Comparison of TGF-β/BMP Pathways Signaled by Demineralized Bone Powder and BMP-2 in Human Dermal Fibroblasts

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Abstract
Demineralized bone induces chondrogenic differentiation of human dermal fibroblasts in vitro. Analyses of signaling gene expression showed that DBP and BMP-2 regulate common and distinct pathways. Although BMP-2 was originally isolated as a putative active factor in DBP, rhBMP-2 and DBP do not affect all the same genes or in the same ways. Demineralized bone powder (DBP) induces chondrogenic differentiation of human dermal fibroblasts (hDFs) in 3D culture, but the initiating mechanisms have not been identified. We tested the hypotheses that DBP would affect expression of signaling genes and that DBP's effects would differ from the effects of bone morphogenetic proteins (BMPs). A chondroinduction model was used in which hDFs were cultured with and without DBP in a porous collagen sponge. BMP-2 was delivered in a square of absorbable collagen felt inserted into a collagen sponge. Total RNA was isolated after 3 days of culture, a time that precedes expression of the chondrocyte phenotype. Gene expression was evaluated with two targeted macroarray screens. Effects of DBP and rhBMP-2 were compared by macroarray, RT-PCR, and Northern hybridization analysis of selected genes in the transforming growth factor (TGF)-beta/BMP signaling pathways. By macroarray analysis of 16 signal transduction pathways, the following pathways were modulated in hDFs by DBP: TGF-beta, insulin/LDL, hedgehog, PI3 kinase/AKT, NF-kappaB, androgen, retinoic acid, and NFAT. There was convergence and divergence in DBP and rhBMP-2 regulation of genes in the TGF-beta/BMP signaling pathway. Smad target genes were the predominant group of DBP- or rhBMP-2-regulated genes. Several genes (IGF-BP3, ID2, and ID3) showed similar responses (increased expression) to DBP and rhBMP-2. In contrast, many of the genes that were greatly upregulated by DBP (TGFBI/betaig-h3, Col3A1, TIMP1, p21/Waf1/Cip1) were barely affected by rhBMP-2. These findings indicate that multiple signaling pathways are regulated in fibroblasts by DBP, that one of the major pathways involves Smad target genes, and that DBP and rhBMP-2 elicit different gene expression responses in hDFs. Although BMP-2 was originally isolated as a putative inductive factor in DBP, rhBMP-2 and DBP do not affect all the same genes or in the same ways.