Hamster Cells In Culture Research Articles

Demographic Approaches to the Study of Aging on Cell Cultures

Aging organisms die out in accordance with the “Gompertz law,” i.e., the probability of their death increases with age. Survival curve construction is the main tool for gerontologists to study aging and test antiaging drugs. The analysis of survival curves includes obtaining some indices characterizing aging of the population, for example, the average and maximum lifespan, the mortality rate, and the aging rate. Testing geroprotectors can be correctly performed only by obtaining such curves. The dying out of stationary cell populations—bacteria, yeast, and mammalian cell cultures—also occurs in accordance with the Gompertz equation. In this regard, it is reasonable to use the construction of survival curves and their analysis to study the “aging” of non-subcultured cell cultures and testing anti-aging drugs on them. We used this approach in our experiments, due to which we were able to detect the positive anti-aging effect of the Quinton Marine Plasma on stationary phase aging culture of Chinese hamster cells.

Studies into the Effect of “Mild” Uncoupling with 2,4-Dinitrophenol on the Growth of Chinese Hamster Cell Culture and Its Subsequent Dying out in the Stationary Phase

Partial uncoupling of the processes of oxidative phosphorylation and energy storage in the form of ATP (“mild” uncoupling) helps reduce the production of reactive oxygen species and can also mimic the effect of calorie restriction. A number of studies have shown that uncouplers, such as 2,4-dinitrophenol (DNP), affect the lifespan of Drosophila, yeast, mice, and rats as well as the manifestation of “age-related” changes in cultures of mammalian and human cells undergoing replicative senescence. This paper is devoted to studying the effect of DNP on the growth and subsequent dying out of “stationary phase aging” Chinese hamster cells. Using the method for evaluating the colony-forming efficiency of cells, the maximum permissible concentration was selected, 5 ×10–5 M, in which the substance presumably induces “mild” uncoupling and does not inhibit cell proliferation. At higher concentrations, DNP has a cytotoxic effect on the studied cell culture. Under the influence of DNP in the potentially “mild” uncoupling concentration (5.6 × 10–7 M), the kinetics of cell growth and dying out does not change, and the lifespan of the cell culture does not increase. This effect may be due to the type of cells studied. In addition, there is a probability that the optimal concentration lies in the range from 5 ×10–7 to 5 × 10–5 M or even lower than 5 × 10–7 M.

Impairment of the Viability of Transformed Chinese Hamster Cells in a Nonsubcultured Culture under the Influence of Exogenous Oxidized Guanoside is Manifested Only in the Stationary Phase of Growth

Despite the fact that oxidation products of nucleotides and nucleosides are markers of oxidative stress, reports of the paradoxical ability of these compounds to protect cells from the harmful effects of reactive oxygen species began to appear more often. Among all nitrogenous bases, guanine is most susceptible to the influence of oxidative stress; therefore, guanosine is oxidized more often than other bases. In the present work, the effect of exogenous 8-oxo-2′-deoxyguanosine on the growth and “stationary phase aging” (accumulation of “age-related” changes in cultured cells during cell proliferation slowing down within a single passage and subsequent “aging” in the stationary growth phase) of nonsubcultured transformed Chinese hamster cells was studied. We showed that the nucleoside is rapidly absorbed by the cells from the medium, but it does not affect the growth of the culture, and impairs the viability of the cells in the late stationary growth phase. Thus, no mitogenic or geroprotective effect of 8-oxo-2′-deoxyguanosine was found.

Murine extracellular superoxide dismutase is converted into the inactive fold by the Ser195Cys mutation.

We have previously shown that human extracellular superoxide dismutase (EC-SOD) exists as two variants with differences in their disulfide bridge patterns: one form is the active enzyme (aEC-SOD), and the other is inactive (iEC-SOD). The availability of both active and inactive folding variants significantly reduces the specific activity of EC-SOD in vivo. Both forms are produced during biosynthesis, but the underlying folding mechanisms remain unclear. To address this issue, we expressed EC-SOD in heterologous systems that do not endogenously express iEC-SOD. Rodents express only aEC-SOD because they lack Cys195 (human EC-SOD sequence numbering), which is essential for the formation of iEC-SOD. However, cultured hamster cells and transgenic mice expressing human EC-SOD were able to produce both human a- and iEC-SOD variants, which led us to hypothesize that the folding was sequence-dependent rather than a property of the expression system. To substantiate this hypothesis, we expressed murine EC-SOD in a human cell line, and as expected, only aEC-SOD was produced. Significantly, when Cys195 was introduced, both murine aEC-SOD and a novel murine iEC-SOD were generated, and the specific activity of the murine EC-SOD was significantly reduced by the mutation. Collectively, these data suggest that Cys195 actuates the formation of iEC-SOD, independent of the expression system or host. In addition, the dual-folding pathway most likely requires biosynthesis factors that are common to both humans and rodents.

Novel Models of Avian Leucosis Virus-induced CarcinogenesisNovel Models of Avian Leucosis Virus-induced Carcinogenesis

The LSCC-SF-MC29 cell culture model system was further characterized by studies on the provirus content of the cells, the host range and the subgroup specificity of the produced virus. The transforming potential of the Mc29 virus was evaluated by the focus-forming and colony-forming assays on primary cell cultures and continuous cell lines of avian and mammalian origin. The in ovo effects of the myelocytomatosis virus Mc29 on 15I line White Leghorn chicken embryos were studied by routine histopathological methods. Six avian leucosis virus-specific proviral sequences were detected by PCR analysis in the genome of LSCC-SF-MC29 cells. The presence of a Mc29 provirus-specific sequence located in the gag-myc region was confirmed. Using primers designed to differentiate ALV subgroups, amplification product was obtained with subgroup B/D-specific PCR primers. As it was expected, the subgroup E-specific PCR primers amplified the endogenous ALV sequences. In vitro studies on the host range of Mc29 virus showed that the primary cultures of chicken and hamster cells and a continuous hamster cell line were susceptible, while the cultures of primary quail cells and of a permanent line of duck embryo cells were resistant to the transforming effect of the virus. In ovo, the inoculated Mc29 virus induced hyperplasic and preneoplastic lesions in the embryonal liver and pancreas and myxomas of the neck.

Induction of apoptosis in populations of mammalian cells in vitro under the influence of lectins

A study of the cytogenetic action of lectins that differ in terms of origin and hydrocarbon specification is presented. In a culture of Chinese hamster cells it is shown that at high concentrations (20 μg/ml), all lectins are capable of inducing an increase in the level of apoptosis two days after treatment with a high degree of reliability. In the case of perch roe lectin, such an increase occurs both immediately after treatment as well as two days later. It is found that, unlike lentil and perch roe lectin, in a culture of human cells black elder lectin does not have any effect on the incidence of apoptotic cells. A trend towards stimulation of proliferation of human cells under the influence of perch roe lectin at low concentrations (0.2 μg/ml) is discovered.

Antibiotic amoxicillin induces DNA lesions in mammalian cells possibly via the reactive oxygen species

Amoxicillin is a commonly prescribed drug for anti- bacterial infection. In this study, we are interested in the effect of the drug on the cellular DNA integrity. Amoxicillin was added to the human or hamster cells in culture, and the DNA lesions induced by the drug were assessed by a comet assay with nuclear extract incubation (Wang et al., 2005 Anal Biochem 337: 70–75). Amoxicillin at 5 mM rapidly induced DNA lesions in human AGS cells. The level of DNA lesions attained a maximum at about 1 h, and then declined steadily and reached almost the basal level at 6 h following the drug treatment. Similar induction pattern of DNA lesions was found with amoxicillin-related antibiotics such as ampicillin but not with the unrelated antibiotics such as kanamycin. For studying the repair kinetics, the cells were treated with amoxicillin for only 1 h and continued culture in the absence of the drug for a certain period of time before subsequent analysis. Repair of the amoxicillin-induced DNA lesions was essentially completed within 4 h. Such repair may not involve nucleotide excision repair (NER) pathway because the repair was completed with similar kinetics in both NER proficient Chinese hamster CHO-K1 cells and its isogenic NER deficient UV24 cells. Instead, the repair may involve base excision repair (BER) pathway because immunodepletion of OGG1/2, glycosylases involved in BER rendered the nuclear extract unable to excise DNA lesions induced by amoxicillin in the modified comet assay. Furthermore, amoxicillin induced intracellular reactive oxygen species (ROS) at the tempo similar to that of DNA lesions induction. Thus, we hypothesize that amoxicillin causes oxidative DNA damage in mammalian cells via ROS.

Poly(ADP-Ribose)-polymerase-1 and aging: Experimental study of possible relationship on stationary cell cultures

Experiments on "stationary aging" cultures of B11dii-FAF28 Chinese hamster cells showed that contact inhibition of cell growth and further culturing of cells in the stationary phase led to continuous inhibition of enzymes realizing poly-ADP-ribosylation of chromatin proteins. Cell density in the monolayer and percentage of damaged cells detected by trypan blue staining decreases during this process.

The Detection of Hamster Connexins: A Comparison of Expression Profiles with Wild-Type Mouse and the Cancer-ProneMinMouse

The open reading frames of 17 connexins from Syrian hamster (using tissues) and 16 connexins from the Chinese hamster cell line V79, were fully (Cx30, Cx31, Cx37, Cx43 and Cx45) or partially sequenced. We have also detected, and partially sequenced, seven rat connexins that previously were unavailable. The expression of connexin genes was examined in some hamster organs and cultured hamster cells, and compared with wild-type mouse and the cancer-prone Min mouse. Although the expression patterns were similar for most organs and connexins in hamster and mouse, there were also some prominent differences (Cx29 and 30.3 in testis; Cx31.1 and 32 in eye; Cx46 in brain, kidney and testis; Cx47 in kidney). This suggests that some connexins have species-specific expression profiles. In contrast, there were minimal differences in expression profiles between wild type and Min mice. Species-specific expression profiles should be considered in attempts to make animal models of human connexin-associated diseases.

Enhancement and Inhibition by 2′-O-Hydroxyethyl Residues of Gene Targeting Mediated by Triple Helix Forming Oligonucleotides

Reagents that recognize and bind specific genomic sequences in living mammalian cells would have great potential for genetic manipulation, including gene knockout, strain construction, and gene therapy. Triple helix forming oligonucleotides (TFOs) bind specific sequences via the major groove, but pyrimidine motif TFOs are limited by their poor activity under physiological conditions. Base and sugar analogues that overcome many of these limitations have been described. In particular, 2′-O-modifications influence sugar pucker and third strand conformation, and have been important to the development of bioactive TFOs. Here we have analyzed the impact of 2′-O-hydroxyethyl (2′-HE) substitutions, in combination with other 2′ modifications. We prepared modified TFOs conjugated to psoralen and measured targeting activity in a gene knockout assay in cultured hamster cells. We find that 2′-HE residues enhance the bioactivity of TFOs containing 2′-O-methyl (2′-OMe) modifications, but reduce the bioactivity of TFOs containing, in addition, 2′-O-aminoethyl (2′-AE) residues.

Instability of chromosome structure in cancer cells increases exponentially with degrees of aneuploidy

Structurally altered or marker chromosomes are the cytogenetic hallmarks of cancer cells, but their origins are still debated. Here we propose that aneuploidy, which is ubiquitous in cancer and inevitably unbalances thousands of synergistic genes, destabilizes the structure of chromosomes by catalyzing DNA breaks. Aneuploidy catalyzes such breaks by unbalancing teams of enzymes, which synthesize and maintain DNA and nucleotide pools, and even unbalancing histones via the corresponding genes. DNA breaks then initiate deletions, amplifications, and intra- and interchromosomal rearrangements. Our hypothesis predicts that the rate at which chromosomes are altered is proportional to the degree of aneuploidy: the more abnormal the number and balance of chromosomes, the higher the rate of structural alterations. To test this prediction, we have determined the rates at which clonal cultures of diploid and aneuploid Chinese hamster cells generate new, and thus nonclonal, structurally altered chromosomes per mitosis. Based on about 20 metaphases, the number of new, structurally altered chromosomes was 0 per diploid, 0–0.23 per near-diploid, 0.2–1.4 per hypotriploid, 3.25–4.8 per hypertriploid, and 0.4 per near-tetraploid cell. Thus, instability of chromosome structure increases exponentially with the deviation of ploidy from the normal diploid and tetraploid balances. The particular chromosomes engaged in aneuploidy also affected the rates of chromosome alteration, particularly at low aneuploidy indices. We conclude that aneuploidy is sufficient to cause structural instability of chromosomes. Further, we suggest that certain structurally altered chromosomes encode cancer-specific phenotypes that cannot be generated by unbalancing intact chromosomes. We also extend the evidence for aneuploidy causing numerical instability of chromosomes autocatalytically, and adduce evidence that aneuploidy can cause the many gene mutations of cancer cells that have been attributed to various mutator genes.

Cell studies of the DNA bis-intercalator Delta-Delta [mu-C4(cpdppz)(2)-(phen)(4)Ru(2)](4+): toxic effects and properties as a light emitting DNA probe in V79 Chinese hamster cells.

Coordination complexes of type [Ru(L)(3)](2+), where L is a nitrogen-containing aromatic bidentate ligand, can often be photolytically reduced, making them useful in studies of DNA- or protein-mediated electron transfer and in artifical photosynthesis model systems. Upon binding to DNA some Ru(L) complexes have been found to display strongly increased fluorescence compared with when free in solution, making those compounds interesting to test as DNA probes. Thus, they are becoming widely used in the chemistry community. Here, asynchronous cultures of V79 Chinese hamster cells were exposed to the DNA bis-intercalator Delta-Delta [mu-C4(cpdppz)(2)-(phen)(4)Ru(2)](4+) at 10(-10)-10(-4) M. The extraordinarily strong binding of the compound to DNA was the reason for testing its possible interference with DNA metabolism in intact mammalian cells. Exposure for 1 h to 10(-10)-10(-4) M did not significantly decrease DNA synthesis. Cells exposed to 10(-5) M for 27 h showed no staining of the nucleus, while DNA was stained in cells electroporated in the presence of the compound. However, the Ru dimer was probably taken up by pinocytosis, because numerous minute precipitates could be observed in the cytoplasm. Treatment for 24 h at concentrations of 10(-10)-10(-5) M did not inhibit growth, as indicated by cell density and mitotic activity. Neither did it affect chromosomal arrangements during mitosis. However, at 10(-4) M the density of cultures was reduced by approximately 45% and apoptotic cells were frequent, as opposed to mitoses. We also investigated the properties of the Ru dimer as a fluorescent DNA stain. The compound appears attractive as a red DNA stain when broad excitation in the visible range is desirable and extremely low background staining is essential. The low toxicity of the compound is a favourable trait in this context.

More Than a Painkiller: Ibuprofen's hidden talent thwarts Alzheimer's disease

As the Mona Lisa smiled from the wall, her creator scribbled down ideas for his flying machine. Sometimes a superstar's famous gifts overshadow additional genius. That appears to be the case for a drug that might prevent Alzheimer's disease (AD): According to new work, ibuprofen blocks the formation of brain-gumming amyloid plaques (see "Detangling Alzheimer's Disease" ) by a mechanism unrelated to its starring role as an anti-inflammatory painkiller. Several epidemiological studies have suggested that nonsteroidal anti-inflammatory drugs (NSAIDs) protect people from AD. Recent experiments showed that the NSAID ibuprofen reduces the number of brain plaques in a mouse model of AD when administered in high doses. Because chronic inflammation in the Alzheimer's brain releases neurotoxins and molecules that promote plaque formation, scientists ascribed the effect of NSAIDs to the anti-inflammatory power of the drugs. The new results suggest an alternative mechanism for NSAIDs' AD-fighting power: They directly block the generation of a protein that clumps into plaques in the brain. The results might explain the mixed outcomes of recent NSAID clinical trials for AD and could help drugmakers circumvent the potential side effects of compounds currently in development. In a key event during the development of AD, an enzyme activity known as γ-secretase trims the so-called β-amyloid precursor protein (APP) into a sticky, plaque-forming protein called Aβ 42. Weggen and colleagues tested whether various NSAIDs influence this molecular snipping. The researchers engineered cultured hamster cells to produce large amounts of both APP and a deviant version of a protein called presenilin 1 (PS1), which might be γ-secretase itself and in any case is required for γ-secretase function. When the cells were exposed to either of three NSAIDs--ibuprofen, indomethacin, or sulindac sulfide--the ratio of Aβ 42 to the shorter, more benign Aβ 40 fell as compared with untreated cells. Results from live animals further suggest that ibuprofen dulls the scissors that shape Aβ 42: In mice engineered to produce large amounts of an AD-associated form of APP, those that were treated with ibuprofen carried 40% less Aβ 42 than untreated ones did. NSAIDs reduce inflammation primarily by inhibiting cyclooxygenase (COX) enzymes, which rev up production of inflammatory prostaglandins. To find out whether NSAIDs depend on COX inhibition to deflate Aβ 42 production, the researchers engineered COX-1- and COX-2-deficient mouse cells to manufacture large amounts of normal APP. Sulindac sulfide treatment reduced Aβ 42 levels, even in the absence of COX. Together, the results indicate that certain NSAIDs can hinder a crucial biochemical step in the genesis of AD--Aβ 42 formation--a capability distinct from their usual anti-inflammatory action. Other NSAIDs, however--including naproxen, aspirin, and celecoxib--do not reduce the amount of Aβ 42 in cells that overproduce APP. These combined observations could explain the mixed results of clinical trials with NSAIDs: Some of these drugs have shown promise in preventing AD, whereas others have not. Many drugs currently in development inhibit beneficial activities of γ-secretase, which leads to faulty T cell development. Additional work in the current study suggests that sulindac-sulfide-treated cells do not suffer from the molecular deviations that underlie these side effects. The new work hints that drugs based on the structure of, for instance, ibuprofen might inhibit the production of Aβ 42 but leave the normal functions of γ-secretase intact. Such an agent could transform this obscure forte of ibuprofen into a celebrated talent. --R. John Davenport S. Weggen, J. L. Eriksen, P. Das, S. A. Sagi, R. Wang, C. U. Pietrzik, K. A. Findlay, T. E. Smith, M. P. Murphy, T. Butler, D. E. Kang, N. Marquez-Sterling, T. E. Golde, E. H. Koo, A subset of NSAIDs lower amyloidogenic Aβ42 independently of cyclooxygenase activity. Nature 414 , 212-216 (2001). [Abstract] [Full Text]

Final Report on the Safety Assessment of Cocoyl Sarcosine, Lauroyl Sarcosine, Myristoyl Sarcosine, Oleoyl Sarcosine, Stearoyl Sarcosine, Sodium Cocoyl Sarcosinate, Sodium Lauroyl Sarcosinate, Sodium Myristoyl Sarcosinate, Ammonium Cocoyl Sarcosinate, and Ammonium Lauroyl Sarcosinate

This safety assessment addresses cosmetic ingredients that are N-acyl derivatives of sarcosine and are generally referred to as acyl sarcosines, and those that are salts, known generally as acyl sar-cosinates. Previous assessments have addressed the safety of each of the fatty acids that appear in these acyl sarcosines and sarcosinates (Coconut Acid, Oleic Acid, Lauric Acid, and Myristic Acid). In each case the fatty acid was either safe for use or safe as used in cosmetic formulations. Acyl sarcosines are considered modified fatty acids with greater solubility and increased acidity of the carboxylic acid group compared to the parent fatty acid. They are used in a large number of cosmetic formulations as hair-conditioning agents and surfactant-cleansing agents. In soaps, concentrations are reported to be as high as 12.9%. These ingredients have low oral toxicity in rats. Although cytotoxic to Chinese hamster cells in culture, acyl sarcosines and sarcosinates are not mutagenic in those cells, nor in bacterial cells in culture. Carcinogenicity data were not available. These ingredients are nonirritating and nonsen-sitizing to animal and human skin, although they can enhance the penetration of other ingredients through the skin. For that reason, caution should be exhibited in formulating cosmetic products that contain these ingredients in combination with other ingredients whose safety is based on their lack of absorption or where dermal absorption is a concern (e.g., HC Yellow No. 4, Disperse Yellow 3). Because sarcosine can be nitrosated to form N-nitrososarcosine, a known animal carcinogen, these ingredients should not be used in cosmetic products in which N-nitroso compounds may be formed. With the above caveat, and based on the available data, it was concluded that these acyl sarcosines and sarcosinates are safe as used in rinse-off products. They may be safely used in leave-on products at concentrations up to 5%, the highest concentration tested in clinical irritation and sensitization studies. Oleoyl Sarcosine is used as a corrosion inhibitor in some aerosol products, at extremely low concentrations. In this circumstance, the ingredient is not being used as a cosmetic ingredient and this report is not intended to limit that use. Because of the absence of data on inhalation toxicity, however, it was concluded that the available data were not sufficient to support the safety of acyl sarcosines and sarcosinates as cosmetic ingredients in products where they are likely to be inhaled.

Final Report on the Safety Assessment of Cocoyl Sarcosine, Lauroyl Sarcosine, Myristoyl Sarcosine, Oleoyl Sarcosine, Stearoyl Sarcosine, Sodium Cocoyl Sarcosinate, Sodium Lauroyl Sarcosinate, Sodium Myristoyl Sarcosinate, Ammonium Cocoyl Sarcosinate, and Ammonium Lauroyl Sarcosinate

This safety assessment addresses cosmetic ingredients that are N-acyl derivatives of sarcosine and are generally referred to as acyl sarcosines, and those that are salts, known generally as acyl sar-cosinates. Previous assessments have addressed the safety of each of the fatty acids that appear in these acyl sarcosines and sarcosinates (Coconut Acid, Oleic Acid, Lauric Acid, and Myristic Acid). In each case the fatty acid was either safe for use or safe as used in cosmetic formulations. Acyl sarcosines are considered modified fatty acids with greater solubility and increased acidity of the carboxylic acid group compared to the parent fatty acid. They are used in a large number of cosmetic formulations as hair-conditioning agents and surfactant-cleansing agents. In soaps, concentrations are reported to be as high as 12.9%. These ingredients have low oral toxicity in rats. Although cytotoxic to Chinese hamster cells in culture, acyl sarcosines and sarcosinates are not mutagenic in those cells, nor in bacterial cells in culture. Carcinogenicity data were not available. These ingredients are nonirritating and nonsen-sitizing to animal and human skin, although they can enhance the penetration of other ingredients through the skin. For that reason, caution should be exhibited in formulating cosmetic products that contain these ingredients in combination with other ingredients whose safety is based on their lack of absorption or where dermal absorption is a concern (e.g., HC Yellow No. 4, Disperse Yellow 3). Because sarcosine can be nitrosated to form N-nitrososarcosine, a known animal carcinogen, these ingredients should not be used in cosmetic products in which N-nitroso compounds may be formed. With the above caveat, and based on the available data, it was concluded that these acyl sarcosines and sarcosinates are safe as used in rinse-off products. They may be safely used in leave-on products at concentrations up to 5%, the highest concentration tested in clinical irritation and sensitization studies. Oleoyl Sarcosine is used as a corrosion inhibitor in some aerosol products, at extremely low concentrations. In this circumstance, the ingredient is not being used as a cosmetic ingredient and this report is not intended to limit that use. Because of the absence of data on inhalation toxicity, however, it was concluded that the available data were not sufficient to support the safety of acyl sarcosines and sarcosinates as cosmetic ingredients in products where they are likely to be inhaled.

XRCC2 and XRCC3, New Human Rad51-Family Members, Promote Chromosome Stability and Protect against DNA Cross-Links and Other Damages

The phenotypically similar hamster mutants irs1 and irs1SF exhibit high spontaneous chromosome instability and broad-spectrum mutagen sensitivity, including extreme sensitivity to DNA cross-linking agents. The human XRCC2 and XRCC3 genes, which functionally complement irs1 and irs1SF, respectively, were previously mapped in somatic cell hybrids. Characterization of these genes and sequence alignments reveal that XRCC2 and XRCC3 are members of an emerging family of Rad51-related proteins that likely participate in homologous recombination to maintain chromosome stability and repair DNA damage. XRCC3 is shown to interact directly with HsRad51, and like Rad55 and Rad57 in yeast, may cooperate with HsRad51 during recombinational repair. Analysis of the XRCC2 mutation in irs1 implies that XRCC2's function is not essential for viability in cultured hamster cells.

Retinoblastoma protein associates with SP1 and activates the hamster dihydrofolate reductase promoter.

The dihydrofolate reductase (dhfr) promoter is powerfully activated by the transcription factor Sp1. It has been suggested that Sp1 is a potential target for transcriptional regulation by the cell cycle regulator retinoblastoma protein (Rb), and so we have explored this possibility using the hamster dhfr gene as a model. By the use of DNA probes from the hamster dhfr gene promoter, containing the most proximal GC box (minimal promoter), and nuclear extracts from cultured hamster cells (CHO K1), we show that polyclonal and monoclonal antibodies against Rb supershift the binding of Sp1. Nuclear extract immunoprecipitation with anti-Rb followed by Western analysis using anti-Sp1 also shows that Rb is complexed to Sp1. Complementary Immunoprecipitation/WB analysis shows both forms of Rb protein in the anti-Sp1 immunoprecipitates. Moreover, nuclear extract immunodepletion of Rb abolishes Sp1 gel-shift. The interaction between Rb and Sp1 is maintained in all the phases of the cell cycle. Transient overexpression of Rb in dhfr negative cells co-transfected with a dhfr minigene driven by its minimal promoter increases DHFR activity and potentiates transcription when overexpressing Sp1. Both effects are severely reduced when the co-transfections are performed with a homologous dhfr minigene containing a single point mutation in the GC box. Thus, the activation by Rb of the dhfr gene may be exerted through Sp1. Stable transfectants of pCMVRb in K1 cells show an increase in both mRNA and DHFR activity. It is concluded that Sp1 is physically associated with Rb, and that this association increases Sp1-mediated transcription of the hamster dhfr gene.

Cell-growth regulation of the hamster dihydrofolate reductase gene promoter by transcription factor Sp1.

The dihydrofolate reductase (DHFR) gene (dhfr) promoter contains cis-acting elements for the transcription factors Sp1 and E2F. Given the ability of Sp1 to activate the dhfr promoter, we have evaluated the contribution of Sp1 to the cell-growth regulation of the dhfr gene. Using gel-mobility assays performed with DNA probes from the minimal promoter of the hamster dhfr gene and nuclear extracts from cultured hamster cells (CHO K1) we show that the binding of Sp1 to the dhfr promoter is cell-growth-phase regulated. Accordingly, dhfr transcription and mRNA levels in K1 cells increase upon serum stimulation. Cytological detection of Sp1 by immunofluorescence reveals a decrease of this protein in the process leading to the G0 state, and an increase upon serum stimulation of quiescent cells. These results were confirmed by western blot analysis. It is concluded that Sp1 progressively binds to the hamster dhfr promoter after stimulation of cell proliferation, which can account for the transcriptional regulation of the dhfr gene during the cell cycle. The role of Sp1 in the specific control of dhfr during the cell cycle was confirmed in vivo using cell lines derived from dhfr-negative cells transfected with dhfr plasmids carrying either the wild-type or mutated Sp1-binding or E2F-binding sites in the dhfr minimal promoter.

Inhibition of mitotic-specific histone phosphorylation by sodium arsenite

Synchronized cultures of Chinese hamster cells (line CHO) were used to measure the effects of 10 μ m sodium arsenite on histone phosphorylation. This treatment caused cell proliferation to be temporarily arrested, after which the cells spontaneously resumed cell proliferation in a radiomimetic manner. Immediately following treatment, it was found that sodium arsenite affected only mitotic-specific H1 and H3 phosphorylations. Neither interphase, nor mitotic, H2A and H4 phosphorylations were affected, nor was interphase H1 phosphorylation affected. The phosphorylation of H1 was inhibited only in mitosis, reducing H1 phosphorylation to 38.1% of control levels, which was the level of interphase H1 phosphorylation. The phosphorylation of both H3 variants was inhibited in mitosis, the less hydrophobic H3 to 19% and the more hydrophobic H3 to 24% of control levels. These results suggest that sodium arsenite may inhibit cell proliferation by interfering with the cyclin B/p34 cdc2 histone kinase activity which is thought to play a key role in regulating the cell cycle. It has been proposed that H1 and H3 phosphorylations play a role in restructuring interphase chromatin into metaphase chromosomes. Interference in this process by sodium arsenite may lead to structurally damaged chromosomes, resulting in the increased cancer risks known to be produced by arsenic exposure from the environment.

DNA Damage-Induced Transcriptional Activation of a Human DNA Polymerase .beta. Chimeric Promoter: Recruitment of Preinitiation Complex in Vitro by ATF/CREB

Treatment of hamster cells in culture with the DNA alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) induces DNA polymerase beta (beta-pol) gene expression and cellular levels of the enzyme. Transcriptional activity of a cloned beta-pol promoter in transient expression assays is also stimulated. Among the requirements for these responses are methylation damage to genomic DNA, cellular cAMP-dependent protein kinase, and the ATF/CREB site of the cloned beta-pol promoter. In the present study, HeLa cell nuclear extract from MNNG-treated cells was much more active in an in vitro transcription assay than nuclear extract from normal cells. By using an oligonucleotide affinity column to deplete the nuclear extract of ATF/CREB, we showed that the difference was due to ATF/CREB activator. Purified ATF/CREB activator from MNNG-treated cells was approximately 10-fold more active than ATF/CREB purified from normal cells as a transcriptional activator for the depleted nuclear extract. ATF/CREB in the extract from normal cells is known to activate in vitro transcription by increasing the rate of promoter clearance [Narayan, S., Widen, S. G., Beard, W. A., & Wilson, S. H. (1994) J. Biol. Chem. 269, 12755-12763]. With ATF/CREB from MNNG-treated cells, the amount of preinitiation complex formed was much greater than with ATF/CREB from normal cells, and the kinetics of both the closed to open preinitiation complex isomerization and promoter clearance were altered. These results indicate that the mechanism of transcriptional activation secondary to DNA alkylation damage is recruitment of more preinitiation complex and alteration of the kinetic scheme of transcription initiation.