Activation of a human c-K-ras oncogene

Synthetic sulfoglycolipids targeting the serine-threonine protein kinase Akt

The serine-threonine protein kinase Akt, also known as protein kinase B, is a key component of the phosphoinositide 3-kinase (PI3K)-Akt-mTOR axis. Deregulated activation of this pathway is frequent in human tumors and Akt-dependent signaling appears to be critical in cell survival. PI3K activation generates 3-phosphorylated phosphatidylinositols that bind Akt pleckstrin homology (PH) domain. The blockage of Akt PH domain/phosphoinositides interaction represents a promising approach to interfere with the oncogenic potential of over-activated Akt. In the present study, phosphatidyl inositol mimics based on a β-glucoside scaffold have been synthesized as Akt inhibitors. The compounds possessed one or two lipophilic moieties of different length at the anomeric position of glucose, and an acidic or basic group at C-6. Docking studies, ELISA Akt inhibition assays, and cellular assays on different cell models highlighted 1-O-octadecanoyl-2-O-β-d-sulfoquinovopyranosyl-sn-glycerol as the best Akt inhibitor among the synthesized compounds, which could be considered as a lead for further optimization in the design of Akt inhibitors.

Deficiency of the complex I of the mitochondrial respiratory chain but improved adenylate control over succinate-dependent respiration are human gastric cancer-specific phenomena

The purpose of study was to comparatively characterize the oxidative phosphorylation (OXPHOS) and function of respiratory chain in mitochondria in human gastric corpus mucosa undergoing transition from normal to cancer states and in human gastric cancer cell lines, MKN28 and MKN45. The tissue samples taken by endobiopsy and the cells were permeabilized by saponin treatment to assess mitochondrial function in situ by high-resolution oxygraphy. Compared to the control group of endobiopsy samples, the maximal capacity of OXPHOS in the cancer group was almost twice lower. The respiratory chain complex I-dependent respiration, normalized to complex II-dependent respiration, was reduced that suggests deficiency of complex I, but the respiratory control by ADP in the presence of succinate was increased. Similar changes were observed also in mucosa adjacent to cancer tissue. The respiratory capacity of MKN45 cells was higher than that of MKN28 cells, but both types of cells exhibited a deficiency of complex I of the respiratory chain which appears to be an intrinsic property of the cancer cells. In conclusion, human gastric cancer is associated with decreased respiratory capacity, deficiency of the respiratory complex I of mitochondria, and improved coupling of succinate oxidation to phosphorylation in tumor tissue and adjacent atrophic mucosa. Detection of these changes in endobiopsy samples may be of diagnostic value.

Oncogenic K-ras expression is associated with derangement of the cAMP/PKA pathway and forskolin-reversible alterations of mitochondrial dynamics and respiration

The Warburg effect in cancer cells has been proposed to involve several mechanisms, including adaptation to hypoxia, oncogenes activation or loss of oncosuppressors and impaired mitochondrial function. In previous papers, it has been shown that K-ras transformed mouse cells are much more sensitive as compared with normal cells to glucose withdrawal (undergoing apoptosis) and present a high glycolytic rate and a strong reduction of mitochondrial complex I. Recent observations suggest that transformed cells have a derangement in the cyclic adenosine monophosphate/cAMP-dependent protein kinase (cAMP/PKA) pathway, which is known to regulate several mitochondrial functions. Herein, the derangement of the cAMP/PKA pathway and its impact on transformation-linked changes of mitochondrial functions is investigated. Exogenous stimulation of PKA activity, achieved by forskolin treatment, protected K-ras-transformed cells from apoptosis induced by glucose deprivation, enhanced complex I activity, intracellular adenosine triphosphate (ATP) levels, mitochondrial fusion and decreased intracellular reactive oxygen species (ROS) levels. Several of these effects were almost completely prevented by inhibiting the PKA activity. Short-time treatment with compounds favoring mitochondrial fusion strongly decreased the cellular ROS levels especially in transformed cells. These findings support the notion that glucose shortage-induced apoptosis, specific of K-ras-transformed cells, is associated to a derangement of PKA signaling that leads to mitochondrial complex I decrease, reduction of ATP formation, prevalence of mitochondrial fission over fusion, and thereby opening new approaches for development of anticancer drugs.

Specific regulation of point-mutated K-ras-immortalized cell proliferation by a photodynamic antisense strategy

It has been reported that point mutations in genes are responsible for various cancers, and the selective regulation of gene expression is an important factor in developing new types of anticancer drugs. To develop effective drugs for the regulation of point-mutated genes, we focused on photoreactive antisense oligonucleotides. Previously, we reported that photoreactive oligonucleotides containing 2'-O-psoralenylmethoxyethyl adenosine (2'-Ps-eom) showed drastic photoreactivity in a strictly sequence-specific manner. Here, we demonstrated the specific gene regulatory effects of 2'-Ps-eom on [(12)Val]K-ras mutant (GGT --> GTT). Photo-cross-linking between target mRNAs and 2'-Ps-eom was sequence-specific, and the effect was UVA irradiation-dependent. Furthermore, 2'-Ps-eom was able to inhibit K-ras-immortalized cell proliferation (K12V) but not Vco cells that have the wild-type K-ras gene. These results suggest that the 2'-Ps-eom will be a powerful nucleic acid drug to inhibit the expression of disease-causing point mutation genes, and has great therapeutic potential in the treatment of cancer.

Mitochondrial Complex I decrease is responsible for bioenergetic dysfunction in K-ras transformed cells

Many cancer cells are characterized by high rate of glycolysis and reduced rate of aerobic respiration, whose mechanism is still elusive. Here we investigate the down-regulation of oxidative phosphorylation (OXPHOS) in K-ras transformed mouse fibroblasts as compared to a control counterpart. Transcriptional analysis showed different expression levels of several OXPHOS nuclear genes in the two cell lines. In particular, during the exponential growth phase most genes encoding proteins of Complex I were expressed at lower levels in transformed cells. Consistently, a significant decrease of Complex I content was found in transformed cells. Moreover, analysis of NAD-dependent respiration and ATP synthesis indicated a strong decrease of Complex I activity in the mitochondria from neoplastic cells, that was confirmed by direct assay of the enzyme redox activity. At variance, succinate-dependent respiration and ATP synthesis were not significantly affected. Taken together, our results provide the new insight that the reduction of respiration observed in K-ras transformed cells is specifically due to a Complex I activity decrease.

Glutamine deprivation induces abortive s-phase rescued by deoxyribonucleotides in k-ras transformed fibroblasts

Background

Oncogene activation plays a role in metabolic reprogramming of cancer cells. We have previously shown that K-ras transformed fibroblasts have a stronger dependence on glycolysis and a reduced oxidative phosphorylation ability as compared to their normal counterparts. Another metabolic adaptation of cancer cells, that has long been established, is their propensity to exhibit increased glutamine consumption, although the effects induced by glutamine deprivation on cancer cells are still controversial.

Methodology and Principal Findings

Here, by using nutritional perturbations and molecular physiology, we show that reduction or complete depletion of glutamine availability in K-ras transformed fibroblasts causes a strong decrease of proliferation ability and a slower re-entry of synchronized cells into the cell cycle. The reduced proliferation is accompanied by sustained expression of cyclin D and E, abortive S phase entrance and is dependent on Ras signalling deregulation, since it is rescued by expression of a dominant negative guanine nucleotide exchange factor. The growth potential of transformed cells as well as the ability to execute the G₁ to S transition is restored by adding the four deoxyribonucleotides, indicating that the arrest of proliferation of K-ras transformed cells induced by glutamine depletion is largely due to a reduced supply of DNA in the presence of signalling pathways promoting G₁ to S transition.

Conclusions and Significance

Our results suggest that the differential effects of glutamine and glucose on cell viability are not a property of the transformed phenotype per se, but rather depend on the specific pathway being activated in transformation. For instance, myc-overexpressing cells have been reported to die under glutamine depletion and not under glucose shortage, while the opposite holds for ras-transformed fibroblasts as shown in this paper. These different responses of transformed cells to nutritional stress should be taken into account when designing anti-cancer therapies that aim to exploit metabolic differences between normal and transformed cells.

Expression of transforming K-Ras oncogene affects mitochondrial function and morphology in mouse fibroblasts

K-ras transformed fibroblasts have been shown to have a stronger dependence from glycolysis, reduced oxidative phosphorylation ability and a fragility towards glucose depletion compared to their immortalized, normal counterparts. In this paper, using RNA profiling assays and metabolic perturbations, we report changes in expression of genes encoding mitochondrial proteins and alterations in mitochondrial morphology that correlate with mitochondrial functionality. In fact, unlike normal cells, transformed cells show reduced ATP content and inability to modify mitochondria morphology upon glucose depletion. Being reverted by GEF-DN expression, such morphological and functional changes are directly connected to Ras activation. Taken together with reported partial mitochondrial uncoupling and more sustained apoptosis of transformed cells, our results indicate that activation of the Ras pathway strikingly impacts on energy and signaling-related aspects of mitochondria functionality, that in turn may affect the terminal phenotype of transformed cells.

Two transforming C-RAF germ-line mutations identified in patients with therapy-related acute myeloid leukemia

Mutations leading to activation of the RAF-mitogen-activated protein kinase/extracellular signal-regulated (ERK) kinase (MEK)-ERK pathway are key events in the pathogenesis of human malignancies. In a screen of 82 acute myeloid leukemia (AML) samples, 45 (55%) showed activated ERK and thus were further analyzed for mutations in B-RAF and C-RAF. Two C-RAF germ-line mutations, S427G and I448V, were identified in patients with therapy-related AML in the absence of alterations in RAS and FLT3. Both exchanges were located within the kinase domain of C-RAF. In vitro and in vivo kinase assays revealed significantly increased activity for (S427G)C-RAF but not for (I448V)C-RAF. The involvement of the S427G C-RAF mutation in constitutive activation of ERK was further confirmed through demonstration of activating phosphorylations on C-RAF, MEK, and ERK in neoplastic cells, but not in nonneoplastic cells. Transformation and survival assays showed oncogenic and antiapoptotic properties for both mutations. Screening healthy individuals revealed a <1/400 frequency of these mutations and, in the case of I448V, inheritance was observed over three generations with another mutation carrier suffering from cancer. Taken together, these data are the first to relate C-RAF mutations to human malignancies. As both mutations are of germ-line origin, they might constitute a novel tumor-predisposing factor.

Ras-dependent carbon metabolism and transformation in mouse fibroblasts

Mutational activation of ras genes is required for the onset and maintenance of different malignancies. Here we show, using a combination of molecular physiology, nutritional perturbations and transcriptional profiling, that full penetrance of phenotypes related to oncogenic Ras activation, including the shift of carbon metabolism towards fermentation and upregulation of key cell cycle regulators, is dependent upon glucose availability. These responses are induced by Ras activation, being specifically reverted by downregulation of the Ras pathway obtained through the expression of a dominant-negative Ras-specific guanine nucleotide exchange protein. Our data allow to link directly to ras activation the alteration in energy metabolism of cancer cells, their fragility towards glucose shortage and ensuing apoptotic death.

ras oncogene mutations in diethylnitrosamine-induced hepatic tumors in medaka (Oryzias latipes), a teleost fish

Medaka fish are an established non-mammalian research model for the study of liver carcinogenesis and exposure to environmental pollutants. Studies have emphasized the development of hepatic neoplasms in medaka following exposure to model carcinogens. To date however, little information is known regarding the mechanisms underlying initiation of hepatic tumors in this species. The aim of this study was to relate our understanding of diethylnitrosamine (DEN)-induced tumor formation to ras gene activation in hepatic neoplasms of exposed medaka. Initial studies were conducted to identify medaka ras exons 1 and 2 by reverse transcriptase polymerase chain reaction (RT-PCR). Amplification of ras exons 1 and 2 from untreated medaka liver resulted in the identification of three polymorphic ras sequence variants exhibiting a high degree of homology to other teleost and mammalian ras genes. Exposure of medaka to 159 ppm of DEN resulted in a wide range of hepatic neoplasms including: hepatocellular adenomas, hepatocellular carcinomas, cholangiomas, and mixed hepatocholangiocellular carcinomas. Individual liver tumors were examined for oncogenically activating ras mutations by probing genomic DNA with probes specific for activating point mutations or by direct cloning and sequencing of ras transcripts using RT-PCR. Using allele-specific oligonucleotide (ASO) analysis, a single point mutation was detected in codon 12 position two in 8/25 (32%) tumors examined. Mutated ras alleles were additionally detected in 12 of 39 (30%) medaka liver tumors by sequence analysis. Ten of the 12 mutations identified contained a single point mutation at codon 12 resulting in a Gly to Asp amino acid substitution. Two unique mutations were identified at codon 16 resulting in either Lys to Asn or Lys to Thr amino acid substitutions. Our results show that ras mutations are induced by DEN and are present in over 30% of the fish that developed tumors. A ras mutation incidence of 30% is similar to that reported in mammalian species exposed to DEN. While mutations at codon 12 have previously been reported, the present study is the first in vivo report of ras point mutations at codon 16.

Only wild-type c-Ki-ras codons 12, 13, and 61 in human pancreatic acinar cell carcinomas

Activation of the c-Ki-ras proto-oncogene is more common in carcinomas of the pancreas than in other human carcinomas. Most such carcinomas are of the ductal cell phenotype. Ductal adenocarcinomas of the hamster pancreas have similar mutations, but acinar cell carcinomas of the mouse and rat pancreas lack the common c-Ki-ras mutations. Therefore, we examined 11 acinar cell carcinomas of the human pancreas for evidence of mutations at codons 12, 13, and 61. DNA was isolated from tumor cells in paraffin-embedded sections. The polymerase chain reaction was used to amplify the appropriate DNA sequence, and then allele-specific oligonucleotide hybridization and DNA sequence analysis were used to evaluate c-Ki-ras structure. Only wild-type sequences were found in codons 12, 13, and 61. Thus, in both the human and rodent species, mutations of c-Ki-ras appear to be more important in the genesis of ductal carcinomas than in the genesis of acinar cell carcinomas of the pancreas.

Crystal structures at 2.2 A resolution of the catalytic domains of normal ras protein and an oncogenic mutant complexed with GDP

The biological functions of ras proteins are controlled by the bound guanine nucleotide GDP or GTP. The GTP-bound conformation is biologically active, and is rapidly deactivated to the GDP-bound conformation through interaction with GAP (GTPase Activating Protein). Most transforming mutants of ras proteins have drastically reduced GTP hydrolysis rates even in the presence of GAP. The crystal structures of the GDP complexes of ras proteins at 2.2 A resolution reveal the detailed interaction between the ras proteins and the GDP molecule. All the currently known transforming mutation positions are clustered around the bound guanine nucleotide molecule. The presumed "effector" region and the GAP recognition region are both highly exposed. No significant structural differences were found between the GDP complexes of normal ras protein and the oncogenic mutant with valine at position 12, except the side-chain of the valine residue. However, comparison with GTP-analog complexes of ras proteins suggests that the valine side-chain may inhibit GTP hydrolysis in two possible ways: (1) interacting directly with the gamma-phosphate and altering its orientation or the conformation of protein residues around the phosphates; and/or (2) preventing either the departure of gamma-phosphate on GTP hydrolysis or the entrance of a nucleophilic group to attack the gamma-phosphate. The structural similarity between ras protein and the bacterial elongation factor Tu suggests that their common structural motif might be conserved for other guanine nucleotide binding proteins.

Point mutation at codon 12 of the Ki-ras gene in a primary breast carcinoma and the MDA-MB-134 human mammary carcinoma cell line

We found an activated Kirsten (Ki)-ras gene in the MDA-MB-134 breast carcinoma cell line by transfection of NIH3T3 cells. Oligonucleotide hybridization demonstrated that this cell line carries a single G to C point mutation at position 12 leading to a glycine-arginine substitution. However, only a fraction of the cell population seems to contain this Ki-ras mutation. Since mutations can occur in cell lines during in vitro culture, we searched in breast carcinoma samples for the presence of single mutations at codon 12, but also for the presence of the double mutation previously found in the H-466B breast carcinoma cell line. Using polymerase chain reaction (PCR), we detected one primary tumour carrying a single mutation at codon 12. No double mutation was found in any of the tumours. These results show that Ki-ras gene mutation could be involved in breast carcinogenesis, albeit at a low frequency.

Rapid repair kinetics of pyrimidine(6-4)pyrimidone photoproducts in human cells are due to excision rather than conformational change

UV-induced pyrimidine(6-4)pyrimidone photoproducts in DNA of mammalian cells are apparently repaired much more rapidly than cyclobutane dimers. Since only immunological assays for (6-4) photoproducts have been sensitive enough for repair measurements, it was possible that these apparently rapid repair kinetics reflected a change in physical conformation of antibody-binding sites, resulting in epitope loss rather than excision. To discriminate between these possibilities, we developed a procedure to photochemically convert (6-4) photoproducts to single-strand breaks in UV-irradiated DNA with a background low enough to permit repair measurements. Analysis of a specific DNA sequence indicated that photoinduced alkali-labile sites (PALS) were induced with the same site-specificity as (6-4) photoproducts. Normal human and xeroderma pigmentosum (XP) variant cells rapidly excised (6-4) photoproducts measured as PALS, but little repair was seen in cells from XP complementation group A. These repair kinetics corresponded to those determined in the same samples by radioimmunoassay of (6-4) photoproducts. Thus we conclude that the rapid repair of (6-4) photoproducts observed in UV-irradiated human cells is not the result of a conformational change resulting in epitope loss, but reflects excision of this photoproduct from DNA.

Activation of c-Ki-ras not detectable in adenomas or adenocarcinomas arising in rat pancreas

The objective of this study was to determine whether activation of c-Ki-ras occurs in carcinogen-induced rat pancreatic tumors. DNAs from 27 samples, which included adenomas, carcinomas in situ, and adenocarcinomas arising in azaserine-treated rats and corn oil-gavaged rats along with a nafenopin-induced rat pancreatic adenocarcinoma were examined for mutation of c-Ki-ras at codons 12, 13, and 61 by using the polymerase chain reaction. Our results indicate that activation of c-Ki-ras is not a common event during pancreatic carcinogenesis in the rat.

Ki-ras activation and expression in transformed mouse lung cell lines

The Ki-ras proto-oncogene is activated by specific point mutations and is the transforming gene often identified in rodent and human lung tumors. An in vitro model to aid in the study of the consequences of Ki-ras activation and expression in mouse lung is needed. Accordingly, we have examined cell lines derived from chemically induced mouse lung tumors as well as spontaneous transformants of untreated mouse lung epithelial cells. The specific Ki-ras-activating gene mutations and the level of mRNA expression were examined for each cell line. Polymerase chain reaction and oligonucleotide hybridization were used to demonstrate that five of seven transformed lung cell lines contain codon 61 Ki-ras-activating mutations, resulting in an arginine substitution for wild-type glutamine. One transformed line contained this activating mutation and had also lost, or contained an altered, wild-type codon 61 Ki-ras allele. No codon 12 Ki-ras mutations were observed. Two transformed and two nontransformed epithelial lung cell lines contained only the wild-type codon 12 and 61 Ki-ras alleles. Northern blot analysis demonstrated that the Ki-ras mRNA was present in all the cell lines and was overexpressed in some, but not all, of the transformed lung cell lines. Those transformed lines with the highest levels of Ki-ras mRNA also expressed more H4-histone mRNA, suggesting that these cells have a greater proliferation rate. The level of Ki-ras mRNA increased during the proliferation of the nontransformed lung cells but then decreased upon reaching confluency. In contrast, the level of Ki-ras mRNA in the transformed lung cells was high during both growth and confluency, suggesting a potential defect in the regulation of Ki-ras in these cells. These lung cell lines will help provide a better understanding of the regulation of both the Ki-ras proto-oncogene and oncogene in the lung.

Transformation of diploid human fibroblasts by transfection with the v-sis, PDGF2/c-sis, or T24 H-ras genes

Gene transfection techniques have provided powerful methods to examine the roles of cellular and retroviral oncogenes in the transformation process in rodent fibroblasts. However, the use of such techniques with diploid human fibroblasts has been limited. We have developed transfection procedures to reproducibly transfect such cells with oncogenes, and methods for the biological characterization of the transformants. We have shown that the v-sis and T24 H-ras oncogenes, as well as the platelet-derived growth factor gene (PDGF2/c-sis), are capable of inducing a transformed phenotype in normal diploid human fibroblasts, but are not capable of conferring infinite lifespan or making such cells tumorigenic.

Most human carcinomas of the exocrine pancreas contain mutant c-K-ras genes

Using in vitro gene amplification by the polymerase chain reaction (PCR) and mutation detection by the RNAase A mismatch cleavage method, we have examined c-K-ras genes in human pancreatic carcinomas. We used frozen tumor specimens and single 5 micron sections from formalin-fixed, paraffin-embedded tumor tissue surgically removed or obtained at autopsy. Twenty-one out of 22 carcinomas of the exocrine pancreas contained c-K-ras genes with mutations at codon 12. In seven cases tested, the mutation was present in both primary tumors and their corresponding metastases. No mutations were detected in normal tissue from the same cancer patients or in five gall bladder carcinomas. We conclude from these results that c-K-ras somatic mutational activation is a critical event in the oncogenesis of most, if not all, human cancers of the exocrine pancreas.

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.

Activated Ki-ras genes in bladder epithelial cell lines transformed by treatment of primary mouse bladder explant cultures with 7,12-dimethylbenz[a]anthracene

DNA from five lines of transformed bladder epithelial cells derived from cultures of primary cells that had been treated with 7,12-dimethylbenz[a]anthracene (DMBA) can transform NIH 3T3 mouse fibroblasts in DNA transfection experiments. Southern analysis of DNA from NIH 3T3 primary and secondary transformants established that four of the DMBA-transformed cell lines contained activated cellular Ki-ras, while the remaining cell line contained a transforming gene that is unrelated to Ki-ras, N-ras, and Ha-ras. The point mutations responsible for Ki-ras activation were detected using oligonucleotide probes following selective amplification of Ki-ras specific sequences using the polymerase chain reaction. The results showed that activation of Ki-ras invariably involved a GC----AT transition mutation of the first position of codon 12. Surprisingly, a Ki-ras gene that was activated by a GC----AT transition mutation at the same position was also detected in a single transformed bladder urothelial cell line derived from control cultures of mouse bladder cells. Together, our results indicate that Ki-ras activation in the DMBA-transformed bladder cell lines may not be a direct consequence of interaction of activated DMBA metabolites with the Ki-ras gene.

Expression of the Kirsten ras viral and human proteins in Escherichia coli

The expression vectors pINIII-A and pINIII (lpp p5) were used to construct plasmids which direct the synthesis in Escherichia coli of the Kirsten ras viral (v-Ki-ras) and human cellular (c-Ki-ras) oncogene products as fusion proteins containing 9 and 10 extra amino acids, respectively, at their N termini. Authenticity of the bacterially produced proteins was determined by immunoprecipitation and immunoblot analyses with ras-specific monoclonal antibodies. After induction with isopropyl-beta-D-thiogalactopyranoside, the viral protein represented approximately 20% of the total cellular protein. The majority of the protein was found in the postsonication low-speed centrifugation pellet. The synthesized viral protein was active in GTP binding, as judged by autophosphorylation and photoaffinity labeling assays.

Oncogene amplification during tumorigenesis of established rat fibroblasts reversibly transformed by activated human ras oncogenes

Normal rat fibroblasts of the established cell line Rat 4 were cotransformed with activated human ras oncogenes and with a cloned chicken thymidine kinase (tk) gene. Linkage between tk and ras genes allowed the isolation of oncogene deletion revertants and of cell clones showing varying degrees of malignant phenotype. Southern and Northern experiments in concert with tumorigenicity assays show that the malignant transformation of these cells by mutant ras oncogenes is a gradual but reversible process that depends on the relative abundance of oncogene sequences and their corresponding transcripts. We also show that moderate amplification of a c-K-ras oncogene in these cells results in a clear increase in their tumorigenicity and that the mutant gene present in low copy numbers in cultured cells undergoes amplification in the corresponding in vivo induced tumors.

A human Ki-ras oncogene encodes two transforming p21 proteins

The human Ki-ras gene was previously reported to contain two alternative fourth exons which encode two distinct p21 proteins differing only at their carboxy termini. The present study shows that either p21 protein is able on its own to transform NIH 3T3 cells to a tumorigenic state.

Chromatin structure of the promoter region of the human c-K-ras gene

The chromatin structure of the human c-K-ras gene has been investigated in various cultured normal and tumor human cells and in a rat cell line transformed with the human oncogene. The promoter region is hypersensitive to DNAse I, micrococcal nuclease, endogenous nucleases and to S1 nuclease in supercoiled plasmids. This hypersensitive region is present in the different cell types analyzed and both normal and mutant alleles exhibit similar general sensitivity to DNAse I digestion in the same tumor cells. However, the 5' more distal DNAse I hypersensitive site, which is coincident with a region of the gene containing sequence homologies with known enhancers, exhibits variable sensitivity which appears to be higher in the tumor than in the normal and in the human than in the rat cells which we have analyzed. These data suggest the presence of specific factors interacting with the promoter sequences and delimits the transcription unit of the c-K-ras locus.

Oncogene amplification during tumorigenesis of established rat fibroblasts reversibly transformed by activated human ras oncogenes

Normal rat fibroblasts of the established cell line Rat 4 were cotransformed with activated human ras oncogenes and with a cloned chicken thymidine kinase (tk) gene. Linkage between tk and ras genes allowed the isolation of oncogene deletion revertants and of cell clones showing varying degrees of malignant phenotype. Southern and Northern experiments in concert with tumorigenicity assays show that the malignant transformation of these cells by mutant ras oncogenes is a gradual but reversible process that depends on the relative abundance of oncogene sequences and their corresponding transcripts. We also show that moderate amplification of a c-K-ras oncogene in these cells results in a clear increase in their tumorigenicity and that the mutant gene present in low copy numbers in cultured cells undergoes amplification in the corresponding in vivo induced tumors.

A human Ki-ras oncogene encodes two transforming p21 proteins

The human Ki-ras gene was previously reported to contain two alternative fourth exons which encode two distinct p21 proteins differing only at their carboxy termini. The present study shows that either p21 protein is able on its own to transform NIH 3T3 cells to a tumorigenic state.

Four human carcinoma cell lines with novel mutations in position 12 of c-K-ras oncogene

We have used synthetic oligonucleotides to probe for mutations affecting amino acid 12 of the c-K-ras gene in human cell line DNA. Of seven carcinoma cell lines tested, four were found to contain a mutation at this position. In each the nucleotide G was replaced with an A resulting in a Gly to Asp substitution in three cases (cell lines A427, A1165 and A1663) and Gly to Ser in the fourth (A549). Neither of these substitutions have been previously reported in either human tumor or human tumor-derived cell line DNA's. These results indicate that association between mutations involving position 12 of the human c-K-ras oncogene and carcinomas may be stronger than previously recognized.

A method to detect and characterize point mutations in transcribed genes: amplification and overexpression of the mutant c-Ki-ras allele in human tumor cells

A significant percentage of human tumors contain activated ras oncogenes that have acquired oncogenic potential as a result of somatic point mutations at codon 12 or 61 of the encoded ras gene product. We report here a method to detect and characterize mutations in ras genes that is based on the ability of pancreatic ribonuclease (RNase A; EC 3.1.27.5) to cleave RNA heteroduplexes containing single-base mismatches. Using this method, we show that certain human tumor cells contain mutant c-Ki-ras genes, and we define the nature and position of these mutations. At the same time, we describe the presence and estimate the expression of both normal and mutant c-Ki-ras alleles in the same tumor cells. This method should be useful for the diagnostic detection and characterization of single point mutations in expressed genes.