Inhibition of Osf2/Cbfa1 expression and terminal osteoblast differentiation by PPAR gamma 2

Cells of the bone marrow stroma can reversibly convert among different phen otypes. 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 osteobl asts or adipocytes, or both. Consistent with a stare of plasticity, cell li nes with a mixed phenotype synthesized osteoblast markers like type I colla gen, alkaline phosphatase, osteocalcin, as well as the adipocyte marker lip oprotein lipase, under basal conditions. In the presence of ascorbic acid a nd beta-glycerophosphate-agents that promote osteoblast differentiation-the y formed a mineralized matrix. In the presence of isobutylmethylxanthine, h ydrocortisone, and indomethacin-agents that promote adipocyte differentiati on-they accumulated fat droplets, but failed to express adipsin and aP2, ma rkers of terminally differentiated adipocytes. Furthermore, they were conve rted back to matrix mineralizing cells when the adipogenic stimuli were rep laced with the osteoblastogenic ones. A prototypic cell line with mixed phe notype (UAMS-33) expressed Osf2/ Cbfa 1-a transcription factor required for osteoblast differentiation, but not PPAR gamma 2-a transcription factor re quired for terminal adipocyte differentiation. Stable transfection with a P PAR gamma 2 expression construct and activation with the thiazolidinedione BRL49653 stimulated aP2 and adipsin synthesis and fat accumulation, and sim ultaneously suppressed Osf2/Cbfa1, alpha 1 (I) procollagen, and osteocalcin synthesis. Moreover, it rendered the cells incapable of forming a minerali zed matrix. These results strongly suggest that PPAR gamma 2 negatively reg ulates stromal cell plasticity by suppressing Osf2/Cbfa1 and osteoblast-lik e biosynthetic activity while promoting terminal differentiation to adipocy tes. (C) 1999 Wiley-Liss, inc.