Construction and isolation of a transforming murine retrovirus containing the src gene of Rous sarcoma virus

Mutation of the Lyn tyrosine kinase delays the progression of Friend virus induced erythroleukemia without affecting susceptibility

During the initial phase of Friend virus (FV) induced erythroleukemia, the interaction between the viral envelope glycoprotein gp55, the Erythropoietin receptor (EpoR) and the naturally occurring truncated version of the Mst1r receptor tyrosine kinase, called Sf-Stk, drives the polyclonal expansion of infected progenitors in an erythropoietin independent manner. Sf-Stk provides signals that cooperate with EpoR signals to effect expansion of erythroid progenitors. The latter phase of disease is characterized by a clonal expansion of transformed leukemic cells causing an acute erythroleukemia in mice. Signaling by Sf-Stk and EpoR mediated by gp55 renders erythroid progenitors Epo independent through the activation of the EpoR downstream pathways such as PI3K, MAPK and JAK/STAT. Previous work has shown that Src family kinases also play an important role in erythropoiesis. In particular, mutation of Src and Lyn can affect erythropoiesis. In this report we analyze the role of the Lyn tyrosine kinase in the pathogenesis of Friend virus. We demonstrate that during FV infection of primary erythroblasts, Lyn is not required for expansion of viral targets. Lyn deficient bone marrow and spleen cells are able to form Epo independent FV colonies in vitro. In vivo infection of Lyn deficient animals also results in a massive splenomegaly characteristic of the virus. However, we observe differences in the pathogenesis of Friend erythroleukemia in Lyn-/- mice. Lyn-/- mice infected with the polycythemia inducing strain of FV, FVP, do not develop polycythemia suggesting that Lyn-/- infected erythroblasts have a defect in terminal differentiation. Furthermore, the expansion of transformed cells in the spleen is reduced in Lyn-/- mice. Our data show that Lyn signals are not required for susceptibility to Friend erythroleukemia, but Lyn plays a role in later events, the terminal differentiation of infected cells and the expansion of transformed cells.

Isolation of transformation suppressor genes by cDNA subtraction: lumican suppresses transformation induced by v-src and v-K-ras

We have reported that suppressive factors for transformation by viral oncogenes are expressed in primary rat embryo fibroblasts (REFs). To identify such transformation suppressor genes, we prepared a subtracted cDNA library by using REFs and a rat normal fibroblast cell line, F2408, and isolated 30 different cDNA clones whose mRNA expression was markedly reduced in F2408 cells relative to that in REFs. We referred to these as TRIF (transcript reduced in F2408) clones. Among these genes, we initially tested the suppressor activity for transformation on three TRIF genes, TRIF1 (neuronatin), TRIF2 (heparin-binding growth-associated molecule), and TRIF3 (lumican) by focus formation assay and found that lumican inhibited focus formation induced by activated H-ras in F2408 cells. Colony formation in soft agar induced by v-K-ras or v-src was also suppressed in F2408 clones stably expressing exogenous lumican without disturbing cell proliferation. Tumorigenicity in nude mice induced by these oncogenes was also suppressed in these lumican-expressing clones. These results indicate that lumican has the ability to suppress transformation by v-src and v-K-ras.

Suppression of v-src transformation by the drs gene

Previously, we isolated a novel gene, drs, which was downregulated by retroviral oncogenes such as v-src and v-K-ras, from a cDNA library of primary rat embryo fibroblasts. Experiments using a temperature-sensitive mutant of the v-src gene indicated that downregulation of drs mRNA was dependent on functional expression of v-Src. In addition, expression of drs mRNA was also reduced by serum stimulation of G0-arrested normal rat fibroblast cells. To clarify the function of the drs gene in cell transformation and proliferation, we introduced drs linked to a potent promoter into a normal rat cell line, F2408, and examined the effect of ectopic expression of exogenous drs on the transformation by the v-src gene and growth properties. Cells expressing exogenous drs gene showed significantly decreased efficiency of transformation by v-src irrespective of functional expression of v-Src kinase, while the growth rate and G1/S progression of the cells were not suppressed by expression of exogenous drs gene, indicating that drs has the ability to suppress v-src transformation without disturbing cell proliferation.

Characterization of the avian GLUT1 glucose transporter: differential regulation of GLUT1 and GLUT3 in chicken embryo fibroblasts

Vertebrate cells that are transformed by oncogenes such as v-src or are stimulated by mitogens have increased rates of glucose uptake. In rodent cells, the mechanisms whereby glucose transport is up-regulated are well understood. Stimulation of glucose transport involves an elevation in mRNA encoding the GLUT1 glucose transporter that is controlled at the levels of both transcription and mRNA stability. Cloning and sequencing of chicken GLUT1 cDNA showed that it shares 95% amino acid sequence similarity to mammalian GLUT1s. Nevertheless, unlike mammalian GLUT1 mRNA, it was not induced by v-src, serum addition, or treatment with the tumor promoter 12-O-tetradecanoylphorbol 13-acetate in chicken embryo fibroblasts. Rather, the induction of glucose transport in chicken embryo fibroblasts by v-src, serum, and 12-O-tetradecanoylphorbol 13-acetate was associated with induction of GLUT3 mRNA level and GLUT3 transcription. Rat fibroblasts were also found to express both GLUT1 and GLUT3 isoforms, but v-src induced GLUT1 and not GLUT3. This suggests that animal cells require both a basal and an upregulatable glucose transporter and that these functions have been subsumed by different GLUT isoforms in avian and mammalian cells.

Activated Ki-Ras complements erythropoietin signaling in CTLL-2 cells, inducing tyrosine phosphorylation of a 160-kDa protein

We have previously shown that expression of erythropoietin (EPO) receptor (EPOR) alone failed to confer EPO responsiveness on the interleukin 2-dependent T-cell line CTLL-2, whereas the introduction of the EPOR into interleukin 3-dependent pro-B-cell lines, such as BAF-B03, allowed the cells to proliferate in response to EPO. Here, we report that additional expression of v-Ki-Ras conferred EPO-dependent growth on CTLL-2 cells expressing the EPOR, with additional formation of a high-affinity EPOR. To investigate possible mechanisms of EPOR downstream signaling induced by v-Ki-Ras expression in these CTLL-2-derived cells, we carried out anti-phosphotyrosine immunoblot analysis of the EPOR complex immunoprecipitated with anti-EPOR antibody from lysates of cells with and without cytokine stimulation, revealing two 160-kDa and 130-kDa phosphotyrosyl proteins. An anti-JAK2 antibody did not react with these proteins, suggesting that they may represent cellular components involved in an EPO-EPOR signaling pathway induced by v-Ki-Ras. Similar phosphotyrosyl proteins were present among Friend erythroleukemia cell lines, in which the Friend virus gp55/EPOR complex on the cell surface constitutively sends signals for cell growth.

Experimental metastasis of oncogene-transformed NIH 3T3 cells in chick embryo

By means of a highly sensitive and quantitative assay for specific detection of metastasized tumor cells in chick embryonic organs using the polymerase chain reaction (PCR), we have examined the experimental metastatic ability of individual clones of NIH 3T3 cells, transformed with oncogenes: v-Ki-ras, v-Ha-ras, v-src, v-fos, and v-abl. Such a transformed clone had different metastatic abilities in different embryonic organs. Among them, two clones of NIH 3T3 cells transformed with ras-oncogenes (v-Ki-ras or v-Ha-ras) metastasized to liver and lungs of chick embryo, and grew there more rapidly than the other clones. The parental NIH 3T3 cells were detected as slight bands of PCR products after iv injection, indicating some cells were trapped in chick embryonic organs, but did not grow. These findings indicate that the transformed cells are able to invade the organ tissues and grow in embryonic chick organs, but non-metastatic cells such as the untransformed-NIH 3T3 cells are not able to grow in the secondary sites. These experiments clearly demonstrate the usefulness of this assay system to study genes involved in malignant transformation.

The v-abl, c-fms, or v-myc oncogene induces gamma radiation resistance of hematopoietic progenitor cell line 32d cl 3 at clinical low dose rate

A variety of viral and cellular oncogenes have been described with differing mechanisms of action but with the common property of inducing morphologic alteration of cells in culture. Subclonal lines of oncogene expressing cells have been shown to produce tumors in vivo. Expression of the N-ras oncogene in embryo fibroblast NIH/3T3 cells has been demonstrated to increase radioresistance in vitro, and these results have been confirmed and extended to human cell lines expressing the c-raf oncogene. In the present report, we have examined the effects of expression of the c-fms, v-abl, or v-myc oncogene in a clonal hematopoietic progenitor cell line 32D cl 3. The 32D cell line is nonmalignant in vivo and is dependent upon a source of Interleukin-3 (IL-3) for growth in vitro. The radiation survival of 32D cl 3 cells transfected and expressed in the c-fms oncogene showed significant increase in the radioresistance at both 5 cGy/min and 116 cGy/min. A clone of 32D cl 3 transfected and expressing the v-myc oncogene demonstrated increased radioresistance at both dose rates. Results of split dose experiments suggested significant repair of sublethal irradiation damage of 32D-v-abl cells. Results were compared with expression of the same v-abl oncogene in the NIH/3T3 embryo fibroblast cell line. The data demonstrate that gamma irradiation resistance is significantly increased by each oncogene expressed in 32D cl 3 cells. The data on cell line 32D cl 3 may correlate with the radioresistance of v-abl expressing human hematopoietic cell malignancies treated by irradiation therapy.

Generation of recombinant murine retroviral genomes containing the v-src oncogene: isolation of a virus inducing hemangiosarcomas in the brain

A series of recombinant retroviral genomes was generated by cotransformation of NIH 3T3 cells with a mixture of cloned DNAs: a proviral copy of the wild-type Moloney murine leukemia virus, and Moloney-based vectors containing defective copies of the chicken v-src and the murine v-abl oncogenes. Morphologically transformed foci, appearing at low frequencies in these cultures, released high titers of transforming viruses. Analysis of one group of these viruses showed that the genomes were recombinants containing portions of the viral gag gene juxtaposed to the v-src oncogene. Biologically active cloned DNAs of two of these viruses were obtained and mapped in detail. One of these viruses did not cause disease after inoculation into newborn mice, but the other induced rapidly fatal hemangiosarcomas located exclusively in the brain.

Constitutive phosphorylation of the receptor for insulinlike growth factor I in cells transformed by the src oncogene

Many oncogene products have been shown to bear strong homology to or to interact with components of normal cellular signal transduction. We have previously shown that a glycoprotein band of 95 kilodaltons (kDa) becomes tyrosine phosphorylated in chick cells transformed by Rous sarcoma virus and that tyrosine phosphorylation of this protein band correlates tightly with phenotypic transformation in cells infected with a large and diverse panel of src mutants (L. M. Kozma, A. B. Reynolds, and M. J. Weber, Mol. Cell. Biol. 10:837-841, 1990). In this communication, we report that a component of the 95-kDa glycoprotein band is related or identical to the 95-kDa beta subunit of the receptor for insulinlike growth factor I (IGF-I). We found that the beta subunit of the IGF-I receptor comigrated on polyacrylamide gels with a component of the 95-kDa glycoprotein region from src-transformed cells under both reducing and nonreducing gel conditions and had a very similar partial phosphopeptide map. To further test the hypothesis that the beta subunit of the IGF-I receptor becomes tyrosine phosphorylated in cells transformed by pp60src, a human cell line that expressed the IGF-I receptor was transformed by src. Comparison of IGF-I receptors immunoprecipitated from normal and transformed cells revealed that the beta subunit of the IGF-I receptor became constitutively tyrosine phosphorylated in src-transformed cells. Moreover, IGF-I receptor phosphorylation induced by src was synergistic with that induced by the hormone: IGF-I-stimulated autophosphorylation of the receptor was much greater in src-transformed cells than in untransformed HOS cells even at maximal concentrations of IGF-I. This increased responsiveness to IGF-I was not due to increases in receptor number, time course of phosphorylation, or affinity for hormone. Finally, no IGF-I-like activity could be detected in culture supernatants collected from the src-transformed cells, suggesting that the increased receptor phosphorylation observed in the src-transformed cells may be mediated by an intracellular mechanism rather than an external autocrine stimulation. Our data demonstrate that the IGF-I receptor becomes constitutively tyrosine phosphorylated in src-transformed cells. This finding raises the possibility that pp60v-src alters growth regulation at least in part by phosphorylating and activating this growth factor receptor.

Constitutive phosphorylation of the receptor for insulinlike growth factor I in cells transformed by the src oncogene

Many oncogene products have been shown to bear strong homology to or to interact with components of normal cellular signal transduction. We have previously shown that a glycoprotein band of 95 kilodaltons (kDa) becomes tyrosine phosphorylated in chick cells transformed by Rous sarcoma virus and that tyrosine phosphorylation of this protein band correlates tightly with phenotypic transformation in cells infected with a large and diverse panel of src mutants (L. M. Kozma, A. B. Reynolds, and M. J. Weber, Mol. Cell. Biol. 10:837-841, 1990). In this communication, we report that a component of the 95-kDa glycoprotein band is related or identical to the 95-kDa beta subunit of the receptor for insulinlike growth factor I (IGF-I). We found that the beta subunit of the IGF-I receptor comigrated on polyacrylamide gels with a component of the 95-kDa glycoprotein region from src-transformed cells under both reducing and nonreducing gel conditions and had a very similar partial phosphopeptide map. To further test the hypothesis that the beta subunit of the IGF-I receptor becomes tyrosine phosphorylated in cells transformed by pp60src, a human cell line that expressed the IGF-I receptor was transformed by src. Comparison of IGF-I receptors immunoprecipitated from normal and transformed cells revealed that the beta subunit of the IGF-I receptor became constitutively tyrosine phosphorylated in src-transformed cells. Moreover, IGF-I receptor phosphorylation induced by src was synergistic with that induced by the hormone: IGF-I-stimulated autophosphorylation of the receptor was much greater in src-transformed cells than in untransformed HOS cells even at maximal concentrations of IGF-I. This increased responsiveness to IGF-I was not due to increases in receptor number, time course of phosphorylation, or affinity for hormone. Finally, no IGF-I-like activity could be detected in culture supernatants collected from the src-transformed cells, suggesting that the increased receptor phosphorylation observed in the src-transformed cells may be mediated by an intracellular mechanism rather than an external autocrine stimulation. Our data demonstrate that the IGF-I receptor becomes constitutively tyrosine phosphorylated in src-transformed cells. This finding raises the possibility that pp60v-src alters growth regulation at least in part by phosphorylating and activating this growth factor receptor.

Friend spleen focus-forming virus induces factor independence in an erythropoietin-dependent erythroleukemia cell line

Erythroid cells from mice infected with the polycythemia-inducing strain of Friend spleen focus-forming virus (SFFVP), unlike normal erythroid cells, can proliferate and differentiate in apparent absence of the erythroid hormone erythropoietin (Epo). The unique envelope glycoprotein encoded by SFFV has been shown to be responsible for this biological effect. The recent isolation of an Epo-dependent erythroleukemia cell line, HCD-57, derived from a mouse infected at birth with Friend murine leukemia virus, afforded us the opportunity to study the direct effect of SFFVP on a homogeneous population of factor-dependent cells. The introduction of SFFVP in complex with various helper viruses into these Epo-dependent cells efficiently and reproducibly gave rise to lines which expressed high levels of SFFV and were factor independent. SFFV appears to be unique in its ability to abrogate the factor dependence of Epo-dependent HCD-57 cells, since infection of these cells with retroviruses carrying a variety of different oncogenes had no effect. The induction of Epo independence by SFFV does not appear to involve a classical autocrine mechanism, since there is no evidence that the factor-independent cells synthesize or secrete Epo or depend on it for their growth. However, the SFFV-infected, factor-independent cells had significantly fewer receptors available for binding Epo than their factor-dependent counterparts had, raising the possibility that the induction of factor independence by the virus may be due to the interaction of an SFFV-encoded protein with the Epo receptor.

Fibroblasts transformed with v-src show enhanced formation of an inositol tetrakisphosphate

The tyrosine kinase pp60v-src, encoded by the v-src oncogene, seems to regulate phosphatidylinositol metabolism. The effect of pp60v-src on control points in inositol phosphate production was examined by measuring the amounts of inositol polyphosphates in Rat-1 cells expressing wild-type or mutant forms of the protein. Expression of v-src-resulted in a five- to sevenfold elevation in the steady-state amount of an isomer of inositol tetrakisphosphate, whereas the concentrations of inositol trisphosphates or other inositol tetrakisphosphates were not affected. The activity of a key enzyme in the formation of inositol tetrakisphosphates, inositol (1,4,5)-trisphosphate 3-kinase, was increased six- to eightfold in cytosolic extracts prepared from the v-src-transformed cells, suggesting that this enzyme may be one target for the pp60v-src kinase and that it may participate in the synthesis of novel, higher order inositol phosphates.

Expression of v-src induces a myeloproliferative disease in bone-marrow-reconstituted mice

A recombinant retrovirus, N-TK-src, was used to introduce the v-src oncogene into mouse hematopoietic cells. This vector efficiently expresses both the neo and v-src genes in different hematopoietic lineages in culture as well as in mice reconstituted with infected bone marrow cells. Expression of v-src had no dramatic effect on the proliferative and differentiative capacity of hematopoietic precursors when assayed in methyl cellulose cultures. However, in mice reconstituted with N-TK-src-infected bone marrow cells, expression of v-src leads to the rapid development of a severe myeloproliferative disease, characterized by splenomegaly, anemia, and a shift of hematopoiesis from the bone marrow to the spleen.

Properties of a murine retroviral recombinant of avian acute leukemia virus E26: a murine fibroblast assay for v-ets function

A replication-defective murine retroviral construct, termed pME26, was generated by inserting avian gag-myb-ets sequences derived from the cloned avian acute leukemia virus E26 into an Abelson murine leukemia virus-derived retroviral vector. ME26 virus can be rescued efficiently from transfected NIH 3T3 cells by replicating murine leukemia viruses. Either pME26-transfected nonproducers or ME26 virus-infected NIH 3T3 cells expressed a 135-kilodalton fusion protein (p135) which was detectable by immunoprecipitation with antiserum directed against avian leukemia virus p27gag, myb or ets oncogene protein, or murine leukemia virus p15gag and was principally localized in the nucleus. NIH 3T3 cells infected with ME26 exhibited morphological alterations and increased proliferation in reduced serum and formed small colonies in agar suspension. Discrete foci could be readily recognized in cells maintained in a defined medium containing 0.03 to 0.1% calf serum. In newborn NFS/N mice, ME26 induced a significantly higher mortality and incidence of erythroid and myeloid leukemias. Analysis of a series of mutants affecting the expression of various portions of p135 indicated that the v-ets gene acts to mitogenically stimulate the proliferation of NIH 3T3 fibroblasts and reduces or abolishes their serum dependence. These properties provide an assay system to study functions of the ets gene family.

Cooperative transforming activities of ras, myc, and src viral oncogenes in nonestablished rat adrenocortical cells

Early-passage rat adrenocortical cells were infected with Kirsten murine sarcoma virus and MMCV mouse myc virus, two retroviruses carrying the v-Ki-ras and v-myc oncogenes, respectively. Efficient morphological transformation required coinfection with the two viruses, was dependent on the presence of high serum concentrations, and was not immediately accompanied by growth in soft agar. The doubly infected cells coordinately acquired the capacity for anchorage- and serum-independent growth during passage in culture. The appearance of such highly transformed cells was correlated with the emergence of a dominant clone, as suggested by an analysis of retrovirus integration sites. These results indicate that the concerted expression of v-Ki-ras and v-myc could induce rapid morphological transformation of nonestablished adrenocortical cells but that an additional genetic or epigenetic event was required to permit full transformation by these two oncogenes. In contrast, v-src, introduced by retrovirus infection in conjunction with v-myc, rapidly induced serum- and anchorage-independent growth. Therefore, the p60v-src protein-tyrosine kinase, unlike p21v-ras, is apparently not restricted in the induction of a highly transformed phenotype in adrenocortical cells. This system provides an in vitro model for the progressive transformation of epithelial cells by dominantly acting oncogenes.

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.

Malignant transformation by ras and other oncogenes produces common alterations in inositol phospholipid signaling pathways

The role of ras proteins in signal transduction was assessed by studying inositol phospholipid metabolism and inositol phospholipid-mediated cellular responsiveness to agonists in cells transformed by ras and other oncogenes. Specific alterations were observed in the inositol phospholipid cycle of ras-transformed fibroblasts, but similar changes were also produced by spontaneous transformation or transformation mediated by either membrane-associated oncogenes, such as src, met, or trk, or cytoplasmic oncogenes, mos and raf; the nuclear oncogenes fos and myc did not produce these changes. The alterations included (i) stimulation of phospholipase A2 activity as indicated by elevated levels of glycerophosphoinositol and nonesterified arachidonic acid and (ii) specific uncoupling between surface receptor-mediated stimulation by platelet-derived growth factor, bombesin, or serum and activation of intracellular phospholipase C. These findings suggest the existence of common biochemical pathways for transformation by cytoplasmic and membrane-associated oncogenes and are not consistent with the hypothesis that 21-kDa ras proteins (p21) are direct or distinct regulatory elements of phospholipase C or phospholipase A2 in inositol phospholipid signal transduction pathways.

Transformation by the src oncogene alters glucose transport into rat and chicken cells by different mechanisms

Transformation of both rat and chicken fibroblasts by the src oncogene leads to a four- to fivefold increase in the rate of glucose transport and in the level of the glucose transporter protein. We have previously shown that, with chicken embryo fibroblasts, transformation leads to a reduction in the rate of degradation of the transporter, with little or no increase in the rate of its biosynthesis. We now show that, with the rat-1 cell line, the opposite result was obtained. src-induced transformation led to an increase in transporter biosynthesis, with little effect on turnover. A src-induced increase in transporter mRNA entirely accounted for the increase in biosynthesis of the protein. By contrast, in chicken embryo fibroblasts, the level of transporter mRNA was low and was not induced to rise by src transformation. Thus, src induced an increase in the level of the glucose transport protein by fundamentally different mechanisms in chicken embryo fibroblasts and rat-1 cells. To test whether this difference was due to rat-1 cells being an immortalized cell line, we measured transporter mRNA levels in primary fibroblast cultures from rat embryos and in parallel cultures transformed by src. Transporter mRNA was inducible by src in these cells. Thus, the difference in mRNA inducibility between chicken and rat cells is not due to immortalization.

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.

Comparison of three recombinant murine leukemia viruses carrying the v-src oncogene of avian sarcoma virus: differences in in vitro transformation and in vivo pathogenicity

We previously described a recombinant Moloney murine leukemia virus (Mo-MuLV) carrying the v-src oncogene, Mo-MuLV(src). Mo-MuLV(src) encodes a gag-src fusion protein, transforms cells in culture, and induces fibrosarcomas in vivo. To compare transforming properties of the gag-src fusion protein to pp60src encoded by Rous sarcoma virus, we constructed a new recombinant virus, Mo-MuLV(+ src). Mo-MuLV(+ src) encodes pp60src in the context of Mo-MuLV. Cells transformed by Mo-MuLV(+ src) were round and formed colonies in soft agar, whereas Mo-MuLV(src)-infected cells were fusiform and did not grow in suspension. Thus, the extent of transformation induced by Mo-MuLV(+ src) was greater than that induced by Mo-MuLV(src). Subcutaneous inoculation of either virus into neonatal NIH Swiss mice resulted in fibrosarcomas at the site of injection. Further studies indicated that tumors induced by Mo-MuLV(+ src) grew rapidly but rarely metastasized. In contrast, tumors induced by Mo-MuLV(src) grew somewhat more slowly but metastasized with a high frequency (60%). These viruses may provide a useful model system for tumor metastasis. Another src-containing virus was also studied, MRSV (constructed by Anderson and Scolnick). MRSV also encodes pp60src but in the context of amphotropic MuLV. When injected intravenously into six-week-old mice, MRSV induced splenomegaly and spleen foci but no solid tumors, as reported previously. In contrast, Mo-MuLV(src)-induced fibrosarcomas mostly in the spleen under the same inoculation protocol. These results suggest that the v-src oncogene was the major pathogenic determinant in neonatal mice for all three src-containing viruses; however, variations in the nature of the transforming protein modulated the behavior of the induced tumors. In adult mice, greater differences in pathogenicity were observed.

Temperature-sensitive cellular mutant for expression of mRNA from murine retrovirus

The cellular mutant B812 isolated from a Fisher rat cell line shows temperature sensitivity of focus formation induced by various retroviruses such as recombinant murine retrovirus containing the middle T gene of polyomavirus (PyMLV), Kirsten murine sarcoma virus, Moloney murine sarcoma virus, and recombinant murine retrovirus containing the src gene of Rous sarcoma virus. B812 cells, however, show normal ability to proliferate and synthesize protein at the nonpermissive temperature, suggesting that their mutation is in a gene specifically concerned with the process of transformation by retroviruses. In this work, experiments with hybrids of mutant and wild-type cells showed that the temperature-dependent defect of this mutant was complemented by wild-type cells. To determine the step of transformation that is restricted at the nonpermissive temperature in B812, we examined the expressions of the oncogene (middle T antigen) in no. 7 (wild-type cells) and B812 cultures infected with PyMLV (the chimeric retrovirus containing the middle T gene of polyomavirus) at the permissive and nonpermissive temperatures. Middle T-associated protein kinase activity, the expression of middle T antigen, and PyMLV-specific mRNA were reduced at the nonpermissive temperature in B812 cultures infected with PyMLV. However, integration of PyMLV into the chromosomal DNA of the mutant was not affected at the nonpermissive temperature. These results suggest that B812 cells have a mutation affecting the expression of viral mRNAs from integrated proviral DNA at the nonpermissive temperature.

Transformation by the src oncogene alters glucose transport into rat and chicken cells by different mechanisms

Transformation of both rat and chicken fibroblasts by the src oncogene leads to a four- to fivefold increase in the rate of glucose transport and in the level of the glucose transporter protein. We have previously shown that, with chicken embryo fibroblasts, transformation leads to a reduction in the rate of degradation of the transporter, with little or no increase in the rate of its biosynthesis. We now show that, with the rat-1 cell line, the opposite result was obtained. src-induced transformation led to an increase in transporter biosynthesis, with little effect on turnover. A src-induced increase in transporter mRNA entirely accounted for the increase in biosynthesis of the protein. By contrast, in chicken embryo fibroblasts, the level of transporter mRNA was low and was not induced to rise by src transformation. Thus, src induced an increase in the level of the glucose transport protein by fundamentally different mechanisms in chicken embryo fibroblasts and rat-1 cells. To test whether this difference was due to rat-1 cells being an immortalized cell line, we measured transporter mRNA levels in primary fibroblast cultures from rat embryos and in parallel cultures transformed by src. Transporter mRNA was inducible by src in these cells. Thus, the difference in mRNA inducibility between chicken and rat cells is not due to immortalization.

One- and two-step transformations of rat thyroid epithelial cells by retroviral oncogenes

A system of epithelial cells is described in which it is possible to study the number and the nature of genes capable of conferring the malignant phenotype. Two fully differentiated, hormone-responsive cell lines from rat thyroid glands are presented which are susceptible to one-step or two-step transformation upon infection with several murine acute retroviruses. After infection, both cell lines became independent from their thyrotropic hormone requirement for growth. However, complete transformation was achieved with one of the cell lines (FRTL-5 Cl 2), whereas the other cell line (PC Cl 3) failed to grow in agar and to give rise to tumors in vivo. The latter cell line was susceptible to complete transformation upon cooperation of the v-ras-Ha and the human c-myc oncogenes.

Transformation by oncogenes encoding protein kinases induces the metastatic phenotype

Oncogenes encoding serine/threonine or tyrosine kinases were introduced into the established rodent fibroblast cell line NIH 3T3 and tested for tumorigenic and metastatic behavior in T cell-deficient nude mice. Transforming oncogenes of the ras family were capable of converting fibroblast cell lines to fully metastatic tumors. Cell lines transformed by the kinase oncogenes mos, raf, src, fes, and fms formed experimental metastases and (in some cases) these genes were more efficient at metastatic conversion than a mutant ras gene. In contrast, cells transformed by either of two nuclear oncogenes, myc or p53, were tumorigenic when injected subcutaneously but were virtually nonmetastatic after intravenous injection. These data demonstrate that, in addition to ras, a structurally divergent group of kinase oncogenes can induce the metastatic phenotype.

One- and two-step transformations of rat thyroid epithelial cells by retroviral oncogenes

A system of epithelial cells is described in which it is possible to study the number and the nature of genes capable of conferring the malignant phenotype. Two fully differentiated, hormone-responsive cell lines from rat thyroid glands are presented which are susceptible to one-step or two-step transformation upon infection with several murine acute retroviruses. After infection, both cell lines became independent from their thyrotropic hormone requirement for growth. However, complete transformation was achieved with one of the cell lines (FRTL-5 Cl 2), whereas the other cell line (PC Cl 3) failed to grow in agar and to give rise to tumors in vivo. The latter cell line was susceptible to complete transformation upon cooperation of the v-ras-Ha and the human c-myc oncogenes.

Isolation of a murine retrovirus with a temperature-sensitive src gene

A murine retrovirus containing the src gene of avian sarcoma virus ts LA31A was generated. In contrast to the avian sarcoma virus, our recombinant murine retrovirus can efficiently infect mammalian cells. Transformation of NIH3T3 cells by the recombinant murine retrovirus is temperature sensitive. At the permissive temperature of 34 degrees, cells form foci of rounded cells. At the nonpermissive temperature of 39 degrees, the infected cells remain flat and exhibit contact inhibition. No disease was observed following infection of newborn NFS/n mice with the ts mutant virus. In contrast, infection of newborn NFS/n mice with a recombinant murine retrovirus containing the wild-type src gene causes fibrosarcomas and hepatosplenomegaly.

Down-modulation of EGF receptors in cells transformed by the src oncogene

The effects of src oncogene expression on epidermal growth factor (EGF) receptors have been investigated in mouse 3T3 and rat-1 fibroblasts. Transformation of both cell types with src resulted in marked reductions in cellular EGF receptor levels, as assayed by either 125I-EGF binding or immunoprecipitation of receptor protein from radiolabeled cell lysates. In contrast to cells transformed by other types of retroviral oncogenes, the loss of EGF receptors in the src-transformed cells did not appear to be due to secreted transforming growth factor-alpha (TGF-alpha), since such factors were undetectable in culture fluids from the src-transformed cells. By several criteria of transformation, an EGF-receptorless cell line infected with src was shown to be transformed, suggesting that EGF receptors themselves are not obligatory to the src transformation process. We suggest that pp60src down-modulates EGF receptors by an intracellular mechanism and that the loss of the receptors is symptomatic of more general effects of pp60src on the machinery of growth regulation.

Perturbed hemopoiesis and the generation of multipotential stem cell clones in src-infected bone marrow cultures is an indirect or transient effect of the oncogene

Multipotential stem cell lines, derived specifically from long-term bone marrow cultures infected with a recombinant retrovirus carrying v-src, lack v-src. Stable consequences thus result from transient actions or indirect effects of v-src on other cells, with the latter possibility being favored by its mosaic expression in marrow cultures.

A murine recombinant retrovirus containing the src oncogene transforms erythroid precursor cells in vitro

A murine retrovirus (MRSV) containing the src gene of Rous sarcoma virus has been shown to cause an erythroproliferative disease in mice (S. M. Anderson and E. M. Scolnick, J. Virol. 46:594-605, 1983). We now demonstrate that this same virus can transform erythroid progenitor cells in vitro. Infection of fetal liver cells or spleen and bone marrow cells from phenylhydrazine-treated adult mice gave rise to colonies of erythroid cells which grew in methylcellulose under conditions not favorable for the growth of normal erythroid cells. The presence of pp60src in the transformed erythroid cells was demonstrated by an immune complex protein kinase assay. The time course of appearance and subsequent differentiation of erythroid colonies indicated that the target cell for MRSV was a 6- to 8-day burst-forming unit. Differentiation of the erythroid progenitors was not blocked by the presence of pp60src, and the cells retained sensitivity to the hormone erythropoietin. In fact, the transformed cells exhibited increased hormone sensitivity since the number, the size, and the extent of hemoglobinization of the colonies were all increased by the addition of small amounts of erythropoietin. MRSV was not susceptible to restriction by the Fv-2 locus, as MRSV could transform hematopoietic cells from C57BL/6 mice. These results indicate that (i) the erythroid proliferation observed in vivo is caused by a direct effect of MRSV on erythroid progenitors and (ii) the transformed erythroid precursors acquire a growth advantage over uninfected cells without losing the ability to differentiate and respond to physiologic regulators.

Self-renewal and differentiation of interleukin-3-dependent multipotent stem cells are modulated by stromal cells and serum factors

Interleukin-3 (IL-3)-dependent cell lines (FDCP-mix) were cloned and isolated from long-term bone-marrow cultures infected with src-MoMuLV. These cell lines have many of the characteristics of hematopoietic stem cells. Early isolates of the FDCP-mix cells form spleen colonies in irradiated mice and establish long-term hematopoiesis on irradiated marrow stroma in vitro in the absence of IL-3. These two properties of the cells are lost within 15 weeks of establishing the cell lines, but the cell lines retain their ability to differentiate in a multilineage response to hematopoietic growth factors and to hematopoietic stromal cells, as well as to self-renew in the presence of IL-3. The choice between differentiation and self-renewal in FDCP-mix cells can clearly be modified by culture conditions: in particular, cultures containing horse serum preferentially promote self-renewal, whereas cultures containing fetal calf serum preferentially promote differentiation. The FDCP-mix cell lines are not leukemic, nor do they contain the src oncogene. Their ability to respond to hematopoietic growth factors and stroma in a similar manner to normal hematopoietic cells makes them a valuable model for studying the regulation of hemopoietic cell self-renewal and differentiation.

Perturbed hemopoiesis and the generation of multipotential stem cell clones in src-infected bone marrow cultures is an indirect or transient effect of the oncogene

Multipotential stem cell lines, derived specifically from long-term bone marrow cultures infected with a recombinant retrovirus carrying v-src, lack v-src. Stable consequences thus result from transient actions or indirect effects of v-src on other cells, with the latter possibility being favored by its mosaic expression in marrow cultures.

A murine recombinant retrovirus containing the src oncogene transforms erythroid precursor cells in vitro

A murine retrovirus (MRSV) containing the src gene of Rous sarcoma virus has been shown to cause an erythroproliferative disease in mice (S. M. Anderson and E. M. Scolnick, J. Virol. 46:594-605, 1983). We now demonstrate that this same virus can transform erythroid progenitor cells in vitro. Infection of fetal liver cells or spleen and bone marrow cells from phenylhydrazine-treated adult mice gave rise to colonies of erythroid cells which grew in methylcellulose under conditions not favorable for the growth of normal erythroid cells. The presence of pp60src in the transformed erythroid cells was demonstrated by an immune complex protein kinase assay. The time course of appearance and subsequent differentiation of erythroid colonies indicated that the target cell for MRSV was a 6- to 8-day burst-forming unit. Differentiation of the erythroid progenitors was not blocked by the presence of pp60src, and the cells retained sensitivity to the hormone erythropoietin. In fact, the transformed cells exhibited increased hormone sensitivity since the number, the size, and the extent of hemoglobinization of the colonies were all increased by the addition of small amounts of erythropoietin. MRSV was not susceptible to restriction by the Fv-2 locus, as MRSV could transform hematopoietic cells from C57BL/6 mice. These results indicate that (i) the erythroid proliferation observed in vivo is caused by a direct effect of MRSV on erythroid progenitors and (ii) the transformed erythroid precursors acquire a growth advantage over uninfected cells without losing the ability to differentiate and respond to physiologic regulators.

Polyomavirus middle T protein encoded by a retrovirus transforms nonestablished chicken embryo cells

A murine retrovirus encoding the middle T protein of polyomavirus infected and transformed nonestablished chicken embryo cells. The infected cultures formed colonies in soft agar-containing medium and released infectious transforming virus. Middle T protein expressed in the transformed chicken cells associated with p60c-src and, in immunoprecipitates, enhanced the tyrosine protein kinase activity of p60c-src.

Long terminal repeat sequences impart hematopoietic transformation properties to the myeloproliferative sarcoma virus

The myeloproliferative sarcoma virus not only transforms fibroblasts but also causes extensive expansion of the hematopoietic stem cell compartment on infection of adult mice. Similar to the Moloney sarcoma virus, it carries the mos oncogene. Moloney sarcoma virus, however, does not induce myeloproliferation and leukemia in adult mice. The difference between the two viruses was explored by using their molecularly cloned genomes and the cellular mos oncogene to construct recombinant genomes. It was shown that the U3 region of the viral long terminal repeat (LTR) has a decisive function in determining the target cell specificity of the myeloproliferative sarcoma virus. Any mos gene, whether of cellular or viral origin, is sufficient in conjunction with the proper LTR to induce myeloproliferation. Our results indicate that the pathogenicity of acutely transforming viruses is determined not only by the oncogene but also by sequences in the viral LTR.

Tissue selectivity of murine leukemia virus infection is determined by long terminal repeat sequences

Here we show that the tissue specificity of murine retrovirus infections is determined by the long terminal repeat (LTR) of an otherwise isogenic set of viruses. The isogenic viruses used for this study contain the coding gag, pol, and env genes of the avirulent Akv virus. Recombinant viruses that contain the LTR of a virus that induces T-cell leukemia lymphoma preferentially infect T lymphocytes. Viruses that carry the LTR of a virus that induces erythroleukemia preferentially infect non-T lymphoblastoid cell lines in the marrow and spleen. The Akv virus itself displays no tissue preference for hematopoietic cells. These experiments suggest that retroviruses that carry appropriate enhancer-promoters can be used to infect selectively specific target cells in animals.

Suppression of tropomyosin synthesis, a common biochemical feature of oncogenesis by structurally diverse retroviral oncogenes

To identify proteins whose production may be altered as a common event in the expression of structurally diverse oncogenes, we compared two-dimensional electropherograms of newly synthesized proteins from NIH/3T3 cell lines transformed by a variety of retroviral oncogenes, from cellular revertant lines, and from a line (433.3) which expresses the v-ras oncogene in response to corticosteroids. Most alterations in the synthesis of specific proteins detected by this approach appeared to be the result of selection during prolonged cultivation and were probably unrelated to the transformation process. However, we detected seven proteins whose synthesis was strongly suppressed in cell lines transformed by each of the six retroviral oncogenes we studied and whose production was fully or partially restored in two cellular revertant lines. Suppression of two of these proteins was also correlated with the initial appearance of morphological alteration during corticosteroid-induced oncogene expression in 433.3 cells. These proteins (p37/4.78 and p41/4.75) were identified as tropomyosins, a group of at least five cytoskeletal proteins. Transformation by the papovaviruses simian virus 40 and polyomavirus caused no suppression of synthesis of these tropomyosins. This indicates that suppression of tropomyosin synthesis is not a nonspecific response by cells to being forced to grow with the transformed phenotype but is specifically associated with oncogenesis by diverse retroviral oncogenes. The results are consistent with the hypothesis that the different biochemical processes initiated by expression of structurally diverse retroviral oncogenes may converge on a limited number of common targets, one of which is the mechanism which regulates the synthesis of tropomyosins.

Specific transforming potential of oncogenes encoding protein-tyrosine kinases

Several chimeric murine retroviruses were constructed to test whether the gag sequence of Abelson murine leukemia virus (A-MuLV) could influence the in vitro specificity of two sarcoma-inducing oncogenes: src of Rous sarcoma virus and fps of Fujinami sarcoma virus. Although the src- or fps- containing chimerae could transform fibroblasts, they were unable to mimic the action of A-MuLV in causing lymphoid transformation in vitro. A-MuLV-derived gag sequences could, however, functionally replace the 5' end of src and restore the transformation potential of a 5'-truncated src gene. To investigate this functional similarity, we replaced the gag sequence of an A-MuLV virus with the 5' end of src. This recombinant virus behaved like the A-MuLV virus from which it was derived: it transformed both fibroblasts and lymphoid cells in vitro. Taken together, these results suggest that lymphoid transformation in vitro is a specific property of abl and not of src or fps. Furthermore, it shows that a functional homology exists between the gag sequence of A-MuLV and the 5' end of src.

Myeloid cell transformation by ras-containing murine sarcoma viruses

BALB and Harvey murine sarcoma viruses contain ras transforming genes capable of altering the proliferation and differentiation of cells within the erythroid and lymphoid lineages (W. D. Hankins and E. M. Scolnick, Cell 26:91-97, 1981; J. H. Pierce and S. A. Aaronson, J. Exp. Med. 156:873-887, 1982; E. M. Scolnick et al., Mol. Cell. Biol. 1:68-74). The present studies demonstrate hematopoietic targets of ras-containing viruses within the myeloid lineage. Diffuse colonies were induced by BALB or Harvey marine sarcoma virus infection of murine bone marrow cells. Generally, these colonies were made up of relatively mature macrophages which exhibited increased self-renewal capacity but eventually underwent terminal differentiation in culture. Cells from one BALB murine sarcoma virus-induced colony displayed phenotypic markers of more immature myelomonocytic cells. This colony, designated BAMC1, readily established as a continuous cell line and was highly malignant in vivo. Exposure of these cells to 12-O-tetradecanoylphorbol-13-acetate led to the induction of a more mature myeloid phenotype, which was associated with decreased growth potential in vitro and in vivo. The effects of the inducing agent were not mediated by an alteration in the level of expression of the ras-coded p21 transforming protein. Our present findings extend the spectrum of targets whose growth is altered by ras-containing retroviruses to cells at several stages of differentiation within each of the major hematopoietic lineages.

Generation of a recombinant Moloney murine leukemia virus carrying the v-src gene of avian sarcoma virus: transformation in vitro and pathogenesis in vivo

A Moloney murine leukemia virus (M-MuLV) recombinant carrying the v-src gene of avian sarcoma virus was generated by the introduction of a cloned portion of v-src from Schmidt-Ruppin A avian sarcoma virus into a molecular clone of M-MuLV provirus at the recombinant DNA level. The v-src sequences (lacking a portion of the 5' end of v-src) were inserted into the p30 region of the M-MulV gag gene so that M-MuLV gag and v-src were in the same reading frame. Transfection of this chimeric clone, pMLV(src), into NIH 3T3 cells which were constitutively producing M-MuLV gag and pol protein resulted in the formation of foci of transformed cells. Infectious and transforming virus could be recovered from the transformed cells. This virus was designated M-MuLV(src). M-MuLV(src)-transformed cells contained two novel proteins of 78 and 90 kilodaltons. The 78-kilodalton protein, p78gag-src, contained both gag and src determinants, exhibited kinase activity in an immune kinase assay, and is probably a fusion of Pr65gag and src. The 90-kilodalton protein, which is of the appropriate size to be the gPr80gag fused to src, contained gag determinants as well as a V8 protease cleavage fragment typical of the carboxy terminus of avian sarcoma virus pp60src. However, it could not be immunoprecipitated with an anti-v-src serum. M-MuLV(src)-transformed cells showed elevated levels of intracellular phosphotyrosine in proteins, although the elevation was intermediate compared with cells transformed with wild-type v-src. M-MuLV and amphotropic murine leukemia virus pseudotypes of M-MuLV(src) were inoculated into newborn NIH Swiss mice. Inoculated mice developed solid tumors at the site of inoculation after 3 to 6 weeks, with most animals dying by 14 weeks. Histopathological analysis indicated that the solid tumors were mesenchymally derived fibrosarcomas that were both invasive and metastatic.

Suppression of tropomyosin synthesis, a common biochemical feature of oncogenesis by structurally diverse retroviral oncogenes

To identify proteins whose production may be altered as a common event in the expression of structurally diverse oncogenes, we compared two-dimensional electropherograms of newly synthesized proteins from NIH/3T3 cell lines transformed by a variety of retroviral oncogenes, from cellular revertant lines, and from a line (433.3) which expresses the v-ras oncogene in response to corticosteroids. Most alterations in the synthesis of specific proteins detected by this approach appeared to be the result of selection during prolonged cultivation and were probably unrelated to the transformation process. However, we detected seven proteins whose synthesis was strongly suppressed in cell lines transformed by each of the six retroviral oncogenes we studied and whose production was fully or partially restored in two cellular revertant lines. Suppression of two of these proteins was also correlated with the initial appearance of morphological alteration during corticosteroid-induced oncogene expression in 433.3 cells. These proteins (p37/4.78 and p41/4.75) were identified as tropomyosins, a group of at least five cytoskeletal proteins. Transformation by the papovaviruses simian virus 40 and polyomavirus caused no suppression of synthesis of these tropomyosins. This indicates that suppression of tropomyosin synthesis is not a nonspecific response by cells to being forced to grow with the transformed phenotype but is specifically associated with oncogenesis by diverse retroviral oncogenes. The results are consistent with the hypothesis that the different biochemical processes initiated by expression of structurally diverse retroviral oncogenes may converge on a limited number of common targets, one of which is the mechanism which regulates the synthesis of tropomyosins.

Myeloid cell transformation by ras-containing murine sarcoma viruses

BALB and Harvey murine sarcoma viruses contain ras transforming genes capable of altering the proliferation and differentiation of cells within the erythroid and lymphoid lineages (W. D. Hankins and E. M. Scolnick, Cell 26:91-97, 1981; J. H. Pierce and S. A. Aaronson, J. Exp. Med. 156:873-887, 1982; E. M. Scolnick et al., Mol. Cell. Biol. 1:68-74). The present studies demonstrate hematopoietic targets of ras-containing viruses within the myeloid lineage. Diffuse colonies were induced by BALB or Harvey marine sarcoma virus infection of murine bone marrow cells. Generally, these colonies were made up of relatively mature macrophages which exhibited increased self-renewal capacity but eventually underwent terminal differentiation in culture. Cells from one BALB murine sarcoma virus-induced colony displayed phenotypic markers of more immature myelomonocytic cells. This colony, designated BAMC1, readily established as a continuous cell line and was highly malignant in vivo. Exposure of these cells to 12-O-tetradecanoylphorbol-13-acetate led to the induction of a more mature myeloid phenotype, which was associated with decreased growth potential in vitro and in vivo. The effects of the inducing agent were not mediated by an alteration in the level of expression of the ras-coded p21 transforming protein. Our present findings extend the spectrum of targets whose growth is altered by ras-containing retroviruses to cells at several stages of differentiation within each of the major hematopoietic lineages.

Transformation of erythroid cells by Rous sarcoma virus (RSV)

RSV transforms several nonhematopoietic cell types and as reported here also has the capacity to transform hematopoietic cells of the erythroid lineage. In vitro, the three RSV isolates tested induced erythroblast-like colonies in infected bone marrow cells that were distinguishable by size and cell arrangement from those induced by avian erythroblastosis virus (AEV). Also in contrast to AEV-transformed erythroblast cultures, isolated cell colonies induced by RSV required complex growth conditions in liquid medium similar to the in vitro conditions necessary for erythroblasts transformed by the acute leukemia virus E26. Temperature-shift experiments using temperature-sensitive (ts) NY68 RSV revealed that when grown at the nonpermissive temperature (42 degrees), mutant-infected cells became benzidine positive and partially differentiated into erythrocytes. Wild-type (wt) RSV-transformed cells did not undergo similar changes. However, both wt RSV-, and to a greater extent, ts RSV-transformed cultures at the permissive temperature (37 degrees) did contain populations of spontaneously differentiating erythroid cells signifying that the transforming activity of the virus did not fully arrest erythroid maturation. In addition, the RSV-transformed cells did express tyrosine kinase activity. When injected intravenously into birds, RSV induced an erythroblastosis-like disease similar to AEV but also caused fibrosarcomas and leg paralysis. These results show that RSV can alter the pattern of erythroid differentiation in a manner similar to, but distinct from, AEV and indicate that the tyrosine-specific pp60src kinase is involved in erythroid cell transformation. Since the src and erb B proteins share a significant amino acid homology, these data suggest that both may also share a common functional homology.