Over-expression of the BRMS1 family member SUDS3 does not suppress metastasis of human cancer cells

A Capped Tudor Domain within a Core Subunit of the Sin3L/Rpd3L Histone Deacetylase Complex Binds to Nucleic Acid G-Quadruplexes

Chromatin-modifying complexes containing histone deacetylase (HDAC) activities play critical roles in the regulation of gene transcription in eukaryotes. These complexes are thought to lack intrinsic DNA-binding activity, but according to a well-established paradigm, they are recruited via protein–protein interactions by gene-specific transcription factors and posttranslational histone modifications to their sites of action on the genome. The mammalian Sin3L/Rpd3L complex, comprising more than a dozen different polypeptides, is an ancient HDAC complex found in diverse eukaryotes. The subunits of this complex harbor conserved domains and motifs of unknown structure and function. Here, we show that Sds3, a constitutively-associated subunit critical for the proper functioning of the Sin3L/Rpd3L complex, harbors a type of Tudor domain that we designate the capped Tudor domain. Unlike canonical Tudor domains that bind modified histones, the Sds3 capped Tudor domain binds to nucleic acids that can form higher-order structures such as G-quadruplexes and shares similarities with the knotted Tudor domain of the Esa1 histone acetyltransferase that was previously shown to bind single-stranded RNA. Our findings expand the range of macromolecules capable of recruiting the Sin3L/Rpd3L complex and draw attention to potentially new biological roles for this HDAC complex.

BRMS1: a multifunctional signaling molecule in metastasis

Despite high mortality rates, molecular understanding of metastasis remains limited. It can be regulated by both pro- and anti-metastasis genes. The metastasis suppressor, breast cancer metastasis suppressor 1 (BRMS1), has been positively correlated with patient outcomes, but molecular functions are still being characterized. BRMS1 has been implicated in focal adhesion kinase (FAK), epidermal growth factor receptor (EGFR), and NF-κB signaling pathways. We review evidence that BRMS1 regulates these vast signaling pathways through chromatin remodeling as a member of mSin3 histone deacetylase complexes.

Identification and characterization of a metastatic suppressor BRMS1L as a target gene of p53

The tumor suppressor p53 and its family members, p63 and p73, play a pivotal role in the cell fate determination in response to diverse upstream signals. As transcription factors, p53 family proteins regulate a number of genes that are involved in cell cycle arrest, apoptosis, senescence, and maintenance of genomic stability. Recent studies revealed that p53 family proteins are important for the regulation of cell invasion and migration. Microarray analysis showed that breast cancer metastasis suppressor 1‐like (BRMS1L) is upregulated by p53 family proteins, specifically p53, TAp63γ, and TAp73β. We identified two responsive elements of p53 family proteins in the first intron and upstream of BRMS1L. These response elements are well conserved among mammals. Functional analysis showed that ectopic expression of BRMS1L inhibited cancer cell invasion and migration; knockdown of BRMS1L by siRNA induced the opposite effect. Importantly, clinical databases revealed that reduced BRMS1L expression correlated with poor prognosis in patients with breast and brain cancer. Together, these results strongly indicate that BRMS1L is one of the mediators downstream of the p53 pathway, and that it inhibits cancer cell invasion and migration, which are essential steps in cancer metastasis. Collectively, our results indicate that BRMS1L is involved in cancer cell invasion and migration, and could be a therapeutic target for cancer.

Breast Cancer Metastasis Suppressor 1 (BRMS1): Robust Biological and Pathological Data, But Still Enigmatic Mechanism of Action

Metastasis requires coordinated expression of multiple genetic cassettes, often via epigenetic regulation of gene transcription. BRMS1 blocks metastasis, but not orthotopic tumor growth in multiple tumor types, presumably via SIN3 chromatin remodeling complexes. Although there is an abundance of strong data supporting BRMS1 as a metastasis suppressor, the mechanistic data directly connecting molecular pathways with inhibition of particular steps in metastasis are not well defined. In this review, the data for BRMS1-mediated metastasis suppression in multiple tumor types are discussed along with the steps in metastasis that are inhibited.

Breast cancer metastasis suppressor 1 (BRMS1) attenuates TGF-β1-induced breast cancer cell aggressiveness through downregulating HIF-1α expression

Background

Cancer metastasis is a multi-step event including epithelial-to-mesenchymal transition (EMT). Breast cancer metastasis suppressor 1 (BRMS1) is a novel metastasis suppressor protein without anti-proliferating activity. However, a detailed underlying mechanism by which BRMS1 attenuates cancer cell EMT and invasion remained to be answered. In the present study, we report an additional mechanism by which BRMS1 attenuates Transforming growth factor-beta1 (TGF-β1)-induced breast cancer cell EMT and invasion.

Methods

Experimental analysis involving chromosome immunoprecipitation (ChIP) and luciferase reporter assays were used to validate hypoxia inducible factor-1alpha (HIF-1α) as a transcriptional regulator of TWIST1 and Snail. Quantitative RT-PCR was used to analyze transcript expression. Immunoblotting and immunofluorescence were used to analyze protein expression. Matrigel-coated in vitro invasion insert was used to analyze cancer cell invasion.

Results

BRMS1 strongly inhibited TGF-β1-induced breast cancer cell EMT and invasion. Unexpectedly, we observed that BRMS1 downregulates not only TWIST1 but also Snail expression, thereby inhibiting breast cancer cell invasion. In addition, we provide evidence that HIF-1α is required for Snail and TWIST1 expression. Further, BRMS1 reduced TGF-β1-induced HIF-1α transcript expression through inactivation of nuclear factor kappaB (NF-κB).

Conclusion

Collectively, the present study demonstrates a mechanical cascade of BRMS1 suppressing cancer cell invasion through downregulating HIF-1α transcript and consequently reducing Snail and TWIST1 expression.

Structural insights into the assembly of the histone deacetylase-associated Sin3L/Rpd3L corepressor complex

Acetylation is correlated with chromatin decondensation and transcriptional activation, but its regulation by histone deacetylase (HDAC)-bearing corepressor complexes is poorly understood. Here, we describe the mechanism of assembly of the mammalian Sin3L/Rpd3L complex facilitated by Sds3, a conserved subunit deemed critical for proper assembly. Sds3 engages a globular, helical region of the HDAC interaction domain (HID) of the scaffolding protein Sin3A through a bipartite motif comprising a helix and an adjacent extended segment. Sds3 dimerizes through not only one of the predicted coiled-coil motifs but also, the segment preceding it, forming an ∼ 150-Å-long antiparallel dimer. Contrary to previous findings in yeast, Sin3A rather than Sds3 functions in recruiting HDAC1 into the complex by engaging the latter through a highly conserved segment adjacent to the helical HID subdomain. In the resulting model for the ternary complex, the two copies of the HDACs are situated distally and dynamically because of a natively unstructured linker connecting the dimerization domain and the Sin3A interaction domain of Sds3; these features contrast with the static organization described previously for the NuRD (nucleosome remodeling and deacetylase) complex. The Sds3 linker features several conserved basic residues that could potentially maintain the complex on chromatin by nonspecific interactions with DNA after initial recruitment by sequence-specific DNA-binding repressors.

BRMS1L suppresses breast cancer metastasis by inducing epigenetic silence of FZD10

BRMS1L (breast cancer metastasis suppressor 1 like, BRMS1-like) is a component of Sin3A-histone deacetylase (HDAC) co-repressor complex that suppresses target gene transcription. Here we show that reduced BRMS1L in breast cancer tissues is associated with metastasis and poor patient survival. Functionally, BRMS1L inhibits breast cancer cells migration and invasion by inhibiting epithelial-mesenchymal transition. These effects are mediated by epigenetic silencing of FZD10, a receptor for Wnt signalling, through HDAC1 recruitment and histone H3K9 deacetylation at the promoter. Consequently, BRMS1L-induced FZD10 silencing inhibits aberrant activation of WNT3-FZD10-β-catenin signalling. Furthermore, BRMS1L is a target of miR-106b and miR-106b upregulation leads to BRMS1L reduction in breast cancer cells. RNA interference-mediated silencing of BRMS1L expression promotes metastasis of breast cancer xenografts in immunocompromised mice, whereas ectopic BRMS1L expression inhibits metastasis. Therefore, BRMS1L provides an epigenetic regulation of Wnt signalling in breast cancer cells and acts as a breast cancer metastasis suppressor.

Metastasis suppressor genes

Metastasis is a major cause of cancer mortality. Metastasis is a complex process that requires the regulation of both metastasis-promoting and metastasis suppressor genes. The discovery of metastasis suppressor genes contributes significantly to our understanding of metastasis mechanisms and provides prognostic markers and therapeutic targets in clinical cancer management. In this review, we summarize the methods that have been used to identify metastasis suppressors and the potential clinical impact of these genes.

Metastasis suppression by BRMS1 associated with SIN3 chromatin remodeling complexes

Epigenetic regulation of gene transcription by histone modification and chromatin remodeling has been linked to many biological and pathological events including cancer metastasis. Breast cancer metastasis suppressor 1 (BRMS1) interacts with SIN3 chromatin remodeling complexes, and, upon forced expression in metastatic cells, a nearly complete suppression of metastasis is noted without preventing primary tumor growth. The data for BRMS1-mediated metastasis suppression and SIN3 interaction are clear; however, connecting the inhibition directly to the association of BRMS1 with SIN3 complexes is currently not well defined. Considering the recent advancements in developing epigenetic drugs for cancer therapy, an improved understanding of how the interactions between BRMS1 and SIN3 regulate the process of metastasis should lead to novel therapies specifically targeting the most deadly aspect of tumor progression. In this article, the data for BRMS1-mediated metastasis suppression are reviewed with a focus on how the SIN3 chromatin remodeling complexes may be functionally involved.

The C-terminal putative nuclear localization sequence of breast cancer metastasis suppressor 1, BRMS1, is necessary for metastasis suppression

Breast cancer metastasis suppressor 1 (BRMS1) is a predominantly nuclear protein that suppresses metastasis in multiple human and murine carcinoma cell lines. BRMS1 interacts with several nuclear proteins including SIN3:HDAC chromatin remodeling complexes that are involved in repressing transcription. However, recent reports suggest BRMS1 may function in the cytoplasm. BRMS1 has two predicted nuclear localization sequences (NLS) that are located near the C-terminus (amino acids 198–205 and 238–244, NLS1 and NLS2 respectively). We hypothesized that nuclear localization sequences of BRMS1 were essential for BRMS1 mediated metastasis suppression. Replacement of NLS2 with NLS1 (BRMS1^NLS1,1), truncation at 238 (BRMS1^ΔNLS2), or switching the location of NLS1 and NLS2 (BRMS1^NLS2,1) did not affect nuclear localization; but, replacement of NLS1 with NLS2 (BRMS1^NLS2,2) or truncation at 197 (BRMS1^ΔNLS which removes both NLS) promoted cytoplasmic localization. MDA-MB-231 human metastatic breast cancer cells transduced with BRMS1^NLS1,1, BRMS1^NLS2,2 or BRMS1^NLS2,1 were evaluated for metastasis suppression in an experimental xenograft mouse model. Interestingly, while NLS2 was not necessary for nuclear localization, it was found to be important for metastasis suppression since BRMS1^NLS2,2 suppressed metastasis by 85%. In contrast, BRMS1^NLS2,1 and BRMS1^NLS1,1 did not significantly suppress metastasis. Both BRMS1 and BRMS1^NLS2,2 co-immunoprecipitated with SIN3A in the nucleus and cytoplasm; however, BRMS1^NLS1,1 and BRMS1^NLS2,1 were associated with SIN3A in the nucleus only. Moreover, BRMS1 and BRMS1^NLS2,2, but not BRMS1^NLS1,1 and BRMS1^NLS2,1, down-regulated the pro-metastatic microRNA, miR-10b. Together, these data demonstrate an important role for NLS2 in the cytoplasm that is critical for metastasis suppression and is distinct from nuclear localization.

Recent advances in breast cancer metastasis suppressor 1

Metastasis is a complex process divided into a number of steps including detachment of tumor cells from the primary tumor, invasion, migration, intravasation, survival in the vasculature, extravasation, and colonization of the secondary site. Proteins that block metastasis without inhibiting primary tumor formation are known as metastasis suppressors; examples are NM23, Maspin, KAI1, KISS1, and MKK4. Breast cancer metastasis suppressor 1 (BRMS1) was identified as a suppressor of breast cancer metastasis in the late 1990s. In vitro and in vivo studies have confirmed that BRMS1 is a potent metastasis suppressor not limited to breast cancer. However, conflicting clinical observations regarding its role as a metastasis suppressor and its validity as a diagnostic biomarker warrant more in-depth clinical study. In this review, the authors provide an overview of its biology, function, action mechanism and pathological significance.

Unraveling the enigmatic complexities of BRMS1-mediated metastasis suppression

Expression of BRMS1 causes dramatic suppression of metastasis in multiple in vivo model systems. As we gain further insight into the biochemical mechanisms of BRMS1, we appreciate the importance of both molecular and cellular context for functional metastasis suppression. BRMS1 associates with large chromatin remodeling complexes including SIN3:HDAC which are powerful epigenetic regulators of gene expression. Additionally, BRMS1 inhibits the activity of NFκB, a well-known transcription factor that plays significant roles in tumor progression. Moreover, BRMS1 coordinately regulates the expression of metastasis-associated microRNA known as metastamir. How these biochemical mechanisms and biological pathways are linked, either directly or indirectly, and the influence of molecular and cellular context, are critical considerations for the discovery of novel therapeutic targets for the most deadly aspect of tumor progression-metastasis.

Metastasis suppressor genes at the interface between the environment and tumor cell growth

The molecular mechanisms and genetic programs required for cancer metastasis are sometimes overlapping, but components are clearly distinct from those promoting growth of a primary tumor. Every sequential, rate-limiting step in the sequence of events leading to metastasis requires coordinated expression of multiple genes, necessary signaling events, and favorable environmental conditions or the ability to escape negative selection pressures. Metastasis suppressors are molecules that inhibit the process of metastasis without preventing growth of the primary tumor. The cellular processes regulated by metastasis suppressors are diverse and function at every step in the metastatic cascade. As we gain knowledge into the molecular mechanisms of metastasis suppressors and cofactors with which they interact, we learn more about the process, including appreciation that some are potential targets for therapy of metastasis, the most lethal aspect of cancer. Until now, metastasis suppressors have been described largely by their function. With greater appreciation of their biochemical mechanisms of action, the importance of context is increasingly recognized especially since tumor cells exist in myriad microenvironments. In this chapter, we assemble the evidence that selected molecules are indeed suppressors of metastasis, collate the data defining the biochemical mechanisms of action, and glean insights regarding how metastasis suppressors regulate tumor cell communication to-from microenvironments.

Homotypic gap junctional communication associated with metastasis suppression increases with PKA activity and is unaffected by PI3K inhibition

Loss of gap junctional intercellular communication (GJIC) between cancer cells is a common characteristic of malignant transformation. This communication is mediated by connexin proteins that make up the functional units of gap junctions. Connexins are highly regulated at the protein level and phosphorylation events play a key role in their trafficking and degradation. The metastasis suppressor breast cancer metastasis suppressor 1 (BRMS1) upregulates GJIC and decreases phosphoinositide-3-kinase (PI3K) signaling. On the basis of these observations, we set out to determine whether there was a link between PI3K and GJIC in tumorigenic and metastatic cell lines. Treatment of cells with the well-known PI3K inhibitor LY294002, and its structural analogue LY303511, which does not inhibit PI3K, increased homotypic GJIC; however, we found the effect to be independent of PI3K/AKT inhibition. We show in multiple cancer cell lines of varying metastatic capability that GJIC can be restored without enforced expression of a connexin gene. In addition, while levels of connexin 43 remained unchanged, its relocalization from the cytosol to the plasma membrane was observed. Both LY294002 and LY303511 increased the activity of protein kinase A (PKA). Moreover, PKA blockade by the small molecule inhibitor H89 decreased the LY294002/LY303511-mediated increase in GJIC. Collectively, our findings show a connection between PKA activity and GJIC mediated by PI3K-independent mechanisms of LY294002 and LY303511. Manipulation of these signaling pathways could prove useful for antimetastatic therapy.

Linking metastasis suppression with metastamiR regulation

Cancer metastasis requires the coordinate expression of multiple genes during every step of the metastatic cascade. Molecules that regulate these genetic programs have the potential to impact metastasis at multiple levels. Breast cancer metastasis suppressor 1 (BRMS1) suppresses metastasis by inhibiting multiple steps in the cascade through regulation of many protein-encoding, metastasis-associated genes as well as metastasis-regulatory microRNA, termed metastamiR. In this Feature , we will highlight connections between BRMS1 biology and regulation of metastamiR.

A shift from nuclear to cytoplasmic breast cancer metastasis suppressor 1 expression is associated with highly proliferative estrogen receptor-negative breast cancers

BACKGROUND/AIMS

To determine breast cancer metastasis suppressor 1 (BRMS1) expression in breast cancers and the efficacy of BRMS1 as a prognostic indicator, BRMS1 expression was assessed in two sets of breast cancer tissues.

METHODS

Epithelial cells from 36 frozen samples of breast cancers and corresponding normal breast were collected by laser capture microdissection and assessed for BRMS1 by quantitative RT-PCR and immunohistochemistry. BRMS1 was also evaluated by immunohistochemistry in a tissue microarray of 209 breast cancers and correlated with indicators of prognosis [estrogen receptor (ER), progesterone receptor (PR), ErbB2, p53, p27(Kip1), Bcl2 and Ki-67].

RESULTS

BRMS1 mRNA and protein were higher in 94 and 81%, respectively, of breast cancers than in corresponding normal epithelium. BRMS1 localization was predominantly nuclear, but 60-70% of cancers also exhibited cytoplasmic immunostaining. Breast cancers with lower nuclear than cytoplasmic BRMS1 (nuclear score - cytoplasmic score < or =0; 11% of cancers) had lower ER, lower PR and higher Ki-67 expression. There was also a trend toward poorer overall survival in this group of cancers, but this was only of borderline significance (p = 0.073). In Cox proportional hazards models, loss of nuclear BRMS1 was not a significant predictor of overall survival.

CONCLUSIONS

Loss of nuclear BRMS1 was associated with ER-negative cancers and a high rate of proliferation, but was not an independent indicator of prognosis.