Regulation of intracellular signal transmission for osteogenesis

骨形态发生蛋白（BMPs）属于转化生长因子（TGF）-β超家族，其中一些在体内和体外均显示出强的成骨活性。已经阐明了涉及细胞膜上的BMP受体和细胞内信使（Smads）的BMP信号级联，但BMP信号的调节机制尚未阐明。我们以前发现，pentylfylline（PETx），磷酸二酯酶（PDE）的非特异性抑制剂，和rolipram，PDE-4特异性抑制剂，增强BMP-4诱导的间充质细胞的成骨分化，可能是通过提高细胞内环磷酸腺苷（cAMP）的积累和BMP信号通路的调制，作为增强的BMP-4的作用是通过添加二丁基-cAMP（dbcAMP）再现。然而，这些试剂对BMP信号传导的增强作用的确切机制尚未完全揭示。如已经报道的，BMP利用特异性细胞内信号传导， ...更多信息 通过R-Smads（Smad 1，5，8）、Co-Smads（Smad 4）和I-Smads（Smad 6，7）级联至靶基因。cAMP介导的对BMP信号传导的作用的一种可能性可能是抑制I-Smads表达，因为这些蛋白质在BMP信号传导中形成负反馈环。为了检验这种可能性，在骨髓来源的成骨细胞系（ST 2）中检验了添加dbcAMP或PeTx时I-Smad（Smad 6）表达的变化。BMP-4处理（300 ng/ml）可持续诱导ST 2细胞中碱性磷酸酶活性，BMP-4处理也可诱导Smad 6 mRNA表达。虽然，同时处理的ST 2细胞与BMP-4和dbcAMP引起碱性磷酸酶的进一步激活，此外dbcAMP减少BMP-4诱导的Smad 6表达的剂量依赖性的方式。此外，检测磷酸化Smad 1/5/8蛋白质印迹分析延长，这表明通过抑制Smad 6的表达延长BMP受体的激酶活性。因此，升高的细胞内cAMP可能通过抑制Smad 6诱导和延长细胞内BMP信号传导来增强BMP信号传导。少

Bone morphogenetic proteins (BMPs) belong to the transforming growth factor (TGF)-〓 super-family, and some display potent osteogenic activity both in vivo and in vitro. The BMP signaling cascade involving BMP receptors at the cell membrane and intracellular messengers (Smads) has been elucidated, but the regulatory mechanisms of BMP signaling have not been clarified. We previously found that pentoxifylline (PeTx), a nonspecific inhibitor of phosphodiesterase (PDE), and rolipram, a PDE-4-specific inhibitor, enhance BMP-4-induced osteogenic differentiation of mesenchymal cells, probably through the elevation of intracellular cyclic adenosine monophosphate (cAMP) accumulation and modulation of BMP signaling pathways, as enhanced BMP-4 action was reproduced by addition of dibutylyl-cAMP (dbcAMP). However, the precise mechanisms underlying the enhancing effects of those agents on BMP signaling were not completely revealed. As already reported, BMPs utilize a specific intracellular signaling … More cascade to target genes via R-Smads (Smad1,5,8), Co-Smad (Smad4) and I-Smads (Smad6,7). One possibility for cAMP-mediated effects on BMP signaling might be suppression of I-Smads expression, since these proteins form a negative feedback loop in BMP signaling. To examine this possibility, changes in I-Smad (Smad6) expression on addition of dbcAMP or PeTx were examined in a bone marrow-derived osteogenic cell line (ST2). Alkaline phosphatase activity in ST2 cells was consistently induced by BMP-4 treatment (300 ng/ml), and Smad6 mRNA expression was also induced by BMP-4 treatment. Although, concurrent treatment of ST2 cells with BMP-4 and dbcAMP elicited further activation of alkaline phosphatase, addition of dbcAMP reduced BMP-4 induced-Smad6 expression in a dose-dependent manner. Furthermore, detection of phosphorylated Smad1/5/8 on Western blotting analysis was prolonged, suggesting prolonged kinase activity of BMP receptors through suppressed expression of Smad6. Elevated intracellular cAMP might thus enhance BMP signaling by suppressing Smad6 induction and prolonging intracellular BMP signaling. Less