The Prostate: Anatomy and Epithelial Organization
The prostate is part of the male reproductive system that secretes part of the fluid that makes up semen. The urethra runs through the prostate from the bladder, and semen progresses from the prostate into the urethra via the ductus deferens. The prostate epithelium has two cell layers: the luminal layer, which is composed of columnar epithelial cells, and the basal layer, which is composed of cuboidal epithelial cells. Either epithelial cell type can become transformed and give rise to prostate cancer.
Advances in culture techniques have allowed researchers to grow prostate cells in a 3D matrix, where they form structures termed prostaspheres. This culture system relies on the presence of stem and progenitor cells, which regenerate the prostate glandular structure in this 3D in vitro system.
Identification of Prostate Stem Cells using Lifeline® Prostate Epithelial Cells
Many tissues in the body retain adult stem cells that are responsible for maintaining tissue homeostasis and tissue renewal. The stem cell population is typically composed of rare cells that can be identified based on two primary characteristics: self-renewal, and the capacity to give rise to all the differentiated cell types of the tissue in which they reside. Adult stem cell populations are notoriously difficult to identify in vivo and are often defined by unique cell surface markers. In animal models, or in vitro, stem cells can also be identified using lineage tracing or lineage labeling techniques.
To enable DNA labeling, BrdU (a DNA-intercalating molecule) is added; upon cell division, it is incorporated into DNA and can be detected experimentally with antibody staining. This technique capitalizes on the fact that stem cells often divide slower than other cell populations and is based on the immortal strand hypothesis, which posits that during stem cell division, the parental (labeled) DNA strand is retained in the new daughter stem cell, while the newer DNA strand (unlabeled) is passed on to the new daughter cell that becomes a more committed progenitor cell. In this way, the BrdU is diluted out of differentiation-committed daughter cell populations, and BrdU positive staining should identify a slower cycling stem cell.
The identity of prostate epithelial stem cells is debated, and while some cell surface markers have been identified, the results are inconsistent. In a new study from this year, Hu et al. set out to identify prostate stem cells using BrdU labeling of prostaspheres derived from Lifeline® human prostate epithelial cells grown in ProstaLife™ medium. Following a 10-day BrdU labeling period and a 5-day washout, the authors found that BrdU-labeled single cells were present, and could also be labeled with the CFSE pro-dye and sorted using FACS analysis. These labeled single cells exhibited stem cell properties, including the ability to generate prostaspheres, as well as prostate-like tissue grafts in vivo. In contrast, cells lowly expressing CFSE could not form spheres or tissue grafts. Importantly, using a paired-cell assay (two neighboring cells derived from one cell division), the authors demonstrated that the putative BrdU-labeled prostate stem cells most often underwent asymmetric cell division, in which cell division yields one daughter stem cell and one more committed progenitor daughter cell that has less BrdU label.
To further characterize this stem cell population, the authors found that the proposed stem cell markers KRT14 and WNT10B were present in the BrdU+ population, but not in BrdU- daughter cells. Additionally, increased evidence of autophagy was found in BrdU+ cells, supporting their identity as stem cells. To determine differences on a more global scale, the authors performed RNASeq on FACS-isolated CFSEhi and CFSElo populations, and found significant gene expression patterns that characterized the CFSEhi stem cell population, including increased translational regulation and estrogen receptor signaling, as well as decreased anabolic metabolism.
The authors also investigated the roles of two significantly altered genes, KRT13 and PRAC1, in prostate stem cell identity. Using knockdown studies, they found that both genes were important for stem cell maintenance. Finally, the authors showed that this labeling technique could be used to label stem-like cells from human prostate cancer samples, as well as stem cells in sphere cultures of colorectal (HCT116) and breast (MCF-7) cancer cell lines. Together, this study has uncovered a method to isolate prostate cancer stem cells (and potential stem cells from other tissue types), and has defined important identifying characteristics of this population. Furthermore, the results from this study confirm that Lifeline® prostate epithelial cell samples contain prostate stem cells and can be used to study how this population contributes to prostate homeostasis and how they may become transformed to initiate prostate tumorigenesis.
Lifeline® prostate epithelial cells are optimized for growth in ProstaLife™ medium, and, as illustrated by the study above, and others, can be grown in 3D matrices as spheroid cultures.
Are you using Lifeline® cells or media in your research? Let us know and your study could be featured in a future blog post!