Renal capsule implantation of Lgr cells further supported

Renal troglitazone implantation of Lgr5+ cells further supported the high repopulation capacity of single Lgr5+ cells within the regressed prostate. A limiting-dilution analysis indicated that every prostatic Lgr5+ cell could be a stem cell in regressed prostates, meaning that both luminal and basal Lgr5+ cells in regressed prostates could obtain a bipotent capacity in the transplantation setting. Importantly, cell types have been shown to acquire a facultative plasticity in single-cell ex vivo and renal-capsule assays that is not readily observable in vivo (Choi et al., 2012; Wang et al., 2013, 2014b). This is consistent with recent work that identified multipotent basal and luminal cells in a human organoid culture system (Karthaus et al., 2014). It seems that prostatic Lgr5+ cells are capable of bipotentiality depending on the environmental cues or assay conditions, i.e., these cells can generate luminal progeny in response to a demand for luminal epithelial cells in androgen manipulation studies in situ, or give rise to both luminal and basal progeny to repopulate the prostate structure from single cells in transplantation studies. Moreover, in this study we demonstrate that Lgr5+ cells are required for prostate regeneration after castration and subsequent androgen substitution, as depletion of Lgr5+ cells led to the generation of significantly smaller prostates. Considering the low frequency of Lgr5+ cells in the prostate and the profound impact of Lgr5+ depletion on prostate regeneration, we speculated that Lgr5+ cells could give rise to other progenitor cells to mediate the regeneration process. Indeed, we observed an increase in labeled cells during two rounds of regression and regeneration. The average unit size in the second-round regressed prostates was two cells, compared with one in the first-round regressed prostate. The additional labeled cell is most likely derived from the single Lgr5+ cell from the first round of regression. This cell is also castration resistant and its expansion rate is similar to that of the original Lgr5+ cell, suggesting that Lgr5+ stem cells in the regressed prostate are self-renewing and the newly generated stem cell also takes part in the regeneration process. More studies will be required to elucidate the networking and the symmetry of divisions of stem cells and progenitor cells in the prostate. With regard to human prostate cancer, tumor cells are largely characterized by luminal phenotypes, and loss of basal cells is the hallmark of prostate adenocarcinoma. However, there is evidence for both basal and luminal cell lineages initiating the disease. Initially, prostate tumors respond to androgen deprivation therapy well, but eventually tumors progress in the absence of androgen, suggesting the possible presence of resistant cells that are responsible for repopulating the tumor. Lgr5+ cells have been shown to be a cell of origin for intestinal tumors (Barker et al., 2009). Although LGR5 is expressed in both human prostate tissues and prostate tumors (Figure S5), it is unclear what role LGR5+ cells play in prostate cancer initiation or maintenance. Interestingly, the majority of castration-resistant Lgr5 cells are of luminal origin, and it was recently demonstrated that luminal cells are the preferred cell of origin for preclinical prostate murine tumor models (Wang et al., 2014b). If castration-resistant Lgr5 cells can serve as the cell of origin for castration-resistant prostate cancer (CRPC), then selectively targeting these Lgr5 cells during androgen-deprivation treatment should be considered as a therapeutic strategy to prevent CRPC. It will be important to continue to explore the role of Lgr5+ cells in prostate cancer biology to assess their potential for tumor initiation, especially given that our studies demonstrate that Lgr5+ cells are castration-resistant, long-lived stem cells that are required for complete prostate regeneration following castration.
Experimental Procedures