Evidence for the formation of a complex between osteopontin and osteocalcin

Abstract

We hypothesize that the mechanisms governing bone formation and remodeling involve the assembly of some of the components of the extracellular matrix into supramolecular complexes. We have examined the associations of osteopontin (OPN) with other proteins isolated from demineralized rat long bones. Three ligand binding techniques were used to demonstrate the formation of complexes between osteopontin and osteocalcin (OCN). Using gel overlay assays, the binding between soluble 125I-OPN and OCN immobilized in acrylamide gels was visualized. Competition for 125I-OPN-OCN complexes was demonstrated when unlabeled OCN-enriched bone extract was included in gel overlay solutions. Also, gel overlay assays showed 125I-OCN binding to OPN. Saturable binding was shown in solid-phase filter binding assays, which yielded an equilibrium binding constant of moderately high affinity (approximately 10(-8) M). Specificity of OPN-OCN complex formation was confirmed by measuring binding in the presence of unlabeled OPN and OCN versus a bone-localized serum protein, alpha 2HS-glycoprotein. Finally, the formation of soluble complexes were demonstrated in a modified Hummel-Dreyer gel filtration assay. These results indicate that OPN and OCN form complexes in vitro. The possible functions of OPN-OCN complexes in osteoclast recruitment and attachment are discussed.

Dilatational band formation in bone

“…At the nanoscale, diffuse damage formation ensues from dilatational bands. On the basis of the results presented in this study, we hypothesize that dilatational bands result from the tensile loading of noncollageneous matrix proteins, in particular, osteocalcin and osteopontin, both of which are known to bind adjacent mineral aggregates through calcium ion-mediated complexes (18). In the absence of continuous tensile loading, the deformed protein complexes may "close" or completely recover the deformation to the initial unloaded state in a time-dependent manner (anelastic behavior).…”

“…We believe that these interactions result from sacrificial bonding involving mineral and collagen. The strength of the interaction between osteocalcin and osteopontin is of the order of 10 −8 (18). Assuming that all Ca 2+ -protein interactions are broken during the formation of a single dilatational band, we estimate a dissipation of over 5 eV per band.…”

“…OC and OPN are directly associated with mineral or mineral-collagen complexes (18,19). Osteocalcin has been shown to bind strongly to the bone mineral hydroxyapatite (20) and it complexes with and links to collagen through osteopontin (18).…”

Associations of Serum Osteocalcin and Polymorphisms of<b> </b>the<b><i> Osteocalcin</i></b> Gene with Bone Mineral Density in Postmenopausal and Elderly Chinese Women

“…Based on former studies, the precise role of osteocalcin in bone mineralization remains unclear. Other studies have led to the hypothesis that osteocalcin binds hydroxyapatite tightly and forms a complex with collagen through osteopontin, so it could bridge the matrix and mineral fractions of bone tissue [53]. Another bone-derived hormone, fibroblast growth factor 23 (FGF23), helps to maintain homeostasis in phosphorus and vitamin D metabolism [54] by directly acting on kidney and parathyroid.…”

Osteopontin Promotes Invasion, Migration and Epithelial-Mesenchymal Transition of Human Endometrial Carcinoma Cell HEC-1A Through AKT and ERK1/2 Signaling

“…This protein has a functional thrombin cleavage site and is a substrate for tissue transglutaminase [21]. OPN binds with type I collagen [23], fibronectin [24], and osteocalcin [25]. Several highly metastatic transformed cells synthesize higher level of OPN than the nontumorigenic cells [26].…”