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Mitochondrial Dysfunction & Antioxidants

Primary open angle glaucoma (POAG) is generally associated with elevations in intraocular pressure (IOP) caused by abnormal resistance of aqueous outflow through the trabecular meshwork, a specialized tissue lining the eye's outflow pathway.

A key suspect in the progression of POAG is local oxidative stress. Oxidative free radicals and reactive oxygen species (ROS) are reported to trigger degeneration in the trabecular meshwork, subsequently leading to increases in IOP and glaucoma. There is mounting evidence that ROS play a fundamental role in reducing local antioxidant activities in the region of the trabecular meshwork. The mitochondria are the main source of cellular ROS and adenosine triphosphate (ATP), and are key regulators of mechanisms controlling cell survival and death. A spectrum of mitochondrial abnormalities in patients with POAG has recently been reported.

The authors of a newly published paper (1), now provide evidence that mitochondrial dysfunction is a possible mechanism for the loss of trabecular meshwork cells in POAG. They also report that the antioxidants vitamin E and n-acetyl cysteine (NAC), as well as mitochondrial permeability transition (MPT) inhibitors, can reduce the progression of this condition.

Study Design and Methods

Trabecular meshwork from patients with POAG and age-matched subjects without disease were obtained by standard surgical trabeculectomy. Primary cultures of trabecular meshwork were treated with 3 different respiratory chain inhibitors specific for Complex I, II and III. The protective effect of vitamin E, NAC and cyclosporine A was examined by adding these antioxidants or the MPT inhibitor to the cells 30 minutes before treatment with the respiratory chain inhibitors. Mitochondrial function was determined by changes in mitochondrial membrane potential and ATP production with fluorescent probes and a luciferin/luciferase-based ATP assay, respectively. ROS levels were determined by H2-DCF-DA, and cell death was measured by lactate dehydrogenase activity.

Results

The trabecular meshwork cells of patients with POAG exhibited senescence and degeneration compared with those of controls. There was spontaneous generation of ROS, decreased mitochondrial membrane potential, decreased ATP production, and loss of cell viability in primary cell cultures of patients with POAG compared with those of control subjects. ROS generation was associated with dysfunction at the level of mitochondrial complex I.

It was also determined that vitamin E, NAC (fig. A below), and the MPT inhibitor cyclosporine A protected POAG trabecular meshwork cells from cytotoxicity by attenuating ROS production and cytochrome c release from the mitochondria and by inhibiting the mitochondrial permeability transition opening.

Comments

Taken together, the results support the hypothesis that a defect in mitochondrial complex I contributes to progressive loss of tabecular meshwork cells in patients with POAG by promoting excessive mitochondrial ROS production. The finding of a protective effect of vitamin E and NAC (a key component of glutathione) adds to the growing evidence that antioxidants are beneficial in POAG and are worthy of further investigation.

(A) Reduction of ROS production in GTM cells by antioxidants Vit E and NAC. Treatment with antioxidants vitamin E (Vit E; 200 µM) or N-acetylcysteine (NAC; 10 mM) induced a decrease in ROS production in GTM cells. Pretreatment with Vit E or NAC for 30 minutes also significantly reduced ROT-induced ROS production in GTM cells. Data are expressed as a fold change in fluorescence levels of GTM to NTM. Results are expressed as the mean SE of six separate experiments performed in triplicate. *Significant differences from untreated control at P = 0.05.

Reference

1. Yuan He, et al. Mitochondrial Complex I defect induces ROS release and degeneration in trabecular meshwork cells of POAG patients: Protection by antioxidants, Invest Ophthalmol Vis Sci 49:1447-58, 2008.