Mast cell growth factor (c-kit ligand) supports the growth of human multipotential progenitor cells with a high replating potential

The replating capability of human multipotential (colony-forming unit-granulocyte-erythrocyte-macrophage-megakaryocyte [CFU-GEMM]) and erythroid (burst-forming unit-erythroid [BFU-E]) progenitors was assessed in vitro as a potential measure of self-renewal using purified, recombinant (r) human (hu) or murine (mu) mast cell growth factor (MGF), a ligand for the c-kit proto-oncogene receptor. Primary cultures of human umbilical cord blood or adult human bone marrow cells were initiated in methylcellulose with erythropoietin (Epo) alone or in combination with rhu interleukin-3 (IL-3) or MGF. Individual day 14 to 18 CFU-GEMM or BFU-E colonies were removed from primary cultures and reseeded into secondary methylcellulose cultures containing a combination of Epo, MGF, and rhu granulocyte-macrophage colony-stimulating factor (GM-CSF). The data showed a high replating efficiency of cord blood and bone marrow CFU-GEMM in response to Epo + MGF in terms of the percentage of colonies that could be replated and the number of secondary colonies formed per replated primary colony. The average number of hematopoietic colonies and clusters apparent from replated cultures of cord blood or bone marrow CFU-GEMM stimulated by Epo + MGF was greater than with Epo + rhuIL-3 or Epo alone. Replated cord blood CFU-GEMM gave rise to CFU-GEMM, BFU-E, and GM colony-forming units (CFU-GM) in secondary cultures. Replated bone marrow CFU-GEMM gave rise mainly to CFU-GM in secondary cultures. A more limited capacity for replating of cord blood and bone marrow BFU-E was observed. These studies show that CFU-GEMM responding to MGF have an enhanced replating potential, which may be promoted by MGF. These studies also support the concept that MGF acts on more primitive progenitors than IL-3.

Diamond-Blackfan anemia: heterogenous response of hematopoietic progenitor cells in vitro to the protein product of the steel locus

Diamond-Blackfan anemia is a congenital disorder of erythropoiesis in humans, characterized by a macrocytic anemia often associated with physical anomalies. Mutations at either the W or Steel loci in the mouse also leads to a severe macrocytic anemia, as well as other developmental abnormalities. The W locus encodes the proto-oncogene c-kit, a member of the receptor tyrosine kinase family, while the Steel locus encodes a potent hematopoietic growth factor that is the ligand for c-kit. Growth of clonogenic marrow erythroid progenitor cells in vitro in the presence of the recombinant hematopoietic growth factors interleukin-3 (IL-3) and Steel was used to characterize this disease at the cellular level. Three patterns of in vitro marrow response to both recombinant IL-3 or Steel were observed among 10 Diamond-Blackfan patients: those that responded quantitatively and qualitatively almost as well as cells from normal marrow, those that responded at an intermediate level, and those that did not respond at all. These results provide evidence for cellular heterogeneity underlying the pathogenesis of this disorder and therefore raise the possibility that there may be more than one underlying molecular basis for the disease. No gross abnormalities in the structure of either the c-kit or Steel loci were observed in these patients. The normal response in culture of the progenitor cells from at least some patients to Steel with or without IL-3 raises the possibility of using this novel growth factor as a therapeutic agent in Diamond-Blackfan anemia.

Hybrid cytokines as hematopoietic growth factors

A large body of in vitro and in vivo data suggests that combinations of cytokines provide the most effective mechanism for stimulating multilineage acceleration of hematopoiesis. Creation of a granulocyte-macrophage colony-stimulating factor (GM-CSF)/interleukin 3 (IL-3) fusion protein has yielded a single therapeutic which has enhanced biological activity in comparison to the individual cytokines from which it is composed. In vivo studies with this fusion protein (PIXY321) suggest that it may provide a means to accelerate both neutrophil and platelet recovery in clinical settings in which hematopoiesis is suppressed. The biology of PIXY321 and the potential for other fusion proteins is discussed.

Cytokine regulation of hematopoietic stem cells

Regulation of hematopoietic stem cell proliferation and differentiation is likely to be controlled by the local concentration of both inhibitory and stimulatory cytokines. A newly described regulator of early haematopoietic cells called mast cell growth factor, stem cell factor or kit ligand (referred to here as the Sl factor) has been described. This factor is the gene product of the murine Steel locus and is the ligand for the c-kit proto-oncogene, the product of the murine W locus. The effects of Sl factor on primitive hematopoietic cells suggest that this growth factor is a major stimulator of basal hemopoiesis. Further, data indicates that Sl factor acts in synergy with virtually all of the later acting growth factors to enhance the proliferative and differentiative potential of these cells.

Macrophage inflammatory protein (MIP)-1 beta abrogates the capacity of MIP-1 alpha to suppress myeloid progenitor cell growth

The effects of recombinant murine macrophage inflammatory protein (MIP)-1 beta and MIP-2 on the suppressive activity of MIP-1 alpha were tested using colony formation by human and murine bone marrow burst-forming unit-erythroid (BFU-E), colony-forming unit-granulocyte erythroid macrophage, megakaryocyte (CFU-GEMM), and colony-forming unit-granulocyte macrophage (CFU-GM) progenitor cells. MIP-1 beta, but not MIP-2, when added with MIP-1 alpha to cells, blocked the suppressive effects of MIP-1 alpha on both human and murine BFU-E, CFU-GEMM, and CFU-GM colony formation. Similar results were observed regardless of the early acting cytokines used: human rGM-CSF plus human rIL-3, and two recently described potent cytokines, a genetically engineered human rGM-CSF/IL-3 fusion protein and MGF, a c-kit ligand. The more potent the stimuli, the greater the suppressive activity noted. Pulse treatment of hu bone marrow cells with MIP-1 alpha at 4 degrees C for 1 h was as effective in inhibiting colony formation as continuous exposure of cells to MIP-1 alpha, and the pulsing effect with MIP-1 alpha could not be overcome by subsequent exposure of cells to MIP-1 beta. Also, pulse exposure of cells to MIP-1 beta blocked the activity of subsequently added MIP-1 alpha. For specificity, the action of a nonrelated myelosuppressive factor H-ferritin, was compared. MIP-1 alpha and H-ferritin were shown to act on similar target populations of early BFU-E, CFU-GEMM, and CFU-GM. MIP-1 beta did not block the suppressive activity of H-ferritin. Also, hemin and an inactive recombinant human H-ferritin mutein counteracted the suppressive effects of the wildtype H-ferritin molecule, but did not block the suppressive effects of MIP-1 alpha. These results show that MIP-1 beta's ability to block the action of MIP-1 alpha is specific. In addition, the results suggest that MIP-1 alpha and MIP-beta can, through rapid action, modulate early myeloid progenitor cell proliferation.

Evidence for complex formation between rabbit lung flavin-containing monooxygenase and calreticulin

Rabbit lung flavin-containing monooxygenase (FMO, EC 1.14.13.8) was denatured, reduced, carboxymethylated, digested with endoproteinase Glu-C or trypsin, and subjected to mass spectrometric analysis. The amino acid sequences of selected peptides were determined by tandem mass spectrometry. Over 90% of rabbit lung FMO was mapped by liquid secondary ion mass spectrometry (LSIMS). The FMO N-terminal amino acid was found to be N-acetylated, and the N-terminal 23 amino acid peptide contained an FAD binding domain consisting of Gly-X-Gly-X-X-Gly. Another peptide was found to contain a NADP+ binding domain consisting of Gly-X-Gly-X-X-Ala. The mapped and/or sequenced peptides were found to be completely consistent with the peptide sequence deduced from the cDNA data and the previously published gas-phase sequencing data. Further mass spectrometry and protein analytical work unambiguously showed that rabbit lung FMO existed in tight association with a calcium-binding protein, calreticulin. Over 68% of rabbit lung calreticulin was mapped by LSIMS. Tandem mass spectrometric and gas-phase sequencing studies provided direct evidence for the identification of the N-terminal and other rabbit lung calreticulin-derived peptide sequences that were identical to other previously reported calreticulins. The complexation of calreticulin to rabbit lung FMO could account for some of the unusual physical properties of this FMO enzyme form.

Macrophage inflammatory protein (MIP)-1 beta abrogates the capacity of MIP-1 alpha to suppress myeloid progenitor cell growth

The effects of recombinant murine macrophage inflammatory protein (MIP)-1 beta and MIP-2 on the suppressive activity of MIP-1 alpha were tested using colony formation by human and murine bone marrow burst-forming unit-erythroid (BFU-E), colony-forming unit-granulocyte erythroid macrophage, megakaryocyte (CFU-GEMM), and colony-forming unit-granulocyte macrophage (CFU-GM) progenitor cells. MIP-1 beta, but not MIP-2, when added with MIP-1 alpha to cells, blocked the suppressive effects of MIP-1 alpha on both human and murine BFU-E, CFU-GEMM, and CFU-GM colony formation. Similar results were observed regardless of the early acting cytokines used: human rGM-CSF plus human rIL-3, and two recently described potent cytokines, a genetically engineered human rGM-CSF/IL-3 fusion protein and MGF, a c-kit ligand. The more potent the stimuli, the greater the suppressive activity noted. Pulse treatment of hu bone marrow cells with MIP-1 alpha at 4 degrees C for 1 h was as effective in inhibiting colony formation as continuous exposure of cells to MIP-1 alpha, and the pulsing effect with MIP-1 alpha could not be overcome by subsequent exposure of cells to MIP-1 beta. Also, pulse exposure of cells to MIP-1 beta blocked the activity of subsequently added MIP-1 alpha. For specificity, the action of a nonrelated myelosuppressive factor H-ferritin, was compared. MIP-1 alpha and H-ferritin were shown to act on similar target populations of early BFU-E, CFU-GEMM, and CFU-GM. MIP-1 beta did not block the suppressive activity of H-ferritin. Also, hemin and an inactive recombinant human H-ferritin mutein counteracted the suppressive effects of the wildtype H-ferritin molecule, but did not block the suppressive effects of MIP-1 alpha. These results show that MIP-1 beta's ability to block the action of MIP-1 alpha is specific. In addition, the results suggest that MIP-1 alpha and MIP-beta can, through rapid action, modulate early myeloid progenitor cell proliferation.

Leukemia inhibitory factor improves survival of retroviral vector-infected hematopoietic stem cells in vitro, allowing efficient long-term expression of vector-encoded human adenosine deaminase in vivo

Low recovery and poor retroviral vector infection efficiency of hematopoietic stem cells has hindered application of gene therapy for disease affecting blood-forming tissues. Developmental restriction (or death) of stem cells during ex vivo infection has contributed to these difficulties. In these studies we report that the cytokine leukemia inhibitory factor (LIF) directly or indirectly supported the survival of hematopoietic stem cells during culture of bone marrow with vector-producing fibroblasts, resulting in efficient recovery of stem cells able to compete for engraftment in irradiated recipient animals. The infection efficiency of hematopoietic stem cells recovered from these cultures was approximately 80%; and all recipients (20/20) of the LIF-treated marrow were stably engrafted with the progeny of provirus-bearing stem cells. Expression of vector-encoded human adenosine deaminase (hADA) was detected in all recipients at levels averaging 15-50% of endogenous murine ADA in all their hematolymphoid tissues. Survival of stem cells in untreated cultures was approximately 10% of that observed from LIF-treated cultures, resulting in poor engraftment of recipient animals with transplanted cells. The infection efficiency of the few stem cells recovered from untreated cultures, however, was high (approximately 80%), suggesting that LIF did not have an effect on infection efficiency per se, but acted at the level of stem cell survival. Consistent with the poor engraftment observed in the control animals, expression of vector-encoded ADA was only approximately 4-20% of the endogenous levels. These results support the postulated role of LIF as a regulator of hematopoiesis and suggest that cytokine stimulation can positively affect inefficient retroviral vector transduction in hematopoietic stem cells.

Embryonic RNA expression patterns of the c-kit receptor and its cognate ligand suggest multiple functional roles in mouse development

Mutations at the dominant white spotting (W) and Steel (Sl) loci in mouse exert deleterious effects on three migratory cell lineages (primordial germ cells, melanocytes and hematopoietic stem cells) resulting in loss of pigmentation, reduced fertility and anemia. The W locus encodes the c-kit protein tyrosine kinase (TK) receptor. More recently, the Sl locus has been shown to encode a ligand for c-kit, which is variously known as mast cell growth factor (MGF), stem cell growth factor and c-kit ligand. Here we report an in situ hybridization analysis comparing the expression profiles of MGF and c-kit transcripts during mouse embryogenesis. The data are consistent with the c-kit receptor-ligand complex providing a homing mechanism during stem cell migration in early development and in stem cell proliferation, differentiation, or survival in late development. In the nervous system, an unexpected and complex pattern of expression is uncovered that suggests involvement of the W and Sl gene products in the organization of the neural tube and brain.

The effects of interleukin 7 (IL-7) on human bone marrow in vitro

Interleukin 7 (IL-7) stimulates the proliferation of pre-B cells from long-term murine lymphoid cultures and normal bone marrow. In addition, IL-7 stimulates the proliferation of murine T cells, including fetal and adult thymocytes as well as peripheral T cells. Flow cytometry and cell enumeration analyses were carried out on light-density human bone marrow cells incubated in the presence or absence of IL-7. The data showed no evidence for a proliferative effect of IL-7 on B-lineage cells expressing CD24 or on myeloid cells expressing CD15; however, IL-7 did stimulate the growth of T cells expressing CD3. After 16 days of stimulation the number of CD3+ cells in marrow cultures increased 350% in the presence of IL-7. In contrast, cultures incubated in the absence of IL-7 showed a 50% decrease in the number of T cells, with a preponderance of myeloid lineage cells. Flow cytometry indicated that cells from IL-7-stimulated cultures were mature T cells because they also expressed cell surface antigens for either CD4 or CD8. These studies show that in contrast to the murine system, IL-7 does not appear to stimulate the growth of human pre-B cells from adult human bone marrow. This is consistent with other experiments that suggest that human pro-B cells and not human pre-B cells respond to IL-7. It appears that IL-7 preferentially promotes the growth of T cells from human marrow.

Signal transduction by normal isoforms and W mutant variants of the Kit receptor tyrosine kinase

Germline mutations at the Dominant White Spotting (W) and Steel (Sl) loci have provided conclusive genetic evidence that c-kit mediated signal transduction pathways are essential for normal mouse development. We have analysed the interactions of normal and mutant W/c-kit gene products with cytoplasmic signalling proteins, using transient c-kit expression assays in COS cells. In addition to the previously identified c-kit gene product (Kit+), a second normal Kit isoform (KitA+) containing an in-frame insertion, Gly-Asn-Asn-Lys, within the extracellular domain, was detected in murine mast cell cultures and mid-gestation placenta. Both Kit+ and KitA+ isoforms showed increased autophosphorylation and enhanced association with phosphatidylinositol (PI) 3' kinase and PLC gamma 1, when stimulated with recombinant soluble Steel factor. No association or increase in phosphorylation of GAP and two GAP-associated proteins, p62 and p190, was observed. The two isoforms had distinct activities in the absence of exogenous soluble Steel factor; Kit+, but not KitA+, showed constitutive tyrosine phosphorylation that was accompanied by a low constitutive level of association with PI-3' kinase and PLC gamma 1. Introduction of the point substitutions associated with W37 (Glu582----Lys) or W41 (Val831----Met) mutant alleles into c-kit expression constructs abolished (W37) or reduced (W41) the Steel factor-induced association of the Kit receptor with signalling proteins in a manner proportional to the overall severity of the corresponding W mutant phenotype. These data suggest a diversity of normal Kit signalling pathways and indicate that W mutant phenotypes result from primary defects in the Kit receptor that affect its interaction with cytoplasmic signalling proteins.

Requirement for mast cell growth factor for primordial germ cell survival in culture

Mast-cell growth factor (MGF) is encoded by the murine steel (Sl) locus and is a ligand for the tyrosine kinase receptor protein encoded by the proto-oncogene c-kit at the murine dominant white spotting (W) locus. Mutations at both these loci affect mast cells, primordial germ cells (PGCs), haemopoietic stem cells and melanocytes. In many Sl and W mutants, the rapid proliferation of PGC that normally occurs between day 7 and 13.5 of embryonic development fails to occur. As c-kit is expressed in PGCs while MGF is expressed in the surrounding mesenchyme, MGF might promote the proliferation of PGCs. Here we report that MGF is essential for PGC survival in culture, but does not stimulate PGC proliferation. Moreover, whereas both the transmembrane and soluble proteolytic cleavage forms of MGF stimulate mast-cell proliferation, soluble MGF has a relatively limited ability to support survival of PGCs in culture, thus explaining the sterility in mice carrying the steel-dickie (Sld) mutation, which encodes only a soluble form of MGF, and providing a functional role for a transmembrane growth factor.

Alternate splicing of mRNAs encoding human mast cell growth factor and localization of the gene to chromosome 12q22-q24

Human mast cell growth factor (MGF) complementary DNAs (cDNAs) were cloned from HeLa cells using the polymerase chain reaction with oligonucleotides corresponding to murine and human MGF sequences. Sequencing of the cloned human MGF polymerase chain reaction products revealed two types of cDNA: a full length form corresponding in size to the murine cDNA, and an alternately spliced clone with a deletion of the sixth exon of the gene. Since membrane-bound MGF is predicted to be proteolytically cleaved within the sequences encoded by exon 6 to generate a soluble protein, this alternately spliced cDNA would likely encode a noncleavable, membrane-bound form of MGF. No difference in biological activity on human bone marrow cells was observed with recombinant, soluble forms of both types of human MGF protein. Our previous localization of the murine MGF gene to the Sl locus on chromosome 10 suggested (via conserved linkage groups) that the human MGF gene would be located on human chromosome 12. Therefore, rodent-human somatic cell hybrids with or without an entire human chromosome 12 and hybrids retaining partial 12 were tested by Southern blot analysis and used to show the presence of the human Mgf locus at chromosome region 12q. Chromosomal in situ hybridization localized the gene to 12q22-q24 in the region predicted by the comparative mapping of the murine Mgf/Sl locus.

OCI/AML-4 an acute myeloblastic leukemia cell line: regulation and response to cytosine arabinoside

This paper describes the properties of a continuous cell line derived from the blast cells of a patient with acute myeloblastic leukemia (AML), secondary to the treatment of Hodgkin's disease. The line grows slowly without stimulation but responds to interleukin-3 (IL-3), GM-CSF and mast cell growth factor (MGF), a ligand for the receptor encoded by the c-kit oncogene. When OCI/AML-4 cells are exposed to MGF with IL-3 or GM-CSF, additive or synergistic effects are seen. Combinations of MGF and G-CSF, IL-6 or CSF-1 give less growth than MGF alone. OCI/AML-4 cells are sensitive to retinoic acid; a dose related decrease in clonogenic cells is observed when OCI/AML-4 cells are exposed to retinoic acid in suspension culture. OCI/AML-4 cells are sensitive to cytosine arabinoside (ara-C), but the ara-C dose-response curve can be changed by altering the regulatory milieu in suspension culture. The cells are more ara-C sensitive in MGF or G-CSF than in IL-3 or GM-CSF. Following a 24 h exposure to retinoic acid, the ara-C sensitivity increases; in contrast, after a similar exposure to hydrocortisone, the cells become less ara-C sensitive. These changes in ara-C sensitivity occur in cells that are actively making DNA, as indicated by the reduction in colony formation after exposure to tritiated thymidine. Since OCI/AML-4 cells respond to many of the regulators that affect the growth of freshly obtained AML blast cells, it is proposed that this cell line may be useful for the study of regulation on AML in general and the interaction between different regulators in particular.

Hormonal regulation of microsomal flavin-containing monooxygenase: tissue-dependent expression and substrate specificity

The substrate- and tissue-dependent hormonal regulation of flavin-containing monooxygenase (EC 1.14.13.8) was studied in male and female rats. Hypophysectomy of males reduced liver microsomal N,N-dimethylaniline N-oxidation, thiobenzamide S-oxidation, and imipramine N-oxidation, although the reduction was not as marked with the latter substrate. Castration also reduced flavin-containing monooxygenase-dependent activities, but not to the same extent as hypophysectomy. Administration of growth hormone or testosterone to hypophysectomized males only partially restored basal activities. In female rats, hypophysectomy had no effect on N,N-dimethylaniline N-oxidation or thiobenzamide S-oxidation and actually stimulated imipramine N-oxidation (98%). These effects were demonstrated to be tissue- and sex-dependent. For example, hypophysectomy markedly (300%) enhanced imipramine N-oxidation in male kidney and significantly decreased the same activity in male and female lung. Correlations between levels of the enzyme determined by immunoquantitation (with antibody to the rat liver enzyme) and activities toward these three substrates, in male and female liver, lung, and kidney, also provide evidence for the existence of multiple forms of flavin-containing monooxygenase, which appear to be under different hormonal regulation.

Derivation of mixed phenotype cell lines with ras-myc- and raf-myc-containing retroviral vectors after infection of murine long-term bone marrow cultures

Murine long-term bone marrow cultures were infected with retroviral vectors expressing the viral ras, raf, and myc oncogenes, alone and in combination. Stably transformed clonal cell lines were obtained after infection with ras-myc and raf-myc retroviruses but not by vectors expressing ras, myc, or raf alone. Northern blot analysis demonstrated that the two clonal cell lines expressed high levels of vector-specific transcripts. Phenotypic analysis of the cell lines by flow microfluorimetry and histochemical staining suggested that both cell lines expressed markers associated with cells of the megakaryocyte differentiation pathway. Histochemical staining demonstrated that these cell lines also expressed cytoplasmic enzymes associated with granulocytes and/or monocytes/macrophages. These cell lines, despite their clonal origin, are therefore of a mixed phenotype.

CD45 cell surface antigens are linked to stimulation of early human myeloid progenitor cells by interleukin 3 (IL-3), granulocyte/macrophage colony-stimulating factor (GM-CSF), a GM-CSF/IL-3 fusion protein, and mast cell growth factor (a c-kit ligand)

CD45 antigens are protein tyrosine phosphatases. A possible link was evaluated between expression of CD45 antigens on human myeloid progenitor cells (MPC) (colony-forming unit-granulocyte/macrophage [CFU-GM], burst-forming unit-erythroid [BFU-E], and colony-forming unit-granulocyte/erythroid/macrophage/megakaryocyte [CFU-GEMM]) and regulation of MPC by colony-stimulating factors (CSF) (interleukin 3 [IL-3], GM-CSF, G-CSF, M-CSF, and erythropoietin [Epo]), a GM-CSF/IL-3 fusion protein, and mast cell growth factor (MGF; a c-kit ligand). Treatment of cells with antisense oligodeoxynucleotides (oligos) to exons 1 and 2, but not 4, 5, or 6, of the CD45 gene, or with monoclonal anti-CD45, significantly decreased CFU-GM colony formation stimulated with GM-CSF, IL-3, fusion protein, and GM-CSF + MGF, but not with G-CSF or M-CSF. It also decreased GM-CSF, IL-3, fusion protein, and MGF-enhanced Epo-dependent BFU-E and CFU-GEMM colony formation, but had little or no effect on BFU-E or CFU-GEMM colony formation stimulated by Epo alone. Similar results were obtained with unseparated or purified (greater than or equal to one of two cells being a MPC) bone marrow cells. Sorted populations of CD343+ HLA-DR+ marrow cells composed of 90% MPC were used to demonstrate capping of CD45 after crosslinking protocols. Also, a decreased percent of CD45+ cells and CD45 antigen density was noted after treatment of column-separated CD34+ cells with antisense oligos to exon 1 of the CD45 gene. These results demonstrate that CD45 cell surface antigens are linked to stimulation of early human MPC by IL-3, GM-CSF, a GM-CSF/IL-3 fusion protein, and MGF.

Flavin-containing monooxygenase: a major detoxifying enzyme for the pyrrolizidine alkaloid senecionine in guinea pig tissues

Evidence based on optimal pH, thermal stability, and enzyme inhibition data suggests that the NADPH-dependent microsomal N-oxidation of the pyrrolizidine alkaloid senecionine is carried out largely by flavin-containing monooxygenase in guinea pig liver, lung, and kidney. In contrast, the hepatic microsomal conversion of senecionine to the pyrrole metabolite (+/-)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP) is catalyzed largely by cytochrome P450. However, the rate of senecionine N-oxide formation (detoxication) far exceeded the rate of DHP formation (activation) in guinea pig liver microsomes over a range of pHs (pH 6.8 to 9.8). In guinea pig lung and kidney microsomes, N-oxide was the major metabolite formed from senecionine with little or no production of DHP. The high rate of detoxication coupled with the low level of activation of senecionine in liver, lung, and kidney may help explain the apparent resistance of the guinea pig to intoxication by senecionine and other pyrrolizidine alkaloids.

Enhanced hematopoietic activity of a human granulocyte/macrophage colony-stimulating factor-interleukin 3 fusion protein

Granulocyte/macrophage colony-stimulating factor-interleukin 3 (GM-CSF-IL-3) fusion proteins were generated by construction of a plasmid in which the coding regions of human GM-CSF and IL-3 cDNAs were connected by a synthetic linker sequence followed by subsequent expression in yeast. Both GM-CSF-IL-3 and IL-3-GM-CSF fusion proteins were purified to homogeneity and shown to bind to cell-surface receptors through either their GM-CSF or IL-3 domains. The fusion proteins exhibited enhanced receptor affinity, proliferative activity, and hematopoietic colony-stimulating activity compared with either IL-3 and/or GM-CSF alone. This suggests that GM-CSF-IL-3 fusion proteins may hold future promise as therapeutic agents.

Purification of hepatocyte couplets by centrifugal elutriation

An initial preparation of rat hepatocytes containing approximately 30% couplets was enriched by centrifugal elutriation. Of the couplets loaded onto the elutriator, 87% were eluted at medium flow rates of 60 to 80 ml/min at a rotor speed of 1,100 rpm; cells eluted in this range maintained a viability of more than 95%. Peak fractions were enriched in couplets to 84.5% +/- 2.5%. After elutriation, couplets retained the ability to secrete fluorescent cholephiles into sealed canalicular vacuoles. The preparation can now be used in hepatobiliary and hepatotoxicity studies not possible with preparations in which they are minor components.

Purification and characterization of a microsomal cytochrome P-450 IIB enzyme from sheep lung

1. Cytochrome P-450 was purified to apparent homogeneity from pulmonary microsomes of female sheep. 2. Sheep lung P-450 was isolated in the low spin state with a lambda max in the CO-reduced vs CO difference spectrum of 450nm with no detectable absorbance at 420 nm. 3. The minimum mol. wt, determined by SDS-PAGE, was 51,400 daltons. Examination of substrate specificity of sheep lung P-450 indicated N-demethylation of benzphetamine, cocaine and N-dimethylnitrosamine. Nicotine was also metabolized by sheep lung P-450, but little turnover was observed with benzo[a]pyrene. 4. The amino acid composition and N-terminal sequence was highly similar to rat or rabbit members of the P-450 IIB gene subfamily. Antibodies raised in rabbit to sheep lung P-450 cross-reacted on Western blots with rabbit lung P-450 IIB4. 5. The P-450 isolated in this study comprises 75% of total P-450 in sheep pulmonary microsomes. Small amounts of this isozyme were also detected in sheep liver. These results show that, as is the case in rabbit and rat, the major constitutive P-450 in sheep is a member of the IIB gene subfamily.

The Steel/W transduction pathway: kit autophosphorylation and its association with a unique subset of cytoplasmic signaling proteins is induced by the Steel factor

The W/c-kit and Steel loci respectively encode a receptor tyrosine kinase (Kit) and its extracellular ligand, Steel factor, which are essential for the development of hematopoietic, melanocyte, and germ cell lineages in the mouse. To determine the biochemical basis of the Steel/W developmental pathway, we have investigated the response of the Kit tyrosine kinase and several potential cytoplasmic targets to stimulation with Steel in mast cells derived from normal and mutant W mice. In normal mast cells, Steel induces Kit to autophosphorylate on tyrosine and bind to phosphatidylinositol 3'-kinase (PI3K) and phospholipase C-gamma 1 but not detectably to Ras GTPase-activating protein. Additionally, we present evidence that Kit tyrosine phosphorylation acts as a switch to promote complex formation with PI3K. In mast cells from mice homozygous for the W42 mutant allele, Kit is not tyrosine phosphorylated and fails to bind PI3K following Steel stimulation. In contrast, in the transformed mast cell line P815, Kit is constitutively phosphorylated and binds to PI3K in the absence of ligand. These results suggest that Kit autophosphorylation and its physical association with a unique subset of cytoplasmic signaling proteins are critical for mammalian development.

Mast cell growth factor, a ligand for the receptor encoded by c-kit, affects the growth in culture of the blast cells of acute myeloblastic leukemia

The c-kit proto-oncogene encodes a transmembrane receptor with a tyrosine kinase internal domain. C-kit has been mapped to the W locus in the mouse, and the gene encoding the ligand has been shown to be the product of the murine SI locus. Previous genetic studies have shown that the murine W and SI loci play important roles in the normal function of hemopoietic stem cells. As these stem cells have been identified as the origins of abnormal clones in acute myeloblastic leukemia (AML), a study was begun of c-kit in AML. By Northern blot analysis, it was shown that all of 21 blast populations from AML patients were kit expression positive, but some AML cell lines did not transcribe detectable c-kit mRNA. This study is now extended to the responses of freshly obtained AML cells and cell lines to the ligand, mast-cell growth factor (MGF). In culture, fresh cells usually responded to added ligand with increases in both self-renewal and terminal divisions. The most obvious effects were seen when MGF was combined with either IL-3 or G-CSF. The response of cell lines to MGF mirrored their expression of c-kit; expression positive lines responded in culture with patterns similar to those seen for fresh cells. C-kit expression negative cells did not respond to MGF. RNA prepared from the cells giving rise to one such line, OCI/AML-5, was available for study. mRNA for c-kit could not be detected in this RNA sample by Northern blot analysis or the polymerase chain reaction. Thus the heterogeneity found in AML blast populations extends to the involvement of c-kit and its ligand in growth regulation, although blast populations without this regulatory apparatus appear to be rare.

Steel-Dickie mutation encodes a c-kit ligand lacking transmembrane and cytoplasmic domains

Mice homozygous for the viable Sl allele steel-Dickie (Sld) are sterile, severely anemic, and black-eyed white. The nature of the Sld mutation was investigated at the molecular level and was found to be due to a 4.0-kilobase intragenic deletion in mast cell growth factor (MGF) genomic sequences, providing conclusive evidence that Sl encodes MGF. As a consequence of this deletion, Sld is only capable of encoding a soluble truncated growth factor that lacks both transmembrane and cytoplasmic domains. Northern analysis indicates that Sld mRNA is expressed at approximately wild-type levels in adult tissues, and yeast expression studies suggest that the Sld protein is as biologically active as wild-type soluble MGF. These studies provide a molecular basis for explaining the Sld phenotype, a description of a germ-line mutation in the transmembrane and cytoplasmic domains of a membrane-bound growth factor, and in vivo evidence for the importance of membrane-bound forms of growth factors in mammalian development.