Transformation of MMC-E epithelial cells by acute 3611-MSV: inhibition of collagen synthesis and induction of novel polypeptides

Mutational activation of c-raf-1 and definition of the minimal transforming sequence

A series of wild-type and mutant raf genes was transfected into NIH 3T3 cells and analyzed for transforming activity. Full-length wild-type c-raf did not show transforming activity. Two types of mutations resulted in oncogenic activity similar to that of v-raf: truncation of the amino-terminal half of the protein and fusion of the full-length molecule to gag sequences. A lower level of activation was observed for a mutant with a tetrapeptide insertion mapping to conserved region 2 (CR2), a serine- and threonine-rich domain located 100 residues amino-terminal of the kinase domain. To determine essential structural features of the transforming region of raf, we analyzed point and deletion mutants of v-raf. Substitutions of Lys-56 modulated the transforming activity, whereas mutation of Lys-53, a putative ATP binding residue, abolished it. Deletion analysis established that the minimal transforming sequence coincided precisely with CR3, the conserved Raf kinase domain. Thus, oncogenic activation of the Raf kinase can be achieved by removal of CR1 and CR2 or by steric distortion and requires retention of an active kinase domain. These findings are consistent with a protein structure model for the nonstimulated enzyme in which the active site is buried within the protein.

Mutational activation of c-raf-1 and definition of the minimal transforming sequence

A series of wild-type and mutant raf genes was transfected into NIH 3T3 cells and analyzed for transforming activity. Full-length wild-type c-raf did not show transforming activity. Two types of mutations resulted in oncogenic activity similar to that of v-raf: truncation of the amino-terminal half of the protein and fusion of the full-length molecule to gag sequences. A lower level of activation was observed for a mutant with a tetrapeptide insertion mapping to conserved region 2 (CR2), a serine- and threonine-rich domain located 100 residues amino-terminal of the kinase domain. To determine essential structural features of the transforming region of raf, we analyzed point and deletion mutants of v-raf. Substitutions of Lys-56 modulated the transforming activity, whereas mutation of Lys-53, a putative ATP binding residue, abolished it. Deletion analysis established that the minimal transforming sequence coincided precisely with CR3, the conserved Raf kinase domain. Thus, oncogenic activation of the Raf kinase can be achieved by removal of CR1 and CR2 or by steric distortion and requires retention of an active kinase domain. These findings are consistent with a protein structure model for the nonstimulated enzyme in which the active site is buried within the protein.

Characterization of murine A-raf, a new oncogene related to the v-raf oncogene

A 1.6-kilobase cDNA (A-raf) has been isolated from a murine spleen cDNA library which encodes part of a protein related to the raf oncogene. Its amino acid sequence has 85% homology to raf in a central portion of 100 amino acids. In contrast to raf, A-raf shows a highly restricted tissue distribution of expression, with highest levels observed in epididymis, followed by intestine. When incorporated into a retrovirus, the resulting gag-A-raf fusion gene causes transformation in vitro and induces tumors in newborn mice. Thus, A-raf represents a new proto-oncogene. Transformation by A-raf is independent of ras gene function, as is the case for raf and mos but not other oncogenes.

Characterization of murine A-raf, a new oncogene related to the v-raf oncogene

A 1.6-kilobase cDNA (A-raf) has been isolated from a murine spleen cDNA library which encodes part of a protein related to the raf oncogene. Its amino acid sequence has 85% homology to raf in a central portion of 100 amino acids. In contrast to raf, A-raf shows a highly restricted tissue distribution of expression, with highest levels observed in epididymis, followed by intestine. When incorporated into a retrovirus, the resulting gag-A-raf fusion gene causes transformation in vitro and induces tumors in newborn mice. Thus, A-raf represents a new proto-oncogene. Transformation by A-raf is independent of ras gene function, as is the case for raf and mos but not other oncogenes.

Detection of the myristylated gag-raf transforming protein with raf-specific antipeptide sera

The post-translational modifications of the gag-raf fusion proteins of the 3611 murine sarcoma virus (MSV) have been examined by inhibiting glycosylation with tunicamycin and by in vivo labeling with [3H]myristic acid. The results show that P75gag-raf is myristylated but not glycosylated and that P90gag-raf is glycosylated but not myristylated (and is now termed gP90gag-raf). gP90gag-raf expression appeared to become lost during passage of the transformed cells, and consequently does not appear to be necessary for the maintenance of transformation. raf-specific sera for detecting gag-raf fusion proteins have been obtained from synthetic peptides made from different regions of the predicted v-raf sequence. Immunoprecipitation of P75gag-raf with raf-specific sera directly confirmed the deduced v-raf sequence. The fact that P75gag-raf is both myristylated and precipitated by antiserum to a predicted carboxyl-terminal peptide of the v-raf gene established that the mature protein represents the entire coding region. The gP90gag-raf thus appears to be a glycosylated form of P75gag-raf specified by the gag sequences of the fusion protein, in analogy with Pr65gag and gPr80gag of murine leukemia viruses. Antiserum to the carboxyl-terminal P75gag-raf peptide was the most efficient in immunoprecipitation, and will be useful for detecting the product of the c-raf gene.

MMC-E cells--origin and changes in karyotype accompanying malignant transformation

Karyotype analysis of an established nontumorigenic cell line (MMC-E) indicated that the cells are of rat origin instead of mouse, as interpreted earlier. This cell line, now termed RE (rat epithelial) had a karyotype of 39,X,-5,-15,-?16,+t(3q11q) in the stem line. There was also one subline that had a karyotype of 40,X,-5,-15,-?16,+t(3q11q), +?t(5;?). Chromosome changes and frequency of sister chromatid exchanges (SCEs) were studied from seven malignantly transformed RE cell lines. These included cells transformed by Moloney murine sarcoma virus, the acute transforming retrovirus 3611-MSV, murine leukemia virus, or ethylnitrosourea. Changes in chromosomes #3 and #5 seemed to be associated with malignant transformation of RE cells. In addition to the t(3q11q) and monosomy for chromosome #5 seen in the parent cell line, monosomy for chromosome #3 and the t(5;?) were observed in all malignant cell lines. The latter changes either were absent (monosomy for chromosome #3) or present only occasionally [t(5;?)] in the parental cells. The results of SCEs showed that the malignantly transformed cell lines do not have increased frequency of SCEs as compared with that of the parental cell line.

Transformation of cultured epithelial cells by ethylnitrosourea. Altered expression of type I procollagen chains

Cultured epithelial rodent cells were transformed in vitro using ethylnitrosourea as a carcinogen either alone or in combination with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). The frequency of transformation in the absence of TPA was 5 X 10(-4) at 10 micrograms/ml ethylnitrosourea. Growth of ethylnitrosourea-treated cells in TPA-substituted medium increased the transformation frequency 8-fold. Colonies of transformed cells were isolated from soft agar and analyzed for the production of pericellular matrix glycoproteins. The ethylnitrosourea-transformed cells retained pericellular matrix structures, typical of the nontransformed control cells. Parent cells produced into their culture media fibronectin and procollagen types I and III as their major pericellular glycoproteins. The ethylnitrosourea-transformed cells synthesized and secreted altered procollagen polypeptides. The procollagen of ethylnitrosourea-transformed cells apparently consisted mainly of homotrimeric pro alpha 1 molecules, with smaller amounts of basement membrane procollagen-like chains. Fibronectin synthesis or secretion was not affected by ethylnitrosourea-induced transformation, but the production of fibronectin was enhanced in the transformed cultures treated with TPA. Also, the deposition of procollagen and fibronectin into the pericellular matrix was not affected by ethylnitrosourea-transformation. Very similar changes had previously been observed in murine sarcoma virus-transformed cells. The change of procollagen type I thus appears to be a correlate of malignant transformation of cultured epithelial cells. The results indicate that ethylnitrosourea can induce malignant transformation of epithelial cells in culture and modify production and deposition of pericullular glycoproteins.

Homologous cell-derived oncogenes in avian carcinoma virus MH2 and murine sarcoma virus 3611

Retroviral oncogenes (v-onc) are derived from cellular genes (c-onc) which are highly conserved among different species. Retrovirus-transduced oncogenes are most commonly associated with the induction of haematopoietic tumours and sarcomas. The avian retrovirus Mill Hill no. 2 (MH2) was isolated from a spontaneous ovarian tumour of a chicken and is distinguished by the predominant induction of liver and kidney carcinomas in fowl. MH2 also induces transformation of fibroblasts, macrophages and epithelial cells in culture. The genome of MH2 contains two unrelated and independently expressed cell-derived oncogenes, v-mil and v-myc. Three other viral isolates among avian acute transforming retroviruses contain the v-myc oncogene, but only MH2 contains both v-myc and v-mil. Hence, some of the pathogenic specificities of MH2 may be due to the simultaneous expression of two oncogenes. The murine sarcoma virus 3611 (3611-MSV) isolated from a mouse carrying lung carcinoma and peritoneal tumours, induces fibrosarcomas in newborn mice and the transformation of fibroblasts and epithelial cells in culture. The oncogenic properties of 3611-MSV are due to the presence in its genome of a cell-derived oncogene termed v-raf. We report here that the two independently transduced oncogenes v-mil and v-raf are closely related and that they were apparently derived from homologous cellular genes of avian and mammalian species.

Spreading of non-transformed and transformed cells

Mechanisms of cellular reactions responsible for the spreading of non-transformed cultured tissue cells on the surface of various substrata and relationships of these reactions to the control of cell proliferation are reviewed; the special role of the membrane-cytoskeleton interactions leading to extension and attachment of pseudopods is stressed. Transition of cells from non-transformed to transformed phenotype is characterized by decreased spreading and by decreased dependence of proliferation on spreading. Manifestations of both of these spreading-associated changes are reviewed and their possible mechanisms are discussed. It is suggested that cell transition to transformed phenotype involves shift of an equilibrium between the reactions induced by the two groups of membrane-bound ligands: those attached and those not attached to the substratum.