Transforming growth factor-beta inhibits CCAAT/enhancer-binding protein expression and PPARgamma activity in unloaded bone marrow stromal cells.

The molecular mechanisms regulating the adipogenic differentiation of bone marrow stromal cells in vivo remain largely unknown. In this study, we investigated the regulatory effects of transforming growth factor beta-2 (TGF-beta2) on transcription factors involved in adipogenic differentiation induced by hind limb suspension in rat bone marrow stromal cells in vivo. Time course real-time quantitative reverse-transcription polymerase chain reaction (RT-PCR) analysis of gene expression showed that skeletal unloading progressively increases the expression of CCAAT/enhancer-binding protein (C/EBP)alpha and C/EBPbeta alpha at 5 days in bone marrow stromal cells resulting in increased peroxisome proliferator-activated receptor gamma (PPARgamma2) transcripts at 7 days. TGF-beta2 administration in unloaded rats corrected the rise in C/EBPalpha and C/EBPbeta transcripts induced by unloading in bone marrow stromal cells. This resulted in inhibition of PPARgamma2 expression that was associated with increased Runx2 expression. Additionally, the inhibition of C/EBPalpha and C/EBPbeta expression by TGF-beta2 was associated with increased PPARgamma serine phosphorylation in bone marrow stromal cells, a mechanism that inhibits PPARgamma transactivating activity. The sequential inhibitory effect of TGF-beta2 on C/EBPalpha, C/EBPbeta, and PPARgamma2 resulted in reduced LPL expression and abolition of bone marrow stromal cell adipogenic differentiation, which contributed to prevent bone loss induced by skeletal unloading. We conclude that TGF-beta2 inhibits the excessive adipogenic differentiation of bone marrow stromal cells induced by skeletal unloading by inhibiting C/EBPalpha, C/EBPbeta, and PPARgamma expression and activity, which provides a sequential mechanism by which TGF-beta2 regulates adipogenic differentiation of bone marrow stromal cells in vivo.