Cancer Stem Cells (CSCs) are a sub-population of cells, identified in most tumors, responsible for the initiation, recurrence, metastatic potential, and resistance of different malignancies. In prostate cancer (PCa), CSCs were identified and thought to be responsible for the generation of the lethal subtype, commonly known as Castration-Resistant Prostate Cancer (CRPC). In vitro models to investigate the properties of CSCs in PCa are highly required. Sphere-formation assay is an in vitro method commonly used to identify CSCs and study their properties. Here, we report the detailed methodology on how to generate and propagate spheres from PCa cell lines and from murine prostate tissue. This model is based on the ability of stem cells to grow in non-adherent serum-free gel matrix. We also describe how to use these spheres in histological and immuno-fluorescent staining assays to assess the differentiation potential of the CSCs. Our results show the sphere-formation Assay (SFA) as a reliable in vitro assay to assess the presence and self-renewal ability of CSCs in different PCa models. This platform presents a useful tool to evaluate the effect of conventional or novel agents on the initiation and self-renewing properties of different tumors. The effects can be directly evaluated through assessment of the sphere-forming efficiency (SFE) over five generations or other downstream assays such as immuno-histochemical analysis of the generated spheres.

Prostate cancers display a range of clinical behavior, from slow-growing tumors of minor clinical significance to locally aggressive and ultimately metastatic disease. Human prostate adenocarcinoma has a mature luminal phenotype characterized by cytokeratin 8 (CK8) and androgen receptor (AR) expression and prostate-specific antigen (PSA) production. Progressive prostate cancer is almost always treated with androgen deprivation therapy; however, despite such treatment, approximately 10% of prostate cancers progress to metastatic disease.1 Defining mechanisms of resistance to androgen deprivation and progression to metastasis would be significantly aided by the availability of genetically defined models of prostate cancer progression.One of the most common genetic alterations in prostate cancer is deletion of at least one copy of the PTEN tumor suppressor, which occurs in approximately 70% of human prostate cancers. Biallelic deletion of PTEN and the associated increase in AKT phosphorylation, which occurs in roughly 25% of prostate cancers, is correlated with resistance to androgen deprivation therapy.2 A recent genomic profiling study of mostly primary prostate cancers demonstrated that 24% of cases had either a heterozygous or homozygous copy number loss of TP53.3 Other large-scale studies using combined immunohistochemistry (IHC) and sequencing approaches have shown that TP53 mutations occur in approximately 5% of primary tumors and at much higher frequencies in lymph node metastases (16%) and castrate-resistant (26%) tumors. 4,5 In addition, TP53 mutations were found to be independent predictors of tumor recurrence in low-and intermediate-grade cancers. Thus, loss of PTEN and aberrations of TP53 are implicated in aggressive forms of human prostate cancer.

Dysregulation of the EGFR signaling axis enhances bone metastases in many solid cancers. However, the relevant downstream effector signals in this axis are unclear. miR-1 was recently shown to function as a tumor suppressor in prostate cancer cells, where its expression correlated with reduced metastatic potential. In this study, we demonstrated a role for EGFR translocation in regulating transcription of miR-1-1, which directly targets expression of TWIST1. Consistent with these findings, we observed decreased miR-1 levels that correlated with enhanced expression of activated EGFR and TWIST1 in a cohort of human prostate cancer specimens and additional datasets. Our findings support a model in which nuclear EGFR acts as a transcriptional repressor to constrain the tumor-suppressive role of miR-1 and sustain oncogenic activation of TWIST1, thereby leading to accelerated bone metastasis.

Aberrant activation of Ras and WNT signaling are key events that have been shown to be up-regulated in prostate cancer that has metastasized to the bone. However, the regulatory mechanism of combinatorial Ras and WNT signaling in advanced prostate cancer is still unclear. TCF7, a WNT signaling-related gene, has been implicated as a critical factor in bone metastasis, and here we show that TCF7 is a direct target of miR-34a. In samples of prostate cancer patients, miR-34a levels are inversely correlated with TCF7 expression and a WNT dependent gene signature. Ectopic miR-34a expression inhibited bone metastasis and reduced cancer cell proliferation in a Ras-dependent xenograft model. We demonstrate that miR-34a can directly interfere with the gene expression of the anti-proliferative BIRC5, by targeting BIRC5 3′UTR. Importantly, BIRC5 overexpression was sufficient to reconstitute anti-apoptotic signaling in cells expressing high levels of miR-34a. In prostate cancer patients, we found that BIRC5 levels were positively correlated with a Ras signaling signature expression. Our data show that the bone metastasis and anti-apoptotic effects found in Ras signaling-activated prostate cancer cells require miR-34a deficiency, which in turn aids in cell survival by activating the WNT and anti-apoptotic signaling pathways thereby inducing TCF7 and BIRC5 expressions.

Activation of EGFR signaling pathway leads to prostate cancer bone metastasis; however, therapies targeting EGFR have demonstrated limited effectiveness and led to drug resistance. miR-203 levels are down-regulated in clinical samples of primary prostate cancer and further reduced in metastatic prostate cancer. Here we show that ectopic miR-203 expression displayed reduced bone metastasis and induced sensitivity to tyrosine kinase inhibitors (TKIs) treatment in a xenograft model. Our results demonstrate that the induction of bone metastasis and TKI resistance require miR-203 down-regulation, activation of the EGFR pathway via altered expression of EGFR ligands (EREG and TGFA) and anti-apoptotic proteins (API5, BIRC2, and TRIAP1). Importantly, a sufficient reconstitution of invasiveness and resistance to TKIs treatment was observed in cells transfected with anti-miR-203. In prostate cancer patients, our data showed that miR-203 levels were inversely correlated with the expression of two EGFR ligands, EREG and TGFA, and an EGFR dependent gene signature. Our results support the existence of a miR-203, EGFR, TKIs resistance regulatory network in prostate cancer progression. We propose that the loss of miR-203 is a molecular link in the progression of prostate cancer metastasis and TKIs resistance characterized by high EGFR ligands output and anti-apoptotic proteins activation.

Prostate cancers of luminal adenocarcinoma histology display a range of clinical behaviors. Although most prostate cancers are slow-growing and indolent, a proportion is aggressive, developing metastasis and resistance to androgen deprivation treatment. One hypothesis is that a portion of aggressive cancers initiate from stem-like, androgen-independent tumor-propagating cells. Here we demonstrate the in vitro creation of a mouse cell line, selected for growth as self-renewing stem/progenitor cells, which manifests many in vivo properties of aggressive prostate cancer. Normal mouse prostate epithelium containing floxed Pten and TP53 alleles was subjected to CRE-mediated deletion in vitro followed by serial propagation as protospheres. A polyclonal cell line was established from dissociated protospheres and subsequently a clonal daughter line was derived. Both lines demonstrate a mature luminal phenotype in vitro. The established lines contain a stable minor population of progenitor cells with protosphere-forming ability and multi-lineage differentiation capacity. Both lines formed orthotopic adenocarcinoma tumors with metastatic potential to lung. Intracardiac inoculation resulted in brain and lung metastasis, while intra-tibial injection induced osteoblastic bone formation, recapitulating the bone metastatic phenotype of human prostate cancer. The cells showed androgen receptor dependent growth in vitro. Importantly, in vivo, the deprivation of androgens from established orthotopic tumors resulted in tumor regression and eventually castration-resistant growth. These data suggest that transformed prostate progenitor cells preferentially differentiate toward luminal cells and recapitulate many characteristics of the human disease.

Bone metastasis is the hallmark of progressive and castration-resistant prostate cancers. MicroRNA 1 (miR-1) levels are decreased in clinical samples of primary prostate cancer and further reduced in metastases. SRC has been implicated as a critical factor in bone metastasis, and here we show that SRC is a direct target of miR-1. In prostate cancer patient samples, miR-1 levels are inversely correlated with SRC expression and a SRC-dependent gene signature. Ectopic miR-1 expression inhibited extracellular signal-regulated kinase (ERK) signaling and bone metastasis in a xenograft model. In contrast, SRC overexpression was sufficient to reconstitute bone metastasis and ERK signaling in cells expressing high levels of miR-1. Androgen receptor (AR) activity, defined by an AR output signature, is low in a portion of castration-resistant prostate cancer. We show that AR binds to the miR-1-2 regulatory region and regulates miR-1 transcription. Patients with low miR-1 levels displayed correlated low canonical AR gene signatures. Our data support the existence of an AR-miR-1-SRC regulatory network. We propose that loss of miR-1 is one mechanistic link between low canonical AR output and SRC-promoted metastatic phenotypes.

The chemokine CC motif ligand 2 (CCL2) is important in recruiting tumor-associated macrophages and is involved in the development of castration-resistance prostate cancer (CRPC) after androgen-deprivation therapy (ADT); however, the underlying mechanism remains unclear. We found that inactivation of the androgen receptor (AR) reduces a transcriptional repressor (SAM pointed domain-containing ETS transcription factor, SPDEF) of CCL2, which mediates epithelial-to-mesenchymal transition (EMT) of prostate tumor cells. Cell lines derived from a prostate-specific Pten/Trp53-null mouse and capable of a spontaneous EMT were utilized for identification of CCL2, and showed that reduced SPDEF expression was associated with an elevated CCL2-activated EMT. AR signaling inhibits CCL2 through a SPDEF-mediated mechanism in that the SPDEF recognizes the CCL2 promoter and transcriptionally represses its activity. Ectopically expressed SPDEF reduced the EMT and rescued expression of CCL2 in SPDEF-expressing cells, which induced the EMT and promotes malignant functions of prostate cancer cells. In tissues from prostate cancer patients with ADT, low SPDEF levels were correlated with high CCL2 expression compared to patients without ADT. We present a novel mechanism that contributes to the EMT and metastatic phenotype observed in a subset of ADT-resistant prostate cancer, where the CCL2 is stimulated through the inactivated of AR-mediated SPDEF.