Genesis of Kirsten murine sarcoma virus: sequence analysis reveals recombination points and potential leukaemogenic determinant on parental leukaemia virus genome

A perspective on the early days of RAS research

The name of the oncogene, ras, has its origin in studies of murine leukemia viruses in the 1960s by Jenny Harvey (H-ras) and by Werner Kirsten (K-ras) which, at high doses, produced sarcomas in rats. Transforming retroviruses were isolated, and its oncogene was named ras after rat sarcoma. From 1979, cellular ras sequences with transforming properties were identified by transfection of tumor DNA initially by Robert Weinberg from rodent tumors, and the isolation of homologous oncogenes from human tumors soon followed, including HRAS and KRAS, and a new member of the family named NRAS. I review these discoveries, placing emphasis on the pioneering research of Christopher Marshall and Alan Hall, who subsequently made immense contributions to our understanding of the functions of RAS and related small GTPases to signal transduction pathways, cell structure, and the behavior of normal and malignant cells.

Analysis of retrotransposon families in genomic DNA by two-dimensional restriction mapping: detection of VL30 insertions in mouse thymic lymphoma

Reinsertion of mammalian retrotransposable genetic elements is known to be causally associated with tumourigenesis, typically through mechanisms involving insertional deregulation of cellular protooncogene expression. We report here on the application of a two-dimensional restriction mapping-Southern hybridisation approach for analysis of retrotransposon families of low to moderate genetic complexity, which is particularly suited to pairwise comparisons between DNA samples. By using this method, non-constitutional mink-cell-focus-forming type retro-elements were readily detectable in AKR mouse thymic lymphomas against a background of approx. 30 related elements in control DNA. However, in the WEHI 3B myeloid leukaemia cell line, the resolution of two-dimensional mapping permitted detection of only occasional reinsertions of intracisternal A particle retro-elements (genetic complexity: 10(3)). In analysing the VL30 family of retrotransposon (genetic complexity: 150) we developed a strategy for identifying the known transcriptionally active sub-set of these elements in genomic DNA through the generation of an internal, diagnostic restriction fragment. Moreover, in some cases of thymic lymphoma, several candidate re-insertions of VL30 elements were detected, consistent with a suggested role for retrotransposition of this class of element in lymphomagenesis of retroviral aetiology.

Structure of a Moloney murine leukemia virus-virus-like 30 recombinant: implications for transduction of the c-Ha-ras proto-oncogene

Tumor progression locus 2 (Tpl-2) encodes a novel serine-threonine protein kinase which is activated by provirus integration in the late stages of oncogenesis in Moloney leukemia virus (MoMuLV) induced rat T-cell lymphomas. In this report, we present evidence that the provirus integrated in the Tpl-2 locus in 1 of 10 T-cell lymphomas harboring a Tpl-2 rearrangement (2779) is a recombinant between MoMuLV and virus-like 30 (VL30) sequences (Mo-VL30). Recombination between MoMuLV and VL30 may contribute to the transduction of ras, as suggested by the finding that VL30 flanks the ras oncogene in all of the ras transducing viruses isolated from rats to date. The Mo-VL30 recombinant described here represents evidence that recombination between MoMuLV and VL30 can be uncoupled from the transduction of ras, and it may precede the transduction. Sequence comparison between clones of Mo-VL30, Harvey sarcoma virus (Ha-MSV), and genomic c-Ha-ras revealed that all three share a 124-bp region of 87.3% homology. This region was detected at nucleotide positions -1845 to -1720 of c-Ha-ras and 20 bp 5' of the recombination breakpoint between VL30 and ras in Ha-MSV. On the basis of the sequence comparison between VL30, Ha-MSV, and c-Ha-ras, we are proposing a model which explains how VL30 may have facilitated the transduction of c-Ha-ras and perhaps the other ras proto-oncogenes. According to this model, the sequence homology between VL30 and c-Ha-ras targets this gene for transduction by promoting the integration of the provirus in this locus through homologous recombination.

Reduced stimulation of helper T cells by Ki-ras transformed cells

A number of viral genes and cellular oncogenes inhibit major histocompatibility complex (MHC) antigen expression at the cell surface. In the case of inhibition of class I MHC antigens by viral genes this results in a reduced recognition by antigen-specific cytotoxic T cells. The activated Ki-ras cellular oncogene carried by the Ki-murine sarcoma virus (Ki-MuSV) in contrast inhibits class II MHC (or Ia) antigen expression on transformed cells. We have studied how transformation with Ki-ras affects recognition by alloreactive helper T cells. We found that the Ki-ras inhibition of class II MHC antigen expression led to greatly reduced stimulation of alloreactive T cells to proliferate and to secrete interferon-gamma (IFN-gamma). These findings support our hypothesis that the ability of an oncogene to reduce class II MHC antigen expression is crucial to its ability to produce tumour cells.

Inducibility of class II major histocompatibility complex antigens by interferon gamma is associated with reduced tumorigenicity in C3H mouse fibroblasts transformed by v-Ki-ras

Paired lines of C3H mouse fibroblasts transformed with murine sarcoma virus (Kirsten strain) were prepared that express high or low levels of class II major histocompatibility complex antigen after treatment with interferon gamma (IFN-gamma). Here, we described a comparison of the tumorigenicity of these lines in euthymic syngeneic and thymus-deficient nu/nu mice and in mice depleted of IFN-gamma. The class II-inducible cells are clearly less tumorigenic than the noninducible cells in syngeneic mice, but of similar tumorigenicity in nu/nu mice and in mice treated with antibodies to deplete IFN-gamma. We propose that in this system, IFN-gamma induction of class II antigens on the tumor cell surface operates to limit tumor growth; ras expression, which inhibits induction of class II antigens, prevents this and so allows tumor growth.

Major histocompatibility complex antigens in v-Ki-ras transformed cells: the different antigens are expressed and induced by interferons independently of one another and of the anti-viral state

A series of cell lines, which differed in their expression of class I (H-2K) or II (I-A) major histocompatibility complex (MHC) antigens, was derived from a line of C3H/He (H-2k) mouse embryo fibroblasts transformed by the v-Ki-ras oncogene. These were prepared by fluorescence-activated cell sorting of cells stained for these antigens either without interferon (IFN) treatment or after induction of antigen expression by either IFN-alpha beta or IFN-gamma, selecting for low or high staining. Cells selected for low (undetectable) constitutive H-2Kk expression were still strongly inducible by either IFN; cells selected for high constitutive expression were induced by IFN to express still higher levels. In all cell lines induction of H-2Kk with one IFN type was paralleled by induction with the other. Expression of H-2Kk appeared largely independent of H-2Dk; in lines which were selected for low H-2Kk expression (constitutive or induced), H-2Dk expression was not much reduced, and lines selected for high H-2Kk expression showed only modest augmentation of H-2Dk. Varying inducibility of I-Ak by IFN-gamma was not closely paralleled by H-2K inducibility by either IFN-alpha beta or -gamma, with again only weak correlation of high and low expression of H2-Kk and I-Ak. On the other hand, expression of I-Ak seemed to be correlated with I-Ek. None of these variable effects could be attributed to differing sensitivity to IFN-alpha beta or -gamma since all the lines showed about the same sensitivity to the anti-viral effects of IFN.

Independent regulation of mouse VL30 retrotransposon expression in response to serum and oncogenic cell transformation

The nucleotide sequence of the long terminal repeats (LTRs) of retrovirus-transmissible mouse VL30 cDNA clones, NVL-1 and NVL-2 were determined and compared with that of the prototype NVL-3. Both shared the typical U3 R U5 structure together with unusual features of redundancy in the tRNAgly primer binding site and adjacent inverted repeat. NVL-1 and NVL-2 LTRs were almost identical and differed from the NVL-3 LTR in the U3 domain harbouring transcriptional regulatory determinants. S1 nuclease analysis of cellular and virus-encapsidated RNA suggested that NVL-1/2 and NVL-3 elements retrotranspose with comparable efficiency but that in contrast to transformation-regulated VL30 expression which affects all types of NVL element, only NVL-1/2 elements were found to be serum responsive. Both modes of VL30 regulation were found to be coupled through protein kinase C-independent pathways. Expression of N-ras transactivated U3 enhancer determinants in all classes of LTR. However the same region of NVL-1/2 LTR did not confer serum responsiveness implying that cis regulatory determinants of VL30 elements mediating growth factor responsiveness are at least in part dissociable from those responsible for cell transformation-regulated expression.

Alignment of U3 region sequences of mammalian type C viruses: identification of highly conserved motifs and implications for enhancer design

We aligned published sequences for the U3 region of 35 type C mammalian retroviruses. The alignment reveals that certain sequence motifs within the U3 region are strikingly conserved. A number of these motifs correspond to previously identified sites. In particular, we found that the enhancer region of most of the viruses examined contains a binding site for leukemia virus factor b, a viral corelike element, the consensus motif for nuclear factor 1, and the glucocorticoid response element. Most viruses containing more than one copy of enhancer sequences include these binding sites in both copies of the repeat. We consider this set of binding sites to constitute a framework for the enhancers of this set of viruses. Other highly conserved motifs in the U3 region include the retrovirus inverted repeat sequence, a negative regulatory element, and the CCAAT and TATA boxes. In addition, we identified two novel motifs in the promoter region that were exceptionally highly conserved but have not been previously described.

Instability of expression of major histocompatibility antigens in fibroblasts expressing activated ras oncogene: constitutive and interferon-gamma induced class I and class II antigens in a series of clonal isolates of murine fibroblasts transformed by v-Ki-ras

We have examined the expression of major histocompatibility complex (MHC) antigens, constitutive or induced with interferon gamma (IFN-gamma), in a line of C3H mouse embryo fibroblasts (C3H 201) transformed with a helper-virus-free preparation of the Kirsten strain of murine sarcoma virus. C3H 201 cells expressed some class I antigen (H-2Kk) in the absence of added interferon, unlike the parental C3H 10T1/2 cells from which they were derived. However, this declined with (in vitro) passage level after transformation. Treatment with IFN-gamma induced very high expression of H-2Kk at all passage levels. There was no constitutive expression of class II antigen (I-Ak); however, this could be induced by IFN-gamma. Inducibility of I-Ak was found also to be related to the number of passages after transformation; at early passage levels after transformation more I-Ak was induced than after the cells had been allowed to grow for several passages, until at high passage levels little or no I-Ak was induced. This was not due to the presence of a subpopulation of untransformed cells since when the cells were cloned shortly after infection all the resulting clones were transformed. In addition, IFN-gamma at any passage level induced clearly less I-Ak than was found in C3H 10T1/2 cells, in which I-Ak inducibility was high and stable. Twenty-one clones were derived from C3H 201 cells at early passage (less than 8) either from soft agar or from liquid culture. These clones showed a wide variation in MHC antigen phenotype. Many expressed H-2Kk in the absence of IFN-gamma, and all were strongly inducible for H-2Kk. None showed I-Ak in the absence of IFN-gamma. All but two expressed I-Ak after IFN-gamma treatment but, with four exceptions, clearly less than the untransformed line. Four clones derived at late passage (40) resembled the late passage line. The expression of the ras oncogene and tumorigenicity was studied in representative clones; there was no obvious correlation with MHC phenotype, nor with the method of cloning. We conclude from these studies that the expression of MHC antigens by fibroblasts expressing the v-Ki-ras oncogene, either with or without exposure to interferon gamma, is unstable, varying with the number of cell generations from transformation and from clone to clone.

Regulation of IFN-gamma-induced host cell MHC antigen expression by Kirsten MSV and MLV. I. Effects on class I antigen expression

We have reported previously that the Kirsten murine sarcoma virus (Ki-MSV) that carries the v-Ki-ras oncogene prevents C3H10T 1/2 fibroblasts from being able to respond to interferon-gamma (IFN-gamma) with the expression of the class II major histocompatibility complex (MHC) antigen, H-2A. In this report we investigate further as to whether MSV or its parent virus Kirsten murine leukaemia virus (Ki-MLV) is able to reduce host class I MHC antigen expression. The results demonstrate that class I expression is diminished in MSV-infected cells over a time-course of 7 days after exposure to IFN-gamma and over a range of IFN-gamma concentrations. The optimal concentration of IFN-gamma for maximal class I expression remained unchanged. Cells infected with Ki-MLV, which failed to abolish the induction by IFN-gamma of class II antigens, also expressed lower levels of class I antigens, similar to those for cells infected with Ki-MSV, after exposure to IFN-gamma. It is likely therefore that the inhibition of class I induction is due to genetic material shared between the viruses, principally in the long terminal repeats (LTR), and hence that the mechanism of action is distinct from that responsible for the abolition of class II induction by Ki-MSV alone. Since class I antigens are required for CD8+ T cells (mainly cytotoxic T cells) to recognize (foreign) antigen this reduction in class I expression might lead to reduced visibility of infected cells to T cells and thus might contribute to the tumorigenicity of Ki-MSV-infected cells.

Interaction of v-Ki-ras oncogene and interferon-gamma in the control of histocompatibility antigen expression in mouse fibroblasts

C3H10T1/2 fibroblasts when transformed with Kirsten murine sarcoma virus lose the ability to be induced to express class II major histocompatibility complex antigens when induced with interferon-gamma (IFN-gamma). Sublines were derived from transformed lines by cell sorting after treatment with IFN-gamma, sorting for low or high expression of H-2Ak. These sublines remained stably noninducible or inducible for class II antigen for several passages after sorting. In all other respects tested, viz, sensitivity to IFN-gamma for the generation of an antiviral state or the induction of class I antigen, content of ras gene products, the sorted sublines were very similar. We conclude that ras oncogene expression in these cells can influence the induction of class II antigen but that because ras expression in the sorted lines is similar the effect of ras expression is indirect and presumably involves interaction with other cellular factors.

Regulation of IFN-gamma-induced host cell MHC antigen expression by Kirsten MSV and MLV. II. Effects on class II antigen expression

We have reported previously that the Kirsten murine sarcoma virus (Ki-MSV), which carries the v-Ki-ras oncogene, prevents the induction of the class II MHC antigen H-2A and reduces the induction of class I MHC antigens by interferon-gamma (IFN-gamma) on C3H10T 1/2 fibroblasts. It is here shown that the abolition by the virus of H-2A expression extends also to class II antigen H-2E and that this is maintained for at least 7 days after IFN treatment. In addition no concentration of IFN-gamma tested, including supra-optimal concentrations for class I antigen expression, induced class II antigens on MSV-infected cells. Thus MSV inhibits the induction by IFN-gamma of class II MHC antigens by a mechanism other than via a change in kinetics of response to, or in the sensitivity of the cells to, IFN. The possibility that transformation by MSV could result in the (selective) outgrowth of cells unresponsive to IFN was refuted by the observation that clones of C3H10T 1/2, when infected with Ki-MSV, expressed no or dramatically reduced levels of H-2A or H-2E. One C3H10T 1/2 clone chosen for high class II expression, when transformed with Ki-MSV, did express low levels of class II antigens at optimal concentrations of IFN-gamma, suggesting that the degree of the reduction of class II expression varies with the cells that are infected. Comparison with mechanisms whereby other viruses inhibit MHC antigen display revealed an interesting possibility: IFN response sequences (IRS) identified in the virus genomes might act in trans to (down) regulate MHC antigen expression. This could be an important mechanism determining the tumourigenicity of, and immune evasion by, Ki-MSV and other viruses.

Accelerated malignant conversion of human HBL-100 cells by the v-Ki-ras oncogene

The human epithelial HBL-100 cell line harbors SV40 genetic information and has an unlimited growth potential. Despite displaying properties characteristic of transformation since its early in vitro passages, it is capable of producing progressively growing tumors in nude mice only after long-term culture. This is a reproducible phenomenon and apparently not the consequence of a selection of preexisting malignant cells. Superinfection of early passage nontumorigenic HBL-100 cells with Kirsten murine sarcoma virus, which contains a Ki-ras oncogene having undergone multiple activating events, induces morphologic alterations and rapidly converts the cells to neoplastic cells, further supporting the hypothesis of multistep carcinogenesis. The HBL-100 cell line might be useful in defining the oncogenes representative of different families, which are able to complement SV40 in this system.

Integration and loss of a single v-Ki-ras gene affects tumorigenic potential of human osteosarcoma cells

The human osteosarcoma cell line Te85 clone F-5 is not tumorigenic in vivo. Its transformation with Kirsten murine sarcoma virus (KiMSV) (KHOS) confers full malignant properties and stable non-tumorigenic revertants of this KHOS cell line have been obtained. Here we show that integration and expression of a single copy of the KiMSV proviral DNA, which is totally lost in the HOS 240S revertant, is responsible for the acquisition of tumorigenicity. Cytogenetic analysis and the absence of a residual LTR copy in the revertant cellular genome suggest that the loss of KiMSV provirus is caused either by chromosomal segregation or by recombination not involving the LTR. In addition analysis of the expression of ras proteins revealed no changes in the pattern of c-ras products and the expression of v-ras only in the KHOS cells. All these data suggest that Te85 and HOS 240S cell lines could represent a human alternative recipient system to rodent cells in studies with oncogenes.

Kirsten murine sarcoma virus abolishes interferon gamma-induced class II but not class I major histocompatibility antigen expression in a murine fibroblast line

The effect of infecting fibroblasts with Kirsten murine sarcoma virus/murine leukemia virus (Ki-MSV/MLV) on constitutive and IFN-gamma-induced H-2 antigen expression was investigated. The fibroblasts used were two established cell lines (C3H10T1/2 and BALB/c3T3) and fresh embryo fibroblasts from C3H mice. Class I antigens were expressed constitutively by BALB/c3T3; infection with MLV, MSV or the two together had little effect on this constitutive expression. Class I antigens (H-2K, H-2D) were strongly induced on all three types of fibroblast by rIFN-gamma, and infection had little effect on this. None of the fibroblasts expressed constitutively detectable levels of class II antigen; however, C3H10T1/2 fibroblasts could be induced for both H-2A and H-2E by IFN-gamma. Infection of C3H10T1/2 with helper-free Ki-MSV, or MSV together with MLV, completely abolished this induction of class II antigens, while infection with MLV alone had little effect, implying that the abolition of class II induction was due to genomic regions of Ki-MSV not shared with Ki-MLV, probably the v-Ki-ras gene.

Analysis of ras genes and linked viral sequences in rat hepatocarcinogenesis

After long-term feeding of a choline-devoid diet to rats, the authors analyzed rasK, rasH, and rasN transcripts and gene structure in livers and liver tumors. They controlled their analysis by studying cell lines derived from chemically induced hepatomas. Transcripts from all three genes were elevated in all tumors, but not in the livers from which they arose. The transcript elevations may represent an effect of active cell proliferation in the tumors. Clone HiHi-3, derived from the Kirsten murine sarcoma virus, detected a large number of hybridization bands, most of which were not part of the rasK-p21 gene. Most tumors had an altered band at 2.6 kb; some had other altered bands. No alterations were seen in liver DNA, and none of the cell lines showed the 2.6 kb band. At low stringency, a rasH probe, which contains a short segment of a similar viral sequence, also detected altered bands in tumors and a single treated liver. These changes in endogenous viral sequences of the rat genome appear to be characteristic of carcinogenesis by a choline-devoid diet.

Expression and transmission of a rodent retrovirus-like (VL30) gene family

A transcriptionally active sub-set of the dispersed mouse VL30 family of proviral genetic elements was shown to be highly transmissible as a murine leukaemia virus pseudotype. Newly acquired VL30 proviruses (present at 1 to 2 copies per cell) were shown to be transcriptionally active. These data substantiate the hypothesis that this process of duplicative transposition may have played a major role in the evolution of the gene family and also demonstrate that VL30 elements would be capable of mediating oncogene activation by a promoter-insertion-type mechanism during leukaemia virus-induced tumourgenesis.