The effects of PPAR-γ inhibition on gene expression and the progression of induced osteogenic differentiation of human mesenchymal stem cells

Microfibrillar-associated protein 5 suppresses adipogenesis by inhibiting essential coactivator of PPARγ

Femoral head necrosis is responsible for severe pain and its incidence is increasing. Abnormal adipogenic differentiation and fat cell hypertrophy of bone marrow mesenchymal stem cells increase intramedullary cavity pressure, leading to osteonecrosis. By analyzing gene expression before and after adipogenic differentiation, we found that Microfibril-Associated Protein 5 (MFAP5) is significantly down-regulated in adipogenesis whilst the mechanism of MFAP5 in regulating the differentiation of bone marrow mesenchymal stem cells is unknown. The purpose of this study was to clarify the role of MAFP5 in adipogenesis and therefore provide a theoretical basis for future therapeutic options of osteonecrosis. By knockdown or overexpression of MFAP5 in C3H10 and 3T3-L1 cells, we found that MFAP5 was significantly down-regulated as a key regulator of adipogenic differentiation, and identified the underlying downstream molecular mechanism. MFAP5 directly bound to and inhibited the expression of Staphylococcal Nuclease And Tudor Domain Containing 1, an essential coactivator of PPARγ, exerting an important regulatory role in adipogenesis.

CHIR99021-Treated Osteocytes with Wnt Activation in 3D-Printed Module Form an Osteogenic Microenvironment for Enhanced Osteogenesis and Vasculogenesis

Finding a bone implant that has high bioactivity that can safely drive stem cell differentiation and simulate a real in vivo microenvironment is a challenge for bone tissue engineering. Osteocytes significantly regulate bone cell fate, and Wnt-activated osteocytes can reversely regulate bone formation by regulating bone anabolism, which may improve the biological activity of bone implants. To achieve a safe application, we used the Wnt agonist CHIR99021 (C91) to treat MLO-Y4 for 24 h, in a co-culture with ST2 for 3 days after withdrawal. We found that the expression of Runx2 and Osx increased, promoted osteogenic differentiation, and inhibited adipogenic differentiation in the ST2 cells, and these effects were eliminated by the triptonide. Therefore, we hypothesized that C91-treated osteocytes form an osteogenic microenvironment (COOME). Subsequently, we constructed a bio-instructive 3D printing system to verify the function of COOME in 3D modules that mimic the in vivo environment. Within PCI3D, COOME increased the survival and proliferation rates to as high as 92% after 7 days and promoted ST2 cell differentiation and mineralization. Simultaneously, we found that the COOME-conditioned medium also had the same effects. Therefore, COOME promotes ST2 cell osteogenic differentiation both directly and indirectly. It also promotes HUVEC migration and tube formation, which can be explained by the high expression of Vegf. Altogether, these results indicate that COOME, combined with our independently developed 3D printing system, can overcome the poor cell survival and bioactivity of orthopedic implants and provide a new method for clinical bone defect repair.

Role of HSP90α in osteoclast formation and osteoporosis development

Osteoporosis (OP) is a systemic metabolic bone disease that occurs most frequently in the elderly. The main pathogenesis of OP is excessive proliferation and differentiation of osteoclasts, in which the peroxisome proliferator-activated receptor γ (PPARγ) pathway has a pivotal role. Recently, heat shock protein (HSP)90α has been identified as an important molecular chaperone with PPARγ, which regulates the effect of the PPARγ pathway. The aim of the present study was to investigate the role of HSP90α involved in the regulation of osteoclast formation and the process of osteoporosis. Firstly, the expression of HSP90α in osteoclast differentiation was detected by western blotting in vitro, then the effect of HSP90α inhibition on the formation and differentiation of osteoclasts was examined. Furthermore, the nuclear import of PPARγ was also assessed to confirm the synergistic effect of HSP90α. Finally, the inhibitory effect of HSP90α in vivo was explored, using a mouse model of osteoporosis. As a result, in the process of osteoclast differentiation and proliferation, the expression of HSP90α was upregulated. Inhibition of HSP90α could block the formation and differentiation of osteoclasts, and remit osteoporosis in mice. Regarding the underlying mechanism, inhibition of HSP90α could block the nuclear import of PPARγ to inhibit osteoclast differentiation and proliferation. In conclusion, these data indicated that the inhibition of HSP90α could block osteoclast formation and remit osteoporosis by reducing the nuclear import of PPARγ.

Sal B targets TAZ to facilitate osteogenesis and reduce adipogenesis through MEK-ERK pathway

Salvianolic acid B (Sal B), a major bioactive component of Chinese herb, was identified as a mediator for bone metabolism recently. The aim of this study is to investigate the underlying mechanisms by which Sal B regulates osteogenesis and adipogenesis. We used MC3T3-E1 and 3T3-L1 as the study model to explore the changes of cell differentiation induced by Sal B. The results indicated that Sal B at different concentrations had no obvious toxicity effects on cell proliferation during differentiation. Furthermore, Sal B facilitated osteogenesis but inhibited adipogenesis by increasing the expression of transcriptional co-activator with PDZ-binding motif (TAZ). Accordingly, TAZ knock-down offset the effects of Sal B on cell differentiation into osteoblasts or adipocytes. Notably, the Sal B induced up-expression of TAZ was blocked by U0126 (the MEK-ERK inhibitor), rather than LY294002 (the PI3K-Akt inhibitor). Moreover, Sal B increased the p-ERK/ERK ratio to regulate the TAZ expression as well as the cell differentiation. In summary, this study suggests for the first time that Sal B targets TAZ to facilitate osteogenesis and reduce adipogenesis by activating MEK-ERK signalling pathway, which provides evidence for Sal B to be used as a potential therapeutic agent for the management of bone diseases.

Peroxisome Proliferator-Activated Receptor-γ Knockdown Impairs Bone Morphogenetic Protein-2-Induced Critical-Size Bone Defect Repair

The Food and Drug Administration-approved clinical dose (1.5 mg/mL) of bone morphogenetic protein-2 (BMP2) has been reported to induce significant adverse effects, including cyst-like adipose-infiltrated abnormal bone formation. These undesirable complications occur because of increased adipogenesis, at the expense of osteogenesis, through BMP2-mediated increases in the master regulatory gene for adipogenesis, peroxisome proliferator-activated receptor-γ (PPARγ). Inhibiting PPARγ during osteogenesis has been suggested to drive the differentiation of bone marrow stromal/stem cells toward an osteogenic, rather than an adipogenic, lineage. We demonstrate that knocking down PPARγ while concurrently administering BMP2 can reduce adipogenesis, but we found that it also impairs BMP2-induced osteogenesis and leads to bone nonunion in a mouse femoral segmental defect model. In addition, in vitro studies using the mouse bone marrow stromal cell line M2-10B4 and mouse primary bone marrow stromal cells confirmed that PPARγ knockdown inhibits BMP2-induced adipogenesis; attenuates BMP2-induced cell proliferation, migration, invasion, and osteogenesis; and escalates BMP2-induced cell apoptosis. More important, BMP receptor 2 and 1B expression was also significantly inhibited by the combined BMP2 and PPARγ knockdown treatment. These findings indicate that PPARγ is critical for BMP2-mediated osteogenesis during bone repair. Thus, uncoupling BMP2-mediated osteogenesis and adipogenesis using PPARγ inhibition to combat BMP2's adverse effects may not be feasible.

IRS-1 targets TAZ to inhibit adipogenesis of rat bone marrow mesenchymal stem cells through PI3K-Akt and MEK-ERK pathways

Gene modification of mesenchymal stem cells (MSCs) offers a promising approach for clinical stem cell therapy. Transcriptional co-activator with PDZ-binding motif (TAZ) plays a vital role in MSCs' differentiation. We aim to explore the interaction of insulin receptor substrate-1 (IRS-1) with TAZ to regulate MSCs' adipogenesis in this study. Initially, IRS-1 and TAZ followed similar decreasing expression pattern at the early stage of adipogenesis. And, overexpression of IRS-1 decreased the CCAAT/enhancer binding protein β (C/EBPβ) and peroxi-some proliferator-activated receptor gamma (PPARγ) expression with TAZ upregulation. Accordingly, knockdown of IRS-1 induced the upexpression of C/EBPβ and PPARγ with TAZ downregulation. Indeed, IRS-1 targeted TAZ to downregulate the C/EBPβ and PPARγ expression, while knockdown of TAZ attenuated the IRS-1 inhibited adipogenesis. Furthermore, both LY294002 (the PI3K-Akt inhibitor) and U0126 (the MEK-ERK inhibitor) blocked the regulation of IRS-1 on TAZ during adipogenesis. Additionally, IRS-1 and TAZ influenced the cell proliferation in the above process. Taken together, this study suggests for the first time that IRS-1 is a key regulator of the MSCs' adipogenesis and may serve as a potential therapeutic target for differential alterations in bone marrow.

Tanshinol alleviates impaired bone formation by inhibiting adipogenesis via KLF15/PPARγ2 signaling in GIO rats

Glucocorticoid (GC)-induced osteoporosis (GIO) is characterized by impaired bone formation, which can be alleviated by tanshinol, an aqueous polyphenol isolated from Salvia miltiorrhiza Bunge. In this study we investigated the molecular mechanisms underlying GC-induced modulation of osteogenesis as well as the possibility of using tanshinol to interfere with GIO. Female SD rats aged 4 months were orally administered distilled water (Con), prednisone (GC, 5 mg·kg-1·d-1), GC plus tanshinol (Tan, 16 mg·kg-1·d-1) or GC plus resveratrol (Res, 5 mg·kg-1·d-1) for 14 weeks. After the rats were sacrificed, samples of bone tissues were collected. The changes in bone formation were assessed using Micro-CT, histomorphometry, and biomechanical assays. Expression of Kruppel-like factor 15 (KLF15), peroxisome proliferator-activated receptor γ 2 (PPARγ 2) and other signaling proteins in skeletal tissue was measured with Western blotting and quantitative RT-PCR. GC treatment markedly increased the expression of KLF15, PPARγ2, C/EBPα and aP2, which were related to adipogenesis, upregulated FoxO3a pathway proteins (FoxO3a and Gadd45a), and suppressed the canonical Wnt signaling (β-catenin and Axin2), which was required for osteogenesis. Thus, GC significantly decreased bone mass and bone quality. Co-treatment with Tan or Res effectively counteracted GC-impaired bone formation, suppressed GC-induced adipogenesis, and restored abnormal expression of the signaling molecules in GIO rats. We conclude that tanshinol counteracts GC-decreased bone formation by inhibiting marrow adiposity via the KLF15/PPARγ2/FoxO3a/Wnt pathway.

P-glycoprotein overexpression in bone marrow-derived multipotent stromal cells decreases the risk of steroid-induced osteonecrosis in the femoral head

P‐glycoprotein (P‐gp) plays a role in steroid‐induced osteonecrosis of the femoral head (ONFH), but the underlying mechanism remains unknown. We hypothesized that P‐gp overexpression can prevent ONFH by regulating bone marrow–derived multipotent stromal cell (BMSC) adipogenesis and osteogenesis. BMSCs from Sprague–Dawley rats were transfected with green fluorescent protein (GFP) or the multidrug resistance gene 1 (MDR1) encoding GFP and P‐gp. Dexamethasone was used to induce BMSC differentiation. Adipogenesis was determined by measuring peroxisome proliferator‐activated receptor (PPAR‐γ) expression and the triglyceride level. Osteogenesis was determined by measuring runt‐related transcription factor 2 (Runx2) expression and alkaline phosphatase activity. For in vivo experiments, rats were injected with saline, BMSCs expressing GFP (GFP‐BMSCs) or BMSCs expressing GFP‐P‐gp (MDR1‐GFP‐BMSCs). After dexamethasone induction, adipogenesis was determined by measuring PPAR‐γ expression and fatty marrow, whereas osteogenesis was detected by measuring Runx2 expression, trabecular parameters and the mineral apposition rate, followed by evaluation of the incidence of ONFH. Overexpression of P‐gp in BMSCs resulted in markedly decreased expression of adipogenic markers and increased expression of osteogenic markers. Compared with rats injected with saline, rats injected with GFP‐BMSCs showed reduced ONFH, and the injected GFP‐positive BMSCs attached to trabecular surfaces and exhibited an osteoblast‐like morphology. Compared with the rats injected with BMSCs expressing GFP alone, rats injected with BMSCs overexpressing GFP and P‐gp showed lower adipocytic variables, higher osteogenic variables and lower incidence of ONFH. Overexpression of P‐gp inhibited BMSC adipogenesis and promoted osteogenesis, which reduced the incidence of steroid‐induced ONFH.

FGF8 signaling sustains progenitor status and multipotency of cranial neural crest-derived mesenchymal cells in vivo and in vitro

The cranial neural crest (CNC) cells play a vital role in craniofacial development and regeneration. They are multi-potent progenitors, being able to differentiate into various types of tissues. Both pre-migratory and post-migratory CNC cells are plastic, taking on diverse fates by responding to different inductive signals. However, what sustains the multipotency of CNC cells and derivatives remains largely unknown. In this study, we present evidence that FGF8 signaling is able to sustain progenitor status and multipotency of CNC-derived mesenchymal cells both in vivo and in vitro. We show that augmented FGF8 signaling in pre-migratory CNC cells prevents cell differentiation and organogenesis in the craniofacial region by maintaining their progenitor status. CNC-derived mesenchymal cells with Fgf8 overexpression or control cells in the presence of exogenous FGF8 exhibit prolonged survival, proliferation, and multi-potent differentiation capability in cell cultures. Remarkably, exogenous FGF8 also sustains the capability of CNC-derived mesenchymal cells to participate in organogenesis such as odontogenesis. Furthermore, FGF8-mediated signaling strongly promotes adipogenesis but inhibits osteogenesis of CNC-derived mesenchymal cells in vitro. Our results reveal a specific role for FGF8 in the maintenance of progenitor status and in fate determination of CNC cells, implicating a potential application in expansion and fate manipulation of CNC-derived cells in stem cell-based craniofacial regeneration.