Targeting sphingosine kinase 2 (SphK2) by ABC294640 inhibits colorectal cancer cell growth in vitro and in vivo
© Xun et al. 2015
Received: 13 May 2015
Accepted: 12 August 2015
Published: 4 September 2015
Colorectal cancer (CRC) is a major health problem in China and around the world. It is one of the leading causes of cancer-related deaths. Research groups are thus searching for novel and more efficient anti-CRC agents.
Here we demonstrated that ABC294640, a novel SphK2 inhibitor, induced growth inhibition and apoptosis in transformed and primary CRC cells. The SphK activity was remarkably inhibited by ABC294640, accompanied by sphingosine-1-phosphate (S1P) depletion and ceramide incensement in CRC cells. Exogenously-added S1P inhibited ABC294640-induced HT-29 cell lethality. While C6 ceramide and SphK1 inhibitor SKI-II facilitated ABC294640-induced cytotoxicity against HT-29 cells. ABC294640 inhibited AKT-S6K1, but activated JNK signaling in transformed and primary CRC cells. JNK inhibitors (SP600125 and JNKi-II) alleviated ABC294640-induced CRC cell apoptosis. Moreover, a low concentration of ABC294640 sensitized the activity of 5-FU and cisplatin in vitro. In vivo, ABC294640 oral administration dramatically inhibited HT-29 xenografts growth in nude mice.
Targeting of SphK2 by ABC294640 potently inhibits CRC cell growth both in vitro and in vivo, ABC294640 could be developed as a novel therapeutic for the treatment of CRC.
KeywordsColorectal cancer Sphingosine kinase ABC294640 Ceramide and signaling
Colorectal cancer (CRC) is a major health problem in China and around the world [1, 2]. It is one of the leading causes of cancer-related deaths [1, 2]. Over the past decades, significant improvements have been accomplished in chemotherapy treatments for CRC [1–3]. However, for those with advanced/malignant CRC, the overall survival has not been remarkably prolonged . Research groups are thus searching for novel and more efficient anti-CRC agents [1, 2, 5–7].
Existing evidences have confirmed sphingosine kinase (SphK) as an important therapeutic target for CRC and other solid tumors . SphK controls the balance of cellular sphingolipids [9–11]. Activation of SphK leads to generation of sphingosine-1-phosphate (S1P), which is a known lipid signaling molecule promoting several pro-cancer behaviors, including migration, differentiation, survival, angiogenesis and immune cell modulation . On the other hand, SphK inactivation will induce accumulation of S1P precursors, including sphingosine and ceramide, causing cell apoptosis and growth arrest .
Thus far, there are at least two isoforms of SphK, SphK1 and SphK2, have been identified . The oncogenic role of SphK1 has been extensively studied in CRC and other cancers . Studies have demonstrated the pivotal role of SphK1 in cellular proliferation, survival, and its ability to reverse chemoresistance in CRC . However, few is known about the role of SphK2 in CRC. It has been shown that the ablation of SphK2 by RNA interference (RNAi) inhibited cell proliferation and migration more effectively than that of SphK1 . In CRC cells, SphK2 siRNA downregulation facilitated sodium butyrate-induced apoptosis .
Recent studies have characterized a novel SphK2 inhibitor, ABC294640, which was shown to suppress growth of breast, kidney, and pancreatic cancer cells [18–20]. ABC294640 is a non-lipid competitive inhibitor of SphK2, and exhibited chemotherapeutic pharmacological efficacy in animal models without systemic toxicity [18–20]. However, the potential effect of ABC294640 in CRC, and the underlying signaling mechanisms have been largely unknown. In this study, we used ABC294640 as a pharmacological tool to determine the efficacy of targeting SphK2 as a novel therapeutic intervention in the treatment of CRC.
Chemicals and antibodies
ABC294640 was purchased from DC Chemicals (Shanghai, China). SKI-II (4-[[4-(4-Chlorophenyl)-2-thiazolyl]amino]phenol) was purchased from Tocris Bioscience (Ellisville, Mo). Cell permeable short-chain C6 ceramide was obtained from Avanti Polar Lipids, Inc. (Alabaster, AL). S1P was purchased from Cayman Chemical Co. (Ann Arbor, MI). 5-fluorouracil (5-FU) and cisplatin were purchased from Sigma (Shanghai, China). SP600125 and JNK inhibitor II (JNKi-II) were obtained from Selleck (Shanghai, China). Antibodies against phospho (p)-AKT (Ser 473), p-AKT (Thr 308), p-ribosomal protein S6 kinase 1 (S6K1) (Thr-389), p-JNK1/2 (Thr 183/Tyr 185) and p-c-Jun (Ser73) were purchased form Cell Signaling Tech (Denver MA). Anti-AKT1, SphK2, c-Jun and tubulin antibodies were obtained from Santa Cruz (Santa Cruz, CA).
CRC cell lines and culture
CRC cell lines, including HT-29, HCT-116 and DLD-1, were from the Cell Bank of CAS (Shanghai, China), cells were cultured in RPMI/DMEM medium, with a 10 % FBS in a CO2 incubator at 37 °C.
Primary colon cancer cell isolation and culture
Surgery-isolated colon cancer tissues were thoroughly washed, non-cancerous surrounding tissues, if any, were separated carefully under microscopy, and were discarded. Clinical pathology reports confirmed those tissues were indeed colon cancer tissues. Tissues were then minced. The pellets were thoroughly washed, then re-pelleted at 400 g for 5 min, and were subjected to 0.15 % (w/v) collagenase I digestion for 1 hour. Primary cells were pelleted and rinsed twice with DMEM, cells were then cultured in medium (DMEM, 15 % FBS, 10 mg/ml transferrin, 2 mM glutamine, 1 mM pyruvate, 10 mM HEPES, 100 units/ml penicillin/streptomycin, 0.1 mg/ml gentamicin, 0.2 units/ml insulin, 0.1 mg/ml hydrocortisone, and 2 g/liter fungizone) . Written informed consents were obtained from all enrolled patients. All clinical investigations were in accordance with principles expressed in the Declaration of Helsinki.
MTT assay of cell proliferation
CRC cell proliferation was analyzed by the 3-[4, 5-dimethylthiazol-2-yl]-2, 5 diphenyltetrazolium bromide (MTT) assay. Briefly, 3,000 cells/well were plated in 96-well plates. After treatment, 20 μl/well of MTT (5 mg/ml, Sigma) was added to culture medium for 2 hours. Absorbance was measured on a microplate reader (Bio-Rad, Basel, Switzerland) at 570 nm.
Clonogenic survival assay
SKOV3 cells were plated in 6-well plates at 1000 cells per well. Cells were then treated with gradient concentrations of ABC294640. Ten days after treatment, survival colonies were fixed with 3 % glutaraldehyde, 0.2 % crystal violet and 20 % methanol, and were manually counted.
Annexin V FACS assay of cell apoptosis
Apoptosis was detected by an Annexin-V-FITC apoptosis detection kit (BD Pharmingen, San Diego, CA). Briefly, CRC cells were harvested and washed twice (with PBS), and then incubated for 15 min with Annexin-V-FITC and propidium iodide (PI). Both early (Annexin V+/PI−) and late (Annexin V+/PI+) apoptotic cells were sorted by the fluorescence activated cell sorter (FACS) machine (Becton Dickinson FACS Calibur, San Jose, CA). The percentage of Annexin V stained cells was gated as a quantitative measurement of cell apoptosis.
Fragmented DNA detection by ELISA
Nucleosomal DNA fragmentation is one of the biological markers for apoptosis. Fragmented DNA was assessed by measuring DNA-associated with nucleosomal histones using a specific two-site ELISA with an anti-histone primary antibody and a secondary anti-DNA antibody, according to the manufacturer's instructions (Roche Applied Science, Shanghai, China). ELISA OD at 450 nm was recorded to measure cell apoptosis.
Lactate dehydrogenase (LDH) assay
LDH content released into conditional medium indicates the level of toxicity. LDH content was assayed by a LDH detection kit from Roche Applied Science (Shanghai, China). % LDH release = LDH released in conditional medium/(LDH released in conditional medium + LDH in cell lysates)* 100 %.
Thirty μg of proteins per sample were separated by 10 % SDS-PAGE gel, and were transferred to polyvinylidene difluoride (PVDF) membranes (Millipore, Bedford, MA). After blocking with 10 % non-fat dry milk (in PBST) for 1 hour, membranes were incubated with designed antibodies (in PBST) overnight at 4 °C, followed by incubation with secondary antibodies (in PBST) for 1–2 hours at room temperature. The blots were visualized with enhanced chemiluminescence (ECL) kit (Pierce, Shanghai, China). The intensity of each band was quantified through ImageJ software after normalized to corresponding loading control.
Assay of SphK activity and S1P content
After treatment, 20 μg of cell lysates were incubated with 25 μmol/L D-erythrosphingosine dissolved in 0.1 % Triton X-100, 2 mmol/L ATP, and [γ-32P] ATP for 30 minutes at 37 °C in a final volume of 200 μL. The reaction was stopped by adding 20 μL of HCl (1 N), followed by 800 μL of chloroform/methanol/HCl (100:200:1, v/v). After vigorous vortex, phases were separated by centrifugation. Radio-labeled S1P was separated by 60 thin-layer chromatography (TLC) on silica gel G60-plates with chloroform/acetone/methanol/acetic acid/water (10:4:3:2:1, v/v) as solvent, and phosphate incorporation was visualized and quantified using a scintillation counter (LS-6500, Beckman, Shanghai, China) . The sphingosine kinase activity was valued as pmol/hour/g protein, and was expressed as percentage of the control group.
Enzymatic measurement of ceramide
Cellular ceramide content in CRC cells was analyzed by the protocol reported in , and was valued as fmol by nmol of phospholipid. Its level in the treatment group was expressed as the percentage change of the control cells.
HT-29 tumor bearing nude mice
A nude mice HT-29 xenograft experiment was performed to evaluate the in vivo activity of ABC294640. Animal care and procedures were in accordance with guidelines and regulations of the Institutional Animal Care and Use Committee (IACUC). This study is approved by the ethics committee of authors’ institutions. SKOV3 cells (2 × 106 per mice) were implanted subcutaneously in right flanks, and tumor volumes were calculated by use of the equation: (L × W2)/2. When the tumors reached around 100 mm3, mice were randomly assigned to three groups. Treatment was then administered every day thereafter, consisting of oral doses of 5 or 20 mg of ABC294640/kg body weight or vehicle (0.375 % Polysorbate-80). Mice body weight and tumor volume measurements were performed every week. On week 6, tumors were excised, and weighted.
Experiments in this study were repeated at least three times. Data were expressed as mean values ± standard deviations (SD). Statistics were analyzed by ANOVA followed by the Tukey’s multiple comparison (SPSS 18.0, Chicago, CA); The level of significance was P < 0.05. Cell doubling time was also calculated by SPSS software.
ABC294640 induces growth inhibition and apoptosis in human CRC cell lines
ABC294640 decreases SphK activity, causing S1P depletion and ceramide accumulation in CRC cells
ABC294640 was cytotoxic to primary human CRC cells
ABC294640 inactivates AKT-S6K1, but activates JNK signaling in cultured CRC cells
ABC294640 sensitizes the activity of 5-FU and cisplatin
Oral administration of ABC294640 inhibits HT-29 xenografts growth in nude mice
SphK has been recognized as an important therapeutic target in various solid tumors, mainly due to its roles in cell proliferation, survival, apoptosis, differentiation, and cell senescence [8, 14, 25, 26]. Thus far, at least two isoforms of SphK have identified, including SphK1 and SphK2 . The two have various tissue expression, and subcellular localization [25, 27]. It has been shown that SphK1 expression is highest in lung, spleen, kidney, and blood; Meanwhile, overexpression of SphK2 is found in liver, kidney, brain, and heart [28, 29]. In the current study, we showed that SphK2 expression level was inconsistent in different primary colon cancer cells, which negatively correlated to ABC294640’s sensitivity. Although the role of SphK1 in cancer progression has been extensively studied in the literature , the function of SphK2 on cell survival, growth and drug resistance has only recently been elucidated [30, 31].
In the current study, our results showed that ABC294640, the novel, specific and competitive SphK2 inhibitor, suppressed SphK activation in CRC cells, causing S1P depletion and ceramide accumulation. ABC294640 inhibited CRC cell growth, and induced CRC cell apoptosis. Further, it sensitized 5-FU- and cisplatin-mediated anti-HT-29 cell activity. In vivo, oral administration of ABC294640 remarkably inhibited HT-29 xenografts growth in nude mice, without inducing system toxicity. These preclinical results suggest that ABC294640 might be an efficient anti-CRC agent.
One of the novel findings of this study is that ABC294640 significantly inhibited AKT-mTOR (S6K1) activation in both transformed and primary CRC cells. Existing evidence have confirmed the pivotal role of AKT-mTOR signaling in regulating CRC cancer cell growth, proliferation, migration and survival. This pathway is frequently dysregulated in CRC [24, 32, 33]. Thus, AKT-mTOR inhibition by ABC294640 might be responsible, at least in part, for its cytotoxic effects against CRC cells. These results are not surprising, since SphK2 blockage by ABC294640 caused ceramide accumulation in CRC cells. Ceramide is able to activate phosphatase 1A (PP1A) or PP2 to directly de-phosphorylate AKT [34, 35]. S1P was shown to activate AKT through different mechanisms , and decreased S1P in ABC294640-treated cells might also be the reason of AKT-mTOR inactivation. The detailed signaling mechanisms of AKT-mTOR inhibition by this SphK2 inhibitor require further characterizations.
Another important signaling discovery of this study is JNK activation by ABC294640 in tested CRC cells, which is also involved in ABC294640-mediated activity. Inhibition of JNK by two different JNK inhibitors alleviated growth inhibition and apoptosis by ABC294640. Although the detailed signaling mechanisms need further investigation, there are possible speculations to explain JNK activation by ABC294640. Chen et al., showed that ceramide alone induced JNK activation and subsequent cell apoptosis through thioredoxin-interacting protein-mediated pathway . SiRNA-mediated silencing of JNK largely reversed ceramide-induced apoptosis . Based on these information, it is possible that blockage of SphK2 by ABC294640 leads to ceramide accumulation, which actives JNK signaling to promote CRC cell apoptosis.
One advantage of using of ABC294640, other than it is a novel, highly-efficient and competitive SphK2 inhibitor, is its oral availability. In the current study, our results demonstrated that oral administration of a single dose of ABC294640 (5 or 20 mg/kg, daily) dramatically inhibited HT-29 xenograft growth in nude mice, leading to remarkable tumor recession. That’s being said, however, certain off-target effects of ABC294640 should be considered. For example, it has been previously shown that ABC294640 binds to the E2 estrogen receptor (ER) in an antagonistic manner, and similar to tamoxifen, this novel SphK2 inhibitor represses ER signaling in both cellular and animal models . Although this is somehow advantageous to ER-expressing breast cancer cells , yet ER inhibition and other potential side effects should be considered fully when given to human, and close monitoring of patients is thus recommended.
Thus, targeting of SphK2 by ABC294640 potently inhibits CRC cell growth both in vitro and in vivo, ABC294640 could be developed as a novel therapeutic for the treatment of CRC.
This research was supported by grants from the National Natural Science Foundation.
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