Abstract

Blood is a tissue with high cellular turnover, and its production is a tightly orchestrated process that requires constant replenishment. All mature blood cells are generated from hematopoietic stem cells (HSCs), which are the self-renewing units that sustain lifelong hematopoiesis. HSC behavior, such as self-renewal and quiescence, is regulated by a wide array of factors, including external signaling cues present in the bone marrow. The transforming growth factor-β (TGF-β) family of cytokines constitutes a multifunctional signaling circuitry, which regulates pivotal functions related to cell fate and behavior in virtually all tissues of the body. In the hematopoietic system, TGF-β signaling controls a wide spectrum of biological processes, from homeostasis of the immune system to quiescence and self-renewal of HSCs. Here, we review key features and emerging concepts pertaining to TGF-β and downstream signaling pathways in normal HSC biology, featuring aspects of aging, hematologic disease, and how this circuitry may be exploited for clinical purposes in the future.

Introduction

Blood cell production takes place in the bone marrow (BM) of adult individuals, where it originates from a rare pool of tissue-specific stem cells known as hematopoietic stem cells (HSCs). HSCs function to sustain and regenerate the entire blood system in an unbroken fashion throughout life, as they have the dual capacity to self-renew and differentiate to all blood cell lineages.1,2  Self-renewal pertains to the ability of a stem cell to duplicate itself without losing developmental potential. This capacity is crucial for maintenance of the stem cell pool and may occur in a symmetric or asymmetric fashion.3  Furthermore, quiescence, or withdrawal from the cell cycle, is a feature of HSCs that provides the blood system with a dormant HSC reservoir that retains the ability to self-renew and replenish all blood lineages. Hematopoiesis is hierarchically organized, with the most immature pool of HSCs at the top, giving rise to various progenitor cells that progressively lose self-renewal capacity as they form differentiating progeny that proliferate extensively, finally generating functional blood cells at the bottom of the hierarchy (Figure 1). To rapidly tailor blood production to acute changes, such as bleeding and infection, HSC behavior is tightly regulated yet highly flexible.4  Signals that promote quiescence, and cues that stimulate proliferation and differentiation, provide a fine-balanced system that safeguards against depletion and overproduction of HSCs. Disruption of these regulatory mechanisms can result in blood disease, such as BM failure or leukemia. Transforming growth factor-β (TGF-β) is the founding member of a large family of secreted polypeptide growth factors, consisting of over 30 members in humans, including activins, bone morphogenetic proteins (BMPs), and others.5  The TGF-β family constitutes a multifunctional set of cytokines that regulate a bewildering array of cellular processes during development and beyond. In the adult organism, TGF-β members regulate tissue homeostasis and regeneration. With respect to hematopoiesis, TGF-β plays an important role in regulating HSC behavior, particularly quiescence and self-renewal. Although many members of this family function to regulate hematopoiesis, we focus this review mainly on TGF-β.

Figure 1

The hematopoietic hierarchy. Hematopoiesis is organized in a hierarchical manner, with rare HSCs at the top that give rise to various types of progenitor cells, which proliferate extensively, finally generating mature blood cells at the bottom of the hierarchy. Red arrows indicate stimulation of proliferation by TGF-β, whereas inhibition signs point to TGF-β’s growth inhibitory effect, in specific cell types or lineages. CLP, common lymphoid progenitor; CMP, common myeloid progenitor; GMLP, granulocyte-macrophage-lymphocyte progenitor; GMP, granulocyte-macrophage progenitor; LT-HSC, long-term HSC; MEP, megakaryocyte-erythrocyte progenitor; MPP, multipotent progenitor; NK, natural killer; ST-HSC, short-term HSC.

Figure 1

The hematopoietic hierarchy. Hematopoiesis is organized in a hierarchical manner, with rare HSCs at the top that give rise to various types of progenitor cells, which proliferate extensively, finally generating mature blood cells at the bottom of the hierarchy. Red arrows indicate stimulation of proliferation by TGF-β, whereas inhibition signs point to TGF-β’s growth inhibitory effect, in specific cell types or lineages. CLP, common lymphoid progenitor; CMP, common myeloid progenitor; GMLP, granulocyte-macrophage-lymphocyte progenitor; GMP, granulocyte-macrophage progenitor; LT-HSC, long-term HSC; MEP, megakaryocyte-erythrocyte progenitor; MPP, multipotent progenitor; NK, natural killer; ST-HSC, short-term HSC.

The basic elements of TGF-β signaling

TGF-β ligands signal through cell surface serine/threonine kinase receptors, known as type I and type II receptors.6  In vertebrates 7 different type I receptors (activin receptor-like kinase [ALK1-7]) and 5 distinct type II receptors have been identified, serving the entire family of ligands.6  TGF-β signals mainly via the type I receptor, ALK5, and the type II receptor, TβRII, both of which are required for signaling activation. Following ligand binding and receptor phosphorylation, the SMAD signaling circuitry becomes activated (Figure 2).6  The SMAD proteins are a family of transcription factors consisting of 8 members, SMAD1-8, which are further subdivided into 3 classes based on structural and functional properties.7  Receptor-regulated SMADs (R-SMADs), SMAD1, 2, 3, 5, and 8, are the only SMADs directly phosphorylated and activated by the kinase domain of type I receptors. Upon phosphorylation, R-SMADs form a complex with the common SMAD, SMAD4, resulting in nuclear accumulation of activated complexes. In the nucleus, R-SMAD–SMAD4 complexes cooperate with transcriptional coregulators that further define target gene recognition and transcriptional regulation.7  The inhibitory SMADs, SMAD6 and SMAD7, constitute the third class, which function to inhibit TGF-β signaling. TGF-β/activin/nodal and BMP/growth differentiation factor use different subsets of R-SMADs. R-SMAD2/3 specifically relay signals from TGF-β and activin receptors, whereas R-SMAD1/5/8 primarily operate downstream of BMP receptors.6 

Figure 2

TGF-β signaling pathways. TGF-β ligands bind type I and type II receptors at the cell surface. Subsequently, the type I receptor (ALK5) becomes phosphorylated by the type II receptor. This leads to phosphorylation of SMAD2 and SMAD3, which form a complex with SMAD4. Activated complexes accumulate in the nucleus where they cooperate with DNA-binding cofactors to regulate target gene transcription. SMAD2 and SMAD3 also bind to TIF1γ. In embryonic stem cells, SMAD2/3-TIF1γ recognizes specific chromatin marks, promoting access of SMAD2/3-SMAD4 to otherwise repressed targets. TIF1-γ–SMAD2/3 promotes erythroid differentiation whereas SMAD4-SMAD2/SMAD3 complexes inhibit proliferation. In certain cell types, JNK and p38 are phosphorylated by TAK1 and constitute, together with the PI3K-AKT-FOXO axis, ERK, and PAR6, so-called noncanonical signaling responses to TGF-β. The dashed line indicates unclear molecular mechanism.

Figure 2

TGF-β signaling pathways. TGF-β ligands bind type I and type II receptors at the cell surface. Subsequently, the type I receptor (ALK5) becomes phosphorylated by the type II receptor. This leads to phosphorylation of SMAD2 and SMAD3, which form a complex with SMAD4. Activated complexes accumulate in the nucleus where they cooperate with DNA-binding cofactors to regulate target gene transcription. SMAD2 and SMAD3 also bind to TIF1γ. In embryonic stem cells, SMAD2/3-TIF1γ recognizes specific chromatin marks, promoting access of SMAD2/3-SMAD4 to otherwise repressed targets. TIF1-γ–SMAD2/3 promotes erythroid differentiation whereas SMAD4-SMAD2/SMAD3 complexes inhibit proliferation. In certain cell types, JNK and p38 are phosphorylated by TAK1 and constitute, together with the PI3K-AKT-FOXO axis, ERK, and PAR6, so-called noncanonical signaling responses to TGF-β. The dashed line indicates unclear molecular mechanism.

Alternative pathways

The SMAD pathway forms a fundamental signaling module and is the most well-studied circuitry downstream of TGF-β. However, activation of other pathways, most notably TGF-β–activated kinase 1 (TAK1), a component of the mitogen-activated protein kinase pathway, has been observed in other cell types.8,9  Depending on context, TAK1 activates a host of downstream transducers, including p38 and c-Jun N-terminal kinase (JNK).10  Conditional deletion of TAK1 in mice, using the Mx-Cre driver, results in hematopoietic failure, but the link to TGF-β has not been firmly established.11  Other non-SMAD circuitries regulated by TGF-β include extracellular signal-regulated kinase (ERK), phosphatidylinositol 3-kinase (PI3K)-AKT-FOXO, and RHO-like small guanosine triphosphatasess, though the contributions of most of these mechanisms are not well defined in HSCs (Figure 2).10  In addition, transcriptional intermediary factor-1γ (TIF1γ) partners with SMAD2/3 (see “Variations on TGF-β signaling: a role for TIF1-γ”).12  The TGF-β pathway is intimately linked with other signaling circuitries (for readers interested in crosstalk mechanisms, we refer to Nishita et al,13  Labbé et al,14  Itoh et al,15  Emmrich et al,16  and Chabanon et al17 ).

TGF-β: inducer of quiescence

TGF-β is cataloged as one of the most potent inhibitors of HSC growth in vitro; a variety of culture systems support this notion.18-21  During homeostatic conditions, the majority of HSCs are in a quiescent cell-cycle state.22,23  Quiescence is related to maintenance of the HSC pool and loss thereof results in exhaustion and erosion of HSC function.24  Naturally, TGF-β has been hypothesized to be a cardinal regulator of HSC dormancy, maintaining a pool of quiescent HSCs in vivo. Indeed, neutralization of TGF-β in vitro releases early hematopoietic progenitor cells from quiescence.25-27  Several molecular mechanisms have been proposed to account for TGF-β–mediated growth inhibition, including alterations in cytokine receptor expression and upregulation of cyclin-dependent kinase inhibitors, such as p15Ink4b, p21Cip1, and p27Kip1.26,28-34  However, TGF-β can exert growth inhibitory actions independent of p21Cip1 and p27Kip1.35  In human CD34+ cells, TGF-β–mediated cell-cycle arrest occurs through upregulation of p57Kip2, another member of the cyclin-dependent kinase inhibitor family.36  Similarly, p57Kip2 is highly enriched in mouse CD34Kit+LinSca1+ (KLS) cells as opposed to the more mature and actively cycling CD34+KLS fraction.37  Interestingly, a high level of p57Kip2 correlates with the activation status of SMAD2/3, which are uniquely phosphorylated in freshly isolated CD34KLS cells but not in CD34+KLS progenitors.38  Additionally, TGF-β upregulates p57Kip2 in CD34KLS cells in vitro.38  These findings point to a mechanism where TGF-β functions to induce p57Kip2 within the most primitive HSC compartment, thus promoting their quiescent state in vivo (Figure 3).

Figure 3

Bidirectional effects of TGF-β. Low concentrations of TGF-β stimulate proliferation of My-HSCs, whereas Ly-HSCs are growth inhibited. A higher dose of TGF-β inhibits proliferation, irrespective of HSC subtype, and induces quiescence via SMAD2/3-SMAD4-dependent expression of p57Kip2.

Figure 3

Bidirectional effects of TGF-β. Low concentrations of TGF-β stimulate proliferation of My-HSCs, whereas Ly-HSCs are growth inhibited. A higher dose of TGF-β inhibits proliferation, irrespective of HSC subtype, and induces quiescence via SMAD2/3-SMAD4-dependent expression of p57Kip2.

Heterogeneity of the HSC pool

Recently, evidence has accumulated suggesting that the adult HSC compartment consists of a number of functionally distinct subsets with diverse self-renewal and differentiation potentials.23,39-41  Interestingly, discrete HSC subtypes respond differently to TGF-β, according to a model proposed by Challen and colleagues.42  Specifically, TGF-β stimulates proliferation of myeloid-biased HSCs (My-HSCs) whereas lymphoid-biased HSCs (Ly-HSCs) are growth inhibited (Figure 3).42  Thus, the effect of TGF-β on HSCs is more nuanced than previously thought, such that proliferation is induced in certain HSC subtypes at certain concentrations. However, though TGF-β represents a potential signal for differential regulation between HSC subtypes, there is currently no tangible explanation for the underlying molecular mechanism. Another study found that the RNA-binding protein MUSASHI-2 is required for HSCs to mount a proliferative response to TGF-β at low concentrations.43  Loss of MUSASHI-2 leads to impaired HSC quiescence, reduced My-HSC numbers, and myeloid output in vivo. These findings are coupled to significant reductions in p57Kip2 and phosphorylated SMAD2/3 in HSCs, suggesting that MUSASHI-2 modulates the TGF-β signaling circuitry.43  Most work regarding TGF-β in hematopoiesis has focused on TGF-β1. However, TGF-β exists in 3 isoforms that are encoded by separate genes, Tgfb1-3. Although TGF-β1-3 share significant sequence homology and signal through the same receptor complex,44,45  differences in receptor affinity exist and varying responses have been reported. Most notably, KLS cells exhibit a biphasic response to TGF-β2, being growth inhibited at high doses and stimulated at low concentrations.46  The bidirectional effects of TGF-β on proliferation vs quiescence further substantiate the complexity of this signaling pathway, and it remains to be clarified whether the SMAD pathway is differentially regulated at high and low doses and between diverse HSC subtypes. Additionally, it is conceivable that the bidirectional effects are a function of contrasting TGF-β receptor expression among HSC subtypes, translating to dosage effects that generate qualitatively different responses. This theory is corroborated by a recent study, which shows that a high level of ALK5 correlates with increased myeloid output.47 

TGF-β and the aging hematopoietic system

The hematopoietic system declines functionally with age, a phenomenon that can be traced back to alterations in the HSC compartment. The clonal composition of the HSC pool changes over time, such that My-HSCs become more abundant at the expense of Ly-HSCs.48  Physiologically, this translates to a myeloid-biased hematopoietic system with reduced lymphoid potential. Furthermore, the risk of hematologic malignancies, anemia, autoimmunity, and inflammatory disorders increases with age.48  As TGF-β differentially regulates My- and Ly-HSCs, the connection to aging is evident, especially because TGF-β is implicated in the aging process of nonhematopoietic tissues.49,50  A recent study (which profiled the transcriptome, DNA methylome, and histone modifications in highly purified young and old HSCs) found significant changes in the TGF-β pathway at the transcriptional level.51  In fact, the TGF-β pathway represented 19% of differentially expressed genes in young vs old HSCs. The study shows changes in multiple layers of the TGF-β pathway, from extracellular regulators, to SMAD transcription factors, and cofactors, resulting in an overall reduction of TGF-β signaling in aged HSCs.51  Because low concentrations of TGF-β stimulate proliferation of My-HSCs in vitro, downsizing the TGF-β circuitry may provide a more proliferative environment for My-HSCs, which can expand over time. Similarly, the stimulatory effect of TGF-β2 on the much cruder KLS population increases with age.52  This is logical, as the aged KLS compartment should contain a larger proportion of My-HSCs. However, the relationship between TGF-β and aging is complex, as evidenced by a study of TIF1-γ. In HSCs, the expression of TIF1-γ decreases with age, consistent with an accelerated aging phenotype of TIF1-γ knockout mice.47  Additionally, TIF1-γ controls the turnover of ALK5, such that ALK5+ HSCs become more abundant with age.47  Interestingly, aged HSCs are more sensitive to the cytostatic effect of TGF-β in vivo, possibly as a reflection of more generous ALK5 expression. As TIF1-γ is itself part of the signal transduction machinery downstream of TGF-β, the balance between TIF1-γ and SMAD4-mediated transcriptional responses may play an additional role, which was not investigated. In sum, perturbations of TGF-β signaling represent a potential mechanism that contributes to HSC aging.

TGF-β and the BM niche

The quest to decipher the role of TGF-β in the regulation of HSCs requires an understanding of the BM niches that house HSCs. Due to the anatomical complexity of the BM, much of our knowledge concerning the HSC microenvironment has for long remained nebulous and it is only in the last decade that we have gained a more detailed appreciation of the regulatory elements of the BM environment, although Ray Schofield proposed the concept of the BM niche already as far back as 1978.53  An increasing number of niche components have now been identified, revealing a complex network of cell-cell interactions, extracellular elements, signaling cues, and structures.54,55  Together, these entities create specialized niches that function to maintain and control self-renewing HSCs in a coordinated manner. TGF-β is part of this microenvironment, as it is produced by a variety of cell types in the BM, and large quantities of latent TGF-β are deposited into bone matrix.56,57  However, according to 1 study, surprisingly few BM cells exhibit significant phosphorylation of SMAD2/3 at steady state, suggesting that activation of latent TGF-β is tightly regulated spatially.58  Nonmyelinating Schwann cells, which ensheath peripheral nerves and lay in parallel with blood vessels in the BM, are proposed as one of the major sources for TGF-β activation in the BM.58  These glial cells regulate the activation process of TGF-β by binding latent TGF-β via integrin β8 on the cell surface, thus promoting activation by exposing TGF-β to proteolytic cleavage by metalloproteinases.58  A sizable portion of HSCs is in direct contact with nonmyelinating Schwann cells in the BM, and denervation results in loss of HSCs and increased cell cycling. Another major source of TGF-β is megakaryocytes, a cell type that physically associates with ∼20% of HSCs in the BM.59,60  HSCs in close proximity to megakaryocytes exhibit activation of SMAD2/3, and ablation of megakaryocytes results in reduced phosphorylation of SMAD2/3 as well as loss of quiescence and increased HSC proliferation.60  Importantly, conditional deletion of Tgfb1 in megakaryocytes results in increased cell cycling of HSCs.60  The anatomical relationship between megakaryocytes and Schwann cells has not been investigated, but the conclusion from these studies is that, indeed, TGF-β provides an important quiescence signal to HSCs in the BM niche. Additionally, the conclusion must be that >1 cell type in the BM contributes to TGF-β production and signaling in HSCs (Figure 4).

Figure 4

TGF-β in the BM niche. HSCs reside in the BM, in specialized niches that support and regulate HSC fate options. TGF-β is activated by Schwann cells that ensheath sympathetic nerves. Large amounts of TGF-β are also produced by megakaryocytes. HSCs in close proximity to Schwann cells and megakaryocytes become exposed to TGF-β and exhibit activation of SMAD2/3.

Figure 4

TGF-β in the BM niche. HSCs reside in the BM, in specialized niches that support and regulate HSC fate options. TGF-β is activated by Schwann cells that ensheath sympathetic nerves. Large amounts of TGF-β are also produced by megakaryocytes. HSCs in close proximity to Schwann cells and megakaryocytes become exposed to TGF-β and exhibit activation of SMAD2/3.

Lessons from in vivo models

TGF-β can affect most cell types throughout the hematopoietic hierarchy, but the response is modulated by context and differentiation stage of the target cell. Therefore, TGF-β generates highly variable biological outcomes and can affect proliferation, differentiation, and apoptosis in both positive and negative directions.21,61-63  Much of our knowledge of the physiological relevance of TGF-β has been gained by studying knockout mouse models. Based on these reports, we know that TGF-β is a principal regulator of immune cell homeostasis and function in vivo. Importantly, both Tgfb1-ligand and receptor knockout mice develop a lethal inflammatory disorder.64-66  With respect to hematopoiesis, Tgfb1-null mice exhibit enhanced myelopoiesis, suggesting that TGF-β acts as a negative regulator of myelopoiesis in vivo.65  Upon analysis before the onset of inflammation, a host of HSC properties are altered in Tgfb1 knockout mice.67  Most significantly, BM cells from Tgfb1-deficient neonates exhibit impaired reconstitution ability upon transplantation, a finding attributed to defective homing.67  However, mice deficient in ALK5 display normal HSC self-renewal and regenerative capacity in vivo, even under extreme hematopoietic stress with no defects in homing capacity.68,69  In contrast, TβRII conditional knockout mice show increased HSC cell cycling in vivo, and reduced regenerative capacity upon transplantation.58  Because of the severe inflammatory disease that develops in TGF-β signaling–deficient mouse models, most studies have used different strategies to overcome disease progression, including a variety of immune-deficient genetic backgrounds. This may account for the differences observed between knockout models. Additionally, signals may emanate from TβRII independently of ALK5, as has been shown in nonhematopoietic cell types where the cell polarity regulator partitioning defective 6 (PAR6) is phosphorylated directly by TβRII.70,71  Furthermore, the fact that TβRII is more highly expressed within HSCs compared with ALK5 might be an additional contributing factor.57  Thus, there are both overlapping and nonoverlapping phenotypes between knockout models and it appears to be critically important at which level and for how long TGF-β signaling is disrupted. This notion is supported by a recent study in which a neutralizing antibody against active TGF-β was administered to mice following 5-fluorouracil treatment.72  TGF-β blockade after chemotherapy results in enhanced hematopoietic regeneration by delaying the return of HSCs to quiescence. These findings imply that the TGF-β–SMAD2–p57Kip2 signaling axis is important for reestablishing quiescence of HSCs following stress, once sufficient regeneration of the hematopoietic system has been attained. Additionally, overexpression of SMAD4 sensitizes human cord blood–derived candidate HSCs to TGF-β, resulting in reduced regenerative capacity in vivo.73  Together, these findings suggest that transient inhibition of TGF-β might be a feasible strategy to enhance hematopoietic recovery in patients following chemotherapy. Due to the multifaceted nature of TGF-β coupled with a potentially complex set of redundant mechanisms, its role as a critical regulator of HSC quiescence in vivo has been difficult to unveil. However, current dogma now suggests that TGF-β is indeed an important quiescence signal in vivo.

A role for SMAD signaling in self-renewal of HSCs

To block the SMAD signaling network downstream of TGF-β, 2 approaches have been used: overexpression of the inhibitory SMAD7 and deletion of Smad4. In murine HSCs, forced expression of SMAD7 results in increased self-renewal of HSCs, indicating that the SMAD pathway negatively regulates self-renewal in vivo.74  Importantly, differentiation was unperturbed in this model, suggesting that self-renewal is regulated independently of differentiation by SMAD signaling. In contrast, overexpression of SMAD7 in human severe combined immunodeficiency repopulating cells results in altered differentiation from lymphoid-dominant engraftment toward increased myeloid contribution.75  Thus, in the xenograft model system, forced expression of SMAD7 modulates differentiation of multipotent human severe combined immunodeficiency repopulating cells. Using a conditional knockout mouse model, disruption of the SMAD pathway at the level of SMAD4 was investigated. Intriguingly, Smad4-deficient HSCs display a reduced repopulative capacity of primary and secondary recipients, indicating that SMAD4 is critical for HSC self-renewal in vivo.76  Because overexpression of SMAD7 vs deletion of Smad4 is anticipated to yield similar hematopoietic phenotypes, it is conceivable that SMAD4 functions as a positive regulator of self-renewal independently of its role in the TGF-β pathway.13-15  Alternatively, SMAD7 may have unanticipated functions in an overexpression setting that impinge positively on self-renewal.

Variations on TGF-β signaling: a role for TIF1γ

SMAD4 has traditionally been viewed as the nexus of SMAD signaling as it functions as a core component of both TGF-β/activin and BMP signaling branches. However, SMAD2/3 can also partner with TIF1γ.12  In human hematopoietic stem/progenitor cells, TGF-β balances erythroid differentiation with growth inhibition in a mechanism dependent on competitive binding between SMAD4 and TIF1γ to SMAD2/3.12  In response to TGF-β, the TIF1γ-SMAD2/3 complex stimulates erythroid differentiation whereas SMAD2/3 in association with SMAD4 leads to growth inhibition of human hematopoietic progenitors (Figure 2). Interestingly, the zebrafish homolog of TIF1γ, encoded by moonshine, is essential for blood formation with mutants displaying severe red cell aplasia, indicating that TIF1γ is required for erythroid development.77  Furthermore, TIF1γ is implicated in regulation of transcription elongation of erythroid genes. The proposed model suggests that TIF1γ functions to release paused Pol II at erythroid genes by recruiting positive elongation factors to the blood-specific transcriptional complex, thus promoting transcription.78  TIF1γ also functions at the erythroid/myeloid lineage bifurcation by modulating GATA1 and PU.1 expression.79  The link between TIF1γ and TGF-β signaling has been delineated in detail in embryonic stem cells. There, TIF1γ functions as a chromatin reader where TIF1γ-SMAD2/3 recognizes certain repressive histone marks, promoting a transition to open chromatin that allows SMAD4-SMAD2/3 to gain access to DNA.80  Thus, TIF1γ is needed for certain SMAD4-SMAD2/3 transcriptional responses.

TGF-β in hematopoietic disease

Leukemia

Despite TGF-β’s pronounced cytostatic effect on HSCs in vitro and the fact that mutations in genes encoding components of the TGF-β pathway are frequently found in other neoplasms, such as pancreatic and colon cancer,81,82  inactivating mutations are relatively uncommon for this pathway in hematologic malignancies.83  Nevertheless, a number of cases have been reported involving SMAD4 and TGFBR2 in patients with acute myelogenous leukemia (AML).84-87  For example, in a study investigating human AML genomes, various copy number alterations were found recurrently modified, 1 of which represented the deletion of SMAD4.88  Furthermore, reports of sporadic mutations in both TGFBR1 and TGFBR2 in lymphoid neoplasms exist, and loss of SMAD3 is associated with T-cell acute lymphocytic leukemia.89-91  The mechanism for the SMAD3 deficiency is not known because the SMAD3 messenger RNA was present and no mutations could be detected in the MADH3 gene, which encodes SMAD3.91  However, T-cell leukemogenesis is promoted in mice with haploinsufficiency of Smad3 and a complete deficiency of p27Kip1.91  These findings are interesting because the p27Kip1 gene is frequently mutated in pediatric acute lymphocytic leukemia, due to translocations and deletions or germline mutations.92,93  Impaired TGF-β signaling in hematologic malignancies can also be caused by suppression of SMAD-dependent transcriptional responses by oncoproteins like TAX, EVI-1, and AML1-ETO.94-96  Similarly, downregulation of the transcription factor ZEB1 and overexpression of SMAD7 contribute to resistance to TGF-β1–mediated growth suppression in adult T-cell leukemia/lymphoma without known mutations in TGF-β pathway genes.97  In addition, there are several reports on oncoproteins, which generate leukemia and simultaneously neutralize the growth inhibitory signal of the SMAD pathway by binding to or interacting with SMADs. Fusion oncoproteins, like TEL-AML1 and AML1-EVI1, bind to SMAD3, impairing both TGF-β signaling and apoptosis of transduced HSCs in vitro.95,98-100  Furthermore, SMAD4 physically associates with HOXA9, reducing its ability to regulate transcriptional targets in hematopoietic cells in vitro.101  In vivo studies show that in wild-type mice overexpressing HOXA9 or NUP98-HOXA9, SMAD4 binds the oncoproteins and sequestrates them to the cytoplasm, suggesting that SMAD4 plays a protective role against further promotion and growth of leukemic cells.102  Therefore, SMAD signaling can be reduced or neutralized in hematopoietic malignancies, but in a majority of cases this is not due to mutations in genes of the circuitry itself, but rather through altered expression or function of cofactors and oncoproteins, or alternatively via loss of TGF-β target genes.

The diseased BM microenvironment

Although TGF-β plays a major role as tumor suppressor, TGF-β can paradoxically facilitate tumor growth, particularly in the later stages of disease. This is due to effects on the tumor microenvironment and the immunosuppressive function of TGF-β. Interestingly, recent findings show that there is considerable interplay between leukemic cells and the BM microenvironment. In fact, the BM microenvironment undergoes substantial remodeling by signals from leukemic cells.103  Similarly, alterations in niche cells can trigger hematopoietic disease and promote leukemia.104,105  Furthermore, activation of the parathyroid hormone receptor specifically in osteoblastic cells results in remodeling of the BM microenvironment.106  The remodeled niche attenuates BCR-ABL1 oncogene-induced chronic myeloid leukemia–like disease, via upregulation of TGF-β1, whereas MLL-AF9 oncogene-induced AML is enhanced. These data differ from the findings by Naka et al, in which TGF-β, via regulation of AKT and FOXO3a, instead maintains leukemia-initiating cells in chronic myeloid leukemia.107  The discrepancies may be attributed to differences in TGF-β dosage and context, indicating that TGF-β sensitivity across different types of leukemia varies. Nevertheless, the findings point to the fact that TGF-β is part of the leukemic BM niche and strategies to target the BM microenvironment may be a promising approach to reduce certain types of leukemic stem cells in the future. On this note, TGF-β has been proposed to contribute to myelofibrosis, which is a chronic myeloproliferative disease characterized by clonal proliferation and BM fibrosis.108  Several cytokines are thought to contribute toward accumulation of reticulin fibers in the BM of patients with myelofibrosis. These include TGF-β, fibroblast growth factor 2 (FGF-2), and platelet-derived growth factor.109-112  Some of the most compelling evidence for a prominent role of TGF-β in myelofibrosis comes from a study using Tgfb1-null mice. To induce myelofibrosis, irradiated mice were transplanted with BM cells transduced with vectors containing the thrombopoietin gene. Importantly, prominent myelofibrosis developed in mice receiving transduced wild-type cells but not Tgfb1-null cells.111  These data indicate that TGF-β1, produced by hematopoietic cells, is a crucial component in the development of myelofibrosis, and underlines further the interplay between hematopoietic cells and the BM microenvironment.

TGF-β in the pathogenesis of MDS

Myelodysplastic syndromes (MDSs) encompass a spectrum of clonal stem cell disorders characterized by inefficient hematopoiesis and reduced peripheral blood counts. Excessive activation of inhibitory pathways, such as TGF-β, has been proposed to amplify the inefficient blood production inherent to MDS.113  For example, SMAD2 is upregulated and overactivated in CD34+ BM progenitors from MDS patients. Importantly, pharmacologic inhibition of the TGF-β pathway in vivo, using a small-molecule inhibitor of ALK5, alleviates anemia in a mouse model of MDS.114  Furthermore, SMAD7 is reduced in BM progenitors from MDS patients, leading to overactivation of TGF-β signaling.115  The reduction of SMAD7 is attributed to increased levels of microRNA-21, which binds to the 3′ untranslated region of SMAD7.116  Administration of a chemically modified inhibitor of microRNA-21 results in increased red blood cell counts, using the same MDS mouse model mentioned in “The diseased BM microenvironment.”116  Together, these studies implicate TGF-β in the pathogenesis of MDS and suggest that this pathway may be a realistic therapeutic target in a subset of MDSs.

Therapeutic avenues and future perspectives

HSCs are the therapeutic units of BM transplantations and, as such, are by far the most widely used application of stem cell therapy to date. Despite this, strategies to improve efficiency and applicability of HSC transplantations are needed, including expansion of HSCs ex vivo and techniques to improve engraftment and regeneration posttransplantation. Although BMPs have been reported to contribute to HSC expansion ex vivo, manipulating SMAD signaling alone is unlikely to result in effective expansion.117  Another approach, with substantial promise that should be investigated further, is to transiently block TGF-β signaling in vivo to boost regeneration and recovery of hematopoiesis following chemotherapy.72  Similarly, a number of recent studies highlight the feasibility of manipulating the TGF-β pathway in hematopoietic disorders like MDS and anemia.115,118,119  In addition, the complexity of the SMAD pathway continues to be exposed as new layers of regulatory mechanisms are found. For example, the linker region of SMADs is subject to negative regulation by glycogen synthase kinase 3, FGF, and epidermal growth factor.120-123  The importance of this type of crosstalk in hematopoietic cells is unknown, but it is worth mentioning as megakaryocytes were recently shown to balance TGF-β and FGF output, thereby regulating hematopoietic regeneration following 5-fluorouracil challenge in vivo.60  Thus, manipulating TGF-β signaling is relevant to several aspects of hematopoiesis. In the future, more detailed mechanistic studies are required to precisely define how TGF-β signaling may be manipulated in relation to other signaling circuitries, ultimately improving HSC regeneration and hematopoietic disease in vivo.

Acknowledgments

The authors apologize to those whose work was not cited due to space limitations.

This work was supported by the European Commission (Stemexpand); Hemato-Linné and Stemtherapy program project grants from the Swedish Research Council; a project grant from the Swedish Research Council (S.K.); the Swedish Cancer Society; the Swedish Children Cancer Foundation; a Clinical Research Award from Lund University Hospital; and a grant from The Tobias Foundation awarded by the Royal Academy of Sciences (S.K.).

Authorship

Contribution: U.B. and S.K. wrote the review, and U.B. designed the figures.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Correspondence: Stefan Karlsson, Division of Molecular Medicine and Gene Therapy, BMC A12, 221 84 Lund, Sweden; e-mail: Stefan.karlsson@med.lu.se; and Ulrika Blank, Division of Molecular Medicine and Gene Therapy, BMC A12, 221 84 Lund, Sweden; e-mail: ulrika.blank@med.lu.se.

References

References
1
Ogawa
 
M
Differentiation and proliferation of hematopoietic stem cells.
Blood
1993
, vol. 
81
 
11
(pg. 
2844
-
2853
)
2
Orkin
 
SH
Zon
 
LI
Hematopoiesis: an evolving paradigm for stem cell biology.
Cell
2008
, vol. 
132
 
4
(pg. 
631
-
644
)
3
Zon
 
LI
Intrinsic and extrinsic control of haematopoietic stem-cell self-renewal.
Nature
2008
, vol. 
453
 
7193
(pg. 
306
-
313
)
4
Rossi
 
L
Lin
 
KK
Boles
 
NC
, et al. 
Less is more: unveiling the functional core of hematopoietic stem cells through knockout mice.
Cell Stem Cell
2012
, vol. 
11
 
3
(pg. 
302
-
317
)
5
Massagué
 
J
TGFβ signalling in context.
Nat Rev Mol Cell Biol
2012
, vol. 
13
 
10
(pg. 
616
-
630
)
6
Massagué
 
J
TGF-beta signal transduction.
Annu Rev Biochem
1998
, vol. 
67
 (pg. 
753
-
791
)
7
Massagué
 
J
Seoane
 
J
Wotton
 
D
Smad transcription factors.
Genes Dev
2005
, vol. 
19
 
23
(pg. 
2783
-
2810
)
8
Blank
 
U
Brown
 
A
Adams
 
DC
Karolak
 
MJ
Oxburgh
 
L
BMP7 promotes proliferation of nephron progenitor cells via a JNK-dependent mechanism.
Development
2009
, vol. 
136
 
21
(pg. 
3557
-
3566
)
9
Yamaguchi
 
K
Shirakabe
 
K
Shibuya
 
H
, et al. 
Identification of a member of the MAPKKK family as a potential mediator of TGF-beta signal transduction.
Science
1995
, vol. 
270
 
5244
(pg. 
2008
-
2011
)
10
Mu
 
Y
Gudey
 
SK
Landström
 
M
Non-Smad signaling pathways.
Cell Tissue Res
2012
, vol. 
347
 
1
(pg. 
11
-
20
)
11
Tang
 
M
Wei
 
X
Guo
 
Y
, et al. 
TAK1 is required for the survival of hematopoietic cells and hepatocytes in mice.
J Exp Med
2008
, vol. 
205
 
7
(pg. 
1611
-
1619
)
12
He
 
W
Dorn
 
DC
Erdjument-Bromage
 
H
Tempst
 
P
Moore
 
MA
Massagué
 
J
Hematopoiesis controlled by distinct TIF1gamma and Smad4 branches of the TGFbeta pathway.
Cell
2006
, vol. 
125
 
5
(pg. 
929
-
941
)
13
Nishita
 
M
Hashimoto
 
MK
Ogata
 
S
, et al. 
Interaction between Wnt and TGF-beta signalling pathways during formation of Spemann’s organizer.
Nature
2000
, vol. 
403
 
6771
(pg. 
781
-
785
)
14
Labbé
 
E
Letamendia
 
A
Attisano
 
L
Association of Smads with lymphoid enhancer binding factor 1/T cell-specific factor mediates cooperative signaling by the transforming growth factor-beta and wnt pathways.
Proc Natl Acad Sci USA
2000
, vol. 
97
 
15
(pg. 
8358
-
8363
)
15
Itoh
 
F
Itoh
 
S
Goumans
 
MJ
, et al. 
Synergy and antagonism between Notch and BMP receptor signaling pathways in endothelial cells.
EMBO J
2004
, vol. 
23
 
3
(pg. 
541
-
551
)
16
Emmrich
 
S
Rasche
 
M
Schöning
 
J
, et al. 
miR-99a/100∼125b tricistrons regulate hematopoietic stem and progenitor cell homeostasis by shifting the balance between TGFβ and Wnt signaling.
Genes Dev
2014
, vol. 
28
 
8
(pg. 
858
-
874
)
17
Chabanon
 
A
Desterke
 
C
Rodenburger
 
E
, et al. 
A cross-talk between stromal cell-derived factor-1 and transforming growth factor-beta controls the quiescence/cycling switch of CD34(+) progenitors through FoxO3 and mammalian target of rapamycin.
Stem Cells
2008
, vol. 
26
 
12
(pg. 
3150
-
3161
)
18
Garbe
 
A
Spyridonidis
 
A
Möbest
 
D
Schmoor
 
C
Mertelsmann
 
R
Henschler
 
R
Transforming growth factor-beta 1 delays formation of granulocyte-macrophage colony-forming cells, but spares more primitive progenitors during ex vivo expansion of CD34+ haemopoietic progenitor cells.
Br J Haematol
1997
, vol. 
99
 
4
(pg. 
951
-
958
)
19
Batard
 
P
Monier
 
MN
Fortunel
 
N
, et al. 
TGF-(beta)1 maintains hematopoietic immaturity by a reversible negative control of cell cycle and induces CD34 antigen up-modulation.
J Cell Sci
2000
, vol. 
113
 
Pt 3
(pg. 
383
-
390
)
20
Sitnicka
 
E
Ruscetti
 
FW
Priestley
 
GV
Wolf
 
NS
Bartelmez
 
SH
Transforming growth factor beta 1 directly and reversibly inhibits the initial cell divisions of long-term repopulating hematopoietic stem cells.
Blood
1996
, vol. 
88
 
1
(pg. 
82
-
88
)
21
Fortunel
 
NO
Hatzfeld
 
A
Hatzfeld
 
JA
Transforming growth factor-beta: pleiotropic role in the regulation of hematopoiesis.
Blood
2000
, vol. 
96
 
6
(pg. 
2022
-
2036
)
22
Cheshier
 
SH
Morrison
 
SJ
Liao
 
X
Weissman
 
IL
In vivo proliferation and cell cycle kinetics of long-term self-renewing hematopoietic stem cells.
Proc Natl Acad Sci USA
1999
, vol. 
96
 
6
(pg. 
3120
-
3125
)
23
Wilson
 
A
Laurenti
 
E
Oser
 
G
, et al. 
Hematopoietic stem cells reversibly switch from dormancy to self-renewal during homeostasis and repair.
Cell
2008
, vol. 
135
 
6
(pg. 
1118
-
1129
)
24
Passegué
 
E
Wagers
 
AJ
Giuriato
 
S
Anderson
 
WC
Weissman
 
IL
Global analysis of proliferation and cell cycle gene expression in the regulation of hematopoietic stem and progenitor cell fates.
J Exp Med
2005
, vol. 
202
 
11
(pg. 
1599
-
1611
)
25
Soma
 
T
Yu
 
JM
Dunbar
 
CE
Maintenance of murine long-term repopulating stem cells in ex vivo culture is affected by modulation of transforming growth factor-beta but not macrophage inflammatory protein-1 alpha activities.
Blood
1996
, vol. 
87
 
11
(pg. 
4561
-
4567
)
26
Fortunel
 
N
Batard
 
P
Hatzfeld
 
A
, et al. 
High proliferative potential-quiescent cells: a working model to study primitive quiescent hematopoietic cells.
J Cell Sci
1998
, vol. 
111
 
Pt 13
(pg. 
1867
-
1875
)
27
Hatzfeld
 
J
Li
 
ML
Brown
 
EL
, et al. 
Release of early human hematopoietic progenitors from quiescence by antisense transforming growth factor beta 1 or Rb oligonucleotides.
J Exp Med
1991
, vol. 
174
 
4
(pg. 
925
-
929
)
28
Dao
 
MA
Hwa
 
J
Nolta
 
JA
Molecular mechanism of transforming growth factor beta-mediated cell-cycle modulation in primary human CD34(+) progenitors.
Blood
2002
, vol. 
99
 
2
(pg. 
499
-
506
)
29
Dao
 
MA
Taylor
 
N
Nolta
 
JA
Reduction in levels of the cyclin-dependent kinase inhibitor p27(kip-1) coupled with transforming growth factor beta neutralization induces cell-cycle entry and increases retroviral transduction of primitive human hematopoietic cells.
Proc Natl Acad Sci USA
1998
, vol. 
95
 
22
(pg. 
13006
-
13011
)
30
Dubois
 
CM
Ruscetti
 
FW
Palaszynski
 
EW
Falk
 
LA
Oppenheim
 
JJ
Keller
 
JR
Transforming growth factor beta is a potent inhibitor of interleukin 1 (IL-1) receptor expression: proposed mechanism of inhibition of IL-1 action.
J Exp Med
1990
, vol. 
172
 
3
(pg. 
737
-
744
)
31
Dubois
 
CM
Ruscetti
 
FW
Stankova
 
J
Keller
 
JR
Transforming growth factor-beta regulates c-kit message stability and cell-surface protein expression in hematopoietic progenitors.
Blood
1994
, vol. 
83
 
11
(pg. 
3138
-
3145
)
32
Ducos
 
K
Panterne
 
B
Fortunel
 
N
Hatzfeld
 
A
Monier
 
MN
Hatzfeld
 
J
p21(cip1) mRNA is controlled by endogenous transforming growth factor-beta1 in quiescent human hematopoietic stem/progenitor cells.
J Cell Physiol
2000
, vol. 
184
 
1
(pg. 
80
-
85
)
33
Jacobsen
 
SE
Ruscetti
 
FW
Dubois
 
CM
Lee
 
J
Boone
 
TC
Keller
 
JR
Transforming growth factor-beta trans-modulates the expression of colony stimulating factor receptors on murine hematopoietic progenitor cell lines.
Blood
1991
, vol. 
77
 
8
(pg. 
1706
-
1716
)
34
Sansilvestri
 
P
Cardoso
 
AA
Batard
 
P
, et al. 
Early CD34high cells can be separated into KIThigh cells in which transforming growth factor-beta (TGF-beta) downmodulates c-kit and KITlow cells in which anti-TGF-beta upmodulates c-kit.
Blood
1995
, vol. 
86
 
5
(pg. 
1729
-
1735
)
35
Cheng
 
T
Shen
 
H
Rodrigues
 
N
Stier
 
S
Scadden
 
DT
Transforming growth factor beta 1 mediates cell-cycle arrest of primitive hematopoietic cells independent of p21(Cip1/Waf1) or p27(Kip1).
Blood
2001
, vol. 
98
 
13
(pg. 
3643
-
3649
)
36
Scandura
 
JM
Boccuni
 
P
Massagué
 
J
Nimer
 
SD
Transforming growth factor beta-induced cell cycle arrest of human hematopoietic cells requires p57KIP2 up-regulation.
Proc Natl Acad Sci USA
2004
, vol. 
101
 
42
(pg. 
15231
-
15236
)
37
Yamazaki
 
S
Iwama
 
A
Takayanagi
 
S
, et al. 
Cytokine signals modulated via lipid rafts mimic niche signals and induce hibernation in hematopoietic stem cells.
EMBO J
2006
, vol. 
25
 
15
(pg. 
3515
-
3523
)
38
Yamazaki
 
S
Iwama
 
A
Takayanagi
 
S
Eto
 
K
Ema
 
H
Nakauchi
 
H
TGF-beta as a candidate bone marrow niche signal to induce hematopoietic stem cell hibernation.
Blood
2009
, vol. 
113
 
6
(pg. 
1250
-
1256
)
39
Dykstra
 
B
Kent
 
D
Bowie
 
M
, et al. 
Long-term propagation of distinct hematopoietic differentiation programs in vivo.
Cell Stem Cell
2007
, vol. 
1
 
2
(pg. 
218
-
229
)
40
Sieburg
 
HB
Cho
 
RH
Dykstra
 
B
Uchida
 
N
Eaves
 
CJ
Muller-Sieburg
 
CE
The hematopoietic stem compartment consists of a limited number of discrete stem cell subsets.
Blood
2006
, vol. 
107
 
6
(pg. 
2311
-
2316
)
41
Copley
 
MR
Beer
 
PA
Eaves
 
CJ
Hematopoietic stem cell heterogeneity takes center stage.
Cell Stem Cell
2012
, vol. 
10
 
6
(pg. 
690
-
697
)
42
Challen
 
GA
Boles
 
NC
Chambers
 
SM
Goodell
 
MA
Distinct hematopoietic stem cell subtypes are differentially regulated by TGF-beta1.
Cell Stem Cell
2010
, vol. 
6
 
3
(pg. 
265
-
278
)
43
Park
 
SM
Deering
 
RP
Lu
 
Y
, et al. 
Musashi-2 controls cell fate, lineage bias, and TGF-β signaling in HSCs.
J Exp Med
2014
, vol. 
211
 
1
(pg. 
71
-
87
)
44
Shi
 
Y
Massagué
 
J
Mechanisms of TGF-beta signaling from cell membrane to the nucleus.
Cell
2003
, vol. 
113
 
6
(pg. 
685
-
700
)
45
Piek
 
E
Heldin
 
CH
Ten Dijke
 
P
Specificity, diversity, and regulation in TGF-beta superfamily signaling.
FASEB J
1999
, vol. 
13
 
15
(pg. 
2105
-
2124
)
46
Langer
 
JC
Henckaerts
 
E
Orenstein
 
J
Snoeck
 
HW
Quantitative trait analysis reveals transforming growth factor-beta2 as a positive regulator of early hematopoietic progenitor and stem cell function.
J Exp Med
2004
, vol. 
199
 
1
(pg. 
5
-
14
)
47
Quéré
 
R
Saint-Paul
 
L
Carmignac
 
V
, et al. 
Tif1γ regulates the TGF-β1 receptor and promotes physiological aging of hematopoietic stem cells.
Proc Natl Acad Sci USA
2014
, vol. 
111
 
29
(pg. 
10592
-
10597
)
48
Beerman
 
I
Rossi
 
DJ
Epigenetic regulation of hematopoietic stem cell aging.
Exp Cell Res
2014
, vol. 
329
 
2
(pg. 
192
-
199
)
49
Blaney Davidson
 
EN
Scharstuhl
 
A
Vitters
 
EL
van der Kraan
 
PM
van den Berg
 
WB
Reduced transforming growth factor-beta signaling in cartilage of old mice: role in impaired repair capacity.
Arthritis Res Ther
2005
, vol. 
7
 
6
(pg. 
R1338
-
R1347
)
50
Loffredo
 
FS
Steinhauser
 
ML
Jay
 
SM
, et al. 
Growth differentiation factor 11 is a circulating factor that reverses age-related cardiac hypertrophy.
Cell
2013
, vol. 
153
 
4
(pg. 
828
-
839
)
51
Sun
 
D
Luo
 
M
Jeong
 
M
, et al. 
Epigenomic profiling of young and aged HSCs reveals concerted changes during aging that reinforce self-renewal.
Cell Stem Cell
2014
, vol. 
14
 
5
(pg. 
673
-
688
)
52
Henckaerts
 
E
Langer
 
JC
Orenstein
 
J
Snoeck
 
HW
The positive regulatory effect of TGF-beta2 on primitive murine hemopoietic stem and progenitor cells is dependent on age, genetic background, and serum factors.
J Immunol
2004
, vol. 
173
 
4
(pg. 
2486
-
2493
)
53
Schofield
 
R
The relationship between the spleen colony-forming cell and the haemopoietic stem cell.
Blood Cells
1978
, vol. 
4
 
1-2
(pg. 
7
-
25
)
54
Lo Celso
 
C
Scadden
 
DT
The haematopoietic stem cell niche at a glance.
J Cell Sci
2011
, vol. 
124
 
Pt 21
(pg. 
3529
-
3535
)
55
Wang
 
LD
Wagers
 
AJ
Dynamic niches in the origination and differentiation of haematopoietic stem cells.
Nat Rev Mol Cell Biol
2011
, vol. 
12
 
10
(pg. 
643
-
655
)
56
Pfeilschifter
 
J
Diel
 
I
Scheppach
 
B
, et al. 
Concentration of transforming growth factor beta in human bone tissue: relationship to age, menopause, bone turnover, and bone volume.
J Bone Miner Res
1998
 
13(4):716-730
57
Utsugisawa
 
T
Moody
 
JL
Aspling
 
M
Nilsson
 
E
Carlsson
 
L
Karlsson
 
S
A road map toward defining the role of Smad signaling in hematopoietic stem cells.
Stem Cells
2006
, vol. 
24
 
4
(pg. 
1128
-
1136
)
58
Yamazaki
 
S
Ema
 
H
Karlsson
 
G
, et al. 
Nonmyelinating Schwann cells maintain hematopoietic stem cell hibernation in the bone marrow niche.
Cell
2011
, vol. 
147
 
5
(pg. 
1146
-
1158
)
59
Bruns
 
I
Lucas
 
D
Pinho
 
S
, et al. 
Megakaryocytes regulate hematopoietic stem cell quiescence through CXCL4 secretion.
Nat Med
2014
, vol. 
20
 
11
(pg. 
1315
-
1320
)
60
Zhao
 
M
Perry
 
JM
Marshall
 
H
, et al. 
Megakaryocytes maintain homeostatic quiescence and promote post-injury regeneration of hematopoietic stem cells.
Nat Med
2014
, vol. 
20
 
11
(pg. 
1321
-
1326
)
61
Söderberg
 
SS
Karlsson
 
G
Karlsson
 
S
Complex and context dependent regulation of hematopoiesis by TGF-beta superfamily signaling.
Ann N Y Acad Sci
2009
, vol. 
1176
 (pg. 
55
-
69
)
62
Larsson
 
J
Karlsson
 
S
The role of Smad signaling in hematopoiesis.
Oncogene
2005
, vol. 
24
 
37
(pg. 
5676
-
5692
)
63
Ruscetti
 
FW
Akel
 
S
Bartelmez
 
SH
Autocrine transforming growth factor-beta regulation of hematopoiesis: many outcomes that depend on the context.
Oncogene
2005
, vol. 
24
 
37
(pg. 
5751
-
5763
)
64
Yaswen
 
L
Kulkarni
 
AB
Fredrickson
 
T
, et al. 
Autoimmune manifestations in the transforming growth factor-beta 1 knockout mouse.
Blood
1996
, vol. 
87
 
4
(pg. 
1439
-
1445
)
65
Letterio
 
JJ
Geiser
 
AG
Kulkarni
 
AB
, et al. 
Autoimmunity associated with TGF-beta1-deficiency in mice is dependent on MHC class II antigen expression.
J Clin Invest
1996
, vol. 
98
 
9
(pg. 
2109
-
2119
)
66
Levéen
 
P
Larsson
 
J
Ehinger
 
M
, et al. 
Induced disruption of the transforming growth factor beta type II receptor gene in mice causes a lethal inflammatory disorder that is transplantable.
Blood
2002
, vol. 
100
 
2
(pg. 
560
-
568
)
67
Capron
 
C
Lacout
 
C
Lécluse
 
Y
, et al. 
A major role of TGF-beta1 in the homing capacities of murine hematopoietic stem cell/progenitors.
Blood
2010
, vol. 
116
 
8
(pg. 
1244
-
1253
)
68
Larsson
 
J
Blank
 
U
Helgadottir
 
H
, et al. 
TGF-beta signaling-deficient hematopoietic stem cells have normal self-renewal and regenerative ability in vivo despite increased proliferative capacity in vitro.
Blood
2003
, vol. 
102
 
9
(pg. 
3129
-
3135
)
69
Larsson
 
J
Blank
 
U
Klintman
 
J
Magnusson
 
M
Karlsson
 
S
Quiescence of hematopoietic stem cells and maintenance of the stem cell pool is not dependent on TGF-beta signaling in vivo.
Exp Hematol
2005
, vol. 
33
 
5
(pg. 
592
-
596
)
70
Ozdamar
 
B
Bose
 
R
Barrios-Rodiles
 
M
Wang
 
HR
Zhang
 
Y
Wrana
 
JL
Regulation of the polarity protein Par6 by TGFbeta receptors controls epithelial cell plasticity.
Science
2005
, vol. 
307
 
5715
(pg. 
1603
-
1609
)
71
Viloria-Petit
 
AM
David
 
L
Jia
 
JY
, et al. 
A role for the TGFbeta-Par6 polarity pathway in breast cancer progression.
Proc Natl Acad Sci USA
2009
, vol. 
106
 
33
(pg. 
14028
-
14033
)
72
Brenet
 
F
Kermani
 
P
Spektor
 
R
Rafii
 
S
Scandura
 
JM
TGFβ restores hematopoietic homeostasis after myelosuppressive chemotherapy.
J Exp Med
2013
, vol. 
210
 
3
(pg. 
623
-
639
)
73
Rörby
 
E
Hägerström
 
MN
Blank
 
U
Karlsson
 
G
Karlsson
 
S
Human hematopoietic stem/progenitor cells overexpressing Smad4 exhibit impaired reconstitution potential in vivo.
Blood
2012
, vol. 
120
 
22
(pg. 
4343
-
4351
)
74
Blank
 
U
Karlsson
 
G
Moody
 
JL
, et al. 
Smad7 promotes self-renewal of hematopoietic stem cells.
Blood
2006
, vol. 
108
 
13
(pg. 
4246
-
4254
)
75
Chadwick
 
K
Shojaei
 
F
Gallacher
 
L
Bhatia
 
M
Smad7 alters cell fate decisions of human hematopoietic repopulating cells.
Blood
2005
, vol. 
105
 
5
(pg. 
1905
-
1915
)
76
Karlsson
 
G
Blank
 
U
Moody
 
JL
, et al. 
Smad4 is critical for self-renewal of hematopoietic stem cells.
J Exp Med
2007
, vol. 
204
 
3
(pg. 
467
-
474
)
77
Ransom
 
DG
Bahary
 
N
Niss
 
K
, et al. 
The zebrafish moonshine gene encodes transcriptional intermediary factor 1gamma, an essential regulator of hematopoiesis.
PLoS Biol
2004
, vol. 
2
 
8
pg. 
E237
 
78
Bai
 
X
Kim
 
J
Yang
 
Z
, et al. 
TIF1gamma controls erythroid cell fate by regulating transcription elongation.
Cell
2010
, vol. 
142
 
1
(pg. 
133
-
143
)
79
Monteiro
 
R
Pouget
 
C
Patient
 
R
The gata1/pu.1 lineage fate paradigm varies between blood populations and is modulated by tif1γ.
EMBO J
2011
, vol. 
30
 
6
(pg. 
1093
-
1103
)
80
Xi
 
Q
Wang
 
Z
Zaromytidou
 
AI
, et al. 
A poised chromatin platform for TGF-β access to master regulators.
Cell
2011
, vol. 
147
 
7
(pg. 
1511
-
1524
)
81
Grady
 
WM
Myeroff
 
LL
Swinler
 
SE
, et al. 
Mutational inactivation of transforming growth factor beta receptor type II in microsatellite stable colon cancers.
Cancer Res
1999
, vol. 
59
 
2
(pg. 
320
-
324
)
82
Villanueva
 
A
García
 
C
Paules
 
AB
, et al. 
Disruption of the antiproliferative TGF-beta signaling pathways in human pancreatic cancer cells.
Oncogene
1998
, vol. 
17
 
15
(pg. 
1969
-
1978
)
83
Kim
 
SJ
Letterio
 
J
Transforming growth factor-beta signaling in normal and malignant hematopoiesis.
Leukemia
2003
, vol. 
17
 
9
(pg. 
1731
-
1737
)
84
Imai
 
Y
Kurokawa
 
M
Izutsu
 
K
, et al. 
Mutations of the Smad4 gene in acute myelogeneous leukemia and their functional implications in leukemogenesis.
Oncogene
2001
, vol. 
20
 
1
(pg. 
88
-
96
)
85
Yang
 
L
Wang
 
N
Tang
 
Y
Cao
 
X
Wan
 
M
Acute myelogenous leukemia-derived SMAD4 mutations target the protein to ubiquitin-proteasome degradation.
Hum Mutat
2006
, vol. 
27
 
9
(pg. 
897
-
905
)
86
Molenaar
 
JJ
Gérard
 
B
Chambon-Pautas
 
C
, et al. 
Microsatellite instability and frameshift mutations in BAX and transforming growth factor-beta RII genes are very uncommon in acute lymphoblastic leukemia in vivo but not in cell lines.
Blood
1998
, vol. 
92
 
1
(pg. 
230
-
233
)
87
Scott
 
S
Kimura
 
T
Ichinohasama
 
R
, et al. 
Microsatellite mutations of transforming growth factor-beta receptor type II and caspase-5 occur in human precursor T-cell lymphoblastic lymphomas/leukemias in vivo but are not associated with hMSH2 or hMLH1 promoter methylation.
Leuk Res
2003
, vol. 
27
 
1
(pg. 
23
-
34
)
88
Walter
 
MJ
Payton
 
JE
Ries
 
RE
, et al. 
Acquired copy number alterations in adult acute myeloid leukemia genomes.
Proc Natl Acad Sci USA
2009
, vol. 
106
 
31
(pg. 
12950
-
12955
)
89
Knaus
 
PI
Lindemann
 
D
DeCoteau
 
JF
, et al. 
A dominant inhibitory mutant of the type II transforming growth factor beta receptor in the malignant progression of a cutaneous T-cell lymphoma.
Mol Cell Biol
1996
, vol. 
16
 
7
(pg. 
3480
-
3489
)
90
Schiemann
 
WP
Pfeifer
 
WM
Levi
 
E
Kadin
 
ME
Lodish
 
HF
A deletion in the gene for transforming growth factor beta type I receptor abolishes growth regulation by transforming growth factor beta in a cutaneous T-cell lymphoma.
Blood
1999
, vol. 
94
 
8
(pg. 
2854
-
2861
)
91
Wolfraim
 
LA
Fernandez
 
TM
Mamura
 
M
, et al. 
Loss of Smad3 in acute T-cell lymphoblastic leukemia.
N Engl J Med
2004
, vol. 
351
 
6
(pg. 
552
-
559
)
92
Komuro
 
H
Valentine
 
MB
Rubnitz
 
JE
, et al. 
p27KIP1 deletions in childhood acute lymphoblastic leukemia.
Neoplasia
1999
, vol. 
1
 
3
(pg. 
253
-
261
)
93
Takeuchi
 
C
Takeuchi
 
S
Ikezoe
 
T
Bartram
 
CR
Taguchi
 
H
Koeffler
 
HP
Germline mutation of the p27/Kip1 gene in childhood acute lymphoblastic leukemia.
Leukemia
2002
, vol. 
16
 
5
(pg. 
956
-
958
)
94
Jakubowiak
 
A
Pouponnot
 
C
Berguido
 
F
, et al. 
Inhibition of the transforming growth factor beta 1 signaling pathway by the AML1/ETO leukemia-associated fusion protein.
J Biol Chem
2000
, vol. 
275
 
51
(pg. 
40282
-
40287
)
95
Kurokawa
 
M
Mitani
 
K
Imai
 
Y
Ogawa
 
S
Yazaki
 
Y
Hirai
 
H
The t(3;21) fusion product, AML1/Evi-1, interacts with Smad3 and blocks transforming growth factor-beta-mediated growth inhibition of myeloid cells.
Blood
1998
, vol. 
92
 
11
(pg. 
4003
-
4012
)
96
Lee
 
DK
Kim
 
BC
Brady
 
JN
Jeang
 
KT
Kim
 
SJ
Human T-cell lymphotropic virus type 1 tax inhibits transforming growth factor-beta signaling by blocking the association of Smad proteins with Smad-binding element.
J Biol Chem
2002
, vol. 
277
 
37
(pg. 
33766
-
33775
)
97
Nakahata
 
S
Yamazaki
 
S
Nakauchi
 
H
Morishita
 
K
Downregulation of ZEB1 and overexpression of Smad7 contribute to resistance to TGF-beta1-mediated growth suppression in adult T-cell leukemia/lymphoma.
Oncogene
2010
, vol. 
29
 
29
(pg. 
4157
-
4169
)
98
Mitani
 
K
Molecular mechanisms of leukemogenesis by AML1/EVI-1.
Oncogene
2004
, vol. 
23
 
24
(pg. 
4263
-
4269
)
99
Ford
 
AM
Palmi
 
C
Bueno
 
C
, et al. 
The TEL-AML1 leukemia fusion gene dysregulates the TGF-beta pathway in early B lineage progenitor cells.
J Clin Invest
2009
, vol. 
119
 
4
(pg. 
826
-
836
)
100
Sood
 
R
Talwar-Trikha
 
A
Chakrabarti
 
SR
Nucifora
 
G
MDS1/EVI1 enhances TGF-beta1 signaling and strengthens its growth-inhibitory effect but the leukemia-associated fusion protein AML1/MDS1/EVI1, product of the t(3;21), abrogates growth-inhibition in response to TGF-beta1.
Leukemia
1999
, vol. 
13
 
3
(pg. 
348
-
357
)
101
Wang
 
N
Kim
 
HG
Cotta
 
CV
, et al. 
TGFbeta/BMP inhibits the bone marrow transformation capability of Hoxa9 by repressing its DNA-binding ability.
EMBO J
2006
, vol. 
25
 
7
(pg. 
1469
-
1480
)
102
Quere
 
R
Karlsson
 
G
Hertwig
 
F
, et al. 
SMAD4 sequestrates HOXA9 to protect hematopoietic stem cells against leukemia transformation [abstract].
Blood
2010
, vol. 
116
 
21
 
Abstract 3153
103
Schepers
 
K
Pietras
 
EM
Reynaud
 
D
, et al. 
Myeloproliferative neoplasia remodels the endosteal bone marrow niche into a self-reinforcing leukemic niche.
Cell Stem Cell
2013
, vol. 
13
 
3
(pg. 
285
-
299
)
104
Raaijmakers
 
MH
Mukherjee
 
S
Guo
 
S
, et al. 
Bone progenitor dysfunction induces myelodysplasia and secondary leukaemia.
Nature
2010
, vol. 
464
 
7290
(pg. 
852
-
857
)
105
Walkley
 
CR
Olsen
 
GH
Dworkin
 
S
, et al. 
A microenvironment-induced myeloproliferative syndrome caused by retinoic acid receptor gamma deficiency.
Cell
2007
, vol. 
129
 
6
(pg. 
1097
-
1110
)
106
Krause
 
DS
Fulzele
 
K
Catic
 
A
, et al. 
Differential regulation of myeloid leukemias by the bone marrow microenvironment.
Nat Med
2013
, vol. 
19
 
11
(pg. 
1513
-
1517
)
107
Naka
 
K
Hoshii
 
T
Muraguchi
 
T
, et al. 
TGF-beta-FOXO signalling maintains leukaemia-initiating cells in chronic myeloid leukaemia.
Nature
2010
, vol. 
463
 
7281
(pg. 
676
-
680
)
108
Dong
 
M
Blobe
 
GC
Role of transforming growth factor-beta in hematologic malignancies.
Blood
2006
, vol. 
107
 
12
(pg. 
4589
-
4596
)
109
Shehata
 
M
Schwarzmeier
 
JD
Hilgarth
 
M
Hubmann
 
R
Duechler
 
M
Gisslinger
 
H
TGF-beta1 induces bone marrow reticulin fibrosis in hairy cell leukemia.
J Clin Invest
2004
, vol. 
113
 
5
(pg. 
676
-
685
)
110
Le Bousse-Kerdilès
 
MC
Chevillard
 
S
Charpentier
 
A
, et al. 
Differential expression of transforming growth factor-beta, basic fibroblast growth factor, and their receptors in CD34+ hematopoietic progenitor cells from patients with myelofibrosis and myeloid metaplasia.
Blood
1996
, vol. 
88
 
12
(pg. 
4534
-
4546
)
111
Chagraoui
 
H
Komura
 
E
Tulliez
 
M
Giraudier
 
S
Vainchenker
 
W
Wendling
 
F
Prominent role of TGF-beta 1 in thrombopoietin-induced myelofibrosis in mice.
Blood
2002
, vol. 
100
 
10
(pg. 
3495
-
3503
)
112
Vannucchi
 
AM
Bianchi
 
L
Paoletti
 
F
, et al. 
A pathobiologic pathway linking thrombopoietin, GATA-1, and TGF-beta1 in the development of myelofibrosis.
Blood
2005
, vol. 
105
 
9
(pg. 
3493
-
3501
)
113
Bachegowda
 
L
Gligich
 
O
Mantzaris
 
I
, et al. 
Signal transduction inhibitors in treatment of myelodysplastic syndromes.
J Hematol Oncol
2013
, vol. 
6
 pg. 
50
 
114
Zhou
 
L
Nguyen
 
AN
Sohal
 
D
, et al. 
Inhibition of the TGF-beta receptor I kinase promotes hematopoiesis in MDS.
Blood
2008
, vol. 
112
 
8
(pg. 
3434
-
3443
)
115
Zhou
 
L
McMahon
 
C
Bhagat
 
T
, et al. 
Reduced SMAD7 leads to overactivation of TGF-beta signaling in MDS that can be reversed by a specific inhibitor of TGF-beta receptor I kinase [published correction appears in Cancer Res. 2011;71(7):2806].
Cancer Res
2011
, vol. 
71
 
3
(pg. 
955
-
963
)
116
Bhagat
 
TD
Zhou
 
L
Sokol
 
L
, et al. 
miR-21 mediates hematopoietic suppression in MDS by activating TGF-β signaling.
Blood
2013
, vol. 
121
 
15
(pg. 
2875
-
2881
)
117
Bhatia
 
M
Bonnet
 
D
Wu
 
D
, et al. 
Bone morphogenetic proteins regulate the developmental program of human hematopoietic stem cells.
J Exp Med
1999
, vol. 
189
 
7
(pg. 
1139
-
1148
)
118
Suragani
 
RN
Cadena
 
SM
Cawley
 
SM
, et al. 
Transforming growth factor-β superfamily ligand trap ACE-536 corrects anemia by promoting late-stage erythropoiesis.
Nat Med
2014
, vol. 
20
 
4
(pg. 
408
-
414
)
119
Dussiot
 
M
Maciel
 
TT
Fricot
 
A
, et al. 
An activin receptor IIA ligand trap corrects ineffective erythropoiesis in β-thalassemia.
Nat Med
2014
, vol. 
20
 
4
(pg. 
398
-
407
)
120
Kretzschmar
 
M
Doody
 
J
Massagué
 
J
Opposing BMP and EGF signalling pathways converge on the TGF-beta family mediator Smad1.
Nature
1997
, vol. 
389
 
6651
(pg. 
618
-
622
)
121
Kretzschmar
 
M
Doody
 
J
Timokhina
 
I
Massagué
 
J
A mechanism of repression of TGFbeta/ Smad signaling by oncogenic Ras.
Genes Dev
1999
, vol. 
13
 
7
(pg. 
804
-
816
)
122
Fuentealba
 
LC
Eivers
 
E
Ikeda
 
A
, et al. 
Integrating patterning signals: Wnt/GSK3 regulates the duration of the BMP/Smad1 signal.
Cell
2007
, vol. 
131
 
5
(pg. 
980
-
993
)
123
Pera
 
EM
Ikeda
 
A
Eivers
 
E
De Robertis
 
EM
Integration of IGF, FGF, and anti-BMP signals via Smad1 phosphorylation in neural induction.
Genes Dev
2003
, vol. 
17
 
24
(pg. 
3023
-
3028
)