Transforming growth factor beta, pleiotropic regulator of hematopoietic stem cells: potential physiological and clinical relevance

'Nomadic' Hematopoietic Stem Cells Navigate the Embryonic Landscape

Hematopoietic stem cells are a unique population of tissue-resident multipotent cells with an extensive ability to self-renew and regenerate the entire lineage of differentiated blood cells. Stem cells reside in a highly specialized microenvironment with surrounding supporting cells, forming a complex and dynamic network to preserve and maintain their function. The survival, activation, and quiescence of stem cells are largely influenced by niche-derived signals, with aging niche contributing to a decline in stem cell function. Although the role of niche in regulating hematopoiesis has long been established by transplantation studies, limited methods in observing the process in vivo have eluded a detailed understanding of the various niche components. Danio rerio (zebrafish) has emerged as a solution in the past few decades, enabling discovery of cellular interactions, in addition to chemical and genetic factors regulating HSCs. This review reiterates zebrafish as a suitable model for studies on vertebrate embryonic and adult hematopoiesis, delving into this temporally and spatially dissected multi-step process. The critical role played by epigenetic regulators are discussed, along with contributions of the various physiological processes in sustaining the stem cell population. Stem cell niche transcends mere knowledge acquisition, assuring scope in cell therapy, organoid cultures, aging research, and clinical applications including bone marrow transplantation and cancer. A better understanding of the various niche components could also leverage therapeutic efforts to drive differentiation of HSCs from pluripotent progenitors, sustain stemness in laboratory cultures, and improve stem cell transplantation outcomes.

Transforming growth factor-β1/Thrombospondin-1/CD47 axis mediates dysfunction in CD34+ cells derived from diabetic older adults

Aging with diabetes is associated with impaired vasoprotective functions and decreased nitric oxide (NO) generation in CD34+ cells. Transforming growth factor- β1 (TGF-β1) is known to regulate hematopoietic functions. This study tested the hypothesis that transforming growth factor- β1 (TGF-β1) is upregulated in diabetic CD34+ cells and impairs NO generation via thrombospondin-1 (TSP-1)/CD47/NO pathway. CD34+ cells from nondiabetic (ND) (n=58) or diabetic older adults (DB) (both type 1 and type 2) (n=62) were isolated from peripheral blood. TGF-β1 was silenced by using an antisense delivered as phosphorodiamidate morpholino oligomer (PMO-TGF-β1). Migration and proliferation in response to stromal-derived factor-1α (SDF-1α) were evaluated. NO generation and eNOS phosphorylation were determined by flow cytometry. CD34+ cells from older, but not younger, diabetics have higher expression of TGF-β1 compared to that observed in cells derived from healthy individuals (P<0.05, n=14). TSP-1 expression was higher (n=11) in DB compared to ND cells. TGFβ1-PMO decreased the secretion of TGF-β1, which was accompanied with decreased TSP-1 expression. Impaired proliferation, migration and NO generation in response to SDF-1α in DB cells were reversed by TGF-β1-PMO (n=6). TSP-1 inhibited migration and proliferation of nondiabetic CD34+ cells that was reversed by CD47-siRNA, which also restored these responses in diabetic CD34+ cells. TSP-1 opposed SDF-1α-induced eNOS phosphorylation at Ser1177 that was reversed by CD47-siRNA. These results infer that increased TGF-β1 expression in CD34+ cells induces dysfunction in CD34+ cells from diabetic older adults via TSP-1/CD47-dependent inhibition of NO generation.

Transforming growth factor β1 accelerates and enhances in vitro red blood cell formation from hematopoietic stem cells by stimulating mitophagy

Background

Generation of red blood cells (RBCs) from hematopoietic stem cells (HSCs) in vitro takes about 21 days, making it unaffordable for clinical applications. Acceleration of the in vitro erythropoiesis process by using small molecules could eventually make the large-scale production of these cells commercially viable. Transforming Growth Factor β1 (TGF-β1) has been shown to have a dose-dependent activity on the HSCs: at high concentration it inhibits, whereas at low concentration it stimulates the HSCs growth. At high concentration, it also inhibits erythropoiesis but accelerates terminal erythroid differentiation of cell lines and erythroid progenitors. Here we examined whether the use of low concentration of TGF-β1 would be beneficial for increasing RBC production by stimulating HSC growth and also supporting erythroid differentiation. Such a strategy could make RBC production in vitro more efficient and cost-effective for clinical applications.

Methods

HSCs isolated from Apheresis samples were differentiated into mature RBCs by the sequential addition of specific combinations of growth factors for 21 days. In the control set, only EPO (3 IU/ml) was added whereas, in the test set, TGF-β1 at a concentration of 10 pg/ml was added along with EPO (3 IU/ml) from day 0.

Results

We found that a low concentration of TGF-β1 has no inhibitory effect on the proliferation of the early stages of erythropoiesis. Additionally, it significantly accelerates terminal stages of erythroid differentiation by promoting BNIP3L/NIX-mediated mitophagy.

Conclusions

Incorporation of TGF-β1 at 10 pg/ml concentration in the differentiation medium accelerates the in vitro erythropoiesis process by 3 days. This finding could have potential applications in transfusion medicine.

Electronic supplementary material

The online version of this article (10.1186/s13287-020-01603-z) contains supplementary material, which is available to authorized users.

Thrombopoietin Metabolically Primes Hematopoietic Stem Cells to Megakaryocyte-Lineage Differentiation

During acute myelosuppression or thrombocytopenia, bone marrow (BM) hematopoietic cells respond rapidly to replenish peripheral blood platelets. While the cytokine thrombopoietin (Thpo) both regulates platelet production and maintains HSC potential, whether Thpo controls megakaryocyte (Mk)-lineage differentiation of HSCs is unclear. Here, we show that Thpo rapidly upregulates mitochondrial activity in HSCs, an activity accompanied by differentiation to an Mk lineage. Moreover, in unperturbed hematopoiesis, HSCs with high mitochondrial activity exhibit Mk-lineage differentiation in vitro and myeloid lineage-biased reconstitution in vivo. Furthermore, Thpo skewed HSCs to express the tetraspanin CD9, a pattern correlated with mitochondrial activity. Mitochondria-active HSCs are resistant to apoptosis and oxidative stress upon Thpo stimulation. Thpo-regulated mitochondrial activity associated with mitochondrial translocation of STAT3 phosphorylated at serine 727. Overall, we report an important role for Thpo in regulating rapid Mk-lineage commitment. Thpo-dependent changes in mitochondrial metabolism prime HSCs to undergo direct differentiation to an Mk lineage.

TGFBI Expressed by Bone Marrow Niche Cells and Hematopoietic Stem and Progenitor Cells Regulates Hematopoiesis

The interactions of hematopoietic stem and progenitor cells (HSPCs) with extracellular matrix (ECM) components and cells from the bone marrow (BM) microenvironment control their homeostasis. Regenerative BM conditions can induce expression of the ECM protein transforming growth factor beta-induced gene H3 (TGFBI or BIGH3) in murine HSPCs. In this study, we examined how increased or reduced TGFBI expression in human HSPCs and BM mesenchymal stromal cells (MSCs) affects HSPC maintenance, differentiation, and migration. HSPCs that overexpressed TGFBI showed accelerated megakaryopoiesis, whereas granulocyte differentiation and proliferation of granulocyte, erythrocyte, and monocyte cultures were reduced. In addition, both upregulation and downregulation of TGFBI expression impaired HSPC colony-forming capacity of HSPCs. Interestingly, the colony-forming capacity of HSPCs with reduced TGFBI levels was increased after long-term co-culture with MSCs, as measured by long-term culture-colony forming cell (LTC-CFC) formation. Moreover, TGFBI downregulation in HSPCs resulted in increased cobblestone area-forming cell (CAFC) frequency, a measure for hematopoietic stem cell (HSC) capacity. Concordantly, TGFBI upregulation in HSPCs resulted in a decrease of CAFC and LTC-CFC frequency. These results indicate that reduced TGFBI levels in HSPCs enhanced HSC maintenance, but only in the presence of MSCs. In addition, reduced levels of TGFBI in MSCs affected MSC/HSPC interaction, as observed by an increased migration of HSPCs under the stromal layer. In conclusion, tight regulation of TGFBI expression in the BM niche is essential for balanced HSPC proliferation and differentiation.

Thrombospondin-1 regulation of latent TGF-β activation: A therapeutic target for fibrotic disease

Transforming growth factor-β (TGF-β) is a central player in fibrotic disease. Clinical trials with global inhibitors of TGF-β have been disappointing, suggesting that a more targeted approach is warranted. Conversion of the latent precursor to the biologically active form of TGF-β represents a novel approach to selectively modulating TGF-β in disease, as mechanisms employed to activate latent TGF-β are typically cell, tissue, and/or disease specific. In this review, we will discuss the role of the matricellular protein, thrombospondin 1 (TSP-1), in regulation of latent TGF-β activation and the use of an antagonist of TSP-1 mediated TGF-β activation in a number of diverse fibrotic diseases. In particular, we will discuss the TSP-1/TGF-β pathway in fibrotic complications of diabetes, liver fibrosis, and in multiple myeloma. We will also discuss emerging evidence for a role for TSP-1 in arterial remodeling, biomechanical modulation of TGF-β activity, and in immune dysfunction. As TSP-1 expression is upregulated by factors induced in fibrotic disease, targeting the TSP-1/TGF-β pathway potentially represents a more selective approach to controlling TGF-β activity in disease.

The role of novel and known extracellular matrix and adhesion molecules in the homeostatic and regenerative bone marrow microenvironment

Maintenance of haematopoietic stem cells and differentiation of committed progenitors occurs in highly specialized niches. The interactions of haematopoietic stem and progenitor cells (HSPCs) with cells, growth factors and extracellular matrix (ECM) components of the bone marrow (BM) microenvironment control homeostasis of HSPCs. We only start to understand the complexity of the haematopoietic niche(s) that comprises endosteal, arterial, sinusoidal, mesenchymal and neuronal components. These distinct niches produce a broad range of soluble factors and adhesion molecules that modulate HSPC fate during normal hematopoiesis and BM regeneration. Adhesive interactions between HSPCs and the microenvironment will influence their localization and differentiation potential. In this review we highlight the current understanding of the functional role of ECM- and adhesion (regulating) molecules in the haematopoietic niche during homeostatic and regenerative hematopoiesis. This knowledge may lead to the improvement of current cellular therapies and more efficient development of future cellular products.

TGF-β signaling and its role in the regulation of hematopoietic stem cells

Transforming growth factor-betas (TGF-βs) and their family members that include bone morphogenic proteins and activins have been implicated in the regulation of proliferation, hibernation, quiescence and differentiation of hematopoietic stem cells (HSCs). Increasing evidence suggests that the superfamily of TGF-βs play an integral role in the intercellular cross-talk between the stem cells and their microenvironment as well as within the cells at an intracellular level. Active sites of hematopoiesis, such as fetal liver and bone marrow are known to have abundant presence of TGF-β indicating their importance in the maintenance and regulation of hematopoiesis. One of the striking features of TGF-β superfamily is the variety of effects they evoke, contingent on the developing history of the responding cells. In the present review, we discuss the Smad-dependent and Smad-independent TGF-β signaling pathways in order to understand and underscore their role in the regulation of HSCs.

TGIF1 is a negative regulator of MLL-rearranged acute myeloid leukemia

Members of the TALE (three-amino-acid loop extension) family of atypical homeodomain-containing transcription factors are important downstream effectors of oncogenic fusion proteins involving the mixed lineage leukemia (MLL) gene. A well-characterized member of this protein family is MEIS1, which orchestrates a transcriptional program required for the maintenance of MLL-rearranged acute myeloid leukemia (AML). TGIF1/TGIF2 are relatively uncharacterized TALE transcription factors, which, in contrast to the remaining family, have been shown to act as transcriptional repressors. Given the general importance of this family in malignant hematopoiesis, we therefore tested the potential function of TGIF1 in the maintenance of MLL-rearranged AML. Gene expression analysis of MLL-rearranged patient blasts demonstrated reduced TGIF1 levels, and, in accordance, we find that forced expression of TGIF1 in MLL-AF9-transformed cells promoted differentiation and cell cycle exit in vitro, and delayed leukemic onset in vivo. Mechanistically, we show that TGIF1 interferes with a MEIS1-dependent transcriptional program by associating with MEIS1-bound regions in a competitive manner and that the MEIS1:TGIF1 ratio influence the clinical outcome. Collectively, these findings demonstrate that TALE family members can act both positively and negatively on transcriptional programs responsible for leukemic maintenance and provide novel insights into the regulatory gene expression circuitries in MLL-rearranged AML.

Transient inhibition of transforming growth factor-beta1 in human diabetic CD34+ cells enhances vascular reparative functions

OBJECTIVE

Peripheral blood CD34(+) cells from diabetic patients demonstrate reduced vascular reparative function due to decreased proliferation and diminished migratory prowess, largely resulting from decreased nitric oxide (NO) bioavailability. The level of TGF-beta, a key factor that modulates stem cell quiescence, is increased in the serum of type 2 diabetic patients. We asked whether transient TGF-beta1 inhibition in CD34(+) cells would improve their reparative ability.

RESEARCH DESIGN AND METHODS

To inhibit TGF-beta1 protein expression, CD34(+) cells were treated ex vivo with antisense phosphorodiamidate morpholino oligomers (TGF-beta1-PMOs) and analyzed for cell surface CXCR4 expression, cell survival in the absence of added growth factors, SDF-1-induced migration, NO release, and in vivo retinal vascular reparative ability.

RESULTS

TGF-beta1-PMO treatment of diabetic CD34(+) cells resulted in increased expression of CXCR4, enhanced survival in the absence of growth factors, and increased migration and NO release as compared with cells treated with control PMO. Using a retinal ischemia reperfusion injury model in mice, we observed that recruitment of diabetic CD34(+) cells to injured acellular retinal capillaries was greater after TGF-beta1-PMO treatment compared with control PMO-treated cells.

CONCLUSIONS

Transient inhibition of TGF-beta1 may represent a promising therapeutic strategy for restoring the reparative capacity of dysfunctional diabetic CD34(+) cells.

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.

Levels of Smad7 regulate Smad and mitogen activated kinases (MAPKs) signaling and controls erythroid and megakaryocytic differentiation of erythroleukemia cells

Smad and MAPK signaling cascades are involved in erythroid and megakaryocytic differentiation. The inhibitory Smad for TGF-beta/activin signaling, Smad7, may directly or indirectly affect these signaling pathways. By modulating Smad7 expression, we attempted to delineate the relevance of Smad7 during erythro-megakaryocytic (E/M) differentiation of human erythroleukemia cells. Smad7 transcripts were detected at low levels in different erythroleukemia cell lines (TF-1, HEL and K562). Reduction of expression of endogenous Smad7 by RNA interference enhanced erythroid differentiation of K562 cells in response to physiological doses of activin-A/TGF-beta1. Stable over-expression of Smad7 in K562 cells (K562/7) prevented activation of Smad2/3 and MAPK (ERK1/2, p38 and JNK1/2) proteins by activin-A/TGF-beta1 and subsequent induction of erythroid differentiation. High levels of Smad7 also interfered with hydroxyurea- and butyrate-, but not hemin-induced erythroid differentiation. Interestingly, K562/7 cells were found to harbor a significant proportion (about 35%) of large ploy nucleated cells compared to fewer than 12% in control cells. K562/7 cells treated with phorbol 12-myristate 13-acetate (PMA), showed a great shift in ploidy towards high ploidy classes (> or =8N) accompanied with an increase in the expression of the maturation marker CD42b. We showed here that: (a) low levels of endogenous Smad7 in erythroleukemia cells are physiologically relevant, and (b) high levels of Smad7 interferes with TGF-beta/activin-induced Smad/MAPK signaling and erythro-differentiation and promotes megakaryocytic differentiation, possibly by blocking autocrine TGF-beta.

Gene expression profiling and analysis of signaling pathways involved in priming and differentiation of human neural stem cells

Human neural stem cells have the ability to differentiate into all three major cell types in the CNS including neurons, astrocytes and oligodendrocytes. The multipotency of human neural stem cells shed a light on the possibility of using stem cells as a therapeutic tool for various neurological disorders including neurodegenerative diseases and neurotrauma that involve a loss of functional neurons. We have discovered previously a priming procedure to direct primarily cultured human neural stem cells to differentiate into almost pure neurons when grafted into adult CNS. However, the molecular mechanism underlying this phenomenon is still unknown. To unravel transcriptional changes of human neural stem cells upon priming, cDNA microarray was used to study temporal changes in human neural stem cell gene expression profile during priming and differentiation. As a result, transcriptional levels of 520 annotated genes were detected changed in at least at two time points during the priming process. In addition, transcription levels of more than 3000 hypothetical protein encoding genes and EST genes were modulated during the priming and differentiation processes of human neural stem cells. We further analyzed the named genes and grouped them into 14 functional categories. Of particular interest, key cell signal transduction pathways, including the G-protein-mediated signaling pathways (heterotrimeric and small monomeric GTPase pathways), the Wnt signaling pathway and the TGF-beta pathway, are modulated by the neural stem cell priming, suggesting important roles of these key signaling pathways in priming and differentiation of human neural stem cells.

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

Transforming growth factor-beta (TGF-beta) is a pleiotropic regulator of all stages of hematopoieis. The three mammalian isoforms (TGF-beta1, 2 and 3) have distinct but overlapping effects on hematopoiesis. Depending on the differentiation stage of the target cell, the local environment and the concentration and isoform of TGF-beta, in vivo or in vitro, TGF-beta can be pro- or antiproliferative, pro- or antiapoptotic, pro- or antidifferentiative and can inhibit or increase terminally differentiated cell function. TGF-beta is a major regulator of stem cell quiescence, at least in vitro. TGF-beta can act directly or indirectly through effects on the bone marrow microenvironment. In addition, paracrine and autocrine actions of TGF-beta have overlapping but distinct regulatory effects on hematopoietic stem/progenitor cells. Since TGF-beta can act in numerous steps in the hematopoietic cascade, loss of function mutations in hematopoeitic stem cells (HSC) have different effects on hematopoiesis than transient blockade of autocrine TGF-beta1. Transient neutralization of autocrine TGF-beta in HSC has therapeutic potential. In myeloid and erythroid leukemic cells, autocrine TGF-beta1 and/or its Smad signals controls the ability of these cells to respond to various differentiation inducers, suggesting that this pathway plays a role in determining the cell fate of leukemic cells.

The role of Smad signaling in hematopoiesis

The TGF-beta family of ligands, including TGF-beta, bone morphogenetic protein (BMP) and activin, signal through Smad pathways to regulate the fate of hematopoietic progenitor and stem cells during development and postnatally. BMP regulates hematopoietic stem cell (HSC) specification during development, while TGF-beta1, 2 and 3 are not essential for the generation of HSCs. BMP4 can increase proliferation of human hematopoietic progenitors, while TGF-beta acts as a negative regulator of hematopoietic progenitor and stem cells in vitro. In contrast, TGF-beta signaling deficiency in vivo does not affect proliferation of HSCs and does not affect lineage choice either. Therefore, the outcome of Smad signaling is very context dependent in hematopoiesis and regulation of hematopoietic stem and progenitor cells is more complicated in the bone marrow microenvironment in vivo than is seen in liquid cultures ex vivo. Smad signaling regulates hematopoiesis by crosstalk with other regulatory signals and future research will define in more detail how the various pathways interact and how the knowledge obtained can be used to develop advanced cell therapies.

Identification and characterization of genes involved in embryonic crystal cell formation during Drosophila hematopoiesis

Parallels between vertebrate and Drosophila hematopoiesis add to the value of flies as a model organism to gain insights into blood development. The Drosophila hematopoietic system is composed of at least three classes of terminally differentiated blood cells: plasmatocytes, crystal cells, and lamellocytes. Recent studies have identified transcriptional and signaling pathways in Drosophila involving proteins similar to those seen in human blood development. To identify additional genes involved in Drosophila hematopoiesis, we have conducted a P-element-based genetic screen to isolate mutations that affect embryonic crystal cell development. Using a marker of terminally differentiated crystal cells, we screened 1040 P-element-lethal lines located on the second and third chromosomes and identified 44 individual lines that affect crystal cell development. Identifying novel genes and pathways involved in Drosophila hematopoiesis is likely to provide further insights into mammalian hematopoietic development and disorders.

Transforming growth factor-{beta}1 modulates responses of CD34+ cord blood cells to stromal cell-derived factor-1/CXCL12

Disruption of stromal cell-derived factor-1 (SDF-1/CXCL12 [CXC chemokine ligand 12]) interaction leads to mobilization of stem/progenitor cells from bone marrow to circulation. However, prolonged exposure of CD34+ cells to SDF-1 desensitizes them to SDF-1. So how do cells remain responsive to SDF-1 in vivo when they are continuously exposed to SDF-1? We hypothesized that one or more mechanisms mediated by cytokines exist that could modulate SDF-1 responsiveness of CD34+ cells and the desensitization process. We considered transforming growth factor-beta1 (TGF-beta1) a possible candidate, since TGF-beta1 has effects on CD34+ cells and is produced by stromal cells, which provide niches for maintenance and proliferation of stem/progenitor cells. TGF-beta1 significantly restored SDF-1-induced chemotaxis and sustained adhesion responses in cord blood CD34+ cells preexposed to SDF-1. Effects of TGF-beta1 were dependent on the dose and duration of TGF-beta1 pretreatment. Phosphorylation of extracellular signal-regulated kinase 1 (Erk1)/Erk2 was implicated in TGF-beta1 modulation of migratory and adhesion responses to SDF-1. Our results indicate that low levels of TGF-beta1 can modulate SDF-1 responsiveness of CD34+ cells and thus may facilitate SDF-1-mediated retention and nurturing of stem/progenitor cells in bone marrow.

Molecular Mechanisms Behind the Dose-Dependent Differential Activation of MAPK Pathways Induced by Transforming Growth Factor-β1 in Hematopoietic Cells

Transforming growth factor-beta (TGF-beta) controls a wide range of cellular responses, including cell proliferation, lineage determination, differentiation, and apoptosis, and figures prominently in animal development. It is considered as a pleiotropic factor because it can exert a positive or negative effect on various cellular processes depending on developmental stage of the target cell, its microenvironment, and also its biochemical make up. It has been shown to have a strong inhibitory effect on hematopoietic stem cell proliferation and differentiation. We have earlier shown that TGF-beta1 exerts a bidirectional effect on hematopoietic cell proliferation as a function of its concentration. Although it acted as an inhibitor at high concentrations, at low concentrations it stimulated the stem/progenitor cells. We also provided evidence that the differential activation of mitogen-activated protein kinase pathways was responsible for the observed bidirectional effect. In the present study, we examined the molecular mechanism behind this phenomenon. We observed that the high inhibitory concentrations of TGF-beta1 induced a strong phosphorylation of SMAD 3 and also activated stress kinase-related transcription factors, namely c-Jun and ATF-2. On the other hand, low stimulatory concentrations acted in a SMAD 3-independent pathway and activated STAT proteins. Our results clearly show that differential activation of signal transduction pathways by TGF-beta1 as a function of its concentration underlies its bidirectional effect on hematopoietic cells.

Differential activation of MAPK signaling pathways by TGF-beta1 forms the molecular mechanism behind its dose-dependent bidirectional effects on hematopoiesis

We have earlier reported that transforming growth factor-beta1 (TGF-beta1), a well-known inhibitor of hematopoiesis, stimulated colony formation from adult human bone marrow mononuclear cells (BM MNC) when used at low concentrations. We examined the possible molecular mechanism behind this bidirectional effect using CD34+ cells isolated from human BM for clonal assays and the KG1a cell line as a model system for analysis of proteins for signaling pathways by immunoblotting. We found that TGF-beta1 at low doses (picogram levels) stimulated the colony formation from CD34+ cells, indicating that these progenitors form the direct target of stimulatory action of TGF-beta1. CD34+ cells were found to be more sensitive to the TGF-beta1 concentration than the total MNC. We used the KG1a cell line as a model system for identification of mitogen-activated protein kinase (MAPK) and AKT signaling pathways involved in the process. Low doses strongly induced p44/42 MAPK phosphorylation, whereas high doses induced p38 activation. Use of specific p44/42 MAPK inhibitor PD 98059 in the colony assay abrogated the stimulatory effect of low TGF-beta1. On the other hand, use of p38 MAPK inhibitor SB 203580 along with low TGF-beta1 concentrations had a synergistic effect on stimulation of colony formation. Treatment of BM MNC with Anisomycin, which activates stress kinases, resulted in a dose-dependent inhibition of colony formation. This inhibition could not be rescued by stimulatory doses of TGF-beta1. Phosphorylation of AKT was found to occur in a dose-dependent way but declined slightly at the highest concentration used (10 ng/ml). Inhibition of the AKT pathway by LY 294002 strongly suppressed colony formation. These data indicate clearly that sustained activation of p44/42 MAPK perhaps forms the stimulatory signal induced by low TGF-beta1, whereas activation of p38 forms the inhibitory pathway.

Tuberous Sclerosis Complex 2 Gene Product Interacts with Human SMAD Proteins

Tuberin (TSC2) is a tumor suppressor gene. At the cellular level, tuberin is required as a critical regulator of cell growth, neuronal differentiation, and tumor suppression. Here we report a critical role for tuberin in late stage myeloid cell differentiation. Tuberin strongly augments transforming growth factor (TGF)-beta1 signal transduction pathways, including SMAD activation. We also demonstrate that the amino-terminal region of tuberin interacts specifically with the MH2 domain of SMAD2 and SMAD3 proteins to regulate TGF-beta1-responsive genes such as p21(CIP). Inhibition of tuberin expression by Tsc2 antisense greatly reduces the ability of TGF-beta to transcriptionally regulate p21(CIP), p27(KIP), and cyclin A leading to an abrogation of the antiproliferative effects of TGF-beta1. Also, inhibition of tuberin expression during stimulation of monocytic differentiation with vitamin D(3) and TGF-beta1 significantly impaired myeloid cell growth inhibition and differentiation. Together, the data demonstrate the presence of a novel activation process following TGF-beta1 stimulation that requires tuberin-dependent activity.

Neutralization of autocrine transforming growth factor-beta in human cord blood CD34(+)CD38(-)Lin(-) cells promotes stem-cell-factor-mediated erythropoietin-independent early erythroid progenitor development and reduces terminal differentiation

Transforming growth factor (TGF)-beta1 exerts autocrine and paracrine effects on hematopoiesis. Here, we have attempted to evaluate the effect of endogenous TGF-beta1 on early erythroid development from primitive human hematopoietic stem cells (HSCs) and to assess the effects of TGF-beta1 on different phases of erythropoiesis. Cord blood CD34(+)CD38(-) lineage-marker-negative (Lin(-)) cells were cultured in serum-free conditions using various combinations of stem cell factor (SCF), erythropoietin (Epo), and TGF-beta-neutralizing antibody. Generation of erythroid progenitors was assessed using colony assay and flow cytometry. Terminal erythroid differentiation was examined when SCF/Epo-stimulated cells were recultured in the presence of Epo with and without TGF-beta1. Anti-TGF-beta augmented the proliferation of CD34(+)CD38(-)Lin(-) cells (day 21) in SCF-stimulated (6.4-fold +/- 1.5-fold) and SCF/Epo-stimulated (2.9-fold +/- 1.2-fold) cultures. Cells stimulated by SCF/Epo underwent similar levels of erythroid differentiation with and without anti-TGF-beta. While SCF alone stimulated the production of tryptase-positive mast cells, cells stimulated by SCF/anti-TGF-beta were predominantly erythroid (CD36(+)CD14(-) and glycophorin A positive). A distinct expansion of erythroid progenitors (CD34(+)CD36(+)CD14(-)) with the potential to form erythroid colonies was seen, revealing early Epo-independent erythroid development. In contrast, the kinetics of erythroid progenitor generation from primitive HSCs indicate that TGF-beta1 is not inhibitory in late erythropoiesis, but it accelerated the conversion of large BFU-E into colony-forming units-erythroid. Finally, TGF-beta1 accelerated Epo-induced terminal erythroid differentiation and resulted in a greater level of enucleation (22% +/- 6% versus 7% +/- 3%) in serum-free conditions. Serum addition stimulated enucleation (54% +/- 18%), which was lower (26% +/- 14%) with anti-TGF-beta, suggesting that optimal erythroid enucleation is Epo dependent, requiring serum factors including TGF-beta1.

IL-3-Dependent Early Erythropoiesis Is Stimulated by Autocrine Transforming Growth Factor Beta

Autocrine/paracrine transforming growth factor beta (TGF-beta) is an important regulator of stem cell quiescence and generally suppresses stem cell proliferation. However, we show here that during the first few days of an erythroid cell culture from adult blood stem cells, the presence of neutralizing antibodies against TGF-beta had a suppressive effect on subsequent erythropoiesis, indicating a stimulatory action of autocrine TGF-beta. The suppression occured in the form of a delay in erythroblast proliferation rather than a reduction in final erythroid colony numbers. The inhibitory effect of anti-TGF-beta occured in the presence of interleukin-3 (IL-3) but not in cultures with only stem cell factor and erythropoietin. Erythroblasts expressing gamma-globin (gamma+) were more strongly suppressed than erythroblasts expressing only beta-globin (gamma-beta+), so that stem cell treatment with anti-TGF-beta caused a decrease in the proportion of gamma+ cells. Anti-TGF-beta had an inhibitory effect on erythropoiesis only when administered during the first 4 days of culture, that is, before the onset of globin expression and dependence on erythropoietin. The decreasing effect of anti-TGF-beta with delayed addition coincided with a decreasing dependence on IL-3. CD133+ stem cells were more strongly suppressed by anti-TGF-beta than the complementary CD133-CD34+ stem cells, and the latter were also much less dependent on IL-3. The treatment of very early stem cell cultures with a pulse of added TGF-beta1 in the presence of IL-3 increased the subsequent proliferation of erythroblasts. Taken together, the data suggest that IL-3-driven early erythropoiesis from immature peripheral blood stem cells is stimulated by autocrine TGF-beta.