Ubiquitin Ligase Cbl-b Downregulates Bone Formation Through Suppression of IGF-I Signaling in Osteoblasts During Denervation

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Abstract
Unloading can prevent bone formation by osteoblasts. To study this mechanism, we focused on a ubiquitin ligase, Cbl-b, which was highly expressed in osteoblastic cells during denervation. Our results suggest that Cbl-b may mediate denervation-induced osteopenia by inhibiting IGF-I signaling in osteoblasts. Introduction: Unloading, such as denervation (sciatic neurectomy) and spaceflight, suppresses bone formation by osteoblasts, leading to osteopenia. The resistance of osteoblasts to growth factors contributes to such unloading-mediated osteopenia. However, a detailed mechanism of this resistance is unknown. We first found that a RING-type ubiquitin ligase, Cbl-b, was highly expressed in osteoblastic cells after sciatic neurectomy in mice. In this study, we reasoned that Cbl-b played an important role in the resistance of osteoblasts to IGF-I. Materials and Methods: Cbl-b–deficient (Cbl-b–/–) or wildtype (Cbl-b+/+) mice were subjected to sciatic neurectomy. Bone formation in these mice was assessed by calcein labeling and histomorphometric analyses. We examined IGF-I signaling molecules in femora of these mice by Western blot and immunohistochemical analyses. We also examined the mitogenic response of Cbl-b–overexpressing or –deficient osteoblastic cells to various growth factors. Results: In Cbl-b+/+ mice, denervation decreased femur mass and bone formation, whereas it increased the expression of Cbl-b protein in osteoprogenitor cells and in osteocalcin-positive cells (osteoblastic cells) in hindlimb bone. In contrast, in Cbl-b−/− mice, bone mass and bone formation were sustained during denervation. Denervation inhibited the mitogenic response of osteoprogenitor cells most significantly to IGF-I. Therefore, we focused on Cbl-b–mediated modification of IGF-I signaling. Denervation decreased the amounts of insulin receptor substrate-1 (IRS-1), phosphatidly inositol 3-phosphate kinase (PI3K), and Akt-1 proteins in femora of Cbl-b+/+ mice, whereas the amounts of these IGF-I signaling molecules in femora of Cbl-b–/– mice were constant after denervation. On a cellular level, primary osteoblastic cells from Cbl-b–/– mice were more stimulated to proliferate by IGF-I treatment compared with those from Cbl-b+/+ mice. Furthermore, overexpression of Cbl-b increased ubiquitination and degradation of IRS-1 in primary Cbl-b–/– osteoblastic cells, leading to their impaired mitogenic response to IGF-I. Conclusions: These results suggest that Cbl-b induces resistance of osteoblasts to IGF-I during denervation by increasing IRS-1 degradation and that Cbl-b–mediated modification of IGF-I signaling may contribute to decreased bone formation during denervation.

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