Fibronectin is a survival factor for differentiated osteoblasts

TLDR

The osteoblast‑derived extracellular matrix contains fibronectin, which is essential for adhesion, differentiation, and function of adherent cells, and its interaction is required for osteoblast differentiation in vitro. The study aimed to test whether fibronectin is required for survival of fully differentiated osteoblasts by treating mature cultures with fibronectin antagonists. Fibronectin antagonists triggered apoptosis in over 95 % of mature osteoblasts within 24 h, while immature cells were resistant; intact fibronectin and its domains induced apoptosis only in mature cells, and TGF‑β1 partially protected against this effect, indicating that osteoblasts shift from fibronectin‑dependent differentiation to fibronectin‑dependent survival during maturation.

Abstract

ABSTRACT The skeletal extracellular matrix produced by osteoblasts contains the glycoprotein fibronectin, which regulates the adhesion, differentiation and function of various adherent cells. Interactions with fibronectin are required for osteoblast differentiation in vitro, since fibronectin antagonists added to cultures of immature fetal calvarial osteoblasts inhibit their progressive differentiation. To determine if fibronectin plays a unique role in fully differentiated osteoblasts, cultures that had already formed mineralized nodules in vitro were treated with fibronectin antagonists. Fibronectin antibodies caused >95% of the cells in the mature cultures to display characteristic features of apoptosis (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 were those associated with mature nodules. Intact plasma fibronectin, as well as fragments corresponding to the amino-terminal, cell-binding, and carboxy-terminal domains of fibronectin, independently induced apoptosis of mature (day-13), but not immature (day-4), osteoblasts. Finally, transforming growth factor-β1 partially protected cells from the apoptotic effects of fibronectin antagonists. Thus, in the course of maturation cultured osteoblasts switch from depending on fibronectin for differentiation to depending on fibronectin for survival. These data suggest that fibronectin, together with transforming growth factor-β1, may affect bone formation, in part by regulating the survival of osteoblasts.