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Transcription factor PU.1 mediates induction of c-fms in vascular smooth muscle cells: a mechanism for phenotypic change to phagocytic cells.

The macrophage colony-stimulating factor receptor encoded by the c-fms gene is expressed in vascular intimal smooth muscle cells isolated from atherosclerotic lesions. A combination of platelet-derived growth factor-BB and epidermal growth factor induces stable expression of c-fms in normal vascular medial smooth muscle cells. The mechanism by which these growth factors induce c-fms expression has now been investigated in an attempt to gain insight into the events that underlie the phenotypic conversion of vascular smooth muscle cells in atherosclerosis. Deletion analysis of the c-fms promoter revealed that the region including a binding site for transcription factor PU.1 was required for transcriptional activity in human aortic medial smooth muscle cells. Mutation in the PU.1 binding site markedly reduced promoter activity. Northern (RNA) blot analysis demonstrated that growth factors induced the expression of PU.1 mRNA in vascular medial smooth muscle cells and that PU.1 mRNA was expressed in vascular intimal smooth muscle cells. PU.1 antisense oligonucleotides inhibited growth factor-induced c-fms expression and foam cell formation. These results suggest that transcription factor PU.1 plays an essential role in the phenotypic conversion of vascular smooth muscle cells to macrophagelike cells by mediating the induction of c-fms.

Aorta↗

Transcriptional regulation of the c-fms (CSF-1R) proto-oncogene in human breast carcinoma cells by glucocorticoids.

Expression of the macrophage colony stimulating factor CSF-1 and its receptor, the c-fms proto-oncogene, has been observed in macrophages, trophoblast and in a variety of neoplasms of epithelial origin including those of the breast. We have reported earlier (Oncogene, 1991, 6: 941-952) that c-fms transcript and protein expression were dramatically increased in several breast carcinoma cell lines by glucocorticoids which are essential humoral regulators of normal mammary epithelial cell differentiation. In this communication, we demonstrate that levels of c-fms transcript and protein increased significantly within the first few hours of glucocorticoid treatment, and that these increases were completely abolished by pretreatment of cells with mifepristone (RU486). We also demonstrate that such early increases in c-fms transcript levels could not be attributed to prolongation of transcript half-life. Both promoters of the c-fms gene were found to exhibit some basal activity in breast carcinoma cell lines and both were stimulated 2-3-fold by glucocorticoids. However the first promoter was shown to be responsible for more than 95% of the observed c-fms transcription. Sequence upstream of both promoters was found to contain potential 'glucocorticoid response elements' (GREs), and in each case, elimination of the GRE closest to the promoter abolished glucocorticoid stimulation. Our observations suggest that one mechanism by which glucocorticoids regulate the proliferation and differentiation of neoplastic mammary epithelial cells is through their regulation of transcription of the gene for the receptor of a ubiquitous cytokine, CSF-1.

Base Sequence↗

Posttranscriptional regulation of colony-stimulating factor-1 (CSF-1) and CSF-1 receptor gene expression during inhibition of phorbol-ester-induced monocytic differentiation by dexamethasone and cyclosporin A: potential involvement of a destabilizing protein.

Colony stimulating factor-1 (CSF-1) and its receptor (encoded by the c-fms proto-oncogene) have long been recognized as playing an important role in monocytic differentiation. However, the regulation of expression of the CSF-1 and c-fms genes during inhibition of monocytic differentiation has not been fully characterized. The present studies demonstrate that dexamethasone (dex) and cyclosporin A (CsA) resulted in inhibition of 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced monocytic differentiation of HL60 cells, as well as TPA induction of c-fms and CSF-1 transcripts. These agents also blocked TPA-induced adherence, alpha-naphthyl acetate esterase staining, and the development of a more differentiated morphology. Nuclear run-off analyses revealed no effect of either of these agents on transcription of either c-fms or CSF-1 genes in TPA-treated HL60 cells. Measurements of c-fms transcript half-life confirmed post-transcriptional regulation of c-fms transcript levels after the addition of dex or CsA to TPA, both of which resulted in a decrease in c-fms mRNA half-life. Others have suggested that TPA results in the stabilization of c-fms mRNA in HL60 cells through induction of a labile mRNA stabilizing protein. We observed, however, that the inhibition of protein synthesis by cycloheximide (CH) in this setting of early monocytic differentiation increased both c-fms and CSF-1 steady-state transcript levels. While CH had no effect on the transcription of c-fms and CSF-1 genes in TPA/dex- or TPA/CsA-treated HL60 cells, c-fms mRNA was stabilized after the addition of CH to TPA/dex-treated cells. Taken together, our results suggest the existence of a labile mRNA regulatory protein or proteins, whose actions include destabilization of both c-fms and CSF-1 transcripts after inhibition of TPA-induced monocytic differentiation by dex or CsA.

Cell Differentiation↗

KRAS2 oncogene overexpression in myelodysplastic syndrome with translocation 5;12.

The factors that initiate and maintain the abnormal hematopoietic clone in the myelo-dysplastic syndromes (MDS) remain largely unknown. We describe a patient with MDS associated with an abnormal karyotype, 46,XY,t(5;12)(q31;p12). According to the FAB cooperative group classification, the patient was classified as chronic myelomonocytic leukemia. Because of the particular chromosomal translocation, the structure-function relationship of three genes relevant to the translocation breakpoints, CSF2, FMS, and KRAS2, was studied in bone marrow and peripheral blood lymphocytes in this patient. No major structural alterations were observed at these three genetic loci. Although the levels of expression of the CSF2 and FMS genes remained unaltered, the KRAS2 oncogene was overexpressed approximately six-fold in bone marrow cells from the MDS patient compared with normal donors. We postulate that the RAS oncogene activation may be instrumental in the genesis of MDS.

Adult↗

Macrophage colony-stimulating factor-induced tyrosine phosphorylation of c-fms proteins expressed in FDC-P1 and BALB/c 3T3 cells.

The c-fms protein is a receptor for macrophage colony-stimulating factor (M-CSF) with intrinsic protein-tyrosine kinase activity. We investigated the tyrosine phosphorylation of murine c-fms proteins expressed from a retroviral vector in factor-dependent myeloid FDC-P1 cells and in BALB/c 3T3 fibroblasts transformed by the expression of the c-fms gene. FDC-P1 cells expressing c-fms were able to grow and differentiate in response to M-CSF. Their c-fms proteins were normally phosphorylated on serine and became phosphorylated on tyrosine residues contained in five tryptic peptides when the cells were exposed to M-CSF. A subset of these peptides was constitutively phosphorylated in BALB/c cells expressing c-fms, consistent with the production of M-CSF by these cells. All the peptides detected in vivo were also phosphorylated in vitro. These peptides were analyzed by susceptibility to proteases, comparison with synthetic peptides, and site-directed mutagenesis. The identities of four of the tryptic peptides were determined; they arise from three unique tyrosine phosphorylation sites. One major site of tyrosine phosphorylation at residue 697 accounted for two of the tryptic peptides. A second major site was identified at tyrosine residue 706. These two tyrosine phosphorylation sites are located within the tyrosine kinase insert region. Tyrosine 807, which has homology to the major autophosphorylation site of the p60v-src tyrosine kinase, is a minor autophosphorylation site. Possible functional roles for these phosphorylations of the c-fms protein include interactions with substrate proteins, catalytic activity, and ligand-induced degradation.

Amino Acid Sequence↗

Resistance to phorbol ester-induced differentiation of a U-937 myeloid leukemia cell variant with a signaling defect upstream to Raf-1 kinase.

Previous studies have demonstrated that treatment of human U-937 myeloid leukemia cells with 12-O-tetradecanoylphorbol-13-acetate (TPA) is associated with growth arrest and induction of monocytic differentiation. The present work describes the isolation of a U-937 cell variant, designated TUR, which is unresponsive to the growth-inhibitory effects of this agent. The results demonstrate that, in contrast to U-937 cells, the TUR line fails to respond to TPA with induction of the c-jun, junB, c-fos, and EGR-1 early response genes. The finding that these cells also fail to exhibit adherence or induction of the tumor necrosis factor and c-fms genes further supports their resistance to TPA-induced differentiation. In contrast, TUR cells responded to 1,25-dihydroxyvitamin D3, another inducer of monocytic differentiation, with growth arrest and induction of early response gene and c-fms transcripts. TUR cells also responded to okadaic acid, an inhibitor of type 1 and 2A protein phosphatases, with similar changes in gene expression. Further characterization of TUR cells has demonstrated decreased expression of protein kinase C as compared to wild-type U-937 cells. We also demonstrate that although treatment of U-937 cells with TPA is associated with activation of the Raf-1 serine/threonine kinase, there was no detectable decrease in electrophoretic mobility of this protein in TPA-treated TUR cells. Taken together, these findings indicate that the TUR variant is defective in TPA-induced signaling events upstream to activation of Raf-1 kinase.

Cell Differentiation↗

The role of the CSF-1 receptor gene (C-fms) in cell transformation.

The macrophage colony stimulating factor, CSF-1 (M-CSF) exerts its pleiotropic effects on hematopoietic cells of the mononuclear phagocyte series by binding to a single class of high affinity receptors encoded by the c-fms proto-oncogene. Binding of CSF-1 to its receptor activates an intrinsic tyrosine kinase activity, resulting in autophosphorylation of the receptor on tyrosine, rapid receptor down modulation, and phosphorylation of as yet unidentified physiologic substrates that initiate a mitogenic response. Transduction of a human CSF-1 receptor cDNA into mouse fibroblasts enables them to proliferate in response to human recombinant CSF-1, suggesting that the receptor gene contains all the information necessary to elicit a mitogenic response, even in cells which do not normally respond to the growth factor. The v-fms oncogene product has undergone genetic alterations which constitutively activate the receptor kinase in the absence of CSF 1. Insertion of the v-fms gene into macrophage or immature myeloid cell lines abrogates their dependence on hematopoietic growth factors and renders them tumorigenic in nude mice. Reconstitution of lethally irradiated mice with bone marrow stem cells containing the v-fms oncogene also induces clonal proliferation and, ultimately, frank malignancies of multiple hematopoietic lineages. Thus, constitutive activation of the CSF-1 receptor gene, either by mutation or gene rearrangement, might be expected to contribute to leukemia.

Animals↗

c-fms expression is a molecular marker of human acute myeloid leukemias.

The c-fms protooncogene product was identified as the CSF-1 or M-CSF receptor, a polypeptide growth factor that plays a major role in myelomonocytic differentiation. This led us to look for expression of c-fms in fresh acute myeloid leukemia (AML) cells, using Northern blot analysis. c-fms expression was found in the leukemic cells of 28 AML patients, regardless of their stage of differentiation, which was assessed in the French-American-British (FAB) classification. However, the level of c-fms expression was especially high in AML of the M5 stage. High levels of expression were not accompanied by either amplification or rearrangements of the c-fms gene in AML cell DNAs. In contrast, c-fms expression was not found in acute lymphoid leukemias, whether of T or B origin. Thus, c-fms expression appears as a specific molecular marker of leukemogenesis in the myeloid lineage.

Biomarkers, Tumor↗

c-FMS chromatin structure and expression in normal and leukaemic myelopoiesis.

The macrophage colony-stimulating factor receptor is encoded by the c-FMS gene, and it has been suggested that altered regulation of c-FMS expression may contribute to leukaemic transformation. c-FMS is expressed in pluripotent haemopoietic precursor cells and is subsequently upregulated during monocytic differentiation, but downregulated during granulopoiesis. We have examined transcription factor occupancy and aspects of chromatin structure of the critical c-FMS regulatory element located within the second intron (FIRE - fms intonic regulatory element) during normal and leukaemic myelopoiesis. Granulocytic differentiation from normal and leukaemic precursors is accompanied by loss of transcription factors at FIRE and downregulated c-FMS expression. The presence of AML1-ETO in leukaemic cells does not prevent this disassembly. In nonleukaemic cells, granulocytic differentiation is accompanied by reversal to a chromatin fine structure characteristic of c-FMS-nonexpressing cells. In addition, we show that low-level expression of the gene in leukaemic blast cells and granulocytes does not associate with increased CpG methylation across the c-FMS locus.

Base Sequence↗

Regulation of CSF-1 receptor expression.

Cells of the mononuclear phagocyte lineage possess receptors for macrophage colony-stimulating factor (CSF-1) encoded by the c-fms protooncogene and respond to CSF-1 with increased survival, growth, differentiation, and reversible changes in function. The c-fms gene is itself a macrophage differentiation marker. In whole mount analyses of mRNA expression in embryos, c-fms is expressed at very high levels on placental trophoblasts. It is detectable on individual cells in the yolk sac around 8.5 to 9 days postcoitus, appears on isolated cells in the head of the embryo around 9.5 dpc, and appears on numerous cells throughout the embryo by day 10.5. The extent of c-fms expression is much greater than for other macrophage-specific genes including lysozyme and a macrophage-specific protein tyrosine phosphatase. Our studies of the cis-acting elements of the c-fms promoter have indicated a key role for collaboration between the macrophage-specific transcription factor, Pu.1, which functions in determining the site of transcription initiation, and other members of the Ets transcription factor family. This is emerging as a common pattern in macrophage-specific promoters. We have shown that two PU box elements alone can function as a macrophage-specific promoter. The activity of both the artificial promoter and the c-fms promoter is activated synergistically by coexpression of Pu.1 and another Ets factor, c-Ets-2. A 3.5kb c-fms exon 2 promoter (but not the 300bp proximal promoter) is also active in a wide diversity of tumor cell lines. The interesting exception is the melanoma cell line K1735, in which the promoter is completely shut down and expression of c-fms causes growth arrest and cell death. The activity of the exon 2 promoter in these nonmacrophages is at least as serum responsive as the classic serum-responsive promoter of the c-fos gene. It is further inducible in nonmacrophages by coexpression of the c-fms product. Unlike other CSF-1/c-fms-responsive promoters, the c-fms promoter is not responsive to activated Ras even when c-Ets-2 is coexpressed. In most lines, production of full length c-fms is prevented by a downstream intronic terminator, but in Lewis lung carcinoma, read-through does occur, and expression of both c-fms and other macrophage-specific genes such as lysozyme and urokinase becomes detectable in conditions of serum deprivation.

Animals↗

PU.1 regulates both cytokine-dependent proliferation and differentiation of granulocyte/macrophage progenitors.

PU.1 is a unique regulatory protein required for the generation of both the innate and the adaptive immune system. It functions exclusively in a cell-intrinsic manner to control the development of granulocytes, macrophages, and B and T lymphocytes. We demonstrate that mutation of the PU.1 gene causes a severe reduction in myeloid (granulocyte/macrophage) progenitors. PU.1 -/- myeloid progenitors can proliferate in vitro in response to the multilineage cytokines interleukin-3 (IL-3), IL-6 and stem cell factor but are unresponsive to the myeloid-specific cytokines granulocyte-macrophage colony-stimulating factor (GM-CSF), G-CSF and M-CSF. The failure of PU.1 -/- progenitors to respond to G-CSF is bypassed by transient signaling with IL-3. In the presence of IL-3 and G-CSF, PU.1 -/- progenitors can differentiate into granulocytic precursors containing myeloperoxidase-positive granules. Thus PU.1 is not essential for specification of granulocytic precursors, but is required for their further differentiation. The failure of PU.1 -/- progenitors to respond to M-CSF is due to lack of c-fms gene transcription. Transduction of c-fms into PU.1 -/- myeloid progenitors bypasses the block to M-CSF-dependent proliferation but does not induce detectable macrophage differentiation. Therefore, PU. 1 appears to be essential for specification of monocytic precursors. Importantly, retroviral transduction of PU.1 into mutant progenitors restores responsiveness to myeloid-specific cytokines and development of mature granulocytes and macrophages. Thus PU.1 controls myelopoiesis by regulating both proliferation and differentiation pathways.

Animals↗

Hormonal regulation of the c-fms proto-oncogene in breast cancer cells is mediated by a composite glucocorticoid response element.

We have previously reported that glucocorticoids markedly increase and anti-glucocorticoids (such as RU-486) block c-fms RNA and protein expression in some breast cancer cell lines, but not in others, and that this increase is the consequence of increased transcription from the first, epithelial cell-specific promoter of the c-fms gene (encoding CSF-1R, macrophage colony-stimulating factor receptor). Employing DNaseI protection and electrophoretic mobility shift assays (EMSA), we now demonstrate that DNA-transcription factor protein complexes are formed on the c-fms first promoter at a composite regulatory element containing overlapping binding sites for AP-1 proteins, bHLH factors, and the glucocorticoid receptor (GR). Competition studies indicate that transcription factor proteins bind the AP-1 site and the GR element (GRE) and both GR and AP-1 proteins are involved in DNA-protein complex formation. The complexes differ in quantity and glucocorticoid inducibility in the different breast cancer cell lines studied depending on whether the promoter responds to glucocorticoid stimulation. Transient transfection of promoter/reporter gene constructs resulted in reduced basal transcription activity of this promoter and lack of glucocorticoid stimulation when the AP-1 site was mutated. We conclude that AP-1 proteins, GR and associated co-factors regulate transcription from the c-fms first promoter and that differences in recruitment of the various components are responsible for cell specific repression and activation of this gene in breast carcinoma cell lines.

Antibodies↗

Molecular cloning of CSF-1 receptor from rat myoblasts. Sequence analysis and regulation during myogenesis.

We have isolated and sequenced a cDNA (mrfms) encoding rat c-fms gene (CSF-1 receptor) from proliferating L6 alpha 1 myoblasts. The predicted amino acid sequence was highly identical with the c-fms protein found in monocytes and macrophages (98, 76 and 84% identity from mouse, cat and human c-fms proteins, respectively). The mechanisms responsible for the regulation of mrfms gene expression during myogenesis were examined. Mrfms products were observed during proliferation of L6 alpha 1 myoblasts and were downregulated during differentiation. Run-on transcription assays demonstrated that the mrfms gene was transcriptionally active only in undifferentiated myoblasts. These findings suggested that mrfms levels in L6 alpha 1 myoblasts are controlled by transcriptional mechanisms. The half-life of mrfms transcripts was found to be at least 5 hr while inhibition of protein synthesis with cycloheximide (CHX) decreased this half-life to 30 min without changes in the rate of mrfms gene transcription. In addition oncogenic transformation of L6 alpha 1 myoblasts by the v-fms induced constitutive upregulation of mrfms mRNAs, and nuclear run-on assays demonstrated that mrfms transcription was not growth-factor dependent. Furthermore, these findings with others previously published indicate that mrfms gene products may play a role in the normal and neoplastic growth of muscular cells.

Amino Acid Sequence↗

Expression of Flt3 tyrosine kinase receptor gene in mouse hematopoietic and nervous tissues.

The Flt3 gene encodes a tyrosine kinase receptor highly related to the Kit and Fms gene products. We have studied the expression of Flt3 by using in situ hybridization of mouse tissue sections. The results show that Flt3 RNAs are present in certain regions of lymphohematopoietic organs, placenta and nervous system. Flt3 is expressed in the medullary area of fetal and newborn thymus, in the paracortical regions of lymph nodes and in the red pulp of spleen. In placenta, labyrinthine trophoblasts express Flt3. Finally, Flt3 RNAs are found in several regions of the brain and in cerebellar Purkinje cells. Western-blot analysis showed that the FLT3 protein is present in the tissues positive for Flt3 RNA expression. Our observations allow for a comparison with the distribution of the Kit gene and analysis of a possible redundancy between KIT and FLT3 receptors.

Animals↗

A macrophage colony-stimulating factor receptor-green fluorescent protein transgene is expressed throughout the mononuclear phagocyte system of the mouse.

The c-fms gene encodes the receptor for macrophage colony-stimulating factor (CSF-1). The gene is expressed selectively in the macrophage and trophoblast cell lineages. Previous studies have indicated that sequences in intron 2 control transcript elongation in tissue-specific and regulated expression of c-fms. In humans, an alternative promoter was implicated in expression of the gene in trophoblasts. We show that in mice, c-fms transcripts in trophoblasts initiate from multiple points within the 2-kilobase (kb) region flanking the first coding exon. A reporter gene construct containing 3.5 kb of 5' flanking sequence and the downstream intron 2 directed expression of enhanced green fluorescent protein (EGFP) to both trophoblasts and macrophages. EGFP was detected in trophoblasts from the earliest stage of implantation examined at embryonic day 7.5. During embryonic development, EGFP highlighted the large numbers of c-fms-positive macrophages, including those that originate from the yolk sac. In adult mice, EGFP location was consistent with known F4/80-positive macrophage populations, including Langerhans cells of the skin, and permitted convenient sorting of isolated tissue macrophages from disaggregated tissue. Expression of EGFP in transgenic mice was dependent on intron 2 as no lines with detectable EGFP expression were obtained where either all of intron 2 or a conserved enhancer element FIRE (the Fms intronic regulatory element) was removed. We have therefore defined the elements required to generate myeloid- and trophoblast-specific transgenes as well as a model system for the study of mononuclear phagocyte development and function.

Animals↗

Murine c-fms cDNA: cloning, sequence analysis and retroviral expression.

We have isolated and sequenced a cDNA clone coding for the murine c-fms gene. The 3677 nucleotide cDNA clone codes for a protein of 976 amino acids, which has 76% and 75% homology to the v-fms and human proteins, respectively. The predicted membrane protein, c-fms, has an amino terminal signal sequence and external, transmembrane and cytoplasmic domains. The homology between murine c-fms and v-fms or human c-fms is the strongest (90%-95%) in the cytoplasmic domain and weakest (59%-63%) in the external domain. The c-fms clone was inserted into a retroviral vector containing a neomycin resistance gene and cell lines resistant to G418 were isolated. These cell lines expressed protein which was immunoprecipitated by anti-c-fms antibodies and of the same size as murine c-fms. These cells were also found to specifically bind human CSF-1.

Amino Acid Sequence↗

Deletion of c-ets1 and T3 gamma loci from the 11q- chromosome in the human monoblastic cell line U937.

The 11q- chromosome in the human monoblastic cell line U937 appears to be derived from an interstitial deletion in bands 11q21-23/24 or a translocation with an unknown chromosome. We show here by in situ hybridisation that this chromosome has lost the c-ets1 and T3 gamma loci. C-ets1 and T3 gamma sequences were not detected on any chromosomes besides the normal 11, indicating either that they were lost from the genome or that the chromosome to which they were translocated was not present in a high enough proportion of the cells to be detected by in situ hybridisation. No DNA rearrangements were found with three different restriction enzymes in the c-ets1, N-CAM, Thy-1 and c-sea genes detected by our probes. There was also no detectable rearrangement in the c-fms gene which was shown to be translocated from chromosome 5 to chromosome 1 in a subline of U937. The size of the c-fms and c-ets1 messages were normal. The possible significance of these findings is discussed.

Blotting, Southern↗

Post-transcriptional regulation of c-fms proto-oncogene expression by dexamethasone and of CSF-1 in human breast carcinomas in vitro.

The c-fms proto-oncogene encodes the receptor for a hematopoietic growth factor, CSF-1. Recently, the importance of c-fms and its ligand CSF-1 in malignancies of non-hematopoietic origin, such as breast, ovarian, endometrial, pulmonary, and trophoblastic cancers has been recognized. We have previously shown that glucocorticoids induce a large increase in c-fms mRNA and protein levels in breast carcinoma cell lines. In this report, we investigate the mechanism underlying such c-fms overexpression by dexamethasone. We show that dexamethasone treatment of two breast carcinoma cell lines (BT20-c-fms expressor, and SKBR3-co-expressor of both c-fms and CSF-1) does not increase the rate of c-fms gene transcription, suggesting a post-transcriptional mechanism of regulation of c-fms expression by dexamethasone. The effect of protein synthesis inhibition was studied to help determine whether there was a role for intermediary regulatory proteins in the regulation of c-fms expression. We find that several protein synthesis inhibitors interfere with dexamethasone induction of c-fms transcripts, suggesting the existence of regulatory proteins. These regulatory proteins do not appear to be constitutively expressed, as we show no effect of protein synthesis inhibition on c-fms transcript expression in resting BT20 cells. These findings suggest that the putative regulatory proteins are induced by dexamethasone. Furthermore, the addition of a protein synthesis inhibitor, pactamycin, to dexamethasone-treated BT20 cells results in a decrease in c-fms mRNA stability.(ABSTRACT TRUNCATED AT 250 WORDS)

Amidohydrolases↗