Oleanane triterpenoid CDDO-Me induces apoptosis in multidrug resistant osteosarcoma cells through inhibition of Stat3 pathway
© Ryu et al. 2010
Received: 26 August 2009
Accepted: 10 May 2010
Published: 10 May 2010
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© Ryu et al. 2010
Received: 26 August 2009
Accepted: 10 May 2010
Published: 10 May 2010
The activation of signal transducer and activator of transcription 3 (Stat3) pathway correlates with tumor growth, survival, drug resistance and poor prognosis in osteosarcoma. To explore the potential therapeutic values of this pathway, we assessed both the expression and the activation of Stat3 pathway in several pairs of multidrug resistant (MDR) osteosarcoma cell lines, and tissues. To explore the potential therapeutic values of this pathway, we analyzed the ability of the synthetic oleanane triterpenoid, C-28 methyl ester of 2-cyano-3,12-dioxoolen-1,9-dien-28-oic acid (CDDO-Me), to inhibit Stat3 expression and activation as well as its effects on doxorubicin sensitivity in osteosarcoma cells.
Expression of Stat3, phosphorylated Stat3 (pStat3) and Stat3 targeted proteins, including Bcl-XL, Survivin and MCL-1 were determined in drug sensitive and MDR osteosarcoma cell lines and tissues by Western blot analysis. The effect of CDDO-Me on osteosarcoma cell growth was evaluated by MTT and apoptosis by PARP cleavage assay and caspase-3/7 activity.
Stat3 pathway was activated in osteosarcoma tissues and in MDR cell lines. CDDO-Me inhibited growth and induced apoptosis in osteosarcoma cell lines. Treatment with CDDO-Me significantly decreased the level of nuclear translocation and phosphorylation of Stat3. The inhibition of Stat3 pathway correlated with the suppression of the anti-apoptotic Stat3 targeted genes Bcl-XL, survivin, and MCL-1. Furthermore, CDDO-Me increased the cytotoxic effects of doxorubicin in the MDR osteosarcoma cell lines.
Stat3 pathway is overexpressed in MDR osteosarcoma cells. CDDO-Me significantly inhibited Stat3 phosphorylation, Stat3 nuclear translocation and induced apoptosis in osteosarcoma. This study provides the framework for the clinical evaluation of CDDO-Me, either as monotherapy or perhaps even more effectively in combination with doxorubicin to treat osteosarcoma and overcome drug resistance.
Osteosarcoma is the most common malignant tumor of bone, which mainly affects children and adolescents [1, 2]. Current treatment of osteosarcoma consists of multi agent chemotherapy and surgical resection. The advancement in intensive chemotherapy has significantly improved the 5-year survival rate from 10% with surgery alone to approximately 60-70% when combined with chemotherapy [1–4]. Even so, for 20 years, survival rate has not changed and nearly 30-40% of the patients still experience local recurrence or metastasis, possibly because of development of multidrug resistance (MDR). Overcoming drug resistance is one approach to improve the survival rate of osteosarcoma patients. The development of drug resistance is associated with many events, such as activation of transcription factors, overexpression of antiapoptotic proteins, and overexpression of multidrug resistance gene 1 (MDR1) [5–8]. Successful management of osteosarcoma might be greatly aided by the use of novel agents that could overcome drug resistance.
Signal Transducer and Activator of Transcription 3 (Stat3) is one of the transcription factors that play an important role in tumor cell growth, survival, proliferation, differentiation, apoptosis, metastasis, angiogenesis and drug resistance [9–17]. Stat3 is activated (phosphorylated) by Janus-activated kinase (JAK)-1 or JAK-2 in response to interleukin-6 (IL-6) family of cytokines and growth factors . Stat3 then forms homodimers that translocate to the cell nucleus and binds to promoters of target genes, activating oncogenes such as c-myc and cyclin D, and antiapoptotic proteins . Constitutive activation of Stat3 pathway has been found in many cancer cells including osteosarcoma [9–11, 13, 14, 16, 17, 20]. Furthermore, constitutively activated Stat3 pathway correlates with malignant tumor phenotype, resistance to chemotherapeutic drugs, and poor prognosis in some cancers [8, 15, 17, 21–27].
Several reports have shown inhibition of Stat3 pathways in cancer cells, resulting in a dramatic increase of apoptosis [13, 17, 20, 28–32]. The novel synthetic oleanane triterpenoid, C-28 methyl ester of 2-cyano-3,12-dioxoolen-1,9-dien-28-oic acid (CDDO-Me), is a promising new class of agents for the prevention and treatment of cancer [33–35]. When CDDO-Me is applied at low concentrations, it demonstrated a variety of anti-inflammatory effects. At higher concentrations, the compound inhibits cancer cell growth and proliferation in a wide variety of cell lines including ovarian, cervical, breast, liver, leukemia, and lung cancer [17, 32–38]. CDDO-Me is currently in phase I/II clinical trials for cancer treatment [36–39]. CDDO-Me-induced apoptosis is associated with the activation of caspase 3 and 8, cytochrome c, SOCS-1, and SHP-1, and inhibition of NF-kB, Cox2 and VEGF [32, 33, 35, 40]. To date, the effect of CDDO-Me on MDR osteosarcoma cells is unclear.
In this study, we investigated the molecular mechanism of CDDO-Me induced apoptosis and the effects of combinations of CDDO-Me with doxorubicin which is known to have established activity in the clinical therapy for osteosarcoma.
Human osteosarcoma cells KHOS, U-2OS, SaOS were obtained from the American Type Tissue Collection (Rockville, MD) and a normal human hipbone osteoblast cell line HOB-c was obtained from the Promo Cell GmbH (Heidelberg, Germany). The MDR cell line U-2OSTR was established as previously reported [1, 2]. Briefly, the U-2OS cell lines were selected over a period of 8 months by continuous culturing in medium containing step-wise increases in drugs. Dr. Efstathios Gonos (Institute of Biological Research and Biotechnology, Athens, Greece) kindly provided the MDR KHOSR2 cell line . In addition, osteosarcoma tissue samples were obtained from the Massachusetts General Hospital Sarcoma Tissue Bank. Surgically treated patients diagnosed with osteosarcoma were identified and utilized for the study and were used in accordance with the policies of the Institutional Review Board of the institution. Doxorubicin was obtained through unused residual clinical material at the Massachusetts General Hospital. CDDO-Me was kindly provided by Dr. Jeff Supko (Massachusetts General Hospital). The stock solution of drugs were prepared according to the drug specifications and stored at -20°C. The rabbit polyclonal antibodies to Stat3, Bcl-XL, MCL-1, PARP, and the mouse monoclonal antibodies to phosphorylated Stat3 (pStat3), survivin were purchased from Cell Signaling Technologies (Cambridge, MA). The mouse monoclonal antibody to β-actin and MTT reagent were purchased from Sigma-Aldrich (St. Louis, MO). The Pgp1 monoclonal antibody C219 was purchased from Signet (Dedham, MA). Goat antimouse-HRP and goat antirabbit-HRP were purchased from BIO-Rad (Hercules, CA). SuperSignal® West Pico Chemiluminescent Substrate was purchased from PIERCE (Rockford, IL).
Osteoblast cells HOB-c were cultured in Osteoblast Growth Medium (Promo Cell), and all other cell lines were cultured in RPMI 1640 supplemented with 10% fetal bovine serum, 100 U/ml penicillin, and 100 μg/ml streptomycin (all obtained from Invitrogen, Carlsbad, CA). All cells were incubated at 37°C in 5% CO2-95% air atmosphere and passaged when near confluent monolayers were achieved using trypsin-EDTA solution. Resistant cell lines were continuously cultured in 0.1 μM doxorubicin. Cells were free of mycoplasma contamination as tested by MycoAlert® Mycoplasma Detection Kit from Cambrex (Rockland, ME).
Protein lysates from osteosarcoma cells and tissues were generated through lysis with 1× radioimmunoprecipitation assay lysis buffer (Upstate Biotechnology, Charlottesville, VA). The concentration of the protein was analyzed by DC Protein assay (Bio-Rad Laboratories, Hercules, CA) and spectrophotometer (Beckman DU-640, Beckman Instruments, Inc., Columbia, MD). Thirty micrograms of total protein was processed on Nu-Page 4-12% Bis-Tris Gel (Invitrogen) and transferred to a pure nitrocellulose membrane (Bio-Rad) then immunoblotted with specific antibodies. Primary antibodies were incubated at 1:1000 dilution in Tris-buffered saline, pH 7.4, with 0.1% Tween-20 and 5% nonfat milk (Bio-Rad) overnight at 4°C. Horseradish peroxidase-conjugated secondary antibodies (Bio-Rad) were incubated in Tris-buffered saline, pH 7.4, with 0.1% Tween-20 and 5% nonfat milk, at 1:2,000 dilutions for 1 hour at room temperature. Positive immunoreactions were detected by using SuperSingal West Pico Chemiluminescent Substrate (Pierce, Rockford, IL).
The In vitro cell viability analysis was performed by MTT assay as described previously . For analyzing effects of CDDO-Me on KHOS, KHOSR2, U-2OS, U-2OSTR and HOB-c, 2 × 103 cells per well were plated in 96-well plates in culture medium containing increasing concentrations of CDDO-Me. For the effect of varying concentrations of CDDO-Me on U-2OSTR and KHOSR2, 2 × 103 cells per well were plated in 96-well plates in culture medium containing increasing concentrations of doxorubicin and CDDO-Me at final concentrations of 0.1 and 0.3 μmol/L, respectively. After 7 days of culture at 37°C, 10 μl MTT (5 mg/ml in PBS, obtained from Sigma) was added to each well and the plates were incubated for 3 h. The resulting formazan product was dissolved with acid-iso-propanol and the absorbance at a wavelength of 490 nm (A490) was read on a SPECTRAmax Microplate Spectrophotometer (Molecular Devices, Sunnyvale, CA). The absorbance values were normalized by assigning the value of the control line in the medium without drug to 1.0 and the value of the no cell control to 0. Experiments were performed in triplicate. The half inhibitory concentration (IC50) was defined as the compound or chemo drug concentration required decreasing the A490 to 50% of the control value.
Lysates from whole-cell were immunoblotted with specific antibodies to PARP (Cell Signaling Technology) and its cleavage products. Positive immunoreactions were detected by using Super Signal West Pico Chemiluminescent Substrate. As a second parameter of apoptotic cell death, we measured caspase-3/7 activities in HOB-c, KHOS and KHOSR2 after treatment with CDDO-Me by using Apo-ONE Homogenous caspase-3/7 system according to the manufacturer's instructions (Promega, Madison, WI). The intensity of the emitted fluorescence was determined at a wavelength of 521 nm with the use of a SPECTRAmax® Microplate Spectrofluorometer (Molecular Devices).
To study the effects of CDDO-Me on Stat3 nuclear translocation in live cells, a real-time cell based assay was used as described below. Stable transfectants expressing EGFP-Stat3 were generated with the osteosarcoma cell line U-2OS using standard lipofectmine transfection techniques with G418 selection. EGFP-Stat3 expressing cells were seeded at a density of 4 × 103 cells per well in 96 well plates and incubated overnight at 37°C. The cells were then treated with either 1 μM CDDO-Me with 30 ng/ml human recombinant IL-6 (R&D Systems, Minneapolis, MN) or IL-6 alone. To counterstain the nuclei, the cells were incubated with 1 μg/ml Hoechst 33342 (Invitrogen, Carlsbad, CA) for 1 minute. IL-6 dependent nuclear translocation of EFGP-Stat3 was analyzed using an Olympus 1X71 fluorescence microscope and the pictures were captured as digital images using IPLab Software from Scanalytics (Rockville, MD).
Values shown are representative of triplicate determinations in two or more experiments. The IC50 was calculated and the effects of treatment were evaluated using a two-sided Student's t test (GraphPad PRISM® 4 software, GraphPad Software, San Diego, CA). Errors are a SD (standard deviation) of averaged results and with value p < 0.05 was considered as a significant difference between means.
The combination index (CI) for experimental treatment combinations was calculated to determine the synergistic, additive, or antagonistic effects using the Chou-Talalay method and GraphPad PRISM® 4 software. When CI = 1, the equation represents the conservation isobologram and indicates additive effects. CI values of <1.0 indicates synergistic effect, > 1.0 indicates antagonistic effect.
While CDDO-Me significantly inhibits Stat3 phosphorylation in KHOSR2, U-2OSTRcells, there was no significant change in Pgp1 expression as observed in the Pgp1-over expressing cell lines (Fig. 5B and 6B).
To investigate the inhibition of Stat3 pathway's effect on drug sensitive osteosarcoma cell lines KHOS, U-2OS and MDR osteosarcoma cell lines KHOSR2, U-2OSTR, they were exposed to varying concentrations of doxorubicin and CDDO-Me for 7 days. MTT demonstrated that CDDO-Me has a synergistic effect on doxorubicin-induced antiproliferative effect in KHOSR2 and U-2OSTR (Fig 7B and 7C). In addition, MTT assay demonstrated that CDDO-Me increases doxorubicin's effect and partially overcomes doxorubicin-resistance. The IC50 of each cells were KHOS: 0.07 μM, KHOSR2/CDDO-Me(0.3 μM): 0.11 μM, KHOSR2/CDDO-Me(0.1 μM): 0.29 μM, KHOSR2: 0.86 μM (Fig 6B), and U-2OS: 0.06 μM, U-2OSTR/CDDO-Me(0.3 μM) 0.12 μM, U-2OSTR/CDDO-Me(0.1 μM): 0.24 μM, U-2OSTR: 0.93 μM (Fig 7C).
Stat3 pathway plays an important role in tumor cell growth and proliferation, and is constantly activated in many human cancer cell lines and tumor tissues [9–14, 17, 20]. Constitutive activation of Stat3 pathway could be an early indicator of drug resistance. Activation of Stat3 pathway was also reported to be present in osteosarcoma cells and tissues [14, 20]. In this study, we observed that elevated levels of Stat3 and pStat3 are detected in osteosarcoma drug sensitive cell lines KHOS, U-2OS, SaOS and osteosarcoma MDR cell lines KHOSR2, U-2OSTR. The data is consistent with previous studies that Stat3 pathways play an important role in not only drug sensitive but also drug resistant osteosarcoma cells [14, 20]. The downstream effect of Stat3 activation is the Stat3-dependent regulation of several antiapoptotic genes including Bcl-XL, survivin, and MCL-1. Overexpressions of these survival-promoting genes have been shown to be highly expressed and prevent apoptosis in human cancer cells, especially in high-grade tumors [13, 44–46]. In our study, these antiapoptotic genes were highly expressed in both drug sensitive and resistant osteosarcoma cell lines, but not in normal human osteoblast cells.
Nuclear translocation of pStat3 is a crucial event for its transcriptional function. Blocking the phosphorylation and translocation of Stat3 is a rational approach for the inhibition of Stat3 activation. Recently, Stat3 has been implicated as a promising target for cancer therapy [12, 13, 20, 28–31]. Stat3 inhibitors, SD-1029 and SD-1008, greatly induce apoptosis in drug resistant ovarian cancer cells by blocking Stat3 nuclear translocation [29, 30]. Stat3, pStat3 and Stat3 targeted antiapoptotic proteins are over expressed in drug resistance osteosarcoma cells. Inhibition of the Stat3 pathway and interruption of antiapoptotic response may also play an important role for treatment of these cells. Our result is consistent with recent reports showing that the novel Stat3 inhibitor, Indirubin, significantly induces apoptosis in human breast cancer cells . In addition, LLL3, Stat3 DNA binding and transcription activities inhibitor, and Stat3 siRNA significantly decrease cell proliferation and viability, ultimately inducing apoptosis in osteosarcoma cells.
CDDO is widely used in Asian herbal medicine and was originally identified as an active compound for anti-inflammatory and anti-carcinogenic treatments [34, 39]. The novel compound CDDO-Me has been shown to be highly effective in vivo models for the prevention and treatment of cancer [33–35]. CDDO-Me is able to induce the differentiation of tumor cells, suppress the growth of tumor cells, and induce apoptosis in cancer cells that are resistant to conventional chemotherapeutic agents . Recent report has shown that CDDO-Me inhibits activation of the JAK/Stat3 pathway by forming adducts with both JAK1 and Stat3 in human cervical and breast cancer cells . Previously, we also demonstrated that CDDO-Me inhibits Stat3 pathway in ovarian cancer cells by down-regulation of antiapoptotic genetic expression, and resulted in a dramatic induction of apoptosis . Furthermore, recent study showed that the novel Stat3 target gene Bcl-XL inhibitor, ABT-737, greatly enhanced the activities of paclitaxel in lung cancer cells . In this study, CDDO-Me inhibits Stat3 phosphorylation and down-regulates Stat3-mediated antiapoptotic proteins Bcl-XL, survivin and, MCL-1, in both dose and time dependent manner. This was through apoptotic cell death in both drug sensitive and resistant osteosarcoma cells. Furthermore, the apoptotic threshold also increases the sensitivity of these cells to the cytotoxic effects of doxorubicin. The results are consistent with a series of studies showing that CDDO-related compounds are not monofunctional drugs that strictly target single steps in signal transduction pathways [32, 40, 48]. Biochemical and cellular assays support the hypothesis that the molecule CDDO-Me inhibits Stat3 activity with resultant inhibition of Stat3 phosphorylation, nuclear translocation, and decrease in Stat3-dependent transcription, leading to apoptosis and enhanced chemosensitivity.
Our results demonstrated that Stat3 pathways were constantly activated in osteosarcoma and MDR osteosarcoma cells. CDDO-Me significantly inhibits Stat3 phosphorylation and consequently Stat3 nuclear translocation, and eventually resulted in inducing apoptosis in these cells. In addition, this compound is able to overcome drug resistance in these cells.
This project was supported, in part, by a grant from the Gaetagno and Wechsler funds. KR is supported by Department of Orthopaedic Surgery, Nihon University School of Medicine. ZD is supported, in part, by a grant from Sarcoma Foundation of America, and a grant from the National Cancer Institute, NIH (Nanotechnology Platform Partnership), R01-CA119617. EC is supported by the Jennifer Hunter Yates Foundation.
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