In vivo production of nitric oxide in rats after administration of hydroxyurea

Hydroxyurea inhibits proliferation and stimulates apoptosis through inducible nitric oxide synthase in erythroid cells

Hydroxyurea (hydroxycarbamide, HU) arrests cells in the S-phase by inhibiting ribonucleotide reductase and DNA synthesis, significantly contributing to the release of nitric oxide (NO). We investigated the involvement of inducible NO synthase (NOS2) in the cytostatic effect of HU using in vitro shRNA-induced knockdown of the NOS2 transcript (NOS2kd) or a specific NOS2 inhibitor (1400W) in human erythroleukemic HEL92.1.7 cells, as well as murine erythroid progenitors (mERPs) from HU-treated wild-type (WT) and Nos2 knockout (Nos2-/-) mice. Over the long-term, HU increased NOS2 expression in HEL92.1.7 cells (via nuclear factor kappa B [NFκB] signaling) and in mERP. In the short-term, HU increased the activity of human recombinant and erythroleukemic cell-derived NOS2, as confirmed by NO metabolite nitrite/citrulline production. In silico molecular docking predicted that HU binds to the NOS2 active site and substrate L-arginine via hydrogen bonds. Molecular dynamics simulations showed reduced rigidity of the NOS2 active site upon interaction with HU, indicating stabilization of the enzyme-substrate complex. Both 1400W and NOS2kd prevented the in vitro reduction in proliferation and induction of apoptosis in HEL92.1.7 cells by HU. NOS2kd preferentially blocked early apoptosis and HU-induced S-phase arrest in HEL92.1.7 cells. The HU-induced decrease in proliferation and stimulation of early apoptosis in mERP were prevented in Nos2-/- mice and by 1400W in WT mice. This study demonstrated that HU induces NOS2 activity through direct interaction and increased protein expression via NFκB signaling. Moreover, NOS2 mediates the HU-induced inhibition of proliferation and stimulation of apoptosis in erythroid cells.

Hydroxyurea does not reverse functional alterations of the nitric oxide-cGMP pathway associated with priapism phenotype in corpus cavernosum from sickle cell mouse

Sickle cell disease (SCD) is a genetic disorder that has been associated with priapism. The role of hydroxyurea, a common SCD therapy, in influencing the nitric oxide (NO)-cGMP pathway and its effect on priapism is unclear. To investigate the effect of hydroxyurea treatment on smooth muscle relaxation of corpus cavernosum induced by stimulation of the NO-cGMP pathway in SCD transgenic mice and endothelial NO synthase gene-deficient (eNOS-/-) mice, which are used as model of priapism associated with the low bioavailability of endothelial NO. Four-month-old wild-type (WT, C57BL/6), SCD transgenic, and eNOS-/- male mice were treated with hydroxyurea (100 mg/Kg/day) or its vehicle (saline) daily for three weeks via intraperitoneal injections. Concentration-response curves for acetylcholine (ACh), sodium nitroprusside (SNP), and electrical field stimulation (EFS) were generated using strips of mice corpus cavernosum. The SCD mice demonstrated an amplified CC relaxation response triggered by ACh, EFS, and SNP. The corpus cavernosum relaxation responses to SNP and EFS were found to be heightened in the eNOS-/- group. However, the hydroxyurea treatment did not alter these escalated relaxation responses to ACh, EFS, and SNP in the corpus cavernosum of the SCD group, nor the relaxation responses to EFS and SNP in the eNOS-/- group. In conclusion, hydroxyurea is not effective in treating priapism associated with SCD. It is likely that excess plasma hemoglobin and reactive oxygen species, which are reported in SCD, are reacting with NO before it binds to GCs in the smooth muscle of the corpus cavernosum, thus preventing the restoration of baseline NO/cGMP levels. Furthermore, the downregulation of eNOS in the penis may impair the pharmacological action of hydroxyurea at the endothelial level in SCD mice. This study emphasize the urgency for exploring alternative therapeutic avenues for priapism in SCD that are not hindered by high plasma hemoglobin and ROS levels.

Effects of Hydroxyurea on Skeletal Muscle Energetics and Function in a Mildly Anemic Mouse Model

Hydroxyurea (HU) is a ribonucleotide reductase inhibitor most commonly used as a therapeutic agent in sickle cell disease (SCD) with the aim of reducing the risk of vaso-occlusion and improving oxygen transport to tissues. Previous studies suggest that HU may be even beneficial in mild anemia. However, the corresponding effects on skeletal muscle energetics and function have never been reported in such a mild anemia model. Seventeen mildly anemic HbAA Townes mice were subjected to a standardized rest-stimulation (transcutaneous stimulation)-protocol while muscle energetics using 31Phosphorus magnetic resonance spectroscopy and muscle force production were assessed and recorded. Eight mice were supplemented with hydroxyurea (HU) for 6 weeks while 9 were not (CON). HU mice displayed a higher specific total force production compared to the CON, with 501.35 ± 54.12 N/mm3 and 437.43 ± 57.10 N/mm3 respectively (+14.6%, p < 0.05). Neither the total rate of energy consumption nor the oxidative metabolic rate were significantly different between groups. The present results illustrated a positive effect of a HU chronic supplementation on skeletal muscle function in mice with mild anemia.

Repurposing of Hydroxyurea Against COVID-19: A Promising Immunomodulatory Role

Cytokine release syndrome, a prominent mechanism of morbidity and mortality in patients with coronavirus disease 2019 (COVID-19), can cause multiple bodily reactions, including excessive release of proinflammatory mediators, with tumor necrosis factor-α (TNF-α) being the most prevalent cytokine combined with persistently elevated D-dimer levels that are indicative of potential thrombotic events, low levels of endogenous nitric oxide (NO) generation, and progressive decrease in hemoglobin production. In our argument, the conceptual repurposing of hydroxyurea (HU) for managing COVID-19 can provide a promising therapeutic option originating from a rich history of investigational antiviral activity. HU as a proposed supportive therapeutic agent for treating COVID-19 can exemplify a successful remedial choice through its anti-inflammatory activity along with an intrinsic propensity to control the circulatory levels of key cytokines including TNF-α. HU has the ability to undergo in vivo NO conversion acting as NO donor together with being a prominent inducer of fetal hemoglobin (HbF) production. The combination of the mentioned two properties allows HU to possess evident capability of protecting against thrombotic events by controlling D-dimer levels. The implication of our hypothetical argument sheds light on the curative potential of HU, which can be strategically harnessed against COVID-19.

Nitric Oxide Mediation in Hydroxyurea and Nitric Oxide Metabolites' Inhibition of Erythroid Progenitor Growth

In several systems, hydroxyurea has been shown to trigger nitric oxide (NO) release or activation of NO synthase (NOS). To elucidate this duality in its pharmacological effects, during myelosuppression, we individually examined hydroxyurea's (NO releasing agent) and NO metabolites' (stable NO degradation products) effects on erythroid colony growth and NOS/NO levels in mice using NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO). Hydroxyurea and nitrite/nitrate decreased the bone marrow cellularity that was blocked by PTIO only for the NO metabolites. Hydroxyurea inhibition of colony-forming unit-erythroid (CFU-E) formation and reticulocytes was reversed by PTIO. Moreover, hydroxyurea, through a negative feedback mechanism, reduced inducible NOS (iNOS) expressing cells in CFU-E, also prevented by PTIO. Nitrate inhibition of burst-forming units-erythroid (BFU-E) colony growth was blocked by PTIO, but not in mature CFU-E. The presented results reveal that NO release and/or production mediates the hydroxyurea inhibition of mature erythroid colony growth and the frequency of iNOS immunoreactive CFU-E.

HPMA Copolymer-Based Nanomedicines in Controlled Drug Delivery

Recently, numerous polymer materials have been employed as drug carrier systems in medicinal research, and their detailed properties have been thoroughly evaluated. Water-soluble polymer carriers play a significant role between these studied polymer systems as they are advantageously applied as carriers of low-molecular-weight drugs and compounds, e.g., cytostatic agents, anti-inflammatory drugs, antimicrobial molecules, or multidrug resistance inhibitors. Covalent attachment of carried molecules using a biodegradable spacer is strongly preferred, as such design ensures the controlled release of the drug in the place of a desired pharmacological effect in a reasonable time-dependent manner. Importantly, the synthetic polymer biomaterials based on N-(2-hydroxypropyl) methacrylamide (HPMA) copolymers are recognized drug carriers with unique properties that nominate them among the most serious nanomedicines candidates for human clinical trials. This review focuses on advances in the development of HPMA copolymer-based nanomedicines within the passive and active targeting into the place of desired pharmacological effect, tumors, inflammation or bacterial infection sites. Specifically, this review highlights the safety issues of HPMA polymer-based drug carriers concerning the structure of nanomedicines. The main impact consists of the improvement of targeting ability, especially concerning the enhanced and permeability retention (EPR) effect.

The effects of nitric oxide or oxygen on the stable products formed from the tyrosine phenoxyl radical

Tyrosine is a critical component of many proteins and can be the subject of oxidative posttranslational modifications. Furthermore, the oxidation of tyrosine residues to phenoxyl radicals, sometimes quite stable, is essential for some enzymatic functions. The lifetime and fate of tyrosine phenoxyl radicals in biological systems are largely driven by the availability and proximity of oxidants and reductants. Tyrosine phenoxyl radicals have extremely low reactivity with molecular oxygen whereas reactions with nitric oxide are diffusion controlled. This is in contrast to equivalent reactions with tryptophanyl and cysteinyl radicals where reactions with oxygen are much faster. Despite, the quite disparate apparent reactivity of tyrosine phenoxyl radicals with oxygen and nitric oxide being known, the products of the reactions are not well established. Changes in the levels from expected basal concentrations of stable products resulting from tyrosine phenoxyl radicals, for example naturally occurring 3,3'-dityrosine, 3-nitrotyrosine, and 3-hydroxytyrosine, can be indicative of oxidative and/or nitrosative stress. Using the radiolytic generation of specific oxidizing radicals to form tyrosine phenoxyl radicals in an aqueous solution at a known rate, we have compared the products in the absence and presence of nitric oxide or oxygen. Possible reactions of the phenoxyl radicals with oxygen remain unclear although we show evidence for a small decrease in the yield of dityrosine and loss of tyrosine in the presence of 20% oxygen. Low concentrations of nitric oxide in anoxic conditions react with tyrosine phenoxyl radicals, by what is most probably through the formation of an unstable intermediate, regenerating tyrosine and forming nitrite.

Tolerability and age-dependent toxicokinetics following perinatal hydroxyurea treatment in Sprague Dawley rats

Hydroxyurea (HU) is a valuable therapy for individuals with sickle cell anemia. With increased use of HU in children and throughout their lives, it is important to understand the potential effects of HU therapy on their development and fertility. Thus, studies were conducted to identify appropriate doses to examine long‐term effects of prenatal and early postnatal HU exposure and to understand kinetics of HU at various life stages. Pregnant Sprague Dawley dams were administered HU (0–150 mg/kg/day) via oral gavage from gestation days 17 to 21 and during lactation. Pups were dosed with the same dose as their respective dam starting on postnatal day (PND) 10 and up to PND 34. There was minimal maternal toxicity, and no significant effects on littering at any dose of HU. Starting on ~PND 16, offspring displayed skin discoloration and alopecia at doses ≥75 mg/kg/day and lower body weight compared to controls at doses ≥100 mg/kg/day. Gestational transfer of HU was observed, but there was minimal evidence of lactational transfer. Our toxicokinetic studies suggest that the internal dose in offspring may be altered due to age, but not due to sex. The plasma area under the curve, a measure of systemic exposure, at doses tolerated by offspring was threefold to sevenfold lower than the internal therapeutic dose in humans. Therefore, strategies to establish clinically relevant exposures in animal studies are needed. Overall, these data are useful for the design of appropriate nonclinical studies in the future to evaluate the consequences of long‐term HU treatment starting in childhood.

Increased use of hydroxyurea (HU) to treat sickle cell disease in children and throughout their lives augments the importance of understanding potential effects of HU on development and fertility. To inform the design of studies to evaluate long‐term safety of HU use in children, tolerability of prenatal and early postnatal HU treatment was evaluated in Sprague Dawley rats. Additionally, studies investigating gestational and lactational transfer of HU and how toxicokinetics of HU vary with age were conducted.

Deferoxamine produces nitric oxide under ferricyanide oxidation, blood incubation, and UV-irradiation

Deferoxamine (DFO), an iron chelator, is used therapeutically for the removal of excess iron in multiple clinical conditions such as beta thalassemia and intracerebral hemorrhage. DFO is also used as an iron chelator and hypoxia-mimetic agent in in vivo and in vitro basic research. Here we unexpectedly discover DFO to be a nitric oxide (NO) precursor in experiments where it was intended to act as an iron chelator. Production of NO from aqueous solutions of DFO was directly observed by ozone-based chemiluminescence using a ferricyanide-based assay and was confirmed by electron paramagnetic resonance (EPR). DFO also produced NO following exposure to ultraviolet light, and its incubation with sheep adult and fetal blood resulted in considerable formation of iron nitrosyl hemoglobin, as confirmed by both visible spectroscopy and EPR. These results suggest that experiments using DFO can be confounded by concomitant production of NO, and offer new insight into some of DFO's unexplained clinical side effects such as hypotension.

Exploiting the dynamics of the EPR effect and strategies to improve the therapeutic effects of nanomedicines by using EPR effect enhancers

The enhanced permeability and retention (EPR) effect is a unique phenomenon of solid tumors that is related to their particular anatomical and pathophysiological characteristics, e.g. defective vascular architecture; large gaps between endothelial cells in blood vessels; abundant vascular mediators such as bradykinin, nitric oxide, carbon monoxide, and vascular endothelial growth factor; and impaired lymphatic recovery. These features lead to tumor tissues showing considerable extravasation of plasma components and nanomedicines. These data comprise the basic theory underlying the development of macromolecular agents or nanomedicines. The EPR effect is not necessarily valid for all solid tumors, because tumor blood flow and vascular permeability vary greatly. Tumor blood flow is frequently obstructed as tumor size increases, as often seen clinically; early stage, small tumors show a more uniform EPR effect, whereas advanced large tumor show heterogeneity in EPR effect. Accordingly, it would be very important to apply enhancers of EPR effect in clinical setting to make EPR effect more uniform. In this review, we discuss the EPR effect: its history, factors involved, and dynamics and heterogeneity. Strategies to overcome the EPR effect's heterogeneity may guarantee better therapeutic outcomes of drug delivery to advanced cancers.

The Aspartic Protease Ddi1 Contributes to DNA-Protein Crosslink Repair in Yeast

Naturally occurring or drug-induced DNA-protein crosslinks (DPCs) interfere with key DNA transactions if not repaired in a timely manner. The unique family of DPC-specific proteases Wss1/SPRTN targets DPC protein moieties for degradation, including stabilized topoisomerase-1 cleavage complexes (Top1ccs). Here, we describe that the efficient DPC disassembly requires Ddi1, another conserved predicted protease in Saccharomyces cerevisiae. We found Ddi1 in a genetic screen of the tdp1 wss1 mutant defective in Top1cc processing. Ddi1 is recruited to a persistent Top1cc-like DPC lesion in an S phase-dependent manner to assist in the eviction of crosslinked protein from DNA. Loss of Ddi1 or its putative protease activity hypersensitizes cells to DPC trapping agents independently from Wss1 and 26S proteasome, implying its broader role in DPC repair. Among the potential Ddi1 targets, we found the core component of Pol II and show that its genotoxin-induced degradation is impaired in ddi1. We propose that the Ddi1 protease contributes to DPC proteolysis.

Augmentation of the Enhanced Permeability and Retention Effect with Nitric Oxide-Generating Agents Improves the Therapeutic Effects of Nanomedicines

Enhanced permeability and retention (EPR) effect-based nanomedicine is a promising strategy for successful anticancer therapy. The EPR effect is based on tumor blood flow. Because advanced large tumors, as frequently seen in clinical settings, are heterogeneous, with regions of defective vasculature and blood flow, achieving the desired tumor drug delivery is difficult. Here, we utilized the EPR effect to increase drug delivery. To augment the EPR effect for improved therapeutic effects of nanomedicine, we exploited vascular mediators-the nitric oxide (NO) generators nitroglycerin (NG), hydroxyurea, and l-arginine. These compounds generate NO in tumors with relatively high selectivity. Using different nanosized drugs in our protocol significantly increased (1.5-2 times) delivery of nanomedicines to different solid tumor models, along with markedly improving (2-3-fold) the antitumor effects of these drugs. Also, in 7,12-dimethylbenz[a]anthracene-induced advanced end-stage breast cancer, often seen in clinical settings, 2 mg/kg polymer-conjugated pirarubicin (P-THP) with NG (0.2 mg/mouse) showed better effects than did 5 mg/kg P-THP, and 5 mg/kg P-THP used with NG resulted in cures or stable tumors (no tumor growth) for up to 120 days. Moreover, in a murine autochthonous azoxymethane/dextran sulfate sodium-induced colon cancer model, NO donors markedly improved the therapeutic effects of P-THP even after just one injection, results that were comparable with those achieved with three weekly P-THP treatments. These findings strongly suggest the potential usefulness of NO donors as EPR effect enhancers to improve the therapeutic efficacy of nanomedicines.

Serum of sickle cell disease patients contains fetal hemoglobin silencing factors secreted from leukocytes

BACKGROUND

The mechanisms that regulate fetal hemoglobin (HbF) expression in sickle cell disease (SCD) remain elusive. We previously showed that steady-state SCD patients with high HbF levels due to a γ-globin gene mutation demonstrate strong inverse correlations between HbF levels and leukocyte counts, suggesting that leukocytes play a role in regulating HbF in SCD.

MATERIALS AND METHODS

To further investigate the role of leukocytes in HbF expression in SCD, we examined the presence of HbF silencing factors in the serum of 82 SCD patients who received hydroxyurea (HU) therapy.

RESULTS

HU-mediated HbF induction was associated with elevated total hemoglobin levels and improved red blood cell parameters, but there was no correlation with reticulocyte or platelet counts. Importantly, we again found that HU-induced HbF levels correlated with reductions in both neutrophils and lymphocytes/monocytes, indicating that these cell lineages may have a role in regulating HU-mediated HbF expression. Our in vitro studies using CD34+-derived primary erythroblasts found that patient serum preparations include HbF silencing factors that are distinct from granulocyte-macrophage colony-stimulating factor, and the activity of such factors decreases upon HU therapy.

CONCLUSION

Together, these results demonstrate the importance of leukocyte numbers in the regulation of HbF levels for SCD patients both in steady state and under HU therapy, and that leukocytes secrete HbF silencing factors that negatively affect HbF expression in erythroid-lineage cells in SCD.

Impact of hydroxycarbamide and interferon-α on red cell adhesion and membrane protein expression in polycythemia vera

Polycythemia vera is a chronic myeloproliferative neoplasm characterized by the JAK2V617F mutation, elevated blood cell counts and a high risk of thrombosis. Although the red cell lineage is primarily affected by JAK2V617F, the impact of mutated JAK2 on circulating red blood cells is poorly documented. Recently, we showed that in polycythemia vera, erythrocytes had abnormal expression of several proteins including Lu/BCAM adhesion molecule and proteins from the endoplasmic reticulum, mainly calreticulin and calnexin. Here we investigated the effects of hydroxycarbamide and interferon-α treatments on the expression of erythroid membrane proteins in a cohort of 53 patients. Surprisingly, while both drugs tended to normalize calreticulin expression, proteomics analysis showed that hydroxycarbamide deregulated the expression of 53 proteins in red cell ghosts, with overexpression and downregulation of 37 and 16 proteins, respectively. Within over-expressed proteins, hydroxycarbamide was found to enhance the expression of adhesion molecules such as Lu/BCAM and CD147, while interferon-α did not. In addition, we found that hydroxycarbamide increased Lu/BCAM phosphorylation and exacerbated red cell adhesion to its ligand laminin. Our study reveals unexpected adverse effects of hydroxycarbamide on red cell physiology in polycythemia vera and provides new insights into the effects of this molecule on gene regulation and protein recycling or maturation during erythroid differentiation. Furthermore, our study shows deregulation of Lu/BCAM and CD147 that are two ubiquitously expressed proteins linked to progression of solid tumors, paving the way for future studies to address the role of hydroxycarbamide in tissues other than blood cells in myeloproliferative neoplasms.

DNA replication inhibitor hydroxyurea alters Fe-S centers by producing reactive oxygen species in vivo

Redox homeostasis is tightly controlled in cells as it is critical for most cellular functions. Iron-Sulfur centers (Fe-S) are metallic cofactors with electronic properties that are associated with proteins and allow fine redox tuning. Following the observation that altered Fe-S biosynthesis is correlated with a high sensitivity to hydroxyurea (HU), a potent DNA replication blocking agent, we identified that oxidative stress response pathway under the control of the main regulator Yap1 attenuates HU deleterious effects, as it significantly increases resistance to HU, Fe-S biosynthesis and DNA replication kinetics in the presence of HU. Yap1 effect is mediated at least in part through up-regulation of two highly conserved genes controlling cytosolic Fe-S biosynthesis and oxidative stress, Dre2 and Tah18. We next observed that HU produces deleterious effects on cytosolic Fe-S clusters in proteins in vivo but not in vitro, suggesting that HU’s impact on Fe-S in vivo is mediated by cellular metabolism. Finally, we evidenced that HU exposure was accompanied by production of reactive oxygen species intracellularly. Altogether, this study provides mechanistic insight on the initial observation that mutants with altered Fe-S biosynthesis are highly sensitive to HU and uncovers a novel mechanism of action of this widely used DNA replication inhibitor.

Nitric Oxide-cGMP Signaling Stimulates Erythropoiesis through Multiple Lineage-Specific Transcription Factors: Clinical Implications and a Novel Target for Erythropoiesis

Much attention has been directed to the physiological effects of nitric oxide (NO)-cGMP signaling, but virtually nothing is known about its hematologic effects. We reported for the first time that cGMP signaling induces human γ-globin gene expression. Aiming at developing novel therapeutics for anemia, we examined here the hematologic effects of NO-cGMP signaling in vivo and in vitro. We treated wild-type mice with NO to activate soluble guanylate cyclase (sGC), a key enzyme of cGMP signaling. Compared to untreated mice, NO-treated mice had higher red blood cell counts and total hemoglobin but reduced leukocyte counts, demonstrating that when activated, NO-cGMP signaling exerts hematopoietic effects on multiple types of blood cells in vivo. We next generated mice which overexpressed rat sGC in erythroid and myeloid cells. The forced expression of sGCs activated cGMP signaling in both lineage cells. Compared with non-transgenic littermates, sGC mice exhibited hematologic changes similar to those of NO-treated mice. Consistently, a membrane-permeable cGMP enhanced the differentiation of hematopoietic progenitors toward erythroid-lineage cells but inhibited them toward myeloid-lineage cells by controlling multiple lineage-specific transcription factors. Human γ-globin gene expression was induced at low but appreciable levels in sGC mice carrying the human β-globin locus. Together, these results demonstrate that NO-cGMP signaling is capable of stimulating erythropoiesis in both in vitro and vivo settings by controlling the expression of multiple lineage-specific transcription factors, suggesting that cGMP signaling upregulates erythropoiesis at the level of gene transcription. The NO-cGMP signaling axis may constitute a novel target to stimulate erythropoiesis in vivo.

A systematic review of known mechanisms of hydroxyurea-induced fetal hemoglobin for treatment of sickle cell disease

AIM

To report on molecular mechanisms of fetal hemoglobin (HbF) induction by hydroxyurea (HU) for the treatment of sickle cell disease.

STUDY DESIGN

Systematic review.

RESULTS

Studies have provided consistent associations between genomic variations in HbF-promoting loci and variable HbF level in response to HU. Numerous signal transduction pathways have been implicated, through the identification of key genomic variants in BCL11A, HBS1L-MYB, SAR1 or XmnI polymorphism that predispose the response to the treatment, and signal transduction pathways that modulate γ-globin expression (cAMP/cGMP; Giα/c-Jun N-terminal kinase/Jun; methylation and miRNA). Three main molecular pathways have been reported: i) Epigenetic modifications, transcriptional events and signaling pathways involved in HU-mediated response, ii) Signaling pathways involving HU-mediated response and iii) Post-transcriptional pathways (regulation by miRNAs).

CONCLUSIONS

The complete picture of HU-mediated mechanisms of HbF production in Sickle Cell Disease remains elusive. Research on post-transcriptional mechanisms could lead to therapeutic targets that may minimize alterations to the cellular transcriptome.

Toxicity of Hydroxyurea in Rats and Dogs

The toxicity of hydroxyurea, a treatment for specific neoplasms, sickle-cell disease, polycythemia, and thrombocytosis that kills cells in mitosis, was assessed in repeat-dose, oral gavage studies in rats and dogs and a cardiovascular study in telemetered dogs. Hydroxyurea produced hematopoietic, lymphoid, cardiovascular, and gastrointestinal toxicity with steep dose response curves. In rats dosed for 10 days, 50 mg/kg/day was tolerated; 500 mg/kg/day produced decreased body weight gain; decreased circulating leukocytes, erythrocytes, and platelets; decreased cellularity of thymus, lymph nodes, and bone marrow; and epithelial degeneration and/or dysplasia of the stomach and small intestine; 1,500 mg/kg/day resulted in deaths on day 5. In dogs, a single dose at ≥250 mg/kg caused prostration leading to unscheduled euthanasia. Dogs administered 50 mg/kg/day for 1 month had decreased circulating leukocytes, erythrocytes, and platelets; increased bone marrow cellularity with decreased maturing granulocytes; increased creatinine kinase activity; and increased iron pigment in bone marrow and hepatic sinusoidal cells. In telemetered dogs, doses ≥15 mg/kg decreased systolic blood pressure (BP); 50 mg/kg increased diastolic BP, heart rate, and change in blood pressure over time (+d P/d t), and decreased QT and PR intervals and maximum left ventricular systolic and end diastolic pressures with measures returning to control levels within 24 hr.

Endothelium-derived nitric oxide production is increased by ATP released from red blood cells incubated with hydroxyurea

Red blood cells (RBCs) release adenosine triphosphate (ATP) in response to a variety of stimuli, including flow-induced deformation. Hydroxyurea (HU), a proven therapy for individuals with sickle cell disease (SCD), is known to improve blood flow. However, the exact mechanism leading to the improved blood flow is incomplete. Here, we report that the incubation of human RBCs with HU enhances ATP release from these cells and that this ATP is capable of stimulating nitric oxide (NO) production in an endothelium. RBCs incubated with HU were pumped through micron-size flow channels in a microfluidic device. The release of ATP from the RBCs was measured using the luciferin-luciferase assay in detection wells on the device that were separated from the flow channels by a porous polycarbonate membrane. NO released from a layer of bovine artery endothelial cells (bPAECs) cultured on the polycarbonate membrane was also measured using the extracellular NO probe DAF-FM. ATP release from human RBCs incubated with 100 μM HU was observed to be 2.06±0.37-fold larger than control samples without HU (p<0.05, N ≥ 3). When HU-incubated RBCs were flowed under a layer of bPAECs, NO released from the bPAEC layer was measured to be 1.34±0.10-fold higher than controls. An antagonist of the P2Y receptor established that this extra 30% increase in NO release is ATP mediated. Furthermore, when RBCs were incubated with L-NAME, a significant decrease in endothelium-derived NO production was observed. Control experiments suggest that RBC-generated NO indirectly affects endothelial NO production via its effects on RBC-derived ATP release.

Arterialization of Venous Blood for Differentiation of Sickle Cell Subjects in Vaso-occlusive Crisis

These studies were designed as two experiments. Experiment 1 was performed to validate the hypothesis that oxygen saturation of the venous blood may be a marker for vaso-occlusive crisis (VOC) in sickle cell patients undergoing hydroxyurea (HU) treatments. Experiment 2 was performed to test the hypothesis that an acute increase in the blood nitric oxide (NO) concentration by administering HU modulates the perception of pain in sickle cell subjects in VOC. The percent saturations of oxyhemoglobin (%O<PRE>2</PRE>Hb), reduced hemoblogin (%RHb), carboxy-hemoglobin (%COHb), met-hemoglobin (%MHb), fetal hemoglobin (HbF), and nitric oxide metabolites were measured in venous blood samples collected from sickle cell disease (SCD) who were on and off HU and O<PRE>2</PRE> at steady state and during VOC. The results showed the ratio of %O<PRE>2</PRE>Hb/RHb in VOC+HU was significantly higher than patients in the steady state who were on and off of HU (p<0.05). The %COHb was higher in all SCD groups, %COHb values were significantly different in SCD at steady state who were on HU. HU and O<PRE>2</PRE> treatment did not play important role on venous blood %O<PRE>2</PRE>Hb and pain scores in SCD during VOC. A single oral dose of HU was associated with a significant increase in the venous concentration of nitric oxide metabolites (NOx), p<0.05. These findings suggest that the ratio %O<PRE>2</PRE>Hb/RHb in venous blood and pain scores differentiate HU-untreated and HU-treated at steady state subjects from HU-treated subjects in VOC; however, the acute increase in venous NOx produced by administering HU to HU-treated subjects in VOC does not explain this difference.

Patients with sickle cell disease taking hydroxyurea in the Hemocentro Regional de Montes Claros

Background

The development of therapies for sickle cell disease has received special attention, particularly those that reduce the polymerization of hemoglobin S. Hydroxyurea is a commonly used medication because it has the ability to raise levels of fetal hemoglobin, decrease the frequency of vaso-occlusive episodes and thus improve the clinical course of sickle cell disease patients.

Objective

To study hematological data and the clinical profile of sickle cell disease patients taking hydroxyurea in a regional blood center.

Methods

From the charts of 20 patients with sickle cell anemia, the clinical outcomes and a number of hematological variables were analyzed before and during treatment with hydroxyurea.

Results

The patients' ages ranged from 6 to 41 years old, most were dark skinned and there was a predominance of women. The main symptom that defined whether patients were prescribed hydroxyurea was painful crises followed by hospitalizations. During treatment with hydroxyurea there were significant increases in hemoglobin, fetal hemoglobin, mean corpuscular volume and mean corpuscular hemoglobin. The reticulocyte and white blood cell counts dropped significantly with treatment. A positive correlation was found between fetal hemoglobin and mean corpuscular volume before and during treatment. Additionally, a correlation was found between the white blood cell and reticulocyte counts before treatment with hydroxyurea.

Conclusion

Most patients showed improvements with treatment as demonstrated by increases in hemoglobin, fetal hemoglobin and mean corpuscular volume, as well as by reductions in the reticulocyte and white blood cell counts. Clinically, more than 50% of patients had a significant reduction of events.

Evaluation of the Forrester-Hepburn mechanism as an artifact source in ESR spin-trapping

Nitrone spin traps such as 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) are commonly used for free radical detection. Though proven examples are rare, artifact formation must be considered. For example, the Forrester-Hepburn mechanism yields the same radical adduct as that formed by genuine radical trapping. A hydroxylamine is formed by nucleophilic attack of the substrate on DMPO and subsequently oxidized to the respective nitroxide radical. One potential candidate for this artifact is the sulfur trioxide radical adduct (DMPO/(•)SO(3)(-)), as detected in spin-trapping experiments with horseradish peroxidase and sulfite. It has previously been shown by NMR experiments that the hydroxylamine intermediate does indeed form, but no direct proof for the ESR artifact has been provided. Here, we used isotopically labeled DMPO with horseradish peroxidase and ferricyanide to test for the Forrester-Hepburn artifact directly in a spin-trapping experiment. Besides sulfite, we investigated other nucleophiles such as cyanide, cysteine, and glutathione. Neither sulfite nor biological thiols produced detectable spin-trapping artifacts, but with cyanide the relatively weak signal originated entirely from the nucleophilic reaction. The hydroxylamine intermediate, which is more abundant with cyanide than with sulfite, was identified as cyano-hydroxylamine by means of 2D NMR experiments. Although our study found that spin trapping provided authentic free radical signals with most of the substrates, the occurrence of the Forrester-Hepburn mechanism artifact with cyanide emphasizes the importance of isotope measurements with nucleophile substrates.

The proinflammatory cytokine GM-CSF downregulates fetal hemoglobin expression by attenuating the cAMP-dependent pathway in sickle cell disease

Although reduction in leukocyte counts following hydroxyurea therapy in sickle cell disease (SCD) predicts fetal hemoglobin (HbF) response, the underlying mechanism remains unknown. We previously reported that leukocyte counts are regulated by granulocyte-macrophage colony-stimulating factor (GM-CSF) in SCD patients. Here we examined the roles of GM-CSF in the regulation of HbF expression in SCD. Upon the analysis of retrospective data in 372 patients, HbF levels were inversely correlated with leukocyte counts and GM-CSF levels in SCD patients without hydroxyurea therapy, while HbF increments after hydroxyurea therapy correlated with a reduction in leukocyte counts, suggesting a negative effect of GM-CSF on HbF expression. Consistently, in vitro studies using primary erythroblasts showed that the addition of GM-CSF to erythroid cells decreased HbF expression. We next examined the intracellular signaling pathway through which GM-CSF reduced HbF expression. Treatment of erythroid cells with GM-CSF resulted in the reduction of intracellular cAMP levels and abrogated phosphorylation of cAMP response-element-binding-protein, suggesting attenuation of the cAMP-dependent pathway, while the phosphorylation levels of mitogen-activated protein kinases were not affected. This is compatible with our studies showing a role for the cAMP-dependent pathway in HbF expression. Together, these results demonstrate that GM-CSF plays a role in regulating both leukocyte count and HbF expression in SCD. Reduction in GM-CSF levels upon hydroxyurea therapy may be critical for efficient HbF induction. The results showing the involvement of GM-CSF in HbF expression may suggest possible mechanisms for hydroxyurea resistance in SCD.

Simplified synthesis of isotopically labeled 5,5-dimethyl-pyrroline N-oxide

5,5-Dimethylpyrroline N-oxide (¹⁵N) and 5,5-di(trideuteromethyl)pyrroline N-oxide were synthesized from the respective isotopically labeled 2-nitropropane analogs obtained from the reaction of sodium nitrate with 2-halopropanes. This facile, straightforward process allows synthesizing isotopically labeled DMPO analogs in a 4-step reaction without special equipment.

Hydroxyurea stimulates the release of ATP from rabbit erythrocytes through an increase in calcium and nitric oxide production

Hydroxyurea, a proven therapy for sickle cell disease, is known to improve blood flow and reduce vaso-occlusive crises, although its exact mechanism of action is not clear. The objective of this study was to determine if hydroxyurea results in an increase of ATP release from the red blood cell (RBC) via the drug's ability to stimulate nitric oxide (NO) production in these cells. A system enabling the flow of RBCs through microbore tubing was used to investigate ATP release from the RBC. Incubation of rabbit RBCs (7% hct) with 50 microM hydroxyurea resulted in a significant increase in the release of ATP from these cells. This level of ATP release was not detected in the absence of flow. Studies also showed that increments in hydroxyurea and NO (from spermine NONOate) resulted in an initial increase in ATP release, followed by a decrease in this release at higher concentrations of hydroxyurea and the NO donor. Incubation with L-NAME abolished the effect of the hydroxyurea, suggesting that NO production by the RBC was involved. Indeed, in the presence of 50 microM hydroxyurea, the amount of total Ca(2+) measured (by atomic absorption spectroscopy) in a 7% solution of RBCs increased from 363+/-47 ng/ml and 530+/-52 ng/ml. Finally, EPR studies suggest that an increase in nitrosylated Hb in the RBC is only measured for those studies involving hydroxyurea and a Ca(2+)-containing buffer.

Detection of nitric oxide by electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy has been used in a number of ways to study nitric oxide chemistry and biology. As an intrinsically stable and relatively unreactive diatomic free radical, the challenges of detecting this species by EPR are somewhat different from those of transient radical species. This review gives a basic introduction to EPR spectroscopy and discusses its uses to assess and quantify nitric oxide formation in biological systems.

The Clinical Application of Ozonetherapy

The reader may be eager to examine in which diseases ozonetherapy can be proficiently used and she/he will be amazed by the versatility of this complementary approach (Table 9 1). The fact that the medical applications are numerous exposes the ozonetherapist to medical derision because superficial observers or sarcastic sceptics consider ozonetherapy as the modern panacea. This seems so because ozone, like oxygen, is a molecule able to act simultaneously on several blood components with different functions but, as we shall discuss, ozonetherapy is not a panacea. The ozone messengers ROS and LOPs can act either locally or systemically in practically all cells of an organism. In contrast to the dogma that “ozone is always toxic”, three decades of clinical experience, although mostly acquired in private clinics in millions of patients, have shown that ozone can act as a disinfectant, an oxygen donor, an immunomodulator, a paradoxical inducer of antioxidant enzymes, a metabolic enhancer, an inducer of endothelial nitric oxide synthase and possibly an activator of stem cells with consequent neovascularization and tissue reconstruction.

The inhibitors of histone deacetylase suberoylanilide hydroxamate and trichostatin A release nitric oxide upon oxidation

Suberoylanilide hydroxamic acid (SAHA, vorinostat, Zolinza) is the lead compound of a new class of histone deacetylase (HDAC) inhibitors used as anticancer drugs that have been shown to affect multiple proteins associated with gene expression, cell proliferation, and migration. Studies have also demonstrated the essential role of the hydroxamate moiety of SAHA in HDAC inhibition. The ability of SAHA and its structural analog trichostatin A (TSA) to generate NO upon oxidation was tested directly, by spin trapping of NO using electron paramagnetic resonance spectroscopy, and also indirectly, via the determination of nitrite using the Griess assay. H2O2/metmyoglobin was used to oxidize SAHA and TSA. These studies demonstrate, for the first time, the release of NO from SAHA and its structural analog TSA. We tested the protective effects of SAHA, TSA, and valproic acid (VPA) in mammalian Chinese hamster V79 cells exposed to a bolus of H2O2 for 1 h and monitored the clonogenic cell survival. Both SAHA and TSA afforded significant cytoprotection when co-incubated with H2O2, whereas VPA was ineffective. These studies provide evidence for the release of NO by hydroxamate-containing HDAC inhibitors and their antioxidant effects. Such roles may be an added advantage of this class of HDAC agents used for epigenetic therapies in cancer.

Hydroxyurea generates nitric oxide in human erythroid cells: mechanisms for gamma-globin gene activation

Hydroxyurea (HU) induces fetal hemoglobin synthesis through activation of cyclic guanine monophosphate (cGMP) signaling. Studies in sickle cell patients demonstrated increased circulating nitric oxide (NO) levels after oral HU treatment. However, the direct measurement of NO in erythroid cells and its role in fetal hemoglobin induction have not been defined. Therefore, we quantified the level of nitrate and nitrite (NOx) generated by HU in human erythroid progenitors in the presence of three nitric oxide synthase inhibitors (NOS), including N(G)-monomethyl-L-arginine (L-NMMA). In addition, cGMP levels were measured in the presence or absence of the pathway inhibitor 1H-(1,2,4)ox-adiazolo(4,3-a)quinoxalin-1-one, which blocks soluble guanylyl cyclase formation. HU treatment increased NOx levels and gamma-globin transcription in K562 and primary erythroid cells, which was augmented when HU was combined with L-NMMA. Pretreatment with the cGMP pathway inhibitor reversed gamma-gene activation by HU. These data demonstrate the direct stimulation of cellular NO and cGMP signaling in erythroid progenitors by HU as a possible mechanism for gamma-globin gene activation.

Cyanide, peroxide and nitric oxide formation in solutions of hydroxyurea causes cellular toxicity and may contribute to its therapeutic potency

Hydroxyurea (HU) is a potent remedy against a variety of ailments and an efficient inhibitor of DNA synthesis, yet its pharmacology is unclear. HU acts in Escherichia coli by the same mechanism as it does in eukaryotes, via inhibition of ribonucleotide reductase. When examining a controversy about concentrations of HU that prevent thymineless death in E. coli, we found instability in HU solutions that avoided prior detection due to its peculiar nature. In contrast to freshly dissolved HU, which did not affect respiration and was bacteriostatic, 1-day-old HU solutions inhibited respiration and were immediately bactericidal. Respiration was inhibited by two gases, hydrogen cyanide (HCN) and nitric oxide (NO), whose appearance we detected in "aged" HU stocks by gas chromatography-mass spectrometry; however, neither gas was bactericidal. While determining the cause of toxicity, we found that HU damages DNA directly. We also demonstrated accumulation of peroxides in HU solutions by enzymatic assays, which explains the toxicity, as both NO and HCN are known to kill bacteria when combined with hydrogen peroxide. Remarkably, we found that bactericidal effects of NO+H(2)O(2) and HCN+H(2)O(2) mixtures were further synergistic. Accumulation of decomposition products in solutions of HU may explain the broad therapeutic effects of this drug.

Increase in brain tumor permeability in glioma-bearing rats with nitric oxide donors

PURPOSE

The blood-brain tumor barrier (BTB) significantly limits the delivery of chemotherapeutics to brain tumors. Nitric oxide (NO) is involved in the regulation of cerebral vascular permeability. We investigated the effects of NO donors, L-arginine and hydroxyurea, on BTB permeability in 9L gliosarcoma-bearing Fischer rats.

EXPERIMENTAL DESIGN

The rats implanted with 9L gliosarcoma were dosed orally with hydroxyurea and L-arginine. BTB permeability, defined by the unidirectional transport constant, Ki, for [14C]sucrose was measured. The expression of neural and endothelial NO synthase (NOS) in tumors and normal brain tissue was examined. Further, the levels of NO, L-citrulline, and cGMP in the tumor and normal brain tissue were measured.

RESULTS

Oral administration of l-arginine or hydroxyurea significantly increased BTB permeability when compared with the nontreated control. The selective effects were abolished by iberiotoxin, an antagonist of calcium-dependent potassium (KCa) channel that is a cGMP pathway effector. The expression of endothelial NOS, but not neural NOS, was higher in tumor vessels than in those of normal brain. Moreover, the levels of NO, L-citrulline, a byproduct of NO formation from L-arginine, and cGMP were enhanced in the tumor tissue by oral administration of L-arginine and/or hydroxyurea.

CONCLUSIONS

Oral administration of L-arginine or hydroxyurea selectively increased tumor permeability, which is likely mediated by alteration in cGMP levels. The findings suggest that use of oral NO donors may be a strategy to enhance the delivery of chemotherapeutics to malignant brain tumors.

An electron paramagnetic resonance investigation of the oxygen dependence of the arterial-venous gradient of nitrosyl hemoglobin in blood circulation

Whether there is a nitrosyl hemoglobin (HbNO) gradient between the venous and the arterial parts of the circulatory system is a very controversial issue in nitric oxide research. We have carefully evaluated the measurement of HbNO concentration in blood using EPR generated in vivo by the NO donor DEANO under various oxygen tensions. We found that the absolute concentrations of HbNO in venous and arterial blood were the same within experimental error, independent of hemoglobin saturation; only the ratios of 5-coordinate and 6-coordinate HbNO differed. The HbNO concentration increased when the oxygen concentration breathed by the rats decreased in a manner that was linear in hemoglobin saturation. These results do not support the existence of an arterial-venous gradient of HbNO under our experimental conditions.

Hydroxyurea chemotherapy in the treatment of meningiomas

✓ Meningiomas are slow growing, extraaxial tumors that derive from the arachnoidal cap cells of the meninges. Resection remains the main modality of treatment and can be curative in some cases. External-beam radiotherapy and radiosurgery can benefit selected patients. The role of chemotherapy continues to be defined, but should be considered for patients with inoperable or frequently recurring meningiomas. Hydroxyurea, an inhibitor of ribonucleotide reductase, is one of the most active agents and is known to induce apoptosis in meningioma cells in vitro and in mouse xenografts. Results of preliminary clinical studies suggest that hydroxyurea has modest activity against recurrent and inoperable meningiomas, and can induce long term stabilization in some patients. However, the results are conflicting and a few clinical trials did not show positive results. Further clinical trials with larger patient cohorts and longer follow-up periods will be necessary to confirm the activity of hydroxyurea.

Platelet aggregation profile as a marker of hydroxyurea bioavailability through nitric oxide generation in chronic myelogenous leukemia

Platelet aggregation profiles were studied in chronic myelogenous leukemia patients who were undergoing hydroxyurea therapy. Nitric oxide (NO) generation induced by hydroxyurea was measured from the altered aggregatory response, in which the platelet suspension exhibits a de-aggregatory behaviour. NO caused platelet de-aggregation by generation of cyclic guanidine monophosphate through the activation of soluble guanylate cyclase (SGC). The fact that the observed response is specific to NO was confirmed by the reversal of the de-aggregatory behaviour in the presence of (1)H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), an inhibitor of SGC. Among the subjects studied, one subset showed an hydroxyurea-induced de-aggregatory effect that was inhibited by ODQ, whereas another subset did not show any such effect. The observed inter-individual variability in platelet aggregometric response after the ingestion of drugs may be an indicator for NO generation from hydroxyurea, and this may help to explain the drug efficacy encountered in such cases.

Expression of the gamma-globin gene is sustained by the cAMP-dependent pathway in beta-thalassaemia

The present study found that the cyclic adenosine monophosphate (cAMP)-dependent pathway efficiently induced gamma-globin expression in adult erythroblasts, and this pathway plays a role in gamma-globin gene (HBG) expression in beta-thalassaemia. Expression of HBG mRNA increased to about 46% of non-HBA mRNA in adult erythroblasts treated with forskolin, while a cyclic guanosine monophosphate (cGMP) analogue induced HBG mRNA to levels <20% of non-HBA mRNA. In patients with beta-thalassaemia intermedia, cAMP levels were elevated in both red blood cells and nucleated erythroblasts but no consistent elevation was found with cGMP levels. The transcription factor cAMP response element binding protein (CREB) was phosphorylated in nucleated erythroblasts and its phosphorylation levels correlated with HBG mRNA levels of the patients. Other signalling molecules, such as mitogen-activated protein kinases and signal transducers and activators of transcription proteins, were phosphorylated at variable levels and showed no correlations with the HBG mRNA levels. Plasma levels of cytokines, such as erythropoietin, stem cell factor and transforming growth factor-beta were increased in patients, and these cytokines induced both HBG mRNA expression and CREB phosphorylation. These results demonstrate that the cAMP-dependent pathway, the activity of which is augmented by multiple cytokines, plays a role in regulating HBG expression in beta-thalassaemia.

Rat liver-mediated metabolism of hydroxyurea to nitric oxide

Hydroxyurea is an approved treatment for sickle cell disease. Oxidation of hydroxyurea results in the formation of nitric oxide (NO), which also has drawn considerable interest as a sickle cell disease therapy. Although patients on hydroxyurea demonstrate elevated levels of nitric oxide-derived metabolites, little information regarding the site or mechanism of the in vivo conversion of hydroxyurea to nitric oxide exists. Chemiluminescence detection experiments show the ability of crude rat liver homogenate to convert hydroxyurea to nitrite/nitrate, evidence for NO production. Nitrite/nitrate form at therapeutic concentrations of hydroxyurea in a clinically relevant time frame. Electron paramagnetic resonance (EPR) studies show the formation of iron nitrosyl complexes during this incubation and experiments with labeled hydroxyurea show the NO derives from the drug. Gas chromatography-mass spectrometry measurements indicate the hydrolysis of hydroxyurea to hydroxylamine in this system. Incubation of hydroxylamine with crude rat liver homogenate also generates nitrite/nitrate and iron nitrosyl complexes. A line of evidence including inhibitor studies, EPR spectroscopy, and nitrite/nitrate detection identifies catalase as a possible oxidant for the conversion of hydroxyurea to NO. These results reveal the ability of liver tissue to convert hydroxyurea to nitric oxide and provide insight into the metabolism of this drug.

Nitric oxide generation from hydroxyurea: significance and implications for leukemogenesis in the management of myeloproliferative disorders

The use of myelosuppressive agents to reduce the risk of thrombosis in patients with polycythemia vera (PV) and essential thrombocythemia (ET) has been associated with an increased risk of transformation to acute myeloid leukemia (AML). Whereas chlorambucil, busulfan, and radiophosphorus (32P) have been demonstrated to increase the risk of transformation, the leukemogenic potential of hydroxyurea (HU) continues to be a matter of debate. Clinical studies have suggested that HU may cause a small increase in the risk of AML, but it has proven difficult to establish whether AML is actually caused by HU or arises during the natural progression of PV and ET. Reports that HU undergoes metabolic activation to species that induce mutation appear to support the notion that it is leukemogenic. Here, we suggest that the ability of HU to induce mutation in cell culture studies results from the generation of nitrogen dioxide via the autoxidation of nitric oxide, a product of HU metabolism. However, we argue that autoxidation would not occur in vivo, leading to the conclusion that generation of the mutagen nitrogen dioxide is peculiar to cell culture systems and has little relevance to the use of HU in the management of PV and ET.

Rates of nitric oxide dissociation from hemoglobin

Nitric oxide (NO) plays a major role in human physiology and in many pathological states. Although oxyhemoglobin is known to destroy NO activity, NO activity can, in principle, be conserved through iron nitrosylation at vacant hemes. In order for this NO activity to be delivered, the NO must dissociate from the heme. Despite its study over the past few decades, our understanding of NO dissociation from hemoglobin is incomplete. In principle, there are at least four NO dissociation rates: kR(alpha), kR(beta), kT(alpha), and kT(beta), where the subscript refers to the quaternary state and the superscript to the hemoglobin chain. In the T-state, a proportion of the proximal histidine bonds break forming pentacoordinate alpha-nitrosyl hemoglobin. In vivo, alpha-nitrosyl hemoglobin predominates over beta-nitrosyl hemoglobin. In this study we have used a fast NO trap, Fe(II)-proline-dithiocarbamate, to measure NO dissociation rates from hemoglobin. We have varied solution conditions so the rate of dissociation from pentacoordinate alpha-nitrosyl hemoglobin could be definitively measured for the first time; kT(alpha) = 4.2 +/- 1.5 x 10(-4) s(-1). We have also found that the fastest NO dissociation rate is on the order of 10(-3) s(-1) and that NO dissociation from sickle cell hemoglobin is the same as that from normal adult hemoglobin.

The hydroxyurea-induced small GTP-binding protein SAR modulates gamma-globin gene expression in human erythroid cells

Hydroxyurea (HU), a drug effective in the treatment of sickle cell disease, is thought to indirectly promote fetal hemoglobin (Hb F) production by perturbing the maturation of erythroid precursors. The molecular mechanisms involved in HU-mediated regulation of gamma-globin expression are currently unclear. We identified an HU-induced small guanosine triphosphate (GTP)-binding protein, secretion-associated and RAS-related (SAR) protein, in adult erythroid cells using differential display. Stable SAR expression in K562 cells increased gamma-globin mRNA expression and resulted in macrocytosis. The cells appeared immature. SAR-mediated induction of gamma-globin also inhibited K562 cell growth by causing arrest in G1/S, apoptosis, and delay of maturation, cellular changes consistent with the previously known effects of HU on erythroid cells. SAR also enhanced both gamma- and beta-globin transcription in primary bone marrow CD34+ cells, with a greater effect on gamma-globin than on beta-globin. Although up-regulation of GATA-2 and p21 was observed both in SAR-expressing cells and HU-treated K562 cells, phosphatidylinositol 3 (PI3) kinase and phosphorylated ERK were inhibited specifically in SAR-expressing cells. These data reveal a novel role of SAR distinct from its previously known protein-trafficking function. We suggest that SAR may participate in both erythroid cell growth and gamma-globin production by regulating PI3 kinase/extracellular protein-related kinase (ERK) and GATA-2/p21-dependent signal transduction pathways.

Mechanisms of vasodilatation induced by nitrite instillation in intestinal lumen: possible role of hemoglobin

It has been shown that nitrite can be reduced to nitric oxide (NO) in intestine and a number of other tissues and released into the blood to form nitrosylhemoglobin (NO-Hb), existing in an equilibrium with S-nitrosohemoglobin. The latter has been suggested to be an NO transporter to distant organs. The aim of this study was to define the pathway of nitrite reduction to form NO in intestinal wall and to estimate whether this pathway has an effect on peripheral circulation. We have shown that in rat intestine at pH 7.0 70% of nitrite is converted to NO in mitochondria. At pH 6.0, nonenzymatic nitrite reduction becomes as efficient as the mitochondrial pathway. To prove whether the NO formed from nitrite in intestine can induce vasodilatation, sodium nitrite was instilled into intestinal lumen and the concentration of NO formed and diffused into the blood was followed by measuring of NO-Hb complex formation. We found that the concentration of NO-Hb gradually increases with the increase of nitrite concentration in intestinal lumen. However, it was not always accompanied by a decrease in systemic blood pressure. Blood pressure dropped down only after NO-Hb reached a threshold concentration of approximately 10 microM. These data show that NO-Hb cannot provide enough NO for vasodilatation if the concentration of NO bound to Hb is < 10 microM. The exact mechanism underlying vasodilatation observed when the concentration of NO-bound Hb was > 10 microM is, however, not clear yet and requires further studies.