High-Dose TGF-β1 Impairs Mesenchymal Stem Cell–Mediated Bone Regeneration via Bmp2 Inhibition

Abstract

Transforming growth factor‐β1 (TGF‐β1) is a key factor in bone reconstruction. However, its pathophysiological role in non‐union and bone repair remains unclear. Here we demonstrated that TGF‐β1 was highly expressed in both C57BL/6 mice where new bone formation was impaired after autologous bone marrow mesenchymal stem cell (BMMSC) implantation in non‐union patients. High doses of TGF‐β1 inhibited BMMSC osteogenesis and attenuated bone regeneration in vivo. Furthermore, different TGF‐β1 levels exhibited opposite effects on osteogenic differentiation and bone healing. Mechanistically, low TGF‐β1 doses activated smad3, promoted their binding to bone morphogenetic protein 2 (Bmp2) promoter, and upregulated Bmp2 expression in BMMSCs. By contrast, Bmp2 transcription was inhibited by changing smad3 binding sites on its promoter at high TGF‐β1 levels. In addition, high TGF‐β1 doses increased tomoregulin‐1 (Tmeff1) levels, resulting in the repression of Bmp2 and bone formation in mice. Treatment with the TGF‐β1 inhibitor SB431542 significantly rescued BMMSC osteogenesis and accelerated bone regeneration. Our study suggests that high‐dose TGF‐β1 dampens BMMSC‐mediated bone regeneration by activating canonical TGF‐β/smad3 signaling and inhibiting Bmp2 via direct and indirect mechanisms. These data collectively show a previously unrecognized mechanism of TGF‐β1 in bone repair, and TGF‐β1 is an effective therapeutic target for treating bone regeneration disability. © 2019 American Society for Bone and Mineral Research.

Mechanism of traumatic heterotopic ossification: In search of injury‐induced osteogenic factors

“…In vitro studies have shown that TGF-β1 inhibits stem cell osteogenesis. [76][77][78] We have also shown that in vitro supplementation of TGF-β1, at a concentration similar to that in plasma, almost completely blocked BMP-7-induced osteogenesis in cultures of MDSCs, 65 muscle resident stem cells that have been considered as putative HO cell source in vivo. 40 This finding confirms that TGF-β1 may play an inhibitory role on MDSC osteogenesis, which supports our systemic osteogenic protective factor/differentiation threshold hypothesis (Figure 3).…”

“…In vitro studies have shown that TGF‐β1 inhibits stem cell osteogenesis 76‐78 . We have also shown that in vitro supplementation of TGF‐β1, at a concentration similar to that in plasma, almost completely blocked BMP‐7‐induced osteogenesis in cultures of MDSCs, 65 muscle resident stem cells that have been considered as putative HO cell source in vivo 40 .…”

“…Endochondral ossification progresses through a cartilaginous precursor stage which requires TGF-β signalling for chondrogenesis, whereas in cases of intramembranous ossification, TGF-β signalling always exerts a direct inhibitory effect on stem cell osteogenesis. 77,84 We also acknowledge that our postulate may be somewhat simplistic, as there are other cell types that participate in HO formation which could be the direct target of TGF-β1. Macrophages represent such a target, given that both glucocorticoids and TGF-β family proteins can modulate their phenotype (Figure 4), which has been reported to affect stem cell osteogenesis.…”

“…74 TGF-β1 is also a key factor involved in muscle regeneration 75 and usually plays an antagonistic role with BMPs in the musculoskeletal system. 76,77 We therefore hypothesize that TGF-β1 may function as the physiological protective factor that maintains the osteoblastic differentiation threshold to control aberrant osteogenesis following injury. In our recent study, we have found that glucocorticoid treatment results in lowered circulating TGF-β1 level, and supplementation of recombinant TGF-β1 markedly reduced the glucocorticoid-induced ectopic bone volume following muscle injury.…”

“…Transforming growth factor‐beta 1 (TGF‐β1) is a well‐known proteolytic target of plasmin, which releases active TGF‐β1 from its latency‐associated peptide 74 . TGF‐β1 is also a key factor involved in muscle regeneration 75 and usually plays an antagonistic role with BMPs in the musculoskeletal system 76,77 . We therefore hypothesize that TGF‐β1 may function as the physiological protective factor that maintains the osteoblastic differentiation threshold to control aberrant osteogenesis following injury.…”