Role of IGF-I signaling in regulating osteoclastogenesis.
ABSTRACT We showed that IGF-I deficiency impaired osteoclastogenesis directly and/or indirectly by altering the interaction between stromal/osteoblastic cells and osteoclast precursors, reducing RANKL and M-CSF production. These changes lead to impaired bone resorption, resulting in high BV/TV in IGF-I null mice.
Although IGF-I has been clearly identified as an important growth factor in regulating osteoblast function, information regarding its role in osteoclastogenesis is limited. Our study was designed to analyze the role of IGF-I in modulating osteoclastogenesis using IGF-I knockout mice (IGF-I(-/-)).
Trabecular bone volume (BV/TV), osteoclast number, and morphology of IGF-I(-/-) or wildtype mice (IGF-I(+/+)) were evaluated in vivo by histological analysis. Osteoclast precursors from these mice were cultured in the presence of RANKL and macrophage-colony stimulating factor (M-CSF) or co-cultured with stromal/osteoblastic cells from either genotype. Osteoclast formation was assessed by measuring the number of multinucleated TRACP+ cells and pit formation. The mRNA levels of osteoclast regulation markers were determined by quantitative RT-PCR.
In vivo, IGF-I(-/-) mice have higher BV/TV and fewer (76% of IGF-I(+/+)) and smaller osteoclasts with fewer nuclei. In vitro, in the presence of RANKL and M-CSF, osteoclast number (55% of IGF-I(+/+)) and resorptive area (30% of IGF-I(+/+)) in osteoclast precursor cultures from IGF-I(-/-) mice were significantly fewer and smaller than that from the IGF-I(+/+) mice. IGF-I (10 ng/ml) increased the size, number (2.6-fold), and function (resorptive area, 2.7-fold) of osteoclasts in cultures from IGF-I(+/+) mice, with weaker stimulation in cultures from IGF-I(-/-) mice. In co-cultures of IGF-I(-/-) osteoblasts with IGF-I(+/+) osteoclast precursors, or IGF-I(+/+) osteoblasts with IGF-I(-/-) osteoclast precursors, the number of osteoclasts formed was only 11% and 48%, respectively, of that from co-cultures of IGF-I(+/+) osteoblasts and IGF-I(+/+) osteoclast precursors. In the long bones from IGF-I(-/-) mice, mRNA levels of RANKL, RANK, M-CSF, and c-fms were 55%, 33%, 60%, and 35% of that from IGF-I(+/+) mice, respectively.
Our results indicate that IGF-I regulates osteoclastogenesis by promoting their differentiation. IGF-I is required for maintaining the normal interaction between the osteoblast and osteoclast to support osteoclastogenesis through its regulation of RANKL and RANK expression.
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ABSTRACT: The osteoclast is the cell that resorbs bone. It has been known for many years that its formation and function are regulated by cells of the osteoblastic lineage. Recently the molecular basis for this regulation was identified; osteoblastic cells induce osteoclastic differentiation and resorptive activity through expression of tumour necrosis factor (TNF) activation-induced cytokine (TRANCE) (also known as RANKL, ODF, OPGL, and TNFSF11), a novel membrane-inserted member of the TNF superfamily. Osteoclastic regulation is assisted through secretion of an inhibitor, osteoprotegerin (OPG) (OCIF, TNFRSF11B), a soluble (decoy) receptor for TRANCE. Osteoclast formation and survival also depend on and are substantially enhanced by transforming growth factor-beta (TGF-beta), which is abundant in bone matrix. Surprisingly, not only TRANCE but also TNF-alpha can induce osteoclast formation in vitro from bone marrow-derived mononuclear phagocytes, especially in the presence of TGF-beta. Whether or not TNF-alpha does the same in vivo, its ability to generate osteoclasts in vitro has significant implications regarding the nature of osteoclasts and their relationship to other mononuclear phagocytes, and a possible wider role for TRANCE in macrophage pathobiology. A hypothesis is presented in which the osteoclast is a mononuclear phagocyte directed towards a debriding function by TGF-beta, activated for this function by TRANCE, and induced to become specifically osteoclastic by the characteristics of the substrate or signals from bone cells that betoken such characteristics.The Journal of Pathology 10/2000; 192(1):4-13. · 7.59 Impact Factor
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ABSTRACT: In the adult skeleton, bone formation is regulated by an event referred to as the coupling of formation to resorption (i.e., formation is linked to resorption), which is thought to be mediated in part by locally produced growth factors. Although human bone cells produce and human bones contain a variety of growth factors, there is sufficient evidence to document an important role for the insulin-like growth factor (IGF) system in mediating this coupling process in bone. Studies on the basic aspects of the IGF system in bone reveal that it is complex and involves a number of components which include the IGF-binding proteins (IGFBPs; mainly IGFBP-3, -4, and -5), specific extracellular IGFBP proteases, and receptors (types 1 and 2). Based on recent experimental evidence from a number of laboratories, we propose the following models of IGF action on the regulation of the coupled increase in bone formation in response to bone resorption: (1) IGF release from bone during bone resorption promotes osteoblasts to initiate cavity refilling; (2) IGF production by osteoclasts creates a population of osteoblasts in proportion to the volume of bone tissue resorbed; and (3) IGF production by stromal cells and osteoblasts predominantly regulates the extent of cavity refill. The amount of growth factor production by osteoblasts and contemporary cells of osteoblast lineage can be further controlled by both systemic and local factors which together determine the eventual level of fill-in of the resorption cavity.(ABSTRACT TRUNCATED AT 250 WORDS)Bone 09/1995; 17(2 Suppl):93S-98S. · 3.82 Impact Factor
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ABSTRACT: Osteoclast differentiation from hematopoietic precursors is controlled by the tumor necrosis factor family member tumor necrosis factor-related activation-induced cytokine (TRANCE) via induction of various transcription factors, including nuclear factor of activated T cells (NFAT) c1. During osteoclast differentiation, NFATc1 is further activated via calcium signaling when costimulatory receptors expressed on osteoclast precursors, such as osteoclast-associated receptor (OSCAR), are stimulated. Here we show that NFATc1 expression precedes that of OSCAR during TRANCE-mediated osteoclastogenesis and that inhibition of NFATc1 by cyclosporin A abolishes TRANCE-induced OSCAR expression and subsequent osteoclast differentiation. Moreover, we show that the 1.0-kb promoter region of the OSCAR gene contains three potential NFATc1-binding sites. Induction of an OSCAR promoter-luciferase reporter is significantly increased when transiently transfected into 293T cells in combination with NFATc1 expression plasmid. Deletion and site-directed mutant constructs confirmed that NFATc1-binding sites are both functional and NFATc1-specific. Furthermore, NFATc1 synergistically activates an OSCAR reporter construct together with microphthalmia transcription factor and PU.1, transcription factors previously shown to be critical for osteoclast differentiation. In addition, a plasmid expressing constitutively active MAP kinase kinase 6 enhances the transactivation activity of NFATc1/microphthalmia transcription factor/PU.1 on the OSCAR promoter. Taken together, our results indicate that NFATc1 is an important transcription factor in the induction of OSCAR during osteoclastogenesis. Elucidation of NFATc1 as a transcription factor for OSCAR expression implies the presence of a positive feedback circuit of TRANCE-induced activation of NFATc1, involving NFATc1-mediated OSCAR expression and its subsequent activation of NFATc1, necessary for efficient differentiation of osteoclasts.Journal of Biological Chemistry 11/2005; 280(42):35209-16. · 4.65 Impact Factor