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Regulation of L-Cystine Transport and Intracellular GSH Level by a Nitric Oxide Donor in Primary Cultured Rabbit Conjunctival Epithelial Cell Layers

¹From the Departments of Pharmaceutical Sciences, ²Medicine, ³Ophthalmology, ⁴Physiology and Biophysics, ⁵Biomedical Engineering, and ⁶Molecular Pharmacology and Toxicology and the ⁷Will Rogers Institute Pulmonary Research Center, Schools of Pharmacy, Medicine, and Engineering, University of Southern California, Los Angeles, California.

PURPOSE. Metabolism and transport of cysteine are critical for maintenance of the intracellular glutathione (GSH) level. In this study, transport mechanisms of L-cystine and regulation of GSH biosynthesis in the absence or presence of NO-induced oxidant stress were investigated in primary cultured rabbit conjunctival epithelial cells (RCECs).

METHODS. RCECs were grown in membrane filters to exhibit tight barrier properties. Uptake and transepithelial transport of L-cystine were determined in the presence or absence of extracellular Na⁺. Uptake was determined at 10 minutes after ¹⁴C-L-cystine instillation into apical (a) or basolateral (b) bathing fluid. The effect of nitric oxide (NO) on L-cystine uptake, cellular GSH level, and expression level of two subunits of the rate-limiting enzyme -glutamylcysteine synthetase (GCS) was examined after a 24-hour incubation of primary cultured RCECs with an NO donor, S-nitroso-N-acetylpenicillamine (SNAP; N-acetyl-3-(nitrosothio)-D-valine.

RESULTS. Cellular uptake of L-cystine by RCECs occurred through both Na⁺-dependent and -independent mechanisms. Uptake from apical fluid was higher than that from basolateral fluid, except for the highest concentration of L-cystine tested (200 µM). Transepithelial permeability (P_app) of L-cystine (at 2.5 µM) was three times higher in the a-to-b direction than in the b-to-a direction in the presence of Na⁺, whereas the reverse was true in the absence of Na⁺. Na⁺-dependent L-cystine uptake from apical fluid was significantly elevated in primary cultured RCECs treated for 24 hours with various concentrations (0.1–2.0 mM) of SNAP, with maximum uptake observed at 1 mM. A similar pattern of SNAP-induced increase of Na⁺-independent L-cystine uptake from apical fluid was observed, whereas no significant difference was observed for basolateral uptake. Concomitantly, a significant elevation of intracellular GSH (up to fivefold versus the control) was recorded, with the highest increase occurring at 0.1 to 0.25 mM SNAP. A parallel increase in the expression levels of both catalytic and regulatory subunits of GCS was observed by Western blot analysis of lysates from RCECs treated with 0.25 mM SNAP for 24 hours.

CONCLUSIONS. L-Cystine is transported by both Na⁺-dependent and -independent amino acid transport systems in RCECs. At low substrate concentrations, L-cystine uptake was higher from apical than basolateral fluid. Permeability studies indicated net absorption of L-cystine across RCECs. SNAP caused significant increases in both L-cystine uptake and intracellular GSH level, which occurred concomitantly with elevation of both catalytic and regulatory subunits of GCS. Understanding sulfur amino acid precursor-dependent cellular mechanisms of GSH homeostasis would be of value in devising GSH-based treatment for conjunctival or other ocular disorders.

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