Studies on regulation of the ascorbic acid transporter in a cell line derived from rabbit non-pigmented ciliary epithelium

A cell line was derived from rabbit non-pigmented ciliary epithelium. The non-pigmented ciliary epithelium is one of the two cell layers which secrete aqueous humor into the eye and concentrate ascorbic acid in the newly-formed fluid. The cultured non-pigmented epithelial cells accumulated ascorbic acid at a rate of 3-5 pmol/μg protein per h. As in freshly-isolated native tissue, the ascorbate uptake mechanism was sodium-dependent and could be inhibited by phloretin (apparent K_i = 2-10^-5 M). Phorbol 12,13-dibutyrate (PDBu), a protein kinase C activator, reduced the ascorbate uptake rate. The PDBu effect was concentration-dependent; at a concentration of 10^-6 M, PDBu reduced the ascorbate uptake rate to 65% of the control value. PDBu reduced the maximal rate of ascorbate uptake (determined at 200-500 μM external ascorbate) but caused no detectable change in the K_m for ascorbic acid (approx. 80 μM). The PDBu-induced inhibition of ascorbate uptake persisted in the presence of ouabain and in low sodium (25 mM Na) medium, suggesting that the effect is not secondary to a change in the sodium gradient. Furthermore, no detectable elevation of cell sodium content was seen in cells equilibrated with ²²Na prior to PDBu treatment. The PDBu-induced inhibition of ascorbate uptake was apparently mediated by protein kinase C because the effect was not observed in the presence of staurosporine (10^-6M), a protein kinase C inhibitor, or in cells in which protein kinase C was downregulated. These observations suggest that activation of protein kinase C causes inhibition of the ascorbate transporter in this cultured cell line.