Detection and regulation of cationic amino acid transporters in healthy and diseased ocular surface.

To evaluate the presence and role of human cationic amino acid transporters (hCATs) at the ocular surface in healthy and pathologic states and under experimental inflammatory conditions. Expression of mRNA for hCATs 1, 2, and 3 (SLC7A1, SLC7A2, and SLC7A3) was analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR) in healthy lacrimal gland, conjunctiva, cornea, and nasolacrimal ducts and in an SV40 immortalized human corneal epithelial (HCE) cell line. Localization of hCAT1 and hCAT2 was determined by immunohistochemistry in healthy tissues and in sections of different corneal abnormalities, including keratoconus, Fuchs dystrophy, and herpetic keratitis. Cultured corneal epithelial cells were stimulated with proinflammatory cytokines and supernatants of Staphylococcus aureus and Pseudomonas aeruginosa and were analyzed by real-time PCR. Expression of hCAT1 and hCAT2 mRNA, but not of hCAT3 mRNA, was detected in healthy conjunctiva, cornea, and nasolacrimal ducts. Human lacrimal gland revealed only hCAT2 mRNA expression. Immunohistochemistry demonstrated the presence of hCAT1 and hCAT2 in epithelial cells of cornea, conjunctiva, and nasolacrimal ducts. Goblet cells revealed no reactivity. Moreover, hCAT2 was visible in acinar cells of lacrimal gland. No changes in staining reactivity were obtained for hCAT1 in different corneal abnormalities. In contrast, hCAT2 showed increased subjective staining intensity in all corneal abnormalities. Cell culture experiments revealed that TNF-alpha and supernatant of S. aureus increased hCAT1 and hCAT2 expression significantly. Supernatant of P. aeruginosa led to an increase in hCAT2-expression only. These results show for the first time the distribution of hCATs in tissues of the ocular surface and lacrimal apparatus. Both transporters seem to be differently regulated under pathologic conditions of the ocular surface. Their physiological functions in amino acid transport make them potential candidates for therapeutic intervention.