Purification and characterization of a cytosolic, 42-kDa and Ca2+-dependent phospholipase A2 from bovine red blood cells: its involvement in Ca2+-dependent release of arachidonic acid from mammalian red blood cells

Immune and Metabolic Interactions of Human Erythrocytes: A Molecular Perspective

Apart from their main function as oxygen carriers in vertebrates, erythrocytes are also involved in immune regulation. By circulating throughout the body, the erythrocytes are exposed and interact with tissues that are damaged as a result of a disease. In this study, we summarize the literature regarding the contribution of erythrocytes to immune regulation and metabolism. Under the circumstances of a disease state, the erythrocytes may lose their antioxidant capacity and release Damage Associated Molecular Patterns, resulting in the regulation of innate and adaptive immunity. In addition, the erythrocytes scavenge and affect the levels of chemokines, circulating cell-free mtDNA, and C3b attached immune complexes. Furthermore, through surface molecules, erythrocytes control the function of T lymphocytes, macrophages, and dendritic cells. Through an array of enzymes, red blood cells contribute to the pool of blood's bioactive lipids. Finally, the erythrocytes contribute to reverse cholesterol transport through various mechanisms. Our study is highlighting overlooked molecular interactions between erythrocytes and immunity and metabolism, which could lead to the discovery of potent therapeutic targets for immunometabolic diseases.

Marine n-3 fatty acids alter the proteomic response to methylmercury in Atlantic salmon kidney (ASK) cells

Fish based diets have been linked to the amelioration of methylmercury (MeHg) induced symptoms in several epidemiological studies, particularly due to their contents of marine n-3 fatty acids. It has been suggested that n-3 fatty acids may mask the detrimental effects of MeHg due to their beneficial effect on the same biological functions which are negatively affected by MeHg. However, in vitro studies have implied that there may be direct interactions between the marine n-3 FAs and MeHg, which ameliorates MeHg toxicity through interactions at a biological level. To understand how marine n-3 FAs and MeHg interact in fish as a biological system, we wanted to investigate molecular interaction in a fish cell system. Atlantic salmon kidney (ASK) cells were pre-incubated with the marine n-3 FAs docosahexaenoic acid (22:6n-3, DHA) and eicosapentaenoic acid (20:5n-3, EPA) before exposing them to MeHg. Modulating effects of the marine FAs on MeHg toxicity were subsequently assessed using the exploratory technique of proteomics, in a factorial design. Thirty-four differentially regulated proteins were identified. From these; twenty-seven were shown to be differentially regulated by MeHg, twelve were regulated by the fatty acids, and another eight showed interaction effects between MeHg and the FAs. Several of the proteins were concomitantly affected by MeHg- and FA-main effects, as well as interaction effects. Functional annotations and pathway analysis of the proteins revealed that marine n-3 FAs and MeHg concurrently affected the abundance of protein markers relating to such molecular mechanisms as: cell signaling, calcium homeostasis, structural integrity, apoptosis, and energy metabolism. In conclusion, both marine n-3 FAs and MeHg can differentially affect the abundances of the same proteins, indicating modulating effects of EPA and DHA on MeHg metabolism, and possibly on its toxicity.

Red blood cells (RBCs), epoxyeicosatrienoic acids (EETs) and adenosine triphosphate (ATP)

In addition to serving as carriers of O(2), red blood cells (RBCs) regulate vascular resistance and the distribution of microvascular perfusion by liberating adenosine triphosphate (ATP) and epoxyeicosatrienoic acids (EETs) upon exposure to a low O(2) environment. Therefore, RBCs act as sensors that respond to low pO(2) by releasing millimolar amounts of ATP, a signaling molecule, and lipid mediators (EETs). The release of EETs occurs by a mechanism that is activated by ATP stimulation of P2X(7) receptors coupled to ATP transporters, which should greatly amplify the circulatory response to ATP. RBCs are reservoirs of EETs and the primary sources of plasma EETs, which are esterified to the phospholipids of lipoproteins. Levels of free EETs in plasma are low, about 3% of circulating EETs. RBC EETs are produced by direct oxidation of arachidonic acid (AA) esterified to glycerophospholipids and the monooxygenase-like activity of hemoglobin. On release, EETs affect vascular tone, produce profibrinolysis and dampen inflammation. A soluble epoxide hydrolase (sEH) regulates the concentrations of RBC and vascular EETs by metabolizing both cis- and trans-EETs to form dihydroxyeicosatrienoic acids (DHETs). The function and pathophysiological roles of trans-EETs and erythro-DHETs has yet to be integrated into a physiological and pathophysiological context.

Stimulation of rat erythrocyte P2X7 receptor induces the release of epoxyeicosatrienoic acids

BACKGROUND AND PURPOSE

Red blood cells (RBCs) are reservoirs of vasodilatory, antiaggregatory, and antiinflammatory lipid mediators-epoxyeicosatrienoic acids (EETs). This study addresses the formation and release of erythrocyte-derived EETs in response to ATP receptor stimulation that may represent an important mechanism regarding circulatory regulation.

EXPERIMENTAL APPROACH

Erythrocyte EET formation and release were investigated by incubating rat RBCs in physiological salt solution with agents that effected ATP release via P2 receptor stimulation of phospholipase A2 and epoxygenase-like activities with activation of the ATP secretory mechanism. EETs were analyzed by gas and liquid chromatography-mass spectrometry.

KEY RESULTS

EETs were released from rat RBCs: 14,15-, 11,12-, 8,9- and 5,6-EETs in a ratio of 1.2:1.0:0.9:0.8. EETs were produced by epoxidation of arachidonic acid catalyzed by hemoglobin. Spontaneous release of EETs, 0.66+/-0.14 ng per 10(9) RBCs, was dose-dependently increased by an ATP analog, BzATP, and inhibited by P2X(7) receptor antagonists. 5 microM ATP increased release of EETs over 20% to 0.83+/-0.15 ng per 10(9) RBCs; 10 microM BzATP tripled the amount of EET release to 1.87+/-0.20 ng per 10(9) RBCs. EET release by ATP or BzATP was not associated with hemolysis. Carbenoxolone, a gap junction inhibitor that inhibits ATP release, and glibenclamide, an inhibitor of the cystic fibrosis transmembrane conductance regulator (CFTR), which is required for ATP release, inhibited the spontaneous and stimulated EET release from RBCs.

CONCLUSIONS AND IMPLICATIONS

EETs are produced and released from RBCs via a mechanism that is mediated by ATP stimulation of P2X(7) receptors coupled to ATP transporters, pannexin-1 and CFTR.

Erythrocyte-derived epoxyeicosatrienoic acids

Red blood cells (RBCs) are reservoirs for cis- and trans-epoxyeicosatrienoic acids (EETs) that can be released. The sources of EET release from RBCs include direct synthesis from arachidonic acid, peroxidation of phospholipids and EETs esterified into cellular phospholipids. The release of EETs from RBCs can be through cytosolic phospholipase A2 (PLA2), secretory PLA2 and other responses associated with ATP release from RBCs. The erythrocyte ATP, purinergic receptors, ATP-binding cassette transporters, PLA2 and cytoskeleton rearrangement may all participate in EET release in the microcirculatory deformation of RBCs. EETs are vasodilatory and are candidate endothelium-derived hyperpolarizing factors. Due to the anti-hypertensive, fibrinolytic, and anti-thrombotic properties of EETs, their release from RBCs is replete with implications for the control of circulation and rheological characteristics of the circulating blood.

Stimulation of erythrocyte ceramide formation by platelet-activating factor

Osmotic erythrocyte shrinkage leads to activation of cation channels with subsequent Ca2+ entry and stimulates a sphingomyelinase with subsequent formation of ceramide. Ca2+ and ceramide then activate a scramblase leading to breakdown of phosphatidylserine asymmetry of the cell membrane. The mediators accounting for activation of erythrocyte sphingomyelinase and phosphatidylserine exposure remained elusive. The study demonstrates that platelet-activating factor (PAF) is released from erythrocytes upon hyperosmotic cell shrinkage. The experiments further disclose the presence of PAF receptors in erythrocytes and show that PAF stimulates the breakdown of sphingomyelin and the release of ceramide from erythrocytes at isotonic conditions. PAF further triggers cell shrinkage (decrease of forward scatter) and phosphatidylserine exposure (annexin binding) of erythrocytes. The stimulation of annexin-binding is blunted by a genetic knockout of PAF receptors, by the PAF receptor antagonist ABT491 or by inhibition of sphingomyelinase with urea. In conclusion, PAF activates an erythrocyte sphingomyelinase and the then formed ceramide leads to the activation of scramblase with subsequent phosphatidylserine exposure.

Modulation of the expression and activity of cyclooxygenases in normal and accelerated erythropoiesis

OBJECTIVE

The present study was aimed at characterizing the expression and activity of cyclooxygenase (COX) isoenzymes in erythropoiesis.

METHODS

The expression and activity of cyclooxygenase (COX) and prostaglandin (PG) synthases were investigated in: 1) erythroblasts developed in culture from human CD34(+) hematopoietic progenitors, 2) erythroblasts in bone marrow specimens, and 3) peripheral erythrocytes isolated from healthy donors and from patients with a high regeneration rate of erythrocytes.

RESULTS

While COX-1 protein was observed at each stage of erythroblast development, COX-2 protein was induced at later stages through a p38/MAPK-dependent pathway. Both COX isoforms were also observed in mature erythroblasts of the bone marrow. Erythroblasts developed in culture synthesized significantly more PGE(2) than TXB(2) and indomethacin delayed erythroid maturation. COX-1 and COX-2 were also observed in erythrocytes by immunostainings, although COX expression was confined to a fraction of circulating erythrocytes. Peripheral erythrocytes synthesized low but detectable amounts of PGE(2) and TXB(2). Similarly to erythroblast progenitors, PGE(2) was the prevalent prostanoid released by erythrocytes. This biosynthetic capacity was significantly increased in erythrocytes from patients with accelerated erythropoiesis as compared to controls.

CONCLUSIONS

Both COX isoforms are present and enzymatically active during human erythropoiesis, although with different kinetics, and COX-derived prostanoids may play a role in erythroid maturation. Furthermore, peripheral erythrocytes retain in part the capacity of expressing COX and synthesizing prostanoids, which may contribute to the hemostatic/thrombotic response to vascular injury in different diseases, including congenital hemolytic disorders.

Cyclooxygenase-2 Expression and Inhibition in Atherothrombosis

Arachidonic acid metabolism plays an important role in acute ischemic syndromes affecting the coronary or cerebrovascular territory, as reflected by biochemical measurements of eicosanoid biosynthesis and the results of inhibitor trials in these settings. Two cyclooxygenase (COX)-isozymes have been characterized, COX-1 and COX-2, that differ in terms of regulatory mechanisms of expression, tissue distribution, substrate specificity, preferential coupling to upstream and downstream enzymes, and susceptibility to inhibition by the extremely heterogeneous class of COX-inhibitors. Although the role of platelet COX-1 in acute coronary syndromes and ischemic stroke is firmly established through ≈20 years of thromboxane metabolite measurements and aspirin trials, the role of COX-2 expression and inhibition in atherothrombosis is substantially uncertain, because the enzyme was first characterized in 1991 and selective COX-2 inhibitors became commercially available only in 1998. In this review, we discuss the pattern of expression of COX-2 in the cellular players of atherothrombosis, its role as a determinant of plaque “vulnerability,” and the clinical consequences of COX-2 inhibition. Recent studies from our group suggest that variable expression of upstream and downstream enzymes in the prostanoid biosynthetic cascade may represent important determinants of the functional consequences of COX-2 expression and inhibition in different clinical settings.