Identification of cancer stem cell-like cells from human epithelial ovarian carcinoma cell line

Ovarian cancer stem cells: are they real and why are they important?

The cancer stem cell hypothesis has been put forward as a paradigm to describe varying levels of aggressiveness in heterogeneous tumors. Specifically, many subpopulations have been clearly demonstrated to possess increased tumorigenicity in mice, broad differentiating capacity, and resistance to therapy. However, the extent to which these experimental findings are potentially clinically significant is still not clear. This review will describe the principles of this emerging hypothesis, ways in which it may be appropriate in ovarian cancer based on the clinical course of the disease, and how we might exploit it to improve outcomes in ovarian cancer patients.

Aiming to immune elimination of ovarian cancer stem cells

Ovarian cancer accounts for only 3% of all cancers in women, but it causes more deaths than any other gynecologic cancer. Treatment with chemotherapy and cytoreductive surgery shows a good response to the therapy. However, in a large proportion of the patients the tumor grows back within a few years. Cancer stem cells, that are less responsive to these treatments, are blamed for this recurrence of disease. Immune therapy either cellular or humoral is a novel concept to treat cancer. It is based on the notice that immune cells invade the tumor. However, the tumor invest heavily to escape from immune elimination by recruiting several immune suppressive mechanisms. These processes are normally in place to limit excessive immune activation and prevent autoimmune phenomena. Here, we discuss current knowledge about the immune (suppressive) status in ovarian cancer. Moreover, we discuss the immunological targets of ovarian cancer stem cells.

Ovarian cancer stem cells are enriched in side population and aldehyde dehydrogenase bright overlapping population

Cancer stem-like cells (CSCs)/cancer-initiaiting cells (CICs) are defined as a small population of cancer cells that have self-renewal capacity, differentiation potential and high tumor-initiating ability. CSCs/CICs of ovarian cancer have been isolated by side population (SP) analysis, ALDEFLUOR assay and using cell surface markers. However, these approaches are not definitive markers for CSCs/CICs, and it is necessary to refine recent methods for identifying more highly purified CSCs/CICs. In this study, we analyzed SP cells and aldehyde dehydrogenese bright (ALDH^Br) cells from ovarian cancer cells. Both SP cells and ALDH^Br cells exhibited higher tumor-initiating ability and higher expression level of a stem cell marker, sex determining region Y-box 2 (SOX2), than those of main population (MP) cells and ALDH^Low cells, respectively. We analyzed an SP and ALDH^Br overlapping population (SP/ALDH^Br), and the SP/ALDH^Br population exhibited higher tumor-initiating ability than that of SP cells or ALDH^Br cells, enabling initiation of tumor with as few as 10² cells. Furthermore, SP/ADLH^Br population showed higher sphere-forming ability, cisplatin resistance, adipocyte differentiation ability and expression of SOX2 than those of SP/ALDH^Low, MP/ALDH^Br and MP/ALDH^Low cells. Gene knockdown of SOX2 suppressed the tumor-initiation of ovarian cancer cells. An SP/ALDH^Br population was detected in several gynecological cancer cells with ratios of 0.1% for HEC—1 endometrioid adenocarcinoma cells to 1% for MCAS ovary mucinous adenocarcinoma cells. Taken together, use of the SP and ALDH^Br overlapping population is a promising approach to isolate highly purified CSCs/CICs and SOX2 might be a novel functional marker for ovarian CSCs/CICs.

Cancer stem cells, epithelial-mesenchymal transition, and drug resistance in high-grade ovarian serous carcinoma

Although epithelial ovarian cancer cells are eliminated by debulking surgery and chemotherapy during initial treatment, it is believed that only a subset of cancer cells, that is, cancer stem cells, may be an important source of tumor recurrence and drug resistance. This review highlights our current understanding of high-grade serous carcinoma, ovarian cancer stem cells, common methods for enrichment of ovarian cancer stem cells, mechanisms involved in drug resistance, and potential strategies for overcoming drug resistance, with associated potential controversies and pitfalls. We also review the potential relationship between epithelial-to-mesenchymal transition and cancer stem cells and how we can induce cancer cells to differentiate into benign stromal fibroblasts in response to certain chemotherapy drugs.

Cyclin D1 affects epithelial-mesenchymal transition in epithelial ovarian cancer stem cell-like cells

Background

The association of cancer stem cells with epithelial–mesenchymal transition (EMT) is receiving attention. We found in our previous study that EMT existed from CD24− phenotype cells to their differentiated cells. It was shown that cyclin D1 functioned in sustaining self-renewal independent of CDK4/CDK6 activation, but its effect on the EMT mechanism in ovarian cancer stem cells is unclear.

Methods

The anchorage-independent spheroids from ovarian adenocarcinoma cell line 3AO were formed in a serum-free medium. CD24− and CD24+ cells were isolated by fluorescence-activated cell sorting. Cell morphology, viability, apoptosis, and migratory ability were observed. Stem-related molecule Bmi-1, Oct-4 and EMT-related marker E-cadherin, and vimentin expressions were analyzed. Cyclin D1 expression in CD24− phenotype enriched spheroids was knocked down with small interfering RNA, and its effects on cell proliferation, apoptosis, migration ability, and EMT-related phenotype after transfection were observed.

Results

In our study, CD24− cells presented stronger proliferative, anti-apoptosis capacity, and migratory ability, than CD24+ cells or parental cells. CD24− cells grew with a scattered spindle-shape within 3 days of culture and transformed into a cobblestone-like shape, identical to CD24+ cells or parental cells at 7 days of culture. CD24− cells or spheroids highly expressed cyclin D1, Bmi-1, and vimentin, and seldom expressed E-cadherin, while CD24+ or parental cells showed the opposite expression. Furthermore, cyclin D1-targeted small interfering RNA resulted in decreased vimentin expression in spheroids. Transfected cells also exhibited an obvious decrease in cell viability and migration, but an increase in cell apoptosis.

Conclusion

Cancer stem cell-like cells possess mesenchymal characteristics and EMT ability, and cyclin D1 involves in EMT mechanism, suggesting that EMT of cancer stem cell-like cells may play a key role in invasion and metastasis of ovarian cancer.

ALDH1-high ovarian cancer stem-like cells can be isolated from serous and clear cell adenocarcinoma cells, and ALDH1 high expression is associated with poor prognosis

Cancer stem-like cells (CSCs)/cancer-initiating cells (CICs) are defined as a small population of cancer cells that have high tumorigenicity. Furthermore, CSCs/CICs are resistant to several cancer therapies, and CSCs/CICs are therefore thought to be responsible for cancer recurrence after treatment and distant metastasis. In epithelial ovarian cancer (EOC) cases, disease recurrence after chemotherapy is frequently observed, suggesting ovarian CSCs/CICs are involved. There are four major histological subtypes in EOC, and serous adenocarcinoma and clear cell adenocarcinoma are high-grade malignancies. We therefore analyzed ovarian CSCs/CICs from ovarian carcinoma cell lines (serous adenocarcinoma and clear cell adenocarcinoma) and primary ovarian cancer cells in this study. We isolated ovarian CSCs/CICs as an aldehyde dehydrogenase 1 high (ALDH1(high)) population from 6 EOC cell lines (3 serous adenocarcinomas and 3 clear cell adenocarcinomas) by the ALDEFLUOR assay. ALDH1(high) cells showed greater sphere-forming ability, higher tumorigenicity and greater invasive capability, indicating that ovarian CSCs/CICs are enriched in ALDH1(high) cells. ALDH1(high) cells could also be isolated from 8 of 11 primary ovarian carcinoma samples. Immunohistochemical staining revealed that higher ALDH1 expression levels in ovary cancer cases are related to poorer prognosis in both serous adenocarcinoma cases and clear cell adenocarcinoma cases. Taken together, the results indicate that ALDH1 is a marker for ovarian CSCs/CICs and that the expression level of ALDH1 might be a novel biomarker for prediction of poor prognosis.

CD44 standard form expression is correlated with high-grade and advanced-stage ovarian carcinoma but not prognosis

Single-chain glycoprotein CD44 is a major cell surface receptor for hyaluronan and mediates epithelial cell adhesion by its involvement in cell-cell and cell-matrix interactions. Recently, CD44 has been identified as a biomarker of cancer stem cells in many malignancies including ovarian carcinoma. However, its clinical significance in human ovarian carcinoma has been controversial until recently. The aim of our current study was to clarify the clinical role of CD44 expression in human ovarian carcinoma. Immunohistochemical staining of 483 primary ovarian carcinoma and 27 paired primary and recurrent ovarian carcinoma samples for CD44 standard form (CD44s) was performed using tissue microarray. The associations between CD44s expression and clinical factors (histologic types, tumor grade, International Federation of Gynecology and Obstetrics stage, and response to chemotherapy), and overall or disease-free survivals were analyzed. We observed CD44s expression in 38% of the ovarian carcinoma samples. Results of the Fisher exact test suggested that CD44s expression was associated with high-grade carcinoma (P = .013), advanced International Federation of Gynecology and Obstetrics stage (III-IV; P < .001), age at diagnosis less than 60 years (P = .011), and transitional cell carcinoma (P = .039). However, CD44s expression was not associated with overall survival (P = .529) or disease-free survival (P = .218) by the log-rank test. Moreover, there was no statistical difference in CD44s expression between the primary and recurrent ovarian carcinomas. Our results showed that CD44s expression is not a prognostic predictor in ovarian cancer.

Regulation of ovarian cancer stem cells or tumor-initiating cells

Cancer stem cells or tumor-initiating cells (CSC/TICs), which can undergo self-renewal and differentiation, are thought to play critical roles in tumorigenesis, therapy resistance, tumor recurrence and metastasis. Tumor recurrence and chemoresistance are major causes of poor survival rates of ovarian cancer patients, which may be due in part to the existence of CSC/TICs. Therefore, elucidating the molecular mechanisms responsible for the ovarian CSC/TICs is required to develop a cure for this malignancy. Recent studies have indicated that the properties of CSC/TICs can be regulated by microRNAs, genes and signaling pathways which also function in normal stem cells. Moreover, emerging evidence suggests that the tumor microenvironments surrounding CSC/TICs are crucial for the maintenance of these cells. Similarly, efforts are now being made to unravel the mechanism involved in the regulation of ovarian CSC/TICs, although much work is still needed. This review considers recent advances in identifying the genes and pathways involved in the regulation of ovarian CSC/TICs. Furthermore, current approaches targeting ovarian CSC/TICs are described. Targeting both CSC/TICs and bulk tumor cells is suggested as a more effective approach to eliminating ovarian tumors. Better understanding of the regulation of ovarian CSC/TICs might facilitate the development of improved therapeutic strategies for recurrent ovarian cancer.

High expression of Lewis y antigen and CD44 is correlated with resistance to chemotherapy in epithelial ovarian cancers

Objectives

To measure Lewis y antigen and CD44 antigen expression in epithelial ovarian carcinoma and to correlate the levels of these antigens with clinical response to chemotherapy.

Methods

The study cases included 34 cases of ovarian carcinoma with resistance to chemotherapeutic drugs, 6 partially drug-sensitive cases, and 52 drug-sensitive cases (92 total).

Results

The rates of expression of Lewis y antigen and CD44 antigen were significantly greater in the drug-resistant group than that in the partially-sensitive or sensitive groups. Surgical stage, residual tumor size and expression of CD44 and Lewis y antigen in ovarian carcinoma tissues were independent risk factors for chemotherapeutic drug resistance.

Conclusions

Over-expression of Lewis y and CD44 antigen are strong risk factors for chemotherapeutic drug resistance in ovarian carcinoma patients.

Cancer stem cells: a shifting subpopulation of cells with stemness?

Carcinogenesis and tumor cells had long been thought to be produced by genetic mutations, until the emergence of the cancer stem cell (CSC) model. This relatively new model reveals another layer of complexity in malignancy formation and maintenance. However, these hard-to-track subpopulations have not only been challenging to understand at the molecular level but have also prevented the development of novel CSC-based cancer therapies. Accumulating evidence from previous studies has suggested that CSCs might be derived from either somatic stem cells (SCCs) or differentiated progenitor cells and that they constantly reestablish their niches; these factors underlie the variability in their frequencies and biological marker expression. Therefore, we present the hypothesis that CSCs possess constantly evolving features. During tumorigenesis, the characteristics of the CSCs may change, thus presenting a "moving target". We also suggest that a multifaceted, integrated strategy combining traditional cell surface markers and transcriptional factor targeting methods to identify "stemness" should lead to the better isolation and characterization of CSCs. Corresponding laboratory approaches to test this hypothesis are also provided.

Ovarian cancer stem cells: working towards the root of stemness

Despite medical advances made over the past decade, ovarian cancer remains one of the more lethal gynecologic cancers in the United States. While current therapeutic strategies are relatively effective, there is a high incidence of recurrent chemoresistant disease. This has been attributed, in part, to a regenerative tumor cell sub-population that has acquired stem cell properties which allows these cells to escape standard chemotherapeutics and drive recurrent disease. To date, a number of laboratories have identified these cancer stem cell (CSC) sub-populations in ovarian cancer cell lines, tumors or ascites and the collective findings suggest ovarian CSCs are likely to be as heterogeneous as the disease itself. Moreover, the multiple ovarian histophenotypes and possible sites of disease origin together with the potential for differential hierarchal contributions of multiple CSCs populations represent significant challenges to the identification, functional characterization and therapeutic targeting of ovarian CSC. This review will highlight the markers and methodology currently used to identify and isolate these cells. We will discuss some of the underlying ovarian CSC biology, the signaling pathways implicated in their survival, replication and differentiation and potential therapeutic targeting strategies.

CD44+/CD24- ovarian cancer cells demonstrate cancer stem cell properties and correlate to survival

Cancer cells with the surface marker profile CD44+/CD24- have previously been described to possess cancer stem cell-like properties. This manuscript evaluates those properties in ovarian cancer cell lines. The proportion of CD44+/CD24- cells corresponded to the clinical aggressiveness of each ovarian cancer cell line histologic subtype. CD44+/CD24- cells demonstrated enhanced progressive differentiation as well as showing a 60-fold increase in Matrigel invasion in both SKOV3 and OV90 cell lines (p < 0.001 each) compared to other phenotypes. CD44+/CD24- demonstrated significant resistance to all chemotherapy agents used in all cell lines, with a 71-93 % increase in resistance compared with baseline. Using a threshold of 25 % CD44+/CD24- ovarian cancer cells found in ascites, patients with >25 % CD44+/CD24- were significantly more likely to recur (83 vs. 14 %, p = 0.003) and had shorter median progression-free survival (6 vs. 18 months, p = 0.01). In conclusion, the CD44+/CD24- phenotype in ovarian cancer cells demonstrate cancer stem cell-like properties of enhanced differentiation, invasion, and resistance to chemotherapy. This CD44+/CD24- phenotype correlates to clinical endpoints with increased risk of recurrence and shorter progression-free survival in patients with ovarian cancer.

Efficient enrichment of hepatic cancer stem-like cells from a primary rat HCC model via a density gradient centrifugation-centered method

BACKGROUND

Because few definitive markers are available for hepatic cancer stem cells (HCSCs), based on physical rather than immunochemical properties, we applied a novel method to enrich HCSCs.

METHODOLOGY

After hepatic tumor cells (HTCs) were first isolated from diethylinitrosamine-induced F344 rat HCC model using percoll discontinuous gradient centrifugation (PDGC) and purified via differential trypsinization and differential attachment (DTDA), they were separated into four fractions using percoll continuous gradient centrifugation (PCGC) and sequentially designated as fractions I-IV (FI-IV). Morphological characteristics, mRNA and protein levels of stem cell markers, proliferative abilities, induced differentiation, in vitro migratory capacities, in vitro chemo-resistant capacities, and in vivo malignant capacities were determined for the cells of each fraction.

FINDINGS

As the density of cells increased, 22.18%, 11.62%, 4.73% and 61.47% of primary cultured HTCs were segregated in FI-FIV, respectively. The cells from FIII (density between 1.041 and 1.062 g/ml) displayed a higher nuclear-cytoplasmic ratio and fewer organelles and expressed higher levels of stem cell markers (AFP, EpCAM and CD133) than cells from other fractions (P<0.01). Additionally, in vitro, the cells from FIII showed a greater capacity to self-renew, differentiate into mature HTCs, transit across membranes, close scratches, and carry resistance to chemotherapy than did cells from any other fraction; in vivo, injection of only 1×10(4) cells from FIII could generate tumors not only in subcutaneous tissue but also in the livers of nude mice.

CONCLUSIONS

Through our novel method, HCSC-like cells were successfully enriched in FIII. This study will greatly contribute to two important areas of biological interest: CSC isolation and HCC therapy.

Identification of a potential ovarian cancer stem cell gene expression profile from advanced stage papillary serous ovarian cancer

Identification of gene expression profiles of cancer stem cells may have significant implications in the understanding of tumor biology and for the design of novel treatments targeted toward these cells. Here we report a potential ovarian cancer stem cell gene expression profile from isolated side population of fresh ascites obtained from women with high-grade advanced stage papillary serous ovarian adenocarcinoma. Affymetrix U133 Plus 2.0 microarrays were used to interrogate the differentially expressed genes between side population (SP) and main population (MP), and the results were analyzed by paired T-test using BRB-ArrayTools. We identified 138 up-regulated and 302 down-regulated genes that were differentially expressed between all 10 SP/MP pairs. Microarray data was validated using qRT-PCR and17/19 (89.5%) genes showed robust correlations between microarray and qRT-PCR expression data. The Pathway Studio analysis identified several genes involved in cell survival, differentiation, proliferation, and apoptosis which are unique to SP cells and a mechanism for the activation of Notch signaling is identified. To validate these findings, we have identified and isolated SP cells enriched for cancer stem cells from human ovarian cancer cell lines. The SP populations were having a higher colony forming efficiency in comparison to its MP counterpart and also capable of sustained expansion and differentiation in to SP and MP phenotypes. 50,000 SP cells produced tumor in nude mice whereas the same number of MP cells failed to give any tumor at 8 weeks after injection. The SP cells demonstrated a dose dependent sensitivity to specific γ-secretase inhibitors implicating the role of Notch signaling pathway in SP cell survival. Further the generated SP gene list was found to be enriched in recurrent ovarian cancer tumors.

Isolation and characterization of stem-like cells from a human ovarian cancer cell line

Increasing evidence supports the existence of a subpopulation of cancer cells capable of self-renewal and differentiation into diverse cell lineages. These cancer stem-like or cancer-initiating cells (CICs) also demonstrate resistance to chemo- and radiotherapy and may function as a primary source of cancer recurrence. We report here on the isolation and in vitro propagation of multicellular ovarian cancer spheroids from a well-established ovarian cancer cell line (OVCAR-3). The spheroid-derived cells (SDCs) display self-renewal potential, the ability to produce differentiated progeny, and increased expression of genes previously associated with CICs. SDCs also demonstrate higher invasiveness, migration potential, and enhanced resistance to standard anticancer agents relative to parental OVCAR-3 cells. Furthermore, SDCs display up-regulation of genes associated with epithelial-to-mesenchymal transition (EMT), anticancer drug resistance and/or decreased susceptibility to apoptosis, as well as, down-regulation of genes typically associated with the epithelial cell phenotype and pro-apoptotic genes. Pathway and biological process enrichment analyses indicate significant differences between the SDCs and precursor OVCAR-3 cells in TGF-beta-dependent induction of EMT, regulation of lipid metabolism, NOTCH and Hedgehog signaling. Collectively, our results indicate that these SDCs will be a useful model for the study of ovarian CICs and for the development of novel CIC-targeted therapies.

Deconstructing the mechanisms and consequences of TGF-β-induced EMT during cancer progression

Transforming growth factor-β (TGF-β) is a potent pleiotropic cytokine that regulates mammalian development, differentiation, and homeostasis in essentially all cell types and tissues. TGF-β normally exerts anticancer activities by prohibiting cell proliferation and by creating cell microenvironments that inhibit cell motility, invasion, and metastasis. However, accumulating evidence indicates that the process of tumorigenesis, particularly that associated with metastatic progression, confers TGF-β with oncogenic activities, a functional switch known as the "TGF-β paradox." The molecular determinants governing the TGF-β paradox are complex and represent an intense area of investigation by researchers in academic and industrial settings. Recent findings link genetic and epigenetic events in mediating the acquisition of oncogenic activity by TGF-β, as do aberrant alterations within tumor microenvironments. These events coalesce to enable TGF-β to direct metastatic progression via the stimulation of epithelial-mesenchymal transition (EMT), which permits carcinoma cells to abandon polarized epithelial phenotypes in favor of apolar mesenchymal-like phenotypes. Attempts to deconstruct the EMT process induced by TGF-β have identified numerous signaling molecules, transcription factors, and microRNAs operant in mediating the initiation and resolution of this complex transdifferentiation event. In addition to its ability to enhance carcinoma cell invasion and metastasis, EMT also endows transitioned cells with stem-like properties, including the acquisition of self-renewal and tumor-initiating capabilities coupled to chemoresistance. Here, we review recent findings that delineate the pathophysiological mechanisms whereby EMT stimulated by TGF-β promotes metastatic progression and disease recurrence in human carcinomas.

Comparison of expression profiles in ovarian epithelium in vivo and ovarian cancer identifies novel candidate genes involved in disease pathogenesis

Molecular events leading to epithelial ovarian cancer are poorly understood but ovulatory hormones and a high number of life-time ovulations with concomitant proliferation, apoptosis, and inflammation, increases risk. We identified genes that are regulated during the estrous cycle in murine ovarian surface epithelium and analysed these profiles to identify genes dysregulated in human ovarian cancer, using publically available datasets. We identified 338 genes that are regulated in murine ovarian surface epithelium during the estrous cycle and dysregulated in ovarian cancer. Six of seven candidates selected for immunohistochemical validation were expressed in serous ovarian cancer, inclusion cysts, ovarian surface epithelium and in fallopian tube epithelium. Most were overexpressed in ovarian cancer compared with ovarian surface epithelium and/or inclusion cysts (EpCAM, EZH2, BIRC5) although BIRC5 and EZH2 were expressed as highly in fallopian tube epithelium as in ovarian cancer. We prioritised the 338 genes for those likely to be important for ovarian cancer development by in silico analyses of copy number aberration and mutation using publically available datasets and identified genes with established roles in ovarian cancer as well as novel genes for which we have evidence for involvement in ovarian cancer. Chromosome segregation emerged as an important process in which genes from our list of 338 were over-represented including two (BUB1, NCAPD2) for which there is evidence of amplification and mutation. NUAK2, upregulated in ovarian surface epithelium in proestrus and predicted to have a driver mutation in ovarian cancer, was examined in a larger cohort of serous ovarian cancer where patients with lower NUAK2 expression had shorter overall survival. In conclusion, defining genes that are activated in normal epithelium in the course of ovulation that are also dysregulated in cancer has identified a number of pathways and novel candidate genes that may contribute to the development of ovarian cancer.

Drug treatment of cancer cell lines: a way to select for cancer stem cells?

Tumors are generally composed of different cell types. In recent years, it has been shown that in many types of cancers a subset of cells show peculiar characteristics, such as the ability to induce tumors when engrafted into host animals, self-renew and being immortal, and give rise to a differentiated progeny. These cells have been defined as cancer stem cells (CSCs) or tumor initiating cells. CSCs can be isolated both from tumor specimens and established cancer cell lines on the basis of their ability to exclude fluorescent dyes, express specific cell surface markers or grow in particular culture conditions. A key feature of CSCs is their resistance to chemotherapeutic agents, which could contribute to the remaining of residual cancer cells after therapeutic treatments. It has been shown that CSC-like cells can be isolated after drug treatment of cancer cell lines; in this review, we will describe the strategies so far applied to identify and isolate CSCs. Furthermore, we will discuss the possible use of these selected populations to investigate CSC biology and develop new anticancer drugs.

Cancer stem cells and cancer therapy

Cancer stem cells (CSCs) are a subpopulation of tumour cells that possess the stem cell properties of self-renewal and differentiation. Stem cells might be the target cells responsible for malignant transformation, and tumour formation may be a disorder of stem cell self-renewal pathway. Epigenetic alterations and mutations of genes involved in signal transmissions may promote the formation of CSCs. These cells have been identified in many solid tumours including breast, brain, lung, prostate, testis, ovary, colon, skin, liver, and also in acute myeloid leukaemia. The CSC theory clarifies not only the issue of tumour initiation, development, metastasis and relapse, but also the ineffectiveness of conventional cancer therapies. Treatments directed against the bulk of the cancer cells may produce striking responses but they are unlikely to result in long-term remissions if the rare CSCs are not targeted. In this review, we consider the properties of CSCs and possible strategies for controlling the viability and tumourigenecity of these cells, including therapeutic models for selective elimination of CSCs and induction of their proper differentiation.

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.

Ehrlichiosis: a cause of bone marrow hypoplasia in humans

Cancer stem cell population is a subset of cells capable of dictating invasion, metastasis, heterogeneity, and therapeutic resistance in tumours. Eradication of this rare population is a new insight in cancer treatment. However, prospective identification, characterization, and isolation of these CSCs have been a major challenge. Many studies were performed on surface markers for potential identification and isolation of CSCs. Lack of universal expression of surface markers limits their usage and no best combination of markers has yet been confirmed to identify CSCs capable of initiating and metastasizing tumours. CD44, a hyaluronic acid receptor, is one of the most commonly studied surface markers, which is expressed by almost every tumour cell. CD24, a heat stable antigen, is another surface marker expressed in many tumour types. However, their expression and prognostic value in isolating CSCs are still an enduring ambiguity. In this critical review, we assess the role of CD44 and CD24 in tumour initiation, development, and metastasis. We mainly focus on analysing the significance of CD44 and CD24 as CSC surface markers in combination or with other putative markers in different types of cancer.