Prostaglandin transporter (OATP2A1/SLCO2A1) contributes to local disposition of eicosapentaenoic acid-derived PGE3

N-glycosylation modifies prostaglandin E2 uptake by reducing cell surface expression of SLCO2A1

SLCO2A1 functions as a prostaglandin (PG) influx transporter to facilitate intracellular oxidation of PGs and its defect causes dysregulation of PG signaling and metabolism. This study aimed to clarify effects of N-glycosylation on functional SLCO2A1 expression. Putative N-glycosylation site(s) (N134, N478, and/or N491) of human SLCO2A1 were mutated to Q and wild-type (WT) and mutant forms were expressed in HEK293 and human epithelial cells. Molecular weight of WT decreased to nearly 55 kDa by PNGase F treatment and was identical to that of triple mutant (TM, i.e., N134Q/N478Q/N491Q). Transport affinity of TM for PGE₂ (K_m of 392.7 nM) was comparable to that of WT (K_m of 328.5 nM); however, immunoassays showed that TM cell surface expression remained at 24% of WT in HEK293 cells, resulting in a reduced cellular PGE₂ uptake. These results suggest N-glycosylation modifies cellular PGE₂ uptake by decreasing SLCO2A1 localization to the plasma membrane.

Functional analysis of mutant SLCO2A1 transporters found in patients with chronic enteropathy associated with SLCO2A1

BACKGROUND AND AIM

Chronic enteropathy associated with the solute carrier organic anion transporter family member 2A1 (SLCO2A1), or CEAS, causes anemia and hypoalbuminemia in young people. Dysfunction of the SLCO2A1 transporter protein is thought to involve genetic mutation, but mutant proteins have not been functionally characterized. We examined the prostaglandin E₂ (PGE₂ ) transport ability of recombinant SLCO2A1 proteins containing 11 SLCO2A1 mutations found in CEAS patients.

METHODS

Wild-type and mutant SLCO2A1 proteins were forcibly expressed in Xenopus laevis oocytes, and measurements of PGE₂ uptake and transport capacity were compared. The membrane protein topology and functionality of the eight SLCO2A1 mutations involving single-nucleotide substitutions were predicted using computer analysis.

RESULTS

The extent of functional disruption of the 11 SLCO2A1 mutations identified in CEAS patients was variable, with 10 mutations (421GT, 547GA, 664GA, 770GA, 830dupT, 830delT, 940 + 1GA, 1372GT, 1647GT, and 1807CT) resulting in loss or reduction of PGE₂ transport, excluding 97GC.

CONCLUSION

PGE₂ transport ability of recombinant SLCO2A1 in X. laevis oocytes was hindered in 10/11 SLCO2A1 mutations identified in patients with CEAS. Further studies on the relationships between the different mutations and PGE₂ transport and clinical features, such as severity, are needed.

Increased/Targeted Brain (Pro)Drug Delivery via Utilization of Solute Carriers (SLCs)

Membrane transporters have a crucial role in compounds’ brain drug delivery. They allow not only the penetration of a wide variety of different compounds to cross the endothelial cells of the blood–brain barrier (BBB), but also the accumulation of them into the brain parenchymal cells. Solute carriers (SLCs), with nearly 500 family members, are the largest group of membrane transporters. Unfortunately, not all SLCs are fully characterized and used in rational drug design. However, if the structural features for transporter interactions (binding and translocation) are known, a prodrug approach can be utilized to temporarily change the pharmacokinetics and brain delivery properties of almost any compound. In this review, main transporter subtypes that are participating in brain drug disposition or have been used to improve brain drug delivery across the BBB via the prodrug approach, are introduced. Moreover, the ability of selected transporters to be utilized in intrabrain drug delivery is discussed. Thus, this comprehensive review will give insights into the methods, such as computational drug design, that should be utilized more effectively to understand the detailed transport mechanisms. Moreover, factors, such as transporter expression modulation pathways in diseases that should be taken into account in rational (pro)drug development, are considered to achieve successful clinical applications in the future.

Assessment of hepatic prostaglandin E2 level in carbamazepine induced liver injury

Objective. Carbamazepine (CBZ), a widely used antiepileptic drug, is one major cause of the idiosyncratic liver injury along with immune reactions. Conversely, prostaglandin E₂ (PGE2) demonstrates a hepatoprotective effect by regulating immune reactions and promoting liver repair in various types of liver injury. However, the amount of hepatic PGE₂ during CBZ-induced liver injury remains elusive. In this study, we aimed to evaluate the hepatic PGE₂ levels during CBZ-induced liver injury using a mouse model. Methods. Mice were orally administered with CBZ at a dose of 400 mg/kg for 4 days, and 800 mg/kg on the 5th day. Results. Plasma alanine transaminase (ALT) level increased in some of mice 24 h after the last CBZ administration. Although median value of hepatic PGE₂ amount in the CBZ-treated mice showed same extent as vehicle-treated control mice, it exhibited significant elevated level in mice with severe liver injury presented by a plasma ALT level >1000 IU/L. According to these results, mice had a plasma ALT level >1000 IU/L were defined as responders and the others as non-responders in this study. Even though, the hepatic PGE₂ levels increased in responders, the hepatic expression and enzyme activity related to PGE₂ production were not upregulated when compared with vehicle-treated control mice. However, the hepatic 15-hydroxyprostaglandin dehydrogenase (15-PGDH) expression and activity decreased significantly in responders when compared with control mice. Conclusions. These results indicate that elevated hepatic PGE₂ levels can be attributed to the downregulation of 15-PGDH expression under CBZ-induced liver injury.

Engineered nasal cartilage for the repair of osteoarthritic knee cartilage defects

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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.

Network pharmacology combined with metabolomics approach to investigate the protective role and detoxification mechanism of Yunnan Baiyao formulation

BACKGROUND

Yunnan Baiyao (YNBY) is a traditional Chinese medicine formulae, which has the functions of hemostasis, activating blood circulation and removing blood stasis, anti-inflammation, etc. Although the presence of Caowu (CW, Aconiti Kusnezoffii Radix), the detoxification mechanism of YNBY is still unclear.

PURPOSE

In current study, network pharmacology, toxicological methods and metabolomics technique were applied to explore YNBY in attenuating toxicity of CW.

METHODS

Prediction of targets and pathways of CW were carried out by commonly used network pharmacological method. Simultaneously, SD rats were orally administrated with CW, processed CW (ZCW), YNBY, and YNBY which lack of CW (QCW) for 15 days. Tissue samples were observed with histopathology. Urine samples were analyzed with ultra-performance liquid chromatography-mass spectrometry to screen differential metabolites and related metabolic pathways associated with toxicity of CW. Furthermore, by comparing the changes of the metabolite contents, focused the attenuated metabolic pathway. Finally, the network pharmacological and experimental data were integrated to investigate detoxification mechanism of YNBY.

RESULTS

A total of 44 potential toxicity biomarkers were identified and 14 related pathways were involved in the toxicity of CW. Furthermore, 5 core toxicity biomarkers (2-keto-6-acetamidocaproate, γ-glutamylleucine, prostaglandin E₃, 4-hydroxy-5-(3'-hydroxyphenyl)-valeric acid-3'-O-sulphate, and 3,4-dihydroxy- phenylglycol O-sulfate) were regulated to normal condition in YNBY group. Lysine degradation was locked as the core metabolic pathway of detoxification of YNBY. Integrating the predicted results of network pharmacology, ACHE, SLC6A3, SLC6A4 might be the target of protective role of other herbs in YNBY.

CONCLUSION

Network pharmacology combined with metabolomics exhibited a powerful mean to investigate the herbal toxicity and probed into the detoxification mechanism of formulae, which contributes to its safety evaluation.

Toxicological implication of prostaglandin transporter SLCO2A1 inhibition by cigarette smoke in exacerbation of lung inflammation

We reported that bleomycin (BLM)-induced pulmonary fibrosis was exacerbated in the prostaglandin transporter gene (Slco2a1)-deficient mice (Slco2a1(-/-)). Because cigarette smoke (CS) contributes to creating a profibrotic milieu in the respiratory region, the present study aimed to investigate the impact of CS on SLCO2A1-associated pathogenesis in the lungs of BLM-instilled mice. Bronchoalveolar lavage (BAL) fluid cell analysis indicated more severe inflammation in Slco2a1(-/-) on day 5 after BLM intratracheal instillation, and Slco2a1 deletion increased mRNA expression of pro-inflammatory cytokines (Tnf-α and Il-1β) and chemokine (Ccl5) in BAL cells. Male Slco2a1(-/-) exhibited significantly higher amounts of released Il-1β in BAL fluid, compared with female Slco2a1(-/-), male or female Slco2a1(+/+) group. The amount of PGE₂ collected in BAL fluid tended to increase in Slco2a1(-/-) compared with Slco2a1(+/+) group, whereas the PGE₂ concentrations in lung tissues were comparable between both groups. Besides, PGE₂ accumulated more in BAL fluid of male than that of female mice. Therefore, Slco2a1-deficient male mice were found to be more susceptible to BLM-treatment. Moreover, CS extracts (CSE) significantly reduced initial PGE₂ uptake by rat type1 alveolar epithelial cell-like (AT1-L) cells and human SLCO2A1-transfected cells. Exposure of AT1-L cells to CSE resulted in decreased mRNA expression of Slco2a1, suggesting that CS modulates SLCO2A1 function. These results indicate that exacerbated lung inflammation is attributed to an increase in Il-1β peptide and PGE₂ accumulation in the alveolar space, which exhibits a male predominance. SLCO2A1 inhibition by CSE is considered to be a new rationale for the lung toxicity of CS.

UHPLC-MS-Based Metabolomics Analysis Reveals the Process of Schistosomiasis in Mice

Metabolomics, as an emerging technology, has been demonstrated to be a very powerful tool in the study of the host metabolic responses to infections by parasites. Schistosomiasis is a parasitic infection caused by schistosoma worm via the direct contact with the water containing cercaria, among which Schistosoma japonicum (S. japonicum) is endemic in Asia. In order to characterize the schistosome-induced changes in the host metabolism and further to develop the strategy for early diagnosis of schistosomiasis, we performed comprehensive LC-MS-based metabolomics analysis of serum from mice infected by S. japonicum for 5 weeks. With the developed diagnosis strategy based on our metabolomics data, we were able to successfully detect schistosomiasis at the first week post-infection, which was 3 weeks earlier than "gold standard" methods and 2 weeks earlier than the methods based on 1H NMR spectroscopy. Our metabolomics study revealed that S. japonicum infection induced the metabolic changes involved in a variety of metabolic pathways including amino acid metabolism, DNA and RNA biosynthesis, phospholipid metabolism, depression of energy metabolism, glucose uptake and metabolism, and disruption of gut microbiota metabolism. In addition, we identified seventeen specific metabolites whose down-regulated profiles were closely correlated with the time-course of schistosomiasis progression and can also be used as an indicator for the worm-burdens. Interestingly, the decrease of these seventeen metabolites was particularly remarkable at the first week post-infection. Thus, our findings on mechanisms of host-parasite interaction during the disease process pave the way for the development of an early diagnosis tool and provide more insightful understandings of the potential metabolic process associated with schistosomiasis in mice. Furthermore, the diagnosis strategy developed in this work is cost-effective and is superior to other currently used diagnosis methods.

Effects of single and repetitive valproic acid administration on the gene expression of placental transporters in pregnant rats: An analysis by gestational period

The use of valproic acid (VPA), an antiepileptic drug, during pregnancy, is known to increase various fetal risks. Since VPA has been known to inhibit histone deacetylases (HDACs); its administration could alter gene transcription levels. However, in vivo effects of VPA administration on placental transporters have not been fully elucidated. The purpose of the present study was to comprehensively evaluate the effects of single and repetitive VPA administration on the expression of placental transporters and analyze them by gestational day. We investigated 18 transporters (8 ATP-binding cassette (ABC) and 10 solute carrier (SLC) transporters) in the placentas of pregnant rats that were orally administered 400 mg/kg/day VPA for one or four days, during mid- or late gestation. In the control rats, 4 ABC transporter genes (Abcb1a, 1b, Abcc2, Abcc4) were upregulated, 3 (Abcc3, Abcc5, Abcg2) downregulated through gestation, whereas 1 (Abcc1) was not changed. Regarding SLC transporters, 6 genes (Slc7a5, Slc16a3, Slc22a3, Slc22a4, Slco2b1, Slco4a1) were increased, 1 (Slc29a1) decreased through gestation, whereas 3 (Slc7a8, Slc22a5, Slco2a1) showed no significant change. Single VPA administration altered the expression of 9 transporters and repetitive administration, 13 transporters. In particular, VPA remarkably decreased Abcc4 and Slc22a4 in late gestation and increased Abcc5 during mid-gestation. Our findings indicated that VPA administration changed transporter expression levels in rat placenta, and suggested that sensitivity to VPA differs across gestational stages.

The regulatory mechanism involved in the prostaglandin E2 disposition in carbon tetrachloride-induced liver injury

Prostaglandin E₂ (PGE₂) exhibits hepatoprotective effects against various types of liver injury. However, there is little information on the disposition of endogenous PGE₂ during liver injury. In the present study, we attempted to elucidate the mechanism involved in regulating PGE₂ distribution during liver injury. Carbon tetrachloride (CCl₄) was used to establish a liver injury mouse model. PGE₂ was measured by LC-MS/MS. The plasma and hepatic PGE₂ levels were significantly increased at 6 to 48 h after CCl₄ treatment. The ratio of plasma levels of 13,14-dihydro-15-ketoPGE₂ (PGEM), a major PGE₂ metabolite, to PGE₂ decreased significantly after CCl₄ treatment. PGE₂ synthesis and expression of enzymes related to PGE₂ production were not induced, while the activity and mRNA expression of 15-prostaglandin dehydrogenase (15-PGDH/Hpgd), a major enzyme for PGE₂ inactivation, decreased significantly in the liver of CCl₄-treated mice compared to that of vehicle-treated control. The plasma and hepatic PGE₂ levels were negatively correlated with the hepatic mRNA expression levels of Hpgd. Although the mRNA expression of organic anion transporting polypeptide 2A1 (OATP2A1/Slco2a1), a major PGE₂ transporter, was upregulated, other hepatic OATPs decreased significantly at 24 h after CCl₄ treatment. Immunohistochemical analysis indicated that 15-PGDH was mainly expressed in endothelial cells and that OATP2A1 was expressed at least in endothelial cells and Kupffer cells in the liver. These results suggest that the decreased 15-PGDH expression in hepatic endothelial cells is the principal mechanism for the increase in hepatic and plasma PGE₂ levels due to the CCl₄-induced liver injury.

Experimental Evidence for Resecretion of PGE2 across Rat Alveolar Epithelium by OATP2A1/SLCO2A1-Mediated Transcellular Transport

Prostaglandin transporter Oatp2a1/Slco2a1 is expressed at the apical (AP) membranes of type-1 alveolar epithelial (AT1) cells. To investigate the role of OATP2A1 in prostaglandin E2 (PGE2) handling by alveolar epithelium, we studied PGE2 transport across and secretion from monolayers of rat AT1-like (AT1-L) cells obtained by trans-differentiation of type-2 alveolar epithelial cells isolated from male Wistar rats. Rat AT1-L cells expressed Oatp2a1/Slco2a1, together with smaller amounts of Mrp4/Abcc4 and Oct1/Slc22a1 PGE2 uptake was saturable with Km 43.9 ± 21.9 nM. Transcellular transport of PGE2 across AT1-L cells grown on permeable filters in the AP-to-basolateral (BL) direction was 5-fold greater than that in the reverse direction and was saturable with Km 118 ± 26.8 nM; it was significantly inhibited by OATP inhibitors bromosulfophthalein (BSP) and suramin, and an MRP4 inhibitor, Ceefourin 1. We simultaneously monitored the effects of BSP on the distribution of PGE2 produced by bradykinin-treated AT1-L cells and PGE2-d4 externally added on the AP side of the cells. In the presence of BSP, PGE2 increased more rapidly on the AP side, whereas PGE2-d4 decreased more slowly on the AP side. The decrease in PGE2-d4 from the AP side corresponded well to the increase on the BL side, indicating that intracellular metabolism did not occur. These results suggest that Oatp2a1 and Mrp4 mediate transepithelial transport of PGE2 in the AP-to-BL direction. Therefore, OATP2A1 may be an important regulator of PGE2 in alveolar epithelium by reducing secretion of PGE2 and facilitating "resecretion" of PGE2 present in the alveolar lumen to the interstitial space or blood.

Quantification of membrane transporter proteins in human lung and immortalized cell lines using targeted quantitative proteomic analysis by isotope dilution nanoLC-MS/MS

Information is needed on the expression of transporters in lung to inform drug development and therapeutic decisions. Much of the information currently available is from semiquantitative gene expression or immunometric densitometry studies reported in the literature. NanoLC-MS/MS (MRM mode) isotope dilution targeted quantitative proteomics was used here to quantify twelve selected transporters in fresh human lung membrane fraction samples and in the membrane fraction of selected immortalized human lung epithelial cell line samples. Fractionation was undertaken by homogenization in crude membrane lysis buffer followed by differential centrifugation of the homogenate. In lung membranes we found OATPs to be the most highly expressed transporters of those measured, followed by PEPT2 and ABCs (P-gp & BCRP). SLC22A transporters (OCTs 2 & 3 and OCTN1) were also found to be expressed. OATP2A1, also known as the prostaglandin transporter, was the most highly expressed transporter, being low in two subjects who were at least occasional smokers. One subject, a non-smoker, had an OATP2A1 concentration that was 8.4 times higher than the next nearest concentration, which itself was higher than the concentration of any other transporter. OATP2A1 is known, from gene expression and animal functional studies, to be present in lung. These results inform the understanding of xenobiotic disposition in the lung and show the distinct profile of transporters in lung compared to other tissues.

Prostaglandin Transporter OATP2A1/SLCO2A1 Is Essential for Body Temperature Regulation during Fever

Prostaglandin E₂ (PGE₂) in the hypothalamus is a principal mediator of the febrile response. However, the role of organic anion transporting polypeptide 2A1 (OATP2A1/SLCO2A1), a prostaglandin transporter, in facilitating this response is unknown. Here, we investigated the effect of Slco2a1 deficiency on the body core temperature (Tc) and on the PGE₂ concentration in hypothalamus interstitial fluid (C_isf) and CSF (C_csf) of lipopolysaccharide (LPS; 100 μg/kg, i.p.)-treated mice of both sexes. Slco2a1^-/- mice did not develop a febrile response. C_csf was increased in Slco2a1^+/+ and Slco2a1^-/- mice, and C_csf of Slco2a1^-/- mice was well maintained at 5 h after LPS injection (1160 pg/ml) compared with Slco2a1^+/+ mice (316 pg/ml). A microdialysis study revealed that C_isf peaked at 2 h after LPS injection in Slco2a1^+/+ mice (841 pg/ml), whereas the increase in C_isf was negligible in Slco2a1^-/- mice. The PGE₂ plasma concentration in Slco2a1^-/- mice (201 pg/ml) was significantly higher than that in Slco2a1^+/+ mice (54 pg/ml) at 1 h after LPS injection, whereas the two groups showed similar PGE₂ concentrations in the hypothalamus. Strong Oatp2a1 immunoreactivity was observed in F4/80-positive microglia and perivascular cells and in brain capillary endothelial cells. The changes in Tc and C_isf seen in LPS-injected Slco2a1^+/+ mice were partially attenuated in monocyte-/macrophage-specific Slco2a1^-/- (Slco2a1^Fl/Fl/LysM^Cre/+) mice. Thus, OATP2A1 facilitates the LPS-induced febrile response by maintaining a high level of C_isf, possibly by regulating PGE₂ secretion from F4/80-positive glial cells and/or facilitating PGE₂ transport across the blood-brain barrier. These findings suggest that OATP2A1 is a useful therapeutic target for neuroinflammation.SIGNIFICANCE STATEMENT Fever is a physiological response caused by pyrogen-induced release of prostaglandin E₂ (PGE₂) in the hypothalamus, which plays a central role in regulating the set-point of body temperature. However, it is unclear whether the prostaglandin transporter OATP2A1/SLCO2A1 is involved in this response. We show here that LPS-induced fever is associated with increased PGE₂ concentration in hypothalamus interstitial fluid (C_isf), but not in CSF (C_csf), by means of a microdialysis study in global Slco2a1-knock-out mice and monocyte-/macrophage-specific Slco2a1-knock-out mice. The results suggest that OATP2A1 serves as a regulator of C_isf in F4/80-positive glial cells. OATP2A1 was detected immunohistochemically in brain capillary endothelial cells and, therefore, may also play a role in PGE₂ transport across the blood-brain barrier.

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.

A novel role for OATP2A1/SLCO2A1 in a murine model of colon cancer

Prostaglandin E₂ (PGE₂) is associated with proliferation and angiogenesis in colorectal tumours. The role of prostaglandin transporter OATP2A1/SLCO2A1 in colon cancer tumorogenesis is unknown. We evaluated mice of various Slco2a1 genotypes in a murine model of colon cancer, the adenomatous polyposis (APC) mutant (Apc^∆716/+) model. Median lifespan was significantly extended from 19 weeks in Slco2a1^+/+/Apc^Δ716/+ mice to 25 weeks in Slco2a1^−/−/Apc^Δ716/+ mice. Survival was directly related to a reduction in the number of large polyps in the Slco2a1^−/−/Apc^∆716/+ compared to the Slco2a1^+/+/Apc^Δ716/+ or Slco2a1^+/−/Apc^Δ716/+mice. The large polyps from the Slco2a1^−/−/Apc^∆716/+ mice had significant reductions in microvascular density, consistent with the high expression of Slco2a1 in the tumour-associated vascular endothelial cells. Chemical suppression of OATP2A1 function significantly reduced tube formation and wound-healing activity of PGE₂ in human vascular endothelial cells (HUVECs) although the amount of extracellular PGE₂ was not affected by an OATP2A1 inhibitor. Further an in vivo model of angiogenesis, showed a significant reduction of haemoglobin content (54.2%) in sponges implanted into Slco2a1^−/−, compared to wildtype mice. These studies indicate that OATP2A1 is likely to promote tumorogenesis by PGE₂ uptake into the endothelial cells, suggesting that blockade of OATP2A1 is an additional pharmacologic strategy to improve colon cancer outcomes.

Impact of FDA-Approved Drugs on the Prostaglandin Transporter OATP2A1/SLCO2A1

To understand interaction of drugs with the prostaglandin transporter OATP2A1/SLCO2A1 that regulates disposition of prostaglandins, we explored the impact of 636 drugs in an FDA-approved drug library on 6-carboxyfluorescein (6-CF) uptake by OATP2A1-expressing HEK293 cells (HEK/2A1). Fifty-one and 10 drugs were found to inhibit and enhance 6-CF uptake by more than 50%, respectively. Effect of the 51 drugs on 6-CF uptake was positively correlated with that on PGE₂ uptake (r = 0.64, p < 0.001). Among those, 5 drugs not structurally related to prostaglandins, suramin, pranlukast, zafirlukast, olmesartan medoxomil, and losartan potassium, exhibited more than 90% PGE₂ uptake inhibition. Inhibitory affinity of suramin to OATP2A1 was the highest (IC_50,2A1 of 0.17 μM), and its IC₅₀ values to MRP4-mediated PGE₂ transport (IC_50,MRP4) and PGE₂ synthesis in human U-937 cells treated with phorbol 12-myristate 13-acetate (IC_50,Syn) were 73.6 and 336.7 times higher than IC_50,2A1, respectively. Moreover, structure-activity relationship study in 29 nonsteroidal anti-inflammatory drugs contained in the library displayed inhibitory activities of anthranilic acid derivatives, but enhancing effects of propionic acid derivatives. These results demonstrate that suramin is a potent selective inhibitor of OATP2A1, providing a comprehensive information about drugs in clinical use that interact with OATP2A1.

Multispecific Organic Cation Transporter 1 (OCT1) from Bos taurus Has High Affinity and Slow Binding Kinetics towards Prostaglandin E2

Organic cation transporter 1 (OCT1, SLC22A1), like many solute carrier 22 (SLC22) family members, is important for the disposition of clinically important drugs, metabolites and signaling molecules. Several studies suggest that SLC22 family (eg. organic anion transporters or OATs and OCTs) bind and possibly transport prostaglandins with relatively high affinity (submicromolar). The affinities of OCT1 and OATs toward PGE2 and PGF2a reported in these cell-based transport studies are considerably greater than for xenobiotics and natural metabolite substrates--in many cases over 100-fold higher. This raises the possibility that prostaglandins are key endogenous substrates and/or that they act on the transporter in a manner different from other substrates such as xenobiotics and lower affinity metabolites. To further investigate OCT1-prostaglandin interactions, we designed biophysical studies using purified bovine OCT1 (Bos taurus, btOCT1/SLC22A1) with PGE2 analogs, in fluorescently labeled and label-free formats. Using fluorescence polarization (FP), we detected a binding of btOCT1 to the PGE2-Rhodamine conjugate at submicromolar affinity, consistent with affinity data for PGE2 from cells over-expressing the related human OCT1. Using purified native btOCT1 as analyte and biotinylated PGE2 analog as ligand, our data from surface plasmon resonance (SPR) revealed that btOCT1 specifically interacts to PGE2 with KD values in the hundred nanomolar range. BtOCT1 also demonstrated a slow association (ka) in the range of 103 M(-1) s(-1) and an even slower dissociation rate (kd) in the range of 10-4 s(-1) for PGE2, suggesting the possibility of a different mode of binding compared to other structurally unrelated transported substrates of low-affinity (eg. drugs, metabolites). Our results complement in vitro transport studies and provide direct evidence that OCT1--which is normally expressed in liver and other tissues--interacts with prostaglandin analogs. While it is not entirely clear from the published literature whether OCTs function as major prostaglandin transporters, the tight binding of the naturally occurring PGE2, as well as the slow dissociation rate, could conceivably affect the transport of lower affinity substrates such as drugs and metabolites by SLC22 transporters. More research is necessary to establish the extent to which individual SLC22 family members actually function as PG transporters in vitro and in vivo and to investigate whether PGs can, independent of being directly transported, alter the ability of SLC22 transporters to handle drugs and other substrates.