Apoptosis in erythroid progenitors deprived of erythropoietin occurs during the G1 and S phases of the cell cycle without growth arrest or stabilization of wild-type p53.

Erythropoietin (Epo) inhibits apoptosis in murine proerythroblasts infected with the anemia-inducing strain of Friend virus (FVA cells). We have shown that the apoptotic process in FVA cell populations deprived of Epo is asynchronous as a result of a heterogeneity in Epo dependence among individual cells. Here we investigated whether apoptosis in FVA cells correlated with cell cycle phase or stabilization of p53 tumor suppressor protein. DNA analysis in nonapoptotic FVA cell subpopulations cultured without Epo demonstrated little change in the percentages of cells in G1,S, and G2/M phases over time. Analysis of the apoptotic subpopulation revealed high percentages of cells in G1 and S, with few cells in G2/M at any time. When cells were sorted from G1 and S phases prior to culture without Epo, apoptotic cells appeared at the same rate in both populations, indicating that no prior commitment step had occurred in either G1 or S phase. Steady-state wild-type p53 protein levels were very low in FVA cells compared with control cell lines and did not accumulate in Epo-deprived cultures; however, p53 protein did accumulate when FVA cells were treated with the DNA-damaging agent actinomycin D. These data indicate that erythroblast apoptosis caused by Epo deprivation (i) occurs throughout G1 and S phases and does not require cell cycle arrest, (ii) does not have a commitment event related to cell cycle phase, and (iii) is not associated with conformational changes or stabilization of wild-type p53 protein.

Cation binding to the integrin CD11b I domain and activation model assessment.

BACKGROUND: The integrin family of cell-surface receptors mediate cell adhesion through interactions with the extracellular matrix or other cell-surface receptors. The alpha chain of some integrin heterodimers includes an inserted 'I domain' of about 200 amino acids which binds divalent metal ions and is essential for integrin function. Lee et al. proposed that the I domain of the integrin CD11b adopts a unique 'active' conformation when bound to its counter receptor. In addition, they proposed that the lack of adhesion in the presence of Ca2+ ion reflected the stabilization of an 'inactive' I-domain conformation. We set out to independently determine the structure of the CD11 b I domain and to evaluate the structural effects of divalent ion binding to this protein. RESULTS: We have determined the X-ray structure of a new crystal form of the CD11 b I domain in the absence of added metal ions by multiple isomorphous replacement (MIR). Metal ions were easily introduced into this crystal form allowing the straight-forward assessment of the structural effects of divalent cation binding at the metal ion dependent adhesion site (MIDAS). The equilibrium binding constants for these ions were determined by titration calorimetry. The overall protein conformation and metal-ion coordination of the I domain is the same as that observed for all previously reported CD11 a I-domain structures and a CD11 b I-domain complex with Mn2+. These structures define a majority conformation. CONCLUSIONS: Addition of the cations Mg2+, Mn2+ and Cd2+ to the metal-free I domain does not induce conformational changes in the crystalline environment. Moreover, we find that Ca2+ binds poorly to the I domain which serves to explain its failure to support adhesion. We show that the active conformation proposed by Lee et al, is likely to be a construct artifact and we propose that the currently available data do not support a dramatic structural transition for the I domain during counter-receptor binding.

Loss of p53 tumor suppressor function is required for in vivo progression of Friend erythroleukemia.

A role for p53 in the in vivo progression of Friend virus-induced erythroleukemia has been suggested but not clearly defined. We developed a Friend virus-sensitive, p53-deficient mouse model to directly address the role of p53 in Friend erythroleukemia. When infected with the polycythemia-inducing strain of Friend virus (FVP), p53 null mice exhibited accelerated progression to erythroleukemia and accelerated death following diagnosis when compared to wild type mice. Confirmation that p53 mutations were required for disease progression was provided by sequence analysis of p53 transcripts in leukemic wild type and heterozygous mice. All transcripts evaluated had point mutations, deletions or insertions in the p53 gene. The ability to grow tumor colonies in vitro and derive cell lines was enhanced in FVP-infected p53 null animals. Although PU.1 oncogene overexpression is a common mutation observed in cell lines derived from Friend virus-infected p53 wild type mice, it was not a universal finding in cell lines derived from p53 null animals. Our data conclusively demonstrate that loss of p53 function is a requirement for progression of Friend erythroleukemia in vivo. Further, the data demonstrate that erythroleukemias arising in Friend virus-infected p53 null mice are biologically and genetically distinct from those that occur in wild type animals, suggesting that the temporal order of PU.1 and p53 mutations is an important parameter in the pathogenesis of leukemic development.

Endogenous p53 regulation and function in early stage Friend virus-induced tumor progression differs from that following DNA damage.

Erythroleukemia induced by the anemia strain of Friend virus occurs in two stages. The first stage results in rapid expansion of pre-leukemic proerythroblasts (FVA cells) dependent on erythropoietin (Epo) for differentiation and survival in vitro. The second stage is characterized by emergence of erythroleukemic clones (MEL cells) which typically bear activation of the ets-oncogene, PU.1/spi.1, and loss of functional p53. We developed a Friend virus-sensitive, p53-deficient mouse model to investigate the biological advantage conferred by p53-loss during tumor progression. Here we report p53 was not required for cell survival or growth arrest during differentiation of FVA cells, nor was p53 required for induction of apoptosis upon Epo withdrawal. However, we detected induction of the p21Cip1 cyclin-dependent kinase inhibitor gene during differentiation, which was markedly enhanced in the presence of p53. p53-dependent expression of p21Cip1 occurred in the absence of an increase in p53 mRNA and protein levels and was specific for p21Cip1, since expression of gadd45, mdm-2, cyclin G and bax were unaffected by p53. In contrast, treatment of FVA cells with DNA damaging agents led to rapid accumulation of p53 protein resulting in transcription of multiple p53-regulated genes, leading to either apoptosis or growth arrest, depending on the agent used. These data demonstrate that p53-dependent activities during differentiation of preleukemic erythroblasts are distinct from those observed in response to genotoxic agents. We propose that enhancement of p53-dependent gene expression during differentiation may represent a tumor suppressor function which is necessary to monitor differentiation of preleukemic cells and which is selected against during tumor progression.

Modulation of radiation-induced apoptosis by thiolamines.

Exposure to the thiolamine radioprotector N-(2-mercaptoethyl)-1,3-propanediamine (WR-1065) induced apoptosis in the mouse TB8.3 hybridoma after a 60-min (LD50 = 4.5 mM) or during a 20-h (LD50 = 0.15 mM) exposure. In contrast, a 20-h exposure to 17 mM L-cysteine or 10 mM cysteamine was required to induce 50% apoptosis within 20 h. Apoptosis was not induced by either a 60-min or 20-h exposure to 10 mM of the thiazolidine prodrugs ribose-cysteine (RibCys) or ribose-cysteamine (RibCyst). Thiolamine-induced apoptosis appeared to be a p53-independent process since it was induced by WR-1065 exposure in human HL60 cells. Exposure to WR-1065 (4 mM for 15 min) or cysteine (10 mM for 60 min) before and during irradiation protected cells against the induction of both DNA double-strand breaks and apoptosis, while exposure to RibCys (10 mM for 3 h) did not. Treatment with either WR-1065, cysteine, RibCys or RibCyst for 60 min beginning 60 min after irradiation did not affect the level of radiation-induced apoptosis. In contrast, treatment with either cysteine, cysteamine or RibCys for 20 h beginning 60 min after irradiation enhanced radiation-induced apoptosis. Similar experiments could not be conducted with WR-1065 because of its extreme toxicity. Our results indicate that thiolamine enhancement of radiation-induced apoptosis is not involved in their previously reported capacity to reduce radiation-induced mutations.

Regulation of programmed death in erythroid progenitor cells by erythropoietin: effects of calcium and of protein and RNA syntheses.

Erythropoietin (EPO) retards DNA breakdown characteristic of programmed cell death (apoptosis) and promotes survival in erythroid progenitor cells. The mechanism by which EPO inhibits programmed death is unknown. In the well-characterized model of glucocorticoid-treated thymocytes, activation of a Ca2+/Mg(2+)-sensitive endonuclease and new protein and RNA syntheses have been found necessary for apoptosis. We examined the effects of EPO on the free intracellular calcium ion concentration ([Ca2+]i), and the roles of Ca2+ and RNA and protein syntheses on DNA cleavage in erythroid progenitor cells. The murine model of erythroid differentiation using Friend leukemia virus-infected proerythroblasts (FVA cells) was used. EPO did not affect the [Ca2+]i in FVA cells. Decreasing [Ca2+]i by extracellular Ca2+ chelation with EGTA facilitated DNA breakdown. Increasing [Ca2+]i with the calcium ionophore 4-bromo-A23187 increased DNA cleavage; however, DNA fragments generated by high [Ca2+]i were much larger than those seen in the absence of EPO or presence of EGTA. Increased [Ca2+]i also inhibited DNA breakdown to small oligonucleosomal fragments characteristic of cells cultured without EPO. However, no concentration of ionophore protected the high molecular weight DNA as did EPO. Cycloheximide inhibited DNA breakdown in a dose dependent manner in cultures lacking EPO, but two other protein synthesis inhibitors, pactamycin and puromycin, did not prevent DNA breakdown. Inhibition of RNA synthesis with actinomycin D did not prevent DNA breakdown. Cells with morphological characteristics similar to those reported in other cells undergoing programmed death accumulated in EPO-derived cultures. These studies demonstrate that although DNA cleavage and morphological changes are common to apoptotic cells, the roles for Ca2+ and protein and RNA syntheses are not universal and suggest that apoptosis can be regulated by different biochemical mechanisms in different cell types.

Production of inositol pentakisphosphate in a human T lymphocyte cell line.

The human T lymphocyte cell line, Jurkat, produced five distinct water soluble, inositol-containing compounds following a period of labeling with 3H-myo-inositol and several hours of incubation in non-radioactive complete medium. The less polar four peaks had been previously shown to be inositol phosphates, InsP through InsP4. Here, we demonstrate that the prominent fifth, very polar, peak was inositol pentakisphosphate, InsP5. The pattern of incorporation of 3H-myo-inositol into InsP5 differed from that of incorporation into other inositol phosphates. InsP5, unlike the second messengers, InsP3 and InsP4, was not increased by perturbation of the T cell receptor/T3 complex.

Agents that raise cAMP in human T lymphocytes release an intracellular pool of calcium in the absence of inositol phosphate production.

Prostaglandins (PGs) of the E series are recognized by specific receptors on T lymphocytes which lead to an increase in cAMP. The role of cAMP in modulation of T lymphocyte function is unknown. Here, we demonstrate that agents which increase cAMP in human T cells raise the intracellular free calcium concentration ([Ca2+]i). This increase in [Ca2+]i occurred following receptor stimulation with PGEs or by bypassing the receptor with the cell-permeant analog 8-(4-chlorophenylthio)-cAMP or forskolin, a direct activator of adenylyl cyclase. The calcium response to a submaximally stimulatory concentration of PGE2 was potentiated by the cAMP phosphodiesterase inhibitor isobutylmethylxanthine. A time course of cAMP production in response to PGE2 stimulation closely resembled the calcium response and suggested that the two events were coincident. The PGE2 concentrations required to achieve 50% maximum effect of cAMP production and increases in [Ca2+]i were similar, 0.07 and 0.15 microM respectively. Chelation of extracellular Ca2+ did not abolish the PGE2-stimulated Ca2+ response, suggesting that an intracellular source of calcium was sensitive to cAMP. Significant inositol phosphate production was not detected in response to PGE2 over a wide concentration range. The PGE2-induced calcium response curves were of lesser magnitude with shorter times to peak than those of a known inositol 1,4,5 trisphosphate-producing agonist, anti-CD3, suggesting distinct Ca2+ release mechanisms. However, the cAMP-releasable store appeared to be contained within the inositol trisphosphate-releasable store since no response could be seen with cAMP-elevating agents following emptying of the inositol trisphosphate-sensitive pool of Ca2+.

Survival or death of individual proerythroblasts results from differing erythropoietin sensitivities: a mechanism for controlled rates of erythrocyte production.

Murine erythroid progenitors infected with the anemia-inducing strain of Friend virus (FVA cells) undergo apoptosis when deprived of erythropoietin (EPO). When cultured with EPO, they survive and complete terminal differentiation. Although cell volume is decreased and nuclear chromatin is condensed during both apoptosis and terminal differentiation, morphologic and biochemical distinctions between these two processes were observed. In apoptosis, homogeneous nuclear condensation with nuclear envelope loss occurred in cells that had not reached the stage of hemoglobin synthesis. In terminal erythroid differentiation, nuclear condensation with heterochromatin, euchromatin, and nuclear envelope preservation occurred simultaneously with hemoglobin synthesis. Cells with apoptotic morphology appeared asynchronously in EPO-deprived cultures, indicating that only a portion of the cells were undergoing apoptosis at any given time. The percentages of apoptotic cells and cleaved DNA increased with time in EPO-deprived cultures. Inhibition of DNA cleavage was directly proportional to EPO concentration over a wide physiologic range, demonstrating a heterogeneity in susceptibility to apoptosis based on variability in the EPO sensitivity of individual cells. A subpopulation of FVA cells with increased EPO sensitivity (decreased EPO requirement) was isolated from EPO-deprived cultures. This increased EPO sensitivity did not result from differences in EPO receptor number, affinity, or structure, suggesting that the differences are in the signal transduction pathway. These results indicate that control of red blood cell production involves both prevention of apoptosis by EPO and heterogeneity in the EPO requirement of individual progenitor cells.

Cell cycle exit during terminal erythroid differentiation is associated with accumulation of p27(Kip1) and inactivation of cdk2 kinase.

Progression through the mammalian cell cycle is regulated by cyclins, cyclin- dependent kinases (CDKs), and cyclin-dependent kinase inhibitors (CKIs). The function of these proteins in the irreversible growth arrest associated with terminally differentiated cells is largely unknown. The function of Cip/Kip proteins p21(Cip1) and p27(Kip1) during erythropoietin-induced terminal differentiation of primary erythroblasts isolated from the spleens of mice infected with the anemia-inducing strain of Friend virus was investigated. Both p21(Cip1) and p27(Kip1) proteins were induced during erythroid differentiation, but only p27(Kip1) associated with the principal G(1) CDKs-cdk4, cdk6, and cdk2. The kinetics of binding of p27(Kip1) to CDK complexes was distinct in that p27(Kip1) associated primarily with cdk4 (and, to a lesser extent, cdk6) early in differentiation, followed by subsequent association with cdk2. Binding of p27(Kip1) to cdk4 had no apparent inhibitory effect on cdk4 kinase activity, whereas inhibition of cdk2 kinase activity was associated with p27(Kip1) binding, accumulation of hypo-phosphorylated retinoblastoma protein, and G(1) growth arrest. Inhibition of cdk4 kinase activity late in differentiation resulted from events other than p27(Kip1) binding or loss of cyclin D from the complex. The data demonstrate that p27(Kip1) differentially regulates the activity of cdk4 and cdk2 during terminal erythroid differentiation and suggests a switching mechanism whereby cdk4 functions to sequester p27(Kip1) until a specified time in differentiation when cdk2 kinase activity is targeted by p27(Kip1) to elicit G(1) growth arrest. Further, the data imply that p21(Cip1) may have a function independent of growth arrest during erythroid differentiation. (Blood. 2000;96:2746-2754)

Activation of phospholipase D: a signaling system set in motion by perturbation of the T lymphocyte antigen receptor/CD3 complex.

A number of cellular signaling systems are called into play by interaction of the T lymphocyte antigen receptor/CD3 complex with its cognate antigen. Well-described signaling systems include phosphoinositide turnover, tyrosine phosphorylation, protein kinase C activation, and increased cytosolic calcium. We have explored the possibility that another recently described signaling system, activation of phospholipase D, may be operative. Data presented here demonstrate that stimulation of Jurkat T cells with anti-CD3 antibodies or phorbol esters resulted in activation of phospholipase D, as measured by production of phosphatidylethanol and phosphatidic acid. The combination of anti-CD3 antibody plus phorbol ester led to a greater than additive production of phosphatidylethanol and to the additive production of phosphatidic acid (in the absence of ethanol). Phorbol esters as a second stimulus with anti-CD3 antibody led to a additive increase in cellular diacylglycerol content but provided no increased production of inositol phosphates, suggesting that diacylglycerol production in these cells results from hydrolysis of noninositol containing lipids as well as from phosphinositides. Exogenous addition of phosphatidic acid led to increases in cytosolic calcium that, depending on the concentration used, resulted from release of an intracellular store of calcium and influx of extracellular calcium. Changes in cytosolic calcium occurred in the absence of inositol phosphates production. These studies establish a role for increased phospholipase D activity in T lymphocyte activation.

Cytokeratin-positive cells in PBSC collections from normal donors and patients with non-epithelial cell-derived tumors.

BACKGROUND: It is possible that post-transplant relapse in patients with breast cancer may result, in part, from residual tumor in the autologous PBSC product. It is unclear from the literature what effect residual breast tumor cells have on clinical outcome and whether purging tumor cells would be beneficial. We hypothesized that lack of standardization of assays for detection of residual breast tumor may be responsible for the inconclusive clinical data. METHODS: We compared two assays routinely for detection of cytokeratin (CK)-positive cells in stem-cell grafts, immunohistochemistry (IHC) and flow cytometry (FCM). The patient population consisted of individuals with breast cancer, non-epithelial cell-derived tumors and normal donors. A rigorous statistical model was developed for evaluation of the data. RESULTS: We found that the IHC assay out-performed the FCM assay. Importantly, both assays detected CK-positive cells in PBSC collections of patients with non-epithelial cell-derived tumors and in normal donors. No distinguishing morphological characteristics could be identified to differentiate potentially malignant from non-malignant CK-positive cells. Due to the inability to distinguish true positive from false positive results, we developed a statistical model to establish a quantitative threshold to discriminate positive from negative samples. Among the patients tested, no clinical outcome differences were detected, regardless of where the threshold of CK-positive cells was set. DISCUSSION: We conclude the more stringent criteria and more specific markers, rather than the presence or absence of CK-positive cells, need to be established to determine the clinical significance of minimal residual disease in autologous breast-cancer