Isolation of Stem-like Cells from 3-Dimensional Spheroid Cultures.

Despite advances in adult stem cell research, identification and isolation of stem cells from tissue specimens remains a major challenge. While resident stem cells are relatively quiescent with niche restraints in adult tissues, they enter the cell cycle in anchor-free three-dimensional (3D) culture and undergo both symmetric and asymmetric cell division, giving rise to both stem and progenitor cells. The latter proliferate rapidly and are the major cell population at various stages of lineage commitment, forming heterogeneous spheroids. Using primary normal human prostate epithelial cells (HPrEC), a spheroid-based, label-retention assay was developed that permits the identification and functional isolation of the spheroid-initiating stem cells at a single cell resolution. HPrEC or cell lines are two-dimensionally (2D) cultured with BrdU for 10 days to permit its incorporation into the DNA of all dividing cells, including self-renewing stem cells. Wash out commences upon transfer to the 3D culture for 5 days, during which stem cells self-renew through asymmetric division and initiate spheroid formation. While relatively quiescent daughter stem cells retain BrdU-labeled parental DNA, the daughter progenitors rapidly proliferate, losing the BrdU label. BrdU can be substituted with CFSE or Far Red pro-dyes, which permit live stem cell isolation by FACS. Stem cell characteristics are confirmed by in vitro spheroid formation, in vivo tissue regeneration assays, and by documenting their symmetric/asymmetric cell divisions. The isolated label-retaining stem cells can be rigorously interrogated by downstream molecular and biologic studies, including RNA-seq, ChIP-seq, single cell capture, metabolic activity, proteome profiling, immunocytochemistry, organoid formation, and in vivo tissue regeneration. Importantly, this marker-free functional stem cell isolation approach identifies stem-like cells from fresh cancer specimens and cancer cell lines from multiple organs, suggesting wide applicability. It can be used to identify cancer stem-like cell biomarkers, screen pharmaceuticals targeting cancer stem-like cells, and discover novel therapeutic targets in cancers.