New Advances in Breast Cancer Metastasis

“…the formation of distant metastases may be due to the presence of cells within the primary tumor that have the ability to form metastases very early in the disease process…”

Breast cancer is the number one cause of cancer death in women in Europe and North America [1], and mortality is related to metastatic spread of the primary tumor, with approximately 50% of all cancer patients demonstrating evidence of metastasis at first presentation [2]. Metastasis is a multistep process involving both invasion and migration. It begins with migration from the primary tumor site followed by intravasation of the invading tumor cells to vessels of the circulatory systems, from where the surviving cells extravasate into a distant organ establishing a secondary tumor [3,4]. It is suggested that the formation of distant metastases may be due to the presence of cells within the primary tumor that have the ability to form metastases very early in the disease process [5,6] and, recently, it has been demonstrated that the SATB1 gene may play an important role in this process [7].

“Once SATB1 is expressed in breast cancer cells, it coordinates the expression of a large number of genes to induce metastasis.”

SATB1 binds to specialized DNA sequences in the genome [8–10] and functions as a genome organizer by tethering these DNA sites to a functional nuclear architecture that has a cage-like protein distribution surrounding heterochromatin [11]. SATB1 regulate a large number of genes during T-cell development and activation [12,13]. Once SATB1 is expressed in breast cancer cells, it coordinates the expression of a large number of genes to induce metastasis. Removal of SATB1 from aggressive breast cancer cells not only reverses metastatic phenotypes, but also inhibits tumor growth, indicating its key role in breast cancer progression. The prognostic significance of SATB1 was determined by assessing its nuclear staining using tissue micro-arrays containing 1318 breast cancer specimens with known clinical follow-up records. Using the Kaplan–Meier survival analysis, a correlation between higher SATB1 expression levels and shorter overall survival times (p < 0.001) has been identified. This correlation was also observed with all breast cancer types. SATB1 expression in primary tumors serves as a prognostic marker independent of other established prognostic factors for breast cancer, including tumor stage, the histological grade and nodal stage [7].

SATB1's role in breast cancer as a global gene regulator was clearly demonstrated by using a breast cancer cell line (MDA-MB-231) in which silencing of SATB1 induces up- or down-regulation of genes that are associated with poor prognoses in breast tumors. Among the upregulated genes are those known to have important functions in promoting metastasis, including meta-stasin (S100A4) [14] and VEGF-b (VEGFB) [15], which play roles in metastasis and angiogenesis; matrix metalloproteases, which degrade the extracellular matrix (ECM) and promote tumor invasion; TGF-b1 (TGFB1), which stimulates invasion [16]; and CTGF, which mediates angiogenesis and bone metastasis [17,18]. It is worth noting that, SATB1 upregulated genes involved in EGF signaling [19], such as the EGF-receptor subfamily members ErbB1, ErbB2 (HER2 or NEU), ErbB3 and ErbB4, and the ligands neuregulin and amphiregulin. ErbB2, the most oncogenic member of the ErbB family, is an important regulator of breast cancer progression [19], and drugs that intercept ErbB2 signaling are in routine clinical application [20].

We and others have postulated that epithelial mesenchymal transition (EMT) occurs during carcinoma progression [21], particularly at specific stages such as invasion and intravasation, where tumor cells disassemble and migrate to tissue/organ sites distant from the primary tumors. As the epithelial cells lose their polarity and cell–cell junctions, regulated in part by the expression of E-cadherin, they acquire characteristics of mesenchymal cells, which lack stable intercellular junctions [22]. This EMT leads to enhanced motility and invasiveness in many cell types, and is often considered a prerequisite for tumor infiltration and metastasis [21,22]. Of particular importance to this pathological process is that SATB1 could be an important

player that was not previously sought. The first indication is that SATB1 expression induces a marked change in the gene-expression pattern in cancer cells and promotes their acquisition of aggressive phenotypes. SATB1 depletion blocks the upregulation of cell-structure genes typical in invasive breast cancers, consistent with our observations that SATB1 depletion from MDA-MB-231 cells restores normal cell morphology [7]. Such cell structure genes include the ECM protein fibronectin, the intermediate filament protein vimentin and the cell–ECM interacting protein β4 integrin. Dysregulated expression of cadherin and catenins, which mediate cell–cell adhesion, has been associated with breast cancer.

“…SATB1 is an important step in the cascade that initiates EMT and, therefore, a justified target for therapeutic intervention.”

Osteoblast-cadherin, vascular endothelial-cadherin and N-cadherin, often upregulated in invasive breast cancer, were all upregulated by SATB1. By contrast, genes downregulated by SATB1 included claudin 1, a tight junction protein that is commonly lost or mislocalized in invasive tumors; β-catenin, a critical member of the canonical Wnt pathway [23]; and E-cadherin, an adherens junction protein and tumor suppressor [24]. Loss of E-cadherin is a hallmark for epithelial–mesenchymal transition, a process whereby epithelial cells lose polarity, cell–cell contacts and cytoskeletal integrity contributing to the dissemination of carcinoma cells from epithelial tumors [21,25]. On SATB1 depletion, the observed upregulation of E-cadherin and downregulation of fibronectin and repressors of E-cadherin, such as SNAIL and SIP1, indicate that the epithelial–mesenchymal transition process is reversed, resulting in the restoration of acinar-like morphology. Altogether, this novel information clearly indicates that SATB1 is an important step in the cascade that initiates EMT and, therefore, a justified target for therapeutic intervention.

Since SATB1 expression in cancer cells necessarily alters the gene-expression profile to promote tumor growth and metastasis, SATB1 can serve as a sentinel, indicating that cells have acquired the aggressive phenotype. The mechanism by which SATB1 globally reprograms gene expression during metastasis could be by tethering hundreds of gene loci onto its regulatory network, assembling them with chromatin-modifying and transcription factors. SATB1 expression is not restricted to late clinical stages of disease, but is also observed in a subset of primary breast tumors at early clinical stages before lymph node metastasis. The SATB1 protein levels in the nuclei of cancer cells has high prognostic significance, independent of the lymph node status (p < 0.0001), indicating its utility in predicting the likelihood of disease progression in patients with early-stage breast cancer. Silencing SATB1 in those tumors that are highly expresser of this protein may open a new target for cancer therapy.