Defining a population of stem-like human prostate cancer cells that can generate and propagate castration-resistant prostate cancer

Purpose: We have shown that the phenotypically undifferentiated (PSA ^-/lo ) prostate cancer cell population harbors long-term self-renewing cancer stem cells (CSC) that resist castration, and a subset of the cells within the PSA ^-/lo population bearing the ALDH^hi CD44⁺ α2β1⁺ phenotype (Triple Marker⁺ /TM⁺ ) is capable of robustly initiating xenograft tumors in castrated mice. The goal of the current project is to further characterize the biologic properties of TM? prostate cancer cell population, particularly in the context of initiating and propagating castration-resistant prostate cancer (CRPC). Experimental Design: The in vivo CSC activities were measured by limiting-dilution serial tumor transplantation assays in both androgen-dependent and androgen-independent prostate cancer xenograft models. In vitro clonal, clonogenic, and sphere-formation assays were conducted in cells purified from xenograft and patient tumors. qPCR, Western blot, lentiviral-mediated gene knockdown, and human microRNA arrays were performed for mechanistic studies. Results: By focusing on the LAPC9 model, we show that the TM⁺ cells are CSCs with both tumor-initiating and tumor-propagating abilities for CRPC. Moreover, primary patient samples have TM⁺ cells, which possess CSC activities in "castrated" culture conditions. Mechanistically, we find that (i) the phenotypic markers are causally involved in CRPC development; (ii) the TM⁺ cells preferentially express castration resistance and stem cell-associated molecules that regulate their CSC characteristics; and (iii) the TM⁺ cells possess distinct microRNA expression profiles and miR-499-5p functions as an oncomir. Conclusions: Our results define the TM? prostate cancer cells as a population of preexistent stem-like cancer cells that can both mediate and propagate CRPC and highlight the TM⁺ cell population as a therapeutic target.