Cbl-mediated ubiquitination of α5 integrin subunit mediates fibronectin-dependent osteoblast detachment and apoptosis induced by FGFR2 activation

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Abstract
Fibroblast growth factor receptor signaling is an important mechanism regulating osteoblast function. To gain an insight into the regulatory role of FGF receptor-2 (FGFR2) signaling in osteoblasts, we investigated integrin-mediated attachment and cell survival in human calvarial osteoblasts expressing activated FGFR2. FGFR2 activation reduced osteoblast attachment on fibronectin. This was associated with reduced expression of the α5 integrin subunit normally expressed in human calvarial osteoblasts in vivo. Treatment with lactacystin, a potent inhibitor of proteasome, restored α5 integrin levels in FGFR2 mutant osteoblasts. Immunoprecipitation analysis showed that α5 integrin interacts with both the E3 ubiquitin ligase Cbl and ubiquitin. Immunocytochemistry revealed that α5 integrin colocalizes with FGFR2 and Cbl at the leading edge in membrane ruffle regions. Transfection with the 70Z-Cbl mutant lacking the RING domain required for Cbl-ubiquitin interaction, or with the G306E Cbl mutant that abolishes the binding ability of Cbl phosphotyrosine-binding domain restored α5 integrin levels. This suggests that Cbl-mediated ubiquitination plays an essential role in α5 integrin proteasome degradation induced by FGFR2 activation. Reduced α5 integrin expression was associated with an increased Bax/Bcl-2 ratio and increased caspase-9 and -3 activities in FGFR2 mutant osteoblasts. Forced expression of α5 integrin rescued cell attachment and corrected both the Bax/Bcl-2 ratio and caspase-3 and caspase-9 activities in FGFR2 mutant osteoblasts. We show that Cbl recruitment induced by FGFR2 activation triggers α5 integrin degradation by the proteasome, which results in reduced osteoblast attachment on fibronectin and caspase-dependent apoptosis. This identifies a functional role of the α5 integrin subunit in the induction of apoptosis triggered by FGFR2 activation in osteoblasts, and reveals that a Cbl-dependent mechanism is involved in the coordinated regulation of cell apoptosis induced by α5 integrin degradation.