Inhibition of glucocorticoid-induced changes of Na+, K+-ATPase in rat lens by a glucocorticoid receptor antagonist RU486

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. Na+, K+-ATPase has long been recognized for its role in regulating electrolyte concentration in the lens, contributing to lens transparency. We previously reported that the inactivation of Na+, K+-ATPase induced by a glucocorticoid in rat lens. Therefore, the question is whether the changes of Na+, K+-ATPase can be induced through the specific GR activation in glucocorticoid-induced cataract formation. Clear rat lenses were cultured in vitro and were treated with or without dexamethasone (Dex) or RU486 (a GR antagonist). The lenses were cultured for 7 days and photographed daily to record the development of opacity. The activity of Na+, K+-ATPase was determined by using spectrophotometric analysis. The mRNA and protein level expressions of Na+, K+-ATPase α1 were examined by reverse transcription polymerase chain reaction (RT-PCR), Western blot and immunohistochemistry analysis, respectively. Our findings are presented in this study and show that mist-like opacity of the lens was observed as early as 5 days after incubation with dexamethasone. The opacity was more obvious at day 7 in the Dex group. The lenses of the untreated group and the RU486+Dex group remained transparent throughout the incubation. The activity of Na+, K+-ATPase in the Dex-treated group decreased in a time-dependent manner. There was no significant loss of enzyme activity in either the control or the RU486+Dex group throughout the incubation period. Both the protein and mRNA expression levels of Na+, K+-ATPase α1 in the capsule-epithelium of lenses decreased in the Dex-treated group. The GR antagonist RU486 inhibited the decrease of the expression of Na+, K+-ATPase α1 induced by Dex. All of the above results suggested that the GR-mediated reduction of Na+, K+-ATPase may contribute to the formation of steroid-induced cataract. Intervention in this pathway maybe helpful to avoid glucocorticoids-cataract formation.