Cultured porcine trabecular meshwork cells display altered lysosomal function when subjected to chronic oxidative stress.

Glaucomas comprise a group of eye diseases that ultimately lead to optic nerve damage and consequent irreversible blindness. Although the specific cause of most forms of the disease has not yet been determined, aging of the trabecular meshwork (TM), the tissue responsible for maintaining proper levels of intraocular pressure (IOP), is suspected to be a major risk factor in the development or progression of the most common form of the disease, primary open-angle glaucoma (POAG).1 Aging is a complex phenomenon associated with a progressive accumulation of deleterious changes that results in a gradual decline in cellular and physiological function, as well as a diminished capacity to respond to stress, thus increasing the probability of degenerative disease.2 A free radical theory of aging has been proposed, connecting the oxidative environment and aging process.3 According to this theory, reactive oxygen species (ROS) resulting from normal aerobic metabolism are responsible for continuous macromolecular damage. Although most damaged biomolecules and organelles are successfully removed by the cellular proteolytic systems, the recycling machinery is inherently imperfect and is affected by the aging process. Thus, many types of cells and tissues accumulate oxidatively damaged material during aging and age-related disorders. A slow accumulation and age-associated accumulation of materials that cannot be degraded primarily occurs in long-lived postmitotic or terminally differentiated cells such as TM cells, leading to the build-up of considerable amounts of waste material.4,5 The validity of the free radical theory of aging has been extensively supported by numerous in vivo and in vitro studies showing that age-related changes accelerate under the influence of oxidative stress, whereas various antioxidants slow aging.6 – 8 Several groups have reported increased expression of oxidative markers, such as oxidative DNA damage and peroxidized lipids, as well as diminished antioxidant potential in the TM tissue of donors with glaucoma, supporting a key role of oxidative stress in the morphologic and physiological alterations observed in the outflow pathway tissue in aging and POAG.9 –11 In addition, we have recently demonstrated an increased number of cells with abnormal β-galactosidase activity at pH 6 (SA-β-gal) in the glaucomatous outflow pathway.12 Although the nature of such activity remains unknown, SA-β-gal has been proposed to be a manifestation of residual lysosomal β-galactosidase activity at a suboptimal pH that becomes detectable due to the increased lysosomal content in aged cells.13–15 The lysosomal compartment is the terminal degradative compartment responsible for the turnover of long-lived proteins and “worn out” organelles. Probably because of such disposal function, lysosomes are the major proteolytic system affected by age.16 –19 Alterations of lysosomal function have been implicated in the pathogenesis of several age-related disorders.20 –26 The purposes of this study were to evaluate hyperoxia as an in vitro model for studying the molecular mechanisms underlying aging of postmitotic TM cells and to examine the effect of chronic oxidative stress/aging in the TM lysosomal system. The data presented provide evidence of impaired lysosomal function as a result of the accumulation of biological debris in aged/oxidized TM cells.