Introduction
Breast cancer is a leading cause of cancer-related death in women. There are five major intrinsic breast cancer subtypes each with its own molecular characteristics, prognosis, incidence, and response to treatment [
1]. Claudin-low tumors are mainly triple-negative invasive ductal carcinomas with a high frequency of metaplastic and medullary differentiation. There are conflicting reports as to the prevalence of these tumors, ranging from as low as 1.5% of tumors [
2] to 5 to 14% of breast tumors [
3]-[
5]. Claudin-low tumor cells are enriched for characteristics of tumor-initiating cells and across a differentiation spectrum are most similar to mammary epithelial stem cells [
5]. Claudin-low breast cancers are characterized by low expression levels of cell-cell adhesion molecules including E-cadherin and several of the tight junction claudin proteins, claudin 3, 4, and 7. This subtype is molecularly similar to cells that have undergone an epithelial-to-mesenchymal transition (EMT) and overlaps with the recently characterized mesenchymal and mesenchymal stem-like subclassifications of triple-negative breast cancer [
6],[
7]. Little is known about molecular therapeutic targets in this highly aggressive subtype of breast cancer.
EMT cells undergo a morphological transition from the epithelial polarized phenotype to the mesenchymal fibroblastoid phenotype. This process is marked by loss of cell-cell adhesion molecules, such as E-cadherin, downregulation of epithelial differentiation markers, and upregulation of mesenchymal markers. In cancer, it is hypothesized that EMT cells gain migratory potential at the expense of proliferative ability. EMT has therefore been implicated in the process of metastasis. There is a close association between the EMT core signature and the signatures that define the claudin-low and metaplastic breast cancer subtypes [
7].
In vertebrates, canonical Hedgehog (Hh) pathway signal transduction occurs when one of the three ligands, Sonic, Indian, or Desert hedgehog, binds to the receptor Patched-1 (
PTCH1) or its homolog, Patched-2. In the off-state, PTCH1 inhibits the activity of Smoothened (SMO). When stimulated by ligand this repression is lifted due to internalization and degradation of PTCH1. SMO then promotes the dissociation of the Suppressor of fused-Gli complex through an unknown mechanism. This allows for translocation of glioma-associated oncogene 1 (GLI1) and GLI family zinc finger 2 (GLI2) to the nucleus and degradation of the repressor form of GLI family zinc finger 3 (GLI3). In the nucleus, activated GLI proteins stimulate the transcription of Hh target genes, including
PTCH1 and
GLI1. PTCH1 is a Gli target, providing a negative feedback mechanism whereby the pathway is regulated. GLI1 is the key final output of the Hh pathway, and
GLI1 transcription is the most reliable marker of pathway activation [
8]. The Hh pathway plays a critical role in vertebrate development, and is responsible for controlling cell fate, patterning, survival, proliferation and differentiation. In the adult organism Hh is active in the maintenance of stem cells [
9]. Deregulation of this pathway can result in cancer.
There is evidence of a role for the Hh pathway in breast cancer. Some tumors exhibit loss of chromosomal regions containing
PTCH1 or amplification of regions containing
GLI1[
10], and Hh expression in the stroma is important [
11]. Additionally, there is evidence for loss of
PTCH1 expression due to promoter methylation in human breast cancer, which correlated with decreased expression in samples from human ductal carcinomas
in situ (DCIS) and in invasive ductal carcinomas [
12]. Similarly, SMO has been found to be ectopically expressed in approximately 70% of DCIS samples, and 30% of invasive breast cancers [
13]. Despite strong evidence for Hh pathway activation in breast cancer, overall few mutations in Hh pathway components have been identified [
14].
GLI1 is amplified in glioblastoma and has been implicated in other cancers.
GLI1 expression in mice causes mammary tumors with a basal-like phenotype [
15]. Additionally, mammary stem cells are regulated by Gli transcription factors [
16], and GLI1 has been associated with poorer outcome in ERα
- tumors [
17] and overall [
18],[
19].
The nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) pathway plays a role in inflammation, cell survival, and transformation in response to stimuli including stress, cytokines, and microbial antigens. NFκB proteins are transcription factors, and inappropriate regulation of this family has been implicated in inflammatory and autoimmune diseases as well as cancer. Subunits of NFκB include v-rel avian reticuloendotheliosis viral oncogene homolog (Rel) family members RELA/p65, RELB, and c-REL, and NFκB subfamily members p105/p50 and p100/p52. NFκB family members associate with nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha (IκBα), which sequesters them in the cytoplasm, and they are generally not active unless they dimerize with Rel subfamily members. NFκB has been implicated in the progression of breast cancer. For example, NFκB promotes cell migration and metastasis by upregulating expression of chemokine receptor CXCR4 [
20]. CXCR4 is highly expressed in metastases from breast cancer patients, and is thought to play a role in homing of tumor cells to the bone marrow.
In order to identify possible drivers of proliferation in mesenchymal stem-like breast cancer, we conducted an inhibitor screen of human mammary epithelial cells (HMLE) induced to undergo an EMT. The results indicate the importance of GLI1 signaling in these cells, which further extended to a panel of claudin-low cancer cell lines. We identified non-canonical NFκB activation of GLI1 in these cells, indicating crosstalk between GLI1 signaling and NFκB pathways in claudin-low and EMT breast cancer cells and suggesting a therapeutic route for claudin-low breast cancer.
Methods
The experiments described did not include human subjects. All tumor data analyzed came from published expression datasets, which sought and obtained ethical approval and used Institutional Review Board approved protocols [
5]. All animal studies were conducted in accordance with international, national, and university approved laws and policies. The animal studies received ethical approval from Yale University’s Institutional Animal Care and Use Committee.
Cell culture
HMLE-EMT and control cell lines were a gift of Robert A. Weinberg (Massachusetts Institute of Technology, Cambridge, MA) and were propagated as previously described [
21],[
22]. MTSV1-7 lines were a gift from Joyce Taylor-Papadimitriou [
23]. Claudin-low cell lines, BT549, HS578T, MDA.MB.157, MDA.MB.231, and MDA.MB.436 as well as MCF10a, were obtained from American Type Culture Collection (ATCC) and propagated according to instructions. All experiments were done on low-passage cells.
High-throughput screen and dose-response curves
A total of 750 HMLE-shEcad cells per well were plated in 384-well plates in 20 μl of growth media and allowed to adhere overnight. The following day 10 nl of compounds from stock plates were added to each well (Table S1 in Additional file
1). The stock plates contained each agent at 16 concentrations from 10 mM to 0.3 nM. Seventy-two hours after drug addition, viability was assayed using CellTiter-Glo reagent (Promega, Madison, WI, USA). The protocol for the screen has been previously described by our laboratory [
24].
For generating dose-response curves manually, 1,000 cells/well in 100 μl of media were plated in 96-well plates. Drug treatment and viability analysis was conducted as described above. Dose-response curves were generated using Graphpad Prism with Michaelis-Menten kinetics (Graphpad Software, Inc., San Diego, CA, USA).
RNA isolation and real-time PCR
RNA was isolated using the RNeasy Plus kit (Qiagen, Germantown, MD, USA) and cDNA synthesized using the iScript kit (Bio-Rad Laboratories, Hercules, CA, USA) according to manufacturers’ protocols. Real-time PCR was performed on a Bio-Rad iCycler after combining the cDNAs with TaqMan universal PCR master mix and premixed FAM-labeled TaqMan probes (Applied Biosystems, Foster City, CA, USA). Abundance of mRNAs relative to GAPDH controls was calculated using the 2-ΔΔCt method.
Cell lysis and immunoblotting
Lysates were prepared from subconfluent cells using NP40 lysis buffer (1.0% NP40, 150 mM NaCl, 50 mM Tris-HCl pH 7.4, 5 mM EDTA, 10% glycerol) with phophatase and protease inhibitors added. Lysates were transferred to polyvinylidine fluoride membranes and blocked in 5% nonfat milk in phosphate-buffered saline (PBS) with 0.1% Tween-20 (PBST). The primary antibodies used recognize GLI1, pEGFR, and ERBB2 (Cell Signaling Technology, Danvers, MA, USA) or epidermal growth factor receptor (EGFR), glyceraldehyde phosphate dehydrogenase (GAPDH) (Santa Cruz Technology, Santa Cruz, CA, USA). Membranes were washed with PBST and incubated with horseradish peroxidase-conjugated secondary antibodies. Gli1 blots were blocked in 5% bovine serum albumin (BSA).
Gene expression analysis of tumors
Analysis of published expression data (Gene Expression Omnibus: GSE18229) from 337 mammary tumors and primary tissue was conducted using the UNC337 dataset [
5]. The expression level of
GLI1 across the predefined subtypes was determined for each dataset following median centering using GEO2R. Results were analyzed by one-way analysis of variance (ANOVA).
shRNA and retroviral infection
The following short hairpin (sh)RNA) sense sequences were cloned into the pSIREN-RetroQ (BD Biosciences, Franklin Lakes, NJ, USA) shRNA-expressing retroviral vector:
GLI1 #1: CCCAGATGAATCACCAAATTCAAGAGATTT [
25] and #2: AAGCGTGAGCCTGAATCTGTG [
26].
RELA: GCTGTGTTCACAGACCTGGCATCCGTCGA [
27].
NFKB1: GCCAGAGTTTACATCTGA [
28]. The negative control vector contains a scrambled sequence of a luciferase-directed shRNA (BD Biosciences).
For inducible shRNA-expressing viruses targeting
RELA, the RHS4430-200223785 and RHS4430-200229897 GIPZ clones were purchased from Thermo Fisher Scientific (Waltham, MA, USA) and cloned in to pInducer10 via MluI and XhoI. Non-targeting sense sequence was 5′-GGATTCCAATTCAGCGGGAGCCTG-3′ [
29]. Virus was produced by co-transfection of the pInducer10 constructs and packaging plasmids into 293 T cells. Virus was harvested and concentrated, and cells were infected and selected as described above. To induce expression of the hairpin, cells were treated with 1 μg/ml doxycycline for three days.
shRNA-expressing retroviruses were produced by co-transfection of the retroviral plasmids, pVSV-G and pCL-ECO (Clontech, Mountain View, CA, USA), into HEK 293 T cells (ATCC) using FuGene6 (Roche, Basel, Switzerland). Retrovirus was harvested in OptiMEM (Invitrogen) for five days, pooled, and concentrated with Centricon plus-20 columns (Millipore, Billerica, CA USA). Cells were infected at a multiplicity of infection of approximately five and selected in medium containing 0.6 μg/ml puromycin for at least three days before use.
Proliferation assay
Cells infected with retrovirus were plated in 6-well dishes in medium containing 0.6 μg/ml puromycin. Every 24 hours, one well was trypsinized and counted using a Countess tissue culture counter (Invitrogen, Carlsbad, CA, USA).
Migration assay
Retrovirally infected cells were selected for three days in medium containing 0.6 μg/ml puromycin, and plated in an 8.0 μm pore cell culture insert (BD Biosciences) in medium containing 1% fetal bovine serum (FBS), above medium containing 10% FBS. After 12 h, cells were scraped from the inside of the insert, and the insert was stained using Diff-Quik (Siemens, Erlangen, Germany). Cells in at least five fields of view were counted for each insert.
Retrovirally infected cells were selected for three days in medium containing 0.6 μg/ml puromycin and plated at limiting dilutions in growth medium containing 0.6 μg/ml puromycin. Colonies were grown for 12 to 14 days, stained with Diff-Quik (Siemens), and counted under light microscopy.
A total of 40,000 cells were plated on 60 mM ultra-low attachment plates (Corning, Inc., Corning, NY, USA) in 4 mL of mammosphere growth medium (mammary epithelial basal medium (Lonza Group Ltd, Basel, Switzerland) with B27 supplement (Invitrogen), EGF (20 ng/mL), bFGF (20 ng/mL), heparin (7 μg/ml), and penicillin/streptomycin with 0.5% methylcellulose, adapted from [
30]). Medium was replaced every 48 hours for 12 days, and spheres were counted under light microscopy.
For secondary sphere formation, primary sphere cultures were filtered using a 70 μm nylon cell strainer, to retain spheres of larger than 70 μm in diameter. Spheres were trypsinized until they dissociated to single cells. Cells were counted, and 20,000 cells were plated on the ultra-low attachment plates, following the same conditions as listed above for primary sphere formation.
Flow cytometry
Cells were plated at 5 × 104 cells/well in 6-well format and allowed to adhere overnight. The following day, cells were treated with 0.02 μM JK184 or 0.0002% DMSO vehicle control. After four days, floating cells were combined with adherent cells harvested by trypsinization and analyzed by flow cytometry using the BD Biosciences Pharmingen (San Diego, CA, USA) FITC Annexin V Apoptosis Detection Kit I. Samples were analyzed with the BD FACScaliburS flow cytometer with recording of 15,000 events per sample. Each line was treated independently and analyzed in three biological replicates. Gates were based on negative control signals, and plots generated using FlowJo 8.8.2 (Tree Star, Inc., San Carlos, CA, USA).
Orthotopic xenograft studies
MDA.MB.436 cells with shGLI1 or control shRNA were trypsinized, washed twice in sterile 1 × PBS, and resuspended 1:1 in growth factor reduced matrigel (BD Biosciences). Cells were maintained on ice until injection. 2 × 106 control (NT) cells were injected into the right fourth mammary fat pad of five-week-old female NOD/SCID mice (NOD.CB17-Prkdcscid/J, The Jackson Laboratory, Bar Habor, ME, USA). A contralateral injection was conducted, with 2 × 106 shGLI1 cells injected into the left fourth mammary fat pad of each mouse.
Tumor growth was monitored using digital calipers, and tumor volume was calculated using the formula: W2 × L × 0.5, where L is the longer dimension and W the shorter. After seven weeks, animals were sacrificed and the tumors excised and weighed.
Immunofluorescence
Cells were plated on glass chamber slides. The next day cells were washed twice with PBS and fixed in 2% paraformaldehyde with 0.1% Triton-X-100 in PBS for 15’ at room temperature (RT). Cells were washed twice in PBS, and quenched in 100 mM glycine in PBS for 5’ at RT. The slides were washed in PBS and permeabilized in 0.1% Triton-X-100 in PBS for 15’ at RT with humidification, followed by blocking in 5% BSA in PBST for 30’ at 37°C with humidification. Primary antibodies against p50 and p65 (Cell Signaling Technology) were diluted 1:250 with PBST and incubated overnight at 4°C with humidity. Slides were washed thrice with PBST, and incubated with Alexa Fluor 594-conjugated secondary antibody (Invitrogen) diluted 1:1000 in PBST for 1 h at RT. Slides were washed thrice with PBST and PBS, and mounted with Prolong Gold (Invitrogen).
Chromatin immunoprecipitation (ChIP) assay
ChIP was performed according to published protocols with minor modifications [
31]. Briefly, cells grown to 80% confluency were fixed in 1% formaldehyde for 10’ at RT, followed by 0.125 M glycine quench. Plates were rinsed twice with PBS, and adherent cells were scraped in ice-cold PBS and collected by centrifugation. Cells were lysed on ice in buffer with protease inhibitors (5 mM PIPES pH 8, 85 mM KCl, 1% v/v igepal) using a dounce homogenizer. After centrifugation, the nuclei pellet was lysed on ice in buffer containing protease inhibitors (50 mM Tris-HCl pH 8, 10 mM EDTA, 1% w/v SDS). The chromatin was sonicated on ice for 15’ with a 30″-on/30″-off cycle using a Biorupter UCD-200 (Diagenode Inc., Denville, NJ, USA) set to high. The chromatin was cleared by centrifugation, and for each ChIP from 10
7 cells was diluted to 1 mL with buffer containing protease inhibitors (50 mM Tris-HCl pH 7.4, 150 mM NaCl, 1% v/v igepal, 0.25% w/v deoxycholic acid, 1 mM EDTA, pH 8). Antibody was added (Histone H3, p65, and IgG from Cell Signaling Technology) and incubated at 4°C overnight on a rotating platform.
A total of 30 μl of magnetic protein G beads (Cell Signaling Technology) were added to each ChIP, and incubated on a rotating platform for 2 h at 4°C. Beads were washed twice with dilution buffer, three times with 100 mM Tris-HCl pH 9, 500 mM LiCl, 1% v/v igepal, 1% w/v doxycholic acid, and once with 100 mM Tris-HCl pH 9, 500 mM LiCl, 150 mM NaCl, 1% v/v igepal, 1% w/v doxycholic acid. Antibody/chromatin complexes were eluted in 100 μl buffer (50 mM NaHCO3, 1% w/v SDS) for 1 h at RT. The samples were adjusted to 0.54 M NaCl, and incubated at 67°C overnight to reverse the crosslinks.
Samples were treated with RNaseA, and purified using a PCR cleanup kit (Qiagen). Real-time PCR was conducted using iQ SYBR Green Supermix (Bio-Rad Laboratories) on a CFX96 Real-Time System thermal cycler (Bio-Rad Laboratories) with 15 μl reactions, in triplicate. The following primer sets were used: GLI1 site 1: 5′-GGGTAAGGGCTGTTGAGGTA-3′, 5′-AAATGCTTGTCTCCCAGTGG-3′; 2: 5′-GAGCTAGGATGTGGGAGGTC-3′, 5′-TGAGAAACGGAGAGGCAGAG-3′; 3: 5′-AGGGTCGGAATAAGTGTGGT-3′, 5′-GTGTGTATGGGGAGGAGGAG-3′; 4: 5′-CTCCTCCTCCCCATACACAC-3′, 5′-CTCTCAGCACATCCGGAAAG-3′; 5: 5′-ACGCCATGTTCAACTCGATG-3′, 5′-GAGATCTGCCAAATCCTCAAGG-3′; 6: 5′-GCCCAATCCTTCCTGAGACT-3′, 5′-CGGGCAGAGTCATGGGGA-3′; GAPDH: 5′-TACTAGCGGTTTTACGGGCG-3′, 5′-TCGAACAGGAGGAGCAGAGAGCGA-3′.
Discussion
Currently, there are no targeted treatment options for patients with claudin-low breast cancer, a particularly aggressive type of breast cancer. We used mammary carcinoma cell lines with induced EMT as surrogates for cells with stem-like characteristics and screened them for growth sensitivity to 150 targeted agents. Selective sensitivity of these cells to inhibition of GLI1 implicated GLI1 as a vulnerable target. The transcriptional similarities of induced EMT mammary cells to claudin-low breast cancer suggested the potential importance of GLI1 for this breast cancer subset. Reduced GLI1 expression impeded migration, clonogenicity, primary and secondary mammosphere formation and tumor formation by claudin-low breast cancer cells. These characteristics are associated with stem-like, invasive, and aggressive aspects of breast cancer, and suggest that inhibiting GLI1 may be an effective treatment strategy for patients with claudin-low breast cancer.
Our work reveals novel SMO-independent activation of
GLI1 by the NFκB pathway, in which the p65 subunit of NFκB binds directly to the
GLI1 promoter in EMT and claudin-low cells (Figure
7C). We have only observed binding of the p65 subunit to one κB binding site in the
GLI1 promoter, but this does not preclude binding of NFκB subunits to the remaining putative κB binding sites in the
GLI1 promoter, perhaps following cytokine stimulation. Knockdown of NFκB subunits resulted in decreased
GLI1 expression (Figure
7D-E), indicating transcriptional regulation of
GLI1 by NFκB.
GLI1 levels were not completely abrogated following knockdown of NFκB subunits, indicating either that residual NFκB activity is sufficient to sustain
GLI1 expression, or that other pathways contribute to
GLI1 transcription.
We also found that NFκB is activated through a non-canonical pathway in EMT and claudin-low cells. Typically, in the absence of an inflammatory signal NFκB dimers are sequestered in the cytoplasm by IκBα. However, we observed NFκB in the nucleus of EMT and claudin-low cells (Figure
7A and Figure S5 in Additional file
1) without stimulation, indicating that NFκB is present in an activated form in the nucleus. Interestingly, we observed less nuclear NFκB in claudin-low cell lines, which express less
GLI1, notably HS578T and MDA.MB.231 cells. This association fits with our data indicating transcriptional regulation of
GLI1 by NFκB, and speaks to the molecular heterogeneity of the claudin-low subclass. Indeed, while claudin-low tumors express more
GLI1 than the basal, human epidermal growth factor receptor 2 (Her2), and luminal B subtypes overall, there still existed heterogeneity within this subset (Figure
2D). Activated NFκB [
53] and expression of
GLI1[
17]-[
19] have been associated with poor prognosis in breast cancer. It will be interesting to see if NFκB activity and
GLI1 expression are correlated in mammary tumors. Recently, nuclear
GLI1 expression was shown to be closely correlated with nuclear expression of NFκB in pancreatic cancer, and both were associated with shorter overall survival and worse outcome [
54]. It will be interesting to determine if a similar phenomenon occurs in breast cancer, and if patients with tumors that co-express NFκB and
GLI1 have a worse outcome.
Constitutive activation of NFκB in nuclear lysates from breast cancer cells has been observed [
55], and it will be interesting to determine the responsible factors that contribute to NFκB pathway activation in EMT and claudin-low cells. One possibility is ERBB3, since recent work has revealed that the ERBB3 ligand heregulin increases mammosphere formation in breast cancer cell lines, which was attenuated by NFκB pathway inhibition [
56]. It will be interesting to explore the role of ERBB3 on NFκB in claudin-low cells and EMT, especially given our findings with EGFR in EMT cells (Figure S1 in Additional file
1) and the known interactions among ERBB family members in breast cancer [
57]. Recently Yamamoto
et al. identified activated NFκB in basal and claudin-low tumors, and a correlation between NFκB activity and JAG1 expression, which was associated with poor prognosis in the basal subset [
52]. These results combined with our findings suggest that NFκB could affect different downstream targets depending on subtype, namely JAG1 in basal and GLI1 in claudin-low tumors.
Our work supports earlier studies implicating GLI1 signaling in some breast cancer cell lines, albeit through different mechanisms.
GLI1 expression is elevated in SUM1315 cells, and knockdown of
GLI1 in MDA.MB.231 cells reduces cell growth, invasion, and metastasis [
41]. Targeting
GLI1 in inflammatory breast cancer has been shown to decrease the migratory ability of these cells, and to increase apoptosis [
38]. We have extended these findings to the claudin-low subtype as a class, and our findings lend evidence to the potential of GLI1 as a therapeutic target in breast cancer.
There is increasing evidence for non-canonical Hh pathway activation in a variety of cancers [
46] including breast [
38],[
42]. Recently, Goel
et al. demonstrated a contribution of Neuropilin-2 (
NRP2), a vascular endothelial growth factor (VEGF) co-receptor, to
GLI1 levels in claudin-low cell lines [
40]. A non-canonical mechanism was implicated, since
SMO knockdown did not interfere with the process. It will be of interest to determine if NRP2 exerts its effects on GLI1 through NFκB. Although the NFκB and Hh pathways are implicated in breast cancer, and these pathways share some common downstream targets [
58], to our knowledge direct transcriptional crosstalk between the two pathways in breast cancer has not yet been reported.
Although our studies focused on
GLI1,
GLI2 expression is also elevated in EMT and claudin-low cells (Figure
2A-B). While
GLI1 is itself a GLI1 regulatory target [
34], in basal cell skin carcinoma cells, activation of
GLI2 by
GLI1 is indirect, and perhaps context dependent [
59]. There are no consensus Gli or κB binding sites in the
GLI2 promoter. Therefore, it is likely that the regulation of
GLI2 expression in these cells occurs via a different mechanism than that described here for
GLI1. Similarly, while some GLI1 targets have been identified [
37], the activity of the Gli proteins is highly context dependent [
34], and it will be of great interest to determine the effectors through which Gli1 mediates the biological phenotypes we observed in claudin-low lines.
EMT cells and claudin-low cells are closely related to cancer stem cells [
5]-[
7]. There is evidence for Hh signaling in normal and malignant human mammary stem cells, and upregulation of
GLI1 in mammospheres [
16]. We observed upregulation of
GLI1 in mammospheres, and a decrease in primary and secondary sphere formation after
GLI1 knockdown (Figure
5C), strongly suggesting a role for
GLI1 in maintenance of breast cancer stem cells/progenitor cells. Elevated levels of
GLI1 transcripts were also seen in a published dataset of mammospheres grown from primary patient material (Figure
2D). We have shown here evidence of crosstalk between the GLI1 signaling and NFκB pathways in both EMT and claudin-low cell lines, indicating that activated
GLI1, could be a mechanism that also operates in breast cancer stem cells. Recent reports substantiating the existence of cancer stem cells in solid tumors [
60] reinforce the potential importance of these findings for breast cancer therapy.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
SAC carried out and designed the experiments and drafted the manuscript. MRZ aided in conception and execution of the animal experiments. QY aided in the conception and design of the animal experiments. DXN analyzed published cell line expression data to create the gene expression heat map, made substantial contributions to the design of the experiments, and reviewed the manuscript critically for important intellectual content. DFS significantly contributed to the design and conception of the study, and critically reviewed the manuscript for intellectual content. All authors read and approved the final manuscript and agree to be accountable for the integrity of the work.