Higher order genomic organization and epigenetic control maintain cellular identity and prevent breast cancer.

Cells establish and sustain structural and functional integrity of the genome to support cellular identity and prevent malignant transformation. In this review, we present a strategic overview of epigenetic regulatory mechanisms including histone modifications and higher order chromatin organization (HCO) that are perturbed in breast cancer onset and progression. Implications for dysfunctions that occur in hormone regulation, cell cycle control, and mitotic bookmarking in breast cancer are considered, with an emphasis on epithelial-to-mesenchymal transition and cancer stem cell activities. The architectural organization of regulatory machinery is addressed within the contexts of translating cancer-compromised genomic organization to advances in breast cancer risk assessment, diagnosis, prognosis, and identification of novel therapeutic targets with high specificity and minimal off target effects.

Oncogenic targeting of BRM drives malignancy through C/EBPß-dependent induction of α5 integrin.

Integrin expression and activity are altered in tumors, and aberrant integrin signaling promotes malignancy. However, how integrins become altered in tumors remains poorly understood. We discovered that oncogenic activation of MEK signaling induces cell growth and survival, and promotes the malignant phenotype of mammary epithelial cells (MECs) by increasing α5 integrin expression. We determined that MEK activates c-Myc to reduce the transcription of the SWI/SNF chromatin remodeling enzyme Brahma (BRM). Our studies revealed that reduced BRM expression and/or activity drives the malignant behavior of MECs by epigenetically promoting C/EBPß expression to directly induce α5 integrin transcription. Consistently, we could show that restoring BRM levels normalized the malignant behavior of transformed MECs in culture and in vivo by preventing C/EBPß-dependent α5 integrin transcription. Our findings identify a novel mechanism whereby oncogenic signaling promotes malignant transformation by regulating transcription of a key chromatin remodeling molecule that regulates integrin-dependent stromal-epithelial interactions.

The SWI/SNF chromatin remodeling subunit BRG1 is a critical regulator of p53 necessary for proliferation of malignant cells.

The tumor suppressor p53 preserves genome integrity by inducing transcription of genes controlling growth arrest or apoptosis. Transcriptional activation involves nucleosomal perturbation by chromatin remodeling enzymes. Mammalian SWI/SNF remodeling complexes incorporate either the Brahma-related gene 1 (BRG1) or Brahma (Brm) as the ATPase subunit. The observation that tumor cell lines harboring wild-type p53 specifically maintain expression of BRG1 and that BRG1 complexes with p53 prompted us to examine the role of BRG1 in regulation of p53. Remarkably, RNAi depletion of BRG1, but not Brm, led to the activation of endogenous wild-type p53 and cell senescence. We found a proline-rich region unique to BRG1 was required for binding to the histone acetyl transferase protein, CBP, as well as to p53. Ectopic expression of a proline-rich region deletion mutant BRG1 that is defective for CBP binding inhibited p53 destabilization. Importantly, RNAi knockdown of BRG1 and CBP reduced p53 poly-ubiquitination in vivo. In support of p53 inactivation by the combined activities of BRG1 and CBP, we show that DNA damage signals promoted disassociation of BRG1 from CBP, thereby allowing p53 accumulation. Our data demonstrate a novel function of the evolutionarily conserved chromatin remodeling subunit BRG1, which cooperates with CBP to constrain p53 activity and permit cancer cell proliferation.

Temporal recruitment of CCAAT/enhancer-binding proteins to early and late adipogenic promoters in vivo.

The CCAAT/enhancer-binding protein (C/EBP) family of transcriptional regulators is critically important for the activation of adipogenic genes during differentiation. The C/EBPbeta and delta isoforms are rapidly induced upon adipocyte differentiation and are responsible for activating the adipogenic regulators C/EBPalpha and peroxisome proliferator activated receptor (PPAR)gamma2, which together activate the majority of genes expressed in differentiating adipocytes. However, mitosis is required following the induction of adipogenesis, and the activation of C/EBPalpha and PPARgamma2 gene expression is delayed until cell division is underway. Previous studies have used electromobility shift assays to suggest that this delay is due, at least in part, to a delay between the induction of C/EBPbeta protein levels and the acquisition of DNA binding capacity by C/EBPbeta. Here we used in vivo chromatin immunoprecipitation analysis of the C/EBPalpha, PPARgamma2, resistin, adiponectin, and leptin promoters to examine the kinetics of C/EBP protein binding to adipogenic genes in differentiating cells. In contrast to prior studies, we determined that C/EBPbeta and delta were bound to endogenous regulatory sequences controlling the expression of these genes within 1-4 h of adipogenic induction. These results indicated that C/EBPbeta and delta bind not only to genes that are induced early in the adipogenic process but also to genes that are induced much later during differentiation, without a delay between induction of C/EBP protein levels and DNA binding by these proteins. We also showed that each of the genes examined undergoes a transition in vivo from early occupancy by C/EBPbeta and delta to occupancy by C/EBPalpha at times that correlate with the induction of C/EBPalpha protein levels, demonstrating the generality of the transition during adipogenesis and indicating that the binding of specific C/EBP isoforms does not correlate with timing of expression from each gene. We have concluded that C/EBP family members bind to adipogenic genes in vivo in a manner that follows the induction of C/EBP protein synthesis.

MyoD can induce cell cycle arrest but not muscle differentiation in the presence of dominant negative SWI/SNF chromatin remodeling enzymes.

Cell cycle arrest is critical for muscle differentiation, and the two processes are closely coordinated but temporally separable. SWI/SNF complexes are ATP-dependent chromatin-remodeling enzymes that have been shown to be required for muscle differentiation in cell culture and have also been reported to be required for Rb-mediated cell cycle arrest. We therefore looked more closely at how SWI/SNF enzymes affect the events that occur during MyoD-induced myogenesis, namely, cell cycle regulation and muscle-specific gene expression, in cells that inducibly express dominant negative versions of Brahma (BRM) and Brahma-related gene 1 (BRG1), the ATPase subunits of two distinct SWI/SNF complexes. Although dominant negative BRM and BRG1 inhibited expression of every muscle-specific regulator and structural gene assayed, there was no effect on MyoD-induced activation of cell cycle regulatory proteins, and thus, cells arrested normally. In particular, in the presence or absence of dominant negative BRM or BRG1, MyoD was able to activate expression of p21, cyclin D3, and Rb, all of which are critical for cell cycle withdrawal in the G1/G0 phase of the cell cycle. These findings suggest that at least one basis for the distinct mechanisms that regulate cessation of cell proliferation and muscle-specific gene expression during muscle differentiation is that SWI/SNF-mediated chromatin-remodeling enzymes are required only for the latter.

Disruption of Ini1 leads to peri-implantation lethality and tumorigenesis in mice.

SNF5/INI1 is a component of the ATP-dependent chromatin remodeling enzyme family SWI/SNF. Germ line mutations of INI1 have been identified in children with brain and renal rhabdoid tumors, indicating that INI1 is a tumor suppressor. Here we report that disruption of Ini1 expression in mice results in early embryonic lethality. Ini1-null embryos die between 3.5 and 5.5 days postcoitum, and Ini1-null blastocysts fail to hatch, form the trophectoderm, or expand the inner cell mass when cultured in vitro. Furthermore, we report that approximately 15% of Ini1-heterozygous mice present with tumors, mostly undifferentiated or poorly differentiated sarcomas. Tumor formation is associated with a loss of heterozygocity at the Ini1 locus, characterizing Ini1 as a tumor suppressor in mice. Thus, Ini1 is essential for embryo viability and for repression of oncogenesis in the adult organism.

Purification and characterization of mSin3A-containing Brg1 and hBrm chromatin remodeling complexes.

Alteration of nucleosomes by ATP-dependent remodeling complexes represents a critical step in the regulation of transcription. The human SWI/SNF (hSWI/SNF) family is composed of complexes that contain either Brg1 or hBrm as the central ATPase; however, these separate complexes have not been compared functionally. Here we describe the establishment of cell lines that express epitope-tagged Brg1 and hBrm and a characterization of the complexes associated with these two ATPases. We show that Brg1 fractionates into two complexes that differ in activity and subunit composition, whereas hBrm is found in one complex with lower activity than the Brg1 complexes. These three complexes can remodel nucleosomal arrays, increase restriction enzyme accessibility, and hydrolyze ATP in a DNA-dependent manner. The three complexes differ markedly in their ability to remodel mononucleosomal core particles. We also show that the hBrm complex and one of the Brg1 complexes contain components of the mammalian Sin3 (mSin3) complex. In addition, we have found that Brg1, hBrm, and BAF155 can interact specifically with mSin3A in vitro, showing a direct association of hSWI/SNF complexes with proteins involved in gene repression. These unexpected functional characteristics indicate that these hSWI/SNF complexes play diverse regulatory roles.

Mammalian SWI/SNF complexes promote MyoD-mediated muscle differentiation.

Mammalian SWI/SNF complexes are ATP-dependent chromatin remodeling enzymes that have been implicated in the regulation of gene expression, cell-cycle control and oncogenesis. MyoD is a muscle-specific regulator able to induce myogenesis in numerous cell types. To ascertain the requirement for chromatin remodeling enzymes in cellular differentiation processes, we examined MyoD-mediated induction of muscle differentiation in fibroblasts expressing dominant-negative versions of the human brahma-related gene-1 (BRG1) or human brahma (BRM), the ATPase subunits of two distinct SWI/SNF enzymes. We find that induction of the myogenic phenotype is completely abrogated in the presence of the mutant enzymes. We further demonstrate that failure to induce muscle-specific gene expression correlates with inhibition of chromatin remodeling in the promoter region of an endogenous muscle-specific gene. Our results demonstrate that SWI/SNF enzymes promote MyoD-mediated muscle differentiation and indicate that these enzymes function by altering chromatin structure in promoter regions of endogenous, differentiation-specific loci.

Human SWI/SNF nucleosome remodeling activity is partially inhibited by linker histone H1.

The physical structure and the compact nature of the eukaryotic genome present a functional barrier for any cellular process that requires access to the DNA. The linker histone H1 is intrinsically involved in both the determination of and the stability of higher order chromatin structure. Because histone H1 plays a pivotal role in the structure of chromatin, we investigated the effect of histone H1 on the nucleosome remodeling activity of human SWI/SNF, an ATP-dependent chromatin remodeling complex. The results from both DNase I digestion and restriction endonuclease accessibility assays indicate that the presence of H1 partially inhibits the nucleosome remodeling activity of hSWI/SNF. Neither H1 bound to the nucleosome nor free H1 affected the ATPase activity of hSWI/SNF, suggesting that the observed inhibition of hSWI/SNF nucleosome remodeling activity depends on the structure formed by the addition of H1 to nucleosomes.

BRG-1 is required for RB-mediated cell cycle arrest.

The antiproliferative action of the retinoblastoma tumor suppressor protein, RB, is disrupted in the majority of human cancers. Disruption of RB activity occurs through several disparate mechanisms, including viral oncoprotein binding, deregulated RB phosphorylation, and mutation of the RB gene. Here we report disruption of RB-signaling in tumor cells through loss of a critical cooperating factor. We have previously reported that C33A cells fail to undergo cell cycle inhibition in the presence of constitutively active RB (PSM-RB). To determine how C33A cells evade RB-mediated arrest, cell fusion experiments were performed with RB-sensitive cells. The resulting fusions were arrested by PSM-RB, indicating that C33A cells lack a factor required for RB-mediated cell cycle inhibition. C33A cells are deficient in BRG-1, a SWI/SNF family member known to stimulate RB activity. Consistent with BRG-1 deficiency underlying resistance to RB-mediated arrest, we identified two other BRG-1-deficient cell lines (SW13 and PANC-1) and demonstrate that these tumor lines are also resistant to cell cycle inhibition by PSM-RB and p16ink4a, which activates endogenous RB. In cell lines lacking BRG-1, we noted a profound defect in RB-mediated repression of the cyclin A promoter. This deficiency in RB-mediated transcriptional repression and cell cycle inhibition was rescued through ectopic coexpression of BRG-1. We also demonstrate that 3T3-derived cells, which inducibly express a dominant-negative BRG-1, arrest by PSM-RB and p16ink4a in the absence of dominant-negative BRG-1 expression; however, cell cycle arrest was abrogated on induction of dominant-negative BRG-1. These findings demonstrate that BRG-1 loss renders cells resistant to RB-mediated cell cycle progression, and that disruption of RB signaling through loss of cooperating factors occurs in cancer cells.

Functional delineation of three groups of the ATP-dependent family of chromatin remodeling enzymes.

ATP-dependent chromatin remodeling enzymes antagonize the inhibitory effects of chromatin. We compare six different remodeling complexes: ySWI/SNF, yRSC, hSWI/SNF, xMi-2, dCHRAC, and dNURF. We find that each complex uses similar amounts of ATP to remodel nucleosomal arrays at nearly identical rates. We also perform assays with arrays reconstituted with hyperacetylated or trypsinized histones and isolated histone (H3/H4)(2) tetramers. The results define three groups of the ATP-dependent family of remodeling enzymes. In addition we investigate the ability of an acidic activator to recruit remodeling complexes to nucleosomal arrays. We propose that ATP-dependent chromatin remodeling enzymes share a common reaction mechanism and that a key distinction between complexes is in their mode of regulation or recruitment.

Phosphoaminoglycosides inhibit SWI2/SNF2 family DNA-dependent molecular motor domains.

Members of the SWI2/SNF2 family of proteins participate in an array of nucleic acid metabolic functions, including chromatin remodeling and transcription. The present studies identify a novel strategy to specifically inhibit the functional DNA-dependent adenosinetriphosphatase (ATPase) motor domain common to SWI2/SNF2 family members. We have identified preparations of phosphoaminoglycosides, which are natural products of aminoglycoside-resistant bacteria, as inhibitors of the in vitro activities of three SWI2/SNF2 family members. These compounds inhibit the ATPase activity of the active DNA-dependent ATPase A domain (ADAAD) by competing with respect to DNA and thus have no effect on DNA-independent ATPases or on RNA-dependent ATPases. Within the superfamily of DNA-dependent ATPases, these compounds are most potent toward SWI2/SNF2 family members and less potent toward other DNA-dependent ATPases. We demonstrate that it is feasible to target DNA-dependent ATPases of a particular type without affecting the function of other ATPases. As the SWI2/SNF2 proteins have been proposed to function in all aspects of DNA metabolism, this paper provides an archetype for development of DNA metabolic inhibitors.

Mammalian SWI-SNF complexes contribute to activation of the hsp70 gene.

ATP-dependent chromatin-remodeling complexes are conserved among all eukaryotes and function by altering nucleosome structure to allow cellular regulatory factors access to the DNA. Mammalian SWI-SNF complexes contain either of two highly conserved ATPase subunits: BRG1 or BRM. To identify cellular genes that require mammalian SWI-SNF complexes for the activation of gene expression, we have generated cell lines that inducibly express mutant forms of the BRG1 or BRM ATPases that are unable to bind and hydrolyze ATP. The mutant subunits physically associate with at least two endogenous members of mammalian SWI-SNF complexes, suggesting that nonfunctional, dominant negative complexes may be formed. We determined that expression of the mutant BRG1 or BRM proteins impaired the ability of cells to activate the endogenous stress response gene hsp70 in response to arsenite, a metabolic inhibitor, or cadmium, a heavy metal. Activation of hsp70 by heat stress, however, was unaffected. Activation of the heme oxygenase 1 promoter by arsenite or cadmium and activation of the cadmium-inducible metallothionein promoter also were unaffected by the expression of mutant SWI-SNF components. Analysis of a subset of constitutively expressed genes revealed no or minimal effects on transcript levels. We propose that the requirement for mammalian SWI-SNF complexes in gene activation events will be specific to individual genes and signaling pathways.

Energy-dependent chromatin remodelers: complex complexes and their components.

Chromatin structure is dynamically regulated such that it can be modified by a number of factors in response to a variety of signals. One class of factors that can mediate changes in chromatin structure is the ATP-dependent nucleosome remodeling complexes. Genetic and biochemical evidence supports the idea that a family of related multisubunit complexes hydrolyzes ATP in order to facilitate the rearrangement of chromatin structure. These complexes are conserved from yeast to mammals and apparently have diverse functions in modifying chromatin structure; ATP-dependent chromatin remodelers have been implicated in nucleosome deposition, nucleosome assembly, and disruption of nucleosome structure to facilitate transcriptional activation. In addition, individual components of these complexes have been linked to control of cell growth, cell cycle regulation, development, and differentiation, and they may also be targets for viral regulatory proteins. The diversity of subunit functions likely relates to effects on chromatin structure, suggesting that the regulation of chromatin structure by ATP-dependent remodelers is important in many different aspects of cellular metabolism.

SWI/SNF complexes and facilitation of TATA binding protein:nucleosome interactions.

It has become increasingly apparent that eukaryotic cells possess machinery that modifies chromatin structure and that this machinery contributes to the regulation of gene expression. Identification of factors that alter chromatin structure has made possible biochemical analyses that have begun to define what structural changes each factor can cause as well as what consequences these changes have on transcription factor function. Here, a protocol that has facilitated study of energy-dependent chromatin remodeling complexes containing SWI/SNF proteins is described. Rotationally phased mononucleosome particles were assembled in vitro and used to demonstrate that human SWI/SNF complexes and the yeast RNA polymerase II holoenzyme, which contains yeast SWI/SNF proteins, can directly alter nucleosome structure in an ATP-dependent manner. A functional consequence of this nucleosome disruption is that the pol II general transcription factor, TATA binding protein (TBP), which cannot bind to unaltered nucleosomal DNA, can bind to its site on the altered nucleosome. This experimental system has been invaluable for characterization of both nucleosome alteration and facilitated transcription factor binding mediated by SWI/SNF complexes. These procedures should also be useful to examine other factors that interact with or structurally affect nucleosome particles.

Accessibility of nucleosomal DNA to V(D)J cleavage is modulated by RSS positioning and HMG1.

B and T cell receptor gene assembly by V(D)J recombination is tightly regulated during lymphoid development. The mechanisms involved in this regulation are poorly understood. Here we show that nucleosomal DNA is refractory to V(D)J cleavage. However, the presence of HMG1, a chromatin-associated nonhistone DNA-binding protein, stimulates V(D)J cleavage of nucleosomal templates. This HMG1 stimulation is differentially affected by the rotational or translational positioning of the recombination signal sequence on the histone octamer, with cleavage of the 12 bp spacer RSS showing sensitivity to rotational position and the 23 bp spacer RSS affected by its displacement from the dyad. These results suggest that V(D)J recombination can be modulated by controlling substrate accessibility and cleavage at the level of an individual nucleosome.

Nucleosome disruption by human SWI/SNF is maintained in the absence of continued ATP hydrolysis.

We have examined the requirement for ATP in human (h) SWI/SNF-mediated alteration of nucleosome structure and facilitation of transcription factor binding to nucleosomal DNA. hSWI/SNF-mediated nucleosome alteration requires hydrolysis of ATP or dATP. The alteration is stable upon removal of ATP from the reaction or upon inhibition of activity by excess ATPgammaS, indicating that continued ATP hydrolysis is not required to maintain the altered nucleosome structure. This stable alteration is sufficient to facilitate binding of a transcriptional activator protein; concurrent ATP hydrolysis was not required to facilitate binding. These data suggest sequential steps that can occur in the process by which transcription factors gain access to nucleosomal DNA.

Activator-dependent regulation of transcriptional pausing on nucleosomal templates.

Promoter-proximal pausing during transcriptional elongation is an important way of regulating many diverse genes, including human c-myc and c-fos, some HIV genes, and the Drosophila heat shock loci. To characterize the mechanisms that regulate pausing, we have established an in vitro system using the human hsp7O gene. We demonstrate that nucleosome formation increases by >100-fold the duration of a transcriptional pause on the human hsp7O gene in vitro at the same location as pausing is observed in vivo. Readthrough of this pause is increased by an activator that contains the human heat shock factor 1 (HSF1) transcriptional activation domains. Maximal effect of the activator requires that the system be supplemented with fractions that have hSWI/SNF activity, which has been shown previously to alter nucleosome structure. No significant readthrough is observed in the absence of activator, and neither the activator nor the hSWI/SNF fraction affected elongation on naked DNA; therefore, these results suggest that an activator can cause increased readthrough of promoter-proximal pausing by decreasing the inhibitory effect of nucleosomes on transcriptional elongation.

Repression and activation by multiprotein complexes that alter chromatin structure.

Recent studies have provided strong evidence that macromolecular complexes are used in the cell to remodel chromatin structure during activation and to create an inaccessible structure during repression, Although there is not yet any rigorous demonstration that modification of chromatin structure plays a direct, causal role in either activation or repression, there is sufficient smoke to indicate the presence of a blazing inferno nearby. It is clear that complexes that remodel chromatin are tractable in vitro; hopefully this will allow the establishment of systems that provide a direct analysis of the role that remodeling might play in activation. These studies indicate that establishment of functional systems to corroborate the elegant genetic studies on repression might also be tractable. As the mechanistic effects of these complexes are sorted out, it will become important to understand how the complexes are regulated. In many of the instances discussed above, the genes whose products make up these complexes were identified in genetic screens for effects on developmental processes. This implies a regulation of the activity of these complexes in response to developmental cues and further implies that the work to fully understand these complexes will occupy a generation of scientists.