Down-regulation of the fetal stem cell factor SOX17 by H33342: a mechanism responsible for differential gene expression in breast cancer side population cells.

Human solid tumors contain rare cancer side population (SP) cells, which expel the fluorescent dye Hoechst 33342 (H33342) and display cancer stem cell characteristics. Transcriptional profiling of cancer SP cells isolated by H33342 fluorescence analysis is a newly emerging approach to discover cancer stem cell markers and aberrant differentiation pathways. Using Affymetrix expression microarrays and quantitative reverse transcription-PCR, we investigated differential gene expression between SP and non-SP (NSP) cells isolated from human mammary carcinoma cell lines. A total of 136 genes were up-regulated in breast cancer SP relative to NSP cells, one of which was the fetal stem cell factor and Wnt/beta-catenin signaling pathway target SOX17. Strikingly, we discovered that SOX17 was down-regulated by H33342 in a dose-dependent manner. In SP cells, which expel H33342, down-regulation of SOX17 was less pronounced than in NSP cells, which retain H33342. As a result of this, SOX17 displayed a 10-20-fold overexpression in cancer SP relative to NSP cells. Similar results were obtained for further stemness-related genes, namely EPC1 and SPRY1. These findings establish a previously unidentified gene-regulatory impact of H33342 as a novel mechanism responsible for differential gene expression in cancer SP cells. This has significant implications for the future interpretation of cancer SP cells.