The prostaglandin transporter is widely expressed in ocular tissues

Expression and Function of Organic Anion Transporting Polypeptides in the Human Brain: Physiological and Pharmacological Implications

The central nervous system (CNS) is an important pharmacological target, but it is very effectively protected by the blood–brain barrier (BBB), thereby impairing the efficacy of many potential active compounds as they are unable to cross this barrier. Among others, membranous efflux transporters like P-Glycoprotein are involved in the integrity of this barrier. In addition to these, however, uptake transporters have also been found to selectively uptake certain compounds into the CNS. These transporters are localized in the BBB as well as in neurons or in the choroid plexus. Among them, from a pharmacological point of view, representatives of the organic anion transporting polypeptides (OATPs) are of particular interest, as they mediate the cellular entry of a variety of different pharmaceutical compounds. Thus, OATPs in the BBB potentially offer the possibility of CNS targeting approaches. For these purposes, a profound understanding of the expression and localization of these transporters is crucial. This review therefore summarizes the current state of knowledge of the expression and localization of OATPs in the CNS, gives an overview of their possible physiological role, and outlines their possible pharmacological relevance using selected examples.

Recent advances in studies of SLCO2A1 as a key regulator of the delivery of prostaglandins to their sites of action

Solute carrier organic anion transporter family member 2A1 (SLCO2A1, also known as PGT, OATP2A1, PHOAR2, or SLC21A2) is a plasma membrane transporter consisting of 12 transmembrane domains. It is ubiquitously expressed in tissues, and mediates the membrane transport of prostaglandins (PGs, mainly PGE₂, PGF_2α, PGD₂) and thromboxanes (e.g., TxB₂). SLCO2A1-mediated transport is electrogenic and is facilitated by an outwardly directed gradient of lactate. PGs imported by SLCO2A1 are rapidly oxidized by cytoplasmic 15-hydroxyprostaglandin dehydrogenase (15-PGDH, encoded by HPGD). Accumulated evidence suggests that SLCO2A1 plays critical roles in many physiological processes in mammals, and it is considered a potential pharmacological target for diabetic foot ulcer treatment, antipyresis, and non-hormonal contraception. Furthermore, whole-exome analyses suggest that recessive inheritance of SLCO2A1 mutations is associated with two refractory diseases, primary hypertrophic osteoarthropathy (PHO) and chronic enteropathy associated with SLCO2A1 (CEAS). Intriguingly, SLCO2A1 is also a key component of the Maxi-Cl channel, which regulates fluxes of inorganic and organic anions, including ATP. Further study of the bimodal function of SLCO2A1 as a transporter and ion channel is expected to throw new light on the complex pathology of human diseases. Here, we review and summarize recent information on the molecular functions of SLCO2A1, and we discuss its pathophysiological significance.

Alternative Drug Delivery for Patients Undergoing Cataract Surgery as Demonstrated in a Canine Model

PURPOSE

(1) To determine ketorolac concentrations in selected ocular tissues following the intracameral administration of phenylephrine and ketorolac injection 1%/0.3% (OMIDRIA^®) delivered in irrigation solution during lens replacement surgery in beagle dogs. (2) To compare the ketorolac initial dose and resultant concentrations from the above study to those achieved in aqueous and vitreous by topical administration in patients undergoing cataract surgery or vitrectomy, respectively.

METHODS

Lens replacement surgery with phacoemulsification was performed in 20 female beagle dogs. A fixed combination of phenylephrine and ketorolac injection 1%/0.3% was diluted 125-fold into the balanced salt solution and delivered intracamerally during the phacoemulsification procedure. Ketorolac concentration was determined by liquid chromatography/mass spectrometry.

RESULTS

Concentrations of ketorolac when administered by the intracameral route in the dosing solution in dogs were found to be considerably higher in both aqueous and vitreous compared to what is achieved with topical dosing in patients.

CONCLUSIONS

Adequate therapeutic concentrations of ketorolac in aqueous and vitreous humor were achieved even at 10 h postdose. Critical concentrations in the aqueous that envelopes the iris/ciliary body, which are sites of prostaglandin E₂ synthesis, and the vitreous are not achieved by topical dosing in clinical studies after the surgery, but are by direct intracameral dosing as determined in this study. Based on these studies and clinical data, phenylephrine and ketorolac injection 1%/0.3% delivered during surgery as an irrigation solution may preclude the need for topically administered pre- and postoperative NSAIDs.

Roles of Organic Anion Transporting Polypeptide 2A1 (OATP2A1/SLCO2A1) in Regulating the Pathophysiological Actions of Prostaglandins

Solute carrier organic anion transporter family member 2A1 (OATP2A1, encoded by the SLCO2A1 gene), which was initially identified as prostaglandin transporter (PGT), is expressed ubiquitously in tissues and mediates the distribution of prostanoids, such as PGE₂, PGF_2α, PGD₂ and TxB₂. It is well known to play a key role in the metabolic clearance of prostaglandins, which are taken up into the cell by OATP2A1 and then oxidatively inactivated by 15-ketoprostaglandin dehydrogenase (encoded by HPGD); indeed, OATP2A1-mediated uptake is the rate-limiting step of PGE₂ catabolism. Consequently, since OATP2A1 activity is required for termination of prostaglandin signaling via prostanoid receptors, its inhibition can enhance such signaling. On the other hand, OATP2A1 can also function as an organic anion exchanger, mediating efflux of prostaglandins in exchange for import of anions such as lactate, and in this context, it plays a role in the release of newly synthesized prostaglandins from cells. These different functions likely operate in different compartments within the cell. OATP2A1 is reported to function at cytoplasmic vesicle/organelle membranes. As a regulator of the levels of physiologically active prostaglandins, OATP2A1 is implicated in diverse physiological and pathophysiological processes in many organs. Recently, whole exome analysis has revealed that recessive mutations in SLCO2A1 cause refractory diseases in humans, including primary hypertrophic osteoarthropathy (PHO) and chronic non-specific ulcers in small intestine (CNSU). Here, we review and summarize recent information on the molecular functions of OATP2A1 and on its physiological and pathological significance.

Emerging role of advanced glycation-end products (AGEs) in the pathobiology of eye diseases

Advanced glycation end products (AGEs) have been implicated in vision loss associated with macula degeneration, cataract formation, diabetic retinopathy and glaucoma. This pathogenic potential is mainly attributed to their accumulation in ocular tissues where they mediate aberrant crosslinking of extracellular matrix proteins and disruption of endothelial junctional complexes that affects cell permeability, mediates angiogenesis and breakdown of the inner blood-retinal barrier. Furthermore, AGEs severely affect cellular metabolism by disrupting ATP production, enhancing oxidative stress and modulating gene expression of anti-angiogenic and anti-inflammatory genes. Elucidation of AGE-induced mechanisms of action in different eye compartments will help in the understanding of the complex cellular and molecular processes associated with eye diseases. Several pharmaceutical agents with anti-glycating and anti-oxidant properties as well as AGE crosslink 'breakers' have been currently applied to eye diseases. The role of diet and the beneficial effects of certain nutriceuticals provide an alternative way to manage chronic visual disorders that affect the quality of life of millions of people.

Localization of organic anion transporting polypeptides in the rat and human ciliary body epithelium

PURPOSE

To identify and localize the expression of multispecific organic anion transporting polypeptides (Oatps/OATPs) in the ciliary body epithelium and to investigate their possible involvement in the transport of the antiglaucoma agent unoprostone.

METHODS

Oatps/OATPs were detected by immunoblot analysis and by immunofluorescence microscopy in homogenized and fixed rat and human ciliary body samples using specific polyclonal antibodies. Transport of 3H-labelled unoprostone was measured in Oatp/OATP expressing Xenopus laevis oocytes.

RESULTS

Immunoblots of ciliary body extracts were positive for rat Oatp1a4, Oatp1a5 and Oatp1b2 and for human OATP1A2, OATP1C1, OATP2B1, OATP3A1 and OATP4A1. Confocal immunofluorescence microscopy localized Oatp1a4 and Oatp1b2 as well as all immunoblot positive human OATPs at the basolateral plasma membrane of the non-pigmented rat and human ciliary body epithelium, respectively. However, for human OATPs additional regional differences in expression were found with OATP1A2 and OATP1C1 being expressed only in the pars plana of human ciliary body epithelium. Furthermore, OATP1C1, OATP3A1 and OATP4A1 were also expressed at the basolateral plasma membrane of the pars plana pigmented epithelium. And finally, deesterified unoprostone carboxylate was found to be transported by OATP1A2, OATP2B1 and OATP4A1 with approximate K(m)-values of 93, 91 and 132 microm, respectively.

CONCLUSIONS

Several multispecific organic anion transporting polypeptides are expressed at the basolateral plasma membrane of the non-pigmented, and to a lesser extent also of the pigmented, epithelium in rat and human ciliary body. These Oatps/OATPs can account for the previously suggested 'liver-like' transport functions of mammalian ciliary body epithelium.

Ocular and systemic pharmacokinetics of latanoprost in humans

The ocular pharmacokinetics of latanoprost (13,14-dihydro-17-phenyl-18, 19,20-trinor-PGF(2alpha)-isopropyl ester; Xalatan [Pharmacia-Upjohn, Peapack, NJ]) was studied in patients undergoing cataract surgery using radio-immunoassay, and the systemic pharmacokinetics of latanoprost was studied in healthy human volunteers with 3H-latanoprost as well as radioimmunoassay. After topical application, latanoprost was rapidly hydrolysed in the cornea and blood. The maximum concentration of the active drug, latanoprost acid, was detected in the aqueous humor 1-2 hours after topical administration of the clinical dose and amounted to 15-30 ng/ml. The half-life of latanoprost acid in the aqueous humor was 2-3 hours. In the systemic circulation the peak concentration of latanoprost acid appeared 5 minutes after topical application and reached a level of 53 pg/ml with an elimination half-life of 17 minutes. In patients that had been on the drug continuously for more than 1 year, 5 out of 10 had plasma levels of latanoprost acid below the limit of detection (<30 pg/ml). The mean plasma clearance was 0.40 +/- 0.04 l/h. kg, and the volume of distribution was 0.16 +/- 0.02 l/kg after intravenous administration. The corresponding figures after ocular administration were 0.88 l/h. kg, and 0.36 l/kg. The majority of the radioactivity was recovered in urine (88%) and the rest was found in feces. In the eye the main metabolism of latanoprost was the ester hydrolysis. The only prominent chromatographic peak in plasma corresponded to latanoprost acid. In urine no latanoprost or latanoprost acid was detected. Before excretion latanoprost acid was beta oxidized to 1,2-dinor and 1,2,3,4-tetranor latanoprost acid. These metabolites accounted for approximately 66% of the radioactivity in urine. In conclusion, latanoprost is rapidly hydrolyzed in the eye and blood to latanoprost acid. Minimal further metabolism occurs in the eye, but latanoprost acid undergoes beta oxidation and other metabolism outside the eye. After topical application the peak concentration in aqueous humor was approximately 10(-7) M, whereas that in plasma was about 10(-10) M or less.

Prostaglandin transport

Newly synthesized prostaglandins (PGs) efflux from cells by simple diffusion, driven by pH and the membrane potential. Metabolic clearance requires energy-dependent uptake across the plasma membrane, followed by cytoplasmic oxidation. Several PG carriers have been cloned and characterized. PGT is broadly expressed in cyclooxygenase (COX)-positive cells, appears to be a lactate/PG exchanger, and is coordinately regulated with COX. By analogy with neurotransmitter release and re-uptake, PGT may regulate pericellular PG levels via re-uptake. PGT may also direct PGs towards and/or away from specific sets of PG receptors. Other members of the OATP transporter family also catalyze PG uptake; these are variably expressed and have variable affinities for PGs. The OATs are alpha-ketoglutarate/organic anion exchangers that accept PGs; these probably represent the uptake step in renal and hepatic PG degradation and excretion. Finally, certain glutathione-conjugated leukotrienes and PGs are actively extruded from cells by the MRPs; these may also play a role in metabolic clearance of PGs.

Comparison between isopropyl unoprostone and latanoprost by prostaglandin E(2)induction, affinity to prostaglandin transporter, and intraocular metabolism

The pharmacological differences between isopropyl unoprostone (referred to as unoprostone) and latanoprost, concerning their induction of endogenous prostaglandin E(2)(PGE(2)) and affinity to a human prostaglandin transporter (PGT), were investigated. Freshly dissected bovine iris tissues were incubated with major intraocular metabolites of unoprostone, M1 and M2, acid of latanoprost, or PGF(2 alpha), and PGE(2)induction was measured. Affinities of M1, M2, latanoprost, acid of latanoprost, and PGF(2 alpha)to PGT molecule were measured using PGT-cDNA transfected HeLa cells by an isotopic influx assay.(3)H-unoprostone was incubated with freshly prepared serum, aqueous humor, or frozen stored fetal bovine serum (FBS), and the radioactivity of supernatants was measured to investigate their metabolism of(3)H-unoprostone.M2, acid of latanoprost, and PGF(2 alpha)significantly increased a release of PGE(2)compared with the control. 10 microM indomethacin completely inhibited PGE(2)induction by acid of latanoprost and PGF(2 alpha), while 100 microM indomethacin was required to inhibit PGE(2)induction completely by M1 and M2. Unoprostone, M1, M2, and latanoprost showed little affinity to PGT, while acid of latanoprost had an affinity to PGT. Freshly prepared serum and aqueous humor metabolized unoprostone, but frozen stored FBS did not. The release of endogenous PGE(2)may play an important role of action by means of PG analogs, and differences in indomethacin-related inhibition of PGE(2)release and in affinities to PGT may in part cause their different actions.

Constitutive expression and localization of COX-1 and COX-2 in rabbit iris and ciliary body

Prostaglandins are involved in the regulation of intraocular pressure and the blood-aqueous barrier of the eye, and are used for the treatment of glaucoma. The decrease of the constitutively expressed PG-synthesizing enzyme cyclooxygenase-2 (COX-2) has been demonstrated in the ciliary non-pigmented epithelial layer of patients with primary open angle glaucoma. Little is known about the distribution of COX-1 and COX-2 in animals. We investigated this in the iris and ciliary body of the normal rabbit eye. The presence of COX-1 and COX-2 in freshly excised iris and ciliary body tissue from adult New Zealand White albino rabbits was demonstrated by real-time RT-PCR, and Western blot analysis. The localization of both isoforms and of the neuron-specific protein gene product 9.5 was determined by indirect immunofluorescence. Both enzymes are expressed in the iris and the ciliary body. Immunofluorescence studies including double staining techniques localized COX-1 and COX-2 to about 50% of cells in the stromal tissue of iris and ciliary body, mainly on the corneal side. They were co-localized in about 75% of these cells. Whereas all stained cells were positive for COX-1, COX-2 showed a gradient-like distribution in the stroma, with some restriction of expression near the epithelial layers, which we clearly showed to be completely negative for both COX-1 and COX-2. Also, neuronal elements did not show COX-1 or COX-2 immunoreactivity. These results establish the presence of COX-1 and COX-2 on the RNA and protein levels in normal, unstimulated rabbit iris and ciliary body. The pattern of distribution suggests a role for both enzymes in maintaining the physiology of the eye. In contrast to our results in man, non-pigmented epithelial cells of the ciliary body did not express immunoreactivity. This could account for differences in the regulation of intraocular pressure and/or blood-aqueous barrier between human and rabbit eyes.

Synthetic modification of prostaglandin f(2alpha) indicates different structural determinants for binding to the prostaglandin F receptor versus the prostaglandin transporter

Several principles governing the binding of E series prostaglandins to EP receptors have emerged in recent years. The C-1 carboxyl group binds electrostatically to a conserved arginine residue in the seventh transmembrane segment of the receptor. Prostaglandin E analogs involving bioisosteric replacements of the carboxyl group, such as acylsulfonamide, are also active. In addition, structurally similar esters may also exhibit similar affinity, presumably by virtue of hydrogen bonding. Other regions of the substrate molecule appear to bind to other domains of EP receptors, either via hydrophobic interactions or by hydrogen bonding. Less information is available about the structural requirements for substrate binding to FP receptors. Prostanoids also bind to the prostaglandin transporter PGT. In this case, a conserved C-1 carboxyl group is critically important, since C-1 esters exhibit little affinity. Here we examined the binding of chemically diverse PGF(2alpha) structural analogs to the FP receptor and compared these with binding by the PG transporter. PGT recognized a wide range of anionic substituents. In contrast, the carboxylic acid group was essential for optimal binding to the FP receptor, since replacement by larger moieties with a similar pK(a), such as acylsulfonamide and tetrazole, substantially decreased binding affinity. Interestingly, insertion of cyclic substituents in the omega chain increased binding to the FP receptor but reduced affinity for PGT, and substitution for the 15-hydroxyl group produced only a modest reduction in FP receptor binding, but eliminated binding by PGT. Because extracellular PGF(2alpha) may compete for binding between FP receptors and PGT, these findings have implications for designing PGF(2alpha) analogs for treating disease states.

Microvascular effects of selective prostaglandin analogues in the eye with special reference to latanoprost and glaucoma treatment

Prostaglandin F(2alpha) analogues have recently been introduced on the market for glaucoma treatment. While these drugs have a well-documented intraocular pressure reducing effect only a limited number of studies have been published regarding their effects on the microvasculature in the eye. Since many naturally occurring prostaglandins have marked effects on the cardiovascular system it is conceivable that synthetic prostaglandins used as glaucoma drugs may exert microvascular effects in the eye, even if they exhibit receptor selectivity. Latanoprost, the active principle of Xalatan((R)) eye drops, is a selective FP prostanoid receptor agonist, and much of the paper is focused on the microvascular effects of latanoprost and some closely related prostaglandin analogues. The purpose of the paper is to review the literature on the microvascular effects of prostaglandins in the eye, and to present some unpublished data on the effects of selective prostaglandin analogues. Most of the prostaglandin analogues studied exhibit selectivity for the FP prostanoid receptor. Results from studies with the following prostaglandin analogues are presented in the paper: PGF(2alpha)-isopropyl ester (PGF(2alpha)-IE), 17-phenyl-18,19,20-trinor-PGF(2alpha)-isopropyl ester (17-phenyl-PGF(2a)-IE), 15-keto-17-phenyl-18,19, 20-trinor-PGF(2alpha)-isopropyl ester (15-keto-17-phenyl-PGF(2a)-IE), 13,14-dihydro-17-phenyl-18,19,20-trinor-PGF(2alpha)-isopropy l ester (latanoprost), 13,14-dihydro-15R,S-17-phenyl-18,19, 20-trinor-PGF(2alpha)-isopropyl ester (PhXA34), 17-phenyl-18,19, 20-trinor-PGE(2)-isopropyl ester (17-phenyl-PGE(2)-IE), and 19R-hydroxy-PGE(2) (19R-OH-PGE(2)). The regional blood flow has been determined with radioactively labelled microspheres, the blood volume with (51)Cr labelled erythrocytes and the capillary permeability to albumin with (125)I and (131)I labelled albumin. PGF(2alpha)-IE has been shown to exert marked microvascular effects in the rabbit anterior segment including vasodilatation, increased capillary permeability, and a breakdown of the blood-aqueous barrier. 17-phenyl-PGF(2alpha)-IE, 15-keto-17-phenyl-PGF(2alpha)-IE, and PhXA34/latanoprost exerted significantly less vasodilatory effect, and little effect on capillary permeability was seen with the FP receptor agonists when studied with Evans blue. Intravenous administration of PhXA34 at a dose range of 1-100 microg/kg b.w. had no consistent effect on the regional blood flow in the eye indicating that FP receptors in the ocular blood vessels are not expressed in the rabbit, or alternatively are not functionally coupled to regulation of vascular tone. In cats topical application of PGF(2alpha)-IE had no significant effect the on the regional blood flow in cannulated eyes. No blood flow experiments were performed in intact eyes with PGF(2alpha)-IE. 17-phenyl-PGF(2alpha)-IE and latanoprost caused some vasodilation in the anterior segment. None of the analogues had any significant effect on the blood volume in the ocular tissues, but an increase in capillary permeability to albumin was seen in several tissues of the eye. However, in the eyelid, nictitating membrane and conjunctiva exposed to high concentrations of the prostaglandins no or only little leakage of albumin was detected. It appears that the intraocular microvasculature in the cat exhibits some sensitivity to FP prostanoid receptor agonists. (ABSTRACT TRUNCATED)

Identification of PKD2L, a human PKD2-related gene: tissue-specific expression and mapping to chromosome 10q25

Mutations in PKD2 cause autosomal dominant kidney disease (ADPKD). Polycystin-2, the PKD2 gene product, is an integral membrane glycoprotein of unknown function. We have identified PKD2L, another member of the PKD2 gene family. PKD2L is expressed in adult heart and skeletal muscle, brain, spleen, testis, and retina, and alternative transcripts of 2.4, 2.7, and 3.0 kb are seen. PKD2L shows 56% identity and 76% similarity with polycystin-2 over a 581-amino-acid span; however, the COOH-terminal 65 residues of PKD2L are unrelated to PKD2. PKD2L is localized to chromosome 10q25 and is excluded as a candidate gene for autosomal recessive polycystic kidney disease, autosomal dominant polycystic liver disease, and the third form of ADPKD. Given the high degree of homology between PKD2L and PKD2, it is likely that the respective functions of these proteins are also closely related.

Molecular cloning of the gene for the human prostaglandin transporter hPGT: gene organization, promoter activity, and chromosomal localization

Prostaglandins (PGs) play diverse and important roles in human health and disease. We recently identified the first known PG transporter cDNA in the rat (PGT) and human (hPGT). To aid in the analysis of any possible human disease caused by mutations in PGT, we have cloned and characterized the hPGT gene. The gene exists as a single copy in the human genome and is comprised of 14 exons distributed over approximately 95 kb. Two introns disrupt putative trans-membrane spans of the coding region; each of these sites is near a highly conserved charged residue. The approximately 250 bp immediately 5' to the start of exon 1 contain a TATAAA sequence (TATA box), a transcription initiation (Inr) consensus (CTCANTCT), two Sp 1 sequences (GGGCGG), and a cAMP response element (CGGCGTCA). Ligation of approximately 3.5 kb of 5' flanking sequence to a luciferase reporter yielded > 15-fold activity above background when expressed in A549 human lung epithelial cells. PCR-based monochromosomal somatic cell hybrid mapping and fluorescence in situ hybridization localized hPGT to chromosome 3q21. Three microsatellites were identified, one of which was demonstrated to be polymorphic in unrelated individuals and may be useful in evaluating PGT as a candidate gene in human disease.

Prostaglandins: a new approach to glaucoma management with a new, intriguing side effect

This introductory overview considers the advantages of a class of local hormones-the prostaglandins (PGs)-for the management of intraocular pressure (IOP) in glaucoma, over agonists and antagonists of neurotransmitters that dominated this field in the 20th century. PGs and PG analogues, in particular esterified prodrug forms of PGF2 alpha, are effective ocular hypotensive agents, but cause some conjunctival hyperemia and corneal sensory irritation at higher concentrations. Based on structure-activity studies, a 17-phenyl PGF2 alpha prodrug, latanoprost (PhXA41), was found to have a greatly improved therapeutic index, without compromising the ocular hypotensive potency of PGF2 alpha prodrugs. The IOP lowering mechanism of such PGF2 alpha s, increased uveoscleral outflow, can be expected to have great physiologic advantages, especially with respect to normal tension glaucoma, over most currently used ocular hypotensive drugs. The introduction of this new approach has already led to a new insight into the control and clinical significance of this outflow route. Similarly, the newly discovered ocular side effect, PG-induced increase in iridial pigmentation, can be expected to provide insight into the oculo-protective role of iridial melanocytes and into the punative association between a decline in the ocular melanin system and the vulnerability of the eye to some age-related diseases.