Primary care diabetes assessment when HbA1c and other measures of glycemia disagree

AIMS

Although diabetes management decisions in primary care are typically based largely on HbA1c, mismatches between HbA1c and other measures of glycemia that are increasingly more available present challenges to optimal management. This study aimed to assess a systematic approach to identify the frequency of mismatches of potential clinical significance amongst various measures of glycemia in a primary care setting.

METHODS

Following screening to exclude conditions known to affect HbA1c interpretation, HbA1c, and fructosamine were obtained and repeated after ∼90 days on 53 adults with prediabetes or type 2 diabetes. A subset of 13 participants with repeat labs wore continuous glucose monitoring (CGM) for 10 days.

RESULTS

As expected, HbA1c and fructosamine only modestly correlated (initial R2 = 0.768/repeat R2 = 0.655). The HbA1c/fructosamine mismatch frequency of ± 0.5% (using the following regression HbA1c = 0.015 *fructosamine + 2.994 calculated from the initial sample) was 27.0%. Of the 13 participants with CGM data, HbA1c and CGM-based Glucose Management Indicator correlated at R2 = 0.786 with a mismatch frequency of ± 0.5% at 46.2% compared to a HbA1c/fructosamine mismatch frequency of ± 0.5% at 30.8%.

CONCLUSIONS

HbA1c is frequently mismatched with fructosamine and CGM data. As each of the measures has strengths and weaknesses, the utilization of multiple different measures of glycemia may be informative for diabetes assessment in the clinical setting.

Antibodies against biotin-labeled red blood cells can shorten posttransfusion survival

BACKGROUND

In hematologic and transfusion medicine research, measurement of red blood cell (RBC) in vivo kinetics must be safe and accurate. Recent reports indicate use of biotin-labeled RBC (BioRBC) to determine red cell survival (RCS) offers substantial advantages over ⁵¹ Cr and other labeling methods. Occasional induction of BioRBC antibodies has been reported.

STUDY DESIGN AND METHODS

To investigate the causes and consequences of BioRBC immunization, we reexposed three previously immunized adults to BioRBC and evaluated the safety, antibody emergence, and RCS of BioRBC.

RESULTS

BioRBC re-exposure caused an anamnestic increase of plasma BioRBC antibodies at 5-7 days; all were subclass IgG₁ and neutralized by biotinylated albumin, thus indicating structural specificity for the biotin epitope. Concurrently, specific antibody binding to BioRBC was observed in each subject. As biotin label density increased, the proportion of BioRBC that bound increased antibody also increased; the latter was associated with proportional accelerated removal of BioRBC labeled at density 6 μg/mL. In contrast, only one of three subjects exhibited accelerated removal of BioRBC density 2 μg/mL. No adverse clinical or laboratory events were observed. Among three control subjects who did not develop BioRBC antibodies following initial BioRBC exposure, re-exposure induced neither antibody emergence nor accelerated BioRBC removal.

DISCUSSION

We conclude re-exposure of immunized subjects to BioRBC can induce anamnestic antibody response that can cause an underestimation of RCS. To minimize chances of antibody induction and underestimation of RCS, we recommend an initial BioRBC exposure volume of ≤10 mL and label densities of ≤18 μg/mL.

Enhanced Efficacy of Combination of Gemcitabine and Phosphatidylserine-Targeted Nanovesicles against Pancreatic Cancer

Phosphatidylserine (PS) is often externalized in viable pancreatic cancer cells and is therapeutically targetable using PS-selective drugs. One of the first-line treatments for advanced pancreatic cancer disease, gemcitabine (GEM), provides only marginal benefit to patients. We therefore investigated the therapeutic benefits of combining GEM and the PS-targeting drug, saposin C-dioleoylphosphatidylserine (SapC-DOPS), for treating pancreatic ductal adenocarcinoma (PDAC). Using cell-cycle analyses and a cell surface PS-based sorting method in vitro, we observed an increase in surface PS as cells progress through the cell cycle from G1 to G2/M. We also observed that GEM treatment preferentially targets G1 phase cells that have low surface PS, resulting in an increased median surface PS level of PDAC cells. Inversely, SapC-DOPS preferentially targets high surface PS cells that are predominantly in the G2/M phase. Finally, combination therapy in subcutaneous and orthotopic PDAC tumors in vivo with SapC-DOPS and GEM or Abraxane (Abr)/GEM (one of the current standards of care) significantly inhibits tumor growth and increases survival compared with individual treatments. Our studies confirm a surface PS and cell cycle-based enhancement of cancer cytotoxicity following SapC-DOPS treatment in combination with GEM or Abr/GEM. Thus, PDAC patients treated with Abr/GEM may benefit from concurrent administration of SapC-DOPS.

When HbA1c and Blood Glucose Do Not Match: How Much Is Determined by Race, by Genetics, by Differences in Mean Red Blood Cell Age?

Commentary placing genetic ancestry markers and racial difference in HbA1c in the context of more common variations in the HbA1c-average glucose relationship and their clinical implications.

Enhanced phosphatidylserine-selective cancer therapy with irradiation and SapC-DOPS nanovesicles

Normal living cells exhibit phosphatidylserine (PS) primarily within the intracellular leaflet of the plasma membrane. In contrast, viable cancer cells have high levels of PS on the external surface, and exhibit a broad range of surface PS, even within specific types of cancer. Agents that target surface PS have recently been developed to treat tumors and are expected to be more effective with higher surface PS levels. In this context, we examined whether surface PS is increased with irradiation. In vitro irradiation of cancer cell lines selected surviving cells that had higher surface PS in a dose- and time-dependent manner. This was more pronounced if surface PS was initially in the lower range for cancer cells. Radiation also increased the surface PS of tumor cells in subcutaneous xenografts in nude mice. We found an inverse relationship between steady state surface PS level of cancer cell lines and their sensitivity to radiation-induced cell death. In addition, serial irradiation, which selected surviving cells with higher surface PS, also increased resistance to radiation and to some chemotherapeutic drugs, suggesting a PS-dependent mechanism for development of resistance to therapy. On the other hand, fractionated radiation enhanced the effect of a novel anti-cancer, PS-targeting drug, SapC-DOPS, in some cancer cell lines. Our data suggest that we can group cancer cells into cells with low surface PS, which are sensitive to radiation, and high surface PS, which are sensitive to SapC-DOPS. Combination of these interventions may provide a potential new combination therapy.

Development, validation, and potential applications of biotinylated red blood cells for posttransfusion kinetics and other physiological studies: evidenced-based analysis and recommendations

The current reference method in the United States for measuring in vivo population red blood cell (RBC) kinetics utilizes chromium-51 (⁵¹ Cr) RBC labeling for determining RBC volume, 24-hour posttransfusion RBC recovery, and long-term RBC survival. Here we provide evidence supporting adoption of a method for kinetics that uses the biotin-labeled RBCs (BioRBCs) as a superior, versatile method for both regulatory and investigational purposes. RBC kinetic analysis using BioRBCs has important methodologic, analytical, and safety advantages over ⁵¹ Cr-labeled RBCs. We critically review recent advances in labeling human RBCs at multiple and progressively lower biotin label densities for concurrent, accurate, and sensitive determination of both autologous and allogeneic RBC population kinetics. BioRBC methods valid for RBC kinetic studies, including successful variations used by the authors, are presented along with pharmacokinetic modeling approaches for the accurate determination of RBC pharmacokinetic variables in health and disease. The advantages and limitations of the BioRBC method-including its capability of determining multiple BioRBC densities simultaneously in the same individual throughout the entire RBC life span-are presented and compared with the ⁵¹ Cr method. Finally, potential applications and limitations of kinetic BioRBC determinations are discussed.

Antibodies to biotinylated red blood cells in adults and infants: improved detection, partial characterization, and dependence on red blood cell-biotin dose

BACKGROUND

Biotin-labeled red blood cells (BioRBCs) are used for in vivo kinetic studies. Because BioRBC dosing occasionally induces antibodies, a sensitive and specific anti-BioRBC detection assay is needed.

STUDY DESIGN AND METHODS

Aims were to 1) develop a gel card assay to evaluate existing, naturally occurring and BioRBC-induced plasma antibodies, 2) compare gel card and tube agglutination detection results, and 3) test for a relationship of antibody induction and BioRBC dose. Reagent BioRBCs were prepared using sulfo-NHS biotin ranging from densities 18 (BioRBC-18) to 1458 (BioRBC-1458) µg/mL RBCs.

RESULTS

Among BioRBC-exposed subjects, gel card and tube agglutination results were concordant in 21 of 22 adults and all 19 infant plasma samples. Gel card antibody detection sensitivity was more than 10-fold greater than tube agglutination. Twelve to 16 weeks after BioRBC exposure, induced anti-antibodies were detected by gel card in three of 26 adults (12%) at reagent densities BioRBC-256 or less, but in none of 41 infants. Importantly, induced anti-BioRBC antibodies were associated with higher BioRBC dose (p = 0.008); no antibodies were detected in 18 subjects who received BioRBC doses less than or equal to BioRBC-18. For noninduced BioRBC antibodies, six of 1125 naïve adults (0.3%) and none of 46 naïve infants demonstrated existing anti-BioRBC antibodies using reagent BioRBC-140 or -162. Existing anti-BioRBCs were all neutralized by biotin compounds, while induced antibodies were not.

CONCLUSIONS

The gel card assay is more sensitive than the tube agglutination assay. We recommend reagent BioRBC-256 for identifying anti-BioRBCs. Use of a low total RBC biotin label dose (≤ BioRBC-18) may minimize antibody induction.

Biochemical surrogate markers of hemolysis do not correlate with directly measured erythrocyte survival in sickle cell anemia

Hemolysis is a key feature of sickle cell anemia (HbSS). Direct quantitation of hemolysis could be used as an objective outcome in clinical trials of new therapeutics for HbSS and would also enable better human studies of the pathogenesis of complications of HbSS that are ostensibly hemolysis-related, such as pulmonary hypertension. However, contemporary human studies in HbSS have used only surrogate markers of hemolysis rather than direct measurements of RBC survival. We directly quantified hemolysis in HbSS by measuring survival of an age cohort of RBCs labeled with a stable isotope, administered orally as ¹⁵ N-glycine, a metabolic precursor of heme. The atomic excess of ¹⁵ N in heme extracted from blood was monitored by mass spectrometry over time. We performed 13 labeling experiments in 11 individuals with HbSS. Mean RBC survival was 31.9 days (range 14.1-53.6). Both HbF level, a known determinant of hemolysis, and absolute reticulocyte count (ARC), an index of the marrow's response to hemolysis, correlated with directly measured RBC survival (r = 0.61, P < 0.002; r = -0.84, P < 0.001). However, commonly used biochemical surrogates of hemolysis (LDH, AST, bilirubin, and plasma free hemoglobin) did not correlate with directly measured RBC survival. These biochemical surrogates should be interpreted cautiously, at best, in clinical trials and human physiologic studies in HbSS. ARC was the best correlate of total hemolysis, but only 70% of the variation in RBC survival was reflected in this marker. If greater accuracy is required in human studies, ¹⁵ N-glycine RBC labeling can directly and accurately quantify hemolysis. Am. J. Hematol. 91:1195-1201, 2016. © 2016 Wiley Periodicals, Inc.

Variation in human cancer cell external phosphatidylserine is regulated by flippase activity and intracellular calcium

Viable cancer cells expose elevated levels of phosphatidylserine (PS) on the exoplasmic face of the plasma membrane. However, the mechanisms leading to elevated PS exposure in viable cancer cells have not been defined. We previously showed that externalized PS may be used to monitor, target and kill tumor cells. In addition, PS on tumor cells is recognized by macrophages and has implications in antitumor immunity. Therefore, it is important to understand the molecular details of PS exposure on cancer cells in order to improve therapeutic targeting. Here we explored the mechanisms regulating the surface PS exposure in human cancer cells and found that differential flippase activity and intracellular calcium are the major regulators of surface PS exposure in viable human cancer cells. In general, cancer cell lines with high surface PS exhibited low flippase activity and high intracellular calcium, whereas cancer cells with low surface PS exhibited high flippase activity and low intracellular calcium. High surface PS cancer cells also had higher total cellular PS than low surface PS cells. Together, our results indicate that the amount of external PS in cancer cells is regulated by calcium dependent flippase activity and may also be influenced by total cellular PS.

Red Blood Cell Dysfunction Induced by High-Fat Diet: Potential Implications for Obesity-Related Atherosclerosis

BACKGROUND

High-fat diet (HFD) promotes endothelial dysfunction and proinflammatory monocyte activation, which contribute to atherosclerosis in obesity. We investigated whether HFD also induces the dysfunction of red blood cells (RBCs), which serve as a reservoir for chemokines via binding to Duffy antigen receptor for chemokines (DARC).

METHODS AND RESULTS

A 60% HFD for 12 weeks, which produced only minor changes in lipid profile in C57/BL6 mice, markedly augmented the levels of monocyte chemoattractant protein-1 bound to RBCs, which in turn stimulated macrophage migration through an endothelial monolayer. Levels of RBC-bound KC were also increased by HFD. These effects of HFD were abolished in DARC(-/-) mice. In RBCs from HFD-fed wild-type and DARC(-/-) mice, levels of membrane cholesterol and phosphatidylserine externalization were increased, fostering RBC-macrophage inflammatory interactions and promoting macrophage phagocytosis in vitro. When labeled ex vivo and injected into wild-type mice, RBCs from HFD-fed mice exhibited ≈3-fold increase in splenic uptake. Finally, RBCs from HFD-fed mice induced increased macrophage adhesion to the endothelium when they were incubated with isolated aortic segments, indicating endothelial activation.

CONCLUSIONS

RBC dysfunction, analogous to endothelial dysfunction, occurs early during diet-induced obesity and may serve as a mediator of atherosclerosis. These findings may have implications for the pathogenesis of atherosclerosis in obesity, a worldwide epidemic.

Phosphatidylserine-selective targeting and anticancer effects of SapC-DOPS nanovesicles on brain tumors

Brain tumors, either primary (e.g., glioblastoma multiforme) or secondary (metastatic), remain among the most intractable and fatal of all cancers. We have shown that nanovesicles consisting of Saposin C (SapC) and dioleylphosphatidylserine (DOPS) are able to effectively target and kill cancer cells both in vitro and in vivo. These actions are a consequence of the affinity of SapC-DOPS for phosphatidylserine, an acidic phospholipid abundantly present in the outer membrane of a variety of tumor cells and tumor-associated vasculature. In this study, we first characterize SapC-DOPS bioavailability and antitumor effects on human glioblastoma xenografts, and confirm SapC-DOPS specificity towards phosphatidylserine by showing that glioblastoma targeting is abrogated after in vivo exposure to lactadherin, which binds phosphatidylserine with high affinity. Second, we demonstrate that SapC-DOPS selectively targets brain metastases-forming cancer cells both in vitro, in co-cultures with human astrocytes, and in vivo, in mouse models of brain metastases derived from human breast or lung cancer cells. Third, we demonstrate that SapC-DOPS nanovesicles have cytotoxic activity against metastatic breast cancer cells in vitro, and prolong the survival of mice harboring brain metastases. Taken together, these results support the potential of SapC-DOPS for the diagnosis and therapy of primary and metastatic brain tumors.

Activation of protein kinase C by phorbol ester increases red blood cell scramblase activity and external phosphatidylserine

Externalization of phosphatidylserine (PS) is thought to contribute to sickle cell disease (SCD) pathophysiology. The red blood cell (RBC) aminophospholipid translocase (APLT) mediates the transport of PS from the outer to the inner RBC membrane leaflet to maintain an asymmetric distribution of PL, while phospholipid scramblase (PLSCR) equilibrates PL across the RBC membrane, promoting PS externalization. We previously identified an association between PS externalization level and PLSCR activity in sickle RBC under basal conditions. Other studies showed that activation of protein kinase C (PKC) by PMA (phorbol-12-myristate-13-acetate) causes increased external PS on RBC. Therefore, we hypothesized that PMA-activated PKC stimulates PLSCR activity in RBC and thereby contributes to increased PS externalization. In the current studies, we show that PMA treatment causes immediate and variable PLSCR activation and subsequent PS externalization in control and sickle RBC. While TfR+ sickle reticulocytes display some endogenous PLSCR activity, we observed a robust activation of PLSCR in sickle reticulocytes treated with PMA. The PKC inhibitor, chelerythrine (Chel), significantly inhibited PMA-dependent PLSCR activation and PS externalization. Chel also inhibited endogenous PLSCR activity in sickle reticulocytes. These data provide evidence that PKC mediates PS externalization in RBC through activation of PLSCR.

SapC-DOPS nanovesicles induce Smac- and Bax-dependent apoptosis through mitochondrial activation in neuroblastomas

BACKGROUND

High toxicity, morbidity and secondary malignancy render chemotherapy of neuroblastoma inefficient, prompting the search for novel compounds. Nanovesicles offer great promise in imaging and treatment of cancer. SapC-DOPS, a stable nanovesicle formed from the lysosomal protein saposin C and dioleoylphosphatidylserine possess strong affinity for abundantly exposed surface phosphatidylserine on cancer cells. Here, we show that SapC-DOPS effectively targets and suppresses neuroblastoma growth and elucidate the molecular mechanism of SapC-DOPS action in neuroblastoma in vitro.

METHODS

In vivo targeting of neuroblastoma was assessed in xenograft mice injected intravenously with fluorescently-labeled SapC-DOPS. Xenografted tumors were also used to demonstrate its therapeutic efficacy. Apoptosis induction in vivo was evaluated in tumor sections using the TUNEL assay. The mechanisms underlying the induction of apoptosis by SapC-DOPS were addressed through measurements of cell viability, mitochondrial membrane potential (ΔΨM), flow cytometric DNA fragmentation assays and by immunoblot analysis of second mitochondria-derived activator of caspases (Smac), Bax, Cytochrome c (Cyto c) and Caspase-3 in the cytosol or in mitochondrial fractions of cultured neuroblastoma cells.

RESULTS

SapC-DOPS showed specific targeting and prevented the growth of human neuroblastoma xenografts in mice. In neuroblastoma cells in vitro, apoptosis occurred via a series of steps that included: (1) loss of ΔΨM and increased mitochondrial superoxide formation; (2) cytosolic release of Smac, Cyto c, AIF; and (3) mitochondrial translocation and polymerization of Bax. ShRNA-mediated Smac knockdown and V5 peptide-mediated Bax inhibition decreased cytosolic Smac and Cyto c release along with caspase activation and abrogated apoptosis, indicating that Smac and Bax are critical mediators of SapC-DOPS action. Similarly, pretreatment with the mitochondria-stabilizing agent bongkrekic acid decreased apoptosis indicating that loss of ΔΨM is critical for SapC-DOPS activity. Apoptosis induction was not critically dependent on reactive oxygen species (ROS) production and Cyclophilin D, since pretreatment with N-acetyl cysteine and cyclosporine A, respectively, did not prevent Smac or Cyto c release.

CONCLUSIONS

Taken together, our results indicate that SapC-DOPS acts through a mitochondria-mediated pathway accompanied by an early release of Smac and Bax. Specific tumor-targeting capacity and anticancer efficacy of SapC-DOPS supports its potential as a dual imaging and therapeutic agent in neuroblastoma therapy.

Use of an oral stable isotope label to confirm variation in red blood cell mean age that influences HbA1c interpretation

HbA1c is commonly used to monitor glycemic control. However, there is growing evidence that the relationship between HbA1c and mean blood glucose (MBG) is influenced by variation in red blood cell (RBC) lifespan in hematologically normal individuals. Correction of HbA1c for mean RBC age (MRBC ) requires a noninvasive, accurate, and affordable method to measure RBC survival. In this study, we evaluated whether a stable isotope approach would satisfy these requirements. RBC lifespan and MRBC were determined in a group of nine hematologically normal diabetic and nondiabetic subjects using oral (15) N-glycine to label heme in an age cohort of RBC. The MRBC was 58.7 ± 9.1 (2SD) days and RBC lifespan was 106 ± 21 (2SD) days. This degree of variation (±15-20%) is consistent with previous studies using other techniques. In a subset of seven subjects, MRBC determined with the biotin label technique were available from approximately five years prior, and strongly correlated with the stable isotope values (R(2) = 0.79). This study suggests that the MRBC is stable over time but varies substantially among individuals, and supports the importance of its variation in HbA1c interpretation. The characteristics of the stable isotope method support its suitability for studies to directly evaluate the impact of variation in MRBC on the interpretation of HbA1c.

Targeting and cytotoxicity of SapC-DOPS nanovesicles in pancreatic cancer

Only a small number of promising drugs target pancreatic cancer, which is the fourth leading cause of cancer deaths with a 5-year survival of less than 5%. Our goal is to develop a new biotherapeutic agent in which a lysosomal protein (saposin C, SapC) and a phospholipid (dioleoylphosphatidylserine, DOPS) are assembled into nanovesicles (SapC-DOPS) for treating pancreatic cancer. A distinguishing feature of SapC-DOPS nanovesicles is their high affinity for phosphatidylserine (PS) rich microdomains, which are abnormally exposed on the membrane surface of human pancreatic tumor cells. To evaluate the role of external cell PS, in vitro assays were used to correlate PS exposure and the cytotoxic effect of SapC-DOPS in human tumor and nontumorigenic pancreatic cells. Next, pancreatic tumor xenografts (orthotopic and subcutaneous models) were used for tumor targeting and therapeutic efficacy studies with systemic SapC-DOPS treatment. We observed that the nanovesicles selectively killed human pancreatic cancer cells in vitro by inducing apoptotic death, whereas untransformed cells remained unaffected. This in vitro cytotoxic effect correlated to the surface exposure level of PS on the tumor cells. Using xenografts, animals treated with SapC-DOPS showed clear survival benefits and their tumors shrank or disappeared. Furthermore, using a double-tracking method in live mice, we showed that the nanovesicles were specifically targeted to orthotopically-implanted, bioluminescent pancreatic tumors. These data suggest that the acidic phospholipid PS is a biomarker for pancreatic cancer that can be effectively targeted for therapy utilizing cancer-selective SapC-DOPS nanovesicles. This study provides convincing evidence in support of developing a new therapeutic approach to pancreatic cancer.

Changes in the properties of normal human red blood cells during in vivo aging

The changes in red blood cells (RBC) as they age and the mechanisms for their eventual removal have been of interest for many years. Proposed age-related changes include dehydration with increased density and decreased size, increased membrane IgG, loss of membrane phospholipid asymmetry, and decreased activity of KCl cotransport. The biotin RBC label allows unambiguous identification of older cells and exploration of their properties as they age. Autologous normal human RBC were labeled ex vivo and, after reinfusion, compared with unlabeled RBC throughout their lifespan. RBC density increased with age, with most of the change in the first weeks. Near the end of their lifespan, RBC had increased surface IgG. However, there was no evidence for elevated external phosphatidylserine (PS) even though older RBC had significantly lower activity of aminophospholipid translocase (APLT). KCl cotransport activity persisted well past the reticulocyte stage, but eventually decreased as the RBC became older. These studies place limitations on the use of density fractionation for the study of older human RBC, and do not support loss of phospholipid asymmetry as a mechanism for human RBC senescence. However, increased levels of IgG were associated with older RBC, and may contribute to their removal from the circulation.

Pharmacological inhibition of calpain-1 prevents red cell dehydration and reduces Gardos channel activity in a mouse model of sickle cell disease

Sickle cell disease (SCD) is a globally distributed hereditary red blood cell (RBC) disorder. One of the hallmarks of SCD is the presence of circulating dense RBCs, which are important in SCD-related clinical manifestations. In human dense sickle cells, we found reduced calpastatin activity and protein expression compared to either healthy RBCs or unfractionated sickle cells, suggesting an imbalance between activator and inhibitor of calpain-1 in favor of activator in dense sickle cells. Calpain-1 is a nonlysosomal cysteine proteinase that modulates multiple cell functions through the selective cleavage of proteins. To investigate the relevance of this observation in vivo, we evaluated the effects of the orally active inhibitor of calpain-1, BDA-410 (30 mg/kg/d), on RBCs from SAD mice, a mouse model for SCD. In SAD mice, BDA-410 improved RBC morphology, reduced RBC density (D(20); from 1106 ± 0.001 to 1100 ± 0.001 g/ml; P<0.05) and increased RBC-K(+) content (from 364 ± 10 to 429 ± 12.3 mmol/kg Hb; P<0.05), markedly reduced the activity of the Ca(2+)-activated K(+)channel (Gardos channel), and decreased membrane association of peroxiredoxin-2. The inhibitory effect of calphostin C, a specific inhibitor of protein kinase C (PKC), on the Gardos channel was eliminated after BDA-410 treatment, which suggests that calpain-1 inhibition affects the PKC-dependent fraction of the Gardos channel. BDA-410 prevented hypoxia-induced RBC dehydration and K(+) loss in SAD mice. These data suggest a potential role of BDA-410 as a novel therapeutic agent for treatment of SCD.

Measurement of red cell lifespan and aging

SUMMARY

The measurement of red blood cell (RBC) survival has a long history, and a wide variety of methods have been utilized for this purpose. Current methods are of 2 types. First, those that label a representative sample of RBCs of all ages from the blood and then measure their rate of disappearance upon reinfusion. This category includes the (51)Cr and biotin labels. Second, those that use a metabolic precursor or product to determine the turnover of hemoglobin. Examples of these are carbon monoxide production and incorporation of labeled glycine. Recent studies with the covalent, nonradioactive biotin label show its unique suitability for both the accurate measurement of red cell survival and the determination of changes in red cell properties as they age in vivo.

Splice variants of tissue factor promote monocyte-endothelial interactions by triggering the expression of cell adhesion molecules via integrin-mediated signaling

BACKGROUND

TF is highly expressed in cancerous and atherosclerotic lesions. Monocyte recruitment is a hallmark of disease progression in these pathological states.

OBJECTIVE

To examine the role of integrin signaling in TF-dependent recruitment of monocytes by endothelial cells.

METHODS

The expression of flTF and asTF in cervical cancer and atherosclerotic lesions was examined. Biologic effects of the exposure of primary microvascular endothelial cells (MVEC) to truncated flTF ectodomain (LZ-TF) and recombinant asTF were assessed.

RESULTS

flTF and asTF exhibited nearly identical expression patterns in cancer lesions and lipid-rich plaques. Tumor lesions, as well as stromal CD68(+)  monocytes/macrophages, expressed both TF forms. Primary MVEC rapidly adhered to asTF and LZ-TF, and this was completely blocked by anti-β1 integrin antibody. asTF- and LZ-TF-treatment of MVEC promoted adhesion of peripheral blood mononuclear cells (PBMCs) under orbital shear conditions and under laminar flow; asTF-elicited adhesion was more pronounced than that elicited by LZ-TF. Expression profiling and western blotting revealed a broad activation of cell adhesion molecules (CAMs) in MVEC following asTF treatment including E-selectin, ICAM-1 and VCAM-1. In transwell assays, asTF potentiated PMBC migration through MVEC monolayers by ∼3-fold under MCP-1 gradient.

CONCLUSIONS

TF splice variants ligate β1 integrins on MVEC, which induces the expression of CAMs in MVEC and leads to monocyte adhesion and transendothelial migration. asTF appears more potent than flTF in eliciting these effects. Our findings underscore the pathophysiologic significance of non-proteolytic, integrin-mediated signaling by the two naturally occurring TF variants in cancer and atherosclerosis.

K-Cl cotransporter gene expression during human and murine erythroid differentiation

The K-Cl cotransporter (KCC) regulates red blood cell (RBC) volume, especially in reticulocytes. Western blot analysis of RBC membranes revealed KCC1, KCC3, and KCC4 proteins in mouse and human cells, with higher levels in reticulocytes. KCC content was higher in sickle versus normal RBC, but the correlation with reticulocyte count was poor, with inter-individual variability in KCC isoform ratios. Messenger RNA for each isoform was measured by real time RT-quantitative PCR. In human reticulocytes, KCC3a mRNA levels were consistently the highest, 1-7-fold higher than KCC4, the second most abundant species. Message levels for KCC1 and KCC3b were low. The ratios of KCC RNA levels varied among individuals but were similar in sickle and normal RBC. During in vivo maturation of human erythroblasts, KCC3a RNA was expressed consistently, whereas KCC1 and KCC3b levels declined, and KCC4 message first increased and then decreased. In mouse erythroblasts, a similar pattern for KCC3 and KCC1 expression during in vivo differentiation was observed, with low KCC4 RNA throughout despite the presence of KCC4 protein in mature RBC. During differentiation of mouse erythroleukemia cells, protein levels of KCCs paralleled increasing mRNA levels. Functional properties of KCCs expressed in HEK293 cells were similar to each other and to those in human RBC. However, the anion dependence of KCC in RBC resembled most closely that of KCC3. The results suggest that KCC3 is the dominant isoform in erythrocytes, with variable expression of KCC1 and KCC4 among individuals that could result in modulation of KCC activity.

Comparison of 2-Methoxyestradiol and Methotrexate Effects on Non-Hodgkin's B-Cell Lymphoma

PURPOSE

Methotrexate (MTX) is the most commonly used chemotherapeutic agent to treat primary central nervous system lymphoma (PCNSL) and intraocular lymphoma (IOL). 2-methoxyestradiol (2ME2) is a potent antitumor and anti-angiogenesis agent which, unlike other cytotoxic drugs, has minimal toxicity. In this study, anti-proliferative, apoptotic, and cell-cycle effects of 2ME2 and MTX were compared to evaluate 2ME2 efficacy in human lymphoma cells, models for non-Hodgkin B cell lymphomas.

METHODS

The cells were cultured and incubated with varying concentrations of 2ME2 or MTX. A tetrazolium-based colorimetric assay was used to quantify the anti-proliferative effects of 2ME2 and MTX using a microplate reader. To detect apoptotic and cell cycle distribution changes induced by 2ME2 and MTX, the cells were stained with Annexin V-FITC and/or propidium iodide (PI) and analyzed by flow cytometry.

RESULTS

Lymphoma cell proliferation was inhibited by 50% at concentrations ranging from 0.4 to 1 microM for 2ME2 and 0.06 to 0.2 microM for MTX. Induction of apoptosis by 2ME2 and MTX was observed in the tested cells. 2ME2 was a G2/M-phase specific blocker whereas MTX was an S-phase specific blocker in cell cycle analyses. At 1 microM concentration, 2ME2 and MTX showed similar anti-proliferative effect on the lymphoma cell lines. In previously reported studies, for normal endothelial cells, 1 microM 2ME2 showed no appreciable toxicity, while MTX at this same concentration exhibited significant cytotoxicity. 2ME2 at a therapeutic target concentration of 1 mu M may be an effective and relatively non-toxic drug for the treatment of PCNSL with IOL.

CONCLUSIONS

Our study of the effect of 2ME2 and MTX on anti-proliferation, apoptosis, and cell cycling suggests that 2ME2 is a potential agent for treating PCNSL and IOL.

The measurement and importance of red cell survival

The measurement of red blood cell survival in the circulation has progressed from the original differential agglutination technique of Ashby to current isotopic and flow cytometric methods. While occasionally useful in the clinic, these methods find widespread use in a number of important research areas, including the evaluation of new red cell storage media in transfusion medicine and studies of the pathophysiology of sickle cell disease and diabetes. In this review, measurement techniques are placed in historical perspective and examined for relative merits and suitable application.

Evidence for interindividual heterogeneity in the glucose gradient across the human red blood cell membrane and its relationship to hemoglobin glycation

OBJECTIVE

To determine whether interindividual heterogeneity in the erythrocyte (red blood cell [RBC]) transmembrane glucose gradient might explain discordances between A1C and glycemic control based on measured fructosamine.

RESEARCH DESIGN AND METHODS

We modeled the relationship between plasma glucose and RBC glucose as the concentration distribution (C(i)-to-C(o) ratio) of a nonmetabolizable glucose analog (14)C-3-O-methyl glucose ((14)C-3OMG) inside (C(i)) and outside (C(o)) RBCs in vitro. We examined the relationship between that distribution and the degree of glycation of hemoglobin in comparison with glycation of serum proteins (fructosamine), the glycation gap. A1C, fructosamine, and in vitro determination of the (14)C-3OMG distribution in glucose-depleted RBCs were measured in 26 fasted subjects.

RESULTS

The C(i)-to-C(o) ratio 0.89 +/- 0.07 for 3-O-methyl-d-glucopyranose (3OMG) ranged widely (0.72-1.04, n = 26). In contrast, urea C(i)-to-C(o) (1.015 +/- 0.022 [range 0.98-1.07], P < 0.0001) did not. Concerning mechanism, in a representative subset of subjects, the C(i)-to-C(o) ratio was retained in RBC ghosts, was not dependent on ATP or external cations, and was reestablished after reversal of the glucose gradient. The 3OMG C(i)-to-C(o) ratio was not correlated with serum fructosamine, suggesting that it was independent of mean plasma glucose. However, C(i)-to-C(o) did correlate with A1C (R(2) = 0.19) and with the glycation gap (R(2) = 0.20), consistent with a model in which differences in internal glucose concentration at a given mean plasma glucose contribute to differences in A1C for given level of glycemic control.

CONCLUSIONS

The data demonstrate interindividual heterogeneity in glucose gradients across RBC membranes that may affect hemoglobin glycation and have implications for diabetes complications risk and risk assessment.

A method for the continuous calculation of the age of labeled red blood cells

New methods for labeling red blood cells (RBC) and monitoring their survival have made it possible to explore changes in the properties of RBC as they age in the circulation. We have adapted a method, originally developed for studying wild animals, to calculate the age of a random sample of labeled RBC from their survival curve. We also show how this method can be expanded to allow continuous calculation of the mean age of the labeled RBC population at any time after labeling. It is expected that this analytical approach will be useful in the study of age-dependent RBC changes.

Fibrinogen deficiency, but not plasminogen deficiency, increases mortality synergistically in combination with sickle hemoglobin SAD in transgenic mice

Patients with sickle cell disease exhibit both acute and chronic activation of the coagulation and fibrinolytic systems. To test the relationship between sickle cell pathology and activation of the hemostatic system, mice with targeted deletions of plasminogen (Plg) or fibrinogen (Fib) were crossed with transgenic mice expressing Hb SAD [beta(6Glu-Val) (HbS), beta(23Val-Ile) (HbAntilles), and beta(121Glu-Gln) (HbD-Punjab)]. Fibrinogen deficiency dramatically reduced the survival of mice with Hb SAD to a much greater degree than mice with normal hemoglobin. The combination of Hb SAD and fibrinogen deficiency had a greater effect on mortality than that obtained by adding the mortality risks of each defect alone. The deleterious effect of the combination of Hb SAD and fibrinogen deficiency on mortality was accelerated by hypoxia. The excess mortality associated with plasminogen deficiency was identical in SAD and control mice. The adverse effect of fibrinogen deficiency on mortality in SAD mice is not consistent with the simple hypothesis that fibrin deposition is uniformly deleterious in the context of vaso-occlusive sickle cell disease. Rather, our findings suggest that the contribution of fibrinogen to tissue repair may in some contexts limit sickle cell disease pathophysiology.

Urea stimulation of KCl cotransport induces abnormal volume reduction in sickle reticulocytes

KCl cotransport (KCC) activity contributes to pathologic dehydration in sickle (SS) red blood cells (RBCs). KCC activation by urea was measured in SS and normal (AA) RBCs as Cl-dependent Rb influx. KCC-mediated volume reduction was assessed by measuring reticulocyte cellular hemoglobin concentration (CHC) cytometrically. Urea activated KCC fluxes in fresh RBCs to levels seen in swollen cells, although SS RBCs required lower urea concentrations than did normal (AA) RBCs. Little additional KCC stimulation by urea occurred in swollen AA or SS RBCs. The pH dependence of KCC in "euvolemic" SS RBCs treated with urea was similar to that in swollen cells. Urea triggered volume reduction in SS and AA reticulocytes, establishing a higher CHC. Volume reduction was Cl dependent and was limited by the KCC inhibitor, dihydro-indenyl-oxyalkanoic acid. Final CHC depended on urea concentration, but not on initial CHC. Under all activation conditions, volume reduction was exaggerated in SS reticulocytes and produced higher CHCs than in AA reticulocytes. The sulfhydryl-reducing agent, dithiothreitol, normalized the sensitivity of KCC activation to urea in SS RBCs and mitigated the urea-stimulated volume decrease in SS reticulocytes, suggesting that the dysfunctional activity of KCC in SS RBCs was due in part to reversible sulfhydryl oxidation.

The effect of fetal hemoglobin on the survival characteristics of sickle cells

The determinants of sickle red blood cell (RBC) life span have not been well-defined but may include both intrinsic factors (eg, the tendency to sickle) and extrinsic factors (eg, the capacity of the reticuloendothelial system to remove defective RBCs). Fetal hemoglobin (HbF) is heterogeneously distributed among sickle RBCs; F cells contain 20% to 25% HbF, whereas the remainder have no detectable HbF (non-F cells). Autologous sickle RBCs were labeled with biotin and reinfused to determine overall survival, non-F- and F-cell survival, and time-dependent changes in HbF content (%HbF) for the surviving F cells. A total of 10 patients were enrolled, including 2 who were studied before and after the percentage of F cells was increased by treatment with hydroxyurea. As expected, F cells survived longer in all subjects. Non-F-cell survival correlated inversely with the percentage of F cells, with the time for 30% cell survival ranging from 6 days in patients with more than 88% F cells to 16 days in patients with less than 16% F cells. As the biotin-labeled RBCs aged in the circulation, the HbF content of the surviving F-cell population increased by 0.28%/d +/- 0.21%/d, indicating that within the F-cell population those with higher HbF content survived longer.

KCl cotransport mediates abnormal sulfhydryl-dependent volume regulation in sickle reticulocytes

KCl cotransport (KCC) activation by cell swelling and pH was compared in sickle (SS) and normal (AA) red blood cells (RBCs). KCC fluxes had the same relationship to mean corpuscular hemoglobin concentration (MCHC) in SS and AA RBCs when normalized to the maximal volume-stimulated (VSmax) flux (MCHC &lt; 270 g/L [27 g/dL]). Acid-stimulated (pH 6.9) KCC flux in SS RBCs was 60% to 70% of VSmax KCC versus 20% in AA RBCs. Density gradients were used to track changes in reticulocyte MCHC during KCC-mediated regulatory volume decrease (RVD). Swelling to MCHC of 260 g/L (26 g/dL) produced Cl-dependent RVD that resulted in higher MCHC in SS than AA reticulocytes. In acid pH, RVD was also greater in SS than AA reticulocytes. Sulfhydryl reduction by dithiothreitol (DTT) lowered VSmax KCC flux in AA and SS RBCs by one third but did not alter swelling-induced RVD. DTT lowered acid-activated KCC in SS RBCs by 50% and diminished acid-induced RVD in SS reticulocytes. Thus, swelling activation of KCC is normal in SS RBCs but KCC-mediated RVD produces higher MCHC in SS than AA reticulocytes. Acid activation of KCC is exaggerated in SS RBCs and causes dehydration in SS reticulocytes. KCC response to acid stimulation was mitigated by DTT, suggesting that it arises from sulfhydryl oxidation.

Phosphatidylserine externalization in sickle red blood cells: associations with cell age, density, and hemoglobin F

Phosphatidylserine (PS) is normally confined to the cytoplasmic leaflet of the red blood cell (RBC) membrane, but some sickle RBCs expose PS in the outer leaflet (PS+ cells). This study examined the relationships among PS externalization, fetal hemoglobin content, hydration state, and cell age. Sickle RBCs exhibit a wide range of PS externalization. Those with low-level exposure (type 1 PS+) include many young transferrin-receptor-positive (TfR+) cells. This is not specific for sickle cell disease because many nonsickle TfR+ cells are also PS+. RBCs with higher PS exposure (type 2 PS+) appear to be more specific for sickle cell disease. Their formation is most likely sickling dependent because type 2 PS+ dense sickle cells have a lower percentage of fetal hemoglobin (HbF) than PS- cells in the same density fraction (1.7 vs 2.9; n = 8; P <.01). In vivo experiments using biotin-labeled sickle cells showed a sharp decrease in the percentage of circulating, labeled PS+ cells in the first 24 hours after reinfusion. This decrease was confined to type 1 PS+ cells and was thus consistent with the reversal of PS exposure in very young cells. As the labeled cells aged in the circulation, the percentages of type 1 and type 2 PS+ cells increased. These studies indicate that PS externalization in sickle cells may be low level, as observed in many immature cells, or high level, which is associated with dehydration and appears to be more specific for sickle RBCs.

Rehydration of high-density sickle erythrocytes in vitro

Recent studies have identified older, low-density sickle red blood cells (SSRBCs) that were resistant to dehydration by valinomycin, a K(+) ionophore. These cells, thought to derive from dense SSRBCs that have rehydrated, may represent a terminal cellular phase. To study rehydration, we subjected dense SSRBCs (rho > 1.107 g/cc) to either oxygenated incubation or rapid oxygenated/deoxygenated (oxy/deoxy) cycling (70 seconds per cycle). Light cells (rho < 1.087 g/cc) were generated during both oxy incubation (2.9% +/- 2.1%; n = 42) and oxy/deoxy cycling (5.3% +/- 2.4%; n = 42). The rehydrated cells were K(+)-depleted (K(+) = 20 +/- 14 mmol/kg hemoglobin [Hb]) and Na(+)-loaded (Na(+) = 394 +/- 106 mmol/kg Hb), and had high levels of external phosphatidylserine. In the presence of external calcium, the generation of rehydrated SSRBCs was inhibited during oxy/deoxy cycling, but the percentage with external phosphatidylserine increased. The calcium-mediated inhibition of rehydration was reversed by charybdotoxin, implying that rehydration was delayed in some cells by the Ca(++)-activated K(+) channel. Preincubation of dense SSRBCs with DIDS (4,4'-di-isothiocyanato-2,2'-disulfostilbene) inhibited the generation of light cells during fast oxy/deoxy cycling, but not during oxy incubation. These results suggest that the sickling-induced pathway, previously implicated in SSRBC dehydration, may be involved in the deoxy-dependent component of rehydration for dense, K(+)-depleted cells. Light-cell generation was inhibited by 1 mM bumetanide during both oxy incubation and oxy/deoxy cycling, providing evidence that a bumetanide-sensitive, deoxy-independent pathway, previously described in circulating light SSRBCs, also contributes to the rehydration of high-density SSRBCs.

Dipyridamole inhibits sickling-induced cation fluxes in sickle red blood cells

Sickling-induced cation fluxes contribute to cellular dehydration of sickle red blood cells (SS RBCs), which in turn potentiates sickling. This study examined the inhibition by dipyridamole of the sickling-induced fluxes of Na(+), K(+), and Ca(++) in vitro. At 2% hematocrit, 10 microM dipyridamole inhibited 65% of the increase in net fluxes of Na(+) and K(+) produced by deoxygenation of SS RBCs. Sickle-induced Ca(++) influx, assayed as (45)Ca(++) uptake in quin-2-loaded SS RBCs, was also partially blocked by dipyridamole, with a dose response similar to that of Na(+) and K(+) fluxes. In addition, dipyridamole inhibited the Ca(++)-activated K(+) flux (via the Gardos pathway) in SS RBCs, measured as net K(+) efflux in oxygenated cells exposed to ionophore A23187 in the presence of external Ca(++), but this effect resulted from reduced anion conductance, rather than from a direct effect on the K(+) channel. The degree of inhibition of sickling-induced fluxes was dependent on hematocrit, and up to 30% of dipyridamole was bound to RBC membranes at 2% hematocrit. RBC membrane content of dipyridamole was measured fluorometrically and correlated with sickling-induced flux inhibition at various concentrations of drug. Membrane drug content in patients taking dipyridamole for other clinical indications was similar to that producing inhibition of sickling-induced fluxes in vitro. These data suggest that dipyridamole might inhibit sickling-induced fluxes of Na(+), K(+), and Ca(++) in vivo and therefore have potential as a pharmacological agent to reduce SS RBC dehydration. (Blood. 2001;97:3976-3983)

The survival characteristics of dense sickle cells

Sickle red blood cells (RBCs) become depleted of potassium, leading to dehydration and abnormally elevated cellular density. The increased sickling that results is important for both hemolysis and vasocclusion. In this study, sickle cells were subjected to high-speed centrifugation, and the bottom 15% were isolated. This procedure removed light cells and to a variable degree enriched cells that were denser than normal to produce a high-density-enriched (HDE) population of sickle cells. Autologous HDE cells from 3 subjects were labeled with biotin and re-infused. The following determinations were performed: (1) the survival and density changes of HDE cells; (2) the amount of fetal hemoglobin (HbF) in labeled cells after magnetic isolation; (3) the percentage of labeled F cells; (4) the percentage of labeled cells displaying external phosphatidylserine (PS). For patients with 3.5%, 4.5%, and 24% HbF in the HDE RBCs, the circulation half-time was 40, 80, and 180 hours, respectively. The percentage of HbF (measured in all 3 subjects) and of F cells (measured in 2 subjects) in labeled RBCs increased with time after re-infusion, indicating that HDE F cells have longer in vivo survival than HDE non-F cells. The percentage of PS(+), biotin-labeled HDE cells showed no consistent increase or decrease with time after re-infusion. These data provide evidence that HDE sickle cells, especially those that do not contain HbF, have a very short in vivo survival, and that the percentage of PS(+) cells in a re-infused HDE population does not change in a consistent manner as these cells age in the circulation.

Dehydration of mature and immature sickle red blood cells during fast oxygenation/deoxygenation cycles: role of KCl cotransport and extracellular calcium

Sickle red blood cells (RBC) become dehydrated as a consequence of potassium loss. This process depends at least partly on deoxygenation and may be influenced by the presence of oxygenation/deoxygenation cycles and the frequency of cycling. In this study, sickle RBC were subjected to approximately 180 oxygenation/deoxygenation cycles during 4 hours to evaluate RBC dehydration with cycle periods more similar to in vivo cycles than those in previous studies. A continuous-flow, steady-state apparatus circulated a dilute RBC suspension through gas-permeable silicone tubing with segments that were exposed to either nitrogen or ambient oxygen. The percentage of sickling and partial pressure of oxygen were measured by means of sampling ports in the deoxygenation and oxygenation regions. The density increase (dehydration) of young (transferrin receptor-positive) and mature (transferrin receptor-negative) RBC and the requirements for calcium and chloride were evaluated. Density increase correlated with the percentage of sickled cells at the deoxygenation sampling port and was observed only in the presence of calcium, thereby implicating the calcium-dependent potassium channel (Gardos pathway). Density increase was not dependent on the presence of chloride, making it unlikely that KCl cotransport was an important pathway under these conditions. (Blood. 2000;95:2164-2168)

Megakaryocytic differentiation of HIMeg-1 cells induced by interferon gamma and tumour necrosis factor alpha but not by thrombopoietin

Activated macrophage-conditioned medium (M-CM) induces megakaryocytic differentiation of HIMeg-1 cells. The megakaryocytic differentiation activity (MDA) is proteinaceous since it is susceptible to treatments by proteinases, heat, and reducing agents. MDA is not thrombopoietin (TPO) since (1) TPO alone or in conjunction with several other recombinant cytokines fails to induce any degree of HIMeg-1 cell differentiation; and (2) a neutralizing antibody against TPO or an antibody against the extracellular domain of c-mpl is unable to abolish M-CM-induced CD41 expression on HIMeg-1 cells. Reverse transcriptase-mediated polymerase chain reaction shows that HIMeg-1 cells express c-mpl but not TPO. Additional neutralizing antibody studies suggest that MDA is not one of the cytokines known to induce some degree of megakaryopoiesis in vitro or in vivo including interleukin 3 (IL-3), IL-6, IL-11, granulocyte-macrophage colony-stimulating factor, erythropoietin, or stem cell factor. On the other hand, MDA appears to be a combination of interferon gamma (IFN-gamma) and tumour necrosis factor alpha (TNF-alpha), since neutralizing antibodies against these two cytokines completely abolish MDA-induced CD41 expression. In addition, either recombinant human IFN-gamma or TNF-alpha alone is capable of inducing CD41 and CD42 expression on HIMeg-1 cells. In combination, IFN-gamma and TNF-alpha induce a maximal level of CD41 and CD42 expression which is also accompanied by an increase in cell size and DNA ploidy level. Thus, our studies indicate that IFN-gamma/TNF-alpha is capable of inducing megakaryocytic differentiation of the HIMeg-1 cell line and that HIMeg-1 is a good system for studying the molecular mechanism mediating megakaryocytic differentiation.

Time-dependent changes in the density and hemoglobin F content of biotin-labeled sickle cells

Sickle red blood cells (RBC) are subject to a number of important cellular changes and selection pressures. In this study, we validated a biotin RBC label by comparison to the standard 51Cr label, and used it to study changes that occur in sickle cells as they age. Sickle RBC had a much shorter lifespan than normal RBC, but the two labels gave equivalent results for each cell type. A variable number of sickle, but not normal, RBC disappeared from the circulation during the first few hours after reinfusion. The number of biotinylated sickle reticulocytes was decreased by 50% after 24 h and 75% after 48 h, with a gradual decrease in the amount of reticulum per cell. The labeled sickle cells exhibited major density increases during the first 4-6 d after reinfusion, with smaller changes thereafter. A small population of very light, labeled sickle RBC was essentially constant in number after the first few days. Fetal hemoglobin (HbF) content was determined in isolated biotinylated sickle RBC after reinfusion, allowing an estimate of lifespan for RBC containing HbF (F cells) and non-F cells. The lifespan of sickle biotinylated RBC lacking HbF was estimated to be approximately 2 wk, whereas F cells survived 6-8 wk.

Deoxygenation of sickle red blood cells stimulates KCl cotransport without affecting Na+/H+ exchange

KCl cotransport activated by swelling of sickle red blood cells (SS RBC)is inhibited by deoxygenation. Yet recent studies found a Cl--dependent increase in sickle reticulocyte density with cyclic deoxygenation. This study sought to demonstrate cotransporter stimulation by deoxygenation of SS RBC in isotonic media with normal pH. Low-density SS RBC exhibited a Cl--dependent component of the deoxygenation-induced net K+ efflux, which was blocked by two inhibitors of KCl cotransport, [(dihydroindenyl)oxy]alkanoic acid and okadaic acid. Cl--dependent K+ efflux stimulated by deoxygenation was enhanced 2.5-fold by clamping of cellular Mg2+ at the level in oxygenated cells using ionophore A-23187. Incubating cells in high external K+ or Rb+ minimized inhibition of KCl cotransport by internal Mg2+, and under these conditions deoxygenation markedly stimulated KCl cotransport in the absence of ionophore. Activation of KCl cotransport by deoxygenation of SS RBC in isotonic media at normal pH is consistent with the generalized dephosphorylation of membrane proteins induced by deoxygenation and activation of the cotransporter by a dephosphorylation mechanism. Na+/H+ exchange activity, known to be modulated by cytosolic Ca2+ elevation and cell shrinkage, remained silent under deoxygenation conditions.

The formation of transferrin receptor-positive sickle reticulocytes with intermediate density is not determined by fetal hemoglobin content

Erythrocyte dehydration is an important feature of sickle cell disease, leading to increased sickle hemoglobin polymerization and decreased red blood cell survival. Substantial in vivo dehydration appears to occur in reticulocytes or in an even younger subset of reticulocytes that are positive for transferrin receptor. Previous studies have suggested both sickling-dependent and sickling-independent components of dehydration for these cells. Two types of investigations are reported here. The first series of experiments explored the possibility that fetal hemoglobin (HbF) content influences the in vivo dehydration of very young, transferrin receptor-positive (T+) cells. These studies confirmed that in most patients the T+ cells in the densest fraction lacked HbF (T+ F-). However, T+ F- and T+ F+ cells appeared to have the same tendency to become moderately dense. The second type of investigation examined moderately dense T+ cells with normalized K+ content and determined the effect of HbF content on KCl cotransport-mediated dehydration in oxygenated incubations. Under these conditions, both T+ F- and T+ F+ cells had an equal tendency to become more dense by this pathway. Taken together, these studies indicate that at least some young sickle cells become moderately dense due to higher KCl cotransport activity independent of HbF content (and by inference, independent of sickling). However, to become very dense, it appears that further dehydration through a sickling-mediated pathway is required. We suggest that the dehydration of young sickle cells occurs in two steps, with the first dominated by KCl cotransport and the second having an important sickling-dependent component.

Dehydration of transferrin receptor-positive sickle reticulocytes during continuous or cyclic deoxygenation: role of KCl cotransport and extracellular calcium

The K+ efflux that mediates sickle-cell dehydration may occur through several pathways, including two with a high capacity for mediating rapid K+ loss, KCl cotransport and the Ca(2+)-dependent K+ channel [K(Ca2+)]. The rate and pathway of red blood cell (RBC) dehydration most likely depends on cell age and hemoglobin (Hb) composition, with the presence of HbF playing an important role. Oxygenated sickle RBCs have relatively stable cell volume during incubation in vitro, whereas deoxygenated cells become dehydrated, and therefore more dense, due to activation of one or more K+ efflux pathways. In this investigation, sickle RBCs were deoxygenated either continuously or in 15-minute cycles for 4 hours, and the density increases of very young, transferrin receptor-positive (TfR+) cells and the remaining TfR- cells were determined. The contribution of KCl cotransport was estimated by replacing Cl- with NO3-. K(Ca2+) was inhibited by removal of Ca2+ or addition of charybdotoxin (ChTX). For both continuous and cyclic deoxygenation, TfR+ cells had a greater density increase when compared with TfR- cells. The lower percentage of HbF found in the TfR+ population may contribute to this difference. With continuous deoxygenation, the density shift was decreased by inhibition of K(Ca2+), but not by inhibition of KCl cotransport. With cyclic deoxygenation, the density shift was decreased in an independent, additive manner by inhibition of both pathways. Thus, cyclic deoxygenation of sickle cells under these conditions appears to activate both K(Ca2+) and the KCl cotransporter.

Increased cation permeability in mutant mouse red blood cells with defective membrane skeletons

Cellular cation homeostasis in mouse erythrocytes with defective membrane skeletons was examined in three mouse mutants, hemolytic anemia (sphha/sphha), spherocytosis (sph/sph), and normoblastosis (nb/nb), and compared with reticulocytes produced by repetitive bleeding of congenic normal mice. To assess reticulocyte maturity, nucleic acid and transferrin receptor contents were measured by fluorescence flow cytometry; mutant cells were somewhat more mature than normal reticulocytes by these criteria. Red blood cell (RBC) sodium contents (Nac+) in homozygous sphha/sphha, sph/sph, and nb/nb animals were 30.1 +/- 0.9, 28.9 +/- 0.3, and 26.9 +/- 1.5 mmol/L cell, respectively, whereas cellular potassium (Kc+) was 102 +/- 2.6, 101 +/- 7.8, and 97.4 +/- 3.0. Nac+ and Kc+ in normal reticulocyte preparations were 11.3 +/- 0.7 and 123 +/- 10, respectively. Net Na+ and K+ fluxes in the presence of ouabain were markedly increased in mutant RBCs. Sodium uptake was 14.8 +/- 1.6, 15.4 +/- 3.3, and 14.7 +/- 3.1 mmol/L cell/h in sphha/sphha, sph/sph, and nb/nb mutants, respectively, whereas K+ loss was 17.0 +/- 4.0, 15.0 +/- 3.8, and 14.1 +/- 2.6. Normal mouse reticulocytes gained Na+ at a rate of 3.9 +/- 1.0 mmol/L cell/h and lost K+ at 6.0 +/- 2.1, rates indistinguishable from those in mature mouse RBCs. Potassium loss from sphha/sphha and nb/nb cells was not dependent on the presence of a Na+ gradient, and net cation movements were insensitive to bumetanide (sphha/sphha and nb/nb RBCs) and to chloride replacement with sulfamate (nb/nb cells). We conclude that mutant mouse RBCs with dysfunctional membrane skeletons have increased passive permeability to monovalent cations. These findings support a role of the membrane skeleton in the maintenance of the membrane permeability barrier and suggest that the abnormal permeability associated with human hereditary spherocytosis and elliptocytosis may be a consequence of the membrane skeleton defects reported in these disorders.

Deoxygenation-induced cation fluxes in sickle cells. IV. Modulation by external calcium

Net cation movements were measured in low-density sickle red blood cells (SS RBC) in the presence and absence of oxygen. External Ca2+ (Ca2+o) partially inhibited deoxygenation-induced fluxes of both Na+ and K+. Deoxygenation-induced Na+ influx was reduced by 2 mM Ca2+o to 0.71 +/- 0.04 (SE) of its value in Ca(2+)-free solutions, whereas this ratio was 0.90 +/- 0.05 for K+ efflux (P < 0.01 by paired t-test). Because Ca2+o inhibited Na+ influx more than K+ efflux, net cation loss in deoxygenated SS RBC was higher in the presence of Ca2+o. In separate experiments, Ca2+o reduced deoxygenation-induced Na+ influx to 0.66 +/- 0.03 of its Ca(2+)-free value compared with 0.77 +/- 0.03 for Rb+ influx (P < 0.001), indicating relative selectivity of this effect for Na+ over Rb+. However, this effect is not specific for Ca2+ because other divalent cations also inhibited deoxygenation-induced Na+ and K+ fluxes. Under the conditions of these experiments, no evidence for K+ channel activation was found, indicating that K+ loss measured in deoxygenated SS RBC was mediated by the deoxygenation-induced pathway. These studies show that in the presence of Ca2+o deoxygenation-induced Na+ influx and K+ efflux are unbalanced. This pathway can, therefore, mediate cation loss and contribute directly to cellular dehydration in SS RBC.

KCl cotransport activity in light versus dense transferrin receptor-positive sickle reticulocytes

A subset of sickle cells becomes K(+)-depleted and dehydrated before or soon after leaving the bone marrow. These young cells may be identified in blood as transferrin receptor-positive (TfR+) dense reticulocytes. KCl cotransport, which is normally active in young erythroid cells with a maximum at pH 6.8, is a candidate pathway for K+ depletion of sickle reticulocytes. In this investigation, KCl cotransport activity was evaluated in young, TfR+ cells which had become dense in vivo and in age-matched cells which had retained normal hydration. Sickle erythrocytes were first separated into three primary density fractions, with care taken to preserve the in vivo hydration state. After normalization of intracellular hemoglobin concentration with nystatin, the cells were incubated at 37 degrees C for 20 min at pH 6.8 and 7.4. Before and after incubation, each primary fraction was separated into four secondary density fractions. The percentage of TfR+ cells in each secondary fraction was measured and a density distribution for TfR+ cells was determined for each primary fraction before and after incubation. The density shift during incubation was a measure of KCl cotransport. TfR+ cells from the denser primary fractions II and III had significantly more density shift than TfR+ cells from the light fraction I. Although the shifts were larger at low pH, differences between primary fractions were also observed at pH 7.4. These data indicate that the cells which become dense quickly in vivo have more KCl cotransport activity than those which remain light in vivo, and support this pathway as a primary mechanism for dehydration of young sickle cells.

Fetal hemoglobin and potassium in isolated transferrin receptor-positive dense sickle reticulocytes

A subset of sickle cells have an increased density at the reticulocyte stage of development, indicating that they are either abnormally dense upon release from the bone marrow or become dense quickly in the circulation. These cells are of interest because they most likely have severely disrupted cation regulation and a short lifespan. Based on the distribution of fetal hemoglobin (HbF) in the density fractions of sickle red blood cells (RBCs) and in vitro studies of cellular K+ loss, it seems likely that HbF content is an important in vivo determinant of dense cell formation. In this study, we tested the hypothesis that young, dense cells have low HbF content. Sickle RBCs were first separated into light and dense fractions. Reticulocytes were isolated from unfractionated cells and from each density fraction with an immunomagnetic technique directed against transferrin receptors (TfR) and assayed for the percentage of HbF and K+/Hb ratio. TfR+ reticulocytes isolated from unfractionated cells had a much lower HbF content when compared with all the unfractionated RBCs. This is most likely caused by enrichment of F cells because of a longer circulation life span. Heavy TfR+ reticulocytes had a K+/Hb ratio similar to that measured in the entire dense population and contained very low levels of HbF, averaging 2.5% of the level in all RBCs, 11.7% of the level in all TfR+ reticulocytes, and 4.0% of the level in all dense RBCs. These findings suggest that TfR+ dense cells derive predominantly from non-F cells. Furthermore, the amount of HbF in the circulating dense cells suggests that many of these cells do not derive from the TfR+ dense cells.

Use of bi-level biotinylation for concurrent measurement of in vivo recovery and survival in two rabbit platelet populations

BACKGROUND

The objectives of this research were 1) to determine whether two populations of platelets may be labeled with different levels of biotin and followed concurrently in vivo by flow cytometry and 2) to determine whether the level of biotinylation affects the in vivo platelet recovery and survival.

STUDY DESIGN AND METHODS

Two platelet aliquots were biotinylated under conditions that resulted in either a lower or a higher number of biotin molecules per platelet. After transfusion, the two populations were distinguished and quantitated by flow cytometry.

RESULTS

In five animals, recoveries were 69.8 +/- 27.0 percent for low-biotin platelets and 72.6 +/- 26.7 percent for high-biotin platelets. For each animal, the recoveries agreed closely. Life span, determined by the multiple-hit method, was 2.68 +/- 0.63 days for low-biotin platelets and 2.58 +/- 0.69 days for high-biotin platelets. These values for recovery and life span are consistent with those measured in rabbits by using radioisotope labels.

CONCLUSION

Platelet biotinylation offers a nonisotopic method for direct comparison of alternative harvest and storage conditions. It also offers the potential for simultaneous evaluation of the in vivo characteristics of platelets from at least two donors.