Three dammarane-type saponins from Gynostemma pentaphyllum

Three new dammarane-type saponins, compounds 1 - 3, were isolated from the aerial parts of Gynostemma pentaphyllum. Their structures were determined using NMR analysis and acid hydrolysis. The structure and stereochemistry of compound 1 were confirmed by X-ray crystallography.

Nuclear beta catenin as a potential prognostic and diagnostic marker in patients with colorectal cancer from Hong Kong

AIMS

To study the expression of nuclear beta catenin in patients with colorectal cancer, colorectal adenoma, and colorectal polyps to elucidate its role in carcinogenesis, and its potential for prognosis and diagnosis.

METHODS

The expression of nuclear beta catenin was studied by immunohistochemistry using paraffin wax embedded specimens. Sixty specimens each of colorectal carcinoma, colorectal adenoma, colorectal polyp, and normal colorectal specimens were analysed. The potential for prognosis was assessed by correlating nuclear beta catenin expression in 60 and 75 patients with colorectal cancer with lymph node metastasis and survival, respectively. The diagnostic capacity was explored by comparing nuclear beta catenin expression in 60 patients with colorectal cancer with other cytokeratin 20 (CK20) positive adenocarcinomas, namely: 30 colonic mucinous adenocarcinomas, 30 gastric adenocarcinomas, 27 pancreatic adenocarcinomas, and 12 ovarian mucinous adenocarcinomas.

RESULTS

Nuclear beta catenin expression was highly associated with progression of colorectal tissue from normal epithelial tissue, polyps, adenomas, to carcinomas (r = 0.875; p < 0.0001). Nineteen patients with colorectal adenoma who subsequently developed colorectal carcinoma had higher nuclear beta catenin expression than those with colorectal adenomas alone (p < 0.0001). Moreover, those patients with colorectal cancer and high nuclear beta catenin expression had a higher incidence of lymph node metastasis (chi(2) = 16.99; p < 0.005) and shorter overall survival (p < 0.0001). Finally, nuclear beta catenin expression in colorectal adenocarcinomas was significantly higher than in other CK20 positive adenocarcinomas.

CONCLUSIONS

Nuclear beta catenin expression is a potential prognostic factor in patients with colorectal cancer, and together with CK20, it could be used to identify colorectal carcinoma in the Hong Kong population.

Transcriptional activity of the promoter region of rat frizzled-related protein gene

Frizzled-related protein (Frp) is a new family of secreted proteins involved in tumorigenesis and Wnt-signaling pathway. Previous study has shown that rat Frp (rFrp) gene was found to be differentially expressed in Rat 6 fibroblast cell line overexpressing p53(val135) (R6#13-8). The rFrp gene was otherwise silent in normal parental Rat 6 cells. To elucidate the molecular basis of the transcriptional activation of rFrp, we have isolated and analyzed a 2-kilobase pair promoter region of the rFrp gene. Mapping of transcription initiation sites of rFrp showed the existence of multiple initiation sites. Transfection studies of serial deletion constructs in both Rat 6 and CHOK1 cell lines revealed that the region from -202 to -144 contains cis-acting elements essential for the efficient transcription of rFrp. This work provides a transcriptional regulation basis for Frp and gives insight into its implication in tumorigenesis.

Differential expression of p16/p21/p27 and cyclin D1/D3, and their relationships to cell proliferation, apoptosis, and tumour progression in invasive ductal carcinoma of the breast

In order to understand the intricate relationship of cell proliferation and apoptosis in tumour development, proliferation markers (Ki-67 and c-myc), apoptosis, cell-cycle inducers cyclin D1 and D3, and cell-cycle inhibitors p16(INK4), p21(CIP1), and p27(KIP1) were evaluated in ductal breast carcinoma. The heterogeneous nature of breast tumours provides a system by which the changes in cell-cycle genes can be explored under a wide range of proliferation and apoptotic indices. To address the above issues, immunohistochemical studies were conducted in 40 pairs of tumours and adjacent normal ductal tissues. The TUNEL method was used to identify apoptotic cells. Except for p27/KIP1, the proliferation (Ki-67, c-myc) and the apoptotic indexes together with levels of p16/INK4a, p21/CIP1, cyclin D1, and cyclin D3, were clearly elevated among tumour tissues, while absent in the adjacent normal tissues. Spearman correlation analysis indicated strong associations among apoptotic index, Ki-67, c-myc, and tumour grade. In addition, p21/CIP1 and cyclin D3 were positively correlated, while p16/INK4a, p27/KIP1, and cyclin D1 were negatively correlated with tumour grade. There was clear decoupling between p21 and p27, as well as decoupling between cyclin D1 and cyclin D3, in terms of their relationship to cell proliferation and apoptosis, indicating differential roles in tumour progression.

Incorporating the ABI GeneScan analysis to a RACE-based technique for mapping multiple transcription initiation sites

Determination of transcription initiation sites has commonly been performed by primer extension and RNase protection assay using radioactively labeled oligonucleotides. Recently, a protocol based on modified 5' rapid amplification of cDNA ends (RACE) with the use of fluorescently labeled primer was developed. Here, we describe the use of RACE-based technique in conjunction with the GeneScan analysis for the determination of transcription initiation sites of genes of interest. The RACE technique is based on the ligation of an adapter to both ends of the cDNAs. The gene of interest was first amplified by PCR using a gene-specific and a 5' adapter primer. Subsequently, nested PCR was performed using an internal gene-specific primer paired with a fluorescently end-labeled adapter primer. The size of the fluorescently labeled PCR products was directly determined by the ABI PRISM 377 GeneScan Analyzer. This novel approach provides an accurate, sensitive, and convenient method for mapping transcription initiation sites, especially for genes with multiple transcription initiation sites, for genes expressed at low levels, and for splice variants that display alternative splicing farther than a few hundred nucleotides downstream from the transcription initiation site. This article describes the application of this new method in the mapping of transcription initiation sites of two splice variants of rat frizzled related protein transcripts.

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.

Cytotoxicity of PM(2.5) and PM(2.5--10) ambient air pollutants assessed by the MTT and the Comet assays

Ambient air particulate matters are classified into two distinct modes in size distribution, namely the coarse and fine particles. Correlation between high particulate concentration and adverse effects on human populations has long been recognized, however, the toxicology of these adverse effects has not been clarified. In the current report, the cytotoxic effects of the solvent-extractable organic compounds (SEOC) from fine particles smaller than 2.5 microm (PM(2.5)) and from coarse particles between 2.5-10 microm (PM(2.5-10)) were studied. Nine 24h consecutive monthly samples were tested to determine the correlation between cytotoxicity and total SEOC in two size fractions of particulate air pollution. Cytotoxicity of SEOC was measured by two micro-scale mammalian cells-based bioassays: the MTT cell proliferation assay, and the Comet assay for the detection of DNA damage. A well-defined mammalian cell line - Rat 6 rodent fibroblast was employed in the study. The SEOC extracts of air particulate matters were sub divided into two equal parts. One part was dissolved in DMSO, the other in KOH/hexane and then conjugated with bovine serum albumin to produce a lipid-soluble fraction for testing. The DMSO fraction would contain mainly the polycyclic aromatic hydrocarbons (PAH), alkanes and alkanols, while the lipid-soluble fraction would be enriched with fatty acids. The results from MTT assay showed that cytotoxicity of the PM(2.5) was much more severe than the PM(2.5-10), suggesting that toxic SEOC were confined to the fine particles. By and large, the DMSO solubles were much more toxic than the lipid solubles. The degree of cytotoxicity of the DMSO soluble samples is positively correlated to the amount of particulates present in the ambient air. For the PM(2.5), the winter samples were significantly more toxic than the summer samples in terms of cell killing, which seemed to be a direct reflection of the total loading of organic matter in the samples. Results from Comet assays showed that SEOC samples of PM(2.5) derived from winter months induced DNA damage at dosages resulting in no obvious cell killing in the MTT assay. Thus, long-term exposure to non-killing dosage of air pollutants may lead to the accumulation of DNA lesions, which may be one of the mechanisms responsible for the chronic adverse health effects of particulate air pollution.

Molecular cloning and functional analysis of the promoter region of rat nonmuscle myosin heavy chain-B gene

Rat nonmuscle myosin heavy chain-B (r-nmMHC-B) mRNA was previously found downregulated in Rat 6 fibroblasts transformed by mutant p53(val135) [J. W. P. Yam, J. Y. Zheng, and W. L. W. Hsiao (1987) Biochem. Biophys. Res. Commun. 266, 472-480]. Overexpression of exogenous r-nmMHC-B could partially reverse the transforming phenotypes both in vitro and in vivo. The downregulation of r-nmMHC-B was also observed in Rat 6 transformed by c-H-ras and v-myc oncogenes. We cloned a 5.2-kb r-nmMHC-B promoter region. Sequence analysis of -1248 to +1 revealed no TATA box, but did show that it contained CAAT boxes, E12/E47, MyoD, MEF, E2F, CREB, and SP1 binding sites. Based on transient reporter assays, the promoter/enhancer activities were unusually extended to the entire 5.2 kb region in normal Rat 6 cultures, but markedly suppressed in p53(val135)-, and c-H-ras-transformed cells. The activity detected by the reporter assay corresponded to levels of mRNA as analyzed previously by Northern blots in each respective cell line. Thus, the switch-off of the r-nmMHC-B in the transformed cells is very likely controlled by upstream transcriptional factors, which might have been altered in the course of neoplastic transformation.

[Cloning of cancer-related genes of Rat6 fibroblasts by using an improved differential display method]

The p53 gene is the most frequently mutated gene identified so far in human cancers. When a mutant p53(135)-val gene was allowed to be over-expressed in Rat6(R6) cells, a high incidence of spontaneous transformation was observed in long-term culture of this cell line(R6 # 13-8). To identify genes involved in cell transformation, parental p53 over-expressing cell, R6 # 13-8, and its spontaneous transformant T2, were analyzed by an improved mRNA differential display technique, which was reproducible, simpler, and was able to clone cDNA longer than 500 bp, and was with less false positives. When 33 10-mer or 12-mer single primers with arbitrary but defined sequence were used for PCR, over 90 discrete cDNAs were obtained from R6 # 13-8 and T2 cells. Three differentially expressed cDNAs were identified, one of them is highly expressed in T2 cells, while the other two, 0.8 kb and 0.9 kb long, are highly expressed in R6 # 13-8 cells. The latter were cloned and confirmed by Northern hybridization. Both cloned fragment were not homologous with any published sequence. Our results suggest that the activation and inactivation of genes are involved in the process of the spontaneous transformation from R6 # 13-8 to T2.

Identification and characterization of genes whose expressions are altered in rat 6 fibroblasts transformed by mutant p53(val135)

The wild-type tumor suppressor gene p53 is known as a transcription factor in activating or suppressing target genes that encode proteins in regulating genome stability, DNA damage, cell arrest, and apoptosis. However, the role of mutant p53 in the process of cell transformation is still unclear. Our recent work indicated that overexpression of mutant p53(val135) induced high incidence of spontaneous transformation in prolonged cultures of Rat 6 fibroblasts. In order to identify genes related to neoplastic transformation induced by the mutant p53, the p53(val135)-overexpressor R6#13-8 and its derived spontaneously transformed cell line T2 were analyzed by mRNA differential display. In a systematic screening with 80 primer sets of RT-PCR reactions, three genes were found to be differentially expressed between R6#13-8 and T2 cells. Two genes, identified as homologues of the growth factor inducible immediate-early gene Cyr61 and the human nonmuscle myosin heavy chain-B, were down-regulated in T2 cells. Interestingly, both genes were also suppressed in Rat 6 cells transformed by c-H-ras and v-myc, but not by v-src genes. The third gene is a homologue of the frizzled related protein, a gene family that acts, in some cases, as an antagonist to the Wnt signaling pathway. It is intriguing that the rat homologue of the frizzled related protein was only expressed in p53(val135)-overexpressing cells, but not in the parental Rat 6 cells. However, the same gene was also highly expressed in ras-transformed Rat 6 cells, and moderately expressed in v-src-transformed Rat 6 cells. This is the first study in which the association of mutant p53 to these three genes is revealed. Our current report may provide new clues to the role of mutant p53 in the process of cell transformation.

Comparison of transformation by manganese sulfate and 5-azacytidine in Rat 6 cells overexpressing the c-myc oncogene

Rat 6 cells are not transformed by treatment with the well-known carcinogens benzo[a]pyrene (BP) or N-methyl-N-nitro-N'-nitrosoguanidine (MNNG). Upon retroviral transduction of the mouse c-myc gene, Rat 6 cells showed mildly altered morphology and formed microcolonies in soft agar; furthermore, they could be transformed by BP and MNNG to form large colonies in agar (Hsiao et al. (1992) Mol. Carcinogenesis, 5, 140-154). In the current report, we tested the sensitivity of the c-myc-overexpressing cells (Rat 6/c-myc) to two additional chemicals: 5-azacytidine and MnSO4. These chemicals differ from the direct-acting mutagens tested previously. 5-Azacytidine, a potent DNA methylation inhibitor, induced growth of large colonies in soft agar cultures of Rat 6 or Rat 6/c-myc cells. On the other hand, MnSO4 only induced transformation in Rat 6/c-myc cells, but not the parental Rat 6 cells. Transformants induced by 5-azacytidine lost c-myc-induced apoptotic cell death, whereas MnSO4-induced transformants showed a higher degree of apoptosis than the parental Rat 6/c-myc cells. These results suggest that MnSO4 co-operates with overexpressed c-myc in inducing transformation, while 5-azacytidine transformation is independent of c-myc overexpression and may involve alterations in the regulation of apoptosis.

Differential spontaneous transformation in vitro of newly established mouse fibroblast lines carrying or lacking the viable yellow mutation (Avy) of the mouse agouti locus

The pleiotropic effects of the viable yellow mutation (Avy), an allele of the mouse agouti coat-color locus, include increased susceptibility to spontaneous and chemically induced tumors that affect a wide variety of tissues. As a first step toward understanding the molecular basis of this phenomenon, we established permanent fibroblast-like cell lines from newborn Avy/a and control congenic a/a mice and compared their growth characteristics in vitro. From the VY/WffC3Hf/Nctr and YS/WffCH3f/Nctr-Avy inbre strains, each of which carries the Avy allele on a congenic background, 38 clonal Avy/a and 16 clonal a/a lines were established. Regardless of inbred strain, all Avy/a cell lines exhibited a significant degree of spontaneous transformation, as assessed by focus formation in monolayer culture, whereas none of the a/a cell lines formed foci in prolonged cultures. To test whether changes in dosage of the Avy- or a-bearing chromosomes were related to these events, we analyzed each cell line with a closely linked molecular probe from the Emv-15 locus, which in the VY strain detects a restriction fragment length variant (RFLV) informative for the Avy- and a-bearing chromosomes. Most of the transformed foci maintained heterozygosity for RFLVs detected by the probe, but two of the transformants lost the a-associated RFLV, and at least one of the transformants exhibited amplification of the Avy-associated RFLV. When the transformants were analyzed with 5' sequences derived from the recently cloned agouti gene, three of eight transformants lost the a-associated RFLV, and two of the transformants showed amplification of the Avy-associated RFLV. Reverse transcriptase-polymerase chain reaction assays indicated that agouti RNA was detected in Avy/a, not a/a cell lines. Surprisingly, some of the Avy/a transformants lacked agouti RNA. These results suggest that deregulated expression of the Avy allele is required for the initiation but not for the maintenance of transformation of the Avy/a cell cultures. These cell lines may provide an in vitro culture system for studying the effect of the agouti gene on tumorigenicity as well as to potentially study other pleiotropic phenotypes.

Acquisition of responsiveness to chemical carcinogens by rodent embryo fibroblasts expressing high levels of the c-myc proto-oncogene

We investigated the ability of overexpression of the c-myc proto-oncogene to potentiate in vitro transformation by model chemical carcinogens. A mouse c-myc gene was introduced to C3H 10T1/2 and Rat 6 embryo fibroblast cell lines via a retroviral vector containing the gene for neomycin resistance. Our present work extends previous findings by showing that individual vectored C3H 10T1/2 clones have enhanced (two-fold to sevenfold) sensitivity to benzo[a]pyrene (BP) and N-methyl-N-nitro-N'-nitrosoguanidine (MNNG). Rat 6 clones acquiring the c-myc gene display various degrees of altered morphology. They form orderly but densely packed cells, grow to higher saturation density, and yield microcolonies in soft agar. The degree of altered growth properties is directly correlated with the level of c-myc expression. Transient exposure of c-myc-expressing clones to BP and MNNG induced the formation of distinct, large colonies in soft agar, whereas the untreated cells formed microcolonies and the parental Rat 6 cells remained single cells in soft agar. We also demonstrated that the degree of responsiveness to chemical carcinogens of the clones correlates with their ability to form microcolonies in soft agar. These cells overexpressing c-myc may be used as a model system to study the interaction between oncogenes and chemical carcinogens in the process of multistage carcinogenesis.

Transformation by a ras oncogene causes increased expression of protein kinase C-alpha and decreased expression of protein kinase C-epsilon

Rat embryo fibroblasts and liver epithelial cell lines normally express two isoforms of protein kinase C (PKC), PKC alpha and PKC epsilon. Derivatives of these cells transformed by an activated human c-H-ras oncogene display a several-fold increase in expression of PKC alpha and a concomitant decrease in PKC epsilon, at both the protein and mRNA levels. Similar changes are seen when the transformed phenotype is induced by Zn2+ in cells carrying the activated ras oncogene under the control of a metallothionein promoter. Studies using cell lines that express very high levels of PKC beta 1, studies using a specific inhibitor of PKC (CGP 41251), and studies in which PKC activity is down-regulated by treatment with a phorbol ester tumor promoter provide evidence that the effects of the ras oncogene on the expression of PKC alpha and PKC epsilon are mediated mainly through a PKC-independent pathway. The present results provide the first evidence that transformation of cells by an oncogene can alter the relative expression of specific isoforms of PKC. It is possible that these changes contribute to the malignant phenotype of these cells.

Effects of specific fatty acids on cell transformation induced by an activated c-H-ras oncogene

An increase in dietary lipid has been associated with an increase in the development of certain forms of cancer, notably breast and colon cancer, both in experimental animal studies and in human epidemiology studies. The underlying mechanisms are not, however, known with certainty. In the present studies we have examined whether certain specific fatty acids (FA) might act by enhancing the role of an activated oncogene in a model cell culture system. We found that when the rat fibroblast cell line Rat 6 was transfected with an activated human c-H-ras oncogene and the cells subsequently grown in medium supplemented with myristic acid, palmitic acid or stearic acid (20-80 microM) there was a marked enhancement of the number of transformed foci obtained. On the other hand arachidonic acid had a marked inhibitory effect in this transformation assay. However, this inhibitory effect can be partially reversed by indomethacin, an inhibitor of cyclo-oxygenase, at dose response manner. Control studies indicated that these results were not simply due to the effects of the FAs on growth of the Rat 6 cells or the process of transfection per se. Lipid analyses of cells grown in the presence of stearic acid indicated that the added FA was extensively incorporated into the major lipid classes of the cell and produced transient changes in lipid composition. This simple cell culture system may be useful for elucidating the mechanisms by which various dietary lipids and nutritional factors influence the carcinogenic process.

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.

Effects of triiodothyronine and tamoxifen on cell transformation induced by an activated c-Ha-ras oncogene

We have found that the thyroid hormone 3,5',3'-triiodo-L-thyronine stimulates the transformation of Rat 6 fibroblasts when these cells are transfected with an activated human c-Ha-ras oncogene (T24). 3,5',3'-Triiodo-L-thyronine did not further augment the stimulation of oncogene-induced transformation obtained with a phorbol ester tumor promoter (12-O-tetradecanoylphorbol-13-acetate) or a factor from fetal calf serum. On the other hand, tamoxifen, an antiestrogen that also inhibits protein kinase C, markedly inhibited Ha-ras-induced cell transformation in the presence of 12-O-tetradecanoylphorbol-13-acetate or fetal calf serum. Time course studies and Southern blot analyses of DNAs isolated from transformed foci provided evidence that 3,5',3'-triiodo-L-thyronine and tamoxifen do not exert their effects simply by enhancing or inhibiting integration of the transfected oncogene into cellular DNA. These findings indicate that hormonal factors can modulate the ability of an activated Ha-ras oncogene to transform cells. They may be relevant to the process of multistage carcinogenesis in vivo.

Effects of various chemical agents on the transformation of rat fibroblasts by an activated c-Ha-ras oncogene

We have previously reported that the potent tumor-promoting agent 12-O-tetradecanoylphorbol-13-acetate (TPA) and a factor from fetal calf serum (FCS) markedly enhance the transformation of mouse C3H 10T1/2 and Rat 6 fibroblasts, when added to cultures following transfection with plasmid pT24 DNA that contains an activated c-Ha-ras oncogene. In the present study, we examined possible enhancing or inhibiting effects of various chemicals on the transformation of Rat 6 fibroblasts by T24 DNA when tested in the presence of calf serum, calf serum plus TPA or FCS. We found that, like TPA, the chemicals mezerein, 1-oleoyl-2-acetylglycerol, and phospholipase C increased the yield of T24-induced foci, thus further implicating protein kinase C as a critical constituent in this process. Low concentrations (10(-6)-10(-7)M) of retinoic acid (both trans and 13-cis) also stimulated cell transformation. Several compounds inhibited T24-induced transformation. These included nontoxic concentrations of the calcium ionophore A23187, indomethacin, and epsilon-amino-n-caproic acid. Compounds that failed to exert a significant reproducible effect included vasopressin, vitamin D3, selenium, antipain, Bowman-Birk inhibitor, vitamin B12, epidermal growth factor, platelet-derived growth factor, insulin, and transferrin. These findings suggest that this simple in vitro system might be useful for detecting enhancers and inhibitors of ras oncogene-induced cell transformation and also elucidating their mechanisms of action.

Inhibition of c-H-ras oncogene induced transformation of rat embryo fibroblasts by cotransfected polynucleotides containing alternating purine-pyrimidine sequences

When Rat 6 cultures were cotransfected with an activated c-H-ras oncogene (pT24) and poly(dG-m5dC), a synthetic polymer that has the potential to form Z DNA, there was marked inhibition of cell transformation. Cotransfection of pT24 DNA with poly(dG-dC) caused somewhat less inhibition, poly(dA-dC). (dG-dT) caused moderate inhibition, and poly(dG). (dC) exerted negligible inhibition. Evidence was obtained that the inhibition seen with poly(dG-m5dC) was not simply due to an inhibition of cellular uptake of the pT24 DNA. Our results suggest that certain polymers that have the potential to form Z DNA can inhibit the integration and expression of a transfected oncogene.

Overproduction of protein kinase C causes disordered growth control in rat fibroblasts

We have generated a series of rat fibroblast cell lines that stably overexpress a full-length cDNA encoding the beta 1 form of protein kinase C (PKC). These cell lines contain a 20- to 53-fold increase in PKC activity and exhibit dramatically enhanced morphologic changes following exposure to the tumor promoter 12-O-tetradecanoyl phorbol-13-acetate (TPA). They grow to a high saturation density in monolayer cultures and, when maintained at postconfluence, develop small, dense foci. In contrast to control cells, which display complete anchorage dependence, PKC-overproducing cells form small colonies in soft agar in the absence of TPA and large colonies in the presence of TPA. Thus, the mere overproduction of a single form of PKC is sufficient to confer multiple growth abnormalities in rat fibroblasts. These results provide direct evidence that PKC plays a critical role in growth control and that it mediates several of the cellular effects of the phorbol ester tumor promoters. They also suggest that the activation of PKC may be of central importance in the process of multistage carcinogenesis.

Structural and functional studies of protein kinase C

It has now become clear that protein kinase C (PKC) plays a fundamental role in cellular growth control in higher eukaryotes. In addition, since PKC has also been shown to be the high-affinity intracellular receptor for several classes of tumor promoters, the study of PKC has also become a central focus of current work in cancer research. We have isolated cDNA clones encoding several forms of this enzyme, and we have used these clones to begin to study in detail the role of PKC in growth control and tumor promotion. The complete primary structure of one of these forms, designated PKC beta 1, exhibits structural and functional characteristics which are shared among all of the currently identified forms of PKC. These include an amino terminal cysteine-rich domain which mediates Ca2+ and phospholipid binding, tumor promoter binding, and membrane association, and a carboxy terminal catalytic domain which possesses serine/threonine protein kinase activity. To further characterize the function of PKC, we have generated a series of rat fibroblast cell lines which stably overexpress a full-length cDNA encoding the beta 1 form of this enzyme. These cell lines contain a 20- to 53-fold increase in PKC activity, and also have an increase in high affinity phorbol ester receptors, relative to control cells. They also exhibit dramatically enhanced morphologic changes in response to treatment with the tumor promoter 12-0-tetradecanoyl phorbol-13-acetate (TPA). These cell lines grow to a higher saturation density in monolayer culture and, when maintained at post-confluence, develop small, dense foci. In contrast to the control cells, which display complete anchorage dependence, the cell lines that overproduce PKC form small colonies in soft agar in the absence of TPA, and larger colonies in the presence of TPA. Thus, the mere overproduction of a single form of PKC is sufficient to confer anchorage independent growth and other growth abnormalities in rat fibroblasts. Taken together, these results provide direct evidence that PKC plays a critical role in normal cellular growth control and that it mediates several, and perhaps all of the cellular effects of the phorbol ester tumor promoters.

Differences between normal and transformed murine fibroblasts in the expression of various promoter/enhancer-chloramphenicol acetyltransferase constructs

High levels of poly(A)+ RNAs homologous to certain endogenous retrovirus-related DNA sequences are frequently seen in carcinogen-transformed rodent cells. To explore the underlying mechanism, transient expression assays were done to determine whether carcinogen- or radiation-transformed C3H 10T 1/2 cell lines differ from normal 10T 1/2 cells in terms of their ability to express the chloramphenicol acetyltransferase (CAT) gene when it is linked to various promoter/enhancer sequences, including two independently isolated intracisternal A particle (IAP) long terminal repeat sequences designated prcm and pMIA6. We found that with several constructs, CAT activity was always 3- to 10-fold higher in the transformed 10T 1/2 cell lines than in the normal 10T 1/2 cells. The prcm-CAT construct displayed the highest CAT activity in both the normal and transformed C3H 10T 1/2 cells. Studies with 32P-labeled prcm-CAT DNA and Southern blot analyses indicated that the differences in CAT activity between normal and transformed cells were not due to greater uptake or retention of the transfected DNA by the transformed cells. Competition studies provided evidence that factors required for the expression of the prcm-CAT construct are present in limited amounts in normal 10T 1/2 cells and in excess amounts in transformed 10T 1/2 cells. These putative factors may play a role in the increased expression of endogenous retrovirus-related sequences in the transformed cells.

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.

Expression of retroviral sequences and oncogenes in rat liver tumors induced by diethylnitrosamine

The expression of three cellular oncogenes (c-myc, c-Ha-ras, and c-delta-raf), the epidermal growth factor receptor gene, and two endogenous retrovirus-like sequences [rat leukemia virus (RaLV) and 30S] was examined in control rat livers and in 16 liver tumors. The tumors were induced in Sprague-Dawley male and female rats by a single i.p. injection of diethylnitrosamine at 1 or 2 days after birth, followed by dietary exposure to phenobarbital beginning at weaning. Increased expression of c-myc was seen in most of the tumors, but there was no consistent increase or decrease in expression of c-Ha-ras or c-delta-raf. It is of interest that a number of the tumor samples showed a decrease in epidermal growth factor receptor RNA. In all of the tumors, including both hepatocellular adenomas and carcinomas, there was a marked increase in expression of the endogenous RaLV sequence, and over 90% of the tumors displayed increased expression of the 30S endogenous retroviral-like sequence. No or a very low level of expression of the RaLV and 30S sequences was found in the control livers. The extent of expression of the RaLV and 30S sequences in individual tumors did not correlate with the extent of expression of c-myc or c-Ha-ras. Although increased expression of certain endogenous retrovirus-related sequences appears to be a common finding during rat liver carcinogenesis, the significance of this finding remains to be determined.

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

Rat embryo fibroblast cell line 6 was transfected with plasmid pT24, which contains the activated human bladder c-Ha-ras oncogene, and the cells were grown continuously in the absence or presence of the tumor promoters 12-O-tetradecanoyl phorbol-13-acetate (TPA) or teleocidin. The presence of TPA or teleocidin led to a 6- to 14-fold increase in the number of morphologically transformed foci. No transformed foci were seen when rat 6 cells were transfected with the normal c-Ha-ras oncogene in the absence or presence of TPA, or in cells simply treated with TPA or teleocidin. Enhancement of pT24-induced foci was seen even when the addition of TPA was delayed until day 16. In transfection studies with the drug resistance genes gpt and neo, TPA and teleocidin did not increase the number of Gpt+ or Neo+ colonies. When rat 6 cells were cotransfected with pT24 and neo genes and grown in the absence or presence of TPA, the presence of TPA did not increase the yield of Neo+ colonies but caused a fivefold increase in the number of Neo+ colonies that displayed a transformed morphology. Southern blot analyses of DNAs obtained from these clones indicated that TPA treatment did not influence the extent of integration of either the pT24 or neo gene. DNA samples from all of the morphologically transformed cells displayed a characteristic 2-kilobase SacI fragment homologous to pT24 DNA and expressed relatively high levels of the corresponding mRNA. Our findings indicate that in this system tumor promoters do not simply enhanced the process of DNA transfection per se. Thus, this model system may be useful for analyzing synergistic interactions between tumor promoters and activated oncogenes during multistage carcinogenesis. It may also serve as a simple screening test for detecting new tumor promoters.

Oncogene-induced transformation of C3H 10T1/2 cells is enhanced by tumor promoters

The tumor promoters 12-O-tetradecanoyl-phorbol-13-acetate and teleocidin markedly enhanced the transformation of C3H 10T1/2 mouse fibroblasts when these cells were transfected with the cloned human bladder cancer c-rasH oncogene. Transfection studies with the drug resistance marker gpt and time course studies indicate that this enhancement is not simply an effect on the process of DNA transfection. These findings, together with parallel studies with NIH 3T3 fibroblasts, also indicate that the competence of animal cells for DNA transfection is a function of the recipient cell line, the transfected marker, and the growth conditions. Our findings suggest that during multistage carcinogenesis tumor promoters may complement the function of activated cellular oncogenes.

Effects of 5-azacytidine on methylation and expression of specific DNA sequences in C3H 10T1/2 cells

The present study indicates that the transient exposure of C3H 10T1/2 mouse embryo fibroblasts to 5-azacytidine leads to extensive loss of methylation of the protooncogene c-mos and the beta-globin locus at the cell population level and in at least 40 isolated subclones. These changes persisted, even when the cells were serially passaged for many generations without further exposure to the drug. Even though the amount of demethylation, assessed through differential digestion by the restriction enzymes HpaII and MspI, was quite extensive, neither locus was transcribed at a detectable level. This nonselective analysis suggests, therefore, that loss of DNA methylation is not sufficient per se to induce the expression of certain loci. Presumably, the expression of these loci requires additional factors, some of which may be related to cell lineage and differentiation.

Rearranged c-mos locus in a MOPC 21 murine myeloma cell line and its persistence in hybridomas

Studies of a number of normal and carcinogen-transformed murine cell lines, and a variety of murine tissues, have indicated that, in contrast to several other cellular oncogenes, the oncogene c-mos gene is usually transcriptionally silent. The recent report by Rechavi et al. indicating that in the mouse myeloma XRPC24 originally induced by pristane (2,6,10-14-tetramethylpentadecane) the c-mos gene is rearranged and transcriptionally active, and that it can transform murine fibroblasts in a transfection assay, is therefore of considerable interest. Here we show that the c-mos locus has undergone a similar rearrangement, and is also transcriptionally active, in the cell line P3-X63-Ag8-653, a derivative of the mouse myeloma MOPC 21 which was induced by mineral oil. This line is widely used for making hybridomas that synthesize monoclonal antibodies. We also demonstrate that the rearranged c-mos sequence is maintained in three different hybridomas derived by fusion of this cell line with normal murine spleen lymphocytes, suggesting that it may play a role in the continuous growth and/or constitutive immunoglobulin production by these hybridomas.

Resistance to 5-fluoroorotic acid and pyrimidine auxotrophy: a new bidirectional selective system for mammalian cells

We have isolated a clone of murine erythroleukemic cells which will yield a population when a single cell is suspended in a medium composed of dialyzed sera and small molecules. We report that it is feasible--in one experiment--to screen more than 10 of these cells for growth under selective media containing 10(-4) M 5-fluoroorotic acid and 10(-4) M uridine. Cells capable of sustained growth in such media were eventually recovered. Clones of these cells, unlike clones of the parental population, required uridine for growth and contained only 0.39% as much orotate phosphoribosyltransferase, less than 0.5% as much 5-fluoroorotate phosphoribosyltransferase, and 0.013% as much orotidine-5'-monophosphate decarboxylase activity as the parental clones. The parental and variant clones had similar levels of activity for six other enzymes that participate in pyrimidine metabolism.

Effects of natural pyrimidines and of certain related compounds on the spontaneous activity of the mouse

Uracil and perhaps other natural pyrimidines may effect the level of arousal of the mammalian brain since: 1) heterocyclic 6-membered rings, which resemble uracil, form part of the structure of many hypnotics; and 2) 6-azauracil (and its riboside) have shown to be hypnotic for several mammalian species, including man. The parenteral administration of uridine, 6-azauridine, cytidine or thymidine depressed the spontaneous activity of adult male C-57 mice. 6-Azauridine was much less potent than the other ribosides tested. Cytosine, barbituric acid, 2-thiobarbituric acid, 2,4-dihydroxypyridine and a variety of pyrimidine catabolites had no effect on activity. Thymine, uracil, 6-azauracil, barbital and phenobarbital increased activity at lower doses and decreased activity at higher ones. 6-Azauracil and uracil were about equally potent as stimulants of activity, but 6-azauracil had about twice the potency of uracil as a depressant of activity. Thymine, which was more active than uracil, had about 10% the potency of barbital, both as a stimulant and as a depressant of activity. For thymine and the two barbiturates the ED50 (for depression of activity) was of the same magnitude as the LD50, while the dose which caused 50% stimulation of activity was about an order of magnitude less than the LD50. These results suggest that the barbiturates might affect arousal by simulating the structure of thymine or uracil at some receptor.