The formation of erythrocyte membrane proteins during erythropoietin-induced differentiation

Red blood cell lactate transport in sickle disease and sickle cell trait

This study determined and compared rates and mechanisms of lactate transport in red blood cells (RBCs) of persons with 1) sickle cell disease (HbSS), 2) sickle cell trait (HbAS), and 3) a control group (HbAA). Blood samples were drawn from 30 African-American volunteers (10 HbSS, 10 HbAS, 10 HbAA). Lactate influx into RBCs was measured by using [14C]lactate at six (2, 5, 10, 15, 25, and 40 mM) unlabeled lactate concentrations. The monocarboxylate transporter pathway was blocked by p-chloromercuriphenylsulfonic acid to determine its percent contribution to total lactate influx. Generally, total lactate influx into RBCs from the HbSS group was significantly greater than influx into RBCs from HbAS or HbAA, with no difference between HbAS and HbAA. Faster influx into HbSS RBCs was attributed to increased monocarboxylate transporter activity [increased apparent Vmax (V'max)]. V'max (4.7 +/- 0.6 micromol x ml(-1) x min(-1)) for HbSS RBCs was significantly greater than V'max of HbAS RBCs (2.9 +/- 1.5 micromol x ml(-1) x min(-1)) and HbAA RBCs (2.0 +/- 0.5 micromol x ml(-1) x min(-1)). Km (42.8 +/- 8 mM) for HbSS RBCs was significantly greater than Km (27 +/- 12 mM) for HbAA RBCs. We suspect that elevated erythropoietin levels in response to chronic anemia and/or pharmacological treatment (erythropoietin injections, hydroxyurea ingestion) is the underlying mechanism for increased lactate transport capacity in HbSS RBCs.

Cystic fibrosis transmembrane conductance regulator in human and mouse red blood cell membranes and its interaction with ecto-apyrase

Elevated blood ATP and increased red blood cell (RBC) ATP transport is associated with cystic fibrosis (CF). In this report, we demonstrate the presence of the wild-type and the DeltaF508 mutant form of the CF transmembrane conductance regulator protein in RBC membranes and its putative interaction with ecto-apyrase, an ATP hydrolyzing enzyme also present in the RBC membrane. RBC membranes of control and DeltaF508 individuals and of wild-type and CF transmembrane conductance regulator-knockout mice were examined by immunoblot using several antibodies directed against different epitopes of this protein. These experiments indicated that human RBC membranes contain comparable amounts of the wild-type CF transmembrane conductance regulator protein and the DeltaF508 mutant form of the protein, respectively. CF transmembrane conductance regulator protein was also detected in wild-type mouse RBC membranes but not in the gene knockout mouse RBC membranes. Antibodies directed against ecto-apyrase co-immunoprecipitated CF transmembrane conductance regulator protein of human RBC membranes indicating a physical interaction between these two membrane proteins consistent with ATP transport and extracellular hydrolysis. We conclude that RBCs are a significant repository of CF transmembrane conductance regulator protein and should provide a novel system for evaluating its expression and function.

Injections of recombinant human erythropoietin increases lactate influx into erythrocytes

Previous studies showed that erythropoietin not only increases erythrocyte production but is also essential in both the synthesis and the good functioning of several erythrocyte membrane proteins, including band 3. It is still unknown whether anion and/or H(+) fluxes are modified by erythropoietin. This study aimed to evaluate the effect of recombinant human erythropoietin (rHuEPO) injections on lactate transport into erythrocytes via band 3 and H(+)-monocarboxylate transporter MCT-1, two proteins involved in lactate exchange. Nine athletes received subcutaneous rHuEPO (50 U/kg body mass 3 times a week for 4 wk), and seven athletes received a saline solution (placebo group). All subjects were also supplemented with oral iron and vitamins B(9) and B(12). Lactate transport into erythrocytes was studied before and after the rHuEPO treatment at different lactate concentrations (1.6, 8.1, 41, and 81.1 mM). After treatment, MCT-1 lactate uptake was increased at 1.6, 41 (P < 0.01), and 81.1 mM lactate concentration (P < 0.001) although lactate uptake via band 3 and nonionic diffusion were unchanged. MCT-1 maximal velocity increased in the rHuEPO group (P < 0.05), reaching higher values than in the placebo group (P < 0.05) after treatment. Our results show that rHuEPO injections increased MCT-1 lactate influx at low and high lactate concentrations. The increase in MCT-1 maximal velocity suggests that rHuEPO may stimulate MCT-1 synthesis during erythrocyte formation in bone marrow.

The mechanism of action of erythropoietin

The proliferation and differentiation of committed erythroid progenitor cells is regulated by the glycoprotein hormone erythropoietin. Erythropoietin increases the number of developing erythroid precursors and accelerates the release of reticulocytes from the marrow without markedly altering the cell cycle length or number of mitotic divisions involved in the differentiation process. Although the hormone has been purified, molecularly cloned and sequenced, its secondary and tertiary structure and active site have not been defined. Erythropoietin has both mitogenic and differentiation functions, and whether an erythroid progenitor cell responds to the hormone by proliferating or differentiating appears to depend on its level of maturation. Erythroid progenitor cells are responsive to a variety of growth and developmental agents but only erythropoietin appears obligatory in vivo for terminal differentiation. Erythropoietin interacts with its target cells through specific high-affinity receptors and Ca2+ may be involved in the receptor-ligand interaction. Ca2+ may also be involved in the induction of differentiation by erythropoietin. An increase in RNA synthesis due to activation of transcription is one of the earliest recognized effects of the hormone and appears not to require protein or DNA synthesis but the initial sequence of biochemical events triggered by erythropoietin is still undefined.

The role of erythropoietin in the production of principal erythrocyte proteins other than hemoglobin during terminal erythroid differentiation

Erythropoietin (EP) controls the terminal phase of differentiation in which proerythroblasts and their precursors, the colony forming units-erythroid (CFU-e), develop into erythrocytes. Biochemical studies of this hormone-directed terminal differentiation have been hindered by the lack of a homogeneous population of erythroid cells at the developmental stages of CFU-e and proerythroblasts that will synchronously differentiate in response to EP. Such a population of cells can be prepared from the spleens of mice with the acute erythroblastosis resulting from infection with anemia-inducing Friend virus (FVA). Using these FVA-infected erythroid cells, which were induced to differentiate with EP, four proteins other than hemoglobin that have key functions in mature erythrocytes were monitored during the 48-hour period of terminal differentiation. Synthesis of spectrin and membrane band 3 proteins were determined by immunoprecipitation and SDS-polyacrylamide gel electrophoresis; accumulation of the cytoskeletal protein band 4.1 was monitored by immunoblotting; carbonic anhydrase activity was measured electrometrically. Band 3 synthesis and band 4.1 accumulation could be detected only after exposure of the cells to EP. Spectrin synthesis was ongoing prior to culture with EP, but it did increase after exposure to the hormone. Carbonic anhydrase-specific activity changed very little throughout the terminal differentiation process. These results reveal at least three patterns of production of principal erythrocyte proteins during EP-mediated terminal differentiation of FVA-infected erythroid cells. Depending on the specific protein examined, de novo synthesis can be induced by EP, an ongoing production can be enhanced by EP, or the production of a protein can be completed at a developmental stage prior to EP-mediated differentiation in these cells.

Induction of glycophorin gene expression in cultured murine erythroleukemia cells

In order to identify the mRNA for mouse glycophorin, mRNA was isolated from immature erythroid cells obtained from the spleens of anemic mice, translated in vitro in mRNA-dependent rabbit reticulocyte lysate, and then immunoprecipitated with a specific antiserum. Glycophorin mRNA was shown to be present only in erythroid cells. In immunofluorescent and in vitro translation studies, it was shown that glycophorin mRNA is absent in uninduced murine erythroleukemia (MEL) cells, but is induced in dimethylsulfoxide-treated differentiating cells.

Effect of amines on erythropoietin-stimulated heme synthesis in fetal mouse liver cells

Amines, diamines and polyamines inhibit the erythropoietin-stimulated incorporation of 59Fe into newly-synthesized heme by fetal mouse liver cells in short-term culture. As assessed by cellular retention of lactic dehydrogenase, most of these compounds affect the viability of the cells very little at concentrations that substantially inhibit heme synthesis. In the families of amines tested, compounds of chain length greater than five carbons are more effective inhibitors the longer the chain, and monoamines are more inhibitory than diamines. The naturally-occurring polyamines spermine and spermidine are among the most potent amine inhibitors tested.

K562 cell anion exchange differs markedly from that of mature red blood cells

Human K562 leukemic cells exhibit several erythroid properties, including synthesis and expression of the major red blood cell sialoglycoprotein, glycophorin. This has led us to ask if these cells express a functional anion transport system analogous to that which is associated with the other major erythrocyte glycoprotein, band 3. The chloride-36 exchange flux in K562 cells is less than 0.6% of that which would be expected in mature erythrocytes under similar conditions. Unlike red blood cells, K562 cells do not exhibit a high chloride-sulfate selectivity, and various agents that inhibit red blood cell chloride exchange are all much less effective in K562 cells. On the basis of these flux measurements, K562 cells probably contain less than 600 fully functional red blood cell-like band 3 molecules per cell, in contrast to about a million molecules in the mature red blood cell. The possible-existence of greatly altered band 3 molecules with a reduced turnover rate and/or a reduced affinity for chloride and for various inhibitors is unlikely but cannot be completely excluded. Anion transport was also measured in K562 cells that had been induced to increase hemoglobin synthesis by various chemical agents. Even under these conditions, chloride fluxes indicated no substantial increase in the number of functional anion transport sites or their chloride transport rate.

Inhibition of heme synthesis in bone marrow cells by succinylacetone: effect on globin synthesis

The effects of 4,6-dioxoheptanoic acid (succinylacetone, SA), an inhibitor of delta-aminolevulinic acid dehydratase, on total iron uptake, heme synthesis, and globin synthesis were studied in rat marrow cells in culture in order to examine the coordination of heme and globin synthesis. SA inhibited heme synthesis in both control and erythropoietin-stimulated cells in a dose-dependent fashion; at 10(-3) M, inhibition was complete, whereas at 10(-7) M, there was no significant effect. Inhibition of total iron uptake was also dose-dependent although, at 10(-3) M, it was not complete. The inhibition of heme synthesis by SA was partially overcome by addition of 10(-4) M porphobilinogen or protoporphyrin IX. SA caused an almost complete suppression of globin formation in both erythropoietin-stimulated and unstimulated cells as early as five hours after the addition of the inhibitor. When inhibition of heme synthesis was incomplete, globin synthesis was partially inhibited. These results indicate that heme synthesis is required for erythropoietin-mediated induction of globin synthesis in cultured bone marrow cells.