Synergistic interactions allow colony formation in vitro by murine haemopoietic stem cells

The influence of p53 functions on radiation-induced inflammatory bystander-type signaling in murine bone marrow

Radiation-induced bystander and abscopal effects, in which DNA damage is produced by inter-cellular communication, indicate mechanisms of generating damage in addition to those observed in directly irradiated cells. In this article, we show that the bone marrow of irradiated p53(+/+) mice, but not p53(-/-) mice, produces the inflammatory pro-apoptotic cytokines FasL and TNF-α able to induce p53-independent apoptosis in vitro in nonirradiated p53(-/-) bone marrow cells. Using a congenic sex-mismatch bone marrow transplantation protocol to generate chimeric mice, p53(-/-) hemopoietic cells functioning in a p53(+/+) bone marrow stromal microenvironment exhibited greater cell killing after irradiation than p53(-/-) hemopoietic cells in a p53(-/-) microenvironment. Cytogenetic analysis demonstrated fewer damaged p53(-/-) cells in a p53(+/+) microenvironment than p53(-/-) cells in a p53(-/-) microenvironment. Using the two different model systems, the findings implicate inflammatory tissue processes induced as a consequence of p53-dependent cellular responses to the initial radiation damage, producing cytokines that subsequently induce ongoing p53-independent apoptosis. As inactivation of the p53 tumor suppressor pathway is a common event in malignant cells developing in a stromal microenvironment that has normal p53 function, the signaling processes identified in the current investigations have potential implications for disease pathogenesis and therapy.

The RNA helicase p68 (DDX5) is selectively required for the induction of p53-dependent p21 expression and cell-cycle arrest after DNA damage

The RNA helicase p68 (DDX5) is an established co-activator of the p53 tumour suppressor that itself has a pivotal role in orchestrating the cellular response to DNA damage. Although several factors influence the biological outcome of p53 activation, the mechanisms governing the choice between cell-cycle arrest and apoptosis remain to be elucidated. In the present study, we show that, while p68 is critical for p53-mediated transactivation of the cell-cycle arrest gene p21(WAF1/CIP1), it is dispensable for induction of several pro-apoptotic genes in response to DNA damage. Moreover, p68 depletion results in a striking inhibition of recruitment of p53 and RNA Pol II to the p21 promoter but not to the Bax or PUMA promoters, providing an explanation for the selective effect on p21 induction. Importantly, these findings are mirrored in a novel inducible p68 knockout mouse model in which p68 depletion results in a selective inhibition of p21 induction in several tissues. Moreover, in the bone marrow, p68 depletion results in an increased sensitivity to γ-irradiation, consistent with an increased level of apoptosis. These data highlight a novel function of p68 as a modulator of the decision between p53-mediated growth arrest and apoptosis in vitro and in vivo.

Long-lived inflammatory signaling in irradiated bone marrow is genome dependent

Ionizing radiation is carcinogenic, but genotype is a key determinant of susceptibility. Mutational DNA damage is generally attributed to cause disease, but irradiation also affects multicellular interactions as a result of poorly understood bystander effects that may influence carcinogenic susceptibility. In this study, we show that the bone marrow of irradiated mice will retain the ability to kill hemopoietic clonogenic stem cells and to induce chromosomal instability for up to 3 months after irradiation. Chromosomal instability was induced in bone marrow cells derived from CBA/Ca mice, a strain that is susceptible to radiation-induced acute myeloid leukemia (r-AML), but not in C57BL6 mice that are resistant to r-AML. Similarly, clonogenic cell lethality was exhibited in C57BL/6 mice but not CBA/Ca mice. Mechanistic investigations revealed that these genotype-dependent effects involved cytokine-mediated signaling and were mediated by a cyclooxygenase-2-dependent mechanism. Thus, our results suggested that inflammatory processes were responsible for mediating and sustaining the durable effects of ionizing radiation observed on bone marrow cells. Because most exposures to ionizing radiation are directed to only part of the body, our findings imply that genotype-directed tissue responses may be important determinants of understanding the specific consequence of radiation exposure in different individuals.

Chromosomal instability in unirradiated hemaopoietic cells induced by macrophages exposed in vivo to ionizing radiation

The tumorigenic potential of ionizing radiation has conventionally been attributed to DNA damage in irradiated cells induced at the time of exposure. Recently, there have been an increasing number of reports of damage in unirradiated cells that are either neighbors or descendants of irradiated cells, respectively, regarded as bystander effects and genomic instability and collectively termed nontargeted effects. In this study, we show that descendants of normal murine hemaopoietic clonogenic stem cells exposed to bone marrow-conditioned medium derived from gamma-irradiated mice exhibit chromosomal instability unlike the descendants of directly gamma-irradiated cells. The instability is expressed in bone marrow cells of the radiation-induced acute myeloid leukemia (r-AML) susceptible strain (CBA/Ca) but not in mice resistant to r-AML (C57BL/6). Furthermore, crossgenetic experiments show the induction of the instability phenotype requires both the producer and responder cells to be of the susceptible CBA/Ca genotype. Macrophages are the source of the bystander signals, and the signaling mechanism involves tumor necrosis factor-alpha, nitric oxide, and superoxide. The findings show a genotype-dependent chromosomal instability phenotype induced by radiation-induced macrophage-mediated bystander signaling. As the majority of accidental, occupational, and therapeutic exposures to ionizing radiation are partial body exposures, the findings have implications for understanding the consequences of such exposure.

Bystander-mediated genomic instability after high LET radiation in murine primary haemopoietic stem cells

Communication between irradiated and unirradiated (bystander) cells can result in responses in unirradiated cells that are similar to responses in their irradiated counterparts. The purpose of the current experiment was to test the hypothesis that bystander responses will be similarly induced in primary murine stem cells under different cell culture conditions. The experimental systems used here, co-culture and media transfer, are similar in that they both restrict communication between irradiated and bystander cells to media borne factors, but are distinct in that with the media transfer technique, cells can only communicate after irradiation, and with co-culture, cells can communication before, during and after irradiation. In this set of parallel experiments, cell type, biological endpoint, and radiation quality and dose, were kept constant. In both experimental systems, clonogenic survival was significantly decreased in all groups, whether irradiated or bystander, suggesting a substantial contribution of bystander effects (BE) to cell killing. Genomic instability (GI) was induced under all radiation and bystander conditions in both experiments, including a situation where unirradiated cells were incubated with media that had been conditioned for 24h with irradiated cells. The appearance of delayed aberrations (genomic instability) 10-13 population doublings after irradiation was similar to the level of initial chromosomal damage, suggesting that the bystander factor is able to induce chromosomal alterations soon after irradiation. Whether these early alterations are related to those observed at later timepoints remains unknown. These results suggest that genomic instability may be significantly induced in a bystander cell population whether or not cells communicate during irradiation.

A310 helical turn is essential for the proliferation-inhibiting properties of macrophage inflammatory protein-1 alpha (CCL3)

Despite possessing marked structural similarities, the chemokines macrophage inflammatory protein-1alpha (MIP-1alpha; CCL3) and RANTES (CCL5) display differential activity in hematopoietic progenitor-cell-inhibitory assays, with MIP-1alpha being active and RANTES inactive in this context. We have sought to identify the key structural determinants of this property of MIP-1alpha. This has involved constructing MIP-1alpha/RANTES chimeras by swapping structural domains between the 2 proteins. Results indicate that, in contrast to other chemokine functions, neither the N nor the C termini are key determinants of inhibitory activity. The motif that appears to be most important for this activity lies between the second and fourth cysteines of MIP-1alpha and further domain swap analysis has narrowed this down to the 3 10 helical turn preceding the first beta-strand in MIP-1alpha. More detailed analysis has highlighted the role played by a specific dipeptide motif in the proliferation-inhibitory activity of chemokines. The involvement of the 3 10 helical-turn motif in chemokine function is unprecedented and this study therefore identifies a novel, functionally essential motif within chemokines. In addition, this study further attests to the alternative mechanisms of action used by MIP-1alpha in inhibition of hematopoietic progenitor-cell proliferation and regulation of leukocyte migration.

Chromosomal instability in unirradiated hemopoietic cells resulting from a delayed in vivo bystander effect of gamma radiation

Untargeted effects of ionizing radiation (de novo effects in the unirradiated descendants or neighbors of irradiated cells) challenge widely held views about the mechanisms of radiation-induced DNA damage with implications for the health consequences of radiation exposures particularly in the context of the induction of malignancy. To investigate in vivo untargeted effects of sparsely ionizing (low linear energy transfer) radiation, a congenic sex-mismatch bone marrow transplantation protocol has been used to repopulate the hemopoietic system from a mixture of gamma-irradiated and nonirradiated hemopoietic stem cells such that host-, irradiated donor- and unirradiated donor-derived cells can be distinguished. Chromosomal instability in the progeny of irradiated hemopoietic stem cells accompanied by a reduction in their contribution to the repopulated hemopoietic system is consistent with a delayed genomic instability phenotype being expressed in vivo. However, chromosomal instability was also shown in the progeny of the nonirradiated hemopoietic stem cells implicating a bystander mechanism. Studies of the influence of irradiated recipient stromal microenvironment and experiments replacing irradiated cells with irradiated cell-conditioned medium reveal the source of the in vivo bystander effect to be the descendants of irradiated cells, rather than irradiated cell themselves. Thus, it is possible that a radiation-induced genomic instability phenotype in vivo need not necessarily be a reflection of intrinsically unstable cells but the responses to ongoing production of inflammatory-type damaging signals as a long-term unexpected consequence of the initial single radiation exposure.

Genotype-Dependent Induction of Transmissible Chromosomal Instability by γ-Radiation and the Benzene Metabolite Hydroquinone

Although it is well established that ionizing radiation and benzene are epidemiologically linked to acute myeloid leukemia (AML), the underlying mechanisms are not understood. We have shown that gamma-radiation can induce a persisting genomic instability in the clonal descendants of hemopoietic stem cells manifested as a high frequency of nonclonal chromosome and chromatid aberrations. A strikingly similar instability is shown after exposure to the benzene metabolite hydroquinone. The CBA/Ca but not the C57BL/6 genotype is susceptible to the induction of instability by both ionizing radiation and hydroquinone and exposure of CBA/Ca, but not C57BL/6, mice to either agent is known to be associated with the development of AML. The results are consistent with the proposal that chromosomal instability induced by either agent may contribute to AML development by increasing the number of genetic lesions in hemopoietic cells. Genotype-dependent chromosomal instability can be induced by hydroquinone doses that are not acutely stem cell toxic and this may have important implications for current assessment of safe levels of exposure to benzene as well as for mechanistic understanding of the hemotoxic and leukemogenic effects.

Generation of a conditionally immortalized myeloid progenitor cell line requiring the presence of both interleukin-3 and stem cell factor to survive and proliferate

The H-2Kappab temperature-sensitive (ts) A58 transgenic (Immorto) mouse has been used previously to generate conditionally immortalized cells from a number of tissues. The present study aimed to investigate characteristics of primitive myeloid precursor cells derived from H-2Kappab-tsA58 bone marrow. Cell populations were enriched for granulocyte/macrophage progenitors by centrifugal elutriation, and were cultured in the presence and absence of cytokines at the permissive and restrictive temperatures for the A58 oncogene. Cells derived from H-2Kappab-tsA58 mice required both A58 activation and the growth factors, stem cell factor (SCF) and interleukin-3 (IL-3), for long-term cell survival and growth; cells were maintained for > 300 d in culture under these conditions. IL-3- and SCF-dependent clonal cell lines were derived with a phenotype (lin-, Sca-1+, CD34+, ER-MP 58+, ER-MP 12+, ER-MP 20-) characteristic of primitive myeloid progenitors. These cells differentiated on addition of granulocyte/macrophage colony-stimulating factor (GM-CSF) or macrophage colony-stimulating factor (M-CSF) and acquired mature cell morphology with some upregulation of differentiation markers. In conclusion, the A58 oncogene can immortalize haemopoietic progenitor cells. These cells require two cytokines for growth, IL-3 and SCF; as such, they constitute a useful resource for the study of synergistic interactions between growth factors. The ability to develop monocytic cell characteristics also permits the investigation of cytokine-mediated early haemopoietic progenitor cell development.

Radioprotective effects of ginsan, an immunomodulator

We previously reported that ginsan, a purified polysaccharide isolated from Panax ginseng, had a mitogenic activity, induced LAK cells, and increased levels of several cytokines. In an effort to identify other immunostimulatory effects, we evaluated the protective effects of ginsan injected in vivo against radiation by measuring its effects on the CFU-S bone marrow cells and spleen cells. Ginsan was found to significantly increase the number of bone marrow cells, spleen cells, granulocyte-macrophage colony-forming cells (GM-CFC), and circulating neutrophils, lymphocytes and platelets in irradiated mice. In addition, ginsan induced the endogenous production of cytokines such as Il1, Il6, Ifng and Il12, which are required for hematopoietic recovery, and was able to enhance Th1 function while interfering with the Th2 response in irradiated mice. We demonstrated that pretreatment with ginsan protected mice from the lethal effects of ionizing radiation more effectively than when it was given immediately after or at various times after irradiation. A significant increase in the LD(50/30) from 7.54 Gy for PBS injection to 10.93 Gy for mice pretreated with 100 mg/kg ginsan was observed. These findings indicate that ginsan may be a useful agent to reduce the time necessary for reconstituting hematopoietic cells after irradiation.

C-terminal truncation of WT1 delays but does not abolish hematopoiesis in embryoid bodies

The effect of mutations that truncate the WT1 protein on in vitro hematopoietic differentiation from embryonal stem cells has been examined by CFU-A assay, o-dianisidine staining for heme, and RT-PCR analysis of the expression of fetal and adult globins. In two independently isolated ES cell lines the mutations delay but do not abolish hematopoiesis. Analysis of replated CFU-A colonies indicates that the delay occurs prior to the formation of hematopoietic stem cells. The results demonstrate a role for WT1 at the onset of hematopoiesis.

Macrophage inflammatory protein-1alpha uses a novel receptor for primitive hemopoietic cell inhibition

Macrophage inflammatory protein-1alpha (MIP-1alpha) is a member of the chemokine family of proinflammatory mediators. In addition to its inflammatory roles, MIP-1alpha has been shown to be active as an inhibitor of primitive hemopoietic cell proliferation. Indeed, a dysfunction in this inhibitory process has been postulated to contribute to leukemogenesis. Research has been aimed at characterizing the receptor involved in cellular inhibition by MIP-1alpha. This study demonstrates that of all the beta-chemokines tested, only MIP-1alpha is capable of inhibiting primitive hemopoietic cell proliferation. Because no MIP-1alpha-specific receptors have been identified, this suggests that inhibition is mediated by an uncharacterized receptor. Further evidence for the involvement of a novel receptor in this process is the equivalent potencies of MIP-1alphaS and MIP-1alphaP variants of human MIP-1alpha and the fact that primitive cells from bone marrow derived from individual MIP-1alpha receptor null mice display a full response to MIP-1alpha inhibition.

A PCR-based clonal analysis of radiation-induced loss of heterozygosity in haemopoietic stem cells

CBA mouse strains have been used for many years as a model of radiation-induced acute myeloid leukaemia and the leukaemias in CBA and their F1 hybrids are characterised by a specific loss of heterozygosity involving one homologue of chromosome 2. Previous cytogenetic studies of transplanted irradiated bone marrow, or of bone marrow obtained from irradiated mice significantly before the appearance of leukaemia, have been interpreted as the chromosome 2 deletion being a high frequency, possibly initiating event. However, these studies had not specifically addressed the question of whether the characteristic deletion was induced at a high frequency in stem cells. Using a PCR-based technique, we have studied the induction of chromosome 2 LOH in the progeny of (CBA/H x C57BL/6)F1 stem cells after a potentially leukaemogenic radiation exposure. Whilst chromosome 2 LOH can be induced directly by irradiation and there is a preferential loss of the CBA allele, the frequency is no greater than LOH induced in other chromosomal regions studied. The data do not support radiation-induced deletion involving one homologue of chromosome 2 in long-term repopulating stem cells (<1 in 200) being as high a frequency event as might be inferred by previous cytogenetic studies of total bone marrow.

In vivo chromosomal instability and transmissible aberrations in the progeny of haemopoietic stem cells induced by high- and low-LET radiations

PURPOSE

To study stable and unstable chromosomal aberrations in the haemopoietic cells of CBA/H mice after exposure to both high- and low-LET radiations.

MATERIALS AND METHODS

Chromosomal aberrations were scored in the clonal progeny of X-, alpha- or non-irradiated short-term repopulating stem cells using the spleen colony-forming unit (CFU-S) assay, 12 days post-transplantation and in the bone marrow reconstituted by X-, neutron- or non-irradiated exogenous (transplanted) or endogenous (X- or neutron whole-body-irradiated) long-term repopulating stem cells for up to 24 months.

RESULTS

Chromosomal instability was demonstrated in 3-6% of cells in all cases. After transplantation of X- or neutron-irradiated bone marrow approximately 8% of cells with stable aberrations were recorded at all times. After 3Gy X- or 0.5 Gy neutron- whole-body irradiation stable aberrations were detected in approximately 17 and 5% of cells respectively.

CONCLUSIONS

Chromosomal instability induced in vitro can be transmitted in vivo by transplantation of haemopoietic stem cells exposed to high- or low-LET radiations. Comparable instability can be induced and shown to persist for the remaining lifetime after whole-body irradiation. There was no direct relationship between the expression of stable and unstable aberrations and significant interanimal variation in the expression of both stable and unstable aberrations.

An improved micro-method for obtaining chromosome preparations from individual haemopoietic colonies

A robust method for obtaining chromosome preparations from individual haemopoietic colonies in semi-solid media is described. The accumulation of metaphases and the hypotonic treatment of cells were carried out in the culture dish and individual colonies were transferred onto poly-L-lysine-treated slides and fixed stepwise prior to staining. The technique produced high yields of well-spread metaphases facilitating clonal cytogenetic analysis.

Chromosomal instability in the descendants of unirradiated surviving cells after alpha-particle irradiation

We have demonstrated chromosomal instability in the clonal descendants of hemopoietic stem cells after irradiating murine bone marrow with alpha-particles. However, because cells that are irradiated by alpha-particles are defined by a Poisson distribution of individual particle traversals, there is an inevitable proportion of unirradiated cells in the surviving population. The calculated expected proportions of irradiated and nonirradiated cells indicate that the number of clonogenic cells transmitting chromosomal instability is greater than the number expected to be hit and survive. To investigate further this discrepancy, we studied the effects of interposing a grid between the cells and the alpha-particle source so that the surviving population consists predominantly of untraversed stem cells. Comparison with the same irradiation conditions without the grid reveals that the same level of instability is induced. The data confirm that alpha-particles induce chromosomal instability but instability is demonstrated in the progeny of nonirradiated stem cells and must be due to unexpected interactions between irradiated and nonirradiated cells. This untargeted effect has important implications for mechanistic studies of radiation action and for assessment of radiation risk.

Radiation-induced transmissable chromosomal instability in haemopoietic stem cells

Heritable radiation-induced genetic alterations have long been assumed to be "fixed" within the first cell division. However, there is a growing body of evidence that a considerable fraction of cells surviving radiation exposure appear normal, but a variety of mutational changes arise in their progeny due to a transmissible genomic instability. In our investigations of G-banded metaphases, non-clonal cytogenetic aberrations, predominantly chromatid-type aberrations, have been observed in the clonal descendants of murine and human haemopoietic stem cells surviving low doses (approximately l track per cell) of alpha-particle irradiations. The data are consistent with a transmissible genetic instability induced in a stem cell resulting in a diversity of chromosomal aberrations in its clonal progeny many cell divisions later. Recent studies have demonstrated that the instability phenotype persists in vivo and that the expression of chromosomal instability has a strong dependence on the genetic characteristics of the irradiated cell. At the time when cytogenetic aberrations are detected, an increased incidence of hprt mutations and apoptotic cells have been observed in the clonal descendants of (alpha-irradiated murine haemopoietic stem cells. Thus, delayed chromosomal abnormalities, delayed cell death by apoptosis and late-arising specific gene mutations may reflect diverse consequences of radiation-induced genomic instability. The relationship, if any, between these effects is not established. Current studies suggest that expression of these delayed heritable effects is determined by the type of radiation exposure, type of cell and a variety of genetic factors.

Genetic factors influencing alpha-particle-induced chromosomal instability

Alpha-particle-induced chromosomal instability in haemopoietic cells obtained from the CBA/H, DBA/2 and C57BL/6 inbred strains of mouse has been demonstrated at frequencies dependent on genotype. The CBA/H and DBA/2 strains may be regarded as 'sensitive' and the C57BL/6 strain as 'resistant'; resistance was dominant in cells from F1 hybrids. Previously, in cultures where we demonstrated radiation-induced chromosomal instability we also demonstrated an enhanced and persisting oxyradical activity. Quantitative differences in superoxide generation have now been correlated with genetically determined differences in the expression of chromosomal instability. Our findings demonstrate an important influence of genetic factors in alpha-particle-induced chromosomal instability.

DNA ligase I is required for fetal liver erythropoiesis but is not essential for mammalian cell viability

Four distinct DNA ligase activities (I-IV) have been identified within mammalian cells. Evidence has indicated that DNA ligase I is central to DNA replication, as well as being involved in DNA repair processes. A patient with altered DNA ligase I displayed a phenotype similar to Bloom's syndrome, being immunodeficient, growth retarded and predisposed to cancer. Fibroblasts isolated from this patient (46BR) exhibited abnormal lagging strand synthesis and repair deficiency. It has been reported that DNA ligase I is essential for cell viability, but here we show that cells lacking DNA ligase I are in fact viable. Using gene targeting in embryonic stem (ES) cells, we have produced DNA ligase I-deficient mice. Embryos develop normally to mid-term when haematopoiesis usually switches to the fetal liver. Thereupon acute anaemia develops, despite the presence of erythroid-committed progenitor cells in the liver. Thus DNA ligase I is required for normal development, but is not essential for replication. Hence a previously unsuspected redundancy must exist between mammalian DNA ligases.

Feedback inhibitors in normal and tumor tissues

Negative feedback represents the principal mechanism for regulating growth in biological systems. Over the past 20 years, our understanding of the role played by inhibitory factors governing this process has advanced considerably. This is particularly well illustrated in the field of experimental hematology with the recognition of hemopoietic progenitor cell proliferation inhibitors, an expanding group of unrelated peptides that act to limit proliferation in hemopoietic precursor cells. The characterization and subsequent production of these molecules by chemical synthesis or recombinant DNA technology has enabled investigators to explore their role in normal hemopoiesis and define a potential role in clinical medicine. A number of inhibitory factors, including macrophage inflammatory protein-1 alpha (MIP-1 alpha) and the tetrapeptide AcSDKP appear to share a relative specificity to hemopoietic progenitor cell subsets. Others, such as interferon and tumor necrosis factor, have a more complex action and their hemopoietic effects are likely to be indirect and nonspecific. In addition to the role of inhibitors in normal steady state, it has become increasingly evident that loss of sensitivity to the normal feedback inhibitory signals may be of central importance in carcinogenesis and tumor promotion. This presumably represents a developmental strategy that allows the neoplastic cell to maintain a growth advantage over its normal cell counterpart. The underlying mechanisms that terminate in inhibitor-resistance are yet to be elucidated, but in some instances they may be associated with aberrant tumor suppressor gene function.

Efficient isolation of human CD34 positive hemopoietic progenitor cells by immune panninga

In this study we have assessed the use of soybean agglutinin (SBA) and CD34 microcellector devices for the selection of CD34 positive hemopoietic progenitor cells. Burst forming unit-erythroid (BFU-E), colony forming unit-granulocyte/macrophage (CFU-GM) and the recently developed multipotential human colony forming unit-type A (CFU-A) clonogenic assays were used to measure progenitor numbers in the starting mononuclear cell (MNC), the SBA negative, the nonadherent CD34 negative and the adherent CD34 positive fractions during panning. CFU-A progenitors were present at a relatively high incidence in the MNC fraction (220 per 10(5) MNC) and were enriched 15-fold in the adherent CD34 positive fraction. This progenitor incidence and enrichment were similar to those of CFU-GM and BFU-E. The mean recovery for CD34 positive cells was 2.3 x 10(6) cells per marrow aspirate. Analyses by flow cytometry demonstrated that 1-5% of input MNC were CD34 positive, that the purity of the CD34 fraction was approximately 80% and that the calculated recovery for CD34 positive cells was 61%. Recoveries for CFU-GM, BFU-E and CFU-A were between 18 and 40%. CFU-A progenitors were found exclusively in the adherent CD34 positive fraction, whereas a significant proportion of both CFU-GM and BFU-E were present in the nonadherent CD34 negative fraction. We propose that the Applied Immune Sciences (AIS) flasks preferentially bind the cells which express CD34 most strongly and that this is reflected in the finding of primitive CFU-A only in the CD34 positive fraction, with lineage-restricted progenitors found in both CD34 positive and negative fractions. This hypothesis is strengthened by data on long-term bone marrow cultures in which the CD34 positive fraction is better able to maintain output of CFU-GM compared with the CD34 negative fraction. In conclusion, relatively pure populations of CD34 positive cells may be rapidly and efficiently isolated from bone marrow samples with good recovery. The isolated cells show enhanced colony forming capacity in standard clonogenic assays and in the multipotential CFU-A assay.

Contrasting effects of rh-MIP-1 alpha and TGF-beta 1 on chronic myeloid leukemia progenitors in vitro

In chronic myeloid leukemia (CML) an abnormality at the stem cell level results in unregulated expansion of myeloid progenitors. The mechanism underlying this uncontrolled proliferation remains unclear. An in vitro clonogenic assay which detects the human counterpart of the murine colony forming unit (CFU) CFU-A/CFU-S day 12 was described in a report of our recent findings. CML bone marrow samples were found to proliferate in the CFU-A assay, producing colonies morphologically indistinguishable from normal controls. The bcr/abl transcripts were sought in the RNA from individual colonies using the polymerase chain reaction (PCR). For the five CML samples tested to date, the majority of CFU-A colonies at diagnosis or in early chronic phase were found to be bcr/abl positive. For normal controls both macrophage inflammatory protein-1 alpha (MIP-1 alpha) and transforming growth factor-beta 1 (TGF-beta 1) inhibited the proliferation of CFU-A colonies when directly added to the assay. In contrast, CML progenitors responded normally to TGF-beta 1, but showed no response to MIP-1 alpha. In suicide assays, for five normal bone marrow samples, CFU-A progenitors induced into S-phase returned to a quiescent state after treatment with MIP-1 alpha. CML progenitors demonstrated inherently high cycle status which showed no definite response to MIP-1 alpha. However, TGF-beta 1 resulted in quiescence of CML progenitor cycling. In conclusion, the primitive progenitors from CML samples were inhibited normally by TGF-beta 1 but showed no response to MIP-1 alpha.(ABSTRACT TRUNCATED AT 250 WORDS)

Transforming growth factor beta: is it a downregulator of stem cell inhibition by macrophage inflammatory protein 1 alpha?

Transforming growth factor beta 1 (TGF-beta 1) and macrophage inflammatory protein 1 alpha (MIP-1 alpha) have recently been identified as potent inhibitors of hemopoietic stem cell proliferation. From previous studies, these molecules appear to have similar functions in the control of stem cell proliferation. This study was designed to investigate the relationship, if any, between these two negative regulators in an attempt to elucidate possible distinctive roles for each within the hemopoietic system. We report here that both MIP-1 alpha and TGF-beta are capable of inhibiting the same stem cell population (colony-forming unit [CFU]-A/CFU-S) with similar potencies. We further show that TGF-beta potently inhibits MIP-1 alpha gene expression in bone marrow-derived macrophages, the presumed source of MIP-1 alpha in the bone marrow. This inhibition is not specific to MIP-1 alpha in that expression of MIP-1 beta, a related molecule that does not exhibit potent stem cell inhibitory properties, is inhibited in a similar manner. The inhibition of MIP-1 alpha gene expression is also seen as a reduction in MIP-1 alpha protein production, which markedly decreases 24 h after treating RAW 264.7 cells, a murine macrophage cell line, with TGF-beta. These in vitro results suggest that in the presence of active TGF-beta in vivo, and in the absence of upregulators of MIP-1 alpha transcription, very little MIP-1 alpha will be produced. To address how MIP-1 alpha's target cells, the stem cells, would respond to TGF-beta, and the consequently low levels of MIP-1 alpha produced, we analyzed the effect of TGF-beta on MIP-1 alpha receptor levels on FDCP-MIX cells, a murine stem cell line. We show that TGF-beta (100 pM) reversibly downregulates MIP-1 alpha receptor levels on these cells to a maximum of 50-70% after 24 h. This level of downregulation does not change upon increasing the concentration of TGF-beta or the length of exposure of the cells to TGF-beta. Scatchard analysis shows that TGF-beta downregulates MIP-1 alpha receptor numbers with no change in affinity of the remaining receptors. These results suggest that TGF-beta may be capable of interfering with MIP-1 alpha's role as a stem cell inhibitor. Indeed, they suggest that in the presence of active TGF-beta in vivo, MIP-1 alpha is at best a weak contributor to the overall physiological inhibition of stem cells.

Transmission of chromosomal instability after plutonium alpha-particle irradiation

When investigating the biological effects of ionizing radiation on the haemopoietic system, a confounding problem lies in possible differences between the biological effects of sparsely ionizing, low linear energy transfer radiation such as X-, beta- or gamma-rays, and densely ionizing, high linear energy transfer radiation such as alpha-particles. To address this problem we have developed novel techniques for studying haemopoietic cells irradiated with environmentally relevant doses of alpha-particles from a plutonium-238 source. Using a clonogenic culture system, cytogenetic aberrations in individual colonies of haemopoietic cells derived from irradiated stem cells have been studied. Exposure to alpha-particles (but not X-rays) produced a high frequency of non-clonal aberrations in the clonal descendants, compatible with alpha-emitters inducing lesions in stem cells that result in the transmission of chromosomal instability to their progeny. Such unexpected instability may have important implications for radiation leukaemogenesis.

Purification and biochemical characterisation of human and murine stem cell inhibitors (SCI)

We have recently characterised an inhibitor of haemopoietic stem cell proliferation (SCI/MIP-1 alpha) and report here on its purification and initial biological and biochemical characterisation. The activity can be detected by direct addition to the CFU-A stem cell assay and this simple test for inhibitory activity has greatly facilitated the purification of the molecule. The purification involves a combination of Mono Q ion exchange chromatography, heparin-sepharose affinity chromatography and Blue Sepharose affinity chromatography. The purified stem cell inhibitor is an 8 kD peptide which is identical to the previously described peptide macrophage inflammatory protein 1 alpha. The peptide has a natural tendency to form large self-aggregates and appears, in physiological buffers, to have a native molecular weight of around 90 kD. SCI is a heat stable, protease sensitive protein which is half maximally active at between 10 and 25 pM in the CFU-A assay. The self-aggregates can be disrupted by dilute solutions of acetic acid and it appears that disruption increases the specific activity of SCI preparations. We also report the characterisation of the human homologue of the stem cell inhibitor (human SCI/MIP-1 alpha) which is 74% identical to murine MIP-1 alpha and which shares all the above features of the murine inhibitor.

The architecture of bone marrow cell populations

Marrow is a loosely bound tissue in which hemopoiesis has frequently been considered to be randomly distributed. The case is presented, however, for an organized and structured marrow in which close relationships exist between hemopoietic tissue and a regulatory microenvironment. Distributions of myeloid cells in the mouse femur are described, and a dynamic picture of their movement, with differentiation and maturation from the endosteal surface of the bone to their release via the central venous sinus, is painted. It is also shown that this structure is established within three weeks of birth. By contrast, mature lymphoid cells (but not their progenitors) are uniformly distributed. Regulatory stromal elements in the marrow are also structured and their localization is found to correspond closely to the properties of the progenitor populations. Such structure has potential practical importance, particularly in the field of medical, industrial or accidental radiation exposure where bone may introduce non-uniform dose distributions in the marrow.

Negative regulators of haemopoiesis--current advances

The overall control of the haemopoietic system is ultimately articulated at the level of stem cell proliferative regulation. An understanding of the control processes involved is central to a full understanding of the regulation of haemopoiesis in health and disease. We describe here the recent advances in understanding of the negative regulation of primitive haemopoietic cells. The possible involvement of inhibitory factors in the development of haemopoietic malignancy is discussed. The known biological functions of many of these inhibitory molecules suggests a therapeutic potential for negative regulators.