Transforming growth factor-β (TGF-beta) 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. The TGF-beta family constitutes a multifunctional set of cytokines that regulate a bewildering array of cellular processes during development and beyond. In the adult organism, TGF-beta members regulate tissue homeostasis and regeneration. This large family of paracrine factors regulates functions that guide the exit of embryonic stem cells from the pluripotent state, and the subsequent differentiation of committed progenitors to more restricted cell fates for the establishment of body axes, mature tissues, and whole organs. In most cell types, TGF-beta signaling additionally controls the expression of a plethora of homeostatic genes whose activity determines cell proliferation, extracellular matrix production, paracrine factor secretion, cell–cell contacts, immune function, and tissue repair. Pathway feedback and crosstalk responses are also built into the transcriptional program of TGF-beta in most cell types.
A critical role for TGF-beta in the regulation of hematopoietic stem cells and progenitor cells was demonstrated more than 15 years ago. The original findings showed a potent inhibition by TGF-beta on the growth of early multiple progenitor populations (MPPs), while more mature progenitors were unaffected. A large number of studies on both human and murine cells have supported these original findings of potent growth inhibitory actions on early hematopoietic progenitors. Although the mechanism of TGF-beta action on hematopoietic progenitors is not fully understood, certain studies reveal that the effects are in part due to down regulation of cytokine receptors [like receptors of interleukin (IL) 1, granulocyte monocyte-colony stimulating factor (GM-CSF), IL3, granulocyte-colony stimulating factor (G-CSF) and stem cell factor (SCF)] and modulation of genes involved in cell cycle. A study by Scandura et al. (2004) has shown that TGF-beta induces cell cycle arrest in human hematopoietic cells by an upregulation of the cyclin-dependent kinase inhibitor, p57KIP2. This is supported by the findings demonstrating reversibility in the growth inhibitory actions of TGF-beta, suggesting that TGF-beta delays the proliferation rather than exerting an irreversible negative effect such as induction of apoptosis. However, a number of reports have shown the involvement of TGF-beta in apoptosis of bone marrow (BM) progenitors. In fact, both apoptotic and anti-apoptotic effects of TGF-beta have been described. Thus, TGF-beta may regulate growth of hematopoietic progenitors through effects on both cell cycling and apoptosis. Furthermore, neutralization studies using TGF-beta monoclonal antibodies has shown to recruit early progenitor cells into cell cycle determining the role of endogenous TGF-beta signaling in maintaining the quiescence of hematopoietic stem cells. TGF-beta is now documented as a potent inhibitor of hematopoietic stem cell proliferation in vitro, while its role in vivo is largely unknown. In a recent study conducted by Park et al., they showed that Mushashi-2 (Msi2) is an important regulator of the hematopoietic stem cell translatome that controls cell fate, lineage bias and TGF-beta signaling in hematopoietic stem cells.
Signaling by TGF-beta in mesenchymal stem cells occurs through the SMAD family of signal transduction proteins. TGF-beta binds to two major types of membrane-bound serine/threonine kinase receptors, TβRI and TβRII. TβRII transphosphorylates TβRI, and phosphorylated TβRI, in association with either ALK1 or ALK5, phosphorylates receptor-regulated SMADs (R-SMADs; including SMAD1–SMAD3, SMAD5, and SMAD8). R-SMADs then rapidly dissociate from the receptor to form complexes with SMAD4 and migrate into the nucleus to regulate transcription of target genes. Using a series of animal models, inhibitors, siRNAs, and retrovirus-mediated expression, we have found that TGF-beta mesenchymal stem cell migration occurs through the ALK5-SMAD2/3-SMAD4 pathway, with SMAD3 playing a more prominent role and SMAD2 playing a compensatory role. Cell type–specific effects of TGF-beta signaling are largely determined by the interaction of SMAD2/3 proteins with cell type–specific master transcription factors that specify and maintain specific effects. Whereas the master transcription factors in embryonic stem cells, myotubes, and pro-B cells have been determined (Oct4, MyoD1, and PU.1, respectively, MSC master transcription factors have not yet been identified.
Active TGF-beta released into the microenvironment can exert specific effects, including proliferation, differentiation, migration, and apoptosis, depending on the cell type and duration of action. For example, although TGF-beta signaling induces mesenchymal stem cell migration, it does not induce osteoblast differentiation. The gradient of TGF-beta created during osteoclast bone resorption can limit further osteoclast activity. In the short term, low concentrations of active TGF-beta can induce macrophage migration via activation of RhoA; however, high concentrations or prolonged exposure of macrophages/monocytes to active TGF-beta have been shown to inhibit migration of osteoclast precursors. Both high concentrations of and prolonged exposure to TGF-beta activate SMAD3-SMAD4 complexes, which in turn activate PKA, resulting in phosphorylation and inactivation of RhoA. Thus, the gradient of TGF-beta generated at the resorption sites likely prohibits further recruitment of osteoclast precursors, protecting it from further resorption during the reversal phase of bone remodeling.
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