Differentiated human colorectal cancer cells protect tumor-initiating cells from irinotecan

Decreased camptothecin sensitivity of the stem-cell-like fraction of Caco2 cells correlates with an altered phosphorylation pattern of topoisomerase I

The CD44+ and CD44− subpopulations of the colorectal cancer cell line Caco2 were analyzed separately for their sensitivities to the antitumor drug camptothecin. CD44+ cells were less sensitive to camptothecin than CD44− cells. The relative resistance of CD44+ cells was correlated with (i) reduced activity of the nuclear enzyme topoisomerase I and (ii) insensitivity of this enzyme to camptothecin when analyzed in extracts. In contrast, topoisomerase I activity was higher in extracts from CD44− cells and the enzyme was camptothecin sensitive. Topoisomerase I from the two subpopulations were differentially phosphorylated in a manner that appeared to determine the drug sensitivity and activity of the enzyme. This finding was further supported by the fact that phosphorylation of topoisomerase I in CD44+ cell extract by protein kinase CK2 converted the enzyme to a camptothecin sensitive, more active form mimicking topoisomerase I in extracts from CD44− cells. Conversely, dephosphorylation of topoisomerase I in extracts from CD44− cells rendered the enzyme less active and camptothecin resistant. These findings add to our understanding of chemotherapy resistance in the Caco2 CD44+ cancer stem cell model.

MET signaling in colon cancer stem-like cells blunts the therapeutic response to EGFR inhibitors

Metastatic colorectal cancer remains largely incurable, although in a subset of patients, survival is prolonged by new targeting agents such as anti-EGF receptor (anti-EGFR) antibodies. This disease is believed to be supported by a subpopulation of stem-like cells termed colon cancer-initiating cell (CCIC), which may also confer therapeutic resistance. However, how CCICs respond to EGFR inhibition has not been fully characterized. To explore this question, we systematically generated CCICs through spheroid cultures of patient-derived xenografts of metastatic colorectal cancer. These cultures, termed "xenospheres," were capable of long-term self-propagation in vitro and phenocopied the original patient tumors in vivo, thus operationally defining CCICs. Xenosphere CCICs retained the genetic determinants for EGFR therapeutic response in vitro and in xenografts; like the original tumors, xenospheres harboring a mutated KRAS gene were resistant to EGFR therapy, whereas those harboring wild-type RAS pathway genes (RAS(wt)) were sensitive. Notably, the effects of EGFR inhibition in sensitive CCICs could be counteracted by cytokines secreted by cancer-associated fibroblasts. In particular, we found that the MET receptor ligand hepatocyte growth factor (HGF) was especially active in supporting in vitro CCIC proliferation and resistance to EGFR inhibition. Ectopic production of human HGF in CCIC xenografts rendered the xenografts susceptible to MET inhibition, which sensitized the response to EGFR therapy. By showing that RAS(wt) CCICs rely on both EGFR and MET signaling, our results offer a strong preclinical proof-of-concept for concurrent targeting of these two pathways in the clinical setting.

Organoid cultures for the analysis of cancer phenotypes

Preclinical models of cancer are essential for a basic understanding of cancer biology and its translation into efficient treatment options for affected patients. Cancer cell lines and xenografts derived directly from primary human tumors have proven very valuable in fundamental oncology research and anticancer drug discovery. Both models inherently comprise advantages and caveats that have to be accounted for. We will outline in these and discuss primary patient derived organoids as third preclinical cancer model. We propose that cancer organoids could potentially fill the gap between simple cancer cell lines suitable for high-throughput screens and complicated, but physiologically relevant xenografts. The resulting applications for cancer organoids range from basic research to drug screens and patient stratification.

Can nanomedicines kill cancer stem cells?

Most tumors are heterogeneous and many cancers contain small population of highly tumorigenic and intrinsically drug resistant cancer stem cells (CSCs). Like normal stem cell, CSCs have the ability to self-renew and differentiate to other tumor cell types. They are believed to be a source for drug resistance, tumor recurrence and metastasis. CSCs often overexpress drug efflux transporters, spend most of their time in non-dividing G0 cell cycle state, and therefore, can escape the conventional chemotherapies. Thus, targeting CSCs is essential for developing novel therapies to prevent cancer relapse and emerging of drug resistance. Nanocarrier-based therapeutic agents (nanomedicines) have been used to achieve longer circulation times, better stability and bioavailability over current therapeutics. Recently, some groups have successfully applied nanomedicines to target CSCs to eliminate the tumor and prevent its recurrence. These approaches include 1) delivery of therapeutic agents (small molecules, siRNA, antibodies) that affect embryonic signaling pathways implicated in self-renewal and differentiation in CSCs, 2) inhibiting drug efflux transporters in an attempt to sensitize CSCs to therapy, 3) targeting metabolism in CSCs through nanoformulated chemicals and field-responsive magnetic nanoparticles and carbon nanotubes, and 4) disruption of multiple pathways in drug resistant cells using combination of chemotherapeutic drugs with amphiphilic Pluronic block copolymers. Despite clear progress of these studies the challenges of targeting CSCs by nanomedicines still exist and leave plenty of room for improvement and development. This review summarizes biological processes that are related to CSCs, overviews the current state of anti-CSCs therapies, and discusses state-of-the-art nanomedicine approaches developed to kill CSCs.

LGR5-positive colon cancer stem cells interconvert with drug-resistant LGR5-negative cells and are capable of tumor reconstitution

The cancer stem cell (CSC) concept has been proposed as an attractive theory to explain cancer development, and CSCs themselves have been considered as targets for the development of diagnostics and therapeutics. However, many unanswered questions concerning the existence of slow cycling/quiescent, drug-resistant CSCs remain. Here we report the establishment of colon cancer CSC lines, interconversion of the CSCs between a proliferating and a drug-resistant state, and reconstitution of tumor hierarchy from the CSCs. Stable cell lines having CSC properties were established from human colon cancer after serial passages in NOD/Shi-scid, IL-2Rγ(null) (NOG) mice and subsequent adherent cell culture of these tumors. By generating specific antibodies against LGR5, we demonstrated that these cells expressed LGR5 and underwent self-renewal using symmetrical divisions. Upon exposure to irinotecan, the LGR5(+) cells transitioned into an LGR5(-) drug-resistant state. The LGR5(-) cells converted to an LGR5(+) state in the absence of the drug. DNA microarray analysis and immunohistochemistry demonstrated that HLA-DMA was specifically expressed in drug-resistant LGR5(-) cells, and epiregulin was expressed in both LGR5(+) and drug-resistant LGR5(-) cells. Both cells sustained tumor initiating activity in NOG mice, giving rise to a tumor tissue hierarchy. In addition, anti-epiregulin antibody was found to be efficacious in a metastatic model. Both LGR5(+) and LGR5(-) cells were detected in the tumor tissues of colon cancer patients. The results provide new biological insights into drug resistance of CSCs and new therapeutic options for cancer treatment.

Stem cell differentiation and lumen formation in colorectal cancer cell lines and primary tumors

Single cancer stem-like cells (CSC) from colorectal cancers can be functionally identified by their ability to form large lumen-containing colonies in three-dimensional Matrigel cultures. These colonies contain the three types of differentiated colorectal epithelial cells, and single cells obtained from them can reproduce themselves and form tumors efficiently in immunodeficient mice. In this study, we show how hypoxia affects these CSC-derived lumens to control differentiation of stem-like cells and enterocytes via the homeobox gene CDX1. Lumens were identified by F-actin staining and they expressed many characteristics associated with normal differentiated intestinal epithelium, including brush border enzymes, polarization, and tight junctions. RNA interference-mediated silencing of CDX1 reduced lumen formation. Inhibitory effects of hypoxia on lumen formation and stem cell differentiation, including suppression of CDX1 expression, could be mimicked by inhibiting prolyl-hydroxylases that activate HIF1, suggesting that HIF1 is a critical mediator of the effects of hypoxia in this setting. Cell line-derived lumens were phenotypically indistinguishable from colorectal tumor glandular structures used by pathologists to grade tumor differentiation. Parallel results to those obtained with established cell lines were seen with primary cultures from fresh tumors. This in vitro approach to functional characterization of CSCs and their differentiation offers a valid model to study colorectal tumor differentiation and differentiation of colorectal CSCs, with additional uses to enable high-throughput screening for novel anticancer compounds.

The secretome of colon cancer stem cells contains drug-metabolizing enzymes

Drug-resistant cancer stem cells (CSCs) have been implicated in tumor recurrence following chemotherapy. However, the contribution of CSCs to drug-resistance in colorectal cancer is unclear and CSC-intrinsic drug-resistance mechanisms are ill-defined. Here, we address these issues by proteomic analysis of the secretomes of CSCs and isogenic differentiated tumor cells (DTCs) isolated from three distinct metastasized colon tumors. Mass spectrometry-based proteomics identified 1254 unique proteins in the conditioned media of the paired CSC and DTC cultures. Ingenuity Pathway Analysis revealed that proteins governing 'Cell Death' were most significantly enriched in the CSC secretome. The vast majority of these (37/43) promote cell survival. The CSC secretome is also characterized by a pro-survival Nrf2 antioxidant signature. Interestingly, proteome-maintenance networks are highly enriched in the CSC secretome. CSCs also secrete high levels of drug-metabolizing enzymes, including aldehyde dehydrogenase 1 (ALDH1A1) and bleomycin hydrolase (BLMH). We show that these enzymes cause extracellular detoxification of maphosphamide and bleomycin respectively. We conclude that colorectal CSCs are characterized by extensive survival and anti-oxidant networks, which are likely to contribute to CSC-intrinsic drug-resistance. In addition, CSCs may modulate drug responses in nearby tumor cells by detoxifying chemotherapeutic drugs in the extracellular space.

BIOLOGICAL SIGNIFICANCE

Cancer stem cells are thought to play an important role in mediating drug resistance and tumor recurrence following chemotherapy. Therefore, it is important to identify the factors that are secreted by them. Our results provide novel insights into the pathways that govern the intrinsic resistance of CSCs to chemotherapy and, furthermore, demonstrate that they can also inactivate chemotherapeutic drugs in the extracellular space. A better understanding of the pathways that govern drug resistance in CSCs may help in developing effective CSC-targeting drugs.

A microRNA miR-34a-regulated bimodal switch targets Notch in colon cancer stem cells

microRNAs regulate developmental cell-fate decisions, tissue homeostasis, and oncogenesis in distinct ways relative to proteins. Here, we show that the tumor suppressor microRNA miR-34a is a cell-fate determinant in early-stage dividing colon cancer stem cells (CCSCs). In pair-cell assays, miR-34a distributes at high levels in differentiating progeny, whereas low levels of miR-34a demarcate self-renewing CCSCs. Moreover, miR-34a loss of function and gain of function alter the balance between self-renewal versus differentiation both in vitro and in vivo. Mechanistically, miR-34a sequesters Notch1 mRNA to generate a sharp threshold response where a bimodal Notch signal specifies the choice between self-renewal and differentiation. In contrast, the canonical cell-fate determinant Numb regulates Notch levels in a continuously graded manner. Altogether, our findings highlight a unique microRNA-regulated mechanism that converts noisy input into a toggle switch for robust cell-fate decisions in CCSCs.

A short-term colorectal cancer sphere culture as a relevant tool for human cancer biology investigation

BACKGROUND

Ex vivo colospheres have been previously characterised as a colorectal cancer (CRC) well-rounded multicellular model, exclusively formed by carcinoma cells, and derived from fresh CRC tissue after mechanical dissociation. The ability to form colospheres was correlated with tumour aggressiveness. Their three-dimensional conformation prompted us to further investigate their potential interest as a preclinical cancer tool.

METHODS

Patient-derived CRC xenografts were used to produce numerous colospheres. Mechanism of formation was elucidated by confocal microscopy. Expression analysis of a panel of 64 selected cancer-related genes by real-time qRT-PCR and hierarchical clustering allowed comparison of colospheres with parent xenografts. In vitro and in vivo assays were performed for migration and chemosensitivity studies.

RESULTS

Colospheres, formed by tissue remodelling and compaction, remained viable several weeks in floating conditions, escaping anoikis through their strong cell-cell interactions. Colospheres matched the gene expression profile of the parent xenograft tissue. Colosphere-forming cells migrated in collagen I matrix and metastasised when subrenally implanted in nude mice. Besides, the colosphere responses to 5-fluorouracil and irinotecan, two standard drugs in CRC, reproduced those of the in vivo original xenografts.

CONCLUSION

Colospheres closely mimic biological characteristics of in vivo CRC tumours. Consequently, they would be relevant ex vivo CRC models.

Proteomics in studying cancer stem cell biology

Normal multipotent tissue stem cells (SCs) are the driving force behind tissue turnover and repair. The cancer stem cell theory holds that tumors also contain stem-like cells that drive tumor growth and metastasis formation. However, very little is known about the regulation of SC maintenance pathways in cancer and how these are affected by cancer-specific genetic alterations and by treatment. Proteomics is emerging as a powerful tool to identify the signaling complexes and pathways that control multi- and pluri-potency and SC differentiation. Here, the authors review the novel insights that these studies have provided and present a comprehensive strategy for the use of proteomics in studying cancer SC biology.

Chemokine 25-induced signaling suppresses colon cancer invasion and metastasis

Chemotactic cytokines (chemokines) can help regulate tumor cell invasion and metastasis. Here, we show that chemokine 25 (CCL25) and its cognate receptor chemokine receptor 9 (CCR9) inhibit colorectal cancer (CRC) invasion and metastasis. We found that CCR9 protein expression levels were highest in colon adenomas and progressively decreased in invasive and metastatic CRCs. CCR9 was expressed in both primary tumor cell cultures and colon-cancer-initiating cell (CCIC) lines derived from early-stage CRCs but not from metastatic CRC. CCL25 stimulated cell proliferation by activating AKT signaling. In vivo, systemically injected CCR9+ early-stage CCICs led to the formation of orthotopic gastrointestinal xenograft tumors. Blocking CCR9 signaling inhibited CRC tumor formation in the native gastrointestinal CCL25+ microenvironment, while increasing extraintestinal tumor incidence. NOTCH signaling, which promotes CRC metastasis, increased extraintestinal tumor frequency by stimulating CCR9 proteasomal degradation. Overall, these data indicate that CCL25 and CCR9 regulate CRC progression and invasion and further demonstrate an appropriate in vivo experimental system to study CRC progression in the native colon microenvironment.

Lack of ABCG2 shortens latency of BRCA1-deficient mammary tumors and this is not affected by genistein or resveratrol

In addition to their role in drug resistance, the ATP-binding cassette (ABC) transporters ABCG2 and ABCB1 have been suggested to protect cells from a broad range of substances that may foster tumorigenesis. Phytoestrogens or their metabolites are substrates of these transporters and the influence of these compounds on breast cancer development is controversial. Estrogen-like properties might accelerate tumorigenesis on the one hand, whereas their proposed health-protective properties might antagonize tumorigenesis on the other. To address this issue, we used a newer generation mouse model of BRCA1-mutated breast cancer and examined tumor latency in K14cre;Brca1(F/F); p53(F/F), Abcb1a/b(-/-);K14cre;Brca1(F/F); p53(F/F), or Abcg2(-/-);K14cre;Brca1(F/F); p53(F/F) animals, fed with genistein- or resveratrol-supplemented diets. Ovariectomized K14cre;Brca1(F/F); p53(F/F) animals were included to evaluate whether any estrogen-mimicking effects can restore mammary tumor development in the absence of endogenous estrogens. Compared with the ABC transporter proficient model, ABCG2-deficient animals showed a reduced median tumor latency of 17.5 days (P < 0.001), whereas no significant difference was observed for ABCB1-deficient animals. Neither genistein nor resveratrol altered this latency reduction in Abcg2(-/-);K14cre;Brca1(F/F); p53(F/F) animals. Ovariectomy resulted in nearly complete loss of mammary tumor development, which was not restored by genistein or resveratrol. Our results show that ABCG2 contributes to the protection of genetically instable epithelial cells against carcinogenesis. Diets containing high levels of genistein or resveratrol had no effect on mammary tumorigenesis, whether mice were lacking ABCG2 or not. Because genistein and resveratrol only delayed skin tumor development of ovariectomized animals, we conclude that these phytoestrogens are no effective modulators of mammary tumor development in our mouse model.

MicroRNA-451 is involved in the self-renewal, tumorigenicity, and chemoresistance of colorectal cancer stem cells

Many antitumor therapies affect rapidly dividing cells. However, tumor proliferation may be driven by cancer stem cells (CSCs), which divide slowly and are relatively resistant to cytotoxic drugs. Thus, many tumors may progress because CSCs are not sensitive to the treatment. In this work, we searched for target genes whose expression is involved in proliferation and chemoresistance of CSCs. Both of these processes could be controlled simultaneously by cell regulators such as microRNAs (miRNAs). Therefore, colonospheres with properties of CSCs were obtained from different colon carcinoma cells, and miRNA profiling was performed. The results showed that miR-451 was downregulated in colonspheres versus parental cells. Surprisingly, expression of miR-451 caused a decrease in self-renewal, tumorigenicity, and chemoresistance to irinotecan of colonspheres. We identified cyclooxygenase-2 (COX-2) as an indirect miR-451 target gene involved in sphere growth. Our results indicate that miR-451 downregulation allows the expression of the direct target gene macrophage migration inhibitory factor, involved in the expression of COX-2. In turn, COX-2 allows Wnt activation, which is essential for CSC growth. Furthermore, miR-451 restoration decreases expression of the ATP-binding cassette drug transporter ABCB1 and results in irinotecan sensitization. These findings correlate well with the lower expression of miR-451 observed in patients who did not respond to irinotecan-based first-line therapy compared with patients who did. Our data suggest that miR-451 is a novel candidate to circumvent recurrence and drug resistance in colorectal cancer and could be used as a marker to predict response to irinotecan in patients with colon carcinoma.

Cancer drug pan-resistance: pumps, cancer stem cells, quiescence, epithelial to mesenchymal transition, blocked cell death pathways, persisters or what?

Although chemotherapy of tumours has scored successes, drug resistance remains the major cause of death of cancer patients. Initial treatment often leaves residual disease, from which the tumour regrows. Eventually, most tumours become resistant to all available chemotherapy. I call this pan-resistance to distinguish it from multi-drug resistance, usually describing resistance caused by upregulation of drug transporters, such as P-glycoprotein. In this review, I discuss mechanisms proposed to explain both residual disease and pan-resistance. Although plausible explanations are at hand for residual disease, pan-resistance is still a mystery. My conclusion is that it is time for a major effort to solve this mystery using the new genetically modified mouse tumour models that produce real tumours resembling cancer in human patients.

Regulation of intestinal cancer stem cells

Colorectal tumours harbour a sub-population of cells with stem like properties termed 'cancer stem cells', which are believed to ultimately drive cancer growth. This review discusses recent advances in our understanding of both normal and cancer intestinal stem cells, with emphasis on similarities and differences. Specifically we discuss the role of the Wnt, Notch and BMP pathways and their roles in both stem cell proliferation and differentiation. Furthermore we discuss the emerging role of microRNA and the influence of environmental factors such as tumour associated myofibroblasts and hypoxia on cancer stem cell regulation.

ALDH+ tumor-initiating cells exhibiting gain in NOTCH1 gene copy number have enhanced regrowth sensitivity to a γ-secretase inhibitor and irinotecan in colorectal cancer

The Notch signaling pathway has been shown to be upregulated in colorectal cancer (CRC) and important for the self-renewal of cancer stem cells. In this study, we evaluated the efficacy of PF-03084014, a γ-secretase inhibitor, in combination with irinotecan to identify the effects of treatment on tumor recurrence and the tumor-initiating population in our CRC preclinical explant model. The combination of PF-03084014 and irinotecan had the greatest effect at reducing tumor growth on four CRC tumors when compared with treatment with PF-03084014 or irinotecan alone. The combination significantly reduced tumor recurrence in two CRC explants (CRC001 and CRC036) after treatment was discontinued. Both of these tumors exhibited elevated baseline levels of Notch pathway activation as well as an increase in NOTCH1 gene copy number when compared with the two CRC explants (CRC026 and CRC027) where tumors reappeared quickly after termination of treatment. Isolation and injection of aldehyde dehydrogenase (ALDH(+) and ALDH(-)) cells in an in vivo explant model demonstrated that the ALDH(+) cell population were tumorigenic. Evaluation of the ALDH(+) cells after 28 days of treatment showed that the combination reduced the ALDH(+) population in the tumors that did not regrow. Furthermore, ALDH(+) cells from CRC001 and CRC027 were injected in vivo and treated immediately for 28 days. Two months after treatment, tumors were evident in the combination treatment group for CRC027 but not for CRC036. These results indicate the combination of PF-03084014 and irinotecan may be effective in reducing tumor recurrence in CRC patients whose tumors exhibit elevated levels of the Notch pathway.

Drug resistance in the mouse cancer clinic

Drug resistance is one of the most pressing problems in treating cancer patients today. Local and regional disease can usually be adequately treated, but patients eventually die from distant metastases that have become resistant to all available chemotherapy. Although work on cultured tumor cell lines has yielded a lot of information on potential drug resistance mechanisms, it has proven difficult to translate these results to clinical drug resistance in patients. The controversy regarding the contribution of ABC transporters to drug resistance in patients is one example. The study of genetically engineered mouse models (GEMMs), which closely resemble cancer in human patients, can help to bridge this gap. In models for BRCA1- or BRCA2-associated breast cancer, we observed a substantial synergy between the defect in homology-directed DNA repair and sensitivity to DNA-targeting drugs. Nevertheless, tumors are not easily eradicated and eventually drug resistance develops. In this review we will discuss the use of the new generation mouse models to address major clinical problems, such as mechanisms of drug resistance, predicting chemotherapy response or characterizing the nature of residual tumor cells that escape eradication. Moreover, we will address the contribution of ABC transporters to drug resistance in our model.

Understanding cancer stem cell heterogeneity and plasticity

Heterogeneity is an omnipresent feature of mammalian cells in vitro and in vivo. It has been recently realized that even mouse and human embryonic stem cells under the best culture conditions are heterogeneous containing pluripotent as well as partially committed cells. Somatic stem cells in adult organs are also heterogeneous, containing many subpopulations of self-renewing cells with distinct regenerative capacity. The differentiated progeny of adult stem cells also retain significant developmental plasticity that can be induced by a wide variety of experimental approaches. Like normal stem cells, recent data suggest that cancer stem cells (CSCs) similarly display significant phenotypic and functional heterogeneity, and that the CSC progeny can manifest diverse plasticity. Here, I discuss CSC heterogeneity and plasticity in the context of tumor development and progression, and by comparing with normal stem cell development. Appreciation of cancer cell plasticity entails a revision to the earlier concept that only the tumorigenic subset in the tumor needs to be targeted. By understanding the interrelationship between CSCs and their differentiated progeny, we can hope to develop better therapeutic regimens that can prevent the emergence of tumor cell variants that are able to found a new tumor and distant metastases.

Comparative proteomics of colon cancer stem cells and differentiated tumor cells identifies BIRC6 as a potential therapeutic target

Patients with liver metastases from colon carcinoma show highly variable responses to chemotherapy and tumor recurrence is frequently observed. Therapy-resistant cancer stem cells have been implicated in drug resistance and tumor recurrence. However, the factors determining therapy resistance and tumor recurrence are poorly understood. The aim of this study was to gain insight into these mechanisms by comparing the proteomes of patient-derived cancer stem cell cultures and their differentiated isogenic offspring. We established colonosphere cultures derived from resection specimens of liver metastases in patients with colon cancer. These colonospheres, enriched for colon cancer stem cells, were used to establish isogenic cultures of stably differentiated nontumorigenic progeny. Proteomics based on one-dimensional gel electrophoresis coupled to nano liquid chromatography tandem MS was used to identify proteome differences between three of these paired cultures. The resulting data were analyzed using Ingenuity Pathway Software. Out of a total data set of 3048 identified proteins, 32 proteins were at least twofold up-regulated in the colon cancer stem cells when compared with the differentiated cells. Pathway analysis showed that "cell death " regulation is strikingly different between the two cell types. Interestingly, one of the top-up-regulated proteins was BIRC6, which belongs to the class of Inhibitor of Apoptosis Proteins. Knockdown of BIRC6 sensitized colon cancer stem cells against the chemotherapeutic drugs oxaliplatin and cisplatin. This study reveals that differentiation of colon cancer stem cells is accompanied by altered regulation of cell death pathways. We identified BIRC6 as an important mediator of cancer stem cell resistance against cisplatin and oxaliplatin. Targeting BIRC6, or other Inhibitors of Apoptosis Proteins, may help eradicating colon cancer stem cells.

Comparative proteomics of colon cancer stem cells and differentiated tumor cells identifies BIRC6 as a potential therapeutic target

Patients with liver metastases from colon carcinoma show highly variable responses to chemotherapy and tumor recurrence is frequently observed. Therapy-resistant cancer stem cells have been implicated in drug resistance and tumor recurrence. However, the factors determining therapy resistance and tumor recurrence are poorly understood. The aim of this study was to gain insight into these mechanisms by comparing the proteomes of patient-derived cancer stem cell cultures and their differentiated isogenic offspring. We established colonosphere cultures derived from resection specimens of liver metastases in patients with colon cancer. These colonospheres, enriched for colon cancer stem cells, were used to establish isogenic cultures of stably differentiated nontumorigenic progeny. Proteomics based on one-dimensional gel electrophoresis coupled to nano liquid chromatography tandem MS was used to identify proteome differences between three of these paired cultures. The resulting data were analyzed using Ingenuity Pathway Software. Out of a total data set of 3048 identified proteins, 32 proteins were at least twofold up-regulated in the colon cancer stem cells when compared with the differentiated cells. Pathway analysis showed that "cell death " regulation is strikingly different between the two cell types. Interestingly, one of the top-up-regulated proteins was BIRC6, which belongs to the class of Inhibitor of Apoptosis Proteins. Knockdown of BIRC6 sensitized colon cancer stem cells against the chemotherapeutic drugs oxaliplatin and cisplatin. This study reveals that differentiation of colon cancer stem cells is accompanied by altered regulation of cell death pathways. We identified BIRC6 as an important mediator of cancer stem cell resistance against cisplatin and oxaliplatin. Targeting BIRC6, or other Inhibitors of Apoptosis Proteins, may help eradicating colon cancer stem cells.

Targeting miRNAs involved in cancer stem cell and EMT regulation: An emerging concept in overcoming drug resistance

Although chemotherapy is an important therapeutic strategy for cancer treatment, it fails to eliminate all tumor cells due to intrinsic or acquired drug resistance, which is the most common cause of tumor recurrence. Emerging evidence suggests an intricate role of cancer stem cells (CSCs) and epithelial-mesenchymal transition (EMT)-type cells in anticancer drug resistance. Recent studies also demonstrated that microRNAs (miRNAs) play critical roles in the regulation of drug resistance. Here we will discuss current knowledge regarding CSCs, EMT and the role of regulation by miRNAs in the context of drug resistance, tumor recurrence and metastasis. A better understanding of the molecular intricacies of drug-resistant cells will help to design novel therapeutic strategies by selective targeting of CSCs and EMT-phenotypic cells through alterations in the expression of specific miRNAs towards eradicating tumor recurrence and metastasis. A particular promising lead is the potential synergistic combination of natural compounds that affect critical miRNAs, such as curcumin or epigallocatechin-3-gallate (EGCG) with chemotherapeutic agents.