Bone mineralization and osteoblast differentiation are negatively modulated by integrin alpha v beta 3

Numerous bone matrix proteins can interact with alphav-containing integrins including alphav beta3. To elucidate the net effects of the interaction be tween these proteins and alphav beta3 on osteoblast function, we developed a murine osteoblastic cell line that overexpressed human alphav beta3. Huma n alphav beta3-integrin was expressed on cell membrane, in which its presen ce did not alter the surface level of endogenous mouse alphav beta3. The ex pressed human alphav beta3 was functional because cell adhesion to osteopon tin was increased and this increment was abolished by antibody against huma n alphav beta3. The proliferation rate of cells overexpressing alphav beta3 (alphav beta3-cells) was increased whereas matrix mineralization was decre ased. To elucidate the mechanisms leading to inhibition of matrix mineraliz ation, the expression of proteins important for mineralization was analyzed . Alkaline phosphatase activity and the expression of osteocalcin, type I c ollagen, and bone sialoprotein (BSP) were decreased whereas osteopontin was stimulated in alphav beta3-cells. The regulation of osteopontin, osteocalc in, and BSP expression was mediated via transcriptional mechanism because t heir promoter activities were altered. Examination of molecules involved in integrin signaling indicated that activator protein-1 (AP-1) and extracell ular signal-regulated kinase (Erk) activities mere enhanced whereas c-jun N -terminal kinase (JNK) activity was decreased in alphav beta3-cells. The ac tivity of p38 and the levels of focal adhesion kinase (FAK) and vinculin we re not altered, Moreover, the adhesions of alphav beta3-cells to type I col lagen and fibronectin were inhibited, which was attributed to decreased bet a1-integrin levels on cell surface. In conclusion, overexpressing alphav be ta3-integrin in osteoblasts stimulated cell proliferation but retarded diff erentiation, which mere derived via altered integrin-matrix interactions, s ignal transduction, and matrix protein expression.