Transport of eicosapentaenoic acid-derived PGE₃, PGF(3α), and TXB₃ by ABCC4

Comparing ATPase activity of ATP-binding cassette subfamily C member 4, lamprey CFTR, and human CFTR using an antimony-phosphomolybdate assay

Introduction: ATP-binding cassette (ABC) transporters use the hydrolysis of ATP to power the active transport of molecules, but paradoxically the cystic fibrosis transmembrane regulator (CFTR, ABCC7) forms an ion channel. We previously showed that ATP-binding cassette subfamily C member 4 (ABCC4) is the closest mammalian paralog to CFTR, compared to other ABC transporters. In addition, Lamprey CFTR (Lp-CFTR) is the oldest known CFTR ortholog and has unique structural and functional features compared to human CFTR (hCFTR). The availability of these evolutionarily distant orthologs gives us the opportunity to study the changes in ATPase activity that may be related to their disparate functions. Methods: We utilized the baculovirus expression system with Sf9 insect cells and made use of the highly sensitive antimony-phosphomolybdate assay for testing the ATPase activity of human ABCC4 (hABCC4), Lp-CFTR, and hCFTR under similar experimental conditions. This assay measures the production of inorganic phosphate (Pi) in the nanomolar range. Results: Crude plasma membranes were purified, and protein concentration, determined semi-quantitatively, of hABCC4, Lp-CFTR, and hCFTR ranged from 0.01 to 0.36 μg/μL. No significant difference in expression level was found although hABCC4 trended toward the highest level. hABCC4 was activated by ATP with the equilibrium constant (Kd) 0.55 ± 0.28 mM (n = 8). Estimated maximum ATPase rate (Vmax) for hABCC4 was about 0.2 nmol/μg/min when the protein was activated with 1 mM ATP at 37°C (n = 7). Estimated maximum ATPase rate for PKA-phosphorylated Lp-CFTR reached about half of hCFTR levels in the same conditions. Vmax for both Lp-CFTR and hCFTR were significantly increased in high PKA conditions compared to low PKA conditions. Maximum intrinsic ATPase rate of hABCC4 in the absence of substrate was twice that of hCFTR when activated in 1 mM ATP. Conclusion: The findings here suggest that while both ABCC4 and hCFTR bear one consensus and one degenerate ATPase site, the hCFTR exhibited a reduced intrinsic ATPase activity. In addition, ATPase activity in the CFTR lineage increased from Lp-CFTR to hCFTR. Finally, the studies pave the way to purify hABCC4, Lp-CFTR, and hCFTR from Sf9 cells for their structural investigation, including by cryo-EM, and for studies of evolution in the ABC transporter superfamily.

Metabolomic characterization of Liancheng white and Cherry Valley duck breast meat and their relation to meat quality

Liancheng white duck is a typical local duck breed in Fujian Province famous for its meat traits. To better understand how meat quality varies with breed, the chemical composition of breast meats of Liancheng white ducks (LD) and Cherry Valley ducks (CD) were examined using ultra-high-performance liquid chromatography quadrupole time-of-flight mass spectrometry (UHPLC-QTOF-MS).The correlation between meat quality and the differential metabolites was further analyzed. The results showed that the effects of breed on duck breast meat were significant for pH, color, cooking loss, and shear force. Liancheng white duck breast meat exhibited a higher shear force and pH, and lower cooking loss and lightness (L*24), redness (a*24), and yellowness (b*24) than CD. Metabolomic analysis revealed significant differences between the meat extracts from the 2 duck breeds. A total of 49 and 57 significantly different metabolites were identified in positive and negative ion modes, respectively. These differentially accumulated metabolites (DAMs) could be divided into 28 classes, of which the 4 main categories were carbohydrates, amino acids, fatty acids, and eicosanoids. Liancheng white duck might have better nutritional and medicinal value considering the higher content of (4Z,7Z,10Z,13Z,16Z,19Z)-4,7,10,13,16,19-docosahexaenoic acid (DHA), docosapentaenoic acid (DPA), eicosapentaenoic acid (EPA), and prostaglandinF3α (PGF3α), having anti-inflammatory orantioxidant effects. Carbohydrate concentration negatively correlated with pH24. The 4 metabolites positively correlated with the shear force. These results provide an overall perspective for bridging the gap between variation of duck meat quality and metabolites with respect to breed.

Honokiol affects the composition of gut microbiota and the metabolism of lipid and bile acid in methionine-choline deficiency diet-induced NASH mice

Honokiol (HNK), one of the main active components of Magnolia officinalis, has a positive effect on non-alcoholic steatohepatitis (NASH). However, the effects of HNK on the composition of serum lipids and bile acids (BAs) and gut microbiota (GM) of NASH mice are still unknown.C57BL/6 mice were fed with methionine-choline deficiency (MCD) diet and gavaged with HNK (20 mg/kg/d) for 8 weeks, then the serum lipids and BAs were detected by LC-MS, the composition of ileum microflora and the mRNA expression of hepatic BAs homeostasis related genes were analyzed by 16S rDNA sequencing and RT-qPCR, respectively. HNK treatment decreased the degree of hepatic lipid drops, inflammatory cell infiltration and fibrosis. Meantime, the serum levels of 34 lipids and 4 BAs in MCD mice were significantly altered by HNK treatment, as well as the increased abundance of Ruminococcaceae, Caulobacteraceae and Brevundimonas, and the decreased abundance of Firmicutes and Dubosiella. Besides, HNK treatment increased the hepatic mRNA expression of Oatp1b2 in MCD mice. The ameliorating effect of HNK on NASH may be partly related to its correction on the disorders of GM, serum lipids and BAs of MCD mice.

Binding mode analysis of ABCA7 for the prediction of novel Alzheimer's disease therapeutics

The adenosine-triphosphate-(ATP)-binding cassette (ABC) transporter ABCA7 is a genetic risk factor for Alzheimer's disease (AD). Defective ABCA7 promotes AD development and/or progression. Unfortunately, ABCA7 belongs to the group of 'under-studied' ABC transporters that cannot be addressed by small-molecules. However, such small-molecules would allow for the exploration of ABCA7 as pharmacological target for the development of new AD diagnostics and therapeutics. Pan-ABC transporter modulators inherit the potential to explore under-studied ABC transporters as novel pharmacological targets by potentially binding to the proposed 'multitarget binding site'. Using the recently reported cryogenic-electron microscopy (cryo-EM) structures of ABCA1 and ABCA4, a homology model of ABCA7 has been generated. A set of novel, diverse, and potent pan-ABC transporter inhibitors has been docked to this ABCA7 homology model for the discovery of the multitarget binding site. Subsequently, application of pharmacophore modelling identified the essential pharmacophore features of these compounds that may support the rational drug design of innovative diagnostics and therapeutics against AD.

Effects of Sodium Selenite Injection on Serum Metabolic Profiles in Women Diagnosed with Breast Cancer-Related Lymphedema-Secondary Analysis of a Randomized Placebo-Controlled Trial Using Global Metabolomics

In our previous study, intravenous (IV) injection of selenium alleviated breast cancer-related lymphedema (BCRL). This secondary analysis aimed to explore the metabolic effects of selenium on patients with BCRL. Serum samples of the selenium-treated (SE, n = 15) or the placebo-controlled (CTRL, n = 14) groups were analyzed by ultra-high-performance liquid chromatography with Q-Exactive Orbitrap tandem mass spectrometry (UHPLC-Q-Exactive Orbitrap/MS). The SE group showed a lower ratio of extracellular water to segmental water (ECW/SW) in the affected arm to ECW/SW in the unaffected arm (arm ECW/SW ratio) than the CTRL group. Metabolomics analysis showed a valid classification at 2-weeks and 107 differential metabolites were identified. Among them, the levels of corticosterone, LTB4-DMA, and PGE₃—which are known anti-inflammatory compounds—were elevated in the SE group. Pathway analysis demonstrated that lipid metabolism (glycerophospholipid metabolism, steroid hormone biosynthesis, or arachidonic acid metabolism), nucleotide metabolism (pyrimidine or purine metabolism), and vitamin metabolism (pantothenate and CoA biosynthesis, vitamin B₆ metabolism, ascorbate and aldarate metabolism) were altered in the SE group compared to the CTRL group. In addition, xanthurenic acid levels were negatively associated with whole blood selenium level (WBSe) and positively associated with the arm ECW/SW. In conclusion, selenium IV injection improved the arm ECW/SW ratio and altered the serum metabolic profiles in patients with BCRL, and improved the anti-inflammatory process in lipid, nucleotide and vitamin pathways, which might alleviate the symptoms of BCRL.

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.

Elucidation of underlying molecular mechanism of 5-Fluorouracil chemoresistance and its restoration using fish oil in experimental colon carcinoma

Latest strategies for cancer treatment primarily focus on the use of chemosensitizers to enhance therapeutic outcome. N-3 PUFAs have emerged as the strongest candidate for the prevention of colorectal cancer (CRC). Our previous studies have demonstrated that fish oil (FO) rich in n-3 PUFAs not only increased therapeutic potential of 5-Fluorouracil(5-FU) in colon cancer but also ameliorated its toxicity. Henceforth, the present study is designed to elucidate mechanistic insights of FO as a chemosensitizer to circumvent drug resistance in experimental colon carcinoma. The colon cancer was induced by 1,2-dimethylhydrazine(DMH)/dextran sulfate sodium(DSS) in male Balb/c mice and these animals were treated with 5-FU(12.5 mg/kg b.w.), FO(0.2 ml), or 5-FU + FO(12.5 mg/kg b.w + 0.2 ml) orally for 14 days. The molecular mechanism of overcoming 5-FU resistance using FO in colon cancer was delineated by estimating expression of cancer stem cell markers using flowcytometric method and drug transporters by immunohistochemistry and immunoblotting. Additionally, distribution profile of 5-FU and its cytotoxic metabolite, 5-FdUMP at target(colon), and non-target sites (serum, kidney, liver, spleen) was assessed using high-performance liquid chromatography(HPLC) method. The observations revealed that expression of CSCs markers was remarkably reduced after using fish oil along with 5-FU in carcinogen-treated animals. Interestingly, the use of FO alongwith 5-FU also significantly declined the expression of drug transporters (ABCB1,ABCC5) and consequently resulted in an increased cellular uptake of 5-FU and its metabolite, 5-FdUMP at target site (colon). It could be possibly associated with change in permeability of cell membrane owing to the alteration in membrane fluidity. The present study revealed the mechanistic insights of FO as a MDR revertant which successfully restored 5-FU-mediated chemoresistance in experimental colon carcinoma.

ATP-Binding Cassette Transporter C4 is a Prostaglandin D2 Exporter in HMC-1 cells

ATP-binding cassette transporter C4 (ABCC4) is associated with multidrug resistance and the regulation of cell signalling. Some prostaglandins (PGs), including: PGE₂, PGF_2α, PGE₃, and PGF_3α are known substrates of ABCC4, and are released from some types of cells to exert their biological effects. In the present study, we demonstrate that PGD₂ is a novel substrate of ABCC4 using a transport assay based on inside-out membrane vesicles prepared from ABCC4-overexpressing cells. Then, we used two types of cell lines with confirmed ABCC4 mRNA and PGD₂ release capacity (human mast cell lines HMC-1 cells and human rhabdomyosarcoma cell lines TE671 cells) to evaluate the contribution of ABCC4. The extracellular levels of PGD₂ were unchanged following addition of a selective ABCC4 inhibitor in TE671 cells. Pharmacological inhibition and knockdown of ABCC4 significantly reduced the extracellular levels of PGD₂ by at least 53% in HMC-1 cells. Moreover, the extracellular levels of PGD₂ decreased by at least 20% using the selective ABCC4 inhibitor in the other mast cell line RBL-2H3 cells. Therefore, our results suggest that ABCC4 functions as a PGD₂ exporter in HMC-1 cells.

Involvement of H+-gradient dependent transporter in PGE2 release from A549 cells

The purpose of this study was to identify the transporter involved in the release of prostaglandin E₂ (PGE₂). In the present study, transport assays were conducted using membrane vesicles prepared from human lung adenocarcinoma A549 cells, thus enabling identification of the novel exporter present in A549 cells. PGE₂ transport into A549 vesicles was higher in the presence of a proton (H⁺)-gradient, thus suggesting the involvement of PGE₂H⁺ symporter in PGE₂ transport. Results from our experiments showed enhanced PGE₂ release in A549 cells in the presence of H⁺-gradient ([H⁺]_{extracellular} < [H⁺]_{intracellular}). Moreover, in vesicular transport assays, H⁺-gradient-dependent transport of PGE₂ did not show saturation up to 500 μM PGE_2, and 10 mM aromatic monocarboxylic acids (acetylsalicylic acid, salicylic acid, and p-nitrobenzoic acid) significantly inhibited PGE₂ transport by 62-70%. These results suggest, the involvement of monocarboxylate transporters in the H⁺-gradient-dependent PGE₂ export.

MRP4/ABCC4 As a New Therapeutic Target: Meta-Analysis to Determine cAMP Binding Sites as a Tool for Drug Design

MRP4 transports multiple endogenous and exogenous substances and is critical not only for detoxification but also in the homeostasis of several signaling molecules. Its dysregulation has been reported in numerous pathological disorders, thus MRP4 appears as an attractive therapeutic target. However, the efficacy of MRP4 inhibitors is still controversial. The design of specific pharmacological agents with the ability to selectively modulate the activity of this transporter or modify its affinity to certain substrates represents a challenge in current medicine and chemical biology. The first step in the long process of drug rational design is to identify the therapeutic target and characterize the mechanism by which it affects the given pathology. In order to develop a pharmacological agent with high specific activity, the second step is to systematically study the structure of the target and identify all the possible binding sites. Using available homology models and mutagenesis assays, in this review we recapitulate the up-to-date knowledge about MRP structure and aligned amino acid sequences to identify the candidate MRP4 residues where cyclic nucleotides bind. We have also listed the most relevant MRP inhibitors studied to date, considering drug safety and specificity for MRP4 in particular. This meta-analysis platform may serve as a basis for the future development of inhibitors of MRP4 cAMP specific transport.

Analysis of membrane transport mechanisms of endogenous substrates using chromatographic techniques

Membrane transporters are expressed in various bodily tissues and play essential roles in the homeostasis of endogenous substances and the absortion, distribution and/or excretion of xenobiotics. For transporter assays, radioisotope-labeled compounds have been mainly used. However, commercially available radioisotope-labeled compounds are limited in number and relatively expensive. Chromatographic analyses such as high-performance liquid chromatography with ultraviolet absorptiometry and liquid chromatography with tandem mass spectrometry have also been applied for transport assays. To elucidate the transport properties of endogenous substrates, although there is no difficulty in performing assays using radioisotope-labeled probes, the endogenous background and the metabolism of the compound after its translocation across cell membranes must be considered when the intact compound is assayed. In this review, the current state of knowledge about the transport of endogenous substrates via membrane transporters as determined by chromatographic techniques is summarized. Chromatographic techniques have contributed to our understanding of the transport of endogenous substances including amino acids, catecholamines, bile acids, prostanoids and uremic toxins via membrane transporters.

Multiple drug resistance-associated protein (MRP4) exports prostaglandin E2 (PGE2) and contributes to metastasis in basal/triple negative breast cancer

Cyclooxygenase-2 (COX-2) and its primary enzymatic product, prostaglandin E2 (PGE2), are associated with a poor prognosis in breast cancer. In order to elucidate the factors contributing to intratumoral PGE₂ levels, we evaluated the expression of COX-2/PGE₂ pathway members MRP4, the prostaglandin transporter PGT, 15-PGDH (PGE₂ metabolism), the prostaglandin E receptor EP4, COX-1, and COX-2 in normal, luminal, and basal breast cancer cell lines. The pattern of protein expression varied by cell line reflecting breast cancer heterogeneity. Overall, basal cell lines expressed higher COX-2, higher MRP4, lower PGT, and lower 15-PGDH than luminal cell lines resulting in higher PGE₂ in the extracellular environment. Genetic or pharmacologic suppression of MRP4 expression or activity in basal cell lines led to less extracellular PGE₂. The key finding is that xenografts derived from a basal breast cancer cell line with stably suppressed MRP4 expression showed a marked decrease in spontaneous metastasis compared to cells with unaltered MRP4 expression. Growth properties of primary tumors were not altered by MRP4 manipulation. In addition to the well-established role of high COX-2 in promoting metastasis, these data identify an additional mechanism to achieve high PGE₂ in the tumor microenvironment; high MRP4, low PGT, and low 15-PGDH. MRP4 should be examined further as a potential therapeutic target in basal breast cancer.

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

Eicosapentaenoic acid (EPA)-derived prostaglandin E3 (PGE3) possesses an anti-inflammatory effect; however, information for transporters that regulate its peri-cellular concentration is limited. The present study, therefore, aimed to clarify transporters involved in local disposition of PGE3. PGE3 uptake was assessed in HEK293 cells transfected with OATP2A1/SLCO2A1, OATP1B1/SLCO1B1, OATP2B1/SLCO2B1, OAT1/SLC22A6, OCT1/SLC22A1 or OCT2/SLC22A2 genes, compared with HEK293 cells transfected with plasmid vector alone (Mock). PGE3 uptake by OATP2A1-expressing HEK293 cells (HEK/2A1) was the highest and followed by HEK/1B1, while no significantly higher uptake of PGE3 than Mock cells was detected by other transporters. Saturation kinetics in PGE3 uptake by HEK/2A1 estimated the Km as 7.202 ± 0.595 μM, which was 22 times higher than that of PGE2 (Km=0.331 ± 0.131 μM). Furthermore, tissue disposition of PGE3 was examined in wild-type (WT) and Slco2a1-deficient (Slco2a1(-/-)) mice after oral administration of EPA ethyl ester (EPA-E) when they underwent intraperitoneal injection of endotoxin (e.g., lipopolysaccharide). PGE3 concentration was significantly higher in the lung, and tended to increase in the colon, stomach, and kidney of Slco2a1(-/-), compared to WT mice. Ratio of PGE2 metabolite 15-keto PGE2 over PGE2 concentration was significantly lower in the lung and colon of Slco2a1(-/-) than that of WT mice, suggesting that PGE3 metabolism is downregulated in Slco2a1(-/-) mice. In conclusion, PGE3 was found to be a substrate of OATP2A1, and local disposition of PGE3 could be regulated by OATP2A1 at least in the lung.