Platelet-derived growth factor enhances in vitro erythropoiesis via stimulation of mesenchymal cells

Platelet-derived growth factors and their receptors in normal and malignant hematopoiesis

Platelet-derived growth factors (PDGF) bind to two closely related receptor tyrosine kinases, PDGF receptor α and β, which are encoded by the PDGFRA and PDGFRB genes. Aberrant activation of PDGF receptors occurs in myeloid malignancies associated with hypereosinophilia, due to chromosomal alterations that produce fusion genes, such as ETV6-PDGFRB or FIP1L1-PDGFRA. Most patients are males and respond to low dose imatinib, which is particularly effective against PDGF receptor kinase activity. Recently, activating point mutations in PDGFRA were also described in hypereosinophilia. In addition, autocrine loops have been identified in large granular lymphocyte leukemia and HTLV-transformed lymphocytes, suggesting new possible indications for tyrosine kinase inhibitor therapy. Although PDGF was initially purified from platelets more than 30 years ago, its physiological role in the hematopoietic system remains unclear. Hematopoietic defects in PDGF-deficient mice have been reported but appear to be secondary to cardiovascular and placental abnormalities. Nevertheless, PDGF acts directly on several hematopoietic cell types in vitro, such as megakaryocytes, platelets, activated macrophages and, possibly, certain lymphocyte subsets and eosinophils. The relevance of these observations for normal human hematopoiesis remains to be established.

The absence of platelet-derived growth factor-B in circulating cells promotes immune and inflammatory responses in atherosclerosis-prone ApoE-/- mice

Both innate and adaptive immunity contribute to the progression of inflammatory-fibrotic lesions of atherosclerosis. Although platelet-derived growth factor (PDGF)-B has been investigated as a stimulant of smooth muscle cells in vascular diseases, its effects on the immune response during disease have not been evaluated in vivo. We used hematopoietic chimeras generated after lethal irradiation of ApoE-/- recipients to test the role of PDGF in atherosclerosis. Monocyte accumulation in early atherosclerotic lesions increased 1.9-fold in ApoE-/-/PDGF-B-/- chimeras. Lymphocytes from null chimeras showed a 1.6- to 2.0-fold increase in the number of activated CD4(+) T cells and a 2.5-fold elevation of interferon-gamma-secreting CD4(+) T cells on ex vivo challenge with modified low-density lipoprotein. Splenocyte transcript levels were also altered with a twofold decrease in interleukin-10 and 1.7- and 3.0-fold increases in interleukin-18 and CCR 5, respectively. These cellular and molecular changes were consistent with a shift to a proinflammatory phenotype in null chimeras. Our data also demonstrated for the first time the presence of a recently discovered family of negative regulators of innate and adaptive immunity, the suppressors of cytokine signaling (SOCS), in developing atherosclerotic lesions. Thus, our studies identify two independent negative immune regulatory pathways-PDGF-B and SOCS-that may help limit lesion expansion.

Expression of angiogenic factors in chronic myeloid leukaemia: role of the bcr/abl oncogene, biochemical mechanisms, and potential clinical implications

Chronic myeloid leukaemia (CML) is a stem cell disease characterized by an increased production and accumulation of clonal BCR/ABL-positive cells in haematopoietic tissues. The chronic phase of CML is inevitably followed by an accelerated phase of the disease, with consecutive blast crisis. However, depending on genetic stability, epigenetic events, and several other factors, the clinical course and survival appear to vary among patients. Recent data suggest that angiogenic cytokines such as vascular endothelial growth factor (VEGF), are up-regulated in CML, and play a role in the pathogenesis of the disease. These factors appear to be produced and released in leukaemic cells in patients with CML. In line with this notion, increased serum-levels of angiogenic growth factors are measurable in CML patients. In this study we provide an overview of angiogenic growth factors expressed in CML cells, discuss the possible pathogenetic role of these cytokines, the biochemical basis of their production in leukaemic cells, and their potential clinical implications.

Platelet-derived growth factor up-regulates the expression of transcription factors NF-E2, GATA-1 and c-Fos in megakaryocytic cell lines

Platelet-derived growth factor (PDGF) is a platelet alpha-granule protein. In previous reports, we demonstrated the expression of PDGF receptors on platelets and megakaryocytic cells and that PDGF enhanced the proliferation of megakaryocytic progenitor cells. In this study, we investigated the effects of PDGF on mRNA and protein expressions of megakaryocyte-associated transcription factors, c-Fos, GATA-1, NF-E2 and PU.1, in two human megakaryocytic cell lines CHRF-288-11 and DAMI. RT-PCR/Southern blot analysis and Real-time PCR demonstrated that PDGF increased the mRNA expression of c-Fos, GATA-1 and NF-E2, but not PU.1 in a dose- and time-dependent manner. The activation was confirmed at the protein level by Western blot analysis of both total cell and nuclear lysates. The addition of increasing concentrations of Tyrphostin AG1295, an inhibitor of PDGF receptor kinase, blocked the stimulatory effect of PDGF on the mRNA and protein expressions of these transcription factors. The up-regulation of c-Fos, GATA-1 and NF-E2 protein by PDGF was inhibited by actinomycin D and cycloheximide, suggesting that mRNA and protein synthesis might be involved in the mechanism. Our data suggest a direct stimulatory effect of PDGF on c-Fos, GATA-1 and NF-E2 expressions and we speculate that these transcription factors might be involved in the signal transduction of PDGF on the regulation of megakaryocytopoiesis.

Platelet-derived growth factor promotes ex vivo expansion of CD34+ cells from human cord blood and enhances long-term culture-initiating cells, non-obese diabetic/severe combined immunodeficient repopulating cells and formation of adherent cells

Platelet-derived growth factor (PDGF) is a major mitogen for connective tissue cells. In this study, we investigated the effects and mechanism of PDGF on the ex vivo expansion of cord blood CD34+ cells. Our data demonstrated that among various cytokine combinations of thrombopoietin (TPO), interleukin 1 beta (IL-1beta), IL-3, IL-6 and Flt-3 ligand (Flt-3L), TPO + IL-6 + Flt-3L was most efficient in promoting the expansion of CD34+ cells, CD34+CD38- cells, mixed-lineage colony-forming units (CFU-GEMM) and long-term culture-initiating cells (LTC-IC) by 21.7 +/- 5.00-, 103 +/- 27.9-, 10.7 +/- 7.94- and 6.52 +/- 1.51-fold, respectively, after 12-14 d of culture. The addition of PDGF increased the yield of these early progenitors by 45.0%, 66.5%, 45.1% and 79.8% respectively. More significantly, PDGF enhanced the engraftment of human CD45+ cells and their myeloid subsets (CD33+, CD14+ cells) in non-obese diabetic (NOD)/severe-combined immunodeficient (SCID) mice. The expression of PDGF receptor (PDGFR)-beta was not detectable in fresh CD34+ cells but was upregulated after culture for 3 d. PDGF also enhanced the development of adherent cells/clusters that expressed the endothelial markers VE-cadherin and CD31. These findings suggest that PDGF is an effective cytokine for the ex vivo expansion of early stem and progenitor cells. The mechanism could be mediated by PDGFR-beta on committed CD34+ progenitor cells and/or secondary to the stimulation of autologous, stromal feeder cells.

Tyrosine kinase oncogenes in normal hematopoiesis and hematological disease

Tyrosine kinase oncogenes are formed as a result of mutations that induce constitutive kinase activity. Many of these tyrosine kinase oncogenes that are derived from genes, such as c-Abl, c-Fes, Flt3, c-Fms, c-Kit and PDGFRbeta, that are normally involved in the regulation of hematopoiesis or hematopoietic cell function. Despite differences in structure, normal function, and subcellular location, many of the tyrosine kinase oncogenes signal through the same pathways, and typically enhance proliferation and prolong viability. They represent excellent potential drug targets, and it is likely that additional mutations will be identified in other kinases, their immediate downstream targets, or in proteins regulating their function.

Platelet-derived growth factor (PDGF) in human acute myelogenous leukemia: PDGF receptor expression, endogenous PDGF release and responsiveness to exogenous PDGF isoforms by in vitro cultured acute myelogenous leukemia blasts

We investigated effects of Platelet-derived growth factor (PDGF) and Platelet factor 4 (PF-4) on the functional characteristics of native, human acute myelogenous leukemia (AML) blasts. AML blast expression of the PDGF-receptor alpha-chain was detected for a subset of patients (45%), whereas PDGF-receptor beta-chain expression was detected for most patients (90%). Constitutive AML blast release of the PDGF-AB isoform (the major form also derived from normal platelets) was detected for 43% of patients, whereas PDGF-BB release was not detected for any patient. The PDGF isoforms AA, AB and BB had dose-dependent and divergent effects on spontaneous and cytokine-dependent AML blast proliferation, whereas for constitutive cytokine secretion (IL-1beta, IL-6, TNF-alpha) inhibitory effects were rare and all three isoforms usually had no effect or enhanced the constitutive secretion. The PDGF effects were caused by a direct effect on the AML blasts and were not dependent on the presence of serum. The PDGF effects could also be detected after in vitro culture of AML cells in the presence of IL-4+ granulocyte-macrophage colony stimulating factor. PF-4 had divergent effects on proliferation and cytokine secretion by native AML blasts. Our results suggest that exogenous (e.g. platelet-secreted) PDGF and PF-4 can function as regulators of leukemic hematopoiesis and possibly also modulate the function of residual AML cells in peripheral blood stem cell grafts. On the other hand, endogenous release of PDGF-AB by native blasts may modulate the function of normal cells in the bone marrow microenvironment (e.g. bone marrow stromal cells).

Platelet-derived growth factor (PDGF) in human acute myelogenous leukemia: PDGF receptor expression, endogenous PDGF release and responsiveness to exogenous PDGF isoforms by in vitro cultured acute myelogenous leukemia blasts

We investigated effects of Platelet-derived growth factor (PDGF) and Platelet factor 4 (PF-4) on the functional characteristics of native, human acute myelogenous leukemia (AML) blasts. AML blast expression of the PDGF-receptor alpha-chain was detected for a subset of patients (45%), whereas PDGF-receptor beta-chain expression was detected for most patients (90%). Constitutive AML blast release of the PDGF-AB isoform (the major form also derived from normal platelets) was detected for 43% of patients, whereas PDGF-BB release was not detected for any patient. The PDGF isoforms AA, AB and BB had dose-dependent and divergent effects on spontaneous and cytokine-dependent AML blast proliferation, whereas for constitutive cytokine secretion (IL-1beta, IL-6, TNF-alpha) inhibitory effects were rare and all three isoforms usually had no effect or enhanced the constitutive secretion. The PDGF effects were caused by a direct effect on the AML blasts and were not dependent on the presence of serum. The PDGF effects could also be detected after in vitro culture of AML cells in the presence of IL-4+granulocyte-macrophage colony stimulating factor. PF-4 had divergent effects on proliferation and cytokine secretion by native AML blasts. Our results suggest that exogenous (e.g. platelet-secreted) PDGF and PF-4 can function as regulators of leukemic hematopoiesis and possibly also modulate the function of residual AML cells in peripheral blood stem cell grafts. On the other hand, endogenous release of PDGF-AB by native blasts may modulate the function of normal cells in the bone marrow microenvironment (e.g. bone marrow stromal cells).

Basis of hematopoietic defects in platelet-derived growth factor (PDGF)-B and PDGF beta-receptor null mice

Platelet-derived growth factor (PDGF)-B and PDGF beta-receptor (PDGFR beta) deficiency in mice is embryonic lethal and results in cardiovascular, renal, placental, and hematologic disorders. The hematologic disorders are described, and a correlation with hepatic hypocellularity is demonstrated. To explore possible causes, the colony-forming activity of fetal liver cells in vitro was assessed, and hematopoietic chimeras were demonstrated by the transplantation of mutant fetal liver cells into lethally irradiated recipients. It was found that mutant colony formation is equivalent to that of wild-type controls. Hematopoietic chimeras reconstituted with PDGF-B(-/-), PDGFR beta(-/-), or wild-type fetal liver cells show complete engraftment (greater than 98%) with donor granulocytes, monocytes, B cells, and T cells and display none of the cardiovascular or hematologic abnormalities seen in mutants. In mouse embryos, PDGF-B is expressed by vascular endothelial cells and megakaryocytes. After birth, expression is seen in macrophages and neurons. This study demonstrates that hematopoietic PDGF-B or PDGFR beta expression is not required for hematopoiesis or integrity of the cardiovascular system. It is argued that metabolic stress arising from mutant defects in the placenta, heart, or blood vessels may lead to impaired liver growth and decreased production of blood cells. The chimera models in this study will serve as valuable tools to test the role of PDGF in inflammatory and immune responses. (Blood. 2001;97:1990-1998)

Platelet-derived growth factor enhances granulopoiesis via bone marrow stromal cells

Platelet-derived growth factor (PDGF), a growth factor for connective tissue cells, stimulates erythropoiesis and megakaryocytopoiesis in vitro but the effect of PDGF on granulocyte proliferation remains unknown. The effect of the recombinant human PDGF-BB isoform on granulopoiesis was investigated in this study. The results show that PDGF significantly stimulated murine colony-forming unit-granulocyte-monocyte (CFU-GM) proliferation in a dose-dependent manner (1 to 100 ng/mL) using murine bone marrow cells (n = 4). Maximum stimulation was obtained with 50 ng/mL of PDGF (P < .01). The effect of PDGF on murine CFU-GM proliferation was compared with that of interleukin (IL)-3, IL-6, granulocyte-monocyte colony-stimulating factor (GM-CSF), and acidic fibroblast growth factor (aFGF) at their optimal doses. The stimulating activity of PDGF was higher than that of aFGF but lower than that of IL-3, IL-6, or GM-CSF. There is no synergistic effect between PDGF and IL-3 or IL-6, but a significant enhancing effect was observed in IL-3 plus IL-6. PDGF also stimulated the growth of CFU-GM with CFU-megakaryocyte in the presence of bone marrow stromal cells. We also found that PDGF had similar a effect on human CFU-GM proliferation using bone marrow mononuclear cells (MNC). However, the increase in PDGF-stimulated CFU-GM proliferation was inhibited by anti-GM-CSF, anti-IL-3, and anti-IL-6 antibodies (n = 4), suggesting that endogenously produced GM-CSF, IL-3, and IL-6 may play a role in the PDGF-induced CFU-GM proliferation. Furthermore, PDGF (1 to 100 ng/mL) did not show any effect on CFU-GM proliferation when replacing bone marrow MNC with immunomagnetic selection-enriched CD34+ cells from human cord blood (n = 5; purity, 91% +/- 6.5%). This study indicates that PDGF may indirectly enhance CFU-GM proliferation by inducing the bone marrow stromal cells to produce GM-CSF, IL-3, or IL-6.

Effects of vascular endothelial and platelet-derived growth factor receptor inhibitors on long-term cultures from normal human bone marrow

Endothelial cells and fibroblasts are important constituents of the haemopoietic microenvironment. Growth and function of these cells are controlled by a variety of cytokines, including vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF). We analysed the effects of novel tyrosine kinase inhibitors targeting the VEGF and PDGF receptors (compounds SU5614 and SU5768) on the performance of long-term cultures from normal human bone marrow. In developing cultures, the inhibitors induced a dose-dependent reduction in stromal fibroblasts, macrophages and endothelial cells with a concomitant decrease in blood cell production and an increase in fat cells. For SU5614, the concentration inhibiting stroma formation by 50% (IC50) was 123nM, and the IC50 for haemopoietic colony forming cell output was 186 nM. For SU5768, the respective values were 871 nM and 331 nM. Changes in stroma composition and inhibition of haemopoietic cell production were also demonstrable after delayed addition of the inhibitors to established cultures. By contrast, haemopoietic colony formation in clonogenic agar cultures was unimpaired (IC50 not reached at 100 microM). Immunofluorescence studies and time course analyses suggested that the primary effect of the inhibitors was interference with the proliferation and function of fibroblasts and endothelial cells which in turn resulted in decreased haemopoiesis and increased adipogenesis. This was associated with decreased levels in conditioned media of granulocyte-macrophage colony-stimulating factor, interleukin-6 and leptin. VEGF and PDGF may play a hitherto underestimated role in the control of blood cell formation. VEGF/PDGF receptor inhibitors may have therapeutic potential in stroma diseases such as myelofibrosis. Since they weaken the stimulatory signals provided by the microenvironment, they may also be of value in the treatment of leukaemia and other neoplastic bone marrow diseases.

Properties of primary murine stroma induced by macrophage colony-stimulating factor

The ability of purified human macrophage colony-stimulating factor (M-CSF) to accelerate the formation of stromal cells from murine bone marrow cells was investigated. The liquid culture of the marrow cells with M-CSF resulted in the formation of monolayers of macrophages on day 7. When the M-CSF was removed on that day and the residual adherent cells were cultured in the absence of M-CSF for an additional 7 days, many colonies appeared with cells that were morphologically distinguishable from M-CSF-derived macrophages. The appearance of the colonies was dependent on the concentration of M-CSF used at the beginning of the culture. Each colony was isolated as a single clone and analyzed. All clones were negative for esterase staining. These cells did not express M-CSF receptor mRNA and did not show a mitogenic response to M-CSF. On the contrary, these cells could be stimulated to proliferate by fibroblast growth factor and platelet-derived growth factor. The polymerase chain reaction analysis of these cells demonstrated constitutive expression of mRNA for M-CSF, stem cell factor, and interleukin (IL)-1, but not IL-3. Some clones expressed mRNA for granulocyte/M-CSF and IL-6. We also examined the ability of the cells to maintain murine bone marrow high proliferative potential colony-forming cells (HPP-CFC) in a coculture system. Most of the clones showed a significant increase in total HPP-CFC numbers after 2 weeks of coculture, although the extent of stimulation differed among clones. These results suggested that the colonies established by M-CSF were composed of functional stromal cells that were phenotypically different from macrophages.

Basic fibroblast growth factor and epidermal growth factor downmodulate the growth of hematopoietic cells in long-term stromal cultures

The bone marrow microenvironment consists of stromal cells and extracellular matrix components which act in concert to regulate the growth and differentiation of hematopoietic stem cells. There is little understanding of the mechanisms which modulate the regulatory role of stromal cells. This study examined the hypothesis that mesenchymal growth factors such as basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF) modulate stromal cell activities and thereby influence the course of hematopoiesis. Both bFGF and EGF were potent mitogens for marrow stroma. However, both factors proved to be inhibitory to hematopoiesis in primary long-term marrow cultures. Inhibition was also observed when hematopoietic cells and bFGF or EGF were added to subconfluent irradiated stromal layers, demonstrating that the decline of hematopoiesis was not due to overgrowth of the stromal layer. Loss of hematopoietic support in bFGF and EGF was dose-dependent. Removal of bFGF and EGF permitted stromal layers to regain their normal capacity to support hematopoiesis. In stroma-free long-term cultures, neither factor affected CFU-GM expansion. Basic FGF slightly enhanced granulocyte-macrophage colony forming unit (CFU-GM) cloning efficiency in short-term agarose culture. Basic FGF did not reduce the levels of interleukin-6 (IL-6), GM-CSF, or G-CSF released by steady state or IL-1-stimulated stroma. Similarly, the constitutive levels of steel factor (SF) mRNA and protein were not affected by bFGF. Basic FGF did not alter the level of TGF-beta 1 in stromal cultures. We conclude that bFGF and EGF can act as indirect negative modulators of hematopoietic growth in stromal cultures. The actual mediators of regulation, whether bound or soluble, remain to be identified.

Recombinant PDGF enhances megakaryocytopoiesis in vitro

The effect of recombinant platelet-derived growth factor (PDGF) on both murine and human megakaryocyte colony formation was studied in the plasma clot culture system. PDGF significantly stimulates megakaryocyte colony formation in a dose-dependent manner. The minimum concentration which had a significant stimulating effect on colony forming unit megakaryocyte (CFU-MK) was 10 ng/ml and maximum stimulation occurred at 50 ng/ml. The effect of PDGF was compared with that of interleukin (IL)-3, IL-6, granulocyte-macrophage colony stimulating factor (GM-CSF), erythropoietin (EPO) and acid fibroblast growth factor (aFGF) on megakaryocyte colony formation. The results showed that megakaryocyte colony stimulating activity of PDGF was slightly higher than those of GM-CSF and aFGF, but lower than those of IL-3, IL-6 and EPO. The effect of PDGF in combination with IL-3 or IL-6 on megakaryocyte colony formation was also investigated. No synergistic action was found between PDGF and IL-3 or IL-6, but an additive effect was observed with IL-3 plus IL-6. We also studied the effects of PDGF in combination with anti-IL6, anti-IL-3 or anti-GM-CSF antibody. The increase of megakaryocyte colony formation induced by PDGF was partially inhibited by anti-IL-6 or anti-GM-CSF antibody but not by anti-IL-3 antibody. These results indicate that PDGF is a positive regulator for megakaryocytopoiesis in vitro and IL-6 and GM-CSF may play a role in the mechanism whereby PDGF stimulates megakaryocytopoiesis.

Regulation of platelet-derived growth factor A and B chain gene expression in bone marrow stromal cells

MBA-2, bone marrow-derived endothelial stromal cells, express platelet-derived growth factor (PDGF) A and B chain mRNAs and secrete PDGF activity that is induced by TGF-beta. Either chain of the PDGF molecule could modulate hematopoiesis and stromal cell growth. Intracellular pathways that regulate PDGF expression in the marrow microenvironment are unknown. In the present study, we examined the mechanisms that mediate PDGF A and B chain mRNA induction by TGF-beta and the role of protein kinase C (PKC) and cyclic AMP in PDGF regulation. TGF-beta was tested in parallel with PMA, an activator of phorbol ester-dependent PKC isoforms. Both PMA (10(-7)M) and TGF-beta (2.5 ng/ml) increased PDGF A and B chain mRNA levels. The serine/threonine protein kinase inhibitor, H7, blocked PDGF A and B chain mRNA induction in response to TGF-beta. However, down-regulation of PKC by prolonged incubation with PMA failed to abolish TGF-beta induction of PDGF A and B chain mRNAs. These findings indicate that induction of PDGF A and B chain mRNAs can be mediated via phorbol ester-dependent PKC pathway. In contrast, H7-sensitive protein kinase(s) other than phorbol ester-sensitive protein kinase C mediate the effect of TGF-beta. Agents that increase cAMP were also tested for their effect on PDGF gene expression. TGF-beta-mediated induction of PDGF A and B chain mRNAs was markedly inhibited by cAMP. cAMP also blocked stimulation of PDGF A chain mRNA by PMA. The positive and negative signaling mechanisms involved in modulating PDGF in the microenvironment may be important for determining hematopoietic and stromal cell responses in vivo.

Regulated production of a pleiotropic cytokine-platelet-derived growth factor--by differentiating erythroid cells in vitro and in vivo

Erythroid progenitor growth in vitro is stimulated by exogenous platelet-derived growth factor (PDGF). We now report that both normal and transformed erythroid progenitor cells produce authentic PDGF in vitro and in vivo. Importantly, this production is highly regulated during erythropoiesis. Addition of soluble lysates from Rauscher murine erythroleukemia cells--an erythropoietin-responsive model progenitor cell line--to quiescent BALB/c 3T3 fibroblasts resulted in a mitogenic response identical to that observed with the addition of authentic recombinant PDGF. Polyclonal and monoclonal anti-PDGF antibodies immunoabsorbed 50-100% of this activity. Induction of Rauscher cell differentiation in vitro with dimethyl sulfoxide or erythropoietin for 48-72 hr markedly upregulated PDGF production by 17- to 18-fold and 14- to 38-fold, respectively. Importantly, stimulation of normal erythropoiesis in vivo in mice treated either with phenylhydrazine or with erythropoietin increased PDGF levels in the spleen by 11- to 48-fold and 20- to 34-fold, respectively. These results strongly suggest a role for erythroid cell-derived PDGF in normal erythropoiesis and provide documentation of the regulated production of a pleiotropic cytokine by erythroid cells.

In vitro p53 and/or Rb antisense oligonucleotide treatment in association with growth factors induces the proliferation of peripheral hematopoietic progenitors

In this work we intended to determine whether p53 and/or retinoblastoma (Rb) tumor suppressor genes are involved at specific stages in the process of in vitro human peripheral stem cell hematopoiesis. Mononuclear peripheral blood cells were depleted of adherent cells and T lymphocytes (A-T-PMCs). Cells were then cultured in semisolid medium, under conditions that favor the growth of specific progenitor cell types. A-T-PMCs were exposed to p53 and/or Rb sense, scrambled DNA and antisense oligodeoxynucleotides. p53 and/or Rb antisenses (but not their senses or scrambled DNA) treatment of A-T-PMCs resulted in a significantly increase in the number of granulocyte/macrophage colony-forming units (CFU-GM) in the presence of interleukin-3 (IL-3) and/or granulocyte/macrophage colony-stimulating factor (GM-CSF). After antisense treatment, blast forming units/erythroblasts (BFU-E) derived from A-T-PMCs cultured in the presence of IL-3 + erythropoietin (Epo) were also increased whereas colony forming units/erythroblasts (CFU-E) were not markedly affected in the presence of Epo only. Megakaryocytic colony (CFU-Meg) formation from A-T-PMCs in the presence of interleukin-6 (IL-6) + IL-3 + Epo was also increased after antisense oligodeoxynucleotide treatment. These results are consistent with the hypothesis that p53 and Rb tumor suppressor gene products are involved in the control of distinct signal pathways in different peripheral progenitor cells.

Synthesis of platelet-derived growth factor by cells of splenic red pulp in normal rats

A population of cells in the spleens of normal rats was found to contain platelet-derived growth factor (PDGF) B chain mRNA. These cells were found predominantly in the red pulp and nuclear morphology of some was consistent with that of macrophages. Similar cells were also shown by immunocytochemical staining to contain PDGF-AB/BB. These PDGF-positive cells were also found almost exclusively in the red pulp. It has been suggested by others that PDGF plays an important role in the function of the lymphohemopoietic microenvironment.

Fetal lung epithelial cells express receptors for platelet-derived growth factor

There is increasing evidence to suggest that platelet-derived growth factor (PDGF) or PDGF-like molecules play a role in fetal lung morphogenesis. Our previous studies demonstrated that fetal lung epithelial cells respond mitogenically to exogenous PDGF, while fetal lung fibroblasts respond with increased glycosaminoglycan synthesis. To further study the target cells of PDGF in fetal rat lung, we investigated the presence and nature of PDGF receptors in fetal lung cells. Functional PDGF receptors were expressed on normal epithelial cells of fetal rat lung. All three isoforms of PDGF (AA, AB, and BB) were mitogenic for quiescent epithelial cells. Northern blot and protein analysis demonstrated the presence of PDGF alpha-receptor and PDGF beta-receptor. All isoforms of PDGF enhanced tyrosine kinase activity and stimulated receptor autophosphorylation. In contrast, fetal lung fibroblasts expressed only the PDGF beta-receptor. PDGF-AB and PDGF-BB, but not PDGF-AA, stimulated tyrosine kinase activity. No PDGF isoform was mitogenic for quiescent fibroblasts. However, PDGF-BB stimulated fibroblast proliferation on a collagen type I substratum in the presence of transferrin. Binding experiments with [125I]PDGF-AA and [125I]-PDGF-BB to epithelial cells and fibroblasts confirmed these observations.

The cloning and biological characterization of recombinant human interleukin 11

Interleukin 11 (IL-11) is a pleiotropic hematopoietic growth factor with stimulatory effects on multiple hematopoietic progenitor cells. An immortalized primate bone marrow stromal cell line was established to facilitate the analysis of interactions between hematopoietic progenitors and the microenvironment. This line was found to produce a novel growth factor that directed the proliferation of a murine plasmacytoma cell line. A cDNA encoding this activity was isolated through functional expression cloning in mammalian cells. Preliminary characterization of the cytokine revealed that IL-11 stimulated T cell-dependent B cell immunoglobulin secretion and synergized with IL-3 in murine megakaryocyte formation. Subsequent analysis of the clone has demonstrated that it also synergizes with IL-3 and steel factor in the stimulation of early progenitors and affects early megakaryocyte formation and maturation. In addition, IL-11 induces secretion of acute phase proteins in the liver and may function in the hematopoietic microenvironment as an adipogenic antagonist in a paracrine manner. Further in vitro and in vivo analysis of IL-11 will be necessary to determine the biologic function and potential therapeutic use of the cytokine.

Chick limb bud mesodermal cell chondrogenesis: inhibition by isoforms of platelet-derived growth factor and reversal by recombinant bone morphogenetic protein

Platelet-derived growth factor (PDGF) influences the proliferation and differentiation of a variety of cells. In this study, we have investigated the effect of PDGF isoforms on chondrogenesis by stage 24 chick limb bud mesoderm cells in culture. Synthesis of sulfated proteoglycans, an index of chondrogenesis, was inhibited by all three PDGF isoforms (PDGF-AA, PDGF-AB, and PDGF-BB). Application of PDGF isoforms during the first 2 days of culture, before the cells were overtly differentiating, resulted in decreased synthesis of sulfated proteoglycans. This was similar to when PDGF isoforms were present throughout the culture period. However, application of PDGF isoform during only the last 2 days of culture, did not inhibit cartilage matrix production. When chondrogenic and nonchondrogenic cells were separated from the cultures and replated, PDGF-AB and PDGF-BB inhibited the incorporation of sulfate by the chondrogenic cells. Recombinant bone morphogenetic protein 2B reversed the inhibitory effects of PDGF on sulfated proteoglycan synthesis and DNA synthesis. PDGF receptor binding analysis indicated that beta-receptors were predominant receptors present on the chondrogenic and nonchondrogenic cells of the stage 24 mesoderm. PDGF isoforms increased thymidine incorporation by 48 h in both high and low density cultures. However, at later periods, cell proliferation was inhibited by PDGF-AA and PDGF-AB but not by PDGF-BB. PDGF acted as a bifunctional modulator of mesodermal cell proliferation and thus may regulate chondrogenesis during limb differentiation and morphogenesis.

Growth factor-dependent inhibition of normal hematopoiesis by N-ras antisense oligodeoxynucleotides

To determine whether N-ras expression is required at specific stages of the process of in vitro normal human hematopoiesis, adherent- and T lymphocyte-depleted mononuclear marrow cells (A-T-MNC) or highly purified progenitors (CD34+ cells) were cultured in semisolid medium, under conditions that favor the growth of specific progenitor cell types, after exposure to N-ras sense and antisense oligodeoxynucleotides. N-ras antisense, but not sense, oligodeoxynucleotide treatment of A-T-MNC and CD34+ cells resulted in a significantly decreased number of granulocyte/macrophage colony-forming units (CFU-GM) induced by interleukin 3 (IL-3) or granulocyte/macrophage colony-stimulating factor (GM-CSF) and of macrophage colonies (CFU-M) induced by M-CSF, but not of granulocytic colonies induced with G-CSF or IL-5. However, the same treatment significantly inhibited colony formation induced by each of the above factors in combination with IL-3. Megakaryocytic colony (CFU-Meg) formation from A-T-MNC or CD34+ cells in the presence of IL-6 + IL-3 + erythropoietin (Epo) was also markedly decreased after antisense oligodeoxynucleotide treatment. Erythroid colonies derived from A-T-MNC in the presence of Epo (CFU-E) were not inhibited upon antisense treatment, whereas those arising from A-T-MNC or CD34+ cells in the presence of IL-3 + Epo (BFU-E) were markedly affected. These results are consistent with the hypothesis that distinct signal transduction pathways, involving N-ras or not, are activated by different growth factors in different hematopoietic progenitor cells.

Tissue inhibitor of metalloproteinase-2 (TIMP-2) has erythroid-potentiating activity

Tissue inhibitor of metalloproteinase (TIMP) was purified and molecularly cloned on the basis of its erythroid-potentiating activity (EPA). TIMP/EPA appears to be a bifunctional molecule with both growth factor and anti-enzymatic activity. Recently, a second TIMP-related molecule was identified and we have investigated its possible erythroid-potentiating activity. Native, purified human TIMP-2 was assayed for erythroid-potentiating activity using an in vitro erythroid burst formation assay and was compared with that of previously characterized recombinant EPA/TIMP-1. The results demonstrate that both members of the tissue inhibitor of metalloproteinase family, TIMP-1 and TIMP-2, possessed erythroid potentiating activity which was inhibited by antibodies developed to neutralize EPA. These results suggest that TIMP-2 shares a common structural domain with EPA/TIMP-1 that is responsible for the erythroid-potentiating activity of these inhibitors. Therefore, TIMP-1 and TIMP-2, with both anti-protease activity and growth factor activity, join a family of bifunctional molecules such as fibroblast growth factor and thrombin which have both enzymatic and growth factor activity.

Platelet-derived growth factor and its role in atherogenesis: a brief review

A number of locally acting growth factors are now widely understood to be important in the regulation of cellular activity. Platelet-derived growth factor (PDGF) is one of the best known and is an important mitogenic stimulant of vascular tissues. PDGF is stored primarily by platelets but other cells, including endothelium and smooth muscle, can act as sources. The mitogenic properties of PDGF may be influential in the pathogenesis of atherosclerosis. PDGF has access to the vascular intima and may act upon smooth muscle cells to produce proliferation, a primary feature of atherosclerosis. In this brief review we examine the structure, function and influences of PDGF with special emphasis on its potential role in the pathogenesis of atherosclerosis.

Molecular analysis of primitive hematopoietic cell proliferation control mechanisms

Cells at two distinct early stages in the development of mature human blood cells from primitive totipotent hematopoietic stem cells can now be defined and quantitated by separate in vitro assays. Current evidence suggests that most, if not all, colony-forming cells--that is, cells that give rise to colonies of mature progeny within one to three weeks in semisolid culture systems, represent an intermediate stage of hematopoietic progenitor. These cells are not self-sustaining; if they are used to initiate hematopoiesis on competent marrow stromal layers, they rapidly disappear as they differentiate or die. However, clonogenic cells can be generated in such cultures from another cell type over a period of four to eight weeks. We have, therefore, assigned the term long-term culture initiating cell (LTC-IC) to this latter type of clonogenic precursor cell. The production and differentiation of cells in both of these compartments in LTC are dependent on, and regulated by, nonhematopoietic "stromal" cells that form a heterogeneous adherent layer in which close-range interactions with hematopoietic cells take place. The use of separate endpoints to monitor the maintenance, differentiation, and reversible activation or arrest of cycling of these cells has recently revealed different molecular mechanisms regulating their respective functions. However, an important common feature appears to be the relative local concentration of positive and negative regulators to which the target hematopoietic cell is exposed. Both gene expression and growth factor release measurements as well as results obtained using genetically engineered stroma and repeated soluble growth factor addition implicate G-CSF as an endogenous positive regulator of primitive hematopoietic cells. Similarly, gene expression, factor production, factor addition, and neutralizing antibody experiments implicate TGF-beta as an endogenous inhibitor of primitive hematopoietic cells.

Expression of a negative regulator of human erythropoiesis by fluidized lymphocyte plasma membranes

Regulation of hematopoiesis by paracrine molecules occurs in vitro. In some cases, hematopoietic paracrine factors have been localized to the plasma membrane of accessory cells. We have purified a unique integral membrane glycoprotein from normal human B cells that functions in vitro as a paracrine factor whose activity is directed toward erythroid progenitor cells. This factor is also spontaneously exfoliated from the cell surface as a component of extracellular vesicles. Analysis of the lipid and protein compositions and membrane lipid order of these extracellular vesicles reveals them to be biochemically distinct and more fluid than their parent membranes. Evidence in nonhematopoietic culture systems indicates that cell membrane function may be altered by modifying membrane fluidity. In an effort to accelerate growth factor release, plasma membranes of B cells were fluidized by incubation with an emulsion of Liposin II, phosphatidylcholine, and phosphatidylethanolamine. Fluidity assessed by steady-state fluorescence polarization was reduced in lipid-treated cells. Exfoliation was 3-4-fold higher from lipid-treated cells relative to untreated cells. Unexpectedly, a negative signal for burst-forming unit-erythroid (BFU-E) proliferation was expressed in membranes, in shed extracellular vesicles, and in supernatants of medium conditioned by the fluidized cells. Purification of the inhibitor is under way. The data are consistent with the hypothesis that accessory cell plasma membranes may positively or negatively regulate erythroid differentiation, depending upon the exchange of cholesterol and phospholipids between plasma membrane and ambient lipid pools.

Culture perfusion schedules influence the metabolic activity and granulocyte-macrophage colony-stimulating factor production rates of human bone marrow stromal cells

The metabolic function and GM-CSF production rates of adherent human bone marrow stromal cells were investigated as functions of medium and serum feeding rates. A range of medium exchange schedules was studied, ranging from a typical Dexter culture protocol of one weekly medium exchange to a full media exchange daily, which more closely approximates what bone marrow cells experience in situ. Glucose consumption was found to be significantly higher at full daily exchange rate than at any other exchange schedule examined. However, the lactate yield on glucose was a constant, at 1.8 mol/mol, under all conditions considered. Differential serum vs. medium exchange experiment showed that both serum supply and medium nutrients were responsible for the altered behavior at high exchange rates. Glutamine consumption was found to be insignificant under all culture conditions examined. A change in exchange schedule from 50% daily medium exchange to full daily medium exchange after 14 days of culture was found to result in a transient production of GM-CSF and a change in metabolic behavior to resemble that of cultures which had full daily exchange from day one. These results suggest that both stromal cell metabolism and GM-CSF production are sensitive to medium exchange schedules. Taken together, the data presented indicate that attempts to model the function of human bone marrow in vitro may be well served by beginning with medium exchange schedules that more closely mimic the in vivo physiologic state of bone marrow.

Intracellular regulation of the production and release of human erythroid-directed lymphokines

Erythroid burst-promoting activity (BPA) is released from B lymphocytes in soluble (sBPA) and membrane-bound (mBPA) forms. To study intracellular processes involved in production of these physically separable factors, we measured their time course release into serum-free medium from B cells that were pulse-exposed for 5-240 min to nonmitogenic base medium or inhibitors of energy-dependent metabolism (2,4-dinitrophenol, sodium azide, and 2-deoxy-D-glucose), transcription and translation (actinomycin D and cycloheximide), replicative DNA synthesis (cytosine arabinoside), or posttranslational processing (monensin). mBPA and sBPA were initially detectable after 1 and 2 h, respectively. Maximum cumulative levels of 8 +/- 0.6 and 9 +/- 1.0 U/ml, respectively, were reached after 8 h. In contrast, cumulative mBPA and sBPA levels in medium prepared from cells treated with metabolic inhibitors were reduced by up to 90%. Both surface exfoliation and mBPA expression by intact plasma membranes were diminished. Whereas pulse-exposure to cytosine arabinoside had no effect, treatment with actinomycin D or cycloheximide abolished BPA expression. Exposure to monensin reduced mBPA and sBPA levels to zero in a concentration-and time-dependent fashion. We conclude that production and release of BPA is an energy-dependent process, requiring mRNA synthesis and translation and posttranslational remodeling of the protein but not replicative DNA synthesis.

The role of colony-stimulating factors in acute leukemia

This article summarises the effects of colony-stimulating factors and related molecules on leukemia blasts by focussing on autocrine and paracrine growth control. This information may lead to a better understanding of the pathobiology of this highly malignant disorder, and may have therapeutic implications.

Platelet-derived growth factor and its role in health and disease

Platelet-derived growth factor (PDGF) was first discovered in platelets because they are the principal source of mitogenic activity in whole blood serum for mesenchymal cells in culture. PDGF is ubiquitous in that it can be formed by a large number of normal cells as well as many varieties of transformed cells. However, its expression and biological activity appear to be controlled at a number of different levels. The molecule consists of two peptide chains (termed 'A' and 'B') and is found as one of at least three possible isoforms, (AB, AA or BB). Each of these isoforms binds to a high-affinity cell-surface receptor that is composed of two different subunits, each of which has specificity for one or the other of the peptide chains of PDGF. The two receptor subunits are present in differing amounts on different cell types, and therefore the capacity of the different isoforms of PDGF to induce mitogenesis depends on the specific PDGF isoform and the relative numbers of receptor subunits present on the responding cell. In addition to inducing cell replication, PDGF elicits a number of intracellular signals related to mitogenesis, is chemotactic, is a vasoconstrictor, activates leukocytes, and modulates extracellular matrix turnover. This growth factor is probably involved in a number of biologically important events including wound repair, embryogenesis and development, and inflammation, leading to fibrosis, atherosclerosis and neoplasia.

Insulin-like growth factors promote DNA synthesis and support cell viability in fetal hemopoietic tissue by paracrine mechanisms

There is significant evidence that the insulin-like growth factors (IGF) play a role in both murine and human hemopoiesis. In order to better define the nature and mechanisms of these effects, we have used a serum-free system to examine DNA synthesis and cell replication in murine hemopoietic cells. Cell preparations from 13-day fetal mice livers were incubated in serum-free DMEM alone or with erythropoietin (Epo) 0.5 U/ml, recombinant human IGF-I, purified IGF-II, or recombinant human growth hormone (GH) in various doses, and [3H]thymidine added for the last 3 hr of 21-hr incubation. Cell distribution was over 80% erythroid or erythroblasts. IGF-I and IGF-II promoted thymidine incorporation into cells at a half-maximal dose of 3 and 1 nM respectively, IGF-II with a maximum potency 65% of IGF-I; insulin stimulated at a half-maximum dose of 100 nM, with similar maximum effect to IGF-I, and their effects were not additive. GH was stimulatory at 1 microM. Epo was 2-9 times as effective as IGF-I and their effects were not additive. A monoclonal antibody to IGF-I reduced the effect of IGF-I by 50-80%, had no effect on Epo, and abolished the GH effect. Separation of erythroid cells and precursors from accessory and other liver cells did not alter the response to IGF-I. Cell counts increased in response to IGF-I or Epo, and cell viability was maintained by IGF-I compared to control medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of biologically active, platelet-derived growth factor-like molecules produced by murine erythroid cells in vitro and in vivo

Platelet-derived growth factor (PDGF) is an important serum regulator of erythropoiesis in vitro. We have now obtained evidence suggesting that PDGF-like molecules may also modulate erythropoiesis in vivo. Western blot analysis of cytoplasmic extracts from Rauscher murine erythroleukemia cells and phenylhydrazine-treated mouse splenic erythroid cells revealed the presence of several PDGF-like proteins. The presence of PDGF-like proteins in the cytoplasm of these two erythroid cell types was confirmed by immunohistochemical staining. Using a serum-free biologic assay, PDGF-like biological activity was found in cell lysates and conditioned medium of both Rauscher cells and phenylhydrazine-treated mouse erythroid cells. Subcellular localization experiments revealed the biological activity to be concentrated in the cytosolic fraction. Using a series of antibodies to hematopoietic growth factors we demonstrated that PDGF-like biological activity was specifically immunoprecipitated by both monoclonal and polyclonal anti-human PDGF antibodies but not by antibodies to burst-promoting activity, granulocyte-macrophage colony-stimulating factor, IL-3, or erythropoietin. Taken together, the data are consistent with the hypothesis that PDGF-like molecules play a role in the regulation of mammalian erythropoiesis in vivo.

Normal mouse serum contains two erythroid stimulating activities differing by their mode of action

We have previously shown that normal mouse serum contains two activities that increase the number of erythroid colonies obtained from murine bone marrow colony-forming units-erythroid (CFU-E) cultured in the presence of saturating Epo concentrations (0.2 U/ml). These activities could be separated by gel filtration and were referred to as peak I and peak II. We report here that peak I increases the number of erythroid colonies in cultures of adult bone marrow cells depleted of cells expressing Mac-1 antigen. On the contrary, peak II has no stimulating effect in these cultures unless Mac-1-positive cells are added to the culture. This synergistic effect is mediated by an activity released in the culture medium by Mac-1-positive cells.

Human vascular endothelial cells, granulopoiesis, and the inflammatory response

We have carried out a series of in vitro studies designed to characterize the role of mononuclear phagocytes as regulators of hematopoiesis. The results of these studies have demonstrated that mononuclear phagocytes produce factors, including interleukin-1 (IL-1), that induce the expression of multilineage hematopoietic growth factors by human vascular endothelial cells. In more recent studies we and others have identified these induced factors as G-CSF, GM-CSF, IL-6, and IL-1. Interleukin 1 stimulates expression of these genes by inducing the accumulation of gene transcripts. Moreover, transcript accumulation, at least with GM-CSF, results from prolongation of mRNA half-life. Based on preliminary studies in a cell-free system, we propose that the inductive capacity of IL-1 results from its activation of ribonuclease inhibitors in the cytoplasm of IL-1-induced cells and hypothesize that this may be a general mechanism by which IL-1 induces gene expression.

Interaction of interferon with other cytokines

Interferons interact with other cytokines to exert their antiviral, cell growth regulatory and immunomodulatory activities. Growth factors, tumor necrosis factors, colony stimulating factors, interleukins and interferons have pleiotropic effects and form a parallel network of intercellular signals. These signals are transduced at the cell surface through specific receptors with intrinsic enzymatic activity or with the capacity to regulate intracellular enzymes through interactive effects with G-proteins. This leads to regulated gene transcription of intracellular and secreted, functional and structural proteins. Although much is known about the interaction of cytokines with their receptors and about the regulation of transcription at the genomic level the various steps linking these two phenomena deserve further research.

Effects of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) on single CD34-positive hemopoietic progenitors from human bone marrow

To determine the extent accessory cells mediate the effects of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) on human hemopoietic progenitors in vitro, we added this hemopoietin to liquid cultures of single CD34-positive marrow cells. These were selected on a fluorescence-activated cell sorter using the HPCA-1 (My10) antibody. Myeloid, erythroid and a few mixed clones developed in 13% of wells in the apparent absence of accessory cells at the beginning of culture. Although accessory cells were generated quickly from the myeloid progenitors and could have mediated the action of rhGM-CSF, this was not the case in the majority of the erythroid clones in which no other cell types were recorded. We conclude that rhGM-CSF can act directly on a subset of erythroid progenitors and probably induces a substantial number of myeloid clones directly.

Vascular endothelial cells and hematopoietic regulation

Vascular endothelial cells elaborate growth factors which support the proliferation and differentiation of hematopoietic progenitor cells. Both the regulation and potential biological significance of growth factor production by endothelial cells are reviewed.

3'-Azido-3'-deoxythymidine (AZT) inhibits proliferation in vitro of human haematopoietic progenitor cells

Peripheral blood cytopenias are a serious, dose-limiting toxicity of AZT therapy in patients infected by HIV. To evaluate the mechanism by which cytopenias develop, AZT effects of haematopoietic differentiation and growth were measured in serum-free, nucleoside-depleted cultures of normal human bone marrow. In contrast to native thymidine, AZT suppressed the proliferation of erythroid, granulocyte/macrophage and primitive haematopoietic stem cells in a dose-related and time-dependent fashion. Relative progenitor sensitivity varied, with half-maximal concentrations of 1-5 microM and 20-40 microM AZT for inhibition of erythroid and nonerythroid progenitor cell growth, respectively. Inhibition was observed over full ranges of concentrations of haematopoietic tissue-specific regulators (human erythropoietin, colony-stimulating activity, interleukin-3 and lymphocyte conditioned medium) and of platelet-derived growth factor (PDGF), an agent that enhances erythropoiesis in vitro via accessory marrow stromal elements. Furthermore, suppression was similar in cultures of marrow cells that were depleted of accessory populations, suggesting that its action is directed at progenitors. Finally, when deoxythymidine was added in increasing amounts to cultures with a half-maximal concentration of AZT, inhibition was abrogated. We conclude that AZT is a potent inhibitor of haematopoiesis in vitro, and that erythroid progenitors are particularly sensitive to its action. These results may explain the marrow hypoplasia that occurs during AZT administration in vivo.

Soluble factor(s) released by the PF-382 T-cell line enhances the stimulatory effect of monocytes on the BFU-E growth

PF-382 is a human T-cell line that has been shown to elaborate factors that modulate normal hemopoiesis in vitro. In the present study we report that this cell line constitutively releases in both serum-containing and serum-free supernatants a potent enhancer of BFU-E growth. The factor(s), partially purified by gel filtration, is a heat-stable molecule(s) degradable by trypsin and 2-mercaptoethanol treatments, equally active on bone marrow and peripheral blood erythroid progenitor cells, but not on CFU-GM. Unlike other sources of BPA, this stimulatory factor(s) exerts its effect in the presence of mononuclear adherent cells. In fact, the addition of conditioned medium obtained by 48 hr preincubation of isolated monocytes with 10% PF-382 supernatant (M-CM2) or the concomitant addition of supernatant from PF-382 cells (PF-382-CM) and from unstimulated monocytes (M-CM1) are capable of fully replacing the presence of monocytes in the BFU-E assay. Since the independent addition of PF-382-CM or of M-CM1 is devoid of stimulatory function, we suggest that the PF-382 derived BFU-E growth inducer, which differs from IL-1, IL-3, IL-4, GM and G-CSF, exerts its activity "via" a synergistic mechanism with a monokine.

Erythroid colony formation by human CFU-E is stimulated by compatible human serum but impaired by blood group antibodies

Normal human serum was shown to stimulate the proliferation of human CFU-E when added into in vitro cultures of bone marrow cells. However, this was observed only when donors of serum and bone marrow cells were compatible on the basis of their blood groups. Absorption of incompatible sera removed the inhibition. Monoclonal anti-blood group antibodies exhibited the same pattern of CFU-E inhibition. The inhibition was complement-independent. Using suspension bone marrow cultures, it was shown that this blockage took place at an earlier differentiation step than the CFU-E level. This effect should be taken into account in studies of the inhibiting activities present in human anaemic sera.

Acetylated lipoproteins impair erythroid growth factor release from endothelial cells

Endothelial cells are a known source of hematopoietic growth-enhancing factors, including platelet-derived growth factor (PDGF). In addition, endothelium interacts directly with plasma lipoproteins which have been shown to modulate hematopoiesis. To determine the relationship of these properties, we measured the release of an erythroid growth-enhancing factor from bovine endothelial cells under lipid-loaded and control conditions. Human bone marrow cells cultured under serum-free conditions form more erythroid, granulocyte/macrophage, and mixed hematopoietic colonies when supplemented with endothelial cell-conditioned medium (ECCM) than do controls (P less than 0.05). The activity is expressed over a wide range of erythropoietin, lymphocyte-conditioned medium (LCM), recombinant human interleukin-3, and colony-stimulating factor (CSF) concentrations, and is related to ECCM dose. In contrast, enhancing activity in ECCM prepared with 0-400 micrograms/ml acetylated low density lipoproteins (AcLDL) or native LDL is diminished to 0% in a dose-dependent fashion (relative to ECCM from unexposed cells or from cells incubated with very low density lipoproteins, P less than 0.05). Upon dilution, medium prepared from cells incubated with LDL shows a rightward shift in the dose-response curve for erythroid colony formation, while that prepared from AcLDL loaded cells demonstrates a downward shift, indicating that the inhibitory activities are kinetically distinct. Delipidation of ECCM prior to addition to marrow culture removes the inhibitory action of native LDL (P less than 0.05) but not that of AcLDL (P greater than 0.10). Immunochemical analysis suggests that the erythropoietic activity in ECCM is unrelated to that of PDGF, recombinant human CSF, and erythroid burst-promoting activity (BPA) present in LCM. This conclusion is supported by Northern blot analysis of endothelial cells using a cDNA probe for the v-sis homologue of the PDGF beta chain and by immunoprecipitation of metabolically labeled PDGF. The relative amounts of c-sis transcripts and of secreted PDGF were similar in endothelial cells incubated with or without AcLDL. We conclude that AcLDL impair the synthesis or release of an erythropoietic growth-enhancing factor(s) which is biologically distinct from PDGF and BPA present in LCM.

Characterization of the human erythroid progenitor cells responsive to the erythropoietic-enhancing activity found in normal human serum

Normal human serum significantly increased the growth of erythroid colonies from bone marrow colony-forming units-erythroid (CFU-e) which were enriched by using a set of monoclonal antibodies in a panning technique. This activity was still observed in cultures of enriched CFU-e plated near the limiting cell dilution. When the addition of erythropoietin was delayed so that only early CFU-e could survive, we observed that the growth of erythroid colonies was less affected in cultures containing erythropoietin and normal serum than in those containing erythropoietin only. We have concluded from this study that normal human serum acts on in vitro erythropoiesis by directly stimulating the growth of a population of early CFU-e.

Effect of platelet-derived growth factor on enriched populations of haemopoietic progenitors from patients with chronic myeloid leukaemia

The effect of pure platelet-derived growth factor and fresh serum on the in-vitro growth of purified haemopoietic progenitors from the peripheral blood of 12 patients with CML was studied. Purified haemopoietic progenitors were prepared using Percoll separation followed by cell sorting with the monoclonal antibody BI.3C5. Both pure PDGF at a concentration of 20 ng/ml and fresh serum significantly increased the numbers of BFU-E (p less than 0.01) and CFU-GEMM (p less than 0.014), but not the CFU-GM. That the PDGF effect was not mediated to any significant extent via prostaglandins, was shown by the lack of inhibitory effect of indomethacin on the growth of purified progenitor cells in the presence of fresh serum. Increased amounts of pure PDGF were required to give maximal stimulation of purified CML peripheral blood progenitors compared to normal bone marrow progenitors. These results show that CML progenitors are capable of responding to PDGF. Whether the quantitative difference in response is due to a reduced proportion of mesenchymal cells in CML peripheral blood compared to normal marrow, or whether CML progenitors are most likely already stimulated by autocrime PDGF or other growth factors remains to be elucidated.

Platelet-derived growth factor stimulates growth of highly enriched multipotent haemopoietic progenitors

Platelet-derived growth factor (PDGF) has been shown to stimulate growth of normal and malignant fibroblasts, glial cells and smooth muscle cells. A growth promoting effect on human haemopoietic precursors has also been described, but the interpretation of this haemopoietic proliferative response to PDGF has been hampered by the lack of purity of the target population. In this study we show that PDGF promotes growth of early bone marrow haemopoietic progenitors depleted of either monocytes or T lymphocytes which are known to influence haemopoiesis. Moreover, the action of PDGF is even increased on a highly enriched BI-3C5 early bone marrow population. BI-3C5 is a novel monoclonal antibody which recognizes an antigen present on all multilineage colony-forming cells (CFU-mix) (Tindle et al. 1985). BI-3C5 positively and negatively sorted fractions were obtained by fluorescence activated cell sorting (FACS) and PDGF was found to stimulate growth of CFU-mix in the BI-3C5-positive fraction (consisting of only 4-6% of the marrow population), the effect being more marked than that on unsorted bone marrow. The results suggest that the product of the cellular proto-oncogene c-sis (the putative structural gene for the beta chain of PDGF) may play a regulatory role in the in vivo proliferation of multipotent haemopoietic progenitors.