Investigation of a Putative Prostate Stem Cell Niche

Although the development of new drugs such as abiraterone and enzalutamide have greatly improved the treatment options for prostate cancer, patients treated with these drugs will usually relapse with more aggressive forms of the disease. One of the most common forms of this relapsed disease is known as treatment-emergent neuroendocrine prostate cancer, in which the tumors now contain many neuroendocrine cells, which are not found at earlier stages of the disease. Despite the increasing clinical importance of neuroendocrine cells in prostate cancer, there is remarkably little known about neuroendocrine cells in the normal prostate. Since they are extremely rare cells in the normal prostate, little is known about their function, or even where they are primarily located in the tissue. Furthermore, due to their rarity, it is difficult to purify these cells to study their function. In our preliminary work, we have found that neuroendocrine cells in the mouse prostate are highly localized to the region near to where the prostate joins with the urethra. In addition, we have developed methods using genetically engineered mice to label these neuroendocrine cells using fluorescent proteins, which enable their purification from prostate tissue. Furthermore, we have developed an innovative new method to grow normal prostate cells as well as tumor cells as clumps of tissue known as organoids in laboratory culture dishes. These organoids contain cell types resembling those found in the corresponding normal or tumor tissue, can be grown for months in the laboratory, and can be preserved for future study by freezing in liquid nitrogen. Using this approach, we can now study important questions regarding the function of neuroendocrine cells in both the normal prostate and in prostate tumors. Based on these and other findings, we now propose that neuroendocrine cells function by regulating the properties of prostate stem cells. Stem cells are of fundamental importance since they can divide and form all of the cell types found within a normal tissue, whereas in the context of tumors, cancer stem cells can divide endlessly to generate the bulk of tumor cells. Niche cells are cells that are distinct from stem cells and cancer stem cells, but regulate their fundamental properties, including how often they divide and whether they form different cell types. Thus, in our proposal, we plan to investigate the hypothesis that neuroendocrine cells are an important part of the stem cell niche, both in the normal prostate and in treatment-emergent neuroendocrine prostate cancer. To investigate this hypothesis, our studies will have two primary aims. First, we will determine whether neuroendocrine cells can promote the growth of stem cells in the normal prostate, as might be expected if they function as a niche. For this purpose, we will use organoid culture to grow combinations of prostate cells that are either enriched or depleted for stem cells together with purified populations of prostate neuroendocrine cells. We will perform similar studies using tissue grafts grown in the kidney of recipient mice, which is a more traditional but labor-intensive assay for stem cell activity. Secondly, we will pursue similar studies using prostate tumor cells from genetically engineered mice that develop treatment-emergent neuroendocrine prostate cancer. We will use organoid culture as well as tissue grafts to determine whether purified neuroendocrine cells will stimulate tumor growth, consistent with a role as a niche for cancer stem cells. We expect that our studies will provide a new conceptual model for the role of neuroendocrine cells in prostate cancer and may ultimately guide the design of therapies for neuroendocrine prostate cancer. In particular, we anticipate that the lethality of treatment-emergent neuroendocrine prostate cancer may be a consequence of the niche function of increasing numbers of neuroendocrine cells, which in tu