On the mechanism of erythropoietin-induced differentiation. VI. Induced accumulation of iron by marrow cells

Utilization of Intracellular Ferritin Iron for Hemoglobin Synthesis in Developing Human Erythroid Precursors

Ferritin (Ft) plays an important role in cellular iron metabolism. It can store substantial amounts of iron in a nontoxic soluble form. However, its ability to donate iron for cellular needs, in particular for hemoglobin (Hb) synthesis in human erythroid cells, is still controversial. We studied the role of intracellular Ft-iron in Hb synthesis and the involvement of lysosomal proteolysis in iron release from Ft. Ft-iron release and its subsequent incorporation into heme was investigated in normal human erythroid precursors developing in culture. Dual staining flow cytometry with antibody (Ab)-specific for Ft and for Hb showed a decrease in cellular Ft content in erythroid cells during their maturation. Cellular Ft-iron participation in heme synthesis was studied by labeling cells with 59Fe. Cells were incubated with 59Fe-labeled human diferric transferrin (Tf), then chased, and intracellular radioiron distribution between Ft and Hb was determined on subsequent days by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and/or Ft immunoprecipitation and heme extraction. On day 6, most of the 59Fe accumulated in Ft. Thereafter, a progressive decrease of radioiron in Ft and a corresponding increase of the label in Hb was observed. Inhibition of heme synthesis with succinylacetone caused radioiron to remain in Ft and prevented its redistribution. Addition of unlabeled diferric Tf to the culture medium did not prevent radioiron from appearing in Hb. Chloroquine repression of lysosomal function prevented radio-iron redistribution between Ft and Hb. Inhibition of proteolysis by chymostatin and/or leupeptin led to Ft-protein accumulation in the cells and also prevented radioiron transfer from Ft to Hb. The results of the present study suggest that intracellular Ft donates iron for heme synthesis and that proteolytic Ft degradation in a lysosmal-like compartment is necessary for iron release and its transfer to heme.

Utilization of Intracellular Ferritin Iron for Hemoglobin Synthesis in Developing Human Erythroid Precursors

Ferritin (Ft) plays an important role in cellular iron metabolism. It can store substantial amounts of iron in a nontoxic soluble form. However, its ability to donate iron for cellular needs, in particular for hemoglobin (Hb) synthesis in human erythroid cells, is still controversial. We studied the role of intracellular Ft-iron in Hb synthesis and the involvement of lysosomal proteolysis in iron release from Ft. Ft-iron release and its subsequent incorporation into heme was investigated in normal human erythroid precursors developing in culture. Dual staining flow cytometry with antibody (Ab)-specific for Ft and for Hb showed a decrease in cellular Ft content in erythroid cells during their maturation. Cellular Ft-iron participation in heme synthesis was studied by labeling cells with 59Fe. Cells were incubated with 59Fe-labeled human diferric transferrin (Tf), then chased, and intracellular radioiron distribution between Ft and Hb was determined on subsequent days by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and/or Ft immunoprecipitation and heme extraction. On day 6, most of the 59Fe accumulated in Ft. Thereafter, a progressive decrease of radioiron in Ft and a corresponding increase of the label in Hb was observed. Inhibition of heme synthesis with succinylacetone caused radioiron to remain in Ft and prevented its redistribution. Addition of unlabeled diferric Tf to the culture medium did not prevent radioiron from appearing in Hb. Chloroquine repression of lysosomal function prevented radio-iron redistribution between Ft and Hb. Inhibition of proteolysis by chymostatin and/or leupeptin led to Ft-protein accumulation in the cells and also prevented radioiron transfer from Ft to Hb. The results of the present study suggest that intracellular Ft donates iron for heme synthesis and that proteolytic Ft degradation in a lysosmal-like compartment is necessary for iron release and its transfer to heme.

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.

Circulating erythroid progenitors in normal and anemic rabbits

The position of circulating blood erythroid progenitors in the erythroid maturation sequence was determined in rabbits. The erythroid progenitors in blood appeared to be the same as the earliest class of erythroid progenitors detectable in bone marrow on the basis of time course studies of colony formation by blood and marrow cells in vitro. This result was confirmed by the ability of circulating erythroid progenitors to respond to the early erythroid regulator burst-promoting activity with an increase in colony number and 59Fe incorporation into heme. Induction of anemia with phenylhydrazine resulted in large increases in early erythroid progenitor frequency in blood and an influx of more mature progenitors (intermediate BFU-E and CFU-E). Circulating erythroid progenitors in rabbits may be an interesting model for the study of human disorders of erythropoiesis.

Red cell aplasia and chronic granulocytic leukaemia

Two patients with chronic granulocytic leukaemia developed red cell aplasia during the course of their disease. In one of them, cell culture studies demonstrated the presence in the patient's serum of an IgG inhibitor of haemoglobin synthesis by his own mature erythroblasts and erythroblasts grown in vitro from his erythroid colony forming cells. The IgG fraction was also found to be cytotoxic for the patients' marrow erythroblasts that were present after disappearance of the red cell aplasia. Treatment with corticosteroids resulted in reappearance of the erythroblasts in the marrow and decrease in the transfusion requirement. Red cell aplasia can occur before, at the same time or after the onset of chronic granulocytic leukaemia and may have the same immune pathogenesis as chronic idiopathic pure red cell aplasia. It occurs without busulphan treatment and seems to have no direct relation to the terminal metamorphosis. Treatment of the red cell aplasia with corticosteroids would appear worthwhile as it may reduce the transfusion requirement without affecting the course of the underlying leukaemia.

Methemoglobin in hypoxic rats

Methemoglobin levels have been found to vary with altitude and to shift the hemoglobin-oxygen dissociation curve. In this study, hematocrits and methemoglobin levels were monitored in rats exposed to hypoxia (420 torr absolute) for various intervals. Hematocrits gradually increased throughout the period of hypoxia, while methemoglobin levels rose by 12 h, peaked at 24 h and returned to control level by day 6. These data, in the context of other work, suggest that increased methemoglobin is important in acclimation to hypoxia.

Somatic cell hybrids between Friend erythroleukemia cells and mouse hepatoma cells

Somatic cell hybrids between hepatoma and Friend erythroleukemia parental cells were studied for the expression of liver-specific and erythroid properties. Several independent clones were isolated using HAT selection and were shown to be true hybrids by isozyme and chromosome analysis. All displayed a complete extinction of hemoglobin and globin mRNA production, but a retention of albumin and transferrin secretion. The data suggest that erythroid differntiation is being actively inhibited by the hepatoma genome. Possible mechanisms that might explain these results are discussed in the light of current hypotheses regarding the mechanism of cell differentiation.

Comparative ultrastructure of late rabbit-embryo erythroid cells in liver and peripheral blood

A morphological study of hemoglobin biosynthesis activity in rabbit-embryo liver and peripheral blood was comparatively developed. Orthochromatic erythroblasts and reticulocytes of the same maturing degree were analysed through thin sections, as to their organellar constitution and behaviour regarding iron incorporation. It was found that peripheral blood erythroid cells contain mainly hemosomes, organelles taken as sites of final hemoglobin molecule biosynthesis. The ratio between the mean number of hemosomes in blood erythroid cell sections and that in liver erythroid cell sections reaches a little more than 6:1. Inversely, late liver erythroid cells are predominantly constituted by mitochondria that directly or indirectly participate in hemosome formation. The ratio between the mean number of mitochondria in blood erythroid cell sections and that in liver erythroid cell sections reaches 1:11. Besides this, the iron incorporation activity is higher in peripheral blood erythroid cells than in liver erythroblasts and reticulocytes. It is apparent that erythroid cells in the liver accumulate heme, given the ratio of the means of iron incorporation activity per cell to hemosome per cell (3.67), while blood erythroid cells, with a 1.08 ratio, synthesize heme, which is immediately afterwards integrated into the globin chains. The sudden increase in the formation of hemosomes when liver orthochromatic erythroblasts and reticulocytes enter the peripheral blood, reflecting an enhancement of hemoglobin synthesis, agrees with biochemical findings of other authors.

Quantitation of Friend spleen focus-forming virus by a nine-day 59Fe assay

A previously described 3-day 59Fe assay for quantitation of Friend spleen focus-forming virus has been modified to produce a 200-fold more sensitive 9-day 59Fe assay. A characterization of this assay is reported here. Male BALB/c mice received intravenous injections of appropriately diluted Friend polycythemia virus (FVP); control mice received virus diluent. All mice were allowed food and water ad libitum for 6 days, and on day 6 after virus injection were fasted by removal of food but not water. On day 3 of the fast (the 9th day after virus injection) each mouse received an intraperitoneal injection of 1 muCi of 59Fe. Six hours later the mice were sacrificed and the splenic radioactivity was determined. The percent splenic incorporation of 59Fe was directly related to the logarithm of spleen focus-forming units (SFFU) of FVP injected in a range of approximately 25 to 1,000 SFFU. Using a standard FVP preparation in a dose range of 25 to 1,000 SFFU, it was possible to determine the SFFU titers of unknown samples by extrapolation of the percent splenic 59Fe incorporation to the logarithm of SFFU. SFFU titers obtained by the 9-day 59Fe assay were similar to those obtained by the enumerative-response assay. Advantages of the 9-day 59Fe assay over the enumerative-response assay include a 50-fold greater virus dose range, an easier and a more objective counting procedure, and a reduced coefficient of variation.

Bone marrow erythropoiesis in the anemia of infection, inflammation, and malignancy

A major factor in the anemia of infection, inflammation, and malignancy is a relative failure of the bone marrow to increase erythropoiesis in response to a shortened red cell survival. The possible causes for this diminished marrow response are: (a) a reduced production of erythropoietin, or, (b) impaired bone marrow response to erythropoietin. In this report studies were performed on 6 normals, 13 patients with anemia from infection or inflammation, and 18 patients with anemia caused by advanced malignancy. Serum erythropoietin activity was measured using the posthypoxic, polycythemic mouse assay. Assessment of bone marrow response to erythropoietin was made by measuring (59)Fe-heme synthesis in bone marrow suspensions cultured for 3 days with and without the addition of erythropoietin. The results showed that marrow heme synthesis was increased in erythropoietin-treated cultures as compared with saline control cultures by 66+/-8% (mean +/-SE) in normals, 101+/-10% in patients with infection or inflammation, and 31+/-5% in malignancy. Serum erythropoietin levels were consistently diminished relative to expected levels for the degree of anemia in the infection-inflammatory group, but not in malignancy. In these patients, plasma inhibitors to the biological activity of erythropoietin were not detected in vitro. These studies suggest that another factor to consider in the anemia of malignancy is a decreased bone marrow response to erythropoietin. In the anemia of infection-inflammation, marrow response to erythropoietin is normal, but serum levels of erythropoietin are decreased relative to the degree of anemia.

Studies on red cell aplasia. V. Presence of erythroblast cytotoxicity in G-globulin fraction of plasma

The marrow cells of a patient with pure red cell aplasia markedly increased their rate of heme synthesis when they were freed from the host environment and were incubated in vitro. When the red cell aplasia was treated with cyclophosphamide and prednisone, marrow cell incorporation of (59)Fe into heme in vitro increased several weeks before a reticulocytosis was apparent, and was the earliest effect noted. The plasma gammaG-globulins of this patient inhibited heme synthesis by normal marrow cells or the patient's own marrow cells obtained after remission of the disease. Since the inhibition of heme synthesis could be the result of damage to erythroblasts, the patient's posttreatment marrow cells or normal marrow cells were labeled with (59)Fe and were then incubated with the patient's pretreatment, treatment, and posttreatment gammaG-globulins as well as normal gammaG-globulins. At the end of this incubation the supernatant and cells were separated and counted. Heme was extracted and also was counted. Treatment of the cells with the patient's pretreatment gammaG-globulins resulted in a release of 40% of the radioactive heme from the cells. This represented the loss of radioactive hemoglobin and was an index of erythroblast cytotoxicity. A progressive disappearance of the cytotoxic factor in the gammaG-globulins occurred in the 3 wk period preceding the onset of reticulocytes in the patient's blood. Posttreatment and normal gammaG-globulins did not produce this effect and increased injury of red cells and lymphocytes was not produced by the patient's pretreatment gammaG-globulins. These studies demonstrate a method for measuring erythroblast cytoxicity and show that red cell aplasia is associated with gammaG-globulins that specifically damage erythroblasts. Whether interference with new erythroblast development also occurs and contributes to the inhibition of heme synthesis has not yet been ascertained.