Cancer Stem Cells (CSCs) are a small subpopulation of cells within tumors with capabilities of self-renewal, differentiation, and tumorigenicity when transplanted into an animal host. A number of cell surface markers such as CD44, CD24, and CD133 are often used to identify and enrich CSCs. A regulatory network consisting of microRNAs and Wnt/β-catenin, Notch, and Hedgehog signaling pathways controls the CSC properties. The clinical relevance of CSCs has been strengthened by emerging evidence, demonstrating that CSCs are resistant to conventional chemotherapy and radiation treatment and that CSCs are very likely to be the origin of cancer metastasis. CSCs are believed to be an important target for novel anti-cancer drug discovery. Herein we summarize the current understanding of CSCs, with a focus on the role of miRNA and epithelial mesenchymal transition (EMT), and discuss the clinical application of targeting CSCs for cancer treatment.

Decreased expression of specific microRNAs (miRNAs) occurs in human tumors, which suggests a function for miRNAs in tumor suppression. Herein, levels of the miR-17-5p/miR-20a miRNA cluster were inversely correlated to cyclin D1 abundance in human breast tumors and cell lines. MiR-17/20 suppressed breast cancer cell proliferation and tumor colony formation by negatively regulating cyclin D1 translation via a conserved 3′ untranslated region miRNA-binding site, thereby inhibiting serum-induced S phase entry. The cell cycle effect of miR-17/20 was abrogated by cyclin D1 siRNA and in cyclin D1–deficient breast cancer cells. Mammary epithelial cell–targeted cyclin D1 expression induced miR-17-5p and miR-20a expression in vivo, and cyclin D1 bound the miR-17/20 cluster promoter regulatory region. In summary, these studies identify a novel cyclin D1/miR-17/20 regulatory feedback loop through which cyclin D1 induces miR-17-5p/miR-20a. In turn, miR-17/20 limits the proliferative function of cyclin D1, thus linking expression of a specific miRNA cluster to the regulation of oncogenesis.

microRNAs are thought to regulate tumor progression and invasion via direct interaction with target genes within cells. Here the microRNA17/20 cluster is shown to govern cellular migration and invasion of nearby cells via heterotypic secreted signals. micro-RNA17/20 abundance is reduced in highly invasive breast cancer cell lines and node-positive breast cancer specimens. Cell-conditioned medium from microRNA17/20-overexpressing noninvasive breast cancer cell MCF7 was sufficient to inhibit MDA-MB-231 cell migration and invasion through inhibiting secretion of a subset of cytokines, and suppressing plasminogen activation via inhibition of the secreted plasminogen activators (cytokeratin 8 and α-enolase). microRNA17/20 directly repressed IL-8 by targeting its 3′ UTR, and inhibited cytokeratin 8 via the cell cycle control protein cyclin D1. At variance with prior studies, these results demonstrated a unique mechanism of how the altered microRNA17/20 expression regulates cellular secretion and tumor microenvironment to control migration and invasion of neighboring cells in breast cancer. These findings not only reveal an antiinvasive function of miR-17/20 in breast cancer, but also identify a heterotypic secreted signal that mediates the microRNA regulation of tumor metastasis.M icroRNAs (miRNAs) are 21-to 22-nucleotide molecules that regulate cellular phenotype through altering the stability or translational efficiency of targeted mRNAs (1). Important associations between miRNA gene expression and human cancer have implicated miRNA in human tumorigenesis (2-4). miRNA encoding genes are frequently located at fragile sites, common breakpoints, or minimal regions of amplification (5). The human miR-17/20 cluster, located on chromosome 13q31, undergoes loss of heterozygosity in several different cancers, including breast cancer. In the human B cell line P493-6, miR-17/20 inhibits cell growth via suppression of E2F1 expression (6). In mice, transgenic mice overexpressing miR-17 alone showed overall tissue growth retardation, smaller organs, and greatly reduced hematopoietic cell lineages (7). Analysis of miRNAs regulated in cyclin D1 transgenic mammary tumors and reciprocally regulated in cyclin D1-knockout mice identified the miR-17/20 cluster as an important regulatory switch in mammary tumor growth. miR-17/20 inhibited breast cancer tumor growth through repression of cyclin D1 expression via the 3′ UTR binding site (8). Recent evidence implicates miRNA in breast cancer metastasis by inhibiting target genes. miR-335 inhibited human breast cancer cell metastasis via repression of SOX4, a regulator of progenitor cell development and migration (9). miR-10b promotes cell migration in vitro and initiates breast tumor invasion in vivo by targeting gene HOXD10 (10). miR-200 family prevented epithelial to mesenchymal transition of breast cancer cells by repressing ZEB1 and SIP1 (11). However, the role of the miR-17/20 cluster in regulation of breast cancer metastasis remains unknown.Tumor cell migration and invasion is in par...

MicroRNAs play important roles in regulating development at both transcriptional and posttranscriptional levels. Here, we report 29 microRNAs from mouse testis that are differentially expressed as the prepubertal testis differentiates to the adult testis. Using computational analyses to identify potential microRNA target mRNAs, we identify several possible male germ cell target mRNAs. One highly conserved sequence in the 3'-untranslated region (UTR) of transition protein 2 (Tnp2) mRNA, a testis-specific and posttranscriptionally regulated mRNA in postmeiotic germ cells, is complementary to Mirn122a. Mirn122a is enriched in late-stage male germ cells and is predominantly on polysomes. Mirn122a, but not another noncomplementary microRNA, inhibits the activity of a luciferase reporter construct containing the 3'-UTR of Tnp2. Site-directed mutations of Mirn122a indicate that base pairing of the 5'-region of Mirn122a to its complementary site in the 3'-UTR of Tnp2 mRNA is essential for the downregulation of luciferase activity. Real-time reverse transcription-polymerase chain reaction and ribonuclease protection assays reveal that the Mirn122a-directed decrease of the Tnp2 reporter gene activity results from mRNA cleavage. We propose that specific microRNAs, such as Mirn122a, could be involved in the posttranscriptional regulation of mRNAs such as Tnp2 in the mammalian testis.

Signaling proteins driving the proliferation of stem and progenitor cells are often encoded by proto-oncogenes. EphB receptors represent a rare exception; they promote cell proliferation in the intestinal epithelium and function as tumor suppressors by controlling cell migration and inhibiting invasive growth. We show that cell migration and proliferation are controlled independently by the receptor EphB2. EphB2 regulated cell positioning is kinase-independent and mediated via phosphatidylinositol 3-kinase, whereas EphB2 tyrosine kinase activity regulates cell proliferation through an Abl-cyclin D1 pathway. Cyclin D1 regulation becomes uncoupled from EphB signaling during the progression from adenoma to colon carcinoma in humans, allowing continued proliferation with invasive growth. The dissociation of EphB2 signaling pathways enables the selective inhibition of the mitogenic effect without affecting the tumor suppressor function and identifies a pharmacological strategy to suppress adenoma growth.