Adult stem cells and cancer stem cells: tie in or tear apart?

Cancer Stem Cells and Their Vesicles, Together with Other Stem and Non-Stem Cells, Govern Critical Cancer Processes: Perspectives for Medical Development

Stem cells, identified several decades ago, started to attract interest at the end of the nineties when families of mesenchymal stem cells (MSCs), concentrated in the stroma of most organs, were found to participate in the therapy of many diseases. In cancer, however, stem cells of high importance are specific to another family, the cancer stem cells (CSCs). This comprehensive review is focused on the role and the mechanisms of CSCs and of their specific extracellular vesicles (EVs), which are composed of both exosomes and ectosomes. Compared to non-stem (normal) cancer cells, CSCs exist in small populations that are preferentially distributed to the niches, such as minor specific tissue sites corresponding to the stroma of non-cancer tissues. At niches and marginal sites of other cancer masses, the tissue exhibits peculiar properties that are typical of the tumor microenvironment (TME) of cancers. The extracellular matrix (ECM) includes components different from non-cancer tissues. CSCs and their EVs, in addition to effects analogous to those of MSCs/EVs, participate in processes of key importance, specific to cancer: generation of distinct cell subtypes, proliferation, differentiation, progression, formation of metastases, immune and therapy resistance, cancer relapse. Many of these, and other, effects require CSC cooperation with surrounding cells, especially MSCs. Filtered non-cancer cells, especially macrophages and fibroblasts, contribute to collaborative cancer transition/integration processes. Therapy developments are mentioned as ongoing preclinical initiatives. The preliminary state of clinical medicine is presented in terms of both industrial development and future treatments. The latter will be administered to specific patients together with known drugs, with the aim of eradicating their tumor growth and metastases.

miR-106b-5p promotes stem cell-like properties of hepatocellular carcinoma cells by targeting PTEN via PI3K/Akt pathway

Background

The miRNA miR-106b-5p has been previously reported to be increased in hepatocellular carcinoma (HCC) tissues compared to cirrhotic tissues. The purpose of this study was to detect its expression in HCC cell lines with distinct metastatic potentials and to explore the molecular mechanisms underlying HCC stemness and migration.

Methods

miR-106b-5p expression was studied in HCC tissues and cell lines. In vitro cancer stem cell (CSC)-like properties, cell migration and invasion were compared between HCC cell lines with upregulation or downregulation of miR-106b-5p. In vivo tail vein injection models were established to evaluate the role of miR-106b-5p in lung metastasis. Bioinformatics programs, luciferase reporter assay and rescue experiments were used to validate the downstream targets of miR-106b-5p. The relationship between the expression of the targeted gene and clinicopathological parameters was also analyzed.

Results

miR-106b-5p expression was higher in HCC tissues and cell lines than that in non-tumor tissues and hepatocyte Chang liver, respectively. Upregulation of miR-106b-5p exhibited a promoting role in CSC properties, cell migration and activation of phosphatidylinositol-3 kinase (PI3K)/Akt signaling in vitro, as well as in lung metastasis in vivo. However, downregulation of miR-106b-5p exhibited the opposite effect. Furthermore, PTEN was verified as a direct target of miR-106b-5p. Upon clinicopathological analysis, lower level of PTEN was significantly associated with more aggressive characteristics. Patients with high PTEN expression had longer overall survival and disease-free survival.

Conclusion

miR-106b-5p promotes HCC stemness maintenance and metastasis by targeting PTEN via PI3K/Akt pathway. Inhibition of miR-106b-5p might be effective therapeutic strategies to treat advanced HCC.

Identification of a tumor-initiating stem cell population in human renal carcinomas

The purpose of the present study was to search for the presence of a tumor-initiating stem cell population in renal carcinomas. Based on the recent identification of mesenchymal stem cells in normal kidneys, we sorted cells expressing the mesenchymal stem cell marker CD105 from 5 human renal carcinomas. Because the CD105(+) but not the CD105(-) population showed enhanced tumorigenicity when injected in severely compromised immunodeficient (SCID) mice, we cloned and characterized CD105(+) cells and evaluated their stemness, differentiative ability, and serial tumor generation. Characterization of the phenotype of CD105(+) clones revealed several stem cell properties: 1) clonogenic ability, 2) expression of nestin, Nanog, Oct4 stem cell markers, and lack of differentiative epithelial markers, 3) ability to grow in nonadhesive spheroids, 4) in vitro differentiation into epithelial and endothelial cell types, and 5) generation in vivo of serially transplantable carcinomas containing an undifferentiated CD105(+) tumorigenic and a differentiated CD105(-) nontumorigenic population. In addition, some vessels present in carcinomas generated from CD105(+) clones were of human origin, suggesting the capability of tumor-initiating stem cells to in vivo differentiate also in endothelial cells. In conclusion, we demonstrate that CD105(+) cells and clones derived from renal carcinomas were enriched in tumor-initiating cells with stem characteristics.

Plasma membrane proteomics of human embryonic stem cells and human embryonal carcinoma cells

Human embryonic stem cells (hESCs) are of immense interest in regenerative medicine as they can self-renew indefinitely and can give rise to any adult cell type. Human embryonal carcinoma cells (hECCs) are the malignant counterparts of hESCs found in testis tumors. hESCs that have acquired chromosomal abnormalities in culture are essentially indistinguishable from hECC. Direct comparison of karyotypically normal hESCs with hECCs could lead to understanding differences between their mechanisms of growth control and contribute to implementing safe therapeutic use of stem cells without the development of germ cell cancer. While several comparisons of hECCs and hESCs have been reported, their cell surface proteomes are largely unknown, partly because plasma membrane proteomics is still a major challenge. Here, we present a strategy for the identification of plasma membrane proteins that has been optimized for application to the relatively small numbers of stem cells normally available, and that does not require tedious cell fractionation. The method led to the identification of 237 and 219 specific plasma membrane proteins in the hESC line HUES-7 and the hECC line NT2/D1, respectively. In addition to known stemness-associated cell surface markers like ALP, CD9, and CTNNB, a large number of receptors, transporters, signal transducers, and cell-cell adhesion proteins were identified. Our study revealed that several Hedgehog and Wnt pathway members are differentially expressed in hESCs and hECCs including NPC1, FZD2, FZD6, FZD7, LRP6, and SEMA4D, which play a pivotal role in stem cell self-renewal and cancer growth. Various proteins encoded on chromosome 12p, duplicated in testicular cancer, were uniquely identified in hECCs. These included GAPDH, LDHB, YARS2, CLSTN3, CSDA, LRP6, NDUFA9, and NOL1, which are known to be upregulated in testicular cancer. Distinct HLA molecules were revealed on the surface of hESCs and hECCs, despite their low abundance. Results were compared with genomic and proteomic data sets reported previously for mouse ESCs, hECCs, and germ cell tumors. Our data provides a surface signature for HUES-7 and NT2/D1 cells and distinguishes normal hESCs from hECCs, helping explain their 'benign' versus 'malignant' nature.

Stem cell biology and the cellular pathways of carcinogenesis

Tissues in the body are maintained by somatic stem cells. This has been demonstrated both in organs with high cell turnover rate, such as the bone marrow, colon and skin, and in organs with low cell turnover rate, such as the brain. To maintain homeostasis in the body it is important to keep tight control over stem cell fate. Stem cells are under strict control from both intrinsic and extrinsic factors and loss of this control has been postulated to be a key step in the carcinogenic process. There is increasing evidence that cancer initiation results from accumulative oncogenic mutations (intrinsic loss of control) in long-lived stem cells or their immediate progenitor, followed by modification of the surrounding microenvironment (loss of extrinsic control). Decades ago, studies on teratocarcinoma led to the hypothesis that a small subset of self-renewing cancer stem cells with differentiation potential exists within tumors. These studies showed that teratocarcinomas contain undifferentiated embryonic carcinoma cells that are able to give rise to differentiated cells which belong to all three germ layers. More recent studies have confirmed cancer stem cells in such diverse cancers as leukemia, brain and breast cancer. It is, however, unclear whether cancer stem cells originate from resident stem cells or whether they arise as a result of an acquired gain of self-renewal capacity in tissue progenitor cells or even more differentiated cells. The characterization of a cancer stem cell profile within diverse cancer types may open up new avenues for cancer treatment. In this review we discuss the concept of cancer stem cells and focus on examples where these cells have been identified.