Cell surface expression of v-fms-coded glycoproteins is required for transformation

The metastasis suppressor gene C33/CD82/KAI1 induces apoptosis through reactive oxygen intermediates

Here we describe the isolation of C33/CD82/KAI1 in a screen for apoptosis-inducing genes. C33 is a gene that is downregulated in many metastatic tumor cells and the expression of which can attenuate the process of metastases formation in a variety of tumors. In accordance, we observed cell death induction by C33 in many different cell types. C33 seems to promote cell death by the generation of reactive oxygen intermediates (ROIs). These ROIs, however, are not derived from the mitochondrial respiratory chain as in most other scenarios leading to apoptosis. We observed that C33 renders cells sensitive to ROIs by causing the specific release of the intracellular antioxidant glutathione (GSH) from cells. Moreover, C33 activates the GTPase Cdc42, which mediates GSH release and apoptosis induction and allows to detect the formation of ROIs.

The tumor suppressor cybL, a component of the respiratory chain, mediates apoptosis induction

A genetic screen was established to clone apoptosis-inducing genes in a high-throughput format. It led to the isolation of several proapoptotic genes whose proteins are localized to mitochondria. One of the isolated genes is cytochrome bL (cybL also known as SDHC, CII-3, or QPs-1), a component of the respiratory chain complex II. It was further investigated because both cybL and another component of complex II, cybS, have recently been identified as tumor suppressor proteins, some of which act by controlling apoptosis. Our studies reveal that cell death induction by cybL expression is concomitant with a transient inhibition of complex II and the generation of reactive oxygen species. Importantly, cells that are constitutively deficient in cybL are resistant to a variety of proapoptotic cytostatic drugs and to the effects of the Fas receptor. Our results therefore identify complex II as a sensor for apoptosis induction and could explain the unexpected observation that complex II is inactivated in tumors.

Spike, a novel BH3-only protein, regulates apoptosis at the endoplasmic reticulum

We have isolated Spike, a novel and evolutionary conserved BH3-only protein. BH3-only proteins constitute a family of apoptosis inducers that mediate proapoptotic signals. In contrast to most proteins of this family, Spike was not found to be associated with mitochondria. Furthermore, unlike the known BH3-only proteins, Spike could not interact with all tested Bcl-2 family members, despite its BH3 domain being necessary for cell killing. Our findings indicate that Spike is localized to the endoplasmic reticulum. The endoplasmic reticulum is an organelle that has only recently been implicated in regulation of apoptosis. At this locale, Spike interacts with Bap31, an adaptor protein for pro-caspase-8 and Bcl-XL. In doing so, Spike is able to inhibit the formation of a complex between Bap31 and the antiapoptotic Bcl-XL protein. Furthermore, Spike transmits the signal of specific death receptors. Its down-regulation in certain tumors suggests that Spike may also play a role in tumorigenesis. Our findings add new insight for how BH3-only and antiapoptotic Bcl-2 proteins regulate cell death.

C-Cbl binds the CSF-1 receptor at tyrosine 973, a novel phosphorylation site in the receptor's carboxy-terminus

The colony-stimulating factor-1 (CSF-1) receptor is a protein-tyrosine kinase that regulates the proliferation and differentiation of monocyte and macrophage precursors. Binding of CSF-1 to its receptor results in activation of the kinase domain and autophosphorylation on a number of tyrosine residues. Phosphorylated tyrosine residues function as binding sites for SH2 domain-containing signaling proteins. It is known that activated receptors are internalized and degraded, but the mechanics of this process remain largely unknown. Recently, evidence has started to emerge that the ubiquitin-protein ligase c-Cbl is involved in CSF-1 receptor degradation. In addition, there is evidence that the CSF-1 receptor carboxy-terminus is involved in down regulation of the receptor. Here we show that the c-Cbl tyrosine kinase-binding (TKB) domain binds in vitro and in vivo to the CSF-1 receptor. Binding is dependent on the receptor's protein-kinase activity. Deletion of the carboxy-terminus or mutation of Tyr 973 blocks binding. We further provide evidence that the CSF-1 receptor's carboxy-terminus is a substrate for autophosphorylation. Our observations are consistent with a model in which receptor autophosphorylation at Tyr 973 creates a binding site for c-Cbl. Association of c-Cbl with the receptor leads to ubiquitination, followed by receptor degradation.

Tyrosine phosphorylation of proteins in primary human myeloid leukemic cells stimulated by macrophage colony-stimulating factor: analysis by disease type and comparison with normal human hematopoietic cells

We investigated tyrosine phosphorylation of proteins in primary human leukemic cells stimulated by macrophage colony-stimulating factor (M-CSF) in 60 patients with acute myeloid leukemia (AML) and 5 patients with chronic myelomonocytic leukemia and compared the findings for leukemic cells with those of normal human monocytes and bone marrow immature hematopoietic cells. M-CSF induced tyrosine phosphorylation of p140-200, p110, p60, p44, and p42 frequently, and that of p95 and p55 less frequently, in primary myeloid leukemic cells, whereas M-CSF-induced phosphorylation of proteins was not detected in the normal human hematopoietic cells tested. Of these phosphoproteins, p140-200 was phosphorylated in all patients who responded to M-CSF and was considered to be almost identical to Fms, a product of the c-fms proto-oncogene. M-CSF-induced tyrosine phosphorylation was observed frequently (89%) in AML of French-American-British class M4 and infrequently in all other subtypes of AML, including M5. In chronic myelomonocytic leukemia, M-CSF-induced protein phosphorylation was prominent in blast crisis but was not detected in the chronic phase. Both bone marrow immature cells and mature monocytes showed low responsiveness to M-CSF. These findings for responsiveness to M-CSF were correlated with the amount of Fms in each type of cell. We also identified tyrosine phosphorylation of Vav, Shc, and extracellular signal-regulated kinase by M-CSF in some cases. These findings clarified an M-CSF-specific pattern of protein tyrosine phosphorylation and the responsiveness to M-CSF of primary human myeloid cells. Particularly, enhanced phosphorylation responses to M-CSF and increased amounts of Fms protein were observed in restricted human hematopoietic cells with a premature myelomonocytic character.

Adenine nucleotide translocase-1, a component of the permeability transition pore, can dominantly induce apoptosis

Here, we describe the isolation of adenine nucleotide translocase-1 (ANT-1) in a screen for dominant, apoptosis-inducing genes. ANT-1 is a component of the mitochondrial permeability transition complex, a protein aggregate connecting the inner with the outer mitochondrial membrane that has recently been implicated in apoptosis. ANT-1 expression led to all features of apoptosis, such as phenotypic alterations, collapse of the mitochondrial membrane potential, cytochrome c release, caspase activation, and DNA degradation. Both point mutations that impair ANT-1 in its known activity to transport ADP and ATP as well as the NH(2)-terminal half of the protein could still induce apoptosis. Interestingly, ANT-2, a highly homologous protein could not lead to cell death, demonstrating the specificity of the signal for apoptosis induction. In contrast to Bax, a proapoptotic Bcl-2 gene, ANT-1 was unable to elicit a form of cell death in yeast. This and the observed repression of apoptosis by the ANT-1-interacting protein cyclophilin D suggest that the suicidal effect of ANT-1 is mediated by specific protein-protein interactions within the permeability transition pore.

Cell specific transformation by c-fms activating loop mutations is attributable to constitutive receptor degradation

Expression of a receptor for human macrophage-colony stimulating factor (M-CSF or CSF-1), containing a point mutation which changes an aspartate to a valine at position 802 of the activating loop of the kinase domain, potently transforms the haemopoietic cell line FDC-P1 yet prevents Rat-2 fibroblast transformation. In order to understand this apparent paradox, aspartate 802 was changed by cassette mutagenesis to each of the other 19 amino acids. All hydrophobic amino acid substitutions were transforming when tested in FDC-P1 cells yet inactivating when tested in Rat-2 fibroblasts. These same amino acid substitutions also activated receptor degradation, strongly suggesting a causal relationship between receptor degradation and inactivation in fibroblasts. Point mutations or small deletions of Y708 within the kinase insert region of the mutant D802V receptor partly inhibited receptor degradation. The more stable D802V receptor derivatives were able to transform both FDC-P1 cells and Rat-2 fibroblasts, so establishing that the cell specific effect of the c-fmsD802V activating loop mutation is attributable to receptor degradation which accompanies kinase activation and prevents the transformation of Rat-2 but not of FDC-P1 cells.

Features of Macrophage Differentiation Induced by p19INK4d, a Specific Inhibitor of Cyclin D–Dependent Kinases

The mitogen-dependent induction of cyclin D–dependent kinase activity is required for cells to enter the DNA synthetic (S) phase of their division cycle. Immature 32Dcl3 myeloid cells (32D) proliferating in the presence of interleukin-3 (IL-3) normally express cyclins D2 and D3, which assemble into binary holoenzyme complexes with their catalytic subunits, CDK4 and CDK6. When 32D cells are switched to medium containing granulocyte colony-stimulating factor (G-CSF ) instead of IL-3, D-type cyclins are degraded and, in the absence of their associated kinase activity, the cells arrest in the first gap phase (G1 ) of the cell cycle and differentiate to neutrophils. We derived 32D cells in which the expression of p19INK4d, a specific polypeptide inhibitor of CDK4 and CDK6, is regulated by the heavy metal-inducible sheep metallothionein promoter. Induction of p19INK4d in response to zinc prolonged cell survival in the absence of growth factor treatment. When maintained in medium containing both IL-3 and zinc, these cells lost cyclin D–dependent kinase activity, underwent G1 phase arrest, and acquired certain morphologic, antigenic, and functional properties of mononuclear phagocytes. Cells induced to express p19INK4d did not synthesize receptors for macrophage colony-stimulating factor (M-CSF/CSF-1) and reverted to an immature myeloid phenotype when shifted back into medium containing IL-3 alone. These cells exhibited accelerated differentiation to neutrophils in response to G-CSF but also gave rise to macrophage-like cells when maintained in medium containing both G-CSF and zinc. Therefore, the acquisition of macrophage properties in response to zinc treatment neither depended upon IL-3 nor upon G1 phase arrest per se and instead reflects some ability of p19INK4d, and presumably cyclin D–dependent kinases, to affect myeloid differentiation.

Features of Macrophage Differentiation Induced by p19INK4d, a Specific Inhibitor of Cyclin D–Dependent Kinases

The mitogen-dependent induction of cyclin D–dependent kinase activity is required for cells to enter the DNA synthetic (S) phase of their division cycle. Immature 32Dcl3 myeloid cells (32D) proliferating in the presence of interleukin-3 (IL-3) normally express cyclins D2 and D3, which assemble into binary holoenzyme complexes with their catalytic subunits, CDK4 and CDK6. When 32D cells are switched to medium containing granulocyte colony-stimulating factor (G-CSF ) instead of IL-3, D-type cyclins are degraded and, in the absence of their associated kinase activity, the cells arrest in the first gap phase (G1 ) of the cell cycle and differentiate to neutrophils. We derived 32D cells in which the expression of p19INK4d, a specific polypeptide inhibitor of CDK4 and CDK6, is regulated by the heavy metal-inducible sheep metallothionein promoter. Induction of p19INK4d in response to zinc prolonged cell survival in the absence of growth factor treatment. When maintained in medium containing both IL-3 and zinc, these cells lost cyclin D–dependent kinase activity, underwent G1 phase arrest, and acquired certain morphologic, antigenic, and functional properties of mononuclear phagocytes. Cells induced to express p19INK4d did not synthesize receptors for macrophage colony-stimulating factor (M-CSF/CSF-1) and reverted to an immature myeloid phenotype when shifted back into medium containing IL-3 alone. These cells exhibited accelerated differentiation to neutrophils in response to G-CSF but also gave rise to macrophage-like cells when maintained in medium containing both G-CSF and zinc. Therefore, the acquisition of macrophage properties in response to zinc treatment neither depended upon IL-3 nor upon G1 phase arrest per se and instead reflects some ability of p19INK4d, and presumably cyclin D–dependent kinases, to affect myeloid differentiation.

An apoptosis-inducing isoform of neu differentiation factor (NDF) identified using a novel screen for dominant, apoptosis-inducing genes

Apoptosis is a genetically programmed series of events that results in cell death. As a consequence, it is difficult to identify dominant genes that play a role in this process using genetic selections in conventional cell culture systems. Accordingly, we have established an efficient expression screen to isolate dominant, apoptosis-inducing genes. The assay is based on the apoptotic morphology induced in the human kidney cell line 293 after transient transfection of small plasmid pools from normalized cDNA expression libraries. Using this assay, we isolated a novel isoform of the proto-oncogene Neu differentiation factor (NDF), a ligand for erbB receptor tyrosine kinases. Several lines of experimental evidence indicate that this gene kills in a cell-autonomous fashion and independently of known erbB receptors. This apoptotic property of an NDF isoform is readily contrasted with NDF's transforming potential and might balance the tendency to tumorigenesis in cells that overexpress NDF.

Oncogenes, Protein Tyrosine Kinases, and Signal Transduction

Many oncogenes encode protein tyrosine kinases (PTKs). Oncogenic mutations of these genes invariably result in constitutive activation of these PTKs. Autophosphorylation of the PTKs and tyrosine phosphorylation of their cellular substrates are essential events for transmission of the mitogenic signal into cells. The recent discovery of the characteristic amino acid sequences, of the src homology domains 2 and 3 (SH2 and SH3), and extensive studies on proteins containing the SH2 and SH3 domains have revealed that protein tyrosine-phosphorylation of PTKs provides phosphotyrosine sites for SH2 binding and allows extracellular signals to be relayed into the nucleus through a chain of protein-protein interactions mediated by the SH2 and SH3 domains. Studies on oncogenes, PTKs and SH2/SH3-containing proteins have made a tremendous contribution to our understanding of the mechanisms for the control of cell growth, oncogenesis, and signal transduction. This review is intended to provide an outline of the most recent progress in the study of signal transduction by PTKs. Copyright 1994 S. Karger AG, Basel

Growth factor independence and growth regulatory pathways in human melanoma development

This review concentrates on growth autonomy of tumor cells in relation to tumor progression. Human malignant melanoma serves as an example for progressive growth factor independence at subsequent stages of tumor progression. Mechanisms by which malignant cells acquire growth factor independence are discussed. In melanoma, deregulation of growth regulatory pathways has been described on four levels: 1) aberrant production of autocrine growth factors that substitute for exogenous growth factors (basic fibroblast growth factor [bFGF]); 2) alterations in the response to negative autocrine growth factors (interleukin [IL]-6 and transforming growth factor [TGF]-beta); 3) overexpression of epidermal growth factor receptors (EGF-R); and 4) alterations of cellular protooncogenes involved in signal transduction (RAS, MYB) and growth suppression (p53). In addition to bFGF and IL-6, multiple other growth factor genes are activated in malignant melanoma cells but not normal melanocytes. These include both chains of platelet-derived growth factor (PDGF), TGF-alpha, IL-1, IL-8, and tumor necrosis factor (TNF)-alpha. Of these, PDGF-B has been investigated in more detail. Melanoma-derived PDGF clearly does not act in a direct autocrine mode, but has important paracrine effects on normal tissue constituents, notably fibroblasts and endothelial cells, that are essential for tumor development in vivo. It is speculated that other melanoma-derived growth factors with as yet undefined functions similarly exert such paracrine or 'indirect' autocrine effects that cannot be sufficiently addressed in studies on cultured cells.

Myeloid malignancies and chromosome 5 deletions

Deletions of chromosome 5 were initially reported as a consistently occurring chromosomal abnormality in 5q- syndrome. They have since been recognized to occur in other myeloid malignancies such as therapy-related leukaemia and de novo AML as well. The variability of the deletions, and the heterogeneity of the clinical syndromes, have made it difficult to describe a single clinical-molecular entity such as we see with chromosomal translocations described elsewhere in this volume. Translocations in leukaemogenesis often have a dominant effect leading to activation of oncogenes or the production of a modified protein. Consistently occurring chromosomal deletions in human tumours, however, have been regarded as evidence that the affected regions contain tumour suppressor genes. Loss of function of these tumour suppressor genes or 'recessive oncogenes' leads to cancer. Deletions in the long arm of chromosome 5 in myeloid malignancies are thought to signal the existence of a recessive oncogene on 5q, which is homozygously inactivated in these malignancies. Here we describe the clinical and molecular features of the diseases associated with deletions of chromosome 5 in an attempt to propose a unified approach to identifying the genes on 5q which are involved in leukaemogenesis. It is likely that the clinical heterogeneity of these disorders will not be understood until the relevant genes are cloned and their role in the initiation or progression of leukaemia is known.

Enhancement of transforming potential of human insulinlike growth factor 1 receptor by N-terminal truncation and fusion to avian sarcoma virus UR2 gag sequence

The human insulinlike growth factor 1 (hIGF-1) receptor (hIGFR) is a transmembrane protein tyrosine kinase (PTK) molecule which shares high sequence homology in the PTK domain with the insulin receptor and, to a lesser degree, the ros transforming protein of avian sarcoma virus UR2. To assess the transforming potential of hIGFR, we introduced the intact and altered hIGFR into chicken embryo fibroblasts (CEF). The full-length hIGFR cDNA (fIGFR) was cloned into a UR2 retroviral vector, replacing the original oncogene v-ros. fIGFR was able to promote the growth of CEF in soft agar and cause morphological alteration in the absence of added hIGF-1 to medium containing 11% calf and 1% chicken serum. The transforming ability of hIGFR was not further increased in the presence of 10 nM exogenous hIGF-1. The 180-kDa protein precursor of hIGFR was synthesized and processed into alpha and beta subunits. The overexpressed hIGFR in CEF bound hIGF-1 with high affinity (Kd = 5.4 x 10(-9) M) and responded to ligand stimulation with increased tyrosine autophosphorylation. The cDNA sequence coding for part of the beta subunit of hIGFR, including 36 amino acids of the extracellular domain and the entire transmembrane and cytoplasmic domains, was fused to the 5' portion of the gag gene in the UR2 vector to form an avian retrovirus. The resulting virus, named UIGFR, was able to induce morphological transformation and promote colony formation of CEF with a stronger potency than did fIGFR. The UIGFR genome encodes a membrane-associated, glycosylated gag-IGFR fusion protein. The specific tyrosine phosphorylation of the mature form of the fusion protein, P75, is sixfold higher in vitro and threefold higher in vivo than that of the native IGFR beta subunit, P95. In conclusion, overexpression of the native or an altered hIGFR can induce transformation of CEF with the gag-IGFR fusion protein possessing enhanced transforming potential, which is consistent with its increased in vitro and in vivo tyrosine phosphorylation.

An activating mutation in the murine erythropoietin receptor induces erythroleukemia in mice: a cytokine receptor superfamily oncogene

A point mutation at codon 129 of the murine erythropoietin receptor (cEpoR) results in constitutive activation. We have generated a recombinant spleen focus-forming retrovirus in which the env gene is replaced by the cEpoR cDNA. Mice infected with this virus (but not by viruses expressing the wild-type EpoR) develop erythrocytosis and splenomegaly. From the spleen of infected animals we have isolated clonal, growth factor-independent, proerythroblast cell lines that express cEpoR, do not express the putative oncogene spi-1, and have rearranged and inactivated expression of the p53 suppressor oncogene. These cells induce erythroleukemia upon injection into mice. This demonstrates that oncogenic point mutations exist in a member of the cytokine receptor superfamily. The activated erythropoietin receptor does not transform cultured fibroblasts, suggesting why oncogenic mutations in other members of this receptor superfamily have not been detected.

Two point mutations in the transmembrane domain of P68gag-ros inactive its transforming activity and cause a delay in membrane association

The transforming protein of the avian sarcoma virus UR2 is a 68-kDa transmembrane tyrosine protein kinase. We examined the relationship between membrane localization and transforming activity of P68 by changing Val-168-Val-169 in its hydrophobic domain into Asp-168-Glu-169. The resulting transmembrane (TM) mutant (P68TM) lost transforming activity toward chicken embryo fibroblasts (CEF). We found that the mutant protein was expressed and rapidly degraded into a smaller form which was still membrane associated and kinase active. The instability of the TM mutant protein is a phenomenon only manifested in CEF, because the same mutant protein was expressed with efficiency and stability similar to those of the wild-type protein in a transient expression system in COS cells. However, there are several differences between the wild-type and the TM mutant proteins in COS cells. The wild-type protein is more heavily phosphorylated and associated with membrane fractions in a cotranslational manner. It is enzymatically active when recovered from membrane fractions. The TM mutant protein is less phosphorylated, more labile toward protease degradation, and delayed in membrane association, with a lag period of 30 min or longer, and has little kinase activity when recovered from membrane fractions. Most of the kinase-active TM mutant protein was found in the cytosol fractions. Despite the delay, most of the TM protein in COS cells was found to be membrane associated, and its orientation on the cell surface was similar to that of the wild-type protein. It is probable that loss of the CEF-transforming activity of the TM mutant protein is due to its susceptibility to protease degradation resulting from improper membrane association of the newly synthesized product. The differences in the kinetics of membrane association and the distribution of kinase activity in COS cells might not be directly applicable in explaining the inability of the TM mutant to transform CEF but are intriguing as regards protein biosynthesis and translocation. The difference between CEF and COS cells implies that different factors or pathways are involved in the biosynthesis and processing of the TM mutant protein in these two cellular environments. Changes of P68TM in the kinetics of membrane association indicate that the transmembrane domain of ros, besides functioning as a membrane anchor, also plays a role in directing initial membrane association.

Transformation of chicken fibroblasts by the v-fms oncogene

The v-fms oncogene of the McDonough strain of feline sarcoma virus (SM-FeSV) encodes a plasma-membrane-associated tyrosine kinase (gp140v-fms) which is closely related, both structurally and functionally, to the c-fms-specified receptor for the macrophage colony stimulating factor (CSF-1). In mammalian fibroblasts, the natural producers of CSF-1, expression of v-fms leads to cell transformation. To study the interaction between CSF-1 and gp140v-fms molecules in a cell system that does not produce endogenous cross-reactive CSF-1, we have expressed the entire v-fms gene as well as a nontransforming deletion mutant (SC2) in chicken embryo cells (CEC). For this purpose the avian retroviral vectors pDS3 and pREP, based on Rous sarcoma virus, were used to isolate recombinant virus particles. CEC infected with virus that carried the entire v-fms gene expressed high amounts of gp140v-fms, comparable to those in SM-FeSV transformed NRK cells. However, these CEC remained flat, retained their fibronectin network, and did not produce enhanced levels of plasminogen activator. The cells grew faster than control CEC for more than 8 weeks but failed to form colonies in soft agar. Within 2 days after addition of CSF-1 to the growth medium, a transformed cell phenotype was induced, as judged by loss of the fibronectin network, again with a growth rate fourfold faster than that of the parental cells and with colony formation in soft agar. Moreover, human CSF-1 caused a rapid tyrosine phosphorylation of v-fms molecules detectable within 5 min after addition of the growth factor. In contrast, CSF-1 had none of the above effects on cells that expressed the SC2 v-fms deletion mutant.

Functional expression of the human receptor for colony-stimulating factor 1 (CSF-1) in hamster fibroblasts: CSF-1 stimulates Na+/H+ exchange and DNA-synthesis in the absence of phosphoinositide breakdown

The human CSF-1 receptor (c-fms protooncogene product) was introduced into CSF-1-unresponsive Chinese hamster lung fibroblasts (CCL39 cell line) in order to study its coupling to biochemical signal-transducing systems and to compare the growth-regulating properties of CSF-1 to those of other growth factors. Independent clones expressing different levels of CSF-1 receptors were isolated and characterized. CSF-1 increased [3H]thymidine incorporation in serum-starved cells and potentiated the mitogenic effects of FGF and thrombin. As already observed for other growth factors activating receptor tyrosine kinases (EGF, FGF, IGF-I), CSF-1 alone did not trigger inositol phosphate formation, but slightly enhanced the activity of phospholipase C agonists (thrombin, A1F4- complex). Activation of the CSF-1 receptor by its ligand was evidenced by the rapid activation of the Na+/H+ exchanger resulting in amiloride-sensitive cytoplasmic alkalinization (0.1-0.2 pH units) within minutes after stimulation. Whereas pertussis toxin does not affect the action of EGF, FGF, or IGF-I in CCL39 cells, it partially inhibited both DNA synthesis reinitiation and activation of Na+/H+ exchange by CSF-1, indicating that the CSF-1 receptor can communicate with a signal-transducing GTP binding protein. A point-mutated form of the c-fms gene product, in which Tyr 969, a residue negatively modulating signal transduction, had been replaced with Phe [fms (F969)], did not generate responses significantly different from those obtained with the wild-type c-fms gene product. In the absence of CSF-1, cells expressing either wild-type or fms (F969) showed a considerably higher basal level of thymidine incorporation and decreased anchorage dependence compared with parental CCL39 cells. Monoclonal antibodies that interfere with signal transduction by the human CSF-1 receptor inhibited both basal [3H]thymidine incorporation and soft agar colony formation, indicating that relaxation of growth control was dependent on CSF-1 receptor expression.

Malignant cell glycoproteins and glycolipids

The cell surface is involved in cell growth and division, cell-cell interaction, communication, differentiation and migration, and other processes likely to be involved in malignant transformation and/or the metastatic spread of cancer. Although there are many alterations of glycoproteins and glycolipids on the malignant cell surface, it is unclear whether these alterations are epiphenomena or an integral part of the malignancy process. This article reviews the recent literature and some earlier studies relevant for understanding emerging concepts and trends with respect to malignant cell glycoconjugates. Emphasis is on structural alterations of the carbohydrate portions of malignant cell glycoproteins and glycolipids and on the enzymes (glycosyltransferases and glycosidases) involved in their metabolism. Practical applications derived from malignant cell glycoconjugate studies are discussed briefly with respect to the diagnosis, staging, monitoring, and treatment of malignant disease. The review concludes by indicating which research areas on malignant cell glycoconjugates are likely to be fruitful in increasing our basic understanding of, and ability to deal effectively with, malignant disease.

Activation of the feline c-fms proto-oncogene: multiple alterations are required to generate a fully transformed phenotype

The v-fms oncogene is capable of producing tumors in vivo and transforming cells in culture; in contrast, the c-fms proto-oncogene is nontransforming. In this report we present the complete nucleotide sequence of a feline c-fms cDNA, the progenitor of the v-fms oncogene. Comparison of this sequence with that of v-fms shows that the proteins encoded by these two genes differ by nine amino acid substitutions and the replacement of 50 C-terminal amino acids present in c-fms by 11 unrelated residues in v-fms. Using chimeric fms genes and site-directed mutagenesis, we have determined that the C-terminal modification present in v-fms is sufficient to generate a partially transforming phenotype, but that mutations at amino acid positions 301 and 374 are required (in addition to the C-terminal modification) to generate a fully transforming fms gene.

A point mutation in the extracellular domain of the human CSF-1 receptor (c-fms proto-oncogene product) activates its transforming potential

A human CSF-1 receptor containing an "activating" mutation in its extracellular domain (serine for leucine 301) induced morphologic transformation, anchorage-independent growth, and tumorigenicity in mouse NIH 3T3 cells. A second regulatory mutation within the receptor's intracytoplasmic carboxy-terminal tail (phenylalanine for tyrosine 969) augmented transforming efficiency but was itself insufficient to induce transformation. Like the v-fms oncogene product, receptors bearing the activating mutation retained high-affinity binding sites for CSF-1 but were retarded in transport to the cell surface and were phosphorylated on tyrosine in the absence of ligand. Although the activating mutation does not affect the CSF-1 binding site in the receptor extracellular domain, it must induce a conformational change that mimics the effect of ligand binding, resulting in CSF-1-independent signals for cell growth.

Colony-stimulating factor-1 receptor (c-fms)

The macrophage colony-stimulating factor, CSF-1 (M-CSF), is a homodimeric glycoprotein required for the lineage-specific growth of cells of the mononuclear phagocyte series. Apart from its role in stimulating the proliferation of bone marrow-derived precursors of monocytes and macrophages, CSF-1 acts as a survival factor and primes mature macrophages to carry out differentiated functions. Each of the actions of CSF-1 are mediated through its binding to a single class of high-affinity receptors expressed on monocytes, macrophages, and their committed progenitors. The CSF-1 receptor (CSF-1R) is encoded by the c-fms proto-oncogene, and is one of a family of growth factor receptors that exhibits an intrinsic tyrosine-specific protein kinase activity. Transduction of c-fms sequences as a viral oncogene (v-fms) in the McDonough (SM) and HZ-5 strains of feline sarcoma virus has resulted in alterations in receptor coding sequences that affect its activity as a tyrosine kinase and provide persistent signals for cell growth in the absence of its ligand. The genetic alterations in the c-fms gene that unmask its latent transforming potential abrogate its lineage-specific activity and enable v-fms to transform a variety of cells that do not normally express CSF-1 receptors.

Transformation by the oncogene v-fms: the effects of castanospermine on transformation-related parameters

The effects of castanospermine on various parameters associated with transformation were examined in cells expressing the viral oncogene v-fms. Fischer rat embryo (FRE) cells transformed by the oncogene v-fms and grown in the presence of castanospermine reverted to a more normal cell morphology and accumulated fms protein within the endoplasmic reticulum. Treated cells attained contact inhibition of cell growth at a much lower cell density compared to the untreated controls. No effect of castanospermine on cell growth was observed for FRE cells transformed by a different oncogene v-fgr. Castanospermine-treated SM-FRE (v-fms transformed) cells reexpressed extracellular matrix fibronectin and exhibited an extensive actin-containing cytoskeleton similar to that of normal nontransformed FRE cells. Castanospermine treatment of SM-FRE cells resulted in a sixfold decrease in [3H]deoxyglucose uptake compared to that of the nonreverted SM-FRE cells. Again, no effect was observed in FRE cells transformed by the oncogene v-fgr (GR-FRE). These results further characterize the reversion caused by castanospermine and indicate that cell surface expression coordinately controls anchorage independent growth, cell morphology, contact inhibition of growth, and hexose uptake.

Multilineage hematopoietic disorders induced by transplantation of bone marrow cells expressing the v-fms oncogene

Mouse bone marrow cells infected with a helper-free retrovirus containing v-fms were engrafted into lethally irradiated mice. Dominant provirus-positive clones emerged in the spleens of some recipients within 1 month. When spleen cells were transplanted into lethally irradiated secondary recipients, clonal erythroleukemias or B cell lymphomas expressing the v-fms-coded glycoprotein developed. Other secondary recipients repopulated by "unmarked" progenitor cells or by cryptic provirus-positive precursors present in the spleens of the same donor mice did not develop disease; thus cells expressing v-fms did not invariably have a proliferative advantage after transplantation. Several primary engrafted recipients developed myeloproliferative disorders that were provirus-positive without evidence of clonality. Although expression of the c-fms product (CSF-1 receptor) is normally restricted to cells of the mononuclear phagocyte series, the v-fms-coded glycoprotein can contribute to proliferative abnormalities of multiple hematopoietic lineages.

Transforming potential of the c-fms proto-oncogene (CSF-1 receptor)

The c-fms proto-oncogene encodes a transmembrane glycoprotein that is probably identical to the receptor for the macrophage colony stimulating factor, CSF-1. Forty C-terminal amino acids of the normal receptor are replaced by 11 unrelated residues in the feline v-fms oncogene product, deleting a C-terminal tyrosine residue (Tyr969) whose phosphorylation might negatively regulate the receptor kinase activity. We show that the human c-fms gene stimulates growth of mouse NIH 3T3 cells in agar in response to human recombinant CSF-1, indicating that receptor transduction is sufficient to induce a CSF-1 responsive phenotype. Although cells transfected with c-fms genes containing either Tyr969 or Phe969 were not transformed, cotransfection of these genes with CSF-1 complementary DNA induced transformation, with c-fms(Phe969) showing significantly more activity than c-fms(Tyr969). In the absence of CSF-1, chimaeric v-fms/c-fms genes encoding the wild-type c-fms C terminus were poorly transforming, whereas chimaeras bearing Phe969 were as transforming as v-fms. Thus, the Phe969 mutation, although not in itself sufficient to induce transformation, activates the oncogenic potential of c-fms in association with an endogenous ligand or in conjunction with mutations elsewhere in the c-fms gene that confer ligand-independent signals for growth.

Transformation by the v-fms oncogene product: an analog of the CSF-1 receptor

The product of the c-fms proto-oncogene is related to, and possibly identical with, the receptor for the macrophage colony-stimulating factor, M-CSF (CSF-1). Unlike the product of the v-erbB oncogene, which is a truncated version of the EGF receptor, the glycoprotein encoded by the v-fms oncogene retains an intact extracellular ligand-binding domain so that cells transformed by v-fms express CSF-1 receptors at their surface. Although fibroblasts susceptible to transformation by v-fms generally produce CSF-1, v-fms-mediated transformation does not depend on an exogenous source of the growth factor, and neutralizing antibodies to CSF-1 do not affect the transformed phenotype. An alteration of the v-fms gene product at its extreme carboxyl-terminus represents the major structural difference between it and the c-fms-coded glycoprotein and may affect the tyrosine kinase activity of the v-fms-coded receptor. Consistent with this interpretation, tyrosine phosphorylation of the v-fms products in membranes was observed in the absence of CSF-1 and was not enhanced by addition of the murine growth factor. Cells transformed by v-fms have a constitutively elevated specific activity of a guanine nucleotide-dependent, phosphatidylinositol-4,5-diphosphate-specific phospholipase C. We speculate that the tyrosine kinase activity of the v-fms/c-fms gene products may be coupled to this phospholipase C, possibly through a G regulatory protein, thereby increasing phosphatidylinositol turnover and generating the intracellular second messengers diacylglycerol and inositol triphosphate.

Fibroblast and hematopoietic cell transformation by the fms oncogene (CSF-1 receptor)

The c-fms proto-oncogene encodes the receptor for the mononuclear phagocyte colony stimulating factor, CSF-1. Although the tyrosine kinase activity of the CSF-1 receptor is stimulated by its ligand, the viral oncogene, v-fms, encodes a constitutive receptor kinase that can transform both fibroblasts and hematopoietic cells by a nonautocrine mechanism. Mutations in the c-fms gene as well as a critical alteration of the distal 3' coding sequences appear to be responsible for fully activating its latent transforming potential. The v-fms gene can convert CSF-1 or IL-3 dependent hematopoietic cell lines to factor independence and render them tumorigenic. Expression of the v-fms gene product does not transmodulate the normal receptors for CSF-1 or IL-3 and affects neither their affinity, number, nor potential to be independently down-regulated by their ligands or by phorbol esters. The ability of v-fms to transform hematopoietic target cells suggests that critical alterations in the c-fms proto-oncogene might similarly contribute to leukemia.

The v-fms oncogene induces factor independence and tumorigenicity in CSF-1 dependent macrophage cell line

The McDonough strain of feline sarcoma virus (SM-FeSV) transforms fibroblast cell lines in culture and produces fibrosarcomas in domestic cats. SM-FeSV does not induce haematopoietic malignancies in spite of the fact that its viral oncogene, v-fms, codes for a glycoprotein related to the receptor for the mononuclear phagocyte colony stimulating factor, CSF-1. The v-fms-coded polypeptide includes the complete extracellular domain of the c-fms proto-oncogene product and retains the ability to bind CSF-1 specifically. The two molecules have very similar sequences except at their extreme carboxyl terminal ends where 40 amino acids of the c-fms-coded glycoprotein are replaced by 11 unrelated residues in the v-fms product. Autophosphorylation of the c-fms gene product on tyrosine is enhanced by CSF-1 addition, whereas phosphorylation of the v-fms-coded glycoprotein appears to be constitutive. We now show that introduction of the v-fms gene into simian virus-40 (SV40)-immortalized, CSF-1 dependent macrophages renders them independent of CSF-1 for growth and tumourigenic in nude mice. These factor-independent cell lines express unaltered levels of the c-fms product which is down-modulated in response to either CSF-1 or the tumour promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA). The induction of factor independence by a non-autocrine mechanism suggests that the v-fms product is an unregulated kinase that provides growth stimulatory signals in the absence of ligand.

The amino-terminal domain of the v-fms oncogene product includes a functional signal peptide that directs synthesis of a transforming glycoprotein in the absence of feline leukemia virus gag sequences

The nucleotide sequence of a 5' segment of the human genomic c-fms proto-oncogene suggested that recombination between feline leukemia virus and feline c-fms sequences might have occurred in a region encoding the 5' untranslated portion of c-fms mRNA. The polyprotein precursor gP180gag-fms encoded by the McDonough strain of feline sarcoma virus was therefore predicted to contain 34 v-fms-coded amino acids derived from sequences of the c-fms gene that are not ordinarily translated from the proto-oncogene mRNA. The (gP180gag-fms) polyprotein was cotranslationally cleaved near the gag-fms junction to remove its gag gene-coded portion. Determination of the amino-terminal sequence of the resulting v-fms-coded glycoprotein, gp120v-fms, showed that the site of proteolysis corresponded to a predicted signal peptidase cleavage site within the c-fms gene product. Together, these analyses suggested that the linked gag sequences may not be necessary for expression of a biologically active v-fms gene product. The gag-fms sequences of feline sarcoma virus strain McDonough and the v-fms sequences alone were inserted into a murine retroviral vector containing a neomycin resistance gene. Both constructs were biologically active when transfected into NIH 3T3 cells and produced morphologically transformed foci at equivalent efficiencies. When transfected into a cell line (psi 2) expressing complementary viral gene functions, G418-resistant (Neor) cells containing either of these vector DNAs produced high titers of transforming viruses. Analysis of proteins produced in cells containing the vector lacking gag gene sequences showed that gP180gag-fms was not synthesized, whereas normal levels of both immature gp120v-fms and mature gp140v-fms were detected. The glycoprotein was efficiently transported to the cell surface, and it retained wild-type tyrosine kinase activity. We conclude that a cryptic hydrophobic signal peptide sequence in v-fms was unmasked by gag deletion, thereby allowing the correct orientation and transport of the v-fms gene product within membranous organelles. It seems likely that the proteolytic cleavage of gP180gag-fms is mediated by signal peptidase and that the amino termini of gp140v-fms and the c-fms gene product are identical.

Antibodies to distal carboxyl terminal epitopes in the v-fms-coded glycoprotein do not cross-react with the c-fms gene product

The product of the v-fms oncogene is an integral transmembrane glycoprotein that is closely related to the cell surface receptor for the macrophage colony stimulating factor, CSF-1. A fragment of the v-fms gene encoding a major portion of the extracellular amino terminal domain, the membrane-spanning segment, and the entire carboxyl terminal tyrosine kinase domain of the glycoprotein was molecularly cloned into an inducible prokaryotic expression plasmid. Polypeptide products consisting only of v-fms-coded amino acids were produced in bacteria and were used to prepare immune reagents that precipitated the v-fms-coded glycoproteins expressed in transformed cells. Whereas rabbit antisera to recombinant polypeptides detected antigenic determinants of the c-fms proto-oncogene product, seven mouse monoclonal antibodies to these same antigens reacted only with v-fms-specific epitopes. Proteolytic mapping experiments and studies with a mutant v-fms-coded glycoprotein lacking the 37 carboxyl terminal amino acids of the wild-type product showed that the monoclonal antibodies were restricted in their reactivity to epitopes at the extreme carboxyl terminus of the glycoprotein. The v-fms and c-fms gene products must differ significantly in this region.

Analysis of the autophosphorylation activity of transformation defective mutants of avian erythroblastosis virus

The v-erb B protein of avian erythroblastosis virus (AEV) possesses an associated protein kinase activity in vitro. Analysis of temperature-sensitive mutants, and nonconditional host range mutants of AEV demonstrated that there was no simple correlation between this autophosphorylation activity and the transformation ability of the various AEV mutants. These data suggest that although this kinase activity may be central to transformation by AEV it is in itself insufficient.

A new acute transforming feline retrovirus and relationship of its oncogene v-kit with the protein kinase gene family

A new acute transforming feline retrovirus, the Hardy-Zuckerman 4 feline sarcoma virus (HZ4-FeSV), has been isolated from a feline fibrosarcoma. The viral genome of HZ4-FeSV contains a new oncogene designated v-kit, has the structure 5' delta gag-kit-delta pol-delta env 3' and specifies a gag-kit polyprotein of relative molecular mass 80,000. The predicted kit amino-acid sequence displays partial homology with tyrosine-specific protein kinase oncogenes. HZ4-FeSV appears to have been generated by transduction of feline c-kit sequences with feline leukaemia virus.

Fractionation of membrane proteins on immobilized lectins by high-performance liquid affinity chromatography

Detergent-solubilized glycoproteins were fractionated on high-performance affinity columns employing A concanavalin and Pisum sativum agglutinin as ligands immobilized on microparticulate silica via a propyl spacer. The separations were characterized by high recovery (90-95%) and high specificity (enrichment factor 6- and 58-fold for the bound fractions on A concanavalin and P. sativum agglutinin columns, respectively, compared with the crude extract), as estimated by enzyme-linked lectin assay and chromatographic criteria. In addition, the short running times (30-40 min) make this method highly useful for a first characterization of complex glycoprotein samples and of individual molecules.

Expression of the human c-fms proto-oncogene in hematopoietic cells and its deletion in the 5q- syndrome

The c-fms proto-oncogene was shown to be expressed in human bone marrow and in differentiated blood mononuclear cells, suggesting that its gene product plays a role in hematopoietic maturation. The c-fms mRNA was not detected in HL-60 cells, an established promyelocytic line, whereas c-fms expression appeared 48 hr after induction when most cells had differentiated into macrophages. An acquired deletion of chromosome 5 (5q-) in bone marrow cells is associated with abnormalities in blood cell production. The normal 5 and 5q- chromosomes were segregated by construction of cell hybrids between bone marrow and rodent cells. A selective system was used that requires retention of the structural gene for dihydrofolate reductase, located on human chromosome 5. Analysis of DNA from individual hybrid clones revealed that the 5q- deletion had removed the c-fms gene. We postulate that hemizygosity at the c-fms locus leads to abnormalities in hematopoietic maturation.

Expression of the c-fms proto-oncogene during human monocytic differentiation

The McDonough strain of the feline sarcoma virus contains a transforming gene (v-fms) which contains partial nucleotide homology with proto-oncogenes encoding tyrosine kinases. One of the v-fms-encoded products, gp140fms, is a cell surface transmembrane glycoprotein that may function as a growth factor receptor. Although c-fms transcripts have been detected in placental trophoblasts and normal human bone marrow, the role of the c-fms gene product is unknown. We now report that induction of monocytic, but not granulocytic, differentiation of human HL-60 leukaemic cells is associated with expression of c-fms, preceded by that of c-myc and c-fos. Because c-fms transcripts are also detectable in peripheral blood monocytes and in blasts from certain patients with myelomonocytic leukaemia, the c-fms gene product may play a role in monocytic differentiation.

Down-modulation of an oncogene protein product and reversion of the transformed phenotype by monoclonal antibodies

Exposure of neu-oncogene-transformed NIH 3T3 cells to monoclonal antibodies reactive with the neu gene product, p185, results in the rapid and reversible loss of both cell-surface and total cellular p185. Although not directly cytotoxic, monoclonal anti-p185 antibody treatment causes neu-transformed NIH 3T3 cells to revert to a nontransformed phenotype, as determined by anchorage-independent growth. Isotype matched control antibodies of an unrelated specificity do not affect p185 levels or colony formation in soft agar by neu-transformed NIH 3T3 cells. Soft agar colony formation by NIH 3T3 cells transformed by ras oncogenes is not affected by anti-p185 antibody treatment. Anchorage-independent growth of cells from the ethylnitrosourea-induced rat neuroblastoma line in which neu was originally detected by DNA transfection is also inhibited in the presence of anti-p185 monoclonal antibodies. Collectively, these results suggest that p185 is required to maintain transformation induced by the neu oncogene.