Novel role of interleukin 7 in myelopoiesis: stimulation of primitive murine hematopoietic progenitor cells.

Interleukin 7 (IL-7) has been demonstrated to be an important regulator of the growth of B and T cell precursors as well as mature T cells, whereas IL-7 has been reported to have no direct myeloproliferative effects. Here we show that IL-7 potently and directly enhances colony stimulating factor-induced myeloid colony formation from Lin-Sca-1+ murine bone marrow progenitor cells, increasing the cloning frequency up to ninefold and cell numbers up to 50-fold, without affecting their ability to differentiate along the myeloid lineages In contrast, IL-7 has no effect on proliferation of committed Lin- myeloid progenitors. Thus, in addition to its established lymphopoietic potential, this study implicates a novel role of IL-7 in early myelopoiesis.

Tumor necrosis factor-alpha inhibits stem cell factor-induced proliferation of human bone marrow progenitor cells in vitro. Role of p55 and p75 tumor necrosis factor receptors.

Stem cell factor (SCF), a key regulator of hematopoiesis, potently synergizes with a number of hematopoietic growth factors. However, little is known about growth factors capable of inhibiting the actions of SCF. TNF-alpha has been shown to act as a bidirectional regulator of myeloid cell proliferation and differentiation. This study was designed to examine interactions between TNF-alpha and SCF. Here, we demonstrate that TNF-alpha potently and directly inhibits SCF-stimulated proliferation of CD34+ hematopoietic progenitor cells. Furthermore, TNF-alpha blocked all colony formation stimulated by SCF in combination with granulocyte colony-stimulating factor (CSF) or CSF-1. The synergistic effect of SCF observed in combination with GM-CSF or IL-3 was also inhibited by TNF-alpha, resulting in colony numbers similar to those obtained in the absence of SCF. These effects of TNF-alpha were mediated through the p55 TNF receptor, whereas little or no inhibition was signaled through the p75 TNF receptor. Finally, TNF-alpha downregulated c-kit cell-surface expression on CD34+ bone marrow cells, and this was predominantly a p55 TNF receptor-mediated event as well.

A novel mammalian protein, p55CDC, present in dividing cells is associated with protein kinase activity and has homology to the Saccharomyces cerevisiae cell division cycle proteins Cdc20 and Cdc4.

A novel protein, p55CDC, has been identified in cycling mammalian cells. This transcript is readily detectable in all exponentially growing cell lines but disappears when cells are chemically induced to fall out of the cell cycle and differentiate. The p55CDC protein appears to be essential for cell division, since transfection of antisense p55CDC cDNA into CHO cells resulted in isolation of only those cells which exhibited a compensatory increase in p55CDC transcripts in the sense orientation. Immunoprecipitation of p55CDC yielded protein complexes with kinase activity which fluctuated during the cell cycle. Since p55CDC does not have the conserved protein kinase domains, this activity must be due to one or more of the associated proteins in the immune complex. The highest levels of protein kinase activity were seen with alpha-casein and myelin basic protein as substrates and demonstrated a pattern of activity distinct from that described for the known cyclin-dependent cell division kinases. The p55CDC protein was also phosphorylated in dividing cells. The amino acid sequence of p55CDC contains seven repeats homologous to the beta subunit of G proteins, and the highest degree of homology in these repeats was found with the Saccharomyces cerevisiae Cdc20 and Cdc4 proteins, which have been proposed to be involved in the formation of a functional bipolar mitotic spindle in yeast cells. The G beta repeat has been postulated to mediate protein-protein interactions and, in p55CDC, may modulate its association with a unique cell cycle protein kinase. These findings suggest that p55CDC is a component of the mammalian cell cycle mechanism.

The alpha-subunit mRNAs for Gs and Go2 are differentially regulated by protein kinase A and protein kinase C in rat Sertoli cells.

In the present study, we have examined regulatory effects of protein kinase A and protein kinase C activation by 8-CPTcAMP and TPA, respectively, on mRNAs for various G protein alpha-subunits and corresponding immunoreactive proteins in rat Sertoli cells. Gs alpha and Go alpha mRNA levels were transiently increased 1.5-fold and 4-fold, respectively, by 8-CPTcAMP in cultured Sertoli cells. This up-regulation of mRNAs for Gs alpha and Go alpha was also observed when Sertoli cells were incubated in the presence of FSH. When protein synthesis was inhibited by cycloheximide, the cAMP-mediated stimulation of Gs alpha mRNA was abolished, whereas Go alpha mRNA was superinduced to a 50- to 100-fold higher level than basal. Activation of protein kinase C with TPA had a strong, synergistic effect on cAMP-mediated stimulation of Gs alpha mRNA, whereas the cAMP-mediated stimulation of Go alpha mRNA was completely blocked. Surprisingly, changes in mRNA levels were not accompanied by any alterations in the levels of immunoreactive Gs alpha and Go alpha proteins.

Platelet factor 4 mRNA expression in human erythroleukemic cells: regulation by phorbol esters and certain cytokines.

Developing megakaryocytes are distinguished from progenitor cells by the appearance of platelet proteins such as platelet factor 4 (PF 4). The human erythroleukemic cell line HEL can also be induced to produce PF 4 by incubation in phorbol esters. HEL cells were used here as a model system in which to study the phenomenon of inducible PF 4 production at both the mRNA and protein levels. The cytokines interleukin 1 beta (IL-1 beta), interleukin 3 (IL-3), interleukin 6 (IL-6), granulocyte-macrophage colony-stimulating factor (GM-CSF), erythropoietin (EPO), and transforming growth factor-beta (TGF-beta) were also evaluated for their effects on PF 4 mRNA induction in HEL cells.

Direct synergistic effects of IL-4 and IL-11 on proliferation of primitive hematopoietic progenitor cells.

The present studies have investigated, for the first time, the synergistic effects of interleukin-4 (IL-4) and IL-11 on the growth of single murine bone marrow progenitor cells. These studies suggest that IL-4 and IL-11 are synergistic hematopoietic growth factors, enhancing colony formation of bone marrow progenitors from normal mice in the presence of colony-stimulating factors or stem cell factor, whereas neither IL-4 nor IL-11, alone or in combination, resulted in colony formation. However, in the presence of a neutralizing anti-TGF-beta antibody, IL-11 plus IL-4 induced clonal growth of primitive Lin-Sca1+ progenitors. Furthermore, here we report several observations extending the knowledge about IL-4 and IL-11 as synergistic factors. In addition to the established ability of IL-11 to enhance IL-3- and GM-CSF-induced colony formation, IL-11 also enhanced the number of G-CSF- and CSF-1-stimulated colonies of mature (Lin-) and primitive (Lin-Sca-1+) hematopoietic progenitors cultured at the single-cell level. In contrast, IL-4 bifunctionally regulated the growth of Lin- progenitors, whereas the growth of single Lin-Sca=1+ progenitors was unaffected or enhanced in the presence of IL-4. Finally, IL-4 and IL-11, in combination, potently synergized to enhance the high-proliferative-potential colony-forming cell colony formation of Lin-Sca-1+ progenitors in response to all four CSFs and to SCF.

Canine stem cell factor (c-kit ligand) supports the survival of hematopoietic progenitors in long-term canine marrow culture.

The cDNA for canine stem cell factor (cSCF, c-kit ligand) was cloned and expressed in Escherichia coli. The recombinant protein (rcSCF), 165 amino acids in length, is very similar structurally to the soluble form of previously cloned and sequenced rodent and human SCFs. The biological effects of rcSCF were studied in a day-10 granulocyte-macrophage colony-forming unit (CFU-GM) clonogenic assay and in long-term liquid bone marrow culture of non-adherent hematopoietic cells in the absence of a stromal underlayer. Synergism in the stimulation of growth of CFU-GM was demonstrated between rcSCF and both recombinant human (rh) granulocyte-macrophage colony-stimulating factor (GM-CSF) and naturally occurring colony-stimulating activity present in the serum of a neutropenic dog. Alone, rcSCF was nonstimulatory for committed marrow precursors in methylcellulose cultures and had minimal effect on hematopoietic progenitor cell survival in stromaless, liquid cultures. When rcSCF was combined with phytohemagglutinin-stimulated canine lymphocyte-conditioned medium (PHA-LCM) or rh interleukin 6 (IL-6), with or without rhGM-CSF, CFU-GM survived for up to 5 weeks. The combination of rcSCF and rhGM-CSF, without rhIL-6, led to an early increase in CFU-GM in liquid cultures that declined more rapidly than in flasks that included rhIL-6. Survival of progenitor cells was negligible beyond 1 week in flasks with growth factor combinations lacking rcSCF. Sustained production of nonadherent cells in long-term cultures also was dependent on rcSCF in combination with canine PHA-LCM or recombinant human growth factors. It appears that rcSCF, like that from rodent and primate species, has the ability to influence the survival and proliferation of CFU-GM, and perhaps earlier progenitor cells, in hematopoietic tissues. In a long-term liquid culture system in which growth factor production by stromal cells is limited, rcSCF possesses a unique ability to maintain the viability of progenitor cells for up to 5 weeks.

Long-term inhibitory effect of cAMP on beta-adrenoceptor acquisition and nonselective attenuation of adenylyl cyclase in hepatocytes.

Long-term effects of cAMP on the surface expression of beta-adrenoceptors and adenylyl cyclase activity were investigated in primary cultures of rat hepatocytes. beta-Adrenoceptor density and catecholamine-responsive adenylyl cyclase activity increased during culturing in a biphasic manner, with a plateau of 10-20 h duration occurring approximately 10 h after plating. Treatment of hepatocyte cultures with 8-bromo-cAMP during the plateau period did not affect the density of beta-adrenoceptors. In contrast, addition of 8-bromo-cAMP, 8-chlorophenylthio-cAMP, forskolin or glucagon during a period of active recruitment of surface beta-adrenoceptors resulted in a suppression of the acquisition of beta-adrenoceptors. In both experimental situations there was a partial decrease in hormone-stimulated and basal adenylyl cyclase activity. The results suggest that cAMP exerts at least two types of long-term regulation of adenylyl cyclase in hepatocytes: a suppressive effect on beta-adrenoceptor acquisition, and a partial, nonselective decrease in adenylyl cyclase activity not involving beta-adrenoceptor down-regulation.

IL-7 stimulates CSF-induced proliferation of murine bone marrow macrophages and Mac-1+ myeloid progenitors in vitro.

The role of IL-7 as an important stimulator of the growth of B and T cell precursors, as well as mature T cells, is well established. In contrast, the role of IL-7 in myelopoiesis has not been characterized thoroughly, and thus, IL-7 has been regarded as a lymphoid lineage-restricted cytokine. However, we have recently reported that IL-7 enhanced CSF-induced myeloid proliferation of primitive murine hematopoietic (Lin-Sca-1+) progenitors, whereas IL-7 did not affect significantly the proliferation of a population of more mature (Lin-) progenitors. The present study was initiated to investigate further whether IL-7 might affect CSF-induced proliferation of subpopulations of committed myeloid progenitors as well as mature bone marrow macrophages. IL-7 enhanced macrophage colony-stimulating factor (CSF-1)-induced colony formation of single bone marrow macrophages 90%, whereas IL-7 alone had no effect. Furthermore, IL-7, in a concentration-dependent manner, increased the proliferation of mononuclear cells expressing the Mac-1 Ag (Mac-1+ mononuclear cells (MNC); CD11b) up to fivefold in response to CSF-1, granulocyte macrophage-CSF (GM-CSF), or IL-3. In contrast, no effect of IL-7 was observed on Mac-1- MNC. The synergistic effect of IL-7 on Mac-1+ MNC was caused by an increase in macrophage colonies (CFU-M) and mixed granulocyte-macrophage colonies (CFU-GM), whereas the total number of granulocyte colonies (CFU-G) was not affected. This suggests that IL-7 can provide proliferative signals to Mac-1+ progenitors with a macrophage potential, but not to progenitors committed to pure granulocyte differentiation.

Modes of determining beta-adrenoceptor number in human mononuclear leucocytes.

Inhibition of total 125I-ICYP binding to intact human mononuclear leucocytes at 32 degrees by propranolol and (+/-) CGP-12177 was biphasic. The high affinity component of 125I-ICYP binding, representing approximately 30% of total, was stereospecific, while the low affinity binding site was inhibited without stereospecificity. (-) Isoproterenol inhibited the high affinity component of 125I-ICYP binding only, with low affinity. By performing binding studies in intact cells at 4 degrees or in broken cell preparations at 37 degrees, the fraction of total 125I-ICYP binding representing specific binding was increased, and agonist affinity was high. Inhibition of 3H-CGP-12177 binding to intact cells at 32 degrees demonstrated a high fraction of specific binding and high agonist affinity. Computer-assisted analysis of total radioligand binding determined over a broad concentration range revealed two populations of saturable 125I-ICYP binding sites in intact cells as well as in broken cell preparations, while 3H-CGP-12177 binding demonstrated only one saturable binding site. The number of high affinity 125I-ICYP binding sites was comparable to the number of saturable 3H-CGP-12177 binding sites. Receptor numbers determined by analysis of total radioligand binding were comparable to receptor numbers determined by subtraction of non-specific binding, determined in the presence of a high concentration of competing ligand. Analysis of total radioligand binding was found to be a better procedure because it eliminates the use of an arbitrary concentration of unlabelled ligand and improves the accuracy of the assay.

Transforming growth factor-beta potently inhibits the viability-promoting activity of stem cell factor and other cytokines and induces apoptosis of primitive murine hematopoietic progenitor cells.

In contrast with the extensively characterized effects of transforming growth factor-beta (TGF-beta) on proliferation and differentiation of hematopoietic progenitors, little is known about the effects of TGF-beta on viability of normal hematopoietic progenitors. In the present report, we demonstrate that TGF-beta potently counteracts hematopoietic growth factor (HGF)-induced survival of individually cultured primitive Lin-Sca-1+ bone marrow progenitors. Specifically, 74% of single Lin-Sca-1+ cells cultured for 40 hours in the presence of stem cell factor (SCF) survived, whereas only 16% survived in the presence of SCF plus TGF-beta. Similarly, the enhanced survival of primitive hematopoietic progenitors in response to granulocyte colony-stimulating factor (G-CSF), interleukin (IL)-1, IL-6, or IL-11 was also potently opposed by TGF-beta. Furthermore, it is demonstrated that neutralization of endogenous TGF-beta present in the cultures enhances survival of Lin-Sca-1+ progenitors in the absence, as well as in the presence, of HGFs such as SCF and IL-6. The reduced HGF-induced survival of primitive hematopoietic progenitors in the presence of TGF-beta was associated with increased apoptosis, as detected by an in situ terminal deoxynucleotidyl transferase (TdT) assay. After 16 hours of incubation in the absence of HGFs, 61% +/- 6% of the hematopoietic progenitors had DNA strand breaks characteristic of apoptosis. The presence of SCF reduced the frequency of apoptic cells to 27% +/- 5%, whereas 55% +/- 3% of the cells had signs of apoptosis in the presence of SCF plus TGF-beta.

8-bromo-cAMP and 8-CPT-cAMP increase the density of beta-adrenoceptors in hepatocytes by a mechanism not mimicking the effect of cAMP.

Addition of 8-bromo-adenosine 3',5'-cyclic monophosphate (8-bromo-cAMP) or 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (8-CPT-cAMP) to hepatocytes at the time of plating enhanced the acquisition of beta-adrenoceptors that occurs spontaneously upon culturing as primary monolayers. This effect was partially suppressed by the phosphodiesterase inhibitor isobutyl methylxanthine, and was mimicked by 8-bromo-AMP, 8-bromo-adenosine, and the adenosine kinase inhibitor 5'-amino-5'-deoxyadenosine. Agents that elevated the intracellular level of cAMP, such as glucagon and forskolin, and Sp-8-bromo-adenosine 3',5'-monophosphorothioate (Sp-8-bromo-cAMPS), a cAMP analogue that is resistant towards metabolic breakdown, did not significantly enhance beta-adrenoceptor expression when used alone, but glucagon enhanced the effect of 8-bromo-adenosine. 8-bromo-cAMP and 8-bromo-adenosine decreased cellular ATP-levels. These observations suggest that the enhanced beta-adrenoceptor acquisition was mediated mainly through the action of metabolites of 8-bromo-cAMP and 8-CPT-cAMP, although there may be a cAMP-mediated component in the effect. Several mechanisms, including depletion of ATP, are probably involved, and might affect beta-adrenoceptor degradation.

Direct synergistic effects of interleukin-7 on in vitro myelopoiesis of human CD34+ bone marrow progenitors.

Interleukin-7 (IL-7) is an important growth factor in B and T lymphopoiesis in mouse and human, whereas IL-7 has been regarded to lack proliferative effects on cells within the myeloid lineage. However, we have recently reported that IL-7 potently can enhance colony stimulating factor (CSF)-induced myelopoiesis from primitive murine hematopoietic progenitors, showing a novel role of IL-7 in early murine myelopoiesis. Using CD34+ human hematopoietic progenitor cells, we show here a similar role of IL-7 in human myelopoiesis, although interesting differences between the two species were found as well. Although purified recombinant human (rh)IL-7 alone did not induce any proliferation of CD34+ cells, IL-7 in a concentration-dependent manner enhanced the colony formation induced by all four CSFs up to threefold. Furthermore, stem cell factor (SCF)-induced granulocyte-macrophage (GM) colony formation was increased fourfold in the presence of IL-7. Single-cell cloning assays showed that these synergistic effects of IL-7 were directly mediated on the targeted progenitors, and that IL-7 increased the number, as well as the size of the colonies formed. Morphological examination showed that IL-7 affected the progeny developed from CD34+ cells stimulated by G-CSF or IL-3, increasing the number of CFU-M (colony forming unit-macrophage) and CFU-granulocyte-macrophage, whereas the number of CFU-granulocyte were unaltered.

TNF-alpha bidirectionally modulates the viability of primitive murine hematopoietic progenitor cells in vitro.

It is well established that TNF-alpha can induce apoptosis in many normal and transformed cell types. The effects of TNF-alpha on cytokine-induced proliferation and differentiation of normal hematopoietic progenitors have been characterized extensively, whereas little is known about how TNF-alpha can affect their viability. The present studies suggest, based on experiments using delayed addition of growth-promoting cytokines as well direct viability assays, that TNF-alpha bidirectionally affects the survival of individually cultured primitive Lin- Sca-1+ hematopoietic progenitors, in that stem cell factor (SCF)-, granulocyte-CSF-, IL-6-, and IL-11-induced survival is potently counteracted by TNF-alpha (42-86%), whereas TNF-alpha synergistically enhances IL-1alpha-induced survival up to threefold. The bidirectional effects of TNF-alpha on hematopoietic growth factor-induced survival of hematopoietic progenitors were reflected in that TNF-alpha enhanced apoptosis of Lin- Sca-1+ cells when combined with SCF, whereas TNF-alpha synergistically suppressed apoptosis in response to IL-1alpha.

The flt3 ligand promotes the survival of primitive hemopoietic progenitor cells with myeloid as well as B lymphoid potential. Suppression of apoptosis and counteraction by TNF-alpha and TGF-beta.

The recently cloned fIt3 ligand (FL) potently enhances hemopoietic growth factor-induced growth of primitive hemopoietic progenitors. In agreement with previous reports, we found FL alone to be a weak mitogen for primitive Lin-Sca-1+ murine bone marrow progenitors. Using delayed addition of growth-promoting cytokines, we demonstrate that FL potently promotes the in vitro survival of Lin-Sca-1+ progenitors responsive to a potent myeloid growth factor combination (FL, stem cell factor (SCF), granulocyte CSF (G-CSF), and IL-1 alpha). Whereas no such progenitors survived in cultures supplemented with medium alone, 33% survived in FL compared with 75 and 13% in the presence of SCF and IL-1 alpha, respectively. These results were obtained when cells were plated individually, suggesting that the viability-promoting effect of FL is mediated directly on the progenitors. Whereas SCF was superior to FL in promoting the survival of FL-, SCF-, G-CSF-, and IL-1 alpha-stimulated Lin-Sca-1+ progenitors, FL was more efficient than SCF at promoting the survival of progenitors with a B cell potential, as measured by their ability to produce B220+ cells in response to delayed addition of FL, SCF, and IL-7. Seventy-one percent of the B220+ cell production could be recovered following 40-h incubation with FL compared with 2% in response to SCF. Analysis of day 12 spleen CFU content after 40-h preincubation of Lin-Sca-1+ cells in FL or SCF demonstrated that SCF maintained 64% of the day 12 spleen CFU, whereas only 16% survived in the presence of FL. Finally, there was no significant difference between the ability of FL and SCF to maintain the viability of long-term culture-initiating cells (25 and 32%, respectively). The ability of FL to promote the survival of Lin-Sca-1+ progenitor cells was reflected by the finding that FL also suppressed apoptosis. Finally, TGF-beta abrogated and TNF-alpha potently counteracted the survival-promoting effect of FL. Thus, FL promotes the survival of primitive hemopoietic progenitor cells, in particular those with an inherent B lymphoid potential.

TNF-alpha, the great imitator: role of p55 and p75 TNF receptors in hematopoiesis.

The clinical application of tumor necrosis factor-alpha (TNF-alpha) has so far been limited due to the severe adverse effects associated with its systemic use. Recently, two distinct TNF receptors with molecular weights of 55 kDa (TNFR55) and 75 kDa (TNFR75) have been cloned and characterized. The subsequent development of TNF-alpha mutants with selective activity on either TNFR55 or TNFR75 suggest that such mutants might maintain the therapeutic (anti-tumor) potential of wild type TNF-alpha, but exhibit reduced toxicity (proinflammatory effects). In the present article we discuss previous studies on the effects of TNF-alpha in in vitro and in vivo hematopoiesis. In addition, we summarize more recent data from our laboratory as well as others, elucidating the role of TNF-alpha as a direct bifunctional regulator of in vitro hematopoiesis. Specifically, TNF-alpha is a potent inhibitor of the clonal growth of primitive and committed murine and human bone marrow progenitors in combination with multiple cytokines, including granulocyte colony-stimulating factor (G-CSF), CSF-1, erythropoietin (Epo), stem cell factor (SCF), and flt3 ligand (FL). In contrast, TNF-alpha at low concentrations can synergistically and directly enhance the clonal growth of primitive and more mature human CD34+ bone marrow progenitors when combined with GM-CSF or interleukin (IL)-3. Thus, a critical determinant of whether TNF-alpha elicits a stimulatory or inhibitory effect on the in vitro growth of hematopoietic progenitors appears to be the specific growth factors with which it interacts, rather than the maturity of the targeted progenitor. Furthermore, we describe the involvement of the two TNF receptors in signaling in vitro hematopoietic effects of TNF-alpha. Whereas TNFR55 is involved in most observed responses to TNF-alpha, signaling of TNFR75 appears to be restricted to inhibitory effects on primitive progenitors. Finally, we discuss the complexity of direct and indirect actions of TNF-alpha in in vivo hematopoiesis, and the potential clinical applications of TNF-alpha or TNF mutants.

Construction of a partial rabbit spleen cDNA library and identification of immunoglobulin clones.

A partial cDNA library was constructed from total poly A(+)-RNA isolated from the spleen of a rabbit (kappa allotype b5; heavy chain allotypes a3d12e15) that had been hyperimmunized with Streptococcus pneumoniae (type III). In spite of the absence of either specific DNA probes for rabbit immunoglobulin (Ig) sequences or cross-hybridizing mouse Ig DNA probes, recombinant clones containing cDNA sequences of rabbit gamma heavy chain and kappa light chains were identified by a combination of screening techniques: (a) colony hybridization using labeled mRNA; (b) mRNA hybridization selection and translation and (c) hybridization to electrophoretically fractionated poly A(+)-RNA ("Northern" analysis). Sequencing of three kappa light chain recombinant DNA sequences, including part of the 3' untranslated (UT) region, has confirmed the fact that recombinant DNA for kappa light chain mRNA has been identified. An unexpectedly high degree of homology between the 3' UT region sequence of this DNA from a rabbit of b5 allotype and the published 3' UT sequence from a b4 rabbit was found. It appears that 3' UT sequences from b4 and b5 alleles have diverged less than the coding sequences for the constant regions. The functional significance of this conservation of 3' UT sequences remains to be elucidated.

Stem cell factor influences the proliferation and erythroid differentiation of the MB-02 human erythroleukemia cell line by binding to a high-affinity c-kit receptor.

Stem cell factor (SCF) acts in synergy with other growth factors such as erythropoietin (Epo), granulocyte-macrophage colony-stimulating factor (GM-CSF), or interleukin-3 (IL-3), to stimulate the growth of primitive hematopoietic cells. Because of the prominent role of CSF in the maintenance of normal erythropoiesis in vivo, we examined the effects of SCF on the Epo-inducible human erythroleukemia cell line MB-02, and characterized the c-kit receptor in these cells. MB-02 cells cultured in serum-containing media do not survive in the absence of exogenous growth factors, but the addition of SCF, Epo, or IL-3 as a single factor enhanced MB-02 survival. Furthermore, in the presence of Epo, SCF (5 to 25 ng/mL) enhanced MB-02 proliferation in a dose-dependent manner, and increased the relative and absolute number of benzidine-positive cells generated. SCF also enhanced cell proliferation in the presence of either IL-3 or low concentrations of GM-CSF. A neutralizing anti-c-kit receptor monoclonal antibody (SR-1) blocked binding of 125I-SCF to MB-02 cells by 98%, and the effect of SCF on MB-02 growth, c-kit receptor-binding parameters were quantitated by equilibrium-binding experiments with 125I-SCF. MB-02 cells display a single class of high-affinity (50 pmol/L) c-kit receptors, with approximately 8,000 receptors per cell. The molecular weight of the c-kit receptor was determined by affinity cross-linking 125I-SCF to MB-02 cells. 125I-SCF-c-kit receptor complexes of approximately 155,000 and approximately 310,000 daltons were found, likely representing the monomeric and dimeric forms of the c-kit receptor. The binding affinity and molecular weight of the c-kit receptor on MB-02 cells are similar to those of normal human marrow cells. These results suggest that SCF synergizes with Epo to influence not only the proliferation but the erythroid differentiation of MB-02 cells. Thus, the MB-02 cell line may be a useful model in which to investigate the molecular mechanisms of SCF action.

Inhibition of stem cell factor-induced proliferation of primitive murine hematopoietic progenitor cells signaled through the 75-kilodalton tumor necrosis factor receptor. Regulation of c-kit and p53 expression.

TNF-alpha is a pleiotropic cytokine with stimulatory as well as inhibitory effects on hematopoiesis. We have previously demonstrated that TNF-alpha directly inhibits CSF-induced proliferation of primitive murine lineage-negative bone marrow progenitors (Lin-) and stem cell antigen-1 hematopoietic progenitors through the 75-kDa TNF receptor (TNF-R2), whereas TNF-alpha-induced inhibition of more committed Lin- progenitors is mediated through the 55-kDa TNF-R (TNF-R1), indicating a differential role of the two TNF-Rs in hematopoiesis. Numerous studies have demonstrated the ability of stem cell factor (SCF), a key regulator of hematopoiesis signaling through c-kit, to synergize with other hematopoietic growth factors, but little is known about cytokines capable of inhibiting hematopoiesis induced by SCF. While TNF-alpha has been demonstrated to enhance SCF-induced proliferation of myeloid leukemia blasts, the present report demonstrates that TNF-alpha, by signaling through TNF-R2, inhibits SCF-induced proliferation of normal murine Lin- and stem cell antigen-1 hematopoietic progenitors. SCF-stimulated proliferation of the hematopoietic cell line FDC-P1 was also potently inhibited by TNF-alpha and was accompanied by down-regulation of c-kit cell surface expression as well as c-kit mRNA levels. Finally, treatment of the FDC-P1 cell line with TNF-alpha resulted in increased levels of the tumor suppressor p53 mRNA, suggesting another mechanism by which hematopoietic effects of TNF-alpha may be mediated.

Nonhematopoietic tumor cell lines express stem cell factor and display c-kit receptors.

Human stem cell factor (SCF) acts in the presence of other growth factors to stimulate the growth of primitive hematopoietic progenitor cells. These effects are performed by activation of the SCF receptor, c-kit. Because of the potential use of SCF in patients undergoing chemotherapy and bone marrow transplantation, the effect of SCF on nonhematopoietic tumors requires investigation. To determine whether human tumor cell lines display c-kit receptors, we performed binding experiments with 125I-SCF on a breast carcinoma cell line (Du4475), a gastric carcinoma cell line (KATO III), a melanoma cell line (HTT144), as well as two small cell lung carcinoma cell lines (H69 and H128). The biologic effect of SCF on tumor cell lines was assessed by its ability to stimulate tritiated thymidine uptake and to enhance colony growth in methylcellulose. The breast carcinoma cell line, Du4475, as well as two small cell lung carcinoma cell lines, H69 and H128, exhibit high-affinity c-kit receptors with approximate binding affinities of 40, 100, and 90 pmol/L, respectively. The number of high-affinity receptors per cell ranged from 700 to 9,500. The gastric carcinoma cell line, as well as the melanoma cell line, showed trace binding of 125I-SCF. In the presence of SCF alone, or in combination with granulocyte-macrophage colony-stimulating factor or interleukin-3, there was less than a 17% increase in the colony growth of Du4475, H69, or H128 cell lines. Postulating that the lack of growth response could be secondary to endogenous SCF production by the tumor cell lines, we used an RNAse protection assay to determine whether the tumor cell lines contain SCF messenger RNA (mRNA). In addition, we tested tumor cell line supernatants for the presence of secreted SCF protein by enzyme immunoassay, and analyzed the tumor cell lines for membrane-bound SCF by indirect immunofluorescence. Our results show that the Du4475, H69, and H128 cell lines, as well as a melanoma cell line (HTT144), have multiple copies of SCF mRNA. Soluble SCF protein was detected in the cell supernatants in the Du4475 and H69 cell lines and SCF was found on the surface of all four cell lines. These data show that some human solid tumor cell lines display high-affinity c-kit receptors and produce SCF, which can be detected on the cell surface. These results suggest the possibility that autocrine production of SCF by c-kit receptor-bearing tumor cells may enhance cell growth in tumor cell lines.