The Mechanism Of The Glucocorticoid-induced Cataract Formation Through Glucocorticoid Receptor

PURPOSE: Cataract formation can be induced by prolonged use of glucocorticoids. The underlying mechanism is not fully understood yet. The presence of the functional glucocorticoid receptor (GR) in human and rat lens epithelial cells suggests that glucocorticoids target lens epithelial cells directly and specifically. Glucocorticoids exert their effects by binding to a specific intracellular receptor, the glucocorticoid receptor (GR), which acts as a ligand dependant transcription factor. The ligand-receptor complex dimerizes, translocates to the nucleus, and binds to a cis acting element, the glucocorticoid response element (GRE), to modulate the expression of target genes. The glucocorticoid–GR binding to a GRE located on that many target genes to directly modulate transcription, which indicate that a broad range of transcripts are altered. The main groupings of transcripts identified were involved in cell structure/extracellular matrix and adhesion, catalysis, transport protein activity, apoptosis, cell growth, and development and cell cycle. Na+, K+-ATPase has long been recognized for its role in regulating electrolyte concentration in the lens, contributing to lens transparency. Vimentin is an important cytoskeletal protein in the epithelial cells of the lens, which plays important roles in maintaining the normal morphology and function of the lens. It's interesting to know whether the changes of Na+, K+-ATPase and vimentin can be induced through the specific GR activation in glucocorticord-induced cataract formation in rat lens in vitro.METHODS: Rat clear lenses were cultured in vitro and were treated with or without dexamethasone (Dex) or RU486(a glucocorticoid receptor antagonist). The lenses were cultured for 7 days and photographed daily to record the development of opacity. The changes of morphology were examined by HE staining. The activity of Na+, K+-ATPase was determined by using spectrophotometric analysis. The mRNA and protein expression of Na+, K+-ATPaseα1 and vimentin were examined by RT-PCR, and Western blot analysis and immunohistochemistry were conducted, respectively.RESULTS: Mist-like opacity of the lenses was observed as early as 5 days after incubation with dexamethasone (P<0.05). The opacity was more obvious at day 7 in the Dex group (P<0.01). At day 7, HE staining showed an orderly arrangement of fiber cells in the control group and the RU486 group. However, in the Dex group, the arrangement of fiber cells was disrupted and the lenses exhibited expanded extracellular lacunae. The activity of Na+, K+-ATPase in the only Dex-treated group decreased in a time-dependent manner. There was no significant loss of enzyme activity in either the control or the RU486 group throughout the incubation peroid (P<0.001). Both the protein and mRNA expression level of Na+, K+-ATPaseα1 decreased in the Dex-treated group. The expression of vimentin protein decreased in the Dex-treated group but its expression of mRNA maintained nomal.CONCLUSIONS: These results suggest that the GR-mediated reduction of Na+, K+-ATPase and vimentin may contrubite to the formation of steriod-induced cataract.