Resistance to oncogenic transformation in revertant R1 of human ras-transformed NIH 3T3 cells

Sensitivity of transformed fibroblasts for intercellular induction of apoptosis is determined by their transformed phenotype

Intercellular induction of apoptosis defines a potential control mechanism of oncogenesis. It is based on induction of apoptosis in transformed fibroblasts by neighboring nontransformed fibroblasts. Transforming growth factor type beta (TGF-beta) represents the initial triggering molecule to induce nontransformed cells to release apoptosis-inducing factors. To test whether sensitivity for intercellular induction of apoptosis is directly dependent on the transformed phenotype, v-src-transformed rat fibroblasts and emerging revertants were tested for their sensitivity. All transformed cell clones were sensitive, whereas all revertant clones had lost their sensitivity in parallel with the loss of the transformed phenotype. In addition, revertants had regained the potential to induce apoptosis in transformed cells. Sensitivity to intercellular induction of apoptosis is therefore directly dependent on the transformed phenotype, whereas the ability to induce apoptosis is a specific feature of nontransformed fibroblasts.

Invasion and metastasis

The invasive character of squamous cell carcinoma of the head and neck represents a major challenge to the clinician since most often these tumors require extensive surgical resection impairing important physiological functions including speech and swallowing. Additionally, in many cases costly reconstructive surgery is required to repair the adverse cosmetic effects of the resective surgery. Thus, there is an urgent need to understand the molecular mechanism(s) which underlie the local and regional spread of this disease. Since the ability of tumor cells to invade into surrounding structures requires hydrolytic action much effort has been spent on identifying the hydrolases involved in this process. Some of the enzymes which have been implicated in the spread of head and neck cancer include the urokinase-type plasminogen activator and several members of the collagenase family such as type I and IV collagenases and the stromelysins synthesized either by the tumor cells or in the surrounding fibroblasts. More recent studies have addressed the mechanism(s) by which these hydrolases are overexpressed in invasive cancer. In the tumor cells themselves, work has focused on defining the transcriptional requirements for enzyme synthesis and addressing how the appropriate transcription factors are activated by signal transduction pathways. In contrast, where the hydrolases (e.g. stromelysin-2 and stromelysin-3) are produced by the fibroblasts, current investigations are directed at identifying tumor-derived growth factors which lead to the inducible expression of the enzymes in the stromal cells. The ultimate goal of these studies is to develop novel therapeutic interventions which decrease the invasive capacity of head and neck cancer leading to longer survival times and enhanced quality of life for patients afflicted with this disease.

Involvement of a mitogen-activated protein kinase signaling pathway in the regulation of urokinase promoter activity by c-Ha-ras

The expression of the urokinase-type plasminogen activator, which plays a crucial role in tissue remodeling by controlling the synthesis of the broadly acting plasmin serine protease, is regulated by several tyrosine kinases. Since the actions of these tyrosine kinases is dependent on the activation of ras proteins, we undertook a study to identify signaling events downstream of ras responsible for the stimulation of urokinase promoter activity. Transient expression of an activated c-Ha-ras in OVCAR-3 cells, which do not harbor the mutated oncogene, led to a dose-dependent trans-activation of the urokinase promoter. A sequence residing between -2109 and -1964 was critical for the stimulation of the urokinase promoter by c-Ha-ras. Mutation of an AP-1 and a PEA3 site at -1967 and -1973, respectively, or the co-expression of a transactivation domain-lacking c-jun substantially impaired the ability of c-Ha-ras to stimulate urokinase promoter activity. The induction of the urokinase promoter by ras was completely blocked by expression of a dominant negative c-raf expression vector and substantially reduced in cells made to co-express a catalytically inactive mitogen-activated protein kinase kinase. Further, the expression of an ERK1/ERK2-inactivating phosphatase (CL100) abrogated the stimulation of the urokinase promoter by c-Ha-ras. These data argue for a role of a mitogen-activated protein kinase-dependent signaling pathway in the regulation of urokinase promoter activity by ras.

Functions of [His321]gelsolin isolated from a flat revertant of ras-transformed cells

A mutant gelsolin, [His321]gelsolin, was isolated from R1, a flat revertant of human activated c-Ha-ras oncogene-transformed NIH/3T3 cells (EJ-NIH/3T3) produced by ethylmethanesulfonate treatment. [His321]Gelsolin has a histidine instead of a proline residue at position 321 and suppresses the tumorigenicity of EJ-NIH/3T3 cells when it is constitutively expressed [Müllauer, L., Fujita, H., Ishizaki, A. & Kuzumaki, N. (1993) Oncogene 8, 2531-2536]. To investigate the biochemical consequences of the amino acid substitution of His321, we expressed the [His321]gelsolin and wild-type gelsolin in Escherichia coli, purified them, and analyzed their effects on actin, polyphosphoinositol lipids and phospholipase C. [His321]Gelsolin has decreased actin-filament-severing activity and increased nucleating activity compared with wild-type gelsolin in vitro. Furthermore, compared to wild-type gelsolin both nucleation and severing by [His321]gelsolin are inhibited more strongly by the phosphoinositol lipids phosphatidylinositol 4-phosphate (PtdInsP) and phosphatidylinositol 4,5-bisphosphate (PtdInsP2). In addition, [His321]gelsolin inhibits PtdInsP2 hydrolysis by phospholipase C gamma 1 more strongly than wild-type gelsolin in vitro because of its higher binding capacity for phosphoinositol lipid. Gelsolin has six homologous amino acid repeats called S1-S6. Our results suggest that the segment S3 which contains the mutation is functionally relevant for regulation of gelsolin's activities even though the relevant actin-binding domains are in segments 1, 2, and 4-6, and that the region around the residue 321 may contain a phosphoinositol-lipid-binding site. Altered functions of [His321]gelsolin might be important for the loss of tumorigenicity of the ras-transformed cells.

Suppression of ras-mediated transformation. Differential expression of genes encoding extracellular matrix proteins in normal, transformed and revertant cells

A subtraction hybridization technique was used to identify genes specifically expressed in phenotypic revertants derived from cells transformed by the H-ras oncogene. The expression of genes coding for components of the extracellular matrix appears to be frequently down-regulated in transformed cells. Partial restoration is associated with the reexpression of the normal phenotype in revertants.

Simple system for isolation of cellular and viral mutants for transformation by retrovirus

To investigate the cellular mechanism of transformation by retroviruses, we established a system for isolation of cellular and viral mutants for transformation of a rat cell line. Mutagenized untransformed cells of this line were infected with recombinant murine retrovirus containing the src gene of Rous sarcoma virus and the selective marker gene, neo. After reaching confluence, cells transformed by the src gene tend to overgrow and die. Utilizing this property of src transformed rat cells and the selective marker gene, we could easily select untransformed cell clones containing the retrovirus genome. Expression of the src gene product in the flat clones selected was examined by in vitro assay of src kinase activity. To determine whether the mutations of these flat clones were viral or cellular, the susceptibilities of the clones to transformation were examined after superinfection with the wild-type virus and also characterized the retroviruses recovered from these clones. With this system, two novel clones were isolated. One had a defect in viral information affecting the transformed phenotype, but still retained src kinase activity like fully transformed cells. The other showed low src kinase activity but retained wild-type transforming virus, suggesting that a cellular gene involved in viral gene expression was mutated.

Re-transformation of non-transformed hybrids between c-myc-activating mouse plasmacytoma cells and normal fibroblasts by transfection with activated c-Ha-ras but not c-myc

In a mouse plasmacytoma S194, c-myc oncogene is rearranged with Ig gene by chromosomal translocation and is consequently activated. We previously reported that transformation of phenotype and expression of rearranged c-myc were repressed in independently isolated hybrid clones, I-1 and IV-10, between S194 and normal fibroblasts. In order to investigate the relationship between transformation of phenotype and oncogene expression, transcriptionally enhanced c-myc or activated c-Ha-ras was transfected into I-1 or IV-10I, a subclone of IV-10. Transfectants expressing high levels of c-myc were found to retain the non-transformed phenotypes. On the other hand, transfectants expressing activated c-Ha-ras showed the transformed phenotypes. These results suggest that enhanced expression of c-myc is not sufficient for re-transformation of the non-transformed hybrid clones between c-myc-activating plasmacytoma cells and normal fibroblasts, but expression of activated c-Ha-ras could diminish or overcome the tumor-suppressive activity of normal fibroblasts.

Identification of genes that exhibit increased expression after flat reversion of NIH/3T3 cells transformed by human activated Ha-ras oncogene

By differential hybridization, we have isolated 14 cDNA clones corresponding to genes that are more highly expressed in the flat revertant cell line R1 than in the parental human Ha-ras oncogene-transformed NIH/3T3 cell line (EJ-NIH/3T3). From cross-hybridization experiments, we determined that 5 sequence families accounted for the 14 clones. DNA sequencing revealed that four out of five selected cDNA clones represented mitochondrial genes (cytochrome b, cytochrome c oxidase subunit II, NADH dehydrogenase subunits 1 and 4, respectively), whereas one cDNA clone was homologous to the alpha 2 (type I collagen gene. Although a Southern blot analysis of the studied cell lines showed similar copy numbers of mitochondrial genomes, the transcript levels of the mitochondrial genes were high in R1, intermediate in NIH/3T3 and low in EJ-NIH/3T3 and partially revertant R2 cell lines. alpha 2 (type I) collagen mRNA levels were high in R1 and NIH/3T3, intermediate in R2 and low in EJ-NIH/3T3 cells. These results suggest that a complex alteration of the expression of mitochondrial and extracellular matrix components may be closely associated with the flat reversion of the transformed cells.

Isolation of cellular revertants from a rat cell line transformed by the E6 and E7 genes of human papillomavirus type 16

Three revertants defective in the ability to form colonies in semisolid medium were isolated from a rat cell line transformed by the E6 and E7 genes of human papillomavirus type 16 (HPV16). These revertants appeared to be defective in a cellular factor(s) necessary for transformation by HPV16-E6E7 genes since they still expressed a comparable amount of HPV16-E6E7 mRNA and E7 protein to the parental cells, harbored rescuable transforming virus, and were resistant to retransformation by HPV16-E6E7 genes. All these reverted phenotypes of the three mutants were recessive on somatic cell hybridization with normal cells, because all the hybrids showed transformed phenotypes.

Elevated gelsolin and alpha-actin expression in a flat revertant R1 of Ha-ras oncogene-transformed NIH/3T3 cells

Expressions of gelsolin and alpha-actin have been investigated in a revertant cell line R1 and compared with the parental human activated Ha-ras oncogene-transformed NIH/3T3 (EJ-NIH/3T3), untransformed NIH/3T3 and partially revertant R2 cells. Gelsolin mRNA expression was strongest in R1 cells, intermediate in R2 and NIH/3T3 cells, and low in EJ-NIH/3T3 cells. Southern blot analysis gave neither signs of gross rearrangements nor amplification of the gelsolin gene. alpha-actin mRNA expression was restored in R1 cells to the level of NIH/3T3 cells. In R2 and EJ-NIH/3T3 cell lines, no alpha-actin transcript was detected. High gelsolin expression and restoration of alpha-actin expression may be associated with the acquirement of flat morphology and ordered cell growth pattern, which imply loss of tumorigenicity of R1 cells.

A specific protein, p92, detected in flat revertants derived from NIH/3T3 transformed by human activated c-Ha-ras oncogene

Total proteins from a mouse embryo fibroblast cell line NIH/3T3, NIH/3T3 cells transformed by human activated c-Ha-ras (EJ-ras) oncogene (EJ-NIH/3T3), and the two flat revertant cell lines, R1 and R2, were analyzed by two-dimensional gel electrophoresis (IEF and NEPHGE). Several hundred polypeptides were resolved as seen by silver staining. Common alterations in four polypeptide spots were observed in the revertants when compared with NIH/3T3 and EJ-NIH/3T3 cells. In these alterations, a new polypeptide spot p92-5.7 (designated by molecular weight x 10(-3) and pI) was detected only in the revertants and not in NIH/3T3 and EJ-NIH/3T3 cells. Furthermore, the expression level of p92-5.7 seemed to be associated with the flat morphology and the reduced tumorigenicity of the revertants. Polypeptide p92-5.7 was also not detected in the total proteins extracted from BALB/3T3 cells, NIH Swiss mouse primary embryo fibroblasts, NRK (normal rat kidney) cells, and L6 (rat myoblast). Subcellular fractionation of total protein from R1 cells revealed that the p92-5.7 was present in the cytosol. Western blot analysis using an anti-gelsolin antibody demonstrated that the p92-5.7 might be a variant form of gelsolin which is thought to be an actin regulatory protein or a gelsolin-like polypeptide. These results may suggest that the expression of p92-5.7 detected only in the revertants is associated, at least in part, with the reversion. This may be the first demonstration of specific protein expression in the flat revertants.

Reduced induction of c-fos but not of c-myc expressions in a nontumorigenic revertant R1 of Ej-ras-transformed NIH/3T3 cells treated with 12-O-tetradecanoylphorbol-13-acetate (TPA)

It has been reported that both c-fos and c-myc mRNAs are induced in NIH/3T3 cells after 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment. We have studied the effect of TPA on the expression of c-fos and c-myc in EJ-ras-transformed NIH/3T3 and its nontumorigenic flat revertant R1 cells. Although TPA treatment induces c-myc mRNA, as in the case of NIH/3T3 cells, the induced level of c-fos mRNA is greatly reduced not only in slow-growing EJ-ras-transformed NIH/3T3 but also in quiescent R1 cells. In addition, serum-induced c-fos expression is also reduced in EJ-ras-transformed NIH/3T3 and R1 cells. These observations suggest that the pathway from TPA to c-fos gene is different from that to c-myc gene and that the former pathway is down-regulated in association not with the transformed phenotype, but with EJ-ras expression, and it is possible that this reduced induction of c-fos is not specific to TPA.