Characterization of side population cells in human malignant mesothelioma cell lines

Side population (SP) assay composed of Hoechst 33342 staining and subsequent flow cytometric analysis has been widely utilized for characterizing putative cancer stem cells (CSCs) in various human malignancies. The present study was designed to evaluate the SP assay as a research tool for mesothelial CSCs. A distinct fraction of SP cells was identified in various human malignant mesothelioma (HMM) cell lines, ranging from 0.05 to 1.32%. The sorted mesothelial SP cells exhibited enhanced proliferation potentials and higher expression of stem-cell genes, compared to non-SP (NSP) cells. Cisplatin treatment increased percentage of SP cells in the HMM cell lines. However, tumorigenic potential of SP cells in immunodeficient mice was similar to that of the NSP cells. These data indicated that SP assay may not be appropriate for enriching putative CSCs in HMM cell lines, and thus warrants the development of a novel tool for mesothelial CSC study.