Connexin 43 maintains tissue polarity and regulates mitotic spindle orientation in the breast epithelium.

Cell-cell communication is essential for tissue homeostasis, but its contribution to disease prevention remains to be understood. We demonstrate the involvement of connexin 43 (Cx43, also known as GJA1) and related gap junction in epithelial homeostasis, illustrated by polarity-mediated cell cycle entry and mitotic spindle orientation (MSO). Cx43 localization is restricted to the apicolateral membrane of phenotypically normal breast luminal epithelial cells in 3D culture and in vivo Chemically induced blockade of gap junction intercellular communication (GJIC), as well as the absence of Cx43, disrupt the apicolateral distribution of polarity determinant tight junction marker ZO-1 (also known as TJP1) and lead to random MSO and cell multilayering. Induced expression of Cx43 in cells that normally lack this protein reestablishes polarity and proper MSO in 3D culture. Cx43-directed MSO implicates PI3K-aPKC signaling, and Cx43 co-precipitates with signaling node proteins ß-catenin (CTNNB1) and ZO-2 (also known as TJP2) in the polarized epithelium. The distribution of Cx43 is altered by pro-inflammatory breast cancer risk factors such as leptin and high-fat diet, as shown in cell culture and on tissue biopsy sections. The control of polarity-mediated quiescence and MSO may contribute to the tumor-suppressive role of Cx43.

ErbB2, but not ErbB1, reinitiates proliferation and induces luminal repopulation in epithelial acini.

Both ErbB1 and ErbB2 are overexpressed or amplified in breast tumours. To examine the effects of activating ErbB receptors in a context that mimics polarized epithelial cells in vivo, we activated ErbB1 and ErbB2 homodimers in preformed, growth-arrested mammary acini cultured in three-dimensional basement membrane gels. Activation of ErbB2, but not that of ErbB1, led to a reinitiation of cell proliferation and altered the properties of mammary acinar structures. These altered structures share several properties with early-stage tumours, including a loss of proliferative suppression, an absence of lumen, retention of the basement membrane and a lack of invasive properties. ErbB2 activation also disrupted tight junctions and the cell polarity of polarized epithelia, whereas ErbB1 activation did not have any effect. Our results indicate that ErbB receptors differ in their ability to induce early stages of mammary carcinogenesis in vitro and this three-dimensional model system can reveal biological activities of oncogenes that cannot be examined in vitro in standard transformation assays.

Segmentation of nuclei and cells using membrane related protein markers.

Segmenting individual cell nuclei from microscope images normally involves volume labelling of the nuclei with a DNA stain. However, this method often fails when the nuclei are tightly clustered in the tissue, because there is little evidence from the images on where the borders of the nuclei are. In this paper we present a method which solves this limitation and furthermore enables segmentation of whole cells. Instead of using volume stains, we used stains that specifically label the surface of nuclei or cells: lamins for the nuclear envelope and alpha-6 or beta-1 integrins for the cellular surface. The segmentation is performed by identifying unique seeds for each nucleus/cell and expanding the boundaries of the seeds until they reach the limits of the nucleus/cell, as delimited by the lamin or integrin staining, using gradient-curvature flow techniques. We tested the algorithm using computer-generated objects to evaluate its robustness against noise and applied it to cells in culture and to tissue specimens. In all the cases that we present the algorithm gave accurate results.

High resolution protein localization using soft X-ray microscopy.

Soft X-ray microscopes can be used to examine whole, hydrated cells up to 10 microm thick and produce images approaching 30 nm resolution. Since cells are imaged in the X-ray transmissive "water window", where organic material absorbs approximately an order of magnitude more strongly than water, chemical contrast enhancement agents are not required to view the distribution of cellular structures. Although living specimens cannot be examined, cells can be rapidly frozen at a precise moment in time and examined in a cryostage, revealing information that most closely approximates that in live cells. In this study, we used a transmission X-ray microscope at photon energies just below the oxygen edge (lambda = 2.4 nm) to examine rapidly frozen mouse 3T3 cells and obtained excellent cellular morphology at better than 50 nm lateral resolution. These specimens are extremely stable, enabling multiple exposures with virtually no detectable damage to cell structures. We also show that silver-enhanced, immunogold labelling can be used to localize both cytoplasmic and nuclear proteins in whole, hydrated mammary epithelial cells at better than 50 nm resolution. The future use of X-ray tomography, along with improved zone plate lenses, will enable collection of better resolution (approaching 30 nm), three-dimensional information on the distribution of proteins in cells.

Cell nucleus in context.

The molecular pathways that participate in regulation of gene expression are being progressively unraveled. Extracellular signals, including the binding of extracellular matrix and soluble molecules to cell membrane receptors, activate specific signal transducers that process information inside the cell leading to alteration in gene expression. Some of these transducers when translocated to the cell nucleus may bind to transcription complexes and thereby modify the transcriptional activity of specific genes. However, the basic molecules involved in the regulation of gene expression are found in many different cell and tissue types; thus, the mechanisms underlying tissue-specific gene expression are still obscure. In this review we focus on the study of signals that are conveyed to the nucleus. We propose that the way in which extracellular signals are integrated may account for tissue-specific gene expression. We argue that the integration of signals depends on the nature of the structural organization of cells (i.e., extracellular matrix, membrane proteins, cytoskeleton, nucleus) that defines a particular cell type within a tissue. Thus, gene expression can be envisioned as being regulated by the mutual influence of extracellular and intracellular organizations, i.e., in context.

AZU-1: a candidate breast tumor suppressor and biomarker for tumor progression.

To identify genes misregulated in the final stages of breast carcinogenesis, we performed differential display to compare the gene expression patterns of the human tumorigenic mammary epithelial cells, HMT-3522-T4-2, with those of their immediate premalignant progenitors, HMT-3522-S2. We identified a novel gene, called anti-zuai-1 (AZU-1), that was abundantly expressed in non- and premalignant cells and tissues but was appreciably reduced in breast tumor cell types and in primary tumors. The AZU-1 gene encodes an acidic 571-amino-acid protein containing at least two structurally distinct domains with potential protein-binding functions: an N-terminal serine and proline-rich domain with a predicted immunoglobulin-like fold and a C-terminal coiled-coil domain. In HMT-3522 cells, the bulk of AZU-1 protein resided in a detergent-extractable cytoplasmic pool and was present at much lower levels in tumorigenic T4-2 cells than in their nonmalignant counterparts. Reversion of the tumorigenic phenotype of T4-2 cells, by means described previously, was accompanied by the up-regulation of AZU-1. In addition, reexpression of AZU-1 in T4-2 cells, using viral vectors, was sufficient to reduce their malignant phenotype substantially, both in culture and in vivo. These results indicate that AZU-1 is a candidate breast tumor suppressor that may exert its effects by promoting correct tissue morphogenesis.

Tissue structure, nuclear organization, and gene expression in normal and malignant breast.

Because every cell within the body has the same genetic information, a significant problem in biology is to understand how cells within a tissue express genes selectively. A sophisticated network of physical and biochemical signals converge in a highly orchestrated manner to bring about the exquisite regulation that governs gene expression in diverse tissues. Thus, the ultimate decision of a cell to proliferate, express tissue-specific genes, or apoptose must be a coordinated response to its adhesive, growth factor, and hormonal milieu. The unifying hypothesis examined in this overview is that the unit of function in higher organisms is neither the genome nor the cell alone but the complex, three-dimensional tissue. This is because there are bidirectional connections between the components of the cellular microenvironment (growth factors, hormones, and extracellular matrix) and the nucleus. These connections are made via membrane-bound receptors and transmitted to the nucleus, where the signals result in modifications to the nuclear matrix and chromatin structure and lead to selective gene expression. Thus, cells need to be studied "in context", i.e., within a proper tissue structure, if one is to understand the bidirectional pathways that connect the cellular microenvironment and the genome. In the last decades, we have used well-characterized human and mouse mammary cell lines in "designer microenvironments" to create an appropriate context to study tissue-specific gene expression. The use of a three-dimensional culture assay, developed with reconstituted basement membrane, has allowed us to distinguish normal and malignant human breast cells easily and rapidly. Whereas normal cells become growth arrested and form organized "acini," tumor cells continue to grow, pile up, and in general fail to respond to extracellular matrix and microenvironmental cues. By correcting the extracellular matrix-receptor (integrin) signaling and balance, we have been able to revert the malignant phenotype when a human breast tumor cell is cultured in, or on, a basement membrane. Most recently, we have shown that whereas beta1 integrin and epidermal growth factor receptor signal transduction pathways are integrated reciprocally in three-dimensional cultures, on tissue culture plastic (two-dimensional monolayers), these are not coordinated. Finally, we have demonstrated that, rather than passively reflecting changes in gene expression, nuclear organization itself can modulate cellular and tissue phenotype. We conclude that the structure of the tissue is dominant over the genome, and that we may need a new paradigm for how epithelial-specific genes are regulated in vivo. We also argue that unless the structure of the tissue is critically altered, malignancy will not progress, even in the presence of multiple chromosomal mutations.

Tissue phenotype depends on reciprocal interactions between the extracellular matrix and the structural organization of the nucleus.

What determines the nuclear organization within a cell and whether this organization itself can impose cellular function within a tissue remains unknown. To explore the relationship between nuclear organization and tissue architecture and function, we used a model of human mammary epithelial cell acinar morphogenesis. When cultured within a reconstituted basement membrane (rBM), HMT-3522 cells form polarized and growth-arrested tissue-like acini with a central lumen and deposit an endogenous BM. We show that rBM-induced morphogenesis is accompanied by relocalization of the nuclear matrix proteins NuMA, splicing factor SRm160, and cell cycle regulator Rb. These proteins had distinct distribution patterns specific for proliferation, growth arrest, and acini formation, whereas the distribution of the nuclear lamina protein, lamin B, remained unchanged. NuMA relocalized to foci, which coalesced into larger assemblies as morphogenesis progressed. Perturbation of histone acetylation in the acini by trichostatin A treatment altered chromatin structure, disrupted NuMA foci, and induced cell proliferation. Moreover, treatment of transiently permeabilized acini with a NuMA antibody led to the disruption of NuMA foci, alteration of histone acetylation, activation of metalloproteases, and breakdown of the endogenous BM. These results experimentally demonstrate a dynamic interaction between the extracellular matrix, nuclear organization, and tissue phenotype. They further show that rather than passively reflecting changes in gene expression, nuclear organization itself can modulate the cellular and tissue phenotype.

Communication between the cell membrane and the nucleus: role of protein compartmentalization.

Understanding how the information is conveyed from outside to inside the cell is a critical challenge for all biologists involved in signal transduction. The flow of information initiated by cell-cell and cell-extracellular matrix contacts is mediated by the formation of adhesion complexes involving multiple proteins. Inside adhesion complexes, connective membrane skeleton (CMS) proteins are signal transducers that bind to adhesion molecules, organize the cytoskeleton, and initiate biochemical cascades. Adhesion complex-mediated signal transduction ultimately directs the formation of supramolecular structures in the cell nucleus, as illustrated by the establishment of multi complexes of DNA-bound transcription factors, and the redistribution of nuclear structural proteins to form nuclear subdomains. Recently, several CMS proteins have been observed to travel to the cell nucleus, suggesting a distinctive role for these proteins in signal transduction. This review focuses on the nuclear translocation of structural signal transducers of the membrane skeleton and also extends our analysis to possible translocation of resident nuclear proteins to the membrane skeleton. This leads us to envision the communication between spatially distant cellular compartments (i.e., membrane skeleton and cell nucleus) as a bidirectional flow of information (a dynamic reciprocity) based on subtle multilevel structural and biochemical equilibria. At one level, it is mediated by the interaction between structural signal transducers and their binding partners, at another level it may be mediated by the balance and integration of signal transducers in different cellular compartments.

Extracellular matrix signaling from the cellular membrane skeleton to the nuclear skeleton: a model of gene regulation.

It is well established that cells must interact with their microenvironment and that such interaction is crucial for coordinated function and homeostasis. However, how cells receive and integrate external signals leading to gene regulation is far from understood. It is now appreciated that two classes of cooperative signals are implicated: a soluble class including hormones and growth factors and a class of insoluble signals emanating from the extracellular matrix (ECM) directly through contact with the cell surface. Using 3-dimensional culture systems and transgenic mice, we have been able to identify some of the elements of this ECM-signaling pathway responsible for gene regulation in rodent mammary gland differentiation and involution. Our major observations are 1) the requirement for a laminin-rich basement membrane; 2) the existence of a cooperative signaling pathway between basement membrane and the lactogenic hormone prolactin (PRL);3) the importance of beta 1-integrins and bHLH transcription factor(s) and the presence of DNA response elements (exemplified by BCE-1, located on a milk protein gene, beta-casein); and 4) the induction of mammary epithelial cell programmed cell death following degradation of basement membrane. We hypothesize that this cooperative signaling between ECM and PRL may be achieved through integrin- and laminin-directed restructuring of the cytoskeleton leading to profound changes in nuclear architecture and transcription factor localization. We postulate that the latter changes allow the prolactin signal to activate transcription of the beta-casein gene. To further understand the molecular mechanisms underlying ECM and hormonal cooperative signaling, we are currently investigating ECM regulation of a "solid-state" signaling pathway including ECM fiber proteins, plasma membrane receptors, cytoskeleton, nuclear matrix and chromatin. We further postulate that disruption of such a pathway may be implicated in cell disorders including transformation and carcinogenesis.

Altered topoisomerase I and II activities in suramin-resistant lung fibrosarcoma cells.

To better understand the molecular basis for the cytotoxic effects of suramin, we have developed suramin-resistant DC-3F/SU 1000 cells by continuous exposure of fibrosarcoma cells to increasing concentrations of suramin. The suramin resistance (approximately 10-fold) is not associated with changes in uptake or intracellular distribution of the drug. The sensitivity to actinomycin D, cytarabine, aphidicolin, hydroxyurea, vincristine, and 5-fluorouracil is unaltered. In contrast, DC-3F/SU 1000 cells are about 2-fold resistant to classical DNA topoisomerase II inhibitors such as doxorubicin, amsacrine, and etoposide, whereas the cells are 1.5-fold more sensitive to the topoisomerase I inhibitor camptothecin. The cross-resistance to topoisomerase II inhibitors occurred earlier than the collateral sensitivity to camptothecin. Amsacrine- and etoposide-induced DNA-protein complex formation is reduced about 2-fold in DC-3F/SU 1000 cells, compared with DC-3F cells, whereas camptothecin-induced DNA-protein complex formation is increased 1.5-fold. Western blot analysis of cellular lysates from the two cell lines shows no significant differences in the level of topoisomerase II, whereas the level of topoisomerase I is increased 2.5-fold in DC-3F/SU 1000 cells. The catalytic activities of topoisomerases I and II in nuclear extracts from DC-3F/SU 1000 cells are both about 2-fold higher than those in extracts from DC-3F cells, whereas amsacrine- and etoposide-induced DNA-protein complex formation is comparable between the two cell lines. Taken together, our results support the involvement of DNA topoisomerases in the cytotoxic activity of suramin. We further believe that the DC-3F/SU 1000 cells may be a useful model for the elucidation of factors that lead to low, clinically relevant, levels of resistance to topoisomerase II inhibitors.

Development and characterization of suramin-resistant Chinese hamster fibrosarcoma cells: drug-dependent formation of multicellular spheroids and a greatly enhanced metastatic potential.

Suramin-resistant DC-3F/SU 1,000 Chinese hamster fibrosarcoma cells were obtained by continuous exposure of parental DC-3F cells to increasing concentrations of suramin (1 mg/ml final concentration). These cells are 10-fold more resistant to suramin compared to the parental cell line as determined by colony formation in the continuous presence of drug; the 50% effective dose for DC-3F is 35 micrograms/ml whereas the 50% effective dose for DC-3F/SU 1,000 is 380 micrograms/ml. The resistance is not due to reduced drug accumulation and is stable for at least 10 months in the absence of drug. Sensitive and resistant cells show comparable growth rate, cell size, and DNA content. In the presence of suramin, DC-3F/SU 1,000 cells form big multicellular spheroids which regrow as monolayer cultures when the drug is removed. Similar morphological changes are not observed for sensitive DC-3F cells exposed to isotoxic doses of suramin but appeared early on during the development of resistance. Inoculation of DC-3F or DC-3F/SU 1,000 cells s.c. into nude mice results in 100% tumor take within 1 week for both groups. Although the tumor size increases at the same rate, only animals given injections of DC-3F/SU 1,000 cells show extensive and persistent s.c. hemorrhages around the tumor. By 3 weeks, 30% of DC-3F-injected mice (9 of 30) show approximately 5 metastases/lung compared to -262 metastases/lung in 100% of DC-3F/SU 1,000-inoculated mice (30 of 30). These findings have several important implications: (a) given the fact that suramin is currently used clinically, special precaution may be warranted in patients undergoing suramin treatment; (b) the drug may possess an unusual potential to interfere with processes essential to invasion and metastasis which, when properly used, may result in the development of antimetastatic therapies; and (c) suramin may serve as a model compound for other molecules of the antiangiogenic and/or antimetastatic type.

Interaction of skeletal-muscle myosin subfragment-1 with the actin-(338-348) peptide.

The data presented here confirm and provide further experimental evidence that rabbit skeletal-muscle myosin subfragment-1 (S-1) binds to the postulated actin-(338-348) hydrophobic segment [Kabsch, Mannherz, Suck, Pai and Holmes (1990) Nature (London) 347, 37-44] with high affinity in the absence and presence of MgATP. The apparent dissociation constant of the S-1 interaction (5.5 x 10(-7) M) with the actin-(338-348) peptide was of the same order of magnitude as that of the actin-(18-28) binding site (2 x 10(-6) M). In similar conditions, fragmented (27 kDa-50 kDa-20 kDa) S-1 also bound to the peptide. Antibodies directed to the vicinal sequence 348-358 were rapidly eliminated from actin by S-1 interaction and weakened S-1 binding to monomeric or filamentous actin. The antigenic site (348-358) is located very close to the C-terminal S-1-binding site (360-369) and encompasses some residues (Leu-349 and Phe-352) included in the hydrophobic S-1-binding region [Schröder, Manstein, Jahn, Holden, Rayment, Holmes and Spudich (1993) Nature (London) 364, 171-174]. It was observed that anti-[actin-(348-358)] antibodies were also unable to decrease actomyosin ATPase activity, in contrast with previous results obtained with anti-[actin-(18-28)] antibodies [Adams and Reisler (1993) Biochemistry 32, 5051-5056]. The hydrophobic actin-(338-348) peptide used in considerable excess was unable to perturb acto-S-1 and S-1 activities in contrast with results obtained with the N-terminal actin peptide [Kôgler, Moir, Trayer and Ruegg (1991) FEBS Lett. 294, 31-34].

Suramin is an inhibitor of DNA topoisomerase II in vitro and in Chinese hamster fibrosarcoma cells.

The antitrypanosomal and antifiliarial drug suramin is currently under investigation for treatment of advanced malignancies including prostatic cancer, adrenocortical cancer, and some lymphomas and sarcomas. Here we show that suramin is a potent inhibitor of the nuclear enzyme DNA topoisomerase II. Suramin inhibited purified yeast topoisomerase II with an IC50 of about 5 microM, as measured by decatenation or relaxation assays. Suramin did not stabilize the covalent DNA-topoisomerase II reaction intermediate ("cleavable complex"), whereas other inhibitors of this enzyme, such as amsacrine, etoposide, and the ellipticines, are known to stabilize the intermediate. In contrast, the presence of suramin strongly inhibited the cleavable-complex formation induced by amsacrine or etoposide. Accumulation of the endogenous cleavable complex was also inhibited. Suramin entered the nucleus of DC-3F Chinese hamster fibrosarcoma cells exposed to radiolabeled suramin for 24 hr as shown by both optic and electron microscopy. The suramin present in the nucleus seemed to interact with topoisomerase II, since suramin reduced the number of amsacrine-induced protein-associated DNA strand breaks in DC-3F cells and protected these cells from the cytotoxic action of amsacrine. Cells resistant to 9-hydroxyellipticine, which have been shown to have an altered topoisomerase II activity, are about 7-fold more resistant to suramin than the sensitive parental cells as shown by 72-hr growth inhibition assay. Our results suggest that DNA topoisomerase II is a target of suramin action and that this action may play a role in the cytotoxic activity of suramin.

Dichromated polyvinyl alcohol films used as a novel polarization real time holographic recording material.

Real time recording and reading of volume transmission holograms were performed with dichromated polyvinyl alcohol thin solid films (31 microm) that were easy to prepare and use. Real time holograms of high diffraction efficiency were produced without any chemical development, but these films were not erasable. The present holographic study of dichromated polyvinyl alcohol films permits the consideration of many engineering applications.