ras oncogenes in chemical carcinogenesis

Influence of DNA repair on nonlinear dose-responses for mutation

Recent evidence has challenged the default assumption that all DNA-reactive alkylating agents exhibit a linear dose-response. Emerging evidence suggests that the model alkylating agents methyl- and ethylmethanesulfonate and methylnitrosourea (MNU) and ethylnitrosourea observe a nonlinear dose-response with a no observed genotoxic effect level (NOGEL). Follow-up mechanistic studies are essential to understand the mechanism of cellular tolerance and biological relevance of such NOGELs. MNU is one of the most mutagenic simple alkylators. Therefore, understanding the mechanism of mutation induction, following low-dose MNU treatment, sets precedence for weaker mutagenic alkylating agents. Here, we tested MNU at 10-fold lower concentrations than a previous study and report a NOGEL of 0.0075 µg/ml (72.8nM) in human lymphoblastoid cells, quantified through the hypoxanthine (guanine) phosphoribosyltransferase assay (OECD 476). Mechanistic studies reveal that the NOGEL is dependent upon repair of O⁶-methylguanine (O⁶MeG) by the suicide enzyme O⁶MeG-DNA methyltransferase (MGMT). Inactivation of MGMT sensitizes cells to MNU-induced mutagenesis and shifts the NOGEL to the left on the dose axis.

Frequency and spectrum of K-RAS codons 12 and 13 mutations in colorectal adenocarcinomas from Taiwan

Mutations in codons 12 and 13 of the K-RAS oncogene are detected at a remarkably high frequency in colorectal adenocarcinoma and are believed to be a critical event in oncogenesis. In the present study, we evaluated colorectal tumor specimens from Taiwan for mutations in K-RAS codons 12 and 13 using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis and direct DNA sequencing. Mutations were found in 48 of 181 (26.5%) tumors, 30 mutations were G-->A transitions (62.5% of all mutations), 14 were G-->T transversions (29.2%), and only 4 were G-->C transversions (8.3%). Similar relative mutation frequencies and spectra were found regardless of the sex of the patient, the tumor grade, or the tumor stage. The high frequency of transitions among K-RAS mutation suggests that G/T mismatches play an important role in the oncogenesis of colorectal adenocarcinoma, implying that alkylating carcinogens may be involved in the colorectal carcinogenesis. Although the frequency of mutation (26.5%) appears to be lower than those reported in the United States (40%), France (49%), and the Netherlands (34%), the spectrum of point mutations in codons 12 and 13 of the K-RAS gene in the Taiwan Chinese population appears to be similar. The reason for these results may be that diet and ethnicity are not rate limit factors in controlling the spectra of mutations but influence on the frequency of K-RAS mutations in human colorectal adenocarcinomas.

The growth hormone-deficient Spontaneous Dwarf rat is resistant to chemically induced mammary carcinogenesis

Recent epidemiologic studies have suggested that the growth hormone (GH)/insulin-like growth factor I axis plays an important role in human breast cancer. The purpose of the present study was to evaluate the function of GH in rat mammary carcinogenesis, a model that closely recapitulates human breast cancer biology. The Spontaneous Dwarf rat (SDR) arose from the Sprague-Dawley rat and harbors a mutation in its GH gene yielding undetectable levels of a severely truncated protein not capable of binding to the GH receptor. When female rats of either strain were exposed to the direct-acting carcinogen N-methyl-N-nitrosourea, all wild-type rats (n = 10) developed multiple mammary cancers (5.3/rat). In contrast, SDR rats (n = 15) developed only three cancers (0.2/rat) and these were very small (<6 mm3). In another experiment, SDRs were backcrossed with wild-type Sprague-Dawley rats and the progeny were exposed to the indirect-acting carcinogen 7,12-dimethylbenz[a]anthracene. Progeny that were either homo- or heterozygous for the wild-type GH gene developed approximately 4 mammary tumors/rat, respectively. In contrast, SDR progeny developed only 0.21 tumors/rat. Mammary glands of SDRs had substantially less alveolar development compared with wild-type, yet ductal branching was similar in the two strains. Infusion of rat GH to SDRs induced mammary epithelial cell proliferation and alveolar development similar to that of wild-type rats. Taken together, these results demonstrate an important role for GH in alveolar development in the virgin rat, and provide the first direct evidence that GH plays a critical role in mammary carcinogenesis.

Relating aromatic hydrocarbon-induced DNA adducts and c-H-ras mutations in mouse skin papillomas: the role of apurinic sites

Mouse skin tumors contain activated c-H-ras oncogenes, often caused by point mutations at codons 12 and 13 in exon 1 and codons 59 and 61 in exon 2. Mutagenesis by the noncoding apurinic sites can produce G-->T and A-->T transversions by DNA misreplication with more frequent insertion of deoxyadenosine opposite the apurinic site. Papillomas were induced in mouse skin by several aromatic hydrocarbons, and mutations in the c-H-ras gene were determined to elucidate the relationship among DNA adducts, apurinic sites, and ras oncogene mutations. Dibenzo[a,l]pyrene (DB[a,l]P), DB[a,l]P-11,12-dihydrodiol, anti-DB[a,l]P-11,12-diol-13,14-epoxide, DB[a,l]P-8,9-dihydrodiol, 7,12-dimethylbenz[a]anthracene (DMBA), and 1,2,3,4-tetrahydro-DMBA consistently induced a CAA-->CTA mutation in codon 61 of the c-H-ras oncogene. Benzo[a]pyrene induced a GGC-->GTC mutation in codon 13 in 54% of tumors and a CAA-->CTA mutation in codon 61 in 15%. The pattern of mutations induced by each hydrocarbon correlated with its profile of DNA adducts. For example, both DB[a,l]P and DMBA primarily form DNA adducts at the N-3 and/or N-7 of deoxyadenosine that are lost from the DNA by depurination, generating apurinic sites. Thus, these results support the hypothesis that misreplication of unrepaired apurinic sites generated by loss of hydrocarbon-DNA adducts is responsible for transforming mutations leading to papillomas in mouse skin.

Aberrant function of the Ras signal transduction pathway in human breast cancer

Although ras mutations are infrequent (approximately 5%) in breast cancers, there is considerable evidence that suggests that the pathways which Ras services may still be deregulated in breast cancer cells. The recent identification of many of the components of the Ras signal transduction pathway has defined a network of proto-oncogene proteins controlling diverse signaling events that regulate cell growth and differentiation. Consequently, mutations that perturb the function of any one component of this signal pathway may trigger the same oncogenic events as mutation of ras itself. Moreover, several Ras-related proteins have recently been demonstrated to possess the ability to trigger malignant transformation via signaling pathways shared with Ras proteins. Thus, it is possible that the aberrant function of Ras-related proteins may contribute to breast cancer development. Consequently, it is important not to dismiss the Ras pathway in the development of breast cancer merely because of the infrequent detection of mutations in ras itself, but rather to consider the influence of aberrations upstream or downstream of Ras and of certain Ras-related proteins in the development of breast cancer. Finally, the critical importance of components upstream and downstream of Ras provides additional targets for rational drug design approaches to block the aberrant function of Ras signaling in human tumors.

N-methylnitrosourea-induced Ki-ras codon 12 mutations: early events in mouse thymic lymphomas

N-Methylnitrosourea (NMU)-induced codon 12 Ki-ras mutations were analyzed in premalignant thymic lymphomas from C57BL/6J mice by using a selective polymerase chain reaction amplification strategy. The frequency of codon 12 Ki-ras mutations was 67% (16 of 24) in NMU-treated animals with premalignant stage I disease. Previously, animals with different stages of disease had been analyzed for cytogenetic changes and for mutations in the p53 tumor suppressor gene. The genetic changes observed were early-activating codon 12 G35-->A transition mutations of the Ki-ras gene, followed closely by trisomy 15 and infrequent mutation of the p53 gene late in tumor development. The consistent and early detection of Ki-ras mutations in NMU-treated animals but not in untreated controls suggests that the mutations result from direct carcinogen exposure. Alternate pathways of NMU-induced thymic lymphomagenesis were implicated. One pathway involved putative NMU-induced mutations in other, non-ras oncogenes that cooperate with trisomy 15 to produce similar T-cell tumors. The frequency of p53 gene mutations in human and murine T-cell tumors is similar but low.

Transformation of type 1 astrocytes with N-ethyl-N-nitrosourea: establishment of an in vitro system and the role of the p53 gene

N-ethyl-N-nitrosourea (ENU)-induced gliomas, animal models of human gliomas, are most frequently oligodendrocytic, while human gliomas tend to be astrocytic. To facilitate a detailed study of human glial carcinogenesis, we developed an in vitro system using type 1 astrocyte transformation with ENU. Type 1 astrocytes from fetal Wistar rat brain were treated by a single dose of ENU. Transformed colonies appeared 50 days after exposure to single doses of ENU greater than 150 micrograms/mL. Cloned cells from these colonies retained the immunohistochemical characteristics of type 1 astrocytes. They showed rapid growth and high saturation densities, colony formation in low (2%) serum medium and gave rise to tumors when injected into nude mice. When p53 expression was studied at each passage, a single cell positive for mutant p53 protein emerged 40 days after ENU treatment. In the next 1-3 passages, the mutant p53 positive cell formed piled-up colonies and exhibited dominant growth. Northern blot analysis showed markedly increased accumulations of p53 mRNA in transformed cells. This in vitro transformation system of type 1 astrocytes provides a valuable tool for further investigations of astrocyte carcinogenesis.

The Ras signal transduction pathway

Considerable progress has been made over the past year in elucidating the mechanisms by which extracellular signals are transduced via cell surface receptors to trigger changes in gene expression which determine the growth and differentiated state of a cell. In particular, Ras proteins have been implicated as key intermediates that mediate the signal from upstream tyrosine kinases to a downstream cascade of serine/threonine kinases, which then activate nuclear factors that control gene expression and protein synthesis. How Ras proteins function is regulated in this role as a molecular switch, and how the signal is transmitted between the various components of the pathway, are now being determined. Finally, the Rho family of Ras-related proteins, which regulate the actin cytoskeleton, have also been implicated as mediators of oncogenic Ras transformation. The brisk pace at which the key components of Ras-mediated signal transduction pathways are being identified hold great promise that new targets for therapeutic intervention in cancer may now be identified.

Allele loss and point mutation in codons 12 and 61 of the c-Ha-ras oncogene in carcinogen-transformed human breast epithelial cells

There is significant evidence that the ras oncogene plays a role in experimental mammary carcinogenesis; the evidence in human breast cancer, however, is more limited. We induced the expression of transformation phenotypes in the human breast epithelial cell line MCF-10F with the chemical carcinogens 7,12-dimethylbenz[a]anthracene, N-methyl-N-nitrosourea, N-methyl-N-nitro-N'-nitrosoguanidine, and benzo[a]pyrene. This work was designed to clarify whether chemically induced neoplastic transformation correlates with alterations in the ras gene. MCF-10F cells have two c-Ha-ras alleles, identified by 1.0-kb and 1.2-kb restriction fragments. Treatment with carcinogens resulted in the loss of one of the alleles (1.0 kb). Polymerase chain reaction-amplified DNA from all carcinogen-treated cells was analyzed for point mutations in c-Ha-ras at codons 12 and 61. All of the carcinogens induced a mutation of the remaining allele at the first position of codon 12 (GGC-->AGC). Another frequent mutation occurred at the first position of codon 61 (CAG-->GAG). The changes in c-Ha-ras were associated with the emergence of colony formation in agar-methocel, but no specific changes in this gene correlated with the emergence of invasiveness or tumorigenesis, indicating that other genes may be involved in the process.

Spontaneous glucose intolerance in the progeny of low dose streptozotocin-induced diabetic mice

Multiple low doses of streptozotocin (LDS) induce low-incidence diabetes mellitus in Balb/cHan and high-incidence diabetes in CD-1 mice. We studied offspring of diabetic parents in both strains. Group 1 consisted of litters from control mice with no streptozotocin treatment. Group 2 litters had an LDS diabetic mother and a control father, group 3 litters had control mother with LDS diabetic father, and group 4 litters had both, LDS diabetic mother and father. Diabetes was induced by 5 x 40 mg streptozotocin per kg on five consecutive days. Progeny of diabetic mothers showed a state of reduced glucose tolerance associated with reduced glucose disappearance during intravenous glucose tolerance test and increased insulin secretion of isolated islets of Langerhans. These metabolic abnormalities predominated in the male litters of both strains of mice. Amniotic insulin was increased in diabetic mothers during pregnancy. No histologic abnormalities were observed in group 2 progeny. Pancreases in male offspring of LDS diabetic CD-1 fathers (group 3) were studied for insulitis. Insulitis was found in 40% of mice with normal glucose tolerance. A single subdiabetogenic dose of streptozotocin (40 mg/kg) induced insulitis in 90% of pancreases accompanied by reduced insulin release of isolated islets. By contrast, male Balb/cHan progeny of diabetic fathers failed to develop insulitis. In conclusion, we found (1) parental LDS diabetes was transmitted more often to male offspring, (2) maternal LDS diabetes was associated with hyperinsulin secretion and glucose intolerance in the offspring and (3) paternal LDS diabetes was accompanied by insulitis and insulin secretion deficiency in CD-1 progeny.

Mutations of the Ki-ras oncogene in endometrial carcinoma

OBJECTIVE

The purpose of this study was to assess the extent of involvement of the ras oncogene in endometrial carcinoma.

STUDY DESIGN

Genomic deoxyribonucleic acid from 30 samples of endometrial carcinoma was examined for point mutations in codons 12, 13, and 61 from the Ha-ras, Ki-ras, and N-ras genes by means of the polymerase chain reaction, slot-blotting, and deoxyribonucleic acid sequencing procedures.

RESULTS

An apparent somatic mutation of Ki-ras codon 12 in one of 10 paraffin-embedded tumors was confirmed by deoxyribonucleic acid sequence analysis. Two of 20 frozen endometrial carcinoma specimens were also shown to contain a point mutation in Ki-ras codon 12. No correlation between ras mutation and a number of histologic or clinical parameters was observed.

CONCLUSIONS

These data suggest a potential role for Ki-ras codon 12 mutations in the development of some (10%) endometrial cancers.

Regulation of tetradecanoyl phorbol acetate-induced responses in NIH 3T3 cells by GAP, the GTPase-activating protein associated with p21c-ras

Proteins of the ras family of oncogenes have been implicated in signal transduction pathways initiated by protein kinase C (PKC) and by tyrosine kinase oncogenes and receptors, but the role that ras plays in these diverse signalling systems is poorly defined. The activity of ras proteins has been shown to be controlled in part by a cellular protein, GAP (GTPase-activating protein), that negatively regulates p21c-ras by enhancing its intrinsic GTPase activity. Thus, overexpression of GAP provides a tool for determining the step(s) in signal transduction dependent on p21c-ras activity. In this paper, we report that overexpression of GAP blocks the phorbol ester (tetradecanoyl phorbol acetate [TPA])-induced activation of p42 mitogen-activated protein kinase (p42mapk), c-fos expression, and DNA synthesis. GAP overexpression did not block responses to serum or fluoroaluminate. Moreover, not all biochemical events elicited by TPA were affected by GAP overexpression, as increased glucose uptake and phosphorylation of MARCKS, a major PKC substrate, occurred normally. Reduction of GAP expression to near normal levels restored the ability of the cells to activate p42mapk in response to TPA. These findings suggest that ras and GAP together play a key role in a PKC-dependent signal transduction pathway which leads to p42mapk activation and cell proliferation.

Regulation of tetradecanoyl phorbol acetate-induced responses in NIH 3T3 cells by GAP, the GTPase-activating protein associated with p21c-ras

Proteins of the ras family of oncogenes have been implicated in signal transduction pathways initiated by protein kinase C (PKC) and by tyrosine kinase oncogenes and receptors, but the role that ras plays in these diverse signalling systems is poorly defined. The activity of ras proteins has been shown to be controlled in part by a cellular protein, GAP (GTPase-activating protein), that negatively regulates p21c-ras by enhancing its intrinsic GTPase activity. Thus, overexpression of GAP provides a tool for determining the step(s) in signal transduction dependent on p21c-ras activity. In this paper, we report that overexpression of GAP blocks the phorbol ester (tetradecanoyl phorbol acetate [TPA])-induced activation of p42 mitogen-activated protein kinase (p42mapk), c-fos expression, and DNA synthesis. GAP overexpression did not block responses to serum or fluoroaluminate. Moreover, not all biochemical events elicited by TPA were affected by GAP overexpression, as increased glucose uptake and phosphorylation of MARCKS, a major PKC substrate, occurred normally. Reduction of GAP expression to near normal levels restored the ability of the cells to activate p42mapk in response to TPA. These findings suggest that ras and GAP together play a key role in a PKC-dependent signal transduction pathway which leads to p42mapk activation and cell proliferation.

Ha-ras oncogene activation in mammary glands of N-methyl-N-nitrosourea-treated rats genetically resistant to mammary adenocarcinogenesis

A single dose of N-methyl-N-nitrosourea given to sexually immature female Buf/N rats produces a high incidence of mammary adenocarcinomas. A large percentage of these tumors contain the Ha-ras oncogene, activated by a G----A transition at the second nucleotide of codon 12. Copenhagen rats, on the other hand, are completely resistant to mammary tumor induction by a number of carcinogens, including N-methyl-N-nitrosourea. Here we show, using a sensitive method involving PCR, that codon 12 Ha-ras mutations occur in the mammary glands of both Buf/N and Copenhagen rats 30 days after N-methyl-N-nitrosourea treatment. These mutations were evenly distributed among individual mammary glands and were present in purified mammary epithelial cells. In Buf/N rats, the fraction of cells containing a mutated Ha-ras allele increased by a factor of 10-100 between 30 and 60 days, whereas in Copenhagen rats, there was no such increase during this time period. We conclude that the resistance of the Copenhagen rat to mammary carcinogenesis is not due to a defect in initiation but rather appears to be due to the inability of cells containing a mutated ras allele to undergo sustained clonal expansion.

ras oncogene activation in mammary carcinomas induced by N-methyl-N-nitrosourea in Copenhagen rats

The occurrence of Ha-ras and Ki-ras oncogenes was investigated in mammary tumors produced by treating genetically resistant Copenhagen (Cop) rats with N-methyl-N-nitrosourea. G35-->A codon 12 mutations in both Ha-ras and Ki-ras genes were analyzed by a polymerase chain reaction/liquid hybridization and gel retardation assay. More than half of the adenocarcinomas analyzed contained an activated Ha-ras gene. This was also the predominant mutation in similar tumors from susceptible Buf/N rats, suggesting a common mechanism of initiation. In contrast, only two of 15 mammary adenosquamous carcinomas from the Cop rats contained an activated Ha-ras gene, suggesting a different initiation mechanism for most of these tumors. Ki-ras activation was found in none of five and one of five adenocarcinomas from Buf/N and Cop rats, respectively, and in none of 13 adenosquamous carcinomas from Cop rats. These results suggest that Ki-ras activation does not play a major role in the initiation of the mammary tumors.

Role of mutations at codon 61 of the c-Ha-ras gene during diethylnitrosamine-induced hepatocarcinogenesis in C3H/He mice

Liver tumors of certain strains of mice frequently contain mutations at codon 61 of the c-Ha-ras gene. In our study, we investigated the significance of these mutations in the carcinogenic process. Male C3H/He mice received a single injection of diethylnitrosamine (DEN) on day 15 after birth, and groups of animals were killed at various time intervals between 11 and 52 wk after treatment. At the earlier time points (11-29 wk), we analyzed microdissected tissue from precancerous glucose-6-phosphatase-deficient liver lesions larger than approximately 200 microns in diameter, for the presence and pattern of c-Ha-ras codon 61 mutations. In parallel, the growth characteristics of these liver lesions were studied by pulse labeling with [3H]thymidine and by determining the size distribution of the lesions. At the later time points (42-52 wk after DEN treatment), liver tumors were dissected and also analyzed for the presence of c-Ha-ras mutations. We found mutations to be already present in some of the enzyme-altered liver lesions at weeks 11-29, suggesting that the mutations occurred early in the carcinogenic process. Whereas about 10% of the precancerous focal liver lesions showed mutations in the c-Ha-ras gene, the mutation frequency was increased to about 50% in the later-appearing hepatocellular adenomas and carcinomas, suggesting that c-Ha-ras codon 61 mutations may provide a selective advantage to the mutated cell clones.

Frequent activation of the Ki-ras oncogene at codon 12 in N-methyl-N-nitrosourea-induced rat prostate adenocarcinomas and neurogenic sarcomas

Rat neoplasms induced by methylating carcinogens frequently contain ras genes activated by a single point mutation. Rat prostatic tumors induced by a combination of a single injection of N-methyl-N-nitrosourea (MNU) and long-term treatment with testosterone were examined for the presence of such activating point mutations in ras genes. These tumors, which arose exclusively in the dorsolateral prostate, included both adenocarcinomas and sarcomas. Activating mutations in codon 12 of the Ki-ras gene were found in 7 of 10 carcinomas and 4 of 5 sarcomas, using selective oligonucleotide hybridization analysis of DNA amplified by the polymerase chain reaction (PCR). However, no mutated Ha-ras oncogenes were detected. The presence of PCR-engineered Hphl restriction sites created by the existence of a G35----A mutation in the rat Ki-ras oncogene identified the mutation as a GC----AT transition at the second position of codon 12. Production of O6-methylguanine adducts in the Ki-ras codon 12 followed by base mispairing during replicative DNA synthesis is thus the likely molecular mechanism of initiation of prostatic carcinogenesis by MNU in the rat. Three of the four sarcomas positive for the Ki-ras G35----A mutation were immunohistochemically defined as of Schwann cell origin, indicating that involvement of the ras gene family is possible in tumorigenesis of this cell lineage. Loss of the wild-type Ki-ras allele was also observed in all four of these sarcomas.

Ki-ras mutations in spontaneous and chemically induced renal tumors of the rat

A high frequency of point mutations at codon 12 of the Ki-ras gene has previously been reported for rat kidney mesenchymal tumors induced by methylating N-nitroso compounds. In this study, we analyzed renal tumors with divergent histogenesis, i.e., mesenchymal tumors (sarcomas), cortical epithelial tumors (carcinomas), and embryonal tumors (nephroblastomas). Renal mesenchymal tumors and carcinomas were induced in juvenile or young adult Wistar rats by a single dose of N-nitrosodimethylamine (NDMA) while nephroblastomas were induced in Nb hooded rats by a single transplacental dose of N-nitrosoethylurea (NEU). Nephroblastomas developing spontaneously in WAB/Not rats were also examined. Amplification of Ki-ras sequences from formalin-fixed, paraffin-embedded tissue by the polymerase chain reaction was followed by direct DNA sequencing. GGT----GAT point mutations at codon 12 of the Ki-ras gene were found in 9 of 12 (75%) renal mesenchymal tumors and in 9 of 12 (75%) cortical epithelial tumors induced by NDMA. Even higher incidences were observed in nephroblastomas (8/8; 100%) induced by NEU and in spontaneous nephroblastomas (10/11; 91%). These results indicate that Ki-ras mutations are frequent events during the development of kidney tumors irrespective of their histogenesis and suggest that they may play an important role in renal carcinogenesis in rats. These data further indicate that mutational activation of Ki-ras proto-oncogenes in carcinogen-induced rat kidney tumors occurs in a tissue-specific, rather than cell-specific, manner.

Mouse transgenes in human cells detect specific base substitutions

We describe a system of transgenic human cell lines that detects and identifies specific point mutations at defined positions within a gene. The target transgenome is a mouse adenine phosphoribosyltransferase (APRT) gene rendered nonfunctional by introduction of a substitution at either of two bases that comprise a splice acceptor site. Reversion at a mutated site results in the expression of wild-type mouse APRT and consequent growth of APRT+ transgenic cell colonies. Site-specific reversion to wild-type sequence is confirmed by regeneration of a previously destroyed diagnostic Pst I site. Two independent cell clones, each with mutant transgenomes bearing an A----G transition, exhibited an up to 7500-fold, dose-dependent induction of reversion following treatment with ethyl methanesulfonate. Treatment of these clones with 2-aminopurine resulted in no induction of revertants. In contrast, another transgenic cell clone, bearing a G----A transition, reverted as a consequence of 2-aminopurine, but not ethyl methanesulfonate, treatment. These data confirm for human cells the proposed mechanisms of action of these mutagens and provide evidence for the utility of our site-specific reversion method for mutagenesis studies.