Absence of proximal duct apoptosis in the ventral prostate of transgenic mice carrying the C3(1)-TGF-beta type II dominant negative receptor

Notch and TGFβ form a reciprocal positive regulatory loop that suppresses murine prostate basal stem/progenitor cell activity

The role of Notch signaling in the maintenance of adult murine prostate epithelial homeostasis remains unclear. We found that Notch ligands are mainly expressed within the basal cell lineage, while active Notch signaling is detected in both the prostate basal and luminal cell lineages. Disrupting the canonical Notch effector Rbp-j impairs the differentiation of prostate basal stem cells and increases their proliferation in vitro and in vivo, but does not affect luminal cell biology. Conversely, ectopic Notch activation in adult prostates results in a decrease in basal cell number and luminal cell hyperproliferation. TGFβ dominates over Notch signaling and overrides Notch ablation-induced proliferation of prostate basal cells. However, Notch confers sensitivity and positive feedback by upregulating a plethora of TGFβ signaling components including TgfβR1. These findings reveal crucial roles of the self-enforced positive reciprocal regulatory loop between TGFβ and Notch in maintaining prostate basal stem cell dormancy.

DHT selectively reverses Smad3-mediated/TGF-beta-induced responses through transcriptional down-regulation of Smad3 in prostate epithelial cells

Androgens suppress TGF-β responses in the prostate through mechanisms that are not fully explored. We have recently reported that 5α-dihydrotestosterone (DHT) suppresses the ability of TGF-β to inhibit proliferation and induce apoptosis of prostatic epithelial cells and provided evidence that such suppression was fueled by transcriptional down-regulation of TGF-β receptor II (ΤβRII). We now show that androgen receptor (AR) activated by DHT suppresses the TGF-β-induced phosphorylation of Sma- and Mad-related protein (Smad)3 in LNCaP cells overexpressing TβRII under the control of a cytomegalovirus promoter, which is not regulated by DHT, suggesting that transcriptional repression of TβRII alone does not fully account for the impact of DHT on TGF-β responses. Instead, we demonstrate that such suppression occurs through loss of total Smad3, resulting from transcriptional suppression of Smad3. We provide evidence that DHT down-regulates the promoter activity of Smad3 in various prostate cancer cell lines, including NRP-154+AR, DU145+AR, LNCaP, and VCaP, at least partly through androgen-dependent inactivation of Sp1. Moreover, we show that overexpression of Smad3 reverses the ability of DHT to protect against TGF-β-induced apoptosis in NRP-154+AR, supporting our model that loss of Smad3 by DHT is involved in the protection against TGF-β-induced apoptosis. Together, these findings suggest that deregulated/enhanced expression and activation of AR in prostate carcinomas may intercept the tumor suppressor function of TGF-β through transcriptional suppression of Smad3, thereby providing new mechanistic insight into the development of castration-resistant prostate cancer.

Relative roles of TGF-beta1 and Wnt in the systemic regulation and aging of satellite cell responses

Muscle stem (satellite) cells are relatively resistant to cell-autonomous aging. Instead, their endogenous signaling profile and regenerative capacity is strongly influenced by the aged P-Smad3, differentiated niche, and by the aged circulation. With respect to muscle fibers, we previously established that a shift from active Notch to excessive transforming growth factor-beta (TGF-β) induces CDK inhibitors in satellite cells, thereby interfering with productive myogenic responses. In contrast, the systemic inhibitor of muscle repair, elevated in old sera, was suggested to be Wnt. Here, we examined the age-dependent myogenic activity of sera TGF-β1, and its potential cross-talk with systemic Wnt. We found that sera TGF-β1 becomes elevated within aged humans and mice, while systemic Wnt remained undetectable in these species. Wnt also failed to inhibit satellite cell myogenicity, while TGF-β1 suppressed regenerative potential in a biphasic fashion. Intriguingly, young levels of TGF-β1 were inhibitory and young sera suppressed myogenesis if TGF-β1 was activated. Our data suggest that platelet-derived sera TGF-β1 levels, or endocrine TGF-β1 levels, do not explain the age-dependent inhibition of muscle regeneration by this cytokine. In vivo, TGF-β neutralizing antibody, or a soluble decoy, failed to reduce systemic TGF-β1 and rescue myogenesis in old mice. However, muscle regeneration was improved by the systemic delivery of a TGF-β receptor kinase inhibitor, which attenuated TGF-β signaling in skeletal muscle. Summarily, these findings argue against the endocrine path of a TGF-β1-dependent block on muscle regeneration, identify physiological modalities of age-imposed changes in TGF-β1, and introduce new therapeutic strategies for the broad restoration of aged organ repair.

Androgenic control of transforming growth factor-beta signaling in prostate epithelial cells through transcriptional suppression of transforming growth factor-beta receptor II

The androgen receptor cross-talks with transforming growth factor-beta (TGF-beta) through mechanisms that remain poorly understood. Here we provide strong evidence that 5alpha-dihydrotestosterone (DHT) intercepts the ability of prostate epithelial cells to undergo TGF-beta-induced apoptosis, and present a new model for this androgenic effect. We report that DHT decreases the level of TGF-beta receptor II (TbetaRII) through a transcriptional mechanism, leading to suppression of the ability of TGF-beta to down-regulate expression of Bcl-xL and cyclin Ds, activate caspase-3, and induce apoptosis. Promoter analysis, DNA pulldown, and electrophoretic mobility shift assays support that transcriptional down-regulation of TbetaRII by DHT occurs through Sp1/Sp3 response elements, with the binding of Sp1 to the TbetaRII promoter being suppressed by DHT, largely driven by loss of Sp1 protein and/or activity. These results provide fresh insight on the mechanism of growth control by androgens and the progression of prostate cancer to androgen independence. [Cancer Res 2008;68(19):8173-82].

Cytokine knockouts in reproduction: the use of gene ablation to dissect roles of cytokines in reproductive biology

Cytokines play many diverse and important roles in reproductive biology, and dissecting the complex interactions between these proteins and the different reproductive organs is a difficult task. One approach is to use gene ablation, or 'knockout', to analyse the effect of deletion of a single cytokine on mouse reproductive function. This review summarizes the essential roles of cytokines in reproductive biology that have been revealed by gene knockout studies, including development and regulation of the hypothalamo-pituitary-gondal axis, ovarian folliculogenesis, implantation and immune system modulation during pregnancy. However, successful utilization of this approach must consider the caveats associated with gene ablation studies, e.g. embryonic lethality, systemic effects of cytokine ablation on local reproductive processes and the limited exposure to pathogens in mice housed in laboratory conditions. New sophisticated technology that temporally or spatially regulates gene ablation can overcome some of these limitations. Discoveries on the roles of cytokines in reproductive function uncovered by gene ablation studies can now be applied to improve in vitro fertilization for infertile couples and in the development of contraceptive therapies.

ELAC2, a putative prostate cancer susceptibility gene product, potentiates TGF-beta/Smad-induced growth arrest of prostate cells

Transforming growth factor-beta (TGF-beta) elicits a potent growth inhibitory effect on many normal cells by binding to specific serine/threonine kinase receptors and activating specific Smad proteins, which regulate the expression of cell cycle genes, including the p21 cyclin-dependent kinase (CDK) inhibitor gene. Interestingly, cancer cells are often insensitive to the anti-mitogenic effects of TGF-beta for which the molecular mechanisms are not well understood. In this study, we found that the candidate prostate cancer susceptibility gene ELAC2 potentiates TGF-beta/Smad-induced transcriptional responses. ELAC2 associates with activated Smad2; the C-terminal MH2 domain of Smad2 interacts with the N-terminal region of ELAC2. Small interfering siRNA-mediated knock-down of ELAC2 in prostate cells suppressed TGF-beta-induced growth arrest. Moreover, ELAC2 was shown to specifically associate with the nuclear Smad2 partner, FAST-1 and to potentiate the interaction of activated Smad2 with transcription factor Sp1. Furthermore, activation of the p21 CDK inhibitor promoter by TGF-beta is potentiated by ELAC2. Taken together our data indicate an important transcriptional scaffold function for ELAC2 in TGF-beta/Smad signaling mediated growth arrest.

Reduction of hypertrophic scar via retroviral delivery of a dominant negative TGF-beta receptor II

Effective blockade of the pluripotent cytokine transforming growth factor (TGF)-beta as a means of cutaneous scar reduction is a strategy with great potential. This desired effect may be achieved through the overexpression of mutant TGF-beta receptors within the wound milieu. Our goal was to examine the effects of dominant negative mutant TGF-beta receptor II (TGFbetaRIIdn) protein expression in a well-established rabbit ear model of hypertrophic scarring. Serial injections of a retroviral construct encoding a truncated TGFbetaRII and the marker green fusion protein (pMSCV-rIIdn-GFP) were performed in 7mm punch wounds at day 10 and day 12 (two-day injection group) or days 8, 10, 12 (three-day injection group) post-wounding. Delivery of an empty vector (pMSCV-GFP) at the same time points served as a negative control. Histomorphometric analysis of wounds harvested at day 28 revealed a modest, though statistically significant reduction (20%, p=0.027) in the scar elevation index (SEI) in two-day treated and a more modest reduction in SEI (12%) in the three-day treated arm compared to null-treated controls. Confocal microscopy confirmed stable, yet variable transfection of the construct in both peri-wound tissue as well as rabbit dermal fibroblasts transfected in vitro. Optimisation of this novel application in retroviral gene therapy could lead to effective anti-scarring strategies.

Truncating mutations in the ACVR2 gene attenuates activin signaling in prostate cancer cells

Activins are classified as members of the TGFbeta superfamily of signaling molecules and both activin and TGFbeta ligands signal through structurally and functionally related serine/threonine kinase receptors. Defects in these signaling pathways have been associated with the initiation and progression of the cancer phenotype. Inactivating mutations in the TGFbeta type II receptor gene, TGFbetaR2, have been identified in a variety of tumors and cell lines, particularly those with microsatellite instability (MSI). More recently, mutations in the activin type II receptor gene, ACVR2, were identified in colon and pancreatic cell lines and tumors with MSI. Because prostate tumors appear to have a high incidence of MSI, we analyzed prostate cancer cell lines, with and without MSI, for ACVR2 and TGFbetaR2 mutations. Our analysis of 6 prostate cell lines revealed mutations in the ACVR2 gene in 22Rv-1, LAPC-4, DU145, and LNCaP cells and mutations in the TGFbetaR2 gene in 22Rv-1 and LAPC-4. PC3 and H660 cells were wild-type for ACVR2 and TFGbetaR2. All of the ACVR2 mutations were truncating mutations, and using an activin response assay, we demonstrate that truncating mutations of the ACVR2 gene result in a significant reduction in activin mediated cell signaling. Inactivation of ACVR2 is a common event in prostate cancer cells suggesting it may play an important role in the development of prostate cancer.

Recent advances on multiple tumorigenic cascades involved in prostatic cancer progression and targeting therapies

Recent advances on differently-expressed gene products and their functions during the progression from localized androgen-dependent states into androgen-independent and metastatic forms of prostate cancer are reported. The expression levels of numerous oncogenes and tumor suppressor genes in distinct prostatic cancer epithelial cell lines and tissues relative to normal prostate cells are described. This is carried out to identify the signaling elements that are altered during the initiation, progression and metastatic process of prostate cancer. Additional information on the interactions between certain deregulated signaling pathways such as androgen receptor (AR), estrogen receptors, epidermal growth factor receptor (EGFR), hedgehog and Wnt/beta-catenin cascades in controlling the proliferation, survival and invasion of tumor prostate epithelial cells during the disease progression is described. The emphasis is on the critical functions of the AR and EGF-EGFR systems at all stages during prostate carcinogenesis. Of therapeutic interest, new strategies for the diagnosis and treatment of localized and metastatic forms of prostate cancer by targeting multiple tumorigenic signaling elements are also reported.

Disruption of the TGF-beta pathway and modeling human cancer in mice

There is considerable complexity underlying the mechanisms through which the TGF-beta signaling pathway regulates the initiation and progression of cancer. Analysis of this pathway and the role that it plays in human malignancy continues to elucidate novel mechanisms through which various genetic and epigenetic events subvert the controls that TGF-beta exerts over cell growth, differentiation, and malignant transformation. Modeling these events in the mouse represents an important goal, as the relevant preclinical models are essential not only for improving our understanding of the role of the TGF-beta pathway in the molecular pathogenesis of cancer, but also as tools for evaluating the impact of novel therapeutics on TGF-beta signaling and the role they may play in the prevention and treatment of malignancies. Here, we consider highlights from a number of in vivo murine model systems and relate a few of the significant observations to what we know about TGF-beta signaling in human cancer.

TGF-{beta} maintains dormancy of prostatic stem cells in the proximal region of ducts

We have previously shown that prostatic stem cells are located in the proximal region of mouse prostatic ducts. Here, we show that this region responds differently to transforming growth factor (TGF)-beta than the distal ductal region and that under physiological conditions androgens and TGF-beta are crucial overall regulators of prostatic tissue homeostasis. This conclusion is supported by the observations showing that high levels of TGF-beta signaling are present in the quiescent proximal region of ducts in an androgen-replete animal and that cells in this region overexpress Bcl-2, which protects them from apoptosis. Moreover, androgen ablation reverses the proximal-distal TGF-beta signaling gradient, leading to an increase in TGF-beta signaling in the unprotected distal region (low Bcl-2 expression). This reversal of TGF-beta-mediated signaling accompanies apoptosis of cells in the distal region and gland involution after androgen withdrawal. A physiological TGF-beta signaling gradient (high proximally and low distally) and its functional correlates are restored after androgen replenishment. In addition to highlighting the regulatory role of androgens and TGF-beta, these findings may have important implications for the deregulation of the stem cell compartment in the etiology of proliferative prostatic diseases.

Functions and regulation of transforming growth factor-beta (TGF-β) in the prostate

The prostate is a highly androgen-dependent tissue that in humans exhibits marked susceptibility to carcinogenesis. The malignant epithelium generated from this tissue ultimately loses dependence on androgens despite retention or amplification of the androgen receptor. Accumulating evidence support that transforming growth factor-beta (TGF-beta) plays key roles in the control of androgen dependence and acquisition of resistance to such hormonal control. Although TGF-beta functions as a key tumour suppressor of the prostate, it can also promote malignant progression and metastasis of the advanced disease, through undefined mechanisms. In addition to giving an overview of the TGF-beta field as related to its function in prostate cancer, this Review focuses on novel findings that support the tumour suppressor function of TGF-beta is lost or altered by changes in the activity of the androgen receptor, insulin-like growth factor-I, Akt, and mTOR during malignant progression. Understanding the mechanisms of cross-talk between TGF-beta and such growth modulators has important implications for the rational therapeutics of prostate cancer.

Novel Function of Androgen Receptor-associated Protein 55/Hic-5 as a Negative Regulator of Smad3 Signaling

Androgen receptor-associated protein 55 (ARA55/Hic-5) belongs to the LIM protein superfamily and is featured by three or four N-terminal LD motifs and four C-terminal zinc finger-like LIM domains. Both LD motifs and LIM domains can serve as protein-protein interaction interfaces. Recently, we found that enforced expression of ARA55 inhibits transforming growth factor-beta-mediated up-regulation of Smad binding element-luciferase reporter activity in NRP-154 and NRP-152 rat prostate and LNCaP human prostate cell lines. Moreover, ARA55 also inhibits the induction of Smad-binding element 4-luciferase and 3TP-luciferase (a plasminogen activator inhibitor-1 (PAI-1) promoter construct) reporters by constitutively active (CA)-Smad3 in these cell lines. Co-immunoprecipitation studies suggest an interaction between ARA55 and either CA-Smad3 or wild-type Smad3 in HEK293 cells that occurs through the MH2 domain of Smad3 and the C terminus of ARA55 with wild-type Smad3 having stronger affinity than CA-Smad3 to ARA55. Glutathione S-transferase pull-down assays demonstrate that this interaction can occur in a cell-free system. These results are consistent with the luciferase data showing that the C terminus of ARA55 is critical for suppression of Smad3 activity. Furthermore, using a mammalian two-hybrid system, we confirmed that ARA55 interacts with the MH2 domain of Smad3 and suppresses CA-Smad3-induced transcriptional responses. In conclusion, these results support that ARA55 selectively intercepts transforming growth factor-beta signaling through an interaction of the LIM domain of ARA55 with the MH2 domain of Smad3.

Prostate stem cells and prostate cancer

Understanding prostate stem cells (PSCs) may provide insight for the design of therapeutics for prostate cancer. We have developed a quantitative in vivo colony-forming assay and have demonstrated that the Sca-1 antigen is present on the surface of a prostate cell subpopulation that possesses multiple stem cell properties. Immunofluorescent analysis demonstrates that Sca-1 is expressed by both basal and luminal cells in the proximal region of the adult prostate, but is not expressed by either lineage in more distal regions. The proximal region has been suggested as the PSC niche based on BrdU label-retention studies and the presence of distinct smooth-muscle cells that produce high levels of TGF-beta. Sca-1 is also expressed by nearly all cells within fetal prostate epithelial chords, suggesting Sca-1 may be conserved on PSCs throughout development. Malignant epithelial cells from TRAMP mice, as well as normal prostate cells with lentiviral-mediated alteration of the PTEN/AKT signaling pathway, give rise to PIN lesions and prostate cancer in vivo. Alteration of PTEN/AKT signaling in Sca-1-enriched PSCs also results in PIN lesions, suggesting that PSCs can serve as one target for prostate carcinogenesis.

Epithelial stem cells in human prostate growth and disease

Benign prostatic hyperplasia and prostate cancer arise as a consequence of changes in the balance between cell division and differentiation. Little, however, is known about the control of this process. Stem cells are a small population of cells that divide occasionally to produce transit-amplifying cells that in turn produce the differentiated cell types of the tissue. It is believed that cancer cell proliferation is also driven by stem cells. We have shown that around one in 200 prostate epithelial cells have characteristics of stem cells and that these cells are contained within a population with a distinct keratin expression pattern. Work is now ongoing to identify markers for these cells that will allow us to study the role they play in prostatic disease.

Fibroblast-specific expression of a kinase-deficient type II transforming growth factor beta (TGFbeta) receptor leads to paradoxical activation of TGFbeta signaling pathways with fibrosis in transgenic mice

To better understand the role of disrupted transforming growth factor beta (TGFbeta) signaling in fibrosis, we have selectively expressed a kinase-deficient human type II TGFbeta receptor (TbetaRIIDeltak) in fibroblasts of transgenic mice, using a lineage-specific expression cassette subcloned from the pro-alpha2(I) collagen gene. Surprisingly, despite previous studies that characterized TbetaRIIDeltak as a dominant negative inhibitor of TGFbeta signaling, adult mice expressing this construct demonstrated TGFbeta overactivity and developed dermal and pulmonary fibrosis. Compared with wild type cells, transgenic fibroblasts proliferated more rapidly, produced more extracellular matrix, and showed increased expression of key markers of TGFbeta activation, including plasminogen activator inhibitor-1, connective tissue growth factor, Smad3, Smad4, and Smad7. Smad2/3 phosphorylation was increased in transgenic fibroblasts. Overall, the gene expression profile of explanted transgenic fibroblasts using cDNA microarrays was very similar to that of littermate wild type cells treated with recombinant TGFbeta1. Despite basal up-regulation of TGFbeta signaling pathways, transgenic fibroblasts were relatively refractory to further stimulation with TGFbeta1. Thus, responsiveness of endogenous genes to TGFbeta was reduced, and TGFbeta-regulated promoter-reporter constructs transiently transfected into transgenic fibroblasts showed little activation by recombinant TGFbeta1. Responsiveness was partially restored by overexpression of wild type type II TGFbeta receptors. Activation of MAPK pathways by recombinant TGFbeta1 appeared to be less perturbed than Smad-dependent signaling. Our results show that expression of TbetaRIIDeltak selectively in fibroblasts leads to paradoxical ligand-dependent activation of downstream signaling pathways and causes skin and lung fibrosis. As well as confirming the potential for nonsignaling receptors to regulate TGFbeta activity, these findings support a direct role for perturbed TGFbeta signaling in fibrosis and provide a novel genetically determined animal model of fibrotic disease.

New perspectives on growth factor-sex steroid interaction in the prostate

Many organs respond to both sex steroids and growth factors. Regulation of these pathways is integral to cell-cell communications during development and aberrant changes cause disease pathogenesis. Traditionally, paracrine and endocrine actions of growth factors and steroid hormones are considered independently. Recently, new data indicated that activin/TGFbeta and sex steroid signalling are linked; explicitly, that the pathways cross-talk intracellularly. Here we present new perspectives on these interactions, using examples predominantly from the prostate, as it is a well-characterised organ in this context. While this information provides insight to the potential mechanisms behind these interactions, it also presents a new challenge; the action of any of these factors cannot be considered exclusively without considering the impact on the other biological pathways.

Defining the actions of transforming growth factor beta in reproduction

Members of the transforming growth factor beta (TGFbeta) family are pleiotropic cytokines with key roles in tissue morphogenesis and growth. TGFbeta1, TGFbeta2 and TGFbeta3 are abundant in mammalian reproductive tissues, where development and cyclic remodelling continue in post-natal and adult life. Potential roles for TGFbeta have been identified in gonad and secondary sex organ development, spermatogenesis and ovarian function, immunoregulation of pregnancy, embryo implantation and placental development. However, better tools must now be employed to map more precisely essential functions and the regulatory networks governing their activity. Gene ablation and transgenic models are expected to provide novel insights into distinct physiological activities for each TGFbeta isoform in normal reproductive function and reproductive pathologies. It is also necessary to consider the mechanisms controlling TGFbeta activation from latent precursor forms, and receptor and binding protein expression. Smad intracellular signalling circuitry and modulation by environmental stimuli through cross-talk with other signal transduction pathways will further constrain TGFbeta action. This review examines existing evidence for TGFbeta1, TGFbeta2 and TGFbeta3 regulation of male and female reproductive biology, and highlights prospects for future research.

Transforming growth factor-beta 2 heterozygous mutant mice exhibit Cowper's gland hyperplasia and cystic dilations of the gland ducts (Cowper's syringoceles)

Analyses of mutant mice with a deletion for the transforming growth factor beta 2 (Tgfbeta2) gene revealed cysts in the perineal/scrotal region of male mice. We present evidence from in situ, light and electron microscopy that the cysts observed in Tgfbeta2+/- heterozygous mice males derive from Cowper's gland tissue. The Cowper's glands of Tgfbeta2+/- heterozygous mutant mice display all steps of glandular hyperplasia and cystic dilation. TGF-beta isoforms and TGF-beta receptor (TbetaR-II) were localized immunocytochemically in sections of Cowper's glands. TGF-beta2 and TGF-beta3 were located predominantly in myoepithelial cells of the Cowper's gland whereas the TbetaRII was found in the plasma membrane of the acinar cells. TUNEL-assays revealed that apoptotic cell death is significantly reduced in Cowper's glands of TgfbetaB2+/- heterozygous mutant mice. The fact that Tgfbeta2+/- heterozygous mutant mice exhibit hyperplasia of Cowper's gland epithelium and Cowper's gland cysts suggests a disturbance of epithelial-stromal interaction most likely due to reduced TGF-beta2 level, accompanied by a significant decrease in apoptosis.

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.

Autochthonous mouse models for prostate cancer: past, present and future

Prostate cancer continues to be the second leading cancer related death among men. In order to more fully develop effective prevention and intervention strategies for this prevalent disease, the underlying molecular mechanisms of initiation, progression and metastatic spread must be understood. To this end mouse models have an essential role in prostate cancer research in that they can closely mimic the pathological and biochemical features of the disease. In this review we discuss the history of autochthonous murine models of prostate cancer, the essentials of the idealized mouse model for prostate cancer and speculate on approaches towards this goal.