Bone mineralization and osteoblast differentiation are negatively modulated by integrin alpha(v)beta3.
Numerous bone matrix proteins can interact with alpha(v)-containing integrins including alpha(v)beta3. To elucidate the net effects of the interaction between these proteins and alpha(v)beta3 on osteoblast function, we developed a murine osteoblastic cell line that overexpressed human alpha(v)beta3. Human alpha(v)beta3-integrin was expressed on cell membrane, in which its presence did not alter the surface level of endogenous mouse alpha(v)beta3. The expressed human alpha(v)beta3 was functional because cell adhesion to osteopontin was increased and this increment was abolished by antibody against human alpha(v)beta3. The proliferation rate of cells overexpressing alpha(v)beta3 (alpha(v)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 alpha(v)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 were enhanced whereas c-jun N-terminal kinase (JNK) activity was decreased in alpha(v)beta3-cells. The activity of p38 and the levels of focal adhesion kinase (FAK) and vinculin were not altered. Moreover, the adhesions of alpha(v)beta3-cells to type I collagen and fibronectin were inhibited, which was attributed to decreased beta1-integrin levels on cell surface. In conclusion, overexpressing alpha(v)beta3-integrin in osteoblasts stimulated cell proliferation but retarded differentiation, which were derived via altered integrin-matrix interactions, signal transduction, and matrix protein expression.
Cheng
S L
SL
Department of Internal Medicine, Washington University School of Medicine, Barnes-Jewish Hospital, St. Louis, Missouri 63110, USA.
Lai
C F
CF
Blystone
S D
SD
Avioli
L V
LV
D000469:Alkaline Phosphatase / Q000378:metabolism; D000818:Animals; D001483:Base Sequence; D015153:Blotting, Western; D002113:Calcification, Physiologic; D002448:Cell Adhesion; D002454:Cell Differentiation; D017931:DNA Primers; D006801:Humans; D051379:Mice; D010006:Osteoblasts / Q000166:cytology*; D010641:Phenotype; D019030:Receptors, Vitronectin / Q000235:genetics / Q000502:physiology*; D014162:Transfection
D017931:DNA Primers; D019030:Receptors, Vitronectin; D000469:Alkaline Phosphatase