Nuclear architecture supports integration of physiological regulatory signals for transcription of cell growth and tissue-specific genes during osteoblast differentiation

Lower osteocalcin and osteopontin contents of the femoral head in hip fracture patients than osteoarthritis patients

In patients with femoral neck fracture, clinical factors, bone metabolism markers (in serum, urine, and bone), bone mineral density, radiographic parameters, and bone histomorphometric parameters were investigated to detect determinants of fragility fracture. The osteocalcin/deoxypyridinoline ratio and osteopontin/calcium ratio of cortical bone were selected as significant predictors.

INTRODUCTION

Measurement of bone mineral density is widely used to assess bone strength, but this also depends on other bone components and on bone structure. The objective of this study was to investigate risk factors for fracture related to bone quality, the patient's history, and the patient's lifestyle.

METHODS

Twenty-one patients with femoral neck fracture and 18 patients with osteoarthritis were enrolled. Blood and urine samples were collected on admission to hospital, and bone samples were obtained from femoral necks resected during surgery. Multivariate logistic regression analysis was performed using osteoarthritis and femoral neck fracture as combined variables to assess the influence of alcohol or coffee intake, eating natto (fermented soybeans), osteocalcin and calcium concentrations, the osteocalcin/deoxypyridinoline ratio and osteopontin/calcium ratios of cortical bone and cancellous bone, various bone histomorphometric parameters, the bone mineral density of the lumbar spine and the intact contralateral femoral neck, and various radiographic parameters of the spine

RESULTS

By forward stepwise multivariate analysis, the osteocalcin/deoxypyridinoline and osteopontin/calcium ratios of cortical bone were selected as significant factors for fracture (the odds ratios were 0.493 and <0.001, respectively; both P<0.001).

CONCLUSIONS

A decrease of osteopontin and osteocalcin in bone is important for promoting vulnerability to hip fracture.

Hairless contains a novel nuclear matrix targeting signal and associates with histone deacetylase 3 in nuclear speckles

Hair follicle cycling is a highly regulated and dynamic cellular process consisting of phases of growth, regression, and quiescence. The hairless (hr) gene encodes a nuclear factor that is highly expressed in the skin, where it appears to be an essential regulator during the regression in the catagen hair follicle. In hairless mice, as well as humans with congenital atrichia, the absence of hr protein initiates a premature and abnormal catagen due to defects in the signaling required for hair follicle remodeling. Here, we report that hr protein is a nuclear protein that is tightly associated with the nuclear matrix scaffold. Using a series of deletion constructs of the mouse hr gene, we monitored the sub-cellular localization of the recombinant protein by in situ immunolocalization and biochemical fractionation after nuclear matrix extraction of transiently transfected cells. We identified a novel nuclear matrix-targeting signal (NMTS) in the hr protein and mapped the domain to amino acid residues 111-186 of the mouse hr sequence. Furthermore, we provide evidence that this region not only mediates the interaction of hr with components of the nuclear architecture, but also specifies the sub-nuclear location of the hr protein to nuclear domains containing deacetylase activity. The N-terminal region directs hr to a speckled nuclear pattern that co-localizes with the histone deacetylase 3 (HDAC), but not with HDAC1 or HDAC7. Based on our findings, we propose that hr protein is part of a specific multi-protein repressor complex and that hr may be involved in chromatin remodeling.

Gene expression and nuclear architecture during development and differentiation

Development requires a precise program of gene expression to be carried out. Much work has focussed on the regulatory networks that control gene expression, for example in response to external cues. However, it is important to recognize that these regulatory events take place within the physical context of the nucleus, and that the physical position of a gene within the nuclear volume can have strong influences on its regulation and interactions. The first part of this review will summarize what is currently known about nuclear architecture, that is, the large-scale three-dimensional arrangement of chromosome loci within the nucleus. The remainder of the review will examine developmental processes from the point of view of the nucleus.

Estimation of nuclear volume as an indicator of maturation of glial precursor cells in the developing rat spinal cord: a stereological approach

Studies on nuclear volume have shown that it is an indication of the state of differentiation of cells. This study provides evidence indicating increasing nuclear volume during cell maturation. Using unbiased stereological techniques, nuclear volume of both proliferating and non-proliferating glial cells was analysed in the developing spinal cord. Proliferating glial precursor cells were identified using a 5-bromo-2'-deoxyuridine (BrdU) incorporation assay. The nuclear volume of BrdU-labelled cells and unlabelled cells was determined in both periventricular regions and the white matter of the cord at different embryonic ages. In the periventricular region BrdU-labelled nuclei were smaller than unlabelled nuclei at all ages examined. These labelled cells represent dividing undifferentiated progenitors. The unlabelled neighbouring cells with larger nuclei represent a more differentiated population. In the white matter BrdU-labelled nuclei were of similar volume to the unlabelled nuclei. Both of these groups represent glial precursor cells that have migrated from deeper regions and are at similar stages of differentiation, perhaps with different proliferative potential. These findings indicate that the nuclear volume of early glial cells increases as these cells migrate and differentiate.

Intranuclear organization of RUNX transcriptional regulatory machinery in biological control of skeletogenesis and cancer

RUNX (AML/CBFA/PEBP2) transcription factors serve as paradigms for obligatory relationships between nuclear structure and physiological control of phenotypic gene expression. The RUNX proteins contribute to tissue restricted transcription by sequence-specific binding to promoter elements of target genes and serving as scaffolds for the assembly of coregulatory complexes that mediate biochemical and architectural control of activity. We will present an overview of approaches we are pursuing to address: (1) the involvement of RUNX proteins in governing competency for protein/DNA and protein/protein interactions at promoter regulatory sequences; (2) the recruitment of RUNX factors to subnuclear sites where the machinery for expression or repression of target genes is organized; and (3) the trafficking and integration of regulatory signals that control RUNX-mediated transcription.

Intranuclear trafficking of transcription factors: Requirements for vitamin D-mediated biological control of gene expression

The architecturally associated subnuclear organization of nucleic acids and cognate regulatory factors suggest functional interrelationships between nuclear structure and gene expression. Mechanisms that contribute to the spatial distribution of transcription factors within the three-dimensional context of nuclear architecture control the sorting of regulatory information as well as the assembly and activities of sites within the nucleus that support gene expression. Vitamin D control of gene expression serves as a paradigm for experimentally addressing mechanisms that govern the intranuclear targeting of regulatory factors to nuclear domains where transcription of developmental and tissue-specific genes occur. We will present an overview of molecular, cellular, genetic, and biochemical approaches that provide insight into the trafficking of regulatory factors that mediate vitamin D control of gene expression to transcriptionally active subnuclear sites. Examples will be presented that suggest modifications in the intranuclear targeting of transcription factors abrogate competency for vitamin D control of skeletal gene expression during development and fidelity of gene expression in tumor cells.

Enhanced osteobonding by negative surface charges of electrically polarized hydroxyapatite

Negative surface charges of electrically polarized hydroxyapatite ceramics have been proven to enhance osteobonding in canine bone tissues. Even in a wide gap of 0.2 mm between hydroxyapatite and bone, the negatively charged hydroxyapatite conducted formation of unidirectionally oriented bone layer into direct contact with the hydroxyapatite surface 7 days after implantation. By day 14, the gap was filled with maturing bones linked to each other while conventional hydroxyapatite required at least 28 days for direct bone-to-bone contact. The electrostatic force was deduced to have affected both the activation of bone formation by myeloid cells and the orientation of the bone domain on negatively charged surfaces.

Insight into regulatory factor targeting to transcriptionally active subnuclear sites

Mechanisms that coordinate the spatial organization of genes and regulatory proteins within the three-dimensional context of nuclear architecture contribute to the sorting of regulatory information as well as the assembly and activity of sites within the nucleus that support gene expression. In this article we will present an overview of experimental approaches that provide insight into the trafficking of the hematopoietic and bone-specific AML/CBF family of regulatory factors to transcriptionally active subnuclear sites.

Implications for interrelationships between nuclear architecture and control of gene expression under microgravity conditions

Components of nuclear architecture are functionally interrelated with control of gene expression. There is growing appreciation that multiple levels of nuclear organization integrate the regulatory cues that support activation and suppression of genes as well as the processing of gene transcripts. The linear representation of genes and promoter elements provide the potential for responsiveness to physiological regulatory signals. Parameters of chromatin structure and nucleosome organization support synergism between activities at independent regulatory sequences and render promoter elements accessible or refractory to transcription factors. Association of genes, transcription factors, and the machinery for transcript processing with the nuclear matrix facilitates fidelity of gene expression within the three-dimensional context of nuclear architecture. Mechanisms must be defined that couple nuclear morphology with enzymatic parameters of gene expression. The recent characterization of factors that mediate chromatin remodeling and identification of intranuclear targeting signals that direct transcription factors to subnuclear domains where gene expression occurs link genetic and structural components of transcriptional control. Nuclear reorganization and aberrant intranuclear trafficking of transcription factors for developmental and tissue-specific control occurs in tumor cells and in neurological disorders. Compromises in nuclear structure-function interrelationships can occur as a consequence of microgravity-mediated perturbations in cellular architecture.

Changes in osteoblast phenotype during differentiation of enzymatically isolated rat calvaria cells

Osteoblasts enzymatically isolated from newborn rat calvariae show various phenotypes including formation of mineralized bone nodules in culture. We investigated the temporal changes in osteoblast phenotype in these cells up to day 20 in culture. These cells formed unmineralized nodules by day 5. Mineralization was observed at the center of nodules by day 10, and nodules became larger on day 15. The nodules were surrounded by numerous alkaline phosphatase (ALP)-positive cells. ALP activity gradually increased by day 20. Parathyroid hormone (PTH) responsiveness increased with time in culture. Osteoblasts produced no osteocalcin by day 10, but its synthesis was detected from day 15. These cells expressed substantial levels of ALP and PTH/PTHrP receptor mRNAs as early as day 5 in culture, but very weak expression of osteocalcin mRNA on day 5. The levels of expression of these transcripts increased with time in culture. In situ hybridization demonstrated that PTH/PTHrP receptor and osteocalcin mRNAs were strongly expressed in nodules, but the former appeared much earlier than the latter. BMP-2 and BMP-4 mRNAs also appeared in the cells forming nodules. Immunohistochemical analysis demonstrated that cells expressing either BMP-2/4 or their receptors (BMPR-IA, BMPR-IB, and BMPR-II) preferentially appeared in nodules. These observations suggested that BMPs play an important role in the formation of mineralized bone nodules in an autocrine and/or paracrine fashion in these cells. The present study confirmed that osteoblasts enzymatically isolated from newborn rat calvariae are a useful tool for studying the differentiation process of osteoblasts.

Localization of human cytomegalovirus structural proteins to the nuclear matrix of infected human fibroblasts

The intranuclear assembly of herpesvirus subviral particles remains an incompletely understood process. Previous studies have described the nuclear localization of capsid and tegument proteins as well as intranuclear tegumentation of capsid-like particles. The temporally and spatially regulated replication of viral DNA suggests that assembly may also be regulated by compartmentalization of structural proteins. We have investigated the intranuclear location of several structural and nonstructural proteins of human cytomegalovirus (HCMV). Tegument components including pp65 (ppUL83) and ppUL69 and capsid components including the major capsid protein (pUL86) and the small capsid protein (pUL48/49) were retained within the nuclear matrix (NM), whereas the immediate-early regulatory proteins IE-1 and IE-2 were present in the soluble nuclear fraction. The association of pp65 with the NM resisted washes with 1 M guanidine hydrochloride, and direct binding to the NM could be demonstrated by far-Western blotting. Furthermore, pp65 exhibited accumulation along the nuclear periphery and in far-Western analysis bound to proteins which comigrated with proteins of the size of nuclear lamins. A direct interaction between pp65 and lamins was demonstrated by coprecipitation of lamins in immune complexes containing pp65. Together, our findings provide evidence that major virion structural proteins localized to a nuclear compartment, the NM, during permissive infection of human fibroblasts.

Comparison of nuclear matrix protein composition in colon cancer and dysplasia

BACKGROUND

Abnormal nuclear morphology associated with cancer may reflect changes in the proteins of the nuclear matrix.

METHODS

Nuclear matrix (NM) proteins were isolated from colonic tissue and analyzed by two-dimensional gel electrophoresis.

RESULTS

Several matrix proteins that were found in ulcerative colitis (UC) dysplasia (n = 5) and/or UC cancer (n = 4) were not identified in normal colonic tissue. UC dysplasia tissue showed three specific NM proteins with molecular masses of 49.2 kDa, 20.0 kDa, and 19.0 kDa, whereas 29.0-kDa and 32.0-kDa proteins were specific to UC cancer. Three proteins with 59.5-kDa (pI 6.3 and 6.6) and 33.75-kDa (pI 7.5) masses were common to both dysplasia and cancer tissue.

CONCLUSIONS

These data suggest that NM proteins may have a role in the transition of tissue towards the malignant phenotype.

Nuclear matrix associated DNA-binding proteins of ocular lens epithelial cells

Association of transcription factors with the nuclear matrix represents a mechanism by which nuclear architecture may influence transcriptional control of gene expression. This investigation examines nuclear matrix associated proteins (NMP's) isolated from ocular lens epithelial cells by monitoring DNA binding activities using consensus oligonucleotides recognized by the transcription factors YY1, AML-1, AP-1, SP-1 and ATF. The nuclear matrix fractions tested included an immortilized human lens epithelial cell line containing the SV40 large T-antigen, and two mouse lens epithelial cell lines derived from either a normal mouse or a cataract mouse. A rabbit epidermal epithelial cell line and HeLa cells were also included in this study for comparison. The data from these experiments reveal that ubiquitously represented and tissue restricted regulatory proteins are associated with nuclear matrix of lens epithelial cells. The functional significance of the nuclear matrix association of these transcription factors remains to be determined. However, our findings raise the possibility that the transcription factors associated with the nuclear matrix could have specific roles in gene regulation and eye tissue development.

A DNA-binding element for a steroid receptor-binding factor is flanked by dual nuclear matrix DNA attachment sites in the c-myc gene promoter

The receptor-binding factor (RBF) for the avian oviduct progesterone (Pg) receptor (PR) has previously been shown to be a unique 10-kDa nuclear matrix protein that generates high affinity PR-binding sites on avian DNA. This paper describes the use of Southwestern blot and DNA gel shift analyses with RBF protein to identify a minimal 54-base pair RBF-binding element in the matrix-associated region (MAR) of the Pg-regulated c-myc gene promoter. This element contains a 5'-GC-rich domain and a 3'-AT-rich domain, the latter of which has a homopurine/homopyrimidine structure. The gel shift assays required the generation of an RBF-maltose fusion protein (RBF-MBP), which specifically binds this element and is supershifted when the anti-RBF polyclonal antibody is added. Computer analysis of the full-length amino acid sequence for RBF predicts a DNA-binding motif involving a beta-sheet structure at the N-terminal domain. Southern blot analyses using nuclear matrix DNA suggests that there are dual MAR sites in the c-myc promoter, which flank an intervening domain containing the RBF element. The co-transfection of this MAR sequence, containing the RBF element and cloned into a luciferase reporter vector, together with an RBF expression vector construct, into steroid treated human MCF-7 cells, results in a decrease of the c-myc promoter activity relative to control transfections containing only the parent vector of the RBF expression construct. These data suggest that a unique chromatin/nuclear matrix structure, composed of the RBF-DNA element complex which is flanked by nuclear matrix attachment sites, serves to bind the PR and repress the c-myc promoter.

The matrix attachment region-binding protein SATB1 participates in negative regulation of tissue-specific gene expression

The nuclear matrix has been implicated in several cellular processes, including DNA replication, transcription, and RNA processing. In particular, transcriptional regulation is believed to be accomplished by binding of chromatin loops to the nuclear matrix and by the concentration of specific transcription factors near these matrix attachment regions (MARs). A number of MAR-binding proteins have been identified, but few have been directly linked to tissue-specific transcription. Recently, we have identified two cellular protein complexes (NBP and UBP) that bind to a region of the mouse mammary tumor virus (MMTV) long terminal repeat (LTR) previously shown to contain at least two negative regulatory elements (NREs) termed the promoter-proximal and promoter-distal NREs. These NREs are absent from MMTV strains that cause T-cell lymphomas instead of mammary carcinomas. We show here that NBP binds to a 22-bp sequence containing an imperfect inverted repeat in the promoter-proximal NRE. Previous data showed that a mutation (p924) within the inverted repeat elevated basal transcription from the MMTV promoter and destabilized the binding of NBP, but not UBP, to the proximal NRE. By using conventional and affinity methods to purify NBP from rat thymic nuclear extracts, we obtained a single major protein of 115 kDa that was identified by protease digestion and partial sequencing analysis as the nuclear matrix-binding protein special AT-rich sequence-binding protein 1 (SATB1). Antibody ablation, distamycin inhibition of binding, renaturation and competition experiments, and tissue distribution data all confirmed that the NBP complex contained SATB1. Similar types of experiments were used to show that the UBP complex contained the homeodomain protein Cux/CDP that binds the MAR of the intronic heavy-chain immunoglobulin enhancer. By using the p924 mutation within the MMTV LTR upstream of the chloramphenicol acetyltransferase gene, we generated two strains of transgenic mice that had a dramatic elevation of reporter gene expression in lymphoid tissues compared with reporter gene expression in mice expressing wild-type LTR constructs. Thus, the 924 mutation in the SATB1-binding site dramatically elevated MMTV transcription in lymphoid tissues. These results and the ability of the proximal NRE in the MMTV LTR to bind to the nuclear matrix clearly demonstrate the role of MAR-binding proteins in tissue-specific gene regulation and in MMTV-induced oncogenesis.

Nuclear matrix, dynamic histone acetylation and transcriptionally active chromatin

The nuclear matrix, the RNA-protein skeleton of the nucleus, has a role in the organization and function of nuclear DNA. Nuclear processes associated with the nuclear matrix include transcription, replication and dynamic histone acetylation. Nuclear matrix proteins, which are tissue and cell type specific, are altered with transformation and state of differentiation. Transcription factors are associated with the nuclear matrix, with the spectra of nuclear matrix bound factors being cell type specific. There is compelling evidence that the transcription machinery is anchored to the nuclear matrix, and the chromatin fiber is spooled through this complex. Transcriptionally active chromatin domains are associated with dynamically acetylated histones. The energy exhaustive process of dynamic histone acetylation has several functions. Acetylation of the N-terminal tails of the core histones alters nucleosome and higher order chromatin structure, aiding transcriptional elongation and facilitating the binding of transcription factors to nucleosomes associated with regulatory DNA sequences. Histone acetylation can manipulate the interactions of regulatory proteins that bind to the N-terminal tails of the core histones. Lastly, dynamic acetylation may contribute to the transient attachment of transcriptionally active chromatin to the nuclear matrix. Reversible histone acetylation is catalyzed by histone acetyltransferase and deacetylase, enzymes associated with the nuclear matrix. The recent isolation and characterization of histone acetyltransferase and deacetylase reveals that these enzymes are related to transcriptional regulators, providing us with new insights about how these enzymes are targeted to nuclear matrix sites engaged in transcription.

Identification of a nuclear matrix targeting signal in the leukemia and bone-related AML/CBF-alpha transcription factors

Transcription factors of the AML (core binding factor-alpha/polyoma enhancer binding protein 2) class are key transactivators of tissue-specific genes of the hematopoietic and bone lineages. Alternative splicing of the AML-1 gene results in two major AML variants, AML-1 and AML-1B. We show here that the transcriptionally active AML-1B binds to the nuclear matrix, and the inactive AML-1 does not. The association of AML-1B with the nuclear matrix is independent of DNA binding and requires a nuclear matrix targeting signal (NMTS), a 31 amino acid segment near the C terminus that is distinct from nuclear localization signals. A similar NMTS is present in AML-2 and the bone-related AML-3 transcription factors. Fusion of the AML-1B NMTS to the heterologous GAL4-(1-147) protein directs GAL4 to the nuclear matrix. Thus, the NMTS is necessary and sufficient to target the transcriptionally active AML-1B to the nuclear matrix. The loss of the C-terminal domain of AML-1B is a frequent consequence of the leukemia-related t(8;21) and t(3;21) translocations. Our results suggest this loss may be functionally linked to the modified interrelationships between nuclear structure and gene expression characteristic of cancer cells.

Parathyroid hormone exerts disparate effects on osteoblast differentiation depending on exposure time in rat osteoblastic cells

It has been reported that PTH exerts bone-forming effects in vivo when administered intermittently. In the present study, the anabolic effects of PTH(1-34) on osteoblast differentiation were examined in vitro. Osteoblastic cells isolated from newborn rat calvaria were cyclically treated with PTH(1-34) for the first few hours of each 48-h incubation cycle. When osteoblastic cells were intermittently exposed to PTH only for the first hour of each 48-h incubation cycle and cultured for the remainder of the cycle without the hormone, osteoblast differentiation was inhibited by suppressing alkaline phosphatase activity, bone nodule formation, and mRNA expression of alkaline phosphatase, osteocalcin, and PTH/PTHrP receptor. Experiments using inhibitors and stimulators of cAMP/protein kinase A (PKA) and Ca2+/PKC demonstrated that cAMP/PKA was the major signal transduction system in the inhibitory action of PTH. In contrast, the intermittent exposure to PTH for the first 6 h of each 48-h cycle stimulated osteoblast differentiation. Both cAMP/ PKA and Ca2+/PKC systems appeared to be involved cooperatively in this anabolic effect. Continuous exposure to PTH during the 48-h incubation cycle strongly inhibited osteoblast differentiation. Although both cAMP/PKA and Ca2+/PKC were involved in the effect of continuous exposure to PTH, they appeared to act independently. A neutralizing antibody against IGF-I blocked the stimulatory effect on alkaline phosphatase activity and the expression of osteocalcin mRNA induced by the 6-h intermittent exposure. The inhibitory effect induced by the 1-h intermittent exposure was not affected by anti-IGF-I antibody. These results suggest that PTH has diverse effects on osteoblast differentiation depending on the exposure time in vitro mediated through different signal transduction systems. These in vitro findings explain at least in part the in vivo action of PTH that varies with the mode of administration.

Mode of transfection influences the stability of ectopically expressed mRNA

In the study of mRNA metabolism, modified mRNAs are often analyzed after corresponding mRNA expression vectors have been transfected, either transiently or stably, into cells. Two differently transfected templates might be localized in distinct nuclear compartments: in transient transfection they remain in the nucleoplasm while in stable transfection they are integrated in the chromatin. Consequently, nascent transcripts may encounter different environments which may affect the physical state of mRNA and its fate. In this work, we addressed the question whether the two different modes of transfection affect the stability of expressed mRNA. We compared globin mRNA, which is characteristically stable, and globin-delta AU mRNA, which contains the 3' untranslated region of urokinase-type plasminogen activator mRNA and is unstable. In stably transfected cells, these mRNAs were degraded in a manner which mimicked the endogenous mRNA, whereas in transiently transfected cells, the regulated degradation of both mRNAs was impaired. However, when lower amounts of template DNA were used in transient transfection, mRNA was degraded in a manner similar to that of stably expressed mRNA, indicating that mRNA levels affect its stability. To monitor potential differences in the physical state of mRNAs in vivo, we developed a method based on a combination of chemical modification of cellular RNA and a modified RT-PCR. We found that patterns of chemical modification vary with the levels of mRNA expressed. Our results suggest that a proper interaction of mRNA with specific cellular proteins is important for regulated degradation and that overexpression of mRNA destroys such proper stoichiometry.

Estrogen regulation of nuclear matrix-intermediate filament proteins in human breast cancer cells

The tissue matrix consists of linkages and interactions of the nuclear matrix, cytoskeleton, and extracellular matrix. This system is a dynamic structural component of the cell that organizes and processes structural and functional information to maintain and coordinate cell function and gene expression. We have studied estrogen regulation of nuclear matrix associated proteins, including the intimately connected cytoskeletal intermediate filaments, in T-47D5 human breast cancer cells. Three proteins (identified as cytokeratins 8, 18, and 19) present in the nuclear matrix-intermediate filament fraction (NM-IF) of cells grown in estrogen-replete conditions were dramatically reduced when the cells were grown in acute (1 week) estrogen-depleted conditions. Replacing estrogen in the medium of acute estrogen-depleted cells restored expression of these proteins. T-47D5 cells that are chronically depleted of estrogen (T5-PRF) are estrogen-nonresponsive in culture. These cells overexpressed these three proteins, compared to parent cells grown in the presence of estrogen. Treatment of the T5-PRF cells with estrogen did not lead to further up-regulation of these proteins. Treating T-47D5 cells in estrogen-replete conditions with the antiestrogens 4-hydroxytamoxifen and ICI 164 384 (100 nM, 3 days) resulted in a significant reduction in these proteins, while no effect was seen in long-term chronic estrogen-depleted T-47D5 cells. In conclusion, we have identified NM-IF proteins (cytokeratins 8, 18, and 19) in human breast cancer cells that are estrogen regulated and may play a role in estrogen action in human breast cancer cells.

hnRNP proteins and B23 are the major proteins of the internal nuclear matrix of HeLa S3 cells

The nuclear matrix is the structure that persists after removal of chromatin and loosely bound components from the nucleus. It consists of a peripheral lamina-pore complex and an intricate internal fibrogranular structure. Little is known about the molecular structure of this proteinaceous internal network. Our aim is to identify the major proteins of the internal nuclear matrix of HeLa 53 cells. To this end, a cell fraction containing the internal fibrogranular structure was compared with one from which this structure had been selectively dissociated. Protein compositions were quantitatively analyzed after high-resolution two-dimensional gel electrophoresis. We have identified the 21 most abundant polypeptides that are present exclusively in the internal nuclear matrix. Sixteen of these proteins are heterogeneous nuclear ribonucleoprotein (hnRNP) proteins. B23 (numatrin) is another abundant protein of the internal nuclear matrix. Our results show that most of the quantitatively major polypeptides of the internal nuclear matrix are proteins involved in RNA metabolism, including packaging and transport of RNA.

ATP-dependent release of glucocorticoid receptors from the nuclear matrix

Glucocorticoid receptors (GRs) have the capacity to shuttle between the nuclear and cytoplasmic compartments, sharing that trait with other steroid receptors and unrelated nuclear proteins of diverse function. Although nuclear import of steroid receptors, like that of nearly all other karyophilic proteins examined to date, requires ATP, there appear to be different energetic requirements for export of proteins, including steroid receptors, from nuclei. In an attempt to reveal which steps, if any, in the nuclear export pathway utilized by steroid receptors require ATP, we have used indirect immunofluorescence to visualize GRs within cells subjected to a reversible ATP depletion. Under conditions which lead to >95% depletion of cellular ATP levels within 90 min, GRs remain localized within nuclei and do not efflux into the cytoplasm. Under analogous conditions of ATP depletion, transfected progesterone receptors are also retained within nuclei. Importantly, GRs which accumulate within nuclei of ATP-depleted cells are distinguished from nuclear receptors in metabolically active cells by their resistance to in situ extraction with a hypotonic, detergent-containing buffer. GRs in ATP-depleted cells are not permanently trapped in this nuclear compartment, as nuclear receptors rapidly regain their capacity to be extracted upon restoration of cellular ATP, even in the absence of de novo protein synthesis. More extensive extraction of cells with high salt and detergent, coupled with DNase I digestion, established that a significant fraction of GRs in ATP-depleted cells are associated with an RNA-containing nuclear matrix. Quantitative Western blot (immunoblot) analysis confirmed the dramatic increase in GR binding to the nuclear matrix of ATP-depleted cells, while confocal microscopy revealed that GRs are bound to the matrix throughout all planes of the nucleus. ATP depletion does not lead to wholesale collapse of nuclear proteins onto the matrix, as the interaction of a subpopulation of simian virus 40 large tumor antigen with the nuclear matrix is not quantitatively altered in ATP-depleted Cos-1 cells. Nuclear GRs which are not bound to the nuclear matrix of metabolically active cells (i.e., a DNA-binding domain deletion mutant and a beta-galactosidase chimera possessing the GR nuclear localization signal sequence) are not recruited to the matrix upon depletion of cellular ATP. Thus, it appears that ATP depletion does not expose the GR to nuclear matrix interactions which are not normally encountered in cells but merely alters the dynamics of such interactions. The dynamic association of steroid receptors with the nuclear matrix may provide a mechanism which is utilized by these regulable transcription factors to facilitate their efficient scanning of the genome.

Dynamic remodeling of nuclear architecture during the cell cycle

The nuclear matrix is an integral part of nuclear structure which undergoes a profound reorganization during the cell cycle reflecting major changes in functional requirements. This includes the processes of DNA replication and gene expression at interphase and partitioning of the nuclear contents during mitosis. Using a monoclonal antibody (mAb2A) which specifically stains a novel nuclear meshwork which reorganizes during the cell cycle in Drosophila, we have initiated a study to: 1) more closely analyze this structural reorganization; 2) clone and characterize the antigens recognized by this antibody; and 3) isolate other interacting proteins in order to gain insight into the regulation of this process. The mAb2A-labeled structure changes from what appears as a diffuse meshwork at interphase to a distinct spindle-like scaffold at prophase. Since at metaphase the microtubules of the mitotic apparatus co-localize with the mAb2A spindle structure, a model is considered whereby the nuclear mAb2A-labeled scaffolding reorganizes during the cell cycle to provide a guide for the establishment of the mitotic apparatus. The mAb2A has identified two separate antigens, each of which shows similar distribution patterns. One of these antigens has been partially cloned and contains an unusual tandem ser-thr kinase domain. The association of this kinase homologue with a nuclear scaffold which reorganizes during the cell cycle suggests that it may be involved in regulating changes in nuclear architecture during the cell cycle and/or in mediating the downstream consequences of such changes.

The architectural organization of nuclear metabolism

Nucleic acid metabolism is structurally organized in the nucleus. DNA replication and transcription have been localized to particular nuclear domains. Additional domains have been identified by their morphology or by their composition; for example, by their high concentration of factors involved in RNA splicing. The domain organization of the nucleus is maintained by the nuclear matrix, a nonchromatin nuclear scaffolding that holds most nuclear RNA and organizes chromatin into loops. The nuclear matrix is built on a network of highly branched core filaments that have an average diameter of 10 nm. Many of the intermediates and the regulatory and catalytic factors of nucleic acid metabolism are retained in nuclear matrix preparations, suggesting that nucleic acid synthesis and processing are structure-bound processes in cells. Tissue-specific and malignancy-induced variations in nuclear structure and metabolism may result from altered matrix architecture and composition.

Nuclear domains and the nuclear matrix

This overview describes the spatial distribution of several enzymatic machineries and functions in the interphase nucleus. Three general observations can be made. First, many components of the different nuclear machineries are distributed in the nucleus in a characteristic way for each component. They are often found concentrated in specific domains. Second, nuclear machineries for the synthesis and processing of RNA and DNA are associated with an insoluble nuclear structure, called nuclear matrix. Evidently, handling of DNA and RNA is done by immobilized enzyme systems. Finally, the nucleus seems to be divided in two major compartments. One is occupied by compact chromosomes, the other compartment is the space between the chromosomes. In the latter, transcription takes place at the surface of chromosomal domains and it houses the splicing machinery. The relevance of nuclear organization for efficient gene expression is discussed.

The nuclear matrix protein NMP-1 is the transcription factor YY1

NMP-1 was initially identified as a nuclear matrix-associated DNA-binding factor that exhibits sequence-specific recognition for the site IV regulatory element of a histone H4 gene. This distal promoter domain is a nuclear matrix interaction site. In the present study, we show that NMP-1 is the multifunctional transcription factor YY1. Gel-shift and Western blot analyses demonstrate that NMP-1 is immunoreactive with YY1 antibody. Furthermore, purified YY1 protein specifically recognizes site IV and reconstitutes the NMP-1 complex. Western blot and gel-shift analyses indicate that YY1 is present within the nuclear matrix. In situ immunofluorescence studies show that a significant fraction of YY1 is localized in the nuclear matrix, principally but not exclusively associated with residual nucleoli. Our results confirm that NMP-1/YY1 is a ubiquitous protein that is present in both human cells and in rat osteosarcoma ROS 17/2.8 cells. The finding that NMP-1 is identical to YY1 suggests that this transcriptional regulator may mediate gene-matrix interactions. Our results are consistent with the concept that the nuclear matrix may functionally compartmentalize the eukaryotic nucleus to support regulation of gene expression.

A hierarchy of ECM-mediated signalling regulates tissue-specific gene expression

A dynamic and reciprocal flow of information between cells and the extracellular matrix contributes significantly to the regulation of form and function in developing systems. Signals generated by the extracellular matrix do not act in isolation. Instead, they are processed within the context of global signalling hierarchies whose constituent inputs and outputs are constantly modulated by all the factors present in the cell's surrounding microenvironment. This is particularly evident in the mammary gland, where the construction and subsequent destruction of such a hierarchy regulates changes in tissue-specific gene expression, morphogenesis and apoptosis during each developmental cycle of pregnancy, lactation and involution.