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Biomedical subjects

J C Hansen

Publications and source records attributed to J C Hansen.

At least 19 recordsLinked to original sources
PubMed1999

Functional interaction between GCN5 and polyamines: a new role for core histone acetylation.

Polyamines are organic polycations essential for a wide variety of cellular functions, including nuclear integrity and chromosome condensation. Here we present genetic evidence that depletion of cellular polyamines partially alleviates the defects in HO and SUC2 expression caused by inactivation of the GCN5 histone acetyltransferase. In addition, the combination of polyamine depletion and a sin(-) allele of the histone H4 gene leads to almost complete bypass of the transcriptional requirement for GCN5. In contrast, polyamine depletion does not alter the transcriptional requirements for the SWI/SNF chromatin remodeling complex nor does depletion lead to global defects in transcriptional regulation. In addition to these genetic studies, we show that polyamines facilitate oligomerization of nucleosomal arrays in vitro, and that polyamine-mediated condensation requires intact core histone N-terminal domains and is inhibited by histone hyperacetylation. Our studies suggest that polyamines are repressors of transcription in vivo, and that one role of histone hyperacetylation is to antagonize the ability of polyamines to stabilize highly condensed states of chromosomal fibers.

Acetylation↗
PubMed1999

The core histone N-terminal domains are required for multiple rounds of catalytic chromatin remodeling by the SWI/SNF and RSC complexes.

SWI/SNF and RSC are large, distinct multi-subunit complexes that use the energy of ATP hydrolysis to disrupt nucleosome structure, facilitating the binding of transcription factors or restriction enzymes to nucleosomes [Cote, J., Quinn, J., Workman, J. L., and Peterson, C. L. (1994) Science 265, 53-60 (1); Lorch, Y., Cairns, B. R., Zhang, M., and Kornberg, R. D. (1998) Cell 94, 29-34 (2)]. Here we have used a quantitative assay to measure the activities of these ATP-dependent chromatin remodeling complexes using nucleosomal arrays reconstituted with hypoacetylated, hyperacetylated, or partially trypsinized histones. This assay is based on measuring the kinetics of restriction enzyme digestion of a site located within the central nucleosome of a positioned 11-mer array [Logie, C., and Peterson, C. L. (1997) EMBO J. 16, 6772-6782 (3)]. We find that the DNA-stimulated ATPase activities of SWI/SNF and RSC are not altered by the absence of the histone N-termini. Furthermore, ATP-dependent nucleosome remodeling is also equivalent on all three substrate arrays under reaction conditions where the concentrations of nucleosomal array and either SWI/SNF or RSC are equivalent. However, SWI/SNF and RSC cannot catalytically remodel multiple nucleosomal arrays in the absence of the histone termini, and this catalytic activity of SWI/SNF is decreased by histone hyperacetylation. These results indicate that the histone termini are important for SWI/SNF and RSC function; and, furthermore, our data defines a step in the remodeling cycle where the core histone termini exert their influence. This step appears to be after remodeling, but prior to intermolecular transfer of the remodelers to new arrays.

Acetylation↗
PubMed1999

High serum coenzyme Q10, positively correlated with age, selenium and cholesterol, in Inuit of Greenland. A pilot study.

Greenlanders (Eskimos) have low prevalence of ischaemic heart disease, partly explained by a lower extent of atherosclerosis and a low n-6/n-3 ratio of polyunsaturated fatty acids. As atherosclerosis is also a result of oxidative stress, the total antioxidative readiness could have a substantial impact. From a health survey we chose the subpopulation from the most remote area, where the traditional Greenlandic diet with high intake of sea mammals and fish predominates. The mean (SD) of S-CoQ10 in males was 1.495 (0.529) nmol/ml and 1.421 (0.629) nmol/ml in females, significantly higher (p < 0.001) compared to a Danish population. In a linear multiple regression model the S-CoQ10 level is significantly positively associated with age and S-selenium in males, and S-total cholesterol in females. The high level of CoQ10 in Greenlanders probably reflects diet, since no bioaccumulation takes place, and it could probably be a substantial part of the antioxidative defense.

Adult↗
PubMed1999

Internal carotid artery patency following common carotid artery occlusion: management of the asymptomatic patient.

Rarely, internal carotid artery (ICA) patency is maintained above a common carotid artery occlusion (CCAO) through collateral flow of the ipsilateral external carotid artery or an aberrant ICA branch. Several small series report excellent results of ICA revascularization in relieving cerebral ischemic symptoms in these patients. The natural history of CCAO with patent ICA in the asymptomatic patient, however, is unknown. The Greenville Memorial Hospital Vascular Teaching Service registry and all carotid duplex scans done in the Greenville Hospital System vascular laboratory from January 1994 through December 1997 were reviewed. Data collection included chart review, phone interviews, and the review of angiograms and duplex scans. This study suggests that carotid duplex is more sensitive for detecting ICA flow after CCAO than routine contrast angiography. It also suggests that while rare in presentation, asymptomatic patients with CCAO and a patent ICA appear to have a benign neurologic course and can probably be observed without a high risk of stroke.

Carotid Artery, Common↗
PubMed1999

Concentration of organochlorines in human brain, liver, and adipose tissue autopsy samples from Greenland.

Organochlorines are persistent lipophilic compounds that accumulate in Inuit people living in circumpolar countries. Organochlorines accumulate as a result of the Inuits' large consumption of sea mammal fat; however, available data are limited to blood lipids, milk fat, and adipose tissue. We report results of organochlorine determination in liver, brain, omental fat, and subcutaneous abdominal fat samples collected from deceased Greenlanders between 1992 and 1994. Eleven chlorinated pesticides and 14 polychlorinated biphenyl congeners were measured in tissue lipid extracts by high-resolution gas chromatography with electron capture detection. Mean concentrations of polychlorinated biphenyls, 2, 2'-bis(4-chlorophenyl)-1,1-dichloroethylene, ss-hexachlorocyclohexane, hexachlorobenzene, mirex, trans-nonachlor, and oxychlordane in adipose tissue samples from Greenlanders were 3-34-fold higher than those measured using the same analytical method in samples from Canadians in Quebec City, Quebec. Brain lipids contained lower concentrations of all organochlorines than lipids extracted from other tissues. Organochlorine residue levels in lipid extracts from liver, omental fat, and subcutaneous abdominal fat samples were similar, with the exception of ss-hexachlorocyclohexane, which reached a greater concentration in liver lipids than in lipids from both adipose tissues (4-fold; p < 0. 05). Comparisons with available international data on adipose tissue levels reveal that the organochlorine body burden in the Inuit population of Greenland is presently among the highest resulting from environmental exposure.

Adipose Tissue↗
PubMed1999

Mercury accumulations in brains from populations exposed to high and low dietary levels of methyl mercury. Concentration, chemical form and distribution of mercury in brain samples from autopsies.

Autopsy samples from 17 Greenlanders and 12 Danes were analysed for total and organic mercury by atomic absorption spectrophotometry, and for cellular localisation by autometallography. The concentration of total mercury in the Greenlanders (median: 174 micrograms Hg/kg wet weight brain tissue, range 59-4782, highest in cerebellum: 492) was found to be significantly higher than in the Danish group (3.7 micrograms Hg/kg w.w., range 1.2-11.8). Furthermore, the total concentration of mercury was positively correlated to age (rho = 0.56, p < 0.05), and the fraction of methyl mercury was negatively correlated to age (rho = -0.66, p < 0.01) among the Greenlanders. This suggests an age dependent accumulation of total mercury and a slow transformation of methyl mercury to inorganic mercury in the brain. The autometallographically demonstrable mercury was primarily located in glia cells.

Adult↗
PubMed1999

High blood levels of persistent organic pollutants are statistically correlated with smoking.

Persistent Organic Pollutants (11 pesticides and 14 PCB-congeners), and heavy metals (Cd, Cu, Hg, Pb, Se, and Zn) were determined in 175 pregnant women and 160 newborn infants (umbilical cord blood) from Disko Bay, Greenland, 1994-96. Among these, 135 women filled out questionnaires about drinking, smoking and intake of traditional Inuit food. Multiple linear regression analyses showed highly significant positive associations between the mothers' smoking status (never, previous, present) and plasma concentrations of all the studied organic pollutants both in maternal blood and umbilical cord blood. Traditional food and not the tobacco is known to be the source of the contaminants. But smoking may influence the enzymatic turnover of toxic substances.

Adolescent↗
PubMed1998

Structure and function of the core histone N-termini: more than meets the eye.

For two decades, the core histone N-termini generally have been thought of as unstructured domains whose function is to bind to DNA and screen negative charge. New data indicates that both the molecular mechanisms of action and biological functions of the core histone N-termini in chromatin are considerably more complex. At the level of the chromatin fiber, multiple distinct functions of the N-termini are required to achieve higher order chromatin condensation, two of which apparently involve protein-protein rather than protein-DNA interactions. In addition, the N-termini have been documented to participate in specific interactions with many chromatin-associated regulatory proteins. Here, we discuss evidence supporting the new concepts that when functioning in their natural chromatin context, (1) the N-termini are engaged primarily in protein-protein interactions, (2) as a consequence of these interactions the N-termini adopt specific secondary structure, (3) posttranslational modifications such as acetylation disrupt the ability of the N-termini to form secondary structure, and (4) because the N-termini perform essential roles in both chromatin condensation and also bind specific chromatin-associated proteins, the global structure and function of any given region of the genome will be determined predominantly by the core histone N-termini and their specific interaction partners.

Amino Acid Sequence↗
PubMed1998

Gcn5p, a transcription-related histone acetyltransferase, acetylates nucleosomes and folded nucleosomal arrays in the absence of other protein subunits.

Gcn5p is the catalytic subunit of several type A histone acetyltransferases (HATs). Previous studies performed under a limited range of solution conditions have found that nucleosome core particles and nucleosomal arrays can be acetylated by Gcn5p only when it is complexed with other proteins, e.g. Gcn5-Ada, HAT-A2, and SAGA. Here we demonstrate that when assayed in buffer containing optimum concentrations of either NaCl or MgCl2, purified yeast recombinant Gcn5p (rGcn5p) efficiently acetylates both nucleosome core particles and nucleosomal arrays. Furthermore, under conditions where nucleosomal arrays are extensively folded, rGcn5p acetylates folded arrays approximately 40% faster than nucleosome core particles. Finally, rGcn5p polyacetylates the N termini of free histone H3 but only monoacetylates H3 in nucleosomes and nucleosomal arrays. These results demonstrate both that rGcn5p in and of itself is catalytically active when assayed under optimal solution conditions and that this enzyme prefers folded nucleosomal arrays as a substrate. They further suggest that the structure of the histone H3 N terminus, and concomitantly the accessibility of the H3 acetylation sites, changes upon assembly into nucleosomes and nucleosomal arrays.

Acetylation↗
PubMed1998

Nucleosomes, linker DNA, and linker histone form a unique structural motif that directs the higher-order folding and compaction of chromatin.

The compaction level of arrays of nucleosomes may be understood in terms of the balance between the self-repulsion of DNA (principally linker DNA) and countering factors including the ionic strength and composition of the medium, the highly basic N termini of the core histones, and linker histones. However, the structural principles that come into play during the transition from a loose chain of nucleosomes to a compact 30-nm chromatin fiber have been difficult to establish, and the arrangement of nucleosomes and linker DNA in condensed chromatin fibers has never been fully resolved. Based on images of the solution conformation of native chromatin and fully defined chromatin arrays obtained by electron cryomicroscopy, we report a linker histone-dependent architectural motif beyond the level of the nucleosome core particle that takes the form of a stem-like organization of the entering and exiting linker DNA segments. DNA completes approximately 1.7 turns on the histone octamer in the presence and absence of linker histone. When linker histone is present, the two linker DNA segments become juxtaposed approximately 8 nm from the nucleosome center and remain apposed for 3-5 nm before diverging. We propose that this stem motif directs the arrangement of nucleosomes and linker DNA within the chromatin fiber, establishing a unique three-dimensional zigzag folding pattern that is conserved during compaction. Such an arrangement with peripherally arranged nucleosomes and internal linker DNA segments is fully consistent with observations in intact nuclei and also allows dramatic changes in compaction level to occur without a concomitant change in topology.

Animals↗
PubMed1998

Linker histones stabilize the intrinsic salt-dependent folding of nucleosomal arrays: mechanistic ramifications for higher-order chromatin folding.

Defined nucleosomal arrays reconstituted from core histone octamers and twelve 208 bp tandem repeats of Lytechinus 5S rDNA (208-12 nucleosomal arrays) possess the ability to form an unstable folded species in MgCl2 whose extent of compaction equals that of canonical higher-order 30 nm diameter chromatin structures [Schwarz, P. M., and Hansen, J. C. (1994) J. Biol. Chem. 269, 16284-16289]. To address the mechanistic functions of linker histones in chromatin condensation, purified histone H5 has been assembled with 208-12 nucleosomal arrays in 50 mM NaCl. Novel purification procedures subsequently were developed that yielded preparations of 208-12 chromatin model systems in which a majority of the sample contained both one histone octamer per 5S rDNA repeat and one molecule of histone H5 per histone octamer. The integrity of the purified 208-12 chromatin has been extensively characterized under low-salt conditions using analytical ultracentrifugation, quantitative agarose gel electrophoresis, electron cryomicroscopy, and nuclease digestion. Results indicate that histone H5 binding to 208-12 nucleosomal arrays constrains the entering and exiting linker DNA in a way that produces structures that are indistinguishable from native chicken erythrocyte chromatin. Folding experiments performed in NaC1 and MgC12 have shown that H5 binding markedly stabilizes both the intermediate and extensively folded states of nucleosomal arrays without fundamentally altering the intrinsic nucleosomal array folding pathway. These results provide new insight into the mechanism of chromatin folding by demonstrating for the first time that distinctly different macromolecular determinants are required for formation and stabilization of higher-order chromatin structures.

Animals↗
PubMed1998

Enhanced transcription factor access to arrays of histone H3/H4 tetramer.DNA complexes in vitro: implications for replication and transcription.

Defined model systems consisting of physiologically spaced arrays of H3/H4 tetramer.5S rDNA complexes have been assembled in vitro from pure components. Analytical hydrodynamic and electrophoretic studies have revealed that the structural features of H3/H4 tetramer arrays closely resemble those of naked DNA. The reptation in agarose gels of H3/H4 tetramer arrays is essentially indistinguishable from naked DNA, the gel-free mobility of H3/H4 tetramer arrays relative to naked DNA is reduced by only 6% compared with 20% for nucleosomal arrays, and H3/H4 tetramer arrays are incapable of folding under ionic conditions where nucleosomal arrays are extensively folded. We further show that the cognate binding sites for transcription factor TFIIIA are significantly more accessible when the rDNA is complexed with H3/H4 tetramers than with histone octamers. These results suggest that the processes of DNA replication and transcription have evolved to exploit the unique structural properties of H3/H4 tetramer arrays.

Animals↗
PubMed1998

Mechanism of Rab geranylgeranylation: formation of the catalytic ternary complex.

Rab proteins are geranylgeranylated on one or two C-terminal cysteines by Rab geranylgeranyl transferase (RabGGTase). The reaction is dependent on a Rab-binding protein, termed Rab escort protein (REP). Here, we studied the role of REP in the geranylgeranylation reaction. We first characterized the interaction between REP and ungeranylgeranylated Rab using analytical ultracentrifugation and a fluorescence-based assay. We measured an equilibrium dissociation constant of 0.2 microM for the formation of a 1:1 REP-Rab complex and showed that this interaction relies mostly on ionic bonds and does not involve the two C-terminal cysteine residues. Second, we show that REP is required for recognition of Rab by RabGGTase and therefore that the REP-Rab complex is the true substrate for RabGGTase. Third, we show that free REP inhibits the geranylgeranylation reaction, suggesting that the complex is recognized by RabGGTase primarily via a REP-binding site. Our data suggest a model whereby REP behaves kinetically as an essential activator of the reaction.

Adaptor Proteins, Signal Transducing↗
PubMed1998

Elements in autopsy liver tissue samples from Greenlandic Inuit and Danes. I. Sulphur, chlorine, potassium and bromine measured by X-ray fluorescence spectrometry.

The purpose of this study was to measure the content of the elements Sulphur (S), Chlorine (C1), Potassium (K) and Bromine (Br) in normal liver tissue samples from Greenlandic Inuit using X-ray fluorescence spectrometry, and compare the results with those obtained in normal liver tissue samples from Danes. Liver tissue sample were obtained at autopsy from 50 Greenlandic Inuit (27 men, 23 women) with a median age of 61 years (range 20-83) and from 74 Danes (44 men, 30 women) with a median age of 52 years (range 15-87). In Inuit, the content of elements given as median and (5-95 percentile) was: sulphur, 108.07 mmol/kg dry liver (86.78 - 169.44); chlorine, 92.16 mmol/kg dry liver (45.39-128.42); potassium, 181.66 mmol/kg dry liver (146.41-236.35); bromine, 0.0901 mmol/kg dry liver (0.0563-0.1589). In Danes, the corresponding values were: sulphur, 147.58 mmol/kg dry liver (70.41-236.81); chlorine, 96.95 mmol/kg dry liver (54.01-162.52); potassium, 198.40 mmol/kg dry liver (150.68-256.37); bromine, 0.1101 mmol/kg dry liver (0.0701 - 0.4203). None of the elements displayed any significant gender difference, neither in Inuit nor in Danes. Inuit had a lower liver content of sulphur (p < 0.0001), potassium (p < 0.008) and bromine (p < 0.002) as compared with Danes.

Adult↗
PubMed1998

Disruption of higher-order folding by core histone acetylation dramatically enhances transcription of nucleosomal arrays by RNA polymerase III.

We have examined the effects of core histone acetylation on the transcriptional activity and higher-order folding of defined 12-mer nucleosomal arrays. Purified HeLa core histone octamers containing an average of 2, 6, or 12 acetates per octamer (8, 23, or 46% maximal site occupancy, respectively) were assembled onto a DNA template consisting of 12 tandem repeats of a 208-bp Lytechinus 5S rRNA gene fragment. Reconstituted nucleosomal arrays were transcribed in a Xenopus oocyte nuclear extract and analyzed by analytical hydrodynamic and electrophoretic approaches to determine the extent of array compaction. Results indicated that in buffer containing 5 mM free Mg2+ and 50 mM KCl, high levels of acetylation (12 acetates/octamer) completely inhibited higher-order folding and concurrently led to a 15-fold enhancement of transcription by RNA polymerase III. The molecular mechanisms underlying the acetylation effects on chromatin condensation were investigated by analyzing the ability of differentially acetylated nucleosomal arrays to fold and oligomerize. In MgCl2-containing buffer the folding of 12-mer nucleosomal arrays containing an average of two or six acetates per histone octamer was indistinguishable, while a level of 12 acetates per octamer completely disrupted the ability of nucleosomal arrays to form higher-order folded structures at all ionic conditions tested. In contrast, there was a linear relationship between the extent of histone octamer acetylation and the extent of disruption of Mg2+-dependent oligomerization. These results have yielded new insight into the molecular basis of acetylation effects on both transcription and higher-order compaction of nucleosomal arrays.

Acetylation↗
PubMed1998

Effects of dietary seal oil on fat metabolism.

In the 1970s, Bang and Dyerberg demonstrated that a high intake of n-3 acids in Greenland protected against ischemic heart disease. This started the interest in fish oil as a preventive component in cardiac disease. The fatty acid composition in Greenlandic diet is quite different from the one in the Danish diet, being lower in saturated fatty acids and polyunsaturated fatty acids (PUFAs) and higher in monounsatured fatty acids (MUFAs). During the last decade, evidence has suggested that the MUFA (18:1) is not a neutral dietary component but has a positive effect of its own. This paper reports on a current project undertaken to study the effect of marine MUFAs and PUFAs on atherosclerosis-related parameters and to evaluate their possible synergistic effect in the prevention of atherosclerosis. The project has recently started, so at the moment results cannot be presented. The study will be carried out as an intervention study on 50 healthy volunteers. Capsules of Greenlandic halibut oil (rich in MUFAs) and "Biomarin" (rich in n-3 PUFAs) will be tested against seal oil.

Adult↗