Suppression of tumorigenicity in hybrids of normal and oncogene-transformed CHEF cells

A genetic basis for tumour suppression

The technique of somatic cell hybridization has established the phenomenon of tumour suppression and provided evidence for a genetic basis for suppression. Further refinements aimed at eventually identifying 'tumour suppressor' genes include the use of monochromosome transfer via microcell hybridization. The application of this technique to the study of tumour suppression in tumorigenic HeLa cell x fibroblast hybrids, Wilms' tumour, retinoblastoma and osteosarcoma cells is described. The issue of whether tumour suppression involves a direct effect on expression of activated oncogenes is discussed. Transformation of normal human cells in culture by activated cellular oncogenes is an extremely rare event. This may be due to a relatively greater genomic stability of human cells compared to rodent cells. We describe the use of a spontaneously immortalized human keratinocyte cell line, HaCaT, for studies of the effects of introduction of activated c-Ha-ras oncogene into these cells, with particular reference to tumorigenic conversion.

Predisposition to neoplastic transformation caused by gene replacement of H-ras1

Homologous recombination was used to introduce a nominally transforming mutation into an endogenous H-ras1 gene in Rat1 fibroblasts. Although both the mutant and the remaining normal allele were expressed equally, the heterozygous cells were not neoplastically transformed. Instead, spontaneously transformed cells arose from the heterozygotes at a low frequency, and the majority of these cells had amplified the mutant allele. Thus, the activated H-ras1 allele was not by itself dominant over the normal allele but predisposed cells to transformation by independent events, such as amplification of the mutant allele.

Spontaneous metastatic potential of rat hepatocarcinoma cells after cell fusion or DNA transfection

Eight LF x ICIG cell hybrid clones, isolated upon fusion of normal ICIG-7 human fibroblasts with tumorigenic, non-metastatic LF Cl.2A cells derived from a DAB-induced rat hepatocarcinoma, were studied. They were all highly tumorigenic and were capable of developing spontaneous lung metastases in syngeneic animals. All the hybrids were characterized by a rapid loss of human chromosomes. However, in long-term culture, they all revealed a persistence of human genetic information as assessed by Southern blotting. In hybrid lines in which human chromosomes were still visible, the most recurrent were numbers 7 and 9. Neither chromosome 7, previously reported to bear some of the genes controlling metastasis in human X mouse T-cell hybrids, nor chromosome 9 appeared to be correlated with the metastatic potential of LF X ICIG hybrids. The same conclusion applied (1) to a human 3.3-kb EcoRI DNA fragment which was amplified (approx. 10-fold) only in metastases induced by one out of 3 metastatic hybrids tested; (2) to the transcription level of c-Ha-ras and c-Ki-ras genes which was enhanced (approx. 4-fold) in metastatic and non-metastatic lines as well. Co-transfection of LF Cl.2A cells with pHSG 272 selectable marker DNA and genomic DNA from normal ICIG-7 human cells or from a hybrid-induced metastasis, reproducibly gave rise to geneticin-resistant transfectants capable of producing spontaneous lung metastases. Neither transfectants nor transfectant-induced metastases harbored detectable human DNA sequences but all harbored pHSG 272 DNA. These results again call for caution in gene transfer studies of the metastatic process.

Differential retention of tumor- and differentiation-suppressor functions in cells derived from a human squamous cell carcinoma

Three morphologically distinct cell lines--F.2a, V, and B.2--were isolated from a single human squamous cell carcinoma. Although all three cell lines can grow indefinitely in culture, they differ in a number of important transformation-related phenotypes. Only B.2 is strongly tumorigenic when injected into the flanks of nude mice, and only V can efficiently grow in semisolid media. The dominance of these traits was investigated by generating somatic cell hybrids among the three cell lines. F.2a was able to suppress the tumorigenicity of B.2 cells, whereas B.2 inhibited the capacity for anchorage-independent growth of V, the latter trait being a function of the ability of these epithelial cells to differentiate when deprived of support. The influence of exogenously added growth factors was also evaluated. This study indicates that the particular tumor we examined consisted of a heterogeneous population of cells with distinct growth and differentiation capacities.

Short-term treatment with gamma interferon induces stable reversion of ras-transformed mouse fibroblasts

Persistent revertants have been generated from NIH 3T3 cells transformed by an activated human Ha-ras gene after short-term gamma interferon treatment in the presence of the cardiac aminoglycoside ouabain. Normal fibroblastlike morphology and anchorage dependence are restored in revertants. Tumorigenicity in nude mice is abolished. The revertants continue to express high steady-state levels of the ras oncogene. Partial retransformation of reverted cells is induced after 5-azacytidine treatment or after infection with retrovirus vectors carrying the v-abl, v-fes, v-myc, or v-src oncogene. The revertants resist the transforming activities of the v-Ha-ras and v-mos oncogenes.

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.

Gene expression and metastasis of somatic cell hybrids between murine fibroblast cell lines of different malignant potential

We have used somatic cell hybrids to study the relationship between ras sensitivity, metastasis, and the expression of ras-responsive or "metastasis-associated" genes. We have previously shown that NIH 3T3 cells are nontumorigenic, but are made metastatic by transfection and expression of activated ras (i.e., they are ras-sensitive). LTA cells, however, are initially tumorigenic, but nonmetastatic, and are not altered in malignancy by ras (i.e., they are ras-resistant). We also have shown that patterns of expression of ras-responsive and "metastasis-associated" genes differ markedly between these two cell types. In the present work, we have constructed three sets of somatic cell hybrids: NIH 3T3 X LTA cells (designated NL), NIH 3T3 X ras-transfected LTA cells (designated NLR), and LTA X ras-transfected NIH 3T3 cells (designated LNR). In all three sets of cell hybrids, pooled clones were found to be highly metastatic in the chick embryo assay, suggesting complementation had occurred. Those cell hybrids that contained ras (NLR and LNR hybrids) were significantly more metastatic than those that did not (NL hybrids). Selected clones of low and high metastatic ability from both NL and LNR fusions were examined for tumorigenicity and "experimental" metastatic ability in nude mice, as well as for expression of several genes thought to be involved in ras-induced progression and malignancy. Patterns of expression of these genes showed a relationship to level of malignancy of the hybrids and demonstrated a responsiveness to the expression of activated ras. These results suggest that the complementation of phenotype observed in the hybrids may arise through a gene regulatory factor(s) supplied by the NIH 3T3- to the LTA-derived parent.

Loss of a tumor suppressor function during neoplastic progression of epithelial cells in vitro

In vitro transformation of rat urothelial cells is a multi-step process. We have used cell fusion to analyse the role of recessive events during in vitro progression of an immortal urothelial cell line. Somatic cell hybrids were made between the transformed cell line RM2T and a series of immortal urothelial cell lines, including the progenitor line RM2AD, from which RM2T was isolated. The ability to produce colonies in soft agar (anchorage independence) was used as an in vitro marker of transformation, and a series of 10 hybrid clones and 4 mass populations of hybrids were assessed for suppression of this phenotype. Hybrids between early-passage (less than passage 35, anchorage-dependent) RM2AD cells and late-passage (greater than passage 35, anchorage-independent) RM2T cells, showed suppression of anchorage independence when tested early after fusion (4/4 mass populations, 7/10 clones). This indicates that in vitro progression of this cell line is associated with loss of a function which can suppress growth in soft agar. Fusions between anchorage-independent RM2T cells and a series of other anchorage-dependent immortal urothelial cell lines generated hybrids which showed no suppression of anchorage independence, indicating that these anchorage-dependent cells have lost the suppressor function identified in RM2AD. Our results indicate that loss of a suppressor function can contribute to urothelial transformation in vitro and that clonal populations of immortal cells, at apparently the same stage of transformation, differ in their ability to suppress anchorage independence of the cell line RM2T. These differences provide the basis for suppressor-gene cloning experiments based on gene transfer.

Normal human colon cells suppress malignancy when fused with colon cancer cells

Normal human colon mucosa cells and cells obtained from histologically normal tissues near that cancer were fused with human colon cancer cells. Resultant hybrid populations of normal and malignant cell fusions behaved as nonmalignant cells in culture, were unable to grow in soft agar, did not express tumor-associated antigens, and were nontumorigenic in nude mice. Autofusion of the cancer cell population led to a phenotype intermediate between normal and malignant cells. That is, the cultures had a much lower plating efficiency in soft agar, and the tumors had a longer latency and slower growth rate in nude mice. This is the first cell culture system to demonstrate that normal epithelial cells can suppress malignancy of their autologous cancer cells, and is a prelude to more extensive studies of genetic events involved in malignant conversion of human colonic epithelium.

Down regulation by p60v-src of genes specifically expressed and developmentally regulated in postmitotic quail neuroretina cells

The avian neuroretina (NR) is composed of photoreceptors and different neurons that are derived from proliferating precursor cells. Neuronal differentiation takes place after terminal mitosis. We have previously shown that differentiating NR cells can be induced to proliferate by infection with Rous sarcoma virus (RSV) and that cell multiplication requires expression of a functional v-src gene. We speculated that the quiescence of NR cells could be determined by specific genes. Cell proliferation could then result from the negative regulation of these genes by the v-src protein. By differential hybridization of a cDNA library, we isolated eight clones corresponding to genes expressed in postmitotic NR cells from 13-day-old quail embryos, transcriptional levels of which are significantly reduced in NR cells induced to proliferate by tsNY68, an RSV mutant with temperature-sensitive mitogenic activity. Partial sequencing analysis indicated that one RNA encoded the calmodulin gene, whereas the other seven showed no similarity to known sequences. By using v-src mutants that induce NR cell proliferation in the absence of transformation, we showed that transcription of six genes was negatively regulated by the v-src protein and that of four genes was correlated with NR cell quiescence. We also report that a subset of genes are specifically transcribed in neural cells and developmentally regulated in the NR. These results indicate that the v-src protein regulates expression of genes likely to play a role in the control of neural cell growth or differentiation.

Down regulation by p60v-src of genes specifically expressed and developmentally regulated in postmitotic quail neuroretina cells

The avian neuroretina (NR) is composed of photoreceptors and different neurons that are derived from proliferating precursor cells. Neuronal differentiation takes place after terminal mitosis. We have previously shown that differentiating NR cells can be induced to proliferate by infection with Rous sarcoma virus (RSV) and that cell multiplication requires expression of a functional v-src gene. We speculated that the quiescence of NR cells could be determined by specific genes. Cell proliferation could then result from the negative regulation of these genes by the v-src protein. By differential hybridization of a cDNA library, we isolated eight clones corresponding to genes expressed in postmitotic NR cells from 13-day-old quail embryos, transcriptional levels of which are significantly reduced in NR cells induced to proliferate by tsNY68, an RSV mutant with temperature-sensitive mitogenic activity. Partial sequencing analysis indicated that one RNA encoded the calmodulin gene, whereas the other seven showed no similarity to known sequences. By using v-src mutants that induce NR cell proliferation in the absence of transformation, we showed that transcription of six genes was negatively regulated by the v-src protein and that of four genes was correlated with NR cell quiescence. We also report that a subset of genes are specifically transcribed in neural cells and developmentally regulated in the NR. These results indicate that the v-src protein regulates expression of genes likely to play a role in the control of neural cell growth or differentiation.

A growth arrest-specific (gas) gene codes for a membrane protein

A set of growth arrest-specific (gas) genes whose expression is negatively regulated by serum has recently been identified. We report on the detailed analysis of one of these genes (gas3). The kinetics of regulation by the presence and absence of serum were investigated, and it was found that this gene is regulated at the post-transcriptional level. The encoded protein deduced from the nucleotide sequence showed some similarity to a mitochondrial oxyreductase, and in vitro translation established that the protein product is a transmembrane glycoprotein.

Spontaneous and cAMP-dependent induction of a resting phase and neurite formation in cell hybrids between human neuroblastoma cells and thymidine auxotrophs of rat nerve-like cells

Cell hybrids (BIM) were produced between human neuroblastoma cells (IMR-32) and thymidine auxotrophs (B3T) of rat nerve-like cells (B103) in order to obtain cell lines undergoing stable neuronal differentiation. BIM cells exhibited the growth properties of partial transformation: 1) When the cell growth reached a plateau, BIM cells ceased to proliferate and expressed a differentiated phenotype. The shape of the cells changed from flat to round and they extended neurites. 2) When cultured in methylcellulose, BIM cells formed colonies, indicating that BIM cells have the ability of anchorage-independent growth. By Southern blot analysis, BIM cells had both human and rat types of N-myc genes. The human N-myc genes were amplified, but the extent of the amplification was lower in BIM cells than that in the parental cell line IMR-32. The rat N-myc gene was detected at a similar level in BIM, B3T, B103, and rat fibroblastic cells 3Y1. Therefore, the decrease in amplification of human N-myc genes may be involved in the properties of partial reverse-transformation in BIM cells. When treated with various drugs such as db-cAMP, forskolin, and cAMP with isobutyl-methylxanthine, BIM cells expressed a nerve-like phenotype. These findings indicate that cell hybridization yielded partial normalization of transformed nerve-like cells.

A growth arrest-specific (gas) gene codes for a membrane protein

A set of growth arrest-specific (gas) genes whose expression is negatively regulated by serum has recently been identified. We report on the detailed analysis of one of these genes (gas3). The kinetics of regulation by the presence and absence of serum were investigated, and it was found that this gene is regulated at the post-transcriptional level. The encoded protein deduced from the nucleotide sequence showed some similarity to a mitochondrial oxyreductase, and in vitro translation established that the protein product is a transmembrane glycoprotein.

Regulation of expression of growth arrest-specific genes in mouse fibroblasts

The suppression of growth arrest-specific (gas) gene expression by serum appeared to be independent of protein synthesis, but expression in resting cells was sensitive to 2-aminopurine, an inhibitor of intracellular protein kinases. Although accumulation of gas gene mRNA was reduced by serum, nuclear transcription of the gas-2, -3, and -5 genes was observed in serum-stimulated cells, indicating that posttranscriptional events may regulate mRNA levels. Growth induction by serum, on the other hand, led to suppression of transcription of the gas-1 gene. Cell cycle regulation and the serum response of gas-1 were lost in ras-transformed cells.

Regulation of expression of growth arrest-specific genes in mouse fibroblasts

The suppression of growth arrest-specific (gas) gene expression by serum appeared to be independent of protein synthesis, but expression in resting cells was sensitive to 2-aminopurine, an inhibitor of intracellular protein kinases. Although accumulation of gas gene mRNA was reduced by serum, nuclear transcription of the gas-2, -3, and -5 genes was observed in serum-stimulated cells, indicating that posttranscriptional events may regulate mRNA levels. Growth induction by serum, on the other hand, led to suppression of transcription of the gas-1 gene. Cell cycle regulation and the serum response of gas-1 were lost in ras-transformed cells.

Extinction and activation of the thyroglobulin promoter in hybrids of differentiated and transformed thyroid cells

Thyroglobulin gene expression was repressed in a rat thyroid cell line transformed with Kirsten murine sarcoma virus. Expression of a dominant selectable marker driven by the thyroglobulin promoter was also inhibited. Somatic cell hybridization of transformed and differentiated thyroid cells resulted in extinction of thyroglobulin gene expression. When transformed cells carrying a dominant selectable marker driven by the thyroglobulin promoter were fused to differentiated cells and expression of this marker was selected, we obtained stable hybrid cell lines expressing both the endogenous and the exogenous thyroglobulin promoters. Although the expression of v-ras remained unchanged compared with expression in the parental transformed cells, transformation was suppressed in the hybrid cell lines. The other thyroid differentiation markers, iodide uptake and thyroid-stimulating hormone-dependent growth, were inhibited in all the hybrids tested. We show that activity of the thyroglobulin promoter correlates with the presence of a thyroid nuclear factor that binds the promoter at position -60 from the transcription start site. Loss of this factor accompanies the extinction of thyroglobulin gene expression in hybrids selected for expression of a non-thyroid-specific promoter.

Extinction and activation of the thyroglobulin promoter in hybrids of differentiated and transformed thyroid cells

Thyroglobulin gene expression was repressed in a rat thyroid cell line transformed with Kirsten murine sarcoma virus. Expression of a dominant selectable marker driven by the thyroglobulin promoter was also inhibited. Somatic cell hybridization of transformed and differentiated thyroid cells resulted in extinction of thyroglobulin gene expression. When transformed cells carrying a dominant selectable marker driven by the thyroglobulin promoter were fused to differentiated cells and expression of this marker was selected, we obtained stable hybrid cell lines expressing both the endogenous and the exogenous thyroglobulin promoters. Although the expression of v-ras remained unchanged compared with expression in the parental transformed cells, transformation was suppressed in the hybrid cell lines. The other thyroid differentiation markers, iodide uptake and thyroid-stimulating hormone-dependent growth, were inhibited in all the hybrids tested. We show that activity of the thyroglobulin promoter correlates with the presence of a thyroid nuclear factor that binds the promoter at position -60 from the transcription start site. Loss of this factor accompanies the extinction of thyroglobulin gene expression in hybrids selected for expression of a non-thyroid-specific promoter.

Tumor suppression involves down-regulation of interleukin 3 expression in hybrids between autocrine mastocytoma and interleukin 3-dependent parental mast cells

Interleukin 3 (IL-3)-dependent PB-3c mouse mastocytes can be transformed by the v-Ha-ras oncogene to generate autocrine IL-3-producing mastocytomas. Hybrid cell lines were constructed by fusing an IL-3-producing mastocytoma cell line with its IL-3-dependent normal parental cell. Unlike the mastocytoma parent cell line, hybrid cell lines required growth factor for in vitro proliferation, indicating that the IL-3-dependent phenotype is dominant. IL-3 mRNA, expressed at high levels in the tumor cells, appeared down-regulated in the cell hybrids. In contrast, p21v-Ha-ras levels were not reduced in the hybrids. The hybrid lines generated tumors in vivo with drastically prolonged latency times when compared to the tumor parent (10 versus 2 weeks). We propose that down-regulation of IL-3 mRNA production after cell fusion is responsible for the loss of growth autonomy in the hybrids and is likely to play a role in the partial suppression of tumor formation in vivo. Our data are consistent with the hypothesis that a tumor suppressor, present in PB-3c cells, acts as a negative regulator of IL-3 expression.

Role of a tumor-suppressor gene in the negative control of anchorage-independent growth of Syrian hamster cells

Tumor-suppressor genes control the neoplastic phenotype of tumor cells, but the function of these genes in normal cells is unknown. In this report we show that the loss of a tumor-suppressor gene function releases negative controls on the growth of cells in agar. This conclusion is based on observations of cell hybrids and studies of cell variants that have retained or lost a tumor-suppressor gene function. Nontumorigenic cell hybrids between normal Syrian hamster embryo cells and a benzo[a]pyrene-transformed tumor-cell line (BP6T) continued to secrete autocrine and/or paracrine growth factors produced by the tumor cells but failed to respond to these factors by growing in agar. Normal diploid cells also failed to grow in agar in response to the growth factors produced by the tumor cells. Clonal variants of nontumorigenic, immortal Syrian hamster cell lines were isolated that either retained (termed supB+) or had lost (termed supB-) the ability to suppress tumorigenicity of BP6T tumor cells after cell hybridization. Neither supB+ nor supB- variants grew in agar under conditions that allowed efficient growth of the tumor cells. However, supB- cells were reversibly induced to grow in agar with high colony-forming efficiencies in the presence of tumor cell-conditioned medium or by supplementation of the medium with a combination of growth factors. Under the same conditions, the supB+ cells failed to grow in agar. This enhanced growth-factor responsiveness in agar was used to select for supB- variants existing at a low frequency in the supB+ population. These two phenotypes, loss of tumor-suppressor function and enhanced growth-factor responsiveness in agar, were seen to cosegregate. These results indicate the tumor-suppressor gene function in these cells negatively regulates the growth response of cells in agar to mitogenic stimuli. This growth regulation may depend on cell shape or adhesion because supB+ and supB- cells grown attached to plastic responded similarly to growth factors.

Chromosomal aberrations as a contributing factor for tumor promotion in the mouse skin

Tumor promotion in mouse skin can be dissected in two stages: stage I (conversion) and stage II. Whereas for stage II clonal expansion of transformed cells is believed to play a major role, the mechanism(s) underlying conversion is still a matter of debate. Because conversion can be achieved upon treatment with phorbol ester tumor promoters prior to initiation, it is unlikely to represent simply proliferative stimulation of initiated cells (due to epigenetic changes induced). Since tumor promoters exert clastogenic activities and, on the other hand, the clastogen methyl methanesulfonate proved to be convertogenic, the possibility arises that chromosomal changes are involved in conversion. Based on this hypothesis, several findings concerning the action of tumor promoters and the process of tumor promotion in the mouse skin system are discussed and interpreted: the frequency, reversibility, and transient nature of conversion, dependence of tumor promotion on DNA synthesis, induction of DNA breaks by tumor promoters, and the protecting effect of scavengers of free radicals. A model is presented suggesting tumor formation in mouse skin (and other systems) to proceed in discrete, genetically determined steps. Initiation is considered to be due to the induction of point mutations in a dominant-acting oncogene that becomes thereupon activated, whereas the decisive event in the conversion stage of tumor promotion is the induction of numerical and/or structural chromosomal changes with the consequence of loss or inactivation of gene(s) involved in suppression of the tumor phenotype.

A recessive cellular mutation in v-fes-transformed mink cells restores contact inhibition and anchorage-dependent growth

A contact-inhibited revertant of mink cells transformed by the Gardner-Arnstein strain of feline sarcoma virus was isolated by fluorescence-activated sorting of cells stained with the mitochondria-specific dye rhodamine 123. The revertant cell line exhibited a decrease in its proliferative rate and saturation density and a complete loss of its capacity for anchorage-independent growth, but it remained tumorigenic when inoculated into nude mice. The revertant cells retained a rescuable Gardner-Arnstein feline sarcoma provirus, expressed high levels of the v-fes oncogene product and its associated tyrosine kinase activity, manifested elevated levels of phosphotyrosine-containing cellular proteins similar to those observed in v-fes-transformed cells, and were refractory to retransformation by retroviruses containing the v-fes, v-fms, and v-ras oncogenes. Fusion of the revertant and parental cells generated somatic cell hybrids which formed colonies in semisolid medium, indicating that the block in transformation was recessive. These data together with the observation that the revertant phenotype is unstable in continuous culture suggest that the loss of transformation is due to the presence of limiting quantities of a gene product which functions downstream of the v-fes-coded kinase in the mitogenic pathway.

Recessive mechanisms of malignancy

It is increasingly recognised that recessive mutations play an important role in the pathogenesis of many forms of malignancy. Some of the affected loci may prove to be recessively-activated proto-oncogenes, but others are now known to be tumorigenic solely by virtue of their loss or inactivation and therefore form a distinct and novel family of tumour genes. Preliminary evidence suggests that such genes are likely to be functionally heterogeneous and to encode molecules involved in the inhibition of cellular proliferation and/or the induction of differentiation. Their further study is likely to illuminate fundamental mechanisms of normal cellular growth and differentiation as well as having important implications for the pathogenesis and management of cancer.

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

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

A recessive cellular mutation in v-fes-transformed mink cells restores contact inhibition and anchorage-dependent growth

A contact-inhibited revertant of mink cells transformed by the Gardner-Arnstein strain of feline sarcoma virus was isolated by fluorescence-activated sorting of cells stained with the mitochondria-specific dye rhodamine 123. The revertant cell line exhibited a decrease in its proliferative rate and saturation density and a complete loss of its capacity for anchorage-independent growth, but it remained tumorigenic when inoculated into nude mice. The revertant cells retained a rescuable Gardner-Arnstein feline sarcoma provirus, expressed high levels of the v-fes oncogene product and its associated tyrosine kinase activity, manifested elevated levels of phosphotyrosine-containing cellular proteins similar to those observed in v-fes-transformed cells, and were refractory to retransformation by retroviruses containing the v-fes, v-fms, and v-ras oncogenes. Fusion of the revertant and parental cells generated somatic cell hybrids which formed colonies in semisolid medium, indicating that the block in transformation was recessive. These data together with the observation that the revertant phenotype is unstable in continuous culture suggest that the loss of transformation is due to the presence of limiting quantities of a gene product which functions downstream of the v-fes-coded kinase in the mitogenic pathway.

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

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

Partial reversion of the transformed phenotype in HRAS-transfected tumorigenic cells by transfer of a human gene

The transformed phenotype of rat FE-8 cells transfected by an activated human HRAS gene was suppressed upon fusion with normal cells. An experimental approach was developed to identify and isolate a human gene capable of suppressing the transforming activity of the HRAS oncogene in FE-8 cells. Genomic DNA from human placenta was introduced into FE-8 cells by cotransfection with the plasmid pY3 conferring hygromycin B resistance. Transfectants were selected in medium containing hygromycin B. HRAS-transformed FE-8 cells showed an increased sensitivity toward ouabain when compared to their normal counterparts. Therefore, the population of transfected hygromycin B-resistant cells was treated with ouabain to eliminate cells with a transformed phenotype. Ouabain selection resulted in a small number of cell clones exhibiting a more normal phenotype. The clones had lost the morphology of transformed cells and required anchorage for growth. The tumorigenicity of transfectants in nude mice was reduced but not completely abolished. FE-8 revertants continued to express the p21 RAS protein. Human repetitive sequences contained in the DNA of a secondary transfectant were used for isolation of the suppressor gene from reverted FE-8 cells. The cloned DNA fragment was transfected into tumorigenic FE-8 cells and conferred a partial reversion of the transformed phenotype.

Spontaneous fusion between metastatic mammary tumor subpopulations

This study describes a differential frequency of spontaneous fusion between metastatic and nonmetastatic subpopulations derived from a single mouse mammary tumor. Subpopulations 66, 66c14 (a variant of 66 which is resistant to both thioguanine and ouabain), 410.4, and 44FTO (a thioguanine-resistant, ouabain-resistant derivative of 410.4) spontaneously metastasize from subcutaneous and mammary fatpad sites. Subpopulations 168, 168FARO (a diaminopurine-resistant, ouabain-resistant derivative of 168), 67, 68H, and 410 do not. The ability of these subpopulation lines to fuse spontaneously in vitro was determined after coculturing a drug-resistant line with a wild-type line in nonselective media. After 16-20 h of coculture, cells were plated in the appropriate media to select for fusion products--either HAT (hypoxanthine, aminopterin, thymidine) plus ouabain or AA (alanosine, adenine) plus ouabain--to determine the number of colony-forming cells (fusion products) present per 10(4) cells plated. When both subpopulations of the pair in the fusion mixture were metastatic, a significantly greater number of fusion products was recovered than if one or both of the subpopulations in the fusion mixture was nonmetastatic, with one exception: line 410 readily fused with both 66c14 and 44FTO. Subline 410 was highly metastatic when originally isolated but lost its metastatic competence after a brief time in tissue culture.

Genetic analysis of tumorigenesis. XXXI: Retention of short arm of chromosome 3 in suppressed CHEF cell hybrids containing c-Ha-ras (EJ) gene

Hybrids between nontransformed Chinese hamster embryo fibroblast (CHEF) cells and their c-Ha-ras (EJ) -transformed derivatives are suppressed for tumor-forming ability when tested at early passage. Hybrid subclones with suppressed (fibroblastic) or transformed appearance have now been selected by multiple recloning. Morphology, but not serum or anchorage requirement, was a sensitive indicator of suppression: Subclones with normal morphology were nontumorigenic, subclones with transformed morphology were highly tumorigenic, and intermediate subclones (7-70% normal colonies) formed tumors with a frequency of 17-50%. Suppressed lines retained the short arm of chromosome 3, but transformed and tumor-derived lines had lost this region (greater than or equal to 1 copy). Transformed and tumor-derived cells exhibited additional chromosome changes, including the loss of at least one copy of chromosomes 7 and/or 8. These findings suggest that a tumor suppressor gene lies on the short arm of chromosome 3, consistent with prior studies from this laboratory. Other suppressor genes may be located on chromosomes 7 and 8.

Oncogenes and tumor suppressor genes

The artificial selection of the directly acting or acute RNA tumor viruses for high transforming ability has led to the isolation of defective retroviral genomes that have picked up, by accidental recombination, some of the important genes that influence, trigger or regulate cell division. These genes belong to at least four functionally different groups. Each of them can contribute to tumor development and/or progression after activation by structural or regulatory changes. Growth factor genes may act as oncogenes following constitutive activation in a cell that normally responds to, but does not produce, the corresponding growth factor (the autocrine model, exemplified by sis). Growth factor receptors may be fixed in a state of continuous, faulty signalling by the truncation of their external, ligand binding portion (examples: erb-B, fms). Genes coding for proteins involved in signal transduction may be activated by point mutations in certain, important domains (example: the ras-family). DNA binding proteins, presumably involved in DNA replication may drive cell division after constitutive activation by retroviral insertion, chromosomal translocation or gene amplification (example: the myc-family).

The approaching era of the tumor suppressor genes

Genes that can inhibit the expression of the tumorigenic phenotype have been detected by the fusion of normal and malignant cells, the phenotypic reversion of in vitro transformants, the induction of terminal differentiation of malignant cell lineages, the loss of "recessive cancer genes," the discovery of regulatory sequences in the immediate vicinity of certain oncogenes, and the inhibition of tumor growth by normal cell products. Such tumor suppressor genes will probably turn out to be as, if not more, diversified as the oncogenes. Consideration of both kinds of genes may reveal common or interrelated functional properties.

Genetic analysis of tumorigenesis: a conserved region in the human and Chinese hamster genomes contains genetically identified tumor-suppressor genes

Regional chromosome homologies were found in a comparison of human 11p with Chinese hamster 3p. By use of probes that recognize six genes of human 11p (INS, CAT, HBBC, CALC, PTH, and HRAS), the corresponding genes were localized by in situ hybridization on Chinese hamster chromosome 3. INS and CAT were located close to the centromere on 3p, whereas HBBC, CALC, and PTH were at 3q3-4 and HRAS at 3q4. Extensive prior data from chromosome studies of tumorigenic and tumor-derived Chinese hamster cells have suggested the presence of a tumor-suppressor gene on 3p. Two tumor-suppressor genes have been described on human 11p, one linked to CAT and one to INS. The present study raises the possibility that the Chinese hamster suppressor may be closely linked to INS or CAT.

The Wellcome Foundation lecture, 1986. The molecular regulators of normal and leukaemic blood cells

The development of a cell-culture system for the cloning and clonal differentiation of different types of blood cell has made it possible to identify: (i), the proteins that regulate growth and differentiation of different cell lineages in normal and leukaemic blood cells; (ii), the molecular basis of normal and abnormal control of cell development in blood-forming tissue; and (iii), how to suppress malignancy in leukaemic cells. By using myeloid blood cells as a model system, it has been shown that normal blood cells require different proteins to induce cell viability and multiplication (growth-inducers) and differentiation (differentiation-inducers), that there is a hierarchy of growth-inducers which act at various stages of cell development, and that a growth-inducer can switch on production of a differentiation-inducer. Gene cloning has established a multigene family for these proteins. Identification of these proteins and their interaction has shown how growth and differentiation are regulated in normal development and demonstrated the mechanisms that uncouple growth and differentiation so as to produce malignant cells. Normal cells require an external source of growth-inducing protein for cell viability and multiplication. Cells can become leukaemic by genetically changing this normal requirement for growth without blocking response to normal differentiation-inducers. The mature cells induced by adding these normal protein-inducers are then no longer malignant. Other genetic changes which inhibit differentiation by the normal blood-cell regulatory proteins can occur in the evolution of leukaemia. But even these leukaemic cells may still be induced to differentiate by other compounds that can induce differentiation by alternative pathways. The differentiation of leukaemic to mature cells, which stops the cells from multiplying, results in the suppression of malignancy by bypassing genetic changes that produce the malignant phenotype. The activity of blood-cell growth- and differentiation-inducing proteins has been shown in culture and in the body. They can, therefore, be clinically useful to correct defects in the development of normal and leukaemic blood cells.

Interferon-induced revertants of ras-transformed cells: resistance to transformation by specific oncogenes and retransformation by 5-azacytidine

Prolonged alpha/beta interferon (IFN-alpha/beta) treatment of NIH 3T3 cells transformed by a long terminal repeat-activated Ha-ras proto-oncogene resulted in revertants that maintained a nontransformed phenotype long after IFN treatment had been discontinued. Cloned persistent revertants (PRs) produced large amounts of the ras-encoded p21 and were refractile to transformation by EJras DNA and by transforming retroviruses which carried the v-Ha-ras, v-Ki-ras, v-abl, or v-fes oncogene. Transient treatment either in vitro or in vivo with cytidine analogs that alter gene expression by inhibiting DNA methylation resulted in transformation of PR, but not of NIH 3T3, cells. The PR retransformants reverted again with IFN, suggesting that DNA methylation is involved in IFN-induced persistent reversion.

Specific growth inhibitory sequences in genomic DNA from quiescent human embryo fibroblasts

We used HeLa cells as recipients in a gene transfer assay to characterize DNA sequences that negatively regulate mammalian cell growth. In this assay, genomic DNA from quiescent human embryo fibroblasts was more inhibitory for HeLa replication than was DNA from either Escherichia coli or HeLa cells. Surprisingly, growth inhibitory activity depended on the growth state of the cells from which genomic DNA was prepared; it was strongest in DNA prepared from serum-deprived, quiescent embryo fibroblasts. This latter observation implies a role for DNA modification(s) in regulating the activity of the inhibitory sequences detected in our assay. The level of the observed growth inhibitory activity was sometimes high, suggesting that the relevant sequences may be abundantly represented in the mammalian genome. We speculate that these findings may provide new insights into the molecular mechanisms involved in cellular quiescence and in vitro senescence.

Interferon-induced revertants of ras-transformed cells: resistance to transformation by specific oncogenes and retransformation by 5-azacytidine

Prolonged alpha/beta interferon (IFN-alpha/beta) treatment of NIH 3T3 cells transformed by a long terminal repeat-activated Ha-ras proto-oncogene resulted in revertants that maintained a nontransformed phenotype long after IFN treatment had been discontinued. Cloned persistent revertants (PRs) produced large amounts of the ras-encoded p21 and were refractile to transformation by EJras DNA and by transforming retroviruses which carried the v-Ha-ras, v-Ki-ras, v-abl, or v-fes oncogene. Transient treatment either in vitro or in vivo with cytidine analogs that alter gene expression by inhibiting DNA methylation resulted in transformation of PR, but not of NIH 3T3, cells. The PR retransformants reverted again with IFN, suggesting that DNA methylation is involved in IFN-induced persistent reversion.

Suppression of transformed phenotypes in intraspecific somatic cell hybrid clones between the c-myc activating mouse plasmacytoma line and normal cells

The role of normal-cell-derived chromosome 15 in suppressing transformed phenotypes was studied in intraspecific hybrid clones between the c-myc oncogene activating BALB/c mouse plasmacytoma (S194) cells and normal spleen cells or fibroblasts from CBA/H-T6 mice. All the hybrid clones between S194 and normal spleen cells grew very rapidly in suspension and formed colonies in soft agar. In contrast, the hybrid clones between S194 and normal fibroblasts grew slowly in an attached form. They were divided into 2 groups on the basis of their morphology and growth properties: most clones showed flat type morphology, and no colony formation was seen in soft agar, while some clones grew in a piled-up fashion and formed colonies in soft agar. The hybrid clones between S194 and normal spleen cells lost some normal-cell-derived chromosomes but retained most tumor-derived marker chromosomes including the t(12;15) chromosome which carried the activated c-myc oncogene. On the other hand, hybrid clones between S194 cells and normal fibroblasts retained almost all chromosomes from both parental cells. With respect to retention of normal-cell-derived chromosome 15, both the flat and piled-up type clones retained 2 copies each of the t(14;15) and T6 marker chromosomes, the normal counterparts of the t(12;15) chromosomes. Our results suggest that the transformed phenotypes of the hybrid clones between S194 cells and normal fibroblasts are negatively modulated by normal-cell-derived chromosomes but not by normal-cell-derived chromosome 15 alone.

Suppression of tumorigenicity in somatic cell hybrids does not involve quantitative changes in transcription of cellular Ha-ras, Ki-ras, myc, and fos oncogenes

The transcriptional activity of ten cellular oncogenes was analyzed in somatic cell hybrids that had been obtained after fusion of tumorigenic Chinese hamster cells and normal mouse fibroblasts. The hybrids showed either the tumorigenic or the nontumorigenic phenotype (suppression of tumorigenicity). Out of ten c-onc genes analyzed, four (c-Ha-ras, c-Ki-ras, c-myc, and c-fos) were found to be transcriptionally active at similar levels in tumorigenic as well as in nontumorigenic (suppressed) hybrids. Thus we conclude that suppression of tumorigenicity in Chinese hamster X mouse somatic cell hybrids does not correlate with quantitative changes in expression of these cellular oncogenes. The remaining six cellular oncogenes (c-abl, c-erb A and B, c-fes, c-myb, and c-sis) were not transcriptionally active in these hybrids.

The Ha-ras-induced transformed phenotype of rat-1 cells can be suppressed in hybrids with rat embryonic fibroblasts

Somatic cell hybrids were isolated from fusions of diploid embryonic rat fibroblasts with transformed Rat-1 cells which contained 4 to 5 copies of the transforming human Ha-ras 1 gene. In contrast to their transformed parental cells four hybrid clones showed normal morphology, long latency periods of tumorigenicity in newborn rats, anchorage requirement of proliferation, and an eightfold-reduced amount of secreted transforming growth factor activity. Thus these hybrids are called suppressed with regard to expression of the Ha-ras-induced transformed phenotype. Tumorigenic derivatives of the suppressed hybrids that had segregated chromosomes were isolated. Since two of the tumorigenic hybrid clones showed the similar low level of secreted transforming growth factors as the suppressed hybrids, decreased production of transforming growth factor activity is unlikely to be a sufficient criterion for suppression of malignancy. Whereas one of the suppressed hybrids expressed the transforming gene product p21 at a level similar to that of the transformed parental cells, other suppressed hybrids expressed less p21. This suggests that the suppressed phenotype can be regulated at the posttranslational level of p21 but that additional controls of expression of p21 are likely to exist. DNA of the suppressed hybrids transformed Rat-1 cells to proliferation in the presence of semisolid agar. Thus the activated human Ha-ras gene in the suppressed hybrids retained its biological activity even though it did not transform these cells to tumorigenicity.

Specific growth inhibitory sequences in genomic DNA from quiescent human embryo fibroblasts

We used HeLa cells as recipients in a gene transfer assay to characterize DNA sequences that negatively regulate mammalian cell growth. In this assay, genomic DNA from quiescent human embryo fibroblasts was more inhibitory for HeLa replication than was DNA from either Escherichia coli or HeLa cells. Surprisingly, growth inhibitory activity depended on the growth state of the cells from which genomic DNA was prepared; it was strongest in DNA prepared from serum-deprived, quiescent embryo fibroblasts. This latter observation implies a role for DNA modification(s) in regulating the activity of the inhibitory sequences detected in our assay. The level of the observed growth inhibitory activity was sometimes high, suggesting that the relevant sequences may be abundantly represented in the mammalian genome. We speculate that these findings may provide new insights into the molecular mechanisms involved in cellular quiescence and in vitro senescence.

Coinfection with viruses carrying the v-Ha-ras and v-myc oncogenes leads to growth factor independence by an indirect mechanism

The concomitant expression of certain oncogenes can transform normal diploid rodent cells into transplantable tumorigenic cells. The mechanism by which these oncogenes collaborate is unclear. Recent findings (M. Oshimura, T. M. Gilmer, and J. C. Barrett, Nature [London] 316:636-639, 1985) raise the possibility that karyotypic changes, including monosomy for chromosome 15, are required to induce tumorigenicity in Syrian hamster embryo cells transfected in vitro with v-Ha-ras and v-myc DNAs. We studied the effect of the oncogenes v-Ha-ras and v-myc, introduced by viral infection, on murine hematopoietic cells. The induction of growth factor independence by the two oncogenes was used as an in vitro correlate of tumorigenicity. After a period of reduced growth rate reminiscent of the growth rate of cells in crisis, the doubly infected cells became growth factor independent. These cells showed a great variability in their karyotypes.

Mitogenic effects of the proto-oncogene and oncogene forms of c-H-ras DNA in human diploid fibroblasts

Nuclear microinjection of c-H-ras DNA induced DNA synthesis in reversibly nonproliferating quiescent human cells. The proto-oncogene and oncogene forms were equally effective inducers. In contrast, c-H-ras DNA either alone or in combination with the adenovirus E1A gene did not cause terminally nondividing senescent cells to synthesize DNA.

The molecular genetics of cancer

The search for genetic damage in neoplastic cells now occupies a central place in cancer research. Diverse examples of such damage are in hand, and they in turn hint at biochemical explanations for neoplastic growth. The way may be open to solve the riddles of how normal cells govern their replication and why cancer cells do not.