Carvalho R S, Schaffer J L, Gerstenfeld L C
The Laboratory for the Study of Skeletal Disorders and Rehabilitation, Children's Hospital and Harvard Medical School, Boston, Massachusetts 02115, USA.
J Cell Biochem. 1998 Sep 1;70(3):376-90. doi: 10.1002/(sici)1097-4644(19980901)70:3<376::aid-jcb11>3.0.co;2-j.
Osteopontin is a predominant integrin binding protein of bone and its expression has been shown to be induced by mechanical stimuli within osteoblasts (Toma et al. [1997] J. Bone Miner. Res. 12:1626-1636). The present studies examined if the cell adhesion would mimic the mechano-transduction that stimulated opn mRNA expression and whether integrin receptors were involved in these processes. Osteopontin mRNA expression was induced three- to four-fold, 24 hours after embryonic chicken calvaria osteoblast attachment to fibronectin (FN), however no induction was observed if the cells were plated on tissue culture plastic alone. Osteopontin mRNA induction in response to cell attachment on FN was dependent on new protein synthesis and the activation of a tyrosine protein kinase(s) but unlike mechano-induction was independent of the maintenance of the cell's microfilament structure. Integrin receptor(s) were shown to be involved in mediating the signal transduction processes of both cell attachment and mechanical stimulation since incubation of osteoblasts with the integrin binding peptide RGDS partially blocked the induction of opn expression in response to both stimuli. Interestingly, incubation of the osteoblasts that were adherent on tissue culture plastic alone with the RGDS peptide lead to an induction in opn expression suggesting that integrin occupancy by itself was sufficient to initiate the signal transduction process that induced opn expression. In order to assess the role of integrin occupancy vs. focal adhesion complex formation that accompanies cell attachment, in the signal transduction process that induces opn expression, receptor clustering was stimulated pharmacologically with bombesin or lysophosphatidic acid in osteoblasts attached to tissue culture plastic. Neither compound in the absence of occupancy of the integrin receptors was capable of stimulating opn expression in attached cells, however if the cells were placed in suspension pharmacological mediation of receptor clustering and integrin occupancy were additive in their effect of inducing opn expression. These data demonstrate that induction of opn expression by mechanical stimuli and cell attachment are commonly mediated through integrin receptor(s). However, when cells are attached receptor clustering alone which accompanies focal adhesion formation was incapable of mediating signal transduction suggesting that receptor occupancy was a prerequisite to the signal transduction process that leads to the induction of opn mRNA expression.
骨桥蛋白是骨骼中一种主要的整合素结合蛋白，其表达已被证明可由成骨细胞内的机械刺激诱导（托马等人[1997年]《骨与矿物质研究杂志》12:1626 - 1636）。本研究考察了细胞黏附是否会模拟刺激骨桥蛋白mRNA表达的机械转导，以及整合素受体是否参与这些过程。胚胎鸡颅骨成骨细胞附着于纤连蛋白（FN）24小时后，骨桥蛋白mRNA表达增加了三到四倍，然而，如果细胞仅接种在组织培养塑料上，则未观察到诱导现象。响应细胞在FN上的附着，骨桥蛋白mRNA的诱导依赖于新蛋白质的合成和酪氨酸蛋白激酶的激活，但与机械诱导不同的是，它不依赖于细胞微丝结构的维持。整合素受体被证明参与介导细胞附着和机械刺激的信号转导过程，因为用整合素结合肽RGDS孵育成骨细胞可部分阻断对两种刺激的骨桥蛋白表达诱导。有趣的是，仅用RGDS肽孵育附着在组织培养塑料上的成骨细胞会导致骨桥蛋白表达的诱导，这表明整合素占据本身足以启动诱导骨桥蛋白表达的信号转导过程。为了评估整合素占据与伴随细胞附着的粘着斑复合物形成在诱导骨桥蛋白表达的信号转导过程中的作用，用蛙皮素或溶血磷脂酸在附着于组织培养塑料的成骨细胞中进行药理学刺激以促进受体聚集。在整合素受体未被占据的情况下，这两种化合物都不能刺激附着细胞中的骨桥蛋白表达，然而，如果将细胞置于悬浮状态，受体聚集和整合素占据的药理学介导在诱导骨桥蛋白表达的作用上是相加的。这些数据表明，机械刺激和细胞附着对骨桥蛋白表达的诱导通常是通过整合素受体介导的。然而，当细胞附着时，仅伴随粘着斑形成的受体聚集无法介导信号转导，这表明受体占据是导致骨桥蛋白mRNA表达诱导的信号转导过程的先决条件。
