Inhibition of Osf2/Cbfa1 Expression and Terminal Osteoblast Differentiation by PPARgamma2

Cells of the bone marrow stroma can reversibly convert among different phenotypes. Based on this and on evidence for a reciprocal relationship between osteoblastogenesis and adipogenesis, we have isolated several murine bone marrow-derived clonal cell lines with phenotypic characteristics of osteoblasts or adipocytes, or both. Consistent with a state of plasticity, cell lines with a mixed phenotype synthesized osteoblast markers like type I collagen, alkaline phosphatase, osteocalcin, as well as the adipocyte marker lipoprotein lipase, under basal conditions. In the presence of ascorbic acid and beta-glycerophosphate-agents that promote osteoblast differentiation-they formed a mineralized matrix. In the presence of isobutylmethylxanthine, hydrocortisone, and indomethacin-agents that promote adipocyte differentiation-they accumulated fat droplets, but failed to express adipsin and aP2, markers of terminally differentiated adipocytes. Furthermore, they were converted back to matrix mineralizing cells when the adipogenic stimuli were replaced with the osteoblastogenic ones. A prototypic cell line with mixed phenotype (UAMS-33) expressed Osf2/Cbfa1-a transcription factor required for osteoblast differentiation, but not PPARgamma2-a transcription factor required for terminal adipocyte differentiation. Stable transfection with a PPARgamma2 expression construct and activation with the thiazolidinedione BRL49653 stimulated aP2 and adipsin synthesis and fat accumulation, and simultaneously suppressed Osf2/Cbfa1, alpha1(I) procollagen, and osteocalcin synthesis. Moreover, it rendered the cells incapable of forming a mineralized matrix. These results strongly suggest that PPARgamma2 negatively regulates stromal cell plasticity by suppressing Osf2/Cbfa1 and osteoblast-like biosynthetic activity, while promoting terminal differentiation to adipocytes.