Epithelial to mesenchymal transition is mechanistically linked with stem cell signatures in prostate cancer cells

[From tumor tissue via primary cultures to xenograft models: a functional approach in prostate cancer research]

The clinical course of prostate cancer, the most common cancer in men, is very variable. Despite intense research activities over the years and besides histopathological criteria, prognostic markers that reliably predict tumor behavior and the necessity for treatment are still missing. A likely explanation for this fact is the lack of good tumor models, mimicking the in vivo situation. These models are not only essential for a better understanding of the pathogenesis of prostate cancer but also play an important role in the development of new therapeutic strategies. Since results of permanent cell culture experiments reflect only in part real tumor behavior and primary cultures from patient material cannot be grown indefinitely, novel approaches need to be developed to achieve reliable and clinically relevant prostate cancer research.In this work the development of several approaches for culturing primary prostate cancer tissue is illustrated and a forecast of future research plans utilizing xenograft models in mice is made.

Epithelial-to-mesenchymal transition in prostate cancer: paradigm or puzzle?

The lethal consequences of prostate cancer are related to its metastasis to other organ sites. Epithelial-to-mesenchymal transition (EMT) has received considerable attention as a conceptual paradigm to explain invasive and metastatic behavior during cancer progression. EMT is a normal physiologic process by which cells of epithelial origin convert into cells bearing mesenchymal characteristics. It has been proposed that EMT is co-opted by cancer cells during their metastatic dissemination from a primary organ to secondary sites, but the extent to which this recapitulates physiologic EMT remains uncertain. However, there is ample evidence that EMT-like states occur in, and may contribute to, prostate cancer progression and metastasis, and so has become a very active area of research. Here we review this evidence and explore recent studies that have aimed to better define the role and mechanisms of EMT in prostate cancer. While definitive evidence of something akin to physiologic EMT is still lacking in human prostate cancer, this area of research has nonetheless provided new avenues of investigation into the longstanding puzzles of metastasis, therapeutic resistance, and prognostic biomarkers.

The aldehyde dehydrogenase enzyme 7A1 is functionally involved in prostate cancer bone metastasis

High aldehyde dehydrogenase (ALDH) activity can be used to identify tumor-initiating and metastasis-initiating cells in various human carcinomas, including prostate cancer. To date, the functional importance of ALDH enzymes in prostate carcinogenesis, progression and metastasis has remained elusive. Previously we identified strong expression of ALDH7A1 in human prostate cancer cell lines, primary tumors and matched bone metastases. In this study, we evaluated whether ALDH7A1 is required for the acquisition of a metastatic stem/progenitor cell phenotype in human prostate cancer. Knockdown of ALDH7A1 expression resulted in a decrease of the α2^hi/αv^hi/CD44⁺ stem/progenitor cell subpopulation in the human prostate cancer cell line PC-3M-Pro4. In addition, ALDH7A1 knockdown significantly inhibited the clonogenic and migratory ability of human prostate cancer cells in vitro. Furthermore, a number of genes/factors involved in migration, invasion and metastasis were affected including transcription factors (snail, snail2, and twist) and osteopontin, an ECM molecule involved in metastasis. Knockdown of ALDH7A1 resulted in decreased intra-bone growth and inhibited experimentally induced (bone) metastasis, while intra-prostatic growth was not affected. In line with these observations, evidence is presented that TGF-β, a key player in cancer invasiveness and bone metastasis, strongly induced ALDH activity while BMP7 (an antagonist of TGF-β signaling) down-regulated ALDH activity. Our findings show, for the first time, that the ALDH7A1 enzyme is functionally involved in the formation of bone metastases and that the effect appeared dependent on the microenvironment, i.e., bone versus prostate.

Activated K-ras and INK4a/Arf deficiency cooperate during the development of pancreatic cancer by activation of Notch and NF-κB signaling pathways

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is the fourth leading cause of cancer-related death in the United States, suggesting that novel strategies for the prevention and treatment of PDAC are urgently needed. K-ras mutations are observed in >90% of pancreatic cancer, suggesting its role in the initiation and early developmental stages of PDAC. In order to gain mechanistic insight as to the role of mutated K-ras, several mouse models have been developed by targeting a conditionally mutated K-ras(G12D) for recapitulating PDAC. A significant co-operativity has been shown in tumor development and metastasis in a compound mouse model with activated K-ras and Ink4a/Arf deficiency. However, the molecular mechanism(s) by which K-ras and Ink4a/Arf deficiency contribute to PDAC has not been fully elucidated.

METHODOLOGY/PRINCIPAL FINDINGS

To assess the molecular mechanism(s) that are involved in the development of PDAC in the compound transgenic mice with activated K-ras and Ink4a/Arf deficiency, we used multiple methods, such as Real-time RT-PCR, western blotting assay, immunohistochemistry, MTT assay, invasion, EMSA and ELISA. We found that the deletion of Ink4a/Arf in K-ras(G12D) expressing mice leads to PDAC, which is in part mediated through the activation of Notch and NF-κB signaling pathways. Moreover, we found down-regulation of miR-200 family, which could also play important roles in tumor development and progression of PDAC in the compound transgenic mice.

CONCLUSIONS/SIGNIFICANCE

Our results suggest that the activation of Notch and NF-κB together with the loss of miR-200 family is mechanistically linked with the development and progression of PDAC in the compound K-ras(G12D) and Ink4a/Arf deficient transgenic mice.

On the intrinsic inevitability of cancer: from foetal to fatal attraction

The cracks in the paradigm of oncogenic mutations and somatic evolution as driving force of tumorigenesis, lucidly exposed by the dynamic heterogeneity of "cancer stem cells" or the diffuse results of cancer genome sequencing projects, indicate the need for a more encompassing theory of cancer that reaches beyond the current proximate explanations based on individual genetic pathways. One such integrative concept, derived from first principles of the dynamics of gene regulatory networks, is that cancerous cell states are attractor states, just like normal cell types are. Here we extend the concept of cancer attractors to illuminate a more profound property of cancer initiation: its inherent inevitability in the light of metazoan evolution. Using Waddington's Epigenetic Landscape as a conceptual aid, for which we present a mathematical and evolutionary foundation, we propose that cancer is intrinsically linked to ontogenesis and phylogenesis. This explanatory rather than enumerating review uses a formal argumentation structure that is atypical in modern experimental biology but may hopefully offer a new coherent perspective to reconcile many conflicts between new findings and the old thinking in the categories of linear oncogenic pathways.

Diosgenin suppresses hepatocyte growth factor (HGF)-induced epithelial-mesenchymal transition by down-regulation of Mdm2 and vimentin

Substantial activation of the hepatocyte growth factor (HGF)/c-Met pathway leads to cancer cell scattering and invasion and has been observed in several types of cancers, including prostate and colorectal cancers. The phosphorylation cascade downstream of HGF, particularly PI3K/Akt signaling, regulates epithelial-to-mesenchymal transition (EMT). How this signaling governs EMT and whether specific kinases respond to particular EMT effectors remain unclear. This study found specific increases in Mdm2 and vimentin rather than the coregulation of an array of EMT marker proteins in response to HGF-induced EMT in DU145 prostate cancer cells. Importantly, it was further found that diosgenin abrogated HGF-induced DU145 cell scattering and invasion. Moreover, diosgenin effectively inhibited the HGF-induced increases in Mdm2 and vimentin by down-regulating phosphorylated Akt and mTOR. In summary, the results suggest that diosgenin may be a potential compound for use in prostate cancer therapy to target the major HGF-induced EMT pathway.

Frequent gene products and molecular pathways altered in prostate cancer- and metastasis-initiating cells and their progenies and novel promising multitargeted therapies

Recent gene expression profiling analyses and gain- and loss-of-function studies performed with distinct prostate cancer (PC) cell models indicated that the alterations in specific gene products and molecular pathways often occur in PC stem/progenitor cells and their progenies during prostate carcinogenesis and metastases at distant sites, including bones. Particularly, the sustained activation of epidermal growth factor receptor (EGFR), hedgehog, Wnt/β-catenin, Notch, hyaluronan (HA)/CD44 and stromal cell-derived factor-1 (SDF-1)/CXC chemokine receptor 4 (CXCR4) during the epithelial-mesenchymal transition (EMT) process may provide critical functions for PC progression to locally invasive, metastatic and androgen-independent disease states and treatment resistance. Moreover, an enhanced glycolytic metabolism in PC stem/progenitor cells and their progenies concomitant with the changes in their local microenvironment, including the induction of tumor hypoxia and release of diverse soluble factors by tumor myofibroblasts, also may promote the tumor growth, angiogenesis and metastases. More particularly, these molecular transforming events may cooperate to upregulate Akt, nuclear factor (NF)-κB, hypoxia-inducible factors (HIFs) and stemness gene products such as Oct3/4, Sox2, Nanog and Bmi-1 in PC cells that contribute to their acquisition of high self-renewal, tumorigenic and invasive capacities and survival advantages during PC progression. Consequently, the molecular targeting of these deregulated gene products in the PC- and metastasis-initiating cells and their progenies represent new promising therapeutic strategies of great clinical interest for eradicating the total PC cell mass and improving current antihormonal treatments and docetaxel-based chemotherapies, thereby preventing disease relapse and the death of PC patients.

Metastatic progression of prostate cancer and e-cadherin regulation by zeb1 and SRC family kinases

Expression of E-cadherin is used to monitor the epithelial phenotype, and its loss is suggestive of epithelial-mesenchymal transition (EMT). EMT triggers tumor metastasis. Exit from EMT is marked by increased E-cadherin expression and is considered necessary for tumor growth at sites of metastasis; however, the mechanisms associated with exit from EMT are poorly understood. Herein are analyzed 185 prostate cancer metastases, with significantly higher E-cadherin expression in bone than in lymph node and soft tissue metastases. To determine the molecular mechanisms of regulation of E-cadherin expression, three stable isogenic cell lines from DU145 were derived that differ in structure, migration, and colony formation on soft agar and Matrigel. When injected into mouse tibia, the epithelial subline grows most aggressively, whereas the mesenchymal subline does not grow. In cultured cells, ZEB1 and Src family kinases decrease E-cadherin expression. In contrast, in tibial xenografts, E-cadherin RNA levels increase eight- to 10-fold despite persistent ZEB1 expression, and in all ZEB1-positive metastases (10 of 120), ZEB1 and E-cadherin proteins were co-expressed. These data suggest that transcriptional regulation of E-cadherin differs in cultured cells versus xenografts, which more faithfully reflect E-cadherin regulation in cancers in human beings. Furthermore, the aggressive nature of xenografts positive for E-cadherin and the frequency of metastases positive for E-cadherin suggest that high E-cadherin expression in metastatic prostate cancer is associated with aggressive tumor growth.

Androgen regulation of epithelial-mesenchymal transition in prostate tumorigenesis

Prostate cancer patient mortality is ascribed to the spread of cancerous cells to areas outside the prostate gland and the inability of current treatment strategies to effectively block progression to metastasis. Understanding the cellular mechanisms contributing to the dissemination of malignant cells and metastasis is critically significant to the generation of effective therapeutic modalities for improved patient survival while combating therapeutic resistance. In recent years, the phenomenon of epithelial-mesenchymal transitions (EMTs) has received considerable attention due to accumulating evidence indicating a role for this developmentally conserved process in tumorigenesis. Cancer cells at the invasive edges of tumors undergo EMT under the influence of contextual signals that they receive from the microenvironment, such as TGF-β. Also derived from developmental studies is the fact that EMT induction is reversible; thus, upon removal of EMT-inducing signals, cells occasionally revert to the epithelial state of their cellular ancestors via the process of mesenchymal-epithelial transition. This article discusses the current evidence supporting a central role for EMT and its reverse process, mesenchymal-epithelial transition, in the metastatic progression of prostate cancer to advanced disease and the involvement of androgen signaling in its regulation towards the development of castration-resistant prostate cancer.

Cancer Stem Cells and Epithelial-to-Mesenchymal Transition (EMT)-Phenotypic Cells: Are They Cousins or Twins?

Cancer stem cells (CSCs) are cells within a tumor that possess the capacity to self-renew and maintain tumor-initiating capacity through differentiation into the heterogeneous lineages of cancer cells that comprise the whole tumor. These tumor-initiating cells could provide a resource for cells that cause tumor recurrence after therapy. Although the cell origin of CSCs remains to be fully elucidated, mounting evidence has demonstrated that Epithelial-to-Mesenchymal Transition (EMT), induced by different factors, is associated with tumor aggressiveness and metastasis and these cells share molecular characteristics with CSCs, and thus are often called cancer stem-like cells or tumor-initiating cells. The acquisition of an EMT phenotype is a critical process for switching early stage carcinomas into invasive malignancies, which is often associated with the loss of epithelial differentiation and gain of mesenchymal phenotype. Recent studies have demonstrated that EMT plays a critical role not only in tumor metastasis but also in tumor recurrence and that it is tightly linked with the biology of cancer stem-like cells or cancer-initiating cells. Here we will succinctly summarize the state-of-our-knowledge regarding the molecular similarities between cancer stem-like cells or CSCs and EMT-phenotypic cells that are associated with tumor aggressiveness focusing on solid tumors.

Evidence for mesenchymal-epithelial transition associated with mouse hepatic stem cell differentiation

Mesenchymal-epithelial transition events are related to embryonic development, tissue construction, and wound healing. Stem cells are involved in all of these processes, at least in part. However, the direct evidence of mesenchymal-epithelial transition associated with stem cells is unclear. To determine whether mesenchymal-epithelial transition occurs in liver development and/or the differentiation process of hepatic stem cells in vitro, we analyzed a variety of murine liver tissues from embryonic day 11.5 to adults and the colonies derived from hepatic stem/progenitor cells isolated with flow cytometry. The results of gene expression, immunohistochemistry and Western blot showed that as liver develops, the expression of epithelial markers such as Cytokeratin18 and E-cadherin increase, while expression of mesenchymal markers such as vimentin and N-cadherin decreased. On the other hand, in freshly isolated hepatic stem cells, the majority of cells (65.0%) co-express epithelial and mesenchymal markers; this proportion is significantly higher than observed in hematopoietic cells, non-hematopoietic cells and non-stem cell fractions. Likewise, in stem cell-derived colonies cultured over time, upregulation of epithelial genes (Cytokeratin-18 and E-cadherin) occurred simultaneously with downregulation of mesenchymal genes (vimentin and Snail1). Furthermore, in the fetal liver, vimentin-positive cells in the non-hematopoietic fraction had distinct proliferative activity and expressed early the hepatic lineage marker alpha-fetoprotein.

CONCLUSION

Hepatic stem cells co-express mesenchymal and epithelial markers; the mesenchymal-epithelial transition occurred in both liver development and differentiation of hepatic stem/progenitor cells in vitro. Besides as a mesenchymal marker, vimentin is a novel indicator for cell proliferative activity and undifferentiated status in liver cells.

Analysis of epithelial and mesenchymal markers in ovarian cancer reveals phenotypic heterogeneity and plasticity

In our studies of ovarian cancer cells we have identified subpopulations of cells that are in a transitory E/M hybrid stage, i.e. cells that simultaneously express epithelial and mesenchymal markers. E/M cells are not homogenous but, in vitro and in vivo, contain subsets that can be distinguished based on a number of phenotypic features, including the subcellular localization of E-cadherin, and the expression levels of Tie2, CD133, and CD44. A cellular subset (E/M-MP) (membrane E-cadherin(low)/cytoplasmic E-cadherin(high)/CD133(high), CD44(high), Tie2(low)) is highly enriched for tumor-forming cells and displays features which are generally associated with cancer stem cells. Our data suggest that E/M-MP cells are able to differentiate into different lineages under certain conditions, and have the capacity for self-renewal, i.e. to maintain a subset of undifferentiated E/M-MP cells during differentiation. Trans-differentiation of E/M-MP cells into mesenchymal or epithelial cells is associated with a loss of stem cell markers and tumorigenicity. In vivo xenograft tumor growth is driven by E/M-MP cells, which give rise to epithelial ovarian cancer cells. In contrast, in vitro, we found that E/M-MP cells differentiate into mesenchymal cells, in a process that involves pathways associated with an epithelial-to-mesenchymal transition. We also detected phenotypic plasticity that was dependent on external factors such as stress created by starvation or contact with either epithelial or mesenchymal cells in co-cultures. Our study provides a better understanding of the phenotypic complexity of ovarian cancer and has implications for ovarian cancer therapy.

miR-200 Inhibits lung adenocarcinoma cell invasion and metastasis by targeting Flt1/VEGFR1

The microRNA-200 (miR-200) family is part of a gene expression signature that predicts poor prognosis in lung cancer patients. In a mouse model of K-ras/p53-mutant lung adenocarcinoma, miR-200 levels are suppressed in metastasis-prone tumor cells, and forced miR-200 expression inhibits tumor growth and metastasis, but the miR-200 target genes that drive lung tumorigenesis have not been fully elucidated. Here, we scanned the genome for putative miR-200 binding sites and found them in the 3'-untranslated region (3'-UTR) of 35 genes that are amplified in human cancer. Mining of a database of resected human lung adenocarcinomas revealed that the levels of one of these genes, Flt1/VEGFR1, correlate inversely with duration of survival. Forced miR-200 expression suppressed Flt1 levels in metastasis-prone lung adenocarcinoma cells derived from K-ras/p53-mutant mice, and negatively regulated the Flt1 3'-UTR in reporter assays. Cancer-associated fibroblasts (CAFs) isolated from murine lung adenocarcinomas secreted abundant VEGF and enhanced tumor cell invasion in coculture studies. CAF-induced tumor cell invasion was abrogated by VEGF neutralization or Flt1 knockdown in tumor cells. Flt1 knockdown decreased the growth and metastasis of tumor cells in syngeneic mice. We conclude that miR-200 suppresses lung tumorigenesis by targeting Flt1.

Cancer stem cells, models of study and implications of therapy resistance mechanisms

There is now compelling evidence for tumour initiating or cancer stem cells (CSCs) in human cancers. The current evidence of this CSC hypothesis, the CSC phenotype and methods of identification, culture and in vitro modelling will be presented, with an emphasis on prostate cancer. Inherent in the CSC hypothesis is their dual role, as a tumour-initiating cell, and as a source of treatment-resistant cells; the mechanisms behind therapeutic resistance will be discussed. Such resistance is a consequence of the unique CSC phenotype, which differs from the differentiated progeny, which make up the bulk of a tumour. It seems that to target the whole tumour, employing traditional therapies to target bulk populations alongside targeted CSC-specific drugs, provides the best hope of lasting treatment or even cure.

Pancreatic cancer: understanding and overcoming chemoresistance

Pancreatic cancer is a highly aggressive malignancy. This feature is believed to be partly attributable to the chemotherapy-resistant characteristics of specific subgroups of pancreatic cancer cells, namely those with an epithelial-mesenchymal transition (EMT) phenotype and cancer stem cells. Accumulating evidence suggests that several new and emerging concepts might be important in the drug-resistant phenotype of these cell types. An understanding of the molecular mechanisms underlying drug resistance in patients with pancreatic cancer might help researchers to devise novel strategies to overcome such resistance. In particular, microRNAs (miRNAs) seem to be critical regulators of drug resistance in pancreatic cancer cells. Selective and targeted elimination of cells with an EMT phenotype and cancer stem cells could be achieved by regulating the expression of specific miRNAs.

Molecular signature of epithelial-mesenchymal transition (EMT) in human prostate cancer bone metastasis

Prostate cancer (PCa) has a predilection to metastasize to bone. Before metastasis can occur there is transition of the sessile epithelial cancer cells to become motile and invasive mesenchymal phenotypes by an important albeit transient process called Epithelial-to-Mesenchymal Transition (EMT). The cascade of molecular events triggered by this process is clinically relevant as they are associated with cancer stem-like cells (CSC), decreased senescence and eventual drug resistance phenotype. We interrogated some EMT markers that have been implicated in primary and bone metastasis of PCa using archived patient samples. Using an immunohistochemical approach, E-cadherin, Vimentin, ZEB1, Notch-1, PDGF-D and NF-κB were analyzed. Cases were microscopically scored using intensity (0, +1, +2, +3) and percentage of positive cells. Data was statistically analyzed using Fisher's Exact Test. Aberrant expression of EMT markers E-cadherin, Vimentin, PDGF-D, NF-κB, Notch-1 and ZEB1 was observed in PCa (primary tumor specimen) and bone metastasis tissues. The aberrant expression pattern varied according to the location within the tumor with higher expression was observed more at the invasive tumor front (ITF) vs. the center of the tumor. Notch-1 was significantly over-expressed in bone metastasis compared to primary PCa (p=0.057). The expression levels, intensity and % of positive cells of the remaining markers were not statistically significant in PCa vs. bone metastasis. In conclusion, protein expression analysis revealed the existence of EMT phenotype in the PCa and bone metastasis. Variation in the aberrant expression patterns at the invasive tumor front indicates the role of EMT markers in tumor invasion. Our results suggest that Notch-1 could play a role in PCa bone metastasis. Studies in larger patient cohorts are warranted before these EMT molecular markers can be translated to the clinical use.