Localization of the globin gene in the template active fraction of chromatin of Friend leukemia cells

Distribution of globin genes and histone variants in micrococcal nuclease-generated subfractions of chromatin from Friend erythroleukemia cells at different malignant states

The distribution of the alpha and beta-globin genes and histone variants was examined in micrococcal nuclease-generated chromatin fractions of three Friend murine erythroleukemia cell types differing in malignant potential and inducibility to erythroid differentiation. A preferential concentration of globin gene sequences, as compared to satellite DNA, was noted in a physiological salt-soluble, histone H1-depleted, mononucleosomal chromatin fraction (Sup 120) in all Friend cell types, even those in which the globin gene was not capable of transcriptional activation by chemical induction. The level of globin gene enrichment in the Sup 120 fraction was highest in the most malignant and inducible cell type. The chemical induction of erythroid differentiation in this cell line did not change the distribution of globin genes in the chromatin fractions. The Sup 120 chromatin fraction prepared from mouse brain nuclei was not enriched in globin genes. Besides the previously reported low H2A. 1/H2A.2 ratio [Blankstein and Levy: Nature 260:638-640, 1976], chromatin from the most tumorigenic cell type showed the lowest H2B.2 to H2B.1 ratio, highest levels of histone H4 acetylation, and the most pronounced change in relative amounts of two major electrophoretic bands of histone H1 variants as compared to the less malignant cell types. The histone variant content of the micrococcal nuclease-generated chromatin fractions from the three Friend cell types reflected the core histone variant differences for the respective intact nuclei. However, the electrophoretic separation of mononucleosomes by size revealed several classes with different H2A variant ratios. The results demonstrate the existence of structural differences in globin gene and histone variants in erythroleukemia cell chromatin associated with distinguishable phenotypes during malignant cell progression.

Oxidative stress as a causal factor in differentiation and aging: a unifying hypothesis

In this article, the authors have pointed out flaws in the current version of the free radical hypothesis of aging and have advanced a new hypothesis that reconciles and encapsulates existing information. The main premise of this hypothesis is that aging is a continuation of development and is thus influenced by genetically programmed phenomena. Completion of various genetic programs and the duration of life are linked to a metabolic potential which is itself a genetically determined sum of energy expenditure. Nevertheless, the rate at which metabolic potential is reached is linked to the rate of metabolism and the level of oxidative stress both of which are influenced by epigenetic stimuli. The current version of the free radical hypothesis postulates that partially reduced oxygen species are produced in aerobic cells in an uncontrolled fashion and do not play any useful physiological function. The principle tenet of the free radical hypothesis is that molecular damage is the underlying cause of aging and that O2- radicals and derivatives induce most of the damage sustained by cells during aging. The authors regard this hypothesis as flawed because it fails to explain either low randomly occurring damage can lead to age-associated changes that are species-specific, or the sequential nature of the changes that occur in aging organisms. In contrast to the free radical hypothesis, our hypothesis can explain the specific and sequential nature of aging-related changes because they are postulated to be neither dependent upon uncontrolled damage nor the cellular capacity to prevent it. Instead, the authors suggest that the damage accumulated during aging is a secondary effect rather than a direct cause of senescence. The authors have shown that cells exert control not only on their level of antioxidant defense but also on their rate of oxidant production. The authors postulate that aging is the terminal stage of development, and as such is influenced genetically. The authors also postulate that a definite sum of energy is required to complete the genetic programs associated with aging. Thus, the rate of aging is linked to the level of oxidative stress; the rate of energy utilization is postulated to determine the level of oxidative stress. Oxidative stress is one of the factors which appears to govern changes in gene expression during differentiation and we suggest that it causes alterations in gene expression during aging. In the authors revised hypothesis, free radicals promote aging by affecting specific genetic programs and the incidental damage they inflict in cells is only a by-product of this process.(ABSTRACT TRUNCATED AT 400 WORDS)

Oxidative influence on development and differentiation: an overview of a free radical theory of development

Metabolic gradients exist in developing organisms and are believed to influence development. It has been postulated that the effects of these gradients on development result from differential oxygen supplies to tissues. Oxygen has been found to influence the course of development. Cells and tissues in various stages of differentiation exhibit discrete changes in their antioxidant defenses and in parameters of oxidation. Metabolically generated oxidants have been implicated as one factor that directs the initiation of certain developmental events. Also implicated as factors that modulate developmental processes are the cellular distribution of ions and the cytoskeleton both of which can be influenced by oxidants. The interaction of oxidants with ion balance and cytoskeleton is discussed.

Activation of sea urchin actin genes during embryogenesis. Measurement of transcript accumulation from five different genes in Strongylocentrotus purpuratus

The number of molecules of mRNA transcribed from each of five different actin genes are reported for developing embryos of the sea urchin Strongylocentrotus purpuratus. Transcripts of the cytoskeletal actin genes CyI, CyIIa, CyIIb and CyIIIa, and of the muscle actin gene M, were measured in unfertilized egg and embryo RNAs of cleavage, blastula, gastrula and pluteus stages. The measurements were obtained by probe excess titrations of these RNAs, using a set of single-stranded RNA probes each identifying the mRNA transcripts of a specific actin gene. These mRNAs can be identified by their distinct 3' non-translated trailer sequences. We confirm prior observations that the prevalence of actin mRNA in the unfertilized egg is low. Cytoskeletal actin genes CyI and CyIIIa each contribute 1 X 10(3) to 2 X 10(3) maternal mRNA molecules, and CyIIb contributes less than 2 X 10(2) mRNA molecules, while no detectable maternal mRNAs derive from cytoskeletal actin gene CyIIa or the muscle actin gene M. During certain periods of development, transcripts derived from the individual cytoskeletal actin genes accumulate rapidly, with kinetics specific to each mRNA. Transcripts of the muscle actin gene are absent until after gastrulation, when the initial muscle progenitor cells are formed. At late stages of development, each of the five genes studied is represented by 10(4) to 10(5) mRNA molecules per embryo. The present measurements permit calculation of the levels of each actin mRNA species in the particular cell types in which each gene functions in the fully differentiated embryo.

Oxygen free radicals play a role in cellular differentiation: an hypothesis

Evidence from a variety of sources supports the view that oxygen free radicals play a role in cellular differentiation. It is postulated that cellular differentiation is accompanied by changes in the redox state of cells. Differentiated cells have a relatively more prooxidizing or less reducing intracellular environment than the undifferentiated or dedifferentiated cells. Changes in the redox balance during differentiation appear to be due to an increase in the rate of O2- generation. Differentiated cells, in general, exhibit higher rates of cyanide-resistant respiration, cyanide-insensitive SOD activity, and peroxide concentration and lower levels of GSH as compared to undifferentiated cells. The effects of free radicals on cellular differentiation may be mediated by the consequent changes in ionic composition.

Non-random interaction of N-2-acetylaminofluorene and its N-hydroxy metabolite with DNA of rat hepatic parenchymal and non-parenchymal cell nuclei following in vivo administration of carcinogen

Treatment of rats with the hepatocarcinogen, N-2-acetylaminofluorene (AAF) results in development of malignant tumors derived primarily from hepatic parenchymal cells. Following administration of AAF, or its N-hydroxy derivative, in vivo nuclei from parenchymal cell and non-parenchymal cell populations (NI and NII nuclei populations, respectively) were isolated and treated with the endonuclease DNase I. The binding of carcinogen residues to the DNA of nuclease-accessible vs. nuclease-resistant regions of chromatin was evaluated on the basis of the selectivity of DNase I for transcriptionally active DNA. Under the experimental conditions employed DNase I digested approx. 50-60% of the genome of NI nuclei while only 10-20% of the DNA from non-parenchymal cell nuclei (NII) was susceptible to this enzyme. When the DNA of NI and NII nuclei were nick translated following limited digestion with DNase I, a greater degree of transcriptional activity (nuclease accessibility) was found in parenchymal cell nuclei (NI). Following a single injection of rats with [ring-3H]AAF or its N-hydroxy metabolite (N-[ring-3H]-OH-AAF) (1.8 mumol carcinogen/100 g), adducts were preferentially associated with DNA of DNase I resistant regions of target cell nuclei (NI), while preferentially associated with nuclease-accessible regions of non-target cell nuclei (NII). Damage following a single injection persisted for up to 3 days in DNase I-resistant DNA of NI nuclei, carcinogen adducts were rapidly lost from DNase I-accessible DNA of NII nuclei. These studies stress the importance of investigating specificity of carcinogens for particular regions of the genome of cell subpopulations within the target organ.

[Structural arrangement of chromatin (author's transl)]

Recent observations and hypotheses on the structure of chromatin are reviewed. Elementary "subunit" for higher structural orders is the nucleosome, consisting of a histone octamer and doublehelical DNA wrapped around it. During the last years details of the nucleosomal structure could be deduced down to a resolution of 2 nm. In the chromatin fiber, built up by (mono-)nucleosomes, the superhelical DNA has a tertiary structure, from which structures of still higher order (quaternary structure of DNA) can be formed. The correlation of these structures to the terms euchromatin and heterochromatin are discussed. Finally, some functional aspects, especially transcription and replication, are discussed in view of the new knowledge of chromatin structure.

Release of a globin gene enriched chromatin fraction from chicken erythrocyte nuclei following DNase II digestion

Mild digestion of chicken erythrocyte nuclei with deoxyribonuclease II results in the release of a chromatin fraction which is 4- to 13-fold enriched for the globin coding sequences when compared to total chicken DNA. The remaining nuclear pellet is depleted in these sequences. A maximum of 25% of the globin genes have been recovered in the released fraction. The addition of 5 mM sodium butyrate to the digestion buffer is required to obtain reproducible globin gene enrichment. The released fraction contains equimolar amounts of the four core histones and a subset of the nonhistone chromosomal proteins. The globin genes are released as large chromatin fragments which exceed the 1.6 kilobase size of the transcribed portion of the gene.

Restriction endonuclease sensitivity of DNA containing globin genes in different murine erythroleukemia cell lines

The arrangement of the globin structural genes has been examined in murine erythroleukemia cells, strain DS19, and several related inducer-resistant variant cell lines. One fragment larger than 20 kilobases and six globin gene-containing fragments between 10 and 1.9 kilobases in size are detected in EcoRI-cleaved purified DNA prepared from strain DS19. By comparison, when isolated nuclei are digested with EcoRI, only two globin gene-containing fragments are detected, one greater than 20 kilobases and the other 1.9 kilobases. Of seven cell lines derived from DS19 and resistant to inducers, six had similar patterns to DS19 of globin gene-containing EcoRI-generated DNA fragments from nuclei and from purified DNA. One cell line, DR10, a DMSo-resistant cell line, lacks the 1.9 kilobase fragment after digestion of either nuclei or purified DNA. The 1.9 kilobase fragment hybridizes with alpha-globin cDNA but not with the beta-globin cDNA, suggesting either rearrangement or deletion of an alpha-globin gene-like fragment in DR10 DNA.

Distribution of high mobility group and other acid-soluble proteins in fractionated chromatin

Chromatin was fractionated by digestion with deoxyribonuclease II and precipitation with MgCl2. The Mg2+-soluble fraction, known to be enriched in transcribed DNA sequences, was enriched also in high mobility group proteins 1 and 2 and contained almost all other acid-soluble nonhistone proteins.

Localization of globin gene replication in Friend Leukemia cells to a specific interval of the S phase

Murine erythroblastic leukemia cells, infected with Friend leukemia virus, were grown and synchronized in suspension culture by a double-block procedure involving medium depletion and treatment with excess thymidine. Replicating cultures were then caused to synthesize DNA during the early, middle, or late third of the S period with bromodeoxyuridine as a precursor. The bromodeoxyuridine density-labeled DNA and normal DNA were isolated by sedimentation in a cesium chloride density gradient and analyzed for the level of globin-specific DNA by hybridization with radioactive cDNA of the globin messenger RNA. Globin genes were found to be replicated near the end of the middle third of the S phase. The incorporation of bromodeoxyuridine into DNA also resulted in a lowered induction of hemoglobin synthesis in dimethyl sulfoxide-treated cells. In this case, the sensitivity was correlated with the introduction of bromodeoxyuridine into DNA replicated in early S phase. The possibility that this fraction of DNA, which is low in globin gene content, carries genes regulating the erythroid differentiation or the expression of globin genes is discussed.

Fractionation of native and reconstituted chromatin by digesting with deoxyribonuclease ii

Native and reconstituted chromatin from Ehrlich ascites tumor cells were fractionated into template-active and inactive fractions by the DNAase II/Mg2+-solubility method of Gottesfeld et al. (Gottesfeld, J.M., Garrard, W.T., Bagi, G., Wilson, R.F. and Bonner, J. (1974) Proc. Natl. Acad. Sci. U.S.A. 71, 2193-2197). The Mg2+-soluble (template-active) fractions were compared in respect to sedimentation behavior in sucrose gradients and the relative content of specific transcribed (ribosomal) and non-transcribed (satellite) DNA sequences. It was found that the Mg2+-soluble fraction of the native chromatin was enriched in ribosomal DNA while almost completely devoid of satellite DNA; the nucleoprotein monomer of this fraction sedimented in sucrose gradient at 14 S. Similar-results were obtained if chromatin was fractionated in the presence of 3 M urea. With reconstituted chromatin, however, neither the sedimentation profile, nor the relative content of ribosomal and satellite DNA sequences were recovered, thus indicating that reconstitution did not yield nucleoprotein structurally similar to native chromatin.

Corticosterone effects on rat brain template active region chromatin

The effect of prenatal exposure to corticosterone on brain chromatin was examined. Pregnant Fischer inbred rats were administered corticosterone or saline on Days 17 and 18 of gestation and their offspring examined at 0 (day of birth), 2, 3, 4 or 6 days of age. In utero exposure to corticosterone was associated with a 24 hr delay of a developmental peak in the percentage of brain template active region chromatin. Brain and body weights of the steroid and saline-treated animals were similar, but corticosterone led to a temporary decrease in body and brain weight at 2 days of age which was reversed at 6 days of age. These results of these studies suggest an impact of corticosterone on brain gene expression.

In vivo binding of N-2-acetylaminofluorene and its N-hydroxy derivative to the DNA of fractionated rat liver chromatin

The in vivo binding of radioactive N-2-acetylaminofluorene (AAF) and N-hydroxy-2-acetylaminofluorene (N-OH-AAF) to the DNA of rat liver chromatin was examined. The chromatin was fractionated into putative transcriptionally active and inactive fractions by hydrodynamic shearing and subsequent glycerol gradient centrifugation, DNAase II digestion followed by MgCl2 aggregation of transcriptionally inactive chromatin, or mild digestion with micrococcal nuclease. Carcinogens were administered for various times prior to sacrifice. Irrespective of the duration of exposure, no preferential binding of either carcinogen to DNA was detected in any of the fractions prepared by hydrodynamic shearing of DNAase II digestion. When micrococcal nuclease was utilized, a 2-fold increase in carcinogen bound to the DNA of that chromatin fraction containing the smallest molecular weight fragments was detected. These small molecular weight fragments produced by micrococcal nuclease have been postulated to be derived from in vivo transcriptional units. Additionally, when DNAase II was used to probe chromatin from rat livers which had been exposed to a carcinogenic regimen of AAF, no preferential binding of radioactive N-OH-AAF to the DNA of any chromatin fraction was detected.

Quantitative and qualitative aspects of the binding of N-hydroxy-2-acetylaminofluorene to hepatic chromatin fractions

The binding of a chemical carcinogen to components of hepatic chromatin in male rats was examined. After a single injection of N-[3H]hydroxy-2-acetylaminofluorene ([3H]OH-AAF) covalent binding to chromatin RNA, protein, and DNA occurs. The amount of carcinogen bound to RNA was approximately 5 times greater than to DNA, and 10 times that of the protein. However, loss of carcinogen from RNA with time was rapid, whereas a persistent binding to DNA equal to 15% of the initial values was observed. To localize the initial and persistent DNA-bound carcinogen, the genome was fractionated using two different chromatin fractionation procedures. The procedures used yielded 3 chromatin fractions based on physical characteristics, degree of association with nascent RNA and in vitro template capacity. Based on those parameters, these chromatin fractions have been tentatively classified as template expressed euchromatin, a repressed heterochromatin, and a highly condensed pelleted heterochromatin. With both the glycerol gradient chromatin fractionation procedure and the selective MgCl2 chromatin precipitation procedure, the initial (2 h) binding of carcinogen was greatest on the euchromatin DNA. Loss of carcinogen from the DNA, however, was also significantly faster from the euchromatin when compared to the heterochromatin and the pelleted heterochromatin. By 10 days after a single injection of the carcinogen, the largest amount of bound fluorene residues was located on the pelleted heterochromatin DNA, an apparently repressed portion of the genome, while less than 5% of the initial values were found on either the eu- or heterochromatin. When the rats were fed a 2-acetylaminofluorene-containing diet, loss of carcinogen from the pelleted heterochromatin DNA was enhanced, while loss from the euchromatin DNA was reduced. The covalent nature of the carcinogen modification of DNA was confirmed by thin-layer chromatography (TLC). These studies also demonstrated 2 separate carcinogen-purine base adducts which were identified as N-(guanin-8-yl)-N-AF and 3-(guanin-N2-yl)-N-AAF based on either co-chromatography with an authentic standard or on published Rf-values, respectively. The pelleted heterochromatin DNA had a significantly greater proportion of the 3-guanine-N2 adduct when compared to DNA from either the eu- or heterochromatin.

Evolutionary change in the repetition frequency of sea urchin DNA sequences

The frequency of occurrence of particular repetitive sequence families has been estimated in the DNA of the three sea urchin species Strongylocentrotus purpuratus. Strongylocentrotus franciscanus and Lytechinus pictus using individual cloned S. purpuratus repetitive sequence elements. Cloned repetitive sequence elements as described by Scheller et al. (1977a) were prepared by reassociation of S. purpuratus DNA fragments to repetitive Cot, digestion with single-strand-specific nuclease S1 and ligation of synthetic restriction sites to their ends. The sequences were cloned by insertion at the Eco RI site of plasmid RSF2124, labeled, strand-separated and reassociated with 800--900 nucleotide long unlabeled DNA. Both kinetic (genomic DNA excess) and saturation (cloned DNA excess) estimates of frequencies were made. For nine cloned fragments, the ratio of the repetition frequency in S. purpuratus DNA to that in S. franciscanus DNA ranges from about 20 to about 1. In the four cases examined, only a few copies were detected in the DNA of L. pictus. Estimates have also been made of frequency changes in many repetitive families by measuring the reassociation of labeled repetitive DNA fractions of each species with total DNA from other species. In each reciprocal comparison, the labeled repetitive sequences reassociate more slowly with DNA of other species than with DNA of the species from which they were prepared. Thus it appears that the dominant repetitive sequence families in the DNA of each species are present at lower frequencies in the DNA of closely related species. Measurements of thermal stability have been made of S. purpuratus cloned repetitive sequences reassociated with S. franciscanus DNA or S. purpuratus DNA. Most families have changed both in frequency and sequence, although some have changed little in sequence but show great changes in frequency.

Specific representation of cloned repetitive DNA sequences in sea urchin RNAs

Nine cloned repetitive sequences were labeled, strands-separated and individually hybridized with RNA extracted from the nuclei of gastrula stage sea urchin embryos and of adult sea urchin intestine cells. The concentration of transcripts complementary to each cloned sequence was measured by RNA excess hybridization kinetics and by a DNA excess titration method. Transcripts of certain of the repeat families are present at over 100 times the concentration of transcripts of other families in each RNA. The set of repetitive sequence families highly represented in intestine nuclear RNA is different from that highly represented in gastrula nuclear RNA. Together with the results obtained with mature oocyte RNA and presented in the accompanying paper by Costantini et al. (1978), these findings show that quantitative patterns of repetitive sequence representation in RNA are specific to each cell type. Both strands of all of the nine cloned repeats are represented at some level in all the RNAs studied. Usually, though not always, the concentration of transcripts complementary to the two strands of each repeat do not differ by more than a factor of two. The cloned tracers do not react with polysomal messenger RNA, and the nuclear RNA molecules with which they hybridize are many times larger than the repetitive sequences themselves.