Dynamic behavior of histone H1 microinjected into HeLa cells

Histone H1 was purified from bovine thymus and radiolabeled with tritium by reductive methylation or with 125I using chloramine-T. Red blood cell-mediated microinjection was then used to introduce the labeled H1 molecules into HeLa cells synchronized in S phase. The injected H1 molecules rapidly entered HeLa nuclei, and a number of tests indicate that their association with chromatin was equivalent to that endogenous histone H1. The injected molecules copurified with HeLa cell nucleosomes, exhibited a half-life of approximately 100 h, and were hyperphosphorylated at mitosis. When injected HeLa cells were fused with mouse 3T3 fibroblasts less than 10% of the labeled H1 molecules migrated to mouse nuclei during the next 48 h. Thus, the intracellular behavior of histone H1 differs markedly from that of high mobility group proteins 1 and 2 (HMG1 and HMG2), which rapidly equilibrate between human and mouse nuclei after heterokaryon formation (Rechsteiner, M., and L. Kuehl, 1979, Cell, 16:901-908; Wu, L., M. Rechsteiner, and L. Kuehl, 1981, J. Cell Biol, 91: 488-496). Despite their slow rate of migration between nuclei, the injected H1 molecules were evenly distributed on mouse and human genomes soon after mitosis of HeLa-3T3 heterokaryons. These results suggest that although most histone H1 molecules are stably associated with interphase chromatin, they undergo extensive redistribution after mitosis.     

Preparation and characterization of chick erythrocyte nuclei from heterokaryons

A method for the isolation of reactivated chick erythrocyte nuclei from heterokaryons was developed. The heterokaryons were produced by fusing chick erythrocytes with HeLa or L cells in the presence of inactivated Sendai virus. At various time intervals after fusion nuclei were isolated directly from the monolayer by treatment with an acidic detergent solution. Chick erythrocyte nuclei were then separated from other nuclei (HeLa or L cell) by centrifugation on sucrose gradients. The purified preparation of reactivated chick erythrocyte nuclei was shown to be free from other nuclei and cytoplasmic contamination. By using L cells which had been labelled with ³H-leucine before fusion or heterokaryons labelled after fusion it was demonstrated that labelled mouse proteins migrate from the cytoplasm of the heterokaryons into the reactivating chick erythrocyte nuclei. ³H-uridine labelling of heterokaryons made by fusing UV-irradiated chick erythrocytes with L cells failed to reveal any significant migration of mouse RNA into the chick erythrocyte nuclei.     

Immortalized phenotype and the presence of active oncogenes correlate with the capacity of culture cells to induce reactivation of DNA synthesis in macrophage nuclei in heterokaryons

Several types of culture cells with limited life span (rat embryo fibroblasts, rat chondrocytes and mouse premacrophages) were found to be unable to induce the reactivation of DNA synthesis in the nuclei of non-dividing differentiated cells (mouse peritoneal resident macrophages) in heterokaryons. By contrast, malignant HeLa cells have this ability. In heterokaryons formed by fusion of mouse macrophages with HE239 cells (Syrian hamster fibroblasts transformed with a ts mutant of the SV40 virus), DNA synthesis in macrophage nuclei is reactivated only at the permissive temperature (33° C), at which viral T antigen is stable. Immortalization of rat chondrocytes by transfection with p53 gene enables to induce DNA synthesis in macrophage nuclei upon fusion. All the evidence indicates that the function of immortalizing oncogenes is necessary for the resumption of the DNA synthesis in macrophage nuclei in heterokaryons.     

Suppressive effect of protease inhibitors on heterokaryons containing chick erythrocyte nuclei

Nuclei of active cells (HeLa, mouse fibroblasts) partnered with chick erythrocyte nuclei in heterokaryons are suppressed, as judged by a decreased rate of ³H-uridine incorporation and diminished nuclear binding of ³H-actinomycin D. The extent to which active partner nuclei are suppressed, the extent to which erythrocyte nuclei are reactivated, and the degree of sensitivity of heterokaryons towards certain inhibitors of proteolytic enzymes, all correlate strongly with the ratios of erythrocyte nuclei to active nuclei. Thus, reactivation of individual erythrocyte nuclei is reduced progressively and active nuclei are suppressed progressively as the ratio of erythrocyte nuclei per active nucleus in heterokaryons increases. This erythrocyte nuclear-dose dependent suppression is markedly amplified when heterokaryons are grown in the presence of protease inhibitors. The protease inhibitors found to affect heterokaryons are low molecular weight (<400) inhibitors of trypsin-like enzymes: -1-tosylamide-2-leucyl chloromethyl ketone (TLCK), N-α-tosyl- -arginine methyl ester (TAME) and N-benzoyl- -arginine amide (BAA). They affect heterokaryons at concentrations comparable to the minimal concentrations at which they inhibit trypsin. Nonfused HeLa cells, mouse fibroblasts, or their homokaryons are refractory to protease inhibitors at these concentrations.Reactivation of chick erythrocyte nuclei in a heterokaryon may involve release of suppressors ordinarily confined to the erythrocyte nucleus, with subsequent redistribution of suppressor among all the nuclei of the heterokaryon. Under these circumstances the state of nuclear activity will depend on the quantity of suppressor per individual nucleus; within the erythrocyte nucleus the suppressors will decrease its rate of reactivation, when they migrate into an active nucleus they will suppress it. These suppressors, either in transit between the nuclei, or within the nuclei, may be hydrolysed by intracellular proteases.     

Haploid genome reactivation and recovery by cell hybridization

DNA replication in haploid spermatid nuclei has been induced by hybridization of mouse early spermatids to proliferating HeLa cells. Use of polyethylene glycol rather than inactivated Sendai virus as the cell fusion agent was found to be essential to the production of large numbers of heterokaryons containing spermatid nuclei. DNA replication was detected in the heterokaryons by autoradiography. Density of silver grains over spermatid nuclei closely approximated the grain density over labelled HeLa nuclei in the same heterokaryons. Mouse centromeric heterochromatin appeared to be labelled last during the spermatid DNA synthetic period. On the average, HeLa nuclei in heterokaryons began DNA synthesis before spermatid nuclei. Results indicated, however, that DNA synthesis by HeLa nuclei might not be a prerequisite for spermatid DNA synthesis. These experiments demonstrate induction of DNA synthesis in spermatid nuclei, the first major step toward reactivation and recovery of their haploid genome by cell hybridization.     

ADP-ribosylation of nucleolar proteins in HeLa tumor cells

ADP-ribosylation reactions in nucleoli of exponentially growing HeLa cells were studied. Isolated nuclei or nucleoli were labeled with ³²P-NAD; then the nucleolar proteins were analyzed by 1-dimensional and 2-dimensional polyacrylamide gel electrophoresis (PAGE) and modified proteins were detected by autoradiography. The labeled nucleolar proteins were also chromatographically fractionated on DEAE-cellulose. Electrophoretic analysis of total nucleolar and chromatographically purified proteins revealed that besides nuclear ADP-ribosyltransferase and histones two characteristic nucleolar phosphoproteins numatrin/B23 and nucleolin/C23 were modified by ADP-ribosylation.     

The chemical composition of nuclei and chromosomes isolated by the Langmuir trough technique

The pattern of staining for DNA, histone, and nonhistone protein has been studied in whole cells and in nuclei and chromosomes isolated by surface spreading. In whole interphase cells from bovine kidney tissue culture, nuclear staining for DNA and histones reveals numerous small, intensely stained clumps, surrounded by more diffusely stained material. Nuclei in whole cells stained for nonhistone proteins also contain intensely stained regions surrounded by diffuse stain. These intensely stained regions also stain for RNA, indicating that the regions contain nucleolar material. Electron microscopy of kidney cells confirms that multiple nucleoli are present. Kidney nuclei isolated by surface spreading show an even distribution of stain for DNA, histones, and nonhistone proteins, indicating that the surface forces disperse both condensed chromatin and nucleoli. DNA and protein staining was also studied in metaphase chromosomes from testes of the milkweed bug, Oncopeltus fasciatus. Staining for DNA and histones in metaphase chromosomes is essentially the same in sections of fixed and embedded testes as in preparations isolated by surface spreading. However, striking differences are noted in the distribution of nonhistone proteins. In sections, nonhistone stain is concentrated in extrachromosomal areas; metaphase chromosomes do not stain for nonhistone proteins. Chromosomes isolated by surface spreading, however, stain intensely for nonhistone proteins. This suggests that nonhistone proteins are bound to the chromosomes as a contaminant during the isolation procedure. The relationship of these findings to current work with chromosomes isolated for electron microscopy is discussed.     

Complementation in heterokaryons derived from a thymidine kinase deficient cell line and senescent human diploid cells.

Incorporation of [3H]thymidine was observed in both parental nuclei in heterokaryons resulting from the fusion of post-mitotic, "senescent" human diploid cells and a thymidine kinase-deficient murine cell line (3T3der-4E). The senescent nuclei displayed a sudden increase of activity approximately 4--6 hours after fusion. A moderate increase of thymidine incorporation was observed in 3T3der-4E cells cocultivated with but not fused to senescent cells, consistent with metabolic cooperation. Chromosome preparations of cultures fixed approximately 24 hr after fusion revealed the presence of hybrid metaphase cells containing almost the entire human complement. All of the identifiable human chromosomes were bi-armed, suggesting that the senescent nuclei were stimulated to reinitiate replicative DNA synthesis rather than repair synthesis in these heterokaryons.     

ACQUISITION OF CHICK CYTOSOL THYMIDINE KINASE ACTIVITY BY THYMIDINE KINASE-DEFICIENT MOUSE FIBROBLAST CELLS AFTER FUSION WITH CHICK ERYTHROCYTES

Chick-mouse heterokaryons were obtained by UV-Sendai virus-induced fusion of chick erythrocytes with thymidine (dT) kinase-deficient mouse fibroblast [LM(TK^-)] cells. Autoradiographic studies demonstrated that 1 day after fusion, [³H]dT was incorporated into both red blood cell and LM(TK^-) nuclei of 23% of the heterokaryons. Self-fused LM(TK^-) cells failed to incorporate [³H]dT into nuclear DNA. 15 clonal lines of chick-mouse somatic cell hybrids [LM(TK^-)/CRB] were isolated from the heterokaryons by cultivating them in selective hypoxanthine-aminopterin-thymidine-glycine medium. LM(TK^-) and chick erythrocytes exhibited little, if any, cytosol dT kinase activity. In contrast, all 15 LM(TK^-)/CRB lines contained levels of cytosol dT kinase activity comparable to that found in chick embryo cells. Disk polyacrylamide gel electrophoresis and isoelectric focusing analyses demonstrated that the LM(TK^-)/CRB cells contained chick cytosol, but not mouse cytosol dT kinase. The LM(TK^-)/CRB cells also contained mouse mitochondrial, but not chick mitochondrial dT kinase. Hence, the clonal lines were somatic cell hybrids and not LM(TK^-) cell revertants. The experiments demonstrate that chick erythrocyte cytosol dT kinase can be activated in heterokaryons and in hybrid cells, most likely as a result of functions supplied by mouse fibroblast cells.     

The chemical composition of nuclei and chromosomes isolated by the Langmuir trough technique

The pattern of staining for DNA, histone, and nonhistone protein has been studied in whole cells and in nuclei and chromosomes isolated by surface spreading. In whole interphase cells from bovine kidney tissue culture, nuclear staining for DNA and histones reveals numerous small, intensely stained clumps, surrounded by more diffusely stained material. Nuclei in whole cells stained for nonhistone proteins also contain intensely stained regions surrounded by diffuse stain. These intensely stained regions also stain for RNA, indicating that the regions contain nucleolar material. Electron microscopy of kidney cells confirms that multiple nucleoli are present. Kidney nuclei isolated by surface spreading show an even distribution of stain for DNA, histones, and nonhistone proteins, indicating that the surface forces disperse both condensed chromatin and nucleoli. DNA and protein staining was also studied in metaphase chromosomes from testes of the milkweed bug, Oncopeltus fasciatus. Staining for DNA and histones in metaphase chromosomes is essentially the same in sections of fixed and embedded testes as in preparations isolated by surface spreading. However, striking differences are noted in the distribution of nonhistone proteins. In sections, nonhistone stain is concentrated in extrachromosomal areas; metaphase chromosomes do not stain for nonhistone proteins. Chromosomes isolated by surface spreading, however, stain intensely for nonhistone proteins. This suggests that nonhistone proteins are bound to the chromosomes as a contaminant during the isolation procedure. The relationship of these findings to current work with chromosomes isolated for electron microscopy is discussed.     

The Effect of Azidothymidine on Reactivation of DNA Synthesis in Macrophage Nuclei Contained in Heterokaryons

In heterokaryons, DNA synthesis is reactivated in macrophage nuclei only in the case of fusion with immortal cells. Assuming that telomerase is responsible for reactivation, the effect of its inhibitor azidothymidine (AZT) was studied in heterokaryons of mouse resident peritoneal macrophages and immortal 3T3 Swiss cells. AZT suppressed reactivation of DNA synthesis in macrophage nuclei and had no effect on DNA synthesis in 3T3 Swiss cell nuclei, suggesting that telomere structure is impaired in normal mouse macrophages.     

Effect of azidothymidine on reactivation of DNA synthesis in macrophage nuclei contained in heterokaryons

EIn heterokaryons, DNA synthesis is reactivated in macrophage nuclei only in the case of fusion with immortal cells. Assuming that telomerase is responsible for reactivation, the effect of its inhibitor azidothymidine (AZT) was studied in heterokaryons of mouse resident peritoneal macrophages and immortal 3T3 Swiss cells. AZT suppressed reactivation of DNA synthesis in macrophage nuclei and had no effect on DNA synthesis in 3T3 Swiss cell nuclei, suggesting an altered telomere structure in normal mouse macrophages.     

DNA synthesis in the heterokaryons of non-dividing differentiated cells and culture cells with various proliferative potentials

Resident peritoneal mouse macrophages (non-dividing differentiated cells) were fused with mouse embryo fibroblasts (cells with a limited lifespan), NIH 3T3 and C3H 10T 1/2 cells ('immortal' cell lines) and SV 3T3 cells (a malignant cell line). DNA synthesis was investigated in the resultant heterokaryons. No inhibitory effect upon the transition of NIH 3T3 and mouse embryo fibroblasts nuclei to the S-phase was observed. C3H 10T 1/2, NIH 3T3 and SV 3T3 cells induced the reactivation of DNA synthesis in the macrophage nuclei in the heterokaryons. At the same time, no replication was detected in the macrophage nuclei after fusion with mouse embryo fibroblasts.     

Activity banding of human chromosomes as shown by histone acetylation

The expression of genes in mammalian cells depends on many factors including position in the cell cycle, stage of differentiation, age, and environmental influences. As different groups of genes are expressed, their packaging within chromatin changes and may be detected at the chromsomal level. The organization of DNA within a chromosome is determined to a large extent by the positively charged, highly conserved histones. Histone subtypes and the reversible chemical modifications of histones have been associated with gene activity. Active or potentially active genes have been associated with hyperacetylated histones and inactive genes with nonacetylated histones. Sodium butyrate increases the acetylation levels of histones in cell cultures and acts as both an inducer of gene activity and as a cell-cycle block. We describe a method to label the interphase distribution of DNA associated with various histone acetylation stages on chromosomes. Nucleosomes from untreated and butyrate-treated HeLa cells were fractionated by their acetylation level and the associated DNA labeled, and hybridized to normal human chromosomes. In the sodium butyrate-treated cells the resulting banding patterns of the high- and low-acetylated fractions were strikingly different. DNA from low-acetylated chromatin labeled several pericentric regions, whereas hybridization with DNA from highly acetylated chromatin resulted in a pattern similar to inverse G-bands on many chromsomes. The results from noninduced cells at both high and low acetylation levels were noticeably different from their induced counterparts. The capture and hybridization of DNA from interphase chromatin at different acetylation states provides a “snap-shot” of the distribution of gene activity on chromosomes at the time of cell harvest. Edited by: P.B. Moens     

Role of nuclear proteins on [3H]actinomycin D binding during lymphocyte mitogenesis

The uptake of [³H]actinomycin D ([³H]AD) by ConA-stimulated lymphocytes was followed during 96 h of incubation and correlated with the level of nuclear proteins in the nucleus, DNA synthesis and the degree of AD-induced inhibition of RNA and DNA synthesis. During the first 48 h there is a parallel increase of drug binding to cells and a rising level of non-histone proteins (NHP) in the nucleus. During the next 48 h, DNA synthesis occurs, drug uptake decreases and the nuclear level of NHP continues to rise. The level of histones remains constant during 96 h. The variations in cellular [³H]AD uptake during 96 h are not due to changes in cell membrane permeability, since similar variations in drug binding are observed in isolated cell nuclei. NHP, obtained as 0.25 M NaCl extracts of cell nuclei, increase binding of [³H]AD to nuclei isolated from non-stimulated lymphocytes, while histones have no such effect. NHP extracted with phenol, after washing the nuclei with salt and acid solutions, or extracted with 0.25 M NaCl from non-stimulated and stimulated lymphocytes and Chang liver cells are equally active to bind [³H]AD to nuclei of non-stimulated lymphocytes. NHP from Chang cells, purified by DNA-cellulose chromatography using calf thymus DNA, stimulated [³H]AD binding to lymphocyte nuclei, indicating that the drug-binding activity is due to proteins binding to DNA. NHP increase binding of [³H]AD to pure DNA in the absence of histones. The degree of [³H]AD binding to ConA-stimulated lymphocytes during 96 h correlated with the degree of inhibition of RNA and DNA synthesis by AD.     

Hepatocytes in heterokaryons do not retard the nuclei of stimulated fibroblasts entering the S period]

Serum-deprived (0.2%) resting and serum-stimulated (10%) proliferating NIH 3T3 mouse fibroblasts were fused with hepatocytes from intact, regenerating and embryonic mouse livers to elucidate mechanisms of liver cell proliferation, DNA synthesis being investigated in nuclei of heterokaryons and non-fused cells using radioautography. Hepatocytes in heterokaryons were found to have no inhibitory effect on the entry of stimulated fibroblast nuclei into the S-period, but on the contrary they were involved in DNA synthesis. In addition, the nuclei in heterokaryons mutually stimulated each other to enter the S-period. In their turn, the resting fibroblasts did not prevent the proliferating hepatocytes from the regenerating and embryonic livers to enter the S-period. Possible reasons of the absence of inhibitory effect of differentiated cells in heterokaryons are discussed. The data obtained enable us to conclude that the mechanism of proliferative process control in resting immortalized cells differs from that in differentiated cells where proliferation seems to be stopped without affecting the endogenous inhibitor postulated for the resting and ageing fibroblasts.     

DNA synthesis in heterokaryons obtained by the fusion of polymorphonuclear leukocytes and cultured cells possessing different proliferative potentials

Heterokaryons between terminally differentiated polymorphonuclear leukocytes (PL) and culture cells of different proliferative potentials: mouse and rat embryo fibroblasts (EFM, EFR); immortal cells NIH 3T3 and E2; malignant cells NCC2, L929, He239 and SV 3T3,--were obtained by means of electrofusion. Radioautographic study of 3H-thymidine incorporation in the nuclei of heterokaryons showed that all the cells taken for fusion were able to induce reactivation of DNA synthesis in PL nuclei, however, with different rates: 7-37% for EFM and NIH 3T3 and 20-40% for malignant cells. The presence of oncogenes Elan in E2 cells and ras in NCC2 cells increased the rate of PL reactivation approximately twice as compared with the cells of original lines (EFR and NIH 3T3, correspondingly). In parallel to reactivation of DNA synthesis in PL nuclei inhibition of the synthesis in culture cell nuclei in the same heterokaryons was found. The rate of inhibition was about 70% for non-malignant and 23, 40 and 18% for NCC2, L and SV 3T3 cells, respectively. He239 cells, transformed by a temperature-dependent mutant of virus SV40 showed at permissive temperature the increased capacity of inducing reactivation of PL nuclei, though He239 cells susceptibility to inhibitory action of PL nuclei did not change with temperature. According to the behaviour in heterokaryons PL were found to be similar to chick erythrocytes, but differing from them by a pronounced inhibiting effect upon DNA synthesis in the nuclei of malignant cells.     

Role of nuclear proteins on [H]actinomycin D binding during lymphocyte mitogenesis

The uptake of [³H]actinomycin D ([³H]AD) by ConA-stimulated lymphocytes was followed during 96 h of incubation and correlated with the level of nuclear proteins in the nucleus, DNA synthesis and the degree of AD-induced inhibition of RNA and DNA synthesis. During the first 48 h there is a parallel increase of drug binding to cells and a rising level of non-histone proteins (NHP) in the nucleus. During the next 48 h, DNA synthesis occurs, drug uptake decreases and the nuclear level of NHP continues to rise. The level of histones remains constant during 96 h. The variations in cellular [³H]AD uptake during 96 h are not due to changes in cell membrane permeability, since similar variations in drug binding are observed in isolated cell nuclei. NHP, obtained as 0.25 M NaCl extracts of cell nuclei, increase binding of [³H]AD to nuclei isolated from non-stimulated lymphocytes, while histones have no such effect. NHP extracted with phenol, after washing the nuclei with salt and acid solutions, or extracted with 0.25 M NaCl from non-stimulated and stimulated lymphocytes and Chang liver cells are equally active to bind [³H]AD to nuclei of non-stimulated lymphocytes. NHP from Chang cells, purified by DNA-cellulose chromatography using calf thymus DNA, stimulated [³H]AD binding to lymphocyte nuclei, indicating that the drug-binding activity is due to proteins binding to DNA. NHP increase binding of [³H]AD to pure DNA in the absence of histones. The degree of [³H]AD binding to ConA-stimulated lymphocytes during 96 h correlated with the degree of inhibition of RNA and DNA synthesis by AD.