Hybridization of two biochemically marked human cell lines

The CGL1 (HeLa × Normal Skin Fibroblast) Human Hybrid Cell Line: A History of Ionizing Radiation Induced Effects on Neoplastic Transformation and Novel Future Directions in SNOLAB

Cellular transformation assays have been utilized for many years as powerful in vitro methods for examining neoplastic transformation potential/frequency and mechanisms of carcinogenesis for both chemical and radiological carcinogens. These mouse and human cell based assays are labor intensive but do provide quantitative information on the numbers of neoplastically transformed foci produced after carcinogenic exposure and potential molecular mechanisms involved. Several mouse and human cell systems have been generated to undertake these studies, and they vary in experimental length and endpoint assessment. The CGL1 human cell hybrid neoplastic model is a non-tumorigenic pre-neoplastic cell that was derived from the fusion of HeLa cervical cancer cells and a normal human skin fibroblast. It has been utilized for the several decades to study the carcinogenic/neoplastic transformation potential of a variety of ionizing radiation doses, dose rates and radiation types, including UV, X ray, gamma ray, neutrons, protons and alpha particles. It is unique in that the CGL1 assay has a relatively short assay time of 18-21 days, and rather than relying on morphological endpoints to detect neoplastic transformation utilizes a simple staining method that detects the tumorigenic marker alkaline phosphatase on the neoplastically transformed cells cell surface. In addition to being of human origin, the CGL1 assay is able to detect and quantify the carcinogenic potential of very low doses of ionizing radiation (in the mGy range), and utilizes a neoplastic endpoint (re-expression of alkaline phosphatase) that can be detected on both viable and paraformaldehyde fixed cells. In this article, we review the history of the CGL1 neoplastic transformation model system from its initial development through the wide variety of studies examining the effects of all types of ionizing radiation on neoplastic transformation. In addition, we discuss the potential of the CGL1 model system to investigate the effects of near zero background radiation levels available within the radiation biology lab we have established in SNOLAB.

Cytogenetic evidence for the absence of an inactivated X chromosome in a human female (XX) breast carcinoma cell line

Barr body staining procedures were applied directly to the chamber slide cultures of female amniotic cells. WI38 fibroblasts, normal female kidney cells, and a human breast carcinoma cell line, Elco. A high frequency of Barr bodies was found in all the normal female control cells; however, no Barr bodies were observed in the Elco cells. By trypsin G-banding analysis, two normal X chromosomes were identified in all Elco cells. The late DNA replication pattern of the cell line was then studied with the terminal BrdU pulse method. Both X chromosomes in the Elco cell line were found to be euchromatic with a characteristic R-banding pattern; no late-replicating X chromosome was observed. The absence of both a Barr body and a late-replicating heterocyclic X chromosome provides strong cytogenetic evidence that an inactivated X chromosome is absent in the human breast carcinoma cells bearing two X chromosomes.

Establishment of a cell line retaining Langerhans cell characteristics

A discontinuous gradient of polyethylene glycol (PEG) with dimethyl sulfoxide (DMSO) was used to induced fusion between guinea pig epidermal cells (GPEC) enriched in Langerhans cells (LC) and cells of a human fibrosarcoma line (T-1080) deficient in hypoxanthine phosphoribosyl transferase (HPRT). The rapidly proliferating carrier T-1080 cells lack plasma membrane ATPase activity, Fc receptors, C3b receptors, guinea pig Ia-antigens, and nonspecific esterase activity, all characteristic of LC, and absent on other guinea pig epidermal cells. GPE/T-1080 hybrid cells were selected in hypoxanthine/aminopterin/thymidine (HAT) medium and by C3b and Fc rosette formation. A resultant continuous cell line was characterized karyotypically, histochemically, ultrastructurally and immunologically.

Autonomous gene expression on the human inactive X chromosome

Local derepression of the hpt locus on the human inactive X chromosome obtained in human female fibroblast x mouse L cell somatic cell hybrids was not correlated with the presence or absence of any specific human chromosome in the hybrids. Loss of the human active X, in particular, did not result in observable derepression of genes on the inactive X. Introduction of an active X, via a second hybridization of human cells having an active X with hybrid cells containing a locally derepressed X chromosome, did not restore repression of the derepressed hpt allele. The rate of hpt locus derepression in hybrid cells was estimated to be 10(-6) per inactive X chromosome per cell generation.

Human X chromosomes: synchrony of DNA replication in diploid and triploid fibroblasts with multiple active or inactive X chromosomes

We have analyzed patterns of DNA replication in X chromosomes from diploid cultured human fibroblasts and from three triploid 69,XXY fibroblast strains, using BrdU--33258 Hoechst--Giemsa techniques. Both X chromosomes in each of these Barr body-negative triploid strains were early-replicating. The results of gene dosage studies using (1) a histochemical stain to measure X-linked glucose-6-phosphate dehydrogenase (G6PD) activity in single cells and (2) cellulose acetate electrophoresis of G6PD activity in cell extracts also indicated that both Xs in these strains were genetically active. When we compared the synchrony of X chromosome DNA replication kinetics both between cells and within cells containing multiple inactive Xs, a marked variability and asynchrony was observed for late-replicating X chromosomes. In a culture of 47,XXX fibroblasts administered an 8-h terminal pulse of dT after growth in BrdU-containing medium, asynchrony was detected between the two late-replicating Xs in approximately 70% of cells examined. No such asynchrony was observed between the two early-replicating Xs in similarly cultured 69,XXY cells; in the triploid strains, the two Xs were distinguished by asynchronous replication in only approximately 15% of cells. The striking variability in late X chromosome replication kinetics appears, then, to be a property unique to inactive Xs and is not inherent to all X chromosomes.

Kinetics of human chromosome loss from 3T3-human hybrid cells

Human chromosomes were lost from populations of 3T3-HeLa and 3T3-diploid human fibroblast (HF) hybrids with similar first-order kinetics. Whereas loss began immediately in 3T3-HF hybrids, there was a lag of 5-10 cell divisions before chromosome loss began in 3T3-HeLa hybrids. Human chromosome loss was not affected by aminopterin selection, the use of polyethylene glycol rather than Sendai virus as fusagen, or by the presence of one or two 3T3 genomes. However, when cell division was retarded by growing 3T3-HF hybrids in low serum or at low temperatures, fewer human chromosomes were lost. This suggests that cell cycle traverse is important in chromosome loss. The distribution of human chromosomes among hybrid metaphases indicated that gradual chromosome loss occurred in all hybrids rather than extensive loss from a portion of the hybrids. During the period of chromosome loss, increased numbers of individual asynchronously condensed human chromosomes were randomly distributed among hybrid metaphases.

Mutant alleles for hypoxanthine phosphoriboxyltransferase: codominant expression, complementation, and segregation in hybrid Chinese hamster cells

Chinese hamster ovary cell mutants resistant to the purine analogs 6-thioguanine or 8-azaguanine have been isolated following mutagenesis with ethyl methane sulfonate. The activities of hypoxanthine phosphoribosyltransferase (HPRT) in three such mutants have been found to exhibit an increased Km for the substrate 5-phosphoribosyl-1-pyrophosphate. The isoelectric point of the mutant enzyme activity has also changed in two mutants. Hybrid cells containing one mutant and one wild-type allele express both genes. Segregants that have lost only the wild-type allele can be selected on the basis of drug resistance. Two mutants exhibiting different alterations in HPRT activity can complement in a hybrid cell to yield a wild-type growth pattern and enzyme activity with intermediate electrophoretic and kinetic properties. The results suggest intracistronic complementation between structural gene mutants of HPRT.

The selection of somatic cell hybrids between human lymphoma cell lines

Somatic cell hybrids have been selected between three pairs of established human lymphoid cell lines producing pure lines of proliferating hybrid cells: Raji/Namalwa, Raji/Daudi, and Raji/BJAB. The hybrid cell lines have been characterized with respect to isozyme pattern, volume, and karyotype.

Localized Derepression on the Human Inactive X Chromosone in Mouse-Human Cell Hybrids

Evidence for derepression of the gene for hypoxanthine phosphoribosyltransferase (HPRT; IMP: pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) on the human inactive X chromosome was obtained in hybrids of mouse and human cells. The mouse cells lacked HPRT and were also deficient in adenine phosphoribosyltransferase (APRT; AMP: pyrophosphate phosphoribosyltransferase; EC2.4.2.7). The human female fibroblasts were HPRT-deficient as a consequence of a mutation on the active X but contained a normal HPRT gene on the inactive X. The two human X chromosomes were further distinguished by differences in morphology: the inactive X was morphologically normal while the active X included most of the long arm of autosome no. 1 translocated to the distal end of the X long arm. Forty-one hybrid clones were first isolated by selection for the presence of APRT; when these clones were selected for HPRT, six of them yielded derivatives having human HPRT with incidences of about 1 in 10-6 APRT-selected hybrid cells. The HPRT-positive derivatives contained a normal-appearing X chromosome indistinguishable from the inactive X of the parental human fibroblasts. The active X with the translocation was not found in any of the HPRT-positive hybrid cells. Human phosphoglycerokinase (ATP:3-phospho-D-glycerate 1-phosphotransferase. EC 2.7.2.3) and glucose-6-phosphate dehydrogenase (D-glucose 6-phosphate: NADP 1-oxidoreductase, EC 1.1.1.49), which are specified by X-chromosomal loci, were not detected in the hybrids expressing HPRT even though they contained an apparently intact X chromosome. The observations are most simply explained by the infrequent, stable derepression of inactive X chromosome segments that include the HPRT locus but not the phosphoglycerokinase and glucose-6-phosphate dehydrogenase loci.

Hereditary orotic aciduria: evidence for a structural gene mutation

Orotic aciduria is a rare autosomal recessive disease in man due to a deficiency of orotate phosphoribosyltransferase (EC 2.4.2.10; orotidine-5'-phosphate:pyrophosphate phosphoribosyltransferase) and orotidine-5'-phosphate decarboxylase (EC 4.1.1.23; orotidine-5'-phosphate carboxy-lyase). We have compared certain physicochemical properties of orotidine-5'-phosphate decarboxylase from normal and mutant fibroblasts grown under identical conditions. Orotidine-5'-phosphate decarboxylase from homozygous mutant cells was more thermolabile and exhibited a different electrophoretic mobility when compared to the enzyme from normal cells; orotidine-5'-phosphate decarboxylase from one heterozygous cell strain exhibited an intermediate thermolability while the other heterozygote displayed a thermal inactivation curve indistinguishable from normal. The enzyme from both normal and mutant cells exhibited biphasic kinetics with the same apparent Michaelis constants. These data suggest that the molecular defect in the enzyme of this patient with orotic aciduria is due to a mutation in a gene that affects the structure of either orotate phosphoribosyltransferase or orotidine-5'-phosphate decarboxylase and cannot be attributed to a mutation in a regulatory gene, as previously suggested.

Isolation and analysis of somatic hybrids derived from two human diploid cells

Lesch-Nyhan fibroblasts and normal human leukocytes with different glucose-6-phosphate dehydrogenase genotypes were fused by Sendai virus. Clones were isolated on the basis of their resistance to a medium containing hypoxanthine, amethopterin, and thymidine and ability to proliferate in monolayer culture. These mononuclear cells (1) incorporated [(3)H]hypoxanthine, (2) expressed the glucose-6-phosphate dehydrogenase heteropolymer, and (3) were polyploid. Therefore, hybrids can originate from the fusion of two diploid human cells. X chromosome inactivation did not occur in these hybrid cells of male origin. The hybrids expressed both parental genomes and exhibited the senescence and contact feeding characteristic of the human skin fibroblast.

Human-mouse cell hybrids: a suggestion of structural mutation for dipeptidase-2 deficiency in mouse cells

The dipeptidase-2 enzyme is inactive in certain cultured cell lines from the mouse. In somatic cell hybrids between such deficient cells and diploid human fibroblasts, the mouse deficiency was complemented when the homologous human peptidase-A was retained. The results suggested that the murine peptidase deficiency was the result of a structural mutation, rather than a regulatory one.

The preparation and properties of macrophage-L cell hybrids

The plasma membrane of the mouse peritoneal macrophage has specific receptors which enable the cell to bind IgG or complement-coated sheep red cells and is also rich in a divalent cation-dependent adenosine triphosphatase (ATPase) activity. L cells lack these macrophage membrane markers. The question of macrophage membrane receptor expression was investigated in DBA/2 mouse macrophage x mouse LMTK(-) cell hybrids produced with the aid of Sendai virus. Three independent clones and one mass culture were isolated by their ability to grow in hypoxanthine, aminopterin, and thymidine (HAT) selection medium. These hybrids retained 85-100% of the sum of two parent cells' chromosomes and expressed several genes derived from both parents, including glucose phosphate isomerase isozymes and H-2 antigens. The hybrids displayed ATPase activity which was intermediate between that of the macrophage and L cell. The macrophage specific receptors for antibody or complement-coated red cells could not be demonstrated on hybrid cells. The selective absence of these receptors is probably because of a failure in gene expression rather than to loss of genes.

Interallelic complementation in hybrid cells derived from human diploid strains deficient in galactose-1-phosphate uridyl transferase activity

Galactose-1-phosphate uridyl transferase activity has been demonstrated in hybrid cells formed from the fusion of human diploid fibroblasts obtained from different patients with galactosemia. The enzyme produced by the hybrid cells was similar to the normal enzyme in regard to K(m), pH optimum, and electrophoretic mobility on starch gel, but differed in specific activity, V(max) and thermal stability. This is the first reported example of interallelic complementation in fused human diploid cells.