A novel PPARγ 2 modulator sLZIP controls the balance between adipogenesis and osteogenesis during mesenchymal stem cell differentiation

Mesenchymal stem cells (MSCs), also known as multipotent stromal cells, are used in clinical trials. However, the use of MSCs for medical treatment of patients poses a potential problem due to the possibility of transdifferentiation into unwanted tissues. Disruption of the balance during MSC differentiation leads to obesity, skeletal fragility, and osteoporosis. Differentiation of MSCs into either adipocytes or osteoblasts is transcriptionally regulated by the two key transcription factors PPARγ 2 and Runx2. PPARγ 2 is highly expressed during adipocyte differentiation and regulates expression of genes involved in adipogenesis. Runx2 induces osteogenic gene expression and, thereby, increases osteoblast differentiation. Although transcriptional modulation of PPARγ 2 has been investigated in adipogenesis, the underlying molecular mechanisms to control the balance between adipogenesis and osteogenesis in MSCs remain unclear. In this study, the role of sLZIP in regulation of PPARγ 2 transcriptional activation was investigated along with sLZIP's involvement in differentiation of MSCs into adipocytes and osteoblasts. sLZIP interacts with PPARγ 2 and functions as a corepressor of PPARγ 2. sLZIP enhances formation of the PPARγ 2 corepressor complex through specific interaction with HDAC3, resulting in suppression of PPARγ 2 transcriptional activity. We found that sLZIP prevents expression of PPARγ 2 target genes and adipocyte differentiation both in vitro and in vivo. sLZIP also upregulates Runx2 transcriptional activity via inhibition of PPARγ 2 activity, and promotes osteoblast differentiation. sLZIP transgenic mice exhibited enhanced bone mass and density, compared with wild-type mice. These results indicate that sLZIP has a critical role in the regulation of osteogenesis and bone development. However, sLZIP does not affect chondrogenesis and osteoclastogenesis. We propose that sLZIP is a novel PPARγ 2 modulator for control of the balance between adipogenesis and osteogenesis during MSC differentiation, and that sLZIP can be used as a therapeutic target molecule for treatment of obesity, osteodystrophy, and osteoporosis.