Identification of endothelin converting enzyme-1 in human non-pigmented ciliary epithelial cells

Proteomic analysis of underlying apoptosis mechanisms of human retinal pigment epithelial ARPE-19 cells in response to mechanical stretch

Our previous study demonstrated mechanical stretch (MS) could induce the apoptosis of retinal pigment epithelial (RPE) cells, but the related mechanisms remained unclear. This study was to characterize the protein expression profile in RPE cell line ARPE-19 exposed to MS, cytochalasin D (CD; an inhibitor of actin polymerization) or CD + MS at 2-time points (6, 24 hr; n = 3, at each time point) by using proteomics technique. Our data highlighted that compared with control, ECE1 was continuously downregulated in ARPE-19 cells treated by MS or CD + MS from 6 to 24 hr. Function and protein-protein interaction network analyses showed ATAD2 was downregulated in all three treatment groups compared with control, but successive upregulation of RPS13 and RPL7 and downregulation of AHSG were specifically induced by MS. ATAD2 was enriched in cell cycle; AHSG was associated with membrane organization; RPS13 and RPL7 participated in ribosome biogenesis. Furthermore, transcription factor CREB1 that was upregulated in MS group at 24 hr after treatment, may negatively regulate ATAD2. The expressions of all crucial proteins in ARPE-19 cells were confirmed by western blot analysis. Overexpression of ATAD2 and AHSG were also shown to reverse the apoptosis of ARPE-19 cells induced by MS or CD + MS, with significantly decreased apoptotic rates and caspase-3 activities. Accordingly, our findings suggest downregulation of ATAD2 and AHSG may be potential contributors to the apoptosis of RPE cells induced by MS. Overexpression of them may represent underlying preventive and therapeutic strategies for MS-induced retinal disorders.

Endothelin antagonism as an active principle for glaucoma therapy

Endothelin, the most potent vasoactive peptide known to date, has been suggested to play a potential role in the pathogenesis of open-angle glaucoma. Open-angle glaucoma is the most common optic nerve head neuropathy and is associated with a loss of retinal ganglion cells and visual field damage. Although an increased intraocular pressure is a major risk factor for glaucomatous optic neuropathy, other factors such as a reduced ocular blood flow play an important role for appearance of the disease. Thus, treatment of glaucoma is focused on lowering of intraocular pressure and preventing the occurrence or progression of glaucomatous optic neuropathy. Endothelin participates in the regulation of intraocular pressure by an effect on trabecular outflow, the main route for aqueous humour outflow from the eye. Trabecular outflow is modulated by trabecular meshwork contractility which is affected by endothelin. In addition to the effects of endothelin in the anterior part of the eye, the vasoconstrictor causes a decrease in ocular blood flow followed by pathological changes in the retina and the optic nerve head which is assumed to contribute to the degeneration of retinal ganglion cells. In sum, inhibition of endothelin signalling leads to lowering of intraocular pressure and exerts neuroprotective effects. Thus, endothelin antagonism in the eye represents a promising approach for pharmacological treatment of glaucoma.

Endothelin-1, nitric oxide, and glutamate in the normal and glaucomatous dog eye

PURPOSE

To document differences in the levels of the endothelin-1 peptide, nitric oxide, and glutamate in aqueous humor and vitreous in the dog eye with spontaneous glaucoma compared to the normal dog eye.

METHODS

Samples of aqueous humor and vitreous from enucleated normal eyes (n = 21) of 14 dogs and glaucomatous eyes (n = 8) of eight dogs were collected. Levels of endothelin-1, nitric oxide, and glutamate were measured by enzyme immunoassay.

RESULTS

Endothelin-1 aqueous humor levels (mean +/- SD) increased significantly from 3.05 (+/- 1.66) pg/mL for the normal eyes to 6.22 (+/- 2.83) pg/mL for the glaucomatous eyes (P = 0.0054). The increase in vitreous from 1.83 (+/- 1.66) pg/mL for the normal eyes to 2.86 (+/- 1.31) pg/mL for the glaucomatous eyes was not significant (P = 0.0840). Nitric oxide levels (mean +/- SD) increased significantly in aqueous humor from 4.12 (+/- 2.64) microM for the normal eyes to 12.95 (+/- 14.42) microM for the glaucomatous eyes (P = 0.0141). The vitreous levels increased from 4.86 (+/- 3.92) microM for the normal eyes to 15.33 (+/- 16.22) microM for the glaucomatous eyes (P = 0.0179). Glutamate levels (mean +/- SD) decreased nonsignificantly in aqueous humor from 2.35 (+/- 3.84) microM for the normal eyes to 1.61 (+/- 0.74) microM for the glaucomatous eyes (P = 0.9377) and in vitreous from 1.37 (+/- 1.89) microM for the normal eyes to 1.02 (+/- 1.11) microM for the glaucomatous eyes (P = 0.3303).

CONCLUSION

Endothelin-1 and nitric oxide increased in aqueous humor and vitreous of dogs with spontaneous glaucoma while the changes in glutamate varied.

Localization of endothelin-converting enzyme in bovine optic nerve and retina

A significant loss and remodeling of the lamina cribrosa tissue leading to the excavation of the optic nerve is seen in glaucoma. Elevated endothelin-1 (ET-1) levels are detected in the aqueous humor of patients of open-angle glaucoma and in the plasma of patients with normal- tension glaucoma. Optic nerve damage, including axonal loss, can be mimicked by ET-1 injection near the optic nerve. ET-1 is produced from its precursor Big ET-1 (38 amino acids) by endothelin-converting enzyme (ECE). Although ET-1 and its receptors have been identified in the retina, little is known of the distribution of ECE at the optic nerve. Presently, ET-1 receptors and Big ET-1 converting activities were characterized in bovine optic nerve and the retina. The ET(B) receptor was detected in both the optic nerve and retina by immunoblotting and cross-linking, using 125I-ET-1. However, the ET(A) receptor was detected only in the retina. Big ET-1 conversion activities were detected in the plasma membrane (PM) of bovine retina, but not in the PM of the optic nerve. The retinal PM Big ET-1 converting activity was inhibited by phosphoramidon, thiorphan, and acidification. Furthermore, ECE cytosolic activities were detected in both the optic nerve and retina. Unlike the PM-ECE, cytosolic Big ET-1 converting activities were activated by acidification (pH 6.4), suggesting the involvement of ECE-2-like activity and/or cathepsin activity. Pepstatin, a potent inhibitor of cathepsins, inhibited the optic nerve (ON) cytosolic conversion of Big ET-1 peptide by 50%, and the combination of pepstatin and phosphoramidon, a potent inhibitor of ECE, inhibited the ON cytosolic activity by 86%. By contrast, the combination of both inhibitors weakly inhibited the cytosolic retinal Big ET-1 converting activity. Western blotting revealed the presence of ECE-1 at the PM of the retina not the ON. ECE-2 and cathpesins B, D, and L were detected only in the cytosol of both the retina and ON. In summary, it appears that ET-1 could be produced in the retina and optic nerve by at least two ECE subtypes and, perhaps, cathepsins. Big ET-1 converting activity may be an important target in preventing ET-1-induced optic nerve pathology.

Characterization of Endothelin-Converting Enzyme Activities in ARPE-19 Cells, a Human Retinal Pigmented Epithelial Cell Line

Elevated endothelin-1 (ET-1) levels are detected in patients with glaucoma. ET-1 is produced from its precursor, Big ET-1, by endothelin-converting enzyme (ECE). Characterization of ET- 1 secretion and ECE activity was performed in ARPE-19 cells, a human retinal pigmented epithelial cell-line. The ET(B) receptor but not the ET(A) receptor was detected by immunoblotting and cross-linking using 125I-ET-1 at the plasma membrane (PM). Tumor necrosis factor-alpha (10 nmol/L) induced a 700% increase in ET-1 levels and such an effect was further potentiated by BQ788, an ET(B) receptor antagonist, suggesting the involvement of ET(B) receptor in ET-1 clearance. Big ET-1-converting activities were detected in both the PM and cytosol. Phosphoramidon, thiorphan, acidification, and phenanthroline inhibited PM ECE activity; the cytosolic ECE activity was not affected by phenanthroline but was inhibited by the others. In contrast, ECE cytosolic activities were activated by acidification (pH 6.4), suggesting the involvement of ECE-2 or cathepsin-like activity. Pepstatin, a potent inhibitor of cathepsins, and phosphoramidon, a potent inhibitor of ECE, inhibited the cytosolic conversion of Big ET-1 peptide by 46% and 35%, respectively, whereas the combination of both inhibited the cytosolic activity by 93%. Based on immunoblotting, ECE-1 was detected only at the PM, whereas ECE-2 and cathpesins B and D were detected in the cytosol. In summary, ET-1 production in RPE is regulated by at least two isoforms of ECE, (cytosolic and PM) as well as cathepsins.

Endothelin: is it a contributor to glaucoma pathophysiology?

Endothelin is a vasoactive peptide that has been shown to play an important role in vascular homeostasis. Recently, endothelin and its receptors have been found in ocular tissues where it appears to have a regulatory function. Endothelin is found in both the aqueous and vitreous humors and its concentration is elevated in glaucoma patients and in animal models of glaucoma. In the current review, the authors present information about the distribution of endothelin and endothelin receptors in the eye and the ocular actions of endothelins. Specifically, endothelin/aqueous humor dynamics, endothelin/nitric oxide interactions, endothelin and ischemia, and endothelin/optic nerve head effects. Observations concerning the potential role of endothelin in glaucoma pathophysiology is presented and discussed relative to its effects on the optic nerve head and in relation to glaucoma theories.

Endothelin 1 levels in the aqueous humor of dogs with glaucoma

PURPOSE

Endothelin 1 is a small peptide that is involved in regulation of intraocular pressure and modulation of ocular circulation. To investigate the role of endothelin 1 in canine glaucoma, the authors measured aqueous humor levels of endothelin 1 in healthy dogs and in dogs with hypertensive glaucoma.

METHODS

Aqueous humor samples were obtained with general anesthesia from the eyes of healthy dogs (n = 5) and dogs with hypertensive glaucoma (n = 10). Measurements were made by enzyme immunoassay for endothelin 1.

RESULTS

The endothelin 1 aqueous humor range was 1.12 - 3.63 pg/mL for healthy dogs and 1.97 - 14.97 pg/mL for glaucomatous dogs. The healthy and glaucomatous canine endothelin 1 aqueous levels (mean +/- SD) were 2.33 +/- 0.90 and 8.11 +/- 5.03 pg/mL, respectively. A two-way analysis of variance indicated that this difference was significant (P = 0.0084). The effect of age on endothelin 1 levels was not significant (P = 0.6283). The large variability found within the glaucomatous group could be explained by the degree of damage of the retina (P = 0.0006). There was no significant correlation between intraocular pressure and endothelin 1 aqueous humor levels within the glaucomatous group (P = 0.29).

CONCLUSIONS

The aqueous humor of dogs with hypertensive glaucoma contains significantly higher levels of endothelin 1 than that of healthy dogs.