ALBERT

Description: Abstract 1515 Poster Board I-538 BACKGROUND Acute chest syndrome (ACS) is a common cause of morbidity and mortality for individuals with sickle cell disease (SCD). The treatment of ACS is mainly supportive. Prior studies have shown that intravenous pulse-dose corticosteroids, such as dexamethasone, can decrease the duration and morbidity of ACS. However, such treatment may also precipitate ‘rebound‘ episodes of vaso-occlusive pain in some individuals that might negate the overall benefit of corticosteroids. Tapered corticosteroid therapy might decrease the rebound effect while maintaining therapeutic benefit. Therefore, we designed a prospective study to begin to test this hypothesis and to assess biomarkers that might clarify the therapeutic and toxic mechanisms of corticosteroids in SCD, predict outcome, and guide therapy. METHODS We conducted a multi-center, placebo-controlled pilot study to test the feasibility and safety of high-dose oral dexamethasone followed by a taper for ACS. Children and adults with SCD and ACS of any severity were randomized to dexamethasone (0.3 mg/kg q12h x 2, 0.3 mg/kg q24h x 2, 0.2 mg/kg q24h x 2, 0.1 mg/kg q24h x2, then stop) or placebo to start within 24 hours of diagnosis. All subjects received standard, protocol-directed supportive care for ACS. We defined the duration of ACS using a novel, objective tool that assessed rate and effort of breathing, oxygen saturation in room air, thoracic pain, and use of supplemental oxygen and ventilatory support. The primary outcomes were the duration of ACS and the duration of hospitalization for ACS. We also measured a panel of biomarkers before, during and after therapy. Inflammatory biomarkers included high sensitivity C-reactive protein and secretory phospholipase A2 (sPLA2). White cell and endothelial activation markers included sVCAM-1, sICAM-1, vWF:Ag and vWF:RCoF, sE-selectin, sP-selectin, sL-selectin, nitric oxide metabolites (NO), and whole blood tissue factor. We used generalized linear mixed models controlling for age to test for differences between treatment groups. RESULTS We enrolled 12 subjects (9 children, 3 adults; mean age 17.3 years, range 5 - 45) with homozygous sickle cell anemia at 4 centers and randomized 11 (1 drop-out) to either dexamethasone (N=5) or placebo (N=6). The objective ACS assessment tool was completed on all subjects without difficulty. In this pilot study, dexamethasone reduced the duration of hospitalization (41.5 vs 62.3 hrs; P=0.024), but not the duration of ACS (log of duration 2.4 vs 3.5 hrs; P=0.127), supplemental oxygen (17.5 vs 41.2 hrs; P=0.876), hypoxemia (13.8 vs 34.3 hrs; P=0.770), or total opioid usage in morphine equivalents (54.4 vs 68.8 mg; P=0.885). There were no statistically significant differences in adverse events between arms. However, 3 patients treated with dexamethasone had a painful event in the 2 weeks after hospital discharge (1 required re-hospitalization), compared to 1 patient in the placebo group (0 re-hospitalizations). No marked leukocytosis occurred in either treatment group, but the leukocyte count at 1-week follow-up had decreased less from baseline in the dexamethasone group compared to placebo (-17.7 vs -37.6%; P=0.028). The baseline sPLA2 concentration was 3 100 ng/mL in 4/5 and 4/6 patients in the dexamethasone and placebo arms. The white cell activation marker, sL-selectin was significantly decreased at 1-week follow-up in the dexamethasone group compared to placebo (573.8 ng/mL vs 742.8; change from baseline -121.2 vs 57.2; P

Description: Phosphatidylserine (PS) exposure contributes to recognition and removal of aged erythrocytes. PS exposure can only arise in cells in which the aminophospholipid translocase, or flippase, is defunct, or otherwise PS would be transported back into the inner cell monolayer. To provide evidence that flippase activity is reduced in old erythrocytes, we studied mouse erythrocytes that were biotinylated in vivo. This way we were able to distinguish the aging, biotinylated, RBC population from the newly formed, unbiotinylated RBC. Detection of the biotinylation with fluorescent streptavidin was combined with a flow cytometric assay for flippase activity using NBD-PS. We found that flippase activity decreased over time during the life-span of the RBC, as shown by a decrease in flippase activity of the biotinylated RBC compared to the newly formed RBC. A particularly high flippase activity was found in CD71-positive reticulocytes. Murine sickle cells show a much higher erythrocyte turnover than normal erythrocytes, with a 25–50% newly formed population each day and complete elimination of all biotinylated cells in 10 days. As in control mice, flippase activity decreased during the (short) life-span of sickle RBC and at all times, a subpopulation of sickle cells had a markedly reduced flippase activity. High flippase activity was only found in newly released erythrocytes. Our data indicate that the flippase activity decreases throughout the life-span of erythrocytes and can be markedly different in subpopulations of RBC in vivo. This decrease reduces the cell’s potential to maintain its phospholipid asymmetry, which results in PS exposure as a removal signal for the damaged cell. We conclude that the inability of sickle RBC to maintain flippase activity is related to the decreased survival time of those cells.

Description: Membrane-spanning proteins may interact with a variety of other integral and peripheral membrane proteins via a diversity of protein-protein interactions. Not surprisingly, defects or mutations in any one of these interacting components can impact the physical and biological properties on the entire complex. Here we use quantum dots to image the diffusion of individual band 3 molecules in the plasma membranes of intact human erythrocytes from healthy volunteers and patients with defects in one of their membrane components, leading to well-known red cell pathologies (hereditary spherocytosis, hereditary elliptocytosis, hereditary hydrocytosis, Southeast Asian ovalocytosis, and hereditary pyropoikilocytosis). After characterizing the motile properties of the major subpopulations of band 3 in intact normal erythrocytes, we demonstrate that the properties of these subpopulations of band 3 change significantly in diseased cells, as evidenced by changes in the microscopic and macroscopic diffusion coefficients of band 3 and in the compartment sizes in which the different band 3 populations can diffuse. Because the above membrane abnormalities largely arise from defects in other membrane components (eg, spectrin, ankyrin), these data suggest that single particle tracking of band 3 might constitute a useful tool for characterizing the general structural integrity of the human erythrocyte membrane.

Description: Since 1–2% of the red cell’s oxyhemoglobin undergoes spontaneous heterolytic dissociation into metHb and superoxide every day, and superoxide is readily converted to H2O2 by superoxide dismutase, the red cell is unavoidably exposed to high levels of reactive oxygen species (ROS). We have used a series of gene-disrupted mice to examine the oxidative defenses of the red cell. Work with glutathione peroxidase deficient [GSHPx(−/−)] red cells has been reported earlier. We here add results with red cells from mice that were catalase deficient (−/−), catalase heterozygotes (+/−), double knockouts [GSHPx(−/−) and catalase (−/−)], or peroxiredoxin II deficient. Catalase(−/−) cells were readily oxidized by exogenous H2O2, as monitored by methemoglobin formation, but no methemoglobin was formed when these cells were exposed to organic peroxides. Catalase heterozygotes were not distinguishable from wild type cells in these assays. This is consistent with the clinical finding that partial catalase deficiency is a benign condition. Red cells lacking both GSHPx and catalase exhibited more metHb formation in response to an H2O2 generating system than did either single knockout, indicating that both enzymes contribute to the defense against exogenous H2O2. Adding catalase deficiency to GSHPx deficiency did not increase Hb oxidation by organic peroxides. With peroxiredoxin-deficient cells, PrxII(−/−), preliminary results found an increased flux of the endogenously generated H2O2 through catalase, which could be detected using 3-amino-1,2,4-triazole (3-AT) inhibition. Earlier work (Lee et al, Blood101:5033, 2003) has demonstrated increased ROS with cell and membrane damage in PrxII(−/−) red cells, but exposure to a flux of H2O2 generated by glucose oxidase produced no increase in metHb formation over that seen in wild type cells. Thus, Hb may be less sensitive to oxidation than is the cell membrane. Prx(−/−) red cells were not more sensitive than wild type red cells to Hb oxidation by organic peroxides. These findings show that all three oxidant defense enzymes play multiple roles in the erythrocyte. All participate in detoxifying endogenously generated H2O2. Calculations with a model of erythrocyte oxidative reactions (Johnson et al, Free Rad. Biol. Med.39:1407, 2005) are consistent with the experimental findings, predicting H2O2 disposal by all three enzymes. Thus, it is not accurate to identify any one of these enzymes as the sole oxidant defense mechanism in the erythrocyte, as all participate in H2O2 removal. GSHPx in addition has a unique role in detoxifying organic peroxides. The data indicate that peroxiredoxin plays a minor role in the red cell’s defense against Hb oxidation. However, PrxII(−/−) mice are anemic, in contrast with the hematologically normal GSHPx(−/−) and catalase(−/−) mice. This suggests that the primary role of PrxII in the red cell may lie in protecting the cell membrane against oxidative damage.

Description: The formation, distribution and utilization of acyl-CoA plays a crucial role in plasma membrane phospholipid turnover in red blood cells (RBC). Upon de-acylation of glycero-phospholipids (PL) via the action of phospholipase, re-acylation of the lysophospholipids (LPL) requires activity of two enzymes of the Lands pathway. Long-chain acyl-CoA synthetases (ACSL) activate fatty acids to acyl-CoA which are subsequently ligated to LPL by LysoPhosphoLipid Acyl Transferase (LPLAT) a family of enzymes with exclusive specificity for the polar group of LPL (phosphatidic acid, choline, serine and ethanolamine). While formation and utilization of acyl-CoA takes place at the membrane, Acyl-CoA Binding Domain containing proteins (ACBD) in RBC cytosol bind acyl-CoA, limiting product feedback inhibition on ACSL and distribute Acyl-CoA to the various acyl-utilizing enzymes while protecting the cells against its potent detergent character. We have identified ACSL6 as the enzyme responsible for the activation of fatty acid in RBC, a protein with several isoforms that acts as a dimer. To relate its structure to activity, we report the expression of different modified forms in E. coli. Our data indicate that, despite the observed activity, enzyme studies of these mammalian membrane proteins in the host E. coli are strongly hampered by their aggregation into inclusion bodies. While activity can be measured, data on enzyme kinetics and specificity are questionable. Oleoyl-CoA formation from oleic acid, CoA and ATP reveled that the two transmembrane spanning segments predicted at the amino-terminus of the protein are not essential for its activity. Moreover, they are not essential for dimer formation and strong association with membranes. ACSL6 appears to be an integral membrane protein. One of the five spliced isoforms of ACSL6 reported, lacks the so-called fatty acid Gate-domain, and appears to be unable to activate long chain fatty acids. We hypothesize that this form modulates activity of the other active isoforms of ACSL6 through hetero-dimer formation. An EST clone of erythroid precursor cells identified ACBD6 as a potential AcylCoA binding protein in RBC. This modular protein contains an Acyl-CoA binding domain at the amino-terminus and two Ankyrin-repeat motifs (ANK) at the C-terminus. Both the full-length protein and the N-terminus domain were soluble when expressed and purified in E. coli. Expression of the C-terminus domain by itself rendered an insoluble protein. We report that ACBD6 binds long-chain acyl-CoA with a preference for C18:1-CoA over C20:4 and C16:0-CoA and does not bind fatty acid. Truncation of the ANK domain had no effect on the binding activity of the N-terminus domain. Together our findings implicate ACBD6 as part of the Acyl-CoA turnover mechanism in RBC, its actual role on the kinetics of ACSL and/or LPLAT activity needs to be established. The description of these proteins involved in Acyl-CoA turnover in RBC will aid to better understand the maintenance of plasma membrane lipid composition of all mammalian cells.

Description: The formation, distribution and utilization of acyl-CoA plays a crucial role in plasma membrane phospholipid turnover in red blood cells (RBC). Upon de-acylation of glycero-phospholipids (PL) via the action of phospholipase, re-acylation of the lysophospholipids (LPL) requires activity of two enzymes of the Lands pathway. Long-chain acyl-CoA synthetases (ACSL) activate fatty acids to acyl-CoA which are subsequently ligated to LPL by LysoPhosphoLipid Acyl Transferase (LPLAT) a family of enzymes with exclusive specificity for the polar group of LPL (phosphatidic acid, choline, serine and ethanolamine). We recently identified ACSL6 as the enzyme responsible for the activation of fatty acid in RBC. None of the family members of LPLAT have been identified in RBC to date. LPC, either generated in the RBC or taken up from plasma, is rapidly acylated by RBC suggesting an important role for Lysohosphatidylcholine-acyl transferase (LPCAT) in RBC. We report the identification and characterization of LPCAT, the enzyme that generates PC from LPC and acylCoA. We identified the RNA expression of LPCAT, an approximately 60kD protein, in reticulocytes, confirming proteomic studies suggesting the presence of this protein in adult RBC membranes.). It is a modular protein containing an acyltransferase domain at the amino-terminus, three predicted membrane spanning domains, and a putative calcium binding site at the C-terminus, distinguishing it from the lysophosphatidic acid acyltransferease (LPAAT). The putative LPCAT was expressed in E. coli. It was found in the E. coli membrane fraction, and was able to use oleoyl-CoA and LPC as substrates to generate PC. Lysophosphatidic acid (LPA) was not acylated by this protein. In contrast the previously identified LPAAT (1) expressed in E. coli, utilized LPA but not LPC, indicating LPL specificity of these enzymes. Radioactive fatty acid added to RBC is also incorporated in phosphatidyl ethanolamine (PE) and phosphatidyl serine (PS). Sequence analysis suggests that two other proteins present in the genome of mammals are homologues of LPCAT. We hypothesize that these putative acyltransferases are responsible for the acylation of lysophosphatidyl ethanolamine (LPE) and lysophosphatidyl serine (LPS). These proteins are essential to maintain a proper glycerophospholipid composition of the RBC membrane and thereby viability of the cells. A dysfunction of this system may underlie the observed differences in phospholipid molecular species composition in subpopulations of sickle cells contributing to sickle cell pathology. A complete description of these proteins involved in the maintenance of glycerophospholipid composition of RBC will aid to better understand the maintenance of plasma membrane lipid composition of all mammalian cells.

Description: Abstract 573 BACKGROUND: Vaso-occlusion is the principal factor in the morbidity of sickle cell disease (SCD). Vaso-occlusive painful episodes (VOE) are common, debilitating, and the leading cause of hospitalizations, emergency room visits, and are associated with an increased mortality rate. There is no effective therapy that targets the underlying mechanisms of VOE. Symptomatic relief with analgesics is the only availabel treatment. Nitric oxide (NO) is a potent vasodilator that contributes to a variety of vaso-occlusive events in SCD. We have found that an arginine deficiency and low NO bioavailability occurs during VOE in SCD. Since arginine is the obligate substrate for NO production, and an acute deficiency is associated with VOE, we hypothesized that arginine supplementation may be a safe and beneficial treatment for sickle cell pain. PATIENTS AND METHODS: Hospitalized SCD patients 〉 3 years diagnosed within 24 hours with VOE and without associated complications were eligible; written informed consent was obtained. A total of 56 patients completed randomization in this double-blinded, placebo controlled trial. A standardized treatment and monitoring program for VOE was followed. Average age was 13.9 ± 4 years (range 3.6-19 years), and 52% were female. Patients received intravenous (IV) or oral arginine (0.1 gram/kg TID, n=28) or placebo (n=28) for 5 days or until discharge from the hospital. Narcotic records for 2 patients (randomized to placebo arm) were incomplete and were not included in the narcotic use analysis. An intention to treat analysis was performed for narcotic use applying an unpaired t-test with Welch's correction to adjust for unequal variance. RESULTS: Age was equally distributed between treatment and placebo groups. 57% of the arginine treatment group and 46% of the placebo group were female. A significant reduction in narcotic use (defined as total morphine use over the course of the hospital stay in mg/kg) by 56% was observed in the treatment arm receiving IV or oral arginine compared to placebo (mean ± SEM: 1.8 ± 0.4 mg/kg; n=28 vs. 4.1 ± 0.8mg/kg; n=26, p=0.01). Average length of hospitalization was 4.5 ± 0.4 days, and there was no significant difference between the 2 groups (4.1 ± 0.3 vs. 4.8 ± 0.5 days, p = 0.27; arginine vs. placebo arm). Four episodes of acute chest syndrome (ACS) developed during the study, three in the treatment arm and one in the placebo arm. There was one patient who experienced clinical deterioration associated with ACS requiring emergent transfusion and a transfer to the pediatric intensive care unit (PICU) in the placebo arm. No clinical deterioration or PICU transfers occurred in the arginine arm. Five in the treatment arm received transfusion vs. four in the placebo arm. No drug-related adverse events were observed. No significant differences were observed between pre and post therapy liver or renal function, or hematological parameters in the arginine treatment group vs. placebo. Two patients admitted for pain management ultimately did not receive IV narcotics. Both had been randomized into the arginine-treatment arm and received arginine therapy per protocol throughout their hospital stay and required only oral narcotics and non-steroidal analgesia. Reduction in narcotic use in the treatment arm remained significant even when these 2 patients were excluded from the analysis (p=0.02). CONCLUSIONS: IV arginine therapy represents a novel nutritional intervention for the treatment of pain in hospitalized patients with SCD. Use of IV arginine should also be considered in the treatment of VOE in the emergency department setting prior to hospitalization, although further investigation is warranted. A reduction of narcotic use by over 50% observed in this study is remarkable, as this is the first successful intervention for sickle cell-related pain that targets the underlying mechanism of vaso-occlusion through a promising NO-based therapy. Arginine is a safe and inexpensive intervention with narcotic-sparing effects that should be considered as an adjunct to standard therapy for VOE requiring hospitalization. Disclosures: Off Label Use: Arginine for treatment of sickle cell vaso-occlusive pain episodes.