On the mechanism of erythropoietin-induced differentiation. XV. Induced transcription restricted by cytosine arabinoside

Inhibitory effect of benzene metabolites on nuclear DNA synthesis in bone marrow cells

Effects of endogenously produced and exogenously added benzene metabolites on the nuclear DNA synthetic activity were investigated using a culture system of mouse bone marrow cells. Effects of the metabolites were evaluated by a 30-min incorporation of [3H]thymidine into DNA following a 30-min interaction with the cells in McCoy's 5a medium with 10% fetal calf serum. Phenol and muconic acid did not inhibit nuclear DNA synthesis. However, catechol, 1,2,4-benzenetriol, hydroquinone, and p-benzoquinone were able to inhibit 52, 64, 79, and 98% of the nuclear DNA synthetic activity, respectively, at 24 microM. In a cell-free DNA synthetic system, catechol and hydroquinone did not inhibit the incorporation of [3H]thymidine triphosphate into DNA up to 24 microM but 1,2,4-benzenetriol and p-benzoquinone did. The effect of the latter two benzene metabolites was completely blocked in the presence of 1,4-dithiothreitol (1 mM) in the cell-free assay system. Furthermore, when DNA polymerase alpha, which requires a sulfhydryl (SH) group as an active site, was replaced by DNA polymerase I, which does not require an SH group for its catalytic activity, p-benzoquinone and 1,2,4-benzenetriol were unable to inhibit DNA synthesis. Thus, the data imply that p-benzoquinone and 1,2,4-benzenetriol inhibited DNA polymerase alpha, consequently resulting in inhibition of DNA synthesis in both cellular and cell-free DNA synthetic systems. The present study identifies catechol, hydroquinone, p-benzoquinone, and 1,2,4-benzenetriol as toxic benzene metabolites in bone marrow cells and also suggests that their inhibitory action on DNA synthesis is mediated by mechanism(s) other than that involving DNA damage as a primary cause.

Cell growth and hemoglobin synthesis in murine erythroleukemic cells propagated in high density microcapsule culture

A new microencapsulation technology, developed for the encapsulation of living cells, has been demonstrated to be useful for the study of growth and differential gene expression using Friend erythroleukemic cells cultured at high cell densities. Using this technology, cultures of FL Clone 745 cells were encapsulated within semipermeable membranes composed of cross-linked alginic acid and poly-l-lysine. Cell growth studies measuring total cell number demonstrated an average generation time of 8.5 h in 5% (vol/vol) microcapsule cultures vs. 8.0 h in suspension cultures. Similar microcapsule cultures were serially propagated for more than 90 cell generations (13 sequential passages) with no significant change in this growth rate. In addition, final culture densities of greater than 1.0 X 10(8) cells/ml of intracapsular volume were attained using a 3% (vol/vol) microcapsule culture in conjunction with a standard refeeding schedule. Comparison of the level of dimethyl sulfoxide-induced hemoglobin production in suspension and microcapsule cultures demonstrated that the total amount of hemoglobin produced on a per cell basis was comparable in both systems. Due to the retention characteristics of the semipermeable membrane, the concentration of detergent-released hemoglobin, relative to other released protein, was approximately twofold higher in microcapsule cultures than in control suspension cultures.

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.

Cell kinetic behaviour of a synchronized population of erythroid precursor cells in vitro

Erythropoiesis in vitro was studied with practically pure erythroid progenitor cells: CFU-E (colony-forming-units-erythroid). The isolation of CFU-e from spleens of thiamphenicol pretreated anaemic mice with the combined methods of centrifugal elutriation and Percoll density gradient centrifugation was monitored by flow cytometry. The ultimate CFU-e preparation with a density of 1.070 g/ml contained a high percentage of cells in the S phase of the cell cycle (80%). CFU-e occasionally found at a lower density of 1.065 g/ml were predominantly in the G2 + M and G1 phases. When CFU-e were cultured, the number of cells in the distinctive phases of the cell cycle changed periodically, so the cells were partly synchronized. Four periods up to 27 hr were observed by flow cytometrical screening of the cultured cells at hourly intervals. Cell-cycle times between 6 and 7 hr were found for all erythroid cell divisions. This was in agreement with results obtained from colony growth curves. Without the addition of erythropoietin cells start to degenerate after the second cell division. This experimental approach can be used for the cell kinetic modelling of erythropoiesis.

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.

The biological properties of endotoxin-free human erythropoietin

The biological effects formerly attributed to erythropoietin were generally observed using impure preparations of the hormone containing, among the impurities, bacterial endotoxin, a contaminant that has recently been shown to affect the haemopoietic system. Erythropoietin, purified to apparent homogeneity and freed of trace contamination by endotoxin (1) promotes erythroid burst formation from mouse marrow cells in semi-solid culture medium, (2) stimulates haemoglobin synthesis in rat marrow cell cultures, (3) stimulates transcription in mouse and rat bone marrow cell cultures and (4) causes the formation of the cytoplasmic mediator protein in rat marrow cells, which enhances transcription in isolated nuclei.