Regulation of Na,K-ATPase Function in the Lens

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.

KCC isoforms in a human lens epithelial cell line (B3) and lens tissue extracts

We recently reported potassium-chloride cotransporter activity in human lens epithelial B3 (HLE-B3) cells. The purpose of the present study was to demonstrate in these cells as well as in human lens tissue the potassium-chloride cotransport (KCC) isoforms by reverse transcriptase-polymerase chain reaction (RT-PCR), Western blotting and immunofluorescence microscopy. Of the four KCC genes known to encode the respective proteins and their spliced variants, RT-PCR with both rat and human primers revealed the predicted cDNA fragments of KCC1, KCC3a, KCC3b, and KCC4 but not KCC2 in both HLE-B3 cells and in human lens tissue extracts from cataractous patients. Polyclonal rabbit (rb) anti-rat (rt) and anti-human (hm) antibodies against rtKCC1 and hmKCC3, respectively, and a commercially available rb-anti-mouse (ms) KCC4 antibody were used. Rb anti-rtKCC1-ECL3 [against epitopes within the large extracellular loop 3 (ECL3)] revealed a 150kDa band in HLE-B3 cells consistent with the known molecular weight of KCC1. Rb anti-hmKCC3-ECL3 yielded three bands of 150, 122 and 105kDa, evidence for the presence of KCC3a, KCC3b and possibly KCC3c isoforms. The 122 and 112kDa bands were also demonstrated by rb anti-hmKCC3-CTD [the C-terminal domain (CTD)]. Rb anti-msKCC4 antibody only showed a 100kDa band in HLE-B3 cells. In the human lens tissues, a 115kDa protein was detected with rb anti-rtKCC1-ECL3 and a 100kDa band with rb anti-msKCC4, however, no bands with rb anti-hmKCC3-ECL3 or rb anti-hmKCC3-CTD. Fluorescence microscopy revealed immunocytochemical cytoplasmic and membrane labeling of HLE-B3 cells with anti-KCC1, -KCC3 (laser confocal microscopy) and -KCC4 antibodies and a Cy3-tagged secondary antibody. Hence HLE-B3 cells expressed proteins of the KCC1, KCC3a, b, and KCC4 isoforms, whereas surgically removed cataractous lens tissue expressed only those of KCC1 and KCC4.

Regulation of potassium transport in human lens epithelial cells

The major K influx pathways and their response to thiol modification by N-ethylmaleimide (NEM) and protein kinase and phosphatase inhibitors were characterized in human lens epithelial B3 (HLE-B3) cells with Rb as K congener. Ouabain (0.1 mM) and bumetanide (5 microM) discriminated between the Na/K pump ( approximately 35% of total Rb influx) and Na-K-2Cl cotransport (NKCC) ( approximately 50%). Cl-replacement with nitrate or sulfamate revealed <10% residual [ouabain+bumetanide]-insensitive K-Cl cotransport (KCC). At 0.3-0.5 mM, NEM stimulated the Na/K pump by 2-fold independent of external Na, KCC between 2 and 4-fold, and abolished approximately 90% of NKCC. Calyculin-A, a serine/threonine protein phosphatase-1 inhibitor, did not affect NKCC but inhibited KCC, whereas 10 microM staurosporine, a serine/threonine kinase inhibitor, abolished NKCC, and stimulated KCC only when followed by NEM treatment. The tyrosine-kinase inhibitor genistein, at concentrations >100 microM, activated the Na/K pump and abolished NKCC but did not affect KCC. The data suggest at least partial inverse regulation of KCC and NKCC in HLE-B3 cells by signaling cascades involving serine, threonine and tyrosine phosphorylation/dephosphorylation equilibria.