Upon termination of bone matrix synthesis, osteoblasts either undergo apoptosis or differentiate into osteocytes or bone lining cells. In this study, we investigated the role of matrix metalloproteinases (MMPs) and growth factors in the differentiation of osteoblasts into osteocytes and in osteoblast apoptosis. The mouse osteoblast cell line MC3T3-E1 and primary mouse calvarial osteoblasts were either grown on two-dimensional (2-D) collagen-coated surfaces, where they morphologically resemble flattened, cuboidal bone lining cells, or embedded in three-dimensional (3-D) collagen gels, where they resemble dendritic osteocytes constituting a network of cells. When MC3T3-E1 osteoblasts were grown in a 3-D matrix in the presence of an MMP inhibitor (GM6001), the cell number was dose-dependently reduced by approximately 50%, whereas no effect was observed on a 2-D substratum. In contrast, the murine mature osteocyte cell line, MLO-Y4, was unaffected by GM6001 under all culture conditions. According to TUNEL assay, the osteoblast apoptosis was increased 2.5-fold by 10 M GM6001. To investigate the mechanism by which MMPs mediate the survival of osteoblasts, we examined the effect of GM6001 on MC3T3-E1 osteoblasts in the presence of extracellular matrix components and growth factors, including tenascin, fibronectin, laminin, collagenase-cleaved collagen, gelatin, parathyroid hormone, basic fibroblast growth factor, vascular epidermal growth factor, insulin-like growth factor, interleukin-1, and latent and active transforming growth factor-␤ (TGF-␤). Only active TGF-␤, but not latent TGF-␤ or other agents tested, restored cell number and apoptosis to control levels. Furthermore, we found that the membrane type MMP, MT1-MMP, which is produced by osteoblasts, could activate latent TGF-␤ and that antibodies neutralizing endogenous TGF-␤ led to a similar decrease in cell number as GM6001. Whereas inhibitors of other protease families did not induce osteoblast apoptosis, an inhibitor of the p44/42 mitogen-activated protein kinase showed the same but non-synergetic effect as GM6001. These findings suggest that MMP-activated TGF-␤ maintains osteoblast survival during trans-differentiation into osteocytes by a p44/42-dependent pathway.The skeleton is a dynamic tissue that is continuously remodeling to sustain calcium homeostasis, repair microfractures, and react to strain and stress of the skeleton. The remodeling process is a complex process and relies on the coupling between bone resorption and formation that involves osteoclasts, osteoblasts, and osteocytes.The constant regeneration of bone emphasizes the delicate balance between bone resorption and bone formation, which otherwise may lead to pathological conditions such as osteoporosis or osteopetrosis. The investigation of the cellular actions of the major players of bone remodeling may therefore contribute significantly to the discovery of new and better drugs for the treatment of osteoporosis (1).The major pharmaceutical interventions for treatment and prevention of osteop...

The mechanism of positive regulation of cytokine signalling pathways has been well investigated, whereas our knowledge of negative regulation is relatively sparse. Here we review recent literature on important negative regulators: the family of suppressors of cytokine signalling, SOCS, consisting of eight members (SOCS-1 to SOCS-7 and CIS) all sharing a central SH2 domain and a C-terminal SOCS box. Expression of CIS, SOCS-1, SOCS-2 and SOCS-3 is induced by various cytokines, and overexpression studies in various cell lines have demonstrated their inhibitory roles. These family members have been implicated in the negative regulation of several pathways, particularly the JAK/STAT pathway, and since this signalling pathway is responsible for their induction, they form part of a classical negative feedback circuit. To date, at least three different modulating mechanisms have been demonstrated: through the SH2 domain they bind to phosphotyrosines on the target protein, leading to inhibition of signal transduction by N-terminal inactivation of JAK, by blocking access of STAT to the receptor sites, or by SOCS box-targeting bound proteins to proteasomal degradation. In gene modification studies in mice, it has been demonstrated that SOCS-1 plays an important role in IFNgamma-regulation and T-cell differentiation, while SOCS-2 seems necessary for normal growth regulation. SOCS-3(-/-) mice die during embryogenesis for a reason still not fully understood, but insufficient control of fetal erythropoiesis or defects in placental development may be involved. The physiological role for the other family members, as well as their molecular regulation mechanisms, remain to be revealed.