Uptake and inactivation of a-type prostanglandins by human red cells

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 two-step model of prostaglandin signal termination: in vitro reconstitution with the prostaglandin transporter and prostaglandin 15 dehydrogenase

Termination of prostaglandin (PG) signaling has been proposed to involve carrier-mediated uptake across the plasma membrane followed by cytoplasmic oxidation. Here, we tested this hypothesis directly by coexpressing the PG uptake carrier prostaglandin transporter (PGT) in various cell types with and without human PG 15 dehydrogenase (PG15DH). In HeLa cells, which express neither PGT nor PG15DH, exogenously added PGE2 or PGF2alpha were rapidly oxidized to the 13, 14-dihydro, 15-keto metabolites only when PGT and PG15DH were coexpressed, directly confirming the two-step hypothesis. Cells expressing PG15DH that were broken open formed more PG metabolites than cells in which the PGs could gain access to PG15DH only via PGT. Similar results were obtained using the human prostate cancer cell line LNCaP, in which endogenous PG15DH is induced after exposure to dihydrotestosterone. Because PGT in vivo is expressed in renal collecting duct epithelia, we also expressed PGT in Madin-Darby canine kidney cells grown on filters, where it mediated both the active uptake of PGE2 across the apical membrane and the transepithelial transport of PGE2 to the basolateral compartment. When PG15DH was coexpressed with PGT in these epithelial monolayers, about half of the PGE2 taken up apically was oxidized to 13, 14-dihydro, 15-keto-PGE2, which in turn exited the cells nondirectionally into both the apical and basolateral compartments. Our data represent reconstitution of the longstanding model of PG metabolism consisting of sequential carrier-mediated PG uptake, cytoplasmic oxidation, and diffusional efflux of the PG metabolite.

Production of prostaglandins E1 and E2 by adult human red blood cells

We showed that human adult red blood cells (RBCs) produce prostaglandin E1 (PGE1) and E2 (PGE2). RBCs that were mechanically stressed in the presence of extracellular Ca2+ by being injected rapidly through a fine needle produced PGE1 and PGE2 within 30 min after this mechanical stress. The amounts of PGE1 and PGE2 produced by 1 x 10(9) mechanically stressed RBCs were approximately 50 pg and 100 pg, respectively, which were determined in the cytosolic fraction from sonicated RBCs using a competitive enzyme immunoassay method. A Western blot analysis using anti-cyclooxygenase-2 antibody revealed a band at the 70-kDa position in the samples from RBCs producing PGE1 and PGE2. Treatment with 10 micrograms/mL indomethacin completely inhibited the productions of PGE1 and PGE2. The present results may indicate a new role of RBCs in microcirculation.

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.

Formation of a prostaglandin A2-glutathione conjugate in L1210 mouse leukemia cells

Prostaglandins containing a cyclopentenone moiety are potent antiviral and antigrowth compounds. Some evidence indicates that these prostaglandins are conjugated to glutathione by cells. However, the metabolism of one group, the prostaglandins of the A type, is unclear due to conflicting reports. We studied the uptake and metabolism of prostaglandin A2 (PGA2) in mouse L1210 leukemia and L929 fibroblast cell lines in which this prostaglandin has antiviral and antigrowth effects. Both cell types took up the PGA2 and then metabolized it to a more polar compound. Inside L1210 cells, PGA2 was initially conjugated to glutathione and then reduced at the 9-keto position to form 9-OH-PGA2-GSH. The 9-OH-PGA2-GSH was then secreted from the cells and apparently degraded to form the CysGly and Cys derivatives. Intracellular glutathione was decreased markedly by the addition of the PGA2 in L1210 and L929 cells. This result confirms that conjugation of PGA2 to glutathione occurs in both cell types. Formation of the 9-OH-PGA2-GSH and other glutathione-related conjugates was prevented when glutathione was depleted by growth in buthionine sulfoximine. The glutathione-depleted cells were insensitive to the cytotoxicity of the PGA2, suggesting that one of the glutathione-related conjugates may be involved in the cytotoxicity of PGA2. These results end the controversy over the metabolism of PGA2 and suggest mechanisms for its antiviral and antigrowth actions.

Synthesis of prostaglandin E2 and prostaglandin F2 alpha by toadfish red blood cells

Saline washed red blood cells of the toadfish convert [1-14C] arachidonic acid to products that cochromatograph with prostaglandin E2 and prostaglandin F2 alpha. This synthesis is inhibited by indomethacin (10 micrograms/ml). Conversion of arachidonic acid to prostaglandin E2 was confirmed by mass spectrometry. When saline washed toadfish red blood cells were incubated with a mixture of [1-14C]-arachidonic acid and [5,6,8,9,11,12,14,15,-3H]-arachidonic acid, comparison of the isotope ratios of the radioactive products indicated that prostaglandin F2 alpha was produced by reduction of prostaglandin E2. The capacity of toadfish red blood cells to reduce prostaglandin E2 to prostaglandin F2 alpha was confirmed by incubation of the cells with [1-14C] prostaglandin E2.

Stability of prostaglandin I2 in human blood

Prostaglandin I2 (PGI2) is hydrolyzed more slowly in human plasma than in buffer at the same pH; this stabilization appears to be due to binding of PGI2 to albumin. The stability of the antiaggregatory potency of PGI2 is similar in human blood and in plasma buffered to pH 7.55. No measurable conversion of [3H]-PGI2 to radioactive or nonradioactive metabolites occurred during incubation with blood.

Metabolism of prostaglandin E1 and of glutathione conjugate of prostaglandin A1 (GSH-prostaglandin A1) by prostaglandin 9-ketoreductase from rabbit kidney

Rabbit kidney prostaglandin 9-ketoreductase was found to metabolize the glutathione conjugate of prostaglandin A1 (GSH-prostaglandin A1). Apparent Km (GSH-prostaglandin A1) 13 microM and apparent Km (prostaglandin E1) 200 microM. The cytosolic preparation was subjected to gelfiltration and isoelectric focusing, which revealed that metabolism of prostaglandin E1 and GSH-prostaglandin A1 occurs by means of the same fractions. Furthermore, prostaglandin E1 and GSH-prostaglandin A1 are competitive inhibitors of the enzyme, when GSH-prostaglandin A1 and prostaglandin E1 are tested as substrates, respectively. It si concluded, that GSH-prostaglandin A1 is a much better substrate for prostaglandin 9-ketoreductase from rabbit kidney than is prostaglandin E1.