Continuous hypoxic culturing maintains activation of Notch and allows long-term propagation of human embryonic stem cells without spontaneous differentiation

Chromosomal variability of human mesenchymal stem cells cultured under hypoxic conditions

Bone marrow derived human mesenchymal stem cells (hMSCs) have attracted great interest from both bench and clinical researchers because of their pluripotency and ease of expansion ex vivo. However, these cells do finally reach a senescent stage and lose their multipotent potential. Proliferation of these cells is limited up to the time of their senescence, which limits their supply, and they may accumulate chromosomal changes through ex vivo culturing. The safe, rapid expansion of hMSCs is critical for their clinical application. Chromosomal aberration is known as one of the hallmarks of human cancer, and therefore it is important to understand the chromosomal stability and variability of ex vivo expanded hMSCs before they are used widely in clinical applications. In this study, we examined the effects of culturing under ambient (20%) or physiologic (5%) O₂ concentrations on the rate of cell proliferation and on the spontaneous transformation of hMSCs in primary culture and after expansion, because it has been reported that culturing under hypoxic conditions accelerates the propagation of hMSCs. Bone marrow samples were collected from 40 patients involved in clinical research. We found that hypoxic conditions promote cell proliferation more favourably than normoxic conditions. Chromosomal aberrations, including structural instability or aneuploidy, were detected in significantly earlier passages under hypoxic conditions than under normoxic culture conditions, suggesting that amplification of hMSCs in a low-oxygen environment facilitated chromosomal instability. Furthermore, smoothed hazard-function modelling of chromosomal aberrations showed increased hazard after the fourth passage under both sets of culture conditions, and showed a tendency to increase the detection rate of primary karyotypic abnormalities among donors aged 60 years and over. In conclusion, we propose that the continuous monitoring of hMSCs will be required before they are used in therapeutic applications in the clinic, especially when cells are cultured under hypoxic conditions.

Defined xenogeneic-free and hypoxic environment provides superior conditions for long-term expansion of human adipose-derived stem cells

Development and implementation of therapeutic protocols based on stem cells or tissue-engineered products relies on methods that enable the production of substantial numbers of cells while complying with stringent quality and safety demands. In the current study, we aimed to assess the benefits of maintaining cultures of adipose-derived stem cells (ASCs) in a defined culture system devoid of xenogeneic components (xeno-free) and hypoxia over a 49-day growth period. Our data provide evidence that conditions involving StemPro mesenchymal stem cells serum-free medium (SFM) Xeno-Free and hypoxia (5% oxygen concentration) in the culture atmosphere provide a superior proliferation rate compared to a standard growth environment comprised of alpha-modified Eagle medium (A-MEM) supplemented with fetal calf serum (FCS) and ambient air (20% oxygen concentration) or that of A-MEM supplemented with FCS and hypoxia. Furthermore, a flow cytometric analysis and in vitro differentiation assays confirmed the immunophenotype stability and maintained multipotency of ASCs when expanded under xeno-free conditions and hypoxia. In conclusion, our data demonstrate that growth conditions utilizing a xeno-free and hypoxic environment not only provide an improved environment for the expansion of ASCs, but also set the stage as a culture system with the potential broad spectrum utility for regenerative medicine and tissue engineering applications.

Delineating nuclear reprogramming

Nuclear reprogramming is described as a molecular switch, triggered by the conversion of one cell type to another. Several key experiments in the past century have provided insight into the field of nuclear reprogramming. Previously deemed impossible, this research area is now brimming with new findings and developments. In this review, we aim to give a historical perspective on how the notion of nuclear reprogramming was established, describing main experiments that were performed, including (1) somatic cell nuclear transfer, (2) exposure to cell extracts and cell fusion, and (3) transcription factor induced lineage switch. Ultimately, we focus on (4) transcription factor induced pluripotency, as initiated by a landmark discovery in 2006, where the process of converting somatic cells to a pluripotent state was narrowed down to four transcription factors. The conception that somatic cells possess the capacity to revert to an immature status brings about huge clinical implications including personalized therapy, drug screening and disease modeling. Although this technology has potential to revolutionize the medical field, it is still impeded by technical and biological obstacles. This review describes the effervescent changes in this field, addresses bottlenecks hindering its advancement and in conclusion, applies the latest findings to overcome these issues.

Large-scale expansion and exploitation of pluripotent stem cells for regenerative medicine purposes: beyond the T flask

Human pluripotent stem cells will likely be a significant part of the regenerative medicine-driven healthcare revolution. In order to realize this potential, culture processes must be standardized, scalable and able to produce clinically relevant cell numbers, whilst maintaining critical biological functionality. This review comprises a broad overview of important bioprocess considerations, referencing the development of biopharmaceutical processes in an effort to learn from current best practice in the field. Particular focus is given to the recent efforts to grow human pluripotent stem cells in microcarrier or aggregate suspension culture, which would allow geometric expansion of productive capacity were it to be fully realized. The potential of these approaches is compared with automation of traditional T-flask culture, which may provide a cost-effective platform for low-dose, low-incidence conditions or autologous therapies. This represents the first step in defining the full extent of the challenges facing bioprocess engineers in the exploitation of large-scale human pluripotent stem cell manufacture.

Hypoxia-dependent inhibition of tumor cell susceptibility to CTL-mediated lysis involves NANOG induction in target cells

Hypoxia is a major feature of the solid tumor microenvironment and is known to be associated with tumor progression and poor clinical outcome. Recently, we reported that hypoxia protects human non-small cell lung tumor cells from specific lysis by stabilizing hypoxia-inducible factor-1α and inducing STAT3 phosphorylation. In this study, we show that NANOG, a transcription factor associated with stem cell self renewal, is a new mediator of hypoxia-induced resistance to specific lysis. Our data indicate that under hypoxic conditions, NANOG is induced at both transcriptional and translational levels. Knockdown of the NANOG gene in hypoxic tumor cells is able to significantly attenuate hypoxia-induced tumor resistance to CTL-dependent killing. Such knockdown correlates with an increase of target cell death and an inhibition of hypoxia-induced delay of DNA replication in these cells. Interestingly, NANOG depletion results in inhibition of STAT3 phosphorylation and nuclear translocation. To our knowledge, this study is the first to show that hypoxia-induced NANOG plays a critical role in tumor cell response to hypoxia and promotes tumor cell resistance to Ag-specific lysis.

The full spectrum of physiological oxygen tensions and step-changes in oxygen tension affects the neural differentiation of mouse embryonic stem cells

The beneficial impact of lowering oxygen tension to physiological levels has been demonstrated in a number of stem cell differentiation protocols. The majority of these studies compare normal laboratory oxygen tension with one physiological condition (typically 2-5% O(2) ). In this article, we investigated whether the full spectrum of physiological oxygen tensions (0-20% O(2) ) and step-changes in oxygen tension could enhance the production of neural populations from of embryonic stem cells (ESCs). We used a model system for the conversion of mouse ESCs into cells expressing one neuroectoderm stem cell marker (nestin) and two neural markers (βIII tubulin and microtubule-associated protein (MAP2)). 4-10% O(2) was associated with large increases in the total production of viable cells and the highest number of cells expressing Nestin, βIII tubulin, and MAP2. However, 4-10% O(2) also caused a reduction in the percentage of cells expressing all three markers. Step changes in oxygen tension at the mid-point of the differentiation process affected the total production of viable cells and the percentage of cells expressing all three markers. We found that the initial oxygen tension and the magnitude of the step-change were critical variables. A step increase from 0 to 2% O(2) mid-way through the protocol resulted in the highest percentage of cells expressing βIII tubulin (86.5%). In conclusion, we have demonstrated that the full spectrum of physiological oxygen tensions and step changes in oxygen tension represent a powerful tool for the optimisation of neural differentiation processes.

Hypoxia and adipose-derived stem cell-based tissue regeneration and engineering

INTRODUCTION

Realization that oxygen is one of the key regulators of development and differentiation has a profound significance on how current cell-based and tissue engineering applications using adipose-derived stem cells (ASCs) can be further improved.

AREAS COVERED

The article provides an overview of mechanisms of hypoxic responses during physiological adaptations and development. Furthermore, a synopsis of the hypoxic responses of ASCs is provided, and this information is presented in context of their utility as a major source of stem cells across the regenerative applications explored to date.

EXPERT OPINION

The reader will obtain insight into a highly specific area of stem cell research focusing on ASCs and hypoxia. In order to enhance the level of comprehension, a broader context with other stem cell and experimental systems is provided. It is emphasized that the pericellular oxygen tension is a critical regulatory factor that should be taken into account when devising novel stem cell-based therapeutic applications along with other parameters, such as biochemical soluble factors and the growth substrates.

Severe Hypoxia: Consequences to Neural Stem Cells and Neurons

BACKGROUND

Multiple neurological diseases result from a pathological hypoxia in the brain, resulting in various motor, sensory or cognitive sequelae. Understanding the response of neural stem cells (NSCs) and differentiated neurons to hypoxia will help better treat such diseases.

METHODS

We exposed mouse embryonic primary neurons (PN) and neural stem cells to 1% O₂ in vitro.

RESULTS

Both cell types survived and retained their immunocyto-chemical markers, and neurons showed no obvious morphological changes. Microarray analysis showed that the number of genes with significantly altered expression levels was almost five-fold higher in NSCs compared to PN. NSCs displayed a clear block in G1/S phase of the cell cycle and a number of down-regulated cytokine genes. Various growth factors (e.g. neural growth factor, prolactin), involved in survival and proliferation, genes of the Notch pathway, and genes involved in glial differentiation, and cell-matrix adhesion were up-regulated. PN displayed a down-regulation of a number of genes involved in neuron-specific functions, in particular, transmitter-related (e.g. synaptic transmission, neurotransmitter transport and release, learning, adult behavior).

CONCLUSIONS

We conclude that hypoxia 1-down-regulates genes involved in multiple neuronal functions which can negatively impact learning and memory; 2-induces a cell cycle block in NSCs; 3-can precondition NSC towards a particular differentiation potential while maintaining them fully undifferentiated.

Induction of mesenchymal progenitor cells with chondrogenic property from mouse-induced pluripotent stem cells

Despite recent cell-engineering advances, treatment and repair of cartilage remains challenging. Although stem cell transplantation therapy using mesenchymal stem cells (MSCs) is considered a prominent strategy, the major problem of limited proliferative capacity of autologous cells has been unsolved. Recently, an induced pluripotent stem (iPS) cell line was suggested as an alternative way to cure various human diseases due to their potential proliferating infinitely while possessing the capacity to form all types of cells. However, the method to induce lineage-restricted differentiation has not been well examined or established. Here, we suggest a simple method to induce mesenchymal progenitors possessing chondrogenic property from mouse iPS cells. The MSC-like cells produced in our study expressed some MSC markers, and could also differentiate to osteoblast and adipocyte. The present study demonstrates the property of iPS cells as an alternative candidate for treatment of articular disorders, and suggests an effective approach for preparing chondrocyte from iPS cells.

Increased proliferation and analysis of differential gene expression in human Wharton's jelly-derived mesenchymal stromal cells under hypoxia

Multipotent mesenchymal stromal cells (MSCs) from Wharton's jelly (WJ) of umbilical cord bear higher proliferation rate and self-renewal capacity than adult tissue-derived MSCs and are a primitive stromal cell population. Stem cell niche or physiological microenvironment plays a crucial role in maintenance of stem cell properties and oxygen concentration is an important component of the stem cell niche. Low oxygen tension or hypoxia is prevalent in the microenvironment of embryonic stem cells and many adult stem cells at early stages of development. Again, in vivo, MSCs are known to home specifically to hypoxic events following tissue injuries. Here we examined the effect of hypoxia on proliferation and in vitro differentiation potential of WJ-MSCs. Under hypoxia, WJ-MSCs exhibited improved proliferative potential while maintaining multi-lineage differentiation potential and surface marker expression. Hypoxic WJ-MSCs expressed higher mRNA levels of hypoxia inducible factors, notch receptors and notch downstream gene HES1. Gene expression profile of WJ-MSCs exposed to hypoxia and normoxia was compared and we identified a differential gene expression pattern where several stem cells markers and early mesodermal/endothelial genes such as DESMIN, CD34, ACTC were upregulated under hypoxia, suggesting that in vitro culturing of WJ-MSCs under hypoxic conditions leads to adoption of a mesodermal/endothelial fate. Thus, we demonstrate for the first time the effect of hypoxia on gene expression and growth kinetics of WJ-MSCs. Finally, although WJ-MSCs do not induce teratomas, under stressful and long-term culture conditions, MSCs can occasionally undergo transformation. Though there were no chromosomal abnormalities, certain transformation markers were upregulated in a few of the samples of WJ-MSCs under hypoxia.

Empowering self-renewal and differentiation: the role of mitochondria in stem cells

Stem cells are characterized by their multi-lineage differentiation potential (pluripotency) and their ability for self-renewal, which permits them to proliferate while avoiding lineage commitment and senescence. Recent studies demonstrate that undifferentiated, pluripotent stem cells display lower levels of mitochondrial mass and oxidative phosphorylation, and instead preferentially use non-oxidative glycolysis as a major source of energy. Hypoxia is a potent suppressor of mitochondrial oxidation and appears to promote "stemness" in adult and embryonic stem cells. This has lead to an emerging paradigm, that mitochondrial oxidative metabolism is not just an indicator of the undifferentiated state of stem cells, but may also regulate the pluripotency and self-renewal of stem cells. The identification of specific mitochondrial pathways that regulate stem cell fate may therefore enable metabolic programming and reprogramming of stem cells.

Extracellular matrix isolated from foreskin fibroblasts supports long-term xeno-free human embryonic stem cell culture

Human embryonic stem (hES) cells hold great promise for application of human cell and tissue replacement therapy. However, the overwhelming majority of currently available hES cell lines have been directly or indirectly exposed to materials containing animal-derived components during their derivation, propagation, and cryopreservation. Unlike feeder-based cultures, which require the simultaneous growth of feeder and stem cells, resulting in mixed cell populations, stem cells grown on feeder-free systems are easily separated from the surface, presenting a pure population of cells for downstream applications. In this study, we have developed a novel method to expand hES cells in xeno-free, feeder-free conditions using 2 different matrices derived from xeno-free human foreskin fibroblasts (XF-HFFs). Using XF-HFF-derived extracellular matrix, together with 100 ng/mL recombinant bFGF-supplemented HEScGRO Basal Medium, long-term xeno-free expansion of hES cells is possible. Resulting hES cells were subjected to stringent tests and were found to maintain ES cell features, including morphology, pluripotency, stable karyotype, and expression of cell surface markers, for at least 20 passages. Xeno-free culturing practices are essential for the translation of basic hES cell research into the clinic. Therefore, the method presented in this study demonstrates that hES cells can be cultured in complete xeno-free conditions without the loss of pluripotency and furthermore, without the possibility of contamination from exogenous sources.

Derivation of pre-X inactivation human embryonic stem cells under physiological oxygen concentrations

The presence of two active X chromosomes (XaXa) is a hallmark of the ground state of pluripotency specific to murine embryonic stem cells (ESCs). Human ESCs (hESCs) invariably exhibit signs of X chromosome inactivation (XCI) and are considered developmentally more advanced than their murine counterparts. We describe the establishment of XaXa hESCs derived under physiological oxygen concentrations. Using these cell lines, we demonstrate that (1) differentiation of hESCs induces random XCI in a manner similar to murine ESCs, (2) chronic exposure to atmospheric oxygen is sufficient to induce irreversible XCI with minor changes of the transcriptome, (3) the Xa exhibits heavy methylation of the XIST promoter region, and (4) XCI is associated with demethylation and transcriptional activation of XIST along with H3K27-me3 deposition across the Xi. These findings indicate that the human blastocyst contains pre-X-inactivation cells and that this state is preserved in vitro through culture under physiological oxygen.

Adaptation to oxygen deprivation in cultures of human pluripotent stem cells, endothelial progenitor cells, and umbilical vein endothelial cells

Hypoxia plays an important role in vascular development through hypoxia-inducible factor-1alpha (HIF-1alpha) accumulation and downstream pathway activation. We sought to explore the in vitro response of cultures of human embryonic stem cells (hESCs), induced pluripotent stem cells (iPSCs), human endothelial progenitor cells (hEPCs), and human umbilical cord vein endothelial cells (HUVECs) to normoxic and hypoxic oxygen tensions. We first measured dissolved oxygen (DO) in the media of adherent cultures in atmospheric (21% O(2)), physiological (5% O(2)), and hypoxic oxygen conditions (1% O(2)). In cultures of both hEPCs and HUVECs, lower oxygen consumption was observed when cultured in 1% O(2). At each oxygen tension, feeder-free cultured hESCs and iPSCs were found to consume comparable amounts of oxygen. Transport analysis revealed that the oxygen uptake rate (OUR) of hESCs and iPSCs decreased distinctly as DO availability decreased, whereas the OUR of all cell types was found to be low when cultured in 1% O(2), demonstrating cell adaptation to lower oxygen tensions by limiting oxygen consumption. Next, we examined HIF-1alpha accumulation and the expression of target genes, including VEGF and angiopoietins (ANGPT; angiogenic response), GLUT-1 (glucose transport), BNIP3, and BNIP3L (autophagy and apoptosis). Accumulations of HIF-1alpha were detected in all four cell lines cultured in 1% O(2). Corresponding upregulation of VEGF, ANGPT2, and GLUT-1 was observed in response to HIF-1alpha accumulation, whereas upregulation of ANGPT1 was detected only in hESCs and iPSCs. Upregulation of BNIP3 and BNIP3L was detected in all cells after 24-h culture in hypoxic conditions, whereas apoptosis was not detectable using flow cytometry analysis, suggesting that BNIP3 and BNIP3L can lead to cell autophagy rather than apoptosis. These results demonstrate adaptation of all cell types to hypoxia but different cellular responses, suggesting that continuous measurements and control over oxygen environments will enable us to guide cellular responses.

The effect of human embryonic stem cells (hESCs) long-term normoxic and hypoxic cultures on the maintenance of pluripotency

The maintenance of pluripotency of human embryonic stem cells (hESCs) requires a high efficiency of self-renewal. During in vitro propagation, however, spontaneous differentiation occurs frequently, and there is also a risk of chromosomal changes. In this study, we assessed the properties of hESCs after long-term culture at ambient air and 5% oxygen growth conditions. The hESC lines were grown for up to 42 and 18 mo in normoxic and hypoxic conditions, respectively, and their proliferation; expression of Oct4, SSEA1, Nanog, and Notch1; karyotype; telomerase activity; and differentiation potential in vitro were evaluated. In contrast to cultures at 20% oxygen, where the central zones of the colonies underwent spontaneous differentiation, during exposure to 5% oxygen, the hESC colonies maintained a homogenous and flat morphology that was consistent with the presence of Oct4-positive undifferentiated phenotype. Irrespective of oxygen concentration, the undifferentiated cells expressed high levels of Nanog and Oct4 transcripts, normal karyotype, and high telomerase activity. When assayed for differentiation potential, they yielded derivatives of all three embryonic germ layers. Our data thus indicate that hypoxic exposure has the capacity to sustain enhanced long-term self-renewal of hESCs. The hESC lines described in the current paper can be obtained for research purposes from the Laboratory for Stem Cell Research, Aalborg University.

The role of hypoxia and neurogenic genes (Mash-1 and Prox-1) in the developmental programming and maturation of pulmonary neuroendocrine cells in fetal mouse lung

Pulmonary neuroendocrine cells (PNECs) are the first cell type to differentiate within the primitive airway epithelium, suggesting a possible role in lung development. The differentiation of PNECs in fetal lung is governed by proneural genes such as the mammalian homolog of the achaete-scute complex (Mash-1) and a related transcription factor, hairy and enhancer of split1 (Hes-1). We examined the expression of Mash-1 and a downstream transcription factor Prox-1 in the developing mouse lung of wild-type and respective knockout mouse models. During early stages (embryonic day 12, E12) of development, only some PNECs expressed Mash-1 and Prox-1, but by E15, all PNECs coexpressed both transcription factors. PNECs failed to develop in Mash-1 but not in Prox-1-null mice, indicating that Mash-1 is essential for the initiation of the PNEC phenotype, whereas Prox-1 is associated with the development of this phenotype. As lung develops within a low O(2) environment (fetal euoxia, pO(2) approximately 20 to 30 mm Hg), we examined the effects of hypoxia on PNEC differentiation. Organ cultures of fetal mouse lungs at E12 and E16 were maintained under either 20% O(2) (normoxia, Nox) or 5% O(2) (hypoxia, Hox) and were examined every 24 h for up to 6 days in culture. In E12 explants, Hox enhanced branching morphogenesis and increased cell proliferation, but PNEC numbers and Mash-1 expression were significantly reduced. This effect could be reversed by switching the explants back to Nox. In contrast, Hox had no apparent effect on Hes-1 expression. Similarly, Hox had no effect on airway branching, PNEC numbers, or Mash-1 expression in E16 explants, indicating locked-in developmental programming. We suggest that during early stages of lung development, pO(2) concentration in concert with neurogenic gene expression modulates PNEC phenotype. Thus, disturbances in intrauterine pO(2) homeostasis could alter the functional maturation of the PNEC system and hence be involved in the pathogenesis of various perinatal pulmonary disorders.

[Ex vivo expansion of hematopoietic stem cells: concept and clinical benefit]

A new discipline was born and grew up over the last 4 decades of 20th century: Experimental Hematology. In addition to yield the concept of Stemness, a paradigm later applied for the other tissues than hematopoietic one, it provided the results allowing a preclinical development and a therapeutic exploitation. The concept of ex vivo expansion of hematopoietic cells for transplantation is directly issued from this knowledge. It enabled us to realize that a critical quantity of different sub-populations of stem and progenitor cells are necessary to obtain a rapid and sustained hematopoietic reconstitution. These principles, transposed to human cells (originating from: bone marrow, peripheral blood, cord blood) required some important technological innovations (conception of the specific media, recombinant technology of cytokine production...), to achieve, after several attempts, the first efficient clinical trials (at the moment for cells mobilized in peripheral blood). This goal remains to be achieved for cord blood cells too. The developments in this field as well as its actual state are the subjects of this review.

Generating specificity and diversity in the transcriptional response to hypoxia

The sensing of oxygen levels and maintenance of oxygen homeostasis is crucial for cells. The hypoxic-sensitive regulation of gene expression allows information about the oxygen status to be converted into appropriate cellular responses. Although there is a core transcriptional pathway, the signalling cascade can be modified to allow diversity and specificity in the transcriptional output. In this Review, we discuss recent advances in our understanding of the mechanisms and factors that contribute to the observed diversity and specificity. A deeper knowledge about how hypoxic signalling is tuned will further our understanding of the cellular hypoxic response in normal physiology and how it becomes derailed in disease.

Cellular cardiac regenerative therapy in which patients?

Cell-based myocardial regenerative therapy is undergoing experimental and clinical trials in order to limit the consequences of decreased contractile function and compliance of damaged ventricles owing to ischemic and nonischemic myocardial diseases. A variety of myogenic and angiogenic cell types have been proposed, such as skeletal myoblasts, mononuclear and mesenchymal bone marrow cells, circulating blood-derived progenitors, adipose-derived stromal cells, induced pluripotent stem cells, umbilical cord cells, endometrial mesenchymal stem cells, adult testis pluripotent stem cells and embryonic cells. Current indications for stem cell therapy concern patients who have had a left- or right-ventricular infarction or idiopathic dilated cardiomyopathies. Other indications and potential applications include patients with diabetic cardiomyopathy, Chagas heart disease (American trypanosomiasis), ischemic mitral regurgitation, left ventricular noncompacted myocardium and pediatric cardiomyopathy. Suitable sources of cells for cardiac implant will depend on the types of diseases to be treated. For acute myocardial infarction, a cell that reduces myocardial necrosis and augments vascular blood flow will be desirable. For heart failure, cells that replace or promote myogenesis, reverse apoptopic mechanisms and reactivate dormant cell processes will be useful. It is important to note that stem cells are not an alternative to heart transplantation; selected patients should be in an early stage of heart failure as the goal of this regenerative approach is to avoid or delay organ transplantation. Since the cell niche provides crucial support needed for stem cell maintenance, the most interesting and realistic perspectives include the association of intramyocardial cell transplantation with tissue-engineered scaffolds and multisite cardiac pacing in order to transform a passive regenerative approach into a 'dynamic cellular support', a promising method for the creation of 'bioartificial myocardium'.

The role of hypoxia in stem cell differentiation and therapeutics

Stem cells differentiate into a variety of cell lines, making them attractive for tissue engineering and regenerative medicine. Specific microenvironmental cues regulate self-renewal and differentiation capabilities. Oxygen is an important component of the cellular microenvironment, serving as both metabolic substrate and signaling molecule. Oxygen has been shown to have a variety of effects on embryonic and adult stem cells. This review examines the role of hypoxia in regulating stem cell biology, specifically focusing on growth, maintenance of pluripotency, differentiation, and production of growth factors. Particular attention is paid to hypoxia and stem cells in relation to therapeutic angiogenesis. We conclude that further study is needed to optimize the use of hypoxia as a stimulus for various stem cell functions, including its potential role in therapeutic angiogenesis.

Enhanced proliferation capacity of porcine tenocytes in low O2 tension culture

Tenocytes reside in relatively avascular tissue and are difficult to expand due to phenotype drift and functional loss. Thus low O(2) tension culture was employed to enhance the expansion capability. The results demonstrated that low O(2) tension (2% O(2)) culture could significantly enhance the expansion of newborn pig tenocytes with 275-473% greater cell yield per cell passage that that of regular O(2) cultured (21% O(2)) cells. Importantly, low O(2) culture did not change the gene expression of functional molecule such as collagens I and III, decorin, prolyl 4-hydroxylase (P4H), lysyl oxidase (LOX), TIMP-1 and TIMP-2, but could significantly down regulate the gene expression of MMP-1 and IL-6. In conclusion, low O(2) tension culture can significantly enhance the expansion capacity of tenocytes without affecting their phenotype and functions.

Hypoxia and pluripotency in embryonic and embryonal carcinoma stem cell biology

Low oxygen availability (hypoxia) is a hallmark of rapidly proliferating tumors and has been suggested to be a characteristic of the embryonic and adult stem cell niche. The idea of relating cancer to stem cells is increasingly popular due to the identification of specific cancer stem cells sharing the typical plasticity and motility of pluripotent stem cells. Hypoxia plays a critical role in early embryonic development and in tumor progression, participating in processes such as angiogenesis, apoptosis, cell migration, invasion and metastasis. Some of the molecular pathways that have been shown to mediate these hypoxia-induced responses, such as the hypoxia inducible factor (HIF)-1alpha and Notch signaling, appear to be active in both embryonic and neoplastic pluripotent stem cells. Nevertheless, the mechanisms underlying these regulatory processes are not yet fully understood. In this review, we attempt to shed some light on the mechanisms involved in hypoxia-dependent processes related to stem cell features and tumor progression, such as the maintenance of the undifferentiated state, cell proliferation, tumor neovascularization, extra-cellular matrix degradation and motility factor up-regulation. With this purpose in mind, we summarize recent observations in embryonic, adult and cancer stem cells that demonstrate the parallelism existing in their hypoxia responses. Finally, based on the observations of our own laboratory and others, we suggest that the comparative analysis of the response to low oxygen levels of embryonic stem cells and cancer stem cells (such as embryonal carcinoma cells), may throw fresh light on our understanding of the mechanisms underlying hypoxia-induced invasiveness and the resistance to anticancer treatments, thereby stimulating the development of novel therapeutic strategies.

Propensity of human embryonic stem cell lines during early stage of lineage specification controls their terminal differentiation into mature cell types

Human embryonic stem cells (hESCs) are able to stably maintain their characteristics for an unlimited period; nevertheless, substantial differences among cell lines in gene and protein expression not manifested during the undifferentiated state may appear when cells differentiate. It is widely accepted that developing an efficient protocol to control the differentiation of hESCs will enable us to produce adequate numbers of desired cell types with relative ease for diverse applications ranging from basic research to cell therapy and drug screening. Hence of late, there has been considerable interest in understanding whether and how hESC lines are equivalent or different to each other in their in vitro developmental tendencies. In this study, we compared the developmental competences of two hESC lines (HUES-9 and HUES-7) at molecular, cellular and functional levels, upon spontaneous differentiation without any added inducing agents. Both cell lines generated the three embryonic germ layers, extra-embryonic tissues and primordial germ cells during embryoid body (EB) formation. However HUES-9 showed a stronger propensity towards formation of neuroectodermal lineages, whereas HUES-7 differentiated preferentially into mesoderm and endoderm. Upon further differentiation, HUES-9 generated largely neural cells (neurons, oligodendrocytes, astrocytes and gangliosides) whereas HUES-7 formed mesendodermal derivatives, including cardiomyocytes, skeletal myocytes, endothelial cells, hepatocytes and pancreatic cells. Overall, our findings endorse the hypothesis that independently-derived hESCs biologically differ among themselves, thereby displaying varying differentiation propensity. These subtle differences not only highlight the importance of screening and deriving lines for lineage-specific differentiation but also indicate that individual lines may possess a repertoire of capabilities that is unique.