Oncogene-induced transformation of a rat embryo fibroblast cell line is enhanced by tumor promoters

Suppression of the tumorigenicity of mutant p53-transformed rat embryo fibroblasts through expression of a newly cloned rat nonmuscle myosin heavy chain-B

In our previous study, a rat homolog of human nonmuscle myosin heavy chain-B (nmMHC-B) was identified by mRNA differential display comparing of transformed against nontransformed Rat 6 cells overexpressing mutant p53val135 gene. The nmMHC-B was found to be expressed in normal Rat 6 embryo fibroblast cell line, but markedly suppressed in the mutant p53val135-transformed Rat 6 cells. To examine the possible involvement of nmMHC-B in cell transformation, we first cloned and sequenced the full length cDNA of rat nmMHC-B, which was then cloned into an ecdysone-expression vector. The resulting construct was introduced into the T2 cell line, a mutant p53val135-transformed Rat 6 cells lacking the expression of the endogenous nmMHC-B. The clonal transfectants, expressing muristerone A-induced nmMHC-B, displayed a slightly flatter morphology and reached to a lower saturation density compared to the parental transformed cells. Reconstitution of actin filamental bundles was also clearly seen in cells overexpressing the nmMHC-B. In soft agar assays, nmMHC-B transfectants formed fewer and substantially smaller colonies than the parental cells in response to muristerone A induction. Moreover, it was strikingly effective in suppressing the tumorigenicity of the T2 cells when tested in nude mice. Thus, the nmMHC-B, known as a component of the cytoskeletal network, may act as a tumor suppressor gene. Our current finding may reveal a novel role of nmMHC-B in regulating cell growth and cell signaling in nonmuscle cells. Oncogene (2001) 20, 58 - 68.

Novel revertants of H-ras oncogene-transformed R6-PKC3 cells

Rat 6 fibroblasts that overproduce protein kinase C beta 1 (R6-PKC3 cells) are hypersensitive to complete transformation by the T24 H-ras oncogene; yet T24 H-ras-transformed R6-PKC3 cells are killed when exposed to 12-O-tetradecanoylphorbol-13-acetate (TPA) (W.-L. W. Hsiao, G. M. Housey, M. D. Johnson, and I. B. Weinstein, Mol. Cell. Biol. 9:2641-2647, 1989). Treatment of an R6-PKC3 subclone that harbors a T24 H-ras gene under the control of an inducible mouse metallothionein I promoter with ZnSO4 and TPA is extremely cytocidal. This procedure was used to isolate rare revertants that are resistant to this toxicity. Two revertant lines, R-1a and ER-1-2, continue to express very high levels of protein kinase C enzyme activity but, unlike the parental cells, do not grow in soft agar. Furthermore, these revertants are resistant to the induction of anchorage-independent growth by the v-src, v-H-ras, v-raf, and, in the case of the R-1a line, v-fos oncogenes. Both revertant lines, however, retain the ability to undergo morphological alterations when either treated with TPA or infected with a v-H-ras virus, thus dissociating anchorage independence from morphological transformation. The revertant phenotype of both R-1a and ER-1-2 cells is dominant over the transformed phenotype in somatic cell hybridizations. Interestingly, the revertant lines no longer induce the metallothionein I-T24 H-ras construct or the endogenous metallothionein I and II genes in response to three distinct agents: ZnSO4, TPA, and dexamethasone. The reduction in activity of metallothionein promoters seen in these revertants may reflect defects in signal transduction pathways that control the expression of genes mediating specific effects of protein kinase C and certain oncogenes in cell transformation.

Novel revertants of H-ras oncogene-transformed R6-PKC3 cells

Rat 6 fibroblasts that overproduce protein kinase C beta 1 (R6-PKC3 cells) are hypersensitive to complete transformation by the T24 H-ras oncogene; yet T24 H-ras-transformed R6-PKC3 cells are killed when exposed to 12-O-tetradecanoylphorbol-13-acetate (TPA) (W.-L. W. Hsiao, G. M. Housey, M. D. Johnson, and I. B. Weinstein, Mol. Cell. Biol. 9:2641-2647, 1989). Treatment of an R6-PKC3 subclone that harbors a T24 H-ras gene under the control of an inducible mouse metallothionein I promoter with ZnSO4 and TPA is extremely cytocidal. This procedure was used to isolate rare revertants that are resistant to this toxicity. Two revertant lines, R-1a and ER-1-2, continue to express very high levels of protein kinase C enzyme activity but, unlike the parental cells, do not grow in soft agar. Furthermore, these revertants are resistant to the induction of anchorage-independent growth by the v-src, v-H-ras, v-raf, and, in the case of the R-1a line, v-fos oncogenes. Both revertant lines, however, retain the ability to undergo morphological alterations when either treated with TPA or infected with a v-H-ras virus, thus dissociating anchorage independence from morphological transformation. The revertant phenotype of both R-1a and ER-1-2 cells is dominant over the transformed phenotype in somatic cell hybridizations. Interestingly, the revertant lines no longer induce the metallothionein I-T24 H-ras construct or the endogenous metallothionein I and II genes in response to three distinct agents: ZnSO4, TPA, and dexamethasone. The reduction in activity of metallothionein promoters seen in these revertants may reflect defects in signal transduction pathways that control the expression of genes mediating specific effects of protein kinase C and certain oncogenes in cell transformation.

A novel, plasmid-based system for studying gene rearrangements in mammalian cells

We have developed a plasmid-based system for isolating gene rearrangements in mammalian cells by selection for reversion of a promoterless drug resistance gene. pNH4 contains the selectable marker gene neo under the control of the herpes simplex virus, thymidine kinase (tk) promoter and, upstream and in the opposite orientation, a dormant promoterless hygromycin B resistance gene (hph) that can be expressed following rearrangement events. An NIH 3T3 cell line stably transfected with pNH4 that has a spontaneous frequency of generation of Hphr colonies of approximately 10(-8) was isolated. Treatment of this line with ethyl methanesulfonate raised the frequency of Hphr colony formation approximately 100-fold. Approximately 60% (21 of 35) of ethyl methanesulfonate-induced Hphr clones showed rearrangements detectable by Southern blot analysis within a 40-kb region surrounding the integrated construct, including a nonhomologous recombination event and, possibly, a large insertion. Additionally, three Hphr clones showed evidence of gene amplification. Northern (RNA) blot analysis of hph mRNA suggests that the rearrangements may provide a function that allows the tk promoter to initiate transcription off the opposite strand, thus yielding hph transcripts. Cell lines harboring pNH4, or modifications of it, may be valuable for studying recombination mechanisms responsible for the various types of genetic rearrangements found in cancer cells.

A novel, plasmid-based system for studying gene rearrangements in mammalian cells

We have developed a plasmid-based system for isolating gene rearrangements in mammalian cells by selection for reversion of a promoterless drug resistance gene. pNH4 contains the selectable marker gene neo under the control of the herpes simplex virus, thymidine kinase (tk) promoter and, upstream and in the opposite orientation, a dormant promoterless hygromycin B resistance gene (hph) that can be expressed following rearrangement events. An NIH 3T3 cell line stably transfected with pNH4 that has a spontaneous frequency of generation of Hphr colonies of approximately 10(-8) was isolated. Treatment of this line with ethyl methanesulfonate raised the frequency of Hphr colony formation approximately 100-fold. Approximately 60% (21 of 35) of ethyl methanesulfonate-induced Hphr clones showed rearrangements detectable by Southern blot analysis within a 40-kb region surrounding the integrated construct, including a nonhomologous recombination event and, possibly, a large insertion. Additionally, three Hphr clones showed evidence of gene amplification. Northern (RNA) blot analysis of hph mRNA suggests that the rearrangements may provide a function that allows the tk promoter to initiate transcription off the opposite strand, thus yielding hph transcripts. Cell lines harboring pNH4, or modifications of it, may be valuable for studying recombination mechanisms responsible for the various types of genetic rearrangements found in cancer cells.

Transfected human beta-polymerase promoter contains a ras-responsive element

beta-Polymerase is a vertebrate cellular DNA polymerase involved in gap-filling synthesis during some types of genomic DNA repair. We report that a cloned human beta-polymerase promoter in a transient expression assay is activated by p21v-rasH expression in NIH 3T3 cells. A decanucleotide palindromic element, GTGACGTCAC, at positions -49 to -40 in the promoter is required for this ras-mediated stimulation.

Transfected human beta-polymerase promoter contains a ras-responsive element

beta-Polymerase is a vertebrate cellular DNA polymerase involved in gap-filling synthesis during some types of genomic DNA repair. We report that a cloned human beta-polymerase promoter in a transient expression assay is activated by p21v-rasH expression in NIH 3T3 cells. A decanucleotide palindromic element, GTGACGTCAC, at positions -49 to -40 in the promoter is required for this ras-mediated stimulation.

Cells that overproduce protein kinase C are more susceptible to transformation by an activated H-ras oncogene

We recently developed rat fibroblast cell lines that stably overproduce high levels of the beta 1 form of protein kinase C (PKC). These cells display several disorders in growth control and form small microscopic colonies in agar. In the present study we demonstrate that one of these cell lines, R6-PKC3, is extremely susceptible to transformation by an activated human bladder cancer c-H-ras oncogene (T24). Compared with control cell line R6-C1, T24-transfected R6-PKC3 cells yielded a 10-fold increase in the formation of large colonies in agar. Cell lines established from these colonies displayed a highly transformed morphology, expressed the T24-encoded p21 ras protein, continued to express high levels of PKC, and were highly tumorigenic in nude mice. These results provide genetic evidence that PKC mediates some of the effects of the c-H-ras oncogene on cell transformation. Data are also presented suggesting that optimum synergistic effects between c-H-ras and PKC require critical levels of their respective activities. These findings may be relevant to the process of multistage carcinogenesis in tissues containing cells with an activated c-H-ras oncogene.

Cells that overproduce protein kinase C are more susceptible to transformation by an activated H-ras oncogene

We recently developed rat fibroblast cell lines that stably overproduce high levels of the beta 1 form of protein kinase C (PKC). These cells display several disorders in growth control and form small microscopic colonies in agar. In the present study we demonstrate that one of these cell lines, R6-PKC3, is extremely susceptible to transformation by an activated human bladder cancer c-H-ras oncogene (T24). Compared with control cell line R6-C1, T24-transfected R6-PKC3 cells yielded a 10-fold increase in the formation of large colonies in agar. Cell lines established from these colonies displayed a highly transformed morphology, expressed the T24-encoded p21 ras protein, continued to express high levels of PKC, and were highly tumorigenic in nude mice. These results provide genetic evidence that PKC mediates some of the effects of the c-H-ras oncogene on cell transformation. Data are also presented suggesting that optimum synergistic effects between c-H-ras and PKC require critical levels of their respective activities. These findings may be relevant to the process of multistage carcinogenesis in tissues containing cells with an activated c-H-ras oncogene.

Multistage carcinogenesis: implications for risk estimation

In undertaking a quantitative estimation of carcinogenesis risk, it is essential to keep in mind that carcinogenesis is a multistage process, and that each stage can be affected by different classes of risk factors. Furthermore, different mechanisms are involved in the various stages of carcinogenesis. Thus, a dose-response analysis of one given factor cannot provide an accurate estimation of carcinogenic risk. Carcinogenic risk estimation is usually undertaken for a specific chemical or group of chemicals; however, the concept of multistage carcinogenesis is based on biological processes and not on the mechanisms of action of the agents involved. It is therefore important to consider three related, but different, factors involved in carcinogenesis: stage, agent, and activity of agent. This is especially important in developing a short-term test for stage-related risk factors, such as tumor-promoting agents. For this reason, carcinogens should not be classified according to only one chemical activity. This article briefly reviews the cellular and molecular mechanisms involved in multistage carcinogenesis, and discusses their implications for risk estimation. Special consideration is given to the effect of treatment frequency on the response of tumor-promoting agents, as seen in long-term tests in experimental animals. It is proposed that exposure frequency be taken into account together with exposure dose.

A factor present in fetal calf serum enhances oncogene-induced transformation of rodent fibroblasts

Our previous studies indicated that addition of the tumor promoters 12-O-tetradecanoylphorbol-13-acetate (TPA) or teleocidin to Dulbecco modified Eagle medium supplemented with calf serum enhanced T24-induced focus formation in both the murine C3H 10T1/2 and rat 6 embryo fibroblast cell lines. In the present studies we have found that fetal calf serum (FCS) is more potent than 12-O-tetradecanoylphorbol-13-acetate in enhancing T24-induced focus formation, in terms of the number and size of the foci, in both C3H 10T1/2 and rat 6 cells. Time course studies indicate that FCS can exert this enhancing effect when it is added several days after the transfection with T24 DNA. In rat 6 cells, an 11-fold increase in T24-induced focus formation occurred when the transfected cultures were maintained for only 1 day in 5% FCS, starting 4 days after the transfection. Several known growth factors, including epidermal growth factor, transforming growth factors alpha and beta, insulin, and platelet-derived growth factor, did not enhance T24-induced transformation in these cell systems. Fractionation studies indicate that the factor present in FCS has a molecular weight of about 1,300, is not lipid soluble, and is acid, base, and heat stable. These findings suggest that a factor(s) normally present in serum may enhance the emergence of tumor cells in vivo, by acting in concert with an activated oncogene, during the multistage carcinogenic process.

Neoplastic transformation of mouse mammary epithelial cells by in vitro exposure to N-methyl-N-nitrosourea

High-efficiency neoplastic transformation of mouse mammary epithelial cells in primary collagen gel culture was induced by N-methyl-N-nitrosourea (MNU). Mammary epithelial cells, isolated from virgin BALB/c mice, were embedded within collagen gels and grown in a serum-free medium containing prolactin, progesterone, and linoleic acid. The cells were then treated with MNU on day 3 of culture and subsequently at weekly intervals for up to 4 weeks. Eleven to 14 days after the final carcinogen treatment, the cells were removed from the collagen gels and injected into the cleared mammary fat pads of syngeneic hosts to assay for transformed cell populations. A single exposure or multiple exposures of these cells to MNU was effective in inducing tumorigenic cells that produced palpable tumors as early as 6 weeks after transplantation. Two treatments with MNU (100 micrograms/ml) were optimal for neoplastic transformation and produced tumors in 79% of the injected fat pads. All the tumors originated at the site of injection and had extensive central necroses. Histological examination indicated that the tumors were mammary carcinomas. Secondary transplantation of tumor pieces into intact mammary glands produced palpable carcinomas of the same histology within 1-8 weeks. Control cells cultured for the same periods of time as MNU-treated cells produced only ductal outgrowths that were morphologically similar to those found in the mammary glands of adult virgin hosts. This system provides a distinct means to study the mechanism of mammary neoplastic transformation at cellular and molecular levels.

Inducible gene expression from multiple promoters by the tumor-promoting agent, PMA

Phorbol ester tumor promoters affect a broad scope of changes in mammalian cells. This report describes the activation of expression of an introduced chloramphenicol acetyltransferase (CAT) reporter gene by the phorbol ester, phorbol 12-myristate 13-acetate (PMA), in a variety of fibroblast and hematopoietic cell lines. PMA-mediated activation appears to be promoter region specific, yet widespread. Enhanced gene expression is observed for four out of five promoter systems tested, and, in some cases, is dependent on the cellular environment. Further experiments indicate that PMA mediates elevated gene expression by rapidly increasing steady state levels of CAT mRNA. The broad range of promoters affected by PMA may help explain the high potency of this agent in tumor production.

A factor present in fetal calf serum enhances oncogene-induced transformation of rodent fibroblasts

Our previous studies indicated that addition of the tumor promoters 12-O-tetradecanoylphorbol-13-acetate (TPA) or teleocidin to Dulbecco modified Eagle medium supplemented with calf serum enhanced T24-induced focus formation in both the murine C3H 10T1/2 and rat 6 embryo fibroblast cell lines. In the present studies we have found that fetal calf serum (FCS) is more potent than 12-O-tetradecanoylphorbol-13-acetate in enhancing T24-induced focus formation, in terms of the number and size of the foci, in both C3H 10T1/2 and rat 6 cells. Time course studies indicate that FCS can exert this enhancing effect when it is added several days after the transfection with T24 DNA. In rat 6 cells, an 11-fold increase in T24-induced focus formation occurred when the transfected cultures were maintained for only 1 day in 5% FCS, starting 4 days after the transfection. Several known growth factors, including epidermal growth factor, transforming growth factors alpha and beta, insulin, and platelet-derived growth factor, did not enhance T24-induced transformation in these cell systems. Fractionation studies indicate that the factor present in FCS has a molecular weight of about 1,300, is not lipid soluble, and is acid, base, and heat stable. These findings suggest that a factor(s) normally present in serum may enhance the emergence of tumor cells in vivo, by acting in concert with an activated oncogene, during the multistage carcinogenic process.