Cyclopamine is a plant-derived alkaloid product used as an Hedgehog (Hh) antagonist (1–7) with potential effectiveness in tumor therapies in several xenograft models of medulloblastoma (8), prostate (9), small cell lung cancer (10), and digestive tract tumors (11). It has been previously shown that the combination of low-dose chemotherapy and cyclopamine results in enhanced cell killing (8). Interestingly, the effects of oncogenic mutations in the Hh pathway can also be reversed by cyclopamine, which inhibits Hh signaling by targeting the product of the proto-oncogene Smoothened (SMO) (12).
Hh signaling pathway regulates cell proliferation, tissue polarity, and cell differentiation during normal development, and abnormal signaling of this pathway has been associated with cancer. Human tumors such as those of basal cell carcinoma, medulloblastoma, rhabdomyosarcoma, and others, are associated with activating mutations of SMO (13) or with inactivating mutations of Patched (PTCH) (14). SMO and PTCH are involved in the Hh pathway, and mutations of these proteins may lead to an increased activity of the Hh response pathway (15). It has been suggested that Hh pathway activity may play a role in the maintenance of the capacity of tumor stem cells, favoring self-renewal, and proliferation of their progeny (16). Because Hh signaling regulates progenitor cell fate in normal development and homeostasis, aberrant pathway activation might be involved in the maintenance of such populations in cancer. Moreover, it has also been proposed that in stem cell populations, downregulation of PTCH1 in the presence of high-levels of SMO expression renders such cells sensitive to Hh ligand and results in pathway activation, leading to stem cell population expansion (17). Thus, Hh pathway inhibition would deplete the clonogenic tumor fraction through terminal differentiation rather than cytotoxicity, and such activation of Hh signaling would have the capacity to expand cancer stem cells without triggering terminal differentiation.
On this basis, a number of different research groups have explored the potential role of cyclopamine in Hh signaling in cancer stem cell biology, showing that the Side Population (SP) can be depleted following cyclopamine treatment (18). SP cells are rare stem cells with expression of ABCG2, a multidrug ABC transporter discovered using mitoxantrone-resistant breast cancer cells (19). Although many ABC transporters have been identified as drug-resistance proteins, they are all expressed in normal tissues (20). ABCB1 and ABCG2 transporters are respectively highly conserved in normal CD34-positive (21) and CD34-negative (22) stem cells and may play a role not only in protecting these cells from cytotoxic agents, but also from endogenous or physiological substrates. ABCG2 confers the SP phenotype and enhances hypoxic cell survival by reducing heme or porphyrin accumulation and having a key role in hematopoietic stem cells concentrated in hypoxic areas (23). Moreover, enforced expression of ABCG2 inhibits hematopoietic development and it has been proposed as a regulator in extracellular signals that influence stem cell interactions with the microenvironment (22). Possible roles for ABC transporters expressed in normal cells have been suggested by disruption of their function (24).
Here, we show that ABCG2, independently of how Ptch regulates Smo phosphorylation, conformation, and trafficking, is likely to be modulating the Hh pathway, and maybe orchestrating some other gene expression programs by means of an active efflux of substrates involved in intercellular and/or intracellular cell signaling. The regulation of stem cell signaling by ABC transporters would be an additional way of understanding how complex cell signaling is, adding more complexity at the upstream signaling pathway.
Materials and Methods
Cell Lines and ABCG2 cDNA
The human KB line (ATCC, Rockville, MD) was originally thought to be derived from an epidermal carcinoma of the mouth, but was subsequently found as a HeLa contaminant. Parental cells and the derived resistant sublines were maintained in complete medium consisting of DMEM (Gibco BRL, Grand Island, NY) supplemented with 20% fetal calf serum (Gibco BRL), 2 mM L-glutamine, 1 mM sodium pyruvate, 15 U ml−1 penicillin, and 15 ng ml−1 streptomycin (Biological Industries, Kibbutz Beth Haemek, Israel). In this study we have used different human brain astrocytoma cell lines. MOG-G-CCM and U87-MG cells were obtained from the European Collection of Cell Cultures (ECACC). GOS3 cells were obtained from the Deutshche Sammlung von Mikroorganismen und Zellkulturen GMBH (DSMZ). LN405 and SW1783 were obtained from the American Type Culture Collection (ATCC). For the transfection experiments, KB cells were transfected with the full-length ABCG2 cDNA (R482 variant; MXRA cells) by a cationic liposome-mediated method (GeneShuttle-20, Q-BIOgene) (following the manufacturer's instructions). KB-transfected cells were continuously maintained in complete medium containing the selected drug. Prior to the flow cytometry experiments, exponentially growing cells were detached using trypsin/EDTA (PAA Laboratories, Pasching, Austria), rinsed twice at 1,500 rpm for 5 min, resuspended in cold PBS supplemented with 0.1% NaN3 and 2% bovine serum albumin (PBS-BSA) and kept on ice. The ABCG2 DNA construct above mentioned was kindly provided by Dr. S. E. Bates (National Cancer Institute, Bethesda, MD).
Chemicals, Antibodies, and Drugs
Mitoxantrone (MTX), Hoechst 33342 (Ho342), propidium iodide, cyclopamine, temozolomide (TMZ), staurosporinone (K252c), 11-ketocyclopamine (jervine), 20(alpha)-hydroxycholesterol, 22(S)-hydroxycholesterol and 25-hydroxycholesterol were purchased from Sigma-Aldrich (St. Louis, MO). The 24(S)-hydroxycholesterol was purchased from Enzo Life Sciences. ABCG2 siRNAs, RIPA buffer, acrylamide/bys acrylamide 30% BioReagent 29:1 solution and Tween® 20, were purchased from Sigma Aldrich (St. Louis, MO). Lipofectamine RNAiMAX and Opti-MEM® I Medium were purchased from Invitrogen Life Science. Monoclonal anti-ABCG2 (BXP-53, ab24115), polyclonal anti-PTCH (ab39266), polyclonal anti-SMO (ab38686), rabbit polyclonal secondary antibody to rat IgG - H&L (HRP, ab6734), and Goat polyclonal Secondary Antibody to Rabbit IgG-H&L (HRP, ab6721), were purchased from Abcam plc (Cambridge, UK). ECL Plus Western Blotting Detection System and Full Range Rainbow™ Molecular Weight Markers were purchased from Amersham (GE Healtchare). Pierce® BCA Protein Assay Kit was purchased from Thermo Scientific (Rockford, IL).
Cells were detached using trypsin/EDTA (PAA laboratories, Pashing, Austria), rinsed at 400g for 5 min at room temperature, and the pellet was frozen with dry ice and kept at −80°C until their use. Pellet was thawed and RIPA buffer was immediately added (1 ml for each 10 × 106 cells) with freshly added protease inhibitor cocktail and PMSF (10 μl of 10 mg ml−1 PMSF stock for each ml of RIPA buffer). Samples were gently resuspended and incubated at 4°C for 30 min under shaking. Next, samples were centrifuged at 12,000g for 20 min at 4°C. The supernatant represented the total cell lysate and was transferred to a new microfuge tube (previously cooled at 4°C). Total protein concentration was measured in the extract by Pierce® BCA Protein Assay Kit (Thermo Scientific, Rockford, IL).
Samples were prepared in loading buffer containing 50 mM Tris pH 6.8, 2% (w/V) SDS, 10 mM DTT, 10% (v/v) glycerol, and 0.2% (w/v) β-mercaptoethanol, incubated at 95°C for 5 min and centrifuged (short spin at 14,000 rpm) for suitable collection of the sample in the tube. Samples were applied to a SDS-PAGE (SDS acrylamide gel electrophoresis 10%) and electrophoresis was performed for ∼60 min at 160 V. Stained proteins of known molecular mass (range 12–225 kDa) were run simultaneously as standards. The electrophoretically separated proteins were transferred onto PVDF membranes (BioRad) by humid electrotransference with the Mini Trans-Blot Cell System (BioRad) at 300 mA for 1 h. After blotting, PVDF membranes were dried and stained with Ponceau red Staining solution and immediately washed twice with TBS-Tween (0.1%) (TBST). Membranes were blocked for 2 h at room temperature in TBST (containing 8% of non-fat dried milk) and then washed three times during 10 min in TBST at room temperature. Membranes were incubated with the appropriate dilution of the corresponding primary antibody in TBST (1% of non-fat dried milk). Incubation dilutions were as follows: Polyclonal anti-SMO (1:1,000 dilution), Polyclonal anti-Patched/PTCH (1:1,000 dilution) and monoclonal anti-ABCG2 (1:250 dilution). All incubations were performed overnight at 4°C. After washing three times during 10 min in TBST, membranes were incubated with a secondary antibody in TBST (1% of non-fat dried milk), HRP-labelled anti-rabbit IgG (1:8,000 dilution) for Polyclonal anti-Smo and Polyclonal anti-PTCH and with secondary antibody HRP-labeled anti-rat IgG (1:10,000) for Monoclonal anti-ABCG2 at room temperature for 1 h. Membranes were washed with TBST again before detecting bound antibodies using an ECL Plus Western Blotting Detection System (GE Healthcare) according to the manufacturer's recommended procedure. Immunoreaction signals were captured and analyzed using an ImageQuant™ LAS 4000 system by Fujifilm Life Science, USA.
siRNA Forward Transfection
Cells were seeded in a six-well tissue culture test plates (Nunc™) at a final concentration of 5 × 104 cells per well in 2.5 ml antibiotic-free normal growth medium supplemented with FBS. Cells were incubated at 37°C in a CO2 incubator until 50–60% confluence for 24 h. The following solutions were prepared: Solution A: For each transfection, 1.5 μl of each siRNA duplex (Mix of three siRNA duplexes) was diluted into 250 μl of Opti-MEM® I Medium without serum (Invitrogen Life Science), Solution B: For each transfection, 5 μl of lipofectamine RNAiMAX (Invitrogen Life Science) was diluted into 250 μl Opti-MEM® I Medium. The siRNA duplex solution (Solution A) was then added directly to the diluted transfection reagent (Solution B), mixed gently and then incubated for 20 min at room temperature. After incubation, the final volume for each transfection (500 μl) was added to the cells (final RNA duplex concentration of 50 nM) in a total volume of 3 ml of antibiotic-free normal growth medium supplemented with FBS. Cells were incubated at 37°C in a CO2 incubator and the medium was changed after 6 h of treatment. Finally, cells were subsequently incubated for 48 h, at which time the medium was aspirated and cells were tripsinized immediately for RNA isolation.
MISSION siRNA Universal negative control #1 was used for the siRNA transfection assay.
Data were collected using a MoFlo® (Beckman-Coulter) equipped with three lasers operating at 351, 488, and 633 nm. Filter combination for Ho342 analysis was: 440DLP, BP405/30 (blue), and BP670/40 (red). Ho342 signal was measured using linear scale at 30 mW. Acquisition was stopped when 100,000 live-gated events were collected. Propidium iodide was collected through a band pass filter of 630/30. MXR fluorescence was measured using a 740LP filter. For drug uptake and retention assays, acquisition was stopped when 20,000 gated events were collected in the fluorescence-cell count histogram. Gating was based on forward scatter and side scatter dot plots, by encircling populations with amorphous regions and then excluding dead cells (life-gate) by propidium iodide counter-staining. Data were stored as listmode files and analyzed using FlowJo Software (Tree Star).
Cells were resuspended in prewarmed Dulbecco's modified Eagle Medium (DMEM) supplemented with 2% heat-inactivated fetal calf serum (FCS) and 10 mM HEPES at a concentration of 1–2 × 106 cells ml−1. Ho342 was added at a concentration of 5 μg/106 cells. Samples were incubated in a water bath at 37°C for 2 h in agitation, in the dark. Cells were centrifuged at 483 rcf for 6 min at 4°C and resuspended in cold Hank's Buffered Salt Solution (HBSS)/FCS/HEPES at a concentration of 1–2 × 107 cells ml−1. Samples were kept in the dark at 4°C until analysis. Propidium iodide was added at a concentration of 5 μg ml−1 to exclude dead cells. In order to remove cell aggregates, samples were filtered through a 50-μm nylon mesh prior to acquisition.
Drug Uptake and Retention Assays
MXRA cells were seeded in six-well tissue culture testplates (Nunc™) at a final concentration of 105 cells ml−1, 1 ml total volume per well, 24 h before the dye uptake/retention experiments to allow attachment to the surface and to achieve the conditions allowing for optimal growth. Drug uptake was determined by adding either Ho342 or mitoxantrone to the culture medium for 3 h at 37°C, in the presence or absence of staurosporinone (K252c), 11-ketocyclopamine (jervine), 20 (alpha)-hydroxycholesterol, 22(S)-hydroxycholesterol, 24(S)-hydroxycholesterol, and 25-hydroxycholesterol in separate wells, at different concentrations. After 3-h incubation, cells were washed and resuspended in dye-free culture medium and the other drugs were maintained to evaluate their effect on Ho342 and mitoxantrone (MTX) retention. Concentrations used were 0.2–20 μM for K252c, and the rest of the molecules ranged from 1 to 10 μM. All experiments were run in triplicate.
Real-Time PCR Analysis
RNA was isolated using Illustra RNAspin Mini RNA Isolation Kit (GE Healthcare) and converted to cDNA using SuperScript™ II RNase H Reverse Transcriptase (Invitrogen). Quantitative rtPCR was performed using an IQ5 Multicolor Real-Time PCR Detection System (BioRad), IQTM SYBR Green Supermix (BioRad) and gene specific primers. All real-time data was normalized respect to the housekeeping gene HPRT. Primers sequences used were as follows:
ABCG2, sense: 5′-CGACAGCTTCCAATGACCTGA-3′
ABCG2, antisense: 5′-ACCAGGTTTCATGATCCCATTGAT-3′
SMO, sense: 5′- CAGCTTCCGGGACTATGTGCTATG-3′
SMO, antisense: 5′-GAAGGCTCGGGCGATTCTTG-3′
PTCH, sense: 5′-GGCAGCGGTAGTAGTGGTGTTC-3′
PTCH antisense: 5′-TGTAGCGGGTATTGTCGTGTGG-3′
To further validate the suitability of the primers, the PCR products were sequenced.
Cyclopamine Modulates ABCG2 Efflux Activity
We first evaluated the modulatory effect of cyclopamine on ABCG2 function by measuring the intracellular accumulation of two fluorescent substrates of ABCG2: Ho342 and mitoxantrone. As shown in Figure 1, 10 and 30 μM cyclopamine blocked the efflux function of ABCG2 and thereby increased the accumulation of Ho342 and MTX in ABCG2-expressing MXRA cells. We used cyclopamine as a potential substrate competitor with Ho342 and MTX in independent experiments. Thus, we confirmed that cyclopamine is modulating ABCG2 activity.
Cyclopamine Effect on SP Studies
The effect of cyclopamine on the accumulation of Ho342 was tested to characterize its modulatory effect on Ho342 mediated transport using functional flow cytometry. The accumulation of Ho342 was initially studied in ABCG2 overexpressing MXRA cells. The cells were incubated as we (25) and others (26) previously described in the presence of either 10 or 30 μM of cycloplamine. This drug (30 μM) and temozolomide (100 μM) were added to MXRA cells during the whole incubation process (2 h) in the presence of Ho342, and showed that only cyclopamine had an effect on the accumulation of Ho342 in MXRA cells (Fig. 2). Since MXRA cells were enforced to express ABCG2, the observed SP cells were not true stem cells. Hence, we resorted to characterizing the accumulation of Ho342 using human brain cancer cell lines previously described to be enriched in stem-like cells. We used GOS3, LN405, MOG-G-CCM, SW1783, and U87-MG cells for cell sorting experiments. Primary cells were stained with Ho342 as described. Relative numbers of initially detected SP cells ranged from 0.002 to 2.86% of the PI-negative cells within the life gate. Subsequently, cells were sorted to obtain enriched products in SP cells (ranging from 40 to 80%). Accordingly, for the enrichment or purification of SP cells, a representative experiment using MOG-G-CCM cells is shown (Fig. 3). SP cells were initially detected in low numbers in a small percentage (0.99%) prior to cell sorting experiments. Isolated SP cells were cultured for 2 weeks following sorting under the same conditions as unsorted cells, giving 5.42% of SP cells. Sorted cells were cultured again for 2 weeks, giving 27.8% of SP cells.
For competitive experiments using cyclopamine, MOG-G-CCM and LN405 cells were sorted again. Only cell cultures with a minimum of 40% of SP cells were prepared in order to study the effect of cyclopamine on Ho342 uptake. SP-enriched MOG-G-CCM and LN405 cells were stained with Ho342 as described in the presence or absence of 30 μM cyclopamine or with the same concentration of this drug and 100μM temozolomide. MOG-G-CCM cells incubated with Ho342 gave 41.2% SP cells, whereas cells incubated either with cyclopamine alone or in combination with temozolomide, gave 11.2 and 15.6% of SP cells respectively (Fig. 4a). LN405 cells incubated with Ho342 gave 67.7% SP cells, whereas cells incubated either with cyclopamine alone or in combination with temozolomide, gave 6.22 and 5.8% of SP cells, respectively (Fig. 4b). Hence, this significant decrease of SP cells was a consequence of adding cyclopamine, whereas temozolomide had no effect on Ho342 efflux. This phenomenon probably occurred independently of the cytotoxic mechanisms of the drugs we used, indicating that the decrease in MOG-G-CCM and LN405 SP cells was not a consequence of the depletion of the SP compartment, but also a direct consequence of the molecular mechanisms involved in the affinity of cyclopamine and Ho342 for ABCG2. Furthermore, we analyzed the effect of cyclopamine in unsorted populations with a low content of SP cells. We incubated SW1783 cells again with Ho342 in the presence or absence of 30 μM cyclopamine or with the same concentration of this drug and 100 μM temozolomide. SW1783 cells incubated with Ho342 gave 0.48% SP cells, whereas cells incubated either with cyclopamine alone or in combination with temozolomide both gave 0.16% of SP cells (Fig. 5a). Taken together, our results showed a significant decrease in the SP compartment as a consequence of adding 30 μM cyclopamine, ranging from 57.47 to 90.81% (median = 69.74%) (Fig. 5b). These results were consistent and reproducible even when low numbers of SP cells were detected. Cells were incubated again in the presence of temozolomide alone (not in combination with cyclopamine), confirming that temozolomide had no effect on Ho342 efflux (Fig. 5c).
Effect of Hydroxycholesterols (HCs), Jervine, and K252c on ABCG2 Activity
We next evaluated the modulatory effect of 20(alpha)-HC, 22(S)-HC, 24(S)-HC, 25-HC, 11-ketocyclopamine (jervine), staurosporinone (K252c) on ABCG2 function by measuring the intracellular accumulation of mitoxantrone in ABCG2-expressing MXRA cells. Each of the drugs tested had an effect on mitoxantrone transport. As shown in Figure 6, 25-HC, jervine and K252c blocked the efflux function of ABCG2 and thereby increased the accumulation of MTX in ABCG2-expressing MXRA cells. We used these drugs as potential substrate competitors with MTX and Ho342 (data not shown) in independent experiments. Thus, we confirmed that HCs, 11-ketocyclopamine and staurosporinone also modulate ABCG2 activity.
ABCG2, SMO, and PTCH1 Expression Levels in Isolated SP Cells
LN405, MOG-G-CCM, and SW1783 cells were used to determine whether ABCG2 expression (mRNA) was increased in SP cells isolated using flow cytometric sorting. Real-time PCR (rtPCR) using specific primers for the transporter was carried out, showing that ABCG2 was expressed at higher concentrations in SW1783 SP-cells, and progressively decreasing in MOG-G-CCM and LN405 SP-cells. Decreased expression levels of Smo and Ptch were observed, and appeared to be dependent on the differential expression pattern of ABCG2 (Figs. 7a and 7b). These results support the hypothesis that ABCG2 regulates Smo levels, whereas the decline in the expression of Ptch may represent a compensatory mechanism as a result of Smo downregulation. Furthermore, LN405, MOG-G-CCM, and SW1783 SP cells were incubated in the presence of 10 and 30 μM cyclopamine for 24 h. Using rtPCR, ABCG2, SMO, and PTCH1 expression levels remained almost unchanged after cyclopamine treatment (Fig. 7c). In addition, we obtained near identical flow cytometric cell cycle profiles of LN405, MOG-G-CCM, and SW1783 SP cells (Fig. 7d). For comparison, parallel cell cycle analyses were performed on identical conditions to those used for rtPCR. All together, these experiments suggest that ABCG2 may play a role in cyclopamine resistance.
Reversion of ABCG2 Levels by Forward Transfection Using siRNAs
For ABCG2 knock-down experiments, Smo mRNA levels of KB and MXRA cells were also measured, since this would show how a controlled increase of ABCG2 expression affects Smo levels. First of all, SP cells were transfected with siRNAs and resulted in a partial decrease in ABCG2 protein expression, with a decrease in the mean fluorescence intensity from 36.35 ± 2.59 to 23.4 ± 1.88 a.u. (arbitrary units; Fig. 7e). Treated cells were then processed in parallel for the isolation of total RNA. Both KB and MXRA cells gave very low Smo mRNA levels and remained unchanged for KB treated cells when compared with its siRNA negative control used for the transfection assay. LN405, MOG-G-CCM, and SW1783 SP-cells gave the higher concentrations of Smo mRNA when compared with KB and MXRA cells, but Smo levels remained almost unchanged following siRNA treatment (Fig. 7f).
Protein Levels for Smo and Ptch by Western Blot Analysis
Since mRNA levels are not necessarily reliable indicators of true expression profiles, the protein levels of SMO and PTCH were also measured in KB, MXRA, LN405, MOG-G-CCM, and SW1783 cells. No significant differences in protein expression levels for KB and MXRA cells were observed. Protein levels were also tested in SP cells isolated using flow cytometric sorting. PTCH was overexpressed in MOG-SP and SW1783-SP cells. SMO was overexpressed in MOG cells and slightly overexpressed in SW1783 cells. In general, Smo protein was underexpressed in SP cells, whereas Patch was overexpressed, with the exception of LN405 cells (Fig. 7g).
It has been reported that cyclopamine may act to deplete the SP compartment (17, 18, 27). Consistent with this idea is our demonstration that cyclopamine can markedly reduce the SP, an observation that would support the argument that SP cells are Hh pathway-dependent. Testing this concept, we looked at the possibility that cyclopamine might interact with ABCG2 and limit the optimal examination of SP cells, using the Hoechst-based assay. Our results clearly show that ABCG2 efflux function is modulated dose-dependently by cyclopamine when human cell models are used, either with an enforced expression of wild-type ABCG2 or with true SP cells. However, it cannot be ruled out that cyclopamine could have a dual mode of action, independently inhibiting Hh signaling and ABCG2-dependent efflux at the same time (28). Furthermore, this observation not only has a direct effect on SP measurements, but may also have a long after-effect through the modulation of signaling cascades in stem cells.
To date, SP cells can only be accurately identified using functional flow cytometry, based on the efflux of a fluorescent dye known as Ho342. Although cyclopamine and other drugs have long served for inhibiting the Hh pathway, there is no doubt that ABC transporters may play an important role apart from regulating drug toxicity in both normal and cancer cells (29). The steroidal nature of cyclopamine and its ability to disrupt cholesterol synthesis or transport suggested to us that it might affect the action of ABCG2. Cyclopamine can inhibit Hh pathway activity in the absence of Ptch function (12). Thus, ABCG2 may protect the proto-oncogene SMO, being a new potential “mechanism-based” associated regulator with the Hh pathway. In addition to some activating mutations that render Smo proteins resistant, the oncogenic SmoA1 protein has been reported as resisting suppression by Ptch (30), indicating that oncogenic Smo proteins may not be subject to normal regulation. Under these circumstances, our results support the idea that ABCG2 also regulates Smo proteins by inhibiting the mechanism of action of cyclopamine and maybe other molecules that may act upon Smo.
Although it has been suggested that Ptch might regulate lipid trafficking to repress Smo, no endogenous lipid regulators of Smo have yet been identified, nor has it ever been shown that Ptch controls lipid trafficking. In fact, Ptch may regulate Smo degradation by modifying the lipid composition of the endosomes through which Smo passes (31). Alternatively, we propose that ABCG2 is likely to be operating as a regulator of Smo independently of how its structural form is preserved. Therefore, a hypothetical role for ABCG2 in retranslocating Smo proteins to intracellular compartments cannot be ruled out. Whether or not ABCG2 also protects Ptch and regulates lipid trafficking was not evaluated in this work and further studies will be needed to discover how ABCG2 may also regulate the binding of different substrates to Ptch.
On the basis of our cyclopamine measurements, we explored the effects of different compounds classically used for Hh signaling pathway studies. In particular, oxysterols represent a class of potent regulatory molecules with remarkably diverse and important biological actions (32). Interestingly, oxysterols are activators of the Hh signaling pathway in pluripotent mesenchymal cells (33), which have not been reported to contain SP cells (34). In addition to the oxysterols that we tested, jervine and K252c also modulated mitoxantrone transport. Given the effect of oxysterol 20(S)-HC (35) and curcumin (36) in Notch pathway signaling, and given that oxysterols and curcumin are ABCG2 substrates (37), it is possible that ABCG2 may act not only as a firewall to regulate Hh pathway signaling, but also to regulate Notch signaling as well as other important signaling pathways highly conserved in stem cells, such as Wnt. Our results seem to contradict the findings of Bleau et al (38) regarding glioma tumor stem-like cells, and a reasonable alternative explanation of their observations could be that ABCG2 may also act as a regulator of the PTEN/PI3K/Akt pathway in SP cells.
The following model (Fig. 8) would be consistent with what is already known about Hh regulation and with our new observations. ABCG2 may efflux different substrates able to interact with Smo and maybe alters lipid trafficking through cell membranes. Hence, the mechanisms by which ABCG2 differentially activates low- or high-level Hh signaling may rely on the efflux of substrates able to bind Smo and other proteins, and even explain different activation levels in the presence of mutated or truncated Smo proteins. The association of Hh and ABCG2 would be an additional way of understanding how complex cell signaling is in stem cell regulation, adding more complexity at the upstream signaling pathway.
We propose that Hh pathway activity in SP stem-like cells can be regulated through the activity of ABCG2. Thus, the ABC transporters expressed in stem cells, such as ABCG2 in SP cells or ABCB1 in human hematopoietic CD34-positive cells may protect not only against xenobiotics or anticancer compounds, but also be able to protect the stem compartment from a series of ligands involved in transduction signaling. In fact, ABC transporters may have an additional and unexpected role in preserving stem cells and cancer stem cells from physiological (i.e., oxysterols) and synthetic substrates resulting in pathway activation or inactivation. This hypothesis should be relevant for preclinical and clinical studies of Hh antagonist therapies (and others) aimed at avoiding self-renewal and long-term survival of tumor stem cells. If true, ABCG2 expression will be crucial for maintaining the SP compartment undifferentiated by inhibiting transduction signaling through ABCG2 activity. These results can be relevant for understanding how stem cells but also how cancer stem cells can proliferate while maintaining the stem cell phenotype.
The authors thank Drs. S. Bates and Rob Robey (National Cancer Institute, Bethesda, MD, USA) for providing the ABCG2 DNA constructs and for all their help. They also thank Noèlia Purroy for technical assistance in preparing samples.