Osteoblast-Targeted suppression of PPARγ increases osteogenesis through activation of mTOR signaling

Abstract

Nuclear receptor peroxisome proliferator-activated receptor gamma (PPARγ) is an essential transcription factor for adipocyte differentiation. In mesenchymal stem cells, PPARγ has been assumed to play a negative role in osteoblastic differentiation, by working in an adipogenesis dependent manner, due to the reciprocal relationship between osteoblast and adipocyte differentiation. However, the direct role of PPARγ in osteoblast function is not fully understood, due in part to inadequate model systems. Here, we describe an adenoviral-mediated PPARγ knock-out system in which suppression of PPARγ in mesenchymal stem cells enhanced osteoblast differentiation and inhibited adipogenesis in vitro. Consistent with this in vitro observation, lipoatrophic A-ZIP/F1 mice, which do not form adipocytes, displayed a phenotype in which both cortical and trabecular bone was significantly increased compared to wild type mice. We next developed an inducible osteoblast-targeted PPARγ knock-out (Osx Cre/flox- PPARγ) mouse to determine the direct role of PPARγ in bone formation. Data from both in vitro cultures of mesenchymal stem cells and in vivo μCT analysis of bones suggests that suppression of PPARγ activity in osteoblasts significantly increased osteoblast differentiation and trabecular number. Endogenous PPARγ in mesenchymal stem cells and osteoblasts strongly inhibited Akt/mTOR/p70S6k activity and led to decreased osteoblastic differentiation. Therefore, we conclude that PPARγ modulates osteoblast differentiation and bone formation through both direct and indirect mechanisms. The direct mode, as shown here, involves PPARγ regulation of the mTOR pathway, while the indirect pathway is dependent on the regulation of adipogenesis.

Structural design and synthesis of arylalkynyl amide-type peroxisome proliferator-activated receptor γ (PPARγ)-selective antagonists based on the helix12-folding inhibition hypothesis

“…6.1.9. N-((5-amino-2-propoxyphenyl)methyl)-4-pyrimidin-2-ylbenzamide (20) Compound 19 (500 mg, 1.27 mmol), 10% palladium on carbon (250 mg, 50 wt%) and 300 mL of methanol/ethyl acetate (1:2 v/v) were mixed and catalytic hydrogenation was carried out at an initial hydrogen pressure of 400e500 kPa. After completion of the reaction, the catalyst was removed by filtration and the filtrate was concentrated to afford 455 mg (98%) of the title compound as a yellow amorphous solid, which was used in the next step without further purification.…”

“…Several reports have indicated that suppression of PPARg promotes osteoblast formation, but inhibits adipocyte differentiation from mesenchymal stem cells, which are the common progenitor of osteoblasts and adipocytes [19,20]. Therefore, antagonists or inverse agonists targeting PPARg are considered to be candidates for treatment of osteoporosis and obesity [21,22].…”

IGF-1 regulation of key signaling pathways in bone

“…This signaling pathway should be extensively studied to define the relative contribution of the different complexes that are involved in osteoblast differentiation. 49 IGF-1 and energy metabolism. All the studies to date with the skeletal IGF regulatory components have produced in vivo phenotypes through deletion or overexpression of the IGF-1 system in mice.…”

GATA2 regulates differentiation of bone marrow-derived mesenchymal stem cells

“…57,58 Furthermore, in murine MSC, the propensity for differentiation toward osteoblasts or adipocytes was affected by various factors including Maf, Runx2, Cebpb and Pparg. 59,60 Nevertheless, our data clearly demonstrate that GATA2 could be one of the important factors that determine immaturity and differentiation toward adipocytes in BM-MSC.…”