Transcriptional profiling of mammary gland side population cells

Similar to the bone marrow, the mammary gland contains a distinct population of Hoechst-effluxing side population cells, mammary gland side population cells (MG-SPs). To better characterize MG-SPs, their microarray gene profiles were compared to the remaining cells, which retain Hoechst dye (mammary gland non-side population cells [MG-NSPs]). For analysis, Gene Ontology (GO) that describes genes in terms of biological processes and Ontology Traverser (OT) that performs enrichment analysis were used. OT showed that MG-SP-specific genes were enriched in the GO categories of cell cycle regulation and checkpoints, multidrug-resistant transporters, organogenesis, and vasculogenesis. The MG-NSP-upregulated genes were enriched in the GO category of cellular organization and biogenesis, which includes basal epithelial markers, p63, smooth muscle actin, myosin, alpha 6 integrin, cytokeratin (CK) 14, and luminal markers CK8 and CD24. Additional studies showed that a higher percentage of MG-SPs exist in the G1 phase of the cell cycle compared with the MG-NSPs. G1 cell cycle block of MG-SPs may be explained by higher expression of cell cycle-negative regulatory genes such as transforming growth factor-beta 2, insulin-like growth factor binding protein-5, P18(INK4C), and wingless-5a (Wnt-5a). Accordingly, a smaller percentage of MG-SPs expressed nuclear beta-catenin, possibly as a consequence of the higher expression of Wnt-5a. In conclusion, microarray gene profiling suggests that MG-SPs are a lineage-deficient mammary gland subpopulation expressing key genes involved in cell cycle regulation, development, and angiogenesis.