Observations on the extraction of prostaglandins from blood

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.

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.

The distribution of tritium-labelled prostaglandin E(1) injected in amounts sufficient to produce central nervous effects in cats and chicks

1. After injection of tritiated prostaglandin E(1) into the lateral cerebral ventricle, the carotid and vertebral arteries and the aorta in cats, only a small proportion of the radioactivity was recovered from the central nervous system.2. Similar results were obtained when the prostaglandin was given intra-venously and intra-arterially to chicks.3. The results of this study confirm work in other species (Hansson & Samuelsson, 1965; Samuelsson, 1964, 1965) and suggest that prostaglandin E(1) exerts its central pharmacological actions in small concentrations.

Molecular mechanisms of prostaglandin transport

Despite the fact that prostaglandins (PGs) have low intrinsic permeabilities across the plasma membrane, they must cross it twice: first upon release from the cytosol into the blood, and again upon cellular uptake prior to oxidation. Until recently, there were no cloned carriers that transported PGs. PGT is a broadly-expressed, 12-membrane-spanning domain integral membrane protein. When heterologously expressed in HeLa cells or Xenopus oocytes, it catalyzes the rapid, specific, and high-affinity uptake of PGE2, PGF2 alpha, PGD2, 8-iso-PGF2 alpha, and thromboxane B2. Functional studies indicate that PGT transports its substrate as the charged anion. The PGT substrate specificity and inhibitor profile match remarkably well with earlier in situ studies on the metabolic clearance of PGs by rat lung. Because PGT expression is especially high in this tissue, it is likely that PGT mediates the membrane step in PG clearance by the pulmonary circulation. Evidence is presented that PGT may play additional roles in other tissues and that there may be additional PG transporters yet to be identified molecularly.

Characterization of leucocyte phagocytic stimulatory material released by activated human platelets

The action of material released by activated human platelets on the phagocytic activity of human neutrophils and monocytes has been examined under conditions where there was a low background level of exposure of the leucocytes to platelet release products ( PRPr ). Incubation with PRPr produced an approximate three-fold stimulation of particle ingestion by both cell types. Material with stimulatory activity passed through ultrafiltration membranes capable of retaining molecules greater than 500 Daltons. This low molecular weight stimulator was inactivated by ATP diphosphopyrolase to suggest it could be one or more of the nucleotide di- or triphosphates in PRPr . Comparison with the action of specific nucleotides revealed only ADP and ATP exerted equivalent stimulatory effects at concentrations consistent with those of nucleotides normally present in PRPr . This effect was mediated by induction of a change in leucocyte behaviour which was sustained in the absence of continued exposure to the stimulatory agent. Stimulation of phagocytosis was also produced by high molecular weight material in PRPr , but this appeared to be mediated by dissociation of low molecular weight stimulator from a high molecular weight complex. These observations extend the range of biological functions known to respond to nucleotides in PRPr , and have implications for the interpretation of leucocyte phagocytic activity assessed in vitro in view of the potential modification of this process by exposure of leucocytes to PRPr during the isolation of leucocytes from peripheral blood.

Prostaglandin E1 in normal human skin: methodological evaluation, topographical distribution and data related to sex and age

A methodological evaluation of a radioimmunoassay technique for PGE1 measurement was applied to normal human skin. The detection limit of the assay was 15 pg and recovery (mean +/- SD) was 90 +/- 6%. The mean value of PGE1 (+/- SEM) in nine punch biopsy specimens (4 mm in diameter) from a piece of skin surgically removed from one person was 117 +/- 14 pg/mg dry weight. The individual variation of PGE1 activity in biopsy specimens from the same topographical area in ten persons belonging to the same sex and age group was 33 +/- 4 pg/mg dry weight. The influence has been elucidated of temperature and tissue processing, local anaesthesia, sex, age and topographical distribution on endogenous PGE1 activity.

Prostaglandin-mediated inhibition of lymphokine secretion in normal individuals and patients with progressive systemic sclerosis (scleroderma, PSS)

The sensitivity of peripheral blood lymphocytes to E-type prostaglandin-mediated inhibition of lymphokine secretion was examined in 3 groups of individuals; normal controls, hospitalized patients, and patients with progressive systemic sclerosis (PSS, scleroderma). Leukocytes were stimulated by a polyclonal T-cell activator, phytohemagglutinin, and the release of the lymphokine, leukocyte migration inhibitory factor (LIF), was measured in the presence or absence of exogenous PGE2 using a direct agarose droplet migration inhibition technique. Leukocytes of scleroderma patients were found to be hyporesponsive to E-type prostaglandin (i.e., lymphokine secretion by these cells was not inhibited at concentrations of PGE2 between 2.8 X 10(-8) and 2.8 X 10(-5) M). In addition, a marked sex difference in PGE responsiveness was found to exist among normal controls, whereby females were hyporesponsive during the latter half of the menstrual cycle. It is possible that this deficit may facilitate, in part, the development of connective tissue diseases in women of childbearing age. The inability to suppress lymphokine production and arrest persistent immune reactivity, coupled with the known ability of lymphokines to augment fibroblast collagen production, offers a a reasonable explanation for the accumulation of tissue collagen in scleroderma.

Prostaglandin-like substances appearing in canine renal venous blood during renal ischemia. Their patial characterization by pharmacologic and chromatographic procedures

Renal prostaglandins (PC s ) might mediate an antihypertensive function of the kidney. The blood-superfused organ technique possesses the sensitivity (threshold < 0.4 ng/ml blood) and specificity required for identification of PGs in blood. Induction of unilateral renal ischemia in 14 chloraloseanesthetized dogs reduced renal blood flows from a mean value of 257 to 109 ml/min on the ischemic side and from 250 to 209 ml/min on the contralateral side. Concomitantly, PG-like substances were detected by assay organs in the venous blood of ischemic (13 experiments) and contralateral (11 experiments) kidneys. In one experiment, in a spontaneously hypertensive dog, PGs were not detected during renal ischemia.

Renal venous blood and renal medullary tissue were extracted for acidic lipids and assayed for PG-like substances. Extracts of venous blood collected during renal ischemia and extracts of renal medulla yielded substances with biological activity indistinguishable from PG-like substances or PG standards. Chromatographic characterization of PG-like substances suggests that they are predominantly a mixture of PGE 2 and PGF 2α .

Prostaglandin uptake and metabolism by the perfused rat liver

1. The prostaglandins are C20 unsaturated fatty acids which exhibit diverse physiological effects of short duration. We have investigated the speed of removal of PGE(1) and PGF(1alpha) from the circulating blood and their subsequent metabolism by the isolated perfused rat liver.2. Following either a single injection of radiolabelled PGE(1) or PGF(1alpha) into the hepatic artery or portal vein, or recirculation of prostaglandins through the liver for 2.5 h, the distribution of radioactivity within extracts of bile, blood and liver was determined. The nature of the radioactive products of meta-bolism was inferred by comparison of the distribution of radioactivity after injecting carbon and tritium labelled standards, and by thin-layer chromatography, gas-liquid chromatography, ultraviolet and bioassay analysis.3. A single injection of 1-(14)C PGE(1) indicated that the liver could efficiently remove 89-95% of circulating PGE(1) on a single passage. Biliary excretion was excluded as a major route for elimination of unchanged PGE(1), because only 0.3-0.8% of the injected radioactivity was detected in the bile. During recirculation of 1-(14)C PGE(1), 11-19% of the injected radioactivity was detected as exchanged (14)CO(2). The radioactivity detected within liver was identified with further fragments resulting from decarboxylation of PGE(1), which were incorporated into fatty acids and then phospholipids.4. Studies with 5,6-(3)H PGE(1), and comparison with the results obtained using 1-(14)C PGE(1), revealed a 30-fold increase in the percentage of radioactivity excreted into the bile, suggesting that biliary excretion may be a major route for elimination of compounds smaller than C20 prostaglandin. Evidence that the cyclopentane ring was intact was inferred by formation of a PGB compound on treatment with alkali; similar biliary excretion of 9-(3)H PGF(1alpha) also occurred. In addition, the increased radioactivity detected within the liver (37%) and blood (43%) after a single injection of 5,6-(3)H PGE(1) had the solvent partition and thin-layer chromatography properties of PGE(1), but were associated with a less polar compound smaller than the C20 parent structure.5. These results indicate rapid uptake of circulating prostaglandins by the rat liver. Decarboxylation of prostaglandins results in pharmacological inactivation. The products are excreted into the bile and venous effluent. These processes would curtail the duration of effects following prostaglandin injection.In addition, we infer from these results that any physiological action of these ubiquitous endogenous substances is likely to be localized within their tissue of origin.