Reduced expression of SOX7 in ovarian cancer: a novel tumor suppressor through the Wnt/β-catenin signaling pathway
© Liu et al.; licensee BioMed Central Ltd. 2014
Received: 2 July 2014
Accepted: 28 August 2014
Published: 5 September 2014
Products of the SOX gene family play important roles in the life process. One of the members, SOX7, is associated with the development of a variety of cancers as a tumor suppression factor, but its relevance with ovarian cancer was unclear. In this study, we investigated the involvement of SOX7 in the progression and prognosis of epithelial ovarian cancer (EOC) and the involved mechanisms.
Expression profiles in two independent microarray data sets were analyzed for SOX7 between malignant and normal tissues. The expression levels of SOX7 in EOC, borderline ovarian tumors and normal ovarian tissues were measured by immunohistochemistry. We also measured levels of COX2 and cyclin-D1 to examine their possible involvement in the same signal transduction pathway as SOX7.
The expression of SOX7 was significantly reduced in ovarian cancer tissues compared with normal controls, strongly indicating that SOX7 might be a negative regulator in the Wnt/β-catenin pathway in ovarian cancer. By immunohistochemistry staining, the protein expression of SOX7 showed a consistent trend with that of the gene expression microarray analysis. By contrast, the protein expression level of COX2 and cyclin-D1 increased as the tumor malignancy progressed, suggesting that SOX7 may function through the Wnt/β-catenin signaling pathway as a tumor suppressor. In comparison between the protein expression levels of SOX7 with pathological features of the cancer, we found that SOX7 was down-regulated mainly in serous cystadenocarcinoma and advanced stages of the cancers.
The expression of SOX7 correlates with tumor progression as a tumor suppressor, possibly through the Wnt/β-catenin signaling pathway in ovarian cancers, suggesting that SOX7 may be a promising prognostic marker.
KeywordsSOX7 Tumor suppressor Ovarian cancer Wnt/β-catenin signaling pathway
Ovarian cancer is one of the three most common cancers in female with high morbidity and mortality rates . The disease is also among the most lethal cancers in females, primarily because of its aggressive metastasis within the peritoneal cavity. It is hardly detectable at the early stage, largely due to the lack of specific symptoms and reliable screening ,. Among all types, epithelial ovarian cancer (EOC) is the most common, accounting for 85% to 90% of ovarian cancers , and this type is highly lethal. This situation calls for the investigation of pathogenesis of EOC and the identification of molecular markers for early diagnosis or for use as targets in treatment.
SOX7, a member of subfamily SOXF along with SOX17 and SOX18, has been identified as a developmental regulator in hematopoiesis and cardiogenesis –. The human SOX7 gene is located at the chromosomal region of 8p23.1 and is approximately 7.7 kbps in length ; a frequently methylated CpG island on the promoter region of the SOX7 gene plays a role in regulating SOX7 gene expression. SOX7 is frequently down-regulated in many human cancers, such as prostate, colon, lung, and breast cancers, and its reduced expression often correlates with poor prognoses –. It is reported that SOX7 can directly bind β-catenin and negatively regulate its activity ,.
The Wnt/β-catenin signal transduction pathway is widely acknowledged as playing a dominating role in human diseases, especially in the occurrence and development of cancer . Although the molecular mechanism by which SOX7 produces its tumor suppressive effects has yet to be fully determined, SOX7 has been shown to interact with β-catenin and inhibit cell proliferation mediated by the Wnt signaling pathways. Evidence is accumulating to show that SOX7 may disrupt the transcriptional function of the β-catenin-TCF/LEF complex and inhibit the activity of Wnt target genes including cyclin D1, c-Myc and COX-2 ,. Additionally, recent studies indicate that SOX7 may act as a tumor suppressor ,,. Ectopic SOX7 expression is reported to have a regulating function against cell growth and promote apoptosis; meanwhile, SOX7 gene knockdown may lead to neoplastic transformation ,. As SOX7 negatively regulates the Wnt/β-catenin signaling pathway by impeding the transcriptional machinery of β-catenin/TCF/LEF-1, inactivation of SOX7 may be associated with the pathogenesis of cancers, e.g., endometrial cancer and prostate cancer ,. Additionally, SOX7 may be involved in aspirin-mediated growth inhibition of COX2 colorectal cancer cells, in which aspirin may up-regulate the expression of SOX7 by activating the MAPK . There are also reports indicating that the decreased expression of SOX7 is an important feature of lung adenocarcinoma . Based on whole genome analysis to measure the expression of SOX7 on a series of non-small cell lung cancers (NSCLC), investigators have shown that SOX7 is a novel tumor suppressor gene silenced in the majority of NSCLC . Numerous studies also suggest that promoter methylation might be an important mechanism of SOX7 down-regulation in breast cancer and MDS ,. On the other hand, reports are also available to show that SOX7 mRNA is significantly up-regulated in cancer, such as in pancreatic, gastric, and esophageal cancer cell lines, as well as in primary gastric cancer . To date, nevertheless, the contributions and molecular mechanisms of SOX7 in ovarian cancer are largely unknown.
In this study, we investigated the changes and possible roles of SOX7 in ovarian cancer using the microarray gene expression techniques and validated the results with clinical tissues. Both kinds of data showed that the expression of SOX7 was significantly reduced in ovarian cancer and the levels of expression were correlated with tumour progression. This effect might be mediated by the down-regulation of Wnt target genes cyclinD1 and COX2.
Chemicals and antibodies
Peroxidase-labeled rabbit-anti human IgG SOX7 (sc-20093) was purchased from Santa cruz biotechnology. Peroxidase-labeled rabbit-anti human IgG COX2 (BA3708) and rabbit-anti human IgG cyclinD1 (BA0770) used in immunohistochemistry were purchased from BOSTER Bio, Wuhan, China. Histostain-Plus Kit (SP-9001) and 3, 3’-diaminobenzidine tetrahydrochoride Substrate Kit (ZLI-9032) used in immunohistochemistry were purchased from ZSGB Bio, Beijing, China.
Datasets and preprocessing
Two sets of normalized microarray gene expression data with accession numbers GSE12470 and GSE27651 were downloaded from the Gene Expression Omnibus data repository (GEO, http://www.ncbi.nlm.nih.gov/geo/). Probe sets that did not match any known Gene ID or that matched multiple Gene IDs were abandoned. For each of the samples, the expression values of the probe sets matched to the same Entrez Gene ID were averaged as the expression value of that Entrez Gene ID. Genes in GSE27651, the expression of which significantly correlated (Pearson correlation) with that of SOX7, were defined as coexpression genes with SOX7. The pathway information of Wnt/β-catenin was documented in Kyoto Encyclopedia of Genes and Genomes (KEGG) website.
Functional enrichment analysis
The functional enrichment analysis was performed using the Database for Annotation, Visualization and Integrated Discovery (DAVID) v6.7 , and the biological processes (BPs) within Gene Ontology (GO) were checked for overrepresented function entities.
Clinical specimens were collected from the Department of Pathology of the Third Affiliated Hospital of Harbin Medical University from January to June 2008. All patients were informed of the purpose of the study and gave written informed consents. This work was reviewed by the Harbin Medical University Ethics Committee and was approved, consistent with the 1975 Declaration of Helsinki. The stage and histological grades of all the cases were determined according to the criteria of FIGO. None of the patients received any chemotherapy or radiotherapy before operation. Clinical information of patients was obtained from medical records and pathology reports.
The samples were fixed in 10% neutral buffered formalin and subsequently embedded in paraffin. The paraffin-embedded tissues were cut at 3 μm, deparaffinized with xylene and rehydrated for further peroxidase (DAB) immunohistochemistry staining. After trypsinization, the tissue slides were blocked with peroxidase and then were incubated overnight with the primary antibodies against respective target proteins (SOX7 Ab., Santa Cruz #sc-20093; Rabbit Anti-COX2, ZSGB-BIO, Rabbit Anti-CyclinD1, ZSGB-BIO) at a dilution of 1:100 at 4°C. After washing, peroxidase labeled polymer and substrate-chromogen were applied to visualize the staining of the target proteins. Finally, the slides were counterstained with hematoxylin.
Standard for evaluation
Immunostaining was scored by two independent experienced pathologists, who were blinded to the clinicopathological data and clinical outcomes of the patients. The scores of the two pathologists were compared and any discrepant scores were trained through re-examining the staining by both pathologists to achieve a consensus score. Tumor specimens were scored in a semi-quantitative manner in consideration of the homogenicity of the staining of the target proteins. The percentage scoring of immunoreactive tumor cells was as follows: 0 (0%), 1 (1-10%), 2 (11-50%) and 3 (>50%). The staining intensity was externally scored and stratified as follows: 0 (negative), 1 (weak), 2 (moderate) and 3 (strong). A final immunoreactivity score (IRS) was obtained for each of the cases by multiplying the percentage and the intensity score. Protein expression levels were further analyzed by classifying IRS values as low (based on an IRS value less than 4) or high (based on an IRS value greater than 4).
The differentially expressed genes of ovarian cancers vs. normal controls in GSE12470 were calculated by student-t test and the false discovery rate was controlled using the Benjamini–Hochberg procedure . The gene expression level of SOX7 was compared in groups of different malignant states in GSE27651 using one-way analysis of variance (one-way ANOVA). The difference between each pair of the groups was tested by multiple comparison tests and the statistical significance level was given by a student-t test.
For the immunohistochemistry experiment, SPSS version 17.0 was used. Statistical analysis was performed with Fisher’s exact test, chi square test and Spearman’s Rank correlation analysis. Differences with p < 0.05 were considered statistically significant.
Expression levels of SOX7 in ovarian cancer and normal tissues
Correlation of reduced SOX7 expression with tumor progression
SOX7 as a negative regulator in Wnt/β-catenin pathway in ovarian cancer
Biological processes enriched with genes dysregulated to a larger extent in ovarian cancer
ribonucleoprotein complex biogenesis
mRNA metabolic process
RNA splicing, via transesterification reactions
RNA splicing, via transesterification reactions with bulged adenosine as nucleophile
nuclear mRNA splicing, via spliceosome
The Pearson correlation coefficients between the expression levels of 13 genes and SOX7
Expression of SOX7, cyclin-D1 and COX2 proteins in normal ovarian tissues, borderline ovarian tumors and ovarian cancer
Relations between SOX7 and clinical or pathological characteristics in patients with epithelial ovarian carcinoma
Correlations of the SOX7 protein expression with the clinicopathological features of ovarian cancer and COX2/cyclinD1
High (2 + 3)
rs = –0.583, p < 0.001
rs = –0.618, p < 0.001
Ovarian cancer remains to be a leading cause of death from gynecological malignancies. It is a huge challenge of current basic and clinical research to seek novel molecular markers for more accurate and efficient use in early diagnosis, treatment or prognosis of ovarian cancer. In this study, we chose ovarian cancer to work on primarily due to the fact that this disease is so devastating in females and that to date relatively little has been done on SOX7 in ovarian cancer. Our results obtained from different platforms indicate that the expression levels of SOX7 were significantly reduced in all types of ovarian cancers studied here, though at different extents. LG is the most malignant among ovarian cancer and may progress from SBOT, while HG, much less malignant than LG, likely develops from other kinds of precursors, such as normal epithelium of ovary or distal fallopian tube . HG is a well-differentiated neoplasm and closely resembles normal tissues in many ways, so its prognosis is generally much better than LG . As a result, the gene expression levels of SOX7 showed opposite tendencies to malignancy degrees. Based on the findings in this study, we propose that SOX7 is a key factor during ovarian cancer progression and is a useful prognostic marker.
Of great significance, we found that SOX7 was negatively correlated with Cyclin D1 and could be a negative regulator of the Wnt/β-catenin pathway. Cyclin D1, which controls the cell cycle, is the target gene of β-catenin and plays an important role in ovarian cancer. It is known that the Wnt/β-catenin signaling pathway is activated in epithelial ovarian cancer  and strongly involved in ovarian cancer development . As SOX7 has been reported to block the transcriptional function of the Wnt/β-catenin signaling pathway and inhibit the activity of Wnt target genes including cyclin D1, c-Myc and COX-2 ,, we wondered whether there might be a correlation between SOX7 and these supposed target genes. Our results demonstrated that the expression levels of SOX7 and its targets, COX-2 and cyclin D1, have an inverse relationship, further supporting our hypothesis that SOX7 is a negative regulator in the Wnt/β-catenin signaling pathway in ovarian cancer. Although our data showed a marginal correlation of cyclin D1 with SOX7 (p = 0.0555) at the mRNA level, the correlation at the protein level was strong, so both had similar trends.
SOX7, a member of the SOX-F subfamily encoding transcription factors that have a pivotal role in cardiovascular development ,has been implicated as a tumor suppressor in a variety of human cancers, e.g., colorectal, prostate, breast, liver and lung cancers ,,,. Additionally, SOX7 may have potential usage as an independent prognostic marker in prostate and lung cancers as well as MDS patients ,,. Several lines of evidence indicate SOX7 as a negative regulator of the Wnt/β-catenin signaling pathway ,. Of great interest, a recent report based on whole genomic copy number analysis demonstrates SOX7 as a novel tumor suppressor, which is silenced in the majority of non-small cell lung cancer (NSCLC) samples . Molecules modulating SOX7 expression, such as miR-184 in hepatocellular carcinoma (HCC), have also been reported . Another study published recently also showed that SOX7 could suppress HCC in vivo and in vitro. There are also contradicting reports to demonstrate that SOX7 mRNA was significantly up-regulated in several human cancer cells . These results have prompted us to hypothesize that SOX7 might be a true tumor suppressor but behaves differently in different cancers.
Our work reported here suggests, for the first time, that SOX7 may play an important role as a tumor suppressor in ovarian cancer progression. Our results also revealed SOX7 as a negative regulator in the Wnt/β-catenin signaling pathway in ovarian cancer. Although further studies are needed to elucidate the underlining mechanisms of interactions between SOX7 and the Wnt/β-catenin signaling pathway, our result demonstrates the suppressive function of SOX7 in the carcinogenic process of ovarian cancer.
HL designed the study, BL carried out bioinformatics analysis and drafted the manuscript, HYL and ZQY participated in sample collection and experimental process, QS, SYY, JJK, DY and KF contributed reagents/materials/analysis tools. SLL finalized the manuscript. All authors read and approved the final manuscript.
Epithelial ovarian cancer
Non-small cell lung cancers
Gene Expression Omnibus
Kyoto Encyclopedia of Genes and Genomes
Database for Annotation, Visualization and Integrated Discovery
False discovery rate
Human ovarian surface epithelia
Serous borderline ovarian tumors
Low-grade serous ovarian carcinomas
High-grade serous ovarian carcinomas
The authors thank all the colleagues of genomics research center for comments on earlier versions of this manuscript. This work was supported by grants of the National Natural Science Foundation of China (NSFC30970119, 81030029, 81271786, NSFC-NIH 81161120416).
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