The presence of cell-free microRNAs (miRNAs) has been detected in a range of body fluids. The miRNA content of plasma/serum in particular has been proposed as a potential source of novel biomarkers for a number of diseases. Nevertheless, the quantification of miRNAs from plasma or serum is made difficult due to inefficient isolation and lack of consensus regarding the optimal reference miRNA. The effect of haemolysis on the quantification and normalisation of miRNAs in plasma has not been investigated in great detail. We found that levels of miR-16, a commonly used reference gene, showed little variation when measured in plasma samples from healthy volunteers or patients with malignant mesothelioma or coronary artery disease. Including samples with evidence of haemolysis led to variation in miR-16 levels and consequently decreased its ability to serve as a reference. The levels of miR-16 and miR-451, both present in significant levels in red blood cells, were proportional to the degree of haemolysis. Measurements of the level of these miRNAs in whole blood, plasma, red blood cells and peripheral blood mononuclear cells revealed that the miRNA content of red blood cells represents the major source of variation in miR-16 and miR-451 levels measured in plasma. Adding lysed red blood cells to non-haemolysed plasma allowed a cut-off level of free haemoglobin to be determined, below which miR-16 and miR-451 levels displayed little variation between individuals. In conclusion, increases in plasma miR-16 and miR-451 are caused by haemolysis. In the absence of haemolysis the levels of both miR-16 and miR-451 are sufficiently constant to serve as normalisers.
Prostaglandins are involved in a wide variety of physiological and pathophysiological processes, but the mechanism of prostaglandin release from cells is not completely understood. Although poorly membrane permeable, prostaglandins are believed to exit cells by passive diffusion. We have investigated the interaction between prostaglandins and members of the ATP-binding cassette (ABC) transporter ABCC [multidrug resistance protein (MRP)] family of membrane export pumps. In inside-out membrane vesicles derived from insect cells or HEK293 cells, MRP4 catalyzed the time-and ATP-dependent uptake of prostaglandin E 1 (PGE1) and PGE2. In contrast, MRP1, MRP2, MRP3, and MRP5 did not transport PGE 1 or PGE 2. The MRP4-mediated transport of PGE1 and PGE2 displayed saturation kinetics, with K m values of 2.1 and 3.4 M, respectively. Further studies showed that PGF 1␣, PGF2␣, PGA1, and thromboxane B 2 were high-affinity inhibitors (and therefore presumably substrates) of MRP4. Furthermore, several nonsteroidal antiinflammatory drugs were potent inhibitors of MRP4 at concentrations that did not inhibit MRP1. In cells expressing the prostaglandin transporter PGT, the steady-state accumulation of PGE 1 and PGE2 was reduced proportional to MRP4 expression. Inhibition of MRP4 by an MRP4-specific RNA interference construct or by indomethacin reversed this accumulation deficit. Together, these data suggest that MRP4 can release prostaglandins from cells, and that, in addition to inhibiting prostaglandin synthesis, some nonsteroidal antiinflammatory drugs might also act by inhibiting this release. P rostaglandins are key mediators in the regulation of many physiological processes. They are involved in inflammatory responses and tumorigenesis, and their synthesis and metabolism are tightly regulated (1). The first step in prostaglandin synthesis is the production of arachidonic acid, which is released from membrane lipid primarily by cytosolic phospholipase A 2 (1). Arachidonic acid is then oxidized to the intermediate prostaglandin H 2 (PGH 2 ) by PGH synthases, also known as cyclooxygenase (COX)-1 and -2, and the recently identified COX-3 (2). These enzymes are known clinically as the targets of aspirin and other nonsteroidal antiinf lammatory drugs (NSAIDs) (3). Moreover, several recent studies have shown a link between COX-2 expression and carcinogenesis. Prostaglandins are overproduced by a variety of tumors, leading to the suggested prophylactic use of COX-2 inhibitors to decrease the incidence of colon cancer (4, 5). After COX-mediated synthesis, PGH 2 is further converted by tissue-specific prostaglandin synthases into PGE 2 , PGF 2␣ , PGD 2 , prostacyclin, or thromboxane B 2 , the biologically active molecules (1).Prostaglandins are formed and secreted by most cells, and act as autocrine-or paracrine-signaling molecules. In many cases they exert their effects extracellularly via interaction with a family of G protein-coupled membrane receptors (reviewed in ref. 6), although some prostaglandins interact with the nuclear horm...
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