Selective pressures and ras activation in carcinogenesis

Green tea polyphenols reverse cooperation between c-Rel and CK2 that induces the aryl hydrocarbon receptor, slug, and an invasive phenotype

Exposure to and bioaccumulation of lipophilic environmental pollutants, such as polycyclic aromatic hydrocarbons (PAHs), has been implicated in breast cancer. Treatment of female rats with the prototypic xenobiotic PAH 7,12-dimethylbenz(a)anthracene (DMBA) induces mammary tumors with an invasive phenotype. Here, we show that green tea prevents or reverses loss of the epithelial marker E-cadherin on the surface of DMBA-induced in situ cancers. To investigate the mechanism(s) leading to a less invasive phenotype, the effects of the green tea polyphenol epigallocatechin-3 gallate (EGCG) on mammary tumor cells were assessed. EGCG reversed epithelial to mesenchymal transition (EMT) in DMBA-treated NF-kappaB c-Rel-driven mammary tumor cells and reduced levels of c-Rel and the protein kinase CK2. Ectopic coexpression of c-Rel and CK2alpha in untransformed mammary epithelial cells was sufficient to induce a mesenchymal gene profile. Mammary tumors and cell lines derived from MMTV-c-Rel x CK2alpha bitransgenic mice displayed a highly invasive phenotype. Coexpression of c-Rel and CK2, or DMBA exposure induced the aryl hydrocarbon receptor (AhR) and putative target gene product Slug, an EMT master regulator, which could be reversed by EGCG treatment. Thus, activation of c-Rel and CK2 and downstream targets AhR and Slug by DMBA induces EMT; EGCG can inhibit this signaling.

Role of Ha-ras activation in superficial papillary pathway of urothelial tumor formation

Urothelial tumors develop along two distinctive phenotypic pathways (superficial papillary non-invasive tumors versus flat carcinoma in situ lesions), with markedly different biological behavior and prognosis. Although multiple genetic alterations have been identified in human bladder cancer, their cause-effect relationship with the two pathways has not been firmly established. Using a urothelium-specific promoter of the uroplakin II gene, we showed earlier in transgenic mice that the urothelial expression of SV40T antigen, which inactivates p53 and pRb, induced carcinoma in situ and invasive and metastatic bladder cancer. In striking contrast, we demonstrate here that the urothelial expression of an activated Ha-ras in transgenic mice caused urothelial hyperplasia and superficial papillary non-invasive bladder tumors. These results provide strong, direct experimental evidence that the two phenotypical pathways of bladder tumorigenesis are caused by distinctive genetic defects. Our results indicate that Ha-ras activation can induce urothelial proliferation in vivo; and that urothelial hyperplasia is a precursor of low-grade, superficial papillary bladder tumors. Our transgenic models provide unique opportunities to study the detailed molecular events underlying different types of bladder neoplasms, and can serve as useful preclinical models for evaluating the in vivo efficacy of preventive and therapeutic agents that act on various signaling pathways in bladder cancer.

Analysis of ras gene mutational spectra in epidermal papillomas from K6/ODC transgenic mice

In standard mouse strains, a high proportion (more than 90%) of epidermal tumors produced by initiation with 7,12-dimethylbenz[a]anthracene and promotion with a variety of chemical agents contain an activating mutation in codon 61 (A182-->T) of the c-Ha-ras gene. We analyzed the ras mutational spectra in 69 tumors induced by DMBA in a unique transgenic model, the K6/ODC mouse. In this model, low-dose DMBA treatment is sufficient per se for tumor induction, so tumor promotion with chemical agents is not required. In contrast to previous studies in standard mouse strains, our study showed that less than 50% of epidermal tumors from K6/ODC mice contained an activating codon 61 c-Ha-ras mutation (A182-->T). This result was obtained in mice initiated either as newborns (when the transgene is not expressed) or as adults (when the transgene is fully expressed). Analysis of other codon hot-spots and other ras genes revealed the presence of three codon 12 and 20 codon 61 (A182-->T) mutations in the c-Ki-ras gene in the 36 tumors that did not have c-Ha-ras mutations. We concluded that promotion in this model, by means of constitutive ornithine decarboxylase expression, causes the clonal expansion of a population of initiated cells not promoted by chemical agents.

Overexpression of elongation factor-1gamma protein in colorectal carcinoma

BACKGROUND

Elongation factor-1 (EF-1) is a cellular protein that plays a role in protein synthesis by mediating the transfer of aminoacyl-tRNA to 80S ribosomes. It is comprised of four subunits: alpha, beta, gamma, and delta. EF-1gamma is a substrate for the maturation-promoting factor, which determines entry into the M-phase of the cell cycle in all eukaryotic cells. Previously, the authors showed that EF-1gamma RNA is overexpressed in a high proportion of colorectal carcinomas. At that time, there were no antibodies to EF-1gamma, so the EF-1gamma protein could not be examined. Because levels of RNA do not always parallel the levels of the protein it encodes, it was important to develop antibodies to EF-1gamma to examine its expression at the protein level in colorectal carcinoma.

METHODS

Twenty-nine patients undergoing surgical resection for colorectal adenocarcinoma were studied. A polyclonal antibody to EF- 1gamma in rabbit was prepared. Tumors and normal-appearing mucosa distant from the tumor (> or = 10 cm) were obtained from each patient. Cytosolic proteins were extracted from the tissues and examined by Western blot analysis with the EF-1gamma antibody. Colonic tumors also were studied by immunohistochemical analysis with another EF-1gamma polyclonal antibody.

RESULTS

Using Western blot analysis, the authors observed greater expression of EF-1gamma in the tumors than in the more distal normal-appearing mucosa. Overexpression was not observed in the patients with the two Dukes Stage A tumors, but was observed in four of ten patients with Dukes Stage B tumors, seven of eight patients with Dukes Stage C tumors, and six of nine patients with Dukes Stage D tumors. Overall, 17 of 29 patients (59%) were found to have overexpression of EF-1gamma. Using immunohistochemical analysis, EF-1gamma protein was shown to be located predominantly in tumor epithelium rather than the stroma or infiltrating mononuclear cells.

CONCLUSIONS

Previous studies showed that EF-1gamma mRNA frequently is overexpressed in colorectal adenocarcinoma. This study showed that EF-1gamma also was overexpressed at the protein level in colorectal adenocarcinoma relative to more distal normal-appearing mucosa from the same patient. Immunohistochemical analysis demonstrated that this protein was expressed predominantly in the tumor epithelial cells and therefore was not derived from cells involved in the desmoplastic response.

Effect of dietary fat on codon 12 and 13 Ha-ras gene mutations in 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine-induced rat mammary gland tumors

Activating mutations in and expression of the Ha-ras gene were examined in benign and malignant female Sprague-Dawley rat mammary gland tumors induced by the heterocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and promoted by a diet high in polyunsaturated fat. Ha-ras mutations were detected in codons 12 and 13 by selective polymerase chain reaction amplification of mutated sequences and nucleotide sequencing. The percentage of Ha-ras mutations in carcinomas from PhIP-treated rats was significantly higher in rats on a low-fat diet than in rats on a high-fat diet (82% (nine of 11) vs 26% (seven of 27), respectively, P < 0.01). In addition, whereas 56% of the carcinomas with Ha-ras mutations from rats on a low-fat diet carried double Ha-ras mutations, none of the carcinomas from rats on a high-fat diet had double mutations. Ha-ras mutations were also detected in benign tumors (largely adenomas) induced by PhIP in rats on different diets; two of eight and three of four benign tumors examined from rats on low-fat and high-fat diets, respectively, had Ha-ras mutations, suggesting that activating Ha-ras mutations alone are not sufficient for PhIP-induced tumors to become malignant. No differences were observed in the level of Ha-ras mRNA expression in the different groups. In our animal model, a high-fat diet increased the incidence and percentage of malignant PhIP-induced mammary gland tumors yet decreased the percentage of carcinomas showing Ha-ras mutations. Thus, the complement of genetic alterations associated with PhIP-induced mammary gland carcinogenesis is probably altered by the level of dietary fat.

Effects of mutationally activated Ha-ras on c-fos expression kinetics in rat tracheal epithelial cells

The rat tracheal implant model was used to characterize the role of activated Ha-ras in the neoplastic progression of heterogeneous rat tracheal epithelial (RTE) cell populations. An activated Ha-ras-containing cell line, RTE 2-2, and its subclone, RTE 2-2n, which possesses only Ha-ras proto-oncogene alleles, were studied to determine whether activated ras could interact with the downstream signal transduction targets fos and myc and alter their cell-cycle-dependent expression in vitro. Transformed RTE cell lines with activated Ha-ras displayed earlier fos expression, with a peak at 15 min after serum stimulation. These cell lines also displayed a more accelerated loss of fos mRNA than seen in cells without activated Ha-ras. The effects on fos expression kinetics were seen only in cell lines with activated ras and were not related to the transformed phenotype of the cells. No change in myc expression kinetics were observed in any RTE cell line. These results suggest that mutations in ras can lead to alterations in nuclear components of the ras signaling pathway at the level of gene transcription.

ras may mediate mammary cancer promotion by high fat

High fat consumption has been implicated as a risk factor for breast cancer. Experimental mammary carcinogenesis studies have demonstrated that the effect of high fat consumption is mainly exerted on the postinitiation stage of the disease process. We report data that have recalled in the formulation of a new hypothesis about the effect of dietary fat on mammary carcinogenesis, namely, that it promotes the development of a subpopulation of cells lacking a specific pathogenetic characteristic. In comparison with animals fed a low-fat diet, female Sprague-Dawley rats fed high-fat diets during the promotional stage developed significantly more (number and proportion) 1-methyl-1-nitrosourea-induced mammary adenocarcinomas that did not contain a codon 12 GGA-->GAA mutation in the c-Ha-ras protooncogene. The effect was independent of the types of fat fed, i.e., corn oil vs. fish oil. A model is presented to account for the preferential promotional effect of high fat consumption on 1-methyl-1-nitrosourea-initiated mammary epithelial cells. The hypothesis that the level of dietary fat consumed affects the proportion of mammary carcinomas that occur with a particular pathogenetic characteristic, in this case, the presence or absence of a Ha-ras point mutation, has important implications on the direction of future investigations concerning fat and cancer risk.

Role of H-ras in the malignant progression of rat tracheal epithelial cells

The effect of an activated H-ras oncogene on the progression of neoplasia was studied in transformed rat tracheal epithelial cells. Nude mouse tumours produced by injection of these cells exhibited a higher fraction of cells containing the mutant ras gene that did the injected cells, while a subclone that lacked the ras mutation was much less tumorigenic than parental cells. Serial passage of one cell line containing a ras mutation resulted in an increase in the fraction of ras-mutated cells, which suggests that, in this line, ras activation may confer a selective advantage in vitro as well. However, this was not seen in another ras-containing line, suggesting the importance of alternative pathways in malignant progression of rat tracheal epithelial cells.

Spontaneous mutation at codon 61 of the Ha-ras gene in the nascent liver of B6C3F1, C3H/He and C57BL/6 mice

DNA was isolated from the liver of young B6C3F1, C3H/He and C57BL/6 mice, 6-9 weeks old. A portion of exon 2 of Ha-ras was amplified by PCR allele-specific amplification. The PCR product was identified by (a) size, (b) presence of a diagnostic restriction site, and (c) direct sequencing. Our results indicate that nascent mouse liver bears a subpopulation of cells which contain a mutation in codon 61 of Ha-ras, specifically an A to G transition at position 2. Therefore, the detection of this mutation in chemically induced mouse liver tumors does not demonstrate that the chemical in question acts as a mutagen. It might act by a nongenotoxic mechanism, i.e., by facilitating a clonal expression of cells bearing this spontaneous mutation.

7,12-Dimethylbenz[a]anthracene-induced mouse keratinocyte malignant transformation independent of Harvey ras activation

Independent clones of mouse keratinocytes initiated in vitro gave rise to tumor phenotypes typical of mouse skin multistage carcinogenesis and histologically similar to human tumors of the skin, and head and neck. High-molecular-weight genomic DNAs isolated from two 7,12-dimethylbenz[a]anthracene (DMBA)-initiated murine epithelial carcinoma cell lines and one papilloma cell line were examined for transforming activity by transfection into NIH3T3 cells. DNAs from each of these cell lines resulted in the formation of foci morphologically unlike foci containing an activated c-Ha-ras oncogene. Following polymerase chain reaction amplification of the c-Ha-ras gene, Xba I restriction analysis and oligonucleotide differential hybridization did not detect 61st, 12th, or 13th codon mutations. Southern and Northern analysis confirmed that the normal c-Ha-ras gene was not activated by amplification or overexpression. These results provide evidence that 7,12-dimethylbenz[a]anthracene-induced malignant transformation of murine keratinocytes occurred independent of point mutations associated with c-Ha-ras activation. The absence of an activated c-Ha-ras oncogene in these cell lines distinguishes our model from other mouse models of carcinogenesis and may provide a model for functional genetic changes during initiation and progression of human epithelial cancers.

Comparison of K-ras oncogene activation in pancreatic duct carcinomas and cholangiocarcinomas induced in hamsters by N-nitrosobis(2-hydroxypropyl)amine

The presence of K-ras point mutations in pancreatic duct carcinomas and cholangiocarcinomas induced by N-nitrosobis(2-hydroxypropyl)amine (BHP) in Syrian hamsters was investigated by single-strand conformation polymorphism analysis of polymerase chain reaction products from frozen fresh materials in order to clarify the K-ras mutation rates in those two carcinomas induced simultaneously by one carcinogen, BHP. In the examined pancreatic duct carcinomas, 10 out of 16 were positive for a mutation in codon 12 while 3 out of 12 cholangiocarcinomas demonstrated mutation of K-ras gene. G-to-A transition was detected in the second position of codon 12 in both pancreatic carcinomas and cholangiocarcinomas. These results suggest that the role of genetic alteration in carcinogenesis may differ with the target organ, even when initiation is with the same carcinogen.

Ki-ras mRNA regulation in untransformed mouse lung cells

Although the Ki-ras gene is an often-observed transforming gene in lung tumors, little is understood of the factors that regulate the expression of the normal gene in lung cells. Therefore, we used untransformed mouse lung epithelial cells to determine the effect of serum, growth factors, and cell confluence on the regulation of Ki-ras mRNA expression. In subconfluent cells synchronized by 24-h serum deprivation, the refeeding of media containing serum resulted in the expression of both Ki-ras and H4 histone mRNA. No change in the expression of either gene was observed in cells refed with media alone. In confluent cell cultures, the refeeding of media with serum had no effect on the expression of these genes, suggesting that cell-density-dependent mechanisms can override the serum-induced stimuli for Ki-ras and H4 histone mRNA expression. Confluent cells expressed low but detectable Ki-ras mRNA levels consistent with constitutive expression of this gene independent of its role in mitogenic stimuli. EGF (10 ng/mL) and TGF-alpha (10 ng/mL) were found to induce transient increases in Ki-ras mRNA but large increases in H4 histone mRNA levels. Similarly, 48-h-conditioned media obtained from two transformed mouse lung cells containing activated Ki-ras genes and overexpressing Ki-ras mRNA were observed to increase both Ki-ras and H4 histone mRNA in the untransformed mouse lung cells. The expression of these genes in mouse lung cells therefore appeared to be linked to stimuli provided by specific growth factors as well as by autocrine factors elaborated by lung tumor cells. The studies reported here provide insight into the regulation of the Ki-ras mRNA in untransformed lung cells and the factors that may contribute to the elevated levels of Ki-ras mRNA often observed in transformed lung cells.

ras protein p21 processing enzyme farnesyltransferase in chemical carcinogen-induced murine skin tumors

Farnesylation of ras protein p21 is crucial for the protein's membrane localization, which is essential for its cell-transforming activity, which in turn is thought to be critical for the ultimate induction of cancer. The cytosolic enzyme farnesyltransferase plays a major role in posttranslational modification of p21, but the level of farnesyltransferase activity in mammalian tumors and its relationship to the processing of cytosolic p21 that leads to tumorigenesis are unknown. We report here that farnesyltransferase activity was significantly higher in chemical carcinogen-induced benign skin papillomas in SENCAR mice than in the epidermises of control animals. The enzyme is primarily epidermal in origin, and kinetic studies with cytosol from epidermis and papillomas showed that the reaction was linear with respect to time, substrate concentration, and protein content. Skin papillomas showed significantly elevated levels of both cytosolic and membrane-bound Ha-ras p21, whereas far lesser cytosolic and almost negligible amounts of membrane-bound p21 were present in the epidermis of control mice. There was a positive correlation between increased enzyme activity in papilloma cytosol and the processing of overexpressed cytosolic Ha-ras p21 for its localization to membrane.

Further analysis of c-Ha-ras mutations in papillomas initiated by several polycyclic aromatic hydrocarbons and papillomas from uninitiated, promoter-treated skin in SENCAR mice

In this study we analyzed the mutations in c-Ha-ras from skin papillomas initiated with benzo[a]pyrene (B[a]P), 7-methylbenz[a]anthracene (7-MBA), and 10-fluoro-7-methylbenz[a]anthracene (10-F-7-MBA) and from papillomas induced by treatment with tumor promoter alone. Among the papillomas induced by treatment with tumor promoter alone, 56% (nine of 16) had mutations in c-Ha-ras. These mutations were found primarily in codon 61 and included both A182-->T and A182-->G mutations. In addition, one promoter-induced tumor had a G35-->A mutation in codon 12, and one had a G37-->C mutation in codon 13. The other promoter-induced papillomas did not have detectable mutations in codons 12, 13, or 61. Most of the B[a]P-initiated papillomas (77%; 10 of 13) did not have detectable mutations in c-Ha-ras codons 12, 13, or 61. However, three of these B[a]P-initiated papillomas had c-Ha-ras codon 13 mutations; one had a G37-->C transversion and two had G38-->T transversions. Most of the 7-MBA-initiated tumors and all of the 10-F-7-MBA-initiated tumors had an activated c-Ha-ras gene [nine of 10 (90%) and 11 of 11 (100%), respectively]. These mutations were almost exclusively A182-->T transversions in codon 61 except for two 7-MBA-initiated papillomas that had G37-->C transversions in codon 13. The results suggest that more than one mechanism may contribute to activation of c-Ha-ras by polycyclic aromatic hydrocarbons (PAHs) in mouse skin. Furthermore, the absence of c-Ha-ras mutations in most B[a]P-initiated papillomas, as well as in a significant fraction of those induced by tumor promoter alone, suggests that there may be other molecular targets involved in tumor initiation by PAHs in mouse skin.