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

Authors

  • Hongli Sun,

    1. Department of Biologic and Materials Sciences, University of Michigan School of Dentistry, Ann Arbor, Michigan, USA
    Search for more papers by this author
  • Jin Koo Kim,

    1. Department of Biologic and Materials Sciences, University of Michigan School of Dentistry, Ann Arbor, Michigan, USA
    Search for more papers by this author
  • Richard Mortensen,

    1. Department of Physiology, University of Michigan School of Medicine, Ann Arbor, Michigan, USA
    Search for more papers by this author
  • Lorraine P. Mutyaba,

    1. Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine and Department of Orthopaedic Surgery, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA
    Search for more papers by this author
  • Kurt D. Hankenson,

    1. Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine and Department of Orthopaedic Surgery, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA
    Search for more papers by this author
  • Paul H. Krebsbach

    Corresponding author
    1. Department of Biologic and Materials Sciences, University of Michigan School of Dentistry, Ann Arbor, Michigan, USA
    • Correspondence: Paul H. Krebsbach, D.D.S., Ph.D., Department of Biologic and Materials Sciences, School of Dentistry, University of Michigan, 1011 North University Ave, Ann Arbor, Michigan 48109, USA. Telephone: +734–936-2600; Fax: +734-647-2110; e-mail: paulk@umich.edu.

    Search for more papers by this author

  • Author contributions: H.S.: conception and design, collection and/or assembly of data, data analysis and interpretation, manuscript writing, and final approval of manuscript; J.K.K.: collection and/or assembly of data, data analysis and interpretation, manuscript writing, and final approval of manuscript; R.M.M. and P.L.M.: provision of study material, collection and/or assembly of data, data analysis and interpretation, and final approval of manuscript; K.D.H.: conception and design, financial support, data analysis and interpretation, and final approval of manuscript; P.H.K.: conception and design, financial support, data analysis and interpretation, manuscript writing and final approval of manuscript.

ABSTRACT

Nuclear receptor peroxisome proliferator-activated receptor-γ (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 knockout 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 with wild-type mice. We next developed an inducible osteoblast-targeted PPARγ knockout (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 suggest 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/mammalian target of rapamycin (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. Stem Cells 2013;31:2183–2192

Ancillary