TGF-β signaling regulates differentiation of MSCs in bone metabolism: disputes among viewpoints

Abstract

Mesenchymal stem cells (MSCs) are multipotent cells that can differentiate into cells of different lineages to form mesenchymal tissues, which are promising in regard to treatment for bone diseases. Their osteogenic differentiation is under the tight regulation of intrinsic and extrinsic factors. Transforming growth factor β (TGF-β) is an essential growth factor in bone metabolism, which regulates the differentiation of MSCs. However, published studies differ in their views on whether TGF-β signaling regulates the osteogenic differentiation of MSCs positively or negatively. The controversial results have not been summarized systematically and the related explanations are required. Therefore, we reviewed the basics of TGF-β signaling and summarized how each of three isoforms regulates osteogenic differentiation. Three isoforms of TGF-β (TGF-β1/β2/β3) play distinct roles in regulating osteogenic differentiation of MSCs. Additionally, other possible sources of conflicts are summarized here. Further understanding of TGF-β signaling regulation in MSCs may lead to new applications to promote bone regeneration and improve therapies for bone diseases.

Keywords: Bone metabolism; MSCs; Osteogenic differentiation; TGF-β.

Fig. 1

Flow charts of the studies selection process. I. II. and III. represents selection process for three isoforms TGF-β1, 2 and 3 separately. The template of flow charts is referred to PRISMA (http://www.prisma-statement.org)

Fig. 2

Schematic of the large triple complex (LCC) and its activation mechanisms by αVβ6 integrin. The prodomain consists of an arm domain and a straitjacket. Two prodomains noncovalently bond with a TGF-β homodimer. Activation of TGF-β requires the binding of αV integrin with the RDG sequence (Arg-Gly Asp) in the prodomain and the exertion of force on this domain. After the straitjacket is unfastened, the TGF-β dimer is released and can bind with receptors. SS, disulfide bonds.

Fig. 3

Interaction between TGF-β ligands and receptors. (I) TGF-β dimer (blue parallelogram) first binds with betaglycan (blue receptor) with 1:1 stoichiometry and is potentiated to bind to TR2 (green receptor). One modular of TR2 is allowed to bind with the complex. TR2 is constitutively phosphorylated (red circles) independent of ligands. (II) TR2 binds with the ligands and potentiates TR1 (orange receptor) to interact with the TGF-β/TR2 complex. (III) TR1 binds to the complex and stabilizes the interaction between TR2 and the ligand. (IV) TR1 is phosphorylated by TR2 and activated to phosphorylate (red circles) downstream substrates (yellow parallelogon). However, betaglycan must be displaced for TR1 binding. Betaglycan is totally displaced due to lower affinity caused by the modified binding state with the TGF-β dimer. Finally, a ligand–receptor complex consisting of a TGF-β homodimer, a pair of TR2, and a pair of TR1 is formed.

Fig. 4

Canonical signaling of TGF-β. SMAD2 or SMAD3 can be activated by TR1 phosphorylation (red circles). Activated SMAD2/3 form SMAD complexes with SMAD4, which are transported into the nucleus to regulate gene expression. SMAD proteins undergo nucleoplasm shuttling without stimulation.

Fig. 5

Noncanonical signaling of TGF-β. TGF-βs can activate many other intracellular pathways independent of the SMAD proteins, including MAPKs, Rho-like GTPases, and PI3K. (Red circle: phosphorylation of TR1).

Fig. 6

The distribution of literature that supports promotion or inhibition of TGF-β signaling in osteogenic differentiation of MSCs. Considering number of studies, more studies tend to support that TGF-β signaling displays an inhibition role in osteogenic differentiation of MSCs. During 2000–2012, more studies tended to support that TGF-β signaling inhibits osteogenic of MSCs. However, studies that support both views are almost equal in recent years.

Fig. 7

Mechanisms of TGF-β signaling. TGF-βs are secreted growth factors that function by binding to receptors on the cytomembrane. Latent TGF-β must be activated before binding to the receptors. SMADs and other intracellular proteins such as MKKKs, PI3K, and Rho-like GTPases can be activated by TGF-β signaling mainly through phosphorylation by TR1 (red circles), thus activating different signaling pathways. As a result, transcription factors are activated or repressed, the expression of specific genes is regulated, and the functions of cells are changed.

Fig. 8

Mechanisms underlying how TGF-β1 regulates osteogenic differentiation. TGF-β1 induces expression of Runx2 through SMADs and p38, and RUNX2 is sufficient to induce fibronectin and type I collagen. SMAD5 induced by BMP signaling is needed to induce Alp expression. However, expression of Ocn, essential for matrix mineralization, is suppressed by TGF-β1. Distinct dosages of TGF-β1 influence the osteogenesis of MSCs by regulating BMP2 expression.