists can reduce IL-1- or TNFα-induced uveitis, the present investigation was undertaken to determine whether cyclooxygenase metabolites of arachidonic acid and PAF are important mediators of IL-1β/TNFα-induced uveitis.• Methods: The cyclooxygenase inhibitor indomethacin and two structurally dissimilar PAF-receptor antagonists, SRI 63-441 and WEB 2086, were used to investigate the importance of cyclooxygenase metabolites and PAF in IL-1β/TNFα-induced uveitis.• Results: Based upon the effectiveness of indomethacin, the anterior uveitis induced by IL-1β/TNFα could be divided into two phases; a primary phase dependent upon generation of cyclooxygenase metabolites (the first 24 h) and a secondary phase largely independent of cyclooxygenase metabolite production (24–48 h). Posterior uveitis was also apparent at 48 h and was reduced by indomethacin. SRI 63-441 reduced the anterior uveitis at 24 h and to a lesser extent at 48 h; it also reduced the posterior uveitis at 48 h. However, although WEB 2086 was as effective as SRI 63-441 in reducing PAF-induced platelet aggregation, ex vivo, it did not significantly reduce IL-1β/TNFα-induced uveitis.• Conclusions: Although the findings do not support an important role for PAF in TNFa/IL-1β-induced uveitis, it cannot be ruled out that more intensive treatment with a specific and long-acting PAF-receptor antagonist might yield more positive results.