Faculty Bibliography

We previously showed that KLF4, a gene highly expressed in murine prostate stem cells, blocks the progression of indolent intraepithelial prostatic lesions into aggressive and rapidly growing tumors. Here, we show that the anti-tumorigenic effect of KLF4 extends to PC3 human prostate cancer cells growing in the bone. We compared KLF4 null cells with cells transduced with a DOX-inducible KLF4 expression system, and find KLF4 function inhibits PC3 growth in monolayer and soft agar cultures. Furthermore, KLF4 null cells proliferate rapidly, forming large, invasive, and osteolytic tumors when injected into mouse femurs, whereas KLF4 re-expression immediately after their intra-femoral inoculation blocks tumor development and preserves a normal bone architecture. KLF4 re-expression in established KLF4 null bone tumors inhibits their osteolytic effects, preventing bone fractures and inducing an osteogenic response with new bone formation. In addition to these profound biological changes, KLF4 also induces a transcriptional shift from an osteolytic program in KLF4 null cells to an osteogenic program. Importantly, bioinformatic analysis shows that genes regulated by KLF4 overlap significantly with those expressed in metastatic prostate cancer patients and in three individual cohorts with bone metastases, strengthening the clinical relevance of the findings in our xenograft model.

There is a considerable need to identify those individuals with prostate cancer who have indolent disease. We propose that genes that control adult stem cell homeostasis in organs with slow turnover, such as the prostate, control cancer fate. One such gene, KLF4, overexpressed in murine prostate stem cells, regulates their homeostasis, blocks malignant transformation, and controls the self-renewal of tumor-initiating cells. KLF4 loss induces the molecular features of aggressive cancer and converts PIN lesions to invasive sarcomatoid carcinomas; its re-expression in vivo reverses this process. Bioinformatic analysis links these changes to human cancer. KLF4 and its downstream targets make up a gene signature that identifies indolent tumors and predicts recurrence-free survival. This approach may improve prognosis and identify therapeutic targets for advanced cancer.

There is evidence that stem cells and their progeny play a role in the development of the prostate. Although stem cells are also considered to give rise to differentiated progeny in the adult prostate epithelium ex vivo, the cohort of adult prostate stem cells in vivo as well as the mechanisms by which the adult prostate epithelium is maintained and regenerated remain highly controversial. We have attempted to resolve this conundrum by performing in vivo tracing of serially replicating cells after the sequential administration of two thymidine analogues to mice. Our results show that, during normal prostate homeostasis, sequentially proliferating cells are detected at a rate that is consistent with a stochastic process. These findings indicate that in vivo, under steady-state conditions, most adult prostate epithelial cells do not represent the progeny of a small number of specialized progenitors that generate sequentially replicating transit-amplifying (TA) cells but are formed by stochastic cell division. Similarly, no rapidly cycling TA cells were detected during regeneration following one cycle of androgen-mediated involution/regeneration of the prostate epithelium. These findings greatly enhance our understanding of the mechanisms regulating prostate epithelial cell renewal and may have significant implications in defining the cell of origin of proliferative prostatic diseases.

The adult mouse prostate has a seemingly endless capacity for regeneration, and sonic hedgehog (SHH) signaling has been implicated in this stem cell-driven process. However, it is not clear whether SHH acts on the epithelium or stromal cells that secrete factors required for epithelial expansion. Because little is known about stromal stem cells compared with their epithelial counterparts, we used in vivo mouse genetics tools to characterize four prostate stromal subtypes and their stem cells. Using knockin reporter alleles, we uncovered that SHH signals from prostate basal epithelial cells to adjacent stromal cells. Furthermore, the SHH target gene Gli1 is preferentially expressed in subepithelial fibroblast-like cells, one of four prostate stromal subtypes and the subtype closest to the epithelial source of SHH. Using Genetic Inducible Fate Mapping to mark adult Gli1- or Smooth muscle actin-expressing cells and follow their fate during regeneration, we uncovered that Gli1-expressing cells exhibit long-term self-renewal capacity during multiple rounds of androgen-mediated regeneration after castration-induced involution, and depleted smooth muscle cells are mainly replenished by preexisting smooth muscle cells. Based on our Genetic Inducible Fate Mapping studies, we propose a model where SHH signals to multiple stromal stem cells, which are largely unipotent in vivo.

BACKGROUND: Stem cells are located in specific regulatory environments termed niches, which modulate the survival and proliferation of the cells through a variety of both mitogenic and inhibitory cytokines. In the murine prostate, stem cells are located in the proximal region of prostatic ducts. We examined the regulation of murine prostate cells in the stem cell niche by transforming growth factor beta (TGF-beta) and stem cell factor (SCF). METHODS: Prostate cells from the proximal and distal regions of prostatic ducts were cultured in the presence and absence of TGF-beta and SCF, both on collagen-coated wells and in collagen gels. Cell growth on collagen was assessed by determining cell number. Cell growth in collagen gels was quantified by determining the number, size and complexity of prostatic ducts. The basal and luminal phenotype of the cells was determined by immunohistochemistry. RESULTS: Endogenous TGF-beta inhibited proliferation and promoted differentiation of proximal cells towards a luminal phenotype. It also inhibited duct-forming capacity and promoted differentiation of prostatic ducts towards a luminal phenotype. Addition of SCF enhanced proximal cell proliferation on collagen-coated wells and duct formation in collagen gels. Proliferation was further increased by ablation of endogenous TGF-beta. CONCLUSION: Proliferation and the basal/luminal cell composition of cells isolated from the proximal region of prostatic ducts, the stem cell niche, is regulated in part by opposing effects of SCF and endogenous TGF-beta. Prostate 72:998-1005, 2012. (c) 2011 Wiley Periodicals, Inc.

BACKGROUND: Signals between stem cells and stroma are important in establishing the stem cell niche. However, very little is known about the regulation of any mammalian stem cell niche as pure isolates of stem cells and their adjacent mesenchyme are not readily available. The prostate offers a unique model to study signals between stem cells and their adjacent stroma as in the embryonic prostate stem cell niche, the urogenital sinus mesenchyme is easily separated from the epithelial stem cells. Here we investigate the distinctive molecular signals of these two stem cell compartments in a mammalian system. METHODOLOGY/PRINCIPAL FINDINGS: We isolated fetal murine urogenital sinus epithelium and urogenital sinus mesenchyme and determined their differentially expressed genes. To distinguish transcripts that are shared by other developing epithelial/mesenchymal compartments from those that pertain to the prostate stem cell niche, we also determined the global gene expression of epidermis and dermis of the same embryos. Our analysis indicates that several of the key transcriptional components that are predicted to be active in the embryonic prostate stem cell niche regulate processes such as self-renewal (e.g., E2f and Ap2), lipid metabolism (e.g., Srebp1) and cell migration (e.g., Areb6 and Rreb1). Several of the enriched promoter binding motifs are shared between the prostate epithelial/mesenchymal compartments and their epidermis/dermis counterparts, indicating their likely relevance in epithelial/mesenchymal signaling in primitive cellular compartments. Based on differential gene expression we also defined ligand-receptor interactions that may be part of the molecular interplay of the embryonic prostate stem cell niche. CONCLUSIONS/SIGNIFICANCE: We provide a comprehensive description of the transcriptional program of the major regulators that are likely to control the cellular interactions in the embryonic prostatic stem cell niche, many of which may be common to mammalian niches in general. This study provides a comprehensive source for further studies of mesenchymal/epithelial interactions in the prostate stem cell niche. The elucidation of pathways in the normal primitive niche may provide greater insight into mechanisms subverted during abnormal proliferative and oncogenic processes. Understanding these events may result in the development of specific targeted therapies for prostatic diseases such as benign prostatic hypertrophy and carcinomas

The epithelium that lines the surface of prostate glands contains several cell types, including luminal secretory cells and basal cells of unclear function. Despite the fact that prostate tumors contain cells with a luminal phenotype and lack basal cells, a recent report indicates that the cell of origin for human prostate cancer is a basal cell and not a luminal cell. In contrast, another study indicates the reverse. It is possible that both basal and luminal stem/progenitor cells may independently give rise to prostate cancer; a comparison of the molecular signatures of the target cells of transformation with those of prostate tumors may aid in predicting the phenotypes of tumors with aggressive characteristics

We have shown previously that prostatic stem/progenitor cells can be purified from isolated prostate ducts, based on their high expression of the Sca-1 surface antigen. We now report that high levels of aldehyde dehydrogenase (ALDH) activity are present in a subset of prostate epithelial cells that coexpress a number of antigens found on stem/progenitor cells of other origins (CD9, Bcl-2, CD200, CD24, prominin, Oct 3/4, ABCG2, and nestin). Almost all of these cells expressing high levels of ALDH activity also express Sca-1 and a third of them express high levels of this antigen. The cells with high levels of ALDH activity have greater in vitro proliferative potential than cells with low ALDH activity. Importantly, in an in vivo prostate reconstitution assay, the cells expressing high levels of ALDH activity were much more effective in generating prostatic tissue than a population of cells with low enzymatic activity. Thus, a high level of ALDH activity can be considered a functional marker of prostate stem/progenitor cells and allows for simple, efficient isolation of cells with primitive features. The elucidation of the role of ALDH in prostate stem/progenitor cells may lead to the development of rational therapies for treating prostate cancer and benign prostatic hyperplasia.

BACKGROUND: The global gene expression profiles of adult and fetal murine prostate stem cells were determined to define common and unique regulators whose misexpression might play a role in the development of prostate cancer. METHODOLOGY/PRINCIPAL FINDINGS: A distinctive core of transcriptional regulators common to both fetal and adult primitive prostate cells was identified as well as molecules that are exclusive to each population. Elements common to fetal and adult prostate stem cells include expression profiles of Wnt, Shh and other pathways identified in stem cells of other organs, signatures of the aryl-hydrocarbon receptor, and up-regulation of components of the aldehyde dehydrogenase/retinoic acid receptor axis. There is also a significant lipid metabolism signature, marked by overexpression of lipid metabolizing enzymes and the presence of the binding motif for Srebp1. The fetal stem cell population, characterized by more rapid proliferation and self-renewal, expresses regulators of the cell cycle, such as E2f, Nfy, Tead2 and Ap2, at elevated levels, while adult stem cells show a signature in which TGF-beta has a prominent role. Finally, comparison of the signatures of primitive prostate cells with previously described profiles of human prostate tumors identified stem cell molecules and pathways with deregulated expression in prostate tumors including chromatin modifiers and the oncogene, Erg. CONCLUSIONS/SIGNIFICANCE: Our data indicate that adult prostate stem or progenitor cells may acquire characteristics of self-renewing primitive fetal prostate cells during oncogenesis and suggest that aberrant activation of components of prostate stem cell pathways may contribute to the development of prostate tumors

BACKGROUND: We previously reported that prostatic stem/progenitor cells are concentrated in the proximal region of prostatic ducts and express stem cell antigen 1 (Sca-1). As Wnt signaling is important for the maintenance of stem cells, we determined whether Sca-1 expressing cells also express Axin2, as Axin2 expression is highly suggestive of active Wnt signaling. METHODS: Axin2 promoter reporter mice were used for whole mount and fluorescence activated cell sorting (FACS) analysis to determine its expression in the prostate. Axin2 expressing cells were also examined for the co-expression of Sca-1. We also used a chemical activator of Wnt signaling, BIO, to determine the effects of Wnt signaling on the growth of primary prostate cells in vitro. RESULTS: We show that Axin2 expression is present in all lobes and is regulated by androgens with the highest Axin2 expression in the lateral and dorsal prostate. Furthermore, a fraction of Axin2 expressing cells co-express Sca-1, suggesting that some progenitor cells have active Wnt signaling. Lastly, we demonstrate that activation of the Wnt pathway may result in increased growth, consistent with a role for Wnt signaling in maintenance and/or expansion of the progenitor cell population. CONCLUSION: Axin2 expressing cells that co-express Sca-1 are present in all prostate lobes suggesting that progenitor cells reside within the Wnt active population. An understanding of the basic biology of signaling pathways mediating growth in the prostate may lead to rational therapies to treat benign prostatic hyperplasia and prostate cancer