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Received: 8 June 2016 Accepted: 31 May 2017 Published online: 9 September 2017

SATB2/β-catenin/TCF-LEF pathway induces cellular transformation by generating cancer stem cells in colorectal cancer Wei Yu1, Yiming Ma1, Sharmila Shankar1,2,4 & Rakesh K. Srivastava1,3,4 Recent studies have demonstrated the involvement of colorectal cancer (CRC) stem cells (CSC) in transformation, cancer progression and metastasis. The main goal of this paper was to examine the molecular mechanisms by which SATB2 induced malignant transformation of colorectal epithelial cells. SATB2 induced malignant transformation and these transformed cells gained the characteristics of CSCs by expressing stem cell markers (CD44, CD133, LGR5 and DCLK1) and transcription factors (c-Myc, Nanog and Sox2). Overexpression of SATB2 in normal colorectal epithelial cells increased cell motility, migration and invasion, which were associated with an increase in N-cadherin and Zeb1, and decrease in E-cadherin expression. SATB2 overexpression also upregulated XIAP and cyclin D1, suggesting its role in cell survival and cell cycle. Furthermore, the expression of SATB2 was positively correlated with β-catenin expression in CRC. In contrary, depletion of SATB2 inhibited cell proliferation, colony formation, cell motility and expression of β-catenin, Snail, Slug, Zeb1 and N-cadherin, and upregulated E-cadherin. Furthermore, SATB2 silencing inhibited the expression of stem cell markers, pluripotency maintaining transcription factors, cell cycle and cell proliferation/survival genes and TCF/LEF targets. Finally, β-catenin/TCF-LEF pathway mediated the biological effects of SATB2 in CSCs. These studies support the role of SATB2/β-catenin/TCF-LEF pathway in transformation and carcinogenesis. Colorectal cancer (CRC) is the third most common malignancy worldwide, and accounts for nearly 1 million newly diagnosed cases and half a million deaths each year1. I majority of cases CRC is incurable because of late detection and metastasis2. The current clinical treatment mainly includes surgery, chemotherapy, and targeted therapy, but the disease ultimately relapse and is associated with low 5-year survival3. There is a significant increase in overall survival for metastatic CRC since the late 1990s coinciding with the introduction and dissemination of new treatment3, 4. The colon cancer initiation, progression and metastasis are related to many factors such as genetics, lifestyle, and environmental pollution4–7. Most of the CRC develops through hyperplasia, and adenoma. Mounting evidence exists to suggest that CSCs are capable of inducing malignant transformation leading to cancer progression and metastasis8–11. Since there are no reliable biomarkers for detection of colon cancer, the management of the disease becomes very difficult. Therefore, improved understanding of the molecular mechanisms underlying CRC carcinogenesis are urgently needed. SATB2 (special AT-rich binding protein-2), a transcription factor and epigenetic regulator12, 13, influences gene expression both by modulating chromatin architecture and by functioning as a transcriptional co-factor12, 14–17. SATB2 gene is conserved in humans and mouse. In humans, there are three transcripts which encodes for SATB2 protein. SATB2−/− mice are defective in bone development and osteoblast differentiation15. SATB2 is linked to craniofacial patterning and osteoblast differentiation15, and in development of cortical neurons12, 16–18. SATB2 is over expressed in 85% of CRC tumors, suggesting its use as a diagnostic marker for colon cancer19. The Cancer Genome Atlas (TCGA) data confirmed the overexpression of SATB2 gene in CRC20. In breast cancer, SATB2 1

Kansas City VA Medical Center, 4801 Linwood Boulevard, Kansas City, MO, 66128, USA. 2Department of Pathology, University of Missouri-School of Medicine, Kansas City, MO, 64108, USA. 3Department of Pharmaceutical Sciences, University of Missouri-Kansas City, Kansas City, MO, 64108, USA. 4Present address: Stanley S. Scott Cancer Center, Department of Genetics, Louisiana State University Health Sciences Center, 1700 Tulane Avenue, New Orleans, LA 70112, United States. Correspondence and requests for materials should be addressed to R.K.S. (email: rsrivastava. [email protected]) Scientific Reports | 7: 10939 | DOI:10.1038/s41598-017-05458-y

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www.nature.com/scientificreports/ mRNA expression is significantly associated with increasing tumor grade and poorer survival21. However, the tumor initiating, promoting and metastatic roles of SATB2 in colorectal carcinogenesis have never been examined. The pluripotency maintaining factors (Nanog, Oct4, c-Myc, Sox2 and Klf4) regulate self-renewal and survival of stem cells. By promoter analysis, we have identified the SATB2 binding sites in the promoter regions of Nanog, Oct4, SOX-2 and Klf-4, which suggest that SATB2 can act as a “master regulator” of pluripotency in CSCs. Based on these analysis it appears that SATB2 can also serve as an oncogene to promote colon carcinogenesis. However, the clinicopathological significance of SATB2, and its possible mechanism in CRC tumorigenesis and progression is still unclear. Since SATB2 is not expressed in human normal colon epithelial cells, but highly expressed in transformed cells, CSCs and CRC cell lines, it can be used as a diagnostic biomarker for CRC. During embryonic development Wnt/β-catenin signaling pathway plays a crucial role in regulating cell proliferation and differentiation, whereas in adults it regulates tissue homeostasis and injury repair through generation of stem cells22–24. Wnt ligands activate signaling pathway leading to β-catenin stabilization, nuclear translocation, TCF/LEF transcription and induction of β-catenin/TCF target genes25, 26. The pathway is also activated by loss or mutations of certain genes. Loss of function of the tumor suppressors APC or Axin2 lead to accumulation of nuclear β-catenin, resulting in the formation of intestinal adenomas27–29. Oncogenic point mutations in β-catenin that prevent its degradation also activate this pathway with similar outcomes28, 30. Expression of Wnt inhibitor Dickkopf-1 (DKK1)31, 32 or deletion of genes encoding β-catenin or Tcf4 blocks crypt proliferation33. Some of the targets of TCF/LEF includes pluripotency maintaining factors (c-Myc, Sox-2, Oct-4, Nanog), stem cell marker (CD44), cell cycle and cell survival genes (Cyclin D1 and Survivin), EMT- and metastasis-related genes (Twist, E-cadherin, MMP2, MMP7 and MMP9), and angiogenesis regulator (VEGF)34. However, the regulation Wnt/β-catenin signaling pathway by SATB2 has not been examined. The roles of SATB2 in colon cancer initiation, progression and metastasis are poorly understood. The main goal of the paper is to examine the molecular mechanisms by which SATB2 induces malignant transformation and promotes CRC carcinogenesis. Our data demonstrate that SATB2 is highly expressed in colon cancer cell lines, and primary CRC tissues, and but not in human normal colon epithelial cells and human normal colon, (ii) overexpression of SATB2 can transform human normal colon epithelial cells to progenitor-like cells/CSCs, (iii) inhibition of SATB2 by shRNA attenuates CRC cell proliferation, colony formation and EMT, and (iv) SATB2 expression correlates with nuclear β-catenin expression, and silencing of SATB2 inhibits TCF/LEF activity and its targets. These data suggest that SATB2 can transform normal colon epithelial cells to CSCs/progenitor-like cells which play significant roles in cancer initiation, promotion and metastasis.

Experimental Procedures

Cell culture conditions and reagents.  Colorectal cancer Colon-320, HT-29, HCT-116, and human normal colon epithelial cells (CRL-1831) were purchased from American Type Culture Collection (ATCC), Manassas, VA. Colon cancer cell lines were grown in Dulbecco’s Modified Eagle’s Medium with 10% Fetal Bovine Serum with antibiotics. The CSCs were isolated from primary tumors and grown in well-defined cell culture medium from Celprogen (Torrance, CA). Antibodies against CD44, CD133, Nanog, Oct4, Sox2, c-Myc, Bcl-2, Survivin, XIAP, Cyclin D1, E-cadherin, N-cadherin, Snail, Slug, Zeb1, and β-catenin were purchased from Cell Signaling Technology (Danvers, MA). Antibodies against SATB2 and β-actin were purchased from Abcam (Cambridge, MA). Enhanced chemiluminescence (ECL) Western blot detection reagents were purchased from Amersham Life Sciences Inc. (Arlington Heights, IL). A set of GIPZ lentiviral shRNA plasmids against human SATB2 and scrambled shRNA plasmids were purchased from Open Biosystems (Lafayette, CO). pLKO.1 puro shRNA β-catenin plasmid was from Addgene. T-cell factor/lymphoid enhancer factor (TCF/LEF)-responsive pGreenFire Lenti-Reporter construct was purchased from SBI System Biosciences (Palo Alto, CA). SATB2 cDNA was PCR amplified from human cDNA and cloned into NheI/EcoRI digested pSicoR (Addgene). Matrigel was purchased from BD Bioscience (San Jose, CA). Crystal violet was purchased from Sigma-Aldrich (St. Louis, MO). TRIZOL was purchased from Invitrogen (Grand Island, NY). Luciferase assay kit was purchased from Promega (Madison, WI). Lentiviral particle production and transduction.  The protocol for lentivirus production and trans-

duction have been described elsewhere35, 36. In brief, lentivirus was produced by triple transfection of HEK 293 T cells. Packaging 293 T cells were plated in 10-cm plates at a cell density of 5 × 106 a day prior to transfection in DMEM containing 10% heat-inactivated fetal bovine serum. 293 T cells were transfected with 4 µg of plasmid and 4 µg of lentiviral vector using lipid transfection (Lipofectamine-2000/Plus reagent, Invitrogen) according to the manufacturer’s protocol. Viral supernatants were collected and concentrated by adding PEG-it virus precipitation solution (SBI System Biosciences) to produce virus stocks with titers of 1 × 108 to 1 × 109 infectious units per ml. Viral supernatant was collected for three days by ultracentrifugation and concentrated 100-fold. Titers were determined on 293 T cells. Cells were transduced with lentiviral particles expressing gene of interest.

Motility assay.  We used scratch motility assay to monitor the horizontal movement of cells as described elsewhere37.

Transwell migration assay.  Transwell migration assay was performed as described elsewhere35. Transwell invasion assay.  Transwell invasion assay was performed as described elsewhere35. Western blot analysis.  The western blot analysis was performed as we described earlier38. In brief, cell lysates were subjected to SDS-PAGE, and gels were blotted on nitrocellulose membrane (Amersham Biosciences, Piscataway, NJ, USA). The membranes were blocked with 5% BSA in Tris-Tween buffered saline at 37 °C for 2 h

Scientific Reports | 7: 10939 | DOI:10.1038/s41598-017-05458-y

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www.nature.com/scientificreports/ and then incubated with primary antibody diluted in tris-buffered saline (1:1000 dilutions) overnight at 4 °C, with gentle shaking. The membranes were then washed three times with tris-buffered saline-T (TBS-T) and incubated with secondary antibody linked to horseradish peroxidase (1:5000) for 1 h. After incubation with secondary antibody, the membranes were washed again three times with TBS-T. Finally, protein antibody complexes were detected by the addition of ECL substrate (Thermo Fisher Scientific, Rockford, IL).

Quantitative real-time PCR.  Total RNA was isolated using an RNeasy Mini Kit (Qiagen, Valencia, CA).

Briefly, cDNA was synthesized using a high capacity cDNA reverse transcription kit (Applied Biosystems). Primers specific for each of the signaling molecules were designed using NCBI/Primer-BLAST and used to generate the PCR products. For the quantification of gene amplification, Real-time PCR was performed using an ABI 7300 Sequence Detection System in the presence of SYBR- Green. The sequence of gene-specific primers are given in supplemental information. Target sequences were amplified at 95 °C for 10 min, followed by 40 cycles of 95 °C for 15 s and 60 °C for 1 min. HK-GAPD was used as endogenous normalization control. All assays were performed in triplicate and were calculated on the basis of ΔΔCt method. The fold change in mRNAs expression was determined according to the method of 2−ΔΔCT.

Immunofluorescence and Immunohistochemistry.  For immunofluorescence staining, cells were grown on fibronectin-coated coverslips (Becton Dickinson, Bedford, MA). Cells were fixed with methanol, permeabilized with 1% NP-40, and blocked with 10% BSA, followed by incubating with primary antibody. After washing, cells were incubated with FITC-labeled secondary antibody. Finally, coverslips were washed and mounted using Vectashield (Vector Laboratories, Burlington, CA). Stained slides were examined under a fluorescence microscope. Cells without primary antibody, or with Isotype-specific control IgG, were used as negative controls. Immunohistochemistry of tissues was performed as described elsewhere35. TCF/LEF reporter assay.  Lentiviral particles expressing cop-GFP and luciferase genes (TCF/

LEF-mCMV-EF1-Neo) were prepared as described elsewhere39. Cells were transduced with lentiviral particles. Transduced cells (5–10,000 cells per well) were seeded in 96-well plates for 48 hrs. At the end of incubation period, luciferase reporter activity was measured as per the manufacturer’s instructions (Promega Corp., Madison, WI).

Statistical analysis.  The mean and SD were calculated for each experimental group with replicates. Differences between groups were analyzed by ANOVA, followed by Bonferoni’s multiple comparison tests using PRISM statistical analysis software (GrafPad Software, Inc., San Diego, CA). Significant differences among groups were calculated at P 

TCF-LEF pathway induces cellular transformation by generating cancer stem cells in colorectal cancer.

Recent studies have demonstrated the involvement of colorectal cancer (CRC) stem cells (CSC) in transformation, cancer progression and metastasis. The...
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