A nuclear address with influence

SATB family chromatin organizers as master regulators of tumor progression

SATB (Special AT-rich binding protein) family proteins have emerged as key regulators that integrate higher-order chromatin organization with the regulation of gene expression. Studies over the past decade have elucidated the specific roles of SATB1 and SATB2, two closely related members of this family, in cancer progression. SATB family chromatin organizers play diverse and important roles in regulating the dynamic equilibrium of apoptosis, cell invasion, metastasis, proliferation, angiogenesis, and immune modulation. This review highlights cellular and molecular events governed by SATB1 influencing the structural organization of chromatin and interacting with several co-activators and co-repressors of transcription towards tumor progression. SATB1 expression across tumor cell types generates cellular and molecular heterogeneity culminating in tumor relapse and metastasis. SATB1 exhibits dynamic expression within intratumoral cell types regulated by the tumor microenvironment, which culminates towards tumor progression. Recent studies suggested that cell-specific expression of SATB1 across tumor recruited dendritic cells (DC), cytotoxic T lymphocytes (CTL), T regulatory cells (Tregs) and tumor epithelial cells along with tumor microenvironment act as primary determinants of tumor progression and tumor inflammation. In contrast, SATB2 is differentially expressed in an array of cancer types and is involved in tumorigenesis. Survival analysis for patients across an array of cancer types correlated with expression of SATB family chromatin organizers suggested tissue-specific expression of SATB1 and SATB2 contributing to disease prognosis. In this context, it is pertinent to understand molecular players, cellular pathways, genetic and epigenetic mechanisms governed by cell types within tumors regulated by SATB proteins. We propose that patient survival analysis based on the expression profile of SATB chromatin organizers would facilitate their unequivocal establishment as prognostic markers and therapeutic targets for cancer therapy.

miR-23a suppresses proliferation of osteosarcoma cells by targeting SATB1

Accumulating evidence has shown that microRNAs are involved in multiple processes in cancer development and progression. Recent studies have shown that miR-23a functions as an oncogene in various human cancer types, but its role in osteosarcoma remains poorly understood. Here, we demonstrated that miR-23a is frequently downregulated in osteosarcoma specimens and cell lines compared with adjacent noncancerous tissues and cell line. Bioinformatics analysis further revealed SATB1 as a potential target of miR-23a. Data from luciferase reporter assays showed that miR-23a directly binds to the 3'UTR of SATB1 messenger RNA (mRNA). Furthermore, we found that expression patterns of miR-23a were inversely correlated with those of SATB1 in osteosarcoma tissues and cell lines, and overexpression of miR-23a suppressed SATB1 expression at both transcriptional and translational levels in osteosarcoma cell lines. In functional assays, miR-23a inhibited osteosarcoma cell proliferation, which could be reversed by overexpression of SATB1. Furthermore, knockdown of SATB1 reduced osteosarcoma cell proliferation, which resembled the inhibitory effects of miR-23a overexpression. Taken together, our data provide compelling evidence that miR-23a functions as a tumor suppressor in osteosarcoma, and its inhibitory effect on tumor are mediated chiefly through downregulation of SATB1.

Silencing SATB1 inhibits the malignant phenotype and increases sensitivity of human osteosarcoma U2OS cells to arsenic trioxide

In a previous study, we found that the global genome organizer Special AT-rich binding protein 1 (SATB1) is highly expressed in mesenchymal-derived human osteosarcoma U2OS cells and that the knock-down of SATB1 results in the inhibition of cell proliferation. The present study was aimed at investigating the effect of silencing SATB1 on cell migration, invasion, apoptosis and resistance to the chemotherapeutic drug arsenic trioxide. Cell migration and invasion were detected by wound-healing assays and trans-well invasion assays, respectively. Cell apoptosis was analyzed by an in situ Cell Death Detection POD Kit, based on terminal deoxynucleotydyl transferase mediated dUTP nick-end labeling (TUNEL) staining and mRNAs were analyzed by real time qRT-PCR. We found that cell migration and invasion were inhibited and that the proportion of apoptotic cells and sensitivities to the chemotherapeutic drug arsenic trioxide were enhanced by knockdown of SATB1 in U2OS cells. Furthermore, mRNA of ABCC1 and ABCG2 were decreased strikingly after SATB1 silencing. It was concluded that the elevated expression of SATB1 in U2OS cells contributes to maintenance of the malignant phenotype and resistance to chemotherapeutic drugs ATO, suggesting that silencing SATB1 in the cells might improve the effects of arsenic trioxides in the treatment of osteosarcoma in which SATB1 is over-expressed and that ABCC1 and ABCG2 were involved in SATB1 mediated resistance of U2OS cells to ATO.

Silencing SATB1 inhibits proliferation of human osteosarcoma U2OS cells

It has been shown that over-expression of Special AT-rich binding protein 1 (SATB1) in breast cancer predicts a poor prognosis. This study was aimed at investigating the effects of silencing SATB1 on mesenchymal derived human osteosarcoma U2OS cells and the underlying mechanisms. The expressions of SATB1 and the related genes in the cells were detected by qRT-PCR and/or Western Blotting. SATB1 silencing was achieved by stable transfection with the vectors expressing small hairpin RNA versus SATB1. Cell proliferation was detected in a microplate reader with Cell Counting Kit-8 and the cell cycle was analyzed by flow cytometry using a cell cycle detection kit. The study found that SATB1 was particularly over-expressed in human osteosarcoma U2OS. Silencing SATB1 inhibited the proliferation of U2OS. It was found that inhibition of cell proliferation resulted from cell cycle arrest due to down-regulated expression of CFGF and JunB. The over-expression of SATB1 is responsible for abnormal proliferation of mesenchymal derived human Osteosatcoma U2OS cells, indicating that silencing SATB1 expression in the cells might be developed as an efficient osteosarcoma therapy. CTGF and JunB were involved in SATB1-mediated proliferation of U2OS cells.

SUMO conjugation to the matrix attachment region-binding protein, special AT-rich sequence-binding protein-1 (SATB1), targets SATB1 to promyelocytic nuclear bodies where it undergoes caspase cleavage

SATB1 (special AT-rich sequence-binding protein-1) provides a key link between DNA loop organization, chromatin modification/remodeling, and association of transcription factors at matrix attachment regions (MARs). To investigate the role of SATB1 in cellular events, we performed a yeast two-hybrid screen that identified SUMO-1, Ubc9, and protein inhibitor of activated STAT (PIAS) family members as SATB1 interaction partners. These proteins, working in concert, enhanced SUMO conjugation to lysine-744 of SATB1. Overexpression of SUMO or PIAS in Jurkat cells, which express high levels of endogenous SATB1, exhibited enhanced caspase cleavage of this MAR-associating protein. Sumoylation-deficient SATB1 (SATB1(K744R)) failed to display the characteristic caspase cleavage pattern; however, fusion of SUMO in-frame to SATB1(K744R) restored cleavage. A SUMO-independent interaction of inactive caspase-6 and SATB1 was noted. A subset of total cellular SATB1 localized into promyelocytic leukemia nuclear bodies where enhanced SATB1 cleavage was detected subsequent to caspase activation. These results reveal a novel sumoylation-directed caspase cleavage of this key regulatory molecule. The role of regulated proteolysis of SATB1 may be to control transcription in immune cells during normal cell functions or to assist in efficient and rapid clearance of nonfunctional or potentially damaging immune cells.

Gelsolin gene silencing involving unusual hypersensitivities to dimethylsulfate and KMnO4 in vivo footprinting on its promoter region

We previously reported that gelsolin gene expression is reduced in various tumors. In an effort to gain further insights into the mechanism of gelsolin downregulation in tumors, we examined the in vivo properties of the gelsolin promoter in urinary bladder cancer cell lines. Neither mutation nor hypermethylation was responsible for gene silencing at the promoter. After exposure to trichostatin A (TSA), a histone deacetylase inhibitor, gelsolin promoter activity was markedly enhanced in the cancer cells, not in cells derived from normal tissue. Chromatin immunoprecipitation assays revealed that both histones H3 and H4 were hypoacetylated in the promoter region of the cancer cells, and the accumulation of acetylated histones was detected by TSA treatment. In vivo footprinting analysis revealed the presence of dimethylsulfate (DMS) hypersensitive site in the untranslated region around nucleotide--35 only in the cancer cells but not in cells derived from normal tissue, and analysis of KMnO4 reactive nucleotides showed that the stem loop structure could be formed in vivo of the cancer cells. This novel stem loop structure may play a part in regulating the transcription of the gelsolin gene in the cancer cells. These results suggest that nucleosome accessibility through histone deacetylation and structural changes (DMS hypersensitivity and stem loop structure) in the promoter region form the basis of the mechanism leading to the silencing of gelsolin gene in human bladder cancer.

MINT, the Msx2 interacting nuclear matrix target, enhances Runx2-dependent activation of the osteocalcin fibroblast growth factor response element

Msx2 promotes osteogenic lineage allocation from mesenchymal progenitors but inhibits terminal differentiation demarcated by osteocalcin (OC) gene expression. Msx2 inhibits OC expression by targeting the fibroblast growth factor responsive element (OCFRE), a 42-bp DNA domain in the OC gene bound by the Msx2 interacting nuclear target protein (MINT) and Runx2/Cbfa1. To better understand Msx2 regulation of the OCFRE, we have studied functional interactions between MINT and Runx2, a master regulator of osteoblast differentiation. In MC3T3E1 osteoblasts (with endogenous Runx2 and FGFR2), MINT augments transcription driven by the OCFRE that is further enhanced by FGF2 treatment. OCFRE regulation can be reconstituted in the naïve CV1 fibroblast cell background. In CV1 cells, MINT synergizes with Runx2 to enhance OCFRE activity in the presence of activated FGFR2. The RNA recognition motif domain of MINT (which binds the OCFRE) is required. Runx2 structural studies reveal that synergy with MINT uniquely requires Runx2 activation domain 3. In confocal immunofluorescence microscopy, MINT adopts a reticular nuclear matrix distribution that overlaps transcriptionally active osteoblast chromatin, extensively co-localizing with the phosphorylated RNA polymerase II meshwork. MINT only partially co-localizes with Runx2; however, co-localization is enhanced 2.5-fold by FGF2 stimulation. Msx2 abrogates Runx2-MINT OCFRE activation, and MINT-directed RNA interference reduces endogenous OC expression. In chromatin immunoprecipitation assays, Msx2 selectively inhibits Runx2 binding to OC chromatin. Thus, MINT enhances Runx2 activation of multiprotein complexes assembled by the OCFRE. Msx2 targets this complex as a mechanism of transcriptional inhibition. In osteoblasts, MINT may serve as a nuclear matrix platform that organizes and integrates osteogenic transcriptional responses.