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The potential role of SOX2 in the anti-apoptosis property of cancer cells
Weijun Su1, 4, 5, Si Chen2,, Na Li3, 4, 5
1.
Department of Pathology, 2. Department of Immunology, 3.Department of Pathophysiology,
School of Medicine, Nankai University, 94 Weijin Road, Tianjin 300071, China
4.
Tianjin Key Laboratory of Tumor Microenvironment and Neurovascular Regulation, Tianjin,
China 300071
5
. Collaborative Innovation Center for Biotherapy, School of Medicine, Nankai University,
Tianjin, China 300071

Current Address: Division of Biochemical Toxicology, National Center for Toxicological
Research, FDA, 3900 NCTR Road, Jefferson, AR 72079, USA
Running Title: Regulatory effect of SOX2 on cell survival

Correspondence: Na Li, Email: [email protected]
Keywords: SOX2, apoptosis, cancer, cancer stem cells
1
Abstract
SOX2, a member of the SOX family, plays important roles in the maintenance of
pluripotency and self-renewal of embryonic stem cells, as well as adult tissue progenitor cells.
Recent studies have revealed ectopic expression of SOX2 in a number of cancer types and
disclosed vital contributions of SOX2 to the stemness property of cancer stem cells and tumor
progression. Despite with some controversy, most researchers have reached a consensus that
SOX2 works as an anti-apoptosis factor in human cancer cells. This review will discuss recent
advances in elucidating the regulatory effect of SOX2 on survival and anti-apoptosis property
of cancer cells. These studies support the concept that SOX2 regulates intricate
apoptosis-related proteins and signal pathways. Delineation of these pathways may help to
develop novel therapy strategies targeting cancer stem cells.
2
Introduction
The transcription factor SOX2 is a member of the SOX (Sex-related HMG Box) family,
and it has been found to play a significant role in the maintenance of the unique characters
such as clonogenicity, pluripotency and self-renewal of embryonic stem cells (ESCs) as well
as adult tissue progenitor cells, and is also indispensable in the process of organogenesis
[1-5]
.
Moreover, SOX2 is one of the key transcription factors capable of generating induced
pluripotent stem cells (iPSCs) from differentiated somatic cells [6-8].
Recent studies pointed out the potential contribution of SOX2 to cancer progression.
Although there is still some controversy [9], elevated SOX2 expression has been detected in a
number of cancer types, such as breast cancer, lung cancer, ovarian cancer and prostate cancer
et al.
[10-13]
. According to our previous studies, SOX2 expression is correlated with the
histological grade and TNM stage in human breast cancer
prostate cancer
[13]
, and Gleason scores in human
[12]
. In addition, predominantly higher expression of SOX2 was found in both
non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC) than in normal or
paracarcinoma tissues
[10]
. Cancer stem cells (CSCs), which have been proposed as
tumor-initiating cells in recent years, possess a series of the same characteristics as ESCs [14-16].
CSCs exhibit greater resistance to apoptosis than differentiated cancer cells, and this
chemoresistance nature of CSCs makes them to be the source of cancer recurrence
[17-19]
. It is
found that SOX2 is highly expressed in isolated CSCs, and contribute to tumorigenesis and
recurrence of certain cancers
[20-23]
. Several groups including us have revealed that SOX2
plays an important role in the anti-apoptosis property of cancer cells using different tumor
models [12, 23, 24]. Nevertheless, the mechanism by which SOX2 regulates apoptosis signals still
3
need to be extensively explored and discussed. In this brief review, we will summarize the
recent studies focusing on the potential role of SOX2 in the anti-apoptosis property of human
cancer, and also further discuss our recent published results on the regulation of apoptosis by
SOX2 through MAP4K4-Survivin signaling pathway.
Role of SOX2 in apoptosis of cancer cells
In our recent published results, we found that suppression of SOX2 expression in distinct
human lung cancer cell models similarly results in apoptosis
[24]
. This observation suggests
SOX2 to be a critical gene for the survival of human cancer cells, which is consistent with our
previous finding that SOX2 increases prostate cancer cells’ resistance against apoptotic
stimuli and chemotherapy
[12]
. Similar results have been discovered by several other research
groups using different tumor models independently. In ovarian cancer, SOX2 overexpression
conferred enhanced resistance to apoptosis in response to conventional chemotherapies and
TRAIL
[25]
. However, in their study, modulation of SOX2 expression did not alter cultured
ovarian cancer cell proliferation, which is different to our results in prostate cancer
[12]
. In
human gastric cancer, there are controversial reports on the role of SOX2. Yuasa’s group
reported that SOX2 expression is downregulated in some human gastric carcinomas compared
to normal gastric mucosae
[9]
. And they further demonstrated that exogenous expression of
SOX2 suppresses cell proliferation in gastric cancer cell lines and induces apoptosis in gastric
cell lines
[9]
. Hütz et al. also verified that most gastric cancer cell lines show no or marginal
expression of SOX2 mRNA [26]. Distinct to Yuasa’s group, however, Hütz et al. chose AZ-521
cell line for their further investigation, which expresses the highest SOX2 transcriptional
4
activity in all the gastric cancer cell lines they screened. Using this SOX2 high-expression cell
line, they came to the very opposite conclusion that blocking SOX2 transcriptional activity
leads to the inhibition of cell proliferation, partly due to the induction of apoptosis
[26]
. The
different observations may be due to the fact that SOX2 is only expressed in a subset of
gastric cancers, and different cell lines were used in different studies.
The mechanism by which SOX2 regulates cancer cell survival and prevents apoptosis
Despite with some controversy, most researchers have reached a consensus that SOX2
mainly works as an anti-apoptosis factor in human cancer. However, the results in different
models indicate that SOX2 may regulate different sets of downstream target genes depending
on cancer cell models.
In our study, we found silencing of SOX2 in human NSCLC induced apoptosis via
activation of both ‘extrinsic’ and ‘intrinsic’ apoptotic signaling pathways, which was reflected
by the activation of p53, overexpression of TNF-, Bax, Bad, release of cytochrome C from
mitochondria to the cytosol, reduced expression of Survivin and cleavage of Procaspase 3 and
Procaspase 8
[24]
. Similar to our results, it was also reported that SOX2 regulates apoptosis
signals by direct regulation of Bcl-2 family members [27]. Chou et al discovered that silencing
of SOX2 in lung adenocarcinoma cells causes marked apoptosis as well as autophagy with
down-regulation of BCL2L1 [27]. They further demonstrated the direct binding of SOX2 to the
promoter of BCL2L1 to regulate cell survival by preventing apoptosis and autophagy
[27]
.
Besides these finding, our previous study showed that overexpression of SOX2 in human
prostate cancer cells reduces the store-operated calcium entry activity, which may contribute
5
to the anti-apoptotic effect of SOX2
[12]
. SOX2 was also reported to increase the tamoxifen
resistance of breast cancer cells through activation of WNT signal pathway [22]. Those
studies revealed the possible mechanisms and signal pathways that contribute to
anti-apoptotic property of SOX2.
SOX2 gene has been shown to be the amplification target at chromosome 3q26.3 in
esophageal squamous cell carcinoma (ESCC) and lung squamous cell cancer (LSCC)
[28, 29]
. It
has been reported that SOX2 is correlated with a series of signaling pathways which are
significant in cancer cell survival and proliferation. Gen et al. assayed multiple signaling
pathways using phosphoprotein assay, and determined that SOX2 can activate AKT/mTORC1
signaling pathway, and the ability of SOX2 to promote the proliferation of ESCC cells
depends on AKT activation
[30]
. Justilien et al. focused on the cooperation effect of PRKCI
and SOX2 on Hedgehog (Hh) signaling pathway activation in LSCC, which are coamplified
on chromosome 3q26 [31]. SOX2 can be phosphorylated by protein kinase C (PKC), and the
PKC-mediated SOX2 phosphorylation is required for the expression of Hedgehog
acyltransferase (HHAT), which catalyzes the rate-limiting step in Hh ligand production [31]. In
primary LSCC tumors, PRKCI and SOX2 are frequently coamplified and coordinately
overexpressed, and the PKC-SOX2-Hh signaling is important for maintenance and survival
of primary human LSCC stem-like cells
[31]
. In primary melanoma cells, SOX2 is reported to
be regulated by Hh signaling via direct binding of transcription factors GLI1 and GLI2 to the
proximal promoter region of SOX2 [32]. It remains unclear whether there is a positive feedback
loop between SOX2 and Hh signaling. Watanabe et al. further compared SOX2-regulated
gene profiles in SCC cell lines to ES cell profiles using ChIP-seq analysis, they found that
6
SOX2 preferentially interacts with p63 in SCCs, but not OCT4, a preferred SOX2 binding
partner in ES cells [33]. Moreover, SOX2 and p63 jointly regulated a series of genes, including
ETV4, which mediated the proliferation and anchorage-independent growth of SCC cells [33].
SOX2 regulates apoptosis through MAP4K4-Survivin signaling pathway
In our recent study, we uncovered the activation of both mitochondrial and death
receptor signal pathways after downregulation of SOX2 in human NSCLC
[24]
. And
unexpectedly, MAP4K4 was revealed to be one of the key proteins in the apoptotic cascade
following SOX2 silencing in our research. We found that the mRNA expression level of
MAP4K4 and its downstream genes such as MAP2K4, ERK1 and JNK1 were all increased in
human NSCLC cell lines (including A549, H1299 and H460) after down-regulation of SOX2,
supporting the activation of MAP4K4 signals. Blocking the expression of MAP4K4 can
effectively inhibit the apoptosis of NSCLC induced by SOX2 silencing, with reduced
expression of p53, P-p53, cytosolic cytochrome C and TNF-. We also found that activated
MAP4K4 induced down-regulation of Survivin, and ectopic expression of Survivin in A549
cells rescued the apoptotic effect induced by SOX2 silencing. Consistent with our finding,
Lin’s study disclosed that SOX2 regulates the expression of Survivin in human prostate
cancer cells [34]. Given that the traditional opinion viewed MAP4K4 to be implicated in cancer
cell growth in hepatocellular carcinoma and pancreatic cancer [35, 36], we tended to believe that
MAP4K4-triggered apoptosis may occur in a context-dependent manner in NSCLC.
Although obvious cell death were observed in human NSCLC, down-regulation of SOX2
did not showed significant apoptosis effect but only cell growth inhibition in many other
7
cancer cell types, including certain types of prostate and breast cancer cell lines, such as
DU145, MCF7, 4T1, EO771 et al. from our published or unpublished studies [12, 13] , possibly
due to different SOX2 silencing efficiency and different gene background in these cells.
Concluding remarks
In summary, mounting evidences have identified SOX2 as an important anti-apoptotic
gene that plays an important role in tumor recurrence in many cancer types both in vitro and
in vivo. Our results showed activation of both death receptor and mitochondria signal pathway
in NSCLC after SOX2 silencing and highlighted the unexpected context-dependent role of
MAP4K4 in SOX2 silencing-induced apoptosis. However, the signal pathway mediating the
apoptotic effect of SOX2 silencing may vary depending on cancer cell types, and a further
disclosure of the molecular mechanism by which SOX2 regulates apoptosis may lead to
effective strategy to induce the apoptosis in cancer stem cells and prevent recurrence of
cancers.
8
Acknowledgment
This work was funded by: Natural Science Foundation of China (81372308 to N. Li).
9
Conflicting interests
The authors declare that they have no conflicting interests.
10
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