Abelson virus drives the differentiation of Harvey virus-infected erythroid cells

Transforming pathways activated by the v-Abl tyrosine kinase

The Abelson Murine Leukemia Virus (A-MuLV) is the acute transforming retrovirus encoding the v-abl oncogene. Two isolates of the virus encoding proteins of p120 Kd and 160 Kd have been extensively studied. These viral isolates have been found to transform both hematopoietic and fibroblastic cells in vitro, while inducing predominantly pre-B cell leukemias in vivo. Both p120(v-Abl) and p160(v-Abl) are plasma membrane-associated non-receptor tyrosine kinases and the transforming activity of these proteins requires their tyrosine kinase activity. A-MuLV infection of hematopoietic cells has often been found to result in the abrogation of their cytokine-dependence for growth. In addition, v-Abl expressing hematopoietic cells often lose their ability to differentiate in response to appropriate cytokines. This review discusses some of the early transformation studies of A-MuLV, as well as some of the findings concerning the structure and biochemical activity of the v-Abl protein. Finally, we discuss the mechanisms associated with v-Abl mediated transformation through examination of the various signal transduction pathways activated by this oncogene.

Modulation of p210(BCR-ABL) activity in transduced primary human hematopoietic cells controls lineage programming

Retroviral transduction of primary hematopoietic cells with human oncogenes provides a powerful approach to investigating the molecular mechanisms controlling the normal proliferation and differentiation of these cells. Here we show that primitive human CD34(+) cord blood cells, including multipotent as well as granulopoietic- and erythroid-restricted progenitors, can be efficiently transduced with a MSCV-BCR-ABL-IRES-GFP retrovirus, resulting in the sustained expression by their progeny of very high levels of tyrosine phosphorylated p210(BCR-ABL). Interestingly, even in the presence of growth factors that supported the exclusive production of granulopoietic cells from green fluorescent protein (GFP)-transduced control cells, BCR-ABL-transduced progenitor subpopulations generated large numbers of erythropoietin-independent terminally differentiating erythroid cells and reduced numbers of granulopoietic cells. Analyses of individual clones generated by single transduced cells in both semisolid and liquid cultures showed this BCR-ABL-induced erythroid differentiation response to be elicited at a high frequency from all types of transduced CD34(+) cells independent of their apparent prior lineage commitment status. Additional experiments showed that this erythroid differentiation response was largely prevented when the cells were transduced and maintained in the presence of the BCR-ABL-specific tyrosine kinase inhibitor, STI-571. These findings indicate that overexpression of BCR-ABL in primary human hematopoietic cells can activate an erythroid differentiation program in apparently granulopoietic-restricted cells through a BCR-ABL kinase-dependent mechanism, thus providing a new molecular tool for elucidating mechanisms underlying lineage fate determination in human hematopoietic cells and infidelity in human leukemia.

Philadelphia chromosome-positive leukaemia: the translocated genes and their gene products

Overwhelming evidence indicates a role for the deregulated ABL protein tyrosine kinase in the aetiology of CML and Ph-positive acute leukaemia. These disorders are characterized by the generation of BCR/ABL fusion proteins with elevated tyrosine kinase activity. Although much is known concerning the transforming potential of ABL proteins in various systems, very little is understood of the normal function and mode of regulation of ABL activity. The mechanism of oncogenic activation is therefore also obscure. In spite of this, our understanding of the molecular details of these chromosomal translocations allows the design of therapies directed against their unique, leukaemia-specific proteins and RNA products.

Elevated activities of protein kinase C and tyrosine kinase correlate to leukemic cell aggressiveness

We report a linkage between cell aggressiveness, protein kinase C (PKC) activity, tyrosine kinase (PTK) activity and serum requirement. We used 2 leukemic cell lines induced by Moloney murine leukemia virus (MLV). One line was highly aggressive (BS-24-1) and required low serum concentrations (3%) for optimal growth in comparison to the less aggressive line (RO2T) that needed 10% serum for optimal growth. The more malignant cells exhibited higher PKC and PTK activity. This activity was independent of serum concentration between 0.01-10%. In contrast, the weakly malignant cells need a high serum concentration (10%) for optimal PKC or PTK activity. Immunoblot analysis revealed a higher level of PKC protein in the BS-24-1 cells than in the RO2T cells. Serum induction of PKC activity did not change the amount of PKC protein in the cytosol or the membrane fractions, indicating post-translational mechanism regulation of PKC. We suggest that the aggressiveness of BS-24-1 resulted from its ability to become independent of growth regulation by serum factors, via autocrine stimulation of PKC and PTK.

Differences in oncogenic potency but not target cell specificity distinguish the two forms of the BCR/ABL oncogene

Two forms of activated BCR/ABL proteins, P210 and P185, that differ in BCR-derived sequences, are associated with Philadelphia chromosome-positive leukemias. One of these diseases is chronic myelogenous leukemia, an indolent disease arising in hematopoietic stem cells that is almost always associated with the P210 form of BCR/ABL. Acute lymphocytic leukemia, a more aggressive malignancy, can be associated with both forms of BCR/ABL. While it is virtually certain that BCR/ABL plays a central role in both of these diseases, the features that determine the association of a particular form with a given disease have not been elucidated. We have used the bone marrow reconstitution leukemogenesis model to test the hypothesis that BCR sequences influence the ability of activated ABL to transform different types of hematopoietic cells. Our studies reveal that both P185 and P210 induce a similar spectrum of hematological diseases, including granulocytic, myelomonocytic, and lymphocytic leukemias. Despite the similarity of the disease patterns, animals given P185-infected marrow developed a more aggressive disease after a shorter latent period than those given P210-infected marrow. These data demonstrate that the structure of the BCR/ABL oncoprotein does not affect the type of disease induced by each form of the oncogene but does control the potency of the oncogenic signal.

Differences in oncogenic potency but not target cell specificity distinguish the two forms of the BCR/ABL oncogene

Two forms of activated BCR/ABL proteins, P210 and P185, that differ in BCR-derived sequences, are associated with Philadelphia chromosome-positive leukemias. One of these diseases is chronic myelogenous leukemia, an indolent disease arising in hematopoietic stem cells that is almost always associated with the P210 form of BCR/ABL. Acute lymphocytic leukemia, a more aggressive malignancy, can be associated with both forms of BCR/ABL. While it is virtually certain that BCR/ABL plays a central role in both of these diseases, the features that determine the association of a particular form with a given disease have not been elucidated. We have used the bone marrow reconstitution leukemogenesis model to test the hypothesis that BCR sequences influence the ability of activated ABL to transform different types of hematopoietic cells. Our studies reveal that both P185 and P210 induce a similar spectrum of hematological diseases, including granulocytic, myelomonocytic, and lymphocytic leukemias. Despite the similarity of the disease patterns, animals given P185-infected marrow developed a more aggressive disease after a shorter latent period than those given P210-infected marrow. These data demonstrate that the structure of the BCR/ABL oncoprotein does not affect the type of disease induced by each form of the oncogene but does control the potency of the oncogenic signal.

Expression of ras-like proteins in embryonic and adult cells of Xenopus laevis

A polyclonal antibody raised against v-Ha-ras p21 was purified and its specificity was checked on Ha-ras transformed cell lines. It was used to immunoprecipitate p21 from different Xenopus laevis cell types: brain cells, blood cells, and embryonic material. By one-dimensional Western blot analysis, we show that ras p21 is synthesized very early in oogenesis and accumulates throughout vitellogenesis. The ras p21 content, estimated to be 1.1 ng in the full-grown oocyte, remains constant during oocyte maturation and egg cleavage. Increase in the amount of ras p21 occurs at the beginning of neurulation. Two-dimensional Western blot patterns reveal the presence of multiple molecular forms of p21 in all Xenopus cell types studied. The numerous resolved polypeptides were ascribed to the expression of at least two different ras genes. Furthermore, specific charge modifications of the ras polypeptides are observed in brain, blood, and embryonic cells. During oogenesis and early embryonic development, differences in two-dimensional patterns mainly concern variations in the relative amounts of the different polypeptides. The results are discussed in relation to the well documented synthesis activities of the growing oocyte and of the early developing embryo.

Serum independence of transcription from the promoter of an avian retrovirus in v-src-transformed cells is a primary, intracellular effect of increased tyrosine phosphorylation

We found that transcription from the promoter in the long terminal repeat of Rous sarcoma virus in rat 3Y1 cells is dependent on the presence of serum in the culture. However, this serum dependence of transcription was relieved when 3Y1 cells were transformed by the oncogene v-src. Crossfeeding experiments showed no evidence for the production of a serum-substituting extracellular growth factor by the transformed cells. Using 3Y1 cells transformed with temperature-sensitive Rous sarcoma virus, we showed that the tyrosine kinase activity of pp60v-src was responsible for the serum-sparing effect on the level of RNA expressed from the viral promoter. Sodium orthovanadate, an inhibitor of phosphotyrosine phosphatases that nonspecifically elevates the level of phosphotyrosine-containing proteins in cells, stimulated transcription from the viral promoter. The effects of both pp60v-src and orthovanadate were resistant to cycloheximide. These results suggest that the serum independence of transcription from the viral promoter in v-src-transformed cells was probably due to the constitutive activation of intracellular growth-factor pathways by the tyrosine kinase activity of pp60v-src.

Competition among leukemic cells

We investigated competition among leukemic cells induced by Moloney murine leukemia virus (MoLV) in order to understand the mechanisms involved in the generation of leukemia. We used six leukemic cells lines from Balb/C mice infected with MoLV. Each line had a unique genetic marker which enabled us to trace it in mixtures of cells either in vivo or in vitro. The markers were a unique rearrangement of T-cell receptors, the integration sites of the retrovirus and rearrangements in the Pim-1 oncogene. A mixture of two cell lines (1:1) injected into intact Balb/C mice usually produced a monoclonal tumor originating from one cell line. In most cases, the cell lines that were aggressive in vivo were also dominant in mixing experiments in vitro. In some lines, we could correlate the aggressiveness of the tumor to its superior growth rate and lower requirement for serum factors in vitro. In others, this correlation did not hold, and we assumed that host factors like the immune system contribute to the malignant potential of the leukemic cell.

Abelson murine leukemia virus induces platelet-derived growth factor-independent fibroblast growth: correlation with kinase activity and dissociation from full morphologic transformation

Abelson murine leukemia virus (A-MuLV) encodes a single protein product, a tyrosine-specific protein kinase, whose activity is necessary for cell transformation by this retrovirus. Using a defined medium culture system, we demonstrate that transformation of NIH 3T3 fibroblasts by A-MuLV abrogates their normal requirement for platelet-derived growth factor (PDGF) for cell growth. Analysis of constructed insertional mutant viruses revealed an absolute correlation between A-MuLV-encoded tyrosine kinase activity and PDGF-independent fibroblast growth. Sequences of the provirus not required for kinase activity appeared unnecessary for abrogating the fibroblast requirement for PDGF. Conversely, sequences required for kinase activity appeared necessary, suggesting that induction of PDGF-independent fibroblast growth, like cell transformation, is a function of this tyrosine kinase. Fibroblasts transformed by a partially transformation-defective mutant demonstrated incomplete morphological transformation but were still independent of PDGF for growth. Thus, the processes of full morphological transformation and growth factor independence can be partially dissociated.

Abelson murine leukemia virus induces platelet-derived growth factor-independent fibroblast growth: correlation with kinase activity and dissociation from full morphologic transformation

Abelson murine leukemia virus (A-MuLV) encodes a single protein product, a tyrosine-specific protein kinase, whose activity is necessary for cell transformation by this retrovirus. Using a defined medium culture system, we demonstrate that transformation of NIH 3T3 fibroblasts by A-MuLV abrogates their normal requirement for platelet-derived growth factor (PDGF) for cell growth. Analysis of constructed insertional mutant viruses revealed an absolute correlation between A-MuLV-encoded tyrosine kinase activity and PDGF-independent fibroblast growth. Sequences of the provirus not required for kinase activity appeared unnecessary for abrogating the fibroblast requirement for PDGF. Conversely, sequences required for kinase activity appeared necessary, suggesting that induction of PDGF-independent fibroblast growth, like cell transformation, is a function of this tyrosine kinase. Fibroblasts transformed by a partially transformation-defective mutant demonstrated incomplete morphological transformation but were still independent of PDGF for growth. Thus, the processes of full morphological transformation and growth factor independence can be partially dissociated.

Growth factor-induced phosphorylation of c-ras p21 in normal hemopoietic progenitor cells

Normal murine hemopoietic progenitor cells (colony-forming cells, CFC), representing 0.2% of the bone marrow cell population, were purified to homogeneity by fluorescence-activated cell sorting. CFC require the presence of the murine hemopoietic regulator, granulocyte-macrophage-colony stimulating factor (GM-CSF) for survival, proliferation, and differentiation along the myeloid pathway. An analysis of protein phosphorylation in GM-CSF-stimulated CFC over a 20-hr period demonstrated three phosphoproteins of approximate MW 21 kd and pI 6.2, 5.7 and 5.2 p21-6.2 persisted for 14 hr, while p21-5.7 and p21-5.2 were only detected during the first 5 hr of the analysis. The phosphate turnover time in all three p21 proteins was less than 3 hr and p21-5.2 contains an alkaline-resistant phosphorylation site. Low levels of p21-6.2 were also detected in unstimulated CFC. The observation of these phosphoproteins led us to investigate c-ras p21 in CFC. Immune precipitation with the anti-Ha/Ki-ras p21 monoclonal antibody (Y13-259) showed that expression of c-ras p21 in CFC was independent of GM-CSF stimulation, but that phosphorylated c-ras p21 was present only after GM-CSF stimulation. CFC contained one-tenth of the amount of phosphorylated c-ras p21 per cell compared with v-Ha-ras-transformed fibroblasts. It is possible that the phosphorylation of c-ras p21 in CFC has a significant role in the growth factor-directed molecular cascade responsible for normal hemopoietic development.

Selective transformation of primitive lymphoid cells by the BCR/ABL oncogene expressed in long-term lymphoid or myeloid cultures

The BCR/ABL gene, formed by the Philadelphia chromosome translocation (Ph1) of human chronic myelogenous leukemia, encodes an altered ABL gene product, P210. P210 is strongly implicated in the malignant process of chronic myelogenous leukemia, but it precise role is unknown. Infection of long-term bone marrow cultures enriched for B-lymphoid cell types with a Moloney murine leukemia virus retroviral vector containing the BCR/ABL cDNA resulted in clonal outgrowths of immature B-lymphoid cells which expressed abundant P210 kinase activity. Surprisingly, infection of long-term myeloid lineage-enriched cultures also resulted in clonal outgrowths of immature B-lymphoid cells. The P210-expressing lymphoid cell lines resulting from either type of culture were resistant to the lethal effects of corticosteroids. These findings indicate that high levels of P210 expressed from a Moloney murine leukemia virus long terminal repeat preferentially stimulate the growth of immature B-lineage cells, and this effect is apparent even in myeloid lineage-enriched cultures, in which few if any lymphoid cells can be detected prior to infection.

In vitro-derived leukemic erythroid cell lines induced by a raf- and myc-containing retrovirus differentiate in response to erythropoietin

In vitro infection of murine fetal liver cells with a retrovirus containing v-raf and v-myc oncogenes has produced continuous lines of immature erythroid cells that are leukemogenic. These cells synthesized a factor that stimulated their growth in vitro before autonomous variants emerged. Approximately 1000 high-affinity erythropoietin receptors could be detected per cell, and the hormone induced terminal differentiation in these cells. The lines were generated at an extremely low frequency (approximately 1 in 10(7) cells), suggesting that the combination of raf and myc is insufficient to develop erythroid cell lines and that additional events are necessary for transformation.

Selective transformation of primitive lymphoid cells by the BCR/ABL oncogene expressed in long-term lymphoid or myeloid cultures

The BCR/ABL gene, formed by the Philadelphia chromosome translocation (Ph1) of human chronic myelogenous leukemia, encodes an altered ABL gene product, P210. P210 is strongly implicated in the malignant process of chronic myelogenous leukemia, but it precise role is unknown. Infection of long-term bone marrow cultures enriched for B-lymphoid cell types with a Moloney murine leukemia virus retroviral vector containing the BCR/ABL cDNA resulted in clonal outgrowths of immature B-lymphoid cells which expressed abundant P210 kinase activity. Surprisingly, infection of long-term myeloid lineage-enriched cultures also resulted in clonal outgrowths of immature B-lymphoid cells. The P210-expressing lymphoid cell lines resulting from either type of culture were resistant to the lethal effects of corticosteroids. These findings indicate that high levels of P210 expressed from a Moloney murine leukemia virus long terminal repeat preferentially stimulate the growth of immature B-lineage cells, and this effect is apparent even in myeloid lineage-enriched cultures, in which few if any lymphoid cells can be detected prior to infection.

Differentiation of erythroid progenitor (CFU-E) cells from mouse fetal liver cells and murine erythroleukemia (TSA8) cells without proliferation

Erythropoietin (epo) appears to play a significant role in influencing the proliferation and differentiation of erythroid progenitor (CFU-E) cells. To determine the mechanism of action of epo, the effect of drugs on the in vitro colony formation of CFU-E cells induced from a novel murine erythroleukemia cell line, TSA8, was examined. While cytosine arabinoside inhibited colony formation and terminal differentiation of the CFU-E cells responding to epo, herbimycin, which is a drug that inhibits src-related phosphorylation, inhibited colony formation only. The same effect of herbimycin was observed with normal CFU-E cells from mouse fetal liver cells. These results suggest that epo induces two signals, one for proliferation and the other for differentiation, and that the two signals are not linked in erythroid progenitor cells.

[Cell proliferation and cooperation of v-mil and v-myc oncogenes]

Retroviruses which possess the property to recombine with genetic material from the cell, have cloned and activated some oncogenes and hence are a privileged source for the study of these genes. Cellular oncogene activation can occur following two non mutually exclusive ways: (i) by over-expression of their products; (ii) by modifications of their products through mutations. Retroviruses can combine these two ways of activation leading to the over-expression of a modified product. In this paper, we present results obtained in the study of MH2, a retrovirus containing two oncogenes. We have shown that the two oncogenes of MH2 (v-mil and v-myc) cooperate in vitro to transform neuroretina cells from chicken embryos. These cells which normally do not grow in a defined medium, are induced to proliferate and become transformed upon infection by MH2. Our data enabled us to show that in MH2 v-mil was responsible for the induction of proliferation and v-myc for the transformation of the proliferating cells. Using in vitro constructs we located two regions in the protein encoded by v-mil which are important for its mitogenic property. We have also cloned the cellular counterpart of v-mil and the study of its biological activity on neuroretina cells enabled us to propose a mechanism of activation of the cellular gene by truncation of its 5' part.

Differentiation of erythroid progenitor (CFU-E) cells from mouse fetal liver cells and murine erythroleukemia (TSA8) cells without proliferation

Erythropoietin (epo) appears to play a significant role in influencing the proliferation and differentiation of erythroid progenitor (CFU-E) cells. To determine the mechanism of action of epo, the effect of drugs on the in vitro colony formation of CFU-E cells induced from a novel murine erythroleukemia cell line, TSA8, was examined. While cytosine arabinoside inhibited colony formation and terminal differentiation of the CFU-E cells responding to epo, herbimycin, which is a drug that inhibits src-related phosphorylation, inhibited colony formation only. The same effect of herbimycin was observed with normal CFU-E cells from mouse fetal liver cells. These results suggest that epo induces two signals, one for proliferation and the other for differentiation, and that the two signals are not linked in erythroid progenitor cells.

Proliferation and differentiation of Abelson virus-infected murine myeloid leukemia cell lines does not involve an autocrine mechanism

Culture in agar of cloned promonocytic leukemia cell lines derived from Abelson virus-infected mice produced colonies of both a compact and diffuse morphology. Diffuse colonies contained fewer cells capable of forming colonies when recultured in agar than did compact colonies. Serial subcloning of cells from diffuse, but not compact, colonies ultimately led to the complete loss of colony-forming cells, i.e., to clonal extinction. The production of both compact and diffuse agar colonies was independent of the cell density of either the static liquid culture from which cells were taken for culture in agar, or the number of cells per agar culture. Furthermore, bioassays of culture supernatants indicated the leukemia cells did not secrete hemopoietic growth factors active on normal hemopoietic cells, transforming growth factors active on adherent cell lines, or factors that influenced the growth of the leukemic cells themselves. Collectively, these data suggest neither growth-factor independent replication nor the spontaneous differentiation of Abelson virus-infected myeloid cells involves autocrine secretion of growth regulators.

The viral and cellular forms of the Abelson (abl) oncogene

The precision of molecular biology has allowed a better definition of the components of the Abelson system. We know the gene structures and gene products for the cellular and viral forms of this family of related tyrosine kinases. However, many basic issues first identified in the early biological observations of Abelson, Rabstein, and others remain unanswered. The precise pathway for transformation in biochemical terms remains unknown for Ab-MLV and all of its relatives. Relatively little can be said to explain the preferential growth stimulation for certain hematopoietic cell types by the viral and other altered forms of the oncogene, and no clear insights into the function of the normal cellular forms of the abl oncogene are available. Future progress will certainly depend on the intensive efforts by many workers in the broader field of cellular growth control mechanisms.

Differential expression of the ras gene family in mice

We compared the expression of the ras gene family (H-ras, K-ras, and N-ras) in adult mouse tissues and during development. We found substantial variations in expression among different organs and in the amounts of the different transcripts originating from each gene, especially for the N-ras gene. The expression patterns were consistent with the reported preferential tissue activation of ras genes and suggested different cellular functions for each of the ras genes.

Structure and origins of the HZ2-feline sarcoma virus

The HZ2-feline sarcoma virus (HZ2-FeSV) is a replication-defective acute transforming feline retrovirus with oncogene homology to Abelson murine leukemia virus (A-MuLV) (P. Besmer, W.D. Hardy,Jr., E. E. Zuckerman, P. J. Bergold, L. Lederman, and H. W. Snyder, Jr. (1983) Nature (London) 303, 825-828). In contrast to A-MuLV which was isolated from a hematopoietic tumor, the HZ2-FeSV derives from a multicentric fibrosarcoma. We have molecularly cloned the HZ2-FeSV provirus from mink HZ2-FeSV nonproducer cells. The molecularly cloned HZ2-FeSV provirus is biologically active upon transfection of NIH 3T3 indicator cells. The genetic structure of the HZ2-FeSV provirus was determined by EM heteroduplex and Southern blot analysis. The HZ2-FeSV has a 6.8 kb-viral genome with the structure: 5' delta gag-abl-delta pol-delta env 3'. The abl insert, which is 1.4 kb, is located 1.9 kb from the 5' end and 3.5 kb from the 3' end of the viral genome. The 5' 1.9 kb in the HZ2-FeSV are colinear with 5' FeLV sequences, and the 3' 3.5 kb are colinear with 3' FeLV sequences, with the exception of a 0.85-kb deletion in the env gene. HZ2-FeSV v-abl and A-MuLV v-abl share 1.2 kb of abl sequences which are known to specify the protein kinase domain of the abl gene product and are necessary for fibroblast transformation in vitro. The DNA from several tumor tissues of cat 3590 from which the HZ2-FeSV was obtained was found to contain several HZ2-FeSV-related proviruses including the HZ2-FeSV. The variant HZ2-FeSVs have indistinguishable 5' gag-abl sequences; however, they differ in 3' sequences which likely do not include any abl sequences. The DNAs from fibrosarcomas obtained by inoculation of kittens with tumor extract were found to contain variant HZ2-FeSV proviruses as well. Taken together these results indicate a role for the HZ2-FeSVs in sarcomagenesis.

Nucleic acid sequence and oncogenic properties of the HZ2 feline sarcoma virus v-abl insert

Hardy-Zuckerman 2 feline sarcoma virus (HZ2-FeSV), isolated from a multicentric feline fibrosarcoma is a replication-defective acute transforming feline retrovirus which originated by transduction of feline c-abl sequences with feline leukemia virus (FeLV) and is known to encode a 110-kilodalton gag-abl fusion protein with tyrosine-specific protein kinase activity (P. Besmer, W. D. Hardy, E. E. Zuckerman, P. J. Bergold, L. Lederman, and H. W. Snyder, Nature (London) 303:825-828, 1983). The nucleotide sequence of the abl segment in the HZ2-FeSV genome was determined and compared with the murine and human v-abl and c-abl sequences. The predicted transforming protein consists of 344 amino acids (aa) of FeLV gag origin, 439 aa of abl origin, and at least 200 aa of FeLV pol origin (p110gag-abl-pol). The 1,317-base-pair HZ2-FeSV v-abl segment (fv-abl) corresponds to 5' abl sequences which include the region known to specify the protein kinase domain. The 5' 189 base pairs of fv-abl correspond to 5' c-abl sequences not contained in Abelson murine leukemia virus (MuLV) v-abl. The mouse c-abl exon which contains these segments was identified, and its nucleotide sequence was determined. Comparison of the predicted amino acid sequence of fv-abl with those of Abelson MuLV v-abl and c-abl revealed five aa differences. The 5' junction between FeLV and abl was found to involve a preferred region in FeLV gag p30 (P. Besmer, J. E. Murphy, P. C. George, F. H. Qiu, P. J. Bergold, L. Lederman, H. W. Snyder, D. Brodeur, E. E. Zuckerman, and W. D. Hardy, Nature (London) 320:415-421, 1986). A six-base homology exists at the recombination site between the parental FeLV and the c-abl sequences. The 3' junction between fv-abl and FeLV pol predicts an in-frame fusion of fv-abl and FeLV pol. A transformed cell line containing a truncated gag-abl-pol protein, p85, that lacks most of the FeLV pol sequences was obtained by transfection of NIH 3T3 mouse cells. This result implies that the pol sequences of the p110gag-abl-pol protein are dispensable for fibroblast transformation. To assess whether the fv-abl segment specifies the unique biological properties of HZ2-FeSV, we constructed a Moloney MuLV-based version of HZ2-FeSV, Mo-MuLV(fv-abl), in which the fv-abl sequences were contained in a genetic context similar to that in HZ2-FeSV.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential expression of the ras gene family in mice

We compared the expression of the ras gene family (H-ras, K-ras, and N-ras) in adult mouse tissues and during development. We found substantial variations in expression among different organs and in the amounts of the different transcripts originating from each gene, especially for the N-ras gene. The expression patterns were consistent with the reported preferential tissue activation of ras genes and suggested different cellular functions for each of the ras genes.

Deregulated expression of c-myc by murine erythroleukaemia cells prevents differentiation

Friend murine erythroleukaemia (F-MEL) cells are a permanent line of primitive erythroid precursors originally derived from the spleens of mice infected with the Friend strain of murine leukaemia virus. F-MEL cells differentiate in vitro in response to various chemical inducers. Concomitantly with induction, a biphasic regulation of c-myc oncogene transcripts is observed. Within one hour of the addition of dimethyl sulphoxide (DMSO) or hypoxanthine (Hyp), the levels of c-myc transcripts fall dramatically and remain virtually undetectable for the next few hours. Between 8 and 24 hours after induction, c-myc transcripts return to pre-induction levels and then decline again between 3 and 5 days as most of the cells undergo terminal differentiation. To explore the potential relationship between c-myc expression and F-MEL terminal differentiation, we have investigated here whether reversing the early fluctuations in c-myc transcript levels affects the ability of F-MEL cells to differentiate. We therefore constructed an amplifiable plasmid vector containing a full-length mouse c-myc complementary DNA and introduced it stably into recipient F-MEL cells. The exogenous c-myc sequences are transcribed in F-MEL cells and the transcript levels do not change significantly in response to inducing agents. The net result is continued c-myc expression following DMSO or Hyp induction and a complete or partial inhibition of F-MEL differentiation.