P66. Label retaining cells and stem cell marker expression in the developing and adult mouse prostate

Introduction and objectives An increasing number of reports suggest that the resistance of cancer stem cells (CSCs) to conventional therapies may account for the inability to cure most cancers, including prostate cancer (PC). It is however still uncertain whether CSCs are derived from normal prostate stem cells (SCs), or if normal mature cells acquire SC characteristics during the malignant process. Additionally, there is experimental evidence to suggest that not only the basal layer, but also the luminal layer, may contain epithelial SCs, and that the stroma contains mesenchymal SCs. To identify and localize slow cycling prostate SCs, labelling of cells during organogenesis of the mouse prostate was performed. Material and methods Male CD1 mice embryos were BrdU labeled in utero for two consecutive days on embryonic day E12.5 and 13.5 (urogenital sinus formation), E14.5 and 15.5 (mesenchymal expansion), and E16.5 and 17.5 (epithelial budding). Male offspring were euthanized at postnatal day 5 and at 6 months of age. Labeling retaining cells were then assayed in conjunction with proliferative and putative SC and lineage markers using immunohistochemistry and immunofluorescence. Results BrdU was washed out in the majority of cells, and only rare cells retained the BrdU label incorporated in the nuclei during embryonic growth. Actively proliferating cells as detected by Ki67/MIB immunohistochemistry were not or only rarely BrdU positive, and decreased with age. Label retaining cells were differentially expressed in postnatal and adult mice, and in the epithelial and stromal compartments. Identification of label retaining cells together with candidate SC markers including c-kit, nestin, meis1, and epithelial and stromal transit amplifying and lineage markers including cytokeratins, chromogranin A, p63, androgen receptor and smooth muscle actin allowed characterization of candidate SC subsets in the prostate gland. Morphometric results also showed that the distribution of epithelial label retaining cells in postnatal mice were greater proximal to the urethra, consistent with previously described SC marker expression. Conclusions A vital goal in PC research is to identify the cells that initiate or maintain tumour growth. The development of PC cells resistant to androgen deprivation therapy has led to the conclusion that the prostate contains a subpopulation of malignant cells that do not depend on androgens for their survival. It has been proposed that prostate CSCs arises from normal androgen-independent epithelial SCs within the prostate, and is the culprit behind castration resistant PC. In order to identify the normal prostate SC subsets, label-retention with BrdU, which is incorporated into the DNA during cell division, was here employed during formation of the urogenital anlagen and budding of the glandular cells. Identification and characterization of the normal prostate SCs may help explain any role they may have in prostate carcinogenesis and progression to the castration resistant stage, and may ultimately aid in the development of novel treatment modalities of PC.