Identification, purification, and biological characterization of hematopoietic stem cell factor from buffalo rat liver--conditioned medium

C-kit receptors in childhood malignant lymphoblastic cells

The product of the protooncogene c-kit is the receptor for the hematopoietic cytokine stem cell factor (SCF). C-kit is expressed on leukemic cells of the erythroid, myeloid, and mast-cell lineage and SCF has a proliferative effect on some of these cells. The role of SCF and c-kit in lymphoid malignancies is much less clear. Here we review the role of c-kit in normal lymphopoiesis and summarize its role in lymphoid malignancies. C-kit is expressed in normal lymphopoiesis and its ligand SCF synergizes with IL-7 to enhance the proliferation of B- and T-cell progenitors. In malignant lymphopoiesis, c-kit can also be expressed in B and T-lymphoblastic cells from children with non Hodgkin's lymphoma (NHL) or acute lymphoblastic leukemia (ALL) when analyzed by the highly sensitive reverse transcriptase polymerase chain reaction (RT-PCR). While c-kit receptors were detected by flow cytometric (FCM) analysis on about 40% of fresh T-lymphoblastic biopsy tumor cell preparations or T-lymphoblastic cell lines, no receptors were detected on B-lymphoblastic fresh cells or cell lines from children with B-ALL or Burkitt's lymphoma (BL). Almost all of the lymphoblastic cells expressing c-kit protein responded to recombinant human (rh)SCF with a downregulation of c-kit receptors. A proliferative response was detected only in a minority of these cells. B-ALL or BL cell lines showed no response to rhSCF. Upregulation of c-kit in T-lymphoblastic cells could be demonstrated by the addition of IL-1 beta, TNF-alpha, TGF-beta or A23187, and downregulation by rhSCF or phorbol myristate acetate (PMA). Despite upregulation of c-kit mRNA, protein remained undetectable on B-ALL or BL cells in the presence of A23187. The metabolic state of the cells seemed to influence c-kit expression, since c-kit was upregulated in T-lymphoblastic cells by the addition of new medium. C-kit appears to play a role in the growth of some malignant T-lymphoblastic but not B-lymphoblastic cells.

In Vitro Expansion of Hematopoietic Progenitors and Maintenance of Stem Cells: Comparison Between FLT3/FLK-2 Ligand and KIT Ligand

The effects of FLT3/FLK-2 ligand (FL) and KIT ligand (KL) on in vitro expansion of hematopoietic stem cells were studied using lineage-negative (Lin−)Sca-1–positive (Sca-1+) c-kit–positive (c-kit+) marrow cells from 5-fluorouracil (5-FU)–treated mice. As single agents, neither FL nor KL could effectively support the proliferation of enriched cells in suspension culture. However, in combination with interleukin-11 (IL-11), both FL and KL enhanced the production of nucleated cells and progenitors. The kinetics of stimulation by FL was different from that by KL in that the maximal expansion by FL of the nucleated cell and progenitor pools required a longer incubation than with KL. We then tested the reconstituting abilities of cells cultured for 1, 2, and 3 weeks by transplanting the expanded Ly5.1 cells together with “compromised” marrow cells into lethally irradiated Ly5.2 mice. Cells that had been expanded with either cytokine combination were able to maintain the reconstituting ability of the original cells. Only cells that had been incubated with KL and IL-11 for 21 days had less reconstituting ability than fresh marrow cells. These results indicate that there can be significant expansion of progenitors in vitro without compromising the reconstituting ability of stem cells. Addition of IL-3 to permissive cytokine combinations significantly reduced the ability of cultured cells to reconstitute the hematopoiesis of irradiated hosts. These observations should provide a basis for a rational approach to designing cytokine combinations for in vitro expansion of hematopoietic stem cells.

Thrombopoietin is synergistic with other hematopoietic growth factors and physiologic platelet agonists for platelet activation in vitro

Thrombopoietin (TPO) is the primary physiologic regulator of platelet production. The effect of TPO on platelet function, both alone and in combination with other hematopoietic growth factors, adenosine diphosphate (ADP), and epinephrine, was investigated using fluorescent-labeled antibodies to the activation-dependent antigen CD62 (P-selectin) and flow cytometry. TPO stimulated CD62 expression on normal human platelets, and this expression was completely inhibited by the soluble extracellular domain of the TPO receptor, MPL. The growth factors granulocyte colony-stimulating factor (G-CSF) and erythropoietin (EPO), but not interleukin-3 (IL-3) or stem-cell factor (SCF), also stimulated platelet activation. The combination of EPO, SCF, ADP, and epinephrine with TPO were synergistic for platelet CD62 expression. These data further support a role for TPO in modulating platelet function.

Functional and ultrastructural studies on the burst of c-kit ligand containing secretory cytoplasmatic vesicles

A fibroblast-like cell line (D064) was established from canine marrow long-term cultures. These cells start to differentiate into hematopoietic progenitor precursors under the influence of c-kit ligand, functioning in an autocrine and paracrine fashion. The message for c-kit ligand is transcribed in the fibroblast-like cell line and the ligand for the tyrosine-kinase c-kit is expressed on the cell surface as well as secreted in its soluble form. With a monoclonal antibody directed against all isoforms of c-kit ligand, the expression of c-kit ligand was investigated functionally and topographically by electron microscopy. Ultrastructural analysis revealed c-kit ligand in electron dense vesicles which were transported to the cell surface and burst to release the soluble, non-membrane bound isoform of c-kit ligand. A membrane bound isoform of c-kit ligand was also present on the cell membrane.

Stem cell factor alters membrane potential of purified peritoneal mast cells in culture

The membrane potential (E(m)) was used as an indicator to evaluate the effect of stem cell factor (SCF) on the membrane integrity of peritoneal mast cells (PMCs). PMCs were harvested from the peritoneal lavage of Sprague-Dawley rats, purified more than 95% and cultured with or without the presence of SCF (2 x 10(-8) M). E(m) values were measured with conventional intracellular recording techniques. Results from day 1 to day 4 in culture were compared. Significant differences in average E(m) (aE(m)) (P < 0.01, analysis of variance) were seen on days 3 and 4 (means +/- SE in millivolts): -67.4 +/- 8.0 and -59.4 +/- 4.8 with SCF vs. -24.8 +/- 7.9 and -7.6 +/- 3.9 without SCF, respectively. Moreover, after culture with SCF for >1 wk, the aE(m) values of purified PMCs had a tendency to reach plateau values similar to that of unpurified PMCs on day 1 (at -20 mV). The morphological appearances of PMCs can be correlated with the results of aE(m) measurements. PMCs with a smooth spherical shape and highly refractive appearance, and better tolerance to electrode impalement, showed E(m) with greater negative values and lesser fluctuations. These results indicate that SCF can maintain the membrane properties and viability of purified PMCs in a long-term culture.

Factors influencing fetal macrophage development: II. Effects of the PDGF subfamily of protein-tyrosine kinase receptor ligands as studied in organ-cultured rat lungs

BACKGROUND

Macrophage precursors in pseudoglandular rat lungs rapidly differentiate into phagocytes in organ culture, although this occurs only gradually in vivo. Macrophage colony-stimulating factor is vital for the process, but the possible importance of other ligands in the platelet-derived growth factor (PDGF) subfamily is scarcely appreciated.

METHODS

Macrophage development was compared in 15-day fetal rat lungs cultured on solid, serum-containing media with and without added stem cell factor (SCF) (100 ng/mL) or antibodies to PDGF-AA and -BB (10-15 micrograms/mL each). In addition, organ cultures and intact lungs were immunostained for PDGF-AA and -BB to confirm their presence in the tissues. Macrophage population growth was measured by coronal area assay.

RESULTS

SCF initially stimulated macrophage production. Thereafter, results varied depending on baseline production by control cultures: where this was vigorous, SCF-exposed explants performed similarly; where this was moderate, the SCF explants outperformed them 1.5-2.6 times over (P < 0.01-0.001). Inhibition of macrophage production by pyrrolidine dithiocarbamate (100 microM) was not significantly diminished in the presence of SCF (10 ng/mL). Immunoreactivity for PDGF-AA and -BB was prevalent in cells of the airway epithelium and stroma during the period macrophage precursors were converting, and both isoforms were detected in differentiating macrophages as early as 2 days in vitro. Nonetheless, exposure of cultures to anti-PDGFs had no significant effect on macrophage population growth.

CONCLUSIONS

Ligands of the PDGF subfamily differ greatly in their influence over development of fetal macrophages. Whereas the PDGFs are ineffective, SCF stimulates growth of macrophage precursors and early differentiating forms and enhances survival of older cells. It appears to act mainly in synergy with other growth factors present in fetal lungs. Furthermore, in the hierarchy of hematopoietic progenitors, the macrophage precursors may be ranked on a par with burst-forming units in the red cell lineage.

Effect of stem cell factor on leukemic progenitor cell growth and sensitivity to cytosine-arabinoside

Recruitment of quiescent, clonogenic blasts from patients with acute myeloid leukemia (AML) by hematopoietic growth factors (HGFs) may improve the cytotoxic effects of cell-cycle-specific drugs like cytosine-arabinoside (Ara-C). Using the culture methods described by Nara and McCulloch and making a distinction between self-renewing and post-deterministic mitoses, we analyzed the effects of stem cell factor (SCF), a growth factor acting on early hematopoietic progenitor and stem cells. First, we demonstrated that SCF, used in combination with other HGFs included in fetal calf serum (FCS) and/or in 5637 cell line supernatant (5637-CM), stimulated both colony formation and self-renewal of blast progenitors from 10 patients, unlike SCF alone. We tested the effects of SCF on the recruitment of cells in the S-phase by using a bromodeoxyuridine/DNA (BrdUrd/DNA) staining method in flow cytometry (FCM). We showed that SCF stimulated proliferation of AML cells significantly in 9/18 patients with AML. Second, we tested the influence of SCF on the sensitivity to Ara-C of self-renewing leukemic cells from 18 patients with AML. We showed that SCF was efficient in increasing the toxicity of Ara-C on the self-renewing blast progenitors, especially with high concentrations of Ara-C. However, a large patient-to-patient heterogeneity was found and the activity of SCF was not correlated with its effect on the cell cycle. These data indicate that SCF can enhance sensitivity to Ara-C of some leukemic cells with self-renewing capacity.

Expression of stem cell factor receptor (c-kit) by the malignant mast cells from spontaneous canine mast cell tumours

Stem cell factor receptor (SCFR, c-kit), normally expressed on haematopoietic and mast cells, plays a regulatory role in cellular growth and differentiation. Dysregulated expression of SCFR may contribute to neoplastic transformation. We investigated expression of SCFR on malignant canine mast cells obtained directly from spontaneous canine mast cell neoplasms, in an attempt to determine whether these undifferentiated cells maintained expression of this growth-promoting cytokine receptor. Malignant mast cells (histological grade 2) from skin tumours or lymph node metastases were collected from canine patients, and SCFRs were detected by flow cytometric analysis of these cells. All of the tumours bound mouse and canine recombinant stem cell factor (SCF), indicating that the cells not only expressed SCFRs, but that the receptors possessed the functional property of ligand binding. Immunoglobulin Fc receptors for canine IgE were identified on these cells by flow cytometry, a further indication that the cells analysed were mast cells and retained some differentiated features. Immunohistochemical analysis of formalin-fixed, paraffin wax-embedded mast cell tumour biopsies confirmed expression of SCFRs by malignant cells from each tumour. The relative binding of SCF to suspensions of tumour cells, as assessed by flow cytometry, correlated with the intensity of immunolabelling for SCFR in sections of the same tumours, suggesting variability in SCFR expression between tumours. Agarose gel electrophoresis of the products of SCFR reverse transcription-polymerase chain reaction derived from each tumour had the molecular weight predicted for canine SCFR cDNA on the basis of the mouse and human counterparts. This further confirmed SCFR expression by malignant canine mast cells. Taken together, these results show that a membrane receptor capable of triggering cell growth is expressed by malignant canine mast cells, suggesting a role for this receptor in the aetiology of canine mast cell cancer. This relatively common malignancy of the dog would seem to present an opportunity for the investigation of the potential role of the SCF/SCFR pathway in the development of spontaneous malignancies of mast cells.

flt-3 ligand is more potent than c-kit ligand for the synergistic stimulation of ex vivo hematopoietic cell expansion

The c-kit and flt-3 tyrosine kinase receptors are expressed on primitive hematopoietic cells, and ligands for both receptors have been cloned. In this study, the effects of c-kit ligand (KL) and flt-3 ligand (FL) were compared in the presence of IL-3, GM-CSF, and erythropoietin (3/GM/EPO), using frequent medium exchange cultures of human bone marrow mononuclear cells (BMMNC) and CD34-enriched cells. In MNC cultures, KL increased cell output by 1.7-fold (p < 10(-4), n = 13) and CFU-GM output by 2.4-fold (p < 10(-3)) as compared with control cultures containing only 3/GM/EPO. Analogously, FL increased cell output by 1.3-fold (p < 10(-3)) and CFU-GM output by 4.4-fold (p < 10(-6)). Therefore, FL was more potent on CFU-GM output than KL, but neither altered the lineage composition (granulocyte, monocyte, macrophage) of the colonies produced. Direct addition of KL or FL to colony assays resulted in only a 1.2-fold increase in CFU-GM outgrowth, suggesting that the effects on increased CFU-GM output were at the preprogenitor stage. In CD34-enriched cell cultures, the effects of KL and FL on CFU-GM output were similar (9-fold above control). Nevertheless, MNC cultures (containing an equivalent number of CD34+lin- cells) always generated more cells (2-fold to 4-fold) and CFU-GM (3-fold to 6-fold) than did parallel cultures of CD34-enriched cells. The greater effect of FL (over KL) in MNC cultures was probably due to synergy with endogenously produced growth factors that were absent in CD34-enriched cell cultures. FL-containing cultures (+/-KL) generated cells that formed larger colonies, and these cells had more proliferative potential on replating into secondary and tertiary cultures. Furthermore, FL increased the output of LTC-IC by 2.1-fold (p < 0.01) and CD34+lin- cells by 6-fold (p < 0.05) as compared with 3/GM/EPO cultures. In contrast, KL did not affect the output of LTC-IC and only slightly increased CD34+lin- cell output (by 1.4-fold). Erythrocytes were increased by KL (2.8-fold) and decreased by FL (0.6-fold), whereas granulocytes and monocytes were increased by both KL (1.4-fold) and FL (2.0-fold). When used together, KL and FL were completely additive with respect to cell, CFU-GM, and LTC-IC output, as well as lineage composition. The results indicate that FL is a more potent synergistic growth factor than KL for MNC expansion and that KL and FL act in an independent, direct, additive manner.

Cytokines at the research-clinical interface: potential applications

Developments in the characterization of growth factors and the recognition of their potential for clinical use has advanced through a number of stages. The development of clonogenic haemopoietic colony assays in the 1960s led to the discovery of colony-stimulating activity in the conditioned medium produced by certain cell lines. This activity was then purified and the colony-stimulating factors were identified. With rapid progress in molecular biology techniques in the 1980s, many further growth factors were cloned and produced on an industrial scale. Although erythropoietin, interferons, G-CSF, GM-CSF and IL-2 were all introduced into clinical practice as single agents, cytokines have more recently been investigated for use either in combination, or sequentially. Clinical trials are currently in progress to examine the optimum combinations and timing of administration. Current clinical applications include optimization of methods for mobilization of peripheral blood progenitor cells and amelioration of cytopenias following chemotherapy and bone-marrow transplantation. In the future, cytokines will be employed to expand stem and progenitor cells ex vivo, to improve gene transduction strategies, possibly to protect the gastrointestinal epithelium and as immunomodulators, both in vivo and in vitro. This review will focus on recently characterized growth factors including c-kit ligand/stem cell factor, flt3 ligand, c-mpl ligand/thrombopoietin and interleukins-11, 4, 7, 10, 12 and 13.

Characterization of the growth factor activity of amniotic fluid on cells from hematopoietic and lymphoid organs of different life stages

We investigated the proliferation-promoting effects of murine amniotic fluid (MAF) on in vitro cultured cells originally obtained from murine hematopoietic and lymphoid organs at different life stages. MAF promoted proliferation of the fetal liver cells (FLC), newborn spleen cells and adult bone marrow cells. The proliferation-promoting activity of MAF was extended to liver cells and spleen cells from mice younger than 2 weeks old. MAF did not, however, promote the proliferation of newborn or adult thymocytes, or of spleen cells, liver cells or peritoneal cells, liver cells or peritoneal cells from 2-week-old or older mice. Rather, it partially inhibited the proliferation of spleen cells, thymocytes and peritoneal cells from 1-year-old mice. These results suggest that MAF contains growth factors for hematopoietic stem cells but not for either mature or immature T lymphocytes. Supporting this view, the MAF activity was partially neutralized by a polyclonal anti-mouse stem cell factor (SCF) antibody. Moreover, the immunoblotting of MAF against anti-mouse SCF antibody revealed a band at 30-32 kDa corresponding to the previously reported SCF. Interestingly, MAF was able to maintain FLC and adult bone marrow cells alive in culture for a relatively long time (2 weeks). The MAF activity was further shown to be partially and cell type-dependently antagonized by TNF-alpha and TGF-beta. These results provided evidence that MAF contains potentially multiple growth factors preferentially affecting the early stage of hematopoiesis, one of which is SCF.

Promotion of proliferation of murine hematopoietic stem cells by growth factors in murine amniotic fluid

It has been shown that murine amniotic fluid (MAF) displays immunosuppressive activities. We examined the effect of MAF obtained from normal mice on a murine fetal liver cell (FLC) primary culture in vitro for the detection of the possible existence of cytokines that affect hematopoiesis. MAF promoted proliferation of murine FLC and adult bone marrow cells. Proliferation-promoting activity of MAF was observed throughout the period between days 12 and 15 of pregnancy, peaking at day 12. Pooled MAF was subjected to purification procedures to isolate the active molecule(s). After ion exchange and size exclusion chromatography, one of the active molecules was shown to react with anti-mouse stem cell growth factor (SCF) antibody and the molecular weight was determined as 40-45 kDa by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot. The proliferation-promoting activity of MAF was partially neutralised by the anti-SCF antibody. These results provide evidence that MAF may contain multiple growth factors for FLC, one of which may be a unique molecule related to SCF.

Release of stem cell factor from a human keratinocyte line, HaCaT, is increased in differentiating versus proliferating cells

Stem cell factor, a recently discovered growth factor for hematopoietic stem cells, mast cells, and melanocytes, was initially reported to be produced by fibroblasts. In this study, we investigated the secretion of this factor from human HaCaT cells during in vitro culture and compared it to synthesis by cells in the skin. Release of stem cell factor from freshly cultured keratinocytes was comparable to that of HaCaT cells and was nearly half that produced by fibroblasts and umbilical vein endothelial cells. No stem cell factor was detectable in culture supernatants of melanocytes. HaCaT cells underwent spontaneous differentiation after a period of proliferation until confluency. Depending on duration of culture, they released increasing amounts of stem cell factor (approximately 150 pg/10(6) cells on day 3 (proliferating cells) vs approximately 450 pg/10(6) cells on day 14 (differentiating cells) measured by enzyme-linked immunosorbent assay. Stimulation for 24 h with the calcium ionophore A 23187 (10(-6) to 10(-8) M) further enhanced release. Western blot analysis of HaCaT cell lysates with a stem cell factor antibody revealed two proteins with the known molecular weights of membrane-bound and soluble stem cell factor. By semiquantitative reverse transcriptase polymerase chain reaction, full-length as well as spliced type stem cell factor mRNA was found to be increased in differentiating versus proliferating HaCaT cells. Keratinocytes are thus potentially important sources of stem cell factor in human skin, and HaCaT cells provide a useful model for further studies of stem cell factor from keratinocytes.

Epitope mapping and immunoneutralization of recombinant human stem-cell factor

The epitope regions of three anti-[stem-cell factor (SCF)]g have been mapped by characterization of immunoreactivities against truncated forms of SCF in immunoblots and against synthetic peptides in solution-phase competition ELISA. Two of the antibodies, mAb 7H6 and mAb 8H7A, were raised against Escherichia coli-derived human SCF-(1-164) while the third, polyclonal antibody (pAb) 1337, was raised against a peptide corresponding to residues 3-31 of human SCF. The epitopes of mAbs 7H6 and 8H7A have been mapped to residues 61-95 and 95-110, respectively. The epitope of pAb 1337 has been mapped to residues 21-31. The ability of the anti-SCF Ig to recognize E. coli-derived human SCF presented in various formats, i.e. partially denatured (fixed in standard ELISA or on a western blot) or native (in solution), was studied, mAb 7H6 recognized its epitope in partially denatured or native SCF with equally high affinity, while mAb 8H7A and pAb 1337 recognized their epitopes only when SCF was at least partially denatured, mAb 7H6 was found to neutralize in vitro SCF-mediated cell proliferation and SCF binding to its receptor, when present in equimolar concentrations relative to the ligand, suggesting that the epitope region is functionally significant. Evidence that the mAb 7H6 epitope is represented by discontinuous regions (residues within sequences 61-65 and 91-95 are critically involved) is presented. The observation that the mAb 7H6 epitope is discontinuous has implications for the structure of SCF.

Regulatory roles of the ligand for Flk2/Flt3 tyrosine kinase receptor on human hematopoiesis

The biological activities of the ligand for the Flk2/Flt3 receptor tyrosine kinase (FL) on human hematopoietic cells are reviewed. In in vitro studies, FL shows relatively few effects by itself on the proliferation and differentiation of hematopoietic cells, but exhibits a potent costimulatory activity in enhancing the proliferation of progenitor cells of multiple lineages. FL promotes the growth of clonogenic myeloid progenitor cells in the presence of other cytokines known to be active on myeloid progenitors, including GM-CSF, interleukin 3 (IL-3), kit ligand (KL), M-CSF and G-CSF. In addition, FL synergizes with IL-7 in inducing the proliferation of pro-B cells, whereas FL has little effect on the growth of clonogenic erythroid progenitors. Furthermore, FL induces the in vitro expansion of the high proliferative potential colony-forming cells (HPP-CFC) and stimulates the proliferation of long-term culture-initiating cells (LTC-IC), suggesting an activity on the proliferation of putative stem cells. Thus, FL plays important roles in regulating the proliferation of hematopoietic progenitor cells and, therefore, may have therapeutic applications.

Aging and haemopoiesis. Implications for treatment with haemopoietic growth factors

Senescence of the lympho-haemopoietic system is associated with an increased incidence of neoplasia, autoimmune diseases and infections. Myelosuppression, either in the context of cancer chemotherapy or in the face of severe infections, commonly manifests as pancytopenia, and has an adverse impact on the prognosis of the elderly cancer patient by increasing infection and bleeding-related morbidity. The physiological basis of this blunted haemopoietic response is unclear, and has been ascribed to age-related deficits in marrow progenitor cell numbers, changes in the marrow microenvironment, decreased production of regulatory growth factors, or a combination of these mechanisms. These age-related deficits tend to be subtle and are only of clinical importance either when present cumulatively or under conditions of extreme haemopoietic stress. Furthermore, some of these deficits can be circumvented with the use of haemopoietic growth factors (HGFs). Thus, the availability in the clinic of various HGFs has had a tremendous impact on the care of the elderly cancer patient. The HGFs currently approved for use are: granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor and epoetin-alpha (recombinant human erythropoietin). However, we still need to better elucidate age-related changes in the early stages of haemopoiesis. The question of haemopoietic exhaustion, particularly under prolonged growth factor stimulation, is real and still unanswered.

In vitro methionine oxidation of Escherichia coli-derived human stem cell factor: effects on the molecular structure, biological activity, and dimerization

The effect of oxidation of the methionine residues of Escherichia coli-derived recombinant human stem cell factor (huSCF) to methionine sulfoxide on the structure and activity of SCF was examined. Oxidation was performed using hydrogen peroxide under acidic conditions (pH 5.0). The kinetics of oxidation of the individual methionine residues was determined by quantitation of oxidized and unoxidized methionine-containing peptides, using RP-HPLC of Asp-N endoproteinase digests. The initial oxidation rates for Met159, Met-1, Met27, Met36, and Met48 were 0.11 min-1, 0.098 min-1, 0.033 min-1, 0.0063 min-1, and 0.00035 min-1, respectively, when SCF was incubated in 0.5% H2O2 at room temperature. Although oxidation of these methionines does not affect the secondary structure of SCF, the oxidation of Met36 and Met48 affects the local structure as indicated by CD and fluorescence spectroscopy. The 295-nm Trp peak in the near-UV CD is decreased upon oxidation of Met36, and lost completely following the oxidation of Met48, indicating that the Trp44 environment is becoming significantly less rigid than it is in native SCF. Consistent with this result, the fluorescence spectra revealed that Trp44 becomes more solvent exposed as the methionines are oxidized, with the hydrophobicity of the Trp44 environment decreasing significantly. The oxidations of Met36 and Met48 decrease biological activity by 40% and 60%, respectively, while increasing the dissociation rate constant of SCF dimer by two- and threefold. These results imply that the oxidation of Met36 and Met48 affects SCF dimerization and tertiary structure, and decreases biological activity.

Refolding and oxidation of recombinant human stem cell factor produced in Escherichia coli

Oxidative folding of recombinant human stem cell factor (rhSCF) produced in Escherichia coli was investigated in vitro. Folding of denatured and reduced rhSCF involves at least five intermediate forms, I-1 to I-5, detectable by their differences in hydrophobicity using reverse-phase high performance liquid chromatography. Both I-1 and I-2 contain a native-like disulfide bond, Cys4-Cys89 and Cys43-Cys138, respectively, and I-3 forms a mispaired disulfide, Cys43-Cys89. These forms appear to reach steady state equilibrium and are important folding intermediates. I-1 was found to be the prominent intermediate that directly folds into native rhSCF (N); and the thermodynamically less stable I-2 favors rearrangment into I-1. I-3 may serve as an intermediate for disulfide rearrangment between I-1 and I-2. I-4 and I-5, which are disulfide-linked dimers, are in equilibrium with reduced rhSCF and other intermediates and may not play an important role in rhSCF folding. Both trifluoroacetic acid-trapped I-1 and I-2, after isolation by high performance liquid chromatography, proceed with the remaining oxidative folding process after reconstitution. Iodoacetate-trapped I-1 and I-2 contain low alpha-helical content and some tertiary structure, while I-3 and reduced rhSCF have little ordered structure. Gel filtration/light-scattering experiments indicate that reduced rhSCF and iodoacetate-trapped I-1, I-2, and I-3 exist as dimeric forms, indicating that rhSCF dimerization precedes formation of disulfide bonds. I-1, I-2, I-3, and the C43,138A analog lacking Cys43-Cys138 bond are not biologically active or exhibit significantly lower activity. The two disulfide bonds in rhSCF seem to be essential for the molecule to maintain an active conformation required for its receptor binding and biological activities.

Isolation and characterization of a disulfide-linked human stem cell factor dimer. Biochemical, biophysical, and biological comparison to the noncovalently held dimer

Distinct from the noncovalently linked recombinant human stem call factor (rhSCF) dimer, we report here the isolation and identification of an SDS-nondissociable dimer produced during folding/oxidation of rhSCF. Experimental evidence using various cleavage strategies and analyses shows that the isolated dimer is composed of two rhSCF monomers covalently linked by four disulfide bonds. The cysteines are paired as in the noncovalently associated dimer except that all pairings are intermolecular rather than intramolecular. Other structural models, involving intertwining of intramolecular disulfide loops, are ruled out. The molecule behaves similarly to the noncovalently associated dimer during ion-exchange or gel permeation chromatography. However, the disulfide-linked dimer exhibits increased hydrophobicity in reverse-phase columns and in the native state does not undergo spontaneous dimer dissociation-association as seen for the noncovalent dimer. Spectroscopic analyses indicate that the disulfide-linked and noncovalently associated rhSCF dimers have grossly similar secondary and tertiary structures. In vitro, the disulfide-linked dimer exhibits approximately 3-fold higher biological activity in supporting growth of a hematopoietic cell line and stimulating hematopoietic cell colony formation from enriched human CD34+ cells. The molecule binds to the rhSCF receptor, Kit, with an efficiency only half that of the noncovalently associated dimer. Formation of intermolecular disulfides in the disulfide-linked dimer with retention of biological activity has implications for the three-dimensional structure of noncovalently held dimer and disulfide-linked dimer.

C-kit ligand (Stem Cell Factor) affects neuronal activity, stimulates pituitary-adrenal axis and prolactin secretion in rats

Stem Cell Factor (SCF) is a potent growth factor affecting proliferation, differentiation and migration as well as secretory functions of various cells of different origin and function. The effect of SCF on neuronal activity and on neuroendocrine regulation has been studied by injecting SCF i.v. into conscious male rats. Administration of SCF elevated adrenocorticotropin (ACTH), corticosterone and prolactin (PRL) plasma levels in a dose-dependent manner. Adrenocorticotropin and corticosterone secretions were maximal after injecting 10 micrograms SCF, while prolactin secretion only reached a maximum at a dose of 20 micrograms. Hormone secretions were maximal at 15 min (ACTH and PRL) or at 30 min (CORT) and declined to the baseline between 90 and 180 min. The functional anatomical approach, using immediate-early gene product c-Fos as a marker of activated neurones, revealed that systemic administration of SCF activated neurosecretory neurones of the hypothalamus in a time-dependent manner. Somatosensory afferent pathways and extrahypothalamic areas--which are involved in the regulation of hypothalamic neurosecretory cells--including various parts of the lamina terminalis, bed nucleus of stria terminalis, central amygdala, locus coeruleus, parabrachial nucleus, nucleus of the solitary tract and ventrolateral medulla also became Fos-positive. As SCF-induced mediators have been suggested to be involved in brain pathophysiology, and as SCF might have been of potential therapeutic use in certain anaemias and leukaemias, the complex interaction between SCF-induced cell-specific actions and its effect on the neuroendocrine system should also be considered.

Hormonal regulation of the ligand for c-kit in the rat ovary and its effects on spontaneous oocyte meiotic maturation

Kit ligand (KL, c-kit ligand) mRNA was detected in the ovaries of 26-day-old prepubertal rats using in situ hybridization. In antral follicles there was a gradient in the intensity of the hybridization signal across the layers of granulosa cells, with greatest intensity observed in the cumulus granulosa cells enclosing the oocyte, and less signal occurring in the granulosa cells furthest from the oocyte. In age-matched rats 40 hr after injection of pregnant mare serum gonadotropin (PMSG), the pattern of distribution of KL resembled that in the untreated ovaries, although the intensity of the hybridization signal was greater in the PMSG-primed ovaries. This morphological observation was confirmed using Northern blot analysis, which indicated that granulosa cells of PMSG-treated rats had 3.5-fold greater abundance of KL mRNA compared to untreated rats. The abundance of KL mRNA further increased to 7-fold over control levels at 6 hr after PMSG-primed rats were treated with human chorionic gonadotropin (hCG). By contrast, treatment of rats with diethylstilbestrol to stimulate follicular growth did not cause any change in the abundance of KL transcripts. To investigate a potential role for KL in oocyte meiotic maturation, fully grown oocytes were cultured for 24 hr with or without KL (50 or 500 ng/ml). The presence of KL resulted in a significant, albeit transient, delay in the progression of spontaneous meiotic maturation, using the indices of germinal vesicle breakdown and polar body formation. The inhibitory effects of KL were specifically blocked by ACK2, an antibody to the extracellular domain of the c-kit receptor. These results indicate that KL is produced in rat granulosa cells at particularly high levels in the cells closest to the oocyte and that this production may be regulated directly by gonadotropic hormones. Furthermore, KL inhibits the progression of meiosis in cultured oocytes, which suggests a possible role in the maintenance of meiotic arrest that occurs throughout oocyte growth.

Stem cell factor (SCF) synergizes with megakaryocyte colony stimulating activity in post-irradiated aplastic plasma in stimulating human megakaryocytopoiesis

Plasma obtained from lethally irradiated animals contains a megakaryocyte (MK) growth factor which has recently been identified as the ligand for the c-mpl receptor and has been named thrombopoietin (TPO). We demonstrate that post-irradiation aplastic canine plasma (PICS-J) and plasma from a human subject (ML) who was accidentally exposed to lethal irradiation, contain high levels of this activity, which support both MK proliferation and maturation in a dose-dependent manner. These plasma were far more active in stimulating human MK colony formation than other types of thrombocytopenic plasma or a number of exogenously added human recombinant cytokines and their combinations. The addition of stem cell factor (SCF), which alone has a minimal stimulatory affect, to post lethal-irradiation plasma provided a synergistic stimulation of megakaryocytopoiesis both in colony assays and liquid cultures. In colony assays, the combination of SCF with PICS-J or ML almost doubled the number of burst forming units (BFU-MK) and provided a 1.5-fold increase in colony forming units (CFU-MK). A 1.6-fold increase in the number of CD34+ BM cell-derived MK colonies was also elicited. In liquid cultures, the presence of both SCF and PICS-J or ML induced the appearance of a high proportion of CD34+ (6.56% vs 0.6% control) and CD41+ (3.5% vs 1.2% control) cells after 3 days in culture. By day 10, 66.8 x 10(4) CD41+ cells and 29.8 x 10(4) CD34+ cells were derived from 2 x 10(6) BMMC originally seeded. We propose that these unique plasma, which do not contain elevated level of IL-6, IL-3, GM-CSF, IL-1 beta, erythropoietin or SCF, probably contain high levels of TPO. The addition of SCF to the post-irradiation plasma provides a synergistic stimulation of megakaryocytopoiesis which may become relevant for future clinical application.

Origin of the c-kit-positive interstitial cells in the avian bowel

Interstitial cells of Cajal (ICC) aroused much interest among neuroanatomists at the beginning of the century. These small cells, organized into networks, are intercalated between nerve fibers and muscle cells, and are now considered by many authors to be responsible for the pacemaker activity of the gut. Renewed interest in these cells arose recently when the receptor tyrosine kinase, c-kit, was shown to be associated with their functional activity. The embryonic origin of interstitial cells has remained a controversial issue ever since their discovery. Some authors consider them to be of neural or glial nature and thus of neural crest origin. Others consider them to be of fibroblastic or muscular nature. We have applied the quail-chick marker system to solve this problem. ICC were identified by means of a chicken-c-kit nucleic probe which cross-reacts with the quail c-kit gene product. We constructed chimeric bowels by grafting isotopically quail vagal neural crest into chick embryos at embryonic day 2 (E2). The enteric innervation of the chimeras was then of quail origin. In situ hybridization of the chimeric bowels showed that all the c-kit-positive cells were of the chick type, and therefore belonged to the gut mesenchyme and were not neural crest-derived cells. This observation was confirmed by culturing aneural chick guts on the chorio-allantoic membrane. Typical ICC, as defined at the EM level and by their expression of the c-kit receptor, developed in the gut wall in the complete absence of enteric innervation. One can conclude the ICC are of mesodermal origin and develop independently from enteric neurons with which they later establish anatomical and functional relations.

Culture of in vitro fertilized bovine embryos with bovine oviductal epithelial cells, Buffalo rat liver (BRL) cells, or BRL-cell-conditioned medium

Co-culture with various cell types can enhance development of bovine embryos, especially through the transition from maternal to embryonic mRNA utilization, a stage of growth refractory to most in vitro methods. Bovine oviductal epithelial (BOE) cells have been particularly successful for culturing embryos through the refractory stage; however, Buffalo rat liver (BRL) cells are a readily available, long-lived, easy-to-care-for alternative. This study compared the embryotrophic activity of BOE to BRL cells with particular emphasis on the transition stage of growth. A total of 7158 immature bovine oocytes, matured and fertilized in vitro, were divided into 4 different culture treatments: Treatment 1: BRL conditioned medium for 72 h then BRL co-culture; Treatment 2: BRL co-culture; Treatment 3: BOE co-culture for 72 h in 5% oxygen then BRL co-culture; and Treatment 4: BOE co-culture for 72 h in 5% oxygen followed by BOE co-culture in air. Those same treatments were used to evaluate embryotrophic differences of early (4 to 5) versus late (14 to 15) passage BRL cells maintained in M-199 medium with 10% serum. Two bulls were also evaluated to determine if there exists a bull-by-culture system interaction. Treatment 3 resulted in the best development after 9 d; 9.1% of selected immature oocytes developed to expanded blastocyst. Early passage BRL cells were significantly more embryotrophic than later passage cells; this was most pronounced for Treatment 2. There was a treatment-by-bull interaction, which should be considered when comparing results among similar studies.

Clinical advances in mastocytosis

Mastocytosis is a disease characterized by an abnormal proliferation of tissue mast cells. The events primarily responsible for mast cell proliferation in mastocytosis are largely unknown, but a derangement of the network involving c-kit receptor and its natural ligand (stem cell factor, which promotes mast cell growth and differentiation in man) is likely to have a primary role in this disease. Mastocytosis comprises a wide spectrum of clinical conditions determined by the degree of mast cell proliferation, the organ systems involved, the age at onset and the association with hematologic diseases. Mastocytosis can occur in a pediatric or an adult form. In both groups of patients, the disease may be limited to the skin (cutaneous mastocytosis) or be systemic, involving predominantly the bone marrow and the gastrointestinal tract. The symptoms in patients with mastocytosis are generally related to the increased release of mast-cell-derived mediators, such as histamine, prostaglandin D2, peptide leukotrienes, platelet-activating factor, heparin and proteolytic enzymes. The measurement of these chemical mediators (histamine, tryptase and prostaglandin D2 and their metabolites) in body fluids is useful for the diagnosis and the laboratory evaluation of patients with systemic mastocytosis. As little is known about the pathogenesis of the different forms of mastocytosis, the treatment of the majority of these patients is largely symptomatic.

Recombinant human stem cell factor does exert minor stimulation of growth in small cell lung cancer and melanoma cell lines

We have previously reported on the stimulation of clonal growth of a glioblastoma cell line by rhSCF (Berdel et al., Cancer Res 1992, 52, 3498-3502). Within an extensive screening programme of haematopoietic growth factor activity on malignant cells, the effects of rhSCF were further tested on the growth of 29 different human cell lines derived from a wide range of solid tumours, among them six lung cancers and five melanomas. RhSCF (0, 1, 10, 100 ng/ml) was tested in a human tumour cloning assay (HTCA) which reliably detects growth modulation of tumour cells by cytokines. Additionally, a tritiated thymidine uptake test was used. Growth of 27 of the 29 cell lines tested was not affected by rhSCF. However, growth of the small cell lung cancer (SCLC) cell line HTB 120 was slightly stimulated (1.5 fold that of controls), and that of the melanoma cell line MeWo was stimulated up to 1.3-fold. This activity was eliminated dose-dependently by the tyrosine kinase inhibitor, genistein. We further analysed the cell lines for expression of the proto-oncogene C-KIT and its ligand SCF. All melanoma and lung cancer cell lines expressed SCF as assessed at the mRNA level. Northern blotting also revealed clear C-KIT mRNA expression in three melanoma (HAS, MeWo, SK-MEL-28), one NSCLC (HTB 53), and four SCLC cell lines (HTB 119, HTB 120, HTB 171, HTB 175). Furthermore, C-KIT protein expression was detected by flow cytometric analysis on the cell surface of MeWo, HTB 119 and HTB 120 cells. Our data indicate that SCF can be operative in growth modulation of non-haematopoietic malignant cells, especially SCLC and melanoma. However, our extensive screening of SCF/tumour cell interaction shows that this interaction is rare and makes potential hazards, such as tumour stimulation upon clinical use of rhSCF in conjunction with chemotherapy in cancer patients, unlikely for the majority of other tumour histologies.

Selective potentiation of IGE-dependent histamine release from rat peritoneal mast cells by stem cell factor

Effect of stem cell factor on histamine release from rat peritoneal mast cells was studied. Although stem cell factor did not evoke histamine release by itself, it clearly potentiated histamine release from sensitized mast cells caused by antigen, anti-IgE and concanavalin A. However, stem cell factor did not affect histamine release caused by compound 48/80, calcium ionophore A23187 and substance P. Although maximum potentiation of antigen-induced histamine release by stem cell factor was accomplished after 1-10 minute-preincubation, potentiation was decline after a longer incubation period. Potentiation of histamine release by phosphatidylserine and non-mast cells in the rat peritoneal cavity was incubation time-dependent. Potentiation by stem cell factor was additive to that by phosphatidylserine or non-mast cells. These results indicate that stem cell factor selectively potentiates IgE-dependent histamine release from rat peritoneal mast cells, and suggest that the mechanism involved is distinct from that of phosphatidylserine or non-mast cells in the rat peritoneal cavity.

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.

Isolation and characterization of a monoclonal antibody binding to the extracellular domain of the flk-2 tyrosine kinase receptor

The flk-2 tyrosine kinase receptor is expressed on hematopoietic stem cells and on acute leukemias (AML and ALL). We have isolated a rat monoclonal antibody (71E1) that binds to this receptor with a relative affinity of 5 nM. The antibody immunoprecipitates both murine and human forms of flk-2 and can block receptor activation by its cognate ligand. In addition, 71E1 inhibits the in vitro proliferation of the murine leukemic cell line, M1, that expresses high levels of flk-2. These results suggest that 71E1 may have utility as both a reagent for elucidating the biological role of flk-2 in hematopoiesis and as an immunotherapeutic in the treatment of acute leukemias.

The effect of mast cell growth factor on peripheral blood granulocyte-macrophage colony-forming cells in methylcellulose in myeloproliferative disorders

Clonogenic cell culture assay was used to evaluate the effect of mast cell growth factor (MGF) on peripheral blood granulocyte-macrophage (GM) progenitors in 26 patients with myeloproliferative disorders (MPDs). MGF alone had a statistically significant stimulatory effect on GM colony formation, as also did interleukin-3 (IL-3) and GM colony-stimulating factor (GM-CSF), although the progenitors could form colonies spontaneously as well. When MGF was combined with either IL-3 or GM-CSF the effect was additive and was as great as that achieved with a mixture of IL-3, GM-CSF, G-CSF and IL-6. The highest colony-forming capacity of all was seen when MGF was added to the above mixture. Within the subgroups of MPDs, the stimulatory effect of MGF was significant in polycythemia vera (PV), essential thrombocythosis (ET) and chronic myelogenous leukemia (CML). MGF was the most potent single factor in PV, while GM-CSF was most effective in idiopathic myelofibrosis and both IL-3 and GM-CSF in CML. The fact that the ability of MGF to induce colony growth varied between the subgroups of MPDs may mean that the target progenitors in these diseases are biologically different. In conclusion, MGF, either alone or with others, was a potent growth factor for GM progenitors in MPDs.

Expression of stem cell factor in cutaneous mastocytosis

Stem cell factor has recently been identified as a potent growth factor for bone marrow stem cells, melanocytes and mast cells. In order to evaluate its possible role in human mastocytosis, skin lesions from 13 patients with urticaria pigmentosa and five patients with mastocytomas, and normal skin specimens from five healthy donors were studied by immunohistochemistry, using polyclonal and monoclonal (hkl-12) antibodies against stem cell factor, and a monoclonal antibody (YB5.B8) against its receptor, the c-kit proto-oncogene product. Stem cell factor expression was noted in all sections studied, with an equal distribution pattern for both antibodies, but a weaker intensity with the hkl-12 reagent. Cytoplasmic staining was noted in keratinocytes, Langerhans cells, sweat gland ductal lining cells, mast cells, endothelial cells and spindle-shaped dermal stromal cells. An intense, diffusely granular reaction pattern was noted in all cells, except for a sparse, coarsely granular pattern in mast cells and stromal cells. In urticaria pigmentosa, staining was weaker in keratinocytes, but more prominent in Langerhans cells. In all sections, toluidine blue-positive mast cells and TA 99-positive basal epidermal melanocytes were the only cells to react with the c-kit antibody. Mastocytomas and urticaria pigmentosa lesions thus exhibit different patterns of stem cell factor expression. However, a possible pathogenetic role of this factor in mastocytosis remains to be determined.

Regulation of cell-matrix adhesion by receptor tyrosine kinases

Cell-cell and cell-matrix adhesive interactions mediated by integrins play crucial roles in leukocyte migration to inflamed tissues, and also in cell migration during embryogenesis. Much remains to be learned about the molecular mechanisms of regulation of adhesion mediated by integrins. Recently we found that steel factor and c-kit induce adhesion to fibronectin by VLA-5 in mast cells. Activation of adhesiveness is transient, and occurs at concentrations of steel factor 100-fold lower than required for growth stimulation. This suggests that regulation of adhesion is an important biological function of steel factor and c-kit. Other receptor tyrosine kinases such as the PDGF receptor can substitute for c-kit. Signaling through receptor tyrosine kinases may offer a general mechanism for the regulation of integrin avidity.

Synergistic effects of stem cell factor and interleukin 6 or interleukin 11 on the expansion of murine hematopoietic progenitors in liquid suspension culture

The synergistic effects of stem cell factor (SCF) in combination with other growth factors including interleukin (IL)-6, IL-11, IL-3, GM-CSF, G-CSF, IL-1 alpha and interferon-gamma (IFN-gamma) on the expansion of murine hematopoietic progenitors were studied in a short-term liquid suspension culture system. Bone marrow (BM) cells obtained 2 days after 5-fluorouracil (5-FU) injection were cultured for up to 18 days in serum-containing and serum-free cultures in the presence of combinations of various cytokines. The numbers of nucleated cells, total colony-forming cells (CFC), mixed-colony forming units (CFU-Mix) and high-proliferative potential colony-forming cells (HPP-CFC) before and after liquid suspension cultures were measured in the presence of different combinations of cytokines. Combinations of SCF with IL-11, IL-6 or IL-1 alpha markedly increased the numbers of total CFC, CFU-Mix and HPP-CFC. A combination of SCF and IL-3 also expanded the number of total CFC; however, the fold increase was smaller than those of SCF plus IL-11, IL-6 or IL-1 alpha. Three or four factor combinations including SCF with IL-3, IL-6 and IL-11 did not yield increased numbers of total CFC over that supported by SCF plus either IL-6 or IL-11. The addition of IFN-gamma to the culture containing SCF plus IL-11 resulted in a decrease of the expansion efficiency. However, this difference is not statistically significant. In contrast, the addition of IFN-gamma to the cultures containing SCF plus IL-6 did not affect the expansion efficiency. Interestingly, the addition of IL-1 alpha in the culture containing SCF plus IL-3 significantly increased the number of HPP-CFC over that supported by SCF plus IL-3 (p < 0.01). In contrast, IL-1 alpha did not significantly affect the expansion efficiency in the presence of SCF plus IL-6 or IL-11. These results suggest that combinations of SCF plus either IL-6 or IL-11 or a combination of SCF, IL-3 and IL-1 alpha can most effectively expand murine hematopoietic progenitors derived from day-2 post-5-FU BM cells in vitro.

c-KIT expression enhances the leukemogenic potential of 32D cells

The growth of human leukemic cells in culture and in vivo is dependent upon the presence of hematopoietic growth factors. Most populations of human leukemic acute myeloblastic leukemia (AML) cells express c-Kit on their surface and respond to Kit ligand (KL) in culture. To determine if this interaction was of potential significance in vivo we used a mouse model system. 32D cells, a murine IL-3-dependent myeloid cell line, were rendered KL responsive by transfection of the murine c-Kit. After injection of 32D or 32D-Kit cells into syngeneic hosts, animals bearing 32D-Kit cells, but not 32D cells, became moribund and were killed. These animals had circulating leukemic blast cells, infiltration of bone marrow, spleen, brain, liver, lung, and kidney. Cells recovered from some of the animals continued to be dependent upon IL-3 or KL for growth while in other cases the cells were factor independent. This model illustrates that the constitutive expression of c-Kit enhances the leukemic potential of 32D cells. The model will be useful for studying the progression of leukemia in vivo and testing whether interruption of the interaction of Kit and KL can affect the growth of leukemic cells.

The effect of stem-cell factor, interleukin-3 and erythropoietin on in vitro erythropoiesis in myelodysplastic syndromes

An inherent defect of erythroid differentiation at the colony-forming unit blast (CFU-blast) compartment and (or) an impaired response of early erythroid progenitors (BFU-E) to growth stimulation are both considered to contribute to anemia in myelodysplastic syndromes (MDS). With the intention of improving survival and growth of early erythroid progenitors we investigated the effect of stem-cell factor (SCF) and interleukin-3 (IL-3) alone and in combination with erythropoietin, on the in vitro erythropoiesis of 13 patients with MDS and of three normal controls. SCF and IL-3 alone did not promote erythroid colony growth in MDS, while 3 cases responded to erythropoietin alone. In each of these, BFU-E colony growth could be increased by SCF, which was also found in all normal bone marrows. Altogether 6 cases showed a significant enhancement of BFU-E colony numbers by the combination of SCF and erythropoietin as compared to erythropoietin alone (P = 0.036). Out of the 6 responding cases, 5 belonged to the FAB-classified subgroups refractory anemia (RA) and refractory anemia with ringed sideroblasts (RA/RS) (5/5), while 1 patient was classified as having refractory anemia with excess of blasts (RAEB) (1/4). No patient with refractory anemia with excess of blasts in transformation (RAEB-T) (0/4) responded. In spite of these positive effects, the absolute number of BFU-E colonies remained reduced in all MDS cases when compared to normal controls. IL-3 proved ineffective in increasing the response to erythropoietin in MDS although it increased erythropoietin-induced BFU-E formation in normal controls significantly. We conclude that the striking synergistic effect of SCF and erythropoietin on erythroid colony formation seen with normal bone marrow is conserved in most cases with RA and RA/RS. In RAEB and RAEB-T the intrinsic defect of the erythroid differentiation pathway cannot be overcome by SCF.

TGF alpha can act as a chemoattractant to perioptic mesenchymal cells in developing mouse eyes

Growth factors are believed to play an important role in regulating cell fate and cell behavior during embryonic development. Transforming growth factor alpha (TGF alpha), a member of the epidermal growth factor (EGF) superfamily, is a small polypeptide growth factor. Upon binding to its receptor, the EGF receptor (EGFR), TGF alpha can exert diverse biological activities, such as induction of cell proliferation or differentiation. To explore the possibility that TGF alpha might regulate cell fate during murine eye development, we generated transgenic mice that express human TGF alpha in the lens under the control of the mouse alpha A-crystallin promoter. The transgenic mice displayed multiple eye defects, including corneal opacities, cataracts and microphthalmia. At early embryonic stages TGF alpha induced the perioptic mesenchymal cells to migrate abnormally into the eye and accumulate around the lens. In situ hybridization revealed that the EGFR mRNA is highly expressed in the perioptic mesenchyme, suggesting that the migratory response is mediated by receptor activation. In order to test this model, the TGF alpha transgenic mice were bred to EGFR mutant waved-2 (wa-2) mice. We found that the eye defects of the TGF alpha transgenic mice are significantly abated in the wa-2 homozygote background. Because the EGFR mutation in the wa-2 mice is located in the receptor kinase domain, this result indicates that the receptor tyrosine kinase activity is critical for signaling the migratory response. Taken together, our studies demonstrate that TGF alpha is capable of altering the migratory decisions and behavior of perioptic mesenchyme during eye development.

Steel and c-kit in the development of avian melanocytes: a study of normally pigmented birds and of the hyperpigmented mutant silky fowl

We describe here the expression of c-kit and Steel (Sl) genes during the development of melanocytes in normally pigmented strains of chick and quail compared to unpigmented (White Leghorn) and hyperpigmented (Silky Fowl) strains of chickens. By using the quail/chick chimera system, we found that the neural crest cells, which migrate dorso-laterally in the subectodermal mesenchyme to give rise to the melanocytes, express c-kit as early as E4, that is about 2 days after they have left the neural primordium. The Sl gene is expressed from E4 onward in the epidermis but not at all in the dermis at any developmental stage. As feather buds develop, Sl mRNA becomes restricted to the apical region of the feather filaments. During formation of the barbs and barbules of the down feather, production of the Steel factor is restricted to the external epidermal cells of the barbules. The cell bodies of the c-kit-positive melanocytes are then located in the internal border of the epidermal ridges and extend their processes toward the source of the Steel factor. We propose that the spatial restriction of Sl gene activity at that stage accounts for the morphology of the melanocytes and their vectorial secretion of melanin to the external barbule cells. As a whole, these results show that during skin development c-kit positive cells are present in the Steel factor-producing areas at the time when melanoblasts proliferate and differentiate. Interestingly, in the mouse, previous studies showed that the Sl gene is activated in the dermis where melanoblasts undergo most of their expansion (Nishikawa et al. [1991] EMBO J. 10:2111-2118). In the unpigmented and hyperpigmented mutants that we studied, expression of the Sl message, as judged quantitatively in Northern blots (for the SF embryos) or spatially by in situ hybridization, is similar to that observed in normal birds. In SF embryos the c-kit expressing melanoblasts migrate initially in the dorso-lateral migration pathway as in normal birds. However their number increases considerably in the dermis from E5 onward. From E7, they invade mesodermally derived organs that do not express the Sl gene. This suggests that another, still unknown, factor(s) is responsible for the survival, the proliferation, and the extensive spreading of melanocytic cells within the mesoderm of this mutant.

Hematologic effects of stem cell factor (SCF) and leukemia inhibitory factor (LIF) in vivo: LIF-induced thrombocytosis in SCF-primed mice

Stem cell factor (SCF) administered as daily bolus injections in dose-response experiments in mice causes a progressive and dramatic dose-dependent panleukocytosis characterized by neutrophilia, eosinophilia, monocytosis, and lymphocytosis. SCF causes circulating platelet numbers to be dose-dependently increased after 2 weeks of daily injections. Leukemia inhibitory factor (LIF) administered as daily bolus injections in mice causes a peripheral leukopenia that is largely due to peripheral lymphopenia. LIF causes thrombocytosis peaking after approximately 1 w. Coinjection of SCF and LIF for 1 to 2 wk in mice does not cause a much greater thrombocytosis than the maximum thrombocytosis achievable with SCF or LIF alone. On the other hand, daily injection of SCF for 5 days followed by daily injection of LIF for 5 to 6 d in mice causes a very substantial increase in platelets that was lineage-specific in terms of not being accompanied by a generalized leukocytosis. In contrast, only a very modest thrombocytosis was noted in SCF-primed LIF-treated rats. LIF causes a large increase in the cytoplasmic volume of splenic megakaryocytes in mice, but not in rats. In conclusion, SCF-induced priming followed by LIF-induced maturation of megakaryocytes causes a substantial selective increase in the numbers of circulating platelets in mice.

Turtle lung cells produce a melanization-stimulating activity that promotes melanocytic differentiation of avian neural crest cells

We found previously that neural crest cells in turtle embryos migrated into the lung buds and melanocytes were located in the lungs. The finding suggested to us that the lungs provide a stimulatory factor(s) to the differentiation of neural crest cells into melanocytes. We have established lung cell lines to facilitate analysis of the interactions of neural crest cells with the environment in melanocyte development. One cell line, TLC-2, was found to produce a putative melanization-stimulating activity (MSA), which promoted the melanocyte differentiation in vitro of avian neural crest cells. The TLC-2-derived MSA was different from that of basic fibroblast growth factor (bFGF), alpha-melanocyte stimulating hormone (alpha-MSH), and steel factor (SLF). Its molecular weight was estimated to be within the range of 150 kD. Our findings suggest that MSA may be a novel factor exercising a positive control over melanocyte differentiation.

The FLT3 ligand potently and directly stimulates the growth and expansion of primitive murine bone marrow progenitor cells in vitro: synergistic interactions with interleukin (IL) 11, IL-12, and other hematopoietic growth factors

The recently cloned murine flt3 ligand (FL) was studied for its ability to stimulate the growth of primitive (Lin-Sca-1+) and more committed (Lin-Sca-1-) murine bone marrow progenitor cells, alone and in combination with other hematopoietic growth factors (HGFs). Whereas FL was a weak proliferative stimulator alone, it potently synergized with a number of other HGFs, including all four colony-stimulating factor (CSF), interleukin (IL) 6, IL-11, IL-12, and stem cell factor (SCF), to promote the colony formation of Lin-Sca-1+, but not Lin-Sca-1- or erythroid progenitor cells. The synergistic activity of FL was concentration dependent, with maximum stimulation occurring at 250 ng/ml, and was observed when cells were plated at a concentration of one cell per culture, suggesting that its effects are directly mediated. 2 wk of treatment with FL in combination with IL-3 or SCF resulted in the production of a high proportion of mature myeloid cells (granulocytes and macrophages), whereas the combination of FL with G-CSF, IL-11, or IL-12 resulted predominantly in the formation of cells with an immature blast cell appearance. Accordingly, FL in combination with G-CSF or IL-11 expanded the number of progenitors more than 40-fold after 2 wk incubation. Thus, FL emerges as a potent synergistic HGF, that in combination with numerous other HGFs, can directly stimulate the proliferation, myeloid differentiation, and expansion of primitive hematopoietic progenitor cells.

Soluble c-kit levels in acute GVHD after allogeneic bone marrow transplantation

Serum soluble c-kit concentrations were measured in 11 patients with or without acute graft-versus-host disease (GVHD) after allogeneic bone marrow transplantation. The serum soluble c-kit levels in the patients with moderate to severe acute GVHD (grade II-IV) were significantly lower than those in the patients with no or mild acute GVHD (grade 0-I) following the onset. The data from this study indicate that measurement of serum soluble c-kit concentration is a useful indicator of severe acute GVHD.

A Xenopus c-kit-related receptor tyrosine kinase expressed in migrating stem cells of the lateral line system

The mammalian c-kit receptor tyrosine kinase gene is required during embryogenesis for the survival and/or proliferation of three migrating stem cell populations: primordial germ cells, haematopoietic stem cells and neural crest-derived melanoblasts. We have cloned a Xenopus gene, XKrk1, whose closest relative is c-kit. Differences in the expression pattern suggest that XKrk1 is not the Xenopus homologue of c-kit; however, it is expressed in a migrating stem cell population, the precursor cells for the mechanosensory lateral line system. XKrk1 is the first reported marker for lateral line stem cells.

AM5, a protein-associated polysaccharide, stimulates hematopoiesis and modulates the expression of endogenous hematopoietic growth factors in murine long-term bone marrow cultures

We have investigated the mechanism behind the hematopoietic stimulation mediated by AM5, a protein-associated polysaccharide that stimulates in vivo the murine hematopoietic system. A dose-dependent increase in hematopoietic progenitors was observed in long-term bone marrow cultures (LTBMCs) treated in vitro with AM5. The stimulatory effect was more marked in colony-forming units-granulocyte-macrophage (CFU-GM) than in CFU-spleen (CFU-S) progenitors and also more significant in the supernatant than in the adherent layer. This stimulatory effect was reversible, and continuous stimulation with high doses of AM5 was conductive to a progressive exhaustion of the culture. The analysis of the CFU-GM stimulating activity present in the supernatant of AM5-treated cultures revealed a dose-dependent induction of granulocyte-macrophage colony-stimulating activity (GM-CSA), in contrast with control cultures in which no CSA was detected. Northern blots of LTBMC-adherent layers obtained after in vitro treatment with AM5 revealed a significant mRNA expression for interleukin 1 alpha (IL-1 alpha), granulocyte-macrophage colony-stimulating factor (GM-CSF), macrophage CSF (M-CSF) and granulocyte CSF (G-CSF), in contrast with adherent layers from untreated cultures which only expressed, in detectable levels, M-CSF and stem cell factor (SCF). The SCF expression was down-modulated in AM5-treated cultures. Our results strongly suggest that the hematopoietic stimulation induced by AM5 is mediated by the modulated expression of endogenous hematopoietic growth factors.

Ultrastructural analysis of the development of human basophils and mast cells in vitro

The ultrastructural analysis of a variety of culture systems of human cord blood mononuclear cells (spanning a 10-year research effort) is reviewed. Human basophils, eosinophils and mast cells reliably developed from their agranular precursors that are present in human cord blood. Suspension cultures and cocultures with fibroblasts were used to examine the effects on differentiation and maturation of full (fibroblast), interleukin-2-depleted (human T cells), and murine inducer T cell culture supernatants, partially purified mouse fibroblast factor(s), recombinant human interleukins 3 and 5, and recombinant human and murine c-kit ligands (stem cell factor, mast cell growth factor). Together, these studies allowed us to define the differentiation and full maturation of the basophil and eosinophil lineages and provided evidence for the induction of a form of secretion (termed piecemeal degranulation) of the basophil and eosinophil lineages in interleukin-3- or -5-supplemented cultures. Mast cells were absent from interleukin-3- or -5-containing cultures. The development of fully mature mast cells occurred regularly in fibroblast-containing cocultures; partially mature mast cells developed in fibroblast culture supernatant-, partially purified mouse fibroblast factor(s)-, and either recombinant human or murine c-kit ligand-supplemented suspension cultures. Small numbers of basophils and eosinophils were present in the suspension cultures that received c-kit ligand in its recombinant or naturally occurring forms. Ultrastructural immunogold analyses confirmed that basophils and eosinophils contained the Charcot-Leyden crystal protein (in different subcellular locations) but that mast cells did not. In both cocultures and suspension cultures, the primary event recorded for mast cells was that of differentiation and maturation, with the ultrastructural correlates of synthetic activity and granule building prevailing. Spontaneous secretory events, recognizable by ultrastructural analysis, were not evident in either mature or partially mature mast cells developing in these cultures.

Extramedullary erythropoiesis in human liver grafts

Extramedullary erythropoiesis in the adult is very rare and is generally confined to situations of severe bone marrow irritation or replacement. In this study, we describe the occurrence of intrahepatic erythropoiesis in patients who have received a liver allograft and who have no evidence of bone marrow dysfunction. By routinely performed transplant aspiration cytology (TAC), marked intrahepatic erythropoiesis could be detected in 39 of 312 patients (12.5%) with liver allograft. In 19 patients, including 5 of 8 (63%) after combined liver and kidney transplantation, intrahepatic erythropoiesis occurred within the first 3 weeks after surgery. Twenty patients showed intrahepatic erythropoiesis between 3 weeks and 4 months after transplantation. Erythropoiesis was usually transient, lasting between 1 and 3 weeks. Cytologically, mature as well as immature erythroblasts of GlyA+ CD36+ CD45- phenotype could be detected in the grafts, whereas they were absent in blood; histologically, the cells could be localized to the sinusoids of the liver. There was no clear correlation of preoperative or postoperative hemoglobin levels, graft function, kidney function, and immunosuppressive medication with the presence or absence of erythropoiesis. Moreover, serum levels of erythropoietin (EPO) and stem cell factor (SCF) in patients with and without intrahepatic erythropoiesis in the early postoperative phase did not show significant differences. These findings show that intrahepatic erythropoiesis can occur transiently in human liver allografts and suggest that systemic stimuli as well as local factors may contribute to it.

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.