TGF-β1 can regulate osteoblast differentiation not only positively but also negatively. However, the mechanisms of negative regulation are not well understood. We previously established the reproducible model for studying the suppression of osteoblast differentiation by repeated or high dose treatment with TGF-β1, although single low dose TGF-β1 strongly induced osteoblast differentiation. The mRNA expression and protein level of insulin-like growth factor-1 (IGF-1) were remarkably decreased by repeated TGF-β1 administration in human periodontal ligament cells, human mesenchymal stem cells, and murine preosteoblast MC3T3-E1 cells. Repeated TGF-β1 administration subsequently decreased alkaline phosphatase (ALP) activity and mRNA expression of osteoblast differentiation marker genes, such as RUNX2, ALP, and bone sialoprotein (BSP). Additionally, repeated administration significantly reduced the downstream signaling pathway of IGF-1, such as Akt phosphorylation in these cells. Surprisingly, exogenous and overexpressed IGF-1 recovered ALP activity and mRNA expression of osteoblast differentiation marker genes even with repeated TGF-β1 administration. These facts indicate that the key mechanism of inhibition of osteoblast differentiation induced by repeated TGF-β1 treatment is simply due to the down-regulation of IGF-1 expression. Inhibition of IGF-1 signaling using small interfering RNA (siRNA) against insulin receptor substrate-1 (IRS-1) suppressed mRNA expression of RUNX2, ALP, BSP, and IGF-1 even with single TGF-β1 administration. This study showed that persistence of TGF-β1 inhibited osteoblast differentiation via suppression of IGF-1 expression and subsequent down-regulation of the PI3K/Akt pathway. We think this fact could open the way to use IGF-1 as a treatment tool for bone regeneration in prolonged inflammatory disease.

We have previously reported that repeated treatment of human periodontal ligament cells and murine pre-osteoblast MC3T3-E1 cells with transforming growth factor-beta 1 (TGF-β1) inhibited their osteoblastic differentiation because of decreased insulin-like growth factor-1 (IGF-1) secretion. We also found that IGF-1/PI3K signaling plays an important role in osteoblast differentiation induced by TGF-β1 treatment; however, the downstream signaling controlling this remains unknown. The aim of this current study is to investigate whether Akt activation is required for osteoblast differentiation. MC3T3-E1 cells were cultured in osteoblast differentiation medium (OBM) with or without 0.1 ng/mL TGF-β1. OBM containing TGF-β1 was changed every 12 h to provide repeated TGF-β1 administration. MC3T3-E1 cells were infected with retroviral vectors expressing constitutively active (CA) or dominant-negative (DN)-Akt. Alkaline phosphatase (ALP) activity and osteoblastic marker mRNA levels were substantially decreased by repeated TGF-β1 treatment compared with a single TGF-β1 treatment. However, expression of CA-Akt restored ALP activity following TGF-β1 treatment. Surprisingly, ALP activity increased following multiple TGF-β1 treatments as the number of administrations of TGF-β1 increased. Activation of Akt significantly enhanced expression of osteocalcin, but TGF-β1 treatment inhibited this. Mineralization of MC3T3-E1 cells was markedly enhanced by CA-Akt expression under all medium conditions. Exogenous IGF-1 restored the down-regulation of osteoblast-related gene expression by repeated TGF-β1 administration. However, in cells expressing DN-Akt, these levels remained inhibited regardless of IGF-1 treatment. These findings indicate that Akt activation is required for the early phase of osteoblast differentiation of MC3T3-E1 cells induced by TGF-β1. However, Akt activation is insufficient to reverse the inhibitory effects of TGF-β1 in the late stages of osteoblast differentiation. TGF-β1 could be an inducer or an inhibitor of osteoblastic differentiation of MC3T3-E1 cells depending on the state of Akt phosphorylation. Our results indicate that Akt is the molecular switch for TGF-β1-induced osteoblastic differentiation of MC3T3-E1 cells.

Gorlin syndrome is a skeletal disorder caused by a gain of function mutation in Hedgehog (Hh) signaling. The Hh family comprises of many signaling mediators, which, through complex mechanisms, play several important roles in various stages of development. The Hh information pathway is essential for bone tissue development. It is also the major driver gene in the development of basal cell carcinoma and medulloblastoma. In this review, we first present the recent advances in Gorlin syndrome research, in particular, the signaling mediators of the Hh pathway and their functions at the genetic level. Then, we discuss the phenotypes of mutant mice and Hh signaling-related molecules in humans revealed by studies using induced pluripotent stem cells.

We recently reported a new method to purify the induced pluripotent stem (iPS)-derived osteoprogenitors (iPSop). In this paper, we optimized the procedure and characterized cells at each process step. We observed that 10 days of treatment with FGF-2, IGF-1 and TGF-β (FIT) resulted in early-phase osteoblasts and 14 days of treatment resulted in late-phase osteoblasts. We found that treatment with 1,25(OH) ₂ vitamin D ₃ increased expression of osteocalcin and decreased expression of tissue-non-specific alkaline phosphatase and runt-related transcription factor 2 (RUNX2) in iPSop-day14 cells (cells treated with FIT for 14 days). Therefore, iPSop-day14 cells were promoted to mature osteoblasts by 1,25(OH) ₂ vitamin D ₃ treatment. In addition, we found that 1,25(OH) ₂ vitamin D ₃ treatment for 14 days enhanced not only mineralization but also expression of osteocyte markers, including dentin matrix protein-1 and fibroblast growth factor-23, in iPSop cells. Therefore, 1,25(OH) ₂ vitamin D ₃ is a potent promoter of osteoblast–osteocyte transition. The results of this study suggest that it is possible to evaluate both early- and late-phase osteoblasts and to apply cells to drug screening for anabolic drugs that stimulate bone formation.

Objectives/Aims: The requisite conditions for successful bone tissue engineering are efficient stem cell differentiation into osteogenic cells and a suitable scaffold. In this study, we investigated in vivo bone regeneration from transplanted induced pluripotent stem cells (iPSCs). Materials and Methods: Two critical-sized calvarial bone defects were created in 36 rats. The surgical sites were randomly assigned to one of three treatments to test the healing effectiveness of the scaffold alone, scaffold with iPSCs or a salt solution as a control. The effectiveness of the treatments was evaluated after 2 or 4 weeks using radiographic and histological analyses of bone regeneration in the six groups. Results: Micro-computed tomography (CT) analysis of the bone defects found minimal bone regeneration with the salt solution and nanofiber scaffold and increased bone regeneration in defects repaired with iPSCs delivered in the nanofiber scaffold. Conclusion: Transplanted iPSCs encapsulated in a nanofiber scaffold can regenerate bone in critical-sized defects.