Origin of thymidine kinase deficient (TK-) haploid frog cells via an intermediate thymidine transport deficient (TT-) phenotype

Genetics and biochemistry of 5-Bromodeoxyuridine resistance inPhysarum polycephalum

5-Bromodeoxyuridine (BrdU)-resistant mutants ofPhysarum polycephalumwere isolated as colonies of myxamoebae growing on BrdU-substituted bacteria after exposure to long-wave ultraviolet light (UV). Twenty-four such mutants were studied. They all show Mendelian segregation in crosses with wild type. Plasmodia constructed from mutant amoebae were all deficient in deoxythymidine incorporation. Extracts made from selected plasmodia showed that all except one had low thymidine kinase activity.

Genetical and biochemical complementation studies revealed two complementation groups: 23 mutants,burA, had low thymidine kinase while 1 mutant,burB, had normal thymidine kinase levels.

Zidovudine induces the expression of cellular resistance affecting its antiviral activity

We have previously shown that multidrug-resistant cells expressing the multidrug transporter P-glycoprotein are less sensitive to the antiviral activity of AZT. Subsequently, we addressed the question whether AZT itself is able to induce cellular resistance to the drug. Indeed, CEM cells propagated in the presence of increasing concentrations of AZT become resistant to the antigrowth and antiviral activity of AZT but do not express detectable level of P-glycoprotein. Sensitivity of these cells to other compounds, such as vinblastine, vincristine, ddI, and ddC remained unchanged, indicating that, in contrast to P-glycoprotein-positive cells, AZT-induced resistance is specific for AZT. Interestingly, in AZT-induced resistant cells the intracellular accumulation of AZT and exogenous deoxythymidine, as well as thymidine kinase activity, are significantly reduced when compared with the parental cell line. Our findings show that AZT itself may directly induce the expression of cellular mechanisms leading to the acquisition of specific cellular resistance that can affect its antiviral activity.

Membrane transport and the antineoplastic action of nucleoside analogues

This article summarizes recent studies characterizing nucleoside transport in mammalian cells and discusses evidence for a role of membrane transport in the pharmacologic action of nucleoside analogues. Some of these studies have also addressed the controversy concerning the multiplicity in transport routes. It seems clear that erythrocytes and, perhaps, some other mammalian cells possess a single, broadly specific system for transporting nucleosides. However, substantial evidence from valid studies discriminating between transport and intracellular metabolism suggests that at least some mammalian cells, including some tumor cells, possess more than a single system. Evidence now exists for a determining role of membrane transport of nucleoside analogues in their cytotoxicity and, in the case of one pyrimidine nucleoside (AraC), in therapeutic responsiveness in leukemic patients. There are also numerous examples of transport-related resistance to nucleoside analogues. Included in this article are the results of studies from the authors' laboratory pertaining to the therapeutic activity of the purine nucleoside, FAraA, in murine tumor models. These studies provide evidence for a determining role of both membrane transport and intracellular phosphorylation in the selective antitumor action of this agent against murine leukemia. Substantially increased transport inward of FAraA occurs at pharmacologically achievable concentrations of this agent in tumor cells as compared to drug-limiting, normal proliferative epithelium of the small intestine. The basis for this differential appears to be the kinetic duality of FAraA and adenosine transport inward found in tumor cells, but not in proliferative intestinal epithelial cells. Tumor cells have highly saturable (low influx Km) and poorly saturable (high influx Km) systems for adenosine transport, both of which are shared by FAraA. In contrast, proliferative epithelial cells have only a poorly saturable system for these substrates. If a similar kinetic duality of nucleoside transport is found in other tumor cells certain implications arise concerning the significance of the duality to neoplastic transformation.

Isolation of mutants of CHO cells resistant to 6(p-hydroxyphenylazo)-uracil I. A novel BrdU cross-resistant phenotype

Three classes of mutants resistant to the drug 6(p-hydroxyphenylazo)-uracil have been isolated from mutagenized cultures of CHO cells. One class of these mutants designated HPURA exhibits a unique form of cross-resistance to bromodeoxyuridine in that it is resistant to this drug only in the presence of thymidine. The molecular basis of the BrdU resistance is unknown but does not appear to involve the known targets of the drug. An interesting feature of these mutants is that they give rise, at a high frequency, to a subpopulation of cells which are much more resistant to BrdU.

Thymidine resistance in Chinese hamster V79 cells in vitro

Thymidine resistance in V79 Chinese hamster cells has been investigated. Phenotypically stable variant resistant lines occurred at a high frequency, and the mutation rate (2.67 x 10(-3) per cell per generation) to 400 micrograms/ml thymidine resistance as measured by the standard Luria--Delbrück fluctuation analysis was extremely high. Populations of cells maintained for extended periods in F-10 medium spontaneously increased in resistance, possibly as a result of selective pressures due to the thymidine present in F-10 medium since this change was not observed in Dulbecco's medium. The degree of resistance for a given variant was correlated with the amount of thymidine employed in its selection. Metabolic cooperation, resulting in the suppression of the resistant phenotype, was demonstrated in artificial mixtures of sensitive and resistant clonal lines. Clones isolated in high levels of thymidine possessed lowered uptake of [3H]thymidine and the depression in uptake was related to the level of resistance of the particular clone. Although thymidine kinase specific activity levels were slightly depressed in variant cell lines, growth rate and uridine uptake were unaffected. We conclude that thymidine resistance is due to a genetically controlled depression of external thymidine uptake.

The effects of 5-bromodeoxyuridine on the growth and morphology of transformed rat liver cells

The effects of bromodeoxyuridine (BrdUrd) on the growth, morphology, and tumorigenicity of the spontaneously transformed rat liver cell line R72/3 were studied. These cells grow either in suspension or in a monolayer and are tumorigenic. In monolayer cultures, cells treated with low concentrations (2.5 micrograms/ml) of BrdUrd were larger, more spread out, and more firmly attached to the substratum than were untreated controls. Treated cells failed to grow in suspension or on confluent monolayers of 3T3 cells and did not form colonies in soft agar. Scanning electron microscopy revealed extensive flattening of treated cells and a dramatic reduction in the number of microvilli on the cell surface. Transmission electron microscopy showed an increase in polyribosomes and rough endoplasmic reticulum, as well as an enlargement of endoplasmic reticulum cisternae and a complete absence of the bundles of intermediate size filaments that were conspicuous in untreated cells. The persistence of these changes required the continuous presence of BrdUrd in the medium. The effects of BrdUrd were readily reversed by withdrawal of BrdUrd and were not expressed in the presence of excess thymidine.

Alteration of human breast tumor cell membrane functions by chromosome-mediated gene transfer

BOT-2 cells (human breast tumor origin) have an impaired ability to utilize exogenous thymidine. Previous studies revealed this deficiency to be the permeation event rather than phosphorylation, since the cells have active thymidine kinase. Chromosome-mediated gene transfer was used to transfer genetic information in the form of metaphase chromosomes, from HeLa-65 cells to the BOT-2 cells, correcting the permease deficiency. Poly-L-ornithine or lipochromes were used for facilitation of chromosome uptake. After selection on HAT medium, transferant clones were isolated at a frequency of 4 x 10(-5) and 1 x 10(-5), respectively. Transferants MGP-1 and MGL-1 are stable after 18 months and have been characterized on the bases of purine and pyrimidine nucleoside uptake, relative thymidine kinase activities, alkaline phosphatase activities, and hydrocortisone-induced alkaline phosphatase activity. MGP-1 demonstrates positive thymidine uptake and incorporates radiolabeled thymidine into DNA. MGL-1 remains thymidine transport-deficient and surveys on HAT by increasing endogenous dihydrofolate reductase activity. Alkaline phosphatase activity in MGL-1 is similar to HeLa-65, 2% of that in BOT-2, and in addition, is inducible 25-30-fold by 3 micro M hydrocortisone. We have separated, genetically, a thymidine permease function from phosphorylation in cells of human origin and have transferred genetic information for the regulation of alkaline phosphatase.

Isolation of UV-sensitive clones from a haploid frog cell line

An isolation procedure has been developed which yielded five clones of haploid frog cells which are sensitive to ultraviolet light. This procedure employed a conventional mutagenesis, followed by time for phenotypic expression and then an enrichment for UV-sensitive mutants. The enrichment relies upon the uptake of bromodeoxyuridine (BrdU) by repairing cells following UV-induced damage, rendering repair-proficient cells differentially sensitive to photolysis by black light. The photolysis is potentiated by use of the bisbenzimidazole dye Hoechst 33258. The enriched population was screened for radiation-sensitive isolates resulting in 5 sensitives from 96 tested. No mutants were obtained from 300 isolates tested from a population which had not undergone enrichment.

Transfer of genetic information via isolated amphibian metaphase chromosomes

The metaphase chromosome transfer system of McBride and Ozer (1973) has been adapted to a haploid, euploid, frog cell line. Genes coding for a deoxypyrimidine kinase and an enzyme responsible for a thymidine-specific saturable transport system have each been transferred at frequencies between 10(-6) and 10(-5) transferents per cell treated. Revertants for each of these two genes were observed at frequencies between 10(-8) and 10(-7) revertants per cell tested. Selfing controls showed no transferents. Two colonies were obtained in which cotransfer of both genes may have occurred. Activities of the transferred genes were assayed by incorporation of [3H]thymidine into alkali-stable, acid-precipitable material. Growth properties of 13 transferents in various media were also determined and presence of the appropriate enzymes inferred. These transferents were tested for stability early (25 generations) after transfer and were found to be stable. All 13 transferents possess the normal haploid number of chromosomes (n = 13) with no cytologically detectable chromosomal fragments.

Effects of an acridine half-mustard (ICR 191) on growth and ploidy of frog cells in culture

The effects of an acridine half-mustard, ICR 191, on the growth rate and ploidy of four haploid and two diploid lines of Rana pipiens cells in culture were studied. Growth curves indicate that the haploid and diploid cell lines were equally resistant to a 4-hour exposure of this drug (0.1 micrometer to 10 micrometer. ICR 191 treatment induced the haploid cell cultures to become diploid. The proportion of diploid cells increased progressively with respect to time after the 4-hour exposure period. The greater the concentration of ICR 191 applied, the more rapid the rate of conversion. Autoradiographic determinations of percent labelled nuclei indicate that DNA synthesis was not inhibited in haploid or in diploid cells. Therefore, the increased proportion of diploid cells did not originate from the small percentage of diploid cells in the initial population. Instead the haploid cells were converted to diploid cells. Time lapse cinematography indicated that the conversion mechanism was other than cell fusion. Conversion to higher ploidy did not occur when diploid cell cultures were exposed to ICR 191.

Prospects for evaluating genetic damage in mammalian cells in culture

Starting from some general considerations on cultured mammalian cells as a biological material for the detection of genetic changes, information is given on cell lines and genetic markers that have been the subject of extensive research. The experimental variables of mutation assays are then considered, with special reference to those involved in the system resistant to 8-azaguanine. Work now in progressin the field of environmental mutagenesis is mentioned at the end of the article.

Defective transport of thymidine by cultured cells resistant to 5-bromodeoxyuridine

A line of HeLa cells resistant to 5-bromo-2'-deoxyuridine (BUdR) was established by continuous culture in growth medium containing BUdR; during the selection period, BUdR concentrations, initially 15 micrometer, were gradually increased to 100 micrometer. Cells of a clone (HeLa/B5) established from this line were also resistant to 5-fluoro-2'-deoxyuridine (FUdR), but not to the free base, 5-fluorouracil. Although extracts of HeLa/B5 cells exhibited levels of thymidine kinase activity comparable to those of parental cells, rates of uptake of BUdR, FUdR, and thymidine into intact cells were much reduced. The kinetics of uptake of uridine and adenosine, nucleosides which appear to be transported independently of thymidine in HeLa cells, were similar for HeLa/B5 and the parental line (HeLa/O). Relative to thymidine uptake by HeLa/O cells, that by HeLa/B5 cells was distinctly less sensitive to nitrobenzylthioinosine (NBMPR), a specific inhibitor of nucleoside transport in various types of animal cells. Despite this difference in NBMPR sensitivity, both cell lines possessed the same number of high affinity NBMPR binding sites per mg cell protein. The altered kinetics of thymidine uptake and the NBMPR insensitivity of that function in HeLA/B5 cells suggest that resistance to BUdR is due to an altered thymidine transport mechanism.

Transport and countertransport of thymidine in ATP depleted and thymidine kinase-deficient Novikoff rat hepatoma and mouse L cells: evidence of a high Km facilitated diffusion system with wide nucleoside specificity

Incubation of cultured Novikoff rat hepatoma and mouse L cells in a glucose-free basal medium containing 5 mM KCN and 5 mM iodoacetate for about 10 minutes resulted in a complete depletion of the cells of ATP. ATP-depleted wild type cells or thymidine kinase-deficient sublines of Novikoff or L cells took up thymidine rapidly from the medium without concentrating it intracellularly, and exhibited countertransport of thymidine. Thus uptake was by facilitated diffusion. This transport system differs from the substrate-specific, low-Km (0.5 muM] thymidine transport system previously described for various types of cultured cells in that it exhibits an at least 100-fold higher Km and transports equally well various ribo- and deoxyribonucleosides. The results suggest that the rate-limiting step in thymidine incorporation into the nucleotide pool by wild type cells is phosphorylation rather than transport, or that the cells possess two transport systems, a facilitated diffusion system with low substrate specificity and a second system which involves substrate phosphorylation by thymidine kinase.

Ploidy level and mutation to hypoxanthine-guanine-phosphoribosyl-transferase (HGPRT) deficiency in Chinese hamster cells

The X-ray induction of 8-azaguanine (AG) resistent mutants in two sets of diploid and tetraploid Chinese hamster cells (DON and V79) was investigated. It was found that (i) the induced mutant frequencies in diploid and tetraploid cells appeared to be of the same order of magnitude and (ii) all mutants showed almost complete loss of hypoxanthine-guanine-phosphoribosyl-transferase (HGPRT) activity except that in the tetraploid V79 cells 50--100% of activity was retained. The gene--dosage effect for glucose-6-phosphate dehydrogenase (G6PD) in these cells make it possible to determine the number of chromosomes bearing the HGPRT-gene in mutants by measuring the G6PD activity per cell. The results show that the spontaneous and induced mutants from the diploid V79 and DON as well as the tetraploid DON cells retained the G6PD activity of the parental cells, whereas the induced mutants from the tetraploid V79 cells had about 35% of the parental G6PD activity. With 6-thioguanine (TG) as selective agent, the induced mutant frequencies in diploid and tetraploid DON cells and in diploid V79 cells appeared to be of the same order of magnitude but no mutants could be recovered from tetraploid V79 cells in a single step. TG-resistant tetraploid V79 cells could only be obtained from the AG-resistant mutants after a second selection. The HGPRT activity was lost in these mutants and some of them showed an increase in G6PD activity. The combined data cannot be explained on the basis of a single genetic mechanism.

Directed chromosome loss by laser microirradiation

In this article I have presented data that indicate the feasibility of attaining the five objectives outlined in the introduction. It should be possible to assign genes to specific chromosome regions by (i) selective DNA deletion of a 0.25- to 0.5-micro.m segment of one or both homologous chromosomes, (ii) deletion of one or both entire homologous chromosomes, or (iii) combining cell fusion with selective deletion of whole chromosomes and then deletion of chromosome segments. By laser microirradiation it should be possible to determine which chromosomes and chromosome regions are essential for immediate cell survival by removing from individual cells whole chromosomes, and chromosome segments from each of the chromosomes in the karyotype, and then assessing the cloning efficiency of each cell. For example, we have already determined that removal of one large chromosome No. 1 from PTK(2) cells does not prevent the cell from undergoing a subsequent mitosis. It should also be possible to generate new classes of mutants by damaging small selected areas of DNA with the laser beam and then cloning the irradiated cells-but this has yet to be demonstrated. This procedure might reveal recessive alleles on the nonirradiated homolog, or might result in the direct production of a genetic mutation. Irradiation of identical places on both homologous chromosomes could result in deletion of a genetic locus which ultimately might be detected as a deficiency in a metabolic pathway or some other cellular abnormality. Studies on chromosome stability and DNA constancy can be conducted with laser irradiated cells. For example, the karyotypic analysis of chromosome No. 1 suggests that a cellular mechanism exists to maintain the constancy of this chromosome in both the diploid and tetraploid cell lines. The same approach could be used with each of the chromosomes in the karyotype. Various cytochemical procedures could be used for making quantitative DNA studies of the cells, and chromosome and DNA analyses could be performed at varying times following laser microirradiation. It might also be possible to study the repair of chromosomal damage caused by laser irradiation. The cells could be examined by autoradiographic, cytochemical, and electron microscopy procedures at varying times after irradiation, and because the precise location, time, and nature of the mutational event would be known, subsequent analysis of repair and alteration would be facilitated.

An analysis of survival in haploid and diploid cell cultures after exposure to ICR acridine half-mustard compounds mutagenic for bacteria

Population changes induced by acridine mustards in haploid and diploid cultured cell lines from Rana pipiens were compared to test the expectation that recessive mutations will be expressed in diploid cells with a frequency equal to the square of that in haploid cells, and to investigate the usefulness of such comparison for the screening of possible mutagens. The differences in survival frequency after treatment were much smaller than predicted on the basis of the expression of lethal (recessive) mutations alone. Survival was also affected by culture conditions, drug-resistance phenomena, and other cell properties. It is suggested that with the evolution of epigenetic processes for the production of stable phenotypes, the vertebrate cell also acquired more efficient means to prevent the expression of gene mutation and that the acridine compounds may affect both epigenetic and genetic changes.

Receptors from glucocorticoid-sensitive lymphoma cells and two clases of insensitive clones: physical and DNA-binding properties

Mouse lymphoma tissue culture cells (S49.1A) are normally killed by dexamethasone, a synthetic glucocorticoid hormone. Dexamethasone-resistant clones have been selected from this line, some of which retain the ability to specifically bind dexamethasone. Addition of [(3)H]dexamethasone to cultures, followed by cell fractionation, reveals that the nuclear transfer of hormone-receptor complexes in some of these variant clones is deficient (nt(-)), while others show increased nuclear transfer (nt(i)) relative to the parental line. Two independently selected members of each class have been studied here, in an effort to elucidate the molecular determinants involved in the receptor-nucleus interaction in vivo. The labeled receptors in cell-free extracts bind to DNA-cellulose, but only after previous incubation of the extract at 20 degrees , similar to the treatment required for cell-free interaction of receptors with nuclei. More importantly, the apparent DNA-binding affinity of the nt(-) receptors is lower than the wild type, whereas the nt(i) receptors bind DNA with an affinity higher than the parental molecules. The parallelism of nuclear and DNA binding, together with the observations that the receptors from the variants have sedimentation properties different from the wild-type cells, lead us to conclude that (i) these variants may contain altered receptor molecules and (ii) DNA is probably the primary nuclear binding site for steroid receptors in vivo.