Glucocorticoid (GC) administration has been associated with an increase in intraocular pressure (IOP), particularly in patients with primary open-angle glaucoma (POAG) where greater than 90% are considered glucocorticoid responders as compared to 33% of the general population. The reason for this difference in responsiveness among the population is not clear and the molecular mechanism for the hyper-responsiveness among glaucoma patients still remains unknown. It has been reported that the GC receptor beta (GRbeta), primarily located in the nucleus, functions as a dominant negative regulator of the transcriptional activity of GRalpha, possibly forming inactive heterodimers. Such an action is consistent with recent reports that elevated GRbeta is associated with glucocorticoid resistance in asthma patients. Preliminary observations in our laboratory shows that normal trabecular meshwork (TM) cell lines express relatively higher amounts of GRbeta than glaucomatous TM cell lines and that GRbeta is highly concentrated within the nucleus in these normal but not glaucomatous TM cell lines. This suggests a potential role of GRbeta in contributing to glucocorticoid responsiveness in TM cells. It is proposed that decreased nuclear amounts of GRbeta could result in the enhanced transcriptional activity of GRalpha and contribute to glucocorticoid hyper-responsiveness seen in glaucoma patients. Consistent with this hypothesis has been the finding that the peripheral vascular response to GCs is enhanced in patients with POAG. The overall goal of this proposal is to demonstrate that GRbeta is a key component in the regulation of the GC GRalpha response in ocular tissues and the relative expression of GRbeta is important for GC action. The hypothesis to test is that high expression of GRbeta in TM cells leads to GC resistance, and that low levels of GRbeta expression, as found in glaucomatous TM, leads to GC hyper-responsiveness and ocular hypertension.
Four aims are designed to address this hypothesis and include: 1) determine the mRNA, protein expression and subcellular distribution of GC receptors, GRalpha and GRbeta, in normal and glaucomatous TM cell lines and intact tissue; 2) determine if GRbeta expression and overexpression inhibits GC-induced gene expression in TM cells and the increase in GC-induced IOP in isolated perfusion cultured human anterior segments; does GRbeta knockout with siRNA induce hyper responsiveness in normal cells; 3) identify the trafficking mechanism responsible for moving GRbeta from the cytoplasm to the nucleus and determine the role trafficking may have on the accumulation of nuclear GRbeta in normal and glaucomatous TM cells; 4) determine whether polymorphisms in GRbeta splice sites or in the splicesome factor gene SRp30c are associated with glaucoma or with GC-induced ocular hypertension and if in these patients there is a generalized sensitivity to cutaneous vasoconstriction.
These specific aims are designed to determine if GRbeta regulates GC responsiveness and if decreases in its expression contribute to ocular hypertension often seen in patients with POAG.
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