Evidence of stem cells in the adult prostatic epithelium based upon responsiveness to mesenchymal inductors

Heterogeneity and complexity of the prostate epithelium: New findings from single-cell RNA sequencing studies

The recent advent of single-cell RNA-sequencing technology has provided new fundamental insights into the heterogeneity of the prostate epithelium. Several independent studies have described extensive heterogeneity of the luminal epithelial compartment, including a major division between a novel population of luminal cells located in the proximal region of the prostate ducts versus luminal cells located more distally. Proximal luminal cells as well as novel periurethral cells display increased progenitor potential in organoid culture and tissue reconstitution assays, but not in lineage-tracing analyses during prostate homeostasis, suggesting context-dependent plasticity of these populations. Here we describe and synthesize recent findings regarding the epithelial cell populations in the mouse prostate, draw comparisons to the human prostate, and address the relevance of these findings to prostate diseases and cancer.

Progenitors in prostate development and disease

The prostate develops by epithelial budding and branching processes that occur during fetal and postnatal stages. The adult prostate demonstrates remarkable regenerative capacity, with the ability to regrow to its original size over multiple cycles of castration and androgen administration. This capacity for controlled regeneration prompted the search for an androgen-independent epithelial progenitor in benign prostatic hyperplasia (BPH) and prostate cancer (PCa). BPH is hypothesized to be a reawakening of ductal branching, resulting in the formation of new proximal glands, all while androgen levels are decreasing in the aging male. Advanced prostate cancer can be slowed with androgen deprivation, but resistance eventually occurs, suggesting the existence of an androgen-independent progenitor. Recent studies indicate that there are multiple castration-insensitive epithelial cell types in the proximal area of the prostate, but not all act as progenitors during prostate development or regeneration. This review highlights how recent cellular and anatomical studies are changing our perspective on the identity of the prostate progenitor.

Prostate progenitor cells proliferate in response to castration

Androgen-deprivation is a mainstay of therapy for advanced prostate cancer but tumor regression is usually incomplete and temporary because of androgen-independent cells in the tumor. It has been speculated that these tumor cells resemble the stem/progenitor cells of the normal prostate. The purpose of this study was to examine the response of slow-cycling progenitor cells in the adult mouse prostate to castration. Proliferating cells in the E16 urogenital sinus were pulse labeled by BrdU administration or by doxycycline-controlled labeling of the histone-H2B GFP mouse. A small population of labeled epithelial cells in the adult prostate localized at the junction of the prostatic ducts and urethra. Fluorescence-activated cell sorting (FACS) showed that GFP label-retaining cells were enriched for cells co-expressing stem cell markers Sca-1, CD133, CD44 and CD117 (4- marker cells; 60-fold enrichment). FACS showed, additionally, that 4-marker cells were androgen receptor positive. Castration induced proliferation and dispersal of E16 labeled cells into more distal ductal segments. When naïve adult mice were administered BrdU daily for 2 weeks after castration, 16% of 4-marker cells exhibited BrdU label in contrast to only 6% of all epithelial cells (P<0.01). In sham-castrated controls less than 4% of 4-marker cells were BrdU labeled (P<0.01). The unexpected and admittedly counter-intuitive finding that castration induced progenitor cell proliferation suggests that androgen deprivation therapy in men with advanced prostate cancer could not only exert pleiotrophic effects on tumor sub-populations but may induce inadvertent expansion of tumor stem cells.

Prostate cancer stem cells: are they androgen-responsive?

The prostate gland is highly dependent on androgens for its development, growth and function. Consequently, the prostatic epithelium predominantly consists of androgen-dependent luminal cells, which express the androgen receptor at high levels. In contrast, androgens are not required for the survival of the androgen-responsive, but androgen-independent, basal compartment in which stem cells reside. Basal and luminal cells are linked in a hierarchical pathway, which most probably exists as a continuum with different stages of phenotypic change. Prostate cancer is also characterised by heterogeneity, which is reflected in its response to treatment. The putative androgen receptor negative cancer stem cell (CSC) is likely to form a resistant core after most androgen-based therapies, contributing to the evolution of castration-resistant disease. The development of CSC-targeted therapies is now of crucial importance and identifying the phenotypic differences between CSCs and both their progeny will be key in this process.

Epigenetic gene regulation in stem cells and correlation to cancer

Through the classic study of genetics, much has been learned about the regulation and progression of human disease. Specifically, cancer has been defined as a disease driven by genetic alterations, including mutations in tumor-suppressor genes and oncogenes, as well as chromosomal abnormalities. However, the study of normal human development has identified that in addition to classical genetics, regulation of gene expression is also modified by 'epigenetic' alterations including chromatin remodeling and histone variants, DNA methylation, the regulation of polycomb group proteins, and the epigenetic function of non-coding RNA. These changes are modifications inherited during both meiosis and mitosis, yet they do not result in alterations of the actual DNA sequence. A number of biological questions are directly influenced by epigenetics, such as how does a cell know when to divide, differentiate or remain quiescent, and more importantly, what happens when these pathways become altered? Do these alterations lead to the development and/or progression of cancer? This review will focus on summarizing the limited current literature involving epigenetic alterations in the context of human cancer stems cells (CSCs). The extent to which epigenetic changes define cell fate, identity, and phenotype are still under intense investigation, and many questions remain largely unanswered. Before discussing epigenetic gene silencing in CSCs, the different classifications of stem cells and their properties will be introduced. This will be followed by an introduction to the different epigenetic mechanisms. Finally, there will be a discussion of the current knowledge of epigenetic modifications in stem cells, specifically what is known from rodent systems and established cancer cell lines, and how they are leading us to understand human stem cells.

Prostate cancer stem cells

Despite the discovery over 60 years ago by Huggins and Hodges that prostate cancers respond to androgen deprivation therapy, hormone-refractory prostate cancer remains a major clinical challenge. There is now mounting evidence that solid tumours originate from undifferentiated stem cell-like cells coexisting within a heterogeneous tumour mass that drive tumour formation, maintain tumour homeostasis and initiate metastases. This review focuses upon current evidence for prostate cancer stem cells, addressing the identification and properties of both normal and transformed prostate stem cells.

Urothelial transdifferentiation to prostate epithelia is mediated by paracrine TGF-beta signaling

The embryonic urogenital sinus mesenchyme (UGM) induces prostate epithelial morphogenesis in development. The molecular signals that drive UGM-mediated prostatic induction have not been defined. We hypothesized that the TGF-beta signaling directed the prostatic induction. UGM from TGF-beta type II receptor stromal conditional knockout mice (Tgfbr2(fspKO)) or control mice (Tgfbr2(floxE2/floxE2)) was recombined with wild-type adult mice bladder urothelial cells. The resulting urothelium associated with Tgfbr2(floxE2/floxE2) UGM was instructively differentiated into prostatic epithelium, as expected. In contrast, the urothelium associated with Tgfbr2(fspKO) UGM permissively maintained the phenotype of bladder epithelial cells. Microarray analysis of UGM tissues suggested the down-regulation of multiple Wnt ligands and the up-regulation of the Wnt antagonist, Wif 1, by the Tgfbr2(fspKO) UGM compared with Tgfbr2(floxE2/floxE2) UGM. The overexpression of Wif-1 by wild-type UGM resulted in the inhibition of prostatic induction. These data suggest that the stromal TGF-beta activity mediated by paracrine Wnt is necessary for the induction of prostatic differentiation. As Wnt ligands mediate differentiation and maintain the stem cell phenotype, the contribution of mouse stem cells and somatic cells to prostatic epithelium in the tissue recombination models was tested. The directed differentiation of mouse embryonic stem cells by UGM is suggested by a threshold number of mouse stem cells required in prostatic differentiation. To determine the contribution of somatic cells, the adult bladder epithelial compartment was labeled with green-fluorescent vital dye (CMFDA) and the stem-like cells marked by bromodeoxyuridine (BrdU) label-retention. The resulting prostatic epithelia of the tissue recombinants maintained the CMFDA dye, suggesting minimal cell division. Thus, the UGM can induce endoderm-derived epithelia and stem cells to form prostate through a transdifferentiation mechanism that requires stromal TGF-beta signaling to mediate epithelial Wnt activity.

Stem cells: The root of prostate cancer?

The cancer stem cell (CSC) model states that tumors contain a reservoir of self-renewing cells that maintain the heterogeneous cell population of the tumor. These cells appear to be resistant to therapy and can therefore survive to repopulate the tumor during progression to therapy resistant disease. The biology of CSCs is still not definitive since it is difficult to isolate them from solid tumors and analyze their characteristics in vitro. Another challenge is to correlate these characteristics with tumor development and progression in vivo. Using the prostate CSC as a model, this review presents the CSC hypothesis, reviews the origin, identification and functions of prostate CSCs, and discusses the clinical implications and therapeutic challenges CSCs have for cancer therapy.

Mesenchymal-epithelial interactions: past, present, and future

This review summarizes the history of research on mesenchymal-epithelial interactions in prostatic development from the first studies in 1970 to the present. From this study we have learned that prostatic development requires a reciprocal interaction between epithelium and mesenchyme in which urogenital sinus mesenchyme induces and patterns epithelial development and differentiation, while developing prostatic epithelium induces and patterns mesenchymal differentiation into smooth muscle and other resident cell types in the stroma. Prostatic development requires androgen action mediated by the androgen receptor (AR). Through analysis of tissue recombinants composed of wild-type and AR-null epithelium and mesenchyme, we have learned that many "androgenic effects" on prostatic epithelium do not require epithelial AR, but instead are elicited by the paracrine action of AR-positive mesenchyme. Present and future studies reviewed in this issue deal with the molecular mechanisms in this developmental communication between epithelium and mesenchyme.

The role of Sox9 in prostate development

The mammalian prostate arises from the urogenital sinus under the influence of testicular androgens. Few factors have been identified to be important in the early stages of prostate development. Here we review the role of the transcription factor Sox9 in prostate development. Sox9 is a member of the Sox gene family that plays an important role during embryogenesis in the cellular differentiation of various tissues, including testicular Sertoli cells, neural crest cells and chondrocytes. This gene is expressed in the epithelia of all mouse prostatic lobes from the initial stages of their development. Mice with a prostate specific deletion of Sox9 showed a lack of ventral prostate development and abnormal anterior prostate differentiation. In depth analysis of these mutant animals suggested that Sox9 is required for the early differentiation of the prostate bud epithelia, consistent with the function of this factor in other developmental processes. These studies also revealed different phases of prostate bud development. These phases were characterized by being dependent on different molecular pathways and having lobe specific properties. Future studies on the identification of pathways regulated by Sox9 will provide insight into the molecular networks required for prostate epithelia differentiation.

Prostate stem cells: the niche and cell markers

Prostate cancer and benign prostatic hyperplasia are common diseases in elderly men worldwide. Identifying the prostate stem cell is an important tool to investigate the mechanism of these prostatic diseases. Although the prostate stem cell has not yet been detected, progress has been made. The 'niche' or place in which the prostate stem cell resides is thought to be located in the proximal region of the murine prostate, near the urethra. Several candidate prostate stem cell markers are currently under investigation. In this review, we summarize the historical approaches and recent evidence regarding the niche and prostate-specific stem cell markers.

Endothelial cells support the growth of prostate tissue in vivo

INTRODUCTION

The contribution of vascular endothelial cells to prostate growth has not been investigated. We examined whether endothelial cells support growth of prostate tissue when co-inoculated with prostate epithelial cells under the renal capsule.

METHODS

Vascular endothelial cells were isolated from mice and co-inoculated under the renal capsule with a prostate luminal or basal epithelial cell line. After 60 days, kidneys were examined for growth of prostate tissue. Prostatic tissues were examined by immunohistochemistry for expression of cytokeratins 5 and 8, and vascular density was determined. To determine if increased expression of VEGF-A would increase prostatic growth, transfected endothelial cells overexpressing VEGF-A were co-inoculated with the prostate luminal or basal epithelial lines.

RESULTS

Co-inoculation of endothelial cells and prostate luminal or basal epithelial cells resulted in significant growth of prostatic tissue, whereas inoculation of any of the cell lines alone resulted in little growth. The growths from co-inoculation of endothelial cells and luminal epithelial cells contained duct-like structures that stained with antibodies to cytokeratin 8, whereas those from co-inoculation of endothelial cells and basal epithelial cells contained cords of cells that stained with antibodies to cytokeratin 5. Overexpression of VEGF-A had no effect on growth of the prostatic tissues.

CONCLUSION

Endothelial cells contribute to the growth of prostatic epithelial cells.

Sox9 is required for prostate development

The mammalian prostate arises from the urogenital sinus and few factors have been identified to be important in the early stages of prostate development. In this study we show that the transcription factor Sox9 is expressed in the epithelia of all mouse prostatic lobes from the initial stages of their development. We used a conditional approach with mice expressing Cre recombinase under the control of Nkx3.1 regulatory sequences to delete Sox9 from the developing prostate. Mice with a prostate specific deletion of Sox9 showed a lack of ventral prostate development and abnormal anterior prostate differentiation. Analysis of these mutant animals revealed an early loss of expression of genes specific to the prostate epithelia such as Nkx3.1 and Shh and a marked reduction in proliferation in the ventral prostate but not in other lobes. Fgf signalling, through the MAPK pathway, has been shown to be important in prostate development and a lobe specific phenotype was reported for a prostate specific Fgfr2 mutant mouse model. Here we show that the levels of Fgfr2 and Sprouty2, a downstream target of Fgf signalling, were severely reduced in the ventral prostate of Sox9 mutant animals but not in other lobes. Prostate organ culture studies with a Mek inhibitor, U0126, and a Fgf receptor inhibitor, SU5402, indicate that the timing of expression of Cre in the mutant animals could account for the lobe specific phenotype in the Sox9 and Fgfr2 mutants. These studies imply that Sox9 is required for the early differentiation of the prostate bud epithelia.

Anchorage-independent culture maintains prostate stem cells

Freshly isolated mouse prostate epithelial cells regenerate fully differentiated prostate tissue when combined with embryonic urogenital sinus mesenchyme and grafted in vivo. We show here that this regenerative capacity, which has been attributed to a small population of pleuripotential progenitor epithelial cells, is rapidly lost when the cells are placed in monolayer culture but can be maintained by culture in anchorage-independent conditions. Epithelial cells placed in anchorage-independent culture formed proliferating spheres that could be serially passaged and exhibited increased expression of putative stem cell markers as compared to cells grown in monolayer culture. Epithelial cells isolated from the fetal urogenital sinus, the newborn, and adult prostate formed spheres with similar efficiency, while cells isolated from the post-castration prostate exhibited significantly higher sphere-forming abilities. When passaged spheres were recombined with E17 rat urogenital sinus mesenchyme and grafted in vivo, they generated fully differentiated mouse prostate glandular epithelium containing both p63+ basal cells and p63- luminal cells and expressing a variety of prostate-specific and terminal differentiation markers.

Self-renewal and multilineage differentiation in vitro from murine prostate stem cells

Murine prostate stem cells express integrin alpha 6, which modulates survival, proliferation, and differentiation signaling through its interaction with the extracellular protein laminin. When plated in vitro in laminin containing Matrigel medium, 1 of 500-1,000 murine prostate cells can grow and form clonogenic spheroid structures that we term prostate spheres. Prostate spheres can be serially passaged individually or in bulk to generate daughter spheres with similar composition, demonstrating that sphere-forming cells are capable of self-renewal. Spheres spontaneously undergo lineage specification for basal and transit-amplifying cell types. P63-expressing cells localized to the outer layers of prostate spheres possess higher self-renewal capacity, whereas cells toward the center display a more differentiated transit-amplifying phenotype, as demonstrated by the expression of the prostate stem cell antigen. When dihydrotestosterone is added to the medium, the androgen receptor is stabilized, is imported to the nucleus, and drives differentiation to a luminal cell-like phenotype. A fraction of sphere cells returned to an in vivo environment can undergo differentiation and morphogenesis to form prostate tubular structures with defined basal and luminal layers accompanied by prostatic secretions. This study demonstrates self-renewal and multilineage differentiation from single adult prostate stem/progenitor cells in a specific in vitro microenvironment.

Prostate cancer stem cells

Prostate cancer is the most frequently diagnosed cancer in men. Despite recent advances in the detection of early prostate cancer there is little effective therapy for patients with locally advanced and/or metastatic disease. The majority of patients with advanced disease respond initially to androgen ablation therapy. However, most go on to develop androgen-independent tumours that inevitably are fatal. A similar response is seen to chemotherapeutic and radiotherapy treatments. As a result, metastatic prostate cancer remains an incurable disease by current treatment strategies. Recent reports of cancer stem cells have prompted questions regarding the involvement of normal stem/progenitor cells in prostate tumour biology, their potential contribution to the tumour itself and whether they are the cause of tumour initiation and progression. Although still controversial, the cancer stem cell is likely to be the most crucial target in the treatment of prostate cancer, and a thorough understanding of its biology, particularly of how the cancer stem cell differs from the normal stem cell, might allow it to be targeted selectively and eliminated, thus improving therapeutic outcome.

Cell-autonomous induction of functional tumor suppressor 15-lipoxygenase 2 (15-LOX2) contributes to replicative senescence of human prostate progenitor cells

Normal human prostatic (NHP) epithelial cells undergo senescence in vitro and in vivo, but little is known about the tissue-specific molecular mechanisms. Here we first characterize young primary NHP cells as CK5(+)/CK18(+) intermediate basal cells that also express several other putative stem/progenitor cell markers including p63, CD44, alpha2beta1, and hTERT. When cultured in serum- and androgen-free medium, NHP cells gradually lose the expression of these markers, slow down in proliferation, and enter senescence. Several pieces of evidence implicate 15-lipoxygenase 2 (15-LOX2), a molecule with a restricted tissue expression and most abundantly expressed in adult human prostate, in the replicative senescence of NHP cells. First, the 15-LOX2 promoter activity and the mRNA and protein levels of 15-LOX2 and its multiple splice variants are upregulated in serially passaged NHP cells, which precede replicative senescence and occur in a cell-autonomous manner. Second, all immortalized prostate epithelial cells and prostate cancer cells do not express 15-LOX2. Third, PCa cells stably transfected with 15-LOX2 or 15-LOX2sv-b, a splice variant that does not possess arachidonate-metabolizing activity, show a passage-related senescence-like phenotype. Fourth, infection of early-passage NHP cells with retroviral vectors encoding 15-LOX2 or 15-LOX2sv-b induces partial cell-cycle arrest and big and flat senescence-like phenotype. Finally, 15-LOX2 protein expression in human prostate correlates with age. Together, these data suggest that 15-LOX2 may represent an endogenous prostate senescence gene and its tumor-suppressing functions might be associated with its ability to induce cell senescence.

Prostate epithelial cell differentiation and its relevance to the understanding of prostate cancer therapies

Prostate cancer is the most common malignancy in males in the western world. However, little is known about its origin and development. This review highlights the biology of the normal prostate gland and the differentiation of basal epithelial cells to a secretory phenotype. Alterations in this differentiation process leading to cancer and androgen-independent disease are discussed, as well as a full characterization of prostate epithelial cells. A full understanding of the origin and characteristics of prostate cancer epithelial cells will be important if we are to develop therapeutic strategies to combat the heterogeneous nature of this disease.

Stromal/epithelial interactions of murine prostatic cell lines in vivo: a model for benign prostatic hyperplasia and the effect of doxazosin on tissue size

BACKGROUND

One of the major constraints in elucidating the mechanisms involved in the etiology of benign prostatic hyperplasia (BPH) is the lack of suitable model systems that are readily manipulable in vitro and in vivo. To address this issue, we have used murine prostatic cell lines to establish a novel in vivo model for studying prostatic cell interactions.

METHODS

Luminal, basal, and smooth muscle (SM) cell lines were inoculated alone or in combinations under the renal capsule of intact or castrated male mice, and the growth and composition of prostatic tissue in the absence or presence of doxazosin was determined.

RESULTS

Both the luminal and basal cell lines reconstituted prostatic tissue if co-inoculated under the renal capsule with normal SM cells, whereas none of the lines formed significant tissue when inoculated alone. Luminal cells produced and secreted prostatic secretory products. The growth of prostatic tissue formed from co-inoculation of basal and SM cells was androgen responsive. In addition, a significant reduction in prostatic tissue was noted in animals treated with doxazosin.

CONCLUSION

We have established an in vivo model that uses prostatic epithelial and SM cell lines for investigating cellular interactions between epithelial and SM cells that regulate prostatic growth and function. This model will be useful for delineating the mechanisms by which prostatic cells interact and in determining the efficacy of new approaches aimed at interfering with prostatic stromal/epithelial interactions that result in abnormal cellular proliferation.

Proximal location of mouse prostate epithelial stem cells: a model of prostatic homeostasis

Stem cells are believed to regulate normal prostatic homeostasis and to play a role in the etiology of prostate cancer and benign prostatic hyperplasia. We show here that the proximal region of mouse prostatic ducts is enriched in a subpopulation of epithelial cells that exhibit three important attributes of epithelial stem cells: they are slow cycling, possess a high in vitro proliferative potential, and can reconstitute highly branched glandular ductal structures in collagen gels. We propose a model of prostatic homeostasis in which mouse prostatic epithelial stem cells are concentrated in the proximal region of prostatic ducts while the transit-amplifying cells occupy the distal region of the ducts. This model can account for many biological differences between cells of the proximal and distal regions, and has implications for prostatic disease formation.

Transforming growth factor-beta(s) and their receptors in aging rat prostate

We hypothesize that rat fetal urogenital sinus mesenchyme (UGM) can induce prostatic growth of growth quiescent adult rat prostate through modulations of TGFbetas and their receptors. To test this hypothesis, prostatic ducts from aging rat prostate (4, 12, 17, 22, and 27 months) were combined with fetal rat UGM and grafted under renal capsule of athymic nude mice. At 1, 3, and 5 months the tissue recombinants were harvested from renal capsule and analyzed for their growth. The gene and protein expression of TGFbeta1, 2, 3 and their receptors, TbetaR-I and TbetaR-II, were analyzed by RT-PCR and immunohistochemistry, respectively. The results of these experiments demonstrate that prostate ducts when combined with rat UGM formed larger grafts as compared to control (prostatic ducts without UGM). The older rat prostate recombinants (17, 22, and 27 months) formed larger grafts (159 mg/graft) as compared to younger rat prostate (4 and 12 months) grafts (51 mg/graft). The mRNA and protein expression for TbetaR-I and TbetaR-II in 22 and 27 months rat prostate tissue recombinants were significantly lower than 4, 12, and 17 month tissue recombinants. However, mRNA expression for TGFbeta1, TGFbeta2, and TGFbeta3 did not change with aging rat tissue recombinants. The protein expression for TGFbeta1 was significantly up-regulated whereas TGFbeta2 and TGFbeta3 were down-regulated with aging prostate tissue recombinants. The present study demonstrates for the first time that rat fetal UGM differentially induces growth of aging rat prostate in a tissue recombinant model. The mechanisms of induction may be through up-regulation of TGFbeta1 and down-regulation of TGFbeta2, and TGFbeta3. However, the action of TGFbetas may be through TbetaR-I and TbetaR-II independent pathways since these receptors were lacking or low in older rat prostate tissue recombinants. These findings are important in understanding the mechanisms of UGM mediated prostatic growth.

Growth, regeneration, and tumorigenesis of the prostate activates the PSCA promoter

The prostate gland undergoes dramatic changes in growth status during normal physiologic development, following androgen administration to castrate animals, and during tumor development. The prostate stem cell antigen (PSCA, named for its strong sequence homology to the thymocyte marker stem cell antigen 2) is a cell surface molecule associated with human and murine prostate cancer. To help define the regulation of this molecule, we created a transgenic mouse strain, which uses the human PSCA promoter region to control the expression of enhanced green fluorescent protein (GFP). Expression of GFP was detected in mid-gestation following the appearance of prostatic buds from the urogenital sinus. In adult mice, GFP expression was restricted to a subset of cells located in the distal tips of the glands. GFP expression increased during puberty and regeneration driven by androgen and associated with expansive growth of the prostate. GFP-positive cells coexpressed markers associated with both basal and secretory cells in the human prostate. Prostate carcinogenesis driven by T antigen in the transgenic adenocarcinoma of the mouse prostate (TRAMP) model results in an increased percentage and intensity level for PSCA promoter-driven GFP-positive cells. This transgenic system helps define the range of cellular changes associated with altered expression of PSCA, shows that transcriptional control is a major component regulating PSCA levels, and provides a useful tool to study subpopulations of prostate epithelial cells and factors that regulate the PSCA promoter.

Changes of phenotypic expression of prostatic antigen in secondary transitional cell carcinoma of the prostate: evidence for induction phenomenon as a mechanism for acquisition of prostatic antigens in prostatic transitional cell carcinoma

BACKGROUND

In vitro and experimental studies of mesenchymal-epithelial interaction for the prostatic stroma have demonstrated that the prostatic stroma is capable of inducing the nonprostatic epithelium to acquire many features of prostatic epithelium. We investigated whether this phenomenon could be observed in vivo in human prostatic stroma. MATERIALS AND METHODS Sixty transitional cell carcinoma (TCC) of the urinary bladder: (a) 20 with glandular lumen; (b) 20 without glandular lumen: (c) 10 mixed TCC-adenocarcinoma (ACA); and (d) 10 with synchronous or metachronous TCC of the prostate; and three primary TCC of the prostate were examined and submitted for immunostaining for prostatic acid phosphatase (PAP) and prostatic specific antigen (PSA).

RESULTS

There was a spectrum of immunostaining for PSA ranging from negative reactivity in TCC without glandular lumen of the urinary bladder, to focal and weak reactivity in single cells with varying degrees of nonmucinous glandular differentiation and to strong reactivity in groups of cells in primary and synchronous or metachronous TCC in the prostate. The areas of carcinoma geographically closest to the prostate and with the most extensive nonmucinous glandular differentiation displayed the most frequent and strongest immunoreactivity for PSA. The immunoreactivity for PAP was usually stronger than for PSA. Four cases of TCC and mixed TCC-ACA were immunoreactive only for PAP. Furthermore, there was a change in the phenotype of TCC in the urinary bladder as it spread into the prostate. For 10 TCC in the urinary bladder with synchronous or metachronous tumor in the prostate, all TCC in the urinary bladder were negative for PAP and PSA, whereas six TCC in the prostate were focally positive.

CONCLUSIONS

The spectrum of immunoreactivity for PAP and PSA and the change in immunoreactivity of TCC of the urinary bladder as it spreads into the prostate are likely induced by the prostatic stroma through the mechanism of mesenchymal-epithelial interaction. Prostate 47:172-182, 2001.

Cultured stromal cells: an in vitro model of prostatic mesenchymal biology

BACKGROUND

Initial efforts to develop in vitro models to study prostatic biology focused on the culture and characterization of epithelial cells. Recently, attention has turned towards inclusion of stromal cells in experimental systems.

METHODS

Improved methods to isolate and culture stromal cells have been developed. An array of markers are employed to characterize subtypes of stromal cells, with particular interest in smooth muscle differentiation.

RESULTS

Defined, serum-free media are available for certain experimental applications. Conditions that promote smooth muscle differentiation have been identified. Investigators have characterized hormonal and peptide factors that regulate the growth of prostatic stromal cells, and have also described paracrine factors produced by stromal cells that influence epithelial biology.

CONCLUSIONS

Prostatic stromal-cell cultures are now widely employed by a large number of investigators for a diverse array of experimental purposes. While further refinement is required to obtain model systems that fully mimic in vivo processes, the availability of stromal- and epithelial-cell cultures provides a valuable resource for studying normal prostatic biology as well as benign prostatic hyperplasia (BPH) and cancer.

New concepts in tissue specificity for prostate cancer and benign prostatic hyperplasia

Of the hundreds of species of mammals, all of which have prostate glands, only humans and dogs are known to suffer from benign prostatic hyperplasia (BPH) and prostate carcinoma. In humans, prostate carcinoma is common, yet carcinomas of other sex accessory tissues are rare. In addition, different anatomic regions within the prostate gland have very different rates of BPH and carcinoma. In this article, we explore ideas and potential mechanisms relating to these paradoxical findings that may help explain the species, organ, and zone specificity of BPH and prostate cancer. We present an evolutionary argument that attempts to relate a high-fat diet, with its potential for generating oxidative DNA damage, to the species selectivity of prostate cancer. In addition, we outline an argument based on our preliminary studies indicating that chronic inflammation and the associated increase in cell turnover in the setting of increased oxidative stress may help to account for the organ selectivity of genitourinary carcinomas.

Stem cell features of benign and malignant prostate epithelial cells

PURPOSE

We present a new hypothesis suggesting that the different malignant potential of benign prostatic hyperplasia (BPH) and high grade prostatic intraepithelial neoplasia may be explained by distinct alterations in stem cell-like properties.

MATERIALS AND METHODS

We used our results and the recent literature to develop this hypothesis in the context of an updated prostate stem cell model.

RESULTS

While high grade prostatic intraepithelial neoplasia is a likely precursor lesion to many prostatic adenocarcinomas, BPH rarely if ever progresses directly to carcinoma. Prostate epithelium contains basal and secretory compartments. Secretory cells appear to differentiate from basal cells. Thus, prostatic stem cells most likely reside in the basal compartment. In BPH there is a slight increase in epithelial proliferation, yet most replicating epithelial cells within BPH maintain their normal restriction to the basal compartment. In high grade prostatic intraepithelial neoplasia there is a marked increase in cell proliferation. In contrast to BPH, the majority of proliferating cells in high grade prostatic intraepithelial neoplasia reside in the secretory compartment. The biological significance of this topographic infidelity of proliferation in high grade prostatic intraepithelial neoplasia remains unclear but may relate mechanistically to down regulation of the cyclin dependent kinase inhibitor, p27kip1. Normal basal cells express GSTP1, an enzyme that inactivates reactive electrophiles and organic hydroperoxides, and that may protect cells from deoxyribonucleic acid damaging agents. In contrast, normal secretory cells and high grade prostatic intraepithelial neoplasia cells do not express this enzyme.

CONCLUSIONS

We propose that topographic infidelity of proliferation produces a population of secretory cells replicating in the absence of key genome protective mechanisms, thus setting the stage for an accumulation of genomic alterations and instability in high grade prostatic intraepithelial neoplasia. This action occurs along with activation of telomerase, resulting in an immortal clone capable of developing into invasive carcinoma. The model predicts that genome protection remains intact in BPH, minimizing its malignant potential.

Defined medium for normal adult human prostatic stromal cells

Stromal-epithelial interactions are pivotal in many aspects of prostatic biology. A defined culture system is critical for the investigation of factors that regulate the growth and differentiation of human prostatic stromal cells. We have identified conditions which promote stromal cell attachment and proliferation in serum-free medium. MCDB 201, originally developed for the clonal growth of chick embryo fibroblasts, proved to be a superior basal medium of those that we tested. Supplementation of MCDB 201 with basic fibroblast growth factor (FGF), insulin-like growth factor (IGF), and platelet-derived growth factor (PDGF) permitted attachment and exponential growth of cells throughout a 7-d period with an initial inoculum as low as 10(3) cells per well of a 96-well microtiter dish. Using these assay conditions, we subsequently verified that basic FGF and IGF, but not PDGF, were required for optimal growth. No activity was found for heparin, transferrin, or the androgen R1881. Epidermal growth factor (EGF) didn't stimulate growth when added to medium containing basic FGF and IGF, but was moderately stimulatory when added to basal medium alone. Cholera toxin inhibited growth. This simple and efficient culture medium provides a suitable assay system for more extensive studies of growth regulation and differentiation of human prostatic stromal cells, and will provide the basis for future development of a defined medium that supports clonal growth. Characterization of stromal-epithelial interactions will be facilitated by the use of this defined culture system for stromal cells in conjunction with the serum-free culture systems previously developed for human prostatic epithelial cells.

Interactions between adult human prostatic epithelium and rat urogenital sinus mesenchyme in a tissue recombination model

Tissue recombinants composed of adult human prostatic epithelium (hPrE) and rat urogenital sinus mesenchyme (rUGM) were grafted beneath the renal capsule of athymic rodent hosts. The pseudostratified human epithelium initially became multilayered, solid epithelial cords emerged, grew into the surrounding mesenchyme and canalized to regenerate a pseudostratified epithelium. Basal cells expressed cytokeratins 5 and 14, while luminal cells expressed cytokeratins 8 and 18, prostate specific antigen and prostatic acid phosphatase. The rat mesenchymal component differentiated into thick sheets of smooth muscle, characteristic of the human but not the rat prostate. These findings indicate that epithelial-mesenchymal interactions were reciprocal. Rat UGM induced adult hPrE to form new ductal-acinar tissue, involving epithelial proliferation, ductal branching morphogenesis and functional cytodifferentiation. Concurrently the epithelium dictated smooth muscle differentiation and patterning. Species-specific reverse transcriptase polymerase chain reaction SC (RT-PCR) analysis of the tissue recombinants was performed to separately examine the expression of epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), epidermal growth factor receptor (EGFR), TGF-beta 1, and TGF-beta 3 in the epithelium, stroma and host components of the graft. All of these genes, except TGF-beta 1, were expressed in all three tissues. Human TGF-beta 1 was not detected, indicating that this gene was not expressed in human prostatic epithelium but was present in stroma.

In utero and lactational exposure of the male rat to 2,3,7,8-tetrachlorodibenzo-p-dioxin impairs prostate development. 2. Effects on growth and cytodifferentiation

In the male Holtzman rat, in utero and lactational 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure decreases prostate weight without inhibiting testicular androgen production or decreasing circulating androgen concentrations. Therefore, the present study sought to characterize effects of TCDD exposure on prostate development, from very early outgrowth from the urogenital sinus (Gestation Day [GD] 20) until rapid growth and differentiation are essentially complete (Postnatal Day [PND] 32). Pregnant Holtzman rats were administered a single dose of TCDD (1.0 microgram/kg po) or vehicle on GD 15 and offspring were exposed via placental transfer (GD 20 euthanasia) or placental and subsequent lactational transfer until euthanasia (if before PND 21) or weaning. Results show that the prostatic epithelial budding process was impaired by in utero TCDD exposure, as evidence by significant decreases in the number of buds emerging from dorsal, lateral, and ventral aspects of the GD 20 urogenital sinus. Ventral prostate cell proliferation index was significantly decreased on PND 1 but was similar to or higher than control at later times, whereas apoptosis was an extremely rare event in ventral prostates from both control and TCDD-exposed animals. Delays were noted in the differentiation of pericordal smooth muscle cells and luminal epithelial cells. In addition, ventral prostates from approximately 40% of TCDD-exposed animals examined on PNDs 21 and 32 exhibited alterations in the histological arrangement of cell types that could not be explained by a developmental delay. Compared to controls, these ventral prostates exhibited a disorganized, hyperplastic epithelium containing fewer luminal epithelial cells and an increased density or continuous layer of basal epithelial cells, as well as thicker periductal smooth muscle sheaths. In addition, in ventral prostates from TCDD-exposed animals, the intensity of androgen receptor staining was relatively low in the central and distal epithelium, and the number of androgen receptor-positive cells was relatively high in the periductal stroma. These data suggest that in utero and lactational TCDD exposure interferes with prostate development by decreasing very early epithelial growth, delaying cytodifferentiation, and, in the most severely affected animals, producing alterations in epithelial and stromal cell histological arrangement and the spatial distribution of androgen receptor expression that may be of permanent consequence.

Osteopontin stimulates a subpopulation of quiescent human prostate epithelial cells with high proliferative potential to divide in vitro

BACKGROUND

Osteopontin (OPN) is a secreted extracellular matrix (ECM) protein found in bone, as well as associated with epithelial cells. The main objective of these studies was to test in vitro the hypothesis that interaction with OPN stimulates proliferation of a quiescent subpopulation of prostate epithelial cells with high proliferative potential.

METHODS

To simulate conditions that restrict proliferation and inhibit terminal differentiation of basal cells in vivo, control cultures grew on substrate coated with collagen (CO) or fibronectin (FN), in medium containing low levels of growth factors.

RESULTS

Under growth-restricting conditions, most prostate epithelial cells with high proliferative potential, seeded in control secondary cultures, were quiescent within the time frame of the studies, as indicated by the small number of large colonies in these cultures. Growing prostate epithelial cells (PR) under the same growth-restricting conditions, but on substrate coated with OPN instead of CO or FN, stimulated proliferation of a subpopulation of single cells with high proliferative ability as indicated by: 1) dose-dependent increase in the percentage of single cells incorporating bromodeoxyuridine, i.e., proliferating PR; and 2) subsequent dose-dependent increase in the percentage of large colonies. The OPN effect was not merely due to preferential attachment to OPN, because PR attachment to OPN, CO, or FN was identical. PR attachment to OPN was inhibited in the presence of GRGDTP or an antibody against the integrin subunit alphav, but not in the presence of an RGES peptide or a nonspecific IgG.

CONCLUSIONS

Integrin-mediated OPN/PR interaction stimulates proliferation of a quiescent subpopulation of prostate epithelial cells with high proliferative potential, possibly stem cells.

Identification of intermediate cell types by keratin expression in the developing human prostate

BACKGROUND

The secretory acini of the adult human prostate contain basal, luminal, and intermediate types of exocrine cells. Intermediate cells are thought to play an important role in normal growth and neoplastic transformation. In this study we investigated whether this cell type is present in early stages of prostate development, using keratin antibodies specific for them.

METHODS

Autoptic tissue from 11 prepubertal and 5 normal adult prostates was immunohistochemically stained with four keratin antibodies capable of specifically detecting basal, luminal, or intermediate cell types.

RESULTS

Morphologically, in fetal prostate cells differentiation was often not evident. However, basally located cells usually displayed a basal-cell keratin-phenotype. Morphologically similar cells with more luminal localization expressed keratins typical of luminal cells, or of intermediate cells.

CONCLUSIONS

1) In early stages of prostate development, cells with intermediate keratin-phenotype can be identified. 2) Their large numbers comply with a hierarchical pathway of cellular differentiation from basal to luminal cells. 3) The presence of intermediate cells at such an early fetal age may reflect their regulatory function in prostate development.

Change in morphological and functional cytodifferentiation induced by seminal vesicle mesenchyme in cell suspensions of rat Dunning prostatic adenocarcinoma cells

Previous experiments have shown that seminal vesicle mesenchyme (SVM) can induce small 0.5 mm fragments of the rat Dunning tumor (DT) to undergo secretory differentiation with a concomitant reduction in tumorigenesis. In the present experiments Dunning tumor epithelial cells (DTE) were purified from DT cell suspensions by Percoll gradient centrifugation and recombined with neonatal rat SVM. The resultant tissue recombinants (SVM + DTE) were grafted under the renal capsule of male athymic mice and grown for 2 months. Under these conditions SVM induced the DTE to exhibit a highly differentiated secretory phenotype by forming ducts lined with tall columnar epithelial cells or large clear cells with pale cytoplasm. Undifferentiated epithelial cells of the parental DT were rarely observed in these tissue recombinants. The loss of tumorigenicity in SVM + DTE recombinants was associated with a striking reduction of epithelial 3H-thymidine labeling index in SVM + DTE recombinants (DT = 8.31%; SVM + DTE recombinants = 1.10%). Differences in putative secretory proteins were also observed by SDS-PAGE in SVM + DTE recombinants in comparison with DT. Testosterone metabolism was examined in epithelial cells recovered from grafts of DT vs. SVM + DTE tissue recombinants by thin layer chromatography and revealed that the major metabolite produced by DTE was androstenedione, whereas in epithelium isolated from SVM + DTE tissue recombinants the major androgen metabolite was 5alpha-DHT. Thus, after induction by SVM the DTE metabolized androgens in a pattern similar to the normal rat dorsal prostate. The SVM-induced changes in DTE suggest the possibility that emerging or established carcinomas might be regulated at least in part by their connective tissue microenvironment.