A mos oncogene-containing retrovirus, myeloproliferative sarcoma virus, transforms rat thyroid epithelial cells and irreversibly blocks their differentiation pattern

A TSH-CREB1-microRNA loop is required for thyroid cell growth

MicroRNA (miRNA or miR) are an important class of regulators that participate in such biological functions as development, cell proliferation, differentiation, and apoptosis. The aim of this study was to elucidate the role of miRNA in cell proliferation using a unique cell system, namely thyroid cells that require thyrotropin for their growth. Here, we report the identification of a set of five specific miRNA (miR-1, miR-28-A, miR-290-5p, miR-296-3p, and miR-297a), whose down-regulation by thyrotropin is required for thyroid cell growth. In fact, overexpression of these miRNA negatively affects cell growth. We show that three of these miRNA target cAMP-responsive element binding protein (CREB)1, a thyrotropin-activated transcription factor, and that CREB1 binds the regulatory regions of the down-regulated miRNA. Hence, these data indicate that a synergistic loop involving thyrotropin, CREB1, and miRNA is required for thyroid cell proliferation.

Transcriptional Profile of Ki-Ras-Induced Transformation of Thyroid Cells

In the last years, an increasing number of experiments has provided compelling evidence for a casual role of Ras protein mutations, resulting in their constitutive activation, in thyroid carcinogenesis. However, despite the clear involvement of Ras proteins in thyroid carcinogenesis, the nature of most of the target genes, whose expression is modulated by the Ras-induced signaling pathways and that are ultimately responsible for Ras-induced cellular transformation, remains largely unknown. To analyze Ras-dependent modulation of gene expression in thyroid cells we took advantage of a differentiated rat thyroid cell line, FRTL-5. As a model for Ras-dependent thyroid transformation, we used FRTL-5 cells infected with the Kirsten murine sarcoma virus, carrying the v-Ki-Ras oncogene. The infected cells (FRTL-5 v-Ki-Ras) have lost expression of the thyroid differentiation markers and also are completely transformed. We hybridized two different Affimetrix chips containing probe sets interrogating both known rat genes and ESTs for a total of more than 17,000 sequences using mRNA extracted from FRTL-5 and FRTL-5 v-Ki-Ras cell lines. We identified about 50 genes whose expression was induced and about 40 genes whose expression was downregulated more than 10-fold by Ras. We confirmed the differential expression of many of these genes in FRTL-5 v-Ki-Ras as compared to parental cells by using alternative techniques. Remarkably, we investigated the expression of some of the Ras-regulated genes in human thyroid carcinoma cell lines and tumor samples, our results, therefore, providing a new molecular profile of the genes involved in thyroid neoplastic transformation.

The rat tyrosine phosphatase η increases cell adhesion by activating c-Src through dephosphorylation of its inhibitory phosphotyrosine residue

The expression of the receptor protein tyrosine phosphatase r-PTPeta is drastically reduced in rat and human malignant thyroid cells, whereas its restoration reverts the neoplastic phenotype of retrovirally transformed rat thyroid cells. Moreover, reduced levels and loss of heterozygosity of DEP-1, the human homolog of r-PTPeta, have been found in many human neoplasias. Here, we report that the r-PTPeta protein binds to c-Src in living cells and dephosphorylates the c-Src inhibitory tyrosine phosphorylation site (Tyr 529), thereby increasing c-Src tyrosine kinase activity in malignant rat thyroid cells stably transfected with r-PTPeta. Tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin was enhanced in r-PTPeta-expressing cells. This was associated with increased adhesion of malignant r-PTPeta-transfected thyroid cells vs both untransfected cells and cells stably transfected with an inactive r-PTPeta mutant. Treatment of rat thyroid cells with the c-Src inhibitor PP2 decreased cell adhesion to a higher extent in r-PTPeta-transfected cells than in mock-transfected or stably transfected cells with the inactive r-PTPeta mutant, indicating that r-PTPeta regulates cell-substratum adhesion by activating c-Src. Interestingly, the extent of both c-Src dephosphorylation at Tyr 529, FAK and paxillin phosphorylation, and the increased cell adhesion were associated with the degree of r-PTPeta expression.

Fra-1 expression in malignant and benign thyroid tumor

BACKGROUND

The differential diagnosis of thyroid nodules is very important in deciding the treatment modality and the fine needle aspiration is the best diagnostic method. But, there are some limitations in use because of inadequate test materials and difficulty in interpreting. According to the study of oncogene and tumor suppressor gene about the origin of thyroid tumor, expression of Fra-1, one of AP-1 complex, is increased in thyroid neoplasm, though not present in the normal tissue. So, there is a possibility that it will be used as a method for the differential diagnosis of thyroid nodules. We tried to know whether presence or absence of Fra-1 expression can be used as a diagnostic method in differential diagnosis of thyroid nodules using the immunohistochemical (IHC) staining method.

METHODS

In 4 types of thyroid tumor that were confirmed by histologic diagnosis after operation (18 cases of adenomatous goiter, 16 cases of follicular adenoma, 30 cases of papillary cancer, 10 cases of follicular cancer), IHC staining method was performed to evaluate the expression of Fra-1.

RESULTS

In papillary and follicular thyroid cancers, the expression of Fra-1 was stronger than in benign thyroid tumor, but there was no difference in Fra-1 expression between the two types of carcinoma. Weak expression of Fra-1 was observed in all cases of follicular adenoma, though it was weaker than in carcinoma, and it was also weakly expressed only in some cases (33%) of adenomatous goiter.

CONCLUSION

The expression of Fra-1 was stronger in thyroid cancer than in benign thyroid tumor, but it was impossible to differentiate thyroid cancer from benign thyroid tumor by presence or absence of Fra-1 expression using IHC staining method.

Rat protein tyrosine phosphatase eta suppresses the neoplastic phenotype of retrovirally transformed thyroid cells through the stabilization of p27(Kip1)

The r-PTPeta gene encodes a rat receptor-type protein tyrosine phosphatase whose expression is negatively regulated by neoplastic cell transformation. Here we first demonstrate a dramatic reduction in DEP-1/HPTPeta (the human homolog of r-PTPeta) expression in a panel of human thyroid carcinomas. Subsequently, we show that the reexpression of the r-PTPeta gene in highly malignant rat thyroid cells transformed by retroviruses carrying the v-mos and v-ras-Ki oncogenes suppresses their malignant phenotype. Cell cycle analysis demonstrated that r-PTPeta caused G(1) growth arrest and increased the cyclin-dependent kinase inhibitor p27(Kip1) protein level by reducing the proteasome-dependent degradation rate. We propose that the r-PTPeta tumor suppressor activity is mediated by p27(Kip1) protein stabilization, because suppression of p27(Kip1) protein synthesis using p27-specific antisense oligonucleotides blocked the growth-inhibitory effect induced by r-PTPeta. Furthermore, we provide evidence that in v-mos- or v-ras-Ki-transformed thyroid cells, the p27(Kip1) protein level was regulated by the mitogen-activated protein (MAP) kinase pathway and that r-PTPeta regulated p27(Kip1) stability by preventing v-mos- or v-ras-Ki-induced MAP kinase activation.

Neoplastic transformation of rat thyroid cells requires the junB and fra-1 gene induction which is dependent on the HMGI-C gene product

The expression of the high mobility group I (HMGI)-C chromatin component was shown previously to be essential for the establishment of the neoplastic phenotype in retrovirally transformed thyroid cell lines. To identify possible targets of the HMGI-C gene product, we have analyzed the AP-1 complex in normal, fully transformed and antisense HMGI-C-expressing rat thyroid cells. We show that neoplastic transformation is associated with a drastic increase in AP-1 activity, which reflects multiple compositional changes. The strongest effect is represented by the dramatic junB and fra-1 gene induction, which is prevented in cell lines expressing the antisense HMGI-C. These results indicate that the HMGI-C gene product is essential for the junB and fra-1 transcriptional induction associated with neoplastic transformation. The inhibition of Fra-1 protein synthesis by stable transfection with a fra-1 antisense RNA vector significantly reduces the malignant phenotype of the transformed thyroid cells, indicating a pivotal role for the fra-1 gene product in the process of cellular transformation.

Transplantable rat thyroid cancer cell line FRTC transformed with muramyl dipeptide

A rat thyroid cancer cell line, FRTC, was established from the normal rat thyroid cell line, FRTL-5. FRTL-5 cells were cultured in vitro with N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) for 4 days and were transplanted intraperitoneally into Fisher rats. Disseminated tumour formation in the peritoneum was found in ten out of ten rats in which MDP-treated FRTL-5 cells were transplanted. Colloid-like structures stained with anti-thyroglobulin (Tg) antibodies were observed in the tumours. On the other hand, no tumour was found in any of the rats in which untreated FRTL-5 cells were transplanted. No morphological changes were observed in FRTL-5 cells after long-term in vitro culture in the presence of MDP. MDP had no effect on thymidine incorporation, the production of cAMP or the expression of c-myc in FRTL-5 cells in vitro. Cells from the tumour (FRTC) secreted Tg in vitro and expressed Tg, thyroid peroxidase (TPO) and thyrotropin (TSH) receptor mRNA. The expression of TSH receptor mRNA increased in FRTC cells after TSH stimulation. FRTC cells produced cAMP in response to TSH stimulation in a dose-dependent manner. However, the growth of FRTC cells was TSH independent. Expression of c-myc and c-fos was observed in FRTC cells in vivo as well as in vitro. FRTC cells formed tumours in Fisher rats when transplanted subcutaneously. FRTC cells have several characteristics of differentiated thyroid cancer cells and may provide a good model for the study of human differentiated thyroid cancers.

Inhibition of HMGI-C protein synthesis suppresses retrovirally induced neoplastic transformation of rat thyroid cells

Elevated expression of the three high-mobility group I (HMGI) proteins (HMGI, HMGY, and HMGI-C) has previously been correlated with the presence of a highly malignant phenotype in epithelial and fibroblastic rat thyroid cells and in experimental thyroid, lung, mammary, and skin carcinomas. Northern (RNA) blot and run-on analyses demonstrated that the induction of HMGI genes in transformed thyroid cells occurs at the transcriptional level. An antisense methodology to block HMGI-C protein synthesis was then used to analyze the role of this protein in the process of thyroid cell transformation. Transfection of an antisense construct for the HMGI-C cDNA into normal thyroid cells, followed by infection with transforming myeloproliferative sarcoma virus or Kirsten murine sarcoma virus, generated cell lines that expressed significant levels of the retroviral transforming oncogenes v-mos or v-ras-Ki and removed the dependency on thyroid-stimulating hormones. However, in contrast with untransfected cells or cells transfected with the sense construct, those containing the antisense construct did not demonstrate the appearance of any malignant phenotypic markers (growth in soft agar and tumorigenicity in athymic mice). A great reduction of the HMGI-C protein levels and the absence of the HMGI(Y) proteins was observed in the HMGI-C antisense-transfected, virally infected cells. Therefore, the HMGI-C protein seems to play a key role in the transformation of these thyroid cells.

Karyotype instability and altered differentiation of rat sarcoma cells after retroviral infection

The karyotypic and phenotypic stability of cultured rat fibrosarcoma cells was challenged by infection with Moloney murine sarcoma virus (MoMuSV). After transformation, the spindle-like morphology of the parental HH-16 cl.2/1 cells had altered to a rounded phenotype, which was maintained in tumors produced by inoculating transformed cells into congenic animals. In contrast to the parental cells, transformed cells lacked cables of cytokeratins 14-16 and 19 and showed reduction of the mesenchymal marker protein vimentin. Additionally, the morphologically altered cell clones tf-1 to tf-3 had lost growth arrest in the presence of dexamethasone. The DNA of the transformed cells contained between four and six randomly integrated proviral copies. Karyotypic alterations were manifested by reduction of morphologically intact chromosomes in the MoMuSV-transformed cells together with increase of structural aberrations. Three additional markers were identified in the virus-transformed cell clones. Karyotypic instability induced by MoMuSV infection appeared closely related to reduction of the cellular differentiation status, although only cells of clone tf-1 had increased metastatic potential.

Demonstration of a heterogeneous transcription pattern of thyroglobulin mRNA in human thyroid tissues

Previous reports on human thyroglobulin (hTg) modifications in thyroid carcinomas prompted us to study hTg mRNA in thyroid adenomas and carcinomas. The quantification of hTg mRNA showed a decrease in its levels of expression in both pathological conditions which differed by a factor of 2 between adenomas and carcinomas. Furthermore, PCR was used to analyse the characteristics of hTg mRNA by amplifying 4 regions of the hTg mRNA. When applied to 2 normal, 17 benign and 13 malignant pathological tissue specimens, PCR showed no modification in the size of Tg mRNA. However, abnormal sized cDNAs appeared in all tissues with no distinction between the pathologies; the Restriction Fragment Length Polymorphism study of these cDNAs suggests the existence of alternate splicing patterns in thyroglobulin mRNAs.

Transformation of a mammary epithelial cell line by the v-raf and v-mos oncogenes

Defective retroviruses were used to investigate the effects of the oncogenes v-raf and v-mos on the behaviour of mammary epithelial cells. Cultures of the mammary epithelial cell lines #43 and NMuMG infected with either virus showed obvious areas of overgrowing cells, which were not seen when cells were infected with control viruses not containing an oncogene. Several clonal cell lines were derived from the areas of overgrowth on #43 cells. Most of these were tumorigenic on subcutaneous inoculation. In culture, several of the cloned lines showed marked morphological abnormalities, had altered rates of growth and saturation densities, and were able to clone in soft agar. Unexpectedly, one raf-containing and one mos-containing line grew more slowly in culture than did control or uninfected cells. Other lines showed only some of these changes. In the mos-containing lines the level of viral transcript corresponded approximately to the extent of transformation, whereas the raf-containing lines showed no such correlation.

Absence of retroviral sequences in Graves' disease

An earlier report of HIV-1 gene sequences in thyroid cell genomic DNA from patients with Graves' disease prompted use of the polymerase chain reaction technique to identify such sequences in Graves' disease thyroid tissue and in white blood-cells from these patients. We were unable to confirm the existence of HIV-1-related DNA sequences in Graves' specimens.

The block of thyroglobulin synthesis, which occurs upon transformation of rat thyroid epithelial cells, is at the transcriptional level and it is associated with methylation of the 5' flanking region of the gene

Transformation of rat thyroid epithelial cells by Kirsten murine sarcoma virus results in the block of certain thyroid differentiated functions, such as synthesis and secretion of thyroglobulin. Our studies, performed by a run-on assay, demonstrate that this block occurs at the transcriptional level. We also demonstrate the de novo methylation of two methylation-sensitive sites, located within the 5' end regulatory sequences of the thyroglobulin gene, in transformed cells, in the absence of any rearrangement of the gene. These two methylation-sensitive sites were methylated also in a rat thyroid cell line transformed by another retrovirus and in two normal cell lines which do not express the thyroglobulin gene.

Changes in in vitro growth behaviour of the mammary epithelial cell line NMuMG caused by the v-fos oncogene

A defective retrovirus was constructed to investigate the effect of the expression of the v-fos oncogene from FBJ-MSV on the in vitro growth properties of the mammary epithelial cell line NMuMG. Clearly visible areas of overgrowth in monolayer cultures of NMuMG were seen in cells infected with the v-fos-containing retrovirus but not in cells infected with control virus which did not contain an oncogene. Two cell lines, representing two morphological types of infected cell, were isolated from a morphologically altered region and further characterized. Fos.3.1.NMuMG grew as very spindly cells, achieving a higher density than control cells in 5% foetal calf serum (FCS) but growing very poorly in 1% FCS or in soft agar. Fos.3.3.NMuMG grew to a high density in 5% FCS and to a limited extent in low serum. This cell line also grew in soft agar. Fos.3.3.NMuMG seemed to be more transformed than fos.3.1.NMuMG using the criteria of growth in soft agar and low serum. All the cells used in this study were shown to retain epithelial characteristics by staining for cytokeratins and to contain at least one viral genome by Southern blotting. fos mRNA expression was raised over control levels in the two transformed cell lines.

Cooperation between the polyomavirus middle-T-antigen gene and the human c-myc oncogene in a rat thyroid epithelial differentiated cell line: model of in vitro progression

Two rat thyroid epithelial differentiated cell lines, PC Cl 3 and PC myc, were infected with the polyoma murine leukemia virus (PyMLV) carrying the Middle-T-antigen gene of polyomavirus. After infection, both cell lines acquired the typical markers of neoplastic transformation; however, the PC myc cells showed a greater malignant phenotype. Furthermore, the thyroid differentiated functions were completely suppressed in PC myc cells transformed by PyMLV, whereas they were, at least partially, retained in PC Cl 3 cells transformed by PyMLV, and in particular, thyroglobulin synthesis and secretion were not affected at all. Since no differences in the expression of the middle-T-antigen gene were observed in the two PyMLV-transformed cell lines, the different properties shown by these two infected cell lines must be ascribed to the expression of the c-myc oncogene.

Cooperation between the polyomavirus middle-T-antigen gene and the human c-myc oncogene in a rat thyroid epithelial differentiated cell line: model of in vitro progression

Two rat thyroid epithelial differentiated cell lines, PC Cl 3 and PC myc, were infected with the polyoma murine leukemia virus (PyMLV) carrying the Middle-T-antigen gene of polyomavirus. After infection, both cell lines acquired the typical markers of neoplastic transformation; however, the PC myc cells showed a greater malignant phenotype. Furthermore, the thyroid differentiated functions were completely suppressed in PC myc cells transformed by PyMLV, whereas they were, at least partially, retained in PC Cl 3 cells transformed by PyMLV, and in particular, thyroglobulin synthesis and secretion were not affected at all. Since no differences in the expression of the middle-T-antigen gene were observed in the two PyMLV-transformed cell lines, the different properties shown by these two infected cell lines must be ascribed to the expression of the c-myc oncogene.

Experimental rat model for human yolk sac tumor

The morphological and biological characteristics of experimentally induced rat yolk sac carcinomas (ysca) are compared to those of human yolk sac tumors. It is shown that the rat ysca shares many morphological and biological properties with its human counterpart although the cellular origin is probably different. Whereas the human yolk sac tumors are believed to be of germ cell origin, the rat visceral yolk sac-derived tumors are not. The hypothesis is formulated that the rat ysca are derived from multipotential cells different from germ cells, and which originate in the extra-embryonic membrane after displacement.

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.

Autocrine stimulation after transfer of the granulocyte/macrophage colony-stimulating factor gene and autonomous growth are distinct but interdependent steps in the oncogenic pathway

Autocrine stimulation of cells by aberrant synthesis of growth factor may lead to malignant transformation, either as a direct consequence of endogenous factor production or as a first step of a series of successive events. Introduction of the granulocyte/macrophage colony-stimulating factor (GM-CSF) cDNA clone into a vector based on the myeloproliferative sarcoma virus allowed efficient transfer and expression of GM-CSF in factor-dependent myeloid cell lines (FDC-P1 and FDC-P2). Factor-independent growth was acquired when the vector was introduced into the GM-CSF-responsive FDC-P1 cell line but not the multi-CSF-dependent FDC-P2 line. Nonlinear clonability in the absence of exogenous growth factor and growth inhibition by GM-CSF antiserum support a model of autocrine stimulation that requires interaction of factor and receptor at the outer membrane. However, many, but not all, infected FDC-P1 cells acquired subsequently a second mutation that abrogated the requirement of GM-CSF secretion and external interaction. The nature of the second step, which presumably leads to tumorigenicity of these cells, is not well understood, but its frequency could be correlated with the level of GM-CSF released by an individual cell clone.

A cell type specific factor recognizes the rat thyroglobulin promoter

We have fused a 900 base pair long DNA segment containing the transcriptional start site of the rat thyroglobulin (Tg) gene to the bacterial gene for chloramphenicol acetyltransferase (cat). The fusion gene has been introduced into three different cell lines derived from the rat thyroid gland and into a rat liver cell line. Expression of the fusion gene was detected only in the one thyroid cell line that is able to express the endogenous Tg gene. The minimum DNA sequence required for the cell type specific expression was determined by deletion analysis; it extends 170 nucleotides upstream of the transcription initiation site. The Tg promoter contains a readily detectable binding sites for a factor present in salt extracts of thyroid cell nuclei. This binding site is not recognized by the nuclear extracts of any other cell type that we have tested, suggesting that it may help mediate the cell type specific expression of the Tg gene.

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.

Changes in cellular gene expression in rat thyroid epithelial cells transformed by the Kirsten murine sarcoma virus

We have exploited a recently characterized system of rat thyroid epithelial cells transformed by the wild-type (wt) and a temperature-sensitive (ts) mutant strain of the Kirsten murine sarcoma virus (Ki-MSV) in order to study the effects of the K-ras oncogene on the gene expression of differentiated thyroid epithelial cells. By using cDNAs isolated from normal thyroid glands as probes, we were able to identify three sets of cellular sequences whose expression is influenced by the v-K-ras oncogene. The first set of genes is irreversibly repressed by transformation with both the wt and the ts viruses. The second set of genes is repressed in the ts-Ki-MSV-transformed cells but not in the same cells grown at the nonpermissive temperature. A third set of genes is present at higher levels at the nonpermissive temperature than at the permissive temperature. This system has allowed us to isolate and characterize a number of cDNA clones belonging to each of these three sets of genes. These specific cDNAs are suitable probes to study phenotypical changes during transformation of epithelial cells.

A temperature-sensitive mutant of the myeloproliferative sarcoma virus, altered by a point mutation in the mos oncogene, has been modified as a selectable retroviral vector

The myeloproliferative sarcoma virus (MPSV) is a mos-oncogenic retrovirus which induces an acute myeloproliferative disease in adult mice. The isolation and molecular cloning of two mutants of MPSV temperature sensitive (ts) for mos transformation (Kollek et al., J. Virol. 50:717-724, 1984) have been described previously. In this report, we describe the biological activity of these clones, the molecular basis of the ts lesion of one clone, and the construction of a selectable vector based on the MPSV ts genome. Both molecular clones, ts159 and ts124, proved to have retained the ts phenotype, the former being tighter for the induction and maintenance of the transformed phenotype. A single transition (G----A) at position 1888 in the mos coding region, resulting in the change of Gly to Arg at position 307, was responsible for the ts phenotype of clone ts159. Substitution of sequences carrying this mutation with the corresponding sequences of the wild-type virus generated a virus that was ts for transformation. Insertion of the dominant selectable marker gene for geneticin resistance (neor) into ts159 did not disrupt mos expression or its ts phenotype. neor-ts159 facilitates the study of mos action by allowing the selection of infected cells at the nonpermissive temperature before mos transformation has been induced. Furthermore, infected cells which show no obvious phenotype alteration due to mos expression can be identified by their Neor phenotype.

Retroviral mutants efficiently expressed in embryonal carcinoma cells

The myeloproliferative sarcoma virus (MPSV) is a unique member of the Moloney murine sarcoma virus family. Due to mutations in the U3 region of its long terminal repeat, MPSV has an expanded host range that includes cells of the hematopoietic compartment. Using a MPSV recombinant containing the gene for neomycin-resistance (NeoR-MPSV), we demonstrate that the host range of MPSV also includes undifferentiated F9 embryonal carcinoma cells. Transfer of G418-resistance with NeoR-MPSV to F9 cells is almost as efficient as G418-resistance transfer to fibroblasts, in contrast to G418-resistance transfer to PCC4 embryonal carcinoma cells, which is at least 3 orders of magnitude lower. To isolate NeoR-MPSV mutants that are efficiently expressed in PCC4 cells, G418-resistant PCC4 cell lines were induced to differentiate, and the provirus was rescued by superinfection with murine leukemia virus. Viral isolates (PCMV-5 and -6; PCMV = PCC4 cell-passaged NeoR-MPSV) were obtained and assayed for expression in embryonal carcinoma cells. The efficiency of NeoR transfer was equally as high in both F9 and PCC4 as in fibroblasts. mos oncogene expression was unaltered as judged by transformation capability. No gross alteration in the coding region and in the long terminal repeat was detectable by restriction enzyme analysis. NeoR-MPSV and its mutants PCMV-5 and -6 can thus be utilized as vectors for the efficient transduction of genes into embryonic cells.