Abstract
IntroductionAberrant expression of the embryonic stem cell marker Sox2 has been reported in breast cancer (BC). We previously identified two phenotypically distinct BC cell subsets separated based on their differential response to a Sox2 transcription activity reporter, namely the reporter-unresponsive (RU) and the more tumorigenic reporter-responsive (RR) cells. We hypothesized that Sox2, as a transcription factor, contributes to their phenotypic differences by mediating differential gene expression in these two cell subsets.MethodsWe used chromatin immunoprecipitation and a human genome-wide promoter microarray (ChIP-chip) to determine the promoter occupancies of Sox2 in the MCF7 RU and RR breast cancer cell populations. We validated our findings with conventional chromatin immunoprecipitation, quantitative reverse transcription polymerase chain reaction (qPCR), and western blotting using cell lines, and also performed qPCR using patient RU and RR samples.ResultsWe found a largely mutually exclusive profile of gene promoters bound by Sox2 between RU and RR cells derived from MCF7 (1830 and 456 genes, respectively, with only 62 overlapping genes). Sox2 was bound to stem cell- and cancer-associated genes in RR cells. Using quantitative RT-PCR, we confirmed that 15 such genes, including PROM1 (CD133), BMI1, GPR49 (LGR5), and MUC15, were expressed significantly higher in RR cells. Using siRNA knockdown or enforced expression of Sox2, we found that Sox2 directly contributes to the higher expression of these genes in RR cells. Mucin-15, a novel Sox2 downstream target in BC, contributes to the mammosphere formation of BC cells. Parallel findings were observed in the RU and RR cells derived from patient samples.ConclusionsIn conclusion, our data supports the model that the Sox2 induces differential gene expression in the two distinct cell subsets in BC, and contributes to their phenotypic differences.Electronic supplementary materialThe online version of this article (doi:10.1186/s13058-014-0470-2) contains supplementary material, which is available to authorized users.
Highlights
Aberrant expression of the embryonic stem cell marker sex-determining region Y-box binding protein-2 (Sox2) has been reported in breast cancer (BC)
To understand the possible biological effects exerted by Sox2 in BC cells, we annotated the functions of the identified genes using the Protein Analysis THrough Evolutionary Relationships (PANTHER) Protein Class classification system software [35]
Using qRT-PCR, we measured and compared the expression levels of the 15 genes of interest described in Overexpression of Sox2 up-regulates target genes in RR cells but not RU cells To demonstrate the direct role of Sox2 in contributing to the differential gene expression between RU and RR, we examined if enforced expression of Sox2 in MCF7 cells results in significant alterations of their expressions
Summary
Aberrant expression of the embryonic stem cell marker Sox has been reported in breast cancer (BC). Sox has been discovered to be aberrantly expressed in cancer cells, including those of the lungs, brain, ovaries, bone, colon, skin, and breasts [8,9,10,11,12,13,14,15]. In many of these studies, Sox was found in the cancer stem cell population [7,12,16,17,18,19,20,21,22], supporting the hypothesis that cancer stemness is related to the aberrant expression of ESC proteins. In a relatively small number of in vitro studies, Sox has been directly implicated in promoting cell proliferation, mammosphere formation, invasion and tumorigenesis in BC [7,8,29]
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