Neutral sphingomyelinase 2 modulates cytotoxic effects of protopanaxadiol on different human cancer cells
© Park et al.; licensee BioMed Central Ltd. 2013
Received: 18 January 2013
Accepted: 16 July 2013
Published: 27 July 2013
Some of ginsenosides, root extracts from Panax ginseng, exert cytotoxicity against cancer cells through disruption of membrane subdomains called lipid rafts. Protopanaxadiol (PPD) exhibits the highest cytotoxic effect among 8 ginsenosides which we evaluated for anti-cancer activity. We investigated if PPD disrupts lipid rafts in its cytotoxic effects and also the possible mechanisms.
Eight ginsenosides were evaluated using different cancer cells and cell viability assays. The potent ginsenoside, PPD was investigated for its roles in lipid raft disruption and downstream pathways to apoptosis of cancer cells. Anti-cancer effects of PPD was also investigated in vivo using mouse xenograft model.
PPD consistently exerts its potent cytotoxicity in 2 cell survival assays using 5 different cancer cell lines. PPD disrupts lipid rafts in different ways from methyl-β-cyclodextrin (MβCD) depleting cholesterol out of the subdomains, since lipid raft proteins were differentially modulated by the saponin. During disruption of lipid rafts, PPD activated neutral sphingomyelinase 2 (nSMase 2) hydrolyzing membrane sphingomyelins into pro-apoptotic intracellular ceramides. Furthermore, PPD demonstrated its anti-cancer activities against K562 tumor cells in mouse xenograft model, confirming its potential as an adjunct or chemotherapeutic agent by itself in vivo.
This study demonstrates that neutral sphingomyelinase 2 is responsible for the cytotoxicity of PPD through production of apoptotic ceramides from membrane sphingomyelins. Thus neutral sphingomyelinase 2 and its relevant mechanisms may potentially be employed in cancer chemotherapies.
KeywordsProtopanaxadiol Lipid raft Neutral sphingomyelinase Ceramide Chemosensitization
Ginsenosides, root extracts from ginseng have been used in many countries for treatment and prevention of a variety of cancers of stomach, lung, liver, pancreas, colon and so on, since they are responsible for most of ginseng’s pharmacological and biological effects . Ginsenosides suppress proliferation and angiogenesis of cancer cells, reduce malignant transformation, and increase cell deaths due to apoptosis and necrosis [2, 3]. They can also be used in combinations with conventional chemotherapeutic agents for treatment of cancers and prevention of their developments in advance.
Recent investigations have suggested that specialized subdomains of cell membrane termed lipid rafts are actively involved in regulation of essential cellular processes such as cell proliferation, differentiation and apoptosis, many of which are altered during development of various tumors [4, 5]. Lipid rafts are enriched in cholesterol, sphingomyelin, gangliosides, caveolins, flotillins, etc. Depending on their lipid and protein compositions, lipid rafts are classified into caveolae and non-caveolar ones . Caveolar lipid rafts are flask-shaped membrane invaginations enriched in cholesterol and caveolins, proteins from caveolin family, while non-caveolar ones are associated with proteins of reggie family, flotillin-1 and flotillin-2 . Due to its high concentrations in lipid rafts, long and saturated sphingolipids make microdomains physicochemically distinct from the remaining of plasma membranes abundant in short and unsaturated phospholipids through their closer interactions with cholesterols . Cholesterol depleting agents such as methyl-β-cyclodextrin (MβCD) can remove cholesterols out of lipid rafts, changing their physical properties and thereby modifying lipid raft-dependent signaling pathways in vital cellular processes . Ginsenosides, phytosterols from Panax ginseng, are natural cholesterol derivatives with hydroxyl groups and oligosaccharide moieties, so many investigators have studied effects on which these ginsenosides might have against crucial functions of lipid rafts.
The ginsenoside Rh2 has been known to increase membrane fluidity and induce apoptosis via lipid raft disruption and thereby caspase-8 dependent Fas activation . Rh2 also reduces the levels of lipid rafts in plasma membranes by increasing their internalization through unknown mechanisms, leading to Akt inactivation and ensuing apoptosis . 25-hydroxyprotopanaxadiol (25-OH-PPD), another ginsenoside from Panax ginseng demonstrated a potent cytotoxicity against a variety of prostate cancer cells, compared to its related ginsenosides . Anti-cancer effects against both prostate and lung cancer cells were repeated with 25-OCH3-protopanaxadiol (25-OCH3-PPD) from Panax notoginseng in vitro and in vivo[10, 11]. These ginsenosides modulated protein regulators in cell proliferation and apoptosis with relatively low toxicity against non-cancer cells. Like Rh2 inhibiting Akt signaling in lipid rafts in favor of apoptosis, 20S-protopanaxadiol (aPPD), another PPD derivative significantly inhibited Akt activities only in lipid rafts with no changes in total Akt levels of plasma membranes .
In present study, we have tested 8 ginsenosides from Panax ginseng for their cytotoxicity using different cancer cell lines. We have demonstrated that cytotoxic effects of PPD on cancer cells were mediated through production of intracellular ceramides from membrane sphingomyelins. Sphingolipids such as sphingomyelins have biologically active roles in cellular functions such as growth and differentiation through production of ceramides, signaling molecules rather than structural elements in cell membrane. In light of the fact that accumulation of intracellular ceramides appeared to be important in chemosensitization of cancer cells during chemotherapies using multiple drugs , potential of the natural product as chemotherapeutic agent are further discussed in terms of chemosensitization of cancer cells to apoptosis. Finally, PPD demonstrated its in vivo anti-tumor activity in mouse xenograft tumor model, confirming its in vitro effects on cancer cells.
Cell lines and reagents
Cancer cells were obtained from American Type Culture Collection (Rockville, MD, USA) and cultured in DMEM or RPMI-1640 medium (Gibco BRL, Grand Island, NY, USA) supplemented with 10% fetal bovine serum (Hyclone, Logan, UT, USA), 2 mM L-glutamine and 1% penicillin/streptomycin (Gibco BRL) at 37°C with 5% CO2. Ginsenosides, including PPD (purity > 98%) were obtained from BTGin (Daejeon, Korea). Propidium Iodide (PI) solution was obtained from Sigma-Aldrich (St. Louis, MO, USA).
Cell viability assay
In XTT assay using Cell Proliferation Kit II (XTT) (Roche Applied Science, Mannheim, Germany), cells were plated into 96 well plates at 0.3 – 1.0 ×104 cells per well overnight and treated with PPD or/and other compounds for 24 or 48 hr. After addition of XTT labeling mixture for 2 hr at 37°C, absorbance was measured at 490 nm with a reference wavelength at 650 nm. In Sulforhodamine B (SRB) assay described previously , after treatment with various ginsenosides for 24 or 48 hr, cells were added with 50 μl 50% trichloroacetic acid (TCA), fixed for 2 h at 4°C, washed with tap water 3 times and air-dried. TCA-fixed cells were stained for 30 min with SRB (0.4% in 1% acetic acid), followed by washing with 1% acetic acid and air-dried. After addition of 100 μl 10 mM Tris, pH 10.5, absorbance was measured at 570 nm. In WST-1 assay with EZ-Cytox Cell Viability Assay kit (Daeillab Service, Seoul, Korea), after incubation with various ginsenosides for 24 or 48 hr, cells were added with 10 μl WST-1 reagent, then incubated for 15 – 60 min at 37°C. Absorbance was determined at 460 nm with microtiter plate reader.
Cell cycle analysis
K562 cells were plated into 6 well plates at 3 × 105 cells per well overnight, then treated with PPD at specified concentrations for 48 h. In chemosensitization experiments, cells were treated with PPD or Cisplatin with or without doxorubicin for 48 hr at indicated concentrations. After treatments, cells were harvested, washed with PBS, fixed in 70% ethanol overnight at -20°C, stained with propidium iodide overnight at 4°C, then analyzed using FACSCalibur flow cytometer (BD Biosciences, San Jose, CA, USA).
Whole cell proteins were separated in 8-15% SDS polyacrylamide gel, transferred to PVDF membranes (Pall Corporation, Pensacola, FL, USA) and probed with anti-Cdk2, anti-Cdk4, anti-caspase-9, anti-β-actin and anti-IGF-1R antibodies (Santa Cruz Biotechnology, Santa Cruz, CA, USA), and anti-Cdk6, anti-Cyclin A, anti-Cyclin B1, anti-α-tubulin, anti-poly (ADP-ribose) polymerase (PARP), anti-Bcl-2, anti-pAkt (Ser473), anti-caspase-8 and anti-Bid (Cell Signalling Technology, Beverly, MA, USA) overnight at 4°C. Next day, membranes were incubated with horseradish peroxidase (HRP)-conjugated goat anti-mouse IgG, bovine anti-goat IgG and bovine anti-rabbit IgG (Santa Cruz) and detected with enhanced chemiluminescent (ECL) detection system (Amerisham Biosciences, Little Chalfont, United Kingdom).
After treatment with PPD overnight as described above, K562 cells were centrifuged and treated with 2 μM BODIPY-C12-SM and 2 μM BODIPY-Cholesterol ester (Invitrogen, Grand Island, NY, USA) respectively in RPMI medium for 30 min at 4°C to label sphingomyelin and cholesterol. After staining, cells were washed with PBS twice and placed onto confocal stage to collect images.
K562 cells were seeded into petri dishes at 1 × 106 cells per dish overnight, then treated with PPD at specified concentrations for 6 or 16 hr. After treatments, cells were harvested, washed twice with ice-cold PBS and extracted with extraction buffer (isopropanol: water: ethyl acetate = 30:10:60), as described previously . Amounts of ceramides were determined with LC/MS/MS system (LC, Thermo surveyor LC pump plus, American Laboratory Trading, East Lyme, CT, USA; MS, LCQ Fleet, Thermo Fisher Scientific, Waltham, MA, USA) using C-17 ceramide as an internal standard.
Inhibition and siRNA interference
In inhibition studies, cells were plated into 96 well plates at 0.3 – 1.0 ×104 cells per well overnight, preincubated for 1 hr with neutral sphingomyelinase 2 (nSMase 2) inhibitor GW4869 at 10 μΜ (Sigma-Aldrich), then treated with PPD at specified concentrations for 48 h. Two nSMase 2 siRNAs were purchased from Bioneer (Daejeon, Korea): siRNA 1141725 sense: GCU ACU UCG AGU ACA UCC U and antisense AGG AUG UAC UCG AAG UAG C and siRNA 1141726 sense: CAC GAA CGG CCU GUA CGA U and antisense: AUG GUA CAG GCC GUU CGU G. Scrambled siRNA (SCR) (Bioneer) was used as negative control: sense CCU ACG CCA CCA AUU UCG U and antisense: ACG AAA UUG GUG GCG UAG G. Real-time PCR was performed to quantify mRNA expressions of nSMase 2 after knockdown with siRNA 1141725 and 1141726 (Additional file 1). Total RNA was isolated from cultured cells using Trizol (Invitrogen). cDNAs were synthesized using oligo (dT) and SuperScript III reverse transcriptase (Promega, Madison, WI, USA), then amplified using iQ SYBR Green PCR Master Mix (Bio-Rad Laboratories, Hercules, CA, USA) in conjunction with Mx3005P QPCR Systems (Agilent Technologies, Santa Clara, CA, USA). Primer sets were as follows: nSMase 2 qPCR primer P109289 (Bioneer); glyceraldehyde-3-phosphate dehydrogenase, 5′-TGGTATCGTGGAAGGACTCATGAC-3′ (forward) and 5′-ATGCCAGTGAGCTTCCCGTTCAGC -3′ (reverse). Fold changes were calculated after normalization to endogenous GAPDH, using comparative 2-ΔΔCt method . In transfections, cells were cultured in complete medium, seeded into 96 well plate at 0.5 – 1.0 × 104 cells per well using culture media without antibiotics overnight along with mixture of Lipofectamine (Invitrogen) and SCR or siRNAs (Bioneer) at 10 nM. Next day, transfected cells were treated with various concentrations of PPD for 48 hr, then were assayed for their viabilities using WST-1, as described above.
Mouse xenograft tumor model
All animal care and experimental procedures were approved by Institutional Animal Care and Use Committee (IACUC) at Gachon University in Incheon, Korea. In mouse xenograft tumor model, female BALB/c nude mice (Charles River, Wilmington, MA, USA) were subcutaneously injected with K562 cells to establish xenografted tumors and provided with food and water ad libitum under standard light/dark cycles. When K562 tumor sizes reached 47.6 mm3, xenografted mice were randomly assigned into 2 groups (n = 4 mice per group) to receive daily intravenous injections with vehicle, Cisplatin (5 mgkg-1) or PPD (25 or 50 mg kg-1 body weight) 5 days per week over 2 weeks. Tumor volumes were measured with calipers over 10 days as well as body weights and were calculated using formula (width2 × length) 2-1, where width is the smaller tumor diameter. Mice were killed on the 11th day and their tumor weights were measured.
The values in figures were expressed as means ± s.d. Unpaired t tests were used to compare two groups. Values of P < 0.05, P < 0.01 and P < 0.001 were considered as statistically significant.
Protopanaxadiol exhibits the most potent cytotoxic effects against a variety of cancer cells among 8 ginsenosides tested in cell survival assays
PPD inhibits cell proliferation through modulation of cell cycle regulators as well as apoptotic proteins, leading to cell death
The ginsenoside mediates anti-cancer activities by disrupting lipid rafts in plasma membrane
PPD exerts its cytotoxic effects on cancer cells through upregulation of neutral sphingomyelinase 2
PPD greatly sensitizes cancer cells to cell death by anti-cancer drugs such as Doxorubicin
The ginsenoside exhibits in vivo anti-tumor activities against xenografted K562 cancer cells
Novel therapeutic agents, giving better efficacy but less toxicity and resistance, have been in great need to treat a variety of cancers refractory to our current cancer therapeutics, thereby resulting in poor outcomes even after intensive treatments. More than 30 ginsenosides identified from ginseng extracts largely consist of four ring structures of 17 carbons derived with multiple sugar moieties . Despite variable effectiveness, their potential as an adjunct to cancer therapeutics has been investigated for a long time due to their potent cytotoxicity against cancer cells but still low toxicity on non-cancer cells. In present investigation, the potent protopanaxadiol illustrated multifaceted aspects of its cytotoxicity through modulation of multiple cell cycle regulators and cell death proteins in cell cycle analyses.
Although PPD disrupts lipid rafts through certain common mechanisms with cholesterol depletion by MβCD, some PPD-specific modulation of essential signaling path-ways seemed to exist in lipid rafts. Previous studies have demonstrated that some lipid raft-associated signaling proteins are modulated by other ginsenosides such as Rp1, aPPD and Rh2 [7, 8, 12, 22]. Rp1 inhibited proliferation of human breast cancer cells such as MCF-7 and MDA-MB-231 through suppression of IGF-1R/pAkt pathway, pAkt of which is also suppressed by Rh2 and aPPD [5, 8, 12, 22]. Activation of IGF pathway is a crucial prerequisite to malignant transformation during development of various cancers as well as differentiation of normal cells such as adipocytes [23, 24]. IGF-1R is overexpressed in most cancer cells, so it’s not surprising that some tumor suppressors exert anti-cancer activity through transcriptional suppression of IGF-1R gene . In Western blot analysis, IGF-1R was downregulated by MβCD and PPD in K562 cells, whereas surprisingly, PPD upregulated IGF-1R in HT29 cells, suggesting that IGF-1R might have a minor role in cytotoxicity of PPD on HT29 cells due to other potent cytotoxic mechanisms. Another ginsenoside Rg1 attenuated cytotoxic effects of neurotoxin 6-OHDA on human neuroblastoma cells through IGF-1R receptor signaling , indicating that ginsenoside-mediated IGF-1R signaling is cell-type specific depending on agents used. Interestingly, MβCD also had a cell-type specific effect in K562 and HT29 cells, since Bid was upregulated in HT29 cells despite its lower level in K562 cells. Since lipid rafts are essential for cancer development, ability of PPD to disrupt the microdomains can be employed as a chemotherapeutic target for cancer treatments, as shown in other ginsenosides such as aPPD and Rh2 [7, 12]. In addition, Rh2-induced Fas activation and its synergism with betulinic acid toward apoptosis of cancer cells through caspase-8 activation made this lipid raft disruptor an adjunct candidate for anti-cancer therapies [7, 27].
Intracellular levels of ceramides are modulated through hydrolysis of sphingomyelins by sphingomyelinases, de novo synthesis involving ceramide synthases and production of pro-survival molecule, sphingosine 1-phosphate from ceramide by sphingosine kinases . Both neutral sphingomyelinases (nSMases) and acid sphingomyelinases (aSMases) mediate formation of ceramides from sphingo-myelins in response to apoptotic inducers including chemotherapeutic agents [18, 20, 29]. In our efforts to identify PPD-specific cytotoxic mechanisms in lipid rafts, we demonstrated that accumulated intracellular ceramides mediate cytotoxic effects of PPD, leading to growth inhibition and apoptosis in distinct cancer cells. Since knockdown or inhibition of neutral sphingomyelinase 2 reduced PPD induced cytotoxic effects against cancer cells to significant extent, this enzyme must be at least partially responsible for growth arrest and apoptosis of cancer cells through increasing intracellular ceramide levels. A recent publication has shown that Withanolide D, an herbal compound from Withania Somnifea, also activated neutral sphing-omyelinase 2/ceramide pathway, inducing apoptosis in leukemia cells . To our knowledge, we have demonstrated for the first time that PPD, one of ginsenosides from Panax ginseng, mediates its cytotoxicity against cancer cells through production of intracellular ceramides by upregulation of neutral sphingomyelinase 2.
We also demonstrated that PPD could significantly enhance Doxorubicin induced cell deaths in two different cancer cell lines as an effort to reduce the usage of toxic anti-cancer drugs. Cisplatin activates acid sphingomy-elinase, followed by production of ceramides, leading to CD95 redistribution in lipid rafts and thereby apoptosis of human colon cancer cells, so its chemosensitization of cancer cells to Doxorubicin could also be acid sphin-gomyelinase/ceramide-dependent [30, 31]. Acid sphingo-myelinase transfection increased intracellular levels of ceramides in glioma cells, thereby sensitizing the cells to chemotherapy via production of reactive oxygen species . This ceramide based synergistic mechanism was also shown by exogenous C6 ceramide which was able to sensitize multiple cancer lines to Doxorubicin induced apoptosis . Given that Doxorubicin induced cancer cell deaths through production of endogenous ceramides as one of its mechanisms , it is quite conceivable that the additive effect or synergism of PPD might come from common pathway or mechanism including ceramides between two chemotherapeutic agents, although the detailed mechanisms will have to wait for further investigations in the future. In mouse xenograft model, doses of PPD tested showed anti-tumor activities comparable to Cisplatin which has shown its in vivo synergistic effects with Doxorubicin , although in vivo chemosensitization of PPD to Doxorubicin remains to be determined.
In conclusion, we have identified the hydrolysis of membrane sphingomyelins into ceramides by neutral sphin-gomyelinase 2 as one of major mechanisms by which PPD exerts its cytotoxic effects against a spectrum of cancer cells, although further investigations are needed to fully understand them. Thus neutral sphingomyelinase 2 and its relevant mechanisms could potentially be employed in cancer chemotherapies. This investigation also demonstrated the potential of PPD as a potent adjunct to conventional anti-cancer drugs in combinational chemotherapies as well as a novel agent by itself. Thus, the clinical use of PPD as adjunct is likely to reduce toxic side effect of anti-cancer drugs and occurrence of drug resistances due to use of single agents at high doses during chemotherapies.
We thank Yeo Jin Park, Jong Hee Choi and A Young Kim for providing technical assistances in preparation of the manuscript. This work was supported in part by grants from National Research Foundation of Korea funded by the Ministry of Education, Science and Technology.
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