Rosiglitazone (Rosi), a member of the thiazolidinedione class of drugs used to treat type 2 diabetes, activates peroxisome proliferator-activated receptor gamma (γ). Using gene expression microarrays, we investigated the transcriptional mechanisms by which the PPARγ2 isoform promotes adipogenesis and suppresses osteoblastogenesis. A total of 4,252 transcriptional changes resulted when Rosi (1 μM) was applied to the U-33 marrow stromal cell line transfected with PPARγ2 (U-33/γ2) compared to non-induced U-33/γ2 cells. By contrast, Rosi stimulation produced a single gene change in U-33 cells transfected with empty vector (U-33/c), indicating that the Rosi-induced differences are specific to PPARγ2. Differences between U-33/γ2 and U-33/c cells comprised 7,928 transcriptional changes, independent of Rosi. Time-specific pathway enrichment analyses identified multiple biological processes important in adipocyte differentiation and skeletal homeostasis. The earliest changes accompanying Rosi activation of PPARγ2 included up-regulation of the specific immune response, the inflammatory response, sequestering and processing of lipids, and negative regulation of apoptosis, as well as adjustments in morphogenesis and signal transduction (Wnt, JAK-STAT, insulin-like growth factor, I-kappaB kinase/NF-kappaB). Expression signatures influenced by longer exposure to Rosi provided evidence for distinct mechanisms governing repression of osteogenesis and induction of adipogenesis. In summary, our results show that PPARγ2 is a major regulator of marrow mesenchymal stem cell differentiation and suggest interactions that could lead to the identification of a master regulatory scheme controlling osteoblast differentiation.

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