Autocrine transforming growth factor-beta regulation of hematopoiesis: many outcomes that depend on the context

Disruption of a Quorum Sensing mechanism triggers tumorigenesis: a simple discrete model corroborated by experiments in mammary cancer stem cells

Background

The balance between self-renewal and differentiation of stem cells is expected to be tightly controlled in order to maintain tissue homeostasis throughout life, also in the face of environmental hazards. Theory, predicting that homeostasis is maintained by a negative feedback on stem cell proliferation, implies a Quorum Sensing mechanism in higher vertebrates.

Results

Application of this theory to a cellular automata model of stem cell development in disrupted environments shows a sharply dichotomous growth dynamics: maturation within 50-400 cell cycles, or immortalization. This dichotomy is mainly driven by intercellular communication, low intensity of which causes perpetual proliferation. Another driving force is the cells' kinetic parameters. Reduced tissue life span of differentiated cells results in uncontrolled proliferation. Model's analysis, showing that under the Quorum Sensing control, stem cell fraction within a steady state population is fixed, is corroborated by experiments in breast carcinoma cells. Experimental results show that the plating densities of CD44+ cells and of CD44+/24lo/ESA+ cells do not affect stem cell fraction near confluence.

Conclusions

This study suggests that stem cell immortalization may be triggered by reduced intercellular communication, rather than exclusively result from somatic evolution, and implies that stem cell proliferation can be attenuated by signal manipulation, or enhanced by cytotoxics targeted to differentiated cells. In vivo verification and identification of the Quorum Sensing mediating molecules will pave the way to a higher level control of stem cell proliferation in cancer and in tissue engineering.

Reviewers

This article was reviewed by Glenn Webb and Marek Kimmel.

Complex and context dependent regulation of hematopoiesis by TGF-beta superfamily signaling

The transforming growth factor (TGF)-beta superfamily of growth factors, including the TGF-betas, activins, and bone morphogenetic proteins (BMPs), provide cells with a broad spectrum of regulatory signals through the intracellular Smad pathway. Since loss-of-function studies of a majority of the TGF-beta superfamily members result in embryonic lethality, much of our current knowledge of the TGF-beta superfamily's role in hematopoiesis is generated from studies performed in vitro, or in very early stages of embryonic development. TGF-beta is well documented as a potent inhibitor of hematopoietic stem cell (HSC) proliferation in vitro, while its role in vivo is largely unknown. BMP signaling is crucial for the initiation of hematopoiesis in the developing embryo, although its role in adult hematopoiesis remains elusive. More recently we and others have used conditional knockout models to unravel the role of several components of TGF-beta family signaling in adult hematopoiesis. Here we review the currently known functions for the major factors of this signaling family in embryonic and adult hematopoietic regulation and discuss the context dependency and complexity that permeate this regulation.

Hematopoietic stem and progenitor cells: their mobilization and homing to bone marrow and peripheral tissue

Hematopoietic stem and progenitor cells (HSPCs) are a rare population of precursor cells that possess the capacity for self-renewal and multilineage differentiation. In the bone marrow (BM), HSPCs warrant blood cell homeostasis. In addition, they may also replenish tissue-resident myeloid cells and directly participate in innate immune responses once they home to peripheral tissues. In this review, we summarize recent data on the signaling molecules that modulate the mobilization of HSPCs from BM and their migration to peripheral tissues.

Detection of microRNA expression in human peripheral blood microvesicles

BACKGROUND

MicroRNAs (miRNA) are small non-coding RNAs that regulate translation of mRNA and protein. Loss or enhanced expression of miRNAs is associated with several diseases, including cancer. However, the identification of circulating miRNA in healthy donors is not well characterized. Microvesicles, also known as exosomes or microparticles, circulate in the peripheral blood and can stimulate cellular signaling. In this study, we hypothesized that under normal healthy conditions, microvesicles contain miRNAs, contributing to biological homeostasis.

METHODOLOGY/PRINCIPAL FINDINGS

Microvesicles were isolated from the plasma of normal healthy individuals. RNA was isolated from both the microvesicles and matched mononuclear cells and profiled for 420 known mature miRNAs by real-time PCR. Hierarchical clustering of the data sets indicated significant differences in miRNA expression between peripheral blood mononuclear cells (PBMC) and plasma microvesicles. We observed 71 miRNAs co-expressed between microvesicles and PBMC. Notably, we found 33 and 4 significantly differentially expressed miRNAs in the plasma microvesicles and mononuclear cells, respectively. Prediction of the gene targets and associated biological pathways regulated by the detected miRNAs was performed. The majority of the miRNAs expressed in the microvesicles from the blood were predicted to regulate cellular differentiation of blood cells and metabolic pathways. Interestingly, a select few miRNAs were also predicted to be important modulators of immune function.

CONCLUSIONS

This study is the first to identify and define miRNA expression in circulating plasma microvesicles of normal subjects. The data generated from this study provides a basis for future studies to determine the predictive role of peripheral blood miRNA signatures in human disease and will enable the definition of the biological processes regulated by these miRNA.

A high confidence, manually validated human blood plasma protein reference set

Background

The immense diagnostic potential of human plasma has prompted great interest and effort in cataloging its contents, exemplified by the Human Proteome Organization (HUPO) Plasma Proteome Project (PPP) pilot project. Due to challenges in obtaining a reliable blood plasma protein list, HUPO later re-analysed their own original dataset with a more stringent statistical treatment that resulted in a much reduced list of high confidence (at least 95%) proteins compared with their original findings. In order to facilitate the discovery of novel biomarkers in the future and to realize the full diagnostic potential of blood plasma, we feel that there is still a need for an ultra-high confidence reference list (at least 99% confidence) of blood plasma proteins.

Methods

To address the complexity and dynamic protein concentration range of the plasma proteome, we employed a linear ion-trap-Fourier transform (LTQ-FT) and a linear ion trap-Orbitrap (LTQ-Orbitrap) for mass spectrometry (MS) analysis. Both instruments allow the measurement of peptide masses in the low ppm range. Furthermore, we employed a statistical score that allows database peptide identification searching using the products of two consecutive stages of tandem mass spectrometry (MS3). The combination of MS3 with very high mass accuracy in the parent peptide allows peptide identification with orders of magnitude more confidence than that typically achieved.

Results

Herein we established a high confidence set of 697 blood plasma proteins and achieved a high 'average sequence coverage' of more than 14 peptides per protein and a median of 6 peptides per protein. All proteins annotated as belonging to the immunoglobulin family as well as all hypothetical proteins whose peptides completely matched immunoglobulin sequences were excluded from this protein list. We also compared the results of using two high-end MS instruments as well as the use of various peptide and protein separation approaches. Furthermore, we characterized the plasma proteins using cellular localization information, as well as comparing our list of proteins to data from other sources, including the HUPO PPP dataset.

Conclusion

Superior instrumentation combined with rigorous validation criteria gave rise to a set of 697 plasma proteins in which we have very high confidence, demonstrated by an exceptionally low false peptide identification rate of 0.29%.

Tail regeneration in Xenopus laevis as a model for understanding tissue repair

Augmentation of regenerative ability is a powerful strategy being pursued for the biomedical management of traumatic injury, cancer, and degeneration. While considerable attention has been focused on embryonic stem cells, it is clear that much remains to be learned about how somatic cells may be controlled in the adult organism. The tadpole of the frog Xenopus laevis is a powerful model system within which fundamental mechanisms of regeneration are being addressed. The tadpole tail contains spinal cord, muscle, vasculature, and other terminally differentiated cell types and can fully regenerate itself through tissue renewal--a process that is most relevant to mammalian healing. Recent insight into this process has uncovered fascinating molecular details of how a complex appendage senses injury and rapidly repairs the necessary morphology. Here, we review what is known about the chemical and bioelectric signals underlying this process and draw analogies to evolutionarily conserved pathways in other patterning systems. The understanding of this process is not only of fundamental interest for the evolutionary and cell biology of morphogenesis, but will also generate information that is crucial to the development of regenerative therapies for human tissues and organs.

Efficient hematopoietic differentiation of human embryonic stem cells on stromal cells derived from hematopoietic niches

Hematopoietic stem cells derived from human embryonic stem cells (hESCs) could provide a therapeutic alternative to bone marrow transplants, but the efficiency of currently available derivation protocols is low. In this study, we investigated whether coculture with monolayers of cells derived from mouse AGM and fetal liver, or with stromal cell lines derived from these tissues, can enhance hESC hematopoietic differentiation. We found that under such conditions hESC-derived differentiating cells formed early hematopoietic progenitors, with a peak at day 18-21 of differentiation that corresponded to the highest CD34 expression. These hESC-derived hematopoietic cells were capable of primary and secondary hematopoietic engraftment into immunocompromised mice at substantially higher levels than described previously. Transcriptional and functional analysis identified TGF-beta1 and TGF-beta3 as positive enhancers of hESC hematopoietic differentiation that can further stimulate this process when added to the culture. Overall, our findings represent significant progress toward the goal of deriving functional hematopoietic stem cells from hESCs.

Tac1 regulation by RNA-binding protein and miRNA in bone marrow stroma: Implication for hematopoietic activity

Hematopoiesis is the process by which immune and blood cells are produced from a finite number of relatively few hematopoietic stem cells (HSCs). In adults, hematopoiesis occurs in the adult bone marrow (BM), with the support of stromal cells. This support partly occurs through the production of hematopoietic regulators belonging to the families of cytokines and neuropeptides/neurotransmitters, which mediate their actions through specific receptors. Thus, stromal cells could be central to the neural-hematopoietic-immune axis. This study focuses on Tac1, which encodes hematopoietic regulators belonging to the tachykinin family of neuropeptides. We examined post-transcriptional regulation of Tac1 in BM stroma. Since this gene is inducible in stroma, we selected cytokines with varying hematopoietic effects: stimulator Stem Cell Factor (SCF), broad-acting IL-11 and suppressive TGF-beta1. RNA shift with Tac1 mRNA and cytoplasmic extracts from IL-11 and SCF-stimulated stroma showed RNA shift after 15min at 37 degrees C, whereas a shift was detected with extracts from TGF-beta1-stimulated stroma after 5min at room temperature. Another level of post-transcriptional regulation was observed by the detection of miRNAs that interact with the 3' untranslated region of Tac1 mRNA. In summary, this study showed that cytokine induced miRNA downregulation and RNA-binding protein(s) are involved in post-transcriptional regulation of Tac1 in BM stroma. The broad categories of cytokines as hematopoietic stimulators or inhibitors might depend on the avidity of RNA-binding protein(s) for Tac1 mRNA, as well as the ability to degrade or stabilize the specific miRNAs.

miR-21 Gene expression triggered by AP-1 is sustained through a double-negative feedback mechanism

miR-21 has been reported to be highly expressed in various cancers and to be inducible in a human promyelocytic cell line, HL-60, after phorbol 12-myristate 13-acetate (PMA) treatment. To examine molecular mechanisms involved in miR-21 expression, we analyzed the structure of the miR-21 gene by determining its promoter and primary transcripts. We show that activation protein 1 (AP-1) activates the miR-21 transcription in conjugation with the SWI/SNF complex, after PMA stimulation, through the conserved AP-1 and PU.1 binding sites in the promoter identified here. The previous findings of enhanced miR-21 expression in several cancers may therefore reflect the elevated AP-1 activity in these carcinomas. A single precursor RNA containing miR-21 was transcribed just downstream from the TATA box in this promoter, which is located in an intron of a coding gene, TMEM49. More important, expression of this overlapping gene is completely PMA-independent and all its transcripts are polyadenylated before reaching the miR-21 hairpin embedding region, indicating that miRNAs could have their own promoter even if overlapped with other genes. By available algorithms that predict miRNA target using a conservation of sequence complementary to the miRNA seed sequence, we next predicted and confirmed that the NFIB mRNA is a target of miR-21. NFIB protein usually binds the miR-21 promoter in HL-60 cells as a negative regulator and is swept off from the miR-21 promoter during PMA-induced macrophage differentiation of HL-60. The translational repression of NFIB mRNA by miR-21 accelerates clearance of NFIB in parallel with the simultaneous miR-21-independent transcriptional repression of NFIB after PMA stimulation. Since exogenous miR-21 expression moderately induced endogenous miR-21, an evolutionarily conserved double-negative feedback regulation would be operating as a mechanism to sustain miR-21 expression.

TGF-beta signaling is required for multiple processes during Xenopus tail regeneration

Xenopus tadpoles can fully regenerate all major tissue types following tail amputation. TGF-beta signaling plays essential roles in growth, repair, specification, and differentiation of tissues throughout development and adulthood. We examined the localization of key components of the TGF-beta signaling pathway during regeneration and characterized the effects of loss of TGF-beta signaling on multiple regenerative events. Phosphorylated Smad2 (p-Smad2) is initially restricted to the p63+ basal layer of the regenerative epithelium shortly after amputation, and is later found in multiple tissue types in the regeneration bud. TGF-beta ligands are also upregulated throughout regeneration. Treatment of amputated tails with SB-431542, a specific and reversible inhibitor of TGF-beta signaling, blocks tail regeneration at multiple points. Inhibition of TGF-beta signaling immediately following tail amputation reversibly prevents formation of a wound epithelium over the future regeneration bud. Even brief inhibition immediately following amputation is sufficient, however, to irreversibly block the establishment of structures and cell types that characterize regenerating tissue and to prevent the proper activation of BMP and ERK signaling pathways. Inhibition of TGF-beta signaling after regeneration has already commenced blocks cell proliferation in the regeneration bud. These data reveal several spatially and temporally distinct roles for TGF-beta signaling during regeneration: (1) wound epithelium formation, (2) establishment of regeneration bud structures and signaling cascades, and (3) regulation of cell proliferation.

Signaling pathways governing stem-cell fate

Hematopoietic stem cells (HSCs) are historically the most thoroughly characterized type of adult stem cell, and the hematopoietic system has served as a principal model structure of stem-cell biology for several decades. However, paradoxically, although HSCs can be defined by function and even purified to near-homogeneity, the intricate molecular machinery and the signaling mechanisms regulating fate events, such as self-renewal and differentiation, have remained elusive. Recently, several developmentally conserved signaling pathways have emerged as important control devices of HSC fate, including Notch, Wingless-type (Wnt), Sonic hedgehog (Shh), and Smad pathways. HSCs reside in a complex environment in the bone marrow, providing a niche that optimally balances signals that control self-renewal and differentiation. These signaling circuits provide a valuable structure for our understanding of how HSC regulation occurs, concomitantly with providing information of how the bone marrow microenvironment couples and integrates extrinsic with intrinsic HSC fate determinants. It is the focus of this review to highlight some of the most recent developments concerning signaling pathways governing HSC fate.

STEM CELL MECHANISMS AND PARACRINE EFFECTS

Heart disease remains the leading cause of death in the industrialized world. Stem cell therapy is a promising treatment modality for injured cardiac tissue. A novel mechanism for this cardioprotection may include paracrine actions. Cardiac surgery represents the unique situation where preischemia and postischemia treatment modalities exist that may use stem cell paracrine protection. This review (1) recalls the history of stem cells in cardiac disease and the unraveling of its mechanistic basis for protection, (2) outlines the pathways for stem cell-mediated paracrine protection, (3) highlights the signaling factors expressed, (4) explores the potential of using stem cells clinically in cardiac surgery, and (5) summarizes all human stem cell studies in cardiac disease to date.

A sub-population of high proliferative potential-quiescent human mesenchymal stem cells is under the reversible control of interferon alpha/beta

Type I interferon (IFN) is shown to control the reversible quiescence of a primitive human bone marrow mesenchymal stem cell (MSC) subpopulation. A 24 h pre-treatment of Stro1+/GlycoA- or CD45-/GlycoA- subpopulations with a monoclonal antibody (mAb) against the IFNAR1 chain of the human type I IFN receptor (64G12), or with a polyclonal anti-IFNalpha antibody, resulted in a marked increase in the number of very large colonies (CFU-F >3000 cells) obtained in the presence of low, but necessary, concentrations of bFGF. Over a 2-month culture period, this short activation promoted a faster and greater amplification of mesenchymal progenitors for adipocytes and osteoblasts. Activation correlated with inhibition of STAT1 and STAT2 phosphorylation and of STAT1 nuclear translocation. A non-neutralizing anti-IFNAR1 mAb was ineffective. We demonstrate that control and activated MSCs express ST3GAL3, a sialyltransferase necessary to produce the embryonic antigens SSEA-3 and -4. Interestingly, activated MSC progeny expressed SSEA-3 and -4 at a higher level than control cultures, but this was not correlated with a significant expression of other embryonic markers. As MSCs represent an essential tool in tissue regeneration, the use of 64G12, which rapidly recruits a higher number of primitive cells, might increase amplification safety for cell therapy.

Canonical and non-canonical Wnts differentially affect the development potential of primary isolate of human bone marrow mesenchymal stem cells

This study examines the role of Wnt signaling events in regulating the differential potential of mesenchymal stem cells (MSCs) from adult bone marrow (BM). Immunohistochemical analysis of BM revealed co-localization of Wnt5a protein, a non-canonical Wnt, with CD45(+) cells and CD45(-) STRO-1(+) cells, while Wnt3a expression, a canonical Wnt, was associated with the underlying stroma matrix, suggesting that Wnts may regulate MSCs in their niche in BM. To elucidate the role of Wnts in MSC development, adult human BM-derived mononuclear cells were maintained as suspension cultures to recapitulate the marrow cellular environment, in serum-free, with the addition of Wnt3a and Wnt5a protein. Results showed that Wnt3a increased cell numbers and expanded the pool of MSCs capable of colony forming unit -- fibroblast (CFU-F) and CFU -- osteoblast (O), while Wnt5a maintained cell numbers and CFU-F and CFU-O numbers. However, when cells were cultured directly onto tissue culture plastic, Wnt5a increased the number of CFU-O relative to control conditions. These findings suggest the potential dual role of Wnt5a in the maintenance of MSCs in BM and enhancing osteogenesis ex vivo. Our work provides evidence that Wnts can function as mesenchymal regulatory factors by providing instructive cues for the recruitment, maintenance, and differentiation of MSCs.

Tgf-Beta signaling in development

The transforming growth factor-beta (TGF-beta) superfamily comprises nearly 30 growth and differentiation factors that include TGF-betas, activins, inhibins, and bone morphogenetic proteins (BMPs). Multiple members of the TGF-beta superfamily serve key roles in stem cell fate commitment. The various members of the family can exhibit disparate roles in regulating the biology of embryonic stem (ES) cells and tumor suppression. For example, TGF-beta inhibits proliferation of multipotent hematopoietic progenitors, promotes lineage commitment of neural precursors, and suppresses epithelial tumors. BMPs block neural differentiation of mouse and human ES cells, contribute to self-renewal of mouse ES cells, and also suppress tumorigenesis. ES cells and tumors may be exposed to multiple TGF-beta members, and it is likely that the combination of growth factors and cross-talk among the intracellular signaling pathways is what precisely defines stem cell fate commitment. This Connections Map Pathway in the Database of Cell Signaling integrates signaling not only from TGF-beta and BMP but also from the ligands nodal and activin, and describes the role of the signaling pathways activated by these ligands in mammalian development. Much of the evidence for the connections shown comes from studies on mouse and human ES cells or mouse knockouts. This pathway is important for understanding not only stem cell biology, but also the molecular effectors of TGF-beta and BMP signaling that may contribute to cancer suppression or progression and thus are potential targets for therapeutic intervention.

Endoglin is not critical for hematopoietic stem cell engraftment and reconstitution but regulates adult erythroid development

Endoglin is a transforming growth factor-beta (TGF-beta) accessory receptor recently identified as being highly expressed on long-term repopulating hematopoietic stem cells (HSC). However, little is known regarding its function in these cells. We have used two complementary approaches toward understanding endoglin's role in HSC biology: one that efficiently knocks down expression via lentiviral-driven short hairpin RNA and another that uses retroviral-mediated overexpression. Altering endoglin expression had functional consequences for hematopoietic progenitors in vitro such that endoglin-suppressed myeloid progenitors (colony-forming unit-granulocyte macrophage) displayed a higher degree of sensitivity to TGF-beta-mediated growth inhibition, whereas endoglin-overexpressing cells were partially resistant. However, transplantation of transduced bone marrow enriched in primitive hematopoietic stem and progenitor cells revealed that neither endoglin suppression nor endoglin overexpression affected the ability of stem cells to short-term or long-term repopulate recipient marrow. Furthermore, transplantation of cells altered in endoglin expression yielded normal white blood cell proportions and peripheral blood platelets. Interestingly, decreasing endoglin expression increased the clonogenic capacity of early blast-forming unit-erythroid progenitors, whereas overexpression compromised erythroid differentiation at the basophilic erythroblast phase, suggesting a pivotal role for endoglin at key stages of adult erythropoietic development.

Immune plasticity of bone marrow-derived mesenchymal stromal cells

Isolated from simple bone marrow aspirates, mesenchymal stromal cells (MSCs) can be easily expanded ex vivo and differentiated into various cell lineages. Because they are present in humans of all ages, are harvested in the absence of prior mobilization and preserve their plasticity following gene modification, MSCs are particularly attractive for cell-based medicine. One of the most fascinating properties of ex vivo expanded MSCs is their ability to suppress ongoing immune responses, both in vitro and in vivo. Although not fully understood, the immunosuppressive properties of MSCs have been reported to affect the function of a broad range of immune cells, including T cells, antigen-presenting cells, natural killer cells and B cells. Whereas successful harnessing of these immunosuppressive properties might one day open the door to the development of new cell-based strategies for the control of graft-versus-host and other autoimmune diseases, recent studies suggest that the immune-modulating properties of MSCs are far more complex than first thought. Reminiscent of the dichotomy of function of dendritic cells (DCs), which can act as potent activators or potent suppressors of immune responses, new studies including our own work has shown that MSCs in fact possess the dual ability to suppress or activate immune responses. In this review, we summarize the different biological properties of MSCs and discuss the current literature on the complex mechanism of immune modulation mediated by ex vivo expanded MSCs.

Smad4-independent, PP2A-dependent apoptotic effect of exogenous transforming growth factor beta 1 in lymphoma cells

B-lymphoid tumor cells are often less sensitive than their normal counterparts or insensitive to transforming growth factor beta1 (TGFb) effects. We studied the apoptotic effect of exogenous TGFb in B-lymphoma cells, focusing on the activity and the role of Smad and protein phosphatase/kinase signals. Recombinant TGFb treatment and Smad4 siRNA transfection were used in HT58 B-NHL lymphoma cells in vitro. Gene expression and apoptosis were detected by RT-PCR, Western blot analysis and flow cytometry. The role of MEK1 kinase and PP2A activity--measured with a phosphatase assay--were assessed with the help of specific inhibitors. Smad4 siRNA treatment completely abolished TGFb-induced early gene upregulation, indicating the absence of the rapid activation of Smad signaling. Moreover, functional inhibition of Smad4 had no influence on TGFb-induced apoptosis, but it was dependent on PP2A phosphatase activation, ERK1/2 and JNK inactivation in lymphoma cells. The results prove that exogenous TGFb uses Smad4-independent, alternative (PP2A/PP2A-like dependent) signaling pathways for apoptosis induction in lymphoma cells. Further studies are needed to clarify the possible role and involvement of Smad4-independent effects of TGFb in normal and malignant lymphoid cells and in cells of the tumor microenvironment.

Elucidating the secretion proteome of human embryonic stem cell-derived mesenchymal stem cells

Transplantation of mesenchymal stem cells (MSCs) has been used to treat a wide range of diseases, and the mechanism of action is postulated to be mediated by either differentiation into functional reparative cells that replace injured tissues or secretion of paracrine factors that promote tissue repair. To complement earlier studies that identified some of the paracrine factors, we profiled the paracrine proteome to better assess the relevance of MSC paracrine factors to the wide spectrum of MSC-mediated therapeutic effects. To evaluate the therapeutic potential of the MSC paracrine proteome, a chemically defined serum-free culture medium was conditioned by MSCs derived from human embryonic stem cells using a clinically compliant protocol. The conditioned medium was analyzed by multidimensional protein identification technology and cytokine antibody array analysis and revealed the presence of 201 unique gene products. 86-88% of these gene products had detectable transcript levels by microarray or quantitative RT-PCR assays. Computational analysis predicted that these gene products will significantly drive three major groups of biological processes: metabolism, defense response, and tissue differentiation including vascularization, hematopoiesis, and skeletal development. It also predicted that the 201 gene products activate important signaling pathways in cardiovascular biology, bone development, and hematopoiesis such as Jak-STAT, MAPK, Toll-like receptor, transforming growth factor-beta, and mTOR (mammalian target of rapamycin) signaling pathways. This study identified a large number of MSC secretory products that have the potential to act as paracrine modulators of tissue repair and replacement in diseases of the cardiovascular, hematopoietic, and skeletal tissues. Moreover our results suggest that human embryonic stem cell-derived MSC-conditioned medium has the potency to treat a variety of diseases in humans without cell transplantation.

Functional network analysis of the transcriptomes of mesenchymal stem cells derived from amniotic fluid, amniotic membrane, cord blood, and bone marrow

Using high-density oligonucleotide microarrays and functional network analyses, we examined whether MSCs derived from four different origins exhibited unique gene expression profiles individually and then compared the gene expression profiles of all MSCs with those of fetal organs. Our results indicated that within each group of MSCs from the same origin, the variability of the gene expression levels was smaller than that between groups of different origins. Functional genomic studies revealed the specific roles of MSCs from different origins. Our results suggest that amniotic fluid MSCs may initiate interactions with the uterus by upregulating oxytocin and thrombin receptors. Amniotic membrane MSCs may play a role in maintaining homeostasis of fluid and electrolytes by regulating the networks of endothelin, neprilysin, bradykinin receptors, and atrial natriuretic peptide. Cord blood MSCs may be involved in innate immune systems as the neonatal defense system against the earliest encountered pathogens. Adult bone marrow MSCs may be an important source not only of all blood lineages but also of bone formation. However, in spite of the different gene expression profiles seen in MSCs derived from different origins, a set of core gene expression profiles was preserved in these four kinds of MSCs. The core signature transcriptomes of all MSCs, when contrasted against those of fetal organs, included genes involved in the regulation of extracellular matrix and adhesion, transforming growth factor-beta receptor signaling, and the Wnt signaling pathways. Disclosure of potential conflicts of interest is found at the end of this article.

Smad4 is critical for self-renewal of hematopoietic stem cells

Members of the transforming growth factor β (TGF-β) superfamily of growth factors have been shown to regulate the in vitro proliferation and maintenance of hematopoietic stem cells (HSCs). Working at a common level of convergence for all TGF-β superfamily signals, Smad4 is key in orchestrating these effects. The role of Smad4 in HSC function has remained elusive because of the early embryonic lethality of the conventional knockout. We clarify its role by using an inducible model of Smad4 deletion coupled with transplantation experiments. Remarkably, systemic induction of Smad4 deletion through activation of MxCre was incompatible with survival 4 wk after induction because of anemia and histopathological changes in the colonic mucosa. Isolation of Smad4 deletion to the hematopoietic system via several transplantation approaches demonstrated a role for Smad4 in the maintenance of HSC self-renewal and reconstituting capacity, leaving homing potential, viability, and differentiation intact. Furthermore, the observed down-regulation of notch1 and c-myc in Smad4^−/− primitive cells places Smad4 within a network of genes involved in the regulation HSC renewal.

HIV-1 negatively affects the survival/maturation of cord blood CD34(+) hematopoietic progenitor cells differentiated towards megakaryocytic lineage by HIV-1 gp120/CD4 membrane interaction

To investigate the mechanisms involved in the human immunodeficiency virus type 1 (HIV-1)-related thrombocytopenia (TP), human umbilical cord blood (UCB) CD34(+) hematopoietic progenitor cells (HPCs) were challenged with HIV-1(IIIb) and then differentiated by thrombopoietin (TPO) towards megakaryocytic lineage. This study showed that HIV-1, heat-inactivated HIV-1, and HIV-1 recombinant gp120 (rgp120) activated apoptotic process of megakaryocyte (MK) progenitors/precursors and decreased higher ploidy MK cell fraction. All these inhibitory effects on MK survival/maturation and platelets formation were elicited by the interaction between gp120 and CD4 receptor on the cell membrane in the absence of HIV-1 productive infection. In fact, in our experimental conditions, HPCs were resistant to HIV-1 infection and no detectable productive infection was observed. We also evaluated whether the expression of specific cytokines, such as TGF-beta1 and APRIL, involved in the regulation of HPCs and MKs proliferation, was modulated by HIV-1. The specific protein and mRNA detection analysis, during TPO-induced differentiation, demonstrated that HIV-1 upregulates TGF-beta1 and downregulates APRIL expression through the CD4 engagement by gp120. Altogether, these data suggest that survival/differentiation of HPCs committed to MK lineage is negatively affected by HIV-1 gp120/CD4 interaction. This long-term inhibitory effect is also correlated to specific cytokines regulation and it may represent an additional mechanism to explain the TP occurring in HIV-1 patients.

Smad5 is dispensable for adult murine hematopoiesis

Smad5 is known to transduce intracellular signals from bone morphogenetic proteins (BMPs), which belong to the transforming growth factor-β (TGF-β) superfamily and are involved in the regulation of hematopoiesis. Recent findings suggest that BMP4 stimulates proliferation of human primitive hematopoietic progenitors in vitro, while early progenitors from mice deficient in Smad5 display increased self-renewal capacity in murine embryonic hematopoiesis. Here, we evaluate the role of Smad5 in the regulation of hematopoietic stem cell (HSC) fate decisions in adult mice by using an inducible MxCre-mediated conditional knockout model. Surprisingly, analysis of induced animals revealed unperturbed cell numbers and lineage distribution in peripheral blood (PB), bone marrow (BM), and the spleen. Furthermore, phenotypic characterization of the stem cell compartment revealed normal numbers of primitive lin–Sca-1+c-Kit+ (LSK) cells in Smad5–/– BM. When transplanted in a competitive fashion into lethally irradiated primary and secondary recipients, Smad5-deficient BM cells competed normally with wild-type (wt) cells, were able to provide long-term reconstitution for the hosts, and displayed normal lineage distribution. Taken together, Smad5-deficient HSCs from adult mice show unaltered differentiation, proliferation, and repopulating capacity. Therefore, in contrast to its role in embryonic hematopoiesis, Smad5 is dispensable for hematopoiesis in the adult mouse.

Smad7 promotes self-renewal of hematopoietic stem cells

The Smad-signaling pathway downstream of the transforming growth factor-beta superfamily of ligands is an evolutionarily conserved signaling circuitry with critical functions in a wide variety of biologic processes. To investigate the role of this pathway in the regulation of hematopoietic stem cells (HSCs), we have blocked Smad signaling by retroviral gene transfer of the inhibitory Smad7 to murine HSCs. We report here that the self-renewal capacity of HSCs is promoted in vivo upon blocking of the entire Smad pathway, as shown by both primary and secondary bone marrow (BM) transplantations. Importantly, HSCs overexpressing Smad7 have an unperturbed differentiation capacity as evidenced by normal contribution to both lymphoid and myeloid cell lineages, suggesting that the Smad pathway regulates self-renewal independently of differentiation. Moreover, phosphorylation of Smads was inhibited in response to ligand stimulation in BM cells, thus verifying impairment of the Smad-signaling cascade in Smad7-overexpressing cells. Taken together, these data reveal an important and previously unappreciated role for the Smad-signaling pathway in the regulation of self-renewal of HSCs in vivo.

Hematopoiesis controlled by distinct TIF1gamma and Smad4 branches of the TGFbeta pathway

Tissue homeostasis in mammals relies on powerful cytostatic and differentiation signals delivered by the cytokine TGFbeta and relayed within the cell via the activation of Smad transcription factors. Formation of transcription regulatory complexes by the association of Smad4 with receptor-phosphorylated Smads 2 and 3 is a central event in the canonical TGFbeta pathway. Here we provide evidence for a branching of this pathway. The ubiquitious nuclear protein Transcriptional Intermediary Factor 1gamma (TIF1gamma) selectively binds receptor-phosphorylated Smad2/3 in competition with Smad4. Rapid and robust binding of TIF1gamma to Smad2/3 occurs in hematopoietic, mesenchymal, and epithelial cell types in response to TGFbeta. In human hematopoietic stem/progenitor cells, where TGFbeta inhibits proliferation and stimulates erythroid differentiation, TIF1gamma mediates the differentiation response while Smad4 mediates the antiproliferative response with Smad2/3 participating in both responses. Thus, Smad2/3-TIF1gamma and Smad2/3-Smad4 function as complementary effector arms in the control of hematopoietic cell fate by the TGFbeta/Smad pathway.

Requirement of Smad3 for mast cell growth

The involvement of the TGF-beta family in cell growth of bone marrow-derived mast cells (BMMC) cultured with medium containing pokeweed mitogen-stimulated spleen cell-conditioned medium (PWM-SCM) was examined. Doubling time of BMMC from Smad3-null mice was longer than that from wild-type (WT) mice, and the differences tended to be larger with time of culture. Consistent with the results, uptake and reduction of [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt; MTS] was lower in Smad3-deficient BMMC. Cell cycle analyses revealed no apparent differences between WT BMMC and Smad3-deficient BMMC, suggesting that longer doubling time in Smad3-deficient BMMC resulted from increased cell death. TGF-beta and activin A were supplied by PWM-SCM rather than by self-production by BMMC. Blocking the TGF-beta pathway by anti-TGF-beta neutralizing antibody or an inhibitor for the type I receptors for ligands including TGF-beta and activin, SB431542, inhibited MTS uptake and reduction in WT BMMC, whereas anti-activin A antibody and SB431542 tended to inhibit them in Smad3-deficient BMMC. The present results suggest that TGF-beta-induced and Smad3-mediated signaling is essential for maximal cell growth in mast cells, and that the activin pathway may be required for it when mast cell context is modulated by Smad3 depletion.

Transforming growth factor-β1 inhibits all-trans retinoic acid-induced apoptosis

The interaction between retinoids and transforming growth factor-beta1 (TGF-beta1) leading to regulation of proliferation, differentiation and apoptosis is not still fully understood. In this study, we demonstrated that a combination treatment with all-trans retinoic acid (ATRA) and TGF-beta1 led to the enhancement of ATRA-induced suppression of cell proliferation, which is accompanied by inhibition of ATRA-induced apoptosis in human leukemia HL-60 cells. This effect was preceded by the arrest of cells in G0/G1 cell cycle phase linked with pRb protein dephosphorylation, continuous accumulation of p21 and transiently increased level of p27, inhibitors of cyclin-dependent kinases. Inhibition of ATRA-induced apoptosis by TGF-beta1 was associated with an increased level of Mcl-1 protein, an anti-apoptotic member of Bcl-2 family, but not with inhibition of mitochondrial membrane depolarization. Levels of other Bcl-2 family proteins (Bcl-2, Bcl-X(L), Bad, Bak, Bax) were unaffected by simultaneous ATRA and TGF-beta1 treatment, when compared to ATRA alone. Upregulation of c-FLIP(L) protein, an inhibitor of apoptosis induced by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), correspond with inhibition of ATRA-induced (autocrine TRAIL-mediated) caspase-8 activation and apoptosis. These results suggest that apoptosis inhibition associated with proliferation block could depend on modulation of the TRAIL apoptotic pathway and regulation of the Mcl-1 protein level. In summary, we demonstrate that the balance of processes leading to regulation of proliferation and differentiation of myeloid cells can modulate cell sensitivity to apoptosis-inducing stimuli.

Role of transforming growth factor-beta in hematologic malignancies

The transforming growth factor-beta (TGF-beta) signaling pathway is an essential regulator of cellular processes, including proliferation, differentiation, migration, and cell survival. During hematopoiesis, the TGF-beta signaling pathway is a potent negative regulator of proliferation while stimulating differentiation and apoptosis when appropriate. In hematologic malignancies, including leukemias, myeloproliferative disorders, lymphomas, and multiple myeloma, resistance to these homeostatic effects of TGF-beta develops. Mechanisms for this resistance include mutation or deletion of members of the TGF-beta signaling pathway and disruption of the pathway by oncoproteins. These alterations define a tumor suppressor role for the TGF-beta pathway in human hematologic malignancies. On the other hand, elevated levels of TGF-beta can promote myelofibrosis and the pathogenesis of some hematologic malignancies through their effects on the stroma and immune system. Advances in the TGF-beta signaling field should enable targeting of the TGF-beta signaling pathway for the treatment of hematologic malignancies.

A road map toward defining the role of Smad signaling in hematopoietic stem cells

The transforming growth factor-beta (TGF-beta) superfamily encompasses the ligands and receptors for TGF-beta, bone morphogenic proteins (BMPs), and Activins. Cellular response to ligand is context-dependent and may be controlled by specificity and/or redundancy of expression of these superfamily members. Several pathways within this family have been implicated in the proliferation, differentiation, and renewal of hematopoietic stem cells (HSCs); however, their roles and redundancies at the molecular level are poorly understood in the rare HSC. Here we have characterized the expression of TGF-beta superfamily ligands, receptors, and Smads in murine HSCs and in the Lhx2-hematopoietic progenitor cell (Lhx2-HPC) line. We demonstrate a remarkable likeness between these two cell types with regard to expression of the majority of receptors and Smads necessary for the transduction of signals from TGF-beta, BMP, and Activin. We have also evaluated the response of these two cell types to various ligands in proliferation assays. In this regard, primary cells and the Lhx2-HPC line behave similarly, revealing a suppressive effect of Activin-A that is similar to that of TGF-beta in bulk cultures and no effect of BMP-4 on proliferation. Signaling studies that verify the phosphorylation of Smad2 (Activin and TGF-beta) and Smad1/5 (BMP) confirm cytosolic responses to these ligands. In addition to providing a thorough characterization of TGF-beta superfamily expression in HSCs, our results define the Lhx2-HPC line as an appropriate model for molecular characterization of Smad signaling.

Transforming growth factor-beta signaling in stem cells and cancer

Transforming growth factor-beta (TGF-beta) and TGF-beta-related proteins, such as the bone morphogenetic proteins, have emerged as key regulators of stem cell renewal and differentiation. These proteins have disparate roles in regulating the biology of embryonic stem cells and tumor suppression, and they help define the selection of cell fate and the progression of differentiation along a lineage. Here we illustrate their roles in embryonic stem cells and in the differentiation of neural, hematopoietic, mesenchymal, and gastrointestinal epithelial stem cells.