Role of IGF-I signaling in regulating osteoclastogenesis.
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.


BACKGROUND
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(-/-)).


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Wang
Yongmei
Y
Department of Medicine, Endocrine Unit, Veterans Affairs Medical Center, San Francisco, California 94121, USA.
Nishida
Shigeki
S
Elalieh
Hashem Z
HZ
Long
Roger K
RK
Halloran
Bernard P
BP
Bikle
Daniel D
DD
D000818:Animals; D001862:Bone Resorption / Q000235:genetics; D001842:Bone and Bones / Q000502:physiology; D002352:Carrier Proteins / Q000378:metabolism; D002454:Cell Differentiation / Q000187:drug effects / Q000235:genetics; D006023:Glycoproteins / Q000378:metabolism; D006412:Hematopoietic Stem Cells / Q000187:drug effects* / Q000378:metabolism; D066298:In Vitro Techniques; D007334:Insulin-Like Growth Factor I / Q000235:genetics / Q000494:pharmacology* / Q000502:physiology*; D016173:Macrophage Colony-Stimulating Factor / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D050778:NFATC Transcription Factors / Q000378:metabolism; D010010:Osteoclasts / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology; D010012:Osteogenesis / Q000187:drug effects* / Q000235:genetics; D053244:Osteoprotegerin; D053245:RANK Ligand; D053246:Receptor Activator of Nuclear Factor-kappa B; D016186:Receptor, Macrophage Colony-Stimulating Factor / Q000378:metabolism; D018003:Receptors, Calcitonin / Q000378:metabolism; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism; D018124:Receptors, Tumor Necrosis Factor / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology
D002352:Carrier Proteins; D006023:Glycoproteins; D008562:Membrane Glycoproteins; D050778:NFATC Transcription Factors; D053244:Osteoprotegerin; D053245:RANK Ligand; D053246:Receptor Activator of Nuclear Factor-kappa B; D018003:Receptors, Calcitonin; D018160:Receptors, Cytoplasmic and Nuclear; D018124:Receptors, Tumor Necrosis Factor; C505687:Tnfrsf11a protein, mouse; C506031:Tnfrsf11b protein, mouse; C505689:Tnfsf11 protein, mouse; D007334:Insulin-Like Growth Factor I; D016173:Macrophage Colony-Stimulating Factor; D016186:Receptor, Macrophage Colony-Stimulating Factor