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

Nuclear receptor peroxisome proliferator-activated receptor-gamma (PPAR gamma) is an essential transcription factor for adipocyte differentiation. In mesenchymal stem cells, PPAR gamma 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 PPAR gamma in osteoblast function is not fully understood, due in part to inadequate model systems. Here, we describe an adenoviral-mediated PPAR gamma knockout system in which suppression of PPAR gamma 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 with wild-type mice. We next developed an inducible osteoblast-targeted PPAR gamma knockout (Osx Cre/flox- PPAR gamma) mouse to determine the direct role of PPAR gamma in bone formation. Data from both in vitro cultures of mesenchymal stem cells and in vivo mu CT analysis of bones suggest that suppression of PPAR gamma activity in osteoblasts significantly increased osteoblast differentiation and trabecular number. Endogenous PPAR gamma in mesenchymal stem cells and osteoblasts strongly inhibited Akt/mammalian target of rapamycin (mTOR)/p70S6k activity and led to decreased osteoblastic differentiation. Therefore, we conclude that PPAR gamma modulates osteoblast differentiation and bone formation through both direct and indirect mechanisms. The direct mode, as shown here, involves PPAR gamma regulation of the mTOR pathway, while the indirect pathway is dependent on the regulation of adipogenesis. Stem Cells 2013;31:2183-2192