The role(s) of Src kinase and Cbl proteins in the regulation of osteoclast differentiation and function

Abstract

The osteoclast resorbs mineralized bone during bone development, homeostasis, and repair. The deletion of the gene encoding the nonreceptor tyrosine kinase c-Src produces an osteopetrotic skeletal phenotype that is the consequence of the inability of the mature osteoclast to efficiently resorb bone. Src-/- osteoclasts exhibit reduced motility and abnormal organization of the apical secretory domain (the ruffled border) and attachment-related cytoskeletal elements that are necessary for bone resorption. A key function of Src in osteoclasts is to promote the rapid assembly and disassembly of the podosomes, the specialized integrin-based attachment structures of osteoclasts and other highly motile cells. Once recruited to the activated integrins, especially alphavbeta3), by the adhesion tyrosine kinase Pyk2, Src binds and phosphorylates Cbl and Cbl-b, homologous multisite adapter proteins with ubiquitin ligase activity. The Cbl proteins in turn recruit and activate additional signaling effectors, including phosphatidylinositol 3-kinase and dynamin, which play key roles in the development of cell polarity and the regulation of cell attachment and motility. In addition, Src and the Cbl proteins contribute to signaling cascades that are activated by several important receptors, including receptor activator of nuclear factor kappaB and the macrophage colony-stimulating factor receptor, and also downregulate the signaling from many of these receptors.

Cbl-b Is a Novel Physiologic Regulator of Glycoprotein VI-dependent Platelet Activation

“…It is in the C-terminal region that c-Cbl and Cbl-b show the greatest divergence of sequence, which can account for different functionality between the two proteins. The fact that c-Cbl and Cbl-b regulate different and often opposing function is not unprecedented, and that has been delineated in the case of osteoclasts by us (53,54). We propose that Cbl-b positively regulates GPVIdependent activation by functioning as an adapter molecule to bring signaling molecules such as BTK and PLC␥2 together.…”

Class IA phosphatidylinositol 3-kinase regulates osteoclastic bone resorption through protein kinase B–mediated vesicle transport

“…(2,7) Osteoclasts are highly motile, but they firmly attach to the bone matrix in the course of bone resorption by means of a ''sealing zone,'' which is effected by an actin-rich structure, the podosome, and consists of a spatial compartment for bone resorption between the bone matrix and the osteoclast. (8) Integrin a v b 3 is important for the recognition and attachment to bone matrix by osteoclasts. (9) Osteoclasts secrete protons and matrix-degrading enzymes, such as cathepsin K and matrix metalloproteinases (MMP), into this space through the ruffled border membrane, thus enabling the efficient decalcification and degradation of the bone matrix.…”

“…This is evident in Src-deficient mice, in which osteoclastic bone resorption is severely impaired and the formation of both of these structures is deleteriously affected. (8,12) The phosphatidylinositol 3-kinases (PI3Ks) are a family of lipid kinases that phosphorylate the 3 0 -hydroxyl group on the inositol ring of phosphatidylinositol. (13)(14)(15) Based on the sequence similarity, composition of the subunits, and substrate specificity, the PI3Ks are divided into three classes: class I, II, and III PI3Ks.…”

“…(14,15) PI3K is known to be stimulated by M-CSF and integrin a v b 3 , both of which are crucial for osteoclast function. (8,9) In vitro inhibitor experiments suggest that PI3K is involved in the differentiation (21) and the bone-resorbing activity (22,23) of osteoclasts, but the level of PI-3,4,5-P 3 has not been quantitated in osteoclasts, and the in vivo function of PI3K in osteoclasts has also not been elucidated. In addition, because the PI3K inhibitors such as wortmannin and LY294002 suppress the catalytic activity of all classes of the PI3Ks, (16,24) the role of each PI3K class needs to be clarified.…”

Rôle de l’ubiquitine ligase c-Cbl dans l’ostéogenèse normale et cancéreuse

“…Cependant, ce phénotype est transitoire et disparaît après la naissance [11]. In vitro, les ostéoclastes c-Cbl -/-ont une adhésion et une mobilité réduites résultant d'anomalies de la fonction des protéines Src et PI3K [12]. L'expression d'une protéine Cbl mutée dans le domaine RING (C 379 ACbl) inhibe l'activité d'ubiquitination de c-Cbl et diminue la mobilité des ostéoclastes, ce défaut de mobilité étant corrigé par l'expression de c-Cbl sauvage [13].…”