Overcoming Multidrug Resistance in Cancer Stem Cells. Biomed Res Int. 2015;2015:635745. [PMID: 26649310 PMCID: PMC4663294 DOI: 10.1155/2015/635745]

The use of natural products to target cancer stem cells

The cancer stem cell hypothesis has been used to explain many cancer complications resulting in poor patient outcomes including induced drug resistance, metastases to distant organs, and tumor recurrence. While the validity of the cancer stem cell model continues to be the cause of much scientific debate, a number of putative cancer stem cell markers have been identified making studies concerning the targeting of cancer stem cells possible. In this review, a number of identifying properties of cancer stem cells have been outlined including properties contributing to the drug resistance and metastatic potential commonly observed in supposed cancer stem cells. Due to cancer stem cells' numerous survival mechanisms, the diversity of cancer stem cell markers between cancer types and tissues, and the prevalence of cancer stem cell markers among healthy stem and somatic cells, it is likely that currently utilized treatments will continue to fail to eradicate cancer stem cells. The successful treatment of cancer stem cells will rely upon the development of anti-neoplastic drugs capable of influencing many cellular mechanisms simultaneously in order to prevent the survival of this evasive subpopulation. Natural compounds represent a historically rich source of novel, biologically active compounds which are able to interact with a large number of cellular targets while limiting the painful side-effects commonly associated with cancer treatment. A brief review of select natural products that have been demonstrated to diminish the clinically devastating properties of cancer stem cells or to induce cancer stem cell death is also presented.

Telomerase and drug resistance in cancer

It is well known that a decreased expression or inhibited activity of telomerase in cancer cells is accompanied by an increased sensitivity to some drugs (e.g., doxorubicin, cisplatin, or 5-fluorouracil). However, the mechanism of the resistance resulting from telomerase alteration remains elusive. There are theories claiming that it might be associated with telomere shortening, genome instability, hTERT translocation, mitochondria functioning modulation, or even alterations in ABC family gene expression. However, association of those mechanisms, i.e., drug resistance and telomerase alterations, is not fully understood yet. We review the current theories on the aspect of the role of telomerase in cancer cells resistance to therapy. We believe that revealing/unravelling this correlation might significantly contribute to an increased efficiency of cancer cells elimination, especially the most difficult ones, i.e., drug resistant.

Epithelial-to-mesenchymal transition in tumor progression

The epithelial-to-mesenchymal transition (EMT) is a biological process in which a non-motile epithelial cell changes to a mesenchymal state with invasive capacities. However, the EMT program is involved in both physiological and pathological processes. Cancer-associated EMT is known to contribute to increase invasiveness and metastasis, resistance to therapies, and generation of cell populations with stem cell-like characteristics and therefore is deeply involved in tumor progression. This process is finely orchestrated by multiple signaling pathways and regulatory transcriptional networks. The hallmark of EMT is the loss of epithelial surface markers, mainly E-cadherin, and the acquisition of mesenchymal phenotype. These events can be mediated by EMT transcription factors which can cooperate with several enzymes to repress the E-cadherin expression and regulate EMT at the epigenetic and post-translational level. A growing body of evidence indicates that cancer cells can reside in various phenotypic states along the EMT spectrum, where cells can jointly retain epithelial traits with mesenchymal ones. This type of phenotypic plasticity endows cancer cells with tumor-initiating potential. The identification of the signaling pathways and modulators that lead to activation of EMT programs during these disease processes is providing new insights into the plasticity of cellular phenotypes and possible therapeutic interventions.

NMI inhibits cancer stem cell traits by downregulating hTERT in breast cancer

N-myc and STAT interactor (NMI) has been proved to bind to different transcription factors to regulate a variety of signaling mechanisms including DNA damage, cell cycle and epithelial–mesenchymal transition. However, the role of NMI in the regulation of cancer stem cells (CSCs) remains poorly understood. In this study, we investigated the regulation of NMI on CSCs traits in breast cancer and uncovered the underlying molecular mechanisms. We found that NMI was lowly expressed in breast cancer stem cells (BCSCs)-enriched populations. Knockdown of NMI promoted CSCs traits while its overexpression inhibited CSCs traits, including the expression of CSC-related markers, the number of CD44⁺CD24⁻ cell populations and the ability of mammospheres formation. We also found that NMI-mediated regulation of BCSCs traits was at least partially realized through the modulation of hTERT signaling. NMI knockdown upregulated hTERT expression while its overexpression downregulated hTERT in breast cancer cells, and the changes in CSCs traits and cell invasion ability mediated by NMI were rescued by hTERT. The in vivo study also validated that NMI knockdown promoted breast cancer growth by upregulating hTERT signaling in a mouse model. Moreover, further analyses for the clinical samples demonstrated that NMI expression was negatively correlated with hTERT expression and the low NMI/high hTERT expression was associated with the worse status of clinical TNM stages in breast cancer patients. Furthermore, we demonstrated that the interaction of YY1 protein with NMI and its involvement in NMI-mediated transcriptional regulation of hTERT in breast cancer cells. Collectively, our results provide new insights into understanding the regulatory mechanism of CSCs and suggest that the NMI-YY1-hTERT signaling axis may be a potential therapeutic target for breast cancers.

Low-density lipoprotein-coupled micelles with reduction and pH dual sensitivity for intelligent co-delivery of paclitaxel and siRNA to breast tumor

Multidrug resistance (MDR) is a major obstacle for the clinical therapy of malignant human cancers. The discovery of RNA interference provides efficient gene silencing within tumor cells for reversing MDR. In this study, a new “binary polymer” low-density lipoprotein–N-succinyl chitosan–cystamine–urocanic acid (LDL–NSC–SS–UA) with dual pH/redox sensitivity and targeting effect was synthesized for the co-delivery of breast cancer resistance protein small interfering RNA (siRNA) and paclitaxel (PTX). In vivo, the co-delivering micelles can accumulate in tumor tissue via the enhanced permeability and retention effect and the specific recognition and combination of LDL and LDL receptor, which is overexpressed on the surface of tumor cell membranes. The siRNA–PTX-loaded micelles inhibited gene and drug release under physiological conditions while promoting fast release in an acid microenvironment or in the presence of glutathione. The micelles escaped from the lysosome through the proton sponge effect. Additionally, the micelles exhibited superior antitumor activity and downregulated the protein and mRNA expression levels of breast cancer resistance protein in MCF-7/Taxol cells. The biodistribution and antitumor studies proved that the siRNA–PTX-loaded micelles possessed prolonged circulation time with a remarkable tumor-targeting effect and effectively inhibited tumor growth. Therefore, the novel dual pH/redox-sensitive polymers co-delivering siRNA and PTX with excellent biocompatibility and effective reversal of MDR demonstrate a considerable potential in cancer therapy.

Anticancer properties of a defensin like class IId bacteriocin Laterosporulin10

Laterosporulin10 (LS10) is a defensin like peptide from Brevibacillus sp. strain SKDU10 that inhibited microbial pathogens. However, in this study, anticancer activity of LS10 was examined against different cancer cell lines and compared with normal cells. LS10 displayed cytotoxicity against cancer cells like MCF-7, HEK293T, HT1080, HeLa and H1299 at below 10 μM concentration, but not against prostate epithelium cells RWPE-1. Additionally, no hemolysis was observed at significantly higher concentration compared to IC₅₀ values observed for different cancer cell lines. Release of lactate dehydrogenase from cancer cell lines at 15 μM concentration upon 120 min treatment indicated the lytic ability of LS10. Accordingly, electron microscopy experiments also confirmed the necrotic effect of LS10 at 15 μM concentration against cancer cells. Furthermore, flow cytometry analysis of treated cancer cell lines revealed that LS10 induce apoptosis even at 2.5 μM concentration. Nevertheless, RWPE-1 cells remained viable even at 20 μM concentration. These results provide evidence that LS10 is an anticancer bacteriocin, which causes apoptotic and necrotic death of cancer cells at lower and higher concentrations, respectively. Taken all results together, the present study signifies that LS10 is an anticancer peptide that could be further developed for therapeutic applications.

Getting to the heart of the matter in cancer: Novel approaches to targeting cancer stem cells

Cancer is one of the leading causes of deaths worldwide. While cancers may initially show good response to chemotherapy or radiotherapy, it is not uncommon for them to recur at a later date. This phenomenon may be explained by the existence of a small population of cancer stem cells, which are inherently resistant to anti-cancer treatment as well as being capable of self-renewal. Therefore, while most of the tumour bulk consisting of cells that are not cancer stem cells respond to treatment, the cancer stem cells remain, leading to disease recurrence. Following this logic, the effective targeting of cancer stem cells holds promise for providing long-term cure in individuals with cancer. Cancer stem cells, like normal stem cells are endowed with mechanisms to protect themselves against a wide range of insults including anti-cancer treatments, such as the enhancement of the DNA damage response and the ability to extrude drugs. It is therefore important to develop new strategies if cancer stem cells are to be eradicated. In this review, we describe the strategies that we have developed to target cancer stem cells. These strategies include the targeting of the histone demethylase jumonji, AT rich interactive domain 1B (JARID1B), which we found to be functionally significant in the maintenance of cancer stem cells. Other strategies being pursued include reprogramming of cancer stem cells and the targeting of a functional cell surface marker of liver cancer stem cells, the aminopeptidase CD13.

Spheroid Culture of Head and Neck Cancer Cells Reveals an Important Role of EGFR Signalling in Anchorage Independent Survival

In solid tumours millions of cells are shed into the blood circulation each day. Only a subset of these circulating tumour cells (CTCs) survive, many of them presumable because of their potential to form multi-cellular clusters also named spheroids. Tumour cells within these spheroids are protected from anoikis, which allows them to metastasize to distant organs or re-seed at the primary site. We used spheroid cultures of head and neck squamous cell carcinoma (HNSCC) cell lines as a model for such CTC clusters for determining the role of the epidermal growth factor receptor (EGFR) in cluster formation ability and cell survival after detachment from the extra-cellular matrix. The HNSCC cell lines FaDu, SCC-9 and UT-SCC-9 (UT-SCC-9P) as well as its cetuximab (CTX)-resistant sub-clone (UT-SCC-9R) were forced to grow in an anchorage-independent manner by coating culture dishes with the anti-adhesive polymer poly-2-hydroxyethylmethacrylate (poly-HEMA). The extent of apoptosis, clonogenic survival and EGFR signalling under such culture conditions was evaluated. The potential of spheroid formation in suspension culture was found to be positively correlated with the proliferation rate of HNSCC cell lines as well as their basal EGFR expression levels. CTX and gefitinib blocked, whereas the addition of EGFR ligands promoted anchorage-independent cell survival and spheroid formation. Increased spheroid formation and growth were associated with persistent activation of EGFR and its downstream signalling component (MAPK/ERK). Importantly, HNSCC cells derived from spheroid cultures retained their clonogenic potential in the absence of cell-matrix contact. Addition of CTX under these conditions strongly inhibited colony formation in CTX-sensitive cell lines but not their resistant subclones. Altogether, EGFR activation was identified as crucial factor for anchorage-independent survival of HNSCC cells. Targeting EGFR in CTC cluster formation might represent an attractive anti-metastatic treatment approach in HNSCC.

Nucleotide specificity of the human terminal nucleotidyltransferase Gld2 (TUT2)

The nontemplated addition of single or multiple nucleotides to RNA transcripts is an efficient means to control RNA stability and processing. Cytoplasmic RNA adenylation and the less well-known uridylation are post-transcriptional mechanisms regulating RNA maturation, activity, and degradation. Gld2 is a member of the noncanonical poly(A) polymerases, which include enzymes with varying nucleotide specificity, ranging from strictly ATP to ambiguous to exclusive UTP adding enzymes. Human Gld2 has been associated with transcript stabilizing miRNA monoadenylation and cytoplasmic mRNA polyadenylation. Most recent data have uncovered an unexpected miRNA uridylation activity, which promotes miRNA maturation. These conflicting data raise the question of Gld2 nucleotide specificity. Here, we biochemically characterized human Gld2 and demonstrated that it is a bona fide adenylyltransferase with only weak activity toward other nucleotides. Despite its sequence similarity with uridylyltransferases (TUT4, TUT7), Gld2 displays an 83-fold preference of ATP over UTP. Gld2 is a promiscuous enzyme, with activity toward miRNA, pre-miRNA, and polyadenylated RNA substrates. Apo-Gld2 activity is restricted to adding single nucleotides and processivity likely relies on additional RNA-binding proteins. A phylogeny of the PAP/TUTase superfamily suggests that uridylyltransferases, which are derived from distinct adenylyltransferase ancestors, arose multiple times during evolution via insertion of an active site histidine. A corresponding histidine insertion into the Gld2 active site alters substrate specificity from ATP to UTP.

Malignancy of Cancers and Synthetic Lethal Interactions Associated With Mutations of Cancer Driver Genes

The mutation status of cancer driver genes may correlate with different degrees of malignancy of cancers. The doubling time and multidrug resistance are 2 phenotypes that reflect the degree of malignancy of cancer cells. Because most of cancer driver genes are hard to target, identification of their synthetic lethal partners might be a viable approach to treatment of the cancers with the relevant mutations.The genome-wide screening for synthetic lethal partners is costly and labor intensive. Thus, a computational approach facilitating identification of candidate genes for a focus synthetic lethal RNAi screening will accelerate novel anticancer drug discovery.We used several publicly available cancer cell lines and tumor tissue genomic data in this study.We compared the doubling time and multidrug resistance between the NCI-60 cell lines with mutations in some cancer driver genes and those without the mutations. We identified some candidate synthetic lethal genes to the cancer driver genes APC, KRAS, BRAF, PIK3CA, and TP53 by comparison of their gene phenotype values in cancer cell lines with the relevant mutations and wild-type background. Further, we experimentally validated some of the synthetic lethal relationships we predicted.We reported that mutations in some cancer driver genes mutations in some cancer driver genes such as APC, KRAS, or PIK3CA might correlate with cancer proliferation or drug resistance. We identified 40, 21, 5, 43, and 18 potential synthetic lethal genes to APC, KRAS, BRAF, PIK3CA, and TP53, respectively. We found that some of the potential synthetic lethal genes show significantly higher expression in the cancers with mutations of their synthetic lethal partners and the wild-type counterparts. Further, our experiments confirmed several synthetic lethal relationships that are novel findings by our methods.We experimentally validated a part of the synthetic lethal relationships we predicted. We plan to perform further experiments to validate the other synthetic lethal relationships predicted by this study.Our computational methods achieve to identify candidate synthetic lethal partners to cancer driver genes for further experimental screening with multiple lines of evidences, and therefore contribute to development of anticancer drugs.

Non-coding RNAs Functioning in Colorectal Cancer Stem Cells

In recent years, the hypothesis of the presence of tumor-initiating cancer stem cells (CSCs) has received a considerable support. This model suggested the existence of CSCs which, thanks to their self-renewal properties, are able to drive the expansion and the maintenance of malignant cell populations with invasive and metastatic potential in cancer. Increasing evidence showed the ability of such cells to acquire self-renewal, multipotency, angiogenic potential, immune evasion, symmetrical and asymmetrical divisions which, along with the presence of several DNA repair mechanisms, further enhance their oncogenic potential making them highly resistant to common anticancer treatments. The main signaling pathways involved in the homeostasis of colorectal (CRC) stem cells are the Wnt, Notch, Sonic Hedgehog, and Bone Morfogenic Protein (BMP) pathways, which are mostly responsible for all the features that have been widely referred to stem cells. The same pathways have been identified in colorectal cancer stem cells (CRCSCs), conferring a more aggressive phenotype compared to non-stem CRC cells. Recently, several evidences suggested that non-coding RNAs (ncRNAs) may play a crucial role in the regulation of different biological mechanisms in CRC, by modulating the expression of critical stem cell transcription factors that have been found active in CSCs. In this chapter, we will discuss the involvement of ncRNAs, especially microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), in stemness acquisition and maintenance by CRCSCs, through the regulation of pathways modulating the CSC phenotype and growth, carcinogenesis, differentiation, and epithelial to mesenchymal transition (EMT).