Evidence indicates that the balance between osteoblastogenesis and adipogenesis of mesenchymal stem cells (MSCs) is regulated by several hormones, growth factors, and their downstream signaling cascades. Previous studies suggest that retinoic acid (RA) plays a role in osteoblastogenesis and adipogenesis. However, it is unknown whether RA regulates commitment of MSCs into osteoblasts and adipocytes. In this study, we investigated the role of RA in differentiation of MSCs using the C3H10T1/2 cell line. RA stimulated activity and expression of alkaline phosphatase (ALP) and upregulated activity of the ALP gene promoter. The effects of RA were further enhanced by bone morphogenetic protein 2 (BMP2) and resultant Smad signaling. Furthermore, overexpression of Runx2 and Msx2, critical transcription factors for bone formation and BMP2-dependent osteoblastogenesis, enhanced RA-dependent ALP activity. In view of these findings, RA likely stimulates osteoblast differentiation through the BMP2–Smad–Runx2/Msx2 pathway. In contrast, RA markedly inhibited BMP2-induced adipocyte differentiation, suppressing expression of peroxisome proliferator-activated receptor-γ (PPARγ), CCAAT/enhancer-binding protein (C/EBP)α and C/EBPδ, and inhibiting adipogenic function of C/EBPβ, C/EBPδ, and PPARγ. In conclusion, our data suggest that RA regulates commitment of MSCs into osteoblasts and adipocytes by controlling transcriptional regulators.

Several studies indicated that a homeobox gene, Msx2, is implicated in regulation of skeletal development by controlling enchondral ossification as well as membranous ossification. However, the molecular basis by which Msx2 conducts chondrogenesis is currently unclear. In this study, we examined the role of Msx2 in chondrocyte differentiation using mouse primary chondrocytes and embryonic metatarsal explants. Treatment with BMP2 up-regulated the expression of Msx2 mRNA along with chondrocyte differentiation in murine primary chondrocytes. Overexpression of wild-type Msx2 stimulated calcification of primary chondrocytes in the presence of BMP2. We also found that constitutively active Msx2 (caMsx2) enhanced BMP2-dependent calcification more efficiently than wild-type Msx2. Consistently, caMsx2 overexpression up-regulated the expression of alkaline phosphatase and collagen type X induced by BMP2. Furthermore, organ culture experiments using mouse embryonic metatarsals indicated that caMsx2 clearly stimulated the maturation of chondrocytes into the prehypertrophic and hypertrophic stages in the presence of BMP2. In contrast, knockdown of Msx2 inhibited maturation of primary chondrocytes. The stimulatory effect of Msx2 on chondrocyte maturation was enhanced by overexpression of Smad1 and Smad4 but inhibited by Smad6, an inhibitory Smad for BMP2 signaling. These data suggest that Msx2 requires BMP2/Smad signaling for its chondrogenic action. In addition, caMsx2 overexpression induced Ihh (Indian hedgehog) expression in mouse primary chondrocytes. Importantly, treatment with cyclopamine, a specific inhibitor for hedgehogs, blocked Msx2-induced chondrogenesis. Collectively, our results indicated that Msx2 promotes the maturation of chondrocytes, at least in part, through up-regulating Ihh expression.

Osterix/Sp7, a member of the Sp1 transcription factor family, plays an essential role in bone formation and osteoblastogenesis. Although Osterix has been shown to be induced by BMP2 in a mesenchymal cell line, the molecular basis of the regulation, expression and function of Osterix during osteoblast differentiation, is not fully understood. Thus we examined the role of BMP2 signaling in the regulation of Osterix using the mesenchymal cell lines C3H10T1/2 and C2C12. Osterix overexpression induced alkaline phosphatase activity and osteocalcin expression in C2C12 cells and stimulated calcification of murine primary osteoblasts. Considering that Runx2 overexpression induces Osterix, these results suggest that Osterix functions as downstream of Runx2. Surprisingly, BMP2 treatment induced Osterix expression and alkaline phosphatase activity in mesenchymal cells derived from Runx2-deficient mice. Furthermore, overexpression of Smad1 and Smad4 up-regulated Osterix expression, and an inhibitory Smad, Smad6, markedly suppressed BMP2-induced Osterix expression in the Runx2-deficient cells. Moreover, overexpression of a homeobox gene, Msx2, which is up-regulated by BMP2 and promotes osteoblastic differentiation, induced Osterix expression in the Runx2-deficient cells. Knockdown of Msx2 clearly inhibited induction of Osterix by BMP2 in the Runx2-deficient mesenchymal cells. Interestingly, microarray analyses using the Runx2-deficient cells revealed that the role of Osterix was distinct from that of Runx2. These findings suggest that Osterix is regulated via both Runx2-dependent and -independent mechanisms, and that Osterix controls osteoblast differentiation, at least in part, by regulating the expression of genes not controlled by Runx2.

Patient: The authors present the case of a 61-year-old woman who requested implant treatment for her completely edentulous maxilla because of difficulty in masticating with a removable complete denture. A treatment plan that involved the immediate loading of implants in the completely edentulous maxilla by using flapless surgery was developed. Soft tissue was punched out, and 10 implants were placed. To avoid sinus bone grafting, tilted implants were placed in front of the wall of the maxillary sinus and in the region of maxillary tuberosity. An immediate provisional restoration was placed on the same day.Results: All the implants achieved good primary stability. Since an immediate provisional restoration was placed on the same day as the implantation, the patient was able to recover immediate jaw function. After surgery, she experienced no postoperative pain and did not need to take any anti-inflammatory and analgesic medications.Discussion: Immediate loading of implants made it possible to reduce the treatment period, achieve immediate jaw function, and decrease surgery time. Thus, this treatment was obviously advantageous for the patient. Flapless implant surgery also contributed to minimizing surgical invasion, reducing the treatment period, and decreasing surgery time.Conclusion: This report presents a case in which the implants were successfully loaded immediately into a patient’s completely edentulous jaw by using flapless surgery.

Bone is a complex tissue which contains osteoclasts, osteoblasts, chondrocytes, adipocytes, hematopoietic cells and immune cells. Since osteoblasts share the same origin with adipocytes in bone marrow cavity, it is assumed that PPAR (peroxisome proliferator-activated receptor) family, which is an important nuclear receptor family for adipocyte differentiation, plays a role in the bone microenvironment. Indeed, recent evidences support the primitive roles of PPAR family in osteoblast differentiation as well as adipocyte differentiation. Furthermore, PPAR family is also implicated in the regulation of differentiation and function of osteoclasts. Here, we summarized the functional roles of PPAR family in bone remodeling and regulation of bone microenvironments. We also discuss the potential mechanisms that regulate expression and function of PPAR family during bone metabolisms.