The hallmarks of ovarian cancer stem cells and niches: Exploring their harmonious interplay in therapy resistance

The concept of a "cancer stem cell" has evolved over the past decades, and research on cancer stem cell biology has entered into a stage of remarkable progress. Cancer stem cells are a major determining factor contributing to the establishment of phenotypic and functional intratumoral heterogeneity in synchronization with their surrounding "cancer stem cell niches." They serve as the driving force for cancer initiation, metastasis, and therapeutic resistance in various types of malignancies. In verity, reciprocal interplay between ovarian cancer stem cells and their niches involves a complex but ingeniously orchestrated tumor microenvironment within the intraperitoneal milieu and especially contribute to chemotherapy resistance in patients with advanced ovarian cancer. Herein, we review the principles of our current understanding of the biological features of ovarian cancer stem cells, focusing mainly on the precise mechanisms underlying acquired chemotherapy resistance. Furthermore, we highlight the specific roles of various cancer-associated stromal and immune cells in creating possible cancer stem cell niches that regulate ovarian cancer stemness.

Molecular pathology underlying the robustness of cancer stem cells

Intratumoral heterogeneity is tightly associated with the failure of anticancer treatment modalities including conventional chemotherapy, radiation therapy, and molecularly targeted therapy. Such heterogeneity is generated in an evolutionary manner not only as a result of genetic alterations but also by the presence of cancer stem cells (CSCs). CSCs are proposed to exist at the top of a tumor cell hierarchy and are undifferentiated tumor cells that manifest enhanced tumorigenic and metastatic potential, self-renewal capacity, and therapeutic resistance. Properties that contribute to the robustness of CSCs include the abilities to withstand redox stress, to rapidly repair damaged DNA, to adapt to a hyperinflammatory or hyponutritious tumor microenvironment, and to expel anticancer drugs by the action of ATP-binding cassette transporters as well as plasticity with regard to the transition between dormant CSC and transit-amplifying progenitor cell phenotypes. In addition, CSCs manifest the phenomenon of metabolic reprogramming, which is essential for maintenance of their self-renewal potential and their ability to adapt to changes in the tumor microenvironment. Elucidation of the molecular underpinnings of these biological features of CSCs is key to the development of novel anticancer therapies. In this review, we highlight the pathological relevance of CSCs in terms of their hallmarks and identification, the properties of their niche—both in primary tumors and at potential sites of metastasis—and their resistance to oxidative stress dependent on system xc (−).

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Intratumoral heterogeneity driven by CSCs is responsible for therapeutic resistance.

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CTCs survive in the distant organs and achieve colonization, causing metastasis.

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E/M hybrid cancer cells due to partial EMT exhibit the highest metastatic potential.

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The CSC niche regulates stemness in metastatic disease as well as in primary tumor.

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Activation of system xc(-) by CD44 variant in CSCs is a promising therapeutic target.

Regulation of heterogeneous cancer-associated fibroblasts: the molecular pathology of activated signaling pathways

Accumulating evidence indicates that intratumoral heterogeneity contributes to the development of resistance to anticancer therapeutics. Fibroblasts, which are components of the paraneoplastic stroma, play a crucial role in the wound-healing process. Activated fibroblasts accumulate in the wound and are involved in many aspects of the tissue remodeling cascade that initiates the repair process and prevents further tissue damage. The pathophysiological roles of cancer-associated fibroblasts (CAFs) in the heterogeneous tumor microenvironment have attracted increasing interest. CAFs play crucial roles in tumor progression and the response to chemotherapy. Several cytokines and chemokines are involved in the conversion of normal fibroblasts into CAFs, and some of these form a feedback loop between cancer cells and CAFs. In addition, the physical force between tumor cells and CAFs promotes cooperative invasion or co-migration of both types of cells. Pro-inflammatory cytokines, such as leukemia inhibitory factor (LIF) and interleukin-6 (IL-6), are secreted by both cancer cells and CAFs, and mediate the epigenetic modification of CAFs. This enhances the pro-tumorigenic function of CAFs mediated by promoting actomyosin contractility and extracellular matrix remodeling to form the tracks used for collective cancer cell migration. The concept of intra-tumoral CAF heterogeneity refers to the presence of inflammatory CAFs with low levels of α-smooth muscle actin (α-SMA) and high levels of IL-6 expression, which are in striking contrast to transforming growth factor-β (TGF-β)-dependent myofibroblastic CAFs with high α-SMA expression levels. CAF populations that suppress tumor growth and progression through stroma-specific Hedgehog (Hh) activation have been detected in different murine tumor models including those of the bladder, colon, and pancreas. A new therapeutic strategy targeting CAFs is the "stromal switch," in which tumor-promoting CAFs are changed into tumor-retarding CAFs with attenuated stromal stiffness. Several molecular mechanisms that can be exploited to design personalized anticancer therapies targeting CAFs remain to be elucidated. Strategies aimed at targeting the tumor stroma as well as tumor cells themselves have attracted academic attention for their application in precision medicine. This novel review discusses the role of the activation of EGFR, Wnt/β-catenin, Hippo, TGF-β, and JAK/STAT cascades in CAFs in relation to the chemoresistance and invasive/metastatic behavior of cancer cells. For instance, although activated EGFR signaling contributes to collective cell migration in cooperation with CAFs, an activated Hippo pathway is responsible for stromal stiffness resulting in the collapse of neoplastic blood vessels. Therefore, identifying the signaling pathways that are activated under specific conditions is crucial for precision medicine.

Beyond the Warburg Effect: N-Myc Contributes to Metabolic Reprogramming in Cancer Cells

Cancer cells generate large amounts of lactate derived from glucose regardless of the available oxygen level. Cancer cells finely control ATP synthesis by modulating the uptake of substrates and the activity of enzymes involved in aerobic glycolysis (Warburg effect), which enables them to adapt to the tumor microenvironment. However, increasing evidence suggests that mitochondrial metabolism, including the tricarboxylic acid (TCA) cycle, oxidative phosphorylation (OXPHOS), and glutaminolysis, is paradoxically activated in MYCN-amplified malignancies. Unlike non-amplified cells, MYCN-amplified cancer cells significantly promote OXPHOS-dependent ATP synthesis. Furthermore, tumor cells are differentially dependent on fatty acid β-oxidation (FAO) according to N-Myc status. Therefore, upregulation of FAO-associated enzymes is positively correlated with both N-Myc expression level and poor clinical outcome. This review explores therapeutic strategies targeting cancer stem-like cells for the treatment of tumors associated with MYCN amplification.

Applications of patient-derived tumor xenograft models and tumor organoids

Patient-derived tumor xenografts (PDXs), in which tumor fragments surgically dissected from cancer patients are directly transplanted into immunodeficient mice, have emerged as a useful model for translational research aimed at facilitating precision medicine. PDX susceptibility to anti-cancer drugs is closely correlated with clinical data in patients, from whom PDX models have been derived. Accumulating evidence suggests that PDX models are highly effective in predicting the efficacy of both conventional and novel anti-cancer therapeutics. This also allows “co-clinical trials,” in which pre-clinical investigations in vivo and clinical trials could be performed in parallel or sequentially to assess drug efficacy in patients and PDXs. However, tumor heterogeneity present in PDX models and in the original tumor samples constitutes an obstacle for application of PDX models. Moreover, human stromal cells originally present in tumors dissected from patients are gradually replaced by host stromal cells as the xenograft grows. This replacement by murine stroma could preclude analysis of human tumor-stroma interactions, as some mouse stromal cytokines might not affect human carcinoma cells in PDX models. The present review highlights the biological and clinical significance of PDX models and three-dimensional patient-derived tumor organoid cultures of several kinds of solid tumors, such as those of the colon, pancreas, brain, breast, lung, skin, and ovary.

Sorafenib with Transarterial Chemoembolization Achieves Improved Survival vs. Sorafenib Alone in Advanced Hepatocellular Carcinoma: A Nationwide Population-Based Cohort Study

We hypothesized that sorafenib plus transarterial chemoembolization (TACE) would confer survival benefits over sorafenib alone for advanced hepatocellular carcinoma (aHCC). We investigated this while using the population-based All-Cancer Dataset to assemble a cohort (n = 3674; median age, 60; 83% men) of patients receiving sorafenib for aHCC (Child-Pugh A) with macro-vascular invasion or nodal/distant metastases. The patients were classified into the sorafenib-TACE group (n = 426) or the propensity score-matched sorafenib-alone group (n = 1686). All of the participants were followed up until death or the end of the study. Time-dependent Cox model and the Mantel-Byar test were used for survival analysis. During the median follow-ups of 221 and 133 days for the sorafenib-TACE and sorafenib-alone groups, 164 (39%) and 916 (54%) deaths occurred, respectively; the corresponding median overall survivals (OS) were 381 and 204 days, respectively (hazard ratio, HR: 0.74; 95% confidence interval, CI, 0.63-0.88; p = 0.021). The one-year and six-month OS were 53.5% and 80.3% in the sorafenib-TACE group and 32.4% and 54.4% in the sorafenib-alone group, respectively. The major complications were comparable between the two groups. The addition of TACE to sorafenib improves survival, with a 26% reduction in mortality. These findings provide strong real-world evidence that supports this combination strategy for eligible Child-Pugh A aHCC patients.

Activated Fibroblast Program Orchestrates Tumor Initiation and Progression; Molecular Mechanisms and the Associated Therapeutic Strategies

: Neoplastic epithelial cells coexist in carcinomas with various non-neoplastic stromal cells, together creating the tumor microenvironment. There is a growing interest in the cross-talk between tumor cells and stromal fibroblasts referred to as carcinoma-associated fibroblasts (CAFs), which are frequently present in human carcinomas. CAF populations extracted from different human carcinomas have been shown to possess the ability to influence the hallmarks of cancer. Indeed, several mechanisms underlying CAF-promoted tumorigenesis are elucidated. Activated fibroblasts in CAFs are characterized as alpha-smooth muscle actin-positive myofibroblasts and actin-negative fibroblasts, both of which are competent to support tumor growth and progression. There are, however, heterogeneous CAF populations presumably due to the diverse sources of their progenitors in the tumor-associated stroma. Thus, molecular markers allowing identification of bona fide CAF populations with tumor-promoting traits remain under investigation. CAFs and myofibroblasts in wound healing and fibrosis share biological properties and support epithelial cell growth, not only by remodeling the extracellular matrix, but also by producing numerous growth factors and inflammatory cytokines. Notably, accumulating evidence strongly suggests that anti-fibrosis agents suppress tumor development and progression. In this review, we highlight important tumor-promoting roles of CAFs based on their analogies with wound-derived myofibroblasts and discuss the potential therapeutic strategy targeting CAFs.

Emerging roles of Myc in stem cell biology and novel tumor therapies

The pathophysiological roles and the therapeutic potentials of Myc family are reviewed in this article. The physiological functions and molecular machineries in stem cells, including embryonic stem (ES) cells and induced pluripotent stem (iPS) cells, are clearly described. The c-Myc/Max complex inhibits the ectopic differentiation of both types of artificial stem cells. Whereas c-Myc plays a fundamental role as a "double-edged sword" promoting both iPS cells generation and malignant transformation, L-Myc contributes to the nuclear reprogramming with the significant down-regulation of differentiation-associated genetic expression. Furthermore, given the therapeutic resistance of neuroendocrine tumors such as small-cell lung cancer and neuroblastoma, the roles of N-Myc in difficult-to-treat tumors are discussed. N-Myc and p53 exhibit the co-localization in the nucleus and alter p53-dependent transcriptional responses which are necessary for DNA repair, anti-apoptosis, and lipid metabolic reprogramming. NCYM protein stabilizes N-Myc, resulting in the stimulation of Oct4 expression, while Oct4 induces both N-Myc and NCYM via direct transcriptional activation of N-Myc, [corrected] thereby leading to the enhanced metastatic potential. Importantly enough, accumulating evidence strongly suggests that c-Myc can be a promising therapeutic target molecule among Myc family in terms of the biological characteristics of cancer stem-like cells (CSCs). The presence of CSCs leads to the intra-tumoral heterogeneity, which is mainly responsible for the therapeutic resistance. Mechanistically, it has been shown that Myc-induced epigenetic reprogramming enhances the CSC phenotypes. In this review article, the author describes two major therapeutic strategies of CSCs by targeting c-Myc; Firstly, Myc-dependent metabolic reprogramming is closely related to CD44 variant-dependent redox stress regulation in CSCs. It has been shown that c-Myc increases NADPH production via enhanced glutaminolysis with a finely-regulated mechanism. Secondly, the dormancy of CSCs due to FBW7-depedent c-Myc degradation pathway is also responsible for the therapeutic resistance to the conventional anti-tumor agents, the action points of which are largely dependent on the operation of the cell cycle. That is why the loss-of-functional mutations of FBW7 gene are expected to trigger "awakening" of dormant CSCs in the niche with c-Myc up-regulation. Collectively, although the further research is warranted to develop the effective anti-tumor therapeutic strategy targeting Myc family, we cancer researchers should always catch up with the current advances in the complex functions of Myc family in highly-malignant and heterogeneous tumor cells to realize the precision medicine.

Therapeutic strategies of drug repositioning targeting autophagy to induce cancer cell death: from pathophysiology to treatment

The 2016 Nobel Prize in Physiology or Medicine was awarded to the researcher that discovered autophagy, which is an evolutionally conserved catabolic process which degrades cytoplasmic constituents and organelles in the lysosome. Autophagy plays a crucial role in both normal tissue homeostasis and tumor development and is necessary for cancer cells to adapt efficiently to an unfavorable tumor microenvironment characterized by hypo-nutrient conditions. This protein degradation process leads to amino acid recycling, which provides sufficient amino acid substrates for cellular survival and proliferation. Autophagy is constitutively activated in cancer cells due to the deregulation of PI3K/Akt/mTOR signaling pathway, which enables them to adapt to hypo-nutrient microenvironment and exhibit the robust proliferation at the pre-metastatic niche. That is why just the activation of autophagy with mTOR inhibitor often fails in vain. In contrast, disturbance of autophagy–lysosome flux leads to endoplasmic reticulum (ER) stress and an unfolded protein response (UPR), which finally leads to increased apoptotic cell death in the tumor tissue. Accumulating evidence suggests that autophagy has a close relationship with programmed cell death, while uncontrolled autophagy itself often induces autophagic cell death in tumor cells. Autophagic cell death was originally defined as cell death accompanied by large-scale autophagic vacuolization of the cytoplasm. However, autophagy is a “double-edged sword” for cancer cells as it can either promote or suppress the survival and proliferation in the tumor microenvironment. Furthermore, several studies of drug re-positioning suggest that “conventional” agents used to treat diseases other than cancer can have antitumor therapeutic effects by activating/suppressing autophagy. Because of ever increasing failure rates and high cost associated with anticancer drug development, this therapeutic development strategy has attracted increasing attention because the safety profiles of these medicines are well known. Antimalarial agents such as artemisinin and disease-modifying antirheumatic drug (DMARD) are the typical examples of drug re-positioning which affect the autophagy regulation for the therapeutic use. This review article focuses on recent advances in some of the novel therapeutic strategies that target autophagy with a view to treating/preventing malignant neoplasms.

The heterogeneity of cancer stem-like cells at the invasive front

Cancer stem-like cells exhibit the multi-functional roles to survive and persist for a long period in the minimal residual disease after the conventional anti-cancer treatments. Cancer stem-like cells of solid malignant tumors which highly express CD44v8-10, the variant isoform of CD44 generated by alternative splicing, has a resistance to redox stress by the robust production of glutathione mediated by ESRP1-CD44v-xCT (cystine/glutamate antiporter) axis. It has been reported that CD44v and c-Myc tend to show the inversed expression pattern at the invasive front of the aggressive tumors. Given that the accumulation of reactive oxygen species triggers the activation of Wnt/β-catenin signal pathway, it is hypothesized that CD44v causes the negative feedback machinery in the regulation of c-Myc expression via the attenuated ROS-induced Wnt signal pathway. To address the fundamental question whether and how both proliferative and quiescent cancer stem-like cells heterogeneously exist at the invasive/metastatic edge, researchers need to investigate into the E3-ubiquitin ligase activity essential for c-Myc degradation. CSCs heterogeneity at the invasive/metastatic front is expected to demonstrate the dynamic tumor evolution with the selective pressure of anti-cancer treatments. Furthermore, the novel molecular targeting therapeutic strategies would be established to disrupt the finely-regulated c-Myc expression in the heterogeneous CSC population in combination with the typical drug-repositioning with xCT inhibitor.

Emerging role of epithelial-mesenchymal transition in hepatic cancer

Accumulating evidence suggests that the phenomenon of epithelial-mesenchymal-transition (EMT) plays a fundamental role in the tumor development. Several research articles have been published from Journal of Experimental and Clinical Cancer Research (JECCR) which have investigated into the molecular machineries underlying the importance of EMT for hepatic cancer. Given those recent publications by JECCR, this commentary focuses on the pathological significance of EMT for liver tumor.

Therapeutic strategies targeting cancer stem cells

Cancer stem cells (CSCs) are undifferentiated cancer cells with a high tumorigenic activity, the ability to undergo self-renewal, and a multilineage differentiation potential. Cancer stem cells are responsible for the development of tumor cell heterogeneity, a key feature for resistance to anticancer treatments including conventional chemotherapy, radiation therapy, and molecularly targeted therapy. Furthermore, minimal residual disease, the major cause of cancer recurrence and metastasis, is enriched in CSCs. Cancer stem cells also possess the property of "robustness", which encompasses several characteristics including a slow cell cycle, the ability to detoxify or mediate the efflux of cytotoxic agents, resistance to oxidative stress, and a rapid response to DNA damage, all of which contribute to the development of therapeutic resistance. The identification of mechanisms underlying such characteristics and the development of novel approaches to target them will be required for the therapeutic elimination of CSCs and the complete eradication of tumors. In this review, we focus on two prospective therapeutic approaches that target CSCs with the aim of disrupting their quiescence or redox defense capability.

Metabolic reprogramming: the emerging concept and associated therapeutic strategies

Tumor tissue is composed of cancer cells and surrounding stromal cells with diverse genetic/epigenetic backgrounds, a situation known as intra-tumoral heterogeneity. Cancer cells are surrounded by a totally different microenvironment than that of normal cells; consequently, tumor cells must exhibit rapidly adaptive responses to hypoxia and hypo-nutrient conditions. This phenomenon of changes of tumor cellular bioenergetics, called “metabolic reprogramming”, has been recognized as one of 10 hallmarks of cancer. Metabolic reprogramming is required for both malignant transformation and tumor development, including invasion and metastasis. Although the Warburg effect has been widely accepted as a common feature of metabolic reprogramming, accumulating evidence has revealed that tumor cells depend on mitochondrial metabolism as well as aerobic glycolysis. Remarkably, cancer-associated fibroblasts in tumor stroma tend to activate both glycolysis and autophagy in contrast to neighboring cancer cells, which leads to a reverse Warburg effect. Heterogeneity of monocarboxylate transporter expression reflects cellular metabolic heterogeneity with respect to the production and uptake of lactate. In tumor tissue, metabolic heterogeneity induces metabolic symbiosis, which is responsible for adaptation to drastic changes in the nutrient microenvironment resulting from chemotherapy. In addition, metabolic heterogeneity is responsible for the failure to induce the same therapeutic effect against cancer cells as a whole. In particular, cancer stem cells exhibit several biological features responsible for resistance to conventional anti-tumor therapies. Consequently, cancer stem cells tend to form minimal residual disease after chemotherapy and exhibit metastatic potential with additional metabolic reprogramming. This type of altered metabolic reprogramming leads to adaptive/acquired resistance to anti-tumor therapy. Collectively, complex and dynamic metabolic reprogramming should be regarded as a reflection of the “robustness” of tumor cells against unfavorable conditions. This review focuses on the concept of metabolic reprogramming in heterogeneous tumor tissue, and further emphasizes the importance of developing novel therapeutic strategies based on drug repositioning.

Dynamic subcellular localization of aquaporin-7 in white adipocytes

Aquaporin-7 (AQP7) is expressed in adipose tissue, permeated by water and glycerol, and is involved in lipid metabolism. AQP7-null mice develop obesity, insulin resistance, and adipocyte hypertrophy. Here, we show that AQP7 is expressed in adipocyte plasma membranes, and is re-localized to intracellular membranes in response to catecholamine in mouse white adipose tissue. We found that internalization of AQP7 was induced by PKA activation and comparative gene identification 58 (CGI-58). This relocation was confirmed by functional studies in 3T3-L1 adipocytes. Collectively, these results suggest that AQP7 makes several contributions to adipocyte metabolism, in both cortical and intracellular membranes.

Abstract B06: EpCAM functions as a sensor that allows prostate cancer cells to become quiescent in response to lack of growth factors

Introduction: Recent studies have shown that intra-tumor heterogeneity is the main cause of therapeutic resistance in cancer. Tumor tissue is composed of heterogeneous cellular populations with a hierarchical organization governed by stem/progenitor-like cells, also referred to as cancer stem-like cells (CSCs). Epithelial cell adhesion molecule (EpCAM), also known as CD326, is over-expressed ubiquitously in CSCs that originate from epithelial lesions. Prostate cancer stem-like cells highly express EpCAM. However, little is known about how EpCAM is involved in the ability of cells to adapt to micro-environmental changes in available growth factors, which is one of the essential biological phenotypes of CSCs.

Methods: EpCAM-high and EpCAM-low subpopulations of cells were established from the prostate cancer cell line PC-3. Signal transduction in response to serum starvation, signaling pathways essential for tumorigenic potential in vivo, and the machinery of serum depletion-induced dormancy and plasticity were analyzed by immunoblot, quantitative RT-PCR, flow cytometry, and immunohistochemical staining.

Results: EpCAM-high and EpCAM-low PC-3 subpopulations showed markedly different responses to serum starvation. EpCAM expression was positively correlated with activation of the mammalian target of rapamycin (mTOR) and epithelial growth factor receptor (EGFR) signaling pathways. Contrary to accepted wisdom, AMP-activated protein kinase (AMPK) was gradually de-activated in EpCAM-low PC-3 cells in the absence of serum. In addition, the Spearman's rank correlation between EpCAM and LAT1 in parental, EpCAM-high, and EpCAM-low PC-3 cells was very high (0.859), which implied that the expression of EpCAM correlates with the potential of leucine uptake. Both EpCAM-high and EpCAM-low PC-3 cells exhibited tumorigenic potential, which depended on mTOR and canonical Wnt signaling, respectively. Finally, serum starvation promoted cell cycle arrest and the acquisition of a quiescent phenotype. It has been well established that there are two major mechanisms that promote G0 entry; the SCF (Fbw7)-c-Myc axis and the Skp2-p27 axis. EpCAM-high PC-3 cancer cells entered the G0 phase via the SCF (Fbw7)-c-Myc axis, while EpCAM-low cells did so via the Skp2-p27 axis. EGF-induced plasticity in EpCAM-high PC-3 cells but not in EpCAM-low PC-3 cells implies that EGFR signaling is likely to be involved in cell cycle re-entry in EpCAM-high PC-3 cells.

Discussion: EpCAM regulates the AMPK signaling pathway, which is essential for the response to growth factors, characterized by EGF, in the tumor microenvironment. Furthermore, EpCAM enhances the ability of cells to enter the G0 phase under long-term serum starvation and promotes cell cycle re-entry from this dormancy upon exposure to growth factors. Taken together, these data suggest that “stemness,” including sensitivity to growth factors and plasticity between dormancy and proliferation, is maintained and regulated by EpCAM.

Significance: The phenotypes induced by EpCAM expression are mainly responsible for the persistence of minimal residual disease (MRD) and the latent relapse of prostate cancers. Therefore, EpCAM may be a promising therapeutic target to block the ability of prostate cancer CSCs to adapt to the lack of growth factors in the tumor microenvironment.

References:

EpCAM expression in the prostate cancer makes the difference in the response to growth factors (Biochemical and Biophysical Research Communications, 2013, in press)

EpCAM functions as a sensor that allows prostate cancer cells to become quiescent in response to lack of growth factors (In preparation)

Citation Format: Go J. Yoshida, Hideyuki Saya. EpCAM functions as a sensor that allows prostate cancer cells to become quiescent in response to lack of growth factors.. [abstract]. In: Abstracts: AACR Special Conference on Cellular Heterogeneity in the Tumor Microenvironment; 2014 Feb 26-Mar 1; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2015;75(1 Suppl):Abstract nr B06. doi:10.1158/1538-7445.CHTME14-B06

Abstract 4967: ESRP1 regulated alternative splicing of CD44mRNA enhances lung colonization of metastatic cancer cell

Cancer metastasis is established by the seeding and successful colonization of stem-like cancer cells which show tumor-initiating ability at distant organs. CD44 is a cell surface marker protein which is highly expressed in stem-like cells of several human cancers including breast cancer. CD44 exists in various isoforms produced through alternative splicing of mRNA. Increased expression of CD44 variant isoform (CD44v) has been shown to correlate with metastatic potential and aggressive clinical behavior of various human cancers. We recently showed that CD44v contributes to reactive oxygen species (ROS)-defense in cancer cells by enhancing cystine uptake by the cystine transporter xCT. However, the functional relevance of CD44v and xCT in the metastatic spread of stem-like cancer cells has remained elusive.

In the present study, we show that metastatic breast cancer 4T1 cells consist of CD44v+ and CD44v- cells. The orthotopic transplantation of CD44v+ population but not that of CD44v-, in mice resulted in efficient lung metastasis. We found that CD44v expressing metastatic cancer cells highly express xCT and thereby maintain high reduced glutathione (GSH) level. A specific xCT inhibitor sulfasalazine suppressed lung metastatsis of CD44v+ cells. Alternative splicing of CD44mRNA in such metastatic cancer cells is regulated by Epithelial Splicing Related Protein1 (ESRP1) and short hairpin RNA (shRNA) targeting ESRP1 switched isoform from CD44v to CD44 standard (CD44s), leading to the suppression of lung metastasis. Furthermore, we found that ESRP1 expression is regulated epigenetically through histone modification by performing chromatin immunoprecipitation (ChIP) sequencing analysis of the ESRP1 locus in CD44v+ and CD44v- cells.

These findings establish a novel role of ESRP1-regulated alternative splicing program in the regulation of antioxidant status of stem-like cancer cells which is crucial for the metastatic ability.

Citation Format: Kenji Tsuchihashi, Osamu Nagano, Toshifumi Yae, Takatsugu Ishimoto, Takeshi Motohara, Momoko Yoshikawa, Go J Yoshida, Takeyuki Wada, Takashi Masuko, Kaoru Mogushi, Hiroshi Tanaka, Tsuyoshi Osawa, Yasuharu Kanki, Takashi Minami, Hiroyuki Aburatani, Mitsuyo Ohmura, Akiko Kubo, Makoto Suematsu, Kazuhisa Takahashi, Eishi Baba, Koichi Akashi, Hideyuki Saya. ESRP1 regulated alternative splicing of CD44mRNA enhances lung colonization of metastatic cancer cell. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4967. doi:10.1158/1538-7445.AM2014-4967

EpCAM expression in the prostate cancer makes the difference in the response to growth factors

INTRODUCTION

Epithelial cell adhesion molecule (EpCAM) is expressed in tumors with an epithelial cell of origin, in a heterogeneous manner. Prostate cancer stem-like cells highly express EpCAM. However, little is known about how EpCAM is involved in the ability of cells to adapt to micro-environmental changes in available growth factors, which is one of the essential biological phenotypes of cancer stem-like cells (CSCs).

METHODS

EpCAM-high and EpCAM-low subpopulations of cells were established from the prostate cancer cell line PC-3. Signal transductions in response to serum starvation, and on the exposure to EGF ligand or the specific inhibitor were analyzed in terms. Furthermore, we analyzed the expression level of amino acid transporters which contribute to the activation of mTOR signal between the two subgroups.

RESULTS

EpCAM-high and EpCAM-low PC-3 subpopulations showed markedly different responses to serum starvation. EpCAM expression was positively correlated with activation of the mTOR and epithelial growth factor receptor (EGFR) signaling pathways. Furthermore, AMP-activated protein kinase (AMPK) was gradually de-activated in EpCAM-low PC-3 cells in the absence of serum.

CONCLUSIONS

EpCAM regulates the AMPK signaling pathway, essential for the response to growth factors characterized by EGF. LAT1, the amino acid transporter stabilized at the cellular membrane by EpCAM, is likely to be responsible for the difference in the susceptibility to EGF between EpCAM-high and EpCAM-low PC-3 cells.

Three-dimensional culture of sebaceous gland cells revealing the role of prostaglandin E2-induced activation of canonical Wnt signaling

BACKGROUND

Prostaglandin E2 (PGE2) is a proinflammatory mediator and activates the canonical Wnt-β-catenin signaling pathway in hematopoietic stem cells. The SZ95 cell line was established from human sebaceous gland cells and is studied as a model system for these cells. Given that 2D culture of SZ95 cells does not recapitulate the organization of sebaceous glands in situ, we developed a 3D culture system for these cells and examined the effects of PGE2 on cell morphology and function.

RESULTS

SZ95 cells maintained in 3D culture formed organoids that mimicked the organization of sebaceous glands in situ, including the establishment of a basement membrane. Organoids exposed to PGE2 were larger and adopted a more complex organization compared with control organoids. PGE2 activated the canonical Wnt signaling pathway as well as increased cell viability and proliferation, mitochondrial metabolism, and lipid synthesis in the organoids.

CONCLUSIONS

Culture of SZ95 cells in 3D culture system recapitulates the structure and susceptibility to PGE2 of sebaceous glands in situ and should prove useful for studies of the response of these glands to inflammation and other environmental stressors. Our results also implicate PGE2-induced activation of canonical Wnt signaling pathway in regulation of the morphology,proliferation, and function of "semi-vivo" sebaceous glands.

Abstract C89: ESRP1-CD44v-xCT axis as a potential therapeutic target for the metastasis

Metastasis of tumor cells is a multi-step phenomenon including dissociation from primary site, intravasation, long-term circulation in the blood, extravasation, and colonization in the pre-metastatic niche in the distant organs. It is quite important that cancer cells are always exposed to redox stress throughout this muti-step metastatic phenomenon. This is why metastasis occurs only when cancer cells are refractory to the high level of reactive oxygen species (ROS) in the tumor microenvironment.

CD44, one of the cancer stem cell (CSC) markers, makes diversity in isofroms due to the alternative splicing via epithelial splicing regulatory protein1 (ESRP1) or transcription factor grainyhead-like 2 (Grhl2). In the clinical perspective, a high expression level of ESRP1 or Grhl-2 in the tumor cells is generally associated with a poor prognosis in breast cancer patients. In addition, epigenetic modifications including DNA and histone methylation at metastasis-related genes have been also considered as a cause of cancer metastasis. This is why the depletion of CD44variant isoform (CD44v) is not necessarily accompanied by epithelial-mesenchymal transition (EMT), given that EMT has widely been recognized to expand the CSC population.

Besides, CD44 is not only a surface marker of CSCs but also a functional molecule; for instance, the interaction between CD44v6 and c-Met, the receptor of hepatocyte growth factor (HGF), activates the Ras-dependent survival signal of tumor cells. We have recently identified that CD44 variant 8-10 (CD44v9) stabilizes the localization of xCT glutamate-cystine transporter at the cellular membrane, thereby increasing the intracellular level of reduced glutathione (GSH). In fact, enrichment of CD44v9-positive tumor cells is frequently recognized in the metastatic foci and recalcitrant cases.

The expression level of CD44v9 is positively correlated with the Akt/PI3K/mTOR signal activity of metastatic breast tumor cells. Akt/PI3K/mTOR signal is related to cholesterol synthesis and to adaptation to the hypo-nutrition, or hypoxia. In the case of metastasis of breast cancer cells to the lungs, tumor-entrained neutrophils (TEN) accumulate in the lung to inhibit lung metastasis through the increased redox stress in the NADPH-dependent manner. Thus, the presence of xCT transporter promotes the GSH synthesis and the protection from tumor stroma-derived ROS, which plays a fundamental role in the adaptive resistance to conventional cancer therapy for the metastasis.

We have also revealed that ESRP1-dependent alternative splicing of CD44 mRNA increases both the intracellular GSH level and the metastatic potential of the breast cancer cells. We have demonstrated that depletion of ESRP1 or Grhl2 in metastatic breast cancer cells adversely affects the metastatic potential, since they fail to express CD44v9. Notably, even the tumor cells which are lack of CD44v9 are able to form the metastasis to lungs, but the ability of survival and proliferation in the microenvironment rich in redox stress are considerably lower. Akt/PI3K/mTOR activity is significantly suppressed compared with the aggressive metastatic foci mainly composed of CD44v9-positive tumor cells.

Therapies targeted to alternative splicing of CD44 as well as those that target CD44v-mediated stabilization of xCT might be expected to imapir the adaptive resistance to ROS in the metastatic stem-like cancer cells and thereby rendering them susceptible to currently available cancer treatments. Considering the high dependency to lipid synthesis In the survival and proliferation of metastatic breast cancer cells, HMG-CoA reductase inhibitors contribute to the synergistic inhibition of the metastatic exacerbation.

Citation Format: Go J. Yoshida, Kenji Tsuchihashi, Toshifumi Yae, Osamu Nagano, Hideyuki Saya. ESRP1-CD44v-xCT axis as a potential therapeutic target for the metastasis. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr C89.

Li-Fraumeni syndrome with simultaneous osteosarcoma and liver cancer: increased expression of a CD44 variant isoform after chemotherapy

Background

Li-Fraumeni syndrome (LFS) is a hereditary cancer predisposition syndrome that is commonly associated with a germline mutation in the tumor suppressor gene p53. Loss of p53 results in increased expression of CD44, a cancer stem cell (CSC) marker, which is involved in the scavenging of reactive oxygen species (ROS). Here, we report a change in the expression of a CD44 variant isoform (CD44v8-10) in an 8-year-old female LFS patient with osteosarcoma and atypical liver cancer after chemotherapy.

Case presentation

The patient visited a clinic with a chief complaint of chronic pain in a bruise on her right knee. Magnetic resonance imaging (MRI) raised the possibility of a bone malignancy. Biochemical testing also revealed significantly elevated levels of AFP, which strongly suggested the existence of a primary malignancy in the liver. MRI imaging showed the simultaneous development of osteosarcoma and liver cancer, both of which were confirmed upon biopsy. Combined therapy with surgical resection after chemotherapy was successful in this patient. Regardless of the absence of a familial history of hereditary cancer, a germline mutation in p53 was identified (a missense mutation defined as c.722 C>T, p.Ser241Phe). To better understand the cancer progression and response to treatment, immunohistochemical (IHC) analysis of biopsy specimens obtained before and after chemotherapy was performed using a specific antibody against CD44v8-10.

Conclusion

This case demonstrates the ectopic up-regulation of CD44v8-10 in a biopsy sample obtained after cytotoxic chemotherapy, which confers high levels of oxidative stress on cancer cells. Because the alternative splicing of CD44 is tightly regulated epigenetically, it is possible that micro-environmental stress resulting from chemotherapy caused the ectopic induction of CD44v8-10 in vivo.

Alternative splicing of CD44 mRNA by ESRP1 enhances lung colonization of metastatic cancer cell

In cancer metastasis, various environmental stressors attack the disseminating cells. The successful colonization of cancer cells in secondary sites therefore requires the ability of the cells to avoid the consequences of such exposure to the stressors. Here we show that orthotopic transplantation of a CD44 variant isoform-expressing (CD44v(+)) subpopulation of 4T1 breast cancer cells, but not that of a CD44v(-) subpopulation, in mice results in efficient lung metastasis accompanied by expansion of stem-like cancer cells. Such metastasis is dependent on the activity of the cystine transporter xCT, and the stability of this protein is controlled by CD44v. We find that epithelial splicing regulatory protein 1 regulates the expression of CD44v, and knockdown of epithelial splicing regulatory protein 1 in CD44v(+) cells results in an isoform switch from CD44v to CD44 standard (CD44s), leading to reduced cell surface expression of xCT and suppression of lung colonization. The epithelial splicing regulatory protein 1-CD44v-xCT axis is thus a potential therapeutic target for the prevention of metastasis.

[Encounter of cancer cells with bone. The significance of cancer stem cells and epithelial-mesenchymal transition in tumor invasion and metastasis]

Cancer stem cells (CSCs) , which are subset of tumor cells resistant to radiation and chemotherapy, are associated with malignant characteristics of tumor and possess both self-renewal ability and pluripotency for tumor formation. In the process of generating non-CSCs from CSCs, gene mutations and epigenetic changes are induced in those cells, resulting in composition of tumor tissue with heterogeneous cell population. CSCs have been recognized as the source of metastatic foci. Epithelial-mesenchymal transition (EMT) is a change in cellular phenotype characterized by the loss of cell-to-cell adhesions and the gain of migratory behaviors,which has been shown to be a critical factor for initiating cancer invasion and metastasis. However, some recent studies suggest that EMT is not essential requirement for tumor invasion and metastasis. Herein, we discuss the biological significance of CSCs and EMT in tumor invasion and metastasis.