Core signaling pathways in ovarian cancer stem cell revealed by integrative analysis of multi-marker genomics data
Core signaling pathways in ovarian cancer stem cell revealed by integrative analysis of multi-marker genomics data
Tianyu Zhang 0 1
Jielin Xu 0 1
Siyuan Deng 0 1
Fengqi Zhou 0 1
Jin Li 0 1
Liwei Zhang 1
Lang Li 0 1
Qi-En Wang 1
Fuhai Li 0 1
0 Department of BioMedical Informatics (BMI), The Ohio State University , Columbus , Ohio, United States of America, 2 School of Mathematical Sciences, Dalian University of Technology , Dalian , China , 3 Department of Radiology, The Ohio State University , Columbus, Ohio , United States of America
1 Editor: Gianpaolo Papaccio, Università degli Studi della Campania , ITALY
Tumor recurrence occurs in more than 70% of ovarian cancer patients, and the majority eventually becomes refractory to treatments. Ovarian Cancer Stem Cells (OCSCs) are believed to be responsible for the tumor relapse and drug resistance. Therefore, eliminating ovarian CSCs is important to improve the prognosis of ovarian cancer patients. However, there is a lack of effective drugs to eliminate OCSCs because the core signaling pathways regulating OCSCs remain unclear. Also it is often hard for biologists to identify a few testable targets and infer driver signaling pathways regulating CSCs from a large number of differentially expression genes in an unbiased manner. In this study, we propose a straightforward and integrative analysis to identify potential core signaling pathways of OCSCs by integrating transcriptome data of OCSCs isolated based on two distinctive markers, ALDH and side population, with regulatory network (Transcription Factor (TF) and Target Interactome) and signaling pathways. We first identify the common activated TFs in two OCSC populations integrating the gene expression and TF-target Interactome; and then uncover up-stream signaling cascades regulating the activated TFs. In specific, 22 activated TFs are identified. Through literature search validation, 15 of them have been reported in association with cancer stem cells. Additionally, 10 TFs are found in the KEGG signaling pathways, and their upstream signaling cascades are extracted, which also provide potential treatment targets. Moreover, 40 FDA approved drugs are identified to target on the up-stream signaling cascades, and 15 of them have been reported in literatures in cancer stem cell treatment. In conclusion, the proposed approach can uncover the activated up-stream signaling, activated TFs and up-regulated target genes that constitute the potential core signaling pathways of ovarian CSC. Also drugs and drug combinations targeting on the core signaling pathways might be able to eliminate OCSCs. The proposed approach can also be applied for identifying potential activated signaling pathways of other types of cancers.
Data Availability Statement: Part of the data
underlying this study are accessible using the
following accession numbers: GSE33874 and
GSE82304, in the NCBI GEO database. The
TFTarget interactions, derived from TRANSFAC
database, are third party data and are available
using the following link: https://genome.cshlp.org/
content/24/11/1869/suppl/DC1. The authors did
not have special access privileges.
Funding: This work is supported in part by the
startup fund from Department of Biomedical
Informatics, Translational Data Analytics, The Ohio
State University to FL. The funder had no role in
study design, data collection and analysis, decision
to publish, or preparation of the manuscript.
Over 90% of ovarian cancers are epithelial in origin, and epithelial ovarian cancer, especially the
most aggressive subtype high-grade serous ovarian cancer (HGSOC), accounts for the majority
of ovarian cancer deaths [
]. Most tumors are initially responsive to the conventional
chemotherapy, and the patients enter into clinical remission after initial treatment. However, tumor
recurrence occurs in more than 70% of patients despite treatment, and the majority eventually
becomes refractory to treatments . Recent research evidences show that tumor is a mixture of
heterogeneous populations of cells with different levels of malignity. A subpopulation of tumor
cells characterized by the capacity of self-renewing, differentiation, and tumor-initiating are
called cancer stem cells (CSCs) or tumor initiating cells (TICs) [
]. CSCs play important roles
in tumor initiation, progression, metastasis, recurrence and drug resistance [4±7]. Thus,
elimination of CSCs is important to overcome drug resistance to improve the prognosis of cancer
patients. The knowledge about CSCs is limited, and one major challenge is that it is difficult
to identify and isolate CSCs with few biomarkers because CSCs are heterogeneous and could
exist a specific hierarchy [8±10]. Ovarian cancer stem cells (OCSCs) have been successfully
identified and isolated based on their expression of distinctive cell surface markers CD44,
CD117, MyD88, and CD133 [
], as well as the activity of ALDH . These CSCs harbor
enhanced tumorigenicity and chemoresistance [
]) and are thought to drive the universal
recurrence of ovarian cancer, as well as responsible for the development of therapeutic
]. Though studies with these markers show evidence in support of OCSCs [
], there is
still a lack of effective drugs to differentiate and eliminate them [
Though common signaling pathways, e.g., WNT, NOTCH, SHH, JAK/STAT, have been
associated with all types of CSCs [
], the core signaling mechanism regulating Ovarian
CSCs remain unclear. The differential gene expression analysis often fails to identify genes
regulate CSCs because it is difficult to identify a few testable targets from a large number of
differentially expressed genes mixed with many passenger genes. Additionally, important proteins
regulating CSCs might be missed because either the fold change is small or the gene expression
data is not available. Therefore, it is necessary to integrate multi-datasets with prior knowledge,
e.g., regulatory network and signaling pathways, to increase the possibility of identifying the
true CSC driver genes in the systems biology perspective. Thus, in this study, we propose an
approach to identify potential core signaling pathways of OCSCs by integrating transcriptome
data of OCSCs isolated based on two distinctive markers, ALDH and side population (Hoechst
33342 stain), with the prior knowledge of regulatory network and KEGG signaling pathways.
Our hypothesis is that the integrative analysis of multi-genomics data sets of OCSCs with
distinct markers could infer more accurate driver-signaling network regulating CSC to generate a
small number of testable biomarkers and drugs. In specific, we first identify the common
activated transcription factors (TFs) in two OCSC populations; and then construct the core
signaling pathways by uncovering the up-stream signaling cascades of the activated TFs, which
constitute the potential core signaling pathways of ovarian CSC. Drugs targeting on the
upstream signaling cascades of activated TFs are selected as potential treatments to eliminate
ovarian cancer stem cells. The details of methodology and datasets are introduced in Section 2;
and analysis results are shown in Section 3, followed by the discussions and conclusion.
Materials and methodology
Transcriptome datasets of OCSCs
In this study, we manually searched the ovarian CSC gene expression datasets in NCBI GEO
(Gene Expression Omnibus) using the aforementioned markers and ªOvarianº as the keywords,
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e.g., ªCD44, Ovarianº, and only two datasets were found, i.e., GSE33874 and GSE82304. The
dataset GSE33874 is the gene expression profile of isolated side population (SP-Hoechst Blue
High and Hoechst Red Low, Ovarian CSCs) and main population (MP) of fresh ascites obtained
from women with high-grade advanced stage papillary serous ovarian adenocarcinoma [
Gene expression profiles (in triplicate) of SKOV3 human ovarian cancer cells of Aldefluor high
(ALDH+, Ovarian CSCs) and Aldefluor low (ALDH-) populations were available in dataset
]. The GEO2R was employed to get the fold change data of individual genes.
KEGG signaling pathways and regulatory network
To obtain the KEGG signaling pathways, the ªPathviewº R package was employed to download
KGMLs of humans pathways [
]. Then the ªKEGGgraphº R package was used to extract
nodes and edges of KEGG signaling pathways from KGMLs [
]. In total, 282 signaling
pathways were collected from seven categories: metabolism, genetic information processing,
environmental information processing, cellular processes, organismal systems, human diseases,
and drug development. The TF-Target regulatory network was downloaded from the
supplemental material of reference [
], which was derived from the TF binding site predictions for
all target genes from TRANSFAC (v7.4) [
]. In summary, the TF-target regulatory network
consists of 230 TFs, 12733 target genes, and 79100 TF-Target interactions.
Signaling pathway construction
First, the Fisher's exact test (using hypo-geometric distribution) [
] was used to identify the
activated TFs by comparing the number of up-regulated targets vs. the number of all target genes,
with the number of all the up-regulated genes vs. the number of all the genes tested. The p-value
threshold, 0.05, was used to select the activated TFs. Second, all 282 signaling pathways from
KEGG were examined, and all the signaling cascades from the starting nodes to the activated TFs
were extracted, and then the top 3 signaling paths were kept to construct the signaling pathway
regulating the given TFs. The python package, NetworkX, was used to screen all the 282 KEGG
signaling pathways to extract signaling cascades starting from the beginning genes of individual
signaling pathways to the given TFs. Then we score each signaling cascades using the average
expression fold change of genes (on the signaling cascades and with fold change > 0). To control
the size of up-stream signaling network of given TFs, the top 3 signaling cascades are kept. The
up-regulated target genes (Fold change > = 2) in both datasets are linked to the given TFs.
Twenty-two activated TFs
In the two gene expression datasets, there are 1988 and 2528 up-regulated genes (fold change
> = 2); and 883 and 2821 down-regulated genes (fold change < = 0.5) respectively. It is
difficult for biologists to identify potential targets associated with Ovarian CSCs from such large
number of differentially expressed genes. With the aim of discovering testable regulatory
signaling networks that maintain Ovarian CSCs, we identify the activated TFs (whose target
genes are up-regulated) in both datasets (CSCs isolated from ALDH+ marker and
side-population) using the Fisher's exact test by integrating the up-regulated genes (Fold_Change > = 2)
and the TF-target interactome (gene regulation network) data. In total, 22 TFs are identified
(see Table 1). As can be seen, some TFs will be missed using only gene expression fold change
because either there is no gene expression data available (NA) or the fold change is small. We
conducted the literature search to evaluate these TFs, and surprisingly 15 TFs have been
reported to play important roles in cancer stem cell regulation (see Table 2). For example,
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FOXO3 is essential for maintenance of CSC properties in pancreatic ductal adenocarcinoma
]; FOXO4 is related to stem cell-like properties of large B-cell lymphoma cells [
]. LEF1 is
able to regulate glioblastoma stem-like cell self-renewal [
]; NFATc2 enhances
tumor-initiating phenotypes in lung adenocarcinoma [
Up-stream signaling cascades regulating activated TFs
We further uncover the up-stream signaling cascades regulating these activated TFs using
KEGG signaling pathways. Out of 22 activated TFs, 10 TFs (ELK1, NRF1, FOXO4, LEF1,
MEF2A, FOXO3, NFATC2, SPI1, TEAD1 and E2F1) are found in KEGG signaling pathways
with up-stream signaling cascades link to them (see Table 1 and red color nodes in Fig 1). As
can be seen in Fig 1, many target genes (cyan color nodes) of transcription factors, FOXO4,
LEF1, NFATC2, SPI1 and TEAD1, are up-regulated (fold_change > = 2) in both datasets. The
yellow color nodes represent the starting proteins activating the signaling cascades in KEGG,
e.g., the MAP kinases that are often activated by mitogenic and environmental stress, the EGF
growth factor, and FLT3 that encodes a class III receptor tyrosine kinase that regulates
hematopoiesis, and PPP3R2 that is related to the calcium signaling. These signaling cascades provide
potential testable biomarkers of Ovarian CSCs for experimental design.
FDA approved drugs targeting on up-stream signaling of activated TFs
To investigate potential drugs that can perturb the Ovarian CSCs, we mapped the FDA
approved drugs on the integrative signaling network (see Fig 2). The target information
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obtained from DrugBank (version 5.0.11) [
]. In total, 40 drugs (pink nodes in Fig 2) were
selected targeting on different signaling cascades. Through the literature search, we found that
15 drugs have been reported to treat cancer stem cells (see Table 3). For example, Palbociclib
can block the propagation of lung, ovarian and breast cancer stem cells by targeting on CDK4
]. Bosutinib and Trametinib targeting on the MAP2K1/2 were used for cancer stem cell and
multi-drug resistance treatment [
]. Celecoxib was used in colon cancer stem cell related
treatment by targeting on PDPK1 . Metformin and Phenformin (targeting on PRKAA1/
PRKAB1 were used for eliminate prostate cancer stem cells [
]. Moreover, Metformin has
been shown to be able to overcome drug resistance to tyrosine kinase inhibitors (TKI) of EGF
receptor (EGFR) in lung cancer . Table 3 shows more details. In addition to the single
drug treatment, combinations of drugs targeting on the different signaling cascades might
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Fig 1. Activated signaling pathways of ovarian CSCs. The color of yellow, green, red and cyan represents signaling starting genes, signaling transduction genes, TFs
and target genes respectively.
have better effects to eliminate cancer stem cells. In summary, this analysis can provide testable
hypothesis and potential drug candidates to eliminate OCSCs.
Discussion and conclusion
Most ovarian cancer tumors are initially responsive to the conventional chemotherapy.
Whereas, more than 70% of patients will experience tumor recurrence, and the majority
eventually becomes treatment resistant. Ovarian cancer stem cells (CSCs) are thought to drive the
universal recurrence of ovarian cancer, as well as responsible for the development of
therapeutic resistance. However, the core signaling pathways regulating Ovarian CSCs remain unclear,
and there is still a lack of effective drugs and drug combinations to differentiate and eliminate
them to improve cancer survival.
In this study, we propose to identify potential core signaling pathways of OCSCs in a
datadriven manner by integrating transcriptome data of OCSCs isolated based on two distinctive
cell surface markers, ALDH and side population, with prior knowledge, e.g., regulatory
network and signaling pathways, to increase the possibility of identifying the true CSC driver
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Fig 2. FDA approved drugs targeting on up-stream signaling of activated TFs of ovarian CSCs. The color of yellow, green, red, cyan and pink represents signaling
starting genes, signaling transduction genes, TFs, target genes, drugs respectively.
genes and signaling pathways. We identified 22 activated transcription factors, and 15 of them
have been reported in the association with cancer stem cells. In addition, 10 transcription
factors were found in the KEGG signaling pathways, and we extracted the up-stream signaling
cascades regulating these transcription factors, which provide potential core signaling
mechanism of ovarian CSC regulation. Moreover, we mapped the FDA approved drugs on these
upstream signaling cascades. Forty FDA approved drugs were identified and 15 of these drugs
have been reported in cancer stem cell treatment. Combinations of these drugs targeting on
different up-stream signaling cascades might be effective to eliminate ovarian cancer stem
The proposed approach can be helpful for discovering synergistic and effective drug
combinations. It is well known that inhibiting a single target does not ensure the success of effective
treatment due to the complicated interplay of multiple signaling pathways [
]. For example,
the activation of Sonic Hedgehog (SHH) signaling and evolution through a mesenchymal
phenotype have been uncovered as a novel mechanism of drug resistance to tyrosine kinase
inhibitors (TKI) of EGF receptor (EGFR) in lung cancer [
], and play important roles in regulating
hepatocellular carcinoma (HCC) [
]. Also, it was reported that the number of CSCs can be
increased by MSCs [
], which could be produced by the activation of SHH signaling [
ovarian cancer. Thus drug combinations blocking the signaling interplay have high possibility
to be synergistic and effective in cancer treatment. For example, Metformin (widely used as
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Targeting cancer stem cell propagation with palbociclib, a CDK4/6 inhibitor: Telomerase drives
tumor cell heterogeneity. [
Arsenic trioxide sensitizes cancer stem cells to chemoradiotherapy. A new approach in the
treatment of inoperable glioblastoma multiforme. [
TP53 IKBKB PRKAG2 PRKAG3 PRKAA1
PRKAA2 PRKAG1 PRKAB1
The therapeutic potential of targeting ABC transporters to combat multi-drug resistance. [
Roles of EGFR and KRAS and their downstream signaling pathways in pancreatic cancer and
pancreatic cancer stem cells. [
Expression Patterns of Cancer Stem Cell Markers During Specific Celecoxib Therapy in
Multistep Rat Colon Carcinogenesis Bioassays. [
Antitumor activity of Cetuximab in combination with Ixabepilone on triple negative breast
cancer stem cells. [
Cancer stem cell-driven efficacy of trastuzumab (Herceptin): towards a reclassification of
clinically HER2-positive breast carcinomas. [
Tyr1068-phosphorylated epidermal growth factor receptor (EGFR) predicts cancer stem cell
targeting by erlotinib in preclinical models of wild-type EGFR lung cancer. [
Cancer Stem Cell-Based Models of Colorectal Cancer Reveal Molecular Determinants of
Therapy Resistance. [
Afatinib radiosensitizes head and neck squamous cell carcinoma cells by targeting cancer stem
Mesalazine inhibits the β-catenin signalling pathway acting through the upregulation of
μprotocadherin gene in colo-rectal cancer cells. [
Cancer Stem Cells in Prostate Cancer: Implications for Targeted Therapy. [
Extracellular ATP reduces tumor sphere growth and cancer stem cell population in
glioblastoma cells. [
Metformin and phenformin deplete tricarboxylic acid cycle and glycolytic intermediates during
cell transformation and NTPs in cancer stem cells. [
Metformin and prostate cancer stem cells: a novel therapeutic target. [
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anti-diabetic drug, also identified in this study) and MEK-inhibitors (Selumetinib/Pimasertib
targeting on the RAF/RAS/MAPK signaling) were discovered to effectively inhibit the
proliferation and metastasis of LKB1 positive Non-Small Cell Lung Cancer (NSCLC) cancer cells [
The synergism of the combination is the down-regulation of GLI1, which is the mediator of
epithelia-to-mesenchymal transition (EMT) signaling, and can be affected by SHH signaling
]. Moreover, the drug combination, Metformin and Erlotinib (EGFR inhibitor), is used in a
phase II study for the treatment of stage IV NSCLC [
There are also some limitations of this integrative data analysis. First, the current TF-target
interactome data might not be complete and accurate. In the future work, we will integrate
new TF-target interaction data resources, e.g., TRUST (text mining) [
] and GTRD
(ChipSeq data) [
], to improve the quality and completeness [
]. Moreover, the tissue specific
regulatory network data might be useful to further refine the TF-target interactome data [
Second, 12 activated TFs are still missed in the up-stream signaling network analysis.
Additional signaling pathway database, e.g., BioGRID [
], STRING [
], Reactome [
], could be
integrated to identify more up-stream signaling cascades of additional active TFs. Thirdly,
other pharmacological data resources, e.g., LINCS (reverse gene signature based data) [
can be integrated to identify more drugs or prioritize drugs to eliminate the Ovarian CSCs.
Also, the drug repositioning [72±74] and drug combination prediction [75±77] are not trivial
tasks. In the future work, we will integrate additional data resources to prioritize targets and
drug combinations to block multiple TF and signaling interplays to eliminate ovarian CSCs.
We would like to thank colleagues in Biomedical Informatics Department for the helpful
Conceptualization: Fuhai Li.
Data curation: Tianyu Zhang, Jielin Xu, Qi-En Wang, Fuhai Li.
Methodology: Tianyu Zhang, Jielin Xu, Siyuan Deng, Fengqi Zhou, Jin Li, Liwei Zhang, Lang
Li, Qi-En Wang, Fuhai Li.
Writing ± original draft: Tianyu Zhang, Qi-En Wang, Fuhai Li.
Writing ± review & editing: Tianyu Zhang, Qi-En Wang, Fuhai Li.
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