Bone mineralization and osteoblast differentiation are negatively modulated by integrin αvβ3

Numerous bone matrix proteins can interact with alphav-containing integrins including alphav beta3. To elucidate the net effects of the interaction between these proteins and alphav beta3 on osteoblast function, we developed a murine osteoblastic cell line that overexpressed human alphav beta3. Human alphav beta3-integrin was expressed on cell membrane, in which its presence did not alter the surface level of endogenous mouse alphav beta3. The expressed human alphav beta3 was functional because cell adhesion to osteopontin was increased and this increment was abolished by antibody against human alphav beta3. The proliferation rate of cells overexpressing alphav beta3 (alphav beta3-cells) was increased whereas matrix mineralization was decreased. To elucidate the mechanisms leading to inhibition of matrix mineralization, the expression of proteins important for mineralization was analyzed. Alkaline phosphatase activity and the expression of osteocalcin, type I collagen, and bone sialoprotein (BSP) were decreased whereas osteopontin was stimulated in alphav beta3-cells. The regulation of osteopontin, osteocalcin, and BSP expression was mediated via transcriptional mechanism because their promoter activities were altered. Examination of molecules involved in integrin signaling indicated that activator protein-1 (AP-1) and extracellular signal-regulated kinase (Erk) activities mere enhanced whereas c-jun N-terminal kinase (JNK) activity was decreased in alphav beta3-cells. The activity of p38 and the levels of focal adhesion kinase (FAK) and vinculin were not altered, Moreover, the adhesions of alphav beta3-cells to type I collagen and fibronectin were inhibited, which was attributed to decreased beta1-integrin levels on cell surface. In conclusion, overexpressing alphav beta3-integrin in osteoblasts stimulated cell proliferation but retarded differentiation, which mere derived via altered integrin-matrix interactions, signal transduction, and matrix protein expression.