Transdifferentiation and nuclear reprogramming in hematopoietic development and neoplasia

Myeloid-Derived Lymphatic Endothelial Cell Progenitors Significantly Contribute to Lymphatic Metastasis in Clinical Breast Cancer

Lymphatic metastasis is a high-impact prognostic factor for mortality of breast cancer (BC) patients, and it directly depends on tumor-associated lymphatic vessels. We previously reported that lipopolysaccharide-induced inflammatory lymphangiogenesis is strongly promoted by myeloid-derived lymphatic endothelial cell progenitors (M-LECPs) derived from the bone marrow (BM). As BC recruits massive numbers of provascular myeloid cells, we hypothesized that M-LECPs, within this recruited population, are specifically programmed to promote tumor lymphatics that increase lymph node metastasis. In support of this hypothesis, high levels of M-LECPs were found in peripheral blood and tumor tissues of BC patients. Moreover, the density of M-LECPs and lymphatic vessels positive for myeloid marker proteins strongly correlated with patient node status. It was also established that tumor M-LECPs coexpress lymphatic-specific, stem/progenitor and M2-type macrophage markers that indicate their BM hematopoietic-myeloid origin and distinguish them from mature lymphatic endothelial cells, tumor-infiltrating lymphoid cells, and tissue-resident macrophages. Using four orthotopic BC models, we show that mouse M-LECPs are similarly recruited to tumors and integrate into preexisting lymphatics. Finally, we demonstrate that adoptive transfer of in vitro differentiated M-LECPs, but not naïve or nondifferentiated BM cells, significantly increased metastatic burden in ipsilateral lymph nodes. These data support a causative role of BC-induced lymphatic progenitors in tumor lymphangiogenesis and suggest molecular targets for their inhibition.

Transdifferentiation of adipogenically differentiated cells into osteogenically or chondrogenically differentiated cells: phenotype switching via dedifferentiation

Reprogramming is a new wave in cellular therapies to achieve the vital goals of regenerative medicine. Transdifferentiation, whereas the differentiated state of cells could be reprogrammed into other cell types, meaning cells are no more locked in their differentiated circle. Hence, cells of choice from abundant and easily available sources such as fibroblast and adipose tissue could be converted into cells of demand, to restore the diseased tissues. Before diverting this new approach into effective clinical use, transdifferentiation could not be simply overlooked, as it challenges the normal paradigms of biological laws, where mature cells transdifferentiate not only within same germ layers, but even across the lineage boundaries. How unipotent differentiated cells reprogram into another, and whether transdifferentiation proceeds via a direct cell-to-cell conversion or needs dedifferentiation. To address such questions, MSC were adipogenically differentiated followed by direct transdifferentiation, and subsequently examined by histology, immunohistochemistry, qPCR and single cell analysis. Direct cellular conversion of adipogenic lineage cells into osteogenic or chondrogenic resulted in mixed culture of both lineage cells (adipogenic and new acquiring osteogenic/chondrogenic phenotypes). On molecular level, such conversion was confirmed by significantly upregulated expression of PPARG, FABP4, SPP1 and RUNX2. Chondrogenic transdifferentiation was verified by significantly upregulated expression of PPARG, FABP4, SOX9 and COL2A1. Single cell analysis did not support the direct cell-to-cell conversion, rather described the involvement of dedifferentiation. Moreover, some differentiated single cells did not change their phenotype and were resistant to transdifferentiation, suggesting that differentiated cells behave differently during cellular conversion. An obvious characterization of differentiated cells could be helpful to understand the process of transdifferentiation.

Evidence for bone marrow adult stem cell plasticity: properties, molecular mechanisms, negative aspects, and clinical applications of hematopoietic and mesenchymal stem cells transdifferentiation

In contrast to the pluripotent embryonic stem cells (ESCs) which are able to give rise to all cell types of the body, mammalian adult stem cells (ASCs) appeared to be more limited in their differentiation potential and to be committed to their tissue of origin. Recently, surprising new findings have contradicted central dogmas of commitment of ASCs by showing their plasticity to differentiate across tissue lineage boundaries, irrespective of classical germ layer designations. The present paper supports the plasticity of the bone marrow stem cells (BMSCs), bringing the most striking and the latest evidences of the transdifferentiation properties of the bone marrow hematopoietic and mesenchymal stem cells (BMHSCs, and BMMSCs), the two BM populations of ASCs better characterized. In addition, we report the possible mechanisms that may explain these events, outlining the clinical importance of these phenomena and the relative problems.

Lineage conversion methodologies meet the reprogramming toolbox

Lineage conversion has recently attracted increasing attention as a potential alternative to the directed differentiation of pluripotent cells to obtain cells of a given lineage. Different means allowing for cell identity switch have been reported. Lineage conversion relied initially on the discovery of specific transcription factors generally enriched and characteristic of the target cell, and their forced expression in cells of a different fate. This approach has been successful in various cases, from cells of the hematopoietic systems to neurons and cardiomyocytes. Furthermore, recent reports have suggested the possibility of establishing a general lineage conversion approach bypassing pluripotency. This requires a first phase of epigenetic erasure achieved by short overexpression of the factors used to reprogram cells to a pluripotent state (such as a combination of Sox2, Klf4, c-Myc and Oct4), followed by exposure to specific developmental cues. Here we present these different direct conversion methodologies and discuss their potential as alternatives to using induced pluripotent stem cells and differentiation protocols to generate cell populations of a given fate.

microRNA-based cancer cell reprogramming technology

Epigenetic modifications play crucial roles in cancer initiation and development. Complete reprogramming can be achieved through the introduction of defined biological factors such as Oct4, Sox2, Klf4, and cMyc into mouse and human fibroblasts. Introduction of these transcription factors resulted in the modification of malignant phenotype behavior. Recent studies have shown that human and mouse somatic cells can be reprogrammed to become induced pluripotent stem cells using forced expression of microRNAs, which completely eliminates the need for ectopic protein expression. Considering the usefulness of RNA molecules, microRNA-based reprogramming technology may have future applications in regenerative and cancer medicine.

Stem cell transplantation for Huntington's disease

By way of commentary on a recent report that transplanted adult neural progenitor cells can alleviate functional deficits in a rat lesion model of Huntington's disease [Vazey, E.M., Chen, K., Hughes, S.M., Connor, B., 2006. Transplanted adult neural progenitor cells survive, differentiate and reduce motor function impairment in a rodent model of Huntington's disease. Exp. Neurol. 199, 384-396], we review the current status of the field exploring the use of stem cells, progenitor cells and immortalised cell lines to repair the lesioned striatum in animal models of the human disease. A remarkably rich range of alternative cell types have been used in various animal models, several of which exhibit cell survival and incorporation in the host brain, leading to subsequent functional recovery. In comparing the alternatives with the 'gold standard' currently offered by primary tissue grafts, key issues turn out to be: cell survival, differentiation prior to and following implantation into striatal-like phenotypes, integration and connectivity with the host brain, the nature of the electrophysiological, motor and cognitive tests used to assess functional repair, and the mechanisms by which the grafts exert their function. Although none of the alternatives yet has the capacity to match primary fetal tissues for functional repair, that standard is itself limited, and the long term goal must be not just to match but to surpass present capabilities in order to achieve fully functional reconstruction reliably, flexibly, and on demand.

Cardiac regeneration by resident stem and progenitor cells in the adult heart

Two main pieces of data have created a new field in cardiac research. First, the traditional view on the heart as a postmitotic organ has been challenged by the finding of small dividing cells in the heart expressing cardiac contractile proteins with stem cell properties and, second, cellular therapy of the diseased heart using a variety of different cells has shown encouraging effects on cardiac function. These findings immediately raise questions like "what is the identity and origin of the cardiac progenitor cells?","which molecular factors are involved in their mobilization and differentiation?", and "can these cells repair the damaged heart?" This review will address the state of current answers to these questions. Emerging evidence suggests that several subpopulations of cardiac stem or progenitor cells (CPCs) reside within the adult heart. CPCs with the ability to differentiate into all the constituent cells in the adult heart including cardiac myocytes, vascular smooth muscle and endothelial cells have been identified. Valuable knowledge has been obtained from the large number of animal studies and a number of small clinical trials that have utilized a variety of adult stem cells for regenerating infarcted hearts. However, contradictory reports on the regenerative potential of the CPCs exist, and the mechanisms behind the reported hemodynamic effects are intensely debated. Besides directly replenishing cardiac tissue, CPCs could also function by stimulating angiogenesis and improving survival of existing cells by secretion of paracrine factors. With this review we suggest that a better understanding of CPC biology will be pivotal for progressing therapeutic cardiac regeneration. This includes an extended knowledge of the molecular mechanisms behind their mobilization, differentiation, survival and integration in the myocardium.

Functional osteoclast-like transformation of cultured human myeloma cell lines

Hyperactive osteoclastogenesis is a hallmark of multiple myeloma, a B cell neoplasia homing to bone marrow and resulting in multiple osteolytic lesions and skeleton devastation. We provide evidence that myeloma cells can themselves act as osteoclasts in vitro. By extending standard cultures of U-266 and MCC-2 myeloma cell lines, we found that subsets of adherent cells also expressed the osteoclast phenotype, including multinuclear morphology, cytoplasmic tartrate-resistant acid phosphatase, the calcitonin receptor and a specific osteoclast antigen. These subsets resorbed bone substrates by producing osteoclast enzymes as well as the characteristic redistribution of F-actin in their cytoskeleton, thus forming the sealing zone that is adopted by adherent osteoclasts to generate the acidified environment essential for bone resorption. Neither the phenotype nor the functional properties of osteoclasts were detected in parental non-adherent cells. In adherent cultures osteoclastogenesis was associated with deregulated expression of both receptor activator of nuclear transcription factor (NF)-kappaB (RANK) and its ligand RANK-L, which triggers cell maturation in osteoclast precursors. Resorption of bone substrates was prevented by a neutralising anti-RANK-L antibody. Our data indicate that osteoclast-like transformation of both U-266 and MCC-2 cellular models of human myeloma is dependent on RANK-L stimulation.

Cardiac stem cells and mechanisms of myocardial regeneration

This review discusses current understanding of the role that endogenous and exogenous progenitor cells may have in the treatment of the diseased heart. In the last several years, a major effort has been made in an attempt to identify immature cells capable of differentiating into cell lineages different from the organ of origin to be employed for the regeneration of the damaged heart. Embryonic stem cells (ESCs) and bone marrow-derived cells (BMCs) have been extensively studied and characterized, and dramatic advances have been made in the clinical application of BMCs in heart failure of ischemic and nonischemic origin. However, a controversy exists concerning the ability of BMCs to acquire cardiac cell lineages and reconstitute the myocardium lost after infarction. The recognition that the adult heart possesses a stem cell compartment that can regenerate myocytes and coronary vessels has raised the unique possibility to rebuild dead myocardium after infarction, to repopulate the hypertrophic decompensated heart with new better functioning myocytes and vascular structures, and, perhaps, to reverse ventricular dilation and wall thinning. Cardiac stem cells may become the most important cell for cardiac repair.

Point mutation in AML1 disrupts subnuclear targeting, prevents myeloid differentiation, and effects a transformation-like phenotype

The multifunctional C terminus of the hematopoietic AML1 transcription factor interacts with coregulatory proteins, supports the convergence and integration of physiological signals, and contains the nuclear matrix targeting signal, the protein motif that is necessary and sufficient to target AML1 to subnuclear sites. The (8;21) chromosomal translocation, which replaces the C terminus of AML1 with the ETO protein, modifies subnuclear targeting of AML1 in acute myeloid leukemia (AML) and results in defective myelopoiesis. We therefore addressed the relevance of AML1 subnuclear targeting and associated functions that reside in the C terminus to myeloid differentiation. A single amino acid substitution that abrogates intranuclear localization was introduced in the AML1 subnuclear targeting signal. Expression of the mutant AML1 protein blocks differentiation of myeloid progenitors to granulocytes in the presence of endogenous AML1 protein, as also occurs in the (8;21) chromosomal translocation, where only one allele of the AML1 gene is affected. The cells expressing the mutant AML1 protein continue to proliferate, maintain an immature blast-like morphology, and exhibit transformed properties that are hallmarks of leukemogenesis. These findings functionally link AML1 subnuclear targeting with competency for myeloid differentiation and expression of the transformed/leukemia phenotype.

Haemopoietic stem cells

Considerable effort has been made in recent years in understanding the mechanisms that govern stem cell generation, proliferation, self-renewal, commitment and lately plasticity. In the development of the haemopoietic system during embryonic and fetal life the notion of different pools of stem cells arising from the endothelium is gaining consensus. Gene expression profiling of populations of stem cells is bringing to light categories of genes important for self-renewal or commitment. Besides the role of transcription factors in lineage decision, the role of soluble factors and transmembrane proteins, very active at the time of embryo development, are taking central stage in the maintenance and in vitro expansion of haemopoietic stem cells (HSCs). The hierarchical model of haemopoietic development is being questioned with reports of lineage switching and plasticity of haemopoietic stem cells to non-haemopoietic cells. Yet the understanding of the overall process is still very fragmented and hypothetical. This is mainly due to the absence of appropriate markers to enable selection of homogeneous stem cell populations and the need to rely on retrospective functional assays, able only to determine the overall behaviour of a population of cells. This review is intended to be an overview of the haemopoietic system and a critical re-visitation of issues such as plasticity and self-renewal important for therapeutic applications of haemopoietic stem cells.