Background
Secreted protein acidic and rich in cysteine (SPARC), also known as osteonectin and BM-40, is a matricellular protein that mediates cell-matrix interaction [
1,
2]. SPARC plays a role in various physiological processes, including cell adhesion, proliferation, migration, morphogenesis and angiogenesis. It is also involved in processes which require extracellular matrix turnover, such as wound healing and tumor progression [
3]. In recent years, the role of SPARC as a modulator in the pathogenesis of different malignancies has become increasingly evident and its role in tumorigenesis appears to be complex, dependent on cell type and tumor microenvironment [
4]. SPARC has been shown to function as a tumor suppressor in neuroblastomas, as well as in ovarian, lung, breast, pancreatic and nasopharyngeal cancers[
5‐
15]. Moreover, in tumor xenograft models, the growth of pancreatic and lung cancers in SPARC
-/- knockout mice was shown to be significantly enhanced compared with wild-type mice [
16,
17]. One mechanism proposed for the anti-tumorigenic properties of SPARC is due to its ability to enhance apoptosis [
18]. Additionally, the up-regulated expression of SPARC was shown to improve effectiveness of radiotherapy [
19] and chemotherapy [
20,
21] in colorectal cancers. Interestingly, SPARC also has a pro-tumorigenic function linking its expression with poor prognosis in certain human cancers such as melanoma, meningioma and prostate cancer [
22‐
25]. Therefore, more studies are warranted to better delineate the regulation of SPARC and its role in tumor progression.
The modulation of chromatin structure is an essential component in the regulation of both transcriptional activation and repression. Brg-1, one of the ATPase subunits of the SWI/SNF chromatin remodeling complex, plays critical functions in SWI/SNF-mediated transcriptional regulation [
26]. It is well established that Brg-1 or Brg-1-containing SWI/SNF complex is involved in either transcriptional activation or transcriptional repression of a subset of genes. For example, Brg-1 is required for the activation of genes such as CD44 [
27], MMP-2 [
28] and MMP-9 [
29], and is required for the repression of genes such as c-fos [
30] and cyclin D1 [
31]. In addition, Brg-1 has been shown to interact with tumor suppressor p53 [
32,
33] and β-catenin [
34], leading to the transcriptional activation of target genes, as well as tumor suppressor prohibitin [
35,
36], TopBP [
37] and HIC1 [
38] mediating transcriptional repression of target genes. As Brg-1 protein does not contain a sequence-specific DNA binding domain, recruitment of Brg-1 or Brg-1-containing SWI/SNF complex to target promoters requires protein-protein interaction between Brg-1 and other transcription factors or transcription regulators. Previous studies have shown that Brg-1 can be recruited to certain gene promoters via its interaction with transcription factor Sp1 [
39,
40]. Meanwhile, another study demonstrated that Sp1 is bound to the SPARC gene promoter and required for activation of the latter [
41]. Taken together, it is not unreasonable to believe that Brg-1 may play an important role in transcriptional regulation of SPARC gene expression.
Fenretinide, a synthetic retinoid with anti-cancer properties, has been widely studied in chemoprevention clinical trials. Prolonged treatment with this drug does not lead to any induction of point mutations or chromosomal aberrations and shows a favorable toxicity profile compared with other classical retinoic acids [
42,
43]. In rat models of breast cancer, fenretinide selectively accumulates in breast tissue; it is thus particularly active in inhibiting rat mammary carcinogenesis [
43,
44]. Moreover, in clinical trials, fenretinide decreases the occurrence of secondary breast cancers with a 50% risk reduction in women aged 40 years or younger treated with a low maintenance dose of fenretinide [
45]. Furthermore, fenretinide inhibits cell growth through the induction of apoptosis rather than differentiation [
46,
47], an effect that is strikingly different from that of the parental compound all-trans retinoic acid; it shows synergistic response with chemotherapeutic drugs such as cisplatin, carboplatin, etoposide or TRAIL/Apo2L [
48‐
50]. All of these properties make fenretinide an attractive candidate for cancer chemoprevention and chemotherapy [
47,
51]. However, the molecular mechanism responsible for these divergent functions of the fenretinide has not yet been fully defined and deserves further investigation.
In this study, we identified Brg-1 as a critical regulator for the constitutive expression of the SPARC gene in mammary carcinoma cell lines. We described, for the first time, the functional importance of the interaction between Brg-1 and Sp1 when binding to the SPARC promoter. We also reported that fenretinide up-regulates the SPARC gene expression via induction of Brg-1. Finally, our results demonstrated that modulation of SPARC is linked to metastatic cancer cell invasion. Overall, our results reveal a novel regulatory mechanism mediating the expression of SPARC and provide new insights for the understanding of the anti-cancer effects of fenretinide.
Discussion
SPARC is a single-copy gene with a high degree of evolutionary conservation. The mouse SPARC gene is 92% identical to the human homologue. The 5'-proximal flanking region of the SPARC gene displays a well-conserved and characterized GGAGG repeats sequence [
52,
53]. The activity of the human SPARC promoter requires a purine-rich region with GGAGG repeats (within the -120/-70 fragment) in human breast cancer MCF7 cell line, and the transactivation of the SPARC promoter is dependent on the transcription factor Sp1/3 in Drosophila SL2 cells [
64]. Furthermore, it was shown that Sp1/3 is required for constitutive activation of the chicken SPARC promoter (-124/+16), by directly binding to the GGA-rich, -92/-57 fragment [
41]. These results suggest that constitutive transcription of SPARC might be regulated by similar mechanisms in various species. Indeed, our present study demonstrated that Sp1 is bound to the GGAGG repeat region within the mouse SPARC promoter, which is consistent with previous findings in chicken and human SPARC genes. What is more, we found for the first time that Sp1 is essential for the recruitment of Brg-1 to the SPARC promoter (within the -130/-56 fragment) via interaction with each other. We demonstrated that inhibition of Brg-1 significantly reduces the expression of the SPARC gene, and Brg-1 cooperates with Sp1 to enhance the SPARC promoter activity. These results suggest that Sp1 and Brg-1 work together to maintain a constitutive expression level of the SPARC gene. We found that there exist significant differences in the levels of endogenous SPARC mRNA and protein expression as well as secreted SPARC among the three tumor cell lines, with these levels being highest in the non-metastatic 67NR cells, intermediate in the partly metastatic 168 FARN cells and lowest in the highly metastatic 4T1 cells. This could be explained, at least in part, by different expressions of Brg-1 as well as different Brg-1 binding levels at the SPARC promoter among the three cell lines.
Fenretinide has been shown to induce apoptosis leading to inhibition of mammary carcinogenesis [
46,
47]; it has also been shown to reduce the occurrence of secondary breast cancer in women aged 40 years or younger [
45]. Our results demonstrated that fenretinide has the ability to induce the expression of Brg-1, which can explain one of the possible mechanisms responsible for the chemopreventive potential of this drug. It has been reported that a variety of human malignancies are associated with mutations of Brg-1, thus suggesting that Brg-1 may play an important role in tumor suppression [
65]. Brg-1 has been shown to interact with the retinoblastoma tumor suppressor gene product (pRB), and induce cell cycle arrest through the repression of E2F target genes such as cyclin E, cyclin A, and CDC2 [
66‐
68]. In addition, Brg-1 is also required for p53-and BRCA-1-mediated transcriptional activation [
32], as well as tumor suppressor prohibitin- and HIC1-mediated transcriptional repression. Brg-1 heterozygous mice display higher susceptibility to mammary tumors [
69], while complete loss of Brg-1 enhances lung cancer development [
70]. Furthermore, Brg-1 has been demonstrated to be silenced or mutated in various human tumor cell lines derived from breast, ovarian, lung, brain and colon cancers [
71], and the loss of Brg-1 expression is associated with a poor prognosis in lung cancer patients [
72]. Another study showed that Brg-1 expression is also lost in 70.6% of established neuroendocrine carcinomas of the cervix [
73]. The findings that Brg-1 is frequent lost in primary and metastatic melanomas and it interacts with the melanoma-associated tumor suppressor p16
INK4a imply an important role for Brg-1 in melanoma [
74]. All these data suggest that Brg-1 may function as a tumor suppressor. Therefore, the anti-cancer effects of fenretinide might be partly due to its ability to induce Brg-1 expression. The induction of Brg-1 expression in response to fenretinide and its enhancing effect on apoptosis and tumor suppression are certainly worthy of more extensive studies.
Besides Brg-1, we also found that SPARC expression and secretion as well as SPARC promoter-driven transcriptional activity were induced by fenretinide in tumor cells. Moreover, our results revealed that knockdown of Brg-1 inhibited the fenretinide-induced SPARC expression and SPARC promoter-driven transcriptional activity. Together with these results, it is suggested that fenretinide up-regulates the SPARC transcription via the induction of Brg-1 expression. SPARC is an important regulator of cell growth and malignancy with complex biological effects that are cell- and tumor-type specific. For example, in certain types of cancers, such as melanomas and gliomas, SPARC is associated with a highly aggressive tumor phenotype, while in others, mainly in neuroblastomas, as well as in ovarian and colorectal cancers, SPARC functions as a tumor suppressor [
18]. The role of SPARC in the development and progression of breast cancer is still not fully elucidated. Dhanesuan et al. [
9] revealed that SPARC can inhibit breast cancer cell proliferation. A report using human MDA-MB231 breast cancer cells demonstrated that overexpression of SPARC inhibited the metastatic capacity of these cells to different organs, including lungs and bones [
11]. Wong and colleagues recently reported that 60% of patients with low SPARC expression had metastases within 5 years of diagnosis, while only 33% of patients with high SPARC-expression developed metastasis in the same period [
75]. Another recent study also revealed that down-regulation of SPARC is correlated with poor prognosis in breast cancer patients[
76]. Therefore, these results seem to support the anti-tumorigenic role of SPARC. In this study, we detected the SPARC expression level and its secretion into milieu using three different mammary carcinoma cell lines with different levels of metastatic potential: highly metastatic 4T1, moderately metastatic 168 FARN and non-metastatic 67NR cell lines. The results showed that both SPARC gene expression and secretion levels were negatively associated with the metastatic capacity of tumor cells. We also found that the invasion activity in 4T1 cells treated with fenretinide was significantly decreased compared with untreated cells. Cancer cells treated with a SPARC antibody resulted in an abrogation of fenretinide-induced decrease in cell invasion. These results suggest that fenretinide is able to induce the expression of SPARC gene, and as a consequence, to inhibit cancer cell invasion.
Materials and methods
Cells and cell culture
Mammary tumor cell lines, 4T1, 168FARN and 67NR, were generously provided by Dr. F. Miller (Barbara Ann Karmanos Cancer Institute, MI, USA), and maintained in DMEM supplemented with 7% fetal bovine serum and 1% streptomycin-penicillin, and incubated in a humidified atmosphere containing 5% CO2 at 37°C.
Plasmid constructs and reagents
pREP4-luc was constructed as described previously [
77]. pREP-SP-luc was constructed by inserting the PCR-amplified SPARC promoter (spanning nucleotides -201/+19 of SPARC gene), using the forward primer (5'-GAGCTAGCTGTCTGGGT AGCACACAGCCTAC-3') and reverse primer (5'-CAAAGCTTCTGAAGGGCTGC AGGAATGTG-3'), into the
Nhe I-
Hin dIII sites of pREP4-luc. Expression plasmid encoding human Brg-1 (pBJ5 BRG1) and the ATPase-defective variant of Brg-1 (pBJ5 BRG1 DN, K798R mutant) [
78] were obtained from AddGene (Cambridge, MA, USA), and human Brg-1 was shown to be expressed correctly and work properly in mouse cells and in frog oocytes [
79,
80]. Fenretinide (2,4,6,8-Nonatetraenamide, N-(4-hydroxyphenyl)-3,7-dimethyl-9-(2,6,6-trimethyl-1-cyclohexen-1-yl-(all-E); 374551) powder was generously provided by Dr. R. Smith (NIH, Bethesda, Maryland, USA).
Western blot analysis
After appropriate treatments, cells were collected and total cellular extracts or nuclear fractions were prepared. Western blot analysis was then performed as described previously [
81]. A monoclonal antibody against β-actin (Sigma, Saint Louis, MO, USA) was used at a 1:5,000 dilution. A monoclonal antibody against SPARC (R&D systems, Minneapolis, MN, USA) was reconstituted at a concentration of 500 μg/ml and used at a 1:2,500 dilution. A rabbit polyclonal antibody against Brg-1 (sc10768×, Santa Cruz Biotechnology, Santa Cruz, CA, USA) or Sp1 (sc14027×, Santa Cruz Biotechnology) was used at a 1:4,000 dilution.
Immobilized-template assays
Immobilized-template assays were performed as described previously [
82]. Two hundred micrograms of Dynabeads M280 streptavidin (Dynal) was prepared, concentrated and resuspended in 20 μl of buffer T (10 mM Tris [pH 7.5], 1 mM EDTA, 1 M NaCl) including 10 pmol of biotinylated GGAGG-rich-containing either probe
a (spanning nucleotides -130/-56 of the
SPARC gene [gene bank accession #M20683]) or probe
b (spanning nucleotides -50/+19 of the
SPARC gene). The mixture was gently agitated for 1 hr at room temperature (RT) and the beads were then washed 4 times in buffer T to remove unbound probes. Bead-coupled probes were equilibrated in buffer R (10 mM Tris [pH 7.5], 1 mM MgCl
2, 0.1% NP-40, 1 mM EDTA, 10 mM DTT, 5% glycerol, 60 mM KCl, 12 mM HEPES [pH 7.9], 0.03% BSA) for 30 min, centrifuged and resuspended in buffer R containing 200 μg of nuclear extract and 40 ng/μl of poly (dG-dC) (120 μl final volume), and agitated for 30 min at RT. After binding reaction, the beads were washed three times using buffer R containing 10 ng/μl of poly (dG-dC). The bound proteins were eluted by boiling them in SDS sample buffer, and the presence of Brg-1, Sp1 and p38 were detected by Western blot analysis.
Chromatin Immunoprecipitation (ChIP) assay, Re-ChIP and ChIP-qPCR
The ChIP assay was performed using a chromatin immunoprecipitation assay kit (Upstate, Lake Placid, NY) according to the manufacturer's instructions. Cells were fixed with 1% formaldehyde for 10 min, washed with ice-cold PBS containing protease inhibitors and lysed with SDS lysis buffer. The lysate was sonicated to yield DNA fragments between 300 and 1000 base pairs, and centrifuged at 13,000 rpm for 10 min. The supernatant was diluted and pre-cleared with salmon sperm DNA/protein A agarose. Immunoprecipitation was performed overnight at 4°C using either non-specific IgG or the antibodies against Brg-1 or Sp1. The immunoprecipitates were washed and eluted, and the cross-links were reversed. The precipitated DNA fragments were purified. The 5'-promoter region spanning nucleotide positions -201 to -23 from the transcription start site of the SPARC gene were amplified by PCR using 5'-TGTCTGGGTAGCACACAGCCTAC-3' and 5'-GCAGGAAGCCTCTT GGAGCTCT-3' primers. Re-ChIP assays utilized a similar protocol, except that the primary immunocomplex obtained with the Sp1 antibody was eluted by 10 mM dithiothreitol with agitation at 37°C for 30 min. The eluate was diluted 50 times with buffer (20 mM Tris-HCl, pH 8.1, 150 mM NaCl, 2 mM EDTA, and 1% Triton X-100) and immunoprecipitated using the second antibodies. For ChIP-qPCR assay, the precipitated DNA fragments were purified and quantified with the Quant-iT™ dsDNA Assay Kit (Molecular Probes, Eugene, Oregon) and were then amplified by real-time qPCR using the same primers as for regular PCR.
Quantitative real-time PCR
Total RNA was extracted from cells using TRIzol Reagent (Invitrogen, Burlington, ON, Canada) according to the manufacturer's instructions. One μg of total RNA was reverse-transcribed with the QuantiTect reverse transcription kit (Qiagen, Mississauga, ON, Canada). An equal amount of cDNA or purified DNA fragment from ChIP was then amplified by real-time PCR using the Stratagene Mx-4000 and Brilliant SYBR Green QPCR Master Mix. Gene expression was normalized to a house-keeping gene (GAPDH) and the relative expression values between the samples were calculated based on the threshold cycle (C
T) value using the 2
-ΔΔCT method [
83]. The following primers were used for cDNA amplification: Brg-1, 5'-TCTGAGGTGGACGCCCGACACATTA-3' (forward) and 5'-TAAGGACCTGC GTCAACTTGCAGTG-3' (reverse); and SPARC, 5'-AGGTGTGTGAGCTGCACG AGA-3' (forward) and 5'-GAAGTGGCAGGA AGAGTCGAA-3'(reverse).
Small RNA interference experiment
The transfection of siRNA into 4T1, 168FARN or 67NR cells was performed in 6-well plates using the Lipofectamine™ RNAiMAX (Invitrogen, Burlington, ON), according to the manufacturer's instructions. One day before transfection, cells were seeded at an appropriate density to give 40~50% confluence at the time of transfection. The siRNAs against Brg-1, Sp1 and control siRNA were purchased from Santa Cruz Biotechnology, Inc. (Santa Cruz, CA). Cells were harvested for assays 48 hrs or 72 hrs after transfection with these siRNAs. To assess luciferase activity, the cells were transfected 24 hrs after siRNA transfection with 1.8 μg of the luciferase reporter constructs and 100 ng of Renilla luciferase control vector (pRL-CMV). 24 hrs later, the cells were treated with fenretinide for another 24 hrs and luciferase activity was measured.
Immunoprecipitation assays
Cells were washed once with ice-cold PBS and lysed in EBMK/0.1% NP-40 buffer (25 mM HEPES, pH 7.6, 5 mM MgCl2, 1.5 mM KCl, 75 mM NaCl, 175 mM sucrose, 0.1% NP-40 and protease inhibitors) on ice for 10 min. The nuclear pellet was collected by centrifugation at 500 × g for 4 min. and washed three times with EBMK buffer (no NP-40). The nuclei were then lysed in 1 ml of RIPA buffer containing protease inhibitors, passed repeatedly through a 22-gauge needle and centrifuged at 10,000 × g for 30 min. The supernatants were pre-cleared with protein A/G agarose for 30 min. Immunoprecipitation was performed overnight at 4°C using the antibody against Brg-1 or Sp1. To precipitate the antigen-antibody complex, protein A/G agarose was added and incubated for 1 hr at 4°C. After washing with RIPA buffer, the precipitated proteins were eluted by boiling in 2× SDS sample buffer and analyzed by immunoblotting using antibodies to Brg-1 or Sp1.
Luciferase activity
Transient transfections of 4T1, 168FARN or 67NR cells were performed using the Lipofectamine™ 2000 and Plus reagent (Invitrogen, Burlington, ON) according to manufacturer's instructions. Briefly, cells were seeded into 12-well plates one day before transfection at a density of 5 × 104 cells (4T1 or 168FARN) or 1 × 105 cells (67NR) per well. Cells were transfected with 1.8 μg luciferase reporter constructs and 100 ng of Renilla luciferase control vector (pRL-CMV). 24 hrs after transfection, cells were treated with fenretinide or left untreated for 24 hrs before harvest. Luciferase reporter assays were performed using the Dual-Luciferase® Reporter Assay System (Promega, Madison, WI) and the luminescence measurements were done with a Turner Designs model TD-20/20 luminometer. Firefly luciferase activity was normalized to Renilla luciferase activity. Each transfection was done in triplicate and repeated three times.
Measuring secreted SPARC
Following the treatment of cells with siRNA or fenretinide, cell media were collected and centrifuged to remove cell debris. Equal volumes (15 μl) of supernatant were used for Western blot analysis. Densitometric quantitation was performed, and to correct for any potential discrepancy in cell number resulting from different experimental conditions, the densitometric results were adjusted for total protein contents of cell lysates.
In vitro wound healing assay
Cells were seeded in 24-well culture plates at 1.5 × 10
5 cells/well. After 18 hrs, the cells were left untreated or pretreated with 2.5 μM or 5.0 μM fenretinide for 6 hrs before wound formation. The
in vitro 'scratch' wounds were created by scraping the confluent cell monolayer with a 200 μl pipette tip and cultures were then washed twice with PBS to remove floating cells. Cells were then cultured in fresh medium or medium containing 2.5 μM or 5.0 μM fenretinide alone or combined with 8 μg/ml goat nonspecific IgG or SPARC antibody (R&D systems). The plates were photographed at 0 hr and 18 hrs after treatment. The wound width was measured using the program Image J
http://rsbweb.nih.gov/ij/ between two certain points on either side of the gap. For proper statistical evaluation, at least three measurements at different points were performed at each image. The wound width at the 18-hr time point was subtracted from that at the 0-hr time point. The distance was normalized to the wound width at 0 hr. The values were then expressed as relative motility, setting the cell motility of untreated cells as 100%. Three independent experiments were done in triplicates.
Invasion assay
Cell invasion ability was assessed using a cell invasion assay kit (Chemicon International, Temecula, CA, USA) according to the manufacturer's instructions. The assay was performed in an invasion chamber, which consists of a 24-well tissue culture plate containing 12 cell culture inserts. The inserts contain an 8-μm pore size polycarbonate membrane coated on the upper side with a thin layer of ECMatrix™. Cells were pretreated with 2.5 μM or 5.0 μM fenretinide or left untreated for 6 hrs and were then collected and counted. 5 × 104 untreated cells were resuspended in 0.3 ml of serum-free medium and pretreated cells were resuspended in 0.3 ml of serum-free medium containing 2.5 μM or 5.0 μM fenretinide only or combined with goat nonspecific IgG (5 μg/ml) or SPARC antibody (5 μg/ml). The lower chamber of the plate was filled with 0.5 ml medium containing 10% FBS with or without fenretinide. The cell suspension was then placed in the upper chamber and incubated at 37°C for 24 hrs. The noninvasive cells on the upper side of the membrane were removed. The invasive cells on the lower surface of the membrane were stained and then lysed. Absorbance was measured with a microplate reader at 560 nm. Each experiment was repeated three times, and the data represent the mean ± SEM of three determinations.
Statistical analysis
All data are presented as means ± SEM of three or four experiments. Analysis was performed using unpaired Student's t test. P < 0.05 was considered significant.
Acknowledgements
We thank Dr. F. Miller, Dr. R. Smith and Dr. G.R. Crabtree for kindly providing the three mammary carcinoma cell lines (4T1, 168 FARN and 67NR), fenretinide and plasmids (pBJ BRG1 and pBJ BRG1 DN), respectively. We are grateful to James Li for help with the wound healing assay, to Gabriella Wojewodka for a critical review of the manuscript. This work was supported by the Canadian Institute of Health Research and NSERC grants (to DR), and Fonds de la recherche en santé du Québec (17734, to YZX).
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
YZX designed the study, performed plasmid construction, ChIP, co-immunoprecipitation, cell transfection, luciferase reporter analysis, cell migration and invasion assays, data analysis as well as prepared the draft version of the manuscript. MH and TT performed cell culture, RNA expression analysis, Western blot analysis, statistical analysis and helped YZX in most experiments. SDM was involved in the design of the study, and specifically performed the immobilized-template assays. TM supervised the study and contributed to the manuscript preparation. DR designed and coordinated the study, and revised the manuscript. All authors have read and approved the final manuscript.