A Renal-Like Organic Anion Transport System in the Ciliary Epithelium of the Eye
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The cornea, blood-retinal barrier and blood-aqueous humor barrier are a significant impediment to ocular drug delivery. These barriers are comprised of endothelial and/or epithelial tissues that act as a physical barrier to drug movement into the intraocular compartment from the outside. There is increasing evidence that these endothelial and epithelial tissues contain drug transporters, and that through active transport they play a functional role in ocular drug disposition as well. Early in vivo and in vitro studies showed that drugs containing a net negative charge at physiological pH (organic anions) are actively eliminated from the eye, with the ciliary body epithelium being a likely site of elimination. An initial screen of drug transporter gene expression by microarray showed that several transporters that contribute to the renal elimination of organic anions are also expressed in the human ciliary body. These included the organic anion transporter 1 (OAT1), organic anion transporter 3 (OAT3), the sodium dicarboxylate cotransporter 3 (NaDC3) and the multidrug resistance-associated protein 4 (MRP4). This led to the hypothesis that the ciliary body epithelium would transport organic anions in the aqueous humor-to-blood direction and that the aforementioned transporters would contribute. The purpose of this study was to determine the direction of organic anion transport across the ciliary body epithelium and the transport proteins that may contribute. Transport of several organic anions across the bovine ciliary body was examined using ciliary body sections mounted in Ussing chambers. RT-PCR, immunoblotting and immunohistochemistry were used to examine organic anion transporter expression in human ocular tissues. mRNA (RT-PCR) and protein (immunoblotting) for OAT1, OAT3, NaDC3 and MRP4 were detected in extracts of human ciliary body from several donors. OAT1 and OAT3 localized to basolateral membranes of non-pigmented epithelial cells and MRP4 to basolateral membranes of pigmented cells in human eye. Para-aminohippurate and estrone-3-sulfate transport across the bovine ciliary body in Ussing chambers was greater in the aqueous humor-to-blood direction than in the blood-to-aqueous humor direction, and active. In contrast, there was little net directional movement of cidofovir. Probenecid (0.1 mM) and novobiocin (0.1 mM) added to the aqueous humor side of the tissue abolished net active para-aminohippurate transport. These data indicate that the ciliary body expresses several organic anion transporters common to the kidney. These transporters are likely involved in clearing potentially harmful endobiotic and xenobiotic organic anions from the eye.