Anti-proliferative effect of LXR agonist T0901317 in ovarian carcinoma cells
© Rough et al; licensee BioMed Central Ltd. 2010
Received: 22 September 2009
Accepted: 26 May 2010
Published: 26 May 2010
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© Rough et al; licensee BioMed Central Ltd. 2010
Received: 22 September 2009
Accepted: 26 May 2010
Published: 26 May 2010
Ovarian cancer is the most common cause of cancer related death from gynecologic tumors in the United States. The insidious nature of the disease precludes early diagnosis, therefore surgical debulking and chemotherapy are considered as standard treatment modalities for advanced stages. We investigated the effect of the LXR agonist, T0901317, on ovarian cancer cell proliferation and apoptosis as a potential therapeutic agent.
T0901317 treatment resulted in a significant (P <0.001) inhibition of cell proliferation in a time- and dose-dependent manner in CaOV3, SKOV3 and A2780 cells. Western blot analysis demonstrated an induction of p21 and p27 with a concominant reduction in phospho-RB protein levels. Cell cycle analysis demonstrated a significant (P <0.001) arrest in the G1 cell cycle phase. Significant induction of Caspase-3 and BAX gene expression occurred with treatment. Induction of apoptosis was confirmed by significant (P < 0.001) elevation of caspase activity on FACS analysis, caspase-glo assay, BAX protein induction and decreased caspase 3 precursor protein expression on Western blot analysis. LXR α/β knockdown experiments did not reverse the anti-proliferative and cytotoxic effects of T0901317.
The LXR agonist, T0901317, significantly suppresses cell proliferation and induces programmed cell death in a dose- and time-dependent manner. Our results indicate that T0901317 induces its anti-proliferative and cytotoxic effects via an LXR-independent mechanism.
Ovarian cancer is the most common cause of cancer related death from gynecologic tumors and the fourth leading cause of death due to cancer in women [1, 2]. The insidious nature of the disease precludes early diagnosis, therefore surgical debulking and chemotherapy are considered as standard treatment modalities for advanced stages . Although the majority of patients with advanced stages of the disease respond to chemotherapy, most will ultimately succumb to the disease due to the development of chemoresistance . For this reason, there is extensive research being performed searching for novel therapies to overcome chemoresistance and to develop more effective chemotherapeutic agents.
Liver X receptor-α (LXRα) and LXRβ (also known as NR1H3 and NR1H2, respectively) were discovered more than a decade ago . LXRα is highly expressed in the liver and at lower levels in the adrenal glands, intestine, adipose, macrophages, lung, and kidney, whereas LXRβ is ubiquitously expressed . LXR receptors and their ligands are involved in the regulation of efflux of cholesterol from atherosclerotic plaques which have led to their interest in their application for the treatment of atherosclerosis [7, 8]. Synthetic LXR ligands have been developed, namely GW3965 and T0901317, and have been observed to have potential therapeutic properties in murine models for the treatment of atherosclerosis, diabetes, and Alzheimer's disease [9, 10]. Over recent years, the antineoplastic properties of LXR agonists have been observed in human carcinomas such as breast and prostate, making the molecule an attractive antineoplastic agent for investigation in the treatment of ovarian cancer [11–15]. In this study we investigated the effects of a synthetic LXR agonist, T0901317, in various human ovarian cancer cell lines. LXR agonist, T0901317 may be a promising therapeutic agent in the treatment of ovarian cancer.
Synthetic non-steroidal LXR agonist N-(2,2,2-trifluoro-ethyl)-N-[4-(2,2,2-tri-fluoro-1-hydroxy-1-trifluoromethyl-ethyl)-phenyl]-benzene sulfonamide (T0901317) was purchased from Sigma (Saint Louis, MO). Dulbecco's Modification of Eagle's Medium (DMEM), Hank's Balanced Salt Solution (HBSS) and Fetal Bovine Serum (FBS) were purchased from Mediatech (Herndon, VA). Protease inhibitor cocktail and enhanced chemiluminescence (ECL) reagents were from Roche Applied Science (Indianapolis, IN). Vybrant FAM Caspase-3 and -7 Assay Kit (V35118, Molecular Probes, Eugene OR). Anti-p27 (sc-528, 1:200), anti-BAX (sc-7480, 1:200), anti-caspase 3 precursor (sc-7148, 1:200), anti-LXRα (sc-1202 1:200), anti LXRβ (sc-130412, 1:200) antibodies were from Santa Cruz Biotechnology (Santa Cruz, CA). Anti-p21 (ab-7960-1, 1:100), and anti-β actin (ab-8229, 1:1000) antibodies were from Abcam (Cambridge, MA). Anti-phospho Rb (Ser 807/811) (#9308, 1:1000) was from Cell Signaling Technology (Danvers, MA).
CaOV3, SKOV3, A2780 (human ovarian carcinoma cell lines) and HS-68 (human foreskin fibroblasts) cell lines were obtained from the American Type Culture Collection (Manassas, VA). CaOV3 and HS-68 cells were maintained in DMEM, and SKOV3 and A2780 cells were maintained in RPMI. Media was supplemented with 10% FBS, 10 mM Hepes buffer, 1 mM Na-pyruvate, 2 mM L-glutamine, 100 units/ml penicillin, 100 μg/ml streptomycin, and cultured at 37°C in an atmosphere of 5% CO2 and 95% oxygen.
CyQuant Cell proliferation assay kit was used according to manufacturer's specifications. CaOV3, SKOV3, and A2780 cells were plated at 1 × 104 cells/well in 100 μL of cell solution in Microtest 96 tissue-culture-treated polystyrene 96-well plates (Falcon; Becton Dickinson, Franklin Lakes, NJ) at 37°C at 5% CO2. Cells were allowed to adhere to the plate surface for 24 h, following adherence the media was aspirated and replaced with treatment media (5, 10, 20, 40 or 50 μM of T0901317 or vehicle alone). Cells were grown under these conditions for 24 to 72 h. At indicated time points, the wells were washed with PBS and subsequently frozen at -70°C overnight. 200 μl of the CyQuant GR dye/cell-lysis buffer was added to each well and incubated for 2 to 5 minutes at room temperature, protected from light. Plates were then measured using a fluorescence microplate reader with filters at 480 nm excitation and 520 nm emission maxima.
1.5 × 106 ovarian carcinoma cells were cultured as above in 100 mm dish in DMEM with above described supplements for 24 h prior to T0901317 treatment. After treatment cells were washed twice in ice-cold HBSS and were lysed in ice-cold lysis buffer (50 mM Tris-HCl, pH 7.4, 150 mM NaCl, 1% Nonidet P-40, and 0.1% SDS), supplemented with protease inhibitors (10 μg/ml leupeptin, 10 μg/ml pepstatin A, 10 μg/ml aprotinin, and 1 mM of 4-(2-aminoethyl) benzenesulfonyl fluoride). Sample protein concentrations were determined via the Biorad Protein assay strictly following the manufacturer's instructions. Proteins (30-40 μg/lane) were separated on a denaturing 8% SDS polyacrylamide gel and transferred to a nitrocellulose membrane. Membranes were blocked in 1% blocking solution in phosphate-buffered saline (PBS) and subsequently incubated overnight at 4°C with primary antibody. After washes, the membranes were incubated with secondary antibody conjugated to horse radish peroxidase for 1 h at room temperature. Chemiluminescence was detected using the ECL reagent according to the manufacturer's protocol. Different exposure times were used to ensure that bands were not saturated. For detection of β-actin, the same membranes were incubated with rabbit polyclonal anti-beta actin antibody overnight at 4°C and processed as described.
Aliquots of cells (1 × 106/ml) were fixed in 70% ethanol for 2 hours at 4°C; cells were then centrifuged at 1500 rpm, and the resulting pellets were resuspended in 1 ml of freshly prepared propidium iodide/RNase solution. Cell cycle distribution was analyzed with the GuavaEasy Cyte mini system by using the Guava CytoSoft Cell Cycle Program according to the manufacturer's instructions (Guava Technologies, Hayward, CA). Based on the intensity of the propidium iodide fluorescence, the flow cytometry program will separate resting cells with one copy of each chromosome (G0/G1), cells that have replicated and contain double DNA content and thus double intensity of fluorescence (G2/M) and cells in S phase.
Vybrant FAM Caspase-3 and -7 Assay Kit V35118, (Molecular Probes, Eugene OR) was used to quantitatively determine the percentage of cells actively undergoing apoptosis according to the manufacturer's instructions. Briefly, ovarian carcinoma cells were seeded overnight in 6 wells plates at a density of 2 × 105 per well. Cells were then treated for 24 h with T0901317 (10 μM) or 0.1% DMSO as negative control. Cells were then trypsinized and collected and 1 × 105 cells per sample were stained with 10 μl of FLICA reagent and 7-AAD and incubated at 37°C in 5% CO2 for one hour. Cells were then washed with 1× wash buffer, centrifuged at 1500 RPM for 5 minutes. The supernatant was discarded, 400 μL of 1× wash buffer was added and samples were analyzed by flow cytometry according to manufacturer's recommendations (Calibur, BD Biosciences).
Caspase-3/7 activation assays were performed using a Caspase-Glo™ 3/7 assay kit (Promega, Madison, WI) according to the manufacturer's instructions. Briefly, ovarian carcinoma cells were seeded in 96-well plates at a density of 1 × 104 cells/well. After 24 h, cells were treated with different concentrations of T0901317 (5, 10, 20, 40 and 50 μM) or 0.1% DMSO as negative control. Caspase-Glo 3/7 reagent (100 μl) was then added to each well including medium alone, untreated control cells or cells treated with T0901317 for 6 h. The plate was then incubated at room temperature for 1 h and the luminescence of each sample was measured with a Veritas Microplate Luminometer (Turner BioSystem, Sunnyvale, CA).
Ovarian carcinoma cells were plated at a density of 1.5 × 105 cells per well in 12 well plates. Allowed to adhere for 24 hours, subsequently the cells were transfected at a confluence of 50-60% with 200 nM of validated LXR-α/LXR-β siRNA (Dharmacon, NR1H3/NR1H2) using the Mirus transfection reagent (Mirus, TransIT-TKO, MIR 2150). Cells remained with transfection complexes for 48 hours and subsequently the knockdown efficiency was assessed via real time RT-PCR.
Total RNA was isolated according to recommendations by the manufacturer using the RNeasy kit (QIAGEN, Valencia CA). The RNA was quantified using the Genequant spectrophotometer and reverse transcription was performed using SuperScript II Reverse Transcriptase and reagents from Invitrogen (USA), strictly following manufacturer's instructions. Real time PCR was performed using Taqman and gene specific primer FAM probe mixes (Applied Biosystems, Foster City CA). Expression of LXR-α, LXR-β, BCL-2, BAX, Caspase-3 and beta-actin as endogenous control was analyzed. The reactions were run in triplicate in the ABI 7500 system (Applied Biosystems) and results were analyzed with SDSv1.3 software that uses the ΔΔCt method for relative quantification.
Cells were plated at a density of 5 × 103cells/well in a 96 well plate, and allowed to adhere overnight. After T0901317 treatment, 100 μL of the fluorogenic, cell permeant reagent GF-AFC, (Promega, Madison WI) and incubated for one hour, following suggested protocol from the manufacturer. Samples were then analyzed using a Wallac Victor microplate Fluorometer.
Each experiment was conducted at least three times with consistent results. All values in the figures are expressed as mean value ± SD. The data were analyzed using student's T test with significance determined as P < 0.05.
Ovarian cancer has an overall poor prognosis especially in the case of chemoresistance; therefore, the development of effective chemotherapeutic agents is of ultimate importance . Our study demonstrates a possible therapeutic mechanism of T0901317 which possesses anti-neoplastic properties in ovarian cancer cells with suppression of proliferation and induction of apoptosis. This is the first study to report these observations in human ovarian carcinoma cells. However, the antineoplastic properties of LXR agonists have been demonstrated in other human carcinomas such as breast and prostate [12–14]. LXRs are nuclear receptors that first were discovered to have a regulatory function in control of lipid metabolism. They were shown to have the ability to induce lipid efflux from atherosclerotic plaques . Subsequently, LXR's were also demonstrated to have an additional regulatory role in immune cell function, specifically modulation of murine macrophage response to inflammatory stimuli .
Interestingly, our study demonstrates that the primary receptor involved in induction of cell death and cell cycle arrest is not LXR. T0901317 has been demonstrated to have agonistic effects on receptors other than LXR, such as the Pregnane X Receptor (PXR) and the Farnesoid X Receptor (FXR) . According to a study by Houck, et al., the principal receptor activated at a dose of 1 μM and below, primarily activates the Liver X Receptor, whereas doses above 1 μM primarily activate the farnesoid X receptor (FXR) . Interestingly, a Phase I pharmacokinetic trial and correlative in vitro Phase II tumor kinetic study of apomine, a FXR agonist, demonstrated inhibition of tumor growth from patients with ovarian cancer . A study by Swales, et al. demonstrated the ability of an FXR agonist, GW4064, to induce apoptosis and inhibit proliferation in breast cancer cells . Therefore, it is likely that FXR activation by T0901317 may lead to induction of apoptosis and cell cycle arrest in ovarian cancer cells. T0901317 has the ability to induce the gene expression of short heterodimer protein (SHP), which is involved in bile acid synthesis regulation, and is reported to be an FXR-dependent gene . Despite T0901317 being a synthetic LXR agonist, the concentration dependent activation of other receptors must be taken into account when studying this compound.
We have demonstrated the effect of T0901317 on ovarian cancer cell morphology and on cellular proliferation. These occur in a time- and dose-dependent manner, which are similar to findings reported in a study by Wente, et al., describing inhibition of cell proliferation in insulinoma cells . Cell cycle analysis indicated that T0901317 induced G0/G1 cell cycle arrest with a concomitant decrease in both the S and G/M2 phases. A study in human prostate cells demonstrated similar findings with a decrease in the percentage of cells in the S-phase after treatment . We analyzed the expression of p21 and p27 which are regulatory proteins involved in G0/G1 phase arrest, via inhibition of cyclin/CDK complexes that are necessary for cell cycle progression . One such mechanism for cell cycle progression into the S-phase is phosphorylation of the retinoblastoma (Rb) protein by cyclin/CDK complexes . Our study demonstrates that upregulation of both p21 and p27 correlates with inhibition of phosphorylation of the Rb protein, therefore causing G0/G1 cell cycle arrest and inhibition of cellular proliferation.
We analyzed the ability of T0901317 to induce apoptosis in ovarian cancer cells. T0901317 has a significant ability to induce the activity of caspase-3 and -7 leading to apoptosis in ovarian carcinoma cells. Further evidence is elucidated by the induction of caspase-3 and BAX gene expression. Induction of the pro-apoptotic protein, BAX, was upregulated in a dose-dependent manner. The BAX protein is a member of the Bcl-2 family, and when over expressed has the ability to accelerate apoptosis .
To our knowledge, this is the first study to report the anti-proliferative and pro-apoptotic activity of T0901317 on ovarian cancer cells mediated via an LXR-independent pathway. We believe that based on our results that synthetic LXR agonists warrant further studies as anti-neoplastic agents in the treatment of ovarian cancer.
The authors declare that they have no competing interests.
We would like to take the opportunity to acknowledge Dr. Mario Rico for his assistance in the acquisition and interpretation of data.
This study was funded by an NIH training grant (T32CA103652-04).
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