OSTEOBLAST PROGRAMMED CELL-DEATH (APOPTOSIS) - MODULATION BY GROWTH-FACTORS AND CYTOKINES

Once osteoblasts have completed their bone-forming function, they are either entrapped in bone matrix and become osteocytes or remain on the surface as lining cells. Nonetheless, 50-70% of the osteoblasts initi ally present at the remodeling site cannot be accounted for after enum eration of lining cells and osteocytes, We hypothesized that the missi ng osteoblasts die by apoptosis and that growth factors and cytokines produced in the bone microenvironment influence this process. We repor t that murine osteoblastic MC3T3-E1 cells underwent apoptosis followin g removal of serum, or addition of tumor necrosis factor (TNF), as ind icated by terminal deoxynucleotidyl transferase-mediated dUTP-nick end labeling and DNA fragmentation studies, Transforming growth factor-be ta and interleukin-6 (IL-6)-type cytokines had antiapoptotic effects b ecause they were able to counteract the effect of serum starvation or TNF, In addition, anti-Pas antibody stimulated apoptosis of human oste oblastic MG-63 cells and IL-6-type cytokines prevented these changes, The induction of apoptosis in MG-63 cells was associated with an incre ase in the ratio of the proapoptotic protein bar to the antiapoptotic protein bcl-2, and oncostatin M prevented this change. Examination of undecalcified sections of murine cancellous bone revealed the presence of apoptotic cells, identified as osteoblasts by their proximity to o steoid seams and their juxtaposition to cuboidal osteoblasts, Assuming an osteoblast life span of 300 h and a prevalence of apoptosis of 0.6 %, we calculated that the fraction that undergo this process in vivo c an indeed account for the missing osteoblasts, These findings establis h that osteoblasts undergo apoptosis and strongly suggest that the pro cess can be modulated by growth factors and cytokines produced in the bone microenvironment.