Cellular Moloney murine sarcoma (c-mos) sequences are hypermethylated and transcriptionally silent in normal and transformed rodent cells

Molecular cloning and sequencing of a guinea-pig pro-opiomelanocortin cDNA

The guinea-pig has high levels of circulating cortisol. Though adrenocorticotropin (ACTH) levels are similar to those in other mammals, guinea-pig ACTH has been reported to have a single amino-acid substitution which results in increased bioactivity of the peptide. Pro-opiomelanocortin (POMC) is the precursor for ACTH, gamma-melanocyte-stimulating hormone (gamma-MSH) and the endogenous opioid peptide beta-endorphin. Both to confirm this substitution in guinea-pig ACTH and to establish whether other non-conservative substitutions occur elsewhere in the precursor we cloned guinea-pig POMC. The guinea-pig alanine for proline substitution at position 24 of ACTH was confirmed. Potentially significant mutations were also identified in gamma-MSH and beta-endorphin. A similar pattern of POMC mRNA expression was obtained for guinea-pig and rat as determined by Northern analysis and in situ hybridization. Southern blot analysis indicated that guinea-pig POMC is a single-copy gene. Cloning and sequencing of guinea-pig POMC thus further demonstrate the divergence of the New World hystricomorph peptides from those in New World primates, and underscore the differences observed in other endocrine axes in the guinea-pig.

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.

Abnormal expression of the MOS proto-oncogene in human thyroid medullary carcinoma

We have been studying the expression of a range of proto-oncogenes in human thyroid tumour tissue by using Northern blot analysis. We have demonstrated the expression of a MOS mRNA of 1 kb in all thyroid samples. Furthermore, in a medullary carcinoma sample we also observed additional mRNA species of 1.7 and 2.2 kb. Southern blot analysis of DNA prepared from the same tumour sample did not reveal a rearrangement of the gene. These findings are the first report of MOS expression in any human tissue, and indicate that MOS oncogene activation might be important in the development of some thyroid tumours.

Amplification of the EGF receptor and c-myc genes in human esophageal cancers

The incidence of esophageal cancer is extremely high in Linxian County and certain other regions of the People's Republic of China. Epidemiologic and laboratory studies suggest that N-nitroso carcinogens and other environmental factors play a causative role. In the present study, employing over 100 DNA samples obtained from Lin-xian patients who underwent surgery for esophageal cancer, we have found a significant frequency of amplification of either the human epidermal growth factor receptor (HER-I) gene or the c-myc oncogene. These changes were found not only in tumor specimens, but also in adjacent non-tumor (grossly normal) tissue specimens obtained from patients with esophageal cancer. RNA samples were also obtained from over 30 tissue samples. These revealed considerable variation in the abundance of HER-I and c-myc transcripts in both the tumor and adjacent non-tumor specimens. A few samples revealed extremely high levels of these transcripts. Thus, changes in gene copy number or level of expression of HER-I or c-myc DNA sequences may play an important role in the pathogenesis of esophageal cancer in this high-risk region.

Nitropyrenes are inducers of polyoma viral DNA synthesis

The biological activity of a series of nitropyrenes was assayed by measuring their ability to induce the asynchronous replication of viral DNA in rat fibroblasts transformed by a ts-a mutant of polyoma virus. Concentrations of 10-30 micrograms/ml of 1-nitropyrene (1-NP) induced viral replication, and this effect was enhanced by addition of rat-liver S9 microsomal fraction (300 micrograms/ml) to the culture medium. The response was less than that obtained with 0.1 micrograms/ml of the activated metabolite of benzo[a]pyrene (BP), BP trans-7,8-dihydrodiol-9,10 epoxide (anti) (BPDE). A series of di-, tri-, and tetra-nitropyrenes were also found to induce polyoma DNA replication, in the absence of exogenous microsomal activation, displaying strongly positive effects at 0.5-2.0 microgram/ml. Dose-response curves with 1,6-dinitropyrene (1,6-DNP) from 0.01 to 0.5 microgram/ml indicated that this compound was approximately equipotent with BPDE for induction of polyoma DNA synthesis. Studies of drug metabolism, DNA binding and DNA adduct formation indicate that 1,6-DNP is metabolized in this cell line, binds to DNA, and forms stable adducts. The level of DNA modification seen with 1,6-DNP is higher than that observed under comparable conditions with an equivalent dose of BPDE. These findings provide additional evidence that the nitropyrene class of compounds can exert biological effects in mammalian cells, and that the dinitropyrenes are more potent than 1-NP.

Structure and expression of mos sequences in spontaneous and Moloney murine sarcoma virus-induced yolk sac carcinomas in rats

Facilitation of yolk-sac carcinoma (YSCa) development in fetectomized rats by the Moloney murine sarcoma virus/murine leukemia virus (Mo-MSV/MLV) complex was found to be closely associated with the presence of Mo-MSV sequences in the genomes of the YSCa cells. The virus-induced YSCas consisted of cells of mono- or oligoclonal origin which always contained in their genomes at least I randomly integrated Mo-MSV provirus. In YSCas which developed in the absence of virus, no rearrangement or amplification of c-mos could be detected. In addition, blot hybridization analysis of cellular RNA failed to detect mos-related RNA in cell lines derived from Mo-MSV-induced as well as from non-virally induced YSCas. The methylation level of c-mos DNA was low in all YSCa cell lines. In contrast, v-mos DNA in cell lines derived from Mo-MSV-induced YSCas was heavily methylated.

c-mos upstream sequence exhibits species-specific enhancer activity and binds murine-specific nuclear proteins

Expression of the c-mos proto-oncogene is detected only in mouse and rat testis and ovaries. Its transcriptionally silent character suggested the involvement of sequences that cause repression of the c-mos expression. Evidence for such repressor sequences has been reported. Here we investigated the possibility that a cis-acting enhancer element(s) is responsible for the cell type-specific c-mos(rat) expression. We localized an enhancer close to (300 bases) the repressor sequences, which acts on heterologous promoters in an orientation-independent manner over large distances. Surprisingly, it shows species preference. Sequence analysis of the enhancer fragment indicated the presence of four blocks of nucleotides homologous to the simian virus 40 enhancer core sequence. Different nuclear proteins, some of which are expressed in murine but not in primate cells, bind to the enhancer DNA. They protect three closely associated areas from DNase I digestion, two of which are homologous to the simian virus 40 enhancer core sequence. The third protected sequence is located downstream from the two enhancer core blocks.

Localization of the human c-mos gene by in situ hybridization in two cases of acute nonlymphocytic leukemia type M2

The t(8;21)(q22.1;q22.3) is specific for the FAB-M2 subtype of acute nonlymphocytic leukemia (ANLL). The human c-mos protooncogene is located near the site of rearrangement on chromosome #8, at a position corresponding to band 8q22. The present in situ hybridization studies were performed in order to establish if c-mos is transposed from chromosome #8 to chromosome #21, in two cases of M2-ANLL showing the typical t(8;21). A statistical analysis of the results revealed that the c-mos oncogene was definitely not translocated from chromosome #8 to #21 in one of these patients, and was inconclusive in the other patient. The findings in the former patient suggest that either c-mos is not involved in the etiology of M2-ANLL or, alternatively, if c-mos is important in the pathogenesis of this disease, it must be activated by some mechanism other than transposition of this oncogene to an aberrant position.

Expression of c-mos proto-oncogene in undifferentiated teratocarcinoma cells

Proto-oncogene c-mos, the cellular homologue of the transforming gene of Moloney murine sarcoma virus, has been characterized by the lack of expression in a variety of differentiated tissues, possibly because of the existence of an inhibitory upstream sequence. We detected mos-related transcripts in undifferentiated embryonal carcinoma cells of pluripotential cell line 311. The sizes of three major transcripts detected were estimated to be 1.8, 4.6 and 6.1-kilobases (kb) by northern analysis. Furthermore, these transcriptions were suppressed when the cell differentiation was induced by retinoic acid. Taken together, the results suggest that mos product plays a role in early stages of development.

Temperature-dependent cytocidal effects of Moloney murine sarcoma virus

Infection of L6E9 rat myoblasts with Moloney murine sarcoma virus (Mo-MuSV) at 37 degrees C resulted in the killing of target cells in less than one week. However, cells infected at 37 degrees C and shifted to 40 degrees C were not killed but underwent morphological transformation and had reduced levels of virus-specific RNA. A marked decrease in a cellular 55 kDa protein, as judged by immunoblotting, correlated with increased expression of Mo-MuSV RNA in infected cells.

Sequences upstream from the mouse c-mos oncogene may function as a transcription termination signal

A region upstream from the mouse c-mos proto-oncogene, termed upstream mouse sequence (UMS), prevents expression of mos transforming activity. Previous studies suggested that the UMS prevented transcription readthrough. In this study, we constructed a recombinant DNA clone, pHTS3MS, with the UMS inserted downstream from both the mos gene and a truncated long terminal repeat containing only the U3 enhancer region. In this position UMS did not inhibit mos transforming activity. We examined cells transformed by pHTS3MS for RNA expression. S1 nuclease analysis showed that the UMS provides two polyadenylation signals to mos-containing RNA and nuclear run-on transcription showed that the primary transcripts terminate in UMS. In addition, using portions of the UMS, we found that a 360-bp fragment containing the UMS polyadenylation signals and sites inserted between the herpes simplex virus type 1 (HSV-1) thymidine kinase gene (tk) and its promoter inhibits tk transforming activity by 99% and prevents detectable expression of this construct in transient expression assays. Thus, the UMS must contain signals for polyadenylation and appears to function as a transcription terminator.

Expression of proto-oncogenes in normal and papovavirus-transformed or -infected rat fibroblasts

Dot blot hybridization was used to determine the transcript levels of 10 cellular oncogenes in Fischer rat fibroblasts transformed or infected in tissue culture by polyoma virus or SV40. The level of these messages was compared to that determined for nontransformed, noninfected control cells. The analysis includes the oncogenes myc, sis, mos, erbB, erbA, Ha-ras, Ki-ras, src, fps, and abl. For all the oncogenes tested, except for mos, detectable expression was observed in all cell lines studied including the normal control cells; when normal and transformed or infected cells were compared, no significant difference in transcript level was found for any of the oncogenes except one. A slight elevation of sis message was observed for some transformants. The results of this study apply to six polyoma and seven SV40 transformants which were chosen with the purpose of analyzing transformants displaying a variety of properties. Thus, the polyoma-transformed cell lines varied in their expression of the transformed phenotype as judged by anchorage-independent growth and cell morphology, in their coding capacity for and expression of early gene products, and included two classes of rat fibroblasts transformed by ts-a mutants: those with a temperature-insensitive transformed phenotype, and those with a temperature-sensitive phenotype, A-type and N-type, respectively. Concerning the latter two types, no differences in oncogene expression were observed between cells grown at low and those grown at high temperatures, or between the two groups of cells at either temperature.

Expression of c-mos proto-oncogene transcripts in mouse tissues

Valuable information about proto-oncogenes and their physiological function has been obtained by studying their expression in normal cells. However, expression of the c-mos gene, the cellular homologue of the transforming gene of Moloney murine sarcoma virus, has not been detected in normal mouse cells or tissues. The conservation of the c-mos open reading frame strongly indicates that the gene must function during some portion of the animal life cycle, and other lines of evidence suggested to us that the c-mos proto-oncogene may be expressed at very low levels in normal tissues. We have used a sensitive S1 nuclease assay to screen RNA preparations from mouse tissues and describe here the detection of c-mos-related transcripts especially in mouse embryos, testes and ovaries. The transcripts found in testis RNA are estimated to be approximately 1.7 kilobases (kb) long by Northern analysis. S1 analysis demonstrated that the entire mos open reading frame is present. In contrast, we detect approximately 1.4-kb transcripts in ovary RNA and at least two major transcripts, approximately 2.3 and approximately 1.3 kb, in embryo RNA. The latter transcripts have in common sequences of at least 1 kb, representing most of the c-mos open reading frame. The variation in size of the mos transcript in different tissues suggests a novel regulatory mechanism for the expression of this proto-oncogene.

Rearranged c-mos locus in a MOPC 21 murine myeloma cell line and its persistence in hybridomas

Studies of a number of normal and carcinogen-transformed murine cell lines, and a variety of murine tissues, have indicated that, in contrast to several other cellular oncogenes, the oncogene c-mos gene is usually transcriptionally silent. The recent report by Rechavi et al. indicating that in the mouse myeloma XRPC24 originally induced by pristane (2,6,10-14-tetramethylpentadecane) the c-mos gene is rearranged and transcriptionally active, and that it can transform murine fibroblasts in a transfection assay, is therefore of considerable interest. Here we show that the c-mos locus has undergone a similar rearrangement, and is also transcriptionally active, in the cell line P3-X63-Ag8-653, a derivative of the mouse myeloma MOPC 21 which was induced by mineral oil. This line is widely used for making hybridomas that synthesize monoclonal antibodies. We also demonstrate that the rearranged c-mos sequence is maintained in three different hybridomas derived by fusion of this cell line with normal murine spleen lymphocytes, suggesting that it may play a role in the continuous growth and/or constitutive immunoglobulin production by these hybridomas.

The transforming protein of Moloney murine sarcoma virus is a soluble cytoplasmic protein

The transforming gene, v-mos, of Moloney murine sarcoma virus (M-MuSV) encodes a 37,000-dalton phosphoprotein, p37mos. Since the biochemical function of this protein is unknown, we have determined the subcellular location of p37mos in M-MuSV 124-transformed cells. Using two different methods of cell lysis and fractionation, we found that newly synthesized as well as mature p37mos is a soluble cytoplasmic protein. In agreement with these results, immunofluorescent staining of cells acutely infected with M-MuSV 124, using an antiserum directed against a synthetic v-mos peptide, produced a diffuse cytoplasmic pattern. Gel filtration experiments and glycerol gradient sedimentation analysis suggest that the bulk of p37mos exists as a monomer and is not involved in a specific association with other cellular proteins. These properties of p37mos are different from those of other characterized retroviral transforming proteins.

Activation of a cellular oncogene by DNA rearrangement: possible involvement of an IS-like element

The cellular oncogene c-mos is rearranged in a mouse myeloma and the tumour mRNA contains transcripts hybridizing with a v-mos probe. The rearranged gene (rc-mos) was cloned in lambda phage and shown to transform mouse fibroblasts in transfection assays, rc-mos differs from its progenitor, c-mos, only at the 5' end of the gene, where c-mos sequences have been substituted by a novel cellular DNA fragment. This fragment contains a 159-base pair (bp) insertion sequence (IS)-like element localized immediately 5' to the junction with c-mos. This is the first demonstration in a non-virally-induced tumour of activation of a cellular oncogene by a mechanism possibly involving DNA transposition.

Tumorigenic transformation of mammalian cells induced by a normal human gene homologous to the oncogene of Harvey murine sarcoma virus

A normal human gene homologous to the p21 ras oncogene of Harvey murine sarcoma virus induced oncogenic transformation and high p21 ras levels in murine fibroblasts when this gene was ligated to a control element (the long terminal repeat) from a murine or feline retrovirus. These results indicate that high levels of a gene product encoded by a normal human oncogene can induce tumorigenic transformation.

Detection of a transforming gene product in cells transformed by Moloney murine sarcoma virus

We identified, in cells transformed by Moloney murine sarcoma virus (M-MuSV clone 124), a protein encoded by the M-MuSV transforming gene, v-mos. An antiserum against a synthetic peptide corresponding to the C terminus of a protein predicted from the v-mos nucleotide sequence specifically recognizes a protein doublet of approximately 37,000 daltons from 35S-methionine-labeled M-MuSV 124-transformed producer cells. By peptide mapping, this protein is almost identical to the 37 kd in vitro translation product from the M-MuSV v-mos gene. Immunoprecipitates from 32P-labeled cells contain a single v-mos-specific phosphoprotein, which has at least six sites of phosphorylation containing phosphoserine. Pulse-chase experiments show that the lower band in the 35S-methionine-labeled doublet is the primary translation product, which is modified, probably by phosphorylation, to yield the upper band. A similar mos protein is immunoprecipitated from HT1-MuSV-transformed cells, but not from uninfected NIH/3T3 cells. These mos proteins are present at very low levels in transformed cell lines. Cells acutely infected with M-MuSV 124, however, transiently contain much higher levels of the mos protein. These high levels coincide with extensive cell mortality.