Architecture of a gamma retroviral genomic RNA dimer

Retroviral genomes contain two sense-strand RNAs that are noncovalently linked at their 5' ends, forming a dimer. Establishing a structure for this dimer is an obligatory first step toward understanding the fundamental role of the dimeric RNA in retroviral biology. We developed a secondary structure model for the minimal dimerization active sequence (MiDAS) for the Moloney murine sarcoma virus in the final dimer state using selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE). In this model, two self-complementary, or palindromic, sequences (PAL1 and PAL2) form extended intermolecular duplexes of 10 and 16 base pairs, respectively. The monomeric starting state was shown previously to contain a flexible domain in which nucleotides do not form stable interactions with other parts of the RNA. In the final dimer state, portions of this initial flexible domain form stable base pairs, while previously base-paired elements lie in a new flexible domain. Thus, partially overlapping and structurally well-defined flexible domains are prominent features of both monomer and dimer states. We then used hydroxyl radical cleavage experiments to characterize the global architecture of the dimer state. Extensive regions, including portions of both PAL1 and PAL2, are occluded from solvent-based cleavage indicating that the MiDAS domain does not function simply as a collection of autonomous secondary structure elements. Instead, the retroviral dimerization domain adopts a compact architecture characterized by close packing of its constituent helices.

The SL1-SL2 (stem-loop) domain is the primary determinant for stability of the gamma retroviral genomic RNA dimer

Retroviral genomes are assembled from two sense-strand RNAs by noncovalent interactions at their 5' ends, forming a dimer. The RNA dimerization domain is a potential target for antiretroviral therapy and represents a compelling RNA folding problem. The fundamental dimerization unit for the Moloney murine sarcoma gamma retrovirus spans a 170-nucleotide minimal dimerization active sequence. In the dimer, two self-complementary sequences, PAL1 and PAL2, form intermolecular duplexes, and an SL1-SL2 (stem-loop) domain forms loop-loop base pairs, mediated by GACG tetraloops, and extensive tertiary interactions. To develop a framework for assembly of the retroviral RNA dimer, we quantified the stability of and established nucleotide resolution secondary structure models for sequence variants in which each motif was compromised. Base pairing and tertiary interactions between SL1-SL2 domains contribute a large free energy increment of -10 kcal/mol. In contrast, even though the PAL1 and PAL2 intermolecular duplexes span 10 and 16 bp in the dimer, respectively, they contribute only -2.5 kcal/mol to stability, roughly equal to a single new base pair. First, these results emphasize that the energetic costs for disrupting interactions in the monomer state nearly balance the PAL1 and PAL2 base pairing interactions that form in the dimer. Second, intermolecular duplex formation plays a biological role distinct from simply stabilizing the structure of the retroviral genomic RNA dimer.

Construction of retroviral vectors with enhanced efficiency of transgene expression

Retroviral vectors have been widely used in gene therapy due to their simple genomic structure and high transduction efficiency. We report a construction of Moloney murine sarcoma virus (MoMSV) and Moloney murine leukemia virus (MoMLV) hybrid-based retroviral vectors with significantly improved efficiency of transgene expression after stable incorporation into the host genome. In these vectors, the residual gag gene coding sequence located in the extended region of packaging signal was removed. These vectors, therefore, contain no coding sequence for the gag, pol, or env gene that can be used for homologous recombination with sequences introduced in the packaging system for a recombinant competent retrovirus (RCR) generation. A strong splice acceptor site obtained from the exon/intron junction of either the chimpanzee EF1-alpha gene or the human CMV major immediate early gene was placed downstream of the MoMSV packaging signal (Psi), significantly improving the efficiency of transgene expression. The 5' LTR U3 sequence was replaced with an extended human CMV major immediate early gene enhancer/promoter for a strong expression of full-length messages from the viral backbone, helping to maintain high levels of viral titer. These newly developed retroviral vectors should facilitate RCR-free gene transfer with significantly improved efficacy in clinical gene therapy trials.

Establishment of Modified Retroviral Vector Targeting X-Linked Severe Combined Immunodeficiency

Gene therapy targeting hematopoietic stem cells has been proposed as a potential therapy for numerous genetic disorders affecting hematopoiesis. Moloney murine leukemia retroviral vectors are now widely used for clinical gene transfer into hematopoietic progenitors and progeny. However, maintaining expression of therapeutic genes inserted via moloney murine leukemia virus (MoMLV)-based vectors has proven to be more difficult than previously expected. In this study, an MND-IL-2R vector containing IL-2Rc gamma cDNA to treat X-linked severe combined immunodeficiency (X-SCID) was constructed from an MND vector that was modified by substituting the myeloproliferative sarcoma virus (MPSV) enhancer for that of MoMLV, deleting the negative control region located in the long terminal repeat (LTR) as an enhancer, and replacing the primer binding site (PBS) of MoMLV with the PBS of the endogenous murine retrovirus dl587rev. This vector was transduced into human CD34 + progenitor cells with comparable efficiency to that of the MoMLV-based vector. The use of this newly created vector may be advantageous for gene therapy of X-SCID.

Quantitative imaging of the T cell antitumor response by positron-emission tomography

We describe a noninvasive, quantitative, and tomographic method to visualize lymphocytes within the whole animal. We used positron-emission tomography (PET) to follow the localization of adoptively transferred immune T lymphocytes. Splenic T cells from animals that had rejected a Moloney murine sarcoma virus/Moloney murine leukemia virus (M-MSV/M-MuLV)-induced tumor were marked with a PET reporter gene, injected into tumor-bearing mice, and imaged in a microPET by using a substrate specific for the reporter. Specific localization of immune T cells to the antigen-positive tumor was detected over time, by sequential imaging of the same animals. Naive T cells did not localize to the tumor site, indicating that preimmunization was required. Autoradiography and immunohistochemistry analysis corroborated the microPET data. The method we have developed can be used to assess the effects of immunomodulatory agents intended to potentiate the immune response to cancer, and can also be useful for the study of other cell-mediated immune responses, including autoimmunity.

A single Glu(62)-to-Lys(62) mutation in the Mos residues of the R7Delta447Gag-tMos protein causes the mutant virus to induce brain lesions

We previously reported that R7Delta447, a 2954-base-pair (bp) laboratory-generated Moloney murine sarcoma virus, induced subcutaneous tumors in about 14% of infected mice but did not induce brain lesions. We now report that R7Delta447K, a spontaneous mutant of R7Delta447, induced brain lesions as well as subcutaneous tumors in all injected mice. The genomes of the two viruses differ in a single base pair: the deduced Glu(62) of the Mos residue of the R7Delta447 Gag-tMos protein is changed to Lys(62). More R7Delta447 than R7Delta447K focus-forming units were detected in both NIH3T3 and mouse cerebral vascular endothelial (MCVE) cells. However, R7Delta447K transformed NIH3T3 and MCVE cells more acutely than did R7Delta447. A distinctive feature that distinguished the morphologic transformation of R7Delta447- and R7Delta447K-infected MCVE cells is the markedly prolonged spindle-shaped phase exhibited by R7Delta447-infected MCVE cells. In addition, R7Delta447K was more efficient in inducing the phosphorylation of ERK1/2 than R7Delta447 in both MCVE and NIH3T3 cells. Moreover morphologic transformation was inhibited, and levels of phosphorylated ERK1/2 were reduced when R7Delta447- or R7Delta447K-infected NIH3T3 or MCVE cells were grown in the presence of the MEK1/2-specific inhibitor PD98095. Thus, we have identified a key residue in the Gag-tMos protein that profoundly affects activation of the Mos/MEK/ERK pathway, virus and cell replication, morphologic transformation in vitro and pathogenicity in vivo.

Effects of homology length in the repeat region on minus-strand DNA transfer and retroviral replication

Homology between the two repeat (R) regions in the retroviral genome mediates minus-strand DNA transfer during reverse transcription. We sought to define the effects of R homology lengths on minus-strand DNA transfer. We generated five murine leukemia virus (MLV)-based vectors that contained identical sequences but different lengths of the 3' R (3, 6, 12, 24 and 69 nucleotides [nt]); 69 nt is the full-length MLV R. After one round of replication, viral titers from the vector with a full-length downstream R were compared with viral titers generated from the other four vectors with reduced R lengths. Viral titers generated from vectors with R lengths reduced to one-third (24 nt) or one-sixth (12 nt) that of the wild type were not significantly affected; however, viral titers generated from vectors with only 3- or 6-nt homology in the R region were significantly lower. Because expression and packaging of the RNA were similar among all the vectors, the differences in the viral titers most likely reflected the impact of the homology lengths on the efficiency of minus-strand DNA transfer. The molecular nature of minus-strand DNA transfer was characterized in 63 proviruses. Precise R-to-R transfer was observed in most proviruses generated from vectors with 12-, 24-, or 69-nt homology in R, whereas aberrant transfers were predominantly used to generate proviruses from vectors with 3- or 6-nt homology. Reverse transcription using RNA transcribed from an upstream promoter, termed read-in RNA transcripts, resulted in most of the aberrant transfers. These data demonstrate that minus-strand DNA transfer is homology driven and a minimum homology length is required for accurate and efficient minus-strand DNA transfer.

Changes in the N- and C-terminal sequences of the murine R7 Gag-tMos protein affect brain lesion induction

Our preliminary studies suggested that the novel gag-truncated mos (tmos) open reading frame (ORF) of R7, a spontaneous deletion mutant of Moloney murine sarcoma virus 124 (MoMuSV124), may be responsible for R7's unique ability to induce brain lesions in all R7-injected mice. However, when we replaced the gag-tmos ORF with either the MoMuSV124 or the homologous myeloproliferative sarcoma virus env-mos gene, we found that both recombinant viruses also induced brain lesions in all injected mice. Although these studies suggested that the critical determinants for brain lesion induction may reside in the tmos sequence common to all three viruses, they did not demonstrate if the N-terminus of Mos was dispensable for this activity. By inserting the FLAG sequence at the 3' end of the R7 gag-tmos ORF, we demonstrated that R7 does synthesize a Gag-tMos fusion protein. Using R7 gag deletion mutants with and without the FLAG sequence, we further demonstrated that (i) deletion of the entire gag sequence abolished R7's transforming activity; (ii) the ability of the virus to transform cultured NIH/3T3 cells was significantly reduced only when most of gag was deleted; (iii) the ability of the virus to induce brain lesions was inversely proportional to the extent of its gag deletions; and (iv) the insertion of FLAG at the Mos C-terminus did not reduce the in vitro transforming activity of the FLAG-tagged viruses but did reduce their ability to induce brain lesions. Thus, we have demonstrated that altering the N- or C-terminus of the R7 Gag-tMos fusion protein can affect disease manifestation.

Studies on the pathology, especially brain hemorrhage and angioendotheliomas, induced by two new mos-containing viruses

Recombinant virus 7 (R7), a spontaneous deletion mutant of SV7, which is itself a molecular clone of Moloney murine sarcoma virus 124 (MoMuSV 124), induces brain lesions and tumors of the subcutaneous tissue and spleen in all infected mice. In contrast, SV7 only induces tumors of the spleen and subcutaneous tissues. One of the genetic differences between R7 and SV7 is that R7 encodes a Gag-Mos protein whereas SV7 encodes an Env-Mos protein. To investigate whether the novel R7 gag-mos oncogene is required for brain lesion induction, two viruses (SV7d1 and SVM1) were constructed in which the R7 gag-mos sequences and the adjacent 53 bp of the 5' noncoding sequence were replaced by either the SV7 or myeloproliferative sarcoma virus (MPSV) env-mos oncogenes, respectively. Like R7, SV7d1 and SVM1 induced brain lesions and tumors in the spleen and subcutaneous tissues. A prominent component of R7-, SV7d1-, and SVM1-induced tumors of the brain, subcutaneous tissues, and spleen was the presence of abnormally enlarged cells with eccentric nuclei lining vessels, scattered singly or in small clusters. Their size, localization to the luminal surface of distended vessels, and binding to Bandeiraea simplicifolia (BS-1) lectin, an endothelial cell (EC) marker, suggest that they are most likely transformed ECs. Our findings therefore indicate that the induction of brain lesions is not limited to the expression of the R7 Gag-Mos protein. However, our findings also indicate that expression of the different forms of the Mos protein results in differences in the relative abundance of ECs in brain angioendotheliomas and subcutaneous and spleen tumors induced by these viruses.

In vivo gene transfer of a suicide gene under the transcriptional control of the carcinoembryonic antigen promoter results in bone marrow transduction but can avoid bone marrow suppression

We constructed the CEA419/CD retrovirus vector carrying the cytosine deaminase (CD) gene directed by the carcinoembryonic antigen (CEA) promoter. pCD2 retrovirus vector carrying the CD gene directed by the retrovirus long terminal repeat promoter was also used. When mice bearing intraperitoneally disseminated colorectal carcinomas (CRCs) were infused intraperitoneally with pCD2 or CEA419/CD retrovirus-producing cells, a CD fragment was detected in CRCs and bone marrow cells. It was shown that the CD gene was expressed both in CRCs and in the bone marrow of animals infused with pCD2 retrovirus-producing cells, while the CD gene was expressed solely in CRCs of animals infused with CEA419/CD retrovirus-producing cells. These results indicate that the use of a tumor-selective promoter may warrant the safety of in vivo gene therapy using suicide genes.

Anti-tumor immunity generated by tumor cells engineered to express B7-1 via retroviral or adenoviral gene transfer

We engineered B7-1 retroviral and adenoviral gene transfer systems and studied them in four immunogenic tumor models. M-MSV tumor cells, but not K-Balb, 38.2 and 205 tumor cells, when expressing B7-1 by retroviral transduction were rejected and conferred protection against a tumor challenge. Transient expression of B7-1 after transduction with adenoviruses was less efficient. We observed enhanced cytotoxic T-lymphocyte activity accompanied by increased secretion of IL-6, IFNgamma and GM-CSF. GM-CSF secretion correlated with tumor rejection. Enhanced IFNgamma but unchanged IL-4 secretion suggested a T-helper 1-mediated anti-tumor immune response.

Studies on the pathology, especially brain lesions, induced by R7, a spontaneous mutant of Moloney murine sarcoma virus 124

We have recently isolated R7, a spontaneous Moloney murine sarcoma virus (MoMuSV) 124 variant. Molecular cloning and sequence analysis showed that, relative to MoMuSV 124, R7 has an extra repeat in each enhancer and a truncated mos gene in frame with the truncated gag coding sequence. This report presents a detailed study on the pathology induced by R7. R7 induced not only sarcomas with well developed angiomatous components but also brain lesions. Brain lesions were observed in all less-than-48-hour-old BALB/c mice inoculated with greater than 2 x 10(5) R7 focus-forming units (FFUs). R7 was detected in all brains examined by day 9 after inoculation, and brain lesions were observed in two of four mice examined by day 14 after inoculation. Light microscopy of brains revealed that approximately 15% of the lesions were unenclosed blood pools of varying sizes containing red blood cells and inflammatory cells spreading into surrounding brain tissues. The remainder of the brain lesions had tumor cells. These lesions ranged from a few enlarged vascular endothelial cells intermixed with blood cells to large circumscribed lesions consisting of well developed tangled masses of vessels surrounded by blood pools. Activated astrocytes surrounded and infiltrated the tumors. In addition, the thymus of R7-infected mice regressed significantly and precipitously due to apoptosis (especially of cortical thymocytes) at the end stage of the disease.

R7, a spontaneous mutant of Moloney murine sarcoma virus 124 with three direct repeats and an in-frame truncated gag-mos gene, induces brain lesions

We have isolated Recombinant 7 (R7), a spontaneous mutant of SV7, a molecular clone of MoMuSV124. Like SV7, R7 induces subcutaneous fibrosarcomas, spleen tumors, and mesentery tumors infiltrated by proliferating vessels lined by transformed endothelial cells. However, it also induces brain lesions. We have molecularly cloned and sequenced the R7 proviral DNA and shown that the R7 genome consists of 3401 bp. It has three direct repeats in each enhancer. Its coding sequence consists of only 176 bp of p15, 263 bp of p30, a 7-bp insertion, and 853 bp of an N-terminally truncated mos gene. From the sequence of R7 we have deduced that the truncated mos sequence is in-frame with all of the gag sequence and the 7-bp insertion. The incorporation of the 3' end of the p15 sequence further suggests that the R7 Gag-Mos is myristylated. We have also shown that the molecularly cloned R7 virus transformed NIH/3T3 fibroblasts about sevenfold better than the parental SV7. We have also confirmed that molecularly cloned R7 induces the same disease phenotype as that induced by the nonmolecularly cloned R7.

Utilization of nonhomologous minus-strand DNA transfer to generate recombinant retroviruses

During reverse transcription, minus-strand DNA transfer connects the sequences located at the two ends of the viral RNA to generate a long terminal repeat. It is thought that the homology in the repeat (R) regions located at the two ends of the viral RNA sequences facilitate minus-strand DNA transfer. In this report, the effects of diminished R-region homology on DNA synthesis and virus titer were examined. A retrovirus vector, PY31, was constructed to contain the 5' and 3' cis-acting elements from Moloney murine sarcoma virus and spleen necrosis virus. These two viruses are genetically distinct, and the two R regions contain little homology. In one round of replication, the PY31 titer was approximately 3,000-fold lower than that of a control vector with highly homologous R regions. The molecular characteristics of the junctions of minus-strand DNA transfer were analyzed in both unintegrated DNA and integrated proviruses. Short stretches of homology were found at the transfer junctions and were likely to be used to facilitate minus-strand DNA transfer. Both minus-strand strong-stop DNA and weak-stop DNA were observed to mediate strand transfer. The ability of PY31 to complete reverse transcription indicates that minus-strand DNA transfer can be used to join sequences from two different viruses to form recombinant viruses. These results suggest the provocative possibility that genetically distinct viruses can interact through this mechanism.

Synergy between the Mos/mitogen-activated protein kinase pathway and loss of p53 function in transformation and chromosome instability

Constitutive activation of mitogen-activated protein kinase (MAPK) is a property common to many oncoproteins, including Mos, Ras, and Raf, and is essential for their transforming activities. We have shown that high levels of expression of the Mos/MAPK pathway in Swiss 3T3 fibroblast cause cells in S phase to undergo apoptosis, while cells in G1 irreversibly growth arrest. Interestingly, cells in G2 and M phases also arrest at a G1-like checkpoint after proceeding through mitosis. These cells fail to undergo cytokinesis and are binucleated. Thus, constitutive overexpression of Mos and MAPK cannot be tolerated, and fibroblasts transformed by Mos express only low levels of the mos oncogene product. Here, we show that p53 plays a key role in preventing oncogene-mediated activation of MAPK. In the absence of p53 (p53-/-), the growth arrest normally observed in wild-type p53 (p53+/+) mouse embryo fibroblasts (MEFs) is markedly reduced. The mos transformation efficiency in p53-/- MEFs is two to three orders of magnitude higher than that in p53+/+ cells, and p53-/- cells tolerate > 10-fold higher levels of both Mos and activated MAPK. Moreover, we show that, like Mos, both v-ras and v-raf oncogene products induce apoptosis in p53+/+ MEFs. These oncogenes also display a high transforming activity in p53-/- MEFs, as does a gain-of-function MAPK kinase mutant (MEK*). Thus, the p53-dependent checkpoint pathway is responsive to oncogene-mediated MAPK activation in inducing irreversible G1 growth arrest and apoptosis. Moreover, we show that the chromosome instability induced by the loss of p53 is greatly enhanced by the constitutive activation of the Mos/MAPK pathway.

The Moloney murine sarcoma virus ts110 5' splice site signal contributes to the regulation of splicing efficiency and thermosensitivity

The 5' splice site signal (5'ss) in Moloney murine sarcoma virus ts110 (MuSVts110) RNA was found to participate in the regulation of its splicing phenotype. This 5'ss (CAG/GUAGGA) departs from the mammalian consensus (CAG/GURAGU) at positions +4 and +6, both of which base pair with U1 and U6 small nuclear RNAs during splicing. A doubling in splicing efficiency and near elimination of the splicing thermosensitivity characteristic of MuSVts110 were observed in 5'ss mutants containing a U at position +6 (termed 5' A6U), even in those in which U1-5'ss complementarity had been reduced. At the permissive temperature (28 degrees C), the 5' A6U mutation increased the efficiency of the second splicing reaction, while at the nonpermissive temperature (39 degrees C), both splicing reactions were positively affected.

Viral interference during simultaneous transduction with two independent helper-free retroviral vectors

The ability to stably transduce a single cell with two independent retroviral vectors would have distinct advantages for gene therapy. We determined that cells can be transduced with two distinct retroviral vectors and have quantitated transduction efficiencies in cells infected sequentially and simultaneously. Two amphotropic, helper virus-free, retroviral vectors, a murine Moloney sarcoma virus-based vector containing the nuclear beta-galactosidase and neomycin resistance genes (MMSVn beta-gal/neoR) and a Harvey virus-derived vector containing the human multidrug resistance gene (HaMDR) were introduced into NIH-3T3 cells, pig keratinocytes, and primary pig fibroblasts simultaneously and sequentially. Analytical flow cytometry was utilized to determine retroviral transduction efficiency by assessing the percentage of cells transduced by either one or both retroviruses, in the absence of selection. Simultaneous retroviral transductions were infrequent events. In addition, transduction of previously infected cells (sequential transductions) occurred at lower than expected frequencies. Our data suggest that there is quantifiable viral interference in sequential retroviral transductions. This interference occurs by a mechanism that appears to be independent of the amphotropic retroviral receptor. Thus, such dual transductions will likely require in vitro selection or the use of a single retrovirus which contains both desired genes on the same genome.

Differentiation of the immortalized adult neuronal progenitor cell line HC2S2 into neurons by regulatable suppression of the v-myc oncogene

A regulatable retroviral vector in which the v-myc oncogene is driven by a tetracycline-controlled transactivator and a human cytomegalovirus minimal promoter fused to a tet operator sequence was used for conditional immortalization of adult rat neuronal progenitor cells. A single clone, HC2S2, was isolated and characterized. Two days after the addition of tetracycline, the HC2S2 cells stopped proliferating, began to extend neurites, and expressed the neuronal markers tau, NeuN, neurofilament 200 kDa, and glutamic acid decarboxylase in accordance with the reduced production of the v-myc oncoprotein. Differentiated HC2S2 cells expressed large sodium and calcium currents and could fire regenerative action potentials. These results suggest that the suppression of the v-myc oncogene may be sufficient to make proliferating cells exit from cell cycles and induce terminal differentiation. The HC2S2 cells will be valuable for studying the differentiation process of neurons.

Branchpoint and polypyrimidine tract mutations mediating the loss and partial recovery of the Moloney murine sarcoma virus MuSVts110 thermosensitive splicing phenotype

Balanced splicing of retroviral RNAs is mediated by weak signals at the 3' splice site (ss) acting in concert with other cis elements. Moloney murine sarcoma virus MuSVts110 shows a similar balance between unspliced and spliced RNAs, differing only in that the splicing of its RNA is, in addition, growth temperature sensitive. We have generated N-nitroso-N-methylurea (NMU)-treated MuSVts110 revertants in which splicing was virtually complete at all temperatures and have investigated the molecular basis of this reversion on the assumption that the findings would reveal cis-acting elements controlling MuSVts110 splicing thermosensitivity. In a representative revertant (NMU-20), we found that complete splicing was conferred by a G-to-A substitution generating a consensus branchpoint (BP) signal (-CCCUGGC- to -CCCUGAC- [termed G(-25)A]) at -25 relative to the 3' ss. Weakening this BP to -CCCGAC- [G(-25)A,U(-27)C] moderately reduced splicing at the permissive temperature and sharply inhibited splicing at the originally nonpermissive temperature, arguing that MuSVts110 splicing thermosensitivity depends on a suboptimal BP-U2 small nuclear RNA interaction. This conclusion was supported by results indicating that lengthening the short MuSVts110 polypyrimidine tract and altering its uridine content doubled splicing efficiency at permissive temperatures and nearly abrogated splicing thermosensitivity. In vitro splicing experiments showed that MuSVts110 G(-25)A RNA intermediates were far more efficiently ligated than RNAs carrying the wild-type BP, the G(-25)A,U (-27)C BP, or the extended polypyrimidine tract. The efficiency of ligation in vitro roughly paralleled splicing efficiency in vivo [G(-25)A BP > extended polypyrimidine tract > G(-25)A,U(-27)C BP > wild-type BP]. These results suggest that MuSVts110 RNA splicing is balanced by cis elements similar to those operating in other retroviruses and, in addition, that its splicing thermosensitivity is a response to the presence of multiple suboptimal splicing signals.

Regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase gene expression in FRTL-5 cells. II. Down-regulation by v-K-ras oncogene

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity and mRNA levels were significantly reduced in FRTL-5 cells transformed with the Kirsten-Moloney sarcoma virus (KiMol); these cells have lost thyrotropin dependence and express high levels of p21ras. FRTL-5 cells, transformed with a temperature-sensitive mutant of the v-K-ras oncogene (Ats cells: 33 degrees C, permissive; 39 degrees C, nonpermissive), showed significant reduction of HMG-CoA reductase expression when exposed to 33 degrees C. In KiMol cells, as well as in Ats cells at 33 degrees C, the transcription driven by cAMP-responsive element was probed by measuring chloramphenicol acetyl transferase (CAT) levels after transfection with a chimeric plasmid containing the reporter gene linked to the rat reductase promoter. Basal CAT activity in KiMol cells transfected with wild-type promoter was lower than in FRTL-5 cells but was increased by forskolin to the levels attained in thyrotropin-stimulated FRTL-5 cells. Forskolin failed to increase CAT activity in KiMol cells transfected with the plasmid harboring a reductase promoter in which the cAMP-responsive element octamer was mutated to a nonpalindromic sequence. The effect of v-K-ras could be mimicked in FRTL-5 cells by tetradecanoyl phorbol acetate and reverted in KiMol and Ats cells, expressing active Ras protein, by increasing intracellular cAMP and/or by protein kinase C inhibition. The data are consistent with the contention that v-K-ras, through protein kinase C and depletion of intracellular cAMP, is inhibitory for the protein kinase A pathway. This is the first demonstration that active v-K-ras down-regulates HMG-CoA reductase expression.

Glucocorticoid receptor phosphorylation in v-mos-transformed cells

Nucleocytoplasmic shuttling of glucocorticoid receptors (GRs) is disrupted in v-mos-transformed cells leading to the redistribution of hormone-bound receptors from the nuclear to cytoplasmic compartments. We show here that GRs from v-mos-transformed cells are hyperphosphorylated on a specific peptide and maintain hormone-induced phosphorylations upon a prolonged hormone treatment that is associated with disruptions in its nucleocytoplasmic shuttling. Since similar effects on GR nucleocytoplasmic shuttling and phosphorylation were exerted upon treatment of nontransformed cells with the protein phosphatase inhibitor okadaic acid, we examined whether hyperphosphorylation of GRs in v-mos-transformed cells resulted from inhibition of receptor dephosphorylation. Protein phosphatase activity, measured using various substrates in vitro, was identical in cell-free extracts prepared from v-mos-transformed and nontransformed cells. Analysis of phosphate turnover in vivo from either the sum of all GR phosphorylation sites or from individual sites using pulse-chase analysis, did not reveal any significant difference between v-mos-transformed cells versus nontransformed cells. Thus, hyperphosphorylation of GR in v-mos-transformed cells does not appear to result from inhibition of GR dephosphorylation, but rather from stimulation of GR phosphorylation.

Mos overexpression in Swiss 3T3 cells induces meiotic-like alterations of the mitotic spindle

High levels of mos protooncogene product are expressed during oocyte meiotic maturation and Mos has been implicated in formation of the spindle and spindle pole. Here, we show that in Swiss 3T3 cells with 4N DNA content, high levels of Mos lead to the production of binucleated cells. The Swiss 3T3 cells in mitosis, before binucleation occurs, are anastral and the spindle poles are juxtaposed to the cell membrane. These phenotypes may be related to the meiotic process of attachment of the spindle pole to the oocyte membrane during polar body formation. The production of binucleated somatic cells could result from attachment of the altered mitotic spindle pole to the cell membrane that interferes with cytokinesis but not karyokinesis. This can explain at least one form of genetic instability that leads to altered DNA content in tumor cells.

Overexpression of mos oncogene product in Swiss 3T3 cells induces apoptosis preferentially during S-phase

When Swiss 3T3 cells are acutely infected with Moloney murine sarcoma virus containing the v-mos oncogene, 90% of the cells round up and detach from the monolayer (floating cells) and express high levels of v-Mos. The majority of the floating cells are generated between 30 and 70 h post infection when the cellular level of Mos reaches approximately 0.1% of the total protein. Seventy percent of the floating cells exclude trypan blue but are growth arrested with 2C or 4C DNA content, whereas the remaining floating cells with < 2C DNA content, are dead or dying, and show characteristic apoptotic phenotypes. The apoptotic cells are most likely generated from cells in S-phase since these cells are absent from the viable floating cell population and the percentage of cells with < 2C DNA approximated the expected S-phase fraction of logarithmically growing cells. In addition, 5'-bromo-2'-deoxyuridine-labeling studies showed that approximately 50% of the floating cells with typical apoptotic phenotypes were metabolically-labelled with the drug. These analyses show that cell populations in different stages of the cell cycle are differently affected by high levels of v-Mos expression and cells in S-phase appear to be uniquely sensitive and undergo apoptosis.

Insertion of the human cytomegalovirus enhancer into a myeloproliferative sarcoma virus long terminal repeat creates a high-expression retroviral vector

A new retroviral vector is characterized in which the enhancer in the long terminal repeat of myeloproliferative sarcoma virus (MPSV) is combined with the strong and universally active immediate-early human cytomegalovirus (hCMV) enhancer. We demonstrate that insertion of the hCMV enhancer increases the amount of vector-specific mRNA in various rodent cell lines and in human diploid fibroblasts, by at least 3-5-fold. The vector may be particularly useful if high expression of two genes is desired.

v-mos-transformed cells fail to enter quiescence but growth arrest in G1 following serum withdrawal

The product of the mos protooncogene normally functions in the induction of meiosis and regulation of cell-cycle progression in oocytes. Here we have investigated the cell-cycle progression of NIH3T3 cells transformed by the v-mos gene. Flow cytometric analysis showed that logarithmically growing v-mos-transformed cells do not differ from their nontransformed counterparts in the distribution of cells in the G1, S, and G2/M phases. Likewise, after serum withdrawal for 48 h, both normal and v-mos-transformed NIH3T3 cells have essentially ceased proliferation, as analyzed by flow cytometry, [3H]thymidine and BrdU incorporation into newly synthesized DNA, and mitotic indexes. However, while the normal NIH3T3 cells are arrested in a quiescent state, the v-mos-transformed cells are arrested in early to mid G1, prior to the point where cells require certain amino acids for proliferation (V point). In agreement with these different arrest points, the v-mos-transformed cells enter S phase following serum stimulation within about 8 h, without the additional 4- to 6-h lag period characteristically displayed by the parental NIH3T3 cells. In addition, we show a lack of expression of a growth arrest-specific gene product, gas1, in the serum-arrested v-mos-transformed cells. These data demonstrated that v-mos-transformed cells display growth characteristics that differ fundamentally from those of normal cells or cells transformed by overexpression of myc [1]. Our results suggest that the v-mos oncoprotein transforms cells, at least in part, by preventing exit from the cell cycle into quiescence.

Functional divergence of the MAP kinase pathway. ERK1 and ERK2 activate specific transcription factors

Growth factor-receptor interactions at the cell surface eventually leading to the transcriptional activation of immediate early genes is mediated by the mitogen-activated protein kinase (MAP kinase/MAPK) cascade. Here we show that overexpression of extracellular signal-regulated kinase 1 (ERK1) cDNA, encoding p44mapk, results in the activation of Elk-1, the serum response factor accessory protein. We also show that overexpression of ERK2, encoding p42mapk, activates Myc, but not Elk-1. Therefore, the MAP kinase cascade diverges with at least one specific target for each MAP kinase isoform and provides a novel mechanism for differential regulation of this signaling pathway.

Characterization of a replication-competent retrovirus resulting from recombination of packaging and vector sequences

A replication-competent retrovirus (RCR) was detected by S+/L- assays in three lots of retroviral vector G1Na that were harvested on consecutive days from a single culture of PA317/G1Na producer cells. Using a number of retrovirus-specific primer pairs, it was shown that this RCR was a novel recombinant created by exchanges between G1Na and helper sequence pPAM3 and was not an existing RCR introduced by cross-contamination. Sequencing of clones of DNA amplified in six independent PCR reactions confirmed that the 3' portion of this RCR was composed of retroviral envelope sequences unique to pPAM3 joined to a 3' long terminal repeat (LTR) unique to G1Na. Comparison of pPAM3 and G1Na sequences at the site corresponding to this junction revealed a short segment of patchy nucleotide identity (8 out of 10 bp), suggesting that these helper and vector sequences were joined by homologous recombination. Generation of RCR by exchanges between helper and vector sequences underscores the necessity of testing by efficient methods all retroviral vectors for the presence of RCR before their use. Production of 171 lots (855 liters) of various retroviral vectors that were free of RCR, including 42 lots of G1Na, however, indicates that the combination of exchanges required to generate an RCR are infrequent in this system.

Serial transplantation shows that early hematopoietic precursor cells are transduced by MDR-1 retroviral vector in a mouse gene therapy model

The administration of high and repeated doses of chemotherapy has been hampered by the bone marrow toxicity imposed by these drugs. This obstacle can be circumvented by the introduction of chemotherapy resistance genes into the normal marrow cells, which are then transplanted back into the patient. To show that this approach can improve our ability to safely deliver high doses of chemotherapy, we used an animal model system to transplant bone marrow cells which have been transduced with a safety-modified retrovirus containing human multiple-drug resistance (MDR-1) cDNA into lethally irradiated mice. These studies produced mice whose bone marrow and peripheral blood displayed an increased level of MDR-1 expression and resistance to the myelotoxic side effects of Taxol. To determine whether sufficient numbers of early hematopoietic precursor cells were transduced with the MDR-1 retrovirus so that durable Taxol-resistant hematopoiesis would result, we serially transplanted the modified bone marrow cells into each of six successive cohorts of BALB/c mice. Taxol-resistant hematopoiesis with little or no myelosuppression was observed in all six of the cohorts. These data suggest that very early precursor cells were transduced by the vector. This animal model may be of use in the development of genetic therapy programs which use bone marrow to introduce therapeutic molecules into the systemic circulation, since it permits in vivo selection of genetically modified hematopoietic progenitor cells. Furthermore, the retroviral vector system we have used could have an immediate impact in the clinical setting, where it can protect patients from the myelosuppressive side effects of Taxol in advanced stages of human epithelial cancers.

Sarcoma viruses containing the mos oncogene induce lesions resembling Kaposi's sarcoma

Kaposi's sarcoma is most accurately described as a multifocal angioproliferative lesion primarily involving the skin, although some studies indicated concomitant and/or exclusive visceral involvement. The mechanism(s) involved in Kaposi's sarcoma development, especially AIDS multiorgan distributed form, is not completely understood. Recent reports have identified a murine retrovirus, MoMuSV-349 that induces multiorgan disseminated angiosarcomatous tumors which resemble Kaposi's sarcoma when virus is inoculated intraperitoneally in newborn BALB/c mice. Performed time point experiment on mice infected with MoMuSV-349 indicated a two stages development of Kaposi's sarcoma-like tumors; sarcomatous and angiosarcomatous. Preliminary studies have demonstrated the pathogenesis of the disease process and the role of angiogenic growth factors released by the spindled cells in the stage development of mouse tumors. It is hypothesized that growth factors such as fibroblast growth factors released by the spindled cells stimulate the proliferation of endothelial cells and neovascularization which is an essential component of Kaposi's sarcoma-like tumors. Understanding the mechanism(s) of endothelial cells proliferation and neovascularization as a response to viral insult and microenvironmental changes will raise the possibilities of developing new therapeutic approaches to block this process.

Endocrine pancreatic tumors in MSV-SV40 large T transgenic mice

Mice carrying a Moloney murine sarcoma virus-(MSV) simian virus 40 large T transgene develop heritable tumors including endocrine pancreatic tumors. We have established several independent transgenic mouse lines expressing this transgene. One of these lines, designated MSV125, is characterized by the development of congenital cataracts and either pancreatic or brain tumors. The development and histopathology of the pancreatic tumors were studied by light microscopy and immunocytochemistry for large T antigen, neuron-specific enolase, insulin, proinsulin, glucagon, somatostatin, pancreatic polypeptide, gastrin, and serotonin. The 23 tumors examined were similar to human endocrine pancreatic tumors with respect to their macroscopic and histological features. We classified 91% of the tumors as insulinomas based on the predominance of insulin immunoreactivity. In newborn and young transgenic animals, nesidioblastosis and islet cell proliferation, consisting mostly of insulin containing beta cells, was obvious and persisted into adulthood. In transgenic animals more than 2 months old, islet hyperplasia and dysplasia predominated from which single tumors developed. Hyperplastic and dysplastic islets were composed mostly of beta cells. Large T antigen was detectable not only in tumor cells, but also in cells of normal and hyperplastic islets and in islet anlagen of newborn transgenic mice, indicating expression of the transgene in the endocrine part of the pancreas. Large T antigen-immunoreactivity was restricted to the beta cells. Insulinomas of the MSV-simian virus 40 T antigen-derived MSV125 transgenic mouse line may represent a valuable model for the study of the development and biology of insulinoma.

En bloc substitution of the Src homology region 2 domain activates the transforming potential of the c-Abl protein tyrosine kinase

Src homology region 2 (SH2) domains are present in many proteins involved in signal transduction. In nonreceptor protein tyrosine kinases the SH2 domain has been implicated in regulation of tyrosine kinase activity and in mediating interactions involved in downstream signaling. Different SH2 domains exhibit distinct binding specificities for both phosphotyrosine- and phosphoserine/phosphothreonine-containing proteins. We show that different SH2 domains are not functionally equivalent within the context of the c-ABL1b protooncogene. c-ABL1b, altered by replacement of its SH2 domain with the N-terminal SH2 domain of Ras GTPase-activating protein, exhibited activated transforming capability, caused intracellular tyrosine phosphorylation of p62, and was relocalized from nucleus to cytoplasm. This en bloc substitution apparently uncouples two distinct functions of the SH2 domain so that c-ABL escapes normal regulatory control while it retains the capability to elicit signals that promote transformation. The SH2 domain of the ARG protein tyrosine kinase, which shares high amino acid-sequence homology with the SH2 domain of ABL, was less effective in activating the oncogenic potential of c-ABL. The effects that the N-terminal SH2 domain of Ras GTPase-activating protein has in the context of c-ABL resemble the effects of deleting the SH3 domain. Thus, the SH2 and SH3 domains may have coordinate roles as regulatory control elements within the context of c-ABL.

Detection of Moloney murine sarcoma virus in tissues and cultured cells by the polymerase chain reaction

The polymerase chain reaction was used for Moloney murine sarcoma virus (MoMuSV) detection in frozen and formalin-fixed, paraffin-embedded tissue sections and cultured cells isolated from MoMuSV-induced tumors. Rapid DNA extraction by proteinase K digestion, followed by CHROMA SPIN + TE-100 column purification proved to be satisfactory. Two pairs of overlapping primers, flanking 1026 base pair (bp) to 221 bp, allowed to choose among four different length of DNA-amplified segments. Although net amplification was obtained for frozen tissue and tumor cultured cells in all combinations of primers, the maximum specificity and sensitivity resulted with 602 bp fragment. This product was fully and adequately digestible using Apa I and Sau3A I restriction endonucleases. DNA extracted from paraffin-embedded sections yielded an amplification product only when the primer pair which delineated a 221-bp segment was used. This reproducible method could be useful for diagnostic and for pathogenetic investigations of MoMuSV infections.

Mos and the cell cycle: the molecular basis of the transformed phenotype

The product of the mos proto-oncogene is a serine/threonine kinase that is expressed at high levels in germ cells. Mos is a regulator of meiotic maturation, and is required for the initiation and progression of oocyte meiotic maturation that leads to the production of unfertilized eggs. Mos is also a component of cytostatic factor, an activity that is believed to arrest oocyte maturation at meiotic metaphase II. There is evidence showing that the Mos protein is associated with tubulin in unfertilized eggs and transformed cells, raising the possibility that it is involved in the microtubular reorganization that occurs during M-phase. Inappropriate expression of its M-phase activity during interphase of the cell cycle may be responsible for its transforming activity.

Regulation of the efficiency and thermodependence of murine sarcoma virus MuSVts110 RNA splicing by sequences in both exons

Efficient splicing of MuSVts110 RNA is restricted to temperatures of 33 degrees or lower. Previously, we have shown that this conditional splicing event is mediated, in part, by cis-acting intronic sequences. We have now examined the role of exon sequences in MuSVts110 RNA splicing. We found that deletion of all but 36 nucleotides of the gag exon (E1) yielded a transcript incapable of supporting splicing. However, inefficient, growth temperature-dependent splicing was recovered after restoration of the 300 nucleotides of E1 proximal to the 5' splice site (5' ss). Increasingly efficient splicing was observed as more E1 was restored. Hence, although MuSVts110 E1 sequences were required for splicing, they were not involved in its thermodependence. Similarly, removal of all but 88 nucleotides of the mos exon (E2) abolished splicing at the usual 3' splice site (3' ss). In contrast to E1, restoration of the 200 nucleotides of E2 adjacent to the 3' ss reactivated efficient, temperature-independent splicing. Thermodependent splicing, however, reappeared with the replacement of E2 sequences located more than 400 nucleotides distal to the 3' splice site. In MuSVts110 mutants containing the minimum amounts of both E1 and E2 which would support splicing, splicing was both far more efficient than predicted and temperature-independent, suggesting that cooperation between E1 and E2 may help to regulate MuSVts110 splicing.

Characterization of activated and normal mouse Mos gene in murine 3T3 cells

We have characterized the mouse Mos proto-oncogene product, pp39Mos, in murine fibroblasts. When expressed in NIH3T3 cells under the influence of the long terminal repeat regulatory element from Moloney murine sarcoma virus [NIH(pTS-1) cells], the Mos protein was present in low levels and had a half-life of about 30 min. In extracts from NIH(pTS-1) cells, we detected additional forms of Mos protein that apparently arose from internal initiation codons (p24Mos and p29Mos) or from upstream non-AUG initiation codons (p42Mos and p44Mos). The Mos protein was found to exist in these cells as a phosphoprotein, pp39Mos, and, when immunoprecipitated with an antiserum specific for the Mos N-terminus [anti-Mos(6-24)], had autophosphorylating kinase activity. We found that anti-Mos(6-24) also detected non-Mos protein kinase activity and non-Mos phosphoproteins in addition to p39Mos. We present evidence, on both the RNA and protein levels, that non-transformed mouse 3T3 cells do not express endogenous Mos.

Memory of butyrate induction by the Moloney murine sarcoma virus enhancer-promoter element

We reported previously that a housekeeping gene could be converted to a butyrate-inducible gene by replacing its cognate promoter with the Moloney murine sarcoma virus enhancer-promoter element. In this study we report that the activated transcriptional state could be propagated from mother to daughter cells after the withdrawal of the inducer.

Substrate preference of metalloproteinases secreted by ts-110 Moloney murine sarcoma virus-transformed normal rat kidney cells

In this study, the two forms of affinity-purified transformation-associated proteins (TAPs) (68 and 64 kD) were shown to have different substrate preferences. For the 68-kD TAP, the order of substrate preference was collagen types I, III, and V; fibronectin; gelatin; and collagen IV. For the 64-kD TAP, the order of substrate preference was collagen I, III, and V and gelatin. The 64-kD TAP did not cleave collagen IV and fibronectin. We also found a 71-kD metalloproteinase in the concentrated purified TAPs that reacted only weakly with a TAP monoclonal antibody and showed this substrate preference: collagen I, III, and V; gelatin; and collagen IV. Whether this 71-kD TAP is a new member of the rat metalloproteinase family will be investigated.

Moloney murine sarcoma virus MuSVts110 DNA: cloning, nucleotide sequence, and gene expression

We have cloned Moloney murine sarcoma virus (MuSV) MuSVts110 DNA by assembly of polymerase chain reaction (PCR)-amplified segments of integrated viral DNA from infected NRK cells (6m2 cells) and determined its complete sequence. Previously, by direct sequencing of MuSVts110 RNA transcribed in 6m2 cells, we established that the thermosensitive RNA splicing phenotype uniquely characteristic of MuSVts110 results from a deletion of 1,487 nucleotides of progenitor MuSV-124 sequences. As anticipated, the sequence obtained in this study contained precisely this same deletion. In addition, several other unexpected sequence differences were found between MuSVts110 and MuSV-124. For example, in the noncoding region upstream of the gag gene, MuSVts110 DNA contained a 52-nucleotide tract typical of murine leukemia virus rather than MuSV-124, suggesting that MuSVts110 originated as a MuSV-helper murine leukemia virus recombinant during reverse transcription rather than from a straightforward deletion within MuSV-124. In addition, both MuSVts110 long terminal repeats contained head-to-tail duplications of eight nucleotides in the U3 region. Finally, seven single-nucleotide substitutions were found scattered throughout MuSVts110 DNA. Three of the nucleotide substitutions were in the gag gene, resulting in one coding change in p15 and one in p30. All of the remaining nucleotide changes were found in the noncoding region between the 5' long terminal repeat and the gag gene. In NIH 3T3 cells transfected with the cloned MuSVts110 DNA, the pattern of viral RNA expression conformed with that observed in cells infected with authentic MuSVts110 virus in that viral RNA splicing was 30 to 40% efficient at growth temperatures between 28 and 33 degrees C but reduced to trace levels above 37 degrees C.

Sodium butyrate selectively induces transcription of promoters adjacent to the MoMSV viral enhancer

The long terminal repeat region of the Moloney murine sarcoma virus (MoMSV) was cloned upstream from the Chinese hamster ovary adenine phosphoribosyltransferase (APRT)-encoding gene (APRT) in order to enhance synthesis of the APRT protein. The replacement of the native promoter with the viral enhancer-promoter increased the enzymatic activity of APRT two- to threefold. Addition of sodium butyrate (NaBu) to the cell growth medium induced APRT activity ten- to 20-fold above wild-type levels in both transient and stable transfectants. The introduction of the APRT native promoter between the MoMSV enhancer-promoter and structural gene reduced the magnitude of the NaBu response. The bacterial cat gene was also stimulated by NaBu when linked to the viral enhancer-promoter. No NaBu response was found in constructs lacking the MoMSV enhancer region. Northern analysis and nuclear run-on experiments indicated that NaBu enhanced transcription of APRT mRNA in both transiently and stably transfected cells, but not in cells inhibited by cycloheximide. Thus, a butyrate-response element (BRE) is associated with the MoMSV enhancer and the action of the MoMSV BRE is promoter-dependent.

Tumours induced by Moloney murine sarcoma virus are clonal in rats, not clonal in mice

Moloney murine sarcoma virus (M-MSV) induces rapidly growing tumours in adult mice of most conventional strains. Rats are less susceptible to M-MSV oncogenesis, but the few rhabdomyosarcomas that do develop after viral inoculation of newborn animals closely resemble conventional malignancies: they develop after a long latency, grow progressively, and metastasize to regional lymph nodes and lungs. Southern blot analysis with a v-mos-specific probe of M-MSV-induced tumours in both species demonstrated an oligo-, monoclonal pattern of exogenous v-mos integration only in the rat system, while mouse tumours were not clonal in origin. Furthermore, the same type of analysis of lymph node and lung metastases showed that cell clones already present in the primary rat lesion colonized secondary sites during tumour progression. Apparently, Moloney murine leukemia virus (M-MuLV) was not involved in rhabdomyosarcoma pathogenesis since M-MuLV-specific DNA sequences could not be demonstrated in three of the six rat tumours. Finally, in all mouse tumours, unintegrated linear M-MSV proviruses could be readily detected.

v-mos proteins encoded by myeloproliferative sarcoma virus and its ts159 mutant

The myeloproliferative sarcoma virus (MPSV) v-mos protein was predicted to be identical in size to p39c-mos because of an observed one-base deletion in the seventh codon of the env-mos open reading frame, which would allow translation to initiate at the methionine equivalent to codon 32 of the env-mos gene. On the basis of published results, p39c-mos is known to have greatly reduced in vitro protein kinase activity compared with p37env-mos encoded by Moloney murine sarcoma virus. Unexpectedly, the relative activity of the MPSV v-mos protein kinase was comparable to that of p37env-mos. Consistent with this finding, the size of MPSV v-mos protein was found to be similar to the size of p37env-mos. Moreover, the pattern and sizes of phosphorylated bands produced by autophosphorylation of the MPSV v-mos protein were similar to those of p37env-mos. These results were confirmed by in vitro transcription-translation of the MPSV v-mos gene. Resequencing portions of the MPSV mos gene failed to show the deletion within codon 7. Except for the codon 262 deletion, other mutations characteristic of MPSV and temperature-sensitive MPSV v-mos genes were confirmed. A glycine-to-arginine mutation at residue 338 of the MPSV env-mos sequence, previously shown to cause thermosensitivity of the mutant virus (termed ts159) transforming function, yielded a v-mos protein that had significantly reduced protein kinase activity in vitro. These findings indicate that MPSV, like other Moloney murine sarcoma virus strains, also encodes a functional env-mos protein.

Midline brain tumors in MSV-SV 40-transgenic mice originate from the pineal organ

Adult transgenic mice expressing the large T-antigen of the Simian virus 40 (SV 40) under the control of the Moloney murine sarcoma virus (MSV) enhancer and the SV 40 promoter develop inheritable uniform midline brain neoplasms showing features of primitive neuroectodermal tumors. The origin and histogenesis of these tumors were investigated in the present study. The brain and pineal organ of fetal and young transgenic mice less than 3 months old displayed normal macroscopic and microscopic features. In 3.5-month-old animals, the pineal organ was considerably enlarged due to hyperplasia, finally leading to tumor formation. Immunocytochemical demonstration of large T-antigen showed that this oncoprotein was already expressed in the nuclei of certain cells in the pineal organ of fetuses (16 and 18 days old) and newborn animals, but was absent from all other parts of the brain. The immunocytochemical demonstration of S-antigen (arrestin), a highly characteristic marker for pinealocytes, was used for further characterization of the large T-antigen-immunoreactive cells. The fetal pineal organ did not contain immunoreactive S-antigen. This first occurred in certain pinealocytes of newborn mice. Double immunostaining revealed that in newborn and older transgenic mice the immunoreactive large T-antigen was exclusively found in nuclei of cells containing S-antigen immunoreaction in their cytoplasm. Thus, transformed pinealocytes appear as stem cells of the experimental tumors. The results of this study suggest that primitive neuroectodermal tumors and the normal tissue from which they originate share certain molecular and immunocytochemical features.

SV7, a molecular clone of Moloney murine sarcoma virus 349, transforms vascular endothelial cells

SV7, a progeny of Moloney murine sarcoma virus 349 cells, was molecularly cloned. SV7 induced sarcomas consisting of vascular and fibrous components. The large blood-filled vascular dilatations appeared grossly as dark red spots in the tumors and constituted up to 50% of the tumor volume. These vascular structures, ranging from small capillaries to cavernous vascular dilatations, were lined by one to several layers of neoplastic endothelial cells. Thick papillary outgrowths of the neoplastic endothelium extended into and often occluded the vessel lumens. The fibrous component consisted mostly of spindle cells and granulocytes, which provided the stroma for the vascular structures. The vascular and fibrous components appeared to have arisen independently. Lymphopenia accompanied by myeloid metaplasia was observed in the spleen of both SV7- and myeloproliferative sarcoma virus (MPSV)-infected mice. The blood of SV7-infected mice had a much higher level of circulating granulocytes than did that of MPSV-infected mice. The latter manifested a more advanced myeloid metaplasia, characterized by aggregates of myelomonocytic blast cells in the spleen.

Temperature-sensitive synthesis of a metalloproteinase in ts110-MSV-M-transformed NRK cells

Previously, we reported that transformation associated protein (TAP) was over-expressed in the 6m2 line, but not in their normal counterparts (1,2). 6m2 is a culture of NRK cells transformed by the ts-110 mutant of MSV-M. The synthesis of TAP and the expression of transformation properties in the 6m2 cells are all temperature-sensitive (2; 3; 4). TAP is secreted as two polypeptides of 64 kD and 68 kD (P64 and P68) (2). Experiments were carried out to determine whether any metalloproteinase (MP) activity was associated with TAP. Results of zymograms indicated that the two forms of purified TAP (P64 and P68) had MP activity, using gelatin or collagen type IV as substrates. Serum-free medium (SFM) of 6m2 cells incubated at 33 degrees C also showed two bands of MP activity, while the corresponding SFM from 6m2 cells at 39 degrees C lacked such MP activity, indicating that the synthesis of MP was temperature-sensitive. The association of MP activity with the P64 and P68 bands of TAP (purified or in SFM) was confirmed by simultaneous Western blot analysis, which showed the reactivity of the two MP bands with monoclonal or polyclonal antibodies to TAP. Accordingly, what we previously designated as TAP is apparently one form of MP, which are known to be involved in tumor cell metastasis.

Inhibition of H-ras expression by the adeno-associated virus Rep78 transformation suppressor gene product

Here the adeno-associated virus Rep78 gene product was found to inhibit the expression of the chloramphenicol acetyltransferase and the bladder cancer-derived EJ-H-ras coding sequences when they were under the control of the natural cellular H-ras regulatory sequences. However, Rep78 had little or no effect on the expression of these same coding sequences when they were under the control of the regulatory sequences of the murine osteosarcoma virus long terminal repeat. These data indicate that the inhibition of H-ras by Rep78 depends upon sequences present within the cellular H-ras upstream regulatory region. Furthermore, these and earlier data indicate that Rep78 functions as an "antioncogene" or transformation suppressor gene, inhibiting H-ras as well as several viral oncogenes.

Proliferin, a prolactin/growth hormone-like peptide represses myogenic-specific transcription by the suppression of an essential serum response factor-like DNA-binding activity

Proliferin (PLF), a protein which has homology to PRL and GH, has been implicated in the regulation of cell growth and differentiation. PLF1 was detected and found to be differentially regulated during myogenesis in the rodent myogenic cell line C2C12. Transient and stable constitutive high level expression of PLF1 repressed expression of the transfected cardiac and skeletal alpha-actin myogenic-specific promoters, but did not affect expression of the cytoskeletal beta-actin and several viral promoters linked to CAT. Stable cotransfection analyses of 5' unidirectionally deleted actin promoters and a PLF expression vector indicated that PLF exerted its effect on transcription down-stream of nucleotide positions -177 and -154 with respect to the start of transcription at 1 in the cardiac and skeletal alpha-actin promoters. Analyses of cells stably transfected with PLF showed reduced levels of MyoD mRNA, a recently identified gene that is sufficient to convert pluripotential 10T1/2 cells into myoblasts. However, transient constitutive expression of MyoD by the Moloney sarcoma virus long terminal repeat did not override the effect of PLF. Electrophoretic mobility shift analysis of nuclear extracts from C2C12 cells stably transfected with a PLF expression vector displayed drastically reduced levels or activity of the CArG-binding factor (CBF) relative to the ubiquitously expressed transcription factor Oct-1. High affinity interaction between CBF and alpha-actin promoter sequences in vitro directly correlates with functional in vivo expression. CBF is a transcription factor that is sufficient and necessary for myogenic-specific transcription, interacts with the promoter sequences targeted by PLF, and is immunologically related to the serum response factor. In conclusion, PLF selectively represses myogenic-specific transcription within the actin multigene family by suppressing the level and/or activity of a trans-acting factor (CBF) that modulates multiple muscle-specific genes. The data provide a molecular explanation for the inhibition of differentiation by an endogenously produced growth factor/hormone that is differentially expressed during myogenesis and a physiologically important antagonistic regulator of muscle-specific transcription.

Production and characterization of a monoclonal antibody to the v-mos oncogene protein

Valuable information about proto-oncogenes and their physiological functions has been obtained by studying their expression in normal cells. However, the protein product of the c-mos gene, the cellular homologue of the transforming gene (v-mos) of Moloney murine sarcoma virus, has not been detected in normal mouse cells or tissues. Here, we have constructed a v-mos expression vector, pRI-delta mos, which directs the synthesis of a truncated v-mos gene product, a protein A fusion protein. Using the truncated v-mos oncoprotein produced in Escherichia coli as immunogen, we prepared anti-v-mos monoclonal antibodies (MAbs). In immunoblotting assays, the MAb was reactive with v-mos oncoprotein and detected bands at 43 KDa or 39 kDa in the tissue extract of mouse testes or ovaries, respectively, in which the c-mos protooncogene mRNA is expressed. These results demonstrate that the v-mos MAb obtained is suitable for elucidating the physiological functions of v-mos gene product and may also be utilized to detect c-mos gene product at the cellular level.

Retroviral vectors related to the myeloproliferative sarcoma virus allow efficient expression in hematopoietic stem and precursor cell lines, but retroviral infection is reduced in more primitive cells

Retroviral vectors are considered to be the most suited vehicles for somatic gene therapy with hematopoietic stem cells as targets. Retrovirus-mediated gene transfer into differentiation-restricted hematopoietic precursor (FDC-P1, FDC-P2) and multipotent progenitor (stem) cell lines (FDC-Pmix) is inefficient. Two cellular restrictions are involved. One is specific for stem but not precursor cells and is at the level of transcription. Due to a unique property of the transcriptional control region of the myeloproliferative sarcoma virus (MPSV), vectors derived from MPSV are not affected by this block. The second restriction occurs before proviral DNA synthesis and integration. This inhibition of effective viral infection depends on the state of differentiation, being more pronounced in multipotent clonogenic blast cells. This block to retroviral infection affects all retroviral vectors tested.

Detection of burst-promoting activity in spleens of myeloproliferative sarcoma virus-infected mice using serum-free cultures

Myeloproliferative syndrome was induced in adult DBA/2 mice by inoculation with myeloproliferative sarcoma virus (MPSV) and Friend murine leukemia virus (F-MuLV) as a helper virus. On day 26 after infection, the spleen weighed a maximum of 2.0 g (about 30 times the control weight). Assay of multipotent stem cells in vitro showed that the more enlarged spleens contained an increased number and concentration of mixed colony-forming units (CFU-mix) (at maximum, 11 times higher than the control). When the supernatant of cultured spleen cells was added to a serum-free bone marrow cell culture with or without erythropoietin (Epo) for detection of burst-promoting activity (BPA), it enhanced erythroid mixed colony (E-mix) formation only in the presence of Epo (p less than 0.05). Even when addition of Epo was delayed, it still induced a significant number of E-mix (p less than 0.05). These findings rule out a mimic effect of Epo resembling BPA and indicate the presence of BPA in the spleen. The culture supernatant also supported the proliferation of interleukin 3 (IL-3)-dependent 32Dcl cells. Therefore, although purification of the BPA substance has not yet been accomplished, BPA in the supernatant seems to depend on the presence of IL-3, which is known to be one of the factors stimulating multipotent hemopoietic stem cells. The presence of BPA- or CFU-mix-stimulating activity in the spleen after infection might be responsible for the development of panmyelosis, which is a characteristic of MPSV-induced myeloproliferative syndrome.