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

Glucagon and p21 ras enhance the phosphorylation of the same 38-kilodalton membrane protein from rat liver cells

We had reported earlier the enhanced phosphorylation of a 38-kilodalton protein (p38) in rat liver plasma membrane by ras proteins. Now we show that glucagon increased the phosphorylation of the same protein. The nature and site(s) of phosphorylation were the same as those for the ras proteins. Both ATP and GTP could donate phosphate for the phosphorylation of p38. The stimulation of p38 phosphorylation by glucagon was guanine nucleotide dependent. This observation, together with our data on the stimulation of p38 phosphorylation by AIF4-, suggest the involvement of G proteins in the reaction. We also showed that glucagon stimulates the phosphorylation of p38 in vivo.

Genetic analysis of the Kirsten-ras-revertant 1 gene: potentiation of its tumor suppressor activity by specific point mutations

Kirsten-ras-revertant 1 (Krev-1) cDNA encodes a ras-related protein and exhibits an activity of inducing flat revertants at certain frequencies (2-5% of total transfectants) when introduced into a v-K-ras-transformed mouse NIH 3T3 cell line, DT. Toward understanding the mechanism of action of Krev-1 protein, we constructed a series of point mutants of Krev-1 cDNA and tested their biological activities in DT cells and HT1080 human fibrosarcoma cells harboring the activated N-ras gene. Substitutions of the amino acid residues in the putative guanine nucleotide-binding regions (Asp17 and Asn116), in the putative effector-binding domain (residue 38), at the putative acylation site (Cys181), and at the unique Thr61 all decreased the transformation suppressor activity. On the other hand, substitutions such as Gly12 to Val12 and Gln63 to Glu63 were found to significantly increase the transformation suppressor/tumor suppressor activity of Krev-1. These findings are consistent with the idea that Krev-1 protein is regulated like many other G proteins by the guanine triphosphate/guanine diphosphate-exchange mechanism probably in response to certain negative growth-regulatory signals.

Infrequent point mutations of ras oncogenes in gastric cancers

Adenocarcinomas of the proximal and distal stomach have significant clinical and biological differences. A study was undertaken to determine if a difference in the incidence of mutated ras oncogenes exists between proximal (gastroesophageal junction or cardia) and distal (body or antrum) gastric tumors, and to assess the overall incidence in gastric cancers from non-Asian patients. Deoxyribonucleic acid from 28 primary gastric adenocarcinomas were analyzed for point mutations in codons 12, 13, and 61 of the Ha-ras, Ki-ras, and N-ras protooncogenes using polymerase-catalyzed chain reaction methodology. Twelve tumors were located at the gastroesophageal junction or cardia, and 16 in the body or antrum. Mutated ras genes were detected in 2 of 28 tumors for an overall incidence of 7%. The mutations occurred in codon 61 of the N-ras gene in a proximal gastric cancer and in codon 12 of the Ki-ras gene in a distal gastric cancer. These data indicate that mutations in ras genes are an uncommon event in primary gastric cancers and that there is no meaningful difference in the incidence of ras mutations in proximal and distal stomach cancers.

Ras gene product expression in blood and marrow smears of patients with acute leukemia: importance of fixation

Activation of ras protooncogenes by any of several possible mutations in codons 12, 13 or 61 has been demonstrated in a variety of human malignancies, including acute non-lymphoblastic leukemia (ANLL). In situ staining for the ras gene product, p21, has been demonstrated in carcinomas of several sites. High levels of p21 expression have been associated with histologic anaplasia in prostate cancer and regional lymph node metastasis in breast cancer. We examined 16 marrow aspirates and blood smears from patients with acute leukemia, predominantly ANLL, and eight controls. Marrow aspirates or blood were smeared on glass slides and fixed immediately in 10% buffered formalin. p21 was examined with avidin-biotin linked immunoperoxidase visualization. Particular attention must be paid to antibody selection and fixation protocol to demonstrate p21, owing to its rapid degradation ex vivo. Three of 16 patients exhibited occasional high p21 expression primarily in leukemic blasts, but in no case were more than 10% of blast cells positive. Normal reticuloendothelial and myeloid cells occasionally exhibited mild to moderately heavy staining, but megakaryocytes, erythroid precursors, lymphocytes and plasma cells were consistently negative. Most patients, 5 normal volunteers and 3 patients with non-malignant disease, exhibited no reactivity, or only a faint blush. These data suggest that while point mutation and concomitant activation of c-N-ras occurs regularly in ANLL, high levels of ras p21 expression are rarely found with this technique.

An immunohistochemical analysis of ras oncogene expression in epithelial neoplasms of the colon

Colonic epithelial tumors (101) including villoglandular adenomas, carcinomas in situ, adenocarcinomas, and neuroendocrine (NE) carcinomas were studied immunohistochemically with monoclonal antibodies (MoAb) RAP-5 and RAS-10 recognizing altered and unaltered ras oncogene products. In addition, 20 samples from multiple polyposis including adenomas with and without dysplasia, carcinomas in situ, and invasive carcinomas were studied. Using immunostaining techniques, normal mucosa was weakly stained, whereas the mucosa in the vicinity of tumors or inflammation showed enhanced staining. More tumors stained intensely with MoAb RAP-5 than with MoAb RAS-10. With MoAb RAP-5, most benign and malignant tumors showed enhanced staining. No significant differences in staining were noted in relation to superficial versus deeply invasive carcinomas or clinical staging. Immunostaining was also noted in some metastases. No significant differences in enhanced staining were found in carcinomas. Interestingly, the most extensive and enhanced immunostaining was noted in the villoglandular adenomas, dysplastic adenomas, and carcinomas in situ. The authors conclude that (1) ras protein expression is detectable in most benign, borderline, and malignant epithelial tumors of the colon as determined with MoAb RAP-5 and RAS-10, whereas enhanced expression is more often detected with RAP-5; (2) enhanced ras product expression in colon carcinomas does not seem to correlate with advanced tumor stages or with exocrine, NE, or phenotypically mixed tumors; and (3) the finding of the most intensely enhanced ras products expression in villoglandular polyps and carcinomas in situ suggests a possibly significant role for the oncogene in the early phases of transformation.

Infection of rat liver epithelial cells with v-Ha-ras: correlation between oncogene expression, gap junctional communication, and tumorigenicity

The role of v-Ha-ras oncogene in tumorigenesis in an in vitro/in vivo model system was studied by investigating the expression of the Ha-ras gene, gap junctional intercellular communication, and tumorigenicity as endpoints. Infection of a Fischer 344 rat liver epithelial cell line (WB 344) with a retrovirus containing the v-Ha-ras oncogene resulted in altered cell morphology and decreased contact sensitivity. Gap junctional intercellular communication in v-Ha-ras infected WB cells (WBHa-ras), assessed by fluorescence redistribution after photobleaching (FRAP), microinjection/dye transfer, and scrape-loading/dye transfer techniques, was markedly decreased compared with the level in control WB cells. Injection of 10(7) WBHa-ras cells into the portal vein of male F344 rats caused multiple focal hepatic lesions within 1 and 2 wk, merging to large invading tumors after 3 and 4 wk. Examination of the methylation pattern of the Ha-ras gene in WBHa-ras and control WB cells showed that the infected Ha-ras gene was relatively hypomethylated in comparison to the normal cellular Ha-ras gene, indicating a greater potential for expression. There was an increased level of Ha-ras mRNA in hepatomas as compared with both adjacent nontumor liver tissue and liver tissue obtained from normal animals. Three cell lines derived from three different primary hepatic tumors induced by an injection of WBHa-ras cells in a F344 rat displayed similar growth characteristics, levels of gap junctional communication, and methylation patterns as the original WBHa-ras cells. The results of these studies have established a strong positive correlation between expression of the Ha-ras oncogene, reduced gap junctional intercellular communication, decreased contact sensitivity, and tumorigenicity of the v-Ha-ras-infected rat liver epithelial cells.

c-ras-Ha gene expression is regulated by insulin or insulinlike growth factor and by epidermal growth factor in murine fibroblasts

Although much is known about the structure of ras-encoded proteins, little is known about how expression is regulated. In serum-stimulated murine fibroblasts, c-ras-Ha mRNA levels fluctuated with the growth state but not with the position in the cell cycle. Two types of growth factors regulated c-ras-Ha expression: insulin (IN) or insulinlike growth factor I, each apparently acting through its cognate receptor, and epidermal growth factor (EGF). In quiescent cells, IN or insulinlike growth factor I induced c-ras-Ha mRNA three- to fivefold within 4 h, but thereafter the mRNA declined. By contrast, EGF had little effect in 4 h but induced the mRNA after 4 to 6 h. When quiescent cells were given serum or IN and EGF simultaneously, c-ras-Ha mRNA rose steadily, beginning 1 to 2 h after stimulation, and reached a stable five- to sevenfold elevation in 16 h. Thus, c-ras-Ha gene expression was sequentially regulated by two growth factors, one of which (IN) does not induce expression of other growth-regulated protooncogenes. A transformed derivative cell line that does not require IN for G1 progression has lost early IN-dependent but not late serum-dependent regulation. The results support the possibility that c-ras-Ha and IN action are functionally linked.

c-Ha-ras gene bidirectional promoter expressed in vitro: location and regulation

Increased transcriptional activity of the c-Ha-ras gene product is correlated with induction of several important human tumor types. For this reason, we have investigated the nature of the c-Ha-ras promoter and the factors that regulate its expression. Using S1 and primer extension analysis of c-Ha-ras RNA from EJ cells, we have identified 18 initiation sites within an upstream exon (exon -1) whose 3' end (the donor splice site [D]) is located 1,105 base pairs (bp) upstream of the ATG codon. The furthest-upstream initiation site is located -191 bp relative to D, and the furthest downstream is located -16 bp relative to D. Transient expression assays, in which a series of mutants spanning this region were ligated to a promoterless chloramphenicol acetyltransferase vector, functionally confirmed the position and extent of this promoter. Mutational analysis further located a 47-bp element located between -243 and -196 relative to D that up-regulated transcriptional activity of the promoter region by 20- to 40-fold. This region contained both a GC box known to bind SP1 and a CCAAT box. Insertion of a simian virus 40 enhancer 5' to the promoter up-regulated transcription from each initiation site by approximately 10- to 20-fold. We have also localized, both by chloramphenicol acetyltransferase assay and by S1 analysis, a strong promoter operating in the direction opposite that of the gene and originating immediately 5' to the 47-bp regulatory region. The reverse promoter was found to have nine initiation sites between -248 and -278 relative to D.

Origin of rare Ha-ras alleles: relationship of VTR length to a 5' polymorphic Xho I site

Amongst the four common Ha-ras alleles in both controls and cancer patients, we detected the presence of a polymorphic Xho I site associated specifically with the 6.6 and 7.7 kb Bam HI fragments but absent from the 7.1 and 8.2 kb alleles, as recently reported by others. We have extended this study and report here, the consistent appearance of this Xho I site in unusual alleles close in size to the two common alleles of 6.6 and 7.7 kb, in control lymphoblastoid DNA samples in a variety of tumor DNAs. Unusual alleles grouped around the 7.1 and 8.2 kb common alleles on the other hand, did not possess the Xho I site. The consistent presence of the Xho I site polymorphism, in the unusual Ha-ras alleles surrounding the 6.6 and 7.7 kb common alleles and its absence in alleles around the 7.1 and 8.2 kb common alleles, suggests that the unusual ones are derived from the corresponding common alleles to which they are closest in size.

c-Ha-ras gene bidirectional promoter expressed in vitro: location and regulation

Increased transcriptional activity of the c-Ha-ras gene product is correlated with induction of several important human tumor types. For this reason, we have investigated the nature of the c-Ha-ras promoter and the factors that regulate its expression. Using S1 and primer extension analysis of c-Ha-ras RNA from EJ cells, we have identified 18 initiation sites within an upstream exon (exon -1) whose 3' end (the donor splice site [D]) is located 1,105 base pairs (bp) upstream of the ATG codon. The furthest-upstream initiation site is located -191 bp relative to D, and the furthest downstream is located -16 bp relative to D. Transient expression assays, in which a series of mutants spanning this region were ligated to a promoterless chloramphenicol acetyltransferase vector, functionally confirmed the position and extent of this promoter. Mutational analysis further located a 47-bp element located between -243 and -196 relative to D that up-regulated transcriptional activity of the promoter region by 20- to 40-fold. This region contained both a GC box known to bind SP1 and a CCAAT box. Insertion of a simian virus 40 enhancer 5' to the promoter up-regulated transcription from each initiation site by approximately 10- to 20-fold. We have also localized, both by chloramphenicol acetyltransferase assay and by S1 analysis, a strong promoter operating in the direction opposite that of the gene and originating immediately 5' to the 47-bp regulatory region. The reverse promoter was found to have nine initiation sites between -248 and -278 relative to D.

Oncogenicity of human N-ras oncogene and proto-oncogene introduced into retroviral vectors

The N-ras gene is the only member of the ras family which has never been naturally transduced into a retrovirus. In order to study the in vitro and in vivo oncogenicity of N-ras and to compare its pathogenicity to that of H-ras, we have inserted an activated or a normal form of human N-ras cDNA into a slightly modified Harvey murine sarcoma virus-derived vector in which the H-ras p21 coding region had been deleted. The resulting constructions were transfected into NIH 3T3 cells. The activated N-ras-containing construct (HSN) induced 10(4) foci per microgram of DNA and was found to be as transforming as H-ras was. After infection of the transfected cells by either the ecotropic Moloney murine leukemia virus or the amphotropic 4070A helper viruses, rescued transforming viruses were injected into newborn mice. Both pseudotypes of HSN virus containing activated N-ras induced the typical Harvey disease with similar latency. However, we found that the virus which contained normal N-ras p21 (HSn) was also pathogenic and induced splenomegaly, lymphadenopathies, and sarcoma in mice after a latency of 3 to 7 weeks. In addition, Moloney murine leukemia virus pseudotypes of N-ras caused neurological disorders in 30% of the infected animals. These results differed markedly from those of previous experiments in which we had inserted the activated form of N-ras in the pSV(X) vector: the resulting SVN-ras virus was transforming on NIH 3T3 cells but was poorly oncogenic in vivo (M. Souyri, C. F. Koehne, P. V. O'Donnel, T. H. Aldrich, M. E. Furth, and E. Fleissner, Virology 158:69-78). However, similarly poor oncogenicity was also observed when the v-H-ras coding sequence was inserted in pSV(X) vector, which indicated that the vector sequences play a crucial role in the pathogenicity of a given oncogene. Altogether, these data demonstrated unequivocally that N-ras is potentially as oncogenic as H-ras and that such oncogenic effect could depend on the vector environment.

Polymorphism of the c-Ha-ras-1 proto-oncogene in sporadic and familial breast cancer

Two studies on breast cancer patients are described. Our aim was to examine whether the combined frequency of rare c-Ha-ras-1 alleles in cancer patients was raised. Firstly, the c-Ha-ras-1 locus in 56 breast cancer patients and 48 controls was examined for restriction fragment length polymorphism (RFLP) by Southern blot analysis of leukocyte DNA. Four predominant allelic fragments were found in both groups together with a variety of rare alleles. The 2 groups did not differ significantly in overall distribution of c-Ha-ras-1 alleles. Rare alleles combined were about as frequent in cases (7.1%) as in controls (6.3%). Secondly, 53 members of 3 families having a high incidence of breast cancer were c-Ha-ras-1 genotyped. None of 10 affected members was found to carry a rare c-Ha-ras-1 allele. The only c-Ha-ras-1 allele common to 11 affected members was a 6.8-kb allele which is found in 72% of the controls. Furthermore, this allele was found with equal frequency in affected and non-affected family members.

Expression of the H-ras proto-oncogene is controlled by alternative splicing

We previously demonstrated that a point mutation in the last intron of the human H-ras oncogene causes a significant increase in its expression and transforming efficiency. Here we establish the basis of this phenomenon. Using gene reconstruction experiments, we have identified a negative-acting element in the intron that is completely inactivated by the mutation. The effects of other nucleotide alterations introduced into this region suggested that the negative element might constitute an alternative exon. Transcripts containing this putative exon were identified and S1 nuclease analysis confirmed that the mutation prevents their synthesis. The abundance of these transcripts is low, apparently due to message instability and/or defective processing. The predicted product of the alternative transcript is suggested to lack transforming potential. Our findings demonstrate that alternative splicing normally operates to suppress p21H-ras expression and that this negative control is abolished by a variety of mutations that interfere with this process.

c-ras-Ha gene expression is regulated by insulin or insulinlike growth factor and by epidermal growth factor in murine fibroblasts

Although much is known about the structure of ras-encoded proteins, little is known about how expression is regulated. In serum-stimulated murine fibroblasts, c-ras-Ha mRNA levels fluctuated with the growth state but not with the position in the cell cycle. Two types of growth factors regulated c-ras-Ha expression: insulin (IN) or insulinlike growth factor I, each apparently acting through its cognate receptor, and epidermal growth factor (EGF). In quiescent cells, IN or insulinlike growth factor I induced c-ras-Ha mRNA three- to fivefold within 4 h, but thereafter the mRNA declined. By contrast, EGF had little effect in 4 h but induced the mRNA after 4 to 6 h. When quiescent cells were given serum or IN and EGF simultaneously, c-ras-Ha mRNA rose steadily, beginning 1 to 2 h after stimulation, and reached a stable five- to sevenfold elevation in 16 h. Thus, c-ras-Ha gene expression was sequentially regulated by two growth factors, one of which (IN) does not induce expression of other growth-regulated protooncogenes. A transformed derivative cell line that does not require IN for G1 progression has lost early IN-dependent but not late serum-dependent regulation. The results support the possibility that c-ras-Ha and IN action are functionally linked.

Oncogenes in hematologic neoplasia

Proto-oncogenes are normal genes involved in cellular proliferation and differentiation. Structural alterations in these genes can convert them to oncogenes involved in the initiation or progression of malignancy. About 50 proto-oncogenes have been described and four different activation mechanisms are known. Proto-oncogene alterations specific for human hematologic malignancies are well characterized.

Activation of the c-H-ras proto-oncogene by retrovirus insertion and chromosomal rearrangement in a Moloney leukemia virus-induced T-cell leukemia

A rearrangement of the c-H-ras locus was detected in a T-cell line (DA-2) established from a Moloney leukemia virus-induced tumor. This rearrangement was associated with the high-level expression of H-ras RNA and the H-ras gene product, p21. DNA from DA-2 cells transformed fibroblasts in DNA transfection experiments, and the transformed fibroblasts contained the rearranged H-ras locus. The rearrangement involved one allele and was present in tissue from the primary tumor from which the cell line was isolated. Cloning and sequencing of the rearranged allele and comparison with the normal allele demonstrated that the rearrangement was complex and probably resulted from the integration of a retrovirus in the H-ras locus between a 5' noncoding exon and the first coding exon and a subsequent homologous recombination between this provirus and another newly acquired provirus also located on chromosome 7. These events resulted in the translocation of the coding exons of the H-ras locus away from the 5' noncoding exon region to a new genomic site on chromosome 7. Sequencing of the coding regions of the gene failed to detect mutations in the 12th, 13th, 59th, or 61st codons. The possible reasons for the complexity of the rearrangement and the significance of the activation of the H-ras locus to T-cell transformation are discussed.

P21 v-ras inhibits induction of c-myc and c-fos expression by platelet-derived growth factor

The viral oncogene v-ras inhibited the platelet-derived growth factor (PDGF)-induced upregulation of c-myc and c-fos proto-oncogene expression in fibroblast monolayers. These v-ras-containing cells proliferated in the absence of c-myc induction and no longer required PDGF to support growth. Fibroblasts expressing v-ras continued to express the same number of functional PDGF receptors on their surface as uninfected cells, yet the usual induction of transcription of the genes c-myc, c-fos, and JE in response to PDGF stimulation did not occur in the presence of newly introduced v-ras or chronic v-ras gene expression, and synthesis of c-myc protein did not occur. This inhibitory effect on growth factor-mediated induction of cellular proto-oncogenes was specific for PDGF in that induction of the c-myc and c-fos genes by certain other factors was not impaired.

Expression of ras oncogene p21 protein in relation to regional spread of human breast carcinomas

The oncogenes most frequently detected in human tumors belong to the ras gene family (Ha-ras, Ki-ras, and N-ras). These genes encode a group of closely related 21,000 dalton proteins termed p21. An immunohistochemical study of ras p21 expression was carried out on paraffin sections of 54 human breast carcinomas using monoclonal antibodies to p21. The control group consisted of ten cases of benign fibrocystic disease. The p21 expression was significantly higher in cancer cells than in epithelial cells of control specimens. No correlations, however, were observed between oncogene product expression and tumor size, histologic type, or grade. As a group, tumors with axillary lymph node metastases expressed higher levels of ras p21 than nonmetastasizing tumors. However, because of the significant overlap in individual p21 values, it is unlikely that the immunohistochemical assay for p21 could be used to predict the behavior of mammary carcinomas.

raf involvement in the simultaneous genetic transfer of the radioresistant and transforming phenotypes

We examined a human Alu+ mouse tertiary transformant derived from a noncancerous skin fibroblast cell line which exhibits the unique characteristic of being resistant to the killing effects of ionizing radiation. This transformed cell line was found to contain activated human c-raf-1, and demonstrated an increased level of radioresistance indicating the simultaneous transfer of both the transforming and radiation-resistant phenotypes. We have also found a relationship between the presence of activated oncogenes, specifically those with serine/threonine kinase activity and the radioresistant phenotype.

Somatic mutation and transcriptional deregulation of myc in endemic Burkitt's lymphoma disease: heptamer-nonamer recognition mistakes?

We examined the structure and expression of the myc protooncogene in DNA extracted from a primary (uncultured) endemic Burkitt's lymphoma sample designated eBL3. Dot and Northern (RNA) blot analyses demonstrated extreme levels of myc RNA in the eBL3 sample. Nearly complete sequence data of the altered myc locus isolated from eBL3 DNA demonstrated extensive mutations (duplications, insertions, and deletions) in critical myc regulatory regions. Taken together, the data support the idea that myc transcriptional deregulation in Burkitt's lymphoma disease may be a consequence of the position and number of mutations produced within and around the myc locus. Furthermore, the myc exon-1-intron-1 hypermutable PvuII site is part of a potential heptamer-nonamer recognition sequence, suggesting a mechanism for mutation in endemic Burkitt's lymphoma disease.

Altered hepatic microsomal function and elevated protooncogene expression as residual effects in rats exposed to delta-9-tetrahydrocannabinol

The microsomal activation of the potent hepatocarcinogen aflatoxin B1 (AFB1) and the expression of selected protooncogenes were investigated in the livers of rats exposed to delta 9-tetrahydrocannabinol (THC). At equimolar levels of cytochrome P-450, the microsome-mediated binding of AFB1 to DNA was significantly lower (56% of the controls) in preparations from drug exposed rats. Hepatic expression of the c-k-ras protooncogene was 3-fold higher in THC exposed animals. These results suggest the possible occurrence of long lasting residual effects in the rats exposed to THC.

Comparison of the computed structures for the phosphate-binding loop of the p21 protein containing the oncogenic site Gly 12 with the X-ray crystallographic structures for this region in the p21 protein and EFtu. A model for the structure of the p21 protein in its oncogenic form

The GTP-binding p21 protein encoded by the ras-oncogene can be activated to cause malignant transformation of cells by substitution of a single amino acid at critical positions along the polypeptide chain. Substitution of any non-cyclic L-amino acid for Gly 12 in the normal protein results in a transforming protein. This substitution occurs in a hydrophobic sequence (residues 6-15) which is known to be involved in binding the phosphate moities of GTP (and GDP). We find, using conformational energy calculations, that the 6-15 segment of the normal protein (with Gly 12) adopts structures that contain a bend at residues 11 and 12 with the Gly in the D* conformation, not allowed energetically for L-amino acids. Substitution of non-cyclic L-amino acids for Gly 12 results in shifting this bend to residues 12 and 13. We show that many computed structures for the Gly 12-containing phosphate binding loop, segment 9-15, are superimposable on the corresponding segment of the recently determined X-ray crystallographic structure for residues 1-171 of the p21 protein. All such structures contain bends at residues 11 and 12 and most of these contain Gly 12 in the C* or D* conformational state. Other computed conformations for the 9-15 segment were superimposable on the structure of the corresponding 18-23 segment of EFtu, the bacterial chain elongation factor having structural similarities to the p21 protein in the phosphate-binding regions. This segment contains a Val residue where a Gly occurs in the p21 protein. As previously predicted, all of these superimposable conformations contain a bend at positions 12 and 13, not 11 and 12. If these structures that are superimposable on EFtu are introduced into the p21 protein structure, bad contacts occur between the sidechain of the residue (here Val) at position 12 and another phosphate binding loop region around position 61. These bad contacts between the two segments can be removed by changing the conformation of the 61 region in the p21 protein to the corresponding position of the homologous region in EFtu. In this new conformation, a large site becomes available for the binding of phosphate residues. In addition, such phenomena as autophosphorylation of the p21 protein by GTP can be explained with this new model structure for the activated protein which cannot be explained by the structure for the non-activated protein.

Co-regulation of metastatic and transforming activity of normal and mutant ras genes

We have analyzed the metastatic properties of NIH-3T3 cells transformed by H-ras activated through over-expression and/or mutation. Our results reveal that elevated expression of H-ras proto-oncogene can induce the complete metastatic phenotype. Cells transformed by the proto-oncogene have a lower metastatic potential than those transformed by a mutated ras gene. ras oncogenes activated through alterations in codon 12 which encode p21 molecules with impaired GTPase activity, or in codon 59 which produce p21 molecules that release bound guanine nucleotide faster, or in codon 61 which produce p21 having impaired GTPase activity and altered nucleotide release properties, are all able to induce the metastatic phenotype. Leucine-61-activated oncogenes with an additional mutation in codons 116, 117 or 119, resulting in a reduced affinity for guanine nucleotides, are also capable of inducing metastatic behavior. These data indicate that ras genes which are capable of transforming are also capable of inducing the full metastatic phenotype in NIH-3T3 cells. This suggests that both phenotypes are induced through an increase in p21-GTP concentration in ras-transformed cells. This established model for ras-mediated transformation can also explain the qualitative and quantitative regulation of metastatic behavior by ras.

Transformation of NIH3T3 cells with synthetic c-Ha-ras genes

Synthetic human c-Ha-ras genes in which amino acid codons were altered to those which are frequently used in highly expressed Escherichia coli genes were ligated to the 3'-end of Rous sarcoma virus long terminal repeat. When NIH3T3 cells were transfected with the plasmids having those genes with valine at codon 12, leucine at codon 61 or arginine at codon 61, transformants were efficiently produced. These results indicated that the synthetic c-Ha-ras genes are expressed in a mammalian system even though their codon usage is altered to correspond with that of E. coli. This expression vector system should be useful for studies on the structure-function relationships of c-Ha-ras, since the synthetic gene can be easily modified to have multiple base alterations, and can also be used simultaneously for the production of large amounts of p21 in E. coli for biochemical and biophysical studies.

Conformational effects of amino acid substitutions at positions 10, 12, and 13 in the P21 protein

Substitutions of amino acids for Gly 12 or Gly 13 in the ras oncogene-encoded P21 proteins have been demonstrated to produce unique structural changes in these proteins that correlate with their ability to produce cell transformation. For example, the P21 proteins with Arg 12 or Val 13 are both known to be actively transforming. Recent site-specific mutagenesis experiments on the transforming Arg 12 protein have found that the substitution of Val for Gly 10 has no effect on transforming activity whereas the substitution of Val for Gly 13 led to a loss of transforming activity. In this study, we examine the structural effects of these substitutions on the amino terminal hydrophobic decapeptide (Leu 6-Gly 15) of P21 using conformational energy analysis. The results show that the transforming proteins with Gly 10 and Arg 12 or Val 10 and Arg 12 can both adopt the putative malignancy-causing conformation, whereas, for the nontransforming protein with Arg 12 and Val 13, this conformation is energetically disallowed. These results further support the theory that due to structural changes the transforming P21 proteins are unable to bind to some regulatory cellular element which may be the recently identified binding protein responsible for the induction of increased GTPase activity in normal P21 compared with transforming mutants.

Spontaneous progression of hyperplastic outgrowths of the D1 lineage to mammary tumors: expression of mouse mammary tumor virus and cellular proto-oncogenes

Mammary cancer in mice is characterized by progression through defined stages of preneoplasia, with the most common preneoplastic stage being the hyperplastic alveolar nodule (HAN). We determined the relative levels of RNA expression of various cellular proto-oncogenes and endogenous mouse mammary tumor virus genes in outgrowths and tumors of three sublines of the transplantable D1 HAN preneoplastic outgrowth line. The three sublines differed in relative tumor-producing capabilities. Subline D1B produced a high incidence of tumors with short latency periods, whereas sublines D1C and D1D produced low incidences of tumors with long latency periods. No consistent alteration in proto-oncogene expression correlated with relative tumorigenicity, although tumors frequently contained higher levels of one or more proto-oncogene transcripts as compared with preneoplastic tissue. Slightly elevated (2- to 6-fold) levels of different oncogene transcripts were detected in 13 of 17 tumors as compared with outgrowth tissue, including abl (2 tumors), fps (5 tumors), Ha-ras (6 tumors), and Ki-ras (8 tumors). One tumor contained 45 times more Ki-ras-specific RNA than outgrowth tissue because of a comparable amplification of Ki-ras DNA sequences. Elevated levels of Ha-ras occurred more frequently in tumors of a high-incidence subline than in a less-aggressive subline (5/10 vs 1/7), but this difference was not statistically significant. However, consistent changes in MMTV expression accompanied progression from preneoplastic tissues to mammary tumors. All 17 tumors displayed reduced levels of the MMTV-specific long terminal repeat (LTR) transcript (1.6 kb) as compared with HAN tissue; tumors with moderate levels of LTR transcript expressed the 3.8-kb envelope message as well, one not detected in HANs. Expression of the LTR transcript is apparently influenced by factors in addition to the methylation status of endogenous mouse mammary tumor virus genes, which was similar in outgrowths and tumors. As the survey of representative proto-oncogenes failed to identify a uniform change between HAN and tumors, it is likely that other genes are involved in tumor progression in the mammary gland.

Somatic mutation and transcriptional deregulation of myc in endemic Burkitt's lymphoma disease: heptamer-nonamer recognition mistakes?

We examined the structure and expression of the myc protooncogene in DNA extracted from a primary (uncultured) endemic Burkitt's lymphoma sample designated eBL3. Dot and Northern (RNA) blot analyses demonstrated extreme levels of myc RNA in the eBL3 sample. Nearly complete sequence data of the altered myc locus isolated from eBL3 DNA demonstrated extensive mutations (duplications, insertions, and deletions) in critical myc regulatory regions. Taken together, the data support the idea that myc transcriptional deregulation in Burkitt's lymphoma disease may be a consequence of the position and number of mutations produced within and around the myc locus. Furthermore, the myc exon-1-intron-1 hypermutable PvuII site is part of a potential heptamer-nonamer recognition sequence, suggesting a mechanism for mutation in endemic Burkitt's lymphoma disease.

Activation of the c-Ha-ras-1 proto-oncogene by methylation in vitro with alpha-acetoxy-N-nitrosodimethylamine

alpha-Acetoxy-N-nitrosodimethylamine, an activated derivative of the carcinogen N-nitrosodimethylamine, methylated in vitro a plasmid containing the human c-Ha-ras-1 proto-oncogene, resulting in the generation of a transforming oncogene, assayed by transfection into NIH 3T3 cells. The resulting transformed cells were tumorigenic and metastatic in immune-deprived mice. Further transfection using tumor DNA led to the formation of three secondary NIH 3T3 transformants. DNA from these secondary transformants contained human ras gene sequences. Two of the three secondary transformants contained G----A mutations at guanine 35 in codon 12, and the third secondary transformant retained the wild-type sequence at codons 12, and 61. For the latter, the activating mutation was not determined. These results demonstrate that a simple methylating agent can activate a normal human ras proto-oncogene to a transforming oncogene.

Characterization of a factor that stimulates hydrolysis of GTP bound to ras gene product p21 (GTPase-activating protein) and correlation of its activity to cell density

The postmicrosomal fraction of the extract from NIH 3T3 and BALB/c 3T3 cells stimulated the hydrolysis of GTP bound to H-ras gene product p21 by severalfold. The stimulation was observed with normal p21 but not with p21 with valine as the 12th residue. This specificity is similar to that of GTPase-activating protein (GAP) for N-ras p21 described by M. Trahey and F. McCormick (Science 238:542-545, 1987). Consistent with this specificity, analysis of p21-bound nucleotides in living cells revealed that almost all normal p21 bound GDP, whereas oncogenic mutant p21s bound both GTP and GDP. Similar activity was also found in various mouse tissues, with brain tissue showing the highest specific activity. When cell extracts were prepared from cultured cells, there was a linear relationship between GAP activity and cell density. These results suggest the factor is involved in the regulation of cell proliferation.

P21 v-ras inhibits induction of c-myc and c-fos expression by platelet-derived growth factor

The viral oncogene v-ras inhibited the platelet-derived growth factor (PDGF)-induced upregulation of c-myc and c-fos proto-oncogene expression in fibroblast monolayers. These v-ras-containing cells proliferated in the absence of c-myc induction and no longer required PDGF to support growth. Fibroblasts expressing v-ras continued to express the same number of functional PDGF receptors on their surface as uninfected cells, yet the usual induction of transcription of the genes c-myc, c-fos, and JE in response to PDGF stimulation did not occur in the presence of newly introduced v-ras or chronic v-ras gene expression, and synthesis of c-myc protein did not occur. This inhibitory effect on growth factor-mediated induction of cellular proto-oncogenes was specific for PDGF in that induction of the c-myc and c-fos genes by certain other factors was not impaired.

Expression of c-myc oncogene product and ras family oncogene products in various human malignant lymphomas defined by immunohistochemical techniques

The authors studied the expression of c-myc and ras family oncogene products in 43 cases of malignant lymphoma (ML) using the immunoperoxidase method. Unfixed frozen sections of lymph nodes from four patients with Hodgkin's disease and 39 with non-Hodgkin's lymphoma, together with normal lymph nodes, were studied by the avidin-biotin-peroxidase complex (ABC) technique. Two monoclonal antibodies, MYC-2 raised against recombinant human c-myc protein (reacting specifically with the c-myc products P62 and P67) and RASK-4 (raised against recombinant P21 and reacting specifically with ras-family product P21) were used. The c-myc product was detected in nuclei of ML cells and some normal, mainly germinal center, lymphocytes. When the staining intensity shown by normal germinal-center lymphocytes was graded as positive (+) or weakly positive (+/-), a very intensely positive reaction ( to ++) was observed in 37 cases (86%) of ML, a positive reaction (+) in four cases (9.3%), and a weakly positive reaction (+/-) in two cases (4.7%). The ras family oncogene product reaction was intensely positive (++) in two cases (4.7%), positive (+) in 16 cases (37.2%), weakly positive (+/-) in 13 cases (30.2%), and negative in 12 cases (27.9%). Western blot analysis confirmed an elevated level of c-myc products in two cases, which showed intense MYC-2 staining, and of ras family products in one case, which demonstrated intense RASK-4 staining. The enhanced expression of these gene products may play an important role in lymphomagenesis of such cases.

Characterization of a factor that stimulates hydrolysis of GTP bound to ras gene product p21 (GTPase-activating protein) and correlation of its activity to cell density

The postmicrosomal fraction of the extract from NIH 3T3 and BALB/c 3T3 cells stimulated the hydrolysis of GTP bound to H-ras gene product p21 by severalfold. The stimulation was observed with normal p21 but not with p21 with valine as the 12th residue. This specificity is similar to that of GTPase-activating protein (GAP) for N-ras p21 described by M. Trahey and F. McCormick (Science 238:542-545, 1987). Consistent with this specificity, analysis of p21-bound nucleotides in living cells revealed that almost all normal p21 bound GDP, whereas oncogenic mutant p21s bound both GTP and GDP. Similar activity was also found in various mouse tissues, with brain tissue showing the highest specific activity. When cell extracts were prepared from cultured cells, there was a linear relationship between GAP activity and cell density. These results suggest the factor is involved in the regulation of cell proliferation.

Serum screening for oncogene proteins in workers exposed to PCBs

A cohort of 16 municipal workers engaged in cleaning oil from old transformers was examined for possible health effects from exposure to polychlorinated biphenyls (PCBs). In addition to the evaluation of routine clinical parameters (history, physical examination, liver function tests, serum triglycerides, serum PCB values), a new screening technique for the presence of oncogene proteins in serum using monoclonal antibodies was used to ascertain the potential carcinogenic risk from exposure in these workers. Except for one individual, serum PCB concentrations were found to be relatively low in this cohort, probably due to the observance of appropriate protective precautions. The results of liver function test were within normal limits and serum triglyceride concentrations showed no consistent relation to PCB concentrations. Six individuals, all of whom were smokers, showed abnormal banding patterns for fes oncogene related proteins. The individual with the highest serum PCB concentration also exhibited significantly raised levels of the H-ras oncogene related P21 protein in his serum. These oncogene protein findings may be indicative of an increased risk for the development of malignant disease in these individuals.

Site-specific integration of H-ras in transformed rat embryo cells

A karyotypic analysis was performed on seven independently derived clones of primary rat embryo cells transformed by the ras oncogene plus the cooperating oncogene myc. The transfected oncogenes were sometimes present in amplified copy number, with heterogeneity in the levels of amplification. Some chromosomal features, such as aberrantly banding regions and double-minute chromosomes, typical of cells carrying amplified genes, were also seen in three of the seven cell lines. Underlying this heterogeneity there was an unexpected finding. All seven lines showed a common integration site for ras on the q arm of rat chromosome 3 (3q12), though some lines also had other sites of integration. In four of the lines integration of ras was accompanied by deletion of the p arm of chromosome 3 or its possible translocation to chromosome 12.

Molecular cloning and characterization of the human dbl proto-oncogene: evidence that its overexpression is sufficient to transform NIH/3T3 cells

We isolated cDNA clones representing the human dbl proto-oncogene transcript. Nucleotide sequence analysis revealed an open reading frame encoding a predicted protein of 925 amino acids. Using peptide antisera directed against specific proto-dbl peptides, a 115-kd protein was detected in COS cells transfected with an expression vector containing the entire coding region of proto-dbl. This mol. wt is consistent with that predicted from the open reading frame. We have previously shown that the dbl oncogene was generated by substitution of the 5' portion of proto-dbl with an unrelated human sequence. In this study we show that this rearrangement resulted in the loss of the 497 amino-terminal codons of the dbl proto-oncogene. Under the influence of a strong promoter proto-dbl could readily transform NIH/3T3 cells but its transforming activity was less than that of the dbl oncogene driven by the same promoter. Proto-dbl overexpression is, therefore, sufficient to transform NIH/3T3 cells, but specific structural alterations of its coding region significantly enhance its transforming activity. No apparent similarity was detected between the predicted proto-dbl product and other known proto-oncogenes. However, a stretch of 300 amino acids within the N-terminal half of proto-dbl showed structural similarity to the intermediate filament vimentin. This region in proto-dbl contains a heptad repeat motif characteristic of an alpha-helical coiled-coil structure. Taken together, these findings indicate that the human proto-dbl represents a new class of cellular oncogenes that may be related to cytoskeletal elements of the cell.

High-level expression of c-H-ras1 fails to fully transform rat-1 cells

Rat-1 cells were transfected with plasmids encoding normal (Gly-12), nonactivated (Pro-12), and activated (Val-12 and Ile-12) p21H-ras in the presence of an amplifiable dihydrofolate reductase marker. The introduced DNA was amplified by selection in methotrexate to establish the relationship between p21H-ras expression and various hallmarks of cellular transformation. The maximum level of p21H-ras (Gly-12) consistent with cell viability was approximately 0.13% of total cell protein (approximately 60,000 molecules per cell); this is 44-fold greater than the level of the endogenous protein. The maximum tolerated level of a second nontransforming form of p21H-ras (pro-12) was about half of this. Amplification in Rat-1 cells of H-ras genes encoding the highly oncogenic Val-12 and Ile-12 forms of p21H-ras could not be achieved by methotrexate selection, providing strong evidence that synthesis of activated p21H-ras above a certain threshold (about 0.02% of total protein) in Rat-1 cells is incompatible with cell viability. Individual cell lines were isolated and their morphology, anchorage-independent growth, tumorigenicity, and response to and production of growth factors were studied. We report that cell lines expressing near-maximum tolerated levels of either the normal or pro-12 form of p21H-ras were not as transformed as cells expressing much more modest levels of the highly oncogenic (Val-12) form, suggesting that the complete elaboration of the transformed phenotype by ras depends, at least in part, on mutations that distinguish the cellular and viral proteins. We found that cells expressing elevated levels of the normal p21(H-ras) could be fully transformed by the activated (Val-12) form and that such cells continued to overexpress p21(H-ras) (Gly-12), arguing against a role for normal ras genes in suppression of the oncogenic potential of their mutationally activated counterparts.

A point mutation in the last intron responsible for increased expression and transforming activity of the c-Ha-ras oncogene

The T24/EJ allele of the Ha-ras proto-oncogene owes its powerful oncogenic activity not merely to the well documented mutation that perturbs the structure of the encoded polypeptide, but in addition to a second single nucleotide alteration in an intron that causes a tenfold increase in expression. This effect on expression is maintained upon transfer of the surrounding DNA to a heterologous gene, and as such defines a novel regulatory element.

ras p21 oncoprotein expression in human colonic neoplasia--an immunohistochemical study with monoclonal antibody RAP-5

Expression of the ras oncogene product p21 (ras p21) in benign and malignant human colonic tissues was studied using the monoclonal antibody RAP-5 and the avidin-biotin-peroxidase technique. Histologically normal colonic mucosa and hyperplastic mucosa adjacent to carcinomas (transitional mucosa) were found, in most cases, to be negative for reactivity with the antibody or showed weak staining of a few epithelial cells. Similar findings were observed in hyperplastic and juvenile polyps. Of the 145 adenomas studied, 47 (32.4%) showed detectable levels of ras p21 expression. RAP-5 immunohistochemical staining was significantly associated with the degree of epithelial dysplasia (P less than 0.01) and the size of adenoma (P less than 0.05), but not with the histological type. Fifty-four of 70 primary adenocarcinomas (77.1%) were reactive with RAP-5 and usually demonstrated a higher percentage of stained cells and more intense cytoplasmic staining than that observed in adenomas. Although metastases often displayed a similar or even higher levels of ras p21 expression compared with the primary carcinomas, in 10 cases one or more metastatic lesions showed lower levels of ras p21. These results suggest that enhanced ras p21 expression may, at times, occur in the early stages of human colon carcinogenesis but are probably not associated with metastatic tumour progression.

Expression of cellular oncogenes in the myocardium during the developmental stage and pressure-overloaded hypertrophy of the rat heart

Proto-oncogenes have been revealed to participate in normal cell proliferation as well as in cell transformation. Since cardiac myocytes are terminally differentiated, they cannot divide except in the fetal period. To determine the role of cellular oncogenes in the growth of the heart, the expression pattern of eight cellular oncogenes during the developmental stage and pressure-overloaded hypertrophy of the rat hearts were examined in vivo. Northern blot analysis was performed with eight oncogene probes (myc, fos, Ha-ras, src, erbA, erbB, sis, myb). Pressure overload increased the levels of cellular (c-) fos, c-myc, and c-Ha-ras. An increase of c-fos and c-myc was detected at 30 minutes and 2 hours, respectively; the levels peaked at 8 hours, and they returned to baseline by 48 hours after aortic constriction. However, the level of c-Ha-ras showed a gradual increase. During the course of development, the expression of c-myc was detectable only in the embryonic stage, whereas the expression of c-fos was not detected in the fetal period, was increased after birth, and peaked in 200-day-old adults. The expression of c-Ha-ras was almost the same throughout the development. Cellular oncogenes were expressed in the heart in response to pressure overload and in a stage-specific manner. These results suggest that cellular oncogenes may participate in the normal developmental process and hypertrophy of hearts and that the cellular hypertrophy induced by pressure overload may share a similar mechanistic pathway with cell proliferation.

Differential potency and trans-activation of normal and mutant T24 human H-ras1 gene promoters

We have employed a short-term transfection assay system in which we monitored the transient expression of the chloramphenicol acetyltransferase (CAT) gene linked to the promoter region of the normal and mutant T24 H-ras1 gene or the human epsilon-globin gene in Chinese hamster lung (CHL) cells or cells derived from them which carry and express one or the other of the polyoma virus early genes. Our findings can be summarized as follows: (i) The mutant T24 H-ras1 promoter region behaves as a stronger promoter than the H-ras1 gene in all these types of cells as well as in rat 208F fibroblast cells. (ii) In CHL cells expressing the polyoma large T antigen the normal and mutant T24 Ha-ras1 promoters are not trans-activated in these cells and only a 2.5-fold activation of the epsilon-globin promoter is observed. (iii) In cells expressing the polyoma middle T antigen both the normal and mutant H-ras1 are trans-activated whereas transcription from the epsilon-globin promoter is not affected when compared to the normal CHL cells. (iv) In cells expressing the polyoma small T antigen the normal and mutant H-ras1 as well as the epsilon-globin promoters are trans-activated. We suggest from these data that a tissue-specific element exists in the promoter region of the H-ras1 gene and that the polyoma middle and small T antigens trigger the expression of proteins that trans-activate these promoters.

Separation of the 24 kDa substrate for botulinum C3 ADP-ribosyltransferase and the cholera toxin ADP-ribosylation factor

Botulinum C3 ADP-ribosyltransferase modifies a approximately 24 kDa membrane protein believed to bind guanine nucleotides. Cholera toxin ADP-ribosylation factors are approximately 19 kDa GTP-binding proteins that directly activate the toxin. To evaluate a possible relationship between C3 ADP-ribosyltransferase substrate and ADP-ribosylation factor, they were partially purified from bovine brain. ADP-ribosylation factor, but not C3 ADP-ribosyltransferase substrate, stimulated auto-ADP-ribosylation of the choleragen A1 subunit whereas C3 ADP-ribosyltransferase substrate, but not ADP-ribosylation factor, was ADP-ribosylated by C3 ADP-ribosyltransferase. Thus, although both may be GTP-binding proteins, no functional similarity between ADP-ribosylation factor and C3 ADP-ribosyltransferase substrate was found.

The ras gene family and human carcinogenesis

It has been well established that specific alterations in members of the ras gene family, H-ras, K-ras and N-ras, can convert them into active oncogenes. These alterations are either point mutations occurring in either codon 12, 13 or 61 or, alternatively, a 5- to 50-fold amplification of the wild-type gene. Activated ras oncogenes have been found in a significant proportion of all tumors but the incidence varies considerably with the tumor type: it is relatively frequent (20-40%) in colorectal cancer and acute myeloid leukemia, but absent or present only rarely in, for example, breast tumors and stomach cancer. No correlation has been found, yet, between the presence of absence of an activated ras gene and the clinical or biological features of the malignancy. The activation of ras oncogenes is only one step in the multistep process of tumor formation. The presence of mutated ras genes in benign polyps of the colon indicates that activation can be an early event, possibly even the initiating event. However, it can also occur later in the course of carcinogenesis to initiate for instance the transition of a benign polyp of the colon into a malignant carcinoma or to convert a primary melanoma into a metastatic tumor. Apparently, the activation of ras genes is not an obligatory event but when it occurs it can contribute to both early and advanced stages of human carcinogenesis.

Induction of neurite formation in PC12 cells by microinjection of proto-oncogenic Ha-ras protein preincubated with guanosine-5'-O-(3-thiotriphosphate)

Rat pheochromocytoma (PC12) cells differentiate to neuronal cells in response to nerve growth factor. It has been shown that microinjection of oncogenic but not proto-oncogenic p21 protein induces morphological differentiation in PC12 cells (D. Bar-Sagi and J. R. Feramisco, Cell 42:841-848, 1985). In this paper we describe a recombinant human proto-oncogenic Ha-ras protein which can effectively induce neurite extension of PC12 cells when microinjected as a complex with guanosine-5'-O-(3-thiotriphosphate). The protein was found to be less effective when complexed with GTP. On the other hand, an oncogenic ras protein coinjected with guanosine-5'-O-(2-thiodiphosphate) was entirely inactive. These results indicate that the binary p21-GTP complex, but not the p21-GDP complex, is effective in inducing differentiation in PC12 cells, irrespective of the oncogenic or the proto-oncogenic protein.

High-level expression of c-H-ras1 fails to fully transform rat-1 cells

Rat-1 cells were transfected with plasmids encoding normal (Gly-12), nonactivated (Pro-12), and activated (Val-12 and Ile-12) p21H-ras in the presence of an amplifiable dihydrofolate reductase marker. The introduced DNA was amplified by selection in methotrexate to establish the relationship between p21H-ras expression and various hallmarks of cellular transformation. The maximum level of p21H-ras (Gly-12) consistent with cell viability was approximately 0.13% of total cell protein (approximately 60,000 molecules per cell); this is 44-fold greater than the level of the endogenous protein. The maximum tolerated level of a second nontransforming form of p21H-ras (pro-12) was about half of this. Amplification in Rat-1 cells of H-ras genes encoding the highly oncogenic Val-12 and Ile-12 forms of p21H-ras could not be achieved by methotrexate selection, providing strong evidence that synthesis of activated p21H-ras above a certain threshold (about 0.02% of total protein) in Rat-1 cells is incompatible with cell viability. Individual cell lines were isolated and their morphology, anchorage-independent growth, tumorigenicity, and response to and production of growth factors were studied. We report that cell lines expressing near-maximum tolerated levels of either the normal or pro-12 form of p21H-ras were not as transformed as cells expressing much more modest levels of the highly oncogenic (Val-12) form, suggesting that the complete elaboration of the transformed phenotype by ras depends, at least in part, on mutations that distinguish the cellular and viral proteins. We found that cells expressing elevated levels of the normal p21(H-ras) could be fully transformed by the activated (Val-12) form and that such cells continued to overexpress p21(H-ras) (Gly-12), arguing against a role for normal ras genes in suppression of the oncogenic potential of their mutationally activated counterparts.

Novel combination of c-myc, N-myc and N-ras oncogene alteration in brain tumors

We have examined forty human brain tumors (neoplasias presenting an important incidence in Mexico), for cellular myc (c-myc), N-myc and N-ras proto-oncogene alterations. An elevated amplification and/or rearrangement of the oncogenes was detected in most samples (60% presenting alteration for c-myc, 54% for N-myc, 6% for N-ras and 60% for ras-related genes). The tumors were of different histological types and for some of them we detected either amplification and/or rearrangement of the oncogenes. We describe, for the first time, the alterations of two related genes (c-myc and N-myc) in the same tumor samples; in 64% of the analyzed samples, oncogene alterations were accompanied by enhanced expression of N-myc and ras-related genes. These results suggest an important role for c-myc, N-myc and N-ras oncogenes, in the development and progression of brain tumors.