Combinatorial activities of Akt and B-Raf/Erk signaling in a mouse model of androgen-independent prostate cancer

Conditional deletion of the Pten gene in the mouse prostate induces prostatic intraepithelial neoplasms at early ages but a slow progression to prostate tumors

The PTEN tumor suppressor gene is frequently inactivated in human prostate cancer. Using Osr1 (odd skipped related 1)-Cre mice, we generated a novel conditional Pten knockout mouse strain, Pten^LoxP:Osr1-Cre. Conditional biallelic and monoallelic Pten knockout mice were viable. Deletion of Pten expression was detected in the prostate of Pten^LoxP/LoxP:Osr1-Cre mice as early as 2 weeks of age. Intriguingly, Pten^LoxP/LoxP:Osr1-Cre mice develop high-grade prostatic intraepithelial neoplasms (PINs) with high penetrance as early as one-month of age, and locally invasive prostatic tumors after 12-months of age. Pten^LoxP/+:Osr1-Cre mice show only mild oncogenic changes after 8-weeks of age. Castration of Pten^LoxP/LoxP:Osr1-Cre mice shows no significant regression of prostate tumors, although a shift of androgen receptor (AR) staining from the nuclei to cytoplasm is observed in Pten null tumor cells of castrated mice. Enhanced Akt activity is observed in Pten null tumor cells of castrated Pten^LoxP/LoxP:Osr1-Cre. This study provides a novel mouse model that can be used to investigate a primary role of Pten in initiating oncogenic transformation in the prostate and to examine other genetic and epigenetic changes that are required for tumor progression in the mouse prostate.

Opposing effects of androgen deprivation and targeted therapy on prostate cancer prevention

Prostate cancer is an ideal target for chemoprevention. To date, chemoprevention clinical trials with 5α-reductase inhibitors have yielded encouraging yet ultimately confounding results. Using a preclinical mouse model of high-grade prostatic intraepithelial neoplasia (HG-PIN) induced by PTEN loss, we observed unprecedented deteriorating effects of androgen deprivation, in which surgical castration or MDV3100 treatment accelerated disease progression of the otherwise stable HG-PIN to invasive castration-resistant prostate cancer (CRPC). As an alternative, targeting the phosphoinositide 3-kinase (PI3K) signaling pathway via either genetic ablation of genes encoding PI3K components or pharmacologic inhibition of the PI3K pathway reversed the PTEN loss-induced HG-PIN phenotype. Finally, concurrent inhibition of the PI3K and mitogen-activated protein kinase (MAPK) pathways was effective in blocking the growth of PTEN-null CRPC. Together, these data have revealed the potential adverse effects of antiandrogen chemoprevention in certain genetic contexts (such as PTEN loss) while showing the promise of targeted therapy in the clinical management of this complex and prevalent disease.

SIGNIFICANCE

Chemoprevention with antiandrogen therapies is attractive for prostate cancer, given its prevalence and established hormonally mediated pathogenesis. However, because PTEN loss has been found in 9% to 45% of HG-PIN in the clinic, the current findings suggest that patients with PTEN-deficient prostate tumors might be better treated with PI3K-targeted therapies.

Adaptation or selection--mechanisms of castration-resistant prostate cancer

Men with advanced prostate cancer are typically treated with hormonal therapy, which leads to tumour shrinkage. However, tumours relapse and develop into the lethal form of the disease, termed castration-resistant prostate cancer (CRPC). Two distinct, but not mutually exclusive, models have been proposed in the literature to describe the onset of CRPC: adaptation and selection. Although some studies indicate that tumour cells acquire new alterations that enable them to survive in the castrated state (adaptation), other research points to the outgrowth of rare, pre-existing cells capable of surviving hormonal therapy (selection). Targeting the cells that survive hormonal therapy--by either adaptation or selection--is necessary to prevent the development of CRPC. Current research is focused on not only understanding the cellular mechanisms of CRPC, but also defining critical pathways that can be targeted with combinatorial therapies in castration-resistant cancer cells.

Sprouty genes function in suppression of prostate tumorigenesis

Expression of Sprouty genes is frequently decreased or absent in human prostate cancer, implicating them as suppressors of tumorigenesis. Here we show they function in prostate tumor suppression in the mouse. Concomitant inactivation of Spry1 and Spry2 in prostate epithelium causes ductal hyperplasia and low-grade prostatic intraepithelial neoplasia (PIN). However, when Spry1 and Spry2 loss-of-function occurs in the context of heterozygosity for a null allele of the tumor suppressor gene Pten, there is a striking increase in PIN and evidence of neoplastic invasion. Conversely, expression of a Spry2 gain-of-function transgene in Pten null prostatic epithelium suppresses the tumorigenic effects of loss of Pten function. We show that Sprouty gene loss-of-function results in hyperactive RAS/ERK1/2 signaling throughout the prostate epithelium and cooperates with heterozygosity for a Pten null allele to promote hyperactive PI3K/AKT signaling. Furthermore, Spry2 gain-of-function can suppress hyperactivation of AKT caused by the absence of PTEN. Together, these results point to a key genetic interaction between Sprouty genes and Pten in prostate tumorigenesis and provide strong evidence that Sprouty genes can function to modulate signaling via the RAS/ERK1/2 and PI3K/AKT pathways. The finding that Sprouty genes suppress tumorigenesis caused by Pten loss-of-function suggests that therapeutic approaches aimed at restoring normal feedback mechanisms triggered by receptor tyrosine kinase signaling, including Sprouty gene expression, may provide an effective strategy to delay or prevent high-grade PIN and invasive prostate cancer.

Persistent androgen receptor-mediated transcription in castration-resistant prostate cancer under androgen-deprived conditions

The androgen receptor (AR) is a ligand-inducible transcription factor that mediates androgen action in target tissues. Upon ligand binding, the AR binds to thousands of genomic loci and activates a cell-type specific gene program. Prostate cancer growth and progression depend on androgen-induced AR signaling. Treatment of advanced prostate cancer through medical or surgical castration leads to initial response and durable remission, but resistance inevitably develops. In castration-resistant prostate cancer (CRPC), AR activity remains critical for tumor growth despite androgen deprivation. Although previous studies have focused on ligand-dependent AR signaling, in this study we explore AR function under the androgen-deprived conditions characteristic of CRPC. Our data demonstrate that AR persistently occupies a distinct set of genomic loci after androgen deprivation in CRPC. These androgen-independent AR occupied regions have constitutively open chromatin structures that lack the canonical androgen response element and are independent of FoxA1, a transcription factor involved in ligand-dependent AR targeting. Many AR binding events occur at proximal promoters, which can act as enhancers to augment transcriptional activities of other promoters through DNA looping. We further show that androgen-independent AR binding directs a gene expression program in CRPC, which is necessary for the growth of CRPC after androgen withdrawal.

Androgen receptor influences on body defense system via modulation of innate and adaptive immune systems: lessons from conditional AR knockout mice

Upon insult, such as infection or tissue injury, the innate and adaptive immune systems initiate a series of responses to defend the body. Recent studies from immune cell-specific androgen receptor (AR) knockout mice demonstrated that androgen and its receptor (androgen/AR) play significant roles in both immune regulations. In the innate immunity, androgen/AR is required for generation and proper function of neutrophils; androgen/AR also regulates wound healing processes through macrophage recruitment and proinflammatory cytokine production. In adaptive immunity, androgen/AR exerts suppressive effects on development and activation of T and B cells. Removal of such suppression causes thymic enlargement and excessive export of immature B cells. Altogether, androgen/AR plays distinct roles in individual immune cells, and targeting androgen/AR may help in treatment and management of immune-related diseases.

Dietary flavonoid fisetin: a novel dual inhibitor of PI3K/Akt and mTOR for prostate cancer management

Epidemiologic and case control population based studies over the past few decades have identified diet as an important determinant of cancer risk. This evidence has kindled interest into research on bioactive food components and has till date resulted in the identification of many compounds with cancer preventive and therapeutic potential. Among such compounds has been fisetin (3,7,3',4'-tetrahydroxyflavone), a flavonol and a member of the flavonoid polyphenols that also include quercetin, myricetin and kaempferol. Fisetin is commonly found in many fruits and vegetables such as apples, persimmons, grapes, kiwis, strawberries, onions and cucumbers. We evaluated the effects of fisetin against melanoma and cancers of the prostate, pancreas and the lungs. Using prostate and lung adenocarcinoma cells, we demonstrated that fisetin acts as a dual inhibitor of the PI3K/Akt and the mTOR pathways. This is a significant finding considering the fact that mTOR is phosphorylated and its activation is more frequent in tumors with overexpression of PI3K/Akt. Dual inhibitors of PI3K/Akt and mTOR signaling have been suggested as valuable agents for treating such cancers. Here, we summarize our findings on the dietary flavonoid fisetin and its effects on cancer with particular focus on prostate cancer. Our observations and findings from other laboratories suggest that fisetin could be a useful chemotherapeutic agent that could be used either alone or as an adjuvant with conventional chemotherapeutic drugs for the management of prostate and other cancers.

B-Raf activation cooperates with PTEN loss to drive c-Myc expression in advanced prostate cancer

Both the PI3K → Akt → mTOR and mitogen-activated protein kinase (MAPK) signaling pathways are often deregulated in prostate tumors with poor prognosis. Here we describe a new genetically engineered mouse model of prostate cancer in which PI3K-Akt-mTOR signaling is activated by inducible disruption of PTEN, and extracellular signal-regulated kinase 1/2 (ERK1/2) MAPK signaling is activated by inducible expression of a BRAF(V600E) oncogene. These tissue-specific compound mutant mice develop lethal prostate tumors that are inherently resistant to castration. These tumors bypass cellular senescence and disseminate to lymph nodes, bone marrow, and lungs where they form overt metastases in approximately 30% of the cases. Activation of PI3K → Akt → mTOR and MAPK signaling pathways in these prostate tumors cooperate to upregulate c-Myc. Accordingly, therapeutic treatments with rapamycin and PD0325901 to target these pathways, respectively, attenuate c-Myc levels and reduce tumor and metastatic burden. Together, our findings suggest a generalized therapeutic approach to target c-Myc activation in prostate cancer by combinatorial targeting of the PI3K → Akt → mTOR and ERK1/2 MAPK signaling pathways.

Genetic variations in a PTEN/AKT/mTOR axis and prostate cancer risk in a Chinese population

Backgroud

Genetic variations in a PTEN/AKT/mTOR signaling axis may influence cellular functions including cell growth, proliferation and apoptosis, and then increase the individual’s risk of cancer. Accordingly, we explore the association between single nucleotide polymorphisms (SNPs) of these genes and prostate cancer (PCa) in our Chinese population.

Methods

Subjects were recruited from 666 PCa patients and 708 cancer-free controls, and eight SNPs in the PTEN/AKT/mTOR axis were determined by the TaqMan assay. Odds ratios (OR) and 95% confidence intervals (95% CI) were evaluated by logistic regression.

Results

We observed significant associations between PCa risk and mTOR rs2295080 [P = 0.027, OR = 0.85, 95%CI = 0.74–0.98], and AKT2 rs7254617 (P = 0.003, OR = 1.35, 95%CI = 1.11–1.64). When estimated these two SNPs together, the combined genotypes with 2–4 risk alleles (rs2295080 T and rs7254617 A alleles) were associated with an increased risk of PCa compared with 0–1 risk alleles, which was more pronounced among subgroups of age >71 years, smokers, drinkers and no family history of cancer. Results of stratified analyses by cliniopathological parameters revealed that the frequencies of the combined genotypes with 2–4 risk alleles in advanced stage were significantly higher than in localized stage(P = 0.022), but there was no significant association in Gleason score and PSA level.

Conclusion

Our results indicate, for the first time that the two variants in AKT2 and mTOR, particularly the joint genotypes with 2–4 risk alleles may influence PCa susceptibility and progression in Chinese, and the association appeared to be more strong in the subgroup of smokers and drinkers.

Consumption of oxygen: a mitochondrial-generated progression signal of advanced cancer

Changes in mitochondrial genome such as mutation, deletion and depletion are common in cancer and can determine advanced phenotype of cancer; however, detailed mechanisms have not been elucidated. We observed that loss of mitochondrial genome reversibly induced overexpression and activation of proto-oncogenic Ras, especially K-Ras 4A, responsible for the activation of AKT and ERK leading to advanced phenotype of prostate and breast cancer. Ras activation was induced by the overexpression of 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGR), the rate-limiting enzyme of the mevalonate pathway. Hypoxia is known to induce proteasomal degradation of HMGR. Well differentiated prostate and breast cancer cells with high mitochondrial DNA content consumed a large amount of oxygen and induced hypoxia. Loss of mitochondrial genome reduced oxygen consumption and increased in oxygen concentration in the cells. The hypoxic-to-normoxic shift led to the overexpression of HMGR through inhibiting proteasomal degradation. Therefore, reduction of mitochondrial genome content induced overexpression of HMGR through hypoxic to normoxic shift and subsequently the endogenous induction of the mevalonate pathway activated Ras that mediates advanced phenotype. Reduction of mitochondrial genome content was associated with the aggressive phenotype of prostate cancer in vitro cell line model and tissue specimens in vivo. Our results elucidate a coherent mechanism that directly links the mitochondrial genome with the advanced progression of the disease.

The genomic landscape of prostate cancer

Prostate cancer is a common malignancy in men, with a markedly variable clinical course. Somatic alterations in DNA drive the growth of prostate cancers and may underlie the behavior of aggressive versus indolent tumors. The accelerating application of genomic technologies over the last two decades has identified mutations that drive prostate cancer formation, progression, and therapeutic resistance. Here, we discuss exemplary somatic mutations in prostate cancer, and highlight mutated cellular pathways with biological and possible therapeutic importance. Examples include mutated genes involved in androgen signaling, cell cycle regulation, signal transduction, and development. Some genetic alterations may also predict the clinical course of disease or response to therapy, although the molecular heterogeneity of prostate tumors poses challenges to genomic biomarker identification. The widespread application of massively parallel sequencing technology to the analysis of prostate cancer genomes should continue to advance both discovery-oriented and diagnostic avenues.

Regulation of androgen receptor-dependent transcription by coactivator MED1 is mediated through a newly discovered noncanonical binding motif

Nuclear receptor (NR) activation by cognate ligand generally involves allosteric realignment of C-terminal α-helices thus generating a binding surface for coactivators containing canonical LXXLL α-helical motifs. The androgen receptor (AR) is uncommon among NRs in that ligand triggers an intramolecular interaction between its N- and C-terminal domains (termed the N/C interaction) and that coactivators can alternatively bind to surfaces in the AR N-terminal or hinge regions. The evolutionary conserved Mediator complex plays a key coregulatory role in steroid hormone-dependent transcription and is chiefly targeted to NRs via the LXXLL-containing MED1 subunit. Whereas MED1 has been demonstrated to serve as a key transcriptional coactivator for AR, the mechanisms by which AR recruits MED1 have remained unclear. Here we show that MED1 binds to a distinct AR N-terminal region termed transactivation unit-1 (Tau-1) via two newly discovered noncanonical α-helical motifs located between MED1 residues 505 and 537. Neither of the two MED1 LXXLL motifs is required for AR binding, whereas loss of the intramolecular AR N/C interaction decreases MED1 binding. We further demonstrate that mitogen-activated protein kinase phosphorylation of MED1 enhances the AR-MED1 interaction in prostate cancer cells. In sum, our findings reveal a novel AR-coactivator binding mechanism that may have clinical implications for AR activity in prostate cancer.

NF-κB signaling in prostate cancer: a promising therapeutic target?

Prostate carcinoma (PCa) displays a wide variety of genetic alterations, versatile expression profiles as well as cell surface markers. Despite this heterogeneity, a common treatment for advanced PCa is androgen deprivation therapy (ADT). ADT targets the androgen receptor-a member of the nuclear receptor superfamily-which is required for development and function of the prostate and critical for PCa growth and survival. After an initial regression of the tumor during ADT, a large fraction of tumors progress to so-called castration-resistant prostate carcinoma (CRPca) which is highly resistant toward chemotherapy. The ensuing high mortality rates illustrate the importance of novel therapeutic targets for CRPCa. The transcription factor NF-κB was recently proposed as such a potential target for therapeutic intervention in CRPCa. Although NF-κB is essential for the regulation of innate and adaptive immunity recent data suggest a role of NF-κB in cancer initiation and progression. However, the exact function of NF-κB signaling in PCa is still a matter of debate. Here, we review known roles of NF-κB signaling in PCa and emphasize the crosstalk of NF-κB and androgen receptor signaling. Finally, we discuss potential therapeutic relevance of blocking NF-κB in PCa.

Conditional expression of the androgen receptor induces oncogenic transformation of the mouse prostate

The androgen signaling pathway, mediated through the androgen receptor (AR), is critical in prostate tumorigenesis. However, the precise role of AR in prostate cancer development and progression still remains largely unknown. Specifically, it is unclear whether overexpression of AR is sufficient to induce prostate tumor formation in vivo. Here, we inserted the human AR transgene with a LoxP-stop-loxP (LSL) cassette into the mouse ROSA26 locus, permitting "conditionally" activated AR transgene expression through Cre recombinase-mediated removal of the LSL cassette. By crossing this AR floxed strain with Osr1-Cre (odd skipped related) mice, in which the Osr1 promoter activates at embryonic day 11.5 in urogenital sinus epithelium, we generated a conditional transgenic line, R26hAR(loxP):Osr1-Cre+. Expression of transgenic AR was detected in both prostatic luminal and basal epithelial cells and is resistant to castration. Approximately one-half of the transgenic mice displayed mouse prostatic intraepithelial neoplasia (mPIN) lesions. Intriguingly, four mice (10%) developed prostatic adenocarcinomas, with two demonstrating invasive diseases. Positive immunostaining of transgenic AR protein was observed in the majority of atypical and tumor cells in the mPIN and prostatic adenocarcinomas, providing a link between transgenic AR expression and oncogenic transformation. An increase in Ki67-positive cells appeared in all mPIN and prostatic adenocarcinoma lesions of the mice. Thus, we demonstrated for the first time that conditional activation of transgenic AR expression by Osr1 promoter induces prostate tumor formation in mice. This new AR transgenic mouse line mimics the human disease and can be used for study of prostate tumorigenesis and drug development.

FGFR-4 Arg³⁸⁸ enhances prostate cancer progression via extracellular signal-related kinase and serum response factor signaling

PURPOSE

Increased expression of FGFR-4 and its ligands have been linked to lethal prostate cancer (PCa). Furthermore, a germ line polymorphism in the FGFR-4 gene, resulting in arginine at codon 388 (Arg³⁸⁸) instead of glycine (Gly³⁸⁸), is associated with aggressive disease. The FGFR-4 Arg³⁸⁸ variant results in increased receptor stability, sustained receptor activation, and increased motility and invasion compared with Gly³⁸⁸. However, the impact of sustained signaling on cellular signal transduction pathways is unknown.

EXPERIMENTAL DESIGN

Expression microarray analysis of immortalized prostatic epithelial cells lines expressing FGFR-4 Arg³⁸⁸ or Gly³⁸⁸ was used to establish a gene signature associated with FGFR-4 Arg³⁸⁸ expression. Transient transfection of reporters and inhibitors was used to establish the pathways activated by FGFR-4 Arg³⁸⁸ expression. The impact of pathway knockdown in vitro and in an orthotopic model was assessed using inhibitors and/or short hairpin RNA (shRNA).

RESULTS

Expression of the FGFR-4 Arg³⁸⁸ protein leads to increased activity of the extracellular signal-related kinase (ERK) pathway, increased activity of serum response factor (SRF) and AP1, and transcription of multiple genes that are correlated with aggressive clinical behavior in PCa. Increased expression of SRF is associated with biochemical recurrence in men undergoing radical prostatectomy. Consistent with these observations, knockdown of FGFR-4 Arg³⁸⁸ in PCa cells decreases proliferation and invasion in vitro and primary tumor growth and metastasis in vivo.

CONCLUSIONS

These studies define a signal transduction pathway downstream of FGFR-4 Arg³⁸⁸ that acts via ERK and SRF to promote PCa progression.

Modulation of protein phosphatase 2A activity alters androgen-independent growth of prostate cancer cells: therapeutic implications

Earlier we identified PPP2CA, which encodes for the α-isoform of protein phosphatase 2A (PP2A) catalytic subunit, as one of the downregulated genes in androgen-independent prostate cancer. PP2A is a serine/threonine phosphatase and a potent tumor suppressor involved in broad cellular functions; however, its role in prostate cancer has not yet been determined. Here, we have investigated the effect of PP2A activity modulation on the androgen-independent growth of prostate cancer cells. Our data show that the PPP2CA expression and PP2A activity is downregulated in androgen-independent (C4-2) prostate cancer cells as compared with androgen-dependent (LNCaP) cells. Downregulation of PP2A activity by pharmacologic inhibition or short interfering RNA-mediated PPP2CA silencing sustains the growth of LNCaP cells under an androgen-deprived condition by relieving the androgen deprivation-induced cell-cycle arrest and preventing apoptosis. Immunoblot analyses reveal enhanced phosphorylation of Akt, extracellular signal-regulated kinase (ERK), BAD, increased expression of cyclins (A1/D1), and decreased expression of cyclin inhibitor (p27) on PP2A downregulation. Furthermore, our data show that androgen receptor (AR) signaling is partially maintained in PP2A-inhibited cells through increased AR expression and ligand-independent phosphorylation. Pharmacologic inhibition of Akt, ERK, and AR suggest a role of these signaling pathways in facilitating the androgen-independent growth of LNCaP cells. These observations are supported by the effect of ceramide, a PP2A activator, on androgen-independent C4-2 cells. Ceramide inhibited the growth of C4-2 cells on androgen deprivation, an effect that could be abrogated by PP2A downregulation. Altogether, our findings suggest that modulation of PP2A activity may represent an alternative therapeutic approach for the treatment of advanced androgen-independent prostate cancer.

TPL2/COT/MAP3K8 (TPL2) activation promotes androgen depletion-independent (ADI) prostate cancer growth

BACKGROUND

Despite its initial positive response to hormone ablation therapy, prostate cancers invariably recur in more aggressive, treatment resistant forms. The lack of our understanding of underlying genetic alterations for the transition from androgen-dependent (AD) to ADI prostate cancer growth hampers our ability to develop target-driven therapeutic strategies for the efficient treatment of ADI prostate cancer.

METHODOLOGY/PRINCIPAL FINDINGS

By screening a library of activated human kinases, we have identified TPL2, encoding a serine/threonine kinase, as driving ADI prostate cancer growth. TPL2 activation by over-expressing either wild-type or a constitutively activated form of TPL2 induced ADI growth, whereas the suppression of TPL2 expression and its kinase activity in ADI prostate cancer cells inhibited cell proliferation under androgen-depleted conditions. Most importantly, TPL2 is upregulated in ADI prostate cancers of both the Pten deletion mouse model and the clinical prostate cancer specimens.

CONCLUSIONS/SIGNIFICANCE

Together these data suggest that TPL2 kinase plays a critical role in the promotion of ADI prostate cancer progression. Furthermore, the suppression of TPL2 diminishes ADI prostate cancer growth and a high frequency of TPL2 overexpression in human ADI prostate cancer samples validates TPL2 as a target for the treatment of this deadly disease.

Crosstalk of the EphA2 receptor with a serine/threonine phosphatase suppresses the Akt-mTORC1 pathway in cancer cells

Receptor tyrosine kinases of the Eph family play multiple roles in the physiological regulation of tissue homeostasis and in the pathogenesis of various diseases, including cancer. The EphA2 receptor is highly expressed in most cancer cell types, where it has disparate activities that are not well understood. It has been reported that interplay of EphA2 with oncogenic signaling pathways promotes cancer cell malignancy independently of ephrin ligand binding and receptor kinase activity. In contrast, stimulation of EphA2 signaling with ephrin-A ligands can suppress malignancy by inhibiting the Ras-MAP kinase pathway, integrin-mediated adhesion, and epithelial to mesenchymal transition. Here we show that ephrin-A1 ligand-dependent activation of EphA2 decreases the growth of PC3 prostate cancer cells and profoundly inhibits the Akt-mTORC1 pathway, which is hyperactivated due to loss of the PTEN tumor suppressor. Our results do not implicate changes in the activity of Akt upstream regulators (such as Ras family GTPases, PI3 kinase, integrins, or the Ship2 lipid phosphatase) in the observed loss of Akt T308 and S473 phosphorylation downstream of EphA2. Indeed, EphA2 can inhibit Akt phosphorylation induced by oncogenic mutations of not only PTEN but also PI3 kinase. Furthermore, it can decrease the hyperphosphorylation induced by constitutive membrane-targeting of Akt. Our data suggest a novel signaling mechanism whereby EphA2 inactivates the Akt-mTORC1 oncogenic pathway through Akt dephosphorylation mediated by a serine/threonine phosphatase. Ephrin-A1-induced Akt dephosphorylation was observed not only in PC3 prostate cancer cells but also in other cancer cell types. Thus, activation of EphA2 signaling represents a possible new avenue for anti-cancer therapies that exploit the remarkable ability of this receptor to counteract multiple oncogenic signaling pathways.

Regulation of NF-E2-related factor 2 signaling for cancer chemoprevention: antioxidant coupled with antiinflammatory

Cancer chemoprevention is a process of using either natural or synthetic compounds to reduce the risk of developing cancer. Observations that NF-E2-related factor 2 (Nrf2)-deficient mice lack response to some chemopreventive agents point to the important role of Nrf2 in chemoprevention. Nrf2 is a member of basic-leucine zipper transcription factor family and has been shown to regulate gene expression by binding to a response element, antioxidant responsive element. It is generally believed that activation of Nrf2 signaling is an adaptive response to the environmental and endogenous stresses. Under homeostatic conditions, Nrf2 is suppressed by association with Kelch-like ECH-associated protein 1 (Keap1), but is stimulated upon exposure to oxidative or electrophilic stress. Once activated, Nrf2 translocates into nuclei and upregulates a group of genes that act in concert to combat oxidative stress. Nrf2 is also shown to have protective function against inflammation, a pathological process that could contribute to carcinogenesis. In this review, we will discuss the current progress in the study of Nrf2 signaling, in particular, the mechanisms of Nrf2 activation by chemopreventive agents. We will also discuss some of the potential caveats of Nrf2 in cancer treatment and future opportunity and challenges on regulation of Nrf2-mediated antioxidant and antiinflammatory signaling in the context of cancer prevention.

Characterization of the small RNA transcriptomes of androgen dependent and independent prostate cancer cell line by deep sequencing

Given the important roles of miRNA in post-transcriptional regulation and its implications for cancer, characterization of miRNA facilitates us to uncover molecular mechanisms underlying the progression of androgen-independent prostate cancer (PCa). The emergence of next-generation sequencing technologies has dramatically changed the speed of all aspects of sequencing in a rapid and cost-effective fashion, which can permit an unbiased, quantitive and in-depth investigation of small RNA transcriptome. In this study, we used high-throughput Illumina sequencing to comprehensively represent the full complement of individual small RNA and to characterize miRNA expression profiles in both the androgen dependent and independent Pca cell line. At least 83 miRNAs are significantly differentially expressed, of which 41 are up-regulated and 42 are down-regulated, indicating these miRNAs may be involved in the transition of LNCaP to an androgen-independent phenotype. In addition, we have identified 43 novel miRNAs from the androgen dependent and independent PCa library and 3 of them are specific to the androgen-independent PCa. Function annotation of target genes indicated that most of these differentially expressed miRNAs tend to target genes involved in signal transduction and cell communication, epically the MAPK signaling pathway. The small RNA transcriptomes obtained in this study provide considerable insights into a better understanding of the expression and function of small RNAs in the development of androgen-independent prostate cancer.

Molecular genetics of prostate cancer: new prospects for old challenges

Despite much recent progress, prostate cancer continues to represent a major cause of cancer-related mortality and morbidity in men. Since early studies on the role of the androgen receptor that led to the advent of androgen deprivation therapy in the 1940s, there has long been intensive interest in the basic mechanisms underlying prostate cancer initiation and progression, as well as the potential to target these processes for therapeutic intervention. Here, we present an overview of major themes in prostate cancer research, focusing on current knowledge of principal events in cancer initiation and progression. We discuss recent advances, including new insights into the mechanisms of castration resistance, identification of stem cells and tumor-initiating cells, and development of mouse models for preclinical evaluation of novel therapuetics. Overall, we highlight the tremendous research progress made in recent years, and underscore the challenges that lie ahead.

Tuberous sclerosis complex 1: an epithelial tumor suppressor essential to prevent spontaneous prostate cancer in aged mice

The phosphoinositide 3-kinase (PI3K) pathway regulates mammalian cell growth, survival, and motility and plays a major pathogenetic role in human prostate cancer (PCa). However, the oncogenic contributions downstream of the PI3K pathway made by mammalian target of rapamycin complex 1 (mTORC1)-mediated cell growth signal transduction in PCa have yet to be elucidated in detail. Here, we engineered constitutive mTORC1 activation in prostate epithelium by a conditional genetic deletion of tuberous sclerosis complex 1 (Tsc1), a potent negative regulator of mTORC1 signaling. Epithelial inactivation was not immediately tumorigenic, but Tsc1-deficient mice developed prostatic intraepithelial neoplasia (mPIN) in lateral and anterior prostates by 6 months of age, with increasing disease penetrance over time. Lateral prostate lesions in 16- to 22-month-old mutant mice progressed to two types of more advanced lesions, adenomatous gland forming lesion (Type 1) and atypical glands embedded in massively expanded reactive stroma (Type 2). Both Type 1 and Type 2 lesions contained multiple foci of microinvasive carcinoma. Epithelial neoplastic and atypical stromal lesions persisted despite 4 weeks of RAD001 chemotherapy. Rapalogue resistance was not due to AKT or extracellular signal-regulated kinase 1/2 activation. Expression of the homeobox gene Nkx3.1 was lost in Tsc1-deficient mPIN, and it cooperated with TSC1 loss in mPIN initiation in doubly mutant Tsc1:Nkx3.1 prostatic epithelial knockout mice. Thus, TSC1 inactivation distal to PI3K and AKT activation is sufficient to activate a molecular signaling cascade producing prostatic neoplasia and focal carcinogenesis.

From man to mouse and back again: advances in defining tumor AKTivities in vivo

AKT hyperactivation is a common event in human cancers, and inhibition of oncogenic AKT activation is a major goal of drug discovery programs. Mouse tumor models that replicate AKT activation typical of human cancers provide a powerful means by which to investigate mechanisms of oncogenic signaling, identify potential therapeutic targets and determine treatment regimes with maximal therapeutic efficacy. This Perspective highlights recent advances using in vivo studies that reveal how AKT signaling supports tumor formation, cooperates with other mutations to promote tumor progression and facilitates tumor-cell dissemination, focusing on well-characterized prostate carcinoma mouse models that are highly sensitive to AKT activation. The implications of these findings on the therapeutic targeting of AKT and potential new drug targets are also explored.

Inhibition of androgen receptor and Cdc25A phosphatase as a combination targeted therapy in molecular apocrine breast cancer

Molecular apocrine breast cancer is an estrogen receptor negative subtype characterized by the over-expression of steroid response genes. In this study we investigate the therapeutic effects of persistent ERK phosphorylation using a Cdc25A phosphatase inhibitor, PM-20 in combination with AR inhibition using flutamide in this subtype. Our findings demonstrate a significant synergy with this combination in reducing cell viability and growth. Furthermore, we show that the mechanism of this effect involves a cross-talk between the AR and ERK signalling pathways. Moreover, using a xenograft molecular apocrine model we demonstrate that the combination therapy results in a significantly better therapeutic response compared to monotherapy and control groups manifesting as reductions in tumor growth, proliferation index, and cellularity. This study demonstrates that the combined application of AR and Cdc25A inhibitors is a promising therapeutic strategy in molecular apocrine breast cancer.

Activating BRAF gene mutations are uncommon in hormone refractory prostate cancer in Caucasian patients

Activating mutations in the cytosolic serine/threonine kinase, BRAF, have been reported in a variety of neoplasms. BRAF activation may contribute to tumor growth via activation of the MAP/ERK kinase pathway, and BRAF represents a possible therapeutic target. Activating BRAF mutations were recently reported in approximately 10% of prostate cancer cases in Asian patients. In the present study, 43 hormone refractory prostate cancers were analyzed for BRAF mutations in order to determine whether anti-BRAF therapy is a suitable approach for advanced prostate cancer patients. In all of the studied tumors, BRAF exons 11 and 15 were PCR-amplified and sequenced, including the backward and forward sequences. BRAF mutations were noted only in the positive control tissues, but were not found in any of the 43 analyzed prostate cancers. We conclude that BRAF mutations occur only rarely in prostate cancers in Caucasian patients and are not associated with tumor progression. The application of anti-BRAF therapies may therefore not be beneficial for prostate cancer.

ProstaCaid Induces G2/M Cell Cycle Arrest and Apoptosis in Human and Mouse Androgen-Dependent and-Independent Prostate Cancer Cells

The anticancer effects of ProstaCaid, a novel integrative blend of vitamins, minerals, multiherb extracts, and derivatives, were tested in human and mouse androgen—dependent (AD) and —independent (AI) prostate cancer cell lines. ProstaCaid shows growth inhibitory effects on both human and mouse AD prostate cancer cells (LNCaP and CASP 2.1) and AI prostate cancer cells (PC3 and CASP 1.1) in a dose-/time-dependent manner. Consistently, long-term treatment with ProstaCaid also reduced colony formation capacities of prostate cancer cells. Flow cytometry assays revealed that ProstaCaid induces G2/M arrest and apoptosis in LNCaP and PC3 cells after 72 hours of treatment. Immunoblotting assay demonstrated that 25 µg/mL of ProstaCaid treatment resulted in (1) the reduction of cyclin D1, cyclin B1, and Cdc2 expression in a time-dependent way; (2) increase in p21WAF1/Cip1 as early as 12 hours after the treatments in PC3 cells and reduction to base line at the 72-hour time point; and (3) repression of Bcl-2, BclxL, and induction of Bim as well as the cleavages of caspase-3 and poly(ADP-ribose) polymerase (PARP) at 72 hours of treatment, suggesting caspase-3-dependent apoptosis. Moreover, ProstaCaid suppressed activation of AKT and MAPK signaling pathways in PC3 and LNCaP cells by reducing phosphorylation levels of AKT, its downstream target S6 ribosomal protein and GSK3β, and ERK1/2, respectively. In summary, these findings strongly suggest that ProstaCaid may be a potential chemopreventive and therapeutic agent for both AD and, more importantly, AI prostate cancer.

Epac1-induced cellular proliferation in prostate cancer cells is mediated by B-Raf/ERK and mTOR signaling cascades

cAMP-dependent, PKA-independent effects on cell proliferation are mediated by cAMP binding to EPAC and activation of Rap signaling. In this report, we employed the analogue 8-CPT-2-O-Me-cAMP to study binding to EPAC and subsequent activation of B-Raf/ERK and mTOR signaling in human cancer cells. This compound significantly stimulated DNA synthesis, protein synthesis, and cellular proliferation of human 1-LN prostate cancer cells. By study of phosphorylation-dependent activation, we demonstrate that EPAC-mediated cellular effects require activation of the B-Raf/ERK and mTOR signaling cascades. RNAi directed against EPAC gene expression as well as inhibitors of ERK, PI 3-kinase, and mTOR were employed to further demonstrate the role of these pathways in regulating prostate cancer cell proliferation. These studies were then extended to several other human prostate cancer cell lines and melanoma cells with comparable results. We conclude that B-Raf/ERK and mTOR signaling play an essential role in cAMP-dependent, but PKA-independent, proliferation of cancer cells.

Analysis of the molecular networks in androgen dependent and independent prostate cancer revealed fragile and robust subsystems

Androgen ablation therapy is currently the primary treatment for metastatic prostate cancer. Unfortunately, in nearly all cases, androgen ablation fails to permanently arrest cancer progression. As androgens like testosterone are withdrawn, prostate cancer cells lose their androgen sensitivity and begin to proliferate without hormone growth factors. In this study, we constructed and analyzed a mathematical model of the integration between hormone growth factor signaling, androgen receptor activation, and the expression of cyclin D and Prostate-Specific Antigen in human LNCaP prostate adenocarcinoma cells. The objective of the study was to investigate which signaling systems were important in the loss of androgen dependence. The model was formulated as a set of ordinary differential equations which described 212 species and 384 interactions, including both the mRNA and protein levels for key species. An ensemble approach was chosen to constrain model parameters and to estimate the impact of parametric uncertainty on model predictions. Model parameters were identified using 14 steady-state and dynamic LNCaP data sets taken from literature sources. Alterations in the rate of Prostatic Acid Phosphatase expression was sufficient to capture varying levels of androgen dependence. Analysis of the model provided insight into the importance of network components as a function of androgen dependence. The importance of androgen receptor availability and the MAPK/Akt signaling axes was independent of androgen status. Interestingly, androgen receptor availability was important even in androgen-independent LNCaP cells. Translation became progressively more important in androgen-independent LNCaP cells. Further analysis suggested a positive synergy between the MAPK and Akt signaling axes and the translation of key proliferative markers like cyclin D in androgen-independent cells. Taken together, the results support the targeting of both the Akt and MAPK pathways. Moreover, the analysis suggested that direct targeting of the translational machinery, specifically eIF4E, could be efficacious in androgen-independent prostate cancers.

Monocyte/macrophage androgen receptor suppresses cutaneous wound healing in mice by enhancing local TNF-alpha expression

Cutaneous wounds heal more slowly in elderly males than in elderly females, suggesting a role for sex hormones in the healing process. Indeed, androgen/androgen receptor (AR) signaling has been shown to inhibit cutaneous wound healing. AR is expressed in several cell types in healing skin, including keratinocytes, dermal fibroblasts, and infiltrating macrophages, but the exact role of androgen/AR signaling in these different cell types remains unclear. To address this question, we generated and studied cutaneous wound healing in cell-specific AR knockout (ARKO) mice. General and myeloid-specific ARKO mice exhibited accelerated wound healing compared with WT mice, whereas keratinocyte- and fibroblast-specific ARKO mice did not. Importantly, the rate of wound healing in the general ARKO mice was dependent on AR and not serum androgen levels. Interestingly, although dispensable for wound closure, keratinocyte AR promoted re-epithelialization, while fibroblast AR suppressed it. Further analysis indicated that AR suppressed wound healing by enhancing the inflammatory response through a localized increase in TNF-alpha expression. Furthermore, AR enhanced local TNF-alpha expression via multiple mechanisms, including increasing the inflammatory monocyte population, enhancing monocyte chemotaxis by upregulating CCR2 expression, and enhancing TNF-alpha expression in macrophages. Finally, targeting AR by topical application of a compound (ASC-J9) that degrades AR protein resulted in accelerated healing, suggesting a potential new therapeutic approach that may lead to better treatment of wound healing.

Pharmacokinetics and pharmacodynamics of broccoli sprouts on the suppression of prostate cancer in transgenic adenocarcinoma of mouse prostate (TRAMP) mice: implication of induction of Nrf2, HO-1 and apoptosis and the suppression of Akt-dependent kinase pathway

PURPOSE

In the present study, we have evaluated the pharmacokinetics and the in vivo prostate chemopreventive activity of broccoli sprouts.

METHODS

The in vivo pharmacokinetic profiles of sulforaphane (SFN) and SFN- glutathione (GSH) conjugate in rats after oral administration of 200 mg and 500 mg broccoli sprouts were analyzed. Next, 8-week old TRAMP mice were fed with dietary broccoli sprouts at two dosages (60 and 240 mg/mouse/day) for 16 weeks, and the mice were sacrificed to examine the pharmacodynamic response on prostate tumor and some biomarkers.

RESULTS

SFN was readily released and conjugated with GSH in the rats after oral administration of broccoli sprouts. TRAMP mice fed with 240 mg broccoli sprouts/mouse/day exhibited a significant retardation of prostate tumor growth. Western blot analysis revealed that the expression levels of Nrf2, HO-1, cleaved-Caspase-3, cleaved-PARP and Bax proteins were increased, but that of Keap1 and Bcl-XL proteins were decreased. In addition, the phosphorylation and/or the expression level of Akt and its downstream kinase and target proteins, e.g. mTOR, 4E-BP1 and cyclin D1, were reduced.

CONCLUSIONS

Our findings indicate that broccoli sprouts can serve as a good dietary source of SFN in vivo and that they have significant inhibitory effects on prostate tumorigenesis.

Targeting the PI3K/AKT pathway for the treatment of prostate cancer

Despite recent advances in our understanding of the biological basis of prostate cancer, the management of the disease, especially in the castration-resistant phase, remains a significant challenge. Deregulation of the phosphatidylinositol 3-kinase pathway is increasingly implicated in prostate carcinogenesis. In this review, we detail the role of this pathway in the pathogenesis of prostate cancer and the rapidly evolving therapeutic implications of targeting it. In particular, we highlight the importance of the appropriate selection of agents and combinations, and the critical role of predictive and pharmocodynamic biomarkers.

Parathyroid hormone-related protein regulates cell survival pathways via integrin alpha6beta4-mediated activation of phosphatidylinositol 3-kinase/Akt signaling

Parathyroid hormone-related protein (PTHrP) is expressed by human prostatic tissues and cancer cell lines. PTHrP enhances tumor cell growth and metastasis in vivo and up-regulates proinvasive integrin alpha6beta4 expression in vitro. Hallmarks of malignant tumor cells include resistance to apoptosis and anchorage-independent cell growth. In this study, we used the human prostate cancer cell lines C4-2 and PC-3 as model systems to study the effects of PTHrP on these processes. We report that PTHrP protects these cells from doxorubicin-induced apoptosis and promotes anchorage-independent cell growth via an intracrine pathway. Conversely, autocrine/paracrine PTHrP action increases apoptosis in C4-2 cells and has no effect on apoptosis in PC-3 cells. The intracrine effects of PTHrP on apoptosis are mediated via activation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. PTHrP also affects the phosphorylation state of Akt substrates implicated in apoptosis suppression, including glycogen synthase kinase-3 and Bad. The prosurvival effects of PTHrP are accompanied by increases in the ratio of antiapoptotic to proapoptotic members of the Bcl-2 family and in levels of c-myc. PTHrP also increases nuclear factor-kappaB activity via a PI3K-dependent pathway. Integrin alpha6beta4 is known to activate PI3K. Here, we also show that knockdown of integrin alpha6beta4 negates the PTHrP-mediated activation of the PI3K/Akt pathway. Taken together, these observations provide evidence of a link between PTHrP and the PI3K/Akt signaling pathway through integrin alpha6beta4, resulting in the activation of survival pathways. Targeting PTHrP production in prostate cancer may thus prove therapeutically beneficial.

Mammalian target of rapamycin: a new target in prostate cancer

Molecular targets in prostate cancer are continually being explored, especially in the poor-prognosis androgen-independent phase of the disease, for which there are currently few therapeutic options. One such target is the mammalian target of rapamycin (mTOR) protein. Activation of mTOR results in sequential activation of downstream molecules, which ultimately results in cell division. In this review, we consider the rationale for pursuing mTOR as a therapeutic target in prostate cancer and summarize preclinical and clinical studies of mTOR inhibition in prostate cancer.

A transcription-independent function of FOXO1 in inhibition of androgen-independent activation of the androgen receptor in prostate cancer cells

Increasing evidence suggests that aberrant activation of the androgen receptor (AR) plays a pivotal role in the development and progression of androgen depletion-independent prostate cancer (PCa) after androgen deprivation therapy. Here, we show that loss of the PTEN tumor suppressor gene is associated with hyperactivation of the AR in human PCa cell lines. This effect is mediated primarily by its downstream effector FOXO1. In addition to the inhibition of androgenic activation of the AR, forced expression of FOXO1 in PTEN-negative PCa cells also inhibits androgen-independent activation of the AR in a manner independent of FOXO1 transcriptional function. In contrast, silencing of FOXO1 in PTEN-positive cells not only increases the basal activity of the AR in the absence of androgens, it also markedly sensitizes the AR activation by low levels of androgens or nonandrogenic factors such as interleukin-6. FOXO1-mediated inhibition of the AR is partially attenuated by the histone deacetylase (HDAC) inhibitor trichostatin A. Accordingly, FOXO1 interacts with HDAC3 as shown by coimmunoprecipitation assays, and cotransfection of cells with FOXO1 and HDAC3, but not HDAC1 and HDAC2, results in a greater inhibition of AR activity than in cells transfected with FOXO1 or HDAC3 individually. Together, our findings define a novel corepressor function of FOXO1 in inhibition of androgen-independent activation of the AR.

Antifolate/folate-activated HGF/c-Met signalling pathways in mouse kidneys-the putative role of their downstream effectors in cross-talk with androgen receptor

This in vivo study of mouse kidneys was focused on the identification of protein mediators involved in the cross-talk between two signalling pathways. One pathway was triggered by testosterone via an androgen receptor, AR, and the other induced by CB 3717/folate via HGF, and its membrane receptor c-Met. Sequential activation of these pathways leads to a drastic decrease of testosterone-induced ornithine decarboxylase, ODC, expression. We proved that CB 3717/folate-induced ODC expression is Akt-dependent. CB 3717/folate activates Akt and ERK1/2 kinases, PTEN phosphatase and also up-regulates cyclin D2 and PCNA, but decreases GSK3beta and cyclin D1 protein levels. Testosterone activation of AR induces GSK3beta and PTEN. Results of the sequential activation of the studied signalling pathways suggest that Akt, GSK3beta and possibly ERK1/2 kinases may participate in the negative cross-talk and attenuation of AR transactivity, while the involvement of PTEN and cyclin D1 seems to be doubtful.

BRAF activation initiates but does not maintain invasive prostate adenocarcinoma

Prostate cancer is the second leading cause of cancer-related deaths in men. Activation of MAP kinase signaling pathway has been implicated in advanced and androgen-independent prostate cancers, although formal genetic proof has been lacking. In the course of modeling malignant melanoma in a tyrosinase promoter transgenic system, we developed a genetically-engineered mouse (GEM) model of invasive prostate cancers, whereby an activating mutation of BRAF^V600E–a mutation found in ∼10% of human prostate tumors–was targeted to the epithelial compartment of the prostate gland on the background of Ink4a/Arf deficiency. These GEM mice developed prostate gland hyperplasia with progression to rapidly growing invasive adenocarcinoma without evidence of AKT activation, providing genetic proof that activation of MAP kinase signaling is sufficient to drive prostate tumorigenesis. Importantly, genetic extinction of BRAF^V600E in established prostate tumors did not lead to tumor regression, indicating that while sufficient to initiate development of invasive prostate adenocarcinoma, BRAF^V600E is not required for its maintenance.

Simultaneous haploinsufficiency of Pten and Trp53 tumor suppressor genes accelerates tumorigenesis in a mouse model of prostate cancer

Tumor suppressor gene PTEN is important in the initiation and progression of human prostate carcinoma, whereas the role of TP53 remains controversial. Since Pten/Trp53 double conditional knockout mice show earlier onset and fast progression of prostate cancer when compared to Pten knockout mice, we asked whether heterozygosity of these two tumor suppressor genes was sufficient to accelerate prostatic tumorigenesis. To answer this question we examined prostatic lesion progression of Pten/Trp53 double heterozygous mice and a series of controls such as Pten heterozygous, Pten conditional knockout, Trp53 heterozygous and Trp53 knockout mice. Tissue recombination of adult prostatic epithelium coupled with embryonic rat seminal vesicle mesenchyme was used as a tool to stimulate prostatic epithelial proliferation. In our study, high-grade prostatic intraepithelial neoplasia (PIN) was found with high frequency at 8 weeks post-tissue recombination transplantation. PIN lesions in Pten/Trp53 double heterozygous mice were more severe than those seen in Pten heterozygous alone. Furthermore, morphologic features attributable to Pten or Trp53 loss appeared to be enhanced in double heterozygous tissues. LOH analysis of Pten and Trp53 in genomic DNA collected from high-grade PIN lesions in Pten heterozygous and Pten/Trp53 double heterozygous mice showed an intact wild-type allele for both genes in all samples examined. In conclusion, simultaneous heterozygosity of Pten and Trp53 accelerates prostatic tumorigenesis in this mouse model of prostate cancer independently of loss of heterozygosity of either gene.

Targeting AKT/mTOR and ERK MAPK signaling inhibits hormone-refractory prostate cancer in a preclinical mouse model

The AKT/mammalian target of rapamycin (AKT/mTOR) and ERK MAPK signaling pathways have been shown to cooperate in prostate cancer progression and the transition to androgen-independent disease. We have now tested the effects of combinatorial inhibition of these pathways on prostate tumorigenicity by performing preclinical studies using a genetically engineered mouse model of prostate cancer. We report here that combination therapy using rapamycin, an inhibitor of mTOR, and PD0325901, an inhibitor of MAPK kinase 1 (MEK; the kinase directly upstream of ERK), inhibited cell growth in cultured prostate cancer cell lines and tumor growth particularly for androgen-independent prostate tumors in the mouse model. We further showed that such inhibition leads to inhibition of proliferation and upregulated expression of the apoptotic regulator Bcl-2-interacting mediator of cell death (Bim). Furthermore, analyses of human prostate cancer tissue microarrays demonstrated that AKT/mTOR and ERK MAPK signaling pathways are often coordinately deregulated during prostate cancer progression in humans. We therefore propose that combination therapy targeting AKT/mTOR and ERK MAPK signaling pathways may be an effective treatment for patients with advanced prostate cancer, in particular those with hormone-refractory disease.

Effects of manganese superoxide dismutase silencing on androgen receptor function and gene regulation: implications for castration-resistant prostate cancer

PURPOSE

Advanced prostate cancer is first treated with androgen deprivation therapy. However, tumors become resistant to and grow despite castrate levels of testosterone. Growth and proliferation of CRPC is mediated by gain-of-function changes in the AR and AR reactivation. Expression of manganese superoxide dismutase (SOD2), which regulates cellular ROS, is markedly down-regulated in CRPC when compared with hormone-responsive tumors.

EXPERIMENTAL DESIGN

Here, we knocked down SOD2 expression in AR-expressing LNCaP prostate cancer cells and determined gene expression changes, transcription factor binding, and AR transcription activity in SOD2 knockdown cells.

RESULTS

SOD2 knockdown results in an increase in ROS. Gene expression changes induced by SOD2 knockdown results in the up-regulation of genes that are also androgen responsive and 46% of genes up-regulated 2-fold by the androgen ligand R1881 are also up-regulated to the same extent with SOD2 knockdown. The induction of many of these genes with SOD2 knockdown, such as VEGFA and FKBP5, is reversible with the antioxidant N-acetylcysteine, suggesting that this mechanism is directly linked to ROS. Furthermore, an array for transcription factor DNA-binding activity shows that SOD2 knockdown induces DNA binding by several transcription factors, including AR. SOD2 knockdown-induced AR activation was confirmed by electrophoretic mobility shift assay and luciferase activity, and both were readily reversible with N-acetylcysteine.

CONCLUSIONS

These findings show that down-regulation of SOD2 induces AR activity in a ROS-dependent manner, and suggest that there may be a role for antioxidant therapy in CRPC.

c-Jun NH(2)-terminal kinase mediates leptin-stimulated androgen-independent prostate cancer cell proliferation via signal transducer and activator of transcription 3 and Akt

Obesity is associated with advanced prostate cancer. Here we demonstrate that in mouse prostate cancer TRAMP-C1 cells epididymal fat extracts from high-fat diet-fed obese mice stimulate androgen-independent cell growth more significantly than those from low-fat diet-fed lean mice or genetically obese leptin-deficient ob/ob mice in correlation with leptin concentrations. This result suggests that obesity promotes androgen-independent prostate cancer cell growth via adipose leptin. We have reported that added leptin stimulates androgen-independent prostate cancer cell proliferation through c-Jun NH(2)-terminal kinase (JNK). As with JNK, signal transducer and activator of transcription 3 (STAT3) and Akt are implicated in androgen-independent prostate cancer. In this study, we identify novel interaction of these three molecules in leptin-stimulated androgen-independent cell proliferation. Leptin activates JNK, STAT3 and Akt in a biphasic manner with a similar time-course. Pharmacological JNK inhibition suppresses leptin-stimulated DNA binding activity, as well as Ser-727 phosphorylation, of STAT3. Since JNK upregulates STAT3 activity via Ser-727 phosphorylation, JNK mediates leptin-stimulated STAT3 activation through Ser-727 phosphorylation. Moreover, JNK inhibition impairs leptin-stimulated Ser-473 phosphorylation of Akt that is required for its activation. Thus, JNK is involved in leptin-stimulated Akt activation. These findings together indicate that JNK mediates leptin-stimulated androgen-independent prostate cancer cell proliferation via STAT3 and Akt.

Targeting the androgen receptor pathway in prostate cancer

When prostate cancers progress following androgen depletion therapy, there are currently few treatment options with only one, docetaxel, that has been shown to prolong life. Recent work has shown that castration-resistant prostate cancers (CRPCs) continue to depend on androgen receptor (AR) signaling which is reactivated despite low serum androgen levels. Currently available AR-targeted therapy, including GnRH agonists and antiandrogens, cannot completely shut down AR signaling. Several mechanisms that enhance AR signaling in an androgen-depleted environment have been elucidated. These include AR mutations that allow activation by low androgen levels or by other endogenous steroids, AR overexpression, increased local intracrine synthesis of androgens, and upregulation of tyrosine kinase pathways. This has led to the development of a number of novel agents targeting the AR signaling pathway, including more effective antiandrogens, inhibitors of CYP17, an enzyme required for androgen synthesis, inhibitors of 5alpha-reductase, inhibitors of HSP90 which protects AR from degradation, inhibitors of histone deacetylases which is required for optimal AR-mediated transcription, as well as inhibitors of tyrosine kinase inhibitors. Many of these strategies are currently being tested in clinical trials in CRPC.

Integrating differentiation and cancer: the Nkx3.1 homeobox gene in prostate organogenesis and carcinogenesis

Several tissue-specific regulatory genes have been found to play essential roles in both organogenesis and carcinogenesis. In the prostate, the Nkx3.1 homeobox gene plays an important role in normal differentiation of the prostatic epithelium while its loss of function is an initiating event in prostate carcinogenesis in both mouse models and human patients. Thus, the Nkx3.1 homeobox gene provides a paradigm for understanding the relationship between normal differentiation and cancer, as well as studying the roles of homeobox genes in these processes. Here, we review recent findings concerning the roles of Nkx3.1 in development and discuss how its normal function is disrupted in processes of early prostate carcinogenesis.

Advances in mouse models of prostate cancer

Advances in science and technology have allowed us to manipulate the mouse genome and analyse the effect of specific genetic alterations on the development of prostate cancer in vivo. We can now analyse the molecular basis of initiation, invasion and progression to metastatic disease. The current mouse models utilise knockout, knock-in or conditional regulation of expression using Cre-loxP technology. Genes that have been targeted include homeobox genes, tumour suppressors and oncogenes, growth factors (and their receptors), steroid hormones and cell-cycle regulators, as well as pro- and anti-apoptotic proteins. Bigenic models indicate that that two 'hits' are required for progression from intra-epithelial neoplasia (PIN) to invasion carcinoma, and two to five hits are needed for metastasis. Here, we discuss the numerous models that mimic various aspects of the disease process, such as PIN, locally invasive adenocarcinoma and metastatic disease. Currently the PB-Cre4 x PTEN(loxP/loxP) mouse is the only model that spans the entire continuum from initiation to local invasion and metastasis. Such mouse models increase our understanding of the disease process and provide targets for novel therapeutic approaches. Hopefully, the transgenic models will become inducible and ultimately allow both temporal and spatial gene inactivation. Compound mutational models will also develop further, with double and triple knock-in or knockout systems adding to our knowledge of the interaction between different signalling cascades.

An AKT activity threshold regulates androgen-dependent and androgen-independent PSA expression in prostate cancer cell lines

The androgen receptor (AR) plays an important role in early prostate cancer by activating transcription of a number of genes participating in cell proliferation and growth and cancer progression. However, as the cancer progresses, prostate cancer cells transform from an androgen-dependent to an androgen-independent state. Androgen-independent prostate cancer can manifest itself in several forms, including a percentage of cancers that show reduced levels of prostate-specific antigen (PSA) and can progress without the need for the ligand or active receptor. Therefore, our goal was to examine the role of intracellular signaling pathways in an androgen-independent prostate cancerin vitromodel. Using the cell line PC3(AR)2, we stimulated cells with 5-α-dihydrotestosterone (DHT) and epidermal growth factor (EGF) and then analyzed PSA expression. We observed lower PSA expression when cells were jointly stimulated with DHT and EGF, and this was associated with an increase in AKT activity. We examined the role of AKT in AR activity and PSA expression by creating stable PC3(AR)2cell lines transfected with a PI3K-Ras-effector loop mutant. These cell lines showed lower DHT-stimulated PSA expression that correlated to changes in the phosphorylated state of AR. Therefore, we propose anin vitroandrogen-independent model in which a PI3K/AKT activity threshold and subsequent AR transactivation regulate PSA expression.

The current state of preclinical prostate cancer animal models

Prostate cancer continues to be a major cause of morbidity and mortality in men around the world. The field of prostate cancer research continues to be hindered by the lack of relevant preclinical models to study tumorigenesis and to further development of effective prevention and therapeutic strategies. The Prostate Cancer Foundation held a Prostate Cancer Models Working Group (PCMWG) Summit on August 6th and 7th, 2007 to address these issues. The PCMWG reviewed the state of prostate cancer preclinical models and identified the current limitations of cell line, xenograft and genetically engineered mouse models that have hampered the transition of scientific findings from these models to human clinical trials. In addition the PCMWG identified administrative issues that inhibit the exchange of models and impede greater interactions between academic centers and these centers with industry. The PCMWG identified potential solutions for discovery bottlenecks that include: (1) insufficient number of models with insufficient molecular and biologic diversity to reflect human cancer, (2) a lack of understanding of the molecular events that define tumorigenesis, (3) a lack of tools for studying tumor-host interactions, (4) difficulty in accessing model systems across institutions, and (5) addressing why preclinical studies appear not to be predictive of human clinical trials. It should be possible to apply the knowledge gained molecular and epigenetic studies to develop new cell lines and models that mimic progressive and fatal prostate cancer and ultimately improve interventions.

Activator protein-1 transcription factors are associated with progression and recurrence of prostate cancer

To identify biomarkers that discriminate the aggressive forms of prostate cancer, we performed gene expression profiling of prostate tumors using a genetically engineered mouse model that recapitulates the stages of human prostate cancer, namely Nkx3.1; Pten mutant mice. We observed a significant deregulation of the epidermal growth factor and mitogen-activated protein kinase (MAPK) signaling pathways, as well as their major downstream effectors--the activator protein-1 transcription factors c-Fos and c-Jun. Forced expression of c-Fos and c-Jun in prostate cancer cells promotes tumorigenicity and results in activation of extracellular signal-regulated kinase (Erk) MAPK signaling. In human prostate cancer, up-regulation of c-Fos and c-Jun proteins occurs in advanced disease and is correlated with Erk MAPK pathway activation, whereas high levels of c-Jun expression are associated with disease recurrence. Our analyses reveal a hitherto unappreciated role for AP-1 transcription factors in prostate cancer progression and identify c-Jun as a marker of high-risk prostate cancer. This study provides a striking example of how accurate mouse models can provide insights on molecular processes involved in progression and recurrence of human cancer.