Bone Abstracts

Cholestatic liver disease is often associated with increased fracture risk. We had found that circulating levels of a fibronectin isoform called oncofetal fibronectin (oFN) were elevated in patients with primary biliary cirrhosis. Indeed, injecting oFN to mice suppressed osteoblast differentiation and bone mineral density in vivo, suggesting it is responsible for bone loss in these patients. The aim of this study was to define the mechanism by which oFN affects osteoblast function and evaluate possible modifiers in experimental hepatic osteodystrophy.

The fibronectin isoform oFN is characterized by the presence of O- and N-glycosylations. Enzymatic O-deglycosylation of this isoform prevented the inhibition of osteoblast function usually seen with oFN in vitro. Either introduction of a mutation at AA 33 of the variable region, or binding of this glycosylated site with a specific antibody normalized osteoblast differentiation. The responsible site binds to α4β1 integrin. Indeed, this integrin α4β1 mediates the inhibitory effect of oFN both in vitro as well as in vivo. In a hepatic osteodystrophy mouse model, we demonstrate that liver fibrosis is associated with increased circulating oFN and diminished BMD. In addition, trabecular bone loss induced by oFN injection or fibrosis induction was counteracted by either administering an antibody that binds to α4 integrin (PS/2), or the CS1 peptide, which contains a binding site for α4β1 integrin.

In summary, oFN inhibits osteoblast activity through an O-glycosylation in the variable region that results in decreased integrin-mediated signaling. This deleterious effect is prevented by binding α4β1 integrin. Thus, we have characterized a molecule and the receptor mediating bone loss in patients with hepatic osteodystrophy associated with increased oFN, and evaluated possible therapeutic interventions in a murine model.