FIBRONECTIN IS A SURVIVAL FACTOR FOR DIFFERENTIATED OSTEOBLASTS

The skeletal extracellular matrix produced by osteoblasts contains the glycoprotein fibronectin, which regulates the adhesion, differentiati on and function of various adherent cells. Interactions with fibronect in are required for osteoblast differentiation in vitro, since fibrone ctin antagonists added to cultures of immature fetal calvarial osteobl asts inhibit their progressive differentiation. To determine if fibron ectin plays a unique role in fully differentiated osteoblasts, culture s that had already formed mineralized nodules in vitro were treated wi th fibronectin antagonists. Fibronectin antibodies caused >95% of the cells in the mature cultures to display characteristic features of apo ptosis (nuclear condensation, apoptotic body formation, DNA laddering) within 24 hours. Cells appeared to acquire sensitivity to fibronectin antibody-induced apoptosis as a consequence of differentiation, since antibodies failed to kill immature cells and the first cells killed w ere those associated with mature nodules, Intact plasma fibronectin, a s well as fragments corresponding to the amino-terminal, cell-binding, and carboxy-terminal domains of fibronectin, independently induced ap optosis of mature (day-13), but not immature (day-4), osteoblasts. Fin ally, transforming growth factor-beta 1 partially protected cells from the apoptotic effects of fibronectin antagonists. Thus, in the course of maturation cultured osteoblasts switch from depending on fibronect in for differentiation to depending on fibronectin for survival. These data suggest that fibronectin, together with transforming growth fact or-beta 1, may affect bone formation, in part by regulating the surviv al of osteoblasts.