Effects of Thailanstatins on Glucocorticoid Response in Trabecular Meshwork and Steroid-Induced Glaucoma

PURPOSE. Elevated intraocular pressure (IOP) is a major risk factor in glaucoma. Various changes in the trabecular meshwork (TM) are responsible for elevated IOP. Glucocorticoids (GCs) increase IOP and mediate biochemical changes in the TM, similar to those associated with primary open-angle glaucoma (POAG). There are differences in steroid responsiveness among the population. Approximately 40% of individuals significantly elevate IOP (i.e., responders) upon GC administration, while others do not (i.e., nonresponders). The responders are at higher risk of developing POAG compared to the nonresponders. In addition, almost all POAG patients are steroid responders. GC responsiveness is regulated by the relative levels of the active GC receptor alpha (GR alpha) and the alternatively spliced dominant negative regulator isoform GR beta. Glaucomatous TM cell strains have a lower GR beta-GR alpha ratio compared to normal TM cells, making them more sensitive to GCs. Our purpose was to investigate the role of a special class of natural products called thailanstatins (TSTs) in GR alternative splicing and GC response in cultured human TM cells. METHODS. Quantitative RT-PCR and Western immunoblotting were used to study the effect of TSTs on GR beta-GR alpha ratios in human TM cell strains. Effects of TSTs on dexamethasone (DEX) responsiveness were assessed by GRE-luciferase reporter activity assay and fibronectin (FN) induction in TM cells. RESULTS. TSTs increased the GR beta-GR alpha ratio in TM cells. Increased GR beta-GR alpha ratios were associated with decreased DEX-mediated FN induction and GRE-luciferase activity. CONCLUSIONS. TSTs modulate the GR splicing process to enhance GR beta levels and thereby decrease the GC response in cultured human TM cells. These TSTs, or similar compounds, may potentially be new glaucoma therapeutic agents.