Evidence for pleiotropic changes in lines of Chinese hamster ovary cells resistant to concanavalin A and phytohemagglutinin-P

Precision genome editing: a small revolution for glycobiology

Precise and stable gene editing in mammalian cell lines has until recently been hampered by the lack of efficient targeting methods. While different gene silencing strategies have had tremendous impact on many biological fields, they have generally not been applied with wide success in the field of glycobiology, primarily due to their low efficiencies, with resultant failure to impose substantial phenotypic consequences upon the final glycosylation products. Here, we review novel nuclease-based precision genome editing techniques enabling efficient and stable gene editing, including gene disruption, insertion, repair, modification and deletion. The nuclease-based techniques comprised of homing endonucleases, zinc finger nucleases, transcription activator-like effector nucleases, as well as the RNA-guided clustered regularly interspaced short palindromic repeat/Cas nuclease system, all function by introducing single or double-stranded breaks at a defined genomic sequence. We here compare and contrast the different techniques and summarize their current applications, highlighting cases from the field of glycobiology as well as pointing to future opportunities. The emerging potential of precision gene editing for the field is exemplified by applications to xenotransplantation; to probing O-glycoproteomes, including differential O-GalNAc glycoproteomes, to decipher the function of individual polypeptide GalNAc-transferases, as well as for engineering Chinese Hamster Ovary host cells for production of improved therapeutic biologics.

Complex N-glycans: the story of the "yellow brick road"

The synthesis of complex asparagine-linked glycans (N-glycans) involves a multi-step process that starts with a five mannose N-glycan structure: [Manα1-6(Manα1-3)Manα1-6][Manα1-3]-R where R = Manβ1-4GlcNAcβ1-4GlcNAcβ1-Asn-protein. N-acetylglucosaminyltransferase I (GlcNAc-TI) first catalyzes addition of GlcNAc in β1-2 linkage to the Manα1-3-R terminus of the five-mannose structure. Mannosidase II then removes two Man residues exposing the Manα1-6 terminus that serves as a substrate for GlcNAc-T II and addition of a second GlcNAcβ1-2 residue. The resulting structure is the complex N-glycan: GlcNAcβ1-2Manα1-6(GlcNAcβ1-2Manα1-3)-R. This structure is the precursor to a large assortment of branched complex N-glycans involving four more N-acetylglucosaminyltransferases. This short review describes the experiments (done in the early 1970s) that led to the discovery of GlcNAc-TI and II.

Lack of correlation between deoxyribonucleotide pool sizes, spontaneous mutation rates and malignant potential in Chinese hamster ovary cells

To examine the relationship between altered spontaneous mutation rates and malignant characteristics of cells, two hydroxyurea-resistant Chinese hamster ovary cell lines, with alterations in ribonucleotide reductase, were examined for their rates of spontaneous mutation to 6-thioguanine and ouabain resistance, tumor growth rates and their ability to form experimental lung metastases. The most resistant cell line, HR-R2T, showed no changes in the rate of spontaneous mutation to 6-thioguanine or ouabain resistance compared to the parental wild-type cell line; however, the mutant line formed lung metastases in experimental metastasis assays with BALB/c nu/nu mice, and exhibited metastatic abilities significantly different from the wild-type population. Furthermore, the HR-R2T population did not show imbalances in any of the deoxyribonucleoside triphosphate pool sizes, which are frequently observed in cells altered in ribonucleotide reductase activity. The second hydroxyurea-resistant line, HNR-AT, had gross alterations in dCTP and dGTP pools and although the rate of spontaneous mutation to 6-thioguanione resistance was unaltered, it showed a moderate decrease in the rate of spontaneous mutation to ouabain resistance when compared to the parental wild-type population. Interestingly, the HNR-AT cell line did not form any lung metastases in the experimental metastasis assay. Both mutant cell lines, HR-R2T, and HNR-AT, had increased tumor growth rates in C57 BALB/c "beige" nude (nu/nu) mice as compared to the parental wild-type population. In total, the results obtained with the two mutant cell lines question the association of altered mutation rates with increased metastatic potential. Although several explanations are possible for the altered malignant properties exhibited by HR-R2T and HNR-AT cells, it is interesting to note that the results are consistent with earlier suggestions that changes in ribonucleotide reductase may accompany modifications in the malignant characteristics of cells.

Molecular and cellular characterization of drug resistant hamster cell lines with alterations in ribonucleotide reductase

Ribonucleotide reductase consists of 2 protein components frequently called M1 and M2. Hydroxyurea specifically inhibits DNA synthesis by interacting with the M2 protein and destroying a unique tyrosyl-free radical. We have carried out a molecular and cellular characterization of 2 Chinese hamster ovary cell lines exhibiting either low (HN(R)-AT) or relatively high (H(R)-R2T) resistance to the cytotoxic effects of hydroxyurea. Both drug-resistant lines have an increased level of ribonucleotide reductase activity. EPR measurements for tyrosyl-free radical content and studies with M1-specific antibodies indicated that the elevation in enzyme activity was entirely due to an increase in the M2 component. Studies with M1 cDNA showed that both drug-resistant cell lines contained a wild-type level of M1 mRNA and a wild-type M1 gene copy number. Studies with M2 cDNA indicated that the 2 drug-resistant lines possessed elevated levels of M2 message that could explain the observed increase in M2 component. The elevation of M2 mRNA in the most resistant line, H(R)-R2T, was due to an increase in M2 gene copy number. The low resistant cell line, HN(R)-AT, exhibited a wild-type M2 gene copy number, indicating that the increase in M2 gene message occurred through a process other than gene amplification. Enzyme kinetic studies with partially purified preparations from both drug resistant lines showed reduced sensitivity to hydroxyurea and to the negative allosteric effector, dATP. In addition to hydroxyurea, H(R)-R2T cells were also resistant to several other drugs whose site of action is the M2 component. Furthermore, H(R)-R2T cells were not cross-resistant to colchicine or puromycin, suggesting that hydroxyurea-resistant cells do not share the multi-drug resistance phenotype, which is frequently associated with cross-resistance to these drugs.

Phytohemagglutinin (PHA)-resistant Chinese hamster ovary cell mutants acquire sensitivity to the lectin after fusion with liposomes containing PHA receptor glycoproteins

Phytohemagglutinin receptor glycoproteins have been inserted into phospholipid vesicles and these have been fused with phytohemagglutinin-resistant chinese hamster ovary cells. Our results show that the fused cells acquire "neoreceptors" for the lectin phytohemagglutinin. Fluorescence activated cell sorting analyses show that approximately 40% of the cells fused with the receptor-containing vesicles. Studies with 125I-labelled lectin showed that fused cells bound three times more ligand than untreated mutant cells. Furthermore, lectin receptors were functionally inserted in the mutant cell plasma membrane. Fused cells cultured in the presence of lectin (200 micrograms ml-1) lost rapidly (8 hours or less) their ability to incorporate [3H] thymidine. Whereas mutant cells cultured for 16 hours in the presence of 50-400 micrograms ml-1 of lectin remained viable, fused cells showed a 45% decrease in 3H-labelled nucleotide incorporation. The method described here should be of general applicability for the study of lectin-dependent cytotoxicity in chinese hamster ovary cell lines.

Ribonucleotide reductase activity in intact mammalian cells: stimulation of enzyme activity by MgCl2, dithiothreitol, and several nucleotides

An intact cell assay system based on Tween-80 permeabilization was used to investigate ribonucleotide reductase activity in Chinese hamster ovary cells. Dithiothreitol, a reducing agent, is required for optimum activity. Analysis of dithiothreitol stimulation of CDP and ADP reductions indicated that in both cases the reducing agent served only to increase the reaction rate without altering the affinity of the enzyme for substrates. Magnesium chloride significantly stimulated the reduction of CDP but not ADP; this elevation in CDP reduction was due to an increase in both the affinity of the enzyme for substrate and the Vmax. In addition to ATP and dGTP, well-known activators of CDP and ADP reductase activities, it was found that dCTP and GTP were also able to activate CDP and ADP reductase activities, respectively. For the dCTP-activated reaction the Vmax was 0.158 nmol dCDP formed 5 X 10(6) cells-1 h-1 and the Km was 0.033 mM CDP, while for the GTP-activated reduction a Vmax of 0.667 nmol dADP formed 5 X 10(6) cells(-1) h-1 and Km of 0.20 mM ADP were observed. Kinetic analysis revealed that dCTP, dGTP, and GTP stimulate ribonucleotide reduction solely by increasing the affinity of the enzyme for substrate without affecting the Vmax of the respective reactions. ATP behaves in a different manner as it stimulates CDP reduction by altering both the affinity of the enzyme for substrate and the Vmax. Cellular concentrations of ribo- and deoxyribonucleoside di- and triphosphate pools were measured to help evaluate the relative physiological importance of the nucleotide activators. These determinations, along with the reaction kinetic studies, strongly imply that ATP is a much more important regulator of CDP reduction that dCTP, whereas GTP may serve as well or better than dGTP as the in vivo activator of ADP reduction.

Lectin-resistant mutants of polarized epithelial cells

Two lectin-resistant mutants derived from Madin Darby canine kidney cells, with constitutive alterations in the asparagine-linked carbohydrate moieties, retained the characteristic structural and functional epithelial polarity of the parental cells. A ricin-resistant cell line was unable to incorporate galactose-sialic acid into glycoproteins and, from the pattern of cross-resistance to other lectins, appears to be different from previously described lines resistant to this lectin: the mutation in a concanavalin A-resistant line results, probably, in the production of defective carbohydrate cores of glycoproteins. In spite of glycosylation defects which result in an increased electrophoretic mobility of many cellular glycoproteins, both mutants retained the typical asymmetric structure of the plasma membrane (microvilli on the apical surface, junctional elements on the basolateral surface), functional tight junctions, and unidirectional active transport of electrolytes and water. These results suggest that glycoproteins with terminal galactose-sialic acid moieties are not critically involved in the development and maintenance of polarity in epithelial cells. The mutant cells, particularly the ricin-resistant line, exhibited, however, morphological and electrophysiological changes which suggest a quantitative effect of the mutations on intracellular traffic of membranes and tight junction formation. The cell lines described in this paper, the first lectin-resistant mutants of epithelial lineage, should prove useful tools for studying the peculiarities of glycosylating pathways in polarized cells.

Selection of mutant Chinese hamster ovary cells altered glycoproteins by means of tritiated fucose suicide

Mutant Chinese hamster ovary cells altered in glycoproteins have been isolated by selecting for ability to survive exposure to [6-3H]fucose. Mutagenized wild-type cells were permitted to incorporate [3H]fucose to approximately 1 cpm of trichloroacetic acid-insoluble radioactivity per cell and then frozen for several days to accumulate radiation damage. The overall viability of the population was reduced by 5- to 50-fold. Four consecutive selection cycles were carried out. The surviving cells were screened by replica plating-fluorography for clones showing decreased incorporation of fucose into trichloroacetic acid-insoluble macromolecules. Considerable enrichment for cells deficient in fucose uptake or incorporation into proteins (or both) was found in populations surviving the later selection cycles. Two mutant clones isolated after the fourth selection cycle had the same doubling time as the wild type, but contained only 30 to 40% as much fucose bound to proteins as the wild type. Sialic acid contents of the mutants and the wild type were similar. The mutants differed quantitatively and qualitatively from the wild type and from each other with respect to total glycoprotein profiles as visualized by sodium dodecyl sulfate gel electrophoresis. Differences were also found in resistances to cytotoxicity of lectins such as concanavalin A and wheat germ agglutinin.

Lectin-resistant mutants of polarized epithelial cells

Two lectin-resistant mutants derived from Madin Darby canine kidney cells, with constitutive alterations in the asparagine-linked carbohydrate moieties, retained the characteristic structural and functional epithelial polarity of the parental cells. A ricin-resistant cell line was unable to incorporate galactose-sialic acid into glycoproteins and, from the pattern of cross-resistance to other lectins, appears to be different from previously described lines resistant to this lectin: the mutation in a concanavalin A-resistant line results, probably, in the production of defective carbohydrate cores of glycoproteins. In spite of glycosylation defects which result in an increased electrophoretic mobility of many cellular glycoproteins, both mutants retained the typical asymmetric structure of the plasma membrane (microvilli on the apical surface, junctional elements on the basolateral surface), functional tight junctions, and unidirectional active transport of electrolytes and water. These results suggest that glycoproteins with terminal galactose-sialic acid moieties are not critically involved in the development and maintenance of polarity in epithelial cells. The mutant cells, particularly the ricin-resistant line, exhibited, however, morphological and electrophysiological changes which suggest a quantitative effect of the mutations on intracellular traffic of membranes and tight junction formation. The cell lines described in this paper, the first lectin-resistant mutants of epithelial lineage, should prove useful tools for studying the peculiarities of glycosylating pathways in polarized cells.

Enzymatic basis for a lectin-resistant phenotype: increase in a fucosyltransferase in mouse melanoma cells

In the search for the biochemical basis of the control of glycosylation of cell surface carbohydrates, revertant clones were isolated from previously characterized wheat germ agglutinin-resistant clones of B16 mouse melanoma cells by selection for resistance to Lotus tetragonolobus lectin or to ricin. Comparison of the wheat germ agglutinin-resistant clones with the parent and revertant clones indicated that this phenotype was correlated with an increased sensitivity to the Lotus lectin, a 60- to 70-fold increase in alpha 1 leads to 3 fucosyltransferase activity and a decreased sialic acid content of the N-glycosidic chains of glycoproteins. The results suggest a novel type of control mechanism for lectin resistance, an increase in a glycosyltransferase activity. The presence of alpha 1 leads to 3 bound fucose on N-acetylglucosamine residues would interfere with the addition of sialic acid by alpha 2 leads to 3 linkages to galactose residues in the carbohydrate units, and this change could explain the resistance to wheat germ agglutinin and the increased sensitivity to the Lotus lectin. A change in a regulatory gene for the fucosyltransferase as a possible primary cause for the changed phenotype is discussed.

Selection of mutant Chinese hamster ovary cells altered glycoproteins by means of tritiated fucose suicide

Mutant Chinese hamster ovary cells altered in glycoproteins have been isolated by selecting for ability to survive exposure to [6-3H]fucose. Mutagenized wild-type cells were permitted to incorporate [3H]fucose to approximately 1 cpm of trichloroacetic acid-insoluble radioactivity per cell and then frozen for several days to accumulate radiation damage. The overall viability of the population was reduced by 5- to 50-fold. Four consecutive selection cycles were carried out. The surviving cells were screened by replica plating-fluorography for clones showing decreased incorporation of fucose into trichloroacetic acid-insoluble macromolecules. Considerable enrichment for cells deficient in fucose uptake or incorporation into proteins (or both) was found in populations surviving the later selection cycles. Two mutant clones isolated after the fourth selection cycle had the same doubling time as the wild type, but contained only 30 to 40% as much fucose bound to proteins as the wild type. Sialic acid contents of the mutants and the wild type were similar. The mutants differed quantitatively and qualitatively from the wild type and from each other with respect to total glycoprotein profiles as visualized by sodium dodecyl sulfate gel electrophoresis. Differences were also found in resistances to cytotoxicity of lectins such as concanavalin A and wheat germ agglutinin.

N-carbamoyloxyurea-resistant Chinese hamster ovary cells with elevated levels of ribonucleotide reductase activity

We describe the isolation and characterization of a Chinese hamster ovary cell line selected for resistance to N-carbamoyloxyurea. Using the mammalian cell permeabilization assay developed in our laboratory, a detailed analysis of the target enzyme, ribonucleotide reductase (EC 1.17.4.1), was carried out. Both drug-resistant and parental wild-type cells required the same optimum conditions for enzyme activity. The Ki values for N-carbamoyloxyurea inhibition of CDP reduction were 2.0 mM for NCR-30A cells and 2.3 mM for wild-type cells, while the Ki value for ADP reduction was 2.3 mM for both cell lines. Although the Ki values remained essentially unchanged, the Vmax values for NCR-30A cells were 1.01 nmoles dCDP formed/5 X 10(6) cells/hour and 1.83 nmoles dADP/5 X 10(6) cells/hour, while those for the wild-type cells were 0.49 nmoles dCDP produced/5 X 10(6) cells/hour and 1.00 nmoles dADP/5 X 10(6) cells/hour. This approximate twofold increase in reductase activity as least partially accounts for a 2.6-fold increase in D10 value for cellular resistance to N-carbamoyloxyurea exhibited by NCR-30A cells. The NCR-30A cell line was also cross-resistant to the antitumor agents, hydroxyurea and guanazole. No differences in Ki values for inhibition of CDP and ADP reduction by these two drugs were detected and cellular resistance could be entirely accounted for by the elevation in activity of the reductase in the NCR-30A cell line. The properties of N-carbamoyloxyurea-resistance cells indicate they should be useful for further investigations into the regulation of mammalian enzyme activity.

Ribonucleotide reduction in intact human diploid fibroblasts

There have been very few studies on ribonucleotide reductase activity in human tissue. In this report we describe a rapid and convenient procedure for determining purine and pyrimidine ribonucleotide reduction in normal human diploid fibroblasts and use the method to examine some general properties of the activity in these cells. ADP and CDP reductase was characterized for its response to the positive effectors, ATP and dGTP, the negative effector dATP, and the reducing agent dithiothreitol. Apparent Km values for ADP and CDP were determined to be 0.1 mM and 0.04 mM respectively. THe antitumor agent hydroxyurea inhibited both purine and pyrimidine reductase in a noncompetitive fashion, giving Ki value of 0.40 mM and 0.41 mM for ADP and CDP respectively. These Ki estimates are about four to five times higher than those reported for some permanent cell lines. An examination of the cytotoxic effects of hydroxyurea indicated a close correlation between the concentration of drug which inhibited enzyme activity and decreased colony-forming ability. Clearly the ability to investigate ribonucleotide reduction in low numbers of normal human diploid cells will be useful for genetic and biochemical studies.

Effects of concanavalin A on the antiviral and cell growth inhibitory action of human interferons

Sensitivities of human transformed cell line RSa and its variant cell line IFr to the cytotoxicity of Con A were compared. IFr cells were more resistant than RSa to Con A. Con A-resistant cell lines, Con Ar-1 and Con Ar-3, were isolated from RSa, and they were slightly more sensitive than RSa cells to the cell growth-inhibitory actions of interferons. Agglutinability of RSa, IFr, and Con Ar cells by Con A was compared and found to be almost equal. The combined effects of Con A and interferon upon growth and viability of these cell lines were tested. When RSa and IFr cells were treated simultaneously with Con A and Le-IF, growth of the cells was suppressed more markedly than when treatment was with Con A or Le-IF alone. To clarify the mechanism of this phenomenon, binding of 125I-labeled Con A was examined. Though ther wee some differences, both leukocyte and fibroblast interferon enhanced the binding of Con A to RSa cells and also in Con Ar cells but, in interferon-resistant IFr and HEC cells, enhancement of Con A binding was low or not observed. Therefore, the combined effect of Con A and interferon on the inhibition of cell growth is not considered to be merely due to the enhanced binding of Con A by interferon action. Successive treatment of RSa or Con Ar cells with Con A and interferon did not enhance the antiviral action of interferon at all. On the contrary, simultaneous treatment with Con A and interferon suppressed the antiviral action of interferon, depending on the concentration of Con A used. Thus, the effect of Con A on the antiviral and cell growth-inhibitory actin of interferon seems rather different.

Hydroxyurea-resistant mouse L cells with elevated levels of drug-resistant ribonucleotide reductase activity

We describe the isolation and partial characterization of a mouse L-cell line which is resistant to normally highly cytotoxic concentrations of hydroxyurea. A detailed analysis of the target enzyme ribonucleotide reductase in both wild-type and hydroxyurea-resistant enzyme preparations suggests that the drug-resistant cells form a ribonucleotide reductase enzyme which contains a structural alteration, rendering it less sensitive to inhibition by hydroxyurea. K1 values for hydroxyurea inhibition of ribonucleotide reduction in enzyme preparations from hydroxyurea-resistant cells were significantly higher than corresponding values from preparations from wild-type cells. The Km for CDP reduction in enzyme preparations of drug-resistant cells was approximately threefold higher than the corresponding parental wild-type value. In addition, in vivo enzyme assays detected a major difference between the temperature profiles of ribonucleotide reduction in nucleotide-permeable drug-resistant and wild-type cells. When levels of ribonucleotide reductase activity were measured in vivo, it was found that the drug-resistant cells contained approximately 3 times the wild-type level of CDP reductase activity and twice wild-type level of GDP reductase activity. This combination of enhanced enzyme levels plus an altered sensitivity to drug inhibition can easily account for the drug-resistance phenotype. The properties of these hydroxyurea-resistant cells indicate that they will be useful for genetic and biochemical studies.

Isolation and characterization of tunicamycin resistant mutants from Chinese hamster ovary cells

Stable clones selected for resistance to tunicamycin (TM) have been isolated from Chinese Hamster Ovary (CHO) cells. The TMR phenotype is stable for more than nine months in the absence of the drug. The morphology of TMR mutant varies from epitheloid to abnormally elongate. The mutants do not display cross-resistance for ConA but are slightly cross-resistant to PHA. Biochemically labeled membrane proteins and glycoprotein of Vesicular stomatitis virus (VSV) grown in the TMR mutants revealed that the incorporation of radioactive glucosamine was markedly reduced in the mutants. The results indicate that TMR cells are a novel type of membrane mutant.

Characteristics of concanavalin A-resistant Chinese hamster ovary cells and certain revertants

Clones of Chinese hamster ovary (CHO) cells were isolated by single-step selection for resistance to killing Concanavalin A (ConA) and certain cellular and membrane properties were examined. The ConA-resistant isolates were only about 2-fold more resistant than wild type cells to the selecting lectin, but exhibited pleiotropic temperature-sensitivity for growth, markedly altered morphology and adherence, and significant difference in susceptibility to other agents such as colchicine. Two revertants to full temperature-resistance were isolated from different ConA-resistant mutants. One revertant clone had reacquired wild type sensitivity to ConA while the other revertant remained ConA-resistant. The two series of wild typed, ConA-resistant, and temperature revertant clones were analyzed for altered mobility of cell surface glycoproteins using lactoperoxidase/125I and galactose oxidase/(3H) borohydride labelling procedures. The ConA-resistant clones showed increased mobility on polyacrylamide gels of three classes of labelled proteins, in the molecular weight ranges 225,000, 200,000, and 130,000 daltons. These changes persisted in the temperature-revertant that remained ConA-resistant, while two of the altered protein closses were restored to wild type mobility in the revertant that regained ConA-sensitivity. Cell hybridization experiments indicated that the temperature-sensitivity phenotypes of different ConA-resistant isolates are recessive and noncomplementing, implying that the same gene is affected in each case. The reversions to temperature resistance appear to be recessive suppressor mutation in different genes.

Genetic characterization of hydroxyurea-resistance in Chinese hamster ovary cells

Hydroxyurea is an excellent selective agent for obtaining drug-resistant mutants. At a frequency of approximately 1 X 10(-5) it was possible to select, in a single step, colonies that exhibited significant resistance to the cytotoxic effects of the drug. These hydroxyurea-resistant cell lines maintained their resistant phenotype after extensive cultivation in the absence of the drug. Reconstruction experiments indicated that the expression of hydroxyurea-resistance and the frequency of drug-resistant colonies was independent of cell densities up to 5 X 10(5) cells per 100-mm selection plate. Luria-Delbrück fluctuation analyses indicated that the appearance of hydroxyurea-resistant cells in wild type populations occurred spontaneously and at a rate of 4.8 X 10(-6) per cell per generation in the presence of 0.33 mM drug. Studies with the mutagen, ethyl methane sulfonate indicated that it was capable of increasing the frequency of hydroxyurea-resistant cells by a factor of approximately 10. Also, cell-cell hybridization experiments showed that hydroxyurea-resistance behaves as a dominant or codominant trait and that hydroxyurea-resistance was a useful new genetic marker for selection of somatic cell hybrids. Furthermore, similar to many other drug-resistant cell lines hydroxyurea-resistant cells were found to exhibit an altered sensitivity to a number of non-selective agents (guanazole, N-carbamoyloxyurea, formamidoxime, and hydroxyurethane). Except for guanazole these compounds are structurally very similar to hydroxyurea and may be expected to have similar modes of action. The results presented in this paper support the view that hydroxyurea-resistance is expressed as a normal genetic trait and is a useful genetic marker for somatic cell genetic studies.

A hypotetraploid human T lymphoid cell line established by cell fusion

A human T lymphoid cell line was established by cell hybridization technique from peripheral blood leucocytes of a patient with Sezary syndrome. The cells beared the surface antigens of human T lymphocyte specificity as demonstrated by immune cytolysis tests, but did not form E rosettes with sheep red blood cells. Isozyme patterns of enzymes in this line such as lactate dehydrogenase, glucose 6-phosphate dehydrogenase and esterase were of human type. The line had 79 chromosomes in modal number. This case supports the proposal that the production of tetraploids is favourable for establishment of cell lines.

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.

Pleiotropic phenotype of colchicine-resistant CHO cells: cross-resistance and collateral sensitivity

Colchicine resistant (CHR) mutants of CHO cells with reduced permeability to colchicine display extensive cross-resistance to a number of apparently unrelated compounds including puromycin, daunomycin, emetine, ethidium bromide and gramicidin D. A positive correlation was observed between the level of cross-resistance and the relative hydrophobicity of these compounds. The mutants also showed increased (collateral) sensitivity to local anaesthetics (procaine, tetracaine, xylocaine and propanolol), steroid hormones (1-dehydrotestosterone, corticosterone and 5beta-pregnan-3,20-dione) and some Triton X compounds. In general, the degree of the pleiotropic response (cross-resistance or collateral sensitivity) correlated with the degree of colchicine resistance in mutant lines. These results are consistent with the pleiotropic phenotype being the result of the same mutation(s) which confer colchicine resistance and support a model for resistance in which the reduced permeability is assumed to be the result of an alteration in the modulation of the fluidity of the surface membrane.

Selection and characterization of Chinese hamster ovary cells resistant to the cytotoxicity of lectins

Chinese hamster ovary (CHO) cells selected in a single step for resistance to the cytotoxicity of the lectin from red kidney beans (PHA) behave as authentic somatic cell mutants. The PHA-resistant (Phar) phenotype is stable in the absence of selection; its frequency in a sensitive-population is increased several-fold by mutagenesis; and it behaves recessively in somatic cell hybrids. The activity of a specific glycosyl transferase which transfers N-acetylglucosamine (GlcNAc) to terminal alpha-mannose residues is dramatically reduced (less than or equal to 5% of the activity detected in wild-type CHO cells) in several independent PhaR clones. These clones also exhibit (a) a decreased ability to bind [125I]-PHA; (b) a marked resistance to the cytotoxicity of wheat germ agglutinin (WGA), Ricin (RIC) and Lens culinaris agglutinin (LCA); (c) a 4- to 5-fold increased sensitivity to the cytoxocity of concanavalin A (Con A); (d) an increased ability to bind 125I-Con A; and (e) decreased surface galactose residues - all properties consistent with the specific loss of the GlcNAc transferase activity. The lectins WGA, RIC, LCA and Con A have also been used to select, in a single step, resistance closes from each of two complementary CHO auxitrophic lines. These lectin-resistant clones have been characterized by their ability to survive cytotoxic doses of PHA, Con A, WGA, RIC, or LCA, and 4-5 "lectin-resistance" phenotypes have been demonstrated. Complementation data is being sought by somatic cell hybridization. Preliminary results show that two phenotypically-distinct Con AR mutants are complementary in that hybrid cells formed between them exhibit wild-type sensitivity to Con A.

Variants of hamster fibroblasts resistant to Ricinus communis toxin (ricin)

1. Variant baby-hamster kidney (BHK) cell lines were isolated that grow in the presence of high concentrations of ricin, the toxic lectin of castor beans (Ricinus communis). The variant lines were independently derived from several cultures of normal BHK cells which had been exposed to the mutagen, methyl-N-nitro-N-nitrosoguanidine, before selection by ricin. 2. The cell lines maintain a high degree of resistance to ricin after growth in lectin-free medium for prolonged periods and therefore exhibit stable phenotypes that are different from normal BHK cells. 3. A preliminary classification of the phenotypes was made. Several cell lines bind normal amounts of 125I-labelled ricin, whereas other bind the lectin poorly. 4. A loss of surface receptors for two other lectins, R. communis RCA and Axinella polyploides, which have specificities similar to ricin, was also found in some but not all of the cell lines showing decreased surface concentrations of ricin receptors. 5. The binding to the ricin-resistant cells of lectins of different sugar specificity, namely Lens culinaris lectin and concanavalin A, was similar to, or higher than, to normal BHK cells. 6. Several of the ricin-resistant cell lines were shown to be cross-resistant to the weak cytotoxicity of Phaseolus vulgaris lectin. By contrast, some cell lines were more sensitive to concanavalin A than were normal BHK cells.

Humoral immunostimulation. V. Selection of variant cell lines

A permanent L-cell variant cell line (LC1) was isolated by the growth of the parent L-cell line (L) in the presence of a cytostimulatory dose (1:200) of rabbit anti-L-cell antiserum (AL) for 9 mo. LC1 differed from L in many aspects: (a) it was larger (1,533 mm3 vs. 1,284 mm3), (b) it grew faster (1.5- to 2-fold), (c) it grew in aggregated fashion, (d) its growth was no longer stimulated by AL, (e) it was almost completely resistant to high concentrations of AL in the presence of complement (C), (f) its original membrane antigens (immunogenic for AL) were redistributed in sparse and patchy clumps as noted by fluorescence microscopy, (g) it contained about 65% of the total original 125I-AL membrane-binding sites (1.4 X 10(7)/cell vs. 2.2 X 10(7)/cell), (h) its AL-binding sites displayed a lower average affinity constant (K = 0.9 X 10(5) M-1 vs. 2.8 X 10(5) M-1), (i) it contained a smaller proportion of high affinity (K greater than 10(6) M-1) binding sites (13% vs 21%), and (j) LC1 was fully immunogenic in that it was readily killed by homologous antiserum (ALC1) and C, whereas L was not similarly affected by ALC1 indicating that LC1 contained new membrane antigens not present on L. Another variant (LC2) was produced by growth of LC1 in a 10-fold higher dose (1:20) of AL (cytotoxic for L) for 1 mo. LC2 was even more resistant to AL in the presence of C, contained 0.84 X 10(7) AL-binding sites/cell with an average affinity constant of 1 X 10(5) M-1 (unchanged from LC1), and was less susceptible than LC1 to lysis in the presence of ALC1 and C. These findings confirm and extend our previous in vitro and in vivo observations dealing with the direct stimulation effects of antibody on tumor cell metabolism and suggest that immunostimulation may be a mechanism of tumor escape from immune control in vivo possibly by immunoselection and antigenic modulation as proposed by other investigators.

Selection and characterization of eight phenotypically distinct lines of lectin-resistant Chinese hamster ovary cell

Clones resistant to the lectins phytohemagglutinin (PHA), wheat germ agglutinin (WGA), the agglutinin(s) from Lens culinaris (LCA), and ricin (RIC) have been selected from parental auxotrophic Chinese hamster ovary (CHO) cells. The sensitivity to other lectins of these cells and of CHO cells resistant to concanavalin A (ConA) has been determined, and their activity of UDP-N-acetyl-glucosamine glycoprotein N-acetyl-glucosaminyltransferase (GlcNAc-T) has been measured. At least 8 different phenotypes have been identified on the basis of this analysis, and complementation between 2 of them demonstrated.

Chinese hamster ovary cells selected for resistance to the cytotoxicity of phytohemagglutinin are deficient in a UDP-N-acetylglucosamine--glycoprotein N-acetylglucosaminyltransferase activity

Several clones of Chinese hamster ovary cells resistant to the cytotoxicity of the phytohemagglutinin from Phaseolus vulgaris show decreased binding of 125I-labeled phytohemagglutinin and contain decreased levels of a UDP-N-acetylglucosamine--glycoprotein N-acetylglucosaminyltransferase (EC 2.4.1.51; UDP-2-acetamido-2-deoxy-D-glucose:glycoprotein 2-acetamido-2-deoxy-D-glucosyltransferase) activity when compared to wild-type cells. The decrease in transferase activity varies from 45% to 96%, depending on the exogenous acceptor used in the enzyme assay. No differences between lectin-resistant and wild-type cells were noted for several other glycosyltransferases. The absence of a particular N-acetylglucosaminyltransferase in the lectin-resistant cells apparently results in defective glycosylation of lectin-binding glycoproteins on the cell surface. A phytohemagglutinin-resistant clone which shows decreased binding of 125I-labeled phytohemagglutinin but does not exhibit the enzyme deficiency has also been isolated.