Stress–induced proteasome inhibition and cellular senescence in trabecular meshwork cells, and their potential roles in glaucoma pathophysiology

Abstract

Abstract: : Purpose: To investigate if chronic mechanical stress can produce proteasome inhibition and cellular senescence in trabecular meshwork (TM) cells; if direct chronic inhibition of the proteasome alone can trigger cellular senescence; if there is an increase of senescent cells in the TM over time; and if this increase is higher in glaucoma patients. Methods: Proteasome inhibition and markers for cellular senescence were measured in primary cultured pig TM cells after chronic mechanical stress (15% stretching/sec; 12 hours/day for four days), chronic proteasome inhibition (0.25 nM, 1 nM, and 4 nM Epoxomicin for four days), and in human primary cultured TM cells from three young (ages 9–25) and old (ages 66–73) donors at different passages. Presence of senescence associated beta–galactosidase (sa–beta–gal) staining cells was analyzed in histology sections from the trabecular meshwork of the eyes from 6 donors with glaucoma and 6 normal control eyes. Results: Chronic mechanical stress induced proteasome inhibition and expression of cellular senescence markers in TM cells, while direct chronic inhibition of the proteasome alone was enough to trigger a senescent phenotype. Primary cell cultures from older donors showed lower proteasome activity, lower replicative capacity and higher expression of senescent markers than those from younger donors. TMs from glaucoma donors showed, in average, a 5–fold increase in the number of sa–beta–gal positive cells and a higher overall sa–beta–gal intensity than those from normal controls. Conclusions: Different types of chronic stress affecting the TM result in inhibition of the proteasome, which in turn cause the acquisition of a senescent phenotype in the TM cells. The number of senescent cells in the TM appears to increase over time, and this increase may occur at a faster rate in glaucoma patients. The accumulation of proteasome–impaired/senescent cells in the TM might contribute to the pathophysiology of this tissue in glaucoma.

Keywords: aging • stress response • trabecular meshwork